Langmuir 1995,11, 2195-2200
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Molecular Aggregation, Orientation, and Structure in Langmuir-Blodgett Films of 2,4-Bis[(3,3-dimethyl-l-octadecyl-2,3-dihydro-2-indolylidene)methyl]-l,3-cyclobutadienediylium-1,3-diolate Studied by Visible Absorption and Infrared Spectroscopies Nobuhiro Kuramoto" Organic Materials Division, Osaka Prefectural Industrial Technology Research Institute, Enokojima, Nishi-ku, Osaka 550, Japan
Sinko Enomotot and Yukihiro Ozaki Department of Chemistry, Kwansei Gakuin University, Uegahara, Nishinomiya 662, Japan Received December 22, 1994. I n Final Form: March 20, 1995@ Molecular aggregation, orientaiton, and structure in Langmuir-Blodgett (LB) films of a squarylium dye, 2,4-bis[(3,3-dimethyl-l-octadecyl-2,3-dihydro-2-indolylidene)methyll-1,3-cyclobutadienediylium-1,3diolate (SQ-VI),have been investigated by using visible absorption and infrared (IR) spectroscopies. The visible absorption spectra show that SQ-VI forms H-aggregates as well as monomer in the LB films and that the proportion of the H-aggregates increases significantly with surface pressure. IR spectra of the LB films of SQ-VI are largely different from those of SQ-VI in solid and cast film. The former show bands at 1718 and 1660 cm-' assignable to C=O and C=N+ stretching modes, respectively, while the latter do not give such bands, indicating that SQ-VI takes the structure shown in Figure 1,parts b and a, respectively, in the LB films and the solid and cast film. Vibrational frequencies of CH2 stretching bands are also different between the former and the latter; it is suggested from the frequencies that the hydrocarbon chains of SQ-VI take a nearly trans-zigzag conformation in the solid and cast film, but a few parts of the chains assume a gauche conformation in the LB films. Comparison of the IR transmission and reflectionabsorption (RA)spectra of the LB films of SQ-VIsuggests that the chromophoric part is fairly perpendicular to the substrate surface and that both the hydrocarbon chains and the molecular axis of the chromophore are tilted considerably with respect to the substrate normal. The molecular orientation and structure of the LB films of SQ-VI are also discussed in comparison with those of the LB films of another type of squaryliumdye, 2,4-bis[4-(methyloctadecylamino)-2-methylphenyl]cyclobutadienediylium-1,3-diolate (SQ11), which we previously investigated.
Introduction Langmuir-Blodgett (LB)films of squarylium (SQ)dyes have been a matter of keen interest because of their potential applications such as those for photovoltaic cells, optical storage, and gas In order to understand interesting properties shown by SQ LB films and to design new SQ LB films with more desirable properties, it is very important to explore the structure-function relationship of SQ LB films. Especially, studies on molecular aggregation, orientation, and structure of the chromophoric part and hydrocarbon chains are desirable because, in general, the functional properties are linked closely with them. We therefore have undertaken structural studies of the LB films with SQ chromophores by use of Fourier-
* To whom correspondence should be addressed. FAX: f 8 1 6-443-3137. + Present address: Shimadzu Co. Ltd. Abstract published in Advance ACS Abstracts, J u n e 1, 1995. (1)Roberts, G. G., Ed. Lungmuir-BlodgettFilms;Plenum: New York, @
1990. (2) Ulman, A. Introduction to Ultrathin Organic Films; Academic Press: San Diego, 1991; p 133. (3) Kawabata,Y.; Sekiguchi,T.; Tanaka, M.; Nakamura, T.; Komizu, H.; Matsumoto, M.; manda, E. Thin Solid Films 1985,133, 175. (4)Tanaka, M.; Sekiguchi,T.; Matsumoto, M.; Nakamura, T.: Manda, E.; Kawabata, Y. Thin Solid Films 1988,160, 299. (5)Kawabata, Y. Nippon Kagaku Kaishi 1990,1087. (6) Kim, S.; Furuki, M.; Pu, L. S.; Nakahara, H.; Fukuda, K. Thin Solid Fdms 1988,159,337. (7) Furuki, M.; Ageishi, K.; Kim, S.; Ando, I.; Pu, L. S. Thin Solid Films 1989,180, 193. ( 8 )Furuki, M.; Kim, S.;Pu, L. S.; Nakahara, H.; Fukuda, K. Nippon Kagaku Kaishi 1990,1121.
transform infrared (FT-IR) spectros~opy,~ which is a powerful nondestructive structural probe for ordered thin organic films deposited on solid substrates.1,2J0J1In our first report of a series of IR studies on SQ LB films, the following conclusions could be reached for LB films of 2 4 bis[4-(methyloctadecylamino)-2-methylphenyllcyclobutadienediylium-1,3-diolate (hereafter referred to as SQ-11): (i) SQ-I1 takes a delocalized structure, A (see Figure 10) for the central four-ring-system structure in both the LB films and the solid state; the structures of both the hydrocarbon chains and chromophoric part change a little upon the formation of the LB films; (ii)the hydrocarbon chains are inclined considerablywith respect to the surface normal in the LB films; (iii)both the molecular axis and the phenyl rings are tilted with respect to the surface normal. The purpose of the present study is to investigate the molecular aggregation, orientation, and structure of LB films of another kind of SQ dye, 2,4-bis[(3,3-dimethyl-loctadecyl-2,3-dihydro-2-indolylidene)methyll-l,3-cyclobutadienediylium-1,3-diolate (hereafter referred to as SQVI), by use of visible absorption and IR spectroscopies. The dependence of molecular aggregation, orientation, and structure upon the surface pressure is also studied. It is of particular interest to compare the structure of the LB films of SQ-VI with that of SQ-11. The comparison (9) Kuramoto, N.; Ozaki, Y. Thin Solid Films 1992,209,264. (10) Takenaka, T.; Umemura, J.In VibrationalSpectra anddStructure; Dung, J. R., Ed.; Elsevier: Amsterdam, 1991; Vol. 19, p 215. (11)Swalen, J. D. Annu. Rev. Mater. Sci. 1991,21,373.
0743-746319512411-2195$09.00/00 1995 American Chemical Society
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2196 Langmuir, Vol. 11, No. 6, 1995
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Figure 1. Structure of 2,4-bis[(3,3-dimethyl-l-octadecyl-2,3- Figure 3. Deposition behavior of SQ-VI on CaFz plates under various surface pressures of (a) 15,(b) 20, (c) 24.5, and (d) 31 dihydro-2-indolylidene)methyll-1,3-cyclobutadienediylium1,3mN m-l. Transfer ratios are plotted against each rising and diolate (SQ-VI) (top) and its resonance structure (bottom). dipping cycle (Y-type).
spectrum ofthe powdered sample was obtained by using a micro infrared attachment (JEOLIR-MAU 110);for the measurement the sample was put on a Ge plate as it was. Visible absorption spectra were recorded on a ShimadzuUV-250spectrophotometer.
Figure 2. Surface pressure-area(z-A) isotherm for SQ-VI on water (at 190 K, pH 6.3). Arrows a-d indicate the transferring points on the substrate. between them reveals that two types of SQ dyes show fairly different behavior upon the formation of the LB films; SQ-I1 keeps its structure during film preparation while SQ-VI changes its structure significantly in the conformation of alkyl chains and the electronic structure of the central conjugated system. Experimental Section The squarylium dye (SQ-VI;Figure 1)was synthesized from psquaric acid and l-octadecyl-2,3,3-trimethylindoleninium chlorobenzenesulfonate according to the procedure described in the literature.12J3The analyticaldata obtainedwere satisfactory. The measurement of the surface pressure-area (n-A)isotherm of monolayers at the air-water interface was performed with a Langmuir trough with a Wilhelmybalance (KyowaKaimen Kagaku, Model HBM-AP). A chloroform solution of SQ-VI (1.0 x M) was spread onto an aqueous subphase containing 1.4 x M CdClz and 1.7 x low4M mc03 at pH 6.3(290K). After evaporation of the solvent, the monolayer was compressed at a constantrate of 20 cm2 min-1 up to the desired surface pressure. Figure 2 shows the n-A isotherm of pure SQ-VI. The monolayers of SQ-VI were transferred under various surface pressures onto CaFz (for visible and IR transmission measurements) or gold-evaporated onto glass slides (for IR RA measurements) at dipping and raising speeds of 10 and 5 mm min-l, respectively. A cast film of SQ-VIwas prepared from a chloroform solution on a CaFz plate. Infrared spectra of the LB, cast films and powdered sample were obtained at 4 cm-l resolution with a JEOL JIR-100 FT-IR spectrometer equippedwith an MCT detector. Generally,several hundred scans were accumulated to ensure an acceptable signal t o noise ratio. For the RA measurements, a JEOL IR-RSC 100 reflection attachment was employed at an incidence angle of 80°,together with a JEOL IR-OPT02 polarizer. The infrared (12) Law, K. Y.; Bailey, F. C. Can. J. Chem. 1986, 64, 2267.
(13)Kuramoto, N.; Natsukawa, IC;Asao, K. Dyes Pigments 1989, 11,21.
Results and Discussion Monolayer Behavior and Film Deposition. The n-Aisotherm of the monolayer of SQ-VI depicted in Figure 2 shows that the surface pressure begins to rise at 138A2 molecule-' and it proceeds slowly, while the molecular area decreases, finally reaching the collapse point (34mN m-l). The characteristic transitions at o = 68 and 20 A are from the gas state to the liquid state and from the liquid state to the solid state, respectively. The first state of the monolayer is formed below a surface pressure of 24 mN m-l while the second state appears beyond it. SQ molecular areas under the higher anp lower surface pressure are found to be 45 and 120 A2, respectively. Molecular modeling and X-ray analysis14 show that the SQ dye molecule is approximated by aFectangylar block with dimensions of approximately 17 A x 6.5A x 3.5 A. The surface area of SQ dye should be more than 110 k, if we assume a flat-on conformation of the chromophqre at the air-water interface. The observed value (120A2) for the first state is very close to the value from the molecular model, suggesting that the chromophore is in the flat-on conformation. On the other hand, the value for the second region (45A2)is small, indicating taht the chromophore is in a side-on conformation or tilted structure. LB films of SQ-VI were prepared under various surface pressures, and the deposition type was indicated by the transfer ratio. As shown in Figure 3d, the monolayers of SQ-VI were deposited on CaFz plates by the alternating Y-type model (head-to-tail configuration) with transfer ratios near unity (0.89)for the upward stroke and 0.77 for the downward stroke at 31 mN m-l (point d in Figure 2). However, they were deposited with smaller transfer ratios at the lower surface pressures and, in the case of 15 mN m-l (point a in Figure 21, the ratios were particularly small (about 0.1)for both the upward and downward strokes. The results in Figure 3 reveal that the transfer of the monolayer to the CaF2 plates strongly depends on the surface pressure. Visible Absorption Spectra. Figure 4A shows absorption spectra of the LB films of SQ-VI prepared under surface pressures in the range 15-31 mN m-l. The ~
(14)Kobayashi, Y.; Goto, M.; Kurahashi, M. Bull. Chem. SOC.Jpn. 1986, 59, 311.
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A
la)
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Figure 4. (A) Visible absorption spectra of nine-monolayer LB films deposited on CaFz plates at (a) 15, (b) 20, (c) 24.5, and (d) 31 mN m-l and of an ethanol solution (e) of SQ-VI.(B) A visible absorption spectrum of a cast film of SQ-VI.
spectrum of its chloroform solution is also shown in the same figure for comparison (Figure 4Ae). The solution spectrum, having a n absorption maximum a t 634 nm with a shoulder near 580 nm, does not show a concentration dependency in the range 1.0 x to 5.0 x M. Therefore, the 580 nm band may be attributed to a vibrational component. The absorption spectra of the SQ-VI LB films are largely different from that of the solution. The spectra (Figure 4b-d) of the nine-monolayer LB films consist of two major peaks near 647 and 538 nm. They are probably due to molecular dispersed states and H-aggregates, respectively. Similar spectra consisting of bands due to the molecular dispersed states and H-aggregates were observed for monolayers on the water surface and LB films of bis[4(dialky1amino)phenyllsquarylium derivatives. It is wellknown that mixed LB films of bis[4-(dialkylamino)phenyll squarylium dye with short alkyl chains and a fatty acid form J - a g g r e g a t e ~ . ~ ,For ' ~ the present LB films, the formation of J-aggregates was not observed. It is notable in Figure 4A that the relative intensity of the band a t 538 nm increases with surface pressure. This observation suggests that the stronger the surface pressure, the easier the formation of the H-aggregates. Figure 4B exhibits a visible absorption spectrum of a cast film of SQ-VI. The spectrum of the cast film consists ofthree major peaks near 550,600, and 653 nm which can be attributed to the H-aggregate, dimer, and monomer, respectively. In addition, there is a weak peak near 705 nm. Comparison of Figure 4B with Figure 4Ad elucidates that the aggregates formed in the cast film have a structure different from that in the LB film. In contrast to the latter, the former contains a small contribution from the Haggregate. IR Spectra of SQ-VIin Solid and Cast Film. Spectra a and b in Figure 5 show the IR transmission spectra of SQ-VI in the solid state and cast film, respectively. The spectrum of the solid state was obtained for the powdered microcrystalline state by using a microspectroscopic technique. The two spectra in Figure 5 are very close to each other in terms of both the relative intensities and frequencies, indicating that the structure of SQ-VI changes little between the solid state and cast film. Two intense bands at 2920 and 2850 cm-l are assigned to CH2 antisymmetric and symmetric stretchingmodes of
the hydrocarbon chains, respectively. Vibrational frequencies of these stretching bands are very sensitive to the conformationof a hydrocarbon chain; the bands appear near 2918 and 2848 cm-l, respectively, when the hydrocarbon chain takes a trans-zigzag conformation,while they shift upward up to near 2928 and 2856 cm-l, depending upon the content of gauche conformers, when conformational disorder is included in the chain. The above frequencies (2920 and 2850 cm-') of the CH2 stretching bands indicate that the hydrocarbon chains of SQ-VI take near trans-zigzag conformations in the solid and cast film. Vibrational spectra of squarylium chromophores have not been studied well, and therefore, spectra interpretation in the 1700-700 cm-l region is not straightforward. We have tried to make band assignments by comparing the spectra in Figure 5 with the spectra of SQ-IIgand cyanine dye with the same substituents in the indoline rings as SQ-VI (3,3-dimethyl-2-[3-(3,3-dimethyl-l-octadecyl-2-indolinylidene)-l-propenyl]-l-octadecyl-3H-indoliumiodide;16hereafter we refer to this dye as NK-2665),for both of which we already have a n interpretation. A weak feature a t 1612 cm-' and a strong band a t 1485 cm-' may be due to stretching modes of the indoline rings because NK-2665 shows corresponding bands in the close positions and SQ-11 does not have these bands. On the other hand, a n intense band a t 1591 cm-l is probably assignable to a stretching mode of the central conjugated system because a corresponding intense band is identified a t 1594 cm-l in the IR spectra of SQ-I1 but no band is observed near 1590 cm-l in the spectra of NK-2665. An intense band a t 1500 cm-l does not have its counterpart in the IR spectra of both NK-2665 and SQ-11. Therefore, it seems that the band is due to a stretching mode involving the motions of the central conjugated system and the C=N groups. There is no band in the 1750-1650 cm-l region where C=O and C=N+ stretching modes are expected, suggesting that SQ-VI takes the structure shown in Figure l a in the solid and cast film. Intense bands near 1280 and 1070 cm-l are probably assignable also to stretching modes involving the motions of the central conjugated system and C=N groups because NK-2665 does not show bands near 1280 and 1070 cm-l. IR Spectra of LB Films of SQ-VI. Figure 6 shows IR transmission spectra of one- and nine-monolayerLB films
(15)Kim, S.; Furuki, M.; Pu, L. S.; Nakahara, H.; Fukuda, K. J. Chem. SOC.,Chem. Commun. 1987,1201.
(16) Fujimoto, Y.; Katayama, N.; Ozaki, Y.; Araki,T.; Iriyama, K. Thin Solid Films 1992,210/211, 597.
Kuramoto et al.
2198 Langmuir, Vol. 11, No. 6, 1995 ( a ) Solid
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Figure 6. Infrared transmission spectra of (a) nine- and (b) one-monolayer LB films deposited on CaFz plates under a surface pressure of 31 mN m-l. Throughout this paper a medium feature appearing near 2350 cm-l is due to COZ. of SQ-VI deposited on CaFz plates under a surface pressure of 31 mN m-l. The two spectra are very similar to each other except for the band intensities. It is notable that the spectra of the LB films are largely different from those of the solid and cast film. There are three important differences between them. First, the frequencies of the CH2 antisymmetric and symmetric stretching bands are slightly higher for the spectra of the LB films than for those of the solid and cast film. The differences are small but reproducible. These high-frequency shifts of the CH2
stretching bands suggest that the hydrocarbon chains are less ordered in the LB films than in the solid and cast film. The second notable difference is the appearance of two bands a t 1718 and 1660 cm-l assignable to C=O and C=N+ stretching modes, respectively. These observations indicate that SQ-VI assumes the structure shown in Figure l b in the LB films. The third difference is the relative intensities of the bands: in Figure 5 most bands appearing below 1700 cm-l are stronger than those due to the CH2 stretching modes while the reverse situation is observed in Figure 6 . In IR transmission spectra bands due to vibrational modes, with their transition moments parallel to the surface, are enhanced.l0J1 Therefore, the results in Figure 6 suggest that the chromophore plane is fairly perpendicular to the substrate surface in the LB films. Spectra a-c in Figure 7 present IR RA spectra of nine-, three-, and one-monolayer SQ-VILB films deposited on gold-evaporated glass slides a t 31 mN m-l. The three spectra are very similar to each other except for the band intensities, indicating that the structures of the LB films do not depend upon the number of monolayers. The spectra in Figure 7 have spectral characteristics similar to those of the spectra in Figure 6 except for the band intensities. For example, the spectra in both Figures 6 and 7 give bands near 1720 and 1660 cm-l assignable to C=O and C=N+ stretching modes, respectively. Bands due to the CHZ antisymmetric and symmetric stretching modes are also identified at the same positions for the spectra in both Figures 6 and 7. Therefore, it seems that the conformation of the hydrocarbon chains and the electronic structure of the chromophoric part change little upon going from the LB films deposited on the CaF2 plate to those prepared on the gold-evaporated glass slides.
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