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Photoluminescence Quenching of Colloidal CdSe and CdTe Quantum Dots by Nitroxide Free Radicals Poulami Dutta and Rémi Beaulac* Department of Chemistry, Michigan State University, East Lansing, Michigan 48824-1322, United States S Supporting Information *

ABSTRACT: Quantum dots (QDs) and other quantum confined semiconductor nanomaterials are emerging as an important class of materials for a variety of applications involving electronic processes. Here, we investigate QD−free radical interactions, focusing on II−VI semiconductor QDs (CdSe and CdTe) and TEMPO (2,2,6,6-tetramethylpiperidine-1-oxyl) and 4-amino-TEMPO (4-amino-2,2,6,6-tetramethylpiperidine-1-oxyl). Although these nitroxide free radicals have previously been shown to be efficient QD photoluminescence quenchers, we show here, using a combination of both steady-state and time-dependent photoluminescence studies, that some of these conclusions need to be revised. In particular, we show that TEMPO has no appreciable effect on the photoluminescence of II−VI QDs. Furthermore, we show that whereas 4-amino-TEMPO is indeed an efficient PL quencher for II−VI QDs (as previously reported) it is simultaneously limited by slow diffusion kinetics and low binding affinities to the nanocrystal surfaces. Our results highlight that the nature of ligand passivation of the QD surface is a very important factor in controlling the interaction between nitroxide radicals and QDs, a key aspect to consider in future applications involving such dyads. as organic batteries and photovoltaic devices.41−48 Here, we investigate II−VI semiconductor QDs (CdSe and CdTe) coupled with TEMPO (2,2,6,6-tetramethylpiperidine-1-oxyl) and 4-amino-TEMPO (4-amino-2,2,6,6-tetramethylpiperidine1-oxyl). These radicals are stable in air and are bright-colored species that are easily monitored via UV−vis spectroscopy and electron paramagnetic resonance (EPR), and they have been the focus of a few previous studies involving coupling to QDs.34−37 Using a combination of steady-state and timedependent photoluminescence (PL) quenching measurements, we find that a number of conclusions previously made about this class of materials have to be revised, most importantly concerning the quantitative efficiency of the PL quenching interaction and the qualitative mechanistic aspects underlying the overall process.

1. INTRODUCTION Quantum dots (QDs) and other quantum-confined semiconductor nanomaterials are emerging as a new class of lightemitting, light-harvesting, and charge-separation materials for applications such as solar energy conversion, light-emitting devices, or as labeling reagents in biotechnological applications.1−10 Of interest for many of these applications is the high absorption cross-sections and size-tunable energetic levels of quantum-confined semiconductor nanocrystals;11−13 furthermore, in the colloidal form, QDs are solution-processable, offering cheap, versatile, and scalable approaches for commercial applications. QD-sensitized solar cells can potentially achieve higher efficiencies than dye-sensitized solar cells due to their unique capability of multiple exciton generation.14 Importantly, the quantitative dissociation of photogenerated excitons (bound electron−hole pairs) through interfacial charge (electron or hole) transfer processes is a required step for optimizing the overall efficiency in many of these applications. A wide variety of organic as well as inorganic electron donor− acceptor dyads have been studied to investigate the interfacial electron transfer reaction following photoexcitation of QDs, including QDs coupled to molecular species like organic molecules,15−21 transition-metal complexes,20,22−26 biomolecules,27,28 and wide bandgap semiconductors.4,6,8,29−33 A limited number of studies have also looked at QD−free radical systems.34−37 Organic free radicals are interesting counterparts for studying electron transfer to/from QDs because, unlike closed-shell molecules, these generally undergo single-electron transfer steps in a reversible fashion.38−41 Nitroxides are an interesting class of stable free organic radicals that have recently been demonstrated for use in electrochemical applications such © 2016 American Chemical Society

2. EXPERIMENTAL METHODS 2.1. Chemicals. Cadmium oxide (CdO, 99%), oleylamine (OLA, 70%), hexadecylamine (HDA, 98%), 1-octadecene (ODE, 90%), selenium (Se powder, 99.5%), tellurium (Te powder, 99.8%), tri-noctylphosphine (TOP, 90%), trioctylphosphine oxide (TOPO, 99%), TEMPO (2,2,6,6-tetramethylpiperidine 1-oxyl, 98%), and 4-aminoTEMPO (4-amino-2,2,6,6-tetramethylpiperidine-1-oxyl, 97%) were purchased from Sigma-Aldrich. Stearic acid (SA, 98%) was purchased from TCI chemicals. Tetradecylphosphonic acid (TDPA, >99%) was purchased from PCI Synthesis. Acetone (99.9+% HPLC grade), methanol (99.93%, HPLC grade), hexanes (ACS grade), and toluene Received: November 12, 2015 Revised: January 17, 2016 Published: January 19, 2016 1076

DOI: 10.1021/acs.chemmater.5b04423 Chem. Mater. 2016, 28, 1076−1084

Article

Chemistry of Materials

Figure 1. Absorption spectra of (a) CdSe and (b) CdTe QDs studied here (diameters listed in Table 1). 2.5. Quenching Experiments. For PL measurements, 3−5 μM QD solutions in toluene were used. Stock solutions of TEMPO and 4amino-TEMPO free radicals were prepared in toluene. Small volumes (∼10−50 μL) of the free radical solutions were added to the QD suspensions, and the PL intensity and lifetime were measured. The suspensions were continuously stirred during the experiment, and the data were recorded after the signal stabilized after each radical addition. The raw data obtained from the PL experiment had to be corrected for excitation light screening and reabsorption of emission by the radicals. The correction was conducted by adapting the procedure described by Credi et al.51 Over time, it was observed that the PL quenching depended somewhat on the initial quantum yield (QY) of the QDs (as discussed in the Supporting Information); care was taken to ensure that the initial QY of the QDs used in the study was similar (20−25%). Time-resolved PL decays of these QD and QD−radical solutions were found to be non-single-exponential,52−56 and the decay curves were fit using a Kohlrausch−Williams−Watts (KWW) stretched exponential function (eq 1).57 Here, I(t) and I0 are the PL intensities at time t and time zero, respectively. β and τKWW are the stretch factor and KWW decay time, respectively; Γ(x) is the gamma function. The average excited-state lifetimes ⟨τ⟩ are determined directly from the fit (eq 2); these parameters are taken as meaningful quantifiers for the ensemble average lifetime values without any assumptions regarding the nature of the underlying distribution of rate constants.57

(ACS grade) were purchased from Macron Fine Chemicals. All chemicals were used as received without any further purification. 2.2. Synthesis of CdSe and CdTe Quantum Dots. The syntheses of the QDs were carried out by adapting standard literature protocols49 and modifying the methods to obtain the required QD sizes for each study. The syntheses were conducted under a N2 atmosphere using standard Schlenk-line techniques. We give here the details of a typical synthesis. To a 25 mL round-bottomed flask, 0.19 g (0.4 M) of selenium (Se) was added, and the flask was sealed, evacuated, and then kept under a N2 atmosphere. To this was added 5 mL of TOP. The solution was stirred at room temperature until all of the Se had dissolved in TOP to give a clear solution. To a 100 mL three-necked flask were added 0.051 g (0.4 mmol) of CdO, 2.845 g (10 mmol) of SA, 13 mL (∼42.5 mmol) of OLA, and 9.3 mL of ODE, and the flask was then sealed. The flask was then evacuated for 15 min, after which it was slowly heated to 250 °C under N2 flow. After CdO was completely dissolved (with an optically clear solution obtained), the TOP−Se solution was injected swiftly into the reaction flask. After the injection, the nanocrystals were allowed to grow for different time intervals ranging from 2 to 30 min depending on the desired nanocrystal size. After the reaction was over, the heating mantle was removed and the flask was allowed to cool to the room temperature. The QDs were extracted by centrifuging the contents of the reaction flask after adding toluene and methanol in a 1:4 ratio. The clear supernatant solution was removed as the QDs decanted to the bottom of the flask. The QDs were then suspended into a small volume of hexane and annealed in order to obtain better surface passivation (vide infra). Cadmium telluride QDs were synthesized50 similarly using CdO and TDPA instead of SA and TOP−Te (tellurium dissolved in TOP) instead of TOP−Se. The CdTe QDs were also annealed in the same manner as mentioned below. 2.3. Annealing Procedure. To a 50 mL 3-necked flask were added 2.2 g of TOPO and 1.2 g of HDA, and the mixture was heated at 130 °C under vacuum for 1 h. The QDs suspended in hexane were then injected to the reaction mixture under nitrogen. The flask was again placed under vacuum to remove the hexane, after which 2.5 mL of TOP was added. The QDs were annealed at 130 °C for 2 to 3 days. Finally, the QDs were washed with a toluene/methanol mixture as described above and then suspended in toluene. 2.4. Optical Spectroscopy. Measurements were performed on solutions of nanoparticles dispersed in toluene in 1 × 1 cm cuvettes. Absorption spectra were collected on an OLIS17 UV/vis/NIR spectrophotometer with 1 nm increments and solvent background subtraction. For steady-state luminescence experiments, the emission of the QDs was monitored using a HORIBA Jobin Yvon spectrometer (iHR500, 150 grooves/mm grating blazed at 500 nm) with a CCD detector (SYMPHONY II, liquid N2 cooled). The lifetime measurements were recorded using a time-correlated single-photon counting (TCSPC) set up. The data acquisition card (DAQ) is from Edinburgh Instruments (TCC900). The laser used for the experiment is a 405 nm pulsed laser from Picoquant (LDH-D-C-405M, CW-80 MHz). The detector is a photomultiplier tube (PMT) from Hamamatsu (H742240). Absolute photoluminescence quantum yields were measured using the Hamamatsu absolute PL quantum yield spectrometer (C11347).

⎛ −t ⎞ β I(t ) = I0·exp⎜ ⎟ ⎝ τKWW ⎠ ⟨τ ⟩ =

1 ⎛1⎞ ·Γ⎜ ⎟· τKWW β ⎝β⎠

(1)

(2)

3. RESULTS AND DISCUSSION 3.1. QD Photoluminescence Quenching. The UV−vis absorbance spectra of CdSe and CdTe QDs used for the experiment are shown in Figure 1. The lowest excitonic transition energies, derived from the maximum of the first exciton absorption peak, were used to estimate the average nanocrystal diameters, extinction coefficients, and concentration of the QD suspensions by using empirical calibration curves correlating the exciton peak energy with nanocrystal size.58,59 A list of all QD samples used in this study is given in Table 1. The structures of the two nitroxide free radicals used in this study, TEMPO and 4-amino-TEMPO, are shown in Scheme 1. As shown in Figure 2, these radicals are colored species, weakly absorbing in the green part of the optical spectrum. Upon mixing of both radicals with the QD suspensions in toluene, the QD PL intensity is found to decrease (Figure 3), with different quenching efficiencies for each radical, consistent with previous reports. Whereas 175 mM TEMPO is required to 1077


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nonlinear quenching effect.34,37 In fact, we see here that, after correcting for each effect listed above, TEMPO is not an efficient PL quencher, contrary to conclusions drawn previously.34,35,37 A weak linear quenching effect is still observed at very high TEMPO concentrations, but it is essentially negligible (I0/I ∼ 2 at [TEMPO] ∼ 200 mM, corresponding to a 40000:1 TEMPO/QD ratio) compared to that with 4-amino-TEMPO, which is most likely due to small ligand displacement effects leading to the formation of surface traps rather than direct electronic quenching effects involving TEMPO itself. TD photoluminescence quenching experiments for one sample of CdSe QDs are shown in Figure 5 (other samples are given in Figures S8−S13); for all QD samples, we observed that the overall PL decay rate is essentially unchanged for QDs mixed with TEMPO, whereas a strong reduction of the overall QD PL decay rate is seen upon addition of 4-amino-TEMPO; these effects are consistent with the CW data presented above and will be further discussed in the next section. 3.2. Mechanistic Analysis of the Quenching Processes. Interestingly, the mechanism of PL quenching of QDs by TEMPO and 4-amino-TEMPO has not been determined in previous studies. PL quenching processes can arise from one of two general mechanistic pathways, either energy or electron transfer processes. We note in passing that the third mechanism invoked previously, “spin-flip” quenching, is not a general path but merely a subclass of either energy or electron transfer processes (such as Dexter energy transfer processes); furthermore, with the excitonic recombination in chalcogenide QDs being fully spin-allowed, it is unlikely that spin-dependent processes are required for efficient PL. We begin by quantitatively investigating the likelihood of a dipolar energy transfer mechanism, also known as Förster resonant energy transfer (FRET).9,10,60,61 For each of the five QD samples listed in Table 1, the Förster radius was calculated from the spectral overlap of the donor (CdSe or CdTe QD) PL and acceptor (nitroxide free radical) absorption spectra (Figure 2)

Table 1. QD Suspensions (in Toluene) Used in This Study

i ii iii iv v

QD

diameter (nm)

concentration (μM)

PL quantum yield, ϕQD

CdSe CdSe CdSe CdTe CdTe

2.5 3.4 5.3 3.9 5.0

4.9 3.5 3.7 3.3 3.6

0.24 0.28 0.22 0.19 0.17

Scheme 1

reduce the QD PL intensity to about 40% of its original value, 11 mM 4-amino-TEMPO is sufficient to achieve the same level of PL quenching. For comparison, the red curve in Figure 3a shows the extent of quenching achieved by adding ∼11 mM TEMPO to a 2.5 nm CdSe QD suspension; similar experiments conducted on the other CdSe QD sizes show that in all cases 4amino-TEMPO quenches the QD PL more efficiently than TEMPO (Figures S1−S6). We compare quantitatively the quenching efficiency of the two nitroxide radicals in Figure 4 using Stern−Volmer plots (I0/I)CW as a function of radical concentration, where I0 is the QD PL intensity in the absence of the radical and I is the PL intensity after each addition of the nitroxide radicals; the CW superscript indicates that this ratio is obtained from steady-state PL experiments (as opposed to time-dependent, TD, measurements presented later). Obtaining reliable Stern−Volmer ratios is not a straigthforward task. At the excitation wavelength used here (λexc = 450 nm), the nitroxide free radicals can absorb an appreciable amount of the excitation beam (inner-filter effect). Furthermore, the light emitted by QDs can be reabsorbed to varying extents by the radicals. Finally, because each sample is prepared by adding a precise amount of a radical solution to the QD suspension, the measured PL intensity has to be corrected by a dilution factor. Each of these effects (excitation screening, emission reabsorption, and dilution factor) needs to be properly taken into account. It is interesting to note that the uncorrected Stern−Volmer curve shown for TEMPO in Figure 4 is essentially identical to that reported previously, which was then described as a

R0 =

6

2 9 ln 10 κ ϕQD J 128π 5NA n 4

(3)

where NA is the Avogadro constant, κ2 is the orientation factor (taken as the isotropic limit of 2/3 here), ϕQD is the PL quantum yield of the donor, n is the solvent index of refraction (ntoluene = 1.4941), and J is the spectral overlap between the normalized PL spectrum of the donor and the absorption spectrum (in molar absorptivity units) of the acceptor.

Figure 2. Absorption spectra of radicals TEMPO (T) and 4-amino-TEMPO (AT) and photoluminescence spectra of (a) CdSe and (b) CdTe QDs (diameters listed in Table 1). 1078


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Figure 3. PL intensity (after correction; described in the text) of 2.5 nm CdSe QDs with addition of TEMPO (a, b) or 4-amino-TEMPO (c, d). The broken lines are a guide to the eyes.

Figure 4. Stern−Volmer plot for quenching 2.5 nm CdSe QDs (sample i) with (a) TEMPO and (b) 4-amino-TEMPO, before and after correcting for the inner-filter and reabsorption effects. The broken lines are a guide to the eyes.

Figure 5. PL lifetime decay curves, normalized at t = 0 ns for 2.5 nm CdSe QDs (sample i) with (a) TEMPO (0−175 mM) and (b) 4-aminoTEMPO (0−12 mM).

As listed in Table 2, all QD−free radical dyads present small Förster radii, smaller than the average QD radii in each case, indicating that the FRET mechanism is not likely to be responsible for the efficient PL quenching observed here. The rather inefficient FRET between QDs and the nitroxide radicals investigated here is mostly due to the very small extinction coefficient of these radicals, as well as to the small spectral overlap, which is especially the case for larger CdSe QD sizes. For large CdTe QD samples, the spectral overlap between the QD PL and the nitroxide absorption spectra is almost negligible, implying that FRET cannot operate in this case. Nevertheless, substantial quenching is still observed for 5.0 nm

CdTe QDs and 4-amino-TEMPO (Figure S6), clearly demonstrating that FRET cannot be the dominant mechanism for the observed quenching. Because the transition observed around 475 nm is the lowest electronic excited state in these nitroxide radicals,62,63 other energy transfer mechanisms that do not depend on strong spectral overlap (higher multipole or exchange/Dexter energy transfer mechanisms) cannot be invoked here; furthermore, similar ligands but without the radical functionality (vide infra) do not show any signs of PL quenching, leading us to conclude that an electron transfer process must be the acting mechanism for the PL quenching of QDs by nitroxide free radicals. While further studies to 1079


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quenching processes are characterized by strong variations of the ensemble’s PL decay dynamics as the concentration of the quencher ([R]) is varied. In instances where both static and dynamic are simultaneously acting, both contributions (static and dynamic) to the overall quenching observed in the CW experiments can be from the CW and TD PL measurements

Table 2. FRET Parameters for Different QD−TEMPO/4Amino-TEMPO Dyads QD sample i ii iii iv v

2.5 3.4 5.3 3.9 5.0

nm nm nm nm nm

CdSe CdSe CdSe CdTe CdTe

overlap integral, J (M−1·cm−1·nm4) 4.7 2.1 0.2 0.8

× 1011 × 1011 × 1011 × 1011 −

Förster radius, R0 (nm)a 1.0−1.3 0.9−1.2 0.6−0.8 0.8−1.0 −

⎛ I0 ⎞CW ⎛ I0 ⎞t = 0 ⎛ I0 ⎞TD ⎜ ⎟ =⎜ ⎟ ×⎜ ⎟ ⎝I⎠ ⎝I⎠ ⎝I⎠

(4)

where (I0/I) is obtained by first normalizing the PL decay curves at time zero and then time-integrating the PL decay curves; the static contribution is extracted from the ratio of the CW and TD Stern−Volmer ratios

a

TD

The lower bound value is calculated using eq 3 directly, assuming that all QDs emit with the average quantum yield given in Table 1; the upper-bound value is calculated by assuming that all bright QDs emit with 100% quantum yield, that is, that the average quantum yield values reported in Table 1 correspond to the ratio of bright QDs vs bright + dark QDs.

⎛ I0 ⎞dynamic ⎛ I0 ⎞TD ⎜ ⎟ =⎜ ⎟ ⎝I⎠ ⎝I⎠

elucidate the nature and direction of the charge transfer process (oxidative or reductive quenching process) go beyond the scope of the present article, some insight on the general feature of that process can clearly be obtained from the analysis of the TD quenching data. Mechanistically, electron transfer quenching processes can be classified according to two limiting scenarios, typically designated as the static and dynamic regimes.64 Static quenching processes imply overall quenching rate constants that are several orders of magnitude larger than the intrinsic rate constant for the excited-state decay of the emitter, as well as a preassociation of the quencher to the emitter. In this case, quenching occurs with unity quantum yield, and the CW Stern−Volmer ratio reports on the fraction of the ensemble of emitters that is associated with the quencher. Importantly, the intrinsic rate constant of static quenching processes cannot be extrapolated from PL quenching experiments, as every emitter associated with a quencher becomes experimentally silent (dark). In the dynamic regime, the overall quenching rate constant is of a similar order of magnitude as the intrinsic excited-state lifetime of the emitter. Dynamic quenching processes are often associated with diffusion-limited quenching mechanisms, but it is important to note that uncompetitive quenching processes relative to intrinsic recombination (such as would occur for slow interfacial electron transfer between preassociated donor and acceptor species) would also lead to a dynamical quenching signature in the absence of diffusionlimited dynamics, which is the only scenario under which intrinsic quenching rate constants could be extracted from Stern−Volmer analyses. The two limiting scenarios are straightforwardly differentiated by TD PL measurements: static quenching processes do not lead to changes of the ensemble’s PL decay dynamics (as the fraction of unquenched emitters decays with the same overall rate constant), whereas dynamic

⎛ I0 ⎞static ⎛ I0 ⎞CW ⎛ I0 ⎞TD ⎜ ⎟ = ⎜ ⎟ /⎜ ⎟ ⎝I⎠ ⎝I⎠ ⎝I⎠

(5)

CW

The total quenching (I0/I) obtained from CW PL data is generally a product of the static and dynamic components ⎛ I0 ⎞CW ⎜ ⎟ = (1 + kqτ0[R])(1 + K a[R]) ⎝I⎠

Ka =

[QD‐R] [QD][R] (6)

where the first term involves the dynamic quenching and is equal to the slope obtained from the linear plot of (I0/I)TD vs quencher concentration since ⎛ I0 ⎞TD ⎜ ⎟ = 1 + kqτ0[R] ⎝I⎠

(7)

and the second term involves the static quenching component. kq is the dynamic quenching constant, and τ0 is the intrinsic lifetime of the CdSe QD in the absence of any radical. As predicted from equation above, the CW quenching plot has a [R]2 dependence that leads to the slight upward curvature of the plot at higher radical (quencher) concentrations (as seen in Figure 4). Thus, the static component can be derived from (I0/I )CW (I0/I )TD

= (1 + K a[R ])

(8)

The slope of such a plot yields a relative association constant (Ka) for the formation of the complex between the CdSe QDs and the radicals during static quenching. However, factors like the concentration of the native ligands (free/bound to the QD surface) in solution and the equilibrium between QD-native ligands and QD-radicals play an important role in determining

Figure 6. Stern−Volmer plots for the PL quenching of 2.5 nm CdSe QDs. (a) Total quenching for both TEMPO and 4-amino-TEMPO. (b) Dynamic and static quenching contributions for 4-amino-TEMPO. 1080


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Chemistry of Materials the relative rates of static quenching process, and Ka is expected to depend directly on such factors. For illustration, Stern−Volmer quenching analyses are reported in Figure 6 for one size of CdSe QDs (2.5 nm). As concluded before, TEMPO is not an efficicient quencher and will not be further discussed here. 4-Amino-TEMPO is seen to be a relatively better quencher, but, overall, it does not outcompete intrinsic recombination. Interestingly, the results in Figure 6b show that for this radical both static and dynamic components contribute to the total quenching. The static quenching component observed here implies an ultrafast quenching process (in this case, given the time resolution of our PL setup, 100 mM). Importantly, we note that the addition of excess HDA does not affect the intrinsic QD PL lifetime or the absolute quantum yields (Table S3) significantly, showing that the addition of excess native ligands does not perturb the nature of the QD surface. At very high HDA concentrations (>100 mM), the static component can be practically eliminated, as shown by the data in Figure 9 (300 mM excess HDA), where the Y axis reports

another result seemingly at odds with previous studies on these radicals, where values of ∼106 M−1 were reported.34−36 The values found here are nevertheless consistent with those reported for many other amine-CdSe QDs systems and are in line with the dynamical ligand exchange that is known to occur at the surface of II−VI QDs.65−75 Control experiments were also conducted to compare the quenching efficiency of 4amino-TEMPO against cyclohexylamine (structurally similar to 4-amino-TEMPO but without the NO• free radical) and aniline (Figure S14), which support the values of Ka reported here for 4-amino-TEMPO. Finally, the association constants found here are also fully consistent with electron paramagnetic resonance (EPR) data collected on the same samples used for quenching experiments (Figure S17), which do not demonstrate the presence of any broadening in the radical spectra, no matter the ratio of [radical]/[QD] used; we note, though, that in experiments where traces of phosphonic acid were introduced in the samples, broadening of the EPR spectra of 4-aminoTEMPO was seen to occur, which could perhaps explain the difference between our results and earlier studies where significant EPR broadening was observed and attributed to

Figure 7. Stern−Volmer plots for (a) CdSe QD-TEMPO and (b) CdSe QD-4-amino-TEMPO, measured for different QD sizes: i, 2.5 nm; ii, 3.4 nm; and iii, 5.3 nm. 1081


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Figure 8. Stern−Volmer plots for (a) total PL quenching and (b) dynamic quenching for 4.0 nm CdSe QDs + 4-amino-TEMPO with different amounts of native ligands added to the suspension: i, no excess HDA added; ii, 2 mM HDA added; iii, 20 mM HDA added; and iv, 200 mM HDA added.

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.chemmater.5b04423. Additional PL quenching, EPR, PL quantum yield, and characterization data (PDF)

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Figure 9. Static Stern−Volmer analysis for CdSe QDs + 4-aminoTEMPO with 300 mM excess HDA, for different QD diameters: i, 2.8 nm; ii, 3.6 nm; iii, 4.3 nm; and iv, 7.2 nm.

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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the efficiency of the static quenching component and the X axis is now reported in [R]/[QD] ratio units. Interestingly, under this regime, the Stern−Volmer static quenching rate is clearly independent of the QD size, which further supports the claim that quantum-confinement effects are marginally involved, if at all, in the overall quenching process. From this observation and using NMR spectroscopy to directly quantitate the concentrations of the free (i.e., unbound) ligands in solution, the effective equilibrium constants (Ka) obtained from the Stern− Volmer analysis in Table 3 can be converted into the overall equilibrium constants given by eq 10 (Keq), as detailed in the Supporting Information (Section 9), showing that for all QD sizes Keq ∼ 0.8. This shows that amino-functionalized nitroxide radicals have very similar binding affinities as those of the HDA ligands used to cap the quantum dots in this study and that nanocrystallite size impacts very little, if at all, the equilibrium dynamics studied here.

ACKNOWLEDGMENTS This research was supported by MSU and its Center of Research Excellence on Complex Materials (CORE-CM). We thank Prof. John McCracken (EPR) and Dr. Daniel Holmes (NMR) for their help and for useful discussions.

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REFERENCES

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4. CONCLUSIONS We have studied the quenching of CdSe QDs with two nitroxide free radicals, TEMPO and 4-amino-TEMPO. We have shown that previous reports claiming a high quenching efficiency for TEMPO were wrong, most likely due to PL intensity correction issues at the high quencher concentrations typically used. 4-Amino-TEMPO is indeed an efficient PL quencher, but it is simultaneously limited by slow diffusion kinetics and low surface binding affinities. Our results highlight that the nature of ligand passivation of the QD surface is a very important factor in controlling the interaction of nitroxide radicals with QDs, a key aspect to consider in future applications involving such dyads. 1082


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