Theoretical study of organic molecules containing N or S atoms as receptors for Hg(II) fluorescent sensors

Sixteen organic compounds containing N or S atoms were carefully selected to investigate the binding site of the Hg ²⁺ and their coordination behaviors for the development of high performance Hg ²⁺ sensors via DFT calculations. The binding energies according to optimized structures of the organic-metal complexes and interaction energies based on hard and soft acids and bases theory (HSAB) have been calculated out and compared with each other. Discussions on the NBO charge, ligand-to-metal charge transfer and molecular orbital of the organic receptors were also presented for further understanding of the coordination. It was found that in heterocyclic compounds, the most stable binding site for Hg ²⁺ is N atom instead of S atom, although the S atom may be kinetically favored due to the lower spatial hindrance, larger detection radius with longer Hg ²⁺-S bond length, and closer softness between Hg ²⁺ and S groups. Several rules for the molecular design of excellent Hg ²⁺ receptors and the suitable fluorophore/receptor pair conditions were suggested for the fabrication of high performance off/on Hg ²⁺ fluorescent sensors too.