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Langmuir 1995,11, 4609-4613
Two-DimensionalDynamic Patterns in Illuminated Langmuir Monolayers Yuka Tabe* and Hiroshi Yokoyama Electrotechnical Laboratory, 1-1-4 Umezono, Tsukuba, Ibaraki 305, Japan Received July 31, 1995. I n Final Form: October 9, 1995@
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Purely light-driven spatiotemporalpattern formation has been found to take place in a liquid-crystalline cis Langmuir monolayer consisting of an amphiphilic azobenzene derivative, undergoing the trans photoisomerizations. The Langmuir monolayer is in a smectic-C-likeliquid crystal phase, whose twodimensional orientation is easily perturbed by slight conformation changes in the constituent molecules. On illuminationwith a linearly polarizedlight, a collective and global in-planereorientation ofthe azobenzene chromophoresis induced over an existing static stripe texture, which finally yields polarization-dependent steady state orientational patterns. Prolonged photoexcitation generates sustained traveling and solitary waves, associated with variations in molecular tilt directions. The liquid crystallinity of the monolayer allows these orientational responses to occur even at an extremely weak light power at least 2 orders of magnitude smaller than that known in previous photoreorientation studies of azobenzene chromophores.
Introduction Dynamic patterns in condensed phases, such as Rayleigh-Benard convective rolls and Williams domains in nematic liquid crystals, have been attracting considerable attention as striking examples of the so-called dissipative structures.l The Langmuir monolayer, a monomolecular thick layer of amphiphilic molecules spread a t the airwater interface, is regarded as one of the ideal twodimensional (2D) systems and exhibits a rich variety of intriguing patterns in its mesophase regime.2 These patterns result from a n intricate interplay among competing intermolecular forces, moderate positional and orientational ordering, and the broken mirror symmetry a t the air-water interface.2 Recently, we found that a class of amphiphilic derivatives of azobenzene, a chromophore known for its ability to undergo trans-cis photoisomerization, can form a unique Langmuir monolayer showing a 2D-analog of the smectic C phase.3 Unlike many other 2D liquid crystalline phases, which have been extensively identified in Langmuir monolayers in the past d e ~ a d e the , ~ azobenzene Langmuir monolayer is liquidlike in the in-plane positional ~ r d e r i n gwhile , ~ the molecular long axis is coherently tilted away from the surface normal as illustrated in Figure 1. As a consequence, the monolayer is extremely fluid, yet a t the same time is orientationally ordered in terms ofthe molecular tilt over a distance as large as a few millimeters,
* To whom correspondence may be addressed: Phone, +81298 58 5593; Fax, +81 298 58 5548;e-mail,
[email protected]. Abstract published in Advance A C S Abstracts, November 15, 1995. (1)Gray, P.; Scott, S. K. Chemical Oscillations and Instabilities; Oxford University Press: New York, 1990. (2)Seul, M.;Andelman, D. Science 1996, 267, 476. (3)Tabe, Y.; Yokoyama, H. J. Phys. SOC.Jpn. 1994, 63, 2472. (4)Qiu, X.; Ruia-Garcia, J.; Knobler, C. M. Mater. Res. SOC.Symp. Proc. 1992, 237, 263. (5) Our recent synchrotron X-ray diffraction experiments have revealed no diffraction peaks, despite extensive search in k-space, for both liquid crystalline and solidlike phases ofthe azobenzene derivative monolayer, indicating the possible absence of positional correlation. Quite remarkably, however, a homologue with a longer alkoxy tail, 4-dodecyloxy-4'-(3-carboxytrimethylenoxy)azobenzene,exhibited conspicuous diffraction peaks in its solidlike phase corresponding to the hexagonal lattice tilted to the nearest neighbor with the shortest lattice spacing of d = 0.38 nm and the molecular tilt angle of p = 43".This compound does not have a smectic-C-like phase, but only the solidlike phase which is continuously miscible, in monolayer state, with the solidlike phase of the azobenzene derivative in question. Although their mixed monolayer shows diffraction peaks almost at the same position as above, their intensity is steadily reduced as the fraction ofthe present azobenzene derivative is increased. More details will be reported elsewhere. @
0743-7463/95/2411-4609$09.00/0
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Figure 1. Schematic illustration of azobenzene derivative Langmuir monolayer in the smectic C phase and the experimental setup. The molecules are tilted away from the surface normal to a certain direction, which is specified by a unit vector n,the director. A p-polarized light from a 10 mW He-Ne laser is incident on the monolayer at an incident angle of 20" and s-polarizedcomponent of the reflected light is observed with a CCD camera. The excitation light passed through an interference filter (480 f 12 nm)is incident normally on the monolayer.
thereby manifesting typical schlieren texture^,^ apparently analogous to those of bulk nematic and smectic C under a reflection-mode polarizing m i ~ r o s c o p e . ~How,~ ever, the present Langmuir monolayer has a fundamental difference from the single layer of bulk smectic C as well as from the 2D nematic phase that the mirror symmetry about the layer plane is absent because of the strict distinguishability of the head and the tail of the molecules in Langmuir monolayers. Therefore, the tilt orientational order of the monolayer should be truely vectorial, characterized by a n arrow drawn along the average molecular orientation, represented by a unit vector n, the director, as depicted in Figure 1. Due to this 2D polar ordering, (6)Pindak, R.;Young, C. Y.; Meyer, R. B.; Clark, N. A. Phys. Reu. Lett. 1980, 45, 1193.
(7)Pang, J.;Muzny, C. D.; Clark, N. A. Phys. Rev. Lett. 1992, 69, 2983.
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the liquid crystalline azobenzene monolayer readily develops a stripe pattern associated with a continuous rotation of the azimuthal angle of the director3 in quite a similar manner as found in free-standing smectic fiims.7-9 We show here that, when the constituent molecules are photoexcited to undergo trans-cis isomerizations, the liquid crystalline Langmuir monolayer exhibits an anomalous transient orientational response causing a dramatic change ofthe static pattern, followed by a prolonged spontaneous generation of 2D traveling orientational waves. This is one of the few cases of non-equilibrium dynamic pattern formation purely driven by light-excitation.1°
Materials and Methods The Langmuir monolayer we used is a single layer of amphiphilic derivative of azobenzene, 4-octyl-4'-(3-carboxytrimethylenoxy)azobenzene, spread a t the air-water interface. The monolayer was prepared a s described previously3 from a m o m chloroform solution on a surface of distilled and deionized water. All the observations reported here were performed a t 25 "C. The spread monolayer assumes a two-dimensional (2D) analog of smectic C phase a t surface pressures below about 15 mN/m, above which the monolayer is transformed into a solidlike phase across a clear first order phase transition. The experimental setup is shown in Figure 1. The excitation light passed through an interference filter (480 i 12 nm) is incident normally on the monolayer from below through a quartz window. The liquid crystal textures of the monolayer are observed with a depolarized reflected light microscope (DRLMP which is functionally similar to the polarizing microscope; p-polarized light (electric vector parallel to the incident plane) from a 10 mW He-Ne laser is obliquely incident on the monolayer and the depolarized ( i e . s-polarized) component of the reflected light is observed with a CCD camera. The depolarized reflectivity from a n optically uniaxial monolayer is generally given by1'
where f = nw2tan OJcl t a n /3. Here, a and /3 are t h e azimuthal angle (measured from the incident direction) and the tilt angle from t h e surface normal of the director, respectively, Bt is the angle of refraction in water, n, and €1 are the refractive index of water and the ordinary dielectric constant of the monolayer, respectively, and h is a Fresnel factor independent of a. On the basis of this formula, we can infer the director orientation of the liquid crystalline monolayer from DRLM images. For the present monolayer viewed a t the incident angle of 20°, we h a v e f - 0.25 < 1 assuming /3 x/4 and €1 2.6, so t h a t , during a complete one t u r n of the director azimuth, there appear four extinction bands a t a = 0, I, +cos-', f, and two bright and two less bright bands between each pair of dark bands (shown in Figure 2a).
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Asymmetrical Reorientation of Stripe Structure Even in the dark, the Langmuir monolayer spontaneously develops a static stripe pattern as shown in Figure 2a, in which the director continuously winds up along the normal of the stripe (analyzed by using eq 1). Such orientational pattern, similar to that observed in freestanding smectic and in Lagmuir monolayers of smectic liquid crystal^,^ are basically understood as resulting from competition between the Frank-Oseen curvature elasticity and the surface-induced flexoelectric polarization of the m o n ~ l a y e r . ~ J ~ J ~ Illuminating this liquid-crystalline azobenzene monolayer with a linearly polarized light a t a wavelength ( 8 ) Meyer, R. B.; Pershan, P. S. Solid State Commun. 1973,13,989. ( 9 ) Maclennan, J. E.; Sohling, U.; Clark, N. A,; S e d , M. Phys. Reu.
E 1994,49, 3207. (10) Nitzan, A; Ross, J. J . Chem. Phys. 1973, 59, 241. (11)Tabe, Y.; Yokoyama, H. Langmuir 1995, 12, 699. (12) Hinshaw, G. A. Jr.; Petschek R. G. Phys. Reu. B 1988,37,2133. (13) Selinger,J. V.; Wang, Z-G.;Bruinsma, R. F.; Knobler, C. M.Phy. Reu. Lett. 1993, 70, 1139.
Letters suitable to induce trans-cis isomerizations (350 nm < 3, < 550 nmY4causes a dramatic change of the stripe texture as shown in Figure 2 . Depending on the polarization direction relative to the stripe, only a certain quadrant of the 2x stripe expands, while the rest bunches up to form almost a 2n defect wall, through a transient collective reorientation of molecules toward a light-induced preferred direction. This transient motion terminates in a few seconds on illumination, resulting in a steady-state anisotropic pattern as shown in parts b and c of Figure 2 . Although the photoexcitation itself should be locally nonpolar, the observed macroscopic photoresponse has a clear in-plane polarity, in the sense that it is asymmetrical against the in-plane reversal of the director, i.e. (n,,n,,n,) (-nx,-ny,n,). This stems obviouslyfrom the polar liquidcrystalline nature of the monolayer and presents a sharp contrast with numerous recent studies on light-induced reorientations of azobenzene derivatives bound on solid surfacesl6-l8 o r in polymer m a t r i c e ~ ; l these ~ - ~ ~studies, aimed a t light control of liquid crystal alignment, share a common observation that the molecules containing azobenzene moiety assume a nonpolar angular distribution with a preferred orientation perpendicular to the plane of polarization. In the present monolayer, however, the preferred direction is not necessarily perpendicular to the polarization direction but changes continuously from parallel (Figure 2b) to perpendicular (Figure 2c) to it, associated with the rotation of polarization plane from parallel to perpendicular to the stripe. Also suggesting the importance of liquid crystallinity of the monolayer, the present photoresponse occurs a t an extremely low incident power of 1-10 pW/cm2 a t 2 = 480 nm, which is a t least 2-orders of magnitude smaller than that needed in these previous reorientation studies.16-22 When the incident power exceeds this level, the texture breaks into a granular texture comprised of randomly oriented domains of 10 pm in diameter or less. This granular texture also appears, even when the monolayer is illuminated by an intense unpolarized or circularly polarized light, which otherwise induces no collective photoresponses. When the illumination was shut off, the initial stripe texture was recovered within 10 s, yet definitely more slowly than the rise of the steady-state patterns. In the solidlike phase at surface pressures above 15 mN/m, no such photoresponses were observed. Traveling OrientationalWaves. Quite interestingly, continued excitation of the monolayer after the cessation of the asymmetric transient photoresponse leads to a spontaneous generation of traveling orientational waves in the expanded and uniformly oriented regions as shown in Figure 3. The embryos of the wave (observed as dark
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(14)All the experiments reported here were conducted under illumination at i= 480 nm, although similar photoresponses could be induced at any wavelength in this region as long as the light intensity was properly controlled. The photoresponse seems to occur only when the fraction of cis isomer is not zero but is kept substantially smaller than that of the trans isomer; when too large a fraction is converted to the cis form, as normally happens under UV illumination, the monolayer can no longer stay in the liquid crystalline state and breaks into the granular structure as mentioned in the text. It is therefore much easier in the present situation to work with visible light that automatically keeps the cis fraction relatively low.'j (15)Zimmermann, G.; Chow, L.; Paik, U. J . Am. Chem. SOC.1958, 80, 3528. (16)Ichimura, K.; Hayashi, Y.; Akiyama, H.; Ikeda, T. Appl. Phys. .. Lett. 1993, 63, 449. (17) Ichimura. K.: Hayashi, Y.; Akiyama, H. :Ishizuki, N. Lanemuzr 1993, 9, 3298 and the rkferences therein. (18) Schonhoff, M.; Chi, L. F.; Fuchs, H.; Losche, M. Langmuir 1995, 11, 163. (19)Jones, C.; Day, S. Nature 1991, 351, 15. (20)Eich, M.; Wendofi, J. J . Opt. SOC.A m . 1990, B7, 1428. (21)Anderle, K.; Birenheide, R.; Werner, M. J. A,; Wendorff, J . H. Lio. Crvst. 1991. 9. 691. '(22)Gibbons, W,'M,;Shannon, P. J.;Sun, S. T.; Swetlin, B. J. Nature 1991, 351, 49.
Langmuir, Vol. 11,No. 12, 1995 4611
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Figure 2. Steady-state orientational patterns in the azobenzene Langmuir monolayer (surface pressure 10 mN/m); (b, bottom left) and (c, bottom right) are observed 1s after starting illumination of the initial stripe texture (a, top) with a linearly polarized excitation light (1= 480 nm) at the incident power density of 5 ,uW/cm2: Polarization direction, denoted by the double arrow, is (b)parallel and (c) perpendicular to the stripe. The monitoring He-Ne laser beam is incident from right to left. The small arrows in the insets show the in-plane orientation of the director. The insets show the simulated static (a) and steady-state patterns, (b and c) based on the Ginzburg-Landau free energy with the choice of parameters: W = lO(ZzWL),A = 0.1 (ZzK/L),SO= ZOA, SI = 50A, and n,*k= 0.7 with L = 50 pm being the period of the stripe. The polarization dependence of the light-inducedreorientation is well reproduced, indicating that the photoresponse of the monolayer discriminates the sign of V 2 hrather than the direction of n itself.
domains) spring up inside the expanded area, grow, and regularly propagate in certain directions, which seem to be determinedby the boundary condition without any fixed relation with the polarization direction.23 The wave generation lasts at least for the next 30 min, provided a disturbing flow of monolayer is carefully avoided. In most cases, the wave is associated with a decrease in the tilt angle compared to the uniform background. As shown in Figure 3, the shape of the wave is far from sinusoidal but consists of elongated bands of excited regions, in which (23)Inside a narrow channel of uniformly oriented area, like the ones shown in parts b and c of Figure 2, the waves move always along the stripe.
the tilt angle is virtually fixed near zero, separated from each other by a narrow wall with the steady-state tilt angle of fi d4. The wave travels basically along the short axis of the elongated bands spanning a typical width of 50 pm. The wave velocity is typically 50 pmls, and is only slightly enhanced with an increase in the incident light power up until the disruption of the texture begins to take place a t high excitations as already mentioned. Occasionally, furthermore, we have come across orientational solitary waves in which the azimuthal angle is primarily changed (in contrast to the wave trains), showing a time evolution as given in Figure 4. As
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Figure 3. Orientational traveling waves excited by continued trans-cis photoisomerizations. The ordinate shows the decrease in the tilt angle p from the stationary value of about n/4(the DRLM images were converted to the tilt angle distributions via eq 1).The bright elongated bands are the excited waves characterized by a smaller tilt angle. These wave regions, separated by a crevasse having stationary orientations, propagate in the direction denoted by the arrow, basically preserving their shapes; (b, bottom)was taken 1s after (a, top). On rotation of the polarization direction by n/2,the propagation direction is completely reversed. The monolayer flow was restricted to be no larger than 1p d s by locally reducing the depth of subphase down below 100 pm.
appreciable from Figure 4a, the brightness is enhanced in the wave region from the surrounding with a bright fringe, which should not be observable when the wave involves only the tilt angle changes. The azimuthal angle
a t the center of this wave is estimated to undergo rotation of about 130"from the surrounding. The typical size and velocity of solitary wave are nearly the same as those of the traveling wave.
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Langmuir, Vol. 11, No. 12, 1995 4613
by d2. This suggests a close connection of the wave generation with the asymmetric transient response of the orientational textures.
Discussion
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One possible scenario, available a t present to us, to account for these rather exotic photoresponses is the following: Irradiation of the monolayer with a polarized light induces an anisotropic yet nonpolar local increase in the cis-isomer fraction;20>21 this increase then perturbs the 2D flexoelectric ~ o l a r i z a b i l i t y ~of~ the 9 ~ ~monolayer, the liquid crystal analog of piezoelectricity, in such an asymmetric way that, if the polarizability is enhanced around n, that around the in-plane reversal of n is depressed; finally, either of the two regions, say that around n, is energetically stabilized over the other, due to the coupling between the induced flexoelectric polarization and the surface polarization field indigenous a t the i n t e r f a ~ e , ~ thereby J ~ J ~ creating an effective orientational torque toward n. This story can be transcribed into the orientational Ginzburg-Landau (GL) free energy8,12,13as
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Distance (pm) Figure 4. Snapshot of a solitary orientational wave (a, top) and its time evolution (b, bottom). Unlike the traveling wave trains (shownin Figure 31, the solitary wave consists primarily of the changes in the azimuthal angle. The rotation of the azimuthal angle at the center of the wave is about 130"from the surrounding. The brightness profiles of sequential DRLM images along a fixed section, passing through the center of the wave, parallel to the propagation direction (indicatedby arrow) are shown for every 1 s.
In both types of wave formations, a particularly striking observation is that the propagation direction can be perfectly reversed by switchingthe polarization direction
where V2)is the 2D gradient operator, k is the unit vector along the surface normal, and pc is the cis-isomer fraction which implicitly depends on n through the rate equation.20726The first and the second terms are the Frank elastic energy and the director tilt anchoring energy, respectively, both of which favor uniform orientations (K, Frank constant; W, anchoring energy coefficient; nu, director in the uniform state). The last term, on the other hand, denotes the flexoelectric coupling that tends to stabilize modulated states over uniform ~ r i e n t a t i o n s ; ~ J ~ J ~ A stands for the coupling strength in the absence of illumination, and So and S1are the constantsrepresenting strength of the light-induced perturbations due to the increase in pc. The resultant description of dynamic behaviors of director, in plausible negligence of hydrodynamic back flows in two dimension,27results in a reactiondiffusion type28GL equation for tilt and azimuthal angles. The insets of Figure 2 show the initial and steady-state patterns obtained by numerically equilibrating the director profile based on the GL equation, and the agreement with the experimental results seems quite satisfactory. The presence of traveling waves in this formalism, however, has not yet been worked out due to its highly nonlinear nature. Nevertheless, whatever the real mechanism might be, we believe that the liquid crystallinity of the Langmuir monolayer, which makes the collective reorientations possible, should play an essential role.
Acknowledgment. The authors wish to thank T. Sigehuzi for valuable assistance in image manipulations. LA9506382 (24) Meyer, R. B. Phys. Rev. Lett. 1969,22, 918. (25)de Gennes, P. G. The Physics of Liquid Crystals; Oxford University Press: Oxford, 1974; pp 97-101. (26) The transient behavior of the cis-isomer fraction pc is governed by the photochemical rate equation as described in ref 18. Under a polarized illumination, the rate constants become a function of the relative orientation of the director n and the electric field E via (n*EI2 when uniaxial symmetry along n is assumed. In stationary states, therefore, the cis-isomer fraction is approximatelygiven in the form, pc zz Ql(n*E)2 Qo, where Q1 and QOare constants connected with the orientational order parameter of the liquid crystal (see Radziszewski, et al. J. Am. Chem. SOC.1987, 109, 61). (27) de Gennes, P. G. Symp. Faraday SOC.1971,5, 16. (28) Ross, J.; Muller, S. C.; Vidal, C. Science 1988,240, 460.
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