The molecular structures and electrochemical response of "twisted'' tetra(aryl) benzidenes

The compounds N,N,N', N'-tetra(4-methylphenyl)-(1,1'-biphenyl)-4,4'-diamine (4), N, N, N', N'- tetra(4-methylphenyl)-( 2,2'-dimethyl)-(1,1'-biphenyl)- 4,4'-diamine (5a) and N, N, N', N'-tetra(4-methylphenyl)( 2,2', 6,6'-tetramethyl)-(1,1'- biphenyl)- 4,4'-diamine (6a) undergo two reversible one electron oxidations. The first oxidation potential increases in the order 4 v 5a v 6a, while the separation between the first and second oxidation decreases in the reverse order 4 (0.30 V) > 5a (0.16 V) > 6a (0.00 V), reflecting the decreasing thermodynamic stability of the radical cations [4(+)] > [5(+)] > [6a(+)]. Electronic spectroscopy and spectroelectrochemistry (UV-Vis-NIR) confirm expectations, and the introduction of methyl groups at the 2,2' and 6,6' positions of the 1,1'-biphenyl moiety electronically decouple the arylamine moieties. In contrast, N, N, N', N'- tetra( phenyl)-( 2,2'- dimethyl)-( 1,1'- biphenyl)- 4,4'- diamine (5b) and N, N, N',N'-tetra( phenyl)-(2,2', 6,6'-tetramethyl)-(1,1'-biphenyl)- 4,4'-diamine (6b) give much less kinetically stable radical cations upon oxidation, which oligomerise/polymerise through the 4 positions of the N-phenyl groups via a step-growth process. The molecular and crystal structures of 4 and 6b are also reported.