POSITION-DEPENDENT METHYLATION EFFECTS ON THE TRIPLET-STATES OF TRANS-2,2'-BIPYRIDINE, METAL-FREE CIS-2,2'-BIPYRIDINE AND ZINC(II) COMPLEX WITH 2,2'-BIPYRIDINE - A TIME-RESOLVED EPR STUDY

The anisotropy of the triplet sublevel populating rates was determined for the trans and metal-free cis conformers of 2,2-bipyridine (bpy), methyl-2,2'-bipyridines (Mebpys), symmetrically substituted dimethyl-2 ,2'-bipyridines (Me2bpys) and their Zn2+ complexes in rigid solutions at 77 K by a time-resolved EPR technique. Following the conformational changes of bpy, Mebpys and Me2bpys, the sublevel preferentially popul ated by intersystem crossing (ISC) changes from T(z) in the trans conf ormers to T(y) in the cis conformers (the y and z axes are close to th e long and short in-plane molecular axis, respectively). The out-of-pl ane T(x) sublevels are preferentially populated in the Zn2+ complexes with 6-Me2bpy, 6,6'-Me2bpy and 6,6'-dibutyl-2,2'-bipyridine. The obser ved anisotropy in the ISC of these Zn2+ complexes is anomalous for a l igand-localized 3pipi state.