Using Visible Light to Tune Boronic Acid–ester Equilibria
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We report a series of azobenzene boronic acids that reversibly control the extent of diol binding via photochemical isomerization. When the boronic acid is ortho to the azo group, the thermodynamically-favored E isomer binds weakly with diols to form boronic esters. Isomerization of the E azobenzene to its Z isomer enhances diol binding, and the magnitude of this enhancement is affected by the azobenzene structure. 2,6-Dimethoxy azobenzene boronic acids show over 20-fold enhancement in binding upon E–Z isomerization, which can be triggered with red light. Competition experiments and computational studies suggest that the changes in binding affinity originate from stabilization of the E boronic acids and destabilization of the E boronic esters. We demonstrate a correlation between diol binding and photostationary state: different wavelengths of irradiation yield different quantities of bound diol. Higher binding constants for the Z isomer relative to the E isomer was observed with all diols investigated, including cyclic diols, nitrocatechol, biologically relevant compounds, and polyols. This photoswitch was employed to “catch and release” a fluorescently tagged diol in buffered water. By tethering this photoswitch to a poly(ethylene glycol) star polymer, we can tune the stiffness of covalent adaptable hydrogels using different wavelengths of visible light. This work establishes photoswitchable equilibria as a tool for the reversible ligation of molecular and macromolecular species.