| Abstract
| - The efficiency of the photochemical ring-opening of chromenes (or benzopyrans) depends on the vibronictransition selected by the chosen excitation wavelength. In the present work, ab initio CASPT2//CASSCFcalculations are used to determine the excited-state ring-opening reaction coordinate for 2H-chromene (C)and 2,2-diethyl-2H-chromene (DEC) and provide an explanation for such an unusual mode-dependent behavior.It is shown that excited-state relaxation and decay occur via a multimodal and barrierless (or nearly barrierless)reaction coordinate. In particular, the relaxation out of the Franck−Condon involves a combination of in-plane skeletal stretching and out-of-plane modes, while the second part of the reaction coordinate is dominatedexclusively by a different out-of-plane mode. Population of this last mode is shown to be preparatory withrespect to both C−O bond breaking and decay via an S1/S0 conical intersection. The observed mode-dependentring-opening efficiency is explained by showing that the vibrational mode corresponding to the most efficientvibronic transition has the largest projection onto the out-of-plane mode of the reaction coordinate. To supportthe computationally derived mechanism, we provide experimental evidence that the photochemical ring-opening reaction of 2,2-dimethyl-7,8-benzo(2H)chromene, that similarly to DEC exhibits a mode-dependentphotoreaction, has a low (∼1 kcal mol-1) activation energy barrier.
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