Theoretical study of benzotriazole UV photostability: Ultrafast deactivation through coupled proton and electron transfer triggered by a charge-transfer state

M A Robb, L Blancafort, A D DeBellis, Martin Paterson

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Abstract

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.

Original languageEnglish
Pages (from-to)2912-2922
Number of pages11
JournalJournal of the American Chemical Society
Volume126
Issue number9
DOIs
Publication statusPublished - 10 Mar 2004

Keywords

  • POTENTIAL-ENERGY SURFACES
  • CONSISTENT-FIELD METHOD
  • ULTRAVIOLET STABILIZERS
  • ORGANIC-PHOTOCHEMISTRY
  • CONICAL INTERSECTION
  • MECHANISM
  • BENZENE
  • SOLVENT
  • ABSORPTION
  • DYNAMICS

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