Thiourea-Mediated Photoinduced Electron Transfer in Water.

Thiourea (TU) is a versatile functional scaffold in catalysis, sensing, and medicinal chemistry, owing to its strong electron-donating character. However, the photophysical parameters governing the efficiency of TU derivatives in photoinduced electron transfer (PET) remain poorly understood. Here, we systematically investigated TU-mediated PET by introducing controlled alkyl substitution at the nitrogen centers while preserving the thiocarbonyl framework. Using Rhodamine 6G (Rh6G) as a model fluorophore, we combined quantum chemical calculations with steady-state and time-resolved spectroscopic techniques to elucidate structure-PET relationships. Progressive alkyl substitution leads to a significant enhancement in fluorescence quenching efficiency (67 to ∼90 %). While the free energy change (ΔG) exhibits minor deviation, the donor-acceptor energy gap (ΔE) decreases systematically, identifying ΔE as a key parameter governing PET efficiency in these systems. Time-resolved fluorescence and femtosecond transient absorption measurements reveal the emergence of intrinsic PET dynamics on the ∼220-360 ps time scale and confirm the formation of long-lived Rh6G radical species. Higher alkylated derivatives exhibit mixed static-dynamic quenching, driven by enhanced ground-state association. This behavior arises from a synergistic interplay of increased electron density and hydrophobic interactions, which collectively promote efficient PET. These findings establish a mechanistic framework for tuning electron transfer in thiocarbonyl systems and provide design principles for advanced fluorescent probes and sensing platforms.