Designing interaction hierarchy
The researchers designed a luminescent molecule containing both bromine atoms and methoxy groups. This molecular architecture simultaneously incorporates several intermolecular interactions, including dispersion forces, dipole–dipole interactions, and halogen bonding.
When crystallized under different conditions, the molecule formed two polymorphic crystal phases. One phase emitted yellow light (α phase), while the other emitted green light (β phase). Despite having identical chemical compositions, the two crystals exhibited distinct molecular arrangements and optical properties.
Single-crystal X-ray diffraction analysis revealed that the two crystal forms possess different interaction hierarchies. In the α phase, relatively homogeneous interactions among methoxy groups and aromatic rings dominate the crystal packing. In contrast, the β phase is governed by a network of heterogeneous interactions, particularly halogen bonds formed between bromine atoms and methoxy groups.
Different stimuli trigger different transformation pathways
A key discovery of the study is that the hierarchy of intermolecular interactions controls not only crystal structures but also the pathways through which phase transitions occur.
When heated, the yellow-emissive α crystal transformed directly into the green-emissive β crystal while maintaining its single-crystalline nature through a single-crystal-to-single-crystal phase transition.
In contrast, mechanical stimulation such as grinding induced a completely different pathway. The α crystal first lost its structural order and passed through an amorphous intermediate state before gradually converting into the β phase.
The team successfully visualized these transformation processes through distinct luminescence-color changes from yellow to green, enabling direct optical monitoring of complex solid-state dynamics.
"Our results show that subtle differences in intermolecular interactions can dramatically alter how molecular crystals respond to external stimuli," says Hayashi. "By deliberately designing interaction hierarchies, we may be able to program functional responses in solid-state materials."
Demonstration of rewritable security paper
To explore practical applications, the researchers fabricated a prototype security paper by impregnating paper with the luminescent molecule.
Under UV irradiation, mechanical writing generated visible patterns through a local color change. The recorded information could subsequently be erased by heating, which restored the original emission state. Repeated writing–erasing cycles demonstrated the feasibility of a rewritable optical recording platform.
Toward programmable molecular materials
The researchers believe that the concept of interaction hierarchy extends far beyond luminescent crystals. The framework could accelerate the rational design of mechanically responsive materials, electronic materials, and photonic systems by providing a means to control both functional properties and transformation pathways at the molecular level.
Moreover, understanding how competing intermolecular interactions generate complex structural transformations may lead to the discovery of previously inaccessible phase-transition phenomena and dynamic solid-state functions.
