Photoinduced Charge-Transfer Suppresses Triplet Formation Efficiency in Thiocoumarins: Evidence from Ultrafast Spectroscopy and Theoretical Calculations.

Thiocarbonyl-containing compounds are known for their distinctive photophysical properties, particularly for their rapid intersystem crossing (ISC) and high triplet yield facilitated by enhanced spin-orbit coupling (SOC). However, the ISC efficiency and resulting triplet yields can vary significantly depending on molecular properties. This study explores the role of intramolecular charge transfer (CT) in modulating triplet state generation in thiocoumarins. Specifically, the introduction of a diethylamino group at the 7-position of the thiocoumarin ring (Thiocoumarin 1 or TC1) induces CT character, yielding a moderate singlet oxygen generation efficiency (0.5-0.6) and solvent polarity-dependent fluorescence property. In contrast, the 7-acetoxy-substituted derivative (Acetoxy-TC) achieves remarkably high singlet oxygen yields (0.8-0.9) that are independent of the solvent environment. Time-resolved spectroscopic measurements reveal an ultrashort fluorescence lifetime and concomitant ultrafast triplet state generation in acetoxy-TC across solvents, indicating highly efficient ISC. On the other hand, TC1 having CT behavior has relatively stable singlet excited states and slower ISC dynamics, consistent with its steady-state photophysical behavior. Complementary theoretical calculations further support these observations: acetoxy-TC exhibits solvent-independent high SOC values and a small singlet-triplet energy gap, both conducive to efficient ISC. In contrast, TC1 has less favorable ISC promoting parameters. These findings underscore the importance of molecular design, specifically avoiding CT states, in achieving efficient triplet generation in thiocarbonyl systems.