The Class II/III Transition in Triarylamine Redox Systems

Abstract

The mixed-valence character of a set of six bistriarylamine derivatives with varying π -electron spacers has been investigated. The distances of triarylamine redox centers vary from 0.5 nm ( N, N, N ‘, N ‘-tetra-4-methoxyphenyl- p -phenylenediamine, 6 + ) to 2 nm (1,4-bis{4-[ N, N -di(4-methoxyphenyl)amino]phenylethynyl}benzene, 1 + ). All radical cation species show rather strong intervalence charge-transfer (IV−CT) bands in the NIR as measured by UV/vis/NIR spectroelectrochemistry. Hush analysis was used to derive the electronic coupling V, which is very high and ranges from 500 to 3240 cm - 1 . A detailed band shape analysis revealed a cutoff at 2 V at the low-energy side of the IV−CT bands when the coupling 2 V approaches the band maximum λ . This results in very narrow IV−CT bands much smaller than the high-temperature limit, although even the phenylenediamine derivative 6 + belongs to the Robin/Day class II (localized redox centers) just at the border to class III (delocalized redox centers). Hush theory anticipates this cutoff because the smallest energy transition possible is at 2 V . The ultrafast thermal electron-transfer rate constants were estimated from V and range from about 10 9 s - 1 to 10 12 s - 1 . The electrochemical properties have also been investigated: a linear correlation of redox potential splitting vs electronic coupling was found.