Abstract
Melanins are skin-centered molecular structures that block harmful UV radiation from the sun and help protect chromosomal DNA from UV damage. Understanding the photodynamics of the chromophores that make up eumelanin is therefore paramount. This manuscript presents a multi-reference computational study of the mechanisms responsible for the experimentally observed photostability of a melanin-relevant model heterodimer comprising a catechol (C)–benzoquinone (Q) pair. The present results validate a recently proposed photoinduced intermolecular transfer of an H atom from an OH moiety of C to a carbonyl-oxygen atom of the Q. Photoexcitation of the ground state C:Q heterodimer (which has a π-stacked “sandwich” structure) results in population of a locally excited ππ* state (on Q), which develops increasing charge-transfer (biradical) character as it evolves to a “hinged” minimum energy geometry and drives proton transfer (i.e., net H atom transfer) from C to Q. The study provides further insights into excited state decay mechanisms that could contribute to the photostability afforded by the bulk polymeric structure of eumelanin.
Highlights
The fundamental photochemistry of prototypical organic and biological chromophores is attracting ever more attention [1,2,3]—driven, in part, by ambitions to advance understanding of photoinduced damage in biomolecules [4,5,6,7,8,9,10,11] and to improve the photoprotection offered by sunscreen molecules [12,13,14,15]
This section is sub-divided into sections addressing the minimum energy structures of the heterodimer and its biradical tautomer, the electronic spectroscopy of the former and the topography of the potential energy (PE) surfaces sampled following photoexcitation of the heterodimer
The diabatic 1 ππ* state progressively develops charge transfer (CT) character as QLIIC → 1, which is neutralized by proton transfer from the C to the Q moiety
Summary
The fundamental photochemistry of prototypical organic and biological chromophores is attracting ever more attention [1,2,3]—driven, in part, by ambitions to advance understanding (and prevention) of photoinduced damage in biomolecules [4,5,6,7,8,9,10,11] and to improve the photoprotection offered by sunscreen molecules [12,13,14,15]. (ii) Photostability, wherein the photoexcited molecule decays back to the ground state, rapidly and with high efficiency, without any permanent chemical transformation Such non-radiative decay (generically termed internal conversion) is the desired photophysical response for the DNA/RNA nucleobases [4,39,40,41,42,43,44,45,46,47,48,49,50,51,52,53,54,55,56,57,58,59,60,61,62,63,64,65] and, for example, for derivatives of the p-aminobenzoates, cinnamates, salicylates, anthranilates, camphor, dibenzoyl methanes and/or benzophenones used in commercial sunscreens [13,14]. These points of degeneracy develop into CIs when orthogonal motions are considered, which facilitate non-adiabatic coupling (i.e., the funneling of population) from the photoexcited state to a lower (e.g., the ground) state
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