Structural diversities of squarate-based complexes: photocurrent responses and thermochromic behaviours enchanced by viologens

Abstract

By using squarates as coordination building units, six new complexes formulated as [Mg(C₄O₄)(H₂O)₂]_n (1), [Pr₂(C₄O₄)₂(OA)(H₂O)₄]_n (2), [Eu₂(C₄O₄)₃(H₂O)₈]_n (3) [Cd₂(C₄O₄)₂(bpdo)(H₂O)₄]_n (4), [Cd(C₄HO₄)₂(H₂O)₄]·(MV)₂·(C₄O₄)₂ (5), and [Cd(C₄HO₄)₂(H₂O)₄]·(EV)·(C₄HO₄)₂ (6), where (C₄O₄)²⁻ = squarate, OA = oxalic acid, MV = methyl viologen, EV = ethyl viologen, and bpdo = 4,4′-dipyrdiyl-N,N′-dioxide, have been synthesized under hydro/solvothermal conditions. They present structural dimensions from 3-D networks (1, 2), 2-D layers (3, 4) to 0-D clusters (5, 6) based on the versatile bridged modes of squarates and the introduction of second ligands. In particular, 5 and 6 are the first examples incorporating squarates (electronic-rich species) with viologens (electronic-poor species) in one lattice. Consequently, they exhibit not only photocurrent responses but also reversible thermochromism, which will be beneficial for the design of multi-functional materials.