During the past 16 years, our group has been the leader in the field of molecular self-assembly as indicated by over 8,500 total citations of our works on self-assembly. In particular, the self-assembly of well-defined cage and capsule structures using transition metals is a highly original methodology from our laboratory whose technique has been recently adopted by numerous research groups. This is the reason why many chemists have paid special attention to our review article.

“The present article describes how we designed this molecule and, for the last 15 years, how this chemistry has been developed...”

The paper describes a new concept in self-assembly. In a SciFinder search, the keyword "self-assembly" has been used in more than 5,000 articles during 2006. Going back to 1990, however, it was used in only 100 papers, most of which are in the fields of biology.

Namely, self-assembly was understood as "the way of bio-structure generation," and only a few chemistry groups noticed it as a method for constructing artificial objects—except for non-discrete assemblies such as micelles and bilayers. Our group is one of the very few research groups which had initiated the self-assembly of well-defined structures in chemistry.

To put it more concretely, our continuous study on self-assembly was triggered in 1990 by showing the spontaneous formation of a metal (90 degree-block) cornered square compound; this and related compounds have been termed "molecular squares."

The present article describes how we designed this molecule and, for the last 15 years, how this chemistry has been developed, particularly regarding the construction of highly complicated 3D architectures and the development of new properties and reactions within these hollow architectures.

By using transition metals—particularly, end-capped metals with only two cis coordination sites available—as coordination building blocks, a variety of well-defined 2D and 3D architectures are self-assembled, such as macrocycles, catenanes, cages, tubes, capsules, bowls, spheres, etc. New properties and reactions have been developed in the hollows of these architectures.

We designed and prepared the metal (90 degree-block) cornered square molecule in 1990 without expecting self-assembly, because at that time self-assembly was not familiar to chemists at all. Then, we discovered the principle and power of self-assembly not from the literature but displayed throughout our experiment. One major obstacle was that nobody believed, before publishing a paper describing the result, that the square structure was spontaneously formed quantitatively by simply mixing the components.

Currently, our ministry recognizes that self-assembly is one the most efficient ways for the bottom-up fabrication of nanostructures. The Japan Science and Technology Agency (JST) is promoting CREST (Core Research for Evolutional Science and Technology) programs for many priority areas. I have been the leader of the CREST project on self-assembly (self-organization) for 10 years.