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Artificial Molecular Machines.

Erbas-Cakmak S, Leigh DA, McTernan CT, Nussbaumer AL - Chem. Rev. (2015)

View Article: PubMed Central - PubMed

Affiliation: School of Chemistry, University of Manchester , Oxford Road, Manchester M13 9PL, United Kingdom.

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Hydrogen-bonding solvents have been shownto disrupt macrocycle–thread interactions in single stationrotaxanes, and here addition of 5% [D4]methanol increased the rate of shuttling 100-fold, consistentwith lowering the energy barrier to migration by disrupting station–macrocycleinteractions and thus raising ground-state energies... The effect of water on the rate of shuttlinghas been investigated and was found to be greatly superior to thatof other protic solvents... Therate of escape from the station energy well can then be modeled byan Arrhenius equation with a contribution from a distance-dependentdiffusion factor to the overall rate of shuttling... A quantum mechanicaltreatment of this system has found that, as the lengthening of thespacer has no effect on the activation for breaking the hydrogen bonds,the effect on the rate of shuttling is due to the widening of theoverall potential energy well... Molecular motion in mechanically interlocked and thus kineticallystable rotaxanes can be controlled using multiple binding sites withaffinities for the macrocycle that vary under different conditions.The conditions can be modified by electrochemical redox processes,light, pH, and environmental changes... Whenthe stilbene unit adopted the E form, the macrocyclecould move randomly along the full length of the thread by Brownianmotion, while when the Z form is adopted, the macrocyclewas trapped in one or the other of the two compartments... As the stretching of the PEOtether continued, and the force exerted by the PEO linker exceededthe hydrogen-bonding forces between the macrocycle and the fumaramidestation, the ring moved away from the fumaramide station... Tensionin the tether decreased as a result of the shuttling... Extracting useful work at the molecular scalerequires the restriction of the thermal movement of submolecular componentsor the exploitation of thermal motion with additional ratcheting.Shuttling, switching, and rotation processes in solution can be modulatedexternally, and the directionality of each motion can be controlledin single molecules... Third, to drive the walker away fromequilibrium, that is, to generate directional motion, a ratchetingprocess (either an energy or an information ratchet) must take place.In addition to the requirements of a Brownian motor, certain additionalcharacteristics are necessary for a motor to be defined as a walker... For the design of processive small molecule synthetic molecularwalkers, mechanically interlocked architectures are good candidates,because the walker (macrocycle) is mechanically bonded to the track(thread)... To perform tasks that cannot be accomplishedby conventional chemical means, it will be necessary to design systemswith multiple integrated parts, each component performing a dedicatedrole within the machine ensemble... This will not be straightforwardbecause unlike a watch where the second hand, say, does not interferewith the components in the escapement mechanism, the components ofa chemical machine are not easily isolated from each other (or theenvironment) and interference from one reactive part of a machinewith another will be a significant issue as complexity increases beyondthe current rather trivial systems. (iii) The machines we arefamiliar with in the macroscopic worldare generally stable, operating unchanged through many cycles, andby and large they do not make “mistakes”... Or it may bethat evolution just did not discover these solutions to such problemsand that mankind, with the whole of the periodic table and known syntheticchemistry to work with, can... Perhaps the most productive approachwill ultimately be found by following neither of these lines of investigationtoo closely, for example, by using chemical principles for “molecularrobotics” in which ratcheted motions of molecular components(i.e., biologically inspired mechanisms) are used to perform tasksthat have their origins in innovations introduced to advance developmentsin macroscopic technology (e.g., factory assembly lines).

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(a)Structure of [2]rotaxane 148. (b) Schematic representationof the structural design components used to create the metal–organicframework. (c) X-ray structure of the tetra-ester precursor to [2]rotaxane 148. (d) X-ray structure of a single unit of the mechanicallyinterlocked molecule, coordinated to four Cu(II) paddlewheel clusters.X-ray crystal structure reprinted with permission from ref (1475). Copyright 2012 NaturePublishing Group.
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fig59: (a)Structure of [2]rotaxane 148. (b) Schematic representationof the structural design components used to create the metal–organicframework. (c) X-ray structure of the tetra-ester precursor to [2]rotaxane 148. (d) X-ray structure of a single unit of the mechanicallyinterlocked molecule, coordinated to four Cu(II) paddlewheel clusters.X-ray crystal structure reprinted with permission from ref (1475). Copyright 2012 NaturePublishing Group.

Mentions: Sozzani and co-workers have published several examples ofmolecularrotors in crystals with open porosity, as well as molecular rotorsembedded in porous frameworks such as aromatic or organosilica frameworks.In some of these systems, the rotational motion can be actively regulatedin response to guest molecules such as CO2, I2, tetraethylammonium chloride, and water. These responsive materialsmay find applications spanning from sensors to actuators, which couldcapture and release chemicals on command.1471−1474 Recently, Schurko and Loeb published a metal–organic framework(MOF) material containing dynamically interlocked components.1475 They used [2]rotaxane 148 asthe organic linker and binuclear Cu(II) units as the nodes (Figure 59). Void spacesinside the rigid framework allowed the macrocyclic ring of the rotaxaneto rotate rapidly. Initially the macrocycle is locked in place throughhydrogen bonding from an ether oxygen atom of the macrocycle to acopper-bound H2O. Dynamic motion occurs upon removal ofthe water molecules by heating to 150 °C under vacuum, destroyinghydrogen-bond interactions and allowing rapid circumrotation aboutthe rotaxane axle. Rotation can be quenched by the addition of water.This type of material could be useful for the creation of solid-statemolecular switches and machines.1475 Arotaxane-based molecular shuttle incorporated into the structure ofa MOF has recently been reported.1476


Artificial Molecular Machines.

Erbas-Cakmak S, Leigh DA, McTernan CT, Nussbaumer AL - Chem. Rev. (2015)

(a)Structure of [2]rotaxane 148. (b) Schematic representationof the structural design components used to create the metal–organicframework. (c) X-ray structure of the tetra-ester precursor to [2]rotaxane 148. (d) X-ray structure of a single unit of the mechanicallyinterlocked molecule, coordinated to four Cu(II) paddlewheel clusters.X-ray crystal structure reprinted with permission from ref (1475). Copyright 2012 NaturePublishing Group.
© Copyright Policy
Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC4585175&req=5

fig59: (a)Structure of [2]rotaxane 148. (b) Schematic representationof the structural design components used to create the metal–organicframework. (c) X-ray structure of the tetra-ester precursor to [2]rotaxane 148. (d) X-ray structure of a single unit of the mechanicallyinterlocked molecule, coordinated to four Cu(II) paddlewheel clusters.X-ray crystal structure reprinted with permission from ref (1475). Copyright 2012 NaturePublishing Group.
Mentions: Sozzani and co-workers have published several examples ofmolecularrotors in crystals with open porosity, as well as molecular rotorsembedded in porous frameworks such as aromatic or organosilica frameworks.In some of these systems, the rotational motion can be actively regulatedin response to guest molecules such as CO2, I2, tetraethylammonium chloride, and water. These responsive materialsmay find applications spanning from sensors to actuators, which couldcapture and release chemicals on command.1471−1474 Recently, Schurko and Loeb published a metal–organic framework(MOF) material containing dynamically interlocked components.1475 They used [2]rotaxane 148 asthe organic linker and binuclear Cu(II) units as the nodes (Figure 59). Void spacesinside the rigid framework allowed the macrocyclic ring of the rotaxaneto rotate rapidly. Initially the macrocycle is locked in place throughhydrogen bonding from an ether oxygen atom of the macrocycle to acopper-bound H2O. Dynamic motion occurs upon removal ofthe water molecules by heating to 150 °C under vacuum, destroyinghydrogen-bond interactions and allowing rapid circumrotation aboutthe rotaxane axle. Rotation can be quenched by the addition of water.This type of material could be useful for the creation of solid-statemolecular switches and machines.1475 Arotaxane-based molecular shuttle incorporated into the structure ofa MOF has recently been reported.1476

View Article: PubMed Central - PubMed

Affiliation: School of Chemistry, University of Manchester , Oxford Road, Manchester M13 9PL, United Kingdom.

AUTOMATICALLY GENERATED EXCERPT
Please rate it.

Hydrogen-bonding solvents have been shownto disrupt macrocycle–thread interactions in single stationrotaxanes, and here addition of 5% [D4]methanol increased the rate of shuttling 100-fold, consistentwith lowering the energy barrier to migration by disrupting station–macrocycleinteractions and thus raising ground-state energies... The effect of water on the rate of shuttlinghas been investigated and was found to be greatly superior to thatof other protic solvents... Therate of escape from the station energy well can then be modeled byan Arrhenius equation with a contribution from a distance-dependentdiffusion factor to the overall rate of shuttling... A quantum mechanicaltreatment of this system has found that, as the lengthening of thespacer has no effect on the activation for breaking the hydrogen bonds,the effect on the rate of shuttling is due to the widening of theoverall potential energy well... Molecular motion in mechanically interlocked and thus kineticallystable rotaxanes can be controlled using multiple binding sites withaffinities for the macrocycle that vary under different conditions.The conditions can be modified by electrochemical redox processes,light, pH, and environmental changes... Whenthe stilbene unit adopted the E form, the macrocyclecould move randomly along the full length of the thread by Brownianmotion, while when the Z form is adopted, the macrocyclewas trapped in one or the other of the two compartments... As the stretching of the PEOtether continued, and the force exerted by the PEO linker exceededthe hydrogen-bonding forces between the macrocycle and the fumaramidestation, the ring moved away from the fumaramide station... Tensionin the tether decreased as a result of the shuttling... Extracting useful work at the molecular scalerequires the restriction of the thermal movement of submolecular componentsor the exploitation of thermal motion with additional ratcheting.Shuttling, switching, and rotation processes in solution can be modulatedexternally, and the directionality of each motion can be controlledin single molecules... Third, to drive the walker away fromequilibrium, that is, to generate directional motion, a ratchetingprocess (either an energy or an information ratchet) must take place.In addition to the requirements of a Brownian motor, certain additionalcharacteristics are necessary for a motor to be defined as a walker... For the design of processive small molecule synthetic molecularwalkers, mechanically interlocked architectures are good candidates,because the walker (macrocycle) is mechanically bonded to the track(thread)... To perform tasks that cannot be accomplishedby conventional chemical means, it will be necessary to design systemswith multiple integrated parts, each component performing a dedicatedrole within the machine ensemble... This will not be straightforwardbecause unlike a watch where the second hand, say, does not interferewith the components in the escapement mechanism, the components ofa chemical machine are not easily isolated from each other (or theenvironment) and interference from one reactive part of a machinewith another will be a significant issue as complexity increases beyondthe current rather trivial systems. (iii) The machines we arefamiliar with in the macroscopic worldare generally stable, operating unchanged through many cycles, andby and large they do not make “mistakes”... Or it may bethat evolution just did not discover these solutions to such problemsand that mankind, with the whole of the periodic table and known syntheticchemistry to work with, can... Perhaps the most productive approachwill ultimately be found by following neither of these lines of investigationtoo closely, for example, by using chemical principles for “molecularrobotics” in which ratcheted motions of molecular components(i.e., biologically inspired mechanisms) are used to perform tasksthat have their origins in innovations introduced to advance developmentsin macroscopic technology (e.g., factory assembly lines).

No MeSH data available.