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1,3-Diradicals Embedded in Curved Paraphenylene Units: Singlet versus Triplet State and In-Plane Aromaticity

Miyazawa, Yuki; Wang, Zhe; Matsumoto, Misaki; Hatano, Sayaka; Antol, Ivana; Kayahara, Eiichi; Yamago, Shigeru; Abe, Manabu (2021) 1,3-Diradicals Embedded in Curved Paraphenylene Units: Singlet versus Triplet State and In-Plane Aromaticity. Journal of the American Chemical Society, 143 (19). pp. 7426-7439. ISSN 0002-7863

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Curved π-conjugated molecules and open-shell structures have attracted much attention from the perspective of fundamental chemistry, as well as materials science. In this study, the chemistry of 1, 3-diradicals (DRs) embedded in curved cycloparaphenylene (CPPs) structures, DR- (n+3)CPPs (n = 0–5), was investigated to understand the effects of the curvature and system size on the spin–spin interactions and singlet versus triplet state, as well as their unique characteristics such as in-plane aromaticity. A triplet ground state was predicted for the larger 1, 3-diradicals, such as the seven- and eight-paraphenylene-unit-containing diradicals DR-7CPP (n = 4) and DR-8CPP (n = 5), by quantum chemical calculations. The smaller- sized diradicals DR-(n+3)CPPs (n = 0–3) were found to possess singlet ground states. Thus, the ground-state spin multiplicity is controlled by the size of the paraphenylene cycle. The size effect on the ground-state spin multiplicity was confirmed by the experimental generation of DR- 6CPP in the photochemical denitrogenation of its azo-containing precursor (AZ-6CPP). Intriguingly, a unique type of in-plane aromaticity emerged in the smaller-sized singlet states such as S-DR- 4CPP (n = 1), as proven by nucleus-independent chemical shift calculations (NICS) and an analysis of the anisotropy of the induced current density (ACID), which demonstrate that homoconjugation between the 1, 3-diradical moiety arises because of the curved and distorted bonding system.

Item Type: Article
Uncontrolled Keywords: 1, 3-diradicals ; curved paraphenylenes
Subjects: NATURAL SCIENCES > Chemistry
NATURAL SCIENCES > Chemistry > Physical Chemistry
NATURAL SCIENCES > Chemistry > Organic Chemistry
Divisions: Division of Organic Chemistry and Biochemistry
Depositing User: Ivana Antol
Date Deposited: 07 Dec 2022 14:05
DOI: 10.1021/jacs.1c01329

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