Charge-Transport Properties of F₆TNAP-Based Charge-Transfer Cocrystals

The crystal structures of the charge-transfer (CT) cocrystals formed by the π-electron acceptor 1,3,4,5,7,8-hexafluoro-11,11,12,12-tetracyanonaphtho-2,6-quinodimethane (F₆TNAP) with the planar π-electron-donor molecules triphenylene (TP), benzo[b]benzo[4,5]thieno[2,3-d]thiophene (BTBT), benzo[1,2-b:4,5-b′]dithiophene (BDT), pyrene (PY), anthracene (ANT), and carbazole (CBZ) have been determined using single-crystal X-ray diffraction (SCXRD), along with those of two polymorphs of F₆TNAP. All six cocrystals exhibit 1:1 donor/acceptor stoichiometry and adopt mixed-stacking motifs. Cocrystals based on BTBT and CBZ π-electron donor molecules exhibit brickwork packing, while the other four CT cocrystals show herringbone-type crystal packing. Infrared spectroscopy, molecular geometries determined by SCXRD, and electronic structure calculations indicate that the extent of ground-state CT in each cocrystal is small. Density functional theory calculations predict large conduction bandwidths and, consequently, low effective masses for electrons for all six CT cocrystals, while the TP-, BDT-, and PY-based cocrystals are also predicted to have large valence bandwidths and low effective masses for holes. Charge-carrier mobility values are obtained from space-charge limited current (SCLC) measurements and field-effect transistor measurements, with values exceeding 1 cm² V⁻¹ s¹ being estimated from SCLC measurements for BTBT:F₆TNAP and CBZ:F₆TNAP cocrystals.