Structural and spectral transformation of cationic porphyrin TMPyP4 at adsorption on graphene

Hybrids of graphene nanomaterials with porphyrins possess distinct photo and electronic properties that enable a wide spectrum of applications involving interaction with light. In this work we analyze structural and spectral transformations of cationic meso-5,10,15,20-tetra-(N-methyl-4-pyridyl) porphyrin (ТМРyР4) at adsorption on graphene. It is shown that the binding of TMPyP4 with graphene is stronger as compared to the neutral porphyrin (TPP) with a similar structure (-111.0 vs -22.6 kcal/mol). This is due to a large contribution from the cation-π-electrons interaction. The interaction of TMPyP4 with graphene is accompanied with a distortion of the flat structure of the porphyrin core and twisting of the side rings. As a result, the porphyrin molecule flattens. The cationic porphyrin adsorbed to graphene induces a stronger molecule flattening in comparison with the neutral porphyrin. A comparison of the experimental and calculated (DFT) UV spectra allows estimating the contribution of the π-π stacking and the twisting of the side rings to the observed red-shifting. The TMPyP4 molecule creates an area of negative potential in graphene close to the porphyrin adsorption site. The adsorption of TMPyP4 on graphene is also investigated with DFT calculations and with molecular dynamics simulations in water environment and in vacuum.