Relaxation dynamics and transient behavior of small arenethiol passivated gold nanoparticles

Novel gold nanoparticles, passivated by monolayers of benzenethiol, biphenylthiol, and similar derivatives, have been synthesized and characterized using UV/vis, NMR, and Fourier transform infrared (FTIR) spectroscopies. The nanoparticle sizes have been evaluated using transmission electron microscopy and UV/vis spectroscopy; they show diameters between 2.1 and 4.7 nm, depending on the method of synthesis and the monolayer protecting group. Femtosecond transient absorption measurements show that the nanoparticles possess optical properties on the boundary between molecular and nanoparticle behavior. The smaller systems based on benzenethiol exhibit long-lived excited states with lifetimes on the order of a few nanoseconds, resembling those of small gold molecular type clusters. The larger nanoparticles protected with biphenylthiol and benzylthiol groups relax much more rapidly on a picosecond time scale, similarly to related citrate stabilized systems reported in the literature.