ABSTRACT
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 13C 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.
