EXPERIMENTALLY reported photophysics of 7-mercapto-4-methyl coumarin compounds in methanol (thiol (C-SH) and thione (C=S)) disclose that these materials are weak or non-fluorescent. When the thiol is methylated, the C-S(CH3) material shows strong fluorescence. This work focuses on applying two computational strategies based on DFT and its time-dependent extension TD-DFT to validate the experimental findings. On-the-fly single reference TDDFT computational is first applied, which identified in a simple way that the S1/S0 intersection is the main radiationless channel leading to fluorescence quenching in mercapto derivatives. A second supporting method based on scanning along of C-S bond distance in the S0 and S1 states is used. Both strategies came to similar conclusions. The applied approaches introduce quantitative explanation of the mechanism of radiationless deactivation in mercapto coumarins due to S1/S0 conical intersection (CI) or due to small S1/S0 energy gap in case of thione tautomer, which is the dominant (thermodynamically favorable) tautomer in the S1 state. The results of this insights may assist designing profitable new fluorescent or non-fluorescent materials by changing specific functional group(s) in the molecule.