Light Can Brake a Molecular Rotor by 300,000-Fold, Czech Chemists Report

Chemists in Prague have shown that light can slow the random spinning of one part of a molecule against another by roughly 300,000 times, a control range far beyond what earlier systems achieved and a step toward molecular machines with parts that coordinate rather than simply move.

The work comes from the Institute of Organic Chemistry and Biochemistry of the Czech Academy of Sciences in Prague. The paper, 'Light-Gated Control of Brownian Rotation by a Molecular Motor,' appeared in Angewandte Chemie International Edition, with Lukáš Severa as first author. Another team member, identified as Kaleta, is quoted in the account of the work.

The molecule has two key parts. One is a light-driven molecular motor, the kind of device honored by the 2024 Nobel Prize in Chemistry. The other is a rotor that, driven only by thermal energy, spins at random around its axis, a process known as Brownian rotation. When light strikes the molecule, the motor changes its spatial arrangement. In the 'unbraked' state the rotor turns freely. In the 'braked' state, the motor's moving part physically blocks the rotor, raising the energy barrier it must overcome to turn.

Severa compares it to a hand on a freely turning axle that is jostled by tiny vibrations. Without an obstacle, it spins easily. Put something in its path and the rotation slows sharply.

The numbers are large. The team reports a slowdown of roughly 300,000-fold, which the account describes as up to five orders of magnitude. 'In previous similar systems, it was possible to change the rotation rate by no more than a factor of 100,' Kaleta said. The effect grows stronger as temperature drops. At very low temperature, an unbraked rotor might complete one rotation per second, while a braked rotor would need about 350 billion years for a single turn, roughly 25 times the current age of the universe.

The point is not the extreme figure but the principle. Building a machine at the scale of molecules takes more than a power source. 'You also need elements that can regulate and coordinate the motion of individual components,' Kaleta said. Light-gated control over a rotor's motion offers one way to do that, and the authors frame it as a step toward more complex molecular machines in which a motor alone would not be enough.

The report is brief on practical uses. It does not describe specific devices or the experimental conditions behind the 300,000-fold figure, details that will be found in the full paper. For now, the result is a clean demonstration that a molecule's internal motion can be switched from nearly free to nearly frozen with a flash of light.