Analyzing the Intermolecular Dynamics of Excited States in Molecular Semiconductors

尽管有大量的基础和应用研究，但在分子材料及其复合材料中观察到的一些重要的物理现象尚未完全理解。一个重要的方面，往往被忽视的一阶，是光激发态的分子聚集体与周围的晶格和声子激发的相互作用。因此，拟议的项目旨在直接观察晶格动力学，即绝热重组以及声子的相干和非相干产生，伴随并支持分子晶体中激子态的演化。为此，超快电子衍射（UED）以及超快电子漫散射（UEDS）将被用来允许洞察激子晶格耦合的机制。通过UED直接观察核动力学，通过UEDS观察整个布里渊区的时间分辨声子群，将补充现有的基于间接光谱证据的模型，并结合密度泛函理论计算和分子动力学模拟。为了这个目的，两个双/多分子激发，激基缔合物和电荷转移（CT）状态，强烈耦合到周围的晶格及其动力学，将在不同的模型系统进行研究。泵浦探测实验，以确定光激发时的时间依赖性的结构响应将进行结构高度定义的分子单晶。本文以芘为模型系统，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的衰变通道。这两个过程将被分析为二聚体成分的分子量的函数，利用全氘代芘作为参考，由于其较大的质量，但相似的电子和结构特性。在项目的第二部分中，将研究CT-配合物并四苯：F4 TCNQ（2，3，5，6-tetrafluoro-7，7，8，8-tetracyanoquinodimethane）的晶格动力学以及光激发时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）从低折射率布拉格反射中推导出时间相关的静电势，从而推导出激发态中的电荷（再）分布，（ii）分析光激发时分子间几何结构的变化，以及（iii）识别作为非-这些结果补充了分子系统中的光物理过程的模型，并与未来的光电应用有关，超过了所研究的模型系统。

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.