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Size-Controllable Enhanced Energy Transfer from an Amphiphilic Conjugated-Ionic Triblock Copolymer to CdTe Quantum Dots in Aqueous Medium Chun Fang,† Bao-Min Zhao,† Hao-Ting Lu,† Li-Man Sai,† Qu-Li Fan,‡ Lian-Hui Wang,*,† and Wei Huang*,‡ Laboratory of AdVanced Materials, Fudan UniVersity, 220 Handan Road, Shanghai 200433, People’s Republic of China, and Institute of AdVanced Materials, Nanjing UniVersity of Posts and Telecommunication, Nanjing 210003, People’s Republic of China ReceiVed: August 10, 2007; In Final Form: February 25, 2008
In this work, we have studied the interaction in aqueous solution between a polyfluorene-containing amphiphilic conjugated-ionic triblock copolymer poly(2-(trimethylamino)ethyl methacrylate)-polyfluorene-poly(2(trimethylamino)ethyl methacrylate) (PTMAEMA-PF-PTMAEMA) and CdTe quantum dots with emission maxima at 550, 580, and 610 nm. We demonstrate by steady-state photoluminescence spectra, photoluminescence excitation spectra, and time-resolved fluorescence spectra that efficient fluorescence resonance energy transfer has occurred from the polymer as the energy donor to CdTe quantum dots that served as acceptors. The Fo¨rster radius and average donor-acceptor distances are estimated from experimentally measured energy transfer efficiency. We have observed that a more effective FRET process occurred in the larger-sized QDs and polymer blended systems both from steady-state fluorescence spectra and time-resolved fluorescence spectra.
Introduction π-Conjugated polymers have been receiving increasingly intense research for the past several decades, especially in optoelectronic devices applications like light-emitting diodes,1 photovoltaic cells,2 and chem- or biosensors.3 Conjugated polymers usually have high hole mobility and low electron mobility. In some applications, these properties limit the device performance.4,5 Thus, a second material is usually required to be incorporated to enhance the performance of optoelectronic devices.6 Semiconductor nanocrystals (quantum dots, QDs) are favorable classes of adjusting materials in many conjugated polymer-based devices due to their quantum confinement effect absorption, narrow photoemission, and relatively high electron mobility.7,8 In recent years, combinations of conjugated polymers and semiconductor nanocrystals have been investigated in many optoelectronic areas, such as layered-hybrid constructed lightemitting diodes and blending materials based solar cells, taking advantage of the relative narrow band gap of semiconductor nanocrystals.8-12 However, there is little understood about the interaction between conjugated polymers and semiconductor nanocrystals. Usually, the combination of conjugated polymers with nanocrystals is realized either by covalent bonding or physical mixing. Such hybrid systems would offer a possibility of controlled energy or electron transfer between polymers and nanocrystals, which is important fundamental research in the construction of devices.13,14 Fluorescence resonance energy transfer (FRET) between a fluorescent donor and acceptor is an effective photophysical analytical technique that has been extensively applied in various biological investigations and determinations of molecular distances and structures.15 Water-soluble QDs, like CdTe and † ‡
Fudan University. Nanjing University of Posts and Telecommunication.
CdSe/ZnS, whose surfaces are capped by mercapto alkyl carboxylic acid, have been demonstrated to be an excellent alternative to organic dyes for FRET-based analysis. Many reports have been focused on their wide applications as excellent donors in fluorescence FRET-based investigations.16 Clapp et al. have explored the use of QDs as FRET acceptors in QDMBP conjugates by labeling MBP with various organic dyes and found no evidence of FRET from dyes to QDs. They suggested the results were attributed to the short lifetime of the dyes and relative strong direct excitation of the QDs.17 Others further demonstrated that FRET from long-lived lanthanide complexes of terbium and europium to semiconductor nanocrystals is feasible.18 However, there are also some reports that demonstrated that QDs can be applied as effective energy acceptors with emitting polymer and quantum well (QW) donors.14,19,20 Since the ability of QDs to accept excited-state energy from other fluorescent donors is unclear, it should be necessary to clarify this contention. Here, we take advantage of the large absorption cross section of QDs, detecting the FRET process from an excited conjugated backbone-based polymer to CdTe QDs. Polyfluorene (PF) is a class of blue-light emission conjugated polymer with excellent chemical and thermal stability. A few ionic polyfluorenes have been synthesized, and their sensitivities to analytes have been investigated.21,22 Recently, we have first reported the successful synthesis of a novel well-defined amphiphilic conjugated-ionic triblock copolymer containing PF block and poly(2-(trimethylamino)ethyl methacrylate) (PTMAEMA) as the coil-like block through atom transfer radical polymerization (ATRP).23 In the present paper, we explored the FRET process of the PTMAEMA-PF-PTMAEMA/CdTe QDs system (Scheme 1) using PF block as the FRET donor and CdTe QDs as the acceptor by means of steady-state fluorescence spectra, photoluminescence excitation spectra, and time-resolved fluorescence
10.1021/jp076427a CCC: $40.75 © 2008 American Chemical Society Published on Web 04/16/2008
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SCHEME 1: Chemical Structure of PTMAEMA-PF-PTMAEMA
spectra. To our knowledge, this is the first report on the watersoluble inorganic-organic FRET system, in which QDs act as effective energy acceptors with a conjugated polymer based amphiphilic block copolymer. Experimental Section Materials. A well-defined conjugated-ionic amphiphilic block copolymer, poly(2-(trimethylamino)ethyl methacrylate)polyfluorene-poly(2-(trimethylamino)ethyl methacrylate) (PTMAEMA-PF-PTMAEMA) was synthesized by quaternization of poly(2-(dimethylamino)ethyl methacrylate)-polyfluorenepoly(2-(dimethylamino)ethyl methacrylate) (PDMAEMA-PFPDMAEMA) triblock copolymer. PDMAEMA-PF-PDMAEMA with Mn of 16 000 was prepared through ATRP initialized by a 2-bromoisobutyrate end-capped PF macroinitiator using CuCl/HMTETA (1,1,4,7,10,10-hexamethyltriethylenetetramine) as the catalyst, according to the publication we previously reported.23 The number-averaged degree of polymerization of fluorene in PDMAEMA-PF-PDMAEMA was about 10. Water-soluble CdTe QDs capped with 3-mercaptopropionic acid (MPA) were prepared according to the procedure our group previously reported by the reaction between Cd2+ and NaHTe in the presence of MPA by a method of program process of microwave irradiation (PPMI).24 Three QD populations with emission maxima at 550, 580, and 610 nm were used in our study. The other reagents were obtained from Aldrich Co. and used as received. The solutions were made in Millipore water. Optical Spectra. UV-vis spectra were recorded on a Shimadzu 3150 PC spectrophotometer. The steady-state photoluminescence (PL) spectra and photoluminescence excitation spectroscopy (PLE) were carried out on a Shimadzu RF-5301 PC spectrophotometer with a xenon lamp as a light source. The solutions for the absorption spectra measurements were prepared by diluting the stock solution to the desired concentrations. The quenching studies were carried out in situ by taking the emission spectra of the solutions at a series of quencher concentrations and comparing the PL intensities. The emission spectra (390700 nm) were taken with excitation at 375 nm. The PL and PLE spectra have been corrected for the spectral response, excitation source, and diffraction grating. Time-Resolved Fluorescence Measurement. Time-resolved fluorescence spectra were measured using an Edinburgh Instruments LifeSpec-PS spectrometer at room temperature. The LifeSpec-PS comprises a 371 nm picosecond laser (PicoQuant PDL 800B) operated at 2.5 MHz and a Peltier-cooled Hammamatsu microchannel plate photomultiplier (R3809U-50). Lifetimes were determined from the data using the Edinburgh Instruments software package. Results and Discussion Steady-State PL Spectroscopy. Figure 1 shows the PL changes when 550 nm emitting CdTe QDs are iteratively titrated
in PTMAEMA-PF-PTMAEMA aqueous solution. The blending system is excited at the 375 nm. For this spectrum, the concentration of the polymer is 5 × 10-8 M, while the concentration of QDs varies in the range from 0 to 18.9 × 10-8 M (the concentration is estimated by absorbance spectroscopy, assuming a molar extinction coefficient of 8.3 × 104 M-1‚cm-1). As the QDs are added to a solution of the polymer, we have observed a decrease in the emission of the polymer, with a concomitant increase in the emission of QDs (solid lines). We also show the photoluminescence spectra of CdTe QDs-only solutions with the same concentration used in polymer-QDs mixtures, shown in Figure 1 (dashed lines). Comparison of the QDs PL intensities to those collected from the blending samples reveals that the PL emission of CdTe QDs in polymer-QDs mixtures is enhanced after the polymer is photoexcited. However, after the QDs concentrations are high up to a certain value, the polymer emissions decline slightly. The other two PL quenching experiments using 580 and 610 nm emitting QDs showed similar fluorescence behaviors. It can be clearly seen that the polymer emission has been quenched with the similar emission profiles with no evident appearance of any new peaks, indicating the introduction of QDs cannot cause the conformational change of the polymer. It has been reported that additions of counterions may cause the aggregation of water-soluble conjugated polymers and consequently result in spectral changes and fluorescence decrease.25 However, we add Na2SO4 to PDMAEMA-PF-PDMAEMA aqueous solution, and when the [SO42-] have reached 0.01 M, the ionic strength does not significantly affect both the absorption and emission spectra except that PL intensity decreases by less than 10%, suggesting that aggregation of PDMAEMAPF-PDMAEMA is not very sensitive to ionic strength.
Figure 1. Fluorescence spectra of PTMAEMA-PF-PTMAEMA aqueous solutions by addition of aqueous solution of CdTe QDs emitting at 550 nm (solid line, the concentration of the PTMAEMA-PFPTMAEMA is 5.0 × 10-8 M) and fluorescence spectra of CdTe QDsonly solutions with the same concentration used in the polymer solutions (dashed line).
7280 J. Phys. Chem. C, Vol. 112, No. 18, 2008
Figure 2. Normalized absorption spectrum of three CdTe QD population’s (emitting at 550, 580, 610 nm) aqueous solutions and PL spectrum of the PTMAEMA-PF-PTMAEMA aqueous solution. Inset: the spectral overlap function defined as J ) PLD(λ)λ4A(λ) for three polymer-QDs D-A systems. ) 8.3 × 104, 11.8 × 104, and 14.2 × 104 M-1‚cm-1 for QD-550, QD-580, QD-610, respectively.
Therefore, the quenching of polymer emission in Figure 1 is not likely due to be aggregation-driven quenching. Since the steady-state PL of the polymer has been quenched by the addition of CdTe QDs, there must be a certain interaction between the two components when the blend system is photoexcited. Generally, the excitation interaction between a conjugated polymer and a QD has two mechanisms: (1) photoexcited charge transfer and (2) fluorescence resonance energy transfer.13,14 Charge transfer via an exchange process of charge carriers is a short-range mechanism, whereas FRET based on the Fo¨rster energy exchange mechanism can occur over longer distances.14 The efficiency of both excitation transfer mechanisms relies highly on the separation distance between the donor and acceptor.26 We previously reported that PF segments could self-assemble into hydrophobic aggregates in water due to the high immiscibility of these blocks in water, which was proved by 1H NMR measurement.23 We suggest that, in dilute aqueous solution, the polyelectrolyte coils extend outward from the PF-fold-core to form a hydrophilic outer shell, stabilizing the PF aggregates. And N+(CH3)3, the charged groups, are located in the pendant of the long hydrophilic coil. The most negatively charged CdTe QDs are in the proximity to the outer shell of the polymer micelles. The PTMAEMA chains and mercapto propyl acid coils both become a barrier around the QDs and increase the average distance between the PF segments and the CdTe QDs core. In this case, the efficient charge-transfer process occurring between the two is suppressed. On the other hand, an excellent overlap was observed between the PL emission of PTMAEMA-PF-PTMAEMA and the absorption of various sizes of CdTe dots (emitting at 550, 580, 610 nm) in Figure 2; thus, we would expect efficient FRET between these two species, which can occur over a longer distance. Photoluminescence Excitation Spectroscopy. Photoluminescence excitation spectroscopy is used as a measure to investigate the contribution of donor absorption on the emission from acceptor in a donor-acceptor (D-A) system. Figure 3 shows 550 nm PLE spectra of the composites of PTMAEMAPF-PTMAEMA and CdTe QDs as the 550 nm emitting QDs concentration increased, which are obtained by fitting the region of the QDs excitation to a pure QDs spectrum and then subtracting the fitted spectrum from the entire composite solution spectrum. The inset in Figure 3 is the 550 nm PLE spectra of pristine CdTe QDs showing a defined peak at 528 nm and a broad peak at 290 nm. We have observed that, in the absence
Fang et al.
Figure 3. Photoluminescence excitation spectra (PLE) of 550 nm emission from CdTe QDs in the PTMAEMA-PF-PTMAEMA-QDs composites aqueous solution with increased concentrations of QDs. The spectra are obtained by extracting the QDs excitation spectrum from each spectrum and correcting for background and direct QDs excitation as described in the text.
of CdTe QDs, the PLE spectrum has a negligible peak at 375 nm attributed to PTMAEMA-PF-PTMAEMA absorption. Once CdTe QDs were added into the polymer aqueous solution, the PLE peak at 375 nm increased and a peak at 290 nm associated with the absorption from the 1p2/3(h)-1s(e) exciton transition in the CdTe QDs appeared and increased with increasing QDs concentration. Then, at much higher QDs concentrations, the 375 nm peak is preserved and a new peak emerges in the PLE spectrum at 528 nm, which is attributed to the lowest energy 1s2/3(h)-1s(e) exciton transition. The PLE spectra suggest that at low CdTe QDs concentrations, the 550 nm PL emission of QDs originates from the absorption of PTMAEMA-PF-PTMAEMA. However, at high QDs concentration, the 550 nm emissions are assigned to both direct excitation of QDs and FRET from the polymer to QDs. Similar behaviors are also observed in the PLE spectra of the other two polymer-QDs systems recording 580 and 610 nm CdTe QDs emissions, respectively. These results provide a further evidence for FRET from PTMAEMA-PF-PTAMEMA to CdTe QDs. Time-Resolved PL Spectroscopy. To confirm the proposed FRET process between the polymer donors and CdTe QDs acceptors observed by steady-state PL spectra, we further carry out fluorescence lifetime decay measurements for all three D-A systems used above. The Fo¨rster energy transfer mechanism is a nonradiative process from a photoexcited donor molecule to an acceptor molecule in the close proximity, which may relax to its ground state by emitting a lower energy photon. Figure 4 shows the PL lifetime decays of the polymer donor (measured at 425 nm) for selected QDs to PTMAEMA-PF-PTMAEMA ratios for 550 nm QDs. The lifetime decay of pristine PTMAEMA-PF-PTMAEMA is fit to a biexponential function with two decay time components (τi) with various relative weights (wi). The average lifetime τav(D) ) w1τ1 + w2τ2 ) 1.3 ( 0.1 ns. However, as the QD acceptors are added in, faster decay has been found of the polymer PL lifetime, and the decay rate increases as a function of the QDs concentration. The same time-resolved PL experiments were measured for two other D-A systems using 580 and 610 nm QDs as acceptors, respectively, and we observed similar PL decay behaviors as that measured in the 550 nm QDs D-A system. The average decay lifetime τav of the polymer and relative weights of each lifetime component extracted from the fitting function are summarized in Table 1. We can find that when larger-sized QDs are in close proximity to polymer, the shortening of polymer
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Figure 4. Photoluminescence lifetime decay (probed at 426 nm) of PTMAEMA-PF-PTMAEMA aqueous solution in the presence and absence of QDs emitting at 550 nm of various concentrations.
Figure 5. Photoluminescence lifetime decay (probed at 550 nm) of CdTe QDs alone and at various QDs concentrations.
TABLE 1: Fluorescence Lifetime Measurements of PTMAEMA-PF-PTMAEMA as a Function of QDs Concentrationa
TABLE 2: Fluorescence Lifetime Measurements of CdTe QDs as a Function of QDs Concentration in the Composite Solutiona
w2 (%)
τav (ns)
QD-550 concn (× 10-8 M)
371 ( 17 329 ( 14 289 ( 11 242 ( 7
42.47 45.61 46.49 47.27
1.3 ( 0.1 1.1 ( 0.1 0.9 ( 0.1 0.7 ( 0.1
pristine 6.3 12.6 18.9
2.0 ( 0.1 2.0 ( 0.1 2.4 ( 0.1
20.46 23.57 29.44
w1 (%)
τ2 (ps)
w2 (%)
τav (ns)
QD-580 concn (× 10-8 M)
τ1 (ns)
w1 (%)
1.8 ( 0.1 1.6 ( 0.2 1.3 ( 0.1 1.2 ( 0.1
56.21 52.89 52.24 50.16
387 ( 15 318 ( 10 280 ( 11 268 ( 10
43.79 47.11 47.76 49.84
1.2 ( 0.1 0.9 ( 0.1 0.8 ( 0.1 0.7 ( 0.1
pristine 6.3 12.6 18.9
2.3 ( 0.1 2.4 ( 0.1 2.4 ( 0.1
26.10 26.95 28.83
QD-610 concn (× 10-8 M)
τ1 (ns)
w1 (%)
τ2 (ps)
w2 (%)
τav (ns)
QD-610 concn (× 10-8 M)
τ1 (ns)
w1 (%)
0 6.3 12.6 18.9
2.0 ( 0.1 1.5 ( 0.1 1.3 ( 0.1 1.3 ( 0.1
60.38 55.57 53.91 50.64
395 ( 12 321 ( 11 277 ( 11 273 ( 9
39.62 45.43 46.09 49.36
1.4 ( 0.1 1.0 ( 0.1 0.9 ( 0.1 0.8 ( 0.1
pristine 6.3 12.6 18.9
2.6 ( 0.2 2.8 ( 0.2 2.7 ( 0.1
23.76 25.38 29.16
QD-550 concn (× 10-8 M)
τ1 (ns)
0 6.3 12.6 18.9
w1 (%)
τ2 (ps)
2.0 ( 0.2 1.8 ( 0.1 1.4 ( 0.1 1.1 ( 0.1
57.53 54.39 53.51 52.73
QD-580 concn (× 10-8 M)
τ1 (ns)
0 6.3 12.6 18.9
a τ and w are time constants and relative weights of the decay i i components, respectively, while τav is the average lifetime constant.
PL decay time experienced is more pronounced. The significant decrease observed in the excited-state lifetime of the PTMAEMA-PF-PTMAEMA upon addition of QDs offers convincing evidence that FRET based on the Fo¨rster energy transfer mechanism is dominant in the excited-state interaction between PTMAEMA-PF-PTMAEMA and CdTe QDs. These results also proved that even relatively long insulating chains do not severely interrupt the energy transfer from the polymer to QDs. Investigating on the QDs fluorescence lifetime decay (probed at the wavelength of QDs emitting), for example, we find that the PL decay of 550 nm emitting QDs alone is fit to a monoexponential function with an excited-state lifetime τ(A) ) 10.2 ( 0.1 ns, but it becomes a biexponential and faster decay when the QDs are conjugated to PTMAEMA-PF-PTMAEMA through electrostatic interaction (Figure 5). The time constants and relative weights of each lifetime component are presented in Table 2. The faster decay of QDs in the blends and appearance of a second shorter time component may be a result of the aggregation of QDs in the polymer solution or complexation between the two. The exact causes are unclear due to the complicated environment of the QDs, which requires for further investigation. Moreover, we have observed a little slower decay process as the QDs concentration increased, which should be expected in a FRET process. At [QDs] ) 12.6 × 10-8 M, the
τ1 (ns)
τ2 (ns)
w1 (%)
w2 (%)
τav (ns)
10.2 ( 0.1 100 10.2 ( 0.1 9.3 ( 0.2 79.54 6.8 ( 0.2 9.6 ( 0.3 76.43 7.8 ( 0.2 10.4 ( 0.4 70.54 8.0 ( 0.3 τ2 (ns)
w2 (%)
9.9 ( 0.1 100 8.7 ( 0.3 73.90 9.9 ( 0.2 73.05 10.8 ( 0.3 71.17 τ2 (ns)
w2 (%)
τav (ns) 9.9 ( 0.1 7.0 ( 0.2 7.9 ( 0.2 8.4 ( 0.2 τav (ns)
11.4 ( 0.2 100 11.4 ( 0.2 8.9 ( 0.1 76.24 7.4 ( 0.1 10.2 ( 0.3 74.62 8.3 ( 0.3 11.2 ( 0.4 70.84 8.7 ( 0.3
a τ and w are time constants and relative weights of the decay i i components, respectively, while τav is the average lifetime constant.
fast component has a time constant τ1 ) 2.0 ( 0.1 ns that slightly increased to 2.4 ( 0.1 ns at [QDs] ) 18.9 × 10-8 M, while the time constant of the slow decay component is τ2 ) 9.6 ( 0.3 ns at the low QDs concentration, which increased to 10.4 ( 0.4 ns at a higher concentration. This observation is similar to the previous demonstration that FRET is preponderant. Fluorescence Resonance Energy Transfer between PTMAEMA-PF-PTMAEMA and CdTe QDs. The FRET process is extremely sensitive to the donor-acceptor separation distance. The transfer efficiency E has been widely applied in the measurement of donor-acceptor separation distance r and can be expressed as
E)
nR06 nR06 + r6
)
1 1 r 1+ n R0
()
6
(1)
where R0 is known as the Fo¨rster radius, which is defined as the distance at 50% energy transfer efficiency when n ) 1.27 n is the number of acceptors interacting with a single donor generally in a dye-dye, QD-dye dipole-dipole system. Here, the FRET between PTMAEMA-PF-PTMAEMA and CdTe QDs is studied in bulk solution; thus, n is the acceptor to donor (A-D) ratio.28 Equation 1 can be used to estimate the average
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Figure 6. FRET efficiency vs CdTe QDs to PTMAEMA-PFPTMAEMA ratios for three populations of QDs, respectively, for the full range of ratios we measured.
donor-acceptor central distance for a particular system. For PTMAEMA-PF-PTMAEMA and 550, 580, and 610 nm emitting CdTe QDs D-A systems, the Fo¨rster radii of about 86.2, 122.5, and 147.5 Å are estimated from eq 2, respectively, which is based on the spectral overlap between the normalized emission spectroscopy of the polymer and the absorbance of the QDs, as shown in Figure 2 (the inset plots are the spectral overlap functions defined as J ) PLD(λ)λ4A(λ) for the three polymer-QDs D-A systems).
R0 )
(
9000(ln 10)κp2QD J NA128π5nD4
)
1/6
(2)
QD is the quantum yield of the donor, and J is the integral of the spectral overlap between acceptor absorption and donor emission; NA is Avogadro’s number, nD refers to the refractive index of the medium, and κp is a parameter that depends on the relative orientation of the donor and acceptor dipoles.27 We used κp2 ) 2/3 here for randomly oriented dipoles as a constant. In this study, for CdTe QDs, a luminescence quantum yield of 22% was determined by comparing with the fluorescence emission of 2 µM quinine sulfate (Φpl ) 0.53) in 0.1 M H2SO4 solution. In our experiments, the FRET process was driven by electrostatic interaction between the donors and acceptors. The energy transfer efficiency E can be measured experimentally and is conventionally defined as
E ) 1 - (I/I0)
(3)
where I and I0 are the integrated fluorescence intensity of the donor in the presence and absence of the acceptor.27 Figure 6 shows the FRET efficiency versus CdTe QDs to PTMAEMAPF-PTMAEMA ratios for the three populations of QDs (emitting at 550, 580, and 610 nm). For 550 nm QDs, we have observed a QD concentration-dependent linear increase of FRET efficiency at the low QDs concentration. As the A-D ratios further increased (beyond ∼1.7), the plot appeared downward. However, the data from experiments using 580 and 610 nm QDs acceptors show that as the ratio was increased, the FRET rate enhancement was more pronounced than that of 550 nm QDs at the same ratio, which is consistent with the results observed in fluorescence lifetime measurements. For 580 nm QDs, when the ratio reached 1.25, the plot also becomes downward, whereas it experienced the same behavior at a lower ratio in the case of 610 nm QDs. Since the spectral overlap integrals I are ∼1.4 or 1.8 times larger for the D-A pairs of
Figure 7. Estimated average donor-acceptor separation distance r for the three polymer-QDs D-A systems, respectively.
580 and 610 nm QDs than the one for 550 nm, the much more significant quenching of polymer-larger-sized QDs systems is probably due to each having better spectral overlap. The efficient FRET process is facilitated by electrostatic attraction which brings the QDs acceptor in close proximity of the polymer donor. When QDs are increased in a close approach to the polymer, the FRET rate speeds up. However, as the amounts of QDs increased to a certain concentration, the emission from the polymer is slightly quenched, or nearly unquenched. We assume that the positive groups on the pendant chain of the polymer are neutralized gradually as QDs are increasingly added. This neutralization effect could increase the average separation between polymer and the following added QDs. And the FRET process is suppressed. This assumption is further supported by the average donor-acceptor separation distance r for the three polymer-QDs D-A systems, as shown in Figure 7. The values of r are estimated using the equations above, and the data are provided by the PL quenching experiments for all of the QDs to PF donor ratios we used. This controllable size effect suggests a feasibility to tune the degree of spectral overlap between conjugated polymer donors and QDs acceptors by simply changing the sizes of QDs. Thus, the enhancement of the rate of FRET in D-A pairs could be accomplished by size control. Conclusions In summary, we have investigated the ability of CdTe QDs to act as energy acceptors in the FRET process, with a polyfluorene-containing amphiphilic conjugated-ionic triblock copolymer PTMAEMA-PF-PTMAEMA serving as the donor. The FRET process is driven by electrostatic attraction between the cationic polymer and the anionic charged CdTe QDs. Three populations of QDs emitting at 550, 580, and 610 nm are used to interact with the polymer. The steady-state PL spectra, PLE spectra, and fluorescence lifetime measurement results reveal that an efficient FRET process has occurred from the polymer to the QDs. We have observed a more effective FRET from polymer to larger-sized QD acceptor populations for a given polymer to QDs ratio, due to an increasing spectral overlap. Fluorescence lifetime decay measurements and steady-state PL spectra have provided consistent information for the sizecontrollable enhanced FRET process. The results provide more for clarifying the ability of QDs to accept excited-state energy from a conjugated polymer in future research and also suggest FRET cannot be dramatically inhibited by long insulating chains. Acknowledgment. This work was financially supported by the National Natural Science Foundation of China under Grants
Conjugated Copolymer to CdTe QDs Energy Transfer 60537030, 30425020, 60235412, and 90406021, as well as the Shanghai Commission of Education under Grant 2004SG06 and Shanghai Leading Academic Discipline Project, Project No. B113. References and Notes (1) (a) Bakueva, L.; Musikhin, S.; Hines, M.; Chang, T.; Tzolov, M.; Scholes, G.; Sargent, E. Appl. Phys. Lett. 2003, 17, 263. (b) Bakueva, L.; Konstantatos, G.; Levina, L.; Musikhin, S.; Sargent, E. Appl. Phys. Lett. 2004, 84, 3459. (2) (a) Halls, J. J. M.; Pichler, K.; Friend, R. H.; Moratti, S. C.; Holmes, A. B. Appl. Phys. Lett. 1996, 22, 3120. (b) Rispens, M. T.; Meetsman, A.; Rittberger, R.; Brabec, C. J.; Sariciftci, N. S.; Hummelen, J. C. Chem. Commun. 2003, 2116. (3) (a) Gaylord, B. S.; Heeger, A. J.; Bazan, G. C. J. Am. Chem. Soc. 2003, 125, 896. (b) Fan, L. J.; Jones, W. E., Jr. J. Am. Chem. Soc. 2006, 128, 6784-6785. (4) Mattousssi, H.; Radzilowski, L. H.; Dabbousi, B. O.; Thomas, E. L.; Rubner, M. F. J. Appl. Phys. 1998, 83, 7965. (5) (a) Yu, G.; Heeger, A. J. J. Appl. Phys. 1995, 78, 4510. (b) Dittmer, J. J.; Marseglia, E. A.; Friend, R. H. AdV. Mater. 2000, 12, 1270. (6) Shaheen, S. E.; Brabec, C. J.; Sarcifitci, N. S.; Padinger, F.; Fromherz, T.; Hummelen, J. C. Appl. Phys. Lett. 2001, 78, 841. (7) Huynh, W. U.; Dittmer, J. J.; Alivisatos, A. P. Science 2002, 295, 2425. (8) Colvin, V. L.; Schlamp, M. C.; Alivisatos, A. P. Nature 1994, 370, 354. (9) Lee, J.; Sundar, V. C.; Heine, J. R.; Bawendi, M. G.; Jensen, K. F. AdV. Mater. 2000, 12, 1102. (10) Gaponik, N. P.; Talapin, D. V.; Rogach, A. L.; Eychmuller, A. J. Mater. Chem. 2000, 10, 2163. (11) Greenham, N. C.; Peng, X. G.; Alivisatos, A. P. Phys. ReV. B 1996, 54, 17628. (12) Huynh, W. U.; Dittmer, J. J.; Teclemariam, N.; Milliron, D. J.; Alivisatos, A. P.; Barnham, K. W. J. Phys. ReV. B 2003, 67, 115326.
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