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 ∼2.4 ps. Photoexcitation of mainly the phthalocyanines in the functionalized nanoparticles leads to an electron transfer to the gold core in ∼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.