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Isomerization and Acid-Base Behavior in Polyion Complex Langmuir-Blodgett Films Robert A. Hall,*,† Masahiko Hara, and Wolfgang Knoll Frontier Research Program, RIKEN, Wakoshi, Saitama 351-01, Japan Received October 10, 1995. In Final Form: January 30, 1996X The nature of an azobenzene moiety incorporated into a polyion complex Langmuir-Blodgett (LB) film over a range of subphase pH values was investigated by UV-visible spectroscopy. The azobenzene unit was substituted with both carboxyl, and hydroxyl groups and the charge per polymer segment was varied by altering the pH of the subphase. The trans isomer of the azobenzene is present in films transferred from a neutral pH subphase. At a higher pH where both the carboxyl and hydroxyl groups are ionized in aqueous solution, the complex LB film formed from this subphase exhibits the cis isomer of the singly charged polymer segment. By comparison with a water-soluble polyion-surfactant complex for which the cis isomer is not observed, it is concluded that the LB film has considerable rigidity which stabilizes the cis isomer. The severely limited photoisomerization of the trans isomer in the film is further indication of the restricted molecular motion within the polyion complex LB film.
Introduction Over the past two decades there has been considerable interest in the use of Langmuir-Blodgett films for the fabrication of a range of nanoscale devices.1-3 One factor limiting many potential applications of LB films is the poor chemical and thermal stability. Over the past 10 years, polyion complex monolayers, in which a charged surfactant is spread onto a subphase containing a polyelectrolyte of opposite charge, have been investigated in an attempt to improve film stability. The initial surface pressure-area isotherm studies4,5 showed that formation of a polyion complex can indeed improve the stability of monolayer films. Although some structural information can be garnered from surface pressure-area isotherms, other methods such as XPS,6-8 direct surface force measurement,9 fluorescence microscopy,10 and FTIR,8,11 UV-visible,12-17 and ESR18 spectroscopies have been employed to obtain knowledge of the polyion complex films on the molecular level. * Author to whom correspondence should be addressed. † Present address: Nufarm Limited, P.O. Box 103, Laverton 3028, Australia. X Abstract published in Advance ACS Abstracts, April 15, 1996. (1) Bryce, M. R.; Petty, M. C. Nature 1995, 374, 771. (2) Ulman, A. An Introduction to Ultrathin Organic Films: from Langmuir-Blodgett to Self-Assembly; Academic Press: San Diego, CA, 1991. (3) Hann, R. A., Bloor, D., Eds. Organic Materials for Non-linear Optics; Royal Society of Chemistry: London, 1989. (4) Shimomura, M.; Kunitake, T. Thin Solid Films 1985, 132, 243. (5) Goddard, E. D. Colloids Surf. 1986, 19, 301. (6) Higashi, N.; Kunitake, T. Chem. Lett. 1986, 105. (7) Takahara, A.; Morotomi, N.; Hiraoka, S.; Higashi, N.; Kunitake, T.; Kajiyama, T. Macromolecules 1989, 22, 617. (8) Lee, B.-J.; Kunitkae, T. Langmuir 1992, 8, 2223. (9) Berndt, P.; Kurihara, K.; Kunitake, T. Langmuir 1992, 8, 2486. (10) Shimomura, M.; Fujii, K.; Karg, P.; Frey, W.; Sackmann, E.; Meller, P.; Ringsdorf, H. Jpn. J. Appl. Phys. 1988, 27, L1761. (11) Umemura, J.; Hishiro, Y.; Kawai, T.; Takenaka, T.; Gotoh, Y.; Fujihira, M. Thin Solid Films 1989, 178, 281. (12) Kimizuka, N.; Kunitake, T. Colloids Surf. 1989, 38, 79. (13) Nishiyama, K.; Kurihara, M.-A.; Fujihira, M. Thin Solid Films 1989, 179, 477. (14) Asano, K.; Miyano, K.; Ui, H.; Shimomura, M.; Ohta, Y. Langmuir 1993, 9, 3587. (15) Hall, R. A.; Thistlethwaite, P. J.; Grieser, F.; Kimizuka, N.; Kunitake, T. J. Phys. Chem. 1993, 97, 11974. (16) Hall, R. A.; Thistlethwaite, P. J.; Grieser, F.; Kimizuka, N.; Kunitake, T. Langmuir 1994, 10, 2743. (17) Hall, R. A.; Thistlethwaite, P. J.; Grieser, F.; Kimizuka, N.; Kunitake, T. Langmuir 1994, 10, 3743. (18) Suga, K.; Iwamoto, Y.; Fujihira, M. Thin Solid Films 1994, 243, 634.
Most of the work to date has focused on polyion complex systems in which both the surfactant and the polyion have a certain fixed charge density. Since the formation of the complex is driven by the electrostatic attraction between surfactant headgroup and polyion, the nature of the resulting complex may be altered by variation in the charge densities of either the monolayer material or the polyion. There are many studies in the literature detailing the effect on various bulk properties of photoisomerization of the azobenzene chromophore. For example, reversible changes in alignment of liquid crystals by azobenzene “command surfaces”19-21 and in the solubility22 and isothermal sol-gel transition23 of polymers incorporating the azobenzene moiety into the backbone or pendant groups of the polymer have been reported. In this paper the effect on the nature of the polyion complex film by varying the charge per polyion segment is investigated. The polyion used in this study contains an azobenzene moiety substituted with both a carboxyl and hydroxyl group; thus the charge per polymer segment can be varied by changing the subphase pH. The acidbase behavior of the polyion in aqueous solution and in polyion complex LB films is monitored by UV-visible spectroscopy. For films prepared from neutral pH, the azobenzene is present as the trans isomer, while the cis isomer predominates for films from high pH subphases. This isomerization occurs without irradiation, and it is shown that the greater rigidity of the LB film compared with a water-soluble polyion complex is necessary to stabilize the cis isomer and thus greatly restricts any photoisomerization. Experimental Method The chromophoric polyion, poly{1-[4-(3-carboxy-4-hydroxyphenylazo)benzenesulfoamido]-1,2-ethanediyl, sodium salt}, was purchased from Sigma Chemical Co., and reprecipitated from methanol into ether three times before use. Dimethyldioctadecylammonium bromide (DDOAB) and dodecyltrimethylammonium bromide (DTAB) were obtained from Tokyo Kasei and used without further purification. The chemical structures of the polyion, DDOAB, and DTAB are given in Figure 1. All (19) Seki, T.; Sakuragi, M.; Kawanishi, Y.; Suzuki, Y.; Tamaki, T.; Fukuda, R.; Ichimura, K. Langmuir 1993, 9, 211. (20) Akiyama, H.; Kudo, K.; Ichimura, K.; Yokoyama, S.; Kakimoto, M.; Imai, Y. Langmuir 1995, 11, 1033. (21) Ikeda, T.; Tsutsumi, O. Science 1995, 268, 1873. (22) Irie, M.; Tanaka, H. Macromolecules 1983, 16, 210. (23) Irie, M.; Iga, R. Macromolecules 1986, 19, 2480.
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Figure 1. Chemical structures of the polyion, DDOAB, and DTAB.
Figure 2. Absorption spectra from the titration of the polyion in aqueous solution at pH 10.03, 12.01, 12.47, 12.75, 12.91, 13.05, 13.2, 13.3, and 13.4. The arrow indicates the direction of increasing pH. aqueous solutions and monolayer subphases were prepared from Milli-Q filtered water, and the pH was adjusted by the addition of analytical grade NaOH. In both monolayer and aqueous solution experiments the concentration of the polyion was 1.0 × 10-5 M. All measurements were carried out at room temperature, 21 ( 1 °C. Monolayer experiments were performed using a KSV-5000 Teflon Langmuir trough. Surface pressure was measured by the Wilhelmy plate method, employing a 15 mm wide platinum plate. The spreading solvent for DDOAB was chloroform. Two monolayers were transferred at 25 mN m-1 onto each side of quartz slides which had been treated with dodecanol to render them hydrophobic.24 UV-visible absorption spectra were recorded using a Shimadzu UV-3100PC spectrometer. In general, the angle of incidence was 90°. For the polarization measurements, the angle of incidence was 45°. The films were irradiated by a xenon lamp with either a band pass filter (360 ( 20 nm) or a high pass filter (470 nm).
Results and Discussion Polyion in Aqueous Solution. The ionization behavior of the polyion in aqueous solution is indicated by the absorption spectra given in Figure 2. At pH 10 the spectrum displays absorption maxima at 267 and 363 nm, and with increasing pH the 363 nm band diminishes and a band at 450 nm grows in. The 363 nm band is ascribed to the π,π* transition of the substituted azobenzene bearing the carboxylate anion, while the 450 nm band is assigned to the quinoidal form of the dianion, for which both the carboxylic acid and hydroxyl groups are ionized. Isosbestic points exist at 304 and 389 nm. While there is appreciable ionization of the hydroxyl group up to pH (24) Trau, M.; Murray, B. S.; Grant, K.; Grieser, F. J. Colloid Interface Sci. 1992, 148, 182.
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Figure 3. Surface pressure-area isotherms of DDOAB on water and polyion solution subphases at various pH values.
13, it is apparent that as the ionization proceeds it becomes progressively more difficult to form the dianion. Such a local charge effect is general for ionizable polyelectrolytes25,26 and arises from the increased work to remove a proton from an acid site on the polyion to the bulk aqueous phase as the electrostatic potential on the polyion backbone becomes more negative. In the present system, at pH 10 the hydroxyl group is located adjacent to the carboxylate anion, and in the vicinity of other carboxylate anions along the polymer backbone, and thus experiences considerable negative electrostatic potential. As the titration proceeds, the electrostatic potential sensed by the hydroxyl group becomes more negative, since in addition to the carboxylate anions there are now also ionized hydroxyl groups distributed along the backbone. The consequences of the local charge effect on the ionization of the polyion can be modeled by assuming the polyelectrolyte molecule to be a cylindrical rod with uniform surface charge distribution and applying the Poisson-Boltzmann equation.27,28 For the purpose of this paper, it is sufficient to recognize qualitatively the effect of the electrostatic potential of the polyion backbone in increasing the apparent pK of the hydroxyl group. Isotherms of DDOAB. Shown in Figure 3 are the surface pressure-area isotherms of DDOAB on water and polyion subphases as a function of pH. It has been reported that the isotherm of DDOAB is sensitive to experimental conditions and, in particular, the concentration and type of counterion.29 A comparison of the isotherm of DDOAB on pure water given in Figure 3 with others reported in the literature10,14,30 reveals good agreement. Although there is some difference between the isotherm presented here and that of Marra,29 this may be understood by the different electrolyte concentration and spreading solvent. For a subphase containing the azobenzene-substituted polyion, the molecular area occupied by the DDOAB/ polyion complex is greater than that for DDOAB on pure water. The greater molecular area can be attributed to the adsorption of the polyion with the pendant azobenzene substituent having an orientation close to the surface plane. This is supported by polarized absorption results (presented below) and also the work of Asano et al.14 using polarized reflection spectra of a polyion complex monolayer (25) Morawertz, H. Macromolecules in Solution; Interscience: New York, 1966. (26) Dautzenberg, H.; Jaeger, W.; Ko¨tz, J.; Philipp, B.; Seidel, C.; Stscherbina, D. Polyelectrolytes: Formation, Characterization and Application; Hanser: Munich, 1994. (27) Kotin, L.; Nasagawa, M. J. Chem. Phys. 1962, 36, 873. (28) Nitta, K.; Sugai, S. J. Phys. Chem. 1974, 78, 12. (29) Marra, J. J. Phys. Chem. 1986, 90, 2145. (30) Kimizuka, N.; Tsukamoto, M.; Kunitake, T. Chem. Lett. 1989, 909.
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Langmuir, Vol. 12, No. 10, 1996 2553 Table 1. Polarized Absorption at 370 nm of Polyion Complex LB Films from Subphases of pH 7.5, 12.0, and 13.0
Figure 4. Absorption spectra of DDOAB/polyion complex films transferred from subphases of different pH.
having a similar chromophoric polyion. Some difference is observed in the isotherm as a function of subphase pH, with a shift to smaller areas as the pH increases from 7.5 to 13.0, and a small change in the shape of the isotherm beyond pH 12.0. Surface pressure-area isotherms obtained from subphases of different pH may be used to investigate the degree of ionization by plotting the area per molecule at a certain pressure against pH. Shimomura et al.31 used this technique to investigate the ionization of poly(acrylic acid) bound to a cationic surfactant and suggested that the large change in the area per molecule around pH 6 was related to the pKa of poly(acrylic acid) in solution being 5.6. In the present work, the absorption spectra of the polyion in aqueous solution indicate that at pH 12.0 nearly all of the OH groups are neutral, while at pH 13.0 approximately half are ionized. Thus, if the OH group underwent ionization at the interface, one might anticipate a significant difference in the areas per molecule at pH 12.0 and 13.0. As the areas per molecule at pH 12.0 and 13.0 are very similar, this suggests that ionization of the OH group at the air-water interface does not occur to any appreciable degree below at least pH 13. DDOAB/Polyion Complex Films. Monolayers of DDOAB/polyion complexes were transferred efficiently to hydrophobic substrates, with the transfer ratios being in the range 0.92 to 1.05 in all cases. Figure 4 shows the absorption spectra of the films transferred from subphases of pH 7.5, 12.0, and 13.0. The spectra differ markedly depending upon the subphase pH; however, this difference does not follow the absorption behavior of the polyion in aqueous solution. A strong similarity is observed between the film prepared from a pH 7.5 subphase and the aqueous solution of the polyion at the same pH. The general shape of the spectrum is the same, and there is a red shift in the wavelength maxima (λmax) of the π,π* absorption band, from 362 nm in solution to 374 nm for the LB film. Other workers have reported such a shift in λmax for azobenzene chromophores in a cyclodextrin inclusion complex32 and amphiphilic polyions.33 This red shift has been attributed to a lower polarity of the microenvironment sensed by the azobenzene chromophore and is consistent with the change in polarity from aqueous solution to the “dry” LB film in the present system. For the film from a pH 13 subphase, the λmax is blue shifted compared to the film prepared from a pH 7.5 subphase (from 374 to 368 nm) and there is also a considerable reduction in the absorbance of this band. (31) Shimomura, M.; Kasuga, K.; Tsukada, T. J. Chem. Soc., Chem. Commun. 1991, 845. (32) Bortolus, P.; Monti, S. J. Phys. Chem. 1987, 91, 5046. (33) Morishima, Y.; Tsuji, M.; Kamachi, M.; Hatasa, K. Macromolecules 1992, 25, 4406.
subphase pH
∆Ts/∆Tp
θ (deg)
7.5 12.0 13.0
1.51 1.38 1.27
82 76 72
Furthermore, the reduction in intensity of this band is not accompanied by a commensurate growth of an absorption band at about 450 nm corresponding to the quinoidal from of the dianion, which is observed in aqueous solution at high pH. Both the intensity and wavelength maxima of the π,π* band of the film prepared from a pH 12.0 subphase show intermediate values compared to the spectra of films from pH 7.5 and pH 13.0 subphases, although they are somewhat closer to that observed for the pH 7.5 case. Polarized Absorption Spectra. As the π,π* transition dipole lies close to the long molecular axis of trans azobenzene,34 the difference in the intensity of the π,π* band of the LB films with subphases of pH 7.5 and 13.0 may be due to a change in the chromophore orientation. These spectra are measured at normal incidence, so if the orientation of the chromophore changes to closer to the surface normal, one would anticipate the absorption intensity to be reduced. Absorbance of LB films at 370 nm prepared from subphases of pH 7.5, 12.0, and 13.0 were measured at 45° incidence using both s- and p-polarized light. The ratios of the transmittance change of the s-polarized light with respect to that of the p-polarized, ∆Ts/∆Tp, are given in Table 1. Using the method of Orrit et al.,35 the orientation angles from the surface normal, θ, have been determined and are also presented in the table. It can be seen that while there is a small change with subphase pH, the chromophore is oriented close to the surface plane in all cases, with tilt angles between 82° and 72°. This slight change in orientation is insufficient to account for the large difference in intensity at 370 nm measured in the films from various subphase pH values using normal incidence. Thus some mechanism other than reorientation of the chromophore must be responsible. Shift of pKa of the Hydroxyl Group. It is well established that the pKa of a species located at an interface may be different from that in bulk solution, and this has been reported for micellar systems,36,37 monolayer films,17,38-40 and polyelectrolyte solutions.41,42 Briefly, the two factors which contribute to the shift in pKa at an interface compared to bulk solution are the different dielectric microenvironment and the electrostatic potential at the interface. In the present system, these two factors will tend to have opposing influences on the position of the acid-base equilibrium of the hydroxyl group. For the polyion in bulk solution at pH 13.0, the hydroxyl group is located in the vicinity of ionized carboxylic acid groups and some dissociated hyroxyl groups, and this negative electrostatic potential contributes to an increase in the apparent pKa, as mentioned earlier. The monolayer (34) Beveridge, D. L.; Jaffe, H. H. J. Am. Chem. Soc. 1966, 88, 1948. (35) Orrit, M.; Mo¨bius, D.; Lehmann, U.; Meyer, H. J. Chem. Phys. 1986, 85, 4966. (36) Fernandez, M. S.; Fromherz, P. J. Phys. Chem. 1977, 81, 1755. (37) Grieser, F.; Drummond, C. J. Phys. Chem. 1988, 92, 5580. (38) Lovelock, B.; Grieser, F.; Healy, T. W. J. Phys. Chem. 1985, 89, 501. (39) Petrov, J. G.; Mo¨bius, D. Langmuir 1989, 5, 523. (40) Petrov, J. G.; Mo¨bius, D. Langmuir 1990, 6, 746. (41) Morishima, Y.; Kobayashi, T.; Nozakura, S.-I. Macromolecules 1988, 21, 101. (42) Morishima, Y.; Higuchi, Y.; Kamachi, M. J. Polym. Sci., Part A: Polym. Chem. 1991, 29, 677.
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Figure 5. Absorption spectra of the aqueous solution of the polyion at pH 13.2 with 0, 5 × 10-6, 1 × 10-5, 5 × 10-5, and 2 × 10-4 M DTAB. The arrow indicates the direction of increasing DTAB concentration.
film of DDOAB has considerable positive electrostatic potential, so the complexation of the polyion to the monolayer would tend to reduce the net negative electrostatic potential and, hence, tend to make the ionization of the hydroxyl group more favorable than in bulk solution. With regard to the dielectric nature of the environments, both the aqueous polyelectrolyte solution and the “dry” LB film have dielectric environments different to bulk solution. Morishima et al.41,42 reported dielectric constants in the range 59-70 at the molecular surface of polyions in aqueous solution, while the headgroup region of LB films submerged in water is significantly nonpolar,43 and it would be anticipated that the same region in the “dry” LB film would be even less polar. This considerable difference in polarity of the polyion solution and the dry LB film environments would tend to shift the acid-base equilibrium of the hydroxyl group in the LB film in favor of the neutral form. In order to investigate further the effects of the dielectric environment and the electrostatic potential on the pKa, a water-soluble surfactant-polyion complex was formed by addition of DTAB to the polyion solution. Water-soluble surfactant-polyion complexes have been reported to have microenvironments similar to that of micelles and, hence, appreciably less polar than that of the polyion aqueous solution.44,45 Spectra of the polyion in aqueous solution at pH 13.2 with various amounts of DTAB added are given in Figure 5. As the amount of added surfactant is increased, the equilibrium shifts from favoring the dianion to favoring the monoanion, indicative of a progressively less polar environment. Above 5 × 10-4 M DTAB the complex begins to precipitate. It is clear from a comparison of the spectrum with no added DTAB and that with 2 × 10-4 M DTAB that the complexation produces a shift in the pKa to higher values. This indicates that the shift of the pKa is determined by the dielectric of the environment, and the net reduction in negative electrostatic potential by complexation of the ammonium surfactant has little effect. Since the “dry” LB film is less polar than the water-soluble polyion complex, one would expect that the pKa in the monolayer shifts to a much higher value, making the dianion unobservable in the LB film, even at pH 13.0. Thus the spectral differences observed for the LB films in going from pH 7.5 to 13.0 cannot be attributed to ionization of the monoanion to the dianion. (43) Murphy, A.; Grieser, F.; Furlong, D. N. Thin Solid Films 1993, 227, 211. (44) Chu, D.-Y.; Thomas, J. K. J. Am. Chem. Soc. 1986, 108, 6270. (45) Schild, H. G.; Tirrell, D. A. Langmuir 1990, 6, 1676.
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Trans-Cis Isomerization. As the change in absorption spectra of the films from subphases of differing pH is not commensurate with the titration of the polyion, other factors such as the isomerization of the azobenzene moiety must contribute to the observed spectral change. The absorption spectrum of the LB film from a pH 13 subphase is very similar to that reported for the cis isomer of azobenzene systems in polymeric22,33 and monolayer13 environments. In those systems the spectrum of the cis isomer differs from the trans isomer in several respects. Firstly, the band at about 350 nm shows a blue shift and has an intensity of approximately 40% of that of the trans isomer. Also, in the spectrum of the cis isomer, the presence of a weaker band at about 450 nm produces an appreciable tail at longer wavelengths, and there is a greater intensity in the region 290-300 nm compared with the trans isomer. Since all these characteristics are observed for the LB film transferred from a subphase of pH 13 when compared to that for the case of pH 7.5, it is apparent that the azobenzene moieties in the film prepared from the pH 13 subphase are present predominantly in the cis form. Photochemical reaction cycles of azobenzene and stilbene derivatives have been reported in the literature.46,47 However in those cases, there is no direct chemical or electrochemical transformation from a trans isomer to the corresponding cis isomersirradiation is required for the isomerization, whereas in the present case the formation of the cis isomer occurs without irradiation. The observation of the cis isomer in the LB film from the pH 13 subphase, where none is present in the film from a pH 7.5 subphase, nor in the water-soluble polyion complex at pH 13, may be understood as follows. In the aqueous solution of the polyion at pH 13, the substituted azobenzene moiety has the carboxylic and most of the hydroxylic groups ionized. The dianion, in which both the carboxylic and hydroxyl groups are ionized, converts rapidly to the quinoidal structure allowing free rotation about the now single N-N bond. As complexation to the ammonium surfactant takes place, the polyion experiences a significantly less polar environment, and the dianion is no longer stable with respect to the monoanion. Protonation of the quinoidal species can generate either the trans or cis isomer of the monoanion. The water-soluble polyion complex has significant freedom of molecular motion, and thus the more labile cis isomer is not observed. In the constrained geometry of the monolayer system, the quinoidal species is preferentially converted to the isomer which occupies the least molecular area. The orientational angles given in Table 1 indicate that the long axis of the azobenzene unit lies almost parallel to the interface, so the “least molecular area” of the azobenzene is for this orientation. Previously it has been reported that relaxation in a constrained environment favors the trans isomer,48 and surface pressure area isotherms of azobenzene amphiphiles suggest that the cis isomer is more bulky.49 However in those cases, the long axis of the azobenzene unit was oriented normal to the interface, whereas the current system has the long axis almost parallel. Given the different orientation of the azobenzene unit in the present situation, it is not unreasonable that protonation of the quinoidal structure might favor the cis isomer. (46) Steiner, U.; Abdel-Kader, M. H.; Fischer, P.; Kramer, H. E. A. J. Am. Chem. Soc. 1978, 100, 3190. (47) Liu, Z. F.; Hashimoto, K.; Fujishima, A. Nature 1990, 347, 658. (48) Kumano, A.; Niwa, O.; Kajiyama, T.; Takayanagi, M.; Kunitake, T.; Kano, K. Polym. J. 1984, 16, 461. (49) Sandhu, S. S.; Yianni, Y. P.; Morgan, C. G.; Taylor, D. M.; Zaba, B. Biochim. Biophys. Acta 1986, 860, 253.
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Figure 6. Absorption spectra of DDOAB/polyion complex film from a pH 7.5 subphase as deposited, with 1 h exposure to 360 nm light, and then with 1 h exposure to 470 nm light.
In summary, a significant amount of the cis isomer can only be observed via a reaction pathway which involves deprotonating the trans isomer to form a quinoidal species and the reprotonation of the quinoidal species in an environment which has sufficient steric constraint to favor the cis isomer. Photoisomerization of Polyion Complex Films. The structural rigidity of the hydrophilic region of the LB films has been investigated by irradiation of the film in an attempt to photoisomerize the azobenzene chromophore. Shown in Figure 6 are the absorption spectra of the polyion complex LB film prepared from a pH 7.5 subphase with two periods of exposure. Irradiation of the film for 1 h with 360 nm light produces some reduction in intensity of the π,π* band, a shift in the λmax to 371 nm, and a slight growth in the long wavelength tail. Further irradiation with 360 nm light produces no change in the spectrum. Some recovery of intensity of the π,π* band can be observed if the film is then exposed to 470 nm light, but a spectrum identical to that of the film as deposited is not obtained. Exposure of the film prepared from a pH 13 subphase to 470 nm light (shown in Figure 7) has no significant effect on the spectrum, even with 3 h of irradiation. The cis isomer was not observed in either the polyion aqueous solution or the DTAB-polyion water-soluble complex over the range of solution pH 7-13, even with irradiation at 350 nm. These results show that in the LB
Langmuir, Vol. 12, No. 10, 1996 2555
Figure 7. Absorption spectra of DDOAB/polyion complex film from a pH 13.0 subphase as deposited and with 3 h of exposure to 470 nm light.
film only slight photoisomerization is possible, indicating that the molecular conformation present in the film when it is transferred to the substrate is largely maintained under irradiation. Conclusion The effects on the acid-base and isomerization behavior of polyion complex LB films have been investigated by UV-visible spectroscopy. The trans isomer of a substituted azobenzene dominates in bulk solution at neutral pH, and LB films formed using this subphase also display the trans isomer. At higher pH values of the polyion solution, the ionization of the hydroxyl group produces the quinoidal form. LB films transferred from this subphase show none of the quinoidal form but exhibit the cis isomer. The lower dielectric environment of the monolayer-water interface compared with bulk polyion solution results in a protonation of the quinoidal form, and the restricted geometry of the monolayer interface favors the cis isomer from the protonation of the quinoidal form. The polyion complex LB film has limited molecular motion which prevents any appreciable amount of photoisomerization. Acknowledgment. R.A.H. gratefully acknowledges the receipt of a Science and Technology Agency (Japan) Postdoctoral Fellowship. LA9508553
