An experimental and theoretical study of the photoisomerization and thermal reversion on 5-arylmethylene-2-thioxoimidazolidin-4-one

Abstract

Unraveling the photochemical behaviour of the green fluorescent protein chromophore has lately attracted widespread attention among scientists. In this paper we present the study of the photochemical isomerization Z → E and the back reaction of the chromophore analog, 5-arylmethylene-2- thioxoimidazolidin-4-one. Experimental results are supported with ab initio calculations at the DFT, (B3LYP/6-31+g(d,p)), TD-DFT (B3LYP/6-311++g(3df,3pd)) and CASSCF levels. A first excitation to the S₂ state, where the isomerization occurs, is proposed followed by two conical intersections to S₁ and S₀ respectively. Three different mechanisms were analyzed for thermal reversion, concluding that the preferred channel involves an intersystem crossing between the S₀ and T₁ states with the formation of a biradical.