Fine-Tuning the Motion of Tetrafluoro-1,4-Benzoquinone within Charge-Transfer Cocrystals Through Variation of the Donor: An Entropic and Enthalpic Point of View.

In this work, we report a detailed investigation on the factors affecting the in-plane motion of tetrafluoro-1,4-benzoquinone (TFBQ) within a series of charge-transfer (CT) cocrystals formed with various fused aromatic donors. Six crystalline materials were obtained, including four cocrystals, a polymorph, and a solid solution. These were characterized by single-crystal X-ray diffraction (SC XRD), solid-state NMR, FTIR, UV-Vis spectroscopy, and DFT calculations. All cocrystals exhibited short π-stacking interactions and varied noncovalent interactions (NCI) modulating the in-plane motion of TFBQ. 19F T₁ relaxation measurements allowed the quantification of the activation enthalpies (ΔH‡) and entropies (ΔS‡), with theoretical calculations backing these results. Cocrystal 4 is the solid with the most restricted motions due to the presence of strong hydrogen bonds. In contrast, cocrystal 3a depicts the highest in-plane motional frequencies (up to 2.2 MHz at 300 K), owing to the weak contacts around TFBQ. Surprisingly, a solid solution (5), which resembled a mixture of cocrystals 2 and 3a, displayed no significant in-plane motions, despite having similar packing and relaxation behavior to the binary cocrystals. This work illustrates how subtle variations in donor structure affect the in-plane motions and photophysical properties of CT cocrystals, providing valuable insight into the rational design of functional materials.