Electronic States of Benzene by Photoselection. The Polarization of Luminescence in Benzene, Perdeuterobenzene, and Perchlorobenzene

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Abstract
The polarization of fluorescence and phosphorescence of benzene (φH), perdeuterobenzene (PDB), and perchlorobenzene (PCB) (phosphorescence only) excited in rigid solutions at 77°K is studied in detail. Excitation is achieved in the first absorption band of each molecule using polarized 2537‐Å mercury radiation. The absorption bands and all emission bands (except the phosphorescence of PCB) are highly structured and of a forbidden type of electronic transition. The unstructured phosphorescence of PCB is also interpreted as an electronically (as well as spin) forbidden transition. The polarization of emission is found to be significantly constant over the entire band of each emission after sufficient experimental control is achieved. Precautions against depolarization effects due to concentration depolarization and rotational relaxation are taken. Both effects are found under extreme conditions. The polarization data are converted to intrinsic molecular transition moments. The fluorescences agree with the 100% in‐plane transition required of a 1B2u assignment in D6h symmetry. The fractional transition moments of the phosphorescences are mixed with the out‐of‐plane component dominant but less so in the order PDB, φH, PCB. The wavelength‐independent but mixed polarization is at first contradictory, but in fact is due to the fineness of the zero‐field splitting of the triplet components. These data have already been used to demonstrate that the triplet state achieves suitable singlet character by vibronic coupling in the triplet manifold to triplet states which spin‐orbit couple to dipole allowed singlet states. Although the lifetime of the phosphorescence of PCB is shortened by more than 2500‐fold, the polarization properties are significantly little changed. The heavy atom effect does not appear to disrupt the mechanism for mixing singlet into triplet proposed for φH. This raises interesting questions regarding the importance of σσ* states and the one‐ and two‐electron components of the spin—orbit coupling operator. The triplet‐state assignment is 3B1u (D6h) for φH and PDB and 3A2u (D3d) for PCB.