Facile Construction of Metallo-supramolecular Poly(3-hexylthiophene

Feb 28, 2017 - Idelson, Sterzenbach, Jester, Tschierske, Baumeister, and Höger. 2017 139 (12), pp 4429–4434. Abstract: Molecular spoked wheels with...
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Facile Construction of Metallo-Supramolecular P3HTb-PEO Diblock Copolymers via Complementary Coordination and Their Self-Assembled Nanostructures Yun-Jui He, Tsung-Han Tu, Ming-Kun Su, Chia-Wei Yang, Kien Voon Kong, and Yi-Tsu Chan J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.7b01010 • Publication Date (Web): 28 Feb 2017 Downloaded from http://pubs.acs.org on February 28, 2017

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Facile Construction of Metallo‐Supramolecular P3HT‐b‐PEO Diblock  Copolymers via Complementary Coordination and Their Self‐ Assembled Nanostructures  Yun-Jui He,† Tsung-Han Tu,† Ming-Kun Su, Chia-Wei Yang, Kien Voon Kong, and Yi-Tsu Chan* Department of Chemistry, National Taiwan University, Taipei 10617, Taiwan ABSTRACT: Complementary coordination of two predesigned 2,2′:6′,2″-terpyridine-based ligands to a ZnII ion led to the exclusive formation of a heteroleptic bis(terpyridine) complex under ambient conditions. This highly self-selective process was facilitated by 9-anthracenyl substituents at the 6,6″-positions of a terpyridine, which not only decelerated the formation rate of its homoleptic complex, but also provided π-stacking stabilization in the heteroleptic complex. Facile construction of metallo-supramolecular P3HTb-PEO diblock copolymers was realized using the complementary ligand pair. The morphological studies of the amphiphilic block copolymers in solution were conducted by AFM and TEM, indicating that the self-assembled core-shell morphology such as spherical and fibrillar nanostructures could be controlled by adjusting the rod/coil block ratios. The heteroleptic complexes residing at the junction between two polymer blocks could be readily dissociated by EDTA to afford the unshelled P3HT nanofiber networks, and restored by treatment of bifunctional ZnII-terpyridine-capped PEO to redisperse the aggregates. The presented supramolecular methodology highlights the merits of complementary metal-ligand coordination, and offers a new approach to engineering nanostructures assembled from rod-coil block copolymers.



well-defined rod-coil block copolymers, development of facile synthetic strategies to avoid tedious purification steps still remains a formidable challenge. In contrast to covalent connectivity, supramolecular polymers consist of monomers or segments held together by non-covalent interactions such as metal-ligand coordination,18 π−π stacking,19 host-guest interaction,20 and hydrogen bonding.21 The supramolecular methodology provides an alternative way to readily produce a wide variety of block copolymers possessing dynamic functions.20i,22 Despite the weak bonding strengths, the recent advances have manifested that supramolecular block copolymers constructed through complementary molecular building blocks could preserve the structural integrity and afford well-ordered, self-assembled nanostructures.23 In this aspect, homopolymers end-functionalized with 2,2′:6′,2″-terpyridine (tpy) ligands have been utilized for the construction of various metallo-supramolecular block copolymers.18k For preparation of an AB diblock copolymer, in order to inhibit the formation of two possible homoleptic complexes, generally a two-step complexation protocol is required to connect two different polymer chains to a kinetically inert metal center (e.g., RuII, CoIII, and NiII).24 The attempts to build diblock copolymers based on labile bis(tpy) complexes were impeded by uncontrollable ligand scrambling in solution. In an effort to overcome the limitation, here we design and construct a complementary tpy-based ligand pair that could undergo spontaneous heteroleptic complexation in the presence of ZnII ions at room temperature. This high-fidelity self-sorting approach not only allows us to easily prepare metallosupramolecular diblock copolymers, but also enables us to endow the resultant copolymers with dynamic features. The synthesis and structural characterization of these well-defined homopolymers and copolymers are presented. The selfassembled morphologies of the amphiphilic [P3HT-Zn-PEO] diblock copolymers in polar environments are carefully

INTRODUCTION 

Rod-coil block copolymers are composed of rigid rodlike macromolecules connected with flexible coillike polymer segments. This class of copolymers has received considerable attention due to the intriguing self-assembled morphologies both in solution and in the solid state.1 The rod block typically comprises rigid π–conjugated backbones such as polymers of fluorine,2 phenylene,3 thiophene,4 and quinolone,5 which afford additional functionality and anisotropic stacking factors in comparison with conventional coil-coil block copolymers.1d,6 Among various conjugated polymers, regioregular poly(3hexylthiophene) (P3HT) has been widely studied because of the unique photophysical and electrochemical properties.7 Taking P3HT-b-PEO as examples, the copolymer has been used as an active layer8 or a compatibilizer9 to tune phase-separated morphology of donor-acceptor blends in organic photovoltaic devices. It has also been demonstrated that P3HT-b-PEO exhibited coexistent ionic and electronic conductivity upon electrochemical oxidation with lithium bis(trifluoromethanesulfonyl) imide in a lithium battery.10 Moreover, various self-assembled nanostructures have been achieved and manipulated based on the crystalline and amphiphilic natures.11 On the basis of the controlled polymerizations for synthesis of conjugated polymers,12 a number of P3HT-based copolymers have been established through chain-end functionalization. For instance, the end-modified P3HTs have been utilized as macroinitiators, chain-transfer reagents, or end-capping agents in living radical polymerization,4a,13 ring opening polymerization,14 and anionic polymerization.4b,15 Highly efficient coupling reactions, e.g., Steglich esterification,16 Suzuki-Miyaura coupling,8 and click chemistry,17 have also been conducted to link two distinct segments. Although these methods have been successfully applied in the preparation of

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examined, and their dynamic properties are demonstrated by the decomplexation-recomplexation experiments.



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suggesting the stability of [ZnL1L2] is enhanced by this π-π interaction. Moreover, in order to understand how the anthracenyl groups affect the formation of homoleptic [ZnL12], a mixture of L1 and Zn(OTf)2 in a molar ratio of 2:1 was monitored by 1H NMR spectroscopy (Figure S6). In comparison with [ZnL22] and [ZnL1L2], the formation rate for [ZnL12] was much slower, and an incomplete conversion (66%) was observed after 7 days at room temperature, presumably due to the increased bulkiness of the substituents at terpyridinyl 6,6″positions.

RESULTS AND DISCUSSION 

In 2004, Lehn reported the heteroleptic complexation of nonand 6,6″-pyrenyl-substituted tpy ligands with ZnII ions,25 where the asymmetric geometry of pyrene moieties gave rise to a mixture of two isomeric complexes. Motivated by this pioneering study, we installed 9-anthracenyl units at the 6,6″positions of a tpy, with expectation that the bulky substituents could not only suppress its homoleptic complexation, but also facilitate the formation of the heteroleptic complex by the π-π stacking interactions between the anthracene units and the tpy ligand (Scheme 1). Accordingly, ligand L1 was synthesized from the 6,6″-dibromo tpy precursor via the Suzuki-Miyaura coupling reaction with 9-anthraceneboronic acid in 90% yield (Scheme S1). The complexation reaction was conducted by mixing an equimolar mixture of L1 and L2 in CHCl3 with a MeOH solution of Zn(OTf)2 (1 equiv) at room temperature. 1H NMR analysis of the resultant complex (Figure 1b) showed four significant upfield shifts for h- (δ = 6.55 ppm), g- (δ = 6.14 ppm) protons of L1 (Δδ = -0.83 and -1.62 ppm) and 3,3″- (δ = 6.89 ppm), 3′,5′- (δ = 6.48 ppm) protons of L2 (Δδ = -1.75 and -2.00 ppm) as compared to the uncoordinated ligands, supporting the quantitative formation of [ZnL1L2]. The unusual upfield shifts for the 3,3″- and 3′,5′-tpy protons could be attributed to the strong shielding effect derived from the anthracenes. The assignments were further confirmed by the COSY and NOESY NMR spectra (Figure S4). Besides, the chemical composition of [ZnL1L2] was verified by the ESI-MS peak at m/z = 494.1837 originated from [ZnL1L2]2+ and its explicit isotope pattern (Figure S5). Scheme 1. Synthesis of Complex [ZnL1L2]

Figure 2. X-ray crystal structure of [ZnL1L2].

The presented complementary ligand pair was further applied to construction of metallo-supramolecular diblock rod-coil copolymers (Scheme 2). The anthracenyl-substituted tpy was incorporated at the hydroxyl end of the commercialized poly(ethylene oxide) methyl ether (PEOME). Firstly, PEOMEs were tosylated, and then reacted with 6,6″-di(9-anthracenyl)-4′(4-hydroxyphenyl)-tpy to afford PEO1 and PEO2 in good yields. The presence of terpyridinyl end groups was verified by the characteristic 1H NMR resonances in the aromatic region (Figures 3a and S9). The high level of end-group fidelity was also confirmed by MALDI-TOF-MS analyses (Figures 3d and S10), with one major molecular weight distribution corresponding to the PEO chains bearing the expected end groups.

Figure 1. 1H NMR spectra of (a) L1 and (c) L2 in CDCl3, and (b) [ZnL1L2] in a mixed solvent of CDCl3/CD3CN (1:2, v/v). The peak for CHCl3 is denoted by an asterisk.

The single crystals of [ZnL1L2] were obtained by slow diffusion of diethyl ether into the complex solution in MeCN. X-ray crystallographic analysis (Figure 2) indicated that the hexacoordinated ZnII ion was located at the center of a pseudooctahedron, and L2 was sandwiched between two anthracenyl substituents with a common π-π stacking distance of 3.3 Å,

Figure 3. Partial 1H NMR spectra of (a) PEO1, (b) [P3HT1-ZnPEO1], and (c) P3HT1 in CDCl3, and MALDI-TOF-MS spectra of (d) PEO1 and (e) P3HT1.

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Scheme 2. Synthesis of Terpyridine-Functionalized Homopolymersa for Construction of Metallo-Supramolecular P3HT-bPEO and Self-Assembled Nanostructures

a Reagents and conditions: (a) TsCl, NEt , THF, rt; (b) 6,6″-di(anthracen-9-yl)-4′-(4-hydroxyphenyl)-2,2′:6′,2″-terpyridine, Cs CO , DMF, 3 2 3 80 oC; (c) 1) t-BuMgCl, LiCl, THF, 2) Ni(dppp)Cl2, THF, 0 oC; (d) 4′-(4-boronophenyl)-2,2′:6′,2″-terpyridine, Pd(PPh3)4, toluene/H2O/tBuOH (3:3:1, v/v/v), reflux

(Figures 3e and S16) suggested the high chain-end functionality for P3HT1 and P3HT2. Four kinds of polymer combinations with ZnII ions were examined in order to investigate their self-assembled morphologies in various solvent systems. The copolymers were prepared by mixing an equimolar solution of tpy-capped PEO and P3HT in CHCl3 with a MeOH solution of Zn(OTf)2 (1 equiv). The resultant diblock copolymers were characterized by 1 H and DOSY NMR experiments (Figures S18-S21). For example, in comparison with the uncoordinated PEO1 and P3HT1, the 1H NMR spectrum after complexation with ZnII ions (Figure 3b) revealed four characteristic upfield shifts for h,g-protons of PEO1 (Δδ = -0.83 and -1.66 ppm) and 3,3″,3′,5′protons of P3HT1 (Δδ = -1.44 and -2.01 ppm), strongly supporting the formation of [P3HT1-Zn-PEO1]. The DOSY spectrum exhibited all the relevant peaks have the same diffusion coefficient (D = 2.29 x 10-10 m2s-1) in CDCl3. It is noteworthy that due to the labile connectivity between two blocks, our attempts to determine the molecular weights of the metallo-supramolecular block copolymers by GPC or MALDITOF-MS were unsuccessful.27 The photophysical properties of the diblock copolymers in a mixed solvent of CHCl3 and MeOH were investigated by UVvis and photoluminescence spectroscopy. Because of the amphiphilic nature of the copolymers, the PEO block hampered direct precipitation of block copolymers in MeOH-rich solvents. The copolymer solutions in CHCl3/MeOH (1:9, v/v) still stayed transparent without precipitation after aging for 3 days. Furthermore, the solutions of [P3HT2-Zn-PEO1] displayed solvatochromism (Figures 5a) with respect to solvents of varying polarities. When the CHCl3 content in MeOH was decreased, the solution color changed from light yellow to purple, most likely due to the enhanced intermolecular interactions among P3HT chains in a polar environment.11b,28 The UV-vis spectra (Figure 5b) displayed a significant bathochromic shift from 448 to 519 nm with two vibronic bands at 549 and 599 nm as the CHCl3 content was reduced to 50%.29 The aggregation-caused photoluminescence quenching30 was also observed at the same solvent composition (Figure 5c). The above observations implied that the P3HT

Figure 4. Partial 1H NMR spectra of (a) L3 and (b) P3HT1 in CDCl3.

On the other hand, the well-defined (Mw/Mn < 1.3) monobromine-terminated Br-P3HTs were prepared by Grignard metathesis (GRIM) polymerization method.12 Subsequently, tpy-functionalized P3HT1 and P3HT2 were obtained through the Suzuki-Miyaura coupling reactions of BrP3HTs with 4′-(4-boronophenyl)-tpy in moderate yields. The resultant polymers were characterized by 1H NMR and MALDI-TOF-MS. Notably, due to the isomeric product (a head-to-tail connection at the chain end) of Br-P3HTs, another set of proton signals for the tpy moiety was observed in the 1H NMR spectra of P3HT1 and P3HT2 (Figures 3c and S15). P3HTs prepared by GRIM have two kinds of chain ends, namely head-to-head and head-to-tail orientations.26 The difference could not be distinguished by the 1H NMR spectra of Br-P3HTs due to the overlapping signals, but it could be clearly seen in those of P3HT1 and P3HT2, whose chain-end head-totail contents are about 20% and 18%, respectively. The thiophene-based bis(terpyridine) L3 was synthesized as a model compound to ensure the correct assignments (Figure 4). Indeed, two sets of tpy signals of L3 were in good accord with those of P3HT1 and P3HT2, supporting the presence of two kinds of tpy connections at the polymer chain ends. The single molecular weight distributions in the MALDI-TOF-MS spectra

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controlled by adjusting the rod/coil ratios. The similar phenomena have been seen in covalently bonded analogues.11a,11d In sharp contrast, the assembled morphology of tpy-P3HT homopolymers in the same solvent system exhibited serious aggregation and intertwined networks composed of irregular ribbons (Figure S24).

  Figure 5. (a) Photograph, (b) UV-vis absorption spectra, and (c) photoluminescence spectra of [P3HT2-Zn-PEO1] in a mixed solvent of CHCl3 and MeOH with varying CHCl3 contents (40100%).

Figure 7. TEM images of nanostructures generated from [P3HT2Zn-PEO1] (3 mg/mL, CHCl3/MeOH = 1:1): (a) as-prepared, (b) after 3-day aging, (c) after 6-day aging, and (d) after 3-day aging, a solution (0.1 mL) of [P3HT2-Zn-PEO1] (30 mg/mL in CHCl3) was added and the mixture was aged for additional 3 days.

To investigate the nanofiber formation process, the nanostructures derived from [P3HT2-Zn-PEO1] over a range of time frames were imaged by TEM (Figure 7). The asprepared sample revealed spherical structures mixed with minor short nanofibers (Figure 7a). After aging for 3 days (Figure 7b), the assemblies were grown into longer fibrillar structures with an average length of 236 ± 95 nm, which was not dramatically changed after aging for additional 3 days (241 ± 124 nm, Figure 7c). On the basis of the crystallization-driven self-assembly (CDSA) process developed by Winnik and Manners,31 the crystalline P3HT domains should be active for epitaxial growth upon treatment with supplementary unaggregated [P3HT2-ZnPEO1]. Therefore, to a 3-day aged solution of [P3HT2-ZnPEO1] (3 mg/mL, CHCl3/MeOH = 1:1), a concentrated solution of [P3HT2-Zn-PEO1] in CHCl3 (0.1 mL, 30 mg/mL) was added, and the mixture was allowed to stand for another 3 days. As expected, the TEM micrograph (Figure 7d) showed the average length of nanofibers was significantly increased to 652 ± 184 nm. A more detailed study is underway to investigate the crystalline nature of the P3HT segment in the nanofibers. According to the morphological observations and CDSA experiment, it is suggested that the metallo-supramolecular diblock copolymers in a polar environment can self-assemble into ordered core-shell nanostructures, where the hydrophobic P3HT is folded in the core and the hydrophilic PEO is surrounded as a shell (Scheme 2). In order to prove the existence of core-shell structures, a strong chelating agent, EDTA, was utilized to dissociate the labile ZnII-tpy junction between two blocks. Since the hydrophilic PEO block has better solubility in MeOH, it is supposed to be easily washed away by a MeOH solution of EDTA. The nanofibers generated from [P3HT2-Zn-PEO1] on a silicon wafer was used to verify our assumption. The silicon wafer coated with [P3HT2-Zn-PEO1] nanofibers was immersed in a MeOH solution containing tetrakis(triethylammonium) EDTA (5 mg/mL) for 1 day, and

Figure 6. TEM images of the solutions (3 mg/mL, CHCl3/MeOH = 1:1) of (a) [P3HT1-Zn-PEO1], (b) [P3HT1-Zn-PEO2], (c) [P3HT2-Zn-PEO1], and (d) [P3HT2-Zn-PEO2].

aggregation induced by the increased solvent polarity might give rise to formation of certain nanostructures in the solution. In light of the solvent-induced P3HT aggregation, the selfassembly behavior of the block copolymers in CHCl3/MeOH (1:1, v/v) was studied by transmission electron microscopy (TEM) and atomic force microscopy (AFM). To a solution of diblock copolymers in CHCl3 (0.5 mL, 6 mg/mL) which was filtered with a PTFE syringe filter (0.22 μm), MeOH (0.5 mL) was added slowly. The copolymer solution was allowed to age for 3 days at room temperature, and then diluted to 0.03 mg/mL with MeOH. The dilute solution was spin-coated onto a silicon wafer washed by piranha solution (H2SO4/H2O2 = 3:1, v/v) and drop-casted on a carbon-coated copper grid for AFM and TEM measurements, respectively. The TEM image of [P3HT1-ZnPEO2] (P3HT weight fraction: fP3HT = 60%) showed spherical structures as majority (Figure 6b). As the fP3HT was increased, [P3HT1-Zn-PEO1] (fP3HT = 77%) and [P3HT2-Zn-PEO2] (fP3HT = 70%) revealed a mixture of spheres and short nanofibers. (Figures 6a and 6d). With further increasing the fP3HT to 84%, [P3HT2-Zn-PEO1] exhibited longer nanofibers with a uniform width of 11 ± 1 nm (Figure 6c). The corresponding AFM images (Figure S23) were also consistent with the TEM observations. As the fP3HT was increased from 60% to 84%, the morphological transition from spheres to fibers was clearly observed, indicating the nanostructures could be

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Journal of the American Chemical Society days. After 1 day, the precipitates were formed and collected by centrifugation. In order to thoroughly remove the ZnII-EDTA complex and decomplexed PEO1, the cotton-like precipitates were washed by centrifugation in MeOH (6 mL, 3 times) and resuspended in MeOH. The TEM image (Figure 9b) showed the networked aggregates composed of well-defined nanofibers, again in sharp contrast to the irregular ribbons directly generated from P3HT2 (Figure S24). A bifunctional PEO3 end-capped with two anthracenyl-substituted tpys was synthesized (Scheme S2) for the recomplexation experiment. The suspension (6 mL) of unshelled P3HT2 nanofibers in MeOH was treated with a MeOH solution (0.1 mL) containing PEO3 (10 mg/mL) and 2 equivalents of Zn(OTf)2, and followed by sonication for 30 s. It was found that the precipitates were well dispersed into MeOH and the fragmented fibrillar aggregates were observed by TEM (Figure 9c).



CONCLUSIONS 

In summary, the spontaneous heteroleptic complexation under ambient conditions was successfully achieved by non- and 6,6″anthracenyl-substituted tpy ligands with ZnII ions. The sterically hindered 6,6″-substituents and the π-π stacking interactions between the anthracenyl group and pyridine rings led to this high-fidelity self-sorting. Building on the complementary ligand pair, the well-defined P3HT and PEO chain ends were mono-functionalized with tpy and 6,6″anthracenyl-substituted tpy, respectively. A series of metallosupramolecular rod-coil diblock copolymers could be readily constructed upon treatment with ZnII ions. Due to the amphiphilic nature of the copolymers, the self-assembled morphology in a mixed solvent of CHCl3 and MeOH could be controlled by tuning their rod/coil ratios. As the fP3HT was increased, the morphological evolution from spheres to fibers was clearly observed by TEM and AFM. The time-dependent formation process of nanofibers was elucidated by TEM, and their CDSA behavior was demonstrated. Moreover, the dynamic character of the labile connectivity allowed the PEO shell of the self-assembled nanofibers to be simply removed by a MeOH solution of EDTA without damaging the P3HT core, which was able to be recomplexed with other ZnII-tpy adducts. The complementary coordination approach described here offers opportunities to develop functional block copolymers, which could be arranged into well-defined assemblies with potential applications in sensing and organic electronics.

Figure 8. AFM adhesion images of nanofibers formed from [P3HT2-Zn-PEO1]: (a) before and (b) after removal of the PEO block. (c) Adhesion force profiles.

Figure 9. (a) Schematic illustration of dynamic features of nanofibers assembled from [P3HT2-Zn-PEO1]. TEM images of [P3HT2-Zn-PEO1] solutions in MeOH: (b) after decomplexation by EDTA and (c) recomplexation with PEO3-ZnII bisadduct. Insets are the photographs of the corresponding solutions.



ASSOCIATED CONTENT 

Supporting Information.

then the surface was washed with 2 mL MeOH. The remaining solvent was evaporated in vacuo. In the AFM adhesion images (Figures 8a and 8b), the higher interaction force between the cantilever tip and the sample is indicative of the contact with the softer PEO region, which is located in the fiber periphery.32 On the other hand, the lower interaction area in the middle part of the fiber is composed of P3HT. The force profiles (Figure 8c) clearly demonstrated that the PEO region was selectively trimmed off after decomplexation by EDTA. In addition, the characteristic Raman signals for PEO1 were significantly diminished after selective removal of PEO (Figure S25). To further explore the dynamic features of self-assembled nanofibers, the decomplexation-recomplexation experiments were conducted in solution (Figure 9a). A MeOH solution (0.1 mL) of tetrakis(triethylammonium) EDTA (50 mg/mL) was added into a solution (2 mL) of [P3HT2-Zn-PEO1] in CHCl3/MeOH (1:1, v/v, 3 mg/mL) which has been aged for 3

X-ray crystallographic data for [ZnL1L2] (CIF) Experimental details, synthesis, and characterization data (PDF)



AUTHOR INFORMATION 

Corresponding Author *[email protected]

Author Contributions  †These

authors contributed equally.

Notes  The authors declare no competing financial interest.

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ACKNOWLEDGMENTS 

This work was supported by the Ministry of Science and Technology of Taiwan (MOST105-2923-M-002-008-MY2 and MOST105-2119-M-002-029). We gratefully thank Ms. C.-Y. Chien and Ms. S.-J. Ji at MOST (National Taiwan University) for the assistance in TEM experiments.



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