J. Phys. Chem. B 2000, 104, 10513-10521
10513
Photochemical Behavior and Formation of Surface Relief Grating on Self-Assembled Polyion/Dye Composite Film Jin-An He, Shaoping Bian, Lian Li, Jayant Kumar, and Sukant K. Tripathy* Center for AdVanced Materials, Department of Chemistry and Physics, UniVersity of Massachusetts Lowell, Lowell, Massachusetts 01854
Lynne A. Samuelson Natick Soldier Center, U.S. Army Soldier & Biological Chemical Command, Natick, Massachusetts 01760 ReceiVed: May 8, 2000; In Final Form: August 29, 2000
Holographic surface relief gratings (SRGs) were fabricated on composite films assembled by electrostatic layer-by-layer (ELBL) deposition of a polyelectrolyte, poly(dimethyl diallylammonium chloride) (PDAC), and an azo dye, Congo Red (CR). Surface modulation and first-order diffraction efficiency of the SRG were found to increase with the thickness of the PDAC/CR films. Polarized absorption spectra indicated an oriented growth of CR on the PDAC film. Analysis of the film thickness, FTIR, and FT-Raman results confirmed that the electrostatic attraction between CR and PDAC, as well as the π-π interaction between CR chromophores resulting in the formation of J aggregates, lead to formation of PDAC/CR composite films. Photochemical changes of the PDAC/CR films after irradiation were investigated by UV-vis absorption, FTIR, and FTRaman spectroscopy. The results indicate that in addition to trans S cis photoisomerization of CR in the composite film, an irreversible photochemical degradation of CR also simultaneously occurs. Recording SRG on PDAC/CR films by s- and p-polarized beams show different behavior compared to spin-coated films of polymers containing functionalized azo chromophores. Our results indicate that the volume collapse due to the photodegradation of CR in the polymeric matrix, as well as gradient force-induced migration due to trans S cis isomerization cycling of CR contribute to the formation of SRG on the composite films. This approach provides a methodology to fabricate SRGs for optical information storage applications by using the facile ELBL technique to assemble commercially available azo dyes and polyelectrolytes.
1. Introduction Formation of organic thin films based on molecular selfassembly is one of the important new techniques to form structures for a number of technological applications. Recently developed electrostatic layer-by-layer (ELBL) deposition has been shown to be a simple and versatile method for assembling thin films. Although the ELBL technique was initially applied to the layering of polyelectrolytes to form alternating polycation/ polyanion multilayer assemblies,1 it has been extended to a wide variety of other interesting charged materials such as functionalized polymers (dendrimers, polymeric nanocrystal, and chromophore-containing polymers), metal and semiconductor nanoparticles.2 The ELBL method has even proven to be quite effective for the layering of biological components such as proteins, enzymes, DNA, cell membrane, and viruses.3 Mallouk’s group has utilized the technique to form energyharvesting/electron-transfer supramolecular systems to mimic natural photosynthesis.4 Caruso and co-workers have reported formation of hollow inorganic-hybrid spheres by colloidal template using ELBL technology. They postulate that the hollow reservoirs have potential applications in diverse areas ranging from medical to materials science.5 It has also been demonstrated that a number of low-molecularweight charged compounds could be integrated into polyelectrolyte or polypeptide films by the ELBL technique.6 In the work * Author to whom correspondence should be addressed.
of Ariga et al. and Cooper et al.,6 an azo dye, Congo Red, had been successfully incorporated into polymer films. However, there is no report on the photochemical properties and applications of these azobenzene-functionalized organic films. Azobenzene compounds possess interesting trans S cis photoisomerization characteristics which may have potential applications for holographic information storage and molecular photoswitching;7 it is therefore essential to investigate the photochemistry and other relevant properties of the azo dye/polyelectrolyte composite films. In this paper, we report the detailed characterization and photochemical properties of the PDAC/CR films formed by the ELBL deposition technique. Furthermore, the azo chromophorecontaining films have been used for surface relief grating (SRG) fabrication. The direct formation of holographic SRGs on azobenzene-functionalized polymer films has recently attracted much attention due to the potential applications in optical information storage and fabrication of diffractive optical elements.8 Several different process methods have been used to produce optical quality films for SRG formation. Spin coating is a conventional, simple, and often-used method for SRG fabrication using azo polymers.9 Recently, Langmuir-Blodgett and sol-gel processes have also been utilized to control the thickness and composition of the azo polymer films in order to achieve optimal conditions for SRG formation.10 On the other hand, several research groups have investigated the origin of SRG, and different mechanisms have been
10.1021/jp001715r CCC: $19.00 © 2000 American Chemical Society Published on Web 10/21/2000
10514 J. Phys. Chem. B, Vol. 104, No. 45, 2000 SCHEME 1: Molecular Structures of Congo Red and PDAC Used for the ELBL Assembly
proposed for the SRG formation on the films of azo polymers.11 It is generally accepted, however, that the photoinduced trans S cis isomerization of azo chromophores in the polymers is a prerequisite to the formation of SRGs. Therefore a large number of azo-containing polymers with varying chemical composition have been synthesized over recent years and studied for the photofabrication of SRGs.12 In the present work, it is demonstrated that the ELBL deposition technique using low-molecular-weight, commercially available, water-soluble azo dyes may be used for SRG formation. Multilayer composite films of poly(dimethyl diallylammonium chloride) (PDAC) and Congo Red (CR) were prepared onto planar solid supports via the ELBL adsorption technique. The PDAC/CR composite films were then successfully used to write SRGs where the surface modulation of the grating was directly dependent on the film thickness. Furthermore, the molecular orientation and photochemical behavior of CR in the polyelectrolyte matrix have been extensively investigated in order to understand the mechanism of SRG formation on the PDAC/CR films. 2. Experimental Section 2.1. Materials and Methods. Congo Red (CR) and PDAC (20 wt % in water, Mw 200000-350000) were purchased from Aldrich and used as received. The chemical structures of PDAC and CR are shown in Scheme 1. It is well-known that CR is an acid/base indicator with color change from blue to red in the approximate pH range from 3 to 5.13 The sequential adsorption of CR and PDAC from their dilute solutions to form PDAC/ CR multilayers was carried out using a HMS series programmable slide stainer apparatus (Carl Zeiss, Inc.) under conditions similar to those presented in a previous work.6a Briefly, a CR solution having a concentration of 0.5 mg/mL dissolved in 10 mM Na2HPO4-NaH2PO4 buffer solution (pH 7.8) was used as the anionic solution, and a PDAC solution having a concentration of 0.5 mg/mL in 10 mM citric acid-sodium citrate buffer solution (pH 4.4) containing 0.20 M NaCl was used as the polycationic solution. A negatively charged glass slide (due to partially hydrolyzing the surface by 2% KOH aqueous solution) was first immersed into PDAC aqueous solution for 10 min. After rinsing three times in Milli-Q water (2 min each bath with agitation), the modified substrate was then transferred into the CR solution for 10 min, agitated, and washed three times with water for 2 min. All adsorption procedures were carried out at room temperature (approximately 18 °C).
He et al. Photodegradation experiments of the PDAC/CR composite films were performed under irradiation of a 488-nm Ar+ laser beam with an intensity of 100 mW/cm2. The experimental setup used for the photofabrication of SRGs was reported previously.9 In the present work, we use two 488-nm, 150-mW/cm2 laser beams with either p- or s-polarization from an Ar+ laser as writing beams. For each sample, the recording process is stopped when the first-order diffraction efficiency, which is defined as a ratio of intensity of the first-order diffracted beam from the SRG to that of the totally transmitted (including diffracted) beam, saturates (usually around 40 min). The SRG formation was monitored in real time by using a low-power (0.5 mW), unpolarized 632.8-nm He-Ne laser. The intensity change of the first-order diffracted beam in the transmission mode from the grating during the writing process was detected by a digital power meter (Newport, Model 815 Series), and the data was collected through a lock-in amplifier (SR510, Stanford Research Systems) by a personal computer. The selected incident angle (θ) between the two interference recording beams was 19°, resulting in a grating period of around 1.5 µm. 2.2. Instrumentation. The absorption and polarized absorption spectra were measured on a Perkin-Elmer Lambda-9 UV/ vis/near-infrared spectrophotometer. Reflection absorption FTIR (RA-FTIR) spectra were obtained using a Perkin-Elmer 1720X FTIR spectrometer at 2 cm-1 resolutions with a RA accessory. Usually 512 scans were carried out to achieve an acceptable signal-to-noise (S/N) ratio. The angle of incidence was chosen to be 45° for both RA-FTIR and polarized absorption measurements. FT-Raman spectra were measured at a resolution of 2 cm-1 by a Perkin-Elmer 1700X NIR FT-Raman spectrometer with a liquid-nitrogen-cooled InGaAs detector and a 1064-nm Nd:YAG laser as the excitation source. The excitation laser was normally incident on the films and 1024 scans were performed to obtain a good S/N ratio. Thickness measurements were carried out with a Rudolph Research AutoEL III ellipsometer with a 632.8-nm He-Ne laser as the light source and confirmed by a Sloan Dektak IIA profilometer. Thickness data were average values obtained from at least three different points on a given sample. Atomic force microscopic (AFM) images were obtained using an atomic force microscope (Park Scientific, CA) operated in the contact mode using a standard silicon nitride cantilever (force constant 0.03 N/m) in ambient air. The scan frequency was 1 Hz, and force on the cantilever was set at 40-50 nN. The solid supports for absorption, AFM, and FT-Raman measurements were regular glass slides, and for RA-FTIR were glass slides covered with about 1000 Å aluminum. 3. Results and Discussion 3.1. Formation and Characterization of the PDAC/CR Composite Film. Unlike typical polyanion/polycation adsorption, low-molecular-weight charged dyes desorb from the support surface into the polyion solution, and thus reduce the deposition rate of the polyion/dye films. The amount of dye desorbed is affected by many factors, such as nature of the polyions, as well as concentration, ionic strength, and adsorption time in the polyion solution.6a,b Under our deposition conditions, 0.5 mg/mL PDAC solution (pH 4.4) was used as the polycation solution, to minimize the desorption of CR due to the electrostatic repulsion between protonated CR (amine groups) and polycationic PDAC in solution.13 The solid support was first immersed in the acidic PDAC solution for 10 min. Subsequently, the substrate with one layer of PDAC was immersed into a pH 7.8, 0.5 mg/mL CR solution for varying adsorption times. The change in the absorbance of the PDAC/CR films at 510 nm,
SRG on Self-Assembled Polyion/Dye Composite Film
Figure 1. Relative adsorbed amount of CR, monitored by its absorption at 510 nm, in the PDAC/CR assembly as a function of adsorption time in CR dipping solutions.
Figure 2. Absorption spectra of PDAC/CR multilayer films on glass slides. The solid curves correspond to the CR layer as the outermost layer with different adsorption cycles shown in the figure. The dotted curve shows the absorption of CR dissolved in the aqueous solution of 10 mM Na2HPO4-NaH2PO4 buffer solution (pH 7.8). The inset shows the linear increase of CR absorbance at 350 and 510 nm with the number of bilayers.
which reflects the adsorbed amount of CR on the PDAC layer, with the adsorption time of the substrate in the CR solution, was measured and the results are presented in Figure 1. This figure shows that the absorbance at 510 nm increased with the adsorption time from 0 to 10 min. The amount of absorbed CR saturates after 10 min. Thus 10 min of CR adsorption time was used throughout our deposition procedure for PDAC/CR film formation. Figure 2 shows the UV-vis absorption spectra for the multilayers prepared by sequential deposition of PDAC and CR from their dilute solutions. The dotted curves are the absorption spectra of CR dissolved in the aqueous solution of 10 mM Na2HPO4-NaH2PO4 buffer solution (pH 7.8). As shown in the inset of Figure 2, the characteristic absorption peaks for CR at 350 and 510 nm from the π f π* transitions6c are observed to increase linearly with the number of PDAC/CR deposition cycles. A plot of absorbance at 510 nm vs the number of bilayers
J. Phys. Chem. B, Vol. 104, No. 45, 2000 10515 gives a slope of 0.029 O.D./bilayer. Our results indicate that the sequential growth of PDAC/CR assemblies is linear and reproducible. In the solution, the maximum absorption of CR is located at 494 nm which has a 16-nm blue shift compared with that of the PDAC/CR multilayers. This spectroscopic shift has been observed previously and is attributed to the formation of ordered J aggregates in the films.6a,c Formation of J aggregates of cyanine dyes in polyelectrolyte composite films14 and at the mica/water interface15 has been observed and confirmed by AFM morphological investigation. AFM images show that the J aggregates of cyanine dye possess three-dimensional leaf-like island structures.15 We observed the surface morphology of the PDAC/CR composite films by AFM, and the images are shown in Figure 3. The thickness of the aggregates is ca. 50 Å high for (PDAC/CR)1 (1 bilayer of PDAC/CR) film, as shown in Figure 3a. The lateral dimensions, 500 Å length, 300 Å width interpreted from the AFM image however, may be exaggerated due to tip size considerations. With the increase of the film thickness, an ordered pattern showing three-dimensional scale-like aggregates with higher grain density was observed in the (PDAC/CR)80 film as shown in Figure 3b. The AFM observations directly demonstrate the existence of J aggregates of CR in PDAC/CR films and account for the red shift of the absorption spectra. The AFM images of the PDAC/CR films show ordered selfaggregated structures in the composite films. The orientation of the CR molecules in the films was further investigated by polarized absorption spectroscopy. Figure 4 shows the polarized absorption spectra of a (PDAC/CR)4 film. The absorption at 510 nm of the polarized light parallel to the plane of the film (All) was 1.3 times larger than that of the polarized light perpendicular to the plane of the film (A⊥). The dichroism ratio gave the orientation factor S ) 0.09 according to following equation: S ) (All - A⊥)/(All + 2A⊥).16 The small orientation factor indicated that the transition moment of CR, giving absorption at 510 nm, which has been shown to be parallel to the long axis of CR molecule,16 tended to be dominantly parallel to the plane of the film.6d The thickness of the PDAC/CR films was measured by ellipsometry and confirmed by Dektak profilometer. The thickness of the PDAC layer in the PDAC/CR bilayer was measured to be ca. 10 Å. The dependence of the film thickness on the number of bilayers is shown in Figure 5. The linear increase of the film thickness with the sequential deposition cycles further confirms a uniform multilayer assembly of the PDAC/CR bilayer. The slope of the fitting line is 65 Å/bilayer, which is the average thickness of one PDAC/CR bilayer. By subtracting the thickness of the PDAC layer, the thickness of CR layer is determined to be approximately 55 Å in one PDAC/CR bilayer. The lengths of the long and short axis of CR are 19 and 5 Å, respectively, from molecular simulation.6a Thus thickness measurements also support the formation of CR aggregate in the composite films. The electrostatic interaction between the dimethyl diallylammonium cation of PDAC and the sulfonate anion of CR is a crucial driving force in the formation of the PDAC/CR films. However, the subsequent formation of the J aggregates among CR molecules leads to a considerable increase in the dye content in the composite films. Therefore, it is worthwhile to explore the molecular mechanism of J aggregate formation among CR molecules. The RA-FTIR spectroscopic results of the PDAC/ CR films presented in Figure 6 indicate that electrostatic interaction exists between CR and PDAC. In addition, the intermolecular π-π interaction involving the conjugated ring
10516 J. Phys. Chem. B, Vol. 104, No. 45, 2000
He et al.
Figure 3. Two-dimensional AFM images of the (PDAC/CR)1 film (a) and the (PDAC/CR)80 film (b).
Figure 4. Polarized absorption spectra of the (PDAC/CR)4 film. The plane of the sample is 45° to the excitation beams.
system of the CR molecules is the driving force to form the J aggregates of CR. The details of the experimental evidences are discussed in the following paragraphs. For reference, the RA-FTIR spectra of the cast films of PDAC and CR are shown in Figure 6. The assignments of the peaks in these spectra are summarized in Table 1 by referring to the data in the literature.17 From Figure 6a and Table 1, a broad band at around 1200 cm-1 and a sharp band at 1047 cm-1 are, respectively, attributed to the asymmetric and symmetric stretching vibrations of the sulfonate groups of CR which are sensitive to their environments.17e The obvious shape change of the band at 1200 cm-1, as well as the appreciably bathochromic shift from 1047 to 1042 cm-1 observed for the (PDAC/CR)80 film in Figure 6c, indicates an environmental change around the SO3- groups. On the other hand, one shoulder band at 2946 cm-1 in Figure 6b is assigned to the asymmetric CH3 stretching vibrations of PDAC which are sensitively affected by the polarity of the nitrogen atom in the dimethyl diallylammonium group. In the (PDAC/CR)80 film, the 2946-cm-1 band shifts to 2937 cm-1. The shift reflects the polarity change of the nitrogen atom of PDAC which further affected the vibration frequency of the neighboring CH3 groups.
Figure 5. Thickness dependence of the PDAC/CR composite films on the number of bilayers. The slope of the fitting line gave the average thickness of the PDAC/CR bilayer as 65 Å.
The cooperative changes of the IR features of the SO3- groups in CR and the CH3 groups in PDAC provide direct evidence that the microenvironment changes in the vicinity of the sulfonate and dimethyl diallylammonium groups. This may be explained and established the basis for the ELBL assembly due to the electrostatic coulombic interaction between the anionic sulfonate groups of CR and the cationic dimethyl diallylammonium groups of the polyelectrolyte PDAC. IR results presented in Figure 6 and Table 1 also provide further insight regarding the molecular mechanism for the J aggregate formation of CR in the PDAC/CR composite films. As shown in Figure 6a, a broad band at 1605 cm-1 (overlapping of ν8a, ν8b mode) and a weak band at 1503 cm-1 (ν19a mode) are assigned to the aromatic ring C-C stretching vibrations of the solid-state CR.17c-e The two bands shifted detectably from 1605 to 1611 cm-1 and from 1503 to 1509 cm-1, respectively, in the PDAC/CR film (see Figure 6c). These shifts of the CR
SRG on Self-Assembled Polyion/Dye Composite Film
J. Phys. Chem. B, Vol. 104, No. 45, 2000 10517
Figure 6. RA-FTIR spectra of cast CR (a), PDAC (b), and the (PDAC/CR)80 composite films (c).
TABLE 1: RA-FTIR Peaks and Assignments for CR, PDAC, and PDAC/CR Films
a
PDAC film/cm-1
assignment
3418 3014 2946 2867 1638 1480
ν (O-H)a ν (-CH2CHdCH2)b νas (CH3) νs (CH2) δ (O-H)a δ (CH2)
CR film/cm-1
assignment
3460 1605 1503 1373 1200 1120 1047
ν (N-H) ν8a/b (ring C-C) ν19a (ring C-C) δ (CH2) νas (SO3-) ν (C-N) νs (SO3-)
(PDAC/CR)80 film/cm-1
assignment
3460 3029 2937 2867 1611 1509 1372 1200 1042
ν (N-H) ν (aromatic C-H) νas (CH3) νs (CH2) ν8a/b (ring C-C) ν19a (ring C-C) δ (CH2) νas (SO3-) νs (SO3-)
From water in the casting film of the PDAC. b From the residual monomer [(CH3)2N+(CH2CHdCH2)]Cl- in the PDAC.
ring-mode bands indicate a strong π electron overlap involving the conjugated ring system of CR. More specifically, the intermolecular π-π interaction from the phenyl and naphthyl ring between the CR molecules may contribute to the driving force to form J aggregates of CR in the composite films. The J aggregates of CR in the PDAC/CR films formed by the π-π interaction were further confirmed by comparing the Raman spectrum of CR powder with that of the PDAC/CR composite films. The FT-Raman spectra of the CR powder and a PDAC/CR film are presented in Figure 7. Table 2 lists the main Raman peaks and their assignments based on the literature.17a,b,e,18 We again find that two Raman bands corresponding to the aromatic ring mode of CR, 1591 cm-1 from the phenyl ring and 1353 cm-1 from the naphthyl ring in the solid-state CR, shift, respectively, to 1595 and 1373 cm-1 in the PDAC/CR film. In conclusion, layer-by-layer assembly of PDAC/CR films is attributed to electrostatic attraction and intermolecular π-π interaction. The PDAC layer adsorbs the first monomolecular layer of CR through electrostatic attraction; the subsequently adsorbed chromophores lead to the formation of J aggregates among CR molecules due to π-π interaction involving the conjugated π electron system of CR molecules. The proposed architecture of the PDAC/CR composite film based on the above discussions is presented in Scheme 2.
Figure 7. FT-Raman spectra of the (PDAC/CR)160 film (a) and the solid-state CR powder (b).
3.2. Photochemical Properties of the PDAC/CR Films. It is well-known that azobenzene compounds show reversible trans S cis photoisomerization behavior, and have potential applications in optical switching and information storage fields.7 Recent investigations on SRG formation of azobenzene polymer films also show that the trans S cis isomerization process is closely linked to the appearance of SRG structure.11 CR is an azo
10518 J. Phys. Chem. B, Vol. 104, No. 45, 2000 TABLE 2: Assignments for Raman Bands of the Solid-State CR and PDAC/CR Film solid-state CR/cm-1
(PDAC/CR)160 film/cm-1
assignment
1591 1454 1408 1353 1156
1595 1451 1402 1373 1155
phenyl ring mode -NdN- stretching -NdN- stretching naphthyl ring mode phenyl-Nd stretching mode
aromatic compound that forms optical-quality composite films with a polyelectrolyte and has relevant application based on its photochemical properties. Therefore, it is important to investigate the photochemical behavior of the PDAC/CR films. The photochemical reaction of the PDAC/CR composite films was monitored by the change of absorption as a function of irradiation time using visible and UV light as excitation sources. Figure 8 shows the results obtained by visible light irradiation. It can be seen that the intensities of the characteristic absorption bands at 350 and 510 nm decrease rapidly with irradiation time, while the intensities of three new peaks at 245, 295, and 370 nm increase with irradiation time. Unlike most azobenzene compounds showing reversible light-induced absorption change,7,19 the absorption change in the PDAC/CR film does not exhibit any recovery by leaving the sample in the dark for several weeks, irradiating with longer-wavelength light, or even on heating. The photochemical bleaching process can be seen visually as the color of the film changed from red to yellow. These results imply that the photochemical reaction of CR in the PDAC matrix is irreversible. The inset of Figure 8 showed the change of the absorbance at 510 nm from CR with irradiation time. The photodegradation kinetics of CR is not monoexponential, indicating the coexistence of different photochemical processes in the PDAC/CR composite films. A biexponential decay function can closely fit our experimental data. The result indicates that at least two photodynamic processes, most possibly reversible photoisomerization and irreversible photodegradation of CR, are coexistent in the PDAC/CR films. When the PDAC/ CR film is illuminated with a 360-nm UV light (4 mW/cm2), a similar photodegradation phenomenon was observed except at a different degradation rate (data not shown). The control experiments show that the photobleaching process is not appreciable in cast CR films and CR aqueous solution. As a comparison, we also use poly(allylamine hydrochloride) (PAH) as the polycation to form PAH/CR multilayers by the ELBL technique. The PAH/CR multilayers film shows a much slower rate of photodegradation. These results indicate that the photochemical degradation process of CR is sensitive to the matrix environment. There appears to be some type of molecular interaction between the polyelectrolyte, PDAC, and the CR in these electrostatically grown assemblies that promotes this unusual photochemical behavior. The photodynamic processes of CR in the polymeric PDAC medium are quite interesting and worth further investigation. The photodegradation of CR was verified more clearly by monitoring the change of the RA-FTIR spectrum of the PDAC/ CR film during visible irradiation. The results are presented in Figure 9. As shown, the intensities of the bands at 1611, 1372, 1042, and 834 cm-1 decreased during the irradiation. The change of the intensity during illumination had been used as a signature for trans S cis photoisomerization of azobenzenes.19 Our RA-FTIR results are in accordance with the literature, and indicate that the photoisomerization process proceeds simultaneously with the photodegradation of CR in the PDAC/CR composite films as implied by the UV-vis absorption experi-
He et al. ments. Furthermore, several new IR bands at 1264, 968, and 649 cm-1 appeared after irradiation. Although we cannot assign the origin of these bands at this time, it seems reasonable that these peaks should come from the photodegradation products of CR. The photoisomerization of azo polymer in polyion films had been recently reported by Frank and co-workers.20 In their work, the relationship between isomerization and polyelectrolytes used and film thickness was investigated. The irreversible photodegradation of the azo chromophore should also occur in their system since only partial recovery of absorbance by thermal relaxation was observed. Our results show that the absorption change of the PDAC/CR film upon irradiation is not recoverable at all, which means that the trans S cis photoisomerization process of CR is not the only photodynamic process occurring in the PDAC/CR film. The direct experimental evidence supporting the trans S cis photoisomerization of CR comes from the comparative study of the Raman spectra of the PDAC/CR film before and after irradiation. Figure 10 shows the Raman spectra of the (PDAC/CR)160 film before and after irradiation. On one hand, the Raman intensities after irradiation decreased to about half the value compared to that without irradiation, which is in accordance with the fact that photodegradation of CR took place in the composite film. In addition, a new Raman band at 1482 cm-1 appears after irradiation. This peak is usually assigned as the characteristic Raman scattering from the cis isomer of azobenzene compounds.17a,b,h,18 In our films, the new peak must be related to the buildup of the cis isomer of CR. Usually, the cis isomers of azo compounds are unstable and will rapidly relax to the trans state in the free environment. However, the cis-azobenzene could be more stable in some rigid structures,21 such as nanosized supramolecular cages and crosslinked polymeric matrixes. It can be postulated that the stacking structure of the PDAC/CR films produces a similar confined geometry to also enhance the stability of the cis isomer of CR. In summary, UV-vis absorption and RA-FTIR spectral investigations of the PDAC/CR films after the irradiation indicate that an irreversible photodegradation, as well as reversible trans S cis photoisomerization of CR take place in the composite films. The Raman spectra show the existence of a stable cis isomer of CR after illumination that is explained by the unique architecture of the PDAC/CR composite films. 3.3. Photofabrication and Mechanism of SRG on the PDAC/CR Films. As discussed earlier in the Introduction section, the films of azobenzene-functionalized polymers can be utilized to fabricate holographic SRG structures by a facile one-step all-optical procedure. Since the PDAC/CR composite films exhibit good optical quality22 and show the photoisomerization ability as discussed in Section 3.2, it is tempting to speculate whether these low-molecular-weight azobenzene dye/ polyelectrolyte films can be used for SRG fabrication in a manner similar to azo polymers. To address this question and explore this possibility further, a series of experiments were carried out to write SRG structures on the PDAC/CR composite films. Efficient SRGs so far have not been reported with lowmolecular-weight azo dyes. In fact, from our control experiments, SRGs may not be fabricated in guest-host spin-coated films of polymers such as poly(methyl methacrylate) doped with low-molecular-weight azo dyes. Figure 11 shows the change of the first-order diffraction efficiency of the SRGs fabricated on the PDAC/CR composite films with exposure time. The curves a, b, c, d, and e correspond to 40, 80, 120, 240, and 320 bilayers, respectively, of the PDAC/ CR films. The inset shows the dependence of the first-order
SRG on Self-Assembled Polyion/Dye Composite Film
J. Phys. Chem. B, Vol. 104, No. 45, 2000 10519
SCHEME 2: Schematic of the Configuration of the PDAC/CR Composite Film on a Negatively Charged Solid Support (the electrostatic attraction between the substrate, PDAC, and CR, and the π-π interaction among CR molecules are shown by dotted lines)
Figure 8. Change of absorption spectrum of the (PDAC/CR)20 multilayers with irradiation time. The excitation light source is an Ar+ laser with intensity of 100 mW/cm2 at 488 nm. The inset shows the function of the absorbance at 510 nm from CR with irradiation time. The experimental data can be well fitted (χ2 ) 1.1 × 10-5) with a biexponential function: y ) A * exp (-B * x) + C * exp (-D * x) giving the parameters A ) 0.148 ( 0.007; B ) 2.0 ( 0.2 × 10-3 s-1; C ) 0.332 ( 0.007; and D ) 1.5 ( 0.07 × 10-4 s-1, respectively.
diffraction efficiency on the thickness of the composite films. As shown, the recording time for saturation and the maximum diffraction efficiencies increase with film thickness. Typically, ca. 5 min is needed to obtain a maximum diffraction efficiency of ∼2% on the (PDAC/CR)40 film, and ca. 75 min is needed to obtain a maximum diffraction efficiency of ∼17% on the (PDAC/CR)320 film. The increase in diffraction efficiency does not show a tendency to saturate with film thickness. Thus, it is anticipated that higher diffraction efficiency can be obtained
Figure 9. RA-FTIR spectra of the (PDAC/CR)80 film before (a), after 60 min (b), and after 120 min (c) irradiation using an Ar+ laser with intensity of 150 mW/cm2 at 488 nm as the light source. The circles highlight several obvious IR changes of the film after irradiation.
with thicker PDAC/CR composite films. After recording the SRGs by two p-polarized laser beams at 488 nm, the exposed area was investigated by AFM. The morphological features of the SRGs formed on the PDAC/CR films are shown in Figure 12. The gratings showed regularly spaced sinusoidal surface relief structures with depth modulations ca. 400 Å for the (PDAC/CR)40 film (Figure 12a) and ca. 1200 Å for the (PDAC/ CR)240 film (Figure 12b) with a period of around 1.5 µm. The grating period could be adjusted by changing the incident angle (θ) of the two writing beams and was consistent with the theoretically calculated period according to Bragg’s equation d ) λ/(2 sin θ/2). The surface modulations of SRG formed on
10520 J. Phys. Chem. B, Vol. 104, No. 45, 2000
He et al.
Figure 10. FT-Raman spectra of the (PDAC/CR)160 film before (a) and after (b) 100 min irradiation using an Ar+ laser with intensity of 150 mW/cm2 at 488 nm as the light source. The intensity of curve b was doubled in order to show the details of the bands.
Figure 11. Plot of the first-order diffraction efficiency of the SRGs on the PDAC/CR composite films with exposure time. The curves a, b, c, d, and e correspond to 40, 80, 120, 240, and 320 bilayers, respectively, of the PDAC/CR films. The interference writing beams are two p-polarized 488-nm Ar+ laser beams with an intensity of 150 mW/cm2. The inset shows the dependence of the first-order diffraction efficiency on the thickness of the composite films.
the PDAC/CR composite films were obtained by measurement of the difference between peak and valley of the grating structure in the AFM images. The results are shown in Figure 12c. The modulation of the SRG increases with the number of bilayers, i.e., the thicker the PDAC/CR multilayers are, the larger the SRG modulation that can be achieved which leads to the greater diffraction efficiency of the grating. Typically, the surface modulation is ca. 1400 Å for the (PDAC/CR)320 multilayers. The above results clearly demonstrated that the low-molecularweight CR dye/polyelectrolyte films could be used for SRG fabrication. To our knowledge, this is the first report of large SRG modulation obtained from a low-molecular-weight azobenzene compound. The SRGs formed on the PDAC/CR films show two different features when compared with those using azo polymers. The first difference is that the PDAC/CR films become more transparent after recording SRG, i.e., a photochemical bleaching process is taking place during laser irradiation on the PDAC/ CR films. The second important difference is that there is no appreciable polarization dependence of SRG recording on the PDAC/CR films. The SRG formed on azo polymers usually show strong writing-beam polarization dependence and negli-
Figure 12. Two-dimensional AFM images of the SRG structures formed on the (PDAC/CR)40 film (a) and on the (PDAC/CR)240 film (b). The recording conditions are the same as that in Figure 11. Dependence of the surface modulation of the SRG fabricated using the PDAC/CR films on the number of bilayers is shown in (c).
gible SRG modulation is obtained under s-polarized light recording.9,11a,c,23 In contrast to the behavior of azo polymers, the s-polarization recording SRG on the PDAC/CR films show significant surface modulation. For example, the modulations for the p- and s-polarization recorded SRGs using the (PDAC/
SRG on Self-Assembled Polyion/Dye Composite Film CR)160 film are, respectively, ca. 1100 and 700 Å at the same recording intensity. The different polarization dependence for SRG recording on the PDAC/CR composite films and azo polymers films implies that the mechanism of SRG formation is appreciably different in these two systems. Considering the photodegradation of CR in the PDAC/CR films after photofabricating SRG, we conclude that the volume collapse due to the photodegradation of CR may also contribute to SRG formation in the PDAC/CR composite films in addition to migration of azo dye induced by the optical-field gradient force. In summary, we have demonstrated for the first time the fabrication of SRGs from composite assembly of a polyelectrolyte and a low-molecular-weight azo dye using the facile ELBL deposition technique. A volume collapse due to the irreversible photochemical degradation of CR molecules in the polymeric PDAC matrix, as well as migration of the small azo compound induced by the optical-field gradient force, contributes to the formation of SRGs on the polyelectrolyte/azo dye composite films. The simplicity and versatility of the ELBL method to precisely control the thickness and composition of the films offers exciting new possibilities in the design and optimization of SRG fabrication. The use of these commercially available materials obviates the need for tedious and cost prohibitive synthetic strategies currently used to prepare azo derivativized polymers and opens the door to explore a wide variety of new molecular dyes that were previously considered poor SRG materials because of processing limitations. Acknowledgment. This work was partially supported by ONR-MURI. The authors are indebted to Prof. D. J. Sandman for his help in reading the manuscript and interpretation of the IR results. References and Notes (1) (a) Decher, G.; Hong, J. D.; Schmitt, J. Thin Solid Films 1992, 210/211, 831. (b) Ferreira, M.; Cheung, J. H.; Rubner, M. F. ibid. 1994, 244, 806. (c) Cheung, J. H.; Ferreira, M.; Rubner, M. F. Thin Solid Films 1994, 244, 985. (d) Decher, G. Science 1997, 277, 1232. (e) Schlenoff, J. B.; Ly, H.; Li, M. J. Am. Chem. Soc. 1998, 120, 7626. (2) (a) Fendler, J. H. Chem. Mater. 1996, 8, 1616. (b) He, J.-A.; Valluzzu, R.; Yang, K.; Dolukhanyan, T.; Sung, C. M.; Kumar, J.; Tripathy, S. K.; Samuelson, L.; Balogh, L.; Tomalia, D. A. Chem. Mater. 1999, 11, 1, 3268. (c) Cassagneau, T.; Mallouk, T. E.; Fendler, J. H. J. Am. Chem. Soc. 1998, 120, 7848. (d) Kovtyukhova, N. I.; Ollivier, P. J.; Martin, B. R.; Mallouk, T. E.; Chizhik, S. A.; Buzaneva, E. V.; Gorchinskiy, A. D. Chem. Mater. 1999, 11, 771. (e) He, J.-A.; Yang, K.; Kumar, J.; Tripathy, S. K.; Samuelson, L. A.; et al. J. Phys. Chem. B 1999, 103, 11050. (f) Wu, A.; Yoo, D.; Lee, J.-K.; Rubner, M. F. J. Am. Chem. Soc. 1999, 121, 4883. (g) Cheung, J. H.; Stockton, W. B.; Rubner, M. F. Macromolecules 1997, 30, 2712. (h) Cassagneau, T.; Fendler, J. H. J. Phys. Chem. B 1999, 103, 1789. (i) Li, D. Q.; Lutt, M.; Fitzsimmons, M. R.; Synowicki, R.; Hawley, M. E.; Brown, G. W. J. Am. Chem. Soc. 1998, 120, 8797. (j) Lvov, Y.; Ariga, K.; Onda, M.; Ichinose, I.; Kunitake, T. Langmuir 1997, 13, 6195. (3) (a) Lvov, Y.; Ariga, K.; Ichinose, I.; Kunitake, T. J. Am. Chem. Soc. 1995, 117, 6117. (b) Caruso, F.; Mohwald, H. J. Am. Chem. Soc. 1999, 121, 6039. (c) He, J.-A.; Samuelson, L.; Li, L.; Kumar, J.; Tripathy, S. K. J. Phys. Chem. B 1998, 102, 7067. (d) He, J.-A.; Samuelson, L.; Li, L.; Kumar, J.; Tripathy, S. K. Langmuir 1998, 14, 1674. (e) Caruso, F.; Niikura, K.; Furlong, D. N.; Okahata, Y. Langmuir 1997, 13, 3427. (f) Lvov, Y.; Haas, H.; Decher, G.; Mohwald, H. Langmuir 1994, 10, 4232. (g) Lvov, Y.; Decher, G.; Sukhorukov, G. Macromolecules 1993, 26, 5396. (h) Lvov, Y.; Lu, Z. Q.; Schenkman, J. B.; Zu, X. L.; Rusling, J. F. J. Am. Chem. Soc. 1998, 120, 4073. (4) (a) Kaschak, D. M.; Lean, J. T.; Waraksa, C. C.; Saupe, G. B.; Usami, H.; Mallouk, T. E. J. Am. Chem. Soc. 1999, 121, 3435. (b) Kaschak, D. M.; Mallouk, T. E. J. Am. Chem. Soc. 1996, 118, 4222. (c) Keller, S. W.; Johnson, S. A.; Brigham, E. S.; Yonemoto, E. H.; Mallouk, T. E. J. Am. Chem. Soc. 1995, 117, 12879. (5) (a) Caruso, F.; Caruso, R. A.; Mohwald, H. Science 1998, 282, 1111. (b) Caruso, F.; Lichtenfeld, H.; Giersig, M.; Mohwald, H. J. Am. Chem. Soc. 1998, 120, 8523.
J. Phys. Chem. B, Vol. 104, No. 45, 2000 10521 (6) (a) Ariga, K.; Lvov, Y.; Kunitake, T. J. Am. Chem. Soc. 1997, 119, 2224. (b) Linford, M. R.; Auch, M.; Mohwald, H. J. Am. Chem. Soc. 1998, 120, 178. (c) Cooper, T. M.; Stone, M. O. Langmuir 1998, 14, 6662. (d) Cooper, T. M.; Campbell, A. L.; Crane, R. L. Langmuir 1995, 11, 2713. (e) Okawa, H.; Li, L.; Kumar, J.; Tripathy, S. K.; Wada, T.; Sasabe, H. Mol. Cryst. Liq. Cryst. 1994, 255, 159. (7) (a) Ikeda, T.; Tsutsumi, O. Science 1995, 268, 1873. (b) Fujiwara, H.; Yonezawa, Y. Nature 1991, 351, 724. (c) Evans, S. D.; Johnson, S. R.; Ringsdorf, H.; Williams, L. M.; Wolf, H. Langmuir 1998, 14, 6436. (8) (a) Kim, D. Y.; Li, L.; Kumar, J.; Tripathy, S. K. Appl. Phys. Lett. 1995, 66, 1166. (b) Kim, D. Y.; Li, L.; Jiang, X. L.; Shivshankar, V.; Kumar, J.; Tripathy, S. K. Macromolecules 1995, 28, 8835. (c) Jiang, X. L.; Li, L.; Kim, D. Y.; Shivshankar, V.; Kumar, J.; Tripathy, S. K. Appl. Phys. Lett. 1996, 68, 2618. (d) Jiang, X. L.; Li, L.; Kumar, J.; Kim, D. Y.; Tripathy, S. K. Appl. Phys. Lett. 1998, 72, 2502. (e) Barratt, C. J.; Natansohn, A.; Rochon, P. J. Phys. Chem. 1996, 100, 8836. (f) Ramanujam, P. S.; Holme, N. C. R.; Hilvsted, S. Appl. Phys. Lett. 1996, 68, 1329. (9) Viswanathan, N. K.; Kim, D. Y.; Bian, S. P.; et al. J. Mater. Chem. 1999, 9, 1941. (10) (a) Mendonca, C. R.; Dhanabalan, A.; Balogh, D. T.; Misoguti, L.; Dos Santos, D. S.; Pereira-da-silva, M. A.; Giacometti, J. A.; Zilio, S. C.; Oliveira, O. N. Macromolecules 1999, 32, 1493. (b) Blanc, D.; Pelissier, S.; Saravanamuttu, K.; Najafi, S. I.; Andrews, M. P. AdV. Mater. 1999, 11, 1508. (c) Darracq, B.; Chaput, F.; Lahlil, K.; Levy, Y.; Boilot, J.-P. AdV. Mater. 1998, 10, 1133. (11) (a) Kumar, J.; Li, L.; Jiang, X. L.; Kim, D. Y.; Lee, T. S.; Tripathy, S. K. Appl. Phys. Lett. 1998, 72, 2096. (b) Viswanathan, N. K.; Balasubramanian, S.; Li, L.; Kumar, J.; Tripathy, S. K. J. Phys. Chem. B 1998, 102, 6064. (c) Bian, S.; Li, L.; Kumar, J.; Kim, D. Y.; Williams, J.; Tripathy, S. K. Appl. Phys. Lett. 1998, 73, 1817. (d) Barratt, C. J.; Rochon, P.; Natansohn, A. J. Chem. Phys. 1998, 109, 1505. (e) Pedersen, T. G.; Johansen, P. M.; Holme, N. C. R.; Ramanujam, P. S.; Hilvsted, S. Phys. ReV. Lett. 1998, 80, 89. (f) Lefin, P.; Fiorini, C.; Nunzi, J. M. Pure Appl. Opt. 1998, 7, 71. (12) (a) Ho, M.; Barratt, C. J.; Patersen, J.; Esteghamatian, M.; Natansohn, A.; Rochon, P.; Macromolecules 1996, 29, 4613. (b) Mandal, B.; Jeng, R.; Kumar, J.; Tripathy, S. K. Makromol. Chem., Rapid Commun. 1991, 12, 607. (c) Wang, X. G.; Chen, J.; Marturunkakul, S.; Li, L.; Kumar, J.; Tripathy, S. K. Chem. Mater. 1997, 9, 45. (d) Wang, X. G.; Li, L.; Chen, J.; Marturunkakul, S.; Kumar, J.; Tripathy, S. K. Macromolecules 1997, 30, 219. (e) Sukwattanasinitt, M.; Wang, X. G.; Li, L.; Jiang, X. L.; Kumar, J.; Tripathy, S. K.; Sandman, D. J. Chem. Mater. 1998, 10, 27. (f) Andruzzi, L.; Altomare, A.; Ciardelli, F.; et al. Macromolecules 1999, 32, 448. (g) Rasmussen, P. H.; Ramanujam, P. S.; Hvilsted, S.; Berg, R. H. J. Am. Chem. Soc. 1999, 121, 4738. (13) Lide, D. R.; Frederikse, H. P. R. CRC Handbook of Chemistry and Physics, 76th ed.; CRC Press: Boca Raton, FL, 1996; pp 8-17. (14) Fukumoto, H.; Yonezawa, Y. Thin Solid Films 1998, 327-329, 748. (15) (a) Yao, H.; Ono, S. S.; Kawabata, R.; et al. J. Phys. Chem. B 1999, 103, 4452. (b) Ono, S. S.; Yao, H.; Matsuoka, O.; et al. J. Phys. Chem. B 1999, 103, 6909. (16) Kuzma, M. R.; Skarda, V.; Labes, M. M. J. Chem. Phys. 1984, 81, 2925. (17) (a) Biswas, N.; Umapathy, S. J. Phys. Chem. A 1997, 101, 5555. (b) Armstrong, D. R.; Clarkson, J.; Smith, W. E. J. Phys. Chem. 1995, 99, 17825. (c) Han, S. W.; Kim, C. H.; Hong, S. H.; et al. Langmuir 1999, 15, 1579. (d) Sato, T.; Ozaki, Y.; Iriyama, K. Langmuir 1994, 10, 2363. (e) Sajid, J.; Elhaddaoui, A.; Turrell, S. J. Mol. Struct. 1997, 408/409, 181. (f) Gregoriou, V. G.; Hapanowicz, R.; Clark, S. L.; Hammond, P. T. Appl. Spectrosc. 1997, 51, 470. (g) Jones, T. P.; Porter, M. D. Appl. Spectrosc. 1989, 43, 908. (h) Colthyp, N. B.; Daly, L. H.; Wiberley, S. E. Introduction to Infrared and Raman Spectroscopy, 3rd ed.; Academic Press: New York, 1990. (18) Wu, Y. Q.; Zhao, B.; Xu, W. Q.; et al. Langmuir 1999, 15, 1247. (19) Matsumoto, M.; Miyazaki, D.; Tanaka, M.; et al. J. Am. Chem. Soc. 1998, 120, 1479. (20) Dante, S.; Advincula, R.; Frank, C. W.; Stroeve, P. Langmuir 1999, 15, 193. (21) (a) Kusukawa, T.; Fujita, M. J. Am. Chem. Soc. 1999, 121, 1397. (b) Riehl, D.; Chaput, F.; Roustamian, A.; Levy, Y.; Boilot, J. Nonlinear Optics 1994, 8, 141. (22) The films are optical transparent and smooth. The surface RMS roughness was measured by AFM method using 15 × 15 µm2 scanning area. The results indicate that, for 20, 40, 80, 160, and 320 bilayers of PDAC/CR multilayers, the surface RMS roughness is 43, 78, 91, 190, and 262 Å, respectively. (23) Viswanathan, N. K.; Balasubramanian, S.; Li, L.; Tripathy, S. K.; Kumar, J. Jpn. J. Appl. Phys. 1999, 38, 5928.
