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

一些凯库勒化合物，如醌类烃，具有相对较小的HOMO-LUMO间隙，并表现出极高的化学反应活性。具有代表性的是对喹诺二甲烷，它被定义为双基化合物，可归类为离域单线态双基。为了分离出这种反应性的闭壳双基并研究与双基性质相关的性质，我们引入了对醌二甲烷和2,6-萘醌二甲烷的自旋离域基团。新设计的分子通过多步合成制备，并且在室温下即使在空气中也是稳定的。通过CASSCF(2,2)/6-31G计算，估计化合物的双基性分别为30%和50%。x射线晶体学分析显示π-π接触短的一维π堆积链（3.14-3.16 Å）。压缩颗粒表现出半导电行为（σ_<RT>= ~ 10^<-5> S cm^<-1>1,E_a = 0.2 ~ 0.3 eV），并且根据EHMO波段计算，化合物沿π-π堆积获得了较大的带宽。单晶的偏振反射光谱在π-π叠加方向的近红外区显示出强波段。固态能带的能量比溶液吸收能带低得多。强烈的分子间自旋-自旋相互作用以及分子内相互作用将导致吸收带的大带宽和低能量位移。

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.