STUDY OF CONFORMERS OF TRANS-2,2'-DIPYRIDYLETHYLENE IN THE LOWEST EXCITED TRIPLET-STATES BY TIME-RESOLVED ELECTRON-PARAMAGNETIC-RESONANCE

Time-resolved EPR spectra have been observed for the very weakly phosphorescent tripler state of trans-2,2'-dipyridylethylene in EPA and in methanol-ethanol (1:1 by volume) at 77 K. The assignment of the transient EPR signals was carried out with the aid of the stretched poly(vinyl alcohol) film method. Two sets of time-resolved EPR signals are assigned to the quasi-planar conformers originated by a 180 degrees rotation of the pyridyl ring around the single bond with the ethylenic carbon. The time-resolved EPR spectra of the two conformers were separated from the spectrum of their mixture using the technique of selective excitation. The zero-field splitting (ZFS) E parameter and the anisotropy of the tripler sublevel populating rates are sensitive to the conformational change, while the ZFS D parameters and lifetimes of the two conformers in the lowest excited tripler states are nearly identical.