Electronic structure of the porphyrin ring in an electrostatically bound π-π complex: methylviologen-metallouroporphyrin complexes

Methylviologen-metallouroporphyrin complexes in aqueous alkaline solution are investigated with resonance Raman difference spectroscopy and UV-visible absorption spectroscopy. The complex is found to be a 2:1 π-π complex with a substantial contribution to the binding energy from electrostaic attraction between the anionic porphyrina and cationic viologen molecules. Strong axial ligation disrupts normal π-π interaction, resulting in a 1:1 complex for 5-coordinate metallouroporphyrins and total disruption of the π-π complex for 6-coordinate metals. An analysis of the absorption spectra in terms of the four-orbital model of porphyrin excited states determines the complex-related shifts in relative frontier π-orbital energies. The Raman shifts provide additional information necessary to determine absolute shifts in orbital levels. Complex-induced shifts in the a/sub 1u/(π) and e/sub g/(π) energy levels, but not the a/sub 2u/(π) level, are indicated. A 500-cm-1 change in the electron-electron repulsion interaction between the excited-states configurations also accompanies methylviologen complex formation