The miniaturization of mechanical devices is predicated on the development of new propulsion strategies because it is physically impossible to build working heat engines or electromotors smaller than 1 mm. Therefore, an autonomous micro- and nanomechanical device must be propelled by a fundamentally different mechanism than that employed in bigger systems. A very attractive candidate is conformational actuation (or molecular propulsion), i.e., direct coupling of conformational changes within a reacting (macro)molecule to unidirectional translation of a mesoscopic object. All biological motility relies on this form of actuation but realizing it in synthetic systems has been difficult. We are interested in delineating the upper limit on the force that can be generated by a reactant, understanding what chemical and/or physical processes limit this force and designing new molecular actuators that maximize the accessible force, energy conversion efficiency or power output.
The simplest molecular actuator has two conformationally distinct states which can be interconverted by irradiation but are separated by a high thermal barrier. Examples include stiff stilbene and azobenzene (see figure). The conceptually simplest paradigmatic example of a mesoscopic object translated by conformational changes of a reacting molecule is a single molecule of trans-oligoazobenzene stretched by an AFM. If the restoring force of such a molecule is <400 pN, the molecule contracts when irradiated bringing the tip closer to the glass slide. The contraction is due to trans → cis photoisomerization, which is apparently suppressed when the restoring force of the stretched