Langmuir 2001, 17, 8167-8171
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Polarization and Surface Density Dependence of Pyrenehexadecanoic Acid at the Air-Water Interface under Compression Studied by a Laser Two-Photon Ionization Technique Miki Sato, Hiromi Akagishi, Akira Harata,* and Teiichiro Ogawa Department of Molecular and Material Sciences, Kyushu University, Kasuga-shi, Fukuoka 816-8580, Japan Received May 5, 2001. In Final Form: October 3, 2001 Orientation of pyrenehexadecanoic acid (PyHA) on the water surface has been investigated using surface compression, fluorescence spectra, and two-photon ionization measurements. PyHA stays as a monomer at surface densities lower than 0.025 nmol/cm2, while at high surface densities excimer emission was observed. The two-photon ionization signal had a peak at 90° (p-polarized laser beam) upon the rotation of the polarization of the incident laser beam. The difference in the signal intensity generated by p-polarized and s-polarized lights became smaller as surface density increased. It is suggested that the tilt angle of the transition moment of the pyrene ring is ca. 70° at a low surface density, and it increases with the surface density increase. Problems in determining orientation of the molecules on water are discussed.
1. Introduction Surface properties depend on the chemical and physical details of the structure at the molecular level. Traditionally, the monolayer at the liquid-air interface has been studied with the Langmuir technique by measuring the surface pressure upon compression of the surface monolayer. Recently, new laser spectroscopic methods such as second harmonic and sum-frequency generations have proven to be an invaluable tool in investigating the orientation of molecules at the air-water interface.1-6 Laser two-photon ionization is an important surface technique as well.7 The photoabsorbing molecules in solution,8,9 on the solid surface,10 and on the water surface11,12 can be efficiently photoionized in a two-photon process by laser irradiation. The ejected electrons from the surface molecules can be measured simply and sensitively by conductivity measurements. However, no trials have been reported with this method to investigate molecular orientation on the surface. The escape depth of an electron from the water surface can be estimated13 to be about 1 nm from the escape depth of electrons from D2O ice14 and the Onsager length for (1) Zhao, X.; Subrahmanyan, S.; Eisenthal, K. B. Chem. Phys. Lett. 1990, 171, 558. (2) Castro, A.; Bhattacharyya, K.; Eisenthal, K. B. J. Chem. Phys. 1991, 95, 1310. (3) Eisenthal, K. B. J. Phys. Chem. 1996, 100, 12997. (4) Wang, H.; Borguet, E.; Eisenthal, K. B. J. Phys. Chem. 1997, 101, 713. (5) Tamburello-Luca, A. A.; Hebert, Ph.; Antoine, R.; Brevet, P. F.; Girault, H. H. Langmuir 1997, 13, 4428. (6) Tsukanova, V.; Harata, A.; Ogawa, T. Langmuir 1999, 16, 1167. (7) Ogawa, T. In Photoionization and Photodetachment; Ng, C.-Y., Ed.; World Scientific: River Edge, NJ, 2000; Chapter 11, pp 601-633. (8) (a) Voigtman, E.; Jurgensen, A.; Winefordner, J. D. Anal. Chem. 1981, 53, 1921. (b) Voigtman E.; Winefordner, J. D. Anal. Chem. 1982, 54, 1834. (9) (a) Yamada, S.; Kano, K.; Ogawa, T. Bunseki Kagaku 1982, 31 (1), E247. (b) Yamada, S.; Hino, A.; Kano, K.; Ogawa, T. Anal. Chem. 1983, 55, 1914. (10) Ogawa, T.; Yasuda, T.; Kawazumi, H. Anal. Chem. 1992, 64, 2615. (11) Inoue, T.; Masuda, K.; Nakashima, K.; Ogawa, T. Anal. Chem. 1994, 66, 1012. (12) Sato, M.; Kaieda, T.; Ohmukai, K.; Kawazumi, H.; Harata, A.; Ogawa, T. J. Phys. Chem. B 2000, 104, 9873.
water.15 Thus, a surface layer of water with a thickness of about 1 nm can be probed by the laser photoionization method.12 Therefore, this method also allows sensitive investigation of the thin surface layer (thickness, about 1 nm) of water. The depth resolution is slightly different from that of the second harmonic generation method. Pyrene has been frequently used as a probe molecule to study the structure and dynamics of micelles16,17 and vesicles.18,19 Compared with other probes, pyrene exhibits marked sensitivity to the medium in several types of photoinduced processes such as excimer formation, fluorescence quenching, and exciplex formation. Among pyrene compounds, 1-pyrenehexadecanoic acid (PyHA, shown in Figure 1) is completely insoluble in water because of its long hydrocarbon chain and has been frequently used as a Langmuir-Blodgett (LB) film compound.20,21 Aggregation of PyHA through hydrogen bonding and other interactive forces enhances the excimer formation efficiencies of the pyrene moiety and prevents molecular motions. We have recently reported on distribution constants and surface equilibrium constants of 1-pyrenebutyric acid, which is a partly soluble molecule having a shorter CH chain than PyHA, using the two-photon ionization method.12 In this paper, we report the surface density and polarization dependence of the two-photon ionization signal of the PyHA monolayer at the air-water interface. (13) Sato, M.; Kaieda, T.; Ohmukai, K.; Kawazumi, H.; Ogawa, T. Anal. Sci. 1998, 14, 855. (14) Jo, S. K.; White, J. M. J. Chem. Phys. 1991, 94, 5761. (15) Holroyd, R. A. Radiation Chemistry: principles and Applications; Farhataziz, Rodgers, M. A. J., Eds.; VHC Publishers: New York, 1987; p 201. (16) Waka, Y.; Hamamoto, K.; Mataga, N. Chem. Phys. Lett. 1978, 53, 242. (17) Waka, Y.; Hamamoto, K.; Mataga, N. Photochem. Photobiol. 1980, 32, 27. (18) Kano, K.; Kawazumi, H; Ogawa, T. Chem. Phys. Lett. 1980, 74, 511. (19) Kano, K.; Kawazumi, H; Ogawa, T. J. Phys. Chem. 1981, 85, 2204. (20) Yamazaki, T.; Tamai, N.; Yamazaki, I. Chem. Phys. Lett. 1986, 124, 326. (21) Yamazaki, I.; Tamai, N.; Yamazaki, T. J. Phys. Chem. 1987, 91, 3572.
10.1021/la010673a CCC: $20.00 © 2001 American Chemical Society Published on Web 11/30/2001
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Figure 1. Surface pressure-area isotherm of the PyHA monolayer on the pH 6.8 subphase. The molecular structure of PyHA is shown in the figure.
In a low surface density, we found a clear signal intensity dependence on excitation laser beam polarization, which should be related to the orientation of these molecules on the water surface. 2. Experimental Section The detection system for the photoionization signal has been described previously,11,12 and only brief descriptions are given here. The photoionization was induced by the third harmonic of a Q-switched Nd:YAG laser (Minilite2, 355 nm, 8 mJ, 3-5 ns pulse width). Its polarization was rotated by using a half-wave plate (Sigma WPQ-3550-2M). The laser beam was focused softly by a quartz lens on the solution surface at an incident angle of 85°. A Langmuir trough (surface area, 100 cm2; volume, 70 mL) was filled with a pH 6.8 buffer solution. A disk electrode was placed 6 mm above the solution surface and positively biased at 1.5 kV. The photocurrent signal was fed to a current amplifier (Keithley 427; time resolution, 10 µs) and a digital storagescope (Kenwood DCS-8200). The signal was accumulated for 64 pulses of the laser. PyHA was purchased from Molecular Probes and used without further purification. Water was deionized and purified by a Millipore Milli-Q system. The subphase was freshly prepared by adjusting the pH at 6.8 by NaH2PO4 and NaOH with a Horiba M-8E pH meter. A quantity (0.01-0.6 mL) of the benzene solution of PyHA (2 × 10-5 mol/L) was spread on the buffer solution (pH ) 6.8) in the Langmuir trough, and the benzene was evaporated. The surface density of PyHA was controlled by changing the surface area with compression using a Teflon barrier moving from one side of the trough. The signal showed a steady-state value within 10 min. The fluorescence emission spectra were measured with a Hitachi F-4010 fluorescence spectrophotometer at an excitation wavelength of 280 nm. A quartz cuvette was initially filled with a pH 6.8 buffer solution, before PyHA benzene solution was spread on the buffer surface, and the measurements were performed after benzene was evaporated. The PyHA monolayer was placed in the optical path of the collimated light beam. A Langmuir-Blodgett trough with a Wilhelmy balance (USI system FSD-300) was used to measure the surface pressurearea isotherm. All measurements were done at a steady-state condition at room temperature (22 ( 2 °C).
3. Results and Discussion Isotherm and Fluorescence. Figure 1 shows the surface pressure-area isotherm of PyHA spread on a buffer solution of pH 6.8. This isotherm agrees well with that measured by Yamazaki et al.20,21 They reported that the limiting area of PyHA was 0.21-0.23 nm2, which was in accordance with the space-filling molecular models for vertical packing. Furthermore, they showed that pure PyHA did not actually form a stable compressed monolayer and that this monolayer collapsed at a molecular area of 0.34 nm2, which corresponds to that of the closest packing
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Figure 2. Fluorescence spectra of the PyHA monolayer at four different surface densities: 0.001 nmol/cm2 (1, s), 0.1 nmol/ cm2 (2, - - -), 0.5 nmol/cm2 (3, ‚‚‚), and 1.0 nmol/cm2 (4, - ‚ -). The excitation wavelength was 280 nm.
of planar pyrene rings. Thus, the monolayer coverage corresponds to 0.49 nmol/cm2 of the surface density where the area per molecule is 0.34 nm2. These values are used for discussion below. Fluorescence spectra of the PyHA monolayer at the airwater interface are shown in Figure 2. The spectra are measured at four different surface densities of 0.001, 0.1, 0.5, and 1.0 nmol/cm2, of which the lowest density is far below the monolayer coverage and the highest density is twice the coverage. It has been reported that the fluorescence spectra of PyHA in the LB film consist of two groups of emission bands with intensity maxima at about 377, 397, and 421 nm (monomer bands) and 470 nm (excimer band).20,21 A similar molecule, 1-pyrenedodecanoic acid, has monomer peaks between 375 and 400 nm and an excimer peak at 445 nm in aqueous solution.22 Thus, the weak band around 375-410 nm and the band around 465 nm in Figure 2 can be assigned as a monomer and an excimer, respectively. The peak at 495 nm and the shoulder at 535 nm were frequently observed at a high surface density, but they were less reproducible than the peak at 465 nm and depended on the sample surface preparation. They may come from experimental artifacts because the excimer emission around 430-550 nm should be structureless, but it seems possible that different types of excimer, such as normal type and sandwich type,23 cause the multiple peaks and shoulders. Anyway, the excimer emission appeared only at surface densities above 0.05 nmol/cm2. The fluorescence maximum of the excimer band of the PyHA monolayer shows a small blue-shift as compared to their spectra in the LB film and a large redshift as compared to that in aqueous solution. Generally, the spectra of a disordered monolayer are similar to those obtained from solution. These results imply that the packing of PyHA on the water surface follows a more orderly fashion than that in solution but a more disorderly one than that in an LB film. Two-Photon Ionization. Figure 3 shows the dependence of the photoionization signal of the PyHA monolayer on the laser pulse energy by using the s-polarized and p-polarized pumping laser. The signal using the ppolarized pumping laser is larger than that using the s-polarized pumping laser, and both photoionization signals of PyHA were proportional to the square of the laser pulse energy, indicating that a two-photon ionization is the dominant process for the wavelength of excitation at 355 nm. The saturation at higher laser power is due to electrical or optical problems because the surface concentration is far below the monolayer coverage. All of (22) Kunjappu, J. T.; Somasundaran, P. Langmuir 1995, 11, 428. (23) (a) Tujii, Y.; Itoh, T.; Fukuda, T.; Miyamoto, T.; Ito, S.; Yamamoto, M. Langmuir 1992, 8, 936. (b) Winnik, F. M. Chem. Rev. 1993, 93, 587.
Orientation of Pyrenehexadecanoic Acid on Water
Figure 3. Dependence of the photoionization signal of the PyHA monolayer on the laser pulse energy. The surface densities and polarization angles are as follows: 2, 0.0033 nmol/ cm2 and s-polarization; 4, 0.0033 nmol/cm2 and p-polarization; b, 0.025 nmol/cm2 and s-polarization; O, 0.025 nmol/cm2 and p-polarization; [, 0.24 nmol/cm2 and s-polarization; ], 0.24 nmol/cm2 and p-polarization.
Figure 4. Dependence of the photoionization signal on the surface density of PyHA. The incident laser was either s-polarization ([) or p-polarization (0). The ordinate was normalized to the photocurrent at a laser pulse energy of 100 µJ/pulse.
the following experiments were carried out under a condition that the photoionization signal was quadratically proportional to the laser pulse energy. The mechanism of the photoionization has been discussed previously.11 Figure 4 shows the dependence of the photoionization signal on the surface density when the PyHA monolayer is compressed. The ordinate is normalized to the photocurrent at the laser pulse energy of 100 µJ/pulse. The dependence is essentially identical for the two polarizations of the incident laser, but the s-polarized laser beam gave systematically smaller signals than the p-polarized laser beam, as discussed later. The photoionization signal was linearly proportional to the surface density of the PyHA monolayer below 0.025 nmol/cm2, and the slope was more than 1 at surface densities from 0.025 to 0.1 nmol/cm2. The change of the slope at 0.025 nmol/cm2 approximately corresponded with the excimer formation on the water surface and in the LB film of PyHA.20,21 Therefore, we can conclude that the change was caused by excimer formation. The excimer would probably have lower ionization potential than the monomer, and the ionization efficiency of the former would be larger than that of the latter. Because the photoionization signal is the sum of the signal from the monomer and the excimer, at higher surface densities it would increase faster than that of the pure monomer monolayer. Unfortunately, we can refer to no
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Figure 5. Fluctuation of the photoionization signal of PyHA on the water surface ionized by the p-polarized laser beam. Surface densities: 0.033 nmol/cm2 (×), 0.24 nmol/cm2 (0), and 0.30 nmol/cm2 (b). The ordinate (It/It)0) is shown as the ratio to the initial value (It)0).
reports on the ionization efficiency and threshold energy of the pyrene excimer on the water surface, but the efficiency will be larger than that of the monomer if the excimer has a lower threshold energy than the monomer. This monolayer collapsed in a molecular area of 0.34 nm2 (corresponding to 0.49 nmol/cm2) in Figure 1. The photoionization signal fluctuated at surface densities above 0.1 nmol/cm2, as shown in Figure 5. It was difficult to determine an accurate intensity of the photoionization signal in the vicinity of the film collapse, while signal intensity was stable for tens of minutes at a surface density under 0.1 nmol/cm2. It is known that a similar molecule, 1-pyrenedodecanoic acid, aggregates on aqueous solution by the hydrogen bond between the carboxylic groups, and their pyrene moiety closely interacts.24 Our result indicates that the PyHA molecules on the water surface aggregate and produce domains on the water surface at high surface densities. Observation for a short time and over a small region of the surface caused a fluctuation of the photoionization signal by the Brownian motion of domains.25-27 Figure 5 shows that signal fluctuation caused by such a motion is significantly large at the high surface densities. On the other hand, it is expected that monomers on the water move around the surface so rapidly that the ionization signal intensity well represents a time-averaged value of the surface density. A typical result of the polarization dependence of the photoionization signal of PyHA is shown in Figure 6; 0° and 90° in the abscissa correspond to the s-polarization and to the p-polarization of the excitation laser beam. Results are shown for four different surface densities, and for each density the photoionization signal was normalized to that obtained at the p-polarized beam. All of the signals had a peak at 90° (p-polarization). The solid, dotted, and broken curves are cosine curves fitted to the observed data. The fitting was very good, and we obtained the values for Is/Ip of 0.71, 0.79, 0.80, and 0.86 for surface densities of 0.033, 0.010, 0.025, and 0.040 nmol/cm2, respectively, where Is and Ip represent the relative intensity of the photoionization signal measured with s-polarized and p-polarized laser beams, respectively. A quantitative interpretation of the Is/Ip values connected with molecular orientation requires some assump(24) Grieser, F.; Thistlethwaite, P. J.; Urquhart, R. S. Chem. Phys. Lett. 1987, 141, 108. (25) Matsuzawa, Y.; Seki, T.; Ichimura, K. Thin Solid Films 1997, 301, 162. (26) Dutta, A. K.; Salesse, C. Langmuir 1997, 13, 5401. (27) Li, Y.-Q.; Slyadnev, M. N.; Inoue, T.; Harata, A.; Ogawa, T. Langmuir 1999, 15, 3035.
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Figure 6. Polarization dependence of the photoionization signal of PyHA on the water surface. The s-polarization corresponds to 0°, and the p-polarization corresponds to 90°. The ordinate (I/Ip) was normalized to the photoionization signal under irradiation of the p-polarized laser (90°) and was fitted with a cosine curve. Surface densities: 0.0033 nmol/cm2 (0, - - -), 0.010 nmol/cm2 (O, - - -), 0.025 nmol/cm2 (2, ‚‚‚), and 0.040 nmol/cm2 (×, s).
Figure 7. Dependence of Is/Ip on the surface density of PyHA. Is and Ip represent the photoionization signal when the incident laser was s-polarized and p-polarized, respectively.
tions because of the complexity involved in a two-photon ionization process, especially of molecules on the water surface. Under the assumptions described in the later section, we determined the tilt angle of the transition moment with respect to the surface normal as 70.8°, 72.7°, 73.1°, and 74.5° for surface densities of 0.033, 0.010, 0.025, and 0.040 nmol/cm2, respectively, and 0.28 and 0.63 for Is/Ip values if molecules are randomly oriented in the air side and the water side of the water surface, respectively. If we assume that the transition moment of the pyrene ring lies parallel to the ring, the above results indicate the following. The pyrene ring is not parallel oriented to the water surface; it has a finite angle with respect to the water surface, and the angle becomes smaller as surface density increases. Another possibility remains that the molecular orientation is unchanged with surface density while the ratio of molecules randomly oriented on the surface decreased with the surface density increase (Figure 7). This is because what we observed was ensemble-averaged values over the molecules photoexcited on the water surface and orientationally random molecules have a small Is/Ip value of 0.28. Since the Is/Ip value for orientationally random molecules on the surface depends on the incident angle, this possibility can be eliminated by conducting measurements under different incident angle conditions. A preliminary experiment performed suggested that the possibility was small. We conclude that the tilt angle of the transition moment of the pyrene ring is ca. 70° at a low surface density, and
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it increases with the surface density increase. It has been found that the tilt angle of fluorescein O322, which has a COOH group, an OH group, and a long chain of octadecanes, was about 54° to the normal of the surface at 2.3-0.83 nm2/molecule by the second harmonic generation technique.6 A molecule with a hydrophilic group and a large conjugated hydrophobic group could be expected to stay on a slant orientation while the hydrophilic group is in the water. Polarization Dependence and Molecular Orientation. The assumptions required are described for quantitative interpretation of the Is/Ip values connected with the molecular orientation. First, we need to know the direction of the transition moment for the two-photon process with respect to the molecular axis. Second, orientation distribution of molecules on the water should be assumed. Finally, the strength of the electromagnetic fields induced by the laser radiation should be evaluated at the position of molecules, for which molecular distribution in the depth direction should be known. As for the strength of the electromagnetic fields, Fresnel’s coefficients of reflection and transmission were evaluated to calculate strength and direction of the fields for a given polarization and the incident angle of the excitation laser, where changes in dielectric constants caused by the PyHA molecules on the surface were neglected. Ionization efficiency was assumed to be proportional to E2 cos2 θ where E is the strength of the electric field and θ is the angle between the transition moment of the molecules and the direction of the electric field vector. Dependence of the transition moment on the molecular environment was neglected. Random azimuthal distribution was always assumed of the transition moment, and the ionization efficiency at fixed angles of transition moment was averaged over the possible distribution. The transition moment was assumed to be in the air side of the water surface except for calculating the Is/Ip value for the randomly oriented molecules in water. The E2 cos2 θ proportionality of the ionization efficiency is for the one-photon electric dipole transition. It is the simplest assumption based on a consideration that the transition from the ground state to a resonantly excited state dominates the ionization efficiency. However, the assumption contradicts the observed quadratic proportionality, corresponding to E,4 of the photoionization signal to the laser pulse energy. For a two-photon transition, the proportionality term is expressed as E4Σ(cos2 θi cos2 φi), where θi and φi are the angles between the direction of the electric field vector and transition moments from the ground state to the ith intermediate state and from the intermediate state to the final state, respectively. The summation should be taken for all the possible intermediate states. If we assume that θ ) θi ) φi corresponding to the E4 cos4 θ proportionality, the angles determined of the transition moment with respect to the surface normal are 74.0°, 75.0°, 75.2°, and 75.9° for surface densities of 0.033, 0.010, 0.025, and 0.040 nmol/cm2, respectively, and Is/Ip values are 0.08 and 0.40 for randomly oriented molecules in the air side and the water side of the water surface, respectively. Even in this case, it is unnecessary to reconsider the discussion in the preceding section. It is not easy to know the direction of the transition moment for the two-photon process with respect to the molecular axis. It is reasonable to consider that the transition moment of PyHA is that of the pyrene ring because the ionization efficiency depends mainly on the electronic property of the aromatic ring and has little to do with the hydrocarbon chain.12 It is known that the transition moment of pyrene for 355 nm excitation is in
Orientation of Pyrenehexadecanoic Acid on Water
the plane of the pyrene ring and the dichroic ratio Ez/Ey is 2, where z is the long axis of the pyrene ring.28 The fact that a two-photon ionization process involves many of possible transitions makes the situation complex, especially for a molecule having a resonant absorption band. A molecule may be ionized by simultaneously absorbing two photons; another molecule may be ionized by absorbing one photon from an excited state generated by one photon absorption from the ground state. The molecule can rotate in the excited state for a laser pulse duration of several nanoseconds. An additional problem is the properties of unknown final states which should be dissociative. The assumption that the transition moment of the pyrene ring lies parallel to the ring is the most possible for the resonant two-photon process, but we have no evidence for that. We hope the relation between the directions of the two-photon transition moment and the molecule on the water surface will be clarified in the future. 4. Conclusion We have demonstrated that the laser two-photon ionization technique offers information on orientation of molecules on the water surface. The two-photon ionization
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signal was measured for PyHA on the water surface under a surface density from 0.003 to 0.5 nmol/cm2. It was found that the signal generated by p-polarized light had a larger intensity than that generated by s-polarized light and that the difference in the signal intensity by p-polarized and s-polarized lights became smaller as surface density increased. The tilt angle of the transition moment of the pyrene ring is evaluated as ca. 70° at a low surface density, and it is suggested that the angle increases with the surface density increase. Because the two-photon ionization technique has a high sensitivity to molecules on the water surface and probes the water surface with a depth resolution different from that by a second harmonic generation technique, these two techniques will provide complementary information and much profound understanding on the behavior of molecules on the water surface and on the water surface itself. LA010673A (28) Michl, J.; Thulstrup, E. W. Spectroscopy with Polarized Light: Solute Alignment by Photoselection, in Liquid Crystal, Polymers and Membranes; VHC Publishers: New York, 1995; pp 118-121, 358-361.
