Electron shuttling across the interface of CdSe nanoparticles monitored by femtosecond laser spectroscopy

The formation and decay of the optical hole (bleach) for 4 nm CdSe nanoparticles (NPs) with adsorbed electron accepters (1,4-benzoquinone and 1,2-naphthoquinone) and the rise and decay of the reduced electron accepters formed after interfacial electron transfer from the CdSe NPs were investigated by femtosecond laser spectroscopy. The ultrashort (200-400 fs) rise times of the bleach at the band-gap energy of the CdSe NP as well as of the acceptor radical anion are found to increase with increasing the excitation energy. This suggests that the electron transfer from the CdSe NP to the quinone electron acceptor occurs after thermalization of the excited hot electrons. The decay times of the transient absorption for the electron acceptor radical anions are found to be comparable to that of the CdSe NP bleach recovery time (3 ps). This suggests that the surface quinones shuttle the electron from the conduction band to the valence band of the excited NP. We contrast this behavior with the excited-state dynamics of the recently investigated CdS-MV2+ system in which the electron acceptor does not shuttle the accepted electron back to the hole in CdS.