Vibrational spectroscopy of phthalocyanine and naphthalocyanine in sandwich-type (na)phthalocyaninato and porphyrinato rare earth complexes

This article reviews mainly the authors’ efforts in studying and understanding the IR and Raman vibrational spectroscopic characteristics of (na)phthalocyanine in sandwich-type (na)phthalocyaninato and porphyrinato rare earth complexes. On the basis of systematic studies into the vibrational characteristics of (na)phthalocyanine in a wide range of homoleptic and heteroleptic bis- and tris-[(na)phthalocyaninato] and mixed (porphyrinato)[(na)phthalocyaninato] complexes of the whole series of rare earth metals, the effects of substituents, molecular symmetry, rare earth ionic size, rare earth valence state, and excitation wavelength on the vibrational characteristics of (na)phthalocyanine in sandwich rare earth complexes have been comparatively studied. The assignments of the IR and Raman bands of (na)phthalocyanine in these complexes have been made on the basis of comparison with related monomeric phthalocyanine compounds. Theoretical calculations of vibrational spectra for monomeric phthalocyanine derivatives are also helpful for the assignments. Both spectroscopic techniques reveal that the frequencies of pyrrole stretching, isoindole breathing, isoindole stretching vibrations, aza stretching vibrations, and coupling of pyrrole and aza stretching vibrations depend on the rare earth ionic size, shifting to higher energy along with the lanthanide contraction due to the increased ring–ring interaction across the series. Moreover, from the viewpoint of vibrational spectroscopy, the substitution of alkoxyl groups at the non-peripheral α positions of phthalocyanine rings exerts more influence on the intrinsic properties of phthalocyanines compared with substituents at peripheral β positions.