The "Hierarchy of Molecular Bonds" Determines Solid-State Function and Changes

Mahiro Nakabayashi (then a third-year doctoral student in the Graduate School of Science and Engineering, graduating March 2026) and Professor Shotaro Hayashi (Faculty of Engineering) at Kochi University of Technology have revealed that the "hierarchy of intermolecular interactions" (i.e., the order of bonding) in organic molecular crystals determines their crystal structure, luminescent properties, and even phase transition behavior. Previously, when multiple interactions coexist between molecules, it was considered extremely difficult to predict and control which force would take precedence and how it would govern the whole. This research has been highly acclaimed academically for proposing a new guideline: "design molecules with a priority order (hierarchy) in the strength of forces, enabling precise control over crystal properties and their changes in response to stimuli."

This achievement was published on June 30, 2026, in Angewandte Chemie International Edition, an international academic journal with high impact in the field of chemistry.

[Key Research Findings]

Revealed that in organic molecular crystals, the "types" and "strength hierarchy (hierarchy)" of forces that attract molecules to each other (intermolecular interactions) determine the crystal's behavior.

Discovered that by designing molecules based on the aforementioned "order of bonding," it is possible to create two different crystals (polymorphs) emitting yellow and green light, respectively, from the same molecule.

Successfully visualized changes in crystal state through different processes as changes in emission color, depending on whether heat (thermal stimulus) or rubbing (mechanical stimulus) was applied.

To explore future application possibilities, demonstrated the application by creating "security paper" that changes its emission color to yellow when rubbed after being impregnated with the molecule, and can be erased (reset) by heating.

▲Figure 1 Security paper created. Changes in color due to repeated mechanical and thermal stimuli.

[Background of the Research]

"Organic molecular crystals"*1), which change their structure and properties in response to external stimuli such as light, heat, and force, are being researched worldwide as next-generation materials for sensors and optoelectronic materials. However, many aspects remain unclear regarding how to design molecules to achieve desired phase transitions (changes in state). In recent years, research on controlling crystal structures using intermolecular interactions has advanced, but a general design guideline has not been established for how multiple attractive forces coexisting within a molecule influence each other to govern the overall crystal structure and phase transitions.

[Research Results]

The research team designed a new molecule by simultaneously incorporating a "bromine atom" and a "methoxy group" into a luminescent molecule containing a cyano group. This molecule coexists with multiple interactions of different strengths, including van der Waals forces*4), dipole-dipole interactions*5), and halogen bonds*6).

When this molecule was crystallized, two types of crystals (polymorphs)*7) with different arrangements were obtained depending on the crystallization conditions: one was a "α phase" emitting yellow light, and the other was a "β phase" emitting green light.

▲Figure 2 Structure of the newly designed molecule and the two types of crystals.

Single-crystal X-ray structure analysis*8) revealed differences in the "hierarchy (order)" of intermolecular interactions within these two crystals.

α phase (yellow crystal): Relatively "homogeneous" interactions, such as those between methoxy groups and aromatic rings, are dominant.

β phase (green crystal): A network of "heterogeneous" interactions, such as halogen bonds formed between bromine atoms and methoxy groups, strongly dominates.

This research is characterized by classifying intermolecular interactions into "homogeneous interactions" and "heterogeneous interactions" and focusing on the hierarchical nature (priority order) of their energies. This hierarchical bonding revealed that the route of crystal structure change is entirely different depending on the type of applied stimulus.

Thermal stimulus (heating): Direct change from yellow crystal (α phase) to green crystal (β phase) while maintaining the crystal form [single crystal-single crystal phase transition].

Mechanical stimulus (rubbing, etc.): The yellow crystal (α phase) first passes through an irregular and unstable "amorphous" state before gradually transforming into the green crystal (β phase).

▲Figure 3 Image of the transformation pathway.

The research team succeeded in optically tracking and visualizing these complex transformation routes as a clear color change from "yellow emission to green emission." This indicates that slight differences in intermolecular interactions significantly alter the crystal's responsiveness and phase transition pathway.

Furthermore, utilizing this molecule, they created and demonstrated security paper. When the paper impregnated with the molecule is "rubbed (mechanical stimulus)" while exposed to UV light (ultraviolet light), only the rubbed areas change emission color, allowing text or images to be revealed. Additionally, "heating (thermal stimulus)" returned the emission color to its original uniform state, enabling the erasure (reset) of the drawn information (Figure 1).

[Future Outlook]

The concept of "interaction hierarchy" presented in this research has the potential to serve as a new guideline for accelerating the design of a wide range of molecular materials, not limited to luminescent materials, but also including mechanoresponsive materials, electronic materials, and optoelectronic materials. The demonstrated security paper, capable of information writing via mechanical stimulus and erasure via heat, is expected to be developed in the future into special anti-counterfeiting materials or new information recording media that can be rewritten multiple times.

Furthermore, understanding the complex structural changes arising from the competition of intermolecular interactions is expected to lead to the creation of new phase transition phenomena and dynamic functions in organic crystals.

[Glossary]

*1) Organic molecular crystal: A solid formed by the regular, three-dimensional arrangement of molecules of organic compounds, primarily carbon-based. Familiar examples include sugar crystals and materials for pharmaceuticals.

*2) Hierarchy of intermolecular interactions (interaction hierarchy): The existence of a "priority order (sequence)" in the strength of intermolecular forces that bind molecules together. When multiple molecules gather to form a regular arrangement called a "crystal," the forces attracting molecules are called "intermolecular interactions." A state where there is a "gradation (hierarchy/sequence)" in the strength of these forces is called "hierarchical." It is precisely because of this "hierarchy" in force strength that crystal behavior changes depending on how stimuli are applied.

*3) Phase transition: A phenomenon where the structure and properties of a substance change drastically to another state (phase) due to external stimuli such as temperature or force. A familiar example is the change of water (liquid) to ice (solid) when cooled, or to steam (gas) when heated.

*4) Van der Waals forces: A type of intermolecular interaction that acts between nonpolar molecules (molecules that have no overall electrical polarity).

*5) Dipole-dipole interaction: A type of intermolecular interaction, an electrostatic force that attracts molecules or atoms with charge asymmetry (dipoles).

*6) Halogen bond: A strong bond formed when specific atoms called halogens, such as iodine (I), bromine (Br), or chlorine (Cl), attract specific parts of other molecules. It is a type of intermolecular interaction.

*7) Crystal (polymorph): A phenomenon where substances with the exact same chemical composition exhibit different colors, hardness, or properties due to different arrangements of atoms or molecules (crystal structures). A representative example is carbon, which exists as "diamond" and "graphite" despite having the same composition.

*8) Single-crystal X-ray structure analysis: A technique that precisely determines how invisible molecules are arranged within a crystal by irradiating a single crystal with "X-rays" and analyzing their scattering. It is like a "super-high-performance molecular microscope" capable of observing molecular arrangements at the picometer (one trillionth of a meter) level.

*9) Amorphous: A state where the atoms or molecules constituting a substance lack a regular arrangement and are in a random state.

[Research Funding]

This research was supported by the Japan Science and Technology Agency's Emergent Research Support Program (JPMJFR211W), Grants-in-Aid for Scientific Research (B) (24K01574), Scientific Research on Innovative Areas (A) "Functional Science Spun from the Pursuit of π-Molecular Complexity" (26H01373), and JST SPRING (grant no. JPMJSP2141).

[Publication Information]

Title: Interaction Hierarchy and Polymorphic Structure–Property Dynamics in Luminescent Molecular Crystals

Authors: Mahiro Nakabayashi, Shotaro Hayashi

Journal: Angewandte Chemie International Edition

Publication Date: June 30, 2026

DOI: 10.1002/anie.8807652