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J. Phys. Chem. B 2000, 104, 4148-4154
Modeling the Interface Region of Command Surface 2. Spectroscopic Evaluations of Azobenzene/Liquid Crystal Hybrid Langmuir-Blodgett Films under Illumination Takashi Ubukata, Takahiro Seki,* Shin’ya Morino, and Kunihiro Ichimura Photofunctional Chemistry DiVision, Research Laboratory of Resources Utilization, Tokyo Institute of Technology, 4259 Nagatsuta, Midori-ku, Yokohama 226-8503, Japan ReceiVed: October 11, 1999; In Final Form: February 10, 2000
Langmuir-Blodgett (LB) films composed of the mixture of an amphiphilic polymer containing azobenzene (Az) side chain (6Az10-PVA) and 4′-pentyl-4-cyanobiphenyl (5CB) were prepared to mimic the twodimensional contacting region of the LC/Az interface of the command surface which photochemically switches the LC alignment. UV-visible absorption and Fourier transform infrared spectroscopic measurements were carried out under illumination. These procedures allowed separate and simultaneous evaluations of the static state and dynamic molecular motions of both Az and LC molecules, which probably reflect the initial triggering step of the “domino-mode” response of LC. The spectroscopic data indicated the induction of reversible perpendicular/tilt orientational changes of both the Az side chain and 5CB molecule upon alternative irradiation of 365 and 436 nm light. Thus, 6Az10-PVA/5CB hybrid LB film can be regarded as a satisfactory interface model of a command surface that promotes the homeotropic/planer alignment switching. From the time courses of the photoisomerization of Az and the orientational change, it was clearly shown that the molecular tilt is not governed only by the trans/cis ratio of Az unit, but is strongly process-dependent (forward or back process), indicative of involvement of strong molecular cooperativity. The validity and limitation of the LC research using this model system are also discussed.
1. Introduction Knowledge on the orienting behavior of liquid crystal (LC) molecules at a substrate surface1-3 is of particular importance to comprehend the entire response of LC cells. Experimentally, selective information on the behavior of LC molecules near the surface can be obtained by optical evaluations in the reflection mode.4-7 An alternative way to understand the behavior of LC molecules near the surface is to perform Monte Carlo8 or molecular dynamics9 simulations. Photoswitchable alignment layers dubbed command surfaces10-19 are of great interest in that reversible orientational changes of LC molecules are readily performed by the photoisomerization of the surface azobenzene (Az) layer. This offers a unique and favorable system for observation of orientational anchoring of LC on the surface. The dynamic motions of LC molecules near the alignment surface can be monitored by the above-mentioned optical techniques; however, some limitations still remain. These methods accumulate signals of typically the ranges of a few tens to hundreds nanometers from the surface, and do not provide pure signals from the interface. Furthermore, such optical methods do not inform details of the molecular aspects that can be obtained by UV-visible absorption and infrared spectroscopies. Particularly for command surface systems, knowledge on the molecular contacting (interpenetrating) region of the Az brush and rodlike LC molecules11 at a nanometer scale should be essential. In the above contexts, we constituted an approach to prepare the molecular interface model system for command surfaces via Langmuir-Blodgett (LB) method. The foregoing paper20 showed that the co-spreading of the Az containing amphiphilic * Author to whom correspondence should be addressed. Fax: +81-45924-5247. E-mail:
[email protected].
polymer (6Az10-PVA) and 4′-pentyl-4-cyanobiphenyl (5CB) renders an ideally homogeneous hybrid film (Langmuir monolayer) at the air-water interface. Furthermore, the molecular mixing leads to a highly upright orientation of both molecules as a consequence of strong molecular cooperativity, which is not attained in the Langmuir monolayer of each component. The criteria for the interface modeling with regard to static viewpoints, i.e., the homogeneous mixing and cooperative structuring are already confirmed.20 Based on the above knowledge, the present paper reports the details of the photostimulated dynamic aspects of the Az/LC LB hybrid films deposited onto a substrate. Results concerning the remaining criterion, i.e., the dynamic aspects and reversibility of the molecular orientation under illumination, are described. Here, we will concentrate on the system of the equimolar cospread mixture of 6Az10-PVA and 5CB.21 Through these investigation, new implications for understanding the photoaligning of the LC with the Az command surface are obtained. 2. Experimental Section 2.1. Materials. The materials used here, 6Az10-PVA, 5CB, water, and the spreading solvent, were described in the preceding paper.20 4′-n-Pentyloxy-4-cyanobiphenyl (5OCB) was obtained from Aldrich. 2.2. Methods. All procedures were carried out in dimmed red light. A spreading solution composed of an equimolar mixture of 6Az10-PVA and 5CB ([Az unit]/[5CB] ) 1) in chloroform (1.0 × 10-3 mol dm-3 for each component) was irradiated with 436 nm light to a photoequilibrated state (ca. 70% trans-isomer content) in advance. This solution was then spread onto pure water (Milli-Q grade) filled in a Lauda FW-1 film balance at
10.1021/jp9936042 CCC: $19.00 © 2000 American Chemical Society Published on Web 04/06/2000
Photoresponse of Azobenzene/Liquid Crystal LB Multilayers 20 ( 0.5 °C. Multilayers were fabricated by the successive vertical deposition onto a solid substrate. CaF2 and Auevaporated glass plates were used for the Fourier transform infrared (FTIR) measurements in the transmission mode and that in the reflection-absorption (RA) mode, respectively. The dipping and lifting speed was 24 mm min-1. The first single monolayer on the substrate was dried over silica gel for more than 2 h. This initial drying process was required for successful multilayer deposition. The deposition was achieved in the Y mode, and transfer ratios were essentially unity (1.0 ( 0.1) for all substrates. Samples on the substrate were placed in a drybox overnight to remove residual water and to allow the thermal cis-to-trans isomerization before the spectral measurements. The FTIR spectra were recorded on a Biorad FTS6000 spectrometer equipped with a MCT detector with a resolution of 4 cm-1. The system was purged with dried air. For the RA measurements, the p-polarized infrared incident beam was set at 80° from the surface normal. For the transmission measurements, the experiment was carried out in the normal incidence. The UV-visible absorption spectra of LB films on the CaF2 substrate were taken on a Hewlett-Packard 8452A diode array type spectrometer in the transmission mode. Each spectrum was taken at every 10 s interval with a sampling time of 100 ms. Thickness of the LB films was measured by surface profilometry using a Dektak3ST (Sloan/ULVAC). The stylus force was set to the minimum (0.01 mN) to avoid artificial scratches on the LB film surface. For simultaneous measurements under photoirradiation, the specimen of LB film was mounted in the UV-visible or FTIR spectrometer and irradiated at an angle of ca. 45° of the sample plane using an optical fiber. Light lines of 365 nm (UV) and 436 nm (visible) were obtained from a Hg-Xe lamp (San-ei Electronics, Supercure 203S) passing through a combination of glass filters, Toshiba UV35 + UVD36A and Y43 + V44, respectively. Irradiation energy was measured with an optical power meter (Advantest TQ8210) equipped with a photosensor of TQ821017.
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Figure 1. Molecular structure of materials.
3. Results 3.1. Structure of the Initial Film. 3.1.1. Multilayer Deposition. Throughout this study, transfer of a 6Az10-PVA/5CB monolayer onto a substrate was carried out at a surface pressure of 30 mN m-1. The achievement of successful layer-by-layer deposition was confirmed by the transmission FTIR spectroscopy. The absorbance of the asymmetric CH2 stretching band at 2926 cm-1 increased exactly in a linear manner with the deposition number. There were no appreciable spectral changes in shape and wavenumber of the peak. 3.1.2. Molecular Orientation. The packing state and orientation in the hybrid 6Az10-PVA/5CB LB films were evaluated by UV-visible absorption and FTIR spectroscopy. UV-visible absorption spectra of the deposited monolayer possessed the features which were observed for the monolayer on the water surface,20 indicating that the monolayer was transferred onto the substrate with retention of its molecular orientation and packing state. Polarized UV-visible absorption and transmission IR spectra revealed that there was no in-plane structural anisotropy, indicating that the dipping and lifting process did not impose anisotropic structure in the LB film. With respect to the outof-plane orientation, the data of the transmission and RA IR spectra were compared. Figure 2 shows the transmission and RA IR spectra of an 11-layered hybrid LB film. Assignments of the absorption bands15,22-24 are summarized in Table 1.
Figure 2. FTIR transmission (upper) and RA (lower) spectra of the 11-layered LB film of the 6Az10-PVA/5CB (1:1 mixture). The multilayer was prepared on a CaF2 plate or gold surface for the transmission and RA measurement, respectively.
Relative intensity of each band showed marked changes between the two spectra. The CtN stretching (2226 cm-1 in the transmission mode), the in-plane vibration bands of benzene ring (1603 and 1501 cm-1), the φ-O stretching (1250 cm-1), and φ-N stretching (1153 and 1142 cm-1 in the RA mode) bands showed stronger absorption in the RA spectrum than in the transmission mode. In contrast, the symmetrical and asymmetrical CH2 stretching bands were smaller in the RA spectrum. These results indicate that both the rodlike Az and biphenyl aromatics and alkyl chains of both molecules orient normal to the substrate plane. The frequency shifts of the CH2 stretching bands provide information on the conformational state of alkyl chains. It is known that the low-frequency shift (2918 cm-1) of the asym-
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TABLE 1: FTIR Data and Orientational Parameters Obtained with 6Az10-PVA/5CB Multilayers wavenumber/cm-1 transmission
RA
assignment
AT/ARa
mb
φ/degc
2926 s 2854 m 2226 w 1736 m 1603 w 1583 w 1501 w 1466 w 1250 w
2928 m 2855 m 2224 m 1736 m 1603 s 1582 m 1501 s 1468 w 1256 s 1153 m 1142 m 839 w 812 w
νa (CH2) νs (CH2) ν (CtN) ν (CdO) benzene ring ν8a benzene ring ν8b benzene ring ν19a δ (CH2) ν (φ-O-C) ν (C-N) ν (C-N) π (φ-H) for Az π (φ-H) for 5CB
0.68 0.61 0.066 0.31 0.036
4.52 4.53 4.78 4.93 4.95
68 67 39 60 31
0.053
4.97
36
0.059
4.98
38
a
b
Ratio of the transmission absorbance to the RA absorbance. Intensity enhancement factor. c Estimated tilt angle from the normal.
metric stretching band is characteristics of the trans conformer, while the high-frequency shifts are indicative of the involvement of the gauche conformer.25,26 The CH2 asymmetric stretching band appeared at 2920 cm-1 for a single monolayer film, and it showed a further higher frequency shift for the multilayered film (2926 cm-1). Therefore, the hydrocarbon chains of 6Az10PVA and 5CB are well ordered in the monolayer state while that in the multilayered films contain more disordered alkyl chains. 3.1.3. Estimation of the Tilt Angle. An attempt was made for quantitative evaluation of the molecular orientation. According to the previous report,27 under the conditions of uniaxial orientation of the transition moment around the surface normal z with the angle φ, the orientation of transition moment of the particular band can be evaluated by eq 1:
AT/AR ) tan2 φ/2m
(1)
where AT and AR are absorbances in the transition and RA spectra, respectively, and the m is of intensity enhancement factor in the RA spectrum to the transmission spectrum. The value of m can be calculated precisely by Hansen’s equations28 for optics of thin multilayer films as functions of the complex refractive indices of the LB film and substrates, vibrational frequency, angle of incidence in RA measurement, and film thickness. Here, we assume the complex refractive indices of the LB film as nx ) ny ) nz ) 1.5 + 0.1i throughout the wavelength region examined.27 The wavelength dependence of the complex refractive indices of substrates is obtained by interpolating the data given in a literature.29 The thickness of the 11-layered film was estimated to be 33 nm from a plot of the film thickness versus the number of layers obtained by surface profilometry. The AT/AR values, the calculated m values, and the tilt angles φ from the surface normal of the major bands were also listed in Table 1. Since the transition moments of the asymmetric and symmetric CH2 stretching bands and hydrocarbon chain axis are mutually perpendicular, the tilt angle γ of the hydrocarbon chain axis from the surface normal can be obtained from the corresponding angles (R and β) of the transition moments of the two CH2 stretching bands by eq 2.
cos2R + cos2β + cos2γ ) 1
(2)
By substituting 67° and 68° (see Table 1) in R and β, the tilt angle of the hydrocarbon chain axis γ was calculated to be 33° from the surface normal. As shown in Table 1, the tilt angles
Figure 3. UV-visible spectral changes of a 39-layered LB film of the 6Az10-PVA/5CB (1/1) upon exposure to UV (366 nm, 0.35 mW cm-2) (a) and visible (436 nm, 0.35 mW cm-2) (b) light.
of φ-O stretching, CtN stretching, and the in-plane vibration bands of benzene ring agree well with the hydrocarbon chain axis. Thus, the hydrocarbon chain axis, the Az and biphenyl planes, and the CtN axis all orient perpendicularly to the film surface. 3.2. Light-Induced Orientational Change. 3.2.1. UVVisible Absorption. Figure 3 shows UV-visible absorption spectra for a 39-layered 6Az10-PVA/5CB LB film deposited on both sides of a CaF2 substrate. The film was first irradiated with 365 nm (UV) light (a) and successively with 436 nm (visible) light (b). The solid curve shows the spectrum of the film before irradiation. The features of the spectra of the multilayer are essentially the same as those of the monolayer on quartz as described in our previous report.21a Irradiation with 365 nm light increased the absorption intensity of n-π* transition peaking around 440 nm. This unequivocally indicates the proceeding of the transto-cis photoisomerization of Az. Concomitantly, a large absorption enhancement around 280-290 nm was observed. This cannot be accounted for by an enrichment of the cis-isomer of Az because no absorbance increase was observed at these wavelengths in a dichloromethane solution. Since the normal incidence was adopted in these experiments, the spectral changes can be clearly ascribed to an induction of the orientational change of 5CB molecule from the perpendicular to a tilt state.30 The enhancement of absorption band at 280-290 nm was associated with that of the n-π* band of Az. This indicates that the orientational change of 5CB is induced by the photoisomerization of the Az unit. The reverse changes also proceeded upon visible light (436 nm) irradiation (Figure 3b). The enhanced absorption band around 280 nm almost reverted to the original level upon 436 nm light irradiation. It is noteworthy that essentially the equivalent spectral changes as the monolayer21a were observed for the built-up multilayers. This means that the dynamic molecular processes take place at each layer in the same manner, and the optical signals can be multiplied by the deposition number. Therefore, the subtle changes in the spectroscopic signals of the monolayer (the model of the interfacial region) can be amplified, which
Photoresponse of Azobenzene/Liquid Crystal LB Multilayers
Figure 4. Irradiation time profiles of absorbance changes at 280, 350, and 440 nm bands for a 39-layered LB film of the 6Az10-PVA/5CB (1:1 mixture) upon exposure to UV (366 nm, 0.35 mW cm-2) (a) and visible (436 nm, 0.35 mW cm-2) (b) light.
then allows an accurate time course observation upon continuous illumination. Spectra were accumulated upon continuous illumination, and absorbances at selected wavelengths, i.e., at 280 nm (5CB π-π* long axis), 350 nm (trans-Az π-π* long axis), and 440 nm (Az n-π*), were shown in Figure 4. The profiles obtained upon 365 and 436 nm light illumination are indicated in Figure 4a and b, respectively. A comparison of the two profiles at 280 and 440 nm indicates that the orientational change from the perpendicular to tilted state of 5CB occurs almost concurrently with the transto-cis isomerization of the Az unit upon UV light irradiation with a small induction period of ca. 50 s (Figure 4a). The profile of 350 nm showed a maximum at 400 s. The tilting motion of the trans-Az enlarges the absorbance whereas the photoreaction to the cis form reduces it. The combination of these two factors generates the profile having a maximum. In any event, the tilt promotion upon 365 nm light irradiation was a simple and continuous process. In contrast, the reverse process upon 436 nm light illumination obviously involved two steps (Figure 4b). The photoreaction proceeded immediately after starting illumination judging from the absorbance increase at 350 nm and decrease at 440 nm; however, the tilted state of 5CB (280 nm) was retained until 150 s irradiation which corresponds to ca. 40% proceeding the cis-to-trans isomerization of the photoisomerizable unit (ca. 70% of the total31). A sudden orientational change to the perpendicular state occurred beyond this period. The initiation of absorbance decrease at 280 nm was exactly associated with absorbance decrease at 350 nm, indicating that the orientational change of 5CB to the upright direction occurs simultaneously with trans-Az in a cooperative manner. Even after sufficient irradiation with visible light, absorbances of these bands did not entirely revert to the initial (as-deposited) level. The structure of the LB film should be somewhat disordered after the photocycle. The changes in the absorbance at 280, 350, and 440 nm upon repeated alternate irradiation were shown in Figure 5. As stated above, the absorbance after one photocycle at each wavelength differed from that in the initial nonirradiated state. From the second cycle, on the other hand, the absorbance changes after UV and visible light irradiation repeated with full reproducibility. Thus, the photoinduced perpendicular/tilt orientational changes of 5CB molecule occurred in a perfect reversible manner. These results further suggest that the layered structure was firmly maintained during the photoinduced aligning processes. The same procedure was carried out with another liquid crystal molecule, 5OCB. Reversible enhancement and restoration of the 300 nm band (the π-π* long axis transition of 5OCB)
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Figure 5. Reversible changes in the absorbance of 280 nm (open circles), 350 nm (open squares), and 440 nm (open diamonds) bands for a 39-layered LB film of the 6Az10-PVA/5CB (1/1) upon exposure to UV (366 nm) and visible (436 nm) light.
Figure 6. Infrared transmission spectra of a 15-layered LB film of the 6Az10-PVA/5CB (1:1 mixture) after exposure to UV (366 nm, 5 mW cm-2 for 20 min) (a) and visible light (436 nm, 5 mW cm-2 for 20 min) (b). Difference spectrum is shown in the upper part of each figure.
was also observed upon alternate UV and visible light irradiation. Thus, reversible photoinduced orientational changes took place also with 5OCB whose crystal to nematic phase transition temperature in the bulk is higher than room temperature (5CB: C-23-N-35-I, 5OCB: C-48-N-67-I). 3.2.2. FTIR Spectroscopy. UV-visible absorption spectra reports the structure and orientation of the chromophoric part only. Similar experiments were carried out by FTIR spectroscopy, which provides information regarding other parts of the molecules. Figure 6 shows the transmission FTIR spectra of a 15-layered 6Az10-PVA/5CB LB film on a CaF2 plate irradiated with 365 nm (Figure 6a) and subsequent 436 nm light (Figure 6b). Difference spectra (spectrum after irradiation is subtracted from that before irradiation) are also displayed in each figure. Assignments of the IR absorption bands of hybrid LB film are already summarized in Table 1. As indicated in the difference spectrum of Figure 6a, irradiation with UV light brought about absorbance decreases of the CH2 stretching bands (2926 cm-1 (νas) and 2854 cm-1 (νs)), and absorbance increases of the Ct N stretching (2226 cm-1), the in-plane vibration of benzene ring (1603 cm-1 (ν8a) and 1501 cm-1 (ν19a)), and the φ-O stretching (1250 cm-1) bands. These results indicate the induction of molecular tilt of both the alkyl parts and aromatic parts of Az and 5CB molecule. On the other hand, upon subsequent illumination with 436 nm light, the absorbances of these bands
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Figure 7. Irradiation time profiles of absorbance changes of ν (Cd O), ν (φ-O), and ν (CtN), bands for a 49-layered LB film of 6Az10PVA/5CB (1:1 mixture) upon exposure to UV (366 nm, 0.35 mW cm-2, a) and visible light (436 nm, 0.36 mW cm-2, b).
were reverted almost to the original level (see the difference spectrum in Figure 6b). Thus, the IR data also confirmed that the photoinduced orientational changes of each molecular part of Az side chain and 5CB molecule are reversible. The CdO stretching band (1736 cm-1) which is located close to the PVA backbone showed no absorbance change upon illumination, indicating that the orientation of this bond is unperturbed. This provides a picture that the photoinduced molecular motion takes place only in the Az side chain part and the motion near the PVA backbone is strongly suppressed. Figure 7 shows the time profile of the absorbance change of the φ-O, CtN and CdO stretching bands upon UV (Figure 7a) and successive visible light (Figure 7b) irradiation for a 49layered hybrid LB film on the both sides of a CaF2 substrate. The φ-O and CtN stretching bands are characteristic in the 6Az10-side chain and 5CB molecule, respectively, and the both transition moments are in the direction parallel to the long axis. Thus, these characteristic bands provide the information regarding the two components separately. Both φ-O and CtN bands showed monotonic changes with time upon UV light irradiation (Figure 7a). In contrast, the reverse process upon visible light illumination exhibited twostep changes (Figure 7b). The absorbance of the φ-O stretching band increased at the initial stage, and then showed a sudden decrease. There was an absorption maximum at ca. 150 s. The CtN stretching band correlated with this behavior. The absorbance intensity of the CtN band stayed constant before 150 s, and then decreased in a consorted way as the φ-O band. These data clearly show that the tilt-to-perpendicular orientational change initiated after certain amounts of trans-Az isomers were accumulated. Since Figures 4 and 7 are obtained in the same light exposure energy (0.35-0.36 mW cm-2), the behavior can be compared directly. The time when the orientational change starts coincides with each other (150 s). The initial increase observed in the φ-O band was unexpected. This shows that the phenyl ether bond located at the lower part of Az is tilted further at the initial stage of the tilt-to-perpendicular orientational change. During the UV and visible light irradiation processes, the absorbance of CdO was virtually unchanged, indicative of the fixed situation of the backbone part. To obtain the time correlation between the molecular motion of the Az side chain and 5CB molecule, the degrees of absorbance changes observed in the φ-O and CtN stretching bands are normalized as (A0 - At)/(A0 - A∞), where A0, At, and A∞ represent the absorbances at time zero, t, and at infinite time, respectively. The normalized absorbance changes for the φ-O (solid line) and CtN (dashed line) stretching bands are plotted in Figure 8 as a function of irradiation time with UV (Figure
Figure 8. Irradiation time profiles of relative absorbance changes of ν (φ-O) (solid mine), and ν (CtN) (dotted line) bands for the LB films of 6Az10-PVA/5CB (1:1 mixture) upon exposure to UV (366 nm) (a) and visible light (436 nm) (b). Irradiation powers are noted in the figure.
8a) and visible (Figure 8b) light. This figure contains four sets of data obtained with different illumination powers. In the perpendicular-to-tilt process (Figure 8a), the degree of tilt for the φ-O band proceeded faster than that for the CtN band at any stages (cf. the profiles of solid and dotted lines). This implies that the motion of triggering Az moiety is faster than that of 5CB molecule. With increasing the light exposure energy, the orientational change was accelerated while this rate relation was maintained. In the back process upon visible light illumination (Figure 8b). The four series of profiles obtained in different light exposure powers also showed the common features. The φ-O band showed an initial further tilt (to the negative sign direction in this expression) followed by the orientational change to the perpendicular (approaching unity in the ordinate). The rate observed in the four sets of experiments in Figure 8a was found to be precisely proportional to the irradiation power. The rate was estimated as a reciprocal of time required for 80% change of the total change (τ80). Here, the irradiation power (P) was corrected by eq 3, which takes into account the light loss of 365 nm light absorbed by the hybrid LB film. Where Pobs is measured irradiation power and A365 is absorbance at 365 nm.
P ) (1-10-A365)Pobs/(A365 × ln 10)
(3)
When the τ80-1 was plotted against the irradiation power, it fell into a straight line with a correlation coefficient of 0.997 and 0.999 for φ-O and CtN band, respectively. With an assumption of a constant quantum yield, it is concluded that the response rate is in exact proportion to the irradiation power within the experimental conditions adopted. 4. Discussion 4.1. Nonlinearity in the Orientational Change. Via modeling the command surface interface by the present hybrid LB system, simultaneous observation of the time course of the photoisomerization of the Az layer and orientational change of the LC molecule has become feasible. Then the exact correlation of molecular tilt of 5CB molecule with the trans-to-cis isomerization state of Az can be obtained. The out-of-plane orientational changes of 5CB upon illumination can be monitored from both the absorbance changes at 280 nm (long axis π-π*) in the UV-visible absorption spectrum
Photoresponse of Azobenzene/Liquid Crystal LB Multilayers
Figure 9. Molecular tilt of 5CB versus fraction of the cis-Az isomer in the process of UV (open circles) and visible light (open diamonds) irradiation. The absorbance at 280 nm is correlated to the degree of tilt angle of 5CB molecules.
and that at 2226 cm-1 (CtN stretching) in the FTIR spectrum in the transmission mode. In both cases the transition moment is in parallel to the molecular rod. Essentially the same time profiles were obtained by the two spectroscopic methods in the cases of both UV and visible light illumination, implying the sufficient accuracy of these measurements. Here, comparisons are made on the basis of UV-visible absorption spectra. In random and fixed orientation systems, quantitative estimations of the trans/cis ratio can be readily made from the absorption intensity of the π-π* transition band of Az around 360 nm. Around this wavelength region, the molar extinction coefficient of the cis-Az is negligible. However, spectroscopic estimations of trans/cis ratios using this band are difficult in the present study because the out-of-plane orientation of the chromophore is changed during the observation as obviously shown in Figures 3 and 4. Therefore, the fraction of the cis-Az isomer was estimated from the absorbance at 440 nm (n-π*) whose intensity is less sensitive to the Az chromophore orientation. In the calculation, we assume that the initial state and after photostationary state after 365 nm light illumination in the 6Az10-PVA LB film contain 0% and 70% cis isomer, respectively.31 In Figure 9, the absorbance at 280 nm of 5CB molecule for the 39-layered 6Az10-PVA/5CB film is plotted against the fraction of cis isomer. The absorbance at 280 nm is directly plotted here instead of the tilt angle because the estimation of tilt angle involves large errors and is of minor importance for the argument. As shown in Figure 9, the Az photoisomerization and the perpendicular/tilt orientational change of 5CB molecule are basically reversible. On the alternative irradiation, the curve corresponding to the forward process (UV light irradiation) does not coincide with that of the back process (visible light irradiation). Thus, the tilt angle of the 5CB molecule is not governed by a simple trans/cis isomer ratio of Az but is processdependent. This nonlinear behavior should be attributable to the molecular cooperativity conducted by the Az-side chains and 5CB molecules. 4.2. Validity of Interface Modeling of Command Surface. Alignment controls of nematic LC molecules in LC cells were investigated using LB films of various kinds of Az amphiphilic polymers as the command surface.11,14-19 The trans and cis forms of the surface Az layer generally induce homeotropic and homogeneous or planer alignment, respectively. In the same manner, the 6Az10-PVA/5CB hybrid LB films investigated here exhibit reversible changes in molecular tilt upon alternative UV
J. Phys. Chem. B, Vol. 104, No. 17, 2000 4153 and visible light irradiation. We thus conclude that the hybrid LB film provides a good interfacial model of command surface. We obtained here a great deal of information on the mutual molecular motions in the framework of the so-called commander and soldier concept.32 The validity and advantage of the investigation using this model system are as follows. (1) The photoreaction and motions of both commander and soldier molecules can be monitored simultaneously as argued in the previous section. In the ordinary studies with LC cells, the proceeding of photoreaction of the Az layer and orientational changes of LC molecules are evaluated in separate experiments. This may sometimes be misleading since the photoresponse behavior of the Az layer may be affected by the contact of LC molecules. The orientational change of 5CB in the UV light irradiating process is, as expected, systematically delayed from the motion of the triggering Az side chain (Figure 8a). Such relation was confirmed for the first time through the simultaneous monitoring. (2) In the LB method, the optical signals from the molecular interface can be amplified by the multilayer deposition. The signal intensity is in proportion to the deposition number. In this situation, the optimization of the experimental setup becomes easy. (3) The interface model “sharply” provides information of molecules, which would not be attained by surface optical evaluation methods.4-7,16,17 In particular, FTIR spectroscopy provides information to the level of each chemical bond of the molecule. In this sense, the existence of CtN group in the 5CB molecule is of particular use. Apart from the photoresponsive surface, there are many remaining subjects to be pursued in the alignment behavior of LC molecules on polymer film surfaces. We expect that the modeling approach via preparation of hybrid LB films can be a powerful tool to obtain new prospects in the aligning behavior at a molecular level. 4.3. Comparison with Three-Dimensional LC Cell System. As mentioned, the modeling approach is highly informative; however, it should be meaningful to point out some discrepancies admitted between the model film system and the actual LC cell. During the exposure to UV light, the orientational change from the perpendicular to tilt state of 5CB occurs almost concurrently as the trans-to-cis isomerization proceeds. Only a minor induction period was observed below 5% photoisomerization (Figures 4a or 8a). This behavior apparently differs from the three-dimensional LC cell system that showed a strong nonlinearity in the alignment response using the identical 6Az10PVA LB monolayer.15,16 In these LC cells, cyclohexanecarboxylate type LC (DON-103) was used as the nematic LC instead of 5CB, hence the inconsistency may arise from the difference in the LC material. DON-103 and 5CB have the transverse anisotropy of dielectric constant with each other. However, this explanation seems incorrect judging from the data of Bu¨chel et al.,17 that the threshold is observed for both DON103 and ZLI-1695 whose structure resembles that of 5CB. Thus, the reason for the discrepancy is unclear. With regard to the switching dynamics in the back process with visible light illumination, little knowledge has been obtained so far. Bu¨chel et al.17 showed, with an LB layer composed of a polyglutamate having an Az side group, that visible light irradiation induces the orientational change of LC molecule without a threshold. This is contradictory to the present observation. Also in this case, the comparison is problematic because the command layer is composed of different Az material. It is already shown that the LC aligning ability markedly changes with subtle structural changes in the Az layer
4154 J. Phys. Chem. B, Vol. 104, No. 17, 2000 such as the lateral packing density12,18 and the chemical structure of the Az side chain.11,12 The nonlinear response in the backward process is a new observation, which was obtained for the first time via the interface modeling. Although proper comparisons between the LC cell systems and the model LB systems are not made, the inconsistencies in dynamic properties stated above may indicate the partial limitation of the two-dimensional modeling. The discrepancies probably stem from the difference in the dimensionality of the LC component. The nematic LC state is formed in a threedimensional space where the rodlike molecules are interdigitated with each other and uniaxially aligned to have a long-range orientational order. In the model LB films, on the other hand, LC molecules are forced to be arranged within a layer plane like a smectic state. Therefore, the nematic state is not properly mimicked. Influences which may operate from the bulk nematic LC phase to the Az layer are not realized in the model systems, and this would impede the “complete” understanding of molecular motions at the command interface by the modeling alone. Thus, optical data on the photoresponse of the overall cell obtained in the transmission mode (in the range of micrometers) and that in the reflection mode (tens to hundreds nanometers), and spectroscopic data with the model LB systems (nano- to sub-nanometers) are all complimentary. 5. Conclusion Together with the data described in the foregoing paper of this series of research,20 it is concluded the hybrid LB films of 6Az10-PVA/5CB all fulfill the following criteria for the interface model of command surfaces which can switch the LC alignment; i.e., (1) complete mixing of the two materials without lateral phase separation is attained, (2) strong molecular cooperativity is admitted both in the static orientation and dynamic processes under illumination, and (3) reversible orientational changes are ideally observed. Using these model LB films, simultaneous spectroscopic (both UV-visible and FTIR) evaluations have become possible, which shed light on the initial stage of the “domino-manner” response from the commander (Az) to soldier (LC) molecules. Novel aspects obtained in this approach are stressed in the dynamic process. The orientational change of the 5CB molecule from the perpendicular to tilt state is delayed from that of the triggering Az unit. In the reverse process, the perpendicular orientation is promoted in a highly cooperative way after an obvious induction period (ca. 40% of the cis-totrans isomerization of the Az unit). In the above manners, modeling the command surface interface by the Az/LC hybrid LB film provided new insights on the molecular processes taking place at the interface that would not be obtained with conventional LC cell systems. It is, at the same time, important to remember that the two-dimensional architecture is not a perfect mimic of the three-dimensional LC cell in which LC molecules assembled in the three-dimensional nematic state may influence the structure and motions of the surface Az layer.
Ubukata et al. Acknowledgment. We thank Drs. M. Nakagawa and K. Arimitsu in our laboratory for helpful discussions and technical supports. We also thank Dr. T. Kawai at Science University of Tokyo for helpful discussions on the IR data analysis. References and Notes (1) Cognard, J. Mol. Cryst. Liq. Cryst. 1982, Suppl. Ser. 1, 1. (2) Yokoyama, H.; Kobayashi, S.; Kamei, H. Mol. Cryst. Liq. Cryst. 1983, 99, 39. (3) Je´roˆme, B. Rep. Prog. Phys. 1991, 54, 391. (4) Evans, S. D.; Allinson, H.; Boden, N.; Flynn, T. M.; Henderson, J. R. J. Phys. Chem. B 1997, 101, 2143. (5) Okutani, S.; Kimura. M.; Akahane, T. Jpn. J. Appl. Phys. 1998, 37, L600. (6) Tadokoro, T.; Fukazawa, T.; Toriumi, H. Jpn. J. Appl. Phys. 1997, 36, L1207. (7) Shishido, A.; Tsutsumi, O.; Kanazawa, A.; Shiono, T.; Ikeda, T.; Tamai, N. J. Am. Chem. Soc. 1997, 119, 7791. (8) Dadmun, M. D.; Muthukumar, M. J. Phys. Chem. 1994, 101, 10038. (9) Yoneya, M.; Iwakabe, Y. Liq. Cryst. 1996, 21, 347. (10) Ichimura, K.; Suzuki, Y.; Seki, T.; Hosoki, A.; Aoki, K. Langmuir 1988, 4, 1214. (11) Seki, T.; Sakuragi, M.; Kawanishi, Y.; Suzuki, Y.; Tamaki, T.; Fukuda, R.; Ichimura, K. Langmuir 1993, 9, 211. (12) Aoki, K.; Tamaki, T.; Seki, T.; Kawanishi, Y.; Ichimura, K. Langmuir 1992, 8, 1014. (13) Kawanishi, Y.; Tamaki, T.; Sakuragi, M.; Seki, T.; Suzuki, Y.; Ichimura, K. Langmuir 1992, 8, 2601. (14) Sakuragi, M.; Tamaki, T.; Seki, T.; Suzuki, Y.; Kawanishi, Y.; Ichimura, K. Chem. Lett. 1992, 1763. (15) Seki, T.; Ichimura, K.; Fukuda, R.; Tanigaki, T.; Tamaki, T. Macromolecules 1996, 29, 892. (16) Knobloch, H.; Orendi, A.; Bu¨chel, M.; Seki, T.; Ito, S.; Knoll, W. J. Appl. Phys. 1995, 77, 481. (17) Bu¨chel, M.; Weichart, B.; Minx, C.; Menzel, H.; Johannsmann, D. Phys. ReV. E 1997, 55, 455. (18) Seki, T.; Fukuda, R.; Tamaki, T.; Ichimura, K. Thin Solid Films 1994, 243, 675. (19) Kim, W.; Iwamoto, M.; Ichimura, K. Jpn. J. Appl. Phys. 1996, 35, 5395. (20) Ubukata, T.; Seki, T.; Ichimura, K. J. Phys. Chem. B. 2000, 104, 4141. (21) Preliminary accounts of this research: (a) Ubukata, T.; Morino, S.; Seki, T.; Ichimura. K. Chem. Lett. 1998, 71. (b) Ubukata, T.; Seki, T.; Ichimura, K. Macromol. Symp. 1999, 137, 25. (22) Kawai, T.; Umemura, J.; Takenaka, T. Langmuir 1990, 6, 672. (23) Kawai, T. Thin Solid Films 1997, 301, 225. (24) Katayama, N.; Ozaki, Y.; Seki, T.; Tamaki, T.; Iriyama. K. Langmuir 1994, 10, 1998. (25) Umemura, J.; Cameron, D. G.; Mantsch, H. H. Biochim. Biophys. Acta 1980, 32, 602. (26) Sapper, H.; Cameron, D. G.; Mantsch, H. H. Can. J. Chem. 1981, 59, 2543. (27) Umemura, J.; Kamata, T.; Kawai, T.; Takenaka, T. J. Phys. Chem. 1990, 94, 62. (28) Hansen, W. N. J. Opt. Soc. Am. 1968, 58, 380. (29) Ordal, M. A.; Long, L. L.; Bell, R. J.; Bell, S. E.; Bell, R. R.; Alexander, R. W., Jr.; Ward, C. A. Appl. Opt. 1983, 22, 1099. (30) In this paper, the orientational state after 365 nm light is termed as “tilt” instead of “planar” that is used in LC cell systems because the degree of tilt in the LB films after UV light irradiation is unknown. In an LC cell system, the orientation of LC molecule is almost parallel to the surface,16 and can be properly called planar. (31) Seki, T.; Ichimura, K. Thin Solid Films 1989, 179, 77. (32) Ichimura, K. Supramol. Sci. 1996, 3, 67.
