[2 + 2] cycloaddition reaction and luminescent sensing of Fe and CrO ions by a cadmium-based coordination polymer.

A new cadmium-based coordination polymer (CP), [Cd(bpe)(ceba)(fa)(HO)] (1; fa = formate anions), was constructed by solvothermal reactions of cadmium nitrates with trans-1,2-bis(4'-pyridyl)ethylene ligand (bpe) and 4-(1-carboxy-ethoxy)-benzoic acid (Hceba). The [2 + 2] photodimerization reaction of CP 1 under UV irradiation generated the corresponding photoproduct [Cd(rctt-tpcb)(ceba)(fa)(HO)] (1a; rctt-tpcb = tetrakis(4-pyridyl)-cyclobutane) via a single-crystal-to-single-crystal (SCSC) transformation. Single crystal X-ray diffraction unveiled that CPs 1 and 1a have similar 2D layered structures, except that rctt-tpcb replaced a pair of bpe ligands in the latter. Furthermore, CPs 1 and 1a are good bifunctional luminescent sensors that can detect Fe and CrO ions in a mixed medium of water and DMF and display high sensitivity, selectivity, and anti-jamming capability for the recognition of Fe and CrO ions. Moreover, we also examine in detail the possible luminescent quenching mechanisms of 1 and 1a toward Fe and CrO ions.