How it works!

Exorcising Demons

In formulating his thought experiment, Maxwell was only interested in illustrating the statistical nature of the Second Law, but subsequent generations of inventors and philosophers have been fascinated by its implications for the creation of a perpetual motion machine. A temperature or pressure differential between two compartments can be used to do work, so if one could be established without expending any energy it could form the basis for a ‘something-for-nothing’ device which does work without requiring fuel! Such a machine is impossible, of course, and is NOT what Serreli et al were trying to achieve through the work described in Nature. But why is it not possible for a demon do the necessary sorting task without an input of energy? The solution to this paradox took more than a century to fully resolve^[5] but it was eventually understood through the discovery that no matter how you design your ‘demon’ component, any device that is able to process and act upon information has an inherent energy requirement that always saves the Second Law. This is due to the fundamental relationship between information and entropy – the link that, for example, requires memory erasure in computers to feed entropy into the environment (Landauer’s principle^[6]).

Exercising Demons

Now, chemists at the University of Edinburgh have actually made^[4] a molecular machine that performs the sorting task envisaged for Maxwell’s pressure demon (Figure 1b) but, crucially, it requires an input of external energy to do so and so does not challenge the Second Law of Thermodynamics. Using light energy, the molecule is able to transmit information about the position of a molecular fragment in a manner that allows transport of the same fragment in a particular direction (Figure 2). This information-based system represents a fundamentally new type of motor-mechanism for synthetic nanomachines.

The new nanomachine belongs to a class of molecules known as ‘rotaxanes’. These structures consist of a molecular ring (‘macrocycle’) trapped on a linear molecular thread by bulky ‘stoppers’ at either end. Molecules of this kind have become popular with synthetic molecular machine designers over the past decade because their architecture restricts significant submolecular motions to only two modes, namely random movement of the ring back and forth along the thread (‘shuttling’) and nondirectional rotation around the thread (‘pirouetting’). But random motion–even cleverly restricted random motion–is not enough to create a molecular machine. An input of energy is required to control how the motion occurs. Various methods for the net transport of macrocycles between different regions in rotaxanes have previously been demonstrated in molecules called ‘stimuli-responsive molecular shuttles’.^[7] However, these are simple two-state switches, the most basic and functionally limited type of machine mechanism^[8] in which the ring distribution is always at, or relaxing towards, equilibrium. In contrast, biological motors and machines are able to drive chemical systems away from equilibrium,^[9] just like Maxwell’s Demon. Only a handful of synthetic molecular machines have been made that can claim to do that.^[10-12] Yet, although these structures are relatively diverse (they do not all use rotaxane architectures, for example), they all use similar mechanistic principles in their operation. The latest nanomachine from the Edinburgh group operates using a new and entirely different type of mechanism involving information transfer between the substrate and the machine.

Figure 2. A photo-operated molecular information ratchet. a Irradiation of rotaxane 1 at 350 nm in CD3OD at 298 K interconverts the different forms of 1 and, in the presence of benzil (PhCOCOPh), drives the ring distribution away from its thermodynamic minimum without ever changing the binding strengths of the macrocycle or ammonium binding sites. When the macrocycle is on the mba binding site (green), intramolecular energy transfer (ET) from the macrocycle is inefficient and intermolecular ET from benzil dominates. When the macrocycle is on the dba binding site (blue), both ET mechanisms can operate. The amount of benzil present determines the relative contributions of the two ET pathways and thus the nanomachine’s effectiveness in pumping the macrocycle distribution away from equilibrium. b Cartoon illustration of the operation of 1 as a Maxwellian pressure demon[2c]: i Photo-induced excitation of a particle signals its position in the blue compartment by energy transfer to the demon operating the gate (the demon uses information from the asymmetry in the compartments to distinguish where the excited particles are most likely to be, here through their average distance from the gate). ii & iii The demon opens the gate and the particle shuttles incessantly between the two compartments by Brownian motion until the gate shuts trapping the particle at random in one of them. iv Photo-induced excitation of the particle in the green compartment is ignored by the demon and the energy of the excited state is dissipated as heat.

The new rotaxane molecule has some key novel components in its design. Firstly, the axle is divided into two compartments by a chemical ‘gate’ known as a stilbene. This can exist in two forms: one, the ‘gate-open’ form, allows the macrocycle to pass over it; the other ‘gate-closed’ form blocks motion of the ring. The operating conditions are chosen such that the gate tends to be closed. Each compartment on the axle also contains a ‘binding site’ for the ring – a zone which the ring finds sticky, and so this is where it spends most of its time. In one compartment the sticky region is close to the gate, in the other compartment the binding site is far away from the stilbene. The rings are also special in that an input of light energy gives them the ability to signal their presence to the gate by ‘energy transfer’ (ET, figure 2). This signalling triggers a process that opens of the gate, momentarily allowing the rings to pass, before the gate is returned to its closed state. Because the rings in one compartment spend much more time close to the gate than those in the other, the gate is more likely to be opened by rings moving in one direction than the other. The result is that the number of rings in one compartment increases upon the shining of light on the molecules. We end up with many more rings in this compartment than is statistically expected given its stickiness – the system has been driven away from equilibrium.

Of course, because the information transfer process is powered by the input of light energy this system is certainly not a perpetual motion machine. In fact, modern theoretical physicists had become aware of the possibility of moving Brownian objects by using energy to cause a transfer of information in a mechanism dubbed an ‘information ratchet’. The molecule reported in Nature^[4] represents the first time chemists have designed and made such a device in the laboratory.

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