Tough carbon–carbon bonds can snap in certain large molecules just because the two sides of the molecule cannot agree on which way to go during adsorption. Heresy? The view through the microscope suggests otherwise.
On page 191 of this issue1, Sheiko and colleagues show that the mechanical deformation induced merely by the adhesion of a complex molecule to a surface can trigger the break-up of that molecule. They thus provide convincing support for the seemingly heretical notion that the commonplace and unremarkable process of adsorption to a surface can bring about what otherwise occurs only with the greatest effort: the rupture of the strong, covalent carbon–carbon bond.
The system designed by the authors1 is elegant in its simplicity. They placed brush-like, polymeric macromolecules on various solid and liquid surfaces to which the molecules' side-chains (the ‘bristles’) were strongly attracted. This attraction drove the bristles to spread out so as to maximize their contact with the surface, in turn causing the polymeric backbone of the molecule to stretch until it was eventually strained too far. Direct imaging of the size of the molecules using atomic force microscopy proved that they had been torn apart, just as if the rope had failed in a game of tug of war (Fig. 1).
When a chemical bond snaps, a chemical reaction takes place. But exactly how can a purely mechanical effect have chemical consequences? That is the wider question investigated by a field known as chemomechanics2. Future research in this area might well focus on whether mechanical stress shifts the energy level of the transition state of a chemical reaction, through which reactants must pass before becoming products. This view has long been held for ‘hard’ materials, such as metals, ceramics and semiconductors (steel, for example, corrodes most easily when stressed and bent3). Mechanical stress could thus lower the activation threshold of certain reactions, making it easier to kick-start them, or even, depending on the magnitude of the stress applied, switch them off and on.
Organic systems such as that of Sheiko et al.1 add a new twist: whereas the basic unit of hard materials is the atom, that of an organic material is the molecule. The added complexity of a molecule's internal architecture means that the kind of stress transmission described by the authors could not have been observed in an atomic system. In biology, the fact that a mechanical stimulus has chemical consequences — for example, in cellular processes that sense mechanical change4 or changes in the conformation and function of ion channels in lipid membranes5 — is being increasingly acknowledged. Thus, investigations of the effect of stress on complex molecules have considerable appeal.
In many organic materials and elastomers (rubbers or rubber-like plastic), the internal architecture of the molecule focuses large stresses on weaker chemical bonds, and stress-induced scission of chemical bonds in such materials is a costly problem6. We believe that the work of Sheiko and colleagues1, by pointing the way towards understanding this ubiquitous and deleterious phenomenon, could provide a general model for designing molecular materials that have an architecture better able to cope with mechanical stress. Such research has myriad technological implications, because the ideas suggested here comprise a new paradigm for solving those problems.
What is in our opinion even more exciting is that there is a general proof-of-concept here — that slow or even forbidden chemical reactions can be activated by mechanical stress. Chemical reactivity clearly depends on the relative orientation of the reactants; so could mechanical deformation be used to place molecules in a more favourable alignment for reaction? For example, friction and confinement offer a versatile way to align molecules7, and tribologists, who earn their bread from the study of these things, have known for a long time that friction promotes chemical reactions8. The tribological question7,8 is not directly addressed in Sheiko and colleagues' experiments, but will be an interesting motivation for further work.
And what about using the heat released to accomplish useful chemical change? Carbon–carbon bonds, by dint of their strength, release a lot of energy when they are broken — just as, in a bout of tug of war, energy is lost when the rope fails and competing teams fall to the ground. Although Sheiko et al. did not set out to capture energy from breaking carbon–carbon bonds, there is no reason that molecules could not be designed that use this energy for productive chemical means. In this area, too, Sheiko et al.1 have presented a fundamental proof-of-principle on which further efforts can be built and go beyond the more obvious, unwanted consequences of mechanical degradation.
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Nature Nanotechnology (2009)