Scientists at the Institute for Molecular Science have designed a molecular cage that permanently traps planar molecules through shape matching rather than electronic attraction.

The team, led by Associate Professor Yasutomo Segawa with Assistant Professor Takashi Harimoto and graduate student Haruka Kano, built the cage by connecting two curved π-conjugated molecules called hexa-cata-hexabenzocoronene (c-HBC) at three points using dibenzodiazocine bridges. The resulting structure, which they named "π-Armor," has three side openings and sandwiches guest molecules between its two curved panels.

The cage captures guests in two ways. Small molecules that fit through the three openings can enter and leave reversibly, binding more strongly when their shapes match the cavity precisely. Larger molecules with three substituents cannot pass through the openings but become permanently trapped when the cage is assembled around them, forming structures called carceplexes. These trapped guests do not escape even when heated to 200 degrees Celsius under reduced pressure.

Testing with dichlorobenzene guests showed that the meta isomer, with substituents at the 1,3-positions, bound about 90 times more strongly than the para isomer. Guests with chlorine or bromine atoms at matching positions bound similarly well, while fluorine-substituted guests (too small) and iodine-substituted guests (too large) bound more weakly. Importantly, molecules with electron-donating methyl groups also became trapped, indicating that shape complementarity, not electronic properties, drives the encapsulation.

Quantum-chemical calculations suggested the two panels sit about 6.3 angstroms apart with openings approximately 10 angstroms wide. Dispersion forces primarily support the binding.

The researchers demonstrated that encapsulated molecules can still undergo chemical modification. A larger guest trapped inside had additional aromatic rings attached at its tips while confined within the cage.

The work was published in the Journal of the American Chemical Society on October 13, 2026.