Selective dispersion of single-walled carbon nanotubes with electron-rich fluorene-based copolymers

Abstract

We herein report the design and synthesis of novel fluorene-based π-conjugated copolymers containing electron-donating dithiafulvene (DTF) and π-extended tetrathiafulvalene (exTTF) repeat units through Suzuki coupling polymerization. The resulting copolymers showed a comparable degree of polymerization and retained the redox activity originating from the DTF and exTTF building blocks. Theoretical modeling studies predicted that these electron-rich copolymers could wrap around individual single-walled carbon nanotubes (SWNTs) via intimate π-stacking, while experimental results confirmed that the copolymers formed stable polymer–SWNT supramolecular complexes in organic solvents with high selectivity for semiconducting SWNTs. Various spectroscopic and microscopic analyses were conducted to show that the electronic nature of conjugated polymer backbones plays an important role in tuning the binding preference of the polymers for particular types of SWNTs, offering an efficient and selective non-covalent method to sort SWNTs from as-produced mixtures.

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Introduction

The extraordinary electronic, mechanical, and physical properties of single-walled carbon nanotubes (SWNTs) have captured enormous research attention,^1–15 leading to a wide range of applications in advanced electronics and optoelectronics, such as field-effect transistors (FETs),^7,16 flexible electronics and printed circuits,¹⁷ chemo/biosensors,^18–21 photodetectors,²² photovoltaics,^23–25 touch screens,²⁶ microelectronic interconnects,²⁷ high-strength fibers,^28–30 and others.³¹ Recently, some SWNT-based commercial products have already been developed, including rechargeable batteries, automotive parts, sporting goods, and filters.²⁷ Nevertheless, it is still problematic to directly apply as-produced SWNTs, which are structurally heterogenous, in device fabrication where the precise electrical and optical properties of SWNTs need to be uniform. Commercially available SWNTs are typically composed of metallic and semiconducting tubes with various diameters and chiral indices.^32–34 The strong inter-tube attraction of SWNTs causes them to strongly aggregate into bundles, rendering not only poor solubility but also considerable difficulty in their processing and purification. New methods to coerce as-produced SWNTs in solution to selectively segregate into subsets of particular tube types according to diameter and electronic nature have attracted broad research interest. Recently, significant progress in the purification of SWNTs has been made by using various non-covalent methodologies, including density-gradient ultracentrifugation,³⁴ agarose gel filtration,³⁵ electrophoresis,³⁶ and selective complexation with conjugated polymers.^37–43 Of these, the use of conjugated polymers is relatively inexpensive and easily scalable, hence pointing to promising potential in realistic applications; for instance, extraction of relatively pure and debundled semiconducting SWNTs in solution has been achieved, which in turn led to improved FET perfromance.⁴⁴ Modulation of the relative electron density within the polymer backbone has also been demonstrated recently to enable differentiation of semiconducting and metallic SWNTs.⁴² It has been found that electron-rich polymers generally exhibit a preference for interacting with semiconducting nanotubes, while analogous electron-deficient polymers favour metallic nanotubes.⁴² In this context, we and others have demonstrated that integration of electron-donating dithiafulvene (DTF) or π-extended tetrathiafulvalene (exTTF) units into various conjugated polymers can enhance their efficiency and selectivity in terms of SWNT dispersion in organic solvents.^39,43,45,46 Furthermore, polyfluorenes and related polymers have been successfully used for selective SWNT dispersion according to size and electronic type.⁴⁷ In this report, we designed a new class of fluorene-based conjugated copolymers as effective and selective SWNT dispersants. Two π-electron donors, dithiafulvene (DTF) and anthraquinoidal-type extended tetrathiafulvalene (TTFAQ), were chosen to generate fluorene-based copolymers DTF-PF and TTFAQ-PF (see Fig. 1). Unlike in the previous reports,^43,45,46 these structures enable a systematic comparison of the effect that expanding the number of DTF units directly attached to the polymer backbone has on the interaction selectivity with SWNTs. We expected these structures to exhibit favourable affinity for large-diameter semiconducting SWNTs via π-stacking and/or charge-transfer interactions.

Fig. 1 Structures of fluorene-based polymers as selective dispersants for SWNTs.

Results and discussion

To synthesize the designed copolymers for this study, two diiodo-substituted monomer building blocks, DTF 5 and TTFAQ 8, were first prepared via literature reported procedures with suitable modifications (Scheme 1).^48–51 These compounds were then subjected to Suzuki coupling polymerization with fluorenyl diboronate 6 (ref. 52) under the catalysis of Pd(PPh₃)₄ at elevated temperature, affording the target polymers DTF-PF and TTFAQ-PF in very good yields. For comparative studies, an analogous polyfluorene (PF)⁵³ was also prepared by the Suzuki coupling of diiodofluorene 10 (ref. 54) and diboronate 6 under the same reaction conditions. The three fluorene-based polymers all exhibited good solubility in common organic solvents such as THF, CH₂Cl₂, and toluene. Gel permeation chromatographic (GPC) analysis indicated that their degrees of polymerization (DP) were in the range of 13 to 22 (see the ESI† for detailed GPC characterization data).

Scheme 1 Synthesis of DTF- and TTFAQ-containing fluorene copolymers and a related polyfluorene.

The electronic properties of polymers DTF-PF, TTFAQ-PF, and PF were characterized by UV-Vis absorption spectroscopy (Fig. 2). The spectrum of DTF-PF shows an absorption peak at 372 nm, which can be attributed to the absorption of the fluorenyl moiety in comparison with the absorption band at 385 nm in the spectrum of PF. In addition, a prominent shoulder band at 408 nm in the spectrum of DTF-PF coincides with the maximum absorption band at 426 nm in the spectrum of DTF 5. The low-energy region of the spectrum of TTFAQ-PF has an absorption band at 455 nm and two shoulder bands at 431 nm and 390 nm, which are nearly identical to the spectral features of TTFAQ 9. Notably, the fluorenyl band at 352 nm appears to be significantly blue-shifted relative to those of PF and TTFAQ-PF. Overall, the UV-Vis data indicate that the fluorene-based polymers exhibit a trend toward increasing optical band gap as follows: TTFAQ-PF (2.55 eV) < DTF-PF (2.68 eV) < PF (2.98 eV). Clearly, the UV-Vis absorption properties of the fluorene copolymers DTF-PF and TTFAQ-PF are dictated primarily by the electronic nature of the electron-donating arene moieties embedded in their repeat units.

Fig. 2 UV-Vis absorption spectra of fluorene-based polymers measured in THF and spectra of compounds 5 and 9 measured in CH₂Cl₂ at 298 K.

The electrochemical redox activities of the two electron-rich copolymers were characterized by cyclic voltammetry (CV) and differential pulse voltammetry (DPV). The DPV of DTF-PF (Fig. 3A) shows that this copolymer undergoes two steps of electron transfer, at +0.68 V and +0.98 V, upon oxidation, which are similar to the redox characteristics of DTF 5. However, the oxidation potentials of DTF-PF appear to be considerably lowered relative to those of 5 (Fig. 3B), presumably due to the extended π-conjugation between the DTF and fluorenyl moieties across the copolymer backbone. The redox behaviour of TTFAQ-PF copolymer exhibits a one-step electron transfer process at ca. +0.68 V according to its DPV (Fig. 3C), which is consistent with the behaviour of simultaneous bielectronic transfer shown in the CV and DPV profiles of TTFAQ 9 (Fig. 3D). The nearly identical redox potentials of TTFAQ-PF versus TTFAQ 9 suggest that the electronic energies of the TTFAQ moieties in the copolymer are not altered by the adjacent fluorenyl groups. This is not an unexpected outcome, given the known non-planar saddle-like conformation of TTFAQ in the neutral state, which disrupts the π-electronic communications between the fluorenyl and TTFAQ units even though they are directly connected to one another via a C–C single bond.

Fig. 3 Differential pulse voltammograms (blue traces) and cyclic voltammograms (red traces) of (A) DTF-PF, (B) compound 5, (C) TTFAQ-PF, and (D) compound 9.

It is interesting to note that the cyclic voltammograms of DTF 5 and TTFAQ 9 show reversible or quasi-reversible redox couples, but the CV profiles of copolymers DTF-PF and TTFAQ-PF merely exhibit irreversible redox features. The dramatically different electrochemical properties in terms of redox reversibility can be tentatively ascribed to the aggregation effects of the copolymers on the working electrode. Furthermore, the CV data show that DTF-PF has an onset oxidation potential (+0.60 V) quite similar to that of TTFAQ-PF (+0.61 V), suggesting that the two copolymers possess almost the same HOMO energy level and electron-donating ability.

The electronic and redox behaviour of the fluorene-based polymers were further probed by theoretical modelling of their monomeric π-building blocks 11–13 (Fig. 4) through density functional theory (DTF) calculations using the Gaussian 09 software package.⁵⁵ To reduce computational costs, the alkyl groups appended were replaced by hydrogen atoms in these structures.

Fig. 4 Optimized geometries and FMO properties of (A) compound 11 calculated at the B3LYP/6-31+G(d,p) level, (B) compound 12 calculated at the B3LYP/6-31+G(d,p) level, and (C) compound 13 calculated at the B3LYP/6-31G(d) level.

Fig. 4 depicts the optimized geometries of these compounds and their frontier molecular orbital (FMO) properties. As can be seen, the arene units in each of the three monomers show a moderate degree of nonplanarity, the twist angle (θ) being ca. 35–40°. The π-conjugation between the two fluorenyl groups in compound 11 is still significantly retained, which is evidenced by the fact that both its HOMO and LUMO are evenly distributed along the entire molecular framework. For compounds 12 and 13, the HOMO is dominantly populated at the electron-rich DTF or TTFAQ moiety, while the LUMO is mainly concentrated around the fluorenyl unit. The calculated HOMO energies increase in the order 11 < 12 < 13, which is in contrast to the observed HOMO energy levels of DTF-PF and TTFAQ-PF from electrochemical analysis. Such a discrepancy can be explained by the highly twisted, nonplanar conformation of TTFAQ versus the nearly planar DTF unit (Fig. 4). The electronic communication between the π-conjugated repeat units in DTF-PT is somewhat greater than that of TTFAQ-PF. As such, the two copolymers possess similar electron-donating ability even though the monomeric unit of TTFAQ-PF is a better electron donor than DTF-PF.

The ability of the fluorene-based polymers to interact with SWNTs and form homogeneous dispersion in organic solvents was investigated using previously published protocols.⁵⁶ In each of the dispersion experiments, 1 mg of raw HiPco SWNT powder was added to a solution containing 10 mg of polymer dissolved in 10 mL of toluene. The polymer–SWNT mixture was sonicated for 30 min in a bath sonicator (chilled with ice), followed by 60 min of centrifugation at 8346 g. The supernatant was carefully removed from the centrifuge tube and filtered through a cotton plug to obtain the polymer–SWNT suspension. The isolated suspension from each of the three polymers were homogeneous, dark, and generally stable with no observable flocculation over several months (Fig. 5).

Fig. 5 Molecular modelling of fluorene-based polymers (n = 3) wrapping around (6,5) SWNTs by the MMFF force field and photographic images of the three polymer–SWNT suspensions in toluene. (A) PF–SWNT, (B) DTF-PF–SWNT, (C) TTFAQ-PF–SWNT. Alkyl groups were replaced with hydrogen atoms in the modelling to reduce computational expense.

The dispersion experiments clearly showed that the three polymers could effectively bind to SWNTs and disperse them in organic solvents. The polymer–SWNT interactions were simulated using molecular mechanics (MM) methods.⁵⁷ As shown in Fig. 5, all three polymers show tight π-stacking with the SWNT surface and the polymer backbones effectively wrap individual SWNTs. Of note is that polymers PF and DTF-PF adopt a less twisted conformation than their monomers (see Fig. 4) to enable intimate π–π interactions with the nanotube, whereas for TTFAQ-PF the saddle-like conformation of TTFAQ units allows the polymer to naturally curve around the nanotube without significant conformational variations. Herein, the binding mode between TTFAQ and a SWNT is similar to those previously reported by Martín and co-workers.^58,59 Overall, the modelling results in Fig. 5 suggest that TTFAQ-PF is more capable of wrapping around SWNTs than the other two, and this prediction is corroborated by the experimental observation that TTFAQ-PF actually resulted in the darkest SWNT suspension in toluene among the three fluorene polymers (see the photographic images in Fig. 5).

The thermal stability of the fluorene polymers, and their supramolecular complexes with SWNTs was studied by thermogravimetric analysis (TGA) (Fig. 6). These TGA data indicate that the onset decomposition temperature for PF is ca. 431 °C. PF exhibited a weight loss of 63% when heated to 600 °C. Similar thermal decomposition properties were observed in the TGA profiles DTF-PF and TTFAQ-PF; 64% weight loss occurring at 235 °C for DTF-PF, and 66% weight loss at 203 °C for TTFAQ-PF. These weight losses correspond to cleavage of the side chains (–SC₁₀H₂₁ and/or –C₁₂H₂₅) within these polymers. The polymer–SWNT complexes were also investigated by TGA after removal of excess polymer by thorough rinsing with toluene. The TGA profile for PF–SWNT shows a weight loss of 25% at 600 °C, which is much smaller than that of PF. Given that HiPco SWNTs are thermally stable at this temperature, the weight loss here is mainly due to thermal cleavage of the side-chains of PF adsorbed to the SWNT wall. If such is the case, the PF/SWNT complexes are estimated to be composed of approximately 47 wt% of PF. The TGA profiles for the DTF-PF/SWNT and TTFAQ-PF/SWNT complexes similarly exhibit weight losses of 28% and 34% at 600 °C, respectively, from which the mass fractions of polymers were calculated to be 54 wt% and 56 wt%, respectively. These results are consistent with the observed SWNT dispersions and previous reports of supramolecular interactions between SWNTs and conjugated polymers.^39,56,60,61

Fig. 6 TGA profiles showing mass loss upon heating for PF, DTF-PF, TTFAQ-PF, and their supramolecular complexes with HiPco SWNTs. Measurements were conducted in an argon atmosphere with a heating rate of 10 °C min⁻¹.

Atomic force microscopy (AFM) was carried out to examine the microscopic features of polymer–SWNT complexes. In the AFM experiments, diluted polymer–SWNT suspensions were spin-cast onto freshly cleaved mica substrates and then imaged by tapping mode AFM. Fig. 7 depicts the AFM images for the supramolecular complexes of SWNTs with the three fluorene polymers, wherein filamentous structures are present in all three samples. These filaments are approximately 1.2 to 2.5 nm in height, which is consistent with the height of a single SWNT coated with polymers.^33,39 The AFM results indicate that all three polymers can effectively exfoliate SWNT bundles, though a larger amount of amorphous carbon particles is present in the sample dispersed by PF (Fig. 7A). The microscopic features of the three SWNT–polymer samples are similar and in line with the polymer wrapping modes predicted by the modelling studies.

Fig. 7 AFM images for polymer–SWNT complexes on mica. (A) PF–SWNT, (B) DTF-PF–SWNT, (C) TTFAQ-PF–SWNT. The corresponding height profiles are shown below the images. The dashed white lines indicate the location of the height profile and the black scale bars represent 200 nm.

UV-Vis-NIR absorption spectroscopy was performed on the three suspensions of SWNTs. For all the samples, sharp absorption bands in the NIR region characteristic of the SWNT van Hove singularities can be clearly observed (Fig. 8). Such features offer another piece of evidence for SWNT bundles being effectively exfoliated. The intense signals observed in the 830–1600 nm (S₁₁ transitions) and 600–830 nm (S₂₂ transitions) ranges are indicative of the presence of semiconducting SWNTs, while absorption in 450–650 nm corresponds to the M₁₁ transitions of metallic SWNTs.⁶² Comparatively, the spectra of DTF-PF/SWNT and TTFAQ-PF/SWNT suspensions show much weaker signals in the M₁₁ region than the PF/SWNT suspension does. This outcome hence suggests that the electron-rich polymers, DTF-PF and TTFAQ-PF, exhibit selectivity toward binding with semiconducting SWNTs.

Fig. 8 UV-Vis-NIR absorption spectra for HiPco SWNTs dispersed with fluorene polymers in toluene. The spectra are offset for clarity.

To further analyse the properties of SWNTs dispersed with the three fluorene polymers, Raman spectroscopic analysis was conducted with excitation at 785 and 633 nm. This non-destructive analytical technique has been commonly used to determine SWNT diameters and electronic nature, since both metallic and semiconducting tubes can be separately probed by different excitation wavelengths.⁶³ Herein samples for Raman analysis were prepared by drop-casting polymer–SWNT suspensions onto silicon wafers, followed by air-drying overnight. In addition, a sample of raw HiPco SWNTs was suspended in chloroform by sonication and then cast on the silicon wafer for comparative Raman studies.

Fig. 9 shows the full Raman spectra and radial breathing mode (RBM) regions of the polymer–SWNT samples. All Raman spectra were normalized at the G band (ca. 1590 cm⁻¹) and offset for clarity. Although semiconducting SWNTs are predominantly in resonance with the 785 nm excitation wavelength, some small peaks arising from large diameter metallic tubes can still be seen in the low frequency region (150–168 cm⁻¹) of the raw SWNT sample, as previously described.⁶⁴ In contrast, no peaks are observable below 170 cm⁻¹ for the three polymer–SWNT samples. Furthermore, the most intense signal in the RBM region (100–400 cm⁻¹) of the raw SWNT sample appears at 265 cm⁻¹, and is due to the presence of (10,2) SWNTs in bundles.⁶⁵ This peak is also known as the “bundling peak” and represents a useful marker to assess the degree of bundling within a SWNT sample. It can be seen that this peak is significantly diminished in all three polymer–SWNT samples, indicating effective exfoliation.

Fig. 9 Full Raman spectra of raw SWNT, PF/SWNT, DTF-PF/SWNT, and TTFAQ-PF/SWNT samples measured at (A) 785 nm and (C) 633 nm excitation wavelengths, and expanded RBM regions from the same data at (B) 785 nm and (D) 633 nm excitation. The purple region highlights the spectral region corresponding to metallic nanotubes, while the cyan region corresponds to semiconducting nanotubes.

Excitation of the samples at 633 nm gives Raman signals arising from both metallic and semiconducting SWNTs in the RBM region. With regard to HiPco SWNT samples at this excitation wavelength, semiconducting SWNTs contribute to the Raman bands at approximately 230–300 cm⁻¹ while metallic SWNTs give rise to signals approximately 175–230 cm⁻¹.^62,66 It is worth noting that both the metallic and semiconducting peaks are present in the raw SWNT and PF/SWNT samples (Fig. 9D), while DTF-PF/SWNT and TTFAQ-PF/SWNT mostly show peaks due to semiconducting SWNTs. The Raman results together with UV-Vis-NIR data confirm that the electron-rich fluorene copolymers preferentially bind with semiconducting tubes, a property useful for the separation of SWNTs according to their electronic types.

Photoluminescence (PLE) mapping was conducted on the polymer–SWNT samples to characterize the chiralities of SWNTs dispersed by the three polymers (Fig. 10). The location of the various SWNT fluorescence maxima were assigned to different nanotube chiralities according to previously published data.¹² The high emission intensity observed for the SWNTs dispersed by the three fluorene-based polymers again confirms a high degree of SWNT exfoliation into individual tubes upon interaction with the three polymers. In addition, differences in selectivity for different SWNT types by the three polymers can be observed. The two most intense fluorescence spots in the map of PF/SWNT suspension correspond to the (6,5) and (7,5) tubes. In the sample of DTF-PF/SWNT, the two most intense signals are due to the (7,5) and (7,6) tubes. For TTFAQ-PF/SWNT the selectivity shifts toward the (8,6) and (8,7) tubes. Although SWNTs with other chiralities also appear in the PLE maps of the three samples, their fluorescence intensity is significantly lower than that of the most intense signals. Table 1 summarizes the selectivity of the three fluorene polymers for SWNTs in terms of chirality and diameter. Herein only the SWNT species giving rise to the two most intense fluorescence spots are listed. This data clearly shows a trend in selectivity toward increasing SWNT diameter on going from PF to DTF-PF and TTFAQ-PF. Given that DTF-PF and TTFAQ-PF are more electron-donating than PF, our strategy of incorporating these arene(s) within the conjugated polymer repeat units turns out to be effective in attaining good selectivity not only for semiconducting SWNTs, but also for structures with larger diameters. In view of the fact that DTF-PF and TTFAQ-PF have similar electron-donating ability according the electrochemical analysis above, the electronic nature of the fluorene polymers is not the only dominating factor for the observed diameter selectivity. It is likely that the sterically demanding DTF and TTFAQ units (along with their solubilizing side-chains) within the polymers affect the helical pitch of the corresponding polymer backbones, making them more compatible with larger diameter SWNTs. These results are consistent with other reports of selective interactions between conjugated polymers and SWNTs.^37,67,68

Fig. 10 PLE maps of HiPco SWNTs dispersed in toluene with (A) PF, (B) DTF-PF, and (C) TTFAQ-PF.

Table 1 Diameters of SWNTs dispersed by fluorene-based polymers

Polymer	SWNT chirality	SWNT diameter^a (nm)
a Based on published data.^10
PF	6,5	0.757
	7,5	0.829
DTF-PF	7,5	0.829
	7,6	0.895
TTFAQ-PF	8,6	0.966
	8,7	1.032

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Conclusions

DTF and TTFAQ-containing fluorenone copolymers have been found to exhibit strong interactions with the surface of SWNTs, presumably through a polymer wrapping mode, according to modelling and microscopic studies. These copolymers can effectively debundle HiPco SWNTs into stable suspensions in organic solvents. By means of various spectroscopic analyses, we have demonstrated that increasing the electron-donating ability of conjugated copolymers can lead to good selectivity for large-diameter semiconducting SWNTs. On the other hand, the shape and conformation of the polymer repeat units must be taken into consideration as important factor in the rational design of selective polymer dispersants for different types of SWNTs. Our studies herein have demonstrated the applicability of DTF and exTTF-based molecular building blocks in generating novel functional polymers useful for SWNT processing and supramolecular assemblies.

Acknowledgements

This work was supported by the National Science and Engineering Research Council of Canada (NSERC), the Canada Foundation for Innovation (CFI), and McMaster University. S. L. gratefully acknowledges the Vanier CGS scholarship, M. K. and Y. Z. also acknowledge Memorial University for funding support.

Notes and references

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Footnote

† Electronic supplementary information (ESI) available: Detailed synthetic procedures and characterization data for new compounds and fluorene-based polymers. See DOI: 10.1039/c6ra02524b

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