Preparation and Characterization of Conducting Mixed-Valence 9,9

Article pubs.acs.org/Langmuir

Preparation and Characterization of Conducting Mixed-Valence 9,9′Dimethyl-3,3′-bicarbazyl Rectangular Nanowires Takuya Tokuda,† Katsuyuki Murashiro,∥ Minako Kubo,∥ Hyuma Masu,‡ Mamoru Imanari,‡ Hiroko Seki,‡ Nobuyuki Aoki,† Yuichi Ochiai,† Hirofumi Kanoh,§ and Katsuyoshi Hoshino*,† †

Graduate School of Advanced Integrated Science, ‡Chemical Analysis Center, and §Graduate School of Science, Chiba University, 1-33 Yayoi-cho, Inage-ku, Chiba 263-8522, Japan ∥ Ichihara Research Center, JNC Petrochemical Corp., 5-1 Goikaigan, Ichihara, Chiba 290-8551, Japan S Supporting Information *

ABSTRACT: The facile synthesis of an organic electric conducting nanowire is described. The simple oxidation of 9methylcarbazole by iron(III) perchlorate in a methanol/ acetonitrile mixture under atmospheric pressure and temperature produces abundant nanowires without using a template. The nanowire consists of 9,9′-dimethyl-3,3′-dicarbazyl and has a rectangular nanowire shape with an average diameter of 397 ± 50 nm and length of 17 ± 5 μm. The results of the elemental analysis, 1H NMR, FT-IR, XPS, and ESR measurements revealed that the chemical composition of the nanowire is (dicarbazyl)0.12(dicarbazylium·ClO4−)0.88·H2O. This result, combined with the UV−vis−NIR measurement, demonstrates that 9,9′-dimethyl-3,3′-dicarbazyl stacks in a mixed valence state. The nanowire is electroactive and has an electric conductivity of 3.0 × 10−5 S cm−1.

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formed a film consisting of self-assembled nanofibers by casting the mixed solution of TTF and TTF + ·(organic acid anions).16,17 Quite recently, our group found that conductive nanotubes of a quadratic prism consisting of 9,9′-dimethyl-3,3′bicarbazyl and 9,9′-dimethyl-3,3′-bicarbazylium perchlorate were formed by a self-assembled growth when the electrolytic oxidation of 9-methylcarbazole (MeCz) was carried out in methanol.18 These well-ordered molecular arrays are expected to become important modules for the integration of molecular circuits, since the carbazole molecule is an attention-getting and important building block of new devices such as organic photovoltaic devices, secondary batteries, hole transport materials for organic light-emitting diodes, organic field-effect transistors, transparent conductive materials, electrochromic devices, etc.19−30 However, the method had a drawback of a low yield resulting from the electrosynthesis; thus, it was required to find an alternative method for large-scale synthesis. In this study, we describe the facile chemical preparation of conducting nanofibers consisting of mixed-valence 9,9′dimethyl-3,3′-bicarbazyl in a large quantity. As a result of various chemical analyses and shape observations, it has been found that these nanofibers consisted of nanowires different from the above-mentioned electrosynthesized nanotubes regarding the shape and the doping level. The results of the

INTRODUCTION The preparation of electric conducting nanofibers based on the self-organization of organic molecules has attracted much attention, since these advanced nanofibers can be utilized as electric wires and molecular modules in the next-generation flexible organic nanoelectronics.1−10 In these nanofibers, intermolecular interactions such as hydrogen bonding, CT interactions, π−π stacking interactions, and van der Waals interactions are the driving force of self-aggregation.11,12 In particular, when low-molecular-weight molecules form a nanofiber, these molecules include long alkyl chains in many cases. This is because the conductivity is increased by the fastener effect;13 i.e., van der Waals interactions between alkyl chains introduced to π-conjugated molecules fasten the π-core of the molecular frameworks tightly. However, it is preferable to improve the electric conductivity without using the alkyl chain since the alkyl chain is intrinsically an insulating group. From the viewpoint of electric conductivity, the current small molecule nanofibers are at a level of a semiconductor; thus, it is necessary to further improve the intermolecular interaction. In recent years, examples of electric conductive nanotubes and wires formed by the self-assembly of small molecules without a long-chain alkyl group have been reported. Park et al. reported that the 1-cyano-trans-1,2-bis(3′,5′-bistrifluoromethylbiphenyl)ethylene gelator formed a fiber structure by self-assembly due to the intermolecular force caused by the π−π stacking interactions and four CF3 units.14,15 Chujo et al. formed conductive nanowires of (TTF·Cl0.78)Au0.12 by the reaction of tetrathiafulvalene (TTF) and gold ions. In addition, they © 2012 American Chemical Society

Received: August 31, 2012 Revised: October 29, 2012 Published: October 29, 2012 16430

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electric conductivity measurements of a single nanowire and a pressed pellet of the nanowires are also reported.

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EXPERIMENTAL SECTION

Materials. MeCz (>99%) and Fe(ClO4)3·nH2O (the content as anhydride was 70.7%) were purchased from Tokyo Chemical Industry and Wako Pure Chemical Industries, Ltd., respectively, and were used as supplied. Methanol (>99.8%, reagent grate), acetonitrile (>99.7%, spectroscopic grade), and ethanol (>99.5%) were obtained from Kanto Chemical Co., Inc. Measurements. The identification of the chemical structure of the nanowires was based on 1H NMR, FT-IR, LDI-TOFMS (laser desorption ionization time-of-flight mass spectrometry), XPS (X-ray photoelectron spectroscopy), ESR, and UV−vis−NIR measurements. The 1H NMR spectra were recorded in DMSO-d6 solution by a Varian NMR system. All the chemical shifts (δ in ppm) were referenced to the solvent signal. The LDI-TOFMS spectra were obtained using a Bruker Daltonics autoflex III. The ESR spectrum was recorded by a JEOL JES-TE200 spectrometer. The g factor value was determined using MnO as an external standard (g = 2.003 29). The UV−vis and UV−vis−NIR spectra were recorded by a Hitachi U-3000 spectrophotometer and a Shimadzu UV-3600, respectively. The FTIR spectrum was obtained using a JASCO FT/IR-410. The scanning electron microscopic (SEM) images were obtained using a Topcon ABT-32 operated at an accelerating voltage of 15 kV. The SEM test samples were prepared by putting a powder sample on an adhesive carbon conducting tape attached to a specimen support. The transmission electron microscopic (TEM) images were obtained using a Hitachi H-7650 operated at an accelerating voltage of 100 kV. The redox properties under a nitrogen atmosphere and 22 °C were examined by cyclic voltammetry using an electrochemical analyzer (ALS model 1200A). An electrolytic cell consisting of a main compartment and a subcompartment, which were separated by a G4 glass filter, was used. A working indium−tin oxide coated glass electrode (ITO, Geomatech Co., 10 Ω/sq) and a counter Pt plate electrode were placed in the main compartment. On the other hand, a KCl agar bridge connected to a reference electrode (saturated calomel electrode, SCE) was placed in the subcompartment. The electrolyte was an acetonitrile solution containing 0.1 M tetrabutylammonium perchlorate (Tokyo Chemical Industry, >98%). The electric conductivity of a single nanowire was measured using the method described in the Supporting Information (Figure S9). The electric conductivity of a molded pellet of the nanowires was also measured at room temperature by a four-point-probe method using a Mitsubishi Chemical Analytech model Loresta-GP MCP-T600 with an MCP-TP06P probe.

Figure 1. (a−c) SEM images of the green product prepared by the oxidation of MeCz with Fe(ClO4)3 in a CH3OH/CH3CN mixed solvent at 0 °C. (d) TEM image of the green product prepared by the procedure in (a). (e) SEM image of the cross-sectional view of the product; the same sample as in (a). Scale bars in the images show (a) 2 μm, (b) 1 μm, (c) 5 μm, (d) 500 nm, and (e) 500 nm.

nanowires were 397 ± 50 nm and 17 ± 5 μm, respectively (aspect ratio = 45 ± 18). In order to examine the constituents of the nanowire, various instrumental analyses were done. Figure 2 shows the 1H NMR

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RESULTS AND DISCUSSION To a 20 mL stirred CH3OH/CH3CN solution (3:1 volume ratio) of MeCz (0.050 M) was dropwise added a 20 mL CH3OH/CH3CN solution of Fe(ClO4)3 solution (0.071 M) under a nitrogen atmosphere at 0 °C. The color of the solution changed from transparent colorless to dark green within several minutes. The obtained solution was then stored for 24 h, and the dark green precipitate was isolated by suction filtration. The green product was washed with a CH3OH/CH3CN mixed solvent three times and then dried under vacuum for 90 min at 50 °C. The yield was 0.19 g (20%). The SEM image (parts a−c in Figure 1) of the green product indicated that it has the shape of a straight nanofiber. No difference in the contrast ratio was observed between the center part and the edge of the nanofiber in the TEM image (Figure 1d). This indicates that its shape is not a nanotube but a nanowire. In addition, based on the result of the SEM observations, the cross-sectional shape of the nanowire was a tetragon (Figure 1e). The average diameter and length of the

Figure 2. 1H NMR spectrum of the nanowire sample in DMSO-d6 (a) and the enlarged spectrum extracted from part a (b).

spectrum of the nanowire sample. The spectrum exhibits a peak at 3.93 ppm arising from the N-methyl proton (singlet) and seven peaks (7.22−8.58 ppm) in the region of aromatic protons. Based on the comparison of the area of the proton peaks, six N-methyl protons and 14 aromatic protons are involved in the sample. These peaks, when considered with their splitting, can be assigned to the protons of 9,9′-dimethyl3,3′-bicarbazyl (1, Chart 1).31 This dimeric species was also confirmed by its LDI-TOFMS measurement. The LDI-TOFMS spectrum of the nanowire sample indicated a signal at m/z = 359.894 (Figure S1, Supporting Information). This value coincided with the molecular weight (360.458) of 1. 16431

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Chart 1. Chemical Structure of 1 and 2

Figure 4. FT-IR spectrum of the nanowire sample in a KBr pellet.

The ESR measurement was carried out using a powder sample of the nanowire (Figure S2). Despite the use of a tiny amount of the sample, an extremely large signal with a g value of 2.003 29 was observed. This g value falls under the category of organic radicals. Additionally, a large peak-to-peak line width suggests that the radical species are delocalized in the large πconjugation system and coupled with many nuclei. These results clearly demonstrate the presence of radical species, i.e., 9,9′-dimethyl-3,3′-bicarbazylium (2, Chart 1). The presence of 2 was also demonstrated by the UV−vis spectrum of the sample solution which was prepared by dissolving 0.30 mg of the sample in 5.0 mL of acetonitrile (Figure 3). The spectrum

cm−1 correspond to the CH bending of the 1,2,4-trisubstituted aromatic rings of poly(9-alkylcarbazole), which indicates that the carbazole units are bonded by 3−3′ coupling.36 The signals36,37 caused by ClO4− were observed near 1100 and 620 cm−1. This is an indication that the building block of the nanowire contains 9,9′-dimethyl-3,3′-bicarbazyl doped with ClO4−. The XPS analysis of the nanowire sample clearly showed the presence of ClO4− (Figure S3). The signal at 1552 cm−1 is the ring vibration band characteristics of the oxidized 9,9′-dimethyl-3,3′-bicarbazyl, 2.38 The signals observed near 750 and 720 cm−1 are attributed to the CH bending of the 1,2disubstituted rings.36 All these FT-IR results are consistent with the results of the 1H NMR, LDI-TOFMS, ESR, and UV−vis measurements. The elemental analysis of the nanowire sample, i.e., C, 67.14%; H, 4.49%; N, 5.92%; and Cl, 6.73%, are consistent with the calculated formula for the nanowire, 10.12(2·ClO4−)0.88H2O (C, 67.02%; H, 4.76%; N, 6.01%; Cl, 6.70%). The doping level, defined as the number of ClO4− ions associated with one methylcarbazole unit, was calculated to be 44% as the ratio of the Cl and N compositions. This value is nearly the same as those of the doping level reported for polycarbazoles.39,40 From the UV−vis−NIR absorption spectrum (part a in Figure 5) of the nanowire-dispersed film, it could be inferred that 1 and 2 were stacked in the nanowire. The sample solution was prepared by adding 150 mg of nanowires to 10 mL of a 2butanone solution of poly(methyl methacrylate) (PMMA, Aldrich, average molecular weight = 3.5 × 105) (PMMA:2butanone = 1:5 wt ratio) and stirring for 1 h. The obtained solution was then spread on a glass substrate using a wire bar coater (RK Print Coat Instruments, model K303 multicoater) to make a sample film of 8.4 μm thickness. Part b in Figure 5 is an optical micrograph of the sample film, and part c is a crosssectional SEM image of the film. These nanowires were uniformly dispersed, while nearly keeping their original shape though some of them formed bundles. Broad absorptions centered at 1350 and 1700 nm were observed, which could be due to an intermolecular charge transfer transition between 1 and 2.41−43 This indicates the π stacking of 1 and 2 in a mixed valence state.16,17 The absorbance of the PMMA single film coated on the glass substrate was 0.05 or less for 350−2000 nm. The absorption spectrum of the above-mentioned nanowiredispersed film was obtained by subtracting the absorbance of the PMMA single film. A powder X-ray diffraction (XRD) pattern of the nanowires (Figure 6) provided the characteristic peaks at 2θ = 6.5°, 7.6°, 7.8°, 10.4°, 13.6°, 14.3°, 15.1°, 16.0°, 16.6°, 18.9°, 20.1°, 25.8°,

Figure 3. UV−vis absorption spectrum of the nanowire sample in acetonitrile.

exhibited absorption peaks at 822, 389, and 294 nm, which respectively correspond to the electron transitions from the bonding state of a polaron (cation radical) to the antibonding state, from the bonding state of the polaron to the π conduction band, and from the valence band to the conduction band.32,33 In the poly(3,4-ethylenedioxythiophene)/poly(4-styrenesulfonate) (PEDOT/PSS) conducting polymer systems, it was found that dimethyl sulfoxide (DMSO) induced the strong screening effect between counterions and charge carriers, which reduced the Coulomb interaction between positively charged PEDOT and negatively charged PSS.34,35 From the analogy to this finding, the radical species in 2 may be also screened by DMSO, which may be the possible reason why the NMR spectrum in Figure 2 exhibited sharp signals despite the involvement of radical species in the test solution. Figure 4 shows the FT-IR transmission spectra of the nanowire sample (KBr method). The signals at 798 and 836 16432

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the nanowires with the highest aspect ratio were formed at the mixing ratio of 3:1 (Figure S5 and Table S1). The shape of nanowires was also affected by the molar ratio of iron(III) perchlorate to MeCz (Figure S6 and Table S2). Compound 1 is formed by the following two processes: oxidation of MeCz by Fe(ClO4)3 (eq 1) and dimerization of the produced MeCz radical (eq 2). Compound 1 is further partially oxidized to produce 2·ClO4− (eq 3). The stoichiometry molar ratio of Fe(ClO4)3/MeCz is 1.4 in view of the result of the elemental analysis and eqs 1−3. When the molar ratio is below and above 1.4, the nanowires with larger diameters were obtained. The maximum aspect ratio of the nanowire was achieved under the stoichiometry reaction ratio of 1.4. MeCz + Fe3 + → MeCz• + Fe 2 + + H+

(1)

2MeCz• → 1

(2)

1 + Fe(ClO4 )3 → 2ClO4 − + Fe2 + + 2· ClO4 −

(3)

The morphology of the nanowire was also dependent on the reaction temperature (Figure S7 and Table S3). Nanowires with a similar diameter and length were obtained at relatively low temperatures (0 and 22 °C). However, their diameter increased and the aspect ratio decreased at elevated temperatures (40 and 60 °C). The redox properties of the nanowire were investigated by cyclic volytammetry. Part a in Figure 7 shows the cyclic voltammogram of a deposit cast from a nanowire dispersion which was prepared by adding the nanowire (5 mg) to 1 mL of

Figure 5. (a) UV−vis−NIR spectrum of the nanowire sample dispersed in a PMMA matrix film. (b) Optical micrograph of the film. (c) SEM image of the cross-sectional view of the film. Scale bars in the images show 10 μm.

Figure 6. XRD pattern of the nanowire sample.

and 27.3°, which might originate from the crystal structure formed by 1 and 2·ClO4− in the nanowire. The shape of nanowire was dependent on the solvent used in the synthesis (Figure S4). The conditions to obtain the nanowire shape were to use acetonitrile (dielectric constant ε = 38) and methanol (ε = 33) as the solvent. When ethanol (ε = 24) was used, the product was an amorphous agglomerate, and when propylene carbonate (ε = 64) and dimethyl sulfoxide (ε = 47) were used, the products dissolved. This indicates that the dissolution and association characteristics of MeCz were affected by the polarity of the solvent and that solvents with an ε value of about 30−40 were preferable for the formation of the nanowires. When acetonitorile was used, though wires with small diameters and high aspect ratios were obtained, the surface properties of the wires were comparatively rough due to some dissolution of the wires. On the other hand, when methanol was used, the wires did not dissolve, and wires with larger diameters and smaller aspect ratios were obtained. A mixture of methanol and acetonitrile was then used as the solvent for synthesizing the wires. When the wires were synthesized in the mixed solvent of methanol and acetonitrile by changing the volume ratio from 4:1 to 1:3, it was found that

Figure 7. (a) Cyclic voltammogram of the nanowire sample cast on an indium−tin oxide coated glass plate in an acetonitrile solution containing 0.1 M tetrabutylammonium perchlorate. (b) Cyclic voltammogram of the nanowire sample dissolved in acetonitrile. Sweep rate: 50 mV/s. 16433

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Notes

CH3OH. The voltammogram exhibited two oxidation waves at 0.85 V (peak potential) and 1.11 V (shoulder). Considering the previous reports on the electrochemical measurements of various 3,6- and 9-substituted carbazoles,43−45 these waves should be assignable to the oxidation of 1 to 2 (radical cation of 1) and the oxidation of 2 to the dication of 1 in the nanowire structure, respectively. Controlled-potential electro-oxidation of the nanowire sample was carried out at 1.00 and 1.17 V, in which the electrolysis time or the amount of electricity was varied (Figure S8). The results demonstrated that a part of the nanowires was transformed into sheet structures for a prolonged electrolysis time or a large amount of electricity injected. This transformation may be caused by the conversion of 1 to 2 by the electro-oxidation and the subsequent additional doping of ClO4− in the nanowires. The cyclic voltammogram of the solution, prepared by adding 93 mg of nanowires to a stirred acetonitrile (20 mL), filtering with a syringe filter (Whatman, 0.2 μm pore size), and dissolving 0.1 M TBAP, showed a reversible oxidation wave at 0.86 V (peak potential) and a quasi-reversible wave at 1.16 V (peak potential) which can be assigned to the formation of the molecular radical cation and dication of 1, respectively (part b in Figure 7). The electric conductivity of a single nanowire was measured by the method described in the Supporting Information (Figure S9). The nanowire exhibited a conductivity of 3.0 × 10−5 S cm−1. This value is consistent with that of a molded pellet of the nanowires measured by the four-point-probe method, i.e., 2.8 × 10−5 S cm−1. The former and the latter values involve the contact resistance at the nanowire/electrode interfaces and the contact resistances between the nanowires, respectively, and therefore, the conductivity value might be the lowest limit.

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors thank Shimadzu Corp. for the measurement of the UV−vis−NIR spectrum.

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CONCLUDING REMARKS The chemical oxidation of 9-methylcarbazole in a methanol/ acetonitrile mixture produced highly regular rectangular straight nanowires without using a template at atmospheric pressure and temperature. Their average diameter and length were 0.40 and 17 μm, respectively. Chemical analyses revealed that the wires had a diamond-shaped cross section and consisted of the mixed-valence 9,9′-dimethyl-3,3′-bicarbazyl. The bicarbazyl is electrochemically active and electrically conducting. We are trying to further increase the electric conductivity of the nanowires by redoping in a liquid phase or gas phase. In addition, it is being found that interestingly shaped nanostructures are formed using different carbazole derivatives as the starting materials. As described in the Introduction, since the carbazole skeleton has various intriguing electric and electrochemical functions, the nanowires obtained in this study and the other carbazole-based nanostructures are expected to be used as an important module of microelectronics and optoelectronic devices.19−30

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ASSOCIATED CONTENT

S Supporting Information *

Characterization data and synthesis conditions of 1 and 2. This material is available free of charge via the Internet at http:// pubs.acs.org.

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REFERENCES

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. 16434

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dx.doi.org/10.1021/la303523a | Langmuir 2012, 28, 16430−16435