Role of a carboxyl group in the adsorption of Zn porphyrins on $TiO_{2}$(011)-2 $\times$ 1 surface

Adsorption of carboxylic-substituted Zn porphyrin molecules (COOH-ZnTPP) on the (011) surface of rutile $TiO_{2}$ 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 1D 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