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Inclusion Complex Formation of Cyclodextrin and Polyaniline Ken-ichi Yoshida, Takeshi Shimomura, Kohzo Ito,* and Reinosuke Hayakawa Department of Applied Physics, Graduate School of Engineering, University of Tokyo, Hongo, Bunkyo-ku, Tokyo 113-8656, Japan Received September 14, 1998. In Final Form: December 7, 1998 Inclusion complex formation between polyaniline with emeraldine base and β-cyclodextrin has been studied by the frequency-domain electric birefringence (FEB) spectroscopy in a solution of N-methyl-2pyrrolidone (NMP) and by scanning tunneling microscopy (STM). The FEB results show that polyaniline in the solution with cyclodextrin changes its conformation from coil to rod at low temperature (below 275 K), and some rodlike images are observed on a substrate by STM. These results suggest that cyclodextrins are threaded onto polyaniline and confine the conformation of the polymer chain to a rodlike one. Furthermore, it is found that the threaded cyclodextrins prevent the chemical oxidation, i.e., doping of polyaniline by iodine. This indicates formation of a new inclusion complex, a conjugated conducting polymer covered by insulated cyclic molecules, namely, “insulated molecular wire”.
Introduction Much attention has been recently focused on the design of nanometer-scale (nanoscale) molecular devices. One approach to the molecular devices is the self-assembly of supramolecular structures such as inclusion complexes.1,2 Cyclodextrins (CDs) are cyclic molecules which consist of six to eight glucose units: R-, β-, and γ-cyclodextrins with six, seven, and eight glucose units, respectively (Figure 1a). Their cylindrical structures with cavities of about 0.7 nm deep and 0.5-0.8 nm inside diameter yield various unique properties. In particular, CDs form inclusion complexes with various low molecular weight compounds by including them into the cavities.1 Recently, it was reported that many CDs were threaded onto a polymer chain and formed an inclusion complex named a “molecular necklace”.1,3 Such a structure confines the conformation of the polymer chain to a rodlike one (all trans configuration) owing to very small cavities and close packing of the CD molecules. Hence the inclusion complex formation between CDs and a polymer chain is entropically unfavorable and encouraged at low temperature by noncovalent interaction such as hydrophobic one, similarly to the inclusion complex formation between linear polymer chains and molecular nanotubes synthesized through the molecular necklace.4 In this paper, our aim is to study inclusion complex formation between these cyclic molecules and conjugated conducting polymers by frequency-domain electric birefringence (FEB) spectroscopy and by scanning tunneling microscopy (STM). Since the FEB signal, or the Kerr effect, comes from optical and electrical anisotropy of molecules, rodlike molecules such as liquid crystals, tobacco mosaic virus, polypeptides, and linear polyions yield large electric birefringence but isotropic molecules such as coiled polymer chains and spherical latices exhibit no signal.5 Thus the FEB technique is a useful tool to determine
Figure 1. Schematic diagrams of (a) cyclodextrins, (b) polyaniline with emeraldine base, and (c) inclusion complex formation of cyclodextrins and a conducting polymer chain: insulated molecular wire.
* To whom correspondence should be addressed. (1) Wenz, G. Angew. Chem., Int. Ed. Engl. 1994, 33, 803. (2) Philp, D.; Stoddart, J. F. Angrew. Chem., Int. Ed. Engl. 1996, 35, 1155. (3) Harada, A.; Li, J.; Kamachi, M. Nature 1992, 356, 325; Macromolecules 1994, 27, 4538. (4) Okumura, Y.; Ito, K.; Hayakawa, R. Phys. Rev. Lett. 1998, 80, 5003. (5) O’Konski, C. T. Molecular Electrooptics: Part 1; Marcel Dekker: New York, 1976.
whether the conformation of a polymer chain is rodlike or coiled in solution. In practice, we have investigated the rod-coil transition of a conjugated conducting polymer in solution by FEB.6 As schematically shown in Figure 1c, (6) Shimomura, T.; Sato, H.; Furusawa, H.; Kimura, Y.; Okumoto, H.; Ito, K.; Hayakawa, R.; Hotta, S. Phys. Rev. Lett. 1994, 72, 2073.
10.1021/la9812471 CCC: $18.00 © 1999 American Chemical Society Published on Web 01/20/1999
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it is expected that the inclusion complex formation between CDs and a conducting polymer changes the polymer conformation from coiled to rodlike in solution. Then we can detect the appearance of rodlike polymer chains with high sensitivity by FEB. In this study, we adopt β-CD as a cyclic molecule and polyaniline (PAn) (Figure 1b) as a soluble conducting polymer since it is known that aniline, the monomer of PAn, forms inclusion compounds with β-CD, not R-CD.7,8 The inclusion complex between β-CD and PAn can be regarded as a novel kind of molecular devices: a conjugated conducting polymer covered by insulated cyclic molecules. Very recently, such insulated molecular wire has been synthesized by coupling several threaded conjugated monomer units.9 The present insulated molecular wire is about thousand times as long as the previous one. Experimental Section We purchased β-CD, N-methly-2-pyrrolidone (NMP) as a solvent and iodine as a dopant from Nacalai Tesque and used them without further purification. Polyaniline (PAn) with emeraldine base, synthesized by chemical oxidative polymerization of aniline, was supplied by Nitto Denko Co. A weight-average molecular weight Mw and the polydispersity ratio Mw/Mn of PAn were evaluated as 6.2 × 104 and 8.5, respectively, by gel permeation chromatography (GPC). In the FEB method, we apply the sinusoidal electric field E ) Re[E0 exp(iωt)] with the angular frequency of ω and the amplitude of E0 to the specimen and detect the birefringence signal ∆n of the solution. If the applied field orients molecules in the solution and the molecules have optical anisotropy, the solution exhibits birefringence, which is called the Kerr effect as a second-order nonlinear optical effect. By FEB, we obtain the frequency-dependent Kerr constant K ≡ ∆n/E02 ) Kdc+ Re[K2ω* exp(i2ωt)] where Kdc corresponds to the dc component and K2ω* ()K2ω′ iK2ω′′) is the complex amplitude of the 2ω component. According to the theoretical treatment,10 Kdc (≡Re[ψ*]) affords the information on the anisotropy and dynamics of the electrical polarizability of the polymer chain while K2ω* is given by K2ω* ) ψ* [1 + i(2/3)ωτr]-1 with the rotational relaxation time τr. It is to be noted that the FEB response appears only in solutions of rodlike molecules which have electro-optical anisotropy. We measured the FEB spectra in the temperature range of 250-300 K. The outline of the apparatus was reported in the previous paper.10 STM was performed with NanoscopeII (Digital Instruments) in air using Pt-Ir tips in a constant height mode. The sample was prepared by spin-coating a drop of the solution cooled at 255 K onto fleshly cleaved highly oriented pyrolytic graphite (HOPG). The STM observation was performed with no thermal drift corrections at room temperature. Results and Discussion When we mixed a small amount of NMP solution of PAn (0.05 wt %) into an aqueous solution of β-CD (1.8 wt %) with the ratio of 1:24 and cooled the mixture with a (7) French, D.; Levine, M.; Pazur, J.; Norberg, E. J. Am. Chem. Soc. 1949, 71, 353. (8) Lewis, E. A.; Hansen, L. D. J. Chem. Soc., Perkin Trans. 2 1973, 2081. (9) Anderson, S.; Aplin, R. T.; Claridge, T. D. W.; Goodson, T.; Maciel, A. C.; Rumbles, G.; Ryan, J. F.; Anderson, H. L. J. Chem. Soc., Perkin Trans. 1 1998, 2383. (10) Ookubo, N.; Hirai, Y.; Ito, K.; Hayakawa, R. Macromolecules 1989, 22, 1359.
Figure 2. Typical experimental results of the FEB spectra in the mixture solution of β-CD and PAn in NMP (a) at 255 K and (b) at 300K.
refrigerator, blue precipitation appeared in the solution. Such precipitation was not observed in an aqueous solution of β-CD nor NMP solution of PAn with the same concentration. Next we mixed R-CD and PAn similarly and cooled the mixture solution. In this case, no precipitation was observed in the solution. The appearance of precipitation is considered as evidence of inclusion complex formation between cyclic molecules and a polymer chain.11 Consequently, the present experimental results suggest the inclusion complex formation between β-CD and polyaniline. Figure 2 shows typical experimental results of the FEB spectra in the mixture solution of β-CD (4.4 × 10-3 M) and PAn (2 × 10-3 wt %) in NMP (a) at 255 K and (b) at 300 K. In the solution, the number of β-CD molecules is 20 times more than that of monomer units of PAn. No FEB signal is detected in the mixed solution at room temperature as shown in Figure 2b. As temperature T decreases, the FEB signal appears at 275 K and enlarges. In contrast, NMP solutions of PAn alone show no FEB response even at low temperatures down to 250 K. As mentioned before, the FEB technique was applied to another soluble conducting polymer, poly(3-hexylthiophene), in solution in order to investigate the relation between the main-chain conformation and the intramolecular conduction mechanism.6 When the polymer conformation changed from rod to coil by the rod-coil transition, the FEB signal becomes undetectable in the coiled state. Thus, the present experimental results indicate that PAn in NMP has coiled conformation at least in the temperature range of 250-300 K, while PAn together with β-CD in NMP changes into rodlike conformation at low temperature below 275 K. This also suggests the inclusion complex formation between β-CD and PAn such as the molecular necklace. From the rotational relaxation frequency (fr ) 3/(4πτr)), at which K2ω′′ has a maximum, we can estimate the effective length (Leff) of a rodlike molecule as12
fr )
9kBT
(ln(Leff/dr) + γr) 2π η0 Leff3 2
(11) Harada, A.; Kamachi, M. Macromolecules 1990, 23, 2823. (12) Nakajima, H.; Wada, Y. Biopolymers 1978, 17, 2291.
(1)
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Figure 3. The temperature dependence of the effective length Leff calculated from the rotational relaxation time τr with eq 1. At temperature above 275 K, the FEB signal was not detected.
where η0 is the solvent viscosity, dr the diameter of the rodlike molecule, γr a correction term for the end effect, and kBT the thermal energy. The temperature dependence of η0 was obtained from other experimental data.13-16 The temperature dependence of Leff is shown in Figure 3, where no FEB signal was observed at a temperature higher than 275 K. At a temperature below 275 K, Leff is almost independent of temperature and is estimated to be about 1.5 µm. The tendency of Leff dropping off sharply with increasing temperature is different from the rod-coil transition of poly(3-hexylthiophene),17 where Leff changes gradually from rod to coil. This would reflect the cooperative effect of the inclusion complex formation. The value 1.5 µm of Leff is larger than the length 0.3 µm of PAn evaluated from Mw. The rotational relaxation time in FEB always gives us larger length than GPC because of the polydispersity and the entanglement effect.6 Next, we observed the shape of the inclusion complex between β-CD and PAn by STM, as shown in Figure 4a and Figure 4b. The tip bias voltage was -500 mV and the setting current was 0.30 nA. When a drop of the mixture solution cooled at 255 K was spin-coated onto a substrate of HOPG, almost all of the β-CD molecules and large amount of the inclusion complexes were swept away from the substrate since the surface of HOPG repels NMP solution. But some complexes were caught by roughness on the surface due to exfoliation of graphite layers as shown in Figure 4a. Figure 4b exhibits the magnification of a part of Figure 4a, where a rodlike structure with the length of about 330 nm is seen clearly. This length agrees with the contour length of PAn evaluated from Mw. As shown in Figure 4c, the vertical height of this structure is about 2 nm, which is equal to the outside diameter of β-CD. Incidentally, these rodlike structures were not observed on the substrate where the solution of β-CD or PAn was spin-coated. As further evidence of the inclusion complex formation between β-CD and PAn, we investigated the chemical oxidation or doping effect of PAn by iodine. It is known that iodine oxidizes PAn with emeraldine base18 and changes the color of PAn solution from blue to violet. If PAn is completely covered by β-CD, it is expected that PAn should not be oxidized by iodine. We added 55 µL of a NMP solution of iodine (8 × 10-4 N) to 2 mL of the (13) Ambrosone, L.; D’Errico, G.; Sartorio, R.; Vitagliano, V. J. Chem. Soc., Faraday Trans. 1995, 91, 1339. (14) Chen, G.; Hou, Y.; Knapp, H. J. Chem. Eng. Data 1995, 40, 1005. (15) Garcia, B.; Alcalde, R.; Leal, J. M.; Matos, J. S. J. Phys. Chem. B 1997, 101, 7991. (16) Guarino, G.; Ortona, O.; Sartorio, R.; Vitagliano, V. J. Chem. Eng. Data 1985, 30, 366. (17) Hotta, S., Ito, K. Electronic Properties of Polythiophenes, in Handbook of Oligo- and Polythiophenes; Fichou, D., Eds; VCH: in press. (18) Cao, Y. Synth. Met. 1990, 35, 319.
Figure 4. STM images where fields of view are (a) 1200 nm × 1200 nm and (b) 350 nm × 350 nm. (c) The height profile of the cross section.
mixture solution of β-CD (4.4 × 10-3 M) and PAn (2 × 10-3 wt %) cooled at 255 K and kept the mixture at 273 K. Similarly, we prepared the mixture of PAn cooled at 255 K and iodine without β-CD and kept it at 273 K. It was found that the latter mixture changed the color from blue to violet in a few days although the former still remained blue. At higher temperature, the former mixture including β-CD also changed the color from blue to violet but a considerable delay of the change was observed in comparison to the latter mixture without β-CD. The detailed results will be reported in a forthcoming paper.19 Con(19) Yoshida, K.; Shimomura, T.; Ito, K.; Hayakawa, R., in preparation.
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sequently, it was confirmed that PAn was completely covered by insulated CD molecules at low temperature. This indicates that PAn forms the inclusion complex with β-CD in NMP solution at low temperature as shown in Figure 1c. In conclusion, we observed the inclusion complex formation between β-CD and PAn by FEB, STM, and the doping effect. This inclusion complex, i.e., the insulated molecular wire, is expected to have some unique features (i) the conformation of PAn is confined to a rodlike
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structure (all trans state) and hence the conjugated structure can spread over the whole length of a conducting polymer; (ii) PAn molecules are isolated from each other by insulated CD molecules. Acknowledgment. We thank Professor Toshio Nishi for his support in the STM observation and Nitto Denko Co. for supplying PAn. LA9812471
