Dipole-Dipole Interactions in TiOPc Ad layers on Ag

The molecular organization of a polar phthalocyanine, titanyl phthalocyanine (TiOPc), on the Ag(110) surface was studied with scanning tunneling microscopy (STM). in addition to the intermolecular hydrogen bonding and electrostatic interactions that guide structure for nonpolar Pc's, we find evidence for the impact of dipole interactions on layer organization for TiOPc/Ag(110). TiOPc molecules adsorb on Ag(110) with a tilted orientation that contrasts with the flat-lying orientation typically observed for nonpolar phthalocyanines. At the highest TiOPc coverages, the layer consists of neighboring TiOPc molecules with opposing molecular tilts. Close inspection reveals a higher level organization, consisting of "zigzag" TiOPc molecular chains with molecular spacings along the chain that are smaller than the average within the layer. We suggest an explanation for this behavior based on the dipole-dipole interactions within the chain. At reduced coverage, variable densities of molecular vacancies form, depending on the coverage. The vacancies align to form rows that order into well-organized superstructures. Finally, TiOPc surface structures on Ag(110) are compared and contrasted with those on Ag(111). The different adlayer arrangements on these two surfaces created by molecule-molecule interactions and molecule-substrate interactions lead to a change from a "ferroelectric" orientation for the TiOPc molecular tilts (and in-plane dipole components) on Ag(111) to an "antiferroelectric" arrangement on Ag(110). We show that this change agrees with theoretical predictions of the influence of lattice symmetry on dipole lattice arrangements, providing additional evidence for the impact of dipole-dipole interactions on the TiOPc surface layer organization.