Polymerization effects and localized electronic states in condensed-phase eumelanin

Abstract

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.

Functional K-doping of eumelanin thin films: Density functional theory and soft x-ray spectroscopy experiments in the frame of the macrocyclic protomolecule model

“…Note that the calculated DOS of the IHHH tetramer catches in energy the main feature of the valence band of a drop casted sample, as already remarked in Ref. 27.…”

“…32 The same procedure was already employed in Ref. 27 for estimating the bandgap of synthetic eumelanin dissolved in DMSO and drop casted on ITO substrates, which occurred to be 1.00 eV. This evaluation has been repeated on the present DC sample and the bandgap results to be 1.05 eV, showing that the bandgap value obtained in this way is independent of the eumelanin concentration in the starting solution and the thickness of the eumelanin film (i.e., from the absolute optical density values) as expected.…”

“…In our previous work on undoped eumelanin drop casted on ITO, 27 we have proposed two macrocyclic tetramers as the most recurrent oligomers in synthetic eumelanin, namely QIQI and IHHH (Q = quinone-imine, I = indolequinone, H = hydroquinone), both having two H atoms bound to two of the four N atoms present in the macrocycle. While the former was recognized to be energetically more stable in the gas phase, computed x-ray spectra of the IHHH tetramer better matched the experimental spectra of eumelanin synthesized by the oxidation of tyrosine with hydrogen peroxide.…”

“…Eumelanins are therefore characterized by a chemical disorder (i.e., choice of the basic monomer in the macrocycle) that makes them, in this respect, different from porphyrins. The validity of the macrocycle model has also been independently tested by a comparison of the calculated electronic structure to synchrotron-based electron spectroscopy experiments, as well as to Raman spectroscopy data, 27,28 where the sensitivity of these spectroscopic tools to discriminate among the number of possible macrocycles has been proven and the feasibility of this model to interpret the experimental data has been further confirmed through a comparison with the theoretical predictions for planar dimers.…”

Biologically derived melanin electrodes in aqueous sodium-ion energy storage devices

“…Eumelanins are a subset of naturally occurring melanin pigments that are composed of randomly polymerized tetramer units of 5,6-dihydroxyindole (DHI) and 5,6-dihydroxyindole-2-carboxylic acid (DHICA) monomers (40,(48)(49)(50). These protomolecules use strong π-π stacking and hydrogen bonding interactions that promote self-assembly into spherical nanostructures with an intermolecular spacing of 3.8 Å and characteristic dimensions of 100-300 nm (50,51).…”