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A molecular machine, nanite, or nanomachine^[1] is a molecular component that produces quasi-mechanical movements (output) in response to specific stimuli (input).^[2]^[3] In cellular biology, macromolecular machines frequently perform tasks essential for life, such as DNA replication and ATP synthesis. The expression is often more generally applied to molecules that simply mimic functions that occur at the macroscopic level. The term is also common in nanotechnology where a number of highly complex molecular machines have been proposed that are aimed at the goal of constructing a molecular assembler.^[4]^[5]

For the last several decades, chemists and physicists alike have attempted, with varying degrees of success, to miniaturize machines found in the macroscopic world. Molecular machines are at the forefront of cellular biology research. The 2016 Nobel Prize in Chemistry was awarded to Jean-Pierre Sauvage, Sir J. Fraser Stoddart, and Bernard L. Feringa for the design and synthesis of molecular machines.^[6]^[7]

History

What would be the utility of such machines? Who knows? I cannot see exactly what would happen, but I can hardly doubt that when we have some control of the arrangement of things on a molecular scale we will get an enormously greater range of possible properties that substances can have, and of the different things we can do.
— Richard Feynman, There's Plenty of Room at the Bottom^[8]

Biological molecular machines have been known and studied for years given their vital role in sustaining life, and have served as inspiration for synthetically designed systems with similar useful functionality.^[9]^[10] The advent of conformational analysis, or the study of conformers to analyze complex chemical structures, in the 1950s gave rise to the idea of understanding and controlling relative motion within molecular components for further applications. This led to the design of "proto-molecular machines" featuring conformational changes such as cog-wheeling of the aromatic rings in triptycenes.^[11] By 1980, scientists could achieve desired conformations using external stimuli and utilize this for different applications. A major example is the design of a photoresponsive crown ether containing an azobenzene unit, which could switch between cis and trans isomers on exposure to light and hence tune the cation-binding properties of the ether.^[12] In his seminal 1959 lecture There's Plenty of Room at the Bottom, Richard Feynman alluded to the idea and applications of molecular devices designed artificially by manipulating matter at the atomic level.^[8] This was further substantiated by Eric Drexler during the 1970s, who developed ideas based on molecular nanotechnology such as nanoscale "assemblers",^[13] though their feasibility was disputed.^[14]

Though these events served as inspiration for the field, the actual breakthrough in practical approaches to synthesize artificial molecular machines (AMMs) took place in 1991 with the invention of a "molecular shuttle" by Sir Fraser Stoddart.^[15] Building upon the assembly of mechanically linked molecules such as catenanes and rotaxanes as developed by Jean-Pierre Sauvage in the early 1980s,^[16]^[17] this shuttle features a rotaxane with a ring that can move across an "axle" between two ends or possible binding sites (hydroquinone units). This design realized the well-defined motion of a molecular unit across the length of the molecule for the first time.^[11] In 1994, an improved design allowed control over the motion of the ring by pH variation or electrochemical methods, making it the first example of an AMM. Here the two binding sites are a benzidine and a biphenol unit; the cationic ring typically prefers staying over the benzidine ring, but moves over to the biphenol group when the benzidine gets protonated at low pH or if it gets electrochemically oxidized.^[18] In 1998, a study could capture the rotary motion of a decacyclene molecule on a copper-base metallic surface using a scanning tunneling microscope.^[19] Over the following decade, a broad variety of AMMs responding to various stimuli were invented for different applications.^[20]^[21] In 2016, the Nobel Prize in Chemistry was awarded to Sauvage, Stoddart, and Bernard L. Feringa for the design and synthesis of molecular machines.^[6]^[7]

Artificial molecular machines

Over the past few decades, AMMs have diversified rapidly and their design principles,^[22] properties,^[23] and characterization methods^[24] have been outlined more clearly. A major starting point for the design of AMMs is to exploit the existing modes of motion in molecules.^[22] For instance, single bonds can be visualized as axes of rotation,^[25] as can be metallocene complexes.^[26] Bending or V-like shapes can be achieved by incorporating double bonds, that can undergo cis-trans isomerization in response to certain stimuli (typically irradiation with a suitable wavelength), as seen in numerous designs consisting of stilbene and azobenzene units.^[27] Similarly, ring-opening and -closing reactions such as those seen for spiropyran and diarylethene can also produce curved shapes.^[28] Another common mode of movement is the circumrotation of rings relative to one another as observed in mechanically interlocked molecules (primarily catenanes). While this type of rotation can not be accessed beyond the molecule itself (because the rings are confined within one another), rotaxanes can overcome this as the rings can undergo translational movements along a dumbbell-like axis.^[29] Another line of AMMs consists of biomolecules such as DNA and proteins as part of their design, making use of phenomena like protein folding and unfolding.^[30]^[31]

AMM designs have diversified significantly since the early days of the field. A major route is the introduction of bistability to produce molecular switches, featuring two distinct configurations for the molecule to convert between. This has been perceived as a step forward from the original molecular shuttle which consisted of two identical sites for the ring to move between without any preference, in a manner analogous to the ring flip in an unsubstituted cyclohexane. If these two sites are different from each other in terms of features like electron density, this can give rise to weak or strong recognition sites as in biological systems — such AMMs have found applications in catalysis and drug delivery. This switching behavior has been further optimized to acquire useful work that gets lost when a typical switch returns to its original state. Inspired by the use of kinetic control to produce work in natural processes, molecular motors are designed to have a continuous energy influx to keep them away from equilibrium to deliver work.^[22]^[32]

Various energy sources are employed to drive molecular machines today, but this was not the case during the early years of AMM development. Though the movements in AMMs were regulated relative to the random thermal motion generally seen in molecules, they could not be controlled or manipulated as desired. This led to the addition of stimuli-responsive moieties in AMM design, so that externally applied non-thermal sources of energy could drive molecular motion and hence allow control over the properties. Chemical energy (or "chemical fuels") was an attractive option at the beginning, given the broad array of reversible chemical reactions (heavily based on acid-base chemistry) to switch molecules between different states.^[33] However, this comes with the issue of practically regulating the delivery of the chemical fuel and the removal of waste generated to maintain the efficiency of the machine as in biological systems. Though some AMMs have found ways to circumvent this,^[34] more recently waste-free reactions such based on electron transfers or isomerization have gained attention (such as redox-responsive viologens). Eventually, several different forms of energy (electric,^[35] magnetic,^[36] optical^[37] and so on) have become the primary energy sources used to power AMMs, even producing autonomous systems such as light-driven motors.^[38]

Types

Various AMMs have been designed with a broad range of functions and applications, several of which have been tabulated below along with indicative images:^[23]

Type	Details	Image
Molecular balance	A molecule that can interconvert between two or more conformational or configurational states in response to the dynamic of multiple intra- and intermolecular driving forces,^[39]^[40] such as hydrogen bonding, solvophobic or hydrophobic effects,^[41] π interactions,^[42] and steric and dispersion interactions.^[43] The distinct conformers of a molecular balance can show different interactions with the same molecule, such that analyzing the ratio of the conformers and the energies for these interactions can enable quantification of different properties (such as CH-π or arene-arene interactions, see image).^[44]^[45]
Molecular hinge	A molecular hinge is a molecule that can typically rotate in a crank-like motion around a rigid axis, such as a double bond or aromatic ring, to switch between reversible configurations.^[46] Such configurations must have distinguishable geometries; for instance, azobenzene groups in a linear molecule may undergo cis-trans isomerization^[47] when irradiated with ultraviolet light, triggering a reversible transition to a bent or V-shaped conformation (see image).^[48]^[49]^[50] Molecular hinges have been adapted for applications such as nucleobase recognition,^[51] peptide modifications,^[52] and visualizing molecular motion.^[53]
Molecular logic gate	A molecule that performs a logical operation on one or more logic inputs and produces a single logic output.^[54] Modelled on logic gates, these molecules have slowly replaced the conventional silicon-based machinery. Several applications have come forth, such as water quality examination, food safety examination, metal ion detection, and pharmaceutical studies.^[55]^[56] The first example of a molecular logic gate was reported in 1993, featuring a receptor (see image) where the emission intensity could be treated as a tunable output if the concentrations of protons and sodium ions were to be considered as inputs.^[57]
Molecular motor	A molecule that is capable of directional rotary motion around a single or double bond and produce useful work as a result (as depicted in the image).^[58]^[59] Carbon nanotube nanomotors have also been produced.^[60] Single bond rotary motors^[61] are generally activated by chemical reactions whereas double bond rotary motors^[62] are generally fueled by light. The rotation speed of the motor can also be tuned by careful molecular design.^[63] Carbon nanotube nanomotors have also been produced.^[64]
Molecular necklace	A class of mechanically interlocked molecules derived from catenanes where a large macrocycle backbone connects at least three small rings in the shape of a necklace (see image for example). A molecular necklace consisting of a large macrocycle threaded by n-1 rings (hence comprising n rings) is represented as [n]MN.^[65] The first molecular necklace was synthesized in 1992, featuring several α-cyclodextrins on a single polyethylene glycol chain backbone; the authors connected this to the idea of a "molecular abacus" proposed by Stoddart and coworkers around the same time.^[66] Several interesting applications have emerged for these molecules, such as antibacterial activity,^[67] desulfurization of fuels,^[68] and piezoelectricity.^[69]
Molecular propeller	A molecule that can propel fluids when rotated, due to its special shape that is designed in analogy to macroscopic propellers (see schematic image on right). It has several molecular-scale blades attached at a certain pitch angle around the circumference of a nanoscale shaft.^[70]^[71] Propellers have been shown to have interesting properties, such as variations in pumping rates for hydrophilic and hydrophobic fluids.^[72]
Molecular shuttle	A molecule capable of shuttling molecules or ions from one location to another. This is schematically depicted in the image on the right, where a ring (in green) can bind to either one of the yellow sites on the blue macrocyclic backbone.^[73] A common molecular shuttle consists of a rotaxane where the macrocycle can move between two sites or stations along the dumbbell backbone; controlling the properties of either site and by regulating conditions like pH can enable control over which site is selected for binding. This has led to novel applications in catalysis and drug delivery.^[73]^[74]
Molecular switch	A molecule that can be reversibly shifted between two or more stable states in response to certain stimuli. This change of states influences the properties of the molecule according to the state it occupies at the moment. Unlike a molecular motor, any mechanical work done due to the motion in a switch is generally undone once the molecule returns to its original state unless it is part of a larger motor-like system. The image on the right shows a hydrazone-based switch that switches in response to pH changes.^[75]
Molecular tweezers	Host molecules capable of holding items between their two arms.^[76] The open cavity of the molecular tweezers binds items using non-covalent bonding including hydrogen bonding, metal coordination, hydrophobic forces, van der Waals forces, π interactions, or electrostatic effects.^[77] For instance, the image on the right depicts tweezers formed by corannulene pincers clasping a C60 fullerene molecule, termed "buckycatcher".^[78] Examples of molecular tweezers have been reported that are constructed from DNA and are considered DNA machines.^[79]
Nanocar	Single-molecule vehicles that resemble macroscopic automobiles and are important for understanding how to control molecular diffusion on surfaces. The image on the right shows an example with wheels made of fullerene molecules. The first nanocars were synthesized by James M. Tour in 2005. They had an H-shaped chassis and 4 molecular wheels (fullerenes) attached to the four corners.^[80] In 2011, Feringa and co-workers synthesized the first motorized nanocar which had molecular motors attached to the chassis as rotating wheels.^[81] The authors were able to demonstrate directional motion of the nanocar on a copper surface by providing energy from a scanning tunneling microscope tip. Later, in 2017, the world's first-ever nanocar race took place in Toulouse.^[82]

Biological molecular machines

A ribosome performing the elongation and membrane targeting stages of protein translation. The ribosome is green and yellow, the tRNAs are dark blue, and the other proteins involved are light blue. The produced peptide is released into the endoplasmic reticulum.

The most complex macromolecular machines are found within cells, often in the form of multi-protein complexes.^[83] Important examples of biological machines include motor proteins such as myosin, which is responsible for muscle contraction, kinesin, which moves cargo inside cells away from the nucleus along microtubules, and dynein, which moves cargo inside cells towards the nucleus and produces the axonemal beating of motile cilia and flagella. "[I]n effect, the [motile cilium] is a nanomachine composed of perhaps over 600 proteins in molecular complexes, many of which also function independently as nanomachines ... Flexible linkers allow the mobile protein domains connected by them to recruit their binding partners and induce long-range allostery via protein domain dynamics."^[1] Other biological machines are responsible for energy production, for example ATP synthase which harnesses energy from proton gradients across membranes to drive a turbine-like motion used to synthesise ATP, the energy currency of a cell.^[84] Still other machines are responsible for gene expression, including DNA polymerases for replicating DNA, RNA polymerases for producing mRNA, the spliceosome for removing introns, and the ribosome for synthesising proteins. These machines and their nanoscale dynamics are far more complex than any molecular machines that have yet been artificially constructed.^[85]

These biological machines might have applications in nanomedicine. For example,^[86] they could be used to identify and destroy cancer cells.^[87]^[88] Molecular nanotechnology is a speculative subfield of nanotechnology regarding the possibility of engineering molecular assemblers, biological machines which could re-order matter at a molecular or atomic scale. Nanomedicine would make use of these nanorobots, introduced into the body, to repair or detect damages and infections. Molecular nanotechnology is highly theoretical, seeking to anticipate what inventions nanotechnology might yield and to propose an agenda for future inquiry. The proposed elements of molecular nanotechnology, such as molecular assemblers and nanorobots are far beyond current capabilities.^[89]^[90]

Research

The construction of more complex molecular machines is an active area of theoretical and experimental research. A number of molecules, such as molecular propellers, have been designed, although experimental studies of these molecules are inhibited by the lack of methods to construct these molecules.^[91] In this context, theoretical modeling can be extremely useful^[92] to understand the self-assembly/disassembly processes of rotaxanes, important for the construction of light-powered molecular machines.^[93] This molecular-level knowledge may foster the realization of ever more complex, versatile, and effective molecular machines for the areas of nanotechnology, including molecular assemblers.

Although currently not feasible, some potential applications of molecular machines are transport at the molecular level, manipulation of nanostructures and chemical systems, high density solid-state informational processing and molecular prosthetics.^[94] Many fundamental challenges need to be overcome before molecular machines can be used practically such as autonomous operation, complexity of machines, stability in the synthesis of the machines and the working conditions.^[23]

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 Though these events served as inspiration for the field, the actual breakthrough in practical approaches to synthesize artificial molecular machines (AMMs) took place in 1991 with the invention of a "molecular shuttle" by [[Fraser Stoddart|Sir Fraser Stoddart]].<ref name=anelli>{{cite journal |last1=Anelli |first1=P. L. |last2=Spencer |first2=N. |last3=Stoddart |first3=J. F. |title=A molecular shuttle |journal=Journal of the American Chemical Society |date=1991 |volume=113 |issue=13 |pages=5131–5133 |doi=10.1021/ja00013a096}}</ref> Building upon the assembly of mechanically linked molecules such as [[catenane]]s and [[rotaxane]]s as developed by [[Jean-Pierre Sauvage]] in the early 1980s,<ref>{{cite journal |last1=Dietrich-Buchecker |first1=C. O. |last2=Sauvage |first2=J. P. |last3=Kintzinger |first3=J. P. |title=Une nouvelle famille de molecules : les metallo-catenanes |journal=Tetrahedron Letters |date=1983 |volume=24 |issue=46 |pages=5095–5098 |doi=10.1016/S0040-4039(00)94050-4 |trans-title=A new family of molecules: metallo-catenanes |language=French}}</ref><ref>{{cite journal |last1=Dietrich-Buchecker |first1=C. O. |last2=Sauvage |first2=J. P. |last3=Kern |first3=J. M. |title=Templated synthesis of interlocked macrocyclic ligands: the catenands |journal=Journal of the American Chemical Society |date=May 1984 |volume=106 |issue=10 |pages=3043–3045 |doi=10.1021/ja00322a055}}</ref> this shuttle features a rotaxane with a ring that can move across an "axle" between two ends or possible [[binding site]]s ([[hydroquinone]] units). This design realized the well-defined motion of a molecular unit across the length of the molecule for the first time.<ref name=kay2015/> In 1994, an improved design allowed control over the motion of the ring by [[pH]] variation or [[electrochemistry|electrochemical]] methods, making it the first example of an AMM. Here the two binding sites are a [[benzidine]] and a [[biphenol]] unit; the cationic ring typically prefers staying over the benzidine ring, but moves over to the biphenol group when the benzidine gets protonated at low pH or if it gets electrochemically [[Redox|oxidized]].<ref>{{cite journal |last1=Bissell |first1=R. A |last2=Córdova |first2=E. |last3=Kaifer |first3=A. E. |last4=Stoddart |first4=J. F. |title=A chemically and electrochemically switchable molecular shuttle |journal=Nature |date=1994 |volume=369 |issue=6476 |pages=133–137 |doi=10.1038/369133a0}}</ref> In 1998, a study could capture the rotary motion of a decacyclene molecule on a copper-base metallic surface using a [[scanning tunneling microscope]].<ref>{{cite journal |last1=Gimzewski |first1=J. K. |last2=Joachim |first2=C. |last3=Schlittler |first3=R. R. |last4=Langlais |first4=V. |last5=Tang |first5=H. |last6=Johannsen |first6=I. |title=Rotation of a Single Molecule Within a Supramolecular Bearing |journal=Science |date=1998 |volume=281 |issue=5376 |pages=531–533 |doi=10.1126/science.281.5376.531}}</ref> Over the following decade, a broad variety of AMMs responding to various stimuli were invented for different applications.<ref name="balzani2000">{{cite journal |last1=Balzani |first1=V. |last2=Credi |first2=A. |last3=Raymo |first3=F. M. |last4=Stoddart |first4=J. F. |title=Artificial Molecular Machines |journal=Angewandte Chemie International Edition |date=2000 |volume=39 |issue=19 |pages=3348-3391 |doi=10.1002/1521-3773(20001002)39:19<3348::AID-ANIE3348>3.0.CO;2-X}}</ref><ref name="ec2015">{{cite journal |last1=Erbas-Cakmak |first1=S. |last2=Leigh |first2=D. A. |last3=McTernan |first3=C. T. |last4=Nussbaumer |first4=A. L. |title=Artificial Molecular Machines |journal=Chemical Reviews |date=2015 |volume=115 |issue=18 |pages=10081–10206 |doi=10.1021/acs.chemrev.5b00146}}</ref> In 2016, the [[Nobel Prize in Chemistry]] was awarded to Sauvage, Stoddart, and [[Ben Feringa|Bernard L. Feringa]] for the design and synthesis of molecular machines.<ref name="NP-20161005">{{cite news |author=Staff |title=The Nobel Prize in Chemistry 2016 |url=https://www.nobelprize.org/nobel_prizes/chemistry/laureates/2016/press.html |date=5 October 2016 |work=[[Nobel Foundation]] |access-date=5 October 2016 }}</ref><ref name="NYT-20161005">{{cite news |last1=Chang |first1=Kenneth |last2=Chan |first2=Sewell |title=3 Makers of 'World's Smallest Machines' Awarded Nobel Prize in Chemistry |url=https://www.nytimes.com/2016/10/06/science/nobel-prize-chemistry.html |date=5 October 2016 |work=[[New York Times]] |access-date=5 October 2016 }}</ref>
+==Artificial molecular machines==
-== Types ==
-Molecular machines can be divided into two broad categories; artificial and biological. In general, artificial molecular machines (AMMs) refer to molecules that are artificially designed and synthesized whereas biological molecular machines can commonly be found in nature and have evolved into their forms after [[abiogenesis]] on Earth.<ref name="Erbas-Cakmak 2015 10081–10206">{{Cite journal|last1=Erbas-Cakmak|first1=Sundus|last2=Leigh|first2=David A.|last3=McTernan|first3=Charlie T.|last4=Nussbaumer|first4=Alina L.|title=Artificial Molecular Machines|journal=Chemical Reviews|volume=115|issue=18|pages=10081–10206|doi=10.1021/acs.chemrev.5b00146|pmid=26346838|pmc=4585175|year=2015}}</ref>
+Over the past few decades, AMMs have diversified rapidly and their design principles,<ref name=cheng2016/> properties,<ref name="Erbas-Cakmak 2015 10081–10206"/> and [[Characterization (materials science)|characterization]] methods<ref>{{cite journal |last1=Nogales |first1=E. |last2=Grigorieff |first2=N. |title=Molecular Machines: putting the pieces together. |journal=The Journal of cell biology |date=2001 |volume=152 |issue=1 |pages=F1-10 |doi=10.1083/jcb.152.1.f1 |pmid=11149934}}</ref> have been outlined more clearly. A major starting point for the design of AMMs is to exploit the existing modes of motion in molecules.<ref name=cheng2016/> For instance, [[single bond]]s can be visualized as axes of rotation,<ref>{{cite journal |last1=Jiang |first1=X. |last2=Rodríguez-Molina |first2=B. |last3=Nazarian |first3=N. |last4=Garcia-Garibay |first4=M. A. |title=Rotation of a Bulky Triptycene in the Solid State: Toward Engineered Nanoscale Artificial Molecular Machines |journal=Journal of the American Chemical Society |date=2014 |volume=136 |issue=25 |pages=8871–8874 |doi=10.1021/ja503467e}}</ref> as can be [[metallocene]] complexes.<ref>{{cite journal |last1=Kai |first1=H. |last2=Nara |first2=S. |last3=Kinbara |first3=K. |last4=Aida |first4=T. |title=Toward Long-Distance Mechanical Communication: Studies on a Ternary Complex Interconnected by a Bridging Rotary Module |journal=Journal of the American Chemical Society |date=2008 |volume=130 |issue=21 |pages=6725–6727 |doi=10.1021/ja801646b}}</ref> Bending or V-like shapes can be achieved by incorporating [[double bond]]s, that can undergo ''cis-trans'' isomerization in response to certain stimuli (typically irradiation with a suitable [[wavelength]]), as seen in numerous designs consisting of [[stilbene]] and azobenzene units.<ref>{{cite journal |last1=Kamiya |first1=Y. |last2=Asanuma |first2=H. |title=Light-Driven DNA Nanomachine with a Photoresponsive Molecular Engine |journal=Accounts of Chemical Research |date=2014 |volume=47 |issue=6 |pages=1663–1672 |doi=10.1021/ar400308f}}</ref> Similarly, [[Ring opening|ring-opening]] and -closing reactions such as those seen for [[spiropyran]] and [[diarylethene]] can also produce curved shapes.<ref>{{cite journal |last1=Morimoto |first1=M. |last2=Irie |first2=M. |title=A Diarylethene Cocrystal that Converts Light into Mechanical Work |journal=Journal of the American Chemical Society |date=2010 |volume=132 |issue=40 |pages=14172–14178 |doi=10.1021/ja105356w}}</ref> Another common mode of movement is the circumrotation of rings relative to one another as observed in mechanically interlocked molecules (primarily catenanes). While this type of rotation can not be accessed beyond the molecule itself (because the rings are confined within one another), rotaxanes can overcome this as the rings can undergo translational movements along a dumbbell-like axis.<ref>{{cite journal |last1=Stoddart |first1=J. F. |title=The chemistry of the mechanical bond |journal=Chemical Society Reviews |date=2009 |volume=38 |issue=6 |pages=1802 |doi=10.1039/B819333A}}</ref> Another line of AMMs consists of biomolecules such as [[DNA]] and [[protein]]s as part of their design, making use of phenomena like [[protein folding]] and unfolding.<ref>{{cite journal |last1=Mao |first1=X. |last2=Liu |first2=M. |last3=Li |first3=Q. |last4=Fan |first4=C. |last5=Zuo |first5=X. |title=DNA-Based Molecular Machines |journal=JACS Au |date=2022 |volume=2 |issue=11 |pages=2381–2399 |doi=10.1021/jacsau.2c00292}}</ref><ref>{{cite journal |last1=Saper |first1=G. |last2=Hess |first2=H. |title=Synthetic Systems Powered by Biological Molecular Motors |journal=Chemical Reviews |date=2020 |volume=120 |issue=1 |pages=288–309 |doi=10.1021/acs.chemrev.9b00249}}</ref>
-===Artificial===
+[[File:Molecular machine principle 1.png|thumb|center|upright=2.0|alt=Some common types of motion seen in some simple components of artificial molecular machines. a) Rotation around single bonds and in sandwich-like [[metallocene]]s. b) Bending due to ''cis-trans'' isomerization. c) Translational motion of a ring along the dumbbell-like rotaxane axis. d) Rotation of interlocked rings in a catenane|Some common types of motion seen in some simple components of artificial molecular machines. a) Rotation around single bonds and in sandwich-like [[metallocene]]s. b) Bending due to ''[[Cis–trans isomerism|cis-trans]]'' isomerization. c) Translational motion of a ring (blue) between two possible binding sites (red) along the dumbbell-like rotaxane axis (purple). d) Rotation of interlocked rings (depicted as blue and red rectangles) in a catenane.]]
-A wide variety of artificial molecular machines (AMMs) have been synthesized by [[chemists]] which are rather simple and small compared to biological molecular machines.<ref name="Erbas-Cakmak 2015 10081–10206" /> The first AMM, a [[molecular shuttle]], was synthesized by [[Fraser Stoddart|Sir J. Fraser Stoddart]].<ref name="10.1021/ja00013a096">{{cite journal |last1=Anelli |first1=Pier Lucio |last2=Spencer |first2=Neil |last3=Stoddart |first3=J. Fraser |title=A molecular shuttle |journal=Journal of the American Chemical Society |date=June 1991 |volume=113 |issue=13 |pages=5131–5133 |doi=10.1021/ja00013a096|pmid=27715028 |doi-access=free }}</ref>
-A [[molecular shuttle]] is a [[rotaxane]] molecule where a ring is mechanically interlocked onto an axle with two bulky stoppers. The ring can move between two binding sites with various stimuli such as light, pH, solvents, and ions.<ref>{{cite journal |last1=Bruns |first1=Carson J. |last2=Stoddart |first2=J. Fraser |title=Rotaxane-Based Molecular Muscles |journal=Accounts of Chemical Research |date=30 May 2014 |volume=47 |issue=7 |pages=2186–2199 |doi=10.1021/ar500138u|pmid=24877992 }}</ref> As the authors of this 1991 ''[[Journal of the American Chemical Society|JACS]]'' paper noted: "Insofar as it becomes possible to control the movement of one molecular component with respect to the other in a [2]rotaxane, the technology for building molecular machines will emerge", [[mechanically interlocked molecular architectures]] spearheaded AMM design and synthesis as they provide directed molecular motion.<ref>{{cite journal |last1=Kay |first1=Euan R. |last2=Leigh |first2=David A. |title=Rise of the Molecular Machines |journal=Angewandte Chemie International Edition |date=24 August 2015 |volume=54 |issue=35 |pages=10080–10088 |doi=10.1002/anie.201503375|pmid=26219251 |pmc=4557038 }}</ref> Today a wide variety of AMMs exists as listed below.
+AMM designs have diversified significantly since the early days of the field. A major route is the introduction of [[bistability]] to produce molecular switches, featuring two distinct configurations for the molecule to convert between. This has been perceived as a step forward from the original molecular shuttle which consisted of two identical sites for the ring to move between without any preference, in a manner analogous to the [[ring flip]] in an unsubstituted [[cyclohexane]]. If these two sites are different from each other in terms of features like [[electron density]], this can give rise to weak or strong recognition sites as in biological systems — such AMMs have found applications in [[catalysis]] and [[drug delivery]]. This switching behavior has been further optimized to acquire useful work that gets lost when a typical switch returns to its original state.
-[[File:Overcrowded alkane molecular motor.png|thumb|Overcrowded alkane molecular motor.]]
+Inspired by the use of [[Thermodynamic versus kinetic reaction control|kinetic control]] to produce work in natural processes, molecular motors are designed to have a continuous energy influx to keep them away from [[Thermodynamic equilibrium|equilibrium]] to deliver work.<ref name=cheng2016/><ref name=vincenzo2000/>
+Various energy sources are employed to drive molecular machines today, but this was not the case during the early years of AMM development. Though the movements in AMMs were regulated relative to the random thermal motion generally seen in molecules, they could not be controlled or manipulated as desired. This led to the addition of stimuli-responsive moieties in AMM design, so that externally applied non-thermal sources of energy could drive molecular motion and hence allow control over the properties. Chemical energy (or "chemical fuels") was an attractive option at the beginning, given the broad array of [[Reversible process (thermodynamics)|reversible]] chemical reactions (heavily based on [[Acid–base reaction|acid-base chemistry]]) to switch molecules between different states.<ref>{{cite journal |last1=Biagini |first1=C. |last2=Di Stefano |first2=S. |title=Abiotic Chemical Fuels for the Operation of Molecular Machines |journal=Angewandte Chemie International Edition |date=2020 |volume=59 |issue=22 |pages=8344–8354 |doi=10.1002/anie.201912659}}</ref> However, this comes with the issue of practically regulating the delivery of the chemical fuel and the removal of waste generated to maintain the efficiency of the machine as in biological systems. Though some AMMs have found ways to circumvent this,<ref>{{cite journal |last1=Tatum |first1=L. A. |last2=Foy |first2=J. T. |last3=Aprahamian |first3=I. |title=Waste Management of Chemically Activated Switches: Using a Photoacid To Eliminate Accumulation of Side Products |journal=Journal of the American Chemical Society |date=2014 |volume=136 |issue=50 |pages=17438–17441 |doi=10.1021/ja511135k}}</ref> more recently waste-free reactions such based on [[electron transfer]]s or isomerization have gained attention (such as redox-responsive [[viologen]]s). Eventually, several different forms of energy (electric,<ref>{{cite journal |last1=Le Poul |first1=N. |last2=Colasson |first2=B. |title=Electrochemically and Chemically Induced Redox Processes in Molecular Machines |journal=ChemElectroChem |date=2015 |volume=2 |issue=4 |pages=475–496 |doi=10.1002/celc.201402399}}</ref> magnetic,<ref>{{cite journal |last1=Thomas |first1=C. R. |last2=Ferris |first2=D. P. |last3=Lee |first3=J.-H. |last4=Choi |first4=E. |last5=Cho |first5=M. H. |last6=Kim |first6=E. S. |last7=Stoddart |first7=J. F. |last8=Shin |first8=J.-S. |last9=Cheon |first9=J. |last10=Zink |first10=J. I. |title=Noninvasive Remote-Controlled Release of Drug Molecules in Vitro Using Magnetic Actuation of Mechanized Nanoparticles |journal=Journal of the American Chemical Society |date=2010 |volume=132 |issue=31 |pages=10623–10625 |doi=10.1021/ja1022267}}</ref> optical<ref>{{cite journal |last1=Balzani |first1=V. |last2=Credi |first2=A. |last3=Venturi |first3=M. |title=Light powered molecular machines |journal=Chemical Society Reviews |date=2009 |volume=38 |issue=6 |pages=1542 |doi=10.1039/B806328C}}</ref> and so on) have become the primary energy sources used to power AMMs, even producing autonomous systems such as light-driven motors.<ref>{{cite journal |last1=Balzani |first1=V. |last2=Clemente-León |first2=M. |last3=Credi |first3=A. |last4=Ferrer |first4=B. |last5=Venturi |first5=M. |last6=Flood |first6=A. H. |last7=Stoddart |first7=J. F. |title=Autonomous artificial nanomotor powered by sunlight |journal=Proceedings of the National Academy of Sciences |date=2006 |volume=103 |issue=5 |pages=1178–1183 |doi=10.1073/pnas.0509011103}}</ref>
-====Molecular motors====
-[[Synthetic molecular motor|Molecular motors]] are molecules that are capable of directional rotary motion around a single or double bond.<ref>{{Cite journal |last1=Fletcher|first1=Stephen P.|last2=Dumur|first2=Frédéric|last3=Pollard|first3=Michael M.|last4=Feringa|first4=Ben L.|date=2005-10-07|title=A Reversible, Unidirectional Molecular Rotary Motor Driven by Chemical Energy |journal=Science |volume=310|issue=5745|pages=80–82 |doi=10.1126/science.1117090|issn=0036-8075 |pmid=16210531 |bibcode=2005Sci...310...80F |s2cid=28174183 |url=https://www.rug.nl/research/portal/en/publications/a-reversible-unidirectional-molecular-rotary-motor-driven-by-chemical-energy(50a4c59b-e2fd-413b-a58f-bd37494432e9).html|hdl=11370/50a4c59b-e2fd-413b-a58f-bd37494432e9|hdl-access=free}}</ref><ref>{{Cite journal |last1=Perera|first1=U. G. E.|last2=Ample|first2=F.|last3=Kersell|first3=H.|last4=Zhang|first4=Y.|last5=Vives|first5=G.|last6=Echeverria|first6=J.|last7=Grisolia|first7=M.|last8=Rapenne|first8=G.|last9=Joachim|first9=C.|date=January 2013|title=Controlled clockwise and anticlockwise rotational switching of a&nbsp;molecular motor|journal=Nature Nanotechnology|volume=8|issue=1|pages=46–51|doi=10.1038/nnano.2012.218|pmid=23263725|issn=1748-3395|bibcode=2013NatNa...8...46P}}</ref><ref>{{Cite journal|last1=Schliwa|first1=Manfred|last2=Woehlke|first2=Günther|date=2003-04-17|title=Molecular motors|journal=Nature|volume=422|issue=6933|pages=759–765|doi=10.1038/nature01601|pmid=12700770|bibcode=2003Natur.422..759S|s2cid=4418203}}</ref><ref>{{Cite journal|last1=van Delden|first1=Richard A.|last2=Wiel|first2=Matthijs K. J. ter|last3=Pollard|first3=Michael M.|last4=Vicario|first4=Javier|last5=Koumura|first5=Nagatoshi|last6=Feringa|first6=Ben L.|date=October 2005|title=Unidirectional molecular motor on a gold surface|journal=Nature|volume=437|issue=7063|pages=1337–1340|doi=10.1038/nature04127|pmid=16251960|issn=1476-4687|bibcode=2005Natur.437.1337V|s2cid=4416787|url=https://pure.rug.nl/ws/files/10188130/2005NaturevDelden.pdf}}</ref> Single bond rotary motors<ref>{{cite journal |last1=Kelly |first1=T. Ross |last2=De Silva |first2=Harshani |last3=Silva |first3=Richard A. |title=Unidirectional rotary motion in a molecular system |journal=Nature |date=9 September 1999 |volume=401 |issue=6749 |pages=150–152 |doi=10.1038/43639|pmid=10490021 |bibcode=1999Natur.401..150K |s2cid=4351615 }}</ref> are generally activated by chemical reactions whereas double bond rotary motors<ref>{{cite journal |last1=Koumura |first1=Nagatoshi |last2=Zijlstra |first2=Robert W. J. |last3=van Delden |first3=Richard A. |last4=Harada |first4=Nobuyuki |last5=Feringa |first5=Ben L. |title=Light-driven monodirectional molecular rotor |journal=Nature |date=9 September 1999 |volume=401 |issue=6749 |pages=152–155 |doi=10.1038/43646 |pmid=10490022 |url=https://pure.rug.nl/ws/files/3616669/1999NatureKoumura.pdf |bibcode=1999Natur.401..152K |s2cid=4412610 |hdl=11370/d8399fe7-11be-4282-8cd0-7c0adf42c96f |hdl-access=free }}</ref> are generally fueled by light. The rotation speed of the motor can also be tuned by careful molecular design.<ref>{{cite journal |last1=Vicario |first1=Javier |last2=Meetsma |first2=Auke |last3=Feringa |first3=Ben L. |title=Controlling the speed of rotation in molecular motors. Dramatic acceleration of the rotary motion by structural modification |journal=Chemical Communications |volume=116 |date=2005 |issue=47 |pages=5910–2 |doi=10.1039/B507264F|pmid=16317472 |url=https://www.rug.nl/research/portal/en/publications/controlling-the-speed-of-rotation-in-molecular-motors-dramatic-acceleration-of-the-rotary-motion-by-structural-modification(002a32ff-d6bf-4078-a546-c2a1ace86aa2).html }}</ref> [[Carbon nanotube nanomotor]]s have also been produced.<ref>{{cite journal |last1=Fennimore |first1=A. M. |last2=Yuzvinsky |first2=T. D. |last3=Han |first3=Wei-Qiang |last4=Fuhrer |first4=M. S. |last5=Cumings |first5=J. |last6=Zettl |first6=A. |title=Rotational actuators based on carbon nanotubes |journal=Nature |date=24 July 2003 |volume=424 |issue=6947 |pages=408–410 |doi=10.1038/nature01823|pmid=12879064 |bibcode=2003Natur.424..408F |s2cid=2200106 }}</ref>
-====Molecular propeller====
+=== Types ===
+Various AMMs have been designed with a broad range of functions and applications, several of which have been tabulated below along with indicative images:<ref name="Erbas-Cakmak 2015 10081–10206">{{Cite journal|last1=Erbas-Cakmak|first1=Sundus|last2=Leigh|first2=David A.|last3=McTernan|first3=Charlie T.|last4=Nussbaumer|first4=Alina L.|title=Artificial Molecular Machines|journal=Chemical Reviews|volume=115|issue=18|pages=10081–10206|doi=10.1021/acs.chemrev.5b00146|pmid=26346838|pmc=4585175|year=2015}}</ref>
-A [[molecular propeller]] is a molecule that can propel fluids when rotated, due to its special shape that is designed in analogy to macroscopic propellers.<ref>{{cite journal |last1=Simpson |first1=Christopher D. |last2=Mattersteig |first2=Gunter |last3=Martin |first3=Kai |last4=Gherghel |first4=Lileta |last5=Bauer |first5=Roland E. |last6=Räder |first6=Hans Joachim |last7=Müllen |first7=Klaus |title=Nanosized Molecular Propellers by Cyclodehydrogenation of Polyphenylene Dendrimers |journal=Journal of the American Chemical Society |date=March 2004 |volume=126 |issue=10 |pages=3139–3147 |doi=10.1021/ja036732j |pmid=15012144 }}</ref><ref>{{cite journal |doi=10.1103/PhysRevLett.98.266102|pmid=17678108|title=Chemically Tunable Nanoscale Propellers of Liquids|journal=Physical Review Letters|volume=98|issue=26|pages=266102|year=2007|last1=Wang|first1=Boyang|last2=Král|first2=Petr|bibcode=2007PhRvL..98z6102W}}</ref> It has several molecular-scale blades attached at a certain pitch angle around the circumference of a nanoscale shaft. Also see [[molecular gyroscope]].
+{| class="wikitable"
-[[File:Daisy chain rotaxane.png|thumb|Daisy chain [2]rotaxane. These molecules are considered as building blocks for artificial muscle.]]
+|-
+! Type !! Details !! Image
+|- style="vertical-align: top;"
-====Molecular switch====
+|Molecular balance
-A [[molecular switch]] is a molecule that can be reversibly shifted between two or more stable states.<ref name="10.1021/cr9900228">{{cite journal |last1=Feringa |first1=Ben L. |last2=van Delden |first2=Richard A. |last3=Koumura |first3=Nagatoshi |last4=Geertsema |first4=Edzard M. |title=Chiroptical Molecular Switches |journal=Chemical Reviews |date=May 2000 |volume=100 |issue=5 |pages=1789–1816 |doi=10.1021/cr9900228|pmid=11777421 |s2cid=11740379 |url=https://pure.rug.nl/ws/files/3616486/1996AdvMaterFeringa.pdf }}</ref> The molecules may be shifted between the states in response to changes in pH, light ([[photoswitch]]), temperature, an electric current, microenvironment, or the presence of a ligand.<ref name="10.1021/cr9900228" /><ref>{{cite journal |last1=Knipe |first1=Peter C. |last2=Thompson |first2=Sam |last3=Hamilton |first3=Andrew D. |title=Ion-mediated conformational switches |journal=Chemical Science |date=2015 |volume=6 |issue=3 |pages=1630–1639 |doi=10.1039/C4SC03525A|pmid=28694943 |pmc=5482205 }}</ref><ref name=":0">{{cite journal|title=Hünlich base derivatives as photo-responsive Λ-shaped hinges|journal=Organic Chemistry Frontiers|date=2017|volume=4|issue=2|pages=224–228|doi=10.1039/C6QO00653A|url=http://pubs.rsc.org/-/content/articlehtml/2017/qo/c6qo00653a | last1 = Kazem-Rostami | first1 = Masoud | last2 = Moghanian | first2 = Amirhossein}}</ref>
+|A molecule that can interconvert between two or more conformational or configurational states in response to the dynamic of multiple intra- and intermolecular driving forces,<ref>{{Cite journal|last1=Paliwal|first1=S.|last2=Geib|first2=S.|last3=Wilcox|first3=C. S.|date=1994|title=Molecular Torsion Balance for Weak Molecular Recognition Forces. Effects of "Tilted-T" Edge-to-Face Aromatic Interactions on Conformational Selection and Solid-State Structure|journal=Journal of the American Chemical Society|volume=116|issue=10|pages=4497–4498|doi=10.1021/ja00089a057}}</ref><ref>{{Cite journal|last1=Mati|first1=Ioulia K.|last2=Cockroft|first2=Scott L.|date=2010|title=Molecular balances for quantifying non-covalent interactions|journal=Chemical Society Reviews|volume=39|issue=11|pages=4195–4205|doi=10.1039/B822665M|pmid=20844782|url=https://www.pure.ed.ac.uk/ws/files/10097959/Molecular_balances_for_quantifying_non_covalent_interactions.pdf|hdl=20.500.11820/7ce18ff7-1196-48a1-8c67-3bc3f6b46946|s2cid=263667 |hdl-access=free}}</ref> such as [[hydrogen bonding]], [[solvophobic]] or hydrophobic effects,<ref>{{Cite journal|last1=Y.|first1=Lixu|last2=A.|first2=Catherine|last3=Cockroft|first3=S. L.|date=2015|title=Quantifying Solvophobic Effects in Nonpolar Cohesive Interactions|journal=Journal of the American Chemical Society|volume=137|issue=32|pages=10084–10087|doi=10.1021/jacs.5b05736|pmid=26159869|issn=0002-7863|hdl=20.500.11820/604343eb-04aa-4d90-82d2-0998898400d2|url=https://www.pure.ed.ac.uk/ws/files/24692670/scockroft.docx|hdl-access=free}}</ref>  [[Pi interaction|π interactions]],<ref>{{Cite journal|last1=L.|first1=Ping|last2=Z.|first2=Chen|last3=Smith|first3=M. D.|last4=Shimizu|first4=K. D.|date=2013|title=Comprehensive Experimental Study of N-Heterocyclic π-Stacking Interactions of Neutral and Cationic Pyridines|journal=The Journal of Organic Chemistry|volume=78|issue=11|pages=5303–5313|doi=10.1021/jo400370e|pmid=23675885}}</ref> and steric and dispersion interactions.<ref>{{Cite journal|last1=Hwang|first1=J.|last2=Li|first2=P.|last3=Smith|first3=M. D.|last4=Shimizu|first4=K. D.|date=2016|title=Distance-Dependent Attractive and Repulsive Interactions of Bulky Alkyl Groups|journal=Angewandte Chemie International Edition|volume=55|issue=28|pages=8086–8089|doi=10.1002/anie.201602752|pmid=27159670|doi-access=free}}</ref> The distinct conformers of a molecular balance can show different interactions with the same molecule, such that analyzing the ratio of the conformers and the energies for these interactions can enable quantification of different properties (such as CH-π or arene-arene interactions, see image).<ref>{{cite journal |last1=Carroll |first1=W. R. |last2=Zhao |first2=C. |last3=Smith |first3=M. D. |last4=Pellechia |first4=P. J. |last5=Shimizu |first5=K. D. |title=A Molecular Balance for Measuring Aliphatic CH−π Interactions |journal=Organic Letters |date=2011 |volume=13 |issue=16 |pages=4320–4323 |doi=10.1021/ol201657p}}</ref><ref>{{cite journal |last1=Carroll |first1=W. R. |last2=Pellechia |first2=P. |last3=Shimizu |first3=K. D. |title=A Rigid Molecular Balance for Measuring Face-to-Face Arene−Arene Interactions |journal=Organic Letters |date=2008 |volume=10 |issue=16 |pages=3547–3550 |doi=10.1021/ol801286k}}</ref>
+|[[File:Molecular balance example.png|frameless|alt=An example of a molecular balance]]
+|- style="vertical-align: top;"
-[[File:Molecular switch.png|thumb|Rotaxane based molecular shuttle.]]
+|Molecular hinge
+|A molecular hinge is a molecule that can typically rotate in a [[Crank (mechanism)|crank]]-like motion around a rigid axis, such as a double bond or aromatic ring, to switch between reversible configurations.<ref>{{cite journal |last1=Kassem |first1=Salma |last2=van Leeuwen |first2=Thomas |last3=Lubbe |first3=Anouk S. |last4=Wilson |first4=Miriam R. |last5=Feringa |first5=Ben L. |last6=Leigh |first6=David A. |title=Artificial molecular motors |journal=Chemical Society Reviews |date=2017 |volume=46 |issue=9 |pages=2592–2621 |doi=10.1039/C7CS00245A|pmid=28426052 |url=https://pure.rug.nl/ws/files/49449226/c7cs00245a_1_.pdf }}</ref> Such configurations must have distinguishable geometries; for instance, azobenzene groups in a linear molecule may undergo ''cis''-''trans'' isomerization<ref>{{cite journal |last1=Bandara |first1=H. M. Dhammika |last2=Burdette |first2=S. C. |title=Photoisomerization in different classes of azobenzene |journal=Chemical Society Reviews |date=2012 |volume=41 |issue=5 |pages=1809–1825 |doi=10.1039/c1cs15179g|pmid=22008710 }}</ref> when irradiated with [[Ultraviolet|ultraviolet light]], triggering a reversible transition to a bent or V-shaped conformation (see image).<ref>{{cite journal |last1=Wang |first1=J. |last2=Jiang |first2=Q. |last3=Hao |first3=X. |last4=Yan |first4=H.|last5=Peng |first5=H. |last6=Xiong |first6=B.|last7=Liao |first7=Y. |last8=Xie |first8=X. |title=Reversible photo-responsive gel–sol transitions of robust organogels based on an azobenzene-containing main-chain liquid crystalline polymer |journal=RSC Advances |date=2020 |volume=10 |issue=7 |pages=3726–3733 |doi=10.1039/C9RA10161F|bibcode=2020RSCAd..10.3726W |doi-access=free }}</ref><ref>{{cite journal |last1=Hada |first1=M.|last2=Yamaguchi |first2=D. |last3=Ishikawa |first3=T.|last4=Sawa |first4=T. |last5=Tsuruta |first5=K.|last6=Ishikawa |first6=K. |last7=Koshihara |first7=S.-y. |last8=Hayashi |first8=Y. |last9=Kato |first9=T.|title=Ultrafast isomerization-induced cooperative motions to higher molecular orientation in smectic liquid-crystalline azobenzene molecules |journal=Nature Communications |date=13 September 2019 |volume=10 |issue=1 |pages=4159 |doi=10.1038/s41467-019-12116-6 |pmid=31519876 |pmc=6744564 |bibcode=2019NatCo..10.4159H |language=en |issn=2041-1723|doi-access=free }}</ref><ref>{{cite journal |last1=Garcia-Amorós |first1=J.|last2=Reig |first2=M. |last3=Cuadrado |first3=A. |last4=Ortega |first4=M. |last5=Nonell |first5=S. |last6=Velasco |first6=D. |title=A photoswitchable bis-azo derivative with a high temporal resolution |journal=Chemical Communications |date=2014 |volume=50 |issue=78 |pages=11462–11464 |doi=10.1039/C4CC05331A|pmid=25132052 }}</ref> Molecular hinges have been adapted for applications such as [[nucleobase]] recognition,<ref>{{cite journal |last1=Hamilton |first1=A. D. |last2=Van Engen |first2=D. |title=Induced fit in synthetic receptors: nucleotide base recognition by a molecular hinge |journal=Journal of the American Chemical Society |date=1987 |volume=109 |issue=16 |pages=5035–5036 |doi=10.1021/ja00250a052}}</ref> [[peptide]] modifications,<ref>{{cite journal |last1=Dumy |first1=P. |last2=Keller |first2=M. |last3=Ryan |first3=D. E. |last4=Rohwedder |first4=B. |last5=Wöhr |first5=T. |last6=Mutter |first6=M. |title=Pseudo-Prolines as a Molecular Hinge: Reversible Induction of cis Amide Bonds into Peptide Backbones |journal=Journal of the American Chemical Society |date=1997 |volume=119 |issue=5 |pages=918–925 |doi=10.1021/ja962780a}}</ref> and visualizing molecular motion.<ref>{{cite journal |last1=Ai |first1=Y. |last2=Chan |first2=M. H.-Y. |last3=Chan |first3=A. K.-W. |last4=Ng |first4=M. |last5=Li |first5=Y. |last6=Yam |first6=V. W.-W. |title=A platinum(II) molecular hinge with motions visualized by phosphorescence changes |journal=Proceedings of the National Academy of Sciences |date=2019 |volume=116 |issue=28 |pages=13856–13861 |doi=10.1073/pnas.1908034116}}</ref>
+|[[File:Molecular hinge example.png|frameless|alt=An example of a molecular hinge that can undergo cis-trans isomerization about a double bond]]
+|- style="vertical-align: top;"
-====Molecular shuttle====
-A [[molecular shuttle]] is a molecule capable of shuttling molecules or ions from one location to another.<ref name="10.1038/369133a0">{{cite journal |last1=Bissell |first1=Richard A |last2=Córdova |first2=Emilio |last3=Kaifer |first3=Angel E. |last4=Stoddart |first4=J. Fraser |title=A chemically and electrochemically switchable molecular shuttle |journal=Nature |date=12 May 1994 |volume=369 |issue=6476 |pages=133–137 |doi=10.1038/369133a0|bibcode=1994Natur.369..133B |s2cid=44926804 }}</ref> A common molecular shuttle consists of a rotaxane where the macrocycle can move between two sites or stations along the dumbbell backbone.<ref name="10.1038/369133a0" /><ref name="10.1021/ja00013a096" /><ref>{{Cite journal|last1=Chatterjee|first1=Manashi N.|last2=Kay|first2=Euan R.|last3=Leigh|first3=David A.|date=2006-03-01|title=Beyond Switches: Ratcheting a Particle Energetically Uphill with a Compartmentalized Molecular Machine|journal=Journal of the American Chemical Society|volume=128|issue=12|pages=4058–4073|doi=10.1021/ja057664z|pmid=16551115|issn=0002-7863}}</ref>
+|[[Molecular logic gate]]
-====Nanocar====
+|A molecule that performs a logical operation on one or more logic inputs and produces a single logic output.<ref>{{cite journal |last1=Erbas-Cakmak |first1=S. |last2=Kolemen |first2=S. |last3=Sedgwick |first3=A. C. |last4=Gunnlaugsson |first4=T. |last5=James |first5=T. D. |last6=Yoon |first6=J. |last7=Akkaya |first7=E. U. |title=Molecular logic gates: the past, present and future |journal=Chemical Society Reviews |date=2018 |volume=47 |issue=7 |pages=2228–2248 |doi=10.1039/C7CS00491E}}</ref> Modelled on [[logic gate]]s, these molecules have slowly replaced the conventional silicon-based machinery. Several applications have come forth, such as water quality examination, [[food safety]] examination, metal ion detection, and pharmaceutical studies.<ref>{{cite journal |last1=de Silva |first1=A. P. |title=Molecular Logic Gate Arrays |journal=Chemistry - An Asian Journal |date=2011 |volume=6 |issue=3 |pages=750–766 |doi=10.1002/asia.201000603}}</ref><ref>{{cite journal |last1=Liu |first1=L. |last2=Liu |first2=P. |last3=Ga |first3=L. |last4=Ai |first4=J. |title=Advances in Applications of Molecular Logic Gates |journal=ACS Omega |date=2021 |volume=6 |issue=45 |pages=30189–30204 |doi=10.1021/acsomega.1c02912}}</ref> The first example of a molecular logic gate was reported in 1993, featuring a receptor (see image) where the emission intensity could be treated as a tunable output if the concentrations of protons and sodium ions were to be considered as inputs.<ref>{{cite journal |last1=de Silva |first1=P. A. |last2=Gunaratne |first2=N. H. Q. |last3=McCoy |first3=C. P. |title=A molecular photoionic AND gate based on fluorescent signalling |journal=Nature |date=1993 |volume=364 |issue=6432 |pages=42–44 |doi=10.1038/364042a0}}</ref>
-[[Nanocar]]s are single-molecule vehicles that resemble macroscopic automobiles and are important for understanding how to control molecular diffusion on surfaces. The first nanocars were synthesized by [[James M. Tour]] in 2005. They had an H-shaped chassis and 4 molecular wheels ([[fullerenes]]) attached to the four corners.<ref>{{cite journal |last1=Shirai |first1=Yasuhiro |last2=Osgood |first2=Andrew J. |last3=Zhao |first3=Yuming |last4=Kelly |first4=Kevin F. |last5=Tour |first5=James M. |title=Directional Control in Thermally Driven Single-Molecule Nanocars |journal=Nano Letters |date=November 2005 |volume=5 |issue=11 |pages=2330–2334 |doi=10.1021/nl051915k|pmid=16277478 |bibcode=2005NanoL...5.2330S }}</ref> In 2011, [[Ben Feringa]] and co-workers synthesized the first motorized nanocar which had molecular motors attached to the chassis as rotating wheels.<ref>{{cite journal |last1=Kudernac |first1=Tibor |last2=Ruangsupapichat |first2=Nopporn |last3=Parschau |first3=Manfred |last4=Maciá |first4=Beatriz |last5=Katsonis |first5=Nathalie |last6=Harutyunyan |first6=Syuzanna R. |last7=Ernst |first7=Karl-Heinz |last8=Feringa |first8=Ben L. |title=Electrically driven directional motion of a four-wheeled molecule on a metal surface |journal=Nature |date=10 November 2011 |volume=479 |issue=7372 |pages=208–211 |doi=10.1038/nature10587|pmid=22071765 |bibcode=2011Natur.479..208K |s2cid=6175720 }}</ref> The authors were able to demonstrate directional motion of the nanocar on a copper surface by providing energy from a scanning tunneling microscope tip. Later, in 2017, the world's first ever [[Nanocar Race]] took place in [[Toulouse]].
+|[[File:Molecular logic gate example.png|frameless|alt=The first reported molecular logic gate]]
+|- style="vertical-align: top;"
-====Molecular balance====
+|[[Molecular motor]]
-A molecular balance<ref>{{Cite journal|last1=Paliwal|first1=S.|last2=Geib|first2=S.|last3=Wilcox|first3=C. S.|date=1994-05-01|title=Molecular Torsion Balance for Weak Molecular Recognition Forces. Effects of "Tilted-T" Edge-to-Face Aromatic Interactions on Conformational Selection and Solid-State Structure|journal=Journal of the American Chemical Society|volume=116|issue=10|pages=4497–4498|doi=10.1021/ja00089a057|issn=0002-7863}}</ref><ref>{{Cite journal|last1=Mati|first1=Ioulia K.|last2=Cockroft|first2=Scott L.|date=2010-10-19|title=Molecular balances for quantifying non-covalent interactions|journal=Chemical Society Reviews|volume=39|issue=11|pages=4195–205|doi=10.1039/B822665M|pmid=20844782|issn=1460-4744|url=https://www.pure.ed.ac.uk/ws/files/10097959/Molecular_balances_for_quantifying_non_covalent_interactions.pdf|hdl=20.500.11820/7ce18ff7-1196-48a1-8c67-3bc3f6b46946|s2cid=263667 |hdl-access=free}}</ref> is a molecule that can interconvert between two and more conformational or configurational states in response to the dynamic of multiple intra- and intermolecular driving forces, such as [[hydrogen bonding]], [[solvophobic]]/hydrophobic effects,<ref>{{Cite journal|last1=Yang|first1=Lixu|last2=Adam|first2=Catherine|last3=Cockroft|first3=Scott L.|date=2015-08-19|title=Quantifying Solvophobic Effects in Nonpolar Cohesive Interactions|journal=Journal of the American Chemical Society|volume=137|issue=32|pages=10084–10087|doi=10.1021/jacs.5b05736|pmid=26159869|issn=0002-7863|hdl=20.500.11820/604343eb-04aa-4d90-82d2-0998898400d2|url=https://www.pure.ed.ac.uk/ws/files/24692670/scockroft.docx|hdl-access=free}}</ref>  [[Pi interaction|π interactions]],<ref>{{Cite journal|last1=Li|first1=Ping|last2=Zhao|first2=Chen|last3=Smith|first3=Mark D.|last4=Shimizu|first4=Ken D.|date=2013-06-07|title=Comprehensive Experimental Study of N-Heterocyclic π-Stacking Interactions of Neutral and Cationic Pyridines|journal=The Journal of Organic Chemistry|volume=78|issue=11|pages=5303–5313|doi=10.1021/jo400370e|pmid=23675885|issn=0022-3263}}</ref> and steric and dispersion interactions.<ref>{{Cite journal|last1=Hwang|first1=Jungwun|last2=Li|first2=Ping|last3=Smith|first3=Mark D.|last4=Shimizu|first4=Ken D.|date=2016-07-04|title=Distance-Dependent Attractive and Repulsive Interactions of Bulky Alkyl Groups|journal=Angewandte Chemie International Edition|volume=55|issue=28|pages=8086–8089|doi=10.1002/anie.201602752|pmid=27159670|issn=1521-3773|doi-access=free}}</ref> Molecular balances can be small molecules or macromolecules such as proteins. Cooperatively folded proteins, for example, have been used as molecular balances to measure interaction energies and conformational propensities.<ref>{{Cite journal|last1=Ardejani|first1=Maziar S.|last2=Powers|first2=Evan T.|last3=Kelly|first3=Jeffery W.|date=2017-08-15|title=Using Cooperatively Folded Peptides To Measure Interaction Energies and Conformational Propensities|journal=Accounts of Chemical Research|volume=50|issue=8|pages=1875–1882|doi=10.1021/acs.accounts.7b00195|issn=0001-4842|pmc=5584629|pmid=28723063}}</ref>
+|A molecule that is capable of directional rotary motion around a single or double bond and produce useful work as a result (as depicted in the image).<ref>{{cite journal |last1=Lancia |first1=F. |last2=Ryabchun |first2=A. |last3=Katsonis |first3=N. |title=Life-like motion driven by artificial molecular machines |journal=Nature Reviews Chemistry |date=2019 |volume=3 |issue=9 |pages=536–551 |doi=10.1038/s41570-019-0122-2}}</ref><ref>{{cite journal |last1=Mickler |first1=M. |last2=Schleiff |first2=E. |last3=Hugel |first3=T. |title=From Biological towards Artificial Molecular Motors |journal=ChemPhysChem |date=2008 |volume=9 |issue=11 |pages=1503–1509 |doi=10.1002/cphc.200800216}}</ref> [[Carbon nanotube nanomotor]]s have also been produced.<ref>{{cite journal |last1=Fennimore |first1=A. M. |last2=Yuzvinsky |first2=T. D. |last3=Han |first3=Wei-Qiang |last4=Fuhrer |first4=M. S. |last5=Cumings |first5=J. |last6=Zettl |first6=A. |title=Rotational actuators based on carbon nanotubes |journal=Nature |date=24 July 2003 |volume=424 |issue=6947 |pages=408–410 |doi=10.1038/nature01823|pmid=12879064 |bibcode=2003Natur.424..408F |s2cid=2200106 }}</ref> Single bond rotary motors<ref>{{cite journal |last1=Kelly |first1=T. Ross |last2=De Silva |first2=Harshani |last3=Silva |first3=Richard A. |title=Unidirectional rotary motion in a molecular system |journal=Nature |date=9 September 1999 |volume=401 |issue=6749 |pages=150–152 |doi=10.1038/43639|pmid=10490021 |bibcode=1999Natur.401..150K |s2cid=4351615 }}</ref> are generally activated by chemical reactions whereas double bond rotary motors<ref>{{cite journal |last1=Koumura |first1=Nagatoshi |last2=Zijlstra |first2=Robert W. J. |last3=van Delden |first3=Richard A. |last4=Harada |first4=Nobuyuki |last5=Feringa |first5=Ben L. |title=Light-driven monodirectional molecular rotor |journal=Nature |date=9 September 1999 |volume=401 |issue=6749 |pages=152–155 |doi=10.1038/43646 |pmid=10490022 |url=https://pure.rug.nl/ws/files/3616669/1999NatureKoumura.pdf |bibcode=1999Natur.401..152K |s2cid=4412610 |hdl=11370/d8399fe7-11be-4282-8cd0-7c0adf42c96f |hdl-access=free }}</ref> are generally fueled by light. The rotation speed of the motor can also be tuned by careful molecular design.<ref>{{cite journal |last1=Vicario |first1=Javier |last2=Meetsma |first2=Auke |last3=Feringa |first3=Ben L. |title=Controlling the speed of rotation in molecular motors. Dramatic acceleration of the rotary motion by structural modification |journal=Chemical Communications |volume=116 |date=2005 |issue=47 |pages=5910–2 |doi=10.1039/B507264F|pmid=16317472 |url=https://www.rug.nl/research/portal/en/publications/controlling-the-speed-of-rotation-in-molecular-motors-dramatic-acceleration-of-the-rotary-motion-by-structural-modification(002a32ff-d6bf-4078-a546-c2a1ace86aa2).html }}</ref> [[Carbon nanotube nanomotor]]s have also been produced.<ref>{{cite journal |last1=Fennimore |first1=A. M. |last2=Yuzvinsky |first2=T. D. |last3=Han |first3=Wei-Qiang |last4=Fuhrer |first4=M. S. |last5=Cumings |first5=J. |last6=Zettl |first6=A. |title=Rotational actuators based on carbon nanotubes |journal=Nature |date=24 July 2003 |volume=424 |issue=6947 |pages=408–410 |doi=10.1038/nature01823|pmid=12879064 |bibcode=2003Natur.424..408F |s2cid=2200106 }}</ref>
-====Molecular tweezers====
-[[Molecular tweezers]] are host molecules capable of holding items between their two arms.<ref>{{cite journal |last1=Chen |first1=C. W. |last2=Whitlock |first2=H. W. |title=Molecular tweezers: a simple model of bifunctional intercalation |journal=Journal of the American Chemical Society |date=July 1978 |volume=100 |issue=15 |pages=4921–4922 |doi=10.1021/ja00483a063}}</ref> The open cavity of the molecular tweezers binds items using non-covalent bonding including hydrogen bonding, metal coordination, hydrophobic forces, [[van der Waals force]]s, [[Pi interaction|π interactions]], or electrostatic effects.<ref>{{cite journal |last1=Klärner |first1=Frank-Gerrit |last2=Kahlert |first2=Björn |title=Molecular Tweezers and Clips as Synthetic Receptors. Molecular Recognition and Dynamics in Receptor−Substrate Complexes |journal=Accounts of Chemical Research |date=December 2003 |volume=36 |issue=12 |pages=919–932 |doi=10.1021/ar0200448|pmid=14674783 }}</ref> Examples of molecular tweezers have been reported that are constructed from DNA and are considered [[DNA machine]]s.<ref>{{cite journal |last1=Yurke |first1=Bernard |last2=Turberfield |first2=Andrew J. |last3=Mills |first3=Allen P. |last4=Simmel |first4=Friedrich C. |last5=Neumann |first5=Jennifer L. |title=A DNA-fuelled molecular machine made of DNA |journal=Nature |date=10 August 2000 |volume=406 |issue=6796 |pages=605–608 |doi=10.1038/35020524|pmid=10949296 |bibcode=2000Natur.406..605Y |s2cid=2064216 }}</ref>
+|[[File:MD_rotor_250K_1ns.gif|frameless|alt=Molecular dynamics simulation of a synthetic molecular rotor composed of three molecules in a nanopore (outer diameter 6.7 nm) at 250 K]]
-====Molecular sensor====
-A [[molecular sensor]] is a molecule that interacts with an analyte to produce a detectable change.<ref>{{cite journal |vauthors=Cavalcanti A, Shirinzadeh B, Freitas Jr RA, Hogg T | title = Nanorobot architecture for medical target identification | year = 2008 | journal = [http://wwwARRAYopARRAYrg/EJ/journal/Nano Nanotechnology] | volume = 19 | issue = 1 | pages = 015103(15pp) | doi = 10.1088/0957-4484/19/01/015103 | bibcode=2008Nanot..19a5103C| s2cid = 15557853 }}</ref><ref>{{cite journal |last1=Wu |first1=Di |last2=Sedgwick |first2=Adam C. |last3=Gunnlaugsson |first3=Thorfinnur |last4=Akkaya |first4=Engin U. |last5=Yoon |first5=Juyoung |last6=James |first6=Tony D. |title=Fluorescent chemosensors: the past, present and future |journal=Chemical Society Reviews |date=2017 |volume=46 |issue=23 |pages=7105–7123 |doi=10.1039/C7CS00240H|pmid=29019488 |hdl=11693/38177 |doi-access=free }}</ref> Molecular sensors combine molecular recognition with some form of reporter, so the presence of the item can be observed.
+|- style="vertical-align: top;"
-====Molecular logic gate====
-A [[molecular logic gate]] is a molecule that performs a logical operation on one or more logic inputs and produces a single logic output.<ref>{{cite journal |last1=Prasanna de Silva |first1=A. |last2=McClenaghan |first2=Nathan D. |title=Proof-of-Principle of Molecular-Scale Arithmetic |journal=Journal of the American Chemical Society |date=April 2000 |volume=122 |issue=16 |pages=3965–3966 |doi=10.1021/ja994080m}}</ref><ref>{{cite journal |last1=Magri |first1=David C. |last2=Brown |first2=Gareth J. |last3=McClean |first3=Gareth D. |last4=de Silva |first4=A. Prasanna |title=Communicating Chemical Congregation: A Molecular AND Logic Gate with Three Chemical Inputs as a "Lab-on-a-Molecule" Prototype |journal=Journal of the American Chemical Society |date=April 2006 |volume=128 |issue=15 |pages=4950–4951 |doi=10.1021/ja058295+|pmid=16608318}}</ref> Unlike a molecular sensor, the molecular logic gate will only output when a particular combination of inputs is present.
-====Molecular assembler====
+|Molecular necklace
-A [[molecular assembler]] is a molecular machine able to guide [[chemical reaction]]s by positioning reactive molecules with precision.<ref>{{Cite journal|last1=Lewandowski|first1=Bartosz|last2=De Bo|first2=Guillaume|author-link2=Guillaume De Bo|last3=Ward|first3=John W.|last4=Papmeyer|first4=Marcus|last5=Kuschel|first5=Sonja|last6=Aldegunde|first6=María J.|last7=Gramlich|first7=Philipp M. E.|last8=Heckmann|first8=Dominik|last9=Goldup|first9=Stephen M.|date=2013-01-11|title=Sequence-Specific Peptide Synthesis by an Artificial Small-Molecule Machine|journal=Science|volume=339|issue=6116|pages=189–193|doi=10.1126/science.1229753|issn=0036-8075|pmid=23307739|bibcode=2013Sci...339..189L|s2cid=206544961|url=https://www.research.manchester.ac.uk/portal/en/publications/sequencespecific-peptide-synthesis-by-an-artificial-smallmolecule-machine(dc7b8f29-3915-46cd-93a5-1b2419ee7466).html}}</ref><ref>{{Cite journal|last1=De Bo|first1=Guillaume|author-link1=Guillaume De Bo|last2=Kuschel|first2=Sonja|last3=Leigh|first3=David A.|last4=Lewandowski|first4=Bartosz|last5=Papmeyer|first5=Marcus|last6=Ward|first6=John W.|date=2014-04-16|title=Efficient Assembly of Threaded Molecular Machines for Sequence-Specific Synthesis|journal=Journal of the American Chemical Society|volume=136|issue=15|pages=5811–5814|doi=10.1021/ja5022415|pmid=24678971|issn=0002-7863|doi-access=free}}</ref><ref>{{Cite journal|last1=De Bo|first1=Guillaume|author-link1=Guillaume De Bo|last2=Gall|first2=Malcolm A. Y.|last3=Kitching|first3=Matthew O.|last4=Kuschel|first4=Sonja|last5=Leigh|first5=David A.|last6=Tetlow|first6=Daniel J.|last7=Ward|first7=John W.|date=2017-08-09|title=Sequence-Specific β-Peptide Synthesis by a Rotaxane-Based Molecular Machine|journal=Journal of the American Chemical Society|volume=139|issue=31|pages=10875–10879|doi=10.1021/jacs.7b05850|pmid=28723130|issn=0002-7863|url=http://dro.dur.ac.uk/22931/1/22931.pdf}}</ref><ref>{{Cite journal|last1=Kassem|first1=Salma|last2=Lee|first2=Alan T. L.|last3=Leigh|first3=David A.|last4=Marcos|first4=Vanesa|last5=Palmer|first5=Leoni I.|last6=Pisano|first6=Simone|date=September 2017|title=Stereodivergent synthesis with a programmable molecular machine|journal=Nature|volume=549|issue=7672|pages=374–378|doi=10.1038/nature23677|pmid=28933436|issn=1476-4687|bibcode=2017Natur.549..374K|s2cid=205259758|url=https://www.research.manchester.ac.uk/portal/en/publications/stereodivergent-synthesis-with-a-programmable-molecular-machine(dd2b7aed-b6ff-455a-8fb7-e31851cea5e6).html}}</ref><ref>{{Cite journal|last1=De Bo|first1=Guillaume|author-link1=Guillaume De Bo|last2=Gall|first2=Malcolm A. Y.|last3=Kuschel|first3=Sonja|last4=Winter|first4=Julien De|last5=Gerbaux|first5=Pascal|last6=Leigh|first6=David A.|date=2018-04-02|title=An artificial molecular machine that builds an asymmetric catalyst|journal=Nature Nanotechnology|volume=13|issue=5|pages=381–385|doi=10.1038/s41565-018-0105-3|pmid=29610529|issn=1748-3395|bibcode=2018NatNa..13..381D|s2cid=4624041|url=https://www.research.manchester.ac.uk/portal/en/publications/an-artificial-molecular-machine-that-builds-an-asymmetric-catalyst(569800d8-beb2-4d4a-acd5-84d0369ddabb).html}}</ref>
+|A class of mechanically interlocked molecules derived from catenanes where a large macrocycle backbone connects at least three small rings in the shape of a necklace (see image for example). A molecular necklace consisting of a large macrocycle threaded by ''n''-1 rings (hence comprising ''n'' rings) is represented as [''n'']MN.<ref>{{cite journal |last1=Zhang |first1=Z. |last2=Zhao |first2=J. |last3=Guo |first3=Z. |last4=Zhang |first4=H. |last5=Pan |first5=H. |last6=Wu |first6=Q. |last7=You |first7=W. |last8=Yu |first8=W. |last9=Yan |first9=X. |title=Mechanically interlocked networks cross-linked by a molecular necklace |journal=Nature Communications |date=2022 |volume=13 |issue=1 |pages=1393 |doi=10.1038/s41467-022-29141-7}}</ref> The first molecular necklace was synthesized in 1992, featuring several [[Α-Cyclodextrin|α-cyclodextrin]]s on a single [[polyethylene glycol]] chain backbone; the authors connected this to the idea of a "molecular abacus" proposed by Stoddart and coworkers around the same time.<ref>{{cite journal |last1=Harada |first1=A. |last2=Li |first2=J. |last3=Kamachi |first3=M. |title=The molecular necklace: a rotaxane containing many threaded α-cyclodextrins |journal=Nature |date=1992 |volume=356 |issue=6367 |pages=325–327 |doi=10.1038/356325a0}}</ref> Several interesting applications have emerged for these molecules, such as [[Antibiotic|antibacterial]] activity,<ref>{{cite journal |last1=Wu |first1=G.-Y. |last2=Shi |first2=X. |last3=Phan |first3=H. |last4=Qu |first4=H. |last5=Hu |first5=Y.-X. |last6=Yin |first6=G.-Q. |last7=Zhao |first7=X.-L. |last8=Li |first8=X. |last9=Xu |first9=L. |last10=Yu |first10=Q. |last11=Yang |first11=H.-B. |title=Efficient self-assembly of heterometallic triangular necklace with strong antibacterial activity |journal=Nature Communications |date=2020 |volume=11 |issue=1 |pages=3178 |doi=10.1038/s41467-020-16940-z |url=https://www.nature.com/articles/s41467-020-16940-z}}</ref> [[desulfurization]] of fuels,<ref>{{cite journal |last1=Li |first1=S.-L. |last2=Lan |first2=Y.-Q. |last3=Sakurai |first3=H. |last4=Xu |first4=Q. |title=Unusual Regenerable Porous Metal-Organic Framework Based on a New Triple Helical Molecular Necklace for Separating Organosulfur Compounds |journal=Chemistry - A European Journal |date=2012 |volume=18 |issue=51 |pages=16302–16309 |doi=10.1002/chem.201203093}}</ref> and [[piezoelectricity]].<ref>{{cite journal |last1=Seo |first1=J. |last2=Kim |first2=B. |last3=Kim |first3=M.-S. |last4=Seo |first4=J.-H. |title=Optimization of Anisotropic Crystalline Structure of Molecular Necklace-like Polyrotaxane for Tough Piezoelectric Elastomer |journal=ACS Macro Letters |date=2021 |volume=10 |issue=11 |pages=1371–1376 |doi=10.1021/acsmacrolett.1c00567}}</ref>
+|[[File:Molecular necklace example.png|frameless|alt=An example of a molecular necklace]]
+|- style="vertical-align: top;"
-====Molecular hinge====
+|[[Molecular propeller]]
-{{anchor|molecular hinge}}A molecular hinge is a molecule that can be selectively switched from one configuration to another in a reversible fashion.<ref>{{cite journal |last1=Kay |first1=Euan R. |last2=Leigh |first2=David A. |last3=Zerbetto |first3=Francesco |title=Synthetic Molecular Motors and Mechanical Machines |journal=Angewandte Chemie International Edition |date=January 2007 |volume=46 |issue=1–2 |pages=72–191 |doi=10.1002/anie.200504313|pmid=17133632 }}</ref> Such configurations must have distinguishable geometries; for instance, [[Azobenzene|azobenzene]] groups in a linear molecule may undergo [[Cis–trans isomerism|''cis''-''trans'' isomerizations]]<ref>{{cite journal |last1=Bandara |first1=H. M. Dhammika |last2=Burdette |first2=Shawn C. |title=Photoisomerization in different classes of azobenzene |journal=Chem. Soc. Rev. |date=2012 |volume=41 |issue=5 |pages=1809–1825 |doi=10.1039/c1cs15179g|pmid=22008710 }}</ref> when irradiated with ultraviolet light, triggering a reversible transition to a bent or V-shaped conformation.<ref>{{cite journal |last1=Wang |first1=Jing |last2=Jiang |first2=Qian |last3=Hao |first3=Xingtian |last4=Yan |first4=Hongchao |last5=Peng |first5=Haiyan |last6=Xiong |first6=Bijin |last7=Liao |first7=Yonggui |last8=Xie |first8=Xiaolin |title=Reversible photo-responsive gel–sol transitions of robust organogels based on an azobenzene-containing main-chain liquid crystalline polymer |journal=RSC Advances |date=2020 |volume=10 |issue=7 |pages=3726–3733 |doi=10.1039/C9RA10161F|pmid=35492656 |pmc=9048773 |bibcode=2020RSCAd..10.3726W |doi-access=free }}</ref><ref>{{cite journal |last1=Hada |first1=Masaki |last2=Yamaguchi |first2=Daisuke |last3=Ishikawa |first3=Tadahiko |last4=Sawa |first4=Takayoshi |last5=Tsuruta |first5=Kenji |last6=Ishikawa |first6=Ken |last7=Koshihara |first7=Shin-ya |last8=Hayashi |first8=Yasuhiko |last9=Kato |first9=Takashi |title=Ultrafast isomerization-induced cooperative motions to higher molecular orientation in smectic liquid-crystalline azobenzene molecules |journal=Nature Communications |date=13 September 2019 |volume=10 |issue=1 |pages=4159 |doi=10.1038/s41467-019-12116-6 |pmid=31519876 |pmc=6744564 |bibcode=2019NatCo..10.4159H |language=en |issn=2041-1723|doi-access=free }}</ref><ref>{{cite journal |last1=Garcia-Amorós |first1=Jaume |last2=Reig |first2=Marta |last3=Cuadrado |first3=Alba |last4=Ortega |first4=Mario |last5=Nonell |first5=Santi |last6=Velasco |first6=Dolores |title=A photoswitchable bis-azo derivative with a high temporal resolution |journal=Chem. Commun. |date=2014 |volume=50 |issue=78 |pages=11462–11464 |doi=10.1039/C4CC05331A|pmid=25132052 }}</ref><ref>{{Cite journal|date=2017|title=Design and synthesis of Ʌ-shaped photoswitchable compounds employing Tröger's base scaffold.|journal=Synthesis|volume=49 | issue = 6 |pages=1214–1222|doi=10.1055/s-0036-1588913|last1=Kazem-Rostami|first1=Masoud}}</ref> Molecular hinges typically rotate in a [[Crank (mechanism)|crank]]-like motion around a rigid axis, such as a double bond or aromatic ring.<ref>{{cite journal |last1=Kassem |first1=Salma |last2=van Leeuwen |first2=Thomas |last3=Lubbe |first3=Anouk S. |last4=Wilson |first4=Miriam R. |last5=Feringa |first5=Ben L. |last6=Leigh |first6=David A. |title=Artificial molecular motors |journal=Chemical Society Reviews |date=2017 |volume=46 |issue=9 |pages=2592–2621 |doi=10.1039/C7CS00245A|pmid=28426052 |url=https://pure.rug.nl/ws/files/49449226/c7cs00245a_1_.pdf }}</ref> However, [[Macrocycle|macrocyclic]] molecular hinges with more [[Clamp (tool)|clamp]]-like mechanisms have also been synthesized.<ref>{{cite journal |last1=Jones |first1=Christopher D. |last2=Kershaw Cook |first2=Laurence J. |last3=Marquez-Gamez |first3=David |last4=Luzyanin |first4=Konstantin V. |last5=Steed |first5=Jonathan W. |last6=Slater |first6=Anna G. |title=High-Yielding Flow Synthesis of a Macrocyclic Molecular Hinge |journal=Journal of the American Chemical Society |date=7 May 2021 |volume=143 |issue=19 |pages=7553–7565 |doi=10.1021/jacs.1c02891 |pmid=33961419 |pmc=8397308 |issn=0002-7863|doi-access=free }}</ref><ref>{{cite journal |last1=Despras |first1=Guillaume |last2=Hain |first2=Julia |last3=Jaeschke |first3=Sven Ole |title=Photocontrol over Molecular Shape: Synthesis and Photochemical Evaluation of Glycoazobenzene Macrocycles |journal=Chemistry - A European Journal |date=10 August 2017 |volume=23 |issue=45 |pages=10838–10847 |doi=10.1002/chem.201701232|pmid=28613430 }}</ref><ref>{{cite journal |last1=Nagamani |first1=S. Anitha |last2=Norikane |first2=Yasuo |last3=Tamaoki |first3=Nobuyuki |title=Photoinduced Hinge-Like Molecular Motion: Studies on Xanthene-Based Cyclic Azobenzene Dimers |journal=The Journal of Organic Chemistry |date=November 2005 |volume=70 |issue=23 |pages=9304–9313 |doi=10.1021/jo0513616|pmid=16268603 }}</ref>
+|A molecule that can propel fluids when rotated, due to its special shape that is designed in analogy to macroscopic propellers (see schematic image on right). It has several molecular-scale blades attached at a certain pitch angle around the circumference of a nanoscale shaft.<ref>{{cite journal |last1=Simpson |first1=Christopher D. |last2=Mattersteig |first2=Gunter |last3=Martin |first3=Kai |last4=Gherghel |first4=Lileta |last5=Bauer |first5=Roland E. |last6=Räder |first6=Hans Joachim |last7=Müllen |first7=Klaus |title=Nanosized Molecular Propellers by Cyclodehydrogenation of Polyphenylene Dendrimers |journal=Journal of the American Chemical Society |date=March 2004 |volume=126 |issue=10 |pages=3139–3147 |doi=10.1021/ja036732j |pmid=15012144 }}</ref><ref>{{cite journal |doi=10.1103/PhysRevLett.98.266102|pmid=17678108|title=Chemically Tunable Nanoscale Propellers of Liquids|journal=Physical Review Letters|volume=98|issue=26|pages=266102|year=2007|last1=Wang|first1=Boyang|last2=Král|first2=Petr|bibcode=2007PhRvL..98z6102W}}</ref> Propellers have been shown to have interesting properties, such as variations in pumping rates for hydrophilic and hydrophobic fluids.<ref>{{cite journal |last1=Wang |first1=B. |last2=Král |first2=P. |title=Chemically Tunable Nanoscale Propellers of Liquids |journal=Physical Review Letters |date=2007 |volume=98 |issue=26 |pages=266102 |doi=10.1103/PhysRevLett.98.266102}}</ref>
+|[[File:Molecularpropeller.jpg|frameless|alt=An example of a molecular propeller pumping water molecules due to its hydrophobic surface]]
+|- style="vertical-align: top;"
-===Biological===
+|[[Molecular shuttle]]
+|A molecule capable of shuttling molecules or ions from one location to another. This is schematically depicted in the image on the right, where a ring (in green) can bind to either one of the yellow sites on the blue macrocyclic backbone.<ref name="10.1038/369133a0">{{cite journal |last1=Bissell |first1=Richard A |last2=Córdova |first2=Emilio |last3=Kaifer |first3=Angel E. |last4=Stoddart |first4=J. Fraser |title=A chemically and electrochemically switchable molecular shuttle |journal=Nature |date=12 May 1994 |volume=369 |issue=6476 |pages=133–137 |doi=10.1038/369133a0|bibcode=1994Natur.369..133B |s2cid=44926804 }}</ref> A common molecular shuttle consists of a rotaxane where the macrocycle can move between two sites or stations along the dumbbell backbone; controlling the properties of either site and by regulating conditions like pH can enable control over which site is selected for binding. This has led to novel applications in catalysis and drug delivery.<ref name="10.1038/369133a0" /><ref>{{cite journal|last1=Chatterjee|first1=M. N.|last2=Kay|first2=E. R.|last3=Leigh|first3=D. A.|date=2006|title=Beyond Switches: Ratcheting a Particle Energetically Uphill with a Compartmentalized Molecular Machine|journal=Journal of the American Chemical Society|volume=128|issue=12|pages=4058–4073|doi=10.1021/ja057664z|pmid=16551115}}</ref>
+|[[File:Molecular_shuttle_illustration_commons.png|frameless|alt=An example of a rotaxane-based molecular shuttle]]
+|- style="vertical-align: top;"
+|[[Molecular switch]]
+|A molecule that can be reversibly shifted between two or more stable states in response to certain stimuli. This change of states influences the properties of the molecule according to the state it occupies at the moment. Unlike a molecular motor, any mechanical work done due to the motion in a switch is generally undone once the molecule returns to its original state unless it is part of a larger motor-like system. The image on the right shows a [[hydrazone]]-based switch that switches in response to pH changes.<ref name="kaseem2017">{{cite journal |last1=Kassem |first1=S. |last2=van Leeuwen |first2=T. |last3=Lubbe |first3=A. S. |last4=Wilson |first4=M. R. |last5=Feringa |first5=B. L. |last6=Leigh |first6=D. A. |title=Artificial molecular motors |journal=Chemical Society Reviews |date=2017 |volume=46 |issue=9 |pages=2592–2621 |doi=10.1039/C7CS00245A}}</ref>
+|[[File:Molecular switch example.png|frameless|alt=An example of a molecular switch]]
+|- style="vertical-align: top;"
+|[[Molecular tweezers]]
+|Host molecules capable of holding items between their two arms.<ref>{{cite journal |last1=Chen |first1=C. W. |last2=Whitlock |first2=H. W. |title=Molecular tweezers: a simple model of bifunctional intercalation |journal=Journal of the American Chemical Society |date=July 1978 |volume=100 |issue=15 |pages=4921–4922 |doi=10.1021/ja00483a063}}</ref> The open cavity of the molecular tweezers binds items using non-covalent bonding including hydrogen bonding, metal coordination, hydrophobic forces, [[van der Waals force]]s, [[Pi interaction|π interactions]], or [[electrostatic]] effects.<ref>{{cite journal |last1=Klärner |first1=Frank-Gerrit |last2=Kahlert |first2=Björn |title=Molecular Tweezers and Clips as Synthetic Receptors. Molecular Recognition and Dynamics in Receptor−Substrate Complexes |journal=Accounts of Chemical Research |date=December 2003 |volume=36 |issue=12 |pages=919–932 |doi=10.1021/ar0200448|pmid=14674783 }}</ref> For instance, the image on the right depicts tweezers formed by [[corannulene]] pincers clasping a [[Buckminsterfullerene|C60 fullerene]] molecule, termed "buckycatcher".<ref>{{cite journal |last1=Sygula |first1=A. |last2=Fronczek |first2=F. R. |last3=Sygula |first3=R. |last4=Rabideau |first4=P. W. |last5=Olmstead |first5=M. M. |title=A Double Concave Hydrocarbon Buckycatcher |journal=Journal of the American Chemical Society |date=2007 |volume=129 |issue=13 |pages=3842–3843 |doi=10.1021/ja070616p}}</ref> Examples of molecular tweezers have been reported that are constructed from DNA and are considered [[DNA machine]]s.<ref>{{cite journal |last1=Yurke |first1=Bernard |last2=Turberfield |first2=Andrew J. |last3=Mills |first3=Allen P. |last4=Simmel |first4=Friedrich C. |last5=Neumann |first5=Jennifer L. |title=A DNA-fuelled molecular machine made of DNA |journal=Nature |date=10 August 2000 |volume=406 |issue=6796 |pages=605–608 |doi=10.1038/35020524|pmid=10949296 |bibcode=2000Natur.406..605Y |s2cid=2064216 }}</ref>
+|[[File:Buckycatcher_JACS_2007_V129_p3843.jpg|frameless|alt=An example of molecular tweezers binding a fullerene]]
+|- style="vertical-align: top;"
+|[[Nanocar]]
+|Single-molecule vehicles that resemble macroscopic automobiles and are important for understanding how to control molecular diffusion on surfaces. The image on the right shows an example with wheels made of fullerene molecules. The first nanocars were synthesized by [[James M. Tour]] in 2005. They had an H-shaped chassis and 4 molecular wheels ([[fullerenes]]) attached to the four corners.<ref>{{cite journal |last1=Shirai |first1=Yasuhiro |last2=Osgood |first2=Andrew J. |last3=Zhao |first3=Yuming |last4=Kelly |first4=Kevin F. |last5=Tour |first5=James M. |title=Directional Control in Thermally Driven Single-Molecule Nanocars |journal=Nano Letters |date=November 2005 |volume=5 |issue=11 |pages=2330–2334 |doi=10.1021/nl051915k|pmid=16277478 |bibcode=2005NanoL...5.2330S }}</ref> In 2011, Feringa and co-workers synthesized the first motorized nanocar which had molecular motors attached to the chassis as rotating wheels.<ref>{{cite journal |last1=Kudernac |first1=Tibor |last2=Ruangsupapichat |first2=Nopporn |last3=Parschau |first3=Manfred |last4=Maciá |first4=Beatriz |last5=Katsonis |first5=Nathalie |last6=Harutyunyan |first6=Syuzanna R. |last7=Ernst |first7=Karl-Heinz |last8=Feringa |first8=Ben L. |title=Electrically driven directional motion of a four-wheeled molecule on a metal surface |journal=Nature |date=10 November 2011 |volume=479 |issue=7372 |pages=208–211 |doi=10.1038/nature10587|pmid=22071765 |bibcode=2011Natur.479..208K |s2cid=6175720 }}</ref> The authors were able to demonstrate directional motion of the nanocar on a copper surface by providing energy from a scanning tunneling microscope tip. Later, in 2017, the world's first-ever [[nanocar race]] took place in [[Toulouse]].<ref>{{cite web|url=https://www.ladepeche.fr/article/2015/11/30/2227726-nanocar-race-la-course-de-petites-voitures-pour-grands-savants.html| title=NanoCar Race : la course de petites voitures pour grands savants|trans-title=NanoCar Race: the race of small cars for great scientists|language=French|date=November 30, 2017|newspaper=[[La Dépêche du Midi]]|accessdate=December 2, 2018}}</ref>
+|[[File:Nanocar2.png|frameless|alt=A nanocar with C60 fullerenes as wheels]]
+|}
+==Biological molecular machines==
 [[Image:Protein translation.gif|thumb|300px|A ribosome performing the [[Transcription (biology)#Elongation|elongation]] and membrane targeting stages of [[eukaryotic translation|protein translation]]. The [[ribosome]] is green and yellow, the [[transfer RNA|tRNAs]] are dark blue, and the other proteins involved are light blue. The produced peptide is released into the [[endoplasmic reticulum]].]]
 The most complex macromolecular machines are found within cells, often in the form of [[Protein complex|multi-protein complexes]].<ref>{{Cite book|title=Biochemistry|last=Donald|first=Voet|date=2011|publisher=John Wiley & Sons|others=Voet, Judith G.|isbn=9780470570951|edition= 4th|location=Hoboken, NJ|oclc=690489261}}</ref> Important examples of biological machines include [[motor proteins]] such as [[myosin]], which is responsible for [[muscle]] contraction, [[kinesin]], which moves cargo inside cells away from the [[Cell nucleus|nucleus]] along [[microtubules]], and [[dynein]], which moves cargo inside cells towards the nucleus and produces the axonemal beating of [[cilia#Motile cilia|motile cilia]] and [[flagella]].  "[I]n effect, the [motile cilium] is a nanomachine composed of perhaps over 600 proteins in molecular complexes, many of which also function independently as nanomachines&nbsp;... [[Flexible linker]]s allow the [[Protein domain#Domains and protein flexibility|mobile protein domains]] connected by them to recruit their binding partners and induce long-range [[allostery]] via [[Protein dynamics#Global flexibility: multiple domains|protein domain dynamics]]."<ref name="Satir2008"/> Other biological machines are responsible for energy production, for example [[ATP synthase]] which harnesses energy from [[Proton-motive force|proton gradients across membranes]] to drive a turbine-like motion used to synthesise [[Adenosine triphosphate|ATP]], the energy currency of a cell.<ref>{{Cite journal|last1=Kinbara|first1=Kazushi|last2=Aida|first2=Takuzo|date=2005-04-01|title=Toward Intelligent Molecular Machines: Directed Motions of Biological and Artificial Molecules and Assemblies|journal=Chemical Reviews|volume=105|issue=4|pages=1377–1400|doi=10.1021/cr030071r|pmid=15826015|issn=0009-2665}}</ref> Still other machines are responsible for [[gene expression]], including [[DNA polymerase]]s for replicating DNA, [[RNA polymerase]]s for producing [[Messenger RNA|mRNA]], the [[spliceosome]] for removing [[intron]]s, and the [[ribosome]] for [[Protein synthesis|synthesising proteins]]. These machines and their [[protein dynamics|nanoscale dynamics]] are far more complex than any molecular machines that have yet been artificially constructed.<ref name="pmid21570668">{{cite book |vauthors=Bu Z, Callaway DJ |chapter=Proteins MOVE! Protein dynamics and long-range allostery in cell signaling |volume=83 |pages=163–221 |year=2011 |pmid=21570668 |doi=10.1016/B978-0-12-381262-9.00005-7 |chapter-url=http://linkinghub.elsevier.com/retrieve/pii/B978-0-12-381262-9.00005-7 |series=Advances in Protein Chemistry and Structural Biology |isbn=9780123812629|title=Protein Structure and Diseases |publisher=Academic Press }}</ref>

v t e Nanotechnology
Overview	History Organizations Popular culture Outline
Impact and applications	Nanomedicine Nanotoxicology Green nanotechnology Regulation
Nanomaterials	Fullerenes Nanotubes Carbon Non-carbon Nanoparticles characterization
Molecular self-assembly	Self-assembled monolayer Supramolecular assembly DNA nanotechnology Solid lipid nanoparticle
Nanoelectronics	Molecular scale electronics Nanolithography
Scanning probe microscopy	Atomic force microscope Scanning tunneling microscope
Molecular nanotechnology	Molecular assembler Nanorobotics Mechanosynthesis
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