Twisted Intramolecular Charge Transfer States in 2-Arylbenzotriazoles:  Fluorescence Deactivation via Intramolecular Electron Transfer Rather Than Proton Transfer

Abstract

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.

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.…”

Analysis of Polymeric UV Absorber (Tinuvin 213) Using LDI-TOFMS: Solvent Effect in Sample Preparation

“…They absorb potentially destructive UV radiation and dissipate the energy on a subpicosecond time scale. [1][2][3][4][5][6] The UVA is used to filter off UV radiation and dissipate the energy in the form of heat. UVAs must have a wide range of chemical and physical properties to be useful in applications, which include coatings and a wide variety of polymer products.…”

“…For the benzotriazole class UVAs, the mechanism of excited-state deactivation is thought to be due to an excited-state intramolecular proton transfer (Scheme 2). [1][2][3][4][5][6][7] Analysis of polymer additives by mass spectrometry and its related techniques enhanced the capability of compound identification. Moving toward higher molecular weight additives with less volatility, conventional mass spectrometry techniques such as electron ionization (EI), chemical ionization (CI), and gas chromatography/mass spectrometry (GC/MS) are generally not suitable for their analysis.…”