title: Third-order nonlinear optical properties of one-dimensional open-shell molecular aggregates composed of phenalenyl radicals.
abstract: The impact of intermolecular interactions on the third-order nonlinear optical (NLO) properties of open-shell molecular aggregates has been elucidated by considering one-dimensional aggregates of π-π stacked phenalenyl radicals with different intermolecular distances and the long-range corrected spin-unrestricted density functional theory method. In the phenalenyl dimer, which can be considered as a diradicaloid system, the diradical character strongly depends on the intermolecular distance, and the larger the intermolecular distance is, the larger the diradical character becomes. Then, around the equilibrium stacking distance that corresponds to an intermediate diradical character, its second hyperpolarizability (γ) is maximized and its value per monomer exhibits about a 30-fold enhancement with respect to the isolated phenalenyl monomer. This suggests that equilibrium is an optimal compromise between localization and delocalization of the radical electron pairs in such pancake bonding. No such effect was observed for the closed-shell coronene dimer. Moreover, when going from the dimer (diradical) to the tetramer (tetraradical), the γ-enhancement ratio increases nonlinearly with the aggregate size, whereas switching from the singlet to the highest spin (quintet) state causes a significant reduction of γ. Finally, for the tetramer, another one-order enhancement of γ is achieved for the dicationic singlet relative to its singlet neutral state. These results demonstrate the key role of intermolecular π-π stacking interactions and charge in open-shell (supra)molecular systems to achieve enhanced third-order NLO properties.
authors: Department of Materials Engineering Science, Graduate School of Engineering Science, Osaka University, Toyonaka, Osaka 560-8531 (Japan), Fax: (+81) 6-6850-6268.
authors: Kyohei
authors: K
authors: Yoneda
authors: Masayoshi
authors: M
authors: Nakano
authors: Kotaro
authors: K
authors: Fukuda
authors: Hiroshi
authors: H
authors: Matsui
authors: Shota
authors: S
authors: Takamuku
authors: Yuta
authors: Y
authors: Hirosaki
authors: Takashi
authors: T
authors: Kubo
authors: Kenji
authors: K
authors: Kamada
authors: Benoît
authors: B
authors: Champagne
mesh_terms: D019281:Dimerization; D005609:Free Radicals; D008958:Models, Molecular; D055095:Optics and Photonics; D043803:Phenalenes / Q000737:chemistry*; D011789:Quantum Theory
chemical_list: D005609:Free Radicals; D043803:Phenalenes
keywords: aggregation; density functional calculations; nonlinear optics; pi interactions; radicals