Graphene induced molecular flattening of meso-5,10,15,20-tetraphenyl porphyrin: DFT calculations and molecular dynamics simulations

Noncovalent functionalization of graphene with porhyrins that possess a large conjugated pi-system of bonds can extend the range of practical applications of this nanomaterial. In this work the structures of hybrids formed by porphin and meso-5,10,15,20-tetraphenyl porphyrin (TPP) with graphene and the corresponding intermolecular interaction energies are studied with the DFT and molecular dynamics (MD) calculations. The influence of the peripheral rings arranged around the porphyrin core in the TPP structure on its interaction energy with graphene upon the TPP adsorption on graphene is analyzed. The flattening of the TPP structure occurring due to the adsorption is observed. The interaction of TPP with graphene also leads to distortion of the flat structure of the porphyrin core and to some twisting of its side rings, so the dihedral angle between the planes of the porphyrin core and the side residues decreases from 64 degrees to 50 degrees. In spite of the flattening and the larger size of the TPP structure compared to the porphin structure their interaction energies with graphene are similar (about 20 kcal/mol). It is also shown that TPP alters the uniform distribution of the electrostatic potential on graphene by reducing the zone of small negative potential of graphene located under the porphyrin molecule. The MD simulation performed in this work describes the process of the adsorption of porphin and TPP on graphene in water environment and reveals significant mobility of this molecule on the graphene surface.