7036
Langmuir 2005, 21, 7036-7043
Azobenzene-Containing Polyamic Acid with Excellent Langmuir-Blodgett-Kuhn Film Formation Behavior Suitable for All-Optical Switching Yun Zong,†,‡ Keiko Tawa,†,§ Bernhard Menges,† Ju¨rgen Ru¨he,| and Wolfgang Knoll*,† Max Planck Institute for Polymer Research, Ackermannweg 10, 55128 Mainz, Germany, and Institute for Microsystem Technology, Albert Ludwig University of Freiburg, Georges-Koehler-Allee 103, 79110 Freiburg, Germany Received March 7, 2005. In Final Form: April 27, 2005 To generate command surfaces for all-optical switching, highly ordered polymeric Langmuir-BlodgettKuhn (LBK) multilayers were fabricated onto silicon substrates and gold-coated optical glass slides from novel azobenzene-bearing polyamic acid systems. Pronounced Bragg peaks and well-defined Kiessig fringes observed in the X-ray reflectivity measurement for samples on silicon substrates indicate that these films possess a regularly repeated Y-type LBK multilayer structure and ultrasmooth surfaces. Fourier transform infrared (FT-IR) spectra taken by grazing incidence reflection suggest specific orientations of the functional groups in the layers. The excellent film-forming properties of the polyamic acid allow for a smooth buildup of several hundreds of layers of the LBK films onto gold-coated glass slides, which in turn allows for determining the geometrical thickness and the anisotropic refractive indices of the films by using optical waveguide spectroscopy. Interestingly, the probe laser beam induced a distinct fluorescence signal from the films, which remained even after the film underwent a thermal imidization process at 160 °C for 8 h in vacuo. LBK films fabricated from these compounds can be successfully applied for all-optically switching the alignment of liquid crystals by irradiation with light of different wavelengths.
1. Introduction Polymer matrices with embedded chromophores have attracted much attention from both fundamental aspects and application points of view.1 On one hand, one is interested in how a polymer matrix influences the photochromism of the chromophores embedded. To this aspect, the free volume in the matrix is of importance, which is closely related to the dye concentration and packing state and the matrix polymer structure, morphology, and viscoelastic properties.2 On the other hand, the photoisomerization reaction of the chromophores induces significant motions of the matrix,3-7 which promises a wide range of interesting potential applications. If an azobenzene-containing polymer thin film is in contact with another matrix, for example, a layer of liquid crystals (LCs), the photoisomerization reaction of azobenzene moieties may induce reversible switching of the liquid crystal (LC) alignment between two different aligned states, subject to the formation of preoriented LC mon* Corresponding author: e-mail
[email protected]; fax ++49-6131-379-360. † Max Planck Institute for Polymer Research. ‡ Present address: Institute of Materials Research and Engineering, 3 Research Link, Singapore 117602. E-mail: y-zong@ imre.a-star.edu.sg. § Present address: Research Institute for Cell Engineering, AIST, 1-8-31 Midorigaoka, Ikeda, Osaka 563-8577, Japan. E-mail:
[email protected]. | Albert Ludwig University of Freiburg. (1) Ichimura, K. Chem. Rev. 2000, 100, 1847-1873. (2) Eisenbach, C. D. Ber. Bunsen-Ges. Phys. Chem. 1980, 84, 680690. (3) Natansohn, A.; Rochon, P. Chem. Rev. 2002, 102, 4139-4175. (4) Ikeda, T. J. Mater. Chem. 2003, 13, 2037-2057. (5) Seki, T. Polym. J. 2004, 36, 435-454. (6) Zhao, Y. Pure Appl. Chem. 2004, 76, 1499-1508. (7) Barrett, C. J.; Rochon, P.; Natansohn, A. J. Chem. Phys. 1998, 109, 1505-1516.
odomains.8 Generally, the preorientation of LCs could be realized via different approaches, including applying electric/magnetic fields,9 polarized laser irradiation,10 or shear flow;11 doping dyes into LC assemblies;10,12,13 fabricating azobenzene-polymer networks14,15 or by selfassembly of monolayers;16,17 use of fluoropolymer surfaces,18 rubbed polymer surfaces,19 patterned surfaces,20-22 surface relief gratings,23,24 and surfaces with selective adsorption,25,26 to name but a few. A simple yet efficient approach that integrates the alignment function with a (8) Ichimura, K.; Suzuki, Y.; Seki, T.; Hosoki, A.; Aoki, K. Langmuir 1988, 4, 1214-1216. (9) Bender, M.; Holstein, P.; Geschke, D. Liq. Cryst. 2001, 28, 18131821. (10) Gibbons, W. M.; Shannon, P. J.; Sun, S. T.; Swetlin, B. J. Nature 1991, 351, 49-50. (11) Berghausen, J.; Fuchs, J.; Richtering, W. Macromolecules 1997, 30, 7574-7581. (12) Ubukata, T.; Seki, T.; Ichimura, K. J. Phys. Chem. B 2000, 104, 4141-4147. (13) Matsuzawa, Y.; Ichimura, K. Langmuir 2000, 16, 8390-8395. (14) Zhao, Y.; Chenard, Y.; Galstian, T. V. Appl. Phys. Lett. 2000, 77, 2644-2646. (15) Kurihara, S.; Yoneyama, D.; Nonaka, T. Chem. Mater. 2001, 13, 2807-2812. (16) Gupta, V. K.; Abbott, N. L. Science 1997, 276, 1533-1536. (17) Mouanda, B.; Viel, P.; Blanche, C. Thin Solid Films 1998, 323, 42-52. (18) Patel, J. S.; Yokoyama, H. Nature 1993, 362, 525-527. (19) Toney, M. F.; Russell, T. P.; Logan, J. A.; Kikuchi, H.; Sands, J. M.; Kumar, S. K. Nature 1995, 374, 709-711. (20) Ruetschi, M.; Grutter, P.; Funfschilling, J.; Guntherodt, H. J. Science 1994, 265, 512-514. (21) Robbie, K.; Broer, D. J.; Brett, M. J. Nature 1999, 399, 764-766. (22) Lee, B. W.; Clark, N. A. Science 2001, 291, 2576-2580. (23) Furumi, S.; Nakagawa, M.; Morino, S.; Ichimura, K.; Ogasawara, H. Appl. Phys. Lett. 1999, 74, 2438-2440. (24) Dantsker, D.; Kumar, J.; Tripathy, S. K. J. Appl. Phys. 2001, 89, 4318-4325. (25) Ueda, M.; Kudo, K.; Ichimura, K. Isr. J. Chem. 1996, 36, 371378. (26) Furumi, S.; Nakagawa, M.; Morino, S.; Ichimura, K. J. Mater. Chem. 2000, 10, 833-837.
10.1021/la0506067 CCC: $30.25 © 2005 American Chemical Society Published on Web 06/17/2005
Polyamic Acid Suitable for All-Optical Switching
satisfactory photoswitching performance is to utilize the Langmuir-Blodgett-Kuhn (LBK) technique for fabricating thin layers with supramolecular architectures from azobenzene-containing polymeric amphiphiles. Such layers induce LC monodomain formation by its high molecular order; in addition, they provide reactive centers for switching the alignment of LCs upon photoisomerization reaction of the azobenzene moieties. The photoswitching process is influenced by numerous factors, such as polarity of the molecules,27 packing density (dye concentration)28-30 and flexibility (mobility)27,31-34 of the azobenzene moieties, chemical features of the azobenzene substituents,35,36 chromophore geometry,37 etc. Among these factors, the flexibility and packing density of the azobenzene moieties have been generally taken as key factors that are of vital influence as they determine the free volume needed for the photoisomerization reaction of the chromophores. For polymers with azobenzene groups covalently attached as side chains to the polymer backbones via a flexible spacer, this approach requires control of the length of the spacer as well as the distance between two adjacent azobenzene units along the polymer backbones. Photoreactive LBK films have been fabricated from different azobenzene-derivatized polymer amphiphiles, for example, azobenzene-poly(vinyl alcohol)s,38 azobenzene-polyglutamates,39 and azobenzene-polyamic acids (Az-PAAs).40,41 Among these polymeric amphiphiles, AzPAAs prepared through a condensation reaction of dianhydrides and diamine/dianiline possess good control of dye concentration and their distribution at the molecular level. Moreover, the carboxylic groups along the polymer backbone first serve as hydrophilic units for the LBK film formation but then can be converted to inert imides after the film fabrication. This not only avoids the ionization of protons but also improves the material properties, for example, the thermal stability. These advantages make Az-PAAs excellent precursors for photoswitching alignment layer of nematic liquid crystals. Amphiphilic azobenzene-containing polyamic acids synthesized so far only show reasonable solubility in dimethylacetamide (DMAc) or N-methylpyrridone (NMP), which makes LBK film fabrication rather difficult. Despite (27) (a) Seki, T.; Sakuragi, M.; Kawanishi, Y.; Suzuki, Y.; Tamaki, T.; Fukuda, R.; Ichimura, K. Langmuir 1993, 9, 211-218. (b) Seki, T.; Ichimura, K.; Fukuda, R.; Tanigaki, T.; Tamaki, T. Macromolecules 1996, 29, 892-898. (c) Seki, T.; Kojima, J.; Ichimura. K. Macromolecules 2000, 33, 2709-2717. (28) Aoki, K.; Seki, T.; Suzuki, Y.; Tamaki, T.; Hosoki, A.; Ichimura, K. Langmuir 1992, 8, 1007-1013. (29) Seki, T.; Fukuda, R.; Tamaki, T.; Ichimura, K. Thin Solid Films 1994, 243, 675-678. (30) Wu, Y. L.; Zhang, Q. J.; Kanazawa, A.; Shiono, T.; Ikeda, T.; Nagase, Y. Macromolecules 1999, 32, 3951-3956. (31) Ikeda, T.; Sasaki, T.; Ichimura, K. Nature 1993, 361, 428-430. (32) Wu, Y. L.; Demachi, Y.; Tsutsumi, O.; Kanazawa, A.; Shiono, T.; Ikeda, T. Macromolecules 1998, 31, 1104-1108. (33) Kong, B.; Cui, L.; Xie, P.; Zhang, R. B.; He, C. B.; Chung, N. T. S. Liq. Cryst. 2000, 27, 1683-1689. (34) Rutloh, M.; Stumpe, J.; Stachanov, L.; Kostromin, S.; Shibaev, V. J. Inf. Rec. 2000, 25, 481-486. (35) Seki, T.; Ichimura, K.; Fukuda, R.; Tamaki, T. Kobunschi Ronbunshu 1995, 52, 599-605. (36) Oh, S. K.; Nakagawa, M.; Ichimura, K. J. Mater. Chem. 2001, 11, 1563-1569. (37) Wu, Y. L.; Demachi, Y.; Tsutsumi, O.; Kanazawa, A.; Shiono, T.; Ikeda, T. Macromolecules 1998, 31, 4457-4463. (38) Akiyama, H.; Momose, M.; Ichimura, K.; Yamamura, S. Macromolecules 1995, 28, 288-293. (39) Bu¨chel, M.; Sekkat, Z.; Paul, S.; Weichart, B.; Menzel, H.; Knoll, W. Langmuir 1995, 11, 4460-4466. (40) Yokoyama, S.; Kakimoto, M.; Imai, Y. Langmuir 1993, 9, 10861091. (41) Akiyama, H.; Kudo, K.; Ichimura, K.; Yokoyama, S.; Kakimoto, M.; Imai, Y. Langmuir 1995, 11, 1033-1037.
Langmuir, Vol. 21, No. 15, 2005 7037
Figure 1. Chemical structure of (a) PAA6B and (b) a low molecular weight nonpolar liquid crystal, ZLI-3086.
the great effort in multilayer fabrication, no LBK films with satisfying molecular order were obtained. Recently, we synthesized a series of azobenzene-containing polyamic acids, which permit easy fabrication of highly ordered LBK multilayer assemblies. The solubility of these compounds is improved by introducing sterically demanding subunits into the polymer backbone and alkyl tails at the end of the azobenzene pendants, which reduce the probability of forming strong π-π complexes,42 as this is presumably one of the reasons for the poor solubility of earlier polymers. In this paper, we report on the special properties of LBK multilayer assemblies fabricated from one of these new polyamic acids, abbreviated as PAA6B, which possesses an n-hexyl group at the end of the azobenzene unit and a 6-CH2 spacer bonding the azobenzene pendants onto backbone of a polyamic acid consisting of 1,4-bis(2-phenyl2-propyl)benzene as sterically demanding subunits (Figure 1a). The molecular order in the films is shown by studies of the internal structure by small-angle X-ray reflectometry, grazing incidence reflection Fourier transform infrared (FT-IR) spectroscopy, and fluorescence spectroscopy as well as optical waveguide field-enhanced fluorescence spectroscopy. All-optical switching performance of the photoreactive thin layer to the alignment of LCs was also investigated by studying the photoreorientation behavior in a hybrid LC cell. 2. Experimental Section 2.1. Materials. The synthesis and characterization of PAA6B was reported in a previous publication.43 A general procedure for the synthesis of these Az-PAAs is as followings: 1.0 mmol of azobenzene-functionalized dianhydride and 1.0 mmol of commercially available dianiline were dissolved in 10 mL of dry DMAc under argon atmosphere. After 2 days reaction at room temperature, the reaction mixture was poured in a fine stream into 200 mL of anhydrous methanol with vigorous stirring. The collected precipitate was redissolved in DMAc, and any insoluble residue was filtered off. The filtrate was again precipitated in methanol. The dissolving/precipitating process was repeated a few more times, and the precipitate was collected and dried in vacuo. For PAA6B, a fine yellow powder was obtained in a yield of 47%. 13C NMR (DMSO-d6, dept 135, δ/ppm): 14.3 (CH3 at head of alkyl), 31.1(4CH3 in isopropyl between aromatic rings), 22.4, 25.7, 26.6, 28.7, 28.9, 30.7, 31.4, 34.9, 35.3, 52.4, 54.5 (2), 63.4 (2), 68.2 (15CH2), 115.2, 119.7, 122.6, 124.7, 126.3, 127.0, (42) Dine-Hart, R. S.; Wright, W. W. Makromol. Chem. 1971, 143, 189-206. (43) Zong, Y.; Hees, U.; Knoll, W.; Ru¨he, J. J. Nonlinear Opt. Phys. Mater. 2002, 11, 367-389.
7038
Langmuir, Vol. 21, No. 15, 2005
128.7, 128.8, 129.5, 130.3, 130.6, 132.5 (26CH, aromatic). IR (KBr, cm-1): 1724 (ester), 1674 (amide), 1600 (carboxylic acid), 1244 (C-OR). Glass transition temperature: 68 °C. The low molecular weight nonpolar liquid crystal ZLI-3086 is a commercial product of Merck with its chemical structure shown in Figure 1b. Spacers for controlling the thickness of the liquid crystal layer in the LC cells are biaxially oriented poly(ethylene terephthalate) (PET, Goodfellow) thin films with a thickness of 23 µm, produced by Cambridge Science Park, Cambridge, England. 2.2. Langmuir-Blodgett-Kuhn Film Formation. The LBK film preparation of PAA6B was carried out on a singletrough film balance (FW2 Lauda) with a Milli-Q water (R ) 18.2 MΩ) subphase. PAA6B chloroform solution (100∼120 µL, c0 ) 1.063 mg/mL) was spread onto the subphase. After evaporation of the solvent (ca. 15 min), the floating monolayer was compressed by a moving barrier with a velocity of 5 cm/min. The surface pressure for the film transfer typically was π ) 14 mN/m. Monolayers were transferred onto silicon substrates or goldcoated glass slides by vertical dipping with a dipping speed of 2 or 5 mm/min, respectively. 2.3. Small-Angle X-ray Reflectivity Measurement. PAA6B LBK multilayer assemblies with different numbers of layers on silicon substrates were studied by small-angle X-ray reflectometry. The X-ray reflectometer applied in the study was a homemade instrument and consists of a Rigaku 18 kW rotating anode with a copper target as the X-ray source, a collimation system, sample goniometer, detector assembly, and a computer for automatic control.44 The beam reflected off the sample surface passes through two further slits, with a pyrolytic graphite monochromator between, before it reaches the final NaI scintillation detector. For angular scans, sample and detector rotate in a defined θ/2θ mode. 2.4. FT-IR Measurement. Grazing incidence reflection FTIR spectra were taken on an FT-IR spectrometer (Nicolet Magna 850) from a monolayer of PAA6B deposited onto a glass slide precoated successively with 3 nm chromium and 100 nm gold layers. A substrate coated only with chromium and gold was used as a reference. The spectrum of the bulk PAA6B (KBr pellet) was recorded on a Nicolet FT-IR 320 spectrometer. 2.5. Optical Waveguide Field-Enhanced Fluorescence Measurement. Measurement of the optical waveguide fieldenhanced fluorescence of PAA6B multilayer assemblies were performed on a conventional surface plasmon resonance (SPR) spectrometer in the Kretschmann configuration45 with a set of fluorescence emission detection components on the back side of the prism (facing the film surface). A beam from a helium-neon laser (λ ) 633 nm) was used as the excitation light, with the emission of the fluorescence from the film passing a band filter (670 ( 10 nm) in order to suppress scattered light from the probe beam. The fluorescence background was monitored by a blank gold-coated glass slide with a typical value of ∼1000 cps. 2.6. Fluorescence Measurement. The fluorescence of the polyimide LBK film was measured on a Spex Fluorolog II fluorescence spectrometer. The excitation spectra were recorded at wavelengths of λ ) 670 ( 4 or 620 ( 4 nm, while emission spectra were taken with λ ) 630 ( 4 or 520 ( 4 nm as the excitation light, respectively. A bare BK7/Au glass slide was applied as a reference. 2.7. Switching Alignment of Liquid Crystals. The photoswitching experiments were carried out in an asymmetric sandwich LC cell, which consists of two different windows, a LaSFN9/Ag/SiOx/4-layer-PAA6B slide and a bare quartz slide. PET films (d ) 23 µm) were used as spacers to control the thickness of the LC layer. ZLI-3086, as an example for a typical nonpolar liquid crystal, was filled into the cell by capillary action. The photoisomerization reaction-induced reorientation of the liquid crystals was studied at a fixed angle of observation by monitoring the reflectivity with the LC being in a specific alignment state. The same SPR setup as described above was used, alternating between UV (λ ) 360 ( 15 nm) and visible (λ > 420 nm) light irradiation. (44) Foster, M.; Stamm, M.; Reiter, G.; Hu¨ttenbach, S. Vacuum 1990, 41, 1441-1444. (45) Knoll, W. Annu. Rev. Phys. Chem. 1998, 49, 569-638.
Zong et al.
Figure 2. Deposition trace of the final 10 deposition cycles for the buildup of a 480-layer LBK film on a gold-coated BK7 glass slide. The monotonic loss of the film on the subphase indicates a homogeneous transfer of the film onto substrate.
3. Results and Discussion 3.1. Layer Structure. As a consequence of the introduction of sterically demanding groups into the polymer backbone and alkyl tails to the end of the azobenzene pendants, which suppresses the interaction between individual polymer chains,46 the solubility of these polyamic acids (PAAs) in volatile water-immiscible solvents (e.g., chloroform) was noticeably improved. Accordingly, monolayers with excellent stability readily form at the airwater interface in a Langmuir trough from their chloroform solutions. This defines a clear distinction from those PAAs that had to be dissolved in a mixture of high boiling point DMAc and toxic benzene,40,41 where health hazard and solvent molecule incubation in the transferred film need to be taken into account, with the process being rather time-consuming and effort-demanding. With the introduction of the alkyl spacer between the azobenzene pendant and the polymer backbone and the alkyl tail attached to the other end of the azobenzene moiety, the polymers are sufficiently hydrophobic so that no salt-formation process40,41 by reacting PAAs with longchain alkylamines is necessary for stabilizing the molecules at the air-water interface. The ease of solvent evaporation in this case allows for a sufficient relaxation of the polyamic acid at the water/air interface such that abundant carboxylic and amide groups along the polymer backbone are released to the surface of the polymer coils and directed to the water subphase, while the hydrophobic tails stretch to the air. This, on one hand, stabilized the monolayer at water subphase in the Langmuir trough and, on the other hand, offers the possibility of building very stable LBK multilayer assemblies through hydrogenbond formation by abundant carboxylic and amide groups between adjacent layers with head-to-head contact and through hydrophobic interaction by the alkyl-containing azobenzene side chains for tail-to-tail transfer. With a dipping speed of 2 mm/min for silicon substrates and 5 mm/min for metal-coated glass slides at a surface pressure of 14 mN/m, very regular film transfer was observed. This allows for a smooth buildup of several hundreds of layers (monotonic loss of the film area on the subphase; cf. Figure 2) with a thickness >0.5 µm that can be employed, for example, in optical waveguide spectroscopy, which separately determines the refractive index and geometrical thickness of the film. However, it is noteworthy that the dipping speed and the surface pressure are quite sensitive parameters for similar compounds with a shorter spacer, which will be reported elsewhere. (46) Bower, G. M.; Frost, L. W. J. Polym. Sci. 1963, 1, 3135-3150.
Polyamic Acid Suitable for All-Optical Switching
Langmuir, Vol. 21, No. 15, 2005 7039
Figure 3. X-ray reflectivity data (O) and the corresponding simulation (s) of a 20-layer sample of PAA6B LBK film on a silicon substrate. (Inset) Electron density profile of the film in the direction normal to the substrate surface.
PAA6B LBK assemblies with different numbers of layers deposited on silicon substrates were studied by using small-angle X-ray reflectometry, and regular Kiessig fringes were observed from all the samples. Once the films had a thickness exceeding 9 nm, which is equivalent to the deposition of six layers, a Bragg peak is clearly visible in the X-ray reflectivity spectra. The intensity of this peak is found to increase with increasing number of layers (not shown). In Figure 3, the experimental data of the X-ray reflectivity measurement from 20 layers of PAA6B (O) is shown together with the corresponding simulation data (s), with the pronounced Bragg peak being located at qz ≈ 2.05 nm-1. The appearance of the Bragg peaks is an indication of a regular stacking of two sublayers with significantly different electron densities along the z-direction in the film. Θn, the angular position of the nth Bragg peak, is determined by the thickness (d0) of the repeat unit (a bilayer):
d0 )
nλ 2 sin Θn
(1)
By introducing the angular position value of the first Bragg peak in Figure 3 into eq 1, one can easily obtain the thickness of the bilayer repeat unit as d0 ) 3.13 nm. On the basis of this value, one has d ) 31.3 nm as an overall thickness of the 20 layers of PAA6B LBK film, which is in excellent agreement with the value from the simulation based on the whole curve, d ) 31.5 nm. Besides the prerequisite of a superlatticelike structure, the existence of Bragg peaks also requires smooth interfaces between sublayers with different electron densities. For PAA6B the interfaces are between the layer of polymer backbones (layer A) and the layer of side chains (layer B). They can only be smooth if the molecules possess excellent packing properties. This suggests a structure for the measured PAA6B multilayer assemblies similar to the one schematically depicted in Figure 4. The smoothness of the film surface is proven by the simulation data based on the X-ray experiment results. Interestingly, it was found that with increasing number of deposited layers the rms roughness of the film decreases. The 20-layer PAA6B LBK film shows an extremely flat surface with a rms roughness of about 0.5 Å, with the values of the blank silicon substrate and the 6- and 10layer PAA6B LBK samples being 5.0, 3.0, and 2.5 Å, respectively (not shown).
3.2. Molecular Orientation and Aggregates. To study the orientation of the functional groups in the thin film, grazing incidence reflection FT-IR spectra were recorded from a monolayer of PAA6B deposited on a 3 nm chromium/100 nm gold coated glass slide. It is noteworthy that the freshly prepared gold substrates possess hydrophilic character onto which only a monolayer with a hydrophobic surface was deposited after the first cycle of the film transfer, as indicated by the thickness measurement. This structure is visible if only the top layer remains on the substrate in Figure 4. Moreover, a normal FT-IR spectrum was taken from a bulk PAA6B (KBr pellet) for comparison. When the bulk spectrum is scaled such that the intensity of the peak at 1730 cm-1 is equal to that of the monolayer (assuming that the absorptions attributed to the carbonyl group stretching motions in the ester/ carboxylic groups are at the same level in the bulk and in the monolayer), differences can be seen in some of the other absorption peaks (cf. Figure 5): On one hand, the peaks corresponding to CdC stretching vibrations of the phenyl rings at 1600 and 1475 cm-1, respectively, show lower intensities in the film than those in the bulk, and the absorption peak due to the CdO amide stretching motions at 1674 cm-1, which possesses a medium intensity in the bulk, even disappears in the spectrum of the monolayer. This very probably indicates that the phenyl rings bonding with amide groups in the polymer backbones (in contact with the substrate surface) and the amide groups associated with them are aligned normal to the substrate surface so that the related stretching motions were severely hindered. On the other hand, the absorption attributed to the C-O stretching vibrations in the ester/ phenyl ether groups (1260 cm-1), as well as to the C-N stretching motion in the amide groups (1310 cm-1), displayed an apparently higher intensity in the monolayer compared to that in the bulk sample. This seems to confirm the assumption about the internal structure given by indicating an orientation of the ester groups parallel to the substrate surface in a well-organized way. Thus, the limitation on their stretching motions is reduced. As a consequence of this ordered orientation, the absorption of the -CH3 bending mode (1375 cm-1) also showed a higher intensity. Such orientation effects are found to reduce gradually with the increasing number of layers deposited, indicating an increasingly less dense packing of the polymer molecules (not shown). The alignment of the phenyl rings normal to the substrate surface provides the possibility of forming stacked aggregates,47 thus leading to some change in the absorption/emission spectra in comparison to those of unassociated molecules, for example, a red shift (bathochromic shift) of the UV-vis absorption peak by a headto-tail J-aggregation48 or a blue shift (hypsochromic shift) by a side-by-side H-aggregation.49 Coexistence of H- and J-aggregates leads to a broadening of the absorption bands.50 However, very often the packing state of the aggregated molecules is between the head-to-tail and the side-by-side orientation. In this case a parameter called the angle of slippage, R, denoting the angle between the line of the centers of a column of dye molecules and the molecular axes, is used to describe the aggregation state.51,52 If R > ∼32°, a hypsochromic shift is present; (47) Henker, D. J.; Frank, C. W.; Thomas, J. W. Polymer 1988, 29, 437-447. (48) Bohn, P. W. Annu. Rev. Phys. Chem. 1993, 44, 37-60. (49) Nuesch, F.; Gratzel, M. Chem. Phys. 1995, 193, 1-17. (50) Tong, X.; Cui, L.; Zhao, Y. Macromolecules 2004, 37, 31013112. (51) Katoh, T.; Inagaki, Y.; Okazaki, R. Bull. Chem. Soc. Jpn. 1997, 70, 2279-2286.
7040
Langmuir, Vol. 21, No. 15, 2005
Zong et al.
Figure 4. Schematic representation of the multilayer-structure of PAA6B films: A and B describe regions in the film in z-direction (normal to the substrate surface) occupied by the polymer backbones or the side chains, respectively; d0 corresponds to the thickness of a bilayer repeat unit. The parts in thicker lines in region A represent the hydrogen bonds formed between the oxygen atoms O in carbonyl group and the and hydrogen atoms H of -NH in amide or -OH in carboxylic units. Azobenzene chromophores in region B are also emphasized with thicker lines. An electron density contrast exists between the A layer and the B layer, resulting in the Bragg peak (cf. Figure 3).
Figure 5. FT-IR spectra measured for a PAA6B monolayer on a gold coated glass slide (solid gray line) and bulk PAA6B powder in a KBr pellet (broken black line).
otherwise, a bathochromic shift will be observed. The farther away R is from ∼32°, the larger the absorption band shift. The UV-vis spectrum of 40 layers of PAA6B LBK film on a quartz glass slide and that of the PAA6B tetrahydrofuran (THF) solution are shown in Figure 6. In the wavelength range of λ ) 250-300 nm, one can see that the absorption corresponding to the phenyl rings in the polymer backbone shows a hypsochromic shift (peak will appear at λ < 250 nm). This implies the existence of aggregates with R > ∼32° formed by stacking of these (52) Harrison, W. J.; Mateer, D. L.; Tiddy, G. J. T. J. Phys. Chem. 1996, 100, 2310-2321.
Figure 6. UV-vis spectra measured for a 40-layer PAA6B LBK film on a quartz glass slide (gray line) and PAA6B THF solution (black line). (Inset) Enlarged spectra for absorption due to n-π* transition of the azobenzene moieties.
phenyl rings, which is promoted by the parallel alignment of the phenyl rings within the same layer. The absorption at λ ) 300-410 nm due to the π-π* transition of the azobenzene moieties seems to suggest a broadening of the absorption band rather than a simple bathochromic shift; however, the absorption at λ ) 410-460 nm (cf. inset in Figure 6) excludes the coexistence of a hypsochromic shift. This probably implies that the azobenzene within the same layer or in adjacent layers (tail-to-tail packing in the Y-type LBK film, cf. Figure 4) could yet form some intermolecular aggregates with R < ∼32°, and possibly also a small amount of intramolecular aggregates with a similar configuration when the polymer’s molecular weight is sufficiently large. The formation of aggregates with lower
Polyamic Acid Suitable for All-Optical Switching
Langmuir, Vol. 21, No. 15, 2005 7041
Figure 7. Reflectivity, R, and the optical waveguide field-enhanced fluorescence intensity, Ifl, versus the angle of incidence measured for 480 layers of PAA6B on a gold-coated BK7 slide. A band-pass filter with a transmission at λ ) 670 ( 10 nm was used. (a) p-mode (TM2); (b) s-mode (TE1).
angle of slippage is probably due to the relatively low density of the azobenzene side chains. 3.3. Fluorescence. A rather strong fluorescence signal was observed from the 480-layer PAA6B LBK film deposited on a gold-coated BK7 glass slide if excited by either a p- or s-polarized probe laser beam via excitation of a guided mode (cf Figure 7, panels a and b, respectively) in an optical waveguide field-enhanced fluorescence (OWFS) measurement. This technique is similar to the well-established surface plasmon field-enhanced fluorescence spectroscopy (SPFS).53,54 If the thickness of a dielectric layer on a metal- (e.g., gold or silver) coated substrate is sufficiently large, optical waveguide modes45 are observed in addition to the surface plasmon resonance (when a substrate with a high refractive index is used). At the resonance angles of these modes, the guided electromagnetic wave can be used to excite fluorophores in the thin film. By use of the different waveguide modes measured in reflectivity for p- (TM-) and s- (TE-) polarization, respectively, one can calculate/simulate various properties of the waveguide structure as well as extract key parameters of the guided modes, for example, the angular dependence of the optical fields, Hy, as one sweeps through the various resonances, the field distribution within the waveguide structure, the optical intensity integrated for each mode across the slab waveguide, etc. As an example for such a simulation, we show in Figure 8 the calculated reflectivity curve and the corresponding angular dependence of the optical Hy field for the TM2 mode excited at θ ) 46°. As was seen also in the experimentally measured data presented in Figure 7a, the minimum in reflectivity is slightly displaced to higher angles compared to the maximum intensity of the resonantly excited guided mode and, hence, the observed fluorescence intensity. This reflects the fact that the reflectivity curve in this Kretschmann format represents the coherent superposition of the directly reflected laser light with the reradiated and outcoupled light of the waveguide mode. The minimum in reflectivity occurs if the two interfering modes are phase-shifted relative to each other by 180°, which happens at an angle just slightly higher than the actual resonance angle. This is completely equivalent to the situation in surface plasmon fieldenhanced fluorescence spectroscopy reported earlier.54 Further parameters of the different modes, in particular for the TE1 and TM2 modes shown in Figure 7, are (53) Schmidt, E. K.; Liebermann, T.; Kreiter, M.; Jonczyk, A.; Naumann, R.; Offenhausser, A.; Neumann, E.; Kukol, A.; Maelicke A.; Knoll, W. Biosens. Bioelectron. 1998, 13, 585-591. (54) Liebermann, T.; Knoll, W. Colloids Surf., A 2000, 171, 115130.
Figure 8. Calculated reflectivity and the corresponding angular dependence of the resonantly enhanced optical field, Hy, of the TM2 mode (cf. Figure 7a). Note the slight shift between the maximum in Hy and the minimum in R.
summarized in Table 1. In addition to the thickness and the refractive index values of the film, which are found to be d0 ) 650 nm and nx ) 1.47, ny ) 1.49, and nz ) 1.50, respectively, a few parameters are listed that are relevant for the calibration of the fluorescence intensities measured. The column R gives that fraction of the optical field that is actually guided within the polymer film (and not outside as an evanescent wave) and, hence, can excite chromophores located in the film. The values for Re{Neff} and Im{Neff} compare the real and the imaginary part of the effective mode refractive index, that is, the phase velocity and the damping of the two modes, respectively. The latter parameter is a quantitative measure of dissipative losses of the excited waves and, hence, is much larger for the p mode because this one couples to surface plasmon excitations at the metal/dielectric interface.55 The lower these losses are, the higher is the field that can excite fluorescence. The parameter “coupling efficiency” κ ) (1 - Rmin) empirically takes into account that the experimental situation is not as ideal as in the simulations: deviations of the real waveguide structure, for example, thickness and index fluctuations, roughness, etc., lead to an energy density within the waveguide that is lower than that of the simulated spectra. We take the minimum reflectivity of each mode as a qualitative (relative) measure of κ. By comparing Figure 7 panels a and b, one can see that the coupling efficiency for the TE1 mode is only about 50% that of the TM2 mode. Nonetheless, the fluorescence intensity excited by this TE mode is about twice as strong as that of the TM mode. (55) Hickel, W.; Knoll, W. Appl. Phys. Lett. 1990, 57, 1286-1288.
7042
Langmuir, Vol. 21, No. 15, 2005
Zong et al.
Table 1. Parameters Used for Calculation of Optical Intensities in the Various Waveguide Modesa
a
mode
a
Re{Neff}
Im{Neff} abs/rel
κ ∼ (1 - Rmin) rel
r rel/norm
Ifl, cps abs/rel
TE1 TM2
0.914 0.842
1.2464 1.0986
0.0013/1.0 0.0051/0.260
0.526 1.0
0.599/1.0 0.241/0.40
79 000/1.0 37 800/0.478
Abs, absolute; rel, relative; norm, normalized. For details, see text.
Figure 9. Reflectivity, R, and p-mode (TM1) optical waveguide field-enhanced fluorescence spectra, Ifl, measured for 480 layers of PI6B on a gold-coated BK7 slide. A band-pass filter with a transmission at λ ) 670 ( 10 nm was used.
This experimental finding, however, correlates with the relative energy densities, F, calculated for the two modes according to
1 F ∼ RRe{Neff} κ Im{Neff}
(2)
It is also interesting to see whether a significant change of the internal structure and/or the optical properties occurs during the thermal imidization process. Accordingly, OWFS spectra were taken for the same film after imidization. To this end, the film was heated for 8 h at 160 °C, which was shown in a separate study to be sufficient for a complete imidization leading to a polyimide film, denoted PI6B. The corresponding reflectivity and fluorescence spectra for p-polarized excitation are shown in Figure 9. The resonance signal in the waveguide spectrum shifted and broadened a little, as did the waveguide-excited fluorescence signals, however, with an intensity that remained essentially unchanged. When the fluorescence from the film was studied by use of conventional fluorescence spectroscopy, it was found that a more suitable light for the fluorescence excitation would have a wavelength of λ ) 520 nm, with the maximum of emission intensity being located at about λ ) 620 nm (cf. Figure 10). Though the He-Ne laser can also excite the fluorophores, the emission detected at a wavelength of λ ) 670 nm (OWFS signal) is just the tail of the fluorescence, which possesses a significantly lower intensity in comparison to that at λ ) 620 nm. It is noteworthy that no significant fluorescence signal was observed from the solution of PAA6B within a wide range of concentrations, indicating that the individual single molecules are not fluorescent. The fluorescence measurement on a gold-coated BK7 blank glass slide also excluded the possibility of a fluorescent substrate. The fluorescence observed here must be the consequence of the J-aggregates described above. 3.4. All-Optical Switching. The photoisomerization reaction of azobenzene moieties in the PAA6B LBK films are found to be reversible and can be well controlled, as indicated by the studies with UV-vis and surface plasmon resonance spectroscopy, as well as optical waveguide
Figure 10. Fluorescence spectra of 480 layers of PI6B on a gold-coated BK7 slide. The excitation spectrum (solid black line) was recorded at λ ) 620 ( 4 nm, while that of the emission (broken gray line) was taken with λ ) 520 ( 4 nm as the excitation light.
spectroscopy.43 A hybrid LC cell filled with a typical nonpolar liquid crystal, ZLI-3086, by capillary action was built according to the protocol described in the Experimental Section. It shows under a conoscope that the PAA6B LBK layers with the azobenzene moieties in the film being in the as-prepared trans state induce the formation of an LC monodomain in homeotropic alignment in which the main axes of the liquid crystal molecules align normal to the substrate surface (not shown). The refractive index of the sample in the sample normal direction, nz, evaluated from the TM0 mode of optical waveguide spectrum measured for the same LC cell sample was found equal to the extraordinary refractive index of the liquid crystal, that is, nz ) ne, coinciding with the results from the conoscopy study. Upon irradiation with light of λ ) 360 ( 15 nm, the azobenzene moieties in the PAA6B layers isomerized to the cis state, and the optical waveguide spectrum was recorded again. At this state nz was reduced to a value which matches the ordinary refractive index of the LC ZLI-3086, that is, nz ) no. This corresponds to a planar alignment in which the main axes of the liquid crystal molecules are parallel to the substrate surface. Our observation indicated that in the in-plane orientation the main axes of the liquid crystal molecules are in the direction of the capillary flow, which is orthogonal to the dipping direction. This is different from previously reported results in which the LCs are aligned in the dipping direction.27a,56,57 A possible explanation for this difference is that the capillary flow of the LC molecules in our case plays a similar role as a piece of soft cloth in a rubbing process. Meanwhile, the in-dipping-direction planar alignment mostly takes place if the main chain of the polymer is very stiff.57 It is obviously not the case for PAA6B. The film morphology generated during the capillary flow keeps aligning LC molecules in the rubbing direction as soon as they take on the planar alignment.58 If irradiation of light with λ g 420 nm is applied to the (56) Sekkat, Z.; Bu¨chel, M.; Knobloch, H.; Seki, T.; Ito, S.; Kobersten, J.; Knoll, W. Opt. Commun. 1994, 111, 324-330. (57) Bu¨chel, M.; Weichart, B.; Minx, C.; Menzel, H.; Johannsmann, D. Phys. Rev. E 1997, 55, 455-463. (58) Berreman, D. W. Phys. Rev. Lett. 1972, 28, 1683-1686.
Polyamic Acid Suitable for All-Optical Switching
Figure 11. LC alignment switching measured with an asymmetric sandwich LC cell by using PAA6B as the photoregulation layers. UV (λ ) 360 ( 15 nm) and visible light (λ > 420 nm) were applied alternatively as irradiation sources.
system, azobenzene moieties in the PAA6B LBK films isomerize back to the trans state. In this process nz increases to ne and the homeotropic alignment fully recovers. By recording the reflectivity at a fixed incident angle and alternatively applying UV (λ ) 360 ( 15 nm) or visible (λ > 420 nm) light to irradiate the LC cell sample, reversible changes can be observed (cf. Figure 11). The curves indicate clearly that the LBK layer fabricated from this new azobenzene-containing polymer, PAA6B, reversibly switches the LC alignment between a homeotropic and a planar alignment. No noticeable fatigue phenomenon is visible after many photoisomerization cycles. Irradiation at a power density of 0.14 mW/cm2 for the UV light and a similar power for the visible irradiation leads to a photoswitching rate of ∼10 s for planar to homeotropic alignment switching and ∼20 s for the reverse process. The switching rates are comparable to those results reported previously on other azobenzene-containing polymer LBK films,59 in our case, however, with a light intensity much lower than in the previously reported examples. By heating PAA6B LBK films on a gold-coated LaSFN9 glass to 160 °C in vacuo and keeping the temperature for 8 h, polyimide PI6B thin layers are prepared. Interestingly, under the identical irradiation conditions, the rates of the photoisomerization reaction of azobenzene moieties in PI6B films (Figure 12, gray line) are noticeably faster than those in PAA6B layers (Figure 12, black line). The reason for such acceleration is still unclear. Next, an LC cell was prepared by using PI6B thin layers instead of PAA6B films together with a blank quartz glass slide following the same protocol, and the same operation was performed on the new LC cell. The results reveal that in comparison to the PAA6B films the thermally converted carboxylic group-free PI6B layers reversibly photoswitch the LC alignment at very similar rates by the photoisomerization reaction of the azobenzene moieties upon irradiation with light of different wavelengths.43 (59) Tang, Y. X.; Liu, D. G.; Xie, P.; Zhang, R. B. Macromol. Rapid Commun. 1996, 17, 759-766.
Langmuir, Vol. 21, No. 15, 2005 7043
Figure 12. Photoisomerization kinetics of 480-layer PAA6B LBK films (black line) and the resulted PI6B layers (gray line) studied by optical waveguide spectroscopy. All irradiation conditions were retained.
If one compares the photoisomerization rate of the azobenzene moieties in the polymer films (PAA6B or PI6B) with the rate of the photoswitching process in the LC cell built with such azobenzene-containing layers, it is easy to find that the former is even less than 1% of the latter. This factor of 100 in the acceleration in the photoresponse rate is partially the consequence of a thinner azobenzenecontaining polymer layer, which needs a shorter equilibrium time. More importantly, it profits from the mutual alignment process between the azobenzene side groups and the interdigitating nematic liquid crystal molecules as a part of the LC monodomain formation. 4. Conclusions Azobenzene-containing polyamic acids with good solubility in water-immiscible solvents prepared from condensation polymerization reactions are excellent candidates for producing azobenzene-embedded polymer matrices with well-defined architectures by the Langmuir-Blodgett-Kuhn technique. The sufficient separation of UV-vis absorption bands attributed to the π-π* and n-π* transitions of the azobenzene moieties permits good control of the photoisomerization reaction of the chromophores. This, on one hand, allows for more precise understanding of the impact of the free volume effect; on the other hand, it gives guidelines for fabricating all-optical switching devices with high performance. This work was conducted by studying analogue PAAs with systematically tuned spacer lengths and lateral distribution of the azobenzene side chains along the polymer backbone. It will be reported elsewhere. Acknowledgment. We express our gratitude to Dr. U. Hees, Dr. R. Ku¨gler, Dr. T. Zhu, and Dr. A. Baba for stimulating discussions and to G. Herrmann, W. Scholdei, M. Bach, and H. Menges for their technical support. Y.Z. gratefully acknowledges the Catholic Academic Exchange Service (KAAD) for a Ph.D. fellowship. LA0506067
