EPR and Density Functional Studies of Light-Induced Radical Pairs in a Single Crystal of a Hexaarylbiimidazolyl Derivative.

The intermolecular exchange coupling 2J for the light-induced radical pair in a crystal of 2,2'-di(ortho-chlorophenyl)-4,4',5,5'-tetraphenylbiimidazole (o-Cl-HABI) was measured for the first time on the basis of Curie analyses of the EPR spectral intensities. The coupling constant evaluated from density functional theory calculations compares well with the experimental value. The picture shows the distribution of spin density for the radical pair.