Terpyridine–Thiophene–Triphenylamine Conjugated Ligand Design Enabling Green-Colored, Multistate Electrochromic Fe(II) Metallopolymers

Abstract

Achieving diverse and precisely controlled color outputs remains a key challenge for electrochromic materials, particularly for Fe^II terpyridine-based metallopolymers, which are typically limited to purple hues due to the fixed energy of Fe^II–terpyridine metal-to-ligand charge-transfer transitions. Here, a donor–π–acceptor terpyridine ligand incorporating a thiophene–triphenylamine (Tpy–Th–TPA) unit was employed to construct an Fe-based electrochromic metallopolymer. The [Fe^II(TpyThTPA)₂]²⁺ monomer complex exhibits an uncommon green color arising from a red-shifted MLCT band combined with an intraligand charge-transfer transition band. Oxidative electropolymerization of this complex enables the direct formation of an electroactive metallopolymer film on ITO substrates. Owing to well-separated ligand- and metal-centered redox processes, the resulting Fe^II metallopolymer film displays three distinct and reversibly addressable electrochromic states, undergoing sequential color changes from green to blue and yellow-green. The film exhibits reversible switching, reasonable response times (t_c = 2.3 s, and t_b =1.9 s), a coloration efficiency of 291.6 cm² C^–1, and good cycling stability, demonstrating that rational donor–π–acceptor ligand design is an effective strategy for tailoring the optical properties of Fe metallopolymers and achieving multistate electrochromism.