Light-driven transport of a molecular walker in either direction along a molecular track.

Nature uses bipedal motor proteins that “walk” down intracellular tracks to perform essential tasks in a variety of key biological processes. Although the molecular mechanisms through which these fascinating linear motors operate are beginning to be understood, there are still few synthetic mimics that exhibit the most important characteristics of natural motors, namely repetitive, progressive, processive, and directional walker transport along a molecular track. Several walker–track systems based on DNA have been described, and recently our research group reported a smallmolecule system, in which the migration of a walker unit along a four-foothold track could be biased in one direction through an information ratchet type of Brownian ratchet mechanism. Herein we report the design, synthesis, and operation of a small-molecule walker–track conjugate, in which the walker can be transported in either direction along a four-foothold molecular track (roughly 1.5 times more likely to take a step in one direction than the other), depending on the sequence of four applied stimuli: acid or base for mutually exclusive “foot” dissociation and UV light or visible light (plus iodine) to induce or release ring strain between the walker and the track. The design (Figure 1) is closely related to the previously reported small-molecule walker–track system, which features a walker with one hydrazone foot (labile in acid; locked in base) and one disulfide foot (labile in base; locked in acid). The crucial difference is that a stilbene unit has been added between the internal aldehyde and the disulfide footholds of the track (Scheme 1). The key to achieving directionality lies in the isomerization of the stilbene moiety, through which significant ring strain can be induced in the positional (constitutional) isomer in which the walker unit bridges the stilbene linkage (Figure 1 and Scheme 2b). E!Z isomerization provides a driving force