Photophysical characterization of imidazolium-substituted Pd(II), In(III), and Zn(II) porphyrins as photosensitizers for photodynamic therapy

The photophysical properties of four imidazolium-substituted metalloporphyrins have been assessed to gain insights into the relative efficacy of the compounds for photodynamic therapy (PDT). A set of zinc(II), palladium(II), and chloro-indium(III) porphyrins all bear a net positive charge owing to the diethylimidazolium unit; one zinc chelate bears a negative charge owing to a bis(sulfobutyl)imidazolium unit. The photophysical properties of the cationic and anionic zinc porphyrins are very similar to one another in organic solvents, phosphate-buffered saline, and in the presence of bovine serum albumin. The properties of the zinc and palladium porphyrins bearing charged peripheral groups are generally similar to those of neutral analogs in organic solvents. The palladium porphyrin shows an essentially quantitative yield (≥0.99) of the triplet excited state compared to the zinc porphyrins (∼0.9), and all are quantitatively quenched (at the diffusion limit) by molecular oxygen in air-saturated fluid solution. If the rate constant and yield of quenching of the triplet excited state by energy or electron transfer to molecular oxygen is the same in the cellular environment as in solution, then these processes combined with the triplet yield contribute only a factor of 1.3 to the higher PDT activity of analogous palladium versus zinc porphyrins, which is much smaller than what is observed. Therefore, other factors such as transient reduction of the excited porphyrin or delivery to the target site must predominantly underlie the difference in PDT efficacy of these sensitizers.