Conversion of the Aggregation State of Merocyanine Dye, Modification

Feb 21, 2008 - Yoshiaki Hirano , Asuka Yamazaki , Ari Maio , Yasutaka Kitahama and Yukihiro Ozaki. The Journal of Physical Chemistry B 2010 114 (33), ...
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Langmuir 2008, 24, 3317-3324

3317

Conversion of the Aggregation State of Merocyanine Dye, Modification of the Subcell Packing of Arachidic Acid, and Removal of the Majority of n-Octadecane by Hydrothermal Treatment in the Liquid Phase in a Mixed Langmuir-Blodgett Film of the Ternary System Yoshiaki Hirano, Ari Maio, and Yukihiro Ozaki* Department of Chemistry, School of Science and Technology, Kwansei Gakuin UniVersity, 2-1 Gakuen, Sanda 669-1337, Japan ReceiVed September 25, 2007. In Final Form: January 7, 2008 We have investigated the influence of heat treatment in an air atmosphere (HT) and hydrothermal treatment in the liquid phase (HTTL) on the H-aggregate in a mixed Langmuir-Blodgett (LB) film of merocyanine dye with an octadecyl group (MS18)-arachidic acid (C20)-n-octadecane (AL18) ternary system by means of polarized visible and IR absorption spectroscopy. HT causes the variation from the H-aggregate to the monomer, the increment in the number of gauche conformers in the MS18 hydrocarbon chain, the slight orientation change in the C20 hydrocarbon chain, and the complete evaporation of AL18. The dissociation of MS18 is probably ascribed to the complete evaporation of AL18 from the mixed LB film and the increase in thermal mobility of the long axis of the MS18 hydrocarbon chain during HT. However, HTTL can easily and rapidly induce the conversion of the MS18 aggregation state from H- to J-aggregates, the modification of the C20 subcell packing from hexagonal to orthorhombic, and the removal of most of the AL18 molecules. The conversion of the MS18 aggregation state can be interpreted to consist of two processes from the H-aggregate to the monomer and from the monomer to the J-aggregate. In the initial stage of HTTL, the MS18 aggregation state changes from the H-aggregate to the monomer, which is caused by the removal of almost all of the AL18 molecules from the mixed LB film to warm water via the thermal energy of warm water. Then, the large relative permittivity of warm water is expected to relate strongly to the subsequent variation from the monomer to the J-aggregate. This transformation results in the decrease in the total value of the electrostatic energy based on the MS18 permanent dipole interaction. Moreover, the modification of the C20 subcell packing is possibly due to the hydrophobic effect, where the C20 hydrocarbon chains cohere again in the warm water during HTTL. Consequently, it has been found that HTTL is quite effective to reorganize the chromophore alignment of MS18, to modify the subcell packing of C20 and to erase the majority of AL18 molecules in the mixed LB film of the MS18-C20-AL18 ternary system in a short time.

1. Introduction In recent years, J- and H-aggregates, which possess headto-tail and side-by-side alignments of transition dipole moments of organic synthetic dyes in ultrathin films, have attracted much attention because of their potential applications in the realization of photovoltaic cells, optical waveguides, and ultrafast optical switches with colorful, flexible, and lightweight features.1-4 Indeed, it is well recognized that one of key issues in their higher performance is the precise control of the dye aggregation state in the ultrathin films in a desired manner1-4 because its change has a direct influence on the device properties of photoelectric conversions, complex refractive indexes, and nonlinear susceptibilities of third order.4 * To whom correspondence [email protected].

should

be

addressed.

E-mail:

(1) (a) Kuhn, H. Thin Solid Films 1989, 178, 1. (b) Ulman, A. An Introduction to Ultrathin Organic Films; Academic Press: San Diego, CA, 1991. (c) Law, K. Y. Chem. ReV. 1993, 93, 449. (d) Kuroda, S., Moebius, D., Miller, R., Eds. Organized Monolayers and Assemblies: Structure, Processes, and Function; Studies in Interface Science; Elsevier: Amsterdam, 2002; Vol. 16, Chapter 6. (e) Ozaki, Y.; Morita, S.; Hirano, Y.; Li, X. AdVanced Chemistry of Monolayers at Interfaces; Imae, T., Ed.; Interface Science and Technology; Elsevier: Amsterdam, 2007; Vol. 14, Chapter 12. (2) (a) Jelly, E. E. Nature (London) 1936, 138, 1009. (b) Scheibe, G. Angew. Chem. 1936, 49, 563. (c) Kobayashi, T., Ed. J-Aggregates; World Scientific: Singapore, 1996. (3) (a) Law, K. Y. J. Phys. Chem. 1988, 92, 4226. (b) Law, K. Y.; Chen, C. C. J. Phys. Chem. 1989, 93, 2553. (c) Chen, H.; Herkstroeter, W. G.; Perlstein, J.; Law, K. Y.; Whitten, D. G. J. Phys. Chem. 1994, 98, 5138. (d) Kim, Y. S.; Liang, K. L.; Law, K. Y.; Whitten, D. G. J. Phys. Chem. 1994, 98, 984. (e) Liang, K.; Law, K. Y.; Whitten, D. G. J. Phys. Chem. 1994, 98, 13379. (4) (a) Saito, K. J. Phys. Chem. B 1999, 103, 6579. (b) Saito, K. J. Phys. Chem. B 2001, 105, 4235. (c) Wakamatsu, T.; Watanabe, K.; Saito, K. Appl. Opt. 2005, 44, 906. (d) Zhou, H. S.; Watanabe, T.; Mito, A.; Honma, I.; Asai, K.; Ishigure, K.; Furuki, M. Mater. Sci. Eng. B 2002, 95, 180. (e) Zhou, H. S.; Watanabe, T.; Mito, A.; Honma, I.; Asai, K.; Ishigure, K. Mater. Lett. 2002, 57, 589.

Figure 1. Chemical structure of the merocyanine dye (MSn) with an alkyl chain. The intramolecular charge transfer is schematically represented.

We have been engaged in research on the control of dye aggregation states by using mixed Langmuir-Blodgett (LB) films with merocyanine dye having an octadecyl group (n ) 18, MS18, 3-carboxymethyl-5-[2-(3-octadecylbenzothiazolin-2-ylidene)ethylidene]rhodanine; see Figure 1). In a mixed LB film composed of an MS18-arachidic acid (C20) binary system, a sharp, redshifted J-band with an absorption maximum at 590 nm is formed, which is markedly shifted to a lower-energy side from the MS18 monomer peak near 530 nm when it is prepared under an aqueous subphase containing a cadmium (Cd2+) ion.5-20 In addition, a (5) (a) Sugi, M.; Fukui, T.; Iizima, S.; Iriyama, K. Mol. Cryst. Liq. Cryst. 1980, 62, 165. (b) Nakahara, H.; Fukuda, K.; Moebius, D.; Kuhn, H. J. Phys. Chem. 1986, 90, 614. (c) Ozaki, Y.; Iriyama, K.; Iwasaki, T.; Hamaguchi, H. Appl. Surf. Sci. 1988, 33-34, 1317. (d) Kuroda, S.; Ikegami, K.; Tabe, Y.; Saito, K.; Saito, M.; Sugi, M. Phys. ReV. B 1991, 43, 2531. (e) Kato, N.; Saito, K.; Aida, H.; Uesu, Y. Chem. Phys. Lett. 1999, 312, 115. (f) Kato, N.; Saito, K.; Serata, T.; Aida, H.; Uesu, Y. J. Chem. Phys. 2001, 115, 1473. (g) Kato, N.; Yuasa, K.; Araki, T.; Hirosawa, I.; Sato, M.; Ikeda, N.; Iimura, K.; Uesu, Y. Phys. ReV. Lett. 2005, 94, 136404. (h) Ikegami, K. Colloids Surf., A 2006, 284-285, 212. (6) Minari, N.; Ikegami, K.; Kuroda, S.; Saito, K.; Saito, M.; Sugi, M. J. Phys. Soc. Jpn. 1989, 58, 222. (7) Kato, N.; Yamamoto, M.; Itoh, K.; Uesu, Y. J. Phys. Chem. B 2003, 107, 11917.

10.1021/la7037944 CCC: $40.75 © 2008 American Chemical Society Published on Web 02/21/2008

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blue-shifted band appears at 505 nm by adding a small amount of n-octadecane (AL18) to the above MS18-C20 binary system.13-20 We assigned the 505 nm band to the H-band on the basis of the shift to the higher-energy side, sharpness, and in-plane anisotropy of the band.14-20 Besides the method of AL18 addition, it has been so far reported that the MS18 aggregation state in the mixed LB film can also be controlled by heat treatment in air, acid and base treatments in both gas and liquid phases, and hydrothermal treatment in the gas phase.21,10-12 Among them, no discussion of the correlation between the aggregation state and the intramolecular charge transfer of MS18 and also about the C20 subcell packing has been given before and after the above secondary treatments, except for our previous study on HT of the MS18 J-aggregate.12 Furthermore, the application of these secondary treatments is actually restricted to the MS18 J-aggregate or its monomer.10-12 Therefere, no attempt to apply heat treatment in air (HT) and hydrothermal treatment in the liquid phase (HTTL) to the MS18 H-aggregate has been reported to our knowledge. In particular, HTTL reminds us of the possibility that drastic structural changes may be immediately induced by the contact between warm water with both thermal energy and a large relative permittivity and the mixed LB film of the MS18-C20-AL18 ternary system because we simultaneously and directly give the above two perturbations to some interactions that act on the ternary system. In this article, we report the influence of HT and HTTL on the H-aggregate in a mixed LB film of the MS18-C20-AL18 ternary system studied by polarized visible and IR absorption spectroscopy. The present study provides new insight into the structural transformations induced by HT and HTTL. 2. Experimental Section 2.1. Chemicals and Sample Preparation Procedures. Merocyanine dye with an octadecyl group (MS18), arachidic acid (C20), and n-octadecane (AL18) were purchased from Hayashibara Bio(8) (a) Ikegami, K.; Mingotaud, C.; Lan, M. J. Phys. Chem. B 1999, 103, 11261. (b) Ikegami, K.; Mingotaud, C.; Lan, M. Thin Solid Films 2001, 393, 193. (c) Ikegami, K. J. Chem. Phys. 2004, 121, 2337. (9) Ikegami, K.; Kuroda, S. Chem. Phys. 2003, 295, 205. (10) (a) Sugi, M.; Saito, M.; Fukui, T.; Iizima, S. Thin Solid Films 1983, 99, 17. (b) Sugi, M.; Saito, M.; Fukui, T.; Iizima, S. Thin Solid Films 1985, 129, 15. (11) (a) Mouri, S.; Morita, S.; Miura, Y. F.; Sugi, M. Jpn. J. Appl. Phys. 2006, 45, 7925. (b) Mouri, S.; Moshino, H.; Hasegawa, S.; Miura, Y. F.; Sugi, M. Jpn. J. Appl. Phys. 2007, 46, 1650. (12) Hirano, Y.; Tateno, S.; Ozaki, Y. Langmuir 2007, 23, 7003. (13) (a) Nakahara, H.; Moebius, D. J. Colloid Interface Sci. 1986, 114, 363. (b) Ray, K.; Nakahara, H.; Sakamoto, A.; Tasumi, M. Chem. Phys. Lett. 2001, 342, 58. (c) Murata, M.; Mori, K.; Sakamoto, A.; Villeneuve, M.; Nakahara, H. Chem. Phys. Lett. 2005, 405, 32. (d) Murata, M.; Villeneuve, M.; Nakahara, H. Chem. Phys. Lett. 2005, 405, 416. (14) Hirano, Y.; Sano, H.; Shimada, J.; Chiba, H.; Kawata, J.; Miura, Y. F.; Sugi, M.; Ishii, T. Mol. Cryst. Liq. Cryst. 1997, 294, 161. (15) Hirano, Y.; Kamata, K. N.; Inadzuki, Y. S.; Kawata, J.; Miura, Y. F.; Sugi, M.; Ishii, T. Jpn. J. Appl. Phys. 1999, 38, 6024. (16) Hirano, Y.; Okada, T. M.; Miura, Y. F.; Sugi, M.; Ishii, T. J. Appl. Phys. 2000, 88, 5194. (17) Morita, S.; Miura, Y. F.; Sugi, M.; Hirano, Y. J. Appl. Phys. 2003, 94, 4368. (18) Hirano, Y.; Murakami, T. N.; Nakamura, Y. K.; Fukushima, Y.; Tokuoka, Y.; Kawashima, N. J. Appl. Phys. 2004, 96, 5528. (19) Hirano, Y.; Ohkubo, M. A.; Tokuoka, Y.; Kawashima, N.; Ozaki, Y. Mol. Cryst. Liq. Cryst. 2006, 445, 383. (20) Hirano, Y.; Tokuoka, Y.; Kawashima, N.; Ozaki, Y. Vib. Spectrosc. 2007, 43, 86. (21) As far as we know, the first application of hydrothermal treatment in the gas phase (HHTG) to the LB film of the organic synthetic dye has been reported by Liang et al.3e They used HHTG for the J- and H-aggregates in the LB film of pure squaraine dyes and named HHTG the steam treatment in their paper. According to their results, the H-aggregate is unchanged with HTTG at 65 °C for 2 h, whereas the J-aggregate changes into the H-aggregate under the same condition. However, Mouri et al.11a independently found that the J-aggregate at 590 nm becomes a more developed J-aggregate near 595 nm in the mixed LB film of the merocyanine dye (MS18)-arachidic acid (C20) binary system by HTTG at 30-90 °C for 1 h. They have also investigated the changes in MS18 J-aggregates against the time of HTTG up to 1 h at 60 °C.

Hirano et al. chemical Laboratories, Inc. (Okayama, Japan), Fluka Chemie AG (Buchs, Switzerland), and Kanto Chemical Co., Inc. (Tokyo, Japan), respectively. Deuterated arachidic acid (CD3(CD2)18COOH, D 98%, C20-d) and deuterated n-octadecane (CD3(CD2)16CD3, D 98%, AL18d) were from C/D/N isotopes, Inc. (Quebec, Canada). These were used without further purification. A spreading solution of the MS18C20-AL18 ternary system ([MS18]/[C20 or C20-d]/[AL18 or AL18-d] ) 1:2:1) was prepared using freshly distilled chloroform from Dojindo Laboratories (Kumamoto, Japan). The MS18 concentration in the ternary system was on the order of 10-4 M. An LB trough of the USI system (Fukuoka, Japan) was used. The preparations of the aqueous subphase containing cadmium chloride (CdCl2) and sodium hydrogen carbonate (NaHCO3) were the same as those reported previously.12,14-20 After the chloroform solution was spread on an air-water interface, the monolayers were directly transferred to both sides of 13 × 38 mm2 mica and CaF2 substrates by the standard vertical dipping method at a surface pressure of 25 mN/m. The raising velocity of the mica substrate was 2.5 mm/min for the deposition of the monolayer, whereas the raising and dipping velocities of the CaF2 substrate were 2.5 mm/min for up to 3 × 2 layers and then 25 mm/min afterward. The LB films deposited on the mica and CaF2 substrates were 1 × 2 and 15 × 2 layers, respectively. All of the LB films were of Y-type with a transfer ratio of approximately unity. Three samples of the mixed LB film of MS18-C20 (or C20-d)AL18 (or AL18-d) ternary systems on the CaF2 substrate were prepared to investigate the influence of secondary treatments on individual structures by means of visible and IR absorption spectroscopy. Before the secondary treatments, it was verified that the MS18 aggregation states in as-deposited LB films for three samples are the same, referring to the results of their visible absorption spectra. The secondary treatments were carried out immediately after the sample preparation and its verification to avoid the aging effect of the MS18 LB films. 2.2. Secondary Treatment. The heat treatment in air (HT) of the mixed LB film was executed at 70 °C for 60 min. Then, the LB film for the hydrothermal treatment in the liquid phase (HTTL) was directly immersed in warm water at 70 °C for 2 min. Ultrapure water with a resistivity of greater than 1.82 × 107 Ω cm at 20 °C as determined by an Advantec ultrapure water system was used for HTTL. 2.3. Spectroscopic Measurements. Visible transmission absorption spectra were measured using a Shimadzu UV-3101PC UV-vis spectrometer. IR transmission absorption spectra were measured at 4 cm-1 resolution using a Nicolet Magna 560 FTIR spectrometer equipped with an MCT detector. To generate a high signal-to-noise ratio, 256 interferograms were coadded. Before and after the secondary treatments, A| and A⊥ of visible and IR absorption spectra of the LB film were measured using linearly polarized light with the electric vector parallel and perpendicular to the raising and dipping directions of the CaF2 substrate, respectively. 2.4. AFM and DSC Measurements. Measurements of atomic force microscopy (AFM) and differential scanning calorimetry (DSC) were carried out by using a Shimadzu SPM-9500 and a PerkinElmer Pyris6 DSC system, respectively. AFM images of the monolayer on a mica substrate were measured in contact mode at a constant force to check the phase separation of MS18 and C20 in the monolayer of the MS18-C20-AL18 ternary system before the secondary treatments. The silicon nitride cantilever with a spring constant of 0.15 mN/m was used. DSC of only AL18 powder was measured at a speed of 5 °C/min because we reported the melting points of the powder states of MS18 and C20 and the mixed LB films of pure C20 and MS18-C20 binary systems chelated by the Cd2+ ion on a polyethylene terephthalate (PET) film in our previous paper.12 The melting point of AL18 with the endothermic peak was confirmed to be 28.5 °C.

3. Results and Discussion 3.1. MS18 Aggregation States Before and After Secondary Treatment. Figure 2 shows polarized visible absorption spectra

Mixed Langmuir-Blodgett Film

Figure 2. Polarized visible absorption spectra of the mixed LB film of the merocyanine dye with an octadecyl group (MS18)-deuterated arachidic acid (C20-d)-n-octadecane (AL18) ternary system (a) before treatment, (b) after heat treatment in air (HT), and (c) after hydrothermal treatment in the liquid phase (HTTL). The solid and dotted lines refer to polarized spectra A| and A⊥, respectively.

in a mixed LB film of merocyanine dye with an octadecyl group (MS18)-deuterated arachidic acid (C20-d)-n-octadecane (AL18) ternary system (a) before treatment, (b) after heat treatment in air (HT), and (c) after hydrothermal treatment in the liquid phase (HTTL). The solid and dotted lines denote A| and A⊥, respectively. In Figure 2a, a blue-shifted peak with sharp absorption is observed at 505 nm, which is ascribed to H-aggregation.13-20 The absorption maximum at 505 nm shows the dichroic ratio with R < 1, where R is defined as A|/A⊥. This tendency of R originates from the MS18 side-by-side arrangement in the H-aggregate13-20 and the flow orientation effect.6 In our AFM results, the MS18 and C20 molecules are phase separated from each other (Supporting Information). Then, the size of the H-aggregate is on the order of several hundred nanometers, according to the estimation using the analytical model of flow orientation (Supporting Information). Furthermore, it has been demonstrated by using a simple geometrical model22 that the MS18 transition dipole moments, being parallel to the long axis of the chromophore, lie in the film plane for this H-aggregate.19 In Figure 2b, a broader band with an absorption maximum at 525-530 nm is observable, showing R ) 1 over the visible range. In Figure 2c, a red-shifted peak having a sharper absorption appears at 600 nm, and R almost reaches unity. Thus, the MS18 aggregation states in the ternary system have been found to change drastically by HT and HTTL. 3.2. Variations of the Degree of MS18 Intramolecular Charge Transfer Before and After Secondary Treatments and Assignments of the MS18 Aggregation State. Figure 3 represents polarized IR absorption spectra of the fingerprint region in the mixed LB film of the MS18-C20-d-AL18 ternary systems (a) before treatment, (b) after HT, and (c) after HTTL. The solid and dotted lines refer to A| and A⊥, respectively, and the soliddotted lines correspond to the wavenumber positions of vibrational bands inherent in the MS18 nonaggregation state.12 Assuming the MS4 structure with a butyl group (n ) 4) in Figure 1, the detailed assignments of MS18 IR bands have been already (22) Ikegami, K.; Kuroda, S.; Tabe, Y.; Saito, K.; Sugi, M.; Matsumoto, M.; Nakamura, T.; Kawabata, Y. Jpn. J. Appl. Phys. 1992, 31, 1206.

Langmuir, Vol. 24, No. 7, 2008 3319

Figure 3. Polarized IR absorption spectra in the fingerprint region of the mixed LB film of the MS18-C20-d-AL18 ternary system (a) before treatment, (b) after HT, and (c) after HTTL. The solid and dotted lines correspond to A| and A⊥, respectively.

proposed on the basis of an ab initio calculation with density functional theory (DFT)9 and are listed in Table 1. In Figure 3a, peaks are observed at 1508, 1379, 1317, 1263, 1240, 1217, 1192, and 1147 cm-1. Among them, the wavenumber positions at 1508 and 1192 cm-1 are shifted slightly upward from those in the MS18 nonaggregation state. Then, the positions at 1379, 1317, and 1240 cm-1 are consistent with those of the nonaggregation state within the wavenumber resolution. Besides these results, R < 1 could be observed for the peaks at 1508, 1379, 1317, and 1192 cm-1. However, the R values are slightly larger than that in Figure 2a. This tendency is consistent with that in our previous study20 and may be due to (i) the coupling band involving some vibrational modes except for the central conjugated system and CdS bond in MS18 or (ii) the slight difference in orientation between transition moments in the visible and IR fingerprint regions. In Figure 3b, the IR bands become slightly broader compared to those in Figure 3a, and R also results in unity in the fingerprint region. The shoulder at 1494 cm-1 is well recognized, and the peaks are located at 1381, 1315, and 1186 cm-1. These positions coincide well with those of the nonaggregation state within the resolution. Furthermore, we can also see the diminution of peak heights for the 1508, 1379, 1317, 1240, 1217, 1192, and 1147 cm-1 bands observed in Figure 3a. In Figure 3c, however, sharper peaks appear at 1593, 1489, 1381, 1363, 1313, 1244, 1178, and 1148 cm-1, with R being approximately unity. Among them, the peaks at 1489 and 1178 cm-1 are prominently downward-shifted from those of the nonaggregation state, and the tendency of similar shift is also seen in the 1313 cm-1 band, whereas the upward shift is observed for the 1244 cm-1 band. Another significant piece of information is obtained from the 1800-1600 region. In this region of Figure 3a,c, no peak assigned to the free carboxylic groups of MS18 and C20-d and that due to the free keto group of MS18 is observed.7-9,12,20,23 These results indicate that all of the MS18 and C20-d molecules are completely chelated by the Cd2+ ion. Unlike these results, the peak associated with the MS18 free keto group in Figure 3b is slightly discernible (23) (a) Fujimoto, Y.; Ozaki, Y.; Takayanagi, M.; Nakata, M.; Iriyama, K. J. Chem. Soc., Faraday Trans.1996, 92, 413. (b) Fujimoto, Y.; Ozaki, Y.; Iriyama, K. J. Chem. Soc., Faraday Trans. 1996, 92, 419.

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Hirano et al.

Table 1. Peak Positions (cm-1) and Assignment for MS18 IR Bands in Mixed LB Films Prepared Under a Subphase Containing a Cadmium (Cd2+) Ion non-aggregation statea 1494 1379 1316 1239 1188

H-aggregateb shoulder 1508 1379 shoulder 1317 1263 1240 1217 1192 1147

aggregation state after HTb

aggregation state after HTTLb

shoulder 1494 (shoulder) 1381 shoulder 1315 shoulder 1242d

1593 1489 1381 1363 1313 shoulder 1244

1186 shoulder

1178 1148

assignmentc CRdC2′, C2′-N3′, δ(CH2-alkyl) C2-N3, benzene ring, C3′a-N3′ benzene ring CRdC2′, C-O, CCH (alkyl) C2dS2a, C2-N3, CCH (benzene)

a

Data were taken from ref 12. b Positions of the individual MS18 aggregation states were obtained from Figure 3. c Assigned from the results of an ab initio calculation based on density functionaly theory (DFT), assuming MS4 with a butyl group (n ) 4) and a (π, π, π) conformer in Figure 1 in chloroform solution in ref 9. d This peak height is fairly low.

near 1680 cm-1,7-9,12,23 suggesting the slight dissociation of the Cd2+ ion from the MS18 keto group that is chelated. Moreover, no peak that is assignable to OH-deformation vibrational modes at around 1730 and 1640 cm-1 appears in Figure 3c.24 It is therefore indicated that no hydration by the sorption of liquid water or water vapor occurs with HTTL. We can discuss the degree of MS18 intramolecular charge transfer in each aggregation state based on the results in Figure 3, and we can assign the bands in Figure 2b,c, referring to their degrees as well. Here, intramolecular charge transfer means the delocalization of π electrons in the butadiene group from the rhodanine group to the benzothiazolydine group7-9,12,20,23 and in the CdS bond in the rhodanine group,9,12,20 as shown in Figure 1. In Figure 3a, the sharp peaks with upward shifts at 1508 and 1192 cm-1 are ascribed to the slight decrease in the MS18 intramolecular charge transfer rather than the case of the nonaggregation state. In Figure 3b, it has been suggested that the degree of MS18 intramolecular charge transfer is consistent with that of the nonaggregation state. Therefore, the spectrum in Figure 2b is ascribed to the MS18 monomer on the basis of no shift, broadening, decrease in height ,and in-plane isotropy of the band in visible and fingerprint regions. In Figure 3c, the sharper peaks with remarkable downward shifts at 1489, 1313, and 1178 cm-1, together with the sharpness for the 1593, 1381, and 1148 cm-1 bands, are attributed to the marked increase in the MS18 intramolecular charge transfer. This remarkable increment is characteristic of the J-aggregation in the monolayer and the asdeposited LB film of MS18,7-9,12,20 which has recently been interpreted as the decrease in the total energy in the aggregation by the resonance effect.8,9,12,20 Consequently, the red-shifted band at 600 nm in Figure 2c is assigned to the J-band on the basis of the shift to the lower-energy side, sharpness, increase in height, and outstanding increase in MS18 intramolecular charge transfer. 3.3. Changes in Packing, Conformation, and Orientation of the MS18 Hydrocarbon Chain Before and After Secondary Treatment. Figure 4A depicts polarized IR absorption spectra A| in the 1485-1435 cm-1 region of the mixed LB film of the MS18-C20-d-deuterated n-octadecane (AL18-d) ternary systems (a) before treatment, (b) after HT, and (c) after HTTL. The singlet bands, being assigned to the CH2 in-plane bending mode25,26 of MS18 hydrocarbon chain,9 are observed at 1468 cm-1 in all cases, although the bands possibly couple with the CH2 in-plane bending mode of acetic acid in MS18.9 Therefore, it is implied that no (24) Morita, S.; Tanaka, M.; Ozaki, Y. Langmuir 2007, 23, 3750. (25) Umemura, J.; Kamata, T.; Kawai, T.; Takenaka, T. J. Phys. Chem. 1990, 94, 62. (26) Ren, Y.; Iimura, K.; Kato, T. J. Chem. Phys. 2001, 114, 1949.

Figure 4. Polarized IR absorption spectra A| in the (A) 1485-1435 cm-1 and (B) 3100-2700 cm-1 regions in the mixed LB film of the MS18-C20-d-deuterated n-octadecane (AL18-d) ternary systems (a) before treatment, (b) after HT, and (c) after HTTL.

orthorhombic subcell packing is formed between MS18 hydrocarbon chains. Figure 4B shows the corresponding A| in the 3100-2700 cm-1 region of the MS18-C20-d-AL18-d ternary systems (a) before treatment, (b) after HT and (c) after HTTL. In this region, the values of dichroic ratio R () A|/A⊥) based on linearly polarized light have been determined to be unity for these spectra, suggesting that the long axis of the MS18 hydrocarbon chain is uniformly distributed around the film normal. In Figure 4Ba, peaks at 2954, 2920, and 2850 cm-1 are assigned to the CH3 asymmetric, CH2 antisymmetric, and CH2 symmetric stretching modes of the MS18 hydrocarbon chain, respectively.12,17,20 The positions at 2920 and 2850 cm-1 suggest that the gauche conformation is slightly contained20 because those at 2917 and 2849 cm-1 are well known to be characteristic of the all-trans conformation and the former upward shift is over 2 cm-1, referring to the wavenumber resolution.20 Moreover, we have already determined that the long axis of the MS18 hydrocarbon chain, slightly involving the gauche conformation, is tilted by 38.5° (〈cos2 γ〉 ) 0.613) from the film normal.20 In Figure 4Bb, the peaks appear at 2954, 2924, and 2852 cm-1, and the latter two positions are clearly upward shifted compared to those in Figure 4Ba. The relative ratios of the peak

Mixed Langmuir-Blodgett Film

heights in Figure 4Bb against those in Figure 4Ba are 1.17:1, 0.880:1, and 0.770:1 for the CH3 asymmetric, CH2 antisymmetric, and CH2 symmetric stretching bands, respectively. In addition, the ratios of the full width at half-maximum (fwhm) are 1.32:1 and 1.43:1 for the CH2 antisymmetric and symmetric stretching bands, respectively. Therefore, the results of the peak shifts, increase and decrease in height and increment of the fwhm value suggest that the gauche conformer increases in the MS18 hydrocarbon chain after HT or that not only the increment of the gauche conformer but also the orientation change, where the long axis of the MS18 hydrocarbon chain is slightly tilted to be parallel to the film surface, is induced. For a more detailed discussion, IR reflection absorption spectroscopy (RAS)25 or advanced infrared multiple-angle incidence resolution spectroscopy (IR MAIRS)27 is required because we can accurately investigate the orientation of the long axis of the MS18 hydrocarbon chain. In Figure 4Bc, the peak positions are located at 2953, 2920, and 2850 cm-1. The results are in good agreement with those in Figure 4Ba within the spectral resolution. The ratios of the peak heights in Figure 4Bc against those in Figure 4Ba are 1.03: 1, 1.02:1, and 1.03:1 for the CH3 asymmetric, CH2 antisymmetric, and CH2 symmetric stretching bands, respectively. Then, the fwhm values for two CH2 stretching bands in Figure 4Bc are the same as those in Figure 4Ba. These variations are within 3%. Consequently, it is indicated that no significant change in the orientation and conformation of the MS18 hydrocarbon chain occurs after HTTL. On the basis of the results in Figure 4A,B, it has been found that the structural variation in the MS18 hydrocarbon chain after HT is more remarkable than that after HTTL. 3.4. Variations in Packing, Conformation, Orientation, and Thermal Mobility of the C20-d Hydrocarbon Chain Before and After Secondary Treatment. For the C20-d subcell packing, we have to see the results of the MS18-C20-d-AL18 ternary systems in Figure 3 again. In Figure 3a,b, a single peak assigned to the CD2 in-plane bending mode of the C20-d hydrocarbon chain12 is observed at 1088 cm-1, with a broader shape in Figure 3b. These results suggest the hexagonal packing of C20-d. In Figure 3c, the in-plane bending mode splits into two peaks at 1092 and 1086 cm-1, indicating the orthorhombic subcell packing of C20-d. Figure 5 represents polarized IR absorption spectra A| in the 2235-2025 cm-1 region of the mixed LB film of MS18-C20d-AL18 ternary systems (a) before treatment, (b) after HT, and (c) after HTTL. In Figure 5a, peaks at 2214, 2193, 2156, and 2089 cm-1 are assigned to the CD3 asymmetric, CD2 antisymmetric, CD3 symmetric, and CD2 symmetric stretching bands, respectively.17,18 Then, the peak positions in Figure 5b,c are the same as those in Figure 5a within the resolution. The peaks at 2193 and 2089 cm-1 suggest the all-trans conformation of the C20-d hydrocarbon chain.17,18 To be investigated next is the change in the orientation of the long axis of the C20-d hydrocarbon chain. For the individual spectra, the in-plane isotropy (R ) 1) has been confirmed, suggesting the uniformly distributed orientation of the C20-d hydrocarbon chain around the film normal. In Figure 5a, we have determined that the long axis of the C20-d hydrocarbon chain with the all-trans conformation is parallel to the film normal.17 In Figure 5b, the ratios of the peak heights in Figure 5b against those in Figure 5a are 1.03:1, 0.930:1, 1:1, and 0.930:1 for the CD3 asymmetric, CD2 antisymmetric, CD3 symmetric, and CD2 symmetric stretching bands, respectively. These indicate (27) Hasegawa, T. Anal. Chem. 2007, 79, 4385.

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Figure 5. Polarized IR absorption spectra A| in the 2235-2025 cm-1 region in the mixed LB film of the MS18-C20-d-AL18 ternary system (a) before treatment, (b) after HT, and (c) after HTTL.

that the long axis of the C20-d hydrocarbon chain is slightly tilted from the film normal after HT. Furthermore, in Figure 5c, the ratios of heights in Figure 5c against those in Figure 5a are 0.976:1, 0.984:1, 1.05:1, and 1.04:1 for the CD3 asymmetric, CD2 antisymmetric, CD3 symmetric, and CD2 symmetric stretching bands, respectively. These fluctuations after HTTL are within 5%, although there is tendency for the heights of the asymmetric and antisymmetric stretching bands to decrease and, in contrast, for those of both symmetric bands to increase. Therefore, it seems that no significant variation in the orientation of the C20-d hydrocarbon chain is induced after HTTL. Finally, we examine the degree of thermal mobility of the C20-d hydrocarbon chain, referring to the results of the packing natures and fwhm values. In Figure 5b, the fwhm values for two CD2 stretching bands are the same as those in Figure 5a, suggesting the identical degree of thermal mobility. Furthermore, the ratios of fwhm values in Figure 5c against those in Figure 5a are 0.967:1 and 0.960:1 for the CD2 antisymmetric and symmetric stretching bands, respectively. The changes are within 4%, though we expected that the fwhm values in Figure 5c should be smaller than those in Figure 5a on the basis of the result that the C20-d subcell packing is hexagonal and orthorhombic in nature before and after HTTL, respectively. This result may reflect the fact that the degree of thermal mobility of the C20-d hydrocarbon chain in the hexagonal state is not very large in comparison with that in the orthorhombic one at room temperature. Thus, it has been found that the orientational variation in the C20-d hydrocarbon chain after HT is more marked than that after HTTL and that the change in the C20-d subcell packing from hexagonal to orthorhombic is induced only after HTTL. 3.5. Changes in the Presence, Conformation, Orientation, and Packing of AL18-d Before and After Secondary Treatment. Figure 6A exemplifies polarized IR absorption spectra A| in the 2235-2025 cm-1 region of the mixed LB film of MS18C20-AL18-d ternary systems (a) before treatment, (b) after HT, and (c) after HTTL. In Figure 6Aa, the peaks due to AL18-d appear at 2214, 2195, 2156, and 2089 cm-1, where the assignments and peak positions are the same as those in Figure 5, indicating the all-trans conformation. Then, the in-plane isotropy (R ) 1) has been verified for four bands. In our previous study, the result of surface-pressure isotherm measurements suggests that almost

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Figure 6. Polarized IR absorption spectra A| in the (A) 2235-2025 cm-1 and (B) 1100-1075 cm-1 regions in the mixed LB film of the MS18-C20-AL18-d ternary system (a) before treatment, (b) after HT, and (c) after HTTL.

all of the AL18 molecules tend to fill the MS18 empty space that can accommodate two straight hydrocarbon chains in the monolayer and that the other AL18 molecules are not in MS18 empty space.15 Furthermore, the long axis of all of the AL18 molecules with the all-trans conformation is parallel to the film normal, according to the result investigated by IR absorption spectroscopy.18 In Figure 6Ab, no peak is observed after HT. This shows that the AL18-d molecules completely evaporate from the mixed LB film during HT. In Figure 6Ac, two peaks can be recognized at 2195 and 2089 cm-1, respectively, with R ) 1 for the two bands being confirmed. The ratios of heights in Figure 6Ac against those in Figure 6Aa are 0.145:1 and 0.150:1 for the CD2 antisymmetric and symmetric stretching bands. Therefore, it is indicated that the majority of the AL18-d molecules are removed from the LB film to the warm water during HTTL and that the other AL18-d molecules that remain in the LB film possess the all-trans conformation. For the removal of AL18 molecules by HTTL, we have interpreted that the phenomenon of evaporation, which is similar to the HT case, occurs via the thermal energy of HTTL. In Section 3.9, we discuss the orientation of the AL18 molecules remaining in the film after HTTL. Through these results, it has been found that the AL18 molecules added as a third component to induce the H-aggregate are completely and almost removed by the thermal energy of HT and HTTL, respectively. Finally, we investigate the packing forms between the AL18-d molecules and between the AL18 molecule and the MS18 hydrocarbon chain before and after HTTL. Figure 6B shows A| in the 1100-1075 cm-1 region in the MS18-C20-AL18-d ternary systems (a) before treatment, (b) after HT, and (c) after HTTL. In Figure 6Ba, the singlet band due to AL18-d is discernible at 1090 cm-1. The position is comparable to those in Figure 3a,b. However, no peak is observed in Figure 6Bb as a matter of course, and we could not detect it in Figure 6Bc, reflecting the results of fairly weak signals in Figure 6Ac. Moreover, for the form between MS18 and AL18, we have to see the spectra of the MS18-C20-d-AL18 ternary system in Figure 3a again. Whereas the close-packing interaction works between the MS18 hydrocarbon chains and AL18 molecules in MS18 empty space to

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stabilize the formation energy of the H-aggregate,20 the singlet band is observed at 1468 cm-1. On the basis of these results, it is implied that no orthorhombic packing is formed between the AL18 molecules and between the AL18 molecule and MS18 hydrocarbon chain before HT and HTTL. 3.6. Interpretation of the Change in the MS18 Aggregation State from H-Aggregate to Monomer by Heat Treatment in Air (HT). We have found that the MS18 aggregation state changes from H-aggregate to monomer by HT in Figures 2b and 3b. In Figure 4Bb, the gauche conformation increases in the MS18 hydrocarbon chain after HT. In this respect, if we observed in situ structural changes in the MS18 hydrocarbon chain during HT, the increments not only of the gauche conformation but also of the degree of thermal mobility of the long axis of MS18 hydrocarbon chain would probably be monitored. Furthermore, the AL18 molecules completely evaporate by HT in Figure 6Ab,Bb. With the results of the above structural characterization before and after HT and our prediction taken into consideration, the dissociation of the MS18 aggregation state from H-aggregate to monomer is caused by the complete evaporation of AL18 from the mixed LB film and the increase in the thermal mobility of the MS18 hydrocarbon chain during HT. In addition, the variation in the R value from R < 1 to R ) 1 in Figures 2a,b and 3a,b is mainly caused by the disorder of chromophores in the MS18 domains. 3.7. Interpretation of Variations in the MS18 Aggregation State from H- to J-Aggregates and C20 Subcell Packing from Hexagonal to Orthorhombic by Hydrothermal Treatment in the Liquid Phase (HTTL). We have also found that the conversion from the blue-shifted H- to red-shifted J-bands is induced with HTTL in a short time in Figures 2c and 3c. In Figure 6Ac,Bc, it has been suggested that the almost all of the AL18 molecules are removed from the mixed LB film to warm water by the thermal energy of HTTL. This removal of the majority of AL18 molecules should probably induce the dissociation of the H-aggregate. Therefore, it is predicted that the conversion in the MS18 aggregation state from the H- to J-aggregate consists of the following two processes. Initially, the H-aggregate changes to the monomer accompanied by the removal of the majority of AL18 molecules. Subsequently, the monomer transforms into the J-aggregate, leading to the decrease in the total value of electrostatic energy based on the MS18 permanent dipole interaction. In this respect, it is presumed that the large relative permittivity (r ) 64) of warm water at 70 °C is strongly associated with the latter transformation because the total value of the dipole-diopole intaraction is well known to be inversily proportional to the relative permittivity,28,29 whose value becomes slightly small, depending on the increase in temperature.28,29 Then, we can tentatively interpret that the change in the R value from R < 1 in the H-aggregate in Figures 2a,c and 3a,c is ascribed to the deviation from uniaxial orientation of the long axis of H-aggregates elongated in shape and that the deviation probably originates not only from the variation in the slippage between arrangements of the MS18 chromophore but also from the fluctuation of the long axis of aggregates by the thermal energy of HTTL. Simultaneously, we have found not only the above conversion of the MS18 aggregation state but also the modification of the C20 subcell packing from hexagonal to orthorhombic with HTTL. As already discussed in section 3.1., the MS18 and C20 molecules are phase separated from each other before HTTL. In the C20 (28) Israelachvili, J. N. Intermolecular and Surface Forces: With Applications to Colloidal and Biological Systems; Academic Press: London, 1985. (29) Tanford, C. The Hydrophobic Effect: Formation of Micelles and Biological Membranes; Wiley: New York, 1980.

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Figure 7. Schematic representation of main structural changes in MS18, C20, and AL18 in the mixed LB film of the MS18-C20-AL18 ternary system induced by HTTL.

domains, the hydrophobic interaction operates between the C20 hydrocarbon chains in the hexagonal packing form in Figure 3a, where their long axis is parallel to the film normal in Figure 5a. If the temperature remains constant, then the polarity of warm water is much larger than that of air, referring to the values of their relative permittivities.28,29 Consequently, the modification of C20 subcell packing by HTTL is possibly due to the fact that the hydrophobic effect28,29 operates between the C20 hydrocarbon chains in warm water and that the C20 hydrocarbon chains cohere again in their domains, resulting in the variation from hexagonal to orthorhombic forms with a more stabilized structure in the packing. 3.8. Differences in the Structural Change in the MS18 Hydrocarbon Chain and the Dissociation of Chelation by Cd2+ After HT and HTTL. As we have already discussed in Figure 4Bb,Bc, the structural change in the MS18 hydrocarbon chain after HT is more remarkable than that after HTTL. The result seems to reflect the difference in their fluctuations during HT and HTTL. If we investigated the degree of the thermal mobility of MS18 hydrocarbon chain during both HT and HTTL, the variation during HT would be appreciably greater than that during HTTL. Consequently, we have expected that the hydrophobic effect also acts on the MS18 hydrocarbon chain in warm water, which probably restrain its structural fluctuation by the thermal energy during HTTL. Moreover, the disconnection of Cd2+ from the chelated MS18 keto group is slightly observed after HT in Figure 3b, whereas no dissociation is seen after HTTL in Figure 3c. Taking into consideration the correlation between the structural changes in the MS18 hydrocarbon chain and the dissociation of chelation after HT and HTTL, it is suggested that the increase in the thermal mobility of the MS18 hydrocarbon chain during secondary treatments probably has an influence on the disconnection of chelation by Cd2+ from the keto group of the MS18 chromophore. Figure 7 shows a schematic representation of the main structural changes in MS18, C20, and AL18 in the mixed LB film of the MS18-C20-AL18 ternary system induced by HTTL.

3.9. Reason for the Presence, Position, and Orientation of AL18 Molecules Remaining in the Mixed LB Film After HTTL. As has been mentioned in section 3.5., before both HT and HTTL, almost all of the AL18 molecules tend to fill the MS18 empty space, and the other AL18 molecules are not in the MS18 empty space, with the long axis of all of the AL18 molecules being parallel to the film normal. After HT and HTTL, the AL18 molecules are completely and almost removed by their thermal energy, respectively, in Figure 6A,B. Referring to the above results, we have currently predicted that during HTTL the AL18 molecules on the MS18 empty space are removed from the LB film with its thermal energy and that the other AL18 molecules, not being in the MS18 empty space, stay in the film after HTTL. If the AL18 molecules remained on the MS18 empty space after HTTL, then a component of the H-aggregate would be observed in Figure 2c. Furthermore, we have presumed that the latter AL18 molecules may partially contribute to the C20 orthorhombic subcell packing in C20 domains by the hydrophobic effect, although we have not yet obtained direct evidence. If the remaining AL18 molecules are based on this picture, then the long axis of AL18 molecules with the all-trans conformation after HTTL should probably be parallel to the film normal, referring to less orientational change in the C20 hydrocarbon chain before and after HTTL in Figure 5a,c. For the determination of the orientation of AL18 after HTTL, IR RAS25 or IR MAIRS27 measurements will be needed, but it may be difficult to estimate the orientation because of fairly weak signals in Figure 6Ac. Further studies to reveal why the effect of relative permittivity rather than that of thermal energy during HTTL preferentially operates for (1) the conversion of the MS18 aggregation state from monomer to J-aggregate, (2) the modification of C20 subcell packing from hexagonal to orthorhombic, (3) the MS18 hydrocarbon chain, and (4) the AL18 molecules not being in the MS18 empty space will be needed by adjusting parameters such as the relative permittivity, temperature, and treatment time. In addition, (5) the point regarding whether the MS18 monomer and C20 hexagonal packing form after HT are converted to the J-aggregate

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and the orthorhombic packing form with HTTL, respectively, is also significant.

4. Conclusions The influence of HT and HTTL on the H-aggregate in the mixed LB film of the MS18-C20-AL18 ternary system has been investigated by polarized visible and IR absorption spectroscopy. The MS18 visible bands after HT and HTTL have been assigned, referring to information not only on the changes in the shift, peak height, shape, and dichroic ratio of their bands in the visible region but also on the degree of MS18 intramolecular charge transfer in the fingerprint region. HT gives rise to the variation from H-aggregate to monomer, the increment of gauche conformers in the MS18 hydrocarbon chain, the slight orientational change in the C20 hydrocarbon chain, and the complete evaporation of AL18. The dissociation from H-aggregate to monomer is caused by the complete evaporation of AL18 from the mixed LB film and the increase in the thermal mobility of the long axis of the MS18 hydrocarbon chain during HT. However, HTTL can easily and rapidly cause the conversion from the H- to J-aggregate, the modification of C20 subcell packing from hexagonal to orthorhombic, and the removal of the majority of AL18 molecules with HTTL. The conversion of the MS18 aggregation state by HTTL is expected to be composed of two processes from the H-aggregate to monomer and from the monomer to J-aggregate. The former variation is ascribed to the removal of almost all of the AL18 molecules from the mixed LB film to warm water by the thermal energy of warm water.

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Subsequently, the large relative permittivity of warm water strongly concerns the latter transformation. This becomes the diminution of the total value of the electrostatic energy by the MS18 permanent dipole interaction. Furthermore, the modification of C20 subcell packing is ascribable to the action of the hydrophobic effect on the C20 hydrocarbon chain in the warm water during HTTL. Consequently, HTTL has been found to be quite effective at reorganizing the chromophore alignment of MS18, modifying the subcell packing of C20, and erasing the majority of AL18 in the mixed LB film of the MS18-C20-AL18 ternary system in a short time. Acknowledgment. We are grateful to Dr. Shin-ichi Morita of Riken and Mr. Shinsuke Tateno and Dr. Yasutaka Kitahama of Kwansei Gakuin University for their valuable comments on the present study. This study was supported by the Open Research Center project (Research Center for Near Infrared Spectroscopy) for private universities through a matching funds subsidy from MEXT (Ministry of Education, Culture, Sport, Science and Technology), 2006-2008. Supporting Information Available: AFM result in an asdeposited LB film of the MS18-C20-d-AL18 ternary system and size estimation of the H-aggregate based on an analytical model of flow orientation. This material is available free of charge via the Internet at http://pubs.acs.org. LA7037944