Solvent-Induced Controllable Supramolecular Isomerism: Phase Transformation, CO2 Adsorption, and Fluorescence Sensing toward CrO42-, Cr2O72-, MnO4-, and Fe3.

Abstract

Complexes {[Zn(bpdc)(Cz-3,6-bpy)]·DMF·H2O}n (1α-DMF, H2bpdc = biphenyl-4,4'-dicarboxylic acid, Cz-3,6-bpy = 3,6-bis(pyridine-4-yl)-9H-carbazole, DMF = N,N'-dimethylformamide) and {[Zn(bpdc)(Cz-3,6-bpy)]·2DMAc·H2O}n (1β-DMAc, DMAc = N,N'-dimethylacetamide) as a couple of solvent-induced supramolecular isomers were hydro(solvo)thermally synthesized using DMF/H2O and DMAc-only as reaction media, respectively. Complexes 1α-DMF and 1β-DMAc adopt very similar wavy sql sheet structures but present great differences in the 3-fold interweaving 2D → 2D nets, which result in free voids of 17.6 and 33.4%, respectively. Desolvated 1β has about 2 times the free voids compared to that of desolvated 1α; however, the former displays a CO2 uptake of 87.9 cm3 g-1 STP at 195 K and P/P0 = 1 which is only slight larger than that (73.7 cm3 g-1 STP) of the latter under the same conditions. This is mainly interpreted by activation- and adsorption-induced framework distortion that caused partial transformation of crystal phase from 1β to 1α and thus reduced free voids. The isosteric heat of CO2 adsorption (Qst) at zero loading is 29.8 kJ mol-1 for desolvated 1α and 30.6 kJ mol-1 for desolvated 1β. On the other hand, results from XRPD, IR, and TGA measurements imply that 1α-DMF and 1β-DMAc are highly stable in several different solvents, with the exception of 1α-DMF in DMAc, which experienced complete transformation to convert to 1β-DMAc and 1β-DMAc in acetone and methanol, which would respectively completely and partially transform to the crystal phase of 1α-DMF. Remarkably, 1α-DMF and 1β-DMAc display intense blue and cyan fluorescence emissions, respectively, promising them to be multifunctional sensing platforms in sensitive detection of CrO42-, Cr2O72-, MnO4-, and Fe3+ with high selectivity and a fairly low limit of detection through perceptible fluorescence quenching effect. The possible sensing mechanisms were also investigated.