A molecular motor can do more than turn itself. By shifting into another configuration, it can obstruct a neighboring component and make that component’s random rotation dramatically harder. A newly developed system uses light to control this molecular brake, offering a way to regulate motion without turning the attached rotor into a continuously driven propeller.

The distinction between the two moving components is central to the result. Light changes the motor’s configuration, while thermal energy keeps jostling the attached rotor. Controlling the space around that rotor changes how readily it turns, rather than eliminating the random motion that dominates molecular environments.

Molecules operate under conditions very different from those of familiar machinery. Thermal fluctuations produce incessant, unpredictable motion, including rotation of molecular components around bonds. Building a more elaborate mechanism therefore requires ways to regulate those movements, not simply attach miniature versions of wheels and axles.

“When we shine light on the molecule, the motor changes its spatial arrangement, which in turn affects the motion of the rotor,” explained first author Lukáš Severa. The light does not directly drive the second component around its axis. Instead, the motor acts as a movable obstruction that changes the energy needed for the rotor to pass.

That obstruction is a consequence of molecular geometry. In the less restricted configuration, the rotor has enough clearance to rotate readily. When the motor moves into its path, close encounters between parts of the molecule raise the rotational energy barrier, making successful turns much less frequent at the same temperature.

The design combines a light-driven motor with a rotor made from anthracene, a rigid structure of connected carbon rings. A triptycene scaffold holds the components in a defined arrangement, and an alkynyl linker connects the rotor to that framework. These chemical structures establish the spacing needed for the motor to influence a separate rotational axis.

X-ray crystallography confirmed the structure and the close alignment of the two axes. The researchers also added methyl groups to the anthracene unit, making its orientations easier to distinguish spectroscopically. Those modifications helped them identify whether rotation remained rapid or became slow enough for separate orientations to appear in the measurements.

The motor follows a four-stage cycle involving alternating light-induced changes and thermal rearrangements. The experiments used irradiation centered around 385 nanometers, in the ultraviolet region. Two configurations provide relatively open conditions for the attached rotor, while two impose greater obstruction, according to the experimental interpretation and calculations.

The samples contained equal amounts of mirror-image motor molecules. These counterparts rotate in opposite directions, so the mixture has no overall directional bias. More importantly, the attached anthracene rotor remains thermally driven: the study demonstrates reversible regulation of its random motion, not conversion into a uniformly turning output shaft.

The motor also helped determine how the molecule was assembled. In one configuration, it shielded a reactive site, allowing the researchers to modify a neighboring site selectively. Further reaction under hotter conditions completed the structure, showing how the same physical obstruction could influence both synthesis and subsequent motion.

To follow the motor’s switching cycle, the team combined ultraviolet-visible spectroscopy with nuclear magnetic resonance, or NMR. These methods reveal changes in molecular structure through light absorption and signals from atomic nuclei. Repeated switching and relaxation cycles showed reversible behavior without detectable photochemical fatigue over the experimental timescale.

The researchers then used variable-temperature NMR to compare two stable configurations, one relatively open and one hindered. Measurements extended from 25 degrees Celsius to minus 95 degrees Celsius. In the open configuration, rotation remained fast on the NMR timescale throughout that range, meaning the instrument did not resolve separate rotor orientations.

The hindered configuration behaved differently as the solution cooled. Signals broadened and eventually separated into two sets, revealing distinct orientations of the anthracene rotor. At minus 95 degrees Celsius, the two populations occurred in an approximately two-to-one ratio, providing direct evidence of slower rotational exchange.

These observations establish a difference in motion, but they do not directly measure a 300,000-fold change across both configurations. The experimental estimates of the hindered rotational barrier were also only approximate. To examine the detailed mechanism and estimate relative rates, the researchers turned to calculations of molecular energies and possible rotational pathways.

Density functional theory, a computational method for estimating molecular properties, identified routes by which the rotor could pass the motor. The favored pathway had a calculated barrier of about 20 kilojoules per mole in the open state. In the hindered state, that barrier rose to about 51 kilojoules per mole.

The difference, 31 kilojoules per mole, corresponds to roughly five orders of magnitude in calculated rotational rates at 20 degrees Celsius. IOCB Prague describes the slowdown as approximately 300,000-fold. That figure summarizes the model-based rate contrast, supported by the experimental evidence that the two states behave differently.

Even the braked rotor is not necessarily motionless at room temperature. The calculated timescales remain extremely short, and stronger restriction becomes easier to distinguish as temperature falls. Cooling reduces the thermal energy available to overcome the barrier, magnifying the contrast between the two configurations.

The institute illustrates that temperature dependence with a deliberately extreme comparison. If an unbraked rotor turned once per second at a sufficiently low temperature, its braked counterpart could take roughly 350 billion years per turn. This is a hypothetical comparison of rates, not an observed rotation time or evidence that a molecular device could remain intact for that duration.

Regulating one component with another is an important step toward coordinating more complicated molecular systems. Kaleta describes the need clearly: “If you want to build a more complex mechanism, having a motor alone is not enough.” Brakes and other control elements must manage how separate parts interact, rather than leave every movement independent.

The present experiments also expose a practical limitation. Temperatures low enough to resolve the slower anthracene rotation were too low for the motor’s thermal rearrangement steps to remain operative. Consequently, the researchers could not directly observe continuous four-state, gearbox-like operation while simultaneously resolving that slower rotor motion.

The result therefore establishes a reversible molecular gate rather than a finished nanoscale transmission. It shows how changing one component’s geometry can strongly regulate another component’s random rotation, with experiments and calculations providing complementary evidence. A next challenge is to demonstrate coordinated operation under conditions that allow both the motor cycle and the controlled motion to be followed together.

These resources explain related approaches to controlling molecular movement and the foundations of artificial molecular machines.