Convergent Proton-Transfer Photocycles Violate Mirror-Image Symmetry in a Key Melanin Monomer

Abstract

We present absorption and emission spectra of 5,6-dihydroxyindole-2-carboxylic acid (DHICA), a key melanin monomer, which violate mirror symmetry, and propose that this violation is due to convergent excited-state intramolecular proton-transfer photocycles. Dual features in the absorption spectra arise from excitation into the S1 and S2 states of a catecholate anion form of DHICA. Emission arises from the S1 state of its proton-transfer conjugate following conversion via dual adiabatic and nonadiabatic reaction paths. In support of our postulated mechanism, we offer results from ab initio equation-of-motion coupled cluster (EOM-CCSD) and multistate multireference second-order perturbation theory (MS-MRPT2) calculations. Melanin pigments display an extremely broad, monotonically decreasing absorbance indicative of a dense, coupled manifold of excited states with varying localization character. Our results raise the possibility that intramonomer proton transfer may function as an energy dissipation mechanism from high-lying photoexcited states of the macromolecule.