Self-assembling of Zn porphyrins on a (110) face of rutile TiO2–The anchoring role of carboxyl groups

•COOH-ZnTPP molecules adsorb in a planar configuration and could be divided into two groups: immobilized and mobile.•The lack of any stable not substituted molecules (ZnTPP) indicates that the COOH group plays a crucial role in the single molecule immobilization.•At a monolayer coverage both species form stable islands with planar oriented molecules and rhomboid unit cells characterized by slightly different dimensions.•The STM investigations show that the presence of an anchoring group has an decisive impact on the molecule-substrate binding and the formation of monolayer assemblies.