Biasing reaction pathways with mechanical force

Abstract

During the course of chemical reactions, reactant molecules need to surmount an energy barrier to allow their transformation into products. The energy needed for this process is usually provided by heat, light, pressure or electrical potential, which act either by changing the distribution of the reactants on their ground-state potential energy surface or by moving them onto an excited-state potential energy surface and thereby facilitate movement over the energy barrier. A fundamentally different way of initiating or accelerating a reaction is the use of force to deform reacting molecules along a specific direction of the reaction coordinate. Mechanical force has indeed been shown to activate covalent bonds in polymers, but the usual result is chain scission. Here we show that mechanically sensitive chemical groups make it possible to harness the mechanical forces generated when exposing polymer solutions to ultrasound, and that this allows us to accelerate rearrangement reactions and bias reaction pathways to yield products not obtainable from purely thermal or light-induced reactions. We find that when placed within long polymer strands, the trans and cis isomers of a 1,2-disubstituted benzocyclobutene undergo an ultrasound-induced electrocyclic ring opening in a formally conrotatory and formally disrotatory process, respectively, that yield identical products. This contrasts with reaction initiation by light or heat alone, in which case the isomers follow mutually exclusive pathways to different products. Mechanical forces associated with ultrasound can thus clearly alter the shape of potential energy surfaces so that otherwise forbidden or slow processes proceed under mild conditions, with the directionally specific nature of mechanical forces providing a reaction control that is fundamentally different from that achieved by adjusting chemical or physical parameters. Because rearrangement in our system occurs before chain scission, the effect we describe might allow the development of materials that are activated by mechanical stress fields.

Mechanically activated rupture of single covalent bonds: evidence of force induced bond hydrolysis

“…Moreover, mechanical activation can be used in natural products chemistry, the chemistry of solids, and even in organic synthesis, where force can be used as a control parameter to steer a reaction to a desired end point. [2][3][4][5][6][7][8] With the advancement of single molecule techniques, rupture forces of individual molecular bonds have become experimentally accessible, [9][10][11][12][13] and recently there has been an increasing number of studies focusing on the mechanical stability of covalent chemical bonds. 4,[14][15][16][17][18][19][20][21] In order to understand the underlying chemical reactions at the molecular level, it is necessary to determine structural and kinetic parameters, like depth and width of the binding potential, as well as the Arrhenius pre-factor and compare these parameters to thermodynamic data as well as quantum chemical modelling.…”

Cephalopod-inspired design of electro-mechano-chemically responsive elastomers for on-demand fluorescent patterning

“…We then integrate the EMCR elastomer into a display panel that can be remotely controlled. Voltages applied to the display panel induce various patterns of large deformation on the surface of the EMCR elastomer, and the induced stresses in turn lead to a versatile range of fluorescent patterns including lines, circles and letters on demand [22][23][24][25] . The activation of EMCR elastomer and fluorescent patterning are reversible and repeatable over multiple cycles, in contrast to the irreversible plastic deformation or fracture required for activating most existing mechanoresponsive polymers 21,[26][27][28][29][30][31][32][33][34] .…”

Edge dislocation slows down oxide ion diffusion in doped CeO2 by segregation of charged defects

“…S train engineering offers a new route to tune the reaction and transport kinetics in oxide materials 1 by altering the inherent energy landscape of reactions, as demonstrated also for metals 2,3 and for polymers 4,5 . Strain can be induced in a material by lattice mismatch at the interface of a thin film with a substrate or with neighbouring layers (such as in multilayer composites).…”