Multifunctional Anionic Zn(II)-MOF for Selective CO₂ Adsorption, Cationic Dye Removal, and Luminescence-Based pH Sensing

An anionic 2-fold interpenetrated 3D framework, {(pbisoixH₂)₂[Zn₂(μ₄-TDBAT)₂]·S}_n (ESOGU-2), referred Eskisehir Osmangazi University (TDBAT: 5,5′-((thiophene-2,5-dicarbonyl)bis(azanediyl))diisophthalate, pbisoixH₂: protonated 1,4-bis(2-isopropylimidazol-1ylmethyl)benzene) was prepared and characterized by diverse techniques. X-ray result demonstrated that the anionic framework was formed by Zn(II) ions and TDBAT^4– and its charge balance was kept by protonated pbisoixH₂. Due to the porous structure of ESOGU-2 and stability in water with different pH ranges, CO₂ and dye adsorption studies were performed. The results displayed that ESOGU-2 possessed a high Brunauer–Emmett–Teller (BET) surface area (1112.77 m² g^–1) despite the interpenetration and a selective CO₂ (19.02%) adsorption over N₂ (0.342%) at 100 kPa and 273 K due to dipole–quadrupole interactions. Furthermore, ESOGU-2 removed the cationic methyl violet dye from an aqueous solution, while anionic methyl orange was not adsorbed by ESOGU-2 due to its anionic framework. In addition, ESOGU-2 displayed turn-on and turn-off luminescence behaviors depending on the amount of H⁺ and OH^– ions in the aqueous solution. ESOGU-2 showed high emission intensity in the pH range of 2.5–4.75 but displayed weak emission after pH = 6.