Hydrogen bonding interaction effect on carbazole triplet state photophysics

Abstract

Hydrogen bonding interaction between the first triplet state (T/sub 1/) of carbazole and pyridine was studied by means of the conventional flash photolysis technique. The carbazole triplet lifetime was determined in cyclohexane solutions at low pyridine concentration, that is when hydrogen bonding between carbazole and pyridine occurs neither in the ground state (S/sub 0/) nor in the first excited singlet state (S/sub 1/). Kinetics of carbazole triplet quenching by pyridine were examined on the basis of the Mataga kinetic scheme for hydrogen bond formation and dissociation reactions in the excited state. Formation of the hydrogen bonded complex carbazole-pyridine in the triplet state was found to be diffusion controlled. Equilibrium between free and bonded carbazole was shown to be established during the triplet lifetime. Bonded carbazole triplet lifetime was found to be 0.077 of that of free carbazole. Besides triplet-triplet absorption, carbazyl radical absorption was observed and was found to increase with increasing pyridine concentration. The carbazyl radical formation in the presence of pyridine was attributed to hydrogen atom transfer from carbazole to pyridine through the hydrogen bond. Photocomplexation of protonated and deuterated carbazole was compared and the hydrogen atom transfer process was shown to be responsible for the nonradiative deactivationmore » of the hydrogen bonded triplet.« less