Picosecond to Millisecond Transient Absorption Spectroscopy of Broad-Band Emitting Chiral CdSe Quantum Dots

The relaxation dynamics of photoexcited carriers in water-soluble, penicillamine-capped, optically active and luminescent chiral CdSe quantum dots (QDs) were investigated. Three pump–probe techniques, namely broad-band UV–visible transient absorption (TA), picosecond time-resolved infrared (ps-TRIR) transient absorption, and nanosecond laser flash photolysis spectroscopy, were used to record transient decays from the picosecond up to the millisecond time scale. Picosecond experiments were carried out at a range of energies per unit area down to ca. 30 μJ cm–2, where only single photon excitation is expected. Whereas ps-UV–visible TA spectra show both bleach and transient features which are associated with depopulation of the lowest lying electron quantized state, the infrared transient bands are broad and structureless. The mid-IR transient absorption and excitonic bleach recovery is only partial, and its decay kinetics were found to be multiexponential in nature. The initial picosecond decay component is attributed to exciton-decay processes and to trapping by surface states. Nanosecond laser flash photolysis experiments have provided direct evidence for the presence of deep, long-lived states in the system. The origin of the ultrafast and long-lived species is discussed.