Internal Conversion from Upper Electronic States to the First Excited Singlet State of Benzene, Toluene, p-Xylene, and Mesitylene

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Abstract
The variation with excitation wavelength of the fluorescence efficiencies of some alkyl benzenes has been studied from 2700 to 1600 Å. Measurements have been made in both liquid (pure and dilute solution) and vapor phases. For pure liquids the internal conversion efficiency βλ from upper electronic states to the first excited singlet state has been found to be significantly less than unity and to increase in the order benzene, toluene, p‐xylene, mesitylene when comparisons of βλ are made at comparable excitation wavelengths. Solvent effects on βλ have been noted. No emission is observed from any alkyl benzene studied in the vapor phase for excitations beginning on the long‐wavelength side of the second absorption band and extending to shorter wavelengths. Dissociation, internal conversion to the ground state, and intersystem crossing to a triplet state are considered as mechanisms for the disappearance of the upper electronic singlet states. Measurements of the benzene‐sensitized phosphorescence of biacetyl indicate that if the disappearance mechanism involves upper‐level intersystem crossing to the triplet manifold, then the triplets so produced do not efficiently internally convert to the lowest triplet state.