title: The ring-opening reaction of chromenes: a photochemical mode-dependent transformation.
abstract: The efficiency of the photochemical ring-opening of chromenes (or benzopyrans) depends on the vibronic transition selected by the chosen excitation wavelength. In the present work, ab initio CASPT2//CASSCF calculations 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 dominated exclusively by a different out-of-plane mode. Population of this last mode is shown to be preparatory with respect to both C-O bond breaking and decay via an S(1)/S(0) conical intersection. The observed mode-dependent ring-opening efficiency is explained by showing that the vibrational mode corresponding to the most efficient vibronic transition has the largest projection onto the out-of-plane mode of the reaction coordinate. To support the 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-dependent photoreaction, has a low ( approximately 1 kcal mol(-1)) activation energy barrier.
authors.affiliation: Dipartimento di Chimica, Università di Siena, 53100 Siena, Italy.
authors.forename: Annapaola
authors.initials: A
authors.lastname: Migani
authors.forename: Pier Luigi
authors.initials: PL
authors.lastname: Gentili
authors.forename: Fabrizia
authors.initials: F
authors.lastname: Negri
authors.forename: Massimo
authors.initials: M
authors.lastname: Olivucci
authors.forename: Aldo
authors.initials: A
authors.lastname: Romani
authors.forename: Gianna
authors.initials: G
authors.lastname: Favaro
authors.forename: Ralph S
authors.initials: RS
authors.lastname: Becker