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First experimental evidence of 3D aromaticity in stacked antiaromatic compounds

Date:
December 15, 2016
Source:
Nagoya University
Summary:
Researchers found that antiaromatic planar norcorrole molecules can form close face-to-face interactions to give structures with increased aromaticity. This behavior is quite different from that of typical aromatic materials, which adopt offset interactions to minimize electron repulsion. The results agree with previous theoretical studies proposing that the aromaticity of antiaromatic materials can be increased through suitable interactions to produce materials with interesting electronic structures.
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Antiaromatic planar norcorrole molecules form close face-to-face stacked structures with increased aromaticity. This behavior is quite different from that of planar aromatic molecules. This result is the first experimental proof for the theoretical prediction that the stacking of antiaromatic molecules may result in the formation of materials with three-dimensional aromaticity.
Credit: Image courtesy of Nagoya University

Researchers centered at Nagoya University found that antiaromatic planar norcorrole molecules can form close face-to-face interactions to give structures with increased aromaticity. This behavior is quite different from that of typical aromatic materials, which adopt offset interactions to minimize electron repulsion. The results agree with previous theoretical studies proposing that the aromaticity of antiaromatic materials can be increased through suitable interactions to produce materials with interesting electronic structures.

International collaboration led by Nagoya University uses two-dimensional antiaromatic materials to realize three-dimensional aromatic structures.

Aromatic molecules consist of planar carbon-based rings with alternating single and double (π) bonds. These molecules contain 4n+2 (n = 0, 1, 2 …) π electrons -- π electrons are those involved in π bonds -- which results in high stability because the π electrons delocalize over the ring structure. Aromatic molecules can interact through offset π-π stacking, and the overlap of π orbitals in aromatic structures with π-π stacking can facilitate electron conduction, making such materials attractive for use in electronics. The overlap between π orbitals would be increased if π-π stacking was face-to-face rather than offset. However, face-to-face stacking is energetically unfavorable in aromatic molecules because of the repulsion of π electrons.

Theoretical studies have indicated that face-to-face interactions between molecules may be achieved using antiaromatic materials. Antiaromatic molecules contain 4n (n = 1, 2 …) π electrons, which makes them highly unstable. It has been postulated that the two-dimensional stacking of antiaromatic materials may result in the formation of materials with three-dimensional aromaticity. However, this had not been verified experimentally as antiaromatic materials are difficult to synthesize because of their instability.

Recently, an international collaboration led by researchers at Nagoya University achieved a breakthrough in two-dimensional stacking of antiaromatic materials. They synthesized nickel complexes of antiaromatic planar norcorrole macrocycles. The study was reported in Nature Communications.

"We synthesized stable antiaromatic nickel norcorroles and then investigated their interactions," first author Ryo Nozawa says. X-ray diffraction analysis showed that the norcorrole complex stacked to form a "triple-decker" structure with the norcorrole planes much closer together than observed for typical π-π stacking interactions. The triple-decker structure displayed aromatic characteristics, unlike its norcorrole subunits.

The researchers then fabricated a molecule containing two antiaromatic norcorrole units linked by a flexible bridge.

"Our characterization results indicate that the two norcorrole units assume face-to-face interactions to form a molecule with higher aromaticity than that of the norcorrole subunit," coauthor Hiroshi Shinokubo explains. "That is, there is strong three-dimensional electronic communication between the norcorrole subunits."

The stacking of antiaromatic units gave closer interactions than that achieved when stacking aromatic units together, corroborating theoretical predictions. The resulting materials had extremely close π-conjugated systems, which should result in large intermolecular orbital interactions. As a result, these materials are interesting for application in optoelectronics.

The researchers also found that the stacked antiaromatic materials displayed nonlinear optical properties that were regulated by the formation of supramolecular structures. A material has nonlinear optical properties when it does not respond linearly to the electric field of light. Such materials are attractive for use in nanofabrication and photodynamic therapy, suggesting possible future applications of norcorrole-based compounds.


Story Source:

Materials provided by Nagoya University. Note: Content may be edited for style and length.


Journal Reference:

  1. Ryo Nozawa, Hiroko Tanaka, Won-Young Cha, Yongseok Hong, Ichiro Hisaki, Soji Shimizu, Ji-Young Shin, Tim Kowalczyk, Stephan Irle, Dongho Kim, Hiroshi Shinokubo. Stacked antiaromatic porphyrins. Nature Communications, 2016; 7: 13620 DOI: 10.1038/ncomms13620

Cite This Page:

Nagoya University. "First experimental evidence of 3D aromaticity in stacked antiaromatic compounds." ScienceDaily. ScienceDaily, 15 December 2016. <www.sciencedaily.com/releases/2016/12/161215081023.htm>.
Nagoya University. (2016, December 15). First experimental evidence of 3D aromaticity in stacked antiaromatic compounds. ScienceDaily. Retrieved May 26, 2017 from www.sciencedaily.com/releases/2016/12/161215081023.htm
Nagoya University. "First experimental evidence of 3D aromaticity in stacked antiaromatic compounds." ScienceDaily. www.sciencedaily.com/releases/2016/12/161215081023.htm (accessed May 26, 2017).

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