First experimental evidence of 3-D aromaticity in stacked antiaromatic compounds

December 15, 2016

Nagoya, Japan - 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.
-end-
The article "Stacked antiaromatic porphyrins" was published in Nature Communications (DOI: 10.1038/ncomms13620).

Nagoya University

Related Electrons Articles from Brightsurf:

One-way street for electrons
An international team of physicists, led by researchers of the Universities of Oldenburg and Bremen, Germany, has recorded an ultrafast film of the directed energy transport between neighbouring molecules in a nanomaterial.

Mystery solved: a 'New Kind of Electrons'
Why do certain materials emit electrons with a very specific energy?

Sticky electrons: When repulsion turns into attraction
Scientists in Vienna explain what happens at a strange 'border line' in materials science: Under certain conditions, materials change from well-known behaviour to different, partly unexplained phenomena.

Self-imaging of a molecule by its own electrons
Researchers at the Max Born Institute (MBI) have shown that high-resolution movies of molecular dynamics can be recorded using electrons ejected from the molecule by an intense laser field.

Electrons in the fast lane
Microscopic structures could further improve perovskite solar cells

Laser takes pictures of electrons in crystals
Microscopes of visible light allow to see tiny objects as living cells and their interior.

Plasma electrons can be used to produce metallic films
Computers, mobile phones and all other electronic devices contain thousands of transistors, linked together by thin films of metal.

Flatter graphene, faster electrons
Scientists from the Swiss Nanoscience Institute and the Department of Physics at the University of Basel developed a technique to flatten corrugations in graphene layers.

Researchers develop one-way street for electrons
The work has shown that these electron ratchets create geometric diodes that operate at room temperature and may unlock unprecedented abilities in the illusive terahertz regime.

Photons and electrons one on one
The dynamics of electrons changes ever so slightly on each interaction with a photon.

Read More: Electrons News and Electrons Current Events
Brightsurf.com is a participant in the Amazon Services LLC Associates Program, an affiliate advertising program designed to provide a means for sites to earn advertising fees by advertising and linking to Amazon.com.