Syntheses, Electronic Structures, and Intermolecular Interactions of Singlet Biradical Molecules

一些凯库勒化合物如醌式烃具有相对小的HOMO-LUMO间隙，并显示出极高的化学反应性。代表性的是对醌二甲烷，其已被定义为双自由基化合物，其可被归类为离域单线态双自由基。为了分离这种反应性闭壳双自由基并研究与双自由基特征相关的性质，我们将自旋离域部分引入对醌二甲烷和2，6-萘醌二甲烷。新设计的分子通过多步合成制备，并且被发现即使在室温下在空气中也是稳定的。经CASSCF（2，2）/6- 31 G计算，化合物的双自由基性质分别为30%和50%。X射线晶体学分析揭示了具有短π-π接触（3.14-3.16 π）的一维π堆叠链。化合物的压缩颗粒表现出半导电性（σ_<RT>= 10 ~ 10^<-5>S cm^<-1>1，E_a = 0.2-0.3 eV），并且根据EHMO能带计算，沿π-π堆积获得了沿着大的带宽。单晶的偏振反射光谱在π-π堆积方向的近红外区显示出强的谱带。固态吸收带的能量比溶液吸收带的能量低得多。强的分子间自旋-自旋相互作用以及分子内自旋相互作用是导致吸收带大带宽和低能位移的原因。

Some Kekule compounds such as quinoid hydrocarbons have relatively small HOMO-LUMO gaps and show extremely high chemical reactivity. The representative is p-quinodimethane that has been defined as a biradicaloid compound, which could be categorized as a delocalized singlet biradical. In order to isolate such a reactive closed-shell biradical and to investigate properties associated with the biradical character, we introduced spin-delocalization moieties to p-quinodimethane and 2,6-naphthoquinodimethane. Newly designed molecules were prepared by multistep syntheses, and were found to be stable even in air at room temperature. Biradical character of the compounds was estimated to be 30% and 50% by a CASSCF(2,2)/6-31G calculation, respectively. An X-ray crystallographic analysis revealed one-dimensional π-stacking chains with short π-π contacts (3.14-3.16 Å). Compressed pellets of the compounds exhibited semi-conductive behavior (σ_<RT>=〜10^<-5> S cm^<-1>1,E_a = 0.2-0.3 eV), and large bandwidths were achieved along the π-π stacking according to an EHMO band calculation. Polarized reflection spectra of the single crystals showed intense bands at near-IR regions in the π-π stacking directions. The solid state bands are extraordinary lower in energy than the solution absorption bands. A strong intermolecular spin-spin interaction as well as an intramolecular one would be responsible for the large bandwidths and the lower-energy shifts of the absorption bands.