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(Ln)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 × 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
(ΔT%) 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 (ΔT%) 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.