- Email: [email protected]
Dye aggregates formed in Langmuir–Blodgett films of amphiphilic merocyanine dyes
Current Applied Physics 6 (2006) 813–819 www.elsevier.com/locate/cap www.kps.or.kr
Dye aggregates formed in Langmuir–Blodgett films of amphiphilic merocyanine dyes Keiichi Ikegami
*
Nanotechnology Research Institute, National Institute of Advanced Industrial Science and Technology (AIST), AIST Tsukuba Central-2, 1-1-1 Umezono, Tsukuba 305-8568, Ibaraki, Japan Received 14 June 2004; received in revised form 24 September 2004 Available online 26 May 2005
Abstract The electronic absorption spectra of pure Langmuir (L) films of 6MeDS prepared upon MgCl2, CaCl2, CdCl2, and CuCl2 aqueous solutions have shown that the Mg2+, Ca2+, and Cd2+ cations promote the J-aggregation of the dye, but the Cu2+ cation inhibits it. Instead, the Cu2+ cation promotes the formation of another type of dye aggregate (D-aggregate), which exhibits a split electronic absorption band. These L films have been transferred onto solid substrates by the Langmuir–Blodgett (LB) technique. The vibronic absorption spectra observed for the obtained LB films have shown the similarity between the metal–cation-containing J-aggregate and the metal-free J-aggregate of the dye, the latter of which was characterized in the previous study [K. Ikegami, J. Chem. Phys. 121 (2004) 2337]. These spectra have also indicated that the bimolecular metal chelation plays an important role in the J-aggregation, like the intermolecular hydrogen bonding in the metal-free case. 2005 Elsevier B.V. All rights reserved. PACS: 82.30.Nr; 68.18.g; 78.67.p Keywords: J-aggregates; Langmuir–Blodgett films; Self-assemble; Molecular nanostructure; Merocyanine dyes; Infrared spectroscopy
1. Introduction Dye aggregates are self-assembled nanostructures that can exhibit various functions such as sensitization, photoelectric conversion, and information storage. One type so called J-aggregate [1] is most intensively studied among dye aggregates because of their fascinating optical properties: a red-shifted and narrowed absorption band, enhanced photoluminescence, and a very small Stokes shift. It was known that an amphiphilic merocyanine dye, 3-carboxy methyl-5-[2-(3-octadecyl-6-methyl-2(3H)benzo thia zolylidene) ethylidene]-2-thioxo-4-thia zolidinone, abbreviated as 6MeDS in this paper (Fig. 1, inset),
*
Corresponding author. Tel.: +81 298 61 5545; fax: +81 298 61 5400. E-mail address: [email protected]
1567-1739/$ - see front matter 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.cap.2005.04.046
and its relatives abbreviated as DS (no methyl substitution in the benzothiazolylidene group) and DSe (S in the benzothiazolylidene group of DS is substituted to Se) form J-aggregates in their Langmuir (L) films prepared upon appropriate aqueous solutions of metal cations. These J-aggregates have interested researchers and lots of works have been reported [2–31], partly because the Langmuir–Blodgett (LB) films [32] made of them can be used for prototypes of optoelectronic and storage devices, and partly because their formation mechanism is considered to be exotic. Besides, formation of J-aggregates in pure L films of 6MeDS upon pure water subphase was recently found and the generated metal-free J-aggregates were spectroscopically studied [31]. Electronic absorption band of 6MeDS observed for its dilute chloroform solution has a maximum at mchl = 18,800 cm1 and a shoulder around 20,200 cm1
814
K. Ikegami / Current Applied Physics 6 (2006) 813–819
Fig. 1. Curves (a)–(c) are the electronic absorption spectra observed for solutions of 6MeDS: (a) chloroform solution with the concentration of 8 · 107 mol/l; (b) 4 · 104 mol/l chloroform solution; (c) 4 · 107 mol/l methanol solution. Curves (d) and (e) are, respectively, the spectra of the J- and D-aggregates in metal-free LS films of 6MeDS obtained by using the spectrum separation procedure. Inset: chemical structure of 6MeDS. Three oxygen atoms are labeled by subscripts (A, B, and C) for clarity.
(Fig. 1(a)). This shoulder is assignable to a vibronic structure because its energy difference from the maximum well corresponds to the vibronic frequency of the central conjugated system of 6MeDS. Change in the profile of this band is small when the concentration of 6MeDS is increased (Fig. 1(b)) or the solvent is changed (Fig. 1(c)). In contrast, a drastic change in the spectral profile is seen when a chloroform solution is spread on to a pure-water subphase and an L film of 6MeDS is formed, i.e., three absorption maxima are observed at mJ(H) = 16,640, mx(H) = 19,700, and my(H) = 18,400 cm1 [31]. The time variation of the absorption spectrum shows an isosbestic point and then indicates that the L film consists of two components: one with its absorption maximum at mJ and the other with the maxima at mx and my (this doublet band is abbreviated as ‘‘D-band’’). Judging from the large red shift and the small full width at half maximum (FWHM, 270 cm1) of the band at mJ, it was concluded that this band is a J-band and therefore that one of the components of the L film can be considered as a type of J-aggregate. The J-aggregate and the other component (abbreviated as ‘‘D-aggregate’’) can be transferred on to solid substrates as their mixtures by the Langmuir– Schaefer (LS) method [32]. With a spectral separation
procedure based on the least squares method [31], the electronic absorption band of the J- and D-aggregates in the LS films can be obtained, as shown in Fig. 1(d) and (e), respectively. Although disorders introduced into the system during the deposition process broaden the J-band, its maximum is still located at 16,640 cm1. FT-IR characterization of the metal-free LS films was carried out [31]. IR spectra of the J- and D-aggregates were obtained by applying the spectral separation procedure to the experimentally observed spectra and were compared to each other, to the solution spectrum, and to the ab-initio frequency calculation of this dye [30]. These comparisons provided information about the microscopic environment around the keto C@OA and carboxyl C@OB groups of 6MeDS and the electronic state of its chromophore. Models of molecular arrangements in the J- and D-aggregates were deduced from the geometrical consideration based on the optimized molecular shape and the simulation of the mJ(H), mx(H), and my(H) frequencies taking into account the interaction between transition dipoles. These results suggested importance of intermolecular hydrogen bonds and that of water substrate in the formation of the head-to-tail molecular arrangement, i.e., J-aggregation [31]. The findings in the previous study [31], however, posed new questions. One of them is: whether the presence of metal cations is always more favorable for the Jaggregation than their absence? And if not, does the dye forms metal–cation-containing D-aggregates? It has been well established that some species of metal cations such as Mg2+ promote the J-aggregation of 6MeDS and its relatives [7,11,12,18–21]. As for DS and DSe, other authors reported that some species of metal cations such as Cu2+ does not promote the J-aggregation [18–21]. However, it is not known whether the 12 Cu2þ –ðCOO Þ salt formation disturbs the J-aggregation because the metal-free J-aggregate has not been obtained in pure DS and DSe films. To what extent the molecular arrangement and electronic state of the metal–cationcontaining J-aggregates are similar to those of the metal-free J-aggregate is another question to be answered. Furthermore, when hydrogen bonding is inhibited by the salt formation, what plays as a substitute for the hydrogen bonding? This work is devoted to answer these questions by spectroscopically characterizing Mg2+-, Ca2+-, Cd2+-, and Cu2+-containing L and LB films of 6MeDS.
2. Experimental 6MeDS was purchased from the Japanese Research Institute for Photosensitizing Dyes, Co. and used without further purification. A solution of the dye with the concentration of 5 · 104 mol/l was made using
K. Ikegami / Current Applied Physics 6 (2006) 813–819
815
spectrograde chloroform as the solvent. This solution was spread on aqueous subphases contained in a Lauda Filmwaage trough under air at 20 C. Solutions of MgCl2, CaCl2, CdCl2, and CuCl2 with the concentration of 4 · 104 mol/l were used as the subphases after being added a small amount of KHCO3 to make their pH values 6–7 (concentration of KHCO3 was 3 · 106, 5 · 106, 5 · 105, and 5 · 105 mol/l, respectively, in the MgCl2, CaCl2, CdCl2, and CuCl2 solutions). Pure water with a resistivity greater than 1.8 · 107 X cm was prepared by using a Millipore Milli-Q system. In situ absorption spectra (visible range) of the L films were recorded by a WRM-10TP polychrometer of Jasco Co. Ltd. For the absorption measurement, the initial molecular area of the L films was 1.19 nm2. A relatively rapid compression (ca. 5 · 103 nm2/s) was applied to the L films in order to minimize the change in the water level and possible decomposition of the dye, which are disturbance for the optical measurements. In order to perform IR transmission measurements, L films under p = 25 mN/m were transferred onto plates of CaF2 precoated by three monolayers of deuteride cadmium arachidate. The Langmuir–Blodgett (LB) technique [32] (the conventional vertical dipping-raising method) was employed as the deposition method. Visible and IR absorption spectra of the obtained LB films were recorded with Perkin–Elmer Lambda-900 and System-2000 spectrometers, respectively, by using a precoated CaF2 plate as reference.
3. Results and discussion 3.1. L films of 6MeDS with divalent metal cations L films of 6MeDS with divalent metal cations were prepared by spreading its chloroform solution onto Mg2+–, Ca2+–, Cd2+–, and Cu2+–containing subphases. Electronic absorption spectra of these L films have been observed every 20 s, while the films have been compressed to a condensed state showing the surface pressure of F = 25 mN/m and then kept under that surface pressure. Spectra observed 60 s after spreading (with the molecular area of A = 1.19 nm2) and those 240 s after spreading (under F = 25 mN/m) are shown in Fig. 2. The F–A curves in these experiments are indicated in the inset of Fig. 2. Distinct absorption bands around mJ(M) = 16,560 cm1 have been observed for the Mg2+–, Ca2+–, and Cd2+–containing L films (Fig. 2(a)–(f)). Judging from the large red shifts from mchl of these bands (ca. 2240 cm1), their small FWHM (ca. 320 cm1), and their similarity to the reported J-bands of DS and DSe, it is concluded that these are J-bands. The shoulders observed around 18,000 cm1 can be assigned to the vibronic structure as in the case of the solution spec-
Fig. 2. Electronic absorption spectra observed for L films of 6MeDS prepared upon subphases containing: (a), (b) Mg2+; (c), (d) Ca2+; (e), (f) Cd2+, and (g), (h) Cu2+ cations observed 60 ((b), (d), (f), and (h)) or 240 s ((a), (c), (e), and (g)) after spreading the solution of the material. The angle of incidence of the probing light is 0. Inset: the surface pressure-molecular area curves of the L films recorded during the optical measurements.
tra (Fig. 1). The motional narrowing effect may drastically decrease the intensity of this vibronic structure. The absorption frequency and FWHM of the J-bands observed for the metal-containing L films are slightly different from those of the J-band observed for the metal-free L film, but the differences are insignificant. In addition, like in the metal-free case, the spectral profiles of the J-bands observed for the metal–cation-containing L films are hardly affected by the change in F, as long as F 6 25 mN/m. This fact is consistent with the relatively large elastic moduli observed for these films (Fig. 2, inset). The observed maximum absorbance of the J-bands were 0.13, 0.10, 0.07, respectively, for the Mg2+–, Ca2+–, and Cd2+–containing L films under 25 mN/m, and are not largely different from that for the metal-free L film, 0.09. In contrast to the afore described cases, a doublet band (referred to as ‘‘D-band’’ in this paper) with the
816
K. Ikegami / Current Applied Physics 6 (2006) 813–819
maxima at mx(Cu) = 19470 and my(Cu) = 18250 cm1 has been detected for the Cu2+–containing L films under very low F (Fig. 2(h)). This finding indicates that the presence of metal cations is not always more favorable for the J-aggregation of the present dye than their absence. Formation of coordination bonds due to d-electrons of transition metals may prevent the Jaggregation under certain conditions. Moreover, the observed spectrum demonstrates the formation of a metal-containing D-aggregate of the dye. In fact, mx(Cu) and my(Cu) are close to mx(H) and my(H), respectively, and the spectral line shape of the D-bands in the Cu2+–containing and metal-free cases, shown in Figs. 2(h) and 1(e), respectively, are close to each other. The D-band observed for the Cu2+–containing L films shifts toward the lower-energy side under a relatively high F of 25 mN/m (Fig. 2(g)). At the same time, the split between the two maxima is reduced. This behavior is consistent with the small elastic modulus observed for these films (Fig. 2, inset), suggesting surface pressure-induced deformation of the D-aggregate. The maximum absorbance of the D-band under F = 25 mN/m is ca. 0.02. This value is much smaller than the maximum absorbance of the J-bands and consistent with the larger total line width of the D-band. The fact that the D-aggregate shows smaller A than the J-aggregates (Fig. 2, inset) indicates that the surface density of the dye chromophore is higher in the former than in the latter, but may simultaneously imply that the angle between the water surface and the long axis of the chromophore is larger in the former than in the latter. The small elastic modulus may reflect a continuous increase in this angle during the compression. Therefore, the smaller optical density and smaller A are not always contradictory to each other. 3.2. LB films of 6MeDS with divalent metal cations To perform more detailed study upon the metal–cation-containing J- and D-aggregates, the Mg2+-, Ca2+-, Cd2+-, and Cu2+-containing L films of 6MeDS have been transferred onto solid substrates. The Langmuir– Blodgett method has been found to be applicable to these pure L films, although the quality of the resultant LB films is inferior to that of mixed LB films of 6MeDS and fatty acids and the transfer ratio is not always close to unity. Absence of matrix molecules such as fatty acids is crucial because such molecules give their own vibronic absorption, and because the possibility of dye-matrix complex formation should be taken into account [11,12]. On the contrary, the quality of the LB films and the transfer ratio are secondary in importance to the present work. The electronic absorption spectra observed for the metal–cation-containing LB films are displayed in Fig. 3.
Fig. 3. Electronic absorption spectra observed for LB films of 6MeDS with: (a) Mg2+; (b) Ca2+; (c) Cd2+ and (d) Cu2+ cations. The angle of incidence of the probing light is 45 and the electric field lies in the film plane.
The J-bands observed for the Mg2+-, Ca2+-, and Cd2+-containing LB films are located around 16,460 cm1 (Fig. 3(a)–(c)). This frequency is close to mJ(M), suggesting that the J-aggregates formed in the L films and those transferred onto the solid substrates are approximately isostructural. Although the deposition process increases the absorption in the 17,000– 23,000 cm1 region maybe through the destruction of the J-aggregates and formation of the D-aggregates, it can be concluded that the major compositions of the Mg2+-, Ca2+-, and Cd2+-containing LB films are the Jaggregates, respectively. On the other hand, in the Cu2+-containing LB films the D-band shifts toward the lower-energy side (Fig. 3(d)), indicating deformation of the D-aggregate due to the deposition process. In comparison with the L-film under low F, the degree of the band shift is 1400 (for mx)– 1100 cm1 (for my). It is obvious that more extensive study is necessary for elucidating the nature of this deformation of the D-aggregate, but such a study is beyond the scope of the present work. Anyway, the doublet feature of the line shape is kept in the spectrum observed for the LB films and therefore the destruction of the D-aggregate is denied. Based on the above discussion, both of the J- and D- aggregates can be studied by characterizing the metal–cation-containing LB films. Therefore, their IR absorption spectra have been recorded and are displayed in Fig. 4. In Fig. 4, the IR absorption spectra
K. Ikegami / Current Applied Physics 6 (2006) 813–819
817
Fig. 4. Curves (a), (b), (c), and (e) are vibronic absorption spectra in the range of 2960–2820 and 1800–1100 cm1 observed for LB films of 6MeDS with Mg2+, Ca2+, Cd2+, and Cu2+ cations, respectively. The angle of incidence of the probing light is 45 and the electric field lies in the film plane. Curves (d) and (f) are, respectively, the spectra of the J- and D-aggregates in metal-free LS films of 6MeDS obtained by using the spectrum separation procedure. Curve (g) is the spectrum observed for a chloroform solution of 6MeDS with the concentration of 4 · 104 mol/l.
of the metal-free J- and D-aggregates and that of a chloroform solution are also displayed for comparison. Two significant bands are seen in the C@O stretching region in the solution spectrum (Fig. 4(g), right-hand side). From the comparison with the ab initio calculation [30], the band around 1740 cm1 is assignable to hydrogen bonded carboxylic C@OB [33], and that around 1680 cm1 is assignable to free keto C@OA. The bands originating from the chromophore are seen in the 1600–1100 cm1 region. Among them, the bands at 1564 and 1516 cm1 are mainly due to C@C stretching modes and that at 1184 cm1 is mainly due to the C@S stretching mode. The band at 1382 cm1 is assignable to C–N stretching. The nature of the band at 1309 cm1 is complicated, but anyway this band is due to the central C@CAC@C chain. The band at 1485 cm1 is due to CH2 scissoring and not relevant with the electronic state of the chromophore. In the metal-free LB case, the most important difference between the IR absorption spectra of the J- and Daggregates (Fig. 4(d) and (f), respectively) was seen around 1680 cm1: a prominent signal due to the free keto C@OA stretching mode is seen for the D-aggregate, but not for the J-aggregate at all, suggesting the participation of the keto group to intermolecular hydrogen bonding in the latter case [31]. The counterpart of this intermolecular hydrogen bonding should be the carbox-
ylic group, which was suggested by the difference in the IR spectra of the J- and D-aggregates around 1740 cm1. Comparing the IR spectra of the Mg2+- (Ca2+-, Cd2+)-containing LB films (Fig. 4(a)–(c)) and that of Cu2+-containing LB films (Fig. 4(e)) with each other, on one hand, we recognize that free keto C@OA stretching band is seen for the D-aggregate, but not for the J-aggregate, again. (The weak absorption around 1680 cm1 observed for the Mg2+- (Ca2+-, Cd2+-) containing LB films is due to the minority composition, which is the product of the partial destruction of the J-aggregate in the LB deposition process). On the other hand, the presence of the metal cations largely decreases the absorption due to the carboxylic C@OB stretching in the both films: in the metal–cation-containing films the carboxylic group should be ionized and then hydrogen bonding is not expected. (The broad absorption in the 1640–1580 cm1 range seen in the metal–cationcontaining films can be assigned to the antisymmetric stretching mode of COO). Therefore, the absence of the free keto signal in the Mg2+- (Ca2+-, Cd2+)-containing films suggests the formation of bimolecular metal chelation structure in the J-aggregate formed in these films. Broadening of several IR absorption bands in the Daggregate was another important finding concerning the
818
K. Ikegami / Current Applied Physics 6 (2006) 813–819
metal-free LB films of 6MeDS [31]. For the Cu2+-containing D-aggregate, the bands seen around 1505 and 1180 cm1 are broadened in comparison with the corresponding bands observed for the solution. This observation and the fact that the D-band is a doublet indicate that there are two types of molecular orientation in the D-aggregate, as in the metal-free case [31]. In the metal-free case, an enhancement of intramolecular charge transfer due to the J-aggregation was suggested from the red shifts of the IR absorption bands assigned to the stretching modes of the chromophore [31]. However, such enhancement of intramolecular charge transfer can be induced by other reasons. Actually, the bands observed around 1550, 1505, 1380, and 1180 cm1 for the Cu2+-containing D-aggregate are clearly red-shifted in comparison with the corresponding bands observed for the solution, indicating that the salt formation enhances the intramolecular charge transfer even without the J-aggregation. In other words, one of the hypotheses of Kawaguchi and Iwata [18–20] is denied: the enhancement of the intramolecular charge transfer due to the salt formation may not be the cause of the J-aggregation. The degree of the red shifts of the band around 1505 cm1 is much larger in the metal-free and Mg2+(Ca2+-, Cd2+-) containing J-aggregates than in the Cu2+-containing D-aggregate. This implies that the enhancement of the intramolecular charge-transfer due to the J-aggregation is larger than that due to the salt formation. In addition, the similarity of the IR spectra observed for the metal-free and Mg2+- (Ca2+-, Cd2+)containing J-aggregates in the 1570–1100 cm1 range reflects the similarity between the molecular arrangements in these aggregates and also the similarity between the electronic states of these aggregates. Consequently, it can be concluded that the bimolecular metal chelation in the metal–cation-containing J-aggregate of 6MeDS well plays a substitute for the intermolecular hydrogen bonding in the metal-free J-aggregate. The IR characterization of the metal-free LS films suggested that difference between the structures of the J- and D- aggregates causes a difference in the packing manner of alkyl chains [31]. This hypothesis can be examined by the comparison of the absorption bands due to the CH2 stretching modes observed for the Mg2+-, Ca2+-, Cd2+, and Cu2+-containing LB films (Fig. 4, left-hand side). Although the difference in the frequency of the symmetric mode is insignificant, it is clearly recognized that the frequency of the antisymmetric mode in the Mg2+- (Ca2+-, Cd2+)-containing LB films is lower than that in the Cu2+-containing LB films. Moreover, both the symmetric and antisymmetric bands are narrower in the Mg2+- (Ca2+-, Cd2+) containing LB films than in the Cu2+-containing LB films. These observations are parallel with those observed for the J- and D-aggregates in the metal-free films, and consequently
support the hypothesis that the alkyl chains are more tightly packed in the J-aggregates than in the Daggregates.
4. Conclusion Optical characterization of the Mg2+-, Ca2+-, Cd2+, and Cu2+-containing L and LB films of 6MeDS has provided the answers to the questions that were newly posed by the previous study upon the metal-free Jand D-aggregates of the dye. Firstly, some species of metal cations such as Cu2+ prevent the formation of the J-aggregate of the dye and, rather, promote the formation of the D-aggregate. Secondly, the molecular arrangement and electronic state of the metal–cationcontaining J-aggregate is quite similar to those of the metal-free J-aggregate. This fact may reflect that the bimolecular metal chelation in the metal–cation-containing aggregate well plays a substitute for the intermolecular hydrogen bonding in the metal-free J-aggregate.
References [1] T. Kobayashi (Ed.), J-Aggregates, Word Scientific, Singapore, 1996. [2] M. Sugi, S. Iizima, Thin Solid Films 68 (1980) 199. [3] M. Sugi, T. Fukui, S. Iizima, Mol. Cryst. Liq. Cryst. 62 (1980) 165. [4] T. Fukui, M. Saito, M. Sugi, S. Iizima, Thin Solid Films 109 (1983) 247. [5] K. Sakai, M. Saito, M. Sugi, S. Iizima, Jpn. J. Appl. Phys. 24 (1985) 865. [6] M. Sugi, M. Saito, T. Fukui, S. Iizima, Thin Solid Films 129 (1985) 15. [7] K. Iriyama, F. Mizutani, M. Yoshiura, Chem. Lett. 1980 (1980) 1399. [8] Y. Fujimoto, Y. Ozaki, M. Takayanagi, M. Nakata, K. Iriyama, J. Chem. Soc. Faraday Trans. 92 (1996) 413. [9] Y. Fujimoto, Y. Ozaki, K. Iriyama, J. Chem. Soc. Faraday Trans. 92 (1996) 419. [10] S. Imazeki, M. Takeda, Y. Tomioka, A. Kakuta, A. Mukoh, T. Narahara, Thin Solid Films 134 (1985) 27. [11] H. Nakahara, D. Mo¨bius, J. Colloid, Surf. Sci. 114 (1986) 363. [12] H. Nakahara, K. Fukuda, D. Mo¨bius, H. Kuhn, J. Phys. Chem. 90 (1986) 614. [13] S. Kuroda, M. Sugi, S. Iizima, Thin solid Films 99 (1983) 21. [14] S. Kuroda, M. Sugi, S. Iizima, Thin solid Films 133 (1985) 189. [15] S. Kuroda, K. Ikegami, M. Sugi, S. Iizima, Solid State Commun. 58 (1986) 493. [16] S. Kuroda, K. Ikegami, K. Saito, M. Saito, M. Sugi, J. Phys. Soc. Jpn. 56 (1987) 3319. [17] S. Kuroda, K. Ikegami, Y. Tabe, K. Saito, M. Saito, M. Sugi, Phys. Rev. B 43 (1991) 2531. [18] T. Kawaguchi, K. Iwata, Thin Solid Films 165 (1988) 323. [19] T. Kawaguchi, K. Iwata, Thin Solid Films 180 (1989) 235. [20] T. Kawaguchi, K. Iwata, Thin Solid Films 191 (1990) 173. [21] M. Yoneyama, T. Nagao, T. Murayama, Chem. Lett. 1989 (1989) 397. [22] K. Murata, H. Shin, S. Kuroda, Mol. Cryst. Liq. Cryst. 294 (1997) 113.
K. Ikegami / Current Applied Physics 6 (2006) 813–819 [23] K. Murata, H. Shin, K. Saito, S. Kuroda, Thin Solid Films 327 (1998) 446. [24] K. Ikegami, C. Mingotaud, M. Lan, J. Phys. Chem. B 103 (1999) 11261. [25] M. Lan, K. Ikegami, Thin Solid Films 384 (2001) 120. [26] K. Ikegami, Jpn. J. Appl. Phys. 41 (2002) 5444. [27] M. Lan, K. Ikegami, Synth. Met. 137 (2003) 977. [28] K. Ikegami, Trans. Mater. Res. Soc. Jpn. 28 (2003) 31. [29] K. Ikegami, S. Kuroda, Chem. Phys. 295 (2003) 205. [30] K. Ikegami, Colloids Surf. A 257–258 (2005) 143. [31] K. Ikegami, J. Chem. Phys. 121 (2004) 2337.
819
[32] A. Ulman, An Introduction to Ultrathin Organic Films, Academic Press, Boston, 1991. [33] This fact may suggest formation of dimers of 6MeDS in its concentrated chloroform solution. In solutions the dimer is surrounded by the solvent molecules and then the interaction between the dimers can be neglected. Therefore, the dimer structure must minimize the repulsion between the static dipoles of the two 6MeDS under the restriction that the molecules are interconnected via COOH@HOOC double hydrogen bonds. With such a dimer structure, interaction between two transition dipoles would be small.
Dye aggregates formed in Langmuir–Blodgett films of amphiphilic merocyanine dyes Keiichi Ikegami
*
Nanotechnology Research Institute, National Institute of Advanced Industrial Science and Technology (AIST), AIST Tsukuba Central-2, 1-1-1 Umezono, Tsukuba 305-8568, Ibaraki, Japan Received 14 June 2004; received in revised form 24 September 2004 Available online 26 May 2005
Abstract The electronic absorption spectra of pure Langmuir (L) films of 6MeDS prepared upon MgCl2, CaCl2, CdCl2, and CuCl2 aqueous solutions have shown that the Mg2+, Ca2+, and Cd2+ cations promote the J-aggregation of the dye, but the Cu2+ cation inhibits it. Instead, the Cu2+ cation promotes the formation of another type of dye aggregate (D-aggregate), which exhibits a split electronic absorption band. These L films have been transferred onto solid substrates by the Langmuir–Blodgett (LB) technique. The vibronic absorption spectra observed for the obtained LB films have shown the similarity between the metal–cation-containing J-aggregate and the metal-free J-aggregate of the dye, the latter of which was characterized in the previous study [K. Ikegami, J. Chem. Phys. 121 (2004) 2337]. These spectra have also indicated that the bimolecular metal chelation plays an important role in the J-aggregation, like the intermolecular hydrogen bonding in the metal-free case. 2005 Elsevier B.V. All rights reserved. PACS: 82.30.Nr; 68.18.g; 78.67.p Keywords: J-aggregates; Langmuir–Blodgett films; Self-assemble; Molecular nanostructure; Merocyanine dyes; Infrared spectroscopy
1. Introduction Dye aggregates are self-assembled nanostructures that can exhibit various functions such as sensitization, photoelectric conversion, and information storage. One type so called J-aggregate [1] is most intensively studied among dye aggregates because of their fascinating optical properties: a red-shifted and narrowed absorption band, enhanced photoluminescence, and a very small Stokes shift. It was known that an amphiphilic merocyanine dye, 3-carboxy methyl-5-[2-(3-octadecyl-6-methyl-2(3H)benzo thia zolylidene) ethylidene]-2-thioxo-4-thia zolidinone, abbreviated as 6MeDS in this paper (Fig. 1, inset),
*
Corresponding author. Tel.: +81 298 61 5545; fax: +81 298 61 5400. E-mail address: [email protected]
1567-1739/$ - see front matter 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.cap.2005.04.046
and its relatives abbreviated as DS (no methyl substitution in the benzothiazolylidene group) and DSe (S in the benzothiazolylidene group of DS is substituted to Se) form J-aggregates in their Langmuir (L) films prepared upon appropriate aqueous solutions of metal cations. These J-aggregates have interested researchers and lots of works have been reported [2–31], partly because the Langmuir–Blodgett (LB) films [32] made of them can be used for prototypes of optoelectronic and storage devices, and partly because their formation mechanism is considered to be exotic. Besides, formation of J-aggregates in pure L films of 6MeDS upon pure water subphase was recently found and the generated metal-free J-aggregates were spectroscopically studied [31]. Electronic absorption band of 6MeDS observed for its dilute chloroform solution has a maximum at mchl = 18,800 cm1 and a shoulder around 20,200 cm1
814
K. Ikegami / Current Applied Physics 6 (2006) 813–819
Fig. 1. Curves (a)–(c) are the electronic absorption spectra observed for solutions of 6MeDS: (a) chloroform solution with the concentration of 8 · 107 mol/l; (b) 4 · 104 mol/l chloroform solution; (c) 4 · 107 mol/l methanol solution. Curves (d) and (e) are, respectively, the spectra of the J- and D-aggregates in metal-free LS films of 6MeDS obtained by using the spectrum separation procedure. Inset: chemical structure of 6MeDS. Three oxygen atoms are labeled by subscripts (A, B, and C) for clarity.
(Fig. 1(a)). This shoulder is assignable to a vibronic structure because its energy difference from the maximum well corresponds to the vibronic frequency of the central conjugated system of 6MeDS. Change in the profile of this band is small when the concentration of 6MeDS is increased (Fig. 1(b)) or the solvent is changed (Fig. 1(c)). In contrast, a drastic change in the spectral profile is seen when a chloroform solution is spread on to a pure-water subphase and an L film of 6MeDS is formed, i.e., three absorption maxima are observed at mJ(H) = 16,640, mx(H) = 19,700, and my(H) = 18,400 cm1 [31]. The time variation of the absorption spectrum shows an isosbestic point and then indicates that the L film consists of two components: one with its absorption maximum at mJ and the other with the maxima at mx and my (this doublet band is abbreviated as ‘‘D-band’’). Judging from the large red shift and the small full width at half maximum (FWHM, 270 cm1) of the band at mJ, it was concluded that this band is a J-band and therefore that one of the components of the L film can be considered as a type of J-aggregate. The J-aggregate and the other component (abbreviated as ‘‘D-aggregate’’) can be transferred on to solid substrates as their mixtures by the Langmuir– Schaefer (LS) method [32]. With a spectral separation
procedure based on the least squares method [31], the electronic absorption band of the J- and D-aggregates in the LS films can be obtained, as shown in Fig. 1(d) and (e), respectively. Although disorders introduced into the system during the deposition process broaden the J-band, its maximum is still located at 16,640 cm1. FT-IR characterization of the metal-free LS films was carried out [31]. IR spectra of the J- and D-aggregates were obtained by applying the spectral separation procedure to the experimentally observed spectra and were compared to each other, to the solution spectrum, and to the ab-initio frequency calculation of this dye [30]. These comparisons provided information about the microscopic environment around the keto C@OA and carboxyl C@OB groups of 6MeDS and the electronic state of its chromophore. Models of molecular arrangements in the J- and D-aggregates were deduced from the geometrical consideration based on the optimized molecular shape and the simulation of the mJ(H), mx(H), and my(H) frequencies taking into account the interaction between transition dipoles. These results suggested importance of intermolecular hydrogen bonds and that of water substrate in the formation of the head-to-tail molecular arrangement, i.e., J-aggregation [31]. The findings in the previous study [31], however, posed new questions. One of them is: whether the presence of metal cations is always more favorable for the Jaggregation than their absence? And if not, does the dye forms metal–cation-containing D-aggregates? It has been well established that some species of metal cations such as Mg2+ promote the J-aggregation of 6MeDS and its relatives [7,11,12,18–21]. As for DS and DSe, other authors reported that some species of metal cations such as Cu2+ does not promote the J-aggregation [18–21]. However, it is not known whether the 12 Cu2þ –ðCOO Þ salt formation disturbs the J-aggregation because the metal-free J-aggregate has not been obtained in pure DS and DSe films. To what extent the molecular arrangement and electronic state of the metal–cationcontaining J-aggregates are similar to those of the metal-free J-aggregate is another question to be answered. Furthermore, when hydrogen bonding is inhibited by the salt formation, what plays as a substitute for the hydrogen bonding? This work is devoted to answer these questions by spectroscopically characterizing Mg2+-, Ca2+-, Cd2+-, and Cu2+-containing L and LB films of 6MeDS.
2. Experimental 6MeDS was purchased from the Japanese Research Institute for Photosensitizing Dyes, Co. and used without further purification. A solution of the dye with the concentration of 5 · 104 mol/l was made using
K. Ikegami / Current Applied Physics 6 (2006) 813–819
815
spectrograde chloroform as the solvent. This solution was spread on aqueous subphases contained in a Lauda Filmwaage trough under air at 20 C. Solutions of MgCl2, CaCl2, CdCl2, and CuCl2 with the concentration of 4 · 104 mol/l were used as the subphases after being added a small amount of KHCO3 to make their pH values 6–7 (concentration of KHCO3 was 3 · 106, 5 · 106, 5 · 105, and 5 · 105 mol/l, respectively, in the MgCl2, CaCl2, CdCl2, and CuCl2 solutions). Pure water with a resistivity greater than 1.8 · 107 X cm was prepared by using a Millipore Milli-Q system. In situ absorption spectra (visible range) of the L films were recorded by a WRM-10TP polychrometer of Jasco Co. Ltd. For the absorption measurement, the initial molecular area of the L films was 1.19 nm2. A relatively rapid compression (ca. 5 · 103 nm2/s) was applied to the L films in order to minimize the change in the water level and possible decomposition of the dye, which are disturbance for the optical measurements. In order to perform IR transmission measurements, L films under p = 25 mN/m were transferred onto plates of CaF2 precoated by three monolayers of deuteride cadmium arachidate. The Langmuir–Blodgett (LB) technique [32] (the conventional vertical dipping-raising method) was employed as the deposition method. Visible and IR absorption spectra of the obtained LB films were recorded with Perkin–Elmer Lambda-900 and System-2000 spectrometers, respectively, by using a precoated CaF2 plate as reference.
3. Results and discussion 3.1. L films of 6MeDS with divalent metal cations L films of 6MeDS with divalent metal cations were prepared by spreading its chloroform solution onto Mg2+–, Ca2+–, Cd2+–, and Cu2+–containing subphases. Electronic absorption spectra of these L films have been observed every 20 s, while the films have been compressed to a condensed state showing the surface pressure of F = 25 mN/m and then kept under that surface pressure. Spectra observed 60 s after spreading (with the molecular area of A = 1.19 nm2) and those 240 s after spreading (under F = 25 mN/m) are shown in Fig. 2. The F–A curves in these experiments are indicated in the inset of Fig. 2. Distinct absorption bands around mJ(M) = 16,560 cm1 have been observed for the Mg2+–, Ca2+–, and Cd2+–containing L films (Fig. 2(a)–(f)). Judging from the large red shifts from mchl of these bands (ca. 2240 cm1), their small FWHM (ca. 320 cm1), and their similarity to the reported J-bands of DS and DSe, it is concluded that these are J-bands. The shoulders observed around 18,000 cm1 can be assigned to the vibronic structure as in the case of the solution spec-
Fig. 2. Electronic absorption spectra observed for L films of 6MeDS prepared upon subphases containing: (a), (b) Mg2+; (c), (d) Ca2+; (e), (f) Cd2+, and (g), (h) Cu2+ cations observed 60 ((b), (d), (f), and (h)) or 240 s ((a), (c), (e), and (g)) after spreading the solution of the material. The angle of incidence of the probing light is 0. Inset: the surface pressure-molecular area curves of the L films recorded during the optical measurements.
tra (Fig. 1). The motional narrowing effect may drastically decrease the intensity of this vibronic structure. The absorption frequency and FWHM of the J-bands observed for the metal-containing L films are slightly different from those of the J-band observed for the metal-free L film, but the differences are insignificant. In addition, like in the metal-free case, the spectral profiles of the J-bands observed for the metal–cation-containing L films are hardly affected by the change in F, as long as F 6 25 mN/m. This fact is consistent with the relatively large elastic moduli observed for these films (Fig. 2, inset). The observed maximum absorbance of the J-bands were 0.13, 0.10, 0.07, respectively, for the Mg2+–, Ca2+–, and Cd2+–containing L films under 25 mN/m, and are not largely different from that for the metal-free L film, 0.09. In contrast to the afore described cases, a doublet band (referred to as ‘‘D-band’’ in this paper) with the
816
K. Ikegami / Current Applied Physics 6 (2006) 813–819
maxima at mx(Cu) = 19470 and my(Cu) = 18250 cm1 has been detected for the Cu2+–containing L films under very low F (Fig. 2(h)). This finding indicates that the presence of metal cations is not always more favorable for the J-aggregation of the present dye than their absence. Formation of coordination bonds due to d-electrons of transition metals may prevent the Jaggregation under certain conditions. Moreover, the observed spectrum demonstrates the formation of a metal-containing D-aggregate of the dye. In fact, mx(Cu) and my(Cu) are close to mx(H) and my(H), respectively, and the spectral line shape of the D-bands in the Cu2+–containing and metal-free cases, shown in Figs. 2(h) and 1(e), respectively, are close to each other. The D-band observed for the Cu2+–containing L films shifts toward the lower-energy side under a relatively high F of 25 mN/m (Fig. 2(g)). At the same time, the split between the two maxima is reduced. This behavior is consistent with the small elastic modulus observed for these films (Fig. 2, inset), suggesting surface pressure-induced deformation of the D-aggregate. The maximum absorbance of the D-band under F = 25 mN/m is ca. 0.02. This value is much smaller than the maximum absorbance of the J-bands and consistent with the larger total line width of the D-band. The fact that the D-aggregate shows smaller A than the J-aggregates (Fig. 2, inset) indicates that the surface density of the dye chromophore is higher in the former than in the latter, but may simultaneously imply that the angle between the water surface and the long axis of the chromophore is larger in the former than in the latter. The small elastic modulus may reflect a continuous increase in this angle during the compression. Therefore, the smaller optical density and smaller A are not always contradictory to each other. 3.2. LB films of 6MeDS with divalent metal cations To perform more detailed study upon the metal–cation-containing J- and D-aggregates, the Mg2+-, Ca2+-, Cd2+-, and Cu2+-containing L films of 6MeDS have been transferred onto solid substrates. The Langmuir– Blodgett method has been found to be applicable to these pure L films, although the quality of the resultant LB films is inferior to that of mixed LB films of 6MeDS and fatty acids and the transfer ratio is not always close to unity. Absence of matrix molecules such as fatty acids is crucial because such molecules give their own vibronic absorption, and because the possibility of dye-matrix complex formation should be taken into account [11,12]. On the contrary, the quality of the LB films and the transfer ratio are secondary in importance to the present work. The electronic absorption spectra observed for the metal–cation-containing LB films are displayed in Fig. 3.
Fig. 3. Electronic absorption spectra observed for LB films of 6MeDS with: (a) Mg2+; (b) Ca2+; (c) Cd2+ and (d) Cu2+ cations. The angle of incidence of the probing light is 45 and the electric field lies in the film plane.
The J-bands observed for the Mg2+-, Ca2+-, and Cd2+-containing LB films are located around 16,460 cm1 (Fig. 3(a)–(c)). This frequency is close to mJ(M), suggesting that the J-aggregates formed in the L films and those transferred onto the solid substrates are approximately isostructural. Although the deposition process increases the absorption in the 17,000– 23,000 cm1 region maybe through the destruction of the J-aggregates and formation of the D-aggregates, it can be concluded that the major compositions of the Mg2+-, Ca2+-, and Cd2+-containing LB films are the Jaggregates, respectively. On the other hand, in the Cu2+-containing LB films the D-band shifts toward the lower-energy side (Fig. 3(d)), indicating deformation of the D-aggregate due to the deposition process. In comparison with the L-film under low F, the degree of the band shift is 1400 (for mx)– 1100 cm1 (for my). It is obvious that more extensive study is necessary for elucidating the nature of this deformation of the D-aggregate, but such a study is beyond the scope of the present work. Anyway, the doublet feature of the line shape is kept in the spectrum observed for the LB films and therefore the destruction of the D-aggregate is denied. Based on the above discussion, both of the J- and D- aggregates can be studied by characterizing the metal–cation-containing LB films. Therefore, their IR absorption spectra have been recorded and are displayed in Fig. 4. In Fig. 4, the IR absorption spectra
K. Ikegami / Current Applied Physics 6 (2006) 813–819
817
Fig. 4. Curves (a), (b), (c), and (e) are vibronic absorption spectra in the range of 2960–2820 and 1800–1100 cm1 observed for LB films of 6MeDS with Mg2+, Ca2+, Cd2+, and Cu2+ cations, respectively. The angle of incidence of the probing light is 45 and the electric field lies in the film plane. Curves (d) and (f) are, respectively, the spectra of the J- and D-aggregates in metal-free LS films of 6MeDS obtained by using the spectrum separation procedure. Curve (g) is the spectrum observed for a chloroform solution of 6MeDS with the concentration of 4 · 104 mol/l.
of the metal-free J- and D-aggregates and that of a chloroform solution are also displayed for comparison. Two significant bands are seen in the C@O stretching region in the solution spectrum (Fig. 4(g), right-hand side). From the comparison with the ab initio calculation [30], the band around 1740 cm1 is assignable to hydrogen bonded carboxylic C@OB [33], and that around 1680 cm1 is assignable to free keto C@OA. The bands originating from the chromophore are seen in the 1600–1100 cm1 region. Among them, the bands at 1564 and 1516 cm1 are mainly due to C@C stretching modes and that at 1184 cm1 is mainly due to the C@S stretching mode. The band at 1382 cm1 is assignable to C–N stretching. The nature of the band at 1309 cm1 is complicated, but anyway this band is due to the central C@CAC@C chain. The band at 1485 cm1 is due to CH2 scissoring and not relevant with the electronic state of the chromophore. In the metal-free LB case, the most important difference between the IR absorption spectra of the J- and Daggregates (Fig. 4(d) and (f), respectively) was seen around 1680 cm1: a prominent signal due to the free keto C@OA stretching mode is seen for the D-aggregate, but not for the J-aggregate at all, suggesting the participation of the keto group to intermolecular hydrogen bonding in the latter case [31]. The counterpart of this intermolecular hydrogen bonding should be the carbox-
ylic group, which was suggested by the difference in the IR spectra of the J- and D-aggregates around 1740 cm1. Comparing the IR spectra of the Mg2+- (Ca2+-, Cd2+)-containing LB films (Fig. 4(a)–(c)) and that of Cu2+-containing LB films (Fig. 4(e)) with each other, on one hand, we recognize that free keto C@OA stretching band is seen for the D-aggregate, but not for the J-aggregate, again. (The weak absorption around 1680 cm1 observed for the Mg2+- (Ca2+-, Cd2+-) containing LB films is due to the minority composition, which is the product of the partial destruction of the J-aggregate in the LB deposition process). On the other hand, the presence of the metal cations largely decreases the absorption due to the carboxylic C@OB stretching in the both films: in the metal–cation-containing films the carboxylic group should be ionized and then hydrogen bonding is not expected. (The broad absorption in the 1640–1580 cm1 range seen in the metal–cationcontaining films can be assigned to the antisymmetric stretching mode of COO). Therefore, the absence of the free keto signal in the Mg2+- (Ca2+-, Cd2+)-containing films suggests the formation of bimolecular metal chelation structure in the J-aggregate formed in these films. Broadening of several IR absorption bands in the Daggregate was another important finding concerning the
818
K. Ikegami / Current Applied Physics 6 (2006) 813–819
metal-free LB films of 6MeDS [31]. For the Cu2+-containing D-aggregate, the bands seen around 1505 and 1180 cm1 are broadened in comparison with the corresponding bands observed for the solution. This observation and the fact that the D-band is a doublet indicate that there are two types of molecular orientation in the D-aggregate, as in the metal-free case [31]. In the metal-free case, an enhancement of intramolecular charge transfer due to the J-aggregation was suggested from the red shifts of the IR absorption bands assigned to the stretching modes of the chromophore [31]. However, such enhancement of intramolecular charge transfer can be induced by other reasons. Actually, the bands observed around 1550, 1505, 1380, and 1180 cm1 for the Cu2+-containing D-aggregate are clearly red-shifted in comparison with the corresponding bands observed for the solution, indicating that the salt formation enhances the intramolecular charge transfer even without the J-aggregation. In other words, one of the hypotheses of Kawaguchi and Iwata [18–20] is denied: the enhancement of the intramolecular charge transfer due to the salt formation may not be the cause of the J-aggregation. The degree of the red shifts of the band around 1505 cm1 is much larger in the metal-free and Mg2+(Ca2+-, Cd2+-) containing J-aggregates than in the Cu2+-containing D-aggregate. This implies that the enhancement of the intramolecular charge-transfer due to the J-aggregation is larger than that due to the salt formation. In addition, the similarity of the IR spectra observed for the metal-free and Mg2+- (Ca2+-, Cd2+)containing J-aggregates in the 1570–1100 cm1 range reflects the similarity between the molecular arrangements in these aggregates and also the similarity between the electronic states of these aggregates. Consequently, it can be concluded that the bimolecular metal chelation in the metal–cation-containing J-aggregate of 6MeDS well plays a substitute for the intermolecular hydrogen bonding in the metal-free J-aggregate. The IR characterization of the metal-free LS films suggested that difference between the structures of the J- and D- aggregates causes a difference in the packing manner of alkyl chains [31]. This hypothesis can be examined by the comparison of the absorption bands due to the CH2 stretching modes observed for the Mg2+-, Ca2+-, Cd2+, and Cu2+-containing LB films (Fig. 4, left-hand side). Although the difference in the frequency of the symmetric mode is insignificant, it is clearly recognized that the frequency of the antisymmetric mode in the Mg2+- (Ca2+-, Cd2+)-containing LB films is lower than that in the Cu2+-containing LB films. Moreover, both the symmetric and antisymmetric bands are narrower in the Mg2+- (Ca2+-, Cd2+) containing LB films than in the Cu2+-containing LB films. These observations are parallel with those observed for the J- and D-aggregates in the metal-free films, and consequently
support the hypothesis that the alkyl chains are more tightly packed in the J-aggregates than in the Daggregates.
4. Conclusion Optical characterization of the Mg2+-, Ca2+-, Cd2+, and Cu2+-containing L and LB films of 6MeDS has provided the answers to the questions that were newly posed by the previous study upon the metal-free Jand D-aggregates of the dye. Firstly, some species of metal cations such as Cu2+ prevent the formation of the J-aggregate of the dye and, rather, promote the formation of the D-aggregate. Secondly, the molecular arrangement and electronic state of the metal–cationcontaining J-aggregate is quite similar to those of the metal-free J-aggregate. This fact may reflect that the bimolecular metal chelation in the metal–cation-containing aggregate well plays a substitute for the intermolecular hydrogen bonding in the metal-free J-aggregate.
References [1] T. Kobayashi (Ed.), J-Aggregates, Word Scientific, Singapore, 1996. [2] M. Sugi, S. Iizima, Thin Solid Films 68 (1980) 199. [3] M. Sugi, T. Fukui, S. Iizima, Mol. Cryst. Liq. Cryst. 62 (1980) 165. [4] T. Fukui, M. Saito, M. Sugi, S. Iizima, Thin Solid Films 109 (1983) 247. [5] K. Sakai, M. Saito, M. Sugi, S. Iizima, Jpn. J. Appl. Phys. 24 (1985) 865. [6] M. Sugi, M. Saito, T. Fukui, S. Iizima, Thin Solid Films 129 (1985) 15. [7] K. Iriyama, F. Mizutani, M. Yoshiura, Chem. Lett. 1980 (1980) 1399. [8] Y. Fujimoto, Y. Ozaki, M. Takayanagi, M. Nakata, K. Iriyama, J. Chem. Soc. Faraday Trans. 92 (1996) 413. [9] Y. Fujimoto, Y. Ozaki, K. Iriyama, J. Chem. Soc. Faraday Trans. 92 (1996) 419. [10] S. Imazeki, M. Takeda, Y. Tomioka, A. Kakuta, A. Mukoh, T. Narahara, Thin Solid Films 134 (1985) 27. [11] H. Nakahara, D. Mo¨bius, J. Colloid, Surf. Sci. 114 (1986) 363. [12] H. Nakahara, K. Fukuda, D. Mo¨bius, H. Kuhn, J. Phys. Chem. 90 (1986) 614. [13] S. Kuroda, M. Sugi, S. Iizima, Thin solid Films 99 (1983) 21. [14] S. Kuroda, M. Sugi, S. Iizima, Thin solid Films 133 (1985) 189. [15] S. Kuroda, K. Ikegami, M. Sugi, S. Iizima, Solid State Commun. 58 (1986) 493. [16] S. Kuroda, K. Ikegami, K. Saito, M. Saito, M. Sugi, J. Phys. Soc. Jpn. 56 (1987) 3319. [17] S. Kuroda, K. Ikegami, Y. Tabe, K. Saito, M. Saito, M. Sugi, Phys. Rev. B 43 (1991) 2531. [18] T. Kawaguchi, K. Iwata, Thin Solid Films 165 (1988) 323. [19] T. Kawaguchi, K. Iwata, Thin Solid Films 180 (1989) 235. [20] T. Kawaguchi, K. Iwata, Thin Solid Films 191 (1990) 173. [21] M. Yoneyama, T. Nagao, T. Murayama, Chem. Lett. 1989 (1989) 397. [22] K. Murata, H. Shin, S. Kuroda, Mol. Cryst. Liq. Cryst. 294 (1997) 113.
K. Ikegami / Current Applied Physics 6 (2006) 813–819 [23] K. Murata, H. Shin, K. Saito, S. Kuroda, Thin Solid Films 327 (1998) 446. [24] K. Ikegami, C. Mingotaud, M. Lan, J. Phys. Chem. B 103 (1999) 11261. [25] M. Lan, K. Ikegami, Thin Solid Films 384 (2001) 120. [26] K. Ikegami, Jpn. J. Appl. Phys. 41 (2002) 5444. [27] M. Lan, K. Ikegami, Synth. Met. 137 (2003) 977. [28] K. Ikegami, Trans. Mater. Res. Soc. Jpn. 28 (2003) 31. [29] K. Ikegami, S. Kuroda, Chem. Phys. 295 (2003) 205. [30] K. Ikegami, Colloids Surf. A 257–258 (2005) 143. [31] K. Ikegami, J. Chem. Phys. 121 (2004) 2337.
819
[32] A. Ulman, An Introduction to Ultrathin Organic Films, Academic Press, Boston, 1991. [33] This fact may suggest formation of dimers of 6MeDS in its concentrated chloroform solution. In solutions the dimer is surrounded by the solvent molecules and then the interaction between the dimers can be neglected. Therefore, the dimer structure must minimize the repulsion between the static dipoles of the two 6MeDS under the restriction that the molecules are interconnected via COOH@HOOC double hydrogen bonds. With such a dimer structure, interaction between two transition dipoles would be small.