Probing Atomistic Behavior To Unravel Dielectric Phenomena in Charge Transfer Cocrystals

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.