ADVERTISEMENT

Novel Organic Macrocycles With Unique Super-Ring Structure, Global Aromaticity And Polyradical Character

The concept of aromaticity is a pivotal one in chemistry used to rationalize the notable stability of unsaturated organic molecules, with the benzene ring being a paramount example. Typical π-conjugated molecules obey the aromaticity rule introduced by Hückel, although other types of aromaticity have been also described, that is Möbius aromaticity in annulenes with Möbius conformation and the Baird’s aromaticity for the lowest spin-triplet state, i.e. with two spin-up unpaired electrons.

When benzenoid rings are fused in a polycyclic structure, π-electrons tend to localize forming pseudo-independent aromatic benzene rings (local aromaticity) and very rarely delocalize over the entire molecular framework (global aromaticity).

ADVERTISEMENT

In this context, conjugated molecules with an annulene-within-an-annulene (AWA) super-ring structure are of special interest. The AWA model consists of inner and outer carbon rings linked through transverse bonds. Many attempts have tried to produce an AWA super-ring in which both rings behave as independent annulenes. But, despite a large amount of theoretical design and many experimental efforts, up to now the synthesis of a decoupled AWA super-ring structure has not been achieved. In the present study, we design, synthesize and characterized experimentally and computationally two novel fully π-conjugated polycyclic hydrocarbon stable macrocycles exhibiting AWA structures. The two compounds, named 8MC and 10MC (Figure 1), are built as alternatively fused five and six membered rings. The tendency to form benzenoid rings with local aromaticity (Clar’s sextet rule) of the two compounds serves as a driving force for the generation of unpaired electrons, resulting in important polyradical character in 8MC and 10MC.

Figure 1. Representation of open-shell (left) and closed-shell (right) resonance forms of super-ring systems with AWA structure. Adapted from Chem 4 (2018) 1586-1595. Copyright © 2018 Elsevier Inc.

The 1H NMR spectrum of 8MC exhibits a singlet peak at very low field and another weaker broad signal at very high field (Figure 2). These two singlets could be respectively assigned to the outer and inner-ring protons, in good agreement with global aromaticity. The broadening and weakening of all proton resonances upon increasing the temperature is typical of open-shell structures and is related to the population of low-lying states with higher spin multiplicities.

Figure 2. VT 1H NMR spectrum in THF-d8 of 8MC. Adapted from Chem 4 (2018) 1586-1595. Copyright © 2018 Elsevier Inc.

Superconducting quantum interference device (SQUID) measurements of 8MC in powder shows an increase of the product of molar magnetic susceptibility and temperature in agreement with a ground state spin-singlet state with a gap of 3.08 kcal/mol to the lowest spin-triplet. SQUID measurements on 10MC suggest a spin-triplet ground state with the lowest singlet lying at 0.03 kcal/mol.

Theoretical and computational analysis allow us to confirm and rationalize the global aromaticity of the two synthesized super-rings in terms of polyradicaloid character and by means of the Hückel’s and Baird’s aromaticity rules applied to the inner and outer annulene rings. 8MC molecule holds a mild octaradicaloid character (tendency towards the eight unpaired electron configuration) and his structure can be described as a [24]annulene-within-[32]annulene, and despite containing 4n π-electrons, its ground state spin-singlet exhibits global aromaticity as the inner and outer rings follow Baird’s rule. The larger 10MC molecule possesses some decaradicaloid character (tendency towards the ten unpaired electron configuration) with a lower state being a spin-triplet. The super-aromatic AWA structure of 10MC corresponds to a [30]annulene-within-[40]annulene, with the inner and outer rings formally being a local spin-singlet and a spin-triplet, respectively. The global aromaticity emerges as the result of an inner ring satisfying the Hückel’s rule for spin-singlet states, while the outer ring follows the Baird’s rule for triplet states (Figure 3).

ADVERTISEMENT
Figure 3. ACID plots of individual inner and outer rings of 8MC (a) and 10MC (b) in their singlet and triplet ground states, respectively. Arrows along the inner and outer rings indicate diamagnetic (clockwise) and paramagnetic (counter-clockwise) electron current flows. Adapted from Chem 4 (2018) 1586-1595. Copyright © 2018 Elsevier Inc.

Our study exemplifies how rational design can work in conjunction with advanced synthesis procedures and a variety of characterization techniques to achieve fully decoupled AWA super-ring systems with unique properties. Moreover, the work provides a well-defined strategy to obtain novel organic AWA super-ring molecules with tunable global aromaticity.

These results were presented in the article entitled Macrocyclic Polyradicaloids with Unusual Super-ring Structure and Global Aromaticity, recently published in the journal Chem. The work was the result of the collaborative efforts of N. Aratani and H. Yamada from the Institute of Science and Technology (Japan); Y. Hong and D. Kim from Yonsei University (Korea); M. E. Sandoval-Salinas and D. Casanova from Donostia International Physics Center (Spain); and C. Liu, T. Y. Gopalakrishna, T. S. Herng, J. Ding, H. Phan and J. Wu from the National University of Singapore.

Comments

READ THIS NEXT

Bread Mold: How To Identify Types Of Mold

” Bread mold is a type of fungus and is therefore eukaryotic (contains membrane-bound organelles) and multi-cellular. Mold cells are […]

Autophagy Declares The Clean Bill Of A Cell’s Health

The term “Autophagy”, originating from the Greek words “auto” (meaning self) and “phagy” (meaning eat) is the evolutionarily conserved physiological […]

Easily Calculate 0.875 As A Fraction In The Simplest Form

You can calculate 0.875 as a fraction with just a few steps, leading you to its simplest form will give you the […]

Observing Magnetically-Mediated Behavior In Lab Mice

The ability of animals to sense the Earth’s magnetic field for spatial behaviors such as navigation and orientation is well-established. […]

Using Photoacoustic Moving Sources In Trace Gas Detection

The photoacoustic effect, which was discovered by A. G. Bell in 1881, refers to the generation of sound as a […]

Periodic Table Ionic Charges, Name, And Mass

The periodic table ionic charge can be broken down by metals that are positive and on the left of the […]

Colorful Bioluminescence: Exploring ATP’s Effect On The Firefly Luciferase Reaction

Every spring, the forest near Hualien (Taiwan) gleams with a myriad of flickering lights. Curious visitors stare at these moving […]

Science Trends is a popular source of science news and education around the world. We cover everything from solar power cell technology to climate change to cancer research. We help hundreds of thousands of people every month learn about the world we live in and the latest scientific breakthroughs. Want to know more?