Conjugated Polymer with Polydiacetylene Cross-Links Through

Article Cite This: ACS Appl. Polym. Mater. 2019, 1, 1918−1924

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Conjugated Polymer with Polydiacetylene Cross-Links Through Topochemical Polymerization of 1,3-Butadiyne Moieties Toward Photopatternable Thin Films Audithya Nyayachavadi, Adam Langlois, M. Nazir Tahir, Blandine Billet, and Simon Rondeau-Gagné* Department of Chemistry and Biochemistry, University of Windsor, Advanced Materials Centre of Research (AMCORe), Windsor, Ontario Canada, N9B 3P4 Downloaded via UNIV OF SOUTHERN INDIANA on July 22, 2019 at 06:47:58 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.

S Supporting Information *

ABSTRACT: A new methodology to covalently cross-link πconjugated polymers by generating π-conjugated bridges was developed through the topochemical polymerization of 1,3butadiyne moieties. Isoindigo-based oligomers containing 1,3butadiyne were prepared and amide moieties were incorporated to enable hydrogen bonding which promoted an optimal morphology for cross-linking through 1,4-addition. Moreover, the molecular weight of the semiconducting materials was carefully controlled to promote crystallinity, leading to the formation of a nanofibrillar network. Upon formation of polydiacetylene cross-links, the resulting conjugated polymeric network was characterized using grazing incidence X-ray diffraction, Raman spectroscopy, atomic force microscopy, and optical spectroscopy. Furthermore, transient absorption and fluorescence spectroscopy were used to probe for the effect of the new πconjugated cross-links on photophysical properties. The new π-conjugated bridges were shown to maintain the morphology and the photophysical properties while altering other important properties for polymer processing, such as the solubility. This new, mild, and additive-free photo-cross-linking approach is particularly promising for the patterning and processing of π-conjugated materials and the rigidification of conjugated polymeric network, thus creating new opportunities for the processing of organic semiconductors and fabrication of organic electronic devices. KEYWORDS: conjugated polymers, polydiacetylene, photopatterning, semiconducting polymers, polymer cross-linking

1. INTRODUCTION π-Conjugated polymers are promising materials for the development of new electronics due to their good charge carrier mobility, large scale processability and good mechanical compliance.1−8 Promising applications have emerged from their utilization in layered devices, where the efficiency can be directly correlated to the stability and compatibility of the components.9−13 One important approach to control the compatibility and stability of π-conjugated materials is through covalent or noncovalent cross-linking.14−19 Cross-linking rigidifies the polymer by connecting chains together and can also enhance its robustness.20 Among others, the topochemical polymerization of 1,3-butadiyne moieties is a promising strategy for cross-linking π-conjugated materials.21−25 In contrast to many cross-links, polydiacetylenes (PDA) possess a π-extended conjugation and were predicted to exhibit a very high charge carrier mobility.26−28 Typically achieved through postfunctionalization, the synthesis of cross-linked polymers with conjugated bridges often requires the use of Pd-catalyzed couplings to cross-link the polymer chains, potentially resulting in partial functionalization and/or defects.29,30 Interestingly, the cross-linking of 1,3-butadiyne, occurring through a highly regioselective phototriggered 1,4-addition between adjacent © 2019 American Chemical Society

diacetylene moieties, can be achieved without the utilization of external additives or initiators.31 However, diacetylene moieties require specific structural parameters to undergo topochemical polymerization.32 These structural requirements are often difficult to achieve in polymers, especially due to difficult control over the solid-state morphology. Herein, we report the photopolymerization of diacetylenecontaining isoindigo-based oligomers to generate PDA-crosslinked conjugated materials. Previously shown to properly orient the self-assembly of 1,3-butadiyne moieties, hydrogenbonding moieties were incorporated to the materials as these interactions were shown to be critical for the photo-crosslinking to occur (Figure 1).33 The resulting cross-linked materials were characterized by various techniques, including Raman spectroscopy, X-ray diffraction and atomic force microscopy (AFM). Fluorescence spectroscopy and transient absorption measurements were also used to determine the effect of cross-linking on the electronic state. In addition to considerably reducing the solubility of the π-conjugated Received: June 5, 2019 Accepted: June 20, 2019 Published: June 20, 2019 1918

DOI: 10.1021/acsapm.9b00520 ACS Appl. Polym. Mater. 2019, 1, 1918−1924

Article

ACS Applied Polymer Materials

Figure 1. Topochemical polymerization of isoindigo-based conjugated oligomers incorporating 1,3-butadiyne moieties in the side chains.

Scheme 1. Synthetic Pathway to iI1-3 and O1−O3

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Figure 2. Raman spectra of (a) O1, (b) O2, and (c) O3 before (black) and after irradiation for 30 min (red) and 1 h (blue) at 254 nm. (d) Raman spectra of O3 nonirradiated (black) and irradiated for 1 h on SiO2 (red) and OTS-functionalized SiO2 (blue) substrates

stable after irradiation for 2 h, but degradation was observed after 24 h. Raman spectroscopy was then used after irradiation of the conjugated materials to confirm PDA formation (Figure 2).40 O1, containing no diacetylene moieties, showed a similar spectra after irradiation. Similarly, the Raman spectra for O2 also showed no change after irradiation, which is an additional indication that the structural parameters required for topochemical polymerization are not achieved despite the presence of 1,3-butadiyne. Interestingly, the irradiation of O3, containing both diacetylene and amide moieties, led to the formation of PDA cross-links after 1 h, as confirmed by the appearance of two distinct bands at ∼1600 and ∼2100 cm−1, associated respectively to the alkene and alkyne moieties of a PDA. This confirmed that intermolecular hydrogen bonds between polymer chains are critical to attain the structural parameters for photopolymerization. As shown in Figure 2d, irradiation of O3 was performed on two different substrates, namely SiO2 and octadecyl-functionalized SiO2 (OTS-SiO2), to control the molecular orientation (edge-on versus faceon).41 The formation of PDAs was observed for O3 after 1 h in both cases. Solid-state morphology was investigated by AFM (Figure 3). O1 showed a smooth surface before and after irradiation, whereas O2 showed a rougher surface, attributed to an increased aggregation due to the 1,3-butadiyne side chains. For

materials, particularly desirable for the fabrication of layered architectures, the formation of PDA was shown to preserve the optoelectronic properties of the conjugated materials.

2. RESULTS AND DISCUSSION Synthesis of diacetylene-containing oligomer O1−O3 is depicted in Scheme 1 and detailed in Supporting Information (Figures S1−S4). Synthesis of π-conjugated oligomers was performed by using Carothers’ equation to reduce chain distribution, allowing for better control over the solubility and crystallinity.34−37 Conjugated oligomer O1, bearing dodecyl side chains, was used as reference whereas O2, bearing diacetylene side chains without hydrogen bonding moieties, was prepared to directly probe the influence of intermolecular hydrogen bonding on topochemical photopolymerization. Finally, O3, bearing both diacetylene and hydrogen bonding amide moieties was designed to maximize the efficiency of the cross-linking reaction.38,39 As detailed in Table S1, the soluble fraction after photo-cross-linking (thin film) was determined to qualitatively evaluate the photo-cross-linking reaction after irradiation at 254 nm (Table S1). Moreover, because conjugated polymers are known to suffer from photobleaching upon extended exposure to light, UV−vis spectroscopy was used to evaluate the degradation of the π-conjugated backbone upon irradiation (Figure S12). All materials were shown to be 1920


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Figure 3. AFM height images of oligomers O1−O3 before and after UV irradiation for 1 h. Scale bar is 500 nm.

O3, a different solid-state morphology, attributed to the presence of both diacetylene and amide moieties incorporated in the side chains, was observed. As shown in Figure 3, the oligomer formed a dense array of nanoscale fibers, having a width of few tens of nanometers. This morphology can be explained by the formation of intermolecular hydrogen bonds between the polymeric chains, promoting aggregation and leading to the formation of discrete structures. Similar to the formation of a gel state in a specific solvent, fiberlike structures can arise with a strong preference for anisotropic unidirectional growth directed by intermolecular hydrogen bonds and π−π stacking between the isoindigo-based oligomers. As a consequence, long-range order can originate from nucleation from a small supramolecular unit (oligomeric hydrogenbonded or π−π stack).21,42,43 The combination of amide and 1,3-butadiyne moieties also increases the polarity and rigidity of the side chains, which can help to promote phase aggregation into fiberlike structures. More importantly, the topochemical polymerization of the diacetylene units did not cause a change in morphology with O3 showing a similar nanofibrous network after irradiating the sample. To probe the packing of the oligomers, grazing incident Xray diffraction (GIXRD) was performed on O1 and O3 in both in-plane and out-of-plane directions (Figures 4a,b and S13). O1 showed an amorphous morphology with a weak crystallinity, especially in the plane directions. Characteristics peaks associated with lamellar spacing and π-stacking were observed for O1 before irradiation and the materials became slightly more amorphous after irradiation, as observed in Figure 4a. O3 showed an amorphous and weakly organized morphology in the solid-state despite the formation of intermolecular hydrogen bonds between the polymer chains

(Figure 4b). Upon photoirradiation, the resulting PDA-crosslinked polymer showed no sign of reorganization, indicating that no morphological modification occurs upon topochemical polymerization. It is important to mention that O2 was also found to have a similar amorphous and weakly organized morphology. In order to probe for the influence of PDA cross-links on photophysical properties, UV−vis spectroscopy was first used (Figure 4c). O1 demonstrated a typical absorption for isoindigo-based polymers, with maxima centered at 400 nm (π−π* transition) and 650 nm (D-A charge transfer). As expected, UV-irradiation induced no changes in absorption as O1 does not possess reactive 1,3-butadiyne groups. O2 showed a similar absorption spectrum than O1 (Figure S14). Interestingly, the UV-irradiation of the sample induced a minor change in absorption; a slight blueshift was observed in the band at 650 nm, potentially indicating a decrease in aggregation. However, the absence of guiding interactions prevents cross-linking, which explains the absence of PDAassociated characteristic bands. As shown in Figure 4d, the photoirradiation of O3 in thin film led to major differences in the absorption spectra. The formation of PDA in O3, promoted by intermolecular hydrogen bonding, led to an important increase in the intensity of the band centered at 400 nm. Moreover, a strong absorption band centered at 300 nm, attributed to σ- σ* transitions in PDA systems, was observed. Finally, a hypsochromic shift was observed after irradiation, which can be attributed to reduced aggregation after crosslinking44,45 Further investigation of the photophysical properties of O1−O3 was performed before photoirradiation by examining the steady state absorption and fluorescence (Figure S15). All 1921


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Figure 4. In-plane and out-of-plane grazing incidence X-ray diffraction spectra (GIXRD) of (a) O1 before and after irradiation; (b) O3 before and after cross-linking; UV−vis spectra of (c) O1 after annealing and irradiation for 1 and 2 h, and (d) O3 after annealing and irradiation for 1 and 2 h.

Figure 5. Kinetic decay traces and fits at given wavelengths obtained by transient absorption measurements on O3 thin films (a) before crosslinking, and (b) after cross-linking.

three compounds exhibited a similar behavior as their absorption spectra exhibited a “camel back” structure commonly seen in donor−acceptor polymeric systems.46 They also exhibited similar fluorescence spectra, indicating that the excitation of the π → π* absorption leads to the emission from the intramolecular charge transfer (ICT) state. Time-resolved photoluminescence (PL) and transient absorption (TA) measurements were then carried out on O1−O3 in solution, showing that the ICT state is populated in the ∼10 ps time scale (rise time) after photoexcitation of the π−π* states. The oligomers then relax to their ground states by two different radiative pathways tentatively attributed to the

geminate and nongeminate recombination of the ICT state.46 Full details are provided in Supporting Information (Figures S16−S21, and Table S2). Femtosecond TA measurements were then carried out on O3 before and after cross-linking (Figures S22 and S23). Interestingly the films of O3 exhibited similar behavior before and after cross-linking, indicating that the formation of PDA does not impact the electronic properties. In the solid state, a rise time associated with the formation of the ICT state is no longer observed. Instead, the immediate formation of an exciton absorption (Sn ← S1) band that relaxes at a rate similar to the ground-state bleach is observed. Kinetic slices at 540 1922


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ACS Applied Polymer Materials and 715 nm corresponding to the ground-state bleach and the exciton absorptions can be observed (Figure 5). Furthermore, the relaxation of the O3 in the solid state is dominated by two relaxation pathways with lifetimes of 24 and 2.7 ps and 49 and 4.3 ps, respectively, before and after cross-linking. The similarity of the transient absorption spectra suggests that despite reducing solubility the new cross-linking approach does not influence the photophysical property of the materials, which is particularly important for processability in devices. Finally, the new cross-linking approach was evaluated for photopatterning by photoirradiation through a shadow mask. After irradiation, etching with chlorobenzene was performed to remove uncross-linked materials (Figures S24 and S25). As expected, O1 and O2 were completely removed from the substrate after irradiation because both oligomers do not undergo cross-linking. In contrast, optical microscopy revealed patterning of O3 with good resolution. This result indicates that cross-linking with PDA can be a useful strategy to control the properties and solubility of conjugated polymers.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Adam Langlois: 0000-0001-5999-0414 Simon Rondeau-Gagné: 0000-0003-0487-1092 Author Contributions

All authors contributed to the manuscript. All authors have given approval to the final version of the manuscript. Notes

The authors declare no competing financial interest.

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3. CONCLUSION In conclusion, the preparation of diacetylene-containing isoindigo-based polymers was performed, and their crosslinking to generate π-conjugated PDA bridges has been investigated. Isoindigo-based oligomer O3, containing both amide and 1,3-butadiyne moieties, was shown to undergo photopolymerization upon UV irradiation. The formation of PDA was confirmed by Raman spectroscopy. Moreover, morphological investigation by AFM and X-ray diffraction showed that the semiconducting oligomer formed a fibrous morphology that was maintained after cross-linking. The development of this new additive-free approach generates new opportunities in polymer chemistry. First, the formation of π-conjugated PDA cross-links allows for the design of materials with new photophysical properties and the generation of new charge transport pathways. Moreover, this cross-linking reaction can impact the processing of πconjugated materials in layered devices, by controlling the solubility and morphology of the active components. Finally, the rigidification of semiconducting polymer networks by the topochemical polymerization of diacetylene, is a promising strategy to develop novel nanoarchitectures with well-defined structure, enabling new opportunities for the efficient processing of organic semiconductors and fabrication of organic electronic devices with improved stability and performance.

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O3; ultrafast photoluminescence decay of O1−O3; femtosecond transient absorption measurement of O1− O3; excited state kinetics of isoindigo-based oligomers O1, O2, and O3; femtosecond transient absorption measurement of O3 after photo-cross-linking; optical microscopy of O1−O3 before and after photo-crosslinking; patterning of O3 in thin film (PDF)

ACKNOWLEDGMENTS The authors thank Prof. Jérôme Claverie (Université de Sherbrooke) and Prof. S. Holger Eichhorn (University of Windsor) for help with materials characterization. The authors also thank Prof. Jean-François Morin (Université Laval) and Mr. Pierre Audet (Université Laval) for help with MALDI-ToF experiments. This work was supported by NSERC through a Discovery Grant (RGPIN-2017-06611).

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REFERENCES

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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/acsapm.9b00520. General procedure and materials; procedures for sample preparation and photo-cross-linking through topochemical polymerization of 1,3-butadiyne moieties; synthetic methods and NMR spectra; molecular weight, polydispersity, and energy level of oligomers O1−O3; FTIR spectra for the oligomers; variable-temperature 1H NMR spectra of O3; cyclic voltammetry of O1−O3; UV−vis spectra of O1−O3 before and after irradiation with UV (254 nm) for 24 h; Grazing incidence X-ray diffraction spectra (GIXD) of O1 and O3 after photo-cross-linking; Absorption and fluorescence excitation spectra of O1− 1923


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Conjugated Polymer with Polydiacetylene Cross-Links Through

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