Substituent Controllable Assembly of Anthracene Donors and TCNQ Acceptors in Charge Transfer Cocrystals

A series of charge transfer cocrystals composed of methyl substituted anthracenes (MeAnt or Me2Ant) and FxTCNQ (x = 0, 1, 2) were obtained. All combinations of the donor and acceptor coformers produced 1:1 cocrystals with alternating mixed stacks. The key factors contributing to crystal packing are pi-pi, F center dot center dot center dot H, N center dot center dot center dot H, and F center dot center dot center dot F interactions and the steric effect of the methyl substituents. The introduction of methyl groups into the donor molecules in the series of complexes with F2TCNQ promotes the skewed overlap mode with a large donor-acceptor shift, dipole-dipole interactions between cyano groups, and the three-dimensional (3D) network of intermolecular interactions. Such supramolecular arrangement results in the delocalized electronic structure with a high dispersion of the density of states. The evaluated charge transfer is about 0.16-0.17 e; the donor-acceptor distance within the stacks is 3.30 angstrom, which is smaller than that in cocrystals of F4TCNQ with anthracene and even tetracene. Solid-state electrochemistry and UV/vis/NIR spectroscopy imply the energy gap for this series of cocrystals of about 0.8-1.1 eV; thus, they may be considered small-gap semiconductors. Our work shows how minor changes in donor or acceptor constituents of cocrystals can result in drastic changes of the packing arrangement and the properties of cocrystals with enhanced donor-acceptor interactions.