π-Conjugated molecules with an annulene-within-an-annulene (AWA) super-ring structure have been pursued by chemists for a long time but with limited success. Here, we report the synthesis of two stable cyclopenta-ring-fused oligo(m-phenylene) macrocycles, 8MC-M and 10MC-M, both showing moderate polyradical character. The octaradicaloid 8MC-M exhibits a annulene-within-annulene structure, which is globally aromatic despite the fact that it possesses 4n π-electrons. Theoretical analyses reveal that the π-electrons in the inner and outer rings follow Baird's rule such that both are triplet biradicals, and overall the molecule is a ground-state spin singlet. The decaradicaloid 10MC-M also possesses a super-aromatic AWA structure with 30 and 40 π-electrons in the inner and outer rings, respectively. However, it has a triplet ground state with a singlet inner ring satisfying Hückel's rule and a triplet outer ring following Baird's rule. Therefore, molecules with a decoupled AWA super-ring structure are eventually attained and also show unusual global aromaticity. Organic opto-electronic and magnetic materials play an important role in future affordable electronics and clean energy, and aromaticity is one of the most important concepts that determine their physical and material properties. Over the past nine decades, Hückel's rule and Baird's rule have been developed to describe the (anti)aromaticity of monocyclic π-conjugated molecules. For polycyclic π-conjugated systems, localized aromaticity usually dominates, and there is almost no report of global aromaticity for benzenoid polycyclic hydrocarbons. In this work, we experimentally obtained two hydrocarbon-based macrocyclic polyradicaloids, which show a long-sought-after annulene-within-an-annulene super-ring structure and very unusual global aromaticity. Both Hückel's rule and Baird's rule can be followed for these polyradicaloids. Our study gives some new insight into the unique global aromaticity in polycyclic π-conjugated molecules. Wu and colleagues describe the challenging synthesis and physical characterization of a macrocyclic octaradicaloid and decaradicaloid. They exhibit the long-sought-after “annulene-within-an-annulene” super-ring structure and unusual global aromaticity with individual inner and outer rings following Hückel's rule in the singlet state and Baird's rule in the triplet state, respectively.
Bibliographical noteFunding Information:
J.W. acknowledges financial support from the Ministry of Education Tier 3 programme ( MOE2014-T3-1-004 ). D.C. is thankful to Eusko Jaurlaritza (project no. IT588-13) and the Spanish Government ( MINECO -FEDER project CTQ2016-80955) for financial support. M.E.S. acknowledges CONACyT-México for a PhD fellowship (ref. 591700). The work at Yonsei University was supported by the Global Research Laboratory ( 2013K1A1A2A02050183 ) through the National Research Foundation of Korea , funded by the Ministry of Science, ICT (Information and Communication Technologies), and Future Planning . We thank Dr. Tan Geok Kheng for the X-ray crystallographic analysis of 8MC-M and Dr. Ji'en Wu for his help with 2D NMR measurements.
All Science Journal Classification (ASJC) codes
- Environmental Chemistry
- Chemical Engineering(all)
- Biochemistry, medical
- Materials Chemistry