Rivera Miguel, Stojanović Ljiljana, Crespo-Otero Rachel
School of Biological and Chemical Sciences, Queen Mary University of London, Mile End Road, London E1 4NS, U.K.
J Phys Chem A. 2021 Feb 4;125(4):1012-1024. doi: 10.1021/acs.jpca.0c11072. Epub 2021 Jan 25.
Organic molecular crystals are attractive materials for luminescent applications because of their promised tunability. However, the link between the chemical structure and emissive behavior is poorly understood because of the numerous interconnected factors which are at play in determining radiative and nonradiative behaviors at the solid-state level. In particular, the decay through conical intersection dominates the nonadiabatic regions of the potential energy surface, and thus, their accessibility is a telling indicator of the luminosity of the material. In this study, we investigate the radiative mechanism for five organic molecular crystals which display a solid-state emission, with a focus on the role of conical intersections in their photomechanisms. The objective is to situate the importance of the accessibility of conical intersections with regards to emissive behavior, taking into account other nonradiative decay channels, namely, vibrational decay, and exciton hopping. We begin by giving a brief overview of the structural patterns of the five systems within a larger pool of 13 crystals for a richer comparison. We observe that because of the prevalence of sheet like and herringbone packing in organic molecular crystals, the conformational diversity of crystal dimers is limited. Additionally, similarly spaced dimers have exciton coupling values of a similar order within a 50 meV interval. Next, we focus on three exemplary cases, where we disentangle the role of nonradiative decay mechanisms and show how rotational minimum energy conical intersections in vacuum lead to puckered ones in the crystal, increasing their instability upon crystallization in typical packing motifs. In contrast, molecules with puckered conical intersections in vacuum tend to conserve this trait upon crystallization, and therefore, their quantum yield of fluorescence is determined predominantly by other nonradiative decay mechanisms.
有机分子晶体因其有望实现的可调节性而成为发光应用的有吸引力的材料。然而，由于在固态层面决定辐射和非辐射行为的众多相互关联的因素，化学结构与发光行为之间的联系仍未得到很好的理解。特别是，通过锥形交叉点的衰变主导了势能面的非绝热区域，因此，它们的可达性是材料发光度的一个有力指标。在本研究中，我们研究了五种呈现固态发射的有机分子晶体的辐射机制，重点关注锥形交叉点在其光机制中的作用。目的是在考虑其他非辐射衰变通道（即振动衰变和激子跳跃）的情况下，确定锥形交叉点的可达性对于发光行为的重要性。我们首先简要概述了13种晶体的更大集合中这五个系统的结构模式，以便进行更丰富的比较。我们观察到，由于有机分子晶体中片状和人字形堆积的普遍性，晶体二聚体的构象多样性受到限制。此外，间距相似的二聚体在50毫电子伏特的区间内具有相似量级的激子耦合值。接下来，我们关注三个典型案例，在其中我们解开了非辐射衰变机制的作用，并展示了真空中的旋转最小能量锥形交叉点如何在晶体中导致褶皱的交叉点，增加它们在典型堆积模式下结晶时的不稳定性。相比之下，真空中具有褶皱锥形交叉点的分子在结晶时倾向于保留这一特征，因此，它们的荧光量子产率主要由其他非辐射衰变机制决定。
