Using Visible Light to Tune Boronic Acid–ester Equilibria

Abstract

<p>We report a series of azobenzene boronic acids that reversibly control the extent of diol binding via photochemical isomerization. When the boronic acid is <i>ortho</i> to the azo group, the thermodynamically-favored <i>E</i> isomer binds weakly with diols to form boronic esters. Isomerization of the <i>E</i> azobenzene to its <i>Z</i> 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 <i>E–Z</i> 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 <i>E</i> boronic acids and destabilization of the <i>E</i> 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 <i>Z</i> isomer relative to the <i>E</i> 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.</p>