Investigation of Photosensitive Langmuir− Blodgett Monolayers by in

Figure 2 Change in absorption spectrum of a LB monolayer of SP/APT (molar ratio 1/1): (a) prior to illumination; (b) after ...... Nelson S. Bell and M...
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Langmuir 1998, 14, 7511-7518

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Investigation of Photosensitive Langmuir-Blodgett Monolayers by in Situ Atomic Force Microscopy and Absorption Spectroscopy Samuel Terrettaz, Hiroaki Tachibana, and Mutsuyoshi Matsumoto* National Institute of Materials and Chemical Research, 1-1 Higashi, Tsukuba, Ibaraki 305-8565, Japan Received May 15, 1998. In Final Form: October 13, 1998 Photosensitive monolayers containing both an azobenzene and a spiropyran group were prepared by the Langmuir-Blodgett technique. These films displayed a good reproducibility and a high stability extending up to several months in the dark. Large circular domains with a diameter of several micrometers were seen about 3.5 nm above a homogeneous phase. From geometrical considerations and investigations with different ratios of molecules containing the spiropyran and the azobenzene groups, these domains were interpreted as bilayers of mainly spiropyran molecules. The photoisomerization of both the spiropyran and the azobenzene group was performed. After an initial irradiation with ultraviolet light, the spiropyran transformed to merocyanine and subsequently to J-aggregates. The morphological and the optical changes induced by light were investigated by atomic force microscopy and by absorption spectroscopy on the same layer. The growth of J-aggregates was found to be the best explanation for the observed roughening of the circular domains. In situ atomic force microscopy allowed us to follow the spectacular morphological changes as a function of time and of the irradiation. At the beginning, a small upper structure was seen, usually at the edge of a domain. This upper patch spread progressively within the domain. The resulting multibranched shapes were related to a diffusion-limited aggregation process. Ultraviolet and visible light were seen to induce the presence of nucleation points and to stimulate the roughening process. The morphological change of a domain could also proceed slowly in the dark after the photoinduced nucleation.

Introduction Molecular layers of organic molecules can provide new functions to solid devices.1,2 Materials whose structure and macroscopic properties can be controlled by external stimuli are promising applications of molecular electronics. The photoinduced conversion of a film between several states can be exploited for instance in molecular switches or in the optical storage of information.3,4 Photochromism has been demonstrated in molecular layers containing spiropyrans5-10 or azo dyes.11-29 The morphology of * To whom correspondence should be addressed. Tel: +81-29854-4586. Fax: +81-298-54-4669. E-mail: [email protected]. (1) Ulman, A. An Introduction to Ultrathin Organic Films; Academic Press: New York, 1991. (2) Petty, M. C. Langmuir-Blodgett Films, An Introduction; Cambridge University Press: Cambridge, 1996. (3) Liu, Z. F.; Hashimoto, K.; Fujishima, A. Nature 1990, 347, 658. (4) Xia, W. A.; Huang, C. H.; Ye, X. Z.; Luo, C. P.; Gan, L. B.; Liu, Z. F. J. Phys. Chem. 1996, 100, 2244. (5) Gruda, I.; Leblanc, R. M. Can. J. Chem. 1976, 54, 576. (6) Polymeropoulos, E. E.; Mo¨bius, D. Ber. Bunsen-Ges. Phys. Chem. 1979, 83, 1215. (7) Ando, E.; Miyazaki, J.; Morimoto, K.; Nakahara, H.; Fukuda, K. Thin Solid Films 1985, 133, 21. (8) Doron, A.; Katz, E.; Tao, G.; Willner, I. Langmuir 1997, 13, 1783. (9) Willner, I.; Willner, B. Bioelectrochem. Bioenerg. 1997, 42, 43. (10) Willner, I.; Willner, B. Adv. Mater. 1997, 9, 351. (11) Yabe, A.; Kawabata, Y.; Niino, H.; Tanaka, M.; Ouchi, A.; Takahashi, H.; Tamura, S.; Tagaki, W.; Nakahara, H.; Fukuda, K. Chem. Lett. 1988, 1. (12) Nishiyama, K.; Fujihira, M. Chem. Lett. 1988, 1257. (13) Tachibana, H.; Nakamura, T.; Matsumoto, M.; Komizu, H.; Manda, E.; Niino, H.; Yabe, A.; Kawabata, Y. J. Am. Chem. Soc. 1989, 11, 3080. (14) Tachibana, H.; Goto, A.; Nakamura, T.; Matsumoto, M.; Manda, E.; Niino, H.; Yabe, A.; Kawabata, Y. Thin Solid Films 1989, 179, 207. (15) Seki, T.; Tamaki, T.; Suzuki, Y.; Kawanishi, Y.; Ichimura, K. Macromolecules 1989, 22, 3505. (16) Katayama, N.; Fukui, M.; Ozaki, Y.; Kuramoto, N.; Araki, T.; Iriyama, K. Langmuir 1991, 7, 2827. (17) Tachibana, H.; Azumi, R.; Nakamura, T.; Matsumoto, M.; Kawabata, Y. Chem. Lett. 1992, 173.

Langmuir-Blodgett (LB) films containing azobenzene has been investigated using atomic force microscopy (AFM).21-28 To form stable layers, these photoactive groups are connected to one or several alkyl chains and usually mixed with other amphiphilic molecules. The extent of isomerization in the film is dependent on the geometrical packing of the photoactive molecules. The concomitant structural change has been used to control the lateral electrical conductivity13,14,17,18 and the electrochemical selectivity8-10 of these layers, notably. Applications in the fields of biosensors9 and liquid crystals15 have been proposed. Spiropyrans exist as an equilibrium between a closed colorless spiro form and an open colored photomerocyanine form. Ultraviolet (UV) light shifts the equilibrium toward the merocyanine form while the back-reaction to the spiro form proceeds thermally or under irradiation with visible (18) Tachibana, H.; Manda, E.; Azumi, R.; Nakamura, T.; Matsumoto, M.; Kawabata, Y. Appl. Phys. Lett. 1992, 61, 2420. (19) Maack, J.; Ahuja, R. C.; Mo¨bius, D.; Tachibana, H.; Matsumoto, M. Thin Solid Films 1994, 242, 122. (20) Katayama, N.; Ozaki, Y.; Seki, T.; Tamaki, T.; Iriyama, K. Langmuir 1994, 10, 1898. (21) Geue, T.; Stumpe, J.; Pietsch, U.; Haak, M.; Kaupp, G. Mol. Cryst. Liq. Cryst. 1995, 262, 157. (22) Scho¨nhoff, M.; Chi, L. F.; Fuchs, H.; Lo¨sche, M. Langmuir 1995, 11, 163. (23) Wang, R.; Jiang, L.; Iyoda, T.; Tryk, D. A.; Hashomoto, K.; Fujishima, A. Langmuir 1996, 12, 2052. (24) Matsumoto, M.; Tachibana, H.; Sato, F.; Terrettaz, S. J. Phys. Chem. B 1997, 101, 702. (25) Ve´lez, M.; Mukhopadhyay, S.; Muzikante, I.; Matisova, G.; Vieira, S. Langmuir 1997, 13, 870. (26) Seki, T.; Tanaka, K.; Ichimura, K. Macromolecules 1997, 30, 6401. (27) Matsumoto, M.; Miyazaki, D.; Tanaka, M.; Azumi, R.; Manda, E.; Kondo, Y.; Yoshino, N.; Tachibana, H. J. Am. Chem. Soc. 1998, 120, 1479. (28) Imae, T.; Aoki, K. Langmuir 1998, 14, 1196. (29) Goldenberg, L. M.; Biernat, J. F.; Petty, M. C. Langmuir 1998, 14, 1236.

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light. The high density of merocyanines in a monolayer may result in the formation of aggregates with new optical properties. Aggregates have been reported for many cyanine (or merocyanine) dyes, and their formation is usually critically dependent on the structure and orientation of the cyanine molecules within the layer.30 The J-aggregates (also called Scheibe aggregates) are characterized by a very narrow red-shifted absorption band and a narrow fluorescence band with a small Stokes shift due to the formation of a delocalized excitonic state among the densely packed molecules.31 Models have been established to obtain the structure of the aggregates from the shift of the absorption band.32 The structures of several J-aggregates have been studied by electron microscopy,33 AFM,34 or recently near-field scanning optical microscopy (NSOM).35 Needles in the micrometer range have been found to be the most common in a variety of structures. Since their relatively recent discovery, scanning probe microscopies have been used intensively to gain information on the structure of organized layers.36 In situ AFM allows the investigation of dynamic processes in real-time. Combined information on the optical and the morphological properties of a layer can be obtained by NSOM.37 We have investigated LB films of molecules containing an azobenzene group connected to a bulky charge-transfer group by an alkyl chain.13 The photoisomerization of the azobenzene induces a reorganization of the charge-transfer group which leads to a change of the lateral conductivity of these layers. For a suitable alkyl chain both the photoisomerization and the electrical conductivity can be reversibly controlled by UV and visible light.17 A higher level of complexity can be obtained simply by using two different functional molecules. Using this strategy, a multiple photochemical switching process has been demonstrated.18 These molecules have also been used to promote the self-organization of a cyanine dye. In a mixed LB film, J-aggregate formation of the cyanine dye was triggered by the reversible photoisomerization of the azobenzene.24 In this paper, we report on the structural analysis of a LB monolayer consisting of two types of amphiphilic molecules containing either an azobenzene or a spiropyran group. The morphological change of the monolayer is followed in real-time during the irradiation by AFM. From optical and topographical measurements performed on the same LB film, the observed changes are best interpreted as photoinduced J-aggregation of the photomerocyanines. Experimental Section N-[p-(p-dodecylphenylazo)phenyloxy]dodecylpyridinium bis(7,7,8,8-tetracyanoquinodimethane) or APT12-12 (APT) was synthesized in a similar way as described previously.14 1′Octadecyl-3′,3′- dimethyl-6-nitro-8-[docosanoyloxymethyl]-spiro[2H-1-benzopyran-2,2′-indoline] or SP1822 (SP) was obtained from Japanese Research Institute for Photosensitizing Dyes Co., Ltd. Hexamethyldisilazane (HMDS) was purchased from Nacalai (30) Steiger R.; Kitzing, R.; Junod, P. J. Photogr. Sci. 1973, 21, 107. (31) Mo¨bius, D. Adv. Mater. 1995, 7, 437. (32) (a) Czikkely, V.; Fo¨rsterling, H. D.; Kuhn, H. Chem. Phys. Lett. 1970, 6, 11. (b) Czikkely, V.; Fo¨rsterling, H. D.; Kuhn, H. Chem. Phys. Lett. 1970, 6, 207. (33) Emerson, E. S.; Conlin, M. A.; Rosenoff, A. E.; Norland, K. S.; Rodriguez, H.; Chin, D.; Bird, G. R. J. Phys. Chem. 1967, 71, 2396. (34) (a) Wolthaus, L.; Schaper, A.; Mo¨bius, D. Chem. Phys. Lett. 1994, 225, 232. (b) Tsukruk, V. V.; Reneker, D. H.; Bliznyuk, V. N.; Kirstein, S.; Mo¨hwald, H. Thin Solid Films 1994, 244, 763. (35) Higgins, D. A.; Barbara, P. F. J. Phys. Chem. 1995, 99, 3. (36) Bottomley, L. A.; Coury, J. E.; First, P. N. Anal. Chem. 1996, 68, 185R and references therein. (37) (a) Betzig, E.; Trautman, J. K. Science 1992, 257, 189. (b) Moers, M. H. P.; Gaub, H. E.; van Hulst, N. F. Langmuir 1994, 10, 2774.

Terrettaz et al. Tesque, Inc. Organic solvents were purchased from Dojindo and were of spectroscopic grade. After passing through a milli-Q filter, deionized water was distilled and used as a subphase. The measurements of surface pressure-area isotherms at the air-water interface were performed with a Lauda Film Balance. Solutions of APT and SP (5 × 10-4 M) in a mixture of acetonitrile and benzene (1/1) were spread at the water surface. The temperature of the subphase was 16 ( 1 °C. The monolayers were then compressed with a speed of 2 cm/min until a surface pressure of 20-25 mN/m was reached. The layers were then maintained at this pressure for 5 min before transfer to a quartz plate of 3.8 × 1.3 cm2. Flat areas for AFM measurements were produced by taping pieces of about 1 cm × 1 cm of mica on the quartz (or glass) support. Transfer on hydrophilic substrates (quartz, freshly cleaved mica) was performed by vertical withdrawal of the substrate from the aqueous subphase at a speed of 5 mm/min. In the absence of another specification, we will always refer to monolayers of APT/SP (molar ratio 1/1) transferred vertically to a hydrophilic substrate at a lateral pressure of 2025 mN/m. Under these conditions, transfer ratios of 1.1 were obtained. The absorption spectra were measured at normal incidence with a Shimadzu UV-265FS spectrometer. The sample was placed on a rotating table and could be irradiated in the spectrometer with monochromatic light from a 500 W high-pressure mercury lamp. The illumination time and the interval between illuminations were fixed at 10 and 5 min, respectively. The intensity at the sample level was between 1.5 and 2.5 mW/cm2 for every wavelength used. AFM images were taken with a Seiko SPA 300 microscope operated in the noncontact mode. The oscillation frequency was around 24 kHz. Commercially available silicon cantilevers with a force constant of 1.5-1.8 N/m were used. Square areas of 1-400 µm2 were scanned at a rate of 0.5-1 Hz with a resolution of 256 pixels × 256 pixels or better. The images are presented without any correction other than for flatness. Repetitive scans gave essentially the same image. Quantitative evaluation of the image data was performed using the instrument analysis software. Direct illumination in the atomic force microscope was conveyed from a high-pressure mercury lamp through a monochromator to the sample by means of a fiber optic. These experiments will be referred to as in situ AFM as compared with the usual ex situ AFM measurements. The angle of incidence was 45°, and the intensity of the monochromatic light at the sample level was between 1.5 and 2.5 mW/cm2 for every wavelength used. For in situ AFM, the illumination time and the interval between illuminations were fixed at 8 or 10 min to coincide with the time needed to take an image. For surface ablation, contact mode AFM experiments were performed using silicon nitride cantilevers with a nominal force constant of 0.089 N/m. All measurements were performed under ambient conditions (temperature, 20-25 °C; humidity, 35-45%).

Results Light may interact with films containing APT and SP in a complex (multiwavelength) way. The molecular formulas of both chromophores together with their photoisomerization reactions can be seen in Figure 1. Initial irradiation with UV light induces both the photomerocyanine (PMC) and the cis-APT forms. The backreactions can be selectively enhanced with blue (436 nm) and green (546 nm) light for the APT and the SP systems, respectively. At room temperature, these back-reactions also occur in the dark. J-aggregates of PMC (J-PMC) can be formed under favorable structural conditions. In this work, the effect of an initial irradiation by UV light and a subsequent alternate irradiation by visible (436 nm) and UV light is presented. A quantitative variation of the same effect was obtained when the details of the irradiation were changed (use of UV light of 334 or 365 nm or number of irradiations).38 The exact conditions of illumination are reported in the captions of each Figure. Photoinduced Absorption Change. The main spectroscopic changes due to the illumination of a film of APT/

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Figure 1. Molecular formulas of SP and APT and scheme of their photochromic reactions.

Figure 2. Change in absorption spectrum of a LB monolayer of SP/APT (molar ratio 1/1): (a) prior to illumination; (b) after three successive irradiations with UV (334 nm) light; (c) after 17 h in the dark. Curves b and c are traced with a dotted and a dashed line, respectively. The times of and between individual irradiations were 10 and 5 min, respectively.

SP can be seen in Figures 2 and 3. The photoinduced formation of PMC by UV irradiation and the subsequent evolution of the system in the dark are shown in Figure 2. As shown in curve a, no absorption is seen at wavelengths longer than 500 nm in the absence of a preliminary UV irradiation. The intensity of the bands at 340 and 245 nm is reproducible for different layers prepared under the same experimental conditions. By comparison with the absorption of LB films consisting of only one component, the spectrum can be understood as the superposition of the bands of SP (at ca. 240, 270, and (38) UV light at the two wavelengths 334 and 365 nm from the mercury lamp triggers the photoisomerization of both chromophores and the subsequent formation of J-aggregates. The initial UV irradiation using 334 nm light was more efficient for the growth of J-aggregates. This may be related to the fact that the absorbance of SP is larger at 334 nm than at 365 nm, and vice versa for trans-APT. A triple 10-min illumination with 334 nm light was normally chosen as the initial illumination in order to reach a quasi-stationary state.

Figure 3. Change in absorption spectrum of a LB monolayer of SP/APT (molar ratio 1/1) as a function of the illumination. In addition to the photoisomerization of both chromophores, the growth of a narrow J-band under illumination is clearly seen. The arrow indicates the progression of the J-band with the increased number of film illuminations. After an initial illumination with UV light (3 irradiations with 334 nm light), the film was alternatively irradiated with visible (436 nm) and UV (365 nm) light. Absorption spectra after the initial UV (334 nm) irradiation, after the third and fifth UV (365 nm) irradiations, and after the sixth visible (436 nm) irradiation are shown. The absorption spectrum taken after the sixth visible irradiation was traced with a dashed line. The times of and between individual irradiations were 10 and 5 min, respectively.

340 nm) and trans-APT (ca. 240 and 350 nm). The noisy aspect of the spectra is due to the small number of molecules contained in the monolayer structures. After the initial illumination with UV light, a broad absorption band typical of the PMC form is observed around 600 nm, as shown in curve b.7 The decrease of the SP absorption at 240 and 340 nm and the appearance of a PMC absorption band at 380 nm are another indication of the photoisomerization of SP in the monolayer. The broad PMC absorption band is the combination of at least two bands (at ca. 560 and 590 nm) probably due to the monomeric form and to different aggregates. In addition, the presence

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of a narrow band at 618 nm is easily recognized after keeping the sample in the dark for approximately 15 min, mainly thanks to a decrease of the monomeric absorption. This band is attributed to J-PMC and will be referred to as the J-band.7 The decrease of the monomeric PMC absorption with time is accompanied by an increase of the SP bands, indicating that this is mainly due to the backreaction of PMC to SP. In curve c of Figure 2, the longterm version of this effect is presented. The absorption spectrum taken 17 h after the UV illumination reveals both the back-reaction to SP and the slow thermal growth of J-PMC. No significant change of the J-band is then observed after a layer aging of up to several months in the dark. The isomerization of APT is also occurring under the conditions of Figure 2. UV illumination induces the cis-form of APT, and the trans form is restored thermally. The isomerization of trans- to cis-APT can be monitored by a decrease of the absorption band at ca. 350 nm. Because of a significant overlap with SP absorption, the isomerization of APT is best seen under cyclic conditions such as those in Figure 3. In Figure 3, the isomerization of both chromophores is demonstrated. After the same initial UV exposure, the film was irradiated alternatively by visible (436 nm) and UV (365 nm) light. The spectra taken directly after the UV illumination are shown for the illumination cycles. An additional spectrum taken directly after the last visible irradiation reveals the cis/trans isomerization of APT. The typical variations of the band intensity at about 355 nm were obtained reversibly after UV and visible irradiations for many cycles.13 The conversion of SP to PMC after the initial UV illumination has already been discussed above. The most striking feature was the quick development of a very clear J-band under repetitive illuminations. A more precise analysis shows that UV light was particularly efficient at promoting J-band growth. A J-band increase could be induced by repetitive irradiations with UV light only. A small decrease of the monomeric absorption of PMC was apparent particularly after irradiation with visible light. Once the illumination was stopped (even after a prolonged illumination of more than 6 h), the reaction proceeded in the dark, as described previously. The J-band grew very slowly for a period of approximately 1 day and then remained practically unchanged for several months. The back conversions of PMC to SP and cis- to trans-APT were then clearly observed. Structure of the Layers before Illumination. In addition to its own photochemical sensitivity, APT provides a reproducible and well-controlled organization of SP in these LB films. Monolayers of only SP molecules were highly viscous and not very stable at the air-water interface. After their transfer to a solid support, AFM measurements revealed many small elongated domains and a few three-dimensional structures.39 Mixed monolayers of SP and octadecane were more stable at the airwater interface.7 After transfer on mica at a surface pressure of 25 mN/m, the obtained films consisted mainly of small domains about 4-5 nm higher than a homogeneous surface. The size (and the shape) of the domains depended on the molar ratio of SP but was in the submicron range. The mixed monolayers of APT and SP were stable with time at the air-water interface at the surface pressures investigated. The mean area per molecule at a lateral pressure of 20 mN/m was 0.40 ( 0.03 nm2. This corre(39) These small domains are approximately 4 nm deeper than the major phase and are interpreted as defects. The three-dimensional structures are likely due to multilayers.

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Figure 4. AFM image of a 20 µm × 20 µm area of a LB monolayer of SP/APT (molar ratio 1/1) prior to illumination. The circular domains are interpreted as bilayers of a SP-rich phase over a homogeneous monolayer.

sponds roughly to the cross-sectional area of APT. Because SP has a cross-sectional area of about 0.8 nm2, some special arrangement of the molecules in the layer has to be postulated. The AFM picture of a monolayer of APT and SP (molar ratio 1/1) deposited on mica can be seen in Figure 4. Large circular domains above a homogeneous phase are easily recognized. Contrary to the case of the corresponding single-component monolayer of APT or SP, no large-scale defect can be observed in the mixed films. Domains with double height, such as the two at the extreme right of Figure 4, and very narrow threedimensional structures were occasionally found. The presence of organic molecules in the homogeneous “lower” phase was demonstrated by the easy removal of material when the interaction between the tip and the sample was increased in contact mode.40 The height difference between the two main phases was 3.5 ( 1.0 nm. This is too large for a two-dimensional phase separation between the two constituents. Because the height corresponds to one layer of molecules and for the sake of simplicity, these circular domains will also be referred to as “bilayers”. The surface area of the bilayers increased with the ratio of SP in the spreading solution, indicating that these were SP-rich phases. At 25 mN/m, the area fraction of the bilayer was 40%, 15%, and less than 2.5% for the SP/APT ratios 2/1, 1/1, and 1/2, respectively. Similarly, the area fraction of the bilayer structures was seen to increase with the surface pressure of the transfer. The area fraction of the bilayer structure in a 1/1 LB film of SP and APT is 27%, 15%, and less than 1% for layers transferred at lateral pressures of 30, 25, and 10 mN/m, respectively. Photoinduced Change of Structure. Even if the structure of the films as described in the previous section was stable for several months in the dark, morphological (40) Squares of 1 µm × 1 µm were removed during the first scan with an applied force between the tip and the sample of several nanonewtons and a scan frequency of 1 Hz. Repetitive scans under the same conditions did not lead to any further change of the hole depth.

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Figure 5. AFM images of a 20 µm × 20 µm area of a LB monolayer of SP/APT (molar ratio 1/1) as a function of the illumination. The first UV irradiation was performed with 334 nm light. An alternate irradiation with visible (436 nm) and UV (365 nm) light was then applied. Snapshots of the photoinduced roughening of the upper domains are presented under the following conditions: (A) after the first UV irradiation; (B) after the first visible irradiation; (C) after the second UV irradiation; (D) after the second visible irradiation. The irradiation time and the time between irradiations were 8 min.

changes could be induced easily by light. In situ AFM measurements were particularly well-suited for such an investigation. A short glance at Figure 5 reveals that light induces a roughening effect on some bilayer domains. Figure 5A taken after a single 10 min irradiation with UV light differs only slightly from the corresponding picture in the dark (Figure 4). A small morphological change can be observed only on the double-height domains on the right side of the figure. After further illumination, hills and valleys are visible and the circular shape of the domains is replaced by a somewhat indented shape. On increased irradiation with both UV and visible light, more and more domains are touched and this roughening increases within the bilayer structure. If we concentrate

on the central domain, the evolution of the roughening process can be described. We can see the start of the photoinduced structural change in Figure 5B. Starting from the edge, a small portion of the bilayer seems to be covered by a new structure whose height is about 3 nm. No corresponding holes can be accounted for in this new upper patch. In Figure 5C, the upper structure has spread radially within the domain, and holes can be seen between the branches. The dendrites have extended as far as the domain border in Figure 5D. The phenomenon continues mainly by removing material between the branches. A more precise image of this particular domain after one more illumination can be seen in Figure 6A. Its shape is not as regular as before the irradiation. Some holes can

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Figure 6. Detailed image of two domains after illumination. The morphological change appears as a new structure on the top of the bilayers. The fractal-like shape is limited by the size of the SP-rich domain. Horizontal and vertical bands in the background are the result of the image flattening. In part A, the central domain of Figures 4 and 5 is shown after the third UV irradiation. For part B, the applied illumination consisted of sequential irradiations with light at the following wavelengths: 3 × 365 nm, 1 × 436 nm, 1 × 365 nm, and 1 × 436 nm. Image B is presented after keeping the sample in the dark for 10 h.

be seen within the bilayer structure which are even 1-2 nm deeper than the monolayer. A precise image of another domain after illumination is presented in Figure 6B. The same roughening of the previously planar bilayer can be observed. The photoinduced morphological change started not from the edge but from the interior of this bilayer with a small three-dimensional structure (height ca. 7 nm). This nucleation point is easily recognizable in the center of the morphological effect in Figure 6B. In this case, the photoinduced structure could extend in several directions but remained confined within the bilayer domain. A second nucleation point appeared later in the irradiation process and was the source of the roughening at the bottom of the picture. While the illumination was necessary to initiate the morphological change in any domain, the effect would then proceed slowly in the dark.41 Ex situ AFM measurements have shown that the roughening was still present after a period of at least 4 months. It should be mentioned that only a few domains are morphologically changed under the conditions of Figure 5. Similarly, the J-band observed is rather small. Under prolonged illumination a larger J-band and larger morphological changes can be obtained until most domains are actually roughened. To get a more direct correlation between the two methods, AFM pictures were also taken directly after the measurement of the absorption spectra on the same samples. The topography of the layers could still be obtained only with a lower quality because of the presence of lines from the quartz support. The structure of the monolayers of APT and SP on quartz was the same as that on mica. The patchwork of circular bilayer domains on a monolayer matrix was stable for months in the dark. (41) A similar photoeffect originating from the homogeneous monolayer phase has also been observed. In those rare cases, the extent of the morphological change was quite small.

After illumination the same roughening of some domains was observed as described earlier. Therefore the nature of the underlying substrate was not important for the investigated effects. Because the morphological change was the same under ex situ illumination, the possibility of artifacts due to repeated scanning during the in situ process is disregarded. In short, some morphological change was obtained directly after the initial UV illumination which corresponds to a film with an absorption spectrum like curve b in Figure 2. In the case of a shorter initial UV irradiation such as that in Figure 5A, little or no morphological change could be detected while a significant PMC absorption was still observed. Then the extent of roughening increased with the number of UV/ vis irradiations. As a control experiment, no morphological change and no J-band were obtained when SP was mixed with octadecane in a 1/1 or 1/2 molar ratio even under prolonged UV illumination (up to 70 min) at room temperature. Discussion The main results can be summarized rapidly. APT is a good matrix for the formation of well-defined and reproducible films of SP. Photoinduced topographical and spectroscopic changes have been demonstrated. The precise correlation of the morphological change with one of the photoreactions is necessary to interpret the AFM pictures at the molecular level. Even with the help of dual measurements on the same samples, this question is not trivial because of the different sensing mechanisms of the instruments. The absorption spectra give information on the isomerization extent of both dyes on a global scale while the morphological changes are observed very locally with the microscope. For instance, Ve´lez et al. could detect

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Figure 7. Scheme of the photochromic reactions occurring in LB films.

some photoisomerization of an azobenzene group in a LB film locally with AFM while no change was measured optically.25 The isomerization reactions occurring in LB films (as revealed by absorption spectra) are illustrated in Figure 7. The initial UV illumination induces the PMC and the cis-APT forms. For the longer UV irradiation, J-PMC is already formed at this stage. The alternate vis/UV irradiation stimulates J-PMC growth and induces the reversible isomerization of APT. The thermal process in the dark consists of a slow J-band growth accompanied by the back-reactions to SP and trans-APT. The modification of the film structure can be observed after the initial UV illumination if the exposure is sufficient. Then, the effect increases both thermally and under alternate vis/ UV irradiation. An interpretation based on precise kinetics is difficult because the extent of roughening is a rather qualitative measure. The morphological change is not due to the APT system. In fact, we have observed a modification of structure due to APT isomerization in pure APT films by AFM in preliminary experiments. The effect was fully reversible and much smaller than the one we report in this paper.42 The structural change due to the conversion of SP to PMC is not seen by AFM under our conditions. Indeed, when the initial UV irradiation time was shorter (a single 10 min irradiation), we observed a significant PMC absorption without any detectable morphological change. Similarly, in the absence of layer reorganization, the back-reaction to SP should have an opposite effect on the film structure. Because no detectable change of morphology could be associated with the former isomerization, the back-reaction to SP is also disregarded. By assuming that the morphological effect and the optical effect are closely associated with each other, the roughening process is therefore best related to the irreversible J-aggregate formation. Furthermore, AFM and absorption measurements on the same sample reveal a good qualitative correlation between the intensity of the J-band and the extent of the topographical effect. At a more fundamental level, differences between the total size of the aggregates and the part where the excitonic delocalization occurs should also be considered.35 Particularly supportive of our interpretation are the cases where no J-band and no morphological effect are observed, such as in the illumination of mixed LB films of SP and octadecane. On the other hand, it is still possible to consider the morphological effect as a consequence of some layer reorganization without simple influence on the spectroscopic properties. Some undefined defect would form during one isomerization reaction and would spread within the layer. Tools combining the topographical and the spectroscopic information locally would be necessary for a more precise interpretation. (42) The reversible morphological change of monolayers of APT on mica was observed by in situ AFM at a higher resolution. Small structures were reversibly erased by UV light and restored by visible light. The films were prepared under the same conditions as the SP/ APT mixed films. Large-scale defects and small irregular bilayer domains contributed to a more complex aspect of the images.

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The dendritic morphological change is also reminiscent of some crystallization or aggregation phenomenon. In molecular layers, self-similar (or fractal-like) structure growth has already been observed in the formation of crystalline domains in phospholipids at the air-water interface43 and in the self-assembly of an alkylsilane monolayer on mica.44 The so-called diffusion-limited aggregation model has been used to describe these multibranched shapes mathematically.45 The model is usually associated with the irreversible aggregation of small particles to clusters, which seems very appropriate for our system. In situ AFM is a good tool to study the kinetics of such a process at a molecular scale. No mathematical description will be attempted, but a few characteristics are highlighted in the following. The morphological change starts usually at the edge of a SPrich domain under illumination. Because of the absence of corresponding holes, we think that the upper patch is due to a film expansion and not to an extra layer. At the beginning the film expansion proceeds in a planar dendritic way very similar to other growth phenomena in molecular layers. As the process grows further (mainly through repetitive illuminations), more molecules from the bilayer and even from the underlying monolayer are actually integrated in the new structure. From this moment on, “holes” can be seen mainly between the branches. The indented shape of the domain is actually a consequence of this last phenomenon. The morphological change can extend to several micrometers and seems mainly limited by the size of the SP-rich domains. We have seen that both APT and SP can undergo photoisomerization in the mixed films investigated here. In this paper the structural changes induced by the photochemistry of SP have been studied. One point of interest is to know if the two systems are coupled and especially if the reversible isomerization of APT influences the growth of J-PMC. We have already reported such a case for a cyanine dye and the same molecule of APT.24 The structure of the cyanine dye was not directly affected by the irradiation, and the J-aggregation was clearly due to structural changes of the layer resulting from the reversible APT isomerization. A similar phenomenon is expected but cannot be proven in this analogous system. In this example, the two chromophores are not independent because the same UV illumination triggers both photoisomerizations and the subsequent J-aggregation of PMC. We believe that the reversible isomerization of APT helps the J-aggregation process because, during the alternate UV/vis illumination after the initial UV treatment, morphological changes are induced equally well by visible and UV light. Visible light mainly enhances the back-reaction to trans-APT while having no significant direct effect on the SP system. A quantitative evaluation of this process is difficult. Clearly, more work using amphiphiles with totally independent chromophore units is required to address this point definitively. Irradiations with UV only or UV in alternance with 546 nm light can also stimulate the J-growth. The latter case could be explained by an orientation process assisted by the existing J-PMC, similar to the one reported by Unuma et al.46 Through reversible isomerizations of SP, a favorable orientation of PMC occurs close to the aggregates while (43) (a) Miller, A.; Knoll, W.; Mo¨hwald, H. Phys. Rev. Lett. 1986, 56, 2633. (b) Miller, A.; Mo¨hwald, H. J. Chem. Phys. 1987, 86, 4259. (44) Schwartz, K. D.; Steinberg, S.; Israelachvili, J.; Zasadzinski, J. A. N. Phys. Rev. Lett. 1992, 69, 3354. (45) (a) Witten, T. A.; Sander, L. M. Phys. Rev. Lett. 1981, 47, 1400. (b) Witten, T. A.; Sander, L. M. Phys. Rev. B 1983, 27, 5686. (c) Fleury, V. Nature 1997, 390, 145. (46) Unuma, Y.; Miyata, A. Thin Solid Films 1989, 179, 497.

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PMC molecules within the aggregates are hardly changed. J-PMC formation is probably critically dependent on the structure of the molecular layer. Ando et al. reported a very intense J-band after UV illumination of a LB film of SP and octadecane (molar ratio 1/2) at temperatures higher than 35 °C.7 We did not observe any J-PMC spectroscopically when SP was mixed with octadecane in a 1/1 or 1/2 molar ratio even under prolonged UV illumination at room temperature. The mechanism of J-PMC formation may be dependent on the irradiation conditions, but the resulting spectroscopic and topographic changes are identical. The basic picture is that a sufficient local concentration of PMC with a favorable orientation has to be present at a certain time. Therefore the initial UV illumination is indispensable. A sufficient mobility within the layer is then necessary for the neighboring PMC molecules to diffuse toward the aggregates and to adopt a suitable orientation. Isomerization reactions increase this possibility notably.

Terrettaz et al.

long-term stability are attractive features for high-density storage of the information using scanning probe microscopies. The structural changes of these layers can be modulated by light at several wavelengths and can also be detected optically. The use of two (or more) different molecules is a simple and flexible way of getting multifunctionality. The LB technique provides a good way to mix or to separate the two active components within or between layers. In our example, the photoisomerization of both chromophores has been obtained. The photoisomerization of the APT system has been seen as small and reversible for many cycles. In that particular case, an electrical detection can be added to the optical and morphological ones. The formation of J-aggregates is the main point of the SP/PMC system. The morphological changes are large, irreversible, and confined to some domains. The resulting J-band can be viewed as an amplifier of the optical change. This phenomenon could be used to fabricate a long-lasting memory in the same layer as the reversible (erasable) APT system.

Conclusion We have studied an example of a molecular layer able to display sophisticated functions while retaining a simple and reproducible structure. These conditions along with

Acknowledgment. S.T. gratefully acknowledges the support of the Science Technology Agency of Japan. LA9805802