A study of the effect of pyridine linkers on the viscosity and electrochromic properties of metallo-supramolecular coordination polymers

Abstract

We present the optical, electrochemical, and electrochromic properties of Fe(II)-, Co(II)- and Ru(II)-based metallo-supramolecular polymers (MEPEs) self-assembled from rigid, π-conjugated bis-terpyridines with different numbers of pyridine linkers. By exciting the metal-to-ligand charge transfer (MLCT) transition, Ru-MEPEs show a weak luminescence in the NIR region (λ_lum = 765 nm) at room temperature. The viscosity of Co-MEPEs in water, ethanol and 75% acetic acid is significantly higher in comparison to the analogous Fe-MEPEs. The SEM cross-section images of the highly transparent dip-coated films show smooth and homogenous surfaces with film thicknesses of <200 nm for Fe-, Co- and Ru-MEPEs. The cyclic voltammograms of the Fe- and Ru-MEPEs show a reversible redox couple corresponding to the Fe(II)/Fe(III) and Ru(II)/Ru(III) states, respectively. Co-MEPEs display two metal centered Co(II)/Co(I) and Co(II)/Co(III) reversible redox waves at −1.12 and −0.10 V vs. Fc/Fc⁺, respectively. Fe-MEPE films show a promising high cycle stability over 1000 switching cycles. The coloration efficiencies were also determined for Fe-MEPEs (720 mC cm⁻² at 594 nm). The study of the electrochromic properties reveals that the introduction of pyridine linkers enhances the switching stability and reversibility. Co-MEPEs exhibit a broad absorption band covering the visible and NIR region at a negative potential (−1.4 V vs. Fc/Fc⁺) due to the metal-to-ligand charge transfer from Co(I) to the acceptor terpyridine ligands.