The photochemistry of a model merocyanine-spiropyran system was analyzed theoretically at the MS-CASPT2//SA-CASSCF(14,12) level. Several excited singlet states were studied in both the closed spiropyran and open merocyanine forms, and the paths to the different S(1)/S(0) conical intersections found were analyzed. After absorption of UV light from the spiropyran form, there are two possible ultrafast routes to efficient conversion to the ground state; one involves the rupture of the C(spiro)-O bond leading to the open form and the other involves the lengthening of the C(spiro)-N bond with no photoreaction. From the merocyanine side the excited state can reach a very broad S(1)/S(0) conical intersection region that leads the system to the closed form after rotation of the central methine bond. Alternatively, rotation of the other methine bonds connects the system through different S(1)/S(0) conical intersections to several merocyanine isomers. The present work provides a theoretical framework for the recent experimental results (Buback , J. J. Am. Chem. Soc. 2010, 132, 1610-1619) and sheds light on the complex photochemistry of these kinds of compounds.
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