Azaindole based fluorescent chemosensors for detecting Fe(II/III) ions and molecular logic gate implementation

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A series of tridentate polypyridyl N^N^N/N^C^N-coordinating ligands (R1, L1, L2) were synthesized by copper-catalyzed Ullmann coupling (C–N bond-forming) reaction in moderate yields (∼50 %). Two charged ligands, L3 and L4 were also synthesized by methylation of L1 and L2 in moderate yields (∼45 %). These ligands were characterized by a set of analytical techniques including NMR, HRMS and single-crystal XRD. The electrochemical and photophysical properties of these ligands were examined using cyclic voltammetry, UV–vis absorption and luminescence spectroscopies. Theoretical calculations in 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 R1 and L1 which were highly emissive were further used as selective chemosensors for the detection of Fe(II)/(III) with a “turn-off” fluorescence behavior in a binding stoichiometry of 2:1 = ligand: M(II/III) [M = Fe(II)/Fe(III)] and a limit of detection up to 0.87–1.19 ppm level. L1 was also realized for the successful construction of the molecular logic gate.