Multiblock Bottlebrush Nanofibers from Organic Electronic Materials

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Cite This: J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

Multiblock Bottlebrush Nanofibers from Organic Electronic Materials Christopher M. Tonge,† Ethan R. Sauve,́ † Susan Cheng, Teresa A. Howard, and Zachary M. Hudson* Department of Chemistry, The University of British Columbia, 2036 Main Mall, Vancouver, British Columbia V6T 1Z1, Canada

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solubilities to achieve complex, well-defined nanomaterials. Though each has been used to prepare nanomaterials with impressive complexity and function, the chemical requirements of each technique narrow the potential scope of materials that might be investigated. We therefore sought methods based on efficient and orthogonal covalent chemistry for preparing nanostructures with well-defined components, which would circumvent many of the challenges associated with nanomaterials synthesis by self-assembly. Bottlebrush copolymers (BBCPs) are ideally suited for this goal, and have recently emerged as a powerful route to large macromolecules with molecular weights exceeding 106 Da.15−17 Consisting of polymeric side-chains attached covalently to a linear backbone, these materials have found numerous applications in photonic crystals,18−20 templating,21−23 and nanomedicine24−28 as a result of their unique physical properties. Because of the steric demands of the side chains, bottlebrush polymers occupy a relatively lowentropy conformational space, adopting extended conformations that limit chain entanglement in solution and the solid state.29,30 As a result, this class of polymers has recently been used to prepare nanostructures with complex and unique morphologies, including Janus particles,31 toroids,32 dumbbells,33 and brushes with tailored graft distributions.34−37 As a result of their size, bottlebrush polymers also make unique building blocks for larger structures, and have been investigated in self-assembled materials themselves.38−40 Multiblock bottlebrush copolymers provide a compelling bottom-up approach to the synthesis of hierarchical nanostructures from soft material. Their block-by-block synthesis allows for the preparation of multicompartment structures that remain nanosegregated by virtue of their covalent chemistry, irrespective of environmental conditions such as temperature or solvent. Furthermore, bottlebrush copolymers may be used to form nanostructures from a vast array of monomers, removing the need to consider factors such as crystallization or selective solvation that a self-assembly approach might require. This would facilitate the preparation of hierarchical materials with multiple, complex functionalities difficult to achieve by existing methods. Here we prepare bottlebrush copolymers that mimic the structure of multilayer organic electronic devices on single polymer chains. Bottlebrush polymers based on polythiophenes have recently shown promise as dielectric materials,41−43 and similar structures composed of multiple organic

ABSTRACT: Methods are described for the preparation of fiber-like nanomaterials that mimic the multilayer structure of organic electronic devices on individual polymer chains. By combining Cu(0) reversible-deactivation radical polymerization (RDRP) and ring-opening metathesis polymerization (ROMP), multiblock bottlebrush copolymers are synthesized from ordered sequences of organic semiconductors. Narrowly dispersed fibers are prepared from materials commonly used as the hole transport, electron transport, and host materials in organic electronics, with molecular weights exceeding 2 × 106 Da and dispersities as low as 1.12. Diblock nanofibers are then synthesized from pairs of semiconducting building blocks, giving nanostructures analogous to p−n junctions that exhibit the reversible electrochemistry of their individual parts. Finally, this strategy is used to construct nanofibers with the structure of phosphorescent organic light-emitting diodes (OLEDs) on single macromolecules, such that the photophysical properties of each component of an OLED can be independently observed. These multiblock nanofibers can be formed from arbitrary organic semiconductors without the need for crystallinity, selective solvation, or supramolecular interactions, providing powerful methods for the miniaturization of materials for organic devices.

T

echniques for the assembly of hierarchical nanostructures from soft matter have opened the door to many new applications of nanotechnology. Methods such as crystallization-driven self-assembly (CDSA),1−6 living supramolecular polymerization,7−11 and hierarchical solution self-assembly (HSSA)12−14 have leveraged low-cost solution processing to address scalability challenges, while the complexity of materials prepared in these ways continues to improve. Despite these achievements, synthetic methods in nanotechnology that rely on self-assembly continue to face several challenges. Most importantly, self-assembly can be highly dependent on conditions such as solvent and temperature, which must be kept within certain ranges if the integrity of the nanomaterial is to be maintained. Furthermore, methods for the precise synthesis of softmatter nanomaterials often require building blocks with highly specific properties. For example, CDSA makes use of polymers amenable to epitaxial crystallization, supramolecular polymerization must precisely balance H-bonding and van der Waals forces, and HSSA uses building blocks with differing © XXXX American Chemical Society

Received: July 25, 2018 Published: September 4, 2018 A

DOI: 10.1021/jacs.8b07915 J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

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Journal of the American Chemical Society

low as 1.12 (Figure 2). Competing vinyl addition to the norbornene group during the RDRP process can be suppressed by halting the polymerization at conversions ≤60%,49 after which residual monomer can be successfully removed and recycled. Grafting-through ring-opening metathesis polymerization (ROMP) of each macromonomer was then used to give bottlebrushes with target backbone degrees of polymerization (DPs) of 100 and 400 from EML-MM, ETL-MM, and HTLMM. The grafting-through approach using Grubbs’ thirdgeneration catalyst (G3) was chosen to ensure 100% grafting density of side chains to the bottlebrush backbone, and gave bottlebrushes with Mn > 2 × 106 Da (Table 1). Consistent with

semiconductors would provide a unique tool for the study of charge transport at nanoscale junctions.44,45 Using a series of macromonomers prepared from p- and n-type organic semiconductors, fiber-like nanomaterials of tunable length were prepared with molecular weights from 470 to 2050 kDa. This approach was then used to access diblock nanofibers analogous to organic p−n junctions, which display the reversible electrochemistry of each of their components (Figure 1). Finally, bottlebrush copolymers were prepared in

Table 1. Synthesis of Bottlebrush Homopolymers Entrya

Mn,SECb (kDa)

Mn,theory (kDa)

Đ

Conv. (%)b

HTL100-BB EML100-BB ETL100-BB HTL400-BB EML400-BB ETL400-BB

570 701 473 1530 1990 2050

499 500 504 1770 2040 1620

1.14 1.17 1.12 1.35 1.32 1.31

78 83 71 69 85 57

a Subscript indicates target DP. bDetermined by SEC in THF after purification by preparatory SEC.

Figure 1. Schematic illustration of a diblock bottlebrush copolymer composed of p-type (blue) and n-type (orange) organic semiconductors.

the results of Matson and co-workers,50 the conversion observed during ROMP decreases and dispersity increases as longer bottlebrushes are targeted, particularly given the considerable steric bulk of the semiconductor side chains employed here. Bottlebrushes formed from ETL-MM consistently showed molecular weights higher than those anticipated by theory, perhaps due to interactions from the coordinating nitrogen atoms on the ETL monomer with the ruthenium catalyst. We also note that the addition of a C11 spacer between the norbornene-dicarboxamide and the polyacrylate was critical in achieving high backbone DPs, as earlier efforts using only a C2 linker gave bottlebrushes with a maximum DP of ≈50. Importantly, we determined that residual macromomomer can be completely removed from these mixtures by passing the reaction over a reusable methacrylic resin in THF, a critical detail if the properties of individual nanowires are to be electrically interrogated. Atomic force microscopy (AFM) images of bottlebrushes prepared from each of these organic

which individual polymer chains exhibit the multilayer structure of four-component phosphorescent organic lightemitting diodes (OLEDs), such that the photophysical properties of each of the four organic semiconductors can be independently observed. A series of macromonomers (MMs) were first synthesized from acrylates bearing organic semiconductor moieties as sidechains, representative of materials used in the emissive layer (EML), hole-transport layer (HTL), and electron-transport layer (ETL) of organic devices such as OLEDs or organic thinfilm transistors (OTFTs). MMs were prepared via direct growth from a norbornene-functionalized initiator via Cu(0) reversible-deactivation radical polymerization (RDRP),46−48 which provides a scalable route to multigram quantities of these materials at room temperature with dispersities (Đ) as

Figure 2. Synthesis of norbornene-functionalized macromonomers from a series of organic semiconductors, and their reaction to form bottlebrush polymers by grafting-through ROMP. B


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prepared. Cyclic voltammetry shows the diblock bottlebrushes to be capable of the reversible reduction and oxidation of their individual components, showing promise for investigating these materials as thin films for ambipolar charge transport or as individual nanoscale diode-like structures. Notably, monomodal molecular weight distributions with dispersities of 1.21 and 1.17 are obtained for (EML75-b-HTL75)-BB and (EML75-b-ETL75)-BB respectively, while significant chain death is consistently observed upon addition of the second block for the bulkier (ETL75-b-HTL75)-BB. This indicates that the steric bulk of the reactive macronomomers plays a key role in achieving well-defined diblock copolymers from organic semiconductors via ROMP. To demonstrate the potential of this approach in preparing hierarchical nanostructures from optoelectronic materials, we next prepared triblock BBCPs such that the structure of each individual macromolecule would mimic the design of a fourcomponent OLED, composed of ETL, HTL, a host material, and a phosphorescent iridium dopant. Such nanostructures would allow for control at three levels of hierarchy: (1) the structures of the monomers used; (2) the arrangement of these monomers within macromonomer strands, and (3) the arrangement of macromonomer strands into a multiblock bottlebrush, mimicking the structure of macroscopic devices. To this end, a fourth macromonomer (EML-co-Ir)-MM was synthesized consisting of EML doped with 8 wt % of an iridium-containing monomer based on the commonly used OLED emitter Ir(ppy)2(acac) (Mn = 9300, Đ = 1.22). Sequential ROMP of ETL-MM, (EML-co-Ir)-MM, and HTL-MM gave (ETL50-b-(EML-co-Ir)30-b-HTL50)-BB (Figure 5). This material shows bright green phosphorescence (Figures 5e and S12, τ = 5.0 μs) alongside the reversible reduction and oxidation of the ETL and HTL blocks at −2.32 and +0.48 V, respectively. Though copolymers consisting of multiple OLED materials have been synthesized previously,53 (ETL50-b-(EML-co-Ir)30-b-HTL50)-BB is composed of individual domains with discrete interfaces that remain nanosegregated due to the unique morphology of the bottlebrush structure (Mn = 296 kDa, Đ = 1.21). Furthermore, the energy levels of the four components of the structure match the typical “staircase” arrangement of a four-component OLED, (Figure 5b, Table S2) designed to facilitate charge-transport of both electrons and holes from each end of the structure to the active layer at its center. SEC analysis shows that narrow dispersity is maintained at each stage of the polymerization, indicating that separation of the bulky HTL and ETL blocks with the smaller EML block allows for the synthesis of triblock BBCPs from organic semiconductors with minimal chain death (Table S3). In summary, the grafting-through synthesis of BBCPs has been shown to be an effective route to multiblock structures formed from optoelectronic materials. Multicomponent nanomaterials similar in architecture to block comicelles can be prepared by covalent chemistry, adapting a cylindrical morphology that does not require environment-dependent self-assembly steps. This work opens the door to research on nanoscale wires and junctions from arbitrary organic semiconductors regardless of their crystallinity or solvophilicity, providing methods for the incorporation of diverse optoelectronic materials into hierarchical nanofibers. Future work will examine the electrical properties of individual wires and multiblock structures by conductive scanning probe micros-

semiconductors on HOPG substrates are shown in Figure 3, with fiber-like morphologies up to several hundred nanometers

Figure 3. Schematic illustrations and AFM height images of (a) HTL400-BB, (b) EML400-BB, and (c) ETL400-BB (right) on HOPG substrates (scale bars = 200 nm).

in length in each case. All bottlebrushes are fluorescent with emission maxima of 347, 412, and 469 nm and quantum yields of 0.21, 0.02, and 0.56 for EML100-BB, ETL100-BB, and HTL100-BB respectively, mirroring the photophysical properties of their respective homopolymers.51,52 This Cu(0)-RDRP/ROMP strategy was then used to prepare diblock copolymers with structures analogous to nanoscale junctions between chemically distinct organic semiconductors (Figure 4). By sequential reaction of macromonomers, diblock BBCPs (EML75-b-HTL75)-BB, (EML75-bETL75)-BB, and (ETL75-b-HTL75)-BB were successfully

Figure 4. Schematics (left) and SEC chromatograms (right) of diblock bottlebrushes (a) (EML75-b-ETL75)-BB, (b) (EML75-bHTL75)-BB, and (c) (ETL75-b-HTL75)-BB showing the RI response after the addition of each block, and following purification by preparatory SEC. (d) Cyclic voltammogram of HTL100-BB (blue), (ETL75-b-HTL75)-BB (gray), and ETL100-BB (orange). C


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Figure 5. (a) Schematic illustration of (ETL50-b-(EML-co-Ir)30-b-HTL50)-BB, showing the structure of the EML-co-Ir side chains in the middle block. (b) Energy level diagram for the four components of the triblock BBCP. (c) Cyclic voltammogram of this BBCP showing the reduction of the ETL block (orange) and oxidation of the HTL block (blue). (d) SEC RI chromatograms after the addition of each block. (e) PL decay of the BBCP (λex = 340 nm) showing the phosphorescent decay of the Ir dopant. (f) AFM height images of these BBCPs on HOPG (scale bar = 200 nm). (7) Ogi, S.; Sugiyasu, K.; Manna, S.; Samitsu, S.; Takeuchi, M. Nat. Chem. 2014, 6, 188−195. (8) Endo, M.; Fukui, T.; Jung, S. H.; Yagai, S.; Takeuchi, M.; Sugiyasu, K. J. Am. Chem. Soc. 2016, 138 (43), 14347−14353. (9) Armao, J. J.; Nyrkova, I.; Fuks, G.; Osypenko, A.; Maaloum, M.; Moulin, E.; Arenal, R.; Gavat, O.; Semenov, A.; Giuseppone, N. J. Am. Chem. Soc. 2017, 139 (6), 2345−2350. (10) Adhikari, B.; Yamada, Y.; Yamauchi, M.; Wakita, K.; Lin, X.; Aratsu, K.; Ohba, T.; Karatsu, T.; Hollamby, M. J.; Shimizu, N.; Takagi, H.; Haruki, R.; Adachi, S.; Yagai, S. Nat. Commun. 2017, 8, 15254. (11) Venkata Rao, K.; Miyajima, D.; Nihonyanagi, A.; Aida, T. Nat. Chem. 2017, 9, 1133−1139. (12) Gröschel, A. H.; Walther, A.; Löbling, T. I.; Schmelz, J.; Hanisch, A.; Schmalz, H.; Müller, A. H. E. J. Am. Chem. Soc. 2012, 134 (33), 13850−13860. (13) Gröschel, A. H.; Walther, A.; Löbling, T. I.; Schacher, F. H.; Schmalz, H.; Müller, A. H. E. Nature 2013, 503, 247−251. (14) Löbling, T. I.; Borisov, O.; Haataja, J. S.; Ikkala, O.; Gröschel, A. H.; Müller, A. H. E. Nat. Commun. 2016, 7, 12097. (15) Müllner, M.; Müller, A. H. E. Polymer 2016, 98, 389−401. (16) Verduzco, R.; Li, X.; Pesek, S. L.; Stein, G. E. Chem. Soc. Rev. 2015, 44 (8), 2405−2420. (17) Pelras, T.; Mahon, C. S.; Müllner, M. Angew. Chem., Int. Ed. 2018, 57 (24), 6982−6994. (18) Miyake, G. M.; Weitekamp, R. A.; Piunova, V. A.; Grubbs, R. H. J. Am. Chem. Soc. 2012, 134 (34), 14249−14254. (19) Sveinbjörnsson, B. R.; Weitekamp, R. A.; Miyake, G. M.; Xia, Y.; Atwater, H. A.; Grubbs, R. H. Proc. Natl. Acad. Sci. U. S. A. 2012, 109 (36), 14332−14336. (20) Chae, C.-G.; Yu, Y.-G.; Seo, H.-B.; Kim, M.-J.; Grubbs, R. H.; Lee, J.-S. Macromolecules 2018, 51 (9), 3458−3466. (21) Jiang, B.; He, Y.; Li, B.; Zhao, S.; Wang, S.; He, Y.-B.; Lin, Z. Angew. Chem., Int. Ed. 2017, 56 (7), 1869−1872. (22) Pang, X.; He, Y.; Jung, J.; Lin, Z. Science 2016, 353 (6305), 1268−1272. (23) Yuan, J.; Xu, Y.; Walther, A.; Bolisetty, S.; Schumacher, M.; Schmalz, H.; Ballauff, M.; Müller, A. H. E. Nat. Mater. 2008, 7, 718− 722. (24) Sowers, M. A.; McCombs, J. R.; Wang, Y.; Paletta, J. T.; Morton, S. W.; Dreaden, E. C.; Boska, M. D.; Ottaviani, M. F.; Hammond, P. T.; Rajca, A.; Johnson, J. A. Nat. Commun. 2014, 5, 5460. (25) Müllner, M.; Yang, K.; Kaur, A.; New, E. Polym. Chem. 2018, 9, 3461−3465. (26) Lu, X.; Tran, T.-H.; Jia, F.; Tan, X.; Davis, S.; Krishnan, S.; Amiji, M. M.; Zhang, K. J. Am. Chem. Soc. 2015, 137 (39), 12466− 12469.

copy techniques, as well as assess the use of these materials in semiconducting thin films.

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/jacs.8b07915. Experimental details, synthetic procedures, additional photophysical and electrochemical data, and differential scanning calorimetry traces (PDF)

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AUTHOR INFORMATION

Corresponding Author

*[email protected] ORCID

Ethan R. Sauvé: 0000-0002-0779-6465 Zachary M. Hudson: 0000-0002-8033-4136 Author Contributions †

These authors contributed equally to this work.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We are grateful to Materia Inc. for the donation of ruthenium catalyst. The authors thank NSERC of Canada for financial support, and E.R.S. is grateful for an NSERC Postgraduate Scholarship.

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REFERENCES

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Multiblock Bottlebrush Nanofibers from Organic Electronic Materials

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