An autonomous chemically fuelled small-molecule motor

A system is described in which a small macrocycle is continuously transported directionally around a cyclic molecular track when powered by irreversible reactions of a chemical fuel; such autonomous chemically fuelled molecular motors should find application as engines in molecular nanotechnology. A molecular motor has first to generate movements that are not swamped by Brownian motion, a dominant force at that scale, and cannot exploit angular momentum as a means of directional control. Despite these constraints, David Leigh and colleagues have developed a system that consumes a single chemical fuel to power a molecular machine that achieves continuous rotary motion as long as the fuel is present, and does not require any further chemical input or external stimulus. The motor consists of two interlocked molecular rings, the smaller of which (the macrocycle) is continuously transported directionally around the larger (the cyclic molecular track) when powered by irreversible reactions of a chemical fuel. Directionality is achieved via asymmetry in reaction rates of the chemical fuel added to the track, forcing the macrocycle to continue travelling in the same direction, rather than reversing towards the previous reactive point. Molecular machines are among the most complex of all functional molecules and lie at the heart of nearly every biological process1. A number of synthetic small-molecule machines have been developed2, including molecular muscles3,4, synthesizers5,6, pumps7,8,9, walkers10, transporters11 and light-driven12,13,14,15,16 and electrically17,18 driven rotary motors. However, although biological molecular motors are powered by chemical gradients or the hydrolysis of adenosine triphosphate (ATP)1, so far there are no synthetic small-molecule motors that can operate autonomously using chemical energy (that is, the components move with net directionality as long as a chemical fuel is present)19. Here we describe a system in which a small molecular ring (macrocycle) is continuously transported directionally around a cyclic molecular track when powered by irreversible reactions of a chemical fuel, 9-fluorenylmethoxycarbonyl chloride. Key to the design is that the rate of reaction of this fuel with reactive sites on the cyclic track is faster when the macrocycle is far from the reactive site than when it is near to it. We find that a bulky pyridine-based catalyst promotes carbonate-forming reactions that ratchet the displacement of the macrocycle away from the reactive sites on the track. Under reaction conditions where both attachment and cleavage of the 9-fluorenylmethoxycarbonyl groups occur through different processes, and the cleavage reaction occurs at a rate independent of macrocycle location, net directional rotation of the molecular motor continues for as long as unreacted fuel remains. We anticipate that autonomous chemically fuelled molecular motors will find application as engines in molecular nanotechnology2,19,20.