Microscopic Organization of Long-Chain Rhodamine Molecules in

J. Phys. Chem. B 2002, 106, 4203-4213

4203

Microscopic Organization of Long-Chain Rhodamine Molecules in Monolayers at the Air/Water Interface Valeria Tsukanova,†,‡,§ Hugo Lavoie,‡,§ Akira Harata,| Teiichiro Ogawa,| and Christian Salesse*,‡,§ GREIB, Department Chimie-Biologie, UniVersite´ du Que´ bec a` Trois-RiVie` res, Trois-RiVie` res, Que´ bec, Canada G9A 5H7, CERSIM, UniVersite´ LaVal, Ste-Foy, Que´ bec, Canada G1K 7P4, and Department of Molecular and Material Sciences, Kyushu UniVersity, Kasuga-shi, Fukuoka 816-8580, Japan ReceiVed: October 8, 2001; In Final Form: January 4, 2002

The monolayer behavior of a rhodamine derivative with two C18 aliphatic chains attached to the nitrogen atoms of the xanthene ring system (RhC18) was studied by epifluorescence microscopy, second-harmonic generation (SHG), and absorption and fluorescence spectroscopy. The isotherm of RhC18 exhibits a plateau which, presumably, corresponds to a slow collapse of the monolayer. As observed by epifluorescence microscopy, the RhC18 monolayer remained homogeneous for a large range of molecular areas, and an abrupt change in the monolayer morphology occurred at the end of the surface pressure isotherm. Spectroscopic data showed that both fluorescent J-aggregates and nonfluorescent H-aggregates with a coplanar-inclined configuration were formed within the RhC18 monolayer upon compression. The orientational SHG measurements revealed that, in the expanded region of the isotherm, the RhC18 chromophore was oriented in a way where its xanthene plane made an angle of 36° with respect to the interface. The azimuthal angle SHG measurements revealed that compression induced an anisotropic arrangement of the S0-S1 transition moment of RhC18 chromophores perpendicular to the compression direction. At the end of the isotherm prior to and after the collapse point, the film possessed a fairly regular structure characterized by a C2V symmetry packing and a long-range order in the parallel alignment of the RhC18 chromophores. Microscopic organization of the RhC18 molecules in monolayers is discussed.

1. Introduction Monolayers of amphiphilic molecules bearing a chromophoric moiety, in particular, those belonging to the xanthene dye family, have attracted considerable attention for many years for both scientific and practical reasons.1-13 As their photophysical properties are sensitive to the organization of chromophores at the interface, dye monolayers provide ideal model systems to study orientational arrangement, molecular aggregation, and energy transfer processes between molecules residing in restricted geometry.1-3 Besides, dye monolayers are used as precursors of multilayered films built up with the LangmuirBlodgett (LB) deposition technique.4 Therefore, the study of the chromophore arrangement at the air/water interface, which provides the ability to elucidate, predict, and control 2D molecular organization, is crucial in the intelligent design of well-ordered and defect-free films for various applications in such areas as molecular electronics, optical devices, and sensors. Recently, pure RhC18 monolayer at the air/water interface as well as RhC18-lipid mixed films deposited on glass substrata have been investigated by a variety of techniques.2,5-11,13 In particular, it was found that the RhC18 chromophore adopted a tilted orientation at the air/water interface.11 It was also reported that a variety of aggregated species, whether fluorescent or * To whom correspondence should be addressed. E-mail: [email protected]. † On leave from St. Petersburg State University, Department of Chemistry, Petrodvorets, St. Petersburg 198904, Russia. ‡ Universite ´ du Que´bec a` Trois-Rivie`res. § Universite ´ Laval. | Kyushu University.

nonfluorescent dimers and aggregates, were formed in RhC18 monolayers during compression and in RhC18-lipid LB films.2,5-9,11,13 However, though these data shed some light on the interchromophoric interactions and excitation energy-transfer processes, they did not provide the key to understanding the fundamental properties of the microscopic organization of the RhC18 molecules in the monolayer. It was proposed that compression caused an arrangement of the RhC18 molecules at the air/water interface with a long-range order in tilt and azimuthal orientation of the chromophore polarization axes,10,11 but this hypothesis has not yet been proved by any experimental data. Meanwhile, it was shown that the direct determination of the microstructure and the symmetry of packing of xanthene dye molecules in the monolayer at the air/water interface could be accomplished with the azimuthal angle second-harmonic generation (SHG) method.12 Thus, using the advantage of this method together with other ones, we attempted a systematic study of the RhC18 monolayer behavior. In this paper, we present the results of the study of pure RhC18 monolayer obtained by epifluorescence microscopy, SHG, and absorption and fluorescence spectroscopy in situ at the air/water interface. Epifluorescence micrographs were taken at different areas per molecule to visualize changes in the monolayer morphology throughout compression. To gain the detailed information on the microscopic organization in the RhC18 monolayer, the orientation of the dye chromophore at the interface and the monolayer packing symmetry were determined by SHG. Absorption and fluorescence spectra were also recorded to elucidate the interchromophoric interactions and pressureinduced aggregation of the RhC18 molecules.

10.1021/jp0137367 CCC: $22.00 © 2002 American Chemical Society Published on Web 04/02/2002

4204 J. Phys. Chem. B, Vol. 106, No. 16, 2002

Tsukanova et al. stage. The frequency-doubled output of a Nd3+:YAG-laser (532 nm, s-polarized, 40 ps) was used as the excitation light. The laser beam irradiated the sample with an angle of incidence of 45° and was focused to an area of 0.2 mm2. In the input polarization SHG measurements, the polarization of the laser beam was changed by a λ/2 plate while the polarization of the SH light generated by the monolayer was determined with a Glan-Taylor polarizer. In the azimuthal angle SHG experiments, the SH signal was measured upon rotation of the trough about its surface normal. The laser beam was carefully adjusted to intersect the same spot on the interface upon rotation. Each azimuthal profile of the SH signal was obtained by averaging three series of rotation scannings.

Figure 1. π-A isotherm of the RhC18 monolayer at the air/water interface obtained during compression and expansion. The isotherm was measured at 20 ( 1 °C and a compression speed of 0.05 nm2/ mol‚min. The inset shows the chemical structure of the RhC18 molecule.

2. Experimental Section 2.1. Materials. The dye, RhC18 (bis-(N-ethyl, N-octadecyl) rhodamine perchlorate), was purchased from NKS Kenkyusho Chemical Corp. and was used without further purification. Its structure is shown in Figure 1. Monolayers of the dye were prepared by spreading of RhC18 from a benzene solution onto pure water which was deionozed and purified using a Nanopure water deionizing system. The specific resistivity of water was 18‚106 Ω‚cm with a pH value of 5.6 in equilibrium with carbon dioxide in the atmosphere. Benzene was of spectroscopic grade. Dye solutions were stored in glass vials wrapped in aluminum foil in the refrigerator to prevent photodecomposition. 2.2. Methods. The homemade Teflon-coated trough used to study the monolayer behavior of RhC18 has been previously described.14 The fluorescence micrographs of the RhC18 monolayer at the air/water interface were obtained with the epifluorescence microscope described elsewhere.15 The micrographs were taken in different regions of the π-A isotherm. The filter set used to observe the RhC18 fluorescence was a combination of a green excitation filter (Nikon, M 510-560), a dichroic mirror (Nikon, DM 580), and a barrier filter (Nikon, M 590). The excitation light was focused on the monolayer with a 20× objective (Nikon, MPlan 20). The apparatus used for in situ measurements of the absorption and emission spectra of the RhC18 monolayer at the air/water interface was described elsewhere.14 The light source for the absorption measurements was a temperature-stabilized tungsten lamp. The spectrum of the bare water surface was taken as the reference. To obtain the emission spectra, the monolayer was illuminated by a 150-W high-pressure mercury lamp through a monochromator. The measurements were made with an excitation wavelength of λexc ) 545 nm. The emission from the monolayer was collected with a Photometrics extended UV CCD camera via a monochromator. The experiments were carried out in a dark room and electronic shutters were used to irradiate the monolayer within 1 s. To record the spectra, compression was stopped at a given area per molecule and surface pressure for 15-20 min. When the measurements were done, compression was proceeded again. The RhC18 monolayer was stable in the expanded region where no change in the surface pressure was observed upon stopping compression for 20 min. In the plateau region of the isotherm and above the film was less stable. However, the surface pressure decreased not more than 1.5-2 mN/m within 20 min. In situ SHG measurements from the RhC18 monolayer were carried out in the reflection geometry as described elsewhere.12,16 A Teflon-coated trough of 100 cm2 was mounted on a rotational

3. Results and Discussion 3.1. Surface Pressure (π-A) Isotherm and Unique Changes in Morphology of the RhC18 Monolayer. The π-A isotherm measured upon compression and expansion of the RhC18 monolayer at the air/water interface is presented in Figure 1. The onset of the surface pressure is detected at an area of approximately 2.3 nm2/molecule. Then, the RhC18 monolayer exhibits typical liquid-expanded behavior until the surface pressure breaks off to a plateau at 0.76 nm2/molecule. The plateau extends to an area of 0.4 nm2/molecule at a surface pressure of approximately 26 mN/m, which remains almost unchanged in that region. Upon further compression, a region of low compressibility where the surface pressure increases rapidly is attained. This isotherm is similar to previously reported data.6,11 The striking feature of the isotherm is the region of low compressibility at areas which are, apparently, too small for all RhC18 molecules to remain at the interface. Since RhC18 is not soluble in water, a plausible explanation for the plateau in the isotherm could be that a smooth transformation of a 2D monolayer into a bulk 3D phase takes place. Appearance of a plateau has been reported for many long-chain amphiphilic molecules, and it may be ascribed either to a first-order phase transition between the liquid-expanded and solid-condensed states17,18 or to a slow collapse of the monolayer.18-21 If the former were the case, continuous changes in the monolayer morphology should be visualized by fluorescence microscopy, such as domains regularly distributed over the field, which appear at the onset of the plateau and grow larger in size until an entire condensed structure occurs upon further compression.22 As one can see, the epifluorescence micrographs of the RhC18 monolayer presented in Figure 2 completely rule out the possibility that the plateau corresponds to a liquid-expanded/ solid-condensed phase transition. On the contrary, domains irregularly distributed in the field as those observed in Figure 2c will likely be formed in monolayer, presumably, at the point corresponding to the monolayer collapse.19 If the latter were the case, a hysteresis should also occur in the π-A isotherm upon decompression.21 Besides, being temperature dependent, the plateau in isotherms of monolayers undergoing slow collapse shows an inverse behavior as compared to that of plateaus originating from liquid-expanded/solid-condensed phase coexistence.19,20 The surface pressure at which the plateau in the RhC18 isotherm occurred decreased with increasing temperatures thus giving another indication that it is due to a slow collapse of the monolayer. However, though several studies did report a reversible monolayer collapse causing a plateau in π-A isotherms, this phenomenon was observed previously mostly with monolayers of polymers. As seen in Figure 2, a rather unique behavior of the RhC18 molecules at the air/water interface was visualized by fluores-

Organization of Long-Chain Rhodamine Monolayers

J. Phys. Chem. B, Vol. 106, No. 16, 2002 4205

Figure 2. Epifluorescence micrographs of a pure RhC18 monolayer at the air/water interface taken in different regions of the π-A isotherm: (a) A ) 2 nm2/molecule, π ) 5.8 mN/m; (b) A ) 1 nm2/molecule, π ) 17.5 mN/m; (c) A ) 0.4 nm2/molecule, π ) 26.7 mN/m; (d) A ) 0.33 nm2/molecule, π ) 38.4 mN/m. Scale bars ) 15 µm.

cence microscopy. Interestingly, the monolayer remained homogeneous with no sign of domain formation for significant area per molecule variations in the range of 2.3-0.4 nm2/ molecule (Figure 2a and b), while the overall fluorescence intensity decreased drastically in the 1.5-0.5 nm2/molecule region (compare micrographs in Figure 2a and b). An abrupt change was observed when the monolayer was compressed to an area of 0.4 nm2/molecule (Figure 2c). Indeed, circular bright domains started to appear suddenly in a darker background. Then, sharply in contrast to the well-defined domains in Figure 2c, the micrograph in Figure 2d displays that compression to 0.33 nm2/molecule resulted in an overall brightening of the field and complete reorganization of the film into an entirely condensed structure with well-distinguished anisotropically oriented highly fluorescent stripes. Upon decompression, numerous cracks appeared in the film while the condensed structure texture remained almost unchanged within the islands (figure not shown). Though dispersing very slowly, these islands disappeared to the end of the decompression run. Observation of the cracks is in good correlation with the hysteresis observed during the compression-expansion run. Pronounced hysteresis loops were recorded upon decompression performed after the compression curve had passed the plateau region. Indeed, as seen in Figure 1, the surface pressure dropped as soon as the expansion was started and the isotherm obtained during decompression was always shifted to lower areas indicating that the film remained in a solid-condensed phase structure organization and respread only slowly. However, no hysteresis was observed in the expanded region, and it was much less marked if decompression was started in the plateau region. Moreover, even when a hysteresis was observed, the changes in the isotherm were reversible and this behavior was highly reproducible. The isotherm recorded after the hysteresis cycle did not show a significant difference as compared to an isotherm of a

newly spread monolayer. Thus, altogether the fluorescence microscopy and hysteresis data provide an unequivocal evidence that the film is collapsed at an area of 0.4 nm2/molecule. At the same time, it is not straightforward to say starting from which point the film is no longer a monolayer. It, obviously, happens in the plateau region between 0.76 and 0.40 nm2/ molecule. A better evidence may be obtained by ellipsometric measurements that will be a subject of the forthcoming paper. Meanwhile, as the film remains homogeneous throughout the entire plateau of the π-A isotherm suggesting that at least a part of the plateau corresponds to a monolayer, we focused our study on elucidating the monolayer organization of the RhC18 molecules at the air/water interface. 3.2. Spectroscopic Properties of RhC18 in Monolayer. Significant quenching of the fluorescence in the 1.5-0.5 nm2/ molecule region observed by fluorescence microscopy implies that the interactions between chromophoric moieties become crucial as the area per molecule is reduced and they play, presumably, a dominant role in the organization of RhC18 molecules at the interface. Rhodamine dyes have been studied extensively at a variety of interfaces and the interpretation of their photophysical properties is usually based on the model of exciton splitting in aggregates of dye chromophores which are formed when the surface density exceeds a certain critical value. A great deal of possible aggregate structures, whether fluorescent or nonfluorescent (J- or H-aggregates, respectively), were reported to be formed at interfaces.2,3,5-9,11,23-35 Depending on the respective orientation of the S0-S1 transition moment of monomers in the aggregate and the distance between them, several aggregate geometries are possible in monolayer at the air/water interface such as sandwichlike structures or obliqueor coplanar-inclined dipole arrangements.3,6,23,26,27,31-33,36 These geometries are characterized by different exciton band energy levels37,38 and, therefore, contribution of each type of structure

4206 J. Phys. Chem. B, Vol. 106, No. 16, 2002 can be distinguished by absorption and fluorescence spectroscopy. However, even though the properties of rhodamine monomers, dimers, and larger aggregates have been well documented, the interpretation of the absorption and fluorescence data is still difficult because the spectral features of both J- and H-aggregates resemble those of the monomer. In particular, the absorption spectrum of H-type rhodamine dimers shows two peaks.28,39,40 The higher energy peak has a stronger intensity than the one observed at lower energy, and it coincides with the position of the monomer vibrational shoulder. J-type dimers also show two peaks, but the lower energy peak has a stronger intensity, and both peaks are shifted to the red compared to the vibrational shoulder and the absorption maximum of the monomer, respectively.28,41 As a result, the superposition of the absorption spectra of the dimer or aggregate with that of the monomer creates an overall shape, which looks like the monomer absorption spectrum, although it is broader than the latter. A remarkable band splitting with the appearance of two maxima which are red and blue shifted compared to the typical bands of the monomer has been predicted only for a special case of oblique dimers.23,37 The absorption spectra of the RhC18 monolayer recorded at different areas per molecule are presented in Figure 3a. Spreading of the RhC18 monolayer caused a red shift of 13 nm of the absorption maximum compared to that in benzene solution (spectrum not shown). The spectrum of the monolayer at the onset of the π-A isotherm (curve a in Figure 3a) has a maximum at approximately 565 nm and a shoulder at 528 nm. Compression of the monolayer had relatively small effect on the absorption maximum. These spectra resemble typical monomer absorption spectrum of rhodamine in polar media.28,40,41 Indeed, as compression proceeds, both absorption maximum and shoulder are slightly shifted to longer wavelengths in the 2.3-0.83 nm2/molecule region. In contrast, a blue shift of approximately 5 nm was observed at areas below 0.83 nm2/molecule, corresponding to the plateau of the π-A isotherm. The absorbance of the maximum decreases throughout the 2.4-0.4 nm2/molecule region although a small increase was detected in the 0.4-0.25 nm2/molecule region (spectrum g in Figure 3a). At the same time, the intensity of the shoulder increases continuously. The increase of the absorbance observed at the end of the isotherm is likely another indication of the transformation of the monolayer into a multilayered structure. Since a similar increase was detected in the fluorecence spectra of the RhC18 monolayer at areas below 0.5 nm2/molecule (see spectra f and g in Figure 3b), this observation can be interpreted as an additive effect of several layers. Thus, the increase of both absorbance and fluorescence intensity at areas of 0.50.25 nm2/molecule supports further the hypothesis of the monolayer collapse at the end of the plateau of the π-A isotherm. More detailed study of the monolayer collapse will be presented in a separate paper, and the spectra corresponding to the collapsed film are not discussed below. The appearance of the shoulder in the absorption spectrum can be attributed either to the vibrational band of the monomer or to the absorption of weakly fluorescent H-dimers.8,23,26,30 For a pure monomeric system, the ratio H/M between the absorbance of the shoulder (H) and that of the maximum (M) should be between 0.3 and 0.4.8,26 A larger ratio might indicate the presence of H-dimers (aggregates). The H/M ratios obtained from the absorption spectra are plotted versus the area per molecule and presented in the inset of Figure 3a. As one can see, the ratio has a value of 0.36 at the beginning of the isotherm and remains unchanged upon compression to 1.5 nm2/molecule.

Tsukanova et al.

Figure 3. Absorption (a) and fluorescence emission (b) spectra of RhC18 in monolayer at the air/water interface recorded in different regions of the π-A isotherm: (a) A ) 2.2 nm2/molecule, π ) 1 mN/ m; (b) A ) 1.5 nm2/molecule, π ) 11.7 mN/m; (c) A ) 1.17 nm2/ molecule, π ) 15.9 mN/m; (d) A ) 0.83 nm2/molecule, π ) 25.6 mN/ m; (e) A ) 0.66 nm2/molecule, π ) 26.3 mN/m; (f) A ) 0.4 nm2/ molecule, π ) 26.7 mN/m; (g) A ) 0.33 nm2/molecule, π ) 38.4 mN/ m. The emission spectra were obtained using an excitation wavelength of 545 nm. The spectra were normalized to the maximum fluorescence intensity detected within a set of measurements with the same monolayer. The inset shows the H/M ratio versus area per molecule for RhC18 at the air/water interface.

Then, it increases reaching a value of 0.44 at an area of 0.5 nm2/molecule. Thus, the value of H/M ratio varying from 0.36 to 0.44 enables us to conclude that at areas below 1.5 nm2/ molecule H-dimers or larger aggregates are, obviously, formed in the monolayer. Assignment of the blue shoulder to H-dimers does not, however, rule out the possibility of the formation of J-dimers with either an oblique- or a coplanar-inclined configuration characterized by red-shifted spectral features. As mentioned above, the absorption maximum showed a red shift of 5 nm upon compression in the 2.4-0.83 nm2/molecule region. A red shift of almost the same magnitude was also observed in the fluorescence spectra of RhC18 (spectra a-d, Figure 3b). The fluorescence spectrum of the monolayer at the onset of the π-A isotherm shows a maximum at a wavelength of 583 nm which likely corresponds to the monomer emission band and a shoulder at approximately 635 nm. As the area per molecule was reduced from 2.4 to 0.83 nm2/molecule, the monomer band shifted

Organization of Long-Chain Rhodamine Monolayers slightly to longer wavelengths and a significant quenching of the fluorescence occurred. A systematic red shift of both absorption and fluorescence maxima observed in the expanded region might be induced through environmental polarity effects. Indeed, the expulsion of water molecules penetrating the monolayer and the approach of RhC18 zwitterions next to each other upon compression alter local polarity and may result in a shift of the monomer spectra to the red.23,30,42,43 On the other hand, the polarity effects alone cannot cause the drastic decrease of the fluorescence intensity to less than 20% of the initial level observed in Figure 3b.2,23 The red shift observed with decreasing area per molecule could also suggest that the excited monomers transfer their energy to lower energy monomer sites.2 Assuming a rotation of the phenyl ring relative to the xanthene plane, one can expect a distribution of the S1 electronic energy level among different monomer sites arising from different conformations of rhodamine chromophores. Consequently, as rhodamine chromophores can have different site energies, as shown by Yamazaki et al.,2 the shift and quenching of the fluorescence of RhC18 may take place during the excitation migration to energetically lower monomer sites. Besides, dimers or larger molecular aggregates are supposed to participate in the energy transfer process as trap sites making a further contribution to these types of spectral changes.5,25,29,44 However, neither polarity effects nor excitation energy migration can explain the appearance of the broad and featureless band at the lower energy side of the emission spectra. The position of this emission band at 640 nm (Figure 3b) implies that the dimers or higher aggregates, which absorb at a wavelength shorter than the monomer absorption maximum (Figure 3a), are responsible for the presence of this band. Since H-dimers are only very weakly fluorescent,24,37,39 this relatively strong long wavelength shoulder could be attributed to the fluorescence of J-dimers (aggregates). The absorption spectra of RhC18 have so far been interpreted as the superposition of the spectra of the monomer and a variety of fluorescent and nonfluorescent aggregated species where the shoulder observed at short wavelengths has been ascribed to H-dimers or larger aggregates. Although the higher energy peak of H-dimers having a stronger intensity coincides with the position of the blue shoulder of the overall absorption spectrum, it is not straightforward to say that H-dimers solely cause an increase of the intensity of the shoulder. It was previously found for Rh6G39 that the absorbance of both H-bands decreases when a structural reorganization from the dimer to the larger H-like aggregates takes place. As a result, the larger the aggregates, the lower their contribution to the absorbance of both the shoulder and the maximum. On the contrary, the shorter wavelength band of the J-type aggregates absorbs stronger than the corresponding band of the J-dimer.41,45 In fact, this shorter wavelength J-band is slightly red shifted compared to the vibrational shoulder of rhodamine monomer and, eventually, its growth upon the transition from the dimer to the larger J-aggregates may contribute not merely to the blue shoulder but also to a blue shift in the absorption maximum depending on the aggregate geometry.28,37,45 Therefore, both types of aggregates, in particular, H- and J-aggregates with a coplanarinclined configuration will bring similar changes in the overall absorption spectrum, such as increasing the intensity of the shoulder together with a shift and a decrease of the absorption maximum. On the basis of the discussion above, the remarkable decrease of the absorption maximum seen in the spectra a-f in Figure 3a can be interpreted as an indication that the aggregation of RhC18 chromophores in the monolayer indeed increases to a

J. Phys. Chem. B, Vol. 106, No. 16, 2002 4207 large extent as compression proceeds. The red shift of absorption in the expanded region followed by the blue shift of 5 nm with further decreasing area per molecule implies that aggregates of both J- and H-type coexist in the plateau region of the π-A isotherm. A blue shift of the absorption maximum is usually interpreted as evidence of H-aggregate formation,37,38 but only J-dimers or higher J-aggregates give a broad emission band at the long wavelength side of the spectrum,22 which is also observed in the fluorescence spectra recorded at areas below 0.76 nm2/molecule. Moreover, as seen in Figure 3b, a progressive blue shift of the fluorescence maximum occurs as the area decreased from 0.83 to 0.4 nm2/molecule. A plausible explanation of this blue shift of approximately 5 nm is an increasing contribution of the J-aggregates to the higher energy side of the fluorescence spectrum.32,43,46 As the emission spectrum of rhodamine J-dimer or larger aggregates overlaps with the monomer band, in analogy to the absorption spectrum,36 their fluorescence is overwhelmed by the intense fluorescence of the monomer which is the predominant species in the expanded region of the isotherm. However, when the RhC18 monomer and aggregates are in equilibrium and strongly coupled, which is the case at areas below 0,76 nm2/molecule, the emission intensity of the monomer decreases more significantly during the excitation energy migration to the trap sites than that of J-aggregates.46 Even being still virtually invisible in the absorption spectra (Figure 3a), the J-aggregates become the dominant fluorescent species and, as a result, the fluorescence maximum is shifted to the blue (see Figure 3b). Besides, the broadening of the absorption spectrum with decreasing area per molecule also suggests that there is a certain distribution of aggregate geometry.6 The full bandwidth at two-thirds of the absorption maximum (fwh2/3m) progressively increases from 914 cm-1 at 2.2 nm2/molecule to 1025 cm-1 at 0.4 nm2/molecule. (The bandwidth is taken at two-thirds of the absorption maximum because of the shoulder broadening the band below two-thirds of the maximum.47) Similarly, the emission spectrum of the RhC18 monolayer at the onset of the isotherm has a fwh2/3m of 617 cm-1 while it amounts to 1044 cm-1 as the area per molecule is reduced to 0.5 nm2/molecule. The latter is, obviously, due to the emission of J-aggregates.6 Therefore, one can conclude that the absorption and fluorescence spectra of the RhC18 monolayer include contributions from a variety of aggregated species as well as monomers. 3.3. Orientation of RhC18 at the Air/Water Interface and Interactions between the Dye Chromophoric Moieties. The resemblance of the shape of the absorption and fluorescence spectra of a variety of aggregated and monomeric species to that of the monomer is an indication that the S0-S1 transition dipole moments are aligned parallel to each other.3 It means that, though both H- and J-type aggregates are formed within the monolayer, the RhC18 molecules adopt a narrow distribution of orientation which is restricted by their interaction with the surface and a certain alignment relative to each other in a coplanar-inclined fashion. As it has been found,31-33 the geometry of rhodamine aggregates is determined primarily by the angle Θ of the monomer S0-S1 transition moment, which lies in the xanthene ring system, relative to the surface plane (Figure 4a). When the angle Θ is ranging between 0 and 55°, the formation of J-aggregates is favored while angles Θ > 55° lead to the formation of H-aggregates.27-29 Since the orientation at the interface plays an important role in the intermolecular interactions, the tilt angle of the S0-S1 transition dipole moment of the RhC18 chromophore with respect


Tsukanova et al.

Figure 4. A schematic diagram of the orientation of the RhC18 molecule at the air/water interface (explanations for the angles are given in the text) and possible aggregate geometries: (a) orientation of RhC18 favorable to the J-aggregate formation and a possible geometry of the coplanarinclined J-dimer; (b) orientation of RhC18 favorable to the H-aggregate formation and a possible geometry of the coplanar-inclined H-dimer. As the angle φ approaches 36°, the molecular x′z′ plane becomes perpendicular to the interface (consequently, in eq 3 the angle between the molecular axis y′ and the laboratory frame Y-axis δ ) 0°). For the sake of clarity, the C18 chains are drawn reduced in size and oriented straight upright to the interface. This drawing is made on the basis of ref 11.

to the surface normal was determined using resonant SHG. Recently, the study of the RhC18 orientation by SHG showed that this chromophore adopts a tilt orientation in the gaslike region of the π-A isotherm (at areas much larger than 2.4 nm2/ molecule) in a way where the polar group on the phenyl substituent and one of the nitrogen atoms of the xanthene ring system point toward the water while the long carbon chains point toward the air.11 As drawn schematically in Figure 4a, both xanthene ring and molecular axis z’ tend to tilt with respect to the surface normal (the Z-axis of the laboratory XYZ frame) making angles θ and φ, respectively. To define the chromophore orientation at the interface, both angles θ and φ must be known. One of them, either θ or φ, can be determined from the input polarization SHG measurements, while the value of the other angle can be estimated making appropriate approximations,3,11,30,48,49 as described below. A laser beam of frequency ω and polarization eˆ ω incident on a surface at angle σ with respect to the surface normal will generate second harmonic with polarization eˆ 2ω and an intensity given by3

I2ω )

32π3ω2 2 sec ζ|eˆ 2ω‚χ(2) : eˆ ωeˆ ω|2Iω2 c3

(1)

where π ) 3.1416 and c is the speed of light in a vacuum. The

main term of eq 1 is the monolayer second-order nonlinear susceptibility tensor χ(2). In general, there are 18 independent elements of the monolayer susceptibility tensor χ(2).33 However, if chromophores are distributed randomly in the monolayer plane and, consequently, the monolayer possesses C∞V symmetry, the number of nonvanishing susceptibility tensor elements is reduced to χ(2)zzz, χ(2)zxx ) χ(2)zyy and χ(2)xzx ) χ(2)xxz ) χ(2)yzy ) χ(2)yyz.50,51 Each of them is related to the chromophore intrinsic secondorder polarizability β as

χ(2)ijk ) Ns

∑〈βi′j′k′〉

(2)

i′j′k′

where Ns is the surface density of an SH active chromophore. The brackets denote an average over the chromophore orientation distribution which is determined by the direction cosine between the laboratory frame (XYZ) and molecular (x′y′z′) Cartesian axes. A complete description of the orientation distribution also depends on which components of the nonlinear polarizability tensor contribute to the total chromophore polarizability.49 As discussed previously,11,30,35 there are two dominant elements of rhodamine chromophore nonlinear polarizability, βz′x′x′ and βx′x′z′. Then, calculating eq 2 accordingly, a condition defining the chromophore orientation in terms of the tilt angle θ with respect to the surface normal, or Euler angles δ and φ,


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TABLE 1: Orientational Parameters for the RhC18 Molecule at the Air/Water Interface Calculated from the SHG Measurements A (nm2/molecule)

χ(2)zzz/ χ(2)zxx

χ(2)xzx/ χ(2)zxx

θ (°)

Θ (°)

2.4 2.0 1.5

3.62 3.78 4.15

0.63 0.66 0.72

54 ( 1 54 ( 1 54 ( 1

36 ( 1 36 ( 1 36 ( 1

can be expressed as a linear combination of the experimentally determined monolayer second-order susceptibility tensor elements

cos2 θ ) cos2 δ‚sin2 φ )

χ(2)zzz 2(χ(2)zzz + χ(2)zxx + χ(2)xzx)

(3)

where δ is the angle between the molecular axis y′ and the laboratory frame Y-axis.30 The ratios among the independent elements of χ(2) are usually obtained from the input polarization dependencies of the SH intensity.48-51 The intensity of the p-polarized and s-polarized SH signals generated by an isotropic monolayer depends on the angle of polarization of the laser beam, γ, as given by

Θ ) 90° - θ

Iγ,p ) K[cos2 γ(Aχ(2)zzz - Bχ(2)xzx) + (C cos2 γ + Dsin2 γ)χ(2)zxx]2 (4) Iγ,s ) K[Eχ(2)xzxsin γ cos γ]2

the air/water interface in the present study is thus in good agreement with the previously reported data. In fact, the value of the average tilt angle θ of 54° agrees well with what is expected from the bond length and angle value of the RhC18 chromophore moiety structure and typical bond distances to describe the interactions between the RhC18 molecule and the water surface.30 Given the polar nature of the surface and the positioning of -COOH group, hydrogen bonding will determine the orientation of the RhC18 molecule in a way where the xanthene ring tends to tilt with an angle θ ranging 50-55° as drawn in Figure 4a. Using further molecular modeling, Kikteva et al.30 have found only a certain range of angles φ. From the point of view of the phenyl ring localization on the surface, the value of φ should not exceed 55-60°.30 In fact, the orientation of the rhodamine chromophore characterized by the angles θ ) 54° and 55° e φ e 60° favors the formation of J-dimers and larger aggregates as shown schematically in Figure 4a. The value of 54° for the average tilt angle θ of the S0-S1 transition moment with respect to the surface normal calculated from the SHG data allowed to determine a value of 36° for the angle Θ of the transition moment relative to the surface from the relationship between θ and Θ which is given by

(5)

where A through E and K are coefficients proportional to the Fresnel factors as well as the frequency of the pumping beam, the angle of reflection of the SH light, and the dielectric constants of the medium through which the incident and reflected beam is propagated.51 The input polarization dependencies of both p- and s-polarized SH signals generated by the RhC18 monolayer were obtained at different areas per molecule. These dependencies were measured as the polarization of the laser beam was gradually changed between γ ) 0° (p-polarized excitation) and γ ) 90° (s-polarized excitation). According to eq 5, if the chromophores are randomly distributed about the surface normal, the intensity of the s-polarized SH output generated by p- or s-polarized excitation should be equal to zero, Ips ) Iss ) 0 (for an incident angle of 45°).50 This was indeed observed in the 2.4-1.5 nm2/molecule region of the RhC18 isotherm. Moreover, in this region, the intensity of the SH light was quadratically proportional to Ns, and the SHG from the monolayer was invariant during rotation of the trough about the surface normal. Thus, the C∞V symmetry could be ascribed to the monolayer and, using a procedure explicated in details elsewhere,51 the ratios of χ(2)zzz/χ(2)zxx and χ(2)xzx/χ(2)zxx were recovered from the input polarization dependencies of the SH intensity by fitting the experimental data to eqs 4 and 5. The results are presented in Table 1. Then, the average tilt angle θ of the S0-S1 transition dipole moment of the RhC18 chromophore with respect to the surface normal was calculated using eq 3. A value of 54° was found for the angle θ and it remained unchanged in the 2.4-1.5 nm2/molecule region of the isotherm (Table 1). Measurements for several rhodamine dyes revealed that their orientation is profoundly affected by the interaction of the polar group on the phenyl substituent with the surface and they are all oriented alike at polar interfaces, such as air/water or air/ silica interface, making an angle of θ = 50-55° between the S0-S1 transition dipole moment and the surface normal.3,30,35 The value of θ ) 54° obtained for the RhC18 chromophore at

(6)

As discussed above, since the angle Θ is found in the range 0°e Θ e 55°, one should expect that, in the 2.4-1.5 nm2/ molecule region, the RhC18 molecules will likely form J-aggregates.31-33 The absorption and fluorescence spectra of the RhC18 monolayer recorded in this region strongly support this idea. Thus, formation of J-aggregates seems to be predetermined by the orientation of the rhodamine chromophore at the air/water interface. Unfortunately, the calculation of the angles θ and Θ could not be proceeded at areas lower than 1.5 nm2/molecule because the RhC18 monolayer was no longer isotropic as will be discussed below. Nevertheless, we may assume that further compression induces change in the angle φ rather than θ. Indeed, to occupy a smaller area at the air/water interface, the molecular x′z′ plane must become more perpendicular to the interface as shown in Figure 4b. Given the angle θ ) 54° and the angle δ between the molecular axis y′ and the laboratory frame Y-axis equal to zero, the angle φ defined by eq 3, in conjunction with θ and δ, must decrease from 60° to 36° to satisfy the condition of a minimum projected area as illustrated in Figure 4b. Although the angle θ still remains at 54°, this orientation allows formation of H-dimers or aggregates as shown schematically in Figure 4b. Thus, pressure-induced changes in local orientation of RhC18 will alter interchromophoric interactions so that at areas below 1.5 nm2/molecule the H-type aggregates start to appear in the monolayer. Presumably, the distribution of the angle φ between values of 36° and 60° makes possible the formation of a range of aggregate geometries with varying dipole-dipole distances and angles of inclination. As a result, aggregates with parallel arrangement of the transition moments of both J- and H-types as shown in Figure 4a and b, respectively, coexist in the monolayer and they are seen in all absorption and fluorescence spectra. Though an oblique geometry cannot be excluded completely, the shape of the absorption spectra and the hypsochromic shift of 5 nm observed in the plateau of the π-A isotherm and above (Figure 3a) suggest that there is a rather long-range order in parallel alignment of RhC18 chromophores at the interface.9,37 3.4. Alignment of the Dye Chromophores and Packing Symmetry in the RhC18 Monolayer. To gain more information


Figure 5. Experimental data (filled circles) and simulated dependencies (solid lines) for the p-polarized SH signal (Ipp) as a function of the azimuthal angle φ for the RhC18 monolayer at the air/water interface compressed to areas per molecule of (a) 1.5; (b) 1.17; (c) 0.83; (d) 0.66; (e) 0.4 nm2/molecule. Error bars correspond to the deviation from an average of three series of rotation scannings.

on whether the RhC18 molecules are indeed organized in nonrandomly oriented aggregates with a preferential parallel arrangement of their transition dipole moments, the azimuthal angle dependencies of the SH intensity were measured by rotating the trough about its surface normal. The dependencies of the p-polarized SH signal generated by the p-polarized laser beam (Ipp) obtained at different areas per molecule are presented in Figure 5. The azimuthal angle, φ, was chosen so that φ ) 0° corresponded to the direction in the monolayer plane along which the lowest intensity of Ipp was detected.12 All of the data obtained during rotation scanning were then arranged correspondingly. As seen in Figure 5, the SHG from the monolayer at an area of 1.5 nm2/molecule remained almost unchanged during rotation, indicating that the RhC18 chromophores within the laser spot are randomly oriented about the surface normal. A sign of azimuthal anisotropy in the Ipp intensity appeared for the first time when the monolayer was compressed to an area of 1.17 nm2/molecule (curve b). Upon further compression, the azimuthal anisotropy of Ipp became evident and showed an apparent two-fold pattern in the plateau of the π-A isotherm. The observed two-fold pattern of Ipp reflects a rather narrow distribution of the azimuthal orientation of the major polarization axis of the RhC18 chromophores along a certain direction in the monolayer plane. The strongest intensity of the p-polarized SH signal generated by the p-polarized laser beam was detected along the direction nearly perpendicular to the compression direction. Since the SH signal reaches a maximum when the direction of the transition moment coincides with the polarization direction of the excitation light and p denotes the polarization parallel to the plane of incoming and outgoing beams, the direction along which the highest Ipp signal is observed is referred to the preferential orientation of the electronic transition axis which has the largest second-order polarizability.52 The dominant contribution to the RhC18 second-order polarizability comes from the S0-S1 transition dipole moment3,48 and, consequently, this enables us to conclude that the xanthene planes of the RhC18 chromophores are oriented nearly parallel to each other and perpendicular to the direction of compression.

Tsukanova et al.

Figure 6. Typical azimuthal angle dependencies of the SH signal for the RhC18 monolayer at areas of 0.83-0.4 nm2/molecule: (a) Ipp; (b) Ips; (c) Iss; (d) Isp. The experimental data are filled circles and the simulated dependencies are solid lines. Error bars are as in Figure 5.

The similar anisotropic in-plane orientation has been discovered recently for a long-chain fluorescein monolayer where a fairly regular structure ordering of molecules is found to be induced by compression.12 Using the procedure explicated elsewhere,12 the determination of the packing symmetry in the RhC18 monolayer was also attempted to gain further insight into the organization of the rhodamine dye molecules at the air/water interface. The measurements were done with RhC18 monolayers at areas of 0.83-0.4 nm2/molecule. The azimuthal angle dependence of Iss was measured and compared with that of Ipp. Both have two maxima as shown in Figure 6a and c. Iss intensity exhibits two maxima at azimuthal angles of 0° and 180°, while that of Ipp has the maxima at 90° and 270°. The observation of two-fold symmetric patterns in the rotation angle dependencies of both Ipp and Iss indicates that the RhC18 molecules are organized with a C2 or C2V packing symmetry.50,53 To distinguish between these two possible symmetry classes, the azimuthal angle dependencies of the SH signal polarized perpendicular to the polarization of the laser beam, Ips and Isp, were measured. These data are presented in Figure 6b and d. Both clearly revealed a four-fold pattern. The dependence of Isp consists of two split maxima located at azimuthal angles of 0° and 180° like Iss maxima. The split maxima of Ips intensity appear at 90° and 270° similar to those of Ipp. Given that the four-fold symmetry of Ips and Isp is evident, the symmetry of the orientational distribution of RhC18 molecules in the plateau region of the isotherm can be ascribed to C2V.12 These data on the SHG rotation anisotropy measurements revealed an interesting feature. The solid lines in Figures 5 and 6 are the simulated curves based on the assumption that the monolayer is composed of well-ordered single microcrystallites of RhC18 with a C2V symmetry.12 As can be seen, all experimental data obtained in the plateau of the isotherm fit well the simulated curves. It suggests that both the monolayer approaching the collapse point and a collapsed film bear a crystalline-like structure with a longrange order in the azimuthal angle φ. 3.5. Microscopic Organization of the RhC18 Molecules in the Monolayer. As it was suggested by Mizrahi et al.,53 the lack of evident symmetry in the azimuthal angle dependencies of SH intensity as that observed at areas above 0.83 nm2/ molecule is a sign of greater disorder in the in-plane macroscopic distribution of dye chromophores. Upon compression, the RhC18


Figure 7. A schematic top view of the C2V symmetry packing within the RhC18 monolayer at the air/water interface: (a) ideal brick arrangement; (b) possible structures of the RhC18 aggregates of coupled chromophores. Depending on the angle of inclination and distance between the dye chromophores in the staircase or ladder fragment, either H- (solid blocks) or J-aggregates (dotted blocks) can be formed. Filled circles show the direction along which the chromophore coupling occurs within the dye aggregates. (c) A view of the anisotropic ordering of the monomeric and aggregated species within the RhC18 monolayer induced by compression. The individual blocks correspond to the RhC18 monomers and the arrow in the block denotes the direction of the S0S1 transition dipole moment of the dye chromophore. Filled block arrays represent different types of RhC18 aggregates described in the part b of the figure.

molecules are forced into some aggregated structures with a certain alignment of their S0-S1 transition moments, and the monolayer should be viewed as a coexistence of different kinds of aggregates and monomeric species. Though several geometries might be assumed, the main feature all aggregates have in common is an array of RhC18 chromophores organized in a way where each one is situated at a certain position according to the C2V symmetry packing.53,54 For the sake of clarity, all possible geometries can be generalized as a brick arrangement as drawn in Figure 7a. The S0-S1 transition dipole moments within an aggregate are oriented in the same direction determined by θ, φ, and φ angles, and the coupling of RhC18 chromophores could occur within staircase, ladder, or string fragments of the brick arrangement as shown in Figure 7b. The chromophore coupling in a head-to-tail fashion does not seem possible because of steric hindrance between the C18 chains attached to the terminal nitrogen atoms of the xanthene ring system.7 As the real monolayer structure is not exactly represented by the ideal brick arrangement depicted in Figure 7a, numerous defects are expected to appear because of the

J. Phys. Chem. B, Vol. 106, No. 16, 2002 4211 formation of either H- or J-aggregates depending on the interchromophoric distance and angle of inclination as illustrated by Figure 7c. This schematic diagram of the organization of RhC18 monolayer is in good agreement with our spectroscopic data and explains why the SHG data in Figures 5 and 6 from the monolayer do not exhibit the perfect C2V symmetry pattern. Moreover, even if RhC18 chromophores were perfectly arranged and anisotropically oriented within the aggregates, there would be, obviously, a distribution of the aggregate azimuthal orientations in the monolayer plane causing a macroscopic irregularity in the structure of the monolayer as a whole.55 In particular, in the expanded region, the RhC18 monomeric and aggregated species are oriented randomly resulting in the isotropic SHG (curves a and b, Figure 5.). As compression proceeds, both monomers and aggregates are aligned along a preferred direction in the monolayer plane and the distribution of their azimuthal orientation is becoming narrow. Consequently, the SH signal shows more and more pronounced C2V symmetry properties as observed in the 0.83-0.4 nm2/molecule region of the π-A isotherm (curves c-e, Figure 5). In summary, though the molecular structure determines the orientation of RhC18 at the air/water interface in the way shown in Figure 4a, the in-plane anisotropic organization in the monolayer is, obviously, induced by compression. The tilt orientation of the RhC18 moiety with an angle of 36° of the S0-S1 transition dipole moment relative to the interface (Table 1) was suggested to be favorable to the formation of a variety of aggregate geometries.3,6,23,26,27,31-33 However, our spectroscopic and SHG data demonstrated unequivocally that only those with the parallel alignment of the RhC18 transition dipole moments were formed upon monolayer compression. Therefore, the preferential formation of parallel or other aggregate geometries should be interpreted in terms of pressure-induced interactions between RhC18 chromophores. We argue that the primary mechanism for the in-plane anisotropic ordering within the RhC18 monolayer is intermolecular dipolar interactions between the dye chromophoric moieties driven by compression which causes breakdown of the C∞ symmetry inherent to the monolayer in the expanded state and, eventually, leads to the formation of a regular structure characterized by C2V packing symmetry with a long-range order in tilt and azimuthal orientation of the RhC18 chromophores. This scenario is further supported by the SHG rotation anisotropy measurements where an excess of RhC18 was spread onto the water surface leading to a positive surface pressure prior compression. If the initial surface pressure after monolayer spreading were high (in this case it was approximately 17 mN/m at an area of 1.5 nm2/ molecule), the SHG from these monolayers was invariant during rotation (data not shown). This finding indicates that compression of the monolayer allows a proper organization of the RhC18 molecules in contrast to the spreading at high surface pressure. Isotropy of the SHG from monolayers spread at high surface pressure indicates a random azimuthal orientation of the S0-S1 transition dipole moment of the RhC18 chromophores which is likely resulted from a spontaneous distribution of the dye over the area available. This causes irregularity and a rather chaotic alignment of RhC18 chromophores in the monolayer resulting in a complete cancellation of the symmetry properties of SHG. Thus, one can conclude that the initial conditions of spreading and compression play a crucial role in the structural ordering of RhC18 chromophores at the air/water interface. Finally, though the present study shed light on the microscopic organization of the RhC18 molecules during compression of the monolayer at the air/water interface, some questions still

4212 J. Phys. Chem. B, Vol. 106, No. 16, 2002 remain to be answered. The analysis of our spectroscopic data showed that during compression two types of aggregated species, fluorescent J-aggregates and nonfluorescent H-aggregates, are formed within the RhC18 monolayer and they coexist at all areas per molecule together with a certain amount of the monomeric species. The structure of the monolayer has been elucidated as consisting of a variety of microstructures each of which is a regular array of chromophores situated at an identical site according to C2V packing symmetry. Presumably, two types of fractal microstructures have to be considered: in the first one, the RhC18 chromophores are strongly coupled forming either J-aggregates emitting at a wavelength higher than the monomer or H-aggregates trapping the excitation energy, but in the second one, the chromophores are isolated and they relax to their ground state as monomers.2,23 More information on fractal microstructures can be gained from the fluorescence decay measurements. This study is in progress. Besides, another interesting aspect of the RhC18 monolayer behavior has to be further investigated. As mentioned above, the plateau in the π-A isotherm of the RhC18 monolayer is, obviously, caused by pressure-induced transformation of a 2D monolayer into a bulk 3D phase. However, it is still unclear from which point the film is no longer a monolayer. To clarify the position of the collapse point, a study using other physicochemical methods including ellipsometry has been performed and will be discussed in a forthcoming paper. Conclusions The monolayer behavior of a rhodamine derivative with two C18 chains attached to the nitrogen atoms of the xanthene ring system was studied by epifluorescence microscopy, SHG, and absorption and fluorescence spectroscopy. Input polarization SHG measurements provided the average orientation of the RhC18 chromophore at the air/water interface. A tilt angle of the S0-S1 transition dipole moment of the chromophore with respect to the surface of 36° was found in the 2.4-1.5 nm2/ molecule region of the π-A isotherm. Spectroscopic data revealed the presence of both fluorescent J-aggregates and nonfluorescent H-aggregates in the RhC18 monolayer. Interestingly, among a variety of possible aggregate geometries, exclusively those with the parallel alignment of the S0-S1 transition dipole moments were formed upon compression. As shown by the azimuthal angle SHG, compression caused the breakdown of the C∞V symmetry inherent to the liquid-expanded monolayer and induced an anisotropic arrangement of the dye transition moments mainly perpendicular to the compression direction. In the plateau region, the film possessed a fairly regular structure characterized by the C2V symmetry packing and a long-range order in the parallel alignment of the RhC18 chromophores. However, there are two striking features of the RhC18 monolayer behavior: (1) the abrupt change in the monolayer morphology occurs at the end of the plateau of the π-A isotherm and (2) the condensed region appears at areas to be apparently small for all the RhC18 molecules to remain at the interface. Since RhC18 is not soluble in water, the appearance of the plateau is likely caused by pressure-induced transformation of a 2D monolayer into a bulk 3D phase. This aspect is being further investigated. Acknowledgment. The authors thank the Natural Sciences and Engineering Research Counsil of Canada for the financial support to this study. An invitation fellowship of the Japan Society for the Promotion of Science (No.RC39726104) and from the Centre de Recherche en Sciences et Ingeńierie des

Tsukanova et al. Macromolećules (Quebec, Canada) to V.T. is gratefully acknowledged. C.S. is a Chercheur boursier senior from the Fonds de recherche en sante´ du Que´bec. We also thank Prof. Surat Hotchandani for helpful comments on the rhodamine aggregate spectroscopic properties. References and Notes (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) Peterson, E. S.; Harris, C. B. J. Chem. Phys. 1989, 91, 2683. (4) Kuhn, H.; Mo¨bius, D.; Bu¨cher, H. In Physical Methods of Chemistry; Weisberger, A.; Rossiter, B., Eds.; Wiley: New York, 1972; Vol. 1, Part 3B. (5) Tamai, N.; Yamazaki, T.; Yamazaki, I. Chem. Phys. Lett. 1988, 147, 25. (6) Van der Auweraer, M.; Verschuere, B.; De Schryver, F. C. Langmuir 1988, 4, 583. (7) Verschuere, B.; Van der Auweraer, M.; De Schryver, F. C. Chem. Phys. 1991, 149, 385. (8) Ballet, P.; Van der Auweraer, M.; De Schryver, F. C.; Lemmetyinen, H.; Vuorimaa, E. J. Phys. Chem. 1996, 100, 13701. (9) Pevenage, D.; Van der Auweraer, M.; De Schryver, F. C. Langmuir 1999, 15, 8465. (10) Tsukanova, V.; Slyadneva, O.; Inoue, T.; Harata, A.; Ogawa, T. Chem. Phys. 1999, 250, 207. (11) Slyadneva, O. N.; Slyadnev, M. N.; Tsukanova, V. M.; Inoue, T.; Harata, A.; Ogawa, T. Langmuir 1999, 15, 8651. (12) Tsukanova, V.; Harata, A.; Ogawa, T. J. Phys. Chem. B 2000, 104, 7707. (13) Slyadneva, O. N.; Slyadnev, M. N.; Harata, A.; Ogawa, T. Langmuir 2001, 17, 5329. (14) Gallant, J.; Lavoie, H.; Tessier, A.; Munger, G.; Leblanc, R. M.; Salesse, C. Langmuir 1998, 14, 3954. (15) Maloney, K. M.; Grandbois, M.; Grainer, D. W.; Salesse, C.; Lewis, K. A.; Roberts, M. F. Biochim. Biophys. Acta 1995, 1235, 395. (16) Tsukanova, V.; Harata, A.; Ogawa, T. Langmuir 2000, 16, 1167. (17) Gaines, G. L. Insoluble monolayers at liquid/gas interface; Wiley: New York, 1966. (18) Pallas, N. R.; Pethica B. A. Langmuir 1985, 1, 509. (19) Seitz, M.; Struth, B.; Preece, J. A.; Plesnivy, T.; Brezesinski, G.; Ringsdorf, H. Thin Solid Films 1996, 284-285, 304. (20) Romeu, N. V.; Trillo, J. M.; Conde, O.; Casas, M.; Iribarnegaray, E. Langmuir 1997, 13, 71. (21) Nikomarov, E. S. Langmuir 1990, 6, 410. (22) Kjaer, K.; Als-Nielsen, J.; Helm, C. A.; Laxhuber, L. A.; Mo¨hwald, H. Phys. ReV. Lett. 1987, 58, 2224. (23) Kemnitz, K.; Tamai, N.; Yamazaki, I.; Nakashima, N.; Yoshihara, K. J. Phys. Chem. 1986, 90, 5094. (24) Urquhart, R.; Grieser, F.; Thistletnwaite, P.; Wistus, E.; Almgren, M.; Mukhtar, E. J. Phys. Chem. 1992, 96, 7808. (25) Ohta, N.; Tamai, N.; Kuroda, T.; Yamazaki, T.; Nishimura, Y.; Yamazaki, I. Chem. Phys. 1993, 177, 591. (26) Vuorimaa, E.; Ikonen, M.; Lemmetyinen, H. Chem. Phys. 1994, 188, 289. (27) Tapia Estevez, M. J.; Lopez Arbeloa, F.; Lopez Arbeloa, T.; Lopez Arbeloa, I. J. Colloid Interface Sci. 1994, 162, 412. (28) Fujii, T.; Nishikiori, H.; Tamura, T. Chem. Phys. Lett. 1995, 233, 424. (29) Bojarski, P. Chem. Phys. Lett. 1997, 278, 225. (30) Kikteva, T.; Star, D.; Zhao, Z.; Baisley, T. L.; Leach, G. J. Phys. Chem. B 1999, 103, 1124. (31) Del Monte, F.; Levy, D. J. Phys. Chem. 1998, 102, 8036. (32) Del Monte, F.; Levy, D. J. Phys. Chem. 1999, 103, 8080. (33) Del Monte, F.; Mackenzie, J. D.; Levy, D. Langmuir 2000, 16, 7377. (34) Uchida, T.; Yamaguchi, A.; Ina, T.; Teramae, N. J. Phys. Chem. B 2000, 104, 12091. (35) Ishibashi, K.; Sato, O.; Baba, R.; Tryk, D. A.; Hashimoto, K.; Fujishima, A. J. Colloid Interface Sci. 2001, 233, 361. (36) Kemnitz, K.; Yoshihara, K. J. Phys. Chem. 1991, 95, 2095. (37) Kasha, M.; Rawls, H. R.; Ashraf El-Bayoumi, M. J. Pure Applied Chem. 1965, 11, 371. (38) McRae, E. G.; Kasha, M. J. Chem. Phys. 1958, 28, 71. (39) Lo´pez Arbeloa, F.; Ruiz Ojeda, P.; Lo´pez Arbeloa, I. J. Chem. Soc., Faraday Trans. 2 1988, 84, 1903. (40) Lo´pez Arbeloa, F.; Llona Gonzalez, I.; Ruiz Ojeda, P.; Lo´pez Arbeloa, I. J. Chem. Soc., Faraday Trans. 2 1982, 78, 989. (41) Ruiz Ojeda, P.; Katime Amashta, I. A.; Ramoń Ochoa, J.; Lo´pez Arbeloa, I. J. Chem. Soc., Faraday Trans. 2 1988, 84, 1.

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Microscopic Organization of Long-Chain Rhodamine Molecules in

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