Ordered Phases in Langmuir Monolayers of an Azobenzene

Langmuir 1998, 14, 899-903

899

Ordered Phases in Langmuir Monolayers of an Azobenzene Derivative M. K. Durbin,*,† A. Malik,† A. G. Richter,† C.-J. Yu,† R. Eisenhower,‡ and P. Dutta† Department of Physics and Astronomy, Northwestern University, Evanston, Illinois 60208, and Metals and Ceramics Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37830 Received September 11, 1997. In Final Form: October 31, 1997 We have performed grazing incidence X-ray diffraction studies of Langmuir monolayers of an azobenzene derivative. There are only two phases over a range of temperatures and pressures in which at least seven phases exist in monolayers of simpler molecules (0-30 °C, 0-40 dyn/cm). The low-pressure phase has tilt toward a nearest neighbor, while the high-pressure phase has tilt toward a next-nearest neighbor. Within each phase the tilt angle does not change significantly with pressure. This behavior is very different from that of monolayers with saturated hydrocarbon tails and implies that the azobenzene groups ‘lock’ and prevent the molecules from sliding against each other. However, the lattice parameters change with pressure; this suggests that the molecular conformation changes. The structure of the low-pressure phase is similar to one observed in azo-derivative bilayer membranes and crystals, while the high-pressure phase has a structure not seen before in such systems.

Introduction Azobenzene-containing amphiphiles form a variety of layered systems. Some are known to have thermotropic liquid-crystalline phases,1 while others form bilayer membranes in aqueous media.2-4 The crystalline forms of these amphiphiles,5-9 as well as the crystalline form of pure azobenzene,10,11 are layered. The tendency of the optically active functional group to orient in these layered structures may permit them to be used for secondharmonic generation in molecular devices.12 The orientation and packing of the molecules within a layer affects their optical characteristics,3 which may be important in the eventual design of any such devices. A few azobenzene derivatives have been shown to spread at the air-water interface. These Langmuir films provide the opportunity to study the basic building block of the layered systemsa single monolayer. Constraining molecules to a water surface allows us to manipulate them using lateral compression and may allow the molecules to form structures that do not occur naturally. Such monolayers are also interesting as model twodimensional systems. Langmuir monolayers of simpler † ‡

Northwestern University. Oak Ridge National Laboratory.

(1) Sano, M.; Kunitake, T. Langmuir 1992, 8, 320. (2) Shimomura, M.; Aiba, S. Langmuir 1995, 11, 969. (3) Shimomura, M.; Ando, R.; Kunitake, T. Ber. Bunsen-Ges. Phys. Chem. 1983, 87, 1134. (4) Okuyama, K.; Ikeda, M.; Yokoyama, S.; Ochiai, Y. Chem. Lett. 1988, 1013. (5) Okuyama, K.; Watanabe, H.; Shimomura, M.; Hirabayashi, K.; Kunitake, T.; Kajiyama, T.; Yasuoka, N. Bull. Chem. Soc. Jpn. 1986, 59, 3351. (6) Xu, G.; Okuyama, K.; Shimomura, M. Bull. Chem. Soc. Jpn. 1993, 66, 2182. (7) Okuyama, K.; Mizuguchi, C.; Xu, G.; Shimomura, M. Bull. Chem. Soc. Jpn. 1989, 62, 3211. (8) Xu, G.; Okuyama, K.; Shimomura, M. Bull. Chem. Soc. Jpn. 1991, 64, 248. (9) Xu, G.; Okuyama, K.; Shimomura, M. Mol. Cryst. Liq. Cryst. 1992, 213, 105. (10) DeLange, J. J.; Robertson, J. M.; Woodward, I. Proc. R. Soc. A 1993, 171, 398. (11) Brown, C. J. Acta. Crystallogr. 1966, 21, 146. (12) Sekkat, Z.; Bu¨chel, M.; Orendi, H.; Menzel, H.; Knoll, W. Chem. Phys. Lett. 1994, 220, 497.

molecules, with saturated hydrocarbon tails, have been extensively studied. They have a complicated phase diagram which results from the interplay between a variety of subtle interactions (head-head, tail-tail, headsubphase, etc.) of comparable magnitude.13-18 Azobenzene derivatives, with stronger interactions and more complicated shapes, may have simpler phase diagrams because there are fewer phases consistent with the molecular interactions. There are a number of methods used to study layered systems of azobenzene derivatives. These include UV absorption spectroscopy (UVAS),2-4 Brewster angle microscopy (BAM),19,20 FTIR spectroscopy,21 scanning tunneling microscopy (STM),22,23 and X-ray diffraction.5-9,23 Only two of these methods have so far been adapted to study azo-derivative monolayers at the air-water interface. Kawai et al.24 measured the UVAS of a Langmuir monolayer of the molecule 4-octyl-4′-(carboxytrimethyleneoxy)azobenzene (C8AzoC3, see structure inset in Figure 1). The isotherm for this molecule appears in Figure 1 (we show our isotherm, but it is essentially the same as that observed by Kawai et al.24 and Tabe and Yokoyama19). There is a flat section indicating a first-order phase transition at 12.5 dyn/cm. For all pressures below this transition, Kawai et al. observed a wavelength shift of the primary absorption peak to 332 nm, close to that observed for liquid-crystalline bilayers (330-340 nm).3 At pres(13) Barton, S. W.; Thomas, B. N.; Rice, S. A.; Lin, B.; Peng, J. B.; Ketterson, J. B.; Dutta, P. J. Chem. Phys. 1988, 89, 2257. (14) Lin, B. Ph.D. Thesis, Northwestern University, 1990. (15) Kaganer, V. M.; Peterson, I. R.; Kenn, R. M.; Shih, M. C.; Durbin, M. K.; Dutta, P. J. Chem. Phys. 1995, 102, 9412. (16) Durbin, M. K.; Malik, A.; Ghaskadvi, R.; Shih, M. C.; Zschack, P.; Dutta, P. J. Phys. Chem. 1994, 98, 1753. (17) Shih, M. C.; Durbin, M. K.; Malik, A.; Zschack, P.; Dutta, P. J. Chem. Phys. 1994, 101, 9132. (18) Shih, M. C.; Bohanon, T. M.; Mikrut, J. M.; Zschack, P.; Dutta, P. J. Chem. Phys. 1992, 96, 1556. (19) Tabe, Y.; Yokoyama, H. J. Phys. Soc. Jpn. 1994, 63, 2472. (20) Tabe, Y.; Yokoyama, H. Langmuir 1995, 11, 4609. (21) Kawai, T.; Umemura, J.; Takenaka, T. Langmuir 1990, 6, 672. (22) Sano, M.; Sasaki, D. Y.; Isayama, M.; Kunitake, T. Langmuir 1992, 8, 1893. (23) Loo, B. H.; Liu, Z. F.; Fugushima, A. Surf. Sci. 1990, 227, 1. (24) Kawai, T.; Umemura, J.; Takenaka, T. Langmuir 1989, 5, 1378.

S0743-7463(97)01033-0 CCC: $15.00 © 1998 American Chemical Society Published on Web 01/23/1998

900 Langmuir, Vol. 14, No. 4, 1998

Durbin et al.

Figure 1. π-A isotherm of C8AzoC3. Both the phase transition and the monolayer collapse are indicated. Inset is a phase diagram showing transition pressure and collapse pressure as a function of temperature.

sures above the transition, they observed a greater wavelength shift to 313 nm, which is closer to the 300 nm seen for H-aggregates.3 Tabe and Yokoyama19,20 performed depolarized Brewster angle microscopy (DBAM) on the same monolayers and found textures reminiscent of bulk liquid-crystal Smectic C phases at low pressures and solid-like textures at high pressures. Both of these methods seem to indicate that the monolayer undergoes a first-order transition from a disordered liquid-crystalline state to a more ordered phase. In order to learn more about the actual packing in these phases, we performed an X-ray diffraction study of the Langmuir monolayer of C8AzoC3. Experiment Our experimental apparatus has been described in previous publications.13,14 X-ray diffraction studies were performed at the Oak Ridge National Laboratory Beam Line X-14 of the National Synchrotron Light Source, Brookhaven National Laboratory. In order to decrease background scattering from the water, the beam was focused in the vertical direction and incident at an angle of ∼1.9 × 10-3 radians, which is just below the critical angle for total external reflection from water. The X-ray energy was 8.5 keV. The resolution, determined by two sets of crossed Soller slits in front of the detector, was ∼0.01 Å-1 full width at half maximum (fwhm) horizontally and ∼0.05 Å-1 fwhm vertically. The subphase water was purified to 18 MΩ‚cm using a Barnstead NANOpure system, and pH was adjusted to 2 using HCl for most studies. For studies as a function of pH when BaCl2 (from Sigma Chemical Company) was present in the subphase, we adjusted the pH using HCl and NaOH or NH4OH. The C8AzoC3 (full name 4-octyl-4′-(3-carboxytrimethyleneoxy)azobenzene) was obtained from Dojindo Laboratories and dissolved in HPLC-grade chloroform (from Aldrich Chemical Company) to form the spreading solution. A slight overpressure of helium was maintained in the trough in order to reduce radiation damage and air scattering.

Results and Discussion X-ray Diffraction Data. The flat section in the isotherm (Figure 1) at 12.5 dyn/cm indicates a strongly first-order phase transition. The inset in Figure 1 shows the position of the flat section as well as the collapse pressure as a function of temperature, demonstrating that the isotherm does not vary very much with temperature between 0 and 30 °C. Because we also found very little change in the X-ray diffraction patterns as a function of temperature in this range, we will discuss only the roomtemperature data here.

Figure 2. Sample X-ray diffraction data from the C8AzoC3 monolayer at room temperature: (a) above the transition pressure; (b) below the transition pressure. Data are plotted as intensity contours (arbitrary units) in the Kz/Kxy plane. In each plot the outermost contour is one unit of intensity above background, and successive contours mark intensity increases by the same unit.

We took diffraction data at several pressures above and below the flat section. As expected, we observed a structural transition at 12.5 dyn/cm. Sample contour plots of intensity as a function of in-plane (Kxy) and off-plane (Kz) wave vectors are shown in Figure 2. It is not possible to break Kxy down into the subcomponents Kx and Ky because the monolayers are powders in the horizontal plane. As can be seen in Figure 2, there are two peaks in each phase. At low pressures (Figure 2b), there is one peak in plane (Kz ) 0) and one peak at Kz ∼ 0.8 Å-1. At high pressures (Figure 2a), both peaks are off-plane, at roughly Kz ∼ 0.4 Å-1 and Kz ∼ 0.8 Å-1. The peak at high Kz appears cut off in Figure 2a because it is near the highest Kz that our setup could achieve. This did not significantly affect our ability to locate the peak positions, since the centers of the peak contours were always within the observable range. We did not observe any other peaks, even at low temperatures. With only two peaks, it is impossible to determine the structure unambiguously. We make the reasonable assumption that the molecules are packed locally into a distorted hexagonal (DH) lattice. If the molecules are perpendicular to the surface of the water, then the observed diffraction pattern will have two inplane peaks, with one of the peaks degenerate. If the molecules tilt, one or both of these peaks will be moved out of the plane. The absence of a third first-order peak means that the molecules must be tilted along one of the symmetry directions of the lattice. Tilt toward a nearest neighbor (NN tilt) gives a diffraction pattern with one in-plane peak and one off-plane peak, while tilt toward a next-nearest neighbor (NNN) gives a diffraction pattern with two off-plane peaks, one at twice the Kz of the other. The fact that the Kz of the second peak in Figure 2a is at twice the Kz of the first peak supports our assumption, since it is what would be expected for a DH lattice with NNN tilt. Furthermore, in both phases the assumption

Ordered Phases in Langmuir Monolayers

Langmuir, Vol. 14, No. 4, 1998 901

Figure 3. Schematic diagram showing tilted molecules in a distorted-hexagonal lattice and defining terms used in this paper.

of a DH lattice leads to a calculated area per molecule that agrees reasonably well with the area per molecule expected from the macroscopic isotherm shown in Figure 1. If the lattice were rectangular or oblique, with the same area/molecule, we would see an additional peak in the vicinity of the two observed peaks. No such peak was observed, though we searched reciprocal space from Kxy ) 0.8 Å-1 to Kxy ) 2.0 Å-1 and from Kz ) 0 Å-1 to Kz ) 1.0 Å-1. We fit the data in two dimensions to obtain the peak positions in Kz and Kxy. From these values we can calculate the bond lengths in the horizontal plane, the average molecular tilt orientation and magnitude, and the bond lengths in the plane perpendicular to the molecules (‘tilted plane’). A schematic diagram of tilted molecules in a DH lattice which defines the terms we will use in this paper is shown in Figure 3. Because we see only two first-order peaks, the triangle that defines the lattice must have two sides that are equal (the sides a′ in Figure 3). The values for each of the variables shown are plotted in Figure 4 as functions of the monolayer pressure. Below we compare the calculated structures to layers found in threedimensional systems of similar molecules, and then we compare the phase behavior to that of Langmuir monolayers of simpler molecules. Comparison to Bulk Structures. There are only a few full structural studies of azobenzene-containing layered structures. Most of these were performed by Shimomura and co-workers5-9 and involved azoammonium amphiphiles either in the crystalline state or as cast bilayer films. The most commonly occurring class is the J-aggregate, which is highly tilted.5-8 The area per molecule within such a layer would be much too large to correspond either to the isotherm or to our X-ray data. Further evidence is that the bathochromic UVAS wavelength shift observed in bulk J-aggregates3 was not observed by Kawai et al.24 in the UVAS monolayer study. Instead, both phases give hypsochromic wavelength shifts close to those of the bulk H-aggregate and so-called liquidcrystalline structures.24 The H-aggregate structure has a DH lattice with bond lengths of a ) 6.5 Å and a′ ) 4.9 Å, and the molecules have NN tilt (φ ) 0°) of magnitude θ ∼ 37° from the layer normal.9 As can be seen in Figure 4a and b, the direction of tilt, the magnitude of tilt, and the NN spacing of 6.5 Å for the monolayer just before the transition agree very well with the H-aggregate structure. The only difference is that a′ is larger for the C8AzoC3 monolayer than for the bulk azoammonium bilayer (5.5 Å vs 4.9 Å). This difference may occur for one or more reasons: the molecules have different head groups, the water may influence the structure, and packing is not stabilized by three-dimensional interactions. It is inter-

Figure 4. (a) Bond lengths in the horizontal plane: a (open squares) and a′ (filled squares). (b) Tilt magnitude θ (4) and orientation φ ([) and bond lengths ap (O) and ap′ (b) in the tilted plane, calculated from fits to the diffraction data from a C8AzoC3 monolayer along a room-temperature isotherm. The vertical line marks the transition seen in the isotherm data.

esting to note that the UV wavelength shift for this phase is not as large as would be expected for a 3D crystalline H-aggregate.24 Although this could be due to the differences in bond spacings, it is probably a result of the lack of long-range interactions between the azo moieties. The correlation lengths determined from the peak widths average ∼80 Å. This lack of long-range order is also consistent with the results of Tabe and Yokoyama.19 The high-pressure phase appears to be a novel structure, not seen before in any azobenzene related material. The DH bond lengths at the highest pressure (before collapse), as can be seen in Figure 4, are a ) 5.3 Å and a′ ) 5.5 Å. The tilt is in the NNN direction (φ ) 90°), with a magnitude of θ ∼ 33°. The tilt direction and spacings are wrong for both the J- and H-aggregates.5-9 We also tried comparing the structure to the packing of azobenzene10,11 and azotoluene,25 but those crystals are far more compact than this. Finally, we compared it to the packing of a similar molecule, 4-octyl-4′-(carboxypentamethyleneoxy)azobenzene, studied by Loo et al.23 Their layers were deposited using different spreading methods onto pyrolytic graphite surfaces and then studied using STM. The structure was oblique, with spacings which did not correspond to the peaks we have observed. We therefore conclude that we have observed a new structure, not seen before in layered azobenzene systems. We believe that the change in the tilt direction of the molecules with respect to the lattice (from NN to NNN) is the cause of the observed UVAS shift at the phase transition as well as the change in observed DBAM textures. At the transition from the NN to the NNN phase, we observed only a small anisotropic change in the average correlation lengths (from ∼70 to ∼200 Å in the NNN (25) Brown, C. J. Acta Crystallogr. 1966, 21, 153.

902 Langmuir, Vol. 14, No. 4, 1998

direction; there was no change in the NN direction). This may be enough of a change to explain the change in DBAM textures observed by Tabe and Yokoyama,19 but it implies that both phases are mesophases; the high-pressure phase is not really solid-like. This result is consistent with studies of fatty acid monolayers, which become only slightly more ordered as a function of monolayer pressure.15,16 Monolayer Comparisons. Examining Figures 1 and 4, we immediately note several differences between the behavior of the C8AzoC3 monolayer and that of a typical long-chain fatty acid monolayer. Although fatty acids at certain temperatures undergo a transition from NN to NNN tilt, the transition is only weakly first order.15 In some cases the transition is so weak that it is not observed in the isotherm at all.16 The difference in the strength of the phase transition is an indication of the importance of tail-group packing in determining the monolayer behavior. Also, for alkane chain molecules, the dominant effect of a change in pressure within a given phase is to change the tilt angle;17 the distances between molecules measured in the ‘tilted plane’ do not change. Here, however, the tilt angle is essentially constant, while the spacings in the tilted plane change (Figure 4c). Because we do not know the relative tilt or orientation of the azo moiety with respect to the straight-chain portion of the molecule, Figure 4c only gives a rough idea of the molecule-molecule packing. However, the X-ray (and isotherm) data show a compressibility in both phases that is comparable to or larger than that of fatty acid monolayers. Since this compression is achieved without tilt changes, it is likely that the conformation of the molecules is changing as a function of pressure. We can speculate as to why the reduction in area is accomplished in this manner for the azo derivative. Either because of interactions between the azo groups or because of the shape of the molecule, one molecule does not slide past another as easily as for a straight-chain molecule. This means the tilt angle cannot easily change within a phase (it changes slightly at the phase transition). However, X-ray diffraction from bulk H-aggregatesswhich the low-pressure monolayer structure resemblessshows that the azobenzene molecule is not quite planar in this phase.9 It may be easier to compress the monolayer by increasing the planarity of the molecules than by having them slide over each other. pH Dependence. The above studies were performed at a pH of 2 in order to prevent any residual metal ion contaminants from affecting the monolayer structures. A final difference between the C8AzoC3 monolayer and a fatty acid monolayer can be seen when the monolayers are studied as the pH is varied with metallic counterions (Ca2+, Ba2+, etc.) in the subphase. We originally performed this study in order to gain more insight into the transition between the low-pressure NN phase and the high-pressure NNN phase. Kawai et al.21,24 found that at high pH values, in the presence of Ba2+, the isotherm flat section disappears, and the UVAS primary absorption peak moves gradually from that expected of the low-pressure phase to that expected of the high-pressure phase. The transition between the two phases cannot become continuous because they have different symmetries, but there might be an intermediate phase, with tilt that moves from one symmetry direction to the other and with weak second-order transitions to the two known phases. However, when we performed X-ray diffraction at high pH values with Ba2+ in the subphase, we found that the monolayer retained the low-pressure NN-tilted structure throughout the achievable pressure range. As can be seen in Figure 5,

Durbin et al.

Figure 5. Peak positions (top, Kxy; bottom, Kz) of the C8AzoC3 monolayer as a function of pressure at pH ) 2 (open symbols) and at pH ) 7 (filled symbols). Circles represent the degenerate peak, a′, while squares represent the nondegenerate peak, a. At low pressures the observed positions (hence the structure) are essentially the same. At high pressures the pH ) 7 film has continued the trend of the low-pressure phase, while the pH ) 2 film has undergone a transition to a new phase.

a more compressed version of the structure can be achieved at high pH values without forcing the monolayer into the NNN phase. This contrasts sharply with the behavior of fatty acid monolayers at high pH valuesstheir ‘swiveling transition’ disappears because the high-pressure NNN phase becomes stable even at low pressures.18 It is particularly surprising that the pH dependence should be so different because the head group of C8AzoC3 is the same as that of the fatty acids. The interactions between the counterions and the head groups should be the same. The difference may be in the coupling between tail-group orientation and headgroup orientation. If the phase transition involves a reorientation of the tail groups, it also forces a reorientation of the head groups. The two orientations in a straightchain monolayer may be equally well accommodated by the head groups, while the orientation resulting in the NNN tilted phase in the azo-derivative monolayer may be impossible to achieve for strongly interacting head groups. Conclusions At low pressures and high pH values the azo-derivative monolayer is very similar to an azo-derivative bulk system (the H-aggregate). When the pressure is increased, at first the conformation of the molecules changes while the tilt angle remains locked, and then the molecules are forced into a structure that is not observed in layered azobenzene systems. While this NN f NNN transition is reminiscent of the swiveling transition of fatty acid monolayers, differences between their behaviors as a function of pressure reveal fundamental differences in their intermolecular interac-

Ordered Phases in Langmuir Monolayers

tions. In both cases the structure in the tilted plane is probably determined by the steric and van der Waals interactions of the tail groups, but the mutability of the azobenzene cross section allows the azo derivative to be compressed without changing the molecular tilt. By examining our structural results in light of the UVAS data of Kawai et al.,24 we see that the direction of tilt of the molecules has a significant effect on their optical properties. The high-pressure phase is known to exhibit a high shift of the primary UVAS wavelength,24 yet the X-ray correlation lengths demonstrate that this is not due to high aggregation in the film. If the NNN tilted structure could be achieved with larger domain sizes or in bulk layered systems, the shift might be even larger. A twodimensional pressure cannot normally be applied to bulk

Langmuir, Vol. 14, No. 4, 1998 903

systems, but 3D versions of this structure might be achieved through Langmuir-Blodgett deposition or through humidity control of bilayer membranes. Studies of this new phase in such systems might demonstrate new and interesting ways to affect the optical properties of azo-derivative films through control of their in-plane structure. Acknowledgment. This work was supported by the U.S. Department of Energy under Grant No. DE-FG0284ER45125 and was performed in part at Beam Line X14 of the National Synchrotron Light Source, both of which are supported by the U.S. Department of Energy. LA9710334