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Orientational Arrangement of Long-Chain Fluorescein Molecules within the Monolayer at the Air/Water Interface Studied by the SHG Technique Valeria Tsukanova,† Akira Harata, and Teiichiro Ogawa* Department of Molecular and Material Sciences, Kyushu University, Kasuga-shi, Fukuoka 816-8580, Japan Received February 12, 1999. In Final Form: September 21, 1999 Orientational arrangement of long-chain Fluorescein O322 molecules within the monolayer at the air/ water interface induced by the compression process was studied by surface second harmonic generation. Compression does not change the average tilt angle of the chromophore in any significant way, but it induces the alignment of the chromophore along a preferred direction perpendicular to the compression direction, originating the in-plane optical anisotropy. A model has been proposed on the orientational ordering of Fluorescein O322 molecules within the monolayer induced by the compression process.
1. Introduction Monomolecular layers and ultrathin films of insoluble long-chain amphiphilic molecules possessing second-order nonlinear optic properties are currently gaining interest in many areas as optical switching in communications and optical computing, sensors, and surface modification layers.1-11 Among these materials the ordered film of Fluorescein derivatives has long been a fascinating subject of the study owing to their special chemical and physical properties.12 The Langmuir-Blodgett method is one of the unique processes of obtaining varieties of layered structures of organic compounds. In this method the alignment of the chromophores within the initial monolayer at the air/water interface plays an important role in further deposition process. Therefore, chromophore arrangements at the interface have repeatedly been investigated on a microscopic level in order to estimate, predict, and control film properties. The structure of a film and the orientational ordering of chromophore on the water surface can be adjusted by compression. Compression induces a transition in the monolayer to an ordered phase, which represents supramolecular assemblies with a unique * To whom correspondence should be addressed. † On leave from the Department of Chemistry, St. Petersburg State University, St. Petersburg, 198904 Russia. (1) Lvov, B. F.; Yamada, S.; Kunitake, T. Thin Solid Films 1997, 300, 107. (2) Spano, F. C.; Mukamel, S. Phys. Rev. A 1989, 40, 5783. (3) Marowsky, G.; Chi, L. F.; Mobius, D.; Steinhoff, R.; Shen, Y. R.; Dorsch, D; Rieger, B. Chem. Phys. Lett. 1988, 147, 420. (4) Yamada, S.; Harada, A.; Matsuo, T.; Ohno, S.; Ichinose, I.; Kunitake, T. Jpn. J. Appl. Phys. 1997, 36, L1110. (5) Kajikawa, K.; Takezoe, H.; Fukuda, A. Jpn. J. Appl. Phys. 1991, 30, 1050. (6) Nakano, T.; Yamada, Y.; Matsuo, T.; Yamada, S. J. Phys. Chem. B 1998, 102, 8569. (7) Okada, S.; Matsuda, H.; Masaki, A.; Nakanishi, H.; Abe, T.; Ito, H. Jpn. J. Appl. Phys. 1992, 31, 365. (8) Kajikawa, K.; Anzai, T.; Takezoe, H.; Fukuda, A.; Okada, S.; Matsuda, H.; Nakanishi, H.; Abe, T.; Ito, H. Chem. Phys. Lett. 1992, 192, 113. (9) Bosshard, Ch.; Kupfer, M.; Gunter, P.; Pasquir, C.; Zahir, S.; Seiferd, M. Appl. Phys. Lett. 1990, 56, 1204. (10) Yamada, S.; Shimada, Y.; Kawazu, M.; Matsuo, T. J. Phys. Chem. 1997, 101, 672. (11) Meyers, F.; Marder, S. R.; Pierce, B. M.; Bredas, J. L. Chem. Phys. Lett. 1994, 228, 171. (12) Dutta, A. K.; Salesse, G. Langmuir 1997, 13, 5401.
spatial distribution and orientation of dye molecules. The optical second harmonic generation (SHG) is a convenient tool to analyze orientational and structure ordering of constituent molecules and their changes upon compression. The orientation of tail and headgroups of long-chain hydrocarbon molecules at the air/water interface13-15 and the molecular reorientation near the monolayer liquidexpanded-liquid-condensed phase transition13,16 have recently been investigated using the SHG technique. In the present paper we have determined orientation of long-chain Fluorescein O322 molecules relative to each other and to the surface under compression at the air/ water interface using the SHG technique. One of the most interesting findings is that the dependence of the second harmonic (SH) intensity on the molecular area is similar to the π-A isotherm. Another is that the average tilt angle of the Fluorescein O322 transition moment was kept almost unchanged under compression, although the s-polarized SH signal appeared gradually by compression. The observed variation in monolayer SHG throughout the compression process was attributed to the pressureinduced ordering of dye molecules in an anisotropic structure. The compression process does not change the tilt angle of the chromophore, but it does change the mutual interaction of the transition moment of the chromophore. 2. Experimental Section 2.1. Materials. The dye long-chain Fluorescein O322 (5(octadecanoylamino)fluorescein; Figure 1) was purchased from Molecular Probes, Inc., and used without further purification. The ethanol solution of Fluorescein O322 containing 1 mM compound was stored in a glass vial wrapped in aluminum foil in a refrigerator to prevent photodecomposition. Ethanol was of special grade and was obtained from Kishida Chemicals. Water was deionized and purified using a Millipore Milli-Q system. The pH of the water subphase was approximately 6.2 in equilibrium with carbon dioxide in the atmosphere. 2.2. Methods. The ethanol solution of Fluorescein O322 was spread onto the water surface to prepare the monolayer film. (13) Rasing, Th.; Shen, Y. R.; Kim, M. W.; Grubb, S. Phys. Rev. Lett. 1985, 55, 2903. (14) Eisenthal, K. B. Annu. Rev. Phys. Chem. 1992, 43, 627. (15) Rasing, Th.; Shen, Y. R.; Kim, M. W.; Valint, P., Jr.; Bock, J. Phys. Rev. A 1985, 31, 537. (16) Barmentlo, M.; Vrehen, Q. H. F. Chem. Phys. Lett. 1993, 209, 347.
10.1021/la990151j CCC: $19.00 © 2000 American Chemical Society Published on Web 11/20/1999
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Figure 2. Four polarization components of the SH intensities as a function of area per molecule: (0) Ipp; (9) Isp; (4) Ips; (2) Iss. The first subscript indicates the polarization of the incident beam, and the second subscript indicates that of the SH beam.
Figure 1. π-A isotherm and chemical formula of Fluorescein O322. The monolayer was gradually compressed from the area per molecule of 2.30 to 0.17 nm2 at a rate of 5 mm/min. On each step of the compression the s- and p-polarized SH intensities were measured upon rotation of input polarization over an angle φ from p (φ ) 0°) to s (φ ) 90°). The compression direction was perpendicular to the plane of incoming and outgoing beams. One optical measurement was performed within 1 min. Each result was obtained by averaging three series of independent experiments. The dye films were stable. The series-to-series deviation was usually below 9%. The light source for SHG was the s-polarized frequency-doubled output (532 nm, 0.5 mJ, and 40 ps) of a Nd3+:YAG laser. The polarization of the pumping laser beam was rotated using a halfwave plate. The laser beam was directed onto the sample with the angle of incidence of 45° and was focused to an area of ∼0.2 cm2. The SH light (266 nm) was separated by suitable filters and detected in reflection by a photomultiplier, and its polarization direction was determined with a sheet polarizer. Signal sampling, averaging, and recording took place with an oscilloscope. The signal was normalized by the reference signal from a KH2PO4 (KDP) doubling crystal in order to eliminate the error caused by laser amplitude fluctuations. The SH signal from the pure water surface was negligible. All experiments were performed at 25 °C.
3. Results and Discussion 3.1. π-A Isotherm and SHG Intensity. The π-A isotherm of the Fluorescein O322 monolayer is shown in Figure 1. Three main regions characterizing the π-A isotherm were distinguished well: area of expanded film; expanded-condensed transition region; region of condensed monolayer. The surface pressure increased monotonically in the expanded liquid phase of the 2.30-0.83 nm2/molecule region. Upon further compression, the isotherm showed a well-defined discontinuity in the slope at the area per molecule of about 0.50-0.60 nm2, indicating the onset of phase transition between liquid-expanded and -condensed states. The phase transition is the plateaulike region of the π-A isotherm, where the surface pressure tended to increase very slowly until the region of low compressibility was attained.
The SH signal and its polarization dependence were measured as a function of molecular area of Fluorescein O322 as shown in Figure 2; they were measured during compression. Four sets of measurements were carried out for the polarization (s and p) of both the input and the SH beam, where Ips represents the intensity of the SH beam when the incident laser was p-polarized and the SH beam was s-polarized. The most interesting feature of these SH intensities is their strong resemblance to the π-A isotherm; they tended to increase almost in the same way as the surface pressure increased. There are two regions in the SH signal enhancement and a discontinuity of slope between them. The discontinuity at the ∼0.60 to ∼0.25 nm2/molecule region can be related to a phase transition. While the growth of the SH signal in the ∼0.25 to ∼0.17 nm2/molecule region should be referred to the region of condensed monolayer, similar evolution of the s- and p-polarized SH signals during compression was observed in the case of ruthenium-polypyridine complex monolayers at the air/water interface.6 This finding has unequivocally demonstrated that the second-order nonlinearity is strongly related to phase transitions within the monolayer. Conversely, analysis of the macroscopic second-order susceptibility of the monolayer, χ(2), is useful to clarify the transition process. The major question about the transition process is whether a reorientation of SH active chromophores or a change in structure ordering of constituent molecules is responsible for the observed significant variation of the SH intensity of the monolayer. 3.2. Average Tilt Angle of Fluorescein O322 Transition Moment at the Air/Water Interface. The tilt angle of the chromophore with respect to the surface normal can be obtained through ratios of independent elements of the χ(2) tensor, which were obtainable from a set of input polarization dependence of the SH intensity using a fitting procedure.5 The input polarization dependence of the p-polarized and s-polarized SH signals from the Fluorescein O322 monolayer are presented in Figure 3a,b, respectively, for several compression degrees. If the molecules are randomly distributed about the surface normal (the Z axis), the SH intensity profiles should possess (1) the zero s-polarized SH output generated by p- or s-polarization excitation, Ips ) Iss ) 0 (for the 45°
Orientational Arrangement of Fluorescein Molecules
Langmuir, Vol. 16, No. 3, 2000 1169 Table 1. Orientational Parameters for the Fluorescein O322 Molecules at the Air/Water Interface under Compression Defined by the Area per Molecule area/molecule (nm2/molecule)
(2) (2) χzzz /χzxx
(2) χ(2) xzx/χzxx
Θ (deg)
2.30 1.70 1.17 1.00 0.83
3.51 3.57 3.66 3.54 3.53
1.19 1.11 1.05 1.15 1.10
54 53 51 53 53
shown in Figure 3b; Ips and Iss were comparable with the noise level and negligible. Consequently, the symmetry of the Fluorescein O322 monolayer could be regarded as C∞v in the expanded monolayer region. When the monolayer has the C∞v symmetry with the z axis (the surface normal), the second-order susceptibility tensor χ(2) can be described by three independent tensor (2) (2) (2) (2) (2) (2) 3,5,17 elements: χ(2) zzz, χzxx ) χzyy, and χxzx ) χxxz ) χyzy ) χyyz. (2) The macroscopic susceptibility χ of the Fluorescein O322 monolayer is governed primarily by optical properties of the π-electron system of the chromophore. There is a strong absorption band at ∼482 nm and a substantial two-photon absorption band at ∼325 nm. These bands originate from broad π-electron delocalization in the vicinity of the Fluorescein O322 chromophore moiety, and they were assigned to the S0 f S1 and S0 f S2 transitions, respectively.12 The Fluorescein O322 molecule in the ground state has approximately C2v symmetry about the plane perpendicular to the molecule passing through the oxygen atom in the xanthene skeleton and the carboncarbon bond attaching phenyl substituent group. The S0 f S1 transition dipole moment lies along the molecule axis in the plane of the xanthene ring system, and the S0 f S2 moment lies parallel to the rotation axis.18 Since the intramolecular orientation of the fluorescein transition moments is similar to that of rhodamine dyes, the tilt angle, Θ, of the S0 f S1 transition moment with respect to the surface normal was deduced through the expression derived by Peterson and Harris,19
cos Θ ) where
R)
Figure 3. SHG from the Fluorescein O322 monolayer as a function of input polarization: (a) p-polarized SHG vs input polarization angle; (b) s-polarized SHG vs input polarization angle. The input polarization angle is defined as 0° for the p-polarized pumping beam and 90° for the s-polarized one. Curves presented refer to the various degrees of compression including the following: (O) 2.30 nm2/molecule; (/) 1.70 nm2/ molecule; (0) 1.17 nm2/molecule; (]) 1.00 nm2/molecule; (4) 0.83 nm2/molecule; (b) 0.58 nm2/molecule; (×) 0.50 nm2/ molecule; (9) 0.42 nm2/molecule; ([) 0.25 nm2/molecule; (2) 0.17 nm2/molecule.
incident angle) and (2) the equal intensities of p- and s-polarized outputs generated by the s- and q-polarized pumping beams, correspondingly, Isp ) Iqs, where the polarization direction between p and s is denoted as q, i.e., 45° with respect to the plane of incidence.3 The former relation was indeed observed within the ∼2.30-∼0.83 nm2/molecule region (region of expanded monolayer) as
[
1 xR
(1)
]
(2) (2) 2-3/2(χ(2) (xR + 1) zxx + χzzz - χxzx) and R ) 1 2-1/2χ(2) xR zxx 2
(
)
2
To obtain ratios among the three independent elements of χ(2), the SH intensity was measured in proper optical geometries as explicated in details elsewhere.3-5 The ratios (2) (2) (2) of χ(2) zzz/χzxx and χxzx/χzxx were obtained from the polarization dependence of the SH intensity (Figure 3) by a fitting procedure for five different degrees of the monolayer compression and showed no substantial change throughout the compression process as shown in Table 1. This finding obviously indicates that the average orientation of the transition dipole moment of the molecules within the monolayer should not vary much in the ∼2.30 to ∼0.83 nm2/molecule region. As clearly shown in Table 1, the χ(2) tensor elements are always arranged in a sequence χ(2) zzz > (2) χ(2) zxx ∼ χxzx. This arrangement in the size of the tensor (17) Cnossen, G.; Drabe, K. E.; Wiersma, D. A. J. Chem. Phys. 1992, 97, 4512. (18) Hermann, J. P.; Ducuing, J. Opt. Commun. 1972, 6, 101. (19) Peterson, E. S.; Harris, C. B. J. Chem. Phys. 1989, 91, 2683.
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Figure 4. Tilt angles and ratios of the main SH intensities determining the average orientation: (0) tilt angle vs area per molecule; (b) (Ipp/Isp)1/2 vs area per molecule; (2) (Isp/Iqs)1/2 vs area per molecule.
Figure 5. View of Fluorescein O322 surface arrangement. Here the water surface is shown as the XY plane in the laboratory coordinate. The Z axis is normal to the surface; M is the direction of transition dipole moment of interest. Large circles in the molecular structure denote schematically the functional COOH-, OH-, or Odgroups attaching to the water surface through the hydrogen bonding.
elements indicates that the average direction of the S0 f S1 transition moment neither lies in the surface plane (the X-Y plane; see Figure 5) nor coincides with the surface normal (the Z axis).6,20 The calculated value of the average tilt angle Θ was found within 54-51° and confirmed this estimation as presented in Table 1. The angle stayed approximately constant in the ∼2.30-∼0.83 nm2/molecule region. The s-polarized SH signal was generated from both sand p-polarized pumping beams and increased sharply in the region of less than 0.83 nm2/molecule as indicated in Figures 2 and 3b. The observable values of Ips and Iss indicates that in-plane anisotropy is not negligible. Therefore, the monolayer in the transition and condensed regions lost the C∞v symmetry and cannot be fitted theoretically by the SHG model for isotropic monolayers. Then, it is difficult to determine the exact tilt angle of the (20) Kajikawa, K.; Takezoe, H.; Fukuda, A. Jpn. J. Appl. Phys. 1991, 30, L1525.
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fluorescein chromophore in this region. A determination of the average tilt angle of chromophores in such occasion would require information on the symmetry of monolayer structure. This study is in progress. However, an approximate value of the average tilt angle of the chromophore under higher compression can be estimated on the basis of the analysis of polarization curves and ratios of the SH intensities with various polarization combinations. The observed ratio of the s-polarized SH signal to that of the p-polarized one suggests that the average direction of the chromophore was far from vertical6 in the transition region. The ratios of the SH intensity, Ipp/Isp and Isp/Iqs, tended to decrease monotonically in the transition and condensed regions as shown in Figure 4. The average tilt angle calculated without considering asymmetry is also shown in Figure 4 (curve 0). These findings allow us to estimate that the average tilt angle of the chromophore would not change more than 10° upon compression in the transition and condensed regions. The lack of significant pressure-induced reorientation of chromophores has also been discovered for some cyanine dyes5,8,21 and nitroaniline-terminated compounds.22 These results suggest that molecules possessing a large π-conjugated system and easily transferable electrons take their own orientation at the interface at the time of the monolayer spreading. This orientation makes little change upon compression, because this orientation is profoundly affected by structural characteristics of the dye and polar properties of the interface.4,14,23 3.3. Orientational Arrangement of Dye Molecules within the Monolayer Due to Compression. As mentioned in the previous section, the Fluorescein O322 molecules orient themselves at the interface in accordance with their molecular structure when they are spread on the surface, and their orientation does not change significantly upon compression. If the molecule has a polar functional group such as -COOH, -OH, or dO, they will play a dominant role in determining orientation of the molecule with respect to the polar surface.14,16,23,25 In particular, the hydrogen bonding of these groups to the water surface determines the orientation of Fluorescein O322 in a way where the polar groups point toward the water while the long carbon chain points toward the air. Then, the xanthene ring tends to tilt with respect to the surface normal (the Z axis) with an angle Θ as drawn schematically in Figure 5. Even if a rotation around the bond attaching the phenyl ring to the xanthene skeleton is assumed,26 the overall orientation of the dye molecule within the monolayer is governed strongly by the hydrogen bond to the water surface and the rigid structure of chromophore moiety. Thus, the average tilt angle Θ may be large, and the value of 51-54° would agree with what is expected from the molecular structure of Fluorescein O322 molecules. There is another reason the average tilt angle does not vary drastically throughout the compression process. This is preassociation of Fluorescein O322 molecules at the time of spreading due to pronounced tendency of the long(21) Mayer M. A.; Vanderlick, T. K. J. Chem. Phys. 1995, 103, 9788. (22) Hsiung, H.; Meredith, G. R.; Vanherzeele, H.; Popovitz-Biro, R.; Shavit, E.; Lahav, M. Chem. Phys. Lett. 1989, 164, 539. (23) Gaines, G. L. Insoluble monolayers at liquid/gas interface; Wiley: New York, 1966. (24) Hicks, J. M.; Kemnitz, K.; Eisenthal, K. B.; Heinz, T. F. J. Phys. Chem. 1986, 90, 562. (25) DiLazzaro, P.; Mataloni, P.; DeMartini, F. Chem. Phys. Lett. 1985, 114, 103. (26) Yamazaki, I.; Tamai, M.; Yamazaki, T. J. Phys. Chem. 1990, 94, 516.
Orientational Arrangement of Fluorescein Molecules
chain dyes to form assemblies.23,27 Further compression initiates an orientational arrangement of the dye assemblies at the interface. Sharp asymmetrical growth of s-polarized SH signals, Ips and Iss, below the ∼0.60 nm2/ molecule region showed the evident anisotropy of the polarization direction in the monolayer plane. Together with the large red shift of S0 f S1 absorption bands of Fluorescein O322 in the monolayer,12 this suggests that the lateral pressure leads to the formation of close-packed chains of aggregated dye molecules where the chromophoric moieties maintain the oblique geometry of J-type alignment28 with a quasi-long range order in tilt and azimuthal orientation of chromophore polarization directions. The in-plane direction of preferred orientation of dye chromophores was determined from a relation between p-polarized SH intensities, Ipp and Isp. As shown in Figure 2, Ipp (the incident laser beam is polarized perpendicular to the compression direction) was always larger than Isp (parallel to it). The plane of incoming and outgoing beams was perpendicular to the compression direction in our experimental geometry, and this indicates that the electron transfer axis of the chromophore, which has the largest second-order polarizability, lies perpendicularly to the direction of the compression.9 Thus, one can conclude the compression process causes the breakdown of C∞v symmetry of the monolayer and leads to the anisotropic orientational arrangement of the polarization direction (27) Cazabat, A. M.; Valignat, M. P.; Villette, S.; DeConinck, J.; Louche, F. Langmuir 1997, 13, 4754. (28) Kemnitz, K.; Yoshihara, K. J. Phys. Chem. 1991, 95, 6095.
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of chromophores mainly perpendicular to the compression direction. 4. Conclusions The four polarization components of the SH intensities of Fluorescein O322 were measured during compression as a function of its molecular area. They are similar to the π-A isotherm of Fluorescein O322. This finding indicates a strong correlation between pressure-induced changes in the molecular ordering and resultant macroscopic second-order susceptibility of the monolayer. The average tilt angle of the transition moment of the chromophore with respect to the surface normal has been found within 54-51° in the region of 2.30-0.83 nm2/ molecule. Further compression did not induce any significant change in the ratio of Ipp/Isp and Isp/Iqs, although Ips and Iss appeared and increased. These findings indicate the lack of the pressure-induced reorientation of the transition moment even though the in-plane anisotropy appeared. Thus, the compression process does not cause any significant change in the average tilt angle of chromophores, but it arranges the transition moment of chromophores along a certain preferred direction. The preferred direction was determined from the relationship between Ipp and Isp as perpendicular to the compression direction. Acknowledgment. An invitation fellowship of the Inoue Foundation for Science to V.T. is gratefully acknowledged. LA990151J
