Conformation Diversity of a Fused-Ring Pyrazine Derivative on Au(111) and Highly Ordered Pyrolytic Graphite

Heterocyclic aromatic compounds have attracted considerable attention because of their high carrier mobility that can be exploited in organic field-effect transistors. This contribution presents a comparative study of the packing structure of 3,6-didodecyl-12-(3,6-didodecylphenanthro[9,10-b]phenazin-13-yl)phenanthro[9,10-b]phenazine (DP), an N-heterocyclic aromatic compound, on Au(111) and highly ordered pyrolytic graphite (HOPG). High-resolution scanning tunneling microscopy (STM) combined with atomistic simulations provide a picture of the interface of this organic semiconductor on an electrode that can have an impact on the field-effect transistor (FET) performance. DP molecules adsorb with different conformational isomers (R/S: trans isomers; C: cis isomer) on HOPG and Au(111) substrates. All three isomers are found in the long-range disordered lamella domains on Au(111). In contrast, only the R/S trans isomers self-assemble into stable chiral domains on the HOPG surface. The substrate-dependent adsorption configuration selectivity is supported by theoretical calculations. The van der Waals interaction between the molecules and the substrate dominates the adsorption binding energy of the DP molecules on the solid surface. The results provide molecular evidence of the interface structures of organic semiconductors on electrode surfaces.