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Control of Photoreaction of Amphiphilic Spiropyran/ n-Alkane Langmuir and Langmuir-Blodgett Films Using the Phase Transition of n-Alkane 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 July 8, 2004. In Final Form: September 9, 2004 The structures and photoreactions of Langmuir and Langmuir-Blodgett (LB) films of an amphiphilic spiropyran, 1′,3′-dihydro-3′,3′-dimethyl-6-nitro-1′-octadecyl-8-(docosanoyloxymethyl)spiro[2H-1-benzopyran2,2′-(2H)-indole] (SP), mixed with n-alkane are investigated. The mixing ratio was fixed at 1/2 for SP/ n-alkane. The surface pressure-area isotherms of SP/octadecane are categorized into two regimes: a low-temperature regime where octadecane is packed with the alkyl chains of SP, and a high-temperature regime where the addition of octadecane does not influence the isotherms significantly. The temperature dividing the two regimes is related with the melting point of the n-alkane mixed with SP in the bulk. UV irradiation of the Langmuir film in the high-temperature regime gives rise to light-induced J-aggregation, whereas that in the low-temperature regime causes only the isomerization of SP to the corresponding merocyanine, indicating that J-aggregation is hindered by the presence of n-alkane in the low-temperature regime. IR external reflection spectroscopy of the Langmuir films shows that n-alkane is released from the film during J-aggregation. The structural changes of the mixed Langmuir and LB films during J-aggregation are almost the same with those of the films of pure SP.
Introduction Langmuir and Langmuir-Blodgett (LB) films have welldefined structures with thickness controlled at the molecular level. Because the molecules are oriented in a designed manner, LB films have attracted considerable interest in terms of applications to various functionalized materials such as molecular electronics, switches, sensors, and memories.1 Controlling the functions of LB films using external stimuli is one of the important subjects for the construction of molecular switches. In particular, switches using phase transition have advantages in that the signals can be amplified. Permeation of materials through LB films has been controlled using the phase transition of the films.2 Well-defined structures of Langmuir and LB films have large effects on the photoreactions that occur in them. In many cases, the photoisomerization of azobenzene and other photochromic compounds is assumed to occur only when free volume is available.3 Amphiphilic diacetylenes can be photopolymerized in the LB films because the molecules are oriented in such a manner that the adjacent diacetylene moieties are arranged favorably for the * To whom all correspondence should be addressed. E-mail:
[email protected]. † Tokyo University of Science. ‡ National Institute of Advanced Industrial Science and Technology (AIST). (1) (a) Kuhn, H.; Mo¨bius, D.; Bu¨cher, H. In Physical Methods of Chemistry; Weisserberger, A., Rossiter, B. W., Eds.; Wiley-Interscience: New York, 1972; Vol. 1, Part IIIB, pp 577-702. (b) LangmuirBlodgett Films; Roberts, G. G., Ed.; Plenum Press: New York, 1990. (c)Ulman, A. An Introduction to Ultrathin Organic Filmssfrom Langmuir-Blodgett Films to Self-Assembly; Academic Press: San Diego, 1991. (d) Special issue: A collection of papers presented at the 9th International Conference on Organized Molecular Films. Colloids Surf., A 2002, 198-200. (2) Ariga, K.; Okahata Y. J. Am. Chem. Soc. 1989, 111, 5618-5622.
polymerization.4 The smooth film surface and the thickness at the molecular level enable us to monitor a subtle change of the film accompanied by the photoreactions using scanning probe microscopy. Reversible morphological changes of LB films are accompanied by photoisomerization of azobenzene, which presents counterexamples to the concept of free volume.5 Large morphological changes of LB films occur with light-induced J-aggregation and triggered J-aggregation of dyes.6 Color change from blue to red with photoirradiation of diacetylene LB films is accompanied by morphological changes of the films.4c (3) (a) Whitten, D. G. Angew. Chem., Int. Ed. Engl. 1979, 18, 440450. (b) Yabe, A.; Kawabata, Y.; Niino, H.; Matsumoto, M.; Ouchi, A.; Takahashi, H.; Tamura, S.; Tagaki, W.; Nakahara H.; Fukuda, K. Thin Solid Films 1988, 160, 33-41. (c) Tachibana, H.; Nakamura, T.; Matsumoto, M.; Komizu, H.; Manda, E.; Niino, H.; Yabe, A.; Kawabata, Y. J. Am. Chem. Soc. 1989, 111, 3080-3081. (d) Tachibana, H.; Azumi, R.; Tanaka, M.; Matsumoto, M.; Sako, S.; Sakai, H.; Abe, M.; Kondo, Y.; Yoshino, N. Thin Solid Films 1996, 284-285, 73-75. (e) Matsumoto, M.; Terrettaz, S.; Tachibana, H. Adv. Colloid Interface Sci. 2000, 87, 147-164. (4) (a) Tieke, B. Adv. Polym. Sci. 1985, 71, 79-151. (b) Tachibana, H.; Yamanaka, Y.; Sakai, H.; Abe, M.; Matsumoto, M. Macromolecules 1999, 32, 8306-8309. (c) Tachibana, H.; Yamanaka, Y.; Sakai, H.; Abe, M.; Matsumoto, M. Langmuir 2000, 16, 2975-2977. (5) (a) Seki, T.; Tanaka, K.; Ichimura, K. Macromolecules 1997, 30, 6401-6403. (b) Matsumoto, M.; Miyazaki, D.; Tanaka, M.; Azumi, R.; Manda, E.; Kondo, Y.; Yoshino, N.; Tachibana, H. J. Am. Chem. Soc. 1998, 120, 1479-1484. (6) (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) 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. (f) Matsumoto, M.; Nakazawa, T.; Azumi, R.; Tachibana, H.; Yamanaka, Y.; Sakai, H.; Abe, M. J. Phys. Chem. B 2002, 106, 11487-11491. (g) Matsumoto, M.; Nakazawa, T.; Ajay Mallia, V.; Tamaoki, N.; Azumi, R.; Sakai, H.; Abe, M. J. Am. Chem. Soc. 2004, 126, 1006-1007. (h) Nakazawa, T.; Azumi, R.; Sakai, H.; Abe, M.; Matsumoto, M. Langmuir 2004, 20, 5439-5444.
10.1021/la0482857 CCC: $27.50 © 2004 American Chemical Society Published on Web 10/27/2004
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Figure 1. Structures of SP and photoisomerization to MC.
These findings show that photoreactions can give rise to morphological changes of the films, suggesting that photoreactions can be monitored by observing the morphology of the films in many cases. J-aggregates of dyes have been investigated extensively from the viewpoint of applications to spectral sensitization, optical storage, and nonlinear optics.7-9 When chromophores form J-aggregates and excitonic state is delocalized, the absorption band is red-shifted and very narrow with a small Stokes shift compared to the case in which chromophores are in a monomeric state. Light-induced J-aggregation6d-g,8f,9 and triggered J-aggregation6a-c of dye molecules in Langmuir and LB films is particularly important because the optical properties of selected areas of the films can be modified by patterning the material with J-aggregate by photoirradiation through photomasks. When 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 are irradiated 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, J-aggregates of the corresponding merocyanine (MC in Figure 1) are formed.9 Room-temperature UV irradiation gives rise only to the isomerization of SP into MC without the formation of the J-aggregates of MC. We have investigated the structures and photoreactions of the Langmuir and LB films of SP.6d-h Under isothermal conditions, light-induced J-aggregation proceeds when multilayer circular domains are formed. J-aggregates are formed around the nucleation sites present in the multilayer domains through diffusion of molecules. The Langmuir films of SP can be transferred as LB films onto solid substrates at 30 °C. The LB films consist of multilayer circular domains with a diameter of 10-20 µm and a height of 4-5 nm. Room-temperature illumination of the LB films with UV light produces the J-aggregation of MC with the development of dendritic structures starting from the three-dimensional structures in the domains.6f,h On the other hand, thermal hysteretic behaviors are observed in the Langmuir film of SP under isobaric conditions.6g In this study, we investigate the Langmuir and LB films of SP mixed with n-alkane. The mixing ratio of SP to n-alkane was fixed at 1/2 because the formation of a stable (7) (a) Jelly, E. E. Nature 1936, 138, 1009-1010. (b) Scheibe, G. Angew. Chem. 1937, 50, 51-52. (c) Czikklely, V.; Fo¨rsterling, H. D.; Kuhn, H. Chem. Phys. Lett. 1970, 6, 207-210. (d) Mo¨bius, D. Adv. Mater. 1995, 7, 437-444. (e) J-aggregates; Kobayashi, T., Ed.; World Scientific: Singapore, 1996. (8) Penner, T. L.; Mo¨bius, D. Thin Solid Films 1985, 132, 185-192. (b) Ishimoto, C.; Tomimuro, H.; Seto, J. Appl. Phys. Lett. 1986, 49, 1677-1679. (c) Furuki, M.; Wada, O.; Pu, L. S.; Sato, Y.; Kawashima, H.; Tani, T. J. Phys. Chem. B 1999, 103, 7607-7612. (d) Saito, K. Jpn. J. Appl. Phys. 1999, 38, 2804-2805. (e) Ikegami, K.; Mingotaud, C.; Lan, M. J. Phys. Chem. B 1999, 103, 11261-11268. (f) Shiratori, K.; Nagamura, T. J. Photopolym. Sci. Technol. 2001, 14, 233-238. (g) Kuroda, S. Colloids Surf., A 2002, 198-200, 735-744. (h) Hirano, Y.; Miura, Y. F.; Sugi, M.; Ishii, T. Colloids Surf., A 2002, 198-200, 37-43. (i) Kato, N.; Yamamoto, M.; Itoh, K.; Uesu, Y. J. Phys. Chem. B 2003, 107, 1197-11923. (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.
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monolayer has been reported at this mixing ratio.9a The structures and photoreactions of the mixed films were studied with variations in the temperature and n-alkane species, using Brewster angle microscopy (BAM), UVvis spectroscopy, IR spectroscopy, and atomic force microscopy (AFM). The results show that light-induced J-aggregation of MC is suppressed in the mixed Langmuir films below the melting point of n-alkane mixed with SP whereas J-aggregation proceeds above the melting point. The photoreaction of Langmuir and LB films is influenced by phase transitions of n-alkanes in the films. Experimental Section Chemicals. Amphiphilic spiropyran SP was purchased from Hayashibara Biochemical Laboratories, Inc. Chloroform and toluene were of spectroscopic grade and obtained from Dojindo Laboratoires. N-Octadecane (HC18), n-nonadecane (HC19), and n-tetracosane (HC24) were purchased from Wako Pure Chemical Industries, Ltd. N-Nonadecane-d40 (DC19) and n-tetracosaned50 (DC24) were obtained from Aldrich Chem. Co., Inc. N-Eicosane (HC20) was purchased from Tokyo Chemical Industry. All the chemicals were used as received. Differential scanning calorimetry (DSC) thermograms were obtained using a Rigaku DSC8230 calorimeter with heating and cooling rates of 2 °C min-1. A melting point of n-alkane mixed with SP in the bulk (hereafter abbreviated as n-alkane/SP) was determined in the second and succeeding heating and cooling cycles. Monolayer Measurements. All the monolayer measurements were done using a NIMA 632D1D2 trough equipped with two moving barriers. The advantage of this trough is that the Langmuir films can be compressed to a very small area, which is important for this study. Chloroform was used as a spreading solvent. The mixing ratio of SP to n-alkane was 1 to 2 for all the measurements. Spreading solution at a SP concentration of 1.0 × 10-4 M was spread on an aqueous subphase purified by passing through a milli-Q filter. The molecules were compressed at a speed of 6.6 × 10-2 nm2 (SP molecule)-1 min-1 after 5 min of evaporation time. The number of the SP molecules spread and the compression speed were kept constant for all the measurements because the surface pressure-area (π-A) 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 quartz plates for spectroscopic measurements, onto CaF2 plates for IR measurements, and onto freshly cleaved mica for AFM observations. Characterization. UV-vis reflection spectra of the Langmuir films were measured using an IMUC 700 spectrophotometer (Otsuka Electronics). The Brewster angle microscope equipped with a CCD camera and a video recorder was homemade. A HeNe laser at 632.8 nm was used as the monitoring light. 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. IR spectra of Langmuir and LB films were measured using a Perkin-Elmer Spectrum 2000 FTIR. IR external reflection spectra of Langmuir films were recorded using an accessory for monolayer measurements. For the measurements of IR transmission spectra of singlelayer LB films, the spectrometer was purged with nitrogen gas to minimize the amount of water vapor present in the sample chamber. All the spectra were recorded at a 4-cm-1 resolution by coadding 256 scans in the 4000-750 cm-1 region. UV-vis absorption spectra of single-layer LB films were recorded on a Cary 500 Scan (Varian) spectrophotometer. UV illumination of the LB films was carried out at normal incidence using a 500-W high-pressure mercury lamp with monochromated radiation at 365 nm at room temperature. 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. In situ AFM observations were performed by illuminating the sample through an optical fiber at oblique incidence.
Results and Discussion Surface Pressure-Area Isotherms of SP and SP/ n-Alkane. Figure 2 shows the π-A isotherms of pure SP
Photoreaction of Dye/Alkane Langmuir and LB Films
Figure 2. Surface pressure-area isotherms of (A) pure SP and (B) SP mixed with HC18 (mixing ratio of 1/2) in the temperature region from 7 to 40 °C.
and SP mixed with n-alkane (SP/n-alkane) in the temperature range 7-40 °C. The surface pressure of pure SP rises at ca. 0.4 nm2 (SP molecule)-1 at subphase temperatures equal to or lower than 13 °C. After the onset point, the isotherms are of condensed type and continuous multilayers are formed. At 18 °C, the onset point is shifted to larger molecular area. Above 23 °C, a deflection point appears that is attributed to a phase transition.6f Mixing HC18 with SP causes large changes to the π-A isotherms. The π-A isotherms of SP/HC18 (mixing ratio of 1/2) can be divided into a low-temperature regime and a high-temperature regime. In the low-temperature regime from 7 to 23 °C, the π-A isotherms are similar to each other and are of condensed type. The surface pressure rises steeply at ca. 0.6-0.7 nm2 (SP molecule)-1. No phase transition point appears in this temperature regime. Ando et al. reported that the surface pressure of SP/ octadecane (mixing ratio 1/2) rises steeply at ca. 0.8 nm2 (SP molecule)-1 at 18 °C.9a The present results show similar features in that the isotherms are of condensed type. The smaller area of the onset point in this study compared with the reported value may be due to the differences in the experimental conditions such as the concentration of the spreading solvent, compression speed, and pH of the subphase, considering that the π-A isotherm of SP depends strongly on the experimental conditions. We suggest that the area at which condensed phases
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appear in this study may be smaller than the actual area occupied by each SP molecule because the Langmuir films are not stable considering that the transfer ratio was much larger than unity. The π-A isotherms have common features also in the high-temperature regime from 30 to 40 °C. The surface pressure increases at ca. 1 nm2 (SP molecule)-1. Phase transition is evident in all the isotherms. After the phase transition, the surface pressure increases very slowly though the isotherm with molecular area less than 0.4 nm2 (SP molecule)-1 depends on the temperature. These features are almost the same as those of the isotherms of pure SP.6f Above results should be related to the physical properties of HC18 mixed in the Langmuir films. DSC thermograms showed that the melting point of HC18/SP in the bulk is 27-31 °C, which is similar to that of HC18 in the bulk as 28 °C.10 The π-A isotherms of SP are almost the same as those of SP/HC18 in the high-temperature regime, which coincides with the temperature region above the melting point of HC18/SP. This strongly suggests that SP is melted in the Langmuir films. On the other hand, the π-A isotherms of SP/HC18 are very different from those of pure SP in the low-temperature regime, which is located below the melting point of HC18/SP. The fact that the surface pressure rises more steeply at larger area for SP/ HC18 than for pure SP shows that the structures of the Langmuir films of SP/HC18 should be different from continuous trilayers observed for the Langmuir film of pure SP.6h The most plausible structural model is the one in which HC18 molecules are incorporated in the hydrophobic part of SP monolayer and packed with the alkyl chains of SP considering that the hydrophilic part of SP is much bulkier than the hydrophobic part.9a Incorporation of n-alkane in the hydrophobic regions of monolayers has been proposed also for other amphiphilic dyes with the hydrophilic moieties much bulkier than the hydrophobic moieties.1a,7c These results strongly suggest that the structures of the Langmuir films of SP are affected to a large extent by the addition of n-alkane below the melting point of n-alkane/SP. Photoreaction of the Langmuir Film of SP/nAlkane. SP isomerizes to MC with UV irradiation in the Langmuir film of pure SP. In particular, the conditions of the light-induced J-aggregation almost coincide with those of the formation of trilayer circular domains.6h We investigate the photoreaction of the Langmuir film of SP/ n-alkane to reveal the effect of n-alkane on the lightinduced J-aggregation. Figure 3 shows some representative changes in UVvis reflection spectra of the Langmuir films of SP/n-alkane on UV irradiation. Figure 3A shows the UV-vis reflection spectra of the Langmuir film of SP/HC19 below the melting point of HC19/SP. The development of a broad reflection band ranging from 500 to 600 nm indicates the isomerization of SP to MC without the J-aggregate formation. Light-induced J-aggregation proceeds in the Langmuir film of pure SP at the same surface pressure and temperature. This shows that SP and HC19 are not phaseseparated in the Langmuir film and supports the structural model proposed on the basis of the π-A isotherms.9a Another structural model that SP multilayer and n-alkane monolayer (or multilayer) coexist and form mixed Langmuir films without phase separation seems less plausible because this model requires that one of the ends of n-alkane is attached to the water surface. Figure 3B shows the (10) Dictionary of Organic Compounds, 6th ed.; Cadogan, J. I. G., Ley, S. V., Pattenden, G., Raphael, R. A., Eds.; Chapman & Hall: London, 1996.
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Figure 4. Increase in reflection of the Langmuir film of SP/ n-alkane (at 20 mN m-1) at 618 nm due to light-induced J-aggregation in the photostationary state: dashed line (circles), SP/HC18; dotted line (squares), SP/DC19; dashed-dotted line (triangles), SP/HC19; dashed-dotted-dotted line (inverse triangles), SP/HC20; thick solid line (rhombuses), SP/HC24. The data for the Langmuir film of pure SP are also included: thin solid line (solid triangles). The lines are to guide the eye. Table 1. Melting Point of n-Alkanes in the Bulk, Melting Point of n-Alkane Mixed with SP in the Bulk, and the Onset Temperature of Light-Induced J-Aggregation in Mixed Langmuir Films
Figure 3. Changes in UV-vis reflection spectra of the Langmuir films of SP/n-alkane at 20 mN m-1 with UV irradiation: (A) SP/HC19 at 20 °C for the irradiation times from 0 to 10 min; (B) SP/DC19 at 30 °C for the irradiation times from 0 to 120 min.
changes in reflection spectrum of the Langmuir film of SP/DC19 above the melting point of DC19/SP. It is evident that J-aggregate of MC forms with UV irradiation. The reflection band is positioned at ca. 618 nm with a line width of 25 nm. These values are almost the same as those of the J-band of the Langmuir film of pure SP,6f indicating that the electronic interaction between the chromophores in the J-aggregate remains unchanged on adding n-alkane. These results suggest that the added n-alkane has a large effect on the photoreaction of SP in the Langmuir film below the melting point of the n-alkane/ SP whereas the photoreaction of SP is not influenced significantly above the melting point of the n-alkane/SP. The photoreactions of the Langmuir films of SP/n-alkane were studied with variations in the n-alkane species and the subphase temperature using UV-vis reflection spectroscopy. Figure 4 shows the increase in reflection of the Langmuir film of SP/n-alkane at 618 nm due to lightinduced J-aggregation in the photostationary state. The data of the Langmuir film of pure SP are also included. It is evident that when n-alkane is longer, in other words, when the melting points of n-alkane and of n-alkane/SP are higher, the starting temperature of J-aggregation of
Langmuir film
mp of n-alkane/°C
mp of n-alkane mixed with SP/°C
SP/HC18 SP/DC19 SP/HC19 SP/HC20 SP/HC24 pure SP
28a NDb 32a 37a 51a NAd
27-31 27-31 29-33 31-34 41-47 NAd
onset temp of J-aggregation/°C 23-25 ca. 25 25-26 30-33 noc 18-23
a Reference 10. b Not determined. c No J-aggregation occurs in the film below 40 °C. d Not applicable.
MC is higher. When SP is mixed with HC24, no lightinduced J-aggregation proceeds below 40 °C. The above results are summarized in Table 1 with the melting points of n-alkane and n-alkane/SP in the bulk. The starting temperature of J-aggregation increases with an increase in melting point of n-alkane/SP and melting point of n-alkane. Considering that the starting temperature of light-induced J-aggregation in the Langmuir film of pure SP is lower than the melting point of any n-alkane/ SP used in this study, light-induced J-aggregation is hindered by the presence of n-alkane below the melting point of n-alkane/SP. This strongly suggests that phase transition of n-alkane has a large effect on the photoreaction of Langmuir films. More specifically, when n-alkane is in solid phase, n-alkane should be packed with the alkyl chains of SP as in the aforementioned structural model. SP can be photoisomerized to MC. However, light-induced J-aggregation of MC is hindered because the structure of the Langmuir film is not appropriate for the light-induced J-aggregation. On the other hand, when n-alkane is melted, the interaction between n-alkane and the alkyl chains of SP should be smaller than that in the solid state. SP can be photoisomerized to MC, followed by the spontaneous formation of the J-aggregates of MC because J-aggregation proceeds in the Langmuir film of pure SP at the same surface pressures and temperatures.
Photoreaction of Dye/Alkane Langmuir and LB Films
Figure 5. IR external reflection spectra of the Langmuir film of SP/DC24 at 20 mN m-1 at 23 °C ((A) CH stretching region and (B) CD stretching region) and the Langmuir film of SP/ DC19 at 20 mN m-1 at 30 °C ((C) CH stretching region and (D) CD stretching region): solid lines, before UV irradiation; dotted lines, after UV irradiation for 120 min. Incident angle of IR beam was 45°.
Structural Change in the Langmuir Film of SP/ n-Alkane with Photoreaction. IR spectroscopy has been used for structural analyses of organic ultrathin films.11 In particular, IR external reflection spectroscopy has proved to be a powerful tool for the structural analyses of Langmuir films at the air-water interface.8i,12 We used this technique to monitor structural changes accompanied by photoreactions of the Langmuir films of SP/n-alkane. For structural analyses of mixed films, it is helpful to use deuterated chemical species as one of the components. Because the CH2 stretching vibrations and the CD2 stretching vibrations are positioned at different wavenumbers, we can obtain the information on the orientation of two components from the same spectra.11c,d We used deuterated n-alkanes for the IR external reflection spectroscopy. Parts A and B of Figure 5 show the IR external reflection spectra of the Langmuir film of SP/DC24 at 23 °C (below the melting point of DC24/SP). The bands at 2920 and 2851 cm-1 are assigned to CH2 asymmetric and CH2 symmetric stretching vibrations of the alkyl chains of SP, respectively. The bands at 2193 and 2088 cm-1 are assigned to CD2 asymmetric and CD2 symmetric stretching vibrations of the alkyl chain of DC24, respectively.11d It (11) (a) Rabolt, J. F.; Burns, F. C.; Schlotter, N. E.; Swalen, J. D. J. Chem. Phys. 1983, 78, 946-952. (b) Kimura, F.; Umemura, J.; Takenaka, T. Langmuir 1986, 2, 96-101. (c) Shimomura, M.; Song, K.; Rabolt, J. F. Langmuir 1992, 8, 887-893. (d) Azumi, R.; Matsumoto, M.; Kuroda, S.; Crossley, M. J. Langmuir 1995, 11, 4495-4498. (e) Kawai, T. Bull. Chem. Soc. Jpn. 1997, 70, 771-775. (f) Matsumoto, M.; Tanaka, K.; Azumi, R.; Kondo, Y.; Yoshino, N. Langmuir 2003, 19, 2802-2807. (12) (a) Mitchell, M. L.; Dluhy, R. A. J. Am. Chem. Soc. 1988, 110, 712-718. (b) Sakai, H.; Umemura, J. Bull. Chem. Soc. Jpn. 1997, 70, 1027-1032. (c) Ren, Y.; Hossain, Md. M.; Iimura, K.; Kato, T. J. Phys. Chem. B 2001, 105, 7723-7729.
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is evident that the band intensities change by the UV irradiation, suggesting that the orientation of the alkyl chains of SP and DC24 change during photoisomerization of SP to MC without J-aggregation. Parts C and D of Figure 5 show IR external reflection spectra of the Langmuir film of SP/DC19 at 30 °C (above the melting point of DC19/SP). A striking feature is found in the region of CD2 stretching vibrations. The bands of CD2 stretching vibrations disappear by the UV irradiation. Under these conditions, DC19 is melted in the film, where the interaction between DC19 and the alkyl chains of SP should be smaller than that in the solid state. We consider that n-alkane is eliminated from the Langmuir film through the dispersion into the subphase and/or evaporation, because the local temperature of the Langmuir film should be higher than the subphase temperature during the UV irradiation. BAM has been used to investigate the morphology of the Langmuir films.13 In particular, we have studied the structural changes of the Langmuir film of pure SP during compression and UV irradiation.6f,h At low subphase temperatures, pure SP forms continuous multilayers at the air-water interface. When the film is irradiated with UV light, SP isomerizes to MC and the reflection of the whole part of the film increases with the isomerization of SP to MC without any significant morphological changes. On the other hand, SP forms continuous monolayers before the phase transition during compression at high subphase temperatures. Further compression gives rise to a phase transition in which some part of the monolayer region is converted into trilayer circular domains. J-aggregate forms around the nucleation site on the trilayer domains with UV irradiation, resulting in large morphological changes of the films. We examine the effect of the phase of n-alkane/SP on the morphology of the Langmuir film. Figure 6A shows BAM images of the Langmuir film of SP/HC18 at 10 mN m-1 at 30 °C, which is around the melting point of HC18/ SP. Under these conditions, SP forms trilayer domains as in the case of the Langmuir film of pure SP. Bright regions grow on the circular domains with UV irradiation during the light-induced J-aggregation (see Figure 6B) as in the case of the Langmuir film of pure SP.6f We also obtained BAM images of the Langmuir film of SP/HC24 at 10 mN m-1 at 23 °C, which is lower than the melting point of HC24/SP. Under these conditions, SP formed a continuous film without any trilayer domains. The reflection of the whole part of the film increased when SP isomerized to MC on UV irradiation as in the case in which only the isomerization of SP to MC occurs in the Langmuir film of pure SP.6f,h It is to be noted that the Langmuir film of pure SP at the same surface pressure and temperature contains trilayer circular domains with nucleation sites, around which MC molecules are gathered to form a J-aggregate with UV irradiation. Above results are consistent with the results of the π-A isotherms, UV-vis reflection spectra, and the IR external reflection spectra. When the subphase temperature is higher than the melting point of n-alkane/SP, the interaction between n-alkane and the alkyl chains of the dye should be weak. Light-induced J-aggregation proceeds as in the Langmuir film of pure SP. Morphology of the (13) (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 7. Change in absorption spectrum of the LB film of SP/DC19 transferred at 20 mN m-1 at 30 °C, followed by UV illumination at 23 °C for the irradiation times of 0-120 min.
Figure 6. Change in the BAM image of the Langmuir film of SP/HC18 at 10 mN m-1 at 30 °C with UV illumination for (A) before UV irradiation and (B) after UV irradiation for 10 min.
Langmuir films before and after the J-aggregation is also similar to that of the Langmuir films of pure SP. On the other hand, when the temperature is lower than the melting point of n-alkane/SP, light-induced J-aggregation is inhibited in the Langmuir films of SP/n-alkane at the temperatures and surface pressures at which J-aggregation proceeds in the Langmuir films of pure SP. This can be explained by the structural model in which n-alkane is packed with the alkyl chains of SP. The interaction between n-alkane and the alkyl chains of SP should be sufficiently strong to hinder the formation of trilayer circular domains and nucleation sites that are necessary for the light-induced J-aggregation under isothermal conditions. Photoreaction of the LB Films of SP/n-Alkane. We investigate the photoreactions of the LB films of SP/nalkane to reveal the effect of n-alkane on the light-induced J-aggregation. Figure 7 shows the change in absorption spectrum of the LB film of SP/DC19 transferred at 10 mN m-1 at 30 °C on UV irradiation at room temperature. The J-band is located at ca. 618 nm with a line width of ca. 12 nm. These values coincide with those for the J-aggregates formed in the LB films of pure SP.6f The line width of the J-band in the LB film is smaller than that in the Langmuir films while the peak position is almost the same. Similar phenomena were reported for the Langmuir films of a different merocyanine (spiropyran) derivative.8f If we assume that there are different types of J-aggregates in the mixed Langmuir and LB films in terms of excitonic delocalization, the distribution in the LB films should be much narrower than that in the Langmuir films. On the other hand, only the photoisomerization of SP to MC proceeded by UV irradiation at room temperature of the LB film of SP/HC24 transferred at 10 mN m-1 at 30 °C. This shows that the light-induced J-aggregation of MC is
Figure 8. IR transmission spectra of a single-layer LB film of SP/DC24 transferred at 20 mN m-1 at 23 °C ((A) CH stretching region and (B) CD stretching region) and those of a single-layer LB film of SP/DC19 transferred at 20 mN m-1 at 30 °C ((C) CH stretching region and (D) CD stretching region): solid lines, before UV irradiation; dotted lines, after UV irradiation for 120 min.
hindered by the presence of HC24 as in the Langmuir film of SP/HC24, suggesting that the structure of the film remains essentially unchanged during the transfer onto solid substrates. Structural Changes of the LB Films of SP/nAlkane by UV Irradiation. We investigate structural changes of the LB films of SP/n-alkane on UV irradiation using IR spectroscopy. Parts A and B of Figure 8 show the change in IR transmission spectrum of the LB film of SP/ DC24 with UV irradiation. The LB film was fabricated at 20 mN m-1 at 23 °C. No significant change is recognized
Photoreaction of Dye/Alkane Langmuir and LB Films
in the absorption bands of symmetric and asymmetric CH2 stretching vibrations of the hydrocarbon chain of SP (MC) and in those of symmetric and asymmetric CD2 stretching vibrations of DC24 by the UV irradiation. This indicates that the structure of the hydrophobic region of the LB film is not affected significantly by the UV irradiation when SP photoisomerizes to MC without J-aggregation below the melting point of n-alkane/SP. This is consistent with the structural model where n-alkane molecules are incorporated in the hydrophobic part of SP monolayer and packed with the alkyl chains of SP.9a Parts C and D of Figure 8 show the change in IR transmission spectrum of the LB film of SP/DC19 fabricated at 20 mN m-1 at 30 °C with UV irradiation. A striking feature is that the absorption bands of CD2 stretching vibrations are hardly seen in the spectrum of the LB film even before UV irradiation. This indicates that n-alkane is not incorporated in the LB film when the temperature is much higher than the melting point of n-alkane/SP. This is consistent with the fact that the UV-vis absorption spectrum of the J-aggregates of MC in the LB film fabricated from the mixed Langmuir films above the melting point of n-alkane/SP is the same with that of the J-aggregates formed by the UV irradiation of the LB film of pure SP. No significant change is seen in the IR absorption bands of symmetric and asymmetric CH2 stretching vibrations of the hydrocarbon chains in SP (MC) by the UV irradiation. When the LB film of SP/DC19 was fabricated at 20 mN m-1 at 23 °C, intermediate features were observed. DC19 molecules present in the as-deposited LB film disappeared with the UV irradiation. The intensities of the absorption bands of symmetric and asymmetric CH2 stretching vibrations decreased with the UV irradiation, indicating the orientational change of the alkyl chains of SP (MC) caused by the evaporation of DC19. Under these conditions, a small amount of J-aggregate was formed. We investigate the structural change of the LB film with UV irradiation using AFM. Figure 9 shows the AFM images of the LB film of SP/HC18 fabricated at 20 mN m-1 at 30 °C. SP forms trilayer domains, each with a nucleation site, before UV irradiation. The LB film consists of trilayer domains that merged with each other whereas the structure of the Langmuir film is characterized as trilayer circular domains embedded in a monolayer. The monolayer region that exists between the trilayer circular domains in the Langmuir film should fail to be transferred during deposition. The J-aggregate of MC forms around the nucleation sites on UV irradiation through diffusion of MC molecules as in the case of the LB films of pure SP.6f This is consistent with the finding that n-alkane is not incorporated in this LB film. Not all the domain regions are converted to J-aggregates probably because diffusion of MC molecules is necessary for J-aggregation. This may correspond to the spectroscopic results shown in Figure 3B in that MC monomers are present after UV irradiation though quantitative comparison between the spectroscopic and AFM results is difficult. The AFM images of the LB film of SP/HC18 fabricated at 20 mN m-1 at 23 °C revealed that SP molecules were transferred on the solid substrates as patches of undefined shape with sizes significantly smaller than the trilayer domains in Figure 9A. This is consistent with the fact that the fabrication of the LB film of pure SP is difficult under these conditions. UV irradiation gave rise to a small morphological change of the LB film which should be caused by J-aggregation. This agrees with the formation of a small amount of J-aggregates. On the other hand, the LB film of SP/HC24 fabricated at 20 mN m-1 at 23 °C (below the melting point of HC24/
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Figure 9. AFM images of the LB film of SP/HC18 transferred at 20 mN m-1 at 30 °C: (A) before UV irradiation; (B) after UV irradiation for 120 min.
SP) forms a continuous film. Considering that the transfer of the Langmuir film of pure SP onto solid substrates is difficult at the same surface pressure and temperature, the addition of HC24 facilitates the fabrication of the LB film, suggesting a large structural change of the Langmuir film due to the addition of HC24. No significant structural change of the film with the UV irradiation was observed. This is consistent with the BAM observations, suggesting that the film structure was essentially preserved during the transfer onto solid substrates. These results suggest that the strong interaction between n-alkane and the alkyl chains of SP (MC) inhibits the J-aggregation of MC by hindering the formation of trilayer circular domains and the nucleation sites, both of which are indispensable for the light-induced J-aggregation under isothermal conditions. Structural Model of the Langmuir Films of SP/ n-Alkane before and after UV Irradiation. We propose a structural model of the mixed Langmuir films of SP/ n-alkane in Figure 10 on the basis of the above results. The film structure depends strongly on whether the film fabrication temperature is above or below the melting point of n-alkane/SP.
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Figure 10. Structural model of the Langmuir films of SP/nalkane below and above the melting point of n-alkane/SP before and after UV irradiation.
Below the melting point of n-alkane/SP, the isotherm is of condensed type without a phase transition. The onset of surface pressure occurs at larger molecular area than that of pure SP. The addition of n-alkane gives rise to a significant structural change of the Langmuir film, suppressing the phase transition. BAM observations show that the Langmuir film is a continuous film without trilayer circular domains that are evident in the Langmuir films of pure SP. These results strongly suggest that the Langmuir film is a continuous monolayer where n-alkane is closely packed with the alkyl chains of SP. In other words, n-alkane fills the vacant space in the hydrophobic region of SP monolayer as proposed for other amphiphilic dyes,1a,7c thereby stabilizing relatively the monolayer structure compared with the trilayer circular domains. SP isomerizes to MC on UV irradiation without the formation of the J-aggregate of MC. No significant structural change occurs with the isomerization. n-Alkane is an essential constituent of the films. The strong interaction between n-alkane and the alkyl chains of SP prohibits the formation of trilayer circular domains and the nucleation sites, both of which are indispensable for the light-induced J-aggregation under the isothermal conditions. This structural model of the mixed Langmuir films is consistent with the one proposed by Ando et al.9a and is applicable also to the LB film. When the subphase temperature is sufficiently higher than the melting point of n-alkane/SP, mixed films of SP/
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n-alkane show behaviors similar to those of the films of pure SP. In the mixed Langmuir film, n-alkane is melted, giving rise to the smaller interaction between n-alkane and the alkyl chains of SP than in the films below the melting point of n-alkane/SP. The π-A isotherms are almost the same with those of pure SP. Continuous monolayers are formed after spreading. Compression induces a phase transition, which is characterized as a transition from a monolayer to trilayer circular domains.6h n-Alkane disappears from the Langmuir film on UV irradiation, leading to the formation of J-aggregate of MC. J-aggregate forms around the nucleation sites on the trilayer domains through diffusion of MC, resulting in a large morphological change of the film. The structure of the LB film is almost the same as that of the LB film of pure SP, with trilayer domains and nucleation sites, and is similar to that of the Langmuir film of SP/n-alkane except for the absence of the monolayer region. Jaggregation also occurs in the LB films in a similar manner. This is consistent with the fact that n-alkane molecules are not incorporated in the LB film. These results suggest that n-alkane is located in the monolayer region of the Langmuir film, possibly on top of the monolayer region, failing to be transferred onto solid substrates. n-Alkane is not an essential constituent of the films without any significant contributions to the structures of the films or influences on the photoreactions of the films. Conclusion This study indicates that the photoreaction of SP in the Langmuir film and the LB film can be controlled by mixing with n-alkane. The occurrence of light-induced J-aggregation depends strongly on whether the temperature of the film fabrication is above or below the melting point of n-alkane/SP. n-Alkane serves as a kind of inhibitor that is effective only below the melting point of n-alkane/SP. This shows that the photoreaction of the film can be controlled using the phase transition of n-alkane. The temperature at which the photoreaction of the film is switched depends on n-alkane. This indicates that we can control the switching temperature by the selection of n-alkane. Further the methodology used in the study can be applied to other systems in which the functions of the materials are switched by external stimuli: the switching temperature can be easily changed by using the phase transition of the added species. Acknowledgment. This work was partly supported by a Grant-in-Aid for Science Research (No. 16655060) from the Ministry of Education, Culture, Sports, Science and Technology of Japan. LA0482857
