Abstract
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 ..pi..-..pi.. 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 ..pi..-..pi.. interaction, resulting in a 1:1 complex for 5-coordinate metallouroporphyrins and total disruption of the ..pi..-..pi.. 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 ..pi..-orbital energies. The Raman shifts provide additional information necessary to determine absolute shifts in orbital levels. Complex-induced shifts in the a/sub 1u/(..pi..) and e/sub g/(..pi..) energy levels, but not the a/sub 2u/(..pi..) level, are indicated. A 500-cm/sup -1/ change in the electron-electron repulsion interaction between the excited-states configurations also accompanies methylviologen complex formation.
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