Probing the Intramolecular Hydrogen Bond of 2-(2-Hydroxyphenyl)benzotriazoles in Polar Environment:  A Photophysical Study of UV Absorber Efficiency

Abstract

An in-depth photophysical study is presented for a series of 2-(2-hydroxyphenyl)benzotriazoles (HBzTs); the structural characteristic of all these photostabilizers is their strong intramolecular hydrogen bridge (IMHB). Tinuvin P (TIN P, 11a) and six other HBzTs, with no substituent in the 3'-position ortho to the hydroxy function, show pronounced phosphorescence already in the dark (at 77 K in a polar glass). Upon irradiation, the phosphorescence intensity rises further until an equilibrium value is attained (up to 1.5 fold the dark value). A kinetic model is given which excellently reproduces this phosphorescence evolution: it demonstrates phosphorescence to arise from open conformers where the IMHB has been broken. Phosphorescence excitation spectra match the absorption spectra of the open conformer and also that of the O-methyl homologue 11A which cannot form an IMHB. Fluorescence spectra likewise prove the equilibrium between the closed and open conformer for these HBzTs. In unpolar glasses as well as in the crystalline state, TIN P displays a long-wavelength (red) fluorescence (with an enormous Stokes shift of approximately 10.000 cm-1) which is associated with the excited singlet state of the closed form after proton transfer within the IMHB, S1'(C). In polar matrixes, on the other hand, a blue fluorescence is observed (with a regular Stokes shift) for all those HBzTs which have no 3'-substituent shielding the IMHB against being opened by the polar solvent. This blue fluorescence, just as the characteristic phosphorescence evolution for these compounds, is associated with the open conformer. For HBzTs with an (alkyl) group ortho to the bridging OH group, however, a long-wavelength (red) fluorescence is again observed. The shielding effect of the 3'-substituent shows a fine gradation, cumyl >/= 1,1,3,3-tetramethylbutyl (isooctyl) > t-butyl >/= methyl.

An adduct of Cl-substituted benzotriazole and hydroxy benzophenone as a novel UVA/UVB absorber: Theory-guided design, synthesis, and calculations

“…UV absorbers with an intramolecular hydrogen bridge are widely employed as additives against UV radiation [2,3]. The two dominant classes of commercially available UV absorbers are benzotriazoles (BZTs) and hydroxybenzophenones (HBPs) [4,5].…”

“…In this work, a high efficient UVA/UVB absorber (UV-D) with Clsubstituted benzotriazole ((2-(5-chloro-2H-benzo[d] [1][2][3]triazol-2-yl)phenol, ClBTZ) and hydroxybenzophenone (HBP) anti-UV functional groups was synthesized based on the first principle theory-guided design. Characterization is performed by 1 H NMR, IR and UV spectroscopy.…”

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.…”