Orientation and Aggregation Behavior of Rhodamine Dye in Insoluble

Langmuir 1999, 15, 8651-8658

8651

Orientation and Aggregation Behavior of Rhodamine Dye in Insoluble Film at the Air-Water Interface under Compression. Second Harmonic Generation and Spectroscopic Studies Oksana N. Slyadneva,† Maxim N. Slyadnev,† Valeria M. Tsukanova,† Takanori Inoue,‡ Akira Harata, and Teiichiro Ogawa* Department of Molecular and Material Sciences, Kyushu University, Kasuga-shi, Fukuoka, 816-8580, Japan Received February 5, 1999. In Final Form: July 29, 1999 The absorption and fluorescence spectra and second harmonic generation (SHG) of the insoluble monolayer of bis-(N-ethyl,N-octadecyl)rhodamine (RhC18) at the air-water interface have been measured. These spectra were affected significantly by compression, and the observed changes were ascribed to the formation and structural rearrangement of aggregated species on the water surface during compression. The spectroscopic behavior of the monolayer was explained in accordance with its rheological properties, and the transition from disordered monomers to dimers, from dimers to aggregates, and from aggregates to two-dimensional arrays was proposed. SHG studies revealed that the RhC18 molecules in the expanded film region are oriented with their C2-axis tilted away from the surface normal on angle θ distributed in the range of 31-39°. The rotational distribution around the C2-axis was assumed to be 45-60° according to preferable intermolecular interactions with the water subphase and surrounding molecules. The θ angle distribution became slightly narrow because of the increase of molecular ordering caused by two-dimensional external pressure. The sharp increase of SHG intensity and the phase shift observed at high compression were ascribed to the formation of blue-shifted aggregates with their electronic transition being in resonance with the incident laser frequency. The results of spectroscopic and SHG studies were jointly analyzed, and the structural rearrangement within the monolayer during compression was described.

Introduction Recently, the photophysical behavior of xanthene dyes at different interfaces has been studied intensively, because efficient excitation energy transfers can be observed under the formation of dimers and higher aggregates in systems with restricted molecular arrangement such as Langmuir films and molecular adsorbates on solid surfaces. Artificial organized molecular assemblies reveal excitation energy-transfer processes similar to those in biological photosynthetic systems.1 Despite their low transport efficiency compared with biological systems, the supramolecular assembly of dye molecules provides the key to understanding the mechanism of energy dissipation in relation to the spatial distribution of dye molecules.2 The photoinduced energy transfer from excited dye monomers to dimers and aggregates has been studied in the two-dimensional submono-, mono-, and multilayer of rhodamine adsorbed on fused quartz1-3 and in mixed monolayers with arachidic acid deposited on glass,4,5 with † On leave from St. Petersburg State University, Department of Chemistry, Universitetskij pr.2, 198904, Petergoff, St. Petersburg, Russia. ‡ Present address: Oita University, Department of Applied Chemistry, Oita 870-1192, Japan. * To whom correspondence should be addressed. Tel: +81-(92)583-7557. Fax: +81-(92)-7557. E-mail: [email protected].

(1) Tamai, N.; Yamazaki, T.; Yamazaki, I. Thin Solid Films 1989, 179, 451. (2) Yamazaki, I.; Tamai, N.; Yamazaki, T. J. Phys. Chem. 1990, 94, 516. (3) Kemnitz, K.; Tamai, N.; Yamazaki, Y.; Nakashima, N.; Yoshihara, K. J. Phys. Chem. 1986, 90, 5094. (4) Van der Auweraer, M.; Vershuere, B.; De Schryver, F. C. Langmuir 1988, 4, 583. (5) Vershuere, B.; Van der Auweraer, M.; De Schryver, F. C. Chem. Phys. 1991, 149, 385.

special attention to the geometry of the aggregates formed. The spectroscopic properties of the dimeric species depend very strongly on both the intermolecular distance and the geometric structure of the dimers.3 Therefore, new knowledge about the photophysical properties of the molecular aggregates could be obtained by studying the orientation of the molecules relative to each other and to the surface. When samples are prepared by deposition on the solid surface from a solution of different concentrations, a wide range of dimeric species of different geometries may exist because of the varied nature of the adsorption sites.6 Meanwhile, on the water surface, which is a smooth and homogeneous plane, insoluble molecules are free to rearrange themselves according to their preferable intermolecular interaction. The intermolecular distance between adsorbed molecules can be easily controlled with a movable barrier. Thus, insoluble films at the water surface are perfect systems for studying the orientation and interaction between molecules. The aggregation of rhodamine molecules has been widely investigated in solutions, and both blue-shifted (H-band) and red-shifted (J-band) dimers were formed.7-14 (6) Peterson, E. S.; Harris, C. B. J. Chem. Phys. 1989, 91 (4), 2683. (7) Selwyn, J. E.; Steinfeld, J. I. J. Phys. Chem. 1972, 76 (5), 762. (8) Gal, M. E.; Kelly, J. R.; Kuruksev, T. J. Chem. Soc., Faraday Trans. 2 1973, 69, 395. (9) Muto, J. J. Phys. Chem. 1976, 80 (12), 1342. (10) Rohatgi, K. K.; Singhal, G. S. J. Phys. Chem. 1966, 70 (6), 1695. (11) Ojeda, P. R.; Amashita, I.; Ochoa, J. R.; Arbeloa, I. L. J. Chem. Soc. Faraday Trans. 2 1988, 84, 1. (12) Arbeloa, F. L.; Ojeda, P. R.; Arbeloa, I. L. Chem. Phys. Lett. 1988, 148, 253. (13) Arbeloa, F. L.; Ojeda, P. R.; Arbeloa, I. L. Photochem. Photobiol. 1988, 45, 313. (14) Arbeloa, F. L.; Ojeda, P. R.; Arbeloa, I. L. J. Chem. Soc., Faraday Trans. 2 1988, 84, 1903.

10.1021/la9901229 CCC: $18.00 © 1999 American Chemical Society Published on Web 10/01/1999

8652

Langmuir, Vol. 15, No. 25, 1999

Several structures for sandwich-type three-dimensional dimers in solution have been proposed according to excitonsplitting theory.15,16 On the contrary, different aggregation behavior was observed on surfaces, because only in-plane two-dimensional dimers would form at the interfaces due to geometry restrictions. With the increase of the surface coverage of xanthene dyes, the dimerization causes only a small red shift of absorption maximum, as was reported by Peterson and Harris6 for adsorbed rhodamine B from the water solution on a glass surface. Meanwhile, Kemnitz et al.3 observed both a red-shifted peak (558-565 nm) and an increase in the intensity of the blue-shifted shoulder at about 528 nm in similar experiments but under different conditions of sample preparation. Van der Auweraer et al.4,5 noted only a red shift in absorption (558-565 nm) for the mixed LB films of rhodamine dye with arachidic acid on a glass surface under increase of the dye/matrix ratio. Both Kemnitz et al.3 and Van der Auweraer et al.4 observed a red shift of the fluorescent band in the emission spectra of rhodamine on solid surfaces with increasing surface coverage. These findings agree with the results of Yamazaki et al.2 However, originally prepared Langmuir film on the water surface may have a different structure with resulting LB films on solid substrates, and aggregates of molecules on the water surface would have spectroscopic properties different from those on solid substrates because of the differences in interaction with the subphase. Second harmonic generation (SHG) is a sensitive method for the investigation of the surface, because it is forbidden in electric-dipole approximation in centrosymmetric media. At the surface where the inversion symmetry is broken, SHG becomes electric-dipole allowed.17 The resonant SHG technique is more selective and sensitive to changes in orientation and aggregation of the molecules in the film than the nonresonant SHG method.17 Information about the molecular orientation relative to the surface normal can be obtained by analyzing the polarization dependencies of the SHG response.17 Numerous orientation studies by the SHG of large soluble dye molecules at the submonolayer coverage have been performed for the solid-air interface,18-23 the solidliquid interface,24 and the air-water interface.25 Although several works have focused on the molecular orientation at the air-water surface, most of the molecules studied previously had only a single dominant molecular nonlinear polarizability tensor element (rod-like molecules),26-30 (15) Kasha, M.; Rawls, H. R.; El-Bayoumi, M. A. Pure Appl. Chem. 1965, 11, 371. (16) McRae, E. G.; Kasha, M. J. Chem. Phys. 1958, 28, 721. (17) Corn, R. M.; Higgins, D. A. Chem. Rev. 1994, 94, 107. (18) Heinz, T. F.; Chen, C. K.; Ricard, D.; Shen, Y. R. Phys. Rev. Lett. 1982, 48 (7), 478. (19) Di Lazzaro, P.; Mataloni, P.; De Martini, F. Chem. Phys. Lett. 1985, 114 (1), 103. (20) Higgins, D.; Buerly, S.; Abrams, M.; Corn, R. J. Phys. Chem. 1991, 95, 6984. (21) Inoue, T.; Moriguchi, M.; Ogawa, T. Anal. Sci. 1995, 11, 671. (22) Inoue, T.; Moriguchi, M.; Ogawa, T. Anal. Chim. Acta, 1996, 330, 117. (23) Inoue, T.; Moriguchi, M.; Ogawa, T. Thin Solid Films, submitted for publication, 1999. (24) Campbell, D.; Higgins, D.; Corn, R. J. Phys. Chem. 1990, 94, 3681. (25) Tamburello-Luca, A. A.; Hebert, Ph.; Antoine, R.; Brevet, P. F.; Girault, H. H. Langmuir 1997, 13, 4428. (26) Eisenthal, K. B. J. Phys. Chem. 1996, 100, 12997. (27) Yan, E. C. Y.; Liu, Y.; Eisenthal, K. B. J. Phys. Chem. 1998, 102, 6331. (28) Barnosky, A. A.; Frysinger, G. S.; Gaines, G. L., Jr.; Korenwsky, G. M. Colloids Surf. A 1994, 88, 123. (29) Rasing, Th.; Shen, Y. R.; Kim, M. W.; Valint, P.; Bock, J. Phys. Rev. A 1985, 31 (1), 537. (30) Rasing, Th.; Shen, Y. R.; Kim, M. W.; Grubb, S. Phys. Rev. Lett. 1985, 55 (26), 2903.

Slyadneva et al.

whereby the orientation analysis could be significantly simplified. The application of the resonant SHG method to large planar dye molecules should take into account the interaction between the monomer’s transition dipole moments responsible for the SHG signal. The estimation of the intermolecular interaction can be made from the absorption and fluorescence spectra of the molecular layer. In this article, we have investigated the aggregation and orientation behavior of rhodamine dye with long alkyl chains at the air-water interface under compression using absorption and fluorescence spectroscopies and the SHG technique. We have discussed the spectroscopic properties of rhodamine dye chromophores, the formation of different types of dimers (aggregates), and the structural rearrangement of aggregates upon increasing the molecular density under external pressure. The surface nonlinear susceptibility tensor elements of the rhodamine molecule and the complex dielectric constants of the adsorbate layer were calculated from the observed spectroscopic data. On the basis of these results, a structure of the compressed RhC18 monolayer on the water surface is proposed. Experimental Section Bis-(N-ethyl,N-octadecyl)rhodamine perchlorate (RhC18) was obtained from NKS Kenkyusho Chemical Corp. and used without additional purification. Monolayers of RhC18 were prepared by spreading 5-7 µL of 10-3 M benzene solution on a pure water subphase in a cell with an area of 100 cm2 in the SHG measurements and 10-14 µL of a 10-4 M benzene solution in a cell with an area of 18 cm2 (sample cell constructed from Teflon with a glass bottom) for spectroscopic measurements. Water was deionized and purified using a Millipore Milli-Q system. The surface density of the molecules was controlled by movable Teflon barriers. All experiments were performed at 25 °C. The SHG measurement was carried out in reflection geometry using a frequency-doubled output at 532 nm from a pulsed Nd3+: YAG laser (pulse duration, 200 ps; repetition rate, 10 Hz). The laser beam was softly focused at a sample surface in an area of 0.25 cm2. The incident angle was 38° from the surface normal. Surface energy density was 5 mJ/cm2 per pulse. The polarization of the incident beam was varied continuously from 0° (p-polarized) to 90° (s-polarized) by a λ/2 wave plate. The reflected second harmonic (SH) light from the monolayer was separated from the incident laser beam by cutoff and band-pass filters and detected by a photomultiplier (Hamamatsu R585). The output polarization (p- and s-) of the SH signal was analyzed by a Glan-Taylor polarizer. The SH signal was normalized by the reference signal from a KDP frequency-doubling crystal to eliminate laserintensity fluctuations. The SH signal from the bare water surface was negligible compared with that of the monolayer under investigation. The surface absorption and fluorescence emission spectra of RhC18 were recorded using a liquid nitrogen-cooled CCD camera (Princeton Instruments) attached to the out-port of a monochromator (Thermo Jarell Ash 27). The light source for absorption measurements was a temperature-stabilized tungsten lamp. The spectrum of the bare water surface served as reference. The accumulation time for both sample and reference spectra was set to 20 s. Fluorescence studies of rhodamine on the water surface were performed using a fluorescence microscope setup with ns-pulsed Nd3+:YAG laser excitation (532 nm). Emission spectra were recorded after appropriate correction. The accumulation time for one spectrum was set to 10 s. A Langmuir-Blodgett trough with a Wilhelmy balance (USI System, model FSD-300) was used in the measurement of the surface pressure-area isotherm.

Results and Discussion Characterization of the RhC18 Monolayer at the Air-Water Interface. A surface-pressure isotherm of

RhC18 at Air-Water Interface

Langmuir, Vol. 15, No. 25, 1999 8653

Figure 2. Absorption spectra of the RhC18 monolayer at the air-water interface under compression at different areas per molecule: (a) 570, (b) 233, (c) 133, (d) 66, and (e) 39 Å2/molecule.

Figure 1. Surface pressure versus area-per-molecule isotherm of a film of pure RhC18 at the air-water interface. The inset shows the molecular structure of RhC18.

the RhC18 monolayer on the water surface is shown in Figure 1. The isotherm clearly distinguishes several regions corresponding to the different states of the film. The region from 210 to 70 Å2 per molecule belongs to the expanded film, where molecules are connected by lateral adhesive van der Waals forces between the long chains.31 The alkyl chains start to rise under compression from an initial flat position along the water surface at this region. After the tails occupy a position close to vertical and the distance between the chromophores becomes significantly small, the transition between the expanded and condensed states appears (70-40 Å2/molecule). At this point, the RhC18 xanthene groups undergo essential rearrangement from a random orientation to a preferable geometric array to occupy the minimum space at the surface. The solid and liquid phases coexist in this region.31 At higher pressures, the region of the condensed film (40-20 Å2/ molecule) and a collapse region of the monolayer can be found. Spectroscopic Studies of the RhC18 Monolayer: Aggregation during Compression. Typical absorption spectra of the monolayer RhC18 on the water surface under compression are shown in Figure 2. Changes in the absorption maximum and the intensity ratio are shown in Figure 3. We have divided the observed region into four regions based on the changes shown in Figure 3. No significant shift occurred in the peak at 563 nm at compression up to 300 Å2/molecule (region 1). However, drastic changes in spectra were observed when the film was further compressed. There was a small red shift to 567 nm upon further compression to 100 Å2/molecule (regions 2 and 3), and the band peak was shifted back to 565 nm with decreasing the area per molecule from 100 to 39 Å2 (region 4). At the same time, a shoulder at 528 nm developed to a peak. The ratio of the absorption band intensity at 528 nm and at 563-567 nm significantly changed as shown in Figure 3 (bottom). The ratio remained constant in region 1 and then decreased in region 2. Subsequently, it rapidly increased in region 4. (31) Adam, N. K. The Physics and Chemistry of Surfaces; Oxford University Press: London, Geoffrey Cumberlege, 1939; p 59.

Figure 3. (top) The position of absorption maximum of RhC18 at the air-water interface during film compression. (bottom) The ratio of the absorption bands at 528 nm and the main maximum versus area per molecule for RhC18 at the air-water interface. The observed region was divided into four regions for the purpose of explanation.

Film compression also affected the fluorescence of RhC18 at the air-water interface. The fluorescence spectra of RhC18 on the water surface at different areas per molecule are shown in Figure 4, the inset shows a red shift of the band maximum from 583.1 to 588.0 nm during film compression. The fluorescence peak intensity was initially proportional to the surface density in regions 1 and 2 and decreased sharply at an area of approximately 210 Å2/ molecule, as shown in Figure 5. The intensity of the redshifted shoulder at 650 nm increased simultaneously. The fluorescence intensity fluctuated significantly in regions 1 and 2; 50 spectra were averaged and accumulated for each compression step in Figure 5. Similar intensity fluctuation was observed for absorption spectra (20-40 spectra were recorded and accumulated). These intensity fluctuations should be caused by RhC18 domains floating on the water surface, because the time-averaged absorption spectrum and the time-averaged fluorescence spectrum were linearly proportional to the surface density and revealed no peak shift; the domains probably were formed owing to the van der Waals interaction of long

8654

Langmuir, Vol. 15, No. 25, 1999

Figure 4. The normalized fluorescence spectra of the RhC18 monolayer at the air-water interface under compression at different areas per molecule: (a) 570, (b) 310, (c) 233, and (d) 200 Å2/molecule. The inset shows the position of the fluorescence peak during film compression.

Figure 5. The dependence of fluorescence intensity of RhC18 at the air-water interface on the surface density of dye molecules (dashed line). The solid line is the linear best fit in the region of increasing intensity. Error bars represent the standard deviation of fluorescence intensity.

alkyl chains.26,27,32-34 The fluctuations disappeared at 210 Å2 because the film became a continuous monolayer. A similar domain formation has been found for the longchain fatty-acid films at the air-water interface below 0.1 mN/m.35,36 The observed spectroscopic behavior indicates that RhC18 molecules exist in a monomeric form in region 1, which corresponds to the gas-like phase at the air-water interface. The main absorption band at 563 nm can be (32) Slyadnev, M. N.; Inoue, T.; Harata, A.; Ogawa, T. Colloids Surf. A, in press, 1999. (33) Li, Y.-Q.; Slyadnev, M. N.; Inoue, T.; Harata, A.; Ogawa, T. Langmuir 1999, 15, 3035. (34) Matsuzawa, Y.; Seki, T.; Ichimura, K. Thin Solid Films 1997, 301, 162. (35) Takoshima, T.; Masuda, A.; Mukasa, A. Thin Solid Films 1992, 210/211, 51. (36) Honig, D.; Mobius, D. Thin Solid Films 1992, 210/211, 64. (37) Dutta, A. K.; Salesse, C. Langmuir 1997, 13, 5401.

Slyadneva et al.

assigned to the S0fS1 transition of the monomer, and its transition dipole moment lies along the long axis of the xanthene moiety.6,8 The shoulder at 528 nm was ascribed to a monomer vibrational band.3 This is consistent with the fluorescence behavior in region 1, where RhC18 molecules showed the fluorescence spectrum of monomer species. There are two explanations for the absorption and fluorescence red shifts under compression in region 2. Rhodamine dyes are sensitive to microenvironments whose changes induce a red shift both in absorption and emission spectra with increasing polarity of the surrounding medium,3 which is the case when xanthene moieties approach each other. An alternative explanation is the formation of dimers which have red-shifted absorption and fluorescence bands compared with monomers. Because fluorescence intensity is linear to the surface density of the chromophores in region 2, the former explanation (microenvironment effect) seems to be correct. The small red shift of the fluorescence maximum in regions 1 and 2 may also be attributed to the energy transfer from the excited monomers to energetically lower monomer sites, as it was observed by a time-resolved fluorescence spectroscopy of RhC18 incorporated in the LB films.1,2 The broad weak shoulder centered at 650 nm in the spectra at 200 Å2/molecule may be assigned to the fluorescence from dimers, which have a red-shifted absorption.2,3 The sharp decrease of the fluorescence intensity in region 3 should not be due to the microenvironment effect. It is well-known that rhodamine dyes are self-quenched at higher surface density because of aggregate formation.3,25 Upon compression above 210 Å2/molecule (region 3) rhodamine fluorophores are brought close together, resulting in the formation of dimers and/or aggregates that are mostly nonfluorescent. Not only direct excitation to the nonfluorescent state but also energy transfer from fluorescent monomeric species to these trap sites induce the observed decrease of the fluorescence intensity (Figure 5). A rapid nonradiative relaxation takes place through the trap sites to the medium.1-3 The observation of a small but measurable fluorescence intensity even at the highest compression suggests that monomers and fluorescent dimers still remain in the film. Identical fluorescence quenching with increasing surface concentration of fluorophores due to energy migration was observed for molecules such as fluorescein and rhodamine in mixed LB films and at the air-water interface,37,38 and the formation of rhodamine dimers which are fluorescent at longer wavelengths has also been established.3 The sharp increase in the absorption ratio observed at 100 Å2/molecule, the increasing intensity of the blueshifted side peak at 528 nm, and the main absorption peak shift to 565 nm in region 4 indicate complete rearrangement of chromophore moieties in a close-packed monolayer film characterized by a blue-shifted aggregate structure. The absorption spectra at high compression seem to represent the superposition of the spectra of coexisting species: blue-shifted aggregates, a small amount of red-shifted ones, and the remaining monomers. Dutta et al.37,38 and Kemnitz et al.3 observed the increase in absorbance ratio for rhodamine dyes with a increase of the surface density and explained the results by the formation of dimer/aggregate species. The aggregation behavior of the dye chromophore can be described with the exciton-splitting theory developed by Kasha et al.15,16 This theory shows that the interaction (38) Dutta, A. K.; Lavoie, H.; Ohta, K.; Salesse, C. Langmuir 1997, 13, 801.

RhC18 at Air-Water Interface

of the transition dipole moments of the molecules forming the dimers (aggregates) can result in the spectral shift or splitting of the absorption band, and that the exciton splitting of the excited state in the composite molecules depends on both the chromophore spacing and the relative spatial arrangement of the transition dipole moments. The red or blue shift and its magnitude will depend on the angle R and the distance r of chromophores relative to each other. In addition, the fluorescent or nonfluorescent nature of the dimers formed depends on the angle R.3 Based on the observed shifts in the absorption spectra, we can propose a possible geometry of the dimers and aggregates at the air-water interface according to the exciton-splitting theory. Because, the band peak is shifted to the red and there is no sign of band splitting (Figures 2 and 3) in region 3, the molecules in the dimer should have a parallel dipole moment and the angle of inclination R should be in range 0° < R < 54.7°. It seems likely that in this region the xanthene moieties are favorably oriented to form linear aggregates or an arrayed structure with parallel dipole moments due to dipole-dipole interaction. At further compression in region 4, the external pressure forces the red-shifted species to change their geometry and rearrange themselves, yielding to the blue-shifted ones. This is characterized by a drastic increase in absorbance ratio and by a sudden shift to blue of the band maximum. For the structure of the blue-shifted aggregates, the angle R should be in the range of 54.7° < R < 90°, i.e., the molecules in the dimers change the inclination angle because of some freedom in lateral reorientation at the air-water interface. Because the compression at this region forces the molecule to occupy the smallest possible area, they formed closely packed monolayers with highly ordered chromophore aggregates. Similar processes of chromophore ordering in closely packed monolayers were observed at the air-water interface under high compression37,38 and in films transferred to solid substrates.39-41 The observed changes of the absorption and fluorescence spectra demonstrate that dimer/aggregate species are formed and that their continuous structural rearrangement on the water surface takes place during film compression. There should be a transition from disordered monomers to dimers, from dimers to aggregates, and then to two-dimensional arrays based on the spectroscopic behavior of the monolayer in accordance with its rheological properties. Second Harmonic Generation Studies of the RhC18 Monolayer during Compression. SHG from the adsorbate layer on the surface can be described as an induced nonlinear optical response of the ensemble of molecules to the incident laser-light field. The general expression for the second harmonic intensity I(2ω) may be obtained as developed by Mizrahi and Sipe,42

I(2ω) )

32π3ω2 sec2Ωω[e(2w)χ(2):e(ω)e(ω)]2I2(ω) (1) 3 Ac

where Ωω is the angle of incidence of fundamental light of frequency ω and intensity I(ω), χ(2) is the surface secondorder susceptibility tensor, the vectors e(ω) and e(2ω) are the fundamental and second-harmonic light fields at the (39) Wolthaus, L.; Schaper, A.; Mobius, D. Chem. Phys. Lett. 1994, 225, 322. (40) Nakahara, H.; Fukuda, K.; Mobius, D.; Kuhn, H. J. Phys. Chem. 1986, 90, 6144. (41) Zhai, X.; Kleijn, J. M. Thin Solid Films 1997, 304, 327. (42) Mizrahi, V.; Sipe, J. E. J. Opt. Soc. Am. B 1988, 5 (3), 660.

Langmuir, Vol. 15, No. 25, 1999 8655

surface, A is the illuminated surface area, and c is the light speed. The elements of the surface nonlinear susceptibility tensor χIJK and the elements of the molecular nonlinear polarizability tensor βijk, which describe the nonlinear behavior of the individual molecule, are related through the average of the molecular orientation distribution function and are given by eq 2,

χIJK ) Ns

∑〈T(φ,θ,Ψ)〉 βijk

(2)

where Ns is the number of molecules per unit surface, 〈T(φ,θ,Ψ)〉 is the transformation tensor between the molecular and the laboratory frames, where molecular axes are referenced with Euler’s angles φ, θ, and Ψ, and the bracket denotes an average over all of the molecules on the surface. If there is more than one molecular species, all of them contribute to macroscopic susceptibility, and the sum over the 〈Ti(φ,θ,Ψ)〉 βi(2) for each species i is required. The elements of the χ(2) tensor being known, the average orientation of the molecular axes relative to the laboratory coordinates can be estimated using eq 2. If the surface is rotationally isotropic about the surface normal, there are only three independent nonzero components of the surface macroscopic susceptibility tensor χ(2): χZZZ, χZXX ) χZYY, and χXXZ ) χZXZ ) χYZY,43 where X, Y, Z is the Cartesian coordinate defined with the surface normal as the +Z direction. Because there was no intensity change for a sample rotation around the Z-axis in the range 0-360°, the RhC18 layer on the water surface at 350-100 Å2/molecule is randomly distributed for φ and exhibits rotational isotropy about the surface normal. Then, the intensity of the s-polarized and p-polarized SHG signal can be directly related to these χ(2) elements,17

Is(2ω) ∝ [a1χXXZ sin 2γ]2I2(ω)

(3)

Ip(2ω) ∝ [(a2χXXZ + a3χZXX + a4χZZZ) cos2 γ + a5χZXX sin2 γ]2I2(ω) (4) where γ is the polarization angle of the incident light, and ai includes the dielectric constants of the monolayer under investigation: (ω) and (2ω) and the components of e(ω) and e(2ω), which depend on the incident angle through the Fresnel coefficients. The complex dielectric constants of the monolayer at ω and 2ω can be calculated using the UV-visible absorption spectra of the sample and the adsorbate surface density; their values are given by (ω) ) [η(ω) + iκ(ω)]2, where η(ω) is the refractive index and κ(ω) is the extinction coefficient of the monolayer.20 The value of κ(ω) can be derived directly from the absorbance A(ω), and the value of η(ω) can be obtained using the Kramers-Kronig relation.20 Assuming C2v symmetry for the rhodamine molecule,6 the molecular nonlinear polarizability tensor β(2) has only three independent nonzero elements, βxxz ) βxzx, βzxx, and βzzz, where the z-axis corresponds to the molecular C2axis and x lies in the xanthene ring. Because the distribution on φ is random, the three nonzero elements in eq 2, χXXZ, χZXX, and χZZZ, can be expressed separately in terms of elements βxxz, βzxx, and βzzz and angles θ and Ψ. Because the S0fS1 and S1fS2 transition dipole moments are parallel to the molecular x-axis and the S0fS2 moment is parallel to the z-axis,6,8 the βzzz component is much smaller than βxxz and βzxx, and is negligible. After (43) Mazely, T. L.; Hetherington, W. M. J. Chem. Phys. 1987, 86 (6), 3640.

8656

Langmuir, Vol. 15, No. 25, 1999

Slyadneva et al. Table 1. The Correlated Values of ψ and θ Anglesa ψ

0°

30°

45°

60°

90°

θ

27.1°

29.7°

33.2°

38.5°

47.6°

a

At area per molecule 300 Å2.

Table 2. The Values of θ Angles and the Dielectric Constants E(ω) of RhC18 Monolayer at 532 nm at the Air-Water Interface at a Few Areas Per Moleculea,b area per molecule, Å2 300 266 233 200

Figure 6. Polarization dependencies of the SHG signal for RhC18 on the water surface at different areas per molecule: (top) p-out, (bottom) s-out. γ ) 0° corresponds to p-in and γ ) 90° to s-in of the incident laser. The solid lines are the fits to eqs 3 and 4. Curve: (a) 300, (b) 266, (c) 233, and (d) 200 Å2/ molecule.

Figure 7. Second-harmonic intensities as a function of RhC18 surface density. Curve: (a) Issp, (b) Ippp, and (c) Iqqs signals.

rearrangement, the orientation parameter D was obtained,

D)

2 〈cos3 θ〉 (3〈cos ψ〉 - 1)χZZZ + 2χXXZ + χZXX (5) ) 〈cos θ〉 3〈cos3 ψ〉χZZZ + 2χXXZ + χZXX

where the angle θ defines the position of the molecular C2-axis relative to the surface normal, and the angle Ψ defines the rotation of the molecule around the C2-axis. A distribution about the Ψ angle should be known to obtain the distribution on the θ angle. Figure 6 shows the polarization dependencies of the second harmonic s- and p-out intensities from the expanded monolayer of rhodamine on the water surface. In the range 300-200 Å2/molecule, the SHG intensities Issp, Ippp, and Iqqs (the first two subscripts denote the polarization of the input light and the third one the polarization of the output light, and q represents 45°) have been found to be quadratically proportional to the rhodamine surface density, shown in Figure 7, as expected theoretically (see eqs 1, 3, 4). This finding indicates that only one molecular species contributes to the susceptibility tensor χ(2). According to the spectroscopic studies of the monolayer in this region, this species should be the monomer. Although

dielectric constant 0.86 + 0.24i 0.83 + 0.25i 0.80 + 0.26i 0.77 + 0.27i

θ, degree 33-39° 32-37° 32-36° 31-35°

a The distribution of ψ angle is assumed to be uniform in the interval 45-60°. b The value of (2ω) ) 1.0 + 0.05i at 200 Å2/molecule was used throughout calculations.

red-shifted dimers start to appear in this region, their concentration should be relatively small because the red shift of the absorption band was small and the fluorescence intensity still increased linearly. Another explanation would be that the second-order polarizability of the redshifted dimer is close to that of the monomer in this compression region. To determine the orientation of monomers on the surface, the relative values of the χXXZ, χZXX, and χZZZ tensor elements have been obtained from experimental SHG polarization dependencies by fitting them to eqs 3 and 4 using a nonlinear least-squares method. These values of the tensor elements were used in eq 5 to extract a relationship between the two unknown angles θ and ψ. At an area of 300 Å2 per molecule, we have calculated the relation between θ and ψ as shown in Table 1 as pairs of θ and ψ angles, where the distribution of θ and ψ was assumed as a delta function. Meanwhile, molecular modeling revealed that only a certain range of angles is the most likely in view of the carboxyphenyl ring localization on the surface. The closest position of the carboxylic group to the aqueous phase was obtained for the correlated pairs of θ and ψ when ψ g 45°. In addition, a rough estimation from fluorescence polarization measurements44 excluded orientations such as ψ > 60°. Then, we could assume a uniform distribution of ψ in the interval of 4560°. In this case, corresponding uniform distributions of θ for a few areas per molecule were obtained from eq 5, as shown in Table 2. The complex dielectric constants of monolayer (ω) calculated from the absorption spectra are also given in Table 2. The value of (2ω) ) 1.0 + 0.05i was estimated from the UV-vis spectra at 200 Å2/molecule (due to experimental difficulties) and was used throughout calculations: a variation of 20% in the dielectric constant of (2ω) leads to an error of θ of ∼1°. The accuracy of θ given in Table 2 was estimated to be ∼2° based on experimental errors in all variables of eqs 3-5. Thus, the orientation of RhC18 chromophores at the air-water interface remains almost constant in the gas-like region (regions 1 and 2), although a small narrowing of θ may be indicated. This means that the molecules orient more strictly in terms of θ angle with decreasing area per molecule. The orientation of RhC18 monomers on the water surface obtained here agrees well with that of related (44) The degree of anisotropy r ) (I⊥ - I|)/(I⊥ + 2I|), I⊥ and I| being fluorescence intensities perpendicular and parallel to the polarization of the excitation beam [Piasecki, D. A.; Wirth, M. J. J. Phys. Chem. 1993, 97, 7700], was -0.16 for regions 1 and 2. This results in the angle between the long axis of xanthene moieties and the surface normal of 70 ( 10°, taking into account that the angle between absorption and emission transition dipole moments is 13° [Lieberherr, M.; Fattinger, Ch.; Lukosz, W. Surface Sci. 1987, 189/190, 954].

RhC18 at Air-Water Interface

Langmuir, Vol. 15, No. 25, 1999 8657

Table 3. The Ratio of Components of β(2) Tensor at Different Areas Per Molecule area per molecule, Å2

300

266

233

199

166

133

100

66

50

βxxz/βzxx

0.74

0.73

0.74

0.74

0.55

0.50

0.45

0.53

0.69

Figure 8. Polarization dependencies of the p-polarized SHG signal for RhC18 on the water surface at different areas per molecule. The solid lines are the fits to eq 4. Curve: (a) 166, (b) 133, (c) 100, (d) 66, and (e) 50 Å2/molecule. The phase shift δ between Ippp and Issp is: (a-c) 0°, (d) 45°, and (e) 60°. The dotted lines are the fits for d and e with δ ) 0°.

xanthene dyes.25 As expected from the molecular structure of these dyes, preferable interaction of the hydrophilic groups with the water surface should determine their orientation. The observed distribution of θ and ψ allows the carboxylic group and one of the amino groups to be positioned toward the water surface, whereas hydrophobic aliphatic chains are tilted into air. Further compression in the 200-39 Å2/molecule region led to a nonquadratic increase in Issp, Ippp, and Iqqs second harmonic intensities, as shown in Figure 7. This observation may be attributed either to the change of molecular orientation or to the additional contribution of dimers and aggregates to the χ(2) tensor according to eq 2. To decide which the case was, the ratio of molecular polarizability tensor elements,16

βxxz 2χXXZ + χZZZ ) βzxx 2χZXX + χZZZ was estimated as a function of area per molecule, and the results are shown in Table 3. This ratio is independent of θ and should be uniquely determined by the electronic properties of the species. In the 300-200 Å2/molecule region, the β ratio remained almost constant but changed significantly during further compression, indicating that a new species starts to contribute to the SHG response of the monolayer at 200 Å2/molecule and then at 66 Å2/ molecule. This finding is consistent with the formation of dimers/aggregates observed by spectroscopic studies. Because the susceptibility of each constituent is unknown, eq 2 is insoluble, and an accurate determination of the molecular orientation of every species is impossible. Similar findings on the SHG signal with the increase of the surface coverage were reported for xanthene dyes at the air-water interface45 and for an LB film of a coumarin dye,46 in which signal changes were related to extended conjugation of the chromophores. (45) Tsukanova, V.; Slyadneva, O.; Inoue, T.; Harata, A.; Ogawa, T. Chem. Phys., submitted for publication, 1998. (46) Ashwell, G. J.; Walker, T. W.; Leeson, P. Langmuir 1998, 14, 1525.

Figure 9. A schematic presentation of a monolayer structure, where alkyl substituents at nitrogen atoms are not shown for simplicity of presentation. (a) orientation of monomer RhC18 at low surface coverage; the dashed line shows the direction of the C2-axis; (b) the structure of the dimer with a red shift of absorption; the front view along the X-axis and the top view along the Z-axis of the laboratory frame are shown, respectively; (c) the structure of the condensed monolayer with a blue shift of absorption.

The polarization dependence of the p-polarized SHG intensity revealed a noticeable change in their appearance at higher compression, as shown in Figure 8. One plausible explanation is that the electronic transition of the blueshifted aggregated species, which becomes dominant at high compression, is nearly resonant with the wavelength of the incident laser. The imaginary part of the complex dielectric constant of monolayer Im() in this region was found to increase significantly below 100 Å2/molecule and to become comparable with the real component Re(). This finding suggests that the molecular nonlinear polarizability, β(2), may have a considerable imaginary part, which in turn should cause a phase shift between the various elements of macroscopic susceptibility χ(2).16 Actually, the phase shift δ between Ippp and Issp was derived from p-polarized SHG curves by fitting them to eq 4, considering the coefficients for cos2γ and sin2γ as complex variables. Though the exact phase shift should be measured using an interference technique,20 we could estimate the phase

8658

Langmuir, Vol. 15, No. 25, 1999

shift approximately as δ ) 60° ( 10° at highest compression and δ ) 0° ( 10° at lower compression, as shown in Figure 8. In general, the phase shift characterizes the time-dependent SHG response as a function of the incident laser frequency47 and is governed by the inherent electronic properties of the species under investigation. The observation of an increasing phase shift along with enhancement of SHG intensity at higher compression indicates the appearance of a new species which effectively generates the SHG signal and has a molecular polarizability tensor β(2) different from that of the precursor species. Strong interactions between xanthene moieties significantly change the properties and symmetry of the π-electron conjugated system, and this change induces a large change in the pattern of the β(2) tensor. These considerations are supported by the dependence of the βxxz/βzxx tensorelements ratio on surface density, as shown in Table 3. The new nonzero χ(2) elements may appear due to increased monolayer symmetry upon compression.17,43 Phase shifts at approaching electronic resonances were observed for methylene blue20 and p-nitrophenol monolayer48 on solid surfaces. The analysis of the spectroscopic behavior of the RhC18 monolayer on the air-water interface together with SHG studies have led to the following proposed picture. The monomer of RhC18 stays on the water surface in such a way that its C2-axis is tilted by θ ) 31-39° away from the surface normal, its carboxyl group forms a hydrogen bond with the surface water, and one of the partly positive nitrogen atoms electrostatically interacts with the counterion ClO4- in the water subphase (Figure 9a). The dimer formation forced by compression leads to a certain distribution of red-shifted dimer structures with parallel coplanar dipole moments inclined on angle R (0° < R < 54.7°), where the nitrogen atom favorably interacts with the carboxylic group of the neighboring molecule in the dimer (Figure 9b). The proposed structure permits the formation of a chain arrangement of molecules in which (47) Bloembergen, N. Nonlinear Optics; W. A. Benjamin, Inc.: New York, 1965; p 59. (48) Higgins, D. A.; Abrams, M. B.; Byerly, S. K.; Corn, R. M. Langmuir 1992, 8, 1994.

Slyadneva et al.

the structure unit is the red-shifted dimer. The spacefilling molecular simulation reveals that the transition from the red-shifted dimer to the blue-shifted aggregate requires detachment of the nitrogen atom from the water surface so that the molecules occupy minimum space at the surface. The xanthene moieties become parallel to the water surface. A corresponding structure would be described as presented in Figure 9c. In this structure, the xanthene moiety projection on the surface partly overlaps with that of neighboring molecules, resulting apparently in a lower area per individual molecule than predicted by the space-filling model. A similar arrayed structure of dyes was previously observed by atomic-force microscopy on solid substrates after transferring from air-water interface. Further compression would be accompanied by the rotation of the carboxyphenyl ring to the xanthene plane, and similar behavior was proposed in LB films.2 Conclusions In this article, we present the orientational and aggregation behavior of the rhodamine dye monolayer at the air-water interface. Both absorption and emission spectra are affected significantly by compression. A small red shift of the absorption maximum is followed by the appearance of a new blue-shifted absorption band, whereas a red shift in fluorescence maximum is accompanied by intensity quenching. These findings are explained based on the transition from monomers to dimers, from dimers to aggregates, and then to two-dimensional arrays. SHG studies reveal that RhC18 molecules in the expanded film region are oriented with their C2-axis tilted away from the surface normal on angle θ distributed in the range 31-39°. The increase of the SHG intensity at high compression is explained by the formation of blueshifted aggregates with their electronic transition being in resonance with the incident laser wavelength. The observed phase shift supports the conclusion above. The results of spectroscopic and SHG studies are jointly analyzed in relation to the structural rearrangement within the monolayer during compression. LA9901229