Excited-state dynamics and dye-dye interactions in dye-coated gold nanoparticles with varying alkyl spacer lengths

Gold nanoparticles (ca. 3 nm in diameter) coated with bis(diarylamino)biphenyl-based thiols with two different alkyl spacers (propyl and dodecyl) between the chromophore and the surface-anchoring thiol group have been prepared and characterized with a variety of techniques. The excited-state dynamics of the dyes in close proximity to the nanoparticle surface were studied using the time-correlated single-photon counting technique and near-IR fs transient absorption spectroscopy. The excited states of the dyes in the hybrid metal/organic systems exhibit an ultrafast (<5 ps) deactivation as evidenced by the fs transient absorption measurements. The length of the alkyl spacer between the dye and the thiol group has a profound effect on the ultrafast dynamics of the photoexcited systems. An ultrafast formation (ca. 0.5 ps) of a cation-like species has been recorded for the system incorporating the propyl spacer but not for the dodecyl-linker system. The formation of the cation-like species has been shown to be less efficient in a mixed-ligand system in which the bis(diarylamino)biphenyl-based thiol was diluted on the surface with dodecanethiol. Additionally, the ultrafast formation (ca. 1 ps) of a cation-like species with a similar spectroscopic signature has been observed in the solid state of the dye. A combination of the ultrafast dynamics and (1)H NMR spectroscopic data has been used to discuss the observed behavior in terms of dye-dye interactions in the nanoparticle systems. Due to the surface curvature of the nanoparticle, the propyl spacer imposes a closer dye-dye distance than the dodecyl spacer, thus facilitating dye-dye interactions that lead to the formation of a charge-transfer species involving two or more dye molecules.