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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
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- Marius MorgenrothMarius MorgenrothFaculty IV: School of Science and Technology, Department Chemistry and Biology, Physical Chemistry 2, University of Siegen, Adolf-Reichwein-Str. 2, 57076 Siegen, GermanyMore by Marius Morgenroth
- Thomas Lenzer*Thomas Lenzer*Email: lenzer@chemie.uni-siegen.deFaculty IV: School of Science and Technology, Department Chemistry and Biology, Physical Chemistry 2, University of Siegen, Adolf-Reichwein-Str. 2, 57076 Siegen, GermanyMore by Thomas Lenzer
- Kawon Oum*Kawon Oum*Email: oum@chemie.uni-siegen.deFaculty IV: School of Science and Technology, Department Chemistry and Biology, Physical Chemistry 2, University of Siegen, Adolf-Reichwein-Str. 2, 57076 Siegen, GermanyMore by Kawon Oum
The Journal of Physical Chemistry C
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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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)
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
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。
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
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
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
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
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
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。
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。
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。
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
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。
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
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。
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%]。
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。
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。
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。
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。
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
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。
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
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 kx (eq 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。
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 态,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。
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”
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)
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 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 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 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。
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)。
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。
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)
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)。
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 kx 和 kCPx 的比率(方程 1 和 2)以及速率常数 kn 和 kCPn 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 kx,total = τx,S x 和 kn 的总数–1,总数 = τn,S n 的总数–1 仍然定义明确(以及 kx 和 kn)。 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。
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。
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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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。
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。
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)
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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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)
Understanding Excited-State Relaxation in 1,3-Bis(N‑carbazolyl)benzene, a Host Material for Organic Light-Emitting Diodes
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支持信息
了解 1,3 中的兴奋态弛豫
‐咔唑基)苯,有机物的主体材料
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Author Information
- 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
- NotesThe authors declare no competing financial interest.
Acknowledgments
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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- 3Bagnich, S. A.; Rudnick, A.; Schroegel, P.; Strohriegl, P.; Köhler, A. Triplet Energies and Excimer Formation in Meta- and Para-Linked Carbazolebiphenyl Matrix Materials. Philos. Trans. R. Soc., A 2015, 373, 20140446, DOI: 10.1098/rsta.2014.0446
- 4Hosokai, T.; Matsuzaki, H.; Nakanotani, H.; Tokumaru, K.; Tsutsui, T.; Furube, A.; Nasu, K.; Nomura, H.; Yahiro, M.; Adachi, C. Evidence and Mechanism of Efficient Thermally Activated Delayed Fluorescence Promoted by Delocalized Excited States. Sci. Adv. 2017, 3, e1603282 DOI: 10.1126/sciadv.1603282Google Scholar4https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC1cXkvVCqtrg%253D&md5=a8e7f40b2806362d6bf948fa036cf806Evidence and mechanism of efficient thermally activated delayed fluorescence promoted by delocalized excited statesHosokai, Takuya; Matsuzaki, Hiroyuki; Nakanotani, Hajime; Tokumaru, Katsumi; Tsutsui, Tetsuo; Furube, Akihiro; Nasu, Keirou; Nomura, Hiroko; Yahiro, Masayuki; Adachi, ChihayaScience Advances (2017), 3 (5), e1603282/1-e1603282/9CODEN: SACDAF; ISSN:2375-2548. (American Association for the Advancement of Science)The design of org. compds. with nearly no gap between the first excited singlet (S1) and triplet (T1) states has been demonstrated to result in an efficient spin-flip transition from the T1 to S1 state, i.e., reverse intersystem crossing (RISC), and facilitate light emission as thermally activated delayed fluorescence (TADF). However, many TADF mols. have shown that a relatively appreciable energy difference between the S1 and T1 states (∼0.2 eV) could also result in a high RISC rate. We revealed from a comprehensive study of optical properties of TADF mols. that the formation of delocalized states is the key to efficient RISC and identified a chem. template for these materials. In addn., simple structural confinement further enhances RISC by suppressing structural relaxation in the triplet states. Our findings aid in designing advanced org. mols. with a high rate of RISC and, thus, achieving the max. theor. electroluminescence efficiency in org. light-emitting diodes.
- 5Godumala, M.; Choi, S.; Cho, M. J.; Choi, D. H. Recent Breakthroughs in Thermally Activated Delayed Fluorescence Organic Light Emitting Diodes Containing Non-Doped Emitting Layers. J. Mater. Chem. C 2019, 7, 2172– 2198, DOI: 10.1039/c8tc06293eGoogle Scholar5https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC1MXhsVOntbc%253D&md5=eec30593246b773b9530f67e8537b12fRecent breakthroughs in thermally activated delayed fluorescence organic light emitting diodes containing non-doped emitting layersGodumala, Mallesham; Choi, Suna; Cho, Min Ju; Choi, Dong HoonJournal of Materials Chemistry C: Materials for Optical and Electronic Devices (2019), 7 (8), 2172-2198CODEN: JMCCCX; ISSN:2050-7534. (Royal Society of Chemistry)Thermally activated delayed fluorescence (TADF) materials have attracted significant attention owing to their state-of-the-art performance in the field of org. light-emitting diodes (OLEDs). TADF materials have the capability of harvesting both singlet and triplet excitons through reverse intersystem crossing, thus, the max. internal quantum efficiency can be expected to be 100%. Thanks to TADF materials that are developed from pure org. entities so that they are inexpensive and external quantum efficiencies as high as 38% have been achieved to date. Despite the high performance of doped OLEDs, precise control of the host-guest ratio and phase sepn. at high driving voltages are the foremost challenges. In order to circumvent these demerits, the use of numerous small mols., dendritic mols., and polymers as non-doped emitters has been reported. Non-doped OLEDs consist of only a single mol. emitter in the emissive layer, thus simplifying the device fabrication process. Furthermore, non-doped OLEDs exhibit better device stability compared to doped devices, because the former do not exhibit phase sepn. or crystallinity and maintain a homogeneous surface film morphol. during device operation. As a result, non-doped OLEDs are capable of overcoming the most pivotal issues of device complexity and stability, which makes them effective in rendering OLEDs com. viable. Although max. external quantum efficiencies of nearly 23.0% in blue, 22.0% in green, 22.0% in yellow, 17.0% in orange, and 10.0% in red emitting materials have been realized with non-doped OLEDs, still further progress is essential to reach the doped devices. This review systematically describes the mol. design, photophys. properties, and electroluminescence performance data of non-doped TADF emitters. We strongly believe that this review will be beneficial in terms of presenting a specific direction for the design of a variety of non-doped TADF emitters in the near future.
- 6Wu, X.; Su, B.-K.; Chen, D.-G.; Liu, D.; Wu, C.-C.; Huang, Z.-X.; Lin, T.-C.; Wu, C.-H.; Zhu, M.; Li, E. Y. The Role of Host–Guest Interactions in Organic Emitters Employing MR-TADF. Nat. Photonics 2021, 15, 780– 786, DOI: 10.1038/s41566-021-00870-3Google Scholar6https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BB3MXitFaqtLnO&md5=775a8cad9811a1a829b16b43ef18b781The role of host-guest interactions in organic emitters employing MR-TADFWu, Xiugang; Su, Bo-Kang; Chen, Deng-Gao; Liu, Denghui; Wu, Chi-Chi; Huang, Zhi-Xuan; Lin, Ta-Chun; Wu, Cheng-Ham; Zhu, Mengbing; Li, Elise Y.; Hung, Wen-Yi; Zhu, Weiguo; Chou, Pi-TaiNature Photonics (2021), 15 (10), 780-786CODEN: NPAHBY; ISSN:1749-4885. (Nature Portfolio)Abstr.: Research into org. light emitters employing multiple resonance-induced thermally activated delayed fluorescence (MR-TADF) materials is presently attracting a great deal of attention due to the potential for efficient deep-blue emission. However, the origins and mechanisms of successful TADF are unclear, as many MR-TADF materials do not show TADF behavior in soln., but only as particular pure solids. Here, an investigation into a well-known MR-TADF material, DABNA-1, together with other new MR materials (9H-quinolino[3,2,1-kl]phenothiazin-9-one (QPO) and 9H-quinolino-[3,2,1-kl]-phenothiazin-9-one 5,5-dioxide (QP3O)), yields new insights regarding the origin of TADF. Although a material system may support the concept of MR, inefficiency in both forward and reverse intersystem crossings forbids TADF unless a suitable host material allows an exciplex-like host-emitter interaction that boosts TADF. This boosted-TADF mechanism can be generalized to any fluorescence dye that lacks TADF in the photoluminescence measurement but has a thermally accessible S1-T1 energy gap, opening the way to high-performance org. light-emitting diodes.
- 7Kondo, Y.; Yoshiura, K.; Kitera, S.; Nishi, H.; Oda, S.; Gotoh, H.; Sasada, Y.; Yanai, M.; Hatakeyama, T. Narrowband Deep-Blue Organic Light-Emitting Diode Featuring an Organoboron-Based Emitter. Nat. Photonics 2019, 13, 678– 682, DOI: 10.1038/s41566-019-0476-5Google Scholar7https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC1MXhtlOlsLjK&md5=7ccab1765bbb7ae0e3c9e7430ba2b5a8Narrowband deep-blue organic light-emitting diode featuring an organoboron-based emitterKondo, Yasuhiro; Yoshiura, Kazuki; Kitera, Sayuri; Nishi, Hiroki; Oda, Susumu; Gotoh, Hajime; Sasada, Yasuyuki; Yanai, Motoki; Hatakeyama, TakujiNature 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.
- 8Hwang, 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.1c15659Google Scholar8https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BB3MXivVeitLbL&md5=5afac2299e07ef96ce025ffb1e41fb44Aryl-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 <0.1Hwang, Jinhyo; Koh, Chang Woo; Ha, Jung Min; Woo, Han Young; Park, Sungnam; Cho, Min Ju; Choi, Dong HoonACS 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.
- 9Monkman, 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.1c09189Google Scholar9https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BB3MXhsVygsrjE&md5=93495210e5468a3c084e700181602ad6Why Do We Still Need a Stable Long Lifetime Deep Blue OLED Emitter?Monkman, AndrewACS 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.
- 10Adamovich, 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/b204301gGoogle Scholar10https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BD38XmsVWnsrc%253D&md5=a9703167d89eae3343f3041f88aaa327High-efficiency single dopant white electrophosphorescent light-emitting diodesAdamovich, 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.
- 11Kim, 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/srep07009Google Scholar11https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC2cXhvFansb7J&md5=643044a0417d16c3529e5d4ddabd776aStudy of Sequential Dexter Energy Transfer in High Efficient Phosphorescent White Organic Light-Emitting Diodes with Single Emissive LayerKim, Jin Wook; You, Seung Il; Kim, Nam Ho; Yoon, Ju-An; Cheah, Kok Wai; Zhu, Fu Rong; Kim, Woo YoungScientific 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).
- 12Sun, 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.6b00286Google Scholar12https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC28XkvFCksLs%253D&md5=091f9e9c0ad5c3826c78323fa4fb49bcHighly 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-JooACS 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.
- 13Holmes, 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: 10.1063/1.1568146Google Scholar13https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BD3sXivFamu7c%253D&md5=07c25a6c54a44fb7b5722f86b2bf51f4Blue organic electrophosphorescence using exothermic host-guest energy transferHolmes, 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.
- 14Hayashi, 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: 10.1063/1.4913461Google Scholar14https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC2MXjs1ansbk%253D&md5=fb964a711958f1087945aaa781d80077Suppression of roll-off characteristics of organic light-emitting diodes by narrowing current injection/transport area to 50 nmHayashi, Kyohei; Nakanotani, Hajime; Inoue, Munetomo; Yoshida, Kou; Mikhnenko, Oleksandr; Nguyen, Thuc-Quyen; Adachi, ChihayaApplied 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.
- 15Flender, 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/c6cp05167gGoogle Scholar15https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC28XhsV2ju7bK&md5=3f5a0359ff6ee72cb2d0d4586c0ba0c5Excited-state relaxation of the solar cell dye D49 in organic solvents and on mesoporous Al2O3 and TiO2 thin filmsFlender, Oliver; Scholz, Mirko; Klein, Johannes R.; Oum, Kawon; Lenzer, ThomasPhysical 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.
- 16Oum, 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/jp4072197Google Scholar16https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC2cXktFehsLw%253D&md5=45628dc70338d8dd0e603b6cf0dddbd3Observation of Ultrafast Carrier Dynamics and Phonon Relaxation of Graphene from the Deep-Ultraviolet to the Visible RegionOum, Kawon; Lenzer, Thomas; Scholz, Mirko; Jung, Dae Yool; Sul, Onejae; Cho, Byung Jin; Lange, Jens; Mueller, AndreasJournal 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.
- 17Merker, 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.1c00446Google Scholar17https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BB3MXmtVOrt7g%253D&md5=2ad11b46b933299299ec7c8d4588d7b8Photoinduced Dynamics of (CH3NH3)4Cu2Br6 Thin Films Indicating Efficient Triplet PhotoluminescenceMerker, Alexander; Scholz, Mirko; Morgenroth, Marius; Lenzer, Thomas; Oum, KawonJournal 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.
- 18Oum, 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/c3cp55298eGoogle Scholar18https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC2cXlsFOisb0%253D&md5=3e84cee278dd71f1b5b0abe4e0d4306cElectron and hole transfer dynamics of a triarylamine-based dye with peripheral hole acceptors on TiO2 in the absence and presence of solventOum, Kawon; Flender, Oliver; Lohse, Peter W.; Scholz, Mirko; Hagfeldt, Anders; Boschloo, Gerrit; Lenzer, ThomasPhysical 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).
- 19Morgenroth, 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.202203075Google Scholar19https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BB38Xhs12gtb7J&md5=3bb57dbe045c351bbf1e3adbb398c0f2Spatiotemporal Mapping of Efficient Chiral Induction by Helicene-Type Additives in Copolymer Thin FilmsMorgenroth, Marius; Scholz, Mirko; Guy, Laure; Oum, Kawon; Lenzer, ThomasAngewandte 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.
- 20Mao, 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)08405-3Google Scholar20https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADyaK28XmvVSrtr4%253D&md5=978ad73ab6fdd613fa93ece687b06644Excimer formation and excitation energy transfer in Langmuir-Blodgett films of cellulose derivatives containing carbazoleMao, 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.
- 21Tani, 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/b104101kGoogle Scholar21https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BD3MXns1yms7o%253D&md5=235a117c5154a681a98c943cce15d389Synthesis and photophysical properties of [3.3](3,9)carbazolophanesTani, Keita; Tohda, Yasuo; Takemura, Hiroyuki; Ohkita, Hideo; Ito, Shinzaburo; Yamamoto, MasahideChemical 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.
- 22Winnik, F. M. Photophysics of Preassociated Pyrenes in Aqueous Polymer Solutions and in Other Organized Media. Chem. Rev. 1993, 93, 587– 614, DOI: 10.1021/cr00018a001Google Scholar22https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADyaK3sXhsVygs7w%253D&md5=ed06727222fdb1eb0b15a61fe5ed7792Photophysics of preassociated pyrenes in aqueous polymer solutions and in other organized mediaWinnik, Francoise M.Chemical Reviews (Washington, DC, United States) (1993), 93 (2), 587-614CODEN: CHREAY; ISSN:0009-2665.A review with 155 refs.
- 23Musser, 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/c7tc02655bGoogle Scholar23https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC2sXhtFamsbvO&md5=cfbc25ec5ccd512a1a6488dee8968250Intermolecular states in organic dye dispersions: excimers vs. aggregatesMusser, 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.
- 24Spano, F. C. The Spectral Signatures of Frenkel Polarons in H- and J-Aggregates. Acc. Chem. Res. 2010, 43, 429– 439, DOI: 10.1021/ar900233vGoogle Scholar24https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BD1MXhsFKgurzL&md5=a66d2effbaf73a403bf00e6e198efe0aThe Spectral Signatures of Frenkel Polarons in H- and J-AggregatesSpano, 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.
- 25Mai, 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-zGoogle Scholar25https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BB3cXitlWgu7fI&md5=c20bcfc8366b0a64ac16c053af5cf3fdSolid cyclooctatetraene-based triplet quencher demonstrating excellent suppression of singlet-triplet annihilation in optical and electrical excitationMai, 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-ChunNature 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.
- 26Morgenroth, 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: 10.1080/00268976.2021.1959072Google Scholar26https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BB3MXhsl2ktLnK&md5=35b603aba36564dbd22bf398185bc47dUltrafast excited-state dynamics of thin films consisting of helicene-like molecules based on dibenzo[c,h]acridineMorgenroth, Marius; Scholz, Mirko; Guy, Laure; Oum, Kawon; Lenzer, ThomasMolecular 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.
- 27Morgenroth, 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.0c00976Google Scholar27https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BB3cXnvFCnur8%253D&md5=1be0836c7d470d6f9d2e5dac83eb157dUltrafast UV-Vis Transient Absorption and Circular Dichroism Spectroscopy of a Polyfluorene Copolymer Showing Large Chiral InductionMorgenroth, Marius; Scholz, Mirko; Lenzer, Thomas; Oum, KawonJournal 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.
- 28Lenzer, 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/b925071aGoogle Scholar28https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC3cXpsVGit7Y%253D&md5=c881fc2ca6de32b4bccdb3dc88b4bea1Assignment of carotene S* state features to the vibrationally hot ground electronic stateLenzer, Thomas; Ehlers, Florian; Scholz, Mirko; Oswald, Rainer; Oum, KawonPhysical 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.
- 29Schwarzer, 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: 10.1063/1.472180Google Scholar29https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADyaK28XltFKjurc%253D&md5=cd9ad4dc000ee6e7c3803cb6f36b3069Collisional deactivation of vibrationally highly excited azulene in compressed liquids and supercritical fluidsSchwarzer, 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.
- 30Grahn, H. T.; Maris, H. J.; Tauc, J. Picosecond Ultrasonics. IEEE J. Quantum Electron. 1989, 25, 2562– 2569, DOI: 10.1109/3.40643Google Scholar30https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADyaK3cXosVentg%253D%253D&md5=21efe48d56a06320aa940a5e52e7cefaPicosecond ultrasonicsGrahn, Holger T.; Maris, Humphrey J.; Tauc, JanIEEE 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.
- 31Ruello, 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.2014.06.004Google Scholar31https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC2cXht1Wmu7rM&md5=c3f9ccc802a63be8ba132e8767a610d9Physical mechanisms of coherent acoustic phonons generation by ultrafast laser actionRuello, 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.
- 32Scholz, 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: 10.1063/1.5124438Google Scholar32https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC1MXisVKgu7jM&md5=b1661ff6f0a595baa9f50f584c4d7adfCoherent acoustic phonon dynamics in chiral copolymersScholz, Mirko; Morgenroth, Marius; Cho, Min Ju; Choi, Dong Hoon; Lenzer, Thomas; Oum, KawonStructural 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.
- 33Morgenroth, 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/s41467-021-27886-1Google Scholar33https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BB38XoslOhug%253D%253D&md5=30a9e8d628d2831906f34985c21562e7Mapping the broadband circular dichroism of copolymer films with supramolecular chirality in time and spaceMorgenroth, Marius; Scholz, Mirko; Cho, Min Ju; Choi, Dong Hoon; Oum, Kawon; Lenzer, ThomasNature 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.
- 34Murawski, C.; Leo, K.; Gather, M. C. Efficiency Roll-Off in Organic Light-Emitting Diodes. Adv. Mater. 2013, 25, 6801– 6827, DOI: 10.1002/adma.201301603Google Scholar34https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC3sXhsVSrs73K&md5=e37d55d72cf73907428d74c147c884adEfficiency Roll-Off in Organic Light-Emitting DiodesMurawski, 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.
- 35Klein, 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/c7cp01718aGoogle Scholar35https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC2sXht1ynsrfO&md5=ea217a0f0fb219f5796be14d10030d6bPhotoinduced dynamics of the hole-transport material H101 in organic solvents and on mesoporous Al2O3 and TiO2 thin filmsKlein, Johannes R.; Scholz, Mirko; Oum, Kawon; Lenzer, ThomasPhysical 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.
- 36Förster, T. Zwischenmolekulare Energiewanderung und Fluoreszenz. Ann. Phys. 1948, 437, 55– 75, DOI: 10.1002/andp.19484370105Google ScholarThere is no corresponding record for this reference.
- 37Förster, T. Transfer Mechanisms of Electronic Excitation. Discuss. Faraday Soc. 1959, 27, 7– 17, DOI: 10.1039/DF9592700007Google ScholarThere is no corresponding record for this reference.
- 38Stevens, 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.63.165213Google Scholar38https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BD3MXislyqtrc%253D&md5=5aa1ea8046038c5e50f6fa32a0404e4fExciton 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.
- 39Smith, M. B.; Michl, J. Singlet Fission. Chem. Rev. 2010, 110, 6891– 6936, DOI: 10.1021/cr1002613Google Scholar39https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC3cXhtlKntbbF&md5=4632198f6c4b979ef92822fd6a3be047Singlet FissionSmith, Millicent B.; Michl, JosefChemical 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.
- 40Wachsstock, D. Tenua 2.1─The Kinetics Simulator for Java. 2007. http://bililite.com/tenua/ (accessed Oct 09, 2022).Google ScholarThere is no corresponding record for this reference.
- 41Ruseckas, 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: 10.1063/1.3253422Google Scholar41https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BD1MXhtl2jtLjL&md5=f11538dca764a37639d27209cb5d83e9Singlet energy transfer and singlet-singlet annihilation in light-emitting blends of organic semiconductorsRuseckas, 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.
- 42Hofkens, 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.2235805100Google Scholar42https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BD3sXptFOjurk%253D&md5=a37dbbb55079bce354af604208100ad6Revealing competitive Forster-type resonance energy-transfer pathways in single bichromophoric moleculesHofkens, 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, FransProceedings 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.
- 43Hedley, 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-zGoogle Scholar43https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BB3MXltlyksbY%253D&md5=7de6aaf825d2510487aa052d08bd62ccPicosecond time-resolved photon antibunching measures nanoscale exciton motion and the true number of chromophoresHedley, Gordon J.; Schroder, Tim; Steiner, Florian; Eder, Theresa; Hofmann, Felix J.; Bange, Sebastian; Laux, Dirk; Hoger, Sigurd; Tinnefeld, Philip; Lupton, John M.; Vogelsang, JanNature 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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- 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
引用此内容:J. Phys. Chem. C2023, 127, 9, 4582–4593
发布时间 2023 年 2 月 28 日
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References
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- 1Wu, M.-F.; Yeh, S.-J.; Chen, C.-T.; Murayama, H.; Tsuboi, T.; Li, W.-S.; Chao, I.; Liu, S.-W.; Wang, J.-K. The Quest for High-Performance Host Materials for Electrophosphorescent Blue Dopants. Adv. Funct. Mater. 2007, 17, 1887– 1895, DOI: 10.1002/adfm.2006008001https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BD2sXpvFCrt7w%253D&md5=79037b0c14521dff0ecf184b55c75082The quest for high-performance host materials for electrophosphorescent blue dopantsWu, Min-Fei; Yeh, Shi-Jay; Chen, Chin-Ti; Murayama, Hideyuki; Tsuboi, Taiju; Li, Wan-Sheung; Chao, Ito; Liu, Shun-Wei; Wang, Juen-KaiAdvanced Functional Materials (2007), 17 (12), 1887-1895CODEN: AFMDC6; ISSN:1616-301X. (Wiley-VCH Verlag GmbH & Co. KGaA)We report a systematic comparison study of 3,5-di(N-carbazolyl)tetraphenylsilane (SimCP) and N,N'-dicarbazolyl-3,5-benzene (mCP), which are used as the host materials for phosphorescent blue dopants in org. light-emitting diodes (OLEDs). On the basis of photoexcitation emission spectroscopy, thermal stability anal., photoelectron anal., charge transport measurements, and mol. dynamics (MD) simulations, we conclude that the non-π-conjugated meta-substituted triphenylsilyl moiety of SimCP exerts a unique hindering effect on the mol. packing characteristics in the condensed phase. The chem. origin of the superior performance of SimCP over mCP is revealed, and is expected to be helpful for the mol. design of effective host materials for enhancing the performance of blue phosphorescent OLEDs.
- 2Tsuboi, T.; Murayama, H.; Yeh, S.-J.; Wu, M.-F.; Chen, C.-T. Photoluminescence Characteristics of Blue Phosporescent Ir3+-Compounds FIrpic and FIrN4 Doped in mCP and SimCP. Opt. Mater. 2008, 31, 366– 371, DOI: 10.1016/j.optmat.2008.05.0102https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BD1cXhtlyjs7fL&md5=5b27298b5899aee7597f44a7b476fd4fPhotoluminescence characteristics of blue phosphorescent Ir3+-compounds FIrpic and FIrN4 doped in mCP and SimCPTsuboi, Taiju; Murayama, Hideyuki; Yeh, Shih-Jay; Wu, Min-Fei; Chen, Chin-TiOptical Materials (Amsterdam, Netherlands) (2008), 31 (2), 366-371CODEN: OMATET; ISSN:0925-3467. (Elsevier B.V.)Absorption and photoluminescence (PL) characteristics are investigated in detail for blue phosphors iridium bis (4,6-difluorophenypyridinato-N, C 2') picolinate (FIrpic) and iridium bis (4,6-difluorophenylpyridinato-N, C 2')[5-(2-pyridyl)tetrazolate] (FIrN4) and for UV fluorescent materials 1,3-bis (9-carbazolyl) benzene (mCP) and 3,5-bis (9-carbazolyl) tetraphenylsilane (SimCP) at 10-300 K. The energies of the S1 and T1 states are estd. for FIrpic, FIrN4, mCP and SimCP from the absorption and excitation spectra. The PL intensities of FIrpic and FIrN4 doped in mCP and SimCP films are obsd. to decrease above about 50 and 150 K, resp., with increasing temp. from 10 to 300 K. In all the neat films of mCP and SimCP, the 0-0, 0-1 and 0-2 vibronic bands of the T1 emission split into four and six components, resp., indicating presence of aggregates such as dimer besides monomer. It is suggested that the aggregates lead to reduce the PL intensity of the dopants at high temps. like room temp.
- 3Bagnich, S. A.; Rudnick, A.; Schroegel, P.; Strohriegl, P.; Köhler, A. Triplet Energies and Excimer Formation in Meta- and Para-Linked Carbazolebiphenyl Matrix Materials. Philos. Trans. R. Soc., A 2015, 373, 20140446, DOI: 10.1098/rsta.2014.0446There is no corresponding record for this reference.
- 4Hosokai, T.; Matsuzaki, H.; Nakanotani, H.; Tokumaru, K.; Tsutsui, T.; Furube, A.; Nasu, K.; Nomura, H.; Yahiro, M.; Adachi, C. Evidence and Mechanism of Efficient Thermally Activated Delayed Fluorescence Promoted by Delocalized Excited States. Sci. Adv. 2017, 3, e1603282 DOI: 10.1126/sciadv.16032824https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC1cXkvVCqtrg%253D&md5=a8e7f40b2806362d6bf948fa036cf806Evidence and mechanism of efficient thermally activated delayed fluorescence promoted by delocalized excited statesHosokai, Takuya; Matsuzaki, Hiroyuki; Nakanotani, Hajime; Tokumaru, Katsumi; Tsutsui, Tetsuo; Furube, Akihiro; Nasu, Keirou; Nomura, Hiroko; Yahiro, Masayuki; Adachi, ChihayaScience Advances (2017), 3 (5), e1603282/1-e1603282/9CODEN: SACDAF; ISSN:2375-2548. (American Association for the Advancement of Science)The design of org. compds. with nearly no gap between the first excited singlet (S1) and triplet (T1) states has been demonstrated to result in an efficient spin-flip transition from the T1 to S1 state, i.e., reverse intersystem crossing (RISC), and facilitate light emission as thermally activated delayed fluorescence (TADF). However, many TADF mols. have shown that a relatively appreciable energy difference between the S1 and T1 states (∼0.2 eV) could also result in a high RISC rate. We revealed from a comprehensive study of optical properties of TADF mols. that the formation of delocalized states is the key to efficient RISC and identified a chem. template for these materials. In addn., simple structural confinement further enhances RISC by suppressing structural relaxation in the triplet states. Our findings aid in designing advanced org. mols. with a high rate of RISC and, thus, achieving the max. theor. electroluminescence efficiency in org. light-emitting diodes.
- 5Godumala, M.; Choi, S.; Cho, M. J.; Choi, D. H. Recent Breakthroughs in Thermally Activated Delayed Fluorescence Organic Light Emitting Diodes Containing Non-Doped Emitting Layers. J. Mater. Chem. C 2019, 7, 2172– 2198, DOI: 10.1039/c8tc06293e5https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC1MXhsVOntbc%253D&md5=eec30593246b773b9530f67e8537b12fRecent breakthroughs in thermally activated delayed fluorescence organic light emitting diodes containing non-doped emitting layersGodumala, Mallesham; Choi, Suna; Cho, Min Ju; Choi, Dong HoonJournal of Materials Chemistry C: Materials for Optical and Electronic Devices (2019), 7 (8), 2172-2198CODEN: JMCCCX; ISSN:2050-7534. (Royal Society of Chemistry)Thermally activated delayed fluorescence (TADF) materials have attracted significant attention owing to their state-of-the-art performance in the field of org. light-emitting diodes (OLEDs). TADF materials have the capability of harvesting both singlet and triplet excitons through reverse intersystem crossing, thus, the max. internal quantum efficiency can be expected to be 100%. Thanks to TADF materials that are developed from pure org. entities so that they are inexpensive and external quantum efficiencies as high as 38% have been achieved to date. Despite the high performance of doped OLEDs, precise control of the host-guest ratio and phase sepn. at high driving voltages are the foremost challenges. In order to circumvent these demerits, the use of numerous small mols., dendritic mols., and polymers as non-doped emitters has been reported. Non-doped OLEDs consist of only a single mol. emitter in the emissive layer, thus simplifying the device fabrication process. Furthermore, non-doped OLEDs exhibit better device stability compared to doped devices, because the former do not exhibit phase sepn. or crystallinity and maintain a homogeneous surface film morphol. during device operation. As a result, non-doped OLEDs are capable of overcoming the most pivotal issues of device complexity and stability, which makes them effective in rendering OLEDs com. viable. Although max. external quantum efficiencies of nearly 23.0% in blue, 22.0% in green, 22.0% in yellow, 17.0% in orange, and 10.0% in red emitting materials have been realized with non-doped OLEDs, still further progress is essential to reach the doped devices. This review systematically describes the mol. design, photophys. properties, and electroluminescence performance data of non-doped TADF emitters. We strongly believe that this review will be beneficial in terms of presenting a specific direction for the design of a variety of non-doped TADF emitters in the near future.
- 6Wu, X.; Su, B.-K.; Chen, D.-G.; Liu, D.; Wu, C.-C.; Huang, Z.-X.; Lin, T.-C.; Wu, C.-H.; Zhu, M.; Li, E. Y. The Role of Host–Guest Interactions in Organic Emitters Employing MR-TADF. Nat. Photonics 2021, 15, 780– 786, DOI: 10.1038/s41566-021-00870-36https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BB3MXitFaqtLnO&md5=775a8cad9811a1a829b16b43ef18b781The role of host-guest interactions in organic emitters employing MR-TADFWu, Xiugang; Su, Bo-Kang; Chen, Deng-Gao; Liu, Denghui; Wu, Chi-Chi; Huang, Zhi-Xuan; Lin, Ta-Chun; Wu, Cheng-Ham; Zhu, Mengbing; Li, Elise Y.; Hung, Wen-Yi; Zhu, Weiguo; Chou, Pi-TaiNature Photonics (2021), 15 (10), 780-786CODEN: NPAHBY; ISSN:1749-4885. (Nature Portfolio)Abstr.: Research into org. light emitters employing multiple resonance-induced thermally activated delayed fluorescence (MR-TADF) materials is presently attracting a great deal of attention due to the potential for efficient deep-blue emission. However, the origins and mechanisms of successful TADF are unclear, as many MR-TADF materials do not show TADF behavior in soln., but only as particular pure solids. Here, an investigation into a well-known MR-TADF material, DABNA-1, together with other new MR materials (9H-quinolino[3,2,1-kl]phenothiazin-9-one (QPO) and 9H-quinolino-[3,2,1-kl]-phenothiazin-9-one 5,5-dioxide (QP3O)), yields new insights regarding the origin of TADF. Although a material system may support the concept of MR, inefficiency in both forward and reverse intersystem crossings forbids TADF unless a suitable host material allows an exciplex-like host-emitter interaction that boosts TADF. This boosted-TADF mechanism can be generalized to any fluorescence dye that lacks TADF in the photoluminescence measurement but has a thermally accessible S1-T1 energy gap, opening the way to high-performance org. light-emitting diodes.
- 7Kondo, Y.; Yoshiura, K.; Kitera, S.; Nishi, H.; Oda, S.; Gotoh, H.; Sasada, Y.; Yanai, M.; Hatakeyama, T. Narrowband Deep-Blue Organic Light-Emitting Diode Featuring an Organoboron-Based Emitter. Nat. Photonics 2019, 13, 678– 682, DOI: 10.1038/s41566-019-0476-57https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC1MXhtlOlsLjK&md5=7ccab1765bbb7ae0e3c9e7430ba2b5a8Narrowband deep-blue organic light-emitting diode featuring an organoboron-based emitterKondo, Yasuhiro; Yoshiura, Kazuki; Kitera, Sayuri; Nishi, Hiroki; Oda, Susumu; Gotoh, Hajime; Sasada, Yasuyuki; Yanai, Motoki; Hatakeyama, TakujiNature 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.
- 8Hwang, 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.1c156598https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BB3MXivVeitLbL&md5=5afac2299e07ef96ce025ffb1e41fb44Aryl-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 <0.1Hwang, Jinhyo; Koh, Chang Woo; Ha, Jung Min; Woo, Han Young; Park, Sungnam; Cho, Min Ju; Choi, Dong HoonACS 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.
- 9Monkman, 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.1c091899https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BB3MXhsVygsrjE&md5=93495210e5468a3c084e700181602ad6Why Do We Still Need a Stable Long Lifetime Deep Blue OLED Emitter?Monkman, AndrewACS 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.
- 10Adamovich, 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/b204301g10https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BD38XmsVWnsrc%253D&md5=a9703167d89eae3343f3041f88aaa327High-efficiency single dopant white electrophosphorescent light-emitting diodesAdamovich, 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.
- 11Kim, 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/srep0700911https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC2cXhvFansb7J&md5=643044a0417d16c3529e5d4ddabd776aStudy of Sequential Dexter Energy Transfer in High Efficient Phosphorescent White Organic Light-Emitting Diodes with Single Emissive LayerKim, Jin Wook; You, Seung Il; Kim, Nam Ho; Yoon, Ju-An; Cheah, Kok Wai; Zhu, Fu Rong; Kim, Woo YoungScientific 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).
- 12Sun, 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.6b0028612https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC28XkvFCksLs%253D&md5=091f9e9c0ad5c3826c78323fa4fb49bcHighly 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-JooACS 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.
- 13Holmes, 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: 10.1063/1.156814613https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BD3sXivFamu7c%253D&md5=07c25a6c54a44fb7b5722f86b2bf51f4Blue organic electrophosphorescence using exothermic host-guest energy transferHolmes, 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.
- 14Hayashi, 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: 10.1063/1.491346114https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC2MXjs1ansbk%253D&md5=fb964a711958f1087945aaa781d80077Suppression of roll-off characteristics of organic light-emitting diodes by narrowing current injection/transport area to 50 nmHayashi, Kyohei; Nakanotani, Hajime; Inoue, Munetomo; Yoshida, Kou; Mikhnenko, Oleksandr; Nguyen, Thuc-Quyen; Adachi, ChihayaApplied 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.
- 15Flender, 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/c6cp05167g15https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC28XhsV2ju7bK&md5=3f5a0359ff6ee72cb2d0d4586c0ba0c5Excited-state relaxation of the solar cell dye D49 in organic solvents and on mesoporous Al2O3 and TiO2 thin filmsFlender, Oliver; Scholz, Mirko; Klein, Johannes R.; Oum, Kawon; Lenzer, ThomasPhysical 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.
- 16Oum, 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/jp407219716https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC2cXktFehsLw%253D&md5=45628dc70338d8dd0e603b6cf0dddbd3Observation of Ultrafast Carrier Dynamics and Phonon Relaxation of Graphene from the Deep-Ultraviolet to the Visible RegionOum, Kawon; Lenzer, Thomas; Scholz, Mirko; Jung, Dae Yool; Sul, Onejae; Cho, Byung Jin; Lange, Jens; Mueller, AndreasJournal 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.
- 17Merker, 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.1c0044617https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BB3MXmtVOrt7g%253D&md5=2ad11b46b933299299ec7c8d4588d7b8Photoinduced Dynamics of (CH3NH3)4Cu2Br6 Thin Films Indicating Efficient Triplet PhotoluminescenceMerker, Alexander; Scholz, Mirko; Morgenroth, Marius; Lenzer, Thomas; Oum, KawonJournal 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.
- 18Oum, 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/c3cp55298e18https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC2cXlsFOisb0%253D&md5=3e84cee278dd71f1b5b0abe4e0d4306cElectron and hole transfer dynamics of a triarylamine-based dye with peripheral hole acceptors on TiO2 in the absence and presence of solventOum, Kawon; Flender, Oliver; Lohse, Peter W.; Scholz, Mirko; Hagfeldt, Anders; Boschloo, Gerrit; Lenzer, ThomasPhysical 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).
- 19Morgenroth, 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.20220307519https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BB38Xhs12gtb7J&md5=3bb57dbe045c351bbf1e3adbb398c0f2Spatiotemporal Mapping of Efficient Chiral Induction by Helicene-Type Additives in Copolymer Thin FilmsMorgenroth, Marius; Scholz, Mirko; Guy, Laure; Oum, Kawon; Lenzer, ThomasAngewandte 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.
- 20Mao, 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)08405-320https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADyaK28XmvVSrtr4%253D&md5=978ad73ab6fdd613fa93ece687b06644Excimer formation and excitation energy transfer in Langmuir-Blodgett films of cellulose derivatives containing carbazoleMao, 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.
- 21Tani, 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/b104101k21https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BD3MXns1yms7o%253D&md5=235a117c5154a681a98c943cce15d389Synthesis and photophysical properties of [3.3](3,9)carbazolophanesTani, Keita; Tohda, Yasuo; Takemura, Hiroyuki; Ohkita, Hideo; Ito, Shinzaburo; Yamamoto, MasahideChemical 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.
- 22Winnik, F. M. Photophysics of Preassociated Pyrenes in Aqueous Polymer Solutions and in Other Organized Media. Chem. Rev. 1993, 93, 587– 614, DOI: 10.1021/cr00018a00122https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADyaK3sXhsVygs7w%253D&md5=ed06727222fdb1eb0b15a61fe5ed7792Photophysics of preassociated pyrenes in aqueous polymer solutions and in other organized mediaWinnik, Francoise M.Chemical Reviews (Washington, DC, United States) (1993), 93 (2), 587-614CODEN: CHREAY; ISSN:0009-2665.A review with 155 refs.
- 23Musser, 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/c7tc02655b23https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC2sXhtFamsbvO&md5=cfbc25ec5ccd512a1a6488dee8968250Intermolecular states in organic dye dispersions: excimers vs. aggregatesMusser, 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.
- 24Spano, F. C. The Spectral Signatures of Frenkel Polarons in H- and J-Aggregates. Acc. Chem. Res. 2010, 43, 429– 439, DOI: 10.1021/ar900233v24https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BD1MXhsFKgurzL&md5=a66d2effbaf73a403bf00e6e198efe0aThe Spectral Signatures of Frenkel Polarons in H- and J-AggregatesSpano, 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.
- 25Mai, 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-z25https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BB3cXitlWgu7fI&md5=c20bcfc8366b0a64ac16c053af5cf3fdSolid cyclooctatetraene-based triplet quencher demonstrating excellent suppression of singlet-triplet annihilation in optical and electrical excitationMai, 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-ChunNature 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.
- 26Morgenroth, 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: 10.1080/00268976.2021.195907226https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BB3MXhsl2ktLnK&md5=35b603aba36564dbd22bf398185bc47dUltrafast excited-state dynamics of thin films consisting of helicene-like molecules based on dibenzo[c,h]acridineMorgenroth, Marius; Scholz, Mirko; Guy, Laure; Oum, Kawon; Lenzer, ThomasMolecular 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.
- 27Morgenroth, 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.0c0097627https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BB3cXnvFCnur8%253D&md5=1be0836c7d470d6f9d2e5dac83eb157dUltrafast UV-Vis Transient Absorption and Circular Dichroism Spectroscopy of a Polyfluorene Copolymer Showing Large Chiral InductionMorgenroth, Marius; Scholz, Mirko; Lenzer, Thomas; Oum, KawonJournal 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.
- 28Lenzer, 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/b925071a28https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC3cXpsVGit7Y%253D&md5=c881fc2ca6de32b4bccdb3dc88b4bea1Assignment of carotene S* state features to the vibrationally hot ground electronic stateLenzer, Thomas; Ehlers, Florian; Scholz, Mirko; Oswald, Rainer; Oum, KawonPhysical 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.
- 29Schwarzer, 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: 10.1063/1.47218029https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADyaK28XltFKjurc%253D&md5=cd9ad4dc000ee6e7c3803cb6f36b3069Collisional deactivation of vibrationally highly excited azulene in compressed liquids and supercritical fluidsSchwarzer, 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.
- 30Grahn, H. T.; Maris, H. J.; Tauc, J. Picosecond Ultrasonics. IEEE J. Quantum Electron. 1989, 25, 2562– 2569, DOI: 10.1109/3.4064330https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADyaK3cXosVentg%253D%253D&md5=21efe48d56a06320aa940a5e52e7cefaPicosecond ultrasonicsGrahn, Holger T.; Maris, Humphrey J.; Tauc, JanIEEE 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.
- 31Ruello, 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.2014.06.00431https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC2cXht1Wmu7rM&md5=c3f9ccc802a63be8ba132e8767a610d9Physical mechanisms of coherent acoustic phonons generation by ultrafast laser actionRuello, 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.
- 32Scholz, 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: 10.1063/1.512443832https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC1MXisVKgu7jM&md5=b1661ff6f0a595baa9f50f584c4d7adfCoherent acoustic phonon dynamics in chiral copolymersScholz, Mirko; Morgenroth, Marius; Cho, Min Ju; Choi, Dong Hoon; Lenzer, Thomas; Oum, KawonStructural 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.
- 33Morgenroth, 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/s41467-021-27886-133https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BB38XoslOhug%253D%253D&md5=30a9e8d628d2831906f34985c21562e7Mapping the broadband circular dichroism of copolymer films with supramolecular chirality in time and spaceMorgenroth, Marius; Scholz, Mirko; Cho, Min Ju; Choi, Dong Hoon; Oum, Kawon; Lenzer, ThomasNature 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.
- 34Murawski, C.; Leo, K.; Gather, M. C. Efficiency Roll-Off in Organic Light-Emitting Diodes. Adv. Mater. 2013, 25, 6801– 6827, DOI: 10.1002/adma.20130160334https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC3sXhsVSrs73K&md5=e37d55d72cf73907428d74c147c884adEfficiency Roll-Off in Organic Light-Emitting DiodesMurawski, 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.
- 35Klein, 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/c7cp01718a35https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC2sXht1ynsrfO&md5=ea217a0f0fb219f5796be14d10030d6bPhotoinduced dynamics of the hole-transport material H101 in organic solvents and on mesoporous Al2O3 and TiO2 thin filmsKlein, Johannes R.; Scholz, Mirko; Oum, Kawon; Lenzer, ThomasPhysical 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.
- 36Förster, T. Zwischenmolekulare Energiewanderung und Fluoreszenz. Ann. Phys. 1948, 437, 55– 75, DOI: 10.1002/andp.19484370105There is no corresponding record for this reference.
- 37Förster, T. Transfer Mechanisms of Electronic Excitation. Discuss. Faraday Soc. 1959, 27, 7– 17, DOI: 10.1039/DF9592700007There is no corresponding record for this reference.
- 38Stevens, 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.63.16521338https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BD3MXislyqtrc%253D&md5=5aa1ea8046038c5e50f6fa32a0404e4fExciton 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.
- 39Smith, M. B.; Michl, J. Singlet Fission. Chem. Rev. 2010, 110, 6891– 6936, DOI: 10.1021/cr100261339https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC3cXhtlKntbbF&md5=4632198f6c4b979ef92822fd6a3be047Singlet FissionSmith, Millicent B.; Michl, JosefChemical 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.
- 40Wachsstock, 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.
- 41Ruseckas, 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: 10.1063/1.325342241https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BD1MXhtl2jtLjL&md5=f11538dca764a37639d27209cb5d83e9Singlet energy transfer and singlet-singlet annihilation in light-emitting blends of organic semiconductorsRuseckas, 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.
- 42Hofkens, 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.223580510042https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BD3sXptFOjurk%253D&md5=a37dbbb55079bce354af604208100ad6Revealing competitive Forster-type resonance energy-transfer pathways in single bichromophoric moleculesHofkens, 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, FransProceedings 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.
- 43Hedley, 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-z43https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BB3MXltlyksbY%253D&md5=7de6aaf825d2510487aa052d08bd62ccPicosecond time-resolved photon antibunching measures nanoscale exciton motion and the true number of chromophoresHedley, Gordon J.; Schroder, Tim; Steiner, Florian; Eder, Theresa; Hofmann, Felix J.; Bange, Sebastian; Laux, Dirk; Hoger, Sigurd; Tinnefeld, Philip; Lupton, John M.; Vogelsang, JanNature 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.
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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