Deciphering the broadband absorption of eumelanin in solution phase

Melanin, a bio-polymer responsible for visible pigmentation in the human body, works as a protective layer in our skin by absorbing UV radiation and efficiently dissipating it into heat, preventing DNA damage. The primary structure of eumelanin, one of the two main forms of melanin, is presumed to be a co-polymer of 5,6-dihydroxyindole (DHI) and 5,6-dihydroxyindole- 2-carboxylic acid (DHICA) monomers, with DHI being the major component. The solution phase investigation into the origin of broadband monotonic absorption spectrum of melanin has been unfolded through the polymerization of dopamine and L-DOPA, the molecular precursors of DHI and DHICA, respectively, in an ionic liquid (IL). This study reveals that the evolution of the electronic absorption spectrum of polymerized L-DOPA did not mimic the electronic spectrum of eumelanin, which was attributed to their inability to form stable redox states, as elucidated via structural characterization. Contrary to that, polymerized DA (pDA) mimics the eumelanin absorption spectrum, and the structural characterization of the polymerization process performed with 1H and 13C NMR and DOSY spectroscopy allowed us to identify chromophores responsible for the absorption profile of pDA. Furthermore, identified chromophores were subjected to quantum chemical calculations and their absorption spectra not only supported their presence in the experimental UV-Vis spectra but also allowed us to identify their gradual evolution during polymerisation. The knowledge of chromophores responsible for the optical properties of melanin will undoubtedly result in their increased application in functional materials.