Electronic Spectra of Benzene in Cyclohexane at 77°K

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Abstract
Absorption, fluorescence, and phosphorescence of solid solutions of benzene in cyclohexane at 77°K have been investigated. The bands obtained for slowly cooled samples are attributed to benzene molecules occluded in the low‐temperature monoclinic crystal form of cyclohexane. The additional bands obtained with fast cooling are attributed to benzene molecules occluded in the high‐temperature cubic crystal form of cyclohexane. Detailed analyses of the fluorescence and phosphorescence of slowly frozen samples are given. The benzene molecules occupy sites that possess at least a center of symmetry. The phosphorescence lifetime for benzene in the monoclinic cyclohexane lattice is 4.6 sec for concentrations ≲0.01M and decreases slowly with increasing concentration. The phosphorescence lifetime for benzene in the cubic cyclohexane lattice at a 0.01M concentration is 1.1 sec. Relative intensities of spectra of benzene in the two cyclohexane lattices are reported for various concentrations. For concentrations ≲0.01M, the relative intensities for obsorption and fluorescence are the same, but the phosphorescence from the cubic lattice is less efficient than from the monoclinic lattice. The relative intensities of both fluorescence and phosphorescence from the cubic lattice decrease with increasing benzene concentration. This is attributed to long‐range energy transfer with trapping in the monoclinic lattice.