Preparing Semiconducting Nanoribbons with Tunable Length and

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

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Preparing Semiconducting Nanoribbons with Tunable Length and Width via Crystallization-Driven Self-Assembly of a Simple Conjugated Homopolymer Inho Choi, Sanghee Yang, and Tae-Lim Choi* Department of Chemistry, Seoul National University, Seoul 08826, Korea

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ABSTRACT: Precise control of width and length of one-dimensional (1D) semiconducting nanostructures has attracted much attention owing to its potential for optoelectronic applications. However, regulating both their length and width using conjugated polymers or even block copolymers is a huge challenge. To solve this problem, we synthesized a unique conjugated polyacetylene homopolymer by living cyclopolymerization, which spontaneously formed 1D nanoribbons via in situ nanoparticlization. Interestingly, their widths could be controlled from 8 to 41 nm, which were directly proportional to their degree of polymerization. Furthermore, a self-seeding technique via crystallization-driven self-assembly (CDSA) was used to control the length of the nanoribbons up to 5.2 μm with narrow distributions less than 1.1. Interestingly, adding a block copolymer unimer to these nanoribbons produced triblock comicelles by the living CDSA mechanism. Finally, these nanoribbons were visualized directly by super-resolution optical fluorescence microscopy. Now, one can modulate both length and width of 1D nanoribbons simultaneously.

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INTRODUCTION Self-assembly of conjugated small molecules1−4 or conjugated polymers such as poly(3-hexylthiophene) (P3HT),5−8 poly(para-phenylene ethylene),9 poly(para-phenylenevinylene) (PPV),10 and polyfluorene11 could form one-dimensional (1D) semiconducting nanostructures for use in organic fieldeffect transistors (OFETs),12−15 solar cells,16,17 or phototransistors.18 Several reports indicated that increasing the width12,15 or length13,16 of 1D nanostructures of P3HT enhanced the charge carrier mobility of OFETs or power conversion efficiency of solar cells. Therefore, controlling the width and length of semiconducting nanostructures is very important in polymer self-assembly and device applications. For this reason, the Zhai group attempted to obtain the width-controlled nanowhiskers of P3HT by dissolution in dimethylformamide/anisole cosolvent (2:1) at 90 °C and cooling to room temperature.19 However, the width did not increase further when the molecular weight (Mn) of P3HT exceeded 10 kDa because of chain folding of the polymer. Recently, an innovative strategy called crystallization-driven self-assembly (CDSA)13,20−38 was developed by the Manners group, who were able to control the length of 1D and twodimensional (2D) nanostructures from crystalline block copolymers (BCPs). There are several reports of CDSA using conjugated polymers such as P3HT13,22,30 and PPV,36,37 but most of these BCPs also contain insulating shells, which inevitably degrade their optoelectronic properties compared to those of fully conjugated polymers. Furthermore, there is no © XXXX American Chemical Society

report on polymer self-assembly that controls the length or width of 1D nanostructures from a simple homopolymer. Meanwhile, in situ nanoparticlization of conjugated polymers (INCP) became an effective method to prepare various semiconducting nanostructures from conjugated BCPs without any post-treatments.39−46 The key is to use strong π−π interaction of insoluble second conjugated blocks such as nonsubstituted polyacetylene,39,40 polythiophene,41−44 or PPV,45,46 which leads to in situ nanoparticlization during living polymerization. Although this kinetically trapped selfassembly spontaneously formed stable zero-dimensional nanoparticles, 1D nanocaterpillars, or three-dimensional (3D) networks very efficiently, it was very hard to control the size of the nanostructures during the INCP process because of the rapid aggregation of the conjugated block. We recently demonstrated another example of INCP, forming an interesting large-area 2D nanosheet from crystalline poly(cyclopentenylene-vinylene) (PCPV) having fluorene and neohexyl side chains.47 This was an intriguing result because we could obtain 2D nanosheets from a simple and easily synthesized homopolymer showing marginal solubility. However, controlling its size remained a challenge. Therefore, we focused on developing a new efficient INCP process to overcome this challenge. Herein, we report a new design of a monomer having a tert-butyl phenyl group, which spontaReceived: September 26, 2018

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DOI: 10.1021/jacs.8b10406 J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

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Journal of the American Chemical Society

catalyst in tetrahydrofuran at 0 °C,47,48 but the dispersity was somewhat broad (monomer-to-initiator ratio [M]/[I] of 40, Đ = 1.36, see the details in Figure S2). Fortunately, when the temperature was lowered to −10 °C as to further suppress possible chain-transfer reaction, living polymerization yielded P1 with narrow dispersities of less than 1.2 and Mn values ranging from 5.6 to 28.5 kDa from the [M]/[I] ratios of 10 to 60 (Table 1 and Figure 1a). Then, a purple powder of P1 was obtained with full conversion and high yields. First, we characterized the resulting P1 by UV−vis absorption spectroscopy in chloroform and observed absorption at 270 nm corresponding to the fluorene moiety and another absorption feature between 500 and 650 nm that represents the conjugated backbone of P1 (Figure 1b). As the degree of polymerization (DP) of P1 increased, λmax was redshifted from 608 to 614 nm; more importantly, the spectra showed strong shoulders having higher 0−0 vibronic peaks than 0−1 vibronic peaks.49 This result, along with 1H NMR analysis revealing only a single olefinic signal of the trans isomer at 6.8 ppm and IR spectra showing only bands for the trans configuration, suggested that the backbone of P1 had a rigid and all-trans extended conformation, which was ideal for self-assembly (Figure S3). To check the direct formation of the nanostructures in solution, the hydrodynamic diameter (Dh) in chloroform was measured by dynamic light scattering (DLS); it increased from 225 nm to 6 μm with increasing DP without any post-treatment (Figure 1c). To investigate their nanostructures in detail, we performed transmission electron microscopy (TEM) and atomic force microscopy (AFM) imaging and observed the direct formation of 1D nanoribbons without any post-treatment, except for lowmolecular weight P110, which was too soluble to form selfassembled structures in chloroform (Figures 2a−2e, S4, and S5). This is a much simpler self-assembly process than any other reported process, including the formation of P3HT 1D nanostructures only in specific cosolvent mixtures after a tedious heating and aging process.19 Interestingly, TEM imaging even without staining indicated that not only the length but also the average width of the nanoribbons increased from 11 to 41 nm, which is directly proportional to the increase in the DP of P1 from 20 to 60. One should note that there are 3 to 10% deviations on the widths because of the intrinsic dispersity on P1 (Figure 2a−f).50 However, the height of the nanoribbons measured by AFM seemed to increase to 8 nm, but it seemed saturated above P140 (Figure S6). To prove in situ nanoparticlization, we analyzed TEM images obtained by in situ sampling of the solution during CP

neously formed width-controlled 1D nanoribbons during living polymerization by INCP process. In addition, we used the selfseeding method via the CDSA strategy to control the length of the nanoribbons within a narrow distribution by modulating the annealing temperature. Furthermore, triblock comicelles were prepared by living CDSA via seeded growth. Finally, we could directly visualize the fluorescent nanoribbons using super-resolution optical microscopy.

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RESULTS AND DISCUSSION We designed a new 1,6-heptadiyne monomer (M1) containing a 1-(tert-butyl)-4-ethylbenzene side chain (Table 1, R group) Table 1. Living Cyclopolymerization (CP) of M1 with Various [M]/[I] Ratios

entry

M/I

time (h)

Mn (kDa)a

Đ

conv (%)b

yield (%)c

1 2 3 4 5 6

10 20 30 40 50 60

1 2 3 4 5 6

5.6 12.0 16.0 19.0 22.0 28.5

1.10 1.17 1.15 1.07 1.14 1.17

>99 >99 >99 >99 >99 >99

80 80 89 91 95 95

a Determined by CHCl3 SEC calibrated using polystyrene (PS) standards. bCalculated from 1H NMR. cPrecipitated in methanol at room temperature.

on the 2,7 position of the fluorene moiety. We expected that, compared to the monomer we reported previously,47 the additional phenyl ring in M1 would enhance π−π interaction among the resulting homopolymers and improve the crystallinity thereby facilitating the self-assembly process to create new nanostructures. Initially, living cyclopolymerization (CP) of M1 was attempted using third-generation Grubbs

Figure 1. (a) Plot of [M]/[I] ratio vs Mn and corresponding Đ values for P1. (b) UV−vis absorption spectra of various P1 samples in chloroform (0.05 mg/mL). (c) DLS profiles showing Dh of P1 solution in chloroform (1 mg/mL) (P120, Dh = 225 nm; P130, Dh = 661 nm; P140, Dh = 1231 nm; P150, Dh = 3490 nm; P160, Dh = 5590 nm). B


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Figure 2. TEM images of various nanoribbons (a−e) with increasing DP of P1 and in situ generated nanoribbons in chloroform (1 mg/mL) without any staining process: (a) P120, (b) P130, (c) P140, (d) P150, and (e) P160. (f) Plot of the DP of P1 vs the average width of the corresponding nanoribbons. Error bars represent the standard deviation of the width. (g) In situ generated nanoribbons after 1 min of polymerization of P1, [M]/[I] = 50. (h) Plot of the conversion of P1 vs the average width of the in situ generated nanoribbons. All scale bars are 500 nm.

Figure 3. (a) HR-TEM images with electron diffraction analysis, (b) SAED images of the selected area of P150 nanoribbons, and (c) corresponding film XRD results. (d) Proposed packing model of the P1 nanoribbon with estimated lattice points for three axes. The longest length of M1 was determined by single-crystal XRD (Figure S1). (e) Three-dimensional schematic illustration of the formation of 1D nanoribbon based on intercalation of tert-butyl phenyl side chains to the upper neighboring fluorenes. According to the proposed model, all the d-spacings from three diffraction patterns of the 1D nanoribbons matched the proposed orthorhombic unit cell with the calculated lattice parameters. Inset scale bars (a,b) are 50 nm.

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Figure 4. (a−f) TEM images of self-seeded P150 nanoribbons after annealing at various temperatures: (a) seed, (b) 35 °C, (c) 40 °C, (d) 45 °C, (e) 50 °C, and (f) 60 °C. (g) Plot of annealing temperature vs average length and distributions of the nanoribbons. (h) SR-SIM images of P150 selfseeded at 50 °C using a 561 nm laser on a glass substrate.

resulting in elongation along the a axis (Figure 3d). This effective aryl−aryl interdigitation via π−π interaction supported by the d-spacing of 3.8 Å from the film XRD analysis (Figure 3c) also explains why the well-defined nanoribbons were efficiently formed. To further support the orthorhombic unit cell of the nanoribbon, we estimated its lattice parameters as a = 33.8 Å, b = 8.4 Å, and c = 30.0 Å (Figure 3e). Then, the major diffraction of the d-spacing at 16.9 Å from HR-TEM corresponded to the (200) plane. The distance between a fluorene and the next (n + 2) fluorene, 8.4 Å, corresponded to the (010) plane, whereas the d-spacing of 4.23 Å from the SAED analysis in Figure 3b corresponded to the (020) plane, and 15.0 Å, the distance between the two intercalated fluorenes, corresponded to the (002) plane (Figure 3d). In addition, the d-spacing values of 7.52, 5.10, 4.30, and 3.86 Å from the film XRD in Figure 3c corresponded to the (210), (413), (514), and (023) planes, respectively. In short, all of the d-spacing values obtained from various diffraction analyses support the formation of 1D nanoribbons from the orthorhombic crystal lattice. Using the simple INCP mechanism, we could readily prepare the long nanoribbons with a precisely controlled width. Although it was remarkable that this length also increased roughly linearly with the DP from 200 to 4400 nm (Figures 2 and S4), their length distributions were relatively broad, between 1.35 and 1.62, except that of P120 (Lw/Ln = 1.14, Figure S4). Encouraged by the well-defined crystalline packing of P1, we attempted more precise control of the length by using the CDSA strategy of self-seeding, which involves annealing the seed micelles at various temperatures.13,24,27,30,31,36,37 Initially, we performed mild sonication of P150 in chloroform (0.05 mg/mL) for 30 min at 0 °C and obtained stable seed micelles with a narrow length distribution (Ln = 107 nm, Lw/Ln = 1.08, Figure S19) whose length did not change after 24 h at room temperature (Ln = 118 nm, Lw/Ln = 1.18, Figure S20). Then, the seed solution was annealed at temperatures from 30 to 60 °C for 30 min, followed by cooling and aging at room temperature for 24 h before TEM and AFM analysis. As shown in Figures 4a−f and S34, with increasing temperature, the nanoribbons from P150 grew to 5263 nm and exhibited narrow length distributions of less than 1.1. More

([M]/[I] = 50) without quenching and purification. We observed the same nanoribbon formation as before, regardless of the workup process (Figures 2g,h and S11). In addition, the same linearity appeared between the conversion of polymerization and the width of the in situ generated nanoribbons from 8 to 28 nm. Furthermore, the first-order kinetics of the increase in width indicates that elongation and widening of the nanoribbon indeed occur simultaneously during living polymerization (Figure S11). To understand the mechanism of 1D nanoribbon formation, we applied various electron diffraction techniques using highresolution TEM (HR-TEM) and selected area electron diffraction (SAED) to the P1 nanoribbons (Figure 3a,b), which showed clear diffraction at a d-spacing of 16.9 Å along the 1D length direction (Figure 3a). In addition, film X-ray diffraction (XRD) revealed a d-spacing of 3.8 Å corresponding to the π−π interaction (Figure 3c). Then, by combining the results of the electron diffraction techniques and film XRD analysis, we concluded that the nanoribbon showed orthorhombic crystal lattice packing (Figure 3e). According to the model in Figure 3d, the polymer chains extended parallel to the surface and perpendicular to the nanoribbon direction, and this explained the linear relationship between the DP of P1 and the width of the nanoribbons. To support this conclusion, we calculated the single chain length of P1 having an all-trans configuration. On the basis of the data from our previous report, the distance between a fluorene and the next (n + 2) fluorene was approximately 8.4 Å (Figure 3d),47 and this yielded a chain length of the single P120 including the terminal styrene moiety of approximately 10 nm, which was similar to the measured average width of the P120 nanoribbon, 11.2 nm (±1.07 nm) obtained by TEM. This tendency of agreement between the DP and the width due to the horizontal extension of P1 was different from our previous observation of 2D nanosheets in which the DP of the polymer corresponded to the height because the polymer chain extended perpendicular to the surface.47 Regardless of DP following the height or width, these linearities due to the rigid all-trans configuration of PCPV were contrast to the previous examples of P3HT where its dimension was not directly proportional to its DP due to its chain folding.19 Then, other chains of P1 would be stacked on one another by intercalation of a phenyl group into the gap between two fluorenes on the other polymer chain, D


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Figure 5. (a) Scheme of block copolymerization of P250-b-P150. (b) Schematic illustration of the preparation of narrow disperses P150 micelles via self-seeding method. (c) TEM, (d) AFM, and (e) SR-SIM images of B−A−B BCM formed by adding a unimer solution of P250-b-P150 in chloroform to the seed solution of P150 in chloroform, followed by aging at 10 °C for 24 h. All scale bars are 500 nm.

solubility, we introduced a new BCP containing a well-soluble norbornene block and a crystalline block, P250-b-P150 (Figures 5a and S43), and its unimer behavior was confirmed by TEM control experiments in chloroform (1 mg/mL) (Figure S44a). Then, 0.25 mL of the P250-b-P150 chloroform solution (1 mg/ mL) was added to 1 mL of the P150 seed solution (0.05 mg/ mL, Ln = 428 nm, Lw/Ln = 1.09), and B−A−B BCMs were obtained with a uniform length and distribution (Ln = 1522 nm, Lw/Ln = 1.06, Figure S44d) after aging at 10 °C for 24 h. These BCMs were easily identified by TEM because the central block A, which was composed of the fully conjugated homopolymer of P1 with Ln ≈ 451 nm (Lw/Ln = 1.10, Figure S44e), was darker than both end B blocks because of the lower electron density resulting from partially conjugated P2-b-P1 (Figures 5c and S44b,c). Moreover, the heights of the block micelle were different (8 and 5 nm for the A and B blocks, respectively), so one could also distinguish each block micelle easily in the AFM height image as well (Figure 5d). The height difference of two blocks could be explained by the blocking effect from the additional shell of the P2 block suppressing its growth along the c-axis thereby lowering its height. The length

importantly, the width of P150 remained constant during the self-seeding experiment (Figures S37 and S38). Furthermore, for P140 and P160, the same sonication protocol produced seeds with a similar size (P140 = 110 nm and P160 = 111 nm), and as in the P150 case, the self-seeding experiment with increasing annealing temperature formed longer, width-controlled 1D nanoribbons with lengths of 400 to 6000 nm with narrow distributions (Figures S30 and S32). It is remarkable that widths of all the nanoribbons were still controlled precisely according to the DP of P1 allowing us to modulate both dimensions of the nanoribbons simultaneously. Unfortunately, controlling the length beyond 6 μm was not possible because decomposition seemed to occur above 65 °C (Figure S27). In addition, P120 and P130, which had low DP values, did not undergo self-seeding, presumably owing to lower crystallinity (Figures S28 and S29). In principle, P1 nanoribbons prepared by the self-seeding methods should maintain the living crystalline ends, thereby allowing further epitaxial growth for preparation of block comicelles (BCMs) upon the addition of another unimer.25,28,30,36 Therefore, to provide a unimer with sufficient E


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of the central block was almost identical to that of the initial seeds, and the lengths of both ends were similar, indicating successful seeded growth to BCMs via living CDSA. Lastly, P1 had a semiconducting band gap of 1.90 eV with a highest occupied molecular orbital level of −4.96 eV as measured by cyclic voltammetry (Figure S45), and this matched the optical band gap calculated from UV−vis spectra (Figure S46). Because semiconducting P1 was also fluorescent by super-resolution structured illumination microscopy (SRSIM) analysis, we could directly visualize the uniform nanoribbons prepared by self-seeding methods (Figure 4h). Also, the shape of the B−A−B triblock comicelle was vividly displayed by SR-SIM because it showed stronger fluorescence on the central block compared to both terminal ends (Figure 5e). During the fluorescent imaging, the nanoribbons showed no photobleaching or decomposition, so the photostable nanoribbons could be a potentially useful material for optoelectronic devices (Figure S48). Furthermore, controlling the length and width of 1D semiconducting nanostructures could modulate the optoelectronic properties.12,13,15,16 To see this effect from our nanoribbons, we measured the absolute quantum yield of the P1 having different widths and lengths. As a result, the values increased with both increases in the width and length of nanoribbons, suggesting that controlling the size of nanoribbons was indeed important for the potential device applications (Table S1).

Sanghee Yang: 0000-0001-7944-6635 Tae-Lim Choi: 0000-0001-9521-6450 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank financial support from the Creative Research Initiative Grant and the Nano-Material Technology Program through NRF. S.Y. was supported by a National Research Foundation of Korea grant funded by the Korean government (NRF Fostering Core Leaders of the Future Basic Science Program/Global Ph.D. Fellowship Program, grant no. NRF2015H1A2A1033703).

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DEDICATION This paper is dedicated to Prof. Whanchul Shin on his 70th birthday and life-long achievement in research and education at SNU.

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(1) Briseno, A. L.; Mannsfeld, S. C. B.; Lu, X.; Xiong, Y.; Jenekhe, S. A.; Bao, Z.; Xia, Y. Fabrication of Field-Effect Transistors from Hexathiapentacene Single-Crystal Nanowires. Nano Lett. 2007, 7, 668−675. (2) Kim, D. H.; Lee, D. Y.; Lee, H. S.; Lee, W. H.; Kim, Y. H.; Han, J. I.; Cho, K. High-Mobility Organic Transistors Based on SingleCrystalline Microribbons of Triisopropylsilylethynyl Pentacene via Solution-Phase Self-Assembly. Adv. Mater. 2007, 19, 678−682. (3) Sun, Y.; Tan, L.; Jiang, S.; Qian, H.; Wang, Z.; Yan, D.; Di, C.; Wang, Y.; Wu, W.; Yu, G.; Yan, S.; Wang, C.; Hu, W.; Liu, Y.; Zhu, D. High-Performance Transistor Based on Individual Single-Crystalline Micrometer Wire of Perylo[1,12-b,c,d]thiophene. J. Am. Chem. Soc. 2007, 129, 1882−1883. (4) Xiao, S.; Tang, J.; Beetz, T.; Guo, X.; Tremblay, N.; Siegrist, T.; Zhu, Y.; Steigerwald, M.; Nuckolls, C. Transferring Self-Assembled, Nanoscale Cables into Electrical Devices. J. Am. Chem. Soc. 2006, 128, 10700−10701. (5) Ihn, K. J.; Moulton, J.; Smith, P. Whiskers of poly(3alkylthiophene)s. J. Polym. Sci., Part B: Polym. Phys. 1993, 31, 735− 742. (6) Samitsu, S.; Shimomura, T.; Heike, S.; Hashizume, T.; Ito, K. Effective Production of Poly(3-alkylthiophene) Nanofibers by means of Whisker Method using Anisole Solvent: Structural, Optical, and Electrical Properties. Macromolecules 2008, 41, 8000−8010. (7) Martin, J.; Campoy-Quiles, M.; Nogales, A.; Garriga, M.; Alonso, M. I.; Goñi, A. R.; Martin-González, M. Poly(3-hexylthiophene) nanowires in porous alumina: internal structure under confinement. Soft Matter 2014, 10, 3335−3346. (8) Li, J.-H.; Xi, Y.; Pozzo, L. D.; Xu, J.-T.; Luscombe, C. K. Macroscopically aligned nanowire arrays of p-conjugated polymers via shear-enhanced crystallization. J. Mater. Chem. C 2017, 5, 5128−5134. (9) Dong, H.; Jiang, S.; Jiang, L.; Liu, Y.; Li, H.; Hu, W.; Wang, E.; Yan, S.; Wei, Z.; Xu, W.; Gong, X. Nanowire Crystals of a Rigid Rod Conjugated Polymer. J. Am. Chem. Soc. 2009, 131, 17315−17320. (10) Babel, A.; Li, D.; Xia, Y.; Jenekhe, S. A. Electrospun Nanofibers of Blends of Conjugated Polymers: Morphology, Optical Properties, and Field-Effect Transistors. Macromolecules 2005, 38, 4705−4711. (11) Neher, D. Polyfluorene Homopolymers: Conjugated LiquidCrystalline Polymers for Bright Blue Emission and Polarized Electroluminescence. Macromol. Rapid Commun. 2001, 22, 1365− 1385. (12) Zhang, R.; Iovu, M. C.; Jeffries-EL, M.; Sauvé, G.; Cooper, J.; Jia, S.; Tristram-Nagle, S.; Smilgies, D. M.; Lambeth, D. N.; McCullough, R. D.; Kowalewski, T. Nanostructure Dependence of Field-Effect Mobility in Regioregular Poly(3-hexylthiophene) Thin

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CONCLUSION In summary, we demonstrated in situ synthesis of 1D nanoribbons from a simple homopolymer prepared by living CP of the 1,6-heptadiyne monomer containing a crystalline fluorene with a tert-butyl phenyl side chain. TEM, AFM, and various diffraction analyses demonstrated that self-assembly proceeded spontaneously during polymerization. The polymer chain length corresponded directly to the width of the nanoribbon, and further elongation of the nanoribbons occurred perpendicular to it. Furthermore, using the selfseeding strategy of CDSA from just a simple homopolymer, we could obtain both width- and length-controlled 1D nanoribbons from 23 to 41 nm in width and 400 nm to 5.2 μm in length, with a narrow distribution of less than 1.1. Furthermore, the seeded growth method extended the scope of CDSA to the production of BCM via epitaxial growth from seed micelles using a BCP as a unimer. These nanoribbons showed stable fluorescence that enabled visualization by superresolution optical imaging, and we anticipate that these nanoribbons could open a new opportunity for optoelectronic applications.

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The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/jacs.8b10406.

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REFERENCES

General analytical information, experimental methods, synthetic procedures, and supporting figures (PDF) Crystallographic details for CCDC 1863683 (CIF, TXT)

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Preparing Semiconducting Nanoribbons with Tunable Length and

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