Spectroscopic characterization of triple-decker lanthanide porphyrin sandwich complexes. Effects of strong .pi..pi. interactions in extended assemblies

Abstract

Electrochemical, optical absorption, near-infrared, infrared, resonance Raman, and electron paramagnetic resonance data are reported for several neutral and singly oxidized triple-decker lanthanide porphyrin sandwich complexes Ln{sub 2}(OEP){sub 3} (Ln = La(III), Ce(III), Eu(III); OEP = octaethylporphyrin). The data indicate that there are strong {pi}{pi} interactions between the porphyrin macrocycles in all of the complexes. As a consequence, the hole in the porphyrin {pi} system of the Ln{sub 2}(OEP){sub 3}{sup +} systems is delocalized over all three rings on the vibrational and electronic time scales. The singly oxidized complexes exhibit a characteristic electronic absorption band at very low energies (ca. 2200 nm). This absorption arises as a direct consequence of the strong {pi}{pi} interactions between the rings. A molecular orbital scheme is proposed that accounts for this low-energy feature as well as a number of other properties of the triple-decker sandwich complexes.