Mechanism of an Exceptional Class of Photostabilizers:  A Seam of Conical Intersection Parallel to Excited State Intramolecular Proton Transfer (ESIPT) in o-Hydroxyphenyl-(1,3,5)-triazine

Abstract

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.

Dramatic Pressure-Dependent Quenching Effects in Supercritical CO2 Assessed by the Fluorescence of 4‘-Dimethylamino-3-hydroxyflavone. Thermodynamic versus Kinetics Control of Excited-State Intramolecular Proton Transfer

“…It is now widely recognized that conical intersections are a significant part of such channels. For example, Robb and co-workers found an extended seam of conical intersection with a sloped topology lying parallel to the ESIPT path of o-hydroxyphenyl triazines, 33 which can be accessed at any point along the reaction path. The presence of conical intersections in the vicinity of the Franck-Condon (FC) state formed upon excitation of the normal form, explains the sensitivity of DMA3HF fluorescence to the CO 2 pressure.…”

Computational Modelling of the Steps Involved in Photodynamic Therapy

“…It is important to note however that this minimum energy crossing point might not be the only one of importance when it comes to the generalisation of various reaction paths, but a good characterisation of the full seem is crucial. (Paterson, Robb et al, 2005) The procedure that is needed to find the minimum energy crossing point is actually fairly straight forward, at least in principle, and involves computing both the energy and its g r a d i e n t o n b o t h s u r f a c e s , a s w e l l a s t h e non-adiabatic coupling, which essentially describes the extent to which the electronic states are coupled by nuclear motion (i.e., a break-down of the BO approximation discussed earlier). Multiconfigurational methods are the most common, and appropriate, for the calculations of the IC pathways, as they can handle the multi state character needed near a crossing point.…”

Excited state intramolecular proton transfer fluorescence emission of o-hydroxyphenyl-triazine derivatives

“…A low energy change from enol to keto in the excited state explains ESIPT for the derivatives. xyphenyl-triazine derivatives have alkyl or aryl substituents on the hydroxyphenyl ring [7][8][9][10]. The electron-donating nature of alkyl or aryl groups could reduce the acidity of the hydroxy group and lower the possibility of ESIPT occurrence.…”

“…o-hydroxyphenyl-triazine derivatives are well-known ultraviolet absorbers, that ESIPT is generally thought to occur in despite the lack of solid experimental evidence. Recent theoretical and experimental investigations of o-hydroxyphenyl-triazine derivatives have drawn opposite conclusions on this for the occurrence or absence of ESIPT, respectively [7][8][9][10]. Consequently, new o-hydroxyphenyl-triazine derivatives need to be developed and their spectral data analyzed to determine if ESIPT can occur in these compounds.…”

“…Consequently, new o-hydroxyphenyl-triazine derivatives need to be developed and their spectral data analyzed to determine if ESIPT can occur in these compounds. In general, the reported o-hydro-*Corresponding authors (email: hongruli1972@gmail.com; fanggao1971@gmail.com) xyphenyl-triazine derivatives have alkyl or aryl substituents on the hydroxyphenyl ring [7][8][9][10]. The electron-donating nature of alkyl or aryl groups could reduce the acidity of the hydroxy group and lower the possibility of ESIPT occurrence.…”