Energy deactivation pathways of porphyrins and dopa melanin in polyvinyl alcohol systems

Abstract

Absorption, fluorescence, delayed luminescence and photoacoustic spectra have been measured to learn about the radiative and non-radiative processes in two porphyrins: (4- N,N,N,N-trimethylanilinium) porphyrin (TAP) and (4-sulphonatophenyl) porphyrin (TPPS 4) and their mixtures with a synthetic dihydroxyphenylalanine (dopa) melanin in polyvinyl alcohol (PVA) solution and in PVA film. Due to ionic interactions, the positively charged TAP in its ground state forms a complex with dopa melanin. Absorption and emission spectra show that the TAP—dopa melanin complex is preserved in PVA solution, whereas when the complex is embedded in PVA film a competitive interaction between the PVA and dopa melanin in the dye—polymer complex is observed. In the case of TPPS 4, no strong interaction between dye and polymer has been observed either in PVA solution or in PVA film. Our photoacoustic spectra indicate that dopa melanin in both PVA solution and in buffer solution liberates heat promptly. Evidently, the PVA matrix does not affect, to any significant extent, the thermal properties of dopa melanin. The casting of PVA film changes the interaction of dopa melanin with porphyrins and in consequence their thermal properties. Under such conditions, porphyrin and dopa melanin behave like individual species. The new results are compared with spectroscopic data previously obtained for TAP and TPPS 4 in phosphate buffer solution, and show that phosphate buffer solution and PVA solution are suitable media for porphyrin—dopa melanin complex electron exchange study, whereas the PVA film can be an appropriate system for investigation of the energy transfer process between porphyrin and dopa melanin.