Anomalous Size-Dependent Excited-State Relaxation Dynamics of NanoGUMBOS

The synthesis, characterization, and ultrafast spectroscopy of size-selected nanospheres of ruthenium bipyridine-bis(pentafluoroethylsulfonyl)imide ([Ru(bipy)(3)][BETI](2)) in water are reported. These studies represent the first experimental evidence of phonons with nanosecond lifetimes in organometallic nanomaterials. Thermally stable, crystalline nanoparticles of [Ru(bipy)(3)][BETI](2) are derived from a group of uniform materials based on organic salts (GUMBOS). Excited-state relaxation dynamics are studied using pump-probe time-resolved transient absorption spectroscopy, and the results are compared to corresponding measurements of aqueous Ru(bipy)(3)Cl-2. The nanoGUMBOS show spectral shifts and size-dependent relaxation dynamics for nanoparticle diameters varying from 20 to 100 nm, characterized by excited-state decay dynamics similar to those of the precursor dye at higher pump pulse energies with an additional pathway attributed to intermolecular energy transfer, where all lifetimes increase with increasing nanoparticle size. Long-lived acoustic phonon oscillations with size-dependent frequencies are also observed, where the phonon frequency increases as the nanoparticle size increases, suggesting a very low coupling between electronic and phonon degrees of freedom and a strong hydrophobic interaction with the aqueous solvent. These studies provide new insights into the photodynamics of these novel nanoGUMBOS for potential advances in dye-sensitized solar cells and other optoelectronic devices, including hot-carrier extraction photovoltaics.