Viologen-based photochromic coordination compounds for inkless and erasable prints

Abstract Two new crystalline inorganic-organic hybrid compounds, namely, {[Zn(PCV)0.5(BDC)0.5]·3H2O}n (1) and {[Cd(PCV)0.5(BDC)0.5(H2O)]·3H2O}n (2) have been solvothermally synthesized from the 1,4-benzenedicarboxylate (BDC2−) ligand and the viologen analog 1-(4-carboxyphenyl)-4,4′-bipyridinium (PCV). In these two compounds, PCV ligands coordinate with metal atoms to form 1D zigzag chains, and the chains are further bridged by the BDC2− ligands to yield a wave-like layer with a (4, 4) net topology. For 1, three layers interpenetrate with each other to construct a triple-layer structure, which are stacked along the b direction through van der Waals's force to yield the final 3D structure where the channels along the a direction host the lattice water molecules. In comparison, two layers interpenetrate with each other to construct a double-layer structure, which further interpenetrate to form the final 3D structures where the lattice water molecules locate in channels along a direction. Both compounds showed photochromism accompanied with dramatic decrease of photoluminescence upon irradiation. The powdered sample of 1 can deposit easily on paper by diffusing into ethanol solution, which displays the potential application as an inkless and erasable print medium.