High-Purity Semiconducting Single-Walled Carbon Nanotubes via

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High-Purity Semiconducting Single-Walled Carbon Nanotubes via Selective Dispersion in Solution Using Fully Conjugated Polytriarylamines Po-I Wang, Chou-Yi Tsai, Yung-Jou Hsiao, Jyh-Chiang Jiang,* and Der-Jang Liaw* Department of Chemical Engineering, National Taiwan University of Science and Technology, 10607, Taipei, Taiwan S Supporting Information *

ABSTRACT: A new approach for polytriarylamine (PTAA)assisted selective dispersion for single-walled carbon nanotubes (SWNTs) in a toluene solution has been developed. The triarylamine-based conjugated polymers are able to selectively wrap the SWNTs with specific chiral indices depending on their backbone structures (e.g., PTAA12, PTAA12-P, and PTAA12-BP) and side-chain functionality (e.g., PTAA6, PTAA6-alt-PTAA, and PTAA12-alt-PTAA). PTAA12 exhibits highly selective wrapping for the (6,5) chirality from CoMoCAT (catalytic processes) SWNTs but low selectivity in a dispersion of HiPCO (high-pressure carbon monoxide) SWNTs. Therefore, the selection for HiPCO SWNTs has been further improved via PTAA12-alt-PTAA wrapping with alternating side chains and mainly exhibits a high affinity to (6,5) SWNTs with high chiral angles (≥24.5°). The wrapping conformation and binding energy of the polymer/(6,5) SWNTs were studied via molecular modeling, and the simulated results are in good agreement with the experimental data for the selective dispersion of (6,5) SWNTs.

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INTRODUCTION Single-walled carbon nanotubes (SWNTs), which were first discovered by Iijima et al., are hollow cylinder-shaped macromolecules with diameters ranging from 0.4 to 3 nm and lengths ranging from tens of nanometers to centimeters; their walls are one atomic-layer thick composed of a hexagonal lattice of carbon.1 SWNTs have been widely used in solar cells, transistors, high-performance capacitors, and materials with high thermal conductivities.2−4 SWNTs have attracted much attention due to their outstanding structural, mechanical and optoelectronic properties.5−7 The properties of SWNTs strongly depend on their structure, which is defined in terms of two integers (n,m) (i.e., chiral indices).8 When |n − m| = 3q (where q is an integer), the nanotubes are metallic or semimetallic. The remaining nanotubes are semiconducting. However, commercially available SWNTs that are prepared from chemical vapor deposition (CVD), high-pressure carbon monoxide (HiPCO) or catalytic processes (CoMoCAT) possess various chiralities and impurities, which limit their real applications.9−11 Therefore, efficient purification methods are needed to remove impurities, separate metallic and semiconducting SWNTs, and sort SWNTs from mixtures based on their electronic nature, diameters, and chirality. The chemical functionalizations used to extract SWNTs can be divided into two classes as follows: (1) covalent chemistry (e.g., covalent sidewall chemistry and modification at defect sites or open ends) and (2) noncovalent chemistry (e.g., surfactant encapsulation and polymer wrapping).12,13 Because © XXXX American Chemical Society

of the irreversible change in the properties of SWNTs prepared using covalent chemistry, noncovalent chemistry is widely used for selective functionalization with minimal perturbation of the SWNTs. In particular, noncovalent polymer wrapping of SWNTs exhibits high selectivity as a function of the diameter and electronic type.14 In addition, the intrinsic properties of the SWNTs can be preserved. Various conjugated polymers including poly(3-hexylthiophene), poly(2,7-carbazole), poly(phenylacetylene), poly(phenylene ethynylene), and polyfluorene have been used to selectively wrap SWNTs.15−19 The polyfluorenes and alternating copolymers have been extensively studied. For example, the incorporation of aromatic or heterocyclic moieties, such as naphthalene, anthracene, anthraquinone, benzothiadiazole, triazole, dithieno[3,2b:20,30-d]pyrrole and bipyridine, into the main chain of polyfluorenes results in the ability to wrap SWNTs with different chiralities. Therefore, the selection mechanism is associated with the polymer structures possessing π−π interactions with the SWNTs.20−25 Moreover, the side chain functionalities of conjugated polymers, including chain length, side-chain halogenation, hydrophilic and/or hydrophobic moieties, also play an important role in the SWNT dispersion.26−29 Received: September 9, 2016 Revised: October 26, 2016

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DOI: 10.1021/acs.macromol.6b01991 Macromolecules XXXX, XXX, XXX−XXX

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Macromolecules Scheme 1. Synthesis of Various Conjugated Polymers: PTAA12, PTAA12-P, and PTAA12-BP

chloroform, THF and dichloromethane). The nuclear magnetic resonance (NMR) data of the monomers and polymers are shown in the Supporting Information (Figures S1−S5), and these results indicate that the polymers were successfully synthesized. Figure 1 shows the UV−vis absorption and photoluminescence (PL) spectra of PTAA12, PTAA12-P, and

To the best of our knowledge, fully conjugated polytriarylamines (PTAAs) for wrapping SWNTs have not been previously reported. Therefore, various PTAAs with different main chain structures and side chain functionalities have been synthesized for SWNT extraction. Triarylamine-containing polymers have been widely used in hole-transporting materials, light-emitting diodes and electrochromism due to their ability to form stable cation radicals and high hole mobilities.30−35 In addition, the noncoplanar and twisted structure of a triarylamine-based conjugated polymer results in an amorphous character and a good solubility in organic solvents. The relationship between the polymer structures and the selective wrapping of the SWNTs was investigated using vis−NIR absorption spectra, PLE maps, Raman spectra, and molecular modeling.

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RESULTS AND DISCUSSION Polymer Characterization. As shown in Scheme 1, the dibromo monomer (4-dodecyl-N,N-bis(4-bromophenyl)aniline (1)) was prepared from 4-dodecylaniline and p-bromoiodobenzene via Buchwald-Hartwig amination.31 The bromo group of 1 was substituted with a pinacolboronic ester via lithiation followed by nucleophilic substitution with 2-isopropoxy-4,4,5,5, tetramethyl[1,3,2]dioxaborolane to form 4-dodecyl-N,N-bis[4(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl-phenyl)]aniline (2). The triphenylamine-based conjugated polymers with backbones containing different phenyl groups, such as poly[N-(4-dodecylphenyl)-4,4′-diphenylamine] (PTAA12), poly[N(4-dodecylphenyl)-4,4′-diphenylamine-alt-1,4-phenylene (PTAA12-P), and poly[N-(4-dodecylphenyl)-4,4′-diphenylamine-alt-4,4′-biphenyl] (PTAA12-BP), were synthesized via Suzuki coupling polymerization from dibromo monomer 1 with various diboron ester monomers corresponding to monomer 2, 1,4-benzenediboronic acid bis(pinacol) ester, and 4,4′-biphenyldiboronic acid bis(pinacol) ester. The polymers were completely soluble in common organic solvents (e.g., toluene,

Figure 1. UV−vis absorption and PL spectra of the conjugated polymers PTAA12, PTAA12-P, and PTAA12-BP in a THF solution (ca. 10−5 M).

PTAA12-BP in THF (ca. 10−5 M). The absorption bands of these polymers are located at approximately 374−375 nm and correspond to the π → π* transition of the triphenylamine backbone. In general, the conjugation length increased due to the incorporation of phenyl or biphenyl groups. However, similar absorption peaks were observed for the three polymers due to the twisted conformation of the triarylamine moieties, derived from a theoretical calculation based on our previous B

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Macromolecules Table 1. Basic Properties of the Conjugated Polymers (PTAA12, PTAA12-P, and PTAA12-BP) polymers

λabs (nm)a

λPL (nm)b

Egopt (eV)c

HOMO (eV)d

LUMO (eV)d

PTAA12 PTAA12-P PTAA12-BP

374 374 375

417 425 438

2.93 2.89 2.85

−5.23 −5.04 −5.03

−2.30 −2.16 −2.18

Polymer solution in THF (ca. 1 × 10−5 M). bExcitation wavelength was at the λmax of absorption. cCalculated from the UV absorption spectra of the d ox Fc polymer films using the following equation: band gap (eV) = 1240/λabs onset. Calculated from the following equation: HOMO = −(Eonset − Eonset)−4.8 and LUMO = HOMO + band gap.

a

work, thus hindering conjugation in the ground states.35 PTAA12, PTAA12-P, and PTAA12-BP exhibited emission bands at 417 nm, 425 and 438 nm, respectively, as shown in Figure 1, parts d−f. The bathochromic shift of the PL bands was due to the extended π-electron system of the phenylene-containing backbone in the excited state, resulting in emissions at a longer wavelength.36 The optical bandgap can be derived from the onset absorption wavelength of the polymer films. As shown in Figure S9(a), PTAA12, PTAA12-P, and PTAA12-BP exhibited bandgaps of 2.93, 2.89, and 2.85 eV, respectively, indicating that the lower energy of the low-lying π−π* transition was a result of an extension of the conjugation.37 The electrochemical behaviors of the conjugated polymer films were investigated by cyclic voltammetry (CV) with working electrodes in a dry CH3CN solution containing 0.1 M TBAP as the electrolyte. As shown in Figure S9(b), the onset potentials of PTAA12, PTAA12-P, and PTAA12-BP were 0.83, 0.64, and 0.63 V, corresponding to HOMO levels of −5.23, −5.04, and −5.03 eV, respectively. The LUMO levels were calculated to be −2.30, −2.16, and −2.18 eV for PTAA12, PTAA12-P, and PTAA12-BP, respectively. The characterization data for the conjugated polymers are shown in Table 1. Dispersion of CoMoCAT and HiPCO SWNTs with the Conjugated Polymers. The vis−NIR absorption spectra exhibited features that corresponded to the electronic transitions of the SWNTs with different diameters.38 In addition, the absorbance of polymer/SWNTs nanohybrids could be used for a comparison of the SWNT solubility, because the absorption intensity of SWNTs is proportional to the concentration of the SWNTs.19 On the basis of Van Hove singularities, the electronic transitions occurred between the valence band and the conduction band (e.g., v1-c1 and v2-c2 states), which are traditionally labeled S11 and S22 for semiconducting and M11 for metallic SWNTs.39 For the semiconducting SWNTs (HiPCO and CoMoCAT), the S11 transitions ranged between 900 and 1600 nm, and the S22 transitions were located between 600 and 900 nm. The M11 transitions for the metallic SWNTs are in the range of 400 to 550 nm. Figure 2 shows the vis−NIR absorption spectra of the CoMoCAT SWNTs dispersed by three conjugated polymers in toluene or a nonselective dispersant (SDBS) in D2O as a reference. In comparison to the nonselective SDBS/CoMoCAT dispersion shown in Figure 2d, all the conjugated polymer samples exhibited well-resolved SWNT absorption peaks. As shown in parts a−c of Figure 2, the conjugated polymers/ CoMoCAT exhibited flat backgrounds because the impurities (e.g., C60 type fullerenes, amorphous carbon and graphitic carbon) or metallic SWNTs had been removed. The absorption bands at 456 and 508 nm corresponded to metallic tubes with (6,6) and (7,7) chiralities, respectively.19 The metallic tubes are primarily observed for SDBS/CoMoCAT (Figure 2d) rather than conjugated polymer/CoMoCAT samples (Figure 2a−c). In Figure 2a, PTAA12/CoMoCAT exhibited a dominant

Figure 2. Vis−NIR absorption spectra of CoMoCAT nanotubes dispersed in toluene with (a) PTAA12, (b) PTAA12-P, (c) PTAA12BP, and (d) D2O with SDBS.

absorption corresponding to (6,5) chirality with S11 and S22 located at 991 and 577 nm, respectively. In Figure 2 (a), PTAA12/CoMoCAT was of a higher quality compared to PTAA12-P and PTAA12-BP based on the decreased amorphous carbon background. In contrast, PTAA12-P/CoMoCAT exhibited a high absorbance, indicating that PTAA12-P can efficiently wrap the CoMoCAT SWNTs with various chiralities (e.g., (6,5), (8,3), (7,5), and (8,4)), as shown in Figure 2b. The weak absorbance of the PTAA12-BP/CoMoCAT sample is shown in Figure 2c and indicates that CoMoCAT was not well dispersed, because the incorporation of phenylene groups had a low affinity to the smaller diameter SWNTs.40 Figure 3 shows the vis−NIR absorption spectra of HiPCO SWNTs dispersed by PTAA12, PTAA12-P, and PTAA12-BP in toluene and SDBS in D2O. As shown in Figure 3 (a), PTAA12/ HiPCO exhibited resolved absorption bands with a lower background absorbance compared to those for the PTAA12-P/ HiPCO, PTAA12-BP/HiPCO, and SDBS/HiPCO samples, indicating that the impurities were removed by the PTAA12 wrapping. Moreover, the absorbances corresponding to the metallic SWNTs with (6,6) and (7,7) chiral indices were not observed in the PTAA12/HiPCO dispersion, indicating that only semiconducting SWNTs can be completely extracted. Both PTAA12-P/HiPCO and PTAA12-BP/HiPCO exhibited high absorbances and broad baselines, demonstrating that the incorporation of phenylene moieties in the polymer backbones enhances the dispersibility of HiPCO SWNTs. All the polymer/HiPCO nanohybrids possessed similar behaviors in the vis−NIR absorption spectra, which indicates that many species were dispersed. To identify the chiralities of the polymer/SWNTs nanohybrids, photoluminescence excitation (PLE) mapping was employed for further investigation. C

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the (6,5) tubes could be ascribed to two possible causes. The first is nonplanar triarylamine on backbones showing a high affinity to the (6,5) chiral tubes.35 The second is due to (6,5)rich CoMoCAT SWNTs in a high abundance after the PTAA12 wrapping. To confirm that the PTAA polymers exhibit selectivity to (6,5) tubes due to their structural features, the polymer wrapping for HiPCO SWNTs and molecular modeling were further investigated in the following sections. Figure 5 shows the PLE maps of the HiPCO SWNT dispersion with PTAA12, PTAA12-P, and PTAA12-BP. As shown in Figure 5a, PTAA12 is primarily selective toward the (9,5), (8,7), and (8,6) chiralities. Compared to the PTAA12/ HiPCO sample, both PTAA12-P/HiPCO and PTAA12-BP/ HiPCO exhibit primarily the (9,5) chirality in dispersion. The (9,5) tubes with a diameter of 0.97 nm are sorted by all three triarylamine-based conjugated polymers from the HiPCO SWNTs. The relative contents of the SDBS/HiPCO and polymer/HiPCO samples are derived from the peak intensity in the PLE maps. The peak intensity is calculated using peakfitting software. Fitting procedures along both the excitation and emission axes should be considered. In addition, the peak overlaps and phonon effects were removed.14 The chiral tube with the highest intensity in the PLE map was normalized and is represented as a percentage, shown in Table S1. The PTAA polymers exhibit completely different selectivities for HiPCO and CoMoCAT SWNTs. In particular, the PTAA polymers were unable to selectively sort the pure species from the HiPCO SWNTs because the HiPCO tubes are composed of many species. To improve the chiral selectivity of the PTAA/ HiPCO, modification of the side-chain functionality of the polymer was used for further investigation, as shown in the following sections (Scheme 2). Raman spectroscopy was employed to distinguish the metallic and semiconducting SWNT species, which possess characteristic peaks in the radial breath mode (RBM) region.41 The RBM peaks attributed to the semiconducting and metallic SWNTs were located in the 230−320 cm−1 and 160−230 cm−1 regions, respectively, using 633 nm as the excitation wavelengths. Figure 6 shows the Raman spectra of the CoMoCAT SWNT samples derived from the PTAA12, PTAA12-P, and PTAA12-BP wrapping. The spectra were normalized to the intensity of the (7,5) peak. An excitation wavelength of 633 nm is in resonance with the (10,3), (7,6), (7,5), and (8,3) species of the semiconducting SWNTs as well as the (9,9) and (12,3) chiralities of the metallic SWNTs.14,42 As shown in Figure 6a, the PTAA12/CoMoCAT sample primarily shows a (7,5)-tube RBM band at approximately 284 cm−1. It is important to note that the characteristic peaks corresponding to the metallic tubes were not observed in the PTAA12/CoMoCAT, indicating that the metallic species were completely removed after wrapping.

Figure 3. Vis−NIR absorption spectra of HiPCO nanotubes dispersed in toluene with (a) PTAA12, (b) PTAA12-P, (c) PTAA12-BP, and (d) D2O with SDBS.

PLE maps of the samples were measured within the excitation (550−850 nm) and emission (900−1500 nm) regions for recognition of nanotube species in the dispersion. The emission intensities in the PLE maps are represented in red for the higher intensity and blue for the lower intensity. The chirality assignments for the SWNTs were based on the PLE mapping results and literature data.39 Figure 4 shows the PLE maps for CoMoCAT SWNTs dispersed by three types of triarylamine-containing polymers. Among the three polymer/ SWNTs hybrids, the best selectivity for the (6,5) chirality was observed in the PTAA12/CoMoCAT dispersion (Figure 4 (a)). Compared to the high (6,5) selectivity in PTAA12/CoMoCAT, various species of CoMoCAT SWNTs are wrapped by PTAA12-P and PTAA12-BP (Figure 4, parts b and c). Broadening of the optical transitions was observed in the CoMoCAT dispersion with PTAA12-P (Figure 4b), PTAA12BP (Figure 4c) and SDBS (Figure S10) due to the presence of metallic SWNTs in the dispersion. Figure S11 shows the (6,5) normalized PLE intensity of the polymer/CoMoCAT nanotube species based on the diameter of the SWNTs. The PTAA12 is able to wrap the (6,5) tube; however, PTAA12-P and PTAA12BP tend to interact with the (6,5), (8,3), (7,5), (8,4), and (7,6) chiralities with a preference for larger diameter nanotubes (0.78−0.89 nm).19 The low wrapping selectivity of the PTAA12-P and PTAA12-BP was a result of the phenylene groups in the polymer backbone exhibiting more rotational freedom, which increases the flexibility of the polymer chains to fit the curvature of different SWNT surfaces and results in wrapping various chiralities with a nonselective dispersion for CoMoCAT SWNTs.14 PTAA12 showing a high selectivity of

Figure 4. PLE maps of CoMoCAT nanotubes dispersed in toluene with (a) PTAA12, (b) PTAA12-P, and (c) PTAA12-BP. D

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Figure 5. PLE maps of HiPCO nanotubes dispersed in toluene with (a) PTAA12, (b) PTAA12-P, and (c) PTAA12-BP.

Scheme 2. Synthesis of Conjugated Polymers: PTAA6, PTAA6-alt-PTAA, and PTAA12-alt-PTAA

Figure 7 shows the Raman spectra (RBM frequencies) of the polymer/HiPCO samples. Various semiconducting SWNTs,

Figure 6. Raman spectra of the RBM frequencies for (a) PTAA12/ CoMoCAT, (b) PTAA12-P/CoMoCAT, and (c) PTAA12-BP/ CoMoCAT samples (λex = 633 nm). Figure 7. Raman spectra of the RBM frequencies for (a) PTAA12/ HiPCO (b) PTAA12-P/HiPCO, and (c) PTAA12-BP/HiPCO samples (λex = 633 nm).

In addition, a weak (6,5) RBM peak at 312 cm−1 is observed in the PTAA12/CoMoCAT due to a low resonance between the S22 absorption wavelength of the (6,5) tube (575 nm) and the excitation wavelength (632 nm).43 Compared to PTAA12/ CoMoCAT, the results in Figure 6b demonstrate a lower selectivity of the PTAA12-P/CoMoCAT sample with the additional (10,3) and (7,6) chiralities. As shown in Figure 6c, bands corresponding to metallic tubes were observed in the PTAA12-BP/CoMoCAT sample. The introduction of benzene rings into the main chains of the polymer (i.e., PTAA12-BP) does not allow for separation of the metallic and semiconducting tubes. In contrast, the Raman spectra indicate that PTAA12 is highly selective for sorting semiconducting tubes from the CoMoCAT SWNTs (Figure 6 (a)).

including those with (7,5), (7,6), (8,3) and (10,3) chiral indices, were dispersed by three PTAA polymers. In the RBM range of the metallic tubes, the characteristic bands at approximately 200 and 220 cm−1 were not observed in the PTAA12/HiPCO sample (Figure 7a), indicating that the metallic tubes were nearly removed by the PTAA12 wrapping as opposed to the PTAA12-P and PTAA12-BP wrapping. In addition, the Raman spectra of the polymer/SWNTs using 532 nm as the excitation wavelength are shown in Figure S12. The metallic SWNTs are not observed in the PTAA12/CoMoCAT (Figure S12 (a)) and PTAA12/HiPCO (Figure S12 (c)) samples. However, the metallic tubes, such as (7,4), (9,3), and E

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Macromolecules (8,5), are observed in the PTAA12-P/SWNT and PTAA12-BP/ SWNT systems in Figure S12. According to the vis−NIR absorption spectra (Figure 3), the PLE maps (Figure 5), and the Raman spectra (Figure 7), the individual semiconducting nanotubes could not be sorted from the HiPCO SWNTs via PTAA12, PTAA12-P, and PTAA12-BP wrapping because the HiPCO tubes are composed of many species. Therefore, to improve the wrapping selectivity for the polymer/HiPCO nanohybrids, the PTAA polymers were chosen to investigate the relationship between the side chain structures and the selective extraction of the SWNTs, since PTAA12 has a high affinity for semiconducting SWNTs. As shown in Scheme 2, conjugated polymers with various side chains were prepared by Suzuki coupling. Synthetic procedures and characterization data are provided in the Supporting Information (Figures S6−S8). The selective wrapping is primarily based on π−π interactions between the PTAA backbones and the SWNTs. Therefore, to enhance the selectivity of the polymer/HiPCO SWNT nanohybrids with suitable solubilities, the PTAA side groups were modified with different chain lengths and alternating side chains. The vis−NIR absorption spectra of the polymer/CoMoCAT dispersion are shown in Figure S13. Compared to the PTAA12/ CoMoCAT shown in Figure 2a, the selectivity of the (6,5) tubes do not change upon modification of the PTAA side chains (e.g., PTAA6, PTAA6-alt-PTAA and PTAA12-altPTAA). However, the weak absorbances of the nanohybrids (PTAA 6 /CoMoCAT, PTAA 6 -alt-PTAA/CoMoCAT and PTAA12-alt-PTAA/CoMoCAT) was due to low solubility compared to that of PTAA12/CoMoCAT (Figure 2a). Figure 8 shows the vis−NIR absorption spectra for the HiPCO SWNTs dispersed by PTAA6, PTAA6-alt-PTAA and

side chain length increased with decreasing selectivity of the polymer wrapping due to a nonselective van der Waals force between the alkyl chains and the SWNTs.26 Therefore, the chiral selectivity depends primarily on the main chain structures, and the side chain modification is used to control the solubility of the nanohybrids. For the PTAA12-alt-PTAA/ HiPCO system, the alternating dodecyl groups induce suitable solubility characteristics for the nanohybrid. In addition, the unsubstituted triarylamines on the PTAA12-alt-PTAA backbone are used for selective wrapping via π−π interactions. The conformation of PTAA12-alt-PTAA/HiPCO obtained through molecular modeling is shown in the following section. The PLE maps of PTAA12-alt-PTAA/CoMoCAT and PTAA12-alt-PTAA/HiPCO are shown in Figure 9. In Figure

Figure 8. Vis−NIR absorption spectra of HiPCO SWNTs dispersed with (a) PTAA6, (b) PTAA6-alt-PTAA, and (c) PTAA12-alt-PTAA in a toluene solution.

Figure 9. PLE maps of (a) CoMoCAT and (b) HiPCO nanotube dispersion with PTAA12-alt-PTAA in toluene.

PTAA12-alt-PTAA. As shown in Figure 8a, PTAA6/HiPCO exhibits many absorption peaks that are similar to PTAA12/ HiPCO (Figure 3a), indicating that selection of the HiPCO SWNTs could not be improved by reducing the length of the side chain. The extremely low absorbance of the PTAA6-altPTAA/HiPCO is shown in Figure 8b and results from the low solubility of the hybrid. However, PTAA12-alt-PTAA/HiPCO is the best dispersant for the HiPCO SWNTs and is favorable for (6,5) and (7,5) tubes, as shown in Figure 8c. In general, the

9a, the (6,5) nanotube was primarily sorted from CoMoCAT by PTAA12-alt-PTAA wrapping. In addition, for the HiPCO SWNTs, the (6,5), (7,5), and (9,8) species were selectively dispersed with PTAA12-alt-PTAA. The (6,5) species was the dominant chiral index in the PTAA12-alt-PTAA/HiPCO dispersion, and other species were nearly completely removed compared to those in PTAA12/HiPCO (Figure 5a). The (6,5) chirality affinity of PTAA12-alt-PTAA/HiPCO is the same as that of PTAA12-alt-PTAA/CoMoCAT, indicating that the F

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in Table S1, relative intensities higher than 50% were plotted as solid circles. Relative intensities in the 20% to 50% range were plotted as hollow circles. The low selectivity of PTAA12 may be due to numerous long alkyl chains that can strongly bind the SWNTs. In contrast, PTAA12-alt-PTAA exhibited a high affinity for specific chiral indices, such as (6,5) and (7,5) and (9,8), with high chiral angles (≥24.5°). Compared to the alternating dodecyl chains of PTAA12-alt-PTAA, the dodecyl groups of PTAA12 show higher solubilities with flexible conformations that induce lower wrapping selectivities for HiPCO SWNTs. Compared to SDBS/HiPCO and PTAA12/ HiPCO shown in Figure 11, parts a and b, PTAA12-alt-PTAA/ HiPCO (Figure 11c) exhibits the highest selectivities for the (6,5) and (7,5) chiralities, which is in agreement with the results for PTAA12-alt- PTAA/CoMoCAT. Molecular Modeling Studies. In this study, all of the polymer wrapping calculations for (6,5) SWNTs were performed with the commercial package Gaussian 09.44 The geometries of various conjugated polymers as adsorbents on (6,5) SWNTs were optimized using Universal Force Field (UFF) without the consideration of solvent effect.45 The relationship between the polymer structures and the dispersibility for (6,5) SWNTs can be divided into the following two classes: a main chain effect and a side chain effect. As shown in Figure 12a, the triarylamine moieties of PTAA show better stacking on the surface of the SWNTs compared to PTAA-P (Figure 12b) and PTAA-BP (Figure 12c). The π−π interactions of the PTAA-P/(6,5) and PTAA-BP/(6,5) SWNTs with (6,5) SWNTs are hindered by the incorporation of rigid phenyl and biphenyl groups. As shown in the front view of parts b and c of Figure 12, the free benzene rings of the PTAA-P and PTAA-BP facing outward from the SWNTs are observed, leading to irregular wrapping conformations. The binding energy data of polymer/(6,5) SWNT nanohybrids are recorded in Table 2. The binding energy of PTAA/ (6,5) SWNTs, PTAA-P/(6,5) SWNTs and PTAA-BP/(6,5) SWNTs are −5.01, −4.25, and −4.06 eV, respectively. The larger binding energy of PTAA/(6,5) SWNTs is observed, indicating that PTAA shows a higher affinity for (6,5) SWNTs.

nature of the wrapping selectivity for the (6,5) tube is attributed to the structure of the polytriarylamines. Figure 10 shows the resonant Raman spectra of the HiPCO SWNTs derived from the SDBS/D2O and PTAA12-alt-PTAA/

Figure 10. Raman spectra (RBM region) excited at 633 nm for (a) the SDBS/HiPCO and (b) PTAA12-alt-PTAA/HiPCO samples.

toluene suspensions. As shown in Figure 10a, several peaks for the semiconducting nanotubes (i.e., (6,5), (7,5), (8,3), and (10,3)) and metallic nanotubes (i.e., (9,9) and (12,3)) were observed for the SDBS/HiPCO samples, which is considered a nonselective dispersant. In contrast, the intensity of the metallic peaks of PTAA12-alt-PTAA/HiPCO decreased substantially, as shown in Figure 10b, indicating that the metallic SWNTs were successfully removed by the PTAA12-alt-PTAA wrapping. Therefore, high-purity semiconducting tubes from HiPCO SWNTs can be obtained via PTAA12-alt-PTAA wrapping. Figure 11 shows the wrapping preference of HiPCO SWNTs dispersed in SDBS/D2O, PTAA12/toluene and PTAA12-altPTAA/toluene by graphene sheet maps, as represented by black, red and blue circles, respectively. According to the results

Figure 11. Graphene sheet maps for HiPCO SWNTs dispersed using SDBS/D2O and polytriarylamine derivatives/toluene. The colors of the circles inside the hexagons represent the polymers that are selective for the specific chiral indices. In addition, the solid circles represent a relative intensity of >50%, and the hollow circles represent a relative intensity of 20−50%. G

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Figure 12. A side view and front view of the conformation of the polymer wrapping around a (6,5) SWNT by (a) PTAA, (b) PTAA-P, and (c) PTAA-BP. (The main chains of the polymers are marked in blue; nitrogen atoms are marked in red.)

SWNTs show a binding energy of −9.93 eV, which is larger than that of PTAA12-P/(6,5) SWNTs (−9.09 eV). Compared to the PTAA and PTAA-P systems, an increase in the binding energy by the incorporation of aliphatic chains is due to the Van Der Waals interaction between the SWNTs and side chains. However, the binding energy of the PTAA12-alt-PTAA/ (6,5) SWNTs is −8.26 eV, which is smaller than that of PTAA12/(6,5) SWNTs and PTAA12-P/(6,5) SWNTs. The dodecyl composition could be determined by calculating the molecular weight. For the PTAA12-alt-PTAA/(6,5), the composition of the dodecyl moiety is approximately 25.8%, which is smaller than that of PTAA12 (41.1%) and PTAA12-P (34.7%), resulting in a small binding energy. Therefore, the (6,5) chiral selectivity could be attributed to the selective π−π interaction between the twisted triarylamine on the backbones and the SWNTs instead of the high amount of (6,5) tubes in the CoMoCAT SWNTs, because PTAA12-alt-PTAA shows affinity to the (6,5) tubes in CoMoCAT and HiPCO. On the basis of the molecular modeling results, the high selectivity of (6,5) SWNTs through polytriarylamine derivatives could be well elucidated.

Table 2. Binding Energy (Eb) of Polymer/(6,5) SWNT Nanohybrids and the Inner Diameter (D) of the Polymer

a

polymers

Eb (eV)

D (nm)a

PTAA PTAA-P PTAA-BP PTAA12 PTAA12-P PTAA12-alt-PTAA

−5.01 −4.25 −4.06 −9.93 −9.09 −8.26

1.293 1.358 1.581 1.255 1.280 1.358

Inner diameter of polymer around (6,5) SWNTs.

This phenomenon is in good agreement with the experimental results from the selective wrapping for (6,5) SWNTs. However, PTAA-P/(6,5) SWNTs and PTAA-BP/(6,5) SWNTs with smaller binding energy could be attributed to the hindrance of the π−π interaction mentioned earlier. In addition, the inner diameters of the polymers wrapping around the (6,5) SWNTs are tabulated in Table 2. The PTAA, PTAA-P and PTAA-BP possess diameters of 1.293, 1.358, and 1.581 nm, respectively. Compact wrapping of the PTAA is observed indicating that PTAA has a stronger π−π interaction with (6,5) SWNTs. Moreover, the side chains of conjugated polymers were incorporated for the calculation. Figure 13 shows the conformation of the polymer wrapping (e.g., PTAA12, PTAA12-P, PTAA12-alt-PTAA) for the (6,5) SWNTs. The aliphatic moieties wrapping the SWNT surface are observed in three polymer/SWNT systems. For the front view of the PTAA12/(6,5) SWNTs and PTAA12-P/(6,5) SWNTs shown in parts a and b of Figure 13, the dodecyl groups wholly cover the tube, which could enhance solubility and induce less chirality selectivity.26 In contrast, as shown in the front view of Figure 13c, the side chains of PTAA12-alt-PTAA/(6,5) SWNTs bind to the arc of the SWNT surface, indicating that selective wrapping of PTAA12-alt-PTAA is still dominated by triarylamine moieties. As shown in Table 2, the PTAA12/(6,5)

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CONCLUSIONS

A series of polytriarylamines with different structures were successfully designed and synthesized via Suzuki coupling. These conjugated polymers were used for the selective extraction of semiconducting SWNTs from CoMoCAT and HiPCO SWNTs. PTAA12-alt-PTAA was the best dispersant for the (6,5) chiral index for both CoMoCAT and HiPCO SWNTs. The selective polymer wrapping for (6,5) SWNTs was confirmed via a molecular modeling calculation. The semiconducting PTAA/SWNT hybrids having highly purified chirality are prepared through polymer wrapping and exhibit the potential for transistor and photovoltaic applications. H

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Figure 13. Side view and front view of the conformation of polymer wrapping around a (6,5) SWNT by (a) PTAA12, (b) PTAA12-P, and (c) PTAA12-alt-PTAA. (The main chains of the polymers are marked in blue, nitrogen atoms are marked in red, and aliphatic chains are marked in yellow.)

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(3) Sangwan, V. K.; Ortiz, R. P.; Alaboson, J. M. P.; Emery, J. D.; Bedzyk, M. J.; Lauhon, L. J.; Marks, T. J.; Hersam, M. C. Fundamental Performance Limits of Carbon Nanotube Thin-Film Transistors Achieved Using Hybrid Molecular Dielectrics. ACS Nano 2012, 6, 7480−7488. (4) Engel, M.; Small, J. P.; Steiner, M.; Freitag, M.; Green, A. A.; Hersam, M. C.; Avouris, P. Thin Film Nanotube Transistors Based on Self-Assembled, Aligned, Semiconducting Carbon Nanotube Arrays. ACS Nano 2008, 2, 2445−2452. (5) Odom, T. W.; Huang, J.-L.; Kim, P.; Lieber, C. M. Atomic structure and electronic properties of single-walled carbon nanotubes. Nature 1998, 391, 62−64. (6) Dai, H. Carbon Nanotubes: Synthesis, Integration, and Properties. Acc. Chem. Res. 2002, 35, 1035−1044. (7) Avouris, P.; Freitag, M.; Perebeinos, V. Carbon-nanotube photonics and optoelectronics. Nat. Photonics 2008, 2, 341−350. (8) Samanta, S. K.; Fritsch, M.; Scherf, U.; Gomulya, W.; Bisri, S. Z.; Loi, M. A. Conjugated Polymer-Assisted Dispersion of Single-Wall Carbon Nanotubes: The Power of Polymer Wrapping. Acc. Chem. Res. 2014, 47, 2446−2456. (9) Nikolaev, P.; Bronikowski, M. J.; Bradley, R. K.; Rohmund, F.; Colbert, D. T.; Smith, K. A.; Smalley, R. E. Gas-phase catalytic growth of single-walled carbon nanotubes from carbon monoxide. Chem. Phys. Lett. 1999, 313, 91−97. (10) Bachilo, S. M.; Balzano, L.; Herrera, J. E.; Pompeo, F.; Resasco, D. E.; Weisman, R. B. Narrow (n,m)-Distribution of Single-Walled Carbon Nanotubes Grown Using a Solid Supported Catalyst. J. Am. Chem. Soc. 2003, 125, 11186−11187. (11) He, M.; Chernov, A. I.; Fedotov, P. V.; Obraztsova, E. D.; Sainio, J.; Rikkinen, E.; Jiang, H.; Zhu, Z.; Tian, Y.; Kauppinen, E. I.; Niemelä, M.; Krause, A. O. I. Predominant (6,5) Single-Walled Carbon Nanotube Growth on a Copper-Promoted Iron Catalyst. J. Am. Chem. Soc. 2010, 132, 13994−13996. (12) Hersam, M. C. Progress towards monodisperse single-walled carbon nanotubes. Nat. Nanotechnol. 2008, 3, 387−394. (13) Antaris, A. L.; Seo, J.-W. T.; Green, A. A.; Hersam, M. C. Sorting Single-Walled Carbon Nanotubes by Electronic Type Using Nonionic, Biocompatible Block Copolymers. ACS Nano 2010, 4, 4725−4732.

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.macromol.6b01991. Full experimental details, NMR spectra for monomers and polymers, UV−vis absorption spectra and cyclic voltammograms for conjugated polymers, PLE maps and histograms, Raman spectra, vis−NIR spectra, and a table of PLE mapping intensity for CoMoCAT and HiPCO dispersion (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*(D.-J.L.) E-mail: [email protected], [email protected], or [email protected]. Telephone:+886-2-27376638 or +886-2-27335050. Fax: +886-2-23781441 or +886-2-27376644. *(J.-C.J.) E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors wish to thank the Ministry of Science and Technology (MOST) of Taiwan for financial support of this work. The authors also wish to thank Professor Robin Nicholas (Department of Physics) for valuable discussions several times at the University of Oxford.

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