This study investigates the excited-state intramolecular proton-transfer (ESIPT) reactions of thiol-based hydrogen-bonded molecules, specifically 4'-substituted-7-diethylamino-3-mercaptoflavones (NTFs). The research highlights the unique behavior of thiol hydrogen bonds, which differ from conventional Pauling-type -OH or -NH hydrogen bonds. The thiol-H-bond system exhibits distinct ESIPT dynamics, with the rate of ESIPT not correlating with the strength of the H-bond but rather with the basicity of the proton-accepting site, namely the carbonyl oxygen. The study synthesized a series of NTFs with varying H-bond strengths and examined their photophysical properties. The results show that the ESIPT rate increases with the basicity of the carbonyl oxygen, not the acidity of the thiol. This is supported by experimental data, including fluorescence upconversion measurements and single-crystal XRD analysis, which reveal the structural and electronic characteristics of the NTFs. The ESIPT dynamics of NTFs were further analyzed using time-resolved photophysical techniques, revealing that the rate of ESIPT is influenced by the basicity of the carbonyl oxygen rather than the H-bond strength. The study also demonstrates that the ESIPT process in NTFs is governed by the proton-accepting strength, leading to an unconventional type of ESIPT. The findings suggest that the non-Pauling-type thiol H-bonding system undergoes an unconventional ESIPT mechanism, where the proton-accepting site plays a key role. This research provides new insights into the fundamental differences between thiol-based and conventional hydrogen-bonded systems, offering a deeper understanding of the factors governing ESIPT reactions in non-Pauling-type H-bond systems. The results have implications for the design of functional materials and analytical applications, where controlling ESIPT dynamics is crucial.This study investigates the excited-state intramolecular proton-transfer (ESIPT) reactions of thiol-based hydrogen-bonded molecules, specifically 4'-substituted-7-diethylamino-3-mercaptoflavones (NTFs). The research highlights the unique behavior of thiol hydrogen bonds, which differ from conventional Pauling-type -OH or -NH hydrogen bonds. The thiol-H-bond system exhibits distinct ESIPT dynamics, with the rate of ESIPT not correlating with the strength of the H-bond but rather with the basicity of the proton-accepting site, namely the carbonyl oxygen. The study synthesized a series of NTFs with varying H-bond strengths and examined their photophysical properties. The results show that the ESIPT rate increases with the basicity of the carbonyl oxygen, not the acidity of the thiol. This is supported by experimental data, including fluorescence upconversion measurements and single-crystal XRD analysis, which reveal the structural and electronic characteristics of the NTFs. The ESIPT dynamics of NTFs were further analyzed using time-resolved photophysical techniques, revealing that the rate of ESIPT is influenced by the basicity of the carbonyl oxygen rather than the H-bond strength. The study also demonstrates that the ESIPT process in NTFs is governed by the proton-accepting strength, leading to an unconventional type of ESIPT. The findings suggest that the non-Pauling-type thiol H-bonding system undergoes an unconventional ESIPT mechanism, where the proton-accepting site plays a key role. This research provides new insights into the fundamental differences between thiol-based and conventional hydrogen-bonded systems, offering a deeper understanding of the factors governing ESIPT reactions in non-Pauling-type H-bond systems. The results have implications for the design of functional materials and analytical applications, where controlling ESIPT dynamics is crucial.