Electronic Couplings in the Reduced State Lie at the Origin of Color Changes of Ommochromes
2020-05-14T07:50:53Z (GMT) by
In the colorful world of pigments and dyes, the chemical reduction of chromophores usually leads to bleaching because of π-conjugation interruption. Yet, the natural phenoxazinone-based ommochrome pigment called xanthommatin displays a bathochromic (i.e. red) shift upon two-electron reduction to its corresponding phenoxazine, whose electronic origins are not completely disclosed. In this study, we investigated, at quantum chemical level, a series of phenoxazinone/phenoxazine pairs that was previously explored by UV-Vis spectroscopy (Schäfer and Geyer, 1972), and which displays different hypsochromic and bathochromic shifts upon reduction. Density Functional Theory (DFT) and Time-Dependent DFT (TDDFT) have been applied to compute their optical properties in order to find a rational explanation of the observed photophysical behavior. Based on our results, we propose that the electro-accepting power of auxochromes and their conjugation facilitate intramolecular charge-transfers across the phenoxazine bridge by lowering unoccupied molecular orbitals via electronic and geometric couplings, leading ultimately to bathochromy. Our findings therefore suggest new potential ways to adjust the color-changing ability of phenoxazinones in technological contexts. Overall, this model extends our mechanistic understanding of the many biological functions of ommochromes in invertebrates, from tunable color changes to antiradical behaviors.