Hydrogen-Bonded One-Dimensional Chains of Quinacridone on Ag(100) and Cu(111): The Role of Chirality and Surface Bonding

The adsorption and ordering of the prochiral molecule quinacridone (QA) on the Ag(100) and Cu(111) surfaces were studied by low-energy electron diffraction and scanning tunneling microscopy. Upon adsorption, the molecules form parallel homochiral chains of flat-lying molecules linked together via hydrogen bonds on both surfaces, but these chains show significant surface-dependent differences concerning their lateral order. On both substrates, the chains are not thermodynamically stable but only metastable and stabilized by kinetic barriers. On the Ag(100) surface, annealing induces a phase transition to a highly ordered and heterochiral structure with a reduced density of hydrogen bonds. The related loss of bonding energy is overcompensated by a stronger bonding to the substrate, yielding a commensurate structure. For QA on Ag(100), we propose that during the initial chain formation and the phase transition upon annealing, the molecules can change their handedness by rotating around their long axes. In contrast, the initial chain formation and the phase transitions of QA on the Cu(111) surface appear to be subject to stronger kinetic limitations. These are explained by stronger substrate molecule interactions on Cu(111), which reduce the diffusion and the possibility for a change of handedness in comparison to QA on Ag(100). We discuss how the intermolecular hydrogen bonds, the 2D chirality, and the different chemical reactivities of the two surfaces [Ag(100) and Cu(111)] influence the structural formation of QA aggregates. We compare our results to the results for QA on Ag(111) reported previously by Wagner et al. [JPCC 2014, 118, 10911-10920].