16-atom carousel: the smallest molecular rotor in the world

At the micro- and nanometer level, there are many interesting processes that we don’t even suspect, because they are not so easy to see. What our own body is worth: millions of cells from different subsystems harmoniously perform their functions, supporting the vital functions of the body. Among the great variety of unusual molecular formations, it is worth highlighting molecular motors, which include motor proteins (for example, kinesin). The concept of artificial molecular motors has existed since the middle of the last century, and there have been a lot of attempts to create something similar, and all of them were somewhat different from others. Today we will meet with you a study in which scientists from EMPA (Swiss Federal Laboratory for Materials Science and Technology) created a molecular engine of 16 atoms, which makes it the smallest at the moment. How exactly did scientists create a nano-engine, what are its features and capabilities, and how can this development be put into practice? We learn about this from the report of scientists. Go.

Study basis

In 1959, the American scientist Richard Feynman (1918-1988) expressed the theory that someday we will be able to create molecular motors. To some, this idea at that time might have seemed crazy, but skepticism about science has never stopped anyone.

The main mechanism of molecular motors is rotation, for which they are also called molecular rotors. In 1999, T. Ross Kelly and his colleagues published a report (Unidirectional rotary motion in a molecular system), which described the rotational motion of a molecular system through chemical processes.


Professor of Organic Chemistry at Boston College T. Ross Kelly.

The system consists of a three-bladed trypticene rotor and helical and can perform unidirectional rotation of 120 °. To perform this rotation, the system must go through 5 stages.


Kelly molecular rotor (5-step rotation scheme).

The main problem of this method is that the rotation occurs once. Kelly and his colleagues have long tried to find a solution to this problem, but to no avail.

However, Kelly’s method shows that chemical energy can be used to create artificial molecular motors.

In the same 1999, another molecular motor was created at the University of Groningen (Netherlands) under the leadership of Ben Fering (Light-driven monodirectional molecular rotor) Their version could rotate 360 ​​degrees and consisted of bis-helicin connected by a double axial link and having two stereo centers.


Fering molecular rotor (4-stage rotation scheme).

This time, there were four stages for a complete rotation. But even there were some shortcomings: the Feringa motor was extremely slow. In other words, it took longer to rotate than natural equivalents.

In 2008, at the University of Illinois (USA), Peter Kral and colleagues developed a molecular motor, the movement of which is carried out due to resonant or non-resonant electron tunneling (Nanoscale Rotary Motors Driven by Electron Tunneling)


Kral’s molecular motor (rotation pattern due to electron tunneling).

Electron tunneling provides the motor with the energy it needs to move. The motor itself consists of 3 (or 6) blades, formed on the basis of polymerized acetone. A carbon nanotube is used as the axis of the motor.

This method is quite effective in the laboratory, however, its performance may decrease due to noise and structural defects that are inevitably present in natural conditions.

Each of the above options for a molecular motor is unique and in some ways superior to the other two. However, despite the striking differences in the methods of their creation, all of them served as inspiration for subsequent developments, in particular for the one that we will now consider.

Scientists say that most synthetic molecular machines, although controlled by quantum processes, exhibit classical kinetics, while working with quantum tunneling is largely elusive. Consequently, scanning tunneling microscopy (STM) provides an ideal platform for studying the dynamics of atoms and molecules on surfaces. However, only a few studies were aimed at achieving directional movement (controlled and independent of the position of the needle), which requires a violation of the symmetry of the inversion, which is usually achieved by adsorption of chiral molecules on achiral surfaces.

Scientists decided to use this concept, but slightly changing it. As chiral stator * It was decided to use the surface of noncentrosymmetric PdGa crystals (Pd – palladium, Ga – gallium).

Stator * – the fixed part of the motor interacting with the rotor (moving part of the motor).

This weakens the geometric constraints on the rotor molecule and allows for directional motion even for simple and symmetrical molecules such as C2H2.

On pd3 acetylene molecules adsorb over Pd trimers. During STM imaging at 5 K, they look like dumbbells with a spacing of about 3 Å between the petals in three symmetrically equivalent orientations rotated 120 ° (1E1G) between which they switch quasi-instantly (1C and 1D)


Image No. 1

Acetylene molecules are firmly attached to the trimmer and usually dissociate * before they are pulled out of the trimer using a microscope needle.

Dissociation* – decomposition of complex chemical compounds into constituent components.

Scientists observed the rotation process by recording the time series of tunnel current IT

IT

SMT images show that counterclockwise direction prevails in the movement of the motor.


Image No. 2

Analysis of the parametric dependence of the speed (2A2C) shows that this molecular engine operates in two different modes: tunneling mode (TR), where its rotation frequency νT does not depend on temperature (T <15 K), bias voltage (| VG| <30 mV) and current (IT <200 pA); Classic mode (CR), where the speed depends on these parameters.The experimental data (image No. 1) were recorded in the TR mode, but the scientists decided to first consider the classical mode, where the rotation C2H2 can be selectively fueled by thermal or electrical excitations.

To begin with, the temperature dependence of the rotational speed at low bias was found (2A) to follow the characteristic Arrhenius * (solid line on 2A): ν (T) = νT + νANDexp (- ΔEB / kBT), where νT = 4.5 Hz, νAND = 10 8.7 ± 2.0 Hz, ΔEB = 27.5 ± 7.1 meV.

Arrhenius equation * establishes the dependence of the rate constant of a chemical reaction on temperature.

Above 30 mV, the frequency increases exponentially with VG, regardless of polarity (2B and 2C) Under the same conditions, but with a constant bias voltage, power dependence * (ν ∝ InT at n ≈ 1; 2D) identifies electron-stimulated rotation as a one-electron process. The dependence of the frequency and direction of rotation on the parameters T, VG and IT well reproduced by the kinetic Langevin model (solid lines on 2B and 2C)

Power law * – a relative change in one quantity leads to a proportional relative change in another quantity.

Scientists note that an important role in the analysis of the entire system is played by understanding the influence of the microscope needle, which is necessary for actual observation of the movement. In particular, it is necessary to make sure that the violation of the symmetry of the inversion due to the position of the needle near the engine does not prevail over the influence of the chiral substrate in determining the direction of rotation.

For this, 6400 time series were measured with a constant tip height zT

Additional modeling, during which the configuration of the molecule and the shape of the needle was optimized, made it possible to obtain the ideal sequence (circuit) of signals (2F) Therefore, regardless of the position of the needle, the signal sequence always corresponds to counterclockwise rotation.

Also, as seen on 2G, there is no obvious dependence of νT on the position of the needle. Therefore, we can assume that all three rotational configurations of C2H2 will be energetically equivalent. Three rotational states become energetically non-degenerate only if the needle is brought too close to the substrate.

Estimating 1792 rotation events (nCcw= 1771 and nCw = 21) in the tunneling mode, the directivity dir ≥ 96.7% was determined with a confidence of 2σ. By comparing the results of modeling and experiments, it was possible to determine the rotation C2H2, which can be described as a rotating rotor, the center of mass of which moves in a circle with a radius r = 0.5 ± 0.1 Å and moment of inertia IC2H2 = 5.62 x 10-46 kgm2 (2H)


Image No. 3

Having established the degree of influence of the microscope needle on the rotation of the system, scientists began a detailed examination of the dependence of rotation on the parameters of the system (3A3D) The temperature dependence shows a rapid decrease in directivity when thermally activated rotations begin to make a significant contribution. Solid line on 3A suggests that νT has a 98% directivity, while the thermally activated jumps described by the Arrhenius equation are purely random.

These random thermal rotation events are expected because the substrate, the STM needle and, therefore, the molecules are in thermal equilibrium and, accordingly, unidirectional rotation (which reduces entropy) is prohibited by the second law of thermodynamics.

At T = 5 K, a decrease in directivity is also observed for bias voltages VG above 35 meV (3B) However, unlike thermal rotations, those caused by inelastic electron tunneling (IET) become non-directional gradually. This is clearly observed in the regime when thermal and IET rotations coexist. As shown in 3Cvoltage-independent directivity (10% at T = 19 K and | VG| <30 mV) can be significantly increased at higher | VG | due to additional directional rotations of the IET. However, this increase is effective only in a narrow voltage range, outside of which (upward) directionality decreases rapidly.In contrast, ITThe directivity dependence for a fixed voltage is weak (3D), where a slight decrease in directivity with increasing current is explained by the detection of two rapidly successive counterclockwise rotations as one erroneous continuous rotation. (solid lines on 3D) From this it follows that the directivity remains above 95% at | VG| <40 mV even at high current.To simulate the kinetics of events in this system, it was decided to use the concept of biased Brownian motion *proposed in the study of Astumyan (The Physics and Physical Chemistry of Molecular Machines) and Hanji (Artificial Brownian motors: Controlling transport on the nanoscale)

Brownian motion* – random motion of solid particles caused by thermal motion of liquid or gas particles.

The obtained model of IET-induced rotation assumes a static and periodic, but asymmetric potential U (ϕ) (ϕ = [0.2π] with periodicity π / 3) with potential asymmetry (Rasyminsert on 3E)

One IET event is sufficient for instantaneous excitation of a molecule from its ground state, and its trajectory ϕ

Depending on Rasym and λ, two different minimal kinetic energies EL and ER are required to overcome the barrier on the left (i.e., for clockwise movement) and on the right (i.e., for counterclockwise movement), respectively. These energies are the basis for describing the frequency and directivity using the used kinetic model.

Comparison of the kinetic model and experimental results (2C and 3C) allows you to determine the temperature-dependent EL(T) and ER(T) (3E) As a result, it was found that Rasym equal to 1.25 asym <1.5, assuming that ΔEB = 25 meV.

Decreased λ dissipation from 1.6 x 10-33 kgm2/ s at 5 K up to 1.1 x 10-33 kgm2/ s at 20 K can be explained by a less efficient binding of the molecule to the substrate with increasing temperature.


Image No. 4

On the chart 4A sequence I is shownT

This is a clear relative decrease in νT contrasts with a relatively small relative change in moment of inertia of 1: 1.08: 1.2 and, therefore, indicative of quantum tunneling.

Consideration IT

Quantum tunnel rotations accompanying a high directivity of 97.7% make it possible to estimate the change in the entropy of a single tunnel rotation from the experimentally obtained probabilities of rotation counterclockwise and clockwise, defined as ΔS = −kBln (ppCCW / pCw) ≈ −kBln (100/1) ≈ −0.4 meV / K.

This means that directional rotation in the tunneling mode should be a nonequilibrium process with energy dissipation ΔQ> 2 meV at 5 K and ΔQ> 6 meV at 15 K per rotation.

The maximum dissipation power was 100 meV / s per rotor, and the tunneling frequency was a maximum of 10 Hz. However, the microscope required for observations locally scatters about 3 x 106 MeV / s even at the lowest tunnel current settings. Despite such extreme settings, a constant speed with a stably high directivity is observed.

In conclusion, scientists note that highly directional rotation of C2H2 on the chiral surfaces of PdGa {111} Pd3 demonstrates a rich phenomenology, most markedly characterized by an unprecedentedly high directivity and small motor size.

Rotor (C2H2) and stator (cluster Pd3-Ga6-Pd3) consist of only 16 atoms, forming a unidirectional six-figure cyclic molecular engine (4B), which continuously works, receiving energy exclusively from single electrons.

For a more detailed acquaintance with the nuances of the study, I recommend a look at report of scientists and Additional materials to him.

Epilogue

Miniaturization has become one of the most popular areas in modern science. Different research groups are creating more and more developments, one way or another connected with this concept. In the work we examined today, its authors described the world’s smallest molecular rotor, consisting of 16 atoms. However, the dimensions are not the only hallmark of this motor. In addition, it works continuously, which predecessors could not boast of, capable of performing only one rotation cycle. Another novelty of the molecular motor is the energy with which it is fed. Due to the loss of energy during tunneling, the rotor continues to rotate in one direction.

According to scientists, this development can not only be used to create nanoscale devices for various purposes (medicine, data transfer, research of microstructural samples, etc.), but also help in understanding the processes associated with energy dissipation during quantum tunneling.

Thanks for watching, stay curious and have a great weekend everyone, guys!

Friday off-top:

Every time I read something about molecular motors, I recall this video (yes, it’s not new, but it always causes a smile).

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