Optical and Electronic Properties of Molecule-Assembled and Molecule-Stabilized Gold Nanostructures

This dissertation presents and evaluates the structure-sensitive optical and electronic properties of gold nanostructure systems: plasmonic gold nanoparticle networks and monolayer-protected gold nanoclsuters. Plasmonic nanoparticle assemblies were prepared using the iron(III) tetrakis(1-methyl-4-pyridyl)porphine (FeTMPyP) μ-oxo bridged dimer molecule as a nanoparticle linker. The adsorption orientation of the iron porphyrin and the resulting geometry of FeTMPyP bridged solid and hollow nanoparticle aggregates was studied using linear extinction and surface-enhanced Raman scattering (SERS) measurements. For both the FeTMPyP-induced SGN and HGN structures, addition of FeTMPyP to nanoparticle solutions, caused a red shift of the localized surface plasmon resonance (LSPR) and the formation of a red-shifted coupled plasmon mode relative to the LSPR of the isolated system, indicating formation of aggregate structures. Along with UV-Vis-NIR extinction spectra, wavelength-dependent SERS studies of FeTMPyP further confirmed the orientation of the FeTMPyP dimer in the inter-particle gap of plasmonic nanoparticle aggregates. The dielectric dependent excited state relaxation dynamics of iron (III) tetra-4-N-methylpyridylporphine molecule (FeTMPyP) was investigated in a series of solvents using steady-state and time-resolved spectroscopies. The linear absorption spectrum for FeTMPyP in water was dominated by a multi-component Soret band, corresponding to π → π* and charge-transfer (C-T) electronic transitions. The lower energy charge-transfer component was significantly influenced by the dielectric properties of the dispersing medium. Over the range of solvents used, the charge-transfer component shifted by 70 meV due to C-T stabilization when lower-dielectric solvents were used. Femtosecond time-resolved transient absorption measurements were carried out to determine the influence of dielectric environment on the electronic relaxation dynamics. A relaxation model, which involves 400 nm excitation, followed by femtosecond internal conversion, charge transfer and thermalization of the C-T state explains the excited state dynamics observed for FeTMPyP. The dielectric-dependent time constant, τ2, correlated linearly with the energy shift of the proposed charge-transfer state relative to the initially prepared state. The results from this study suggest that the charge-transfer transitions of this iron porphyrin can be used as a spectroscopic probe in order to extract environmental properties. Studies on the electronic relaxation properties of molecule-stablized gold nanoclusters is presented. Polarization-resolved transient absorption (TA) measurement were employed to study the spin conversion dynamics in the Au25 nanocluster system. Using these methods, we have successfully detected spin relaxation dynamics and quantified picosecond lifetimes. Spin-sensitive transient absorption signal was only observed when pumping systems with open-shell character compared to the lack of spin dynamics observed when excitation occurred from closed shell HOMO states. Spin dynamics were only observed when pumping the open shell HOMO state of the neutral Au25 cluster. Due to splitting of the superatomic P orbitals, the anionic and cationic Au25 species represent closed shell systems, and we report no observed spin-sensitive dynamics in our experiments. The results here are consistent with the previously observed paramagnetism for the neutral Au25 nanocluster. Ligand-induced modifications to the electronic relaxation dynamics of the Au102(SR)44, nanocluster were studied using femtosecond time-resolved transient absorption spectroscopy. Time-resolved data revealed unique relaxation processes for the Au102 nanocluster depending on the thiol molecule used in the oligomeric units. Both Au102 nanocluster systems exhibit "molecule-like" dynamics, having no pulse energy dependent rates and a long-lived nanosecond relaxation component indicative of a significant energy gap. The solution-phase electronic relaxation for the organo-soluble Au102(SPh)44 system following 400-nm excitation proceeds by: (1) relaxation of the initially prepared excited state in 1.70 ± 0.10 ps, (2) vibrational cooling occurring with a lifetime of 7.60 ± 0.60 ps, and (3) low energy radiative decay on the timescale of nanoseconds. The relaxation dynamics obtained for the water soluble Au102(SPhCOOH)44 nanocluster, involved: (1) fast picosecond processes with lifetimes of 0.70 ± 0.10 ps and 2.80 ± 0.50 ps which corresponded to internal conversion from initially prepared excited states, (2) a relaxation component of 70.10 ± 4.30 ps that was assigned to intersystem crossing from the lowest triplet state to the ground state and (3) a longer time constant (> 2 ns) attributed to low energy radiative decay. The observed ligand-dependent relaxation rates of the Au102 nanocluster points to unique interactions occurring between the Au core and the ligand groups. Significant charge-transfer (C-T) interactions at the surface of ligand protected nanoclusters have been examined and were found to strongly influence the electronic relaxation properties of Au102 nanoclusters. A mechanism based on charge transfer can account for the ligand-dependent relaxation rates.