Light excitation powers the reversible shuttling movement of the ring component of a rotaxane between two stations located at a 1.3-nm distance on its dumbbell-shaped component. The photoinduced shuttling movement, which occurs in solution, is based on a ''four-stroke'' synchronized sequence of electronic and nuclear processes. At room temperature the deactivation time of the high-energy charge-transfer state obtained by light excitation is Ϸ10 s, and the time period required for the ring-displacement process is on the order of 100 s. The rotaxane behaves as an autonomous linear motor and operates with a quantum efficiency up to Ϸ12%. The investigated system is a unique example of an artificial linear nanomotor because it gathers together the following features: (i) it is powered by visible light (e.g., sunlight); (ii) it exhibits autonomous behavior, like motor proteins; (iii) it does not generate waste products; (iv) its operation can rely only on intramolecular processes, allowing in principle operation at the single-molecule level; (v) it can be driven at a frequency of 1 kHz; (vi) it works in mild environmental conditions (i.e., fluid solution at ambient temperature); and (vii) it is stable for at least 10 3 cycles. molecular machine ͉ nanoscience ͉ photochemistry ͉ rotaxane ͉ supramolecular chemistry T he miniaturization race is encouraging scientists to design and construct motors on the nanometer scale, that is, at the molecular level (1-5). Such a daring goal finds its scientific origin in the existence of natural molecular motors (6-9).Natural molecular motors, however, are extremely complex, and any attempt to construct systems of such a complexity, using the bottom-up molecular approach (10), would be challenging. What can be done, at present, is to construct simple prototypes of artificial molecular motors and machines (1-5, 11-19), consisting of a few components capable of moving in a controllable way, and to investigate the associated problems posed by interfacing them with the macroscopic world (20-25), particularly as far as energy supply is concerned.Natural motors are ''autonomous'': they keep operating, in a constant environment, as long as the energy source is available. By contrast, apart from a few recent examples (26-28), the fuel-powered artificial motors described so far are not autonomous because, after the mechanical movement induced by a chemical input, they need another, opposite chemical input to reset, which also implies generation of waste products. Addition of a fuel, however, is not the only means by which energy can be supplied to a chemical system. In fact, nature shows that, in green plants, the energy needed to sustain the machinery of life is ultimately provided by sunlight. Energy inputs in the form of photons can indeed cause mechanical movements by reversible chemical reactions without formation of waste products (13,14,16,17).In a previous work (29), we reported on the rotaxane 1 6ϩ (Scheme 1) that was carefully designed and synthesized to perform as a linear molecular...