Photophysical properties of pheophorbide- a-substituted diaminobutane poly-propylene-imine dendrimer

Abstract

Dendrimers are very promising molecules as hosts for guest embedding as well as for multiple covalent coupling of dye molecules to their peripheral groups. The use of such dendrimer–dye conjugates as part of a modular carrier system for photodynamic therapy has recently been discussed. Using covalent coupling we accomplished an average loading of 12–13 molecules of pheophorbide a to one diaminobutane poly-propylene-imine dendrimer molecule of the third generation (DAB dendrimer) having a total number of 16 binding sites. This was confirmed with various photophysical measurements and MALDI-mass spectrometry. As long as the dye molecules are covalently bound to the dendrimer, their photosensitized generation of singlet oxygen is reduced dramatically due to interactions between different dye molecules. The main interaction is supposed to be a Förster-energy transfer along the surface of the dendrimer–dye complex. Nevertheless the remaining photosensitized generated singlet oxygen leads to a destruction of the dendrimer backbone under illumination. The photophysical properties of the covalently linked dye–dendrimer complexes and especially the remarkable influence of illumination on these properties are presented in this paper.