Complexation of a Conjugated Polyelectrolyte and Impact on

Letter Cite This: ACS Macro Lett. 2019, 8, 88−94

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Complexation of a Conjugated Polyelectrolyte and Impact on Optoelectronic Properties Scott P. O. Danielsen,†,‡ Thuc-Quyen Nguyen,§ Glenn H. Fredrickson,*,†,∥,‡ and Rachel A. Segalman*,†,∥,‡ †

Department of Chemical Engineering, ‡Materials Research Laboratory, §Center for Polymers and Organic Solids, Department of Chemistry and Biochemistry, and ∥Materials Department, University of California, Santa Barbara, California 93106, United States

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ABSTRACT: Electrostatic assembly of conjugated polyelectrolytes, which combine a π-conjugated polymer backbone with pendant ionic groups, offer an opportunity for tuning materials properties and a new route for formulating concentrated inks for printable electronics. Complex coacervation, a liquid−liquid phase separation upon complexation of oppositely charged polyelectrolytes in solution, is used to form dense suspensions of πconjugated material. A model system of a cationic conjugated polyelectrolyte poly(3-[6′-{N-butylimidazolium}hexyl]thiophene) bromide and sodium poly(styrenesulfonate) dissolved in tetrahydrofuran−water mixtures was used to investigate this complexation behavior of conjugated polyelectrolytes in terms of electrostatic strength, solvent quality, and polymer concentration. The balance of electrostatic interaction between the oppositely charged polyelectrolytes together with their charge compensating counterions and solvent quality for the hydrophobic π-conjugated backbone leads to a rich phase diagram of soluble complexes, precipitates, and complex coacervates. The conjugated polyelectrolyte in the polyelectrolyte complexes has an increased πconjugation length and enhanced emissivity, with ideal chain configurations due to the reduction of kink sites and torsional disorder. The advantageous photophysical properties in the dense liquid phases makes the scheme attractive for the large-scale processing of optoelectronic devices, chemical sensors, and bioelectronics components.

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compatible with large-scale processing methods, it has to be highly tuned for each application, imparting a need for molecularly designed alternatives. Attempts to design a better aqueous formulation have thus far focused on tailoring the polyelectrolyte scaffold, through variation of the charge moieties, chain stiffness, and concentration of the polyelectrolyte,22,36−39 which have shown that the nature of the scaffold proves crucial not just for the resulting PEDOT:PSS structure, but also in the level of doping and stabilization of the PEDOT.22,36,38,39 An alternative to the dispersion polymerization approach has relied on making the electronically or optically active component intrinsically water-soluble by the use of conjugated polyelectrolytes (CPEs), which combine a π-conjugated polymer backbone with pendant ionic groups.40 This approach leads to better tunability over the solution structures,41−43 film morphologies, and resulting optoelectronic properties by using a single-component material.44 Polyelectrolyte complexation45,46 offers a viable strategy to combine these approaches for the development of controllable, molecularly designed aqueous formulations of semiconducting

queous formulations of conjugated polymers have enabled the printing of flexible, transparent, and biocompatible conducting films, spurring the growth of nextgeneration organic optoelectronic devices,1−15 chemical and biological sensors,16−23 bioelectronics,24−29 and thermoelectric modules,30−33 owing to their ability to conduct electrons or holes, ion permeability, environmental stability, and advantageous mechanical properties. However, the only industrially successful approach toward the development of aqueous semiconducting polymers has been the dispersion of a waterinsoluble conjugated polymer through the use of a p o l y e l e c t r o l y t e s c a ffo l d , P E D O T : P S S ( p o l y [ 3 , 4ethylenedioxythiophene]:poly[styrenesulfonate]). 8 The anionic polyelectrolyte (PSS) acts as a template and counterion for the positively charged, doped π-conjugated material (PEDOT). Conventionally, these materials are prepared through a dispersion electropolymerization, resulting in a colloidal mixture of hierarchical, ill-defined structure34 that is sensitive to synthetic and processing conditions when casting to solid films. Efforts to improve the transport properties of PEDOTbased devices have focused on ad hoc formulation engineering, primarily relying on mixing the heterogeneous dispersion with low-cost additives prior to casting.35 While this method has produced devices with conductivities up to 1000 S cm−1 and is © XXXX American Chemical Society

Received: November 28, 2018 Accepted: January 2, 2019

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DOI: 10.1021/acsmacrolett.8b00924 ACS Macro Lett. 2019, 8, 88−94

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ACS Macro Letters

The system was designed to maximize the width of the coacervate region by using polymers of high degrees of polymerization (N ≈ 150) and high charge densities (f = 1, 4− 5 Å charge spacing), enabling phase separation into a dense liquid phase at low overall concentration. The relatively high degrees of polymerization also simplify the system by minimizing the effects of chain stiffness on mesoscale assembly and phase separation. The contour length, the length of the fully extended polymer chain, of each chain is Lc ≈ 600 Å, much greater than the intrinsic persistence length, the length over which the polymer persists in a direction (a measure of

polymers amenable to large-scale fabrication protocols. A special case is the complexation upon mixing two oppositely charged polyelectrolytes in polar solvent resulting in liquid− liquid phase separation with a dense fluid phase, the coacervate, coexisting in equilibrium with a dilute supernatant phase.47 These dense fluid phases promote the accessibility of viscous, concentrated solutions, with very high loadings of the electroactive material, which are necessary for casting largeareas and in high-throughput. Herein, we demonstrate the ability to achieve fluid phases dense in π-conjugated material by the complexation of a model conjugated polyelectrolyte (poly{3-[6′-(N-butylimidazolium)hexyl]thiophene}bromide, P3BImHT +Br−) with an oppositely charged polyelectrolyte (sodium poly(styrenesulfonate), Na+PSS−; Figure 1). The optoelectronic properties of the

+

−

+

−

Br PSS = 30 Å or lNa = 9.1 Å,48,49 polymer stiffness), lP3BImHT p p,0 indicating expected flexible Gaussian chain behavior at scales exceeding lp. Further, the system was designed to decouple solubility from electrostatic effects, where both the polycation and the polyanion are hydrophobic polyelectrolytes with diffuse, ionic-liquid-like delocalized charges in the form of imidazolium and sulfonate groups. To minimize the importance of the counterions and keep the complexes soluble in high-dielectric media (bulky, organic counterions lower aqueous solubility), the associated counterions for each polyelectrolyte are small and monovalent, Na+ and Br−. Unlike most traditional coacervate systems (particularly the preponderance of peptide-based systems), the coacervate region for these hydrophobic polyelectrolytes does not develop upon the emergence of added ionic strength,50−53 but rather with added organic solvent. In this system, the coacervate region is suppressed at high ionic strength, due to the insolubility of P3BImHT+Br− in the presence of added salt. The state diagram of polyelectrolyte complexes formed at symmetric charge ratios (1:1 Na+PSS−::P3BImHT+Br−) across a wide range of combined monomer concentrations, 10−1000

Figure 1. Chemical structures of the cationic conjugated polyectrolyte, P3BImHT+Br−, and anionic polyelectrolyte, Na+PSS−.

resulting polyelectrolyte complexes can be enhanced by manipulating solvent quality and polymer concentration, to suppress self-aggregation and solid precipitation.

Figure 2. State diagram of 1:1 Na+PSS−:P3BImHT+Br− polyelectrolyte complexes with optical micrographs of isolated states formed. Solvent quality is quantified as the % THF in H2O, the dielectric constant is of the relative dielectric constant of the solvent mixture, and the ρ is the total polymer concentration (i.e., Na+PSS− and P3BImHT+Br−.) Primary samples studied in more detail are represented by large markers. Smaller markers indicate samples used to find the appropriate phase boundaries. The regions are color-coded by observed phase: one-phase solutions (yellow), solution and precipitates (red), solution and complex coacervates (green), and the ambiguous region of solution and metastable coacervate−precipitates (green-red gradient). Dotted lines show the estimated tie lines based off NMR analysis of the THF/H2O content in the coexisting supernatant for the 40% THF, 400 mM coacervate and 20% THF, 100 mM precipitate. High polymer concentration side of state diagram shown schematically. Concentration of coacervate (dense nexus of green tie line) estimated by mass conservation given the volume of coacervate formed. 89

DOI: 10.1021/acsmacrolett.8b00924 ACS Macro Lett. 2019, 8, 88−94

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Figure 3. (a) Normalized absorbance spectra, (b) absolute photoluminescence spectra (λex = 475 nm), (c) normalized photoluminescence spectra (λex = 475 nm), and (d) time-resolved photoluminescence decay curves (λex = 475 nm, λem = 650 nm) for 1:1 Na+PSS−:P3BImHT+Br− polyelectrolyte complexes with P3BImHT+Br− (1 M, 40% THF) shown for reference. Markers are measured data, and lines are exponential fits to the data.

multiphase regions are segregated according to the primary cause of the thermodynamic instability, either hydrophobic or electrostatic interactions, where the separated phases have structural signatures of the responsible interactions (e.g., precipitates, poor solvent quality; coacervates, electrostatics; coacervate−precipitate, both hydrophobic and electrostatic interactions). Samples from multiphase regions are separated into the dilute solution (the supernatant) and the dense phase (precipitate, coacervate, or coacervate−precipitate) with all further structural analysis and characterization done on the isolated phases. The precipitates, formed at low solvent quality (low THF content, high ionic strength), are polymer dense, dry, irregular solids (precipitate micrograph, Figure 2). These precipitates are almost entirely polymer, with little solvent content, and are amorphous without any evident crystalline structures or π−π stacking (Figure S4). The solvent present in the precipitate is primarily H2O (red tie line, Figure 2), likely due to the higher

mM, and solvent conditions, 10−50% (v/v) tetrahydrofuran (THF) in H2O is presented in Figure 2. This solvent range (10−50% THF v/v in H2O) covers a wide breadth of solvent quality for each component but a rather modest decrease in dielectric constant (ϵr = 54.6− 76.4),54 such that all solvent compositions are considered highdielectric media. The Bjerrum length, lB = e2/(4πϵ0ϵrkBT), the separation between two elementary charges at which the electrostatic energy is of order the thermal energy, kBT, ranges from 7−10 Å for these solvent compositions. Importantly, the Bjerrum length remains of order the charge spacing on the backbone, such that the degree of counterion condensation can be considered approximately constant throughout the phase diagram.55 There is a one-phase region of soluble polyelectrolyte complexes (solutions) at low polymer density and high solvent quality, as well as large multiphase regions, where either liquid−liquid or liquid−solid phase separation occurs. These 90

DOI: 10.1021/acsmacrolett.8b00924 ACS Macro Lett. 2019, 8, 88−94

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ACS Macro Letters

of this blue-shift is dependent on the exact phase, varying from 55−100 nm. The precipitate and most dilute solutions display the highest blue-shift of 100 nm, emphasizing the compact globular aggregates of the isolated complexes (Figure 3c). The aggregation of chromophores is likely relatively disordered due to chain collapse from the electrostatic interactions and solvent conditions rather than the formation of H- or J-style aggregates. The compact dimensions result in either a large number of, or more accessible structural defects that quench the electronic state through nonradiative pathways. The homogeneous solution and the precipitate quench the fluorescence relative to P3BImHT+Br− alone (Figure 3b). Notably, however, the dense complexes, coacervate, and coacervate−precipitate, increase fluorescence relative to P3BImHT+Br− (by 1−2 orders of magnitude). This increase in the emission of the P3BImHT+Br− within the complex is likely due to both the higher concentration of electroactive material located in the phase and the reduction of backbone torsional disorder and structural defects from bends and kinks that disrupt the connectivity of the π conjugated domains.57−60 The increased Stokes shift relative to the compact globular precipitate and isolated solution complexes is conventionally attributed to aggregation and resulting delocalization of the electronic wave function over multiple chromophores arising from more percolation and connectivity between different πconjugated domains.56,61 The changes in local concentration could increase fluorescence due to an increased number of fluorophores but also decrease emission due to internal filtering. Since both of these concentration effects may be occurring in these materials, to isolate chain connectivity and conformation effects, timeresolved measurements of the fluorescence lifetime, the amount of time the exciton stays in the excited state, can provide information about the disorder of the π domains (Figure 3d). This lifetime relates to local chain specific information as the emission of the photon results from the excited state losing energy through vibrational relaxation and the molecule returning to the ground state. Interestingly, the measured PL lifetimes are not a function of solvent polarity (as commonly seen due to differences in the polarization and dipolar relaxation), but rather are likely due to phase-specific chain conformation. Most notably, the complexes across a wide range of concentrations and breadth of solvent quality have comparable or longer PL lifetime relative to P3BImHT+Br− alone, at equivalent concentrations. Adding an insulating, nonfluorescent polymer (Na+PSS−) actually increases fluorescence. An analogy can be drawn to majority insulator blends used in organic field effect transistors (OFETs),62 where addition of even large amounts of insulator result in similar conductivities by maintaining ideal chain connectivity and eliminating deleterious self-trapping. Possibly due to higher levels of structural defects (e.g., bends, kinks, and twisting) along the conjugated polymer backbone,56,61 P3BImHT+Br−, solutions, and precipitates have photoluminescence decays that are biexponential with two characteristic time constants, including a short lifetime of ∼100 ps and a slow component of ∼500 ps (Figure 3d and Table S1). The fast PL lifetime is typically attributed to the single chromophore and the slow PL lifetime is from aggregation between multiple chromophores/polymer chain segments (either on a single chain or between multiple chains). The fast PL lifetimes of pristine 1 M P3BImHT+Br− and 100 mM, 20% THF precipitate are 92.1 and 81.3 ps, respectively, which

(in magnitude) energy of solvation of adsorbed H2O on the charged regions of the polyelectrolytes. Increasing in polymer concentration at higher THF content leads to a two-phase region of the phase diagram, where the polyelectrolyte complexation yields a liquid−liquid phase separation with a dilute solution, the supernatant, and a concentrated solution, the coacervate. The coacervate, isolated by centrifugation, appears in Figure 2 as a predominantly homogeneous fluid phase. The π-conjugation of P3BImHT+Br− leads to absorbance in the visible range due to the optical bandgap of the polymer. The intensity of red (or darkness beyond some optical cutoff) in the image is a proxy for the concentration of the polyelectrolyte. Further, the transmitted red color indicates relatively homogeneous distribution of the CPE throughout the phase. Unexpectedly, there is strong preferential partitioning of the THF into the coacervate rather than the supernatant (green tie line, Figure 2). The total THF in the phase scales with polymer concentration, implying that each chain requires some minimal amount of THF to solvate the aromatic segments. Decreasing the solvent quality at these concentrated conditions leads to a seeming overlap of the region of precipitate phase separation with that of electrostatic liquid− liquid phase separation, yielding a dilute solution supernatant in coexistence with a metastable coacervate−precipitate, a dense fluid phase with intense concentration fluctuations that appear as fibrillar-like aggregates (coacervate−precipitate micrograph, Figure 2). Due to the difficulty of identifying the distinction between concentration fluctuations and solid aggregates, we show a shaded region separating this presumed three-phase region from the neighboring two-phase regions. All phases across a wide breadth of solvent quality and polymer concentrations show signatures of complexation on their optical absorbance, even at very low solution concentrations (ρ ∼ 1 mM). The absorption of 1:1 P3BImHT+Br−:Na+PSS− polyelectrolyte complexes at different concentrations in 20% and 40% THF are red-shifted (Figure 3a) up to 50 nm from the λpeak ≈ 470 nm absorption of pristine P3BImHT+Br (40% THF), indicating that the πconjugation length of P3BImHT+Br− is extended as a result of open chain conformations upon the complexation with Na+PSS−. The red-shift increases with concentration and saturates at λpeak ≈ 520 nm at approximately 10 mM total polymer concentration. Complexation is commonly characterized by such a redshift in the absorption, but it is surprising that the degree of this reduction in the optical bandgap is relatively insensitive to the concentration, solvent quality, or structural features of the complex as is seen in CPE solutions or CPE−surfactant complexes.56−58 This data, however, shows that the formation of a polyelectrolyte complex results in an overall structure that is less sensitive to concentration or solvent. The absorbance remains a featureless single broad peak, without any vibronic character, confirming that the polymer remains in a random coil type configuration (recall, no π−π stacking or liquid crystallinity was observed, Figures S4 and S6). Additionally, there exists a broader distribution of chain conformations and conjugation lengths for the phases with aggregate character (precipitates, coacervate−precipitates) presumably due to heterogeneous character of these phases. The Stokes shift (Stokes shift = Δ(λabs − λPL)) is blue-shifted from P3BImHT+Br− for the all 1:1 polyelectrolyte complexes, also indicating the aggregation of chromophores. The degree 91

DOI: 10.1021/acsmacrolett.8b00924 ACS Macro Lett. 2019, 8, 88−94

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ACS Macro Letters are shorter than those of other phases (137−198 ps). The soluble complexes have longer lifetimes than the P3BImHT+Br− alone at the same concentration, reinforcing the notion that polyelectrolyte complexation likely reduces the torsional disorder of the polymer, by extending the πconjugation length upon complexation with Na+ PSS−. However, at increased solvent quality (increased THF content), the complexes show a decrease in the relative percentage of the short lifetime, corroborating an increase in the solvent quality and a reduction in self-quenching, possibly due to the increasing ideality of the chain dimensions. At high concentrations, the coacervate becomes monoexponential with a PL lifetime of 481.5 ps. Conjugated polymer chains take on different conformations in different solution environments and the tendency for conjugated chromophores to aggregate together depends sensitively on the chain conformation. The nature of chromophore interaction in solution depends on a subtle interplay of many factors: polymer−solvent interactions, electrostatics, and the local chain conformations or overlap, much like explored in single component conjugated polymer solutions.56 This sensitivity has been exploited to understand the behavior of P3BImHT+Br− within the formed polyelectrolyte complexes. We have developed aqueous formulations of an organic semiconductor through the complexation of a cationic CPE with an anionic polyelectrolyte. This polyelectrolyte complexation incorporating a CPE yields dense phases of the electroactive material where the fluidity of the dense phase is controlled by manipulation of the solvent quality. The fluidity of the dense coacervates further accentuates the promising nature for casting organic electronic materials, by offering desirable solution properties of accessible viscosity and surface tension, while delivering a high concentration (up to approximately 10 M of active material, significantly higher than would be achievable through conventional solutions of conjugated polymers, which are limited in solubility in many solvents (typically to approximately 0.1 M or lower). The CPE in the polyelectrolyte complexes has an increased π-conjugation length and enhanced emissivity, with reduced self-aggregation possibly due to ideal chain configurations from the reduction of kink sites and torsional disorder. While not explored in this study, memory of the polymer conformation and degree of chromophore interaction has been shown to be carried through the casting process and survive into the solid state film. Specifically, the approach of polyelectrolyte complexation should be considered as a method for the reduction of nonradiative decay losses in organic photovoltaics (OPVs) or as a mechanism to increase light emission in lightemitting electrochemical cells (LEECs). The loose packing, poor energy transfer, and increased luminescence would be desirable for electroluminescent devices where fluorophore aggregations conventionally reduce luminescence and facilitate rapid excitation transport.63,64 Thus, it is expected that manipulating all these variables by polyelectrolyte complexation in solution could lead to improved strategies for processing thin film organic optoelectronic devices.

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Synthetic and characterization procedures, X-ray structural characterization, and photoluminescence decay fits (PDF).

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. ORCID

Scott P. O. Danielsen: 0000-0003-3432-5578 Thuc-Quyen Nguyen: 0000-0002-8364-7517 Glenn H. Fredrickson: 0000-0002-6716-9017 Rachel A. Segalman: 0000-0002-4292-5103 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors gratefully acknowledge support from the U.S. Department of Energy Office of Basic Energy Sciences under Grant No. DE-SC0016390. This research used resources at beamline 11-3 of the Stanford Synchrotron Radiation Lightsource, SLAC National Accelerator Laboratory, 7.3.3 of the Advanced Light Source, Lawrence Berkeley National Laboratory, and 11-BM (CMS) of the National Synchrotron Light Source II, Brookhaven National Laboratory, which are supported by the Office of Basic Energy Sciences, of the U.S. Department of Energy under Contract Nos. DE-AC0276SF00515, DE-AC02-05CH11231, and DE-SC0012704. Use of the Shared Experimental Facilities of the Materials Research Science and Engineering Center at UCSB (MRSEC NSF-DMR-1720256) is gratefully acknowledged. The authors thank Dr. Alexander Mikhailovsky (UCSB) for assistance in obtaining the time-resolved photoluminescence spectroscopy measurements and Dr. Rachel Behrens (UCSB) for assistance in polymer characterization and analysis.

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REFERENCES

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ACS Macro Letters

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