ABSORPTION AND RESONANCE RAMAN-SPECTRA OF EU(III)(PORPHYRIN)2+ COMPLEXES - OXIDATION-INDUCED ENHANCEMENT OF PORPHYRIN-PORPHYRIN PI-PI INTERACTION

Absorption and resonance Raman spectra are reported for the lanthanide porphyrin sandwich complexes EuIII(OEP)2+, EuIII(TPnP)2+, and EuIII(TPP)2+ (OEP = octaethylporphyrin, TPnP = meso-tetrapentylporphyrin and TPP = meso-tetraphenylporphyrin). These cations contain two holes in the porphyrin pi-system; consequently, they are electronically similar to solution cation dimers of the form (MOEP+)2. All three of the double-hole sandwich complexes exhibit spectral features characteristic of strong pi-pi-interactions. In particular, a near-infrared (near-IR) absorption band is observed at 861 nm (EuIII(OEP)2+), 867 nm (EuIII(TPnP)2+, and 1025 nm (EuIII(TPP)2+). The energies of these near-IR bands are significantly higher than those of the corresponding single-hole complexes. The blue shift of the near-IR absorption feature is consistent with enhanced pi-pi-interactions in the double-versus single-hole complexes. The near-IR absorption maximum of all three double-hole sandwiches undergoes a substantial blue shift as the temperature is lowered from room temperature to 11 K. This shift is attributed the systems attaining a conformation which further enhances pi-pi-interaction at low temperatures. The electronic properties of the sandwich complexes are discussed in the context of the type pi-pi-interactions which might occur between the special pair of bacteriochlorophyll molecules in photosynthetic reaction centers.