Donor–Acceptor Interactions of C ₆₀ F ₁₈ with Polycyclic Aromatic Hydrocarbons: Size Effects in Bulk Crystallization and Surface Constraints

An in-depth study of donor-acceptor (D/A) interactions between the high-dipole acceptor C₆₀F₁₈ (A) and polycyclic aromatic hydrocarbon (PAH) donors-pyrene, perylene, and coronene-reveals a surprisingly strong PAH size influence on the D/A complex stoichiometry and ordering in co-crystals. The crystallographic study shows the tendency of D/A mixtures to form stacked layered structures for the larger PAHs, perylene and coronene, while the role of aromatic π-π interactions diminishes, in contrast to the smaller pyrene/C₆₀F₁₈ system. The behavior of the layered-D/A assemblies is investigated by utilizing sequential deposition and co-evaporation of C₆₀F₁₈ and coronene on Au(111) surfaces. Scanning tunneling microscopy shows that the flat lying configuration adopted by coronene on the metal, which forms highly ordered close-packed monolayers stabilized by the interaction between their π electrons and the high density of gold surface states, hinders the formation of the ordered assemblies of the corresponding co-crystal. The influence of the substrate plus the critical role played by electronic and steric effects in the co-crystal formation are believed to cause the lack of viability. However, it is remarkable that, on the surface, adsorbed single C₆₀F₁₈ molecules are well centered on top of one coronene molecule, facilitating charge transfer between D and A molecules.