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<i>n</i>)₂, <i>n</i> = 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 × 10⁴ 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<i>T</i>%) of 40% and a high coloration efficiency of 3823 cm² 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<i>T</i>%) 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