Structure-property relationship studies of photoactive tridentate ligands and their applications in sensing of Cu(II) and Fe(II) ions

A series of tridentate polypyridyl N<^>N<^>N/N<^>C<^>N-coordinating ligands (R1, R2, L1, L2) were synthesized by palladium-catalyzed Suzuki-Miyaura C-C bond-forming and N-methylation reactions in good-to-excellent yields (70-97%). These ligands were characterized by a set of analytical techniques including NMR, HRMS and singlecrystal XRD. The electrochemical and photophysical properties of these ligands were examined using cyclic voltammetry, UV-vis absorption and luminescence spectroscopies. Theoretical calculations at the level of DFT and TD-DFT were also performed for the detailed studies of the optoelectronic properties of these compounds. These ligands displayed either an admixture of singlet ligand-centered (1LC) and singlet charge-transfer (1CT) transitions or pure 1LC/1CT luminescence in some cases. Ligands (L3 and L4) which are charged in nature were further used as selective chemosensors for the detection of Cu(II) and Fe(II) ions with a turn-off fluorescence behavior in a binding stoichiometry of 2:1 = ligand:M(II) [M = Fe(II)/Cu(II)] and a limit of detection up to 4.66-21.71 ppm level.

Automated summary

A series of tridentate polypyridyl N^N^N/N^C^N-coordinating ligands (R1, R2, L1, L2) were synthesized using palladium-catalyzed Suzuki-Miyaura C-C bond-forming and N-methylation reactions with high yields (70-97%). The ligands were characterized using various analytical techniques, and their electrochemical and photophysical properties were investigated. Theoretical calculations were also performed to study the optoelectronic properties of these compounds. Some of the ligands exhibited mixed or pure luminescence transitions. Charged ligands (L3 and L4) were used as selective chemosensors for the detection of Cu(II) and Fe(II) ions with high sensitivity.