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Charge Transfer–Induced State Filling in CdSe Quantum Dot–Alizarin Complexes Lars Dworak, Sina Roth, and Josef Wachtveitl J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.6b12265 • Publication Date (Web): 11 Jan 2017 Downloaded from http://pubs.acs.org on January 20, 2017

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The Journal of Physical Chemistry

Charge Transfer–Induced State Filling in CdSe Quantum Dot–Alizarin Complexes Lars Dworak†, Sina Roth†, Josef Wachtveitl*† †

Institute of Physical and Theoretical Chemistry, Goethe–University Frankfurt am Main, D–

60438 Frankfurt am Main, Germany.

ABSTRACT Ultrafast transient absorption spectroscopy is applied to study the photoinduced processes of inorganic–organic CdSe quantum dot–alizarin hybrid complexes. The formation of a pronounced transient bleaching of the quantum dot excitonic transitions after selective photoexcitation of the surface–bound alizarin indicates an electron transfer from the alizarin excited state to the quantum dot 1S(e)–state. An electron transfer time of 19 ps is determined, which is independent of the alizarin concentration. A derivative–like spectral signature immediately after photoexcitation of the alizarin is explained by an excitation induced level shifting of the QD electronic transitions. Our study demonstrates that the bleaching of the quantum dot excitonic transitions can be used to evaluate the charge transfer dynamics in the investigated hybrid complexes.

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INTRODUCTION The assembly of quantum dots (QD) and organic molecules can yield complexes with appealing properties for the application in various research fields such as biological sensing,1–3 fluorescence modulation4–7 and photovoltaics.8–14 Due to the size tunable properties of QD and the wide range of organic compounds available, QD can act as energy donors or acceptors as well as electron donors or acceptors in QD–organic molecule complexes depending on the energetic situation within the constructed system. In terms of biological sensing and fluorescence modulation, the QD are typically used as energy donors. For systems composed of QD and organic dyes it has been demonstrated that the efficiency and dynamics of the energy transfer from the QD to the dye can be interpreted in the framework of the FRET theory and strongly depend on the molecular acceptor concentration.15,16 QD are frequently applied as electron donors in QD–organic molecule complexes for the investigation of the fundamentals in photoinduced charge separation processes at the semiconductor–organic molecule interface. Former studies showed that the charge separation dynamics after photoexcitation of the QD strongly depend on the acceptor concentration17–19 as well as the diving force of the reaction.20–21 The influence of a passivating shell on the electron transfer (ET) from CdSe/CdS22–23 and CdSe/ZnS24 core–shell heterostructures to molecular acceptors as well as the possibility of multiexciton separation25–27 has been investigated previously. A detailed understanding of the charge transfer processes after photoexcitation of QD can help in the design of more efficient QD–sensitized solar cells. Recently, a power conversion efficiency of 6.33% has been reported for a solar cell composed of CdSe QD doped with Mn2+ and TiO2.13


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In hybrid solar cells made from semiconductor QD blended with conjugated polymers the QD can act as electron acceptors.28–35 In a time resolved absorption study on CdSe QD blended with the low band gap polymer PCPDTBT the photoinduced ET from the polymer to the QD was monitored by the spectral signature of the reduced QD.32 However, the interface between the inorganic semiconductor and the organic moiety can be rather complex with various charge trapping and recombination pathways. Therefore, a detailed understanding of the interfacial processes is necessary to improve the hybrid solar cell performance. Currently, the power conversion efficiency can reach 4.8% in such devices.35 The alizarin dye is frequently used as sensitizer for the large band–gap semiconductor TiO2. In that system the ET from the photoexcited alizarin to the conduction band of TiO2 occurs on the femtosecond time scale.36–40 Other studies used different anthraquinone dyes, including alizarin, as electron acceptors in combination with CdTe,41 as well as type I24 and type II42,43 core–shell QD. Therefore, the alizarin dye is an interesting candidate to study the fundamentals of charge transfer processes at the QD–organic molecule hybrid interface. Herein we study the photophysics of CdSe QD–alizarin hybrid complexes with ultrafast transient absorption spectroscopy in the visible spectral range. In particular, the excited state dynamics of surface–bound alizarin is probed after selective photoexcitation at 600 nm. The spectral signature of state filling in the QD is unambiguously monitored giving clear evidence for the ET from the alizarin excited state to the QD 1S(e)–state. The state filling related transient absorption signal is used to study the influence of the alizarin concentration on the ET dynamics.


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EXPERIMENTAL METHODS Chemicals. Oleic acid (OA; tech. 90%), 1–octadecene (ODE; tech. 90%), trioctylphosphine (TOP; tech. 90%), cadmium oxide (99.5%), selenium (100 mesh, 99.99%) and ethanol (99.8%) were obtained from Aldrich. Toluene (99.95%) and methanol (100%) were purchased from Acros Organics and VWR, respectively. Synthesis of the CdSe QD. The synthesis was performed according to a slightly modified published procedure.44 The selenium precursor was prepared by dissolving 16 mg selenium in 0.5 mL TOP and 1.5 mL ODE using an ultrasonic bath. In a three–neck flask 26 mg cadmium oxide were dissolved in 7 mL ODE and 1 mL OA at 240 °C under argon atmosphere. The temperature of the solution was set to 220 °C and the selenium precursor solution was swiftly injected under vigorous stirring. After a certain time period of crystal growth the heating mantle was removed and the flask was transferred into a water bath. The QDs were precipitated by adding a 3:1 mixture of ethanol/methanol. After centrifugation the QD were dried in vacuum and subsequently redispersed in toluene. The diameters and concentrations of CdSe QD samples were calculated according to formalisms published by Yu et al.45 In the following, the QD samples are named according to the wavelength of the first excitonic transition. The samples CdSe490, CdSe508, CdSe515 and CdSe526 had calculated diameters of 2.26 nm, 2.42 nm, 2.50 nm and 2.64 nm. Preparation of the CdSe QD–Alizarin Complexes. The QD–alizarin complexes were prepared by mixing specific amounts of the single components. In the case of CdSe490–, CdSe515– and CdSe526–alizarin complexes the molar ratio between QD and dye in the preparation procedure has been adjusted to 1:10. For that purpose 200 µL of a 1 mM alizarin solution (toluene) was mixed with 80–130 µL of the QD solution (concentrations 0.15–


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0.25 mM). The mixtures were allowed to react overnight. To remove possible unbound alizarin, the QD–alizarin complexes were precipitated and washed by a 3:1 mixture of ethanol/methanol. After centrifugation the complexes were dried in vacuum and redispersed in toluene. The procedure yielded clear solutions without any indication of inhomogeneity. CdSe508–alizarin complexes at different molar ratios from 1:1 to 1:10 were prepared by the variation of the concentration of the alizarin solution. The spectroscopic determination of the final QD–alizarin molar ratios is challenging since the extinction coefficient of surface–bound alizarin is unknown. Consequently, relative ratios determined from the absorption spectra are used in the discussion of the concentration–dependent investigation. Transient Absorption Setup. The time–resolved experiments were performed using a home– built pump–probe setup.46 An oscillator–amplifier system (Clark, MXR–CPA iSeries) operating at a repetition rate of 1 kHz and a central wavelength of 775 nm provided the laser pulses (pulse duration of 150 fs). The pump pulses at desired wavelengths were generated in a home–built two stage non–collinear optical parametric amplifier (NOPA).47,48 For the direct excitation of alizarin, pump pulses with a central wavelength of 600 nm in combination with a OG590 filter (Schott) were applied. The QD were directly excited at 490 nm or 505 nm, depending on the investigated sample. A prism compressor was used to compress the pump pulses to about 100 fs. The pump pulse energy depended on the sample under investigation. Pulse energies of 75 nJ and 3–5 nJ were used for the direct excitation of the alizarin and the QD, respectively. The low pump pulse energies in case of the QD excitation minimizes the effect of multiexcitations in the TA measurements. Single filament white light (WL) covering a spectral range between 450 nm and 675 nm was generated by focusing the laser fundamental in a 5 mm thick sapphire crystal. The WL pulses were split into a signal and reference part. The signal part was transmitted through the


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sample and subsequently detected via a spectrograph whereas the reference beam was guided directly to a spectrograph. The spectrographs (AMKO Multimode) were equipped with 1200 grooves/mm gratings blazed at 500 nm and photodiode arrays (PDA, Hamamatsu Photonics, S8865–64) combined with driver circuits (Hamamatsu Photonics, C9118). A data acquisition card (National Instruments, NI–PCI–6110) digitized the PDA signals at 12 bits. All experiments were conducted in fused silica cuvettes with 1 mm optical path length under magic angle conditions (54.7 pump–probe polarization angle difference) to eliminate possible anisotropic contributions. The cuvette was continuously moved in the plane perpendicular to the direction of probe pulse propagation. RESULTS AND DISCUSSION Steady State Spectroscopy. Absorption spectra of pure CdSe490, CdSe515 and CdSe526 and the corresponding QD–alizarin complexes are depicted in Figure 1. The absorption bands related to the lowest excitonic transition (1S(e)–1S3/2(h)) of CdSe490, CdSe515 and CdSe526 are thus located at 490 nm, 515 nm and 526 nm, respectively. At > 550 nm the absorption of all pure QD samples under investigation is zero. In contrast the QD–alizarin complexes composed of CdSe490, CdSe515 and CdSe526 exhibit a similar broad absorption feature up to 650 nm which is related to the alizarin. To determine the spectral shape of the alizarin contribution, the spectra of the pure QD and the corresponding complexes have been normalized and subtracted. This subtractive procedure results in a broad alizarin absorption band with a maximum at 532 nm (Figure 1, grey line) which is independent of the QD diameter (see Figure S1 of the SI for a comparison of the alizarin absorption on differently sized QD). The spectral position of the absorption band of surface–bound alizarin is quite different from that of pure alizarin (approx. 430 nm). A spectral shift of the alizarin absorption has been already observed in TiO2– and


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Al2O3–alizarin complexes and explained by the interaction between the inorganic surface and the alizarin molecules.37–40 It is therefore rational to assume the formation of stable QD–alizarin complexes. A bidentate binding mode of the alizarin on metal oxide surfaces has been discussed previously as the energetically most favorable49–51 which may also apply for alizarin on the CdSe surface. Figure 1 further demonstrates that in all investigated complexes, the selective photoexcitation of the surface–bound alizarin at 600 nm should be possible.

Figure 1. Absorption spectra of pure QD samples with different diameters and the corresponding QD–alizarin complexes as well as the spectrum of adsorbed alizarin (grey line) calculated from the difference of CdSe490 and CdSe490–alizarin. Spectra of the pure QD and the corresponding complexes have been normalized at a spectral range (375–400 nm) where the contribution of alizarin is small. Transient Absorption Spectroscopy. Our time resolved studies aim for the investigation of the photoinduced processes after selective excitation of surface–bound alizarin at 600 nm. Steady state absorption measurements described above showed no absorption of the pure QD at that


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wavelengths. In accordance with this observation, the pure QD exhibit negligible absorption changes after excitation at 600 nm. In Figure 2 spectra of CdSe515–alizarin complexes recorded at selected delay times after photoexcitation of the alizarin are depicted. At short delay times the spectra are composed of positive absorption changes at < 500 nm which are related to the alizarin excited state absorption (ESA) and negative contributions at > 575 nm evoked by the alizarin ground state bleach (GSB) and the stimulated emission (SE). At 500–575 nm a negative and a positive band in direct proximity are observed whose origin is discussed later. At later delay times a pronounced negative band at the spectral position of the CdSe515 QD lowest excitonic transitions (1S(e)– 2S3/2(h) and 1S(e)–1S3/2(h)) appears and reaches a maximal amplitude at 50–100 ps. Additionally, a relatively sharp positive absorption at the red side of the negative band is visible. At delay times > 100 ps the described spectral features decay slowly. The spectral signature measured at 50–100 ps is very similar to that typically observed for directly photoexcited QD in transient absorption studies52–56 suggesting that the absorption changes emerge in a comparable fashion (see also Figure S4 of the SI for a comparison with data recorded after QD excitation). Former studies interpreted the TA signals of photoexcited QD by a carrier induced Stark effect57 and state filling.58 The carrier–induced Stark effect leads to a shift of all excitonic transitions after excitation of QD by the pump pulse. Consequently, the probe pulse measures shifted absorption bands leading to difference spectra with derivative–like shape. The state filling is based on the Pauli exclusion principle. The occupation of quantized electronic states leads to a bleaching of the corresponding optical transitions. Based on that, the pronounced bleach observed in our measurements is assigned to the occupation of the QD 1S(e)–state.


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Figure 2. Spectra of CdSe515–alizarin complexes at selected delay times recorded after excitation of alizarin at 600 nm. Charge Transfer–Induced State Filling. The occupation of the QD 1S(e)–state observed in our experiment can be the result of (1) direct photoexcitation of the QD, (2) excitation energy transfer or (3) electron transfer from the photoexcited alizarin. Pump pulses at 600 nm did not lead to a significant excitation the pure QD. Since the QD keep their spectral properties during complexation with alizarin, direct excitation of the QD in the complexes at 600 nm is unlikely. Additionally, direct photoexcitation of the QD populates the 1S(e)–state instantaneously what contradicts with the much slower formation kinetics of the QD bleach observed in the experiment. The energy transfer from the photoexcited alizarin to the 1S(e)–state is energetically an uphill process and hence also unlikely. However, the observed TA data are in agreement with an ET from the photoexcited alizarin to the QD 1S(e)–state. This assignment is corroborated by the quantitative decay of the alizarin signal at > 625 nm where the alizarin SE dominates. Such a quenching of SE is a typical feature of ET reactions.37–40 At the same time the alizarin GSB at 550–625 nm persists throughout the complete investigated timescale demonstrating that the alizarin ground state is not completely repopulated during the investigated process. Additionally,


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positive absorption changes appear at > 625 nm and long delay times. Although the signal is weak, the spectral position coincides well with the oxidized alizarin observed in earlier studies on the TiO2–alizarin system. In these studies an efficient ET from photoexcited alizarin to the conduction band of TiO2 was found. 37–40 The thermodynamic feasibility of the electron transfer from alizarin to the 1S(e)–state of CdSe QD is supported by the energetic positons of the involved states. According to data reported in the literature59,60 the conduction band edges of CdSe and TiO2 are comparable. Considering the photoinduced ET reaction in TiO2-alizarin mentioned above, ET from alizarin to CdSe can be expected. Excitation–Induced Stark Effect. To clarify if the positive band at the red side of the QD bleach is an alizarin excited state feature or related to the QD, additional experiments on complexes containing QD with different diameters (CdSe490–alizarin and CdSe526–alizarin) have been performed (see Figures S3 and S5 of the SI for complete spectra). Spectra of CdSe490–, CdSe515– and CdSe526–alizarin complexes at a delay time of 0.4 ps are depicted in Figure 3 (a). For better comparability the complete sets of transient absorption data are normalized to the same signal amplitude in the range of the alizarin GSB/SE at short delay times. The positive band is observable for all investigated samples but the spectral position clearly shifts with the spectral position of the QD lowest excitonic transition from 524 nm in QD490– alizarin complexes to 557 nm in QD526–alizarin. This observation gives clear evidence that the sharp positive band is related to the QD. This assignment together with the derivative–like signature of the spectra at 0.4 ps leads to the conclusion that the QD undergo an electronic level shift.


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Figure 3. Spectra of CdSe490–, CdSe515– and CdSe526–alizarin complexes recorded at (a) 0.4 ps and (b) 100 ps after excitation of alizarin at 600 nm. Data sets are normalized to the same signal amplitude in the spectral range of the alizarin GSB/SE (600–650 nm) at short delay times. This level shift occurs within the temporal resolution of the applied setup and is therefore most probably induced by the photoexcitation. Therefore, we interpret our observation in terms of an excitation–induced Stark effect. Figure 3 (b) shows spectra of CdSe490–, CdSe515– and CdSe526–alizarin complexes recorded at a delay time of 100 ps. All spectra exhibit the bleaching of the QD lowest excitonic transitions due to the ET from photoexcited alizarin to the QD 1S(e)–state. However, the amplitude of this bleach increases with the size of the QD (CdSe490 < CdSe515 < CdSe526). It is known that the QD extinction coefficient increases with the QD diameter (ε = diameter2.65). According to that, an increase in QD bleach signal with larger QD diameters is expected. Different ET efficiencies for the investigated CdSe–alizarin complexes may also contribute to the different bleach amplitudes.


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Introduction of Fit Model. To test our assignment of the observed TA signals a simple fit model is introduced to describe the TA data of the QD515–alizarin complexes at certain delay times. In this model, Gaussian functions account for the alizarin–related GSB and ESA. The Gaussians are defined by their amplitudes AGSB and AESA, their centers λGSB and λESA as well as their widths wGSB and wESA (with w = FWHM/[2×(ln(4))0.5]). For simplicity contributions of the alizarin SE and oxidized alizarin are neglected in the fit model. The contribution of the QD is approximated by the difference of two Gaussians, which are related to QD in the photoexcited complex and QD in the unexcited complex (for simplicity we neglect that the QD signal arises from different excitonic transitions). The two Gaussians are defined by their amplitudes AQD’ and AQD, their centers λQD’ and λQD as well as their widths wQD’ and wQD. The TA signal at each delay time is expected to be a superposition of all mentioned contributions leading to the formalism: ‫ۍ‬ ‫ۇ‬-0.5·൫λ-λQD' ൯൘w ‫ۊ‬ ‫ۇ‬-0.5·൫λ-λQD ൯൘w ‫ۊ‬ QD' QD ‫ێ‬ ‫ۉ‬ ‫ی‬ ‫ۉ‬ ‫ی‬ ‫ێ‬ ∆Aሺλ,tሻ= AQD' ·e -AQD ·e ‫ێ‬ ‫ێ‬ ‫ۏ‬ 2

‫ۇ‬-0.5·൫λ-λGSB ൯൘w

+AGSB ·e‫ۉ‬

GSB

‫ۊ‬ ‫ی‬

2

‫ۇ‬-0.5·൫λ-λESA ൯൘w

+AESA ·e‫ۉ‬

ESA

‫ۊ‬

2

‫ې‬ ‫ۑ‬ ‫ۑ‬ ‫ۑ‬ ‫ۑ‬ ‫ے‬

2

‫ی‬

(1) The parameters λQD, wQD, λGSB, wGSB, λESA and wESA were determined for the spectrum at 0.2 ps (shortest selected delay time). Since transient changes of these parameters are not expected, the


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obtained values were treated as constants in the fits of the spectra at all other selected delay times (fit parameters and results for each delay time are given in Table S1 and Figure S7 of the SI).

Figure 4. Spectra of CdSe515–alizarin complexes (black circles) and the results of the corresponding fits (red lines) at (a) 0.2 ps and (b) 100 ps. The fits are the superposition of alizarin GSB, alizarin ESA and QD contributions depicted at the bottom of (a) and (b). Comparison of (c) TA data recorded for CdSe515–alizarin complexes and (d) fit results. (e) Simplified reaction scheme of the photoinduced processes observed after excitation of the QD– alizarin complexes.


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Figure 4 (a) demonstrates that the spectrum at 0.2 ps is satisfactorily described if identical amplitudes for the QD in the photoexcited complex (AQD’) and QD in the unexcited complex (AQD) are used in the fit. The derivative–like signature of the QD–related signal at 500–575 nm is fitted solely by a shift of the QD absorption band (λQD → λQD’) indicating that a level shifting in the QD electronic structure is responsible for that transient absorption feature. This observation is surprising since the electronic excitation is not located on the QD but at the surface–bound alizarin molecules. It must be mentioned that trap–state absorption as possible QD excitation pathway cannot be completely excluded. However, experiments on the pure QD do not support this excitation mechanism. At > 625 nm the fit underestimates the measured negative TA signal. At short delay times, this spectral region is dominated by alizarin SE which is neglected in the fitting procedure. The lower panel in Figure 4 (a) shows the contributions of alizarin GSB, alizarin ESA and QD to the fit at 0.2 ps. The alizarin GSB and ESA are present immediately after photoexcitation and the QD contribution exhibits the characteristic derivative– like signature. At 100 ps (Figure 4 (b)) the alizarin SE disappeared whereas the alizarin GSB is still present indicative of a charge transfer reaction. At the same time the QD–related signal turned into a pronounced bleach with an increase of signal amplitude by a factor of 10. The strong QD bleach is a common feature of photoexcited QD and was explained by the population of the QD 1S(e)– state. In our case the state filling is not evoked by direct photoexcitation but by the electron transfer from the photoexcited alizarin to the QD. At > 600 nm the fit deviates from the TA signal. The weak positive TA at these wavelengths suggests the formation of a photoproduct, namely the oxidized alizarin as described above.


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A comparison of the transient absorption data and the fit result in Figures 4 (c) and (d) demonstrates that the fit model describes the measured TA data satisfactorily with a minimal set of transient species in the analyzed temporal range of 0.2–100 ps. We want to emphasize that our model is introduced to describe our data qualitatively. Conclusions based on the absolute values of the fitting parameters derived from the fitting procedure should not be drawn. Figure 4 (e) schematically summarizes the processes which are deduced from the experiments. Immediately after photoexcitation of the alizarin, a level shifting in the QD electronic structure is observed which is explained by an excitation–induced Stark effect. At later delay times the electron transfer from the alizarin to the QD leads to a charge transfer–induced state filling. This effect is not only in line with the generally accepted interpretation of CdSe QD–related transient absorption signals, but could also help to disentangle the photoinduced processes in QD–organic molecule complexes. Such complexes have been frequently applied in hybrid QD–polymer solar cells, and the electron transfer from the photoexcited organic molecule to the QD plays an important role in the functionality of these devices. Influence of Alizarin Concentration. In QD–organic molecule complexes the dynamics of the photoinduced electron transfer from QD to electron accepting molecules strongly depends on the organic molecule concentration. This observation was explained by an increase of the density of accepting states with increasing acceptor concentration.17–19 In our case, the surface–bound alizarin molecules act as electron donors and it is not clear yet, if the alizarin concentration can influence the ET dynamics. To study the influence of alizarin concentration on the ET from photoexcited alizarin to the QD 1S(e)–state, samples at different QD–alizarin ratios (see sample preparation) have been prepared and studied in transient absorption experiments. Absorption spectra of the investigated CdSe508–alizarin complexes are depicted in Figure 5 (a). Based on


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the absorption of alizarin at 570–650 nm, relative ratios of 1:1, 1:1.3, 1:2 and 1:2.5 are determined.

Figure 5. (a) Normalized absorption spectra of CdSe508–alizarin complexes at different relative ratios. (b) Transient traces at λprobe = 514 nm recorded for CdSe508–alizarin complexes at different QD–alizarin relative ratios (1:1 to 1:2.5). (c) Scaled transient traces and fit (sum of two exponentials). Excitation of the complexes at 600 nm led to the transient absorption features already described above (see Figure S6 of the SI for complete spectra). Figure 5 (b) shows the transient traces at 514 nm recorded for CdSe508–alizarin complexes with different relative ratios. As expected, the signal amplitude increases with the alizarin concentration. However, the normalized transient traces depicted in Figure 5 (c) demonstrate that the formation dynamics of the QD bleach at the


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different relative ratios appears to be highly similar. Fitting procedures of the traces with a sum of two exponentials reveal that the bleach formation is satisfactorily described with a time constant of 19 ps and does not depend on the alizarin concentration. Another time constant of 1.2 ns is needed to describe the decrease of the signal at long delay times which is most probably related to a slow recombination between the transferred electron and the alizarin cation. In that process the depopulation of the 1S(e)–state leads to the recovery of the QD ground state. In the investigated range of alizarin concentrations, significant effects on the ET dynamics cannot be observed. This is in sharp contrast to studies on QD in combination with molecular electron acceptors where the increase of accepting states increases the ET rate. Since the photoexcited alizarin acts as donor state in the QD–alizarin complexes the variation of its concentration does not influence the density of accepting states and consequently not the ET dynamics. CONCLUSION Time resolved absorption measurements showed an immediate level shifting in the QD electronic structure after photoexcitation of alizarin. This observation is explained by an excitation–induced Stark effect. The formation of a pronounced bleaching of the QD lowest excitonic transition on the picosecond time scale indicates the population of the QD 1S(e)–state during the electron transfer from the alizarin to the QD. This charge transfer–induced state filling could be a convenient tool to monitor charge transfer reactions in QD–organic molecule complexes and could help to evaluate this process not only qualitatively but also quantitatively. We could show that in the investigated range of QD–alizarin relative ratios, the concentration of the alizarin donor has no influence on the dynamics of the ET reaction.


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ASSOCIATED CONTENT Supporting Information Absorption spectra of alizarin in complexes with different QD diameters; Fluorescence of surface–bound alizarin; Transient absorption spectra of CdSe490, CdSe515 and CdSe526 after direct excitation; Transient absorption spectra of CdSe508–alizarin complexes at different relative ratios; Fit parameters and results for each chosen delay time; This material is available free of charge via the Internet at http://pubs.acs.org.” AUTHOR INFORMATION Corresponding Author *Tel.: +49 (0)69 798 29351, E–mail: [email protected]–frankfurt.de Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT This work was supported by the German Research Foundation (DFG) (WA 1850/6–1). We thank Lea Marie Totzauer for conducting steady state experiments.


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