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Langmuir 1998, 14, 6226-6230
Relationship between Surface Morphology and Electroabsorption Spectrum of an Oxacyanine Chromophore in a Mixed Langmuir-Blodgett Film Nobuhiro Ohta,* Mitsuhiro Nakamura, and Iwao Yamazaki Department of Molecular Chemistry, Graduate School of Engineering, Hokkaido University, Sapporo 060-8628, Japan
Masatsugu Shimomura and Kuniharu Ijiro Molecular Device Laboratory, Research Institute for Electronic Science, Hokkaido University, Sapporo 060-0812, Japan Received February 10, 1998. In Final Form: July 24, 1998 Surface morphology of oxacyanine dye (OC) suddenly changes in mixed Langmuir-Blodgett films composed of OC, arachidic acid, and methyl arachidate when the OC fraction becomes more than 30 mol %. The agglomerates of OC show a ring structure just like a rubber band. The diameter and the shape of the “rubber band” change with different OC fractions, and microcrystalline domains appear when J-type aggregates are formed. The morphological change of the surface monolayer is well-correlated with a change in the electroabsorption spectra of the OC chromophore.
Introduction A complex formation or an aggregation of dye chromophores resulting in the drastic change in optical spectra and/or in electric properties may be tailored in highly ordered Langmuir-Blodgett (LB) monolayers representing restricted geometries.1 In such a case, dye molecules incorporated in LB films provide invaluable insight into the spectra-structure-property relationship in these systems as well as the nature of interaction between the dye molecules and their microenvironment. Molecular interactions among dye chromophores in restricted monolayer films may largely influence the distribution of chromophores and crystal morphology in the monolayer films. In fact, weak intermolecular interaction, for example, hydrogen bonding, van der Waals interaction, or hydrophobic interaction, is considered to be indispensable architectural tools for assembling molecular organizations.2,3 N,N′-Dioctadecyloxacyanine dye (OC) was suggested to form a sandwich or partially overlapping dimer and J-aggregate in LB films.4,5 To study the molecular interactions among OC chromophores, we have varied the OC fractions of the mixed LB monolayer film composed of OC, arachidic acid, and methyl arachidate and examined * To whom correspondence should be addressed. (1) (a) Kuhn, H.; Mo¨bius, D.; Bucher, H. In Physical Methods of Chemistry; Weissberger, A., Rossiter, B. W., Eds.; Wiley: New York, 1972; Vol. 1, p 577. (b) Ulman, A. An Intruduction to Ultrathin Organic Thin Films: From Langmuir-Blodgett Films to Self-Assemblies; Academic Press: New York, 1991. (c) Kuhn, H.; Mo¨bius, D. In Investigation of Surfaces and Interfaces-Part B; Rossiter, B. W., Baetzold, R. C., Eds.; Physical Methods of Chemistry Series; Wiley: New York, 1993; Vol. IXB. (2) Ringsdorf, H.; Schlarb, B.; Venzmer, J. Angew. Chem., Int. Ed. Engl. 1988, 27, 114. (3) Kunitake, T. Angew. Chem., Int. Ed. Engl. 1992, 31, 706. (4) (a) Czikkely, V.; Fo¨rsterling, H. D.; Kuhn, H. Chem. Phys. Lett. 1970, 6, 11. (b) Bu¨cher, H.; Kuhn, H. Chem. Phys. Lett. 1970, 6, 183. (5) Tamai, N.; Matsuo, H.; Yamazaki, T.; Yamazaki, I. J. Phys. Chem. 1992, 96, 6550. (6) Ohta, N.; Okazaki, S.; Yamazaki, I. Chem. Phys. Lett. 1994, 229, 394.
the absorption spectra and their electric field dependence.6 A molecular complex of OC which is completely different from J-aggregates was suggested to newly appear in a mixed LB film at an OC fraction of 30 mol %, on the basis of the Stark shift evaluated from the so-called electrochromism. Then, a question arises whether a new complex formation of OC chromophores in mixed LB films influences two-dimensional (2D) crystal morphology or molecular arrangement. Fluorescence microscopic techniques applied to monolayers provide a unique and versatile method to visually observe the morphology of the surface layer and the crystallization process both on the water-air surface and on the deposited film surface.7,8 In this paper, this technique is used for the morphological observation of 2D molecular assemblies of the mixed LB films of OC to examine molecular distribution and crystal morphology of dye chromophores of OC. We find that both crystal morphology and molecular packing arrangements change with different concentrations. Through a comparison of the fluorescence microscopy image with the electroabsorption spectra under different mixing ratios between OC and the matrix, the morphology was confirmed to be correlated extremely well with the complex formation and aggregation resulting in the drastic change in electric properties. Experimental Section N,N′-Dioctadecyloxacyanine perchlorate (Nippon Kanko Shikiso), denoted by OC, was used without further purification. Sample preparations of two kinds of mixed LB films for the measurements of the electoabsorption spectra (i.e., J-sample and sample-(M)), are the same as reported elsewhere.6 Hexadecane, denoted by HD, was used as a matrix in the J-sample, and a molar mixing ratio between OC and HD was fixed to be 1:1. In (7) (a) von Tscharner, V.; McConnell, H. M. Biophys. J. 1981, 36, 409. (b) McConnell, H. M.; Tamm, L. K.; Weis, R. M. Proc. Natl. Acad. Sci. U.S.A. 1984, 81, 3249. (8) (a) Lo¨sche, M.; Sackmann, E.; Mo¨hwald, H. Ber. Bunsen-Ges. Phys. Chem. 1983, 87, 848. (b) Mo¨hwald, H. Angew. Chem., Int. Ed. Engl. 1988, 27, 728.
S0743-7463(98)00170-X CCC: $15.00 © 1998 American Chemical Society Published on Web 09/12/1998
Surface Morphology and Electroabsorption of OC
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Figure 1. Fluorescence microscopy images taken from a mixed monolayer film composed of OC and AA/MA with OC fractions of 20 mol % (upper left), 30 mol % (upper right), and 35 mol % (lower left), respectively, and composed of OC and HD (lower right). The arrow shown in the picture at 30 mol % shows a dipping direction for the film deposition. The expanded view of the square enclosed with broken lines is shown under magnification three times and twice at 30 and 35 mol %, respectively, at each of the upper right corners. sample-(M), a mixture of arachidic acid (AA) and methyl arachidate (MA) whose molar ratio was 1:1 was used as a matrix, and various mixing ratios between OC and AA/MA were employed. Five mixed monolayers of OC and AA/MA were deposited with a spacer composed of three layers of AA between all the adjacent mixed layers in sample-(M), while one monolayer of a mixture of OC and HD was deposited. Stacking multilayer films of AA were deposited before and after the deposition of a mixed monolayer film both in the J-sample and in sample-(M) in order to make the surface of the substrate uniform and hydrophobic before the deposition of the dye monolayers and in order to prevent the surface from contamination by any impurity, respectively. Note that all the OC chromophores have the same direction with respect to the electrodes in every sample. For the fluorescence microscopy measurements, three monolayers of AA were precoated on the aluminum-coated quartz substrate, and one mixed monolayer film composed of OC and AA/MA or HD was deposited. All the monolayers were deposited as a Y-type. A mixed monolayer film composed of OC and AA/MA as well as a film of AA were deposited with a surface pressure of 25 mN/m, while a mixed film composed of OC and HD was deposited with a pressure of 35 mN/m. At these pressures, the alkyl chains are considered to be oriented perpendicular to the surface. Absorption spectra and steady-state fluorescence spectra were obtained with a Ubest-50 spectrometer (JASCO) and an FP770F fluorescence spectrometer (JASCO), respectively. Plots of the electric field induced change in the absorption intensity of OC as a function of excitation wavelength (i.e., the so-called electroabsorption spectra) were obtained with the same apparatus and same procedures as reported elsewhere.6 The applied field strength was estimated by assuming that the thickness of each layer is 27.5 Å.9 Fluorescence images of the deposited monolayer films were monitored at room temperature using an epifluorescence microscope (Olympus, BH-2UMA) equipped with a CCD cameraimage processor system (Flovel, HCC-3800). A mercury lamp was used as an excitation light source with a band-pass filter (V-filter). Unpolarized light was used for excitation, and (9) Fromherz, P.; Oelschla¨gel, U.; Wilke, W. Thin Solid Films 1988, 159, 421.
fluorescence emissions at wavelengths above 400 nm were monitored without a polarizer.
Results and Discussion Figure 1 shows fluorescence microscopy images observed for the mixed monolayer composed of OC and AA/MA with OC fractions of 20, 30, and 35 mol %, respectively, and for the mixed monolayer composed of OC and HD. The absorption and fluorescence spectra of the samples prepared under the same conditions are also shown in Figure 2. In the mixed film composed of OC and HD, a narrow band with a peak at 404 nm is observed both in the absorption spectrum and in the fluorescence spectrum, indicating that J-type aggregation of OC chromophores occurs in the transferred LB films deposited on quartz substrates.4 Absorption and fluorescence spectra of other samples are essentially the same and have a peak at 366 and 386 nm on absorption and at 427 nm on fluorescence. There is no doubt that the observed images show the morphology of OC chromophores in every case. It is worth mentioning that a catastrophe which suggests a sudden change in the molecular orientation of OC chromophores was not observed in pressure-area isotherms as well as in the polarized absorption spectra observed in the S0fS1 region of OC. As is shown in Figure 1, a bright crystalline domain is observed in the sample composed of OC and HD, where J-aggregates are formed. Their size distribution is rather irregular, but the mean size is roughly estimated to be ∼10 µm. The presence of the single-crystalline domain seems to be common in J-aggregate monolayers of cyanine dyes,10,11 though the domain size depends on the dye (10) Mo¨bius, D. Z. Phys. Chem. Neue Folge 1987, 154, 121.
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Figure 2. Absorption and emission spectra of a mixed monolayer film composed of OC and AA/MA with OC fractions of 20 mol % (chain line), 30 mol % (dotted line), and 35 mol % (broken line), respectively, and the spectra of the film composed of OC and HD (solid line). The employed samples were deposited at the same time under the same conditions as those for the samples for the measurements of the fluorescence images shown in Figure 2. The maximum intensity of the absorption and fluorescence spectra is normalized to unity in every case. Structure formula of OC is also given in the figure.
chromophore. The absorption and fluorescence spectra shown in Figure 2 indicate that a monomer and/or dimer of OC which give absorption peaks at 367 and 386 nm exist besides the J-aggregate in the mixed film composed of OC and HD. In fact, the image of the J-sample reveals that the bright domains superimpose a fluorescence background which is regarded as resulting from the OC chromophores other than the J-aggregates. The fluorescence image at 20 mol % of sample-(M) shows a homogeneous fluorescence background, as is shown in Figure 1. A similar image was observed at a lower OC fraction of 10 mol %. Thus, OC chromophores mixed with AA/MA (i.e., in sample-(M)) are considered to be distributed homogeneously in the deposited film when the OC fraction is less than 20 mol %. To our surprise, the fluorescence image drastically changes when the OC fraction of sample-(M) is increased above 30 mol %, as is shown in Figure 1. The image at 30 mol % shows a dispersion of a lot of bright rings, each of which is dark at the center. The ring is just like a rubber band. The average outer diameter and the average thickness of the “rubber band” are estimated to be about 10 and 2 µm, respectively. These rubber bands very likely correspond to a formation of large macroscopic agglomerates of OC chromophores in a 2D molecular assembly. Careful inspection of the image picture shows that several striae like a fault are located perpendicular to the dipping direction for deposition (see the image at 30 mol % in Figure 1). The presence of the striae may be attributed to a small fluctuation from a continuous scanning of a movable barrier of the trough during transfer of the monolayer to a solid support, though the employed trough was fully computerized. When the OC fraction is increased up to 35 mol %, the size of the rubber bands increases. The average outer diameter was roughly estimated to be 15 µm. The inner
diameter of the rubber band (i.e., the diameter of the dark center) is also increased. The thickness of the rubber band is almost the same, in contrast with the outer and inner diameters. The shape of the rubber band is also influenced by an increase of the OC fraction in the sense that the rubber band whose shape is similar to a flower leaf can be observed at 35 mol % (see the image at 35 mol % in Figure 1). Thus, a fluorescence microscopy image of the mixed LB monolayers composed of OC and AA/MA abruptly and drastically changes as the OC fraction increases above 30 mol %, though the optical spectra are essentially the same. If fluorescence quenching occurs at the center of the rubber band by an aggregation of OC dyes, such aggregations will lead to a drastic change in the absorption spectrum since overall absorption is monitored. As far as the absorption and fluorescence spectra are concerned, however, no significant difference was found in sample(M) with OC fractions from 10 to 35 mol % (see Figure 2), implying the absence of OC aggregates. Therefore, the dark domain of the rubber band at the center is not regarded as being filled with OC molecules both at 30 and 35 mol %. Absorption and linear electroabsorption spectra observed for the J-sample and for sample-(M) with OC fractions of 20, 30, and 35 mol %, respectively, are shown in Figure 3. These spectra are the same as the ones shown in a previous paper except for the spectra at 35 mol %.6 These electroabsorption spectra correspond to the fluorescence images shown in Figure 1, as far as the LB mixed films having the same OC fraction as AA/MA or HD are compared. The electroabsorption spectrum at 30 mol % is essentially the same in shape as the first derivative of the absorption spectrum, indicating the presence of the liner Stark shift, which shows the change in the electric dipole moment following excitation into the S1 state from
(11) Duschl, C.; Kemper, D.; Frey, W.; Meller, P.; Ringsdorf, H.; Knoll, W. J. Phys. Chem. 1989, 93, 4587.
(12) Liptay, W. In Excited States; Lim, E. C., Ed.; Academic Press: New York, 1974; p 129.
Surface Morphology and Electroabsorption of OC
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Figure 3. Linear electroabsorption spectra, the first derivative of the absorption spectra and the absorption spectra (from top to bottom) in the S0 f S1 region of OC chromophores. The spectra correspond to sample-(M) with OC fractions of 20, 30, and 35 mol %, respectively, and to the J-sample (from left to right). The field strength was 1.1 × 106 V cm-1 for sample-(M) and 2.0 × 106 V cm-1 for the J-sample. The field direction was along the normal to the surface from the glass plate to the outside.
the ground state (∆µ).12 The magnitude of ∆µ along the normal to the surface (i.e., |∆µ|) is determined to be 0.21 D at 30 mol %. The electroabsorption spectrum of the J-sample is also quite similar in shape to the first derivative of the absorption spectrum, and |∆µ| of the J-aggregate is estimated to be 0.19 D, as reported previously.6 Thus, both values of |∆µ| are nearly the same, but the directions of ∆µ along the normal to the surface are opposite each other. It is also noted that the electroabsorption spectra of sample-(M) with different fractions of OC are very different from each other, though the absorption spectra are essentially the same in shape (see Figure 3). In contrast with the Stark effect at 30 mol %, for example, the Stark effect at 20 mol % is dominated by a negative first-derivative contribution because of a negative ∆µ. The electroabsorption spectrum at 10 mol % is nearly identical with that at 20 mol %. The value of |∆µ| of sample-(M) with OC fractions below 20 mol % is estimated to be about 0.05 D.6 The electroabsorption spectrum at 35 mol % is complicated, but the spectrum is regarded as an intermediate between the electroabsporption spectrum at 30 mol % and the spectrum of the J-sample. Thus, the electroabsorption spectra of sample(M) show that the electric property of the individual OC chromophores suddenly changes when the OC fraction is increased above 30 mol %, and the electric property is influenced by a further increase of the OC fraction. The J-aggregate of OC is considered to show a brick stone packing arrangement of OC chromophores to form a large domain of aggregates such as a 2D crystal.4,13,14 The presence of 2D crystallization in the J-sample is confirmed by the fluorescence microscopy image shown
in Figure 1, and the molecular plane of OC is considered to be stacked perpendicular to the layer plane with a long axis nearly parallel to the substrate plane. The polarized absorption spectra in the S0 f S1 region of OC in sample(M), which are nearly the same with OC concentrations of 20 and 30 mol %, show that the long axis of the molecular plane of the OC chromophore is also nearly parallel to the layer plane in mixed LB films composed of OC and AA/ MA, though the long axis actually tilts a little against the plane parallel to the substrate.15 In a previous paper,6 the in-plane short axis of the OC chromophore was assumed to be perpendicular to the substrate at any concentration of OC in sample-(M) as well as in the J-sample, and the total magnitude of ∆µ itself changes by a complex formation or by a J-aggregation. However, the validity of this assumption may have to be carefully examined, as will be mentioned below. The magnitude of ∆µ following excitation into S1 of the OC chromophore was theoretically determined to be 0.2 D, on the basis of the free electron model.16 This value agrees quite well with the |∆µ| value determined for the J-aggregates of OC (0.19 D) and for OC in sample-(M) at 30 mol % (0.21 D). On the other hand, the value of |∆µ| at 10 and 20 mol % of sample-(M) is about 4 times smaller than the theoretical one. Then, a possibility is pointed out that the small value of |∆µ| at 10 or 20 mol % comes from a tilt of the in-plane short axis of OC chromophores from the plane normal to the surface and that the inplane short axis of OC in sample-(M) is nearly perpendicular to the substrate only at OC fractions above 30 mol %. In such a case, the value of |∆µ| determined from the electroabsorption spectra at OC fractions below 20 mol %
(13) Duschl, C.; Frey, W.; Knoll, W. Thin Solid Films 1988, 160, 251. (14) Kuroda, S.; Ikegami, K.; Saito, K.; Saito, M.; Sugi, M. J. Phys. Soc. Jpn. 1987, 56, 3319.
(15) Ohta, N.; Matsunami, S.; Okazaki, S.; Yamazaki, I. Langmuir 1994, 10, 3909. (16) Bucher, H.; Kuhn, H. Z. Naturforsch. 1970, 25b, 1323.
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is smaller than the total magnitude of ∆µ, depending on the tilt angle of the in-plane short axis. Note that ∆µ as well as the permanent electric dipole moment is directed along the short axis, as far as OC molecules have a plane structure with C2v symmetry as is shown in Figure 2. The orientation of OC molecular planes perpendicular to the substrate makes the face-to-face interaction effective and therefore gives a sandwich or partially overlapping arrangement of OC chromophores. Thus, the abrupt formation of the macroscopic agglomerates with a rubber band structure with OC fractions above 30 mol % may be attributed to a sudden arrangement of the OC molecules, with which the in-plane short axis of OC chromophores becomes perpendicular to the surface. If the above assumption is correct, the concentration dependence of the electroabsorption spectra shows that the direction cosine between the in-plane short axis of the OC chromophore and the normal to the surface at 30 mol % is opposite in sign to the corresponding direction cosines at OC fractions less than 20 mol %. The electroabsorption spectrum at 35 mol % is regarded as an intermediate between the spectrum at 30 mol % and the spectrum of the J-sample. Then, a change in size
Ohta et al.
and shape of the rubber band structure on going from 30 to 35 mol % seems to be interpreted by assuming that the molecular packing arrangement becomes closer to that of the J-aggregates with increasing the OC fraction in mixed LB films composed of OC and AA/MA. Macroscopic agglomeration of OC chromophores with a rubber band structure is one of the interesting examples of 2D molecular self-assemblies by a specific intermolecular interaction. It is emphasized that organization of dye chromophores and morphological changes in surface monolayers can be well-followed by taking the electroabsorption spectra, even when the absorption and emission spectra do not change. Acknowledgment. We express our sincere thanks to Dr. S. Okazaki for his contribution in the early stage of this work and to Dr. Y. Nishimura for his help in data processing of the observed fluorescence images. This work was partly supported by a Grant-in-Aid for the Scientific Research from the Ministry of Education, Science, Sports and Culture of Japan. LA980170D
