- Email: [email protected]
Determination of dipole moment of azobenzene dendrimer by Maxwell-displacement-current measurement for Langmuir monolayer
25 March 2002
Chemical Physics Letters 355 (2002) 164–168 www.elsevier.com/locate/cplett
Determination of dipole moment of azobenzene dendrimer by Maxwell-displacement-current measurement for Langmuir monolayer Takaaki Manaka, Daisuke Shimura, Mitsumasa Iwamoto
*
Department of Physical Electronics, Graduate School of Science and Engineering, Tokyo Institute of Technology, 2-12-1 O-okayama, Meguro-ku, Tokyo 152-8552, Japan Received 28 August 2001; in final form 25 January 2002
Abstract Using Maxwell-displacement-current (MDC) measurement for Langmuir monolayer, dipole moment of azobenzene dendrimer on the water surface was examined, and analyzed using a simple two-layer model. The dipole moment along molecular long axis of the azobenzene dendrimer molecule increases as the number of constituent azobenzene groups increases, and it also increases with the number of alkyl groups. The dipole moment obtained in the MDC experiments had a similar tendency to the results estimated using molecular orbital (MO) calculation. Ó 2002 Elsevier Science B.V. All rights reserved.
1. Introduction Monolayers on the water surface exhibit very interesting behaviors as two-dimensional systems, and the study of monolayers has been a continuous research subject since the discovery of the preparation technique by Langmuir [1]. A variety of experimental techniques such as X-ray diffraction methods. Brewster Angle Microscopy, electrical techniques and others have been employed to get information on the physico-chemical properties of monolayers [2]. Until now, membranes, polymeric materials, fatty acids and others have
*
Corresponding author. Fax: +81-3-5734-2191. E-mail address: [email protected] (M. Iwamoto).
been examined from the viewpoint of physics, chemistry and biology. On the one hand, organic monolayers on water surface have been paid much attention in electronics, because they can be used to fabricate electrical and optical elements with artificially layered structures. Thus it is important to study the electrical and optical properties of monolayers for practical applications. Among these properties, the dielectric property is one of the most fundamental ones. Using surface potential and Maxwell-displacement-current (MDC) measurements, the dielectric property of various kinds of organic monolayers has been examined [3]. Recently dendritic macromolecules, a novel category of hyper-branched materials, have attracted great interest [4–6], since they have unique
0009-2614/02/$ - see front matter Ó 2002 Elsevier Science B.V. All rights reserved. PII: S 0 0 0 9 - 2 6 1 4 ( 0 2 ) 0 0 2 0 4 - X
T. Manaka et al. / Chemical Physics Letters 355 (2002) 164–168
physical and chemical properties that are different from those of traditional macro-molecules. Azobenzene dendrimer contains several azo-groups along with branched long chains, and they are expected to have large hyperpolarizabilities and can be used in electronics and optoelectronics, especially in nonlinear optics [7,8]. Furthermore, these azobenzene groups undergo reversible trans– cis isomerization by alternating irradiation with ultraviolet (UV) and visible light. Thus, this dendrimer is also expected to be used for the optical data storage media as well as monomolecular memory application. As such, it is important to get information on the electrical property of azobenzene dendrimer monolayers in association with the molecular motion. For a decade, we have been developing a novel electrical method named MDC technique that allows the molecular motion in monolayers to be probed through the displacement current flowing across monolayers [9,10,12]. In this paper, we examine azobenzene dendrimer monolayers on a water surface using the MDC measurement, and determine the dipole moment in association with the molecular motion of the constituent molecules, assuming a two-layer model. We also calculate the dipole moment of azobenzene molecules using the semiempirical molecular orbital (MO) calculation, and compare the results with the experiments.
2. Experimental 2.1. Materials and experimental set-up The materials used here were the first, second and third generations of the azobenzene dendrimer abbreviated as AZ-G1, AZ-G2 and AZ-G3, respectively. As a representative of azobenzene dendrimer, the chemical structure for AZ-G2 is shown in Fig. 1a. The azobenzene dendrimer was provided from elsewhere [11], and it was used in the experiment without further purification. For Langmuir monolayer fabrication, the spreading solution of azobenzene dendrimer was a 1, 0.5 and 0.1 mmol/1 chloroform solution for AZ-G1, AZ-G2 and AZG3, respectively. Using an experimental system schematically shown in Fig. 2, we examined the
165
Fig. 1. (a) Chemical structures of the azobenzene dendrimer, AZ-G2 and (b) optimized structure for AZ-G2 molecules using for the calculation of dipole moment. Molecule consists of three components: one hydrophilic head group (–COOH), the number of 2n 1 azobenzene groups, and the number of 2n alkyl groups.
vertical component of dipole moment of these azobenzene monolayers. Electrode 1 is a roundshape ITO-coated electrode with a working area of 45:8 cm2 and electrode 2 is spiral shaped gold wire. It is placed parallel to the water surface with a spacing of about 1 mm from the water surface. After spreading the solution of AZ-Gn onto the
Fig. 2. Schematic diagram of experimental set-up for the MDC measurement.
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T. Manaka et al. / Chemical Physics Letters 355 (2002) 164–168
water surface of the Langmuir-trough made of polytetrafluoroethylene (150 mm 730 mm in width length, 10 mm in depth), MDC was measured during the course of monolayer compression. Monolayers on the water surface are compressed with aid of two barriers moving at the same speed in opposite direction of 40 mm/min. 2.2. MDC measurement Since detailed theoretical considerations of MDC have been described in our previous papers [10,12], we here treat the fundamentals in brief. The material used here has a dendritic hyperbranched molecular structure, and the molecular structure is very complex. Thus it is helpful to analyze the generation of MDC in association with the contribution of each component of molecules, azo-groups and others. For simplicity, we assume here that the molecule consists of three components, one hydrophilic head group (–COOH), the number of 2n 1 azobenzene groups, and the number of 2n alkyl groups located at the end of long chains of the dendritic molecules shown in Fig 1a. Thus the dipole moment ðlÞ along molecular long-axis (see Fig. 1b) is expressed as l¼
n 1 2X
cos hNN lNN þ i
cos hCH lCH i
i¼1
i¼1
þ cos h
2n X
CO CO
l
¼ ð2n 1Þ cos hNN lNN þ 2n cos hCH lCH þ cos hCO lCO lt þ lw ;
ð1Þ
where lNN , lCH and lCO represent the dipole moments of azo-group, alkyl-group and COOH group, respectively. Here, the abbreviations, lt ¼ ð2n 1Þ cos hNN lNN þ 2n cos hCH lCH ; lw ¼ cos hCO lCO ; are used to avoid the complexity of the equation. hNN , hCH and hCO are angles between each dipole and molecular long-axis. Of course, each dipole orients in different directions due to the spatial restrictions. cos h denotes an orientational average
of each component and n represents the generation orders. As shown in Fig. 2, when a single monolayer is prepared on the water surface, it is postulated that a two-layer film is formed and it works like a capacitor; that is, the hydrophilic head group is immersed in water with a dielectric constant w of 78, and the azobenzene groups and the alkyl groups are located above the water surface in a medium with a dielectric constant of s 1. The induced charge Q on electrode 1 is expressed using lt and lw defined in Eq. (1) as ns B hltz i hlwz i ð2Þ Q¼ þ C/s ; d s w where B is the working area of electrode 1, ns is the surface density of molecules given by 1=A (A: molecular area), d is the distance between electrode 1 and the water surface, C is the capaciatance between electrode 1 and the water surface, and /s is the surface potential of water of the Langmuirtrough and the suffix z represents the vertical component of the dipole moment. In Eq. (2), h i denotes a thermodynamic average of the distributed components of the molecules. The MDC is generated across monolayers due to the change of induced charge Q with respect to time. By monolayer compression, MDC is generated and expressed as t B hlz i hlwz i d I¼ ns þ d s w dt t Bns d hlz i hlwz i d þ ð3Þ þ þ C /s : s w dt d dt In our experimental system, an air-gap between the water surface and electrode 1 works as a good electrical insulator and it eliminates the flow of leakage current. Thus only the MDC current given by Eq. (3) is allowed to flow. The change of induced charge on electrode 1 can be estimated by integrating the MDC current with respect to time. As the dielectric constant of water w ( ¼ 78) is so high, the contribution of hlw i to the generation of MDC is electrically screened off. Furthermore, the change of water surface potential is small in comparison with the contribution of the first and second terms of Eq. (3) [3]. We assume that molecular structure is fixed and tilt angle of molecules
T. Manaka et al. / Chemical Physics Letters 355 (2002) 164–168
167
only gradually increases during monolayer compression. Using the optimized geometry for AZGn molecule, we can estimate the orientational average, cos h. As a result, the MDC current is expressed using orientational order parameter, S NN ¼ hcos hNN i and S CH ¼ hcos hCH i, as d B n ns ð2 1ÞS NN lNN I¼ þ 2n S CH lCH z z s d dt Bns d n ð2 1ÞS NN lNN þ þ 2n S CH lCH : ð4Þ z z s d dt Since we can experimentally obtain the displacement current I, induced charge Q is calculated by Rt integration with the current, Q ¼ 0 I dt. The charge Q is directly related to the dipole moment as Eq. (2). In this experiment, we can assume that the initial value of Q is zero because the displacement current was not observed in the range of 2 ). large molecular area ( 300 A
3. Results and discussion Fig. 3 shows a typical example of the MDC currents generated across monolayer of AZ-G2 molecules together with the dipole moment and surface pressure. Looking at these figures, it is found that two peaks appear during monolayer compression for the MDC current. The first and second peaks appear in the range before and after the initial rise of surface pressure, respectively. Using Eq. (4) we calculated the charge flowing through the circuit and could calculate the vertical component of dipole moment of these molecules, hlz i and then plotted the results in the figure. Of interest is that the hlz i increases gradually by monolayer compression and finally saturates at a 2 . The result indicates that surface area of 120 A these dendritic molecules spread onto the water surface gradually stand up, and they are then packed during monolayer compression. It is interesting to plot the hlz i as a function of the generation order n. Fig. 4 shows the relationship between the number of azo-groups 2n 1 and the obtained hlz i. In the figure, the values of limiting area of these molecules estimated from the surface pressure-area ðp AÞ isotherms and those of the end alkyl-group are plotted, the increase is
Fig. 3. Typical example of the MDC currents generated across AZ-G2. From bottom to top, the surface pressure-area. MDCarea, dipole moment-area are plotted.
Fig. 4. Relationship between the number of azo-groups and limited area (open triangle), dipole moment obtained from experimental (filled circle) and estimated dipole moment by MO calculation (open circle).
almost proportional to the number of azo-group 2n 1. This suggests that the AZ-Gn molecule has a uniaxially dipolar structure. In order to further
168
T. Manaka et al. / Chemical Physics Letters 355 (2002) 164–168
clarify the details, we calculated the dipole moment of azobenzene group and the end group by means of MO calculation. All the MO calculations were done using semiempirical calculation package, MOPAC 2000 (Fujitsu). Molecular properties such as orbital energy and dipole moment strongly depend on the molecular structure. Before the calculation of dipole moment, hence we have optimized the molecular structure with AM1 Hamiltonian. Generally, MO calculation is used for the isolated systems. Although intermolecular interaction affects the molecular structure for the monolayer on the water surface, averaged conformation of molecules on the water surface was assumed to be similar to the optimized conformation obtained by MOPAC calculation. Optimized molecular structure of AZ-G2 is shown in Fig. 1b. This seems fairly elongated and the result is consistent with the nonlinear optical measurements [13]. As a consequence of screening of the water, COOH-group cannot contribute to the dipole moment determined by MDC measurement on the water surface. As mentioned above, we divided AZ-G1 molecule into three parts, COOH-group, azo-group and alkyl-group (see Fig. 1a) and estimate the dipole moment of each group using Mulliken population. As a result, we have obtained dipole moment of 3:523D and 0:05D for azo-group and alkyl-group, respectively. For AZGn, the molecule consists of the number of 2n 1 azo-groups and 2n alkyl-groups. Fig. 4 shows dipole moment estimated by using Eq. (3) and MDC measurement. As shown in figure, dipole moment obtained using MOPAC was qualitatively coincident with the results of the MDC measurement.
4. Conclusion Dipole moment of azobenzene dendrimer on the water surface was examined by means of MDC
technique for Langmuir monolayer. The vertical component of dipole moment of the constituent azobenzene dendrimer molecule increases as the number of azobenzene groups and alkyl-groups in the molecule. The behavior could be also obtained by MO calculation using MOPAC. The increasing of the dipole moment suggests that the azobenzene groups arrange noncentrosymmetrically in the dendrimer.
Acknowledgements We acknowledge Dr. S. Yokoyama, Dr. T. Kubota and Dr. S. Mashiko in Communication Research Laboratory for kindly providing AZ-Gn molecules and useful discussion.
References [1] G.L. Gaines Jr., Insoluble Monolayers at Liquid–Gas interfaces, Interscience, New york, 1966. [2] A. Ulman, Characterization of Organic Thin Films, Butterworths/Heinemann, Boston, MA, 1995. [3] M. Iwamoto, C.X. Wu, The Physical Properties of Organic Monolayers, World Scientific, Singapore, 2001. [4] D.A. Tomalia, A.M. Naylor, W.A. Goddard III, Angew. Chem. Int. Ed. Engl. 29 (1990) 138. [5] M. Frechet, Science 263 (1994) 1710. [6] A. Tomalia, Adv. Mater. 6 (1994) 529. [7] Y. Shiyoshi, T. Nakahara, A. Otomo, S. Mashiko, J. Am. Chem. Soc. 122 (2000) 3174. [8] Y. Shiyoshi, T. Nakahara, A. Otomo, S. Mashiko, Thin Solid Films 331 (1998) 248. [9] M. Iwamoto, Y. Majima, H. Naruse, T. Noguchi, H. Fuwa, Nature 353 (1991) 645. [10] M. Iwamoto, Y. Majima, J. Chem. Phys. 94 (1991) 5135. [11] Y. Shiyoshi, T. Nakahara, A. Otomo, S. Mashiko, Chem. Lett. 137 (1997) 1. [12] M. Iwamoto, Thin Solid Films 244 (1994) 1031. [13] S. Yokoyama, S. Mashiko, Y. Yamaguchi, Y. Yokomichi, Polym. Adv. Technol. 11 (2000) 1.
Chemical Physics Letters 355 (2002) 164–168 www.elsevier.com/locate/cplett
Determination of dipole moment of azobenzene dendrimer by Maxwell-displacement-current measurement for Langmuir monolayer Takaaki Manaka, Daisuke Shimura, Mitsumasa Iwamoto
*
Department of Physical Electronics, Graduate School of Science and Engineering, Tokyo Institute of Technology, 2-12-1 O-okayama, Meguro-ku, Tokyo 152-8552, Japan Received 28 August 2001; in final form 25 January 2002
Abstract Using Maxwell-displacement-current (MDC) measurement for Langmuir monolayer, dipole moment of azobenzene dendrimer on the water surface was examined, and analyzed using a simple two-layer model. The dipole moment along molecular long axis of the azobenzene dendrimer molecule increases as the number of constituent azobenzene groups increases, and it also increases with the number of alkyl groups. The dipole moment obtained in the MDC experiments had a similar tendency to the results estimated using molecular orbital (MO) calculation. Ó 2002 Elsevier Science B.V. All rights reserved.
1. Introduction Monolayers on the water surface exhibit very interesting behaviors as two-dimensional systems, and the study of monolayers has been a continuous research subject since the discovery of the preparation technique by Langmuir [1]. A variety of experimental techniques such as X-ray diffraction methods. Brewster Angle Microscopy, electrical techniques and others have been employed to get information on the physico-chemical properties of monolayers [2]. Until now, membranes, polymeric materials, fatty acids and others have
*
Corresponding author. Fax: +81-3-5734-2191. E-mail address: [email protected] (M. Iwamoto).
been examined from the viewpoint of physics, chemistry and biology. On the one hand, organic monolayers on water surface have been paid much attention in electronics, because they can be used to fabricate electrical and optical elements with artificially layered structures. Thus it is important to study the electrical and optical properties of monolayers for practical applications. Among these properties, the dielectric property is one of the most fundamental ones. Using surface potential and Maxwell-displacement-current (MDC) measurements, the dielectric property of various kinds of organic monolayers has been examined [3]. Recently dendritic macromolecules, a novel category of hyper-branched materials, have attracted great interest [4–6], since they have unique
0009-2614/02/$ - see front matter Ó 2002 Elsevier Science B.V. All rights reserved. PII: S 0 0 0 9 - 2 6 1 4 ( 0 2 ) 0 0 2 0 4 - X
T. Manaka et al. / Chemical Physics Letters 355 (2002) 164–168
physical and chemical properties that are different from those of traditional macro-molecules. Azobenzene dendrimer contains several azo-groups along with branched long chains, and they are expected to have large hyperpolarizabilities and can be used in electronics and optoelectronics, especially in nonlinear optics [7,8]. Furthermore, these azobenzene groups undergo reversible trans– cis isomerization by alternating irradiation with ultraviolet (UV) and visible light. Thus, this dendrimer is also expected to be used for the optical data storage media as well as monomolecular memory application. As such, it is important to get information on the electrical property of azobenzene dendrimer monolayers in association with the molecular motion. For a decade, we have been developing a novel electrical method named MDC technique that allows the molecular motion in monolayers to be probed through the displacement current flowing across monolayers [9,10,12]. In this paper, we examine azobenzene dendrimer monolayers on a water surface using the MDC measurement, and determine the dipole moment in association with the molecular motion of the constituent molecules, assuming a two-layer model. We also calculate the dipole moment of azobenzene molecules using the semiempirical molecular orbital (MO) calculation, and compare the results with the experiments.
2. Experimental 2.1. Materials and experimental set-up The materials used here were the first, second and third generations of the azobenzene dendrimer abbreviated as AZ-G1, AZ-G2 and AZ-G3, respectively. As a representative of azobenzene dendrimer, the chemical structure for AZ-G2 is shown in Fig. 1a. The azobenzene dendrimer was provided from elsewhere [11], and it was used in the experiment without further purification. For Langmuir monolayer fabrication, the spreading solution of azobenzene dendrimer was a 1, 0.5 and 0.1 mmol/1 chloroform solution for AZ-G1, AZ-G2 and AZG3, respectively. Using an experimental system schematically shown in Fig. 2, we examined the
165
Fig. 1. (a) Chemical structures of the azobenzene dendrimer, AZ-G2 and (b) optimized structure for AZ-G2 molecules using for the calculation of dipole moment. Molecule consists of three components: one hydrophilic head group (–COOH), the number of 2n 1 azobenzene groups, and the number of 2n alkyl groups.
vertical component of dipole moment of these azobenzene monolayers. Electrode 1 is a roundshape ITO-coated electrode with a working area of 45:8 cm2 and electrode 2 is spiral shaped gold wire. It is placed parallel to the water surface with a spacing of about 1 mm from the water surface. After spreading the solution of AZ-Gn onto the
Fig. 2. Schematic diagram of experimental set-up for the MDC measurement.
166
T. Manaka et al. / Chemical Physics Letters 355 (2002) 164–168
water surface of the Langmuir-trough made of polytetrafluoroethylene (150 mm 730 mm in width length, 10 mm in depth), MDC was measured during the course of monolayer compression. Monolayers on the water surface are compressed with aid of two barriers moving at the same speed in opposite direction of 40 mm/min. 2.2. MDC measurement Since detailed theoretical considerations of MDC have been described in our previous papers [10,12], we here treat the fundamentals in brief. The material used here has a dendritic hyperbranched molecular structure, and the molecular structure is very complex. Thus it is helpful to analyze the generation of MDC in association with the contribution of each component of molecules, azo-groups and others. For simplicity, we assume here that the molecule consists of three components, one hydrophilic head group (–COOH), the number of 2n 1 azobenzene groups, and the number of 2n alkyl groups located at the end of long chains of the dendritic molecules shown in Fig 1a. Thus the dipole moment ðlÞ along molecular long-axis (see Fig. 1b) is expressed as l¼
n 1 2X
cos hNN lNN þ i
cos hCH lCH i
i¼1
i¼1
þ cos h
2n X
CO CO
l
¼ ð2n 1Þ cos hNN lNN þ 2n cos hCH lCH þ cos hCO lCO lt þ lw ;
ð1Þ
where lNN , lCH and lCO represent the dipole moments of azo-group, alkyl-group and COOH group, respectively. Here, the abbreviations, lt ¼ ð2n 1Þ cos hNN lNN þ 2n cos hCH lCH ; lw ¼ cos hCO lCO ; are used to avoid the complexity of the equation. hNN , hCH and hCO are angles between each dipole and molecular long-axis. Of course, each dipole orients in different directions due to the spatial restrictions. cos h denotes an orientational average
of each component and n represents the generation orders. As shown in Fig. 2, when a single monolayer is prepared on the water surface, it is postulated that a two-layer film is formed and it works like a capacitor; that is, the hydrophilic head group is immersed in water with a dielectric constant w of 78, and the azobenzene groups and the alkyl groups are located above the water surface in a medium with a dielectric constant of s 1. The induced charge Q on electrode 1 is expressed using lt and lw defined in Eq. (1) as ns B hltz i hlwz i ð2Þ Q¼ þ C/s ; d s w where B is the working area of electrode 1, ns is the surface density of molecules given by 1=A (A: molecular area), d is the distance between electrode 1 and the water surface, C is the capaciatance between electrode 1 and the water surface, and /s is the surface potential of water of the Langmuirtrough and the suffix z represents the vertical component of the dipole moment. In Eq. (2), h i denotes a thermodynamic average of the distributed components of the molecules. The MDC is generated across monolayers due to the change of induced charge Q with respect to time. By monolayer compression, MDC is generated and expressed as t B hlz i hlwz i d I¼ ns þ d s w dt t Bns d hlz i hlwz i d þ ð3Þ þ þ C /s : s w dt d dt In our experimental system, an air-gap between the water surface and electrode 1 works as a good electrical insulator and it eliminates the flow of leakage current. Thus only the MDC current given by Eq. (3) is allowed to flow. The change of induced charge on electrode 1 can be estimated by integrating the MDC current with respect to time. As the dielectric constant of water w ( ¼ 78) is so high, the contribution of hlw i to the generation of MDC is electrically screened off. Furthermore, the change of water surface potential is small in comparison with the contribution of the first and second terms of Eq. (3) [3]. We assume that molecular structure is fixed and tilt angle of molecules
T. Manaka et al. / Chemical Physics Letters 355 (2002) 164–168
167
only gradually increases during monolayer compression. Using the optimized geometry for AZGn molecule, we can estimate the orientational average, cos h. As a result, the MDC current is expressed using orientational order parameter, S NN ¼ hcos hNN i and S CH ¼ hcos hCH i, as d B n ns ð2 1ÞS NN lNN I¼ þ 2n S CH lCH z z s d dt Bns d n ð2 1ÞS NN lNN þ þ 2n S CH lCH : ð4Þ z z s d dt Since we can experimentally obtain the displacement current I, induced charge Q is calculated by Rt integration with the current, Q ¼ 0 I dt. The charge Q is directly related to the dipole moment as Eq. (2). In this experiment, we can assume that the initial value of Q is zero because the displacement current was not observed in the range of 2 ). large molecular area ( 300 A
3. Results and discussion Fig. 3 shows a typical example of the MDC currents generated across monolayer of AZ-G2 molecules together with the dipole moment and surface pressure. Looking at these figures, it is found that two peaks appear during monolayer compression for the MDC current. The first and second peaks appear in the range before and after the initial rise of surface pressure, respectively. Using Eq. (4) we calculated the charge flowing through the circuit and could calculate the vertical component of dipole moment of these molecules, hlz i and then plotted the results in the figure. Of interest is that the hlz i increases gradually by monolayer compression and finally saturates at a 2 . The result indicates that surface area of 120 A these dendritic molecules spread onto the water surface gradually stand up, and they are then packed during monolayer compression. It is interesting to plot the hlz i as a function of the generation order n. Fig. 4 shows the relationship between the number of azo-groups 2n 1 and the obtained hlz i. In the figure, the values of limiting area of these molecules estimated from the surface pressure-area ðp AÞ isotherms and those of the end alkyl-group are plotted, the increase is
Fig. 3. Typical example of the MDC currents generated across AZ-G2. From bottom to top, the surface pressure-area. MDCarea, dipole moment-area are plotted.
Fig. 4. Relationship between the number of azo-groups and limited area (open triangle), dipole moment obtained from experimental (filled circle) and estimated dipole moment by MO calculation (open circle).
almost proportional to the number of azo-group 2n 1. This suggests that the AZ-Gn molecule has a uniaxially dipolar structure. In order to further
168
T. Manaka et al. / Chemical Physics Letters 355 (2002) 164–168
clarify the details, we calculated the dipole moment of azobenzene group and the end group by means of MO calculation. All the MO calculations were done using semiempirical calculation package, MOPAC 2000 (Fujitsu). Molecular properties such as orbital energy and dipole moment strongly depend on the molecular structure. Before the calculation of dipole moment, hence we have optimized the molecular structure with AM1 Hamiltonian. Generally, MO calculation is used for the isolated systems. Although intermolecular interaction affects the molecular structure for the monolayer on the water surface, averaged conformation of molecules on the water surface was assumed to be similar to the optimized conformation obtained by MOPAC calculation. Optimized molecular structure of AZ-G2 is shown in Fig. 1b. This seems fairly elongated and the result is consistent with the nonlinear optical measurements [13]. As a consequence of screening of the water, COOH-group cannot contribute to the dipole moment determined by MDC measurement on the water surface. As mentioned above, we divided AZ-G1 molecule into three parts, COOH-group, azo-group and alkyl-group (see Fig. 1a) and estimate the dipole moment of each group using Mulliken population. As a result, we have obtained dipole moment of 3:523D and 0:05D for azo-group and alkyl-group, respectively. For AZGn, the molecule consists of the number of 2n 1 azo-groups and 2n alkyl-groups. Fig. 4 shows dipole moment estimated by using Eq. (3) and MDC measurement. As shown in figure, dipole moment obtained using MOPAC was qualitatively coincident with the results of the MDC measurement.
4. Conclusion Dipole moment of azobenzene dendrimer on the water surface was examined by means of MDC
technique for Langmuir monolayer. The vertical component of dipole moment of the constituent azobenzene dendrimer molecule increases as the number of azobenzene groups and alkyl-groups in the molecule. The behavior could be also obtained by MO calculation using MOPAC. The increasing of the dipole moment suggests that the azobenzene groups arrange noncentrosymmetrically in the dendrimer.
Acknowledgements We acknowledge Dr. S. Yokoyama, Dr. T. Kubota and Dr. S. Mashiko in Communication Research Laboratory for kindly providing AZ-Gn molecules and useful discussion.
References [1] G.L. Gaines Jr., Insoluble Monolayers at Liquid–Gas interfaces, Interscience, New york, 1966. [2] A. Ulman, Characterization of Organic Thin Films, Butterworths/Heinemann, Boston, MA, 1995. [3] M. Iwamoto, C.X. Wu, The Physical Properties of Organic Monolayers, World Scientific, Singapore, 2001. [4] D.A. Tomalia, A.M. Naylor, W.A. Goddard III, Angew. Chem. Int. Ed. Engl. 29 (1990) 138. [5] M. Frechet, Science 263 (1994) 1710. [6] A. Tomalia, Adv. Mater. 6 (1994) 529. [7] Y. Shiyoshi, T. Nakahara, A. Otomo, S. Mashiko, J. Am. Chem. Soc. 122 (2000) 3174. [8] Y. Shiyoshi, T. Nakahara, A. Otomo, S. Mashiko, Thin Solid Films 331 (1998) 248. [9] M. Iwamoto, Y. Majima, H. Naruse, T. Noguchi, H. Fuwa, Nature 353 (1991) 645. [10] M. Iwamoto, Y. Majima, J. Chem. Phys. 94 (1991) 5135. [11] Y. Shiyoshi, T. Nakahara, A. Otomo, S. Mashiko, Chem. Lett. 137 (1997) 1. [12] M. Iwamoto, Thin Solid Films 244 (1994) 1031. [13] S. Yokoyama, S. Mashiko, Y. Yamaguchi, Y. Yokomichi, Polym. Adv. Technol. 11 (2000) 1.