Shpol’skii effect and vibronic spectra of the phthalocyanines

Free base Mg, ClAl, Zn, and Cu phthalocyanines (Pc’s) are successfully incorporated in Shpol’skii matrices, although they are practically insoluble in the open chain normal alkanes which are the conventional Shpol’skii solvents. In analogy with the porphyrins (P’s) the doublet spectra of the 0–0 transitions are observed. They are attributed to removal of the degeneracy of the first excited states in metal Pc’s, and to the existence of two tautomeric states in H2Pc. Relaxation between the doublet components, separated by several tens of cm−1, are slow at the experimental temperature, 4.2 K. Their relative fluorescence intensity is found for the first time to vary with the excitation wavelength. This is attributed to a strong vibronic mixing between the doublet components. The high resolution fluorescence-excitation and fluorescence spectra of the four investigated Pc’s incorporated in the Shpol’skii matrices are structurally very rich. They are characterized by weak electron–phonon and weak vibronic interactions in total accord with our previous findings with PtPc. While the electronic transition a1u(π)→eg(π*), accounts for most of the observed intensity, the numerous vibronic transitions, through weak, are laden with information. Qualitative analysis of the vibronic spectra helps to unravel the mechanism of vibronic coupling as well as to assign the vibronic bands unique to H2Pc and ClAlPc. There is strong indication from the electronic spectrum that the corresponding electronic state in ClAlPc is strongly perturbed by the presence of Cl. It is also found that the vibronic bands in the fluorescence-excitation spectra deviate markedly from their images in the fluorescence spectra in three aspects: relative intensity; extra vibronic activity; and band halfwidth. The causes of these differences are discussed.