Subpicosecond transient dynamics in gold nanoparticles encapsulated by a fluorophore-terminated monolayer

Gold nanoparticles (2.2 nm) protected by a densely packed monolayer of 9-(9-fluorenyl)-nonane-1-thiol (MPC-b) or a mixed monolayer of 9-(9-fluorenyl)-alkane-1-thiol with nonane-1-thiol (MPC-c and MPC-d) were studied by transient spectroscopy upon 305 nm excitation with subpicosecond laser pulses. Electronic relaxation of the gold core took place through a fast electron-phonon interaction within 2.0 ps, followed by a slow phonon-phonon interaction over a period longer than 15 ps. Electronic coupling of the excited fluorenyl groups with the gold core in these composite clusters affected the observed gold electron relaxation dynamics, likely by adding an excitation path for gold electrons by working as a sensitizer. The efficiency of energy transfer from the excited fluorene to the gold particles was affected by surface composition, with the ratio of the fast decay component to the slow decay component decreasing upon increasing the fraction of fluorenyl groups present at the surface. Thus, energy transfer from the excited fluorene to the gold nanocore is a dominant relaxation mode for the fluorene excitons in chromophore-labeled monolayer protected gold clusters, taking place with a rate constant k(EN) estimated to be greater than 10(12) s(-1).