Ultrafast ET with a characteristic time constant of approximately 70 fs between CdSe QDs (mean radii of 1.4 nm) photoexcited in the lowest 1S electron state (lambda(exc) = 539 nm), and the molecular electron acceptor MV(2+) adsorbed on the QD surface was observed. The photophysics of such a system was investigated by time-resolved transient absorbance spectroscopy in the UV-visible spectral region. Our studies for the coupled system as a function of excitation intensity at lambda(exc) = 387 nm show that the ET processes compete efficiently with Auger recombination in CdSe QDs and at least 4 e-h pairs can be separated by ET to the electron acceptor MV(2+).

Exciton separation dynamics in the electron transfer system containing highly photostable CdSe/CdS core/shell nanocrystal quantum dots and adsorbed methylviologen was investigated by means of femtosecond absorption spectroscopy. The experiments revealed that electron extraction from the photoexcited core is possible, and the rate of the ET reaction strongly depends on the CdS shell thickness. A CdS associated exponential decay constant β of 0.33 Å -1 was obtained reflecting the electronic barrier effect of the shell. These findings show that core/shell structures are well suited for the design of optimized QD-based solar cells.

The dynamics of the photoinduced Forster resonance energy transfer (FRET) in a perylene diimide−quantum dot organic−inorganic hybrid system has been investigated by femtosecond time-resolved absorption spectroscopy. The bidentate binding of the dye acceptor molecules to the surface of CdSe/CdS/ZnS multishell quantum dots provides a well-defined dye-QD geometry for which the efficiency of the energy transfer reaction can be easily tuned by the acceptor concentration. In the experiments, the spectral characteristics of the chosen FRET pair facilitate a selective photoexcitation of the quantum dot donor. Moreover, the acceptor related transient absorption change that occurs solely after energy transfer is utilized for the determination of the energy transfer dynamics. Our time-resolved measurements demonstrate that an increase of the acceptor concentration accelerates the donor−acceptor energy transfer. Considering a Poisson distribution of acceptor molecules per quantum dot, the dependence of the energy transfer rate on its mean value is linear. The results of the presented spectroscopic experiments allow for determining the relative and absolute acceptor/donor ratio in the investigated FRET system without any parameters intrinsic to Forster theory. ■ INTRODUCTIONSemiconductor quantum dots (QD) are nanometer sized luminescent particles with remarkable optical properties. Since the QD dimension is in the range of the Bohr exciton radius, the electronic transition energies become size dependent which leads to a tunability of the absorption and emission properties. 1 QD have received broad and multidisciplinary research interest which ranges from the elucidation of their fundamental photophysical properties such as blinking, 2−5 homogeneous line width, 6−8 and exciton relaxation dynamics 9−12 to the implementation as light absorbers or emitters in photovoltaic devices, 13−16 LEDs, 17−19 and lasers. 20−22 The reduced size of QD inevitably leads to a high surface to volume ratio and consequently to a strong impact of the surface composition on the QD properties. The QD fluorescence quantum yield can be significantly increased by growing an inorganic shell on the core nanoparticles. The stronger fluorescence of core/shell particles is explained by the saturation of surface associated trap states, and fluorescence quantum yields as high as 85% have been observed. 23,24 High photostability together with strong and narrow emission has motivated the application of QD in biological sensing 25−29 and imaging. 30,31 Sensing applications are typically based on the modulation of the QD emission as response to the presence of a target molecule. 28 The modulation can be achieved by quenching mechanisms such as Forster resonance energy transfer (FRET) 25−28,32,33 or charge transfer (CT). 34−38 In QD-based FRET systems the inorganic nanoparticles are predominantly applied as energy donors. Yet, in some recent reports QD have been successfully utilized as acceptors in conjugation with naphthalimide dyes 39 as well as with light harves...

Close to the edge: Photoexcitation of alizarin coupled to the surface of mesoporous TiO(2) films leads to ultrafast electron transfer to the TiO(2) conduction band (see picture). Complex kinetics after photoexcitation depend on the excitation energy, and indicate a position of the alizarin excited state close to the TiO(2) conduction band edge, where the density of acceptor states is reduced. The photoinduced dynamics in Al(2)O(3) and TiO(2) mesoporous films sensitized by the strongly coupled alizarin dye is investigated by femtosecond transient absorption spectroscopy in the spectral range from UV to mid-IR. Alizarin/Al(2)O(3) acts as a nonreactive reference system, in which no electron transfer is observed. For comparison, the photoexcitation of the alizarin dye coupled to the surface of TiO(2) films leads to ultrafast electron transfer from the dye to the TiO(2) conduction band on the sub-100-fs timescale. We observe a fast relaxation of the alizarin excited state as well as a fast recombination of injected electrons with the alizarin cation on the picosecond timescale, which gives rise to very complex kinetics at short delay times. The infrared measurements clearly indicate that trapping of injected electrons is the main mechanism responsible for the observed long-lived charge separation in TiO(2) mesoporous films. The experimental findings can be explained by a position of the dye excited state close to the conduction band edge.

We observe the CdSe longitudinal-optical ground-state phonon in the electron transfer system composed of CdSe quantum dots and methylviologen directly by femtosecond absorption spectroscopy. A significant phase shift indicates that the coherent oscillations are triggered by an ultrafast charge migration, which is the consequence of an electron transfer from the photoexcited quantum dot to the molecular acceptor methylviologen. In contrast, the observed coherent phonons in isolated quantum dots stem from the frequency modulation of the quantum dot excited-state spectrum. From the probe wavelength dependence of the longitudinal-optical phonons in the electronic ground state and excited state it is possible to determine a biexciton binding energy of 35 meV.