A Thiophene-Containing Conductive Metallopolymer Using an Fe(II) Bis(terpyridine) Core for Electrochromic Materials

Three Fe(II) bis(terpyridine)-based complexes with thiophene (Fe(L1) 2 ), bithiophene (Fe(L2) 2 ), and 3,4-ethylenedioxythiophene (Fe(L3) 2 ) side chains were designed and synthesized for the purpose of providing two terminal active sites for electrochemical polymerization. The corresponding metallopolymers (poly-Fe(L n ) 2, n = 2 or 3) were synthesized on indium tin oxide (ITO)-coated glass substrates via oxidative electropolymerization of the thiophene-substituted monomers and characterized using electrochemistry, X-ray photoelectron spectroscopy, UV–vis spectroscopy, and atomic force microscopy. The film poly-Fe(L2) 2 was further studied for electrochromic (EC) color-switching properties and fabricated into a solid-state EC device. Poly-Fe(L2) 2 films exhibit an intense MLCT absorption band at 596 nm (ε = 4.7 × 10 4 M –1 cm –1 ) in the UV–vis spectra without any applied voltage. Upon application of low potentials (between 1.1 and 0.4 V vs Fc + /Fc), the obtained electropolymerized film exhibited great contrast with a change of transmittance percentage (Δ T %) of 40% and a high coloration efficiency of 3823 cm 2 C –1 with a switching time of 1 s. The film demonstrates commonplace stability and reversibility with a 10% loss in peak current intensity after 200 cyclic voltammetry cycles and almost no loss in change of transmittance (Δ T %) after 900 potential switches between 1.1 and 0.4 V (vs Fc + /Fc) with a time interval of 0.75 s. The electropolymerization of Fe(L2) 2 provides convenient and controllable film fabrication. Electrochromic behavior was also achieved in a solid-state device composed of a poly-Fe(L2) 2 film and a polymer-supported electrolyte sandwiched between two ITO-coated glass electrodes.