Hollow porphyrin nanospheres

The famous Catalan architect Antoni Gaudí once said: “Everything created by human beings is already in the great book of nature”. Among the various man-made architecture and art, spherical structures and shapes have been the most fantastic geometric shape that has captivated the inventions of the human imagination. Making perfect spherical architectures is challenging due to their geometric purity and technical complexity and therefore these structures are both enchanting and rare. On the one hand, perhaps inspired by enormous celestial bodies, architects such as Fuller have designed geodesic dome structures such as the Montreal Biosphère; on the other hand there are the chemists who are the architects of the most miniature aesthetic structures in the world.

The latter draw most of their inspiration from complex self-assembled structures found in nature such as highly symmetrical hollow spherical viral capsids and protein cages. The realization of hollow molecular spheres or cages, purely organic and atomically precise, is synthetically demanding. Previous approaches to constructing pure organic cages usually allowed for the formation of small organic cages (cavity diameter

Now, a team led by Director KIM Kimoon at the Center for Self-Assembly and Complexity within the Institute for Basic Science (IBS) in Pohang, South Korea, has successfully developed a one-pot synthesis without models of a giant organic porphyrin-based cages composed of multi-porphyrin units (see animation). In general, the progress of a chemical reaction or process is favored by an increase in the randomness or entropy of the system. However, during cage formation, when multiple randomly scattered cage subunits organize to form a single 3-D structure, the process becomes entropically unfavorable. To force multiple molecules to assemble in a spherical 3-D space and amalgamate them into a single spherical molecule through covalent bonds, the researchers previously synthesized and used other molecules specifically to act as models to promote the pre-organization process.

By avoiding these challenges, Kim and colleagues were however able to synthesize P12L24 cages constructed with 36 components, i.e. 12 square-shaped porphyrin (P) units and 24 bent (L) linkers, without the use of a model-based strategy. . “We hypothesized that it would have been possible to synthesize such large organic cages if the shape, stiffness, length and bent angles of the component molecules (porphyrin derivative and bent linker) had been judiciously designed,” explains KOO Jaehyoung, the former. author of this study.

In 2015, the same research group reported porphyrin boxes consisting of 6 four-link porphyrins and 8 three-triamine links (P6L8) with a cube-shaped geometry. This result inspired them to go one step further to build larger porphyrin cages by modifying their synthetic design with four connecting porphyrins and two connecting bent fittings. The currently synthesized P12L24 cage has a truncated cuboctahedron structure with 12 square faces, 8 regular hexagonal faces and 6 regular octagonal faces (see animation). The cage has an external dimension of 5.3 nm and an internal cavity of 4.3 nm in diameter (Figure 1). The overall structure of the P12L24 cage resembles the cage structure of the COPII transport protein, which possesses a cuboctahedral shape and consists of heterotetrameric units other components of the mantle that meet at the tetrameric vertex similar to the porphyrin and linker subunits in P12L24 (Figure 2).

Researchers also explored the potential applicability of such large hollow molecular spheres or cages as encapsulation of host molecules and in photocatalysis. The current results will certainly facilitate the synthesis of large multivariate organic cages in the future, which may be suitable for the transport of large loads, synthesis of uniformly sized nanoparticles, modulation of bound host reactivity, molecular recognition, catalysis and so on.

“This is a major breakthrough in the synthesis of giant sphere-shaped molecules. If we can make the P12L24 cages water soluble, perhaps they can serve as an efficient container for large host molecules such as proteins and assist in their storage, delivery. and other applications. Our study can offer a breakthrough in creating a simple and intelligent way to build a superstructure composed of a large number of building blocks by overcoming the entropy problem, “notes Director Kim. He also adds: “The other meaning of these structures is to exploit the presence of porphyrin subunits, which exhibit interesting photophysical properties such as light collection, energy transfer, electron transfer, etc.”