Synergistic\nEffects of Unconventional Hydrogen Bonds\nand π‑Stacking Interaction and Their Excited-State Dependence:\nThe Origin of Unusual Photophysical Properties of Aromatic Thioketones\nin Acetonitrile and Hydrocarbons

It\nhas been established experimentally that aromatic thioketones\npossess several inherently unique photophysical properties, some of\nwhich are highly sensitive even to common hydrocarbon solvents. However,\nthe deeper reasons and the underlying mechanisms remain unclear up\nto date. In this study, the multistate complete active space second-order\nperturbation theory (MS-CASPT2) has been utilized to investigate the\nfive lowest-lying electronic states (S<sub>0</sub>, T<sub>1</sub>,\nS<sub>1</sub>, T<sub>2</sub>, and S<sub>2</sub>) of 4<i>H</i>-1-benzopyran-4-thione (BPT) in acetonitrile and hydrocarbons. The\nresults show that the S<sub>1</sub>, T<sub>1</sub>, and T<sub>2</sub> states of BPT are close in energy so that the T<sub>2</sub>-state-mediated\nS<sub>1</sub> → T<sub>2</sub> → T<sub>1</sub> and T<sub>1</sub> → T<sub>2</sub> → S<sub>1</sub> transitions\ncould occur in tens of picoseconds, which exhibits little dependence\non the formation of the BPT–solvent complexes and on the bulk-solvent\neffect. This explains why thermally activated delayed fluorescence\nfrom the S<sub>1</sub> state has been observed for many aromatic thioketones\nin both inert media and hydrocarbons. Meanwhile, our calculations\nshow that the intracomplex noncovalent interactions could be automatically\nadjusted by the redistribution of π-electrons in the flexible\naromatic rings. This allows the S<sub>2</sub> → S<sub>1</sub> internal conversion to occur efficiently in the vicinity of the\ntwo-state conical intersection, which results in the remarkable changes\nin the S<sub>2</sub>-state lifetimes and fluorescence quantum yields\nof many aromatic thioketones from inert media to hydrocarbon solvents.\nThe aforementioned inherent photophysical properties could be qualitatively\nunderstood by a simple model of frontier molecular orbitals. This\nmodel could be used to understand photophysical properties of other\naromatic compounds (such as aldehydes, ketones, amines, and carboxylic\nacids) in different solvents.