Porphyrin-Melanins interaction: effect on fluorescence and nonradiative relaxations

Optical techniques and pulsed-laser, time-resolved photoacoustics (PA) were employed to obtain information on the mechanism of interaction between cationic zinc tetrabenzilpyridilporphyrin (ZnTBzPyP) and synthetic l-Dopa melanins. Synthetic eumelanin and pheomelanin strongly quench the fluorescence of ZnTBzPyP, but Stern-Volmer plots suggest a mechanism of interaction quite different for the two pigments. This diversity was confirmed by PA: for eumelanin no thermal relaxation was observed other than prompt heat, whereas for the complexed form of ZnTBzPyP with pheomelanin we were able to detect a heat-emitting species with a non-radiative lifetime in the microsecond range. The involvement of oxygen in the photophysics of the complexes formed between the cationic porphyrin and the two pigments was demonstrated, but its role has yet to be described.