Theoretical study of benzotriazole UV photostability: ultrafast deactivation through coupled proton and electron transfer triggered by a charge-transfer state.
A theoretical CASSCF study of the reaction path for excited-state intramolecular proton transfer (ESIPT) for a model system derived from the UV absorber 2-(2'-hydroxyphenyl) benzotriazole without the fused benzo ring on the triazole has been carried out. A planar reaction path can be optimized but is shown to have no physical significance. The true reaction path involves twisted geometries. Adiabatic proton transfer is triggered by a charge-transfer from the phenol to the triazole group, and is followed by radiationless decay at the keto form. Along the nonplanar reaction path, there is a coupled proton and electron transfer in a manner similar to tryptophan. This rationalizes unexpected experimental results on the effect of electron withdrawing substituent groups on the photostability. The coupled proton and electron transfer is followed by a barrierless relaxation in the ground state to recover the enol form. An alternative photostabilization pathway from a phenyl localized state has also been documented and is similar to the channel 3 decay pathway in benzene photochemistry. Additionally, a long-lived intermediate for a twisted intramolecular charge-transfer (TICT) state has been identified as the species potentially responsible for the increase of blue fluorescence in strongly polar media.
Department of Chemistry, Imperial College London, London, SW7 2AZ, United Kingdom.
Martin J
MJ
Paterson
Michael A
MA
Robb
Lluís
L
Blancafort
Anthony D
AD
DeBellis