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Control of formation and molecular orientation of J-aggregates in Langmuir–Blodgett films of mixed merocyanine dyes
Colloids and Surfaces A: Physicochem. Eng. Aspects 284–285 (2006) 97–102
Control of formation and molecular orientation of J-aggregates in Langmuir–Blodgett films of mixed merocyanine dyes Hisaaki Tanaka∗ , Takehide Mizutani, Shin-ichi Kuroda Department of Applied Physics, Nagoya University, Furo-cho, Chikusa-ku, Nagoya 464-8603, Japan Received 21 July 2005; received in revised form 13 October 2005; accepted 28 October 2005 Available online 20 December 2005
Abstract We have fabricated Langmuir–Blodgett (LB) films of mixed-merocyanine dyes, DS and its methyl-substituted analog (6-Me-DS), diluted with arachidic acid (AA), expressed as [6-Me-DS]1−X [DS]X [AA]2 , in order to control the J-aggregation of dye molecules. For most of X-values, the red-shifted J-band peak appeared in the optical absorption spectra with its peak position depending on the mixing ratio of the dyes. In particular, J-band anomalously disappears around X = 0.9, suggesting the dominance of monomer state. We have also characterized the in-plane preferential orientation of the dye molecules along the dipping direction in the mixed-dye films by the orientation-dependent ESR signals and polarized optical absorption spectra. The optical dichroic ratios were semiquantitatively reproduced by the angular distribution function of the molecular orientation determined by the ESR spectrum simulation. The in-plane orientation of the dye molecules was clearly observed for DS, 6-Me-DS and X = 0.2 films, whereas it was nearly random for X = 0.5 film, suggesting the possibility to control the molecular orientational order of dye molecules by employing mixed-dye system. © 2005 Elsevier B.V. All rights reserved. Keywords: Langmuir–Blodgett films; J-aggregates; Electron-spin-resonance spectroscopy; Molecular orientation
1. Introduction The Langmuir–Blodgett (LB) technique attracts much attention as a tool for arranging various kinds of molecules into the form of monolayer assemblies, which may be suitable for constructing self-assembling nanosized molecular aggregates of various functions [1]. The J-aggregates of the functional dye molecules are characterized by the sharp red-shifted band and a strong photoluminescence with a small Stokes shift [2]. These J-aggregates are indispensable as photographic sensitizers, and there exist continuous and growing interests on Jaggregates both scientifically and technologically [1–3]. As for the structure of J-aggregates, Kuhn et al. has proposed the twodimensional (2D) brick-stone arrangement of the dye molecules based on the analysis of the red-shifted peak from the monomeric peak using the extended dipole model in their pioneering work [3,4]. We have been focusing our attention on the control of the J-aggregate formation of surface-active merocyanine dye
∗
Corresponding author. Tel.: +81 52 789 5165; fax: +81 52 789 3712. E-mail address: [email protected] (H. Tanaka).
0927-7757/$ – see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.colsurfa.2005.10.057
molecules [5]. These dye molecules, shown in Fig. 1 and abbreviated as DX (X = O, S, Se), form the J-aggregates in LB films diluted with arachidic acid (AA) except for DO molecule [5–9]. In order to control the J-aggregation of the dye molecules in the LB film, two approaches have been applied as follows: (i) the aggregation size of the merocyanine dye molecule [6-MeDS in Fig. 1b] has been controlled by varying the mixing ratio of 6-Me-DS with respect to the AA matrix [5,10]. In this case, well-defined J-band has been observed in the optical absorption spectra for the mixing ratio down to [6-Me-DS]:[AA] = 1:120. Monomer bands have been hardly observed in this dilution experiment, indicating that almost all the dye molecule form the J-aggregate and (ii) mutual mixtures of merocyanine dye molecules such as [DS]1−X [DO]X or [6-Me-DS]1−X [DO]X etc. have been examined [5,11–14]. In these mixed-dye LB films, the shift of the J-band peak has been observed depending on the mixing ratio. In particular, mixture of J-forming DS, 6-Me-DS or 5-Cl-DS and non J-forming DO molecules exhibits fascinating results that the DO molecules participate to form the J-aggregate due to the formation of solid solution with the J-forming dye molecules [11–13]. Such a behavior adds a variety of the dye-mixing effect which has been observed for the cyanine dye LB films, where the mixture of the two
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Fig. 1. Chemical structures of the merocyanine-dye molecules. The definition of the molecular coordination axes is also shown. (a) DO, DS, DSe, and (b) 6-Me-DS.
J-forming cyanine dyes with different J-band positions shows continuous shift of J-band peak depending on the mixing ratio [15]. In this paper, we report the mutual mixing effect of the two merocyanine dyes, DS and 6-Me-DS, which have not been studied so far. These two molecules form J-aggregates with different J-band peak positions in the LB films. The mutual mixing effect of these dye molecules to control the J-aggregation is discussed from the dependence of the J-band peak position on the mixing ratio. Surface pressure-area (π–A) isotherm measurements were also carried out to clarify the possible formation of solid solution of these dye species. We also focus our attention on the in-plane preferential orientation of the dye molecules in the LB films, which has been found to be the origin of the observed optical dichroism in the DS LB films [5]. In order to clarify both the in-plane and outof-plane molecular orientations in the film, it has been shown to be crucial to analyze the anisotropic ESR spectra of the stable radicals based on the anisotropic ESR properties of unpaired π-electrons [5,9]. Thus, obtained direction of the pπ orbital of the DS molecule (z-direction in Fig. 1) makes the averaged angle φ0 with respect to the dipping direction of 60–65◦ , giving the angle between the long axis of the dye molecules (y-direction) and the dipping direction, α, to be approximately 25–30◦ [5,9,16]. Schematic illustration of the observed molecular orientation of DS molecules is shown in Fig. 2. This result corresponds well with the brick-stone model of the J-aggregates by Kuhn et al., where one of the possible structure is such that the angle between the direction connecting the center of molecules and the long axis of the dye molecule becomes about 30◦ [1– 3,5,9]. Based on these results, the flow-orientation effect has been proposed as the origin of the in-plane orientation [5,16– 18], which is further reinforced by the theoretical development [19]. In this context, it would be interesting if we can control the in-plane flow-orientation by mixing the dye molecules. Thus, we studied the mutual mixing effect of different dye molecules, DS and 6-Me-DS, on the molecular orientations. The microscopic molecular orientation has been probed by the anisotropy of the ESR spectra.
Fig. 2. Orientation of the stable radical in the film plane of mixed LB films of DS and AA. φ0 Denotes the angle between the pπ orbital axis (z-axis in Fig. 1) and the dipping direction of the substrate. The long axis of the merocyanine dye molecule parallel to the chromophore (y-axis) is expressed by the thick lines.
In the following, after describing the experimental details in the next section, the experimental results of π–A curves, optical absorption and ESR spectra are presented in Section 3. Based on the ESR spectrum-simulation method, in-plane preferential orientation of the dye molecule is discussed in Section 4. An approximate estimation of the optical dichroism, which is calculated from the angular distribution function obtained from the ESR simulation, is also presented in Section 4 to see the consistency of the ESR analysis of in-plane distribution. A brief summary is given in Section 5. 2. Experimental details Dye molecules and AA were purchased from the Japanese Institute for Photosensitizing Dyes, Co. and Aldrich Chemical, Co., respectively. Chloroform solutions containing 1 × 10−3 M DS and 6-Me-DS were mixed with each other with appropriate ratios to give a binary mixed system. The mixed solutions were further diluted by the chloroform solution containing 2 × 10−3 M AA molecule with a ratio of dyes to AA of [dyes]:[AA] = 1:2. Then, hereafter we represent these mixed system as [6-Me-DS]1−X [DS]X [AA]2 (0 ≤ X ≤ 1). The LB films were fabricated by a standard vertical dipping method using a conventional trough. The aqueous subphase containing CdCl2 (3 × 10−4 M) and KHCO3 (5 × 10−5 M) was kept at 20–21 ◦ C. The pH of the subphase was controlled to be 5.8–6.2. Y-type films of mixed dyes and AA were transferred either on glass or mylar substrates precoated with 3 or 5 layers of cadmium arachidate for optical and ESR measurements, respectively. ESR measurements were performed by using a Bruker EMX spectrometer at the X-band (9.4 GHz). The LB films on mylar substrates were cut into pieces with the size of 10 mm × 2 mm
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and stacked in the ESR sample tube. All the measurements were performed at room temperature. 3. Experimental results Fig. 3 shows pressure-area (π–A) isotherms for the [6-MeDS]1−X [DS]X [AA]2 monolayer system. The limiting area for each film is determined by extrapolating the relevant isotherm to the zero pressure as shown in the solid arrow in the case of (a). The limiting area Sπ=0 is expressed by the effective area per AA molecule contained in monolayer, or in other words, the contribution of dye molecules is effectively added to AA with their molar mixing ratios. Then the molecular occupation area per dye molecule, defined as SD , in the dye-AA mixed LB film with a molar ratio of 1:Y is evaluated by the relation SD = Y (Sπ=0 − SAA ), where SAA is the limiting area for the pure AA ˚ 2 in the present experiment). Thus, obtained film (SAA = 21 A ˚ 2 for DS and 6-Memolecular occupation areas were 54 and 63 A DS, respectively. These values coincide well with previously reported values with an experimental error [10]. As discussed previously, the observed areas are consistent with the edge-on structures of the molecules having the face-to-face packing. Fig. 4 shows the optical absorption spectra of the mixeddye LB films, [6-Me-DS]1−X [DS]X [AA]2 , for various mixing ratios. In the case of pure dye systems, the optical absorption spectra show distinct peaks at 591 and 608 nm for DS and 6-MeDS, respectively, red-shifted from the monomer peaks (521 and 528 nm for DS and 6-Me-DS, respectively), indicating the formation of J-aggregates. In the mixed-dye system, well-defined J-band is observed in the absorption spectra for the wide range of mixing ratio X of X = 0.1–0.8 as well as for X = 0.95. In addition, the peak position and the shape of the absorption spectrum changes depending on X as shown in Fig. 4. In particular, J-band peak anomalously disappears around X = 0.9. Instead, the spectral weight corresponding to the monomer peak grows considerably. Thus, the J-aggregation of both DS and 6Me-DS molecules are prevented in this mixing region. In such
Fig. 3. Surface-pressure-area (π-A) isotherms for the [6-Me-DS]1−X [DS]X [AA]2 system. The horizontal axis shows the effective surface area per AA molecule contained in each film. The arrow in (a) represents an extrapolation to the zero pressure.
Fig. 4. Optical absorption spectra of the mixed-dye LB films, [6-Me-DS]1−X [DS]X [AA]2 , for various mixing ratios. Arrows represent the peak positions for the monomer states of 6-Me-DS (upper) and DS (lower).
a case, above mentioned brick-stone arrangement of the dye molecules is expected to be broken, resulting in the enhancement of the molecular occupation area as has been reported in some mixed-dye LB films [5,12]. However, we observed no anomalous enhancement of the molecular occupation area obtained from the π–A curves in Fig. 3 around X = 0.9. This indicates that the face-to-face molecular packing is not significantly modified. More detailed discussion for the molecular packing in the mixed system, however, is beyond the scope of this work. The critical mixing ratio of X = 0.9 may remind us the fact that the aggregation size of the DS is reported to be about 10 molecules with the one-dimensional molecular alignment [17]. At the mixing ratio of X = 0.9, stable formation of the J-aggregate of DS molecules may be prevented owing to the mixture of 6-Me-DS by 10%. This may cause the monomer formation for both DS and 6-Me-DS around X = 0.9. In other mixing ratios, dominance of the J-aggregation of either DS (X > 0.9) or 6-Me-DS (X < 0.9) molecules may cause the cooperative formation of J-aggregates consisting of both molecules. In this context, the absorption spectrum for X = 0.95 is interesting because clear red-shifted J-like band is observed with a shoulder in low-energy-side in addition to the monomer component, although further experiments such as photoluminescence measurement may be necessary to characterize this spectrum.
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4. In-plane orientation of the dye molecules As discussed above, we succeed to control the J-aggregation by mixing the different J-forming dye molecules. The next problem is to clarify the in-plane molecular orientation of the J-forming dyes in the present mixed-dye system. Fig. 5 shows the polarized absorption spectra of the LB films of [DS][AA]2 (upper) and [6-Me-DS][AA]2 (lower) with the electric polarization vectors parallel (solid curves) and perpendicular (dashed curves) to the dipping direction. The larger absorbance is observed for the polarization parallel to the dipping direction, qualitatively indicating that the direction of the transition dipole of the chromophore (N C C C C C O) is preferentially oriented along the dipping direction for both dye molecules. In order to elucidate the detailed molecular orientations, we performed the ESR measurements as typically shown in Fig. 6, where the orientation dependence of the first-derivative ESR spectra are plotted for [6-Me-DS][AA]2 (left) and [6-MeDS]0.5 [DS]0.5 [AA]2 (right). The external magnetic field H is normal to the film plane in (a), whereas it lies in the film plane and makes an angle of 0◦ and 90◦ with the dipping direction of the substrate in (b) and (c), respectively. It is reported for the DS LB film that the π-electron radical originates from the intermolecular charge transfer [17]. The ESR spectra exhibit both in-plane and out-of-plane anisotropies in 6-Me-DS film as reported in the DS film [5]. Important find-
Fig. 5. Polarized optical absorption spectra of [DS][AA]2 (upper) and [6-MeDS][AA]2 (lower) LB films. Linearly polarized electric vector E is parallel (solid curves) and perpendicular (dashed curves) to the dipping direction.
ing is that the in-plane anisotropy in X = 0.5 film is not so clearly observed. Such a difference of the anisotropy reflects the difference of the in-plane molecular orientation between singlecomponent dye films and mixed-dye film of X = 0.5. Studies of other mixing ratios revealed that ESR signals are almost similar as those of X = 0 for X ≤ 0.2, and they become weak for higher X values than X = 0.5. More detailed studies are currently under progress. As has been demonstrated in the case of DS LB films, ESR measurements are powerful tool to elucidate the microscopic molecular orientation by detecting the stable radical as a probe [9,5,16–18]. In the case of diluted spins with negligible interactions with other spins, the ESR spectra may be reproduced by the following spin Hamiltonian [20] H = µB S · g · H +
S · Ai · I i
(1)
i
The first and second terms in the right-hand side represent the Zeeman energy and the hyperfine coupling, with g and Ai being the g tensor and hyperfine tensor with ith nuclear spin, respectively. Other parameters, µB , S and I i denote Bohr magneton, electron spin and ith nuclear spin, respectively. The three-dimensional molecular orientation is usually determined by comparing the angular dependence of the observed spectra with the curves simulated using Eq. (1) together with the orientation distribution function. Typical distribution function is of a Boltzmann-type form, given as follows, sin2 (φ + φ0 ) sin2 (φ − φ0 ) P(φ) ∝ exp − + exp − (2) 2 sin2 δφ 2 sin2 δφ here ±φ0 shows the most probable orientation and δφ shows the width of distribution. It is often the case that the distribution function can be decoupled into the product of the function of each Eulerian angle. Further details of the ESR spectrum simulation methods using three-dimensional angular distribution functions are described elsewhere [5,16]. In the ESR spectra in Fig. 6, a clear triplet structure is observed, which is prominent when the external magnetic field H lies in the substrate (Fig. 6b and c). Such a triplet structure around g = gz (∼ 2.002) indicated by stic lines is reported to originate from the hyperfine splitting of the 14 N nuclei (I = 1) in the chromophore [9,5,16]. In-plane anisotropy is readily visualized as the orientationdependent variation of relative intensities of resolved triplet at g = gz and lower field peak with unresolved triplet at g = gy (∼ 2.009). These peaks are simultaneously observed due to the orientation distribution of the z (or y) axis. In order to determine the in-plane orientation of the dye molecules, we have performed the ESR spectrum simulation based on Eq. (1) using the angular distribution function defined in Eq. (2). For the spectrum simulation, the best results are shown in Fig. 6 by the dotted curves. gi and Ai are almost the same as those reported for DS film [5], independent of X. Typical values for [6-Me-DS][AA]2 film are gx = 2.007, gy = 2.009, gz = 2.002, Ax = 1.1 (G), Ay = 0.95 (G) and Az = 5.5 (G).
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Fig. 6. Orientation dependence of the first-derivative ESR spectra are plotted for [6-Me-DS][AA]2 (left) and [6-Me-DS]0.5 [DS]0.5 [AA]2 (right) films. The external magnetic field H is normal to the film plane in (a), whereas it lies in the film plane and makes an angle of 0◦ and 90◦ with the dipping direction of the substrate in (b) and (c), respectively. Dotted curves represent the results of spectrum simulation. See text for details.
In this simulation, out-of-plane distribution parameters are obtained as θ0 = 87◦ with δθ = 8◦ for z-axis and 0 = 5◦ with δ = 10◦ for y-axis, which are reported to be the best-fit parameters for the DS film [5,18]. As for the in-plane orientation parameters, φ0 and δφ are 55–59◦ and 24–28◦ , respectively, for 6-Me-DS and 60–64◦ and 18–22◦ , respectively, for DS Film. φ0 for DS film corresponds well with the earlier reported value [5]. Thus, we find the clear in-plane orientations for both of the pure films. On the other hand, the best-fitted curves for X = 0.5 film is calculated by putting P(φ) = 1, i.e., the molecular orientation is random in this film. This result is consistent with the fact that the ESR spectra are essentially the same when the external magnetic field lies in the film plane as shown in Figs. 6b and c for X = 0.5. In order to confirm the in-plane orientation of the dye molecules in [6-Me-DS]1−X [DS]X [AA]2 LB films, we have performed the polarized optical absorption measurements. The experimental results of optical dichroic ratio Robs are 1.35 and 1.80 for 6-Me-DS and DS films, respectively. As for the X = 0.5 film, Robs is 1.07, which is nearly unity, confirming the random in-plane molecular orientation in this film. By using the angular distribution function P(φ) in Eq. (2), the optical dichroic ratio is approximately calculated π π as [17]; Rcal = 0 P(φ) sin2 φ dφ/ 0 P(φ) cos2 φ dφ. Calculated dichroic ratios are 1.56 and 2.35 for 6-Me-DS and DS, respectively, and unity for X = 0.5. Thus, the experimentally obtained tendency of R(DS)> R(6-Me-DS)> R(X = 0.5)∼ 1 is semi-quantitatively reproduced by the present calculation. 5. Summary The LB films of the binary mixture of the J-forming merocyanine dyes, DS and 6-Me-DS, have been fabricated
together with the arachidic acid matrix. The J-aggregation of the single-component dye molecules is controlled by the mixing of the different dyes, which is clarified by the shift of the optical absorption spectra for [6-Me-DS]1−X [DS]X [AA]2 films. In particular, the J-band anomalously disappears around X = 0.9. Both DS and 6-Me-DS films exhibit the in-plane preferential orientation of the molecules along the dipping direction as clarified from the ESR spectra and their simulations and the optical dichroism. On the other hand, the 1:1 mixed-dye film exhibits no in-plane molecular orientations, although the J-band peak is clearly observed for this film. Thus, we are successful to control the microscopic orientation of the dye molecules forming J-aggregates by mixed-dye system. Acknowledgement This work has been partially supported by Grant-in-Aid for Scientific Research (17740191 and 17340094) and for Science Research in a Priority Area “Super-Hierarchical Structures” (17067007) from the Ministry of Education Culture, Sports, Science and Technology of Japan. References [1] H. Kuhn, D. M¨obius, H. B¨ucher, A. Weissberger, B.W. Rossiter (Eds.), Techniques of Chemistry, vol. 1, Part IIIB Wiley, New York, 1973, p. 577. [2] T. Kobayashi (Ed.), J-Aggregates, World Scientific, Singapore, 1996. [3] H. Kuhn, C. Kuhn, Chapter 1 of Ref. [2]. [4] V. Czikklely, H.D. Forsterling, H. Kuhn, Chem. Phys. Lett. 6 (1970) 207. [5] S. Kuroda, Adv. Colloid Interface Sci. 111 (2004) 181 and references therein. [6] M. Sugi, S. Iizima, Thin Solid Films 68 (1980) 199. [7] H. Nakahara, K. Fukuda, D. M¨obius, H. Kuhn, J. Phys. Chem. 90 (1986) 6144. [8] T. Inoue, Thin Solid Films 132 (1985) 21.
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[9] S. Kuroda, D. M¨obius, R. Miller (Eds.), Organized Monolayers and Assemblies: Structure, Processes and Function, Studies in Interface Science, vol. 16, Elsevier, Amsterdam, 2002 (Chapter 6). [10] S. Kuroda, H. Ito, Y. Uchiyama, T. Mori, K. Marumoto, I. Hatta, Jpn. J. Appl. Phys. 41 (prt1) (2002) 6223. [11] K. Murata, S. Kuroda, K. Saito, Thin Solid Film 295 (1997) 73. [12] K. Murata, H-.K. Shin, K. Saito, S. Kuroda, Thin Solid Film 327–329 (1998) 446. [13] M. Lan, K. Ikegami, Thin Solid Film 384 (2001) 120. [14] H.K. Shin, Y.S. Kwon, Synth. Met. 102 (1999) 1514. [15] T.L. Penner, D. M¨obius, Thin Solid Films 132 (1985) 185.
[16] S. Kuroda, K. Ikegami, Y. Tabe, K. Saito, M. Saito, M. Sugi, Phys. Rev. B 43 (1991) 2531. [17] S. Kuroda, K. Ikegami, K. Saito, M. Saito, M. Sugi, J. Phys. Soc. Jpn. 56 (1987) 3319. [18] S. Kuroda, Colloids and Surf. A: Physicochem. Eng. Asp. 198–200 (2002) 735. [19] N. Minari, K. Ikegami, S. Kuroda, K. Saito, M. Saito, M. Sugi, J. Phys. Soc. Jpn. 58 (1989) 222. [20] J.E. Wertz, J.R. Bolton, Electron Spin Resonance, McGraw-Hill, New York, 1972.
Control of formation and molecular orientation of J-aggregates in Langmuir–Blodgett films of mixed merocyanine dyes Hisaaki Tanaka∗ , Takehide Mizutani, Shin-ichi Kuroda Department of Applied Physics, Nagoya University, Furo-cho, Chikusa-ku, Nagoya 464-8603, Japan Received 21 July 2005; received in revised form 13 October 2005; accepted 28 October 2005 Available online 20 December 2005
Abstract We have fabricated Langmuir–Blodgett (LB) films of mixed-merocyanine dyes, DS and its methyl-substituted analog (6-Me-DS), diluted with arachidic acid (AA), expressed as [6-Me-DS]1−X [DS]X [AA]2 , in order to control the J-aggregation of dye molecules. For most of X-values, the red-shifted J-band peak appeared in the optical absorption spectra with its peak position depending on the mixing ratio of the dyes. In particular, J-band anomalously disappears around X = 0.9, suggesting the dominance of monomer state. We have also characterized the in-plane preferential orientation of the dye molecules along the dipping direction in the mixed-dye films by the orientation-dependent ESR signals and polarized optical absorption spectra. The optical dichroic ratios were semiquantitatively reproduced by the angular distribution function of the molecular orientation determined by the ESR spectrum simulation. The in-plane orientation of the dye molecules was clearly observed for DS, 6-Me-DS and X = 0.2 films, whereas it was nearly random for X = 0.5 film, suggesting the possibility to control the molecular orientational order of dye molecules by employing mixed-dye system. © 2005 Elsevier B.V. All rights reserved. Keywords: Langmuir–Blodgett films; J-aggregates; Electron-spin-resonance spectroscopy; Molecular orientation
1. Introduction The Langmuir–Blodgett (LB) technique attracts much attention as a tool for arranging various kinds of molecules into the form of monolayer assemblies, which may be suitable for constructing self-assembling nanosized molecular aggregates of various functions [1]. The J-aggregates of the functional dye molecules are characterized by the sharp red-shifted band and a strong photoluminescence with a small Stokes shift [2]. These J-aggregates are indispensable as photographic sensitizers, and there exist continuous and growing interests on Jaggregates both scientifically and technologically [1–3]. As for the structure of J-aggregates, Kuhn et al. has proposed the twodimensional (2D) brick-stone arrangement of the dye molecules based on the analysis of the red-shifted peak from the monomeric peak using the extended dipole model in their pioneering work [3,4]. We have been focusing our attention on the control of the J-aggregate formation of surface-active merocyanine dye
∗
Corresponding author. Tel.: +81 52 789 5165; fax: +81 52 789 3712. E-mail address: [email protected] (H. Tanaka).
0927-7757/$ – see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.colsurfa.2005.10.057
molecules [5]. These dye molecules, shown in Fig. 1 and abbreviated as DX (X = O, S, Se), form the J-aggregates in LB films diluted with arachidic acid (AA) except for DO molecule [5–9]. In order to control the J-aggregation of the dye molecules in the LB film, two approaches have been applied as follows: (i) the aggregation size of the merocyanine dye molecule [6-MeDS in Fig. 1b] has been controlled by varying the mixing ratio of 6-Me-DS with respect to the AA matrix [5,10]. In this case, well-defined J-band has been observed in the optical absorption spectra for the mixing ratio down to [6-Me-DS]:[AA] = 1:120. Monomer bands have been hardly observed in this dilution experiment, indicating that almost all the dye molecule form the J-aggregate and (ii) mutual mixtures of merocyanine dye molecules such as [DS]1−X [DO]X or [6-Me-DS]1−X [DO]X etc. have been examined [5,11–14]. In these mixed-dye LB films, the shift of the J-band peak has been observed depending on the mixing ratio. In particular, mixture of J-forming DS, 6-Me-DS or 5-Cl-DS and non J-forming DO molecules exhibits fascinating results that the DO molecules participate to form the J-aggregate due to the formation of solid solution with the J-forming dye molecules [11–13]. Such a behavior adds a variety of the dye-mixing effect which has been observed for the cyanine dye LB films, where the mixture of the two
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Fig. 1. Chemical structures of the merocyanine-dye molecules. The definition of the molecular coordination axes is also shown. (a) DO, DS, DSe, and (b) 6-Me-DS.
J-forming cyanine dyes with different J-band positions shows continuous shift of J-band peak depending on the mixing ratio [15]. In this paper, we report the mutual mixing effect of the two merocyanine dyes, DS and 6-Me-DS, which have not been studied so far. These two molecules form J-aggregates with different J-band peak positions in the LB films. The mutual mixing effect of these dye molecules to control the J-aggregation is discussed from the dependence of the J-band peak position on the mixing ratio. Surface pressure-area (π–A) isotherm measurements were also carried out to clarify the possible formation of solid solution of these dye species. We also focus our attention on the in-plane preferential orientation of the dye molecules in the LB films, which has been found to be the origin of the observed optical dichroism in the DS LB films [5]. In order to clarify both the in-plane and outof-plane molecular orientations in the film, it has been shown to be crucial to analyze the anisotropic ESR spectra of the stable radicals based on the anisotropic ESR properties of unpaired π-electrons [5,9]. Thus, obtained direction of the pπ orbital of the DS molecule (z-direction in Fig. 1) makes the averaged angle φ0 with respect to the dipping direction of 60–65◦ , giving the angle between the long axis of the dye molecules (y-direction) and the dipping direction, α, to be approximately 25–30◦ [5,9,16]. Schematic illustration of the observed molecular orientation of DS molecules is shown in Fig. 2. This result corresponds well with the brick-stone model of the J-aggregates by Kuhn et al., where one of the possible structure is such that the angle between the direction connecting the center of molecules and the long axis of the dye molecule becomes about 30◦ [1– 3,5,9]. Based on these results, the flow-orientation effect has been proposed as the origin of the in-plane orientation [5,16– 18], which is further reinforced by the theoretical development [19]. In this context, it would be interesting if we can control the in-plane flow-orientation by mixing the dye molecules. Thus, we studied the mutual mixing effect of different dye molecules, DS and 6-Me-DS, on the molecular orientations. The microscopic molecular orientation has been probed by the anisotropy of the ESR spectra.
Fig. 2. Orientation of the stable radical in the film plane of mixed LB films of DS and AA. φ0 Denotes the angle between the pπ orbital axis (z-axis in Fig. 1) and the dipping direction of the substrate. The long axis of the merocyanine dye molecule parallel to the chromophore (y-axis) is expressed by the thick lines.
In the following, after describing the experimental details in the next section, the experimental results of π–A curves, optical absorption and ESR spectra are presented in Section 3. Based on the ESR spectrum-simulation method, in-plane preferential orientation of the dye molecule is discussed in Section 4. An approximate estimation of the optical dichroism, which is calculated from the angular distribution function obtained from the ESR simulation, is also presented in Section 4 to see the consistency of the ESR analysis of in-plane distribution. A brief summary is given in Section 5. 2. Experimental details Dye molecules and AA were purchased from the Japanese Institute for Photosensitizing Dyes, Co. and Aldrich Chemical, Co., respectively. Chloroform solutions containing 1 × 10−3 M DS and 6-Me-DS were mixed with each other with appropriate ratios to give a binary mixed system. The mixed solutions were further diluted by the chloroform solution containing 2 × 10−3 M AA molecule with a ratio of dyes to AA of [dyes]:[AA] = 1:2. Then, hereafter we represent these mixed system as [6-Me-DS]1−X [DS]X [AA]2 (0 ≤ X ≤ 1). The LB films were fabricated by a standard vertical dipping method using a conventional trough. The aqueous subphase containing CdCl2 (3 × 10−4 M) and KHCO3 (5 × 10−5 M) was kept at 20–21 ◦ C. The pH of the subphase was controlled to be 5.8–6.2. Y-type films of mixed dyes and AA were transferred either on glass or mylar substrates precoated with 3 or 5 layers of cadmium arachidate for optical and ESR measurements, respectively. ESR measurements were performed by using a Bruker EMX spectrometer at the X-band (9.4 GHz). The LB films on mylar substrates were cut into pieces with the size of 10 mm × 2 mm
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and stacked in the ESR sample tube. All the measurements were performed at room temperature. 3. Experimental results Fig. 3 shows pressure-area (π–A) isotherms for the [6-MeDS]1−X [DS]X [AA]2 monolayer system. The limiting area for each film is determined by extrapolating the relevant isotherm to the zero pressure as shown in the solid arrow in the case of (a). The limiting area Sπ=0 is expressed by the effective area per AA molecule contained in monolayer, or in other words, the contribution of dye molecules is effectively added to AA with their molar mixing ratios. Then the molecular occupation area per dye molecule, defined as SD , in the dye-AA mixed LB film with a molar ratio of 1:Y is evaluated by the relation SD = Y (Sπ=0 − SAA ), where SAA is the limiting area for the pure AA ˚ 2 in the present experiment). Thus, obtained film (SAA = 21 A ˚ 2 for DS and 6-Memolecular occupation areas were 54 and 63 A DS, respectively. These values coincide well with previously reported values with an experimental error [10]. As discussed previously, the observed areas are consistent with the edge-on structures of the molecules having the face-to-face packing. Fig. 4 shows the optical absorption spectra of the mixeddye LB films, [6-Me-DS]1−X [DS]X [AA]2 , for various mixing ratios. In the case of pure dye systems, the optical absorption spectra show distinct peaks at 591 and 608 nm for DS and 6-MeDS, respectively, red-shifted from the monomer peaks (521 and 528 nm for DS and 6-Me-DS, respectively), indicating the formation of J-aggregates. In the mixed-dye system, well-defined J-band is observed in the absorption spectra for the wide range of mixing ratio X of X = 0.1–0.8 as well as for X = 0.95. In addition, the peak position and the shape of the absorption spectrum changes depending on X as shown in Fig. 4. In particular, J-band peak anomalously disappears around X = 0.9. Instead, the spectral weight corresponding to the monomer peak grows considerably. Thus, the J-aggregation of both DS and 6Me-DS molecules are prevented in this mixing region. In such
Fig. 3. Surface-pressure-area (π-A) isotherms for the [6-Me-DS]1−X [DS]X [AA]2 system. The horizontal axis shows the effective surface area per AA molecule contained in each film. The arrow in (a) represents an extrapolation to the zero pressure.
Fig. 4. Optical absorption spectra of the mixed-dye LB films, [6-Me-DS]1−X [DS]X [AA]2 , for various mixing ratios. Arrows represent the peak positions for the monomer states of 6-Me-DS (upper) and DS (lower).
a case, above mentioned brick-stone arrangement of the dye molecules is expected to be broken, resulting in the enhancement of the molecular occupation area as has been reported in some mixed-dye LB films [5,12]. However, we observed no anomalous enhancement of the molecular occupation area obtained from the π–A curves in Fig. 3 around X = 0.9. This indicates that the face-to-face molecular packing is not significantly modified. More detailed discussion for the molecular packing in the mixed system, however, is beyond the scope of this work. The critical mixing ratio of X = 0.9 may remind us the fact that the aggregation size of the DS is reported to be about 10 molecules with the one-dimensional molecular alignment [17]. At the mixing ratio of X = 0.9, stable formation of the J-aggregate of DS molecules may be prevented owing to the mixture of 6-Me-DS by 10%. This may cause the monomer formation for both DS and 6-Me-DS around X = 0.9. In other mixing ratios, dominance of the J-aggregation of either DS (X > 0.9) or 6-Me-DS (X < 0.9) molecules may cause the cooperative formation of J-aggregates consisting of both molecules. In this context, the absorption spectrum for X = 0.95 is interesting because clear red-shifted J-like band is observed with a shoulder in low-energy-side in addition to the monomer component, although further experiments such as photoluminescence measurement may be necessary to characterize this spectrum.
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4. In-plane orientation of the dye molecules As discussed above, we succeed to control the J-aggregation by mixing the different J-forming dye molecules. The next problem is to clarify the in-plane molecular orientation of the J-forming dyes in the present mixed-dye system. Fig. 5 shows the polarized absorption spectra of the LB films of [DS][AA]2 (upper) and [6-Me-DS][AA]2 (lower) with the electric polarization vectors parallel (solid curves) and perpendicular (dashed curves) to the dipping direction. The larger absorbance is observed for the polarization parallel to the dipping direction, qualitatively indicating that the direction of the transition dipole of the chromophore (N C C C C C O) is preferentially oriented along the dipping direction for both dye molecules. In order to elucidate the detailed molecular orientations, we performed the ESR measurements as typically shown in Fig. 6, where the orientation dependence of the first-derivative ESR spectra are plotted for [6-Me-DS][AA]2 (left) and [6-MeDS]0.5 [DS]0.5 [AA]2 (right). The external magnetic field H is normal to the film plane in (a), whereas it lies in the film plane and makes an angle of 0◦ and 90◦ with the dipping direction of the substrate in (b) and (c), respectively. It is reported for the DS LB film that the π-electron radical originates from the intermolecular charge transfer [17]. The ESR spectra exhibit both in-plane and out-of-plane anisotropies in 6-Me-DS film as reported in the DS film [5]. Important find-
Fig. 5. Polarized optical absorption spectra of [DS][AA]2 (upper) and [6-MeDS][AA]2 (lower) LB films. Linearly polarized electric vector E is parallel (solid curves) and perpendicular (dashed curves) to the dipping direction.
ing is that the in-plane anisotropy in X = 0.5 film is not so clearly observed. Such a difference of the anisotropy reflects the difference of the in-plane molecular orientation between singlecomponent dye films and mixed-dye film of X = 0.5. Studies of other mixing ratios revealed that ESR signals are almost similar as those of X = 0 for X ≤ 0.2, and they become weak for higher X values than X = 0.5. More detailed studies are currently under progress. As has been demonstrated in the case of DS LB films, ESR measurements are powerful tool to elucidate the microscopic molecular orientation by detecting the stable radical as a probe [9,5,16–18]. In the case of diluted spins with negligible interactions with other spins, the ESR spectra may be reproduced by the following spin Hamiltonian [20] H = µB S · g · H +
S · Ai · I i
(1)
i
The first and second terms in the right-hand side represent the Zeeman energy and the hyperfine coupling, with g and Ai being the g tensor and hyperfine tensor with ith nuclear spin, respectively. Other parameters, µB , S and I i denote Bohr magneton, electron spin and ith nuclear spin, respectively. The three-dimensional molecular orientation is usually determined by comparing the angular dependence of the observed spectra with the curves simulated using Eq. (1) together with the orientation distribution function. Typical distribution function is of a Boltzmann-type form, given as follows, sin2 (φ + φ0 ) sin2 (φ − φ0 ) P(φ) ∝ exp − + exp − (2) 2 sin2 δφ 2 sin2 δφ here ±φ0 shows the most probable orientation and δφ shows the width of distribution. It is often the case that the distribution function can be decoupled into the product of the function of each Eulerian angle. Further details of the ESR spectrum simulation methods using three-dimensional angular distribution functions are described elsewhere [5,16]. In the ESR spectra in Fig. 6, a clear triplet structure is observed, which is prominent when the external magnetic field H lies in the substrate (Fig. 6b and c). Such a triplet structure around g = gz (∼ 2.002) indicated by stic lines is reported to originate from the hyperfine splitting of the 14 N nuclei (I = 1) in the chromophore [9,5,16]. In-plane anisotropy is readily visualized as the orientationdependent variation of relative intensities of resolved triplet at g = gz and lower field peak with unresolved triplet at g = gy (∼ 2.009). These peaks are simultaneously observed due to the orientation distribution of the z (or y) axis. In order to determine the in-plane orientation of the dye molecules, we have performed the ESR spectrum simulation based on Eq. (1) using the angular distribution function defined in Eq. (2). For the spectrum simulation, the best results are shown in Fig. 6 by the dotted curves. gi and Ai are almost the same as those reported for DS film [5], independent of X. Typical values for [6-Me-DS][AA]2 film are gx = 2.007, gy = 2.009, gz = 2.002, Ax = 1.1 (G), Ay = 0.95 (G) and Az = 5.5 (G).
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Fig. 6. Orientation dependence of the first-derivative ESR spectra are plotted for [6-Me-DS][AA]2 (left) and [6-Me-DS]0.5 [DS]0.5 [AA]2 (right) films. The external magnetic field H is normal to the film plane in (a), whereas it lies in the film plane and makes an angle of 0◦ and 90◦ with the dipping direction of the substrate in (b) and (c), respectively. Dotted curves represent the results of spectrum simulation. See text for details.
In this simulation, out-of-plane distribution parameters are obtained as θ0 = 87◦ with δθ = 8◦ for z-axis and 0 = 5◦ with δ = 10◦ for y-axis, which are reported to be the best-fit parameters for the DS film [5,18]. As for the in-plane orientation parameters, φ0 and δφ are 55–59◦ and 24–28◦ , respectively, for 6-Me-DS and 60–64◦ and 18–22◦ , respectively, for DS Film. φ0 for DS film corresponds well with the earlier reported value [5]. Thus, we find the clear in-plane orientations for both of the pure films. On the other hand, the best-fitted curves for X = 0.5 film is calculated by putting P(φ) = 1, i.e., the molecular orientation is random in this film. This result is consistent with the fact that the ESR spectra are essentially the same when the external magnetic field lies in the film plane as shown in Figs. 6b and c for X = 0.5. In order to confirm the in-plane orientation of the dye molecules in [6-Me-DS]1−X [DS]X [AA]2 LB films, we have performed the polarized optical absorption measurements. The experimental results of optical dichroic ratio Robs are 1.35 and 1.80 for 6-Me-DS and DS films, respectively. As for the X = 0.5 film, Robs is 1.07, which is nearly unity, confirming the random in-plane molecular orientation in this film. By using the angular distribution function P(φ) in Eq. (2), the optical dichroic ratio is approximately calculated π π as [17]; Rcal = 0 P(φ) sin2 φ dφ/ 0 P(φ) cos2 φ dφ. Calculated dichroic ratios are 1.56 and 2.35 for 6-Me-DS and DS, respectively, and unity for X = 0.5. Thus, the experimentally obtained tendency of R(DS)> R(6-Me-DS)> R(X = 0.5)∼ 1 is semi-quantitatively reproduced by the present calculation. 5. Summary The LB films of the binary mixture of the J-forming merocyanine dyes, DS and 6-Me-DS, have been fabricated
together with the arachidic acid matrix. The J-aggregation of the single-component dye molecules is controlled by the mixing of the different dyes, which is clarified by the shift of the optical absorption spectra for [6-Me-DS]1−X [DS]X [AA]2 films. In particular, the J-band anomalously disappears around X = 0.9. Both DS and 6-Me-DS films exhibit the in-plane preferential orientation of the molecules along the dipping direction as clarified from the ESR spectra and their simulations and the optical dichroism. On the other hand, the 1:1 mixed-dye film exhibits no in-plane molecular orientations, although the J-band peak is clearly observed for this film. Thus, we are successful to control the microscopic orientation of the dye molecules forming J-aggregates by mixed-dye system. Acknowledgement This work has been partially supported by Grant-in-Aid for Scientific Research (17740191 and 17340094) and for Science Research in a Priority Area “Super-Hierarchical Structures” (17067007) from the Ministry of Education Culture, Sports, Science and Technology of Japan. References [1] H. Kuhn, D. M¨obius, H. B¨ucher, A. Weissberger, B.W. Rossiter (Eds.), Techniques of Chemistry, vol. 1, Part IIIB Wiley, New York, 1973, p. 577. [2] T. Kobayashi (Ed.), J-Aggregates, World Scientific, Singapore, 1996. [3] H. Kuhn, C. Kuhn, Chapter 1 of Ref. [2]. [4] V. Czikklely, H.D. Forsterling, H. Kuhn, Chem. Phys. Lett. 6 (1970) 207. [5] S. Kuroda, Adv. Colloid Interface Sci. 111 (2004) 181 and references therein. [6] M. Sugi, S. Iizima, Thin Solid Films 68 (1980) 199. [7] H. Nakahara, K. Fukuda, D. M¨obius, H. Kuhn, J. Phys. Chem. 90 (1986) 6144. [8] T. Inoue, Thin Solid Films 132 (1985) 21.
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