Manipulating dynamic covalent bonds through direct photoisomerization

The wide availability, ease of manipulation, and access to spatiotemporal control make light an attractive stimulus for controlling dynamic covalent chemistries/networks. In this work, a series of photo-isomerizable benzalisoxazolone (BIOx) thia-Michael (tM) acceptors were developed that exhibit an increase in thiol-bonding during irradiation with visible light (455-470 nm). In situ photo-NMR experiments demonstrate significant increases in the overall system K _eq (extent of bonding) for a variety of electronically-substituted BIOx tM acceptors. A combination of experimental and computational studies show that steric interactions between the β-phenyl ring and substituent on the isoxazolone in the E-isomer serve as a driving force for the increased K _eq. In addition, it is demonstrated that the power/intensity of the light can be used to tune the system's response. Incorporation of these photoisomerizable motifs into dynamic polymer networks gives access to organogels that exhibit reversible, on-demand, light-triggered stiffening.