Analyzing the Intermolecular Dynamics of Excited States in Molecular Semiconductors

尽管基础和应用驱动的研究非常激烈，但在分子材料及其复合材料中观察到的一些重要的光物理现象尚未完全理解。分子聚集体中的光激发态与周围晶格及其声子激发的相互作用是一个重要的方面，在一阶中经常被忽视。因此，本项目旨在直接观察晶格动力学，即伴随着并支持分子晶体中激子态演化的绝热重组以及声子的相干和非相干产生。为此，将使用超快电子衍射（UED）和超快电子漫射散射（UEDS）来深入了解激子-晶格耦合的机制。通过UEDS直接观察核动力学和整个布里渊区的时间分辨声子居群，将对现有的基于间接光谱证据、密度泛函理论计算和分子动力学模拟的模型进行补充。为此，将在不同的模型系统中研究与周围晶格及其动力学强烈耦合的两种双/多分子激发，即准分子和电荷转移（CT）态。将在结构高度明确的分子单晶上进行泵浦探针实验，以确定光激发时随时间变化的结构响应。在模型体系芘上研究了准分子态形成过程中的晶格动力学。通过对芘单晶进行UED和UEDS，可以加深对准分子形成过程中晶格重组和声子产生的理解，并可以识别非辐射声子辅助衰变通道。这两个过程都将作为二聚体成分分子质量的函数进行分析，利用过氘化芘作为参考，因为它的质量更大，但电子和结构性质相似。在项目的第二部分，将研究ct配合物四烯：F4TCNQ（2,3,5,6-四氟-7,7,8,8-四氰喹啉二甲烷）在光激发下的晶格动力学和声子辅助的非辐射衰变通道。预期的UED和UEDS研究将提供独特的机会(i)推断出与时间相关的静电势，从而从低指数布拉格反射中推断激发态中的电荷（再）分布，（ii）分析光激发时分子间几何结构的变化，（iii）识别声子模式作为非辐射损失通道。这些结果补充了分子系统中光物理过程的模型，并与未来超越所研究的模型系统的光电应用相关。

Despite intense fundamental as well as application driven research, some important photophysical phenomena observed in molecular materials and their composites are not fully understood, yet. One important aspect, often neglected at first order, is the interaction of photoexcited states in molecular aggregates with the surrounding lattice and its phononic excitations. Hence, the proposed project aims to observe the lattice dynamics directly, i.e. the adiabatic reorganization as well as the coherent and non-coherent generation of phonons, accompanying and supporting the evolution of excitonic states in molecular crystals. For this purpose, ultrafast electron diffraction (UED) as well as ultrafast electron diffuse scattering (UEDS) will be used to allow for insights into the mechanisms of the exciton-lattice coupling. By directly observing the nuclear dynamics via UED and the time resolved phonon population across the entire Brillouin zone via UEDS, already existing models based on indirect spectroscopic evidences in combination with density functional theory calculations and molecular dynamics simulations will be complemented. For this purpose, two bi- /multimolecular excitations, the excimer and the charge transfer (CT) state, which couple strongly to the surrounding lattice and its dynamics, will be investigated on distinct model systems. The pump-probe experiments to determine the time dependent structural response upon photoexcitation will be carried out on structurally highly defined molecular single crystals. The lattice dynamics during the formation of an excimer state will be studied on the model system pyrene. By performing UED and UEDS on pyrene single crystals it is expected that the understanding of the lattice reorganization and phonon generation during excimer formation can be deepened as well as that non-radiative phonon-assisted decay channels can be identified. Both processes will be analyzed as function of the dimer constituents’ molecular masses utilizing perdeuterated pyrene as reference due to its larger mass but similar electronic and structural properties. In the second part of the project, the lattice dynamics as well as phonon-assisted non-radiative decay channels upon photoexcitation of the CT-complex tetracene:F4TCNQ (2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane) will be studied. The intended UED and UEDS studies will offer the unique opportunities (i) to deduce the time dependent electrostatic potential and, hence, the charge (re)distribution in the excited states from the low index Bragg reflections, (ii) to analyze the changes in the intermolecular geometry upon photo excitation and (iii) to identify phonon modes acting as non-radiative loss channels.These results complement the models on photophysical processes in molecular systems and are relevant for future optoelectronic applications exceeding the examined model systems.