From Single Chains to Aggregates, How Conjugated Polymers

Jul 26, 2013 - Dapeng Wang, Yuan Yuan, Yati Mardiyati, Christoph Bubeck, and Kaloian Koynov*. Max-Planck Institute for Polymer Research, ...
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From Single Chains to Aggregates, How Conjugated Polymers Behave in Dilute Solutions Dapeng Wang, Yuan Yuan, Yati Mardiyati, Christoph Bubeck, and Kaloian Koynov* Max-Planck Institute for Polymer Research, Ackermannweg 10, 55128 Mainz, Germany S Supporting Information *

ABSTRACT: Conjugated polymers offer unique combination of easily tailored mechanical, electrical and optical properties that makes them perfect materials for the preparation of various devices such as light-emitting diodes, photovoltaic cells or fieldeffect transistors. However, the design and fabrication of such devices in a controlled and reproducible way are possible only if the behavior and the properties of individual polymer chains are well understood. One major problem in this respect is that aggregation often occurs even in dilute solutions and prevents the single polymer chain studies. To address this issue, in this work we employed fluorescence correlation spectroscopy (FCS) to study the behavior of a model conjugated polymer, poly(2methoxy-5-(2′-ethylhexyloxy)-1,4-phenylenevinylene) (MEH-PPV) in several commonly used solvents. The very high sensitivity of FCS allowed measurements in ultradilute solutions and thus unambiguous determination of the hydrodynamic radius of single polymer chains. The solvent quality for MEH-PPV was then quantitatively evaluated from the measured logarithmic scaling of the single chain hydrodynamic radius versus the polymer molecular weight. Scaling exponents of 0.40, 0.41, and 0.43 were found in toluene, chloroform and 1,2dichlorobenzene, respectively. These values are well below the θ-condition, emphasizing poor solvent quality for MEH-PPV, despite the fact that all studied solvents are commonly regarded as “good” solvents. In addition, by investigating the aggregation behavior of MEH-PPV at higher polymer concentrations, we found a clear relation between aggregates size and solvatochromism that indicates more extended chain conformation in larger aggregates..

1. INTRODUCTION Conjugated polymers, such as poly(p-phenylenevinylene) (PPV) and its derivatives, have been intensively studied for their use in numerous applications including polymer-based light emitting diodes,1,2 field-effect transistors,3 photovoltaics,4,5 all-optical switching,6,7 etc. In particular, poly(2-methoxy-5-(2′ethylhexyloxy)-1,4-phenylenevinylene) (MEH-PPV), consisting of a PPV backbone and flexible side chains (chemical structure shown in the inset of Figure 1), is frequently regarded as a model conjugated polymer due to its outstanding semiconducting, luminescent and nonlinear optical properties. It has been shown, however that these properties may strongly depend on the polymer molecular weight, the type of solvents, the solution concentrations and the film preparation procedures.8−18 Two important factors should be considered here. On one hand, the polymer properties are defined by the molecular structure, conformation and size of the individual chains. Therefore, understanding the behavior of single chains in various solvents, e.g., in terms of solvent quality expressed by the scaling dependence of the hydrodynamic radius versus the polymer molecular weight is very important. On the other hand, the extended π-electron delocalization along conjugated polymer backbones leads to strong tendency toward interchain aggregation.19 The presence of aggregates can also strongly affect the optical and film forming properties of a conjugated polymer.9 In view of this broad spectrum of phenomena influencing the final film properties, it has become increasingly © XXXX American Chemical Society

Figure 1. Normalized experimental autocorrelation curves (symbols) measured for MEH-PPVs in toluene at a concentration of ∼0.5 nM. The solid lines represent the corresponding fits with eq 1. Inset: the chemical structure of MEH-PPV.

evident that only an accurate knowledge of the behavior of single conjugated polymer chains in solutions combined with a good understanding of their aggregation behavior can ensure the well-controlled and reproducible preparation of devices.17,20 Received: June 4, 2013 Revised: July 10, 2013

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GPC experiments performed with MEH-PPV 4 dissolved in THF at a concentration of about 0.005 mg/mL showed that the polymer molecular weight did not change after 30 min ultrasonication. This is in good agreement with a recent study.46 2.2. Fluorescence Correlation Spectroscopy. The FCS measurements were carried out on a commercial setup (Carl Zeiss, Jena, Germany) comprising the module ConfoCor2 and an inverted microscope model Axiovert 200. A Plan Neofluar 40×/NA0.9 multiimmersion objective with oil as immersion liquid was used to focus the excitation light provided by a CW Argon laser (488 nm) to a diffraction limited spot into the studied polymer solutions. The laser power after the objective was typically less than 5 μW. The fluorescence signal was collected by the same objective and passed through a dichroic mirror, a LP505 long pass filter, a confocal pinhole to finally reach the detector, a single photon counting avalanche photodiode. An Attofluor cell chamber (Invitrogen, Leiden, Netherlands) with a microscope glass slide (diameter of 25 mm and thickness of approximately 0.15 mm) was employed as a sample cell. The temporal fluctuations of the detected fluorescence intensity, δF(t) caused by fluorescent species diffusing through the confocal observation volume (around the laser focus) were recorded and evaluated in terms of an autocorrelation function, G(t) = ⟨δF(t)δF(t + τ)⟩/⟨F(t)⟩2. For an ensemble of freely diffusing fluorescent species, the autocorrelation function can be represented by the following analytical equation:39

Unfortunately, the strong tendency toward aggregation even at very low concentrations often prevents the accurate experimental determination of single polymer chain size and properties. Furthermore, the use of some classical techniques is further limited by the strong luminescence of most conjugated polymers and the investigated concentration range. That is why there is a need for development and application of new sensitive and selective techniques that can address properties of the conjugated polymers at a single molecule level. During the past decade, single-molecule spectroscopy techniques have emerged as an extremely powerful tool to obtain important information on the properties of individual conjugated polymer molecules.21−27 In particular, fluorescence correlation spectroscopy (FCS)28−38 can offer a promising alternative for addressing conjugated polymer molecules in solutions. The method is based on detecting and analyzing the fluorescence fluctuations emitted by fluorescent species diffusing one at a time through an extremely small observation volume ( chloroform ≥ toluene. Furthermore, it is clear that all studied solutions are below the θ-condition in which the exponent is 0.5 and the polymer coils act as an ideal Gaussian chain; i.e., the excess chemical potential between a polymer and solvents is thermodynamically approaching zero.53 As such, the MEH-PPVs exhibit a collapsed state as compared to an ideal Gaussian chain in all three solvents. It is tempting to compare our results with previous simulation results. An earlier Monte Carlo simulation used a model which consisted of only several tetrahedral defects on the backbone.54 They suggested that the morphology of a MEH-PPV chain in solutions could be significantly tuned from a tightly folded to an extended conformation by the choice of the solvent, e.g., toluene and dichloromethane. However, our control FCS experiments showed that the hydrodynamic radii of MEH-PPV 4 in chloroform and dichloromethane at the concentration of 1 nM were similar. Furthermore, our FCS data (Figure 4) indicate that the conformation of MEH-PPVs in the three common solvents that we studied is always more collapsed than an ideal Gaussian chain. A recent molecular dynamic calculation showed that a MEH-PPV chain without defects should exhibit a coil conformation in a good solvent.55 With the reasonable assumption of inevitable presence of defects in real chains, one can expect that real chains should be collapsed relative to the random coil.56 Interestingly, a recent coarse-grained simulation with a model comprising of an alternating cis−trans conformation57 showed a good consistency with our observations. The smaller values of scaling exponents found in that study (0.32 for toluene and 0.38 for chloroform), are likely due to an overestimation of the number of the cis conformations because a previous investigation indicated that the MEH-PPV, or at least its oligomers, are all trans in solutions.27,58 Moreover, although one expects more extended chain behavior in an aromatic solvent owing to its better affinity to the polymer backbone, this is against both our FCS results and the coarse-grained simulation in toluene. Rather than the local chemical affinity, global solvent effects appear more important for the conformation of the polymer chain. Comparison to experimental results of other authors is more challenging. There are a large number of studies of isolated conjugated polymer chains as observed mainly by singlemolecule spectroscopy (SMS)22,59−62 indicating that the

Figure 4. RH in the single chain region versus Mn for MEH-PPVs dissolved in diverse solvents. The error bars evaluated from the statistical deviations of the measurements are smaller than the symbol size.

summarized in Table 1, indicate that RH at the single chain plateau range for a given polymer sample is largest in o-DCB and smaller in toluene/chloroform. This is the first direct evidence that MEH-PPV molecules are more swollen in o-DCB than in chloroform and/or toluene. The solvent quality can be quantitatively evaluated from the slope of the logarithmic scaling of the single chain size against the polymer molecular D

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room temperature the minimum binding length is ∼4−6 nm.68,69 Therefore, it is not surprising that MEH-PPV 1 (RH ∼ 2 nm, see Table 1) does not exhibit any aggregation in all solvents studied. As the molecular weight is increased, it is anticipated that the interchain binding energy is gradually increased via extending the available binding length and the aggregates are stabilized. This anticipation is consistent with our experimental results that show the formation of aggregates and increase of their size with the increase of the polymer concentration for MEH-PPVs 2−4 in studied solvents. Direct comparison of the aggregation behavior of the MEHPPVs in the different solvents based simply on the aggregate sizes is difficult. Indeed, as discussed above the single chain size (see Table 1) depends on the type of the solvent. Thus, the density (or structural compactness) of MEH-PPV polymer coils is different in the different solvents and therefore the number of chains in an aggregate may not be directly proportional to its hydrodynamic radius. To partially overcome this problem and to estimate the number of chains per aggregate, we introduced the relative size RH/RH,SC by normalizing the measured RH in a given solvent to the hydrodynamic radius of the single chain RH,SC in the same solvent. Figure 5 shows RH/RH,SC measured for MEH-PPV 4 as a function of the polymer concentration in the three studied

choices of solvent or polymer matrix could strongly influence the conformation and photophysical properties of single MEHPPV molecules. While our results also show that the chain hydrodynamic radius (and therefore conformation) depends on the solvent, a direct comparison to SMS works is difficult. Indeed, most SMS studies are performed with isolated and immobilized MEH-PPV chains on a surface or within a “solid” polymer matrix. In this respect, the conformation and the photophysics of the MEH-PPV chains strongly depend on a rich variety of parameters, for example the preparation history, the choice of the polymer matrix, the solvent evaporation rate and the annealing procedure. On the other hand, the FCS studies are performed with individual MEH-PPV chains freely diffusing in solution, and therefore, they should be considered as complementary to the SMS studies performed on immobilized molecules. Alternatively, our results on the solvent quality evaluated on a single chain level can be compared with the classical approach, which examines the solubility of polymers in solvents based on the Hansen solubility parameters.63,64 The solubility parameter distance Dp,s describes the solubility of a polymer (subscript p) in a solvent (subscript s), where smaller values of Dp,s indicate better solubility. Taking into account the literature data of the Hansen solubility parameters64 and the corresponding parameters of MEH-PPV,65 we calculated the values of Dp,s (see Supporting Information) and displayed them in Table 1. The obtained results show that the solvent quality for MEHPPV is consistently described by the experimentally measured scaling exponent α and the hydrodynamic radius RH and the calculated solubility parameter Dp,s, as their values show the same trend in the three solvents. 3.3. Aggregation Behavior. After evaluating the solvent quality of some common solvents for individual MEH-PPV chains, we explored the effect of solvent on the polymer aggregation by considering the concentration-dependent RH in Figure 3 at concentrations above ∼1 nM. First, it should be emphasized here that, as discussed in the Experimental Section, the FCS measurements yield the value of the diffusion coefficient of the fluorescent species. Consequently the hydrodynamic radius is evaluated through the Stokes−Einstein equation using the viscosity of the pure solvent. However, at high polymer concentrations, above the so-called overlap concentration, the polymer chains start to overlap with each other and their diffusion is slowed down. Thus, one need to make sure that the decrease of diffusion coefficient at the higher concentration, presented in Figure 3 results from the formation and growth of aggregates and not from the overlap of individual polymer chains. The first evidence supporting the aggregation scenario is the increase of the fluorescent brightness of the diffusing species with concentration (Figure 2b). Second, we estimated that the overlap concentration cp*=[3/(4πNARg 3)]66 for the MEH-PPVs 1−4 in the studied solvents is in the ∼100 μM range, which is several orders of magnitude higher than the concentration of all MEH-PPV solutions studied in this work. Thus, we can neglect any possible effects generated from polymer chain overlap. As shown in Figure 3, the MEH-PPV 1 does not form aggregates in the entire concentration range studied. One major reason for the aggregation of conjugated polymers is that it leads to a decrease in the Helmholtz free energy through the interchain π−π stacking.67 A stable aggregate is achieved if the binding energy, corresponding to the binding persistence length, is higher than kBT. For typical conjugated polymers, at

Figure 5. Normalized RH versus concentration in toluene, CHCl3 and o-DCB. The error bars are removed for clarity. The horizontal lines indicate the calculated number of single chains within an aggregate based on the two-dimensional (2D) and three-dimensional (3D) model.

solvents. A rather similar behavior is observed in all cases. After an initial single chain plateau, the RH/RH,SC values rapidly increase with the concentration up to another quasi plateau region that apparently reflects the formation of stable aggregates. The values of RH/RH,SC in this quasi plateau region are of special interests because they reflect the degree of aggregation in the corresponding solvents. Apparently, the aggregates formed in o-DCB contain more polymer chains than that in toluene and chloroform. A recent small angle neutron scattering study revealed that the MEH-PPVs with hairy-rod segments form two-dimensional (2D) aggregates in aromatic solvents and three-dimensional (3D) structure in chloroform.70 Thus, we performed a simple estimation of the number of single chain within an aggregate based on two assumptions: (1) the aggregate is a 2D disk in aromatic solvents and a 3D sphere in chloroform;70 (2) the compactness (density) of single chains and aggregates in a given solvent is equal. In the 3D model, the aggregates comprising of 3 chains should have a 1.44 times E

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larger radius than the single chain. In the 2D model, the calculated radii of aggregates consisting of 3, 4, or 5 chains are respectively 1.73, 2.00, and 2.24 times bigger than the single chain radius. In fact, one should expect that the actual RH of aggregates are slightly smaller than the calculated values in the 2D model because in the dimension perpendicular to the diskshape aggregates, the length cannot be entirely neglected as compared to the size of aggregates. As shown in Figure 5 in the high concentration plateau region, the value of RH/RH,SC in chloroform is close to 1.44, indicating a 3-chain aggregate based on the 3D model, while in toluene RH/RH,SC is close to that of 4-chains aggregate. In o-DCB, the aggregation behavior is even more pronounced and RH/RH,SC shows a higher value, reminiscent of about 5-single chain assemblies. The most notable result from the data summarized in Figure 5 is that the aggregation of MEH-PPVs in the aromatic solvents toluene and o-DCB is more pronounced than that in the nonaromatic solvent chloroform, independent of the solvent quality (α values) measured at the single chain level. This finding is in good agreement with earlier spectroscopic8,71 and SANS observations,70 in which a high degree of aggregation was observed in aromatic solvents. These observations signify the fact that in contrast to common polymers, the solvent quality is not the sole factor in determining the aggregation behavior of MEH-PPVs and other conjugated polymers. Instead, our data indicate that the conjugated polymer-aromatic solvent interactions appears to play a major role, resulting in strong promotion of aggregation in o-DCB, which on the other hand is the best solvent for MEH-PPV single chains (Table 1). This interesting phenomenon was addressed by a recent NMR study, where the complexes formed through the π-electrons stacking along the chain backbone and the delocalization effect with aromatic solvents were considered.72 This study indicated that the plane of aromatic molecules tends to be trapped within two parallel filaments of polymer segments through π−π stacking and the assembled intermolecular structure is accordingly stabilized. This picture is consistent with our results showing that for MEH-PPVs in different aromatic solvents, the polymer conformation is more extended in o-DCB than in toluene, thus offering an increased number of binding sites and promoting the aggregation in o-DCB. The fact that the MEH-PPV chains aggregate even in very dilute solutions, should have also major impact on the interpretation of the commonly measured UV−vis absorption spectra of these polymers. For example, the solvatochromic shift in the wavelengths of the absorption maxima (λmax) is commonly explained considering the conformation change of single polymer chain in solution8,10 However, our results convincingly show that at a typical polymer concentration used for UV−vis solution absorption spectroscopy (typically ∼0.005 mg/mL), only aggregates exist in the solutions and therefore the conformation of single chain within aggregate is the relevant parameter. To illustrate this effect we show in Figure 6 absorption spectra of MEH-PPV 4 measured in chloroform, toluene and o-DCB at concentration of 0.005 mg/mL. The values of λmax for all studied polymers are summarized in Table S2 (Supporting Information). All MEH-PPVs show consistently the same solvatochromic shift in the sequence λmax(chloroform) < λmax(toluene) < λmax(o-DCB). To discuss this solvatochromism, we have to consider first the well-known red-shift of ππ*-transitions at increasing solvent polarity.73 The polarity of the solvents increases in the sequence of the polar solubility parameter

Figure 6. Typical absorption spectra of MEH-PPV 4 with a concentration of about 0.005 mg mL−1 in toluene, chloroform, and o-DCB.

δP(toluene) < δP(chloroform) < δP(o-DCB), see Supporting Information. This way, it is only conceivable that λmax of all MEH-PPVs in o-DCB is always observed at longest wavelengths as compared to the other solvents used here. But the fact that λmax of chloroform solutions is consistently observed at shorter wavelengths than λmax of toluene solutions cannot be explained by this kind of solvatochromic shift. On the other hand, the increase of the solvent shift of λmax from chloroform to toluene and then to o-DCB, perfectly correlates with the increase in the RH/RH,SC values in the higher concentration saturation region seen in Figure 6, i.e., with the increase of aggregate size and the degree of aggregation. This correlation can be readily explained by the fact that in bigger aggregates, the polymer chains can adopt a more extended conformation, thereby increasing the average conjugation length and showing a red shift in λmax. We believe that this physical picture may help to interpret many spectroscopic studies in a consistent way. It is also consistent with recent single molecule spectroscopy studies revealing that even at conditions at which only single chains are commonly expected, aggregates may still be present and may strongly affect the measured photophysical properties.74,75

4. SUMMARY AND CONCLUSIONS Fluorescence correlation spectroscopy was used to measure the size of single polymer chains of a model conjugated polymer, MEH-PPV in toluene, chloroform and o-DCB. The solvent quality of these solvents for MEH-PPV was then evaluated from the logarithmic scaling of single chain hydrodynamic radius vs the molecular weight. Scaling exponents of 0.40, 0.41, and 0.43 were found in toluene, chloroform and o-DCB, respectively. These values are below the θ-condition for an ideal chain as defined by the Flory−Huggins polymer solution theory. We note, however, that a real MEH-PPV chain clearly deviates from an ideal chain and in this respect we consider our results as an experimental finding that may be useful for building a consistent picture of the behavior of individual conjugated polymer chains in solution and can help to tune/verify relevant theoretical studies. Furthermore, the aggregation behavior of MEH-PPV and the evolution of the aggregate size with the increase of polymer concentration were studied. It was found that the aggregation is more pronounced in aromatic solvents than in the nonaromatic solvent, independently of the solvent quality on a single chain F

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level. This phenomenon can be correlated to the solvatochromism that is commonly observed in such systems and can be readily explained by the fact that in bigger aggregates, the polymer chains may adopt more extended conformations, thereby increasing the average conjugation length and showing a red shift in λmax. The method presented here is widely applicable for the characterization of many conjugated polymer systems, permitting a new degree of understanding of the ways, in which solvent environment influences their molecular conformation and aggregation and, in turn, functions.

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ASSOCIATED CONTENT

S Supporting Information *

Calculation of the Hansen solubility parameters and the UV− vis absorption spectra data. 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: (K.K.) [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank Andreas Best and Sandra Seywald for technical assistance. The financial support provided from International Max Planck Research School to Y.Y., and from German Academic Exchange Services (DAAD) to Y.M., respectively is gratefully acknowledged.

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REFERENCES

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dx.doi.org/10.1021/ma4011523 | Macromolecules XXXX, XXX, XXX−XXX