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Preparation of Polymer Langmuir-Blodgett Films Containing Porphyrin Chromophore Fei Feng, Masaya Mitsuishi, and Tokuji Miyashita* Institute for Chemical Reaction Science, Tohoku University, Katahira 2-1-1, Aoba-ku, Sendai 980-8577, Japan
Ichiro Okura Department of Bioengineering, Tokyo Institute of Technology, Nagatsuta, Midori-ku, Yokohama 226-8501, Japan
Keisuke Asai and Yutaka Amao National Aerospace Laboratory, Jindaiji-Higashi, Chofu, Tokyo 182-0012, Japan Received March 10, 1999. In Final Form: August 12, 1999 The polymer Langmuir-Blodgett films containing a large porphyrin chromophore were prepared from a copolymer (pADA-Por) consisting of N-[(N′-aminopropyl)pyridyltriphenylporphyrinate (Zn(II))]acrylamide (Por) and N-adamantylacrylamide (ADA). The spreading behavior of the copolymer on the water surface was investigated by the measurement of surface pressure (π)-surface area (A) isotherm. The isotherm indicates that the pADA-Por copolymer forms a stable condensed monolayer with a steep rise in surface pressure and has a collapse pressure of 30 mN/m where a hydrogen bonding based on amide structure is working as a self-assembling factor. The monolayer can be transferred onto solid supports giving a typical Y-type LB film. A flat orientation of the porphyrin ring in the LB films was discussed from the absorption dichroism measurement.
Introduction The Langmuir-Blodgett (LB) method is one of the most useful techniques for the fabrication of functional ultrathin films with a controlled thickness and an ordered structure similar to biomembranes,1-5 and the technique has been widely applied to various functional devices.6 Thus far, LB films have been prepared from low molecular weight amphiphilic compounds with long hydrocarbon chains. These LB films, however, have poor stability with respect to mechanical and thermal treatment or poor resistance to dissolution by organic solvents. To overcome these drawbacks, polymer LB films have been investigated extensively.7-10 In a classical concept, amphiphilic molecules having a polar headgroup and a long hydrophobic tail can form a stable monolayer and LB multilayers. Recently, various LB films composed of molecules without long hydrophobic chains have been reported.11-13 In polymer LB films, we have reported that N-dodecylacryl(1) Langmuir, I. J. Am. Chem. Soc. 1917, 39, 1848. (2) Blodgett, K. J. Phys. Chem. 1935, 41, 975. (3) Blodgett, K. J. Am. Chem. Soc. 1935, 57, 1007. (4) Blodgett, K.; Langmuir, I. Phys. Rew. 1937, 51, 964. (5) Gains, G. L., Jr. Insoluble Monolayers at Liquid-Gas Interfaces; Wiley: New York, 1972. (6) Markowitz, M. A.; Janout, V.; Castner, D. G.; Regen, S. L. J. Am. Chem. Soc. 1989, 111, 8192. (7) Miyashita, T. Prog. Polym. Sci. 1993, 18, 263. (8) Ringsdorf, H.; Schmidt, G.; Schneider, J. Thin Solid Films 1987, 152, 207. (9) Laschewsky, A.; Ringsdorf, H.; Schmidt, G.; Schneider, J. J. Am. Chem. Soc. 1987, 109, 788. (10) Schneider, J.; Ringsdorf, H.; Rabolt, J. F. Macromolecules 1989, 22, 205. (11) (a) Whitten, D. G. J. Am. Chem. Soc. 1976, 98, 1584. (b) Vandevyer, M.; Barraud, A.; Ruaudel-Teixier, A.; Maillard, P.; Gianotti, C. J. Colloid Interface Sci. 1982, 85, 571. (c) Schick, G. A.; Schreiman, I. C.; Wagner, R. W.; Lindsey, J. S.; Bocian, D. F. J. Am. Chem. Soc. 1989, 111, 1344.
amide polymer (pDDA) forms a stable monolayer on the water surface and stable LB film.14-16 Moreover, the polymers without long alkyl pendant chains, a shortbranched N-alkylacrylamide polymer17 and an cyclic adamantylacrylamide polymer (pADA),18 were also found to form stable monolayer and LB films. It is well-known that porphyrins are highly conjugated organic molecules, and the properties are utilized for photodiodes,19 catalyst,20 artificial solar energy conversion system,21 and particularly for sensor devices.22 Furthermore, ultrathin films of porphyrins are expected to increase sensitivity and sensor response. A porphyrin LB film attached a long alkyl substituent is reported to work as a highly sensitive gas sensor.23 (12) (a) Baker, S.; Petty, M. C.; Roberts, G. G.; Twigg, M. V. Thin Solid Films 1983, 99, 53. (b) Ko, W. H.; Fu, C. W.; Wang, H. Y.; Batzel, D. A.; Kenney, M. E.; Lando, J. B. Sens. Mater. 1990, 2, 39. (13) (a) Markowitz, M. A.; Janout, V.; Castner, D. G.; Regen, S. L. J. Am. Chem. Soc. 1989, 111, 8192. (b) Ishikawa, Y.; Kunitake, T.; Matsuda, T.; Otsuka, T.; Shinkai, S. J. Chem. Soc., Chem. Commun. 1989, 763. (c) Moreira, W. C.; Dutton, P. J.; Aroca, R. Langmuir 1995, 11, 3137. (14) Miyashita, T.; Yoshida, H.; Itoh, H.; Matsuda, M. Polymer 1987, 28, 311. (15) Miyashita, T.; Yoshida, H.; Itoh, H.; Matsuda, M. Nippon Kagaku Kaishi 1987, 2169. (16) Miyashita, T.; Mizuta, Y.; Matsuda, M. Br. Polym. J. 1990, 22, 327. (17) Taniguchi, T.; Yokoyama, Y.; Miyashita, T. Macromolecules 1997, 30, 3646. (18) Feng, F.; Aoki, A.; Miyashita, T. Chem. Lett. 1998, 205. (19) Nishikata, Y.; Fukui, S.; Kakimoto, M.; Imai, Y.; Nishiyama, K.; Fujihira, M. Thin Solid Films 1992, 210, 296. (20) Abatti, D.; Zaniquelli, M. E. D.; Iamamoto, Y. Idemori, Y. M. Thin Solid Films 1997, 310, 296. (21) Lichtin, N. N. CHEMTECH 1980, 10, 254. (22) Porphyrin Gas Sensors. UK Patent Application No. 9802866.5. (23) George, C. D.; Richardson, T.; Hofton, M. E.; Vale, C. M.; Neves, M. G. M.; Cavaleiro, J. A. S. European Conference on Thin Organised Films; Potsdam, 1998.
10.1021/la990283c CCC: $18.00 © 1999 American Chemical Society Published on Web 09/30/1999
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Figure 1. A synthesis route for copolymers of pADA-SuOA and pADA-Por.
In general, it is very difficult to prepare polymers containing porphyrin. One possible way is to polymerize a monomer containing a porphyrin unit; however, the synthesis is too hard. We here overcame the point by utilizing the replacement reaction of ADA-co-SuOA polymer with asymmetric porphyrin. We have tried to introduce the porphyrin chromophore into polymer LB films without long alkyl chains in favor of keeping the film thickness of one layer to a minimum as 1 nm. The asymmetric zinc 5-(4-aminopropylpyridinium)-10,15,20triphenylporphyrin can be incorporated into a copolymer consisting of N-adamantylacrylamide (ADA) and Nacryloxysuccinimide (SuOA) by the replacement reaction with a succinimide group because the SuOA is an excellent leaving group.24 We determined the orientation of the porphyrin chromophore in the polymer LB film as a supplementary work by using polarized UV absorption spectra, and we would like to show how well the orientation of porphyrin chromophore is kept even in polymer LB film. Experimental Section Materials. The synthesis of adamantylacrylamide (ADA) and N-acryloxysuccinimide (SuOA) was reported previously.18,24 5-(4Pyridyl)-10,15,20-triphenylporphyrin (PyTP) was synthesized by the following procedure. Pyridine-4-aldehyde (10.0 g, 0.093 mol) and benzaldehyde (28.8 g, 0.27 mol) were added into 500 mL of boiling propionic acid, and then pyrrole (25.0 g, 0.373 mol) was added and refluxed at 165 °C for 1 h. Metallic purple precipitate was collected by suction filtration and washed with methanol and then dried under vacuum overnight. The crude product obtained was purified by the column chromatography (100-200 mesh of silica gel, eluted with chloroform). The desired product, PyTP, was eluted as a second fraction and dried by evaporation. Purple precipitate was collected, washed with water and methanol, and then dried under vacuum overnight to yield the desired product. The yield was 5.1% (3.0 g). The 1H NMR data were given as follows (CDCl3, ppm): δ -2.90 to -2.70 (m, 2H), 7.70-7.80 (m, 9H), 8.10-8.30 (m, 8H), 8.70-9.10 (m, 10H). Zinc 5-(4-aminopropylpyridinium)-10,15,20-triphenylporphyrin was prepared as follows. PyTP (1.0 g, 0.0016 mol) was quaternized with an excess amount of 1-bromopropylamine (5.0 g, 0.023 mol) in acetonitrile at 100 °C for 48 h to obtain 5-(4-aminopropylpyridinium)-10,15,20-triphenylporphyrin. The yield was 71% (0.95 g). The 1H NMR data were given as follows (DMSO-d6, ppm): δ -2.90 to -2.70 (m, 2H), 2.90 (q, 2H), 3.40 (m, 2H), 5.00 (m, 2H), (24) Arisumi, K.; Feng, F.; Miyashita, T. Langmuir 1998, 14, 5555.
5.10 (t, 2H), 7.80-7.90 (m, 9H), 8.20-8.40 (m, 6H), 8.80-9.20 (m, 16H), 9.60-9.70 (m, 2H). Further, this porphyrin compound (0.5 g, 0.0006 mol) and an excess amount of zinc acetate (5.0 g, 0.023 mol) were refluxed in methanol at 80 °C to obtain zinc 5-(4-aminopropylpyridinium)-10,15,20-triphenylporphyrin. The solvent was removed by vacuum pump. After being washed with water to remove unreacted zinc acetate, a purple precipitate was collected by suction filtration and dried under vacuum overnight to yield the desired product. The yield was 85.1% (0.45 g). The 1H NMR data were given as follows (DMSO-d , ppm): δ 2.89 (q, 6 2H), 3.41 (m, 2H), 5.00 (m, 2H), 5.12 (t, 2H), 7.80-7.90 (m, 9H), 8.20-8.40 (m, 6H), 8.80-9.20 (m, 16H), 9.60-9.70 (m, 2H). MS (TOF): m/z 892.3 [M + H+]. The pADA-SuOA copolymer was synthesized by free-radical copolymerization24 in toluene at 60 °C with 2,2′-azobis(isobutyronitrile) (AIBN) as a thermal initiator (Figure 1). The copolymer pADA-Por was produced by the replacement reaction of pADA-SuOA with the porphyrin compound, (N-aminopropyl)pyridyltriphenylporphyrinate (Zn(II)), in CHCl3 under reflux for 12 h. After the replacement reaction, adamantanamine was added to replace the unreacted SuOA unit. The copolymers were purified by reprecipitation into a large excess of acetonitrile and dried under vacuum at room temperature. The copolymer compositions were determined by 1H NMR24 spectra recorded on a JEOL 400 and UV measurement recorded on a Hitachi U-3000 UV-vis spectrophotometer. The ratio of porphyrin chromophore to the monomer unit was 1.5 mol %. The molecular weight of the copolymer pADA-Por was determined by gel permeation chromatography (GPC), which is 4.2 × 104, and the polydispersity is 2.1. Preparation of Langmuir-Blodgett Films and Measurement. Measurements of surface pressure (π)-area (A) isotherms and deposition of monolayer were carried out with a computer-controlled Langmuir trough (FSD-50, 51, USI). Pure distilled water with a resistivity higher than 17 MΩ cm-1, purified by a Milli-Q II-Millipore system, was used as the subphase. The copolymers are dissolved in chloroform at a concentration of about 1 mM (monomer unit) and spread on a pure water surface. The π-A curves were measured using different compression speeds and different spreading volumes in order to ensure the reproducibility of the isotherms. The quartz slide on which LB multilayers are deposited was cleaned in boiling HNO3, washed with pure water, and made hydrophobic with chlorotrimethylsilane. The monolayer on the water surface was compressed at a speed of 7.0 cm2/min and then transferred onto solid supports with a deposition rate of 10 mm/min at 15 mN/m. UV-vis and absorption dichroism spectra were measured with a Hitachi U-3000 UV-vis spectrophotometer. X-ray diffraction measurement was carried out on M18XHF22-SRA of MAC Science using Cu KR (1.54 Å) as the target. Contact angles were recorded on
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Figure 4. UV absorption spectra of pADA-Por LB films with various deposited layers. Figure 2. Surface pressure-area isotherms of pADA and pADA-Por at 20 °C.
Figure 5. Plots of the absorbance at 427 nm in pADA-Por LB film against the number of layers.
Figure 3. Most probable orientation of pADA-Por in the monolayer at the air-water interface. a contact angle meter, CA-X model, of Kyowa Interface Science Co., Ltd.
Results and Discussion Copolymer Monolayer at the Air-Water Interface and LB Film Formation. The behavior of the polymer monolayers spread on the water surface was examined by measuring π-A isotherms (Figure 2). The π-A isotherms indicate that pADA-Por formed a stable condensed monolayer with a steep rise of surface pressure. The surface area was shifted toward a larger area region than that of the pADA monolayer studied previously.18 The average molecular occupied surface area of the condensed monolayer of pADA-Por was estimated to be 0.30 nm2/ monomer unit by extrapolating the steeply rising part of the π-A curve to zero pressure. The area is in agreement with the value (0.31 nm2) calculated from a CPK model for the copolymer of pADA (0.28 nm2 × 0.985)18 and porphyrin moiety (2.54 nm2 × 0.015), which has a face-on orientation (Figure 3). This orientation of the porphyrin in the monolayer can be held in the LB film, which is
confirmed in the following section. The monolayer of pADA-Por can be transferred onto a hydrophobic quartz slide from the water surface under 15 mN/m at 20 °C in up/downward strokes with transfer ratios of unity, suggesting the formation of a typical Y-type LB film. The thickness of the polymer LB films and the layer structure were investigated by X-ray diffraction (XRD) measurement. The thickness of one layer in the LB film was obtained to be 10.7 Å, which is nearly identical with that of homopolymer pADA.18 It means that the whole layer structure shows no disorder and has been kept in the copolymer LB film. Characterization of the Surface and Molecular Orientation in LB Film. The UV-vis absorption spectra of the pADA-Por LB films with various numbers of deposited layers on quartz are shown in Figure 4. The Soret band due to the π-π* transition of the porphyrin moiety appeared at 427 nm, and the shape of absorption band is resemble to that in chloroform solution. Moreover, the absorbance at 427 nm increases linearly with increasing number of deposited layers (Figure 5). In low molecular weight LB films, porphyrins tend to form aggregates such as J and H forms which act as an energy trap site and make an inefficient energy transfer in some devices; therefore a great effort has been done to reduce the formation of the aggregates.25-27 In the present polymer
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Figure 6. Schematic illustration of the optical system for measuring the absorption dichroism as a function of tilt angle.
LB film, the UV spectra show that there is no special aggregation of the porphyrin chromophores between the adjacent layers and in the same layer. To obtain information about the orientation of porphyrin ring in the LB films, absorption dichroism measurement was carried out for the LB film with 80 layers deposited on a hydrophobic quartz glass.28 The absorption spectra were measured with s- and p-polarized light as a function of tilt angle, as shown in Figure 6. The absorbance of sor p-polarized light at an angle of θ relative to the polarizer was defined as As(θ) or Ap(θ), the absorbance at 0° was defined as As(0) or Ap(0), respectively. When s-polarized light was used as an incident light, no matter what kind of orientation the porphyrin ring takes, As(θ) changes by an increase in optical path length only, as
As(θ)/As(0) ) (cos θ)-1
Figure 7. UV absorption spectra of pADA-Por LB films with 80 layers at different tilt angles using p-polarized light as an incidence light.
(1)
while in the case of p-polarized light, Ap(θ) depends on the chromophore orientation. If the porphyrin ring takes a parallel orientation to the substrate, the absorbance should be proportional to cos2 θ. Considering the effect of the path length, the absorbance would be given as
Ap(θ)/Ap(0) ) cos θ
(2)
If the porphyrin ring has a perpendicular orientation, the absorbance will be independent of the tilt angles. Thus the ratio of Ap(θ)/Ap(0) will fit eq 1 again. The absorption spectra of pADA-Por LB films with 80 layers at different tilt angles using p-polarized light as an incident light and the plots of the absorbance at 427 nm for s- and p-polarized light as a function of tilt angles are shown in Figures 7 and 8, respectively. When we used p-polarized light as an incident light, the absorbance became weaker and weaker as changing the tilt angles from 0° to 30° relative to the polarizer, and the experimental data agreed with theoretical curve calculated from the expression of cos θ (downward curve in Figure 8). Furthermore, when s-polarized light was used, the absorbance became a little stronger and stronger as changing the tilt angles in the same way as p-polarized light, and the experimental data also agreed well with theoretical curve calculated from the expression of cos θ-1 (upward curve in Figure 8). From this result, it can be said that the parallel molecular orientation of the porphyrin ring relative to the substrate as shown in Figure 3 is situated (25) Aramata, K.; Kamachi, M.; Takahashi, M.; Yamagishi, A. Langmuir 1997, 13, 5161. (26) Koon, J. M.; Sudholter, E. J. R.; Schenning, A. P. H. J.; Notel, R. J. M. Langmuir 1995, 11, 214. (27) Zhang, Z. J.; Nakashima, K.; Verma, A. L.; Yoneyama, M.; Iriyama, K.; Ozaki, Y. Langmuir 1998, 14, 1177. (28) Ito, S.; Kanno, K.; Ohmori, S.; Onogi, Y.; Yamamoto, M. Macromolecules 1991, 24, 659.
Figure 8. Tilt angle dependence of UV absorbance at 427 nm with s and p light. The downward and upward lines are the curves for cos θ and cos θ-1, respectively.
in this polymer LB film as we discussed in the π-A isotherm measurement and the polymer LB films could be deposited on the substrate keeping the chromophore orientation. This face-on orientation is favorable to act as a gas sensor because the gas molecule can make a strong coordination from the axial direction of the porphyrin ring. It is of great interest that the orientation of porphyrin chromophore in our polymer LB film is similar to that reported in low molecular weight porphyrin Langmuir monolayers29-31 and LB films.32-34 There are several groups researching porphyrin LB films in which the macrocycle moieties are almost placed at hydrophilic part.26,27,29,32,34 In the case of the present polymer, we conjectured that the porphyrin chromophores (29) Yoneyama, M.; Fujii, A.; Maeda, S.; Murayama, T. J. Phys. Chem. 1992, 96, 8982. (30) Schick, G. A.; Schreiman, I. C.; Wagner, R. W.; Lindsey, J. S.; Bocian, D. F. J. Am. Chem. Soc. 1989, 111, 1344. (31) Orrit, M.; Mo¨bius, D.; Lehmann, U.; Meyer, H. J. Chem. Phys. 1986, 85, 4966. (32) Zhang, Z. J.; Verma, A. L.; Yoneyama, M.; Nakashima, K.; Iriyama, K.; Ozaki, Y. Langmuir 1997, 13, 4422. (33) Azumi, R.; Matsumoto, M.; Kuroda, S.; King, L. G.; Crossley, M. J. Langmuir 1995, 11, 4056. (34) Fukushima, H.; Taylor, D. M.; Morgan, H.; Ringsdorf, H.; Rump, E. Thin Solid Films 1995, 266, 289.
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enter in the adamantyl hydrophobic part in the monolayer on the water surface and in the LB films. The contact angles of water droplet on the polymer LB films were measured to estimate the hydrophobicity of the surface of the present LB films. The surface of the alkyl tail part of the Y-typed pADA-Por LB film was more hydrophobic (θ ) 88°) than that of the homopolymer pADA LB film (θ ) 82°). While the hydrophilic surface of the pADA-Por LB film was the same with that of the homopolymer pADA LB film (θ ) 76°). It is of interest that the surface of the LB film becomes more hydrophobic by incorporation of the porphyrin moiety. A simple reason is due to the replacement with hydrophobic porphyrin ring. It can be presumed that a bumpy roughness at a molecule level is
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produced on the surface of the LB film by difference in length between adamantyl and porphyrin groups, supposed from a orientation shown in Figure 3. Conclusively, we have succeeded in the preparation of a polymer LB film containing a porphyrin chromophore. The present acrylamide polymer has no long alkyl chains that are necessary to form a stable monolayer and LB film formation in a classical concept. Since the cohesive forces based on cyclic adamantyl groups are weaker than long alkyl chains, the present stable LB film formation is attributable to a self-assembled property with a hydrogen bonding based on amide groups. LA990283C
