Light-Driven Molecular Motor Acts as a Brake, Slowing a Neighboring Rotor's Random Motion

A molecule can be made to turn itself, but a team in Prague has now shown it can also be made to hold something else back. Researchers led by Jiří Kaleta at the Institute of Organic Chemistry and Biochemistry of the Czech Academy of Sciences (IOCB Prague) built a single molecule in which a light-driven motor sits next to a rotor. When light flips the motor into a different shape, it blocks the rotor's path and makes the rotor's random turning far harder.

The work was done with colleagues at the University of Chemistry and Technology Prague and Charles University, and it appears in Angewandte Chemie International Edition. The distinction at its center matters. Light changes the motor's configuration, but the attached rotor is never driven around by the light. Thermal energy keeps jostling it, as it does every molecular component. What changes is how much room the rotor has to pass, so the study shows regulation of random motion, not a continuously spinning output shaft.

The design pairs the motor with a rotor made of anthracene, a rigid unit of connected carbon rings. A triptycene scaffold holds the parts in a fixed arrangement, and an alkynyl linker joins the rotor to that framework. X-ray crystallography confirmed the structure and showed the two rotational axes lying close together. The team also added methyl groups to the anthracene so that its different orientations would be easier to tell apart in spectra. The samples held equal amounts of mirror-image motors, which turn in opposite directions, so the mixture has no overall directional bias.

The motor runs a four-stage cycle of light-induced changes alternating with thermal rearrangements, here using ultraviolet irradiation centered near 385 nanometers. Two of its configurations leave the rotor relatively free, and two obstruct it. In one configuration the motor even shielded a reactive site during synthesis, letting the chemists modify a neighboring site selectively before a further reaction under hotter conditions finished the structure. The same physical obstruction therefore shaped both how the molecule was built and how it moves.

To follow the switching, the team combined ultraviolet-visible spectroscopy with nuclear magnetic resonance. Repeated switching and relaxation cycles showed reversible behavior, with no detectable photochemical fatigue over the timescale tested. Variable-temperature NMR then compared one open and one hindered configuration from 25 degrees Celsius down to minus 95. The open form kept rotating fast on the NMR timescale across that whole range, so separate rotor orientations never appeared. The hindered form behaved differently: as it cooled, its signals broadened and split into two sets, and at minus 95 the two orientations appeared in roughly a two-to-one ratio. That is direct evidence of slower rotational exchange.

The headline number comes from calculation, not measurement. Density functional theory identified the favored route for the rotor to get past the motor. The barrier was about 20 kilojoules per mole in the open state and about 51 in the hindered state. That 31 kilojoule gap corresponds to about five orders of magnitude in calculated rates at 20 degrees Celsius, which IOCB Prague describes as roughly 300,000-fold. The experiments show the states differ, but they do not directly measure that factor, and the experimental barrier estimates were only approximate. Even the braked rotor is not necessarily still at room temperature, since the calculated timescales stay extremely short.

The institute offers a deliberately extreme illustration: if an unbraked rotor turned once per second at a sufficiently low temperature, its braked counterpart could take about 350 billion years per turn. That is a comparison of rates, not an observed time. First author Lukáš Severa said that when light hits the molecule, the motor changes its spatial arrangement, which affects the rotor's motion. Kaleta put the motivation plainly: a motor alone is not enough to build a more complex mechanism.

There is a practical limit. Temperatures cold enough to resolve the slow anthracene rotation were too cold for the motor's thermal rearrangement steps to work. The team therefore could not watch continuous four-state operation while also resolving the slower rotor. What they have shown is a reversible molecular gate, not a finished transmission, and the next step is to follow the motor cycle and the controlled motion together.