Langmuir 1993,9, 1110-1114
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Preparation of Polymer Langmuir-Blodgett Films Containing Pyrene Chromophore and Energy Transfer in the Films' Yasushi Mizuta,t Minoru Matsuda,t and Tokuji Miyashita'J Department of Molecular Chemistry and Engineering, Tohoku University, Aoba Aramaki, Aoba-ku, Sendai 980, Japan, and Institute for Chemical Reaction Science, Tohoku University, Katahira 2-1-1, Aoba-ku, Sendai 980, Japan Received July 6,1992. In Final Form: January 11, 1993 Pyrene chromophorecould be introduced into polymer Langmuir-Blodgett (LB) films in the form of copolymersof N-(1-pyrenelacrylamide(PYA)and N-alkylacrylamides(AA) which have an excellentability to form stable monolayers and LB multilayers. The spreading behaviors of the copolymers of PYAwith AA having various alkyl chain lengths (octyl (OA),decyl (DA), dodecyl (DDA),and tetradecyl (TDA))on a water surface were investigated by a measurement of the surface pressure-surfacearea isotherms. The isotherms show that N-dodecylacrylamide copolymers (P(DDA/PyA))form the most stable condensed monolayers,and the pyrene chromophoreis dispersed uniformly having ita own Surface area on the water surface. All the copolymer monolayers of PyA with AA could be transferred onto solid supports as a Y-type film. Regular and uniform deposition was confirmed by a linear relationship between the UV absorbance of the pyrene chromophore and the number of layers. The excimer formation and energy transferof the pyrene chromophorein the LB films have been investigatedby photostationaryfluorescence spectroscopy. In the copolymer LB films of OA, the relative intensity ratio of the excimer emission to the monomer (ZdZm)increases with the number of layers, whereas the ratio in the LB films of DA and DDA was not changed with the numbers. The vertical energy transfer efficiency is discussed from a comparison of the alkyl chain length (thickness of monolayer) with energy transfer critical distance.
Introduction The Langmuir-Blodgett (LB) method is one of the most useful techniques for fabrication of functional ultrathin films. The LB technique makes it possible to prepare a thin film with a controlled thickness at a molecular size and well-defined molecular orientationn2Because of this superior feature, the photophysical processes such as excitation energy transfer and electron transfer can be controlled with the organized molecular assembly using the LB technique. Since Kuhn and co-workers reported attractive photophysicalprocesses in LB films,3extensive studies have been carried out. Yamazaki and co-workersperformed a fine investigation on chromophoredistributions in mixed LB films by means In their system, of picosecond fluorescencedecay analy~is.~ a mixture solution containing both amphiphilic pyrene compounds and long alkyl fatty acids is spread onto a water surface and the mixed monolayer was transferred onto a solid support but the chromophore formed aggregates or a ground-state dimer of pyrene units.5 They have described that a uniform distribution of pyrene moieties in a plane of LB film is difficult to obtain. On the other hand, Yamamoto and Itoh have tried to incorporate probe chromophoresinto polymer LB films by polymer reaction
to obtain the LB films having a uniform chromophore distribution in a plain of LB film. They studied the interlayer energy transfer process as a probing technique of the layered structure of polymer LB films using poly(vinyloctal). The excimer formation and energy trapping processes of the chromophore in the polymer LB films have been investigated by photostationary and timeresolved fluorescence spectroscopy.6 Polymer LB films appear to have the advantage to disperse chromophores uniformly by fixing them to polymer chains. Recently some preformed polymers are found to form a stable monolayer at the air-water interface and to be transferableto solid ~ubstrates.~-~~ We have also continued to study the polymer LB films23and have succeeded in
(6)(a) Ohmori, S.;Ito, S.; Yamamoto, M. Macromolecules 1990,23, 4047. (b) Ohmori, S.;Ito, S.; Yamamoto, M. Macromolecules 1991,24, 2377. ( c ) Ito, S.;Ohmori, S.; Yamamoto, M. Macromolecules 1992,25, 185. (7) (a) Takenaka, T.; Harada, K.; Matsumoto, M. J. Colloid Interface Sci. 1980,73,569.(b)Takeda, F.;Mataumoto, M.;Takenaka, T.; Fujiyoshi, Y. J . Colloid Interface Sci. 1981,84,220. (8)Breton, M. J . Macromol. Sci., Rev. Macromol. Chem. 1981,C21 (I), 61. (9)(a) Tredgold, R. H.; Winter, C. S. J. Phys. D Appl. Phys. 1982, 15,L55. (b) Tredgold, R. H.; Winter, C. H. Thin Solid Films 1983,99, 81. ( c ) Vickers, A. J.; Tredgold, R. H.; Hodge, P.; Khoshdel, E.; Girling, I. Thin Solid Films 1985,134,43. (d) Winter, C. S.; Tredgold, R. H.; Vicers, A. J.; Khoshdel, E.; Hodge, P. Thin Solid Films 1985,134,49.(e) Tredgold, R. H. Thin Solid Films 1987,152, 223. + Institute for Chemical Reaction Science. (10)Naito, K. J . Colloid Interface Sci. 1987,131, 218. 8 Department of Molecular Chemistry and Engineering. (11) (a) Kakimoto, M.; Suzuki, M.; Konishi, T.; Imai, Y.; Iwamoto, M.; (1)Polymer Langmuir-Blodgett Films Containing Photofunctional Hino, T. Chem. Lett. 1986,823. (b) Nishikawa, Y.; Kakimoto, M.; Imai, Groups. Part 6. Y. J . Chem. Soc., Chem. Commun. 1988,1040. (2)Kuhn,H.;MBbious,D.; Bhcher, H.PhysicalMethodsofChemistry; (12)Kawaguchi, T.; Itoh; Nakahara, H.; Fukuda, K. J . Colloid. Weiseberger, A., Roesiter, B., Eds.; Wiley: New York, 1972;Vol. 1, p 577. Interface.Sci. 1985,104,290.(b) Kawaguchi, T.; Nakahara, H.; Fukuda, (3)(a) MBbius, D.D.Ber. Bunsen-Ges. Phys. Chem. 1978,82,848.(b) K. Thin Solid Films 1985,133,29. Kuhn, H. J. Photochem. 1979,10,111.(c) Kuhn, H. Pure Appl. Chem. (13)Mumby, S.J.; Swalen, J. D.; Rabolt, J. F. Macromolecules 1986 1981,53,2105. 19, 1054. (4)(a) Tamai, N.; Yamazaki, T.; Yamazaki, I. J. Phys. Chem. 1987,91, (14)(a) Laschewsky, A.; Ringsdorf, H.; Schmidt, G.;Schneider, J. J. 841. (b) Tamai, N.; Yamazaki, T.; Yamazaki, I. Chem. Phys. Lett. 1988, Am. Chem.SOC.1987,109,788. (b) Ringadorf, H.; Schmidt, G.;Schneider, 147,25. (c)Yamezaki,L;Tamai,N.;Yamazaki,T.;Murakami,A.;Mimuro, J. ThinSolid F i l m 1987,152,207.( c ) Schneider, J.;Ringsdorf,H.;Rabolt, M.; Fujita, Y. J. Phys. Chem. 1988,92,5035. J. F. Macromolecules 1989,22, 205. (d) Schneider, J.; Erdelen, C.; (5)(a) Yamazaki, T.;Tamai, N.;Yamazaki, I. Chem. Phys. Lett. 1986, Ringsdorf, H.; &bolt, J. F. Macromolecules 1989,22,3475. 124,326.(b)Yamazaki,I.;Tamai,N.;Yamazaki,T. J.Phys. Chem. 1987, (15)Carpenter, M. M.; Prasad, P. N.; Griffin, A. C . Thin Solid Films 91,3572. 1988,161,315.
0743-7463/93/2409-1110$04.00/0
0 1993 American Chemical Society
LB Film Containing Pyrene Chromophore Chart I
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/
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the preparation of fairly uniform polymer LB films using N-alkylacrylamide series.24 Moreover, we have proposed a method to introduce various functional groups into polymer LB films as a comonomer of AA, especially DDA, with the help of the excellent ability of AA to form stable LB films.25 In the present work, pyrene chromophore, which is often used as a typical probe chromophore, is introduced into the copolymers (Chart I) of N-(l-pyrene)acrylamide (PyA) and N-alkylacrylamides (AA) having various alkyl side chains from octyl (m= 8) to tetradecyl (m= 14) with the purpose to obtain stableLB films having controlled chromophore distribution in a plane and controlled distance between the plains in the LB films. The spreading behaviors of the copolymers on the water surface were investigated. Moreover, the excimer formation and energy transfer of the pyrene probe were investigatedby photostationaryfluorescencespectroscopy.
Experimental Section Materials. N-Alkylacrylamide (AA) monomers were synthesized by the reaction of acryloylchlorideand the corresponding alliylamines (octylamine, decylamine, dodecylamine, and tetradecylamine) in the presence of triethylamine in 1,2-dichloroethane at 4 "C. The products were purified by column chromatography and by recrystallization from n-hexane. N-(lPyrene)acrylamide (PYA)was synthesized by the reaction of acryloyl chloride and l-aminopyrene (Aldrich Chemical) in the presence of triethylamine in tetrahydrofuran at 4 OC. The copolymers of AA with PyA were prepared by free radical polymerization in benzene at 60 "C with 2,2'-azobis(isobutyronitrile) as a thermal initiator. The copolymers were purified by reprecipitation from filtered benzene solution into a large excess of acetonitrile and dried under vacuum at room temperature. Molecular weights were determined by a TOSO gel permeation chromatography (GPC) using a polystyrenestandard. (16) (a) Seki, T.; Ichimura, K. Polym. Commun. 1989, 30, 108. (b) Seki, T.; Tamaki, T.; Suzuki, Y.; Kawanishi, Y.; Ichimura, K. Thin Solid Films 1989, 179, 77. (17) Era, M.; Kamiyama, K.; Yoshiura, K.; Momii, T.; Murata, H.; Tokito, S.; Tsutsui, T.; Saito, S. Thin Solid Films 1989, 171, 1. (18) Tmamura, M.; Ishida, H.; Sekiya, A. Thin Solid Films 1989,178,
Average surface area ( ndmonomer unit ) Figure 1. Surface pressure-area isotherms for the copolymers of PYA and N-alkylacrylamides with various alkyl chain lengths: (a) TDA (m = 14); (b) DDA (m= 12); (c) DA (m = 10); (d) OA (m = 8).
Table I. Characterization of Copolymer Monolayers of AA and PVA collapse area for PyA pressure PYA AA (mol %) Mn (M,IMn) (nm2/unit) (mN/m) OA ( m = 8) 4 2100 (1.37) 35 4 DA (m= 10) 2800 (1.35) 39 1 DDA(m=12) 6500 (1.55) 0.28 57 4 7900 (1.87) 0.28 55 7 loo00 (1.64) 0.28 54 12 12300 (1.80) 0.28 52 29 12700 (1.41) 0.28 44 TDA(mr14) 4 4400 (1.46) 47 PYAmole fraction in the copolymerswas determined by 'H NMR and UV spectroscopy. Preparation of Langmuir-Blodgett Films. The measurement of surface pressure-area isotherms and the deposition of the monolayers were carried out with an automatically working Langmuir trough (Kyowa Kaimen Kagaku HBM-AP using a Wilhelmy type film balance). Distilled and deionized water (Millipore Milli-Q) was used for the subphase. Chloroform used for spreading monolayer on a water surface was of spectroscopy grade. Glass slides on which LB multilayer was deposited were cleaned in a boiling HzSOd-HNOs (2:l) solution and made hydrophobicwith dichlorodimethylsilane. Spin-coatedfilms were prepared from a toluene solution of copolymer (1 wt %) by a spin-coating method with the first step 500 rpm for 10 s and the second step 2000 rpm for 60 s. Measurements. Fluorescence spectra and UV-visible absorption spectra were measured with a Hitachi 850 spectrofluorophotometer and a Hitachi U-3500 UVvisible spectrophotometer, respectively.
Results and Discussion Monolayer and Multilayer Formation. The copolymers of PyA with AA having various alkyl chain lengths 373. (OA, DA, DDA, and TDA) were spread from chloroform (19) Kumemura, H.;Kasuga, T.; Watanabe, T.; Miyata, S. Thin Solid Films 1989,178, 175. solution onto a pure water. Figure 1 shows the surface (20) Nerger, D.; Ohst, H.; Schopper, H.-C.; Wehrmann, R.Thin Solid pressurearea isotherms at 19 O C for the copolymers with Films 1989, 178, 253. nearly the same PyA content (r = 0.04). Apparently, the (21) Lupo, D.; Praes, W.; Scheunemann, U. Thin Solid F i l m 1989, 178,403. isotherms change with alkyl side-chain length, that is, as (22) (a) Oguchi, K.; Yoden, T.; Sunai, K.; Ogata, N. Polym. J. 1986, alkyl chain length increases from octyl (m= 8) to dodecyl 18,887. (b) Watanabe,M.; Kosaka, Y.; Sunami, K.; Ogata, N.; Yoden, (m= 121, the rise in surface pressure becomes greater and T. Macromolecules 1987,20,452. (c) Watanabe, M.; Kosaka, Y.; Oguchi, the collapse pressure, at which the monolayer films begins K.; Sunai, K.; Ogata, N. Macromolecules 1988,21,2997. (d) Oguchi, K.; Yoden, T.; Kosaka, Y.; Watanabe, M.; Sunai,K.; Ogata, N. Thin Solid to collapse ale0 increases. However, the copolymer monoFilms 1988, 161, 305. layer with tetradecyl side-chain length (m= 14)becomes (23) (a) Murakata, T.; Miyashita, T.; Matsuda, M. Macromolecules unstable and the collapse pressure decreases (Table I). 1988,21,2730. (b) Murakata, T.; Miyashita, T.; Matauda, M. Macromolecules 1989,22, 2706. The spreading behavior of the copolymer monolayers on (24) Miyaehita, T.; Mizuta, Y.; Matsuda, M. Br. Polym. J. 1990,22, the water is similar to that of N-alkylacrylamide ho327. mopolymers.24 (25) (a) Miyashita, T.; Yatsue, T.; Mizuta, Y.; Matsuda, M. Thin Solid Films 1989,179,439. (b) Miyaehita, T.; Yatsue, T.; Matsuda, M. J.Phys. T h e most stable monolayer is formed from the copolymer Chem. 1991, 102, 2448. (c) Mizuta, Y.; Miyashita, T.; Matsuda, M. of PyA and DDA (P(DDA/PyA)). The surface pressureMatauda, Macromolecules 1991,24,5459. (d) Miyaehita, T.; Saito, H.; area isotherms for P(DDA/PyA) were measured as a M.Chem.Lett. 1991,859. (e)Qian,P.;Miyaehita,T.;Tamai,Y.;Miyano, S.; Matauda, M. Polym. J. 1991, 23, 1393. function of PyA content (Figure 2). As the mole fraction
1112 Langmuir, Vol. 9, No. 4, 1993
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Figure 2. Surface pressurearea isotherms for the copolymers of PYAand DDA with various copolymer compositions: (a) 0.01 mol % PYA;(b) 0.04;(c) 0.07; (d) 0.12; (e) 0.29.
5 10 Number of layers Figure 4. UV absorbance at 350 nm of P(DDA/PyA) ( x = 0.04) LB films as a function of the number of layers.
0-
0.25 nm2
Wavelength (nm)
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Figure 3. A probable orientation of the P(DDA/PyA) monolayer on the water surface and the calculated area of cross section for pyrene chromophore.
of PYAincreases from x = 0.01 to x = 0.29,the collapse pressure decreases from 57 to 44 mN/m but the limiting surface area per monomer unit remains constant at 0.28 nm2. Here the limiting surface area is estimated by extrapolating the condensed state of the isotherms to zero pressure. Since the area of a DDA monomer unit has been determined to be 0.28 nm2/monomerfrom the isotherm of DDA homopolymer, the area for the PYA can be calculated to be ca. 0.28 nm2regardless of the mole fraction of PYA in the copolymers. This means that the pyrene chromophoreis homogeneouslydispersed in the copolymer monolayer having its own surface area. If the chromophore forms aggregates or the polymer chain has a folded or coiled form on the water surface, the surface area allotted to the pyrene chromophore unit would become smaller and vary with the copolymer compositions.26The determined surface area (0.28nm2)is consistent with the value estimated from a space-filling molecular (CPK) model where the polymer chain is laid horizontally on the water surface and the dodecyl substituent and the pyrene ring are oriented verticallyto the water surface (Figure3).These copolymer monolayers with various alkyl chain lengths and variou PYAcontents could be transferred onto a solid support by both downwardand upward strokesat a dipping speed of 10 mm/min with a transfer ratio of unity (Y-type deposition). Regular and uniform deposition of the monolayerswas confirmedby a linear relationshipbetween the absorbanceat 350nm due to pyrene chromophoreand the number of layers (Figure 4). It is concluded that the copolymers of AA and PYA form stable condensed monolayers where pyrene chromophore is dispersed uniformly having its own surface area (0.28nm2)on the water (26) Gains, G. L., Jr. Insoluble Monolayer at Liquid-Gas Interfaces; Wiley-Interscience: New York, 1972.
Figure 5. Fluorescence spectra of P(DDA/PyA) LB films with various PYAcontents (a-e): (a) 0.01, (b) 0.04, (c) 0.07, (d) 0.12, (e) 0.29 mol %; (f) the excitation spectrum monitored a t 390, 430,and 470 nm (see text).
surface and the deposition of the monolayer yields the polymer ultrathin LB multilayers. Excimer Formation. Excimer formation of pyrene chromophore is often utilized as a molecular fluorescence probe for aggregation of chrom~phores.~~-~l The emission and the excitation spectra of the LB films (40layers) for the copolymers with various PYA contents are shown in Figure 5. The sharp emission bands at 387 and 410 nm can be assigned to the monomer emission. The broad band around 480nm is for the sandwich excimer emission and the shoulder around 425 nm is for the partial overlappingexcimer, which are observed for the LB films containing PYA contents more than 0.04 and increase proportionally with PYAcontent. The excitation spectra (fin Figure 5)monitored at 390 (monomer),430,and 470 nm (excimer) were completely the same and identified with the absorption spectrum for all the copolymer LB films. In mixed LB films of stearic acid and 16-(l-pyrene)hexadecanoic acid, it is reported that the excitation spectrum of the excimer is shifted to a longer wavelength compared with that of the monomer emi~sion.~ This may be due to the aggregate formation in low molecular weight LB films. The emission spectra of P(DDA/PyA) ( x = 0.07)were measured for comparison in LB film, in spin-coated film, (27) (a) Diiller, E.; Fiirster, Th. 2. Phys. Chem. (Munich) 1954,1,275. (b) Diiller, E.; Fiirster, Th. 2. Phys. Chem. (Munich) 1962, 34, 132. (28) Johnson, G. D. Macromolecules 1980,13,839. (29) (a) Fujihira, M.; Nishiyama, K.; Hamaguchi, Y. J . Chem. SOC., Chem. Commun. 1986, 823. (b) Fujihira, M.; Yamada, H. Thin Solid Films 1988, 160, 125. (c) Fujihira, M.; Nishiyama, K.; Aoki, K. Thin Solid Films 1988, 160,317. (30) Kinnunen, P. K. J.; Tulkki, A.-P.; Lemmetyinen, H.; Paakkola, J.; Virtanen, J. A. Chem. Phys. Lett. 1987,4A, 321. (31) Karcher, W.; Fordham, R. J.; Dubois, J. J.; Glaude, P. G. J. M.;
Ligthart, J. A. M. Spectral Atlas of Polycyclic Aromatic Compounds 1985, 94.
LB Films Containing Pyrene Chromophore
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Wavelength (nm) Figure 6. Fluorescence spectra of P(DDA/PyA) ( x = 0.07) in THF solution (a), LB film (b), and spin-coatedfilms (c). and in tetrahydrofuran (THF) solution (Figure 6). Apparently, the intensity of excimer emission in the LB film is relatively stronger than that in solution and is weaker than that in spin-coated film. The excimer formation in polymer systems can be described as a function of the concentration of excimer sites and energy transfer. The concentration of the potential excimer sites will be proportional to the number of pyrene-pyrene dyad sequences. Moreover,the energy transfer efficiencydepends on the distance between pyrene chromophores in the polymers. The concentration of pyrene-pyrene dyad (excimer formation site) is constant because the same polymer is used. Therefore, the results in Figure 6 would be caused by the difference in the efficiency of energy transfer from the excited monomer to the excimer forming sites in the media. In solution, the energy migration proceeds along the polymer main chain and the migration between polymer chains would be rare in diluted condition. In aY-type LB fii,two-dimensional(intraplane) efficient energy migration is expected because the pyrene chromophores are placed closely at the head-to-head interface of Y-type bilayers and highly oriented. As confirmed by the following experiment, energy migration between the chromophores in the different interface across the hydrophobic alkyl tails which have a longer distance than the critical energy transfer distance of Fiirster type for pyrene-pyrene chromophores would be depressed. This causes the intensity of excimer emission in the LB film to be weaker than that in spin-coated film where threedimensional energy migration is possible. Energy Transfer to the Different Interfaces in Y-TypeLB Films. The preformed N-alkylacrylamide homopolymers with various alkyl chain lengths can be transferred onto a solid support yielding well-defined Y-type polymer LB films. The thickness of monolayer in the LB films can be controlled by varying alkyl chain length. Therefore, the vertical distance between the chromophores linked on the copolymer chains can be controlled by the alkyl chain lengths used. The thickness of monolayer in the DDA homopolymer LB film was already determined to be 1.72 nm.24 Assuming a similar orientation to DDA polymer, the thickness of monolayer in the polymer LB films with various alkyl chain lengths by using the CPK model can be estimated to be roughly 1.12 (OA), 1.37 (DA),and 1.97 (TDA), respectively. The actual distances between chromophores would be somewhat less than these values because the hydrophobic chromophoretends to orient toward the hydrophobic part (Chart 11). The efficiency of vertical energy migration in the copolymer LB films of PYA with OA and DDA is studied with the emission spectra.
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The emission spectra of the copolymer ( x = 0.04) LB films of PyA with OA and with DA were measured as a function of the number of transferred layers (Figure 7). For the N-octylacrylamide copolymer LB film, the intensity around 480 nm due to the excimer emission increaseswith the number of layers, whereas the emission spectra for the N-decylacrylamidecopolymer LB film were not changed with the deposition. No change in the emission spectra with the number of transferred layers
1114 Langmuir, Vol. 9, No.4,1993
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was also observed for the DDA polymer LB film. The relative intensities of the excimer emission at 480 nm to the monomer at 387 nm were plotted against the number of layers for the P(OA/PyA) and P(DA/PyA) LB films, respectively, in Figure 8. Increasing in the excimer intensity with the number of layers observed for the P(OA/ PYA)LB films indicates that vertical interlayer energy transfer to excimer forming sites occurs across the distance
of double octyl chains (ca. 2 nm). On the other hand, the energy transfer across the double decyl chains would be impossible. The distance for energy transfer has been often discussed from F6rster’s model. The criticaltransfer distance (Farster radius) between pyrene chromophores is reported to be 1.0 nm.32 This value is, however, too small to explain the results in the LB f h described above. The critical transfer distance for 1-aminopyrene chromophore employed in the LB films is not known. The value larger than that of pyrene is proposed. Moreover, lateral energy migration between pyrenes existing in the same layer would be possible even at 0.04 mole fraction of pyrene, and the lateral energy migration appears to play a role in promoting the vertical energy transfer. Further study, however, would be necessary for the longdistance energy transfer in the LB assembly. It is important that the energy transfer can be controlled by the alkyl chain lengths of the N-alkylacrylamides in the LB films. Controllingthe energy transfer in ultrathin polymer films shows a possibility of application to various optical devices.
Acknowledgment. T.M. thanks the Ciba-Geigy Foundation (Japan) for the Promotion of Science for financial support. (32)Berlman, I. B. Energy Transfer Parameters of Aromatic Compounds: Academic: New York, 1973.
