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

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 (H(2)FDA) 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 horizontal ellipsis 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 x 10(6) M-1 and 37.5 nM (equivalent to 7.40 ppb) in water.

Automated summary

A new cadmium based MOF Cd_1 was synthesized by a solvothermal reaction, with a two-dimensional structure arranged in an ABAB fashion and exhibiting room temperature phosphorescence at 520 nm. Cd_1 shows high selectivity in detecting Hg2+ ions, with highly selective interactions between Hg2+ ions and S atoms.