Toluene: Phosphorescence spectrum and distortion in the triplet state

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Abstract
Relatively sharp phosphorescence spectra have been obtained for toluene and a few of its deuterated analogs in a crystalline benzene matrix at 4°K. The vibronic fundamentals observed in the phosphorescence are reported for toluene‐h8(C6H5CH3), toluene‐αd3(C6H5CD3), toluene‐d5(C6D5CH3), and toluene‐d8(C6D5CD3). The entire phosphorescence spectra obtained for toluene‐h8 and toluene‐d8 are recorded and assigned. The relative intensities of some important vibronic progressions in all four toluenes are shown to indicate both planar and nonplanar distortions in the lowest triplet state of this molecule. Making use of the results of a normal coordinate analysis, quantitative calculations show that the lowest triplet of toluene has an expanded ring which is quinoidally distorted (two short, four long C–C bonds) with the methyl group lying at the apex of two long bonds and out of the plane of the ring. The side aryl hydrogens appear to be bent in the plane of the molecule. Although some of the intensity of the 0,0 transition is shown to be vibronically induced by the distortion, some of its intensity is derived from the purely electronic transition of the lowest triplet to the ground state.