The conversion of the lactone functionality of porpholactones, porphyrin analogs in which a porphyrin β,β'-double bond was replaced by a lactone moiety, to a thionolactone functionality using Lawesson's reagent is described. The resulting novel thionolactones were spectroscopically characterized and their electronic structure defined using experimental (UV-vis, fluorescence, cyclic voltammetry) and theoretical methods (molecular modelling at the B3LYP/6-31G(d) level). The structures of two benchmark thionolactones were determined by single crystal X-ray diffractometry. The idealized planarity of the chromophores rationalize the bathochromically shifted optical spectra of the thionolactones when compared to the lactones on electronic grounds. The thionolactone moiety can be used as a synthetic handle in the preparation of oxazolochlorins using RANEY® nickel-induced hydrodesulfurization reactions. Importantly, the meso-pentafluorophenyl-based porphothionolactone fluoresces by at least a factor of 60 less compared to the corresponding lactone. The hypochlorite-selective conversion of the thionolactone to the lactone is the basis for the use of this thionolactone as a switch-on chemodosimeter for hypochlorite, a widely used disinfectant and molecule of biological significance in some inflammatory processes.