An electric molecular motor

The study addresses whether a fully synthetic molecular motor can be driven electrically in solution to achieve continuous, unidirectional rotation without producing chemical waste. Building on decades of work on artificial molecular machines, the authors sought to implement an energy-ratchet mechanism controlled by redox potentials in a mechanically interlocked molecule. A [2]catenane design lacking sufficient kinetic asymmetry did not yield directional motion, motivating a [3]catenane architecture with two CBPQT rings whose mutual interactions and a loop engineered with switchable recognition sites and barriers bias motion under alternating redox conditions. The purpose is to demonstrate electrically powered unidirectional circumrotation and elucidate the mechanistic basis via a two-dimensional potential energy landscape shaped by nanoconfined noncovalent interactions.

The work builds on artificial molecular motors and pumps, including photo- and redox-driven systems, Brownian ratchet mechanisms, and rotaxane-based molecular pumps where redox-controlled gates (e.g., PY+-V2+-IPP cassettes) transport rings directionally. Prior [3]catenane motors used multiple stimuli or chemical fuels; other demonstrations of electrically driven motion were on surfaces or single-molecule junctions. The authors leverage radical-pairing interactions between reduced viologen (V•+) units and reduced CBPQT2+ rings, previously shown to enable recognition and assembly, and apply energy-ratchet concepts to sculpt the potential energy surface in response to redox switching. The design contrasts with linear pumps by enabling continuous circumrotation in a closed loop without net production of waste.

• Molecular design: A [3]catenane loop comprising two switchable viologen (V2+/V•+) recognition sites, a triazole (T), a 2,6-dimethylpyridinium (PY+) Coulombic barrier, a bis(4-methylenephenyl)methane (BPM) unit providing a binding well in the oxidized state, and a steric isopropylphenylene (IPP) barrier. Two CBPQT4+ rings encircle the loop in the oxidized state.
• Mechanism: Alternating reduction and oxidation cycles modulate Coulombic repulsion and radical-pairing attractions. Reduction injects six electrons total (both rings to CBPQT2+ and both V2+ to V•+), enabling the rings to move onto V sites. Oxidation reinstates Coulombic repulsion, biasing thermally activated passage of one ring over PY+ rather than IPP, producing a 180° positional exchange per cycle; two cycles give 360° rotation.
• Synthesis: Radical templation to form an intermediate pseudo[3]rotaxane followed by loop closure, yielding [3]CMM13+ (PF6− salt). A deuterium-labelled ring ([D1]-CBPQT) was incorporated to create isotopologues, enabling tracking of ring positions by 1H NMR to quantify directionality.
• Spectroscopy and structure: 1H NMR (500–600 MHz, CD3COCD3 or CD3CN, 298 K) assigned ring positions via shielding of BPM and T protons. Visible/NIR spectroscopy monitored the reduced state via a broad band at 1,122 nm characteristic of trisradical interactions. X-ray single-crystal crystallography confirmed reduced-state co-conformations and radical-pairing between CBPQT2+ rings and V•+ units.
• Electrochemistry: Cyclic voltammetry in MeCN (0.1 M TBAPF6) characterized reversible switching. Controlled potential electrolysis (CPE) powered operation using alternating constant potentials: reduction at −0.5 V (vs Ag/AgCl) for 10 min and oxidation at +0.7 V (vs Ag/AgCl) for 15 min; [3]CMM concentration 30 µM.
• Computations: Quantum mechanical calculations (M06-2X/6-31G**) generated potential energy surfaces (PESs) for oxidized and reduced states, evaluating barrier heights for ring transit over PY+ vs IPP and interaction energies with V sites.
• Kinetics: Observation of a metastable oxidized co-conformation post-oxidation and its relaxation to the stable state by 1H NMR at 298 K in CD3CN. First-order kinetics analysis gave rate constants and activation parameters.
• Controls: Homologous [2]catenane ([2]C*) with a single CBPQT ring was synthesized and probed; quantum calculations and experiments assessed its lack of directional motion due to a BPM-centered energy well and insurmountable IPP barrier under relevant conditions.

• Electrically powered unidirectional rotation: The [3]catenane molecular motor undergoes unidirectional circumrotation of both CBPQT rings around the loop, achieving 180° per redox cycle and 360° after two cycles.
• High directionality: The loop design and ring–ring interactions yield highly (≈85%) unidirectional motion, quantified using a deuterium-labelled CBPQT ring and 1H NMR integrals after one cycle.
• Operational robustness: Vis/NIR absorbance at 1,122 nm reversibly switches between oxidized (colourless) and reduced (purple) states under CPE. Operation was repeated at least five cycles without substantial loss of reversibility; >95% recovery of [3]catenane after cycling with negligible degradation.
• Mechanistic PES features: Calculations show the barrier for CBPQT2+ to traverse PY+ is substantially lower than for IPP, and redox switching modulates electrostatic barriers and interaction energies with V sites. The reduced-state minimum has both rings on V sites; oxidation induces biased Brownian motion favoring the observed positional exchange.
• Kinetics: Post-oxidation metastable state relaxes with first-order kinetics at 298 K: k = (8.6 ± 0.4) × 10−1 s−1 (≈0.86 s−1), ΔG‡ ≈ 21.6 kcal mol−1, in good agreement with computed barriers (≈20.1 kcal mol−1).
• Control system [2]catenane: No directional motion observed; BPM well traps the ring under oxidizing conditions and the IPP barrier (ΔE > 25 kcal mol−1) is too high to traverse on relevant timescales.
• Energy transduction: The system implements a flashing energy ratchet driven by redox potential oscillation, sculpting a two-dimensional energy landscape reminiscent of FoF1-ATP synthase behavior.

The study demonstrates that time-symmetric oscillation of redox potential can drive directional motion in a synthetic molecular machine by exploiting kinetic asymmetry built into a [3]catenane and emergent noncovalent interactions between two mobile rings under nanoconfinement. Computations and experiments confirm that reduction lowers Coulombic barriers and enables radical-pairing-driven localization on V sites, whereas oxidation reinstates repulsive interactions that bias thermally activated passage over PY+ rather than the sterically prohibitive IPP barrier, producing a net clockwise positional exchange each cycle. Deuterium-labelling experiments quantify the motor’s directionality (~85%), and spectroscopic/electrochemical measurements show reversible operation over multiple cycles without degradation. The findings address the core question of electrical actuation of molecular rotation in solution without chemical waste and highlight reciprocal ring–ring interactions as a functional analogue of gearing, enabling the conversion of electrical inputs into continuous mechanical motion. The two-dimensional PES and biased Brownian dynamics provide a mechanistic framework relevant to the broader field of molecular motors.

This work reports an electrically driven [3]catenane molecular motor that achieves highly unidirectional (≈85%) circumrotation of two CBPQT rings around a loop through alternating redox control, without breaking covalent bonds or producing waste. The motor operates robustly under controlled potential electrolysis with rapid cycle times (minutes) and retains structural integrity over repeated cycles, with mechanistic support from quantum calculations, spectroscopy, crystallography, and kinetics. The design offers a chemical-engineering approach to electric molecular motors, establishing principles for sculpting two-dimensional energy landscapes to harness Brownian motion. Future directions include immobilization on electrode surfaces for spatially directed rotation and energy transduction at interfaces, optimizing structural elements to increase directionality and efficiency, and integrating such motors into functional devices.

• Directionality is high but not absolute (≈85%), indicating some backward steps.
• Demonstrations of continuous operation under electrical drive were shown for at least five cycles; longer-term durability and fatigue over many more cycles were not detailed here.
• The system operates in homogeneous solution; while the design is compatible with electrode attachment, surface-bound operation is prospective.
• The homologous [2]catenane did not exhibit directional motion, underscoring sensitivity to architectural features and energy landscape design.