Photochromic Ln-Phosphonates Assembled by an Imidazole Derivative: Construction, Crystal Structures, and Light-Enhanced Proton Conductivity

A series of Ln(III)-based coordination compounds with one-dimensional chains have been synthesized by assembling organic phosphonate ligands hydroxyethylidene diphosphonate (HEDP) and the imidazole derivative 2,4,5-tris(4-pyridyl)-imidazole (TPI) with the lanthanide ions under solvothermal conditions. Based on the electron-donor/-acceptor property, the isomorphic compounds exhibit radical-actuated photochromic behavior induced by the electron transfer process under light illumination at room temperature. In addition, taking advantage of the abundant hydrogen-bond interactions in the structures, the compounds display significant proton conduction behavior in the temperature range of 303–343 K at 100% relative humidity. Importantly, proton conductivities of all compounds increase by about 100% compared to the initial state after light irradiation. As a result, this work shows new hybrid complexes with photochromic and photomodulated proton conduction characteristics, providing a method for the design of intelligent and multifunctional materials with diverse photochromism and tunable proton conduction.