A 3D porous supramolecular architecture via π–π assembly of 2D metal–organic frameworks (MOFs): structure-versus-luminescence reversibility and gas adsorption properties

Abstract

A three-dimensional (3D) porous supramolecular architecture, {[Zn(bdc)(dpds)]·0.62(MeOH)·2H₂O}_n (1) (bdc²⁻ = dianion of benzenedicarboxylic acid and dpds = 4,4′-dipyridyldisulfide), has been synthesized and structurally characterized. Single-crystal X-ray diffraction analysis reveals that each [Zn₂(dpds)₂]⁴⁺ macrocycle is connected by bdc²⁻ ligands to form a two-dimensional (2D) layered metal–organic framework (MOF). Adjacent layers are further assembled via two interlayer π–π interactions in sandwich-type π_pyridyl–π_benzene–π_pyridyl and π_pyridyl–π_pyridyl fashions to afford a 3D porous supramolecular architecture. Controlled heating of the as-synthesized crystal 1 at ~120 °C causes de-solvated species of {[Zn(bdc)(dpds)]}_n (1a). The structural determination of the de-solvated compound shows the same structural characterization as that of 1 with the only difference of the nonexistence of solvated MeOH and water molecules. The de-solvated 1a generates the re-hydrated crystal of {[Zn(bdc)(dpds)]·1.1(H₂O)}_n (1b) upon exposure to water vapor. The water ab-/de-sorption phenomenon by cyclic TG measurement suggests the complete reversibility upon re-/de-hydration between 1a and 1b, associated with reversible temperature-dependent emission properties. Such a reversible switching process of 1 at RT to 120 °C can proceed for at least 20 cycles, implying good thermoluminescence reversibility of 1 during the heating–cooling processes. After removal of the solvent molecules, compound 1a exhibits permanent porosity verified by the N₂ sorption isotherm with a Langmuir surface area of 530.0 m² g⁻¹ and a Brunauer–Emmett–Teller (BET) surface area of 429.4 m² g⁻¹ and also exhibits significant gas storage capacities of 1.09 wt% for H₂ at 77 K and 17 wt% for CO₂ at 195 K. Moreover, 1a also displays interesting reversible water, methanol and ethanol vapor ad-/de-sorption behavior correlated with the polarity of the pore surface in 1a to the corresponding adsorbate molecules.