High-Sensitivity Aminoazobenzene Chemisorbed Monolayers for

Dec 18, 2008 - Department of Physics, Colorado School of Mines, Golden, Colorado 80401, and Department of Chemistry and Biochemistry, University of ...
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Langmuir 2009, 25, 997-1003

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High-Sensitivity Aminoazobenzene Chemisorbed Monolayers for Photoalignment of Liquid Crystals Youngwoo Yi,†,‡ Matthew J. Farrow,§,| Eva Korblova,§ David M. Walba,§ and Thomas E. Furtak*,† Department of Physics, Colorado School of Mines, Golden, Colorado 80401, and Department of Chemistry and Biochemistry, UniVersity of Colorado, Boulder, Colorado 80309 ReceiVed October 20, 2008. ReVised Manuscript ReceiVed NoVember 12, 2008 We describe a new type of optically controlled liquid crystal alignment layer that demonstrates unprecedented performance. It consists of an aminoazobenzene-type material with a very simple molecular structure, which is derived from methyl red by a one-step synthesis. We have devised a method of forming covalently attached monolayers of this material on glass by an amine-assisted condensation reaction involving the triethoxysilane end of the molecule. A nematic liquid crystal (LC) cell made with the monolayer and a rubbed polymer layer was switched from a uniform state to a twisted state with a polarized 450 nm control beam having a dose of 5.5 mJ/cm2. This is equivalent to an average of only one absorbed photon per azobenzene group. Through atomic force microscopy, absorption spectroscopy, spectroscopic ellipsometry, and second harmonic generation experiments, we have confirmed that layers of this type are smooth and uniform with a surface coverage consistent with a monolayer and that the azobenzene groups are tilted, on average, 55° with respect to the surface normal. These characteristics lead to a large interaction energy density between the layer and LC. The monolayer’s rapid response in developing anisotropy in this property can be attributed to a large absorption cross section, as well as the favorable tilt angle, which allows for sufficient photoisomerization free volume in a dense layer.

I. Introduction Light-induced modification of the physical configuration of a molecule has been successfully engineered into thin photoactive organic films to control liquid crystal (LC) alignment.1-11 The covalent structure of the LC molecules (mesogens) leads to the self-assembly of supermolecular ensembles with a wide variety of anisotropic fluid structures known as liquid crystals. The free energy cost of defects is so small that in the absence of external influences LCs are not useful in electro-optic (EO) applications such as information display or telecom switching. In such applications, an LC monodomain over macroscopic length scales is typically required. The common practice in LC device manufacturing is to use a mechanically rubbed polymer as the alignment layer. The resulting grooves and polymer chain reorientations create anisotropic interaction energy between the * To whom correspondence should be addressed. E-mail: tfurtak@ mines.edu. † Colorado School of Mines. ‡ Current address: Department of Physics and Liquid Crystal Materials Research Center, University of Colorado, Boulder, CO 80309. § University of Colorado. | Current address: Sandia National Laboratories, P.O. Box 5800, MS1455, Albuquerque, NM 87185-1455. (1) Ichimura, K.; Suzuki, Y.; Seki, T.; Aoki, K. Langmuir 1988, 4, 1214–1216. (2) Gibbons, W. M.; Shannon, P. J.; Sun, S.; Swetlin, B. J. Nature 1991, 351, 49–50. (3) Schadt, M.; Seiberle, H.; Schuster, A. Nature 1996, 381, 212–215. (4) Ichimura, K.; Akiyama, H.; Kudo, K.; Ishizuki, N.; Yamamura, S. Liq. Cryst. 1996, 20, 423–435. (5) Wu, Y. L.; Demachi, Y.; Tsutsumi, O.; Kanazawa, A.; Shiono, T.; Ikeda, T. Macromolecules 1998, 31, 349–354. (6) Ubukata, T.; Seki, T.; Ichimura, K. J. Phys. Chem. B 2000, 104, 4141– 4147. (7) Yeung, F. S. Y.; Kwok, H. S. Appl. Phys. Lett. 2003, 83, 4291–4293. (8) Stalder, M.; Schadt, M. Liq. Cryst. 2003, 30, 285–296. (9) Zhong, Z. X.; Li, X. D.; Lee, S. H.; Lee, M. H. Appl. Phys. Lett. 2004, 85, 2520–2523. (10) Furumi, S.; Kidowaki, M.; Ogawa, M.; Nishiura, Y.; Ichimura, K. J. Phys. Chem. B 2005, 109, 9245–9254. (11) Niitsuma, J.; Yoneya, M.; Yokoyama, H. Appl. Phys. Lett. 2008, 92, 241120.

surface and LCs. A more advanced LC alignment method would not involve contact with the surface to reduce contamination, could be switched after the device was assembled, and might also be reversible. An alignment layer that is activated by light satisfies these requirements, although there are other issues, such as a low anchoring strength of photoalignment layers for practical use of them in an LC display. Beyond that, a particularly sensitive photoalignment layer would also be the basis for a light-activated optical switch, thus opening up a wide range of interesting and practical photonic applications as well as promoting molecularlevel understanding of LC alignment. Photoalignment technology has matured since the first report of its implementation.1 The most sensitive system relies on optical effects that depend on the orientation of the polarization of the treatment illumination.4 A common component of the most effective materials is an azobenzene group that demonstrates dichroism. The ground state of the group is the trans configuration, which can be converted to the bent (or cis) geometry upon absorption of a photon (photoisomerization).12 Thermal or photoninduced relaxation to the trans state completes an optomechanical cycle that could leave the azobenzene group in a new orientation. The likelihood that the molecule continues to undergo additional cycles is proportional to the square of the cosine of the angle between the transition moment and the actinic polarization. Upon exposure to polarized light, an ensemble of randomly oriented molecules will evolve so that its angular distribution becomes anisotropic, favoring an orientation perpendicular to the treatment polarization.13,14 Azobenzene side-chain polymer films and monolayer films of azobenzene-containing molecules, grown by Langmuir-Blodgett (12) Rau, H. In PhotoreactiVe Organic Thin Films; Sekkat, Z., Knoll, W., Eds.; Academic Press: San Diego, 2002; Chapter 1. (13) Todorov, T.; Nikolova, L.; Tomova, N. Appl. Opt. 1984, 23, 4309–4312. (14) Anderle, K.; Birenheide, R.; Werner, M. J. A.; Wendorff, J. H. Liq. Cryst. 1991, 9, 691–699.

10.1021/la803491g CCC: $40.75  2009 American Chemical Society Published on Web 12/18/2008

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(LB) deposition, have been extensively studied.15-17 Attempts to produce a sensitive photoactive monolayer by LB methods have suffered from either inhibition of photoisomerization due to a lack of free volume in a tightly packed film16 or instability due to poor coverage. However, the most sensitive and interesting photoactive systems would be those designed to be as thin and flat as possible. This will maximizes the effective interaction energy anisotropy and also optimizes the communication between LCs and the conducting coating on a device window, a necessary condition for field-driven switching. Surface-specific covalent chemical bonding of azobenzene species emerges as the most promising synthetic route. Ichimura and co-workers pioneered the use of silane-coupled materials.1,4,18-21 Li and co-workers investigated very thin azobenzene films for photoalignment of LCs.22 In many cases the resulting films were neither characterized nor claimed to be monolayers, which limit our understanding of the interaction of the LC with the layers. In spite of that, the photoactivity of those alignment layers, especially ones with a meta-linked active group containing a push-pull azobenzene chromophore, demonstrated successful light-driven control of a nematic liquid crystal using what was, until now, an unmatched low dose of the actinic illumination.4 We have previously shown that high-quality self-assembled monolayers (SAMs) of covalently attached octadecylsiloxane can be grown on float glass through the action of an amine catalyst.23 Our photoactive monolayers were produced using a similar method. The precursor is simple to synthesize in a onestep coupling of the commercial azo dye methyl red (a carboxylic acid) with (n-propylamino)triethoxysilane, followed by silica gel column chromatography.23,24 In our previous reports of studies involving this aminoazobenzene-based material, its highsensitivity photoalignment performance was not discussed, because we were not initially aware of it at the time.25 We also have previously reported an all optically prepared liquid crystal light valve device made with these same layers.26 Our molecular system is fundamentally different from other photoactive liquid crystal alignment layers that have been investigated in the past. It is a true monolayer that involves a very short surface attachment and a photoactive group that is free from alkyl chains. Therefore, it is important to understand the characteristics of the layer to guide the design of even more responsive systems. Here we report the details of the very high sensitivity for reversible photoalignment of a nematic liquid crystal cell made with the derivatized methyl red monolayer as well as the primary properties of the monolayer, including its surface morphology, optical absorption characteristics, headgroup (15) Ignes-Mullol, J.; Claret, J.; Albalat, R.; Crusats, J.; Reigada, R.; Romero, M. T. M.; Sagues, F. Langmuir 2005, 21, 2948–2955. (16) Scho¨nhoff, M.; Mertesdorf, M.; Lo¨sche, M. J. Phys. Chem. 1996, 100, 7558–7565. (17) Corvazier, L.; Zhao, Y. Macromolecules 1999, 32, 3195–3200. (18) Kawanishi, Y.; Tamaki, T.; Sakuragi, M.; Seki, T.; Suzuki, Y.; Ichimura, K. Langmuir 1992, 8, 2601–2604. (19) Aoki, K.; Seki, T.; Suzuki, Y.; Tamaki, T.; Hosoki, A.; Ichimura, K. Langmuir 1992, 8, 1007–1013. (20) Seki, T.; Saguragi, M.; Kawanishi, Y.; Suzuki, Y.; Tamaki, T.; Fukuda, R.; Ichimura, K. Langmuir 1993, 9, 211–218. (21) Ichimura, K.; Hayashi, Y.; Kawanishi, Y.; Seki, T.; Tamaki, T.; Ishizuki, N. Langmuir 1993, 9, 857–860. (22) Li, X.; Kozenkov, V. M.; Yeung, F. S.-Y.; Xu, P.; Chigrinov, V. G.; Kwok, H.-S. Jpn. J. Appl. Phys. 2006, 45, 203–205. (23) Walba, D. M.; Liberko, C. A.; Ko¨rblova, E.; Farrow, M.; Furtak, T. E.; Chow, B. C.; Schwartz, D. K.; Freeman, A. S.; Douglas, K.; Williams, S. D.; Klittnick, A. F.; Clark, N. A. Liq. Cryst. 2004, 31, 481–489. (24) Barness, Y.; Gershevitz, O.; Sekar, M.; Sukenik, C. N. Langmuir 2000, 16, 247–251. (25) Yi, Y. W.; Furtak, T. E.; Farrow, M. J.; Walba, D. M. J. Vac. Sci. Technol., A 2003, 21, 1770–1775. (26) Yi, Y. W.; Furtak, T. E. Appl. Phys. Lett. 2004, 85, 4287–4288.

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Figure 1. Chemical structure of the aminoazobenzene molecule (dMR) derived from methyl red.

orientation, and surface coverage, all of which help to clarify its unusual performance.

II. Experiments A.. Preparation of Aminoazobenzene Monolayers. The monolayer precursor was prepared by dissolving 5.75 g of methyl red (21 mM) and 4.77 g of 1,3-dicyclohexylcarbodiimide (DCC) in 60 mL of dichloromethane, which was distilled from CaH. Then (3aminopropyl)triethoxysilane (4.73 g, 21 mM) was injected into the system, and the reaction was allowed to progress under argon overnight, with stirring. The resulting solution was filtered and concentrated to produce a dark red oil. This was purified by column chromatography (50% ethyl acetate, 50% hexane) and collected to yield dark red crystals. Figure 1 shows the chemical structure of the derivatized methyl red, called dMR hereafter. The silane-based chemisorbed monolayers were prepared in toluene using butylamine as a catalyst.25 The procedure to make dMR monolayers was as follows: In 70 mL of toluene, 272 mg of dMR was dissolved, yielding an 8.2 mM solution. To this solution was added 0.25 mL of the catalyst n-butylamine. The solution was then heated to 45 °C in a water bath while being stirred. Glass slides were cleaned with soap (Fisher microscope slides or Corning 2947 microslide) and then with a piranha solution (50% hydrogen peroxide, 50% sulfuric acid by volume) for 1 h. (Caution: Piranha oxidizes organic material Very aggressiVely. It should be handled Very carefully.) The slides were rinsed with Milli-Q water and dried with high-purity nitrogen. Then the slides were immersed in the dMR solution at 45 °C. After 100 min, the slides were rinsed with toluene, dried with nitrogen, and annealed in an oven at 115 °C for 4.5 h. It is worthwhile to note that it was possible to use the dMR solution repeatedly, without apparent degradation of the quality of the monolayer, by reactivating the solution with several drops of the catalyst. The butylamine catalyst promotes siloxane bonding by interaction with surface hydroxyl groups, while the slow bulk reaction of triethoxysilanes minimizes the formation of oligomers in the solution, which would contribute to multilayer growth.23 B. Illumination Treatments. Two illumination treatments were used during the course of these investigations. Type A: For the photoswitching of LC cells, light from a xenon arc lamp (450 W) was passed through a water bath and a monochromator with a 20 nm bandpass. Finally, the light went through a dichroic polarizer. Type B: To study photoinduced anisotropy development of the dMR monolayer, a polarized blue light beam was prepared as follows. Light from a mercury arc lamp (100 W) was collected by a lens, reflected by a dichroic mirror, and then passed through a dichroic polarizer. Finally, a narrow band filter centered at 436 nm was used. The intensity of the type B treatment illumination was 0.5 mW/cm2, and the angle of incidence was 45°. C. LC Cell Preparation and Photoswitching Tests. To test the performance of the dMR monolayer in the photoalignment of LC materials, nematic LC cells were made with glass slides coated with dMR monolayers (monolayer windows) and glass slides coated with nylon (nylon windows). The nylon window was prepared as follows. Nylon (Du Pont Elvamide 8023) was dissolved in methanol (0.5%

Aminoazobenzene Monolayers for Photoalignment of LCs

Figure 2. Schemtic diagram of the photoswitching experiment. The probe beam and the treatment beam pass through the LC cell from the left to the right horizontally and from the right to the left obliquely, respectively. The polarization of P1 and P3 is in the page, and that of P2 is perpendicular to the page.

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Figure 4. Photoswitching of an LC cell with a dMR SAM. The SAM initially aligned parallel to the rubbing direction (left) reorients by the polarized UV light illumination and the liquid crystal twist from the bottom surface to the top surface (right). The arrows in the cell represent the rubbing direction on the nylon window.

backside was removed before the SHG measurements with several drops of piranha treatment and Milli-Q water rinsing. The thickness and dichroism of dMR monolayers were measured using a variable-angle spectroscopic ellipsometer (J. A. Woollam VASE) with a rotating analyzer and a semiconductor detector. For the thickness measurement, the monolayers were prepared on silicon wafers with the same procedure as describe above. The measurements for the silicon samples were performed before and after cleaning by exposure to UV light. The dichroism of the monolayers was measured with transmission mode ellipsometry, while the samples were exposed to type B illumination.

III. Results and Discussion Figure 3. Schematic diagram of the second harmonic generation system.

by weight), and several drops of the solution was spun on a clean glass slide for 20 s at a speed of 6000 rpm. The slide was rubbed with a fine cloth. The rubbing direction was guided to ensure its accuracy. An empty cell was made with the monolayer window, the nylon window, and 20 µm spacer films applied with epoxy adhesive. A nematic liquid crystal (5CB) was inserted by capillary action above the transition temperature (TNI ) 35 °C) for the isotropic phase. The intensity of a probing laser beam passed through a polarizer (P1), the LC cell with the nylon window first, and an analyzer (P2) was measured, as shown in Figure 2. The wavelength of the probing beam was 670 nm. The orientation of P1 was parallel to the rubbing direction and perpendicular to P2. The cell was exposed to type A treatment light incident at an angle of 20° on the monolayer window. D. AFM Measurement. Surface topogrophies of the monolayers and bare substrates were obtained with an atomic force microscope operating in the noncontact mode. The samples were cleaned with acetone, then rinsed with Milli-Q water, and then dried with nitrogen before the measurements. E. Optical Characterization of dMR Monolayers. A second harmonic generation (SHG) measurement system, as shown in Figure 3, was used to measure the average tilt angle of the aminoazobenzene groupsinthedMRmonolayers.AQ-switchedNd:YAG(yttrium-aluminum garnet doped with neodymium) pulsed laser beam was used, which produced 6 ns pulses with a 10 Hz repetition rate, as a fundamental (1064 nm) light source. The beam diameter was about 0.5 cm at the sample, and the intensity at the sample was kept at less than 5 mW to minimize degradation of the sample by the beam. A half-wave plate was used to rotate the polarization of the light. The polarized beam went through a filter to remove any visible light and irradiated the sample at a 45° angle of incidence. The reflected beam passed through an infrared filter to block the fundamental light, and a collection lens and dichroic polarization analyzer were set to pass only p-polarized light. Finally, it passed through a narrow-band interference filter centered at 532 nm. The detector was a photomultiplier tube with a high quantum efficiency at 532 nm (Hamamatsu R3896). A preamplifier and a boxcar amplifier registered the detected signal. For each data point, signals from several hundred pulses were averaged in a computer. To avoid the interference of light from the front and backside material of a sample, the monolayer on the

A. Photoswitching of Liquid Crystal Cells. The initial alignment of the nematic director within the cells was arranged to be uniform by treating the cell with a light beam whose polarization was perpendicular to the rubbing direction of the nylon. This caused the preferred directions on the photoactive monolayer and the nylon windows to be parallel as shown in the left sketch of Figure 4. With the cell in this state, the polarization direction of the probe beam remained the same as it passed through the cell. When the monolayer window was treated with a light beam whose polarization was parallel to the rubbing direction of the nylon window, the dominant orientation of molecules in the photoactive monolayer rotated by 90°. This caused the LC alignment at that window to also rotate according to the monolayer rotation as sketched in Figure 4. This produced a near-90° twist of the director within the cell. As a result, the polarization direction of the probe beam was caused to rotate as it passed through the cell. The twist in the LC director could be reduced or removed by illuminating the monolayer window without the polarizer or by orienting the polarizer to be perpendicular to the rubbing direction on the nylon window. Figure 5a shows the relative intensity of the transmitted probing light beam through the cell versus the dose of the 450 nm treatment illumination (type A). Initially, the cell was in the untwisted state and the transmitted probe beam intensity was at a minimum. As the dose of the treatment light beam increased, the cell became twisted. The relative intensity of the probe beam increased until leveling off when the treatment dose reached 6 mJ/cm2. The switching dose, which is the dose required for a change of 90% in the relative intensity of the probe beam, was 5.4 mJ/cm2 with the cell held at 26 °C (TNI-9 °C). The temperature of the cell could be increased with a stream of warm air. In that case the switching dose was only 4.0 mJ/cm2 at a temperature of 30 °C (TNI-5 °C). This dose is an order of magnitude smaller than the dose of ∼70 mJ/cm2 reported by Ichimura and his co-workers for photoswitching of a nematic liquid crystal (EXP-CIL) with a cyanoazobenzene (CNAzC6-m-C5) alignment layer at 25 °C (TNI-6.5 °C).4 The difference in the steepness of the photoswitching behavior shown in Figure 5a is probably due to the smaller elastic constant of the LC at the higher temperature. The cell could be repeatedly optically switched between the low- and high-transmission states by successive rotations of the

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Figure 6. AFM images of a substrate and dMR monolayers: (a) surface of the glass slide, (b) exposed to dMR and butylamine for 90 min, (c) exposed to dMR and triethylamine overnight. The rms roughness for images a, b, and c are 0.215, 0.265, and 0.733 nm, respectively. The circles in images a and b are the structure on a float glass surface. The size of the scale bars is 100 nm.

Figure 5. (a) Relative transmission versus dose of 450 nm light (type A) illumination of a nematic LC cell with a dMR monolayer at 26 and 30 °C. The intensity of the treatment illumination was 2 ( 0.2 mW/cm2. (b) Switching of a nematic LC cell with a dMR monolayer sandwiched between two crossed polarizers by an overhead projector illumination at room temperature.

polarizer in the treatment beam by 90°. Although the monolayers degraded if they were left exposed to air during the treatment illumination, they were stable for many months when sealed in the LC cell.26 Figure 5b shows sequential photos during switching of a nematic LC cell sandwiched between two crossed polarizers placed on an ordinary overhead projector. In this demonstration the overhead projector illumination played the roles of both the treatment and probe beams. It took only 6 s for the cell to switch from the uniform state to the twisted state in this example. While our results exhibit unprecedented sensitivity, we acknowledge that caution must be taken in a direct comparison of the switching performance between our system and the device described by Ichimura, since the LC materials and the cell structures are both different. Our cell had planar alignment at the photoinactive window, while the Ichimura cell had homeotropic alignment at that window. We measured the real-time transmission of the cell between crossed polarizers. Ichimura and his coworkers measured transmission of a probing laser beam through the cell as a function of the polarization direction of the probe beam to determine the photoalignment direction relative to the polarization direction of the treatment light for each dose of the treatment illumination. The cells retained their twisted state in room light in spite of the fact that the photoinduced anisotropy of a bare monolayer without contact to an LC disappeared within 1 h. The transmission of the cell under room lighting decreased by only 2% after 20 h. This interesting behavior confirms that the dMR interacts effectively with 5CB and that the mean field of the nematic helps to maintain the orientation of the aminoazobenzene groups in the monolayer. However, the monolayer alone displays some characteristics of liquid crystal ordering, as revealed by simple temperaturedependent studies. A cell that was in the twisted state was heated to 45 °C, which is above the nematic to isotropic phase transition temperature (35 °C) of 5CB, and subsequently was kept at this temperature for 15 min, followed by a cool-down to room temperature. The transmission dropped and recovered at the nematic-to-isotropic phase transition temperature. The difference between the relative transmissions before and after the heating

treatment of the cell was only 3.4%. This shows that the surface alignment was retained through the temperature cycle. This may indicate that the effective isotropic transition temperature for a dMR monolayer is above 45 °C. To measure the anchoring energy of the dMR monolayer, the twist angle (i.e., the change of director from one surface to the other surface) of a cell was measured as a function of the dose of the 450 nm treatment illumination as follows. After a dose of the illumination, the analyzer was being rotated from the initial position, which is perpendicular to the polarizer, using a stepping motor while the intensity of the probing beam was being recorded. The interval between the illumination and the measurement was 3 min. The rotation angle of the analyzer at minimum probe intensity was the twisted angle of the cell assuming that the director at the nylon surface was fixed along the rubbing direction since the anchoring energy of the nylon film was large. The twist angle increased fast initially and was 80° when the accumulated dose was 8.2 mJ/cm2, and it reached 86° when the dose was 82 mJ/cm2. Assuming the anchoring strength of the nylon surface is infinite and both the nylon and the monolayer windows are planar, the anchoring energy of the monolayer can be calculated using the following torque balance equation:27

Wφ )

2K∆φ d sin[2(Φ - ∆φ)]

(1)

where K ) 4.5 × 10-12 N,28 d ) 20 µm, Φ ) 90°, and ∆φ are the twist distortion constant of 5CB, the cell gap, the twist angle defined by the alignment layers, and the measured twist angle, respectively. The values of the anchoring energy are 1.8 × 10-6 and 4.6 × 10-6 J/m2 for the two doses. The anchoring energy of rubbed nylon, obtained from a twist angle measurement with a nematic cell made with two nylon windows, was 6 × 10-5 J/m2. The anchoring energy of the dMR monolayer, measured in the same way, was about an order of magnitude smaller. This may be related to the absence of grooves in the monolayer compared to the rubbed nylon and the small photoinduced anisotropy of the monolayer, which is described later in the following section. B. Characteristics of the dMR Monolayer. 1. Morphology. The AFM measurements show that the root-mean-square (rms) value of roughness for a glass slide and the dMR monolayer on a glass slide were 2.9 and 3.6 Å, respectively, as shown in Figure 6 a,b. This suggests that the monolayers were uniform at the molecular scale. By contrast, Figure 6c shows the AFM image of a dMR film self-assembled overnight using triethylamine (27) Vorflusev, V. P.; Kitzerow, H.-S.; Chigrinov, V. G. Appl. Phys. A: Mater. Sci. Process. 1997, 64, 615–618. (28) Karat, P. P.; Madhusudana, M. V. Mol. Cryst. Liq. Cryst. 1977, 40, 239– 245.

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Figure 7. SHG of a dMR monolayer. The curve and the dots represent the p-polarization SHG calculation and signals, respectively. In the horizontal axis, 0° and 90° correspond to s- and p-polarized incident polarization, respectively. The inset shows how the tilt angle (θ) and the azimuthal angle (φ) are defined with respect to the orientation of the c-axis in the molecular coordinate system.

(rather than butylamine) as a catalyst. The rms roughness of this image is 7.3 Å, which indicates multilayer islands. An LC cell made with this dMR film needed 34 mJ/cm2 of blue light illumination for switching from the uniform state to the twisted state. This dose was 6 times larger than the switching dose for the regular dMR monolayer. This suggests that the smoothness of the monolayers plays an important role in the efficient photoalignment performance. 2. Tilt Angle of dMR. The average tilt angle of the azobenzene groups in the dMR monolayer was derived from measurement of the p-polarized second harmonic reflection while the incident polarization direction was vaired. The data were fit to a model calculation using the following expression for the SHG signal: 25,29

S ∝ |e2ω · χ(2):eωeω|2

(2)

Here, e2ω and eω represent outgoing and incoming polarization vectors for fundamental and second harmonic light, respectively, and also include corrections for local field effects. In the molecular coordinate system (labeled with orthogonal directions a, b, and c), the second-order nonlinear hyperpolarizability is dominated by a single component, βccc, for rodlike molecules (with the c direction aligned to the azobenzene axis). The nonlinear susceptibility tensor, χ(2), of the monolayer can be written as (2) χijk ) Ns〈(ui · c)(uj · c)(uk · c)〉βccc

(3)

where ui, uj, and uk are unit vectors along the ith, jth, and kth sample coordinates, respectively. The angular brackets designate an ensemble average for all the molecules in the sample using the orientation distribution function f(θ) g(φ). Here, θ and φ are the tilt angle and azimuthal angle of the aminoazobenzene groups (see the inset of Figure 7), respectively. Figure 7 shows the SHG data and the best model fit for a dMR monolayer without any illumination treatment. The average tilt angle was found to be 55°. In the model calculation, a narrow distribution of the tilt angle and a uniform distribution of the azimuthal angle were assumed. The indexes of refraction used for the substrate are 1.50 and 1.51 for the fundamental light and the second harmonic light, respectively. The method of Zhuang et al.29 was followed to estimate the contribution to the local field from molecules in the layer. Our result is consistent with (29) Zhuang, X.; Miranda, P. B.; Kim, D.; Shen, Y. R. Phys. ReV. B 1999, 59, 12632–12640.

Figure 8. Absorbance spectra of dMR in chloroform and a dMR monolayer. The monolayers are on both sides of the glass slide. The shapes are similar except that the spectrum of the monolayer is broader. The absorbance values of the monolayer are multiplied by a factor of 168 for comparison. The concentration of the solution is 5 × 10-5 mol/ L, and the thickness of the cell is 1 cm.

the work of Ekhoff and her co-workers,30 who also studied dMR on glass using SHG. With similar assumptions their data predict an average tilt angle of 50°. If f(θ) is allowed to represent a variety of orientations, as a Gaussian distribution, the predicted average tilt angle increases. With a distribution width of 30° our data predict an average tilt angle of 65°, while Ekhoff’s work, which also included measurement of dichroism anisotropy using photoacouostic absorption, reported an average tilt angle as high as 76° ( 3°.30 3. Absorbance Spectrum. Figure 8 shows the absorbance spectrum, A(λ), for dMR in chloroform (5 × 10-5 mol/L) in a 1 cm cuvette and a dMR monolayer on glass (corrected to represent a single side) as measured using a spectrophotometer (Cary 5G UV-vis-NIR). The reference samples were a cell filled with only the solvent and a bare glass slide, respectively. The π-π* band in the blue region and n-π* band in the green region overlap because dMR is of aminoazobenzene type.12 The absorbance is related to the molecular absorption cross section, σc, along the aminoazobenzene axis, through

A ln 10 ) σcG

d V1

(4)

Here, d is the sample thickness, V1 is the volume occupied by one dMR molecule, and G is the orientational averaging factor. In our solution V1 ) 3.32 × 104 nm3 and Gsol ) 1/3. From this we find that Gsolσc ) 1.06 × 10-16 cm2 (ε ) 27 660 dm3/(mol · cm)) at the absorbance maximum. The monolayer demonstrates strong absorbance, with a maximum of 0.00833 near 435 nm. The spectrum for the monolayer is broader than that of dMR in solution, most likely due to aggregation effects. 4. CoVerage. The area per dMR molecule can be determined from the SHG and absorbance experiments. However, to properly account for local field effects, it is necessary to estimate the effective index of refraction (nj) for the monolayer from the molar refractivity (R):

nj2 - 1 1 R ) nj2 + 2 V0 NA

(5)

where V0 is the volume occupied by one molecule in a hypothetical pure dMR-siloxane layer and NA is Avogadro’s number. The estimated size of dMR, from a Hartree-Fock calculation using (30) Ekhoff, J. A.; Farrow, M.; Walba, D.; Rowlen, K. L. Talanta 2003, 60, 801–808.

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a 3-21G basis set, is 1.4 nm long, 0.4 nm wide on average (the aminoazobenzene group), and 1.0 nm high, leading to V0 ) 0.56 nm3. The width of the azobenzene group varies from 0.24 to 0.54 nm due to the varying angle of the bonding. The surface-attached molecule has a molecular refractivity of R/NA ) 0.151 nm3.31 This leads to nj ) 1.452, a value very close to that of chloroform nsolv ) 1.443. Therefore, comparisons of the absorption in solution and in the monolayer are valid without the need for explicit corrections due to local field effects.32 For the monolayer the ratio d/V1 in eq 4 is replaced by 1/a1, where a1 is the area per molecule, and the orientational averaging factor is GSAM ) 1/2〈sin2 θ〉. Assuming the molecular absorption cross section is the same for the dMR in chloroform and in the monolayer, we can form the following equality:

ASAMa1 σc AsolV1 ) ) ln 10 Gsoldsol GSAM

(6)

If we use the narrow-distribution tilt angle result from the SHG experiment (θ ) 55°), this expression leads to a molecular area of a1 ) 0.55 nm2. This is remarkably close to the geometrical footprint of the aminoazobenzene group viewed from the top of the SAM, which is 0.45 nm2. Therefore, we must conclude that the dMR layer contains a quantity of molecules nearly equivalent to what would be expected from a single monolayer. Since the surface topography is smooth, it is safe to assume that these molecules are uniformly distributed in a single, covalently bound layer (rather than being distributed in multilayer islands). Even though the surface coverage is close to 1, a free volume for the photoisomerization, which is necessary for efficient photoinduced anisotropy development, can still be available because the azobenzene group is tilted and the space normal to the substrate is free. Finite roughness of the substrate may reduce steric hindrance further. 5. Thickness. Additional verification of the single monolayer conclusion comes through ellipsometry. The variable angle spectral data for ∆ and Ψ obtained from a dMR monolayer on silicon was fitted to a multilayer optical model consisting of SiO2 on a silicon substrate. This is justified because the molar refractivity estimate for the effective index of refraction of dMR is very close to that of SiO2 in the spectral region between 300 and 1100 nm. The average difference in the thickness of the layer before and after UV cleaning, which was due to the thickness of the dMR monolayer, was 0.96 nm. This is consistent with a reasonable thickness for the covalently bonded monolayer, considering the average tilt of the aminoazobenzene group and inevitable flexibility of the propyl chain. 6. Photoinduced Anisotropy. To verify that the LC cell switching is caused by the reorientation of the dMR monolayer, the dichroism (∆A) of the monolayer was tested using type B illumination. The ellipsometric parameter in transmission is related to ∆A, from which the surface in-plane order parameter (Qs) can be found:

∆A ) Ax - Ay ) log(tan Ψ)2

(7)

The y-axis is chosen to be parallel to the polarization of the treatment illumination. With polarized detection the orientational averaging factors are Gx ) 〈cos2 φ sin2 θ〉 and Gy ) 〈sin2 φ sin2 θ〉. Therefore, the surface in-plane order parameter is

Qs )

Ax - Ay ∆A ) 〈cos2 φ - sin2 φ〉 ≈ Ax + Ay 2A

(8)

Figure 9 shows the surface order development of a dMR (31) Ghose, K. A.; Crippen, G. M. J. Chem. Inf. Comput. Sci. 1987, 27, 21–35. (32) Servant, L.; Dignam, M. J. Thin Solid Films 1994, 242, 21–25.

Figure 9. Surface order parameter of the dMR monolayer versus the dose of 436 nm (type B) treatment illumination. The monolayer is initially aligned along the polarization of the treatment light. The error bar shows a representative uncertainty in the measurement.

monolayer. The required dose to switch the preferred direction of the monolayer by 90° is about 20 mJ/cm2. Before the measurement, the monolayer was aligned and then rotated by 90° so that the preferred direction of the molecule was parallel to the polarization of the treatment light, explaining the initial negative order parameter in the graph. The small saturated order parameter is related to the resulting weak anchoring energy of the aligned LC. However, we observed that the order parameter also depended on the intensity of the treatment illumination. It was possible to achieve a larger order parameter with higher intensity. Further investigation of this phenomenon is in progress. C. Spectral Efficiency of Photoswitching. To study the spectral dependence of the photoalignment of a nematic cell with a dMR monolayer, the switching dose (incident energy needed for 90% switching, Ds(λ)) for type A illumination with different wavelengths between 400 and 500 nm was measured. As expected, the photoswitching dose was the smallest near the region where the absorbance was at a maximum, as shown in Figure 10a. To compensate for this effect, we determined the absorbed switching dose (the energy absorbed by the monolayer during the switching) (see Figure 10b). The absorbed switching dose can be calculated by subtracting the transmitted portion of the optical energy from the switching dose using [Ds(λ)](1 10-A(λ)). This shows the trend that the longer wavelength region (n-π* band) needed a less absorbed dose in switching the cell than the short-wavelength region (π-π* band). Since the average area per molecule a1 is known (from our calibrated absorption experiments), we can calculate how many absorbed photons per molecule were needed in the switching. This is the absorbed switching dose per unit area times the average area per molecule divided by the energy per photon. The number of absorbed photons per molecule that enabled 90% switching of the LC cell varied from 1.2 to 1.5, as shown in Figure 10c. Considering the measurement uncertainty in determining the dose (∼10%) along with the uncertainty in the calculation of the average area per molecule, it is hard to say that there was any spectral dependence on this quantity. The switching photon number per molecule was as low as 1.1 for the cell whose switching behavior is shown in Figure 5a. This would appear to be the most sensitive photoalignment layer yet reported. D. Comparison with Previously Reported High-Sensitivity Azobenzene Layers. To gain some insight into the performance of our photoalignment layers, we need to compare a dMR monolayer with the m-cyanoazobenzene (CNAzC6-m-C5) layer described by Ichimura and co-workers, which was also shown to be very sensitive.4 The molecular structure of the dMR molecule is simpler than that of CNAzC6-m-C5. The dMR has only a dimethylamino group on a para position of the aminoazobenzene group, while the other has cyano and hexyloxy groups on the two

Aminoazobenzene Monolayers for Photoalignment of LCs

Langmuir, Vol. 25, No. 2, 2009 1003

m-C5 (11 500 dm2/mol, in ethanol), which ensures more efficient absorption of light energy. This may be due to a more efficient push-pull optical transition in dMR, compared to CNAzC6m-C5. The dMR layers were prepared by catalyst-assisted selfassembly to ensure formation of true monolayers, for both efficient isomerization and effective interaction with LCs.23 It is not clear whether the cyanoazobenzene layers were really monolayers because of insufficient information in the published report. However, the π-π* absorbance of the CNAzC6-m-C5 layer on a glass slide (both sides) reported in the paper is similar to what we have observed with glass slides coated on both sides with dMR. Considering the small molar absorption coefficient of CNAzC6-m-C5, it appears that the absorbance was about twice as big as it would have been if it were a single monolayer.

IV. Conclusion

Figure 10. Spectral dependence of the photoswitching of a liquid crystal cell with a dMR monolayer: (a) dose for 90% switching; (b) absorbed dose for the switching shown in (a); (c) absorbed photon number per azobenzene group for the switching in (a). The error bars represent the uncertainty in the measurement of the dose.

para positions. Our results show that effective anisotripic anchoring of a nematic LC is possible with a molecular structure in the active group that contains only a rigid core. It is not necessary for the alignment layer to simulate the LC with both the core and the flexible tail. In addition, dMR has a shorter spacer chain between the azobenzene and silane groups, compared to CNAzC6-m-C5. We speculate that this may help to reduce steric hindrance of photoisomerization of the azobenzene, which may also enhance anisotropic anchoring of LCs. The linking groups of dMR and the cyanoazobenzene-based material are attached at different positions, ortho and meta, respectively. Although the meta configuration was reported to be more effective in photoalignment of LCs than the ortho position,4 our results show that the ortho position is also very effective. The molar absorption coefficient (27 660 dm2/mol, in chloroform) of dMR is more than 2 times larger than that of CNAzC6-

Chemisorbed monolayers of aminoazobenzene molecules, derived from methyl red in a simple process, demonstrate high efficiency in the photoswitching of nematic liquid crystal cells, requiring as little as one absorbed photon per azobenzene group. The characteristics of the layer including its morphology, absorption spectrum, tilt angle of the azobenzene group, thickness, and photoinduced anisotropy (PIA) all support the proposition that the layer is truly a monolayer. The absorption cross section of the layer is large, the area per molecule is very close to the geometric footprint of the azobenzene group, the layer is as smooth as the substrate, and the PIA development needs a very small dose of blue light illumination to produce efficient photoisomerization that aligns a nematic LC. This efficiency in photoisomerization of the azobenzene groups can be explained by the large absorption cross section. The fast thermal relaxation or photorelaxation from cis to trans, which is typical of the aminoazobenzene-type material, seems also to enhance the isomerization cycle. Finally, the monolayer is effective in producing in-plane anchoring of a nematic LC because the surface coverage is high and the layer is smooth. However, the tilt of the azobenzene group still allows for sufficient free volume for effective isomerization without steric hindrance. The dMR monolayer appears to demonstrate the most efficient photoalignment of nematic LCs yet reported. This performance is derived from a simple molecular structure without a terminal alkane chain and a method of attachment that optimizes monolayer formation. These are features of our system that are fundamentally different from those of previously described alignment layers. This system can be used not only for rapid control of responsive optical devices, but also as a model system for further understanding and improvement of photoinduced anisotropy of monolayers and management of the interaction between surfaces and soft materials. Acknowledgment. This work was supported by the National Science Foundation under grants to the Liquid Crystal Material Research Science and Engineering Center (DMR-0213918 and CHE-0079122). LA803491G