Ru(II)-Based Metallo-Supramolecular Polymer with Tetrakis(N-methylbenzimidazolyl)bipyridine for a Durable, Nonvolatile, and Electrochromic Device Driven at 0.6 V

Low-voltage operation, high durability, and long memory time are demanded for electrochromic (EC) display device applications. Metallo-supramolecular polymers (MSPs), composed of a metal ion and ditopic ligand, are one of the recently developed EC materials, and the ligand modification is expected to tune the redox potential of MSP. In order to lower the redox potential of MSP, tetrakis(N-methylbenzimidazolyl)bipyridine (LBip) was designed as an electronically rich ligand. Ru-based MSP (polyRu-LBip) was successfully synthesized by 1:1 complexation of RuCl2(DMSO)4 with LBip. The molecular weight (Mw) was high (8.8 × 106 Da) enough to provide a simple 1H NMR spectrum, of which the proton peaks could be assigned by the comparison with the spectrum of the corresponding mono-Ru complex. The redox potential (E1/2) between Ru(II/III) was 0.51 V versus Ag/Ag+, which was much lower than the redox potential of previously reported Ru-based MSP with bis(terpyridyl)benzene (0.95 V vs Ag/Ag+). The polymer film exhibited reversible, distinct color changes between violet and light green-yellow upon applying very low potentials of 0 and 0.6 V vs Ag/Ag+, respectively. The appearance and disappearance of the metal-to-ligand charge transfer absorption by the electrochemical redox between Ru(II/III) were confirmed using in situ spectro-electrochemical measurement. A solid-state EC device with polyRu-LBip was revealed to have large optical contrast (ΔT 54%), fast response time (1.37 s for bleaching and 0.67 s for coloration), remarkable coloration efficiency (571 cm2/C), and high durability for the repeated color changes more than 20,000 cycles. The device also showed a long optical memory time of up to 19 h to maintain 40% to the initial contrast under the open circuit conditions. It is considered that the stabilization of the Ru(III) state by LBip suppressed the self-coloring to Ru(II) inside the device.