Synthesis, characterisation, water adsorption and proton conductivity of three Cd(II) based luminescent metal–organic frameworks

Three hydrogen bonded three-dimensional (3D) metal–organic frameworks (MOFs) namely [Cd(L-tart)(bpy)(H₂O)]_n·9n(H₂O) (1), [Cd(D-tart)(bpy)(H₂O)]_n·9n(H₂O) (2) and [Cd(DL-tart)(bpy)(H₂O)]_n·6n(H₂O) (3) (tart = tartaric acid, bpy = 4,4-bipyridine) have been synthesized by the solvent diffusion technique at room temperature. Compounds 1 and 2 have been characterized by single crystal X-ray analysis, whereas the powder X-ray diffraction patterns show that the structural integrity of compound 3 is similar to 1 and 2. Structural analysis of 1 and 2 shows H-bonded homochiral 3D MOFs, fabricated by the hydrogen bonding interactions between the nearby 2D pillared-layer frameworks through the metal-bound water, metal-bound carboxylate, free carboxylic acid and the hydroxy group of L-/D- tart. The absolute configuration of all the compounds was investigated by solid state circular dichroism (CD) spectroscopy, which signifies that 1 and 2 are enantiomers whereas 3 is racemic. The adsorption studies reveal that compounds 1–3 show a significant amount of uptake for water vapor (∼239 mL g⁻¹ for 1, ∼240 mL g⁻¹ in 2, whereas 184 mL g⁻¹ for 3 at P/P₀ ≈ 1 bar) over other solvents (MeOH, EtOH) and an impedance measurement indicates that these compounds show proton conduction (1.3 × 10⁻⁶ S cm⁻¹ in 1, 1.3 × 10⁻⁶ S cm⁻¹ in 2 and 4.5 × 10⁻⁷ S cm⁻¹ in 3) at a higher temperature (358 K) and at 95% relative humidity. The observed conductivity is explained by the so-called vehicle mechanism (activation energy (E_a) = 0.63–0.77 eV). Since all the compounds contain H₃O⁺ cations in the interlayer space, the hydronium ions might act as vehicles to transport the protons in the interlayer space. The photoluminescence properties of all the compounds are also reported.