Role of a Carboxyl Group in the Adsorption of Zn Porphyrins on TiO2(011)-2x1 Surface

Adsorption of carboxylic-substituted Zn porphyrin molecules (COOH-ZnTPP) on the (011) surface of rutile TiO2 is examined by scanning tunneling microscopy at room temperature. The molecules are either flat, stabilized by the surface defects, or mobile with their movement limited to a reconstruction row of the substrate. Upon increasing the molecular coverage the molecules form ID structures along the [1-10] direction, in which molecular boards are no longer parallel to the surface. This behavior is contrasted with the structures formed by Zn-TPP lacking the carboxylic group, which remain flat up to a monolayer coverage.