Photochromism of stable crystalline 3D Cd-viologen coordination polymers

Abstract

Recent studies have demonstrated the efficient synthesis of photochromic coordination polymers by assembly of viologens and metal centers. In order to enrich this system, in-depth research should be conducted on the impact of metal centers, but related research is rare. On the basis of two photochromic viologen-based compounds, two new photochromic coordination polymers are prepared by introducing cadmium ions. In this study, four novel host-guest compounds based on viologen cations (EV2+ = 1,1′-bis(ethyl)-4,4′-bipyridinium dication, PV2+ = 1,1′-bis(propyl)-4,4′-bipyridinium dication) and 1,3,5-benzenetricarboxylic acid (H3BTC) have been synthesized and structurally characterized, [(EV)0.5(H2BTC)]·H2O (1), [(PV)(H2BTC)2]·2H2O (2), {(EV)0.5[Cd(BTC)(H2O)]·2H2O}n (3), {(PV)0.5[Cd(BTC)(H2O)]·2H2O}n (4). Compounds 1 and 2 feature the isolated structure, where uncoordinated H2BTC− units connect to viologen cations by supramolecular interactions. Compounds 3 and 4 feature the three-dimensional (3D) anionic MOFs of {[Cd(BTC)(H2O)]-}n with viologen cations inserted in the framework. As expected, incorporating the viologen fragment into the framework results in predefined photochromism of four compounds. It is worth noting that the introduction of cadmium ions will directly improve the photosensitive behaviors and protect the photo-generated radicals.