Resonance Raman spectra of .alpha.-copper phthalocyanine

Abstract

Raman spectra of ..cap alpha..-copper phthalocyanine (..cap alpha..-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. At low temperatures, multimode interactions are reduced and profiles were obtained which can be compared with solution profiles of porphyrins. Both Q/sub x/ and Q/sub y/ 0-0 scattering can be identified and a helper mode is evident. A term enhancement predominates, with B/sub 1g/ and B/sub 2g/ 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 ..cap alpha..-CuPc is largely due to an allowed ..pi..-..pi..* transition associated mainly with the macrocycle and inner five-membered rings of the isoindolemore » groups. 25 references, 5 figures, 2 tables.« less