The development of molecular motors driven by electricity is a timely pursuit, given the significant impact of macroscopic electric motors on modern society. This research aims to address this challenge by designing and synthesizing a novel electric molecular motor. The successful creation of such a device would represent a significant advancement in the field of nanotechnology, opening up new possibilities for the development of miniaturized devices and machines with applications ranging from drug delivery to advanced computing. Previous research has focused on molecular motors powered by light or chemical reactions, but the development of an electric molecular motor offers unique advantages. Electrically driven motors offer precise control over the speed and direction of rotation through modulation of voltage or current, enabling a level of control not easily achieved with other methods. Moreover, the integration of electric molecular motors into existing electronic systems would be straightforward, simplifying the design and operation of complex nanoscale devices. This study builds upon previous work on redox-driven rotaxane-based molecular pumps, which have demonstrated the ability to transport molecules along a linear track. However, these systems lack the continuous rotary motion that characterizes macroscopic motors. The design of a truly rotational molecular motor requires a different approach, one that incorporates features that can break the symmetry of the system and induce unidirectional movement. The researchers hypothesize that a [3]catenane structure, incorporating two interlocked rings and carefully designed structural elements, can achieve this goal. This study seeks to prove the feasibility of creating a continuous rotational motion using an electric stimulus, without the production of waste products. The development of such a motor would have profound implications for nanoscale technologies, opening possibilities for new types of micro and nanorobots, sensors, and actuators.

The field of artificial molecular motors has seen significant progress in recent decades. Early work focused primarily on light-driven motors, such as the light-driven unidirectional molecular rotor developed by Feringa and colleagues. However, the use of light as an energy source can have limitations in terms of controllability and scalability. Chemical fuel-driven motors have also been developed, showing promise in terms of autonomous operation, but controlling these systems with precision can be challenging. A key development in the field has been the use of redox-driven systems, exploiting the reversible changes in electron distribution to induce movement. Redox-driven rotaxane-based molecular pumps have been created successfully, showcasing the potential of electrochemical control. The focus on [2]catenanes and [3]catenanes is noteworthy. While [2]catenanes have been explored as potential molecular motors, their limited kinetic asymmetry often prevents unidirectional rotation. [3]catenanes, on the other hand, with their more complex interlocked structure, offer greater potential for symmetry breaking. The present research builds on these previous efforts, aiming to develop a highly efficient and controllable electric molecular motor using a [3]catenane architecture, incorporating elements from previous rotaxane-based pumps while overcoming limitations observed with [2]catenanes. The research team's prior work on redox-driven molecular pumps provides a foundation for the design of the proposed electric molecular motor. These earlier successes demonstrate that redox chemistry can be harnessed to drive directional molecular motion. Furthermore, the researchers' expertise in the synthesis and characterization of mechanically interlocked molecules provides the necessary tools to create the complex [3]catenane structure required for a rotational motor.

The researchers designed and synthesized a [3]catenane molecular motor ([3]CMM) composed of a 50-membered loop encircled by two cyclobis(paraquat-p-phenylene) (CBPQT⁴⁺) rings. The loop contains several strategically placed structural elements: isopropylphenylene (IPP) steric barriers, a triazole (T) ring, and 2,6-dimethylpyridinium (PY⁺) Coulombic barriers. These features are essential to directing the movement of the CBPQT rings. The synthesis involved a radical templation strategy, initially forming a pseudo[3]rotaxane before final loop closure. The [3]CMM was characterized using various techniques: ¹H NMR spectroscopy confirmed the structure and identified the positions of the CBPQT rings on the loop. The changes in chemical shifts of proton resonances on the BPM unit (H-13 and H-14) and triazole ring (H-28) provided clear evidence of the rings' positioning. X-ray crystallography provided further structural confirmation, particularly regarding the radical-pairing interactions between the reduced CBPQT rings and V^•+ units. Vis/NIR spectroscopy was used to monitor the redox state of the [3]CMM. A broad absorption band centered at 1122 nm is characteristic of the reduced state. The researchers demonstrated the reversible switching between oxidized and reduced states using both chemical (cobaltocene, Zn dust) and electrochemical stimuli (controlled potential electrolysis, CPE). Cyclic voltammetry investigated the electrochemical properties of the [3]CMM, revealing the redox potentials required for switching between states. To evaluate the unidirectionality of the motor, a deuterium-labeled CBPQT ring ([D1]-CBPQT) was introduced during synthesis, producing a deuterium-labeled [3]catenane ([D]-[3]CMM). The relative positions of the labeled and unlabeled CBPQT rings were determined after a redox cycle using ¹H NMR spectroscopy. The ¹H NMR spectra provided information on the relative integrals of the signals associated with the CBPQT rings, which allowed determination of the degree of unidirectionality. Quantum mechanical calculations provided insights into the potential energy surface (PES) of both the [2]catenane and [3]catenane systems, helping to explain the observed unidirectional behavior of the [3]catenane. The calculations also provided information on the energy barriers associated with ring movement, consistent with experimental findings from the kinetic analysis of the metastable state. An electrochemical cell was used to demonstrate electrically driven operation of the molecular motor using CPE, alternating between reduction (-0.5 V) and oxidation (+0.7 V) potentials. The entire process was monitored using Vis/NIR spectroscopy, showing the reversible switching between redox states over multiple cycles. The researchers used high-resolution electrospray ionization mass spectrometry to confirm the presence of the different isotopologues of the [D]-[3]CMM and their presence was confirmed by high-resolution electrospray ionization mass spectrometry (Supplementary Fig. 55) and ¹H NMR spectroscopy (Supplementary Fig. 56).

The researchers successfully demonstrated a continuously rotating electric molecular motor based on a [3]catenane structure. The motor consists of two CBPQT⁴⁺ rings that encircle a 50-membered loop. Unidirectional rotation is achieved by applying an oscillating voltage or by modulating the redox potential of the system. The key to the motor's unidirectional rotation is the unique design of the loop and the interactions between the two CBPQT rings. The presence of both steric and electrostatic barriers, along with the redox-active viologen units, creates a complex potential energy landscape. This landscape directs the rings to traverse the system in a specific order. The researchers showed that the [3]catenane structure is crucial for unidirectional movement. A homologous [2]catenane, with only one CBPQT ring, exhibited no directional motion. The incorporation of a second ring introduces an interaction between the two rings, which breaks the symmetry of the system and allows for unidirectional movement. The unidirectionality of the motor was demonstrated to be 85% efficient, based on the positional exchange of deuterium-labeled CBPQT rings. The rate constants for the relaxation of the metastable state, observed during oxidation, provided further evidence for unidirectional movement. The overall energy of activation (ΔG^‡) is about 21.6 kcal/mol, demonstrating a relatively high energy barrier that is required to switch between the reduced and oxidized states. The electrically driven operation of the motor was demonstrated using controlled potential electrolysis (CPE). The researchers observed that the motor could be operated continuously for at least five cycles without significant degradation. The motor’s operation is fully reversible with more than 95% of the initial [3]catenane recovered at the end of the experiment. The timescale for a full 360° rotation is a few minutes. The design of the [3]catenane allows for potential future modifications to attach the motor to a surface, thereby allowing for spatially directed rotation. The study’s quantum mechanical calculations and the experimental data are in excellent agreement. The calculations provided critical insights into the energy barriers and potential energy landscapes of the system, supporting the experimental findings.

The findings demonstrate the feasibility of constructing an electric molecular motor capable of continuous, unidirectional rotation. This achievement addresses the long-standing challenge of creating artificial molecular machines that mimic the functionality of macroscopic motors. The success hinges on the synergistic interplay between the molecular design and the energy ratchet mechanism. The researchers' choice of a [3]catenane architecture, incorporating strategically placed steric and Coulombic barriers and redox-active components, creates a unique potential energy landscape. The externally applied oscillating redox potential effectively biases Brownian motion, overcoming the principle of microscopic reversibility. The observed unidirectional motion is not simply a consequence of a stepwise, deterministic sequence of events but rather an emergent property of the coupled motions of the two CBPQT rings under nanoconfinement. This emergent behavior is analogous to the “gearing” mechanism observed in macroscopic machines. The 85% unidirectionality achieved is remarkably high for a molecular motor. The long-term stability and reversibility of the motor are also notable features, indicating the robustness of the design. The ability to seamlessly integrate the motor's operation with electrochemical techniques opens up avenues for precise control over rotational speed and direction. The results provide a blueprint for future designs of sophisticated molecular machines for use in various technologies, such as molecular-scale devices and nanomachines.

This research successfully demonstrates an electrically driven molecular motor based on a [3]catenane. This device exhibits high unidirectionality (85%), long-term stability, and reversible operation. The use of electricity as the driving force offers significant advantages over light or chemical fuels. The modular nature of the [3]catenane design allows for potential surface attachment for directed rotation. This work represents a significant advancement in the field of artificial molecular machines, paving the way for the development of advanced nano-scale devices and technologies. Future research could focus on optimizing the design of the molecular motor to further enhance its efficiency and speed, and exploring potential applications in areas such as sensing, actuation, and energy harvesting. Attachment of this motor to a surface for directed rotation will be a key focus area for the next research phase.

While the study demonstrates remarkable progress, several limitations warrant attention. The synthesis of the [3]catenane, although described as four-step, may involve complex purification steps. The current experimental setup involves bulk solution, and scaling this up for broader application will be challenging. The study primarily focuses on the mechanistic understanding of the motor and does not investigate the possibilities of attaching the molecule to the surface of an electrode and exploring different ways to improve the efficiency of the motor’s operation. Also, while the motor shows high unidirectionality (85%), there's still a 15% chance of counterclockwise rotation. Future research should focus on further optimizing the design to enhance unidirectionality and stability even more.