Download Hi-Res ImageDownload to MS-PowerPointCite This:J. Phys. Chem. C 2023, 127, 9, 4582-4593

C: Spectroscopy and Dynamics of Nano, Hybrid, and Low-Dimensional MaterialsFebruary 28, 2023
C：纳米、混合和低维材料的光谱学和动力学2月 28， 2023

Understanding Excited-State Relaxation in 1,3-Bis(N-carbazolyl)benzene, a Host Material for Organic Light-Emitting Diodes
了解 1,3-双（N-咔唑基）苯（有机发光二极管的主体材料）中的激发态弛豫
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Marius Morgenroth 马里乌斯·摩根罗斯
Marius Morgenroth
Faculty IV: School of Science and Technology, Department Chemistry and Biology, Physical Chemistry 2, University of Siegen, Adolf-Reichwein-Str. 2, 57076 Siegen, Germany
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https://orcid.org/0000-0001-8528-3502
Thomas Lenzer* 托马斯·伦策*
Thomas Lenzer
Faculty IV: School of Science and Technology, Department Chemistry and Biology, Physical Chemistry 2, University of Siegen, Adolf-Reichwein-Str. 2, 57076 Siegen, Germany
*Email: lenzer@chemie.uni-siegen.de
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https://orcid.org/0000-0002-0766-709X
Kawon Oum*
Kawon Oum
Faculty IV: School of Science and Technology, Department Chemistry and Biology, Physical Chemistry 2, University of Siegen, Adolf-Reichwein-Str. 2, 57076 Siegen, Germany
*Email: oum@chemie.uni-siegen.de
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https://orcid.org/0000-0001-6137-2236

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The Journal of Physical Chemistry C

Cite this: J. Phys. Chem. C 2023, 127, 9, 4582–4593

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https://doi.org/10.1021/acs.jpcc.2c07440

Published February 28, 2023

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Abstract 抽象

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We present a detailed spectroscopic study of 1,3-bis(N-carbazolyl)benzene (mCP), a prototypical host material for blue emitters in organic light-emitting diodes, using time-resolved emission and ultrafast transient absorption spectroscopy. Upon photoexcitation, mCP dissolved in tetrahydrofuran and mCP dispersed at low number density in poly(methyl methacrylate) (PMMA) films show a monoexponential emission decay in the range 6–7 ns. Transient absorption experiments detect the formation of the T₁ triplet state from S₁ with a quantum yield of ca. 20%, a band maximum at 450 nm, and a lifetime on the microsecond time scale. The strong spectral overlap of S₁ stimulated emission and S₁ excited state absorption suggests that S₁–S₁ singlet–singlet annihilation (SSA) based on Förster resonance energy transfer (FRET) should be feasible at high mCP concentrations in mCP:PMMA and neat mCP films. At these concentrations, the intensity of the mCP emission is strongly quenched and the spectra show a dramatic change exhibiting monomer, aggregate, and excimer emission bands. The small Stokes shift and thus the good overlap of the absorption and emission spectra of mCP open up the possibility for efficient diffusive S₁ singlet hopping in neat mCP films by means of a multi-step homo-FRET mechanism involving S₁ and the respective S₀ nearest neighbor. Fluence-dependent transient absorption measurements find pronounced S₁–S₁ SSA. Kinetic modeling suggests a bimolecular diffusive SSA mechanism with a rate constant k_diff of 1.40 × 10^–8 cm³ s^–1. In contrast, modeling based on direct S₁–S₁ FRET results in an unrealistically large Förster radius of 17 nm. These experiments therefore suggest that mCP molecules in S₁ efficiently diffuse through the mCP film by multi-step homo-FRET and annihilate upon approaching a critical distance to another mCP molecule in S₁. Finally, we find clear spectral evidence for vibrationally hot mCP molecules (S₀^*) in the transient absorption spectra of neat mCP thin films, which cool down on a time scale from nanoseconds to microseconds.
我们使用时间分辨发射和超快瞬态吸收光谱对 1,3-双（N-咔唑基）苯（mCP）进行了详细的光谱研究，1,3-双（N-咔唑基）苯（mCP）是有机发光二极管中蓝色发射器的原型主体材料。光激发后，溶解在四氢呋喃中的 mCP 和以低数密度分散在聚甲基丙烯酸甲酯（PMMA）薄膜中的 mCP 在 6-7 ns 范围内表现出单指数发射衰减。瞬态吸收实验检测到 S₁ 形成 T₁ 三重态，量子产率约为 20%，最段为 450 nm，寿命为微秒级。S₁ 受激发射和 S₁ 激发态吸收的强光谱重叠表明，基于 Förster 共振能量转移（FRET）的 S_1-S ₁ 单线态-单线态湮没（SSA）在 mCP：PMMA 和纯 mCP 薄膜中的高 mCP 浓度下应该是可行的。在这些浓度下，mCP 发射的强度被强烈猝灭，光谱显示出单体、聚集体和准分子发射带的巨大变化。小的斯托克斯位移以及 mCP 的吸收光谱和发射光谱的良好重叠，通过涉及 S₁ 和相应的 S₀ 最近邻的多级同级 FRET 机制，为在整洁的 mCP 薄膜中实现高效扩散的 S₁ 单线态跳跃提供了可能性。通量依赖性瞬态吸收测量发现明显的 S₁–S₁ SSA。动力学模型表明双分子扩散 SSA 机制，速率常数 k_diff 为 1.40 × ^10-8 cm³ ^s-1。相比之下，基于直接 S₁–S₁ FRET 的建模会产生 17 nm 的不切实际的大 Förster 半径。因此，这些实验表明，S₁ 中的 mCP 分子通过多步同 FRET 有效地扩散穿过 mCP 膜，并在接近与 S₁ 中另一个 mCP 分子的临界距离时湮灭。最后，我们在纯 mCP 薄膜的瞬态吸收光谱中找到了振动热 mCP 分子（S₀^*）的明确光谱证据，这些薄膜在纳秒到微秒的时间尺度上冷却。

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Subjects

1. Introduction 1. 引言

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Carbazole derivatives, such as 1,3-bis(N-carbazolyl)benzene (shortly mCP, Scheme 1), are widely used as host materials for organic light-emitting diodes (OLEDs) because of their beneficial photophysical and electrical properties. (1−6) They have high-lying S₁ singlet and T₁ triplet states, which is particularly important for realizing efficient blue-emitting OLEDs. (7−9) This way, triplet excited states of the light-emitting guest chromophores cannot be quenched by triplet energy transfer to the host molecules. In addition, by using an appropriate doping level of the guest, excited-state annihilation processes can be minimized as the emitters are separated far enough inside the host matrix so that processes such as singlet–singlet annihilation (SSA) and triplet–triplet annihilation (TTA) between the emitter molecules can be avoided. In addition, the host ensures appropriate transport of charges by providing low energy barriers for the injection of electrons and holes and, very importantly, efficient energy transfer to the emitter. (8,10−12)
咔唑衍生物，如 1,3-双（N-咔唑基）苯（简称 mCP，方案 1），因其有益的光物理和电学特性而被广泛用作有机发光二极管（OLED）的主体材料。（1−6）它们具有高位 S₁ 单重态和 T₁ 三重态，这对于实现高效的蓝光发射 OLED 尤为重要。（7−9）这样，发光客体发色团的三重态激发态不能被三重态能量转移到宿主分子来淬灭。此外，通过使用适当的客体掺杂水平，可以最大限度地减少激发态湮没过程，因为发射极在主基质内分离得足够远，从而可以避免发射极分子之间的单重态-单重态湮灭（SSA）和三重态-三重态湮灭（TTA）等过程。此外，主机通过为电子和空穴的注入提供低能量屏障来确保电荷的适当传输，并且非常重要的是，可以有效地将能量转移到发射器。（8,10−12）

Scheme 1. Chemical Structure of mCP
方案 1.mCP 的化学结构

Despite their importance for OLED applications, the photophysics and ultrafast spectroscopy of these host materials are severely underexplored and have not kept pace with the enormous developments in the field. Previous studies mostly focused on steady-state absorption and emission spectroscopy at different temperatures as well as time-resolved fluorescence with a time resolution in the several tens of picoseconds range. Even for the steady-state spectroscopy results, there are often conflicting reports: for instance, in the case of mCP thin films, Forrest and co-workers did not observe any emission of the T₁ triplet state of mCP at room temperature and only detected very weak structured emission of T₁ with vibronic peaks at 430 and 460 nm at a very low temperature of 10 K using gated detection starting from 1 μs. (13) In contrast, Wu et al. and Tsuboi et al. assigned a weak band with peaks at 410 and 440 nm in the long-wavelength tail of the emission spectrum to phosphorescence of the T₁ state. (1,2) More recently, Kim and co-workers reported the 0–0 peak of the T₁ emission for a mCP thin film at 455 nm. (11) As the energetic location of the triplet state is relevant to OLED operation, such contradictory findings should be clarified. The detailed dynamic photophysical processes of these compounds after photoexcitation and their time scales are largely unknown as well, especially in thin films, although these are important properties for their application in OLED systems. For instance, SSA and TTA processes between mCP molecules will compete with the desired energy transfer to the guest chromophore intended to be used as a blue emitter. Understanding the time scale of the SSA processes is also highly relevant to modeling efficiency roll-off effects in OLEDs, i.e., a decrease in efficiency with increasing current density, when operating at high brightness levels. (14) Here, we therefore provide a detailed steady-state and time-resolved spectroscopic study of mCP in the solvent tetrahydrofuran (THF), in neat mCP thin films and in thin film blends consisting of mCP and poly(methyl methacrylate) (PMMA) with a wide variation of the mCP:PMMA ratio. We employ time-correlated single-photon counting (TCSPC) and broadband UV–vis–NIR transient absorption experiments to elucidate the complex relaxation processes of mCP in the excited state after photoexcitation, for instance, to determine the complete T₁ absorption spectrum and the time scale of the SSA processes.
尽管它们对 OLED 应用很重要，但这些主体材料的光物理学和超快光谱学尚未得到充分开发，并且没有跟上该领域的巨大发展。以前的研究主要集中在不同温度下的稳态吸收和发射光谱以及时间分辨率在几十皮秒范围内的时间分辨荧光。即使对于稳态光谱结果，也经常存在相互矛盾的报告：例如，在 mCP 薄膜的情况下，Forrest 及其同事在室温下没有观察到 mCP 的 T₁ 三重态的任何发射，并且仅使用从 1 μs 开始的门控检测，在 10 K 的极低温度下检测到 T₁ 的非常弱的结构发射，在 430 和 460 nm 处具有振动峰。（13）相比之下，Wu 等人和 Tsuboi 等人将发射光谱长波长尾部 410 和 440 nm 处峰值的弱带分配给 T₁ 态的磷光。（1,2）最近，Kim 及其同事报道了 mCP 薄膜在 455 nm 处的 T₁ 发射峰为 0-0。（11）由于三重态的能量位置与 OLED 操作有关，因此应澄清这些相互矛盾的发现。这些化合物在光激发后的详细动态光物理过程及其时间尺度在很大程度上也是未知的，尤其是在薄膜中，尽管这些是它们在 OLED 系统中应用的重要特性。例如，mCP 分子之间的 SSA 和 TTA 过程将与所需的能量转移到旨在用作蓝色发射器的客体发色团竞争。了解 SSA 工艺的时间尺度也与模拟 OLED 中的效率滚降效应高度相关，即在高亮度水平下运行时，效率会随着电流密度的增加而降低。（14）因此，在这里，我们提供了溶剂四氢呋喃（THF）、纯 mCP 薄膜以及由 mCP 和聚甲基丙烯酸甲酯组成的薄膜混合物中 mCP 的详细稳态和时间分辨光谱研究，mCP：PMMA 比率变化很大。我们采用时间相关单光子计数（TCSPC）和宽带 UV-vis-NIR 瞬态吸收实验来阐明光激发后激发态 mCP 的复杂弛豫过程，例如，确定完整的 T₁ 吸收光谱和 SSA 过程的时间尺度。

2. Experimental Section 2. 实验部分

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2.1. Sample Preparation 2.1. 样品制备

For steady-state and time-resolved absorption as well as fluorescence measurements, mCP (Lumtec, ≥99.5%) was dissolved in THF (Merck, Uvasol, ≥99.9%) at appropriate concentrations. The solutions were saturated with nitrogen by bubbling them for 15 min. Thin films of neat mCP and mCP:PMMA blends were deposited on fused silica substrates (Tempotec, RW25-25-1UV, JGS1), which were thoroughly cleaned using acetonitrile (Merck, Uvasol, ≥99.9%) and then subjected to ozone treatment for 15 min in a UV-C chamber (Dinies ELG100S) prior to use.
对于稳态和时间分辨吸收以及荧光测量，将 mCP （Lumtec， ≥99.5%）溶于适当浓度的 THF （Merck， Uvasol， ≥99.9%）中。通过鼓泡 15 分钟，用氮气使溶液饱和。将纯 mCP 和 mCP：PMMA 混合物的薄膜沉积在熔融石英基材（Tempotec、RW25-25-1UV、JGS1）上，使用乙腈（Merck、Uvasol≥99.9%）彻底清洁，然后在使用前在 UV-C 室（Dinies ELG100S）中进行臭氧处理 15 分钟。

PMMA (Alfa Aesar) was dissolved in THF (5 mg mL^–1) in a tube shaker for 5 h, and mCP was dissolved in THF (5 mg mL^–1) and treated in an ultrasonic bath for 30 min. The solutions were mixed in appropriate amounts to match the desired mCP:PMMA ratio. The precursor solutions (typically 200 μL per sample) were dropped onto the fused silica substrates using glass pipettes. The spin-coating conditions were 60 s at 500 rpm, and thicker blends, required for optical measurements with low mCP content, were prepared with 30 s loading time. The coated slides were then dried for 10 min at 23 °C under a nitrogen atmosphere to evaporate the solvent.
将 PMMA （Alfa Aesar）溶于 THF （5 mg mL^–1）中，在试管振荡器中溶解 5 h，将 mCP 溶于 THF （5 mg mL^–1）中，并在超声浴中处理 30 min。将溶液以适量混合，以匹配所需的 mCP：PMMA 比率。使用玻璃移液器将前驱体溶液（通常每个样品 200 μL）滴到熔融石英基底上。旋涂条件为 60 s，转速为 500 rpm，低 mCP 含量光学测量所需的较稠混合物，加载时间为 30 s。然后将包被的载玻片在 23 °C 下在氮气气氛下干燥 10 分钟以蒸发溶剂。

2.2. Steady-State Absorption and Fluorescence Experiments
2.2. 稳态吸收和荧光实验

Steady-state absorption spectra of mCP dissolved in THF, neat mCP thin films, and the mCP:PMMA blends were recorded using a Varian Cary 5000 spectrophotometer with a slit width of 0.5 nm. Emission spectra were measured using an Agilent Cary Eclipse spectrometer with excitation and emission slit widths of 5 nm. The emission spectra were corrected by a calibration function considering the wavelength-dependent sensitivity of the instrument.
使用狭缝宽度为 0.5 nm 的 Varian Cary 5000 分光光度计记录溶解在 THF 中的 mCP、纯 mCP 薄膜和 mCP：PMMA 混合物的稳态吸收光谱。使用激发和发射狭缝宽度为 5 nm 的 Agilent Cary Eclipse 光谱仪测量发射光谱。考虑到仪器的波长依赖性灵敏度，通过校准函数对发射光谱进行校正。

2.3. Ultrafast Transient Absorption Spectroscopy
2.3. 超快瞬态吸收光谱

Ultrafast broadband transient absorption spectroscopy was carried out on two setups based on an amplified titanium:sapphire laser system (Coherent Libra USP-HE), which covers the UV–vis (260–700 nm) and NIR spectral ranges (850–1630 nm) with a time-resolution of ca. 80 fs. (15,16) mCP dissolved in THF, neat mCP thin films, and the mCP:PMMA blends were excited at 290 nm by an optical parametric amplifier (Coherent OPerA Solo). For pump–probe experiments up to several microseconds, the samples were excited by the fourth harmonic of a Q-switched Nd:YAG microlaser (Standa-Q1TH, 266 nm), which was electronically synchronized with the amplifier system. (17) The time resolution of these measurements was 420 ps. The fluence of the pump laser beam was adjusted by a combination of a half-wave plate and a polarizer with the help of a calibrated photodiode (Thorlabs S120VC) and a beam profiler (Visulux). The measurements for mCP in THF were performed in quartz cuvettes (Hellma, Suprasil I) with a path length of 1 mm, and the thin films of neat mCP and mCP:PMMA deposited on the fused silica substrates were mounted inside a custom-made nitrogen-flushed aluminum holder. (18) The cuvette and thin-film holder were constantly moved by an x–y piezo stage, sampling an area of 2 × 2 mm².
超快宽带瞬态吸收光谱基于放大钛：蓝宝石激光系统（Coherent Libra USP-HE）在两种设置上进行，该系统覆盖紫外-可见光谱范围（260-700 nm）和 NIR 光谱范围（850-1630 nm），时间分辨率约为 80 fs。（15,16） mCP 溶解在 THF 中，纯 mCP 薄膜和 mCP：PMMA 混合物在 290 nm 处被光学参量放大器（Coherent OPerA Solo）激发。对于长达几微秒的泵浦探针实验，样品由 Q 开关 Nd：YAG 微激光器（Standa-Q1TH， 266 nm）的四次谐波激发，该激光器与放大器系统电子同步。（17）这些测量的时间分辨率为 420 ps。在校准的光电二极管（Thorlabs S120VC）和光束轮廓仪（Visulux）的帮助下，通过半波片和偏振器的组合来调节泵浦激光束的通量。在光程为 1 mm 的石英比色皿（Hellma，Suprasil I）中测量 THF 中的 mCP，并将沉积在熔融石英基底上的纯 mCP 和 mCP：PMMA 薄膜安装在定制的氮气冲洗铝支架内。（18）比色皿和薄膜支架由 x-y 压电载物台不断移动，采样面积为 2 × 2 mm²。

2.4. Time-Resolved Fluorescence Experiments
2.4. 时间分辨荧光实验

The setup for TCSPC has been described in detail previously. (19) Briefly, the samples were excited by a pulsed UV-LED (Becker & Hickl, UVL-FB-270, 273 nm, 20 or 1 MHz repetition frequency). The output of the LED was vertically polarized (0°) by means of a wire-grid polarizer (Thorlabs WP25M-UB), and the emission was measured at an angle of 90° employing another wire-grid polarizer (Thorlabs WP25M-UB, set at the magic angle of 54.7°) and appropriate bandpass filters (Thorlabs, FWHM 10 nm). Photons were detected by a hybrid multialkali photodetector (Becker & Hickl, HPM-100-07) connected to a TCSPC module (Becker & Hickl, SPC-130IN) operating in a reversed start–stop configuration. The instrument response function (IRF) was determined from the LED scattering signal either using a diluted suspension of colloidal silica nanoparticles (Merck, Ludox AM-40) or a UV-grade fused silica diffuser (Thorlabs DGUV10-120 or DGUV10-1500). The time resolution was limited by the LED pulse width of 500 ps. The time constants and amplitudes of the emission signals were obtained from reconvolution using the FAST program (Edinburgh Instruments).
TCSPC 的设置之前已详细介绍过。（19）简而言之，样品由脉冲UV-LED（Becker & Hickl，UVL-FB-270,273 nm，20或1 MHz重复频率）激发。LED 的输出通过线栅偏振器（Thorlabs WP0M-UB）垂直极化（25°），并使用另一个线栅偏振器（Thorlabs WP25M-UB，设置为魔角 54.7°）和适当的带通滤光片（Thorlabs，FWHM 10 nm）以 90° 的角度测量发射。光子由混合多碱光电探测器（Becker & Hickl，HPM-100-07）检测到，该探测器连接到以反向启停配置运行的TCSPC模块（Becker & Hickl，SPC-130IN）。仪器响应函数（IRF）由稀释的胶体二氧化硅纳米颗粒悬浮液（Merck、Ludox AM-40）或 UV 级熔融石英扩散器（Thorlabs DGUV10-120 或 DGUV10-1500）根据 LED 散射信号确定。时间分辨率受 500 ps 的 LED 脉冲宽度限制。发射信号的时间常数和幅度是使用 FAST 程序（Edinburgh Instruments）通过重新卷积获得的。

2.5. Atomic Force Microscopy
2.5. 原子力显微镜

The thickness of the mCP thin films was recorded using an atomic force microscope (PSIA XE-100) in the non-contact mode using a cantilever with a silicon tip. The resolution was 1024 times 64 pixel for an area of 50 × 3.12 μm², with a scan rate of 0.2 Hz. The atomic force microscopy (AFM) traces were evaluated using XEI (Version 4.3.4, Park Systems), and visualization was carried out using OriginPro2022b (OriginLab Corporation). After performing a background correction, the scan lines were averaged and the thickness was determined from the difference between the lower and upper plateaus.
使用原子力显微镜（PSIA XE-100）在非接触式模式下使用带有硅尖端的悬臂记录 mCP 薄膜的厚度。分辨率为 1024 x 64 像素，面积为 50 × 3.12 μm²，扫描速率为 0.2 Hz。使用 XEI（4.3.4 版，Park Systems）评估原子力显微镜（AFM）轨迹，并使用 OriginPro2022b （OriginLab Corporation）进行可视化。进行背景校正后，对扫描线进行平均，并根据上下平台之间的差异确定厚度。

3. Results and Discussion
3. 结果与讨论

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3.1. Steady-State Absorption and Emission of mCP in THF and Thin Films
3.1. THF 和薄膜中 mCP 的稳态吸收和发射

Figure 1a shows steady-state absorption and fluorescence spectra of mCP in different solvation environments as dotted lines and solid lines with shaded areas, respectively. Characteristic spectral properties are summarized in Table S1 of the Supporting Information. In THF (violet), the lowest-energy absorption peak of the S₀ → S₁ absorption band of mCP is located at 338 nm. The mirror-image-like structured fluorescence spectrum has a very small Stokes shift (52 meV, 418 cm^–1), which suggests that only small geometry changes occur upon photoexcitation to the S₁ state. The absorption and emission spectra of the PMMA film with the smallest mCP fraction (mCP:PMMA 2:98, green) look similar to those in THF. They are red-shifted by ca. 1 nm, exhibit a comparable but somewhat smaller Stokes shift (44 meV, 355 cm^–1), and have a slightly increased intensity in the long-wavelength tail of the emission. A further increase of the mCP content leads to a further shift of the spectra (mCP:PMMA ratio 10:90, blue). Importantly, this is accompanied by a distinct relative increase in intensity of the long-wavelength tail of the fluorescence compared with the main emission band. At a high mCP content (mCP:PMMA 90:10, red) and for the neat mCP thin film (gray), the emission spectra change drastically, while the corresponding absorption spectra only show a slight red shift.
图 1a 显示了 mCP 在不同溶剂化环境中的稳态吸收和荧光光谱，分别显示为虚线和带有阴影区域的实线。特征光谱特性总结在支持信息的表 S1 中。在 THF（紫色）中，mCP 的 S₀ → S₁ 吸收带的最低能量吸收峰位于 338 nm。镜像状结构荧光光谱具有非常小的斯托克斯位移（52 meV，418 cm^–1），这表明在光激发到 S₁ 状态时，仅会发生微小的几何变化。具有最小 mCP 分数的 PMMA 薄膜（mCP：PMMA 2：98，绿色）的吸收和发射光谱看起来与 THF 中的光谱相似。它们的红移约为 1 nm，表现出相当但稍小的斯托克斯位移（44 meV， 355 cm^–1），并且在发射的长波长尾部的强度略有增加。mCP 含量的进一步增加导致光谱的进一步偏移（mCP：PMMA 比率 10：90，蓝色）。重要的是，与主发射带相比，这伴随着荧光长波长尾部强度的明显相对增加。在高 mCP 含量（mCP：PMMA 90：10，红色）和纯 mCP 薄膜（灰色）下，发射光谱发生巨大变化，而相应的吸收光谱仅显示轻微的红移。

Figure 1. (a) Normalized steady-state absorption and fluorescence spectra (λ_exc = 290 nm), shown as dotted lines and solid lines with shading, respectively, for mCP dissolved in THF, mCP:PMMA blends with weight ratios of 2:98, 10:90, and 90:10, and a neat mCP thin film (from top to bottom), (b) thin-film spectra with the same color coding as that in panel (a), but compared on an absolute scale (absorption spectra shown down to 200 nm), and (c) superimposed normalized steady-state fluorescence spectra from panel (a) [the same color coding as that in panels (a,b)] including colored areas indicating the wavelength ranges of the bandpass filters employed for the TCSPC experiments: center wavelengths of (i) 370, (ii) 430, and (iii) 560 nm (FWHM in each case is 10 nm). (d) TCSPC data (symbols) recorded at 1 MHz repetition frequency with excitation at 273 nm for mCP in THF, the three different mCP:PMMA blends, and the neat mCP film obtained using the 370 nm bandpass filter [the same color coding as that in panels (a–c)]. Fit lines obtained from reconvolution using mono- or biexponential fit functions. (e) TCSPC data recorded at 1 MHz for the neat mCP film (symbols) and the kinetic fits (lines) are compared for the spectral regions (i) 370 nm (brown range), (ii) 430 nm (sky blue range), and (iii) 560 nm (orange range). (f) Same as in panel (e), but obtained with the 560 nm bandpass filter. In panels (d–f), the IRF is shown as black crosses.
图 1.（a）溶解在 THF 中的 mCP、重量比为 2：98、10：90 和 90：10 的 mCP：PMMA 混合物以及整洁的 mCP 薄膜（从上到下）的归一化稳态吸收和荧光光谱（λ_exc = 290 nm），分别显示为虚线和带阴影的实线，但是在绝对尺度上进行比较（吸收光谱显示低至 200 nm），并且（c）叠加来自图（a）的归一化稳态荧光光谱 [与图（a，b）中的颜色编码相同]，包括表示用于 TCSPC 实验的带通滤光片波长范围的彩色区域：中心波长为（i） 370，（ii） 430，（iii） 560 nm（每种情况下的 FWHM 为 10 nm）。（d）在 1 MHz 重复频率下记录的 TCSPC 数据（符号），在 273 nm 激发下，用于 THF 中的 mCP、三种不同的 mCP：PMMA 混合物以及使用 370 nm 带通滤光片获得的纯 mCP 薄膜 [与图（a-c）中的颜色编码相同]。使用单指数或双指数拟合函数从重新卷积中获得的拟合线。（e）比较光谱区域（i） 370 nm（棕色范围）、（ii） 430 nm（天蓝色范围）和（iii） 560 nm（橙色范围）在 1 MHz 下记录的纯 mCP 薄膜（符号）和动力学拟合（线）的动力学拟合（线）。（f）与图（e）相同，但使用 560 nm 带通滤光片获得。在面板（d-f）中，IRF 显示为黑色叉号。

The emission spectra of the mCP-rich films can be roughly divided into at least three regions: The first one is an emission band in the range 340–380 nm with two distinct peaks, which is also observed for mCP in THF and is therefore present regardless of the fact, if the mCP molecules are isolated in solution or aggregated in films. We therefore assign this feature to mCP monomer emission. Only for the mCP:PMMA film with a high mCP amount (90:10) and the neat mCP film, there is a second structured emission band in the range 380–450 nm with two peaks at 402 and 425 nm, and an additional third band above 450 nm, which is broad and unstructured with a peak at about 515 nm.
富含 mCP 的薄膜的发射光谱可以大致分为至少三个区域：第一个区域是 340-380 nm 范围内的发射带，具有两个明显的峰，在 THF 中也可以观察到 mCP，因此无论事实如何，如果 mCP 分子在溶液中分离或在薄膜中聚集，它都存在。因此，我们将这一特征分配给 mCP 单体发射。仅对于具有高 mCP 量（90：10）的 mCP：PMMA 薄膜和纯 mCP 薄膜，在 380-450 nm 范围内有第二个结构化发射带，在 402 nm 和 425 nm 处有两个峰，在 450 nm 以上还有一个额外的第三个发射带，该发射带很宽且非结构化，峰值约为 515 nm。

The third, most red-shifted band can be safely assigned to mCP excimer emission as it resembles the excimer bands of carbazole-containing cellulose derivatives (20) and carbazolophanes. (21) In contrast, the structured emission of the second band is incompatible with an excimer as the latter has a repulsive ground state and thus shows a broad emission. Instead, such bands were previously assigned to triplet emission. (1,2) Nevertheless, the average lifetime ⟨τ⟩ of 7.2 ns for the emission band of the neat mCP thin film, which we obtained from the time-resolved fluorescence experiments reported below, is incompatible with a triplet state either. Also, the fluorescence excitation spectra detected in this band (λ_det = 400 nm) and in the long-wavelength band (λ_det = 530 nm) are broadened compared with the one recorded in the monomer band (λ_det = 370 nm), cf. Figure S1 (Supporting Information). This is likely a signature of chromophore preassociation (i.e., ground-state aggregates). (22) Similar structured emission bands, which were red-shifted by about 0.3–0.5 eV with respect to the monomer emission, were previously observed for films of certain aggregate-forming BODIPY dyes and were assigned to J-coupled dimers. (23) These species also showed an increased vibronic peak spacing compared with the monomer, and a similar effect is found here for mCP (0.148 eV for the monomer vs. 0.165 eV for the aggregate). In addition, a spectral decomposition of the mCP thin film emission spectrum (Figure S2, Supporting Information) indicated an enhancement of the 0–0/0–1 emission peak ratio, also compatible with the formation of J-aggregates with small delocalization. (23,24) Therefore, we assigned this structured red-shifted mCP emission band to a J-coupled dimer-type structure. In addition, we found that the intensity ratio of the excimer and monomer emission bands was dependent on the excitation wavelength, with more pronounced emission for the long-wavelength excimer band for shorter excitation wavelengths (Figure S3, Supporting Information), thus favoring the formation of this excimer relative to the J-coupled dimer species at higher initial excitation energies of mCP. We note that structured emission tails were previously found for mCP:polystyrene thin films (with 1 wt % mCP) and neat mCP thin films, which were produced via thermal vacuum deposition. (1,2) Nevertheless, in contrast to our films here, the intensity of this excimer emission was weaker than that of the monomer emission. As we produced our films by spin-coating, we suspect that spin-coated thin films have stacking motifs different from those of films obtained via thermal vacuum deposition.
第三个，也是红移最多的条带，可以安全地分配给 mCP 准分子发射，因为它类似于含咔唑的纤维素衍生物（20）和咔唑啡的准分子带。（21）相比之下，第二个波段的结构发射与准分子不兼容，因为后者具有排斥基态，因此表现出较宽的发射。相反，这些频段以前被分配给三重态发射。（1,2）然而，我们从下面报告的时间分辨荧光实验中获得的纯 mCP 薄膜发射带的平均寿命 ⟨τ⟩ 为 7.2 ns，也与三重态不相容。此外，与在单体波段（λ_det = 370 nm）中检测到的荧光激发光谱相比，在该波段（λ_det = 400 nm）和长波长波段（λ_det = 530 nm）中检测到的荧光激发光谱变宽，参见图 S1（支持信息）。这可能是发色团预缔合（即基态聚集体）的特征。（22）先前在某些聚集体形成 BODIPY 染料的薄膜中观察到类似的结构化发射带，相对于单体发射红移约 0.3-0.5 eV，并被分配给 J 偶联二聚体。（23）与单体相比，这些物质也显示出更大的振动峰间距，并且此处对 mCP 也有类似的效果（单体为 0.148 eV，聚集体为 0.165 eV）。此外，mCP 薄膜发射光谱的光谱分解（图 S2，支持信息）表明 0-0/0-1 发射峰比的增强，也与具有小离域的 J 聚集体的形成相兼容。（23,24）因此，我们将这个结构化的红移 mCP 发射带分配给 J 偶联二聚体型结构。此外，我们发现准分子和单体发射带的强度比取决于激发波长，对于较短的激发波长，长波长准分子带的发射更明显（图 S3，支持信息），因此有利于该准分子相对于 J 的形成-mCP 较高初始激发能下的偶联二聚体种类。我们注意到，以前在 mCP：聚苯乙烯薄膜（含 1 wt % mCP）和纯 mCP 薄膜中发现了结构化发射尾部，它们是通过热真空沉积生产的。（1,2）然而，与我们这里的电影相比，这种准分子发射的强度比单体发射的强度弱。由于我们通过旋涂法生产薄膜，我们怀疑旋涂薄膜的堆叠基序与通过热真空沉积获得的薄膜具有不同。

Furthermore, we note that the total emission yield decreased dramatically with increasing mCP content, as shown in Figure 1b. This is even more interesting considering the fact that the absorption of films with high mCP content is of course much larger than in an mCP:PMMA blend with a ratio of, e.g., 2:98. This indicates that the mCP monomer emission is strongly quenched upon reducing the mCP–mCP distance and that the aggregate and excimer emission bands are also very weak.
此外，我们注意到，随着 mCP 含量的增加，总排放产率急剧下降，如图 1b 所示。考虑到具有高 mCP 含量的薄膜的吸收当然比 mCP：PMMA 混合物的吸收大得多，比例为 2：98，这一点就更加有趣了。这表明，在缩短 mCP-mCP 距离时，mCP 单体发射被强烈猝灭，并且聚集体和准分子发射带也非常弱。

3.2. Time-Resolved Fluorescence Experiments
3.2. 时间分辨荧光实验

To understand the physical mechanisms underlying this complex fluorescence behavior in more detail, the lifetime of the emission bands in the different films was investigated using transient fluorescence experiments based on the TCSPC method. Figure 1c compares normalized emission spectra of mCP in the different solvation environments, with the spectral detection ranges for the mCP monomer (370 nm), aggregate (430 nm,) and excimer bands (560 nm) indicated by the brown, sky blue, and orange regions, respectively. Figure 1d shows a comparison of the emission dynamics for the different samples in the monomer emission band recorded at 1 MHz repetition frequency with 126 ps channel width. Lifetimes for these experiments and also complementary results at 20 MHz repetition frequency with 12.2 ps channel width are summarized in Table S2 of the Supporting Information. In THF (violet), a purely monoexponential decay of 5.97 ns is observed, which represents the lifetime of isolated mCP molecules in S₁ and compares favorably with the lifetime of 5.3 ns of mCP in toluene reported previously. (25) The decay of the film with the mCP:PMMA ratio of 2:98 (green) looks quite similar. Nevretheless, the measurements with higher time resolution (Table S2, Supporting Information) find two decay components (τ₁ = 7.21 ns with A₁ = 78.3% and τ₂ = 1.24 ns with A₂ = 21.7%), resulting in an average relaxation time ⟨τ⟩ of 6.94 ns. This already indicates the growing influence of mCP–mCP interactions, even at quite low mCP number densities. With increasing mCP content in the PMMA host, the decay becomes faster, with average relaxation times of 2.64 ns for an mCP:PMMA ratio of 10:90 and ca. 0.8–0.9 ns for an mCP content of 90% and more. The proximity of the mCP molecules enables the formation of the aggregate and excimer species by only slight rearrangements of the local geometry. In addition, closer inspection of Figure 1a shows that the very small Stokes shift of mCP leads to a substantial overlap of the S₀ → S₁ absorption and the S₁ → S₀ emission bands. It is therefore likely that at higher mCP content, mCP(S₁)–mCP(S₀) Förster resonance energy transfer (FRET) is operative. Due to this homo-FRET process (S₁ → S₀ accompanied by S₀ → S₁) involving identical chromophores, the initial excitation efficiently migrates through the mCP film and can also, e.g., reach a site for aggregate or excimer formation. There, the excitation is trapped because the emission spectra of these species have no overlap with the absorption spectrum of mCP.
为了更详细地了解这种复杂荧光行为背后的物理机制，使用基于 TCSPC 方法的瞬态荧光实验研究了不同薄膜中发射带的寿命。图 1c 比较了不同溶剂化环境中 mCP 的归一化发射光谱，mCP 单体（370 nm）、聚集体（430 nm）和准分子带（560 nm）的光谱检测范围分别由棕色、天蓝色和橙色区域表示。图 1d 显示了在 1 MHz 重复频率和 126 ps 通道宽度下记录的单体发射频带中不同样品的发射动态比较。这些实验的寿命以及 20 MHz 重复频率和 12.2 ps 信道宽度下的互补结果总结在支持信息的表 S2 中。在 THF（紫色）中，观察到 5.97 ns 的纯单指数衰减，这代表了分离的 mCP 分子在 S₁ 中的寿命，与之前报道的 mCP 在甲苯中 5.3 ns 的寿命相比是有利的。（25） mCP：PMMA 比率为 2：98（绿色）的薄膜的衰减看起来非常相似。尽管如此，具有较高时间分辨率的测量（表 S2，支持信息）发现了两个衰减分量（τ₁ = 7.21 ns，A₁ = 78.3%，τ₂ = 1.24 ns，A₂ = 21.7%），导致平均弛豫时间 ⟨τ⟩ 为 6.94 ns。这已经表明 mCP-mCP 相互作用的影响越来越大，即使在相当低的 mCP 数密度下也是如此。随着 PMMA 主体中 mCP 含量的增加，衰减变得更快，当 mCP：PMMA 比率为 10：90 且约为 0 时，平均弛豫时间为 2.64 ns。mCP 含量为 90% 及以上时为 8–0.9 ns。mCP 分子的接近性使得仅通过局部几何形状的轻微重排即可形成聚集体和准分子物质。此外，仔细观察图 1a 表明，mCP 的非常小的斯托克斯位移导致 S₀ → S₁ 吸收与 S₁ → S₀ 发射带有很大重叠。因此，在较高的 mCP 含量下，mCP（S₁）–mCP（S₀） Förster 共振能量转移（FRET）可能起作用。由于这种涉及相同发色团的同源 FRET 过程（S₁ → S₀ 伴随着 S₀ → S₁），初始激发有效地迁移通过 mCP 膜，并且还可以到达聚集体或准分子形成的位点。在那里，激发被捕获，因为这些物质的发射光谱与 mCP 的吸收光谱没有重叠。

A comparison of the emission decays in the monomer, aggregate, and excimer emission bands of the neat mCP film is shown in Figure 1e, and a comparison for the neat mCP film and an mCP:PMMA film (90:10) in the excimer emission band is shown in Figure 1f. Fit parameters are included in Tables S3 and S4 of the Supporting Information. For the neat mCP film, we find an average relaxation time of 0.92 ns for the monomer, 3.84 ns for the aggregate, and 7.22 ns for the excimer. As the excimer band shows both a long lifetime and a weak emission, this points toward a small radiative rate constant of the excimer species. Regarding the mCP aggregates, previous studies on BODIPY films (which have a very small Stokes shift of the monomer emission, similar to mCP) suggest that the fluorescence quantum yield for such aggregate species is high. (23) Therefore, we conclude that only a small number of aggregate sites are available in the mCP thin film. We once more stress that the probability for being trapped at an excimer site appears to increase substantially with initial excitation energy. This was indicated by separate steady-state fluorescence experiments for an mCP thin film performed at different excitation wavelengths, which showed a pronounced increase in the amplitude of the excimer emission with increasing initial excess energy (Figure S3, Supporting Information). We also note that we were not able to detect a delayed or slowed-down increase of the aggregate and excimer emission, presumably because our time resolution in the TCSPC experiments was limited to about 500 ps.
纯 mCP 薄膜的单体、聚集体和准分子发射带的发射衰减比较如图 1e 所示，纯 mCP 薄膜和 mCP：PMMA 薄膜（90：10）在准分子发射带中的比较如图 1f 所示。拟合参数包含在支持信息的表 S3 和 S4 中。对于纯 mCP 薄膜，我们发现单体的平均弛豫时间为 0.92 ns，聚集体的平均弛豫时间为 3.84 ns，准分子的平均弛豫时间为 7.22 ns。由于准分子带表现出长寿命和弱发射，这表明准分子种类的辐射速率常数很小。关于 mCP 聚集体，先前对 BODIPY 薄膜（其单体发射的斯托克斯位移非常小，类似于 mCP）的研究表明，此类聚集体种类的荧光量子产率很高。（23）因此，我们得出结论，mCP 薄膜中只有少量聚集位点可用。我们再次强调，被困在准分子位点的可能性似乎随着初始激发能量的增加而大大增加。通过在不同激发波长下对 mCP 薄膜进行的单独稳态荧光实验来证明这一点，该实验显示准分子发射的振幅随着初始过剩能量的增加而显着增加（图 S3，支持信息）。我们还注意到，我们无法检测到聚集体和准分子发射的延迟或减慢增加，这可能是因为我们在 TCSPC 实验中的时间分辨率被限制在大约 500 ps。

3.3. Transient Absorption Spectroscopy of mCP in THF and Thin Films
3.3. THF 和薄膜中 mCP 的瞬态吸收光谱

To obtain further insights into the dynamics of mCP after photoexcitation, UV–vis–NIR broadband transient absorption experiments covering the wavelength range 260–1600 nm range were carried out on femtosecond to nanosecond time scales. Results for mCP dissolved in THF and deposited as a neat thin film on quartz are shown in Figure 2. Panel a of Figure 2 shows a contour plot of the transient absorption experiments for mCP dissolved in nitrogen-saturated THF after excitation at 290 nm. Selected transient absorption spectra are provided in panels b–d. At early times (panel b), we observe the formation of a ground state bleach (GSB) around 280 nm and a broad excited state absorption (ESA) band spanning the entire UV–vis–NIR range, which is assigned to the S₁ → S_n transition. On top of this broad ESA band, several spectral dips are found, which are due to superimposed GSB and stimulated emission (SE) contributions: a characteristic minimum at 340 nm, which agrees with the overlapping 0–0 transitions in the inverted steady-state absorption spectrum and the S₁ → S₀ stimulated emission spectrum (cf. panel e), and two weaker features corresponding to the respective 0–1 transition in the absorption and stimulated emission spectra at 325 and 359 nm. Up to 1400 ps, all bands appear to decay uniformly. However, panel d shows that a broad long-lived ESA band with a peak at 400 nm remains on much longer time scales. We assign this band to the T₁ triplet state, and it is accompanied by a residual GSB feature below 345 nm, cf. the red-filled spectrum at 100 ns. Because of the broad overlapping ESA band, only an approximate quantum yield Φ(T₁) for triplet formation through intersystem crossing (ISC) from S₁ can be estimated, and based on the ratio of the GSB features at 340 nm at around 0.5 ps and at 100 ns, we arrive at a value of (20 ± 5)%. The T₁ → T_n absorption band finally decays on the nanosecond to microsecond time scale.
为了进一步了解光激发后 mCP 的动力学，在飞秒到纳秒的时间尺度上进行了覆盖 260-1600 nm 波长范围的 UV-vis-NIR 宽带瞬态吸收实验。mCP 溶解在 THF 中并在石英上沉积成整齐的薄膜的结果如图 2 所示。图 2 的面板 a 显示了在 290 nm 激发后溶解在氮气饱和 THF 中的 mCP 的瞬态吸收实验的等值线图。选定的瞬态吸收光谱见图 b-d。在早期（图 b），我们观察到在 280 nm 附近形成基态漂白剂（GSB）和跨越整个 UV-vis-NIR 范围的宽激发态吸收（ESA）带，该波段被分配给 S₁ → S_n 跃迁。在这个宽的 ESA 波段之上，发现了几个光谱下降，这是由于叠加的 GSB 和受激发射（SE）贡献造成的：340 nm 处的特征最小值，这与倒置稳态吸收光谱和 S₁ → S₀ 受激发射光谱中重叠的 0-0 跃迁一致（参见图 e），以及两个较弱的特征，对应于 325 nm 和 359 nm 处吸收和受激发射光谱中各自的 0-1 跃迁。在 1400 ps 以下，所有频带似乎都均匀衰减。然而，图 d 显示，峰值为 400 nm 的宽长寿命 ESA 谱带在更长的时间尺度上仍然存在。我们将该波段分配给 T₁ 三重态，它伴随着低于 345 nm 的残余 GSB 特征，参见 100 ns 处的红色填充光谱。由于重叠的 ESA 波段较宽，因此只能估计从 S₁ 开始通过系统间交叉（ISC）形成三重态的近似量子产率 Φ（T₁），并且根据 340 nm 处的 GSB 特征比值（约 0.5 ps）和 100 ns，我们得出的值为（20 ± 5）%。T₁ → T_n 吸收带最终在纳秒到微秒的时间尺度上衰减。

Figure 2. (a) Contour plot of transient absorption (TA) spectra of mCP in THF upon photoexcitation at 290 nm (femtosecond TA) and at 266 nm (nanosecond TA). Note the logarithmic time scale. (b–d) Transient absorption spectra at different delay times, as indicated by the colors. The transient spectrum at 100 ns, which represents the GSB and the ESA of the T₁ state, is filled with red color. (e) Inverted normalized steady-state absorption (red) and stimulated emission (blue). (f) Kinetics at 287 nm (GSB, black), 340 nm (blue), 600 nm (violet) (mostly S₁ ESA), and 1200 nm (brown) from femtosecond TA. (g) Kinetics of the S₁ region (blue, averaged over the range 310–340 nm) and of the T₁ region (red, averaged over the range 390–410 nm) upon nanosecond photoexcitation. (h–n) Corresponding plots for a neat mCP thin film.
图 2.（a）在 290 nm（飞秒 TA）和 266 nm（纳秒 TA）光激发下 mCP 在 THF 中的瞬态吸收（TA）光谱的等值线图。请注意对数时间刻度。（B-D）不同延迟时间的瞬态吸收光谱，如颜色所示。100 ns 处的瞬态光谱（代表 T₁ 状态的 GSB 和 ESA）用红色填充。（e）倒置归一化稳态吸收（红色）和受激发射（蓝色）。（f）飞秒 TA 在 287 nm（GSB，黑色）、340 nm（蓝色）、600 nm（紫色）（主要是 S₁ ESA）和 1200 nm（棕色）处的动力学。（g）纳秒光激发时 S₁ 区（蓝色，在 310-340 nm 范围内平均）和 T₁ 区（红色，在 390-410 nm 范围内平均）的动力学。（h-n）纯 mCP 薄膜的相应绘图。

Panel f shows kinetic traces for time delays up to 500 ps, which clearly indicates the partial decay of the S₁ ESA band (340, 600, and 1200 nm) and the partial recovery of the GSB feature at 287 nm. Panel g highlights the kinetics on long time scales. The blue trace (averaged over the wavelength range 310–340 nm) is strongly dominated by S₁ dynamics (cf. the spectrum at 100 ps in panel d), and the fast spike-like decay is consistent with the S₁ lifetime of 6 ns extracted from the aforementioned TCSPC experiments. The red trace (averaged over the wavelength range 390–410 nm) is completely governed by the slow T₁ decay. A biexponential fit provides two time constants: the initial fast drop is due to the S₁ decay forming the T₁ state via ISC [τ(S₁) = 6 ns, A(S₁) = 14%], and the dominant slow decay arises from ISC/phosphorescence, leading to a triplet lifetime τ(T₁) of 790 ns [A(T₁) = 86%].
图 f 显示了高达 500 ps 的时间延迟的动力学轨迹，这清楚地表明 S₁ ESA 波段（340、600 和 1200 nm）的部分衰减和 GSB 特征在 287 nm 处的部分恢复。图 g 突出显示了长时间尺度上的动力学。蓝色迹线（在 310-340 nm 波长范围内平均）强烈由 S₁ 动力学主导（参见图 d 中 100 ps 的光谱），快速尖峰状衰减与从上述 TCSPC 实验中提取的 S₁ 寿命 6 ns 一致。红色迹线（在 390-410 nm 波长范围内取平均值）完全受缓慢的 T₁ 衰减控制。双指数拟合提供两个时间常数：初始快速下降是由于 S₁ 衰变通过 ISC 形成 T₁ 状态 [τ（S₁） = 6 ns，A（S₁） = 14%]，而主要的慢速衰减由 ISC/磷光产生，导致三元组寿命 τ（T₁）为 790 ns [A（T₁） = 86%]。

Having characterized the dynamics of isolated mCP molecules in THF, we now turn to the dynamics in the neat mCP thin film. The excitation wavelength was 290 nm. Contour plots, transient absorption spectra, and the inverted steady-state absorption and SE spectra are provided in panels h–l of Figure 2. In contrast to that of THF, the decay dynamics of the film is considerably more complex as the proximity of the mCP molecules opens up additional intermolecular relaxation channels. Upon inspection of the decay dynamics shown from 1 to 50 ps (panel i), we identify a broad absorption band above 380 nm, which is due to S₁ → S_n ESA. This band decays much faster than in THF [cf. panel (b)], with substantial contributions within the first 5 ps. As we discuss in more detail below, this fast decay is due to SSA of two mCP molecules in the S₁ state, producing a higher excited S_n state (which repopulates S₁ by internal conversion) and an mCP molecule in S₀. This fast decay of S₁ and the fast recovery of the S₀ population are clearly indicated by the early time portion in the kinetics at 282, 340, 600, and 1260 nm shown in panel m.
在表征了 THF 中分离的 mCP 分子的动力学之后，我们现在转向纯 mCP 薄膜中的动力学。激发波长为 290 nm。等值线图、瞬态吸收光谱以及倒置稳态吸收和 SE 光谱在图 2 的面板 h-l 中提供。与 THF 相比，薄膜的衰变动力学要复杂得多，因为 mCP 分子的接近会打开额外的分子间弛豫通道。在检查从 1 到 50 ps 的衰变动力学（图 i）后，我们确定了 380 nm 以上的宽吸收带，这是由于 S₁ → S_n ESA。该波段的衰减速度比 THF 快得多 [参见图（b）]，在前 5 ps 内有很大的贡献。正如我们在下面更详细地讨论的那样，这种快速衰变是由于两个处于 S₁ 状态的 mCP 分子的 SSA，产生更高激发的 S_n 状态（通过内部转化重新填充 S₁）和 S₀ 的 mCP 分子。S₁ 的快速衰变和 S₀ 群体的快速恢复由图 m 中 282、340、600 和 1260 nm 的动力学早期部分清楚地表明。

Looking at the spectral dynamics below 360 nm, we find a very different spectral relaxation behavior compared with that of THF. While at first sight, the positions of several negative going bands quite closely coincide with peaks of the steady-state absorption spectrum, the bands in the transient absorption spectrum are much sharper (cf. panels i and l). In addition, we find a new absorption band at 350 nm, which is only slightly red-shifted with respect to the S₀ → S₁ (0–0) transition of the steady-state absorption spectrum. Another new absorption band is seen at 303 nm, also just to the red of the strong absorption band in the steady-state spectrum at 297 K. The sharp bleach bands and the extra absorption features are hallmarks of vibrationally “hot” mCP molecules in the ground electronic state (S₀^*), as shown previously for organic molecules in solution and in neat thin films. (26−29) An example comparing a hot absorption spectrum with the steady-state spectrum at 297 K is shown in Figure S4 (Supporting Information). In the present case, these spectral features are long-lived and decay on the nanosecond to microsecond time scale (panel k). The energy transferred from an originally photoexcited mCP molecule heats up adjacent mCP molecules in S₀, which are initially at a temperature of 297 K. Therefore, the deposited photon energy is converted into excess heat and then redistributed inside the mCP film. Eventually, this excess energy will be dissipated to the quartz substrate by heat transfer, but this happens on a slow time scale of hundred nanoseconds to microseconds (for a more detailed discussion, see Section 4 of the Supporting Information). Figure 3 shows the decays of three representative kinetics at 303, 340, and 348 nm, all of them located in the S₀^* spectral region. They can be fitted by a sum of three exponential functions with the time constants τ₁ = 2 ns, τ₂ = 100 ns, and τ₃ = 1000 ns, highlighting that the complex cooling processes in the mCP film are stretched out over several orders of magnitude in time.
观察 360 nm 以下的光谱动力学，我们发现与 THF 相比，光谱弛豫行为非常不同。虽然乍一看，几个负向带的位置与稳态吸收光谱的峰值非常吻合，但瞬态吸收光谱中的能带要清晰得多（参见图 i 和 l）。此外，我们在 350 nm 处发现了一个新的吸收带，相对于稳态吸收光谱的 S₀ → S₁ （0–0）跃迁，它仅略微红移。在 303 nm 处可以看到另一个新的吸收带，也刚好在 297 K 的稳态光谱中强吸收带的红色。尖锐的漂白带和额外的吸收特性是基电子态（S₀^*）下振动“热”mCP 分子的标志，如前所述，溶液和纯薄膜中的有机分子。（26−29）图 S4（支持信息）显示了比较 297 K 时的热吸收光谱和稳态光谱的示例。在目前的情况下，这些光谱特征是长寿命的，并且在纳秒到微秒的时间尺度上衰减（图 k）。从原始光激发的 mCP 分子转移的能量加热 S₀ 中的相邻 mCP 分子，其初始温度为 297 K。因此，沉积的光子能量转化为多余的热量，然后在 mCP 薄膜内重新分配。最终，这些多余的能量将通过热传递耗散到石英衬底上，但这发生在数百纳秒到微秒的缓慢时间尺度上（有关更详细的讨论，请参阅支持信息的第 4 节）。图 3 显示了 303、340 和 348 nm 处三种代表性动力学的衰减，它们都位于 S₀^* 光谱区域。它们可以通过时间常数 τ₁ = 2 ns、τ₂ = 100 ns 和 τ₃ = 1000 ns 的三个指数函数之和进行拟合，这突出了 mCP 薄膜中复杂的冷却过程在时间上延伸了几个数量级。

Figure 3. Decay kinetics of highly vibrationally excited mCP in the spectral region of the hot ground electronic state (S₀^*) for a neat mCP thin film recorded at 303 nm (black), 340 nm (red), and 348 nm (blue) up to 2000 ns. The correspondingly colored lines are triexponential fits with common time constants of τ₁ = 2 ns, τ₂ = 100 ns, and τ₃ = 1000 ns. The inset shows a magnification up to 100 ns.
图 3.在 303 nm（黑色）、340 nm（红色）和 348 nm（蓝色）波长下记录的纯 mCP 薄膜在热地面电子态（S₀^*）光谱区域中的高振动激发 mCP 的衰变动力学，最长可达 2000 ns。相应颜色的线是三指数拟合，常见的时间常数为 τ₁ = 2 ns、τ₂ = 100 ns 和 τ₃ = 1000 ns。插图显示放大倍率高达 100 ns。

In addition, we find a weak broad absorption band with a peak at 450 nm (cf. the red spectrum at 1400 ps in panel j). This band decays very slowly, with a time constant of 66 μs (panel n), and thus much slower than the T₁ absorption band in THF. It is also shifted by about 0.4 eV toward lower energies so that it does probably not arise from the same T₁ triplet state but more likely originates from a charge pair (S₀^•+ and S₀^•–), as discussed in more detail below.
此外，我们发现了一个较弱的宽吸收带，在 450 nm 处有一个峰值（参见图 j 中 1400 ps 处的红色光谱）。该频带衰减非常缓慢，时间常数为 66 μs（图 n），因此比 THF 中的 T₁ 吸收带慢得多。它还向较低的能量移动了大约 0.4 eV，因此它可能不是来自相同的 T₁ 三重态，而更有可能来自电荷对（S₀^•+ 和 S₀^•–），如下文更详细地讨论。

At this point, it is worthwhile to draw a comparison with the transient absorption spectra of a mCP:PMMA film (10:90) shown in Figure 4. In contrast to the neat mCP film, the contour plot and the transient spectra in panels a and b closely resemble the spectral features in THF (Figure 2a,b). For instance, the broad S₁ ESA band has a characteristic minimum at 340 nm, which arises from the overlapping 0–0 transitions in the inverted steady-state absorption spectrum and the S₁ → S₀ stimulated emission spectrum.
在这一点上，值得与图 4 中所示的 mCP：PMMA 薄膜的瞬态吸收光谱（10：90）进行比较。与整洁的 mCP 薄膜相比，图 a 和 b 中的等值线图和瞬态光谱与 THF 中的光谱特征非常相似（图 2a、b）。例如，宽 S₁ ESA 波段在 340 nm 处具有特征最小值，这是由倒置稳态吸收光谱和 S₁ → S₀ 受激发射光谱中重叠的 0-0 跃迁引起的。

Figure 4. (a) Contour plot of ultrafast transient absorption spectra of an mCP:PMMA blend (10:90) upon photoexcitation at 290 nm. Note the logarithmic time axis. (b) Transient absorption spectra at different delay times in the range 0.2–1400 ps, with representative spectra indicated by thick colored lines. (c) Kinetics at 282 nm (GSB, black), 340 nm (red), 600 nm (violet), and 1100 nm (brown) shown up to 500 ps. (d) Same as in panel (c), but showing the dynamics at early times up to 50 ps.
图 4.（a） mCP：PMMA 混合物（10：90）在 290 nm 光激发下的超快瞬态吸收光谱等值线图。请注意对数时间轴。（b） 0.2–1400 ps 范围内不同延迟时间的瞬态吸收光谱，其中代表性光谱由粗彩色线条表示。（c） 282 nm（GSB，黑色）、340 nm（红色）、600 nm（紫色）和 1100 nm（棕色）处的动力学显示高达 500 ps。（d）与图（c）相同，但显示了早期高达 50 ps 的动力学。

We do not see any clear spectral features associated with vibrationally hot mCP molecules because the excess energy is mainly transferred to the PMMA host matrix, which is in large excess and spectrally “silent”. Still, the kinetics in panels c and d already indicate a faster decay compared with mCP in THF, showing that even at this quite low mCP concentration, SSA processes between mCP molecules in the S₁ state are non-negligible.
我们没有看到任何与振动热的 mCP 分子相关的清晰光谱特征，因为多余的能量主要转移到 PMMA 宿主基质上，该基质处于大量过量状态并且光谱上“沉默”。尽管如此，图 c 和 d 中的动力学已经表明，与 THF 中的 mCP 相比，衰变更快，这表明即使在这种相当低的 mCP 浓度下，S₁ 状态下 mCP 分子之间的 SSA 过程也是不可忽略的。

3.4. Fluence-Dependent Transient Absorption Measurements and Their Kinetic Modeling for Neat mCP Thin Films
3.4. 纯 mCP 薄膜的能量依赖性瞬态吸收测量及其动力学建模

As already mentioned, the kinetics in panel m of Figure 2 clearly show the influence of SSA processes in terms of a fast decay of the traces at early times. The kinetics of these higher-order processes will be therefore very sensitive to the initial singlet number density. Therefore, transient absorption experiments at three different pump laser fluences were performed and afterward subjected to a detailed kinetic analysis. Panels a and b of Figure 5 show representative kinetics at 340 nm up to 100 ps and up to 1500 ps, respectively, with initial exciton number densities N(S_x) of 3.15 × 10¹⁸ (blue circles), 6.30 × 10¹⁸ (red circles), and 1.19 × 10¹⁹ cm^–3 (black circles), corresponding to fluences of 0.73, 1.46, and 2.77 mJ cm^–2 (50 fs pulse length). These number densities were determined from accurate measurements of the laser fluence using the optical density and thickness of the film. The latter was determined from AFM experiments, which provided a value of 253 nm (panel c). We also note that we observed oscillations in the kinetics with a period of about 150 ps, which were particularly strong at the S₀ → S₁ absorption edge. Panel d shows an example of a kinetic trace averaged over the wavelength range 345–355 nm. As demonstrated previously, such an oscillation originates from a coherent acoustic phonon induced by the pump beam, which propagates back and forth between the film/quartz and the film/nitrogen interface. (19,26,30−33) Based on the known thickness measured by AFM, we were able to extract the longitudinal sound velocity c_L of the mCP film from the measured oscillation period, which was determined to be 6800 m s^–1.
如前所述，图 2 中图 m 中的动力学清楚地显示了 SSA 过程在早期迹线快速衰减方面的影响。因此，这些高阶过程的动力学对初始单重态数密度非常敏感。因此，在三种不同的泵浦激光通量下进行了瞬态吸收实验，然后进行了详细的动力学分析。图 5 的图 a 和 b 分别显示了 340 nm 至 100 ps 和高达 1500 ps 的代表性动力学，初始激子数密度 N（S_x）为 3.15 × 10¹⁸（蓝色圆圈）、6.30 × 10¹⁸（红色圆圈）和 1.19 ×^{10 19} cm^–3（黑色圆圈），对应于 0.73、1.46、和 2.77 mJ cm^–2（50 fs 脉冲长度）。这些数密度是使用薄膜的光密度和厚度对激光通量进行精确测量来确定的。后者是通过 AFM 实验确定的，该实验提供的值为 253 nm（图 c）。我们还注意到，我们在大约 150 ps 的周期内观察到动力学振荡，在 S₀ → S₁ 吸收边缘尤其强烈。图 d 显示了在 345–355 nm 波长范围内平均的动力学轨迹示例。如前所述，这种振荡源于泵浦光束感应的相干声子，该声子在薄膜/石英和薄膜/氮气界面之间来回传播。（19,26,30−33）根据 AFM 测量的已知厚度，我们能够从测得的振荡周期中提取 mCP 薄膜的纵向声速 c_L，该振荡周期确定为 6800 m ^s-1。

Figure 5. (a) Transient absorption kinetics of a neat mCP thin film up to delay times of 100 ps detected at a probe wavelength of 340 nm after photoexcitation at 290 nm. (b) Same as in panel (a), but shown up to 1500 ps. Initial exciton number densities N(S_x): 3.15 × 10¹⁸ (blue circles), 6.30 × 10¹⁸ (red circles), and 1.19 × 10¹⁹ cm^–3 (black circles). (c) Two AFM measurements for the mCP thin film providing a film thickness of 253 nm. (d) Oscillatory kinetics averaged over the spectral range 345–355 nm originating from a coherent acoustic phonon propagating in the film.
图 5.（a）在 290 nm 光激发后，在 340 nm 探针波长处检测到的高达 100 ps 延迟时间的纯 mCP 薄膜的瞬态吸收动力学。（b）与图（a）中相同，但显示高达 1500 ps。初始激子数密度 N（S_x）：3.15 × 10¹⁸（蓝色圆圈）、6.30 × 10¹⁸（红色圆圈）和 1.19 ×^{10 19} cm^–3（黑色圆圈）。（c） mCP 薄膜的两次 AFM 测量，提供 253 nm 的薄膜厚度。（d）在 345-355 nm 光谱范围内平均的振荡动力学，源自在薄膜中传播的相干声子。

In the following, we outline the kinetic model for describing the relaxation processes monitored by the kinetics in panels a and b. The probe wavelength 340 nm is located in a region, where one expects absorption contributions of the electronically excited singlet states as well as the hot ground state S₀^*. The initial positive spike in each signal comes from the ESA of the S₁ state. Decay of S₁ leads to the formation of the S₀^* state, resulting in a negative signal. We note that similar decays are observed in the S₁ ESA band, e.g., at 400 nm (Supporting Information, Figure S5). The resulting vibrationally hot molecules then cool down to form “cold” S₀ molecules, resulting in the final decay. A scheme of the different pathways for excited-state relaxation is provided in Figure 6. The specific kinetic model derived from this scheme is summarized by eqs 1–11 and explained in more detail below
在下文中，我们概述了用于描述由图 a 和 b 中的动力学监测的弛豫过程的动力学模型。探针波长 340 nm 位于一个区域，人们预期电子激发的单线态以及热接地态 S₀^* 的吸收贡献。每个信号中的初始正峰值来自 S₁ 状态的 ESA。S₁ 的衰减导致 S₀^* 状态的形成，从而产生负信号。我们注意到，在 S₁ ESA 波段中观察到类似的衰减，例如在 400 nm 处（支持信息，图 S5）。由此产生的振动热分子随后冷却形成“冷”S₀ 分子，导致最终衰变。图 6 提供了激发态弛豫的不同途径的方案。从该方案中得出的具体动力学模型由方程 1-11 总结，并在下面更详细地解释

S_{x} \overset{k_{x}}{\to} S_{1}

(1)

S_{x} \overset{k_{C P, x}}{\to} e^{-} + h^{+}

(2)

S_{1} \overset{k_{I C + F l, 1}}{\to} S_{0}^{*}

(3)

S_{1} \overset{k_{I S C, 1}}{\to} T_{1}

(4)

T_{1} \overset{k_{I S C + P h, 2}}{\to} S_{0}^{*}

(5)

2 S_{1} \overset{k_{d i f f}}{\to} S_{n} + S_{0}^{*}

(6)

3 S_{1} \overset{k_{F}}{\to} 2 S_{n} + S_{0}^{*}

(7a) （7a）

3 S_{1} \overset{k_{F}}{\to} S_{n} + 2 S_{0}^{*}

(7b) （7b）

S_{n} \overset{k_{n}}{\to} S_{1}

(8)

S_{n} \overset{k_{C P, n}}{\to} e^{-} + h^{+}

(9)

S_{0}^{*} \overset{k_{C E T}}{\to} S_{0}

(10)

e^{-} + h^{+} \overset{k_{r e c}}{\to} S_{0}

(11)

Figure 6. Overview of the different pathways for the relaxation of mCP in the excited state. Details regarding the kinetic modeling are provided in the text.
图 6.激发态 mCP 松弛的不同途径概述。文中提供了有关动力学建模的详细信息。

Photoexcitation at 290 nm prepares the mCP molecule in a higher excited singlet state S_x. Because we did not observe any pronounced spectral changes within the first picosecond, the lifetime of this state must be very short, presumably on the sub-100 fs time scale, producing the S₁ state by internal conversion (IC) with the rate constant k_x (eq 1). We also assume that the highly excited S_x state in the organic thin film can split into a charge pair (electron and hole, eq. 2, e.g., located on individual mCP molecules as an S₀^•– radical anion and an S₀^•+ radical cation), as discussed previously for molecular OLED compounds, hole transport materials, and polymer thin films. (19,33−35) The time constant for this process is k_CP,x.
290 nm 的光激发使 mCP 分子处于更高激发的单线态 S_x。因为我们在第一个皮秒内没有观察到任何明显的频谱变化，所以这种状态的寿命一定非常短，大概在低于 100 fs 的时间尺度上，通过内部转换（IC）产生 S₁ 状态，速率常数为 k_x （eq 1）。我们还假设有机薄膜中的高激发 S_x 态可以分裂成电荷对（电子和空穴，方程 2，例如，位于单个 mCP 分子上，作为 S₀^•– 自由基阴离子和 S₀^•+ 自由基阳离子），如前所述分子 OLED 化合物、空穴传输材料和聚合物薄膜。（19,33−35）此过程的时间常数为 k _CP，x。

The S₁ state formed by IC from S_x can take several parallel channels: mCP may decay by IC and fluorescence to the vibrationally hot ground state S₀^* with the rate constant k_IC+Fl,1, cf. eq 3, or alternatively to the T₁ state by ISC via eq 4 with the rate constant k_ISC,1. Afterward, the T₁ state decays to S₀^* either by ISC or phosphorescence (eq 5, rate constant k_IC+Ph,2). These are the only options for “isolated” mCP molecules in a solvent (such as THF) or widely distributed mCP molecules in an inert film (such as the example mCP:PMMA 2:98 in Figure 1a), where the S₁ number density is low. At larger S₁ number densities, such as in neat mCP films, higher-order SSA processes need to be considered. One channel is the bimolecular diffusive SSA of two S₁ species, forming a higher excited S_n singlet state and an S₀^* ground state species with the rate constant k_diff, i.e., eq 6 in the kinetic scheme. In this case, the diffusive transport of the S₁ excitation occurs by a hopping process, i.e., repeated homo-FRET between S₁ and its respective nearest S₀ neighbor, which is supported by the favorable overlap of the absorption and emission spectra (Figure 1a). This statistical migration process does not lead to any net population change in S₁ and S₀ but eventually brings two S₁ excitons into a critical contact distance. At this short distance, SSA takes place by homo-FRET of the two mCP molecules in S₁, as explained below. The overall SSA process in eq 6 is of second-order because the diffusive hopping of the two S₁ excitations is rate-determining.
IC 从 S_x 形成的 S₁ 态可以采用多个并行通道：mCP 可以通过 IC 和荧光衰减到振动热的基态 S₀^*，速率常数为 k_IC+Fl，1，参见方程 3，或者通过 ISC 通过方程 4 以速率常数 k ISC 变为 T₁ 态_，1. 之后，T₁ 状态通过 ISC 或磷光衰减到 S₀^*（方程 5，速率常数 k_IC+Ph，2）。对于溶剂中“分离”的 mCP 分子（例如 THF）或惰性膜中广泛分布的 mCP 分子（例如图 1a 中的示例 mCP：PMMA 2：98），其中 S₁ 数密度较低，这些是唯一的选择。在较大的 S₁ 数密度下，例如在纯 mCP 薄膜中，需要考虑更高阶的 SSA 工艺。一个通道是两个 S₁ 物质的双分子扩散 SSA，形成一个更高激发的 S_n 单重态和一个速率常数为 k_diff 的 S₀^* 基态物质，即动力学方案中的方程 6。在这种情况下，S₁ 激发的扩散传输是通过跳跃过程发生的，即 S₁ 与其各自最近的 S₀ 邻居之间重复的同级 FRET，这得到了吸收光谱和发射光谱的有利重叠的支持（图 1a）。这个统计迁移过程不会导致 S₁ 和 S₀ 中的任何净种群变化，但最终将两个 S₁ 激子带入临界接触距离。在这个短距离上，SSA 是通过 S₁ 中两个 mCP 分子的同源 FRET 发生的，如下所述。方程 6 中的整个 SSA 过程是二阶的，因为两个 S₁ 激发的扩散跳跃是速率决定性的。

Alternatively, we consider direct SSA by homo-FRET of two mCP molecules in S₁, i.e., S₁ → S_n accompanied by S₁ → S₀^*, mediated by a nonradiative dipole–dipole coupling mechanism (without prior diffusion), so that one of the electronic excitations is “destroyed”
或者，我们认为 S₁ 中两个 mCP 分子的同质 FRET 直接 SSA，即 S₁ → S_n 伴随着 S₁ → S₀^*，由非辐射偶极-偶极耦合机制（无先验扩散）介导，因此其中一个电子激发被“破坏”

S_{1} \overset{k_{F \ddot{o} r s t e r} (R)}{\to} S_{0}^{*}

(12a) （12a）

S_{1} \overset{k_{F \ddot{o} r s t e r} (R)}{\to} S_{n}

(12b) （12b）

Here, the rate constant k_Förster depends on the distance R of the two interacting S₁ molecules and will thus change over time during the relaxation (36,37)
在这里，速率常数 kFörster 取决于两个相互作用的 S1 分子的距离 R，因此在弛豫期间会随着时间的推移而变化（36,37）

k_{F \ddot{o} r s t e r} = \frac{1}{τ_{1}} {(\frac{R_{0}}{R})}^{6}

(13)

This expression shows a characteristic R^–6 dependence on the donor–acceptor distance R, with τ₁ being the total S₁ lifetime of the donor in the absence of energy transfer. R₀ is the distance, where there is equal probability for the FRET process and the intramolecular decay of S₁. To make this direct S₁–S₁ FRET process tractable within a standard kinetic mechanism with time-independent rate constants, the alternative formulation based on eqs 7a and 7b was deduced previously instead of eqs 12a and 12b. (33,38) There, it was shown that S₁–S₁ FRET can be described as an apparent third-order process. The two separate eqs 7a and 7b are required to correctly account for the overall stoichiometry of the S₁–S₁ FRET process, leading to the apparent third-order rate constant k_FRET = 6 k_F. (33) Such an S₁–S₁ FRET process should be feasible because the S₁ emission spectrum overlaps with strong S₁ ESA, as demonstrated in the transient absorption spectra of Figure 2b.
该表达式显示了特征 ^R-6 对供体-受体距离 R 的依赖性，其中 τ₁ 是在没有能量转移的情况下供体的总 S₁ 寿命。R₀ 是距离，其中 FRET 过程和 S₁ 的分子内衰变的概率相等。为了使这种直接的 S₁–S₁ FRET 过程在具有与时间无关的速率常数的标准动力学机制中易于处理，之前推导出了基于方程 7a 和 7b 的替代公式，而不是方程 12a 和 12b。（33,38）在那里，表明 S_1-S ₁ FRET 可以被描述为明显的三阶过程。需要两个单独的方程 7a 和 7b 来正确解释 S₁–S₁ FRET 过程的整体化学计量，从而得出明显的三阶速率常数 k_FRET = 6 k_F。（33）这样的 S₁–S₁ FRET 过程应该是可行的，因为 S₁ 发射光谱与强 S₁ ESA 重叠，如图 2b 的瞬态吸收光谱所示。

Second-order diffusive and third-order FRET-type SSA processes produce a higher energy S_n state, which should behave similar to the initially prepared S_x state. Therefore, it can either decay to the S₁ state by IC with the rate constant k_n, cf. eq 8, or form a charge pair with the rate constant k_CP,n via eq 9. The vibrationally excited S₀^* molecules are deactivated by collisional energy transfer (CET) with adjacent mCP molecules to cold S₀ species (eq 10, rate constant k_CET). The appropriateness of such a simple first-order description of CET is shown in the Supporting Information (Figure S6). Finally, the electrons and holes of the charge pair states recombine on much longer time scales (eq 11, rate constant k_rec). We do not consider singlet fission (39) because the T₁ state of mCP is located at more than half of the energy of the S₁ state. Also, TTA is not relevant here because the time scale for the formation of the T₁ state is way too slow to accumulate any appreciable T₁ number density in the transient absorption experiments. In addition, we do not include contributions from the excimer and aggregate species observed in the steady-state and transient fluorescence spectra (Figure 1). In the transient absorption experiments, we do not see indications for such species with a lifetime in the several nanosecond range (Tables S3 and S4, Supporting Information), only bands which decay on the microsecond time scale (Figure 2n).
二阶扩散和三阶 FRET 型 SSA 过程产生更高能量的 S_n 状态，其行为应该类似于最初准备的 S_x 状态。因此，它可以通过速率常数 k_n 的 IC 衰减到 S₁ 状态，参见方程 8，也可以通过方程 9 形成速率常数 k _CP，n 的电荷对。振动激发的 S₀^* 分子通过与相邻 mCP 分子的碰撞能量转移（CET）失活，以冷却 S₀ 物质（方程 10，速率常数 k_CET）。这种简单的 CET 一阶描述的适当性显示在支持信息中（图 S6）。最后，电荷对态的电子和空穴在更长的时间尺度上重新组合（方程 11，速率常数 k_rec）。我们不考虑单重态裂变（39），因为 mCP 的 T₁ 态位于 S₁ 态能量的一半以上。此外，TTA 在这里无关紧要，因为形成 T₁ 态的时间尺度太慢了，无法在瞬态吸收实验中积累任何明显的 T₁ 数密度。此外，我们不包括在稳态和瞬态荧光光谱中观察到的准分子和聚集体物质的贡献（图 1）。在瞬态吸收实验中，我们没有看到这种物质的寿命在几纳秒范围内的迹象（表 S3 和 S4，支持信息），只看到在微秒时间尺度上衰减的频带（图 2n）。

In the following, we will turn to the modeling of the kinetic traces in panels a and b of Figure 5, which was performed using the program Tenua. (40) First, the absorbance was converted into a time-dependent S₀^* number density. At the probe wavelength of 340 nm, we detect the formation of S₀^* from S₁, which corresponds to the initial fast formation of the negative signal in panel a, and the subsequent cooling of S₀^* to S₀, which is the much slower decay in panel b. Therefore, the peak in each kinetics near t = 0 was taken as an initial value of zero for the S₀^* number density. The kinetics were then inverted, and the amplitudes were scaled by the known initial number densities for the three different pump laser fluence conditions. The resulting time-dependent number densities of the S₀^* population are displayed in Figure 7a–c for the time ranges of up to 50, 300, and 1500 ps, respectively. This conversion of the transient absorption into a transient S₀^* number density profile is allowed because at the probe wavelength of 340 nm, the differences in absorption coefficients ε(S₁)−ε(S₀) and ε(S₀)−ε(S₀^*) are essentially the same. One can see this in the kinetic transient of Figure 5a: there the maximum positive amplitude (maximum S₁ population) and maximum negative amplitude (maximum S₀^* population, S₁ population practically fully depleted) are essentially identical. This means that the kinetic transient at 340 nm directly tracks the conversion of population from S₁ to S₀^*, or, in other words, the increase of S₀^* number density starting from a value of zero. The subsequent decay monitors the cooling process, i.e., the conversion of S₀^* into S₀.
在下文中，我们将转向图 5 的面板 a 和 b 中动力学轨迹的建模，该建模是使用 Tenua 程序执行的。（40）首先，将吸光度转换为随时间变化的 S₀^* 数密度。在 340 nm 的探针波长处，我们检测到 S₀^* 从 S₁ 的形成，这对应于图 a 中负信号的初始快速形成，以及随后 S₀^* 到 S₀ 的冷却，这是图 b 中慢得多的衰减。因此，将 t = 0 附近每个动力学中的峰作为 S₀^* 数密度的初始值为零。然后反转动力学，并按三种不同泵浦激光通量条件的已知初始数密度缩放振幅。图 7a-c 显示了 S₀^* 种群随时间变化的数字密度，时间范围分别高达 50、300 和 1500 ps。允许将瞬态吸收转换为瞬态 S₀^* 数密度曲线，因为在 340 nm 的探针波长下，吸收系数 ε（S₁）−ε（S₀）和 ε（S₀）−ε（S₀^*）的差异基本相同。在图 5a 的动力学瞬变中可以看到这一点：最大正振幅（最大 S₁ 种群）和最大负振幅（最大 S₀^* 种群，S₁ 种群几乎完全耗尽）基本相同。这意味着 340 nm 处的动力学瞬变直接跟踪种群从 S₁ 到 S₀^* 的转换，或者换句话说，从零值开始 S₀^* 数密度的增加。随后的衰变监测冷却过程，即 S₀^* 转化为 S₀。

Figure 7. Kinetic modeling of the S₀^* number density N for the three different initial S_x number densities of 3.15 × 10¹⁸ (blue), 6.30 × 10¹⁸ (red), and 1.19 × 10¹⁹ cm^–3 (black). Solid lines are for the kinetic model with diffusive SSA, and dashed lines are for the model with SSA based on direct S₁–S₁ homo-FRET. (a) Time range up to 50 ps. (b) Time range up to 300 ps. (c) Time range up to 1500 ps.
图 7.3.15 × 10¹⁸（蓝色）、6.30 × 10¹⁸（红色）和 1.19 ×^{10 19} cm^–3（黑色）三种不同初始 S_x 数密度 N 的 S₀^* 数密度 N 的动力学建模。实线适用于具有扩散 SSA 的动力学模型，虚线适用于基于直接 S₁–S₁ 同叶 FRET 的 SSA 模型。（a）时间范围最大 50 ps. （b）时间范围最大 300 ps. （c）时间范围最大 1500 ps。

As it turned out, successful modeling of the initial fast increase up to 100 ps was exclusively dependent on the correct choice of the type and the rate constant of the S₁–S₁ annihilation process. To determine the mechanism of SSA, we simulated two limiting cases: the solid lines are the results for purely diffusive SSA with the rate constant k_diff = 1.40 × 10^–8 cm³ s^–1, whereas the dashed lines are the results of a simulation using only FRET with a rate constant k_F = 8.00 × 10^–28 cm⁶ s^–1. The diffusive model provides a much better description of the experimental signals across the complete time range. The initial rise is slightly flatter and the curve saturates more slowly than for the simulation based on S₁–S₁ FRET (cf. panel a), which is a result of the bimolecular compared with the apparent third-order mechanism. In addition, the diffusive model nicely follows all of the three curves for different pump laser fluences. Our result for k_diff is larger but still compares very well to the value 3.4 × 10^–9 cm³ s^–1 found for the related material 4′-bis(9-carbazolyl)-2,2′-biphenyl (CBP) by Ruseckas et al. using time-resolved fluorescence. (41)
事实证明，初始快速增加到 100 ps 的成功建模完全取决于 S₁–S₁ 湮灭过程的类型和速率常数的正确选择。为了确定 SSA 的机制，我们模拟了两种极限情况：实线是速率常数 k_diff = 1.40 × 10-8 cm³ s-1 的纯扩散 SSA 的结果，而虚线是仅使用速率常数 k_F = 8.00 × ^10-28 cm⁶ ^s-1 的 FRET 的模拟结果.扩散模型可以更好地描述整个时间范围内的实验信号。与基于 S₁–S₁ FRET 的模拟相比，初始上升略微平坦，曲线饱和得更慢（参见图 a），这是双分子与明显的三阶机制相比的结果。此外，漫射模型很好地遵循了不同泵浦激光通量的所有三条曲线。我们的 k_diff 结果更大，但仍然与 Ruseckas 等人使用时间分辨荧光为相关材料 4'-双（9-咔唑基）-2,2'-联苯（CBP）发现的 3.4 × ^10-9 cm 3 ^s-1 的值相比非常好。（41）

In contrast, if the early-time behavior of the black curve for highest N₀ is modeled using S₁–S₁ homo-FRET only (panel a), the experimental data at longer times for all three initial number densities are consistently underestimated. Importantly, an unrealistically large value for the Förster radius R₀ of 17 nm is required to fit the data, which is far outside the range expected for efficient S₁–S₁ homo-FRET. There, typical values in the range of about 6 nm are expected for smaller organic molecular systems. (42) The final decay of the three curves stems from the vibrational relaxation of the vibrationally hot mCP molecules and occurs on much slower time scales. By 1500 ps, the decay has not yet finished (see Figure 7c), and the visible range of the kinetics is well described by a rate constant k_CET of 5 × 10⁸ s^–1 (τ_CET = 2 ns, cf. also Figure 3).
相比之下，如果仅使用 S_1-S ₁ 同种 FRET 对最高 N₀ 的黑色曲线的早期行为进行建模（图 a），则所有三个初始数密度在较长时间的实验数据始终被低估。重要的是，需要 17 nm 的 Förster 半径 R₀ 的不切实际的大值来拟合数据，这远远超出了高效 S₁–S₁ 同形 FRET 的预期范围。在那里，对于较小的有机分子系统，预计典型值在大约 6 nm 的范围内。（42）这三条曲线的最终衰变源于振动热 mCP 分子的振动弛豫，并且发生在更慢的时间尺度上。到 1500 ps，衰减尚未完成（参见图 7c），动力学的可见范围可以用 5 × 10⁸ s^–1 的速率常数 k_CET 很好地描述（τ_CET = 2 ns，参见图 3）。

We note that the kinetic model described by eqs 1–11 involves several other processes with their accompanying rate constants beside the SSA processes (eqs 6, 7a, and 7b). We would therefore like to provide a few comments regarding the accuracy of the fitted rate constants: The ratio of the rate constants k_x and k_CPx (eqs 1 and 2) and the ratio of the rate constants k_n and k_CPn (eqs 8 and 9) are directly linked to the yield for the formation of the long-lived CP state. An accurate value for this yield is difficult to estimate because at long delay times, the bleach feature below 380 nm (cf., e.g., Figure 2j at 1400 ps) is not a real population bleach but mainly reflects the difference absorption spectrum of S₀^* and S₀ molecules. The weak absorption band of the CP state with a peak at 450 nm suggests that the value is in the few percent range. This is for instance smaller than in copolymer systems, where this yield is in the range 15–25%. (19,33,38) The total rate constants k_x,total = τ_x,total^–1 of S_x and k_n,total = τ_n,total^–1 of S_n are still well-defined (and also k_x and k_n). The generation of S₁ species must be ultrafast (τ_x,total and τ_n,total less than 100 fs) as both of them are relevant to correctly fit the steep rise of the kinetics in Figure 7a, which indicates the fast decay of S₁ population due to SSA.
我们注意到，方程 1-11 描述的动力学模型涉及除 SSA 过程之外的其他几个过程及其伴随的速率常数（方程 6、7a 和 7b）。因此，我们想就拟合速率常数的准确性提供一些评论：速率常数 k_x 和 k_CPx 的比率（方程 1 和 2）以及速率常数 k_n 和 k_CPn 的比率（方程 8 和 9）与形成长寿命 CP 状态的产量直接相关。这个产率的准确值很难估计，因为在较长的延迟时间内，低于 380 nm 的漂白剂特征（参见，例如，图 2j 在 1400 ps 时）不是真正的群体漂白剂，而是主要反映了 S₀^* 和 S₀ 分子的不同吸收光谱。CP 态的弱吸收带在 450 nm 处具有峰值，表明该值在几个百分点范围内。例如，这比共聚物系统小，共聚物系统的收率在 15-25% 的范围内。（19,33,38）总速率常数 k_x，total = τ_x，S _x 和 k_n 的总数^–1，总数 = τ_n，S _n 的总数^–1 仍然定义明确（以及 k_x 和 k_n）。 S₁ 物质的产生必须是超快的（τ_x，total 和 τ_n，total 小于 100 fs），因为它们都与正确拟合图 7a 中动力学的急剧上升有关，这表明由于 SSA 导致 S₁ 种群的快速衰减。

The unimolecular decay channels of S₁ and T₁ (eqs 3–5) have fixed time constants in the nanosecond range, as determined from the TCSPC experiments in combination with the triplet yield for mCP in THF (obtained from transient absorption), which is assumed to be similar in the mCP film. The total S₁ lifetime in the absence of SSA processes is 6 ns for “isolated” mCP in THF and 6.7 ns for the mCP:PMMA 2:98 blend (cf. Table S2, Supporting Information), with an estimated quantum yield of about 20% for T₁ formation (see above). They are therefore way too slow to compete with the SSA processes at the singlet exciton number densities present in the transient absorption experiments. The lifetime of the T₁ state was fixed at the value for mCP in THF. Finally, the recombination of the charge pairs (eq 11) is very slow (66 μs), as seen in the fit provided in Figure 2n, and therefore also does not have an impact on the modeling shown in Figure 7.
S₁ 和 T₁ 的单分子衰变通道（方程 3-5）在纳秒范围内具有固定的时间常数，这是根据 TCSPC 实验结合 THF 中 mCP 的三重态产率（从瞬态吸收获得）确定的，假设它在 mCP 薄膜中相似。在没有 SSA 过程的情况下，THF 中“分离”mCP 的 S₁ 总寿命为 6 ns，mCP：PMMA 2：98 混合物为 6.7 ns（参见表 S2，支持信息），估计 T₁ 形成的量子产率约为 20%（见上文）。因此，它们太慢了，无法在瞬态吸收实验中存在的单重态激子数密度下与 SSA 过程竞争。T₁ 状态的寿命固定在 THF 中的 mCP 值。最后，电荷对（方程 11）的复合非常缓慢（66 μs），如图 2n 中提供的拟合所示，因此对图 7 所示的建模也没有影响。

A summary of the kinetic fit parameters is provided in the Supporting Information (Table S5). In addition, fit results using the same fit parameters are provided for another probe wavelength (318 nm) in the Supporting Information (Figure S7). The fits are as good as for the wavelength 340 nm, which shows the reliability of the fitting approach.
支持信息中提供了动力学拟合参数的摘要（表 S5）。此外，支持信息中提供了另一个探针波长（318 nm）使用相同拟合参数的拟合结果（图 S7）。拟合结果与波长 340 nm 的拟合结果一样好，这表明了拟合方法的可靠性。

4. Conclusions 4. 结论

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In this contribution, we were able to provide a comprehensive picture of the excited state dynamics of mCP, an important organic host material for blue emitters in OLEDs. The transient fluorescence experiments covered an excitation regime corresponding to very low number densities of the S₁ excited singlet state. Under these conditions, the influence of S₁–S₁ annihilation can be excluded. Experiments for mCP in THF solution and in mCP:PMMA films with a very small mCP content showed a monoexponential kinetics with a mCP monomer lifetime in the range 6–7 ns. PMMA films with high mCP content and neat mCP thin films exhibited strongly quenched mCP emission featuring three different bands, assigned to monomer, aggregate, and excimer emission. mCP molecules in the S₁ state can migrate through the film via an S₁–S₀ homo-FRET mechanism (“S₁ + S₀ → S₀ + S₁”) (42,43) and can emit monomer fluorescence “on the way” or be eventually trapped either at an aggregate or an excimer site. The homo-FRET mechanism is also supported by the small Stokes shift and the consequently good overlap of the mCP absorption and emission spectra.
在这篇文章中，我们能够全面提供 mCP 的激发态动力学，mCP 是 OLED 中蓝色发射器的重要有机主体材料。瞬态荧光实验涵盖了对应于 S₁ 激发单重态的极低数密度的激发机制。在这些条件下，可以排除 S_1-S ₁ 湮灭的影响。在 THF 溶液和 mCP 含量非常小的 mCP：PMMA 薄膜中的 mCP 实验显示，mCP 单体寿命在 6-7 ns 范围内具有单指数动力学。具有高 mCP 含量的 PMMA 薄膜和纯 mCP 薄膜表现出强淬火 mCP 发射，具有三个不同的波段，分别分配给单体、聚集体和准分子发射。S₁ 状态的 mCP 分子可以通过 S₁–S₀ 同源 FRET 机制（“S₁ + S₀ → S₀ + S₁”）在薄膜中迁移（42,43），并且可以“途中”发射单体荧光或最终被捕获在聚集体或准分子位点。同质 FRET 机制也得到了小斯托克斯位移的支持，因此 mCP 吸收和发射光谱的良好重叠也得到了支持。

In contrast, the UV–vis–NIR transient absorption experiments for the mCP thin film probed the dynamics for high number densities of the S₁ singlet state. Here, SSA was clearly detected (“S₁ + S₁ → S₀ + S_n”), and the experiments favored a bimolecular diffusive mechanism. In contrast, modeling with a direct S₁–S₁ homo-FRET mechanism resulted in an unrealistically large Förster radius. We therefore suggest that the S₁ excitation migrates through the neat mCP thin film by the aforementioned site-to-site hopping mechanism as there is always a very close mCP partner in S₀ for the excitation transfer. Annihilation takes places, once two mCP molecules in the S₁ state are within a critical contact distance.
相比之下，mCP 薄膜的 UV-vis-NIR 瞬态吸收实验探测了 S₁ 单重态高数密度的动力学。在这里，清楚地检测到了 SSA （“S₁ + S₁ → S₀ + S_n”），并且实验有利于双分子扩散机制。相比之下，使用直接的 S₁–S₁ 同源 FRET 机制进行建模会导致不切实际的大 Förster 半径。因此，我们建议 S₁ 激发通过上述位点跳跃机制通过整洁的 mCP 薄膜迁移，因为 S₀ 中总是有一个非常接近的 mCP 伙伴用于激发转移。一旦两个处于 S₁ 状态的 mCP 分子处于临界接触距离内，就会发生湮灭。

The ultrafast transient absorption experiments for mCP in THF also provided clear evidence for the formation of the T₁ triplet state with a quantum yield of about 20%. The triplet state was spectrally characterized and has a microsecond lifetime in nitrogen-saturated THF. In the transient absorption experiments for the mCP thin film, the triplet yield is small because SSA outcompetes triplet formation. For the initial S_x exciton number densities of 1.19 × 10¹⁹, 6.30 × 10¹⁸, and 3.15 × 10¹⁸ cm^–3 studied here, we obtain a triplet yield of 0.3, 0.5, and 1.0%, respectively. For very low initial S_x exciton number densities (e.g., TCSPC conditions), the T₁ excitons are very far apart, and there is no efficient migration process available for the T₁ species in the film. Thus, ISC and/or phosphorescence will prevail. Although the T₁ state has a long lifetime, the contribution of TTA will be therefore minor, when one initially uses photoexcitation. Looking at the spectra of Figure 2d and e, there is a substantial overlap between the T₁ absorption and the S₁ stimulated emission bands. Under electrical excitation in an OLED (T₁:S₁ ratio 3:1 due to spin statistics), at high S₁ and T₁ exciton number densities, singlet–triplet annihilation between S₁ and T₁ (“S₁ + T₁ → S₀ + T_n”) in addition to TTA should be feasible. These processes are also relevant to the roll-off characteristics of OLED devices. (14)
THF 中 mCP 的超快瞬态吸收实验也为形成 T₁ 三重态提供了明确的证据，量子产率约为 20%。三重态具有光谱特征，在氮饱和 THF 中具有微秒级寿命。在 mCP 薄膜的瞬态吸收实验中，三重态产率很小，因为 SSA 优于三重态形成。对于这里研究的初始 S_x 激子数密度 1.19 × 10¹⁹、6.30 × 10¹⁸ 和 3.15 ×10¹⁸ cm^–3，我们分别获得了 0.3、0.5 和 1.0% 的三重态产率。对于非常低的初始 S_x 激子数密度（例如，TCSPC 条件），T₁ 激子相距很远，并且薄膜中的 T₁ 物质没有有效的迁移过程。因此，ISC 和/或磷光将占上风。尽管 T₁ 状态的寿命很长，但因此当最初使用光激发时，TTA 的贡献将是微小的。查看图 2d 和 e 的光谱，T₁ 吸收和 S₁ 受激发射带之间存在很大重叠。在 OLED 的电激发下（由于自旋统计，T₁：S₁ 比率为 3：1），在高 S₁ 和 T₁ 激子数密度下，除了 TTA 之外，S₁ 和 T₁ 之间的单重态-三重态湮灭（“S₁ + T₁ → S₀ + T_n”）应该是可行的。这些工艺也与 OLED 器件的滚降特性有关。（14）

There are still several open questions, which will be worthwhile exploring in future investigations: here, we deliberately investigated thin films produced by spin-coating as this wet-chemical method has gained increased popularity in OLED fabrication because of the easier and cheaper manufacturing process compared with thermal vacuum deposition. (5,8) In a future study, it will be interesting to see if there are relevant differences regarding the time-resolved spectroscopy of films prepared by spin-coating and thermal evaporation in vacuum, for instance with respect to the time constants of the SSA processes. In addition, transient absorption studies of representative blue emitters doped into mCP films will be of interest to follow processes, such as hetero-FRET between mCP and the blue emitter, in real time.
还有几个悬而未决的问题，值得在未来的研究中进行探索：在这里，我们特意研究了旋涂产生的薄膜，因为与热真空沉积相比，这种湿化学方法的制造工艺更容易、更便宜，因此在 OLED 制造中越来越受欢迎。（5,8）在未来的研究中，看看通过真空中旋涂和热蒸发制备的薄膜的时间分辨光谱是否存在相关差异，例如在 SSA 过程的时间常数方面，将会很有趣。此外，掺杂到 mCP 薄膜中的代表性蓝色发射极的瞬态吸收研究对于实时跟踪过程（例如 mCP 和蓝色发射极之间的异质 FRET）等过程也很有趣。

Supporting Information 支持信息

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The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acs.jpcc.2c07440.
支持信息可在 https://pubs.acs.org/doi/10.1021/acs.jpcc.2c07440 免费获取。

Absorption, fluorescence, and fluorescence excitation spectra, time-resolved fluorescence experiments, spectrum of vibrationally hot mCP molecules at 1400 ps, slow vibrational cooling in mCP thin films, kinetics of mCP at the probe wavelengths of 340 and 400 nm, modeling of collisional energy transfer in mCP thin films, kinetic analysis of transient absorption data, and results of kinetic modeling at the probe wavelength of 318 nm (PDF)
吸收、荧光和荧光激发光谱、时间分辨荧光实验、1400 ps 振动高温 mCP 分子光谱、mCP 薄膜中的缓慢振动冷却、340 和 400 nm 探针波长下的 mCP 动力学、mCP 薄膜中碰撞能量传递的建模、瞬态吸收数据的动力学分析以及 318 nm 探针波长下的动力学建模结果（PDF）

jp2c07440_si_001.pdf (1.05 MB)
jp2c07440_si_001.pdf （1.05 MB）

Understanding Excited-State Relaxation in 1,3-Bis(N‑carbazolyl)benzene, a Host Material for Organic Light-Emitting Diodes

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Supporting Information for

Understanding Excited‐State Relaxation in 1,3‐

Bis(

‐carbazolyl)benzene, a Host Material for Organic

Light‐Emitting Diodes

Marius Morgenroth, Thomas Lenzer,* and Kawon Oum*

University of Siegen, Faculty IV:

School of Science and Technol

ogy, Department Chemistry and

Biology, Physical Chemistry 2, A

dolf‐Reichwein‐Str. 2, 57076 Si

egen, Germany

Email: lenzer@chemie.uni‐siegen.

de, oum@chemie.uni‐siegen.de

Table of Contents

1. Absorption, Fluorescence, and

Fluorescence Excitation Spectr

a ............................................. S2

2. Time‐Resolved Fluor

escence Expe

riments .....................

........................................................... S5

3. Spectrum of Vibrationally H

ot mCP Molecules at 1400 ps .....

.................................................. S8

4. Slow Vibrational Coo

ling in mCP T

hin Films .................

............................................................. S

5. Kinetics of mCP at the Probe Wavelengths 340 nm and 400 nm .

........................................... S10

6. Modeling of Collisional Ener

gy Transfer in mCP Thin Films ..

.................................................. S11

7. Kinetic Analysis of Tran

sient Absorption Data ..............

.......................................................... S13

8. Results of Kinetic Modeling at the Probe Wavelength 318 nm .

............................................. S14

9. Refer

ences .................................................

...............................................................

............... S15

1. Absorption, Fluorescence, and

Fluorescence Excitation Spectr

Table S1 summarizes characteristic spectral parameters of the “

monomer species” in the absorption

and fluorescence spectra of mCP in THF and in thin films. In ge

neral, the spectral shift induced by

the different solvation environments as well as the Stokes shif

t are small.

Table S1.

Peak positions of the 0



0 transition for the “monomer transition” in the absorption and

fluorescence spectra of mCP in

different environments including

Stokes shifts.

Sample

abs









Stokes



Stokes









(nm)

(meV)

(cm

‐1

)

mCP in THF

338.4

343.2

418

mCP:PMMA 2:98

339.4

343.6

355

mCP:PMMA 10:90

339.1

345.5

541

mCP:PMMA 90:10

342.1

349.0

583

mCP:PMMA 98:2

342.3

349.0

565

mCP thin film

342.5

349.5

583

Figure S1 shows fluorescence excitation spectra at different de

tection wavelengths for a neat mCP

thin film. The spectra detected

at the wavelengths 400 nm (red

line) and 530 nm (blue line) are

much broader than the spectrum recorded in the “monomer band” a

t 370 nm (black line). This is

clearly visible in the region of the S



absorption band in the range 320



340 nm. This broadening

of the fluorescence excitation spectrum is characteristic for e

xcimer formation.

Figure S1.

Fluorescence excitation spectra for a neat mCP thin film produ

ced by spin‐coating on

quartz detected at 370 nm (black), 400 nm (red) and 530 nm (blu

e). All spectra were normalized at

341 nm.

Terms & Conditions

Most electronic Supporting Information files are available without a subscription to ACS Web Editions. Such files may be downloaded by article for research use (if there is a public use license linked to the relevant article, that license may permit other uses). Permission may be obtained from ACS for other uses through requests via the RightsLink permission system: http://pubs.acs.org/page/copyright/permissions.html.

Author Information

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Corresponding Authors
- Thomas Lenzer - Faculty IV: School of Science and Technology, Department Chemistry and Biology, Physical Chemistry 2, University of Siegen, Adolf-Reichwein-Str. 2, 57076 Siegen, Germany; https://orcid.org/0000-0002-0766-709X; Email: lenzer@chemie.uni-siegen.de
- Kawon Oum - Faculty IV: School of Science and Technology, Department Chemistry and Biology, Physical Chemistry 2, University of Siegen, Adolf-Reichwein-Str. 2, 57076 Siegen, Germany; https://orcid.org/0000-0001-6137-2236; Email: oum@chemie.uni-siegen.de
Author
- Marius Morgenroth - Faculty IV: School of Science and Technology, Department Chemistry and Biology, Physical Chemistry 2, University of Siegen, Adolf-Reichwein-Str. 2, 57076 Siegen, Germany; https://orcid.org/0000-0001-8528-3502
Notes
The authors declare no competing financial interest.

Acknowledgments

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We thank T. Staedler (Institute of Materials Engineering, University of Siegen) for carrying out and supervising the AFM measurements. We are also thankful to D. H. Choi, M. J. Cho, J. Hwang, and N. Y. Kwon (Department of Chemistry, Korea University) for enlightening discussions regarding OLED materials.

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Kondo, Yasuhiro; Yoshiura, Kazuki; Kitera, Sayuri; Nishi, Hiroki; Oda, Susumu; Gotoh, Hajime; Sasada, Yasuyuki; Yanai, Motoki; Hatakeyama, Takuji
Nature Photonics (2019), 13 (10), 678-682CODEN: NPAHBY; ISSN:1749-4885. (Nature Research)
Luminescent materials that exhibit narrowband emission are vital for full-color displays. Here, we report a thermally activated delayed-fluorescence material that exhibits ultrapure blue emission with full-width at half-max. of just 14 nm. The emitter consists of five benzene rings connected by two boron and four nitrogen atoms and two diphenylamino substituents. The multiple resonance effect of the boron and nitrogen atoms induces significant localization of the highest occupied and lowest unoccupied MOs on different atoms to minimize not only the vibronic coupling between the ground state (S0) and the singlet excited state (S1) but also the energy gap between the S1 state and triplet excited state (T1). Org. light-emitting diode devices employing the emitter emit light at 469 nm with full-width at half-max. of 18 nm with an external quantum efficiency of 34.4% at the max. and 26.0% at 1,000 cd m-2.
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Hwang, J.; Koh, C. W.; Ha, J. M.; Woo, H. Y.; Park, S.; Cho, M. J.; Choi, D. H. Aryl-Annulated [3,2-a] Carbazole-Based Deep-Blue Soluble Emitters for High-Efficiency Solution-Processed Thermally Activated Delayed Fluorescence Organic Light-Emitting Diodes with CIEy <0.1. ACS Appl. Mater. Interfaces 2021, 13, 61454– 61462, DOI: 10.1021/acsami.1c15659
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Aryl-Annulated [3,2-a] Carbazole-Based Deep-Blue Soluble Emitters for High-Efficiency Solution-Processed Thermally Activated Delayed Fluorescence Organic Light-Emitting Diodes with CIEy <0.1
Hwang, Jinhyo; Koh, Chang Woo; Ha, Jung Min; Woo, Han Young; Park, Sungnam; Cho, Min Ju; Choi, Dong Hoon
ACS Applied Materials & Interfaces (2021), 13 (51), 61454-61462CODEN: AAMICK; ISSN:1944-8244. (American Chemical Society)
In this study, we demonstrated two deep-blue TADF emitters, BO-tCzPhICz and BO-tCzDICz, for soln.-processable thermally activated delayed fluorescence org. light-emitting diodes (TADF-OLEDs). They were synthesized by employing an organoboron acceptor and 9-(3,6-di-tert-butyl-9H-carbazol-9-yl)-5-phenyl-5,12-dihydroindolo[3,2-a]carbazole (tCzPhICz) and 12-(3,6-di-tert-butyl-9H-carbazol-9-yl)-15H-diindolo[2,3-b:1',2',3'-lm]carbazole (tCzDICz) as bulky aryl-annulated [3,2-a] carbazole donors, resp. Both emitters showed sufficient soly. in org. solvents, narrow deep-blue emission, and small energy difference (ΔEST) between singlet and triplet states, which can be applied to soln.-processable deep-blue TADF-OLEDs. Soln.-processed OLEDs exploiting these TADF emitters displayed deep-blue electroluminescence with CIEy <0.1, and high external quantum efficiencies of 17.8 and 14.8% were obsd. for BO-tCzPhICz and BO-tCzDICz, resp. The emitter bearing bulky ICz-based donating units shows highly promising potential for high-efficiency soln.-processable deep-blue TADF-OLEDs.
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Monkman, A. Why Do We Still Need a Stable Long Lifetime Deep Blue OLED Emitter?. ACS Appl. Mater. Interfaces 2022, 14, 20463– 20467, DOI: 10.1021/acsami.1c09189
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Why Do We Still Need a Stable Long Lifetime Deep Blue OLED Emitter?
Monkman, Andrew
ACS Applied Materials & Interfaces (2022), 14 (18), 20463-20467CODEN: AAMICK; ISSN:1944-8244. (American Chemical Society)
The need for a high efficiency deep blue org. emitter with narrow emission line width has never been so great. This is driven by the need to simplify the complex OLED stack for displays to enable larger substrate sizes to be used and greatly increase prodn. yields. Here, the merits of using the hyperfluorescence scheme based around new multiresonance boron nitrogen mols. typified by DABNA type emitters are discussed and key requirements for suitable sensitizer hosts described, esp. the photophys. requirements for optimal performance.
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Adamovich, A.; Brooks, J.; Tamayo, A.; Alexander, A. M.; Djurovich, P. I.; D’Andrade, B. W.; Adachi, C.; Forrest, S. R.; Thompson, M. E. High Efficiency Single Dopant White Electrophosphorescent Light Emitting Diodes. New J. Chem. 2002, 26, 1171– 1178, DOI: 10.1039/b204301g
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High-efficiency single dopant white electrophosphorescent light-emitting diodes
Adamovich, Vadim; Brooks, Jason; Tamayo, Arnold; Alexander, Alex M.; Djurovich, Peter I.; D'Andrade, Brian W.; Adachi, Chihaya; Forrest, Stephen R.; Thompson, Mark E.
New Journal of Chemistry (2002), 26 (9), 1171-1178CODEN: NJCHE5; ISSN:1144-0546. (Royal Society of Chemistry)
Efficient white electrophosphorescence has been achieved with a single emissive dopant. The dopant in these white org. light emitting diodes (WOLEDs) emits simultaneously from monomer and aggregate states, leading to a broad spectrum and high-quality white emission. The dopant mols. are based on a series of platinum(2) [2-(4,6-difluorophenyl)pyridinato-N,C2'] β-diketonates. All of the dopant complexes described herein have identical photophysics in dil. soln. with structured blue monomer emission (λmax = 468, 500, 540 nm). A broad orange aggregate emission (λmax ≈ 580 nm) is also obsd., when doped into OLED host materials. The intensity of the orange band increases relative to the blue monomer emission, as the doping level is increased. The ratio of monomer to aggregate emission can be controlled by the doping concn., the degree of steric bulk on the dopant and by the choice of the host material. A doping concn. for which the monomer and excimer bands are approx. equal gives an emission spectrum closest to that for std. white illumination sources. WOLEDs have been fabricated with doped CBP and mCP luminescent layers (CBP = N,N'-dicarbazolyl-4,4'-biphenyl, mCP = N,N'-dicarbazolyl-3,5-benzene). The best efficiencies and color stabilities were achieved when an electron/exciton blocking layer (EBL) is inserted into the structure, between the hole transporting layer and doped CBP or mCP layer. The material used for an EBL in these devices was fac-tris(1-phenylpyrazolato-N,C2')iridium(3). The EBL material effectively prevents electrons and excitons from passing through the emissive layer into the hole transporting NPD layer. CBP-based devices gave a peak external quantum efficiency of 3.3 ± 0.3% (7.3 ± 0.7 lm W-1) at 1 cd m-2, and 2.3 ± 0.2% (5.2 ± 0.3 lm W-1) at 500 cd m-2. MCP based devices gave a peak external quantum efficiency of 6.4% (12.2 lm W-1, 17.0 cd A-1), CIE coordinates of 0.36, 0.44 and a CRI of 67 at 1 cd m-2 (CIE = Commission Internationale de l'Eclairage, CRI = color rendering index). The efficiency of the mCP-based device drops to 4.3 ± 0.5% (8.1 ± 0.6 lm W-1, 11.3 cd A-1) at 500 cd m-2; however, the CIE coordinates and CRI remain unchanged.
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Kim, J. W.; You, S. I.; Kim, N. H.; Yoon, J.-A.; Cheah, K. W.; Zhu, F. R.; Kim, W. Y. Study of Sequential Dexter Energy Transfer in High Efficient Phosphorescent White Organic Light-Emitting Diodes with Single Emissive Layer. Sci. Rep. 2014, 4, 7009, DOI: 10.1038/srep07009
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Study of Sequential Dexter Energy Transfer in High Efficient Phosphorescent White Organic Light-Emitting Diodes with Single Emissive Layer
Kim, Jin Wook; You, Seung Il; Kim, Nam Ho; Yoon, Ju-An; Cheah, Kok Wai; Zhu, Fu Rong; Kim, Woo Young
Scientific Reports (2014), 4 (), 7009CODEN: SRCEC3; ISSN:2045-2322. (Nature Publishing Group)
In this study, we report our effort to realize high performance single emissive layer three color white phosphorescent org. light emitting diodes (PHOLEDs) through sequential Dexter energy transfer of blue, green and red dopants. The PHOLEDs had a structure of; ITO(1500 Å)/NPB(700 Å)/mCP:Firpic-x%:Ir(ppy)3-0.5%:Ir(piq)3-y%(300 Å)/TPBi(300 Å)/Liq(20 Å)/Al(1200 Å). The dopant concns. of FIrpic, Ir(ppy)3 and Ir(piq)3 were adjusted and optimized to facilitate the preferred energy transfer processes attaining both the best luminous efficiency and CIE color coordinates. The presence of a deep trapping center for charge carriers in the emissive layer was confirmed by the obsd. red shift in electroluminescent spectra. White PHOLEDs, with phosphorescent dopant concns. of FIrpic-8.0%:Ir(ppy)3-0.5%:Ir(piq)3-0.5% in the mCP host of the single emissive layer, had a max. luminescence of 37,810 cd/m2 at 11 V and a luminous efficiency of 48.10 cd/A at 5 V with CIE color coordinates of (0.35, 0.41).
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Sun, J. W.; Kim, K.-H.; Moon, C.-K.; Lee, J.-H.; Kim, J.-J. Highly Efficient Sky-Blue Fluorescent Organic Light Emitting Diode Based on Mixed Cohost System for Thermally Activated Delayed Fluorescence Emitter (2CzPN). ACS Appl. Mater. Interfaces 2016, 8, 9806– 9810, DOI: 10.1021/acsami.6b00286
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Highly Efficient Sky-Blue Fluorescent Organic Light Emitting Diode Based on Mixed Cohost System for Thermally Activated Delayed Fluorescence Emitter (2CzPN)
Sun, Jin Won; Kim, Kwon-Hyeon; Moon, Chang-Ki; Lee, Jeong-Hwan; Kim, Jang-Joo
ACS Applied Materials & Interfaces (2016), 8 (15), 9806-9810CODEN: AAMICK; ISSN:1944-8244. (American Chemical Society)
The mixed cohosts of 1,3-bis(N-carbazolyl)benzene and 2,8-bis(diphenylphosphoryl)dibenzothiophene were developed for a highly efficient blue fluorescent org. LED (OLED) doped with a thermally-activated delayed fluorescence (TADF) emitter [4,5-di(9H-carbazol-9-yl)phthalonitrile (2CzPN)]. An external quantum efficiency of 21.8% in blue fluorescent OLEDs was demonstrated, which is identical to the theor. achievable max. electroluminescence efficiency using the emitter. The efficiency roll-off is large even under the excellent charge balance in the device and almost the same as the single host based devices, indicating that the efficiency roll-off in 2CzPN based TADF host is related to the material characteristics, such as low reverse intersystem crossing rate rather than charge imbalance.
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Holmes, R. J.; Forrest, S. R.; Tung, Y.-J.; Kwong, R. C.; Brown, J. J.; Garon, S.; Thompson, M. E. Blue Organic Electrophosphorescence using Exothermic Host–Guest Energy Transfer. Appl. Phys. Lett. 2003, 82, 2422– 2424, DOI: 10.1063/1.1568146
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Blue organic electrophosphorescence using exothermic host-guest energy transfer
Holmes, R. J.; Forrest, S. R.; Tung, Y.-J.; Kwong, R. C.; Brown, J. J.; Garon, S.; Thompson, M. E.
Applied Physics Letters (2003), 82 (15), 2422-2424CODEN: APPLAB; ISSN:0003-6951. (American Institute of Physics)
The authors demonstrate efficient blue electrophosphorescence using exothermic energy transfer from a host consisting of N,N'-dicarbazolyl-3,5-benzene (mCP) to the phosphorescent Ir complex Ir(III)bis[(4,6-difluorophenyl)-pyridinato-N,C2']picolinate (FIrpic). By examg. the temp. dependence of the radiative lifetime and the photoluminescence of a film of mCP doped with FIrpic, the authors confirm the existence of exothermic energy transfer in contrast to the endothermic transfer characteristic of the N,N'-dicarbazolyl-4-4'-biphenyl and FIrpic system. In employing exothermic energy transfer between mCP and FIrpic, a max. external electroluminescent quantum efficiency of (7.5 ± 0.8)% and a luminous power efficiency of (8.9 ± 0.9)lm/W were obtained, representing a significant increase in performance over previous endothermic blue electrophosphorescent devices.
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Hayashi, K.; Nakanotani, H.; Inoue, M.; Yoshida, K.; Mikhnenko, O.; Nguyen, T.-Q.; Adachi, C. Suppression of Roll-Off Characteristics of Organic Light-Emitting Diodes by Narrowing Current Injection/Transport Area to 50 nm. Appl. Phys. Lett. 2015, 106, 093301, DOI: 10.1063/1.4913461
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Suppression of roll-off characteristics of organic light-emitting diodes by narrowing current injection/transport area to 50 nm
Hayashi, Kyohei; Nakanotani, Hajime; Inoue, Munetomo; Yoshida, Kou; Mikhnenko, Oleksandr; Nguyen, Thuc-Quyen; Adachi, Chihaya
Applied Physics Letters (2015), 106 (9), 093301/1-093301/5CODEN: APPLAB; ISSN:0003-6951. (American Institute of Physics)
Using e-beam nanolithog., the current injection/transport area in org. LEDs (OLEDs) was confined into a narrow linear structure with a min. width of 50 nm. This caused suppression of Joule heating and partial sepn. of polarons and excitons, so the charge d. where the electroluminescent efficiency decays to the half of the initial value (J0) was significantly improved. A device with a narrow current injection width of 50 nm exhibited a J0 that was almost 2 orders of magnitude higher compared with that of the unpatterned OLED. (c) 2015 American Institute of Physics.
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Flender, O.; Scholz, M.; Klein, J. R.; Oum, K.; Lenzer, T. Excited-State Relaxation of the Solar Cell Dye D49 in Organic Solvents and on Mesoporous Al₂O₃ and TiO₂ Thin Films. Phys. Chem. Chem. Phys. 2016, 18, 26010– 26019, DOI: 10.1039/c6cp05167g
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Excited-state relaxation of the solar cell dye D49 in organic solvents and on mesoporous Al2O3 and TiO2 thin films
Flender, Oliver; Scholz, Mirko; Klein, Johannes R.; Oum, Kawon; Lenzer, Thomas
Physical Chemistry Chemical Physics (2016), 18 (37), 26010-26019CODEN: PPCPFQ; ISSN:1463-9076. (Royal Society of Chemistry)
We present an ultrafast UV-Vis-NIR transient absorption study of the donor-acceptor solar-cell dye D49 in diisopropyl ether, THF and acetonitrile, as well as on mesoporous Al2O3 and TiO2 thin films. Photoexcitation at 505 nm initially populates the first electronically excited state of the dye having significant intramol. charge transfer character ("S1/ICT"). On Al2O3 and in the three org. solvents, the dynamics are fully explained in terms of S1/ICT stabilization (by reorientation of adjacent solvent or D49 mols. and collisional cooling), intramol. vibrational redistribution and S1/ICT → S0 electronic decay. A substantial decrease of the S1/ICT lifetime is obsd. with increasing polarity of the surrounding medium suggesting an acceleration of internal conversion. In agreement with these results, the addn. of the nonpolar co-adsorbent deoxycholic acid (DCA) to the Al2O3 surface leads to a substantial increase of the S1/ICT lifetime. DCA spacers reduce the local polarity around the dye mols., thus interrupting D49 "self-solvation". These results are in contrast to a recent exptl. study for the indoline dye D131 on Al2O3, where charge transfer from electronically excited D131 to adjacent dye mols. was proposed (Cappel et al., Sci. Rep., 2016, 6, 21276). We do not see evidence for charge transfer processes between D49 mols. and also not for electron injection from D49 into Al2O3 trap states. Charge sepn. is only obsd. for D49 bound to TiO2 thin films, with efficient injection of electrons into the conduction band of the semiconductor via formation of a [D49 +···e-] complex and a transient Stark effect signalling the formation of mobile electrons upon dissocn. of the complex.
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Oum, K.; Lenzer, T.; Scholz, M.; Jung, D. Y.; Sul, O.; Cho, B. J.; Lange, J.; Müller, A. Observation of Ultrafast Carrier Dynamics and Phonon Relaxation of Graphene from the Deep-Ultraviolet to the Visible Region. J. Phys. Chem. C 2014, 118, 6454– 6461, DOI: 10.1021/jp4072197
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Observation of Ultrafast Carrier Dynamics and Phonon Relaxation of Graphene from the Deep-Ultraviolet to the Visible Region
Oum, Kawon; Lenzer, Thomas; Scholz, Mirko; Jung, Dae Yool; Sul, Onejae; Cho, Byung Jin; Lange, Jens; Mueller, Andreas
Journal of Physical Chemistry C (2014), 118 (12), 6454-6461CODEN: JPCCCK; ISSN:1932-7447. (American Chemical Society)
We investigated the ultrafast carrier dynamics and phonon relaxation of CVD-grown monolayer and 9-layer graphene on a quartz substrate. Excitation was performed at 400 and 800 nm. The normalized change in optical d. ΔOD was probed over the range 260-640 nm (1.94-4.77 eV), reaching down into the region of graphene's Fano resonance, previously not investigated in femtosecond broadband pump-probe expts. Time consts. of 160 fs and 4 ps were found and assigned to carrier-optical phonon scattering and slower phonon relaxation processes, resp. The carrier distribution at early times was clearly hotter for 400 nm excitation than for 800 nm excitation. A pronounced spectral bleach feature was obsd. below 300 nm. It immediately formed after photoexcitation and recovered slowly, with a time const. of 35 ps for monolayer and time consts. of 120 and 970 ps for 9-layer graphene. The same dynamics were found for weak transient absorption features above 300 nm, which emerged after ca. 0.5 ps. The slow dynamics were assigned to interfacial heat flow from graphene to the quartz substrate. The bleach and absorption features were well described by a simple model assuming a red-shift of the Fano resonance. This red-shift disappeared with progressive cooling of graphene. We therefore suggest that the red-shift is induced by shrinking of the band sepn. due to lattice heating.
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Merker, A.; Scholz, M.; Morgenroth, M.; Lenzer, T.; Oum, K. Photoinduced Dynamics of (CH₃NH₃)₄Cu₂Br₆ Thin Films Indicating Efficient Triplet Photoluminescence. J. Phys. Chem. Lett. 2021, 12, 2736– 2741, DOI: 10.1021/acs.jpclett.1c00446
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Photoinduced Dynamics of (CH3NH3)4Cu2Br6 Thin Films Indicating Efficient Triplet Photoluminescence
Merker, Alexander; Scholz, Mirko; Morgenroth, Marius; Lenzer, Thomas; Oum, Kawon
Journal of Physical Chemistry Letters (2021), 12 (11), 2736-2741CODEN: JPCLCD; ISSN:1948-7185. (American Chemical Society)
Hybrid org.-inorg. halogenidocuprates based on copper(I) represent materials with rich structural diversity and high photoluminescence (PL) quantum yield, yet the mechanism responsible for their efficient, strongly Stokes-shifted emission is still unclear. Here we report the successful prepn. of (CH3NH3)4Cu2Br6 thin films with a zero-dimensional mol. salt structure featuring "isolated" [Cu2Br6]4- ions. Time-resolved broadband PL measurements provide an excited-state lifetime of 114μs at 298 K. Results from femto- to microsecond UV-vis-NIR transient absorption expts. combined with DFT/TDDFT calcns. suggest the formation of a long-lived structurally relaxed triplet species through intersystem crossing (61 ps), which almost exclusively decays by phosphorescence. In addn., time scales for structural relaxation and cooling processes are extd. from a global kinetic anal. of the transient spectra. Calcns. for the isolated [Cu2Br6]4- anion and the (CH3NH3)4Cu2Br6 crystal suggest a strong impact of the crystal environment on the structure of the anion.
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Oum, K.; Flender, O.; Lohse, P. W.; Scholz, M.; Hagfeldt, A.; Boschloo, G.; Lenzer, T. Electron and Hole Transfer Dynamics of a Triarylamine-Based Dye with Peripheral Hole Acceptors on TiO₂ in the Absence and Presence of Solvent. Phys. Chem. Chem. Phys. 2014, 16, 8019– 8029, DOI: 10.1039/c3cp55298e
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Electron and hole transfer dynamics of a triarylamine-based dye with peripheral hole acceptors on TiO2 in the absence and presence of solvent
Oum, Kawon; Flender, Oliver; Lohse, Peter W.; Scholz, Mirko; Hagfeldt, Anders; Boschloo, Gerrit; Lenzer, Thomas
Physical Chemistry Chemical Physics (2014), 16 (17), 8019-8029CODEN: PPCPFQ; ISSN:1463-9076. (Royal Society of Chemistry)
We studied photoinduced primary charge transfer processes of the sensitizer E6 on TiO2 without solvent and in contact with the org. solvent acetonitrile and the ionic liq. 1-ethyl-3-methylimidazolium tetracyanoborate [C2mim]+[B(CN)4]- using transient absorption spectroscopy, spectroelectrochem., and DFT/TDDFT calcns. E6, which belongs to a family of triarylamine dyes for solar cell applications, features two peripheral triarylamine units which are connected via diether spacer groups to the core chromophore and are designed to act as hole traps. This function was confirmed by spectroelectrochem., where the E6*+ radical cation shows a considerably blue-shifted absorption compared to dyes without these two substituents. This indicates that one of the terminal triarylamine units must carry the pos. charge. After photoexcitation of E6 at 520 nm (S0 S1 band), electrons are injected into TiO2 predominantly within the cross-correlation time (<80 fs), with some subsequent delayed electron injection (τ ca. 250 fs). Importantly, a transient Stark shift (electrochromism) is obsd. (time consts. ca. 0.8 and 12 ps) which is related to a changing elec. field generated by the E6*+ radical cations and injected electrons. This field induces absorption shifts of the dye species on the surface. Interestingly, these dynamics are largely unaffected by solvent mols. However, pronounced differences are obsd. on longer timescales. In contact with solvent, one observes an increase in the E6*+ absorption band above 600 nm with a time const. of 75 ps. This is assigned to hole transfer from the core chromophore to one of the peripheral triarylamine substituents. Electron-cation recombination occurs on much longer timescales and is multiexponential, with time consts. of ca. 100 μs, 1 ms and 15 ms. Because of hole trapping, it is slower than for similar dyes lacking the peripheral triarylamines. Addnl. expts. were performed for E6 attached to the wide band gap semiconductor ZrO2. Here, electron injection occurs into surface trap states with subsequent recombination. Another fraction of non-injecting E6 mols. in S1 quickly decays to S0 (time consts. 1 and 35 ps).
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Morgenroth, M.; Scholz, M.; Guy, L.; Oum, K.; Lenzer, T. Spatiotemporal Mapping of Efficient Chiral Induction by Helicene-Type Additives in Copolymer Thin Films. Angew. Chem., Int. Ed. 2022, 61, e202203075 DOI: 10.1002/anie.202203075
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Spatiotemporal Mapping of Efficient Chiral Induction by Helicene-Type Additives in Copolymer Thin Films
Morgenroth, Marius; Scholz, Mirko; Guy, Laure; Oum, Kawon; Lenzer, Thomas
Angewandte Chemie, International Edition (2022), 61 (31), e202203075CODEN: ACIEF5; ISSN:1433-7851. (Wiley-VCH Verlag GmbH & Co. KGaA)
We obsd. efficient induction of chirality in polyfluorene copolymer thin films by mixing with helicene-type chiral additives based on the dibenzo[c,h]acridine motif. Images obtained from CD (CD) and circularly polarized luminescence (CPL) microscopy provide information about the chiral arrangements in the thin films with diffraction-limited resoln. The CD signal shows a characteristic dependence on the film thickness, which supports a supramol. origin of the strong chiral response of the copolymer. In particular, we demonstrate the discrimination between films of opposite chirality based on their ultrafast transient chiral response through the use of femtosecond broadband CD spectroscopy and a newly developed setup for transient CPL spectroscopy with 28 ps time resoln. A systematic variation of the enantiomeric excess of the chiral additive shows that the "Sergeants and Soldiers" concept and "Majority Rules" are not obeyed.
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Mao, L.; Ritcey, A. M. Excimer Formation and Excitation Energy Transfer in Langmuir-Blodgett Films of Cellulose Derivatives Containing Carbazole. Thin Solid Films 1996, 284–285, 618– 621, DOI: 10.1016/s0040-6090(95)08405-3
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Excimer formation and excitation energy transfer in Langmuir-Blodgett films of cellulose derivatives containing carbazole
Mao, Lijun; Ritcey, Anna M.
Thin Solid Films (1996), 284-285 (), 618-621CODEN: THSFAP; ISSN:0040-6090. (Elsevier)
Fluorescence emission spectra are recorded for Langmuir-Blodgett (LB) films prepd. from cellulose derivs. contg. carbazole as a side chain substituent. Three samples of differing carbazole content are considered. At low carbazole content, only the fluorescence characteristic of monomeric carbazole emission is obsd. Clear excimer emission is, however, evident in the spectra of the more highly substituted derivs. Since similar excimer formation is not obsd. in solns. or cast films of the same polymers, it is concluded that mol. ordering in the LB films favors the parallel alignment of chromophore units. For a given polymer of sufficiently high carbazole content, the relative intensity of the excimer emission increases with the no. of transferred layers, indicating the interaction of chromophores in neighboring layers. Energy transfer between layers is also evident in expts. involving multilayer assemblies composed of a monolayer of 1-pyrenedodecanoic acid, covered with a varying no. of layers of the carbazole-contg. polymer. Excitation of carbazole results in strong emission from pyrene, whereas only weak emission is obsd. from carbazole. It is therefore concluded that energy transfer from carbazole to pyrene occurs in these LB films.
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Tani, K.; Tohda, Y.; Takemura, H.; Ohkita, H.; Ito, S.; Yamamoto, M. Synthesis and Photophysical Properties of [3.3](3,9)Carbazolophanes. Chem. Commun. 2001, 1914– 1915, DOI: 10.1039/b104101k
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Synthesis and photophysical properties of [3.3](3,9)carbazolophanes
Tani, Keita; Tohda, Yasuo; Takemura, Hiroyuki; Ohkita, Hideo; Ito, Shinzaburo; Yamamoto, Masahide
Chemical Communications (Cambridge, United Kingdom) (2001), (19), 1914-1915CODEN: CHCOFS; ISSN:1359-7345. (Royal Society of Chemistry)
Syn- And anti-[3.3](3,9)carbazolophanes, which are suitable model compds. for sandwich and partial-overlap excimers, resp., were synthesized and characterized; the structures of both singlet and triplet carbazole excimer have been described. Intramol. cyclization of ammonium cyanide with 9,9'-(1,3-propanediyl)bis[3-(bromomethyl)-9H-carbazole] gave the title compds.
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Winnik, F. M. Photophysics of Preassociated Pyrenes in Aqueous Polymer Solutions and in Other Organized Media. Chem. Rev. 1993, 93, 587– 614, DOI: 10.1021/cr00018a001
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Photophysics of preassociated pyrenes in aqueous polymer solutions and in other organized media
Winnik, Francoise M.
Chemical Reviews (Washington, DC, United States) (1993), 93 (2), 587-614CODEN: CHREAY; ISSN:0009-2665.
A review with 155 refs.
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Musser, A. J.; Rajendran, S. K.; Georgiou, K.; Gai, L.; Grant, R. T.; Shen, Z.; Cavazzini, M.; Ruseckas, A.; Turnbull, G. A.; Samuel, D. W. Intermolecular States in Organic Dye Dispersions: Excimers vs. Aggregates. J. Mater. Chem. C 2017, 5, 8380– 8389, DOI: 10.1039/c7tc02655b
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Intermolecular states in organic dye dispersions: excimers vs. aggregates
Musser, A. J.; Rajendran, S. K.; Georgiou, K.; Gai, L.; Grant, R. T.; Shen, Z.; Cavazzini, M.; Ruseckas, A.; Turnbull, G. A.; Samuel, I. D. W.; Clark, J.; Lidzey, D. G.
Journal of Materials Chemistry C: Materials for Optical and Electronic Devices (2017), 5 (33), 8380-8389CODEN: JMCCCX; ISSN:2050-7534. (Royal Society of Chemistry)
Rapid excited-state quenching in the solid state is a widespread limitation for org. chromophores. Even when mols. are dispersed in neutral host matrixes, photoluminescence quantum yields decrease sharply with increased concn., pointing to efficient intermol. non-radiative decay pathways that remain poorly understood. Here we study the nature of the intermol. states formed in dispersions of the prototypical BODIPY dyes. Using temp.-dependent and time-resolved photoluminescence measurements, we describe the processes of energy transfer into excimer states and, in materials with suitable chem. structure, excitonically coupled dimers. These dimer states exhibit remarkable near-unity quantum yield.
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Spano, F. C. The Spectral Signatures of Frenkel Polarons in H- and J-Aggregates. Acc. Chem. Res. 2010, 43, 429– 439, DOI: 10.1021/ar900233v
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The Spectral Signatures of Frenkel Polarons in H- and J-Aggregates
Spano, Frank C.
Accounts of Chemical Research (2010), 43 (3), 429-439CODEN: ACHRE4; ISSN:0001-4842. (American Chemical Society)
A review. Electronic excitations in small aggregates, thin films, and crystals of conjugated org. mols. play a fundamental role in the operation of a wide array of org.-based devices including solar cells, transistors, and light-emitting diodes. Such excitations, or excitons, are generally spread out over several mols.: a balance between the delocalizing influence of resonant intermol. coupling and the localizing influence of static and dynamic disorder dets. the coherence range of the exciton. Because of the soft nature of org. materials, significant nuclear relaxation in the participating mols. also accompanies the electronic excitations. To properly understand energy or charge transport, 1 must treat intermol. (excitonic) coupling, electron-vibrational coupling, and disorder on equal footing. In this Account, the authors review the key elements of a theor. approach based on a multiparticle representation that describes electronic excitations in org. materials as vibronic excitations surrounded by a field of vibrational excitations. Such composite excitations are appropriately called Frenkel excitonic polarons. For many conjugated mols., the bulk of the nuclear reorganization energy following electronic excitation arises from the elongation of a sym. vinyl stretching mode with energy ∼1400 cm-1. To appreciate the impact of aggregation, the authors study how the vibronic progression of this mode, which dominates the isolated (solvated) mol. absorption and emission spectra, is distorted when mols. are close enough to interact with each other. As the authors demonstrate in this Account, the nature of the distortion provides a wealth of information about how the mols. are packed, the strength of the excitonic interactions between mols., the no. of mols. that are coherently coupled, and the nature of the disorder. The aggregation-induced deviations from the Poissonian distribution of vibronic peak intensities take on 2 extremes identified with ideal H- and J-aggregates. The sign of the nearest neighbor electronic coupling, pos. for H and neg. for J, distinguishes the 2 basic aggregate forms. For several decades, researchers have known that H-aggregates exhibit blue-shifted absorption spectra and are subradiant while J-aggregates exhibit the opposite behavior (red shifted absorption and superradiance). However, the exact inclusion of exciton-vibrational coupling reveals several more distinguishing traits between the 2 aggregate types: in H(J)-aggregates the ratio of the 1st 2 vibronic peak intensities in the absorption spectrum decreases (increases) with increasing excitonic coupling, while the ratio of the 0-0 to 0-1 emission intensities increases (decreases) with disorder and increases (decreases) with increasing temp. These 2 extreme behaviors provide the framework for understanding absorption and emission in more complex morphologies, such as herringbone packing in oligo(phenylene vinylene)s, oligothiophenes and polyacene crystals, as well as the polymorphic packing arrangements obsd. in carotenoids.
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Mai, V. T. N.; Ahmad, V.; Mamada, M.; Fukunaga, T.; Shukla, A.; Sobus, J.; Krishnan, G.; Moore, E. G.; Andersson, G. G.; Adachi, C. Solid Cyclooctatetraene-Based Triplet Quencher Demonstrating Excellent Suppression of Singlet–Triplet Annihilation in Optical and Electrical Excitation. Nat. Commun. 2020, 11, 5623, DOI: 10.1038/s41467-020-19443-z
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Solid cyclooctatetraene-based triplet quencher demonstrating excellent suppression of singlet-triplet annihilation in optical and electrical excitation
Mai, Van T. N.; Ahmad, Viqar; Mamada, Masashi; Fukunaga, Toshiya; Shukla, Atul; Sobus, Jan; Krishnan, Gowri; Moore, Evan G.; Andersson, Gunther G.; Adachi, Chihaya; Namdas, Ebinazar B.; Lo, Shih-Chun
Nature Communications (2020), 11 (1), 5623CODEN: NCAOBW; ISSN:2041-1723. (Nature Research)
Triplet excitons have been identified as the major obstacle to the realization of org. laser diodes, as accumulation of triplet excitons leads to significant losses under continuous wave (CW) operation and/or elec. excitation. Here, we report the design and synthesis of a solid-state org. triplet quencher, as well as in-depth studies of its dispersion into a soln. processable bis-stilbene-based laser dye. By blending the laser dye with 20 wt% of the quencher, negligible effects on the ASE thresholds, but a complete suppression of singlet-triplet annihilation (STA) and a 20-fold increase in excited-state photostability of the laser dye under CW excitation, were achieved. We used small-area OLEDs (0.2 mm2) to demonstrate efficient STA suppression by the quencher in the nanosecond range, supported by simulations to provide insights into the obsd. STA quenching under elec. excitation. The results demonstrate excellent triplet quenching ability under both optical and elec. excitations in the nanosecond range, coupled with excellent soln. processability.
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Morgenroth, M.; Scholz, M.; Guy, L.; Oum, K.; Lenzer, T. Ultrafast Excited-State Dynamics of Thin Films Consisting of Helicene-Like Molecules Based on Dibenzo[c,h]acridine. Mol. Phys. 2021, 119, e1959072 DOI: 10.1080/00268976.2021.1959072
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Ultrafast excited-state dynamics of thin films consisting of helicene-like molecules based on dibenzo[c,h]acridine
Morgenroth, Marius; Scholz, Mirko; Guy, Laure; Oum, Kawon; Lenzer, Thomas
Molecular Physics (2021), 119 (17-18), e1959072/1-e1959072/11CODEN: MOPHAM; ISSN:0026-8976. (Taylor & Francis Ltd.)
We investigated the excited-state dynamics of bridged and non-bridged chiral helicene-like bidibenzo[c,h]acridines deposited as a thin film. Broadband UV/VIS-NIR transient absorption spectroscopy from the femtosecond to the microsecond time-scale and transient fluorescence measurements were employed to understand the photoinduced processes in these org. thin-film layers. Photoexcitation at 370 nm populates the S1 state, which has a lifetime of 15 and 36 ps for the non-bridged and bridged deriv., resp. In both cases, the decay is governed by internal conversion (78% and 84%), whereas intersystem crossing represents only a smaller channel (21% and 15%), and the remaining 1% accounting for fluorescence. Fast repopulation of the S0 state leads to the generation of hot ground state mols. (S0*), with characteristic finger-type transient absorption features. The T1 state decays back to S0 by intersystem crossing with a time const. of 775 and 5500 ps, resp. Even at high excitation fluence, there are no indications for higher-order processes, such as singlet-singlet annihilation or Forster energy transfer. Photoexcitation also launches coherent acoustic phonons in these films, appearing as damped oscillations in the transient absorption kinetics on the red and blue edge of each S0 absorption band, with a period of about 190 ps.
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Morgenroth, M.; Scholz, M.; Lenzer, T.; Oum, K. Ultrafast UV–Vis Transient Absorption and Circular Dichroism Spectroscopy of a Polyfluorene Copolymer Showing Large Chiral Induction. J. Phys. Chem. C 2020, 124, 10192– 10200, DOI: 10.1021/acs.jpcc.0c00976
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Ultrafast UV-Vis Transient Absorption and Circular Dichroism Spectroscopy of a Polyfluorene Copolymer Showing Large Chiral Induction
Morgenroth, Marius; Scholz, Mirko; Lenzer, Thomas; Oum, Kawon
Journal of Physical Chemistry C (2020), 124 (18), 10192-10200CODEN: JPCCCK; ISSN:1932-7447. (American Chemical Society)
Ultrafast broadband UV-vis transient CD (TrCD) and transient absorption (TA) spectroscopy are employed to study the dynamics of chiral polymer thin films. Helical cholesteric arrangements are formed by mixing chiral binaphthalene derivs. with the intrinsically achiral polyfluorene-based copolymer F8BT. The induced CD (ICD) response of F8BT shows a mirror-image-like shape when the chirality of the additive is changed. Peaks in the CD spectra reach large abs. values of about 800 mdeg. Photoexcitation by ultrashort laser pulses results in TrCD spectra of opposite sign for cholesteric polymer films of different helicity, demonstrating transient chiral discrimination on the femtosecond time scale over the wavelength range 260-700 nm. Differences between the excited-state and ground-state CD spectra considerably depend on the electronic transition. The excited-state processes in these systems are evaluated by a detailed kinetic anal. providing rate consts. for the relaxation of the different singlet exciton states as well as singlet-singlet annihilation processes of F8BT involving diffusive bimol. recombination and Forster resonance energy transfer.
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Lenzer, T.; Ehlers, F.; Scholz, M.; Oswald, R.; Oum, K. Assignment of Carotene S* State Features to the Vibrationally Hot Ground Electronic State. Phys. Chem. Chem. Phys. 2010, 12, 8832– 8839, DOI: 10.1039/b925071a
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Assignment of carotene S* state features to the vibrationally hot ground electronic state
Lenzer, Thomas; Ehlers, Florian; Scholz, Mirko; Oswald, Rainer; Oum, Kawon
Physical Chemistry Chemical Physics (2010), 12 (31), 8832-8839CODEN: PPCPFQ; ISSN:1463-9076. (Royal Society of Chemistry)
The so-called S* state has been suggested to play an important role in the photophysics of β-carotene and other carotenoids in soln. and photosynthetic light-harvesting complexes, yet its origin has remained elusive. The present expts. employing temp.-dependent steady-state absorption spectroscopy and ultrafast pump-supercontinuum probe (PSCP) transient absorption measurements of β-carotene in soln. demonstrate that the spectral features of S* are due to vibrationally excited mols. in the ground electronic state S0. Characteristic spectral signatures, such as a highly structured bleach below 500 nm and absorption in the range 500-660 nm result from the superposition of hot S0 absorption ("S0*") on top of the ground-state bleach of room-temp. mols. Appearance and disappearance of the S0* mols. can be completely described by a global kinetic anal. employing time-dependent species-assocd. spectra without the need to invoke the population of an intermediate electronically excited state.
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Schwarzer, D.; Troe, J.; Votsmeier, M.; Zerezke, M. Collisional Deactivation of Vibrationally Highly Excited Azulene in Compressed Liquids and Supercritical Fluids. J. Chem. Phys. 1996, 105, 3121– 3131, DOI: 10.1063/1.472180
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Collisional deactivation of vibrationally highly excited azulene in compressed liquids and supercritical fluids
Schwarzer, D.; Troe, J.; Votsmeier, M.; Zerezke, M.
Journal of Chemical Physics (1996), 105 (8), 3121-3131CODEN: JCPSA6; ISSN:0021-9606. (American Institute of Physics)
The collisional deactivation of vibrationally highly excited azulene was studied from the gas to the compressed liq. phase. Employing supercrit. fluids like He, Xe, CO2, and ethane at pressures of 6-4000 bar and temps. ≥380 K, measurements over the complete gas-liq. transition were performed. Azulene with an energy of 18 000 cm-1 was generated by laser excitation into the S1 and internal conversion to the S0*-ground state. The subsequent loss of vibrational energy was monitored by transient absorption at the red edge of the S3←S0 absorption band near 290 nm. Transient signals were converted into energy-time profiles using hot band absorption coeffs. from shock wave expts. for calibration and accounting for solvent shifts of the spectra. Under all conditions, the decays were mono-exponential. At densities below 1 mol/L, collisional deactivation rates increased linearly with fluid d. Av. energies 〈ΔE〉 transferred per collision agreed with data from dil. gas phase expts. For Xe, CO2, and C2H6, the linear relation between cooling rate and diffusion coeff. scaled collision frequencies ZD turned over to a much weaker dependence at ZD>0.3 ps-1. Up to collision frequencies of ZD = 15 ps-1 this behavior can well be rationalized by a model employing an effective collision frequency related to the finite lifetime of collision complexes.
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Grahn, H. T.; Maris, H. J.; Tauc, J. Picosecond Ultrasonics. IEEE J. Quantum Electron. 1989, 25, 2562– 2569, DOI: 10.1109/3.40643
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Picosecond ultrasonics
Grahn, Holger T.; Maris, Humphrey J.; Tauc, Jan
IEEE Journal of Quantum Electronics (1989), 25 (12), 2562-9CODEN: IEJQA7; ISSN:0018-9197.
A review with 20 refs. The generation and detection of ultrasonic waves and picosecond time resoln. is described. With picosecond ultrasonics, the frequency range of acoustic waves is extended into the 100 GHz range. It allows the investigation of thin-film materials, microstructures, and materials with large acoustic attenuation. Velocity and attenuation measurements on a variety of thin films are reviewed. The results on complex microstructures including multilayers are presented.
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Ruello, P.; Gusev, V. E. Physical Mechanisms of Coherent Acoustic Phonons Generation by Ultrafast Laser Action. Ultrasonics 2015, 56, 21– 35, DOI: 10.1016/j.ultras.2014.06.004
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Physical mechanisms of coherent acoustic phonons generation by ultrafast laser action
Ruello, Pascal; Gusev, Vitalyi E.
Ultrasonics (2015), 56 (), 21-35CODEN: ULTRA3; ISSN:0041-624X. (Elsevier B.V.)
In this review we address the microscopic mechanisms that are involved in the photogeneration processes of GHz-THz coherent acoustic phonons (CAP) induced by an ultrafast laser pulse. Understanding and describing the underlying physics is necessary indeed for improving the future sources of coherent acoustic phonons useful for the non-destructive testing optoacoustic techniques. Getting more phys. insights on these processes also opens new perspectives for the emerging field of the opto-mechanics where lattice motions (surface and/or interfaces ultrafast displacements, nanostructures resonances) are controlled by light. We will then remind the basics of electron-phonon and photon-phonon couplings by discussing the deformation potential mechanism, the thermoelasticity, the inverse piezoelec. effect and the electrostriction in condensed matter. Metals, semiconductors and oxide materials will be discussed. The contribution of all these mechanisms in the photogeneration process of sound will be illustrated over several examples coming from the rich literature.
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Scholz, M.; Morgenroth, M.; Lenzer, M. J.; Choi, D. H.; Lenzer, T.; Oum, K. Coherent Acoustic Phonon Dynamics in Chiral Copolymers. Struct. Dyn. 2019, 6, 064502, DOI: 10.1063/1.5124438
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Coherent acoustic phonon dynamics in chiral copolymers
Scholz, Mirko; Morgenroth, Marius; Cho, Min Ju; Choi, Dong Hoon; Lenzer, Thomas; Oum, Kawon
Structural Dynamics (2019), 6 (6), 064502/1-064502/7CODEN: SDTYAE; ISSN:2329-7778. (American Institute of Physics)
Coherent phonon oscillations in the UV-Vis transient absorption and CD response of two chiral polyfluorene-based copolymer thin films are investigated. A slow oscillation in the hundred picosecond regime indicates the propagation of a longitudinal acoustic phonon with a frequency in the gigahertz range through cholesteric films of PFPh and PFBT, which allow for the optical detn. of the longitudinal sound velocity in these polymers, with values of (2550 ± 140) and (2490 ± 150) m s-1, resp. The oscillation is induced by a strain wave, resulting in a pressure-induced periodic shift of the electronic absorption bands, as extd. from a Fourier anal. of the transient spectra. The acoustic phonon oscillation is also clearly detected in the transient CD (TrCD) response of PFPh, indicating a transient pressure-induced shift of the CD spectrum and possibly also phonon-induced chirality changes via pitch length modulation of the cholesteric helical polymer stack. (c) 2019 American Institute of Physics.
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Morgenroth, M.; Scholz, M.; Cho, M. J.; Choi, D. H.; Oum, K.; Lenzer, T. Mapping the Broadband Circular Dichroism of Copolymer Films with Supramolecular Chirality in Time and Space. Nat. Commun. 2022, 13, 210, DOI: 10.1038/s41467-021-27886-1
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Mapping the broadband circular dichroism of copolymer films with supramolecular chirality in time and space
Morgenroth, Marius; Scholz, Mirko; Cho, Min Ju; Choi, Dong Hoon; Oum, Kawon; Lenzer, Thomas
Nature Communications (2022), 13 (1), 210CODEN: NCAOBW; ISSN:2041-1723. (Nature Portfolio)
Abstr.: Measurements of the electronic CD (CD) are highly sensitive to the abs. configuration and conformation of chiral mols. and supramol. assemblies and have therefore found widespread application in the chem. and biol. sciences. Here, we demonstrate an approach to simultaneously follow changes in the CD and absorption response of photoexcited systems over the UV-visible spectral range with 100 fs time resoln. We apply the concept to chiral polyfluorene copolymer thin films and track their electronic relaxation in detail. The transient CD signal stems from the supramol. response of the system and provides information regarding the recovery of the electronic ground state. This allows for a quantification of singlet-singlet annihilation and charge-pair formation processes. Spatial mapping of chiral domains on femtosecond time scales with a resoln. of 50μm and diffraction-limited steady-state imaging of the CD and the circularly polarised luminescence (CPL) of the films is demonstrated.
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Murawski, C.; Leo, K.; Gather, M. C. Efficiency Roll-Off in Organic Light-Emitting Diodes. Adv. Mater. 2013, 25, 6801– 6827, DOI: 10.1002/adma.201301603
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Efficiency Roll-Off in Organic Light-Emitting Diodes
Murawski, Caroline; Leo, Karl; Gather, Malte C.
Advanced Materials (Weinheim, Germany) (2013), 25 (47), 6801-6827CODEN: ADVMEW; ISSN:0935-9648. (Wiley-VCH Verlag GmbH & Co. KGaA)
A review. Org. light-emitting diodes (OLEDs) have attracted much attention in research and industry thanks to their capability to emit light with high efficiency and to deliver high-quality white light that provides good color rendering. OLEDs feature homogeneous large area emission and can be produced on flexible substrates. In terms of efficiency, OLEDs can compete with highly efficient conventional light sources but their efficiency typically decreases at high brightness levels, an effect known as efficiency roll-off. In recent years, much effort has been undertaken to understand the underlying processes and to develop methods that improve the high-brightness performance of OLEDs. In this review, we summarize the current knowledge and provide a detailed description of the relevant principles, both for phosphorescent and fluorescent emitter mols. In particular, we focus on exciton-quenching mechanisms, such as triplet-triplet annihilation, quenching by polarons, or field-induced quenching, but also discuss mechanisms such as changes in charge carrier balance. We further review methods that may reduce the roll-off and thus enable OLEDs to be used in high-brightness applications.
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Klein, J. R.; Scholz, M.; Oum, K.; Lenzer, T. Photoinduced Dynamics of the Hole-Transport Material H101 in Organic Solvents and on Mesoporous Al₂O₃ and TiO₂ Thin Films. Phys. Chem. Chem. Phys. 2017, 19, 21748– 21758, DOI: 10.1039/c7cp01718a
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Photoinduced dynamics of the hole-transport material H101 in organic solvents and on mesoporous Al2O3 and TiO2 thin films
Klein, Johannes R.; Scholz, Mirko; Oum, Kawon; Lenzer, Thomas
Physical Chemistry Chemical Physics (2017), 19 (32), 21748-21758CODEN: PPCPFQ; ISSN:1463-9076. (Royal Society of Chemistry)
The authors present a comprehensive steady-state and time-resolved UV-Vis-NIR absorption and fluorescence study of the hole-transport material H101 to characterize its photophysics in different org. solvents and on mesoporous Al2O3 and TiO2 thin films. Photoexcitation of H101 at 400 nm in org. solvents populates the S1 state which shows intramol. relaxation on a picosecond time scale. Branching from the relaxed S1 state leads to population of the T1 triplet state and the ground electronic state S0. Triplet formation is induced by the internal heavy-atom effect of the sulfur atom, and the triplet yield decreases substantially with solvent polarity. On mesoporous Al2O3, intermol. exciton-splitting is obsd. leading to the formation of a radical cation - radical anion pair (H101(S1) + H101(S0) → H101 + + H101-) followed by exciton recombination. On mesoporous TiO2, efficient electron injection is obsd. in addn. to exciton-splitting. Complementary spectroelectrochem. expts. enable a full spectral characterization of the cation species H101+ and H1012+. Extensive DFT/TDDFT calcns. successfully assign the spectral features of all exptl. obsd. species. Implications for the function of H101 in photovoltaic devices are discussed.
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Förster, T. Zwischenmolekulare Energiewanderung und Fluoreszenz. Ann. Phys. 1948, 437, 55– 75, DOI: 10.1002/andp.19484370105
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Förster, T. Transfer Mechanisms of Electronic Excitation. Discuss. Faraday Soc. 1959, 27, 7– 17, DOI: 10.1039/DF9592700007
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Stevens, M. A.; Silva, C.; Russell, D. M.; Friend, R. H. Exciton Dissociation Mechanisms in the Polymeric Semiconductors Poly(9,9-dioctylfluorene) and Poly(9,9-dioctylfluorene-co-benzothiadiazole). Phys. Rev. B 2001, 63, 165213, DOI: 10.1103/physrevb.63.165213
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Exciton dissociation mechanisms in the polymeric semiconductors poly(9,9-dioctylfluorene) and poly(9,9-dioctylfluorene-co-benzothiadiazole)
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Alexandre Malinge, Shiv Kumar, Dongyang Chen, Eli Zysman-Colman, Stéphane Kéna-Cohen. Heavy Atom Effect in Halogenated mCP and Its Influence on the Efficiency of the Thermally Activated Delayed Fluorescence of Dopant Molecules. The Journal of Physical Chemistry C 2024, 128 (3) , 1122-1130. https://doi.org/10.1021/acs.jpcc.3c05567
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The Journal of Physical Chemistry C
物理化学杂志 C

Cite this: J. Phys. Chem. C 2023, 127, 9, 4582–4593
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https://doi.org/10.1021/acs.jpcc.2c07440

Published February 28, 2023
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Abstract
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Scheme 1
Scheme 1. Chemical Structure of mCP
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Figure 1
Figure 1. (a) Normalized steady-state absorption and fluorescence spectra (λ_exc = 290 nm), shown as dotted lines and solid lines with shading, respectively, for mCP dissolved in THF, mCP:PMMA blends with weight ratios of 2:98, 10:90, and 90:10, and a neat mCP thin film (from top to bottom), (b) thin-film spectra with the same color coding as that in panel (a), but compared on an absolute scale (absorption spectra shown down to 200 nm), and (c) superimposed normalized steady-state fluorescence spectra from panel (a) [the same color coding as that in panels (a,b)] including colored areas indicating the wavelength ranges of the bandpass filters employed for the TCSPC experiments: center wavelengths of (i) 370, (ii) 430, and (iii) 560 nm (FWHM in each case is 10 nm). (d) TCSPC data (symbols) recorded at 1 MHz repetition frequency with excitation at 273 nm for mCP in THF, the three different mCP:PMMA blends, and the neat mCP film obtained using the 370 nm bandpass filter [the same color coding as that in panels (a–c)]. Fit lines obtained from reconvolution using mono- or biexponential fit functions. (e) TCSPC data recorded at 1 MHz for the neat mCP film (symbols) and the kinetic fits (lines) are compared for the spectral regions (i) 370 nm (brown range), (ii) 430 nm (sky blue range), and (iii) 560 nm (orange range). (f) Same as in panel (e), but obtained with the 560 nm bandpass filter. In panels (d–f), the IRF is shown as black crosses.
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Figure 2
Figure 2. (a) Contour plot of transient absorption (TA) spectra of mCP in THF upon photoexcitation at 290 nm (femtosecond TA) and at 266 nm (nanosecond TA). Note the logarithmic time scale. (b–d) Transient absorption spectra at different delay times, as indicated by the colors. The transient spectrum at 100 ns, which represents the GSB and the ESA of the T₁ state, is filled with red color. (e) Inverted normalized steady-state absorption (red) and stimulated emission (blue). (f) Kinetics at 287 nm (GSB, black), 340 nm (blue), 600 nm (violet) (mostly S₁ ESA), and 1200 nm (brown) from femtosecond TA. (g) Kinetics of the S₁ region (blue, averaged over the range 310–340 nm) and of the T₁ region (red, averaged over the range 390–410 nm) upon nanosecond photoexcitation. (h–n) Corresponding plots for a neat mCP thin film.
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Figure 3
Figure 3. Decay kinetics of highly vibrationally excited mCP in the spectral region of the hot ground electronic state (S₀^*) for a neat mCP thin film recorded at 303 nm (black), 340 nm (red), and 348 nm (blue) up to 2000 ns. The correspondingly colored lines are triexponential fits with common time constants of τ₁ = 2 ns, τ₂ = 100 ns, and τ₃ = 1000 ns. The inset shows a magnification up to 100 ns.
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Figure 4
Figure 4. (a) Contour plot of ultrafast transient absorption spectra of an mCP:PMMA blend (10:90) upon photoexcitation at 290 nm. Note the logarithmic time axis. (b) Transient absorption spectra at different delay times in the range 0.2–1400 ps, with representative spectra indicated by thick colored lines. (c) Kinetics at 282 nm (GSB, black), 340 nm (red), 600 nm (violet), and 1100 nm (brown) shown up to 500 ps. (d) Same as in panel (c), but showing the dynamics at early times up to 50 ps.
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Figure 5
Figure 5. (a) Transient absorption kinetics of a neat mCP thin film up to delay times of 100 ps detected at a probe wavelength of 340 nm after photoexcitation at 290 nm. (b) Same as in panel (a), but shown up to 1500 ps. Initial exciton number densities N(S_x): 3.15 × 10¹⁸ (blue circles), 6.30 × 10¹⁸ (red circles), and 1.19 × 10¹⁹ cm^–3 (black circles). (c) Two AFM measurements for the mCP thin film providing a film thickness of 253 nm. (d) Oscillatory kinetics averaged over the spectral range 345–355 nm originating from a coherent acoustic phonon propagating in the film.
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Figure 6
Figure 6. Overview of the different pathways for the relaxation of mCP in the excited state. Details regarding the kinetic modeling are provided in the text.
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Figure 7
Figure 7. Kinetic modeling of the S₀^* number density N for the three different initial S_x number densities of 3.15 × 10¹⁸ (blue), 6.30 × 10¹⁸ (red), and 1.19 × 10¹⁹ cm^–3 (black). Solid lines are for the kinetic model with diffusive SSA, and dashed lines are for the model with SSA based on direct S₁–S₁ homo-FRET. (a) Time range up to 50 ps. (b) Time range up to 300 ps. (c) Time range up to 1500 ps.
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References
This article references 43 other publications.
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8. 8
  Hwang, J.; Koh, C. W.; Ha, J. M.; Woo, H. Y.; Park, S.; Cho, M. J.; Choi, D. H. Aryl-Annulated [3,2-a] Carbazole-Based Deep-Blue Soluble Emitters for High-Efficiency Solution-Processed Thermally Activated Delayed Fluorescence Organic Light-Emitting Diodes with CIEy <0.1. ACS Appl. Mater. Interfaces 2021, 13, 61454– 61462, DOI: 10.1021/acsami.1c15659
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  8
  Aryl-Annulated [3,2-a] Carbazole-Based Deep-Blue Soluble Emitters for High-Efficiency Solution-Processed Thermally Activated Delayed Fluorescence Organic Light-Emitting Diodes with CIEy <0.1
  Hwang, Jinhyo; Koh, Chang Woo; Ha, Jung Min; Woo, Han Young; Park, Sungnam; Cho, Min Ju; Choi, Dong Hoon
  ACS Applied Materials & Interfaces (2021), 13 (51), 61454-61462CODEN: AAMICK; ISSN:1944-8244. (American Chemical Society)
  In this study, we demonstrated two deep-blue TADF emitters, BO-tCzPhICz and BO-tCzDICz, for soln.-processable thermally activated delayed fluorescence org. light-emitting diodes (TADF-OLEDs). They were synthesized by employing an organoboron acceptor and 9-(3,6-di-tert-butyl-9H-carbazol-9-yl)-5-phenyl-5,12-dihydroindolo[3,2-a]carbazole (tCzPhICz) and 12-(3,6-di-tert-butyl-9H-carbazol-9-yl)-15H-diindolo[2,3-b:1',2',3'-lm]carbazole (tCzDICz) as bulky aryl-annulated [3,2-a] carbazole donors, resp. Both emitters showed sufficient soly. in org. solvents, narrow deep-blue emission, and small energy difference (ΔEST) between singlet and triplet states, which can be applied to soln.-processable deep-blue TADF-OLEDs. Soln.-processed OLEDs exploiting these TADF emitters displayed deep-blue electroluminescence with CIEy <0.1, and high external quantum efficiencies of 17.8 and 14.8% were obsd. for BO-tCzPhICz and BO-tCzDICz, resp. The emitter bearing bulky ICz-based donating units shows highly promising potential for high-efficiency soln.-processable deep-blue TADF-OLEDs.
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9. 9
  Monkman, A. Why Do We Still Need a Stable Long Lifetime Deep Blue OLED Emitter?. ACS Appl. Mater. Interfaces 2022, 14, 20463– 20467, DOI: 10.1021/acsami.1c09189
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  9
  Why Do We Still Need a Stable Long Lifetime Deep Blue OLED Emitter?
  Monkman, Andrew
  ACS Applied Materials & Interfaces (2022), 14 (18), 20463-20467CODEN: AAMICK; ISSN:1944-8244. (American Chemical Society)
  The need for a high efficiency deep blue org. emitter with narrow emission line width has never been so great. This is driven by the need to simplify the complex OLED stack for displays to enable larger substrate sizes to be used and greatly increase prodn. yields. Here, the merits of using the hyperfluorescence scheme based around new multiresonance boron nitrogen mols. typified by DABNA type emitters are discussed and key requirements for suitable sensitizer hosts described, esp. the photophys. requirements for optimal performance.
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10. 10
  Adamovich, A.; Brooks, J.; Tamayo, A.; Alexander, A. M.; Djurovich, P. I.; D’Andrade, B. W.; Adachi, C.; Forrest, S. R.; Thompson, M. E. High Efficiency Single Dopant White Electrophosphorescent Light Emitting Diodes. New J. Chem. 2002, 26, 1171– 1178, DOI: 10.1039/b204301g
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  10
  High-efficiency single dopant white electrophosphorescent light-emitting diodes
  Adamovich, Vadim; Brooks, Jason; Tamayo, Arnold; Alexander, Alex M.; Djurovich, Peter I.; D'Andrade, Brian W.; Adachi, Chihaya; Forrest, Stephen R.; Thompson, Mark E.
  New Journal of Chemistry (2002), 26 (9), 1171-1178CODEN: NJCHE5; ISSN:1144-0546. (Royal Society of Chemistry)
  Efficient white electrophosphorescence has been achieved with a single emissive dopant. The dopant in these white org. light emitting diodes (WOLEDs) emits simultaneously from monomer and aggregate states, leading to a broad spectrum and high-quality white emission. The dopant mols. are based on a series of platinum(2) [2-(4,6-difluorophenyl)pyridinato-N,C2'] β-diketonates. All of the dopant complexes described herein have identical photophysics in dil. soln. with structured blue monomer emission (λmax = 468, 500, 540 nm). A broad orange aggregate emission (λmax ≈ 580 nm) is also obsd., when doped into OLED host materials. The intensity of the orange band increases relative to the blue monomer emission, as the doping level is increased. The ratio of monomer to aggregate emission can be controlled by the doping concn., the degree of steric bulk on the dopant and by the choice of the host material. A doping concn. for which the monomer and excimer bands are approx. equal gives an emission spectrum closest to that for std. white illumination sources. WOLEDs have been fabricated with doped CBP and mCP luminescent layers (CBP = N,N'-dicarbazolyl-4,4'-biphenyl, mCP = N,N'-dicarbazolyl-3,5-benzene). The best efficiencies and color stabilities were achieved when an electron/exciton blocking layer (EBL) is inserted into the structure, between the hole transporting layer and doped CBP or mCP layer. The material used for an EBL in these devices was fac-tris(1-phenylpyrazolato-N,C2')iridium(3). The EBL material effectively prevents electrons and excitons from passing through the emissive layer into the hole transporting NPD layer. CBP-based devices gave a peak external quantum efficiency of 3.3 ± 0.3% (7.3 ± 0.7 lm W-1) at 1 cd m-2, and 2.3 ± 0.2% (5.2 ± 0.3 lm W-1) at 500 cd m-2. MCP based devices gave a peak external quantum efficiency of 6.4% (12.2 lm W-1, 17.0 cd A-1), CIE coordinates of 0.36, 0.44 and a CRI of 67 at 1 cd m-2 (CIE = Commission Internationale de l'Eclairage, CRI = color rendering index). The efficiency of the mCP-based device drops to 4.3 ± 0.5% (8.1 ± 0.6 lm W-1, 11.3 cd A-1) at 500 cd m-2; however, the CIE coordinates and CRI remain unchanged.
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11. 11
  Kim, J. W.; You, S. I.; Kim, N. H.; Yoon, J.-A.; Cheah, K. W.; Zhu, F. R.; Kim, W. Y. Study of Sequential Dexter Energy Transfer in High Efficient Phosphorescent White Organic Light-Emitting Diodes with Single Emissive Layer. Sci. Rep. 2014, 4, 7009, DOI: 10.1038/srep07009
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  11
  Study of Sequential Dexter Energy Transfer in High Efficient Phosphorescent White Organic Light-Emitting Diodes with Single Emissive Layer
  Kim, Jin Wook; You, Seung Il; Kim, Nam Ho; Yoon, Ju-An; Cheah, Kok Wai; Zhu, Fu Rong; Kim, Woo Young
  Scientific Reports (2014), 4 (), 7009CODEN: SRCEC3; ISSN:2045-2322. (Nature Publishing Group)
  In this study, we report our effort to realize high performance single emissive layer three color white phosphorescent org. light emitting diodes (PHOLEDs) through sequential Dexter energy transfer of blue, green and red dopants. The PHOLEDs had a structure of; ITO(1500 Å)/NPB(700 Å)/mCP:Firpic-x%:Ir(ppy)3-0.5%:Ir(piq)3-y%(300 Å)/TPBi(300 Å)/Liq(20 Å)/Al(1200 Å). The dopant concns. of FIrpic, Ir(ppy)3 and Ir(piq)3 were adjusted and optimized to facilitate the preferred energy transfer processes attaining both the best luminous efficiency and CIE color coordinates. The presence of a deep trapping center for charge carriers in the emissive layer was confirmed by the obsd. red shift in electroluminescent spectra. White PHOLEDs, with phosphorescent dopant concns. of FIrpic-8.0%:Ir(ppy)3-0.5%:Ir(piq)3-0.5% in the mCP host of the single emissive layer, had a max. luminescence of 37,810 cd/m2 at 11 V and a luminous efficiency of 48.10 cd/A at 5 V with CIE color coordinates of (0.35, 0.41).
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12. 12
  Sun, J. W.; Kim, K.-H.; Moon, C.-K.; Lee, J.-H.; Kim, J.-J. Highly Efficient Sky-Blue Fluorescent Organic Light Emitting Diode Based on Mixed Cohost System for Thermally Activated Delayed Fluorescence Emitter (2CzPN). ACS Appl. Mater. Interfaces 2016, 8, 9806– 9810, DOI: 10.1021/acsami.6b00286
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  12
  Highly Efficient Sky-Blue Fluorescent Organic Light Emitting Diode Based on Mixed Cohost System for Thermally Activated Delayed Fluorescence Emitter (2CzPN)
  Sun, Jin Won; Kim, Kwon-Hyeon; Moon, Chang-Ki; Lee, Jeong-Hwan; Kim, Jang-Joo
  ACS Applied Materials & Interfaces (2016), 8 (15), 9806-9810CODEN: AAMICK; ISSN:1944-8244. (American Chemical Society)
  The mixed cohosts of 1,3-bis(N-carbazolyl)benzene and 2,8-bis(diphenylphosphoryl)dibenzothiophene were developed for a highly efficient blue fluorescent org. LED (OLED) doped with a thermally-activated delayed fluorescence (TADF) emitter [4,5-di(9H-carbazol-9-yl)phthalonitrile (2CzPN)]. An external quantum efficiency of 21.8% in blue fluorescent OLEDs was demonstrated, which is identical to the theor. achievable max. electroluminescence efficiency using the emitter. The efficiency roll-off is large even under the excellent charge balance in the device and almost the same as the single host based devices, indicating that the efficiency roll-off in 2CzPN based TADF host is related to the material characteristics, such as low reverse intersystem crossing rate rather than charge imbalance.
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13. 13
  Holmes, R. J.; Forrest, S. R.; Tung, Y.-J.; Kwong, R. C.; Brown, J. J.; Garon, S.; Thompson, M. E. Blue Organic Electrophosphorescence using Exothermic Host–Guest Energy Transfer. Appl. Phys. Lett. 2003, 82, 2422– 2424, DOI: 10.1063/1.1568146
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  13
  Blue organic electrophosphorescence using exothermic host-guest energy transfer
  Holmes, R. J.; Forrest, S. R.; Tung, Y.-J.; Kwong, R. C.; Brown, J. J.; Garon, S.; Thompson, M. E.
  Applied Physics Letters (2003), 82 (15), 2422-2424CODEN: APPLAB; ISSN:0003-6951. (American Institute of Physics)
  The authors demonstrate efficient blue electrophosphorescence using exothermic energy transfer from a host consisting of N,N'-dicarbazolyl-3,5-benzene (mCP) to the phosphorescent Ir complex Ir(III)bis[(4,6-difluorophenyl)-pyridinato-N,C2']picolinate (FIrpic). By examg. the temp. dependence of the radiative lifetime and the photoluminescence of a film of mCP doped with FIrpic, the authors confirm the existence of exothermic energy transfer in contrast to the endothermic transfer characteristic of the N,N'-dicarbazolyl-4-4'-biphenyl and FIrpic system. In employing exothermic energy transfer between mCP and FIrpic, a max. external electroluminescent quantum efficiency of (7.5 ± 0.8)% and a luminous power efficiency of (8.9 ± 0.9)lm/W were obtained, representing a significant increase in performance over previous endothermic blue electrophosphorescent devices.
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14. 14
  Hayashi, K.; Nakanotani, H.; Inoue, M.; Yoshida, K.; Mikhnenko, O.; Nguyen, T.-Q.; Adachi, C. Suppression of Roll-Off Characteristics of Organic Light-Emitting Diodes by Narrowing Current Injection/Transport Area to 50 nm. Appl. Phys. Lett. 2015, 106, 093301, DOI: 10.1063/1.4913461
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  14
  Suppression of roll-off characteristics of organic light-emitting diodes by narrowing current injection/transport area to 50 nm
  Hayashi, Kyohei; Nakanotani, Hajime; Inoue, Munetomo; Yoshida, Kou; Mikhnenko, Oleksandr; Nguyen, Thuc-Quyen; Adachi, Chihaya
  Applied Physics Letters (2015), 106 (9), 093301/1-093301/5CODEN: APPLAB; ISSN:0003-6951. (American Institute of Physics)
  Using e-beam nanolithog., the current injection/transport area in org. LEDs (OLEDs) was confined into a narrow linear structure with a min. width of 50 nm. This caused suppression of Joule heating and partial sepn. of polarons and excitons, so the charge d. where the electroluminescent efficiency decays to the half of the initial value (J0) was significantly improved. A device with a narrow current injection width of 50 nm exhibited a J0 that was almost 2 orders of magnitude higher compared with that of the unpatterned OLED. (c) 2015 American Institute of Physics.
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15. 15
  Flender, O.; Scholz, M.; Klein, J. R.; Oum, K.; Lenzer, T. Excited-State Relaxation of the Solar Cell Dye D49 in Organic Solvents and on Mesoporous Al₂O₃ and TiO₂ Thin Films. Phys. Chem. Chem. Phys. 2016, 18, 26010– 26019, DOI: 10.1039/c6cp05167g
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  15
  Excited-state relaxation of the solar cell dye D49 in organic solvents and on mesoporous Al2O3 and TiO2 thin films
  Flender, Oliver; Scholz, Mirko; Klein, Johannes R.; Oum, Kawon; Lenzer, Thomas
  Physical Chemistry Chemical Physics (2016), 18 (37), 26010-26019CODEN: PPCPFQ; ISSN:1463-9076. (Royal Society of Chemistry)
  We present an ultrafast UV-Vis-NIR transient absorption study of the donor-acceptor solar-cell dye D49 in diisopropyl ether, THF and acetonitrile, as well as on mesoporous Al2O3 and TiO2 thin films. Photoexcitation at 505 nm initially populates the first electronically excited state of the dye having significant intramol. charge transfer character ("S1/ICT"). On Al2O3 and in the three org. solvents, the dynamics are fully explained in terms of S1/ICT stabilization (by reorientation of adjacent solvent or D49 mols. and collisional cooling), intramol. vibrational redistribution and S1/ICT → S0 electronic decay. A substantial decrease of the S1/ICT lifetime is obsd. with increasing polarity of the surrounding medium suggesting an acceleration of internal conversion. In agreement with these results, the addn. of the nonpolar co-adsorbent deoxycholic acid (DCA) to the Al2O3 surface leads to a substantial increase of the S1/ICT lifetime. DCA spacers reduce the local polarity around the dye mols., thus interrupting D49 "self-solvation". These results are in contrast to a recent exptl. study for the indoline dye D131 on Al2O3, where charge transfer from electronically excited D131 to adjacent dye mols. was proposed (Cappel et al., Sci. Rep., 2016, 6, 21276). We do not see evidence for charge transfer processes between D49 mols. and also not for electron injection from D49 into Al2O3 trap states. Charge sepn. is only obsd. for D49 bound to TiO2 thin films, with efficient injection of electrons into the conduction band of the semiconductor via formation of a [D49 +···e-] complex and a transient Stark effect signalling the formation of mobile electrons upon dissocn. of the complex.
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16. 16
  Oum, K.; Lenzer, T.; Scholz, M.; Jung, D. Y.; Sul, O.; Cho, B. J.; Lange, J.; Müller, A. Observation of Ultrafast Carrier Dynamics and Phonon Relaxation of Graphene from the Deep-Ultraviolet to the Visible Region. J. Phys. Chem. C 2014, 118, 6454– 6461, DOI: 10.1021/jp4072197
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  16
  Observation of Ultrafast Carrier Dynamics and Phonon Relaxation of Graphene from the Deep-Ultraviolet to the Visible Region
  Oum, Kawon; Lenzer, Thomas; Scholz, Mirko; Jung, Dae Yool; Sul, Onejae; Cho, Byung Jin; Lange, Jens; Mueller, Andreas
  Journal of Physical Chemistry C (2014), 118 (12), 6454-6461CODEN: JPCCCK; ISSN:1932-7447. (American Chemical Society)
  We investigated the ultrafast carrier dynamics and phonon relaxation of CVD-grown monolayer and 9-layer graphene on a quartz substrate. Excitation was performed at 400 and 800 nm. The normalized change in optical d. ΔOD was probed over the range 260-640 nm (1.94-4.77 eV), reaching down into the region of graphene's Fano resonance, previously not investigated in femtosecond broadband pump-probe expts. Time consts. of 160 fs and 4 ps were found and assigned to carrier-optical phonon scattering and slower phonon relaxation processes, resp. The carrier distribution at early times was clearly hotter for 400 nm excitation than for 800 nm excitation. A pronounced spectral bleach feature was obsd. below 300 nm. It immediately formed after photoexcitation and recovered slowly, with a time const. of 35 ps for monolayer and time consts. of 120 and 970 ps for 9-layer graphene. The same dynamics were found for weak transient absorption features above 300 nm, which emerged after ca. 0.5 ps. The slow dynamics were assigned to interfacial heat flow from graphene to the quartz substrate. The bleach and absorption features were well described by a simple model assuming a red-shift of the Fano resonance. This red-shift disappeared with progressive cooling of graphene. We therefore suggest that the red-shift is induced by shrinking of the band sepn. due to lattice heating.
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17. 17
  Merker, A.; Scholz, M.; Morgenroth, M.; Lenzer, T.; Oum, K. Photoinduced Dynamics of (CH₃NH₃)₄Cu₂Br₆ Thin Films Indicating Efficient Triplet Photoluminescence. J. Phys. Chem. Lett. 2021, 12, 2736– 2741, DOI: 10.1021/acs.jpclett.1c00446
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  17
  Photoinduced Dynamics of (CH3NH3)4Cu2Br6 Thin Films Indicating Efficient Triplet Photoluminescence
  Merker, Alexander; Scholz, Mirko; Morgenroth, Marius; Lenzer, Thomas; Oum, Kawon
  Journal of Physical Chemistry Letters (2021), 12 (11), 2736-2741CODEN: JPCLCD; ISSN:1948-7185. (American Chemical Society)
  Hybrid org.-inorg. halogenidocuprates based on copper(I) represent materials with rich structural diversity and high photoluminescence (PL) quantum yield, yet the mechanism responsible for their efficient, strongly Stokes-shifted emission is still unclear. Here we report the successful prepn. of (CH3NH3)4Cu2Br6 thin films with a zero-dimensional mol. salt structure featuring "isolated" [Cu2Br6]4- ions. Time-resolved broadband PL measurements provide an excited-state lifetime of 114μs at 298 K. Results from femto- to microsecond UV-vis-NIR transient absorption expts. combined with DFT/TDDFT calcns. suggest the formation of a long-lived structurally relaxed triplet species through intersystem crossing (61 ps), which almost exclusively decays by phosphorescence. In addn., time scales for structural relaxation and cooling processes are extd. from a global kinetic anal. of the transient spectra. Calcns. for the isolated [Cu2Br6]4- anion and the (CH3NH3)4Cu2Br6 crystal suggest a strong impact of the crystal environment on the structure of the anion.
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18. 18
  Oum, K.; Flender, O.; Lohse, P. W.; Scholz, M.; Hagfeldt, A.; Boschloo, G.; Lenzer, T. Electron and Hole Transfer Dynamics of a Triarylamine-Based Dye with Peripheral Hole Acceptors on TiO₂ in the Absence and Presence of Solvent. Phys. Chem. Chem. Phys. 2014, 16, 8019– 8029, DOI: 10.1039/c3cp55298e
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  18
  Electron and hole transfer dynamics of a triarylamine-based dye with peripheral hole acceptors on TiO2 in the absence and presence of solvent
  Oum, Kawon; Flender, Oliver; Lohse, Peter W.; Scholz, Mirko; Hagfeldt, Anders; Boschloo, Gerrit; Lenzer, Thomas
  Physical Chemistry Chemical Physics (2014), 16 (17), 8019-8029CODEN: PPCPFQ; ISSN:1463-9076. (Royal Society of Chemistry)
  We studied photoinduced primary charge transfer processes of the sensitizer E6 on TiO2 without solvent and in contact with the org. solvent acetonitrile and the ionic liq. 1-ethyl-3-methylimidazolium tetracyanoborate [C2mim]+[B(CN)4]- using transient absorption spectroscopy, spectroelectrochem., and DFT/TDDFT calcns. E6, which belongs to a family of triarylamine dyes for solar cell applications, features two peripheral triarylamine units which are connected via diether spacer groups to the core chromophore and are designed to act as hole traps. This function was confirmed by spectroelectrochem., where the E6*+ radical cation shows a considerably blue-shifted absorption compared to dyes without these two substituents. This indicates that one of the terminal triarylamine units must carry the pos. charge. After photoexcitation of E6 at 520 nm (S0 S1 band), electrons are injected into TiO2 predominantly within the cross-correlation time (<80 fs), with some subsequent delayed electron injection (τ ca. 250 fs). Importantly, a transient Stark shift (electrochromism) is obsd. (time consts. ca. 0.8 and 12 ps) which is related to a changing elec. field generated by the E6*+ radical cations and injected electrons. This field induces absorption shifts of the dye species on the surface. Interestingly, these dynamics are largely unaffected by solvent mols. However, pronounced differences are obsd. on longer timescales. In contact with solvent, one observes an increase in the E6*+ absorption band above 600 nm with a time const. of 75 ps. This is assigned to hole transfer from the core chromophore to one of the peripheral triarylamine substituents. Electron-cation recombination occurs on much longer timescales and is multiexponential, with time consts. of ca. 100 μs, 1 ms and 15 ms. Because of hole trapping, it is slower than for similar dyes lacking the peripheral triarylamines. Addnl. expts. were performed for E6 attached to the wide band gap semiconductor ZrO2. Here, electron injection occurs into surface trap states with subsequent recombination. Another fraction of non-injecting E6 mols. in S1 quickly decays to S0 (time consts. 1 and 35 ps).
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19. 19
  Morgenroth, M.; Scholz, M.; Guy, L.; Oum, K.; Lenzer, T. Spatiotemporal Mapping of Efficient Chiral Induction by Helicene-Type Additives in Copolymer Thin Films. Angew. Chem., Int. Ed. 2022, 61, e202203075 DOI: 10.1002/anie.202203075
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  19
  Spatiotemporal Mapping of Efficient Chiral Induction by Helicene-Type Additives in Copolymer Thin Films
  Morgenroth, Marius; Scholz, Mirko; Guy, Laure; Oum, Kawon; Lenzer, Thomas
  Angewandte Chemie, International Edition (2022), 61 (31), e202203075CODEN: ACIEF5; ISSN:1433-7851. (Wiley-VCH Verlag GmbH & Co. KGaA)
  We obsd. efficient induction of chirality in polyfluorene copolymer thin films by mixing with helicene-type chiral additives based on the dibenzo[c,h]acridine motif. Images obtained from CD (CD) and circularly polarized luminescence (CPL) microscopy provide information about the chiral arrangements in the thin films with diffraction-limited resoln. The CD signal shows a characteristic dependence on the film thickness, which supports a supramol. origin of the strong chiral response of the copolymer. In particular, we demonstrate the discrimination between films of opposite chirality based on their ultrafast transient chiral response through the use of femtosecond broadband CD spectroscopy and a newly developed setup for transient CPL spectroscopy with 28 ps time resoln. A systematic variation of the enantiomeric excess of the chiral additive shows that the "Sergeants and Soldiers" concept and "Majority Rules" are not obeyed.
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20. 20
  Mao, L.; Ritcey, A. M. Excimer Formation and Excitation Energy Transfer in Langmuir-Blodgett Films of Cellulose Derivatives Containing Carbazole. Thin Solid Films 1996, 284–285, 618– 621, DOI: 10.1016/s0040-6090(95)08405-3
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  20
  Excimer formation and excitation energy transfer in Langmuir-Blodgett films of cellulose derivatives containing carbazole
  Mao, Lijun; Ritcey, Anna M.
  Thin Solid Films (1996), 284-285 (), 618-621CODEN: THSFAP; ISSN:0040-6090. (Elsevier)
  Fluorescence emission spectra are recorded for Langmuir-Blodgett (LB) films prepd. from cellulose derivs. contg. carbazole as a side chain substituent. Three samples of differing carbazole content are considered. At low carbazole content, only the fluorescence characteristic of monomeric carbazole emission is obsd. Clear excimer emission is, however, evident in the spectra of the more highly substituted derivs. Since similar excimer formation is not obsd. in solns. or cast films of the same polymers, it is concluded that mol. ordering in the LB films favors the parallel alignment of chromophore units. For a given polymer of sufficiently high carbazole content, the relative intensity of the excimer emission increases with the no. of transferred layers, indicating the interaction of chromophores in neighboring layers. Energy transfer between layers is also evident in expts. involving multilayer assemblies composed of a monolayer of 1-pyrenedodecanoic acid, covered with a varying no. of layers of the carbazole-contg. polymer. Excitation of carbazole results in strong emission from pyrene, whereas only weak emission is obsd. from carbazole. It is therefore concluded that energy transfer from carbazole to pyrene occurs in these LB films.
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21. 21
  Tani, K.; Tohda, Y.; Takemura, H.; Ohkita, H.; Ito, S.; Yamamoto, M. Synthesis and Photophysical Properties of [3.3](3,9)Carbazolophanes. Chem. Commun. 2001, 1914– 1915, DOI: 10.1039/b104101k
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  21
  Synthesis and photophysical properties of [3.3](3,9)carbazolophanes
  Tani, Keita; Tohda, Yasuo; Takemura, Hiroyuki; Ohkita, Hideo; Ito, Shinzaburo; Yamamoto, Masahide
  Chemical Communications (Cambridge, United Kingdom) (2001), (19), 1914-1915CODEN: CHCOFS; ISSN:1359-7345. (Royal Society of Chemistry)
  Syn- And anti-[3.3](3,9)carbazolophanes, which are suitable model compds. for sandwich and partial-overlap excimers, resp., were synthesized and characterized; the structures of both singlet and triplet carbazole excimer have been described. Intramol. cyclization of ammonium cyanide with 9,9'-(1,3-propanediyl)bis[3-(bromomethyl)-9H-carbazole] gave the title compds.
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22. 22
  Winnik, F. M. Photophysics of Preassociated Pyrenes in Aqueous Polymer Solutions and in Other Organized Media. Chem. Rev. 1993, 93, 587– 614, DOI: 10.1021/cr00018a001
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  22
  Photophysics of preassociated pyrenes in aqueous polymer solutions and in other organized media
  Winnik, Francoise M.
  Chemical Reviews (Washington, DC, United States) (1993), 93 (2), 587-614CODEN: CHREAY; ISSN:0009-2665.
  A review with 155 refs.
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23. 23
  Musser, A. J.; Rajendran, S. K.; Georgiou, K.; Gai, L.; Grant, R. T.; Shen, Z.; Cavazzini, M.; Ruseckas, A.; Turnbull, G. A.; Samuel, D. W. Intermolecular States in Organic Dye Dispersions: Excimers vs. Aggregates. J. Mater. Chem. C 2017, 5, 8380– 8389, DOI: 10.1039/c7tc02655b
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  23
  Intermolecular states in organic dye dispersions: excimers vs. aggregates
  Musser, A. J.; Rajendran, S. K.; Georgiou, K.; Gai, L.; Grant, R. T.; Shen, Z.; Cavazzini, M.; Ruseckas, A.; Turnbull, G. A.; Samuel, I. D. W.; Clark, J.; Lidzey, D. G.
  Journal of Materials Chemistry C: Materials for Optical and Electronic Devices (2017), 5 (33), 8380-8389CODEN: JMCCCX; ISSN:2050-7534. (Royal Society of Chemistry)
  Rapid excited-state quenching in the solid state is a widespread limitation for org. chromophores. Even when mols. are dispersed in neutral host matrixes, photoluminescence quantum yields decrease sharply with increased concn., pointing to efficient intermol. non-radiative decay pathways that remain poorly understood. Here we study the nature of the intermol. states formed in dispersions of the prototypical BODIPY dyes. Using temp.-dependent and time-resolved photoluminescence measurements, we describe the processes of energy transfer into excimer states and, in materials with suitable chem. structure, excitonically coupled dimers. These dimer states exhibit remarkable near-unity quantum yield.
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24. 24
  Spano, F. C. The Spectral Signatures of Frenkel Polarons in H- and J-Aggregates. Acc. Chem. Res. 2010, 43, 429– 439, DOI: 10.1021/ar900233v
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  24
  The Spectral Signatures of Frenkel Polarons in H- and J-Aggregates
  Spano, Frank C.
  Accounts of Chemical Research (2010), 43 (3), 429-439CODEN: ACHRE4; ISSN:0001-4842. (American Chemical Society)
  A review. Electronic excitations in small aggregates, thin films, and crystals of conjugated org. mols. play a fundamental role in the operation of a wide array of org.-based devices including solar cells, transistors, and light-emitting diodes. Such excitations, or excitons, are generally spread out over several mols.: a balance between the delocalizing influence of resonant intermol. coupling and the localizing influence of static and dynamic disorder dets. the coherence range of the exciton. Because of the soft nature of org. materials, significant nuclear relaxation in the participating mols. also accompanies the electronic excitations. To properly understand energy or charge transport, 1 must treat intermol. (excitonic) coupling, electron-vibrational coupling, and disorder on equal footing. In this Account, the authors review the key elements of a theor. approach based on a multiparticle representation that describes electronic excitations in org. materials as vibronic excitations surrounded by a field of vibrational excitations. Such composite excitations are appropriately called Frenkel excitonic polarons. For many conjugated mols., the bulk of the nuclear reorganization energy following electronic excitation arises from the elongation of a sym. vinyl stretching mode with energy ∼1400 cm-1. To appreciate the impact of aggregation, the authors study how the vibronic progression of this mode, which dominates the isolated (solvated) mol. absorption and emission spectra, is distorted when mols. are close enough to interact with each other. As the authors demonstrate in this Account, the nature of the distortion provides a wealth of information about how the mols. are packed, the strength of the excitonic interactions between mols., the no. of mols. that are coherently coupled, and the nature of the disorder. The aggregation-induced deviations from the Poissonian distribution of vibronic peak intensities take on 2 extremes identified with ideal H- and J-aggregates. The sign of the nearest neighbor electronic coupling, pos. for H and neg. for J, distinguishes the 2 basic aggregate forms. For several decades, researchers have known that H-aggregates exhibit blue-shifted absorption spectra and are subradiant while J-aggregates exhibit the opposite behavior (red shifted absorption and superradiance). However, the exact inclusion of exciton-vibrational coupling reveals several more distinguishing traits between the 2 aggregate types: in H(J)-aggregates the ratio of the 1st 2 vibronic peak intensities in the absorption spectrum decreases (increases) with increasing excitonic coupling, while the ratio of the 0-0 to 0-1 emission intensities increases (decreases) with disorder and increases (decreases) with increasing temp. These 2 extreme behaviors provide the framework for understanding absorption and emission in more complex morphologies, such as herringbone packing in oligo(phenylene vinylene)s, oligothiophenes and polyacene crystals, as well as the polymorphic packing arrangements obsd. in carotenoids.
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25. 25
  Mai, V. T. N.; Ahmad, V.; Mamada, M.; Fukunaga, T.; Shukla, A.; Sobus, J.; Krishnan, G.; Moore, E. G.; Andersson, G. G.; Adachi, C. Solid Cyclooctatetraene-Based Triplet Quencher Demonstrating Excellent Suppression of Singlet–Triplet Annihilation in Optical and Electrical Excitation. Nat. Commun. 2020, 11, 5623, DOI: 10.1038/s41467-020-19443-z
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  25
  Solid cyclooctatetraene-based triplet quencher demonstrating excellent suppression of singlet-triplet annihilation in optical and electrical excitation
  Mai, Van T. N.; Ahmad, Viqar; Mamada, Masashi; Fukunaga, Toshiya; Shukla, Atul; Sobus, Jan; Krishnan, Gowri; Moore, Evan G.; Andersson, Gunther G.; Adachi, Chihaya; Namdas, Ebinazar B.; Lo, Shih-Chun
  Nature Communications (2020), 11 (1), 5623CODEN: NCAOBW; ISSN:2041-1723. (Nature Research)
  Triplet excitons have been identified as the major obstacle to the realization of org. laser diodes, as accumulation of triplet excitons leads to significant losses under continuous wave (CW) operation and/or elec. excitation. Here, we report the design and synthesis of a solid-state org. triplet quencher, as well as in-depth studies of its dispersion into a soln. processable bis-stilbene-based laser dye. By blending the laser dye with 20 wt% of the quencher, negligible effects on the ASE thresholds, but a complete suppression of singlet-triplet annihilation (STA) and a 20-fold increase in excited-state photostability of the laser dye under CW excitation, were achieved. We used small-area OLEDs (0.2 mm2) to demonstrate efficient STA suppression by the quencher in the nanosecond range, supported by simulations to provide insights into the obsd. STA quenching under elec. excitation. The results demonstrate excellent triplet quenching ability under both optical and elec. excitations in the nanosecond range, coupled with excellent soln. processability.
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26. 26
  Morgenroth, M.; Scholz, M.; Guy, L.; Oum, K.; Lenzer, T. Ultrafast Excited-State Dynamics of Thin Films Consisting of Helicene-Like Molecules Based on Dibenzo[c,h]acridine. Mol. Phys. 2021, 119, e1959072 DOI: 10.1080/00268976.2021.1959072
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  26
  Ultrafast excited-state dynamics of thin films consisting of helicene-like molecules based on dibenzo[c,h]acridine
  Morgenroth, Marius; Scholz, Mirko; Guy, Laure; Oum, Kawon; Lenzer, Thomas
  Molecular Physics (2021), 119 (17-18), e1959072/1-e1959072/11CODEN: MOPHAM; ISSN:0026-8976. (Taylor & Francis Ltd.)
  We investigated the excited-state dynamics of bridged and non-bridged chiral helicene-like bidibenzo[c,h]acridines deposited as a thin film. Broadband UV/VIS-NIR transient absorption spectroscopy from the femtosecond to the microsecond time-scale and transient fluorescence measurements were employed to understand the photoinduced processes in these org. thin-film layers. Photoexcitation at 370 nm populates the S1 state, which has a lifetime of 15 and 36 ps for the non-bridged and bridged deriv., resp. In both cases, the decay is governed by internal conversion (78% and 84%), whereas intersystem crossing represents only a smaller channel (21% and 15%), and the remaining 1% accounting for fluorescence. Fast repopulation of the S0 state leads to the generation of hot ground state mols. (S0*), with characteristic finger-type transient absorption features. The T1 state decays back to S0 by intersystem crossing with a time const. of 775 and 5500 ps, resp. Even at high excitation fluence, there are no indications for higher-order processes, such as singlet-singlet annihilation or Forster energy transfer. Photoexcitation also launches coherent acoustic phonons in these films, appearing as damped oscillations in the transient absorption kinetics on the red and blue edge of each S0 absorption band, with a period of about 190 ps.
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27. 27
  Morgenroth, M.; Scholz, M.; Lenzer, T.; Oum, K. Ultrafast UV–Vis Transient Absorption and Circular Dichroism Spectroscopy of a Polyfluorene Copolymer Showing Large Chiral Induction. J. Phys. Chem. C 2020, 124, 10192– 10200, DOI: 10.1021/acs.jpcc.0c00976
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  27
  Ultrafast UV-Vis Transient Absorption and Circular Dichroism Spectroscopy of a Polyfluorene Copolymer Showing Large Chiral Induction
  Morgenroth, Marius; Scholz, Mirko; Lenzer, Thomas; Oum, Kawon
  Journal of Physical Chemistry C (2020), 124 (18), 10192-10200CODEN: JPCCCK; ISSN:1932-7447. (American Chemical Society)
  Ultrafast broadband UV-vis transient CD (TrCD) and transient absorption (TA) spectroscopy are employed to study the dynamics of chiral polymer thin films. Helical cholesteric arrangements are formed by mixing chiral binaphthalene derivs. with the intrinsically achiral polyfluorene-based copolymer F8BT. The induced CD (ICD) response of F8BT shows a mirror-image-like shape when the chirality of the additive is changed. Peaks in the CD spectra reach large abs. values of about 800 mdeg. Photoexcitation by ultrashort laser pulses results in TrCD spectra of opposite sign for cholesteric polymer films of different helicity, demonstrating transient chiral discrimination on the femtosecond time scale over the wavelength range 260-700 nm. Differences between the excited-state and ground-state CD spectra considerably depend on the electronic transition. The excited-state processes in these systems are evaluated by a detailed kinetic anal. providing rate consts. for the relaxation of the different singlet exciton states as well as singlet-singlet annihilation processes of F8BT involving diffusive bimol. recombination and Forster resonance energy transfer.
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28. 28
  Lenzer, T.; Ehlers, F.; Scholz, M.; Oswald, R.; Oum, K. Assignment of Carotene S* State Features to the Vibrationally Hot Ground Electronic State. Phys. Chem. Chem. Phys. 2010, 12, 8832– 8839, DOI: 10.1039/b925071a
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  28
  Assignment of carotene S* state features to the vibrationally hot ground electronic state
  Lenzer, Thomas; Ehlers, Florian; Scholz, Mirko; Oswald, Rainer; Oum, Kawon
  Physical Chemistry Chemical Physics (2010), 12 (31), 8832-8839CODEN: PPCPFQ; ISSN:1463-9076. (Royal Society of Chemistry)
  The so-called S* state has been suggested to play an important role in the photophysics of β-carotene and other carotenoids in soln. and photosynthetic light-harvesting complexes, yet its origin has remained elusive. The present expts. employing temp.-dependent steady-state absorption spectroscopy and ultrafast pump-supercontinuum probe (PSCP) transient absorption measurements of β-carotene in soln. demonstrate that the spectral features of S* are due to vibrationally excited mols. in the ground electronic state S0. Characteristic spectral signatures, such as a highly structured bleach below 500 nm and absorption in the range 500-660 nm result from the superposition of hot S0 absorption ("S0*") on top of the ground-state bleach of room-temp. mols. Appearance and disappearance of the S0* mols. can be completely described by a global kinetic anal. employing time-dependent species-assocd. spectra without the need to invoke the population of an intermediate electronically excited state.
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29. 29
  Schwarzer, D.; Troe, J.; Votsmeier, M.; Zerezke, M. Collisional Deactivation of Vibrationally Highly Excited Azulene in Compressed Liquids and Supercritical Fluids. J. Chem. Phys. 1996, 105, 3121– 3131, DOI: 10.1063/1.472180
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  29
  Collisional deactivation of vibrationally highly excited azulene in compressed liquids and supercritical fluids
  Schwarzer, D.; Troe, J.; Votsmeier, M.; Zerezke, M.
  Journal of Chemical Physics (1996), 105 (8), 3121-3131CODEN: JCPSA6; ISSN:0021-9606. (American Institute of Physics)
  The collisional deactivation of vibrationally highly excited azulene was studied from the gas to the compressed liq. phase. Employing supercrit. fluids like He, Xe, CO2, and ethane at pressures of 6-4000 bar and temps. ≥380 K, measurements over the complete gas-liq. transition were performed. Azulene with an energy of 18 000 cm-1 was generated by laser excitation into the S1 and internal conversion to the S0*-ground state. The subsequent loss of vibrational energy was monitored by transient absorption at the red edge of the S3←S0 absorption band near 290 nm. Transient signals were converted into energy-time profiles using hot band absorption coeffs. from shock wave expts. for calibration and accounting for solvent shifts of the spectra. Under all conditions, the decays were mono-exponential. At densities below 1 mol/L, collisional deactivation rates increased linearly with fluid d. Av. energies 〈ΔE〉 transferred per collision agreed with data from dil. gas phase expts. For Xe, CO2, and C2H6, the linear relation between cooling rate and diffusion coeff. scaled collision frequencies ZD turned over to a much weaker dependence at ZD>0.3 ps-1. Up to collision frequencies of ZD = 15 ps-1 this behavior can well be rationalized by a model employing an effective collision frequency related to the finite lifetime of collision complexes.
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30. 30
  Grahn, H. T.; Maris, H. J.; Tauc, J. Picosecond Ultrasonics. IEEE J. Quantum Electron. 1989, 25, 2562– 2569, DOI: 10.1109/3.40643
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  30
  Picosecond ultrasonics
  Grahn, Holger T.; Maris, Humphrey J.; Tauc, Jan
  IEEE Journal of Quantum Electronics (1989), 25 (12), 2562-9CODEN: IEJQA7; ISSN:0018-9197.
  A review with 20 refs. The generation and detection of ultrasonic waves and picosecond time resoln. is described. With picosecond ultrasonics, the frequency range of acoustic waves is extended into the 100 GHz range. It allows the investigation of thin-film materials, microstructures, and materials with large acoustic attenuation. Velocity and attenuation measurements on a variety of thin films are reviewed. The results on complex microstructures including multilayers are presented.
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31. 31
  Ruello, P.; Gusev, V. E. Physical Mechanisms of Coherent Acoustic Phonons Generation by Ultrafast Laser Action. Ultrasonics 2015, 56, 21– 35, DOI: 10.1016/j.ultras.2014.06.004
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  31
  Physical mechanisms of coherent acoustic phonons generation by ultrafast laser action
  Ruello, Pascal; Gusev, Vitalyi E.
  Ultrasonics (2015), 56 (), 21-35CODEN: ULTRA3; ISSN:0041-624X. (Elsevier B.V.)
  In this review we address the microscopic mechanisms that are involved in the photogeneration processes of GHz-THz coherent acoustic phonons (CAP) induced by an ultrafast laser pulse. Understanding and describing the underlying physics is necessary indeed for improving the future sources of coherent acoustic phonons useful for the non-destructive testing optoacoustic techniques. Getting more phys. insights on these processes also opens new perspectives for the emerging field of the opto-mechanics where lattice motions (surface and/or interfaces ultrafast displacements, nanostructures resonances) are controlled by light. We will then remind the basics of electron-phonon and photon-phonon couplings by discussing the deformation potential mechanism, the thermoelasticity, the inverse piezoelec. effect and the electrostriction in condensed matter. Metals, semiconductors and oxide materials will be discussed. The contribution of all these mechanisms in the photogeneration process of sound will be illustrated over several examples coming from the rich literature.
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32. 32
  Scholz, M.; Morgenroth, M.; Lenzer, M. J.; Choi, D. H.; Lenzer, T.; Oum, K. Coherent Acoustic Phonon Dynamics in Chiral Copolymers. Struct. Dyn. 2019, 6, 064502, DOI: 10.1063/1.5124438
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  32
  Coherent acoustic phonon dynamics in chiral copolymers
  Scholz, Mirko; Morgenroth, Marius; Cho, Min Ju; Choi, Dong Hoon; Lenzer, Thomas; Oum, Kawon
  Structural Dynamics (2019), 6 (6), 064502/1-064502/7CODEN: SDTYAE; ISSN:2329-7778. (American Institute of Physics)
  Coherent phonon oscillations in the UV-Vis transient absorption and CD response of two chiral polyfluorene-based copolymer thin films are investigated. A slow oscillation in the hundred picosecond regime indicates the propagation of a longitudinal acoustic phonon with a frequency in the gigahertz range through cholesteric films of PFPh and PFBT, which allow for the optical detn. of the longitudinal sound velocity in these polymers, with values of (2550 ± 140) and (2490 ± 150) m s-1, resp. The oscillation is induced by a strain wave, resulting in a pressure-induced periodic shift of the electronic absorption bands, as extd. from a Fourier anal. of the transient spectra. The acoustic phonon oscillation is also clearly detected in the transient CD (TrCD) response of PFPh, indicating a transient pressure-induced shift of the CD spectrum and possibly also phonon-induced chirality changes via pitch length modulation of the cholesteric helical polymer stack. (c) 2019 American Institute of Physics.
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33. 33
  Morgenroth, M.; Scholz, M.; Cho, M. J.; Choi, D. H.; Oum, K.; Lenzer, T. Mapping the Broadband Circular Dichroism of Copolymer Films with Supramolecular Chirality in Time and Space. Nat. Commun. 2022, 13, 210, DOI: 10.1038/s41467-021-27886-1
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  33
  Mapping the broadband circular dichroism of copolymer films with supramolecular chirality in time and space
  Morgenroth, Marius; Scholz, Mirko; Cho, Min Ju; Choi, Dong Hoon; Oum, Kawon; Lenzer, Thomas
  Nature Communications (2022), 13 (1), 210CODEN: NCAOBW; ISSN:2041-1723. (Nature Portfolio)
  Abstr.: Measurements of the electronic CD (CD) are highly sensitive to the abs. configuration and conformation of chiral mols. and supramol. assemblies and have therefore found widespread application in the chem. and biol. sciences. Here, we demonstrate an approach to simultaneously follow changes in the CD and absorption response of photoexcited systems over the UV-visible spectral range with 100 fs time resoln. We apply the concept to chiral polyfluorene copolymer thin films and track their electronic relaxation in detail. The transient CD signal stems from the supramol. response of the system and provides information regarding the recovery of the electronic ground state. This allows for a quantification of singlet-singlet annihilation and charge-pair formation processes. Spatial mapping of chiral domains on femtosecond time scales with a resoln. of 50μm and diffraction-limited steady-state imaging of the CD and the circularly polarised luminescence (CPL) of the films is demonstrated.
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34. 34
  Murawski, C.; Leo, K.; Gather, M. C. Efficiency Roll-Off in Organic Light-Emitting Diodes. Adv. Mater. 2013, 25, 6801– 6827, DOI: 10.1002/adma.201301603
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  34
  Efficiency Roll-Off in Organic Light-Emitting Diodes
  Murawski, Caroline; Leo, Karl; Gather, Malte C.
  Advanced Materials (Weinheim, Germany) (2013), 25 (47), 6801-6827CODEN: ADVMEW; ISSN:0935-9648. (Wiley-VCH Verlag GmbH & Co. KGaA)
  A review. Org. light-emitting diodes (OLEDs) have attracted much attention in research and industry thanks to their capability to emit light with high efficiency and to deliver high-quality white light that provides good color rendering. OLEDs feature homogeneous large area emission and can be produced on flexible substrates. In terms of efficiency, OLEDs can compete with highly efficient conventional light sources but their efficiency typically decreases at high brightness levels, an effect known as efficiency roll-off. In recent years, much effort has been undertaken to understand the underlying processes and to develop methods that improve the high-brightness performance of OLEDs. In this review, we summarize the current knowledge and provide a detailed description of the relevant principles, both for phosphorescent and fluorescent emitter mols. In particular, we focus on exciton-quenching mechanisms, such as triplet-triplet annihilation, quenching by polarons, or field-induced quenching, but also discuss mechanisms such as changes in charge carrier balance. We further review methods that may reduce the roll-off and thus enable OLEDs to be used in high-brightness applications.
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35. 35
  Klein, J. R.; Scholz, M.; Oum, K.; Lenzer, T. Photoinduced Dynamics of the Hole-Transport Material H101 in Organic Solvents and on Mesoporous Al₂O₃ and TiO₂ Thin Films. Phys. Chem. Chem. Phys. 2017, 19, 21748– 21758, DOI: 10.1039/c7cp01718a
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  35
  Photoinduced dynamics of the hole-transport material H101 in organic solvents and on mesoporous Al2O3 and TiO2 thin films
  Klein, Johannes R.; Scholz, Mirko; Oum, Kawon; Lenzer, Thomas
  Physical Chemistry Chemical Physics (2017), 19 (32), 21748-21758CODEN: PPCPFQ; ISSN:1463-9076. (Royal Society of Chemistry)
  The authors present a comprehensive steady-state and time-resolved UV-Vis-NIR absorption and fluorescence study of the hole-transport material H101 to characterize its photophysics in different org. solvents and on mesoporous Al2O3 and TiO2 thin films. Photoexcitation of H101 at 400 nm in org. solvents populates the S1 state which shows intramol. relaxation on a picosecond time scale. Branching from the relaxed S1 state leads to population of the T1 triplet state and the ground electronic state S0. Triplet formation is induced by the internal heavy-atom effect of the sulfur atom, and the triplet yield decreases substantially with solvent polarity. On mesoporous Al2O3, intermol. exciton-splitting is obsd. leading to the formation of a radical cation - radical anion pair (H101(S1) + H101(S0) → H101 + + H101-) followed by exciton recombination. On mesoporous TiO2, efficient electron injection is obsd. in addn. to exciton-splitting. Complementary spectroelectrochem. expts. enable a full spectral characterization of the cation species H101+ and H1012+. Extensive DFT/TDDFT calcns. successfully assign the spectral features of all exptl. obsd. species. Implications for the function of H101 in photovoltaic devices are discussed.
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36. 36
  Förster, T. Zwischenmolekulare Energiewanderung und Fluoreszenz. Ann. Phys. 1948, 437, 55– 75, DOI: 10.1002/andp.19484370105
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  There is no corresponding record for this reference.
37. 37
  Förster, T. Transfer Mechanisms of Electronic Excitation. Discuss. Faraday Soc. 1959, 27, 7– 17, DOI: 10.1039/DF9592700007
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  There is no corresponding record for this reference.
38. 38
  Stevens, M. A.; Silva, C.; Russell, D. M.; Friend, R. H. Exciton Dissociation Mechanisms in the Polymeric Semiconductors Poly(9,9-dioctylfluorene) and Poly(9,9-dioctylfluorene-co-benzothiadiazole). Phys. Rev. B 2001, 63, 165213, DOI: 10.1103/physrevb.63.165213
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  38
  Exciton dissociation mechanisms in the polymeric semiconductors poly(9,9-dioctylfluorene) and poly(9,9-dioctylfluorene-co-benzothiadiazole)
  Stevens, Mark A.; Silva, Carlos; Russell, David M.; Friend, Richard H.
  Physical Review B: Condensed Matter and Materials Physics (2001), 63 (16), 165213/1-165213/18CODEN: PRBMDO; ISSN:0163-1829. (American Physical Society)
  We present femtosecond transient absorption measurements on the semiconductor conjugated polymers poly(9,9-dioctylfluorene) (F8) and poly(9,9-dioctylfluorene-co-benzothiadiazole) (F8BT). Detailed photophys. modeling reveals that, in F8, sequential excitation, first to the lowest singlet excited state, and then to a higher-energy state resonant with the pump photon energy, is predominantly responsible for the rapid (<150 fs) dissocn. of photoinduced excitons. Resonant sequential excitation accesses high-energy states that can promptly evolve to charged or triplet states. In F8BT, however, we find that sequential excitation plays a lesser role in fast polaron-pair generation, and that exciton bimol. annihilation can explain the charge population. We suggest that the electrophilic benzothiadiazole groups in F8BT facilitate charge formation by dissocn. of the excited state formed by exciton-exciton annihilation. Modeling also reveals that exciton bimol. annihilation can occur via two sep. and competing processes. We find that in F8, the dominant mechanism involves exciton diffusion and collision. In F8BT, however, addnl. annihilation of spatially sepd. excitons occurs when they interact through the Forster transfer mechanism, where the crit. distance for annihilation in F8BT is 4 nm.
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39. 39
  Smith, M. B.; Michl, J. Singlet Fission. Chem. Rev. 2010, 110, 6891– 6936, DOI: 10.1021/cr1002613
  View
  39
  Singlet Fission
  Smith, Millicent B.; Michl, Josef
  Chemical Reviews (Washington, DC, United States) (2010), 110 (11), 6891-6936CODEN: CHREAY; ISSN:0009-2665. (American Chemical Society)
  A review. A theory of singlet fission is discussed along with its relation to photovoltaics. The literature is reviewed dealing with singlet fission in org. chromophores from the time of initial discovery of singlet fission to the present day.
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40. 40
  Wachsstock, D. Tenua 2.1─The Kinetics Simulator for Java. 2007. http://bililite.com/tenua/ (accessed Oct 09, 2022).
  There is no corresponding record for this reference.
41. 41
  Ruseckas, A.; Ribierre, J. C.; Shaw, P. E.; Staton, S. V.; Burn, P. L.; Samuel, I. D. W. Singlet Energy Transfer and Singlet-Singlet Annihilation in Light-Emitting Blends of Organic Semiconductors. Appl. Phys. Lett. 2009, 95, 183305, DOI: 10.1063/1.3253422
  View
  41
  Singlet energy transfer and singlet-singlet annihilation in light-emitting blends of organic semiconductors
  Ruseckas, A.; Ribierre, J. C.; Shaw, P. E.; Staton, S. V.; Burn, P. L.; Samuel, I. D. W.
  Applied Physics Letters (2009), 95 (18), 183305/1-183305/3CODEN: APPLAB; ISSN:0003-6951. (American Institute of Physics)
  Excitation energy transfer from host to guest was studied in spin-cast blends of 4,4'-bis(N-carbazolyl)biphenyl (CBP) and a phosphorescent fac-tris(2-phenylpyridyl)iridium(III)-cored dendrimer using time resolved fluorescence. The kinetics of energy transfer are consistent with homogeneous dispersion of the dendrimers in the CBP host. Diffusion-controlled singlet-singlet exciton annihilation is obsd. in the CBP host at moderate excitation densities, similar to those encountered in high brightness light-emitting devices and org. lasers. The results are important for org. lighting and the understanding of exciton diffusion in guest-host systems for electrophosphorescence. (c) 2009 American Institute of Physics.
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42. 42
  Hofkens, J.; Cotlet, M.; Vosch, T.; Tinnefeld, P.; Weston, K. D.; Ego, C.; Grimsdale, A.; Müllen, K.; Beljonne, D.; Brédas, J. L. Revealing Competitive Förster-Type Resonance Energy-Transfer Pathways in Single Bichromophoric Molecules. Proc. Natl. Acad. Sci. U.S.A. 2003, 100, 13146– 13151, DOI: 10.1073/pnas.2235805100
  View
  42
  Revealing competitive Forster-type resonance energy-transfer pathways in single bichromophoric molecules
  Hofkens, Johan; Cotlet, Mircea; Vosch, Tom; Tinnefeld, Philip; Weston, Kenneth D.; Ego, Christophe; Grimsdale, Andrew; Muellen, Klaus; Beljonne, David; Bredas, Jean Luc; Jordens, Sven; Schweitzer, Gerd; Sauer, Markus; De Schryver, Frans
  Proceedings of the National Academy of Sciences of the United States of America (2003), 100 (23), 13146-13151CODEN: PNASA6; ISSN:0027-8424. (National Academy of Sciences)
  Measurements are demonstrated of the efficiency of competing Forster-type energy-transfer pathways in single bichromophoric systems by monitoring simultaneously the fluorescence intensity, fluorescence lifetime, and the no. of independent emitters with time. Peryleneimide end-capped fluorene trimers, hexamers, and polymers with interchromophore distances of 3.4, 5.9, and on av. 42 nm, resp., served as bichromophoric systems. Because of different energy-transfer efficiencies, variations in the interchromophore distance enable the switching between homo-energy transfer (energy hopping), singlet-singlet annihilation, and singlet-triplet annihilation. Probably similar energy-transfer pathways have to should be considered in the anal. of single-mol. trajectories of donor/acceptor pairs as well as in natural and synthetic multichromophoric systems such as light-harvesting antennas, oligomeric fluorescent proteins, and dendrimers. Visualization is reported selectively of different energy-transfer pathways taking place between identical fluorophores in individual bichromophoric mols.
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43. 43
  Hedley, G. J.; Schröder, T.; Steiner, F.; Eder, T.; Hofmann, F. J.; Bange, S.; Laux, D.; Höger, S.; Tinnefeld, P.; Lupton, J. M. Picosecond Time-Resolved Photon Antibunching Measures Nanoscale Exciton Motion and the True Number of Chromophores. Nat. Commun. 2021, 12, 1327, DOI: 10.1038/s41467-021-21474-z
  View
  43
  Picosecond time-resolved photon antibunching measures nanoscale exciton motion and the true number of chromophores
  Hedley, Gordon J.; Schroder, Tim; Steiner, Florian; Eder, Theresa; Hofmann, Felix J.; Bange, Sebastian; Laux, Dirk; Hoger, Sigurd; Tinnefeld, Philip; Lupton, John M.; Vogelsang, Jan
  Nature Communications (2021), 12 (1), 1327CODEN: NCAOBW; ISSN:2041-1723. (Nature Research)
  The particle-like nature of light becomes evident in the photon statistics of fluorescence from single quantum systems as photon antibunching. In multichromophoric systems, exciton diffusion and subsequent annihilation occurs. These processes also yield photon antibunching but cannot be interpreted reliably. Here we develop picosecond time-resolved antibunching to identify and decode such processes. We use this method to measure the true no. of chromophores on well-defined multichromophoric DNA-origami structures, and precisely det. the distance-dependent rates of annihilation between excitons. Further, this allows us to measure exciton diffusion in mesoscopic H- and J-type conjugated-polymer aggregates. We distinguish between one-dimensional intra-chain and three-dimensional inter-chain exciton diffusion at different times after excitation and det. the disorder-dependent diffusion lengths. Our method provides a powerful lens through which excitons can be studied at the single-particle level, enabling the rational design of improved excitonic probes such as ultra-bright fluorescent nanoparticles and materials for optoelectronic devices.
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Supporting Information
Supporting Information
The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acs.jpcc.2c07440.
- Absorption, fluorescence, and fluorescence excitation spectra, time-resolved fluorescence experiments, spectrum of vibrationally hot mCP molecules at 1400 ps, slow vibrational cooling in mCP thin films, kinetics of mCP at the probe wavelengths of 340 and 400 nm, modeling of collisional energy transfer in mCP thin films, kinetic analysis of transient absorption data, and results of kinetic modeling at the probe wavelength of 318 nm (PDF)
- jp2c07440_si_001.pdf (1.05 MB)
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Understanding Excited-State Relaxation in 1,3-Bis(N-carbazolyl)benzene, a Host Material for Organic Light-Emitting Diodes了解 1,3-双（N-咔唑基）苯（有机发光二极管的主体材料）中的激发态弛豫Click to copy article link点击复制文章链接Article link copied!

The Journal of Physical Chemistry C

Abstract 抽象

1. Introduction 1. 引言

Scheme 1 方案 1

2. Experimental Section 2. 实验部分

2.1. Sample Preparation 2.1. 样品制备

2.2. Steady-State Absorption and Fluorescence Experiments2.2. 稳态吸收和荧光实验

2.3. Ultrafast Transient Absorption Spectroscopy2.3. 超快瞬态吸收光谱

2.4. Time-Resolved Fluorescence Experiments2.4. 时间分辨荧光实验

2.5. Atomic Force Microscopy2.5. 原子力显微镜

3. Results and Discussion3. 结果与讨论

3.1. Steady-State Absorption and Emission of mCP in THF and Thin Films3.1. THF 和薄膜中 mCP 的稳态吸收和发射

Figure 1 图 1

3.2. Time-Resolved Fluorescence Experiments3.2. 时间分辨荧光实验

3.3. Transient Absorption Spectroscopy of mCP in THF and Thin Films3.3. THF 和薄膜中 mCP 的瞬态吸收光谱

Figure 2 图 2

Figure 3 图 3

Figure 4 图 4

3.4. Fluence-Dependent Transient Absorption Measurements and Their Kinetic Modeling for Neat mCP Thin Films3.4. 纯 mCP 薄膜的能量依赖性瞬态吸收测量及其动力学建模

Figure 5 图 5

Figure 6 图 6

Figure 7 图 7

4. Conclusions 4. 结论

Supporting Information 支持信息

Terms & Conditions

Author Information

Acknowledgments

References

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The Journal of Physical Chemistry C物理化学杂志 C

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Abstract

Scheme 1

Figure 1

Figure 2

Figure 3

Figure 4

Figure 5

Figure 6

Figure 7

References

Supporting Information

Supporting Information

Terms & Conditions

Understanding Excited-State Relaxation in 1,3-Bis(N-carbazolyl)benzene, a Host Material for Organic Light-Emitting Diodes
了解 1,3-双（N-咔唑基）苯（有机发光二极管的主体材料）中的激发态弛豫
Click to copy article link
点击复制文章链接Article link copied!

2.2. Steady-State Absorption and Fluorescence Experiments
2.2. 稳态吸收和荧光实验

2.3. Ultrafast Transient Absorption Spectroscopy
2.3. 超快瞬态吸收光谱

2.4. Time-Resolved Fluorescence Experiments
2.4. 时间分辨荧光实验

2.5. Atomic Force Microscopy
2.5. 原子力显微镜

3. Results and Discussion
3. 结果与讨论

3.1. Steady-State Absorption and Emission of mCP in THF and Thin Films
3.1. THF 和薄膜中 mCP 的稳态吸收和发射

3.2. Time-Resolved Fluorescence Experiments
3.2. 时间分辨荧光实验

3.3. Transient Absorption Spectroscopy of mCP in THF and Thin Films
3.3. THF 和薄膜中 mCP 的瞬态吸收光谱

3.4. Fluence-Dependent Transient Absorption Measurements and Their Kinetic Modeling for Neat mCP Thin Films
3.4. 纯 mCP 薄膜的能量依赖性瞬态吸收测量及其动力学建模

The Journal of Physical Chemistry C
物理化学杂志 C