Influence of Intramolecular Hydrogen Bonding on Electronic De-excitation in Amidophenylbenzotriazoles

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Abstract
Infrared absorption spectra show that the ortho isomer of a 2‐(amidophenyl)‐2,1,3‐benzotriazole is intramolecularly hydrogen bonded. The quantum yields for fluorescence and chemical reaction of the ortho isomer are much lower than those of the non‐hydrogen‐bonded meta and para isomers. To establish how the hydrogen bond affects the response of these molecules to electronic excitation, measurements were made from which the rate constants of all deactivation processes have been calculated. Internal conversion from excited singlet to ground state is a hundredfold more rapid in the hydrogen‐bonded species. Most of the photoexcitation energy is dissipated thermally within its singlet system. Hydrogen bonding does not affect the triplet levels nor change the chemical reaction rate constant of the excited singlet state. The rate of intersystem crossing in the compounds examined is unchanged in the temperature interval 77° to 300°K, while the rate of internal conversion within the singlet system is significantly reduced at the lower temperature.