Photoisomerization of Polyionic Layer-by-Layer Films Containing

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Langmuir 1999, 15, 193-201

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Photoisomerization of Polyionic Layer-by-Layer Films Containing Azobenzene Silvia Dante,†,‡ Rigoberto Advincula,‡ Curtis W. Frank,*,‡ and Pieter Stroeve*,† Center on Polymer Interfaces and Macromolecular Assemblies (CPIMA), Department of Chemical Engineering and Materials Science, University of California, Davis, California 95616, and Department of Chemical Engineering, Stanford University, Stanford, California 94305 Received April 29, 1998. In Final Form: November 3, 1998 In this work we employed the layer-by-layer adsorption technique for deposition on solid substrates of polyionic films containing photoactive azobenzene groups. We investigated two systems, each having the same polyanion but using a different polycation. Poly {1-4[4-(3-carboxy-4-hydroxyphenylazo)benzenesulfonamido]-1,2-ethanediyl sodium salt} (PAZO) was employed as the photoactive polyanion; poly(diallyldimethylammonium chloride) (PDDA) and poly(ethyleneimine) (PEI) were used as the polycations. Our phenomenological data show dramatic differences in the behavior of the two systems, although the same experimental conditions were employed in both cases. The assembly of the multilayers was monitored by ellipsometry and X-ray reflectivity via thickness measurements. We observed a considerable difference in the bilayer thickness in the two systems. An average polycation/polyanion bilayer thickness of 5 Å was measured for PDDA/PAZO, while the PEI-containing system resulted in a 36 Å thick bilayer. We used quartz crystal microbalance (QCM) measurements and UV-visible spectroscopy to monitor the adsorption process. QCM measurements showed an influence of the polycation in the polyanion adsorption process of the PAZO molecules. In particular, PEI appears to promote complexation and aggregation of the negatively charged polyion. Aggregates, mainly in the J form, were detected in both PDDA/PAZO and PEI/PAZO systems by UV-visible spectroscopy. We induced trans-to-cis photoisomerization of the azobenzene groups by UV light (340 nm), and we followed the photoreaction by the decrease in the intensity of the π-π* band, which is associated with the trans form of the azo molecules. The photoreaction apparently did not reach completion because the π-π* band did not completely disappear. We found also that the polycations have a significant influence on the molecular orientation of the azobenzene groups in the film and on the photoisomerization kinetics. The kinetics of photoisomerization were not monoexponential, indicating the coexistence of different processes. We investigated also the cis-to-trans reverse isomerization. In particular, we observed a partial recovery of the π-π* band after thermal relaxation that was more significant in the PDDA-containing system. By contrast, cis-to-trans isomerization induced by blue light (460 nm) was not observed. UV light irradiation was responsible for reversible changes in the optical thickness of the films, defined as n × d, where n is the refractive index and d is the overall thickness of the film.

Introduction When a photochemical reaction takes place in a macromolecular environment, factors such as the molecular mobility and the free volume available to the active site influence the reaction kinetics. Cis-to-trans isomerism of azobenzene and its derivatives is a well-studied phenomenon and perhaps is the simplest photochromic reaction known to date.1 This reaction involves the reversible transformation of the more stable trans isomer of an azo group into its less stable cis isomer upon light irradiation.2 The incorporation of a photochromic moiety in polymers is a very attractive molecular design option due to the possibility of creating new light-sensitive materials and optical devices.3 Even though it is known that the nature and morphology of a polymer influence both photo- and thermochromism of a chromophore in a given polymer matrix, no complete theory exists to explain how a * To whom correspondence should be addressed. E-mail: [email protected], Telephone: (530) 752-8778. Fax: (530) 7521031. † University of California, Davis. ‡ Stanford University. (1) Kumar, G. S.; Neckers, D. C. Chem. Rev. 1989, 89, 1915. (2) Rau, H. In Photoisomerization of Azobenzenes in Photochemistry and Photophysics; Rabek, J. F., Eds.; CRC Press: Boca Raton, FL, 1990; Vol. II, pp 119-141. (3) Sekkat, Z.; Knoll, W. SPIE 1997, 2998, 164.

photochromic process is linked to polymer properties.4 For example, the local arrangement of the polymeric environment and its related properties of polarity, viscosity, and Tg may affect the photophysical and photochemical event.4 On the other hand, the conformational change induced by the photochemical reaction can be exploited to change the microscopic and macroscopic properties of the polymer matrix. For instance, photoreversible changes in the bulk properties of spin-cast films (including viscosity,5-8 wettability,9-11 conductivity,12 and permeability13,14 have been reported in the literature. In recent years, much research has been focused on the characterization of the photoresponse of oriented macromolecular assemblies such as Langmuir-Blodgett films (4) Williams, J. L. R.; Daly, R. C. Prog. Polym. Sci. 1977, 5, 61. (5) Irie, M.; Menju, A.; Hayashi, K. Macromolecules 1979, 12, 1176. (6) Irie, M.; Hiramo, K.; Hashimoto, S.; Hayashi, K. Macromolecules 1981, 14, 262. (7) Blair, H. S.; Pogue, H. I.; Riordan, E. Polymer 1980, 21, 1195. (8) Kumar, G. S.; Depra, P.; Neckers, D. C. Macromolecules 1984, 17, 1912. (9) Ishihara, K.; Hawada, N.; Kato, S.; Shinohara, I. J. Polym. Sci. Polym. Chem. Ed. 1983, 21, 1551. (10) Negishi, K. N.; Shinohara, I. J. Appl. Polym. Sci. 1982, 27, 1897. (11) Irie, M.; Iga, R. Makromol. Chem., Rapid. Commun. 1987, 8, 569. (12) Irie, M.; Tanaka, H. Macromolecules 1983, 16, 210. (13) Anzai, J.; Sasaki, H.; Ueno, A.; Osa, T. Chem. Lett. 1984, 7, 1205. (14) Kumano, A.; Niwa, O.; Kajiyama, T.; Takayanagi, M.; Kano, K.; Shinkai, S. Chem. Lett. 1983, 8, 1327.

10.1021/la980497e CCC: $18.00 © 1999 American Chemical Society Published on Web 12/11/1998

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and self-assembled monolayers containing azo dyes.15-19 The assemblies are considered to be very promising materials for optical information storage,20-23 for light switching and nonlinear devices,24 and also for liquid crystal alignment.25-27 More recently, polymer films assembled by layer-by-layer adsorption through a charge reversal mechanism have been shown to provide wellordered layered structures.28-33 In this work, we apply the polyionic layer-by-layer adsorption technique to build up multilayers of two alternating polycation/polyanion systems. The influence of the polycation on the film structure and on the photoresponse of the azo-containing polyanion has been investigated. We have focused particular attention on the thickness measurement and on its change due to UV light irradiation. Experimental Section Materials. Poly{1-4[4-(3-carboxy-4-hydroxyphenylazo)benzenesulfonamido]-1,2-ethanediyl sodium salt} (PAZO; Aldrich), poly(diallyldimethylammonium chloride) (PDDA; Aldrich), and poly(ethyleneimine) (PEI; MW 70 000; Aldrich) were commercially available and used without further purification. The molecular structures of the polyions used are shown in Figure 1. Aqueous solutions (Milli-Q water with 18.2 MΩ resistivity) of the polymers, with a concentration of 1 mM per repeat unit, were freshly prepared for each experiment. The polyions are strongly charged at neutral pH. Hellmanex solution (2%) (Hellma, Germany) was used to clean the substrates. Mercaptopyridine (MW 112.2; Aldrich) and (3-aminopropyl)triethoxysilane (APS; Aldrich) were employed in the substrate coating. All solvents were spectroscopic grade and obtained from Aldrich. Layer-by-Layer Adsorption. The layer-by-layer adsorption of the polyanion and polycation was performed following the Decher approach.28 Quartz slides (25 mm × (15) Roberts, G. G., Ed. Langmuir-Blodgett Films; Plenum Press: New York, 1990. (16) Ulman, A. Introduction To Ultrathin Organic Films, Academic Press: San Diego, CA, 1991, p 133. (17) Polymeropouplous, E. E.; Mobius, D.; Kuhn, H. J. Chem. Phys, 1978, 68, 3918. (18) Aktsipetrov, O. A.; Akhimediev, N. N.; Mishima, E. D.; Novak, V. R. JETP Lett. (Engl. Transl.) 1983, 37, 207. (19) Blinov, L. M.; Dubinin, N. V.; Mikhnev, L. V.; Yudin, S. G. Thin Solid Films 1984, 120, 161. (20) Rochon, P.; Bissonnette, D.; Natansohn, A.; Xie, S. Appl. Opt. 1993, 32 (35), 7277. (21) Ikeda, T.; Tsutsumi, O. Science 1995, 268, 1873. (22) Yokoi, T. M.; Fukuda, R.; Tamaki, T.; Ichimura, K. Advanced Materials ‘93 II/A: Biomaterials, Organic and Intelligent Materials; Aoki, H., et al., Eds.; Trans. Mater. Res. Soc. Jpn. Vol. 15A; Elsevier Science B. V.: New York, 1994; p 361. (23) Gibbons, W. M.; Shannon, P. J.; Swetlin, S. T.; Sun, B. J. Nature 1991, 351, 49. (24) Wang, X.; Balasubramanian, S.; Li, L.; Jiang, X.; Sandman, D. J.; Rubner, M. F.; Kumar, J.; Tripathy, S. K. Macromol. Rapid Commun. 1997, 18, 451. (25) Ichimura, K.; Seki, T.; Kawanishi, Y.; Suzuki, Y.; Sakuragi, M.; Tamaki, T. In Photoreactive Material for Ultrahigh-Density Optical Memory; Irie, M., Ed.; Elsevier: Amsterdam, 1994; p 55. (26) Seki, T.; Sakuragi, M.; Kawanishi, Y.; Suzuki, Y.; Tamaki, T.; Fukuda, R.; Ichimura, K. Langmuir 1993, 9, 211. (27) Seki, T.; Tanigaki, N.; Yase, K.; Kaito, A.; Tamaki, T.; Ueno, K.; Tanaka, Y. Macromolecules 1995, 28, 5609. (28) Decher, G.; Hong, J. D. Ber. Bunsen-Ges. Phys. Chem. 1991, 95, 1430. (29) Decher, G.; Hong, J. D.; Schmitt, J. Thin Solid Films 1992, 210/ 211, 831. (30) Decher, G.; Lvov, Y.; Schmitt, J. Thin Solid Films 1994, 244, 772. (31) Lvov, Y.; Ariga, K.; Ichinose, I.; Kunitake, T. J. Chem. Soc., Chem. Commun. 1995, 2313. (32) Ferreira, M.; Rubner, M. F. Macromolecules 1995, 28, 7107. (33) Fou, A. C.; Rubner, M. F. Macromolecules 1995, 28, 7115.

Figure 1. Chemical formulae of the polyions.

40 mm × 1 mm) were used as substrates for the UVvisible spectroscopy, while silicon wafers (25 mm × 40 mm × 0.5 mm) were used for ellipsometry, and float glass slides (25 mm × 40 mm × 1 mm), for X-ray reflectivity measurements. The substrates were cleaned according to the following procedure:34 sonication in propanol (15 min), rinsing with Milli-Q water (10 times), sonication (two times) in Hellmanex solution (20 min), and copious rinsing with double-distilled water. The substrates were then treated with APS (0.1% acetone solution) and used immediately after preparation. The APS layer was charged by immersion in HCl solution (pH ) 1-2) before deposition. Quartz crystals (Maxtek Inc.; area ) 6.45 cm2) with evaporated gold electrodes (area ) 1.19 cm2) were used as substrates for the quartz crystal microbalance measurements. These substrates were plasma cleaned and immersed for several hours in an ethanol solution of mercaptopyridine at 0.1 mM. Surface charge was induced by dipping the substrate in HCl (pH ) 1-2) before starting the deposition. The polyion deposition was carried out using a HMS Series Programmable Slide Stainer apparatus (Carl Zeiss, Inc). The immersion time in each polymer solution (1 mM of repeat unit) was 15 min, followed by 2 min of immersion in distilled water and then 1 min in a cell with flowing distilled water. The rinsing solutions were changed after adsorption of 5 bilayers. Up to 90 bilayers were deposited for the PDDA/PAZO system, and up to 50 bilayers were deposited for the PEI/PAZO system. Measurements. UV-visible absorption spectra were recorded with a Hewlett-Packard 8452A diode array spectrophotometer. The samples were irradiated using a Hg lamp (Oriel, 50 W) equipped with glass band-pass filters (340 and 460 nm for UV and blue light). To avoid heating effects during long time exposure, the samples were cooled with water from a constant-temperature bath. A Hewlett-Packard E5100A Network Analyzer was used for the quartz crystal microbalance (QCM) measurements. The frequency was followed continuously by a universal frequency counter attached to a microcomputer system. The layer-by-layer deposition was performed in-situ, dipping the QCM holder alternatively in the polyion and (34) Cassier, T. Diploma Thesis, University of Mainz (Germany), 1994.

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Figure 2. X-ray reflectivity spectra for 20, 30, 40, and 50 bilayers of PDDA/PAZO. The best fit curves are reported as solid lines.

Figure 3. Thickness as a function of the number of bilayers measured by ellipsometry (circles) and X-ray reflectivity (squares) for PDDA/PAZO (open) and PEI/PAZ0 (filled) samples. Note the differences in thickness scales.

in the rinsing solutions and gently drying the quartz with a nitrogen gas flow before each frequency measurement. Ellipsometry was performed with a Gaertner L116 C ellipsometer. X-ray reflectivity was carried out with a Rigaku RU-H3R (18 kW) rotating anode high-power X-ray apparatus (Cu KR, λ ) 1.54 Å) equipped with a HV/PHA scintillation counter and operating in the θ-θ geometry at 48 kV/160 mA. Results Layer-by-Layer Adsorption. Figure 2 shows the X-ray reflectivity curves obtained from a series of PDDA/ PAZO samples, ranging from 20 to 50 bilayers. For total external reflection, the critical angle is determined by the average electron density of the sample according to35

Rc ) λ(reFe/π)1/2

(1)

where λ is the wavelength of the X-rays, re is the classical electron radius, and Fe is the average electron density near the surface. From the measured curves Rc was determined to be 0.23 for all samples. This value is in good agreement with the number calculated from the electron density of glass,36 which in this case gives the main contribution to the average value. The main features of the measured reflectivity curves are the Kiessig fringes of increasing periodicity as a function of the number of layers. By calculating the reflectivity from a model structure using the recursion scheme of Parratt,35 we determined the thickness and the average electron density values of the polymer layer on the substrate by a leastsquares fitting procedure. A two-slab model was used for the structure simulation, where the substrate and the polymer film were considered as layers with uniform refractive index. The corresponding reflectivity curves are included in Figure 2 as solid lines. The thickness values obtained from the simulations are reported in Figure 3. Together with electron density and layer thickness, we fitted the interface and the surface roughness. The latter is included in the calculation scheme using the approach of Weber and Lengeler.37 Ellipsometry, as an optical spectroscopic technique, also provides information on the overall structure of the sample, including film thicknesses and density. In most (35) Parrat, L. G. Phys. Rev. 1954, 95, 359. (36) Nevot, L.; Croce, P. Rev. Phys. Appl. 1980, 15, 761. (37) Weber, W.; Lengeler, B. Phys. Rev. B 1992, 46, 7953.

Figure 4. UV-vis spectra of (a) 10-5 M PAZO in aqueous solution, (b) 20 bilayers of PDDA/PAZO, and (c) 20 bilayers of PEI/PAZO. A red shift in the maximum absorbance of the π-π* transition is clearly visible in the PEI/PAZO spectrum.

ellipsometric work on thin organic films, it is assumed that the organic layers are isotropic and nonabsorbing. With this assumption, the thicknesses measured by ellipsometry are reported in Figure 3 along with the thicknesses estimated by the X-ray reflectivity fitting procedure. The thickness increases linearly with the number of bilayers, and there exists good agreement between the values obtained by the two different techniques. As is evident from Figure 3, the two systems result in remarkably different thicknesses per polycation/polyanion bilayer. The average values are 5 and 36 Å/bilayer for PDDA/PAZO and PEI/PAZO, respectively. The deposition of the polymer layers was also monitored by UV-visible spectroscopy and QCM measurements. Figure 4 reports the UV-visible spectra of 20 bilayers of PEI/PAZO and PDDA/PAZO. For comparison, the spectrum of a 10-4 M solution of PAZO in water is reported as well. The strong absorption at approximately 366 nm is due to the π-π* transition of the trans-azobenzene isomer.2 The absorption at 260 nm is attributed to a transition with the electronic transition moment roughly parallel to the short axis of the trans-azobenzene chromophore.2 In the PEI/PAZO film, the maximum, which is centered at 366 nm in the polymer solution, is red shifted by 8 nm. The absorption spectra, obtained from two series of PDDA/PAZO and PEI/PAZO samples with increasing number of bilayers, are shown in Figure 5. When PDDA is used as the polycation, a red shift in the maximum

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Figure 7. Frequency decrease as a function of the number of deposited layers as measured with the QCM apparatus. In the PEI/PAZO system the frequency changes by steps, after each bilayer absorption; in the other system, a frequency increase is observed after both the polyanion (PAZO) and the polycation (PDDA) depositions. A larger amount of PAZO is deposited in each cycle when the polycation is PEI. Note also the linear behavior of the frequency shift against the number of bilayers.

Figure 5. (a) UV-vis spectra of a series of PDDA/PAZO samples containing different numbers of bilayers (20-90). A red shift is observed as the thickness increases. (b) UV-vis spectra of a series of PEI/PAZO (10, 20, 30, 40, 50 bilayers).

direction (to the substrate). On the other hand, in the PDDA/PAZO system a change in the slope is detected after the deposition of 20 bilayers. Since the measured thickness per bilayer remains unchanged with the number of deposited bilayers, this slope change may be due to development of progressive disorder in the arrangement of the azo groups and not due to a decrease in the amount of transferred mass with the number of dipping cycles. Finally, a significantly higher absorbance is observed in the PEI-containing samples. Additional information on the adsorption process of the multilayers comes from the QCM measurements. As is well-known, the frequency shift is correlated to the adsorbed mass by the Sauerbrey equation:39

∆F ) -{2F0/A(Fqµq)1/2}∆m

(2)

absorbance is detected as a function of the film thickness, suggesting the formation of aggregates38 in the films with an increase in the number of deposited layers. In comparison, the position of the same maximum in the PEI/PAZO samples, which is initially located at 372 nm, undergoes a blue shift with an increase in the bilayer number. For the PEI/PAZO system the increase of the maximum absorbance with the number of layers (Figure 6) indicates uniform deposition in the perpendicular

where F0 is the parent frequency of the QCM (5 MHz), A is the electrode area (1.19 cm2), Fq is the density of quartz (2.65 g cm-3), µq is the shear modulus of quartz (2.95 × 1011 dyn cm -2), and m is the deposited mass. Assuming a density of 1.2 ( 0.1 g/cm3 for the polyions40 and taking into account the characteristics of the quartz resonator used, one can estimate the thickness of the deposited films. In our case, a frequency shift of 1 Hz corresponds to an added thickness of 1.47 Å. This calculation is made with the implicit assumption that the layers are uniform in the direction perpendicular to the substrate. To obtain a measurable signal, it was necessary to use polyion solutions at the concentration 5 × 10-3 M per monomer unit. Figure 7 shows the frequency decreases (proportional to mass increases) as a function of the number of adsorption cycles. The systems show two remarkable differences. First, for PDDA/PAZO, adsorption of mass is observed after immersion in both PDDA and PAZO solutions, indicating successful deposition of both polyions. By contrast, in the PEI/PAZO system, the polycation apparently does not produce any mass adsorption and the frequency shift occurs by steps after each bilayer deposition. Second, the frequency shift due to PAZO deposition is higher for PEI/PAZO than PDDA/PAZO. This

(38) Ku¨hn, H.; Mo¨bius, D.; Bu¨cher, H. in Techniques of Chemistry; Weissberger, A., Rossiter, B. Wiley: New York, 1992; Vol. 1, Part 3b, pp 577-702.

(39) Sauerbrey, G. Z. Phys. 1959, 155, 206. (40) Lvov, Y.; Ariga, K.; Ichinose, I.; Kunitake, T. J. Am. Chem. Soc. 1995, 117, 6117.

Figure 6. Increase of the maximum absorbance of the 366 nm transition as a function of the number of polyion bilayers: (a) PDDA/PAZO; (b) PEI/PAZO.

Polyionic Films Containing Azobenzene

Figure 8. Maximum absorbance of the π-π* transition peak during a heating-cooling cycle, as a function of the investigated temperature. The reported data refer (a) to a 90 bilayer sample of PDDA/PAZO and (b) to 50 bilayers of PEI/PAZO.

observation is in agreement with the higher thickness per bilayer and the higher UV absorbance detected in the PEI/PAZO samples than the PDDA/PAZO system. The fact that the mass deposition occurs by steps suggests that an excess of polyanion is deposited on the film and that aggregates are then desorbed during the polycation deposition. Finally, from the total frequency shift it is possible to estimate the overall thickness of the samples. We obtained 238 ( 20 and 492 ( 20 Å for PDDA/PAZO and PEI/PAZO, respectively, in good agreement with the thickness of the same samples measured by ellipsometry (197( 20 Å, 522 ( 21 Å). Due to the different concentrations employed, a direct comparison with the thicknesses reported in Figure 3 is not possible. Thermal Stability. To study the temperature effect on the UV-visible spectra, we performed a heatingcooling cycle in the range 20-120 °C on the samples, and UV-visible spectra were measured after each 10 °C temperature step. The heating (or cooling) rate was 1 °C /min, and the samples were allowed to equilibrate for 30 min after each 10 °C temperature step. Small changes in the intensity of the maximum at 366 nm were observed. The process is reversible with cooling, but as reported in Figure 8, hysteresis occurs in all samples. The change is small and is approximately 7% for PDDA/PAZO, while it is more pronounced in PEI/PAZO (about 15%). This suggests the existence of possible preferential orientations of the chromophore in the multilayers that are lost after heating. A shift of a few nanometers in the maximum absorbance toward longer wavelengths is also observed in the case of thick PDDA/PAZO films (>50 layers) as the

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Figure 9. (a) Changes in the UV-vis spectra of PDDA/PAZO (80 bilayers) induced by UV light irradiation. (b) Photoismerization kinetics for 30 and 80 bilayers of PDDA/PAZO. The curve fits are shown as well (solid lines).

temperature increases (not shown). The initial position of the maximum is completely recovered after the cooling process. Photoisomerization. UV-visible absorption spectroscopy was employed in order to investigate the photoisomerization kinetics. In these experiments, the samples were irradiated with the photoactive light propagating perpendicular to the sample plane. The photoisomerization reaction was studied by irradiating the sample with UV light and recording the spectra over different time intervals until a photostationary state was reached. In case a steady state was not reached, the reaction was stopped after 120 min of irradiation. Since the polycation strongly influences the photoisomerization mechanism, we report separately, for purposes of clarity, the results obtained in the two systems. PDDA/PAZO. The spectra obtained during the photoisomerization of an 80 bilayer PDDA/PAZO sample are reported in Figure 9a. As the photoisomerization reaction is induced by UV light, a spectral change occurs, with a progressive decrease in the π-π* band maximum intensity, denoted as Al. This band does not disappear even after the longest irradiation interval, indicating that the photoreaction does not reach completion. The kinetics of photoisomerization are reported in Figure 9b. The best fit of the experimental points was obtained by simulating the absorbance (A) data with an initial exponential decay of the maximum absorbance Al, followed by a linear intensity decrease:

A(t) ) A1e-t/T1 - A2t + A3

(3)

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Figure 11. As/Al during photoisomerization.

Figure 10. (a) Changes in the UV-vis spectra of PEI/PAZO (50 bilayers) induced by UV light irradiation. (b) Photoismerization kinetics for 10, 20, and 50 bilayer PEI/PAZO samples. The curve fits are shown as well (solid lines).

The fits to the data are shown in the same figure as solid lines. It is possible to see how the kinetics are influenced by the sample thickness; in particular, the second part of the curve is slower for thicker samples. The position of the absorption band is affected by the photoreaction mechanism as well. For instance, in the 80 bilayer sample, the maximum position slowly undergoes a blue shift, from 372 to 366 nm at the end of the UV light irradiation. Further, we have observed that the intensity of the band at 260 nm (denoted as As) is not affected by the photoisomerization. The ratio As/Al, which can be related to the azo group orientation,41 changes as shown in Figure 11. Figure 11 also shows the value of the same ratio in aqueous solution, where isotropic orientation of the azo group is assumed. PEI/PAZO. Figure 10a shows the spectra obtained from a 50 bilayer PEI/PAZO sample. The intensity of the π-π* band progressively decreases, but it does not disappear. The kinetics (Figure 10b) can be fit by a double exponential function:

A(t) ) A1e-t/T1 + A2e-t/T2 + A3

(4)

The time decay constants (T1 and T2) obtained from the best fit are reported in Table 1, along with all the other parameters. The effect of the sample thickness is to increase T1 and T2 monotonically with the number of layers. The dependence on the number of layers is nearly linear for T2. Figure 10 shows that for PEI/PAZO the total (41) Seki, T.; Ichimura, K. Thin Solid Films 1989, 179, 77.

decrease of absorbance is smaller in comparison to that for PDDA/PAZO. The position of the π-π* band changes in this case also but with a more complicated behavior. The initial position of the 372 nm band was observed first red shifting by about 4 nm and subsequently receding to a constant value of 366 nm. For the As/Al ratio, the value calculated at the beginning of the photoreaction is very close to the one calculated in solution (about 0.58), and it increases during UV irradiation to approximately 0.8 (see Figure 11). Back-isomerization. After irradiation by UV light, we investigated the thermal cis-to-trans isomerization of the samples. For this purpose, the samples were left in the dark and the UV-visible absorption spectra were measured after 5, 15, 30, 45, and 60 min, 1 day, and 1 week. Neither system showed a complete recovery in the maximum absorbance. However, the PDDA-containing samples exhibited a much higher intensity recovery (see Figure 12). The back-isomerization kinetics are monoexponential in all samples investigated and very slow when compared to those of the trans-to-cis reaction. In the equation used for the fit of the data points

Al(t) ) A4(1 - e-t/T1) + A5

(5)

the time constant T1 is of the order of hours. The maximum position of this band was 366 nm for all samples and did not change during thermal relaxation. We also attempted to induce the cis-to-trans isomerization of the azobenzene chromophore by blue light irradiation (λ ) 420 nm, for time intervals ranging between 1 and 30 min). No significant change in the spectra occurred. Optical Thickness Changes. The thicknesses of the films were measured by ellipsometry as a function of UV irradiation time and after thermal relaxation in the dark. The reported data were obtained by assuming that the refractive index is equal to 1.5 and the refractive index is not affected by the irradiation. Figure 13 shows the results obtained for a 90 bilayer PDDA/PAZO sample. Analogous behavior was obtained in all the other samples. Additional information on the reversible change of the optical thickness can be obtained from X-ray reflectivity measurements, which we are currently pursuing. Discussion The uniform assembly of the multilayers is demonstrated by the linear increase in the UV-visible absorbance as a function of the number of deposited layers and by the ellipsometry measurements. It is remarkable how the polycation affects the sample thickness. It is well-

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Table 1. Parameters of the Photoisomerization Kinetics Calculated from the Fit Procedure PDDA/PAZO 30 bilayers 80 bilayers PEI/PAZO 10 bilayers 20 bilayers 50 bilayers

A1

A2

A3

T1 (s)

0.17 ( 0.01 0.15 ( 0.01

6.8 × 10-5 ( 2 × 10-6 5.2 × 10-5 ( 2 × 10-6

0.828 ( 0.008 0.844 ( 0.005

83 ( 16 16 ( 11

0.21 ( 0.02 0.25 ( 0.02 0.15 ( 0.04

0.35 ( 0.02 0.22 ( 0.03 0.22 ( 0.07

0.44 ( 0.02 0.53 ( 0.03 0.62 ( 0.08

Figure 12. Thermal relaxation kinetics for (a) a PDDA/PAZO and (b) a PEI/PAZO sample. The maximum absorbance is normalized to the value at UV irradiation time t ) 0 (80 bilayers for part a; 10 bilayers for part b).

Figure 13. Decrease in the optical thickness as measured by ellipsometry, during the UV light irradiation. A recovery in the thickness is observed after a thermal relaxation in the dark for 1 week.

known from the literature42 that the polymer layer thickness is highly affected by the ionic strength of the solution. In our case, although the ionic conditions in solution are the same, it is possible that, due to a different extent of protonation or the branched nature of PEI, the (42) Decher, G. Comprehensive Supramolecular Chemistry; Pergamon Press: New York, 1996, Vol. 9, p 507.

21 ( 0.02 51 ( 13 134 ( 78

T2 (s)

χ2 0.0018 0.0003

1.3 × 10-3 ( 2 × 102 2.5 × 103 ( 9 × 104 4.3 × 103 ( 1 × 103

0.0025 0.0022 0.0079

ionic condition on the film surface and the microenvironment inside the film may be different in the two systems investigated. In particular, the average thickness of each polyanion/polycation adsorption obtained in the case of the PDDA/PAZO system is extremely low. Although singlelayer thicknesses as low as 5-8 Å have been reported in the literature when the adsorption takes place from aqueous solution, the 5 Å value that we measured for each PDDA/PAZO adsorption suggests that only a partial surface coverage takes place. This would allow, for instance, interdigitation of the polymer chains instead of the adsorption of a proper polycation/polyanion bilayer. Moreover, theory43 predicts the thickness increase of an adsorbed polyelectrolyte layer with increasing surface coverage, and this behavior has been observed experimentally.44-46 As a confirmation of the partial surface coverage, preliminary investigation of PDDA/ PAZO samples by transmission electron microscopy shows a patchy structure of the films, with aggregates of the size of 60 nm. The data suggest that PEI promotes greater deposition of PAZO, as deduced by the extremely high absorbance values in comparison to those for the PDDA/PAZO system. Further, the polyanion appears to form complexes in the PEI matrix. The formation of PAZO/PEI complexes and their partial desorption from the surface during the immersion in PEI solution could explain the stepwise frequency decrease in the QCM measurements. Moreover, PEI has already been shown to be less favorable, in terms of a regular deposition, than PDDA in the case of dye/ polyion adsorption.47 Differences in the layer thicknesses and in the molecular conformation can be due to additional factors, like different chain flexibility of the polycations, and, in particular, to the branched conformation of the PEI molecules. The linear PEI structure reported in Figure 1 is in fact purely schematic, and the commercial material is invariably highly branched. This branching leads to complexities in the behavior of the ionized forms of PEI, because it results in the presence of primary, secondary, and tertiary amine groups in the chain; additionally only three-quarters of the amine groups can be protonated in aqueous solution; that is, the effective monomer mass of the ionized form is higher than the one expected from the molecular formula.48 Our data clearly show the complexity of this binary polyelectrolyte. Several factors are known to govern both polyelectrolyte adsorption on charged surfaces and the interaction of polyelectrolytes, for example, the effect of both pH and ionic strength.49-51 Unfortunately, many of the parameters involved in the theoretical calculations, (43) Fleer, G. J.; Lyklema, J. In Adsorption from Solution at the Solid/Liquid Interface; Parfitt, G. D., Rochester, C. H.,Eds.; Academic Press: London, 1983; p 153. (44) Priel, Z.; Sieberberg, A. J. Polym. Sci., Polym. Phys. Ed. 1978, 16, 1917. (45) Killmann, E.; Kuzenko, M. V. Angew. Makromol. Chem., 1974, 35, 39. (46) Gebhart, H. Angew. Makromol. Chem. 1976, 53, 171. (47) Ariga, K.; Kunitake, T. J. Am. Chem. Soc. 1997, 119, 2224. (48) Molyneux, P. Water-Soluble polymers: properties and behavior; CRC Press: Boca Raton, FL, 1984; Vol. II, p 18.

200 Langmuir, Vol. 15, No. 1, 1999

such as segment-surface free energy and polymer chain flexibility, are not known quantitatively for most systems. The same holds for the entropic and enthalpic contributions that govern the interaction between polyectrolytes in aqueous media, parameters that are strongly dependent on the particular system. It is not completely surprising then that the change of the polycation leads to dramatically different behavior, which at present is not predictable. A parameter that can play an important role in the molecular packing is obviously Tg. The difference in Tg can explain also the different behavior of the thermal cycle and, in particular, the higher stiffness showed by the PDDA/PAZO system. The glass transition temperature for the unprotonated form of PEI is known to be -23.5 °C.48 A higher value is expected for the glass transition temperature of PDDA, even though it is not reported in the literature. In fact, Tg values of different ionenes, mainly aliphatic, have been investigated by differential scanning calorimetry (DSC) and found to appear between 27 and 80 °C.52-53 We assume that Tg values for ionene polymers with aromatic rings or cyclic structures in their skeletons, such as PDDA, will be much higher than those of the more flexible aliphatic ionenes. We focused particular attention on the maximum position of the absorption band in the UV-visible spectra. When molecules with a dipole moment are brought close together in an ordered structure (for instance in a LB film), a splitting of the excitation energy levels occurs. As a consequence, shifts in the absorption spectra are observed, depending on the mutual orientation of the interacting dipole moments. When the dipole moments of the molecules in aggregates are parallel, a hypsochromic (blue) shift occurs (H band). If the transition dipoles are in-line, rather than parallel, the spectra exhibit a bathochromic (red) shift and the aggregates are in this case termed J aggregates. In-line and parallel aggregation are of course two ideal extreme cases; in general, the mutual orientation of the interacting dipoles may be at some angle, resulting in more complex spectral changes. Further average orientations may vary from layer to layer or even within layers. As mentioned previously, a red shift is observed with the PDDA/PAZO system as the deposition process proceeds. This may be interpreted as aggregation in the form of J aggregates. A more complex behavior emerges from the PEI/PAZO interaction in which a red shift in the first part of the adsorption process is followed by a blue shift as the number of the deposited bilayers increases. It is possible either that both types of aggregates coexist in this system or that a complex molecular arrangement gives rise to nonmonotonic changes in the spectra. From the photoisomerization kinetics investigation, two main observations appear to be clear. The first is that in the UV-visible spectra no clear band assigned to the cis isomer appears (the cis isomer has a weak absorption band centered at 460 nm). However, the trans-to-cis isomerization reaction does take place, at least to some extent, as demonstrated by the decrease in the π-π* band intensity. In the PDDA/PAZO system, the decrease of the normalized absorbance is more pronounced (Figure 10), demonstrating that the azo group mobility is higher in (49) Petterkorn, E.; Schmitt, A.; Varoqui, R. J. Membr. Sci. 1978, 4, 17. (50) Greene, B. W. J. Colloid Interface Sci. 1971, 37, 144. (51) Awad, N. M.; Morawetz, H. J. Polym. Sci., Polym. Phys. Ed. 1981, 19, 245. (52) Dominguez, L.; Meyer, W. H. Solid State Ionics 1988, 28/29, 941. (53) Dominguez, L.; Meyer, W. H.; Wegner, G. Makromol. Chem. Rapid Commun. 1987, 8, 151.

Dante et al.

this system. The thermal relaxation gives also higher absorbance intensity recovery in the PDDA/PAZO system (about 85%). The shift in the maximum absorption position as a function of the irradiation time may be explained by the progressive isomerization and dissolution of the aggregates present in the polymer matrix. Since the position of the maximum does not change during the thermal relaxation, an irreversible molecular rearrangement takes place during the photoisomerization. The fact that the trans-to-cis photoreaction does not follow first-order kinetics is not surprising, and examples of second-order kinetics are usually observed in azopolymer systems.1 Nevertheless, in those systems the kinetics are orders of magnitude faster compared to those of our system. The kinetics in our case seem to be highly influenced by the steric hindrance and most probably by the Coulombic interaction between the polyions. The electrostatic attraction may also play a role in the slow and incomplete back-isomerization, and irreversible configuration changes may occur. As mentioned earlier, a comparison of the intensities of the absorption bands near 260 and 366 nm gives an indication of the orientation of the azobenzene chromophore in the multilayers. The ratio As/Al calculated in solution was about 0.55, and it is representative of a random chromophore orientation. Since the spectra were measured with a perpendicular incident beam, an upright orientation of the azo group long axis relative to the substrate raises this value. As a general observation, the ratio As/Al in both systems as a function of the number of adsorbed layers indicates isotropic orientation of the azo groups before irradiation. As reported in Figure 11, in the PDDA/PAZO system the As/Al ratio increases during the photoisomerization to a value of 1.5. This means that the trans monomers at the end of the photoreaction are oriented more perpendicular to the substrate. Comparing Figures 10b and 11, we note that the change of the average orientation of the azo groups in the trans configuration is strictly related to the photoisomerization kinetics, since it is purely due to a change of the intensity of the long axis band. A different behavior is observed in the PEI/PAZO system. First of all, the band at 260 nm changes to a higher intensity during the UV irradiation. Moreover, for irradiation time longer than 1 h, an additional absorption at lower wavelength appears in the spectra, interfering with the azo group short axis absorption. The As/Al ratio (see Figure 11) indicates isotropic orientation of the azo group before UV irradiation; at the end of the photoreaction the trans monomers are oriented to some extent but less so than in the PDDA/PAZO film. Conclusions In this work we have used a commercially available azo-containing polymer as the polyanion (PAZO) and two different polycations (PDDA and PEI) to build up multilayered systems. The deposition of the layers has been monitored by UV-visible spectroscopy and by quartz crystal microbalance measurements. A significant effect of the polycation in the film assembly mechanism was found. In particular, PEI seems to promote aggregation and probably orientation of the azo polymer in the films. Trans -to-cis photoisomerization upon UV light illumination was induced. The kinetics of photoisomerization are strongly dependent on the polycation structure as well as on the multilayer thickness. As expected, the kinetics are not of first-order and involve at least two mechanisms characterized by different rate constants. Cis-

Polyionic Films Containing Azobenzene

to-trans reverse isomerization has been attempted both by blue light irradiation and by thermal relaxation. In the time scale employed in our experiments, no reverse isomerization was observed upon blue light irradiation. Thermal relaxation, on the other hand, is responsible for a very slow, partial recovery of the trans configuration. The kinetics are in this case monoexponential. Finally, macroscopic and reversible changes of the film optical thickness have been detected by ellipsometry.

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Acknowledgment. This work was supported by the MRSEC Program of the National Science Foundation (Grant DMR-9400354) through the Center on Polymer Interfaces and Macromolecular Assemblies (CPIMA). S.D. thanks Dr. Kay Kanazawa for his help during the QCM measurements and Dr. Zouhir Sekkat for valuable discussion. LA980497E