Langmuir 1991, 7, 2827-2832
2827
Molecular Aggregation, Orientation, and Structure in Langmuir-Blodgett Films of 2-(4’-(Ethyloctadecylamino)phenylazo)-N-methylbenzothiazolium Perchlorate and 2-(4’-(Dioctadecylamino)phenylazo)-N-methylbenzothiazolium Perchlorate Studied by Infrared, Visible Absorption, and Resonance Raman Spectroscopies Norihisa Katayama, Masahiko Fukui, and Yukihiro Ozaki’ Department of Chemistry, School of Science, Kwansei Gakuin University, Uegahara, Nishinomiya 662, Japan
Nobuhiro Kuramoto Osaka Prefectural Industrial Technology Research Institute, Nishi-ku, Osaka 550, Japan
Toshinari Araki Frontier Technology Research Institute, Tokyo Gas Co. Ltd., Shibaura, Minato-ku, Tokyo 105, Japan
Keiji Iriyama Department of Biochemistry, Institute of Medical Science, The Jikei University School of Medicine, Nishi-Shinbashi, Minato-ku, Tokyo 105, Japan Received March 19, 1991. In Final Form: July 25, 1991 Infrared transmission and reflection-absorption (RA)spectra have been measured for Langmuil-Blodgett (LB) films of 2-(4’-(ethyloctadecylamino)phenylazo)-N-methylbenzothiazolium perchlorate (Azo I) and 2- (4’(dioctadecylamino)phenylazo)-N-methylbnzothiazolium perchlorate (Azo 11) and visible absorption and resonance Raman spectra have been obtained for Azo I to compare their molecular aggregation, orientation, and structure in the LB films. The visible absorption spectrum of the LB film of Azo I shows that in contrast to Azo I1 previously investigated, Azo I does not form an H aggregate in the LB film. A comparison of the infrared transmission and reflection-absorption (RA) spectra indicates that the benzothiazolium rings of Azo I and Azo I1 are fairly perpendicular to the substrate surface but their benzene rings are tilted appreciably. As for the hydrocarbon chains, the infraredtransmission, polarized transmiasion, and RA measurements suggest that the chains of both compounds are neither perpendicular nor parallel to the substrate surface, being tilted with respect to the surface normal, and they are in a hexagonal or pseudohexagonal subcell packing with a uniaxial orientation. Vibrational frequencies of infrared bands due to CHZantisymmetric and symmetric stretching modes indicate that Azo I1 molecules in the LB films have highly ordered (trans-zigzag)hydrocarbon chains, but Azo I molecules in the films have slightly less ordered chains. Probably, Azo I1 with two hydrocarbon chains is more easily ordered in the films than Azo I with one hydrocarbon chain. Resonance Raman spectra of Azo I in the LB and cast films and solutions are very similar to each other in contrast to the previous result for Azo 11, indicating that the conformation and structure of the chromophoric part of Azo I change little among the three states.
Introduction In recent years, interest in Langmuir-Blodgett (LB) films has been increasing greatly because of their possibilities as functional molecular devices such as electronic, nonlinear optical, and pyroelectric elements.’-3 The increase in the interest has accelerated not only functional studies but also structural studies of LR films. A number of physical techniques have been introduced to the structural characterization of the molecular assemblies. We have been investigating the structure of LB films consisting of functionally important dye molecules by * Author to whom correseondence should be directed. (1) Mobius, D., Ed. Langmuir-Blodgett Films 3. Thin-Solid Films
1988,159 (1, 2) and 160 (1, 2). (2) Sugi, M. Molecular Electronic Devices; Carter, F. L., Siatkowsk, R. E.,Wohltjen, H., Eds.; Elaevier: Amsterdam, 1988;p 441. (3) Kuhn, H. Thin Solid Films 1989, 178, 1.
means of infrared and resonance Raman spectroscopy.InfraredS2l and Raman22-33spectroscopy, which are both powerful nondestructive structural probes for various kinds (4) Ozaki, Y.; Iriyama, K.; Iwahashi, K.; Hamapuchi, H. Appl. Surf. Sci. 1988, 33/34, 1317. (5) Katavama.. N.:. Ozaki, Y.: Araki, T.:Iriyama, . K. J. Mol. Struct. 1991,’ 242, i7. (6) Kubota, M.; Ozaki, Y.; Araki,T.;Ohki, S.; Iriyama, K. Longmuir 1991, 7, 774.
(7) Fukui, M.; Katayama, N.; Ozaki, Y.; Araki, T.; Iriyama, K. Chem.
Phys. Lett. 1991, 77,247.
(8)Katayama, N.; Ozaki, Y.; Kurnmoto, N. Chem. Phys. Lett. 1991,
179. 227.
__
(9)Greenler, R. G. J. Chem. Phys. 1966,44, 310. (10) Chollet, P.-A.; Messier, J.; Roeilio, C. J. Chem. Phys. 1976, 64,
in42.
(11) Koyama, Y.; Yanagishita, M.; Toda, 5.;Matsuo, T. J. Colloid Interface Sci. 1977,61,438. (12) Chollet, P.-A. Thin Solid Films 1978,52, 343. (13) Allara, D.L.;Swalen, J. D. J. Phys. Chem. 1982,86, 2700.
0743-7463/91/2407-2827$02.50/0 0 1991 American Chemical Society
Katayama et al.
2828 Langmuir, Vol. 7, No. 11, 1991
of molecular assemblies, provide knowledge about the orientation, conformation, and electronic structure of molecules in the LB films and about the interaction between a substrate and molecules. T h e information derived from the two spectroscopies is from time to time complementary to each other, and therefore a combined infrared and Raman study is, in general, very useful for the structural characterization of the LB films. In our previous papers we presented visible absorption and resonance Raman spectra of LB films of 2-(4’-(dioctadecylamino)phenylazo)-N-methylbenzothidum perchlorate (Azo 11, Figure 1) and discussed molecular aggregation and structure in the films. Here, we report combined infrared and resonance Raman studies on LB films of 2-(4’-(ethyloctadecylamino)phenylazo)-N-methylbenzothiazolium perchlorate (Azo I, Figure 1)and Azo I1 which are new a type of azo-containing LB films.34Azo I and Azo I1 have the same chromophoric part but are different from each other in the number of the hydrocarbon chains bonded; Azo I has one hydrocarbon chain while Azo I1 has two hydrocarbon chains. It is of particular interest to investigate alterations in the molecular aggregation, orientation, and structure induced by the change in the number of hydrocarbon chains. This sort of comparative structural study may offer new insight into molecular design of LB films.
Experimental Section Synthetic methods of Azo I and Azo I1 will be reported elsewhere.” A Kyowa Kaimen Kagaku Model HBM-AP Langmuir trough with a Wilhelmy balance was employed for the r-A isotherm measurements as well as LB film fabrications. The Azo I or Azo I1 monolayer was spread from a dilute chloroform M) on doubly distilled water containing 1.4 x solution (2 x lo4 M CdClp and 1.7 x M NaHCOs (pH 6.3,20 “C). After evaporation of the solvent, the monolayer was compressed at a constant rate of 20 cm2/min up to the surface pressure of 35 mN m-l. The *-A isotherm showed that the monolayers were solid (14) Rabolt, J.F.;Burns,F.C.; Schlotter, N. E.;Swalen, J. D.J.Chem. Phys. 1983, 78, 946. (15) (a) Maoz, R.; Sagiv,J. J.ColloidInterface Sci. 1984,100,465. (b) Gun, J.; Iscovici, R.; Sagiv, J. J. Colloid Interface Sci. 1984, 101, 201. (16) Allara, D. L.; Nuzzo, R. G. Langmuir 1986,1, 52. (17) Naselli, C.; Rabolt, J. F.; Swalen, J. D. J. Chem. Phys. 1986,82, 2136. (18) Kimura, F.; Umemura, J.; Takenaka, T. Langmuir 1986, 2, 96. (19) Umemura,J.;Kamata,T.;Kawai,T.;Takenaka,T. J.Phys. Chem. 1990,94, 62. (20) Kawai, T.; Umemura, J.; Takenaka, T. Langmuir 1990,6, 672. (21) Ancelin, H.; Briody, G.;Yarwood, J.; Lloyd, J. P.; Petty, M. C.; Ahmad, M. M.; Feast, W. J. Langmuir 1990,6, 172. (22) Rabolt, J. F.; Santo, R.; Swalen, J. D. Appl. Spectrosc. 1979,33, 549. (23) Knoll, W.; Philpott, M. R.; Golden, W. G. J.Chem. Phys. 1982, 77, 219. (24) Rabolt, J. F.; Schlotter, N. E.; Swalen, J. D.; Santo, R. J. Polym. Sci. 1983, 21, 1. (25) (a) Aroca, R.; Jennings, C.; Kovacs, G. J.; Loutf R. 0.; Vincett, P. S. J. Phys. Chem. 1986,89,4051. (b) Kovacs, G. Loutfy, R. 0.; Vincett, P. S.; Jennings, C.; Aroca, R. Langmuir 1986,2,689. (26) (a) Uphaus, R.A.; Cotton, T. M.; Mobius, D. Thin Solid Films 1985,132,173. (b) Cotton, T. M.; Upham, R. A.; Mobiue, D. J. Phys. Chem. 1986,90,6071. (27) Bunynski, R.; Prasad, P. N.; Biegajaki, J.; Cadenhead, D. A. Macromolecules 1986,19, 1059. (28) (a) Harrand, M. J. Chem. Phys. 1986,85,2429. (b) Harrand, M.; Masson, M. J. Chem. Phys. 1987,87, 5176. (29) Rabe, J. P.; Swalen, J. D.; Rabolt, J. F. J.Chem. Phys. 1987,86, 1601. (30) Duschl, C.; Knoll, W. J. Chem. Phys. 1988,88, 4062. (31) Dierker, S. B.; Murray, C. A.; Legrange, J. D.; Schlotter, N. E. Chem. Phys. Lett. 1987,137,453. (32) Kawai, T.; Umemura, J. and Takenaka, T. Chem. Phys. Lett. 1989,162, 243.
l;
(33) A r m , P., Jr.; Aroca, R.; Kovacs, G. J.; Loutfy, R. 0. Langmuir
1990, 6, 1050. (34) Kuramoto, N.; Nataukawa, K.; Sakurai, Y.;Fujishima, M. Sub-
mitted for publication in Dyes and Pigments.
CH3
11 CH, ( n = 2 ; A Z O I) ( n = 1 8 ; Azo 1 1 ) Figure 1. Structure of 2-(4’-(ethyloctadecylamino)phenylazo)N-methylbenzothiazolium perchlorate (Azo I) and 2-(4‘-(dioctadecy1amino)phenylazo)-N-methylbenzothiazoliumperchlorate (Azo 11) (top) and their resonance structure (bottom). condensed films at this pressure. They were transferred onto CaFz plates (for infrared transmission measurements), Auevaporated glass slides (for infrared reflection-absorption (RA) measurements), or quartz plates (for resonance Raman measurements) at a dipping and raising speed of about 10 and 5 mm/min, respectively. The transfer ratio was found to be nearly unity (0.95 0.02) throughout the experiments. For the first monolayer deposition the substrates were vertically dipped into the monolayer-covered subphase, but the initial transfer occurred only for Azo I. The initial transfer for Azo I1 occurred at the first upstroke. The LB film assemblies thus prepared have Y-type structure with the first layer in the “tail-on” (Azo I) or “head-on” configuration (Azo 11). Probably, the difference in the number of the hydrocarbon chains bonded produces the difference in the configuration. Infrared spectra of the LB and cast films were recorded with 4 cm-’ resolution on a JEOL JIR-100 FT-IR spectrometer equipped with an MCT detector. Generally, several hundred scans were accumulated for acceptable signal/noise. For the RA measurements, a JEOL IR-RSC 110 reflection attachment was employed at the incidence angle of 80°, together with a JEOL IR-OPT02 polarizer. Polarized spectra were measured with the aid of the same polarizer. Infrared spectra of powdered samples were obtained by using a micro infrared attachment (JEOL IRMAU 110); for the measurements the samples were put on the Ge plate as they were. The instrumentation and sample-handling technique for recording resonance Raman spectra of LB films with a chromophore were described previously.8 Stability of the LB films of Azo I against the laser illumination (488.0 pm, 100 mW)was much lower than that of Azo 11, and therefore spectral quality of the resonance Raman spectra of Azo I was much worse than that of the resonance Raman spectra of Azo 11. Absorption spectra were recorded on a Shimadzu UV-360 spectrophotometer.
*
Results and Discussion Infrared Spectra of Azo I and Azo 11. Figure 2 shows infrared transmission spectra of Azo I and Azo I1 in solid states obtained by using a microspectroscopic technique.
LB Films of Azo I and Azo 11
Langmuir, Vol. 7, No. 11,1991 2829
Ps a
SIm
Pa
a
8m a
3600 3200
2800"1800
1600 1400
1200 loo0
800
WAVENUMBER (cm-7 Figure 2. Infrared transmission spectra of Azo I (a) and Azo I1 (b) in solid states.
Intense bands at 2923 and 2851 cm-l are assigned to CH2 antisymmetric and symmetric stretching modes of the hydrocarbon chains and a medium one at 2953 cm-l is due to their CH3 asymmetric stretching mode. Comparison of the infrared spectra in Figure 2 with those of benzene-,%*% azobenzene-,36-39 and cyanineq0 derivatives enables us to propose vibrational assignments for several bands in the 1700-800-cm-l region. Bands at 1611 and 1520 cm-l may be assigned to a +N=C stretching mode and vlg mode of the benzene ring, respectively, as in the case of resonance Raman spectra of Azo 11.8 A medium feature at 1468cm-' may be due to a CH2 scissoring mode of the hydrocarbon chains because the relative intensity of this band is much stronger in the spectrum of Azo I1 which has two hydrocarbon chains. It is well-known that azobenzene derivatives give a band due to a =N-Ph stretching mode near 1150cm-l.36-39 Therefore, a band at 1163cm-l of Azo I and one of two bands near 1165 cm-l of Azo I1 probably arise from the =N-Ph stretching mode. There is little doubt that a band near 841 cm-l is due to a CH out-ofplane deformation mode of the benzene ring.35y36 An intense band near 1095 cm-l is ascribed to C104-.41 In the resonance Raman study of LB and cast films and solutions of Azo I1we concluded that resonance structure (b)in Figure 1makes a significant contribution to the true structure of Azo I1 since the +N=C stretching frequency (1612 cm-l) of Azo I1 is appreciably lower than usual and itsvlefrequency (1519cm-1) issomewhat higher than usual. Because of the same reasons it is considered that the true structure of Azo I also contains an appreciable contribution from the resonance structure (b) in Figure 1. Figure 3A exhibits infrared transmission spectra of the 2-, 4-, and 10-monolayerAzo I LB films. The three spectra (35) Varsanyi, G . Vibrational Spectra of Benzene Direuatives; Academic Press: New York, 1969. (36)Colthup, N. B.; Daly, L. H.; Wiberley,S. E.Zntroduction tolnfrared and Raman Spectroscopy, 2nd ed.; Academic Press: New York, 1975; pp 257 and 331. (37) Takenaka, T.; Nakanaga, T. J. Phys. Chem. 1976,80,475. (38) Machida, K.; Kim, B.; Saito, Y.; Igarashi, K.; Uno, T. Bull. Chem. soc. JDn. 1974. 47. 78. (3Sj Kumar,' K.; Carey, P. R. Can. J. Chem. 1977,55, 1444. (40) Fujimoto, Y.;Katayama, N.; Ozaki, Y.; Araki, T.; and Iriyama, K. To be submitted for publication. (41) Nakamoto, K. Infrared and Raman Spectra of Inorganic and Coordination Compounds, 4th ed.; John Wiley & Sons: New York, 1986; p 251.
I
3 D
.
I
if
I
.
I
.
I
.
I
.
I
3200 2800111800 1600 1400 1200 lo00
.
m
WAVENWBEFI(Cm'') Figure 3. (A) Infrared transmission spectra of 2-, 4-,and 10monolayer LB films of Azo I. (B) Infrared RA spectrum of 10monolayer LB film of Azo I.
are very similar to each other except for band intensities which increase almost linearly with increasing the number of monolayers, n,suggesting that structure of the LB films changes little as a function of n value. Bands due to CH2 antisymmetric and symmetric stretching modes of the hydrocarbon chains are identified at 2923 and 2852 cm-l, respectively. These frequencies indicate that the hydrocarbon chains of Azo I in the LB films are not fully in an ordered structure but include conformational disorder, i.e., gauche conformers to some extent.42 A band a t 1468 cm-l is assignable to a CH2 scissoring mode of the hydrocarbon chains, which is sensitive to the intermolecular interaction and thus often used to distinguish the lateral packings of the chain^;^^.^ when the hydrocarbon chains crystallize with an orthorhombic subcell packing the band splits into two bands (- 1473and 1463 cm-l), whereas the splitting does not take place when they are packed in a hexagonal or pseudohexagonal subcell.6JlJs The appearance of the singlet scissoring band shows that the hydrocarbon chains of Azo I in the LB films are in a hexagonal or analogous subcell packing. The infrared transmission spectra of the LB films (Figure 3A) resemble that of the solid state (Figure 2a) except for relative intensities of some bands, but it should be noted that the band due to clod-observed strongly a t 1090 cm-' in the solid spectrum almost disappears in the LB film spectra. This observation indicates that most of C104- was lost during the formation of the LB films. The
-
(42) Sapper, H.; Cameron, D. G.;Mantsch, H. H. Can. J. Chem. 1981, 59, 2543. (43) Snyder, R. G. J. Mol. Spectrosc. 1961, 7, 116. (44) Tasumi, M.; Shimanouchi, T.; Miyazawa, T. J. Mol. Spectrorc. 1962, 9, 261.
2830 Langmuir, Vol. 7, No.11, 1991 C104- band was observed strongly in the infrared spectrum of Azo I cast film which was prepared from its chloroform solution. Accordingly, it seems very likely that c104-was dissolved in the aqueous subphase when the chloroform solution was placed on it. Infrared RA spectrum of the 10-monolayer Azo I LB film is shown in Figure 3B. Comparison of band intensities in the infrared transmission and RA spectra of the 10monolayer LB films enables us to discuss molecular orientation of the hydrocarbon chain and chromophore of Azo I because according to the surface selection rule in infrared RA s p e c t r o s c o p y ~ ~ ~vibrational 0 ~ l 2 ~ ~ ~ modes with their transition moments perpendicular to the surface are enhanced in a RA spectrum, while those with their transition moments parallel with the surface give strong bands in a transmission spectrum. The intensities of the bands due to the CH2 antisymmetric and symmetric stretching modes of the hydrocarbon chain, which have their transition moments perpendicular to its molecular axis, are much stronger in the RA spectrum than in the transmission spectrum (for the infrared transmission measurements 10-monolayers of Azo I were deposited on both sides of a CaFz substrate while for the RA measurement they were deposited on one side of an Au-evaporated glass slide, and therefore, the ordinate scale of the RA spectrum was doubled compared with those of the transmission spectra). This result indicates that the hydrocarbon chains are tilted considerably with respect to the surface normal. The bands due to the in-plane vibrational modes of the chromophoric part also gain intensities in the RA spectrum; particularly striking is that the band at 1609 cm-l due to the +N=C stretching mode becomes much stronger in the RA spectrum. However, it should be noted that the intensity enhancement of the 1516-cm-' band due to the v19 mode of the benzene ring is not so remarkable as that of the 1610-cm-l band while the 843-cm-l band arising from the CH outof-plane mode of the benzene ring also gets intensity in the RA spectrum. These observations suggest that the +N=C bond and therefore the benzothiazolium ring are fairly perpendicular to the substrate surface, but the benzene ring is tilted to some extent. Probably, the Azo I molecule in the LB films is twisted in the C-N=N and/or N=N-C single bonds. Since a band due to a N=N stretching mode is not seen in the infrared transmission and RA spectra, it is difficult to discuss the orientation of the N=N bond. Polarized infrared transmission spectra of the 10-monolayer LB film of Azo I measured by using polarized infrared light with the electric vectors parallel and perpendicular to the direction of the dipping showed no significant dichroism for the infrared bands in the 4000-800-~m-~ region, indicating that the hydrocarbon chains in the Azo I LB films are uniaxially oriented with respect to the surface normal. Figure 4A exhibits infrared transmission spectra of the one-, three-, and nine-monolayer Azo I1 LB films. The three spectra are again very similar to each other except for the band intensities, indicating that the structure of the LB films does not depend upon the number of monolayers. Bands due to the CH2 antisymmetric and symmetric stretching modes of the hydrocarbon chains are observed at 2918 and 2849 cm-1, respectively. These frequencies indicate that the Azo I1 LB films have highly ordered (trans-zigzag) hydrocarbon chains.42 Probably, Azo I1 with two hydrocarbon chains is more easily ordered in the LB films than Azo I with one hydrocarbon chain. A band arising from the CH2 scissoring mode of the
Katayama et al.
Figure4. (A) Infrared transmission spectra of 1-,3-, and 9-monolayer LB films of Azo 11. (B) Infrared RA spectrum of 9-monolayer LB film of Azo 11.
hydrocarbon chains is seen a t 1470 cm-', suggesting that the hydrocarbon chains of Azo I1 in the LB films are in a hexagonal or analogous subcell packing. The band due to C104- is very weak in the LB film spectra, again indicating that most of the c104- is removed in the course of LB film preparation. Figure 4B presents infrared RA spectrum of the ninemonolayer Azo I1 LB film. Particularly notable in comparison between the infrared transmission and RA spectra of the nine-monolayer Azo I1 LB films is that the bands due to in-plane vibrational modes of the chromophore, in general, gain intensities greatly in the RA spectrum (compare the relative intensity of two bands at 2918 and 1610 cm-l between the transmission and RA spectra) but the 1516-cm-l band from the benzene ring does not show intensity enhancement and the 843-cm-l band due to the CH out-of-plane mode of the benzene ring exhibits it significantly. These observations suggest that as in the case of Azo I the benzothiazolium ring of Azo I1 is fairly perpendicular to the surface in the LB films but its benzene ring is tilted considerably. The intensities of the bands due to the CH2 antisymmetric and symmetric stretching modes do not show a remarkable change between the two spectra, indicating that the chains are neither perpendicular nor parallel to the surface, being in an intermediate direction. The measurements of infrared polarized transmission spectra of the nine-monolayer Azo I1LB film showed that the hydrocarbon chains in the LB film take a uniaxial orientation with respect to the surface normal. Visible Absorption Spectra of Azo I. It is very important to study visible absorption spectra to know
LB Films
of Azo I
Langmuir, Vol. 7, No. 11, 1991 2831
and Azo II
-.-.-._.-._
, 400
800 Wavelength/ nm
I
700
500
Figure 5. Visible absorption spectra of 10-monolayer LB (a) and cast (b) films and acetone solution (c, 1 x lod M)of Azo 1. fl 11 1
I
I
I
1600
1400
1200
lo00
RAMAN SHIFT (an.')
Figure 7. 514.5-nm excited resonance Raman spectra of 25monolayer LB (a) and cast (b) films of Azo I.
1800
1600
1400
1200
lo00
RAMAN SHIFT (an.')
Figure 6. 514.5-nm excited resonance Raman spectra of Azo I in acetone (a), chloroform (b), and methylene chloride (c) solutions: concentration, 1 x 1V M. aggregation states of dye molecules in the LB films and to analyze their resonance Raman spectra. Absorption spectra of Azo I1 in the LB and cast films and solutions were reported in our previous paper.* Figure 5 compares the absorption spectra of Azo I in the 10-monolayer LB (a) and cast films (b) and the acetone solution (c). The spectrum of Azo I solution having an absorption maximum a t 598 nm with a shoulder near 564 nm does not show concentration dependency in the 1 X lo4 to 5 X 104 M range. Therefore, the 564-nm band may be ascribed to a vibronic component. However, an absorption maximum near 560 nm of Azo I LB film and that near 550 nm of its cast film are probably not due to the vibronic component
but assignable to the dimer because the separation between the 560- and 624-nm bands of the LB film spectrum and that between the 550- and 616-nm bands of the cast film are larger than 1800 cm-l. The cast film spectrum gives the third absorption maximum at 480 nm, probably due to H The absorption spectrum of the Azo I LB film is largely different from that of Azo 11; the latter gives an intense band due to H aggregate but the former does not show a band assignable to it (Figure 5a). It is particularly notable that Azo I1 forms H aggregate extensively in the LB films but Azo I does not form it. There is little doubt that the difference in the number of the hydrocarbons in the chain causes a crucial change of the aggregation state in the LB films. The absorption spectrum of Azo I cast film is also clearly different from that of Azo I1 cast film. The former shows a band a t 480 nm ascribed to H aggregate while the latter provides it a t 446 nm, indicating that different kipds of H aggregates are formed in the Azo I and Azo I1 cast films. Resonance Raman Spectra of Azo I. Figure 6 presents the 514.5-nm excited resonance Raman spectra of Azo I in acetone (a), chloroform (b), and methylene chloride (c) solutions. The 514.5-nm excitation is closer to the dimer absorption of Azo I solutions and therefore yields predominantly resonance Raman spectra of the dimer. The spectra of the three solutions are very close to each other. This result indicates that the contribution of resonance structure alters little with solvents irrespective of the difference of their polarities and the interactions between the dye and solvents are very weak. As in the case of resonance Raman spectra of Azo I1 solutions: bands at 1612, 1520, and 1463 cm-' can be assigned to a +N=C stretching mode, u19 mode of the benzene ring, and N=N stretching mode, respectively. A band a t 1160 cm-', which seems to correspond to the infrared band at 1163 cm-' of Azo I in the solid state, may (45) Kawai, T.; Umemura, J.; Takenaka, T. Langmuir 1989,5,1378. (46) Sturmer, D. M.; Heseltine, D. W. The Theory of Photographic Process; James, T. H., Ed.; Macmillan Publishing: New York, 1977; p 194.
2832 Langmuir, Vol. 7, No.11, 1991 be due to a =N-Ph stretching mode. The appearances of N-N and =N-Ph stretching modes at 1463and 1160 cm-l, respectively, show that the Azo I dimer assumes a trans conformation around the N=N bond in the solutions.3"39 The 514.5-nm excited resonance Raman spectra of the 25-monolayer LB (a) and cast films (b) of Azo I are shown in Figure 7. The 514.5-nm excitation provides chiefly resonance Raman spectrum of the dimer for the LB film because it is close to ita dimer absorption maximum (Figure 5a) while the same excitation enhances both resonance Raman spectrum of the dimer and that of H aggregate for the cast film. The 488.0-nm excited resonance Raman spectrum of the cast film, which mainly consists of the spectrum of the H aggregate, is almost identical with that of the 514.5-nm excited resonance Raman spectrum of the cast film. Therefore, it seems that the resonance Raman spectra of the dimer and H aggregate of the cast film are nearly the same. Vibrational frequencies of bands due to the +N=C stretching mode, vlg-like mode of the benzene ring, N=N stretching mode, and =N-Ph stretching mode change little among the spectra of the LB and cast films and solutions. This result leads us to consider that at least as far as the dimer is concerned, the conformation and electronic structure of Azo I do not change appreciably among the LB and cast films and solutions. In this regard we obtained a different conclusion for Azo I1 H aggregate;s
Katayama et al.
conformation around the -N=N- bond alters slightly between H aggregates in the Azo I1 LB and cast films.
Conclusions This paper demonstrates that combined infrared, visible absorption, and resonance Raman studies are very useful to investigate the molecular aggregation, orientation, and structure in LB films. It was found that Azo I and Azo 11, which have the same chromophoric part but are different from each other in the number of the hydrocarbon chains bonded, give similar but significantly different aggregation, orientation, and structure when the LB films are formed; for example, Azo I does not form an H aggregate in the LB films while Azo I1 forms it as previously investigated; the hydrocarbon chains of Azo I1 have highly ordered (trans-zigzag)structure but that of Azo I is slightly less ordered in the films; the conformation and structure of the Azo I chromophore change little among the LB and cast films and solutions while those of Azo I1 alter significantlybetween the LB films and the latter two states. This sort of study may provide useful information to design new types of functional LB films. Acknowledgment. We are grateful to JEOL, Ltd., for allowing us to test-use the micro infrared attachment (JEOLIR-MAU 110). Registry No. Azo I, 136568-95-1; Azo 11, 136537-76-3; CaF2, 1189-75-5.
