Essentially, all motion in living organisms emerges from the collective action of biological molecular machines transforming chemical energy, originally harvested from light, into ordered activity. As a man-made counterpart to nature’s biomolecular machines, chemists have created artificial molecular machines that display controlled and even directional motion in response to light. However, to be of practical value, the motion of these light-fuelled molecular machines will have to be coupled to the rest of the world. Inspired by the complex functional movement seen in the plant and animal world, chemists have undertaken the challenge to harness molecular motion and, so, they have set artificial molecular motors and switches to work and perform useful mechanical action at the macroscopic level. Here, we review these recent developments. We show how modern research has embraced the full complexity of the molecular world by aiming at the design of autonomous, and sometimes adaptive, molecular systems that work continuously under the effect of illumination. We report evidence that molecular motion can be engineered into highly sophisticated movements and that, from a fundamental point of view, continuous movement can only emerge when man-made molecules cooperate, in space and time. Eventually, unravelling the rules of molecular motion will support the creation of molecular materials that produce work continuously under a constant input of energy.
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The authors acknowledge funding support from the European Research Council (Consolidator Grant Morpheus 30968307) and the Netherlands Organization for Scientific Research (Projectruimte grant 13PR3105).
The authors declare no competing interests.
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- Artificial molecular machines
Man-made molecules or molecular systems that perform useful tasks by converting an energy input into a mechanically relevant motion.
- Feedback loops
Effects that regulate a molecular signal, in space and time. This signal can be a mechanical signal, an electrical signal, an optical signal or, more classically, a concentration of molecules. The regulation mechanism can be based on a network of chemical reactions or on a series of mechanical events.
- Liquid crystals
Molecules and materials that exhibit a liquid-crystalline state in specific conditions of temperature or dilution. The liquid-crystalline state is a consequence of molecular shape anisotropy and is characterized by a fluidity that is inherent to conventional liquids, combined with a long-range molecular orientation that is also found in crystals. This long-range organization can occur in solution (lyotropic phases) or in bulk materials (thermotropic phases).
Colloidal networks, polymer networks or supramolecular assemblies that are expanded throughout their whole volume (e.g. swollen) by a fluid and exhibit no flow when in the steady state.
- Filter effect
The effect that limits the propagation of light through the thickness of a molecular material, by absorption or by scattering.
- Photosalient effect
Accumulation of stress in bulk materials under continuous irradiation that leads to the abrupt release of kinetic energy via bursting, jumping, rolling, etc.
- Liquid-crystal networks
Materials in which the physical properties of polymeric networks and high anisotropy inherent to the liquid-crystalline state are combined.
- Nematic liquid crystals
Liquid crystals in which the long axes of the molecules align, on average, in one direction preferentially. This direction is defined as the director n (see Box 1).
- Cholesteric liquid crystals
Liquid crystals in which the molecules are organized into a helix.
- Photothermal effect
The production of heat by the dissipation of energy, when light is absorbed by molecules in solution or by a molecular material. When coupled to anisotropic media, the increase of temperature can result in the generation of an anisotropic, mechanical strain.
- Planar alignment
The situation in which the long axes of the liquid-crystal molecules align parallel to the interface.
- Homeotropic alignment
The situation in which the long axes of the liquid-crystal molecules align perpendicularly to the interface.
- Supramolecular polymers
Polymers whose monomeric units hold together via highly directional and reversible, non-covalent interactions, including hydrogen bonding, π–π interaction, metal coordination and host–guest interaction.
- Out of equilibrium
Description for processes that occur under a constant input of energy and, thus, remain away from thermodynamic equilibrium.
- Negative feedback loop
A regulation mechanism by which the increase in the concentration of a product inhibits its own production.