Oligonuclear Manganese Complexes with Multiple Redox Properties for High-Contrast Electrochromism

This study is dedicated to the design of multiple redox-active oligonuclear manganese complexes supported with a bis(tetradentate) ligand (TPDP = 1,3-bis(bis(2-pyridinylmethyl)amino)-2-propanol) for high-contrast electrochromism based on the reversible redox process between Mn(II) (colorless) and Mn(III) (dark brown). Pentanuclear Mn₅ complex <b>1</b> (colorless) was synthesized via a one-pot reaction of Mn²⁺ and TPDP, while tetranuclear Mn₄ complex <b>2</b> (brown) was obtained through aerial oxidation of complex <b>1</b>. Mn₅ complex <b>1</b> features a central MnCl₆ unit connected to two Mn₂(<i>μ</i>-TPDP) fragments through <i>μ</i>₃-Cl^- and <i>μ</i>-Cl^-, whereas Mn₄ complex <b>2</b> adopts a symmetric tetranuclear structure with two mixed-valence Mn₂^II,III(<i>μ</i>-TPDP)(<i>μ</i>-Cl) fragments that are further linked by <i>μ</i>-oxo. Electrochemical studies revealed multi-step reversible redox properties for both complexes, attributed to Mn^II/Mn^III processes with significant electronic coupling (Δ<i>E</i>_1/2 = 0.27-0.37 V) between Mn centers. Spectroelectrochemical analysis revealed dynamic optical modulation through the tunable d-d transition and ligand-to-metal charge transfer (LMCT) state through reversible multiple redox processes based on Mn(II) ⇆ Mn(III) interconversion. The fabricated electrochromic device (ECD) exhibited reversible and high optical contrast between the colored state (dark brown) and the bleaching state (colorless). The results highlight the potential of polynuclear manganese complexes as high-contrast electrochromic materials for next-generation smart windows and adaptive optical technologies.