Enhanced rates of electron transport in conjugated-redox polymer hybrids

A series of benzimidazole-based conjugated polymers have been synthesized and their Ru²⁺ and Os²⁺ complexes reported. These novel compounds feature coordination of redox-active metal centres into the quasi-infinite π network of a conjugated polymer to give "conjugated-redox polymer hybrids''. These advanced materials offer technologically useful applications in, for example, electrochromic devices, molecular electronics, and electrocatalysis. -- The focus of this work has been to examine whether the interaction of metal sites through the polymer backbone can lead to enhancement of electron transport rates, measured as De, the electron diffusion coefficient. This parameter was measured by three different methods: rotating disc current mediation, dual electrode voltammetry, and impedance spectroscopy. Good agreement between the three was found, with impedance being the most reliable and versatile approach. -- Conservative estimates of De range up to 1x10⁻⁸ and 2x10⁻⁸ cms⁻¹ respectively for polymers containing ruthenium and osmium. This represents a tenfold to a one-hundredfold increase in the rate of electron transport over conventional redox polymers. Furthermore it was shown that De can be manipulated by controlling the protonation of the polymer, and that backbone degradation also has an influence. -- The special arrangement of the hybrid polymers permits strong electronic communication between metal sites. This in turn leads to the higher rates of electron transport. Intervalence transfer bands in the near infrared spectrum confirmed that the electronic coupling of neighbouring metal sites through the polymer is comparable to that in similar dinuclear model compounds. This coupling is so strong in the pyrazine polymers that it leads to splitting of the redox electrochemistry into a pair of distinct waves, with vanishing conductivity in the 50% oxidized state. This unusual condition provides some insight into the orbital energy relationships in these systems. -- The experimental evidence indicates that charge propagation and the corresponding enhancement of De in the hybrid polymers occurs via two exclusive superexchange mechanisms, extending the model previously invoked in simpler dinuclear systems