Journal
JOURNAL OF PHYSICAL CHEMISTRY A
Volume 117, Issue 22, Pages 4574-4583Publisher
AMER CHEMICAL SOC
DOI: 10.1021/jp4028035
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We report a computational study on the photochemistry of the prototypical aromatic Schiff base salicylideneaniline in the gas phase using static electronic structure calculations (TDDFT, OM2/MRCI) and surface-hopping dynamics simulations (OM2/MRCI). Upon photoexcitation of the most stable cis-enol tautomer into the bright S, state, we find an ultrafast excited-state proton transfer that is complete within tens of femtoseconds, without any C = N double bond isomerization. The internal conversion of the resulting S-1 cis-keto species is initiated by an out-of-plane motion around the C-C single bond, which guides the molecule toward a conical intersection that provides an efficient deactivation channel to the ground state. We propose that the ease of this C-C single bond rotation regulates fluorescence quenching and photocoloration in condensed-phase environments. In line with previous work, we find the SI cis-keto conformer to be responsible for fluorescence, especially in rigid surroundings. The So cis-keto species is a transient photoproduct, while the stable So trans-keto photoproduct is responsible for photochromism. The trajectory calculations yield roughly equal amounts of the So cis-enol and trans-keto photoproducts. Methodologically, full-dimensional nonadiabatic dynamics simulations are found necessary to capture the preferences among competitive channels and to gain detailed mechanistic insight into Schiff base photochemistry.
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