Gold Nanoparticle-Chromophore Systems: Assembly and Photophysical Interactions

Abstract

Thin films of gold nanoparticles and photoactive organic molecules were prepared and studied with photoelectrical, spectroscopic and microscopic methods. Photoinduced electron transfer takes place from a poly(hexylthiophene) layer to the gold nanoparticle layer, and in the case of a porphyrin or a fullerene layer, the gold nanoparticles donate electrons to these chromophores. The photoelectrical measurements indicate that the particles can function either as electron acceptors or donors to the photoexcited chromophores. The highest photoelectrical signal was observed for films combining gold nanoparticles and porphyrin-fullerene dyads. Porphyrin-fullerene dyads are known to undergo intramolecular photoinduced charge transfer via an exciplex intermediate state. A gold nanoparticle layer enhances charge transfer of the dyad, when placed near the porphyrin moieties of the dyads. In addition, fluorescence measurements indicated that the gold nanoparticle layer affects the relaxation of the exciplex state of the dyad. The photoelectrical measurements demonstrated charge transfer in the films of porphyrins and gold nanoparticles, but energy transfer was considered to be possible as well. Time-resolved spectroscopic measurements showed that most, more than 80%, of the photoexcited porphyrins decay by energy transfer to the gold nanoparticles, whereas charge transfer is a minor relaxation route. Porphyrin- and phthalocyanine-functionalized gold nanoparticles were prepared using a ligand exchange method and characterized with steady-state and time-resolved spectroscopic techniques. The photoexcited porphyrins transfer energy to the gold cores very rapidly, in few picoseconds. The packing of the porphyrins on the gold nanoparticle surface and their fluorescence lifetimes are dependent on position of the linkers on the porphyrin core. Time-resolved absorption measurements were used to study the fast photoinduced processes of the phthalocyanine-functionalized gold nanoparticles. The selective excitation of the gold cores leads to energy transfer to the phthalocyanines. Photoexcitation of the phthalocyanines results in energy and electron transfers to the gold cores. As a conclusion, the photoinduced charge and energy transfer processes of the gold nanoparticle-chromophore systems studied are dependent on the choice of the chromophore and on the design of the system.