“We were shocked to see the crystal structure of the molecule,” says Hiroyuki Isobe, a research professor in the Department of Chemistry at the University of Tokyo, Japan, as he describes his latest work to synthesize phenine nanotubes (pNTs). The nanotubes are analogues of carbon nanotubes (CNTs) – rolled up sheets of honeycomb carbon lattice whose discovery by Sumio Ijima in 1990 generated sustained excitement for decades. In pNTs Isobe and collaborators at the University of Tokyo, the Japan Science and Technology Agency, Riken, and Tohoku University in Japan, have replaced the atoms of a CNT with phenine rings – derivatives of benzene where each molecule is a six-membered carbon ring. The result is a nanotube with periodic vacancies giving a porous crystal structure that Isobe describes as “simply and astonishingly, beautiful”.
The pores change the electronic properties of the nanotubes and may have interesting functions for entrapping other molecules – 63% of the material is void and the researchers have already successfully entrapped C70 molecules in pNTs. Furthermore, Isobe highlights the significance of some of the fundamental differences between pNTs and their CNT forebears.
“The pNT is a molecular entity, and when you have it in your hands, all the molecules in the powders have an identical length, diameter and molecular weight. The CNT is not a molecular entity – they are chemical species that comprise a mixture of various structures.” As he explains, this distinction opens up the opportunity to understand chemical characteristics that studies of the properties of CNT mixtures cannot provide.
The chemistry of nanotube pores is of particular interest to Isobe, who published several papers in the 2000s on molecular transport through nanopores based on studies of carbon nanotubes. This kind of transport is fundamental to understanding a wide range of biological, chemical and physical processes.
“At that time, we discussed a lot about the chemistry at the pore, which ended up with very speculative discussion,” he tells Physics World, as he describes how the mixture of various structures inherent to samples of CNTs made it impossible to learn anything about the chemical characteristics of CNT pores. “I was naturally motivated for design of the “pore on CNTs” with molecules.”
Synthesis and beyond
The route the researchers take to produce pNTs begins with commercially available 1,3-dibromobenzene, where bromine atoms replace the hydrogens linked to each carbon atom in the benzene molecule at the first and third positions around the ring. They then follow a concise nine-step procedure resulting in pNTs. Although the molecular structure was the result of tailored design and produced through carefully elaborated synthetic strategies, Isobe was still struck when he saw its crystal structure.
Despite the multiple stages of the process the researchers calculate the average efficiency of each phenine ring they bond to another at 91%. However, this still leaves the overall yield of pNTs from dibromobenzene at 0.7%. Exploiting the recently renovated lab in the University of Tokyo’s Graduate School of Science, Isobe and collaborators have already successfully synthesized milligrams of the product but are looking at ways to improve the procedure so that hundreds of milligrams and even grams may be possible.
Another avenue of interest is the possibility of polymerizing the pNTs. Using the same length index used to describe carbon nanotubes, the pNTs commonly have an index of seven (that is, they are seven benzene rings long). While this is more than twice the longest CNTs with no defects, which were limited to an index of three, polymerization could achieve yet longer pNTs.
“Theoretical investigations of an infinite version of the pNT indicated that the nanotube should have semiconductor characteristics,” says Isobe, explaining that the periodic pores on CNTs open a band gap. “Although it is a challenging subject, “polymerization of pNT” should also be an interesting target.”
Full details are available in Science.