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Reversible Wettability of Photoresponsive Fluorine-Containing Azobenzene Polymer in Langmuir-Blodgett Films Chuan Liang Feng,†,‡ Yan Jie Zhang,§ Jian Jin,§ Yan Lin Song,† Lian Ying Xie,‡ Gui Rong Qu,‡ Lei Jiang,*,† and Dao Ben Zhu† Institute of Chemistry, Chinese Academy of Sciences, Beijing 100080, People’s Republic of China, Department of Chemistry, Henan Normal University, Xinxiang, 453002, People’s Republic of China, and Department of Chemistry, Jilin University, Changchun, 130023, People’s Republic of China Received January 12, 2001. In Final Form: April 17, 2001 A polymer containing 3′-[trifluoromethyl-4,3′-dibenzoazo] dyes in the side chains has been synthesized, and a monolayer of this polymer has been transferred onto solid substrate by the Langmuir-Blodgett (LB) technique. The reversible processes of photoisomerization of the LB films induced by light are observed by UV-vis absorption spectra, in situ atomic force microscopy, and friction force microscopy. In addition, the physisorbed monolayers of the monomer of the polymer at liquid/solid interface are investigated by scanning tunneling microscopy. The films of the polymer show the reversible change of morphological and friction force responses under alternate UV (365 ( 10 nm) and visible (436 ( 10 nm) irradiation. The films also undergo reversible wettability changes with UV and visible light irradiation, which are directly confirmed by contact angle measurement. The largest photoinduced change of contact angle is 11 ( 1°. The larger value of the contact angles measured corresponds to films mainly composed of the trans isomers of the molecules. Evidence shows that the photoinduced surface energy changes have an important influence on the wettability changes.
Introduction There has been a great deal of activity in searching for molecules which may have application as switching devices in areas such as optical storage and molecular recognition.1,2 Azobenzene derivatives are particularly promising candidates and have been widely investigated because of their reversible trans-cis photoisomerization3 and electrochemical activity.4,5 This is because azobenzene chromophores are unique systems since the photoisomerization of this unit is widely reversible with little degradation occurring after many switching cycles. The LangmuirBlodgett (LB) technique has been shown to be a powerful convenient method for the preparation of ultrathin films. LB films of molecules containing azobenzene show potential for improving molecular order and orientation in films at the molecular level.6,7,8 Meanwhile, wettability of films containing photoresponsive molecules has also received a great deal of attention for their application. A change in wettability of * To whom correspondence should be addressed. FAX: 01082627566 E-mail:
[email protected] † Chinese Academy of Sciences. ‡ Henan Normal University. § Jilin University. (1) Astrand, P. O.; Ramanujam, P. S.; Hvilsted, S.; Bak, K.; Sauer Stephan, P. A. J. Am. Chem. Soc. 2000, 122, 3482. (2) Morisue, M.; Nakamura, H.; Ijiro, K.; Shimomura, M. Mol. Cryst. Liq. Cryst. 1999, 337, 457. (3) Nakayama, K.; Jiang, L.; Iyoda, T.; Hashimoto, K.; Fujishima, A. Jpn. J. Appl. Phys. 1997, 36, 3897. (4) Kajikawa, K.; Anzai, T.; Takezoe, H.; Fukuda, A. Thin Solid Films 1994, 243, 587. (5) Sekkat, Z.; Buchil, M.; Orendi, H.; Menzel, H.; Knoll, W. Chem. Phys. Lett. 1994, 220, 497. (6) Wang, R.; Iyoda, T.; Jiang, L.; Hashimoto, K.; Fujishima, A. J. Electroanal. Chem. 1997, 13, 4644. (7) Wang, R.; Jiang, L.; Iyoda, T.; Hashimoto, K.; Fujishima, A. Chem. Lett. 1996, 11, 1005. (8) Wang, R.; Jiang, L.; Iyoda, T.; Tryk, D. A.; Hashimoto, K.; Fujishima, A. Langmuir 1996, 12, 2052.
low-molecular-weight molecules containing photoresponsive chromophores and polymers containing azobenzene either in the backbone or as a pendant group is investigated.9-13 It is due to change in the dipole moment of the two forms of the photochromic species from trans to cis. Generally, the wettability change is mainly detected by the measurements of contact angle on the macroscopic scale. However, few reports have investigated the change of wettability in the film on a microscopic scale. Atomic force microscopy (AFM) is a powerful tool for studying the morphology of molecular films,14 and it has been used to study the morphological change of the LB film of azobenzene accompanied by photoisomerization.6,15 The friction force microscopy (FFM) technique is applicable to a wide range of materials, for the study of material-specific friction and as a method of identifying materials on a small scale from their different frictional responses.16 Here, we investigated the wettability change of an azobenzene polymer LB film by the combination of contact angle measurements and in situ AFM/FFM studies. The reversible change of contact angle reflects directly on the reversible change of topographic and friction properties of the film surface. (9) Ichimura, K.; Oh, S. K.; Nakagawa, M. Science 2000, 288, 1624. (10) Siewierski, L. M.; Brittain, W. J.; Petrash, S.; Foster, M. D. Langmuir 1996, 12, 5838. (11) Abbott, S.; Ralston, J.; Reynold, G.; Hayes, R. Langmuir 1999, 15, 8923. (12) Ishihara, K.; Odazaki, A.; Negishi, N.; Shinohara, I.; Okano, T.; Kataoka, K.; Sakurai, Y. J. Appl. Polym. Sci. 1982, 27, 239. (13) Mo¨ller, G.; Harke, M.; Motschmann, H.; Prescher, D. Langmuir 1998, 14, 4955. (14) Bottomley, L. A.; Coury, J. E.; First, P. N. Anal. Chem. 1996, 68, 185R-230R and references therein. (15) Matsumoto, M.; Miyazaki, D.; Tanaka, M.; Azomi, R.; Manda, E.; Kondo, Y.; Yoshino, N.; Tachibana, H. J. Am. Chem. Soc. 1998, 120, 1479. (16) Overney, R. M.; Meyer, E.; Frommer, J.; Brodbeck, D.; Lthi, R.; Howald, L.; Takano, H.; Gotoh, Y. Nature 1992, 359, 13.
10.1021/la010071r CCC: $20.00 © 2001 American Chemical Society Published on Web 06/22/2001
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Scheme 1. Chemical Structure of the Polymer Used in This Study (x ) 0.1; y ) 0.9)
Feng et al. butyloxy)-3′-trifluoromethylazobenzene with potassium methacrylate. The polymerization was carried out in degassed toluene with AIBN as initiator at 70 °C. The LB trough used in this experiment is a homemade instrument (China). The compound was dissolved in chloroform with a concentration of 5 × 10-4 mol/L. The chloroform solution with a certain volume was spread dropwise on the water subphase (Milli-Q water with 18.2 MΩ) at 22 °C. After 15 min to allow the chloroform to evaporate, the film was slowly compressed at a constant barrier speed of 0.15 cm2/s. The monolayer film was transferred onto solid substrates by a conventional vertical dipping method at a surface pressure of 15 mN/m. Y-type multilayer films were fabricated with the transfer ratio close to 0.7. UV-vis absorption spectra were taken with a U3010 spectrophotometer. UV light irradiation was carried out using a xenon lamp with a wavelength of 365 nm for the trans-cis photoisomerization at a photointensivity of 30 mW/cm2. The scanning tunneling microscopy (STM) experiments were performed with an SPA 3700 STM/AFM system (Seiko Instruments). Tips were electrochemically etched from W wire in a 2 N KOH solution. The STM images were acquired in the variable-current (constant height) mode under ambient conditions. AFM measurements were also achieved with an SPA 3700 STM/AFM system with a 20 µm scanner in contact mode. A triangle-shaped Si3N4 cantilever with a spring constant of 0.02 N/m was used to acquire images under ambient air condition at 22 °C. Contact angle measurements were carried out with a Dataphysics OCI200 contact angle goniometer (Germany), and water was used as the test liquid for all measurements. A drop of 20 µL volume was formed with the use of a micropipet and placed directly onto the sample. The liquid drops were observed with a microscopy 5× magnification. For photoisomerization studies, the contact angles were measured immediately after the exposure to monochromatic UV light (365 ( 10 nm) and visible light (436 ( 10 nm) for several minutes. In this experiment the experimental error in the contactact measurements is (1°.
Results and Discussion Experimental Section The copolymers used in this paper are poly{2-hydroxyethyl methacrylate} y -co-{6-[3-((trifluoromethyl)phenyl)azo]phenoxylhexyl methacrylate}x (Scheme 1). The ratio of x and y is 1:9. The synthesis of the copolymer is chiefly described as follows: The azobenzene-containing monomer was prepared from 4-hydroxy-3′-trifluoromethylazobenzene by etherification with excess 1,4-dibromobutane and reaction of the resulting 4-(4-bromo-
First, we have observed the photoisomerization of a monomer (4-hydroxy-3′-trifluoromethylazobenzene) selfassembly on highly oriented pyrolytic graphite (HOPG) by STM with submolecular resolution. Figure 1A shows the STM images of the resulting monolayer in trans state on HOPG. After UV light irradiation, the monolayer is observed again with the same scanning parameters (shown in Figure 1B). The aromatic groups of molecules can be
Figure 1. (A) STM image of an ordered monolayer of trans isomer of the molecule formed by physisorption at the 1-undecane/ graphite interface. (B) STM image after UV irradiation about 20 min. (C) Proposed molecular model of the trans isomer of the molecule in image A. (D) Proposed molecular model of the cis isomer of the molecule in image B. The solid lines in the image show the lamellar direction.
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Figure 2. Surface pressure-area isotherms of polymer on a water subphase.
clearly recognized as bright spots that correspond to the high measured tunneling current.17 The distance between two trans molecules along the lamellar direction as shown in the image is about 5.5 nm, and the distance in the cis state becomes larger (∼6.0 nm). The models of the trans and cis isomers are proposed and shown in Figure 1C and Figure 1D, respectively. Here, the cis domains without the trans domains are observed, which indicates that this molecule has high conversion ratio from trans to cis isomer after UV light irradiation. To investigate the photoresponsive-wetting behavior, we synthesized the fluorinecontaining azobenzene polymer with hydrophilic group (-OH) and hydrophobic group (-CF3) (shown in Scheme 1), which can be used to make LB films. The π-A isotherm of the polymer on water subphase is presented in Figure 2, which has high collapse pressure (∼32 mN/m). The title molecule can form a reasonable monolayer on the subphase for the stronger hydrophobic interactions between the trifluoromethyl groups and the π-π interactions between the azobenzene groups. Figure 3A is the UV-vis absorption of polymer in chloroform solution and its 19-layer LB film on CaF2 substrate. Generally, UV-vis spectroscopy can offer the information about the aggregation effect that can give us information on the structure characterization of the azobenzene derivative.18 In Figure 3A, the maximal π-π* absorption of molecules is located at about 345 nm in the dilute chloroform solutions and about 355 nm in the LB films, respectively. The obvious red shift in LB film relative to the dilute solution suggests the formation of the J-aggregation.19 In this case, the adjacent azobenzene units parallel each other. The results indicate that the azobenzene moiety is tilting significantly with respect to the normal of the LB films. UV-vis absorption spectra of the LB films before and after illumination are shown in Figure 3B. The trans to cis photoisomerization of polymer occurs as indicated by a distinctive decrease of π-π* absorption at 355 nm when illuminated by UV light. The visible light irradiation causes the π-π* transition absorption to increase, which accompanies cis to trans photoisomerization. On further alternate illumination with UV and visible light, the spectra changes from curve c to b and curve b to c, respectively. Similar results have been reports for polymeric material containing azobenzene LB films.15 Contact angle (CA) changes at various stages of irradiation are given in Figure 4 by the sessile drop (17) Frommer, J. Angew. Chem., Int. Ed. Engl. 1992, 31, 1298. (18) Jin, J.; Li, L. S.; Zhang, Y. J.; Tian, Y. Q.; Jiang, S. M.; Zhao, Y. Y.; Bai, Y. B.; Li, T. J. Langmuir 1998, 14, 5231. (19) Yao, H.; Sugiyama, S.; Kawabata, R.; Ikeda, H.; Matsuoka, O.; Yamamoto, S.; Kitamura, N. J. Phys. Chem. B 1999, 103, 4452.
Figure 3. (A) (a) Absorption spectrum of polymer dissolving in chloroform with concentrations of 5 × 10-5 mol/L. (b) Absorption spectrum of a 19-layer LB film of polymer on CaF2. (B) Absorption spectrum of a 19-layer LB film of polymer on CaF2 before (a) and after illumination with UV light for 5 min (b) and visible light for 5 min (c).
Figure 4. Reversible wettability for a 11-layer LB film on glass. The value of the contact angles was a gradual decrease after several cycles of irradiation.
technique.20 Before UV light irradiation, the CA of the LB films is about 85 ( 1°. The relatively large CA should be attributed to the low surface energy caused by the J-aggregates of trifluoromethyl groups on the surface of the films. After irradiation by UV light, the average CA decreases to 74 ( 1°. The difference of the two CAs is up to 11 ( 1°. It is due to the existence of trifluoromethyl group causing the large change of dipole moment and surface energy during the process of photoisomerization (20) Hunter, R. J. Found. Colloid Sci.; OUP 1987, 1.
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Figure 5. In situ AFM images of a single-layer LB film of polymer on mica before (A) and after illumination with UV light for 10 min (B) and visible light for 10 min (C). The scanned areas are 5 µm × 5 µm. The solid lines in the images show the in situ position. In the images the brighter white dots represent higher areas and the gray images are lower areas.
from trans to cis.21 This contact angle change is almost completely reversible after visible light irradiation. A gradual decrease in the magnitude of the contact angle change was observed after several cycles. This implies that photoisomerization of this unit causes degradation after many switching cycles in LB films. Figure 5 shows the in situ AFM images of a single-layer LB film of polymer on mica before and after UV illumination. Before UV illumination (Figure 5A), a number of holes are observed, which should be associated with defects formed during the transfer process.22 The average depth (21) Matsumoto, K.; Kubota, M.; Matsuoka, H.; Yamaoka, J. Macromolecules 1999, 32, 7122. (22) Fang, J. Y.; Knobler, C. M. J. Phys. Chem. 1995, 99, 10425.
Feng et al.
of the holes is ca. 1.5 nm. Many large grains are also observed on the films. They may arise from the dewetting of the water cotransferred to the substrate because of surface tension.23 The average height of them is ca. 10 nm. After UV light irradiation, the film morphology exhibits an obvious change (Figure 5B). The grains become larger although the height do not change significantly. The average depth of the holes also increases. In addition, the formation of small grains and holes is also seen. The average diameter of the hills and the holes is ca. 50 and 30 nm, respectively. In addition, the average depth of the holes is ca. 1.2 nm. With visible light irradiation for 10 min again, the film morphologies almost reversed and the additional hills and hole almost disappeared (in Figure 5C). However, the morphology is not completely the same as the morphology in Figure 5A and some cracks are formed. It is known that the cis isomer occupies a larger space than the trans isomer. Therefore, the reason for the appearance of the cracks is most probably the molecule volume induced shrinkage from cis isomer to trans isomer after visible light irradiation. Second, the phenomenon is ascribed to the dewetting behavior of the thin films on solid substrates because of the surface tension,22 which breaks up into cylindrical holes and then grow together and touch, leaving rims of liquid that break up into cracks. In addition, after this cycle the distance between the large grains and the diameter of the grains as shown in the image have changed with alternate UV and visible light irradiation, which also proved the conformational change of the molecules in the LB films. Friction force microscopy (FFM) images are made simultaneously with the topography measurements in Figure 5. Figure 6A shows an FFM image of the monolayer before UV light irradiation. There are many bright areas in the image that should belong to the exposed mica substrate because it has a larger friction force.22 After UV light irradiation, many small additional bright areas are observed (Figure 6B), which then disappear with visible light irradiation again (Figure 6C). The reversible changes of the FFM images are related to not only the structure changes of the molecules as described in Figure 5 but also the friction changes in the surface of the monolayer. The reversibility of the morphological changes suggest that the ordered structures exist in the LB films after alternate irradiation with UV and visible light.15 The ordered structures in the self-assembled films have been observed by STM and are shown in Figure 1. According to this result, we propose a model of the structural change induced by photoisomerization. Figure 7 shows a schematic representation of the conformational change of the polymeric monolayer on the mica surface. By absorbing UV light, a free-standing trans molecule changes its form to a shorter and wider cis molecule. Upon visible light irradiation, a reversible reaction from cis to trans may occur. In a condensed state, a large volume change has to be induced during the reaction.3 The trans to cis photoisomerization gives rise to an increase in cross-sectional area of azobenzene, which also is proved by the distance change between two molecules before and after UV light irradiation in the STM image (Figure 1). However, the system has sufficient space in both the vertical and horizontal directions for photoisomerization to occur on the layered structure surface. When this area increase exceeds the molecular area of azobenzene in the LB films, the two-dimensional film structure should be changed. The most probable reason is that it can result in the release (23) Hu, J.; Carpick, R. W.; Salmeron, M.; Xiao, X. D. J. Vac. Sci. Technol., B 1996, 14, 1341.
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Figure 7. Model of the structural change of a single-layer LB film of the polymer accompanied by photoisomerization.
Si3N4 cantilever is hydrophilic.25 Therefore, the lower friction measured with the FFM over the fluorocarbon is attributed to the fluorocarbon moiety because of the low interaction between the hydrophilic tip and hydrophobic fluorocarbon moiety. However, after UV irradiation, many fluorinated sites have disappeared from the surface due to trans to cis isomer changes that also induced a stronger dipole as described in Figure 7. As a result, the surface friction strength became large and the many bright areas are also observed because of the strong interaction between the hydrophilic tip and hydrophilic areas. These observations of friction on the scale of nanometers can be applied to the study of tribology and boundary lubrication.26 However, it should be more adaptive to studying the wettability of the films. Because the tip of the Si3N4 cantilever is hydrophilic, the bright areas in the FFM image should be attributed to the hydrophilic areas and the dark to the hydrophobic areas, which is consistent with the result reported.25 According to this experiment, the wettability change obtained by FFM under alternate irradiation with UV and visible light directly reflects the change of contact angle. It shows that the FFM technique may be an efficient method to investigate the wettability of the films. Conclusion
Figure 6. In situ FFM images measured (also covering 5 µm × 5 µm) simultaneously with the topography image. (A) The image before UV light irradiation. (B) The image after UV light irradiation for 10 min. (C) The image after visible light irradiation for about 10 min. The solid lines in the images show the in situ position. In parts A and C the brighter white areas belong to the exposed mica substrate because it has larger friction, and in part B some additional brighter white dots correspond to the cis state of the molecules because they have strong interaction with the hydrophilic tip. The gray areas in the images correspond to the trans state of the molecules which have low friction.
of stress in the film by giving curvature to the film. This means that the film comes loose at the points where the hills are formed. From the model, the trifluoromethyl groups of the molecules are outside the monolayer in a trans state. Moreover, it is well-known that fluorine-containing materials exhibit unique properties such as low surface energy, high contact angle, reduced coefficient of friction, and oleo- and hydrophobicity,24 and also the tip of the
The LB films of a synthesized copolymer containing azobenzene groups have been studied. The UV-vis absorption spectra and AFM/FFM images indicate that azobenzene photoisomerizes reversibly in the LB films with alternate UV and visible light irradiation. The LB films possess a photocontrollable wetting behavior, which is illustrated by the reversible contact angle changes. In this experiment, the different surface energy in response to wettability changes is directly proved by the FFM, which shows different wetting areas before and after UV irradiation because of the changes of the surface friction in the LB films. It is proposed that FFM technique may be an efficient technique to investigate the wettability of photoresponsive systems. LA010071R (24) Matsumoto, K.; Kubota, M.; Matsuoka, H.; Yamaoka, H. Macromolecules 1999, 32, 7127. (25) Wang, R.; Hashimoto, K.; Fujishima, A.; Chiduni, M.; Kojima, E.; Kitamura, A.; Shimohigoshi, M.; Watanabge, T. Nature 1997, 388, 431. (26) Hardy, W. B. Proc. R. Soc. 1913, A88, 313.
