Effect of molecular packing on the solid state spectra of ruthenium phthalocyanine: anomalous behaviour of a monodimensional stacked assembly

The ruthenium phthalocyanine molecule has never been isolated as a single unit in the solid phase. Any attempt to obtain it leads to the dimeric form (RuPc)(2) with a double bond between Ru-Ru atoms. Owing to the amorphous nature of this compound, its structural characterisation was previously investigated by LAXS (Large Angle X-ray Scattering) and EXAFS (Extended X-ray Absorption Fine Structure) studies. Magnetic and conductivity properties were also discussed in the light of the found structure. The optical spectra of the material in the solid phase and as evaporated films are reported in the present contribution. The obtained results show a marked red shifting absorption when passing from solution to film. Such anomalous behaviour can be explained taking into account the considerable molecular distortion presented by the two macrocycles within the dimer and, overall, considering the strong pi-pi interactions throughout the molecular stack. The attempt to apply the coupling exciton theory in order to clarify the optical data was not successful for the molecular assembly assumed by (RuPc)(2) dimeric units. The EXAFS investigation on films, which supports the suggested interpretation, also shows some degree of flexibility of the molecule which somehow influences the whole molecular packing. Depth profile analysis of (RuPc)(2) films, studied by the XPS (X-ray Photo-emission spectroscopy) technique, clearly indicates a homogeneity and a stable chemical composition throughout the thickness.