A bistable electrochromic material based on a hysteretic molecular switch immobilized on nanoparticulate metal oxide
Combination of an electrochemically bistable Ru polypyridyl complex (2) with the metal oxide semiconductor Sb:SnO2 formed the basis of an electrochromic hybrid material characterised by a hysteretic response to applied potential. The electrochemical bistability of the molecular component arises from redox-triggered linkage isomerisation where an ambidentate ligand changes reversibly between N- and O-coordination in the Ru(II) and Ru(III) states, respectively. Reversible oxidation and reduction result in a pronounced electrochromic effect (change in UV-Vis absorption at the metal-to-ligand-charge-transfer (MLCT) band) and the potentials for interconversion between the states are separated by approx. 0.5 V due to the linkage isomerisation reactions. With a carboxylate anchoring group on the auxiliary ligand, the bistable molecular switch was immobilised on the surface of nanoparticulate Sb:SnO2 films. This resulted in an electrode material featuring a hysteretic electrochemical response where oxidation state and colour of the electrode depended on the electrochemical history of the system.