Enhancing the Conjugation of Nickel(II) Schiff Bases for High-Contrast Electrochromism

In this work, we elaborately designed two nickel(II) Schiff base complexes (NiL and NiL') with different π-conjugated systems (benzene vs. naphthalene) to prepare uniform metallopolymer films with nickel(II) chelates as repeating units on ITO substrates through oxidative electropolymerization. The π-conjugation extending from the benzene moiety to the naphthalene moiety greatly enhances the electron delocalization of the metallopolymer film, resulting in a significant increase in optical contrast from 25% ([NiL]_n) to 80% ([NiL']_n). The solid-state electrochromic devices based on metallopolymer film [NiL']_n achieved a transmittance modulation of 71% and an electrochromic efficiency of 268.58 cm² C^-1. This work provides an effective strategy for developing low-cost and high-performance non-precious metal electrochromic materials through ligand conjugation engineering.