Solvent Vapor Annealing of Single Conjugated Polymer Chains

Perspective pubs.acs.org/JPCL

Solvent Vapor Annealing of Single Conjugated Polymer Chains: Building Organic Optoelectronic Materials from the Bottom Up Jan Vogelsang*,§ and John M. Lupton§,† §

Institut für Experimentelle und Angewandte Physik, Universität Regensburg, Universitätsstrasse 31, 93053 Regensburg, Germany Department of Physics and Astronomy, University of Utah, Salt Lake City, Utah 84112, United States

†

ABSTRACT: Optoelectronic devices based on organic materials show a strong relationship between the morphological structure of the material and the function of the device. One of the grand challenges in improving the efficiencies of these devices is hence achieving morphological control throughout the entire course of processing. One of the most important postprocessing methods is solvent vapor annealing, which has repeatedly demonstrated its utility in improving the efficiency of organic-material-based devices by changing bulk-film morphology. This Perspective discusses the recent impact of single-molecule spectroscopy techniques in unraveling morphological changes and molecular dynamics and presents solvent vapor annealing as a tool to build organic optoelectronic materials from the bottom up. In particular, we discuss examples of how solvent vapor annealing at the single-chain level can be split into two different regimes, (i) the solvation regime, in which intrachain interactions and molecular dynamics during solvent vapor annealing can be probed, and (ii) the aggregation regime, in which the influence of interchain interactions can be probed. Finally, it will be shown that solvent vapor annealing in the aggregation regime can be used to build highly ordered mesoscopic objects with distinct properties such as long-range energy transfer.

S

emergence of intrachain interactions, the conformation of intermediates along the annealing pathway, the dynamics of chain reassembly during annealing, and spatial and temporal inhomogeneities of the annealing process are poorly understood. In this Perspective, we aim to shed some light on intrachain interactions and their impact on photophysical properties by discussing the use of SVA at the single-molecule level. Furthermore, we will discuss the possibilities and challenges of single-molecule spectroscopy in providing a molecular picture of this important postprocessing technique. Finally, we will elucidate the role of SVA in understanding the evolution of conjugated polymer materials from single chains to the first building blocks of a neat solid film, that is, molecular aggregates. The highly complex local molecular chain-packing arrangements and chain conformations, that is, the morphology, of conjugated polymers are known to strongly modulate the useful properties of these materials and must be optimized to produce useful bulk systems.19,24 It has already been demonstrated that the morphology of conjugated polymers is further complicated by a complex patchwork of locally ordered nanoscale domains.25 A few examples of possible chain morphologies with different intrachain interactions that are accessible by SVA are shown in Figure 1.

olution processability is both a blessing and a curse of devices based on organic materials. On the one hand, conjugated polymers have enormous technological potential as low-cost, easily processed materials open to a diverse set of applications, including biomedical sensors, inexpensive solar cells, light-emitting diodes, and printable electronics.1−4 On the other hand, solution-processing methods are highly complex at the molecular level, which prevents the development of rational design approaches to improve these methods. During processing, conjugated polymers are first dissolved in a suitable solvent, in which they can form a random coil structure with few, if any, inter- and intrachain contacts.5−7 Second, the material is processed by different methods, for example, doctorblading,8,9 drop-casting, or spin-casting,10,11 and the conjugated polymers form a neat solid film during the evaporation of the solvent. The film morphology is mainly determined by the second step and depends on a variety of parameters, such as solvent quality, evaporation rate, and substrate surface.11 Third, different postprocessing methods, such as thermal and solvent vapor annealing (SVA), can be applied to equilibrate and change the film morphology toward a more stable and desirable morphology, which can lead to final devices with improved optoelectronic properties.12−19 SVA is an industrially important technique because it causes a rapid morphological equilibration of films at room temperature without generating complications such as thermal damage, one of the drawbacks of hightemperature annealing.20−22 SVA is very well studied on the micrometer length scale as is its impact on interchain interactions and phase separation in polymer blend films.23 In contrast, the effects of SVA on the nanometer length scale, the © 2012 American Chemical Society

Received: March 12, 2012 Accepted: May 10, 2012 Published: May 17, 2012 1503

dx.doi.org/10.1021/jz300294m | J. Phys. Chem. Lett. 2012, 3, 1503−1513

The Journal of Physical Chemistry Letters

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Figure 1. Cartoon of different conjugated polymer morphologies. (a) An unfolded random coil morphology with each segment represented by a bead, without taking twisted segments into account. (b) A folded, ordered rod-like morphology. (c) Aggregate consisting of several conjugated polymer chains (each color represents one chain). (d) A conjugated polymer can be divided into segments between which the π-conjugation is broken by, for example, strong twisting or chemical defects. (e) A conjugated polymer with the same shape as that shown in (d) but with planarized segments (shown in red) leading to extended π-conjugation. Note that these segments are not necessarily straight. (f) Scheme of the experimental approach used to measure the degree of dipole alignment (anisotropy) within a multichromophoric system by excitation polarization fluorescence spectroscopy.

units (shown as gray and red bars). This situation is illustrated by the red segments in Figure 1e, where planarization leads to extended π-conjugation. A whole new set of photophysical properties can be anticipated by extended π-conjugation, such as a shift in the absorption and emission wavelengths, a change in interchromophoric energy-transfer efficiencies,32 different oscillator strengths for excited-state transitions, and a larger overall volume for excitons to screen the environment for traps or quenchers.33 Especially in the system of poly(2-methoxy-5(2′-ethylhexyloxy)-1,4-phenylene-vinylene) (MEH-PPV) studied here, we assume for simplicity that weakly coupled individual chromophores transfer energy along the chain by an incoherent Förster-type mechanism. We note that this picture may constitute an oversimplification because indications of intermediate (coherent) coupling have also been reported.34,35 In the case of polyfluorene, weakly coupled chromophores appear to exist in the glassy phase. In the βphase, instances of individual chains exist where the π-system does not appear to be disrupted,36 a situation that clearly jars with the weak coupling limit. The clearest signatures for single polymer chains which exist in the strong-coupling limit, has been derived from polydiacetylene polymerized in its monomeric crystal. To all extents and purposes, the singlechromophore exciton appears to assume the electronic structure of a J-aggregate.37 Due to the structural diversity of conjugated polymers, single-molecule spectroscopy has become the method of choice to unravel the broad inhomogeneity within these materials and has already proven to be a valuable experimental tool to correlate the conformations of polymer chains with the photophysical properties observed within solid films.24,27,38−42 However, one characteristic that all single-molecule spectroscopy studies have in common is that the material of choice is highly diluted until single chains can be well resolved by their fluorescence using an optical microscope. This approach has two consequences. First, only diffraction-limited spots can be observed in the fluorescence microscope, and structural information must be acquired indirectly, for example, by means of measuring the polarization anisotropy. Even recently

A conjugated polymer molecule can most simply be described as a chain of segments, in which each segment consists of one chromophoric unit that can be represented by a bead, as shown in Figure 1a−c. No interchromophoric interactions, which are due to planarization between neighboring units, are taken into account in this model. However, already in this simple model, a structure−property relationship can be anticipated due to the orientation of the absorption and emission dipoles of each chromophoric unit with respect to each other and the assumption that the orientation of the dipoles is directly related to that of the backbone. We note that this simple assumption is not necessarily founded; chromophores themselves may be bent in space, so that the orientation of the transition dipole moment does not map the spatial orientation of the backbone.26 Figure 1a shows a random coil structure typical of a well-dissolved conjugated polymer chain in a good solvent,5−7 which leads to a random distribution of absorption and emission dipoles and a large average distance between each of these, which impedes energy transfer. In contrast, a rod-like folded conjugated polymer chain, as shown in Figure 1b, is typical of a single conjugated polymer chain embedded in a host matrix.27,28 Here, the dipoles are more aligned and closer to each other. It becomes intuitively clear that interchromophoric energy transfer must be more efficient within the rod-like folded conjugated polymer chain, which leads, for example, to more efficient suppression of emission of the complete conjugated polymer chain or aggregates consisting of several ordered conjugated polymer chains (such as those portrayed in Figure 1c) by a single quencher.29 An example for such an efficient quencher is the hole polaron, which can be formed by photoinduced charge transfer within the molecular structure.30,31 In this folded chain, the structure− property relationship is directly reflected by the connection between the backbone conformation and energy transfer within a conjugated polymer chain or aggregate. Another structure−property relationship is revealed by the spatial orientation and the shapes of neighboring chromophoric units, as shown in Figure 1d,e; a different degree of planarization is implied between neighboring chromophoric 1504



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spectrally discrete transitions of single chromophores within one polymer chain could be identified and differentiated between. In addition, energy-transfer processes between chromophores along the polymer backbone can then be resolved spectrally and extrapolated to a spatial model of the polymer chain. Slight changes in conformation can strongly impact the low-temperature spectroscopy. For example, bending of the π-electron system can promote random fluctuations in the electronic transition energy through coupling with the matrix environment, which leads to strong spectral jitter in the single-chromophore transition line.54 Planarization of the backbone can reduce the level of excited-state relaxation (often referred to as the Stokes shift, which is not entirely accurate), giving rise to strong spectral shifts. A particularly prominent case of spontaneous intrachain planarization, discussed below, is given by polyfluorene. In this material, the planar and twisted conformations, which can be differentiated macroscopically by X-ray scattering, exhibit distinct spectral signatures, which in turn are readily separated out on the singlechain level. SVA provides a crucial tool to tune the conformation of a single polymer chain in situ. The technique can be applied in

emerging subdiffraction-resolution methods (often referred to as “superresolution” microscopy) are limited within these systems43−45 because a fixed and unknown emission dipole orientation restricts the attainable spatial resolution.46 More importantly, independent fluorescent emitters are a main prerequisite of these subdiffraction techniques.43,47,48 In contrast, the first single-molecule spectroscopy studies on conjugated polymers by Barbara and co-workers in 1997 already showed that thousands of repeat units behave as one spectroscopic entity. 49 The effective coupling between chromophores on a conjugated polymer chain limits the spatial resolution of the conformation achievable by superresolution microscopy. Nevertheless, just recently, a detailed picture of energy-transfer processes and at least a rough picture of chain conformation could be obtained by superresolution microscopy, where an external electric perturbation was used to switch the molecular fluorescence.50,51 As a second consequence of the implicit dilution of molecules, only intrachain interactions are probed by single-molecule spectroscopy, which on the one hand can be used as an argument in favor of the technique because intrachain interactions can be accessed selectively, in contrast to ensemble methods where a clear distinction between intrachain and interchain effects is usually not possible. On the other hand, the results of single-molecule spectroscopy can be extended to bulk films only to a limited degree because interchain interactions play an important role in determining the optoelectronic properties of organic materials.52 Both consequences of the spectroscopic technique will be addressed in the following. Because the conformation of conjugated polymer chains cannot be resolved directly by optical microscopy as discussed above, indirect methods are used. One method is shown in Figure 1f, which probes the degree of order between all absorbing dipoles in a single diffraction-limited fluorescent object. Hu et al. showed that polarization excitation fluorescence spectroscopy can be used on single conjugated polymer chains to characterize the conformational order through the polarization excitation anisotropy, A.24 The fluorescence intensity of single conjugated polymer chains was measured while rotating the angle of the linearly polarized excitation light in the x−y plane of the laboratory frame. The modulation depth, M, was obtained by fitting the plot of the intensity versus the polarization angle, θ, to the following equation (see the red curve in Figure 1f) I(θ ) ∝ 1 + M cos 2(θ − Φ)

SVA provides a crucial tool to tune the conformation of a single polymer chain in situ. two regimes, controlling intramolecular (i.e., intrachain) conformation and tuning intermolecular aggregation. In the following, we will discuss the two different regimes of SVA with respect to highly dispersed conjugated polymer chains embedded in a host matrix, which leads to two different areas of application of the method at the single-molecule level, (i) the study of the intrachain interchromophoric interactions and the translational and folding dynamics of single chains and (ii) the assessment of the interchain interactions and aggregation of multiple chains. The two different regimes are mainly determined by the solvents chosen for annealing and the concentration of the polymer chains in a given host matrix. The boundary between these two regimes is determined by the applicability of the Ostwald ripening equation, which describes the size of the critical radius, RC, at which stable aggregates can form56

(1)

RC =

where Φ is the orientation of the net transition dipole moment of the polymer chain for which the emission is maximized. For each conjugated polymer chain, M can be acquired, and a histogram over a large number of chains can be obtained. Additional orientation and detection efficiency factors must be taken into account to convert M to A.27 This technique is particularly and exclusively suitable for testing the backbone conformation using the overall absorption dipole orientation. For instance, it cannot be used to measure planarization effects of the chain between neighboring segments. An alternative approach to derive structural information from a single polymer chain lies in a detailed analysis of the emission37,53−55 or excitation35 spectrum of the material. Because the fluorescence is strongly broadened by temperature-dependent interactions of the excited state with the environment, such measurements are best performed at cryogenic temperatures. Using this approach, individual

C∞ ⎛ 2σ ⎞ ⎜ ⎟ν ⎝ kT ⎠ CCP − C∞

for C∞ < CCP

(2)

where σ is the interphase surface tension between the solute and the solvent, v is the atomic or molecular volume of the solute, CCP is the solute concentration, and C∞ is the saturation limit of the solute concentration, that is, the concentration for which chain entanglement and hence gelation occurs. Here, the conjugated polymer is the solute, and a simplification for the solvent is made such that the surrounding medium, which consists of both the swollen host matrix along with the solvent used for annealing, is considered as the solvent. The solute concentration is fixed and, for single-molecule spectroscopy applications in particular, very low (∼10−12 mol/L), whereas C∞ depends directly on the quality of the solvent used for annealing. Annealing can therefore be split into a regime where C∞ > CCP, which leads to no aggregation due to RC being infinite (referred to as the solvation regime), and a condition 1505



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where C∞ < CCP, which leads to aggregation due to a finite RC (referred to as the aggregation regime). The quality of a solvent is defined by the miscibility of a polymer in that solvent and can be described according to Flory−Huggins theory by the polymer−solvent interaction parameter χ.57,58 This parameter relates the difference in the enthalpy of interactions between the polymer and solvent and the self-interactions experienced by the two. This difference is normalized by temperature such that polymer and solvent are completely miscible over the entire composition range when χ < 0.5.58 A good solvent is thereby defined as having an interaction parameter with χ < 0.5, and a bad solvent is defined as having an interaction parameter with χ > 0.5.58 The transition from the solvation to the aggregation regime is schematically illustrated in Figure 2. C∞ decreases with

annealing with a good solvent (Figure 2c) with that of the same sample after annealing with a bad solvent (Figure 2d).59 In the former case, the single polymer chains are well-dispersed. In the latter, aggregation is clearly visible; the spot density in the image decreases, and the average brightness per spot increases. In the following, we discuss the influence of SVA in the solvation regime on intrachain interactions in the polymer polyfluorene. Polyfluorene is an intriguing conjugated polymer that has been explored extensively as a potential blue emitter for light-emitting diodes. It can exist in two distinct conformations, one where the repeat units are twisted with respect to each other (the glassy phase) and one where all repeat units approximately lie in the same plane (known as the β-phase). These two distinct phases of the polymer were first distinguished by means of ensemble spectroscopy and X-ray diffraction.60 SVA can increase the percentage of chains adopting the spontaneously planarized conformation.60 This planarization leads to a reduction in electronic disorder broadening, resulting in an additional narrow absorption band to the red of the dominant absorption, as well as a narrowing and red shifting of the emission.60−62 Planarized and twisted polymer chains are therefore readily distinguished on the single-molecule level.63 Figure 3 summarizes emission spectra obtained for planarized (panel a), twisted (panel b), and mixed phase (panel c) conformations of the polymer, recorded at 5 K.

Figure 2. Two different regimes of solvent vapor annealing are illustrated for highly diluted, thin polymer films with dispersed single conjugated polymer chains. (a) Qualitative dependence of the saturation concentration, C∞, and the polymer concentration, CCP, on the solvent quality χ during annealing. (b) Dependence of the critical radius for aggregate formation by Ostwald ripening, RC, on ΔC = CCP − C∞ (see eq 2). (c) Wide-field fluorescence image (40 × 20 μm2) of a highly diluted MEH-PPV/PMMA thin film after 30 min of solvent vapor annealing with 100% chloroform vapor. (d) The same film and conditions as those described in (c) but with a 20% acetone, 80% chloroform vapor mixture. The different regimes for the cases in (c) and (d) are illustrated above the images. On the left-hand side, CCP is below C∞, resulting in an infinite RC, thus preventing aggregate formation and giving rise to well-dissolved single chains. The aggregation regime is illustrated on the right side of the figure, where CCP > C∞, resulting in a noninfinite RC and therefore aggregate formation. Figure adapted from ref 59.

Figure 3. PL spectra and excitation polarization anisotropy of single polyfluorene chains at 5 K excited at 400 nm. (a) Single-molecule spectrum of the polyfluorene β-phase, in which all monomer units lie in one plane. The corresponding X-ray diffraction pattern shows a clear structure, indicating long-range intramolecular order. This conformation is reached by solvent vapor annealing of polyfluorene in an inert polymer matrix. (b) Single-molecule spectrum of the glassyphase polymer, in which the monomer units are twisted along the polymer backbone relative to one another. The inset shows a roomtemperature ensemble X-ray diffraction pattern of a drawn fiber, which does not display any significant structure. (c) Dual glassy and β-phase present in one chain. The insets show schematics of chain segments in the different conformations. Note that the side groups (dioctyl units) are not shown. Panels (d−f) display the corresponding histograms of excitation polarization anisotropy that indicate chain conformation, demonstrating that the planarized chains are also the most extended. The blue histogram in panel (e) represents glassy-phase molecules not subjected to the solvent vapor annealing procedure, whereas the red histogram shows residual glassy-phase molecules following annealing. Figure adapted from refs 60 and 64.

decreasing solvent quality (black curve in Figure 2a), whereas the conjugated polymer concentration CCP remains constant (red curve in Figure 2a). As a concequence, the concentration curves intersect at a certain solvent quality, dividing the process of SVA into the two regimes. The left-hand side of the diagram is described by the solvation regime where C∞ > CCP (represented by a random coil chain illustrated in Figure 2b). The right-hand side is understood in terms of the aggregation regime (represented by a folded chain aggregate of several polymer molecules) with C∞ < CCP. The qualitative dependence of RC on ΔC = CCP − C∞ in the aggregation regime is illustrated on the right-hand side of Figure 2b (black curve). As a consequence, no aggregates are formed within the solvation regime, and the difference can be clearly observed by comparing a wide-field fluorescence image of a film of highly diluted conjugated polymer chains in a PMMA matrix after 1506



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consists of only one chromophore. Figure 4 compares PLE and PL spectra of a polyfluorene oligomer and a polymer in the β-

X-ray diffraction images of solvent-annealed drawn (panel a) and pristine (panel b) bulk fibers are shown in the insets, adapted from ref 60. Whereas the pristine fibers show a very low degree of ordering and reveal an amorphous structure in the scattering image, the annealed fibers have a high degree of order displayed by well-resolved scattering maxima. Annealing therefore increases the effective electronic coherence length of the polymer, which implies a reduction of intramolecular disorder best explained by planarization of the polymer chain. In emission, this planarization corresponds to a narrowing and a red shifting of the spectrum (panel a) when compared to the pristine polymer (panel b). The selection rules for the vibronic transitions also change. The mixed phase shown in panel c displays a mixture of both spectroscopic features. It is important to note that planarization, as triggered by SVA, is not necessarily equivalent to a distinct unfolding of the polymer chain, as discussed above. Panels d and e show the polarization anisotropy measured on the single chain level in excitation as described in Figure 1f. The β-phase polymers tend to be much more elongated than the glassy-phase chains; the anisotropy histogram of many single molecules peaks at unity for the βphase in panel d. The histogram of glassy-phase chains in panel e compares molecules in pristine films (solid bars) to those in annealed films (hashed bars). Whereas SVA is required to generate the planarized β-phase in sufficient quantity, it does not affect the conformation of the remaining glassy-phase chains because the anisotropy is unchanged from before annealing. Likewise, the mixed-phase single-chain anisotropy in panel f appears to follow the same distribution of the pure glassy-phase chains in panel e, implying again that SVA does not primarily modify the conformation of the polymer chain but in this case leads to planarization of the polymer chain. What sort of molecule is the planarized β-phase? The X-ray patterns suggest that it forms a sort of one-dimensional crystal, akin to a carbon nanotube, with highly delocalized π-electrons. There is an important precedence for such extraordinarily longrange order in conjugated polymers, namely, in transpolydiacetylene polymerized in a crystal of the monomers.65−67 These materials, although not strictly solution-processable, have shown electronic coherence lengths on the scale of micrometers, on the level of single polymer chains. These polymers are effectively free of defects, suggesting that the πelectron system extends along the entire chain. While the structure of polyfluorene is fundamentally different, swellinginduced planarization of the polymer chain to form the β-phase can be considered as a form of crystallization itself. β-phase polyfluorene exhibits a unique property in that its emission is not linearly polarized on the single-molecule level.64 This result implies that there is no single linear transition dipole moment associated with the optical transition. Instead, it may be concluded that when the β-phase polymer chain is bent, as is the case for some molecules in Figure 3a, the emission can occur from any part of the polymer chain that consists of one πelectron system and hence assumes a range of orientations in terms of polarization. More conclusive evidence for complete delocalization of the π-electron system in β-phase polyfluorene comes from PL excitation (PLE) spectroscopy. This technique can be used to map out the absorption of individual chromophores in a multichromophoric polymer.35 Because multiple units contribute to absorption yet energy transfer on the polymer chain tends to occur to the lowest-energy units, the emission spectrum is usually narrower than the excitation spectrum.35 This conclusion is not correct if the polymer chain

Figure 4. PL excitation (PLE) and PL spectra of single β-phase chains formed by solvent vapor annealing. (a) Illustration of the structure of the planarized polymer chains. Either π-conjugation is disrupted, leading to the formation of chromophores (left), or the π-system extends along the entire chain (right). (b) Excitation and emission spectra of single β-phase fluorene nonamers. The distribution of 0−0 transitions for different single molecules is shown in the inset. (c) The same spectra described in (b) for a single PFO chain. Figure adapted from ref 36.

phase, which is created by prior SVA. The distribution of narrow electronic transitions is indicated in the histogram's inset. In both cases, near-mirror symmetry is observed between emission and excitation. If anything, the oligomer spectra, which can contain only one chromophore at a time due to the limited chain length,32 appear broader than the polymer spectra. This narrowing of the polymer spectra is a result of increased electronic oscillator strength due to increased delocalization in the polymer.37 The fact that near-perfect mirror symmetry is observed in the polymer implies that the structure of the polymer does not consist of multiple chromophores, as depicted in the cartoon on the left side of panel a, but is made up of only one single chromophoric unit (right side of panel a in Figure 4).36 SVA is widely associated with morphological control of molecular packing in the bulk, particularly in blend systems. However, the technique can also play a crucial role in modifying the intrinsic intramolecular properties of a polymer chain, up to the extreme limit of forming select perfect single π-systems.36 The challenge in device engineering now lies in devising schemes to exploit these unique single-molecule structures in the ensemble. The solvation regime of polymer swelling can also be exploited to investigate molecular dynamics during the annealing process. So far, only the start and end points of the SVA process have been studied by single-molecule spectroscopy. However, we are currently venturing into the realm of in situ observations of the annealing process at the singlemolecule level. This approach becomes possible by combining time-resolved single-molecule spectroscopy techniques, such as time-resolved wide-field fluorescence microscopy and fluo1507



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conclusion that a fairly heterogeneous swelling process occurs during SVA.68 This demonstration of in situ observation of transitions between the solid and liquid phases of the polymer chain is the second example of how SVA affects the intrachain conformation of single conjugated polymer molecules. Polarization excitation fluorescence spectroscopy, illustrated in Figure 6a,

rescence correlation spectroscopy (FCS), with a home-built SVA chamber mounted on a microscope.68

We are currently venturing into the realm of in situ observations of the annealing process at the single-molecule level. It has been shown that single conjugated polymer chains of the prototypical polymer MEH-PPV embedded in a poly(methyl-methacrylate) (PMMA) host matrix undergo large translational jumps during SVA, accompanied by fluctuations in the fluorescence quantum yield ϕF.68 These translational jumps correspond to dissolution of the polymer in the matrix, thermal flowing of the liquid, and subsequent spontaneous resetting. A typical example is given in Figure 5, which shows a series of

Figure 6. Experimental histograms of the polarization anisotropy for single MEH-PPV molecules embedded in a PMMA host matrix obtained by fluorescence excitation polarization spectroscopy. (a) Asspin-cast from chloroform and (b) additionally solvent-vapor-annealed for 60 min with 100% toluene vapor. The histograms shown in (a) and (b) describe the polarization anisotropy of 230 and 146 MEH-PPV molecules, respectively. The insets illustrate chain morphologies consistent with the histogram averages. Figure adapted from ref 68.

shows that highly diluted MEH-PPV/PMMA films spin-cast from chloroform (a good solvent) yield a broad variety of ordered and disordered conformations based on the broad distribution of the excitation polarization anisotropy.68,72 This distribution arises due to the fact that a variety of conformations are frozen in in the PMMA matrix upon the rapid drying process following spin coating. After 60 min of SVA with a good solvent (e.g., toluene or chloroform), the majority of conjugated polymer chains reside in an ordered conformation, yielding only high anisotropy values above 0.8 in the histogram in Figure 6b.68 We note that the results in Figure 6 are different from the observations in Figure 3 for the case of polyfluorene, where solvent swelling leads primarily to planarization of the polymer chain without strongly modifying the overall chain conformation. This discrepancy can be explained by the much greater rigidity of polyfluorene in a liquid or solvent-swelled environment when compared to that of MEH-PPV. With the observation of unfolding of the chain during SVA (Figure 5), we can now speculate about whether the chains fold toward a more ordered and equilibrated structure during later stages of the swelling process or remain in an unfolded random coil conformation throughout the entire process and only fold into an ordered conformation during the slow solvent evaporation after the solvent-saturated gas is flushed out by dry gas and the liquid-like polymer film dries into a solid. An obvious question then regards the impact of controlled solvent evaporation rates on intrachain conformations and the quest for direct experimental observations of these folding events. These questions will need to be addressed in future studies to understand the full dynamics of drying following SVA. Furthermore, because MEH-PPV is sensitive to the degree of solvent swelling, as indicated by the increase in ϕF, it can be used as a reporter on the dynamics of the environment. Thus, in combination with fluorescence microscopy, a noninvasive method comes into reach to study the dynamics of solvent percolation in situ during the early stages of SVA;73 possible

Figure 5. Fluorescence intensity transient of a single MEH-PPV chain within a PMMA host matrix during nitrogen purging (no SVA) and SVA with 100% toluene vapor. (a) Wide-field fluorescence images corresponding to the fluorescence transient in (b) at the times indicated. (b) The complete fluorescence transient (black curve) and the velocity, v, (red curve) can be recorded by tracking the position of the diffraction-limited spot in the microscope image. (c) Scheme of the relation between the conjugated polymer morphology, brightness, and solvent vapor annealing. Figure adapted from ref 68.

wide-field fluorescence images of a single MEH-PPV chain with and without solvent vapor swelling active (panel a). The movement of the single chain during annealing is directly visualized by following the diffraction-limited spot in space and time. Information on the annealing process is also encoded in the dynamics of the fluorescence intensity. The intensity of the spot was observed to increase a few seconds after the annealing process was started and continued to rise during the exposure to solvent vapor, as indicated by the black line in Figure 5b. The red line shows the velocity of the spot, corresponding to the lateral motion of the molecule, which also begins shortly after the onset of annealing and continues to accelerate during annealing. Removing the solvent vapor by flushing with clean nitrogen arrests the movement, which correlates with a renewed decrease in fluorescence intensity. This procedure is reproducible, as evidenced by the similarity in the results obtained when running multiple switching cycles in Figure 5b. The ϕF increase is interpreted as an unfolding process of the polymer during SVA, in agreement with previous comparisons of ϕF of MEH-PPV in the liquid and solid phases. The equivalent collapsed and random coil conformations are illustrated schematically in Figure 5c.5,69−71 Furthermore, via FCS, a large heterogeneity of diffusion times could be observed ranging from milliseconds to seconds, which leads to the 1508



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mechanisms of crust-forming and residual solvent accumulation in the film after termination of the annealing process can also be studied using this method.74 Improved understanding of these dynamics at the molecular level paves the way toward the rational design of customized annealing procedures for different materials to achieve the desired intrachain conformations and their respective spectroscopic or optoelectronic properties. In the final part of this Perspective, we depart from SVA in the solvation regime and address the emerging possibilities of annealing to control aggregation. Distinct differences can be Figure 7. Controlled aggregation in highly dilute, thin conjugated polymer guest/polymer host films by solvent vapor annealing. An increase in the chloroform/acetone vapor ratio raises the saturation limit, C∞, of MEH-PPV during solvent vapor annealing, whereas the overall MEH-PPV concentration, CCP, remains constant. The critical radius, RC, which determines the minimum size of stable aggregates, increases until it approaches infinity at C∞ > CCP (here, at ∼60% acetone, 40% chloroform). The evolution of a thin film during solvent vapor annealing is schematically illustrated at different chloroform/ acetone vapor ratios (the red dots represent single conjugated polymer chains, whereas the blue dots depict aggregates). Figure adapted from ref 59.

Distinct differences can be observed between the spectroscopic and optoelectronic properties determined at the singlemolecule level with regards to those obtained in bulk films. observed between the spectroscopic and optoelectronic properties determined at the single-molecule level with regards to those obtained in bulk films. One very important example is the comparison of exciton diffusion lengths in single MEH-PPV chains and in thin MEH-PPV films. On the one hand, the exciton migration length in bulk PPV-based derivatives was reported to be only about 7 nm or less,75,76 whereas recent subdiffraction-resolution measurements featuring single charge injection into single MEH-PPV chains suggest extraordinarily long energy-transfer distances of up to 75 nm.27,50,51 The immediate question arising is what factors are responsible for the deterioration of energy transport in the bulk.77 Two possible culprits are suggested, either the creation of interchain species that become low-energy trapping sites or a strong increase in disorder in the bulk due to random packing of chains as they form a solid.52 The aggregation regime of SVA is currently capable of shedding light on this phenomenon by allowing for the direct observation of the transition from single chains to aggregates, the first building blocks of a bulk film, by monitoring the correlated morphological and spectroscopic evolution. The aggregation regime is reached by reducing the solvent quality in the annealing process. Furthermore, the critical radius, RC, can be tuned to values at which stable aggregates can form by adjusting the quality of the solvent within this regime, for example, by mixing a good solvent with a bad solvent (see the dependence of RC on solvent quality in Figure 2b). This approach was recently demonstrated using the model system MEH-PPV/PMMA and derivatives thereof.59,78 The most important aspect in the experiment is the choice of solvents.59 The bad solvent must not hinder the diffusion of the polymer during solvent swelling, which means that it has to be a selectively good solvent for the host matrix (i.e., for PMMA). Acetone was used as a solvent selective for the host matrix in the model system MEH-PPV/PMMA,79 whereas chloroform was used as a nonselective good solvent for both guest and host polymers.6 RC could then be adjusted by mixing these two solvents, as illustrated in the cartoon in Figure 7. A rather small RC is expected when starting with 100% acetone vapor due to a very low saturation concentration C∞ (see eq 2). This small value will lead to the formation of many small aggregates, as indicated by the blue dots in Figure 7, with no single chains present after SVA (shown as red dots). As RC rises by

increasing the acetone/chloroform vapor ratio up to a value of 60/40, fewer, though larger, aggregates will form until C∞ exceeds the actual conjugated polymer concentration CCP at 100% chloroform vapor concentration. In this final situation, no aggregates can form during annealing. This SVA technique was used to build, from the bottom up, distinct ensembles of aggregates of different sizes. Wide-field fluorescence images of these mesoscopic objects are shown in Figure 8a.59 Using single-molecule spectroscopy techniques, such as those depicted in Figure 1f, the evolution of chain morphology and excitonic transport dynamics can be studied during the transition from single molecules to (small) bulk-like systems. For each single CP chain or aggregate, the polarization anisotropy was acquired, and a histogram (bottom panel of Figure 8a) of a large number (150−200) of particles was obtained. Surprisingly, as the aggregates grow in size, the polarization anisotropy histograms show only a slight shift of the average to lower values (see the red line in Figure 8a). Only if the mean intensity of the particles exceeds ∼45 times the intensity of a single CP chain do the polarization anisotropy histograms show a significant population of particles with low anisotropy values (right histogram in Figure 8a). It was concluded that the highly ordered morphology of single conjugated polymer chains (as shown in Figure 6b) persists up to aggregate sizes at least 25 times larger than a single conjugated polymer chain.59 Panels b and c summarize studies of the depth of spontaneous fluorescence intermittency (blinking) of the aggregates. The aggregate is 25 times brighter than the single chain yet shows nearly complete spontaneous modulation of the fluorescence intensity, presumably due to the formation of a quenching species such as a hole polaron. A similar blinking behavior was also previously observed for PPV clusters formed in poly(vinylalcohol).76 Analysis of the data using a simple energy-transfer model suggests that the longrange energy-transfer mechanism previously reported in single conjugated polymer chains51 is also present in these highly ordered aggregates due to efficient interchain coupling and a reduction in disorder.59 1509



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before they are assembled into larger, ordered objects to achieve a more-ordered morphology in the bulk. Such an ordered morphology evidently results in vastly improved transport of excitation energy. We stress that the discussion presented here focuses on structural and not on electronic intersegment aggregates of conjugated polymers. Even though the latter have been speculated on repeatedly,85,86 assignment of spectral features to electronic aggregation remains controversial, particularly in the case of MEH-PPV.87 However, signatures of strong interchromophore electronic aggregation are clearly manifested in model small-molecule electronic systems such as perylenes.88 For example, it was recently demonstrated that inhibition of spontaneous emission by promotion of strong H-aggregation in perylene nanocrystals can raise the effective exciton diffusion length by orders of magnitude.89 If such forms of electronic aggregation can indeed be conclusively observed in conjugated polymers, it would open a wide range of avenues for further engineering of light harvesting. In conclusion, we have illustrated some of the emerging topics with regard to the impact of SVA on intrachain conformations of conjugated polymers.32,64 A rough picture of polymer chain dynamics during SVA may already be acquired by conventional single-molecule spectroscopy methods,68 and it has been shown that this knowledge already leads to a rational approach to the design of highly ordered mesoscopic objects with distinct optoelectronic properties.59 The deeper inside view of the SVA process may lead to even better control of the different possible morphologies. Optimal morphologies may not always necessarily be characterized by the highest degree of ordering. It has been shown, for example, that a certain degree of structural disorder can lead to improved charge carrier dynamics by connecting ordered domains in polythiophene.16 A large set of state-of-the-art single-molecule spectroscopy methods, developed mainly in the life sciences, are simply waiting to be applied in the near future to investigate, in greater detail, the dynamics of the folding and aggregation processes of conjugated polymer chains during the transition from the dissolved to the solid state. Various FCS techniques, such as two-focus FCS,90 wide-field FCS, or FCS with polarized excitation light, will provide detailed information on the translational and rotational diffusion of polymers during annealing.91,92 Time-resolved dual-emission detection and advanced particle tracking with wide-field excitation can provide additional information on the dynamic photophysical changes, which are correlated to folding or unfolding events of the chains occurring during annealing.93 Finally, improved morphological control and a deeper understanding of the structure−function relationship will open up avenues to bottom-up design of devices, with control over various optoelectronic properties, such as excitonic characteristics, photon statistics, energy transfer, and charge mobility.

Figure 8. Morphological and spectroscopic evolution of MEH-PPV from single chains to aggregates. (a) Wide-field fluorescence images of highly diluted MEH-PPV/PMMA thin films after being subjected to different processing conditions. The aggregate samples were obtained after 30 min of SVA with the chloroform/acetone vapor ratios indicated in parentheses. The polarization anisotropy histograms obtained by fluorescence excitation polarization spectroscopy are shown below the corresponding widefield images and give insight into the conformation of the emitting species (high values correspond to ordered structures). The red line indicates the mean modulation depth. Representative single-particle fluorescence transients are shown for a single MEH-PPV chain in (b) and an MEH-PPV aggregate in (c) under the same experimental conditions. Even though the aggregate is 25 times brighter than the single chain, the object still behaves “as one” and shows digital intermittency. The inset gives a schematic representation of a single conjugated polymer chain and an aggregate consisting of 25 conjugated polymer chains interacting with a fluorescence quencher. Figure adapted from ref 59.

Together with the observations of very low energy-transfer lengths previously reported in bulk PPV-based derivatives75 and the fact that no blinking behavior is seen in disordered polymer nanoparticles,80,81 these results point toward the strong and quite remarkable influence of the morphology of the material on the single-chain level on the underlying bulk photophysics. Furthermore, if the highly ordered morphology of the single polymer molecules can be maintained upon annealing these chains into mesoscopic objects, as in the case of the ordered aggregates in Figure 8, the optoelectronic properties of the single chain can be maintained on the bulk level. Counter to conventional wisdom, bulk ordering is not detrimental and, importantly, does not lead to strong fluorescence quenching if it occurs in thermal equilibrium. The main difference between the conventional spin-casting process and the slow aggregation arising during SVA lies in the self-folding of the polymer chains. Depending on the spin-casting conditions, polymer chains may become entangled before all of the solvent is removed and hence do not have the chance to fold into ordered structures.82−84 However, the chains are already folded into well ordered rod-like structures within highly diluted conjugated polymer/polymer matrix blends (cf. Figure 6), and further annealing with a bad solvent causes them to assemble, preventing the chains from unfolding and becoming entangled. In other words, the chains must fold on themselves

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel: +49 (0) 941 943-2076. Fax: +49 (0) 941 943-4226. Notes

The authors declare no competing financial interest. 1510



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Biographies

(12) Granstrom, M.; Petritsch, K.; Arias, A. C.; Lux, A.; Andersson, M. R.; Friend, R. H. Laminated Fabrication of Polymeric Photovoltaic Diodes. Nature 1998, 395, 257−260. (13) Sirringhaus, H.; Brown, P. J.; Friend, R. H.; Nielsen, M. M.; Bechgaard, K.; Langeveld-Voss, B. M. W.; Spiering, A. J. H.; Janssen, R. A. J.; Meijer, E. W.; Herwig, P.; de Leeuw, D. M. Two-Dimensional Charge Transport in Self-Organized, High-Mobility Conjugated Polymers. Nature 1999, 401, 685−688. (14) Nguyen, T. Q.; Martini, I. B.; Liu, J.; Schwartz, B. J. Controlling Interchain Interactions in Conjugated Polymers: The Effects of Chain Morphology on Exciton−Exciton Annihilation and Aggregation in MEH-PPV Films. J. Phys. Chem. B 2000, 104, 237−255. (15) Ma, W. L.; Yang, C. Y.; Gong, X.; Lee, K.; Heeger, A. J. Thermally Stable, Efficient Polymer Solar Cells with Nanoscale Control of the Interpenetrating Network Morphology. Adv. Funct. Mater. 2005, 15, 1617−1622. (16) Kline, R. J.; McGehee, M. D.; Kadnikova, E. N.; Liu, J. S.; Frechet, J. M. J.; Toney, M. F. Dependence of Regioregular Poly(3hexylthiophene) Film Morphology and Field-Effect Mobility on Molecular Weight. Macromolecules 2005, 38, 3312−3319. (17) Reyes-Reyes, M.; Kim, K.; Carroll, D. L. High-Efficiency Photovoltaic Devices Based on Annealed Poly(3-hexylthiophene) and 1-(3-Methoxycarbonyl)-propyl-1-phenyl-(6,6)C-61 Blends. Appl. Phys. Lett. 2005, 87, 083506. (18) Yang, X. N.; Loos, J.; Veenstra, S. C.; Verhees, W. J. H.; Wienk, M. M.; Kroon, J. M.; Michels, M. A. J.; Janssen, R. A. J. Nanoscale Morphology of High-Performance Polymer Solar Cells. Nano Lett. 2005, 5, 579−583. (19) Kim, Y.; Cook, S.; Tuladhar, S. M.; Choulis, S. A.; Nelson, J.; Durrant, J. R.; Bradley, D. D. C.; Giles, M.; McCulloch, I.; Ha, C. S.; Ree, M. A Strong Regioregularity Effect in Self-Organizing Conjugated Polymer Films and High-Efficiency Polythiophene: Fullerene Solar Cells. Nat. Mater. 2006, 5, 197−203. (20) Hor, A. M.; Loutfy, R. O. Solvent-Induced Dimorphic Transformation in Magnesium Phthalocyanine and Its Effect on the Photoactivity. Thin Solid Films 1983, 106, 291−301. (21) Law, K. Y. Organic Photoconductive Materials  Recent Trends and Developments. Chem. Rev. 1993, 93, 449−486. (22) Miller, S.; Fanchini, G.; Lin, Y. Y.; Li, C.; Chen, C. W.; Su, W. F.; Chhowalla, M. Investigation of Nanoscale Morphological Changes in Organic Photovoltaics During Solvent Vapor Annealing. J. Mater. Chem. 2008, 18, 306−312. (23) Campoy-Quiles, M.; Ferenczi, T.; Agostinelli, T.; Etchegoin, P. G.; Kim, Y.; Anthopoulos, T. D.; Stavrinou, P. N.; Bradley, D. D. C.; Nelson, J. Morphology Evolution via Self-Organization and Lateral and Vertical Diffusion in Polymer:Fullerene Solar Cell Blends. Nat. Mater. 2008, 7, 158−164. (24) Hu, D. H.; Yu, J.; Wong, K.; Bagchi, B.; Rossky, P. J.; Barbara, P. F. Collapse of Stiff Conjugated Polymers with Chemical Defects into Ordered, Cylindrical Conformations. Nature 2000, 405, 1030−1033. (25) Teetsov, J.; Vanden Bout, D. A. Near-Field Scanning Optical Microscopy (NSOM) Studies of Nanoscale Polymer Ordering in Pristine Films of Poly(9,9-dialkylfluorene). J. Phys. Chem. B 2000, 104, 9378−9387. (26) Lupton, J. M. Chromophores in Conjugated PolymersAll Straight? ChemPhysChem 2011, 13, 901−907. (27) Adachi, T.; Brazard, J.; Chokshi, P.; Bolinger, J. C.; Ganesan, V.; Barbara, P. F. Highly Ordered Single Conjugated Polymer Chain Rod Morphologies. J. Phys. Chem. C 2010, 114, 20896−20902. (28) Ebihara, Y.; Vacha, M. Relating Conformation and Photophysics in Single MEH-PPV Chains. J. Phys. Chem. B 2008, 112, 12575− 12578. (29) Traub, M. C.; Lakhwani, G.; Bolinger, J. C.; Vanden Bout, D.; Barbara, P. F. Electronic Energy Transfer in Highly Aligned MEH-PPV Single Chains. J. Phys. Chem. B 2011, 115, 9941−9947. (30) Lim, S. H.; Bjorklund, T. G.; Bardeen, C. J. The Role of LongLived Dark States in the Photoluminescence Dynamics of Poly(phenylene vinylene) Conjugated Polymers. II. Excited-State Quench-

Jan Vogelsang is a university assistant at the University of Regensburg. He received his Ph.D. from the Ludwig-Maximilians University with Philip Tinnefeld (2009) and was a German Science Foundation postdoctoral fellow at UT Austin with Paul Barbara. His current research interests are the structure−function relationships of organic materials at the single-molecule level (http://www.physik.uniregensburg.de/forschung/lupton/lupton/jvogelsang.php). John M. Lupton holds a chaired professorship in Regensburg and is Research Professor at the University of Utah. He received his Ph.D. in physics from the University of Durham, U.K., and pursued various academic engagements in St. Andrews, Mainz, Munich, and Utah. His group works on single-molecule spectroscopy, semiconductor nanoparticles, nanoplasmonics and organic spin physics (http://www. physik.uni-regensburg.de/forschung/lupton/).

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ACKNOWLEDGMENTS The authors are indebted to Paul F. Barbara, David A. Vanden Bout, Takuji Adachi, Johanna Brazard, Joshua C. Bolinger, and Enrico Da Como for many inspiring discussions and collaborations in some of the measurements presented here. J.M.L. acknowledges financial support by the David & Lucile Packard Foundation. J.V. thanks the German Research Foundation (DFG) for a fellowship.

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Solvent Vapor Annealing of Single Conjugated Polymer Chains

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