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Understanding Excited-State Relaxation in 1,3-Bis(N-carbazolyl)benzene, a Host Material for Organic Light-Emitting Diodes
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C: Spectroscopy and Dynamics of Nano, Hybrid, and Low-Dimensional Materials
C:纳米、混合和低维材料的光谱学和动力学

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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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
Copyright © 2023 American Chemical Society

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 kdiff 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 S1-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 kdiff 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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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 方案 1

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 xy 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 图 1

Figure 1. (a) Normalized steady-state absorption and fluorescence spectra (λexc = 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 (λexc = 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 (λdet = 400 nm) and in the long-wavelength band (λdet = 530 nm) are broadened compared with the one recorded in the monomer band (λdet = 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 (λdet = 370 nm) 中检测到的荧光激发光谱相比,在该波段 (λdet = 400 nm) 和长波长波段 (λdet = 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 → Sn 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 → Tn 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 → Sn 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 → Tn 吸收带最终在纳秒到微秒的时间尺度上衰减。

Figure 2 图 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.
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 → Sn 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 Sn 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 → Sn 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 Sn 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 图 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.
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 图 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.
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(Sx) 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 cL 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(Sx) 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 cL,该振荡周期确定为 6800 m s-1

Figure 5 图 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(Sx): 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(Sx):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 111 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
SxkxS1
(1)
SxkCP,xe+h+
(2)
S1kIC+Fl,1S0*
(3)
S1kISC,1T1
(4)
T1kISC+Ph,2S0*
(5)
2S1kdiffSn+S0*
(6)
3S1kF2Sn+S0*
(7a) (7a)
3S1kFSn+2S0*
(7b) (7b)
SnknS1
(8)
SnkCP,ne+h+
(9)
S0*kCETS0
(10)
e+h+krecS0
(11)

Figure 6 图 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.
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 Sx. 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 kx (eq 1). We also assume that the highly excited Sx 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 kCP,x.
290 nm 的光激发使 mCP 分子处于更高激发的单线态 Sx。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 kxeq 1)。我们还假设有机薄膜中的高激发 Sx 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 Sx can take several parallel channels: mCP may decay by IC and fluorescence to the vibrationally hot ground state S0* with the rate constant kIC+Fl,1, cf. eq 3, or alternatively to the T1 state by ISC via eq 4 with the rate constant kISC,1. Afterward, the T1 state decays to S0* either by ISC or phosphorescence (eq 5, rate constant kIC+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 Sn singlet state and an S0* ground state species with the rate constant kdiff, 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 Sx 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 kIC+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 T1,1. 之后,T1 状态通过 ISC 或磷光衰减到 S0*方程 5,速率常数 kIC+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 Sn 单重态和一个速率常数为 kdiff 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 → Sn 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 → Sn 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”
S1kFo¨rster(R)S0*
(12a) (12a)
S1kFo¨rster(R)Sn
(12b) (12b)
Here, the rate constant kFörster 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)
kFo¨rster=1τ1(R0R)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 kFRET = 6 kF. (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 7a7b 的替代公式,而不是方程 12a12b(33,38) 在那里,表明 S1-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 7a7b 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 kFRET = 6 kF(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 Sn state, which should behave similar to the initially prepared Sx state. Therefore, it can either decay to the S1 state by IC with the rate constant kn, cf. eq 8, or form a charge pair with the rate constant kCP,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 kCET). 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 krec). 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 Sn 状态,其行为应该类似于最初准备的 Sx 状态。因此,它可以通过速率常数 kn 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 kCET)。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 krec)。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 图 7

Figure 7. Kinetic modeling of the S0* number density N for the three different initial Sx 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 Sx 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 kdiff = 1.40 × 10–8 cm3 s–1, whereas the dashed lines are the results of a simulation using only FRET with a rate constant kF = 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 kdiff 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 kdiff = 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 kF = 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 kdiff 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 kCET of 5 × 108 s–1CET = 2 ns, cf. also Figure 3).
相比之下,如果仅使用 S1-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 kCET 很好地描述(τCET = 2 ns,参见图 3)。
We note that the kinetic model described by eqs 111 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 kx and kCPx (eqs 1 and 2) and the ratio of the rate constants kn and kCPn (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 kx,total = τx,total–1 of Sx and kn,total = τn,total–1 of Sn are still well-defined (and also kx and kn). The generation of S1 species must be ultrafast (τx,total and τn,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 67a7b)。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 kxkCPx 的比率(方程 12)以及速率常数 knkCPn 的比率(方程 89) 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 kx,total = τx,S xkn 的总数–1,总数 = τn,S n 的总数–1 仍然定义明确(以及 kxkn)。 S1 物质的产生必须是超快的(τx,total 和 τn,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 35) 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 S1-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 + Sn”), 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 + Sn”),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 Sx 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 Sx 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 + Tn”) 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 Sx 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 Sx 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 + Tn”)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。

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  • 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

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

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S1 
Supporting Information for 
Understanding Excited‐State Relaxation in 1,3‐
Bis(
N
‐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
9
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
S2 
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 
00
abs
00
fl
Stokes
Ε
Stokes
 (nm)
 (nm)
(meV)
(cm
‐1
)
mCP in THF
338.4 
343.2 
52 
418 
mCP:PMMA 2:98
339.4 
343.6 
44 
355 
mCP:PMMA 10:90
339.1 
345.5 
67 
541 
mCP:PMMA 90:10
342.1 
349.0 
72 
583 
mCP:PMMA 98:2
342.3 
349.0 
70 
565 
mCP thin film
342.5 
349.5 
72 
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
0
 S
1
 absorption band in the range 320
340 nm. This broadening 
of the fluorescence excitation spectrum is characteristic for e
xcimer formation.
1
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. 

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Author Information

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  • Corresponding Authors
  • 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, GermanyOrcidhttps://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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  1. 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

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
发布时间 2023 年 2 月 28 日
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  • Abstract

    Scheme 1

    Scheme 1. Chemical Structure of mCP

    Figure 1

    Figure 1. (a) Normalized steady-state absorption and fluorescence spectra (λexc = 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.

    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.

    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.

    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.

    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(Sx): 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.

    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.

    Figure 7

    Figure 7. Kinetic modeling of the S0* number density N for the three different initial Sx 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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  • 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)


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