Mixed Langmuir and Langmuir−Blodgett Films of Disperse Red-13

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Langmuir 1998, 14, 3614-3619

Mixed Langmuir and Langmuir-Blodgett Films of Disperse Red-13 Dye-Derivatized Methacrylic Homopolymer and Cadmium Stearate A. Dhanabalan, D. T. Balogh, C. R. Mendonc¸ a, A. Riul, Jr., C. J. L. Constantino, J. A. Giacometti, S. C. Zilio, and O. N. Oliveira, Jr.* Instituto de Fı´sica de Sa˜ o Carlos, Universidade de Sa˜ o Paulo, CP 369, 13560-970, Sa˜ o Carlos, SP, Brazil Received January 5, 1998 Stable mixed monolayers of disperse red-13 dye (DR13)-derivatized methacrylic homopolymer (HPDR13) and cadmium stearate (CdSt) have been obtained and transferred as mixed multilayer Langmuir-Blodgett (LB) films. Langmuir monolayers with different mole percentages of HPDR13 were studied by surface pressure and surface potential isotherms. In contrast to the poor transferability of pure HPDR13 monolayers, mixed monolayers could be successfully transferred as Y-type LB films up to a large number of layers. Fourier transform infrared results on the mixed LB films confirmed the transfer of CdSt and HPDR13, while UV-vis spectra indicated a uniform transfer of HPDR13 during multilayer LB deposition and the possible J-aggregation of HPDR13 molecules. X-ray diffraction results were close to those of pure CdSt films, indicating the existence of CdSt domains in the mixed LB film. As expected from the presence of DR13 chromophores, nonlinear optical effects were observed. However, by using the Z-scan technique for measuring the refractive indices we were able to demonstrate that they were mainly caused by thermal effects, with no clear indication of an electronic origin. This calls for caution when analyzing nonlinear optical effects.

Introduction The reversible photoisomerization characteristics of azobenzene compounds are being considered for a number of molecular electronic devices which include optical memory and optical switching.1,2 The Langmuir-Blodgett (LB) manipulation of azobenzene-functionalized molecules, in particular, allows the study of the cis-trans photoisomerization of the azo functionality in the solid state, especially in the form of ultrathin films whose packing can be controlled at the molecular level.3-6 Several molecular engineering approaches have been used for preparing LB films of these compounds, the most common one being the use of long chain amphiphilic azobenzene derivatives that possess amphipatic properties to form stable and easily transferable monolayers. In an earlier study, we have shown that a stable monolayer of methacrylic homopolymer could be obtained in which disperse red-13 dye (DR13) is attached as a pendent group (HPDR13). There was no need of attaching long alkyl chains to the azobenzene or of mixing with typical filmforming materials.7 Nevertheless, even after optimizing the transfer conditions, only a small number of layers (up * To whom correspondence should be addressed. Fax: +55 (16) 271 3616; e-mail:[email protected]. (1) Liu, Z. F.; Hashimoto, K.; Fujishima, A. Nature 1990, 347, 658. (2) Nahata, A.; Shan, J.; Yardley, J. T.; Wu, C. J. Opt. Soc. Am. B 1993, 10, 1553. (3) Yabe, A.; Kawabata, Y.; Nino, H.; Matsuyoshi, M.; Ouchi, A.; Takahashi, H.; Tamura, S.; Tagaki, W.; Nakahara, H.; Fukuda, K. Thin Solid Films 1988, 160, 33. (4) Laschewsky, A.; Paulus, W.; Ringsdorf, H.; Schuster, A.; Frick, G.; Mathy, A. Thin Solid Films 1992, 210/211, 91. (5) Tachibana, H.; Azume, R.; Tanaka, M.; Matsumoto, M.; Sako, S.; Sakai, H.; Abe, M.; Kondo, Y.; Yoshino,N. Thin Solid Film 1996, 284285, 73. (6) Crosswell, J. P.; Petty, M. C.; Ferguson, I.; Hutchings, M.; Allen, S.; Ryan, T. G.;. Wang, C. H.; Wherrett, B. S. Adv. Mater. Opt. Electron. 1996, 6, 33. (7) Dhanabalan, A.; Balogh, D. T.; Riul, A., Jr.; Giacometti, J. A.; Oliveira, O. N., Jr. Thin Solid Films, in press.

to 15 layers) of HPDR13 could be uniformly transferred. In the attempts to fabricate thick LB films, one generally obtained poor-quality LB films that were visually nonuniform. Yet, particularly for nonlinear optical measurements one needs thick LB films. To enhance the transferability, we have now employed the mixed LB film approach in which the material of interest is transferred along with a typical film-forming material. This mixed LB film approach has proven extremely successful for nonamphiphilic polymeric materials.8 The present paper describes the monolayer characteristics of mixed monolayers of DR13 dye-derivatized methacrylic homopolymer and CdSt as inferred through surface pressure and surface potential measurements. Transferred films that could be 100 layers thick were characterized by UV-vis and Fourier transform infrared (FTIR) spectrocospies, X-ray diffraction (XRD), surface potential, and nonlinear optical measurements. Experimental Section HPDR13 was obtained through chemical synthesis as described in ref 7. It was purified by fractional crystallization and characterized by IR and NMR spectroscopies.7 HPDR13 was soluble in most common organic solvents. Stearic acid (Aldrich, 99%) was used as received without further purification. Spreading solutions for LB experiments were obtained by dissolving different mole percentages of HPDR13 (based on the monomer repeat unit, molecular weight of 416) and stearic acid in chloroform (Merck, HPLC grade). Ultrapure water supplied by a Milli-RO coupled to a Milli-Q purification system from Millipore (resistivity 18.2 MΩ cm-1) was used to prepare subphase solutions. Cadmium chloride (4 × 10-4 mol; Carlo Erba, AR grade, 99.5%) was added to the subphase for complexing with stearic acid molecules, which required the pH to be kept at about 6.0 with the addition of sodium bicarbonate (LABSYNTH, AR grade, 99.7%). Monolayer studies and multilayer LB film (8) Dhanabalan, A.; Riul, A., Jr.; Oliveira, O. N., Jr. Supramol. Sci., in press.

S0743-7463(98)00025-0 CCC: $15.00 © 1998 American Chemical Society Published on Web 06/05/1998

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Figure 2. Surface pressure-mean molecular area isotherms of mixed monolayers containing different mole percentages of HPDR13 and CdSt at a subphase temperature of 22 °C. The area is calculated per stearic acid molecule. ratio, [R(z,f)], was used to eliminate spurious linear effects as a way of increasing the sensitivity of the measurement. The dependence of R(z,f) on f can be used to discriminate between nonlinear processes with different relaxation times.

Figure 1. Structure of the repeat unit for the DR13 dyederivatized methacrylate homopolymer (HPDR13). deposition were carried out with a KSV-5000 LB system placed on an antivibration table in a class 10 000 clean room. All experiments were carried out at a subphase temperature of 22 °C. During the isotherm studies, the monolayer was compressed at a barrier speed of 10 mm/min. The stability of the monolayer was inferred by holding the monolayer in the compressed state and monitoring the change in mean molecular area with time. Isotherm results presented in this paper are based on the number of stearic acid molecules spread at the air-water interface and provide information on the effect of addition of various amounts of HPDR13 on the isotherm of pure CdSt. However, we have also calculated the mean molecular area using the average molecular weight of both HPDR13 (based on the monomer repeat unit as shown in Figure 1) and stearic acid. Materials used as substrates included BK7 glass, thin gold evaporated glass, and calcium fluoride plates that were cleaned thoroughly prior to use. Irrespective of the monolayer composition, the mixed monolayers were transferred at a dipping speed of 3 mm/min. After each layer deposition, the film was dried in air for 10 min. UV-vis and FTIR spectral measurements were carried out with a Hitachi-U2001 spectrophotometer (transmission) and a BOMEM Michelson series instrument (transmission, 16 scans), respectively. For X-ray diffraction (XRD) measurements use was made of a Rigaku Rotaflex (Model RU200B) X-ray diffractometer in the 2θ range of 3-20° using a Cu target. Surface potential measurements of transferred LB films on glass coated with gold were conducted with a Trek 320B electrostatic voltmeter. The details of the experimental setup for investigating nonlinear optical parameters by Z-scan measurements with Fourier analysis have been reported elsewhere,9 and therefore only a brief description is given here. A dye laser operating with dicyanomethylene (DCM) (620-670 nm) was used as the light source. The laser beam was modulated at the frequency f by means of a mechanical chopper and was then focused onto the sample that can be translated along the z direction. The transmission through an aperture located at the far field was monitored as a function of time. The Fourier components at f and 2f measured with two independent lock-in amplifiers are respectively related to the linear and nonlinear refractions. Their (9) Mendonc¸ a, C. R.; Misoguti, L.; Zilio, S. C. Appl. Phys. Lett. 1997, 71, 2094.

Results and Discussions Π-A Isotherms. The homopolymer HPDR13 may form stable Langmuir monolayers, unlike its monomer, the methacrylate derivative of DR13. The HPDR13 monolayers exhibited a pressure-area isotherm with an initial characteristic liquid-expanded region followed by a solid-condensed region with a limiting mean molecular area of about 20 Å2.7 When mixed with CdSt, HPDR13 forms condensed films with a well-defined solid region, as shown in Figure 2, in a fashion similar to that for a pure CdSt monolayer. Because the area per molecule is calculated using the number of stearic acid molecules, the limiting mean molecular areas obtained by extrapolation of the solid region to zero surface pressure increase with increasing mole percentages of HPDR13 in the mixture. The reproducibility observed during hysteresis isotherm experiments ruled out the possibility of any material dissolution. It is worth noting that as the HPDR13 content is increased, the initial liquid-expanded region characteristic of pure HPDR13 monolayers becomes prominent. In contrast to the case of mixed monolayers of typical amphiphilic compounds, no clear dependence of the collapse pressure on the monolayer composition could be observed. This is probably because the polymer does not form a true monomolecular layer. In an attempt to understand the nature of the mixing in the HPDR13/CdSt monolayers, the area per molecule was also calculated using the average molecular weight of both HPDR13 and stearic acid, and plotted against the monolayer composition (Figure 3). Data for pure cadmium stearate and HPDR13 are available in refs 7 and 10. For phase-separated mixed monolayers, an additive behavior is expected in which the mean molecular area varies linearly with the composition,11 as shown by the line in the figure. For HPDR13/CdSt monolayers, however, this does not occur. Deviation from this linearity is generally associated with the attractive/repulsive interaction between distinct types of molecules and/or molecular packing (10) Dhanabalan, A.; Riul, A., Jr.; Mattoso, L. H. C.; Oliveira, O. N., Jr. Langmuir 1997, 13, 4882. (11) Cheung, J. H.; Rubner, M. F. Thin Solid Films 1994, 244, 990.

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Figure 3. Plot of mean molecular area obtained on the basis of the average molecular weight of HPDR13 (using the monomer repeat unit) and stearic acid versus the mole percentage of HPDR13 in the mixture. Table 1. Variation of Monolayer Characteristics with the Amount of Spreading Solution volume spread (µL)

limiting mean molecular area (Å2)

collapse pressure (mN/m)

Ac (Å2)

∆Vmax (mV)

300 400 500 600 700

67 52 44 39 35

68 70 69 70 70

76 60 51 45 39

120 130 110 130 140

rearrangements in one component during the compression process. It is possible that an attractive interaction appears between the nitrogen atom of the amino group in the HPDR13 and the carboxylic acid group of the CdSt, but such an interaction is probably very weak. Changes in the molecular packing of the HPDR13 are therefore the most likely explanation. To check that, we performed a series of isotherms for a given composition by spreading different volumes of the mixture at the air-water interface. It is known that, for a typical monolayer material, such as a fatty acid, no significant variation in the pressure-area isotherms is observed when different amounts of spreading solution are employed. Indeed, isotherms for pure CdSt are not altered by spreading different volumes of chloroform solution. Furthermore, in our previous study7 pure HPDR13 monolayers were not affected by changing the volume of solution spread. Nevertheless, the isotherms for the HPDR13/CdSt mixed monolayers did depend on the volume spread. As Table 1 shows for a typical mixed monolayer containing 24.8 mol % HPDR13, the mean molecular area decreases with the volume of spreading solution (the results of the critical area (Ac) and maximum surface potential will be discussed in the next section). With the reasonable assumption that the packing arrangement of the CdSt molecules is not changed drastically, it is concluded that the HPDR13 molecules may probably assume different packing arrangements depending on the amount of material spread. This could be caused by the formation of stacks rather than a true monolayer of HPDR13, as happens to other polymers.12 According to this reasoning, as the volume spread is increased, the calculated area would decrease, since it is based upon the number of stearic acid molecules in the monolayer. Material dissolution into the subphase (12) Dhanabalan, A.; Dabke, R. B.; Prasanth Kumar, N.; Talwar, S. S.; Major, S.; Lal, R.; Contractor, A. Q. Langmuir 1997, 13, 4395.

Figure 4. Surface potential-mean molecular area isotherms of mixed monolayers containing different mole percentages of HPDR13 and CdSt at a subphase temperature of 22 °C. The area is calculated per stearic acid molecule.

water may be ruled out as the source for the nonlinearity, since reproducible hysteresis isotherms have been obtained. It is also noted that the collapse pressure did not change appreciably with the change of spreading volume. ∆V-A Isotherms. The surface potential-mean molecular area (∆V-A) isotherms of mixed monolayers containing different mole percentages of HPDR13 and CdSt are presented in Figure 4. Analogously to the pressure-area isotherms, the curves are shifted toward larger areas as the HPDR13 content is increased, since the areas plotted are calculated per stearic acid molecule. No quantitative interpretation of surface potentials can be made for complex molecules such as those investigated here, as already pointed out for other polymers.10 Nevertheless, it is clear that in all cases a relatively sharp increase in surface potential is seen at a critical area, which is usually associated with the coming together of domains formed at very early stages of monolayer formation. By the same token, as the content of polymer is increased, it becomes more likely that such domains will be joined together, thus causing the surface potential to be nonzero even at large areas per molecule. This is well illustrated by the surface potential curve for 67.9% HPDR13. The formation of larger continuous domain structures has also been visualized with Brewster angle microscopy (BAM) (results not shown). As expected, the maximum surface potential (∆Vmax) for the mixed monolayers with low mole percentages of HPDR13 (16.8%) is similar to that of a pure CdSt monolayer, while for higher polymer contents, ∆Vmax approaches 160 mV, which is close to that of pure HPDR13.7 The effect of varying the volume of spreading solution on the surface potential isotherms is given in Table 1 along with surface pressure isotherm results. The critical area decreases with increasing volume of spreading solution, similar to the trend observed with the mean molecular area. As can be clearly visualized, the difference between the limiting mean molecular area and the critical area was also found to decrease with increasing amounts of spreading solution. There is no clear dependence of the maximum surface potential with increasing spreading volume. The observed surface potential isotherm results seem to reinforce the possibility of different molecular packing arrangements for HPDR13 with the change in the spreading volume. LB Film Deposition and Characterization. Mixed

Films of Methacrylic Homopolymer and Cadmium Stearate

Figure 5. UV-vis absorption spectra of a mixed LB film containing HPDR13 (54.4%) and CdSt, and of HPDR13 in chloroform.

monolayers were transferred onto different substrates by the vertical dipping method at a constant surface pressure of 31 mN/m. Unlike the pure HPDR13 monolayer, which could be transferred only at very low dipping speeds (1 mm/min),7 mixed monolayers with less than 50% HPDR13 could be transferred uniformly at relatively higher dipping speeds (10 mm/min). However, mixed monolayers with higher HPDR13 content could be transferred with an optimum dipping speed of 3 mm/min. In the case of a pure HPDR13 monolayer, the initial Y-type deposition changed to Z-type after the transfer of typically seven monolayers. In contrast, the mixed monolayers could be transferred up to 100 layers as Y-type LB films with near unity transfer ratios. Therefore, the addition of a builder material like CdSt has significantly enhanced the transferability of the HPDR13. Figure 5 shows the UV-vis spectra of an as-deposited mixed LB film of HPDR13 and CdSt (27 layers on glass). The spectra for HPDR13 in a chloroform solution are also shown. In a control experiment, we have also obtained the solution spectra for mixtures containing equimolar amounts of HPDR13 and stearic acid. Such spectra were very similar to those of pure HPDR13, which indicates a poor molecular interaction between HPDR13 and stearic acid in the solution. As shown in the figure, the absorption maximum at 500 nm corresponding to the π-π* electronic transition was red-shifted in the LB film structure, in comparison to the solution (475 nm). The red shift of the absorption maximum has been correlated to the antiparallel type aggregation (J-aggregation) of chromophores.13 Such J-aggregation has also been observed with pure HPDR13 LB film,7 possibly indicating that the presence of CdSt did not prevent the aggregation of HPDR13. The maximum in absorbance at 500 nm was not shifted when thicker multilayers were built. As Figure 6 shows, the absorbance increased linearly with the number of layers, thus indicating a uniform transfer of HPDR13 during the deposition. Figure 7 shows the FTIR spectrum of an as-deposited mixed LB film of HPDR13 and CdSt (21 layers) on a calcium fluoride plate. The spectrum exhibits characteristic absorption peaks for both CdSt (2917 cm-1 for asymmetric CsH stretching, 2849 cm-1 for symmetric CsH stretching, 1545 cm-1 for CsO stretching of carboxylate groups, and 1465 cm-1 for CH2 bending vibra(13) Menzel, H.; Weichart, B.; Schmidt, A.; Paul, S.; Knoll, W.; Stumpe, J.; Fischer, T. Langmuir 1994, 10, 1926.

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Figure 6. Plot of absorbance at 500 nm versus the number of layers of a mixed LB film containing HPDR13 (54.4%) and CdSt.

Figure 7. FTIR transmission spectrum of a mixed LB film (21 layers) containing HPDR13 (54.4%) and CdSt.

tions) and HPDR13 (1732 cm-1 for CsO stretching of ester groups, 1599 cm-1 for CdC stretching in benzene rings, 1518 cm-1 for NO2 asymmetric stretching, 1336 cm-1 for NO2 symmetric stretching, 1242, 1252, and 1134 cm-1 for asymmetric and symmetric C(dO)sO and OsCsC stretching vibrations of the ester group).14 The CsH stretching vibrations of CH2 of HPDR13 were overlapped with the strong CsH absorption of the stearate. The absence of any absorption peak at 1700 cm-1 indicated the complete ionization of the acid head groups of stearic acid molecules. The superimposed nature of the spectrum confirmed the transfer of both CdSt and HPDR13 in the mixed LB film. For comparison, we have also obtained the FTIR spectrum of bulk HPDR13 in a KBr pellet.7 The intensities of the peaks at 1242 and 1252 corresponding to asymmetric and symmetric C(dO)sO and OsCsC stretching vibrations of the ester group were found to be reversed, thereby indicating the possible preferred orientation of DR13 chromophores in the LB film structure.7 No significant change was noticed in the absorption peaks corresponding to HPDR13 when the composition and the number of layers were varied in the mixed LB films. The XRD pattern of an as-deposited mixed LB film containing 55 mol % HPDR13 and CdSt (21 layers) is (14) Silverstein, R. M.; Bassler, G. C.; Morrill, T. C. Spectrometric Identification of Organic Compounds, 5th ed.; John Wiley & Sons: New York, 1991.

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Figure 8. XRD pattern of a mixed LB film containing HPDR13 (45.2%) and CdSt (21 layers).

Figure 9. Eclipsing Z-scan measurement of a mixed LB film (100 layers) containing HPDR13 (54.4%) and CdSt, at 640 nm and at 400 Hz.

Dhanabalan et al.

Figure 10. Plot of ∆Zpv versus the modulation frequency (The theoretical fit as a function of 1/f is shown as a solid line).

Figure 11. Plot of ∆Rpv versus the modulation frequency (theoretical fit shown as solid line).

shown in Figure 8. A set of weak diffraction peaks with a corresponding bilayer spacing of 49.5 ( 0.4 Å is clearly seen. This bilayer spacing is very close to that measured for pure CdSt LB films,15 which points to the presence of CdSt domains in the mixed LB film. However, in comparison to the LB film of pure CdSt, the mixed LB films exhibited less intense and broader diffraction peaks. Therefore, the HPDR13 molecules have actually influenced the packing order in the CdSt domains. As mentioned in the Introduction, the mixed monolayer approach has been proven excellent for obtaining uniform polymeric LB films. This has been confirmed here through surface potential measurements on the deposited LB films. The surface potential for LB films containing different numbers of layers (3, 7, and 27) of the mixed monolayer with different HPDR13 contents transferred onto goldevaporated glass was 240-250 mV in all cases. When scanning the probe along the film surface, the maximum change detected was 10 mV, which is within the accuracy of the instrument. This is indicative of film uniformity, at least at the macroscopic level. That the surface potential measured was independent of the number of layers is consistent with the centrosymmetric Y-type deposition.

The determination of the nonlinear refractive indices of a mixed LB film (100 layers) on BK7 glass has been made using the eclipsing Z-scan technique16 with a Fourier analysis.9 This technique is adequate to distinguish between thermal and electronic origins for nonlinear optical effects. Figure 9 shows the eclipsing Z-scan measurement at a fixed wavelength of 640 nm, which is below the absorption edge of HPDR13. As the peak occurred at Z < 0, the observed nonlinearity is positive.16 Open-aperture measurements gave no indication of any two-photon absorption effect. To infer about the origin of the nonlinear index, a set of Z-scan measurements with several modulation frequencies were performed. Figure 10 shows the dependence of the peak and valley separation (∆Zpv) on the modulation frequency f at 640 nm. The observed characteristic features indicated that the nonlinear effect is of thermal origin. For lower modulation frequencies, there is a considerable heat diffusion into the film for a given time period depending on the size of the laser spot. As a consequence, the thermally induced lens becomes broader than the Gaussian profile of the laser beam, thereby resulting in a larger ∆Zpv. However, for high modulation frequencies, there is no significant heat diffusion and the thermal lens follows the Gaussian profile of the beam, now leading to a narrower ∆Zpv, as predicted earlier.17 Measurements on thinner LB films

(15) Roberts, G., Ed. Langmuir-Blodgett Films; Plenum Press: New York, 1990.

(16) Xia, T.; Hagan, D.; Sheik-Bahae, J. M.; Van Stryland, E. W. Opt. Lett. 1994, 19, 317.

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A successful and uniform transfer of DR13-derivatized methacrylate polymer (HPDR13) with the aid of CdSt has

been demonstrated. Isotherm studies of the mixed monolayer containing HPDR13 and CdSt revealed a clear transition from the characteristics of CdSt to that of HPDR13 with increasing HPDR13 content in the mixture. The amount of spreading solution seems to affect the monolayer characteristics of the mixed monolayer. FTIR results indicated the transfer of both HPDR13 and CdSt in the mixed LB film. UV-vis studies revealed the J-type aggregation between DR13 molecules in the LB film structure. The presence of CdSt domains could be inferred from the XRD patterns. Preliminary nonlinear optical measurements revealed the dependence of ∆Zpv on the modulation frequency, indicating the thermal origin of the nonlinear effects despite the preferential orientation of the polymer as inferred through the FTIR results. Although the excitation was carried out in a preresonant condition, the small residual absorption in the edge of the band seems to give rise to thermal effects. The inevitable conclusion is then that measured nonlinear optical effects must be analyzed with extreme caution.

(17) Sheik-Bahae, M.; Said, A. A.; Van Stryland, E. W. IEEE J. Quantum Electron. 1990, 26, 760.

LA9800250

gave essentially the same results, but obviously with a weaker signal, as it depends on the thickness of the film. The nonlinear refractive index and the thermal diffusion coefficient were calculated using the method described in ref 9. Defining ∆Rpv as the difference of R(z,f) for the peak and valley positions, we have measured the dependence of ∆Rpv on f, as shown in Figure 11. A good fit between the experimental data and calculated values (shown as a solid line) is achieved. We have obtained the nonlinear coefficient (n2) ) 6.0 × 10-5 cm2/kW and the thermal diffusion coefficient (D) ) 2.5 × 10-3 cm2/s. The observed D value is close to that observed for bare BK7 glass on which the LB film is deposited. As the thermal diffusion in the LB film is comparatively much smaller than that of glass, we may be just observing the heat diffusion to the glass. Further experiments to investigate the existence of electronic nonlinearity are in progress. Conclusions