Mixing of Frenkel and charge transfer excitons in quasi-one-dimensional copper phthalocyanine molecular crystals

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We present an electron energy-loss study of the nature of the low-lying singlet excitations in quasi-one-dimensional copper phthalocyanine molecular crystals. The observed variation of the spectral weight at different excitation energies strongly indicates the presence of two types of excitations near the excitation onset. These are associated with mixed intramolecular Frenkel and intermolecular charge transfer excitons. In particular, the lowest excitation component reveals a significant nondipolar character which can only be rationalized taking into account the admixture of charge transfer excitations.