The synthesis of a novel series of 5,5′-sulfonamide thioindigo derivatives, including a new synthetic methodology (via microwave assisted synthesis), together with a comprehensive electronic spectral and photophysical characterization is described. The study made in solution, as a function of the solvent polarity, showed -rationalized by TDDFT calculations- that in nonpolar solvents the high efficiency of the radiative deactivation channel (50% in dioxane and 70% in benzene) is due to an S1 (of π,π* origin) in the trans form, whereas in polar solvents the low fluorescence quantum yield (1% or less) is due to the adiabatic stabilization of the trans and saddle S1 forms, making the sloped type CI (Conical Intersection) with S0 energetically accessible thus becoming an efficient competitive decay channel to the radiative pathway. It is shown that thioindigo and all the 5,5′-sulfonamide thioindigo derivatives exhibit “on-off” fluorescence that can be tuned by the solvent polarity. The presence of a dark cis form with an S1 (of n,π* origin) was confirmed by the appearance of a new absorption band, upon irradiation, assigned by TDDFT to an S2 (of π,π* origin) state.