Bicyclic Baird-type aromaticity

Abstract

Classic formulations of aromaticity have long been associated with topologically planar conjugated macrocyclic systems. The theoretical possibility of so-called bicycloaromaticity was noted early on. However, it has yet to be demonstrated by experiment in a simple synthetic organic molecule. Conjugated organic systems are attractive for studying the effect of structure on electronic features. This is because, in principle, they can be modified readily through dedicated synthesis. As such, they can provide useful frameworks for testing by experiment with fundamental insights provided by theory. Here we detail the synthesis and characterization of two purely organic non-planar dithienothiophene-bridged [34]octaphyrins that permit access to two different aromatic forms as a function of the oxidation state. In their neutral forms, these congeneric systems contain competing 26 and 34 π-electronic circuits. When subject to two-electron oxidation, electronically mixed [4n+1]/[4n+1] triplet biradical species in the ground state are obtained that display global aromaticity in accord with Baird's rule.

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Figure 1: Molecular structures.
Figure 2: Spectroscopic evidence for dual aromaticity.
Figure 3: NICS calculations support dual aromaticity.
Figure 4: Oxidized forms display [4n] π-electron aromatic character in accord with Baird's rule.

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Acknowledgements

The research at Yonsei University was supported by the Samsung Science and Technology Foundation (SSTF-BA1402-10). The quantum calculations were performed using the supercomputing resources of the Korea Institute of Science and Technology Information. T.K.C. thanks the Department of Science and Technology, New Delhi, India, for a J. C. Bose Fellowship. The work in Austin was supported by the National Science Foundation (grant CHE-1402004) and the Robert A. Welch Foundation. The work at Sookmyung Women's University was supported by the Basic Science Research Program through the National Research Foundation of Korea funded by the Ministry of Science, ICT & Future Planning (NRF-2016R1C1B2014895). This work was also supported by the Japanese Society for the Promotion of Science (grant no. 16H02268 to S.F.).

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D.K. and J.L.S. conceived and designed this work. W.-Y.C., A.G., R.A., J.S.P. and T.K.C. designed and synthesized the materials. Z.Z., X.-S.K. and V.M.L. obtained the single-crystal X-ray diffraction structures. J.J., W.-Y.C. and S.F. performed and analysed the EPR measurements. W.-Y.C. and T.K. performed the computational calculations. W.-Y.C., T.K., W.K. and S.L. performed the spectroscopic measurements. D.K., J.L.S. and W.-Y.C. co-wrote the paper. D.K. supervised the study.

Corresponding authors

Correspondence to Shunichi Fukuzumi or Jung Su Park or Jonathan L. Sessler or Tavarekere K. Chandrashekar or Dongho Kim.

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Cha, W., Kim, T., Ghosh, A. et al. Bicyclic Baird-type aromaticity. Nature Chem 9, 1243–1248 (2017). https://doi.org/10.1038/nchem.2834

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