A cleverly engineered molecule uses light to generate a charge-separated state and so cause one of its components to move. It's the latest study of a molecular machine that exploits nature's most plentiful energy source.
Nature runs the nanomachinery that makes life possible using the last word in clean, free and readily available power sources — sunlight. In photosynthetic bacteria and green plants, photon absorption by chlorophyll generates a charge-separated state, from which the electron is quickly passed down a cascade of electron carriers, ultimately generating energy in a convenient chemical form. Can similar capabilities be engineered? An exemplary effort to do just this is given by Balzani et al. who, writing in Proceedings of the National Academy of Sciences1, describe photochemical experiments on an artificial machine that uses light to displace a fragment of its unimolecular structure.
Those who seek to harness the Sun's energy for synthetic molecular machines find that chemistry is always throwing up obstacles. In particular, charge recombination typically occurs thousands or millions of times faster than the nuclear movements on which such machines rely, making charge-separated states difficult to exploit. This problem can be overcome using bimolecular systems: here, the charged partners quickly diffuse apart so their energy can be used, for example, to achieve switching in a rotaxane2. This class of molecule, consisting of a ring that shuttles randomly and incessantly along a string, stopped only by bulky groups at the string's termini, is also that used by Balzani and colleagues1.
Their rotaxane3 (Fig. 1) incorporates two structurally different bipyridinium sites — ‘stations’ 1 and 2 — that slow the shuttling ring's motion through strong short-range electrostatic interactions. The ring thus divides its time between station 1, station 2 and the rest of the string in the ratio of around 95:5:<1. At room temperature, the ring shuttles between the stations tens of thousands of times per second, but the net flux is zero. So no work can be done, or useful task performed, by the shuttling action (the ‘principle of detailed balance’4).
One of the bulky end-groups of the rotaxane's string is a ruthenium trisbipyridine complex. This can absorb a photon of visible light and so form a reactive, excited state that donates an electron to the more easily reduced of the two bipyridinium sites — station 1, the ring's preferred binding site. One would normally expect the resulting charge imbalance to be corrected by back-transfer of an electron on the sub-microsecond timescale, even given the considerable distance between the ruthenium complex and the stations (which also slows down the rate of the forward electron-transfer reaction). In fact, the charge-separated state has an average lifetime of around 10 microseconds.
Reduced bipyridinium is a very much poorer binding site for the ring, and the charge-separated state survives long enough for about 10% of the rings to undergo significant brownian motion. Detailed balance is broken and a net flux of rings occurs as they shift their allegiance to the unreduced station 2. After 10 microseconds, however, as back electron transfer finally takes place, station 1 regains its stickiness. A net flux of rings occurs back from station 2, the system returns to its original equilibrium, and a machine cycle has taken place. The process can be repeated for at least a thousand cycles.
This is a fascinating system, working (as do most other light-driven shuttles2,5,6,7,8) without the consumption of chemical fuels or the formation of waste products. It can be properly called a machine because component displacements occur in response to an external stimulus. But is it really best described as a ‘nanomotor’? Chemists are still pondering the most useful way to understand the behaviour of the contemporary, early generations of synthetic molecular machines, but physics and biology offer many useful phenomenon-based ideas in this regard. Brownian motors, for example, use mechanisms9 that harness random molecular-level motion like that of the ring motion in rotaxanes.
Such brownian ratchet mechanisms, which are believed to account for the behaviour of some motor proteins10, all require detailed balance to be broken to allow a net, directed flux of particles. Crucially, however, the ‘ratchet’ part of the mechanism ensures that the resulting change in particle distribution is not undone when the motor is reset. This allows the machine to be able to pump the particle distribution further and further away from equilibrium (as with the enzymes that synthesize the currency of intracellular energy transfer, ATP) or move itself progressively down a track (as with the cell's internal packhorse, kinesin).
This feature is missing from Balzani and colleagues' rotaxane1, and other simple molecular shuttles2,5,6,7,8: the work done in breaking detailed balance is undone by the reset step. For such a system to be understood to be a motor by a statistical physicist or biologist, therefore, the rings would have to be diverted along a different track during the reset phase (making a rotary motor), or remain where they are while the machine is reset (making a linear motor or pump). The former is the basis of several wholly synthetic molecular motors11,12,13,14,15; the latter has thus far been achieved only with artificial structures made from DNA16.
Balzani and colleagues' shuttle operates through a fully autonomous photochemical cycle; but can these molecules repetitively do work as long as sunlight is available? The authors did not use sunlight in the experiments they report, but instead treated the rotaxanes with a single 10-nanosecond laser pulse at a visible-light wavelength. If continuously irradiated with sunlight, the distribution of the rings between the two stations would reach a steady state (the exact distribution depending on the intensity of the light) within a few milliseconds. To generate net flows of rings between the stations after that, it appears one would have to switch the light source rapidly on and off.
This is in contrast to the performance of another family of machine molecules that have components that rotate directionally, rather than shuttle linearly12,17. When these rotary molecules reach a steady state under continuous irradiation, as with the rotaxanes the bulk distribution of the machine components no longer changes. But even at the steady state, at a sufficiently high temperature, net fluxes of the rotor components still occur through different pathways between four different rotary isomers that are present. Under constant irradiation, the molecules thus operate continuously as directionally rotating motors17.
Synthetic molecular motors and machines are very much in their infancy, and chemists are still learning the most basic rules for their design and operation. It is a field that can usefully draw on input from physicists, biologists, engineers and materials scientists. For a deeper understanding of molecular machine systems to evolve, therefore, it would be highly beneficial if the terminology used to describe them were to become consistent across all the contributing disciplines. Balzani and colleagues' latest photochemical experiments1 represent a fascinating advance in our understanding of how a charge-separated state can be used to bring about a nuclear displacement in a unimolecular machine. It will doubtless prove an important stage on the route towards autonomous artificial nanomotors powered by sunlight.
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Nature Nanotechnology (2006)