title: Fluoreno[2,3-b]fluorene vs Indeno[2,1-b]fluorene: Unusual Relationship between the Number of π Electrons and Excitation Energy in m-Quinodimethane-Type Singlet Diradicaloids.
abstract: The dimesityl derivative of fluoreno[2,3-b]fluorene (6b) was synthesized and its structure and physical properties were investigated to elucidate the effects of its enhanced open-shell character, which was predicted theoretically in comparison with the smaller congener indeno[2,1-b]fluorene (5b). All structural and physical properties are in accordance with the theoretical predictions and can be interpreted in terms of the resonance contributors. The most remarkable spectroscopic property is the larger excitation energy of 24π-electron hydrocarbon 6b than that of 20π-electron system 5b in their lowest energy absorption bands of electronic spectra, a trend that is contrary to the well-known feature for common, alternant aromatic hydrocarbons. The theoretical basis of this unusual behavior was elucidated on the basis of the balance between the diradical character, exchange integral, and HOMO-LUMO gap and was confirmed by a complete-active-space configuration-interaction method with two electrons in two orbitals for the corresponding parent hydrocarbons 5a and 6a.
authors.forename: Hirokazu
authors.initials: H
authors.lastname: Miyoshi
authors.forename: Masahito
authors.initials: M
authors.lastname: Miki
authors.forename: Shintaro
authors.initials: S
authors.lastname: Hirano
authors.forename: Akihiro
authors.initials: A
authors.lastname: Shimizu
authors.forename: Ryohei
authors.initials: R
authors.lastname: Kishi
authors.forename: Kotaro
authors.initials: K
authors.lastname: Fukuda
authors.affiliation: Department of Chemistry, Graduate School of Science, Osaka City University , Sumiyoshi-ku, Osaka 558-8585, Japan.
authors.forename: Daisuke
authors.initials: D
authors.lastname: Shiomi
authors.affiliation: Department of Chemistry, Graduate School of Science, Osaka City University , Sumiyoshi-ku, Osaka 558-8585, Japan.
authors.forename: Kazunobu
authors.initials: K
authors.lastname: Sato
authors.affiliation: Department of Chemistry, Graduate School of Science, Osaka City University , Sumiyoshi-ku, Osaka 558-8585, Japan.
authors.forename: Takeji
authors.initials: T
authors.lastname: Takui
authors.affiliation: Department of Material and Life Science, Graduate School of Engineering, Osaka University , Suita, Osaka 565-0871, Japan.
authors.forename: Ichiro
authors.initials: I
authors.lastname: Hisaki
authors.forename: Masayoshi
authors.identifier: 0000-0002-3544-1290
authors.initials: M
authors.lastname: Nakano
authors.forename: Yoshito
authors.identifier: 0000-0002-1795-5829
authors.initials: Y
authors.lastname: Tobe