Personal Account
THE CHEMICAL RECORD
Development of an Electrically Driven Molecular Motor Colin J. Murphy and E. Charles H. Sykes*[a] Department of Chemistry, Tufts University, Medford, Massachusetts 02155 (USA) E-mail: [email protected] Homepage: http://ase.tufts.edu/chemistry/sykes/index.html
[a]
Received: February 26, 2014 Published online: July 22, 2014
ABSTRACT: For molecules to be used as components in molecular machinery, methods are required that couple individual molecules to external energy sources in order to selectively excite motion in a given direction. While significant progress has been made in the construction of synthetic molecular motors powered by light and by chemical reactions, there are few experimental examples of electrically driven molecular motors. To this end, we pioneered the use of a new, stable and tunable molecular rotor system based on surface-bound thioethers to comprehensively study many aspects of molecular rotation. As biological molecular motors often operate at interfaces, our synthetic system is especially amenable to microscopic interrogation as compared to solution-based systems. Using scanning tunneling microscopy (STM) and density functional theory, we studied the rotation of surface-bound thioethers, which can be induced either thermally or by electrons from the STM tip in a two-terminal setup. Moreover, the temperature and electron flux can be adjusted to allow each rotational event to be monitored at the molecular scale in real time. This work culminated in the first experimental demonstration of a single-molecule electric motor, where the electrically driven rotation of a butyl methyl sulfide molecule adsorbed on a copper surface could be directionally biased. The direction and rate of the rotation are related to the chirality of both the molecule and the STM tip (which serves as the electrode), illustrating the importance of the symmetry of the metal contacts in atomic-scale electrical devices. DOI 10.1002/tcr.201402007 Keywords: density functional calculations, molecular devices, scanning probe microscopy, singlemolecule studies, thioethers
Introduction Molecular machines are ubiquitous in nature, and exhibit various functions such as organizing the cellular cytoplasm through vesicle transport or powering the motion of cells and even driving whole-body locomotion via muscle contraction.[1,2] In stark contrast current synthetic devices, with the exception of liquid crystals, make no use of nanoscale molecular motion.[3] This is partly due to a gap in the understanding
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of how individual molecular components behave in the face of opposing forces such as friction, thermal fluctuations and coupling to neighboring molecules. A lot of the current understanding of molecular machines has been generated by synthesizing complex organic structures and studying their properties.[4–7] Many of these studies were performed on molecules in solution; however, in nature most
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© 2014 The Chemical Society of Japan and Wiley-VCH, Weinheim
Development of an Electrically Driven Molecular Motor
Fig. 1. Schematic of a flashing temperature ratchet. Particles are driven to the left by repeated heating/cooling cycles.
molecular machines operate at interfaces like those at membrane surfaces or on microtubules. Therefore, mastering the properties of surface-bound systems is essential for harnessing their utility. Studying the motion of molecules bound to surfaces also offers the advantage that a single layer can be assembled and monitored using the tools of surface
Colin Murphy obtained his B.S. (Hons) in Applied Chemistry from Cork Institute of Technology, Ireland. He worked in an industrial setting as a quality control specialist for Millipore Ireland and Pfizer Ireland before moving to Tufts University to begin a Ph.D. under the supervision of Professor Charles Sykes. His interests lie in tuning molecule–surface interactions and molecular devices that work at interfaces.
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science.[7–15] To highlight one example of directed rotary motion: Feringa and co-workers reported a light-driven unidirectional molecular motor that utilized the chiral helicity of a molecule and produced 360° unidirectional motion around a double bond upon irradiation with light and thermally activated relaxation steps.[16] Liquid-crystal films doped with 1% of their light-driven unidirectional molecular motor have been shown to be capable of rotating objects with near-macroscopic dimensions.[17] This experiment was the first demonstration of collective rotations of molecules driving macroscopic motion and illustrates the great potential for incorporation of molecular machines into useful devices.[18] We aim to discover more general approaches to directing molecular rotation of molecules on surfaces that allow ensembles of molecules to act in concert. While we have excited vibrational states of a molecule with a scanning tunneling microscope (STM) tip, global activation of directed rotation of all the molecules on a surface will be possible by exciting the system either with a macroscopic electron or pulsed light source (Figure 1).[19–22] For example, electron guns can be used to macroscopically excite the vibrational modes of molecules,[23] and light irradiation of surface-bound molecules couples to their motion via excited electrons in the metal.[24] To focus on understanding and actuating the rotation of individual molecules on surfaces, we pioneered the use of a new, stable, and tunable molecular rotor system based on surface-bound thioethers. We studied many aspects of molecular rotation, including the effect of temperature,[25,26] molecular structure,[25,27] surface,[28,29] binding site,[28,30] proximity of neighboring molecules,[31] and electric fields/current,[26,32] as well as the influence of molecular[33] and probe chirality[34–36]
Charles Sykes is a Professor of Chemistry at Tufts University. Charles obtained his B.S. and M.S. from Oxford University before moving to Cambridge University for a Ph.D. under the supervision of Professor Richard Lambert. He then relocated to the U.S. to start postdoctoral fellowships with Professor Paul Weiss at Penn State and Professor Mike Fiddy at the University of North Carolina at Charlotte. Research in the Sykes Group at Tufts University is aimed at understanding a range of technologically important systems from molecular motors to chiral and catalytic alloy surfaces. Sykes has been named a Beckman Young Investigator, Research Corporation Cottrell Scholar, IUPAC Young Observer and the Usen Family Career Development Professor. He is also the recipient of a 2009 NSF CAREER award, a 2011 Camille Dreyfus Teacher-Scholar Award and the 2012 AVS Peter Mark Memorial Award.
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Fig. 3. STM images showing how a spinning dibutyl sulfide molecular rotor can be “braked” by physically moving it towards a chain of static molecules. Adapted with permission from reference [25]. Copyright 2009 American Chemical Society. Fig. 2. STM images of a set of thioether rotors: dimethyl, diethyl, dibutyl, and dihexyl sulfide, both static (upper images) and spinning (lower images). Dimethyl sulfide rotates even at 7 K, whereas diethyl, dibutyl and dihexyl sulfide all begin to rotate at a temperature around 16 K. Adapted with permission from reference [25]. Copyright 2009 American Chemical Society.
on directed rotation. Here we describe our scanning tunneling microscopy and density functional theory (DFT) work, which culminated in the first experimental demonstration of a singlemolecule electric motor.[35]
Thioethers as Tunable Molecular Rotors When anchored to a metal surface, thioethers bind through the sulfur atom to a metal atom, and the molecules can rotate like a propeller around this bond.[30,31] When imaging under nonperturbative scanning conditions and at a temperature of 7 K, the molecules are static and appear in the STM images as crescent-shaped protrusions, as shown in the top row of Figure 2. The lower panels show the same molecular rotors at ∼17 K, again under non-perturbative scanning conditions.[27] At these elevated temperatures the molecules rotate via fast interconversion between six equivalent orientations dictated by the high symmetry directions of the Au(111) surface. The molecular rotation occurs much faster than the timescale of STM imaging (ca. 2 min per image); thus, the molecules appear as hexagons due to the time-averaged imaging of all six rotational orientations. Interestingly, the thermal onset to rotation was found to be nearly identical for studied thioether molecules with alkyl tails of two carbons or more. We propose that this plateau in thermal onset is due to interplay between degrees of freedom in the alkyl tail, S–metal bond length, and surface interactions, which was supported by subsequent molecular dynamics calculations.[27] Arrhenius measurements yielded rotational barriers for the ethyl, butyl and hexyl rotors of ∼1 kJ/mol.[25,26] Through a series of single-molecule manipulation experiments, we have mechanically switched the rotation on and off reversibly by moving the molecules toward or away from one another. Figure 3 shows such an experiment, in which a single spinning dibutyl sulfide rotor was moved into contact with a
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chain of three static rotors and its motion was quenched by van der Waals forces between the alkyl tails of adjacent molecules.[25] One of the major goals for the field of molecular rotors is creating ordered arrays with which to study rotational energy propagation. Our mechanical deactivation of dibutyl sulfide molecules demonstrates that there will be a complex interplay between sterics and electrostatics that will mediate the rotational coupling of neighboring molecules. This ability to accurately move and position individual molecular rotors will allow many important aspects of mechanical coupling between individual molecules to be studied in a quantitative manner.
Measuring Individual Molecule Rotational Rate and Directionality In order to measure the rate at which these molecular rotors spin, tunneling current vs. time experiments were performed.[26] As an individual molecule rotates, the distance between the STM tip and alkyl tails varies, causing the tunneling current to fluctuate. Placing the STM tip asymmetrically to the side of one of the six lobes of dibutyl sulfide results in the formation of three discrete states in the tunneling current (see Figure 4). These three current values corresponded to the three inequivalent orientations of the molecule with respect to the STM tip. The tunneling current is inversely dependent on the distance between the tip and the closest point on the molecule. The highest tunneling current corresponded to the orientation of the molecule (shaded in red) where one alkyl chain is directly under the tip (black dot). The next highest tunneling current corresponded to the molecule lying in the orientation shaded in green, and the lowest current reading came from the molecule lying at an angle that resulted in the largest separation to the tip (shaded in blue). These three-state I vs. t plots allowed the direction of rotation of individual molecules to be monitored. As would be expected from the second law of thermodynamics, we have found that thermal excitation leads to a random progression of rotational direction (analysis of >10,000 thermally induced rotational events yielded 0.0 ± 0.2% net directionality). The molecule essentially flips
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© 2014 The Chemical Society of Japan and Wiley-VCH, Weinheim
Development of an Electrically Driven Molecular Motor
Fig. 5. (A) STM image of a 2D layer of Ag on Cu(111) bimetallic surface that self-assembles into a regular hexagonal array of stacking dislocations. (B) Molecular rotors self-organize on the bimetallic array in a manner analogous to placing cogs on a pegboard (C). The white parallelogram shows the unit cell of the Ag/Cu bimetallic surface. Adapted with permission from reference [29]. Copyright 2009 American Chemical Society.
Fig. 4. Plots of tunneling current vs. time reveal that the dibutyl rotors reside in three inequivalent orientations (green, red, and blue) with respect to the STM tip position (black dot). Changes in the position of the thioether alkyl tail result in three distinct levels of tunneling current from which the rotation rate and the direction of rotation can be measured. Adapted with permission from reference [25]. Copyright 2009 American Chemical Society.
between the three orientations with no knowledge of the previous state.[35] Asymmetry in the thioether molecules leads to an increase in the number of distinguishable rotational orientations of the molecule. The asymmetric thioether butyl methyl sulfide (BuSMe) has six discrete states, compared to the three observed for dibutyl sulfide, resulting from the different lengths of the butyl and methyl chains.[35] The three highest current states occur when the butyl chain is closest to the tip, as the tip–molecule distance is minimized, while the lower current states occur when the methyl chain is directed towards the STM tip. The increased number of states allowed both the hop direction and hop length to be investigated.
Two-Dimensional Ordering of Molecular Rotors While an electrical coupling scheme for powering synthetic molecular motors would allow for highly localized injection and offer the ability to address molecules individually, methods for aligning rotary elements are crucial for harnessing and transmitting rotational energy. 2D ordered arrays of dipolar rotors are a key step towards manufacturing molecular devices and investigating the relevant physical properties.[37,38] For example, rotor arrays could be used to propagate waves of rotary motion at speeds much lower than typical phonon velocities. This functionality may play a role similar to surface
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acoustic waves, which are currently used in radio-frequency filters.[5] Collections of dipolar rotors also represent a ferroelectric surface built from individual molecules and may offer many interesting and useful dielectric properties.[5,37,38] We engineered a bimetallic surface system with a regular array of dislocations and studied the adsorption of molecular rotors, namely dibutyl sulfide, onto this surface, as shown in Figure 5. Due to size differences between the Ag and Cu atoms, a single layer of Ag deposited onto Cu(111) reconstructs the Cu surface into a regular array of hexagonal close-packed (hcp) domains with an average spacing of 2.6 ± 0.1 nm surrounded by facecentered cubic close-packed areas.[29] STM imaging was used to investigate the affinity of adsorbates for these different domains. The binding preference for hcp sites of dibutyl sulfide molecules was used to spatially control the distribution of single rotor molecules in a hexagonal pattern, in a manner analogous to placing cogs on a pegboard.[29] These results open up the possibility to study 2D arrays of either sterically or electrostatically interacting rotors with atomic-scale detail.
Powering Rotation Electrically While rotation can be induced by thermal energy, thermodynamics stipulates that in the absence of a thermal gradient, the rotation direction will be random and the system will be incapable of producing useful work.[39] Therefore, for molecules to meet their full potential as components in molecular machines, methods for coupling them to external sources of energy that selectively excite the desired motions must be devised. Our work has focused on coupling electrons from an STM tip to the rotation of individual molecular rotors, as seen in Figure 6.[32] In order to minimize the effects of thermally induced rotation the system was cooled to temperatures below 8 K, where the molecules were static and could be stably imaged for many hours at tunneling voltages less than ±0.36 V. However, either scanning or positioning the STM tip over the molecules at biases above ±0.36 V caused them to switch between their
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Fig. 7. Single-molecule chiral rotors. (a) STM image of the two enantiomeric forms of BuSMe adsorbed on Cu(111). (b) Schematic showing that while BuSMe is achiral in the gas phase, adsorption on a surface results in two enantiomers depending on which of the prochiral lone pairs bonds to the surface. Inset shows the DFT-calculated asymmetric “ratchet-like” rotational barrier of BuSMe. Adapted with permission from reference [33]. Copyright 2011 American Chemical Society. Fig. 6. STM images of (a) static and (b) spinning dibutyl sulfide molecular rotors on Au(111) at 7 and 78 K respectively. Schematics of the static and spinning rotors appear in insets. Panel (c) shows a tunneling current vs. time (I vs. t) plot taken during the electrically induced rotational excitation of the rotor seen in panel (d). The white circles in (d) and (e) mark the electron injection point. Panel (e) shows that after excitation the molecule rotated to a new orientation. Reprinted with permission from reference [32].
three distinct orientations. With isotopic labeling experiments in which deuterated molecules were excited at biases above ±0.28 V, we showed that the mechanism for this electrically induced rotation is the excitation of a C–H/C–D stretch which then decays to the rotation of the molecule.[32]
Creating Ratchet-Like Rotational Barriers by Tuning Molecular Structure In order to construct molecular rotors capable of directed rotation we turned to asymmetric thioethers.[28,33,40] Molecular asymmetry can lead to chirality in the adsorbed state and potentially yield an asymmetric rotational potential energy landscape. BuSMe is achiral in the gas phase but, due to its asymmetric alkyl tails, has two prochiral lone pairs on the central S atom that give rise to chirality in the surface-bound molecules.[33,36] The STM image in Figure 7 reveals that the two enantiomers of BuSMe appear as pinwheels related by mirror symmetry. The left- and right-handed pinwheels are named here by the Cahn–Ingold–Prelog rules as “R” and “S”. DFT calculations revealed that the rotational barrier of BuSMe is asymmetric (as seen in the inset of Figure 7) and hence of interest for directed rotation.[33] This set of DFT calculations
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also revealed that the barrier to inversion between the energetically equivalent enantiomers was 0.24 eV. This relatively high barrier was also evident with STM experiments, as the rotors were not observed to switch chirality under normal scanning conditions at either 5 or 78 K. Only by supplying high-energy electrons (>0.4 eV) with the STM tip was it possible to overcome the high barrier to inversion and switch the chirality of the adsorbed molecules.[36]
Regular STM Tips Can Be Intrinsically Chiral Careful measurements of chiral molecular rotors revealed a particularly surprising result: different tunneling probabilities were observed through right- and left-handed molecules.[34–36] This indicated that the symmetry of the STM tip itself must have an influence on the tunneling process and, in cases in which right- and left-handed molecules have different tunneling probabilities (i.e., apparent heights), the STM tip itself is chiral.[34] About 80% of all STM tips tested (both the etched W and cut Pt/Ir, the most commonly used STM probes) displayed this effect and hence are chiral. This result enabled us to differentiate between individual chiral molecules solely based on their apparent heights in STM.[34,36] Perhaps most importantly we found that the tunneling electrons from a chiral tip have a stronger coupling to either right- or lefthanded molecules, resulting in rates of rotation of R and S rotors that differed by a factor of three under identical electrical excitation conditions.[34,35] Given that all molecular devices require contact electrodes, this result has important consequences for the design of future devices that aim to
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Development of an Electrically Driven Molecular Motor
electrically control molecular motion. We illustrate the importance of both the structure and chirality of the electrode and report the previously unforeseen and unexpected effect on directionality of motion. This relates to many systems that require electrical contacts to molecules that are either chiral or become chiral in the adsorbed complex, including break junctions, nanoscale electrodes, nanopores, nanowires, Hg drop contacts, crosswire assemblies and scanning probes. Up to this point, there are no experimental data in the literature showing such diastereomeric effects in the electrical excitation of molecules in these types of junctions.
Demonstration of a Single-Molecule Motor After two years of careful measurements of the electrical excitation of BuSMe rotors, we reported preferential directionality based on the chirality of the rotor molecules.[35] Such externally driven preferential directional motion is a defining characteristic of molecular motors. Both the rate and direction were measured as a function of STM tip chirality for the R and S surface-bound enantiomers of BuSMe. The rate and directionality were highly STM tip dependent, as evidenced by the different behaviors of opposite enantiomers of the molecular rotors. As a control for these electrically driven rotation experiments, the system was heated from 5 to 8 K to thermally activate BuSMe rotation under non-perturbative tunneling conditions while the rotation was recorded. In each experiment, between 103 and 105 molecular rotational events were recorded, which revealed clockwise and anticlockwise rotations were equally probable. When electrically excited, the molecules exhibited a preferential rotation that depended on the rotor’s chirality and a rate that was related to the chirality of the tip of the microscope (which serves as the electrode). Preferential directionality of up to 5% was observed, which reflects the slight asymmetry of the rotational energy landscape (Figure 7, inset). Most strikingly, the differences in the excitation rates of enantiomeric forms of the motor molecule were up to a factor of three: 30 Hz vs. 90 Hz under identical excitation conditions. Given that all electrical molecular devices require contact electrodes, this work illustrates the importance of both the structure and chirality of these elements. These results illustrate that a two-terminal setup, consisting of an STM tip and a metal surface, when coupled with an asymmetric ratchet-like potential is capable of producing directional rotational motion. These experimental methods and concepts provide a solid basis for future work aimed at directed molecular translational motion.
than using a more traditional macroscopic approach like applying directional external forces to guide molecular motion, we showed that a combination of an asymmetric potential energy landscape and electrical excitation of vibrational modes can be used to bias rotational direction.[35] This new method for electrically directing molecular motion is of broad interest as it should be generalizable to other systems, one example of which was recently shown by Hla and co-workers.[9] Compared with macroscopic machinery, small percentages of directional motion at hop rates on the order of minutes is not particularly impressive. However, all these experiments are performed with pA tunneling currents delivered over the whole scan window to ensure that the excitation event rate is much lower than the image acquisition rate. Given the short excited-state lifetime of molecules on metals (∼ps), higher currents or photon fluxes that yield higher excitation rates (and hence more net directional hops per second) could be applied to boost the rate of directional translation. There is a significant opportunity for single-molecule measurements to answer many of the current questions in the field of molecular machines and have a major impact on efforts to control all types of molecular motion. Development of novel microscopic mechanisms for the coupling of electronic energy to directed molecular rotation will provide important proofof-principle demonstrations that are generalizable in other systems and fields. This type of enabling technology is crucial for the rational design of new molecular machinery with functionalities such as mass transport, propulsion, separations, sensing, signaling and chemical reactions.
Acknowledgements This work was supported by the National Science Foundation under grants CHE-0844343/CHE-1412402.
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Development of an Electrically Driven Molecular Motor Colin J. Murphy and E. Charles H. Sykes*[a] Department of Chemistry, Tufts University, Medford, Massachusetts 02155 (USA) E-mail: [email protected] Homepage: http://ase.tufts.edu/chemistry/sykes/index.html
[a]
Received: February 26, 2014 Published online: July 22, 2014
ABSTRACT: For molecules to be used as components in molecular machinery, methods are required that couple individual molecules to external energy sources in order to selectively excite motion in a given direction. While significant progress has been made in the construction of synthetic molecular motors powered by light and by chemical reactions, there are few experimental examples of electrically driven molecular motors. To this end, we pioneered the use of a new, stable and tunable molecular rotor system based on surface-bound thioethers to comprehensively study many aspects of molecular rotation. As biological molecular motors often operate at interfaces, our synthetic system is especially amenable to microscopic interrogation as compared to solution-based systems. Using scanning tunneling microscopy (STM) and density functional theory, we studied the rotation of surface-bound thioethers, which can be induced either thermally or by electrons from the STM tip in a two-terminal setup. Moreover, the temperature and electron flux can be adjusted to allow each rotational event to be monitored at the molecular scale in real time. This work culminated in the first experimental demonstration of a single-molecule electric motor, where the electrically driven rotation of a butyl methyl sulfide molecule adsorbed on a copper surface could be directionally biased. The direction and rate of the rotation are related to the chirality of both the molecule and the STM tip (which serves as the electrode), illustrating the importance of the symmetry of the metal contacts in atomic-scale electrical devices. DOI 10.1002/tcr.201402007 Keywords: density functional calculations, molecular devices, scanning probe microscopy, singlemolecule studies, thioethers
Introduction Molecular machines are ubiquitous in nature, and exhibit various functions such as organizing the cellular cytoplasm through vesicle transport or powering the motion of cells and even driving whole-body locomotion via muscle contraction.[1,2] In stark contrast current synthetic devices, with the exception of liquid crystals, make no use of nanoscale molecular motion.[3] This is partly due to a gap in the understanding
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of how individual molecular components behave in the face of opposing forces such as friction, thermal fluctuations and coupling to neighboring molecules. A lot of the current understanding of molecular machines has been generated by synthesizing complex organic structures and studying their properties.[4–7] Many of these studies were performed on molecules in solution; however, in nature most
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Fig. 1. Schematic of a flashing temperature ratchet. Particles are driven to the left by repeated heating/cooling cycles.
molecular machines operate at interfaces like those at membrane surfaces or on microtubules. Therefore, mastering the properties of surface-bound systems is essential for harnessing their utility. Studying the motion of molecules bound to surfaces also offers the advantage that a single layer can be assembled and monitored using the tools of surface
Colin Murphy obtained his B.S. (Hons) in Applied Chemistry from Cork Institute of Technology, Ireland. He worked in an industrial setting as a quality control specialist for Millipore Ireland and Pfizer Ireland before moving to Tufts University to begin a Ph.D. under the supervision of Professor Charles Sykes. His interests lie in tuning molecule–surface interactions and molecular devices that work at interfaces.
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science.[7–15] To highlight one example of directed rotary motion: Feringa and co-workers reported a light-driven unidirectional molecular motor that utilized the chiral helicity of a molecule and produced 360° unidirectional motion around a double bond upon irradiation with light and thermally activated relaxation steps.[16] Liquid-crystal films doped with 1% of their light-driven unidirectional molecular motor have been shown to be capable of rotating objects with near-macroscopic dimensions.[17] This experiment was the first demonstration of collective rotations of molecules driving macroscopic motion and illustrates the great potential for incorporation of molecular machines into useful devices.[18] We aim to discover more general approaches to directing molecular rotation of molecules on surfaces that allow ensembles of molecules to act in concert. While we have excited vibrational states of a molecule with a scanning tunneling microscope (STM) tip, global activation of directed rotation of all the molecules on a surface will be possible by exciting the system either with a macroscopic electron or pulsed light source (Figure 1).[19–22] For example, electron guns can be used to macroscopically excite the vibrational modes of molecules,[23] and light irradiation of surface-bound molecules couples to their motion via excited electrons in the metal.[24] To focus on understanding and actuating the rotation of individual molecules on surfaces, we pioneered the use of a new, stable, and tunable molecular rotor system based on surface-bound thioethers. We studied many aspects of molecular rotation, including the effect of temperature,[25,26] molecular structure,[25,27] surface,[28,29] binding site,[28,30] proximity of neighboring molecules,[31] and electric fields/current,[26,32] as well as the influence of molecular[33] and probe chirality[34–36]
Charles Sykes is a Professor of Chemistry at Tufts University. Charles obtained his B.S. and M.S. from Oxford University before moving to Cambridge University for a Ph.D. under the supervision of Professor Richard Lambert. He then relocated to the U.S. to start postdoctoral fellowships with Professor Paul Weiss at Penn State and Professor Mike Fiddy at the University of North Carolina at Charlotte. Research in the Sykes Group at Tufts University is aimed at understanding a range of technologically important systems from molecular motors to chiral and catalytic alloy surfaces. Sykes has been named a Beckman Young Investigator, Research Corporation Cottrell Scholar, IUPAC Young Observer and the Usen Family Career Development Professor. He is also the recipient of a 2009 NSF CAREER award, a 2011 Camille Dreyfus Teacher-Scholar Award and the 2012 AVS Peter Mark Memorial Award.
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Fig. 3. STM images showing how a spinning dibutyl sulfide molecular rotor can be “braked” by physically moving it towards a chain of static molecules. Adapted with permission from reference [25]. Copyright 2009 American Chemical Society. Fig. 2. STM images of a set of thioether rotors: dimethyl, diethyl, dibutyl, and dihexyl sulfide, both static (upper images) and spinning (lower images). Dimethyl sulfide rotates even at 7 K, whereas diethyl, dibutyl and dihexyl sulfide all begin to rotate at a temperature around 16 K. Adapted with permission from reference [25]. Copyright 2009 American Chemical Society.
on directed rotation. Here we describe our scanning tunneling microscopy and density functional theory (DFT) work, which culminated in the first experimental demonstration of a singlemolecule electric motor.[35]
Thioethers as Tunable Molecular Rotors When anchored to a metal surface, thioethers bind through the sulfur atom to a metal atom, and the molecules can rotate like a propeller around this bond.[30,31] When imaging under nonperturbative scanning conditions and at a temperature of 7 K, the molecules are static and appear in the STM images as crescent-shaped protrusions, as shown in the top row of Figure 2. The lower panels show the same molecular rotors at ∼17 K, again under non-perturbative scanning conditions.[27] At these elevated temperatures the molecules rotate via fast interconversion between six equivalent orientations dictated by the high symmetry directions of the Au(111) surface. The molecular rotation occurs much faster than the timescale of STM imaging (ca. 2 min per image); thus, the molecules appear as hexagons due to the time-averaged imaging of all six rotational orientations. Interestingly, the thermal onset to rotation was found to be nearly identical for studied thioether molecules with alkyl tails of two carbons or more. We propose that this plateau in thermal onset is due to interplay between degrees of freedom in the alkyl tail, S–metal bond length, and surface interactions, which was supported by subsequent molecular dynamics calculations.[27] Arrhenius measurements yielded rotational barriers for the ethyl, butyl and hexyl rotors of ∼1 kJ/mol.[25,26] Through a series of single-molecule manipulation experiments, we have mechanically switched the rotation on and off reversibly by moving the molecules toward or away from one another. Figure 3 shows such an experiment, in which a single spinning dibutyl sulfide rotor was moved into contact with a
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chain of three static rotors and its motion was quenched by van der Waals forces between the alkyl tails of adjacent molecules.[25] One of the major goals for the field of molecular rotors is creating ordered arrays with which to study rotational energy propagation. Our mechanical deactivation of dibutyl sulfide molecules demonstrates that there will be a complex interplay between sterics and electrostatics that will mediate the rotational coupling of neighboring molecules. This ability to accurately move and position individual molecular rotors will allow many important aspects of mechanical coupling between individual molecules to be studied in a quantitative manner.
Measuring Individual Molecule Rotational Rate and Directionality In order to measure the rate at which these molecular rotors spin, tunneling current vs. time experiments were performed.[26] As an individual molecule rotates, the distance between the STM tip and alkyl tails varies, causing the tunneling current to fluctuate. Placing the STM tip asymmetrically to the side of one of the six lobes of dibutyl sulfide results in the formation of three discrete states in the tunneling current (see Figure 4). These three current values corresponded to the three inequivalent orientations of the molecule with respect to the STM tip. The tunneling current is inversely dependent on the distance between the tip and the closest point on the molecule. The highest tunneling current corresponded to the orientation of the molecule (shaded in red) where one alkyl chain is directly under the tip (black dot). The next highest tunneling current corresponded to the molecule lying in the orientation shaded in green, and the lowest current reading came from the molecule lying at an angle that resulted in the largest separation to the tip (shaded in blue). These three-state I vs. t plots allowed the direction of rotation of individual molecules to be monitored. As would be expected from the second law of thermodynamics, we have found that thermal excitation leads to a random progression of rotational direction (analysis of >10,000 thermally induced rotational events yielded 0.0 ± 0.2% net directionality). The molecule essentially flips
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Fig. 5. (A) STM image of a 2D layer of Ag on Cu(111) bimetallic surface that self-assembles into a regular hexagonal array of stacking dislocations. (B) Molecular rotors self-organize on the bimetallic array in a manner analogous to placing cogs on a pegboard (C). The white parallelogram shows the unit cell of the Ag/Cu bimetallic surface. Adapted with permission from reference [29]. Copyright 2009 American Chemical Society.
Fig. 4. Plots of tunneling current vs. time reveal that the dibutyl rotors reside in three inequivalent orientations (green, red, and blue) with respect to the STM tip position (black dot). Changes in the position of the thioether alkyl tail result in three distinct levels of tunneling current from which the rotation rate and the direction of rotation can be measured. Adapted with permission from reference [25]. Copyright 2009 American Chemical Society.
between the three orientations with no knowledge of the previous state.[35] Asymmetry in the thioether molecules leads to an increase in the number of distinguishable rotational orientations of the molecule. The asymmetric thioether butyl methyl sulfide (BuSMe) has six discrete states, compared to the three observed for dibutyl sulfide, resulting from the different lengths of the butyl and methyl chains.[35] The three highest current states occur when the butyl chain is closest to the tip, as the tip–molecule distance is minimized, while the lower current states occur when the methyl chain is directed towards the STM tip. The increased number of states allowed both the hop direction and hop length to be investigated.
Two-Dimensional Ordering of Molecular Rotors While an electrical coupling scheme for powering synthetic molecular motors would allow for highly localized injection and offer the ability to address molecules individually, methods for aligning rotary elements are crucial for harnessing and transmitting rotational energy. 2D ordered arrays of dipolar rotors are a key step towards manufacturing molecular devices and investigating the relevant physical properties.[37,38] For example, rotor arrays could be used to propagate waves of rotary motion at speeds much lower than typical phonon velocities. This functionality may play a role similar to surface
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acoustic waves, which are currently used in radio-frequency filters.[5] Collections of dipolar rotors also represent a ferroelectric surface built from individual molecules and may offer many interesting and useful dielectric properties.[5,37,38] We engineered a bimetallic surface system with a regular array of dislocations and studied the adsorption of molecular rotors, namely dibutyl sulfide, onto this surface, as shown in Figure 5. Due to size differences between the Ag and Cu atoms, a single layer of Ag deposited onto Cu(111) reconstructs the Cu surface into a regular array of hexagonal close-packed (hcp) domains with an average spacing of 2.6 ± 0.1 nm surrounded by facecentered cubic close-packed areas.[29] STM imaging was used to investigate the affinity of adsorbates for these different domains. The binding preference for hcp sites of dibutyl sulfide molecules was used to spatially control the distribution of single rotor molecules in a hexagonal pattern, in a manner analogous to placing cogs on a pegboard.[29] These results open up the possibility to study 2D arrays of either sterically or electrostatically interacting rotors with atomic-scale detail.
Powering Rotation Electrically While rotation can be induced by thermal energy, thermodynamics stipulates that in the absence of a thermal gradient, the rotation direction will be random and the system will be incapable of producing useful work.[39] Therefore, for molecules to meet their full potential as components in molecular machines, methods for coupling them to external sources of energy that selectively excite the desired motions must be devised. Our work has focused on coupling electrons from an STM tip to the rotation of individual molecular rotors, as seen in Figure 6.[32] In order to minimize the effects of thermally induced rotation the system was cooled to temperatures below 8 K, where the molecules were static and could be stably imaged for many hours at tunneling voltages less than ±0.36 V. However, either scanning or positioning the STM tip over the molecules at biases above ±0.36 V caused them to switch between their
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Fig. 7. Single-molecule chiral rotors. (a) STM image of the two enantiomeric forms of BuSMe adsorbed on Cu(111). (b) Schematic showing that while BuSMe is achiral in the gas phase, adsorption on a surface results in two enantiomers depending on which of the prochiral lone pairs bonds to the surface. Inset shows the DFT-calculated asymmetric “ratchet-like” rotational barrier of BuSMe. Adapted with permission from reference [33]. Copyright 2011 American Chemical Society. Fig. 6. STM images of (a) static and (b) spinning dibutyl sulfide molecular rotors on Au(111) at 7 and 78 K respectively. Schematics of the static and spinning rotors appear in insets. Panel (c) shows a tunneling current vs. time (I vs. t) plot taken during the electrically induced rotational excitation of the rotor seen in panel (d). The white circles in (d) and (e) mark the electron injection point. Panel (e) shows that after excitation the molecule rotated to a new orientation. Reprinted with permission from reference [32].
three distinct orientations. With isotopic labeling experiments in which deuterated molecules were excited at biases above ±0.28 V, we showed that the mechanism for this electrically induced rotation is the excitation of a C–H/C–D stretch which then decays to the rotation of the molecule.[32]
Creating Ratchet-Like Rotational Barriers by Tuning Molecular Structure In order to construct molecular rotors capable of directed rotation we turned to asymmetric thioethers.[28,33,40] Molecular asymmetry can lead to chirality in the adsorbed state and potentially yield an asymmetric rotational potential energy landscape. BuSMe is achiral in the gas phase but, due to its asymmetric alkyl tails, has two prochiral lone pairs on the central S atom that give rise to chirality in the surface-bound molecules.[33,36] The STM image in Figure 7 reveals that the two enantiomers of BuSMe appear as pinwheels related by mirror symmetry. The left- and right-handed pinwheels are named here by the Cahn–Ingold–Prelog rules as “R” and “S”. DFT calculations revealed that the rotational barrier of BuSMe is asymmetric (as seen in the inset of Figure 7) and hence of interest for directed rotation.[33] This set of DFT calculations
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also revealed that the barrier to inversion between the energetically equivalent enantiomers was 0.24 eV. This relatively high barrier was also evident with STM experiments, as the rotors were not observed to switch chirality under normal scanning conditions at either 5 or 78 K. Only by supplying high-energy electrons (>0.4 eV) with the STM tip was it possible to overcome the high barrier to inversion and switch the chirality of the adsorbed molecules.[36]
Regular STM Tips Can Be Intrinsically Chiral Careful measurements of chiral molecular rotors revealed a particularly surprising result: different tunneling probabilities were observed through right- and left-handed molecules.[34–36] This indicated that the symmetry of the STM tip itself must have an influence on the tunneling process and, in cases in which right- and left-handed molecules have different tunneling probabilities (i.e., apparent heights), the STM tip itself is chiral.[34] About 80% of all STM tips tested (both the etched W and cut Pt/Ir, the most commonly used STM probes) displayed this effect and hence are chiral. This result enabled us to differentiate between individual chiral molecules solely based on their apparent heights in STM.[34,36] Perhaps most importantly we found that the tunneling electrons from a chiral tip have a stronger coupling to either right- or lefthanded molecules, resulting in rates of rotation of R and S rotors that differed by a factor of three under identical electrical excitation conditions.[34,35] Given that all molecular devices require contact electrodes, this result has important consequences for the design of future devices that aim to
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electrically control molecular motion. We illustrate the importance of both the structure and chirality of the electrode and report the previously unforeseen and unexpected effect on directionality of motion. This relates to many systems that require electrical contacts to molecules that are either chiral or become chiral in the adsorbed complex, including break junctions, nanoscale electrodes, nanopores, nanowires, Hg drop contacts, crosswire assemblies and scanning probes. Up to this point, there are no experimental data in the literature showing such diastereomeric effects in the electrical excitation of molecules in these types of junctions.
Demonstration of a Single-Molecule Motor After two years of careful measurements of the electrical excitation of BuSMe rotors, we reported preferential directionality based on the chirality of the rotor molecules.[35] Such externally driven preferential directional motion is a defining characteristic of molecular motors. Both the rate and direction were measured as a function of STM tip chirality for the R and S surface-bound enantiomers of BuSMe. The rate and directionality were highly STM tip dependent, as evidenced by the different behaviors of opposite enantiomers of the molecular rotors. As a control for these electrically driven rotation experiments, the system was heated from 5 to 8 K to thermally activate BuSMe rotation under non-perturbative tunneling conditions while the rotation was recorded. In each experiment, between 103 and 105 molecular rotational events were recorded, which revealed clockwise and anticlockwise rotations were equally probable. When electrically excited, the molecules exhibited a preferential rotation that depended on the rotor’s chirality and a rate that was related to the chirality of the tip of the microscope (which serves as the electrode). Preferential directionality of up to 5% was observed, which reflects the slight asymmetry of the rotational energy landscape (Figure 7, inset). Most strikingly, the differences in the excitation rates of enantiomeric forms of the motor molecule were up to a factor of three: 30 Hz vs. 90 Hz under identical excitation conditions. Given that all electrical molecular devices require contact electrodes, this work illustrates the importance of both the structure and chirality of these elements. These results illustrate that a two-terminal setup, consisting of an STM tip and a metal surface, when coupled with an asymmetric ratchet-like potential is capable of producing directional rotational motion. These experimental methods and concepts provide a solid basis for future work aimed at directed molecular translational motion.
than using a more traditional macroscopic approach like applying directional external forces to guide molecular motion, we showed that a combination of an asymmetric potential energy landscape and electrical excitation of vibrational modes can be used to bias rotational direction.[35] This new method for electrically directing molecular motion is of broad interest as it should be generalizable to other systems, one example of which was recently shown by Hla and co-workers.[9] Compared with macroscopic machinery, small percentages of directional motion at hop rates on the order of minutes is not particularly impressive. However, all these experiments are performed with pA tunneling currents delivered over the whole scan window to ensure that the excitation event rate is much lower than the image acquisition rate. Given the short excited-state lifetime of molecules on metals (∼ps), higher currents or photon fluxes that yield higher excitation rates (and hence more net directional hops per second) could be applied to boost the rate of directional translation. There is a significant opportunity for single-molecule measurements to answer many of the current questions in the field of molecular machines and have a major impact on efforts to control all types of molecular motion. Development of novel microscopic mechanisms for the coupling of electronic energy to directed molecular rotation will provide important proofof-principle demonstrations that are generalizable in other systems and fields. This type of enabling technology is crucial for the rational design of new molecular machinery with functionalities such as mass transport, propulsion, separations, sensing, signaling and chemical reactions.
Acknowledgements This work was supported by the National Science Foundation under grants CHE-0844343/CHE-1412402.
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