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

Abstract

Abstract A 3D porous Zn II metal–organic framework {[Zn 2 (H 2 dht)(dht) 0.5 (azpy) 0.5 (H 2 O)] ⋅ 4 H 2 O} ( 1 ; H 2 dht=dihydroxyterphthalate, azpy=4,4′‐azobipyridine) has been synthesised by employing 2,5‐dihydroxyterephthalic acid (H 4 dht), 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, {[Zn 2 (H 2 dht)(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 H 2 dht linker, thereby resulting in a highly polar pore surface. The dehydrated framework 1′ shows highly selective adsorption of CO 2 over other gases, such as N 2 , H 2 , O 2 and Ar, at 195 K. Photoluminescence studies revealed that compound 1 exhibits green emission ( λ max ≈530 nm) on the basis of the excited‐state intramolecular proton‐transfer (ESIPT) process of the H 2 dht 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 H 2 dht linker in 1′ / 1 .