A metal-organic framework with highly polar pore surfaces: selective CO₂ adsorption and guest-dependent on/off emission properties

Abstract

A 3D porous Zn^II metal–organic framework {[Zn₂(H₂dht)(dht)_0.5(azpy)_0.5(H₂O)]⋅4H₂O} (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 Zn^II 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 195K. Photoluminescence studies revealed that compound 1 exhibits green emission (λmax≈530nm) 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⋅⋅⋅OCO hydrogen‐bonding interaction in the H₂dht linker in 1′/1.