Department of Chemistry, Indian Institute of Science Education and Research Bhopal, Bhopal Bypass Road, Bhauri, Bhopal 462066, Madhya Pradesh, India.
Materials Science Division, Council of Scientific and Industrial Research, National Aerospace Laboratories, Kodihalli, Bengaluru 560017, Karnataka, India.
J Am Chem Soc. 2021 Jan 20;143(2):1024-1037. doi: 10.1021/jacs.0c11459. Epub 2020 Dec 30.
Six new binary charge-transfer (CT) cocrystals have been synthesized by solvent drop-assisted mechanochemical grinding method, and all of them exhibited remarkable color changes during the grinding process. Crystal structure analysis reveals the donor (D) and acceptor (A) molecules have assembled primarily via cofacial π···π stacking interactions displaying mixed D-A-D-A stacked columns. Interestingly these cocrystals exhibited very diverse dielectric response in the presence of an alternating current (ac) external electric field, and their dielectric behavior can be explained from the nature and strength of CT interactions in the cocrystal assembly. Strong CT cocrystals were found to display a rigid supramolecular framework while weakly bound CT complexes allowed its constituent polar molecules to relax and hence the observed rotational dynamics contributed to their dielectric properties. Chemical shift anisotropy parameters, spin-lattice relaxation, and molecular correlation times obtained from C solid-state NMR spectroscopy measurements establish the occurrence of molecular dynamics at the atomistic scale in cocrystals, thereby displaying high permittivity. Furthermore, we also propose a strategy directed toward the design of CT cocrystals that allows us to introduce rotational dynamics in noncentrosymmetric molecules, which significantly enhances their dielectric properties due to orientation polarization. The results indicate that D-A-based organic CT systems, particularly with a mixed stack, have a wide range of potential applications in materials science.
六种新的双电荷转移（CT）共晶通过溶剂滴辅助机械化学研磨方法合成，它们在研磨过程中都表现出显著的颜色变化。晶体结构分析表明，给体（D）和受体（A）分子主要通过面对面的π···π堆积相互作用组装，呈现混合的 D-A-D-A 堆积柱。有趣的是，这些共晶在交流（ac）外电场存在下表现出非常多样化的介电响应，其介电行为可以从共晶组装中 CT 相互作用的性质和强度来解释。强 CT 共晶被发现显示出刚性的超分子框架，而弱结合的 CT 配合物允许其组成的极性分子松弛，因此观察到的旋转动力学对其介电性质有贡献。从 C 固态 NMR 光谱测量获得的化学位移各向异性参数、自旋晶格弛豫和分子相关时间确立了共晶中原子尺度上分子动力学的发生，从而显示出高介电常数。此外，我们还提出了一种设计 CT 共晶的策略，使我们能够在非中心对称分子中引入旋转动力学，由于取向极化，显著提高了它们的介电性能。结果表明，基于 D-A 的有机 CT 体系，特别是具有混合堆积的体系，在材料科学中有广泛的潜在应用。
