A Metal-Organic Framework with Highly Polar Pore Surfaces: Selective CO2 Adsorption and Guest-Dependent On/Off Emission Properties

A 3D porous ZnII metalorganic framework {[Zn2(H2dht)(dht)0.5(azpy)0.5(H2O)]center dot 4?H2O} (1; H2dht=dihydroxyterphthalate, azpy=4,4'-azobipyridine) has been synthesised by employing 2,5-dihydroxyterephthalic acid (H4dht), a multidentate ligand and 4,4'-azobipyridine by solvent-diffusion techniques at room temperature. The as-synthesised framework furnishes two different types of channels: one calyx-shaped along the [001] direction and another rectangle-shaped along the [101] direction occupied by guest water molecules. The dehydrated framework, {[Zn2(H2dht)(dht)0.5(azpy)0.5]} (1') provides 52.7?% void volume to the total unit-cell volume. The pore surfaces of 1' are decorated with unsaturated ZnII sites and pendant hydroxyl groups of H2dht linker, thereby resulting in a highly polar pore surface. The dehydrated framework 1' shows highly selective adsorption of CO2 over other gases, such as N2, H2, O2 and Ar, at 195 K. Photoluminescence studies revealed that compound 1 exhibits green emission (lambda max approximate to 530 nm) on the basis of the excited-state intramolecular proton-transfer (ESIPT) process of the H2dht linker; no emission was observed in dehydrated solid 1'. Such guest-induced on/off emission has been correlated to the structural transformation and concomitant breaking and reforming of the OH center dot center dot center dot OCO hydrogen-bonding interaction in the H2dht linker in 1'/1.