Photophysical studies of a room temperature phosphorescent Cd(ii) based MOF and its application towards ratiometric detection of Hg2+ ions in water

Abstract

A new cadmium based MOF [Cd(C₅N₁S₁H₄)(C₆O₅H₃)] (C₅N₁S₁H₄ = 4MPy, C₆O₅H₃ = HFDA), Cd_1, was synthesized by a solvothermal reaction using 2,5-furandicarboxylic acid (H₂FDA) and 4-mercaptopyridine (4MPyH) formed in situ by the S–S bond rupture of the aldrithiol-4 ligand. Single crystal X-ray data of Cd_1 confirmed a two-dimensional structure arranged in an ABAB… fashion in three dimensions where the nitrogen atom of the pyridine ring of 4MPy remains free. The aqueous dispersion of Cd_1 showed room temperature phosphorescence at 520 nm upon excitation at 330 nm. Luminescence lifetime studies revealed that the emission at 520 nm originates from a triplet state which became populated through intersystem crossing (ISC) from the singlet excited state. A trace amount of Hg²⁺ ions interacted very strongly with Cd_1 and significantly altered its UV-vis absorption and luminescence spectra. After interacting with the Hg²⁺ ions, the emission of Cd_1 at 520 nm gradually disappeared with a concomitant increase of the low intensity fluorescence at 383 nm. The ISC is hampered by the coordination of Hg²⁺ ions with two S atoms of the 4MPy moiety of two adjacent 1D chains (inter-layer) and a huge reduction of the structure's flexibility. On the other hand, the interactions between metal ions and Cd_1via the free N atom of the pyridine moiety resulted in the increase of the fluorescence intensity at 383 nm, however the phosphorescence intensity did not change significantly. These studies indicate that the interactions between Hg²⁺ ions and S atoms are highly selective due to the soft–soft interaction whereas the interactions between the N atoms and all the metal ions are not selective. A ratiometric luminescence approach has been employed to establish Cd_1 as a detector of Hg²⁺ in aqueous medium selectively over other common metal ions. The estimated quenching constant and detection limit of Cd_1 for Hg²⁺ were found to be 1.15 × 10⁶ M⁻¹ and 37.5 nM (equivalent to 7.40 ppb) in water.