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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 13 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.

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The chemical shift anisotropy tensor and site specific spin-lattice relaxation time of folic acid were determined by a ¹³C 2DPASS CP-MAS NMR experiment and Torchia CP experiment respectively.

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Adefovir is regarded as a potential antiviral agent. However, it cannot be considered as a valuable drug candidate due to its high polarity that limits its permeability across the human intestinal mucosa. When the ribose phosphate group of adefovir is replaced by the isopolar phosphonomethyl ether functionality, it neutralizes the negative charge of the drug. This makes the drug lipid-soluble and potent to diffuse across the cell membrane. The prodrug adefovir dipivoxil is regarded as a potent antiviral drug against hepatitis B virus (HBV), human immunodeficiency virus (HIV), Rauscher murine leukemia virus (R-MuLV), murine cytomegalovirus (MCMV), herpes simplex virus (HSV), simian immunodeficiency virus (SIV), and feline immunodeficiency virus (FIV). The correlation between the structure and the dynamics of adefovir dipivoxil is determined by measuring the principal components of chemical shift anisotropy (CSA) tensor, site-specific spin–lattice relaxation time, and molecular correlation time at crystallographically different carbon nuclei sites. The CSA parameters, spin–lattice relaxation time, and molecular correlation time of phosphorous nucleus of the organophosphate group of adefovir dipivoxil molecule are also determined. The spin–lattice relaxation time of carbon nuclei varies from 1 to 107 s. The range of molecular correlation time also varies from 10–4 to 10–8 s. These remarkable diversities of motional dynamics of the molecules imply that there exist various motional degrees of freedom within this valuable drug and these motional degrees of freedom are independent of each other, which may be the reason for the biological activities exhibited by the drug. The correlation between structure and dynamics of such an important antiviral drug adefovir dipivoxil can be visualized by these types of extensive spectroscopic measurements, which will enlighten the path of inventing advanced medicine in the pharmaceutical industry, and it will also illuminate the understanding of the structure–activity relationships of antiviral drug.

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