Single-Molecule Observation of a Mechanically Activated <i>Cis</i>-to-<i>Trans</i> Cyclopropane Isomerization

The mechanochemical activation of <i>cis</i>-<i>gem</i>-difluorocyclopropane (<i>cis</i>-<i>g</i>DFC) mechanophore in toluene was characterized with single-molecule force spectroscopy. Unlike previously reported behavior in methyl benzoate (MB), two transitions are observed in the force vs extension curves of <i>cis</i>-<i>g</i>DFC polymers in toluene. The first transition occurs at the same force of ∼1300 pN observed previously in MB, but a second transition is observed at forces of ∼1800 pN that reveal the partial formation of the <i>trans-g</i>DFC isomer. The behavior is attributed to competing reactions of the <i>cis</i>-<i>g</i>DFC at the 1300 pN plateau: addition of oxygen to a ring-opened diradicaloid intermediate, and isomerization of <i>cis</i>-<i>g</i>DFC to its <i>trans</i> isomer

Force-Rate Characterization of Two Spiropyran-Based Molecular Force Probes

The mechanically accelerated ring-opening reaction of spiropyran to a colored merocyanine provides a useful method by which to image the molecular scale stress/strain distribution within a polymer, but the magnitude of the forces necessary for activation has yet to be quantified. Here, we report single molecule force spectroscopy studies of two spiropyran isomers. Ring opening on the time scale of tens of milliseconds is found to require forces of ∼240 pN, well below that of previously characterized covalent mechanophores. The lower threshold force is a combination of a low force-free activation energy and the fact that the change in rate with force (activation length) of each isomer is greater than that inferred in other systems. Finally, regiochemical effects on mechanochemical coupling are characterized, and increasing force reverses the relative ring opening rates of the two isomers

Solvent Polarity Effects on the Mechanochemistry of Spiropyran Ring Opening

The spiropyran mechanophore (SP) is employed as a reporter of molecular tension in a wide range of polymer matrices, but the influence of surrounding environment on the force-coupled kinetics of its ring opening has not been quantified. Here, we report single-molecule force spectroscopy studies of SP ring opening in five solvents that span normalized Reichardt solvent polarity factors (ETN) of 0.1–0.59. Individual multimechanophore polymers were activated under increasing tension at constant 300 nm s–1 displacement in an atomic force microscope. The extension results in a plateau in the force–extension curve, whose midpoint occurs at a transition force f* that corresponds to the force required to increase the rate constant of SP activation to approximately 30 s–1. More polar solvents lead to mechanochemical reactions that are easier to trigger; f* decreases across the series of solvents, from a high of 415 ± 13 pN in toluene to a low of 234 ± 9 pN in n-butanol. The trend in mechanochemical reactivity is consistent with the developing zwitterionic character on going from SP to the ring-opened merocyanine product. The force dependence of the rate constant (Δx‡) was calculated for all solvent cases and found to increase with ETN, which is interpreted to reflect a shift in the transition state to a later and more productlike position. The inferred shift in the transition state position is consistent with a double-well (two-step) reaction potential energy surface, in which the second step is rate determining, and the intermediate is more polar than the product

Mechanism Dictates Mechanics: A Molecular Substituent Effect in the Macroscopic Fracture of a Covalent Polymer Network

The fracture of rubbery polymer networks involves a series of molecular events, beginning with conformational changes along the polymer backbone and culminating with a chain scission reaction. Here, we report covalent polymer gels in which the macroscopic fracture "reaction" is controlled by mechanophores embedded within mechanically active network strands. We synthesized poly(ethylene glycol) (PEG) gels through the end-linking of azide-terminated tetra-arm PEG (Mn = 5 kDa) with bis-alkyne linkers. Networks were formed under identical conditions, except that the bis-alkyne was varied to include either a cis-diaryl (1) or cis-dialkyl (2) linked cyclobutane mechanophore that acts as a mechanochemical "weak link" through a force-coupled cycloreversion. A control network featuring a bis-alkyne without cyclobutane (3) was also synthesized. The networks show the same linear elasticity (G' = 23-24 kPa, 0.1-100 Hz) and equilibrium mass swelling ratios (Q = 10-11 in tetrahydrofuran), but they exhibit tearing energies that span a factor of 8 (3.4 J, 10.6, and 27.1 J·m-2 for networks with 1, 2, and 3, respectively). The difference in fracture energy is well-aligned with the force-coupled scission kinetics of the mechanophores observed in single-molecule force spectroscopy experiments, implicating local resonance stabilization of a diradical transition state in the cycloreversion of 1 as a key determinant of the relative ease with which its network is torn. The connection between macroscopic fracture and a small-molecule reaction mechanism suggests opportunities for molecular understanding and optimization of polymer network behavior

Accelerating a Mechanically Driven <i>anti</i>-Woodward–Hoffmann Ring Opening with a Polymer Lever Arm Effect

Mechanical forces have previously been used to drive reactions along pathways that violate the orbital symmetry effects captured in the Woodward–Hoffmann rules. Here, we show that a polymer “lever arm effect” can provide a mechanical advantage in accelerating the symmetry forbidden disrotatory ring opening of benzocyclobutene (BCB). Addition of an α-<i>E</i>-alkene to the BCB mechanophore drops the force required to induce reactions on the ∼0.1 s time scale of single-molecule force spectroscopy experiments from 1370 to 920 pN

Accelerating a Mechanically Driven <i>anti</i>-Woodward–Hoffmann Ring Opening with a Polymer Lever Arm Effect

Mechanical forces have previously been used to drive reactions along pathways that violate the orbital symmetry effects captured in the Woodward–Hoffmann rules. Here, we show that a polymer “lever arm effect” can provide a mechanical advantage in accelerating the symmetry forbidden disrotatory ring opening of benzocyclobutene (BCB). Addition of an α-<i>E</i>-alkene to the BCB mechanophore drops the force required to induce reactions on the ∼0.1 s time scale of single-molecule force spectroscopy experiments from 1370 to 920 pN

Mechanistic Insights into the Sonochemical Activation of Multimechanophore Cyclopropanated Polybutadiene Polymers

Structure–activity relationships in the mechanochemistry of <i>gem</i>-dichlorocyclopropane (<i>g</i>DCC)-based polymer solutions triggered by pulsed ultrasound are reported. Insights into the flow-induced mechanochemical transformations of <i>g</i>DCC mechanophores into the corresponding 2,3-dichloroalkenes are obtained by monitoring the mechanochemistry as a function of initial polymer molecular weight and sonication conditions. The competition between <i>g</i>DCC activation and polymer chain scission is invariant to sonication power, temperature, polymer concentration, and solvent but is sensitive to initial polymer molecular weight. The results have practical implications for the use of polymer sonochemistry as a tool for quantifying the relative mechanical strength of scissile polymers and conceptual implications for thinking about the nature of the force distributions experienced during sonochemical experiments

Mechanochemistry of Cubane

We report the mechanochemical reactivity of the highly strained pentacyclic hydrocarbon cubane. The mechanical reactivity of cubane is explored for three regioisomers with 1,2-, 1,3-, and 1,4-substituted pulling attachments. Whereas all compounds can be activated thermally, mechanical activation is observed via pulsed ultrasonication of cubane-containing polymers only when force is applied via 1,2-attachment. The single observed product of the force-coupled reaction is a thermally inaccessible syn-tricyclooctadiene, in contrast to cyclooctatetraene (observed thermally) or a pair of cyclobutadienes that would result from sequential cyclobutane scission. We further quantify the mechanochemical reactivity of cubane by single molecule force spectroscopy, and force-coupled rate constants for ring opening reach ∼33 s–1 at a force of ∼1.55 nN, lower than forces of 1.8–2.0 nN that are typical of conventional cyclobutanes

Catch and Release: Orbital Symmetry Guided Reaction Dynamics from a Freed “Tension Trapped Transition State”

The dynamics of reactions at or in the immediate vicinity of transition states are critical to reaction rates and product distributions, but direct experimental probes of those dynamics are rare. Here, <i>s</i>-<i>trans</i>, <i>s</i>-<i>trans</i> 1,3-diradicaloid transition states are trapped by tension along the backbone of purely <i>cis</i>-substituted <i>gem</i>-difluorocyclopropanated polybutadiene using the extensional forces generated by pulsed sonication of dilute polymer solutions. Once released, the branching ratio between symmetry-allowed disrotatory ring closing (of which the trapped diradicaloid structure is the transition state) and symmetry-forbidden conrotatory ring closing (whose transition state is nearby) can be inferred. Net conrotatory ring closing occurred in 5.0 ± 0.5% of the released transition states, in excellent agreement with ab initio molecular dynamics simulations

A Remote Stereochemical Lever Arm Effect in Polymer Mechanochemistry

Molecular mechanisms by which to increase the activity of a mechano­phore might provide access to new chemical reactions and enhanced stress-responsive behavior in mechano­chemically active polymeric materials. Here, single-molecule force spectroscopy reveals that the force-induced acceleration of the electro­cyclic ring opening of <i>gem</i>-dichloro­cyclo­propanes (<i>g</i>DCC) is sensitive to the stereo­chemistry of an α-alkene substituent on the <i>g</i>DCC. On the ∼0.1 s time scale of the experiment, the force required to open the <i>E</i>-alkene-substituted <i>g</i>DCC was found to be 0.4 nN lower than that required in the corresponding <i>Z</i>-alkene isomer, despite the effectively identical force-free reactivities of the two isomers and the distance between the stereo­chemical permutation and the scissile bond of the mechano­phore. Fitting the experimental data with a cusp model provides force-free activation lengths of 1.67 ± 0.05 and 1.20 ± 0.05 Å for the <i>E</i> and <i>Z</i> isomers, respectively, as compared to 1.65 and 1.24 Å derived from computational modeling