Computational photochemistry of organic molecular crystals

Work package:
Understanding photoinduced processes in organic molecular crystals is central to the design of highly emissive materials such as organic light-emitting diodes,[1] and multicolored photochromic materials with application for photoswitchable full-color displays and multifrequency three-dimensional optical memory media.[2] However, the rationalization of the photochemical behavior is still poorly understood at the solid-state level because of the difficulty to model photochemical pathways in crystal environments. In particular, radiationless decay through conical intersections (CIs) is ubiquitous in photochemical processes.[3] CIs not only provide critical structures acting as funnels for efficient non-radiative transitions between electronic states leading to the formation of photoproducts not reachable by thermal (ground-state) processes, but they are also at the origin of emission quenching depending on their accessibility on the excited-state potential energy surface.[4] While computational photochemistry has been highly successful in studying various photochemical processes in the gas phase, in solution and even in complex biological environments,[5] it has rarely been applied in the context of molecular crystals. This can mainly be explained by the lack of excited-state methodologies tailored for such periodic systems. Recently, embedding approaches based on the Ewald sum have been used in conjunction with excited-state electronic structure methods to model the localized excitations which characterize these materials.[6,7] During this internship, we aim at using different embedding schemes to study aggregation-induced emissions in different organic molecular crystals using either benzylideneimidazolinone[8] or cinnamoyle pyrone[9] derivatives. We will first use the point charge embedding (PCE) approach where only the Coulomb interactions are considered, using point charges, and nonelectrostatic interactions are neglected.[6] Then, we will explore the possibility to increase the size of the quantum mechanical system to several chromophore units by using a cluster approach in combination with the PCE approach. Eventually, if time allows, we will use hybrid QM/QM’ electrostatic embedding schemes in order to account for the polarization of the environment.[7] The purpose of these computational studies will be to rationalize the solid-state luminescence enhancement observed experimentally compared to the behavior in the gas phase or in solution.