Second Harmonic Generation from Mixed Films of Rhodamine Dye

acid (ArAc) at the air-water interface were characterized by surface ... isotherms of mixed RhC18/ArAc layers provided evidence that there was a high ...
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Langmuir 2001, 17, 5329-5336

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Second Harmonic Generation from Mixed Films of Rhodamine Dye and Arachidic Acid at the Air-Water Interface: Correlation with Spectroscopic Properties Oksana N. Slyadneva,* Maxim N. Slyadnev,† Akira Harata, and Teiichiro Ogawa Department of Molecular and Material Sciences, Kyushu University, Kasuga-koen 6-1, Kasuga-shi, Fukuoka 816-8580, Japan Received November 6, 2000. In Final Form: May 25, 2001 Mixed insoluble monolayers of bis(N-ethyl,N-octadecyl)rhodamine perchlorate (RhC18) and arachidic acid (ArAc) at the air-water interface were characterized by surface pressure-area isotherm, second harmonic generation (SHG), and reflection spectroscopy studies. The analysis of surface pressure-area isotherms of mixed RhC18/ArAc layers provided evidence that there was a high degree of miscibility of film components and a strong interaction among them. A new band was found to appear on the blue side of the reflection spectrum after an increase in both the concentration of ArAc in the layer and the applied surface pressure. This new band was assigned to dye aggregates, where spontaneous formation in the gaslike region was promoted by the presence of ArAc and induced by compression in the region of the continuous monolayer. Both in gaslike and in compressed film, the formation of blue-shifted aggregates coincided with the increase of the SHG response of the monolayer. Resonance SHG enhancement was proven to be the main reason for this observation due to electronic resonance of aggregates with the incident light frequency.

Introduction Recently, the nonlinear optical (NLO) properties of organic compounds have been investigated intensively. Practical applications require materials with large optical susceptibility to stimulate the search for ways to improve second harmonic generation (SHG) in organic films of electron-rich molecules.1 The main efforts are aimed at the design of a donor (π-bridge)/acceptor type of nonsymmetric chromophores that has inherent NLO properties and SHG capabilities that depend on the length of the π-electron bridge and the donating/accepting capacity of the terminal groups.1a,2 Another approach that is attracting great interest is a design of highly ordered molecular structures on different substrates.3 In both cases, molecular interactions have a significant influence on the SHG response of the system. Generally, the interactions depend on numerous conditions such as substrate nature, composition, density, and packing order of the films under consideration. Because a molecular second-order nonlinear hyperpolarizability tensor β(2), which governs the SHG response, is related to the molecular electronic structure,4 it is affected by all interactions that change molecular * To whom correspondence should be addressed. † On leave from St. Petersburg State University, Department of Chemistry, Universitetskij pr. 2, 198904, Petergoff, St. Petersburg, Russia. (1) (a) Bishop, M.; Clarke, J. H. R.; Davis, L. E.; King, T. A.; Mayers, F. R.; Mohebati, A.; Munn, R. W.; Shabat, M. M.; West, D.; Williams, J. O. Thin Solid Films 1992, 210/211, 185. (b) Yao, Z. Q.; Liu, P.; Yan, R. Z.; Liu, L. Y.; Liu, X. H.; Wang, W. C. Thin Solid Films 1992, 210/ 211, 208. (2) (a) Takahashi, T.; Chen, Y. M.; Rahaman, A. K.; Kumar, J.; Tripathy, S. K. Thin Solid Films 1992, 210/211, 202. (b) Zhu, D.; Yang, C.; Liu, Y.; Xu, Y. Thin Solid Films 1992, 210/211, 205. (3) (a) Tam, W.; Guerin, B.; Calabrese, J. C.; Stevenson, S. H. Chem. Phys. Lett. 1989, 154, 93. (b) Tang, Q.; Zahir, S. A.; Bosshard, C.; Flo¨rsheimer, M.; Ku¨pfer, M.; Gu¨nter, P. Thin Solid Films 1992, 210/ 211, 195. (c) Dentan, V.; Blanchard-Desce, M.; Palacin, S.; Ledoux, I.; Barraud, A.; Lehn, J.-M.; Zyss, J. Thin Solid Films 1992, 210/211, 221. (d) Kajikawa, K.; Takezoe, H.; Fukuda, A. Chem. Phys. Lett. 1993, 205, 225. (4) Mazely, Y. L.; Hetherington, W. M. J. Chem. Phys. 1987, 86, 3640.

permanent or transition dipole moments.5 For example, Bre´das et al. reported that β(2) depends on microenvironmental polarity.6 If molecular interactions result in the formation of dimers or aggregates, a new species with different β(2) will contribute to SHG intensity. Ashwell et al. reported SHG enhancement from films of squaraine dyes due to the formation of intermolecular chargetransfer aggregates with a noncentrosymmetric structure.7 Some organic polymers deposited as Langmuir-Blodgett (LB) films on a glass substrate have been discovered to be a highly active NLO system under the formation of aggregates.1a,2a,3c,8 The enhancement of the SH (second harmonic) signal due to aggregation has also been found in the surface films of rhodamine9 and fluorescein10 dyes under compression at the air-water interface. Besides a direct change of β(2) magnitude, aggregation may lead to changes in the spectroscopic characteristics of the system. The formation of dimers and aggregates with different absorption bands was reported to significantly affect the SHG response of the rhodamine dye layers on a waterglass interface11 and on a water-air interface.9 An additional contribution to the SH signal may arise from the high ordering of chromophores and a favorable (5) (a) Ye, P.; Shen, Y. R. Phys. Rev. B: Condens. Matter 1983, 28, 4288. (b) Verbiest, T.; Samyn, C.; Persoons, A. Thin Solid Films 1992, 210/211, 188. (6) (a) Cross, G. H.; Healy, D.; Szablewski, M.; Bloor, D.; Malagoli, M.; Kogej, T.; Beljonne, D.; Bre´das, J.-L. Chem. Phys. Lett. 1997, 268, 36. (b) Dehu, C.; Geskin, V.; Persoons, A.; Bre´das, J. L. Eur. J. Org. Chem. 1998, 1267. (7) (a) Ashwell, G. J.; Jefferies, G.; Hamilton, D. G.; Lynch, D. E.; Roberts, M. P. S.; Bahra, G. S.; Brown, C. R. Nature 1995, 375, 385. (b) Ashwell, G. J.; Jefferies, G.; Rees, N. D.; Williamson, P. C.; Bahra, G. S.; Brown, C. R. Langmuir 1998, 14, 2850. (c) Ashwell, G. J.; Roberts, M. P. S.; Rees, N. D.; Bahra, G. S.; Brown, C. R. Langmuir 1998, 14, 5279. (d) Ashwell, G. J.; Dyer, A. N.; Green, A. Langmuir 1999, 15, 3627. (8) Wang, Y. Chem. Phys. Lett. 1986, 126, 209. (9) Slyadneva, O.; Slyadnev, M.; Tsukanova, V.; Inoue, T.; Harata, A.; Ogawa, T. Langmuir 1999, 15, 8651. (10) Tsukanova, V.; Slyadneva, O.; Inoue, T.; Harata, A.; Ogawa, T. Chem. Phys. 1999, 250, 207. (11) Yamaguchi, A.; Uchida, T.; Teramae, N.; Kaneta, H. Anal. Sci. 1997, 13, 85.

10.1021/la0015415 CCC: $20.00 © 2001 American Chemical Society Published on Web 07/19/2001

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orientational distribution in two-dimensional (2D) supramolecular systems such as close-packed monolayers.3c,3d,9,10 High molecular ordering also provides strong favorable coupling of chromophores due to the shortening of the distance between the dye molecules that lead to the delocalization of the electronic excitation over the aggregated 2D structure.3a,3b In the present paper, we investigate the correlation between the spectroscopic behavior and the nonlinear properties of the insoluble mixed films of rhodamine dye with long alkyl chains and fatty acids at the air-water interface by means of absorption spectroscopy and a resonant SHG technique after compression. Because an underlying solid or crystalline substrate may significantly affect the polarizability of adsorbed molecules by magnetic-dipole or electric-quadrupole interaction,5a it is quite difficult to reveal whether the SHG response is influenced by adsorbate-substrate interaction or by chromophore aggregation in layers on solid substrates.12,7d In this respect, the insoluble monolayers at the air-water interface seem to present an attractive model system for studying the influence of intermolecular interaction on the second-order nonlinear properties of compounds. The change of the molecular surface concentration and, in turn, the distances between molecules and the packing order are easily controlled using external pressure. That flexibility allows a liquid-air interface system to be used for fine-tuning a SHG response of film in many applications. As was also demonstrated, the combination of nonlinear and linear spectroscopic techniques provides unique information about the molecular interactions and structures of the aggregates formed.9,13,14 Understanding the correlation between the intermolecular interactions in mixed films and the efficiency of optical SHG is key to the development of a wide area of organic films in which a nonlinear response can be controlled. Experimental Section The dye bis(N-ethyl,N-octadecyl)rhodamine perchlorate (RhC18) and arachidic acid (ArAc) were purchased from the NKS Kenkyusho Chemical Corp. and the Tokyo Kasei Kogyo Company, respectively. Both substances were of spectroscopic grade and used as supplied. Each component was dissolved in benzene (stock solution, CRhC18 ) 10-3 M and CArAc ) 10-2 M). These solutions were diluted as required and used to prepare RhC18/ArAc mixtures with desired mole ratios. The mixtures of the molar ratios of RhC18/ArAc (1:1, 1:2, 1:3, 1:5, and 1:10; corresponding molar parts of RhC18 were 0.5, 0.33, 0.25, 0.17, and 0.09) were prepared. Double-purified and deionized water was obtained with a Millipore Milli-Q system and used as a subphase throughout. The surface pressure-area (π-A) isotherms were recorded using a LB Teflon trough equipped with a Wilhelmy balance and Teflon barriers (USI System, model FSD-300). The solutions were spread on an aqueous subphase, and after solvent evaporation (∼10 min), isotherms were obtained at the compression rate of 5 mm/min. The area per molecule was calculated by dividing the total surface area occupied by the monolayer under the specified pressure by the total number of molecules in the monolayer. Monolayers of RhC18 and mixed RhC18/ArAc films for SHG measurements were prepared by spreading a 5-20-µL benzene solution on a pure water subphase in a stainless steel cell coated (12) (a) Lynch, D. E.; Geissler, U.; Peterson, I. R.; Floersheimer, M.; Terbrack, R.; Chi, L. F.; Fuchs, G.; Calos, N. J.; Wood, B.; Kennard, C. H. L.; Langley, G. J. J. Chem. Soc., Perkin Trans. 2 1997, 827. (b) Lynch, D. E.; Peterson, I. R.; Floersheimer, M.; Essing, D.; Chi, L. F.; Fuchs, G.; Calos, N. J.; Wood, B.; Kennard, C. H. L.; Langley, G. J. J. Chem. Soc., Perkin Trans. 2 1998, 779. (13) Peterson, E. S.; Harris, C. B. J. Chem. Phys. 1989, 91, 2683. (14) Simpson, G. J.; Westerbuhr, S. G.; Rowlen, K. L. Anal. Chem. 2000, 72, 887.

Slyadneva et al. with Teflon over a total area of 100 cm2. The surface density of molecules was controlled manually by carefully moving the Teflon barriers. The SHG measurements were carried out on an experimental setup described elsewhere.9 Briefly, a frequency-doubled s-polarized beam of a pulsed Nd3+:YAG laser (Continuum model PY61, λ ) 532 nm, pulse duration ) 40 ps, repetition rate ) 10 Hz, and max peak energy ) 30 mJ/pulse) was softly focused by a quartz lens (f ) 90 cm) at a sample surface, resulting in a beam waist area of 0.25 cm2. The incident angle was 38° from the surface normal. The surface energy density was 5 mJ/cm2 per pulse. The reflected SH light from the monolayer was separated from the incident laser beam by cutoff and band-pass filters, detected by a photomultiplier (Hamamatsu R585), and recorded by a digital oscilloscope (Tektronix, TDS 380). The SH signal was normalized by the reference signal from a KDP frequencydoubling crystal in order to eliminate laser-intensity fluctuations. The SH signal from a bare water surface was negligible as compared with that of the monolayer under investigation. All experiments were performed at 23 °C. The films at the air-water interface were gradually compressed, and the SH intensity was measured at each step of compression. The compression direction was perpendicular to the plane where the incident and generated beams were located. The SH signal for each measurement was accumulated over 2 min, and several sets of films with specified compositions were repetitively characterized by this procedure. Special attention was given to the accuracy of the measurements in the low surfacepressure region because of the film floating on the water surface.9,10,15 A stable SH signal during the accumulation was considered as a criterion for successful measurement. The reflection spectra of films at the air-water interface were obtained using a deuterium lamp (MC-962A) as a visible light source and a high-sensitivity multichannel detector (Otsuka Electronics Company, “Photal” MCPD-7000). The films were continuously compressed in the Langmuir trough, and spectra were taken at the different points of π-A isotherms. A single spectrum was taken during 20 ms, and 100 spectra were averaged for each measurement.

Results and Discussion Surface Pressure-Area Isotherms of the RhC18/ ArAc Monolayers. The π-A isotherms of pure and mixed RhC18/ArAc layers at the air-water interface were obtained to study the behavior of monolayers and the miscibility of the constituents in the films (Figure 1). The pure monolayer of RhC18 was found to be stable with a collapse pressure of 26 mN/m at 26 Å2/molecule. The π-A isotherm clearly demonstrates several phase regions with a transition between them. The transition from a gaslike phase to an expanded film is indicated by an arrow in Figure 1 and followed by the region of expanded film (230105 Å2/molecule), the transition to condensed film (10550 Å2/molecule), and then the region of condensed film (50-25 Å2/molecule) followed by the collapse point. The isotherm measured is similar to that obtained in previous investigations.9,10,16 For the series of mixed monolayers, the transition to expanded film gradually shifted toward smaller areas per molecule with an increase in the mole fraction of fatty acid, as presented in Table 1. Besides, the stability of the mixed films was found to increase as compared to that of a pure RhC18 monolayer. The collapse-pressure values varied with the composition of the mixture, as shown in Table 1. According to a criterion developed by Gaines,17 the variation of collapse pressure with the composition (15) Li, Q.; Slyadnev, M.; Inoue, T.; Harata, A.; Ogawa, T. Langmuir 1999, 15, 3035. (16) Van der Auweraer, M.; Verschuere, B.; De Schryver, F. C. Langmuir 1988, 4, 583. (17) Gains, G. L., Jr. Insoluble Monolayers at Liquid-Gas Interfaces; Interscience: New York, 1966; Chapter 6.

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Figure 2. Molecular area, at various surface-pressures values, of mixed monolayers as a function of a mole fraction of RhC18. Points represent the experimental data fit by a spline. Figure 1. Surface pressure-area isotherms of mixed RhC18/ ArAc monolayers with a different molar ratio of RhC18/ArAc: (a) pure ArAc; (b) 1:10; (c) 1:5; (d) 1:3; (e) 1:2; (f) 1:1; (g) pure RhC18. Inset shows the molecular structures of RhC18. Table 1. Transition to Expanded Film and Collapse-Pressure Values for Mixed Monolayers of RhC18 and ArAc mole fraction of RhC18 1

0.5 0.33 0.25 0.17 0.09

transition to expanded film, 225 89 Å2/molecule collapse pressure, 26 29 mN/m

0

66

57

46

36

26

34

39

38

37

42

surface-pressure values over a concentration range studied. These deviations indicate the miscibility of the components in the films as well as some sort of attractive molecular interactions that arise from van der Waals forces between long alkyl chains of RhC18 and ArAc molecules. Specific attractive CH-π interactions between an ArAc carbon chain and a xanthene core of dye19 may also facilitate the formation of a mixed monolayer. The Gibbs free energy of mixing (∆GM) calculated directly from π-A isotherms provides information on the thermodynamic properties of the systems under investigation and can be obtained by using the following equations:17

∆GM ) ∆GME + ∆GMI indicates the miscibility of the components in the films. The main features of π-A isotherms of mixed films resemble those of a pure rhodamine curve. The significant difference is observed in the length of the plateau region, which is much shorter than in the pure RhC18 film, probably due to the interaction between the components. This interaction can be characterized by the comparison of the real film behavior and the properties of the ideal two-component 2D mixture. To investigate this aspect, we have applied the method17 commonly used to describe the properties of mixed monolayers18 to the systems studied. Figure 2 shows the experimental values of the area per molecule plotted as a function of the mole fraction of RhC18 at a different surface pressure. The dotted line in the figure represents the ideal concentration dependence for an immiscible two-component system, which obeys the equation:

Aideal ) N1A1 + N2A2

(1)

where N1 and N2 are the mole fractions of components; A1 and A2 are the areas per molecule of pure RhC18 and ArAc, respectively; and Aideal is the average area per molecule in the two-component monolayer. Negative deviations from ideal behavior (Figure 2) have been found for all (18) (a) Pal, P.; Dutta, A. K.; Pal, A. J.; Misra, T. N. Langmuir 1994, 10, 2339. (b) Ray, K.; Misra, T. N. Langmuir 1997, 13, 6731. (c) Seoane, R.; Min˜ones, J.; Conde, O.; Casas, M.; Iribarnegaray, E. Langmuir 1999, 15, 3570. (d) Seoane, R.; Min˜ones, J.; Conde, O.; Iribarnegaray, E.; Casas, M. Langmuir 1999, 15, 5567.

∆GME )

∫0π(Amix - N1A1 - N2A2) dπ

∆GMI ) RT(N1 ln N1 + N2 ln N2)

(2) (3) (4)

where ∆GME is the excess free energy of mixing for a real system; ∆GMI is the free energy of mixing for an ideal system; π is the experimental surface pressure; Amix is the area in the mixed monolayer; R is the universal gas constant; and T is the absolute temperature. The plot of the Gibbs free energy of mixing vs the mole fraction of RhC18 in the mixed monolayers at different surface pressures is shown in Figure 3. As seen in the figure, the experimental data lie below the curve of ideal behavior over the entire range of concentration. This finding also indicates that there is a strong attractive interaction between the constituents of the films.17 The negative values of ∆GM provide thermodynamically stable layers at the air-water interface and suggest the miscibility of components of the films over the entire range of surface pressure studied. Second Harmonic Generation Study. The SH intensity generated by chemical species at the interface can be described in terms of the second-order nonlinear susceptibility χ(2) tensor. The susceptibility χ(2) is a (19) (a) Nishio, M.; Umezawa, Y.; Hirota, M.; Takeuchi, Y. Tetrahedron 1995, 51, 8665. (b) Heaton, N. J.; Bello, P.; Herradon, B.; Campo, A.; Barbero, J. J. J. Am. Chem. Soc. 1998, 120, 12371. (c) Lindeman, S. V.; Kosynkin, D.; Kochi, J. K. J. Am. Chem. Soc. 1998, 120, 13268.

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the appearance of SHG. Most notable is a significant increase in the SH signal in the gaseous film region with an increase of the ArAc molar fraction. Deviation of a normalized experimental SHG response from theoretically expected behavior was estimated by the standard error parameter σ:

σ)

Figure 3. Free Gibbs energy of mixing (∆GM) as a function of RhC18 concentration in mixed monolayers at different ranges of surface pressure.

macroscopic quality that is related to the nonlinear properties of the individual molecular species by

χ(2) )

∑i Ni〈Ti(φ, θ, ψ)〉βi(2)

(5)

where Ni is the surface density; βi(2) is the molecular secondorder polarizability tensor; Ti(φ, θ, ψ) is the orientational distribution over Euler angles φ, θ, and ψ; and the brackets indicate an ensemble average for the ith species. The pattern of the β(2) tensor is sensitive to the local environment of the molecule. With the increase of the microenvironmental polarity, the magnitude of the β(2) tensor increases.6b Because the elements of the β(2) tensor are directly related with permanent and transition dipole moments of the molecule under consideration,4,5a,20 the β(2) tensor pattern can be affected by dipole-dipole and/or dipole-quadrupole intermolecular interactions. In complicated systems such as the supramolecular assembly, where aggregation can occur, several species with distinctive β(2) simultaneously contribute to the χ(2) tensor. In such cases, the separation of individual contributions is significantly complicated. Thus, SH intensity at interfaces is related to the chemical composition of the layer, surface concentrations, orientation of each species, and dielectric properties of the surrounding environment, as eq 5 shows in a generalized form. Figure 4 shows the experimental dependencies of a SH signal as a function of the average area per molecule after the compression of the insoluble films of RhC18 and RhC18/ ArAc monolayers at the air-water interface. Solid lines represent the theoretical dependence of a SHG response on the surface density of noninteracting dye molecules. A theoretical SH signal was calculated as proportional to the second power of RhC18 surface density taking into account the SH intensity of the RhC18 monomer species, which has been found to exist in a gaseous film region of a pure dye layer.9 A SH signal from a pure RhC18 layer (Figure 4a) obeys theoretical behavior in the region of an expanded state of film up to 150 Å2/molecule. It then demonstrates a large positive deviation from the calculated curve in the region of transition between the liquidexpanded state and the solid state of a film, while the solid-state region shows some lack of a signal as compared to the intensity of the signals generated by the monomers. The addition of ArAc into the layer results in changes in (20) Corn, R. M.; Higgins, D. A. Chem. Rev. 1994, 94, 107.

(

1

n

∑

n - 1i)1

)

1/2

[Iiexperimental - Iitheoretical]2

(6)

where Iiexperimental is an experimental value of the SH signal; Iitheoretical is the theoretically expected signal at the ith point; and n is the number of experimental points. The results are summarized in the Table 2. Data in Table 2 show a tendency for the standard error to increase with an increase in the molar part of fatty acids in the film. Films of 1:5 and 1:10 ratios exhibit the largest values of σ that are conditioned mostly by the enhancement of the SH signal in the region of the gaseous film. The rest of the films reveal smaller deviations related to regions where surface pressure exists. SHG deviation from theoretical behavior after film compression can, in our opinion, be caused by forced dye aggregation, which is accompanied by molecular reorientation and changes in the molecular dipole moments as well as the rearrangement of structural ordering within formed aggregates. Molecules of ArAc may facilitate or counteract these processes depending on the molar part in the layer. In our previous investigation,9 we found that distinct dye species (from monomers in the gaslike phase to dimers in the transition expanded-condensed region to molecular aggregates in the condensed layer) correspond to the different regions of a π-A isotherm in the monolayer of pure RhC18. Combining SHG and spectroscopic studies, each type of species formed has been characterized by the distinct ratio of the β(2) tensor elements and the specific absorption bands in the spectrum. The observed similar nonlinear behavior of the pure RhC18 and mixed monolayers in this work (see Figure 4) reflects the sum of intermolecular interactions within films and allows us to conclude that several types of species with different SH activity probably exist on a water surface at every compression step. The RhC18/ArAc mixed film at a molar ratio of 1:10 exhibits quite different behavior from other mixtures (Figure 4f). The SH signal is noticeably higher than it had been for all previous samples, and it remains almost constant in the gaseous film region until the area of 50 Å2/molecule is reached. It then falls sharply and starts to rise again after the point of transition to the expanded state of a monolayer. The high and constant SH intensity in the gaslike region allows us to suggest the presence of some assembled structure formed on the water surface, which is then possibly broken down by applied external pressure. Our conclusion is supported by the fact that fatty acids easily form patterns of self-organized, ordered molecular islands at zero surface pressure.21 In our present experimental work, the confirmation of the formation of islands was observed in the large SH intensity fluctuations in a gaseous film state in the entire molar ratio of the components, which was similar to the results obtained previously.9,22 (21) (a) Takoshima, T.; Masuda, A.; Mukasa, A. Thin Solid Films 1992, 210/211, 51. (b) Ho¨nig, D.; Mo¨bius, D. Thin Solid Films 1992, 210/211, 64. (c) Ulman, A. Chem. Rev. 1996, 96, 3. (22) Slyadnev, M.; Inoue, T.; Harata, A.; Ogawa, T. Colloids Surf., A 2000, 154, 155.

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Figure 4. SH intensities as a function of area per molecule at different molar ratios of RhC18/ArAc: (a) pure RhC18; (b) 1:1; (c) 1:2; (d) 1:3; (e) 1:5; (f) 1:10. The dashed line marks the transition from a gaslike phase to an expanded film. Table 2 film composition, RhC18:ArAc σ

pure RhC18

1:1

1:2

1:3

1:5

1:10

0.196

0.127

0.142

0.152

0.302

0.667

A demonstration of the difference in SHG behavior of monolayers is presented in Figure 5, where the dependence of an SH signal from monolayers at zero surface-pressure values is depicted (constant surface concentration of the dye molecules (CRhC18 ) 1.8 × 1013 mol/cm2) was chosen for a correct comparison). As expected from the SHG theory, the SH signal should remain constant under constant surface concentration, provided that all other conditions remain constant. However, with an increase of the ArAc molar part, the films in a gaslike region generate an increasing SH signal, as can be seen in Figure 5. The SH intensity is enhanced when the molar part of the ArAc in the film becomes larger than 0.66 (RhC18/ArAc molar ratio, 1:2). Several explanations can be offered to interpret the observed behavior. On one hand, the change in the orientation of the dye molecule relative to the water surface can result in the different values of susceptibility χ(2) according to eq 5. However, in the absence of applied pressure, the dye molecules situated on the water surface orient themselves according to the hydrophilicity of their groups. The tilt angle of the rigid xanthene core of RhC18 toward the water surface is strictly determined by its carboxylic group and one amino group,9 while the flexible alkyl chains manage the mutual orientation of dye molecules by rotating them around surface normal. In mixed monolayers, the flexibility of alkyl chains allows the ArAc to interact with RhC18 by means of a change in the chain conformation to fit the layer structure rather

Figure 5. SH intensity as a function of a RhC18 molar part in mixed monolayers in a gaslike region.

than detach hydrophilic groups from the water, which should be done if the tilt angle changed.9 Therefore, it seems unlikely that an increase in the amount of the fatty acid spacer would produce a significant change in the dye orientation with respect to the surface normal, whereas the rotation around the surface normal would not meet any restriction. Another explanation is that the presence of ArAc molecules in the films leads to the generation of a new species at the water surface. The magnitude of the SH signal is then predominantly determined by the value and direction of the dipole moment of formed dimers/ aggregates. To support the validity of our concept,

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Figure 6. Normalized reflection spectra of mixed RhC18/ArAc films at different surface pressures: (a) π ) 0 mN/m; (b) π ) 5 mN/m; (c) π ) 15 mN/m. The solid line shows the spectra of a pure RhC18 layer, while the other curves correspond to mixed films of different molar ratios: 1:3 (dotted line); 1:5 (dashed line); 1:10 (dashed-dotted line).

spectroscopic properties of monolayers at the air-water interface were investigated. Spectroscopic Study. A series of reflection spectra was obtained for RhC18 and RhC18/ArAc mixed films at the air-water interface during gradual compression. Typical reflection spectra of RhC18 and RhC18/ArAc mixed films at the air-water interface are shown in Figure 6 a-c for three surface-pressure values. Surface pressure (π ) 0 mN/m, Figure 6a) corresponds to the transition to expanded film for each monolayer (see Table 1). In this film state, a monolayer is already completed, but the molecules are arranged according to their interactions and not to external pressure. In contrast, Figure 6b,c represents spectra at π ) 5 and 15 mN/m, which correspond to the expanded and condensed film state for each monolayer (see Figure 1), respectively. The structure of the monolayer in those film states is governed by intermolecular interactions and applied pressure. As seen in Figure 6, the spectra are significantly affected by the

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presence of ArAc molecules in the monolayer and show evidence of intermolecular interaction. The increase in the concentration of fatty acid results in an increase in the intensity of the shoulder at the blue side of the main band in the gaseous film region (Figure 6a). Under applied pressure, the shoulder in the shorter wavelength region became more pronounced and reached a peak at λ ) 528 nm in the 1:10 mixed film (Figure 6b,c). Monolayers of different compositions are characterized by shifts in the main-band maximum relative to the RhC18 spectra. Besides, all spectra show an increase in full width at halfmaximum with an increase of the ArAc molar part. Moreover, an interesting feature is revealed by the film of the molar ratio 1:3 in a condensed state (Figure 6c). Its spectrum shows an abrupt diminution in the intensity of the blue shoulder and a large red shift in the main band as compared to the spectrum of pure RhC18. Blue or red shifts in the spectra can be explained by the aggregation, which will cause splitting of the absorption band, according to the exciton theory.23 An alternative explanation for absorption shifts, apart from the aggregation, could be found in the changing polarity of the microenvironment with an increasing amount of fatty acid in the layer. However, an increase in the medium polarity results in only a small red shift of the main absorption band of rhodamine dyes.9,16,24 Van der Auweraer et al.16 have reported a red shift (558-564 nm) for the mixed LB film of RhC18 with ArAc on a glass surface under an increase of the RhC18/ArAc ratio and explained it as a red-shifted dimer formation with a small contribution from microenvironmental polarity. Tamai et al. have also found only a red shift from 560 to 569 nm in similar LB films.25 Moreover, changes in medium polarity cannot induce the appearance of a peak at the short-wavelength side of the main band. Thus, we assigned the peak at 528 nm to the blue-shifted dimers/aggregates, which are formed in the monolayer at the water surface. A well-resolved peak at the blue edge of the main band has been reported for pure RhC18 film at the water surface near the collapse pressure of a monolayer,9 and Kemnitz et al.24 have observed an increase in the intensity of the short-wavelength shoulder at about 528 nm under deposition of Rhodamine B on a glass substrate by adsorption from an aqueous solution. On the other hand, a red shift of spectral maximum caused by aggregation is a common phenomenon for rhodamine dyes. Such shifts have been reported for different surface substrates and arrangements.13,16,24,26 To clarify the origin of the spectral changes, the ratio of the absorption band at 528 nm and at λmax together with the λmax values was plotted vs the molar part of RhC18 at three surface-pressure values (0, 5, and 15 mN/m), as shown in Figure 7a,b, respectively. At π ) 0 mN/m, the ratio A528/Amax increases with the rise of an ArAc molar part (Figure 7a) simultaneously with the shift in the wavelength of the main maximum toward the blue region of the spectrum (Figure 7b) as the ArAc molar part reaches 0.66. At π ) 5 mN/m, a similar dependence of the A528/ Amax ratio is observed, corresponding to the appearance of the blue-side peak in the absorption spectra. At π ) 15 mN/m, a more pronounced blue-side peak is observed for the entire RhC18 molar ratio except for the 1:3 film. The (23) Kasha, M.; Rawls, H. R.; El-Bayoumi, M. A. Pure Appl. Chem. 1965, 11, 371. (24) Kemnitz, K.; Tamai, N.; Yamazaki, Y.; Nakashima, N.; Yoshihara, K. J. Phys. Chem. 1986, 90, 5094. (25) Tamai, N.; Yamazaki, T.; Yamazaki, I. Chem. Phys. Lett. 1988, 147, 25. (26) (a) Lo´pez Arbeloa, F.; Herra´n Martı´nez, J. M.; Lo´pez Arbeloa, T.; Lo´pez Arbeloa, I. Langmuir 1998, 14, 4566. (b) Chaudhuri, R.; Lo´pez Arbeloa, F.; Lo´pez Arbeloa, I. Langmuir 2000, 16, 1285.

Mixed Films of Rhodamine Dye and Arachidic Acid

Langmuir, Vol. 17, No. 17, 2001 5335 Table 3 area per molecule, Å2/molecule λmax, nm A528/Amax SH intensity, mV

Figure 7. (a) Ratio of absorption band intensity at λ ) 528 nm and at λmax vs a molar part of RhC18 in the film; (b) wavelength λmax of main absorption band vs a molar part of RhC18 at three surface-pressure values.

1:3 film represents a sudden red shift in the main band at π ) 5 and 15 mN/m, as depicted in Figure 7b. This film was found to consist of red-shifted dimers/aggregates through the entire range of film states. The observed spectral behavior can be explained by the cooperative influence of intermolecular interactions and pressure on the monolayer structure. At low surface pressure, the mutual orientation of rhodamine molecules is governed by interactions between ArAc and RhC18 molecules, which lead to the formation of either red- or blue-shifted dimers/aggregates depending on the ArAc molar part in the layer. Applied pressure also affects the monolayer structure, and blue-shifted aggregates of rhodamine molecules are observed at high surface pressure even for a low ArAc molar part. Complex relations between film structure and this cooperative influence result in a mixture of multiple species in the layers that are characterized by different spectral properties. Discussion The analysis of the SH signal from mixed monolayers of RhC18/ArAc together with spectroscopic behavior reveals that a rise in SH intensity in gaseous film regions with an increase in the ArAc molar part (see Figure 5) coincides with the appearance of blue-shifted aggregates in the layer (see Figure 6a). An enhanced SHG response from these aggregates is provided by the resonance between the electronic transition of the species formed and the wavelength of the incident light. The 1:10 film is composed mainly of blue-shifted species within almost the entire

51

47

45

37

32

30

28

560 0.57 1800

560 0.63 852

562 0.49 212

562 0.68 520

560 0.72 620

560 0.77 1200

560 0.79 1470

compression region investigated, which results in the enhancement of SH intensity as compared to the theoretical dependence for noninteracting dye molecules. From Figure 7, it seems that we can choose the A528/Amax ratio value of 0.6 and the λmax value of 562 nm as a rough criterion of the blue-aggregates domination. The rest of the films never exceed those criterion values even at high compression. Detailed inspection of the SHG behavior of the 1:10 monolayer (Figure 4f) and spectral changes induced by film compression presented in Table 3 reveal that, with a decrease of the area per molecule, this film shows an evident decrease in the ratio of A528/Amax below 0.6 as well as a small increase in the λmax value, which coincides with the observed drop of the SH signal. The spectral changes near the point of transition to expanded film at the 45 Å2/molecule are probably associated with the rearrangement of the film structure, which is caused by the rise in surface pressure. In this compression region, blue-shifted aggregates are mostly decomposed and restructured, which leads to a sharp decrease in the SH signal. Further compression in the region of the 38-28 Å2/molecule causes the formation of the blue-shifted species, as seen in Table 3, and another increase in the SH signal. Continuous changes in the monolayer structure induced by simultaneous action of RhC18/ArAc molecular interactions and applied pressure cause continuous changes in the linear and nonlinear spectral properties of the system. It was recently shown that a dominant contribution to the SHG enhancement results from blue-shifted rhodamine aggregates, whereas red-shifted ones have a relatively smaller influence on the SHG response.9 However, the SHG response from the red-shifted dye species might be amplified due to an improvement of molecular ordering within aggregates at some steps of compression.3c,d,9,10 This factor seems to play an important role in the SHG enhancement in the film of molar ratio 1:3, which is uniquely composed of red-shifted aggregates through the entire range of surface pressure. Most SHG deviations from theoretical behavior in this film occur through regions of expanded and expanded-condensed states (see Figures 1 and 4d), where structural ordering of a monolayer takes place. Other films in the studied series are composed of a mix of blue- and red-shifted aggregates, whose ratio in the layer determines the appearance of the SH signal at each compression step. Results obtained in this study show that the SHG response of the monolayer at the air-water interface can be increased both by the aggregation of chromophore moieties and by the structuring of layers. Each of these processes is, in turn, governed both by intermolecular interactions and by applied surface pressure. Relative influence of aggregation is higher when the species formed are in resonance with fundamental light frequency, while ordered structuring can accompany aggregation, producing additional contributions to SHG. This complex behavior stimulates our further interest in studying mixed monolayers, especially where intermolecular interaction between film components enhances the SHG response.

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Conclusions In this paper, the influence of ArAc molecules on the NLO response and spectroscopic properties of an RhC18 monolayer at the air-water interface under compression was investigated. ArAc molecules incorporated into a dye monolayer in a wide range of concentrations revealed good miscibility with RhC18 and had a pronounced effect on the dye self-aggregation in a gaslike phase and the aggregation on surface pressure. The appearance of a new band at the blue region of the absorption spectra as well as the red shift of the main-absorption band in mixed films was found after aggregation depending on the molar ratio of RhC18/ ArAc and the value of applied pressure. Formed blueshifted aggregates have revealed improved nonlinear properties, which should arise from their electronic transition being in resonance with a fundamental laser

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beam. From SHG measurements and spectroscopic studies, it can be concluded that structured molecular aggregates of rhodamine dye at the air-water interface demonstrate an advanced capability to generate an SH signal. The influence of molecular ordering within aggregates on a nonlinear response of film is currently under investigation by our group. Acknowledgment. The authors thank Prof. N. Kimizuka for permission to use his laboratory equipment for reflection spectra measurements and Mr. Y. Hatanaka for his assistance in the performance of the experiments. We are grateful to Prof. Y. Hatano for participating in helpful discussions. This work was supported by the Japanese Ministry of Education, Science, and Culture. LA0015415