Proton Conduction, Dielectric Relaxation, Photoluminescence, and Photochromism Governed by Humidity and Alcohol Vapors in a Uranyl-Cobalt Framework with Labile Coordination Sites.

Abstract

Stimuli-responsive coordination polymers serve as an efficient platform for the generation of functional systems, showing tunable magnetic, electrical, optical, and mechanical features, as well as their rich combinations. We report a truly multifunctional material based on a coordination framework employing photoluminescent uranyl(VI) cations bonded through cyanido bridges to cobalt(III) centers and between themselves by hydroxido and 1,4-diazabicyclo[2.2.2]octane-N,N'-dioxide linkers. This material reveals proton conduction, dielectric relaxation, and visible light emission, all switchable by relative humidity (RH). Moreover, the reported system shows affinity to organic solvents, which efficiently quench the emission. The sorption with a RH-reversible shape-memory effect was found for methanol and acetonitrile vapors, while the unusual RH-activated sorption appears for ethanol. More importantly, the adsorption of simple alcohols activates visible-light photochromism, leading to an optical memory effect present after their desorption. The observed adsorption of solvents and the related modification of the optical properties are possible not only due to the flexibility of the framework but also owing to extraordinarily labile coordination sites of uranium(VI) centers involving aqua and bridging hydroxido ligands, which can be selectively exchanged by specific solvent molecules.