Synthetic molecular motor

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Molecular dynamics simulation of a synthetic molecular rotor composed of three molecules in a nanopore (outer diameter 6.7 nm) at 250 K. MD rotor 250K 1ns.gif
Molecular dynamics simulation of a synthetic molecular rotor composed of three molecules in a nanopore (outer diameter 6.7 nm) at 250 K.

Synthetic molecular motors are molecular machines capable of continuous directional rotation under an energy input. [2] Although the term "molecular motor" has traditionally referred to a naturally occurring protein that induces motion (via protein dynamics), some groups also use the term when referring to non-biological, non-peptide synthetic motors. Many chemists are pursuing the synthesis of such molecular motors.

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The basic requirements for a synthetic motor are repetitive 360° motion, the consumption of energy and unidirectional rotation.[ citation needed ] The first two efforts in this direction, the chemically driven motor by Dr. T. Ross Kelly of Boston College with co-workers and the light-driven motor by Ben Feringa and co-workers, were published in 1999 in the same issue of Nature.

As of 2020, the smallest atomically precise molecular machine has a rotor that consists of four atoms. [3]

Chemically driven rotary molecular motors [4]

An example of a prototype for a synthetic chemically driven rotary molecular motor was reported by Kelly and co-workers in 1999. [5] Their system is made up from a three-bladed triptycene rotor and a helicene, and is capable of performing a unidirectional 120° rotation.

This rotation takes place in five steps. The amine group present on the triptycene moiety is converted to an isocyanate group by condensation with phosgene (a). Thermal or spontaneous rotation around the central bond then brings the isocyanate group in proximity of the hydroxyl group located on the helicene moiety (b), thereby allowing these two groups to react with each other (c). This reaction irreversibly traps the system as a strained cyclic urethane that is higher in energy and thus energetically closer to the rotational energy barrier than the original state. Further rotation of the triptycene moiety therefore requires only a relatively small amount of thermal activation in order to overcome this barrier, thereby releasing the strain (d). Finally, cleavage of the urethane group restores the amine and alcohol functionalities of the molecule (e).

The result of this sequence of events is a unidirectional 120° rotation of the triptycene moiety with respect to the helicene moiety. Additional forward or backward rotation of the triptycene rotor is inhibited by the helicene moiety, which serves a function similar to that of the pawl of a ratchet. The unidirectionality of the system is a result from both the asymmetric skew of the helicene moiety as well as the strain of the cyclic urethane which is formed in c. This strain can be only be lowered by the clockwise rotation of the triptycene rotor in d, as both counterclockwise rotation as well as the inverse process of d are energetically unfavorable. In this respect the preference for the rotation direction is determined by both the positions of the functional groups and the shape of the helicene and is thus built into the design of the molecule instead of dictated by external factors.

The prototype of a chemically driven rotary molecular motor by Kelly and co-workers. Kelly chem motor.png
The prototype of a chemically driven rotary molecular motor by Kelly and co-workers.

The motor by Kelly and co-workers is an elegant example of how chemical energy can be used to induce controlled, unidirectional rotational motion, a process which resembles the consumption of ATP in organisms in order to fuel numerous processes. However, it does suffer from a serious drawback: the sequence of events that leads to 120° rotation is not repeatable. Kelly and co-workers have therefore searched for ways to extend the system so that this sequence can be carried out repeatedly. Unfortunately, their attempts to accomplish this objective have not been successful and currently the project has been abandoned. [6] In 2016 David Leigh's group invented the first autonomous chemically-fuelled synthetic molecular motor. [7]

Some other examples of synthetic chemically driven rotary molecular motors that all operate by sequential addition of reagents have been reported, including the use of the stereoselective ring opening of a racemic biaryl lactone by the use of chiral reagents, which results in a directed 90° rotation of one aryl with respect to the other aryl. Branchaud and co-workers have reported that this approach, followed by an additional ring closing step, can be used to accomplish a non-repeatable 180° rotation. [8]

The chemically driven rotary molecular motor by Feringa and co-workers Feringa chem motor.png
The chemically driven rotary molecular motor by Feringa and co-workers

Feringa and co-workers used this approach in their design of a molecule that can repeatably perform 360° rotation. [9] The full rotation of this molecular motor takes place in four stages. In stages A and C rotation of the aryl moiety is restricted, although helix inversion is possible. In stages B and D the aryl can rotate with respect to the naphthalene with steric interactions preventing the aryl from passing the naphthalene. The rotary cycle consists of four chemically induced steps which realize the conversion of one stage into the next. Steps 1 and 3 are asymmetric ring opening reactions which make use of a chiral reagent in order to control the direction of the rotation of the aryl. Steps 2 and 4 consist of the deprotection of the phenol, followed by regioselective ring formation.

Light-driven rotary molecular motors

Rotary cycle of the light-driven rotary molecular motor by Feringa and co-workers. First gen mol motor feringa.png
Rotary cycle of the light-driven rotary molecular motor by Feringa and co-workers.

In 1999 the laboratory of Prof. Dr. Ben L. Feringa at the University of Groningen, The Netherlands, reported the creation of a unidirectional molecular rotor. [10] Their 360° molecular motor system consists of a bis-helicene connected by an alkene double bond displaying axial chirality and having two stereocenters.

One cycle of unidirectional rotation takes 4 reaction steps. The first step is a low temperature endothermic photoisomerization of the trans (P,P) isomer 1 to the cis (M,M) 2 where P stands for the right-handed helix and M for the left-handed helix. In this process, the two axial methyl groups are converted into two less sterically favorable equatorial methyl groups.

By increasing the temperature to 20 °C these methyl groups convert back exothermally to the (P,P) cis axial groups (3) in a helix inversion. Because the axial isomer is more stable than the equatorial isomer, reverse rotation is blocked. A second photoisomerization converts (P,P) cis 3 into (M,M) trans 4, again with accompanying formation of sterically unfavorable equatorial methyl groups. A thermal isomerization process at 60 °C closes the 360° cycle back to the axial positions.

Synthetic molecular motors: fluorene system TBu Helicenemolecularmotor.png
Synthetic molecular motors: fluorene system

A major hurdle to overcome is the long reaction time for complete rotation in these systems, which does not compare to rotation speeds displayed by motor proteins in biological systems. In the fastest system to date, with a fluorene lower half, the half-life of the thermal helix inversion is 0.005 seconds. [11] This compound is synthesized using the Barton-Kellogg reaction. In this molecule the slowest step in its rotation, the thermally induced helix-inversion, is believed to proceed much more quickly because the larger tert-butyl group makes the unstable isomer even less stable than when the methyl group is used. This is because the unstable isomer is more destabilized than the transition state that leads to helix-inversion. The different behaviour of the two molecules is illustrated by the fact that the half-life time for the compound with a methyl group instead of a tert-butyl group is 3.2 minutes. [12]

The Feringa principle has been incorporated into a prototype nanocar. [13] The car synthesized has a helicene-derived engine with an oligo (phenylene ethynylene) chassis and four carborane wheels and is expected to be able to move on a solid surface with scanning tunneling microscopy monitoring, although so far this has not been observed. The motor does not perform with fullerene wheels because they quench the photochemistry of the motor moiety. Feringa motors have also been shown to remain operable when chemically attached to solid surfaces. [14] [15] The ability of certain Feringa systems to act as an asymmetric catalyst has also been demonstrated. [16] [17]

In 2016, Feringa was awarded a Nobel prize for his work on molecular motors.

Experimental demonstration of a single-molecule electric motor

A single-molecule electrically operated motor made from a single molecule of n-butyl methyl sulfide (C5H12S) has been reported. The molecule is adsorbed onto a copper (111) single-crystal piece by chemisorption. [18]

See also

Related Research Articles

<span class="mw-page-title-main">Stereoisomerism</span> When molecules have the same atoms and bond structure but differ in 3D orientation

In stereochemistry, stereoisomerism, or spatial isomerism, is a form of isomerism in which molecules have the same molecular formula and sequence of bonded atoms (constitution), but differ in the three-dimensional orientations of their atoms in space. This contrasts with structural isomers, which share the same molecular formula, but the bond connections or their order differs. By definition, molecules that are stereoisomers of each other represent the same structural isomer.

<span class="mw-page-title-main">Azobenzene</span> Two phenyl rings linked by a N═N double bond

Azobenzene is a photoswitchable chemical compound composed of two phenyl rings linked by a N=N double bond. It is the simplest example of an aryl azo compound. The term 'azobenzene' or simply 'azo' is often used to refer to a wide class of similar compounds. These azo compounds are considered as derivatives of diazene (diimide), and are sometimes referred to as 'diazenes'. The diazenes absorb light strongly and are common dyes. Different classes of azo dyes exist, most notably the ones substituted with heteroaryl rings.

<span class="mw-page-title-main">Chirality (chemistry)</span> Geometric property of some molecules and ions

In chemistry, a molecule or ion is called chiral if it cannot be superposed on its mirror image by any combination of rotations, translations, and some conformational changes. This geometric property is called chirality. The terms are derived from Ancient Greek χείρ (cheir) 'hand'; which is the canonical example of an object with this property.

<span class="mw-page-title-main">Conformational isomerism</span> Different molecular structures formed only by rotation about single bonds

In chemistry, conformational isomerism is a form of stereoisomerism in which the isomers can be interconverted just by rotations about formally single bonds. While any two arrangements of atoms in a molecule that differ by rotation about single bonds can be referred to as different conformations, conformations that correspond to local minima on the potential energy surface are specifically called conformational isomers or conformers. Conformations that correspond to local maxima on the energy surface are the transition states between the local-minimum conformational isomers. Rotations about single bonds involve overcoming a rotational energy barrier to interconvert one conformer to another. If the energy barrier is low, there is free rotation and a sample of the compound exists as a rapidly equilibrating mixture of multiple conformers; if the energy barrier is high enough then there is restricted rotation, a molecule may exist for a relatively long time period as a stable rotational isomer or rotamer. When the time scale for interconversion is long enough for isolation of individual rotamers, the isomers are termed atropisomers. The ring-flip of substituted cyclohexanes constitutes another common form of conformational isomerism.

<span class="mw-page-title-main">Molecular machine</span> Molecular-scale artificial or biological device

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 macromolecular devices such as switches and motors. Naturally occurring or biological molecular machines are responsible for vital living processes such as DNA replication and ATP synthesis. Kinesins and ribosomes are examples of molecular machines, and they often take the form of multi-protein complexes. 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 rotaxane with a ring and two different possible binding sites. In 2016 the 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.

<span class="mw-page-title-main">Axial chirality</span> Type of symmetry in molecules

In chemistry, axial chirality is a special case of chirality in which a molecule contains two pairs of chemical groups in a non-planar arrangement about an axis of chirality so that the molecule is not superposable on its mirror image. The axis of chirality is usually determined by a chemical bond that is constrained against free rotation either by steric hindrance of the groups, as in substituted biaryl compounds such as BINAP, or by torsional stiffness of the bonds, as in the C=C double bonds in allenes such as glutinic acid. Axial chirality is most commonly observed in substituted biaryl compounds wherein the rotation about the aryl–aryl bond is restricted so it results in chiral atropisomers, as in various ortho-substituted biphenyls, and in binaphthyls such as BINAP.

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<span class="mw-page-title-main">Triptycene</span> Chemical compound

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<span class="mw-page-title-main">Nanocar</span> Chemical compound

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A molecular switch is a molecule that can be reversibly shifted between two or more stable states. The molecules may be shifted between the states in response to environmental stimuli, such as changes in pH, light, temperature, an electric current, microenvironment, or in the presence of ions and other ligands. In some cases, a combination of stimuli is required. The oldest forms of synthetic molecular switches are pH indicators, which display distinct colors as a function of pH. Currently synthetic molecular switches are of interest in the field of nanotechnology for application in molecular computers or responsive drug delivery systems. Molecular switches are also important in biology because many biological functions are based on it, for instance allosteric regulation and vision. They are also one of the simplest examples of molecular machines.

<span class="mw-page-title-main">Molecular propeller</span>

Molecular propeller is a molecule that can propel fluids when rotated, due to its special shape that is designed in analogy to macroscopic propellers: it has several molecular-scale blades attached at a certain pitch angle around the circumference of a shaft, aligned along the rotational axis.

<span class="mw-page-title-main">Isomer</span> Chemical compounds with the same molecular formula but different atomic arrangements

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<span class="mw-page-title-main">Technomimetics</span>

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<span class="mw-page-title-main">Chirality</span> Difference in shape from a mirror image

Chirality is a property of asymmetry important in several branches of science. The word chirality is derived from the Greek χείρ (kheir), "hand", a familiar chiral object.

The single-molecule electric motor is an electrically operated synthetic molecular motor made from a single butyl methyl sulphide molecule. The molecule is adsorbed onto a copper (111) single-crystal piece by chemisorption. The motor, the world's smallest electric motor, is just a nanometer across. It was developed by the Sykes group and scientists at the Tufts University School of Arts and Sciences and published online September 4, 2011.

<span class="mw-page-title-main">Ben Feringa</span> Dutch Nobel laureate in chemistry

Bernard Lucas Feringa is a Dutch synthetic organic chemist, specializing in molecular nanotechnology and homogeneous catalysis. He is the Jacobus van 't Hoff Distinguished Professor of Molecular Sciences, at the Stratingh Institute for Chemistry, University of Groningen, Netherlands, and an Academy Professor of the Royal Netherlands Academy of Arts and Sciences. He was awarded the 2016 Nobel Prize in Chemistry, together with Sir J. Fraser Stoddart and Jean-Pierre Sauvage, "for the design and synthesis of molecular machines".

Nathalie Helene Katsonis is a Professor of Active Molecular Systems at the Stratingh Institute for Chemistry, University of Groningen. In 2016 she was awarded the Royal Netherlands Chemical Society Gold Medal.

Chiral inversion is the process of conversion of one enantiomer of a chiral molecule to its mirror-image version with no other change in the molecule.

References

  1. ↑ Palma, C.-A.; Björk, J.; Rao, F.; Kühne, D.; Klappenberger, F.; Barth, J.V. (2014). "Topological Dynamics in Supramolecular Rotors". Nano Letters. 148 (8): 4461–4468. Bibcode:2014NanoL..14.4461P. doi:10.1021/nl5014162. PMID   25078022.
  2. ↑ Kassem, Salma; van Leeuwen, Thomas; Lubbe, Anouk S.; Wilson, Miriam R.; Feringa, Ben L.; Leigh, David A. (2017). "Artificial molecular motors". Chemical Society Reviews. 46 (9): 2592–2621. doi:10.1039/C7CS00245A. PMID   28426052.
  3. ↑ Stolz, Samuel; Gröning, Oliver; Prinz, Jan; Brune, Harald; Widmer, Roland (2020-06-15). "Molecular motor crossing the frontier of classical to quantum tunneling motion". Proceedings of the National Academy of Sciences. 117 (26): 14838–14842. Bibcode:2020PNAS..11714838S. doi: 10.1073/pnas.1918654117 . ISSN   0027-8424. PMC   7334648 . PMID   32541061.
  4. ↑ Mondal, Anirban; Toyoda, Ryojun; Costil, Romain; Feringa, Ben L. (5 September 2022). "Chemically Driven Rotatory Molecular Machines". Angewandte Chemie International Edition. 61 (40): e202206631. doi:10.1002/anie.202206631. PMC   9826306 . PMID   35852813.
  5. ↑ Kelly, T. R.; De Silva, H; Silva, R. A. (1999). "Unidirectional rotary motion in a molecular system". Nature. 401 (6749): 150–2. Bibcode:1999Natur.401..150K. doi:10.1038/43639. PMID   10490021. S2CID   4351615.
  6. ↑ Kelly, T. Ross; Cai, Xiaolu; Damkaci, Fehmi; Panicker, Sreeletha B.; Tu, Bin; Bushell, Simon M.; Cornella, Ivan; Piggott, Matthew J.; Salives, Richard; Cavero, Marta; Zhao, Yajun; Jasmin, Serge (2007). "Progress toward a Rationally Designed, Chemically Powered Rotary Molecular Motor". Journal of the American Chemical Society. 129 (2): 376–86. doi:10.1021/ja066044a. PMID   17212418.
  7. ↑ Wilson, M. R.; Solá, J.; Carlone, A.; Goldup, S. M.; Lebrasseur, N.; Leigh, D. A. (2016). "An autonomous chemically fuelled small-molecule motor". Nature. 534 (7606): 235–240. Bibcode:2016Natur.534..235W. doi:10.1038/nature18013. PMID   27279219. S2CID   34432774. Archived from the original on 9 June 2016.
  8. ↑ Lin, Ying; Dahl, Bart J.; Branchaud, Bruce P. (2005). "Net directed 180° aryl–aryl bond rotation in a prototypical achiral biaryl lactone synthetic molecular motor". Tetrahedron Letters. 46 (48): 8359. doi:10.1016/j.tetlet.2005.09.151.
  9. ↑ Fletcher, S. P.; Dumur, F; Pollard, MM; Feringa, BL (2005). "A Reversible, Unidirectional Molecular Rotary Motor Driven by Chemical Energy". Science. 310 (5745): 80–82. Bibcode:2005Sci...310...80F. doi:10.1126/science.1117090. hdl: 11370/50a4c59b-e2fd-413b-a58f-bd37494432e9 . PMID   16210531. S2CID   28174183.
  10. ↑ Feringa, Ben L.; Koumura, Nagatoshi; Zijlstra, Robert W. J.; Van Delden, Richard A.; Harada, Nobuyuki (1999). "Light-driven monodirectional molecular rotor" (PDF). Nature. 401 (6749): 152–5. Bibcode:1999Natur.401..152K. doi:10.1038/43646. hdl: 11370/d8399fe7-11be-4282-8cd0-7c0adf42c96f . PMID   10490022. S2CID   4412610.
  11. ↑ Vicario, Javier; Walko, Martin; Meetsma, Auke; Feringa, Ben L. (2006). "Fine Tuning of the Rotary Motion by Structural Modification in Light-Driven Unidirectional Molecular Motors" (PDF). Journal of the American Chemical Society. 128 (15): 5127–35. doi:10.1021/ja058303m. PMID   16608348.
  12. ↑ Vicario, Javier; Meetsma, Auke; Feringa, Ben L. (2005). "Controlling the speed of rotation in molecular motors. Dramatic acceleration of the rotary motion by structural modification". Chemical Communications (47): 5910–2. doi:10.1039/b507264f. PMID   16317472.
  13. ↑ Morin, Jean-François; Shirai, Yasuhiro; Tour, James M. (2006). "En Route to a Motorized Nanocar". Organic Letters. 8 (8): 1713–6. doi:10.1021/ol060445d. PMID   16597148.
  14. ↑ Carroll, Gregory T.; Pollard, Michael M.; Van Delden, Richard; Feringa, Ben L. (2010). "Controlled rotary motion of light-driven molecular motors assembled on a gold film". Chemical Science. 1: 97. doi:10.1039/C0SC00162G. S2CID   97346507.
  15. ↑ Carroll, Gregory T.; London, Gábor; Landaluce, Tatiana FernáNdez; Rudolf, Petra; Feringa, Ben L. (2011). "Adhesion of photon-driven molecular motors to surfaces via 1, 3-dipolar cycloadditions: Effect of interfacial interactions on molecular motion" (PDF). ACS Nano. 5 (1): 622–30. doi:10.1021/nn102876j. PMID   21207983. S2CID   39105918.
  16. ↑ Wang, J.; Feringa, B. L. (2011). "Dynamic Control of Chiral Space in a Catalytic Asymmetric Reaction Using a Molecular Motor Science". Science. 331 (6023): 1429–32. Bibcode:2011Sci...331.1429W. doi:10.1126/science.1199844. PMID   21310964. S2CID   24556473.
  17. ↑ Ooi, T. (2011). "Heat and Light Switch a Chiral Catalyst and Its Products". Science. 331 (6023): 1395–6. Bibcode:2011Sci...331.1395O. doi:10.1126/science.1203272. PMID   21415343. S2CID   206532839.
  18. ↑ Tierney, H.; Murphy, C.; Jewell, A. (2011). "Experimental demonstration of a single-molecule electric motor". Nature Nanotechnology. 6 (10): 625–629. Bibcode:2011NatNa...6..625T. doi:10.1038/nnano.2011.142. PMID   21892165.
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