Phthalocyanines: structure and vibrations

Abstract

Raman scattering for a range of metal phthalocyanines using excitation frequencies from 457.9 to 1064 nm have been interpreted in the light of the recent DFT calculation on zinc phthalocyanine. The intention was to determine the features of the spectra that might be used for in situ analysis of specific phthalocyanines. B₁ bands were found to be the most intense. The frequency of one band with a large C–N–C ring displacement is particularly sensitive to the metal ion size. The effect is determined not only by the size of the ion but also by the effect it has on ring shape. First and second order overtone bands of zinc and copper phthalocyanines are broadly similar, with some coupling differences at about 2000 cm⁻¹. The region between 1350 and 1550 cm⁻¹ has been little studied previously. It shows a remarkable sensitivity to the metal ion present and provides a specific signature for each phthalocyanine studied. In contrast, a study of α-, β-, γ- and ε-copper phthalocyanines using 514.5 nm excitation showed very few differences, suggesting that intra- rather than intermolecular markers are most efficiently determined by Raman scattering. The study enables the interpretation of the Raman spectra of the phthalocyanines in terms of molecular structure and due to the resonant enhancement involved will enable the in situ identification of specific phthalocyanines in matrices.