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Directing the Conformation of Oligo(phenylenevinylene) Polychromophores with Rigid, Nonconjugatable Morphons Xinju Zhu,† Beiyue Shao,‡ David A. Vanden Bout,*,‡ and Kyle N. Plunkett*,† †

Department of Chemistry and Biochemistry and the Materials Technology Center, Southern Illinois University, Carbondale, Illinois 62901, United States ‡ Center for Nano- and Molecular Science and Technology, Department of Chemistry, The University of Texas at Austin, Austin, Texas 78712, United States S Supporting Information *

ABSTRACT: The synthesis and morphological investigation of a series of polychromophore polymers composed of oligomeric bis(2-ethylhexyl)-p-phenylenevinylene (BEH-PPV) (trimer, pentamer, and heptamer) monomers copolymerized with rigid morphological directing groups (morphons) are described. The polymerization was carried out using a Sonogashira cross-coupling polymerization between monomers composed of an iodo-terminated BEH-PPV oligomer and a bis(phenylacetylene)-containing morphon. The rigid morphons are prepared from adamantane and diamantane frameworks that are composed solely of sp3 carbons, inhibit conjugation between BEH-PPV oligomers, and direct the local polymer morphology in either a bent or linear vector, respectively. The morphological properties of the polychromophore polymers were interrogated via single molecule fluorescence spectroscopy, thin-film absorption and fluorescence spectroscopy, and atomic force microscopy. Significant morphological variation was found upon substituting the morphon, as well as the chromophore size, with the most ordered structures being accessed with diamantane morphons.

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molecule spectroscopy (SPS)17−19 to probe the morphology of single polymer chains20,21 or small polymer aggregates22−24 as related to polymer backbone composition. Our recently studied polymer system utilized polychromophores with oligomeric bis(2-ethylhexyl)-p-phenylenevinylenes (BEH-PPV) separated with flexible morphons (tetraethylene glycol) that completely inhibit through-chain electronic communication between chromophores and provide ample flexibility between chromophore units.25 We found correlations between the size of the chromophore and the single polymer chain anisotropies with the longest chromophores (heptamers) assembling into higher ordered nanostructures versus the polymers with shorter chromophores (trimer and pentamer).26 Furthermore, the thin film fluorescence data supported these findings in the solid state with red-shifted signals that constituted a distribution of overlapping Franck−Condon progressions.25 These results are significant as highly aligned single polymer nanodomains facilitate ultralong range energy transfer.27−29 These findings spurred further questions about flexibility versus rigidity in the linking morphons of these polychromophore systems. In this contribution, we have utilized the previously synthesized BEHPPV chromophores (trimer, pentamer, and heptamer) but have incorporated them into polychromophore structures (Figure 1)

INTRODUCTION Understanding the role of molecular structure on polymer morphology and then adapting that knowledge to create functional materials through rational design will be important for further improvement of optoelectronic devices based on conjugated polymers (CPs).1,2 Chemical modification of CPs can lead to significant variation in polymer morphology,3,4 which can be accomplished through either side-chain variation or through a multitude of possible main-chain manipulations including atomic substitutions5,6 or changes in regioregularity.7 To gain premium insight into structure−property relationships, systematic variation of the chemical structure parameters is critical. One strategy to address this challenge is to utilize polychromophore polymers8−16 where identical CP chromophore fragments are linked between morphological directing groups (morphons). This strategy is beneficial owing to the defined nature of the chromophore that does not suffer from variation in the effective chromophore length from chemical or physical (torsion angles) defects that typically occur in CPs. Owing to the defined chromophore, the optoelectronic properties of the chromophores are identical, and therefore any new electronic states that are accessed in the film can be identified as interchromophore interactions. One approach to investigate the effect of structure changes on bulk film properties involves a bottom-up approach that attempts to correlate morphology at a single polymer level to thin films. Over the past several years, we have utilized single © XXXX American Chemical Society

Received: January 11, 2016 Revised: April 21, 2016

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DOI: 10.1021/acs.macromol.6b00067 Macromolecules XXXX, XXX, XXX−XXX

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oligomers. To create polychromophores that could be easily compared to previous examples, we required the synthesis of diethynylene functionalized monomers for copolymerization with the already known BEH-PPV oligomers.25 The synthesis of a nonlinear morphon started with the preparation of 1,3dibromoadamantane 7 via bromination of adamantane (Scheme 1). A Friedel−Crafts alkylation to bromobenzene gave 8 which was carried forward via a Sonogashira crosscoupling with TMS−acetylene to give 9. Deprotection of the silyl protecting group gave the desired bis(phenylacetylene)adamantane 10 with the reactive ethynylene groups directed at an angle of 109.5° from each other. The synthesis of the linear diamantane morphon began with the preparation of 4,9dibromodiamantane 11, which was converted into the desired morphon 14 through a series of analogous transformations resulting in a monomer with alkynes directed at an angle of 180° from each other. Polychromophore synthesis was accomplished using a stepgrowth polymerization between morphons 10 or 14 with the previously synthesized iodo-terminated BEH-PPV oligomers 15−17 (trimer, pentamer, or heptamer).25 The polymerization was carried out using Sonogashira cross-coupling conditions with a catalyst system of Pd(PPh3)4, CuI, and NHEt2 in toluene at 85 °C for 3 days (Scheme 2). The molecular weights varied between 15 and 40 kDa allowing access to polymers that contained ∼10−15 chromophores per polymer chain (Table 1). The adamatane-linked polychromophores (1−3) as well as the diamantane-linked heptamer polychromophore 6 were quite soluble in common solvents. However, the diamantane linked trimer 4 and pentamer 5 polychromophores were considerably less soluble and are most likely due to the smaller relative content of solubilizing chains with the polymers adopting more elongated structures in the solid owing to the linear diamantane linker.

Figure 1. Rigid-morphon BEH-PPVs prepared in this work.

with rigid and nonconjugatable morphons based on adamantane (1−3) and diamantane (4−6). The polychromophores were investigated both at the single polymer chain level and in thin films, and we have found correlation between the order of the polymers and the chromophore conjugation length, as well as the choice of rigid morphon, in the polymer systems. Polymer Synthesis. The diamondoid hydrocarbons adamantane and diamantane were selected as rigid, nonconjugatable morphon units owing to their known functionalization chemistry30−32 that provides access to substituent directing angles of 109.5° or 180°, respectively. The all sp3 carbon framework of the diamondoids assures limited throughchain electronic communication between the BEH-PPV Scheme 1. Synthesis of Rigid Morphons 10 and 14

B


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Table 1. Summary of Electrochemical and Optical Properties of 1−6a polymer

Mn

Đ

av no. chromophores

λabs,max,sol (nm)

λabs,max,film (nm)

λem,max (nm)

Eox/onset (V)

HOMO (eV)

LUMOb (eV)

optical Egap (eV)

1 2 3 4 5 6

15 100 21 460 39 260 16 060 21 610 34 390

1.9 2.2 2.7 2.5 3.1 4.3

11 10 14 11 10 13

435 467 485 439 470 481

440 472 490 449 479 494

515 553 566 509 551 570

0.59 0.52 0.50 0.56 0.48 0.48

−5.39 −5.32 −5.30 −5.36 −5.28 −5.28

−2.96 −3.04 −3.09 −2.94 −3.03 −3.10

2.43 2.28 2.21 2.42 2.25 2.18

a

Potentials are measured relative to a ferrocenium/ferrocene redox couple used as an internal standard. Eox/onset is the onset of oxidation potential and Ered/onset is the onset of reduction potential and were used to calculate the HOMO and LUMO via ferrocene reference in a vacuum (4.8 eV). b LUMO calculated via HOMO + Egapopt. Mn determined via GPC vs polystyrene standard. Đ = dispersity.

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RESULTS AND DISCUSSION Thin Film Electrochemistry. Cyclic voltammograms (CV) of the polychromophore films were obtained in acetonitrile with a platinum working electrode (Figure 2). Scanning first at negative potentials, all polymers displayed no reduction waves but did show irreversible oxidation peaks that were similar in shape and potential for a given chromophore size (e.g., 1 similar to 4). Similar trends were observed for solution-based CV of the iodo-containing monomers 15−17 (Supporting

Information). Depending on the specific polymer backbone, PPV polymers typically give reversible or quasi-reversible reductions and oxidation signals.33−35 The shorter chromophore size of 1−6, compared to fully conjugated PPVs homopolymers, results in the absence of reduction signals and reversibility of the oxidation processes. The highest occupied molecular orbital (HOMO) energetic levels were estimated by comparing to a ferrocene standard while the lowest unoccupied molecular orbital (LUMO) was calculated by subtracting the HOMO from the optical band gap (Table 1). Although the difference was small (∼0.03−0.04 eV), the HOMO levels of all linear diamantane (4−6) containing polymers were lower than the corresponding adamantane polymers (1−3). These lower energy states suggest the linear, diamantane morphons provide an environment for more intimate interaction between the chromophore segments in comparison to a more disordered state found in the adamantane system. Fluorescence Excitation Polarization Spectroscopy. To examine the morphological order (i.e., chromophore alignment) of each polymer system, fluorescence intensity modulation depths M have been extracted from fluorescence excitation polarization experiments shown in Figure 3.19 The magnitude of M (ranging from 0 to 1) indicates the degree of alignment of chromophores within a single polymer chain. Higher M values correspond to higher degrees of chromophore alignment within the polymer, which is characteristic of an anisotropic structure. Isotropic polymers give smaller M values that approach 0. Details of the experimental setup and

Figure 2. Thin film cyclic voltammograms of 1−6 in 0.1 M tetrabutylammonium hexafluorophosphate in acetonitrile with platinum working electrode, platinum counter electrode, and an Ag/AgCl reference electrode. Scan rate = 50 mV/s. Ferrocene added to following scan (not shown) for reference. C


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negative skewness value for 6 suggests a larger population of aligned chromophores within 6. Solution and Thin Film Based Photophysics. Figure 4 shows the absorption and fluorescence emission spectra of 1−6

Figure 3. Histograms of fluorescence intensity modulation depth, M, for polymers 1−6 shown in panels A−F, respectively. Polymers 1−3 contain bent adamantane morphons, and polymers 4−6 contain linear diamantane morphons.

mathematical representations can be found in the Supporting Information. Linearly linked polychromophores 4−6 gave higher M values than their nonlinearly linked counterparts 1−3 (Figure 3) with mean M = 0.75 vs 0.59, 0.77 vs 0.68, and 0.69 vs 0.55 for trimer, pentamer, and heptamer, respectively (Table 2). In addition, polymers 4−6 exhibit M values greater

Figure 4. Absorption spectra of 1−6 in toluene (A) and thin film (B). Fluorescence emission spectra of 1−6 in toluene (C) and thin film (D).

Table 2. Summary of Single Molecule Fluorescence Data of 1−6 polymer

M (mean)

M (median)

skewness

1 2 3 4 5 6

0.59 0.68 0.55 0.75 0.77 0.69

0.59 0.72 0.52 0.78 0.79 0.72

0.04 −0.37 0.18 −0.73 −0.66 −0.48

in solution (toluene) and thin film. For both solution and film, the λabs,max shows a red-shift upon increasing the chromophore size from trimer to pentamer to heptamer (Figure 4A,B). The red-shift is owing to increased π-electron delocalization along the backbone with longer chromophore length.36−38 In addition, there is no noticeable difference between the linear and bent polymers with the same chromophore size in solution. Unlike the single molecule data, the morphons do not strongly affect the polymer spectroscopy in solution due to a lack of strong interchromophore interactions between oligomer units along the polymers. The spectra differences are only based on the length of the chromophore, not the linear or bent natures of the linkers.39 The λabs,max of the linear polymers (4−6) are slightly red-shifted compared to their bent counterparts (1−3), which can be attributed to packing induced planarization of polymer backbone and leads to a slight increase in the effective conjugation length. Similarly, λabs,max of all polymers are redshifted going from solution to film. This is likely the result of a combination of the chains adopting a conformation with a longer conjugation length in the films along with efficient energy transfer to the lowest energy segments. The fluorescence emission spectra (Figure 4C,D) also give the expected red-shift in λem,max with increasing chromophore size. Comparisons of the solution and thin film emission spectra (Figure 5B,C) show the pentamer- and heptamercontaining polymers red-shift (∼15 nm or 438 cm−1 and ∼18 nm or 578 cm−1, respectively) in the solid state due to packinginduced intrachain planarization (e.g., more efficient conjugation along the oligomer backbone). In contrast, the trimercontaining polymers (Figure 5A) do not show significant spectral shifting because their shorter chromophore length cannot distort from planarity as much as the longer chromophores can in solution. The shorter length of the trimer chromophore does not allow for significant conformational change from solution to film compared to the longer chromophores. In addition to the spectral shifts, the vibronic structures in fluorescence emission have, in general, changed for all polymers

than 0.5 for a majority of the individual polymer chains whereas 1−3 give a broader distribution of M. This result suggests that the chromophores within the linearly linked diamantane polymers (4−6) are more aligned compared with those within the nonlinear, adamantane linked ones (1−3). This is confirmed by performing further statistical analysis where “skewness” is calculated for each histogram (Table 2). Zero skewness indicates a symmetric distribution of M around 0.5. Negative skewness values suggest a higher probability of having high M values that correspond to well-aligned chromophores within the polymer. Conversely, positive skewness values indicate a higher probability of poorly aligned chromophores. The substantial difference in M between the bent polymers 1− 3 and the linear polymers 4−6 highlight the ramifications of morphon geometry. The incorporation of rigid, linear linkers preserve the stiff, linear nature of the polymer backbones while the bent linkers allow variations in the alignment of the polymer backbones. Interestingly, both the mean and median M values for 6 are found to be smaller than for both 4 and 5. This could be a result of the broader molecular weight distribution of 6 (Đ = 4.3) where higher molecular weight polymer chains are inherently more prone to disorder. In the cases of the bent polymers, 2 is skewed to higher M values than 1 and 3. Compared to any of the linear systems, however, 2 is still skewed more toward lower M values, indicating more poorly aligned chromophores within the bent polymer than the linear ones as expected. Additionally, while the mean and median M values for 2 and 6 are essentially identical, the more D


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Figure 5. A side-by-side comparison of solution and thin film emission spectra of polymers composed of same chromophore size. Spectra normalized to 0−1.

going from solution to film. For the trimer-containing polymers seen in Figure 5A, the film emission bands are broader, accompanied by a loss of some vibronic structures and show a suppressed 0−0 transition. Upon formation of thin films, the short chromophores with bent morphon polymers (e.g., 1) may not stack on top of each other in an ordered fashion due to the broad distribution of their single chain conformations (Figure 3), which leads to a distribution in emission energies. Despite the disorder, some chromophores from neighboring chains can still interact strongly and lead to H-aggregate like emission.40,41 For the pentamer- and heptamer-containing polymers in Figure 5B,C, the larger chromophores lead to a more uniform packing and a smaller distribution of energies. The longer chains also allow for interaction between neighboring chains and similarly result in H-aggregate like emission, which is characterized by a depression in the 0−0 transition. One hypothesis for the more featureless emission spectra for 1 in thin film is that the fast solvent evaporation during spincasting could have trapped the chains to form an array of distinct domains of different energies. To address this possibility, we performed solvent vapor annealing (SVA, see Supporting Information) to allow polymer chains in the films to reorder themselves in a toluene vapor. The absorption and emission spectra of all polymer films do not exhibit any obvious change upon SVA for all trimer and pentamer containing polymers. However, for thin film 6, it appears that the Huang− Rhys factor42−44 has increased slightly while the 0−0 transition is additionally suppressed. This may indicate interactions between the chromophores but the effect is very small. In general, SVA does not have a drastic effect on thin film photophysics and this suggests that the spectral features seen in Figures 4 and 5 are attributed mainly to polymers’ inherent single chain structures. Atomic Force Microscopy. To gain an insight into the correlation between single chain conformation and thin film morphology, atomic force microscopy (AFM) was utilized to investigate the surface properties of thins films composed of two polymers with markedly different single chain structures (1 and 6). After SVA, the thin films were visualized with AFM phase imaging, which is capable of elucidating surface inhomogeneity.45−47 The phase image of 6 (Figure 6B) reveals that highly oriented, closely packed fibril structures prevail throughout the film. By sampling line scans across the scan areas, an average width of ∼80 nm is obtained for the fiber domains. This value is consistent with the length of a fully extended polymer chain of 6, suggesting the fiber growth direction is along the π−π stacking direction. In spite of the fiber structure observed in the AFM, no structural information

Figure 6. AFM phase images (1 × 1 μm) of polymer 1 (A) and 6 (B) thin films and their respective sample cross-section line profiles.

could be obtained with X-ray diffraction (XRD) experiments, suggesting the polymers were not crystalline enough for sufficient X-ray scattering. In contrast, AFM phase images of 1 (Figure 6A) show a drastically different surface morphology where small domains are prevalent. This is in agreement with the inherent bent nature of the material.

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CONCLUSION In summary, we have successfully synthesized a series of oligo(phenylenevinylene) polychromophores with rigid, nonconjugatable morphons. Their single chain conformation, measured with single molecule fluorescence excitation polarization spectroscopy, highlights the effectiveness of the linear diamantane morphon compared to its bent adamantane counterpart in preserving an extended polymer backbone. Further investigations into their solution, thin film photophysics, and thin film morphology suggest that the polymers’ single chain conformation is correlated to the bulk film property. This bottom-up approach of polymer synthesis with controlled rigidness, in addition to the controlled chromophore size, could offer powerful insights into future CP designs using morphons to achieve desired morphology control and, ultimately, energy migration.

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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.macromol.6b00067. Experimental procedures for photophysics measurements; absorption and fluorescence emission spectra; E


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cyclic voltammetry; experimental procedures for synthesis; 1H and 13C NMR spectra for compounds (PDF)

AUTHOR INFORMATION

Corresponding Authors

*(D.A.V.B.) E-mail: [email protected]. *(K.N.P.) E-mail: [email protected]. Present Address

X.Z.: College of Chemistry and Molecular Engineering, Zhengzhou University, Zhengzhou, China. Author Contributions

X.Z. and B.S. contributed equally. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS D.A.V.B. acknowledges support from the National Science Foundation (Grant CHE-1310222). K.N.P. acknowledges support from the National Science Foundation (Grant CHE1352431). The fluorometer was funded by the National Science Foundation (Grant CHE-094750). X.Z. thanks Bob and Beth Gower for a Gower Fellowship and Southern Illinois University for the SIU Doctoral Fellowship.

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