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Sugar-Dependent Spectral Responses of Azobenzene Glycopyranoside Monolayers Masahito Sano,* Masato Amaike, Ayumi Kamino, and Seiji Shinkai* Chemotransfiguration Project-JST, 2432 Aikawa, Kurume, Fukuoka 839-0861, Japan Received February 5, 2001. In Final Form: April 19, 2001 Novel amphiphilic glycopyranosides containing an azobenzene chromophore and an amide-benzene auxochrome are synthesized. A sugar headgroup is attached to the auxochrome unit that links directly to azobenzene. The UV-vis spectral response of the monolayer to a change of the molecular area is found to depend on the chemical structure of the sugar unit, R-D-glucopyranose or R-D-galactopyranose. When spread on the water surface at large molecular areas, both compounds show π-π* long-wavelength peaks at around 460 nm. Upon compression of the films, the peaks for the glucopyranoside exhibits a red shift, whereas that for the galactopyranoside gives a blue shift. A reference compound having a carboxylic acid in place of the sugar unit produces a blue shift because of the formation of H-like aggregates. The pressure isotherm and a morphological study suggest that a similar aggregation is responsible for the blue shift due to the galactopyranoside. FTIR spectroscopy indicates that the amide group of the glucopyranoside contains mostly the -CdN- resonance form. This resonance form increases the planarity of the azobenzeneamide-benzene units, resulting in the observed red shift. This study offers a basic chemical structure for systematic controls of dye aggregates by sugar.
Introduction Controlling the color of dye aggregates is a challenging task. Even a small chemical modification to the chromophore often leads to unpredictable spectral responses. In nature, a sugar is frequently found to coexist with dye molecules and to exhibit a well-defined color. The present work is motivated by the possibility of systematically cataloging color through the introduction of different kinds of sugar. In general, the optical characteristics of dye molecules depend on the aggregation state and the electronic structure. It is particularly important to design a fundamental chemical structure that, when aggregation states are changed externally, its spectral responses can be tuned through the introduction of a different kind of sugar into the dye molecules. In this way, it becomes possible to catalog the spectral responses of a huge library of sugar structures. One of the effective means of changing aggregation states is to make dyes amphiphilic so that techniques of Langmuir monolayers and Langmuir-Blodgett (LB) films can be applied to define molecular orientations.1 In particular, amphiphiles having azobenzene as a chromophore are quite well-suited for such control because their optical properties are known to depend strongly on the relative molecular orientation within an aggregate.2-7 Systematic controls of azobenzene electronic structures, on the other hand, are more difficult because of the high sensitivity of the π* states on the chemical groups attached directly to azobenzene. Here, we introduce an auxochrome group between the azobenzene and the sugar. In this way, * Authors to whom correspondence should be addressed. (1) Gaines, G. Insoluble Monolayers at Liquid-Gas Interfaces; WileyInterscience: New York, 1966. (2) Shimomura, M.; Ando, R.; Kunitake, T. Ber. Bunsen-Ges. Phys. Chem. 1983, 87, 1134. (3) Kunitake, T. Colloids Surf. 1986, 19, 225. (4) Kimizuka, N.; Kunitake, T. Colloids Surf. 1989, 38, 79. (5) Decher, G.; Tieke, B. Thin Solid Films 1988, 160, 407. (6) Shin, D.-M.; Schanze, K. S.; Whitten, D. G. J. Am. Chem. Soc. 1989, 111, 8494. (7) Song, X.; Perlstein, J.; Whitten, D. G. J. Am. Chem. Soc. 1997, 119, 9144.
different sugar groups can be implemented systematically to regulate spectral properties without deviating in a disorderly way from the basic characteristics of a chromophore-auxochrome system. Although many glycosides and related compounds have been studied over the past century, a surprisingly small number of reports are available for monolayers.8-11 Thus far, we are unaware of any report implementing chromophore systems in glycoside monolayers. Based on the above methodology, the novel azobenzene glycopyranosides Azo-Glu (1) and Azo-Gala (2) were synthesized (Scheme 1). These glycopyranosides have three structural components: double alkyl chains, a chromophore-auxochrome unit consisting of (amino-)azobenzene with a benzene linked by an amide, and a glycopyranose head. Double alkyl chains are introduced to balance the bulkiness of the sugar head. The carboxylic acid derivative Azo-COOH (3) is also made as a reference. The surface pressure-area (Π-A) isotherms and UVvis reflection spectra are measured to follow the spectral response to a change of molecular aggregation. Fourier transform infrared (FTIR) spectroscopy and atomic force microscopy (AFM) on LB films are utilized to gain information on the molecular structures. Experimental Section Synthesis of 4-(Di-n-octylamino)-4′-[(4-aminophenyl-r(1). 4-[4-(Di-noctylamino)phenylazo]benzoic acid (210 mg, 0.45 mmol) was dissolved in 20 mL of dry methylene chloride under a nitrogen atmosphere. To this solution was added oxalyl chloride (1.0 mL, 11.5 mmol) and a few drops of DMF. After being stirred for 1 h, the reaction mixture was evaporated to dryness. The residue was dissolved in 10 mL of dry DMF, and this solution was added dropwise to p-aminophenyl-R-D-glucopyranoside (147 mg, 0.54 D-glucopyranoside)carbonyl]azobenzene
(8) Luckham, P.; Wood, J.; Froggatt, S.; Swart, R. J. Colloid Interface Sci. 1993, 156, 164. (9) Marron-Brignon, L.; Morelis, R. M.; Coulet, P. R. J. Colloid Interface Sci. 1997, 191, 349. (10) Emrich, G.; Vollhardt, D.; Gutberlet, T.; Kling, B.; Furhop, J.-H. Prog. Colloid Polym. Sci. 1995, 98, 266. (11) Ma, Z.; Li, J.; Jiang, L. Langmuir 1999, 15, 489.
10.1021/la010187n CCC: $20.00 © 2001 American Chemical Society Published on Web 06/09/2001
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Scheme 1
Figure 1. Surface pressure-area isotherms of Azo-Glu, AzoGala, and Azo-COOH on a pH 11.3 subphase and Azo-COOH on 0.5 mM CdCl2. Table 1. Isothermal Properties of Azobenzene Glycopyranosides
mmol) and triethylamine (0.31 mL, 2.24 mmol) in 10 mL of dry DMF in an ice bath under a nitrogen atmosphere. After the reaction mixture was stirred for 15 h at room temperature, it was poured into 2000 mL of water. The resulting orange precipitate was recovered by filtration and washed with water. The obtained orange solid was dissolved in chloroform, dried over anhydrous magnesium sulfate, and evaporated to dryness. The residue was purified by column chromatography on silica gel with chloroform/methanol (85:15 v/v) as the eluent. Compound 1 was isolated as an orange solid: yield 114 mg (35%); mp 270.3271.0 °C; 1H NMR (DMSO-d6) δ 10.3 (s, 1H, NH), 8.09 (d, J ) 8.4 Hz, 2H, ArH), 7.87-7.79 (m, 4H, ArH), 7.70 (d, J ) 9.0 Hz, 2H, ArH), 7.10 (d, J ) 8.7 Hz, 2H, ArH), 6.80 (d, J ) 9.0 Hz, 2H, ArH), 5.35 (d, J ) 3.6 Hz, 1H, sugar-H), 5.06 (d, J ) 6.3 Hz, 2H, OH), 4.98 (d, J ) 5.4 Hz, 2H, OH), 4.93 (d, J ) 4.8 Hz, 2H, OH), 4.51-4.48 (t, 1H, OH), 3.65-3.18 (m, 10H, sugar-H, N-CH2), 1.57 (m, 4H, CH2), 1.32-1.27 (m, 20H, CH2), 0.89-0.84 (m, 6H, CH3); IR (KBr) νmax 3335, 2924, 1647, 1601, 1510, 1392, 1365, 1313, 1138, 1022, 819, 520 cm-1; MS (positive SIMS, NBA) m/z 719 [M + H]+. Anal. Calcd for (C41H58N4O7): C, 68.50; H, 8.13; N, 7.79. Found: C, 68.00; H, 8.07; N, 7.79. 4-(Di-n-octylamino)-4′-[(4-aminophenyl-r-D-galactopyranoside) carbonyl]azobenzene (2). Compound 2 was synthesized according to a method similar to that used for 1: yield 145 mg (45%); mp 259.3-260.2 °C; 1H NMR (DMSO-d6) δ 10.3 (s, 1H, NH), 8.10 (d, J ) 8.4 Hz, 2H, ArH), 7.87-7.79 (m, 4H, ArH), 7.69 (d, J ) 8.7 Hz, 2H, ArH), 7.09 (d, J ) 9.0 Hz, 2H, ArH), 6.80 (d, J ) 9.0 Hz, 2H, ArH), 5.37 (s, 1H, sugar-H), 4.87 (d, J ) 5.1 Hz, 1H, OH), 4.73 (d, J ) 4.2 Hz, 1H, OH), 4.56-4.53 (m, 2H, OH), 3.81-3.37 (m, 10H, sugar-H, N-CH2), 1.57 (m, 4H, CH2), 1.32-1.27 (m, 20H, CH2), 0.89-0.84 (m, 6H, CH3); IR (KBr) νmax 3308, 2924, 1648, 1601, 1510, 1394, 1366, 1316, 1213, 1138, 1030, 819, 518 cm-1; MS (positive SIMS, NBA) m/z 719 [M + H]+. Anal. Calcd for (C41H58N4O7): C, 68.50; H, 8.13; N, 7.79. Found: C, 67.73; H, 8.10; N, 7.79. 4-[4-(4-Di-n-octylaminophenylazo)benzoylamino]benzoic Acid (3). 4-[4-(4-Di-n-octylaminophenylazo)benzoylamino]benzoic acid methyl ester (195 mg, 0.33 mmol) was dissolved in 25 mL of THF and 7 mL of methanol. To this solution was added 10 mL of 0.1 N KOH. The reaction mixture was stirred for 24 h at room temperature. Then, 0.1 N HCl (15 mL) was added, and the reaction mixture was stirred for 30 min. It was extracted with ethyl acetate, washed with water, dried over anhydrous magnesium sulfate, and evaporated in a vacuum. The
amphiphiles
A(Π > 0) (nm2)
A0 (nm2)
Πc (mN/m)
Ac (nm2)
Azo-Glu Azo-Gala Azo-COOH (pH 11.3) Azo-COOH (Cd2+)
0.83 0.72 0.78 0.72
0.50 0.61 0.65 0.60a
49.2 41.3 41.6 (23.9)b
0.40 0.43 0.37 (0.44)b
a Value estimated by assuming the same curvature as for AzoCOOH (pH 11.3). b Values correspond to the bump in the isotherm and might not have the same meaning as the others.
residue was purified by column chromatography on silica gel with ethyl acetate/hexane (1:1 v/v) as the eluent. Compound 3 was isolated as an orange solid: yield 151 mg (79%); mp 236.7237.4 °C; 1H NMR (DMSO-d6) δ 10.62 (s, 1H, COOH), 8.12 (d, J ) 8.4 Hz, 2H, ArH), 7.95-7.79 (m, 11H, ArH + NH), 6.80 (d, J ) 9.3 Hz, 2H, ArH), 3.37 (t, J ) 6.9 Hz, 4H, N-CH2), 1.57 (m, 4H, CH2), 1.31-1.27 (m, 20H, CH2), 0.87-0.84 (m, 6H, CH3); IR (KBr) νmax 2924, 2662, 2556, 1674, 1597, 1510, 1409, 1370, 1330, 1250, 1136, 858, 776 cm-1; MS (positive SIMS, glycerol + NBA) m/z 585 [M + H]+. Anal. Calcd for (C36H48N4O3): C, 73.94; H, 8.27; N, 9.58. Found: C, 73.93; H, 8.23; N, 9.46. Measurements. The Π-A isotherms were obtained on a monolayer spread from CHCl3 at 20 °C with a compression rate at 3.6 cm2/min. NaOH was used to adjust the water pH to 11.3. UV-vis reflection spectra (MCPD-110, Otsuka Electronics) were obtained during compression. The maximum range of wavelength that we could set to cover the relevant peaks was from 250 to 500 nm. For the FTIR (Nicolet 710, reflection mode) measurements, four layers were transferred onto vacuum-evaporated Ag at 20 mN/m as a LB film, and a drop of CHCl3 solution was dried on vacuum-evaporated Au as a cast film. For the AFM (TopoMetrix, the noncontact mode) observations, a single upstroke transfer was made on mica from pure water at 0.85 nm2/molecule.
Results Figure 1 shows Π-A isotherms of Azo-Glu and AzoGala on pure water and Azo-COOH on pH 11.3 water as well as on 0.5 mM CdCl2. Values of the molecular area at which Π starts to increase [A(Π > 0)], the limiting area obtained by extrapolating to zero pressure (A0), and the collapse pressure Πc with its corresponding collapse area Ac are summarized in Table 1. AFM observations suggest that a sudden drop of Π by Azo-COOH (Cd2+) at 23.9 mN/m is due to instability caused by crystallization and
Spectral Responses of Azobenzene Glycopyranoside Monolayers
Figure 2. Selected UV-vis reflection spectra of the glycopyranoside monolayers on water. Azo-Gala at 0.91 nm2/molecule is shown here to represent the spectra of all compounds at large molecular areas. It is characterized by the long-wavelength π-π* component (λmax) at 460 nm. Upon compression of the films, this peak shifts to a shorter wavelength (blue shift) for Azo-Gala, shown here for one at 0.49 nm2/molecule. In contrast, Azo-Glu induces a red shift to a longer wavelength, an example of which is shown for one at 0.46 nm2/molecule.
associated multilayer formation. Other than this drop, the isotherm of Azo-COOH (Cd2+) is seen as a simple horizontal shift of the Azo-COOH (pH 11.3) curve to the left. Despite the slight difference in curvatures, Azo-Gala and both cases of Azo-COOH have similar monotonically increasing isotherms. On the other hand, Azo-Glu exhibits a totally different type of the isotherm. It starts developing Π at a larger molecular area than other amphiphiles. The isotherm, then, shows a bump at 2.3 mN/m. It is not known whether the bump is due to a phase transition or film instability. AFM observations show no indication of multilayer formation at this pressure. The condensed isotherm gives a much smaller value of A0 than others do. The value of Πc is also the largest. Some examples of the UV-vis reflection spectra are displayed in Figure 2. All compounds show two π-π* transition peaks at 260 and 420-490 nm for the entire range of molecular areas. Upon compression of the monolayers, the 260 nm peak remains stationary, but the longer-wavelength component (λmax) shifts in various ways. Figure 3 shows plots of λmax as a function of the molecular area. When the area is large, all compounds have their maxima at around 460 nm. The same peak wavelength is also observed in the CHCl3 solution. Whereas AzoGala and both Azo-COOH cases show blue shifts, AzoGlu exhibits a red shift upon compression. The onsets of the shifts are close to A(Π > 0), with the exception of Azo-Glu, in which the peak starts red shifting after the bump in the Π-A isotherm. Figure 4 displays the amide region of the FTIR spectra obtained from the LB and cast films of both glycopyranosides. The spectra of both cast films are identical to those of the dilute CHCl3 solutions. The cast films of both compounds and the LB film of Azo-Gala give a peak at 1645 cm-1, whereas the LB film of Azo-Glu hardly shows any sign of this peak. Also, the intensity at 1510 cm-1 of the LB film of Azo-Glu is 15% larger than that at 1600 cm-1, whereas the corresponding peaks for the other films have roughly the same intensities. Three compounds exhibit distinct morphology as dis-
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Figure 3. Plot of λmax versus the molecular area. The measured points of each glycopyranoside are connected by the smooth curves, but those of Azo-COOH are omitted for clearer presentation. For Azo-COOH, the fine dashed curve connects the points for the monolayer on 0.5 mM CdCl2 and the coarse curve for that on a pH 11.3 subphase.
Figure 4. FTIR spectra of the LB and cast films of Azo-Glu and Azo-Gala, respectively. The intensities are normalized by the peak at 1600 cm-1. The arrows indicate the peaks at 1510 and 1645 cm-1.
played in the AFM images of Figure 5. We transferred the films at an area slightly greater than A(Π > 0) purposely to emphasize the difference; otherwise, both compounds produce uniformly flat films at smaller areas. The AzoGlu film features many islands with a small number of holes inside each island, and an island of Azo-Gala contains many holes. Azo-COOH produces highly crystalline islands already at this large area. It is not known if the multilayers seen in the image have been formed on water during compression or on mica during transfer. Discussion The typical cross-sectional areas of each chemical component are about 0.4 nm2 for double alkyl chains, 0.3 nm2 for the azobenzene unit, and 0.4-0.5 nm2 for R-glucopyranose and R-galactopyranose. The crosssectional area of a complete molecule depends on the angle
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Figure 5. AFM images (2.5 × 2.5 µm2) of (a) Azo-Glu, (b) Azo-Gala, and (c) Azo-COOH on mica. The darkest region corresponds to the mica surface.
that each component makes with the next-neighbor segment, as well as the molecular tilt angle from the water surface normal. The observation that Ac is close to the areas of double alkyl chains and the glycopyranose unit suggests that, at the collapse area, a molecule is in an extended conformation and stands nearly straight from the water surface. In the solid crystals of alkanoyl glycopyranosides in which the molecules are packed in bilayers, it is known that the area occupied by a galactopyranose unit is greater than that of a glucopyranose unit.12,13 Also, the present Ac value of Azo-COOH is smaller than those of the glycopyranosides. Thus, Ac of Azo-Glu and Azo-Gala reflects the areas occupied by the glycopyranoses. At pH 11.3, most of the carboxylic acids are ionized to increase repulsive interactions. In contrast, Cd2+ binds strongly to the carboxylic acids to pack molecules more closely. Both cases of Azo-COOH isotherms clearly demonstrate that these repulsive or attractive interactions on the hydrophilic headgroup result in uniform expansion or condensation of the isotherms without altering their shape. Thus, the completely different shapes of the AzoGlu and Azo-Gala isotherms imply that their difference cannot be explained simply by the size or the repulsive/ attractive interactions of the sugar headgroups. Because the λmax value at large molecular areas agrees with that of the chloroform solution, the present amphiphiles are in molecularly disordered states at large molecular areas. The blue shifts observed for both cases of Azo-COOH are commonly seen in various amphiphiles containing azobenzene. Such shifts are usually attributed to formation of H-like aggregates as the film is compressed.4,14 This means that the azobenzene units are stacked in a parallel fashion at high surface pressures, which is consistent with the highly oriented structure indicated by the isotherms. AFM results also support the conclusion that Azo-COOH is in a crystalline state. Most importantly, the spectral response of the glycopyranoside monolayers depends on the sugar unit. AzoGala shows a blue shift upon compression. In general, a blue shift can be induced by the formation of H-like aggregates as in Azo-COOH or the twisting of bonds within the chromophore component, reducing the planarity of the π-electron system.15 Here, similarities in the isothermal properties of Azo-Gala and Azo-COOH (12) Abe, Y.; Harata, K.; Fujiwara, M.; Ohbu, K. Langmuir 1996, 12, 636. (13) Abe, Y.; Fujiwara, M.; Ohbu, K.; Harata, K. J. Chem. Soc., Perkin Trans. 2 2000, 341. (14) Hochstrasser, R. M.; Kasha, M. Photochem. Photobiol. 1964, 3, 317.
Scheme 2
suggest that aggregation might be the major factor. In contrast, Azo-Glu gives a red shift. It is known that the azobenzene units in head-to-tail stacking can produce a red shift.2,14 Such stacking requires the molecules to be in a highly tilted orientation from the surface normal. The isotherm, however, does not support such a large tilt. The observed red shift can be explained by a change in electronic structures. The electronic state of the amide group is usually a mixture of two resonance forms, X and Y, as illustrated in Scheme 2. FTIR spectra of all films other than the LB film of Azo-Glu show a peak at 1645 cm-1. This is a carbonyl amide I band. The amide II band might overlap the amide I band or be hidden under the ring-stretching bands at 1510 and 1600 cm-1. Thus, the amides in these films take the X form. On the other hand, the LB film of Azo-Glu has no amide I band but shows an additional peak around 1510 cm-1 the overlap with the ring-stretching bands. The CdN stretching vibration of a similar compound is reported to appear at around 1500 cm-1.16 This indicates that the Y form contributes mainly to the amide in the LB film of Azo-Glu. Because of the conjugated double-bond character of Ar-CdNAr′, both aromatic groups and CdN lie on the same plane. Because the planarity of the entire π-electron system is increased, the Y form produces a red shift. In contrast, the single-bond character of Ar-C-N-Ar′ in the X form has no such restriction on rotation around each bond and does not guarantee planarity. Red shifts in fluorescence spectra have been reported in amide-linked aromatic systems by a similar mechanism known as twisted intermolecular charge transfer.17 This explanation based on the different amide structures is consistent with the previous argument in terms of the different isothermal shapes. Thus, the contrasting spectral response is exhibited by the glycopyranose units that control the resonance form of the amide. It is amazing to note that this stems from (15) Forber, C. L.; Kelusky, E. C.; Bunce, N. J.; Zerner, M. C. J. Am. Chem. Soc. 1985, 107, 5884. (16) Fillaux, F.; Fontaine, J. P.; Baron, M.-H.; Kearley, G. J.; Tomkinson, J. Chem. Phys. 1993, 176, 249. (17) Anada, T.; Kitaoka, T.; Ota, H.; Kakizawa, Y.; Akita, T.; Morozumi, T.; Nakamura, H. Bunseki Kagaku 1999, 48, 1107.
Spectral Responses of Azobenzene Glycopyranoside Monolayers
only one structural difference in either an equatorial or an axial position of the OH group on a pyranose ring. Although there is a possibility that these sugar headgroups produce different in-plane packing structures of azobenzene molecules,18 the structures and properties of glycopyranosides at the air-water interface are not well enough understood to deduce such in-plane structures. Although the mechanism by which a single OH group affects the resonance form is not known, the present result clearly demonstrates that such a small structural difference is enough to induce contrasting optical behaviors. Additionally, the indirect path to modifying the optical response of the auxochrome unit ensures that the major spectral characteristics are still dominated by the chromophore, as evidenced by the symmetric change in λmax on the (18) Song, X.; Perlstein, J.; Whitten, D. G. J. Am. Chem. Soc. 1995, 117, 7816.
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molecular area starting from the same molecularly dispersed state at 460 nm. Conclusions Glycopyranosides that exhibit sugar-dependent spectral responses to a change in the molecular area were successfully synthesized. The fundamental structure consists of double alkyl chains, an azobenzene chromophore, an amide-benzene auxochrome, and a sugar headgroup. A blue shift was attained by formation of H-like aggregates, whereas a red shift resulted from the increased planarity of the chromophore and auxochrome units through a resonance structure. Because a huge number of structural variations are available for sugar, the present result indicates the feasibility of developing a library for a combinatorial approach to the coloring of ultrathin films. LA010187N
