Modeling of pressure-induced charge transfer character in piezoluminescent pyridylvinylanthracene crystals

Abstract

Mechanoluminescent (ML) materials show color changes resulting from the application of a mechanical constraint such as an external pressure. In these systems, the applied pressure might alter the structure of the molecular units and their supramolecular packing, which translates into changes in emission wavelength and intensity. In this work, we report a computational study of the ML properties of the recently synthesized 9,10-bis((E)-2-(pyrid-2-yl)vinyl)anthracene (BP2VA) crystal. In a previous experimental report, the BP2VA crystal powder has been shown to undergo drastic changes in photoluminescence color upon exertion of external pressure. The evolution of the piezochromic luminescence upon increasing pressure was rationalized by comparison to the emission behavior of the three BP2VA crystal polymorphs, which are characterized by distinct stacking modes, each exhibiting varying strengths of π–π interactions. Combining periodic calculations based on density functional theory (DFT) and time-dependent DFT within a hybrid QM/MM scheme to account for crystal environment effects, we explored the structural and optical features of the BP2VA polymorphs and examined the changes in supramolecular arrangement and luminescence associated with the application of pressure. Decomposition of the electronic excited states of relevant dimers extracted from the optimized crystal cells on a diabatic basis allowed to characterize the electronic transitions according to their local and charge transfer (CT) nature. The results, which overall compare well to experimental data, provide a new mechanistic picture behind the piezochromic behavior of organic crystals. While intermolecular interactions and structural constraints play a crucial role in the photophysical properties of these materials, we demonstrate that external pressure can alter the nature of the low-lying excited states, notably imparting CT character. The gradual redshift of the fluorescence emission in the three polymorphs under external pressure is attributed to the increasing excimer-like character of the emissive state.