Study of Conformers of 2,2′-, 3,3′-, and 4,4′-Dimethylbiphenyls in the Phosphorescent Triplet States by Electron Spin Resonance and Phosphorescence

Abstract

Abstract For the phosphorescent triplet (T1) states of symmetrically-substituted dimethylbiphenyls, the relationship between the twisting angle of the two aromatic rings and the zero-field splitting parameters is discussed in terms of the ESR spectra. Two sets of ESR spectra observed for 2,2′- and 3,3′-dimethylbiphenyls (2DMBP and 3DMBP) are precisely assigned to the corresponding s-cis or s-trans conformer. The results clarify the inadequate assignment of the spectra of these molecular species in the previous literature. The lifetime obtained for the s-cis conformer is different from that of the s-trans one for both 2DMBP and 3DMBP. Examining the T1–T1 energy transfer from one conformer to the other, the relative locations for the T1 states of s-cis and s-trans conformers are obtained, even if phosphorescence measurements do not provide each isolated spectrum: ET (s-cis-3DMBP)<ET(s-trans-3DMBP) and ET(s-cis-2DMBP)>ET(s-trans-2DMBF).