672
Langmuir 1990, 6, 672-676
Molecular Orientation in LB Films of Azobenzene-Containing Long-chain Fatty Acids and Their Barium Salts Studied by FT-IR Transmission and Reflection-Absorption Spectroscopy Takeshi Kawai, Junzo Umemura, and Tohru Takenaka" Institute for Chemical Research, Kyoto University, Uji, Kyoto-fu 611, J a p a n Received August 10, 1989. I n Final Form: October 27, 1989 Molecular orientations in Langmuir-Blodgett films of four kinds of azobenzene-containing long-chain fatty acids and their barium salts are quantitatively evaluated by Fourier transform infrared transmission and reflection-absorption spectroscopy. The effects of carbon chain length of the tail and spacer portions, the number of monolayers, and the head group difference between the carboxyl and carboxylate groups on the molecular orientation are examined. It is concluded that the barium salts are more highly oriented as compared with the corresponding acids. This tendency is remarkable between barium salts with an eight-carbon tail and their acids. These results are consistent with our previous results obtained by UV absorption spectroscopy. Further, the orientation of the hydrocarbon chain and the azobenzene chromophore is found to be almost independent of the number of monolayers, but the tilt angle of the symmetrical axis of the carboxylate group in the first monolayer is larger than those in the upper monolayers. This fact may arise from the difference in the mode of barium ion binding between the first and upper monolayers.
Introduction Ordered thin organic films deposited on solid substrates by the Langmuir-Blodgett (LB) technique' have attracted much attention because of their possibility of being applied to functional molecular devices such as electronic, nonlinear optical, and pyroelectric elements and biological sensors. As the basis of studies of these applications, the structural characterization of the molecular assemblies has become very important. In a previous paper,' we have studied UV absorption spectra of azobenzene-containing long-chain fatty acids (Figure 1, abbreviated as mAnH, m = 8 or 12 and n = 3 or 5 ) and their barium salts (mAnBa) in spread monolayers on the water surface and in LB films on quartz plates. The aggregation and orientation of the chromophore in the LB films have been compared with those in the spread monolayers prior to the transfer. However, since the UV absorption spectra give structural information only about the chromophores, other characterization methods are necessary for investigating the orientation of other parts of the molecule: the hydrocarbon chain and polar group. Several attempts have been made to apply infrared reflection-absorption (RA) s p e c t r o ~ c o p yfor ~ . ~this purFor example, Swalen et al.7310 have qualitatively studied the molecular orientation in cadmium arachidate LB films by comparing band intensities between (1) Blodgett, K. B. J. Am. Chem. SOC.1935,57, 1007. (2) Kawai, T.; Umemura, J.; Takenaka, T. Langmuir 1989, 5, 1378. (3) Greenler, R. G. J. Chem. Phys. 1966, 44, 310. (4) Greenler, R. G. J . Phys. Chem. 1969,50, 1963. (5) Chollet, P. A.; Messier, J.; Rosilio, C. J . Chem. Phys. 1976, 64, 1042. (6) Allara, D. L.; Swalen, J. D. J. Phys. Chem. 1982, 86, 2700. (7) Rabolt, J. F.; Burns, F. C.; Schlotter, N. E.; Swalen, J. D. J .
Chem. Phys. 1983, 78,946. (8) Bonnerot, A.; Chollet, P. A.; Frisby, H.; Hoclet, M. Chem. Phys. 1985, 97, 365. (9) Allara, D. L.; Nuzzo, R. G. Langmuir 1985,1,52. (10) Naselli, C.; Rabolt, J. F.; Swalen, J. D. J. Chem. Phys. 1985, 82,
2136.
0743-7463/90/2406-0672$02.50/0
the RA and transmission spectra. Further, Chollet et al.538and Allara et aL6v9have quantitatively discussed the molecular orientation in LB films using the same technique. Recently, we" developed the method proposed by Chollet et al.598 using a rather rigorous formalism on optics of thin multilayer films given by Hansen." In the present paper, we applied this method to the structure study of thin LB films of mAnH and mAnBa compounds and examined the effects of carbon chain length of the tail and spacer portions, the number of monolayers, and the head group difference between the carboxyl and carboxylate groups on the molecular orientation in LB films. Results were compared with those previously obtained by the UV absorption method.'
Experimental Section All samples of the form mAnH (Figure 1) were the same as those reported previously.' Water was purified with a modified Mitamura Riken Model PLS-DFR automatic lab still consisting of a reverse osmosis module, an ion-exchange column, and a double distiller. A Kyowa Kaimen Kagaku Model HBMAP Langmuir trough with a Wilhelmy balance was used for LB film fabrications. The spread monolayers of acids and their barium salts were prepared by dropwise addition of dilute chloroform solutions of the acids (0.7 mg/mL) on pure water at pH 6.2 and on water containing 2 X M BaCl,, buffered with 4 X M NaHCO, and 5 X lo-' M Na,CO, to pH 7 . 7 , respectively.' After evaporation of the solvent, the monolayers were compressed at a constant rate of 10 cm2/min up to planned surface pressures at 20 "C and then transferred on glass slides covered with the vacuum-evaporated 100-nm-thickAg (for RA measurements) and on ZnSe plates (for transmission measurements). The solid substrates were cleaned prior to use as follows. The glass slides were soaked in a chromic acid mixture for 1-2 days, rinsed with distilled water, ultrasonicated in ethanol for 10 min, and dried in air just prior to the Ag evaporation. The ZnSe plates were cleaned by successive ultrasonica(11)Umemura, J.; Kamata, T.; Kawai, T.; Takenaka, T. J. Phys. Chem., in press. (12) Hansen, W. N. J . Opt. SOC. Am. 1968,58, 380.
0 1990 American Chemical Society
Langmuir, Vol. 6, No. 3, 1990 673
Molecular Orientation in LB Films
R
= N o O - ( C H 2 In-C OOH To.004
R
n
8A3H
:
C8Hi7
3
8A5H
:
CaHi7
5
12A3H
:
C12H250
3
12A5H
:
C12H250
5
Figure 1. Azobenzene-containing long-chain fatty acids used in this work. tions in ethanol, acetone, chloroform, and distilled water for 10 min each. The monolayers of 8AnH and 8AnBa were transferred onto the solid substrates by the standard LB (vertical dipping) method.' In the case of 12AnH and 12AnBa, however, only the first monolayers were transferred by the LB method. The second and subsequent monolayers were transferred by the horizontal lifting method,13because the first monolayers were peeled off when the substrates were dipped in water to deposit the second monolayers. The surface pressures of the transfer were 25 m N / m for 8A3H, 20 m N / m for 8A3Ba, 16 mN/m for 8A5H, 25 m N / m for 8A5Ba, and 18 mN/m for all 12AnH and 12AnBa. At these respective surface pressures, the monolayers were in the solid condensed state, as seen in Figures 2 and 3 of a previous paper.' The transfer ratios were in the range 0.94-1.1 throughout the experiments. All infrared spectra were recorded on a Nicolet Model 6000C FT-IR spectrophotometer equipped with an MCT detector. A Harrick Model RMA-OOG reflection attachment was used for RA measurements. The p-polarized infrared beam by a Hitachi wire-grid polarizer was incident on the sample plane a t 85' from the surface normal. Four-thousand interferograms collected with the maximum optical retardation of 0.25 cm were coadded, apodized with the Happ-Genzel function, and Fourier transformed with one level of zero filling to yield spectra of a high signal-to-noise ratio with the resolution of 4 cm-l. X-ray diffraction patterns were obtained by a Rigaku Denki Model RAD-2B diffractometer with the use of Cu Kcu radiation. The long spacings of 8A3Ba, 8A5Ba, 12A3Ba, and 12A5Ba LB films were found to be 5.33, 5.73,6.09, and 6.50 nm, respectively. Since these values are almost twice the corresponding molecular lengths estimated from the structure model^,'^ these LB films apparently have an alternate head-to-head, tail-totail layer structure (so-called Y-type structure).
Results a n d Discussion I n f r a r e d Transmission a n d RA Spectra. Figures 2 and 3 represent infrared transmission and RA spectra, respectively, of 1-,3-, 5-, 7-, and ll-monolayer LB films of 8A5Ba. The ordinate scale of the RA spectra (Figure 3) is 3 times as large as that of the transmission spectra (Figure 2). It is seen that both transmission and RA spectra have been recorded with high signal-to-noise ratios even for the one-monolayer films. Assignments of the absorption bands are summarized in Table I.6,9*15-'9 Relative intensities of the bands are quite different between the transmission and RA spectra. The sym(13)Fukuda, H.; Nakahara, H.; Kato, T. J. Colloid Interface Sci. 1976,54,430. (14)Xu, X.; Kawarnura, S.; Era, M.; Tsutsui, T.; Saito, S. Nippon Kagaku Kaishi 1987,11,2083. (15)Varsanvi. G. Vibrational Spectra of Benzene Deriuatiues; Academic'Press: New York, 1969. (16)Katritzky, A. R.; Coats, N. A. J. Chem. SOC.1969,2062. (17)Kubler, V. R.; Luttke, W.; Weckherlin, S. 2.Electrochem. 1960, 64, 650. (18)Chaturvedi, G.C.; Rao, C. N. R. Spectrochim. Acta 1971,27,65. (19)Babu, V. A.; Lakshmaiah, B.; Ramulu, K. S.; Rao, G. R. Indian J . Pure Appl. Phys. 1987,25,58. ~
3000
2800 '1800
1600
1400
1200
1000
800
Wavenumber / cm-' Figure 2. Infrared transmission spectra of 1-,3-, 5-, 7-, and 11-monolayer LB films of 8A5Ba on a ZnSe plate.
k
3000
2800
1800
1600
1400
1200
k
1000
800
Wavenumber / cm-'
Figure 3. Infrared RA spectra of 1-, 3-, 5-, 7-, and ll-monolayer LB films of 8A5Ba on an evaporated Ag film. p-Polarized radiation. Angle of incidence is 85'.
metric CH, stretching bands a t 2849 cm-', the antisymmetric COO- stretching band at 1515 cm-', and the 4-H out-of-plane bending band a t 840 cm-' are stronger in the transmission spectra than in the RA spectra. Contrary, the in-plane vibration bands of the benzene ring a t 1604,1585,1501, and 1476 cm-', the symmetric COOstretching band at 1433 cm-', and the 4-0 stretching band a t 1255 cm-l are much stronger in the RA spectra than in the transmission spectra. Since the electric field of the infrared beam is parallel to the LB film in the normally incident transmission measurements, while perpendicular in the RA meas~rements,3'~*'' the above results indicate that the hydrocarbon chains, the benzene plane, and the symmetrical axis of the carboxylate group tend to orient perpendicular to the film surface. The CH, scissoring and rocking bands of the hydrocarbon chains are known to be sensitive to the intermolecular interaction and are often used to distinguish the lateral packings of the Figure 4 shows the transmission spectra of the CH, scissoring and rocking vibration regions of 8A5H, 8A5Ba, 12A5H, and 12A5Ba in the ll-monolayer LB films. The spectra of 8A3H, (20)Snyder, R. G.J.Mol. Spectrosc. 1961,7, 116. (21)Snyder, R. G.J. Chem. Phys. 1979,71,3229.
674 Langmuir, Vol. 6, No. 3, 1990
Kawai et al. incidence in the RA measurements is the x z plane. Here, AT and A, are the absorbances of the band in the transmission and RA spectra and m, and m, the enhancement factors along the z and x axes of the RA intensity to the transmission intensity of hypothetical isotropic LB film. The values of m, and m, can be precisely calculated by Hansen's optical equations for thin multilayer films12 and are known to depend on complex refractive indices of the sample film and substrates, thickness of the film, the angle of incidence in RA measurements, and infrared Since the transition moments of the antisymmetric and symmetric CH, stretching bands and the hydrocarbon chain axis are mutually perpendicular, the tilt angle y of the hydrocarbon chain axis from the surface normal can be obtained from the corresponding angles (a and 0)of the transition moments of the two CH, stretching bands by"
+
cos2a + cos2p cos2y = 1 (2) The orientation of the normal axis to the benzene plane and the symmetrical axis of the carboxylate group may be directly evaluated from the AT/A, values for the 6H out-of-plane bending and symmetric COO- stretching bands, respectively. 8A5H 8A5Bo Before applying this method to the study of the molecular orientation in the mAnH and mAnBa LB films, we have to make some points clear. The first point is whether the molecular orientation is uniaxial around the surface normal. A series of polarized infrared transmission spectra of LB films were recorded by rotating the polarization plane on the film surface. For all compounds examined, no difference was observed among these spectra, indicating that the condition of the uniaxial orientation is fulfilled.22 The second point is whether the molecular orientations on both ZnSe plate and glass slide with the evaporated Ag are the same. The long spacings of the ll-monolayer LB films of 8A5Ba on both substrates were obtained to be 5.73 nm by X-ray analysis. From this fact, we can consider that the molecular orientation $80' 1460 ' ' " 740' ' 720 ' ' Ii80' Id60' '%k%$? is the same on both substrates. The third point is whether Wovenumber / cm-' the anomalous dispersion of the refractive index of the Figure 4. Infrared transmission spectra in the CH, scissoring LB film around the infrared absorption frequency affects and rocking vibration regions of ll-monolayer LB films of 8A5H, the calculated 4 value." If this anomaly occurs, large 8A5Ba, 12A5H, and 12A5Ba. frequency differences of the RA peaks from the transmission peaks should be observed as discussed pre8A3Ba, 12A3H, and 12A3Ba are similar to those of the viously." In the present case, however, these frequency corresponding compounds. In the case of the 8AnH and shifts were very small, as seen in Table I. Hence this 8AnBa films, the scissoring and rocking bands appear as problem can be neglected." singlets at ca. 1466 and ca. 725 cm-', respectively, indiMolecular Orientation in the LB Films. Absorcating that the alkyl chains are in a hexagonal subcell bance values of the major bands in the transmission and packin where each chain is freely rotated around its long axis.20' In the case of the 12AnH and 12AnBa films, RA spectra (AT and AR) of ll-monolayer LB films of 8A5Ba, their ratios, the calculated m, and m, values, and on the other hand, the CH, scissoring band split into a the tilt angles 4 obtained from eq 1 are listed in Table doublet a t ca. 1475 and ca. 1465 cm-l, and the CH, rock11. For the calculation of the m, and m, values," we ing band also appears as a doublet a t ca. 730 and ca. 720 considered a three-phase plane-bounded system of air/ cm-l. This suggests that they are crystallized with the LB film/substrate (ZnSe or Ag) and used the following orthorhombic subcell packing where the alkyl chains are parameters, for example, a t 2919 cm-': the complex refracpacked alternately.20~21 tive indices R, = 1.00 (air), R, = 2.46 (ZnSe for the transQuantitative Evaluation of Molecular Orientamission measurement^),^^ and A, = 0.62 + 25.li (Ag for tion. According to a previous paper,'' the orientation the RA mea~urernents);'~ the thickness of the ll-monoof the transition moment of the particular vibration band is evaluated by
'
'
P'
-AT--
sin24 (1) AR 2m, cos2 4 + m, sin24 under the condition of uniaxial orientation of the transition moment around the surface normal z with the angle 4. The film surface is the xy plane, and the plane of
(22) The uniaxial orientation means the following. The monolayer consists of a number of small crystallites in which all hydrocarbon chain axes are tilted in the same direction with the angle y from the surface normal. But the crystallites are randomly distributed, so their hydrocarbon chains are uniformly oriented around the surface normal. (23) Kudo, K. Kiso Bussei Zuhyo; Kyoritu Shuppan: Tokyo, 1972. (24) Ordal, M. A.; Long, L. L.; Bell, R. J.; Bell, S. E.; Bell, R. R.; Alexander, R. W., Jr.; Ward, C. A. A p p l . O p t . 1983,22, 1099.
Molecular Orientation in LB Films
Langmuir, Vol. 6, No. 3, 1990 675
Table 11. Transmission and RA Absorbances, Their Ratio, Enhancement Factors, and the Tilt Angle of the Transition Moment of Major Absorption Bands of 8A5Ba in the 11-Monolayer LB Film
v, cm-l assignment direction of transition moment ATn AR ATIAR mz m, $ 0.004 70 0.00641 0.73 9.1 0.15 a = 76" v,(CH,) I to carbon chain plane 2919 I to carbon chain axis in carbon chain plane 0.002 27 0.001 89 1.20 9.3 0.15 B = 790b v,(CH,) 2849 1515 v,(COO-) I to bisector of OCO angle 0.01069 0.002 16 4.95 12.9 0.06 86' 1433 v,(COO-) (1 to bisector of OCO angle 0.010 13.2 0.05 27' 0.00043 0.04200 0.05404 0.019 13.6 0.04 44-0) I1 to 6-0 bond 0.001 02 36' 1255 840 *($-HI I to benzene plane 0.002 81 0.002 64 1.06 14.4 0.02 80" 'Absorbances in Figure 4 divided by 2 to convert to the one-side (ll-monolayer) film on a ZnSe window. 'The tilt angle of the hydrocarbon chain axis, y = 18O.
I ,+-36O
I
I
/ I / / / / /////I/////// /// / Substrate
Figure 5. Schematic illustration of the molecular orientation of 8A5Ba in the 11-monolayer LB film.
layer film h, = 31.5 nm; and the angle of incidence in the RA measurements 8, = 85'. The wavelength dependence of the R , values was obtained by interpolating the data in the refs 23 and 24. The R, value for the LB films was fixed a t 1.50 + 0.li throughout the wavelength region The tilt angles of the transition moments of the antisymmetric and symmetric CH, stretching bands are found to be 76' and 79', respectively. Thus, that of the hydrocarbon chain axis, y,is calculated to be 18' from eq 2. The tilt angles of the transition moments of the symmetric and antisymmetric COO- stretching bands are 27" and 86', respectively. The former value, which is much smaller than 40' and was pointed out previously," may be less accurate than the other values, because the A , value is very small. The angle for the 4-H out-of-plane bending band is 80' (fairly close to go'), indicating that the benzene plane orients almost normal to the film surface. These results are schematically illustrated in Figure 5, manifesting a high degree of the molecular orientation in the LB film. Transmission and RA spectra of ll-monolayer LB films of the other seven amphiphiles were also measured, and their orientations were evaluated in the same way. The results of all the compounds examined (including 8A5Ba) are summarized in Table 111. It is generally seen that (25) Dignam, M. J. Appl. Spectrosc. Reu. 1988,24, 99
the tilt angles of the molecular axis of barium salts are smaller than those of corresponding acids. This tendency is most remarkable between 8A5Ba and 8A5H. Examination of the tilt angles of the hydrocarbon chain axis, the 4-0bond, and the benzene normal suggests that the molecular axes of 8A5Ba are most perpendicularly oriented on the film surface of all the amphiphiles examined, while those of 8A5H are least. Further, it is to be noted that all of these angles for the 8A5H molecule are close to 54.7', which is known as the average of the angles between the surface normal and randomly oriented transition moments. The difference in the molecular orientation is also seen between 8A3Ba and 8A3H, though it is not as large as that observed between 8A5Ba and 8A5H. The much smaller but perceptible difference still exists between l2AnBa and 12AnH (n = 3, and 5). In a previous paper,, we studied UV absorption spectra of the same amphiphiles in spread monolayers on the water surface and in LB films on solid substrates. It was concluded that the degree of H - a g g r e g a t i ~ n ~of~8A5H ~,~ and 8A3H was largely lowered during the transfer process of the monolayers from the water surface onto solid substrate. On the contrary, the molecular aggregation of 8A5Ba and 8A3Ba was developed during the transfer process and resulted in a very high H-aggregate in the ll-monolayer LB films. In the case of both 12AnH and 12AnBa, the molecules were in a good order of H-aggregation on the water surface and kept it unchanged during the monolayer transfer. Since the high H-aggregation means a high degree of orientation of the long axis of the chromophore with respect to the film surface (as well as a large number of the aggregated molecule^),^^ there is a good consistency between the results of the present and previous papers. In the last column of Table 111, we added the tilt angles of the long axes of the azobenzene chromophores in the ll-monolayer LB films which have been reported in the previous paper., It is also evident that the tilt angles are smaller for barium salts than for the corresponding acids. This trend is remarkable between 8AnBa and 8AnH, being in a good agreement with the results of the present work. It has been reported that an upper limit of the tilt angle of the hydrocarbon chain axis in cadmium arachidate LB films is 8' f 5°.28 Recently, we have evaluated the corresponding tilt angle of cadmium stearate to be 7O.I' The present results, 18-27' for barium salts of azobenzenecontaining fatty acids (Table 111), are much larger than those obtained for divalent metal salts of long-chain fatty acids. Dependence of Molecular Orientation on Number of Monolayers. From the infrared transmission and (26) H-aggregate means a linearly arranged aggregate of chromophoreswith their transition moments parallel to each other and ordered nearly perpendicularly (>54.7') to the stacking direction. (27) McRae, E. G.; Kasha, M. Physical Processes in Radiation Biology; Academic Press: New York, 1964; p 23. (28) Duschl, C.; Knoll, W. J. Chem. Phys. 1988,88,4062.
676 Langmuir, Vol. 6, No. 3, 1990
Kawai et al.
Table 111. Tilt Angles (Deg) of the Chain Axis and Various Transition Moments from the Surface Normal of mAnH and mAnBa in the 11-Monolayer LB Films m
n
8
3
8
5
12
3 5
12 a
acid Ba salt acid Ba salt acid Ba salt acid Ba salt
Yo 33 23 50 18 29 27 36 27
u,(COO-)
u,(COO-)
85
30
86
27
82
27
85
29
u(4-0) 63 41 53 36 42
The tilt angle of the hydrocarbon chain axis.
40 47 43
d4-H)
long axis of azobenzene moietyb
63 79 56 80 80 81 70 81
45 29 46 28 29 24 35 25
From Table I of ref 2.
Table IV. Tilt Angles (Deg) of the Chain Axis and Various Transition Moments of 8A5Ba and 12ASBa from the Surface Normal as a Function of Number of Monolayers monolayers 1 3 5 7 11
yo
u,(COO-)
20 23 21 20 18
83 82 84 82 86
24 31 28 26 27
80 83 82 85 85
u,(COO-) 8A5Ba 59 31 28 25 27
~(4-0) ~ ( 4 - H ) 42 45 40 41 36
76 77 78 75 80
49 45 48 44 43
83 81
12A5Ba 1
3 5 7 11 a
46 26 25 30 29
80
81 81
The tilt angle of the hydrocarbon chain axis. Substrate
RA measurements of the 1-11-monolayer LB films of 8A5Ba and 12A5Ba, effects of the number of monolayers on the molecular orientation were investigated. The results are listed in Table IV. The tilt angles of the hydrocarbon chain axis and the transition moments of the antisymmetric COO- stretching, the 4-0stretching, and the 6-H out-of-plane bending bands are almost independent of the number of monolayers. However, that of the transition moment of the symmetric COO- stretching band in the 1-monolayer LB films is nearly twice as large as those in the multilayer LB films. These results indicate that the orientations of the hydrocarbon chain and the azobenzene chromophore are the same in all the monolayers examined, while the symmetrical axis of the carboxylate group in the first monolayer is much more tilted as compared with those in the upper monolayers. This difference is probably caused by the difference in the modes of barium ion binding between the first and upper monolayers, as shown in Figure 6. In the upper mono-
Figure 6. Schematic illustration of the orientation of the carboxylate groups in the first three monolayers.
layers, divalent barium ion binds two carboxylate anions in the adjacent layers. In the first monolayer, however, barium ion combines the two neighboring carboxylate anions in the same layer.
Acknowledgment. We are indebted to Dr. K. Kina of Dojindo Laboratories for his kind supply of the samples used in the present work. This work was partly supported by the Grant-in-Aid for Special Project Research from the Ministry of Education, Science and Culture, Japan, to which we are thankful. Registry No. 8A5Ba, 121918-59-0; 8A3Ba, 121887-98-7; 12A3Ba, 121887-99-8; 12A5Ba, 121888-00-4; 8A3H, 112360-084; 8A5H, 112360-09-5; 12A3H, 121887-97-6; 12A5H, 112360-108.
