Substrate dependency of the structure of Langmuir-Blodgett films of

Substrate dependency of the structure of Langmuir-Blodgett films of thia tri- and heptamethine cyanine derivatives studied by infrared and resonance R...
0 downloads 0 Views 617KB Size
Download PDF
2758

Langmuir 1992,8, 2758-2763

Substrate Dependency of the Structure of Langmuir-Blodgett Films of Thia Tri- and Heptamethine Cyanine Derivatives Studied by Infrared and Resonance Raman Spectroscopies Norihisa Katayama, Yasuhiko Fujimoto, and Yukihiro Ozaki' Department of Chemistry, Kwansei Gakuin University, Uegahara, Nishinomiya 662, Japan

Toshinari Araki Frontier Technology Research Institute, Tokyo Gas Co., Ltd., Suehiro-cho, Tsurumi-ku, Yokohama 230, Japan

Keiji Iriyama Department of Biochemistry, Institute of Medical Science, The Jikei University School of Medicine, Nishi-Shinbashi, Minato-ku, Tokyo 105, Japan Received January 17,1992. In Final Form: August 31,1992 Substrate dependency of the structure of LangmeBlodgett (LB) films of thia tri- and heptamethine benzothiazolium cyanine derivatives (3-odadecyl-2-[3-~3-octedecyl-2-benzothiazolinylidene~-l-propenyll iodide (NK-2560)and 3-odadecyl-2-[7-(3-octadecyl-2-be~~~ylidene)-1,3,5heptatrienyl]benzothiazolium perchlorate (NK-2861)) has been studied by infrared and resonance Raman spectroscopies. Frequencies of infrared bands due to CH2 antisymmetric and symmetric stretching modes suggest that the hydrocarbon chains of NK-2560 are highly ordered (trans-zigzag) in the LB f i deposited on CaFz plates but they are less ordered in the f h prepared on Au-evaporated glass slides. Simii substrate dependency of the structure of the hydrocarbon chains is found for the LB films of NK-2861. Infrared spectra of two types of NK-2560 LB films deposited on CaFz plates with "head-on" and "tail-on" configurations are almost identical with each other irrespective of the number of monolayers. This result is contrasted with that previously obtained for the LB f i prepared on Ge substrates. It seems that both the interaction between the substrate and the hydrocarbon chains and that between the substrate and the chromophoric part are very weak in the LB f i on the CaFz substrates. Comparisons of infrared transmission and RA spectra of the LB f i of NK-2560 and NK-2861 indicate that both the hydrocarbon chains and central conjugated systems of the cyanines are neither perpendicular nor parallel to the substrate surface, being in an intermediate direction.

Introduction In recent years Langmuir-Blodgett (LB)f i i of cyanine dyes have been a matter of intensive experimental investigations because of their potential applications such as those for photovoltaic cells, optical information storage, and photoinduced electron transfer.lP2 In order to understand interesting properties which cyanine LB films show and to design new LB f i e based on cyanine dyes with planned properties, it is very important to study the structure-function relationship of cyanine LB f h s . However, structural studies of cyanine LB f i i have fallen far behind to the studies of their physical properties. Therefore, we have started the investigations of structural characterization of LB films of cyanine dyes by use of infrared and resonance Raman spectroscopies,3c which are both very powerful nondestructive techniques for exploring molecular aggregation, orientation, and molecular and electronic structure in LB films.633

* Author to whom all correspondence should be a d d r e d .

(1) Roberta, G. G. In Langmuir-Blodgett Films; Roberta, G. G., Ed.; Plenum: New York, 1990, p 317. (2) uknan,A. Anlntroduction to Ultrathin Organic Films;Academic Press: San Diego, CA, 1991; p 339. (3) Katayama, N.; Ozaki, Y.; Araki, T.; Iriyama. K. J. Mol. Struct. 1991,242,27. (4) Katayama, N.; Fukui, M.; Ozaki, Y.; Araki, T.; Yokoi, S.;Iriyama, K. SPIE-A.OC.1991,1403,147. (6) Takenaka. T.; Umemura,J. In Vibrational SDectra and Structure: ' Durig, J. R., Ed;;Eleevier: Amsterdam, 1991.

In our first report of a series of vibrational spectroscopic studies on cyanine LB films,we obtained the following conclusion^:^ (i) The hydrocarbon chains of 3-odadecyl2- [3-(3-octadecyl-2-benzothiazolinylidene)-l-propenyllbenzothiazolium iodide (Figure 1,NK-2560)have highly ordered (trans-zigzag) structure in ita Y-type LB f i deposited on Ge substrates with "head-on" configuration irrespective of the number of monolayers, n, while the structure of the hydrocarbon chains depends upon the number of monolayers in the Y-type filmsprepared on Ge substrates with "tail-on" configuration; in the latter f i i (6) Greenler, R. G. J. Chem. Phys. 1966,44, 310. (7) Chollet, P.-A.; Meeeienn, J.; Rosilio, C. J. Chem. Phys. 1976,64, 1042. (8)Koyama, Y.; Yanagiahita, M.; Toda, S.; Matauo, T. J . Colloid Interface Sci. 1977,61,438. (9) Chollet, P.-A. Thin Solid Films 1978,62, 943. (10) Allara, D. L.; Swalen, J. D. J. Phys. Chem. 1982,86, 2700. (11) Rabolt, J. F.; Burns,F. C.; Schlotter, N. E.;Swalen, J. D. J.Chem. phy8. 1983, 78, 948. (12) (a) Maw, R.; Sagiv, J. J. Colloid Interface Sci. 1984,100,466. (b) Gun,J.; Iscovici, R.; Sagiv, J. J. Colloid Interface Sci. 1984, 101, 201. (13) Allara, D. L.; Nuzzo, R. G. Langmuir 1985, I , 62. (14) Kimura, F.; Umemura, J.; Takenaka, T. Langmuir 1986, 2, 96. (16) Umemura,J.; Knmata, T.; Kawai,T.; Takenaka,T. J. Phy8. Chem. 1990,94,62. (16) Kubota, M.; O d i , Y.; Araki, T.; Ohki, 5.;Iriyama, K. Langmuir 1991, 7, 774. (17)Katayama, N.; Fukui, M.; Ozaki,Y.; Kuramoto, N.; Araki, T.; Iriyama, K. Langmuir 1991, 7, 2827. (18) Fujimoto,Y.; Katayama, N.; Ozaki, Y.; Araki, T.; Iriyama, K. Thin Solid Films 1992,210/211, 597.

0 1992 American Chemical Society

Substrate Dependency of the Structure of LB Films

Langmuir, Vol. 8, No.11, 1992 2769

examinations,accordingto the same chromatographicconditions as used in our previousreport,%revealedthatthe dyepreparations did not contain any other colored components. Their lH NMR spectra in methylenadz chloride ale0 showed that they did not contain any detectable impurity.86 Chloroform of spectroscopic gradewas purchased from Dojin ChemicalCo. Ltd. (Kumamoto, Japan) and distilled just before use. The LB f i were prepared on a Kyowa Kaimen Kagaku Model HBM-APLangmuirtroughwith a Wilhelmybalance. The dyes dissolved in chloroform (2 x 1od M) were spread onto an aqueoussubphaseof doublydistilledwater (pH5.8,20 OC). After evaporation of the solvent, the monolayer was compressed at a constant rate of 76 cmVmin up to the surface pressure of 20 mN m-l. The P A isotherm showed that the monolayere were solid Figure 1. Structure of I-octadecyl-2-[3-(3-octadecyl-Z-ben- condensed f i i at this pressure. The f i i transfer onto a zothiazolinylidene)-l-propenyl]benzothiazolium iodide (NKsubstrate was carried out at 10 mm/min by the vertical dipping 2560) and 3-octadecyl-2-17-(3-octedecyl-2-be~~~ylidene)- method. The substrates employed were CaFzplates (for infrared 1,3,5-heptatrienyl]benzothiazoliumperchlorate (NK-2861). In transmiasion and resonance Raman measurements) and Authis paper the conformation shown here is termed conformation evaporatedglassslides (forRA measurements). The preparation A. of the Au-evaporatedglass slides was described elsewhere.16 The cleaning procedure of the CaFz plates and Au-evaporated glass the hydrocarbon chains in the first monolayer have slides was successive ultrasonifications in chloroform, acetone, considerable amounta of gauche conformers but they are water, 6% aqueous solution of DCN 90 of Decon Laboratories ordered in and above the second monolayer. (ii) NK-2560 Ltd. (thealkalinesurfactant),water,acetone,and then chloroform for 5 min each. molecules assume conformation A (Figure 1) in the solid The transfer ratio was found to be 0.95 f 0.05 throughout the state, chloroform solution, and LB films; in conformation experiments. T w o types of the LB f i i with head-to-head (tailA the methine chain takes an extended all-trans form, the to-tail) structure were prepared according to the method preS atoms are cis about the chain, and the CC and CN bonds viouslyreported;3in this paper the head-to-head structures with in their central conjugated system have single and a half the f i t layer in the 'head-on" and 'tail-on" confiiations are bond character (CXCand CITNhM termed Y- and Y'-type, respectively. The purpose of the present paper is to provide further Infrared spectra of the LB f i b were measured with a JEOL insight into the structure of LB filmsof NK-2660; substrate JIR-100FT-IR spectrometerequippedwith a MCT detector. All dependency of the structure of the LB films and ita data were collected at a spectralresolution of 4 cm-' and generally dependency upon the number of monolayers are studied severalhundred scanswere accumulated to ensure an acceptable signal-to-noiee ratio. For the RA measurements a JEOL IRby use of infrared and resonance Raman spectroscopies. RSC 110 reflection attachment was employed at the incidence The structure of LB films of 3-octadecyl-2-[7-(3-oc~decylangle of 80°, together with a JEOL IR-OPT02 polarizer. 2-benzothiazolinylidene)-1,3,5-heptatrienyll bemothim The Raman syetem employedconsistsof a triplepolychmator lium perchlorate (Figure 1;NK-2861)which has a larger (Spex 1877~1,an intensified photodiode array detector (PAR chromophoric part than NK-2560is also explored for 1466R-HQ), and a personal computer (NEC PC98O1). The comparison purposes. A comparisonbetween the LB films detector was used at -20 OC. The 488.0" line from an At laser of NK-2560and NK-2861may offer useful information to (Spectra Physics 2016-06) was used for excitation. The laser construct new LB filmsbased upon a cyaninechromophore. power at the sample position was about 30 mW, and an 8-cm-l spectral slit width was employed. Frequency calibration was Experimental Section carried out by measuring Raman spectra of acetone and ethyl acetate, and estimated frequency errors were f 2 cm-' for wellThe cyanine dyes (NK-2560 and NK-2861) employed in the resolved bands. presentstudyweregenerouslyprovidedbyDr. S. Yasui (Japanese NK-2560gradually decomposed even with the illumination of Research Institute for PhotosensitizingDyes Co., La.) and used a loosely focused lase.r beam. Therefore, each sample point of without further purification. The thin-layer chromatographic the LB f i e was illuminated by the laser beam only for 6 s, and to raise the signal-to-noise ratio of the spectrum, the resonance (19)Fukui, M.; Katayama, N.; Ozaki, Y.;Arnki,T.; Iriyama, K. Chem. Raman spectra were measured for 20 sampling points of each Phys. Lett. 1991, 177,247. f i i and the obtained spectra were then coadded. For each (20)Rabolt, J. F.;Santo, R.; Swalen, J. D. Appl. Spectrosc. 1979,33, .ua _". measurement, the exposure time on the diode array was 1s and (21)Knoll, W.;Philpott, M. R.; Golden, W. G. J. Chem. Phys. 1982, the integration time was five. 77,219. Absorption spectra were recorded on a Shimadzu W-360 (22)Rabolt, J. F.;Schlotter, N. E.; Swalen, J. D.; Snnto, R. J.Polym. spectrophotometer. Sci. l98& 21,1. (23)Kovacs, G. J.; Loutfy, R. 0.; Vincett, P. S.; Jenninge, C.; Arm,

R.h n g m u i r 1986,2,689.

(24)Cotton, T. M.; Uphnus, R. A.; Mobiue, D. J. Phys. Chem. 1986, 90,6071. (25) Bunyneki, R.;Prnand, P. N.; Biegajski, J.; Cadenhead, D. A. Macromolecules 1986,19,1059. (26)Harrand, M.; Maeeon, M. J. Chem. Phys. 1987,87,6176. (27)Rabe, J. P.;Swalen, J. D.; Rnbolt, J. F.J. Chem. PhyS. 1987,86, 1601.

(28)Duechl, C.; Knoll, W. J. Chem. Phys. 1988,88,4062. (29)Dierker, S. B.;Murray, C. A.; Legrange, J. D.; Schlotter, N. E. Chem. Phys. Lett. 1987,137,453. (30)Ozaki, Y.; Iriynmn,T.; I w h h i , T . ; Hamnguchi,H. Appl. Surface Sci. 1988,33134,1317. (31)Kawai, T.; Umemura, J.; Takennkn, T. Chem. Phys. Lett. 1989, 162,243. (32)Aroca, P.,Jr.; Aroka, R.; Kovacs, G. J.; Loutfy, R. 0. Langmuir 1990,6,10W. (33)Katayama, N.;Ozaki, Y.;Kurnmoto, N. Chem. Phys. Lett. 1991, 179,227. (34)Wheatley, P. J. J. Chem. SOC.1959,3246.

Results and Discussion Infrared Spectra of the LB Films of NK-2860. Figure 2A shows infrared transmission spectra of (a) one-, (b) two-, (c) three-, and (d) six-monolayerY-type LB films of NK-2560deposited on the CaF2 plates. Note that even one monolayer film gives high-quality infrared spectrum. The four spectra in Figure 2A are very close to each other except for band intensities which increase almost linearly with the number of monolayers, n. This suggesta that the structure and molecular orientation of NK-2660depend (36)Iriyama, K.; Shiraki, K.; Tauda, K.; Oknda, A.; Sugi, M.; Irjima, S.; Kudo, K.; Shioknwa,5.;Moriizumi, T.; Yaeuda, T. Jpn. J.Appl. Phys. Suppl. 1980,19,173. (36)Katayama, N.;Oznki, Y.; Yaeui, S.; Iriyama, K. J. Mol. Stnrct., in prees.

Katayama et al.

2760 Langmuir, Vol. 8,No.11,1992 3

a

LJ l

l

l

.

I

.

I

.

I

.

,

l

'

. ~~~

41

~

~

~~

1 3600 3200 2800 2400 2000 1600 1200

800

WAVENUMBER (cm") WAVENUMBER(C~") Figure 2. (A) Infrared transmission spectra of (a) one-, (b) two-, (e) three-, and (d) six-monolayer Y-type LB films of NK-2560 deposited on CaFl plates. (B) Corresponding spectra of Y'-type LB fiis.

little upon the number of monolayers and interaction between the substrate and the first monolayer is very weak. T w o intense bands at 2918 and 2850 cm-' are assigned to CH2 antisymmetric and symmetric stretching modes of the hydrocarbon chains of NK-2560,respectively, and a weak feature at 2956 cm-' is attributed to an asymmetric stretching mode of their terminal CH3 groups. Vibrational frequenciesof the CH2 stretching bands are very sensitive to the conformation of hydrocarbon chain;37the bands appear near 2918 and 2950 cm-', respectively, when the hydrocarbon chains take trans-zigzag conformation, while they shift upward up to near 2928and 2856cm-' depending upon the content of gauche conformers when conformational disorder is included in the hydrocarbon chains. The above frequencies (2918 and 2850 cm-') of the CH2 stretching bands indicate that NK-2560molecules in the LB films have highly ordered (trans-zigzag) hydrocarbon chains. Three intense bands observed at 1553,1463,and 1421 cm-' may be assigned to stretching modes of the central conjugated system of NK-2560,although the detailed ~ * ~above ~ feature of vibrational modes is u n k n o ~ n . The frequencies are close to those of NK-2560in the solid, in chloroform solution, and in LB films deposited on Ge substrates. This observation suggests that the central conjugated system of NK-2560 in the LB films on the CaF2 substrates takes conformation A (Figure 1) as in the cases of the solid, solution, and LB films on the Ge substrates. Figure 2B exhibits infrared transmission spectra of (a) one-, (b) two-, (c) three-, and (d) six-monolayer Y'-type (37) Sapper, ~. H.; Cameron, D. G.; Mantach, H. H. Can. J. Chem. 1981, 59, 2543. (38) Fujimoto, Y.; Katayama, N.; Ozaki, Y.; Araki, T.; Iriyama, K.

Submitted for publication.

LB films of NK-2560 deposited on CaF2 plates. The spectra in Figure 2B are very close to correspondingspectra in Figure 2A in terms of both the vibrational frequencies and band intensities. This suggests that the structure of LB films alters little upon going from Y- to Y'-type structure. In other words, the CaF2 substrate accepts both head-on and tail-on configuration of the first monolayer. This result is markedly different from that obtained for the NK-2560LB films deposited on the Ge substrate^;^ NK-2560 molecules have highly ordered hydrocarbon chains in the Y-type LB films on the Ge substrates while they have much less ordered hydrocarbon chains in the first monolayer of the Y'-type films on the Ge substrates. Therefore, it seems that the nature of the surface of a substrate is very important to control the structure of hydrocarbon chains. The difference in the nature of the surface between the CaF2 and Ge substrates is reflected in the transfer ratios of the first monolayers of the Y'-type films; it is high for the CaFz substrates (-0.95) but very low for the Ge substrates (~0.601.~ Figure 3 compares infrared RA (a) and transmission (b) spectra of six-monolayer Y-type LB films of NK-2660. The substrates used were an Au-evaporated glass slide (RA spectrum) and CaF2 plate (transmission spectrum), respectively. Particularly striking is that frequencies of bands due to the CH2 antisymmetric and symmetric stretching modes are significantly higher in the RA spectrum than in the transmission spectrum. The CH2 antisymmetric s t r e w band, whose frequencyis thought to be more sensitive to the structure of the hydrocarbon chains than that of the CH2 symmetric stretching band:' is observed at 2918,2920,and 2924 cm-l in the infrared spectra of the NK-2560LB films deposited on the CaF2, Ge,3 and Au-evaporated substrates, respectively. This observationindicatesthat the structure of the hydrocarbon

Substrate Dependency of the Structure of LB Films

Langmuir, Vol. 8, No. 11, 1992 2761

I I

2000

lo00

HkAVEE(UMBER(cm")

Figure 3. Infrared RA (a) and transmission (b) spectra of sixmonolayer Y-type LB filmsof NK-2560 (a)Au-evaporated glass slide; (b) CaFa plate.

.

,

,

,

4000 3600 3200 2800 2400 2000 I600 1200

WAVE"m") Figure4. Infrared transmission spectra of (a) one-, (b)two-, (c) three-, and (d) six-monolayer Y-type LB f i b of NK-2861 deposited on CaFz plates.

of monolayers, indicating that structure of NK-2861 LB films does not depend upon the number of monolayers n. chains depends upon the substrates used; it seems that Vibrationalfrequenciesof bands due to CHZantisymmetric the chains in the LB filmsdeposited on the Au-evaporated and symmetric stretching modes of the hydrocarbon chains glass slides have considerable amountsof gauche rotamera. of the NK-2861 LB filmsare slightly higher than those of Higher frequency shifte of the CH2 antisymmetric and the NK-2560 LB films. This result suggests that the NKsymmetric stretching bands upon going from a CaF2 plate 2861 LB films have slightly less ordered hydrocarbon to an Au-evaporated glass slide were observed for LB films chains. Probably, the large chromophore of NK-2861 with Yhead-on"configuration of 3,3-dimethyl-2-[3-(3,3makes the structure of the hydrocarbon chains less ordered. dimethyl-l-octadecyl-2-indolinylidene)-l-propenyll -laThe band frequencies and intensities of infrared transtadecyl-3i-indolium iodide (NK-2665)18 and tetrakismission spectra of the one-, two-, three-, and six-monolayer (dimethy1dioctadecyla"onium)nickel phthalocyanineY'-type LB films of NK-2861 on the CaF2 plates are also tetrasulfonate (Pc4)19but not observed for thoseof 2-octavery close to those of the spectra of the corresponding decy1-7,7,8,8-tetracyanoq~inodimethane~~ and 2-(4'-di~ d ~ ~ o ) p h e n y l a z o > - N - m e t h y l b e nperz o t Y-type LB films, indicating that the structures of the Y and Y'-type LB films of NK-2861 are almost identical to ~hl0rate.I~It is therefore likely that the interaction each other. between an Au-evaporated glass slide and a chromophoric Bands at 1507,1468,1458,and 1416 cm-'may be assigned part plays an important role in determining the structure to stretching modes of the central conjugated system of of the hydrocarbon chains. NK-2861. The frequencies and intensities of these four Although the structure of the hydrocarbon chains of bands are almost identical with those of the corresponding NK-2560 alters slightly between the LB f i s deposited bands of NK-2861 in solid and solutions.98 Therefore, it on the CaF2 plate and Au-evaporated glass slide, it may seemsthat NK-2861in the LB f h assumes conformation be possible to discuss roughly molecular orientation in A (Figure 1) as in the cases of solid and solutions. the LB filmson the basis of a comparison of the intensities T w o intense bands near 1125and 1090cm-' may be due of bands in the RA and transmission spectra (Figure 3). to a ClodThe intensities of the bands due to CH2 antisymmetric Infrared RA and transmission spectra of six-monolayer and symmetric stretching modes and those of the bands Y-type LB films of NK-2861 are shown in parta a and b assignable to the stretchingmodes of the central conjugated of Figure 5, respectively. Again, infrared frequencies of system do not change largely upon going from the RA bands due to the CHz antisymmetric and symmetric spectrum to the transmission spectrum. According to the surface selection rule in infrared RA spe~troscopy,8~~*~J5stretching modes are higher in the RA spectrum than in the transmission spectrum, suggesting that the Structure this observation indicates that both the hydrocarbon of the hydrocarbon chains depends upon the substrates. chaina and the central conjugated system are neither The intensities of the bands due to CH2 antisymmetric perpendicular nor parallel to the surface, being tilted and symmetric Stretching modes and those of the bands considerably from the surface normal. ascribedto the stretching modes of the central conjugated Infrared Spectra of the LB Films of NK-2861. system do not change dramatically between the two spectra Figure 4 displays infrared transmission spectra of (a) one-, (Figure 5). Accordingly, it seems that both the hydro(b) two-, (c) three-, and (d) six-monolayer Y-type LB films of NK-2861depoeited on the CaF2 plates. The four spectra (39)Nakamoto, K.Infrared and Raman Spectra of Inorganic and are again very similar to each other expect for band Coordination Compounds, 4th 4. John ; Wiley & Sone: New York, 1985; intensities which increase almost linearlywith the number p 261.

Katayama et

2762 Langmuir, Vol. 8, No. 11,1992

3600

3OOO

2000

41.

lo00

WAVENUIHBER(om-') Figure 5. Infrared RA (a) and transmission (b) spectra of sixmonolayerY-typeLB films of NK-2861: (a)Au-evaporatedglass slide; (b) CaF2 plate.

c I

D

ls00

I

I

1400

1200

1( Do

RAMAN SHIFT (an.')

Figure 7. The 488.0-nm excited resonance Raman spectra of NK-2560 in a chloroform solution (a),KBr disk (b),and cast film on a Ge substrate (c).

3

Figure 6. The 488.0-nmexcitedresonance Raman spectra of (a) one-, (b) two-, and (c)three-monolayerY-type NK-2560 LB f i b s deposited on CaFz plates.

carbon chains and the central conjugated system are considerably tilted from the surface normal. Resonance Raman Spectra of the LB Films of NK2560. Figure 6 shows the 488.0-nm excited resonance Raman spectra of (a) one-, (b) two-, and (c) threemonolayer Y-type LB films of NK-2560 deposited on CaF2 substrates. Note that even a one monolayer LB film gives a spectrum with acceptable signal-to-noise ratio. The

spectra in Figure 6 are very similarto each other, indicating that the structure of the chromophoric part of NK-2560 in the LB f i does not depend upon the number of monolayers and the interaction between the chromophoric part and substrate surface is very weak. This conclusion is in good agreement with that obtained from the infrared transmieeion spectra in Figure 2. Raman bands a t 1667 and 1465 cm-' are assignable to stretching modes of the central conjugated system of NK-2560.'~~ However, as discussed elsewhere these bands do not correspond to the infrared bands at 1653 and 1464 cm-' in the spectra of Figure 2.% The 1557-and 1465-cm-' bands in the Raman spectramaybe totdysymmetricstretchingmodeebecause the corresponding bands in the spectrum of NK-2560 in a chloroform solution were polarized.ae Figure 7 shows resonance Raman spectra of NK-2560 in a chloroform solution (a), KBr disk (b), and cast film on a Ge substrate (c). Frequencies of Raman bands in the LB film spectra are almost identicalto those in the solution, KBr disk, and cast film spectra. Thie result suggesta that the molecular and electronic etructure in the ground state of the chromophoricpart of NK-2660 are very close among the LB f i , solution, KBr disk, and cast film. However, it must be noted that the relative intensity of the band at 1556 cm-' is much stronger in the LB film spectra than in the others. This marked intensity enhancement of the band at 1556 cm-1 d m not depend upon the incidence angle of laser beam,indicating that the molecular orientation doea not refer to this phenomenon. We reported visible absorption spectra of NK-2560 in KBr disk and LB f i h 3 The absorption bands near 580 and 535 nm were ascribed to a monomer and dimer of

Substrate Dependency of the Structure of LB F i l m NK-2560, respectively. In addition, the spectrum of LB This shoulder may be due to H-aggregates of NK-2560. The absorption spectra of a KBr disk, chloroform solution, and cast film do not show such a shoulder near 500 nm. Since the excitation wavelength (488.0 nm) of the resonance Raman measurements of the LB films is very close to the wavelength of the shoulder, the resonance Raman spectra of the LB films should reflect largely the electronic and molecular structure of H-aggregates. Therefore, the H-aggregation in the LB filmsis probably responsible for the intensity enhancement of the band at 1556 cm-'.

film had a shoulder around 500

Conclusion The present study provides new insight into the substrate dependency of the structure of LB films of functionaldyes. The following conclusions can be reached (i) The structure of the hydrocarbon chains in the LB and cast films of NK-2560 and NK-2861 depends upon

Langmuir, Vol. 8, No. 11, 1992 2?63 the substrates used; they are more ordered in the CaF2 plate% than in the Au-evaporated glass slides. (ii) In contrast to Ge substrates, the structures of the Y- and Y'-type LB filmsof NK-2560 deposited on the CaFz plates are almost identical to each other. (iii) The hydrocarbon chains of NK-2560 are more ordered than those of NK-2861 in the LB films. It seems that the larger chromophoric group of NK-2861 makes the structure of the hydrocarbon chains less ordered. (iv) Both the hydrocarbon chains and the central conjugated systems of the dyes are neither perpendicular nor parallel to the substrate surface, being tilted considerably in the LB films of NK-2560 and NK-2861. Acknowledgment. The authors express their thanks to Professor K. Nakatsu (KwanseiGakuin University) and Professor J. Umemura (Kyoto University) for useful discussions. They are also grateful to Dr. S. Yasui (JapaneseResearch Institute for PhotosensitizingDye Co., Ltd.) for his donation of the dyes.