Remote-Controlled Exchange Rates by Photoswitchable Internal Catalysis of Dynamic Covalent Bonds

Abstract

The transesterification of boronate esters with diols is tunable over 14 orders of magnitude. Rate acceleration is achieved by internal base catalysis, which lowers the barrier for proton transfer. Here we report a photoswitchable internal catalyst that tunes the rate of boronic ester/diol exchange over 4 orders of magnitude. We employed an acylhydrazone molecular photoswitch, which forms a thermally stable but photoreversible intramolecular H-bond, to gate the activity of the internal base catalyst in 8-quinoline boronic ester. The photoswitch is bidirectional and can be cycled repeatedly. The intramolecular H-bond is found to be essential to the design of this photoswitchable internal catalyst, as protonating the quinoline with external sources of acid has little effect on the exchange rate.

Controlling Dynamics of Associative Networks through Primary Chain Length

“…While some of the first reports of elastomers with exchangeable cross-links date to at least the 1950s, Bowman and co-workers championed the concept of associative CANs in 2005–2009. − Subsequent seminal reports by Leibler and co-workers introduced the term “vitrimers” to describe materials with Arrhenius-like viscosity dependence analogous to vitreous silica. , In the decade since, research concerning these dynamically cross-linked materials has evolved at a rapid pace. Increased understanding of the fundamentals that govern vitrimer rheology over this relatively short timeframe has resulted in a host of strategies to control or predict flow behavior. − Research has largely focused on three synthetic approaches to tune the rheological properties of vitrimers: modifying or compounding the exchange chemistry at the cross-link site(s), − introduction of internal or external catalysts (or inhibitors), − and varying the matrix composition, polarity, and architecture. ,− …”

Structure–Reactivity–Property Relationships in Covalent Adaptable Networks

“…Rather than manipulating both k a and k d in a dissociative dynamic cross-link, in analogy to Rosales’ work, we envisioned that an associative mechanism would enable photoswitchable control over k ex without affecting network topology and thus stiffness. Based on Wulff’s studies of internal catalysis in boronic ester transesterification, we designed a bidirectional hydrazone photoswitch that reversibly gates an internal base catalyst (Figure d) . The resulting k ex is altered by over 4 orders of magnitude based on the photoswitch conformation.…”

Light‐Fueled Transformations of a Dynamic Cage‐Based Molecular System

“…Molecular photoswitches are attractive synthetic tools that use light as an energy source to induce a reversible change in their properties. The development of molecular machines by the incorporation of photoswitches in more complex molecular systems and architectures, such as macrocycles and molecular cages, has the potential to exhibit emergent behaviors that resemble their biological counterparts [30–45] …”

“…The development of molecular machines by the incorporation of photoswitches in more complex molecular systems and architectures, such as macrocycles and molecular cages, has the potential to exhibit emergent behaviors that resemble their biological counterparts. [30][31][32][33][34][35][36][37][38][39][40][41][42][43][44][45] An example of this emergent behavior is the influence of switching in the reactivity of constrained molecular architectures. Particularly, the effect of the geometric change in the switch can introduce strain in the macrocycle, and by doing so, it can promote the reactions in which the strain directly affects the rate limiting step.…”