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)(H 2 O)] n ·9 n (H 2 O) ( 1 ), [Cd( d -tart)(bpy)(H 2 O)] n ·9 n (H 2 O) ( 2 ) and [Cd( dl -tart)(bpy)(H 2 O)] n ·6 n (H 2 O) ( 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 / P 0 ≈ 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 H 3 O + 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.