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Raman spectra of a-copper phthalocyanine (a-CuPc) were recorded at room temperature and at 10 K with excitation wavelengths between 457 and 714 nm. Resonance enhancement was greatest for modes for which the largest displacements were on either the inner five-membered ring of the isoindole groups or the inner macrocycle and consequently assignment of the bands to modes of the entire molecule was possible by comparison with nickel octaethylporphyrin. Four out of five bands resonant in the Q band region and preresonant near the B band absorption region are totally symmetric modes. B band preresonance occurs more strongly with high-frequency modes and, due to the more delocalized nature of the transition, there is a larger contribution from six-membered ring displacements. Q band profiles are divided into two types. Those from high-frequency modes associated mainly with the isoindole ring are broad and involve a number of different vibronic levels of the excited state and those from low-frequency modes associated with the macrocycle are sharper and resonance from the lowest vibronic level of the excited state is the most intense. At low temperatures, multimode interactions are reduced and profiles were obtained which can be compared with solution profiles of porphyrins. Both Q, and Qy 0-0 scattering can be identified and a helper mode is evident. A term enhancement predominates, with B,, and B,, modes enhanced because of a Jahn-Teller distortion of the excited state. The resonance studies, together with electronic absorption spectra and published theoretical studies, confirm that the Q band in a-CuPc is largely due to an allowed T-T* transition associated mainly with the macrocycle and inner five-membered rings of the isoindole groups. In addition, the study illustrates the advantages of the silver disk method of presenting samples and the advantages of low-temperature profiles in solid-state studies.

Resonance excitation profiles have been obtained for cobalt phthalocyanine at both room temperature and 10 K using a compressed disc of silver and the phthalocyanine. Low-frequency vibrations are shown to be influenced mainly by displacements on the bonding nitrogen, but high-frequency vibrations show a more complex pattern with some evidence that the vibrations v3 and vZB depend on C, displacements. By comparison with copper, there appears to be a greater degree of scattering from 0-0 and 0-1 transitions and there is a shift towards increased 0-0 scattering in the sharper low-temperature profiles. It is concluded that a distortion, dynamic at room temperature and static at low temperature, plays a key role in determining the relative scattering and this distortion is larger for copper phthalocyanine than for cobalt phthalocyanine.

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