Paramagnetic resonance in phosphorescent aromatic hydrocarbons

Electron resonance spectra are presented of the phosphorescent state of the mesitylene species C6H3(CD3)3 in a B-trimethylborazole host crystal. The fine structure does not correspond to trigonal symmetry of the spin distribution and it varies with temperature: The z axis is perpendicular to the molecular plane and the y axis along a H-C-C-CD3 diagonal to within a few degrees. Hyperfine structure is only resolved for H‖x, where one obtains a doublet with a splitting of 12–13 g. The data are consistent with a dominant interaction with a single proton bonded to a carbon atom carrying a spin density of about 0·34. The results are discussed in terms of the pseudo Jahn-Teller instability of the 3 B 1u state of the benzene π-electron system.