A tiny molecular machine that can switch circularly polarized light on and off in response to mechanical force has been developed by researchers in Japan. The system, reported by scientists at the Institute of Science Tokyo and Tokyo University of Agriculture and Technology, uses a specially designed rotaxane molecule embedded in a polymer gel. By swelling the gel with different solvents, the researchers can mechanically rearrange the molecule and reversibly control its light emission without breaking a single chemical bond.
The discovery addresses a long-standing challenge in the development of mechanoluminescent materials: how to control circularly polarized luminescence, or CPL, using force alone. CPL is produced when light is emitted with its electric field rotating in a specific direction, either clockwise or counterclockwise. Because this “handedness” carries information beyond ordinary brightness or color, CPL is being explored for advanced three-dimensional displays, optical data technologies, anti-counterfeiting materials, biosensors and other photonic applications.
Many existing molecular systems can switch CPL in response to chemical signals such as acidity, solvent composition or the presence of specific ions. Mechanical control, however, is considerably more difficult. A force must be transmitted to a molecule in a predictable way, and the resulting structural change must be large enough to alter its optical behavior while remaining reversible. The Japanese team overcame this problem by combining a supramolecular mechanophore with a carefully engineered double-network gel.
At the heart of the system is a rotaxane, a molecular architecture that resembles a ring threaded onto a rod-like axle. Unlike conventional molecules, the ring and axle are not connected by a chemical bond. Instead, bulky molecular groups at the ends of the axle prevent the ring from slipping off. This arrangement allows the ring to move along the axle while keeping the entire structure intact. In the new design, the ring carries a chiral, spiral-shaped light-emitting unit capable of producing CPL, while the axle contains a molecular quencher.
When the system is relaxed, the emitting component on the ring remains close to the quencher. At this short distance, the quencher suppresses the excited state of the emitter, preventing CPL from being observed. The researchers describe this state as the optical “off” position. When mechanical force pulls the ring and axle in opposite directions, the ring slides away from the quencher. As the separation increases, the quenching effect weakens and the chiral emitter is able to produce circularly polarized light, switching the system to its “on” state.
Rather than applying force to individual molecules with a microscopic probe, the researchers used a double-network gel to distribute mechanical stress throughout the material. The gel contains two interpenetrating polymer networks, which provide both structural strength and a means of transmitting deformation. The rotaxane mechanophores are incorporated into this network so that changes in the gel’s dimensions generate tension across the molecular components. This design allows many molecules to experience a controlled mechanical environment at the same time.
The crucial trigger is the solvent used to swell the gel. When chloroform enters the polymer network, the chains expand and stretch in a way that produces sufficient force to separate the ring from the quencher. The resulting molecular displacement activates CPL. When methanol is used instead, the gel experiences a much weaker mechanical response, leaving the rotaxane largely in its compact state and keeping the luminescence suppressed. The different swelling behaviors therefore act as a simple chemical route for delivering a mechanical signal to the molecular switch.
Importantly, the researchers demonstrated that the process can be repeated. Alternating between the two solvents switches the CPL emission on and off multiple times, showing that the rotaxane architecture survives the mechanical cycling. Because the system operates through movement rather than the breaking and reforming of covalent bonds, it can respond reversibly while preserving its molecular structure. This durability is a key requirement for practical force-responsive optical materials, which would need to function through many cycles of use.
The gel platform also gives scientists a way to measure force-induced CPL more accurately than conventional bulk materials. In ordinary solid samples, the orientation of many molecules can influence the observed optical signal, making it difficult to determine whether a change comes from molecular behavior or from the alignment of the material as a whole. The double-network gel distributes the strain more uniformly and reduces these orientation-related complications. This enables researchers to evaluate the optical response of individual molecular systems with greater precision.
The study, led by Associate Professor Yoshimitsu Sagara of the Institute of Science Tokyo and Professor Koji Nakano of Tokyo University of Agriculture and Technology, expands the potential of supramolecular mechanophores beyond their more familiar uses in force-induced color changes. The researchers believe the same strategy could eventually be adapted to control other photophysical properties, including fluorescence intensity, emission wavelength or energy-transfer pathways. Their work may also help build a broader library of mechanically responsive molecular devices.
The results suggest that future optical materials could do more than simply reveal when they have been stretched, compressed or damaged. They could encode mechanical information through the direction, intensity or color of emitted light. Such materials might one day be incorporated into smart sensors, flexible photonic components, anti-counterfeiting systems or displays that respond directly to physical manipulation. By turning molecular motion into controllable circularly polarized light, the Japanese team has provided a striking example of how mechanical force can be translated into sophisticated optical behavior.
Subject of Research: Experimental study
Article Title: Force-induced Control of Circularly Polarized Luminescence with Rotaxane Architecture
News Publication Date: 24-Jul-2026
Web References: https://doi.org/10.1002/anie.9590374
References: Angewandte Chemie International Edition; DOI: 10.1002/anie.9590374
Image Credits: Institute of Science Tokyo
Keywords
Circularly polarized luminescence, mechanophores, rotaxane, supramolecular chemistry, force-responsive materials, polymer gels, double-network gels, optical materials, materials science, nanotechnology, polymer chemistry, optics
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