Role of Centrosymmetry and Intermolecular Orbital Interactions in the Photophysics of Molecular Crystals and Aggregates

According to the Cambridge Structural Database, more than 50% of organic molecules belong to a centrosymmetric space group when forming crystals. However, the significance of crystal symmetry in the photophysics of molecules in their aggregated or crystalline form is seldom discussed nowadays. Herein, we utilize group theory and molecular orbital (MO) calculations to analyze the optical properties of centrosymmetric molecular aggregates, including their transition dipole moments and spin–orbit coupling. Symmetry-forbidden electronic transitions are closely related to the ultralong organic phosphorescence (UOP) phenomenon in molecular crystals. These forbidden electronic transitions result in smaller radiative decay rates (kr) and prolonged emission lifetimes, thus favoring the generation of UOP.