Langmuir 2007, 23, 10441-10444
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Mixing Aqueous Ferric Chloride and O-Phenylenediamine Solutions at Room Temperature: A Fast, Economical Route to Ultralong Microfibrils of Assemblied O-Phenylenediamine Dimers Xuping Sun*,† and Matthias Hagner‡ Fachbereich Chemie, and Fachbereich Physic, UniVersita¨t Konstanz, UniVersita¨tsstrasse 10, D-78457 Konstanz, Germany ReceiVed May 12, 2007. In Final Form: August 20, 2007 The direct mix of aqueous ferric chloride and o-phenylenediamine (OPD) solutions at room temperature has been demonstrated for the first time to be an effective, economic, and fast method for preparing microfibrils on a large scale. The formation of such large microfibrils is attributed to the self-assembly of the OPD dimers generated by the oxidation of OPD monomers by ferric chloride. It is also interesting that the resulting microfibrils can be broken into shorter ones by a simple sonication process and the final length of the microfibrils obtained can be controlled by varying the sonication time. The influences of both the amount of ferric chloride and the oxidant type on the size and the morphology of the microstructures are also examined.
1. Introduction During the past years, electrically conducting and intrinsically colored oligomers or polymers have been paid considerable attention due to their unique properties such as good environmental stability, moderately high conductivity upon doping suitable ions, and higher gas separation efficiencies,1 and thus widely used in diverse applications.2 Polyaniline (PANI) is one of the most studied conducting polymers (CPs) by far due to its chemical stability and relatively high conductivity.3 Polymers based on aniline derivatives have also been extensively studied,4 and the research on polymers of aromatic diamines has recently received increasing attention mainly due to their apparently different characteristics compared with those widely researched conducting polymers.1 Among them, the oligomers or polymers based on o-phenylenediamine (OPD), that is, POPD, have been used as catalysts and sensors and for the creation of electrochromic films.1,5 POPD is usually prepared by electrochemical polymerization,6 but chemical polymerization is also an effective preparative route.7 Since the discovery of carbon nanotubes,8 the synthesis and characterization of one-dimensional (1D) structures has attracted considerable attention. Because CPs have long conjugated length and good conductive property, they hold promise as advanced materials in this field.9 Although the methods on the preparation of 1D structures of CPs have been largely explored and reported, only a little attention has been paid to preparing 1D structures of POPD. For example, Curulli et al. have prepared POPD * E-mail:
[email protected]. † Fachbereich Chemie. ‡ Fachbereich Physic. (1) Li, X.; Huang, M.; Duan, W.; Yang, Y. Chem. ReV. 2002, 102, 2925. (2) Curtis, C. L. AdV. Mater. 1994, 6, 688. (3) Pei, Q.; Yu, G.; Zhang, C.; Yang, Y.; Heeger, A. G. Science 1995, 269, 1086. (4) Sulimenko, T.; Stejskal, J.; Prokesˇ, J. J. Colloid Interface Sci. 2001, 236, 328. (5) (a) Malitesta, C.; Palmisano, F.; Torsi, L.; Zambonin, P. G. Anal. Chem. 1990, 62, 2735. (b) Xu, J. J.; Chen, H. Y. Anal. Biochem. 2000, 280, 221. (c) Losito, L.; Palmisano, F.; Zambonin, P. Anal. Chem. 2003, 75, 4988. (6) (a) Ohnuki, Y.; Matsuda, H.; Ohsaka, T.; Oyama, N. J. Eletroanal. Chem. 1987, 219, 117. (b) Dai, H.; Wu, Q.; Sun, S.; Shiu, K. J. Electroanal. Chem. 1998, 456, 47. (7) Ogura, K.; Shiigi, H.; Nakayama, M.; Fujii, A. J. Electrochem. Soc. 1998, 145, 3351. (8) Lijiman, S. Nature (London) 1991, 354, 56. (9) Martin, C. R. Science 1994, 266, 1961.
nanotubes by using polycarbonate nanoporous particle tracketched membranes as templates.10 We have recently developed a quite simple surfactantless, templateless strategy for fabricating POPD nanobelts by directly mixing HAuCl4 and OPD aqueous solutions at room temperature and speculated that the spontaneous formation of the nanobelts was attributed to oriented polymerization of OPD catalyzed by gold nanoparticles formed in this synthesis.11 We have further demonstrated that mixing aqueous AgNO3 and OPD solutions produces hexamers of OPD oligomers, which can then gradually self-assemble into 1D microstructures.12 However, such a nanoparticle-involved strategy suffers from the following disadvantages: (1) The noble metal salt used as an oxidant is expensive. (2) Several hours are required for the formation of the 1D structures. (3) The 1D structures thus formed are not pure due to the coexistence of some Au or Ag particle byproducts, and therefore a post-separation process should be involved to obtain pure 1D structures. In this letter, an economic, fast route to preparing pure microfibrils of OPD dimers on a large scale is demonstrated, carried out by mixing aqueous ferric chloride and OPD solutions at room temperature. The chemical structure of the dimer was examined by NMR spectra. The formation of the microfibrils is attributed to the self-assembly of the OPD dimers formed. More importantly, the microfibrils can be broken into shorter ones by a simple sonication process, and the final length of the microfibrils obtained can be controlled by varying the sonication time. Both the size and the morphology of the microstructures can be heavily influenced by the amount of ferric chloride as well as the oxidant type used. 2. Experimental Section Ferric chloride, OPD, and ammonium persulfate were purchased from Merck and used as received. Sample 1 was prepared as follows: Briefly, 0.09 g OPD was dissolved in 20 mL of water first, and then 2.34 mL of 0.71 M ferric chloride aqueous solution was added rapidly into the solution at room temperature under vigorous stirring. A brown-red turbid solution was obtained within 15 s, and the stirring was continued for another 5 min. After that, the solution (10) Curulli, A.; Valentini, F.; Orlanducci, S.; Terranova, M. L.; Paoletti, C.; Palleschi, G. Sens. Actuators, B 2004, 100, 65. (11) Sun, X.; Dong, S.; Wang, E. Chem. Commun. 2004, 1182. (12) Sun, X.; Dong, S.; Wang, E. Macromol. Rapid Commun. 2005, 26, 1504.
10.1021/la701378y CCC: $37.00 © 2007 American Chemical Society Published on Web 09/14/2007
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Figure 2. A series of optical microscopy images of the microfibrils obtained with different elapsed times of 15 s, 30 s, 2 min, and 30 min after the introduction of the ferric chloride to the OPD aqueous solution. Figure 1. (A) An optical microscopy image of the microfibrils. The inset is an optical photograph of the dispersion. (B) A lowmagnification SEM image of the microfibrils. The inset is a highmagnification SEM image of the microfibrils. was placed at room temperature, and a large quantity of precipitate occurred gradually. The precipitate thus formed was washed with water several times for further use. Some of the precipitate was dispersed in water for microscope observation. For NMR measurement, some of the precipitate was dried in vacuo overnight at 50 °C. Scanning electron microscopy (SEM) measurements were made on a Zeiss DSM 940 microscope operated at an accelerating voltage of 10 kV. Samples for SEM examination were made by placing a drop of the dispersion on a glass slide and air-drying at room temperature. Optical microscopy images were taken with a LEICA DM4000 M microscope, and samples for this characterization were also prepared by placing a drop of the dispersion on a glass slide and air-drying at room temperature. The UV-vis spectrum was collected on a Perkin-Elmer Lambda 18 spectrophotometer. The NMR spectra of sample 1 dissolved in [D6]DMSO were acquired with a VARIAN AS400 spectrometer. The mass spectrometer of sample 1 dissolved in methanol was acquired on a Bruker APEX.
3. Results and Discussion Figure 1A shows an optical microscopy image of as-prepared products. It is clearly seen that the products consist of a large quantity of microfibrils with a length of more than 100 µm (the longest length of the microfibrils could reach 130 µm). The inset is an optical photograph of the dispersion, revealing the brownred nature of the microfibrils. The morphology of the products was also examined by SEM. Figure 1B shows a highmagnification SEM image, which also indicates the formation of microfibrils. The inset is a high-magnification SEM image of the microfibrils, which further reveals that the microfibril is less than 1 µm in width and several hundred nanometers in height (the exact data cannot be measured here). It should be noted that the formation of the microfibrils is very fast. Figure 2 shows a series of optical microscopy images of the fibrils obtained with different elapsed times after the introduction of ferric chloride to OPD aqueous solution. It is found during the synthesis that a brown-red dispersion was formed within 15 s, and after that, no further color change was observed. These images clearly show that an elapsed time of 15 s, 30 s, 2 min, or 30 min gives one-dimensional structures with the same length
Figure 3. SEM images of the microfibrils obtained after sonication of the dispersion of sample 1 for different times: 0 min, 1 min, 4 min, and 20 min. Insets in (A) and (D) are a closer view of the corresponding microfibrils.
as those observed in the final products shown in Figure 1. The densities of the microfibrils obtained with elapsed times of 15 s, 30 s, 2 min, and 30 min after the mix of ferric chloride and OPD aqueous solution are also found by counting the number of the microfibrils within a 500 µm × 500 µm area of each optical image is about 250 microfibrils per 100 cm2, which is nearly the same as that of the final products. These observations suggest that only 15 s is involved in the complete formation of the microfibrils in this synthesis and the generation of the microfibrils is a quite fast process. We have measured the dc conductivity of the pure microfibrils at room temperature using a standard four-probe method and found that the electronic conductivity of product is on the order of 10-8 S/cm, which is lower than that of the product formed by the oxidation of OPD by AgNO3,12 which may be attributed to the fact that the Ag particles cannot be completely removed from the product in that system and, therefore, a better conductivity is expected.
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Figure 4. SEM images of the microstructures obtained with (A) 6:1 molar ratio of ferric chloride to OPD, (B) 1:2 molar ratio of ferric chloride to OPD, and (C) ammonium persulfate as the oxidant, under otherwise identical conditions used for preparing sample 1.
It is quite interesting found that the microfibrils can be broken into shorter ones by the sonication process and the final length of the microfibrils can be controlled by varying the sonication time. Figure 3 shows a series of SEM images of the microfibrils before (Figure 3A) and after sonication for different time under a power intensity of 40 W. When the sample was sonicated for 1 min, the length of the fibrils was reduced from more than 100 µm to about 30 µm (Figure 3B). When the sample was sonicated for 4 min, fibrils with a length below 20 µm (most of them are 15 µm in length) were obtained, as shown in Figure 3C. When 10 min was used for sonicating the sample, much shorter fibrils were obtained (most of them are 5 µm in length) as shown in Figure 3D. Please note that the sonication process has little effect on the width as well as the height of the fibrils, as evidenced by a closer view of the fibrils before (inset in A) and after sonication for 20 min (inset in D). It is found that further sonication can further cut the fibrils into shorter structures. To examine the influence of the molar ratio of the reactant on the formation of the microstructures, another two samples with varied molar ratio of ferric chloride to OPD were prepared, under otherwise identical conditions used for preparing sample 1. When the molar ratio is increased up to 6:1, microfibrils with increased width (>3 µm) and decreased length (2 µm) were obtained, as shown in Figure 4B. The influence of oxidant type on the formation of the microstructures was also examined. Because both ferric chloride and ammonium persulfate are usually used as oxidants for the preparation of many conducting polymers, a control experiment using ammonium persulfate as an oxidant was performed. A clear, deep red solution was formed when ammonium persulfate was introduced into OPD aqueous solution. However, it takes more than 1 day to obtain a small quantity of precipitate. Figure 4C shows the SEM image of the precipitate, indicating that the products consist of microstructures with a length of about 100 µm and a width of more than 30 µm. These observations demonstrate that both the size and the morphology of the microstructures can be heavily influenced by the amount of ferric chloride as well as the oxidant type used. It has been demonstrated that the 1D structures resulting from the oxidation of OPD by AgNO3 are assemblies of hexamers of OPD oligomers formed.12 There are also many papers reporting on the dimerization of OPD to 2,3-diaminophenazine.13 We found that the microfibrils can be easily dissolved in tetrahydrofuran to form a clear, yellow solution whose UV-vis spectrum shows a maximum absorption peak at about 435 nm, which can be assigned to the π-π* transition associated with the phenazine unit conjugated to lone pairs of bridging nitrogens.1 To obtain the mass of the oxidized products of OPD by ferric chloride in our present study, the ESI-FTICR MS technique was used. A strong ion peak at 235 ([M + Na]+) is observed, indicating that
an oligomer in the form of a dimer is generated in this synthesis. Also, note that a rather weak ion peak at 337 ([M + Na]+) is observed. The observations reveal that a dimer is the major oxidation product under applied reaction conditions. To determine the chemical structure of the dimer, the corresponding 1H and 13C NMR spectra of the microfibrils dissolved in [D6]DMSO were acquired. Figure 5A shows the 1H and 13C NMR spectra obtained. The occurrence of one singlet around 3.4 ppm with a broad signal can be due to the substantial amount of water, because DMSO is very hygroscopic and it is very difficult to obtain a spectrum in this solvent without water.14 One singlet around 2.4 ppm can be attributed to the DMSO solvent used.15 One singlet around 7.0 ppm can be attributed to two aromatic hydrogens which are not coupled, and two sets of multiplets around 7.7 and 8.0 ppm can also be assigned to aromatic hydrogens. These chemical shifts are quite consistent with the reported values of 2,3-diaminophenazine in the literature.16 We failed to observe one singlet corresponding to four amine hydrogens around 6.25. However, one small singlet around 3.2 ppm was observed, which may be assigned to the four amine hydrogens and, on the other hand, could result from some minor impurity. Figure 5B shows the corresponding 13C NMR spectrum. The spectrum shows six peaks with the following chemical shifts: 105, 126, 128, 142, 143, and 144 ppm, which are also consistent with the reported values corresponding to 2,3-diaminophenazine,16 providing further evidence to support the dimerization of OPD to 2,3diaminophenazine in our present study. Figure 5C shows the scheme describing the redox reaction for the formation of 2,3diaminophenazine. It is also worthwhile mentioning that a large amount of H+ are released during the oxidative dimerization and, therefore, the amine group of 2,3-diaminophenazine should be protonated. Elemental analysis of sample 1 after it was dried in vacuo overnight at 50 °C reveals that it consists of C, N, H, Fe, and Cl with atom number ratio 12.05:3.98:12.10:0.14:0.36, showing that a small amount of Fe and Cl elements are still contained in the sample, which can be attributed to the fact that washing the sample with water cannot completely remove the Fe salt from the product. The formation of microfibrils can be attributed to the selfassembly of the OPD dimers formed due to the oxidation of (13) See, for example: (a) Hempen, C.; van Leeuwen, S. M.; Luftmann, H.; Karst, U. Anal. Bioanal. Chem. 2005, 382, 234. (b) Niu, S. Y.; Zhang, S. S.; Ma, L. B.; Jiao, K. Bull. Korean Chem. Soc. 2004, 25, 829. (c) Kaizer, J.; Csonka, R.; Speier, D. React. Kinet. Catal. Lett. 2002, 75, 367. (d) Thomas, K. A.; Euler, W. B. J. Electroanal. Chem. 2001, 501, 235. (e) Premasiri, A. H.; Euler, W. B. Macromol. Chem. Phys. 1995, 196, 3655. (f) Tarcha, P. J.; Chu, V. P.; Whittern, D. Anal. Biochem. 1987, 165, 230. (g) Sluka, J.; Zikan, V.; Danek, J. Cesk. Farm. 1987, 36, 25. (h) Neneth, S.; Simandi, L. I. J. Mol. Catal. 1982, 14, 241. (i) Fischer, O.; Hepp, E. Ber. Dtsh. Chem. Ges. 1889, 22, 355. (j) Griess, P. J. Prakt. Chem. 1871, 3, 143. (14) Comments of one reviewer. (15) Gottlieb, H. E.; Kotlyar, V.; Nudelman, A. J. Org. Chem. 1997, 62, 7512. (16) Rosso, N. D.; Szpoganicz, B.; Martell, A. E. Inorg. Chim. Acta 1999, 287, 193.
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Figure 5. (A) 1H NMR and (B) 13C NMR spectra of the microfibrils of sample 1 dissolved in [D6]DMSO, and (C) a scheme describing the redox reaction for the formation of 2,3-diaminophenazine.
OPD by ferric chloride. It is well-known that π-π interactions often exist in π-conjugated materials.17 In water, the stacking interaction between aromatic molecules is mainly caused by the hydrophobic effect. Because water molecules solvating the aromatic surface have a higher energy than bulk water, the aromatic surfaces are stacked together to reduce the total surface exposed to the water. The crystal packing of 2,3-diaminophenazine is quite interesting, and Doyle et al. described the packing manners involved.18 Such a molecule is rich with numerous π-bonding and hydrogen-bond donor and acceptor sites and thus may be expected to act as a particularly versatile supramolecular tecton.18 The π-πstacks, hydrogen bonds, and T-bonded interactions among these dimers contribute to the formation of microfibrils. It should be noted that the mechanism for the formation of the large microfibrils is very complex, and the formation process also depends on the experimental conditions such as oxidative potential or pH,19 which is why quite different assemblies are obtained when another oxidant ammonium persulfate is used in this synthesis. (17) Hoeben, F. J. M.; Jonkheijm, P.; Meijer, E. W.; Schenning, A. P. H. J. Chem. ReV. 2005, 105, 1491. (18) Doyle, R. P.; Kruger, P. E.; Mackie, P. R.; Nieuwenhuyzen, M. Acta Crystallogr. 2001, C57, 104. (19) (a) Malinauskas, A.; Bron, M.; Holze, R. Synth. Met. 1999, 92, 127. (b) Losito, I.; Palmisano, F.; Zambonin, P. G. Anal. Chem. 2003, 75, 4988. (c) Camurri, G.; Ferrarini, P.; Giovanardi, R.; Benassi, R.; Fontanesi, C. J. Electroanal. Chem. 2005, 585, 181.
4. Conclusions In conclusion, the mixture of aqueous ferric chloride and OPD solutions at room temperature has been proven to be an effective method for the fast formation of a large quantity of microfibrils of assemblied OPD dimers formed due to the oxidation of OPD by ferric chloride. The present study is significant for the following reasons: (1) It presents a quite fast method for the preparation of microfibrils of supramolecular assemblies consisting of π-conjugated organic oligomers. (2) Because no expensive oxidant of noble metal salt such as HAuCl4 or AgNO3 is involved and no metal particles are generated in the synthesis, it provides us an economic route to obtain pure microfibrils of OPD oligomers. (3) Because the microfibrils can be not only easily soluble in many organic solvents, but also controllably cut into shorter lengths by sonication, they may hold promise for using as templates for the creation of other structures with controlled length for applications.20 Acknowledgment. X.S. thanks Prof. S. Mecking for being the research host and also appreciates the support from the Alexander von Humboldt Foundation. LA701378Y (20) Caruso, F.; Caruso, R. A.; Mo¨hwald, H. Science 1998, 282, 1111.
