Molecular Motion, Dielectric Response, and Phase Transition of Charge-Transfer Crystals: Acquired Dynamic and Dielectric Properties of Polar Molecules in Crystals

Molecules in crystals often suffer from severe limitations on their dynamic processes, especially on those involving large structural changes. Crystalline compounds, therefore, usually fail to realize their potential as dielectric materials even when they have large dipole moments. To enable polar molecules to undergo dynamic processes and to provide their crystals with dielectric properties, weakly bound charge-transfer (CT) complex crystals have been exploited as a molecular architecture where the constituent polar molecules have some freedom of dynamic processes, which contribute to the dielectric properties of the crystals. Several CT crystals of polar tetrabromophthalic anhydride (TBPA) molecules were prepared using TBPA as an electron acceptor and aromatic hydrocarbons, such as coronene and perylene, as electron donors. The crystal structures and dielectric properties of the CT crystals as well as the single-component crystal of TBPA were investigated at various temperatures. Molecular reorientation of TBPA molecules did not occur in the single-component crystal, and the crystal did not show a dielectric response due to orientational polarization. We have found that the CT crystal formation provides a simple and versatile method to develop molecular dielectrics, revealing that the molecular dynamics of the TBPA molecules and the dielectric property of their crystals were greatly changed in CT crystals. The TBPA molecules underwent rapid in-plane reorientations in their CT crystals, which exhibited marked dielectric responses arising from the molecular motion. An order–disorder phase transition was observed for one of the CT crystals, which resulted in an abrupt change in the dielectric constant at the transition temperature.晶体中的分子往往在动态过程中受到严重限制，尤其是涉及大规模结构变化的动态过程。因此，即便结晶化合物拥有较大偶极矩，通常也难以充分发挥其作为介电材料的潜力。为使极性分子能够发生动态过程，并赋予其晶体介电特性，研究人员开发了弱结合型电荷转移（CT）配合物晶体作为分子架构，其中构成极性分子具备一定的动态过程自由度，该自由度可贡献于晶体的介电性能。研究人员以四溴邻苯二甲酸酐（TBPA）作为电子受体，以苝、蔻等芳香烃作为电子给体，制备了数种含极性TBPA分子的CT晶体。针对上述CT晶体以及TBPA单组分晶体，研究人员在不同温度下对其晶体结构与介电性能展开了系统研究。TBPA分子在单组分晶体中未发生分子重取向，该晶体也未表现出源于取向极化的介电响应。本研究发现，CT晶体的构建为开发分子型介电材料提供了一种简便且通用的方法，结果表明，在CT晶体中，TBPA分子的动态行为及其晶体的介电性能均发生了显著改变。TBPA分子在其CT晶体中发生了快速的面内重取向，该晶体由此表现出由分子运动引发的显著介电响应。其中一种CT晶体出现了有序-无序相变，该相变在转变温度处导致介电常数发生突变。