Dipole-Dipole Interactions in TiOPc Adlayers on Ag
Steven W. Robey, Xianjie Liu, Yinying Wei, Janice E. Reutt-Robey
The molecular organization in layers of a polar phthalocyanine, titanyl phthalocyanine (TiOPc), on the Ag(110) surface was studied with scanning tunneling microscopy (STM). TiOPc molecules adsorb on Ag(110) in a tilted configuration in contrast to the flat-lying orientation typically observed non-polar phthalocyanines. Common to each surface structure are "zigzag" TiOPc molecular chain motifs formed from neighboring TiOPc molecules with opposing molecular tilts. This arrangement reduces repulsion between the intrinsic TiOPc molecular dipoles, as well potentially enabling enhanced attractive interactions between aromatic rings. For the highest TiOPc coverage, the zigzag molecular chains nest to create an adlayer with a packing density of 1 TiOPc molecule/13 Ag atoms with two reflection domains through the substrate  direction. At reduced molecular coverage, TiOPc adlayers with a variable density of molecular vacancies are produced. The point vacancies spontaneously align into vacancy chains parallel to the TiOPc zigzag molecular chains. Multiple vacancy chains adopt regular spacings to generate local organization where the vacancy rows are separated by n zigzag chains, with n = 1 and n = 2 most abundant. Registration shifts between adjacent TiOPc chains in these structures also reduce dipole repulsion between neighboring TiOPc chains. We compare and contrast the TiOPc structures observed on Ag(110) with those observed previously for TiOPc on Ag(111).
Journal of Physical Chemistry C
phthalocyanine, film structure, organic interface, dipole interaction, scanning tunneling microscopy