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

• Published in: The journal of physical chemistry. A, Molecules, spectroscopy, kinetics, environment, & general theory Vol. 106; no. 34; pp. 7680 - 7689
• Main Authors: Maliakal, Ashok, Lem, George, Turro, Nicholas J, Ravichandran, Ravi, Suhadolnik, Joseph C, DeBellis, Anthony D, Wood, Mervin G, Lau, Jacqueline
• Format: Journal Article
• Language: English
• Published: American Chemical Society 29.08.2002
• ISSN: 1089-5639; 1520-5215
• DOI: 10.1021/jp021000j

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 × 1010 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.