Anion sensing has been gaining a tremendous amount of attention over the last 25 years because of the important roles that anions play in both biological and chemical systems. Anion sensors present great potential for application in a wide array of fields, offering new methods of bioanalytical applications in living organisms, as well as the ability to extrapolate chemical pollutants from an environment. Rhenium(I) complexes containing amide subunits have been reported in the literature to interact very strongly with anions through charge-assisted amide hydrogen bonding, displaying binding affinities for anions as high as Ka ~106 M-1 in CH2Cl2 solution. This group had previously prepared a series of amide-based rhenium(I) complexes that are shown to have their emission quenched 10% by as little as 10-8 M cyanide or fluoride. To better understand the electronic properties of these fascinating systems, a series of rhenium(I) polypyridyl-based complexes with a unique recognition site have been made and tested with anions. The structures of the bridging ligand, rhenium(I) monomers, and anion-sensing complexes are confirmed via 1H NMR spectroscopy and IR spectroscopy. The electronic properties of the complexes and their interactions with anions are measured via absorbance spectroscopy and emission spectroscopy. The effect of anion-complexation on the structure of the complexes is measured through anion titrations monitored by 1H NMR spectroscopy. Stern Volmer plots are generated from emission data to extrapolate binding constants for the sensors with anions. The sensors display binding affinities for anions as high as Ka ~106 M-1 in CH2Cl2 solution, having the strongest interactions with fluoride, cyanide, and iodide. The complexes recognize anions between the amide protons and central pyridine but display a unique binding mode with iodide anion. 1H NMR spectra of the sensors titrated with iodide anion display an upfield chemical shift of ~0.05 ppm by the N1 hydrogen of the bipyridine subunit as a 1:1 ratio between the sensor and iodide is met. This data demonstrates a distinct binding mode experienced by the sensors with iodide anion that allows for highly sensitive differentiation between iodide and other respective anions. Future work aims to further accentuate the interaction between the sensors and iodide, potentially allowing for highly sensitive naked-eye detection of iodide. Such a possibility offers many modes of application in a wide array of fields.

Photophysics of rhenium(I) polypyridyl-based complexes and their employment as highly sensitive anion sensors through charge-assisted hydrogen-bond formation

ACS Spring Meeting 2021

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