Near-Infrared and Two-Wavelength Electrochromism Based on Nanocrystalline TiO₂ Films Functionalized with...

Two cyclometalated ruthenium complexes 2 and 4 containing a redox-active amine substituent and three carboxylic acid groups were synthesized, where the ruthenium ion and the amine site were separated by a phenyl or biphenyl bridge for 2 and 4, respectively. Nanocrystalline TiO₂ films on fluorine doped SnO₂ (FTO)/glass substrates anchored with 2 or 4 were prepared. These films display two consecutive anodic waves in the potential region between +0.1 and +1.0 V vs Ag/AgCl. Among four organic solvents examined, the FTO/TiO₂/2 film displays the best cyclic electrochemical stability in ClCH₂CH₂Cl. Both films exhibit promising two-step electrochromism in the near infrared region. A contrast ratio (ΔT%) of 42% at 1000 nm and 40% at 680 nm was achieved for the FTO/TiO₂/2 film in the single- and double-oxidation process, respectively. In contrast, the FTO/TiO₂/4 film shows electrochromism at 1300 nm (ΔT% = 58%) and 780 nm (ΔT% = 59%) in the single- and double-oxidation process, respectively. The coloration efficiency of these processes varies in a range of 150-270 cm²/C. These films are potentially useful as variable optical attenuators for telecommunications.