Noncovalent Functionalization of SWNTs with Azobenzene

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Noncovalent Functionalization of SWNTs with Azobenzene-Containing Polymers: Solubility, Stability, and Enhancement of Photoresponsive Properties Chakkooth Vijayakumar, Bijitha Balan, Mi-Jeong Kim,† and Masayuki Takeuchi* Macromolecules Group, Organic Nanomaterials Center (ONC), National Institute for Materials Science (NIMS), 1-2-1 Sengen, Tsukuba 305-0047, Japan

bS Supporting Information ABSTRACT: Azobenzene-derived photoactive polymers (P1-P3) containing pyrene pendants were designed and synthesized (Mw ∼ 30 000) for the noncovalent functionalization of single-walled carbon nanotubes (SWNTs). P1-P3 were found to be highly effective for the solubilization of SWNTs in common organic solvents, resulting in hybrid materials with enhanced thermal stability. The solubilization process was mostly driven by the π-π stacking interactions of pyrene with SWNTs. It also brings the azobenzene chromophores to the vicinity of nanotube surface, thereby allowing the electronic interactions between them. In addition to that, stacking of the pyrene and subsequent wrapping of the polymer around CNT surface provides more volume for the photoisomerization of azobenzene. These effects eventually accelerate the kinetics of photoisomerization of azobenzene in the polymer-SWNT composite. The photoalignment property of the composite was also increased when compared to that of the parent polymer which was studied by means of photoinduced birefringence.

’ INTRODUCTION Carbon nanotubes (CNTs) constitute a relatively new class of nanostructures with unique and exceptional mechanical, electronic, and optical properties.1 They find several applications in the emerging fields of nanoscience and technologies. However, the poor solubility of CNTs in common organic solvents limits their applications to a large extent. During the past several years, significant research has been carried out on the functionalization and subsequent dissolution of single-walled carbon nanotubes (SWNTs) in various solvents which mainly includes covalent2 and noncovalent3 functionalization. The covalent functionalization perturbs the conjugated π-structure of the CNTs and results in adverse changes to its optoelectronic properties. On the other hand, noncovalent functionalization enables one to tailor their properties while preserving the intrinsic properties of nanotubes and hence attracts particular research interest. Because of this reason, noncovalent attachment of numerous aromatic molecules,4 oligomers,5 and polymers6 to the nanotube surface has been investigated. In many cases, not only dissolution of the CNTs but also modulation of the optoelectronic properties of the solubilizing species was achieved leading to the development of hybrid materials with novel and/or enhanced functional properties. In recent years, the focus has been shifted more toward the latter aspect. Development of smart materials responding to various external stimuli such as light, temperature, and pH is one of the most interesting research topics in the area of CNT-based hybrid systems.7 In this context, photoresponsive polymers consisting of r 2011 American Chemical Society

azobenzene chromophores is important. Because of the unique properties such as light-induced cis-trans isomerization as well as the anisotropic alignment, they find applications in optical information storage devices, switching devices, holographic gratings, and so on.8 A number of examples for azobenzenebased systems are known in the literature for the solubilization of CNTs.9 However, only a few have studied the photoresponsive behavior of the resulting hybrid materials. In one report, Yang et al. have shown that the photoisomerization rates of azobenzene side chains of polyurethanes grafted onto multiwalled carbon nanotubes are slower than those of the corresponding parent polymer, and it was suggested to be due to the steric effects.9c Very recently, Imahori et al. reported the noncovalent functionalization and dispersion of SWNTs using polymers containing azobenzene in the main chain.9d In this case also, the cis-trans isomerization in the composite state was found to be very low, and it was attributed to the low mobility of the wrapped polymers. This is not desirable from an application point of view, and hence it is important to design photoactive polymers which do not exhibit lower rate of photoisomerization when functionalized with CNTs for the development of hybrid materials with improved stimuli-responsive functionalities and novel device applications. Herein, we report the first example of azobenzene-based photoresponsive polymers (P1-P3) which Received: November 25, 2010 Revised: January 24, 2011 Published: February 28, 2011 4533

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The Journal of Physical Chemistry C shows good dispersion of SWNTs as well as enhancement in the kinetics of photoisomerization in the composite state. In our design, the polymer main chain was also incorporated with pyrene pendants, which is known to have great capability of binding to the CNT surface through noncovalent π-π stacking interactions, to assist the solubilization process.4a,b,10 Pyrene can also be utilized as a tool to study the photoelectronic interactions between the polymer and CNT through monitoring its absorption and emission properties. In addition to that, the stacking interactions of pyrene with CNT might bring the azobenzene chromophore to the close vicinity of the CNT surface allowing electronic interactions between them. So the main objective of this work is to use these polymers P1-P3 for solubilizing SWNTs and thus to study the photoresponsive and photoalignment properties of the hybrid materials. From our experimental results we were able to show that the formation of polymer/ SWNT nanocomposites accelerate the kinetics of photoisomerization as well as the photoalignment properties of azobenzene.

’ EXPERIMENTAL METHODS Materials. All chemicals were purchased from Aldrich, Kanto Chemicals, TCI, or Wako and used as received. Air- and watersensitive synthetic steps were performed in an argon atmosphere using standard Schlenk techniques. SWNTs were purchased from Unidym, Inc. (Lot No. P0355). Measurements. Melting points were determined with a Yanaco NP-500P micro melting-point apparatus. 1H and 13C NMR spectra were recorded on a 600 MHz Bruker Biospin DRX600 spectrometer. All the chemical shifts were referenced to (CH3)4Si (TMS; δ = 0 ppm) for 1H or residual CHCl3 (δ = 77 ppm) for 13C. High-resolution (HR) LCMS-TOF was obtained with Shimadzu LCMS-IT-TOF workstation. Electronic absorption spectra were recorded on a Hitachi U-2900 spectrophotometer, and the emission spectra were recorded on a Hitachi F-7000 by using a quartz cuvette with 1 cm path length. For emission studies right angle geometry (excitation source and emission detector placed at an angle of 90
) was used. Transmission electron microscope (TEM) analysis of the samples were carried out using a JEOL JEM-2100F at an accelerating voltage of 200 kV. The molecular weights of the polymers were measured by gel permeation chromatography (GPC, TOSOH, HLC8220) in N,N-dimethylformamide solution calibrated against polystyrene standards. Thermogravimetric analysis (TGA) was carried out on a Seiko SII TG/DTA 6200 at a heating rate of 10
C/min between 20 and 600
C. Differential scanning calorimetry (DSC) measurements were done by using a Seiko SII DSC 6220 under nitrogen with a heating rate of 10
C/min. Photoisomerization studies were done by irradiating the samples using a xenon light source (100 W, ASAHI Spectra, LAX-102) with optical filters. The optical setup for photoinduced birefringence measurement was reported earlier.11 For the birefringence measurements, 532 nm light from a diode-pumped laser (Coherent, Verdi 2 V) was used as incident beam. The intensity of the collimated laser beam was 1 mW/cm2. A 633 nm He-Ne laser (Coherent) was used as analysis beam. Preparation of Polymer/SWNT Nanocomposites. Polymer/SWNT nanocomposites were prepared by dispersing SWNT in a solution of the respective polymers in THF. Typically, 5 mg of the polymer was dissolved in 5 mL of THF and 2.5 mg of SWNT was added to the solution. The suspension was then sonicated for 30 min in a low-power sonic bath.

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Subsequently, the dispersion was centrifuged at 3000 rpm for 30 min; two-thirds of the resultant supernatant liquid was decanted carefully from the settled solid and used as the stock solution. It was stable for several weeks; no subsequent settling of material has been observed during this period. The solvent was removed at room temperature by applying vacuum. The resultant polymer/SWNT nanocomposite was soluble in common organic solvents such as chloroform, toluene, DMF, cyclohexanone, etc. For TEM analysis, the stock solution was drop-casted on a carbon-coated Cu grid, and the solvent was allowed to evaporate by keeping at room temperature. UV-vis absorption including the cis-trans isomerization, and emission studies were done in THF after diluting the stock solution by 100 times. For photoinduced birefringence measurements, the dried composite were redispersed in cyclohexanone, followed by the addition of polymer until the concentration of the latter become 8 wt %. The mixture was then sonicated for 30 min and spin-coated onto glass plates (precleaned with acid, alkali, and water) at 1000 rpm. The samples were then heated above the glass transition temperature of the polymer for 8 h, yielding uniform, amorphous films.

’ RESULTS AND DISCUSSION Synthesis and Characterization of Polymers. The monomers and polymers were prepared according to known methods in the literature,12 and the synthetic route is shown in Scheme 1. Both P1 and P2 contain the same ratio of azobenzene and pyrene (50% each) chromophores, whereas P3 has less amount of pyrene (30%). The latter was considered as a model compound to study the role of pyrene and azobenzene on the solubilization of SWNT. The ratio of each chromophores in the polymer were controlled by varying the molar ratio of the monomers during the synthesis. The reaction time was adjusted in such a way that the molecular weights obtained were in the range of ∼30 000. The detailed experimental procedures and characterization data are given in the Supporting Information. Molecular weights and polydispersity index values were determined and are summarized in Table 1. All polymers exhibited considerable broadening in NMR spectrum when compared to that of the monomers. The thermal stability of the polymers was investigated by thermal gravimetric analysis (TGA), which was conducted with a heating rate of 10
C/min in the presence of air. TGA traces of polymers P1-P3 revealed that they were thermally stable up to 500
C.13 The temperature at which a polymer loses 5% of its weight (T5 value) is shown in Table 1. T5 values of polymers P1 and P2 were nearly same, whereas that of P3 was found to be lower. This implies that, as the ratio of the pyrene units decreases, the thermal stability of the polymers also decreases. Differential scanning calorimetry (DSC) analysis of these polymers13 up to 200
C showed a single glass transition temperature (Tg) and the values of which are given in Table 1. The polymers P1 and P2 showed relatively high Tg values (120.2 and 119.5
C, respectively), whereas P3 exhibited a low value (107.8
C). Low value of Tg for P3 once again confirms that higher amounts of pyrene enhance the thermal properties of the polymers. Formation and Characterization of Polymer/SWNT Nanocomposite. Homogeneous polymer/SWNT nanocomposites were obtained when a solution of polymer P1-P3 in THF were mixed with SWNT followed by sonication and centrifugation. Figure 1a shows the photographs of SWNT, polymer P1, and polymer P1/SWNT composite in THF. The composite solution 4534

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Scheme 1. Synthesis of Monomers and Polymersa

a

Reagents and conditions: (i) NaNO2, HCl, H2O, 0-5
C; (ii) triethylamine, CH2Cl2, 0-rt; (iii) AIBN, THF, 60
C, 2 days.

Table 1. Molecular Weight and Polydispersity Index Values As Obtained from GPC measurements, T5 Values As Obtained from TGA Measurements, and Tg Values As Obtained from DSC Measurements polymer

molecular weight (MW)

polydispersity index (PDI)

T5 value (
C)

Tg value (
C)

P1 P2

31 000 30 000

1.66 1.48

260.7 263.8

120.2 119.5

P3

30 000

1.71

242.6

107.8

exhibited a dark color, whereas that of the polymer alone was orange-red. The dark coloration of the hybrid solution was due to dissolution of SWNTs into the organic media by their interaction with the polymer chain. Transmission electron microscopy (TEM) images showed the presence of SWNTs densely coated with polymers as shown in Figure 1b. The TEM image of the nanocomposite exhibited the characteristic rodlike morphology with a width of 2-50 nm and length of several micrometers. The maximum solubility of SWNTs in THF solutions of polymers P1 and P2 was about 0.4 mg/mL, whereas that of polymer P3 was about 0.2 mg/mL. Comparison of solvating power of polymers P1-P3 shows that P1 and P2 with equal ratio of pyrene and azo groups are highly efficient in solubilizing SWNTs, whereas P3 with lesser ratio of pyrene chromophore is less efficient. This observation clearly implies that the solubilization process was

mainly driven by the interactions of pyrene (π-π stacking) with CNT. The TGA analysis showed that the T5 values of polymer/ SWNT nanocomposites increased when compared to that of the polymer alone (Figure 2), which revealed that thermal stability of the polymer was enhanced after hybridization. The T5 values obtained for the nanocomposites of SWNT with polymers P1, P2, and P3 were 326.5, 323.2, and 291.7
C, respectively. Comparison of the T5 values of the nanocomposites reveals that the effect of CNT was more pronounced in the case of P1 and P2 than that of P3. This observation clearly suggests that the thermal stability of polymers increases with their ability to solubilize CNTs. TGA analysis was carried out with all polymers containing the same amounts of SWNTs (20%) also showed a similar trend,13 reiterating that polymers with higher pyrene content 4535

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Figure 1. (a) Dispersion of SWNT in THF (A), solution of polymer P1 in THF (B), and solution of polymer P1/SWNT nanocomposite in THF (C). (b) TEM image of polymer P1/SWNT nanocomposite with a zoomed image in the inset.

Figure 3. Absorption spectra of polymer P1 (blue) and polymer P1/ SWNT (red) composite in THF(10 μg/mL). Inset shows the zoomed portion of the composite absorption. Figure 2. TGA plots of polymers P1-P3/SWNT nanocomposites and SWNT.

exhibit better interaction with SWNT resulting in higher stability to the composites. The decomposition temperature of the polymer was much lower than that of the SWNTs. Thus, the polymers in the polymer/SWNT nanocomposite may be selectively removed in a relatively slow TGA scan (10
C/min), leaving behind the thermally defunctionalized SWNTs at temperatures above 500
C. According to the TGA traces, the carbon nanotube contents in the polymer P1 and P2/SWNT nanocomposites were 40% and that in polymer P3/SWNT was 20%, which supports the previous observation. The noncovalent binding of polymers on the surface of SWNTs and the subsequent solubilization of the SWNTs was evident from the absorption spectrum in THF. The UV-visNIR absorption spectra of polymer P1 and polymer P1/SWNT nanocomposite are shown in Figure 3. The absorption spectrum of the nanocomposite exhibited typical features of solubilized SWNTs whose characteristic van Hove singularities are discernible in the vis-NIR region with additional features of azobenzene and pyrene chromophores in the shorter wavelength region. λmax at 444 nm corresponds to the stronger π-π* electronic transition of the azo group, and λmax at 345 nm corresponds to the π-π* transitions of the pyrene chromophore. The absorption band corresponding to the benzene ring of azobenzene chromophore at a lower wavelength could not be seen here because of the overlap of absorption of the pyrene chromophore.

The π-π* electronic transition of the azobenzene chromophore showed a marginal red shift (λmax = 449 nm) in the nanocomposite which indicates electronic interactions between the azobenzene chromophore and CNTs. This implies that the stacking interactions of the pyrene chromophores with CNT surface bring the azobenzene units to the close proximity of the latter, thereby allowing electronic interactions between them. Any shift in the absorption of pyrene chromophore was hardly observed. However, the proof for the interaction of pyrene moiety with SWNTs was obtained from the emission experiments. In general, the fluorescence intensities of polymers were found to be very low when compared to that of the pyrene monomer molecule (3). In the case of P2, in addition to the quenching, it exhibited a broad emission around 470 nm corresponding to the pyrene excimer fluorescence,14 as shown in Figure 4a (c = 10 μg/mL). One reason for the lower emission intensity for the polymer could be attributed to the formation of excimer which is absent in 3 at this concentration. However, the dramatic quenching could be mainly due to some other electronic interactions between the azobenzene and pyrene moieties. It is well-known that the azobenzene can act as an energy sink by accepting the excitation energy from suitable acceptors which in turn utilized for the isomerization process. In the case of P2, the LUMO of the azobenzene might be suitable for accepting the pyrene monomer emission but not the excimer emission. When P2 forms composite with SWNT, an enhancement in the excimer emission along with further decrease in monomer emission was 4536

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and 510 nm corresponding to the cis-azobenzene increases slightly as a result of the photoisomerization. Under identical conditions, the absorption spectra of polymer P2/SWNT composite solution showed a similar phenomena as that of the polymer alone. However, the photoisomerization process was found to be faster when compared to that of the polymer alone. The first-order rate constant for the trans-to-cis isomerization, ktc, was determined by fitting the experimental data to eq 19c Figure 4. Comparison of the emission spectrum of (a) P2 and (b) P1 before (blue) and after (red) addition of SWNT with that of 3 (green) in THF (c = 10 μg/mL, l = 1 cm, λex = 345 nm).

Figure 5. (a) Absorption spectra showing the trans-to-cis isomerization of polymer P2 in THF under different irradiation time. (b) Comparison of the first-order plots for the photoisomerization of P2 in the absence (blue) and presence (red) of CNT.

observed. In addition to that, a significant red shift was also observed for the λmax of the excimer emission which indicates strong interactions between the CNT and pyrene. CNT could be acting as a template for the stacking of pyrene chromophore into several layers, thereby enhancing the excimer emission. As in the case of P2, polymer P1 also exhibited weak fluorescence intensity. However, no excimer emission was observed in this case before and after addition of SWNT, as can be seen in Figure 4b. Nevertheless, the monomer emission showed further quenching in the presence of SWNT. This observation could be explained by considering the presence of a cyano group in the azobenzene units of polymer P1. The electron-withdrawing nature of the cyano group can reduce the LUMO level of the azobenzene chromophore and make it suitable to accept the excitation energy from both pyrene monomer as well as the excimer species. The fluorescence behavior of P3 was found to be similar to that of P2. Photoresponsive Properties of Polymer and Polymer/ SWNT Composites. Azobenzene and its derivatives can be transformed between the thermodynamically more stable trans form and the less stable cis form upon exposure to UV or visible light. Figure 5a shows the changes in the UV-vis absorption spectra of solution of polymer P2 in THF (c = 10 μg/mL) under different irradiation times. The sample was irradiated with 405 nm unpolarized UV light until they reached the photostationary state. The absorption spectra of polymer P2 shows a high intensity absorption around 345 nm which is due to the π-π* transition of the pyrene and a broad absorption around 418 nm is due to the π-π* transition of the trans-azobenzene moieties. A low-intensity π-π* and n-π* transition of the cis-isomer was observed at 365 and 510 nm, respectively. Upon photoirradiation, the absorbance at 418 nm corresponding to the transazobenzene decreases significantly while the absorbance at 365

lnðA¥ - At Þ=ðA¥ - A0 Þ ¼ - ktc t

ð1Þ

where At, A0, and A¥ are absorbance at 418 nm at time t, time zero, and infinite time, respectively. The first-order plots according to this equation for the trans-to-cis isomerization of azobenzene moieties for polymer P2 and polymer P2/SWNT nanocomposites are shown in Figure 5b. The slope corresponds to the value of ktc. Clearly, the rate constant for the photoisomerization of the polymer (0.0077 s-1) increased upon the formation of nanocomposite with SWNTs (0.0107 s-1). When the UV-irradiated solutions were kept in dark for several days, thermal cis-to-trans back-isomerization occurred which indicates that the photoisomerization behavior is reversible. The azobenzene chromophore of polymer P1 contains donor-acceptor groups (amino and CN) in its 4,40 -positions and exhibits the absorption corresponding to the trans- and cisisomers at the same region in the visible spectrum.11,15 Such systems are known to exhibit very fast photoisomerization and could not be studied by means of conventional UV-vis absorption spectroscopy. However, when irradiated with a laser beam, trans-to-cis isomerization occurs and the cis-isomer quickly go back to the trans configuration after some reorientation. By repetition of the isomerization and reorientation cycles, a sufficiently large portion of the initially homogeneous distribution of the azobenzene groups becomes anisotropic by aligning perpendicular to the polarization direction of the laser beam. When a ray of light passes through an anisotropic film, it decompose into two rays depending on the polarization, resulting in birefringence.11,15,16 The extent of the photoinduced alignment of the azobenzene polymer could be studied by monitoring the birefringence. The birefringence, Δn, was obtained using eq 211 I ¼ I0 sin2 ðπΔnd=λpÞ sin2 θ

ð2Þ

where I is the intensity of the light transmitted through the crossed polarizer, I0 is the intensity of the excitation laser beam, d is the thickness of the film, and θ is the angle between the polarization axes (45
in the present case) of the exciting laser beam and the analyzing laser beam. Figure 6 shows the kinetics of the induction and relaxation of birefringence for P1 and P1/ SWNT nanocomposites (film thickness = 185.6 and 184.3 nm, respectively) at an excitation wavelength of 532 nm. When the polarized laser beam radiation was introduced, an anisotropic accumulation of the azo chromophores was obtained. As a result of onset of birefringence, the curves gradually increased and saturated with different speed for the polymer and the nanocomposite. As it can be clearly seen, the azobenzene polymer bound to the SWNTs showed faster growth in birefringence than that of the free polymer. The birefringence curves were fitted using a biexponential function17 Δn ¼ Af1 - expð - ka tÞg þ Bf1 - expð - kb tÞg 4537

ð3Þ

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Figure 6. Photoinduced birefringence curves of (a) polymer P1 and (b) polymer P1/SWNT nanocomposites.

where t is the irradiation time, A and B are the contribution parameters, and ka and kb are the rate constants. The fast aligning motion of the azobenzene chromophores is represented by A{1 - exp(-kat)}, whereas that of the slow aligning motion of the polymeric segments is represented by B{1 - exp(-kbt)}. The rate constant for the aligning motion of azobenzene chromophores in the polymer/SWNT (15.79 s-1) was found to be higher than that of polymer alone (14.88 s-1). When the excitation light was turned off, the birefringence exhibited a decay and then reached a stable state within a few minutes. This was attributed to the thermal reorientation of some of the azobenzene chromophores in the polymer chain. Thus, even after the switching of the laser light, the remnant birefringence was higher for P1/SWNT nanocomposites than the polymer alone. This result suggests a better molecular orientation and spatial confinement of the azobenzene moieties on the surface of SWNTs than that of the free polymer. Photoresponsive properties of P1 and P2 suggest that the kinetics of the photoisomerization process of the azobenzene derivatives in the polymer chain increases when hybridized with SWNT. It is known that the rate of photoisomerization of azobenzene chromophores are highly affected by the packing density of the chromophores within the polymer chains. In the present case, we assume that the stacking of pyrene chromophores and the subsequent wrapping of the polymer chains around CNT surface provides more volume for the photoisomerization of azobenzene. This could be the reason for the increase of rate constant for the trans-to-cis isomerization upon the formation of nanocomposite with SWNTs, as shown in Figure 5b. Since the anisotropic accumulation of the azo chromophores is also driven by the trans-cis isomerization, an increase in the latter due to the above reason could also results an enhancement in the former process as shown in Figure 6. In addition to this, the pyrene/SWNT interaction also brings the azobenzene chromophores to the vicinity of nanotube surface, thereby allowing the electronic interactions between them as evident from the UV-vis absorption studies. This might also have some contributions to the enhancement of the photoresponsive properties.

’ CONCLUSIONS We have synthesized three azobenzene-derived photoactive polymers containing pyrene pendants for the noncovalent functionalization of SWNTs. They exhibited effective solubilization of SWNTs in common organic solvents opening up the opportunity

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to fabricate photoresponsive polymer/SWNT nanocomposites. As a result of the functionalization, the thermal stability of the polymers were found to be high in the composite state. The solubilization process was mostly driven by the π-π stacking interactions of SWNTs with pyrene. It also brings the azobenzene chromophores to the vicinity of nanotube surface, thereby allowing the electronic interactions between them. Moreover, wrapping of the polymer chains around CNT surface provides more volume for the photoisomerization of azobenzene. These effects eventually accelerate the kinetics of photoisomerization of azobenzene in the polymer/SWNT composite, which have never been reported prior to this study. These results might be helpful for the development of CNT-based hybrid materials with improved stimuli-responsive functionalities and novel device applications.

’ ASSOCIATED CONTENT

bS

Supporting Information. Details of synthesis and characterization of the monomers and polymers, emission properties, TGA and DSC analysis. This material is available free of charge via the Internet at http://pubs.acs.org.

’ AUTHOR INFORMATION Corresponding Author

*Tel þ81-29-859-2110; Fax þ81-29-859-2101; e-mail TAKEUCHI. [email protected]. Present Addresses †

Materials Research Center, Samsung Advanced Institute of Technology (SAIT), Korea 446-712.

’ ACKNOWLEDGMENT The authors thank “Nanotechnology Network Project” of MEXT. This study was supported partially by a Grant-in-Aid for Scientifc Research for Priority Area “Coordination Programming” (area 2107) to M.T. from MEXT, Japan. ’ REFERENCES (1) (a) Iijima, S. Nature 1991, 354, 56–58. (b) Ajayan, P. M. Chem. Rev. 1999, 99, 1787–1800. (c) Carbon Nanotubes: Basic Concepts and Physical Properties; Reich, S., Thomsen, C., Maultzsch., J., Eds.; VCH: Weinheim, Germany, 2004. (d) Sun, Y.-P.; Fu, K.; Lin, Y.; Huang, W. Acc. Chem. Res. 2002, 35, 1096–1104. (e) Andrews, R.; Jacques, D.; Qian, D.; Rantell, T. Acc. Chem. Res. 2002, 35, 1008–1017. (f) Niyogi, S.; Hamon, M. A.; Hu, H.; Zhao, B.; Bhowmik, P.; Sen, R.; Itkis, M. E.; Haddon, R. C. Acc. Chem. Res. 2002, 35, 1105–1113. (g) Eder, D. Chem. Rev. 2010, 110, 1348–1385. (2) (a) Holzinger, M.; Vostrowsky, O.; Hirsch, A.; Henrich, F.; Kappes, M.; Weiss, R.; Jellen, F. Angew. Chem., Int. Ed. 2001, 40, 4002–4005. (b) Hirsch, A. Angew. Chem., Int. Ed. 2002, 41, 1853–1859. (c) Tasis, D.; Tagmatarchis, N.; Georgakilas, V.; Prato, M. Chem.—Eur. J. 2003, 9, 4000–4008. (d) Li, H. M.; Cheng, F. O.; Duft, A. M.; Adronov, A. J. Am. Chem. Soc. 2005, 127, 14518–14524. (e) Tasis, D.; Tagmatarchis, N.; Bianco, A.; Prato, M. Chem. Rev. 2006, 106, 1105–1136. (f) Karousis, N.; Tagmatarchis, N.; Tasis, D. Chem. Rev. 2010, 110, 5366–5397. (3) (a) O’Connel, M. J.; Boul, P.; Ericson, L. M.; Huffman, C.; Wang, Y.; Haroz, E.; Kuper, C.; Tour, J.; Ausman, K. D.; Smalley, R. E. Chem. Phys. Lett. 2001, 342, 265–271. (b) Zheng, M.; Jagota, A.; Semke, E. D.; Diner, B. A.; Mclean, R. S.; Lustig, S. R.; Richardson, R. E.; Tassi, N. G. Nature Mater. 2003, 2, 338–342. (c) Britz, D. A.; Khlobystov, A. N. 4538

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The Journal of Physical Chemistry C Chem. Soc. Rev. 2006, 35, 637–659. (d) Moniruzzaman, M.; Winey, K. I. Macromolecules 2006, 39, 5194–5205. (4) (a) Chen, R. J.; Zhang, Y.; Wang, D.; Dai, H. J. Am. Chem. Soc. 2001, 123, 3838–3839. (b) Nakashima, N.; Tomonari, Y.; Murakami, H. Chem. Lett. 2002, 638–639. (c) Zhang, J.; Lee, J.-K.; Wu, Y.; Murray, R. W. Nano Lett. 2003, 3, 403–407. (d) Li, H.; Zhou, B.; Lin, Y.; Gu, L.; Wang, W.; Fernando, K. A. S.; Kumar, S.; Allard, L. F.; Sun, Y.-P. J. Am. Chem. Soc. 2004, 126, 1014–1015. (e) Hasobe, T.; Fukuzumi, S.; Kamat, P. V. J. Am. Chem. Soc. 2005, 127, 11884–11885. (f) Chen, J.; Collier, C. P. J. Phys. Chem. B 2005, 109, 7605–7609. (g) Tanaka, H.; Yajima, T.; Matsumoto, T.; Otsuka, Y.; Ogawa, T. Adv. Mater. 2006, 18, 1411–1415. (h) Shen, Y.; Skirtach, A. G.; Seki, T.; Yagai, S.; Li, H.; M€ohwald, H.; Nakanishi, T. J. Am. Chem. Soc. 2010, 132, 8566–8568. (5) (a) Cheng, F.; Zhang, S.; Adronov, A.; Echegoyen, L.; Diederich, F. Chem.—Eur. J. 2006, 12, 6062–6070. (b) Srinivasan, S.; Babu, S. S.; Praveen, V. K.; Ajayaghosh, A. Angew. Chem., Int. Ed. 2008, 47, 5746–5749. (c) Srinivasan, S.; Praveen, V. K.; Philip, R.; Ajayaghosh, A. Angew. Chem., Int. Ed. 2008, 47, 5750–5754. (6) (a) Star, A.; Stoddart, J. F.; Steuerman, D.; Diehl, M.; Boukai, A.; Wong, E. W.; Yang, X.; Chung, S.-W.; Choi, H.; Heath, J. R. Angew. Chem., Int. Ed. 2001, 40, 1721–1725. (b) Petrov, P.; Stassin, F.; Pagnoulle, C.; Jer^ome, R. Chem. Commun. 2003, 2904–2905.(c) Shonaike, G. O.; Advani, S. G. Advanced Polymeric Materials: Structure Property Relationships; CRC Press: Boca Raton, FL, 2003. (d) Lou, X.; Daussin, R.; Cuenot, S.; Duwez, A.-S.; Pagnoulle, C.; Detrembleur, C.; Bailly, C.; Jer^ome, R. Chem. Mater. 2004, 16, 4005–4011. (e) Baskaran, D.; Mays, J. W.; Bratcher, M. S. Chem. Mater. 2005, 17, 3389–3397. (f) Cheng, F.; Adronov, A. Chem.—Eur. J. 2006, 12, 5053–5059. (g) Yuan, W. Z.; Sun, J. Z.; Dong, Y.; H€aussler, M.; Yang, F.; Xu, H. P.; Qin, A.; Lam, J. W. Y.; Zheng, Q.; Tang, B. Z. Macromolecules 2006, 39, 8011–8020. (h) Coleman, J. N.; Khan, U.; Gunko, Y. K. Adv. Mater. 2006, 18, 689–706. (i) Kongkanand, A.; Vinodgopal, K.; Kuwabata, S.; Kamat, P. V. J. Phys. Chem. B 2006, 110, 16185–16188. (j) Fujigaya, T.; Haraguchi, S.; Takahiro, F.; Nakashima, N. Adv. Mater. 2008, 20, 2151–2155. (k) Meuer, S.; Braun, L.; Zentel, R. Chem. Commun. 2008, 3166–3168. (l) Yuan, W. Z.; Mao, Y.; Zhao, H.; Sun, J. Z.; Xu, H. P.; Jin, J. K.; Zheng, Q.; Tang, B. Z. Macromolecules 2008, 41, 701–707. (m) Endo, M.; Strano, M. S.; Ajayan, P. M. In Carbon Nanotubes; Springer: Berlin, 2008; Vol. 111, pp 13-61. (n) Liu, J.; Bibari, O.; Mailley, P.; Dijon, J.; Rouviere, E.; Sauter-Starace, F.; Caillat, P.; Vinet, F.; Marchand, G. New J. Chem. 2009, 33, 1017–1024. (o) Byrne, M. T.; Gun’ko, Y. K. Adv. Mater. 2010, 22, 1672–1688. (7) (a) Hong, C.-Y.; Pan, C. Y. J. Mater. Chem. 2008, 18, 1831–1836. (b) Wang, Z.; Li, Z.-x.; Liu, Z. J. Phys. Chem. C 2009, 113, 3899–3902. (c) Chen, S.; Jiang, Y.; Wang, Z.; Zhang, X.; Dai, L.; Smet, M. Langmuir 2008, 24, 9233–9236. (d) Ikeda, A.; Totsuka, Y.; Nobusawa, K.; Kikuchi, J.-i. J. Mater. Chem. 2009, 19, 5785–5789. (e) Saint-Aubin, K.; Poulin, P.; Saadaoui, H.; Maugey, M.; Zakri, C. Langmuir 2009, 25, 13206–13211. (f) Grunlan, J. C.; Liu, L.; Kim, Y. S. Nano Lett. 2006, 6, 911–915. (g) Wang, D.; Chen, L. Nano Lett. 2007, 7, 1480–1484. (h) Etika, K. C.; Jochum, F. D.; Theato, P.; Grunlan, J. C. J. Am. Chem. Soc. 2009, 131, 13598–13599. (8) (a) Kim, D. Y.; Tripathy, S. K.; Li, L.; Kumar, J. Appl. Phys. Lett. 1995, 66, 1166–1168. (b) Kawata, S.; Kawata, Y. Chem. Rev. 2000, 100, 1777–1788. (c) Yoon, H. C.; Shin, H. K.; Kim, C.; Kwon, Y. S. Synth. Met. 2003, 137, 1427–1428. (d) Matharu, A. S.; Jeeva, S.; Ramanujam, P. S. Chem. Soc. Rev. 2007, 36, 1868–1880. (e) Barada, D.; Fukuda, T.; Sumimura, H.; Kim, J. Y.; Itoh, M.; Yatagai, T. Jpn. J. Appl. Phys. 2007, 46, 3928–3932. (f) Sobolewska, A.; Bartkiewicz, S. Appl. Phys. Lett. 2009, 95, 123302. (g) Sobolewska, A.; Bartkiewicz, S.; Miniewicz, A.; SchabBalcerzak, E. J. Phys. Chem. B 2010, 114, 9751–9760. (h) Yoneyama, S.; Yamamoto, T.; Tsutsumi, O.; Kanazawa, A.; Shiono, T.; Ikeda, T. Macromolecules 2002, 35, 8751–8758. (i) Russew, M.-M.; Hecht, S. Adv. Mater. 2010, 22, 3348–3360. (j) Goldenberg, L. M.; Kulikovsky, L.; Gritsai, Y.; Kulikovska, O.; Tomczyk, J.; Stumpe, J. J. Mater. Chem. 2010, 20, 9161–9171. (9) (a) Costanzo, G. D.; Ledesma, S.; Mondragon, I.; Goyanes, S. J. Phys. Chem. C 2010, 114, 14347–14352. (b) Bluemmel, P.; Setaro, A.;

ARTICLE

Popeney, C. S.; Haag, R.; Reich, S. Phys. Status Solidi B 2010, 247, 2891–2894. (c) Yang, Y.; Wang, X.; Liu, L.; Xie, X.; Yang, Z.; Li, R. K. Y.; Mai, Y.-W. J. Phys. Chem. C 2007, 111, 11231–11239. (d) Umeyama, T.; Kawabata, K.; Tezuka, N.; Matano, Y.; Miyato, Y.; Matsushige, K.; Tsujimoto, M.; Isoda, S.; Takano, M.; Imahori, H. Chem. Commun. 2010, 46, 5969–5971. (10) (a) Gomez, F. J.; Chen, R. J.; Wang, D.; Waymouth, R. M.; Dai, H. Chem. Commun. 2003, 190–191. (b) Artyukhin, A. B.; Bakajin, O.; Stroeve, P.; Noy, A. Langmuir 2004, 20, 1442–1448. (c) Guldi, D. M.; Rahman, G. M. A.; Jux, N.; Tagmatarchis, N.; Prato, M. Angew. Chem., Int. Ed. 2004, 43, 5526–5530. (11) Kim, M. J.; Lee, J.; Chun, C.; Kim, D.-Y.; Higuchi, S.; Nakayama, T. Macromol. Chem. Phys. 2007, 208, 1753–1763. (12) (a) Ho, M.-S.; Natansohn, A.; Barret, C.; Rochon, P. Can. J. Chem. 1995, 73, 1773–1778. (b) Narita, M.; Hoshino, F.; Mouri, M.; Tsuchimori, M.; Ikawa, T.; Watanabe, O. Macromolecules 2007, 40, 623–629. (13) See Supporting Information. (14) Hamai, S. J. Phys. Chem. 1989, 93, 6527–6529. (15) (a) Kumar, G. S.; Neckers, D. C. Chem. Rev. 1989, 89, 1915–1925. (b) Buffeteau, T.; Labarthet, F. L.; Pe’zolet, M.; Souris, C. Macromolecules 2001, 34, 7514–7521. (c) Ivanov, M.; Ilieva, D.; Minchev, G.; Petrova, Ts.; Dragostinova, V.; Todorov, T.; Nikolova, L. Appl. Phys. Lett. 2005, 86, 181902. (d) Kim, J. Y.; Fukuda, T. Jpn. J. Appl. Phys. 2006, 45, 456–459. (16) (a) Natansohn, A.; Rochon, P. Adv. Mater. 1999, 11, 1387–1391. (b) Delaire, J. A.; Nakatani, K. Chem. Rev. 2000, 100, 1817–1845. (c) Hagen, R.; Bieringer, T. Adv. Mater. 2001, 13, 1805–1810. (d) Natansohn, A.; Rochon, P. Chem. Rev. 2002, 102, 4139–4175. (e) Kim, M.-J.; Shin, B.-G.; Kim, J.-J.; Kim, D.-Y. J. Am. Chem. Soc. 2002, 124, 3504–3505. (f) Bo, Q.; Yavrian, A.; Galstian, T.; Zhao, Y. Macromolecules 2005, 38, 3079–3086. (g) Okano, K.; Shishido, A.; Ikeda, T. Adv. Mater. 2006, 18, 523–527. (h) Angiolini, L.; Benelli, T.; Giorgini, L.; Salatelli, E.; Bozio, R.; Dauruu, A.; Pedron, D. Macromolecules 2006, 39, 489–497. (i) Choi, S.-W.; Kawauchi, S.; Ha, N. Y.; Takezoe, H. Phys. Chem. Chem. Phys. 2007, 9, 3671–3681. (j) Ikeda, T.; Mamiya, J.-i.; Yu, Y. Angew. Chem., Int. Ed. 2007, 46, 506–528. (k) Priimagi, A.; Vapaavuori, J.; Rodriguez, F. J.; Faul, C. F. J.; Heino, M. T.; Ikkala, O.; Kauranen, M.; Kaivola, M. Chem. Mater. 2008, 20, 6358–6363. (l) Yu, H.; Naka, Y.; Shishido, A.; Ikeda, T. Macromolecules 2008, 41, 7959–7966. (m) Cipparrone, G.; Pagliusi, P.; Provenzano, C.; Shibaev, V. P. J. Phys. Chem. B 2010, 114, 8900–8904. (n) Kawamoto, M.; Sassa, T.; Wada, T. J. Phys. Chem. B 2010, 114, 1227–1232. (o) Wu, S.; Duan, S.; Lei, Z.; Su, W.; Zhang, Z.; Wang, K.; Zhang, Q. J. Mater. Chem. 2010, 20, 5202–5209. (p) Han, D.; Tong, X.; Zhao, Y.; Galstian, T.; Zhao, Y. Macromolecules 2010, 43, 3664–3671. (q) Hernandez-Ainsa, S.; Alcala, R.; Barbera, J.; Marcos, M.; Sanchez, C.; Serrano, J. L. Macromolecules 2010, 43, 2660–2663. (17) Ho, M. S.; Natansohn, A.; Rochon, P. Macromolecules 1995, 28, 6124–6127.

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