The molecular structures and electrochemical response of “twisted” tetra(aryl)benzidenes

Abstract

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 < 5a < 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.