Authors:

Shoki Matsushima

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Combining dynamic covalent bonds with photoswitches allows the kinetics and thermodynamics of exchange to be controlled with light. However, this two-component strategy introduces synthetic and compatibility challenges. We hypothesized that Donor-Acceptor Stenhouse Adducts (DASAs), a photoswitch that undergoes reversible isomerization between a conjugated open form and a polar, colorless closed form, could act as a dynamic bond in the open form.' Therefore, the material would be expected to undergo faster exchange and greater viscoelasticity in the dark and become more elastic upon irradiation. We discovered that open DASA isomers undergo dynamic covalent exchange of their amine donor via two pathways, reversible dissociation and conjugate transamination.? Exchange can then be arrested upon irradiation to form the closed DASA isomer, offering a handle to gate dynamic behavior. Consequently, incorporating DASAs as cross-linkers in PDMS-based networks yields covalent adaptable networks (CANs) with viscoelastic behavior that can be tuned by light. We also identify degradation pathways that limit the reversibility of this system under extended heating. Overall, DASA exchange represents a synthetically accessible platform for photocontrolled soft materials. Further research on different generations of DASAs would further our understanding of this exchange system. Similar strategies could be utilized for other photoswitches that could go through conjugated addition and elimination reactions. Light-responsive dynamic materials could be applied in drug delivery systems. By tuning the exchange reaction using DASA isomerization, the material could release therapeutics in a controlled manner based on the light responsiveness.*

Source:

Northwestern University

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Co-authors:

Shoki Matsushima