Characterization of Langmuir Monolayers of the Amphiphilic Ru

Ultraviolet photoelectron yield spectroscopy (UPYS) was applied to the characterization of Langmuir layers of the amphiphilic Ru complex at the air/wa...
0 downloads 0 Views 84KB Size
Download PDF
9226

Langmuir 2003, 19, 9226-9230

Characterization of Langmuir Monolayers of the Amphiphilic Ru Complex at the Air/Water Interface by Ultraviolet Photoelectron Yield Spectroscopy Hideaki Monjushiro,*,† Kazumasa Harada,‡ and Masa-aki Haga§ Department of Chemistry, Graduate School of Science, Osaka University, 1-1 Machikaneyama, Toyonaka, Osaka 560-0043, Japan, Department of Radiology, Komazawa Junior College, 1-23-1 Komazawa, Setagaya-ku, Tokyo 154-8525, Japan, and Integrated Inorganic Material Chemistry Laboratory, Department of Chemistry, Faculty of Science and Engineering, Chuo University, 1-13-27 Kasuga, Bunkyo-ku, Tokyo 112-8551, Japan Received February 18, 2003. In Final Form: August 25, 2003 Ultraviolet photoelectron yield spectroscopy (UPYS) was applied to the characterization of Langmuir layers of the amphiphilic Ru complex at the air/water interface in ambient conditions. The UPY spectra were successfully obtained for Langmuir layers of the complex in the surface concentration range of 0.142.1 molecule nm-2, and the photoelectron emission threshold energy, Et, and photoelectron emission efficiency, R, were determined for the respective surface concentrations. The Et value was constant at lower surface concentration and decreased as the surface concentration of the complex was increased. The decrease in the Et value is considered to be due to the decrease in the distance between the Ru complexes in the two-dimensional gas phase and the following aggregation or crystallization considering the polarization effect on the photoionization hole state. The dependence of the R value upon the surface concentration of the complex also indicates the change in structure of the Langmuir layer of the Ru complex on the water surface. It is concluded that UPYS is applicable to the in situ characterization of Langmuir layers of photosensitive molecules at the air/water interface.

Introduction The fabrication of a designed, ordered, and oriented molecular film by the Langmuir-Blodgett (LB) method has attracted recent interest.1 Ruthenium pyridyl complexes are suitable to be the binding blocks for the construction of molecular devices for electron transfer, energy transfer, and proton transfer using the LB method because they have redox-active and photoactive properties and catalytic activities.2-7 We have studied the construction of the ordered multilayers organized by photoactive Ru pyridyl complexes and characterized the properties of the films.8-10 It is well-known that, to control the two-dimensional structure of the LB film and to construct well-defined layered materials, it is important to characterize the Langmuir layer at the air/water * Corresponding author. Telephone: +81-6-6850-5412. Fax: +81-6-6850-5414. E-mail: [email protected]. † Osaka University. ‡ Komazawa Junior College. § Chuo University. (1) Roberts, G. Langmuir-Blodgett Films; Plenum Press: New York, 1990. (2) Endicott, J. F.; Schlegel, H. B.; Uddin, M. J.; Seniveratne, D. S. Coord. Chem. Rev. 2002, 229, 95-106. (3) Rodriguez, M.; Romero, I.; Llobet, A.; Deronzier, A.; Biner, M.; Parella, T.; Evans, H. S. Inorg. Chem. 2001, 40, 4150-4156. (4) Inagaki, A.; Takaya, Y.; Takemori, T.; Suzuki, H.; Tanaka, M.; Haga, M. J. Am. Chem. Soc. 1997, 119, 625-626. (5) Mizushima, K.; Nakaura, M.; Park, S. B.; Nishiyama, H.; Monjushiro, H.; Harada, K.; Haga, M. Inorg. Chim. Acta 1997, 261, 175-180. (6) Ali, M. M.; Sato, H.; Haga, M.; Tanaka, K.; Yoshimura, A.; Ohno, T. Inorg. Chem. 1998, 37, 6176-6180. (7) Carlson, D. L.; Murphy, W. R., Jr. Inorg. Chim. Acta 1991, 181, 61-64. (8) Monjushiro, H.; Harada, K.; Nakaura, M.; Kato, N.; Haga, M.; Ryan, M. F.; Lever, A. B. P. Mol. Cryst. Liq. Cryst. 1997, 294, 15-18. (9) Haga, M.; Kato, N.; Monjushiro, H.; Wang, K.; Hossain, M. D. Supramol. Sci. 1998, 5, 337-342. (10) Wang, K.; Haga, M.; Hossain, M. D.; Shindo, H.; Hasebe, K.; Monjushiro, H. Langmuir 2002, 18, 3528-3536.

interface in situ. Although a number of techniques have been applied to characterize the structure and the morphology of the transferred LB films on the solid substrates, spectroscopic methods, which can be applied to the analysis of the structure of Langmuir films at the air/water interface, are limited. The characteristics required for the spectroscopic method, which applies to the characterization of the Langmuir layer at the air/water interface, have a high sensitivity and high surface selectivity and are capable of operating in ambient air conditions. Analytical techniques employing a photoelectron as a probe are extremely sensitive to the surface because the escape depth of the photoemitted electron in the solid film is very short, typically on the order of several nanometers.11 Therefore, electron spectroscopy is considered to be suitable for the analysis of Langmuir layers. Actually, electron spectroscopic techniques such as X-ray photoelectron spectroscopy (XPS), ultraviolet photoelectron spectroscopy (UPS), and Auger electron spectroscopy are widely used for the characterization of Langmuir layers transferred on solid substrates. However, these techniques require high vacuum conditions because analysis of the electron kinetic energy and of the excitation sources is needed, and this requirement limits the samples to be analyzed. Although several attempts have been made to detect photoelectrons from liquids or liquid surfaces, such experiments are limited to equilibrium vapor pressures not greater than 0.1 Torr.12,13 Some photoemission studies have been reported on the water surface; however, the experiments were carried out under the vacuum condition and with dynamic sample conditions such as a fast flowing (11) Seah, M. P.; Dench, W. A. Surf. Interface Anal. 1979, 1, 2-11. (12) Faubel, M.; Steiner, B.; Toennies, J. P. J. Chem. Phys. 1997, 106, 9013-9031. (13) Baschenko, O. A.; Bokman, F.; Bohman, O.; Siegbahn, H. O. G. J. Electron Spectrosc. Relat. Phenom. 1993, 62, 317-334.

10.1021/la0342767 CCC: $25.00 © 2003 American Chemical Society Published on Web 10/03/2003

UPYS of Langmuir Monolayers

liquid jet,14 liquid films on rotating disks,15,16 or conicalshaped trundles,17 and these experimental conditions are far from those for the preparation of LB films. The only analytical technique that can be applied to the characterization of chemical species on the air/water interface employing a photoelectron as a probe is the laser two-photon ionization method. This technique has been used to determine the concentration of chemical species on the water surface in ambient air.18-20 The laser twophoton ionization method is very sensitive to the surface and actually has been applied to the characterization of the Langmuir layer of pyrene hexadecanoic acid on water.21 With this technique, however, it is difficult to obtain a continuous photoexcitation spectrum,22 which gives information on the structure of the surface species, because the pulsed laser is an essential light source for two-photon excitation. Ultraviolet photoelectron yield spectroscopy (UPYS) is a unique and convenient technique for characterizing the surface electronic properties of materials under ambient conditions. In this technique, ultraviolet light irradiates the sample and photoelectrons emitted from the sample surface are detected by a low-energy electron counter, which works in ambient air.23 One of the principal advantages of this technique is its surface sensitivity. We have shown that the UPY spectrum of a gold film with only a 0.3-nm thickness can be easily obtained,24 and the escape depth of low-energy photoelectrons in a hydrocarbon film is about 1-3 nm.25 Two physical quantities, the photoelectron emission efficiency and photoelectron threshold energy, concerning the surface structure or the surface state of the sample can be obtained by the analysis of an UPY spectrum. The photoelectron emission efficiency reflects the photoelectron emissivity of the sample, which corresponds to the electron density of state of the highest occupied molecular orbital and gives quantitative information on the sample such as the amount and the thickness of the photoemissive materials.24,26 The photoelectron threshold energy, which is the minimum photon energy required to remove an electron from the sample surface, gives information on the surface electronic structure and has been used to characterize the properties of the photosensitive materials.27 In this paper, we apply UPYS to the characterization of Langmuir layers of the amphiphilic photosensitive ruthenium complex at the air/water interface under ambient conditions. Dependences of the photoelectron threshold energy and the photoelectron emission efficiency (14) Faubel, M.; Steiner, B.; Toennies, J. P. J. Electron Spectrosc. Relat. Phenom. 1998, 95, 159-169. (15) Watanabe, I.; Takahashi, N.; Tanida, H. Chem. Phys. Lett. 1998, 287, 714-718. (16) Watanabe, I. Anal. Sci. 1994, 10, 229-239. (17) Lundholm, M.; Siegbahn, H.; Holmberg, S.; Arbman, M. J. Electron Spectrosc. Relat. Phenom. 1986, 40, 163-180. (18) Inoue, T.; Masuda, K.; Nakashima, K.; Ogawa, T. Anal. Chem. 1994, 66, 1012-1014. (19) Sato, M.; Kaieda, T.; Ohmukai, K.; Kawazumi, H.; Ogawa, T. Anal. Sci. 1998, 14, 855-856. (20) Sato, M.; Kaieda, T.; Ohmukai, K.; Kawazumi, H. A. H.; Ogawa, T. J. Phys. Chem. B 2000, 104, 9873-9877. (21) Sato, M.; Akagishi, H.; Harata, A. H.; Ogawa, T. Langmuir 2001, 17, 8167-8171. (22) Ogawa, T.; Chen, H.; Inoue, T.; Nakashima, K. Chem. Phys. Lett. 1994, 229, 328-332. (23) Kirihata, H.; Uda, M. Rev. Sci. Instrum. 1981, 52, 68-70. (24) Monjushiro, H.; Watanabe, I.; Yokoyama, Y. Anal. Sci. 1991, 7, 543-547. (25) Monjushiro, H.; Watanabe, I. Anal. Sci. 1995, 11, 797-800. (26) Monjushiro, H.; Watanabe, I.; Yokoyama, Y. Anal. Sci. 1991, 7 (Supplement), 1395-1398. (27) Kohiki, S.; Nishitani, M.; Negami, T.; Wada, T.; Monjushiro, H.; Watanabe, I.; Yokoyama, Y. Thin Solid Films 1994, 238, 195-198.

Langmuir, Vol. 19, No. 22, 2003 9227 Chart 1. Molecular Structure of the Amphiphilic Ruthenium Complex, trans-RuCN2(C12-bpybim)

Figure 1. Experimental apparatus of UV photoelectron yield spectrometer working in air.

on the surface concentration of the complex are elucidated and discussed in relation to the states of the Langmuir layers of the complex. The applicability of UPYS for the in situ characterization of the Langmuir monolayer at the air/water interface is demonstrated. Experimental Section The amphiphilic ruthenium complex, trans-RuCN2(C12-bpybim) (C12-bpybim ) 6,6′-bis(N-laurylbenzimidazoil)-2,2′-bipyridine), shown in Chart 1, was synthesized by the reaction of [RuCl2(η6-p-cymene)]2 with C12-bpybim in ethanol and the following metathesis with cyanide anion. The details of the synthesis of the complex are given elsewhere.8 The surface pressure-area (π-A) isotherm was recorded on a computer-controlled LB transfer system (USI System Co. FSD300P) at 20 °C. The surface pressure was monitored with filter paper during a constant rate compression. The Ru complex was spread from a chloroform solution of a 0.32 mg cm-3 concentration onto ultrapure water. The system was located in a laminar flow hood with a filter capacity of 99.99% for 0.3-µm particles. UPY spectra of the Ru complex on the water surface were measured with a modified Riken Keiki AC-1M under atmospheric conditions. A schematic diagram of the experimental apparatus is shown in Figure 1. The UV light emitted from a D2 lamp was monochromatized and irradiated onto the sample surface through a quartz optical fiber. The photon energy of the irradiated UV light was swept from 4.0 to 6.5 eV (about 310-190 nm), and the photoelectrons emitted from the sample surface were detected in ambient air with a low-energy electron counter23 set at about 5 cm above the sample surface. Because the mean free pass of electrons in atmospheric air is less than 1 mm, the electron emitted from the sample surface is immediately captured by the

9228

Langmuir, Vol. 19, No. 22, 2003

Monjushiro et al.

O2 molecule in the air and forming the O2- ion. The O2- ions could pass through both the suppressor grid and the quenching grid, whose voltages were kept at 80 and 100 V, respectively, and entered the counter. The anode of the counter kept a constant voltage of about 3.5 kV. An incident O2- ion initiated a counter discharge, causing a pulse signal. The counter pulse derived a quenching circuit and a suppressor. The quenching of the discharge was carried out by supplying a positive square pulse to the quenching grid. During the quenching period, typically 5 ms, the entering of the ions was suppressed by supplying a negative voltage to the suppressor grid. After complete quenching of the discharge, both the voltages of the suppressor grid and the quenching grid were returned to the initial values. The counter repeated the same counting-quenching/suppressing cycle and counted the discharge pulse. The number of counter pulses per second produced at the anode is related to the number of electrons emitted from the sample surface. A loss of counts is, however, introduced by presence of the dead time of the counter, which is equal to the quenching time τ. The number of incident ions per second, I, is then expressed as

I ) N/(1 - τN)

Figure 2. Surface pressure-molecular area isotherm of RuCN2(C12-bpybim) on pure water measured at 20 °C.

(1)

where N is the observed counting rate by the air counter. The UPY spectrum was obtained by plotting the photoelectron yield Y against the incident photon energy hν. The photoelectron yield Y is defined by

Y ) I(hν)/P(hν)

(2)

where I(hν) is the number of detected ions per second and P(hν) is the incident photon flux on sample surface with an energy of hν. The incident photon flux was determined with a calibrated photodiode and was lower than 1.5 × 1010 s-1 in the measured energy range. The irradiation area of the sample surface was about 1 mm2. The apparatus for the UPY measurements were controlled by a personal computer. In the photoemission experiment, the Ru complex was spread on ultrapure water in a glass container dish (62-mm diameter) from the chloroform solution. The concentration of the solution was same as that used in the π-A isotherm experiment. In this case, the surface concentration of the Ru complex was altered by controlling the volume of the spreading solution. To ascertain the equilibrium condition of Langmuir layer, the UPY measurement was started 20 min after the spreading of the solution. The UPY spectra were measured at at least four different positions of the sample surface. The typical spectrum acquisition time was about 20 min. The UPY spectrum of the Ru complex in the solid phase was also measured for the solid samples, which were prepared by spreading the sample solution on a glass slide and evaporating the solvent.

Results and Discussion The proposed Ru complex formed a stable Langmuir film at the air-water interface. The typical π-A isotherm of the complex spread on ultrapure water at 20 °C is shown in Figure 2. The compression rate of the film was 2.0 × 10-3 nm2 molecule-1 s-1. The area for the onset of the pressure rise for the complex occurs at approximately 1.25 nm2 molecule-1, where the complex begins to form the liquid-expanded phase. As the compression continues, a plateau occurs at a surface area of about 1.1 nm2 molecule-1 and the second rise in the pressure appears at about 0.8 nm2 molecule-1. The film collapses at a pressure of approximately 44 mN m-1. The surface area of the first rising point is not affected by the compression rate, whereas the shape of π-A isotherm at the plateau region changed with the compression rate. On the analogy of monolayers of dipalmitoil phosphatidylcholine28 and (28) Kajiyama, T.; Kozuru, H.; Takashima, Y.; Oishi, Y.; Suehiro, K. Supramol. Sci. 1995, 2, 107-116.

Figure 3. UPY spectra of RuCN2(C12-bpybim) spread on water at the surface concentrations of (a) 1.89, (b) 1.45, (c) 1.02, (d) 0.43, and (e) 0.29 molecule nm-2. The blank spectrum for the pure water surface is shown as curve f.

arachidic acid,29 which give π-A isotherms similar to that in Figure 2, the Ru complex monolayer may be twodimensionally crystallized by the compression on the water surface in the plateau region (1.1-0.8 nm2 molecule-1). The second rise in the surface pressure corresponds to the completion of the two-dimensional crystallization of the complex. The UPY spectra of the Langmuir layers of the complex at the air/water interface were successfully measured in ambient conditions. The example of the UPY spectra for Langmuir layers of five different surface concentrations of the complex is shown in Figure 3. It can be seen that the photoelectron yield is monotonically increased with an increasing photon energy, and there are no marked structures in all the spectra and no photoemission was observed from the pure water surface (curve f in Figure 3). It is also clear from Figure 3 that the photoelectron yield increases as the surface concentration of the complex is increased. It is worth noting that the UPY spectrum of the complex with a surface concentration of only 0.14 (29) Kajiyama, T.; Oishi, Y.; Uchida, M.; Takashima, Y. Langmuir 1993, 9, 1978-1979.

UPYS of Langmuir Monolayers

Langmuir, Vol. 19, No. 22, 2003 9229

Figure 4. (Yield)1/2-photon energy plots for the UPY spectra of RuCN2(C12-bpybim) on water at the surface concentrations of (a) 1.45, (b) 0.72, and (c) 0.29 molecule nm-2. The Et value was determined by extrapolating the linear portion of the plot to yield ) 0.

molecule nm-2 on the water surface could be easily detected by this technique. This fact demonstrates the high surface sensitivity of the UPY spectroscopy. Following the interpretation by Kane,30 the photoelectron yield Y is expressed as a power law of the form

Y ) R(hν - Et)n for hν > Et and

Y ) 0 for hν e Et

(3)

where R is a photoelectron emission efficiency, Et is the photoelectron threshold energy, and the value of n is in the range of 1-5/2, depending on the system. The value of Et can be determined from the Y1/n versus hν plot by extrapolating the linear portion of the plot to Y ) 0, and nth power of the slope of the plot gives the R value. In this study, the value of 2 was adopted as an index n because of good linearity in the Y1/2 versus hν plot. In Figure 4 are shown the typical Y1/2 versus hν plots for Langmuir layers of three different surface concentrations of the Ru complex on the water surface. It is clear that the Et value can be determined precisely from the plot. The accuracy of the determined Et values is estimated to be (0.01 eV. It is also recognized from Figure 4 that the value of Et varies with the surface concentration of the Ru complex. Figure 5 shows the dependence of the Et value, determined by the above-mentioned method, upon the surface concentration of the complex. The threshold energy of the complex on the water surface is almost constant at 5.77 ( 0.03 eV for the surface concentration of less than 0.4 molecule nm-2 and decreases to 5.63 ( 0.03 eV as the concentration is increased in the concentration range of 0.4-0.8 molecule nm-2. Then, the Et value decreases slowly with increasing the surface concentration beyond 0.8 molecule nm-2. These features indicate that the Et value determined by UPYS sensitively reflects the two-dimensional surface state of the complex at the air/water interface. The variation of the Et value with the surface concentration shown in Figure 5 could be understood by considering the polarization effect on the photoionization (30) Kane, E. O. Phys. Rev. 1962, 127, 131-141.

Figure 5. Dependence of the Et value on the surface concentration of RuCN2(C12-bpybim) on the water surface.

final state of the complex. It is well-known that the ionization energy for the valence electronic levels in an organic molecular compound decreases on going from the gas phase to the condensed phases as a result of the energetic stabilization effect working on a photoionized molecule by the electrostatic polarization induced on the surrounding molecules.31 In the Langmuir film of the dilute surface concentration phase, where the complex is thought to be in a dilute two-dimensional gas phase, the distance between the complexes is sufficiently large and there is no electrostatic interaction between them. Therefore, the UV photoionized hole state of the complex is relaxed electrostatically only by the polarization of water molecules in the subphase. Whereas in the case of a more concentrated two-dimensional gas phase, the distance between the complexes becomes smaller and the ionized hole state of the complex is relaxed by not only water molecules in the subphase but also the surrounding complexes on the water surface. This final state stabilization results in the lowering of the Et value in the higher surface concentration phase. Therefore, the dependence of the value of Et on the surface concentration in Figure 5 can be interpreted as follows. In the low surface concentration region (0.8 molecule nm-2) is probably due to the formation and the growth of aggregates or microcrystallines of the complex on the water surface because the surface concentration of 0.8 molecule nm-2 corresponds to the molecular area where the surface pressure begins to rise as shown in Figure 2. The formation of the liquid(31) Sato, N. Electrical and Related Properties of Organic Solids; Munn, R. W., Miniewicz, A., Kuchta, B., Eds.; Kluwer Academic Publishers: Norwell, MA, 1997.

9230

Langmuir, Vol. 19, No. 22, 2003

Figure 6. Relationship between the photoelectron emission efficiency, R, and the surface concentration of RuCN2(C12bpybim) on the water surface.

expanded phase is ruled out because the lateral surface pressure is not applied for the film in this batch experiment. Therefore, in the concentrated Langmuir film (0.82.1 molecule nm-2), both the densely packed two-dimensional gas phase and the aggregates or the microcrystallines of the complex are thought to coexist on the water surface. In this case, the obtained Et values are considered to be those for the mixture of the densely packed twodimensional gas phase and the aggregates or the microcrystallines of the complex. It seems reasonable that the threshold energy of the most concentrated phase, 5.61 ( 0.01 eV at 2.1 molecule nm-2, is somewhat higher than that of the complex in the solid state, 5.50 ( 0.03 eV, because the hole state is relaxed almost two-dimensionally in the Langmuir layer, whereas the relaxation occurs three-dimensionally in the solid state. It has been reported that, in the case of evaporated gold on a glass substrate, the threshold energy decreases with increasing the particle size of the gold as a result of the polarization of the final state effect.26 The polarization effects on the photoionization of the final state have been also observed and discussed both on the ionization energies of organic compounds in UPS32,33 and on the core level shift of chemisorbed molecules on Si(111)34 and metals35 in XPS. These reports also support our argument. Figure 6 shows the dependence of the UV photoelectron emission efficiency, the term R in eq 3, upon the surface concentration of the Ru complex. Because the Et value depends on the surface concentration of the complex as discussed above, the square of the slope in the Y1/2 plot, the value of R, instead of the photoelectron yield at a certain excitation energy is utilized for quantitative analysis. It is clear from Figure 6 that the photoelectron emission efficiency is directly proportional to the surface concen(32) Sato, N. Synth. Met. 1994, 64, 133-139. (33) Sato, N.; Yoshikawa, M. J. Electron Spectrosc. Relat. Phenom. 1996, 78, 387-390. (34) Mitsuya, M.; Sato, N. Langmuir 1999, 15, 2099-2102. (35) Crispin, X.; Lazzaroni, R.; Crispin, A.; Geskin, V. M.; Bredas, J. L.; Salaneck, W. R. J. Electron Spectrosc. Relat. Phenom. 2001, 121, 57-74.

Monjushiro et al.

tration of the complex in the range of 0.14-0.9 molecule nm-2, whereas the increment of the efficiency becomes insensitive to the surface concentration above 0.9 molecule nm-2. The slope of the plot in the higher surface concentration is about 1/3 of that in the lower surface concentration range. The change in the slope indicates the structural change of the Langmuir layer of the complex. The surface concentration of the break point, 0.9 molecule nm-2, approximately corresponds to the starting point of the pressure rise in the π-A isotherm of the complex. Therefore, the change in the slope indicates the formation and the growth of the aggregates or the microcrystallines of the complex in the film. The decrease in the slope in the higher surface concentration region is due to the coexistence of dense two-dimensional gas phase and the aggregates or the microcrystallines of the complex. In the two-dimensional gas phase, photoelectrons emitted from the complex can emit into air directly without attenuation and the photoelectron emission efficiency is directly proportional to the surface concentration of the complex, whereas in the case of the aggregation or the microcrystalline of the complex, the over-layered complexes attenuate the photoelectrons emitted from the underlying complexes because the escape depth of UV photoelectrons is in the range of 1-3 nm.25 Therefore, the photoelectron emission efficiency is less sensitive to the surface concentration where the densely packed twodimensional gas phase and the aggregates or the microcrystallines of the complex are thought to coexist. This hypothesis of the change in photoemissivity due to aggregation or crystallization seems to be reasonable by considering the molecular size of the complex and is consistent with the dependence of the Et value on the surface concentration of the Ru complex discussed above. Conclusions In this study, UPYS was applied to the characterization of Langmuir layers of the Ru complex at the air/water interface for the first time. It is demonstrated that UPYS is a sufficiently sensitive technique to obtain the spectra of Langmuir layers of the complex down to the surface concentration of 0.14 molecule nm-2. Both the photoelectron emission threshold energy, Et, and the photoelectron emission efficiency, R, determined by the analysis of the UPY spectrum vary with the surface concentration of the complex, indicating the change in the structure of the Langmuir layer with the surface concentration. Although the UPY spectra were measured on the batch-equilibrium condition in this study, the UPY spectra could be measured as the compression of the Langmuir layer is underway. In conclusion, UPYS is a useful technique for the in situ characterization of Langmuir layers of photosensitive materials on the air/water interface in the ambient air atmosphere. Acknowledgment. The present work was partially supported by the Ministry of Education, Science, Sports and Culture, Grant-in-Aid for Scientific Research (C) 13640605. LA0342767