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The transesterification rate 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. 

ortho‐Phenylenes are one of the simplest classes of aromatic foldamers, adopting helical geometries because of aromatic stacking interactions. The folding and misfolding ofortho‐phenylenes are slow on the NMR timescale at or below room temperature, allowing detection of folding states using¹H NMR spectroscopy. Herein, anortho‐phenylene hexamer is coupled with a RAFT chain transfer agent (CTA) on each repeat unit. A variety of acrylic monomers are polymerized onto the CTA‐functionalizedortho‐phenylene using PET‐RAFT to yield functionalized star polymers withortho‐phenylene cores. The steric bulk of the acrylate monomer units as well as the chain length of each arm of the star polymer is varied.¹H NMR spectroscopy shows that the folding of theortho‐phenylenes do not vary, providing a robust helical core for star polymer systems.

 

Polymer networks crosslinked with spring-like ortho -phenylene ( o P) foldamers were developed. NMR analysis indicated the o P crosslinkers were well-folded. Polymer networks with o P-based crosslinkers showed enhanced energy dissipation and elasticity compared to divinylbenzene crosslinked networks. The energy dissipation was attributed to the strain-induced reversible unfolding of the o P units. Energy dissipation increased with the number of helical turns in the foldamer. 

The self-assembly of foldamers into macrocycles is a simple approach to non-biological higher-order structure. Previous work on the co-assembly of ortho -phenylene foldamers with rod-shaped linkers has shown that folding and self-assembly affect each other; that is, the combination leads to new emergent behavior, such as access to otherwise unfavorable folding states. To this point this relationship has been passive. Here, we demonstrate control of self-assembly by manipulating the foldamers' conformational energy surfaces. A series of o -phenylene decamers and octamers have been assembled into macrocycles using imine condensation. Product distributions were analyzed by gel-permeation chromatography and molecular geometries extracted from a combination of NMR spectroscopy and computational chemistry. The assembly of o -phenylene decamers functionalized with alkoxy groups or hydrogens gives both [2 + 2] and [3 + 3] macrocycles. The mixture results from a subtle balance of entropic and enthalpic effects in these systems: the smaller [2 + 2] macrocycles are entropically favored but require the oligomer to misfold, whereas a perfectly folded decamer fits well within the larger [3 + 3] macrocycle that is entropically disfavored. Changing the substituents to fluoro groups, however, shifts assembly quantitatively to the [3 + 3] macrocycle products, even though the structural changes are well-removed from the functional groups directly participating in bond formation. The electron-withdrawing groups favor folding in these systems by strengthening arene–arene stacking interactions, increasing the enthalpic penalty to misfolding. The architectural changes are substantial even though the chemical perturbation is small: analogous o -phenylene octamers do not fit within macrocycles when perfectly folded, and quantitatively misfold to give small macrocycles regardless of substitution. Taken together, these results represent both a high level of structural control in structurally complex foldamer systems and the demonstration of large-amplitude structural changes as a consequence of a small structural effects. 

While many foldamer systems reliably fold into well‐defined secondary structures, higher order structure remains a challenge. A simple strategy for the organization of folded subunits in space is to link them together within a macrocycle. Previous work has shown thato‐phenylenes can be co‐assembled with rod‐shaped linkers into twisted macrocycles, showing an interesting synergy between folding and thermodynamically controlled macrocyclization. In these systems the foldamer units were largely decoupled from each other both conformationally and electronically. Here, we show that hydrocarbon macrocycles, with very short ethenylene linkers, can be assembled fromo‐phenylenes using olefin metathesis. Characterization by NMR spectroscopy, X‐ray crystallography, and ab initio calculations shows that the products are approximately triangular trimer macrocycles with helicalo‐phenylene corners in a heterochiral configuration. Their photophysics are dominated by the 4,4'‐diphenylstilbene moieties, the longest conjugated segments, with further conjugation broken by the twisting of theo‐phenylenes.