| Abstract
| - We report the first structural data for bis(diarylamine) “bipolarons”: we have isolated andcrystallographically 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-raystructures, together with 1H NMR coupling constants for [2]2+, indicate geometries in which the bond lengthsare shifted toward a quinoidal pattern relative to that in the neutral species, but not to a fully quinoidalextent. In particular, the bond-length alternations across the vinylene bridging groups approach zero. DFTcalculations with closed-shell singlet configurations reproduce the observed structures well. Our resultsindicate that singlet species for which one might expect quinoidal geometries (with differences of ca. 0.1Å between formally single and double bonds) on the basis of a limiting valence-bond representation of thestructure can, in fact, show structures with significantly different patterns of bond lengths.
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