Dual-Branched Dense Hexagonal Fe(II)-Based Coordination Nanosheets With Red-to-Colorless Electrochromism and the Durable Device Fabrication
Sanjoy Mondal, Yoshikazu Ninomiya, Takefumi Yoshida, Taizo Mori, Manas Kumar Bera, Katsuhiko Ariga, Masayoshi Higuchi
Highly dense hexagonal Fe(II)-based coordination nanosheets (CONASHs) were designed by dual-branching, at the metal-coordination moieties and the tritopic ligands, which successfully obtained a liquid/liquid interface by the complexation of Fe(II) ions and the tritopic bidentate ligands. The 1:1 complexation was confirmed by titration. The obtained Fe(II)-based nanosheets were fully characterized by small-angle X-ray scattering (SAXS), atomic force microscopy (AFM), scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDX), and X-ray photoelectron spectroscopy (XPS). A monolayer of the sheets was obtained, Page 1 of 27 ACS Paragon Plus Environment ACS Applied Materials & Interfaces 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 2 employing the Langmuir–Blodgett (LB) method, and the determined thickness was ~2.5 nm. The polymer nanosheets exhibited red-to-colorless electrochromism because the electrochemical redox transformation between Fe(II)/(III) ions controlled the appearance/disappearance of the metal (ion)-to-ligand charge-transfer (MLCT) absorption. The poor π-conjugation in the tritopic ligands contributed to the highly colorless electrochromic state. A solid-state device, with the robust polymer film, exhibited excellent electrochromic (EC) properties, with high optical contrast (∆T > 65%) and high durability after repeated color changes for ˃ 15,000 cycles, upon applying lowoperating voltages (+1.5/0 V).
