J. Phys. Chem. B 2007, 111, 7761-7766
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Kinetics of Photoinduced E to Z Isomerization of Azobenzene in Polystyrene Films: Thickness, Molecular Weight and Temperature Effects Yohei Tateishi, Keiji Tanaka,* and Toshihiko Nagamura* Department of Applied Chemistry, Faculty of Engineering, Kyushu UniVersity, 744 Motooka, Nishi-ku, Fukuoka 819-0395, Japan ReceiVed: January 20, 2007; In Final Form: May 4, 2007
E to Z (trans f cis) photoisomerization of azobenzene (Az) chromophores tagged to polystyrene (PS) in thin films was studied as functions of thickness, PS molecular weight, and temperature, using the change in absorption at 336 nm arising from the Az E isomer remaining upon ultraviolet light irradiation at 350 ( 5 nm. The photoisomerization in solid films exhibited fast and slow modes. The fractional amount of the fast mode (I1) started to increase with decreasing film thickness once the films were thinner than a threshold value. This was explained in terms of a surface layer in which the photoisomerization reaction proceeds quickly, the effect of which becomes more noticeable with decreasing thickness due to a larger surface to volume ratio. The thickness dependence of the I1 fraction was insensitive to the molecular weight of the PS used. The thickness of the surface layer, estimated through a layer model analysis, increased with rising temperature up to 298 K. Interestingly, the surface layer markedly thickened at temperatures at which the molecular motion of PS is on a relatively small scale, namely, at the γ and β relaxation temperatures.
Introduction Polymers containing azobenzene (Az) groups have been investigated for their potential as photoresponsive materials, due to the photoinduced E to Z (trans to cis) isomerization reaction, which may be applicable to a wide variety of technological applications.1-23 For instance, materials containing Az groups have demonstrated intriguing potential in diverse fields, including memory devices,6 surface relief gratings,1,3-5,7-10 and optical switching devices.1-4,11-14 Also, Az chromophores can regulate aggregation states and physical properties in materials by virtue of their liquid crystalline properties and/or photoisomerization reactions.1-5,15-23 However, the molecular origin of such functions is not yet fully understood. From the point of view of local free volume, the physical properties of polymers have been extensively studied using the photoisomerization reaction of Az groups as a probe.24-27 Sung et al. and Mita and co-workers have studied the kinetics of the photoinduced E to Z isomerization in solution and solid states.25,27 While the photoisomerization kinetics of Az chromophores in a dilute solution could be expressed as a firstorder reaction, two time constants, corresponding to fast and slow modes, were necessary to reproduce the experimental data for the reaction in the solid state. This was interpreted by the supposition that the distribution of local free volume was heterogeneous in a solid film. In the past decade, thin and ultrathin polymer films have become widely utilized in cutting-edge applications such as nanocoatings,28 nanoadhesion,29 and nanolubrication,30 biomaterials,31,32 and multilayer devices.33,34 However, it has been revealed that the physical properties of these films are often different from those of the bulk materials. As a typical result, the glass transition temperature (Tg) in the ultrathin state * Corresponding authors. Telephone: +81-92-802-2879 (K.T.); +8192-802-2878 (T.N.). Fax: +81-92-802-2880 (K.T.). E-mail: k-tanaka@ cstf.kyushu-u.ac.jp (K.T.);
[email protected] (T.N.).
becomes lower than the bulk value,35,36,38-42,44,45 if an attraction does not exist between the chains and the substrate.37,43 This could be explained in terms of a surface layer, in which mobility is enhanced in comparison with the bulk,46-52 although an interpretation for the Tg depression without the surface effect has also been attempted.53 In contrast, chain mobility at the substrate interface should likely be less owing to the presence of a hard wall even without an attractive interaction between the chains and the substrate.54 Such a mobility gradient in ultrathin polymer films has been theoretically pursued, using a coupling model, by Ngai.55 The molecular motions of polymers occur at various size scales, and thus, understanding the hierarchy is very important. However, most studies in thin film dynamics are aimed at relatively large scale molecular motions such as segmental motion and center-of-mass diffusion. Therefore, in this study, we have explored the small-scale motions involved in the kinetics of the photoinduced E to Z photoisomerization reaction of Az chromophores tagged to polystyrene (PS) of various molecular weights in thin and ultrathin films at low temperatures and compared them with the rheological properties of the PS matrix. The information obtained from this study enables us to better understand the mechanism of the unique dynamics manifested in ultrathin states as well as to gain insights into regulating the photoisomerization of Az chromophores in nanosystems for industrial applications. Experimental Section Synthesis of Az-Labeled Polystyrenes. Scheme 1 shows the synthetic route to the polystyrene (PS) containing Az groups (PS-Az) used in these experiments. First, monodisperse PS with various molecular weights was synthesized by a living anionic polymerization using sec-butyllithium as an initiator and methanol as a terminator. Then, random aminomethylation of phenyl rings in PS was carried out by the procedure of Mitchell et al.56 with some modifications. The aminomethyl groups were
10.1021/jp0705065 CCC: $37.00 © 2007 American Chemical Society Published on Web 06/16/2007
7762 J. Phys. Chem. B, Vol. 111, No. 27, 2007
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SCHEME 1: Synthetic Route for PS-Az
Figure 1. Time change in UV-visible spectra for 255-nm-thick PSAz film under UV irradiation at room temperature.
onto a sample at a power of 2 mW‚cm-2. The absorption at 336 nm was followed as a function of time at various temperatures by a spectrophotometer (Hitachi U-4100). Dynamic Mechanical Analysis. The thermal molecular motions of a compression-molded PS film with the size of 0.3 × 3 × 30 mm3 was examined by dynamic mechanical analysis (DMA) under a tensile-compression mode using a Rheovibron DDV-01FP (A&D Co., Ltd.) tester. The values of Mn and Mw/ Mn of the PS used for the DMA measurement were 131 500 and 2.19, respectively. Since the film thickness was about 0.3 mm, the results obtained can be regarded as bulk information. The measurement was carried out at a heating rate of 0.5 K min-1 at a frequency of 0.02 Hz under a dry nitrogen purge.
subsequently reacted with an appropriate amount of the Az chromophores to complete the labeling procedure. The Az fraction of PS was about 1 mol % of the styrene monomers in each sample. Characterization of PS-Az. Table 1 shows the numberaverage molecular weight (Mn), the molecular weight distribution (Mw/Mn, where Mw is the weight-average molecular weight), the bulk glass transition temperature (Tgb), and twice the radius of gyration (2Rg) for the PS-Az materials obtained. The values of Mn and Mw/Mn were determined by gel permeation chromatography using tetrahydrofuran (THF) as an eluent, and the Tgb values were found by differential scanning calorimetry under a dry nitrogen purge at a heating rate of 10 K min-1. The values of 2Rg were calculated assuming that the chain dimension is not perturbed by the presence of the Az chromophores. Sample Preparation. PS-Az films with various thicknesses were prepared on quartz plates by a spin-coating method and were annealed at 393-403 K in vacuo for 24 h. It was found to be difficult to experimentally determine the film thickness of a film on a quartz plate by ellipsometry. Hence, the thickness of a film prepared under the same conditions, but on a silicon wafer, was measured by ellipsometry. Because of the chemical similarity of the two substrate surfaces, the thickness on quartz was assumed to be the same as that of the same type of film prepared on silicon. The plausibility of this assumption was confirmed by different thickness determinations based on the ultraviolet (UV) absorbance change and the interference fringes present. Furthermore, the step heights of films on quartz which had been partially scratched by a blade, measured by atomic force microscopy, were in excellent accordance with the thickness determined as mentioned above. The absorbance of the specimen was always kept at approximately 0.1. This was attained by stacking a number of films for a thinner case. On the other hand, in the case of a thick film, it was diluted by mixing the nonlabeled PS. Photoisomerization Kinetics. To study the kinetic behavior of the E to Z photoisomerization of the Az groups, the absorption band at 336 nm arising from the Az E isomer remaining was measured. The PS-Az film was set in a cryostat (Oxford DN1754) and a mercury-xenon lamp (Hamamatsu L8333-01) was used as a light source. Using a band-pass filter, ultraviolet light with wavelength centered at 350 ( 5 nm was irradiated
Results and Discussion Photoisomerization Kinetics for Az Probes. Figure 1 shows the time dependence of the UV-visible spectra for a 255-nmthick PS-Az film upon UV irradiation. An absorption peak observed at 336 nm is assigned to the π-π* transition along the long axis of the Az E isomers.57 In the case of a control PS thin film, an increase in absorbance based on interference due to the film thickness and/or Rayleigh scattering was apparent, even though Az probes were not present in the film. Hence, all spectra shown in Figure 1 have had the corresponding spectrum of the control PS film subtracted. After this subtraction process, the absorption spectra of PS-Az films had basically the same shape as those of a solution. The absorbance near 336 nm decreased with increasing irradiation time of UV light, indicating that the E to Z isomerization of the Az probes was in fact caused by the UV irradiation. Figure 2 shows the UV irradiation time dependence of the absorbance (A(t)) at 336 nm for PS-Az in a dilute solution and in the films. The ordinate was normalized by the value of A at t ) 0 (A(0)). The A value exponentially decayed with increasing t for the PS-Az THF solution. In general, the photoisomerization of chromophores in a dilute solution proceeds on the first-order reaction. Hence, the experimental result was fitted by eq 1.27
A(t) t ) I exp - + Ioffset τ A(0)
( )
(1)
where τ, I, and Ioffset are the time constant for the reaction, the fractional amount of E form photoisomerized after reaching the photostationary state, and an offset, respectively. The offset
TABLE 1: Characteristics of PS-Az sample
Az residue/mol %
Mn
Mw/Mn
Tgb/K
2Rg/nm
PS-Az-22 000 PS-Az-59 000 PS-Az-245 000
1.2 1.1 1.4
22 000 59 000 245 000
1.08 1.06 1.15
375 379 380
8.3 13.6 26.7
E to Z Isomerization of Az in PS Films
J. Phys. Chem. B, Vol. 111, No. 27, 2007 7763
Figure 2. Irradiation time dependence of absorbance at wavelength of 336 nm for PS-Az dilute solution and films. As typical examples, the data sets for the 15-nm-thick, 40-nm-thick, and 1.5-µm-thick films are presented. The ordinate was divided by the absorbance at a time of 0. The solid curves are the best fits to eq 1 or 2.
corresponds to the Z fraction at the photostationary state. The best-fit curve was obtained with τ ) 52 s and Ioffset ) 0.15. On the other hand, in the case of the PS-Az films, the time dependence of the E form fraction could not be well fitted by eq 1. Instead, a double-exponential equation was used to fit the data, as reported elsewhere.25b-e,27 The need for a double exponential means that the photochemical E to Z isomerization can be separated into a fast and a slow process
( )
( )
A(t) t t + I2 exp + I3 + I4 ) I1 exp τ1 τ2 A(0)
(2)
where τ1 and τ2 are time constants for the two modes of the reaction and I1 and I2 denote the fractional amount of the fast and slow modes, respectively. In general, the local free volume in a glassy film is not homogeneous. Thus, it seems most likely that I1 is proportional to the number of regions where local free volumes are greater than a critical size necessary for the Az group.25b-e Mita et al. have also explained the E to Z photoisomerization of Az in polymer films in terms of the local free volume and its fluctuation.27 The slow mode (I2) is a unique process in a solid film, which is probably impeded by a lack of local free volume and/or frozen matrix mobility. The I3 term has a meaning similar to that of I2, and it corresponds to the fraction in which the reaction did not occur because of a lack of local free volume and/or matrix mobility. The I4 term represents the fraction of the E form in the photostationary state. Since the Ioffset value in the PS-Az THF solution was independent of temperature below room temperature, it seems most likely that the fractional amount of the E form is constant below room temperature. Thus, we can simple identify the value of I4 to be the same as Ioffset. According to extensive studies by Sung and co-workers, the fast process in a bulk PS solid is as fast as it is in a dilute solution,25b-e fixing τ1 at 52 s. The time variance of A(t)/A(0) for the 1.5-µm-thick film, which can be regarded as a bulk system, was fitted by eq 2 using τ1 ) 52 s and Ioffset ) 0.15 and taking τ2 and I1 to be fitting parameters. The best-fit curve was obtained with τ2 ) 222 s and I1 ) 0.23, and is drawn in Figure 2. This implies that the time constant for the slow mode, 222 s, was characteristic of the photoisomerization reaction in the bulk solid. Thus, in the following, the two time constants of 52 and 222 s were used to fit the data for thinner PS-Az films. Data fitting with τ1 ) 52 and τ2 ) 222 s worked well, as seen in typical examples such as the 15- and 40-nm-thick films shown in Figure 2. Film Thickness and Molecular Weight Dependences. The fractional amount of the fast mode (I1) contribution to the
Figure 3. Thickness dependence of the fast mode contribution to the photoisomerization reaction of Az probes in films of PS with Mn of 22 000, 59 000, and 245 000 at 298 K. The solid and broken curves were drawn by fitting procedures mentioned in the text.
photoisomerization was examined as functions of film thickness and PS molecular weight. Figure 3 shows the results. The I1 value started to increase with decreasing thickness for films thinner than 100 nm. Then, the value became insensitive to the thickness at approximately 25 nm. The thickness dependence of I1 was also independent of the molecular weight of PS. So far, it has been widely accepted that there exists a liquidlike surface layer of the PS films.38,39 Thus, it is plausible that the photochemical E to Z isomerization of the chromophores in the surface region proceeds as fast as in a dilute solution. If that is the case, the following equation should be used to reproduce the relation in Figure 3. The first and second terms reflect the contribution from the surface layer and the bulk phase to the fractional amount of the fast mode.
(
)
Rs Rs I1(d) ) I1s + I1b 1 d d
(3)
where I1s and I1b are I1 values in the surface and bulk regions, respectively, and Rs and d are surface layer and film thicknesses, respectively. As mentioned before, Ioffset had a value of 0.15; that is, 15% of the Az probes exist as the E form in the photostationary state. Hence, the value of I1s was assumed to be 0.85 ()1 - 0.15). Of course, this maximum value cannot be attained in reality because a film possesses a bulk region with a finite thickness. Also, I1b was estimated by averaging the I1 value over the films thicker than 1 µm, in which the surface effect might be ignored. Fixing I1s ) 0.85 and I1b ) 0.23, the best-fit curve was obtained with Rs ) 11.8 nm, provided that data points for the films thinner than 20 nm were not counted. This is because the data points for the films thinner than 20 nm were not well fitted using eq 3. Recently, it has been revealed that Tg in the interfacial region with the solid substrate is higher than the Tgb.54 As a result, the interfacial effect was incorporated to eq 3 as a third term. Using this equation, the experimental d vs I1 relation could be accurately fitted over the entire thickness range employed, as shown by the broken curve in Figure 3.
(
I1(d) ) I1b 1 -
)
()
Rs Ri R s + Ri + I1s - I1i d d d
2
(4)
Here, Ri and I1i are the interfacial layer thickness and the I1 value at the substrate interface, respectively. The values of I1b, I1s, and I1i were fixed to be 0.23, 0.85, and 1.00 for the fitting analysis. The meaning of the third term is that the presence of the substrate deactivates the E to Z photoisomerization in the films. The best-fit broken curve in Figure 3 corresponds to Rs ) 18.3 and Ri ) 8.3 nm. This three-layer model appears to fit experimental data better than the two-layer model.
7764 J. Phys. Chem. B, Vol. 111, No. 27, 2007
Figure 4. Thickness dependence of I3 for Az probes in films of PS with Mn of 22 000, 59 000, and 245 000 at 298 K.
Figure 4 shows the film thickness dependence of I3. The I3 parameter corresponds to the fraction of the Az E form which cannot be isomerized on account of a lack of local free volume and/or frozen matrix mobility. While the I3 value was invariant with respect to the thickness down to approximately 50 nm, it started to decrease slightly for thinner films, implying that local free volume and/or matrix mobility increases with decreasing film thickness. In general, Tg for the PS thin films on an inert substrate starts to decrease with decreasing thickness mainly due to the presence of the free surface, once the film becomes thinner than 50 nm. For the moment, the relation between the kinetics of the Az photoisomerization reaction and segmental motion of the matrix PS is far from clear. However, once the segmental motion of PS is possible, the local free volume in the system definitely increases. Thus, this coincidence seems to be quite reasonable. Surface Effect. To confirm whether the presence of the surface layer truly accelerates the E to Z photoisomerization, a 160-nm-thick film of PS with Mn of 59 000 was put onto the PS-Az film having a thickness of 40 nm, and the time dependence of the E fraction was examined.58 In this case, the surface effect should be weakened. Although the I1 value for the 40-nm-thick PS-Az film was 0.47 ( 0.04 as shown in Figure 3, it decreased to 0.37 ( 0.02 after the PS top layer was laminated. Moreover, after the bilayer was annealed at 393 K for 24 h to diffuse the interface between PS and PS-Az,59 there was a further decrease in the surface effect as the I1 value decreased to 0.32 ( 0.02. Similarly, the surface effect on I3 was clear. While the I3 value for the 40-nm-thick PS-Az film was 0.27 ( 0.04, after putting the PS top layer, it increased to 0.33 ( 0.02, which is comparable to the corresponding bulk value. These results strongly support our argument that the surface effect is the factor responsible for the behavior seen in Figures 3 and 4. Temperature Dependence. To address the effect of the thermal molecular motion of PS on the photoisomerization kinetics of Az probes, the reaction was examined at various low temperatures. Since the thermal backward reaction from Z to E form cannot be ignored at higher temperatures,27,60 the measurement was truncated at 298 K. Figure 5 shows the film thickness dependence of I1 for the PS-Az film with Mn of 59 000 at 298, 175, and 80 K. In the thickness range thinner than about 200 nm, the I1 value increased with decreasing thickness. Although this was the case even at 80 K, as shown in Figure 5 this tendency became less marked at this temperature. This means that the surface layer, in which the photoinduced E to Z isomerization of the Az probes can proceed as fast as in liquid, became thicker with increasing temperature. Previously, Mita and co-workers have studied photoisomerization kinetics for Az in thick poly(carbonate) (PC) films at
Tateishi et al.
Figure 5. Thickness dependence of I1 for Az probes in films of PS with Mn of 59 000 at 298, 175, and 80 K.
Figure 6. Temperature dependence of Rs for Az probes in PS films with Mn of 59 000. The Rs value was calculated by the three-layer model using eq 4. The temperature dependence of tan δ for a thick PS film obtained at a frequency of 0.02 Hz is also shown. The solid curve was drawn from the data in ref 62. The broken curve was obtained by stretching the solid curve along the ordinate to fit the data points.
various low temperatures, and claimed that the initial rate of the reaction was insensitive to temperature.27 This seems to imply that the rate of the fast mode, related to the photoisomerization, was equal to that in solution and was independent of temperature.27 Hence, we again fixed the two time constants τ1 and τ2 to be 52 and 222 s and the two-layer model was also applied to the data sets at 175 and 80 K. The Rs values so obtained were lowered from 11.8 nm at 298 K to 6.0 and 2.4 nm at 175 and 80 K, respectively, provided that the data points for the films thinner than 20 nm were not counted. The three-layer model was also applied to this analysis. In this case, the Ri value of 9.4 nm was taken for PS with Mn of 59 000 at 298 K on the basis of the best fit, and was assumed to be independent of temperature because the interfacial layer, in which mobility was depressed, should not grow with increasing temperature. The dead mobile layer was also observed in the analysis for the mean square displacement in PS ultrathin films by inelastic and quasielastic neutron scattering, and the thickness was also reported to be approximately 10 nm.61 Figure 6 shows the temperature dependence of surface layer, in which the photoisomerization proceeded faster, for the PS-Az films. The Rs value was calculated by the three-layer model using eq 4. The temperature-tan δ relation for a thick PS film is also shown in Figure 6 in order to discuss the origin of the surface layer on the basis of thermal molecular motion of the PS. Since the time constant of the fast mode for the Az photoisomerization was 52 s, a dynamic viscoelastic measurement should be made at a frequency of 3 mHz [)1/(2πτ1)]. However, that frequency was beyond the lower limit of our instrument. Thus, the
E to Z Isomerization of Az in PS Films
Figure 7. Temperature dependence of I1 for Az probes in 1.5-µmthick and 15-nm-thick films of PS with Mn of 59 000. The broken curve was obtained by stretching the solid curve in Figure 6 along the ordinate to fit the data points.
frequency of 0.02 Hz was chosen. From the viscoelastic data, three relaxation processes, the so-called R, β, and γ processes, were observed at the higher temperatures. The R relaxation process arises from the segmental motion. The β and γ processes are, respectively, assigned to twisting of main chains and restricted rotation of phenyl groups.62 The strength and temperature locations of these relaxation processes agreed well with published data, although the data in the γ process was scattered because of the poorer signal-to-noise ratio at lower frequencies. The Rs value increased with increasing temperature. Interestingly, an increase in Rs became prominent at the onset temperatures for the γ and β processes. Although the absolute value of Rs is strongly dependent on the model used, it seems most likely that the surface layer grows with increasing temperature and markedly thickens at the relaxation temperatures. To confirm whether this idea is correct, the I1 value was plotted as a function of temperature for ultrathin and thick films with the results shown in Figure 7. The temperature dependence of I1, which has basically the same trend as the temperature dependence of Rs shown in Figure 6, was more dominant in the ultrathin film than in the thick one. This is simply because the ratio of the surface area to volume was much larger in the ultrathin film. That is, although the surface layer thickened even in the thick film, the effect could not be discerned due to the small surface ratio to the volume. Therefore, it seems reasonable to conclude that the surface layer, in which the photoisomerization proceeded faster, grew with the aid of molecular motion. However, unfortunately, it was not possible to study how the segmental motion impacts the growth of the surface layer because the backward isomerization reaction of Z to E cannot be ignored at elevated temperatures and thus the kinetics becomes complicated. Finally, we discuss the origin of the surface layer. Based on Figures 6 and 7, it was apparent that the photoinduced E to Z isomerization of Az chromophores proceeded even at temperatures at which rotation of phenyl groups was not allowed.27 Also, this tendency was more dominant at the surface than in the bulk region. If we accept that there exist regions where the local free volumes are larger than the critical size necessary for the Az photoisomerization,25b-e,27 and that the number is larger in the surface region than in the bulk, then the results can be understood. In other words, the density at the surface might be lower than that in the interior region of the film.63 Conclusions We have studied isomerization kinetics for the Az probes in thin and ultrathin films of PS. The reaction became faster with
J. Phys. Chem. B, Vol. 111, No. 27, 2007 7765 decreasing thickness once a film became thinner than a threshold value. Applying layer model analyses to the thickness dependence of photoisomerization kinetics, the thickness of the surface layer, in which the reaction proceeded faster, could be successfully extracted. The value increased with increasing temperature, at least up to 300 K. Interestingly, the surface layer markedly thickened at temperatures at which molecular motion at a relatively small scale became possible, in particular, the γ and β relaxation temperatures. This is consistent with the result that there was little dependence of the kinetics on the molecular weight dependence. Also, the origin of the surface layer is traced to the idea that the density at the surface is lower than that in the interior region of the film. Acknowledgment. This research was partly supported by the Industrial Technology Research Grant Program in 2006 from the New Energy and Industrial Technology Development Organization (NEDO) of Japan, and by the Grants-in-Aid for Young Scientists A (No. 18685014) and Science Research in a Priority Area “Soft Matter Physics” (No. 19031021) from the Ministry of Education, Culture, Sports, Science and Technology, Japan. References and Notes (1) Delaire, J. A.; Nakatani, K. Chem. ReV. 2000, 100, 1817. (2) Ichimura, K. Chem. ReV. 2000, 100, 1847. (3) Natansohn, A.; Rochon, P. Chem. ReV. 2002, 102, 4139. (4) Ikeda, T. J. Mater. Chem. 2003, 13, 2037. (5) Seki, T. Polym. J. 2003, 36, 435. (6) Liu, Z. F.; Hashimoto, K.; Fujishima, A. Nature (London) 1990, 347, 658. (7) Rochon, P.; Batalla, E.; Natansohn, A. Appl. Phys. Lett. 1995, 66, 136. (8) Kim, D.; Tripathy, K.; Li, L.; Kumar, J. Appl. Phys. Lett. 1995, 66, 1166. (9) Kumar, J.; Li, L.; Jiang, X. L.; Kim, D.; Lee, T. S.; Tripathy, S. Appl. Phys. Lett. 1998, 72, 2096. (10) Viswanathan, N.; Kim, D.; Bian, S.; Williams, J.; Liu, W.; Li, L.; Samuelson, L.; Kumar, J.; Tripathy, S. J. Mater. Chem. 1999, 9, 1941. (11) Ikeda, T.; Sasaki, T.; Ichimura, K. Nature (London) 1993, 361, 428. (12) Ikeda, T.; Tsutsumi, O. Science 1995, 268, 1873. (13) Kim, M. J.; Yoo, S. J.; Kim, D. Y. AdV. Funct. Mater. 2006, 16, 2089. (14) Yager, K. G.; Barrett, C. J. J. Photochem. Photobiol., A 2006, 182, 250. (15) Seki, T.; Sekizawa, H.; Morino, S.; Ichimura, K. J. Phys. Chem. B 1998, 102, 5313. (16) Ikeda, T.; Nakano, M.; Yu, Y.; Tsutsumi, O.; Kanazawa, A. AdV. Mater. 2002, 15, 201. (17) Yu, Y.; Nakano, M.; Ikeda, T. Nature (London) 2003, 425, 145. (18) Ichimura, K.; Oh, S. K.; Nakagawa, M. Science 2000, 288, 1624. (19) Morikawa, Y.; Nagano, S.; Watanabe, K.; Kamata, K.; Iyoda, T.; Seki, T. AdV. Mater. 2006, 18, 883. (20) Seki, T.; Sakuragi, M.; Kawanishi, Y.; Suzuki, Y.; Tamaki, T.; Fukuda, R.; Ichimura, K. Langmuir 1993, 9, 211. (21) Seki, T.; Fukuda, K.; Ichimura, K. Langmuir 1999, 15, 5098. (22) Ubukata, T.; Hara, M.; Ichimura, K.; Seki, T. AdV. Mater. 2004, 16, 220. (23) Fukamoto, H.; Nagano, S.; Kawatsuki, T.; Seki, T. AdV. Mater. 2006, 17, 1035. (24) Eisenbach, C. D. Ber. Bunsen-Ges. Phys. Chem. 1980, 84, 680. (25) (a) Sung, C. S. P.; Lamarre, L.; Chung, K. H. Macromolecules 1979, 12, 666. (b) Sung, C. S. P.; Lamarre, L.; Chung, K. H. Macromolecules 1981, 14, 1839. (c) Lamarre, L.; Sung, C. S. P. Macromolecules 1983, 16, 1729. (d) Sung, C. S. P.; Gould, I. R.; Turro, N. J. Macromolecules 1984, 17, 1447. (e) Yu, W.; Sung, C. S. P.; Robertson, R. E. Macromolecules 1988, 21, 355. (26) Victor, J. G.; Torkelson, J. M. Macromolecules 1987, 20, 2241. (27) (a) Mita, I.; Horie, K.; Hirao, K. Macromolecules 1989, 22, 558. (b) Horie, K.; Mita, I. AdV. Polym. Sci. 1989, 88, 77. (28) Shi, D.; Wang, S. X.; van Ooji, W. J.; Wang, L. M.; Zhao, J.; Yu, Z. Appl. Phys. Lett. 2001, 78, 1243. (29) Akabori, K.; Baba, D.; Koguchi, K.; Tanaka, K.; Nagamura, T. J. Polym. Sci., Part B: Polym. Phys. 2006, 44, 3598.
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