Nanotechnologies / Materials
Organic device, Organic magnetism, Organic conductor

Synthesis and characterization of bis-periazulene:
Unprecedented pyrene’s structural isomer
Department of Applied Chemistry, Graduate School of Engineering
Assistant Professor Akihito Konishi		https://researchmap.jp/a-koni?lang=en

Professor Makoto Yasuda				https://researchmap.jp/read0185253?lang=en

Abstract
Bis-periazulene (cyclohepta[def]fluorene), which is an un-known
pyrene isomer, was synthesized as kinetically protected forms. Its
triaryl derivatives exhibited the superimposed electronic structuresof peripheral, polarized, and open-shell π-conjugated systems. In
contrast to previous theoretical predic-tions, bis-periazulene derivatives were in the singlet ground state. Changing an aryl groupcontrolled the energy gap between the lowest singlet‒triplet states.

ripheral, charge-separated, and open-shell π-conjugations. These
characteristics of bis-perizulene are not shared by the previously
reported polycyclic systems that contain azulene-subunit(s).

Nanotechnologies / Materials▍ Background & Results
The chemistry of non-alternant hydrocarbons has recently experienced a considerable resurgence. As the relationship between
naphthalene and azulene shows, an important design concept
for a nonalternant framework is to replace the two hexagons of analternant system with a pentagon and heptagon pair. The pursuitof the non-alternant isomers of pyrene, which has seven possible
non-alternant isomers, has been the most significant related project. Through enormous dedication by many pioneering works,
six of the seven possible non-alternant isomers of pyrene havebeen isolated and characterized as stable aromatic molecules,
differentiating themselves from pyrene in terms of optoelectronicproperties. However, the only unsynthesized isomer, bis-periazulene (cyclohepta[def]fluorene), had remained an uncharacterizedhydrocarbon until 2022, despite many synthetic and theoreticalinvestigations since it was first reported in 1955. we successfully
synthesized and characterized the kinetically protected derivatives,
the remaining non-alternant isomer of pyrene. Triaryl-substituted
derivatives exhibit singlet ground states, contrary to previous theoretical predictions.

Fig1. Bis-periazulene as a one of the non-alternant isomers of pyrene▍ Significance of the research and Future perspective
Despite many previous efforts, the synthesis and characterization of bis-periazulene have not been realized. Previous theoreticalstudies have offered an intriguing hint that bis-periazulene has atriplet open-shell ground state. Presumably, the high reactivity of
bis-periazulene due to an m-quinoidal subunit has made isolationand characterization inaccessible. In this work, details of the synthesis and physical properties of the triaryl derivatives of bis-periazulene are described. The triaryl derivatives, in which three ortho-disubstituted aromatic groups were used to kinetically stabilize
the reactive sites, were realized via an eight-step synthesis fromthe fluorene derivative. The synthesized triaryl derivatives weresuccessfully isolated in their crystalline forms and characterizedby X-ray analysis. Contrary to previous theoretical predictions, thedetermined molecular geometry and the observed magnetic behaviors exhibited singlet open-shell ground states. At the presentstage, it is unclear why the determined ground state is a singlet
state, regardless of the previous theoretical studies, but the introduced substituents may affect the lowest singlet‒triplet energygap. Changing a substituent enabled us to control the energy gap
between the lowest singlet‒triplet states, which is a rare example ofordinary polycyclic hydrocarbons. Notably, our study demonstrated
that bis-periazulene contained three aspects of π-conjugation: pe-Fig2. Molecular structure of the synthesized bis-periazulene derivative

P a t e n tHorii, Koki; Konishi, Akihito; Yasuda, Makoto et al. Bis-periazulene (cyclohepta[def]fluorene) as a nonalternant isomer of pyrene: synthesis andcharacterization of its triaryl derivatives. Journal of the American Chemical Society. 2022, 144 (8), 3370-3375. doi: 10.1021/jacs.2c00476
UR
L http://www.chem.eng.osaka-u.ac.jp/~yasuda-lab/e/K e y w o r d non-alternant hydrocarbon, azulene, open-shell character
Tr e a t i s e