Near-infrared electrochromism of multilayer films of a cyclometalated diruthenium complex prepared by layer-by-layer deposition on metal oxide substrates

A cyclometalated diruthenium complex 2 bridged by 1,2,4,5-tetra(pyrid-2-yl)benzene with six carboxylic acid groups at two ends was synthesized. Monolayer and multilayer films FTO/TiO₂/(2)_n(Zr) (n=1,2) and FTO/SnO₂:Sb/(2)_n(Zr) (n=1–4) have been prepared via interfacial layer-by-layer coordination assembly of 2 with zirconium(IV) ions. All films show two consecutive redox couples in the potential range between 0 and +1.0 V vs. Ag/AgCl. These films exhibit reversible near-infrared electrochromism upon switching of redox potential. The response time of the films on SnO₂:Sb is around a few seconds, while that on TiO₂ is around a few tens of seconds. The film deposition cycles were found to have a great impact on the electrochromic performance. Among six films examined, the two-layered film on SnO₂:Sb displays the best balanced performance with a contrast ratio of 56% at 1,150 nm and good cyclic stability (9% loss of contrast ratio after 1,000 continuous double-potential-switching cycles), which is superior to that of the previously reported electropolymerized films of a related diruthenium complex with the same bridging ligand. In addition, the X-ray photoelectron spectroscopy, scanning electron microscopy, and electron transfer mechanism of these films have been investigated.