The concept of molecular recognition has found a multitude of uses, from sensing a few molecules of analytes to separating radionuclides from nuclear waste. In the preponderance of these applications, the need to produce a recognition agent that can be cycled between bound and unbound states with a target guest species is of significant interest. Generally, such a reversal is accomplished with chemical swings such as pH; while effective, this requires chemical inputs and produces waste. Building upon the precedents already established with imine photoisomerization, the imine bond in iminoguanidinium groups may undergo E–Z photoisomerization that can be exploited for switched binding and release of oxoanions. The pair of N–H groups available for directed oxoanion binding in the E,E isomer may be deactivated by appending 2-pyridyl groups that can act to close off the binding site by intramolecular hydrogen binding. Following this logic, photoswitching between open E,E and closed Z,Z photoisomers provides a binding-release mechanism for efficient oxoanion separations. Herein, we report the first example of photoswitching of anion binding by a bis-2-pyridyldiiminoguanidinium salt as specifically demonstrated with sulfate. Photoisomerization in DMSO with UV-visible light was monitored through 1H NMR spectroscopy and crystal structures of the binding E,E form with sulfate and the nonbinding Z,Z photoisomer. Strong binding of sulfate by the Z,Z form vanishes, illustrating an unprecedented efficient light-stimulated release of the sulfate ion.

Sulfate recognition by a pyridine-functionalized diiminoguanidinium photoswitch: Structural, kinetic, and thermodynamic studies

ACS Spring 2022

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