Adsorption of meso-tetraphenylporphines on thin films of C60 fullerene

Noncovalent porphyrin–fullerene hybrids represent promising materials for organic photovoltaic applications and the construction of novel platforms for the immobilization of diverse molecules. In such hybrids, a structural aspect of particular interest is self-assembled monolayers (SAMs) of porphyrins on fullerene films supported on solid substrates. In the present work, we performed combined theoretical-experimental analysis of the behavior of meso-tetraphenylporphine (or 5,10,15,20-tetraphenyl-21H,23H-porphine, H2TPP) and its complexes with Ni(II) and Co(II) (NiTPP and CoTPP, respectively) when adsorbed from gas phase in vacuum (by means of physical vapor deposition, PVD) onto HOPG-supported fullerene C60 films. The general film morphology was characterized by atomic force microscopy, whereas more detailed molecular structure as well as the adsorption of porphyrins was studied by scanning tunneling microscopy (STM). The results of STM imaging were compared with theoretical models, consisting of graphene sheets covered with a dense monolayer of fullerene cages and accommodating H2TPP molecules in different ways; their geometry was optimized by using MM+ force field. The results showed that NiTPP and CoTPP molecules are able to form ordered surface structures on the C60 thin films. The geometries observed included such basic structural elements as equilateral triangles or parallel rows formed by porphyrin molecules. Nevertheless, the areas of the assemblies on C60 turned out to be very limited, owing to the fact that the C60 thin films are composed of overlapping 20–70 nm-sized clusters, whereas each individual cluster exhibits additional imperfections; altogether resulting in a highly irregular surface, inconvenient for the formation of extended porphyrin arrays.