Motor proteins are nature's solution for directing movement at the molecular level. The field of artificial molecular motors takes inspiration from these tiny but powerful machines. Although directional motion on the nanoscale performed by synthetic molecular machines is a relatively new development, significant advances have been made. In this review an overview is given of the principal designs of artificial molecular motors and their modes of operation. Although synthetic molecular motors have also found widespread application as (multistate) switches, we focus on the control of directional movement, both at the molecular scale and at larger magnitudes. We identify some key challenges remaining in the field.

Modern day factory assembly lines often feature robots that pick up, reposition and connect components in a programmed manner. The idea of manipulating molecular fragments in a similar way has to date only been explored using biological building blocks (DNA). Here we report on a wholly artificial small-molecule robotic arm capable of selectively transporting a molecular cargo in either direction between two spatially distinct, chemically similar, sites on a molecular platform. The arm picks up/releases a 3-mercaptopropanehydrazide cargo by formation/breakage of a disulfide bond, while dynamic hydrazone chemistry controls the cargo binding to the platform. Transport is controlled by selectively inducing conformational and configurational changes within an embedded hydrazone rotary switch that steers the robotic arm. In a three-stage operation 79-85 % of 3-mercaptopropanehydrazide molecules are transported in either (chosen) direction between the two platform sites, without the cargo at any time fully dissociating from the machine nor exchanging with other molecules in the bulk.The mechanical manipulation of matter at atomic length-scales has fascinated scientists since it was proposed by Feynman in 'There's Plenty of Room at the Bottom'.

It has been convincingly argued that molecular machines that manipulate individual atoms, or highly reactive clusters of atoms, with Ångström precision are unlikely to be realized. However, biological molecular machines routinely position rather less reactive substrates in order to direct chemical reaction sequences, from sequence-specific synthesis by the ribosome to polyketide synthases, where tethered molecules are passed from active site to active site in multi-enzyme complexes. Artificial molecular machines have been developed for tasks that include sequence-specific oligomer synthesis and the switching of product chirality, a photo-responsive host molecule has been described that is able to mechanically twist a bound molecular guest, and molecular fragments have been selectively transported in either direction between sites on a molecular platform through a ratchet mechanism. Here we detail an artificial molecular machine that moves a substrate between different activating sites to achieve different product outcomes from chemical synthesis. This molecular robot can be programmed to stereoselectively produce, in a sequential one-pot operation, an excess of any one of four possible diastereoisomers from the addition of a thiol and an alkene to an α,β-unsaturated aldehyde in a tandem reaction process. The stereodivergent synthesis includes diastereoisomers that cannot be selectively synthesized through conventional iminium-enamine organocatalysis. We anticipate that future generations of programmable molecular machines may have significant roles in chemical synthesis and molecular manufacturing.

The sorption of species from solution into and onto solids, surfaces, crystals, gels and other matrices, underpins the sequestering of waste and pollutants, the recovery of precious metals, heterogeneous catalysis, many forms of chemical and biological analysis and separation science, and numerous other technologies. [1][2][3] In such cases the transfer of the substrate between phases tends to proceed spontaneously, in the direction of equilibrium. Molecular ratchet mechanisms, 4-7 where kinetic gating 8-12 selectively inhibits or accelerates particular steps in a process, makes it possible to drive dynamic systems 13-16 out of equilibrium [17][18][19][20][21][22] . Here we report on a small-molecule pump 23 immobilised on and near the surface 24-26 of polymer beads, that uses an energy ratchet mechanism [18][19][20][21][22][27][28][29][30][31] to actively transport substrates from solution onto the beads away from equilibrium. One complete cycle of the pump occurs with each pulse of a chemical fuel, 20,32-36 synchronizing the ratchet dynamics so that the immobilised molecular machines all act in unison. Upon addition of the trichloroacetic acid (CCl3CO2H) fuel, 20,35,37,38 micrometre-diameter polystyrene beads functionalised with an average of ~8×10 10 molecular pumps per bead, 39 sequester from solution crown ethers appended with a fluorescent tag (Fig. 1). Following consumption of the fuel, the rings are mechanically trapped in a higher energy, out-of-equilibrium, state on the beads and cannot be removed by dilution nor by switching the binding interactions off. This differs from dissipative assembled materials that require a continuous supply of energy to persist. [13][14][15][16] Addition of a second pulse of fuel causes the uptake of more macrocycles, which can be labelled with a different fluorescent tag. This drives the system progressively further away from equilibrium and also confers sequence information 40 on the deposited structure. The polymer-bound substrates (and the stored energy) can subsequently be released back to the bulk on demand, 41-43 either emptying one compartment at a time or all at once. Non-equilibrium 44 sorption by using immobilised artificial molecular machines 45-51 to pump substrates from solution onto and into materials 52 , offers potential 53,54 for the transduction of energy from chemical fuels for the storage and release of energy and information.

Melanin and related polyphenolic pigments are versatile functional polymers that serve diverse aesthetic and protective roles across the living world. These polymeric pigments continue to inspire the development of adhesive, photonic, electronic and radiation-protective materials and coatings. The properties of these structures are dictated by covalent and non-covalent interactions in ways that, despite progress, are not fully understood. It remains a major challenge to direct oxidative polymerization of their precursors (amino acids, (poly-)phenols, thiols) toward specific structures. By taking advantage of supramolecular pre-organization of tyrosine-tripeptides and reactive sequestering of selected amino acids during enzymatic oxidation, we demonstrate the spontaneous formation of distinct new chromophores with optical properties that are far beyond the range of those found in biological melanins, in terms of color, UV absorbance and fluorescent emission.