A chemical insulation approach produces highly insulated molecular wires with strong fluorescence.
Insulated molecular wires, which resemble nanoscale coaxial cables in which a conducting polymer chain is wrapped with insulating molecules, could be used for making and connecting electronic devices at the molecular scale. These wires are often made by synthesizing polymer chains and then threading ring-like molecules onto the chains for insulation.
However, this approach leads to imperfect chain coverage, and therefore incomplete insulation. Now, in a novel approach, Jun Terao from Kyoto University and colleagues1 have threaded ring molecules onto individual monomers prior to polymerization. Their approach ensures high coverage of the polymer chains and allows for the synthesis of very rigid wires (Fig. 1) that are strongly fluorescent and processable using a variety of organic solvents.
The researchers started by making monomer precursors of the insulated conducting polymers. The monomer, a π-conjugated diaryl acetylene side-chain, was then linked to a cyclodextrin ring, which acts like an insulating tube. After a process called intramolecular self-inclusion, the monomer slipped into the tube, where it was fixed by chemically attaching a large blocking group to the exposed end.
Once the insulated unit was complete, Terao and his colleagues built polymer chains by linking the units together. This approach, however, only resulted in around 60% insulation coverage. To improve this, they tried doubling the length of the monomer and threading it into two cyclodextrin rings. After polymerization, they were able to achieve 95% insulation coverage.
When the researchers compared the insulated chains with the un-insulated equivalent, they found that the insulation greatly increased the rigidity of the chains. This increase in rigidity resulted in enhanced fluorescence from the polymer, and made it possible to produce a liquid-crystalline phase — the first time this has been achieved for such molecular wires, according to Terao. He also says that the insulated chains are readily soluble in a variety of organic solvents.
“We will wire π-conjugated molecules between nanogap electrodes using a cross-coupling reaction to realize molecular electronics,” says Terao. In this application, the rigidity and resulting high electron mobility of the polymers will be vital.
References
Terao, J., Tsuda, S., Tanaka, Y., Okoshi, K., Fujihara, T., Tsuji, Y. & Kambe, N. Synthesis of organic-soluble conjugated polyrotaxanes by polymerization of linked rotaxanes. J. Am. Chem. Soc. 131, 16004 (2009).
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Molecular wires: Miniature coaxial cables. NPG Asia Mater 2, 43 (2010). https://doi.org/10.1038/asiamat.2010.9
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DOI: https://doi.org/10.1038/asiamat.2010.9