Molecular engineering of a 2,5-dithiophen-2-yl-pyrrole-bridged terpyridine–Fe(ii) coordination polymer toward multi-state electrochromic energy storage

Abstract

Electrochromic energy-storage materials are of great interest due to their ability to simultaneously achieve reversible color modulation and charge storage, with molecular engineering playing a key role in performance optimization. Herein, a terpyridine–Fe-coordinated conjugated polymer (FeSNSTPy) incorporating a 2,5-dithiophen-2-yl-pyrrole (SNS) π-bridge was successfully constructed. The polymer exhibits a dual electrochromic response arising from SNS backbone doping and Fe(II)/Fe(III)-associated MLCT modulation. FeSNSTPy delivers an optical contrast of 34%, fast switching kinetics (t_c/t_b = 2.4/2.5 s), and a high coloration efficiency of 626.54 cm² C⁻¹, together with good cycling stability (91% retention after 100 cycles and 77% after 250 cycles). Moreover, the FeSNSTPy film exhibits an areal capacitance of 2.74 mF cm⁻² at 0.1 mA cm⁻², demonstrating its promising electrochemical energy-storage capability. This work provides a feasible strategy for developing multi-state electrochromic coordination polymers toward advanced electrochromic energy-storage applications.