Charge transfer states govern the excited state dynamics of melanin-like materials

The broadband absorption of melanin is vital for living organisms, functioning as the coloration and photoprotection survival mechanisms. Beside this, a possible carcinogenic action of melanin upon UV irradiation has been extensively debated. Here we investigate the origin of the optical properties of melanin-like materials in order to explain this dual behaviour.

We demonstrate that the optical absorption of melanin-like materials, and in particular of polydopamine (PDA), is due, to a large extent, to delocalize charge-transfer (CT) states involving electron donor and electron acceptor molecular unit.

The ultrafast photophysical dynamics of PDA were investigated in detail by multidimensional spectroscopic techniques. The results pinpoint the importance of a joint model to reconstruct the excited state response, considering CT states and heterogeneity within chromophores. Interestingly, under visible excitation a 40-fs formation time of CT-states is observed, while UV excitation results in at least twofold faster formation. While an opposite trend is observed for the recombination lifetimes as a function of the excitation wavelength. Our data highlight the photoprotective role of the visible transitions and give a hint to the debated possible photodamaging pathways under UV excitation.