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Thermal Behavior of J-Aggregates in Mixed Langmuir-Blodgett Films Composed of Merocyanine Dye and Deuterated Arachidic Acid Investigated by UV-visible and Infrared Absorption Spectroscopy Yoshiaki Hirano, Shinsuke Tateno, and Yukihiro Ozaki* Department of Chemistry, School of Science and Technology, Kwansei Gakuin UniVersity, 2-1 Gakuen, Sanda 669-1337, Japan ReceiVed January 30, 2007. In Final Form: March 10, 2007 We have investigated the thermal behavior of J-aggregates in the mixed Langmuir-Blodgett (LB) films composed of the merocyanine dye (MS18)-deuterated arachidic acid (C20-d) binary system in the temperature range from 25 to 250 °C by means of UV-visible and IR transmission absorption spectroscopy. The temperature-dependent variations in both UV-visible and IR absorption spectra indicate that the MS18 aggregation states are linked with the MS18 intramolecular charge transfer and the behavior of the packing, orientation, conformation, and thermal mobility of the MS18 hydrocarbon chain. The J-aggregate formed at 25 °C in the mixed LB films dissociates in the temperature range from 25 to 110 °C, which is mainly ascribed to the increase in the thermal mobility of MS18 hydrocarbon chain and the dissociation of the chelation by a cadmium ion to the MS18 keto group. A thermally induced blue-shifted band appears at around 515 nm from 110 to 160 °C. This band is attributed to oligomeric aggregation with side-by-side alignment of the MS18 transition dipole moments on the basis of the shift to a higher-energy side, broadening, and temporary increment of the MS18 intramolecular charge transfer of the band. Consequently, the appearance of the thermally induced blue-shifted band indicates the possibility that the MS18 aggregation states can be controlled from the red shift to the blue shift by the annealing method adopted in the present study.
1. Introduction There has been a growing trend of applying organic thin films containing synthetic dyes to various devices such as photovoltaic cells, optical waveguides, and ultrafast optical switches.1-3 In particular, J- and H-aggregates with head-to-tail and side-byside alignments of transition dipole moments of the dyes in the thin films have been considered candidates for applications to the above devices because of their sharp and narrow absorption.1-10 In fact, photoelectric properties, complex refractive indexes, and nonlinear susceptibilities of third order inherent in J- and H-aggregates have been investigated by Saito,6,7 Wakamatsu et al.,8 and Zhou et al.,9,10 respectively. The guidelines indicate the significance of precisely controlling the dye aggregation states.6-10 We have been engaged in research on the control of the dye aggregation states using mixed Langmuir-Blodgett (LB) films with merocyanine dye (MS18, 3-carboxymethyl-5-[2-(3-octadecylbenzothiazolin-2-ylidene)ethylidene]rhodamine; see Figure 1a). Mixed LB films composed of MS18-arachidic acid (C20) binary system exhibit a sharp absorption peak at 590 nm, which is remarkably red-shifted from the absorption maximum of the * To whom correspondence should be addressed. E-mail: ozaki@ kwansei.ac.jp (Y. Ozaki). (1) Kobayashi, T. J-Aggregates; World Scientific: Singapore, 1996. (2) 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. (3) Ozaki, Y.; Morita, S.; Hirano, Y.; Li, X. Monolayer on Air/Solid Interface -Vibrational Spectroscopy and Atomic Force Microscopy-, AdVanced Chemistry of Monolayers at Interfaces; Imae, T., Ed.; Elsevier Science, in press. (4) Jelly, E. E. Nature (London) 1936, 138, 1009. (5) Scheibe, G. Angew. Chem. 1936, 49, 563. (6) Saito, K. J. Phys. Chem. B 1999, 103, 6579. (7) Saito, K. J. Phys. Chem. B 2001, 105, 4235. (8) Wakamatsu, T.; Watanabe, K.; Saito, K. Appl. Opt. 2005, 44, 906. (9) Zhou, H. S.; Watanabe, T.; Mito, A.; Honma, I.; Asai, K.; Ishigure, K.; Furuki, M. Mater. Sci. Eng., B 2002, 95, 180. (10) Zhou, H. S.; Watanabe, T.; Mito, A.; Honma, I.; Asai, K.; Ishigure, K. Mater. Lett. 2002, 57, 589.
Figure 1. Chemical structures of merocyanine dyes with (a) an octadecyl group (MS18) and (b) a butyl group (MS4).
MS18 monomer peak near 530 nm, when they are prepared under an aqueous subphase containing a cadmium (Cd2+) ion. The red-shifted sharp absorption band at 590 nm is termed the J-band and is ascribed to specific alignment of MS18, i.e., the J-aggregate.11-38 In addition, a blue-shifted band appears at 505 nm by adding a small amount of n-hexadecane (AL16) or (11) Sugi, M.; Fukui, T.; Iizima, S.; Iriyama, K. Mol. Cryst. Liq. Cryst. 1980, 62, 165. (12) Kuroda, S.; Ikegami, K.; Tabe, Y.; Saito, K.; Saito, M.; Sugi, M. Phys. ReV. B 1991, 43, 2531. (13) Nakahara, H.; Fukuda, K.; Moebius, D.; Kuhn, H. J. Phys. Chem. 1986, 90, 614. (14) Nakahara, H.; Moebius, D. J. Colloid Interface Sci. 1986, 114, 363. (15) Ozaki, Y.; Iriyama, K.; Iwasaki, T.; Hamaguchi, H. Appl. Surf. Sci. 1988, 33-34, 1317. (16) Minari, N.; Ikegami, K.; Kuroda, S.; Saito, K.; Saito, M.; Sugi, M. J. Phys. Soc. Jpn. 1989, 58, 222. (17) Ikegami, K.; Mingotaud, C.; Lan, M. J. Phys. Chem. B 1999, 103, 11261. (18) Ikegami, K.; Mingotaud, C.; Lan, M. Thin Solid Films 2001, 393, 193. (19) Ikegami, K.; Kuroda, S. Chem. Phys. 2003, 295, 205.
10.1021/la700254p CCC: $37.00 © 2007 American Chemical Society Published on Web 05/26/2007
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n-octadecane (AL18) to the MS18-C20 binary system.14,28-38 We assigned the 505 nm band to the H-band on the basis of the shift to a higher-energy side, sharpness, and in-plane anisotropy of the band.28-35 We characterized the individual structures of the J- and H-aggregates, aiming at the elucidation of the formation mechanism of the H-aggregate.30,32-35 Subsequently, Ikegami20 reported that, for the J-aggregation of an analogous MS18, the slippage of the alignment between adjacent MS18’s in the aggregates can also be regulated by the hydrocarbon chain substituted to MS18. His guideline comes from an ab initio calculation of the optimization for the geometrical structure where the MS18 hydrocarbon chain penetrates into the empty space on the adjacent MS18 chromophore. More recently, we concluded that the origin of the formation of the J- and H-aggregates is ascribed to the MS18 slippage, which is also restricted by the degree of the orientation of MS18 hydrocarbon chain,35 referring to the above guideline20 and the results of the orientation change in the long axis of the MS18 hydrocarbon chain for both aggregates.35 In addition, we interpreted that the close-packing interaction between the AL18 molecule and MS18 hydrocarbon chain on the empty space on the MS18 chromophore induces the stabilization of the formation energy of the H-aggregate as well as the orientation variation in the MS18 hydrocarbon chain.35 The dye aggregation state in the MS18 LB films can be effectively controlled not only by AL18 addition14,28-38 but also by heat treatments.24-27 Sugi et al.24 have found that the J-aggregate in the MS18-C20 binary system gradually dissociates with temperature, and that a broad band appears at around 530 nm at 90 °C. In addition, it has been recently reported by Miyata et al.25,26 that the J-aggregate elongated in shape at 590 nm changes into the two-dimensionally developed J-aggregate at 596 nm with enhanced ordering of the MS18 chromophores in the temperature range 60-70 °C. Moreover, Mouri et al.27 found that the similar color phase transition occurs under the condition of 100% relative humidity from 30 to 90 °C by hydrothermal treatments. These results suggest that the changes in the MS18 aggregation state strongly depend on the annealing methods. (20) Ikegami, K. J. Chem. Phys. 2004, 121, 2337. (21) Kato, N.; Saito, K.; Aida, H.; Uesu, Y. Chem. Phys. Lett. 1999, 312, 115. (22) Kato, N.; Saito, K.; Serata, T.; Aida, H.; Uesu, Y. J. Chem. Phys. 2001, 115, 1473. (23) Kato, N.; Yuasa, K.; Araki, T.; Hirosawa, I.; Sato, M.; Ikeda, N.; Iimura, K.; Uesu, Y. Phys. ReV. Lett. 2005, 94, 136404. (24) Sugi, M.; Saito, M.; Fukui, T.; Iizima, S. Thin Solid Films 1983, 99, 17. (25) Miyata, J.; Morita, S.; Miura, Y. F.; Sugi, M. Jpn. J. Appl. Phys. 2005, 44, 8110. (26) Miyata, J.; Morita, S.; Miura, Y. F.; Sugi, M. Colloids Surf., A 2006, 284-285, 509. (27) Mouri, S.; Morita, S.; Miura, Y. F.; Sugi, M. Jpn. J. Appl. Phys. 2006, 45, 7925. (28) Hirano, Y.; Sano, H.; Shimada, J.; Chiba, H.; Kawata, J.; Miura, Y. F.; Sugi, M.; Ishii, T. Mol. Cryst. Liq. Cryst. 1997, 294, 161. (29) Hirano, Y.; Kawata, J.; Miura, Y. F.; Sugi, M.; Ishii, T. Thin Solid Films 1998, 345, 327-329. (30) Hirano, Y.; Kamata, K. N.; Inadzuki, Y. S.; Kawata, J.; Miura, Y. F.; Sugi M.; Ishii, T. Jpn. J. Appl. Phys. 1999, 38, 6024. (31) Hirano, Y.; Okada, T. M.; Miura, Y. F.; Sugi, M.; Ishii, T. J. Appl. Phys. 2000, 88, 5194. (32) Morita, S.; Miura, Y. F.; Sugi, M.; Hirano, Y. J. Appl. Phys. 2003, 94, 4368. (33) Hirano, Y.; Murakami, T. N.; Nakamura, Y. K.; Fukushima, Y.; Tokuoka Y.; Kawashima, N. J. Appl. Phys. 2004, 96, 5528. (34) Hirano, Y.; Ohkubo, M. A.; Tokuoka, Y.; Kawashima, N.; Ozaki, Y. Mol. Cryst. Liq. Cryst. 2006, 445, 383. (35) Hirano, Y.; Tokuoka, Y.; Kawashima, N.; Ozaki, Y. Vib. Spectrosc. 2007, 43, 86. (36) Ray, K.; Nakahara, H.; Sakamoto, A.; Tasumi, M. Chem. Phys. Lett. 2001, 342, 58. (37) Murata, M.; Mori, K.; Sakamoto, A.; Villeneuve, M.; Nakahara, H. Chem. Phys. Lett. 2005, 405, 32. (38) Murata, M.; Villeneuve, M.; Nakahara, H. Chem. Phys. Lett. 2005, 405, 416.
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We have also been exploring the possibility of conversion from the J-aggregate to the other aggregation states using a slower rate in the temperature-programmed process of annealing to gradually ease the regulation of MS18 slippage by the MS18 hydrocarbon chain using the increase in its thermal mobility. In addition to this investigation, it will be of great interest to examine the correlation among the MS18 aggregation state, the MS18 intramolecular charge transfer, and the packing, orientation, conformation, and thermal mobility of hydrocarbon chains of MS18 and deuterated arachidic acid (C20-d). Furthermore, it may also be valuable to monitor the changes in relation to the decomposition or evaporation of MS18 and C20-d chelated by the Cd2+ ion. In order to perform the study with these motivations, we amply investigate them with a continuous scan by means of UV-visible and IR absorption spectroscopy. So far, no systematic study on the temperature-dependent structural characterization of the J-aggregate in MS18 LB films from the above viewpoints has come to our knowledge. In this paper, we report the thermal behavior of the J-aggregate in mixed LB films of the MS18-C20 binary system in the range from 25 to 250 °C studied by means of UV-visible and IR absorption spectroscopy. The present study provides new insight into the relationship between the MS18 aggregation state and the individual structures in our heat treatment, the origin of dissociation of the MS18 J-aggregate, and the occurrence and assignment of blue-shifted bands induced by this annealing. 2. Experimental Section 2.1. Chemicals and Sample Preparation Procedures. Merocyanine dyes with an octadecyl group (MS18) and with an ethyl group (MS2) were purchased from Hayashibara Biochemical Laboratories, Inc. (Okayama, Japan), and arachidic acid (C20) and deuterated arachidic acid (CD3(CD2)18COOH, D 98%, C20-d) were from Fluka Chemie AG (Buchs, Switzerland) and C/D/N isotopes, Inc. (Quebec, Canada). These were used without further purification. Four types of spreading solutions of pure C20 (and C20-d) and MS18C20 (and MS18-C20-d, [MS18]:[C20 or C20-d] ) 1:2) binary systems were prepared using freshly distilled chloroform from DOJINDO Laboratories (Kumamoto, Japan). The MS18 concentration was on the order of 10-4 M. A USI system trough was used. An aqueous subphase with 5.0 × 10-4 M CdCl2 and (3.33-5.00) × 10-5 M NaHCO3 (pH ) 6.3-6.8) was prepared using distilled water from Kanto Chemical Co., Inc. (Tokyo, Japan). The subphase was kept at 20 °C. After the chloroform solutions were spread on an airwater interface, the monolayers were directly transferred onto the substrates of the mica, CaF2, and polyethylene terephthalate (PET) film having the size of 13 × 38 mm2 by the standard vertical dipping method at the surface pressure of 25 mN/m. The raising velocity of the mica was 25 mm/min for the deposition of the monolayer, whereas the raising and dipping velocities of the CaF2 substrate and PET film were 2.5 mm/min 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. Pure C20 and MS18-C20 binary systems on the PET film were 100 × 2 and 1500 × 2 layers, respectively. All the LB films were of Y-type with a transfer ratio of approximately unity. Two samples of the LB films of the MS18-C20-d binary system on CaF2 substrates were prepared to investigate the thermal behavior of the MS18 LB films by means of UV-visible and infrared absorption spectroscopy. Before the measurements of their thermal behavior, it was confirmed that the MS18 aggregation states of two samples are the same, referring to the results of their visible absorption spectra. The temperature-dependent UV-visible and IR measurements were started immediately after the sample preparation and checked to avoid the aging effects of the MS18 LB films fabricated. 2.2. Spectroscopic Measurements. Measurements of UV-visible absorption spectra were carried out using a Shimadzu UV-3101PC UV-vis spectrometer. IR absorption spectra were measured at a 4
Thermal BehaVior of J-Aggregates in LB Films
Figure 2. Temperature-dependent UV-visible absorption spectra in the mixed LB film of merocyanine dye (MS)-deuterated arachidic acid (C20-d) binary system (25-250 °C). The spectra are shown every 10 °C from 30 °C in addition to the spectrum measured at 25 °C. cm-1 resolution using a Nicolet Magna 870 FT-IR spectrometer equipped with a TGS detector. In the IR measurements, the spectrometer involving a sample chamber was purged with dry air to minimize the absorption of water vapor. To generate a high signalto-noise ratio, 256 interferograms were coadded. For measuring temperature-dependent UV-visible and IR spectra, the LB film deposited on both sides of the CaF2 substrate was inserted into the cell. The temperature of the cell was controlled by thermoelectric device (MODEL SU, CHINO) with an accuracy of (0.1 °C. Both UV-visible and IR spectra were collected at an interval of about 2 °C from 25 to 250 °C at a rate of approximately 0.4 °C /min. 2.3. Spectral Analyses. The analyses in temperature-dependent UV-visible and IR spectra were carried out by using homemade software. The third-derivative spectra were calculated by the SavitzkyGolay method. 2.4. AFM and DSC Measurements. Measurements of atomic force microscopy (AFM) and differential scanning calorimeter (DSC) were preliminarily carried out. AFM images of the one-layer LB film of pure C20 and MS18-C20 binary systems on mica were measured with a Shimadzu SPM-9500 by contact mode with constant force. The cantilever of silicon nitride with a spring constant of 0.15 mN/m was used. DSC measurements were performed on a Perkin-Elmer Pyris6 DSC system. DSC of C20, MS18, and MS2 in powder states was measured at a speed of 5 °C/min, and that of the LB films of pure C20 and MS18-C20 binary systems on PET films was at a speed of 10 °C/min.
3. Results and Discussion 3.1. Temperature-Dependent Changes in the MS18 Aggregation State and Its Domain Size. Figure 2 shows temperature-dependent UV-visible absorption spectra of a mixed LB film of the MS18-C20-d binary system (25-250 °C). At 25 °C, the mixed LB film exhibits a red-shifted sharp absorption peak at 594 nm with a shoulder in the 520-560 nm region. This spectrum is consistent with those reported by earlier studies.11-38 The red-shifted band is ascribed to the J-aggregation,11-38 and the shoulder is due to the formation of the MS18-MS18 dimer
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with Davidov splitting17,18,39 and the MS18-C20-d monomer, referring to our composition ([MS18]:[C20-d] ) 1:2) as well. According to our AFM results, the MS18 and C20 molecules are phase-separated from each other. The domains of the MS18 J-aggregates are fairly ellipsoidal in shape, and the sizes are approximately several hundred nanometers. These are in good agreement with those from the AFM measurement by Kato et al.21,22 Besides these results, the tendency of the flow orientation effect,16 where the long axis of the MS18 J-aggregates elongated in shape is preferentially oriented to the dipping direction of the mica substrate, is seen according to our AFM image. It is also noted that, for this J-aggregate, the MS18 transition dipole moments, being parallel to the chromophore, lie in the film plane.12,14,16,21,22,34 In this figure, it can be seen that remarkable spectral changes appear in the temperature range from 25 to 250 °C. Figure 3 represents (a) normalized peak heights of absorption maxima, (b) wavelength positions of absorption maxima, and (c) normalized absorbances at 810 nm in the UV-visible spectra in Figure 2 plotted as a function of temperature. In Figure 3a, the height decreases from 25 to 90 °C and almost remains constant up to 110 °C. Then, it slightly increases until 120 °C. Afterward, it gradually descends up to 165 °C and makes a downturn with a convex curve until 250 °C. In Figure 3b, the absorption maxima are mainly located in wavelength regions near 590, 515, and 460 nm with the increasing temperature. In Figure 3c, the normalized absorbance at 810 nm monotonically decreases from 25 to 250 °C with the value at 250 °C being 75% of that at 25 °C. The baseline shift in the region with no absorption is an indicator of changes in the size of domain structures composed of functional molecules in LB films.40,41 If the order of each domain size is approximately the same as that of the wavelength of incident light, the incident light should be diffused by the domains, resulting in the increment in the baseline.40,41 Therefore, the result of decrease in the baseline in Figure 3c implies the diminution from the order of several hundred nanometers in the MS18 J-aggregate at 25 °C in the course of the increasing temperature. Here, we can briefly summarize the thermally induced changes in the MS18 aggregation states, domain sizes, and also MS18 chromophores on the basis of the results in Figures 2 and 3a,b,c,. At 25 °C, the MS18 J-aggregate is formed as the dominant aggregation state. From 25 to 110 °C, the J-aggregate dissociates gradually. From 110 to 160 °C, the blue-shifted band with the absorption maximum at around 515 nm is induced, where the component or band near 460 nm gradually appears from approximately 140 °C. This appearance of absorption at around 460 nm can be tentatively understood from our results obtained by the DSC method as follows. An endothermic peak for C20 powder is observed at 77 °C, showing the melting point. In addition, two endothermic peaks appear at 134 and 173 °C for MS18 powder, whereas only one endothermic peak is observable at 290 °C for MS2 powder. These results imply that the peaks at 134 and 173 °C for the MS18 powder mainly correspond to the melting points of the octadecyl group and chromophore of MS18, respectively. Moreover, an endothermic peak due to the melting point is observed at 108 °C for pure CdC20 LB film deposited on a PET film. The result is in good agreement with that (110 °C) reported by Naselli et al.42 Then, the MS18-C20 binary system shows endothermic peaks at around 110, 130, (39) Ikegami, K. Colloids Surf., A 2006, 284-285, 112. (40) Wang, Y.; Nichogi, K.; Terashita, S.; Iriyama, K.; Ozaki, Y. J. Phys. Chem. B 1996, 100, 368. (41) Morita, S.; Nichogi, K.; Ozaki, Y. J. Phys. Chem. B 2000, 104, 1183. (42) Naselli, C.; Rabolt, J. F.; Swalen, J. D. J. Chem. Phys. 1985, 82, 2136.
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Figure 4. Temperature-dependent IR absorption spectra in the region 1800-1000 cm-1 in the mixed LB film of MS18-C20-d binary system (25-250 °C).
Figure 3. (a) Normalized height of absorption maxima in the UVvisible absorption spectra plotted as a function of temperature. (b) Temperature-dependent changes in wavelength positions of the absorption maxima. (c) Normalized absorbance at 810 nm plotted versus temperature.
160, 175, and 205 °C. The peak at 110 °C and those at 130 and 175 °C are probably ascribed to the melting points of CdC20 and MS18 with the insufficient chelation by the Cd2+ ion in the mixed LB film, respectively. The other two peaks at 160 and 205 °C probably originate from the melting points of the hydrocarbon chain and chromophore of MS18 chelated by the Cd2+ ion, respectively. With the above melting points taken into consideration, the appearance of the component near 460 nm from about 140 °C may be associated with the phenomenon in which the partial decomposition of the MS18 chromophore with the insufficient chelation starts to occur as well during the decomposition of its MS18 hydrocarbon chain. From 160 up to 250 °C, furthermore, the decomposition and/or evaporation of the MS18
chromophore with and without the chelation can be interpreted to occur and proceed increasingly, referring to the decrease in the peak height at around 515 nm and the dominance and subsequent decrease in the height at about 460 nm. In addition, the domain size of the MS18 aggregates decreases anytime through this annealing. 3.2. Temperature-Dependent Changes in the Degree of MS18 Intramolecular Charge Transfer. Figure 4 shows temperature-dependent IR absorption spectra in the fingerprint region of a mixed LB film of the MS18-C20-d binary system (25-250 °C). The dotted lines represent the wavenumber positions of vibrational bands inherent in the nonaggregation state of the MS18 chromophore chelated by the Cd2+ ion (Table 1).17,18 The assignments of MS18 IR bands in the fingerprint region have been already accomplished by an ab initio calculation based on density functional theory (DFT) by Ikegami and Kuroda,19 assuming the structure shown in Figure 1b. For the IR spectra at 25 °C, sharp peaks are observed at 1488, 1379, 1315, 1239, 1183, and 1147 cm-1 (Table 1). These peak positions have been determined from the zero crossing point of the third-derivative spectra. The results are in good agreement with those reported earlier and our works within the wavenumber resolution.17-19,35 According to the results of the DFT calculation (Table 1), the band at 1488 cm-1 is assigned to the coupling mode involving the stretching vibrations of CRdC2′ and C2′s N3′, and the CH2 in-plane bending vibration of alkyl chain (See Figure 1b).19 In addition, the 1379, 1239, and 1183 cm-1 bands are attributed to the coupling modes of C2sN3, benzene ring and C3′asN3′, CRdC2′, CsO and CCH (alkyl), and C2dS2a, C2sN3, and CCH (benzene), respectively.19 The 1315 and 1147 cm-1 bands have not yet been assigned, but the tendency of shifts for these bands resembles fairly well that for the 1183 cm-1 band according to the results of the J- and H-aggregates in the MS18C20 binary and MS18-C20-n-octadecane (AL18) ternary systems in our previous paper.35 Therefore, the heights and wavenumber positions for the above six bands are deduced to discuss the degree of MS18 intramolecular charge transfer. Another significant
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Table 1. Peak Positions (cm-1) and Assignment for Infrared Bands of MS18 in Mixed LB Films Prepared under a Subphase Containing a Cadmium (Cd2+) Ion nonaggregation statea
J-aggregateb
aggregation state at 100 °Cc
blue-shifted aggregated
assignmente
(1512) 1381 1317
1488 1379 1315 1239 1183 1147
1494 1379 1316 1239 1188
1494 1379 1313
CRdC2′, C2′sN3′, δ(CH2-alkyl) C2sN3, benzene ring, C3′asN3′
1186
CRdC2′, CsO, CCH (alkyl) C2dS2a, C2sN3, CCH (benzene)
1190
a The data were taken from refs 17 and 18. These wavenumber positions were deduced from the difference spectra using the results of the polarized visible and IR absorption spectra of the MS18 J-band, where the composition is only MS18 without the matrix molecule such as arachidic acid (C20). The description of “nonaggregation state” corresponds to “dimer” in ref 17 and “non-J” in ref 18. In our composition ([MS18]:[C20-d]))1:2), not only MS18 dimer (MS18-MS18) but also monomer (MS18-C20-d) will be formed as a possibility, each of which can be probably produced with the chelation by a Cd2+ ion. In the present study, we assume that the degree of the MS18 intramolecular charge transfer for MS18-C20 is almost the same as that for MS18-MS18. b The positions for the MS18 J-aggregate at 25 °C were obtained from Figure 6. c These were acquired from Figure 6. d The positions at 120 °C were acquired from Figure 6. e These were the assignment from the results of ab initio calculation based on the density functional theory (DFT), assuming MS4 with a butyl group and (π, π, π) conformer in Figure 1b in chloroform solution in ref 19.
piece of information is obtained from the 1800-1600 cm-1 region as follows. At 25 °C, no peak due to free carboxylic groups in MS18 and C20 is observed in the 1800-1700 cm-1 region,17-19,43,44 whereas a slight peak assigned to the free keto group of MS18 appears at around 1680 cm-1.17-19,43,44 These results indicate that most of the MS18 and C20 molecules are chelated and coupled by the Cd2+ ion. In addition, the result of the slight peak at around 1680 cm-1 suggests the existence of the MS18-MS18 dimer with Davidov splitting17,18,39 and the MS18-C20-d monomer where two carboxylic groups are chelated by the Cd2+ ion, taking into consideration our composition ([MS]:[C20-d] ) 1:2) as well. This interpretation is comparable to the assignment for the shoulder at 520-560 nm in the UV-visible spectra at 25 °C in Figure 2. Furthermore, it should be noted that the peak at around 1680 cm-1 seems to slightly increase in the course of temperature from 25 to approximately 200 °C, and that no absorption is observed at 1800-1700 cm-1 in the same course. Consequently, one of the reasons for the decrease in the domain sizes of MS18 aggregates, which is implied from the decrease in the baseline at 810 nm in Figure 3c, is possibly the increment of dissociation of the Cd2+ ion from the keto group of MS18. Figure 5 exemplifies normalized peak heights (a) at around 1490 (O), 1380 (b), and 1315 (0) cm-1, and those (b) near 1240 (O), 1185 (b), and 1145 (0) cm-1 plotted as a function of temperature, where the individual heights have been estimated within the feasible range. In these results, roughly speaking, there is similarity in the fluctuation of all the peak heights as follows. The heights gradually decrease up to about 80 °C and rapidly diminish until 100 °C. Then, they increase drastically up to approximately 120 °C and finally decrease until 250 °C. Figure 6 shows temperature-dependent changes in wavenumber positions of peaks near (a) 1490 (O) and 1380 (b) cm-1, (b) 1315 (0) and 1240 (O) cm-1, and (c) 1185 (b) and 1145 (0) cm-1. All the results reveal the following tendency of the band shifts. (i) The shifts for all the bands are commonly within 1 cm-1 in the temperature range from 25 to 80 °C. (ii) The upward shifts for five bands at around 1490, 1380, 1315, 1240, and 1185 cm-1 are always observed in the range between 80 and 110 °C. (iii) After the upward shift described in (ii), downward shifts for five bands occur between 80 and 150 °C. (iv) After the downward shift in (iii), three bands near 1490, 1315, and 1185 cm-1 shift upward from 150 to 160 °C, except for the 1380 cm-1 band. (v) From 160 °C, there are irregular shifts for the 1490, 1380, 1315, and 1185 cm-1 bands. (43) Fujimoto, Y.; Ozaki, Y.; Takayanagi, M.; Nakata, M.; Iriyama, K. J. Chem. Soc., Faraday Trans.1996, 92, 413. (44) Fujimoto, Y.; Ozaki, Y.; Iriyama, K. J. Chem. Soc., Faraday Trans. 1996, 92, 419.
Figure 5. Normalized height values of peaks at around (a) 1490 (O), 1380 (b), and 1315 (0) cm-1, and (b) 1240 (O), 1185 (b), and 1145 (0) cm-1 in the IR spectra plotted as a function of temperature.
We can discuss temperature-dependent variations in the degree of MS18 intramolecular charge transfer, based on the results in Figures 5 and 6. For the J-aggregation at 25 °C, sharp peaks appear at 1488, 1379, 1315, 1239, 1183, and 1147 cm-1 (Table 1). Among them, the wavenumber positions at 1488 and 1183 cm-1 are markedly downward shifted from those of the MS18 nonaggregation state. In addition, similar downward shifts can
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Figure 7. Temperature-dependent IR spectra in the region 31002700 cm-1 in the mixed LB film of MS-C20-d binary system (25250 °C).
Figure 6. Temperature-dependent changes in wavenumber positions of the peaks at around (a) 1490 (O) and 1380 (b) cm-1, (b) 1315 (0) and 1240 (O) cm-1, and (c) 1185 (b) and 1145 (0) cm-1 determined from zero crossing points using the third-derivative method.
be recognized for the 1379 and 1315 cm-1 bands. These downward shifts are ascribed to the increase in the intramolecular charge transfer of MS18 (See Figure 1b) caused by (i) chelation between the MS18 chromophore and the Cd2+ ion and (ii) J-aggregation.17-19 As shown in Figure 1b, the MS18 intramolecular charge transfer means the delocalization of π electrons in the butadiene group from the rhodamine group to the benzothiazolydine group17,18 and the CdS bond in the rhodamine group in MS18.19 Ikegami et al. interpreted that the MS18 intramolecular charge transfer enhanced by the latter case decreases the total energy in the aggregation by the resonance effect upon formation of the J-aggregate.17-19 In addition, the result, where the degree of the MS18 intramolecular charge transfer for the H-aggregate is quite close to that for the nonaggregation state, also supports the above interpretation.35 In Figures 5 and 6, the results from 25 to 80 °C
and from 80 to 100 °C are attributed to the gradual and drastic decreases in the MS18 intramolecular charge transfer, respectively. From 100 to 120 °C, it is indicated that the temporary increase in the MS18 intramolecular charge transfer is induced. In order of range from 120 to 150 °C, from 150 to 160 °C, and from 160 to 250 °C, it can be interpreted that the degrees in the MS18 intramolecular charge transfer diminish gradually from those at 120 °C. 3.3. Temperature-Dependent Changes in the Packing, Orientation, Conformation, and Thermal Mobility for Octadecyl Group Substituted to MS18. The temperature-dependent changes in the packing for the MS18 hydrocarbon chain have been moreover investigated. In this respect, we have to see the IR spectra in Figure 4 again. At 25 °C, the peak appears at 1468 cm-1. This is assigned to the CH2 in-plane bending mode for the MS18 hydrocarbon chain,17,19 because C20-d is utilized. The singlet band is observed in the range from 25 to 250 °C, suggesting that the long axis of the MS18 hydrocarbon chain possesses thermal mobility. Here, it should be noted that we cannot discuss temperature-dependent changes in the CH2 in-plane bending mode in more detail due to the following reason. While the temperaturedependent variations in the peak height fundamentally resemble those in Figure 8a, which is shown later, the changes in the wavenumber position are fairly similar to those of the MS18 central conjugated system at around 1490 cm-1. This will probably be because their bands are heavily overlapped with each other. Consequently, we have judged that further discussion about the CH2 in-plane bending mode is difficult at the present stage. Figure 7 shows temperature-dependent IR spectra in the region 3100-2700 cm-1. The peaks at around 2920 and 2850 cm-1 are assigned to the CH2 antisymmetric and symmetric stretching modes for the MS18 hydrocarbon chain,32,35,44 respectively. In Figure 8a,b,c, normalized peak heights, wavenumber positions,
Thermal BehaVior of J-Aggregates in LB Films
Figure 8. (a) Normalized heights, (b) wavenumber positions, and (c) normalized full width at the half-maxima (fwhm) for peaks at around 2920 (O) and 2850 (b) cm-1 in the IR spectra plotted against temperature.
and normalized fwhm values for the CH2 antisymmetric (O) and symmetric (b) stretching bands plotted versus temperature are represented to discuss the temperature-dependent variations in its orientation, conformation, and thermal mobility. The results in Figures 7 and 8 can be understood as follows. At 25 °C, the long axis of the MS18 hydrocarbon chains, slightly involving the gauche conformation, is tilted by 43.8° (〈cos2 γ〉 ) 0.522) from the film normal.35 From 25 to 50 °C, the decrease in heights is observed in Figure 8a, which is discussed later referring to the results in Figure 8b,c as well. The upward shifts in Figure 8b are caused by the increment of the gauche conformation to the MS18 hydrocarbon chains. The increase in the fwhm values in Figure 8c is ascribed to the increment of thermal mobility of the long axis of the MS18 hydrocarbon chains.
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Therefore, at the present stage, the decrease in the heights in Figure 8a is probably attributed to the increase in the gauche conformation as well as the orientation change in which the long axis of the MS18 hydrocarbon chain slightly approaches parallel to the film surface by the increase in thermal mobility, in comparison with the case at 25 °C. For further detailed discussion, the temperature-dependent IR reflection absorption spectroscopy (RAS) measurement will be required. From 50 to 80 °C, similar and moderate changes continue. From 80 to 165 °C, moreover, the changes in the orientation of the MS hydrocarbon chain become marked. Here, the remarkable feature is that the changeable point of the slope at 80 °C is well-recognized, which cannot be observed at the identical temperature in the MS18C20-d-AL18-d ternary system.45 In Figure 8b, the upward shifts from 80 to 100 °C and from 100 to 135 °C are ascribed to the drastic and slight increases in the gauche conformation, respectively, whereas the downward shift from 135 to 165 °C is attributed to the gradual decrease. For this decrease, there may be relationships with our presumption based on the results of UV-visible spectra and DSC discussed in section 3.1. in which the decomposition of the hydrocarbon chains of MS18 with insufficient chelation by degrees occurs from about 140 °C. In addition, in Figure 8c, the thermal mobility continues to increase. From 165 to 250 °C, the decrease with a convex curve in Figure 8a and the almost constant position in Figure 8b are probably caused by the decomposition and/or evaporation of the MS18 hydrocarbon chain with the conformation of the MS18 hydrocarbon chain that remains constant in the mixed LB films. Therefore, the increments of the fwhm values, including an upturn in the CH2 symmetric stretching band from 135 °C, are possibly due to not only the mere increase in the thermal mobility but also the apparent rise associated with the decomposition and/or evaporation of the MS18 hydrocarbon chains with and without the chelation. 3.4. Temperature-Dependent Changes in the Packing, Orientation, Conformation, and Thermal Mobility for the Hydrocarbon Chain of C20-d. The structure of the C20-d hydrocarbon chain can be investigated from the results in Figures 4 and 9-11. In Figure 4, the singlet band assigned to the CD2 in-plane bending mode appears at 1088 cm-1 from 25 to 190 °C. This suggests that the C20-d hydrocarbon chains are in the hexagonal packing state and possess thermal mobility. Figure 9 represents IR spectra in the 2300-2000 cm-1 region. The peaks at around 2200 and 2100 cm-1 are assigned to the CD2 antisymmetric and symmetric stretching modes for the C20-d hydrocarbon chain, respectively.32,33 In Figures 10 and 11, (a) normalized heights, (b) wavenumber positions, and (c) normalized fwhm values are shown from a peak near 1090 cm-1 in Figure 4 and two peaks at around 2200 and 2100 cm-1 in Figure 9. The following is the interpretation as to the C20-d hydrocarbon chain. At 25 °C, the long axis of the C20-d hydrocarbon chain with the all-trans conformation is almost parallel to the film normal in mixed LB films of the MS18-C20-d binary system.32 In addition, the packing between the C20-d hydrocarbon chains in this binary system is in hexagonal states from the results in Figure 4. This is different from the orthorhombic subcell packing of the pure C20-d LB film in our preliminary experimental result. From 25 to 80 °C, the height decreases in Figures 10a and 11a are attributed to the long axis of the C20-d hydrocarbon chain tilted by degrees from the film normal. In addition, the downward shift in Figure 10b and the upward shift in Figure 11b are ascribed to the slight decline of the packing interaction between the C20-d hydrocarbon chains and the moderate increase in the gauche (45) Hirano, Y.; Tateno, S.; Ozaki, Y., in preparation.
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Figure 9. Temperature-dependent IR absorption spectra in the region 2300-2000 cm-1 in the mixed LB film of the MS-C20-d binary system (25-250 °C).
conformation, respectively. Moreover, the increment of the fwhm values in Figure 11c is caused by the gradual increase in thermal mobility of the long axis of the C20-d hydrocarbon chain. From 80 to 110 °C, the variations in the orientation, conformation, and thermal mobility become more remarkable in comparison with those from 25 to 80 °C. As already described in section 3.1, the melting point of the CdC20 LB films is around 110 °C. Therefore, the decreases with a convex curve in Figures 10a and 11a from 105 to 200 °C probably originate from the decomposition and/or evaporation of the C20-d hydrocarbon chain. In addition, the downward and upward shifts in Figures 10b and 11b indicate that the lowering of the packing interaction and the increase in the gauche conformation still continue during their decomposition and/or evaporation. Moreover, the sudden upturn in Figure 11c is possibly caused by the outward increase in the thermal mobility associated with decomposition and/or evaporation as well as the real increment, as also discussed in the MS18 hydrocarbon chain in section 3.3. Finally, in Figure 10c, the fwhm value decreases from 25 to 100 °C, showing the minimum at around 100 °C. Then, the value gradually increases from 100 °C. The behavior is contrary to our prediction that the fwhm value monotonically increases with temperature. The tendency in Figure 10c can be understood as follows, referring to our preliminary result of pure C20-d LB film. First, we have checked the number of peaks of the CD2 in-plane bending mode in the raw spectra of pure C20-d LB films. In the raw spectrum at 25 °C, two peaks assigned to CD2 in-plane bending modes are observed at 1092 and 1086 cm-1, indicating the orthorhombic subcell packing. In addition, only one peak appears at 1088 cm-1 from approximately 100 °C in the raw spectrum, suggesting disappearance in the orthorhombic nature. Second, we have estimated the normalized fwhm values of the CD2 in-plane bending mode from 25 to 200 °C. Here, it is noted that the fwhm values including two peaks against the value of
Figure 10. (a) Normalized heights, (b) wavenumber positions, and (c) normalized full width at the half-maxima (fwhm) of peaks at around 1090 cm-1 plotted versus temperature.
the peak top at 1086 cm-1 are estimated in the case of two peaks. As a result, the fwhm value decreases with a convex curve from 25 to 106 °C, showing the minimum at 106 °C, and then it increases with temperature up to 200 °C. Consequently, this tendency can be interpreted to be characteristic of the case with the orthorhombic nature at 25 °C. The result in Figure 10c is fairly similar to that in the above pure C20-d LB film. Therefore, it is considered that the band at 1088 cm-1 in Figure 4 possesses a character not only of the hexagonal nature suggested above but also of partly orthorhombic nature with split components, even if the band peak in the raw spectra is at the position of hexagonal packing. Then, the decrease
Thermal BehaVior of J-Aggregates in LB Films
Figure 11. (a) Normalized heights, (b) wavenumber positions, and (c) normalized full width at the half-maxima (fwhm) of peaks at around 2200 (O) and 2100 (b) cm-1 plotted as a function of temperature.
in the fwhm values from 25 up to 100 °C is possibly caused by the increase in the rotational chain mobility which loses the orthorhombic nature. Moreover, the increase from 100 °C is mainly ascribed to the apparent rise accompanied by the decomposition and/or evaporation as well as the increment of thermal mobility, taking into consideration the melting point of CdC20 LB films (108 °C) in section 3.1. 3.5. Correlation between the MS18 Aggregation State, MS18 Intramolecular Charge Transfer, and Packing, Orientation, Conformation, And Thermal Mobility of MS18 and C20-d Hydrocarbon Chains. From Figures 2 to 11, we have investigated the temperature-dependent variations in the MS18 aggregation
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states, the MS18 intramolecular charge transfer, and the packing, orientation, conformation and thermal mobility of the MS18 and C20-d hydrocarbon chains. Through a series of experimental results, the aggregation state and the degree of intramolecular charge transfer of MS18 can be similarly grouped into the following temperature ranges: (i) 25 °C, (ii) 25-110 °C, (iii) 110-160 °C, and (iv) 160 °C and above. Furthermore, the degree of MS18 intramolecular charge transfer in the range 25-110 °C can be categorized into degrees in the ranges 25-80 °C and 80-110 °C, and that in the range 110-160 °C are into those in the ranges 110-120 °C, 120-150 °C, and 150-160 °C. On the basis of these results, it can be seen that there is a close correlation between the aggregation states and the degree of intramolecular charge transfer of MS18. Consequently, the MS18 intramolecular charge transfer may show the MS18 aggregation states in more detail. Moreover, the change in the orientation of the long axis of the MS18 hydrocarbon chains can be classified into the following temperature ranges: (i) 25 °C, (ii) 25-50 °C, (iii) 50-80 °C, (iii) 80-165 °C, and (iv) 165 °C and above. The change in conformation can be classified into the following ranges: (i) 25 °C, (ii) 25-50 °C, (iii) 50-80 °C, (iii) 80-100 °C, (iv) 100135 °C, (v) 135-165 °C, and (vi) 165 °C and above. In addition, variations in thermal mobility can be classified into the following ranges: (i) 25 °C, (ii) 25-50 °C, (iii) 50-135 °C, and (iv) 135 °C and above. For the behavior of the MS18 hydrocarbon chain only, their linkages can be well-recognized. With reference to the above categorization, there seems to be a relationship among the MS18 aggregation state, degree of MS18 intramolecular charge transfer, and behavior of the MS18 hydrocarbon chain as well. The changes in the packing, orientation, conformation, and thermal mobility of the C20-d hydrocarbon chain can be classified in the following four ranges: (i) 25 °C, (ii) 25-80 °C, (iii) 80-105 °C, and (iv) 105-200 °C. For the difference in the packing of C20-d between MS18-C20-d binary and pure C20-d systems at 25 °C discussed in section 3.4, it can be interpreted that the C20-d molecules in the binary system are influenced by the existence of the MS18 molecules. However, the MS18 and C20-d molecules are phase-separated from each other according to our AFM images discussed in section 3.1. In addition, the temperature-dependent variations in the heights, wavenumber shifts, and fwhm values of C20-d in the binary system in Figure 11a-c are similar to those in the pure C20-d system in our preliminary experimental result. Consequently, the temperaturedependent changes in C20-d seem to be independent of those of MS18 in the binary system. If the MS18 and C20-d molecules were greatly influenced by each other, the temperature-dependent behavior of C20-d in the binary system would deviate considerably from that of the pure C20-d system. On the basis of the above discussion, it is indicated that the thermally induced variations in the MS18 aggregation state are determined by the linkage between the MS18 intramolecular charge transfer and the behavior of the MS18 hydrocarbon chain. Figure 12 shows a schematic representation of individual structural changes in MS18 aggregates in the mixed LB film of the MS18-C20-d binary system in the temperature-programmed process of annealing. 3.6. Origin of Dissociation of J-Aggregate in the Range from 25 to 110 °C. From 25 to 110 °C, the J-aggregate dissociates gradually, as shown in Figure 2, and the domain size of aggregates diminishes by degrees, as implied from the results in Figure 3c. Besides the results, the degree of MS18 intramolecular charge transfer induced for the J-aggregation decreases in the range from 25 to 110 °C, respectively, as shown in Figures 5 and 6. In fact, in Figure 6, the wavenumber positions at 100 °C, indicating the degree of the MS18 intramolecular charge transfer, fairly
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Figure 12. Schematic representation of individual structural changes in MS18 aggregates in the mixed LB film of the MS18-C20-d binary system in the temperature-programmed process of annealing.
approach those of the MS18 nonaggregation state in Table 1, except for the 1490 cm-1 band. For the MS18 hydrocarbon chain, its thermal mobility increases in Figure 8c, and then its long axis approaches to parallel to the film surface in Figure 8a from 25 to 110 °C. Furthermore, there is the tendency of a slight increase in the free keto group of MS18 from 25 to 110 °C in Figure 4. With the results of the above structural changes taken into consideration, the dissociation of the J-aggregates is mainly ascribed to the upturn in thermal mobility of the MS18 hydrocarbon chain and the disconnection of the Cd2+ ion chelated to the MS18 keto group. 3.7. Occurrence and Assignment of the Blue-Shifted Band with Absorption Maximum near 515 nm Observed in the Range from 110 to 160 °C. From 110 to 160 °C, the blueshifted peak appears at around 515 nm in Figures 2 and 3b. The 515 nm band is less blue-shifted than the H-band14,28-38 at 505 nm. Then, the slight increase in the peak height is observed from 110 to 120 °C in Figures 2 and 3a, and the shape of the 515 nm band is fairly broad compared to that of the H-band.14,28-38 In addition, the decrease in the domain size from the J-aggregate with the size of several hundred nanometers is implied from the results in Figure 3c. Moreover, the temporary increase in the degree of MS18 intramolecular charge transfer is observed for the formation of the 515 nm band in Figures 5 and 6. Therefore, the 515 nm band is probably caused by oligomeric aggregation with side-by-side alignment of the MS18 transition dipole moments. If the 515 nm band were to be the MS18-MS18 dimeric or MS18-C20 monomeric band, no increase in the peak height, no blue shift, and no transient increment of the MS18 intramolecular charge transfer would be observed at 120 °C. As shown in Figures 5 and 6, the result of the transitory increment of the MS18 intramolecular charge transfer, with its maximum at approximately 120 °C, is of great interest, since the degree of MS18 intramolecular charge transfer for the H-aggregate in the MS18-C20-AL18 ternary systems at 25 °C is quite close to that for the MS18 nonaggregation states.35 The reason the degree of MS18 intramolecular charge transfer is temporarily induced in the 515 nm band can be tentatively comprehended as follows. In the range from 110 to 160 °C, the MS18 chromophores in the blue-shifted aggregate should be in the side-by-side stacking alignment. Then, the thermal mobility of the long axis of the MS18 hydrocarbon chain remarkably increases in comparison with that at 25 °C. In this case, the formation of blue-shifted
aggregates with increasing thermal mobility should be fairly unstable in correlation of the MS18 geometric structures with their formation energy. Consequently, the temporary increase in the MS18 intramolecular charge transfer probably induces stabilization of the formation energy of the blue-shifted oligomeric aggregates with the 515 nm band. For the formation of the 515 nm band, if it is possible, in a desired manner, to gradually ease the regulation of the slippage of the MS18 chromophore by the MS18 hydrocarbon chain using the increase in thermal mobility from the MS18 J-aggregate, we expected that the change from J-aggregate to blue-shifted aggregates will directly occur. However, a broad band is actually formed at 100 °C in Figure 2. Therefore, the driving force which induces the 515 nm band is currently not clear. Further studies as to whether similar blue-shifted bands appear in the case not only of the MS18-C20 binary but also of MS-C20-AL18 ternary systems and discussion to clarify the origin of the appearance of the blue-shifted band will be needed. In addition, the point regarding whether the 515 nm band is kept in the descending process from 120 to 25 °C is hereafter a significant subject as well. Through the present annealing process, the possibility of control of the MS18 aggregation states from red shift to blue shift has been found by the appearance of the thermally induced blueshifted bands near 515 nm from 110 to 160 °C.
4. Conclusion In the present study, the thermal behavior of J-aggregates in mixed LB films of MS18 and C20-d has been investigated by UV-visible and infrared absorption spectroscopy. In this study, the correlation among the MS18 aggregation states, the MS18 intramolecular charge transfer, and the packing, orientation, conformation, and thermal mobility of the hydrocarbon chains of MS18 and C20-d has been examined. It has been found that there is a close correlation between the aggregation states and the degree of the intramolecular charge transfer of MS18. In addition, there is a relationship among the MS18 aggregation state, the MS18 intramolecular charge transfer, and the behavior of the packing, orientation, conformation, and thermal mobility of the MS18 hydrocarbon chains. On the other hand, it has been suggested that the structural changes in the packing, orientation, conformation, and thermal mobility of the C20-d hydrocarbon
Thermal BehaVior of J-Aggregates in LB Films
chain are independent of the above MS18 behavior. All the results have revealed that the temperature-dependent changes in the MS18 aggregation state are determined by the linkage between the MS18 intramolecular charge transfer and the packing, orientation, conformation, and thermal mobility of the MS18 hydrocarbon chain. Second, the origin of the dissociation of the J-aggregate and the occurrence and assignment of thermally induced blue-shifted aggregates have been discussed. The J-aggregate observed at 25 °C dissociates in the temperature range from 25 to 110 °C. This is mainly attributed to the increase in thermal mobility of the MS18 hydrocarbon chains and the dissociation from the MS18 keto group chelated by the Cd2+ ion to the free keto group. From 110 to 160 °C, it has been found that the thermally induced blue-shifted band appears near 515 nm. This band has been
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ascribed to oligomeric aggregation with side-by-side alignment of the MS18 transition dipole moments. Therefore, the appearance of the 515 nm band induced by the annealing method adopted in the present study has indicated the possibility of control of the MS18 aggregation states from red shift to blue shift. Acknowledgment. The authors are grateful to Dr. Sigeaki Morita and Dr. Akifumi Ikehata of Kwansei Gakuin University for their valuable comments in the present study. This study was supported by “Open Research Center” project (Research Center for Near Infrared Spectroscopy) for private universities: matching fund subsidy from MEXT (Ministry of Education, Culture, Sport, Science and Technology), 2006-2008. This study was also supported by Hyogo Science and Technology Association. LA700254P
