Coumarin Dimer Is an Effective Photomechanochemical AND Gate for Small-Molecule Release

Abstract

Stimulus-responsive gating of chemical reactions is of considerable practical and conceptual interest. For example, photocleavable protective groups and gating mechanophores allow the kinetics of purely thermally activated reactions to be controlled optically or by mechanical load by inducing the release of smallmolecule reactants. Such release only in response to a sequential application of both stimuli (photomechanochemical gating) has not been demonstrated despite its unique expected benefits. Here, we describe computational and experimental evidence that coumarin dimers are highly promising moieties for realizing photomechanochemical control of small-molecule release. Such dimers are transparent and photochemically inert at wavelengths >300 nm but can be made to dissociate rapidly under tensile force. The resulting coumarins are mechanochemically and thermally stable, but rapidly release their payload upon irradiation. Our DFT calculations reveal that both strain-free and mechanochemical kinetics of dimer dissociation are highly tunable over an unusually broad range of rates by simple substitution. In head-to-head dimers, the phenyl groups act as molecular levers to allow systematic and predictable variation in the force sensitivity of the dissociation barriers by choice of the pulling axis. As a proof-of-concept, we synthesized and characterized the reactivity of one such dimer for photomechanochemically controlled release of aniline and its application for controlling bulk gelation.

Sacrificial Mechanical Bond is as Effective as a Sacrificial Covalent Bond in Increasing Cross-Linked Polymer Toughness

“…Calculated Δ G ‡ ( f ) suggests that mechanochemical dissociation of DFSN in a loaded elastomer is likely to occur at local force ≥1.5 nN. This threshold is significantly lower than that derived with COGEF, which adds to the increasing volume of evidence that the superficially appealing simplicity of COGEF requires aphysical assumptions that render its conclusions both conceptually and empirically suspect. Recombination of the product radicals upon dissipation of the local load is diffusion-limited in amorphous solids and melts, but is slower than the diffusion rate in solution (recombination Δ G 0 ‡ = 13.6 and 12.5 kcal/mol), in accordance with the reported observations. , Recombination likely produces approximately equimolar mixtures of the diastereomers.…”

Mechanochemical Approaches to Fundamental Studies in Soft‐Matter Physics

“…Highly non-monotonic ΔG ‡ (f) of R10 at < 1 nN reflects sequential change in the ratedetermining TS, followed by change in the minimum-energy reaction path, with increasing force: refs. [8,32] report the other examples. Table S1 lists the sources and DFT model chemistries of all plotted ΔG ‡ .…”

“…Precluding this mechanistic switch makes the reaction force-insensitive. [32] Since the sequence of bonds defining a backbone is not always unique, an objective, quantitative analysis of the effect of molecular structure, including the location of the chain/mechanophore attachment atoms, on the mechanochemical kinetics relies on restoring forces of individual bonds. [39] These are calculated from the stretching force in harmonic oscillator approximation using technicallystraightforward [40] and theoretically validated [39] approaches.…”

Dynamic Interfaces in Self-Healable Polymers

“…Tetrafunctional monomers with coumarin photoactive groups can form a rigid polymer network via phototriggered [2 + 2] cycloaddition reactions exposed to 365 nm light. Dissociation of coumarin dimers requires activation energies 137 > kJ mol –1 . Thus, exposure to 254 nm UV light cleaves the cross-linked network, forming monomeric or low molecular weight fragments, an interfacial T g reduction, facilitating enhanced segmental mobilities and self-healing.…”