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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) Chemical structures of 115, and the open and closedforms of 114. (b) Schematic representation of light modulatedswitch and K+/18-crown-6-mediated complexation. (c) Fluorescencequenching of the Eu3+ complex upon UV irradiation in 1:1CH3CN/CHCl3. Fluorescence before (I) and after(II) excitation at 390 nm (c inset). Reprinted with permission fromref (1314). Copyright2013 American Chemical Society.
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fig46: (a) Chemical structures of 115, and the open and closedforms of 114. (b) Schematic representation of light modulatedswitch and K+/18-crown-6-mediated complexation. (c) Fluorescencequenching of the Eu3+ complex upon UV irradiation in 1:1CH3CN/CHCl3. Fluorescence before (I) and after(II) excitation at 390 nm (c inset). Reprinted with permission fromref (1314). Copyright2013 American Chemical Society.

Mentions: Liu et al. have developed a [2]pseudorotaxane witha dual stimulusluminescent lanthanide switch using a diarylperfluorocyclopenteneas the photochromic component (Figure 46).1314 The diarylperfluorocyclopentene(114) bears a benzyl ammonium recognition unit and reactsreversibly to generate its closed form upon absorption of UV irradiation(365 nm). The open form can be obtained via irradiation at 614 nm.The Eu3+ complex of terpyridinyldibenzo-24-crown-8 (115) was used to reversibly complex the ammonium moiety of 114. This Eu3+ center fluoresces at 619 nm throughintramolecular energy transfer from an excited terpyridine ligand.Upon complexation of the two compounds through crown ether–ammoniuminteractions, a small amount of quenching of the lanthanide fluorescenceat 619 nm was observed (ca. 10%) due to the poor spectral overlapbetween the donor emission and acceptor absorbance (which is requiredfor resonant energy transfer (RET)). Upon UV irradiation to form theclosed form of compound 114, spectral overlap increasedand fluorescence was quenched by 80%, with an accompanying 3-folddecrease in excited-state lifetime. The pseudorotaxane could be reversiblydisassembled by the addition of potassium cations and regeneratedby the addition of 18-crown-6.


Artificial Molecular Machines.

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

(a) Chemical structures of 115, and the open and closedforms of 114. (b) Schematic representation of light modulatedswitch and K+/18-crown-6-mediated complexation. (c) Fluorescencequenching of the Eu3+ complex upon UV irradiation in 1:1CH3CN/CHCl3. Fluorescence before (I) and after(II) excitation at 390 nm (c inset). Reprinted with permission fromref (1314). Copyright2013 American Chemical Society.
© Copyright Policy
Related In: Results  -  Collection

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

fig46: (a) Chemical structures of 115, and the open and closedforms of 114. (b) Schematic representation of light modulatedswitch and K+/18-crown-6-mediated complexation. (c) Fluorescencequenching of the Eu3+ complex upon UV irradiation in 1:1CH3CN/CHCl3. Fluorescence before (I) and after(II) excitation at 390 nm (c inset). Reprinted with permission fromref (1314). Copyright2013 American Chemical Society.
Mentions: Liu et al. have developed a [2]pseudorotaxane witha dual stimulusluminescent lanthanide switch using a diarylperfluorocyclopenteneas the photochromic component (Figure 46).1314 The diarylperfluorocyclopentene(114) bears a benzyl ammonium recognition unit and reactsreversibly to generate its closed form upon absorption of UV irradiation(365 nm). The open form can be obtained via irradiation at 614 nm.The Eu3+ complex of terpyridinyldibenzo-24-crown-8 (115) was used to reversibly complex the ammonium moiety of 114. This Eu3+ center fluoresces at 619 nm throughintramolecular energy transfer from an excited terpyridine ligand.Upon complexation of the two compounds through crown ether–ammoniuminteractions, a small amount of quenching of the lanthanide fluorescenceat 619 nm was observed (ca. 10%) due to the poor spectral overlapbetween the donor emission and acceptor absorbance (which is requiredfor resonant energy transfer (RET)). Upon UV irradiation to form theclosed form of compound 114, spectral overlap increasedand fluorescence was quenched by 80%, with an accompanying 3-folddecrease in excited-state lifetime. The pseudorotaxane could be reversiblydisassembled by the addition of potassium cations and regeneratedby the addition of 18-crown-6.

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.