Zn-MOFs containing flexible α,ω-alkane (or alkene)- dicarboxylates and 1,2-bis(4-pyridyl)ethane ligands: CO₂ sorption and photoluminescence

The use of five flexible α,ω-alkane (or alkene)-dicarboxylates (succinate (2), fumarate (3), adipate (5), and muconate (6)) produced three-dimensional (3-D) Zn^II frameworks with 1,2-bis(4-pyridyl)ethane pillars, whereas glutarate gave a two-dimensional (2-D) Zn^II compound (4). Structures 2 and 5 are 4-fold interpenetrated networks, and 3 and 6 are 5-fold interpenetrated networks. Both 5 and 6 displayed good CO ₂ sorption capabilities at 196 K, as evidenced by S-shape adsorption isotherms. Both compounds also exhibited selective CO₂ sorption over N₂ at low temperature, while 5 containing an adipate ligand showed higher CO₂ uptake at 273 and 298 K than 6 with a more rigid muconate ligand. The isosteric heats of CO₂ adsorption for 5 and 6 were 23.1 kJ mol^-1 and 30.7 kJ mol^-1, respectively. A photoluminescence study showed the emissions of 2, 4, and 5 to be blue-shifted relative to both the free acids and ligand, while 3 had ligand-based luminescence properties. The thermal stabilities of these complexes were also examined.