Quantum Rotation in a Single Molecular Tunneling Junction

Molecular conformation is crucial to its chemical reactivity, , electron transport, , and photophysical properties. However, it is very challenging to precisely control, as molecules typically switch rapidly between different conformations, which are generally considered to be in thermal equilibrium. This is especially true in multiphenyl systems, where conformational changes induced by σ-bond rotations are both complex and rapid, , making it very challenging to precisely monitor and control conformational switching at the single-molecule level.

In a recent study published in Science Advances, Guo and co-workers employed a graphene-based single-molecule device technique to achieve, for the first time, real-time in situ monitoring of the intramolecular rotational kinetics of a single aromatic molecule (Figure ). The study revealed that, under low-temperature conditions, the intramolecular rotation of benzene rings exhibits quantized characteristics rather than classical quasi-free rotation. This quantized rotational behavior manifests as transitions between discrete metastable states rather than continuous rotation. Through systematic temperature-dependent measurements, the study found that at, low temperatures (<100 K), the rotational process is primarily driven by quantum tunneling, exhibiting significant temperature independence. As the temperature increases above 100 K, the rotation gradually transitions to a thermally activated process.

1.

Schematic of a graphene electrode single-molecule device with a hexaphenyl aromatic molecule featuring a fluorene center, showing three quantized rotational states at low temperature.

Furthermore, the study demonstrated that inelastic electron tunneling (IET) plays an important role in driving intramolecular rotation. At 80 K, by applying a low bias voltage of 20 to 32 mV, only transition between rotational states 1 and 2 was observed, guided by the chiral environment constructed by the graphene electrodes and the molecular structure. The presence of this chiral environment led to opposite rotational directions on either side of the molecule, enabling sustained unidirectional rotation. This unidirectional rotation not only overcomes the tendency of random molecular rotation but also is accompanied by a reduction in entropy, indicating that the molecular junction is driven out of thermodynamic equilibrium under the influence of IET. In contrast, as the bias voltage increased, the complexity of intramolecular rotation gradually emerged and more rotational states were observed. At 38 mV, state 3 occasionally appeared, while at higher bias voltages (e.g., 54 mV), the molecular rotation patterns became increasingly complex, including alternating rotations, single-circle delays, and half-circle delays. These complex rotational modes reflect the coupling between multiple rotational units and external energy, revealing the intricate nature of intramolecular multibody rotationan aspect not yet fully considered at the macroscopic scale. This study provides a new perspective for understanding such complexity at the single-molecule level.

By utilizing graphene-based single-molecule devices, this research achieved highly sensitive monitoring of intramolecular rotation, offering a novel technological platform for single-molecule studies. This technique not only enables real-time monitoring of intramolecular rotational dynamics but also reveals the behavior of molecules in different energy states. The findings provide direct evidence for the quantized nature of intramolecular rotation and lay a foundation for the design of molecular machines.

The authors declare no competing financial interest.