Shi Xueliang, Quintero Estefanía, Lee Sangsu, Jing Linzhi, Herng Tun Seng, Zheng Bin, Huang Kuo-Wei, López Navarrete Juan T, Ding Jun, Kim Dongho, Casado Juan, Chi Chunyan
Department of Chemistry , National University of Singapore , 3 Science Drive 3 , 117543 , Singapore . Email:
Department of Physical Chemistry , University of Malaga , Campus de Teatinos s/n , 229071 Malaga , Spain . Email:
Chem Sci. 2016 May 1;7(5):3036-3046. doi: 10.1039/c5sc04706d. Epub 2016 Jan 19.
Open-shell singlet diradicaloids have recently received much attention due to their unique optical, electronic and magnetic properties and promising applications in materials science. Among various diradicaloids, quinoidal π-conjugated molecules have become the prevailing design. However, the need for a fundamental understanding on how the fusion mode and pro-aromaticity/anti-aromaticity affect their diradical character and physical properties remains unaddressed. In this work, a series of pro-aromatic benzo-thia-fused []thienoacenequinodimethanes ( ( = 1-3) and ) were synthesized and compared with the previously reported anti-aromatic bisindeno-[]thienoacenes (, = 1-4). The ground-state geometric and electronic structures of these new quinoidal molecules were systematically investigated by X-ray crystallographic analysis, variable temperature NMR, ESR, SQUID, Raman, and electronic absorption spectroscopy, assisted by DFT calculations. It was found that the diradical character index () increased from nearly zero for to 2.4% for , 18.2% for , and 38.2% for , due to the enhanced aromatic stabilization. Consequently, with the extension of molecular size, the one-photon absorption spectra are gradually red-shifted, the two-photon absorption (TPA) cross section values increase, and the singlet excited state lifetimes decrease. By comparison with the corresponding anti-aromatic analogues ( = 1-3), the pro-aromatic ( = 1-3) exhibit larger diradical character, longer singlet excited state lifetime and larger TPA cross section value. At the same time, they display distinctively different electronic absorption spectra and improved electrochemical amphotericity. Spectroelectrochemical studies revealed a good linear relationship between the optical energy gaps and the molecular length in the neutral, radical cationic and dicationic forms. Our research work discloses a significant difference between the pro-aromatic and anti-aromatic quinoidal compounds and provides guidance for the design of new diradicaloids with desirable properties.
开壳单重态双自由基类化合物因其独特的光学、电子和磁性性质以及在材料科学中的潜在应用，近年来受到了广泛关注。在各种双自由基类化合物中，醌型π共轭分子已成为主流设计。然而，对于融合模式和亲芳香性/反芳香性如何影响其双自由基特性和物理性质的基本理解仍未得到解决。在这项工作中，合成了一系列亲芳香性的苯并噻吩稠合的[ ]噻吩并苊醌二甲烷（（ = 1 - 3）和 ），并与先前报道的反芳香性双茚并[ ]噻吩并苊（， = 1 - 4）进行了比较。通过X射线晶体学分析、变温核磁共振、电子顺磁共振、超导量子干涉仪、拉曼光谱和电子吸收光谱，并辅以密度泛函理论计算，系统地研究了这些新型醌型分子的基态几何和电子结构。研究发现，由于芳香稳定化增强，双自由基特性指数（）从 的接近零增加到 的2.4%、 的18.2%和 的38.2%。因此，随着分子尺寸的增加，单光子吸收光谱逐渐红移，双光子吸收（TPA）截面值增加，单重激发态寿命缩短。与相应的反芳香性类似物 （ = 1 - 3）相比，亲芳香性的 （ = 1 - 3）表现出更大的双自由基特性、更长的单重激发态寿命和更大的TPA截面值。同时，它们表现出明显不同的电子吸收光谱和改善的电化学两性。光谱电化学研究表明，在中性、自由基阳离子和二价阳离子形式下，光学能隙与分子长度之间存在良好的线性关系。我们的研究工作揭示了亲芳香性和反芳香性醌型化合物之间的显著差异，并为设计具有理想性质的新型双自由基类化合物提供了指导。
