Kochi JK, Rathore R, Magueres PL
Department of Chemistry, University of Houston, Houston, Texas 77204-5641, USA.
J Org Chem. 2000 Oct 20;65(21):6826-36. doi: 10.1021/jo000570h.
The spontaneous assembly of aromatic cation-radicals (D(+)()()) with the parent donor (D) to afford the paramagnetic dimer (D)(2)(+)()() is accompanied by a dramatic color change. For example, spectral (UV-vis and ESR) and X-ray crystal structure analyses establish the molecular association of octamethylbiphenylene cation-radical with its neutral counterpart to produce the mixed-valence or dimeric cation-radical in which the positive charge is completely delocalized over both aromatic moieties. The use of the sterically hindered cation-radicals confirms the new spectral or charge-resonance (CR) band to result in dimeric cation-radicals in which the intermolecular separation occurs at an optimum distance allowed by van der Waals contacts. The striking similarities between the classical donor/acceptor (EDA) complexes and the dimeric cation-radicals (D)(2)(+)()() (both in terms of the geometrical requirement as well as the appearance of new absorption bands) suggest that the latter can be considered as particular examples of Mulliken's charge-transfer complexes in which the positive charge is completely (equally) delocalized over both donor (D) and acceptor (D(+)()()).
芳香族阳离子自由基（D(+)()()）与母体供体（D）自发组装形成顺磁性二聚体（D）2(+)()()，同时伴随着显著的颜色变化。例如，光谱（紫外可见光谱和电子顺磁共振光谱）和X射线晶体结构分析证实了八甲基联苯阳离子自由基与其中性对应物的分子缔合，生成了混合价或二聚阳离子自由基，其中正电荷完全离域在两个芳香部分上。使用空间位阻阳离子自由基证实了新的光谱或电荷共振（CR）带，从而形成二聚阳离子自由基，其中分子间距离在范德华接触允许的最佳距离处。经典供体/受体（EDA）配合物与二聚阳离子自由基（D）2(+)()()之间的显著相似性（在几何要求以及新吸收带的出现方面）表明，后者可被视为穆利肯电荷转移配合物的特殊例子，其中正电荷完全（均匀）离域在供体（D）和受体（D(+)()()）上。
