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Langmuir 1995,11, 623-626
Monolayer Formation Properties of Cholesterol-Based Azobenzene Amphiphiles with the Natural and the Inverted C3 Configuration Hirosuke Kawabata, Kazutaka Murata, Takaaki Harada, and Seiji Shinkai" Chemirecognics Project, Research and Development Corporation of Japan, Aikawa 2432-3, Kurume, Fukuoka 830, Japan Received July 15, 1994. I n Final Form: November 3, 1994@ The monolayer formation properties of cholesterol-based azobenzene amphiphiles with the natural (SIC3 configuration (Is)and the inverted (R)-C3configuration ( 1 ~were ) examined at the air-water interface. The computational study reveals that 1s adopts an extended conformation whereas 1~adopts an L-shaped bent conformation. 1s ave an expanded phase with A0 (limiting area) = 0.60 nm2 molecule-I and A1 (lift-off area) = 0.64 nmQ molecule-I whereas 1~ gave a condensed phase with A0 = 0.49 nm2 molecule-I and A1 = 0.54 nm2 molecule-l. Examination using reflectance spectroscopy established that 1s forms a monolayer with a J-aggregation mode (I,, 407 nm) and with an increase in the compressibility it changes to an H-aggregation mode (I,,, 336 nm) whereas 1~ forms a monolayer having a I,, (362 nm) comparable with the monomeric absorptionmaximum, indicatingthat the electronic interaction among the azobenzene moieties is absent. The morphological changes in the monolayers were directly observed by an optical microscope and reasonably correlated with the spectroscopicchanges. The results indicatethat the absolute configuration at C3 has a crucial influence on the aggregationproperties of cholesterol-based amphiphiles in a monolayer system.
Introduction It is a new, intriguing field of chemistry to apply monolayers formed at the air-water interface to molecular rec0gnition.l Of particular interest is the potential application to chiral discrimination of guest molecules present in the subphase: chiral guest molecules interact at the interface with chiral amphiphiles and the discrimination process can be "read-out"as a change in the n-A i ~ o t h e r m . ~One - ~ may expect the chiral discrimination ability for the monolayer system to be better than that for the solution complex because the chiral amphiphiles are tightly packed and the formed monolayers are more or less similar to the solid state. We previously found that cholesterol derivatives modified with a crown ring and a boronic acid form monolayers at the air-water interface and they show a significant chiral discrimination ability for a-amino acid methyl esters and saccharide^.^^^ This implies that the cholesterol skeleton, having a wide chiral plane, is very useful to create an asymmetric surface on the water-facing side of the monolayer. More recently, we attached a chromophoric azobenzene group to C3-OH of c h o l e ~ t e r o l .In ~~ the ~ synthetic process, the reaction of Abstract published in Advance ACS Abstracts, February 1, 1995. (1)(a)Kurihara, K.; Ohto, K.; Tanaka, Y.; Aoyama, Y.; Kunitake, T. J.h. Chem.Soc. 1991,113,444.(b)Ikeura,Y.;Kurihara,K.; Kunitake, T. J.Am. Chem.SOC.1991,113,7342.(c)Kurihara, K.;Ohto, K.;Honda, Y.; Kunitake, T. J . Am. Chem. SOC.1991,113,5077.(d) Sasaki, D.Y.; Kurihara, K.; Kunitake, T. J . Am. Chem. SOC.1991, 113, 9685. (2)(a)Amett, E.M.; Harvey, N. G.;Rose, P. L.Acc. Chem. Res. 1989, 22, 131. (b) Harvey, N.G.; Mirajovsky, D.; Rose, P. L.; Verbiar, R.; Amett. E. M. J.Am. Chem. SOC.1989.111.1115. (3)Maruyama, A,; Adachi, N.; Takatsuki, T.; Torii, M.; Sanui, K.; Ogata, N. Macromolecules 1990,23,2748. (4)Miyasaka, T.; Nishikawa, N.; Orikasa, A,; Ono, M. Chem. Lett. 1991,969. (5) Qian, P.;Matsuda, M.; Miyashita, T. J . Am. Chem. SOC.1993, 115,5624. (6)Kawabata, H.; Shinkai, S . Chem. Lett. 1994,375. (7)Ludwig, R.;Harada, T.; Ueda, K.; James, T. D.; Shinkai, S . J . Chem. Soc., Perkin Trans. 2 1994,697. (8)Murata, K.; Aoki, M.; Nishi, T.; Ikeda, A,; Shinkai, S. J . Chem. Soc., Chem. Commun. 1991,1715. (9) (a)Murata, K.; Aoki, M.; Shinkai, S. Chem. Lett. 1992,738. (b) Murata, K.; Aoki, M.; Suzuki, T.; Harada, T.; Kawabata, H.; Komori, T.; Ohseto, F.; Ueda, K.; Shinkai, S . J . Am. Chem. Soc. 1994,116,6664.
acid chlorides with cholesterolyielded the ester derivatives with the natural (SI-configuration at C3 whereas the reaction ofcarboxylicacids with cholesterolin the presence of triphenylphosphine and diazenedicarboxylicacid diethyl ester yielded the inverted (R)-configurationa t C3.9 They acted as excellent thermally-reversible gelators of various organic fluids, but their gelation ability and aggregation mode were fairly different.g It thus occurred to us that their monolayer formation properties should also be different. We here report the monolayer studies of 4-methoxy-4'-((cholesteryloxy~carbonyl)azobenzenes with the natural (S)-configuration (1s) and with the inverted . studies may provide a clue to (R)-configuration( 1 ~ ) The explain why nature adopts the (5')-configuration.
@
1R Results and Discussion Previously, we estimated energy-minimized structures ~ shown in of 1s and 1~ by a computational m e t h ~ d .As Figure 1 , l swith the natural (S)-configurationat C3 adopts an extended conformation and the dihedral angle between the cholesterol plane and the azobenzene plane is about 90"whereas 1~with the inverted (R)-configurationat C3 adopts an L-shaped bent conf~rmation.~ It is expected that the structural difference would be reflected by the difference in the aggregation properties. When the cholesterol moieties in 1s are arranged in a cholestericliquid-crystal-like, one-dimensional stack, the azobenzene
0743-746319512411-0623$09.00/0 0 1995 American Chemical Society
624 Langmuir, Vol. 11,No.2, 1995
Kawabata et al.
5 .
*
120
I IO
/
ta)
(h)
Figure! 1. Energy-minimized structures of (a) 1 s and (b) 1 ~ . 250
SO 40
300
350 400 Wavelength I nm
450
500
Figure! 3. Reflectance spectra of 1~ monolayered at the airwater interface (20 "C, on pure water): the surface pressures (from bottom to top) are 0.0,0.4,4.0,19.5,29.5,35.0, and 37.6 mN m-l.
E
.
30
2
; 20
I40
J
2
IO 0 0
0.2
0.4
0.6
0.8
Area I nm2molecule''
Figure 2. Surfacepressure-area(n-A) isotherms of 1s (- -) and 1~ (-1 at 20 "C on pure water. A0 indicates the limiting area defined by extrapolatingthe solidlike region to zero pressure andAI indicates the lift-off area defined as the first point of the x-A isotherms where a monolayershows detectableresistance to compression. A0 and A1 of 1~are shown in this figure to correspond to those for the n-A isotherms on pure water.
moieties are situated as branches around a cholesterol stem and take a face-to-faceorientation. In contrast,when the cholesterol moieties in 1~ have a similar onedimensional stack, the azobenzene moieties cannot take such a face-to-face orientation and the additional stabilization effect arising from the stacking of the azobenzene moieties is not expected. In the gel system, 1s showed a much stronger cohesive nature than 1~ (e.g., in melting point, solubility, sol-to-gel phase transition temperature (Tgel),etc.). It is of great significance to assess how this structural difference appears in a monolayer system. Figure 2shows n-A isothermsof natural 1sand inverted 1 ~ .The limiting molecular area for 1s (A0 = 0.60 nm2 molecule-') is comparable with those for crowned cholesterols with the natural (S)-configuration a t C3 (A0 = 0.67-0.72nm2molecule-1).6J0The collapse pressure (nc) appears a t 14.5 mN m-l, indicating that the monolayer is relatively unstable. In contrast, the A0 for 1~(0.49nm2 molecule-') is much smaller than that for IS,and the nc (36.2mN m-l) is much higher than that for 1s. From CPK molecular models the minimum area occupied by simple cholesterol derivatives is estimated to be about 0.45 nm2 molecule-l. This value implies that the molecular area in l~ reflects that of the cholesterol moiety and the monolayer thus formed from the condensed phase is fairly stable because of the high packing density. On the other hand, the finding that the A0 for 1s is larger than 0.45 nm2 molecule-' means that in 1s the initial collision occurs among the azobenzenemoieties (or a t least both moieties together) and they are packed as an expanded phase. (10)(a) Kimizuka, N.; Kunitake, T. Chem. Lett. 1988, 827. (b) Kimizuka, N.; Kunitake, T. Colloids SUI$ 1989,38, 79.
Inn ."" 1
250
300
350 400 Wavclength I nm
450
500
Figure! 4. Reflectance spectra of 1s monolayered at the airwater interface (20 "C, on pure water): the surface pressures (from bottom to top at 407 nm) are 23.3,23.4,19.0,24.3,17.8, 17.5,0.0, 13.8, 0.2,and 9.1 mN m-*.
To obtain further insights into the aggregation properties we measured the reflectance spectra of monolayered 1s and 1~ as a function of compressibility a t the airwater interface. We previously found that both 1s and 1~ aggregate in 1-butanoland gelatinizethe s o l ~ t i o n .In ~ 1s the absorption maximum in the isotropic solution phase (Amm 360nm) shifts to shorter wavelength (higher energy region) in the gel phase (Am= 310 nm).9 This shift is due to the H-aggregation of the azobenzene m ~ i e t i e s .In ~ l~, in contrast, the absorption maximum always appears a t 360 nm both in the isotropic solution phase and in the gel phase.g This implies that the azobenzene moieties exist separately even in the gel phase. Figure 3 shows reflectance spectra of 1~ monolayered a t the air-water interface. The Am= appears a t 362 nm, which is comparable with that in the isotropic solution phase and is scarcely affected by the increase in the surface pressure. This implies that, as in the gel system, the azobenzene moieties in 1~exist separately in a sparselyscattered state (at n % 0), in a condensed phase (at around nc),and even in a collapsed state (at n > nc). Figure 4 shows reflectance spectra of 1s measured under the same conditions. The
Monolayer Formation of Cholesterol Derivatives -
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Langmuir, Vol. 11, No. 2, 1995 625 I
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H-aggregation. One can thus conclude that 1s forms J-aggregates when dispersed as a monolayer on water whereas it forms H-aggregates as in the gel system when the monolayer collapses and 1s molecules are piled up. The n-A isotherms and the spectroscopic data now allow us to illustrate Figure 6 for the formation of monolayers from natural 1s and inverted 1~ (although it may be somewhat oversimplified to emphasize the difference between 1s and 1~).In ls, the microaggregates with a J-aggregation mode are compressed to a monolayer with a J-aggregation mode. This means that 1s molecules in the aggregate are tilted. At n > nc they collapse to a multilayer with an H-aggregation mode. In 1~ it is unknown if the molecules form the microaggregates at n = 0. When the molecules are compressed to a monolayer, the collision occurs among the cholesterol moieties while the azobenzene moieties do not interact with each other. Since the A,-, is comparable with the molecular area of the cholesterol moiety, 1~molecules in the monolayer are not tilted. Subsequently, we directly observed the monolayers with an optical microscope. For this purpose we mixed 1mol % of octadecyl Rhodamine B (fluorescent amphiphile)with the monolayers.ll As can be seen from photographs in Figure 7, octadecyl Rhodamine B is more miscible with 1~ (less cohesiveg)than with 1s (more cohesive9). This difference is reflected by the n-A isotherms: the n-A isotherm for 1~ plus octadecyl Rhodamine B is not so different from that for 1~ only, whereas the collapse
1
I 10
336 nm
loo 0
5
10
15
20
25
Pressure I mN m-l
Figure 5. Plots of the reflectance intensity (I)a t 336 and 407 nm against the surface pressure (n).The plots were made from the data in Figure 4.
A,
appears at 407 nm even at n = 0 mN m-l. This indicates that the microaggregates with the J-aggregation mode are formed even in a sparsely scattered state (at n = 0). This trend is in line with the conclusion obtained in the gel system that natural 1s is more cohesive than inverted 1 ~ .As shown in Figure 5, the reflectance intensity (I)at 407 nm increases with increasing n and reaches a maximum at around nc (9.1 mN m-l). At n > nc the I at 407 nm abruptly decreases and a new reflectance maximum appears at 336 nm which is assigned to the
- cholesterol
azohenzene
methoxy group
0
Phase transition
__c
no electronic interaction
among azohenzenes
bbhbbbob
Figure 6. Schematic representation for the monolayer formation from 1s (left) and 1~ (right).
Figure 7. Optical microscopic morphologies of 1s and 1~ (containing 1.0 mol % of octodecyl Rhodamine B)at 20 "Con pure water. The micrographs a-e are for 1~ and f-j are for IS. Each letter corresponds to the surface pressure marked in the adjoining n-A isotherms.
626 Langmuir, Vol. 11, No. 2, 1995
pressure for 1s plus octadecylRhodamine B is significantly lower than that for 1s only. In Figure 7, parts a and b are pictures for 1~ above the lift-off area (A1 = 0.54 nm2 molecule-'). The domain structure was not observed immediately after spreading a benzene solution containing l ~but, after a few minutes the domain structure appeared. In the examination with reflectance spectroscopy we already noticed that the reflectance intensity fluctuates with time above AI. The optical microscopic observation suggests that the microaggregates of 1~ are drifting on the water surface, and the reflectance intensity increases when they come into the vision field of the microscope. The findings support the view that 1~ also forms the microaggregates even at 0 mN m-'. At n = 19.0 mN (just below n c ; Figure 7c), the large, nearly homogeneous monolayer of 1~ was observable. At n > nc (Figure 7d,e), on the other hand, long stripes appeared in the homogeneous domain. These wrinkles are formed by a compression that is strong enough to collapse the monolayer. On the other hand, 1s gave a number ofbright spots above A1 (Figure 70. As demonstrated by reflectance spectroscopy, this pattern corresponds to the formation of microaggregates with a J-aggregation mode. When they are compressed to a monolayer, one can observe a welldispersed island structure (Figure 7g). This corresponds to an expanded monolayer phase with a J-aggregation mode. Further increase in the compression changed the island structure to the network structure with large cracks. Conceivably, the monolayer is collapsed here and a crystal (or liquid crystal) with an H-aggregation mode is yielded on the surface (Figure 7g,h).
Conclusion The present study has demonstrated several intriguing aspects of cholesterol-based amphiphiles in a monolayer system, that is, (i) cholesterols linked to an azobenzene moiety via an ester group at C3 form a monolayer at the water-air interface, (ii) the behaviors are profoundly affected by the absolute configuration a t C3: ISwith the natural (5)-configuration forms an expanded phase with a J-aggregation mode whereas 1~ with the inverted (R)configuration forms a condensed phase in which the azobenzene moieties do not interact with each other, (iii) the formation and the collapse of the monolayers can be monitored by an optical microscope, and (iv) there are many common properties with the gel system. The results indicate that cholesterol is a useful skeleton to create a novel monolayer with a chiral surface. Furthermore, the azobenzene acts as a chromophoric reporter to monitor the aggregation mode of the monolayer formed a t the airwater interface. We are now planning to extend the present system to, for example, chiral recognition, photocontrol of the monolayer by cis-trans isomerism, energytransfer to and from the azobenzene moiety. Experimental Section 4-((4-Methoxyphenyl)azo)benzoic Acid. To the mixture of 4-((4-hydroxyphenyl)azo)benzoic acid12(4.0 g, 16.6 mmol) and potassium carbonate (30.0 g, 0.16 mol) in dry DMF (100 mL) was
Kawabata et al. added methyl iodide (25.0 g, 0.16 mol). The solution was allowed to stand for 16 h at reflux temperature with stirring. The desired compound was isolated from the solution in 35% yield: mp 256258 "C; 'H NMR (CDC13)6 3.93 (s,3H, CH30), 6.90-7.38 (d, 2H, ArH), 7.80-8.40 (m, 6H, ArH); IR(Nujo1) 1700 (vc-0) cm-l. Anal. Calcd for C14H1203N2: C, 65.62; H, 4.70; N, 10.94. Found: C, 65.50; H, 4.66; N, 10.90. 4Methoxy-4-((cholesteryloxy~carbonyl)azobenzene (IS). The mixture of 4-((4-methoxyphenyl)azo)benzoic acid (1.00 g, 3.70 mmol) and 4-(dimethylamino)pyridine(0.23 g, 1.90 mmol), cholesterol (1.43 g, 3.70 mmol), and dicyclohexyl carbodiimide (0.84 g, 4.07 mmol) in CHzCl2 (100 mL) was stirred a t room temperature for 4 h. The solvent was removed and the product was purified by silica gel column chromatography: yield 48%, mp 302-306 "C (from DSC); 'H NMR (CDC13) 6 0.30-2.65 (m, 43H, cholesterol), 3.96 (s, 3H, CH30), 4.70-5.26 (m, l H , cholesterol 3-H), 5.37-5.58 (m, l H , cholesterol 6-H), 7.00-7.46 (d, 2H,ArH), 7.85-8.42 (m, 6H, ArH); [a125~ 14" (c 0.0011, CHCl3); , (YC-o methoxy) cm-l. Anal. Calcd IR (Nujol) 1710 ( Y C ~ )1250 for C41H5603N2: C, 78.80; H, 9.03; N, 4.48. Found: C, 78.98; H, 8.98; N, 4.55.
4-Methoxy-4'-((cholesteryloxy)carbonyl)azobenzene The mixture of4-((4-methoxyphenyl)azo)benzoicacid (1.69
(l&13
g, 6.60 mmol), triphenylphosphine (4.57 g, 17.4 mmol), cholesterol (2.55 g, 6.59 mmol), and diazenedicarboxylic acid diethyl ester (6.0 g, 34.5 mmol) in THF (100 mL) was stirred at room temperature for 24 h. The solvent was removed and the product was purified by silica gel column chromatography: yield 55%, mp 110-115 "C (from DSC); IH NMR (CDC13) 6 0.60-2.70 (m, 43H, cholesterol), 3.85 (s, 3H, CH30), 5.18-5.41 (m, 2H, cholesterol 3-H and 6-H), 6.90-7.18 (d, 2H, ArH), 7.78-8.18 (m, 6H, ArH); [ a I z 5-28" ~ (c 0.0012, CHC13); IR (Nujol) 1711 ( Y C - o ) , 1252 (YC-0 methoxy) cm-l. Anal. Calcd for C41H5803Nz: C, 78.80; H, 9.03; N, 4.48. Found: C, 78.57; H, 8.78; N, 4.63. Pressure-Area (mA) Isotherms. A computer-controlled film balance system FSD-20 (US1 system, Fukuoka) was used for measuring the surface pressure as a function of the molecular area. The trough size was 150 x 470 mm2 and the temperature of the aqueous subphase was maintained at 20 f 0.1 "C. The concentration ofthe spreading solution was 10 mg/lO mL benzene. After spreading 50 pL of the solution, the monolayer film was incubated for 10 min and then compressed at a rate of 60 mm2 s-'. The x-A isotherms were very reproducible. Reflectance Spectra. Reflectance spectra were obtained by a photodiode-array-equipped spectrophotometer (Otsuka Electronics, model MCPD 100). The tip of an optical fiber was fixed vertically a t a position 2-3 mm above the water surface. A tilted black plate was placed in the trough below the optical fiber. Optical Microscopic Morphologies. Observations of optical microscopic morphologies of 1s and 1~containing 1.0 mol % of octadecyl Rhodamine B as a fluorescence probe were carried out with an Olympus BH2RFCA microscope with a computercontrolled film balance system FSD-50 (US1 system). Mixed solutions of lipids and a fluorescence probe were prepared by mixing lipid stock solutions (benzene) with a probe solution (methanol).
LA940556Y (11)Shinkai, S.;Tsukagoshi,K.; Ishikawa,Y.; Kunitake, T.J. Chem. Soc., Chem. Commun. 1991,1039. (12) Wallach, 0.;Kiepenheuer, L. Ber. Dtsch. Chem. Ges. 1881,14, 2617.
(13)Bose, A.; Lal, B.; Hoffman, W. A.; Manhas, M. S. Tetrahedron Lett. 1973,18,739.
