Momentum Dependence of the Electronic Excitations in Polyfluorinated Metal Phthalocyanines

We present an experimental investigation of the lowest electronic excitations of F16ZnPc, F16CuPc, and F16CoPc thin films using optical absorption and electron energy loss spectroscopy. Our results show that, despite the similarities to beta-phase metal phthalocyanines, the investigated fluorinated metal phthalocyanines all show a small dispersion of the lowest electronic excitation. Quantitative analysis of the momentum dependence of the spectral weight displays a substantial admixture of quadrupole transitions. Moreover, we point out the importance of charge transfer exciton interaction in order to understand this complex behavior.