Article pubs.acs.org/JPCC
A Study of the Binding of Cyanine Dyes to Colloidal Quantum Dots Using Spectral Signatures of Dye Aggregation Eric A. McArthur, Jacqueline M. Godbe, Daniel B. Tice, and Emily A. Weiss* Department of Chemistry, Northwestern University, 2145 Sheridan Rd., Evanston, Illinois 60208-3113, United States S Supporting Information *
ABSTRACT: This paper describes the use of the spectral signatures of near-infrared (NIR) absorbing cyanine dyes to quantitatively analyze their intermolecular interactions upon adsorption to colloidal CdSe quantum dots (QDs) with diameters of 2−3 nm. Spectroscopic characterization of the disaggregation of two types of sulfonate-functionalized cyanine molecules, IR783 and IR820, from H-aggregate dimers to monomers upon addition of methanol yields spectral signatures of aggregation used to analyze the response of the dyes to exposure to CdSe QDs. The spectrally distinct absorbances of the cyanines and QDs enable a factor analysis procedure that decomposes the absorbance spectrum of the QD/ cyanine mixture into three distinct componentssolution-phase cyanine molecules (in monomer and H-aggregate form), QDbound cyanine monomers, and disordered, QD-bound cyanine aggregatesas a function of the molar ratio of cyanine to QD. The presence of these three distinct components strongly suggests that cyanines initially bind to QDs as either disordered aggregates (for small molar ratios of QD:cyanine) or as monomers (for large molar ratios of QD:cyanine). Quantitative analysis of the adsorption motifs of cyanine dyes on nanocrystalline semiconductors is a first step in understanding the influence of binding geometry on the rate and mechanism of charge transfer across the organic−inorganic interface within cyanine-sensitized photoconversion materials.
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affinity for the Cd2+ ions on the surfaces of the QDs and therefore serve as attachment points for chemisorptions of the dyes onto the QDs,1,2 and (ii) the spectrally distinct absorbances of the cyanines and QDs allow us to use a factor analysis procedure to decompose the cyanine absorption spectrum into three distinct components: solution-phase cyanine molecules, QD-bound cyanine monomers, and disordered, QD-bound cyanine aggregates. Our analysis yields the quantitative relationship between the composition of the QD/cyanine mixture (molar ratio of cyanine to QD) and the relative mole fractions of species in these three aggregation states. Organic dye molecules adsorbed to nanocrystalline semiconductors are critical components of two major types of thirdgeneration solar cells (low-cost alternatives to bulk semiconductor photovoltaic devices): dye-sensitized solar cells (DSSCs)3−8 and nanocrystal-based thin film cells.9,10 These devices rely on efficient photoinduced charge transfer across the nanostructured organic−inorganic interface. The rate and yield of interfacial charge transfer depends on relative orbital energies of and electronic coupling between the donor and acceptor and the lifetime of the photoinduced excited state of the light-harvesting component; these parameters depend on
INTRODUCTION This paper describes the use of the spectral signatures of nearinfrared (NIR) absorbing cyanine dyes to quantitatively analyze their intermolecular interactions upon adsorption to colloidal CdSe quantum dots (QDs) with diameters of 2−3 nm. We spectroscopically observe the disaggregation of two types of sulfonate-functionalized cyanine molecules, IR783 and IR820 (Chart 1), from H-aggregate dimers to monomers upon the addition of CdSe QDs. We chose this particular combination of dyes and QDs because (i) the sulfonate groups have a high Chart 1
Received: January 15, 2012 Revised: February 20, 2012 Published: February 21, 2012 © 2012 American Chemical Society
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Figure 1. Representative data sets for cyanine dye disaggregation induced by either methanol (A, B) or QDs (C, D), where the red spectra correspond to monomers and the green spectra correspond to monomer/H-aggregate dimer mixtures in solution equilibrium. (A) The spectrum of 0.6 μM IR820 in CH2Cl2 (green) evolves to the red spectrum upon addition of methanol (final concentration = 356 mM). (B) The spectrum of 0.8 μM IR783 in CH2Cl2 (green) evolves to the red spectrum upon addition of methanol (final concentration = 260 mM). (C) The spectrum of 1.5 μM IR820 in CH2Cl2 (green) evolves to the red spectrum upon addition of CdSe QDs with λabs = 541 nm (final concentration = 1.3 μM). (D) The spectrum of 1.4 μM IR783 in CH2Cl2 (green) evolves to the red spectrum upon addition of CdSe QDs with λabs = 503 nm (final concentration = 1.0 μM).
combinations of dyes are adsorbed to a semiconductor substrate in order to harvest a broad portion of the solar spectrum.4−7,14,23−25 Charge injection from a variety of cyanine dyes into TiO2, SiO2, and SnO2 nanocrystalline substrates occurs on the time scale of hundreds of femtoseconds to tens of picoseconds,17,30,31 and the short-circuit photocurrents for 4 μm thick solar cells with combinations of dicarboxylated cyanine-sensitized nanocrystalline TiO2 exceed that of Ru coordination complexes.3,4,7,21,22 Cyanine-based DSSC’s using TiO2 and ZnO electrodes have achieved power conversion efficiencies greater than 7%,6,14,32 but making these sensitizers competitive with the Ru complexes in solar energy conversion applications requires a more sophisticated understanding of the mechanisms of dye aggregation on semiconductor nanoparticle surfaces, and how this aggregation affects the light absorption, excited state decay, and photoinduced degradation processes of the dye,14 and the dye-to-semiconductor electron injection process, which depends sensitively on the chemical and electronic structure of the substrate−dye interface.4,23,11,31,33−35 The complexity of the cyanine−nanocrystal binding equilibrium, although advantageous for light harvesting, provides a challenging system for which to map observed interfacial electron transfer rates to the chemical and structural parameters of the interface. Here, we provide a starting point for such studies by using the optical signatures of dye aggregation and disaggregation to isolate various binding motifs of IR783 and IR820 (Chart 1) to CdSe QDs as a function of the QD−cyanine ratio. Our method exploits the sensitivity of the ground state absorption of cyanine dyes to
the geometry of the adsorbate, the mode of the adsorption (chemical or physical), and the intermolecular interactions of adsorbates on the surface.11 Cyanine dyesan important class of organic NIR-absorbing dyes developed originally as photographic sensitizers12−16 and now commercially available for use as laser dyes, mode-lockers, initiators in photopolymerization, and sensitizers in biological imaging of cells and living organisms12,13,15−20are excellent molecules with which to study adsorption motifs because their various aggregation states have distinct spectral signatures. Cyanines have high oscillator strengths ( f = 1.02 for diethylthiacarbocyanine, compared with f = 0.140 calculated for Ru(II)(bipyridyl)3 and f = 0.197 for the N3 Ru(II) dye4) and wide range of oxidation and reduction potentials;13 these properties allow for thinner light-harvesting layers in solar cells than with Ru dyes and, possibly, the use of a redox couple other than iodide/triiodide.3,4,6,7,14,17,21−24 Diverse classes of cyanine dyes functionalized to change their optical properties (one strategy is to lengthen the characteristic methane chain to shift the spectrum to lower energy25) can be produced in good yields by metal-catalyzed couplings26,27 and “click” reactions.28 Cyanine dyes have relatively narrow spectral bandwidths21 but compensate for their lack of spectral coverage as monomers by forming dimers or oligomers, in solution and when adsorbed onto solid substrates. The formation of aggregates shifts and/or broadens their absorption spectra.4,6,24 The ability of cyanines to form hetero-oligomers with other cyanines (mixed dimers are formed in aqueous solutions of certain dye pairs)29 makes them excellent candidates for cosensitization schemes, in which 6137
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Figure 2. Addition of CdSe QDs with λabs = 541 nm to 1.5 μM IR820 in CH2Cl2 (full set of spectra shown in Figure 1C) can be split into two distinct regimes. (A) Spectra acquired in regime 1, where the [QD] is ≤10−7 M, reveal the presence of the composite monomer/H-aggregate dimer spectrum (green) and another, as yet unidentified, component (component 2, blue). These two spectra share two isosbestic points (circles, 766 and 884 nm). (B) Spectra acquired in regime 2, where the [QD] is ≥10−7 M, reveal the presence of component 2 (blue) and a new component (component 3, red). These two spectra share two isosbestic points (circles, 812 and 861 nm).
monomer-like species present at the conclusion of QD addition is not the solution-phase monomer produced by simple disaggregation. We can, however, identify a pair of isosbestic points when we split the set of QD−dye spectra in Figure 1C,D into two regions: [QD] < 10−7 M and [QD] > 10−7 M (the concentration of dye of 1.5 × 10−6 M). Figure 2A,B illustrates this process for IR820, where the blue spectra correspond to [QD] = 10−7 M. This observation suggests that the complex QD−cyanine equilibrium comprises three total components but that at any given concentration of QDs no more than two of those components exist simultaneously. In the regime of [QD] < 10−7 M, the spectrum corresponding to solution phase monomers and H-aggregate dimers (green in Figures 1 and 2) is present along with an as yet unidentified component 2 (blue spectrum in Figure 2A). In the regime of [QD] > 10−7 M, component 2 is present along with an as yet unidentified component 3 (red spectrum in Figure 2B). In order to identify the species contributing to the spectra of the QD/cyanine system, we used a factor analysis scheme in which we decomposed the total absorptivity at each wavelength and QD concentration, ελc, into a sum of n component absorptivities, εi, weighted by their corresponding mole fractions, xi (eq 1).43
their local environment to construct a complete mechanistic picture of cyanine adsorption on colloidal QDs.
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RESULTS AND DISCUSSION IR783 and IR820 Dyes Exist in Equilibrium between Monomers and H-Aggregated Dimers in CH2Cl2. The absorbance spectra of both IR820 and IR783 in CH2Cl2 contain characteristic features6,19,36−38 of aggregation for cyanine dye molecules (Figure 1). Both J- and H-type aggregates have been observed for cyanine dyes in solution and adsorbed on semiconductor surfaces. 6,11,39 In general, the type of aggregation is influenced by the viscosity, polarity, salt content, and temperature of the medium or the nature of the dye− substrate binding.12,17,19,24,29,40−42 In CH2Cl2, cyanines primarily form H-aggregates. The addition of millimolar concentrations of methanol to the cyanine solutions in CH2Cl2 results in controllable disaggregation of the cyanine H-aggregates (Figure 1A,B). The prominent isosbestic points (λmonomer = λdimer) at 739 nm for IR783 and at 761 nm for IR820 indicate the presence of two-state systems composed of monomers (red spectra) and H-aggregate dimers.17 The green spectra in Figure 1A,B contain contributions from both monomer and dimers in solution equilibrium. The QD−Cyanine Equilibrium Includes Three Components. When aliquots of CdSe QDs are added to a solution of IR820 or IR783 in CH2Cl2 (Figure 1C,D), the intensity of the most distinct peak associated with the H-aggregate dimer (693 nm for IR783 and 720 nm for IR820) decreases, and the intensity of the monomer peak (796 nm for IR783 and 835 nm for IR820) increases, as in the case of methanol addition. We can therefore conclude that the QDs induce disaggregation of H-aggregates of the dyes (the spectrum of the QDs in the mixture is unchanged from that of the isolated QDs). The disaggregation behavior upon addition of QDs is different, however, than that observed upon addition of MeOH in that (i) there is no single, distinct isosbestic point over the concentration range shown in Figure 1C,D and (ii) the absorptivity at which the monomer spectrum saturates at the conclusion of QD addition is 20%−30% smaller than that at which the monomer spectrum saturates at the conclusion of the MeOH addition. Observation i indicates that the cyanine dyes and QDs participate in a multicomponent equilibrium that is more complex than the simple disaggregation that takes place upon methanol addition, while observation ii indicates that the
n
ελc =
∑ εi ⊗ xi i=1
(1)
This model must conserve massthat is, the mole fractions must sum to one for each composite spectrum (eq 2). n
∑ xi = 1 i=1
(2)
The Supporting Information contains details of the factor analysis procedure. Briefly, we used spectral components identified in Figure 2 (red, blue, and green spectra) as initial guess components. We then applied target transformations to the initial principal factor data and compared the resulting transformed vector components to the initial test vectors for goodness of fit. The procedure was iterated for each component until we achieved successful reproduction of the experimental QD/cyanine composite spectra and mass conservation for each concentration of added QD. 6138
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Figure 3. (A) Absorptivity spectra of each of the IR820 speciescomponent 1 (green), component 2 (blue), and component 3 (red)present at each point in the stepwise addition of 1.5 μM IR820 in CH2Cl2 with CdSe QDs (λabs = 541 nm). (B) Mole fraction of each species of dye; the colors correspond to those of the absorptivity spectra in (A).
Figure 4. (A) The green spectrum shows 0.9 μM IR783 in CH2Cl2 after addition of 240 mM MeOH; this spectrum represents the completely disaggregated (monomeric) form of the IR783 dye. Upon addition of CdSe QDs with λabs = 503 nm, the absorptivity of the monomer decreases; the red trace is the spectrum of the solution at [QD] = 0.8 μM. (B) The absorptivity of IR783 at λ = 796 nm as a function of the concentration of added QDs; the absorptivity plateaus at 171 000 M−1 cm−1, so we can conclude that the decrease is not due to degradation, but rather adsorption of the dye onto the QD surface.
fraction of component 3 is one (that is, it is the only species contributing to the composite spectrum). Component 3 Is a Surface-Bound Monomer. From our simple disaggregation experiment with methanol (Figure 1A,B), we have already identified component 1 (Figure 3, green) as the spectrum of solution-phase dye (monomer plus Haggregate dimer). Component 3 (Figure 3, red) is identical to the spectrum of the solution-phase monomer, except that it has a lower absorptivity, so we hypothesized that it is a surfacebound monomer. In order to test this hypothesis, we specifically monitored the absorptivity of IR783 monomers as a function of their binding to QD surfaces. To ensure that we initially only had monomers in solution, we prepared a sample of IR783 that contained enough MeOH to completely disaggregate the dye (green spectrum in Figure 4A). To this predisaggregated solution, we added a series of concentrations of QDs (Figure 4A). The absorptivity of the monomer (monitored at 796 nm) decreases initially and then levels off (Figure 4B) at 72% of its original amplitude (Figure 4B). The spectrum maintains the same shape during the QD addition (compare green and red spectra in Figure 4A). This result demonstrates that the decrease in absorptivity of the dye is not due to degradation; if degradation were occurring, the shape of the spectrum would probably change, and the absorptivity at 796 nm would continue to decrease over the course of QD addition. In contrast, here, the absorptivity
We find that three components, as suggested in Figure 2, are satisfactory to reproduce the original absorption data for both IR820/QD and IR783/QD systems. Figure 3A shows the absorptivity spectrum of each component that we calculated from the factor analysis, and Figure 3B shows the calculated mole fractions of the three components as a function of the concentration of QDs added to 1.5 μM IR820 in CH2Cl2. The component dominant at low concentrations of QDs (green in Figure 3A,B) is a composite spectrum of the solutionphase (unbound) monomer and H-aggregate dimer. The mole fraction of this component decreases with the addition of QDs to solution up to 10−7 M QDs, at which point the factor analysis indicates that it no longer contributes to the observed spectrum. The second component (blue in Figure 3A,B) is a broadened monomer-like spectrum. The mole fraction of this component increases to a maximum of one at a QD concentration of 10−7 M. Increasing the QD concentration beyond 10−7 M leads to a decrease in the mole fraction of this component, eventually to zero. The third component (red in Figure 3A,B) has the same shape as the monomer in the MeOH disaggregation experiment, but with smaller amplitude. This component appears only at concentrations of QDs >10−7 M and at the expense of component 2. At the conclusion of QD addition, the mole 6139
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Figure 5. Diagrams of the adsorption process for the QD−cyanine system without addition of methanol: (A) At low QD concentrations (10−6 M), the cyanines redistribute until only the surface-bound monomer (component 3) is present.
Figure 6. (A) Absorption spectra of a sample of 0.7 μM IR783 in CH2Cl2 after addition of 244 mM MeOH to induce complete disaggregation (black), after addition of MeOH and 0.27 μM CdSe QDs (green), and after addition of MeOH, QDs and octanethiol (red). Inset: absorption spectra of the QDs in this sample before (green) and after (red) addition of the octanethiol. (B) Absorption spectra of a sample of 0.8 μM IR783 in CH2Cl2 (no MeOH) showing characteristic H-aggregation (black), after addition of 0.27 μM CdSe QDs (green), and after addition of QDs and octanethiol (red). Inset: absorption spectra of the QDs in this sample before (green) and after (red) addition of the octanethiol.
surface-bound monomer spectrum, (iii) forms at the expense of solution-phase monomers and dimers, and (iv) is the initial adsorption product upon mixing QDs with solution-phase dye. Component 2 forms as a precursor to the surface-bound monomer (component 3) (Figure 3B). We propose that component 2 is a disordered aggregate of chemisorbed and physisorbed dye molecules, where one molecule within the aggregate is chemisorbed to the QD through its sulfonate group and one or more additional molecules are physisorbed to the surface-bound cyanine. The presence of methanol in the QD/cyanine mixture (as in the experiment illustrated in Figure 4) stabilizes the solutionphase monomer state of the dye and removes the energetic driving force for physisorption of these monomers to those bound to the QD surface; we therefore see no evidence of component 2 in the QD−cyanine−methanol system. Without methanol, the dye is more stable as a physisorbed or chemisorbed aggregate than in solution, so we observe component 2 in mixtures with only QDs and cyanine (Figures 1C,D and 2). Figure 5 summarizes the proposed adsorption
remains constant over a period of 6 h after reaching its final amplitude. We therefore conclude that binding to the QD reduces the absorptivity of the monomer through perturbation of the cyanine dye’s transition dipole by the QD surface. This perturbation probably occurs through interaction of the dye with charges from unpassivated surface atoms and/or charged ligand head groups (the native ligands for these QDs are negatively charged alkylphosphonates). Sufficient modifications to the dielectric environment of the surface by the long hydrocarbon chains of native surface bound ligands may also contribute to the perturbation. Regardless of the mechanism of the perturbation, this experiment indicates that component 3 (the red trace in Figures 2 and 3) is dye bound to the surface of a QD as a monomer. Component 2 Is a Surface-Bound, Disordered Aggregate. Upon adding QDs to predisaggregated monomers (as described in the previous section), we observed no sign of component 2 (blue spectrum in Figure 3); rather, the monomer spectrum maintained its narrow line shape throughout QD addition. Component 2, therefore, (i) is not a surface-bound monomer, (ii) is broadened with respect to the 6140
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process for the QD−cyanine system in the absence of methanol at different relative mole fractions of QDs and cyanine. Binding of Cyanine Dye to CdSe QDs Is Reversible. In order to support the proposed assignments of the component spectra for the QD−cyanine system, and eliminate degradation of the dye as a contributor to the observed spectra, we demonstrated that the cyanine adsorption events are reversible. We added octanethiol (OT), a tight-binding ligand for CdSe QDs, to a solution of QD/cyanine dye at a QD concentration at which all cyanines are on the QD surface. Figure 6 shows that when this concentration OT is added to the QD/cyanine solution, the OT ligand displaces the cyanine and causes the cyanine goes back into solution. Figure 6A shows the spectrum of a solution of IR783 in CH 2 Cl 2 that had been predisaggregated by 240 mM methanol (black dashed line). We then added a solution of 0.27 μM QDs to the cyanine/ methanol solution, and as we observed previously (Figure 4), the absorptivity of the dye decreases as it binds to the QDs as a monomer (green). Further addition of 28 mM OT to the solution results in the complete recovery of the disaggregated IR783 spectrum (red) to its original absorptivity values. Simultaneously, the first absorbance feature of the QD (503 nm, inset to Figure 6A) shifts to lower energy due to binding of OT to the QD surface, the characteristic bathochromic shift of the spectra associated with binding of thiols.44,45 The OT ligand has no effect on the IR783 absorptivity when added without QDs, so we can conclude that the binding of IR783 to the QDs is completely reversible by displacement with OT, without degradation of the dye. Figure 6B shows the same experiment repeated without the initial addition of MeOH. The initially solution-phase aggregates of dye (black spectrum) disaggregate and adsorb as monomers in the presence of 0.27 μM QDs (green spectrum). The absorptivity of the dye increases after its displacement from the surface by OT (red spectrum). The difference in absolute absorptivities between the sample with and without MeOH is probably due to the different dielectric constants of the solvent. We note that Figure 6B shows that, once the dye is displaced from the surface of the QD by OT, it exists in solution as a monomer, not an H-aggregate, even in the absence of MeOH. We believe that the displaced dye is stabilized in its monomeric form by excess phosphonic acid ligands (the native ligands of the QDs) that are also displaced by addition of thiol. The Supporting Information contains evidence that, indeed, excess phosphonic acid (in a concentration similar to that present upon addition of thiol) is capable of stabilizing the monomeric form of the dyes.
The presence of these three distinct components strongly suggests that it is not sufficient to assume a single chemical environment for ligands on the surface of a quantum dot. The local surface environment of the cyanine dyes on the surface of QDs is dependent upon the molar ratio of dye to QD in the sample and on the solvent composition. The addition of a small amount of MeOH to a solution of QDs and dye is enough to change the energetics of dye association such that the surface and solution-phase aggregate states are less favorable than the solution monomer state. We will next explore the influence of the local chemical environment of dye molecules at the surface of the QDs on the dynamics of electron exchange between photoexcited dyes and their semiconductor substrates.
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ASSOCIATED CONTENT
S Supporting Information *
Details of cyanine dye purification and CdSe QD synthesis, experimental methods for absorption, ICP, cyanine dye absorptivity measurements, procedure for adding QDs and MeOH to cyanine dye solution, details of the factor analysis procedure, disaggregation of dye by phosphonic acid, and Figures S1 and S2. This material is available free of charge via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was supported by a grant from the Camille and Henry Dreyfus Postdoctoral Program in Environmental Chemistry to E.A.W. and E.A.M. D.B.T. is funded by the IGERT: Quantum Coherent Optical and Matter Systems Program (NSF Award 0801685).
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REFERENCES
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CONCLUSION A factor analysis of the absorption spectra of QD−cyanine mixtures upon serial QD additions to cyanine dye solutions in CH2Cl2 shows that three spectral components are responsible for the observed spectra. These three components correspond to four distinct species of the cyanine dye. The first component corresponds to the unbound dye species in H-aggregate dimer and monomer forms. The second component corresponds to disordered surface-bound aggregates: dye molecules that are loosely bound to the surface of the QD and interact with other dye molecules on the surface of the same QD. The third component corresponds to surface-bound monomers: dye molecules that are tightly bound to the QD and do not interact with other dye molecules. 6141
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dx.doi.org/10.1021/jp300478g | J. Phys. Chem. C 2012, 116, 6136−6142
