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

一些凯库勒化合物，如喹烷烃，具有相对较小的HOMO-LUMO能隙，并显示出极高的化学反应活性。最具代表性的是对奎诺二甲烷，它被定义为双自由基化合物，可以归类为离域的单重态双自由基。为了分离这种反应性的闭壳双自由基，并研究与双自由基性质相关的性质，我们引入了自旋离域基团给对-奎诺二甲烷和2，6-萘奎诺二甲烷。新设计的分子是通过多步合成制备的，发现即使在室温下的空气中也是稳定的。用CASSCF(2，2)/6-31G计算得到化合物的双自由基性质分别为30%和50%。X-射线单晶分析揭示了一维π-堆积链，具有短的π-π接触(3.14-3.16Å)。化合物的压片表现出半导体行为(σ_&lt；RT&gt；=~10^&lt；-5&gt；S cm&lt；-1&gt；1，E_a=0.2-0.3 eV)，并获得了沿π-π堆积的大带宽。单晶的偏振反射光谱在π-π堆积方向的近红外区域有很强的谱带。固态吸收带的能量比溶液吸收带低得多。分子间的自旋-自旋相互作用以及分子内的自旋相互作用将导致吸收带的较大带宽和较低的能量移动。

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.