Limits...
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.

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.


Related in: MedlinePlus

(a) Rotaxane 149-based molecular switch tunnel junctionsand proposed mechanism for the operation. (i) In the ground state,the tetracationic cyclophane (dark blue) mainly encircles the TTFstation (green) and the junction exhibits low conductance. (ii) Applicationof a positive bias results in one- or two-electron oxidation of theTTF units (green → pink), and increases electrostatic repulsioncausing (iii) shuttling of the macrocycle to the DNP station (red).(iv) Returning the bias to near −0 V provides a high conductancestate, in which the TTF units have been regenerated, but translocationof the cyclophane has not yet occurred due to a significant activationbarrier to movement. Thermally activated decay of this metastablestate may occur slowly ((iv) → (i), in a temperature-dependentmanner) or can be triggered by the application of a negative voltage(v), which temporarily reduces the cyclophane to its diradical dicationform (dark blue → orange), allowing facile recovery of thethermodynamically favored coconformation (vi). (b) Example of onedesign of a molecular switch. The coloring of the functional unitscorresponds to that used for the structural diagrams.1479,1482,1483 Reprinted with permission fromref (14). Copyright2007 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.
© Copyright Policy
Related In: Results  -  Collection

License
getmorefigures.php?uid=PMC4585175&req=5

fig60: (a) Rotaxane 149-based molecular switch tunnel junctionsand proposed mechanism for the operation. (i) In the ground state,the tetracationic cyclophane (dark blue) mainly encircles the TTFstation (green) and the junction exhibits low conductance. (ii) Applicationof a positive bias results in one- or two-electron oxidation of theTTF units (green → pink), and increases electrostatic repulsioncausing (iii) shuttling of the macrocycle to the DNP station (red).(iv) Returning the bias to near −0 V provides a high conductancestate, in which the TTF units have been regenerated, but translocationof the cyclophane has not yet occurred due to a significant activationbarrier to movement. Thermally activated decay of this metastablestate may occur slowly ((iv) → (i), in a temperature-dependentmanner) or can be triggered by the application of a negative voltage(v), which temporarily reduces the cyclophane to its diradical dicationform (dark blue → orange), allowing facile recovery of thethermodynamically favored coconformation (vi). (b) Example of onedesign of a molecular switch. The coloring of the functional unitscorresponds to that used for the structural diagrams.1479,1482,1483 Reprinted with permission fromref (14). Copyright2007 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.

Mentions: In a series of ground-breakingbut controversial experiments interfacingswitchable rotaxanes and catenanes with silicon-based electronics,molecular shuttles have been employed in solid-state molecular electronicdevices.1146,1477−1485 Bistable [2]rotaxanes and [2]catenanes have been the subject ofnumerous experimental investigations in the course of the developmentof such molecular electronic devices.1233,1478,1486−1495 Here, the bistable [2]catenanes and [2]rotaxanes feature a cyclobis(paraquat-p-phenylene) (CBPQT4+) macrocycle and two stations,often a tetrathiafulvalene (TTF) site and a dioxynaphtalene site (DNP).Initially the macrocycle preferentially resides over the TTF sitedue to strong aromatic charge-transfer interactions between the components;this is referred to as the ground-state coconformation (GSCC). Electrochemicaloxidation of the TTF station to form TTF2+ generates Coulombicrepulsion between CBPQT4+ and TTF2+, and drivesthe translation of the macrocycle to the DNP station, to give themetastable state coconformation (MSCC). The process can be reversedon reduction of TTF2+ to TTF followed by either thermalrelaxation of the macrocycle to the TTF station, or reduction of thebipyridinium units in the cyclophane ring to the corresponding radicalcations, which reduces the activation barrier to shuttling, restoringthe system to the GSCC. These two mechanically distinguishable statesexhibit different characteristic tunneling currents. On the basisof quantum mechanical computational studies, the MSCC state is predictedto be the more highly conducting state. The switching cycle can bedetected by a number of experimental techniques including time- andtemperature-dependent electrochemistry and spectroscopy. Studies haveshown that current levels on switching are influenced by temperature,the structure of the rotaxane/catenane, and the environment in whichthe molecular machines are embedded.1496−1498 Different environments,including Langmuir–Blodgett (LB) films,1499−1501 self-assembled monolayers (SAMs),1502−1506 and solid-state molecular-switch tunneljunctions (MSTJs), have been extensively studied.1150,1151,1488,1489,1507−1512 In one particular MSTJ, a monolayer of switchable rotaxane 149 was embedded between two conducting electrodes (Figure 60). This MSTJ actsas a gate, which can be opened or closed in response to an appliedvoltage by changes in conductivity and resistance and could be usedin molecular logic gate designs. The reported design showed stableswitching voltages of −2 and +2 V, with reasonable on/off ratiosand low switch-closed currents. Nanometer-scale devices have beenbuilt using this approach and connected to form 2-D crossbar circuitarchitectures.1144 As a next step, theauthors published the design of a 160-kilobit molecular electronicmemory circuit consisting of 400 silicon-nanowire electrodes (16 nmwide) and crossed by 400 Ti electrodes sandwiching a monolayer ofbistable [2]rotaxanes.1513 Despite theinteresting findings, many remain skeptical about the utility of rotaxanesin electronics, and an array of scientific and engineering challengesremain to be addressed such as device robustness, improved etchingtools, and improved switching speed.1514−1516


Artificial Molecular Machines.

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

(a) Rotaxane 149-based molecular switch tunnel junctionsand proposed mechanism for the operation. (i) In the ground state,the tetracationic cyclophane (dark blue) mainly encircles the TTFstation (green) and the junction exhibits low conductance. (ii) Applicationof a positive bias results in one- or two-electron oxidation of theTTF units (green → pink), and increases electrostatic repulsioncausing (iii) shuttling of the macrocycle to the DNP station (red).(iv) Returning the bias to near −0 V provides a high conductancestate, in which the TTF units have been regenerated, but translocationof the cyclophane has not yet occurred due to a significant activationbarrier to movement. Thermally activated decay of this metastablestate may occur slowly ((iv) → (i), in a temperature-dependentmanner) or can be triggered by the application of a negative voltage(v), which temporarily reduces the cyclophane to its diradical dicationform (dark blue → orange), allowing facile recovery of thethermodynamically favored coconformation (vi). (b) Example of onedesign of a molecular switch. The coloring of the functional unitscorresponds to that used for the structural diagrams.1479,1482,1483 Reprinted with permission fromref (14). Copyright2007 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.
© Copyright Policy
Related In: Results  -  Collection

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

fig60: (a) Rotaxane 149-based molecular switch tunnel junctionsand proposed mechanism for the operation. (i) In the ground state,the tetracationic cyclophane (dark blue) mainly encircles the TTFstation (green) and the junction exhibits low conductance. (ii) Applicationof a positive bias results in one- or two-electron oxidation of theTTF units (green → pink), and increases electrostatic repulsioncausing (iii) shuttling of the macrocycle to the DNP station (red).(iv) Returning the bias to near −0 V provides a high conductancestate, in which the TTF units have been regenerated, but translocationof the cyclophane has not yet occurred due to a significant activationbarrier to movement. Thermally activated decay of this metastablestate may occur slowly ((iv) → (i), in a temperature-dependentmanner) or can be triggered by the application of a negative voltage(v), which temporarily reduces the cyclophane to its diradical dicationform (dark blue → orange), allowing facile recovery of thethermodynamically favored coconformation (vi). (b) Example of onedesign of a molecular switch. The coloring of the functional unitscorresponds to that used for the structural diagrams.1479,1482,1483 Reprinted with permission fromref (14). Copyright2007 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.
Mentions: In a series of ground-breakingbut controversial experiments interfacingswitchable rotaxanes and catenanes with silicon-based electronics,molecular shuttles have been employed in solid-state molecular electronicdevices.1146,1477−1485 Bistable [2]rotaxanes and [2]catenanes have been the subject ofnumerous experimental investigations in the course of the developmentof such molecular electronic devices.1233,1478,1486−1495 Here, the bistable [2]catenanes and [2]rotaxanes feature a cyclobis(paraquat-p-phenylene) (CBPQT4+) macrocycle and two stations,often a tetrathiafulvalene (TTF) site and a dioxynaphtalene site (DNP).Initially the macrocycle preferentially resides over the TTF sitedue to strong aromatic charge-transfer interactions between the components;this is referred to as the ground-state coconformation (GSCC). Electrochemicaloxidation of the TTF station to form TTF2+ generates Coulombicrepulsion between CBPQT4+ and TTF2+, and drivesthe translation of the macrocycle to the DNP station, to give themetastable state coconformation (MSCC). The process can be reversedon reduction of TTF2+ to TTF followed by either thermalrelaxation of the macrocycle to the TTF station, or reduction of thebipyridinium units in the cyclophane ring to the corresponding radicalcations, which reduces the activation barrier to shuttling, restoringthe system to the GSCC. These two mechanically distinguishable statesexhibit different characteristic tunneling currents. On the basisof quantum mechanical computational studies, the MSCC state is predictedto be the more highly conducting state. The switching cycle can bedetected by a number of experimental techniques including time- andtemperature-dependent electrochemistry and spectroscopy. Studies haveshown that current levels on switching are influenced by temperature,the structure of the rotaxane/catenane, and the environment in whichthe molecular machines are embedded.1496−1498 Different environments,including Langmuir–Blodgett (LB) films,1499−1501 self-assembled monolayers (SAMs),1502−1506 and solid-state molecular-switch tunneljunctions (MSTJs), have been extensively studied.1150,1151,1488,1489,1507−1512 In one particular MSTJ, a monolayer of switchable rotaxane 149 was embedded between two conducting electrodes (Figure 60). This MSTJ actsas a gate, which can be opened or closed in response to an appliedvoltage by changes in conductivity and resistance and could be usedin molecular logic gate designs. The reported design showed stableswitching voltages of −2 and +2 V, with reasonable on/off ratiosand low switch-closed currents. Nanometer-scale devices have beenbuilt using this approach and connected to form 2-D crossbar circuitarchitectures.1144 As a next step, theauthors published the design of a 160-kilobit molecular electronicmemory circuit consisting of 400 silicon-nanowire electrodes (16 nmwide) and crossed by 400 Ti electrodes sandwiching a monolayer ofbistable [2]rotaxanes.1513 Despite theinteresting findings, many remain skeptical about the utility of rotaxanesin electronics, and an array of scientific and engineering challengesremain to be addressed such as device robustness, improved etchingtools, and improved switching speed.1514−1516

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.


Related in: MedlinePlus