Role of Centrosymmetry\nand Intermolecular Orbital\nInteractions in the Photophysics of Molecular Crystals and Aggregates

According to the Cambridge Structural\nDatabase, more\nthan 50% of\norganic molecules belong to a centrosymmetric space group when forming\ncrystals. However, the significance of crystal symmetry in the photophysics\nof molecules in their aggregated or crystalline form is seldom discussed\nnowadays. Herein, we utilize group theory and molecular orbital (MO)\ncalculations to analyze the optical properties of centrosymmetric\nmolecular aggregates, including their transition dipole moments and\nspin–orbit coupling. Symmetry-forbidden electronic transitions\nare closely related to the ultralong organic phosphorescence (UOP)\nphenomenon in molecular crystals. These forbidden electronic transitions\nresult in smaller radiative decay rates (<i>k</i><sub><i>r</i></sub>) and prolonged emission lifetimes, thus favoring\nthe generation of UOP.