Photophysics of eumelanin: ab initio studies on the electronic spectroscopy and photochemistry of 5,6-dihydroxyindole. |
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Authors: | Andrzej L Sobolewski Wolfgang Domcke |
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Institution: | Institute of Physics, Polish Academy of Sciences, 02668 Warsaw, Poland. sobola@ifpan.edu.pl |
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Abstract: | Excited-state reaction paths and energy profiles of 5,6-dihydroxyindole (DHI), one of the elementary building blocks of eumelanin, have been determined with the approximated singles-and-doubles coupled-cluster (CC2) method. 6-Hydroxy-4-dihydro-indol-5-one (HHI) is identified as a photochromic species, which is formed via nonadiabatic hydrogen migration from the dangling OH group of DHI to the neighboring carbon atom of the six-membered ring. It is shown that HHI is a typical excited-state hydrogen-transfer (ESIHT) system. HHI absorbs strongly in the visible range of the spectrum. A barrierless hydrogen transfer in the (1)pipi* excited state, followed by barrierless torsion of the hydroxyl group, lead to a low-lying S(1)-S(0) conical intersection and thus to ultrafast internal conversion. This very efficient mechanism of excited-state deactivation provides HHI with a high degree of intrinsic photostability. It is suggested that the metastable photochemical product HHI plays an essential role for the photoprotective biological function of eumelanin. |
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Keywords: | ab initio calculations excited states photochromism UV/Vis spectroscopy |
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