Energy Transfer Concerning the Lowest Triplet State of Tetracyanobenzene with Methyl-Substituted Benzenes

Abstract

Abstract The quantum-yield ratios of phosphorescence to fluorescence and the phosphorescence lifetimes were measured for the charge-transfer complexes of tetracyanobenzene with methyl-substituted benzenes. By combining the observed results with the fluorescence quantum yields and lifetimes reported previously, the upper and lower limits of the rate constants for the intersystem crossing and for the radiative and non-radiative energy-transfer processes from the lowest triplet state to the ground state were obtained with the complexes. The intersystemcrossing rate constant of tetracyanobenzene was found to be decreased by the complex formation. This tendency was explained by the mixing of the charge-transfer configuration with the appropriate singlet and triplet states of the component molecules. The phosphorescence radiative rate constant was found to be determined by the contribution of the higher excited singlet configurations of each complex to the phosphorescent state. The temperature dependence of the phosphorescence-quenching rate constant was determined, and the activation energy of the process was interpreted as being due to intramolecular out-of-plane bending vibrations.