Molecular Suppression of the Pimerization of Viologens (=4,4′-Bipyridinium Derivatives) Attached to Nanocrystalline Titanium Dioxide Thin-Film Electrodes

Twelve viologens, i.e., 4,4′-bipyridinium derivatives 1 – 12, were examined for their use as electrochromic material when attached to nanocrystalline titanium dioxide thin-film electrodes. Eight of these (1 – 4, 7 – 8, 10, and 12) are new, and their synthesis is included. The modifier compounds consist of one to four bipyridinium subunits with linear or dendritic architecture, equipped with one to three TiO2-anchoring phosphonate groups. They are tailored for high electrochromic dynamics (large absorbance change upon reduction) and low extent of pimerization (=charge-transfer (CT) complexation of viologen cation radicals). A new graphical method is presented for the discrimination of simple dilution phenomena and more complex structural effects on the extent of pimerization in the surface-attached viologen layer