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-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 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 ...