MURI 24 Engineered Multifunctional Nanophotonic Materials for Ultrafast Optical Switching

Interactions between nonlinear optical organic dyes and assmblies of noble-metal nanostructures may potentially be exploited for optical-switching applications. Our focus has been on studying fundamental aspects of the interactions between conjugated molecules and noble metals. Aus 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 were characterized (collaboration with Perry) with a variety of techniques. The excited-state dynamics of the dyes in close proximity to the nanoparticle surface were studied using the timecorrelated single-photon counting technique and near-IR fs transient absorption spectroscopy. The excited states of the dyes in the hybrid metal/organic systems exhibit ultrafast (<5 ps) deactivation, the length of the alkyl spacer between the dye and the thiol group having a profound effect on the ultrafast dynamics. Ultrafast formation (ca. 0.5 ps) of a cation-like species was seen for the propyl system but not for the dodecyl-linker system, and was attributed to dye–dye interactions leading to the formation of a chargetransfer species involving two or more dye molecules. We have also, in collaboration with Antoine Kahn (Princeton) and Georg Heimel (Humboldt, Berlin), investigated the electronic structure of planar Au surfaces coated with variously substituted stilbene thiolates using photoelectron spectroscopy and comparison with theory. In particular, we confirmed for the first time theoretical predictions that π-donors in self-assembled monolayers have only a minor influence on the hole injection barrier from the metal to the organic, despite significantly affecting the molecular ionization potential. We have also supplied thiol lignads to other groups in the program for nonlinear optical measurements.