Control of chromophore aggregation in mono- and multilayers of long-chain merocyanine dyes

Control of Chromophore Aggregation in Mono- and Multilayers of LongChain Merocyanine Dyes H I R O O N A K A H A R A 1 AND D I E T M A R MOBIUS Max-Planck lnstitut fiir biophysikalische Chemic, D-3400 GOttingen-Nikolausberg, West Germany

Received October 28, 1985; accepted February 17, 1986 Control of the orientation and aggregation of the chromophore in the mixed monolayers of longchain merocyanine dyes with various matrices has been investigated. The mixed monolayers on water were characterized by measuring the surface pressure-area isotherms, and the organization of the chromophorein the transferredlayerswith differentmatriceswas studied by measuringthe polarizedabsorption spectra. As a result of the addition of hexadecane to the binary mixtures of the dye and arachidic acid or methyl arachidate, anisotropic aggregationof the chromophores in the mixed monolayers has been found to occur during the process of loss of hexadecane from the spread films under a constant surface pressure. These mixed films exhibit narrow and strong bands in the absorption and fluorescencespectra, which can be attributed to J aggregates. © 1986AcademicPress,Inc. INTRODUCTION The study of molecular organized systems with artificial or biological functions will be a fascinating paradigm of chemistry, physics, and biology (1-3). For construction of the molecular organizates, monolayer assembly techniques have been widely developed with respect to the designed amphiphatic molecules, accompanied by film characterization and potential applications (4-6). Particularly, for dyes containing long alkyl chains it would be desirable to control the orientation and aggregation of the chromophores in the layered structures with functions such as energy and electron transfer (4, 7). For this purpose, chemical alterations such as the numbers and relative positions of the substituent groups attached on the chromophore systems (8-10), modified procedures of transferring the monolayers onto the solid supports (9, 11, 12), and treatment of mixed monolayers in various matrices of c o m m o n fatty substances (4, 10, 12) have been examined. Merocyanine dyes can be used as indicators On leave of absence from Department of Chemistry, Faculty of Science, Saitama University,Urawa 338, Japan.

for solvent polarity (13, 14) and as probes of the photoelectric field across biomembranes or a lamellar structure (15) since the charge distribution of these dyes and consequently the spectral properties depend on the surrounding polarity. Recently, merocyanine dyes have been applied to solar energy conversion devices (16) and thus their long-chain derivatives are of spectral interest as photocatalysts for the generation of photoelectrons and photoconduction processes in monolayer assemblies (17). Further, photoelectron tunneling junctions h a v e been investigated by measuring the transverse and the lateral conduction in these systems (18, 19). In the present work, mixed monolayers of the long-chain merocyanine dyes with various matrices have been studied in order to control the orientation and aggregation of the chromophore in the layered structures. First, the mixed monolayers on water were characterized by measuring the surface pressure-area isotherms. Then, the organization of the chromophores in transferred layers with different matrices and changes with elapsed time on water have been studied by measuring the polarized absorption spectra in the visible region.

363 0021-9797/86 $3.00 Journal of Colloid and Interface Science, Vol. 114, No. 2, December 1986

Copyright © 1986 by Academic Press, Inc. All rights of reproduction in any form reserved.

364

N A K A H A R A A N D MOBIUS

Particular aggregates of chromophores, the J aggregates, are found in monolayers of cyanine dyes upon incorporating octadecane (20-22). This inert nonvolatile component can fill the gaps present in the hydrocarbon portion of the monolayers and thereby promote the stable arrangement when the monolayer is slowly compressed. J-Aggregate formation in monolayers of cyanine dyes is also observed with coaggregate molecules other than octadecane (23). Further, the important role of nhexadecane added to mixed monolayers of a cyanine dye and a fatty acid for obtaining homogeneous films has been demonstrated with molecular lubricants (24). Thus, homogeneous multicomponent monolayers may be formed by careful control of spreading of the dye mixture solution and of compressing the mixed monolayer on the water surface (25, 26). According to these considerations, the influence of hexadecane on J-aggregate formation and hence dependence of the spectra of transferred layers have been studied in addition to the behavior of dye mixtures with the common amphiphatic compounds.

EXPERIMENTAL

The merocyanine dyes (Mcn) with long hydrocarbon chains used in this work are indicated in Fig. 1. The maximum of the absorption band was at 528 nm in the chloroform solution. These long-chain dyes were synthesized by Dr. S. Yasui, Japanese Research Institute for Photosensitizing Dyes (Okayama, Japan). The components for the formation of mixed monolayers, arachidic acid (AA), methyl arachidate (MA), eicosyl amine (EA), and n-hexadecane (HD) were obtained commercially and were purified by recrystallization. The monolayers were spread from the chloroform solution onto the surface of distilled water (pH 5.6) or on the aqueous subphase with 3 × 10-4 MCdC12 and 5 × 10-s M NaHCO3 (pH 6.3). Surface pressure-area isotherms were measured on a round trough equipped with a Langmuir balance as described by Kuhn et al. (4) and the monolayer was usually compressed at a slow rate of 0.075 A2/s. molecule. The monolayers were trans-

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FIG. 1. Pressure-area curves for long-chain merocyanine dyes (Mc,) (R = CnH2n+I, n = 16, 18, 20; X = CH2COOH). The monolayers were spread on the buffer aqueous subphase containing CdCI2. Journal of Colloid and Interface Science, Vol. 114, No. 2, December 1986

CONTROL

OF CHROMOPHORE

ferred according to the Langmuir-Blodgett method onto half of a glass plate which was previously covered with cadmium arachidate films (3-5 layers) to make the surface hydrophobic. The surface pressure during monolayer transfer was usually 30 dyn/cm, except for monolayers containing HD mixtures which were deposited at 15 dyn/cm. The dipping rate was about 10-2 cm/s in all cases, and the deposition ratio checked simultaneously was about unity. Absorption and fluorescence spectra of the built-up films were measured with spectrometers modified as described in Ref. (4) within an hour after deposition. After the samples were stored for a few days in the dark, the spectra were unchanged. All experiments were performed at room temperature (15-20°C). RESULTS AND DISCUSSION

1. Surface Pressure-Area Isothermsfor the Monolayers of Long-Chain Merocyanine Dyes in VariousMatrices The merocyanine dyes with long hydrocarbons of different chain lengths form stable rigid monolayers according to the surface pressurearea curves (Fig. 1), although collapse is observed at comparatively low surface pressures of 10-20 dyn/cm. The surface pressures of collapse increase with the substituent chain length of the dyes. The compressibility ( - 1 / A) (dA/d~r)T is about 0.003-0.004 cm/dyn in the rigid region, which is larger than that observed for the common fatty substances with saturated straight chains of about 0.001 cm/ dyn in the solid film. This appears to reflect the closely packed arrangement of the chromophores rather than that of the hydrocarbon chains. From the results of X-ray crystal analyses of the planar merocyanine dye analog (14, 27), the molecular dimensions of the chromophore are estimated to be 15.8 A long, 7.7 wide, and 3.6 A thick, as illustrated in Fig. 1. These values lead to three basic cross-sectional areas, namely, 27.7 ~2 along the short axis, 56.7 A2 along the long axis, and 122 A2 in the plane of the ring system. The fact that

AGGREGATION

365

the limiting areas A,~0' obtained from extrapolation of the linear part to zero pressure, are about 63 ~2/molecule suggests the orientation of the merocyanine chromophores with the long axis nearly parallel to the water surface. For well-defined transfer ofmonolayers with the merocyanine dyes, we have studied mixed monolayers of the dye Mcn with AA, MA, and EA. The surface pressure-area isotherms of monolayers of the dye Mc16 mixed with AA in various ratios are shown in Fig. 2. Similar isotherms have been obtained for monolayers of the dyes Mc18 and Mc20 mixed with AA. From these isotherms, it has been found that the long-chain merocyanine dyes and AA are miscible in monolayers and these mixtures show approximately ideal behavior, i.e., the interaction between the dyes and AA appears to be small in monolayers. The isotherms of the dye Mc18/MA and Mc18/EA mixtures are shown in Figs. 3 and 4, respectively. For comparison, the curves calculated for ideal mixing from curves a and fare given as dotted lines. Negative deviations from the ideal behavior, i.e., a condensing effect, are observed for mixed monolayers with MA, whereas positive deviations are seen for mixed monolayers with EA at lower pressures. From these facts, it is suggested that the dye molecules are more closely packed in the MA matrix than in the others, whereas the carboxyl group of the dye may be almost ionized only in the EA matrix with the positive charge, resulting in more expanded films at lower pressures but overall contraction observed at high compression. The homogeneity of the mixed monolayers of Mc18 with AA, MA, or EA has been tested by two methods: (i) interference contrast microscopy of the monolayers coated in high vacuum with a thin silver film; and (ii) the energy transfer method (28) based on quenching of the fluorescence of an excited monolayer of a donor dye by the Mc mixed monolayers as an acceptor with donor and acceptor layers adjacently contacted. In the mixed films without HD, an inhomogeneous domain structure was observed and the donor fluorescence could lournal of Colloid and Interface Science, Vol. 114, No. 2, December 1986

366

NAKAHARA AND MOBIUS

f~

e

\,

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0

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30

40

AREA

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(~f/molecule)

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80

90

FIG. 2. Pressure-area curves of the dye Mc~6 monolayers mixed with AA, spread on the buffer aqueous subphase with CdC12: (a) MCl6, (b) 1:1 in Mc16/AA molar ratio, (c) 1:2, (d) 1:5, (e) 1:10, (f) AA.

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AREA (A /molecule) FIG. 3. Pressure-area curves of the dye Mc~8 monolayers mixed with MA, spread on the buffer aqueous subphase w i t h CdC12: (a) M c l s , (b) 1:1 in M c t 8 / M A m o l a r ratio, (c) 1:2, (d) 1:5, (e) 1:10, (f) MA. C a l c u l a t e d curves for the ideal m i x t u r e s are indicated as dashed lines. Journal of Colloid and Interface Science, Vol. 114, No. 2, December 1986

CONTROL

OF

CHROMOPHORE

367

AGGREGATION

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AREA (A-/molecule) FIG. 4. Pressure-areacurves of the dye Mc18monolayersmixed with EA, spread on the bufferaqueous subphase with CdClz: (a) Mcls, (b) 1:1 in Mc~8/EAmolar ratio, (c) 1:2, (d) 1:5, (e) 1:10, (f) EA. Calculated curves for the ideal mixtures are indicated as dashed lines.

be somewhat detected. When H D was added to the Mc mixtures with the above amphiphatic compounds at the molar ratio 1:1:1, the contrast of lateral structures was found to be very small [method (i)], and the donor fluorescence was almost completely quenched [method (ii)]. Therefore, it is considered that the homogeneity of the dye mixed films is appreciably improved by mixing H D (24). The surface pressure-area isotherms of monolayers for Mc~8/AA/HD (curve a), Mcis/ M A / H D (b), and MCIs/EA/HD (c), molar ratio 1:1:1, obtained at the rate of compression of 0.075 Ae/s. molecule are shown in Fig. 5 (solid lines), as compared with those obtained for the mixed monolayers without H D (dashed lines). Although all these curves are quasiequilibrium isotherms obtained by the very slow continuous compression, the effect of l i D addition is different in each mixed monolayer. The Mc~8/AA/HD mixed monolayer is slightly more expanded at surface pressures below 20 dyn/cm, while the Mc18/MA/HD mixture

gives a slightly more condensed monolayer with a lower surface pressure of collapse. A considerable decrease in the molecular area is observed in Mc~8/EA/HD mixed film as compared to MCls/EA. This may be interpreted by ion-pair formation between the dye anion and the protonated EA cation facilitated in the presence of HD, the degree of ionization being still uncertain. An indication for this is the condensing effect seen by comparison with the dotted line b in Fig. 4, referring to ideal mixing. For a monolayer of an equimolar mixture ofstearic acid and HD, the area becomes constant after 10 min under a surface pressure of 15 dyn/cm by loss of the hydrocarbon from the spread film (25, 29). This process of surface area decrease at a constant surface pressure seems to be more complicated in the case of the dye mixed monolayers containing HD. Figure 6 indicates the decrease in the surface area of the dye Mc~8 mixed monolayers with H D at 15 dyn/cm. The Mc~8/MA/HD mixture Journal of Colloid and Interface Science, Vol. 114, No. 2, December 1986

368

NAKAHARA AND MOBIUS

6C

o

-g

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o

\

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hi

~0

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FIG. 5. Pressure-area curves of the dye Mc18 mixed monolayers with hexadecane, spread on the buffer aqueous subphase containing CdC12: (a) MCls/AA/HD 1:1:1 molar ratio, (b) Mcls/MA/HD ( 1:1:1), (c) Mc18/ EA/HD (1:1:1), as compared with those for each mixture without HD (dashed lines).

(curves b and b') reaches the constant area after 30 min, but the surface areas of other mixtures decrease over periods of more than 50 rain. Assuming ideal behavior of all components in the mixed monolayer, the process of decrease in the surface area could be expressed by the equation

At-Ao~ = e_kl

[1]

A0 - A ~ where A0, A~, and A t a r e the surface areas of the initial stage, the steady end, and the intermediate time t, respectively (25). Equation [1 ] relates the evaporation process of H D from the spread monolayer to a first-order rate law. It is apparent that the curves for the decrease in the area follow approximately Eq. [ 1], except for the Mcls/EA/HD mixture in which the process is somewhat retarded after 5 min. For the initial period, the decays in the area on the buffer solution with CdC12 are slightly faster than those on distilled water. Journal of Colloid and Interface Science, Vol. 114, No. 2, December 1986

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FIG. 6. Rate of decrease in the surface area of the mixed monolayers at 15 dyn/cm: (a) Mc,8/AA/HD (1:1:1 molar ratio), (b) Mc~8/MA/HD (1:1:1), (c) McIs/EA/HD (1:1:1), spread on distilled water, a' and b': each mixed monolayer spread on the buffer aqueous subphase with CdC12. Dotted lines are the curves calculated with Eq. [1], for (a) k = 0.05, (a') k = 0.07, (b) k = 0.20, and (b') k = 0.43 (min-l).

369

CONTROL OF CHROMOPHORE AGGREGATION

. Visible Absorption Spectra for Multilayers of the Long-Chain Merocyanine Dyes in Various Matrices Monolayers of the long-chain merocyanine dyes and of their mixtures with the common long-chain compounds could be transferred onto solid supports by the Langmuir-Blodgett method both the up-trip or down-trip deposition. In the multilayers obtained, the chromophore moieties are in contact with the hydrophilic groups of the adjacent cadmium arachidate layers. Figure 7 shows the visible absorption spectra for the multilayers of the Mcl6/AA mixtures depo,;ited at 30 dyn/cm, together with that of the pure dye MCl6 transferred at 10 dyn/cm (dotted line). For various molar ratios of the dye t,o AA up to 1: I0, the spectra are very

~to'3/

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similar with a characteristic band at 590-593 nm and a shoulder at about 545 nm. From the surface pressure-area isotherms, the areas occupied by the dye and AA at 30 dyn/cm are estimated to be about 50 and 19 A=/molecule, respectively. When the optical densities of both bands are plotted against the surface concentration of the chromophore, calculated from the areas given above, an approximately linear relation is obtained (inset of Fig. 7). This fact seems to support the model of nearly ideal mixing in the monolayers of McI6/AA, described above. As another point of view, however, it has been considered for the merocyanine dye films that the red-shifted and relatively narrow band is due to a J-like aggregate (14, 19, 30). Although the microscopic aspect of the aggregate formation seems to be contradictory to ideal mixing, the stereospecific

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WAVELENGTH ( n m ) FIG. 7. Visible absorption spectra of the dye M c I 6 / A A mixed monolayers deposited at 30 dyn/cm (solid line) and the dye Mci6 monolayer deposited at I0 dyn/cm (dotted line): (a) 1:1 McI6/AA molar ratio, (b) 1: 2, (c) 1:4, (d) 1:6, (e) 1:10. The insert is the plot of the optical density at 593 and 545 nm vs the surface concentration of the dye MCl6. Journal of Colloid and Interface Science, Vol. 114, No. 2, December 1986

370

NAKAHARA AND MOBIUS

characteristics of these films of the dye mixtures are interesting subjects. The dichroism of the absorption spectra has been determined for the dye Mc]6 mixed films with AA or MA, molar ratio 1:10, deposited at 30 dyn/cm, by using plane polarized light with electric vector oscillating either perpendicularly to the dipping direction (in other words, parallel to the plane of the monolayer on water) or parallel to this (in the plane of incidence) under a 30 ° incidence, as seen in Fig. 8. The spectral shape undergoes narrowing and a slight red shift to 605 nm is observed for the dye/MA mixed films, as compared to the dye/AA films. It should be noted that the 590-nm band for the dye/AA film is strongly polarized along the dipping direction, whereas the 605-nm band for the dye/MA is slightly polarized perpendicularly to this. That is, a little difference in the matrix components of the dye mixed films has brought about significant changes in not only the band positions but also the orientation of chromophores.

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Further, in the case of the dye/AA film, the dichroism and the optical density increase with increasing surface pressure at deposition, as indicated in the inset of Fig. 8. The optical anisotropy of the dye/AA mixed film is also observed for the normal incidence, indicating a preferential orientation of the electronic transition moments parallel to the dipping direction. At the present stage, a quantitative analysis of this dichroism is not possible, since the evaluation of the spatial distribution function of the chromophore requires a more detailed measurement of the angular dependence of the dichroism. On the other hand, the spectra of the dye/ EA mixed films are characterized by a broad band with two maxima in the range of 510545 nm, as shown in Fig. 9. This broad band, which shows practically no dichroism under normal incidence, may be attributed to monomeric and dimeric forms of the dye with the carboxyl group ionized in the EA matrix, respectively.

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WAVELENGTH ( n m ) FIG. 8. Polarized absorption spectra for the Mc~6/AA (A) and Mc~6/MA (B) (l:10 in the molar ratio) mixed monolayers deposited at 30 dyn/cm, with electric vectors perpendicular (solid line) and parallel

(dashedline)to dippingdirection,undera 30° incidence.The insertis the plotofopticaldensityand dichroic ratio for the peakabsorbanceof the McI6/AA(1:10) mixedfilmsagainstthe surfacepressuresat deposition. Journal of Colloid andlnterface Science, Vol. 114, No. 2, December 1986

CONTROL OF C H R O M O P H O R E xt°'3 IO

a

0 400

500

600

WAVELENGTH ( nm ) F/G. 9. Visible absorption spectra of the dye Mc]dEA mixed monolayers deposited at 30 dyn/cm: (a) 1:1 Mc]6/ EA molar ratio, (b) 1:2, (c) 1:5, (d) h l 0 .

As another characteristic of these dye mixed films, emission spectra were measured by irradiation of 545 nm. A weak fluorescence

371

AGGREGATION

band was observed at about 610 nm for the dye/AA or dye/MA mixed films which exhibited the red-shifted absorption band at about 600 nm, while no fluorescence could be detected for the dye/EA. This result seems to support the consideration that a J-like aggregate has been formed in the case of the dye/ AA or dye/MA mixed film. Next, for the purpose of following the chromophore rearrangements of the long-chain merocyanine dye in the mixed monolayers with HD as the surface area decreases at constant surface pressure, the monolayers were transferred onto the solid supports at several definite times after spreading the dye mixed solution and compressing the monolayer under a surface pressure of 15 dyn/cm, and the electronic absorption spectra of these built-up films were measured.

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FIG. 10. Spectral change for the dye McIs/AA/HD (l: 1:1 molar ratio) mixed monolayer, spread on distilled water, with time after compression at 15 dyn/cm: 5 rain (--), 15 rain (---), 30 rain ( . . . . . . . ), and 52 min

(...). Journal of Colloid and Interface Science, VoI. 114, No. 2, December 1986

372

NAKAHARA

AND

MOBIUS

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FIG. I 1. Absorption dependencies upon the angle ~6between the polarization plane and the water surface during deposition, under normal (c~= 0 °) and 30° incidences, for the dye Mc~8/AA/HD(1:1:1) mixed film spread on distilled water, with time after compression at 15 dyn/cm: (a) 5 min ([2 510 nm, L~535 nm, © 588 nm) and (b) 15 rain (n 510 nm, A 535 nm, O 598 nra).

For the dye M c l s / A A / H D (molar ratio 1:1:1) mixed monolayer spread on the distilled water, the spectral change with the decrease in the molecular area through squeezing out n-hexadecane from the spread layer is shown in Fig. 10. A band at 588 n m is observed with a shoulder at 598 n m 5 min after the compression. Then, the 588-nm band disappears and the 598-nm band becomes prominent at 15 min. The intensity at the m a x i m u m of this band decreases slightly between 15 and 30 min and stays constant within experimental error. Additionally, the polarization characteristics for the spectra of the mixed dye monolayers prepared 5 and 15 min after the compression have been examined under normal and 30 ° Journal of Colloid and Interface Science, Vol. 114, No. 2, D e c e m b e r 1986

incidences. Figure 11 indicates the dependence of the m a x i m u m absorption upon the angle q~ between the plane of polarization and the water surface during the deposition. At normal incidence, the 510- and 535-nm bands exhibit a small angular dependence or an isotropy, while the longer wavelength bands at both 588 n m (a) and 598 n m (b) show considerable anisotropies in the plane. The m a x i m u m absorptions are obtained at q~ = 15 ° and 105 ° for the 588- and 598-nm bands, respectively. At 30 ° incidence, the 510-, 535-, 588-, and 598-nm bands apparently show m a x i m u m absorption at q~ = 0, 0, 15, and 105 °, respectively. In accordance with K u h n ' s treatment (4) assuming equal refractive indices of the

CONTROL

OF CHROMOPHORE

films; o~, the angle o f incidence; and/3, the angle o f refraction (sin a = n - s i n / 3 ) . F o r 30 ° incidence a n d n = 1.53 (fatty acid layers) (4), we o b t a i n e d rs = 0.251 (@ = 0 °) a n d re = 0.167 (@ = 90°). Therefore, the true a b s o r p t i o n s A , = 0.799As a n d Ap = 0.857Ap. W i t h the 588a n d 5 9 8 - n m b a n d s the c o r r e c t e d a b s o r p t i o n s (4) for the p o l a r i z e d rays o f q5 = 0 a n d 90 ° are d e p e n d e n t u p o n the incident angle o~, as shown in Fig. 12. E a c h d i c h r o i s m o f the 588- a n d 5 9 8 - n m b a n d s exhibits a m i n i m u m a n d a m a x i m u m d e p e n d i n g o n the p o l a r i z a t i o n o f the i n c i d e n t angle, at a = 0 °. Qualitatively, a s s u m i n g that the biaxial a n i s o t r o p y o f chrom o p h o r e o r i e n t a t i o n is r e p r e s e n t e d b y a rot a t i o n a l ellipsoid o f the optical units (31), the axis o f the ellipsoid for the 5 8 8 - n m t r a n s i t i o n is o r i e n t e d nearly p e r p e n d i c u l a r to the film

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373

AGGREGATION

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FIG. 12. Absorption (A) dependencies upon the incidence angle a with the polarized rays of ~b = 0 ° (open plots) and ~b = 90 ° (black plots), for the dye MCIs/AA/ HD (1:1:1) mixed film spread on distilled water, with time after compression at 15 dyn/cm: 5 min (588 nm --) and 15 rain (598 nm ---).

substrate a n d the film, the true a b s o r p t i o n corrected for interference at the s u b s t r a t e / a i r interface is given b y

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where ri is the reflection coefficient. F o r the light with electric v e c t o r p e r p e n d i c u l a r to the plane o f incidence (s polarization) a n d for that parallel to the p l a n e o f incidence ( p p o l a r i z a tion), rs -

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and

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WAVENUMBER

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15

14

( x to3cm -' )

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FIG. 13. Fluorescence spectra of the dye M C l s / A A / H D ( 1:1:1) m i x e d films, spread on distilled water and deposited

respectively, where n is the refractive i n d e x o f

at 5 rain (--) and 15 min (. . . . . . . ) after compression under 15 dyn/cm, irradiated by 545 nm. Dashed and dotted lines are the corresponding absorption spectra.

rp =

17 • COS OZ- - COS/~

n . cos a + cos

JournalofColloidandInterfaceScience,Vol. 114, No. 2, December 1986

374

NAKAHARA AND MOBIUS

XlO~c 2G

t 5 2 rnin.

)'-12 I-Z

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-,400

500 WAVELENGTH

( nm

)

600

FIG, 14. Spectral change for the dye McIs/MA/HD ( h l : l molar ratio) mixed monolayer, spread on the buffer aqueous subphase with CdC12, with time after compression at 15 dyn/cm: 5 min (--), 15 min (. . . . .

•-), 30 min (---), 52 min (. • •), and J aggregate.

plane, while in the case of the 598-nm band the axis is oriented rather parallel. However, quantitative analysis of the biaxial orientation of the transition moments requires further experimental work. Additional information about molecular reorganization is obtained by measuring the fluorescence spectra of these films on excitation of 545 nm. A strong emission band at 610 nm is observed together with a shoulder at 595 n m when the monolayer is transferred 5 min after the compression and subsequently the narrow 610-nm band only with a halfbandwidth of 550 cm -1 at 15 rain, as shown in Fig. 13. Thus, it is concluded that associaJournal of Colloid and Interface Science,

Vo]. 114,No. 2, December1986

tion of the chromophores to the so-called J form occurs through the process of packing the dye molecules with loss of the hydrocarbon from the spread film of the dye Mc18/AA/HD, accompanied by considerable optical anisotropy. In the case of the dye Mc18/MA/HD (molar ratio 1:1:1) mixed monolayer on the buffered aqueous solution containing CdC12, the change of the adsorption spectra with time after compression of the monolayer is indicated in Fig. 14. Three bands at 500, 540, and 600 n m are observed, whose relative intensities change with time after compression. The facts that the integrated intensities over the range

375

CONTROL OF CHROMOPHORE AGGREGATION

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--,4, FIG. 15. Absorption dependencies upon the angle 4, under a normal incidence for the 500-nm (solid symbols) and the 600-nm (open symbols)bands of the dye MCls/MA/HD (1:1:1) mixed film, soread on the buffer aqueous subphasewith CdCI2,with time after compressionunder 15 dyn/cm: 5 min (ZX),15 min ([~), and 30 min (O).

of 400-650 nm have remained constant and an isosbestic point at 525 nm can initially be seen indicate that different aggregate forms of the chromophore are in equilibrium in the monolayer. Occasionally, the rearrangement processes lead to monolayers characterized by a strong and narrow absorption band at 600 nm, correlated with a narrow fluorescence band at 610 nm with a half-bandwidth of 450 cm -1. These characteristics support the conclusion that a J aggregate of the chromophores is formed in the monolayer. As for orientation of the transition moments of the 500- and 600nm bands in these film spectra, the depen-

dence of the absorption upon the angle 4~ under normal incidence is shown in Fig. 15 for different wavelengths. A considerable anisotropy in the layer plane has been observed for the 600-rim band with a small dependence on time after the compression, whereas an almost isotropic orientation was found for the 500n m band. When the dye Mc18/MA/HD mixture was spread onto distilled water the deposition ratio was only about 0.6. In the case of the dye Mc18/EA/HD (molar ratio 1:1:1) mixed monolayers spread on distilled water, the spectra of the deposited films Journal of Colloid and Interface Science, Vol.114,No.2, December1986

376

NAKAHARA AND MOBIUS

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FIG. 16. Spectral change for the dye Mcas/EA/HD (1:1:1 molar ratio) mixed monolayer, spread on distilled water, with time after compression at 15 dyn/cm: 5 min (--), 15 min (---), 30 rain (. . . . . . . ), and 52 min

(...). at definite times after the compression are shown in Fig. 16. In comparison with the other mixed films, the feature of these film spectra is a broader band in the range of 500-550 nm, which is similar to the mixed films without HD. With the time dependence of the spectra, the integrated intensity is found to decrease significantly down to a half for 50 rain in spite of a nearly constant deposition ratio. As any other spectral change has not been detected in the range of 300-800 nm, it is considered that the dye monolayer has been somewhat dissolved into the aqueous subphase together with EA and HD. A similar decrease of the absorption intensity was also observed for the other mixing ratios of the dye Mc18/EA/HD (molar ratio 1:2:2). Several authors (25, 32) have pointed out that the monolayers of long-chain compounds with amino groups are somewhat unstable in the presence of counteranions and other compounds. In this mixed system, the transition moment was found to exhibit comparatively little anisotropic orientation. In conclusion, from the surface pressurearea isotherms of the monolayers and the spectroscopic behavior of transferred monolayers with the long-chain merocyanine dyes Journal of Colloid and Interface Science, Vol. 114,No. 2, December1986

it has been found that chromophore orientation and aggregation in the mono- and multilayers depend significantly upon the matrix components of the mixed films. Particularly, as a result of addition of HD to the binary mixtures of the dye Mcn and AA, MA, or EA, more homogeneous films have been obtained and anisotropic aggregation of the chromophores in the mixed monolayers with AA and MA has been found to occur during the process of loss of liD from the spread films under a constant surface pressure. The absorption and fluorescence spectra of the mixed films of the dye Mcn with AA or MA and HD are characterized by the narrow and strong bands attributed to J aggregates. This type of dye aggregate is expected to have interesting properties in respect to electron and energy transfer. Furthermore, it has been suggested that the monolayer technique is useful to obtain a simplified model for spectroscopic study of the dye aggregates. ACKNOWLEDGMENTS The authors express special thanks to Professor Hans Kuhn for stimulating and fruitful discussions, and Mr. W. Zeiss for technical assistance.

CONTROL OF CHROMOPHORE AGGREGATION REFERENCES 1. Kuhn, H., in "Structural Chemistry and Molecular Biology" (A. Rieh and N. Davidson, Eds.), p. 566. Freeman, San Francisco/London, 1968. 2, Fendler, J. H., and Fendler, E. J., "Catalysis in MiceUar and Maeromolecular Systems." Academic Press, New York, 1975. 3. Fendler, J. H., "Membrane Mimetic Chemistry." Wiley-Interscience, New York, 1982. 4. Kuhn, H., and M6bius, D., Angew. Chem. Int. Ed. Engl. 10, 620 (1972); Kuhn, H., M6bius, D., and BiJcher, H., in "Physical Methods of Chemistry" (A. Weissberger, Ed.), Pt. III-B, Chap. VII, p. 577. Interscience, New York, 1972. 5. Thin Solid Films 99, 1 (1983). 6. Roberts, G. G., Contemp. Phys. 25, 109 (1984). 7. Polymeropoulos, E. E., M6bius, D., and Kuhn, H., J. Chem. Phys. 68, 3918 (1978). 8. Fukuda, K., Nakahara, H., and Kato, T., J. Colloid Interface Sci, 54, 430 (1976). 9. Nakahara, H., and Fukuda, K., J. Colloid Interface Sci. 69, 24 (1979); 93, 530 (1983). 10. Heesemann, J., J. Amer. Chem. Soc. 102, 2167, 2176 (1980). 11. Inacker, O., Kuhn, H., M6bius, D., and Debuch, G., Z. Phys. Chem. Neue Folge 101, 337 (1976). 12. Sagiv, J., J. Amer. Chem. Soc. 102, 337 (1976). 13. Brooker, L. G. S., Keyes, G. H., et aL, J. Amer. Chem. Soc. 73, 5332 (1951). 14. Sturmer, D. M., in "Special Topics in Heterocyclic Chemistry" (A. Weissberger and E. C. Taylor, Eds,), Chap. VIII. Wiley, New York, 1977.

377

15. Lelkes, P. I., and Miller, I. R., J. Membrane Biol. 52, 1 (1980). 16. Gosh, A. K., and Feng, T., J. Appl. Phys. 49, 5982 (1978). 17. Kuhn, H., PureAppL Chem. 51, 341 (1979). 18. Sugi, M., and Iijima, S., Thin Solid Films 68, 199 (198o). 19. Sugi, M., Fukui, T., Iijima, S., and Iriyama, K., Mol. Co,st. Liq. Cryst. 62, 165 (1980). 20. Bficher, H., and Kuhn, H., Chem. Phys. Lett. 6, 183 (1970). 21. Steiger, R., Junod, P., Kilchoer, B., and Schumacher, E., Photogr. Sci. Eng. 17, 107 (1973). 22. O'Brien, D. F., Photogr. Sci. Eng. 18, 16 (1974). 23. Vaidyanathan, S., Patterson, L. K,, M6bius, D., and Gruniger, H. R., J. Phys. Chem. 89, 491 (1985). 24. M6bius, D., Ber. Bunsen-Ges. Phys. Chem. 82, 848 (1978). 25. Gaines, G. L., Jr., "Insoluble Monolayers at LiquidGas Interfaces." Interseience, New York, 1966. 26. Gaines, G. L., Jr., Thin SolidFilms 68, 1 (1980). 27. Smith, D. L., Photogr. Sei. Eng. 18, 309 (1974). 28. Bticher, H., Elsner, O. v., M6bius, D., and Tillmann, P., Z. Phys. Chem. Neue Folge65, 152 (1969). 29. Gaines, G. L., Jr., J. Phys. Chem. 65, 382 (1961). 30. Mizutani, F., Iijima, S., and Tsuda, K., Bull. Chem. Soe. Japan 55, 1295 (1982). 31. Nomura, S., Kawai, H., Kimura, 1., and Kagiyama, M., J. Polym. Sei. Part A-2 5, 479 (1967). 32. Geidel, Th., Hopf, W., and Richter, G., Colloid Polym. Sci. 257, 776 (1979).

Journal of Colloid and Interface Science, Vol. 114, No. 2, December 1986