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.

Figure 1

(Color online) Model protomolecules for eumelanin: (a) a dimer composed of IH) and (b) IHHH tetramer ($\text{gold}=\text{C}$, $\text{red}=\text{O}$, $\text{black}=\text{N}$, and light $\text{blue}=\text{H}$).

Figure 2

(Color online) Measured Raman spectrum of the melanin thin films on (a) ITO compared with the best fit based on a linear combination of (b) monomers and with the calculated spectra of (c) IHHH and (d) QIQI.

Figure 3

(Color online) Absorption spectrum of eumelanin deposited on a ITO in the visible range. In order to estimate the band gap, the ${\text{OD}}^{1/2}$ has been fit with a straight line.

Figure 4

(Color online) (a) RESPES data of melanin films deposited on ITO collected across the $\text{C}1s$ absorption threshold. Labels A–F denote the binding energies of the spectral features identified in spectrum (b), which shows the valence band collected with $h\nu =281.6\text{eV}$ photon energy. (c) Intensity profile, also known as CIS (i.e., constant ionization state), of peak A in the valence band when the photon energy is scanned through the $\text{C}1s$ absorption threshold. (d) $\text{C}1s$ XAS spectrum.

Figure 5

(Color online) Top panel: photoemission spectra of synthetic eumelanin collected by exciting the sample with (a) $h\nu =281.6\text{eV}$ and (b) $h\nu =1486.6\text{eV}$ photon energies. The main bands are labeled from A to F. Calculated density of states of the tetramers (c) IHHH and (d) QIQI, of a weighted mix of (e) IQ and HQ, and of the monomers (f) HQ and (g) IQ. Bottom panel: calculated spectra convoluted with a Gaussian function $\left(\text{FWHM}=1.0\text{eV}\right)$.

Figure 6

(Color online) (a) Experimental $\text{N}1s$ XAS spectrum; calculated XAS spectra of the (b) QIQI, and (c) IHHH tetramers, the IHHH tetramer with four hydrogens in the ring [(d) ${\text{IHHH}}^{\ast }$], the dimer (e) IH, the spectrum resulting from the linear combination (f) ${\text{HQ}}^{\ast }0.7+{\text{IQ}}^{\ast }0.3$, the monomer (g) IQ, and the monomer (h) HQ. The experimental spectrum has been collected across the $\text{N}1s$ $K$ edge at nearly grazing photon incidence with linear polarization of light perpendicular to the thin-film surface.

Figure 7

(Color online) (a) $\text{C}1s$ XAS spectrum compared with the calculated XAS spectra of the (b) IHHH, and (c) QIQI tetramers, the spectrum resulting from the linear combination (d) ${\text{HQ}}^{\ast }0.7+{\text{IQ}}^{\ast }0.3$, the monomer (e) IQ and the monomer (f) HQ. The experimental conditions are those specified for Fig. 6.