HOLE LOCALIZATION OR DELOCALIZATION - AN OPTICAL, RAMAN, AND REDOX STUDY OF LANTHANIDE PORPHYRIN-PHTHALOCYANINE SANDWICH-TYPE HETEROCOMPLEXES

A systematic electrochemical, redox, optical absorption, and resonance Raman spectroscopic study of 22 gadolinium and cerium double- and triple-decker tetraphenylporphyrin-phthalocyanine (TPP-Pc) complexes is reported. The work is aimed at elucidating the extent of the localization or delocalization among the chromophore ligands in the mixed complexes, where there exists a charge imbalance between the metal centers and the dianion Pc/TPP ligands, as well as in different oxidation states for these types of complexes. For mixed double- and triple-decker complexes, \Pc(M)TPP\ and \TPP(M)Pc(M)TPP\ (where M = Gd-III, Ce-IV), in which the metal cannot be oxidized, it appears that the ligand preferentially oxidized is the Pc moiety. This preferentially localized hole in the phthalocyanine results in new near-infrared transitions which show maxima around 1200-1300 nm for the double-decker complexes and shifts to 1800-2300 nm for the triple-decker derivatives. In this case, the removal of an electron from a complex redox orbital which is rich in Pc character is easily identified by the accompanying changes in the optical absorption characteristic of a Pc(.-) species and is also seen in FT Raman spectra (excited at 1064 nm) by a significant (>15 cm(-1)) upshift of a redox-sensitive Raman band at ca. 1500 cm(-1). The case of the cerium(III) derivatives, where the metal can also be oxidized, is more complex. It appears that the metal is preferentially oxidized in the symmetric triple decker with the two outer Pcs. In contrast, the interpretation of the data obtained for the second triple-decker compound with the inner Pc is more complex. This point is thoroughly discussed.