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 tri ple-decker tetraphenylporphyrin-phthalocyanine (TPP-Pc) complexes is r eported. The work is aimed at elucidating the extent of the localizati on or delocalization among the chromophore ligands in the mixed comple xes, where there exists a charge imbalance between the metal centers a nd the dianion Pc/TPP ligands, as well as in different oxidation state s for these types of complexes. For mixed double- and triple-decker co mplexes, \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 prefer entially 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 sh ifts 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 th e optical absorption characteristic of a Pc(.-) species and is also se en 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 ca se of the cerium(III) derivatives, where the metal can also be oxidize d, is more complex. It appears that the metal is preferentially oxidiz ed 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 d iscussed.