Amphiphilic Azo Polymer Spheres, Colloidal Monolayers, and

Publication Date (Web): June 1, 2005 .... Nanoparticle assembly of a photo- and pH-responsive random azobenzene copolymer ... both an azo chromophore ...
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Langmuir 2005, 21, 6567-6571

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Amphiphilic Azo Polymer Spheres, Colloidal Monolayers, and Photoinduced Chromophore Orientation Yaobang Li, Yonghong Deng, Yaning He, Xiaolan Tong, and Xiaogong Wang* Department of Chemical Engineering, School of Materials Science and Engineering, Tsinghua University, Beijing, 100084, P. R. China Received January 11, 2005. In Final Form: April 13, 2005 In this work, azobenzene-containing colloidal spheres have been fabricated and used to construct photoresponsive monolayers. The colloidal spheres were prepared from an amphiphilic azobenzenecontaining random copolymer through hydrophobic aggregation of the polymer chains, which was induced by adding the selective solvent (H2O) into a THF solution of the polymer. The size and size distribution of the spheres depended on the initial concentration of the azo polymer in THF and the H2O/THF ratio. Adjusting those factors and optimizing other preparation conditions, uniform colloidal spheres could be obtained. Monolayers composed of hexagonally close-packed colloidal spheres were prepared by the capillaryforce-driven method. The colloidal monolayers showed obvious dichroism after laser irradiation due to the photoinduced azo-chromophore orientation occurred in the spheres. The orientation order parameter was related to the irradiation time and estimated to be 0.09 at the photostationary state. The colloidal spheres and their monolayers can potentially be used as building blocks or media for reversible optical data storage, photoswitching, sensors, and other photodriven devices.

Introduction Polymeric materials containing azobenzene and its derivatives (azo chromophores) have been actively investigated in recent years.1 One of the most interesting properties of the materials is the photoinduced birefringence and dichroism.2 The photoinduced anisotropy is caused by the repeated cis-trans isomerization of azo chromophores under polarized light irradiation, which forces the chromophores to continually change their orientation and to eventually stabilize at the direction perpendicular to the polarization.3,4 The effect shows potential applications in areas such as reversible optical data storage, optical-switching and sensors.5 The photoinduced orientation and related properties have been extensively investigated for various polymeric materials, such as chromophore-doping polymers (guest-host systems), liquid crystal azo polymers, and amorphous azo polymers.6-10 Recently, the exploration has covered the Langmuir-Blodgett (LB) films and the electrostatic layerby-layer self-assembled multilayer films of azo polymers.11-13 In these systems, polymeric media play an important role to enable or restrict the orientation of the azo chromophores.2 The nature of polymers can also be exploited to offer tremendous richness to the aggregation states and mor* Corresponding author. E-mail: [email protected]. (1) Delaire, J. A.; Nakatani, K. Chem. Rev. 2000, 100, 1817. (2) Natansohn, A.; Rochon, P. Chem. Rev. 2002, 102, 4139. (3) Xie, S.; Natansohn, A.; Rochon, P. Chem. Mater. 1993, 5, 403. (4) Domont, M.; Osman, A. E. Chem. Phys. 1999, 245, 437. (5) Petersen, T. G.; Johansen, P. M.; Holme, N. C. R.; Ramanujam, P. S. J. Opt. Soc. Am. B 1998, 15, 1120. (6) Todorov, T.; Nikolova, L.; Tomova, N. Appl. Opt. 1984, 23, 4309. (7) Eich, M.; Wendorff, J.; Reck, B.; Ringsdorf, H. Makromol. Chem. 1987, 8, 59. (8) Eich, M.; Wendorff, J.; Reck, B.; Ringsdorf, H. Makromol. Chem. 1987, 8, 467. (9) Rochon, P.; Gosselin, J.; Natansohn, A.; Xie, S. Appl. Phys. Lett. 1992, 60, 4. (10) Natansohn, A.; Rochon, P.; Gosselin, J.; Xie, S. Macromolecules 1992, 25, 2268. (11) Yokoyama, S.; Kakimoto, M.; Imai, Y. Mol. Cryst. Liq. Cryst. 1993, 227, 295. (12) Hong, J. D.; Park, E. S.; Park, A. L. Langmuir 1999, 15, 6515. (13) Wang, H. P.; He, Y. N.; Tuo, X. L.; Wang, X. G. Macromolecules 2004, 37, 135.

phologies of the photoresponsive substances.2,14 Colloidal spheres, ranging from micrometer-sized particles down to nanoparticles, have been widely applied in many industrial products.15 Colloidal spheres can be prepared by methods such as controlled precipitation, emulsion polymerization, and aggregation of amphiphilic block copolymers.16-18 Amphiphilic polymers are also wellknown for their ability to form other type structures through self-assembly processes.19 By incorporating azo polymers into colloidal spheres, one can expect that the colloidal spheres will demonstrate the photoinduced anisotropy function of the polymers and other interesting properties related with colloids. Recently, construction of two-dimensional (2D) and three-dimensional (3D) periodic structures from monodispersed colloidal spheres has attracted tremendous attention due to some fascinating applications in sensors, photonic band gap (PBG) crystals, and three-dimensional (3D) porous materials among others.20-23 Some delicate methods, such as transferring the colloidal array formed at the air-liquid interface, exploiting the capillary forces (14) Fo¨rster, S.; Plantenberg, T. Angew. Chem., Int. Ed. 2002, 41, 688. (15) Hiemenz, P. C. Principles of Colloid and Surface Chemistry, 2nd ed.; Marcel Dekker Inc.: New York, 1986. (16) Matijevic, E. Acc. Chem. Res. 1981, 14, 22. (17) Piirma, I. Emulsion Polymerization; Academic: New York, 1982. (18) Halperin, A.; Tirrell, M.; Lodge, T. P. Adv. Polym. Sci. 1992, 100, 31. (19) See, for example: (a) Zhang, L.; Eisenberg, A. Science 1995, 268, 1728. (b) Ma, Y. H.; Cao, T.; Webber, S. E. Macromolecules 1998, 31, 1773. (c) Antonietti, M.; Heinz, S.; Schmidt, M.; Rosenauer, C. Macromolecules 1994, 27, 3276. (d) Sommerdijk, N. A. J. M.; Holder, S. J.; Hirons, R. C.; Jones, R. G.; Nolte, R. J. M. Macromolecules 2000, 33, 8289. (e) Moffit, M.; Khougaz, K.; Eisenberg, A. Acc. Chem. Res. 1996, 29, 95. (20) Xia, Y.; Gates, B.; Yin, Y.; Lu, Y. Adv. Mater. 2000, 12, 693. (21) Ozin, G. A.; Yang, S. M. Adv. Funct. Mater. 2001, 11, 95. (22) See, for example: (a) Denkov, N. D.; Velev, O. D.; Kralchevsky, P. A.; Ivanov, I. B.; Yoshimura, H.; Nagayama, K. Nature (London) 1993, 361, 26. (b) Dimitrov, A. S.; Nagayama, K. Langmuir 1996, 12, 1303. (c) Jiang, P.; Bertone, J. F.; Hwang, K. S.; Colvin, V. L. Chem. Mater. 1999, 11, 2132. (d) Norri, D. J.; Arlinghaus, E. G.; Meng, L. L.; Heiny, R.; Scriven, L. E. Adv. Mater. 2004, 16, 1393. (e) Gu, Z. Z.; Fujishima, A.; Sato, O. Chem. Mater. 2002, 14, 760. (23) Schroden, R. C.; Al-Daous, M.; Blanford, C. F.; Stein, A. Chem. Mater. 2002, 14, 3305.

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to drive the assembly, and using electrophoretic deposition or sedimentation in a force field, have been developed and consummated to obtain ordered colloidal arrays.20-22 The colloidal structures have been predominantly constructed from monodispersed colloidal spheres of silica and few polymers (e.g., PS and PMMA). The colloids are usually isotropic and themselves do not exhibit particularly interesting optical, nonlinear optical, or electrooptical functionality.20-23 More recently, the studies on the functional particles and their ordered structures start to emerge.24-27 Two-dimensional hexagonally aligned patterns of paramagnetic particles at air-water or glasswater interfaces have been induced by using an external magnetic field.25 Two-dimensional crystallization of paramagnetic beads has been assembled and manipulated by movable nanomagnets.26 Arrays of superparamagnetic spheres assembled by a magnetic field have been studied as cooperative microlenses.27 The structures constructed from azobenzene-containing colloidal spheres, such as 2D arrays or monolayers, can show some specifically attractive properties such as photoinduced dichroism and birefringence. At least theoretically, the arrays or monolayers can be photomanipulated to exhibit anisotropy in any predetermined direction. Compared to spin-coated and other films of azo polymers, the 2D arrays or monolayers of the colloidal spheres can show flexibility to further adjust their optical properties by altering the alignment patterns of the colloidal particles. The photoresponsive colloidal arrays or monolayers are of interest for both technological applications and theoretical studies. However, a report on the colloidal spheres and the structures assembled from them is still lacking. In this work, we explored a way to prepare uniform colloidal spheres from an amphiphilic random copolymer that contains donor-and-acceptor type azo chromophores. The factors influencing the particle formation were studied and adjusted to control the size and size distribution of the colloidal spheres. Monolayers of the spheres were prepared by the self-assembly of the colloidal spheres from suspensions driven by the attractive capillary force. The monolayers showed obvious dichroic effect after they were exposed to a polarized Ar+ laser beam due to the photoinduced chromophore orientation occurred in the spheres. The orientation order parameter was estimated to be 0.09 at the photostationary state. Experimental Section Amphiphilic Azo Polymer. The chemical structure of the amphiphilic azo polymer (PNANT) is given as

PNANT was tailored to contain both “pseudo-stilbene” type azo chromophores to render proper photoorientation properties and ionizable carboxyl groups to incorporate hydrophilicity. PNANT was synthesized from a polymeric reagent, poly(acryloyl chloride) (PAC), which was prepared by the radical polymerization of acryloyl chloride.28 The degree of polymerization (DP) and polydispersity index of PAC were estimated by the gel permeation (24) Tian, Z. Y.; Huang, W. T.; Xiao, D. B.; Wang, S. Q.; Wu, Y. S.; Gong, Q. H.; Yang, W. S.; Yao, J. N. Chem. Phys. Lett. 2004, 391, 283. (25) Dimitrov, A. S.; Takahashi, T.; Furusawa, K.; Nagayama, K. J. Phys. Chem. 1996, 100, 3163. (26) Helseth, L. E.; Wen, H. Z.; Hansen, R. W.; Johansen, T. H.; Heining, P.; Fisher, T. M. Langmuir 2004, 20, 7323. (27) Helseth, L. E.; Fisher, T. M. Opt. Express 2004, 12, 3428. (28) Wu, L.; Tuo, X.; Cheng, H.; Chen, Z.; Wang, X. Macromolecules 2001, 34, 8005.

Li et al. chromatography (GPC) (Viscotek TDA 302 GPC). The measurement was performed on a poly(methyl acrylate) sample that was obtained from the reaction of the same-batch PAC with an excess of methanol. DP of PAC used in this study was estimated to be 239 with a polydispersity index of 1.53. The precursor polymer containing anilino moieties was synthesized by the reaction between a proper amount of N-ethyl-N-(2-hydroxyethyl)aniline and PAC through the Schotten-Baumann reaction. Unreacted acyl chloride groups were then hydrolyzed to obtain the COOH groups after the reaction. Finally, PNANT was obtained by the azo-coupling reaction between the precursor polymer and diazonium salt of 4-nitroaniline. In this reaction, the molar ratio of the diazonium salt to the anilino moieties of the precursor polymer was 1.1 in order that the anilino moieties were completely reacted. The details of those reactions can be seen in our previous papers.13,28,29 The degree of functionalization (DF), defined as the percentage of the structure units bearing azo chromophores among the total structure units, was estimated by the elemental analysis to be 20%. The analytical data of PNANT are given as follows. 1H NMR (DMSO-d6), δ: 12.4, 8.3, 7.8, 6.8, 4.2, 3.2-3.8, 1.4-2.4, 1.1; IR (KBr, cm-1): 3500 (board O-H), 1734 (CdO, s), 1600 1510 1460 (benzene ring, s), 1253 (C-O, s). Anal. Found: C, 57.45; H, 5.69; N, 11.04. Tg ) 119 °C. UV-vis (DMF), λmax: 489 nm. Colloidal Sphere Preparation. The PNANT samples were first dissolved in tetrahydrofuran (THF) that is a good solvent of the polymer. Analytical pure THF that was refluxed with cuprous chloride and distilled for dehydration was used in this process. The solutions were prepared to have concentrations in the range from 0.1 to 1.0 mg/mL. Then a suitable amount of Milli-Q water was added dropwise into each solution at a rate about 5-6 drop/min with stirring. The water addition processes were monitored by the turbidity measurement. For each solution, there was little change in the scattering light intensity at the beginning stage of the water addition. When water content reached ca. 25% (% in volume), the scattering light intensity increased sharply as the water content further increased. After the water content reached a required ratio, a large mount of water was added to the suspensions to quench the structures formed. Finally, the suspensions were dialyzed against water for 72 h to remove THF from the systems.30 Monolayer Preparation. Monolayers of the colloidal spheres were prepared by the vertical deposition method.22c,d The glass slides or silicon wafers used for the experiments were first treated with a solution containing 30% hydrogen peroxide and 70% sulfuric acid overnight. The substrates were immersed vertically into beakers containing the sphere suspensions (about 0.2 g/L). When water evaporated slowly (about 0.05 mL/h) in a 30 °C oven, the surface of liquid moved down and the colloidal spheres were gradually assembled to form monolayers on the substrates. Characterization. Transmission electron microscopy (TEM) measurements were performed by using a Hitachi H-800 microscope with an accelerating voltage of 150 kV. The TEM samples were prepared by dropping the diluted sphere suspensions onto the copper grids coated with a thin polymer film and then dried in a 30 °C vacuum oven for 24 h. No staining treatment was performed for the measurement. The average hydrodynamic diameter (Dh) and the polydispersity index of the colloidal spheres were estimated by the dynamic light scattering (DLS) measurements, and the average sizes were also estimated by TEM observation. The DLS measurement was performed on a Marvern Zetasizer 3000 dynamic light scattering instrument equipped with a multi-τ digital time correlation and a 632 nm solid-state laser light source. The scattering angle used for the measurement was 90° and the temperature was controlled to be 25 °C. The scanning electron microscopy (SEM) images of the colloidalsphere monolayers were obtained by using a field emission microscope (JEOL JSM-6301F), which was operated with an accelerating voltage of 5 kV. All the samples prepared for SEM studies were coated with a thin layer of gold (∼15 nm in thickness) (29) Wang, X. G.; Kumar, J.; Tripathy, S. K.; Li, L.; Chen, J.; Marturunkakul, S. Macromolecules 1997, 30, 219. (30) See, for example: (a) Zhang, L.; Eisenberg, A. J. Am. Chem. Soc. 1996, 118, 3168. (b) Zhang, L.; Shen, H.; Eisenberg, A. Macromolecules 1997, 30, 1001. (c) Terreau, O.; Luo, L.; Eisenberg, A. Langmuir 2003, 19, 5601.

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Figure 2. Hydrodynamic diameters (Dh) and the polydispersity index of the spheres as a function of the H2O/THF ratio (v/v). The initial polymer concentration in THF was 1.0 mg/mL.

Figure 1. Typical TEM image of the colloidal spheres formed from PNANT. The initial polymer concentration in THF was 0.3 mg/mL and the H2O/THF ratio (v/v) was 1. before the measurement. The atom force microscopy (AFM) pictures were obtained by using a scanning probe microscope (Nanoscope IIIa) operated in the tapping mode. Photoinduced Dichroism. Dichroism of the colloidal-sphere monolayers was induced by exposure of the samples to a polarized Ar+ laser beam at 488 nm with intensity about 150 mW/cm2. The photoinduced dichroism of the monolayers was measured by using a UV-vis spectrophotometer (Agilent 8453 spectrometer) equipped with a polarizer. In both writing and detecting experiments, the incident beams were all perpendicular to the substrate surfaces.

Results and Discussion 1. Colloidal Sphere Preparation. The colloidal spheres were prepared by a method similar to those reported previously for preparing micelles and other selfassembledaggregatesfromamphiphilicblockcopolymers.19a In current work, THF solutions of PNANT were prepared to have concentrations in the range from 0.1 to 1.0 mg/ mL. The selective solvent (H2O) was added slowly into the THF solutions, which caused the self-assembly of polymeric chains through hydrophobic interaction to form the spherical colloids. The water addition process was divided into two stages. In the first stage, a suitable amount of Milli-Q water was added into the THF solutions to form the colloidal spheres. After that, excess Milli-Q water was added into the suspensions to quench the structures formed. In this paper, the H2O/THF ratio means the ratio of H2O to THF added in the first stage. Under proper conditions discussed in the following part, uniform colloidal spheres could be obtained. The colloidal particles could be observed by TEM in a vivid contrast without staining treatment. Figure 1 shows a typical TEM image of the colloidal spheres. Colloidal spheres with uniform shape can be seen from the picture. The average size and the size distribution of the colloidal spheres depended on the factors such as the initial concentration of the azo polymer in the THF solution, DF of the polymer, and the H2O/THF ratio. When the initial concentration and DF were fixed, the average size and size distribution were mainly related to the H2O/THF ratio. As TEM observations indicated the spherical shape of the colloids, the average sizes and the size distributions were characterized by the average hydrodynamic diameter (Dh) and the polydispersity index obtained from DLS mea-

surements. Figure 2 shows the relationship between Dh, the polydispersity index of the colloidal spheres and the H2O/THF ratio. In this experiment, the initial concentrations of PNANT in the THF solutions were all 1.0 mg/mL. The effect of the initial concentration on the average size and the size distribution will be discussed in the following part. When the H2O/THF ratio increases in a range from 0.4 to 0.6 (v/v), Dh increases and the polydispersity index decreases sharply as the H2O/THF ratio increases. After the ratio reaches a critical value (about 0.6), Dh and the polydispersity index of the spheres no longer significantly change as the H2O/THF ratio further increases. This relationship is closely related with the polymer structure used in this study. PNANT is a random copolymer composed of the hydrophobic azobenzenecontaining units and the hydrophilic acrylic acid units. The polymer possesses distribution both in the molecular weight and the degree of functionalization (i.e., hydrophobicity). The colloidal spheres were formed by the selfassembly of the polymeric chains through the hydrophobic interaction.30 It is believed that the formation of colloidal spheres started from the aggregation and collapse of the most hydrophobic chains. Then other segments and polymeric chains aggregated sequentially according to their hydrophobicity. When the water content was low, only the most hydrophobic chains associated with each other and formed small-size colloidal spheres like a nucleation process in most colloid formation processes.15 As the water content further increased, less hydrophobic chains aggregated on the colloidal spheres already formed and the sphere sizes grew gradually. When the water content reached the critical value, most of the polymer chains had transferred from the solution to the colloidal spheres and the sizes of the colloids no longer significantly increased with the increase of the water content. To obtain colloidal spheres with uniform sizes, the ratios of H2O/ THF were selected to be larger than the critical value (e.g., 0.6 v/v). The average size and the size distribution of the spheres also depended on the initial concentration of PNANT in the THF solutions. The relationship between Dh, the polydispersity index and the initial concentration is shown in Figure 3. It can be seen that the initial concentration has different influence on Dh and the polydispersity index. Dh of colloids increases from 110 to 220 nm as the initial concentration increases from 0.1 to 1.0 mg/mL. When the initial concentration is in a range from 0.1 to 0.5 mg/mL, the polydispersity index of the spheres significantly decreases as the initial concentration increases. On the other hand, when the initial concentration increases between 0.7 and 1.0 mg/mL, the polydispersity index of the spheres slightly increases with the concentration increase. When the initial concentration varies from 0.4 to 1.0 mg/mL, the polydispersity index is lower than 0.04

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Figure 3. Hydrodynamic diameters (Dh) and the polydispersity index of the spheres as a function of the initial concentration of PNANT in THF. The H2O/THF ratio (v/v) was 1.

Figure 4. Typical SEM image of the colloidal monolayer obtained by the vertical deposition method. Dh and the polydispersity index of the spheres were 195 nm and 0.04, respectively.

and shows little variation with the concentration change, but the average size of colloids steadily increases as the initial concentration increases. Therefore, the initial concentration of PNANT in the THF solutions can be used as a parameter to adjust the average size of the colloidal spheres while keeping the size distribution to be relatively narrow. By adjusting those factors discussed above, stable suspensions containing the colloidal spheres with Dh in the range from 160 to 220 nm and the polydispersity index lower than 0.05 were obtained. 2. Monolayers of Colloidal Sphere. The attractive capillary force can cause the colloidal spheres to organize into hexagonally close-packed 2D arrays or monolayers.22 The vertical deposition method is frequently used to realize the capillary-force-driven self-assembly. In the vertical deposition method, a substrate is immersed vertically in a suspension containing colloidal spheres. By evaporation of the dispersion medium or lifting of the substrate out of the suspension at constant speed, the surface of the liquid moves down and the colloids deposit onto the substrate during the decline of the surface.22c,e In the current work, the vertical deposition was carried out from a suspension containing the colloidal spheres with Dh of 195 nm and the polydispersity index of 0.04. The dispersion medium (H2O) was evaporated slowly under the controlled condition. The parameters, such as the concentration of the colloidal suspension and the volatilization speed of the water, were carefully adjusted.22b,c As the colloidal spheres used in this work showed a tendency to aggregate, the concentration of the suspension was controlled to be lower than those reported. A slower volatilization rate was selected as suggested by the computational study.31 Figure 4 shows a typical SEM image of the colloidal monolayers constructed from the azo polymer spheres. It can be seen that the monolayer is mainly composed of the hexagonally close-packed structures of the colloidal (31) Yamaki, M.; Higo, J.; Nagayama, K. Langmuir 1995, 11, 2975.

Figure 5. (a) AFM 3D-view of the colloidal monolayer; (b) 2D fast Fourier transform (FFT) plot of the monolayer obtained from an AFM image of a 10 × 10 µm2 region. Dh and the polydispersity index of the spheres were 195 nm and 0.04, respectively.

spheres. A typical AFM 3D-view of the monolayers is given in Figure 5a. The aligned pattern observed from AFM is consistent with the SEM observation. Figure 5b is a 2D fast Fourier transform (FFT) plot obtained from an AFM image of a 10 × 10 µm2 region. The FFT plot shows distinct spots as one could expect for the hexagonal structures. Within a 2 mm × 2 cm region, the same FFT pattern can be observed. This result indicates that hexagonally closepacked structure is a predominant alignment pattern in the region. Both SEM and AFM observations showed that those colloidal spheres with the approximately same size were self-assembled into the hexagonally close-packed structures and most defects occurred around the spheres with obviously different sizes. The defects could occur during the nucleation or the organizing process driven by the attractive capillary force through a convective transport.22 To obtain ordered colloidal arrays by the vertical deposition, the size distribution of the colloids need to be strictly controlled.22b Compared with the PS latex normally used in previous works, the colloidal spheres used in this study were more difficult to be aligned orderly because of the wider size distribution. The samples, obtained by adjusting the initial concentration and the H2O/THF ratio during the preparation process, still contained a small amount of spheres with different sizes. Here, our main

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Figure 7. Relationship between the orientation order parameter (S) of the colloidal monolayer and the irradiation time.

sharply at beginning and levels off gradually. The curve can be fit by the following first-order exponential decay function33

S ) 1 - exp (-t/τ)

Figure 6. (a) UV-vis spectra of the colloidal monolayer measured after irradiation with a linearly polarized Ar+ laser beam for 10 min. Curve a: perpendicular to the polarization direction of the laser beam. Curve b: parallel to the polarization direction of the laser beam. (b) Absorbance measured at λmax as a function of the rotation angles to the polarization direction of the laser beam.

purpose is to demonstrate a possible way to construct 2D structures with the photoresponsive colloids. To obtain more highly ordered structures such as 2D arrays for real device applications, the monodispersed colloidal spheres might be required. The colloidal spheres with narrower size distribution can be obtained by some well-established methods such as the ultracentrifugation. 3. Photoinduced Dichroism. To study the photoinduced anisotropy, a well-prepared monolayer of the colloidal spheres was irradiated with a linearly polarized Ar+ laser beam (488 nm, 150 mW/cm2) for different periods of time. Dichroism of the monolayer was monitored by the polarized UV-vis spectroscopy. After the colloidal monolayer was exposed to the laser beam for 10 min, significant dichroism was observed (Figure 6a). The absorbance in the direction parallel to the polarization of the laser beam is obviously smaller than that in the direction perpendicular to the polarization of the laser beam. The maximum absorbance measured at different rotation angles to the polarization direction of the writing beam is given in Figure 6b. Both Figure 6a and Figure 6b indicate that the preferential orientation of the azo chromophores is perpendicular to the electric vector of the laser beam. The orientation order parameter S, which is often used to describe the degree of orientation, can be estimated from the dichroic ratio32

S ) (A⊥ - A|)/(A⊥ + 2A|)

(1)

where A⊥ and A| are the maximum absorbance in directions perpendicular and parallel to the polarization. The relationship between the orientation order parameter and the irradiation time is given in Figure 7. As the irradiation time increases, the orientation order parameter increases (32) Wu, Y. L.; Demachi, Y.; Tsutsumi, O.; Kanazawa, A. T.; Shiono, A.; Ikeda, T. Macromolecules 1998, 31, 349. (33) Ho, M. S.; Natansohn, A.; Rochon, P. Macromolecules 1995, 28, 6124.

(2)

The fitting curve is given in the same figure with a correlation coefficient (χ2) of 0.98. The time constant τ obtained from the best fit is 6.3. After irradiating about 10 min, the photoinduced chromophore orientation is almost saturated. The orientation order parameter was estimated to be 0.09 at the photostationary stateswhich is comparable to those reported for spin-coating films.2,13 However, the photoresponsive rate of the colloidal monolayer is obviously slower than those observed for the spincoating films of acrylate-based azo polymers.2,33 The reason to explain this observation is still not clear to us at the current stage. Because of the photoresponsive properties, the colloidal spheres and the structures constructed from them can potentially be used as a new type material. As the colloidal spheres are uniform, the degree of the photoinduced chromophore orientation should approximately be the same for each sphere. With the aid of the methods such as near-field optics and two-photon recording,34,35 the photodriven process can be performed or tested for single or few spheres. Ordered arrays of the spheres can be used promisingly for applications such as date storage and light manipulation. Conclusion In summary, colloidal spheres with photoinduced anisotropy properties have been prepared from an amphiphilic azobenzene-containing random copolymer. The initial concentration of the polymer in THF and the H2O/ THF ratio were explored as parameters to control the average size and the size distribution of the spheres. Monolayers of the spheres were prepared by the capillaryforce-driven method and composed predominantly of the hexagonally close-packed structures. The monolayers showed obviously photoinduced dichroism upon Ar+ laser irradiation. The orientation order parameter of the colloidal spheres was related to the irradiation time and estimated to be 0.09 at the photostationary state. As the spherical colloids were prepared with a random copolymer through a self-assembling method, the spheres can be obtained in a relatively feasible way. With a proper molecular design and adjustment of the preparation conditions, uniform spheres and ordered arrays with required photonic properties can be expected. Acknowledgment. The financial support from the NSFC under Projects 59925309 and 20374033 is gratefully acknowledged. LA050082A (34) Tsujioka, T.; Irie, M. Appl. Opt. 1998, 37, 4419. (35) Strickler, J. H.; Webb, W. W. Opt. Lett. 1991, 16, 1780.