Synergistic Effects of Unconventional Hydrogen Bonds and π-Stacking Interaction and Their Excited-State Dependence: The Origin of Unusual Photophysical Properties of Aromatic Thioketones in Acetonitrile and Hydrocarbons

It has been established experimentally that aromatic thioketones possess several inherently unique photophysical properties, some of which are highly sensitive even to common hydrocarbon solvents. However, the deeper reasons and the underlying mechanisms remain unclear up to date. In this study, the multistate complete active space second-order perturbation theory (MS-CASPT2) has been utilized to investigate the five lowest-lying electronic states (S₀, T₁, S₁, T₂, and S₂) of 4<i>H</i>-1-benzopyran-4-thione (BPT) in acetonitrile and hydrocarbons. The results show that the S₁, T₁, and T₂ states of BPT are close in energy so that the T₂-state-mediated S₁ → T₂ → T₁ and T₁ → T₂ → S₁ transitions could occur in tens of picoseconds, which exhibits little dependence on the formation of the BPT-solvent complexes and on the bulk-solvent effect. This explains why thermally activated delayed fluorescence from the S₁ state has been observed for many aromatic thioketones in both inert media and hydrocarbons. Meanwhile, our calculations show that the intracomplex noncovalent interactions could be automatically adjusted by the redistribution of π-electrons in the flexible aromatic rings. This allows the S₂ → S₁ internal conversion to occur efficiently in the vicinity of the two-state conical intersection, which results in the remarkable changes in the S₂-state lifetimes and fluorescence quantum yields of many aromatic thioketones from inert media to hydrocarbon solvents. The aforementioned inherent photophysical properties could be qualitatively understood by a simple model of frontier molecular orbitals. This model could be used to understand photophysical properties of other aromatic compounds (such as aldehydes, ketones, amines, and carboxylic acids) in different solvents.