Entropy-Driven Heterocomplexation of Conjugated Polymers in Highly

Article Cite This: J. Phys. Chem. C 2019, 123, 16596−16601

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Entropy-Driven Heterocomplexation of Conjugated Polymers in Highly Diluted Solutions Longfei Wu, Belen Nieto-Ortega, Teresa Naranjo, Emilio M. Peŕ ez,* and Juan Cabanillas-Gonzalez* Madrid Institute for Advanced Studies in NanoscienceIMDEA Nanociencia, Calle Faraday 9, Ciudad Universitaria de Cantoblanco, Madrid 28049, Spain

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S Supporting Information *

ABSTRACT: Examples of complexes formed by the association of two dissimilar polymers are scarce. We unravel in this report a rare phenomenon of complexation of two conjugated polymers in highly diluted solutions. Spectroscopy characterization of titrated solutions combined with molecular dynamics and time-dependent density functional theory reveal the presence of strong supramolecular interactions giving rise to the formation of conjugated heterocomplexes of the distinctive charge transfer electronic character. Temperaturedependent studies provide association constants in the 104 to 105 M−1 range and negative van’t Hoff slopes characteristic of entropy-driven association. We explain the entropy gain as the result of solvent molecule release and their subsequent redistribution upon polymer association. Our findings have implications in the field of organic photovoltaics, where the mixing/segregation of polymers of different electronic characters is key to performance.

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transfer,18,19 or to achieve photoinduced electron transfer at electron donor−acceptor interfaces in organic solar cells,20,21 amid other examples. Among the many reported combinations, there are several examples of excited state heterointeractions between the blend components forming exciplexes localized at the interface between the two phases. Exciplexes have been reported, for instance, in blends of some fluorene-based copolymers.22,23 Ground-state heterointeractions in conjugated polymer blends are nevertheless much more unusual. It has been shown that doping of some conjugated polymers with strong electron acceptor molecules such as 2,3,5,6-tetrafluoro7,7,8,8-tetracyanoquinodimethane or 1,5-dinitroanthraquinone results in ground-state charge transfer complexes and a concomitant change in the color of the mixture.24,25 Ground state heterointeractions have also been invoked to explain photocurrent at low photon energies below the band gap in organic heterojunction solar cells,26 the presence of distinctive phase transitions in differential scanning calorimetry thermograms of polymer/small molecule blends,27 or the appearance of new peaks in the small-angle X-ray scattering blend patterns assigned to conjugated polymer/fullerene co-crystals.28 Despite these observations, there has not been a sufficient description into the nature and kinetics of macromolecular heterocomplexation. In this paper, we shed light into an unprecedented phenomenon of strong supramolecular interactions between

INTRODUCTION Supramolecular structures featuring the association of two different molecular species bound by weak intermolecular interactions are an attractive class of arranged matter. The interactions that govern the self-assembly range from electrostatic to π−π or hydrogen bonding.1 Supramolecular structures of two or more compounds that self-assemble into spheres, rods, or sheets have been reported.2−9 In solvating media, supramolecular association requires, apart from geometrical affinity, the adoption of fine equilibrium between the energetics of solvation and the specific molecular interactions.10 However, when it comes to polymers, reports on supramolecular polymer−polymer heterointeractions are rare. As molecules become larger, self-recognition, eventually leading to segregation, is much more likely based on simple “like-dissolves-like” arguments, which are thermodynamically reflected in low entropies of mixing.11−13 In fact, in the case of polymer mixtures, segregation becomes a real problem. This problem can be addressed by purposely building in host−guest recognition motifs.14,15 However, in applications where the structure of the polymer and its function are tightly linked, structural variations to favor heterointeractions are not desirable. In the field of organic optoelectronics, the hectic quest for outperforming devices has triggered the synthesis of a wide range of conjugated polymers and molecules. The use of blend layers of two polymer or molecular components has been extensively applied, in a trial and error fashion, to achieve ambipolar charge transport in a single layer,16,17 to exploit luminescent down conversion via Förster resonant energy © 2019 American Chemical Society

Received: May 20, 2019 Revised: May 27, 2019 Published: June 14, 2019 16596

DOI: 10.1021/acs.jpcc.9b04758 J. Phys. Chem. C 2019, 123, 16596−16601

Article

The Journal of Physical Chemistry C

spectrum was acquired 2 min after addition in order to achieve complete equilibrium. The titrated absorption spectra of toluene mixtures obtained at varied temperatures (283, 288, 298, and 318 K) were fitted with ReactLab EQUILIBRIA (Jplus Consulting) software, which provides different binding models for global analysis. DLS was carried out with a Malvern Zetasizer Nano ZS at room temperature. Intensity-averaged DLS histograms were obtained from the mean of three consecutive measurements, each one involving 15 repeating runs.

two conjugated polymers of interest in optoelectronics, namely, regioregular poly(3-hexylthiophene-2,5-diyl) (P3HT) and poly(9,9-dioctylfluorene-alt-benzothiadiazole) (F8BT). Blend layers of F8BT and P3HT have been exploited for a wide range of applications including light-emitting diodes,29 light-emitting transistors,30 optically pumped lasers,31 and bulk heterojunction solar cells.32 Blends of these two components exhibit a characteristic red photoluminescence (PL) and substantially improved PL quantum efficiency with respect to pristine P3HT.33,34 The performance of F8BT:P3HT blends as active layers in OSCs is, however, rather poor, an observation that has been explained to be due to the large blend miscibility and inability to form interpenetrated F8BT and P3HT rich domains in blends, required for the efficient charge transport and collection.35 Indeed, in a previous report, some of us have shown that these blends show low degree of P3HT crystallinity inferred from substantial reduction in P3HT crystallization enthalpies upon its dispersion in F8BT.34 We hereby report a systematic study of equilibrated F8BT:P3HT mixtures in solution which confirms the formation of F8BT:P3HT heterocomplexes at low molar contents in a nonpolar solvent such as toluene, as confirmed by UV−vis absorption, PL, and dynamic light scattering (DLS). Titration experiments performed at different temperatures provide evidences for an endothermic entropy-driven association of F8BT and P3HT in toluene with binding constants ranging in the 104 to 105 M−1 range. These observations are supported by molecular dynamics (MD) and time-dependent density functional theory (DFT). We explain this phenomenon in terms of weakly interacting toluene molecules that are primarily enclosed by entangled polymer segments and subsequently released upon F8BT:P3HT association, leading to entropy gain. Our studies confirm that F8BT:P3HT complexes are already formed in precursor solutions and subsequently freeze in the solid state when solutions are processed into thin films.

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RESULTS AND DISCUSSION Assessment of Beer−Lambert law compliance is an appropriate starting point to probe for intermolecular interactions between chromophores in solution. Assuming that only two types of chromophores are effectively dissolved, that is, in the absence of complex formation, the total absorbance should exactly match the superposition of the individual concentrations times of their molar extinction coefficients. Figure 1a depicts the

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EXPERIMENTAL METHODS Conjugated polymer solutions with 10−3 to 10−1 mM polymer concentration were prepared upon mixing separated master solutions of regioregular P3HT (BASF Sepiolid P200, >98% regioregularity, Mn ≈ 14 kg mol−1, Mw ≈ 34.6 kg mol−1, PDI = 2.48) with F8BT (American Dye Source, Mn ≈ 33 kg mol−1, Mw ≈ 217 kg mol−1, PDI = 6.6) in a 3:1 P3HT:F8BT monomer molar ratio. Absorption and PL measurements were performed on a Cary 50 UV−vis spectrophotometer (Varian/ Agilent) and a FluoroMax 4 fluorimeter (Jobin Yvon). Temperature-controlled UV−vis titration experiments were carried out on a Cary 5000 UV−vis−NIR spectrophotometer (Varian/Agilent). The titration method consisted the preparation of a F8BT host solution with 0.1 mM F8BT monomer concentration ([F8BT]0) and a separate guest solution composed of a mixture of 1 mM P3HT in 0.1 mM F8BT and subsequent addition of different volumes of guest to host solutions. Given that the F8BT monomer concentrations in host and guest solutions were the same, the total F8BT concentration upon subsequent addition of the guest into the host solution remained constant whilst the P3HT content in the titrated solution ([P3HT]0) increased proportionally. Titrated absorption spectra were measured systematically after addition of guest solution in steps of 0.1 [P3HT]0/ [F8BT]0 equivalents up to a total P3HT content of 3 [P3HT]0/[F8BT]0 equivalents (3:1 molar fraction). Each

Figure 1. (a) Absorption of a F8BT:P3HT mixture (1:3 monomer molar ratio, 10−3 mM monomer concentration) in chloroform (open circles) and toluene (open squares). The absorption spectrum in chloroform was accurately reproduced by a linear combination of the absorption spectra of F8BT and P3HT (dashed line). (b) PL spectra photoexcited with 2.4 eV from the same F8BT:P3HT mixture in chloroform (open circles) and toluene (open squares). Dashed lines stand for Frank−Condon fits. (c,d) Corresponding DLS intensitybased size distributions obtained from mixtures in chloroform and toluene, respectively. The obtained hydrodynamic radii are provided in the insets.

normalized absorbance of F8BT:P3HT dispersions (1:3 monomer molar ratio, 10−3 mM) in chloroform (open circles) and toluene (open squares) as well as a linear combination of the F8BT and P3HT spectral contributions (dotted line), whereas the absorbance in chloroform is closely reproduced by a superposition of F8BT and P3HT absorbance, and clear deviations are evident in toluene. A bathochromic shift of the main absorption band from 2.75 to 2.65 eV is seen together with the appearance of an additional shoulder located in the 2.40−2.10 eV region, features which are unnoticed in the absorbance of individual F8BT and P3HT chloroform and toluene control solutions (Figure S1 in the Supporting Information). These spectral discrepancies have their correspondence in the PL spectra. The PL spectrum of the polymer 16597

DOI: 10.1021/acs.jpcc.9b04758 J. Phys. Chem. C 2019, 123, 16596−16601

Article

The Journal of Physical Chemistry C mixture in chloroform photoexcited with 2.4 eV (Figure 1b) exhibits a maximum at 2.17 eV, and shoulders at 1.99 and 1.81 eV in very good agreement with the 0−0, 0−1, and 0−2 PL transitions (2.16, 1.98, and 1.80) of P3HT in chloroform (Figure S1 in the Supporting Information). In line with absorbance, the PL of the mixture in toluene cannot be ascribed to contributions from the pristine polymers. A Frank− Condon fit of the PL spectrum of the toluene mixture provides a 0.8 Huang−Rhys factor, 2.04 eV 0−0 phonon emission, and 0.17 eV vibronic displacement values which are substantially different from the corresponding 0.96, 2.17, and 0.18 eV of P3HT in toluene and in the chloroform mixture (see Frank− Condon fits of PL spectra from pure P3HT in Figure S1 together with a summary of all fitting parameters in Table S1 in the Supporting Information). These new optical transitions seen in the absorbance and PL in toluene point toward the formation of F8BT:P3HT complexes. Additional proofs for F8BT:P3HT complexation are provided by DLS experiments in chloroform and toluene (Figure 1c,d, respectively). The intensity-averaged size distribution in chloroform depicts a dominant contribution (76%) with a hydrodynamic radius (RH) of 30 nm (PDI = 0.48), a value which approaches 22 (21) and 32 nm (21 nm) found in F8BT (P3HT) in chloroform and toluene dispersions, respectively, (Figure S2 in the Supporting Information). Conversely, the DLS histogram of F8BT:P3HT in toluene reflects a significantly enhanced RH value (109 nm, PDI = 0.50), suggesting the formation of large hydrodynamical scattering centers which are not present in chloroform nor in the single polymer dispersions. Assuming that the distinctive absorbance, PL, and DLS features in toluene are caused by F8BT:P3HT complexation, what could be the driving force responsible for such phenomenon? Additional insights are provided through a quantitative evaluation of the F8BT:P3HT binding constant and its dependence with temperature. For this purpose, we performed UV−vis titration experiments on F8BT:P3HT chloroform and toluene dispersions. Figure 2a,b depicts, respectively, the room-temperature absorbance in toluene and chloroform upon increasing the molar ratio of P3HT in F8BT in 0.1 steps whilst keeping the F8BT concentration constant. In line with Figure 1, gradual absorbance red-shift and a parallel rise of the shoulder at the low-energy tail take place upon P3HT doping, in sharp contrast with the less variable spectral shape in chloroform. The evolution of these complex-driven features was correlated to an appropriate binding model to determine the F8BT:P3HT molar stoichiometry ratio and the association constant (Ka) of the complex at different temperatures. The best fit was provided by a 1:1 complexation ratio and Ka values in the 104 to 105 M−1 range (ln Ka = 10.7 ± 0.6 at room temperature, see Table S2 and Figure S3 in the Supporting Information for titration details, spectral weights, and individual concentrations obtained with global analysis). The absorbance binding isotherms of F8BT:P3HT complexes are shown in Figure 2c, suggesting large agreement with the behavior predicted by the binding model. The concentration of complexes rises steeply evolving toward a highly sublinear increase above the 1:1 molar ratio. The changes in enthalpy and entropy associated to the complexation were evaluated from the temperature dependence of Ka (Figure 2d) according to the van’t Hoff relation.36 The fit provides 57.8 kJ mol−1 and 278.4 J mol−1 K−1 ΔH and ΔS values, respectively, suggesting that the association of

Figure 2. (a) Room-temperature absorption spectra of F8BT:P3HT mixtures with the monomer molar ratio ranging from 1:0 to 1:3 in 0.1 equiv steps in (a) chloroform and (b) toluene. In both cases, the F8BT monomer concentration (0.1 mM) was kept constant in all mixtures. (c) Binding isotherms obtained from the absorbance at 2.2 eV as a function of the F8BT:P3HT monomer molar ratio at 288 K (empty up-triangles), 298 K (filled down-triangles), and 318 K (empty diamonds). The absorbance of pristine F8BT was in all cases subtracted from the total absorbance at 2.2 eV of each mixture. Dashed lines stand for global fits at each temperature. (d) Dependence of Ka with temperature. Values were obtained as an average of three consecutive titration experiments. A linear van’t Hoff fit (R2 = 0.89) yielded ΔH = 57.8 kJ mol−1 and ΔS = 278.4 J mol−1 K−1.

F8BT and P3HT in toluene is an endothermic- and entropydriven process. Previous studies on entropy-driven host−guest encapsulations in inorganic or organic medium have interpreted that the large entropy gain arises from desolvation of the host interior cavity and of the guest surface which compensates for the enthalpic cost.37,38 Likewise, as two single P3HT and F8BT chains encounter to form a heterocomplex, abundant toluene molecules surrounding the polymer chains are liberated to the bulk solvent, giving rise to the observed entropy increase. In order to visualize the role of solvent polarity on polymer interactions, we carried out MD in combination with DFT calculations on chloroform and toluene mixtures with the adoption of continuum solvation models. First, we carried out MD simulation of both polymers in the corresponding solvents. Figure 3a,b depicts representative snapshots obtained with these two models. The most stable configuration in chloroform provokes an edge-to-face arrangement between the respective aromatic moieties present in F8BT and P3HT, which are kept together by the interplays between the aliphatic chains and the aromatic moieties. Meanwhile, a significantly larger degree of face-to-face and π−π interaction is found in toluene. The polymer chains in toluene intertwine to form an analogous heterogeneous double helix rather than more separated assembly in chloroform, which favors cofacial π−π interactions. In the simulations, the solvent was considered as a continuum solvation model (see the Supporting Information for details) for that we are inclined to say that the difference in the aggregation form is mainly due to electronic effects, but the structural effect cannot be discarded. Next, we evaluate the effect of F8BT:P3HT complexation in the absorbance spectra with DFT calculations on relevant F8BT:P3HT snapshots chosen from MD for both models, 16598

DOI: 10.1021/acs.jpcc.9b04758 J. Phys. Chem. C 2019, 123, 16596−16601

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(LUMO) (62%) and HOMO → LUMO + 1 (31%) oneelectron promotions. The HOMO is localized in the conjugated terthiophene skeleton, whereas the LUMO and LUMO + 1 are centered in the F8BT moieties, Figure 4c, confirming the charge transfer nature of this transition. Therefore, the absorption signature band of the F8BT:P3HT complex is referred to as the charge transfer band, P3HT acting as an electron donor and F8BT as an electron acceptor. Previous studies on oligomers with alternating thiophene− benzothiadiazole groups have demonstrated how the donor (acceptor) nature of thiophene (benzothiadiazole) groups results in the formation of an intramolecular charge transfer state.39 Likewise, predominance of polymer/polymer over polymer/solvent interactions in F8BT:P3HT solutions could favor the formation of interchain charge transfer states delocalized across adjacent P3HT and F8BT chromophores.

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CONCLUSIONS In summary, we find clear spectroscopic evidences for the formation of heterocomplexes of two conjugated polymers in solution giving rise to distinctive new optical transitions detectable in the absorption and PL spectra. MD combined with DFT calculations provide additional insights into their electronic nature confirming a strong charge transfer character of the HOMO → LUMO and of the HOMO → LUMO + 1 heterocomplex transitions. Analysis of the underlying equilibria and thermodynamic conditions of complexation at different temperatures reveals association constants in the 104 to 105 M−1 range which increase with temperature, suggesting that entropy is the main driving force for F8BT:P3HT complexation. This unusual conjugated polymer complexation paves the way for engineering of heterointerfaces with advanced photonic and light harvesting properties.

Figure 3. MD snapshots of F8BT:P3HT assembly (a) in chloroform and (b) in toluene. Color code: green carbon atoms belong to F8BT, red carbon atoms belong to P3HT, blue for nitrogen atoms, and yellow for sulfur atoms.

Figure 4a. For each experimental condition, we have analyzed the F8BT:P3HT dimer configurations, selecting the most stable dimers. The corresponding theoretical absorption spectra are very well reproduced by DFT, Figure 4b, compared to our absorption results of polymer mixtures. Remarkably, the DFT calculations predict the existence of an additional band at 2.4 eV for the F8BT:P3HT toluene model, which is reminiscent of the characteristic low-energy band of the F8BT:P3HT complex. The electronic transition computed at 2.4 eV (with oscillator strength f = 0.0597) corresponds to the excitation from the ground state to the first singlet excited state (S0 → S1). A further insight into the nature of this transition can be obtained by analyzing the molecular orbitals. As a result, the electronic transition can be described as the highest occupied molecular orbital (HOMO) → lowest unoccupied molecular orbital

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.9b04758. Absorbance and PL of pristine polymers, dynamic light scattering results in pristine polymer solutions, titration

Figure 4. (a) Optimized F8BT:P3HT structures in chloroform (top) and toluene (bottom) for DFT calculations. Color code: green carbon atoms belong to F8BT, red carbon atoms belong to P3HT, blue for nitrogen, and yellow for sulfur. (b) Theoretical UV−vis spectra of the two models, red for chloroform and black for toluene. The energies and oscillator strengths calculated for the electronic transitions are included as bars. Level of calculations: DFT/CAM-B3LYP/6-31G(d,p). (c) Molecular orbital topologies calculated for (S0 → S1) monoelectronic transition for the F8BT:P3HT complex in toluene [HOMO → LUMO (62%), HOMO → LUMO + 1 (31%)]. Color code: gray for carbon atoms, blue for nitrogen, and yellow for sulfur. 16599

DOI: 10.1021/acs.jpcc.9b04758 J. Phys. Chem. C 2019, 123, 16596−16601

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details, spectral weights, concentration, molecular dynamics, DFT calculations, and Z-matrix (PDF) The optimized cartesian coordinates of F8BT:P3HT dimers (XYZ) (XYZ)

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (E.M.P.). *E-mail: [email protected] (J.C.-G.). ORCID

Belen Nieto-Ortega: 0000-0002-1834-3664 Emilio M. Pérez: 0000-0002-8739-2777 Juan Cabanillas-Gonzalez: 0000-0002-9926-3833 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS L.W. and J.C.-G. acknowledge the China Scholarship Council for financial support through project number: 201206230083. J.C.-G. acknowledges funding from the Regional Government of Madrid through NMAT2D-CM project (S2018/NMT4511) and from MINECO through projects MAT2014-57652C21/2-R (LAPSEN), PCIN-2015-169-C02-01/02 (MOFSENS) and RTI2018-097508-B-I00 (AMAPOLA). E.M.P. acknowledges funding from the European Union (ERC-StG307609), MINECO (CTQ2014-60541-P and CTQ201786060-P), and the Comunidad de Madrid (MAD2D-CM S2013/MIT-3007). IMDEA Nanociencia acknowledges support from the “Severo Ochoa” Programme for Centres of Excellence in R&D (MINECO, grant SEV-2016-0686). We thank Comunidad de Madrid and the European Structural Funds for their financial support through FotoArt-CM project (S2018/NMT-4367). We thank Mariano Vera-Hidalgo for help with DLS measurements.

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DOI: 10.1021/acs.jpcc.9b04758 J. Phys. Chem. C 2019, 123, 16596−16601

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DOI: 10.1021/acs.jpcc.9b04758 J. Phys. Chem. C 2019, 123, 16596−16601