Tuning melanin: theoretical analysis of functional group impact on electrochemical and optical properties

The poly indolequinone eumelanin has been found to be a useful material for organic energy storage systems as a compelling, eco-friendly, and renewable alternative to sparse, inorganic materials. Current research has spotlighted the tuning of electrochemical properties of the material through chemical derivatization. The porphyrin-like tetrameric protomolecule of eumelanin presents an ideal candidate for analyzing the influence of adding functional groups on the electronic and optical properties of the system. In this study, after verifying the stability of the tetrameric protomolecule as a model structure, derivatives of F, Cl, Br, I and NH₂-substituted melanin were analyzed to investigate the impact of different functional groups. Additionally, for the first time, the new protomolecule adsorbed on a graphene surface could be modeled. One-electron properties were obtained with the Green function method combined with screened Coulomb interaction (GW). Excited-state properties were calculated using the GW-BSE (Bethe-Salpeter equation) method. The most significant shift in the gap was observed for di-aminated melanin with a difference of 0.45 eV (16%). The optical absorption spectra for the halogenides exhibited an increasing red-shift with increasing amount of substitution. The aminated melanin shows a much larger shift in the main absorption band of around 100 nm while reducing the absorption band intensity by 50%. These results can serve as a guide for future applications of the derivatives and offer insights on what other eumelanin derivatives may be of interest.