The chapter explores the excited-state intramolecular proton transfer (ESIPT) reactions of thiol-related hydrogen bonds, focusing on the compound 3NTF, which exhibits a 710 nm tautomer emission with a large Stokes shift of 12,230 cm⁻¹. The study investigates the relationship between H-bonding strength and ESIPT dynamics in a series of 4′-substituted-7-diethylamino-3-mercaptoplavones (NTFs). The NTFs were synthesized with varying H-bonding strengths, and their photophysical properties were characterized. The results show that the ESIPT rate does not correlate with H-bonding strength but is influenced by the basicity of the proton acceptor, specifically the carbonyl oxygen. This finding highlights the intrinsic difference between non-classical −SH and Pauling-type −OH/NH-type H-bonds, where the proton-donating site's electron negativity plays a crucial role in ESIPT dynamics and thermodynamics. The study provides insights into the fundamental differences in ESIPT mechanisms for non-Pauling-type H-bonds.The chapter explores the excited-state intramolecular proton transfer (ESIPT) reactions of thiol-related hydrogen bonds, focusing on the compound 3NTF, which exhibits a 710 nm tautomer emission with a large Stokes shift of 12,230 cm⁻¹. The study investigates the relationship between H-bonding strength and ESIPT dynamics in a series of 4′-substituted-7-diethylamino-3-mercaptoplavones (NTFs). The NTFs were synthesized with varying H-bonding strengths, and their photophysical properties were characterized. The results show that the ESIPT rate does not correlate with H-bonding strength but is influenced by the basicity of the proton acceptor, specifically the carbonyl oxygen. This finding highlights the intrinsic difference between non-classical −SH and Pauling-type −OH/NH-type H-bonds, where the proton-donating site's electron negativity plays a crucial role in ESIPT dynamics and thermodynamics. The study provides insights into the fundamental differences in ESIPT mechanisms for non-Pauling-type H-bonds.