title: Isolation and crystal structures of two singlet bis(triarylamine) dications with nonquinoidal geometries.
abstract: We report the first structural data for bis(diarylamine) "bipolarons": we have isolated and crystallographically characterized salts of the dications obtained by two-electron oxidation of E-4,4'-bis[di(p-anisyl)amino]stilbene and E,E-2,5-bis{4-[di(p-anisyl)amino]styryl}-3,4-di(n-butoxy)thiophene, [1](2+) and [2](2+) respectively. ESR, NMR, and magnetometry suggest both species have singlet ground states. X-ray structures, together with (1)H NMR coupling constants for [2](2+), indicate geometries in which the bond lengths are shifted toward a quinoidal pattern relative to that in the neutral species, but not to a fully quinoidal extent. In particular, the bond-length alternations across the vinylene bridging groups approach zero. DFT calculations with closed-shell singlet configurations reproduce the observed structures well. Our results indicate that singlet species for which one might expect quinoidal geometries (with differences of ca. 0.1 A between formally single and double bonds) on the basis of a limiting valence-bond representation of the structure can, in fact, show structures with significantly different patterns of bond lengths.
authors.affiliation: School of Chemistry and Biochemistry and Center for Organic Photonics and Electronics, Georgia Institute of Technology, Atlanta, GA 30332-0400, USA.
authors.forename: Shijun
authors.initials: S
authors.lastname: Zheng
authors.forename: Stephen
authors.initials: S
authors.lastname: Barlow
authors.forename: Chad
authors.initials: C
authors.lastname: Risko
authors.forename: Tiffany L
authors.initials: TL
authors.lastname: Kinnibrugh
authors.forename: Viktor N
authors.initials: VN
authors.lastname: Khrustalev
authors.forename: Simon C
authors.initials: SC
authors.lastname: Jones
authors.forename: Mikhail Yu
authors.initials: MY
authors.lastname: Antipin
authors.forename: Neil M
authors.initials: NM
authors.lastname: Tucker
authors.forename: Tatiana V
authors.initials: TV
authors.lastname: Timofeeva
authors.forename: Veaceslav
authors.initials: V
authors.lastname: Coropceanu
authors.forename: Jean-Luc
authors.initials: JL
authors.lastname: Brédas
authors.forename: Seth R
authors.initials: SR
authors.lastname: Marder
mesh_terms: D000814:Aniline Compounds / Q000737:chemistry* / Q000302:isolation & purification; D002412:Cations; D018360:Crystallography, X-Ray; D004578:Electron Spin Resonance Spectroscopy; D009682:Magnetic Resonance Spectroscopy / Q000379:methods; D015394:Molecular Structure; D013267:Stilbenes / Q000737:chemistry
chemical_list: D000814:Aniline Compounds; D002412:Cations; D013267:Stilbenes