Photochemical Properties and Ultrafast Dynamics of Azobenzene-Functionalized Gold Nanoparticles
The main goal of the present Thesis was the investigation of the photoinduced isomerization processes of azobenzenes (ABs) attached to small gold nanoparticles (AuNPs) by static and femtosecond timeresolved transient absorption spectroscopy. ABs functionalized with linker chains of three different lengths (C3, C7, C11) were brought onto the surface of ~4 nm diameter AuNPs in a mixed self-assembled monolayer (mSAM) with decanethiol or pentanethiol as co-ligand. The AB-functionalized AuNPs show excellent photoisomerization yields independent from the linker chain length. AuNPs functionalized with ABs bearing long alkyl linker chains and short co-ligands reversibly aggregate when switched to their cis state and disaggregate when isomerized back to their trans state. The time-resolved absorption data reveal excited-state absorption of AB and strong signals of the simultaneously excited localized surface plasmon resonance (LSPR). The excited-state dynamics of AB show surprisingly little difference compared to the AB dynamics in solution, leading to the conclusion that the AB photochrome and the AuNP core are efficiently decoupled by the long (C11) linker chain. Furthermore, the excited-state dynamics of Disperse Red 1 in a mSAM with decanethiol on a AuNP surface were investigated. A second goal of this Thesis was the spectroscopic investigation of the ultrafast relaxation dynamics of functionalized AuNPs themselves, especially the hot electron cooling dynamics, and how they are influenced by different ligands. AB-functionalized AuNPs show the same hot electron lifetime of τ~2 ps as purely decanethiol functionalized AuNPs, revealing that the electronic states of AB do not participate in the electron cooling process. A study investigating the influence of (i) the ligand heat capacity and (ii) resonant coupling between ligand absorption band and LSPR was performed.
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