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Optical chirality of citronelloxy-cyanobiphenyl monolayer at an air–water interface studied by the MDC and SHG measurement
Chemical Physics Letters 407 (2005) 337–341 www.elsevier.com/locate/cplett
Optical chirality of citronelloxy-cyanobiphenyl monolayer at an air–water interface studied by the MDC and SHG measurement Ryousuke Tamura a, Takaaki Manaka a, Mitsumasa Iwamoto Tomomichi Itoh b, Junji Watanabe b b
a,*
,
a Department of Physical Electronics, Tokyo Institute of Technology, O-okayama 2-12-1-S3-33, Meguro-ku, Tokyo 152-8552, Japan Department of Organic and Polymeric Materials, Tokyo Institute of Technology, O-okayama 2-12-1, Meguro-ku, Tokyo 152-8552, Japan
Received 25 February 2005; in final form 18 March 2005 Available online 14 April 2005
Abstract Chirality of S- and R-enantiomer of citronelloxy-cyanobiphenyl (CCB) monolayer on a water surface were investigated by Maxwell displacement current (MDC) and optical second harmonic generation (SHG) measurements. While p–s SHG signal could not reveal the presence of chirality, the polarized angle dependence measurement indicated the ratio between chiral and achiral component of the non-linear optical susceptibility as (vxyz + vxzy)/(vxxz + vxzx) 1/40 and 1/23 for S- and R-CCB, respectively. It was confirmed that our MDC–SHG system has the ability to detect the chirality in monolayer level. 2005 Elsevier B.V. All rights reserved.
1. Introduction Since the discovery of the technique for the formation of floating monolayer at a liquid–air interface [1], interesting behaviors of the monolayers have attracted many scientists to intense research for these materials [2]. Monolayers on a water surface are classified as a twodimensional (2D) materials. Since the nature of the 2D materials is different from that of 3D bulk ones, monolayer at the interface between two different media are much important in the area of chemistry, surface physics and biology. Classically, surface pressure–area (p–A) isotherm measurement has been widely employed for the study of monolayers. For the past decades, novel experimental techniques, which are highly surfacespecific, have been developed for the study of monolayers and they have been used in combination with the p–A isotherm measurement. In particular, microscopic
*
Corresponding author. Fax: +81 3 5734 2191. E-mail address: [email protected] (M. Iwamoto).
0009-2614/$ - see front matter 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.cplett.2005.03.083
structures of monolayer have been revealed by the grazing incidence X-ray scattering [3] and non-linear optical (NLO) methods such as optical second harmonic generation (SHG) [4,5]. Microscopic flow behavior and domain formation in monolayers have been visualized by Brewster angle microscope (BAM) and fluorescence microscopy. We have been also developing a novel method named Maxwell displacement current (MDC) technique that allows the molecular motion in monolayers to be probed through the displacement current flowing across monolayers [6,7]. With this MDC measurement, change of spontaneous polarization in monolayers, accompanied by the orientational change of polar molecules has been examined. Recently, we have extended this MDC measurement system by coupling with SHG measurement [8–11]. MDC and SHG processes are related to spontaneous polarization and the second order induced polarization formed in monolayer, respectively. From a dielectric physics point of view, dielectric polarization phenomena truly reflect the specific properties of materials. Thus, these techniques are useful to specify the characterization of monolayers.
338
R. Tamura et al. / Chemical Physics Letters 407 (2005) 337–341
At the air–water interface, amphiphilic molecules tend to align so as that hydrophobic parts direct toward air. Immediately after spreading solution, molecules lie on a water surface due to the electrostatic coulomb attractive force working between molecule and the water surface, and it is reasonable to consider that there is no in-plane anisotropy, i.e., monolayer possesses the D1h symmetry. Since all NLO susceptibilities vanish at this state, SHG signal should not be observed from these molecules. Decreasing a molecular area by a monolayer compression gradually rises up molecules from a water surface. The growth of asymmetrical structure of the monolayer on the water surface reduces its symmetry from D1h to C1v. Therefore, in this region SHG signals should begin to generate and gradually increase, indicating the lack of the inversion symmetry. In our previous paper describing MDC–SHG measurement of 8CB monolayers [9], p–p SHG signal was observed and it gradually increased during monolayer compression. On the other hand, interesting behavior of the structural change was observed for the 7OCB monolayer [11]. In 7OCB monolayer, constituent molecules tilt in one direction with continuous compression after standing up the molecules, due to the shear stress working among molecules. Thus, we could find the phase transition from polar orientational (untilt) phase to planer alignment (tilting) phase. The formation of the planer alignment phase was confirmed by observation of p–s signal in the SHG measurement, indicating that the resultant symmetry changed from C1v to C2v. From a view point of symmetry of a monolayer, introducing chirality into molecules extinguishes the mirror plane of the system. Thus, monolayer symmetry is reduced from C2v to C2 for the tilting phase, and is reduced from C1v to C1 for the untilt phase. In these cases, the number of non-zero component of the NLO susceptibility increases and equations expressing the SHG process are more generalized. Furthermore, since chiral properties play an important role in biology and chemistry, to confirm the ability to detect chiral property of the surface monolayer is of great importance for extending the potentiality of our MDC–SHG systems. The chirality of the molecule can be discriminated using the optical spectroscopic techniques, such as the optical rotary dispersion (ORD) and the circular dichroism (CD). Usually, since the interaction between chiral materials and circularly polarized light are quite small, observation of the optical chirality in a monolayer using typical ORD and CD technique is a quite task. Even in such case, the surface specific nonlinear optical technique, such as the SHG circular dichroism (SHG-CD), also can be a powerful tool for the detection of monolayer chirality, where differential SHG signals from chiral surface are recorded with right and left circularly polarized fundamental incidence. The SHG-CD tech-
nique was first applied for 2,2 0 -dihydroxy-1,1 0 -binaththyl (BN) solid/solution interface by HicksÕ group [12] and by other researchers [13,14]. In our previous paper, we also examined the chiral properties and the molecular orientation of S-citronelloxy-cyanobiphenyl (S-CCB) monolayer on a water surface by MDC and SHG measurement [15], where MDC and SHG were mainly used to detect the phases and the chirality of monolayers, respectively. At that time, the measurement was limited to S-CCB monolayer and the SHG signal which indicates the chirality was rather weak. Thus, there was a possibility remained that the difference in the SHG signal was due to experimental errors. In this Letter, we measured the chiral properties of the both S- and R-enantiomers of CCB, on the basis of the polarized angle dependence measurement and the p–s SHG measurement. As will be discussed later, both measurement can distinguish the optical chirality. Measurement using an opposite enantiomer of CCB strongly supported our previous results [15].
2. Experiment The MDC and SHG experiments were performed using the experimental setup described in our previous paper [8]. Since detailed information of the measurement setup was described in that paper, we briefly introduce here our experimental system. The experimental setup used here consists of Langmuir-trough equipped with a two electrodes arrangement for the MDC measurement and optical arrangement with an optical parametric oscillator (OPO) for the SHG measurement. The rectangular-shape Langmuir-trough (600 · 150 mm in length and width, 10 mm in depth) was made from poly-tetrafluoroethylene (PTFE) and it was filled with pure water (electrical resistivity > 17 MX cm1). One transparent silica glass plate was attached to the bottom of the LB trough for the SHG measurement. For the MDC measurement, transparent glass slide coated with Indium Tin Oxide (ITO) was used as the top electrode (45 cm2). It was placed in air parallel to the water surface at a distance of 1 mm. The bottom electrode was a spiral shape gold wire (1-mm diameter and 500 mm in length) and it was immersed in the water. These two electrodes were connected to each other through an electrometer (Keithley 617) with negligible electrical resistance. The surface pressure of the monolayer was measured by a Wilhelmy plate. For SHG measurement, fundamental light with a wavelength of 560 nm was irradiated onto the monolayer at an energy of 6 mJ with a pulse rate of 10 Hz, and the spot size of the laser was about 56 mm2. In this measurement, fundamental light was not focused and was impinged on a water surface with an incident angle of 60 after passing through an SH-cut filter to
R. Tamura et al. / Chemical Physics Letters 407 (2005) 337–341
CN
Fig. 1. Structure of the molecules used in this experiment, R-CCB. The chiral center for the R-CCB molecule is marked by asterisk.
eliminate unexpected SH light from various optical components. Both transmitted and reflected SH light from the monolayer were detected by photomultiplier tubes after passing through fundamental cut and the interference filters. The molecules used in this experiment were chiral S- and R-CCB. The molecular structure of R-CCB is schematically described in Fig. 1. As figure shows, since the four different groups attached to the carbon atom labeled by asterisk in the R-CCB molecule, this carbon atom becomes a chiral center and hence this molecule has a chirality. Since the configuration of this CCB molecule is R-enantiomer, this molecule is referred as R-CCB in this Letter. The chloroform solution with a molar density of 1 mmol/l was spread on a water surface to form a Langmuir monolayer. The monolayer was compressed with aid of two barriers moving at the same speed in opposite direction of 10 mm min1 (0.056 nm2 min1 molecule1).
3. Results and discussion Hereinafter, the SH intensities for the transmitted geometry are listed for four kinds of input and output polarization combinations, p–p, s–p, p–s and s–s. We can obtain the SH intensity for C1 symmetry as, Ið2xÞTp–p / j s15 sin / cos2 / þ s31 cos2 / sin / þ s33 sin3 /j2 ; Ið2xÞTs–p / js31 cos /j2 ;
Region 4 SH intensity [arb. unit]
O
Fig. 2 shows typical results of the MDC–SHG measurement for R-CCB monolayer during barrier compression, where p-polarized fundamental light was used. From bottom to top, molecular area dependence of the surface-pressure, displacement current, dipole moment calculated from the MDC, transmitted SH intensity for p–p and p–s polarization are plotted. The isotherm is divided into four regions based on the results of molecular area and the MDC measurement as shown in Fig. 2. In Region 1, surface pressure is negligibly small and the MDC signal cannot be detected. It is reasonable to consider that the R-CCB molecules lie on a water surface in this region. Any SHG signals cannot be also detected in Region 1, reflecting the planer molecular orientation. Decreasing a molecular area, the MDC signal begins to generate and gradually increase and p–p SHG signal begins to generate in Region 2. These results indicate that the R-CCB molecules gradually arise from a water surface during monolayer compression. Since molecules can change its orientation freely at this region, intermolecular interaction which contributes to the surface pressure is expected to be small. Thus, the surface pressure is still negligible small in Region 2. The MDC signal suddenly decreases and the dipole moment almost 3
Region 2
Region 1
4
p-p
3
p-s
2 1 0
Dipole [mD]
*
339
600 400 200 0
ð1Þ
where / represents the incident angle. Contracted NLO susceptibilities sij are the function of the single molecular hyperpolarizabilities and the orientation of the molecules. Among them, s14 is expressed as s14 = vxyz + vxzy and represents the effect of chirality on the SHG. Thus, s14 = 0 corresponds to the achiral system. If the symmetry of the monolayer belong to C2v, it is possible to detect the s-output signal such as p–s and s–s signal. Therefore, it is not sufficient to assign the origin of the SHG to the chirality only from the p–s signal. Since s–s SHG signal cannot be detected in chiral system with in-plane symmetry (see above equations), we can distinguish the chiral originated SHG from the SHG due to the in-plane anisotropy.
100 50 0
Surface pressure [mN/m]
Ið2xÞTs–s ¼ 0;
MDC [fA]
Ið2xÞTp–s / j s14 sin / cos /j2 ;
5 4 3 2 1 0
0.4
0.6
0.8
1.0
1.2
Molecular area [nm2] Fig. 2. Experimental results for the SHG and MDC measurement of R-CCB monolayers during the course of monolayer compression. Transmitted SH lights were detected for p–p and p–s polarized conditions.
R. Tamura et al. / Chemical Physics Letters 407 (2005) 337–341
2
Ið2xÞs / js15 sin ci cos ci s14 cos2 ci cos /j
ð2Þ
for the monolayer with C1 symmetry. Here, ci and / represent the input polarized angle and the incident angle, respectively. It is clear from above equation that the polarized angle dependence of the SH intensity becomes symmetrically around ci = p/2 for the monolayer with C1v symmetry (s14 = 0). However, the polarized angle dependence of the SH intensity does not show a symmetrical profile for the monolayer with C1 symmetry (s14 6¼ 0). Thus, we can determine the chirality of the monolayer by measuring the polarized angle dependence of the SH intensity. Moreover, we can distinguish two enantiomers through this measurement. Because s14 of R- and Senantiomer have opposite sign, dependency on the SH intensity of R-enantiomer should be different with those of S-enantiomer. We could only show the polarized angle dependence of S-enantiomer in our previous paper. Thus, observation of the different polarized angle dependence of opposite enantiomer, R-enantiomer, will support strongly our previous interpretation and will show the potentiality to detect the chirality with our system. Fig. 3 shows the result for the polarized angle dependence measurement for: (a) R-CCB and (b) S-CCB monolayer. As shown in Fig. 3, asymmetrical profiles around ci = p/2 was actually observed for both enantiomers, indicating the presence of the molecular chirality. We have confirmed that the polarized angle
1 SH intensity [arb.units.]
reaches the maximum at the end of Region 2. At this point, average tilt angle of the molecule does not change any longer with additional compression and molecules form nearly closed packing state. And the surface pressure begins to increase with the completion of the molecular packing. As shown in Fig. 2, we cannot observe the significant SH signals under the p–s polarization condition, which indicate the presence of the chirality of monolayer, over the entire range of molecular area. These results indicate that the monolayer chirality could not be detected through p–s SHG observation within the constraint of our experiment during monolayer compression. In our previous paper, we could not detect p–s SHG signal from the S-CCB monolayer on a water surface during compression [15]. To get the p–s SHG signals originating from the chirality, relationship between two tensor components, s14 and s15 is effective [15]. According to our estimations, if s14/s15 is smaller than 1/4, it is difficult to detect the p–s SHG signal originated from the molecular chirality during monolayer compression. As discussed later, ratio between two tensor components, s14/s15 was less than 1/20 for these molecules. To detect the chirality of the monolayer on a water surface, we applied the polarized angle dependence measurement. We can obtain the polarized angle dependence of the SH intensity as,
0.5
0 0 (a)
50 100 Polarized angle [deg]
150
50
150
1 SH intensity [arb.units.]
340
0.5
0 0 (b)
100
Polarized angle [deg]
Fig. 3. Experimental results for the polarized angle dependence measurement for the (a) R- and (b) S-CCB monolayer. SH light was detected for transmitted direction. In this method, analyzer angle was fixed and the SH signals were measured as changing the input polarized angle. Filled and open squares represent the experimental results, and solid line shows the fitting curves using appropriate formula, Ið2xÞs / js15 sin 2ci s14 cos2 ci j2 .
dependence of the SH intensity was almost symmetry around ci = p/2 for achiral 8CB monolayer [15]. Moreover, the opposite enantiomer actually reversed the profile of the polarized angel dependence, as shown in figure. It implies that s14 of R and S-CCB have opposite sign. These results indicate that we can confirm the chirality of CCB molecules and distinguish between R- and S-enantiomer through polarized angle dependence of the SH intensity. According to the curves fitting, we evaluated the ratio between two tensor components, s14/s15. We obtained |s14/s15| 1/40 for the S-CCB monolayer and |s14/s15| 1/23 for the R-CCB monolayer. Therefore, it was reasonable that we could not detect the p–s SHG signal originating from the molecular chirality of CCB molecules during monolayer compression (as shown in Fig. 2). Finally, we would like to emphasize the effectiveness of the MDC–SHG system. It is well known that ultra thin layer at the interface between two different
R. Tamura et al. / Chemical Physics Letters 407 (2005) 337–341
materials are much important in the area of cytology and biochemistry, such as phospholipid bilayer of cell membranes. Thus, many artificial membranes were investigated and there are many ways to fabricate such artificial membrane. Among them, the LB technique is suitable for the investigation of the interfacial film between liquid and gas phases. As mentioned above, MDC has a great advantage in the detection of the orientational change of the molecules. Biochemical reactions in the membrane such as a channel-forming will possibly be monitored by the MDC. On the other hand, the SHG-CD can distinguish the presence of chirality in monolayer level. Chirality plays a particularly important role in a living body, because in many biochemical reactions only one enantiomer of the molecules can participate. Thus, the MDC–SHG technique will be a powerful tool for studying the orientation and the chirality of monolayer, and it can be used to investigate the biochemical reaction at the interface.
4. Conclusion Chiral properties of the S- and R-CCB monolayer on a water surface were investigated by the SHG and MDC measurement. With monolayer compression, p–p SHG and MDC signals gradually increased indicating that these molecules stand up from a water surface by a compression. On the other hand, the p–s SHG signal which indicates the chirality of the monolayer was not observed during a compression. However, the polarized angle dependence measurement showed a chiral behavior of these molecules. The ratio between chiral and achiral component of the NLO susceptibility was obtained as |s14/s15| 1/40 for the S-CCB monolayer and |s14/s15| 1/23 for the R-CCB monolayer by theoretical fittings. These small contributions of the chiral component of the NLO susceptibility led to the inactive-
341
ness of the p–s SHG signal during monolayer compression. It was also confirmed the ability to detect the monolayer chirality using this MDC–SHG measurement system.
Acknowledgment This work was partly supported by ÔGrant-in-Aid for Scientific ResearchÕ from the Science and Technology Agency of Japan.
References [1] G.L. Gaines, Insoluble Monolayers at the Liquid–gas Interfaces, Interscience, New York, 1966. [2] A. Ulman, An Introduction to Ultrathin Organic Films, Academic Press, San Diego, 1991. [3] V.M. Kaganer, H. Mo¨hwald, P. Dutta, Rev. Mod. Phys. 71 (1999) 779. [4] Y.R. Shen, Liq. Cryst. 5 (1989) 635. [5] K.B. Eisenthal, Chem. Rev. 96 (1996) 1343. [6] M. Iwamoto, Y. Majima, H. Naruse, T. Noguchi, H. Fuwa, Nature 353 (1991) 645. [7] M. Iwamoto, Y. Majima, J. Chem. Phys. 94 (1991) 5135. [8] A. Tojima, T. Manaka, M. Iwamoto, Rev. Sci. Instrum. 74 (2003) 2828. [9] A. Tojima, T. Manaka, M. Iwamoto, J. Chem. Phys. 115 (2001) 9010. [10] A. Tojima, T. Manaka, M. Iwamoto, Z.C. Ou-Yang, J. Chem. Phys. 118 (2003) 5640. [11] M. Iwamoto, A. Tojima, T. Manaka, Z.C. Ou-Yang, Phys. Rev. E 67 (2003) 041711. [12] T. Patralli-Mallow, T.M. Wong, J.D. Byers, H.I. Yee, J.M. Hicks, J. Phys. Chem. 97 (1993) 1383. [13] J.M. Hicks, T. Patralli-Mallow, Appl. Phys. B 68 (1999) 589. [14] R. Stolle, M. Loddoch, G. Marowsky, Non-linear Opt. 8 (1994) 79. [15] H. Fujimaki, T. Manaka, A. Tojima, M. Iwamoto, J. Chem. Phys. 119 (2003) 7427.
Optical chirality of citronelloxy-cyanobiphenyl monolayer at an air–water interface studied by the MDC and SHG measurement Ryousuke Tamura a, Takaaki Manaka a, Mitsumasa Iwamoto Tomomichi Itoh b, Junji Watanabe b b
a,*
,
a Department of Physical Electronics, Tokyo Institute of Technology, O-okayama 2-12-1-S3-33, Meguro-ku, Tokyo 152-8552, Japan Department of Organic and Polymeric Materials, Tokyo Institute of Technology, O-okayama 2-12-1, Meguro-ku, Tokyo 152-8552, Japan
Received 25 February 2005; in final form 18 March 2005 Available online 14 April 2005
Abstract Chirality of S- and R-enantiomer of citronelloxy-cyanobiphenyl (CCB) monolayer on a water surface were investigated by Maxwell displacement current (MDC) and optical second harmonic generation (SHG) measurements. While p–s SHG signal could not reveal the presence of chirality, the polarized angle dependence measurement indicated the ratio between chiral and achiral component of the non-linear optical susceptibility as (vxyz + vxzy)/(vxxz + vxzx) 1/40 and 1/23 for S- and R-CCB, respectively. It was confirmed that our MDC–SHG system has the ability to detect the chirality in monolayer level. 2005 Elsevier B.V. All rights reserved.
1. Introduction Since the discovery of the technique for the formation of floating monolayer at a liquid–air interface [1], interesting behaviors of the monolayers have attracted many scientists to intense research for these materials [2]. Monolayers on a water surface are classified as a twodimensional (2D) materials. Since the nature of the 2D materials is different from that of 3D bulk ones, monolayer at the interface between two different media are much important in the area of chemistry, surface physics and biology. Classically, surface pressure–area (p–A) isotherm measurement has been widely employed for the study of monolayers. For the past decades, novel experimental techniques, which are highly surfacespecific, have been developed for the study of monolayers and they have been used in combination with the p–A isotherm measurement. In particular, microscopic
*
Corresponding author. Fax: +81 3 5734 2191. E-mail address: [email protected] (M. Iwamoto).
0009-2614/$ - see front matter 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.cplett.2005.03.083
structures of monolayer have been revealed by the grazing incidence X-ray scattering [3] and non-linear optical (NLO) methods such as optical second harmonic generation (SHG) [4,5]. Microscopic flow behavior and domain formation in monolayers have been visualized by Brewster angle microscope (BAM) and fluorescence microscopy. We have been also developing a novel method named Maxwell displacement current (MDC) technique that allows the molecular motion in monolayers to be probed through the displacement current flowing across monolayers [6,7]. With this MDC measurement, change of spontaneous polarization in monolayers, accompanied by the orientational change of polar molecules has been examined. Recently, we have extended this MDC measurement system by coupling with SHG measurement [8–11]. MDC and SHG processes are related to spontaneous polarization and the second order induced polarization formed in monolayer, respectively. From a dielectric physics point of view, dielectric polarization phenomena truly reflect the specific properties of materials. Thus, these techniques are useful to specify the characterization of monolayers.
338
R. Tamura et al. / Chemical Physics Letters 407 (2005) 337–341
At the air–water interface, amphiphilic molecules tend to align so as that hydrophobic parts direct toward air. Immediately after spreading solution, molecules lie on a water surface due to the electrostatic coulomb attractive force working between molecule and the water surface, and it is reasonable to consider that there is no in-plane anisotropy, i.e., monolayer possesses the D1h symmetry. Since all NLO susceptibilities vanish at this state, SHG signal should not be observed from these molecules. Decreasing a molecular area by a monolayer compression gradually rises up molecules from a water surface. The growth of asymmetrical structure of the monolayer on the water surface reduces its symmetry from D1h to C1v. Therefore, in this region SHG signals should begin to generate and gradually increase, indicating the lack of the inversion symmetry. In our previous paper describing MDC–SHG measurement of 8CB monolayers [9], p–p SHG signal was observed and it gradually increased during monolayer compression. On the other hand, interesting behavior of the structural change was observed for the 7OCB monolayer [11]. In 7OCB monolayer, constituent molecules tilt in one direction with continuous compression after standing up the molecules, due to the shear stress working among molecules. Thus, we could find the phase transition from polar orientational (untilt) phase to planer alignment (tilting) phase. The formation of the planer alignment phase was confirmed by observation of p–s signal in the SHG measurement, indicating that the resultant symmetry changed from C1v to C2v. From a view point of symmetry of a monolayer, introducing chirality into molecules extinguishes the mirror plane of the system. Thus, monolayer symmetry is reduced from C2v to C2 for the tilting phase, and is reduced from C1v to C1 for the untilt phase. In these cases, the number of non-zero component of the NLO susceptibility increases and equations expressing the SHG process are more generalized. Furthermore, since chiral properties play an important role in biology and chemistry, to confirm the ability to detect chiral property of the surface monolayer is of great importance for extending the potentiality of our MDC–SHG systems. The chirality of the molecule can be discriminated using the optical spectroscopic techniques, such as the optical rotary dispersion (ORD) and the circular dichroism (CD). Usually, since the interaction between chiral materials and circularly polarized light are quite small, observation of the optical chirality in a monolayer using typical ORD and CD technique is a quite task. Even in such case, the surface specific nonlinear optical technique, such as the SHG circular dichroism (SHG-CD), also can be a powerful tool for the detection of monolayer chirality, where differential SHG signals from chiral surface are recorded with right and left circularly polarized fundamental incidence. The SHG-CD tech-
nique was first applied for 2,2 0 -dihydroxy-1,1 0 -binaththyl (BN) solid/solution interface by HicksÕ group [12] and by other researchers [13,14]. In our previous paper, we also examined the chiral properties and the molecular orientation of S-citronelloxy-cyanobiphenyl (S-CCB) monolayer on a water surface by MDC and SHG measurement [15], where MDC and SHG were mainly used to detect the phases and the chirality of monolayers, respectively. At that time, the measurement was limited to S-CCB monolayer and the SHG signal which indicates the chirality was rather weak. Thus, there was a possibility remained that the difference in the SHG signal was due to experimental errors. In this Letter, we measured the chiral properties of the both S- and R-enantiomers of CCB, on the basis of the polarized angle dependence measurement and the p–s SHG measurement. As will be discussed later, both measurement can distinguish the optical chirality. Measurement using an opposite enantiomer of CCB strongly supported our previous results [15].
2. Experiment The MDC and SHG experiments were performed using the experimental setup described in our previous paper [8]. Since detailed information of the measurement setup was described in that paper, we briefly introduce here our experimental system. The experimental setup used here consists of Langmuir-trough equipped with a two electrodes arrangement for the MDC measurement and optical arrangement with an optical parametric oscillator (OPO) for the SHG measurement. The rectangular-shape Langmuir-trough (600 · 150 mm in length and width, 10 mm in depth) was made from poly-tetrafluoroethylene (PTFE) and it was filled with pure water (electrical resistivity > 17 MX cm1). One transparent silica glass plate was attached to the bottom of the LB trough for the SHG measurement. For the MDC measurement, transparent glass slide coated with Indium Tin Oxide (ITO) was used as the top electrode (45 cm2). It was placed in air parallel to the water surface at a distance of 1 mm. The bottom electrode was a spiral shape gold wire (1-mm diameter and 500 mm in length) and it was immersed in the water. These two electrodes were connected to each other through an electrometer (Keithley 617) with negligible electrical resistance. The surface pressure of the monolayer was measured by a Wilhelmy plate. For SHG measurement, fundamental light with a wavelength of 560 nm was irradiated onto the monolayer at an energy of 6 mJ with a pulse rate of 10 Hz, and the spot size of the laser was about 56 mm2. In this measurement, fundamental light was not focused and was impinged on a water surface with an incident angle of 60 after passing through an SH-cut filter to
R. Tamura et al. / Chemical Physics Letters 407 (2005) 337–341
CN
Fig. 1. Structure of the molecules used in this experiment, R-CCB. The chiral center for the R-CCB molecule is marked by asterisk.
eliminate unexpected SH light from various optical components. Both transmitted and reflected SH light from the monolayer were detected by photomultiplier tubes after passing through fundamental cut and the interference filters. The molecules used in this experiment were chiral S- and R-CCB. The molecular structure of R-CCB is schematically described in Fig. 1. As figure shows, since the four different groups attached to the carbon atom labeled by asterisk in the R-CCB molecule, this carbon atom becomes a chiral center and hence this molecule has a chirality. Since the configuration of this CCB molecule is R-enantiomer, this molecule is referred as R-CCB in this Letter. The chloroform solution with a molar density of 1 mmol/l was spread on a water surface to form a Langmuir monolayer. The monolayer was compressed with aid of two barriers moving at the same speed in opposite direction of 10 mm min1 (0.056 nm2 min1 molecule1).
3. Results and discussion Hereinafter, the SH intensities for the transmitted geometry are listed for four kinds of input and output polarization combinations, p–p, s–p, p–s and s–s. We can obtain the SH intensity for C1 symmetry as, Ið2xÞTp–p / j s15 sin / cos2 / þ s31 cos2 / sin / þ s33 sin3 /j2 ; Ið2xÞTs–p / js31 cos /j2 ;
Region 4 SH intensity [arb. unit]
O
Fig. 2 shows typical results of the MDC–SHG measurement for R-CCB monolayer during barrier compression, where p-polarized fundamental light was used. From bottom to top, molecular area dependence of the surface-pressure, displacement current, dipole moment calculated from the MDC, transmitted SH intensity for p–p and p–s polarization are plotted. The isotherm is divided into four regions based on the results of molecular area and the MDC measurement as shown in Fig. 2. In Region 1, surface pressure is negligibly small and the MDC signal cannot be detected. It is reasonable to consider that the R-CCB molecules lie on a water surface in this region. Any SHG signals cannot be also detected in Region 1, reflecting the planer molecular orientation. Decreasing a molecular area, the MDC signal begins to generate and gradually increase and p–p SHG signal begins to generate in Region 2. These results indicate that the R-CCB molecules gradually arise from a water surface during monolayer compression. Since molecules can change its orientation freely at this region, intermolecular interaction which contributes to the surface pressure is expected to be small. Thus, the surface pressure is still negligible small in Region 2. The MDC signal suddenly decreases and the dipole moment almost 3
Region 2
Region 1
4
p-p
3
p-s
2 1 0
Dipole [mD]
*
339
600 400 200 0
ð1Þ
where / represents the incident angle. Contracted NLO susceptibilities sij are the function of the single molecular hyperpolarizabilities and the orientation of the molecules. Among them, s14 is expressed as s14 = vxyz + vxzy and represents the effect of chirality on the SHG. Thus, s14 = 0 corresponds to the achiral system. If the symmetry of the monolayer belong to C2v, it is possible to detect the s-output signal such as p–s and s–s signal. Therefore, it is not sufficient to assign the origin of the SHG to the chirality only from the p–s signal. Since s–s SHG signal cannot be detected in chiral system with in-plane symmetry (see above equations), we can distinguish the chiral originated SHG from the SHG due to the in-plane anisotropy.
100 50 0
Surface pressure [mN/m]
Ið2xÞTs–s ¼ 0;
MDC [fA]
Ið2xÞTp–s / j s14 sin / cos /j2 ;
5 4 3 2 1 0
0.4
0.6
0.8
1.0
1.2
Molecular area [nm2] Fig. 2. Experimental results for the SHG and MDC measurement of R-CCB monolayers during the course of monolayer compression. Transmitted SH lights were detected for p–p and p–s polarized conditions.
R. Tamura et al. / Chemical Physics Letters 407 (2005) 337–341
2
Ið2xÞs / js15 sin ci cos ci s14 cos2 ci cos /j
ð2Þ
for the monolayer with C1 symmetry. Here, ci and / represent the input polarized angle and the incident angle, respectively. It is clear from above equation that the polarized angle dependence of the SH intensity becomes symmetrically around ci = p/2 for the monolayer with C1v symmetry (s14 = 0). However, the polarized angle dependence of the SH intensity does not show a symmetrical profile for the monolayer with C1 symmetry (s14 6¼ 0). Thus, we can determine the chirality of the monolayer by measuring the polarized angle dependence of the SH intensity. Moreover, we can distinguish two enantiomers through this measurement. Because s14 of R- and Senantiomer have opposite sign, dependency on the SH intensity of R-enantiomer should be different with those of S-enantiomer. We could only show the polarized angle dependence of S-enantiomer in our previous paper. Thus, observation of the different polarized angle dependence of opposite enantiomer, R-enantiomer, will support strongly our previous interpretation and will show the potentiality to detect the chirality with our system. Fig. 3 shows the result for the polarized angle dependence measurement for: (a) R-CCB and (b) S-CCB monolayer. As shown in Fig. 3, asymmetrical profiles around ci = p/2 was actually observed for both enantiomers, indicating the presence of the molecular chirality. We have confirmed that the polarized angle
1 SH intensity [arb.units.]
reaches the maximum at the end of Region 2. At this point, average tilt angle of the molecule does not change any longer with additional compression and molecules form nearly closed packing state. And the surface pressure begins to increase with the completion of the molecular packing. As shown in Fig. 2, we cannot observe the significant SH signals under the p–s polarization condition, which indicate the presence of the chirality of monolayer, over the entire range of molecular area. These results indicate that the monolayer chirality could not be detected through p–s SHG observation within the constraint of our experiment during monolayer compression. In our previous paper, we could not detect p–s SHG signal from the S-CCB monolayer on a water surface during compression [15]. To get the p–s SHG signals originating from the chirality, relationship between two tensor components, s14 and s15 is effective [15]. According to our estimations, if s14/s15 is smaller than 1/4, it is difficult to detect the p–s SHG signal originated from the molecular chirality during monolayer compression. As discussed later, ratio between two tensor components, s14/s15 was less than 1/20 for these molecules. To detect the chirality of the monolayer on a water surface, we applied the polarized angle dependence measurement. We can obtain the polarized angle dependence of the SH intensity as,
0.5
0 0 (a)
50 100 Polarized angle [deg]
150
50
150
1 SH intensity [arb.units.]
340
0.5
0 0 (b)
100
Polarized angle [deg]
Fig. 3. Experimental results for the polarized angle dependence measurement for the (a) R- and (b) S-CCB monolayer. SH light was detected for transmitted direction. In this method, analyzer angle was fixed and the SH signals were measured as changing the input polarized angle. Filled and open squares represent the experimental results, and solid line shows the fitting curves using appropriate formula, Ið2xÞs / js15 sin 2ci s14 cos2 ci j2 .
dependence of the SH intensity was almost symmetry around ci = p/2 for achiral 8CB monolayer [15]. Moreover, the opposite enantiomer actually reversed the profile of the polarized angel dependence, as shown in figure. It implies that s14 of R and S-CCB have opposite sign. These results indicate that we can confirm the chirality of CCB molecules and distinguish between R- and S-enantiomer through polarized angle dependence of the SH intensity. According to the curves fitting, we evaluated the ratio between two tensor components, s14/s15. We obtained |s14/s15| 1/40 for the S-CCB monolayer and |s14/s15| 1/23 for the R-CCB monolayer. Therefore, it was reasonable that we could not detect the p–s SHG signal originating from the molecular chirality of CCB molecules during monolayer compression (as shown in Fig. 2). Finally, we would like to emphasize the effectiveness of the MDC–SHG system. It is well known that ultra thin layer at the interface between two different
R. Tamura et al. / Chemical Physics Letters 407 (2005) 337–341
materials are much important in the area of cytology and biochemistry, such as phospholipid bilayer of cell membranes. Thus, many artificial membranes were investigated and there are many ways to fabricate such artificial membrane. Among them, the LB technique is suitable for the investigation of the interfacial film between liquid and gas phases. As mentioned above, MDC has a great advantage in the detection of the orientational change of the molecules. Biochemical reactions in the membrane such as a channel-forming will possibly be monitored by the MDC. On the other hand, the SHG-CD can distinguish the presence of chirality in monolayer level. Chirality plays a particularly important role in a living body, because in many biochemical reactions only one enantiomer of the molecules can participate. Thus, the MDC–SHG technique will be a powerful tool for studying the orientation and the chirality of monolayer, and it can be used to investigate the biochemical reaction at the interface.
4. Conclusion Chiral properties of the S- and R-CCB monolayer on a water surface were investigated by the SHG and MDC measurement. With monolayer compression, p–p SHG and MDC signals gradually increased indicating that these molecules stand up from a water surface by a compression. On the other hand, the p–s SHG signal which indicates the chirality of the monolayer was not observed during a compression. However, the polarized angle dependence measurement showed a chiral behavior of these molecules. The ratio between chiral and achiral component of the NLO susceptibility was obtained as |s14/s15| 1/40 for the S-CCB monolayer and |s14/s15| 1/23 for the R-CCB monolayer by theoretical fittings. These small contributions of the chiral component of the NLO susceptibility led to the inactive-
341
ness of the p–s SHG signal during monolayer compression. It was also confirmed the ability to detect the monolayer chirality using this MDC–SHG measurement system.
Acknowledgment This work was partly supported by ÔGrant-in-Aid for Scientific ResearchÕ from the Science and Technology Agency of Japan.
References [1] G.L. Gaines, Insoluble Monolayers at the Liquid–gas Interfaces, Interscience, New York, 1966. [2] A. Ulman, An Introduction to Ultrathin Organic Films, Academic Press, San Diego, 1991. [3] V.M. Kaganer, H. Mo¨hwald, P. Dutta, Rev. Mod. Phys. 71 (1999) 779. [4] Y.R. Shen, Liq. Cryst. 5 (1989) 635. [5] K.B. Eisenthal, Chem. Rev. 96 (1996) 1343. [6] M. Iwamoto, Y. Majima, H. Naruse, T. Noguchi, H. Fuwa, Nature 353 (1991) 645. [7] M. Iwamoto, Y. Majima, J. Chem. Phys. 94 (1991) 5135. [8] A. Tojima, T. Manaka, M. Iwamoto, Rev. Sci. Instrum. 74 (2003) 2828. [9] A. Tojima, T. Manaka, M. Iwamoto, J. Chem. Phys. 115 (2001) 9010. [10] A. Tojima, T. Manaka, M. Iwamoto, Z.C. Ou-Yang, J. Chem. Phys. 118 (2003) 5640. [11] M. Iwamoto, A. Tojima, T. Manaka, Z.C. Ou-Yang, Phys. Rev. E 67 (2003) 041711. [12] T. Patralli-Mallow, T.M. Wong, J.D. Byers, H.I. Yee, J.M. Hicks, J. Phys. Chem. 97 (1993) 1383. [13] J.M. Hicks, T. Patralli-Mallow, Appl. Phys. B 68 (1999) 589. [14] R. Stolle, M. Loddoch, G. Marowsky, Non-linear Opt. 8 (1994) 79. [15] H. Fujimaki, T. Manaka, A. Tojima, M. Iwamoto, J. Chem. Phys. 119 (2003) 7427.