It has long been a dream of chemists to carry out the important chemical reactions of living organisms in the laboratory or as practical technology. For instance, in 1912 Italian photochemist Giacomo Ciamician of the University of Bologna wondered whether it would not be advantageous to make better use of radiant energy, such as to make batteries based on photochemical processes.
Molecules continuously recognize and affect one another through chemical and physical interactions. For example, water molecules behave like a gigantic molecule by forming numerous
hydrogen bonds. That water boils at 100C° is not an intrinsic nature of a single water molecule, but is a characteristic of such a gigantic aggregate. Because molecular interactions involve many different forces, certain important reactions such as electron transfer and energy migration occur. But, if one can break some of these interactions or if a molecule can be freed from all of these intermolecular forces, it may eventually exhibit some unexpected properties which are inherent to the molecule itself or its local environment.
Understanding and utilizing this fact was an important step to realizing the dream of Ciamician. In the mid 1980s non-linear hyperbranched macromolecules were first synthesized. Such nanometer-size macromolecules are called dendrimers, which are characterized by a regular tree-like array of branched units. In 1992, Takuzo Aida encapsulated a porphyrin at the center of a cavity consisting of a dendrimer molecule. A porphyrin is a dye-type molecule, like the iron porphyrin that binds molecular oxygen in the heme protein of red blood cells, or the chlorophyll that promotes photosynthesis in green plant cells. In the course of this study, it was found that the cavity of a certain dendrimer, like a biological light-harvesting antenna, traps photons and channels the excitation energy to the interior molecule, whereby certain chemical reactions then take place. A detailed investigation has shown that this reaction occurs only when the dendritic nanocavity has such a closed architecture that it can specially isolate the interior environment from collisional energy dissipation. Such an observation has been extended to the development of novel light-emitting organic diodes for next-generation imaging devices and the potential to utilize photosentizers for photochemical water splitting.
Further, by using a porous inorganic material, Aida and co-workers also succeeded to fabricate crystalline nanofibers of linear polyethylene with an ultrahigh molecular weight (6,200,000) and a diameter of 30 to 50 nanometers. A particular type of silicate material of a honeycomb-like porous framework was utilized as a nanoflask for the polymerization of ethylene, where the produced polymer chains were extruded from the mesopores and then assembled to form extended-chain crystalline polyethylene nanofibers with excellent mechanical properties. The present strategy has been intended to mimic the natural processes for the formation of crystalline fibers, such as cellulose and cocoon, via extrusion through biological nanopores, and has potential for the fabrication of novel polymeric materials from commodity monomers.
These two examples illustrate the great possibilities of restricted nanospaces for developing novel reactions that have never been expected based on the existing knowledge about ordinary bulk systems. Extending this basic idea by changing the size, dimension, and shape of the nanoscopic cavities and pores should result in breakthroughs in science and technology.