Probing Unusual Intrinsic Photophysical Properties of Thiocoumarin from XMS-CASPT2 Calculations

Abstract

Aromatic thioketones have long served as model molecules for studying the fundamental principles of molecular photochemistry because their intrinsic photophysical properties differ markedly from those of aromatic ketones and many other aromatic compounds containing various organic functional groups. Thiocoumarin (TC), however, is an exceptional aromatic thioketone whose excited-state dynamics are distinctly different from those of common aromatic thioketones. Although many experimental and theoretical investigations have been devoted to TC and its derivatives, the intrinsic origin of their unusual photophysical properties remains unclear. In this work, high-level XMS-CASPT2 calculations were performed for the five lowest-lying electronic states, S0, T1, S1, T2, and S2, of TC and benzopyranthione (BPT), with BPT used as a representative common aromatic thioketone for comparison. The calculated results show that the small difference between the molecular structures of BPT and TC leads to substantial changes in the excited-state structures, electronic characters, and energy-level ordering in going from BPT to TC. These changes are identified as the intrinsic origin of the anomalous photophysical properties of TC. Meanwhile, the XMS-CASPT2 calculations reveal that the T1 state of TC can be populated through the two ultrafast S2 → S1 → T1 and S2 → T2 → T1 processes, whereas the subsequent T1 → S0 radiative and nonradiative processes are predicted to be inefficient. The theoretical predictions are in excellent agreement with the experimental findings that the long-lived T1 state of TC is formed with near-unity quantum yield. The T1-state features make TC and its derivatives metal-atom-free photosensitizers applicable in the field of photodynamic therapy.