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.

Ultraviolet absorbers such as Tinuvin P (2-(2-hydroxy-5-methylphenyl)benzotriazole), 1, achieve their exceptional photostabilities as a result of deactivation of excited singlet states through excited state intramolecular proton transfer (ESIPT). Adding a methyl group to the 6′ position of 2-arylbenzotriazoles reveals an additional excited singlet state deactivation mechanism in this class of molecules which does not require intramolecular hydrogen bonding. Steady state fluorescence and fluorescence lifetime measurements for a series of 6′-methyl-2-arylbenzotriazoles provides compelling evidence for a twisted intramolecular charge transfer (TICT) mechanism of excited singlet state deactivation. Due to the steric requirements of the 6′-methyl group, conformations are favored in which the phenyl and triazole rings are no longer coplanar. In the case of compound 11 (2-(6-methoxy-2,3-dimethylphenyl)-2H-benzotriazole), the presence of a 2′-methoxy group enhances nonplanarity and results in large deactivation rates. Compound 12 (2-(6-methoxy-2,3-dimethylphenyl)-5-(trifluoromethyl)-2H-benzotriazole), which possesses both twist and enhanced donor/acceptor properties, undergoes the most efficient fluorescence quenching for the methoxyarylbenzotriazoles. Compounds with both a 6′-methyl and a hydroxy group on the phenyl ring exhibit diffusion controlled quenching (k q ) 2 × 10 10 M -1 s -1) by DMSO. This quenching appears to result from either partial or complete excited state proton transfer to DMSO, which enhances TICT deactivation of the singlet excited state.

We present a detailed CASSCF study of the mechanism of excited-state intramolecular proton transfer (ESIPT) in the o-hydroxyphenyl triazine class of photostabilizers. The valence-bond analysis of the ground state and the two pipi* excited states permits a simple chemical interpretation of the mechanistic information. Our results show that the barrier to enol-keto tautomerism on the ground-state adiabatic surface is high. Following photoexcitation to the charge-transfer state, the ESIPT is predicted to take place without a barrier. Radiationless decay to the ground state is associated with an extended seam of conical intersection, with a sloped topology lying parallel to the ESIPT path, which can be accessed at any point along the reaction path. Our results show that the triazine class of photostabilizers has the photochemical and photophysical qualities associated with exceptional photostability.

The relative proximity of Al atoms substituted in zeolite lattices is an important parameter that influences both hydrothermal stability and catalytic function, but the underlying chemistry that governs Al site proximity is not well understood. Here, we examine relationships between exchanged countercations and different Al–Al arrangements in a chabazite (SSZ-13) zeolite lattice. We report periodic supercell density functional theory (DFT) calculations for structures and energies of SSZ-13 lattices with systematically enumerated and varied Al–Al proximity, both charge-uncompensated and charge-compensated by either proton pairs (H+/H+) or divalent copper cations (Cu2+). Al–Al interactions are electrostatically repulsive without charge compensation, but the relative energies of certain Al–Al site arrangements change upon compensation by countercations. Al–Al interactions are uniformly attractive when compensated by H+/H+ pairs but are attractive at long and repulsive at short Al–Al distances when compensated by Cu2+, highlighting the role of the countercation in stabilizing different Al–Al arrangements. Through descriptor analysis, we find that the Cu2+ energy landscape can be described by models consisting of electrostatics and a binary term that specifies whether or not Cu2+ resides in the six-membered ring (6MR). The H+/H+ and Cu2+ energy landscapes together imply that Cu2+ prefers to reside at 6MR Al–Al pairs. These results shed light on how countercations influence Al distribution and rearrangement during synthesis and postsynthetic treatments of the SSZ-13 zeolite, which potentially influences its susceptibility to dealumination during hydrothermal aging. The systematic DFT computation workflow and descriptor analysis reported here are promising approaches that can be applied generally to examine other combinations of ions and zeotypes of interest.