Langmuir 1992,8, 936-941
936
Multilayer Films of Chromophoric Cellulose Octadecanoates Studied by Fluorescence Spectroscopy Yoshinobu Tsujii, Takahiro Itoh, Takeshi Fukuda, and Takeaki Miyamoto' Institute for Chemical Research, Kyoto University, Uji, Kyoto 611, Japan
Shinzaburo Ito and Masahide Yamamoto Department of Polymer Chemistry, Faculty of Engineering, Kyoto University, Sakyo-ku, Kyoto 606, Japan Received September 16, 1991. In Final Form: January 2, 1992 Multilayer films of pyrene-labeled cellulose octadecanoatewere investigatedby fluorescencespectroscopy. Cellulosetrioctadecanoateforms a homogeneous monolayer at the air/water interface. The surface pressure (r)-area (A) isotherms indicate that the introduction of pyrenyl groups as much as 7 % (by side chain mole base) does not essentially alter the monolayer characteristics. The monolayer on the water surface was transferred successfully onto a substrate by a horizontal lifting method to form a Y-type multilayer film. The fluorescent properties (fluorescencespectrum and decay curve) were independent of the number of layers, which suggests no interlayer excimer formation and no interlayer energy transfer. Analysis of the fluorescent quenching efficiency as a function of pyrenyl content revealed that the pyrenyl groups are two-dimensionallyrandomly distributed within each layer. Furthermore, in the multilayer film with high pyrenyl concentration, the formation of two types of excimers was demonstrated by time-resolved fluorescence spectroscopy. One is a sandwich excimer and the other is a partially overlapped one. The existence of the latter excimer means that the conformational change of side chains in such a system with a condensed structure is appreciably restricted.
Introduction The Langmuir-Blodgett (LB) technique is a very useful method to design high-performance multilayer To prepare a highly functional LB film, a functional group must be introduced without disturbing the monolayer characteristics. It is also very important to control the distribution of the introduced functional group. One method to introduce a functional group into an LB film is to mix amphiphilic chromophores with amphiphiles such as fatty acids to form a surface monolayer. However, Yamazaki et al.,3who studied the chromophore distribution in such a mixed LB film by fluorescence spectroscopy, found that the amphiphilic pyrenyl moieties underwent aggregation in each layer. The LB films of fatty acids have another disadvantage of thermal and mechanical in~tability.~,~ Recently, polymer LB films have attracted much attention as candidates with improved ~ t a b i l i t i e s . ~ ' ~ Ohmori et al. obtained polymer LB films with uniformly (1)Roberta, G. G. Adu. Phys. 1985,34,475. (2)(a) Kuhn, H.; Mobius, D.; Bucher, H. In Physical Methods of Chemistry; Weissberger, A,, Rossiter, B. W., Eds.; Wiley: New York, 1972,Vol. 1, Part 3B, p 577. (b) Mobius, D. Ber. Bunsen-Ges. Phys. Chem. 1978,82,848. (c) Kuhn, H Pure Appl. Chem. 1981,53,2105. (3)(a) Yamazaki, T.; Tamai, N.; Yamazaki, I. Chem. Phys. Lett. 1986, 124,326. (b) Yamazaki, I.; Tamai, N.; Yamazaki, T. J.Phys. Chem. 1987, 91, 3572. (4)Blodgett, K. B. Phys. Reu. 1939,55,391. (5)Fukui, T.; Sugi, M.; Iizima, S. Phys. Reu. B. 1980,22,4898. (6)(a) Takenaka, T.; Harada, K.; Mataumoto, M. J. Colloid Interface Sci. 1979,73,569. (b) Takeda, F.;Matsumoto, M.; Takenaka, T.; Fujiyoshi, Y. J . Colloid Interface Sci. 1981,84,220. (7)(a) Tredgold, R. H.; Winter, C. S.J. Phys. D: Appl. Phys. 1982, 15,L55. (b) Tredgold, R.H.; Winter, C. S.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.; Vickers, A. J. Thin Solid Films 1985.134.49. (e) Tredgold, R. H. Thin Solid Films 1987,152,223. (8)(a) Oguchi, K.; Yoden, T.; Sanui, K.; Ogata, N. Polym. J. 1986,18, 887. (b) Watanabe, M.; Kosaka, Y.; Sanui, K.; Ogata, N.; Oguchi, K.; Yoden, T. Macromolecules 1987,20,452.(c) Oguchi, K.; Yoden, T.; Kosaka, Y.; Watanabe, M.; Sanui, K.; Ogata, N. Thin Solid Films 1988,161, 305. (e) Watanabe, M.; Kosaka, Y.; Oguchi, K.; Sanui, K.; Ogata, N. Macromolecules 1988,21, 2997.
distributed pyrenyl chromophores and studied the excimer formation and energy trapping processes of the chromophores in the film.14c Cellulose, with its three reactive hydroxyl groups per glucose unit, has been known as a potential functional material. Particularly, the recent development of ita nonaqueous solvents1618 has motivated a number of studies on its modification for functional polymers such as lyotropic and thermotropic liquid crystalline derivatives.14-23 One of the interesting attempts was to introduce hydrophobic side chains onto cellulose to yield an amphiphilic (9)(a) Kakimoto, M.; Suzuki, M.; Konishi, T.; Imai, Y.; Iwamoto, M.; Hino,T. Chem.Lett. 1986,823.(b) Nishikata, Y.; Konishi,T.; Morikawa, A.; Kakimoto, M.; Imai, Y. Polym. J. 1988,20,269. (c) Kakimoto, M.; Morikawa, A.; Nishikata, Y.; Suzuki, M.; Imai, Y. J. Colloid Interface Sci. 1988,121,599. (10)Uekita, M.; Awaji, H.; Murata, M. Thin Solid Films 1988,160,21. (11)Shinehara, K.: Hara, M.: Nakahama. H.: Mivata. S.: Murata. Y.; Yamada, A: J. Am. Chem. SOC.1987,109,1237. (12)(a) Murakata, T.; Miyashita, T.; Matsuda, M. Macromolecules 1988,21,2730. (b) Murakata, T.; Miyashita, T.; Matsuda, M. Macromolecules 1989,22,2706. (13)(a) Erdelen, C.;Laschewsky, A.; Ringsdorf, H.; Schneider, J.; Schuster, A. Thin Solid Films 1989,180,153.(b) Schneider, J.; Ringsdorf, H.; Rabolt, J. F. Macromolecules 1989,22,205. (c) Schneider, J.; Erdelen, C.; Ringsdorf, H.; Rabolt, J. F. Macromolecules 1989,22,3475. (d) Mumby, S.J.; Swalen, J. D.; Rabolt, J. F. Macromolecules 1986,19, 1054. (14)(a) Ito, S.;Okubo, H.; Ohmori, S.;Yamamoto,M. Thin Solid Films 1989,179,445. (b) Ohmori, S.;Ito, S.; Yamamoto, M.; Yonezawa, Y.; Hada, H. J.Chem. SOC.,Chem. Commun. 1989,1293.(c) Ohmori, S.;Ito, S.;Yamamoto, M. Macromolecules 1991,23,4047.(d) Ito, S.;Kanno, K.; Ohmori, S.;Onogi, Y.; Yamamoto, M. Macromolecules 1991,24, 659. (15)Naito, K. J. Colloid Interface Sci. 1989,131,218. (16)Hudson, S.M.; Cuculo, J. A. J . Macromol. Sci., Rev. Macromol. Chem. 1980,C18, 1. (17)McCormick, C. L.; Lichatowich, D. K. J.Polym. Sci., Polym. Lett. Ed. 1979,17,479. (18)(a) Isogai, A.; Ishizu, A,; Nakano, J. J.Appl. Polym. Sci. 1984,29, 2097,3873. (19)(a) Kennedy, J. F.,Phillips, G. O., Williams, P. A,, Eds. Cellulose: Structural and Functional Aspects; Ellis Horwood Chichester, 1989. (b) Kennedy, J. F., Phillips, G. O., Williams, P. A,, Eds. WoodProcessing and Utilization; Ellis Horwood Chichester, 1989. (20)Inagaki, H.; Phillips, G. O., Eds. Cellulosics Utilization;Elsevier: London. 1989.
0~43-7463/92/240S-0936$03.oo/o 0 1992 American Chemical Society
Langmuir, Vol. 8, NO.3, 1992 931
Multilayers of Chromophoric Cellulose Octadecanoates polymer that may possibly form a stable monolayer at the aidwater interfa~e.2~ Monolayers of cellulose derivatives are expected to be thermally stable owing to the semiflexibility of the cellulosic main chain. P r e v i ~ u s l ywe ,~~ showed by electron microscopy that the cellulose esters with relatively long alkyl side chains form a perfectly homogeneous monolayer, while the monolayers of cellulose ethers are inhomogeneous with many holes. In this study, we have prepared cellulose trioctadecanoate partially labeled with a fluorescent probe, a (l-pyreny1)butyroyl group, and studied by fluorescence spectroscopy the influence of the chromophore on the monolayer characteristics and the distribution of the chromophore within the LB film. Fluorescence properties (fluorescence spectrum and decay curve) of this system were also examined with particular attention to the homogeneity of the film structure.
Experimental Section Materials. Onto the cellulose regenerated from a cellulose diacetate with a viscosity-averagedegree of polymerization of ca. 200 (Daicel Chem. Ind.), octadecanoyl groups were introduced by the acid chloride-pyridine pro~edure:~6~~6 0.65 g (4 X mol equiv) of the regenerated cellulose was dispersed in 15 mL of toluene, to which pyridine (1.7 X 10-2 mol) was added. 1-Octadecanoyl chloride (ODC,Wako Pure Chem. Ind.) diluted with an equivolume of toluene was added dropwise. The reaction was run under reflux for 24 h. The reaction mixture was poured into a mixture of methanol and water (1O:l by volume), and the precipitate was separated by filtration. This crude material was dissolved in tetrahydrofuran (THF)and insolubleimpuritieswere removed by centrifugation. The product, cellulose octadecanoate (COD) was reprecipitated with hot methanol and filtered off. These procedures were repeated several times. By controlling the amount of ODC, we have obtained COD'S with degrees of substitution (DSOD~) ranging from 1.9 to 3.0. These DSox values were determined by lH NMR spectroscopy, in which the integral intensity of terminal methyl protons was compared with that of glucose-ringprotons.23b Cellulose octadecanoate (1-pyreny1)butyrates were prepared from COD by the method of Hayashi et al.:27a toluenesulfonyl chloride (TsC1)(double molar quantity of the OH group in COD) was dissolved in pyridine (10 times of the OH group), and 4(l-pyreny1)butyric acid (PBA) (3.6 times of TsC1) was added. PBA (Aldrich Chem. Co.) was purified by recrystallization from a toluene solution. Stirring for 15min at room temperatureyields a turbid solution, to which COD was added, and the temperature was kept at 323 K for 24 h. The reaction mixture was poured into a mixture of methanol and water (101 by volume), and the precipitate was separated by filtration and dried. The crude product was purified by reprecipitation from a THF solution into methanol several times and dried in vacuum. Impurities insoluble in THF were removed by centrifugation. In this way, four samples of COD with different pyrenyl contents were synthesized, which will be designated as C1, C2, C3, and C4 in the order of increasing pyrenyl fraction. We let CO denote the fully-substituted COD without pyrenyl group. The DS values (21) (a) Gray, D. G. J. Appl. Polym. Sci., Appl. Polym. Symp. 1983, 37, 179. (b) Gray, D. G. Faraday Discuss. Chem. Soc. 1985, 79,257. (22) Gilbert, R. D.; Patton, P. A. h o g . Polym. Sci. 1983, 9, 115. (23) (a) Miyamoto, T.; Tsujii, Y.; Fukuda, T. J. Korean Fiber Soc. 1990,27,387. (b)Fukuda,T.;Sugiura, M.;Takada,A.;Sato,T.;Miyamoto, T. Bull. Inst. Chem. Res., Kyoto Uniu. 1991, 69, 211. (24) (a) Kawaguchi, T.;Nakahara,H.; Fukuda, K. J . Colloidlnterface Sci. 1985,104,290. (b) Kawaguchi, T.; Nakahara, H.; Fukuda, K. Thin Solid Films 1985, 133, 29. (25) (a) Itoh,T.; Suzuki,H.;Matsumoto,M.;Miyamoto,T.In Cellulose:
Structural and Functtonal Aspects; Kennedy, J. F., Phillips, G. O., Williams, P. A., Eds.; Ellis Horwood: Chichester, 1989; p 409. (b) Matsumoto, M.; Itoh, T.; Miyamoto, T. In Cellulosics Utilization; Inagaki, H., Phillips, G. O., Eds.; Elsevier: London, 1989; p 151. (26) Malm, C. J.; Mench, J. W.; Kendall, D. L.; Hiatt, G. D. Ind. Eng. Chem. 1951,43, 684. (27) (a) Shimizu, Y.; Hayashi, J. Sen-I Gakkaishi 1988, 44, 451. (b) Shimizu, Y.; Hayashi, J. Cellul. Chem. Technol. 1989, 23, 661.
Table I. Overall Degree of Substitution (DS), Degree of Substitution of Pyrene Units (DSpy),and Limiting Areas per Glucose Unit (Ao) sample DS DSpv A&" co 3.0 0 0.71 c1 3.0 0.007 0.70 c2
c3 c4
3.0 3.0
0.038
3.0
1.1
0.21
0.70
0.70 0.84
of the pyrenyl group per glucose unit (DSp,) were estimated on the basis of the UV absorbance in benzene relative to the known molar extinction coefficientsof pyrene in benzene (e = 4.45 X l(r M-I cm-l at 338 nm and e = 1.17 X l(r M-I cm-l at 329 nm). As for C3 and C4, DSb was also determined by lH NMR spectroscopy. These values agreed with those estimated by UV absorbance within experimental errors. In addition, it was confirmed by IR spectroscopy that these cellulose derivatives have no hydroxyl groups, namely the total degree of substitution, DS = DSom + DSp,., is 3.0 in all cases. Preparation of Multilayer Films. The cellulose derivatives (CGC4) were spread from 0.5 to 1mL of benzene solution (ca. 1 X lo-' mol equiv in mL) onto the surface of pure water in a Teflon-coated trough (200 X 500 X 3 mm). Benzene of Dotite Spectrosol was used as received. The water used in the subphase was purified using a Mitamura Riken Model PLS-DFR automatic still, consisting of a reverse osmosis module, an ion-exchange column, and a double distiller. The subphase temperature was kept at 293 K. The surface pressure was measured by using a film balance of a Wilhelmy type. A nonfluorescent quartz plate (18 X 24 mm) waa used as a substrate, which was cleaned in sulfuric acid containing a small amount of potassium permanganate, dipped in a 10% hydrogen peroxide solution, and then washed by pure water. The surface of the substrate was made hydrophobic by dipping it in a 10% toluene solution of trimethylchlorosilane(NacalaiTesque, Inc.) for 1hand then washed by toluene. The surface pressure (+area (A) isotherms were measured at a constant compression rate of 12 cm2.min-' at 293 K. The horizontal lifting method without a frame was used to deposit the surfacemonolayeronto the substrate.28The substrate was lowered in a nearly horizontal position until it just touched the monolayer on the water and then lifted up by using an elevating apparatus. The transfer ratio was found to be ca. 2, showing that a Y-type bilayer was constructed by this operation. During the deposition, the surface pressure was controlled to remain 10"em-'. At this surface pressure the monolayer of CO had been observed to be homogeneous under an electron microscope.% After a bilayer of COwas deposited on the substrate, a chromophoric multilayer was piled up on it, and finally on top of the layers another bilayer of CO was deposited as a protective layer. Namely, the chromophoric multilayer was sandwiched between the two bilayers of CO to suppress possible effect of the substrate and air interfaces. Measurements. Absorption spectra were measured by a Shimadzu UV-200sspectrophotometer. Fluorescence and excitation spectra were recorded on a Hitachi 850 spectrophotofluorometer. A single photon counting method was applied for the measurements of fluorescence decay curves and time-resolved fluorescence s p e ~ t r aThe . ~ photoexcitation ~~~ of pyrenyl groups was made by a 315-nm pulse from a Spectra-Physics picosecond synchronously pumped, mode-locked, cavity-dumped dye laser (Model2020,342A, 375B,and 3448). The time-correlated pulses were obtained by conventional Ortec photon-counting electronics and accumulated on a multichannel analyzer (NorlandIT-5300) controlled with a microcomputer (NEC9801). The full-width at ~
(28) (a) Kamata, T.; Umemura, J.; Takenaka, T. Chem. Lett. 1988, 1231. (b)Umemura, J.; Hishino, Y.; Kawai, T.; Takenaka, T.; Gotoh, Y.; Fujihira, M. Thin Solid Films 1989,178, 281. (c) Kawai, T.; Umemura, J.; Takenaka, T. Langmuir 1990, 6,672. (29) OConnor, P. D.; Phillips, D. Time-Correlated Single Photon Counting; Academic: London, 1984. (30) (a) Ito, S.;Takami, K.; Yamamoto, M. Makromol. Chem., Rapid Commun.1989,10,79. (b) Ito, S.; Takami, K.; Tsujii, Y.; Yamamoto, M. Macromolecules 1990, 23, 2666.
Tsujii et a1.
938 Langmuir, Vol. 8,No. 3,1992 l " " " " " ' 1
0
2
4
6
l " " " ' " " 1
8
1012
Number of Layers
Number of Layers
Figure 2. Fluorescence intensity of the monomer emission at 377 nm for C2 (A) and C3 (B),plotted against the number of
c4
layers.
? " " ~ " " ~ " " ~ ' " ' ~ " " ~ " " I
0
0
0.5
1.0
1.5
Area / nm2per unit Figure 1. Surface pressurearea isotherms of cellulose derivatives of CO through C4 measured at 293 K. half-maximum of the overall excitation pulse was 500 pa. The spectral response was calibrated with a standard tungsten lamp.
Results and Discussion Surface Pressure-Area Isotherms. Figure 1shows the n-A isotherms of the pyrene-labeled cellulose octadecanoates along with that of cellulose trioctadecanoate, CO. For CO,the surface pressure begins to increase a t an area of ca. 0.9nm2 per glucose unit and rises steeply upon compression. The limiting area per glucose unit estimated by extrapolating the steepest tangent of the isotherm to zero surface pressure is 0.71nm2. This isotherm indicates that CO forms a condensed monolayer. According to the electron microscopic observations reported pre~iously,2~ the monolayer of CO transferred a t ?r = 10 mN.m-' was homogeneous. On further compression, the monolayer collapses at n = 49 mN-m-'. As shown in Figure 1,C1,C2, and C3 give almost the same n-A isotherms as CO. The surface pressure increases sharply with compression, and the limiting area is estimated to be ca. 0.70 nm2 in all cases. This suggests that the introduction of pyrenyl group as much as 7% (by side chain mole base) does not essentially alter the monolayer characteristics, giving a condensed and homogeneous monolayer similar to that of
co.
The n-A isotherm of C4 (DSg = 1.1)having a pyrenyl content of 38% (by side chain mole base) is different from those of others. It has a larger limiting area (0.84 nm2) and a small pressure of collapse (ca. 20 mN0m-I). Obviously, this is due to the introduction of too many bulky pyrenyl groups. Nevertheless, the isotherm does suggest that C4 also forms a condensed monolayer. The transferability of the surface film onto the substrate was checked by measuring the fluorescence intensity as a function of the number of layers. Figure 2 shows the relation between the monomer fluorescence intensity at 377 nm and the number of layers for C2 and C3. In each case, the fluorescence intensity increased proportionally to the number of layers. This means that the surface monolayer was transferred successfully and that the CO protecting bilayer worked effectively. Furthermore, the observed linear relationships indicate the absence of interlayer chromophoricinteractions. This is consistent with other observations described later.
D I , , , ,
350
400
450
500
550
600
650
Wavelength / nm Figure 3. Fluorescence spectra of 10-layer films for C4 (A), C3 (B),C2 (C),and C1 (D).
Excimer Formation and Chromophore Distribution. Figure 3 shows the fluorescence spectra of 10-layer films of C1 to C4. These spectra did not depend on the number of layers. Samples of low pyrenyl content,
C1 and C2,give virtually the monomer fluorescence only, with emission bands at 377 and 397 nm. With increasing pyrenyl content (C3),the emission intensities at longer wavelengths increase relative to the monomer fluorescence. This broad emission band is due to both the normal and the partially overlapped excimers as will be demonstrated by the time-resolved fluorescence spectroscopy. In C4, which has the highest DSp,., the emission of the normal excimer is dominantly observed at 480 nm. The excimer formation in the multilayer films of these polymers is highly suppressed compared with that in an LB film of mixed fatty acids having the same surface concentration of pyrenyl groups. This means that the pyrenyl groups are distributed homogeneously in the polymer multilayer film. Pyrenyl groups in an LB film of mixed fatty acids are known to be aggregated.3 The excitation spectra also suggest no interaction between pyrenyl chromophores in the ground state. The sample, C3,gives the same excitation spectra independent of the monitor wavelength. Figure 4 shows the excitation spectrum of C3 monitored at 377 nm. This spectrum is the same as that of sample C1 (monitored at 377 nm), which has the lowest pyrenyl content and gives the monomer fluorescence only. Thus, the C3 film has no aggregation of pyrenyl groups. Even for sample C4,the excitation spectrum was red-shifted only by 1nm or less.
Langmuir, Vol. 8, No. 3, 1992 939
Multilayers of Chromophoric Cellulose Octadecanoates
r 8
x E0
3
E; ,,,,
"'A,,,
Wavelength / nm Figure 4. Fluorescence excitation spectra of C3 monitored at 377 nm. The peak wavelength (nm) of each vibrational band is shown in the figure.
n
- v0u Figure 5. Schematic two-dimensional hexagonal packing of side chains in a monolayer. The closed circle at the center denotes a photoexcited ppenyl group, the distance from which to site i is ri. As for sites a, b, and c, see text.
The value of DSp, of this sample is 1.1,i.e., about one side chain out of three is pyrene-labeled. If all side chains are standing straight and normal to the film surface (cf. Figure 8)and packed in a two-dimensionally hexagonal order (cf. Figure 51, approximately two, on the average, out of the six first neighbors of a pyrenyl group will be pyrenyl groups, even if their distribution is statistical. This probability of first-neighbor contact of pyrenyl groups is quite high. In this regard, the observed (rather small) shift in the excitation spectrum should not be considered as being evidence of an aggregation of pyrenyl groups. Yamazaki et al. have observed a red-shift of as much as 2 nm in the excitation spectrum (monitored a t 474 nm) of a fatty acid film that had a strong aggregation of pyrenyl group^.^ The randomness of the spatial distribution of pyrenyl groups was confirmed by the following simulation. We assumed that side groups (octadecanoyl and (1-pyreny1)butyroyl groups) be packed in a hexagonal structure as shown in Figure 5. In this figure, circles denote a side group (an octadecanoyl or a (1-pyreny1)butyroyl group). The assumption may be reasonable since the observed limiting area per side chain (0.23 nm2) is close to the cross section (0.18 nm2) of a methylene chain in the orthorhombic crystal of p ~ l y e t h y l e n e . When ~~ a pyrenyl group at the center (closed circle in the figure) is excited, it may be quenched statically or dynamically. Quenching of excited state occurs a t a pair of pyrenyl groups to form an excimer. The probability of static quenching may be assumed to be equal to the fraction of the excited pyrenyl ~~~
(31) Wunderlich, B. Macromolecular Physics VoE. 1; Academic: New York, 1973; p 92.
groups which have a pyrenyl group a t an adjacent site. Since each site has six first neighbors, the probability of static quenching is 1- to6, where fo is the fraction of octadecanoyl groups. On the other hand, some of excited pyrenyl groups which did not suffer static quenching will be quenched by energy transfer to an excimer forming site. Because the pyrenyl chromophore has a small Forster radius (Ro= 1.1nm),32the exciton-diffusion by energy migration should be restricted to a relatively small area. Therefore, we consider only one step energy transfer and not energy migration among pyrenyl groups. When Forster type transfer is the probability for the excitation energy to transfer to a pyrenyl group a t site i will be proportional to
f p,(Ro/ri) 1
where fp, ('1 - f o ) is a fraction of (1-pyreny1)butyroyl groups and ri is a distance to the pyrenyl group at site i. Thus, the probability of dynamic quenching is
where the factor off$ indicates the probability of escaping from static quenching and fexi is the probability of site i being an excimer forming site. Second neighbor sites have two kinds of sites, a and b. There is also third and higher order neighbor sites c. It is a prerequisite for dynamic quenching that the excited pyrenyl group has no pyrenyl group in its first neighbor sites. Then, it is required that the site a has two no pyrenyl sites and the site b has one no pyrenyl site, and the probability that a second neighbor site (site a orb) is an excimer forming site is different from that of all other (nonsecond neighbor) sites (e.g., site c)
ft f,,b = 1- ft fe: = 1-
f,," = 1- f,"
for second neighbor site a for second neighbor site b for all other sites
Then we obtain the total quenching efficiency as a sum of the static and the dynamic processes. In Figure 6, the total quenching efficiency calculated in this way is plotted against the pyrenyl content. The hatched area indicates the contribution of the dynamic quenching by energy transfer, whose relative fraction decreases with increasing pyrenyl content. In C4, almost all the excited pyrenylgroups are statically quenched. The closed circles indicate the quenching efficiency experimentally estimated by the monomer fluorescenceintensity. The good fit of the experimental data to the simulation line suggests that the pyrenyl groups are two-dimensionally randomly distributed within each layer, directly reflecting the chemical randomness of the chromophoric side-chain distribution along the cellulosic main chain. This homogeneity is characteristic of a polymer LB film. Interlayer ChromophoricInteraction. As described in the previous section, the emission spectra of C1 through C4 did not depend on the number of layers of the built-up films. This is also the case with fluorescent lifetimes. Figure 7,for example, shows the fluorescence decay of the (32) Berlman, I. B. Energy Transfer Parameters of Aromatic Compounds; Academic: New York, 1973. (33) Forster, Th.2. Naturforsch. 1949, 4A, 321.
940 Langmuir, Vol. 8, No. 3,1992
Tsujii et al.
1.0
V h
8
. I
V
E w
Static Quenching
or, 0.5 E
. I
5
z
0 0
0.5
0
1.0
Fraction of Pyrenyl Group
Figure 6. Quenchingefficiency as a function of pyrenyl fraction.
The closed circles show the experimental results estimated by the monomer fluorescenceintensity. The curve is a simulation line, and the hatched area indicates the contribution of the dynamic quenching.
Figure 8. Molecular model of the monolayer of pyrene-labeled cellulose octadecanoate.
"0
50
100
150
200
250
Channel (13ns/ch.)
Figure 7. Fluorescence decay curves of the monomer emission observed at 377 nm for the C3 films of differing numbers of layers: (a) 2-layer; (b) 4-layer; (C) 6-layer; (d) 8-layer; (e) 10-
layer films. The decay curves ( b e ) are shifted appropriately, but the maximum count number is lo4 in all cases.
monomer emission observed at 377 nm for the C3 films of differing number of layers. Clearly, all the films give the same decay curves, parallel with each other except for a short time region that was affected by an impurity emission.34 Because the Forster radius between pyrenyl moieties is only 1.1 nm, an energy migration between adjacent layers should not occur in a multilayer of cellulose octadecanoates which has a thickness of 2.4 nm per layer. Furthermore, this layer thickness suggests the molecular structure of monolayer in which the glucose ring lies parallel to the substrate and the octadecanoyl groups with all-trans conformationsstand vertically to the glucose ring, as shown in Figure 8. Since the (I-pyrenyl)butyroyl group has a length of 1.6 nm in the most extended conformation, a pyrenyl ring is located about in the middle of a layer. This means that no excimer is formed between chromophores belonging to different layers. This contrasts to (34) Fluorescentmeasurements of the sample without pyrenyl groups (sample CO) confirmed that the initial parts of the decay curves have been disturbed by an impurity emission with a short lifetimeof the order of a few nanoseconds, and this prevented us from analyzingthe fluorescent decay curves on the basis of the Fiirsterenergy-transfermodel. However, since the impurity emission was very weak in absolute intensity, its influence on the photostationary measurement (fluorescence spectra) could be neglected.
the case of poly(viny1 octal) LB films, in which an interlayer energy migration and excimer formation have been observed due to a small layer thickness of 1.05 nm.14c Time-&solved FluorescenceSpectroscopy. Figure 9 shows the time-resolved fluorescence spectra of twolayer films of C4. Each spectrum has been normalized so as to give the same maximum intensity. In each time range, a broad emission band is observed in a long wavelength region in addition to the monomer emission band with a vibrational structure. This broad band exhibits a red-shift with time. We can divide the band into two components, one with a maximum at around 480 nm and the other with a maximum at around 430 nm. The former component is assigned to a normal excimer (E2) with a sandwich arrangement of pyrenyl moieties. This excimer is commonly observed in solutions. We assigned the latter to a partially overlapped excimer (El). This type of excimer has been observed in LB films of fatty acids, in vapor-depositedfilms, in neat liquids, and in single crystals. In a short time region (0-1 ns), the emission of E l is dominant. Since E l is less stable and has a shorter lifetime than E2, the intensity of E2 relative to that of E l increases as time elapses. After 50 ns, the emission of E2 becomes a main component. The apparent red-shift of the excimer band is due to the change of the relative intensities of E l and E2. Since an interlayer energy transfer and an interlayer excimer formation cannot occur in this system, the relaxation of the excitation energy is restricted to proceed within a layer. In C4 with its high content of pyrenyl groups, the monomer excited state is quenched rapidly to form excimers. There are two types of excimer-forming sites; in one type, the normal excimer can be formed easily by a small conformational rearrangement, and in the other type of sites, only a partially overlapped excimer can be formed. The motion of pyrenyl groups in a condensed monolayer should be highly restricted. Therefore, E l presumably will not be converted to the more stable excimer E2, and each excimer will be deactivated by itself. The fraction of E l and E2 depends
Langmuir, Vol. 8, No. 3, 1992 941
Multilayers of Chromophoric Cellulose Octadecanoates
' \ E2
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Wavelength / nm Figure 10. Time-resolved fluorescencespectra of a 2-layer film of C3. The gate times of spectra a, b, c, d, and e are 0-1 ns, 2-4 ns, 5-9 ns, 20-30 ns, and 50-100 ns, respectively.
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Wavelength I nm Figure 9. Time-resolved fluorescence spectra of a 2-layer film of C4. The gate times of spectra A, B, and C are 0-1 ns, 5-9 ns, and 50-100ns, respectively. The symbolsM, El, and E2 indicate the emitting components due to the monomer excited state, partially overlapped excimer, and sandwichexcimer, respectively. on the concentrations of the two types of excimer-forming sites and the rate of energy transfer to each site. Also in a multilayer of C3, the excitation energy relaxes in the same way. The spectra shown in Figure 10 are normalized a t 377 nm. In a short time region (0-1 ns), a shoulder is observed a t around 430 nm, which corresponds to the emission of the partially overlapped excimer El. This emission decreases rapidly with time, but the emission a t around 480 nm due to the normal excimer continues at later stages.
Conclusion The fluorescence behavior in multilayers of cellulose octadecanoate containing pyrenyl chromophores was investigated in detail by fluorescence spectroscopy. The results are summarized as follows.
(1)A homogeneous LB film with chromophoric groups statistically randomly distributed within each layer was prepared by partially substituting the alkanoyl groups of cellulose trioctadecanoate by a (1-pyreny1)butyroylgroup. The homogeneity of this film and the randomness of the chromophore distribution within each layer were reflected in much less formation of excimers than for the LB films of fatty acids. The introduction of pyrenyl groups as much as 7% (by side chain mole base) did not essentially alter the monolayer characteristics. (2) The pyrene-labeled cellulose octadecanoates did not show interlayer chromophoric interaction, such as energy migration and excimer formation. This is because the layer thickness (2.4 nm) is larger than the Forster radius between pyrenyl moieties and pyrenyl groups are located about in the middle of each layer. (3) In multilayers having a pyrenyl fraction higher than 7 % , two types of excimers were identified by the timeresolved fluorescence spectroscopy. One is a normal excimer with a sandwich arrangement of pyrenyl rings, and the other is a partially overlapped excimer. The emission of the latter type of excimers is dominant in a short-time region, while that of the former is dominant in a long-time region. The existence of the partially overlapped excimer indicates that conformational changes of side chains including chromophores in such a condensed system are restricted significantly. Registry No. PBA, 3443-45-6; Cellulose trioctadecanoate, 39320-20-2.
