First experimental validation of a “150-year-old chemical common sense” direct visualization of the molecular structural changes in the ultrafast anthracene [4+4] photocycloaddition reaction

Overview

　Anthracene—a hydrocarbon known for its strong luminescence and high photo-reactivity—has long been considered one of the most iconic and widely studied organic molecules. Its [4+4] photocycloaddition reaction, first reported in 1867, has underpinned the development of countless photo-functional materials, including actuators, adhesives, and chromic systems.

　Despite being a textbook reaction, the structural changes during this photoreaction occur in an extremely short time window (10⁻⁸–10⁻⁶ s), placing the process beyond direct experimental observation for more than a century. Consequently, controlling and visualizing the reaction mechanism has remained a major unresolved challenge—one that has been considered an “unreachable frontier” of fundamental chemistry.

　The present study overcomes this long-standing barrier. By successfully visualizing the reaction pathway, the researchers provide the first experimental validation of a chemical process that has been accepted as “common knowledge” for more than 150 years.

　This work was conducted by the research group of Yuto Hino (PhD candidate), Assistant Professor Takumi Matsuo, and Professor Shotaro Hayashi at Kochi University of Technology. The group has previously pioneered new directions in crystalline photochemistry using precisely designed anthracene derivatives (*3). The current achievement demonstrates a controllable, trans-scale photoinduced reaction system, allowing direct visualization of intermediate states in the classical anthracene [4+4] reaction for the first time.

　The findings were published in Nature Communications on November 26, 2025.

Research Details

　1. Construction of a dual control system using light and heat

　The team found that UV irradiation triggers an ultra-rapid reaction, causing sudden structural changes (phase transitions) that make crystals leap from the substrate. In contrast, weaker light sources—such as LEDs or sunlight—slow the reaction dramatically, enabling optical control of the reaction rate. Additionally, temperature was found to provide an independent regulatory factor (Fig.1,2).

2. Direct visualization of intermediate states via SCXRD

　Under weak light at near-room temperature, the normally ultrafast reaction can be slowed by several orders of magnitude. This led to the formation of a solid solution (*5) consisting of both monomers and dimers (*4).

　Using single-crystal X-ray diffraction, the team successfully determined the three-dimensional molecular arrangement within this intermediate phase (Fig.3).
This represents a milestone achievement—the first direct visualization of the anthracene dimerization pathway, solving a central challenge in photochemical reaction analysis.

3. Control of reaction initiation and termination by incident light direction

　By focusing on the anisotropic orientation of the transition dipole moment μ in the crystal, the team demonstrated that the reaction responds strongly to the angle of incident irradiation (Fig.4). The crystal structure of the target compound, 1,8-bis(pentafluorophenyl)anthracene, features uniformly aligned molecules, making it an ideal platform for studying directional light–matter interactions.

　This discovery shows that orientation serves as a third regulatory factor, in addition to light intensity and temperature, and suggests new mechanisms involving optical cavities and anisotropic excitation within molecular crystals.

　These insights open the door to precise control of light-driven molecular motion, with implications for artificial/biological motion systems, photonic devices, and molecular machines.

This study brings a transformative breakthrough to molecular science by uniting two key achievements: the development of a trans-scale, multi-parameter control system grounded in an interdisciplinary framework, and the long-awaited visualization of a classical photoreaction that has remained enigmatic for over a century.

Glossary

(*1)　 [4+4] photocycloaddition
　A photochemical reaction in which two monomers combine to form a dimer. Widely used in photomechanical, self-healing, and adhesive materials.

(*2) 　Single-crystal X-ray diffraction (SCXRD)
　A powerful method for determining atomic-level structures in crystalline materials.

(*3) 　Prior press release from Kochi University of Technology (Nov. 5, 2021):
　“Realization of single-crystal–single-crystal transitions in molecular crystals resembling a domino effect…”

(*4)　 Monomer / Dimer
　The fundamental molecular unit and its two-unit combination.

(*5)　 Solid solution
　A homogeneous crystalline phase containing two species—in this case, monomers and dimers.

(*6) 　Transition dipole moment (μ)
　A vector that defines the orientation at which a molecule most effectively absorbs light.

(*7)　 Anisotropy
　Directional dependence of material properties.