The Class II/III Transition in Triarylamine Redox Systems

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-bis4-N, N-di (4-methoxyphenyl) aminophenylethynylbenzene, 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 2V at the low-energy side of the IV−CT bands when the coupling 2V 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 2V. The ultrafast thermal electron-transfer rate constants were estimated from V and range from about 109 s-1 to 1012 s-1. The electrochemical properties have also been investigated: a linear correlation of redox potential splitting vs electronic coupling was found.