Excited state dynamics, particularly intersystem crossing, of a set of meso-substituted porphyrins bearing different electron–donor and acceptor groups was studied by pulse train fluorescence technique. Triplet quantum yield was found to be critically dependent on the nature of meso-substituents in the porphyrin system. Porphyrins with meso methoxyphenyl groups were found to show high triplet quantum yields ([Formula: see text] between 0.70 and 0.81). Moreover, the quantity of methoxyphenyl groups and the substitution pattern directly influence [Formula: see text]. Alternatively, porphyrins attached to nitrophenyl group possess low triplet quantum yield values (~0.3). The observed structure-properties relationships suggest new ways for tuning the optical properties of porphyrins via chemical modification.