Origins of crystallisation-induced dual emission of terephthalic and isophthalic acid crystals

Metal-free organic crystals with room-temperature phosphorescence (RTP) present an innovative alternative to conventional inorganic materials for optoelectronic applications and sensing. Recently, substantial attention has been directed towards the design of new phosphorescent crystals through crystal engineering and functionalisation. In this paper, we investigate the excited-state deactivation mechanisms of two simple organic molecules: terephthalic acid (TPA) and isophthalic acid (IPA) using embedding models based on multiconfigurational MS-CASPT2 calculations. These molecules exhibit prompt and delayed fluorescence and RTP in the solid state. We explore intramolecular internal conversion pathways using high-level quantum chemistry methods in both solution and crystalline phases. We analyse deactivation mechanisms involving singlet and triplet states, quantifying direct and reverse intersystem crossing rates from the lowest triplet states, as well as fluorescence and phosphorescence rates. Additionally, our study examines singlet exciton transport in single crystals of TPA and IPA. Our findings clarify the mechanisms underlying the prompt and delayed fluorescence and RTP of crystalline TPA and IPA, revealing distinct differences in their deactivation processes. Notably, we explain the enhanced fluorescence and phosphorescence in IPA compared to TPA, emphasising how the positioning of the carboxylic group influences electronic delocalisation in excited states, (de)stabilising delocalised ππ* states along the reaction coordinate, thereby significantly impacting deactivation mechanisms.