Polymerization effects and localized electronic states in condensed-phase eumelanin

The electronic structure of eumelanin thin films has been investigated by means of x-ray absorption and photoemission spectroscopies. The main features of the experimental data are interpreted on the basis of density-functional calculations for the isolated monomers participating to the eumelanin macromolecule. In order to single out the polymerization effects, we followed a bottom-up scaling approach to establish the minimum supramolecular level of organization that can provide a consistent spectroscopical picture of an altogether complex and highly disordered system. A tetramer macrocycle, made by three hydroquinones and one indolequinone, is found to reproduce the observed polymerization effects at the N K edge, while preserving the experimental spectral weight among the different monomers. This tetramer is different from that predicted for the synthesis from isolated monomers, providing an experimental evidence of the role of the reaction path on the stabilization of macrocycles in condensed-phase eumelanin.