Computational Study of Absorption Spectra of Eumelanin Constituents: Key Role of Oxidized Species in the Red Edge Absorption

We have computed the vertical absorption of the basic components of eumelanin, oligomers of 5,6-dihydroxyindole (DHI) and 5,6-dihydroxyindole-2-carboxylic acid (DHICA), at the TD-DFT level. Here we present a preliminary report on the results for 73 DHI and 58 DHICA oligomers that cover the structural and redox diversity found in eumelanin. We have identified several structures that absorb at wave lengths above 800 nm, all of which contain oxidized DHI units, <i>ie</i> quinones or quinone methides. The transitions also have a pronounced charge transfer character, which suggests that they may give rise to proton-coupled electron transfer radiationless decay mechanisms that are important for the photoprotecting function.