Relaxation dynamics of Au25L18 nanoclusters studied by femtosecond time-resolved near infrared transient absorption spectroscopy

The relaxation dynamics of electronically excited [Au-25(SR)(18)](q), where q = 0 or -1 and SR = S(CH2)(2)Ph, were studied using femtosecond time-resolved transient absorption spectroscopy. Nanoclusters excited by 400 nm light were probed using temporally delayed broad-bandwidth continuum probe pulses. Continuum pulses were generated in both the visible and near infrared (NIR) spectral regions, providing access to a wide range of transient spectral features. The use of NIR probe pulses allowed the relaxation dynamics of the excited states located near the HOMO-LUMO energy gap to be monitored in the probe step via the sp <- LUMO and sp <- LUMO+1 transitions. These NIR measurements yielded excited state absorption (ESA) data that were much less congested than the typical visible transient spectrum. For the neutral nanocluster, the time-domain data were composed of three components: (1) a few-picosecond decay, (2) a slower decay taking a few hundred picoseconds and (3) a non-decaying plateau function. Component 1 reflected energy relaxation to semi-ring ligand states; component 2 was attributed to relaxation via a manifold of states located near the HOMO-LUMO energy gap. Component 3 arose from slow radiative recombination. The dynamics of the anion depended upon the identity of the excited state from which the particle was relaxing. The LUMO+1 state of the anion exhibited relaxation dynamics that were similar to those observed for the neutral nanocluster. By comparison, the time-domain data observed for the LUMO state contained only two components: (1) a 3.3 +/- 0.2 ps decay and (2) a 5 +/- 1 ns decay. The amplitude coefficients of each component were also analyzed. Taken together, the amplitude coefficients and lifetimes were indicative of an activation barrier located approximately 100 meV above the HOMO-LUMO energy gap, which mediated a previously unobserved excited state decay process for [Au-25(SR)(18)](0). These data suggested that NIR ESA measurements will be instrumental in describing the relaxation processes of quantum-confined nanoclusters.