Triplet (T1) state and radical cation resonance Raman investigation of biphenyl derivatives

Abstract

The time‐resolved Raman spectra of the first triplet state—in resonance with the T_n←T₁ absorption at 370 nm—and of the radical cation transient—in resonance with the R^+.° ←R^+.* absorptions at 380 and 690 nm—are reported for biphenyl, fully deuterated biphenyl, p,p’‐bitolyl (4,4’‐dimethyl biphenyl), and m,m’‐bitolyl (3,3’‐dimethyl biphenyl). Complete, reliable vibrational assignments are proposed and a detailed discussion of the resonance Raman activity and enhancement is given based on simple molecular orbital considerations. This investigation confirms the planar, quinoidal character of the triplet and radical cation structures. The structural information available from the analysis of the resonance effects is consistent with the assignment of the 370 nm T_n ←T₁ absorption and 690 nm R^+.* ←R^+. absorption to a φ₆←φ₃ transition (Dewar’s notation), which causes a decrease of the quinoidal distortion and, particularly, a reduction of the interring CC bond strength. The R^+.* ←R^+. absorption at 380 nm is ascribable to a φ₇←φ₆ transition, which leads to a reinforcement of the quinoidal distortion. These results are compared to those available for the biphenyl radical anion and for the radical cation and triplet state of the 4,4’‐diamino derivative of biphenyl, N,N,N’,N’‐tetramethylbenzidine (TMB).