Role of solvent, pH, and molecular size in excited-state deactivation of key eumelanin building blocks: implications for melanin pigment photostability

Ultrafast time-resolved fluorescence spectroscopy has been used to investigate the excited-state dynamics of the basic eumelanin building block 5,6-dihydroxyindole-2-carboxylic acid (DHICA), its acetylated, methylated, and carboxylic ester derivs., and 2 oligomers, a dimer and a trimer in the O-acetylated forms. The results show that (1) excited-state decays are faster for the trimer relative to the monomer; (2) for parent DHICA, excited-state lifetimes are much shorter in aq. acidic medium (380 ps) as compared to org. solvent (acetonitrile, 2.6 ns); and (3) variation of fluorescence spectra and excited-state dynamics can be understood as a result of excited-state intramol. proton transfer (ESIPT). The dependence on the DHICA oligomer size of the excited-state deactivation and its ESIPT mechanism provides important insight into the photostability and the photoprotective function of eumelanin. Mechanistic analogies with the corresponding processes in DNA and other biomols. are recognized.