Understanding the Role of Aggregation in the Broad Absorption Bands of Eumelanin.

In this work, we investigate the relationship between the complex hierarchical assembly structure of eumelanin, its characteristic broad absorption band, and the highly unusual nonlinear dynamics revealed by pump-probe or transient absorption microscopy. Melanin-like nanoparticles (MelNPs), generated by spontaneous oxidation of dopamine, were created with uniform but adjustable size distributions, and kinetically controlled oxidation was probed with a wide range of characterization methods. This lets us explore the broad absorption bands of eumelanin models at different assembly levels, such as small subunit fractions (single monomeric and oligomeric units and small oligomer stacks), stacked oligomer fractions (protomolecules), and large-scale aggregates of protomolecules (parental particles). Both the absorption and pump-probe dynamics are very sensitive to these structural differences or to the size of intact particles (a surprising result for an organic polymer). We show that the geometric packing order of protomolecules in long-range aggregation is key secondary interactions to extend the absorption band of eumelanin to the low energy spectrum and produce drastic changes in the transient absorption spectrum.