An enhanced electronic state in ZnTPP induced by interaction with solvation: A multiple-scattering calculation of x-ray absorption near-edge spectra

Metalloporphyrins (MP) are important for a wide variety of biological processes and photosynthesis and have been intensively investigated by X-ray absorption spectroscopy. The typical metal K-edge XANES spectra of the square-planar MTPP show a very sharp peak on the rising edge of the main absorption peak, which was assigned to 1s -> 4p(z) electronic transition. We carried out a series of multiple-scattering calculations of porphyrin molecules and found that this peak can be greatly enhanced due to the pi-pi stacking of phenyl rings below and above the porphyrin plane. Density of state calculations confirmed that the emergence of this electronic state is due to the interaction of the 4pz orbital of the metal with pi orbitals of an aromatic solvent molecule with its plane parallel with the macrocycle. This XANES feature of metalloporphyrins can be used to investigate the solvent-solute interactions with presumed non-ligating solvent.