Electrochromic properties of asymmetric 4,4′-bipyridinium nanocomposites with inorganic nanoparticles

Asymmetric mono cationic 4,4′-bipyridiniums, 1 -butyl-4,4′- bipyridinium bromide (BUBP) and 1-methyl-4,4′-bipyridinium iodide (MEBP) showed electrochromic properties from blue to transparent yellow. The electrochromic bipyridiniums, BUBP and MEBP, were introduced to solid state electrochromic cells by using a solution of the bipyridinium, TiO ₂, and polymer electrolyte composite. The 4,4′-bipyridinium derivatives was attached to TiO ₂ nanoparticles and decreased aggregation of TiO ₂ to afford EC dye-TiO ₂ nanoparticles with average diameter smaller than 20 nm, as determined by FE-SEM. An all solid state electrochromic cells prepared from the dye-TiO ₂ nanoparticles showed improved electrochromic response compared to that without TiO ₂. Thus the cell from the BUBP- TiO ₂ nanoparticles responded to a step potential of ±2 V within 7 sec with coloration efficiency of 117. The redox cyclability of the bipyridinium-TiO ₂ nanoparticles cell was longer than that of the TiO ₂ free cells. The enhanced properties were attributed to the conductivity and the large surface area of TiO ₂ nanoparticles.