Brewster Angle Microscopic Observations of the Langmuir Films of

Brewster Angle Microscopic Observations of the Langmuir. Films of Amphiphilic Spiropyran during Compression and under UV Illumination. Takahiro Nakaza...
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Langmuir 2004, 20, 5439-5444

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Brewster Angle Microscopic Observations of the Langmuir Films of Amphiphilic Spiropyran during Compression and under UV Illumination Takahiro Nakazawa,† Reiko Azumi,‡ Hideki Sakai,† Masahiko Abe,† and Mutsuyoshi Matsumoto*,‡ Faculty of Science and Technology, Tokyo University of Science, 2641 Yamazaki, Noda 278-8510, Japan, and Nanotechnology Research Institute, National Institute of Advanced Industrial Science and Technology (AIST), Tsukuba Central 5-2, Tsukuba 305-8565, Japan Received February 18, 2004. In Final Form: March 22, 2004 The structure of the Langmuir film of an amphiphilic spiropyran, 1′,3′-dihydro-3′,3′-dimethyl-6-nitro1′-octadecyl-8-(docosanoyloxyme thyl)spiro[2H-1-benzopyran-2,2′-(2H)-indole] (SP), is investigated using Brewster angle microscopy (BAM). The BAM observations show that the Langmuir film of SP can be roughly categorized into three regimes: a low-temperature regime at 7-13 °C; a medium-temperature regime at 23-30 °C; a high-temperature regime at 40 °C. The low-temperature regime is characterized both by the domains that are formed just after the spreading and by the onset of the surface pressure when the domains are merged together to form continuous trilayers. In the medium-temperature regime, a continuous monolayer film is formed after the solvent evaporation, followed by the growth of “embryos” with compression. Around the phase transition point, the “embryos” serve as the “nucleation sites” of the circular trilayer domains. The characteristic features of the high-temperature regime are similar to the ones of the medium-temperature regime except for the absence of a steep rise in surface pressure after the plateau region and the absence of the circular trilayer domains. UV illumination of the Langmuir films leads to the isomerization of SP into merocyanine (MC). However, J-aggregates of MC are formed only when the circular trilayer domains are present. On the basis of the above results, we present a phase diagram of the Langmuir film of SP. The structure and photoreaction depend strongly on the phase of the Langmuir film, indicating that the area/molecule is not the only decisive parameter.

Introduction Molecular aggregates of dyes have attracted much attention due not only to their relevance to the lightharvesting and primary charge separation steps in photosynthesis but also to their interesting properties and great potential in technological applications. In particular, the structures and optical properties of J-aggregates of dyes have been extensively investigated under various conditions1-6 from the viewpoint of applications to the materials for spectral sensitization,7 optical storage,8,9 and nonlinear optics.10-12 Molecules in J-aggregates are arranged one-dimensionally or as in a brick-stone work. When chromophores form J-aggregates and a delocalized * To whom correspondence should be addressed. E-mail: [email protected]. † Tokyo University of Science. ‡ National Institute of Advanced Industrial Science and Technology. (1) (a) Jelly, E. E. Nature 1936, 138, 1009-1010. (b) Scheibe, G. Angew. Chem. 1937, 50, 51-52. (2) (a) Czikklely, V.; Fo¨rsterling, H. D.; Kuhn, H. Chem. Phys. Lett. 1970, 6, 207-210. (b) Steiger, R.; Kitzing, R.; Junod, P. J. Photogr. Sci. 1973, 21, 107-117. (3) Tsukruk, V. V.; Reneker, D. H.; Bliznyuk, V. N.; Kirstein, S.; Mo¨hwald, H. Thin Solid Films 1994, 244, 763-767. (4) Mo¨bius, D. Adv. Mater. 1995, 7, 437-444. (5) Higgins, D. A.; Kerimo, J.; VandenBout, D. A.; Barbara, P. F. J. Am. Chem. Soc. 1996, 118, 4049-4058. (6) J-aggregates; Kobayashi, T., Ed.; World Scientific: Singapore, 1996. (7) Tani, T. In J-aggregates; Kobayashi, T., Ed.; World Scientific: Singapore, 1996; p 209-228. (8) (a) Penner, T. L.; Mo¨bius, D. Thin Solid Films 1985, 132, 185192. (b) Ishimoto, C.; Tomimuro, H.; Seto, J. Appl. Phys. Lett. 1986, 49, 1677-1679. (9) (a) Ando, E.; Miyazaki, J.; Morimoto, K. Thin Solid Films 1985, 133, 21-28. (b) Unuma, Y.; Miyata, A. Thin Solid Films 1989, 179, 497-502.

excitonic state is formed, a new and sharp optical absorption band (J-band) appears, which is positioned at longer wavelengths with respect to the absorption band of the monomer. J-aggregates in two-dimensional molecular assemblies such as Langmuir films, LangmuirBlodgett (LB) films, and self-assembled adsorbed films are of considerable interest because the films have welldefined structures with a low-dimensional nature.4,8,9,12-18 Light-induced J-aggregation9,13e,14-16 and triggered Jaggregation18 of dye molecules is particularly important (10) (a) Kobayashi, T.; Misawa, K. In J-aggregates; Kobayashi, T.. Ed.; World Scientific: Singapore, 1996; p 161-180. (b) Godanos, R. In J-aggregates; Kobayashi, T., Ed.; World Scientific: Singapore, 1996; p 181-197. (11) Feller, K.-H.; Gadonas, R.; Pugzlys, A.; Mo¨bius, D. Laser Chem. 1997, 17, 123-137. (12) Furuki, M.; Wada, O.; Pu, L. S.; Sato, Y.; Kawashima, H.; Tani, T. J. Phys. Chem. B 1999, 103, 7607-7612. (13) (a) Saito, K. Jpn. J. Appl. Phys. 1999, 38, 2804-2805. (b) Ikegami, K.; Mingotaud, C.; Lan, M. J. Phys. Chem. B 1999, 103, 11261-11268. (c) Liu, M.; Lang, J.; Nakahara, H. Colloids Surf., A 2000, 175, 153159. (d) Tachibana, H.; Matsumoto, M. Jpn. J. Appl. Phys. 2000, 39, L884-L886. (e) Shiratori, K.; Nagamura, T. J. Photopolym. Sci. Technol. 2001, 14, 233-238. (f) Kuroda, S. Colloids Surf., A 2002, 198-200, 735-744. (g) Hirano, Y.; Miura, Y. F.; Sugi, M.; Ishii, T. Colloids Surf., A 2002, 198-200, 37-43. (h) Kato, N.; Yamamoto, M.; Itoh, K.; Uesu, Y. J. Phys. Chem. B 2003, 107, 1197-11923. (14) (a) Tachibana, H.; Yamanaka, Y.; Sakai, H.; Abe, M.; Matsumoto, M. Chem. Lett. 2000, 1182-1183. (b) Tachibana, H.; Yamanaka, Y.; Sakai, H.; Abe, M.; Matsumoto, M. Mol. Cryst. Liq. Cryst. 2000, 345, 149-154. (c) Tachibana, H.; Yamanaka, Y.; Sakai, H.; Abe, M.; Matsumoto, M. J. Lumin. 2000, 87-89, 800-802. (d) Tachibana, H.; Yamanaka, Y.; Matsumoto, M. J. Phys. Chem. B 2001, 105, 1028210286. (e) Tachibana, H.; Yamanaka, Y.; Matsumoto, M. J. Mater. Chem. 2002, 12, 938-942. (15) Matsumoto, M.; Nakazawa, T.; Azumi, R.; Tachibana, H.; Yamanaka, Y.; Sakai, H.; Abe, M. J. Phys. Chem. B 2002, 106, 1148711491. (16) Matsumoto, M.; Nakazawa, T.; Ajay Mallia, V.; Tamaoki, N.; Azumi, R.; Sakai, H.; Abe, M. J. Am. Chem. Soc. 2004, 126, 1006-1007.

10.1021/la049582e CCC: $27.50 © 2004 American Chemical Society Published on Web 05/18/2004

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Figure 1. Structure of SP and photoisomerization of SP to MC.

because the optical properties of local areas can be selectively modified. Light-induced J-aggregation of merocyanine (MC in Figure 1) is first reported for multilayer LB films of an amphiphilic spiropyran, 1′,3′-dihydro-3′,3′dimethyl-6-nitro-1′-octadecyl-8-(docosanoyloxymethyl)spiro[2H-1-benzopyran-2,2′-(2H)-indole] (SP in Figure 1), mixed with matrix molecules when illuminated with UV light at high temperatures, e.g., above 35 °C for the mixed LB films of SP and octadecane and above 40 °C for the mixed LB films of SP and stearic acid.9 UV illumination at room temperature results only in the isomerization of SP into MC without the formation of J-aggregates of MC. At room temperature, the transfer of the Langmuir films of SP onto solid substrates does not yield LB films with reproducible structures. We have reported that singlecomponent LB films of SP can be fabricated when the subphase temperature is 30 °C.14 We define a single-stroketransferred LB film as the one fabricated by transferring a Langmuir film by a single upward stroke. A singlestroke-transferred LB film is not always the same as a single-layer LB film because the former includes a case where the Langmuir film is a multilayer. When a singlestroke-transferred LB film of SP fabricated at 30 °C is illuminated with UV light at room temperature, Jaggregates of MC are formed. This suggests that the primary role of the matrix molecules is to facilitate the LB transfer at room temperature. The J-aggregation in LB films is much more complicated than that in Langmuir films due to the necessity of the transfer process. We have studied the photoreaction of the Langmuir film of SP and found that light-induced J-aggregation occurs at high surface pressures and at high temperatures under isothermal conditions.15 On the other hand, thermal hysteretic behaviors are observed in the Langmuir film of SP under isobaric conditions.16 Phase transitions are reported for Langmuir and LB films.16,19 In particular, the functions of the LB films depend strongly on the phase of the films, providing us with means to control the functions of the films using phase transitions triggered by external stimuli.16,19g,h This demonstrates the importance of investigating the phase of the Langmuir films to control the functions of the films. (17) (a) Owens, R. W.; Smith, D. A. Langmuir 2000, 16, 562-567. (b) Kawasaki, M.; Sato, T.; Yoshimoto, T. Langmuir 2000, 16, 54095417. (18) (a) Matsumoto, M.; Tachibana, H.; Sato, F.; Terrettaz, S. J. Phys. Chem. B 1997, 101, 702-704. (b) Matsumoto, M.; Sato, F.; Tachibana, H.; Terrettaz, S.; Azumi, R.; Nakamura, T.; Sakai, H.; Abe, M. Mol. Cryst. Liq. Cryst. 1998, 316, 113-118. (c) Terrettaz, S.; Tachibana, H.; Matsumoto, M. Langmuir 1998, 14, 7511-7518. (d) Matsumoto, M. Recent Res. Devel. Phys. Chem. 1999, 3, 79-94. (e) Matsumoto, M.; Terrettaz, S.; Tachibana, H. Adv. Colloid Interface Sci. 2000, 87, 147164. (19) (a) Gaines, G. L. Insoluble Monolayers at Liquid-Gas Interfaces; Intersciences Publishers: New York, 1966. (b) Bell, G. M.; Combs, L. L.; Dunne, L. J. Chem. Rev. 1981, 81, 15. (c) Naselli, C.; Rabolt, J. F.; Swalen, J. D. J. Chem. Phys. 1985, 82, 2136. (d) Ulman, A. An Introduction to Ultrathin Organic Films; Academic Press: New York: 1991; p 133. (e) Kajiyama, T.; Oishi Y. New Developments in Construction and Functions of Organic Thin Films; Kajiyama, T., Aizawa, M., Eds.; Elsevier: Amsterdam, 1996; p 1. (f) Sakai, H.; Umemura, J. Bull. Chem. Soc. Jpn. 1997, 70, 1027. (g) Nakamura, T.; Tanaka, M.; Sekiguchi, T.; Kawabata, Y. J. Am. Chem. Soc. 1986, 108, 1302. (h) Ariga, K.; Okahata, Y. J. Am. Chem. Soc. 1989, 111, 5618.

Figure 2. Surface pressure-area isotherms of SP: (a) at 7 °C; (b) at 13 °C; (c) at 18 °C; (d) at 23 °C; (e) at 30 °C; (f) at 40 °C.

In this paper, we report on the structure of the Langmuir film of SP by varying surface pressure and subphase temperature under isothermal conditions. Using surface pressure measurements and Brewster angle microscope (BAM), a phase diagram of the Langmuir film of SP is presented. The occurrence of light-induced J-aggregation of MC depends strongly on the phase of the Langmuir film. Experimental Section Chemicals. SP was obtained from Hayashibara Biochemical Laboratories, Inc., and used without further purification. Chloroform was of spectroscopic grade and purchased from Dojindo. Measurements for Langmuir Films and LB Transfer. All the measurements for Langmuir films and the LB transfer were done on a NIMA 632D1D2 trough equipped with two barriers under isothermal conditions. A chloroform solution of SP at a concentration of 1.0 × 10-4 M was spread onto an aqueous subphase purified by passing through a milli-Q filter. After 5 min of evaporation time of the solvent, the film was compressed at a speed of 6.6 × 10-2 nm2 molecule-1 min-1. The number of the SP molecules spread and the compression speed were kept constant for all the measurements because the surface pressurearea isotherms were sensitive to these factors. The Langmuir film was transferred using the vertical dipping method at a withdrawal speed of 4 mm min-1 onto CaF2 for transmission IR (TIR) spectroscopic measurements and onto vacuum-evaporated Au for reflection-absorption IR (RAIR) spectroscopic measurements. Hydrophobic substrates were prepared for AFM observations by immersing Si-wafers with oxidized surfaces in 1,1,1,3,3,3hexamethyldisilazane overnight, followed by rinsing with water and heating at 110 °C for 10 min. Characterization. A BAM equipped with a CCD camera and a video recorder was homemade. A He-Ne laser at 632.8 nm was used as the monitoring light. IR spectra of single-stroketransferred LB films were measured using a Perkin-Elmer Spectrum 2000 FTIR. The spectrometer was purged with nitrogen gas to minimize the amount of water vapor present in the sample chamber. The spectra were recorded at a 4-cm-1 resolution by coadding 256 scans in the 3200-800-cm-1 region. TIR spectra of single-stroke-transferred LB films were obtained at normal incidence. For RAIR measurements, p-polarized light was incident at an angle of 80°. UV illumination of the Langmuir films was done at oblique incidence through an optical fiber using a 500-W high-pressure mercury lamp with monochromated radiation at 365 nm. UV/vis reflection spectra of the Langmuir films were measured using an IMUC 700 spectrophotometer (Otsuka Electronics). AFM observations were made using a Seiko SPA300 microscope in a noncontact mode using a silicon tip with a resonant frequency of 28 kHz and a spring constant of 1.9 N m-1.

Results and Discussion Surface Pressure-Area Isotherms of SP. Figure 2 shows the surface pressure-area isotherms of the Langmuir films of SP in the temperature range of 7-40 °C.

BAM Study of the Langmuir Film of Spiropyran

These results reveal that the Langmuir films of SP can be roughly categorized into three regimes: a low-temperature regime at 7-13 °C; a medium-temperature regime at 23-30 °C; a high-temperature regime at 40 °C. In the low-temperature regime, the isotherm is of condensed type. The slope is steep, and the surface pressure rises at 0.4-0.5 nm2. When the molecules are compressed to 5 mN m-1 or more and expanded again, the isotherm shows a small hysteresis. In the medium-temperature regime, the surface pressure rises at larger area/molecule and the isotherm becomes more expanded at low surface pressures than in the low-temperature regime. A clear inflection point appears at ca. 5 mN m-1 that should be assigned to a phase transition. However, the higher surface pressure regions of the isotherms are almost the same with those in the low-temperature regime. When the molecules are compressed to a surface pressure below the phase transition pressure and expanded again, the isotherm is reversible. On the other hand, with compression to a surface pressure above the phase transition pressure and further expansion, the isotherm shows a hysteretic behavior. The surface pressure-area isotherm at 18 °C shows an intermediate behavior between the low-temperature and medium-temperature regimes. The surface pressure rises at around 0.55 nm2, and a small infection point appears at ca. 10 mN m-1, above which the isotherm is similar to the ones in the low-temperature regime. In the high-temperature regime at 40 °C, the isotherm is almost the same with that at 30 °C except for the absence of a steep rise after the phase transition. These results indicate that the isotherms depend strongly on the subphase temperature and suggest that several phases are present in the Langmuir film of SP. BAM Observations of the Langmuir Films of SP. Structures of Langmuir films have been extensively investigated using BAM.20 BAM has a great advantage over fluorescence microscopy in that no probe molecules are required for the observation of the morphology of ultrathin molecular films. This feature is very important especially in the present system because the isotherm depends strongly on the experimental conditions. The results of BAM observations revealed that the Langmuir films of SP can be categorized into three regimes as indicated by the results of surface pressure-area isotherm measurements: the low-temperature regime at 7-13 °C; the medium-temperature regime at 23-30 °C; the high-temperature regime at 40 °C. The surface pressure-area isotherms in the lowtemperature regime are characterized both by the small area/molecule at the onset of the surface pressure and by the absence of phase transition before the collapse. Typical BAM images of the Langmuir film of SP obtained during the compression in this regime are shown in Figure 3. At 7 °C, domains are evident on the water surface even just after the spreading of the molecules. We consider that no SP molecules exist in the interdomain region. The compression of the molecules induces a gradual fusion of the domains (Figure 3A). A continuous film is formed around the onset of the surface pressure (Figure 3B) at 0.4-0.5 nm2. This small area shows the formation of a multilayer. Further compression does not change the film structure significantly until the film collapses. Similar structural changes occur at 13 °C during compression. (20) (a) Ho¨nig, D.; Mo¨bius, D. J. Phys. Chem. 1991, 95, 4590-4592. (b) Henon, S.; Meunier J. Rev. Sci. Instrum. 1991, 62, 936-939. (c) Mo¨bius, D. Curr. Opin. Colloid Interface Sci. 1998, 3, 137-142. (d) Vollhardt, D. Adv. Colloid Interface Sci. 1999, 79, 19-57. (e) Johonson, M. J.; Anvar, D. J.; Skolimowski, J.; Majda, M. J. Phys. Chem. B 2001, 105, 514-519. (f) Iimura, K.; Yamauchi, Y.; Tsuchiya, Y.; Kato, T.; Suzuki, M. Langmuir 2001, 17, 4602-4609.

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Figure 3. BAM images of the Langmuir film of SP at different surface pressures at 7 °C: (A) at 0 mN m-1 (ca. 0.8 nm2); (B) at 5 mN m-1.

In the medium-temperature regime, the surface pressure rises at a larger area/molecule than that in the lowtemperature regime, followed by a phase transition at ca. 5 mN m-1. Typical BAM images in this regime are shown in Figure 4. During the spreading at 23 °C, the film seems oily and flexible. After the solvent evaporation, the film is continuous though a small number of bright spots are evident. The platelike domains observed in the lowtemperature regime are absent. These bright spots grow and increase in number during compression. The size and the number of the structures increase rapidly near the phase transition point (Figure 4A,B). We suggest that “embryos” are formed during the evaporation of the solvent. Some of the “embryos” are visible using BAM in the early stage of compression, but others are not large enough to be detected. With increasing size of the “embryos” during compression, more and more “embryos” become visible in the BAM images. After passing the phase transition, the film is comprised of bright structures that have grown into circular domains (Figure 4C). These circular domains have multilayer structures.15 Molecules should be present in the interdomain region as well. Closer look at the domains reveals that each domain has a brighter spot inside,15 suggesting that the “embryos” serve as the nucleation sites of the circular domains. Further compression gives rise to enhancement of the reflection from the interdomain region. The reflection from the domain regions does not change significantly. At a certain point around 15-20 mN m-1, the BAM image of the Langmuir film appears homogeneous, showing that the thickness is almost the same in all part of the film. With further compression, the reflection from the interdomain region becomes more intense than that from the domains. These results suggest that the interdomain region start to collapse while the domains preserve defined multilayer structures, until the interdomain region be-

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Figure 5. BAM images of the Langmuir film of SP at different surface pressures at 40 °C: (A) at 10 mN m-1; (B) at 18 mN m-1.

Figure 4. BAM images of the Langmuir film of SP at different surface pressures at 23 °C: (A) at 0 mN m-1 (ca. 0.9 nm2); (B) at 5 mN m-1; (C) at 10 mN m-1.

comes thicker than the domain region. Similar features are also evident at 30 °C. The Langmuir film at 18 °C shows an intermediate behavior between the low-temperature and the mediumtemperature regimes. A small inflection point appears in the surface pressure-area isotherm. After the onset point, the film seems almost homogeneous with some optically thinner regions. After passing the small inflection point, the film is homogeneous without the formation of the domains. The surface pressure-area isotherm in the hightemperature regime (at 40 °C) is similar to the one in the medium-temperature regime except for the absence of a steep rise after the phase transition. BAM images are also similar to those in the medium-temperature regime below the phase transition. However, the “nucleation sites” that are formed around the phase transition do not lead to the formation of the circular multilayer domains (Figure 5A). Only the reflection from the “nucleation sites” becomes more intense. Further compression also fails to develop the circular multilayer domains (Figure 5B). The reason of the absence of the circular multilayer domains may be

due to the fact that this temperature is higher than the melting point of SP in the bulk.16 Structure of Multilayer Domains in the Langmuir Film of SP. The structure of multilayer domains present in the medium-temperature regime is studied. We consider only trilayer and bilayer structures because the height of the multilayer domains in a single-stroke-transferred LB film is 4-5 nm.15 We assume that the surface of the Langmuir film consists of the hydrophobic moiety of SP. This leads to three candidate structures as shown in Figure 6. A Langmuir film of SP at 10 mN m-1 at 30 °C was transferred onto a hydrophobic substrate by an upward stroke. AFM observations revealed that the resultant LB film was a poorly structured film in which only small patches of SP molecules were scattered on the substrate surface. This strongly suggests that the hydrophilic moiety of SP constitutes the lower interface of the Langmuir film. This shows that the structure B is not a plausible model. The finding that the contact between the hydrophilic moiety of SP and a hydrophobic interface is not favored further suggests that structure C should be ruled out. Hence we propose that the trilayer structure shown in Figure 6A is the most plausible model as opposed to a bilayer structure suggested in our previous paper.15 As described in the previous section, the Langmuir film of SP appears homogeneous at 15-20 mN m-1 at 23 °C using BAM. We consider that the whole film should have a trilayer structure under the condition. This corresponds to the area per molecule of 0.30 nm2. This suggests that the molecular area of SP in a monolayer is ca. 0.90 nm2, which roughly coincides with the dimension of the chromophore of 0.8 nm2 estimated from the molecular model.9a We consider that continuous multilayers observed in the low-temperature regime also have trilayer structures because the surface pressure-area isotherms of the

BAM Study of the Langmuir Film of Spiropyran

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Figure 7. Space coordinates for expressing the orientation of the alkyl chains of an amphiphile.

This gives us the expression of the intensity ratios of the bands of νa(CH2) and νs(CH2) as follows:

IT/IRA ) (cos2 θ + 1)/2 m sin2 θ

Figure 6. Three candidate structures of the circular multilayer domain.

condensed phases are similar to those in the mediumtemperature regime. Tilt angle of the alkyl chains of SP can be estimated using the intensities of the bands of νa(CH2) and νs(CH2) at ca. 2920 and 2850 cm-1, respectively, in the TIR and RAIR spectra.21 The enhancement factor m for the RAIR measurement with respect to the TIR measurement can be obtained as follows:

m ) 4n13 sin2 φ/n23 cos φ

(1)

Here n1 and n2 are the refractive indices of air and the film and φ is the incident angle of the RAIR measurements. The value of m ) 6.62 was obtained with the parameters n1 ) 1.0, n2 ) 1.5, and φ ) 80°. In Figure 7, we introduce Cartesian coordinates (X, Y, Z). The X-Y plane is chosen as the surface of the substrate, and Z axis is perpendicular to the substrate. The vector P indicates the direction of the chain axis of either of the two hydrocarbons of the amphiphile and has a polar angle of θ. Space coordinates (X′, Y′, Z′) were also introduced to express the orientation of the alkyl chains of SP in the LB films. The Y′ axis is parallel to the X-Y plane, and the Z′ axis has the same direction as P. M is the transition moments of the CH2 symmetric or antisymmetric stretching mode. M is in the X′-Y′ plane and has an angle ψ with the coordinate X′. In the coordinates (X, Y, Z), the elements of M are expressed as follows:

M ) (M cos ψ cos θ cos φ M sin ψ sin φ, M cos ψ cos θ sin φ + M sinψ cos φ, -M cos ψ sin θ) (2)

(3)

Here IT is the absorbance of the band in the TIR spectrum and IRA is that in the RAIR spectrum. The results of the IR measurements of single-stroketransferred LB films of SP show that the tilt angle of the alkyl chains of SP is ca. 20°. Molecular length of SP was estimated as ca. 3 nm using a MM2 software installed in CS Chem3D Pro 3.5.1. By a combination of these values, the thickness of the trilayer of SP should be ca. 8.5 nm. The smaller value of the height of the domains measured by AFM may be due to (1) the interdigitation of the alkyl chains in the trilayers and/or (2) the presence of gauche structure in the alkyl chains as in single crystals of amphiphiles with more than one alkyl chains.22 Phase Diagram of the Langmuir Films of SP. Considering the above results, we present a phase diagram of the Langmuir film of SP in Figure 8. The phases of “continuous trilayer (CT)”, “monolayer with grains (MoG)”, “monolayer with circular trilayer domains (MoD)”, “multilayer with circular trilayer domains (MuD)”, and “collapsed film (CF)” appear in the Langmuir films of SP under the experimental conditions studied. Photoreactions of SP in the Langmuir films were monitored by measuring UV/vis reflection spectra of the Langmuir films during UV illumination and are summarized in Figure 9. The region of the occurrence of lightinduced J-aggregation roughly coincides with the region where “circular trilayer domains” are present. This shows that the photoreaction of SP depends strongly on the phase in the Langmuir film. These results together with the surface pressure-area isotherms indicate that the molecular area is not the only decisive parameter in the determination of the structure of the Langmuir film. By measuring simply the surface pressure-area isotherms of SP without BAM observation or investigation of photoreactions, one might assume that the structure and the photoreaction of the condensed phase of the Langmuir film should be the same in the lowtemperature and medium-temperature regimes because (21) (a) Umemura, J.; Kamata, T.; Kawai, T.; Takenaka, T. J. Phys. Chem. 1990, 94, 62-67. (b) Azumi, R.; Matsumoto, M.; Kuroda, S.; Crossley, M. J. Langmuir 1995, 11, 4495-4498. (22) (a) Honda, K.; Goto, M.; Kurahashi, M.; Miura, Y.; Nakamura, T.; Matsumoto, M.; Kawabata, Y. Anal. Sci. 1990, 6, 927-928. (b) Miura, Y. F.; Horikiri, M.; Honda, K.; Shiro, M.; Sasaki, K.; Sugi, M. Jpn. J. Appl. Phys., in press.

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Figure 8. Phase diagram of the Langmuir film of SP from 7 to 40 °C: CT, continuous trilayer; CF, collapsed film; MoG, monolayer with grains; MoD, monolayer with circular trilayer domains; MuD, multilayer with circular trilayer domains. Triangles represent the boundaries between the phases determined by the BAM observations. Solid circles show the collapse pressures obtained from the surface pressure-area isotherms. At 40 °C, the collapse pressure was not easily determined due to the absence of a steep rise after the phase transition. Broken lines are to guide the eye.

Figure 9. Conditions of the light-induced J-aggregation of MC in the Langmuir film.

the molecular area is almost the same. This assumption is denied by the present results. In a similar way, although the surface pressure-area isotherms are almost the same up to the end of the plateau region in the mediumtemperature and high-temperature regimes, the structures and the photoreactions of the Langmuir films are very different in the two regimes. The occurrence of light-induced J-aggregation of MC should depend on the presence of the nucleation sites, the concentration of MC, and the mobility of MC under isothermal conditions.15 The absence of J-aggregation in the low-temperature regime should be ascribed to the insufficient mobility of MC as observed for the LB film of SP. On the other hand, the J-aggregation does not proceed in the Langmuir film of SP in the high-temperature regime, which should be related with the fact that SP molecules are melted in this regime. However, we have to consider that light-induced J-aggregation proceeds in the Langmuir films of SP that are fabricated by spreading and compressing in the low-temperature regime, followed by heating to 40 °C under isobaric conditions.16 This strongly suggests that the two melted Langmuir films have different structures.

Nakazawa et al.

Light-Induced Structural Change of the Langmuir Films of SP. Structural change of the Langmuir film with UV illumination was monitored using BAM. The Langmuir film at 10 mN m-1 at 7 °C is located in the phase of “continuous trilayer”. UV illumination gave rise to the isomerization of SP into MC without the formation of J-aggregates of MC. Before UV illumination, the film was homogeneous as shown in Figure 3B. The reflection from the whole part of the film increased with UV illumination. However, the reflection decreased abruptly during the BAM observation. BAM observation for a few tens of seconds was enough to decrease the reflection intensity to that before UV illumination. This is due to the fact that the isomerization of MC to SP proceeds during the BAM observations because the wavelength of the monitoring light falls in the tail region of the absorption band of MC. The reflection change occurred reversibly with UV illumination-BAM observation cycles, indicating that this reflection change was due not to the decomposition but to the reversible isomerization of the dye. Prolonged UV illumination caused no significant change in the BAM images. The Langmuir film at 30 °C at 10 mN m-1, situated in the phase of “monolayer with circular trilayer domains”, was illuminated with UV light. This phase is characterized by the occurrence of the light-induced J-aggregation of MC. Before UV illumination, trilayer domains with grains were evident. With UV illumination, these grains became brighter and increased in size within the circular trilayer domains.15 These bright structures should consist of J-aggregates of MC. The reflection intensity of the bright structures decreased more slowly than that of MC monomers in the low-temperature phase during the BAM observation, indicating that the J-aggregates of MC are more photochemically stable than the latter. The Langmuir film at 40 °C at 10 mN m-1 is in the phase of “monolayer with grains”. The important feature is the absence of the trilayer domains. In this phase, only the isomerization from SP to MC occurred with UV illumination. BAM observation revealed that UV illumination failed to increase the size of the grains or induce large morphological changes. This is consistent with the absence of the light-induced J-aggregation. Conclusions The phase diagram of the Langmuir film of SP shows that several phases are present in the Langmuir film of SP and that mechanical compression gives rise to the phase transition in the medium-temperature regime. Lightinduced J-aggregation occurs when the circular trilayer domains are present. The photoreaction of the Langmuir film of SP is strongly related with the phase of the Langmuir film. The present results also indicate that the molecular area (molecular density) is not the only decisive parameter in the determination of the structure of the Langmuir film. Even if surface pressure-area isotherms behave similarly or area/molecule is similar under different conditions, that alone does not guarantee that the structures of the Langmuir films should be similar or that similar photoreactions should proceed in the Langmuir films. BAM is very efficient in the characterization of the Langmuir films. LA049582E