SUBPICOSECOND EXCITATION OF STRONGLY COUPLED PORPHYRIN PHTHALOCYANINE MIXED DIMERS

Electrostatic heterodimers are formed in the liquid phase by pairing a zinc porphyrin with either a copper or an aluminium phthalocyanine bearing oppositely charged substitutents. The ground-state absorption spectra of such heterodimers are drastically changed with respect to the corresponding monomers, indicating the existence of a strong interaction between the two chromophores. Irrespective of the nature of the metal ions and of the peripheral substituents, the heterodimers present very similar ground-state spectra. The photophysical properties of the two heterodimers are investigated with the use of femto- and nano-second absorption spectroscopies. Excitation of the zinc porphyrin-aluminium phthalocyanine heterodimer at 565 or 620 nm is followed by a very efficient electron transfer from the porphyrin to the phthalocyanine moiety while a very efficient intersystem conversion takes place in the excited zinc porphyrin-copper phthalocyanine, leading to the final 'triplet' excimer. The difference in behaviour is analysed and explained in terms of the thermodynamics of the transfer process and of the peculiar properties of the paramagnetic copper phthalocyanine.