Fullerene-rich dendrimers: divergent synthesis and photophysical properties

Dendrimers containing up to 16 fullerene peripheral subunits have been prepared by a divergent synthetic approach based on the functionalization of polypropyleneimine (PPI) dendrimers with a C60 derivative bearing an activated carboxylic acid function. The absorption and fluorescence properties of these compounds are substantially identical in toluene and benzonitrile. Enhanced absorption in the region between 350 and 500 nm is detected by increasing the generation number, attributable to intramolecular interactions. A size-dependent trend of decreasing singlet lifetimes ( 16%) and fluorescence quantum yields ( 18%) is observed. The fullerene triplet state of the dendrimers was monitored via laser flash-photolysis in toluene and benzonitrile. In both media the transient absorption signal intensity is decreased with the molecular size and the effect is more pronounced in oxygen-free solution ( 60%) compared to air-equilibrated samples ( 37%). In toluene the triplet decay kinetics is unchanged for the whole series, ruling out the possibility of self-quenching effects, whereas in benzonitrile a triplet lifetime increase is recorded as a consequence of fullerene self-protection towards oxygen quenching. No amine-fullerene photoinduced electron transfer is detected because the complex molecular architecture does not allow the establishment of favourable donor-acceptor distances.