Competition between radiative and nonradiative excited state processes in photoluminescent organic molecular crystals
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Organic molecular crystals are attractive materials for luminescent applications due to their promised tunability. However, the link between chemical structure and emissive behaviour is poorly understood due to the numerous interconnected factors which are at play in determining radiative and non-radiative behaviours at the solid state level. In this study, we investigate thirteen luminescent molecular crystals and apply newly implemented tools to study their geometric properties and constituent dimer excitonic coupling values. We then focus on the excited state decay pathways of five of the molecules. The competition between radiative and nonradiative processes was used to rationalise the different fluorescence quantum yields across systems. We found that due to the prevalence of sheet and herringbone packing in organic molecular crystals, the conformational diversity of crystal dimers is limited. Additionally, similarly spaced dimers have exciton coupling values of similar order within a 50 meV interval. Finally, we found that the accessibility of conical intersection geometries was a robust indicator of the role of nonradiative decay in the excited state mechanism of most molecules. The conical intersections all displayed a measure of rotation and puckering, where purely rotational conical intersections in vacuum lead to high energy puckered conical intersections in the crystal phase.