title: Synthesis and structural, electrochemical, and optical properties of Ru(II) complexes with azobis(2,2'-bipyridine)s.
abstract: Two new ditopic ligands, 5,5"-azobis(2,2'-bipyridine) (5,5"-azo) and 5,5"-azoxybis(2,2'-bipyridine) (5,5"-azoxy), were prepared by the reduction of nitro precursors. Mononuclear and dinuclear Ru(II) complexes having one of these bridging ligands and 2,2'-bipyridine terminal ligands were also prepared, and their properties were compared with previously reported Ru(II) complexes having 4,4"-azobis(2,2'-bipyridine) (4,4"-azo). The X-ray crystal structure showed that 5,5"-azo adopts the trans conformation and a planar rodlike shape. The X-ray crystal structure of [(bpy)(2)Ru(5,5"-azo)Ru(bpy)(2)](PF(6))(4) (Ru(5,5"-azo)Ru) showed that the bridging ligand is in the trans conformation and nearly planar also in the complex and the metal-to-metal distance is 10.0 A. The azo or azoxy ligand in these complexes exhibits reduction processes at less negative potentials than the terminal bpy's due to the low-lying pi level. The electronic absorption spectra for the complexes having 5,5"-azo or 5,5"-azoxy exhibit an extended low-energy metal-to-ligand charge-transfer absorption. The ligands, 5,5"-azo and 5,5"-azoxy, and the mononuclear complex, [(bpy)(2)Ru(5,5"-azo)](2+), isomerize reversibly upon light irradiation. The low-energy MLCT state sensitizes the isomerization of the azo moiety in this complex. While [(bpy)(2)Ru(4,4"-azo)Ru(bpy)(2)](PF(6))(4) exhibits light switch properties, namely, significant electrochromism and a large luminescence enhancement, upon reduction, Ru(5,5"-azo)Ru does not show these properties. The radical anion formation upon reduction of these complexes has been confirmed by ESR spectroscopy.
authors.affiliation: College of Science and Technology, Nihon University, 1-8-14 Kanda Surugadai, Chiyoda-ku, Tokyo 101-8308, Japan. otsuki@chem.cst.nihon-u.ac.jp
authors.forename: Joe
authors.initials: J
authors.lastname: Otsuki
authors.forename: Nobuyuki
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authors.lastname: Omokawa
authors.forename: Kenji
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authors.lastname: Yoshiba
authors.forename: Isao
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authors.lastname: Yoshikawa
authors.forename: Tetsuo
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authors.lastname: Akasaka
authors.forename: Tomoyoshi
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authors.lastname: Suenobu
authors.forename: Toshio
authors.initials: T
authors.lastname: Takido
authors.forename: Koji
authors.initials: K
authors.lastname: Araki
authors.forename: Shunichi
authors.initials: S
authors.lastname: Fukuzumi