Photochemical Pathways and Light-Enhanced Radical Scavenging Activity of 1,8-Dihydroxynaphthalene Allomelanin

Melanins play important roles in nature, particularly in coloration and photoprotection, where interaction with light is essential. Biomimetic melanins represent an advantageous alternative to natural melanin for technological applications, sharing the same unique biocompatibility, as well as optoelectronic properties. Allomelanin, derived from 1,8-dihydroxynaphthalene, has been reported to exhibit even better photoprotective and antioxidant properties than the most studied example of biomimetic melanin, polydopamine. However, the interaction of allomelanin with light remains largely unexplored. Here we report the excited state dynamics of allomelanin in a wide range of time windows from femtoseconds to microseconds to minutes, using different experimental techniques, i.e., ultrafast transient absorption, nanosecond transient absorption, X-band electron paramagnetic resonance and radical quenching assays. We find that the photophysics of allomelanin starkly differs from that of the widely studied polydopamine, with broadband excitonically coupled states funneling the absorbed energy to a lower energy species in less than 1 ps. Independent of the excitation wavelength, a long-lived (>450 μs) photoproduct is populated in ≈24 ps. Quantum chemistry calculations suggest that the photoproduct primarily exhibits the character of localized 1,8-naphthoquinone radical anions. This light-driven increase in the anionic semiquinone-like radical concentration enhances the antioxidant activity of allomelanin. These results suggest that the two mechanisms considered at the basis of photoprotection, light-extinction and antioxidant action, are indeed synergistic in allomelanin and not independent, paving the way for new applications of allomelanin in nanomedicine, photocatalysis, energy conversion and environmental remediation.