Arresting consecutive steps of a photochromic reaction: studies of β-thioxoketones combining laser photolysis with NMR detection.

Photochromism of monothiodibenzoylmethane has been studied in a number of environments at different temperatures. Direct laser irradiation of a sample located in the NMR magnet allowed in situ monitoring of the phototransformation products, determining their structure, and measuring the kinetics of the back reaction. These observations, along with the data obtained using electronic and vibrational spectroscopies for rare gas matrix-isolated samples, glasses, polymers, and solutions, as well as the results of quantum-chemical calculations, provide insight into the stepwise mechanism of the photochromism in β-thioxoketones. At low temperature in rigid matrices the electronic excitation leads to the formation of the -SH exorotamer of the (Z)-enethiol tautomer. In solutions, further steps are possible, producing a mixture of two other non-chelated enethiol forms. Photoconversion efficiency strongly depends on the excitation wavelength. Analysis of the mechanisms of the photochromic processes indicates a state-specific precursor: chelated thione-enol form in the excited S2(ππ*) electronic state. The results show the potential of using laser photolysis coupled with NMR detection for the identification of phototransformation products.