title: Electronic stabilization effect of a spin-delocalized neutral radical: synthesis of an 8-cyano-6-oxophenalenoxyl derivative and quantitative evaluation of the electronic spin structure in terms of resonance structures.
abstract: A new 2,5-di-tert-butyl-6-oxophenalenoxyl (6OPO) derivative with a cyano group at the 8-position, where a large spin density resides, has been synthesized. This neutral radical exhibits high stability in the solid state in air despite the low steric protection on the 8-position; the stability is comparable to that of a corresponding 8-tert-butylated 6OPO derivative. EPR/(1)H-ENDOR/TRIPLE (electron paramagnetic resonance/(1)H-electron-nuclear double resonance/TRIPLE) spectroscopy and cyclic voltammetry showed an extended spin delocalization on the cyano group and a significant increase in electron-accepting ability relative to that of the 8-tert-butylated 6OPO derivative. DFT calculations indicated the extension of a singly occupied molecular orbital (SOMO) onto the cyano group and the lower-lying SOMO and LUMO in comparison with those of the 8-tert-butylated 6OPO derivative, which was consistent with experimental results. Furthermore, the extended nature of π conjugation onto the cyano group was quantitatively evaluated by calculating the contributing weights of resonance structures in terms of a molecular orbital (MO)-based valence-bond (VB) method. Herein, the synthesis and physical properties of the 8-cyano-6OPO derivative are described, emphasizing that the high stability arises from the electronic effect of the cyano group. Also, the usefulness of the quantitative resonance structure analysis is shown.
authors.affiliation: Department of Chemistry, Graduate School of Science, Osaka University, Toyonaka, Osaka 560-0043, Japan.
authors.forename: Shinsuke
authors.initials: S
authors.lastname: Nishida
authors.forename: Kazuki
authors.initials: K
authors.lastname: Kariyazono
authors.forename: Azusa
authors.initials: A
authors.lastname: Yamanaka
authors.forename: Kozo
authors.initials: K
authors.lastname: Fukui
authors.forename: Kazunobu
authors.initials: K
authors.lastname: Sato
authors.forename: Takeji
authors.initials: T
authors.lastname: Takui
authors.forename: Kazuhiro
authors.initials: K
authors.lastname: Nakasuji
authors.forename: Yasushi
authors.initials: Y
authors.lastname: Morita