The photosensitizing properties of a novel plithalocyanine analogue, sulfonated dihydroxy phosphorus(V) tetrabenzotriazacorrole [P(OH)(2)TBCSn] and a non-sulfonated one [P(OH)(2)TBC] are reported in this paper. Different from other phthalocyanine derivatives, P(OH)(2)TBCSn shows little aggregation in aqueous solution. The fluorescence quantum yield (Phi(F)) of P(OH)(2)TBCSn is lower than that of the non-sulfonated one. Studies of triplet state photophysics show that the presence of peripheral substituents on the macrocycle enhances the quantum yield of the triplet state. The sulfonated derivative, P(OH)(2)TBCSn, has a longer triplet lifetime (tau(T) = 0.234 ms) and higher singlet oxygen quantum yield (Phi(Delta) = 0.88) than P(OH)(2)TBC. Together with the ground-state absorption properties, the photo sensitizing properties of the new compound suggest that it may be used as an excellent photosensitizer for photodynamic therapy (PDT).