Photoinduced Charge and Energy Transfer in Phthalocyanine-Functionalized Gold Nanoparticles

Photoinduced processes in phthalocyanine-functionalized gold nanoparticles (Pc-AuNPs) have been investigated by spectroscopic measurements. The metal-free phthalocyanines used have two linkers with thioacetate groups for bonding to the gold nanoparticle surface, and the attachment was achieved using a ligand exchange reaction. The absorption spectrum of the Pc-AuNPs shows a broadening of the phthalocyanine Q-band absorption, probably due to a tight packing of the phthalocyanines on the gold nanoparticle surface. For the attached phthalocyanines, fluorescence is strongly quenched, and the fluorescence lifetimes determined by time-correlated single photon counting (TCSPC) are strongly reduced. The quenching mechanisms were studied in detail with time-resolved absorption (pump-probe) measurements. A selective excitation of the gold cores in the pump-probe experiment results in an energy transfer from the gold nanoparticles to the attached phthalocyanines in similar to 2.4 ps. Photoexcitation of mainly the phthalocyanines in the functionalized nanoparticles leads to an electron transfer to the gold core in similar to 3.0 ps. The recombination of charges in the Pc-AuNP takes place on a. picosecond time scale. In addition, there is evidence of energy transfer from the photoexcited phthalocyanines to the gold nanoparticles.