'Molecular armor' can trap shape-matched planar molecules

A research team has designed and synthesized a molecular cage with an internal cavity by connecting two curved π-conjugated molecules face-to-face at three points.

The cage takes up a range of π-conjugated guest molecules through three orifices in its sides and sandwiches them between two π-conjugated panels. Guests small enough to pass through the orifices can enter and leave reversibly, and bind more strongly as their shapes better match the cavity. Larger guests that cannot pass through the orifices are instead incorporated as the cage is assembled, forming carceplexes from which the guests cannot escape even upon heating under reduced pressure. A distinctive achievement of this study is the permanent encapsulation of planar π-conjugated molecules through molecular shape complementarity, irrespective of the guests' electronic properties—a longstanding challenge.

Because the two π-conjugated panels enclose the guest like armor, the researchers named this molecule "π-Armor." The approach may enable the design of molecular capsules with diverse functions, including the protection of unstable molecules and the development of new molecular materials.

The research findings were published online in the journal Journal of the American Chemical Society on Oct. 13, 2026.

The challenge of trapping planar molecules

The field that studies molecules that capture other molecules (hosts) and the molecules they capture (guests) is known as host-guest chemistry. The ability of one molecule to recognize and bind a particular partner is being studied worldwide because of its potential applications in sensing, separation, catalysis, medicine and many other areas.

Hosts capture guests in two principal ways. One is reversible encapsulation, in which a guest repeatedly binds to and dissociates from a host. The other is permanent encapsulation, in which the guest is incorporated while the host is being assembled and cannot escape afterward. Permanent encapsulation was first reported by Dr. Donald Cram in 1985. A pioneer of host-guest chemistry, Cram received the Nobel Prize in Chemistry in 1987. The resulting complexes, known as carceplexes, can perform distinctive functions, such as stably retaining molecules that would otherwise be unstable.

Many hosts capable of reversibly capturing planar π-conjugated molecules have been developed using various types of noncovalent interactions. However, very few carceplexes that permanently encapsulate planar π-conjugated molecules have been reported. Those rare examples depend on the attraction between electron-rich guests and electron-deficient hosts, limiting the types of molecules that can be trapped. They also yield mixtures of guest-filled and empty hosts rather than carceplexes selectively. Permanently trapping planar π-conjugated molecules through shape complementarity, without depending on guest electronic properties, has therefore remained a longstanding challenge.

A cage tailored to molecular shape

The research team focused on hexa-cata-hexabenzocoronene (c-HBC), a π-conjugated molecule composed of many fused benzene rings that adopts a curved, monkey-saddle-shaped structure. The team designed a molecular cage with an internal cavity by connecting two c-HBC panels at three points using dibenzodiazocine (DBDA), a bent eight-membered-ring framework. Reacting precursor molecule 2 under acidic conditions formed all three DBDA bridges in one step, yielding cage molecule 1 in 55% yield.

Quantum-chemical calculations suggested that the two c-HBC panels face one another in parallel, approximately 6.3 Å apart, with three side orifices approximately 10 Å wide.

Cage 1 reversibly captured π-conjugated guests small enough to pass through the orifices, holding them between the two π-conjugated panels. The strength of association varied markedly with guest shape. Among dichlorobenzene guests, the meta isomer, with chlorine atoms at the 1,3-positions of the benzene ring, bound about 90 times more strongly than the para isomer, whose chlorine atoms occupy opposite positions.

Comparing different halogen substituents at the meta positions, chlorine- and bromine-substituted guests showed similarly strong association, whereas guests bearing fluorine (too small) or iodine (too large) bound more weakly. A guest bearing two electron-donating methyl groups at the meta positions was also encapsulated, despite its electronic properties being opposite to those of the halogen-substituted guests.

These guests were able to enter and leave the cage through the orifices, and adding a more strongly binding guest caused the encapsulated molecule to be exchanged. Theoretical calculations indicated that the binding is primarily supported by dispersion forces. Association was strongest when the two meta substituents fit into two orifices and their sizes most closely matched the host cavity.

Assembly locks larger guests inside

In contrast, guests bearing three substituents at alternating 1,3,5-positions could not pass through the orifices and were not taken up by the preformed cage. Noting that their three substituents would fit the three orifices, the researchers synthesized the cage in the presence of these guest molecules. This produced carceplexes containing permanently trapped guests in yields of up to 57%, with no empty cage detected.

The carceplexes did not release their guests even when heated to 200 °C under reduced pressure. Molecules with three electron-donating methyl groups could be trapped in the same way as halogen-containing molecules, indicating that permanent encapsulation depends on shape complementarity rather than the guests' electronic properties.

The team also trapped a larger π-conjugated guest bearing three arms that extended beyond the cavity, forming a structure in which the arms protruded through the three orifices. While the guest remained trapped, chemical reactions at the tips of these arms attached additional aromatic rings, further extending its π-conjugated system. This demonstrates that an encapsulated molecule can still be chemically modified after entrapment.

Molecular armor with adaptable functions

The key advance is the permanent encapsulation of planar π-conjugated molecules based on matching molecular shapes, independent of the guests' electronic properties. Both reversible and permanent encapsulation are stabilized by dispersion arising from shape complementarity. The researchers therefore named this new host "π-Armor": a cage that behaves like armor by sandwiching a π-conjugated guest between π-conjugated panels.

This strategy exploits the cage-assembly process to permanently encapsulate π-conjugated molecules through shape complementarity, independently of the electronic properties of the guest. Because guests can be chemically modified even after entrapment, the molecular structures housed inside the cage can be designed with considerable flexibility. The strategy offers a new design principle for molecular capsules that enclose π-conjugated molecules. By imparting optical or electronic functions derived from the trapped guests, or by protecting unstable molecular species, it is expected to contribute to progress in host-guest chemistry and materials science.

The research team included associate professor Yasutomo Segawa, assistant professor Takashi Harimoto and graduate student Haruka Kano of the Institute for Molecular Science (IMS), National Institutes of Natural Sciences, and The Graduate University for Advanced Studies, Sokendai.