Mechanism and kinetics of excited-state relaxation in internally hydrogen-bonded molecules: 2-(2′-hydroxy-5′-methylphenyl)-benzotriazole in solution

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Abstract
We report the ground state absorption recovery kinetics (at 355 nm) and the fluorescence spectra and kinetics of 2-(2′-hydroxy-5′-methylphenyl)-benzotriazole (I) following excitation at 355 nm in several solvents at room temperature. Measured lifetimes range from 14 to 75 ps. Fluorescence lifetimes are shorter than the corresponding ground state recovery times, indicating the intervention of intermediate forms during the excited state relaxation process. The measured fluorescence quantum yield is lower than one predicted by the usual calculated radiative rate for the near UV, strongly absorbing singlet state of this molecule. Decay rates are slower in ethanol than in nonhydrogen bonding solvents indicating that external interference with the intramolecular hydrogen bond in (I) slows the relaxation rate. The room temperature decay rates are not strongly affected by the solvent viscosity. Deuteration of the molecule produces only a slightly more rapid ground state recovery rate than in the protonated species. A model involving excited state proton transfer is presented for the decay mechanism, rationalizing the known experimental data.