title: Diradical and Ionic Characters of Open-Shell Singlet Molecular Systems.
abstract: The diradical and ionic natures of open-shell singlet systems have been investigated using new definitions of the diradical and ionic characters as well as of their densities within the valence configuration interaction (VCI) model with two electrons in two active orbitals. The two-site symmetric and asymmetric diradical models are examined by using these diradical/ionic characters. For realistic compounds, we investigate a diradicaloid diphenalenyl and a rectangular graphene nanoflake in the presence of an external static electric field, as well as π-stacked phenalenyl-derivative dimers with varying the intermonomer distance, where the central carbon atoms in the phenalenyl rings are substituted by boron (B) and nitrogen (N) atoms, respectively. It is found that the increase of charge asymmetricity induced by the static electric field decreases the diradical character and finally induces an ionic character in the ground state, while the first excited state is changed from pure ionic to diradical-dominant as the field amplitude increases. On the other hand, when increasing the intermonomer distance, the B/N substitution in the phenalenyl dimer changes the electronic state from open-shell singlet with small diradical character to closed-shell with large ionic character. These results indicate that the application of a static electric field to diradicaloids and the asymmetric substitution of a pancake bonded π-dimer combined with the variation of intermonomer distance could tune the diradical/ionic characters and therefore control the nonlinear optical responses.
authors.affiliation: Department of Materials Engineering Science, Graduate School of Engineering Science, Osaka University , Toyonaka, Osaka 560-8531, Japan.|Center for Spintronics Research Network (CSRN), Graduate School of Engineering Science, Osaka University , Toyonaka, Osaka 560-8531, Japan.
authors.forename: Masayoshi
authors.identifier: http://orcid.org/0000-0002-3544-1290
authors.initials: M
authors.lastname: Nakano
authors.affiliation: Department of Materials Engineering Science, Graduate School of Engineering Science, Osaka University , Toyonaka, Osaka 560-8531, Japan.
authors.forename: Kotaro
authors.initials: K
authors.lastname: Fukuda
authors.affiliation: Department of Materials Engineering Science, Graduate School of Engineering Science, Osaka University , Toyonaka, Osaka 560-8531, Japan.
authors.forename: Soichi
authors.identifier: http://orcid.org/0000-0001-6550-702X
authors.initials: S
authors.lastname: Ito
authors.affiliation: Department of Materials Engineering Science, Graduate School of Engineering Science, Osaka University , Toyonaka, Osaka 560-8531, Japan.
authors.forename: Hiroshi
authors.initials: H
authors.lastname: Matsui
authors.affiliation: Department of Materials Engineering Science, Graduate School of Engineering Science, Osaka University , Toyonaka, Osaka 560-8531, Japan.
authors.forename: Takanori
authors.initials: T
authors.lastname: Nagami
authors.affiliation: Department of Materials Engineering Science, Graduate School of Engineering Science, Osaka University , Toyonaka, Osaka 560-8531, Japan.
authors.forename: Shota
authors.initials: S
authors.lastname: Takamuku
authors.affiliation: Department of Materials Engineering Science, Graduate School of Engineering Science, Osaka University , Toyonaka, Osaka 560-8531, Japan.|Center for Spintronics Research Network (CSRN), Graduate School of Engineering Science, Osaka University , Toyonaka, Osaka 560-8531, Japan.
authors.forename: Yasutaka
authors.initials: Y
authors.lastname: Kitagawa
authors.affiliation: Laboratoire de Chimie Théorique, University of Namur , rue de Bruxelles, 61, 5000 Namur, Belgium.
authors.forename: Benoît
authors.identifier: http://orcid.org/0000-0003-3678-8875
authors.initials: B
authors.lastname: Champagne