Lateral Interactions and Order-Disorder Phase Transitions of Metal Phthalocyanines on Ag(111)

Y The intermolecular interaction among phthalocyanine molecules deposited on Ag(111) has been investigated at submonolayer coverages. By means of infrared absorption spectroscopy and spot-profile analysis low-energy electron diffraction, unambiguous evidence for an attractive interaction between neighboring titanyl-phthalocyanine (TiOPc) molecules is found for dilute layers, which is in contrast to the intermolecular repulsion reported for the copper-phthalocyanine (CuPc)/Ag(111) system. Accordingly, the formation of dense two-dimensional islands of TiOPc molecules with a well-defined long-range order (commensurate phase) is observed upon cooling to temperatures below 75 K, while a disordered arrangement is retained for adsorbed CuPc. Using density functional theory (DFT) calculations as a guide, these differences have been traced to the competition of attractive van der Waals and repulsive electrostatic interactions. Specifically, the vertically oriented molecular dipoles are substantially smaller for adsorbed TiOPc as compared to CuPc due to a compensation of the oppositely oriented dipole moments arising from the Pauli push-back effect on the one hand and the internal molecular dipole associated with the axial Ti-O group on the other. A detailed account and comparison of the molecule-substrate interaction, molecular binding geometries, and intermolecular interactions of TiOPc and CuPc on the Ag(111) surface are provided in a theoretical analysis based on DFT.