Potential for a New Kind of “Empty Space”

The turning point came in 1989. Together with two Kindai University students, Kitagawa visited Kyoto University, where a large computer for X-ray structure analysis of single crystals was made available. They input the X-ray diffraction data obtained for a coordination polymer crystal and then waited for modeling and simulating the structure. “Many researchers were using the computer at the same time, so the process took more than an hour until we get our result. If we had been part of Kyoto University, we could have gone back to our own lab and done some other work, but as visitors we had nowhere in particular to go. We just had time on our hands. Eventually we started looking at some preliminary results to try and predict the structure, and one of the students said, ‘Hey, there are holes in it.’” When Kitagawa saw the honeycomb-like network structure, he noticed there are regular array of cavities. Kitagawa instantly made up his mind.

“Most coordination chemists were looking only at the frameworks themselves of the polymers, and paid no attention to the interstices. But I thought those totally empty spaces were something interesting.” That was the moment when his research focus made a crucial shift.

Nonetheless, it would be some time before he arrived at the idea of “porous materials.” Solvent molecules within the pores play an important role in the stability of the crystal, and their removal would cause the framework to collapse. “I wasn’t thinking at first about utilizing the pores.” After two years of trial and error, on the idea that these molecules could be removed and leave a non-collapsing framework, the first porous coordination polymer was completed. Allowing gas to flow freely through the countless minute empty spaces, groundbreaking findings emerged that extended the applications of the porous structure. Yet when first published in 1997, the reception to this finding was cool. “At the time, everyone thought that ‘organic matter is soft, so a framework of porous material is not usable.’ Our work was seen as an adsorption experiment mistake and hardly believed by other scientists. It was even denounced as a lie…” Only when enough other researchers had similar findings, was it accepted that a solid pore structure can be made using organic matter. Suddenly, porous materials research became a fiercely competitive field.

When iCeMS was founded in 2007, Kitagawa was named the deputy director. There are two founding principles of the institute. “One is to describe cellular processes in terms of chemistry and create materials to control them, an aim that contributes to the medical treatment of illness or dysfunction. The other is to create functional materials inspired by cellular processes.” One of Kitagawa’s motivations is to create materials that can accomplish things that natural organisms cannot. For example, a natural cell membrane can transmit a substance both from the more concentrated side to the less concentrated side and also vice versa, yet it will not transmit all substances. “The mechanisms of life are quite well designed, but they do not function beyond their given roles. Human-designed materials may achieve additional functions.”

2017 marked twenty years since the porous coordination polymers were developed. “Our bodies are made up of various proteins that are formed from just 20 different amino acids. The body assemblage is achieved by the widely divergent combinations. Similarly, porous coordination polymers can be synthesized from the combinations of components enormously more numerous than proteins. Even the most powerful computer programs can’t compute them all for exact structures and functions. This is exactly why we find rich veins away from the main road and why we need the patience, from ‘luck, patience, persistence.’ The fun of materials science is that so much may lie buried beneath our feet.”