Isolation and crystal structures of two singlet bis(triarylamine) dications with nonquinoidal geometries.
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.
Zheng
Shijun
S
School of Chemistry and Biochemistry and Center for Organic Photonics and Electronics, Georgia Institute of Technology, Atlanta, GA 30332-0400, USA.
Barlow
Stephen
S
Risko
Chad
C
Kinnibrugh
Tiffany L
TL
Khrustalev
Viktor N
VN
Jones
Simon C
SC
Antipin
Mikhail Yu
MY
Tucker
Neil M
NM
Timofeeva
Tatiana V
TV
Coropceanu
Veaceslav
V
Brédas
Jean-Luc
JL
Marder
Seth R
SR
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
D000814:Aniline Compounds; D002412:Cations; D013267:Stilbenes