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Molecular machines are a class of molecules typically described as an assembly of a discrete number of molecular components intended to produce mechanical movements in response to specific stimuli, mimicking devices such as switches and motors. Naturally occurring or biological molecular machines are responsible for vital living processes such as and . and are examples of molecular machines, and they often take the form of . For the last several decades, scientists have attempted, with varying degrees of success, to miniaturize machines found in the macroscopic world.

The first example of an artificial molecular machine (AMM) was reported in 1994, featuring a with a ring and two different possible . In 2016 the Nobel Prize in Chemistry was awarded to Jean-Pierre Sauvage, , and for the design and synthesis of molecular machines. A major point is to exploit existing motion in proteins, such as rotation about or cis-trans isomerization. Different AMMs are produced by introducing various functionalities, such as the introduction of to create switches. A broad range of AMMs has been designed, featuring different properties and applications; some of these include , , and logic gates. A wide range of applications have been demonstrated for AMMs, including those integrated into , , and systems for varied functions (such as materials research, homogenous catalysis and surface chemistry).


Terminology
Several definitions describe a "molecular machine" as a class of molecules typically described as an assembly of a discrete number of molecular components intended to produce mechanical movements in response to specific stimuli. The expression is often more generally applied to molecules that simply mimic functions that occur at the macroscopic level. A few prime requirements for a molecule to be considered a "molecular machine" are: the presence of moving parts, the ability to consume energy, and the ability to perform a task. Molecular machines differ from other stimuli-responsive compounds that can produce motion (such as cis- trans isomers) in their relatively larger amplitude of movement (potentially due to chemical reactions) and the presence of a clear external stimulus to regulate the movements (as compared to ). , , and other materials that produce a movement due to external stimuli on a macro-scale are generally not included, since despite the molecular origin of the motion the effects are not useable on the molecular scale.

This definition generally applies to synthetic molecular machines, which have historically gained inspiration from the naturally occurring biological molecular machines (also referred to as "nanomachines"). Biological machines are considered to be nanoscale devices (such as molecular ) in a living system that convert various forms of energy to mechanical work in order to drive crucial biological processes such as intracellular transport, muscle contractions, and .


History
Biological molecular machines have been known and studied for decades given their vital role in sustaining life, and have served as inspiration for synthetically designed systems with similar useful functionality. 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 in . 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 containing an unit, which could switch between cis and trans isomers on exposure to light and hence tune the cation-binding properties of the ether. In his seminal 1959 lecture There's Plenty of Room at the Bottom, alluded to the idea and applications of molecular devices designed artificially by manipulating matter at the atomic level. This was further substantiated by Eric Drexler during the 1970s, who developed ideas based on molecular nanotechnology such as nanoscale "assemblers", though their feasibility was disputed.

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 . Building upon the assembly of mechanically linked molecules such as and as developed by Jean-Pierre Sauvage in the early 1980s, this shuttle features a rotaxane with a ring that can move across an "axle" between two ends or possible ( units). This design realized the well-defined motion of a molecular unit across the length of the molecule for the first time. In 1994, an improved design allowed control over the motion of the ring by pH variation or methods, making it the first example of an AMM. Here the two binding sites are a and a 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 . In 1998, a study could capture the rotary motion of a decacyclene molecule on a copper-base metallic surface using a scanning tunneling microscope. Over the following decade, a broad variety of AMMs responding to various stimuli were invented for different applications. In 2016, the Nobel Prize in Chemistry was awarded to Sauvage, Stoddart, and for the design and synthesis of molecular machines.


Artificial molecular machines
Over the past few decades, AMMs have diversified rapidly and their design principles, properties, and characterization methods have been outlined more clearly. A major starting point for the design of AMMs is to exploit the existing modes of motion in molecules. For instance, can be visualized as axes of rotation, as can be complexes. Bending or V-like shapes can be achieved by incorporating , that can undergo cis-trans isomerization in response to certain stimuli (typically irradiation with a suitable ), as seen in numerous designs consisting of and azobenzene units. Similarly, and -closing reactions such as those seen for and can also produce curved shapes. 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. Another line of AMMs consists of biomolecules such as and as part of their design, making use of phenomena like and unfolding.

AMM designs have diversified significantly since the early days of the field. A major route is the introduction of 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 in an unsubstituted . If these two sites are different from each other in terms of features like , this can give rise to weak or strong recognition sites as in biological systems — such AMMs have found applications in and . 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.

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. 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, more recently waste-free reactions such based on electron transfers or isomerization have gained attention (such as redox-responsive ). Eventually, several different forms of energy (electric, magnetic, optical and so on) have become the primary energy sources used to power AMMs, even producing autonomous systems such as light-driven motors.


Types
Various AMMs are tabulated below along with indicative images:

Molecular balanceA molecule that can interconvert between two or more conformational or configurational states in response to the dynamic of multiple intra- and intermolecular driving forces, such as , or hydrophobic effects, , and steric and dispersion interactions. 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).

Molecular hingeA 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. Such configurations must have distinguishable geometries; for instance, azobenzene groups in a linear molecule may undergo cis- trans isomerization when irradiated with , triggering a reversible transition to a bent or V-shaped conformation (see image). Molecular hinges have been adapted for applications such as recognition, modifications, and visualizing molecular motion.

Molecular logic gateA molecule that performs a logical operation on one or more logic inputs and produces a single logic output. Modelled on , these molecules have slowly replaced the conventional silicon-based machinery. Several applications have come forth, such as water quality examination, examination, metal ion detection, and pharmaceutical studies. 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.

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). Carbon nanotube nanomotors have also been produced. Single bond rotary motors are generally activated by chemical reactions whereas double bond rotary motors are generally fueled by light. The rotation speed of the motor can also be tuned by careful molecular design.

Molecular necklaceA 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 nMN. 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. Several interesting applications have emerged for these molecules, such as activity, of fuels, and .

Molecular propellerA 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. Propellers have been shown to have interesting properties, such as variations in pumping rates for hydrophilic and hydrophobic fluids.

Molecular shuttleA 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. 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.

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 -based switch that switches in response to pH changes.

Molecular tweezersHost molecules capable of holding items between their two arms. The open cavity of the molecular tweezers binds items using non-covalent bonding including hydrogen bonding, metal coordination, hydrophobic forces, van der Waals forces, , or effects. For instance, the image on the right depicts tweezers formed by pincers clasping a C60 fullerene molecule, termed "buckycatcher". Examples of molecular tweezers have been reported that are constructed from DNA and are considered .

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 () attached to the four corners. In 2011, Feringa and co-workers synthesized the first motorized nanocar which had molecular motors attached to the chassis as rotating wheels. 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 took place in .


Biological molecular machines
Many macromolecular machines are found within cells, often in the form of .
(2025). 9780470570951, John Wiley & Sons.
Examples of biological machines include such as , which is responsible for contraction, , which moves cargo inside cells away from the along , and , which moves cargo inside cells towards the nucleus and produces the axonemal beating of motile cilia and . "In effect, the motile is a nanomachine composed of perhaps over 600 proteins in molecular complexes, many of which also function independently as nanomachines ... allow the mobile protein domains connected by them to recruit their binding partners and induce long-range via ." Other biological machines are responsible for energy production, for example which harnesses energy from proton gradients across membranes to drive a turbine-like motion used to synthesise ATP, the energy currency of a cell. Still other machines are responsible for , including for replicating DNA, for producing , the for removing , and the for synthesising proteins. These machines and their are far more complex than any molecular machines that have yet been artificially constructed.

Biological machines have potential applications in . For example, they could be used to identify and destroy cancer cells. Molecular nanotechnology is a subfield of nanotechnology regarding the possibility of engineering molecular assemblers, biological machines which could re-order matter at a molecular or atomic scale. would make use of these , introduced into the body, to repair or detect damages and infections, but these are considered to be far beyond current capabilities.


Research and applications
Advances in this area are inhibited by the lack of synthetic methods. In this context, theoretical modeling has emerged as a pivotal tool to understand the or -disassembly processes in these systems.

Possible applications have been demonstrated for AMMs, including those integrated into , , and systems for varied functions. Homogenous catalysis is a prominent example, especially in areas like asymmetric synthesis, utilizing noncovalent interactions and biomimetic allosteric catalysis. AMMs have been pivotal in the design of several stimuli-responsive smart materials, such as 2D and 3D self-assembled materials and -based systems, for versatile applications ranging from 3D printing to drug delivery.

AMMs are gradually moving from the conventional solution-phase chemistry to surfaces and interfaces. For instance, AMM-immobilized surfaces (AMMISs) are a novel class of functional materials consisting of AMMs attached to inorganic surfaces forming features like self-assembled monolayers; this gives rise to tunable properties such as fluorescence, aggregation and drug-release activity.

Most of these "applications" remain at the proof-of-concept level. Challenges in streamlining macroscale applications include autonomous operation, the complexity of the machines, stability in the synthesis of the machines and the working conditions.


See also
  • Supramolecular chemistry

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