Inherent and Cooperative Photomechanical Motions in Monolayers of

The UV irradiation constantly induced increases in ΔV, but the dark adaptation between the illumination periods showed some characteristic features...
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J. Phys. Chem. B 1998, 102, 5313-5321

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Inherent and Cooperative Photomechanical Motions in Monolayers of an Azobenzene Containing Polymer at the Air-Water Interface Takahiro Seki,* Hidehiko Sekizawa, 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: February 25, 1998; In Final Form: May 4, 1998

Photoinduced expansion and contraction response in monolayers consisting of a poly(vinyl alcohol) derivative having an azobenzene (Az) side chain at the air-water interface was investigated in detail by surface potential measurements, Brewster angle microscopy (BAM), UV-visible absorption spectroscopy, and the macroscopic area estimations with a Langmuir film balance. Surface potential measurements confirmed the model of photostimulated motions previously supposed (Seki et al., Bull. Chem. Soc. Jpn. 1996, 69, 2375). The BAM observation combined with a movable minitrough allowed precise evaluations of the mechanical response of an isolated domain monolayer at zero pressure, which can be regarded as providing the inherent attributes. In comparison with the corresponding UV-visible spectroscopic data, the expansion process on UV light (365 nm) illumination was found to show a nonlinear response with photoreaction, which is characterized by the existence of an induction period. Initiation of the film expansion required ca. 40% conversion of the trans-to-cis photoisomerization. This method also confirmed a self-contracting motion on visible light (436 nm) illumination. In macroscopic area evaluations monitored at an applied surface pressure, the photoresponding behavior was found to include large artificial distortions. This work presents, for the first time, the inherent photomechanical response and its cooperativity in monolayers at the air-water interface.

1. Introduction To date, a wide variety of polymers containing photochromic moieties such as azobenzene (Az) or benzospiropyran (Sp) have been synthesized,1-3 and various photofunctional properties have been explored in solutions,2,3 films and membranes,2-4 and solvent-swollen gels2 and so forth. The photoisomerization of the chromophores induces conformational changes in the polymer chain, which in turn lead to macroscopic variations in the physical and chemical properties of surroundings and media. In connection to biological interests, these associating processes can be regarded as mimics of the vision systems in which photoexcitable proteins are operating. From a technological viewpoint, switchings of the material shape,2-4 membrane permeabilities,3,4 optical properties5-7 can be performed mostly in a reversible fashion, and hence these functionalities are expected to constitute potent future applications. Although such photoinduced variations in systems and materials are widely recognized, very little is understood in terms of molecular mechanisms. Most of the materials are composed of random or highly complicated hierarchies, and it is therefore difficult to grasp exact and direct correlations between the triggering photoreaction and associating macrosize effects. In this context, monomolecular films (monolayers) at the air-water interface are a class of fascinating research candidates since they possess the simplest hierarchical structure among many artificial molecular assemblies. The molecules are oriented at the interface to form a polar structure, and the aligned vectors of the photochromic reaction result in uniform and collective motions with retention of the two-dimensionality. Early studies on photomechanical effects in monolayered polymers at the air-water interface probably started from the * To whom correspondence should be addressed. Phone and fax: +8145-924-5247; e-mail: [email protected].

works of Blair’s8-10 and Rondelez’s11-13 groups using poly(methyl methacrylate)s8,11-13 and polyamides.9,10 After 1990, monolayers of Az-pendent polypeptides have been reported. Malcolm and Pieroni14 studied the photomechanical response of Az-containing poly(L-lysine). Az side groups were also introduced by Menzel15 to a so-called “hairy-rod” polymer consisting of helical poly(L-glutamate). Higuchi et al.16 combined two helical rods of poly(L-glutamate) with an Az unit. Since 1993 we have been conducting studies with poly(vinyl alcohol) derivatives having an Az side group.17-20 Despite the above efforts, the following essential understandings still remain ambiguous or unexplored. (1) EValuation of Photomechanical Responses in Microscopic Scales. There exists an enormous scale gap between the molecular event of photoreaction at nanometer levels and the macroscopic response that is estimated by a Langmuir film balance. For bridging these incidents of discrepant scales, information obtained in microscopic scales ranging, for instance, from millimeters to micrometers is highly required. As related investigations, microscopic visualizations by Brewster angle microscopy (BAM)21,22 and fluorescence microscopy23,24 have been performed for low-molecular-mass photochromic amphiphiles focusing on observation of dynamics of photogenerated two-dimensional patterns.25-28 However, no work has been done from a viewpoint of the mechanical properties in a microscopic field. (2) EValuation of PhotoresponsiVe BehaVior at Zero Pressure. So far area estimation of monolayers has been done with a Langmuir film balance applying a surface pressure for area monitoring. Evaluations in this manner should lead to distortions in dynamics and magnitude of the longitudinal motion of the monolayer and are prone to experimental artifacts. It is hence desired to establish a method for evaluation of area changes in a pressure-free state. Given such conditions, the

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CHART 1

“inherent” mechanical responsive behavior can be obtained. Visualization of monolayers by BAM would be promising for this purpose. (3) Precise EValuation of Time Correlation between the Phototrigger and Film Response. Panaiotov et al.13 proposed a unique method to observe a dilatational motion of monolayers using poly(methyl methacrylate) having a pendant Sp unit. By means of the local irradiation onto the monolayer, they analyzed the longitudinal motion taking into account the parameters of the surface viscoelasticity, the viscosity of the subphase, and the photoreaction rate. In their analysis, it was hypothesized that the time necessary for molecular reorientation is much slower than that necessary for the photochemical conversion. More recently, Menzel15 pointed out with his Az-pendant poly(L-glutamate) monolayer that the photomechanical response upon UV irradiation is delayed from the proceeding of the photoreaction basing on the spectral data on water. In both works, however, quantitative experimental results have not been supplied. Our explorations with Az-containing poly(vinyl alcohol) derivatives revealed that the monolayer of 6Az10-PVA (Chart 1) shows ca. 3-fold expansion upon UV light irradiation at low pressures. On the basis of the data of molecular occupying area, we proposed a molecular mechanism for monolayer deformation as illustrated in Figure 1.17-19 In the trans-Az state, the Az moiety in the side chain is confined in the monolayer array on the air side. The trans-to-cis transformation of Az induced by UV light (365 nm) illumination leads to a large increase in the dipole moment of this unit (0.5 D (trans) f 3.1 D (cis) for unsubstituted Az),29 and the Az moiety contacts with the water surface at low pressures. This should give rise to a large expansion of the film. On visible light (436 nm) irradiation, the reverse process takes place with full reproducibility. It was anticipated that these features of the 6Az10-PVA monolayer should be of great help in promoting a precise micro- and macroscopic evaluations. This paper describes our detailed examinations on the photomechanical response of the 6Az10-PVA monolayer at the air-water interface by surface potential measurements, BAM combined with a specially designed trough, UV-visible absorption spectroscopy, and area evaluations using an ordinary Langmuir film balance. Through combination of these data, we evaluated the inherent photomechanical behavior and made adequate comparisons with those obtained by the conventional macroscopic method. With respect to our monolayer systems, the molecular motion model shown in Figure 1 requires further

Figure 1. Schematic representation of the reversible photoinduced area changes of the 6Az10-PVA monolayer on a water surface.

supporting information. This paper starts with presenting some confirming data for this obtained by surface potential measurements. 2. Materials and Methods Synthesis of 6Az10-PVA was described previously.30 Chloroform for monolayer spreading was of UV spectroscopic grade (Uvasol, Ciba-Merck). Unless stated otherwise, all measurements regarding the monolayer on the water surface were achieved at 20 ( 0.5 °C under subdued red light. In the experiments of macroscopic area evaluations, UVvisible absorption spectrum, and surface potential measurements on the water surface, the monolayers were spread on pure water (Milli-Q grade, 18 MΩ cm-1) filled on a Lauda FW1 film balance. For microscopic area evaluation, a home-built mini rectangular trough (3 × 8 cm) made of Teflon placed on an X-Y stage (12 × 12 cm, Sigma Koki Co.) was used. Details of this setup will be described in section 3.2. For both cases, the 6Az10-PVA monolayer was spread from a chloroform solution (1.0 × 10-3 mol dm-3). Surface potential measurements were made with a Face SEP (Kyowa Interface Science) using an air electrode coated with Am241 95 and a Pt electrode in water. Brewster angle microscopic observations were achieved with a NLE-EMM633 (Nippon Laser Electronics) equipped with a 10 mW He-Ne laser. The obtained images were recorded on a videotape and analyzed with the NIH image for area estimation of monolayers. UV-visible absorption spectra for monolayers at the airwater interface were taken on a spectrometric system composed of a photodiode array detector system (MCPD-2000, Ohtsuka Electronics) assembled with a deuterium/halogen lamp (MC2530, Ohtsuka Electronics) and a processing computer. Light power from the lamp was attenuated as possible so as that photoreactions by the probing light can be ignored. Light irradiation was performed with a 150 W Hg-Xe lamp (San-ei UV Supercure-203S) equipped with an optical fiber which favors irradiation to a target part. The 365 and 436 nm lines from the lamp were selected with the combination of Toshiba optical filters, UV-35/UV-D36A and Y-44/V-42, respectively. Irradiation onto the Lauda film balance for

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Figure 3. Schematic illustration of the plausible orientational change of the cis-Az unit in the side chain of 6Az10-PVA upon compression.

Figure 2. Surface pressure (dashed line) and surface potential (solid line) isotherms of the 6Az10-PVA monolayer on a pure water surface at 20 ( 1 °C. The chloroform solution containing 6Az10-PVA was spread in the trans form (a) and in the preirradiated state with UV (365 nm) light (cis content of ca. 90%).

macroscopic evaluations was achieved as follows. The terminus of the optical fiber was set 25 cm above the water surface, and the light beam was spread to allow irradiation of the entire film area in the most expanded state of the monolayer. The exposure energy of UV and visible light irradiation was measured around the center. Light intensities were estimated with an optical power meter (Advantec TQ-8210). 3. Results and Discussion 3.1. Surface Potential. The surface potential is capable of giving information on molecular orientations.31 The surface potential (∆V) is correlated to the effective dipole moment (µ) and the numbers of molecules per cm2 (N) as ∆V ) 4πµZN/D, where D is the dielectric constant (usually assumed as 1). µZ is the perpendicular contribution of dipole moment of the molecule (µ) and can be denoted as µZ ) µ cos θ, in which is θ the tilt angle of the molecular dipole from the normal. Then the above equation can be written as ∆V ) 4πNµ cos θ. Therefore, ∆V is proportional to the number of dipoles at a given area and the magnitude of normal component of the molecular dipole moment. Surface potential measurements on the compression process were made simultaneously to the surface pressure measurements. The results are shown in Figure 2. The surface potential of the monolayer was given as the difference from the control value obtained for a pure water surface before monolayer spreading. The monolayers were spread in the trans form (a) and the UV light irradiated form (ca. 90% cis content) (b). In the trans form, the surface potential started to rise at ca. 0.6-0.8 nm2 per Az unit before the pressure liftoff (0.4 nm2). This can be understood in the terms of critical packing density.32 The surface potential technique allows the Langmuir monolayer to be proved at much earlier stages of monolayer compression in comparison with surface pressure isotherms. Actually, BAM observation of the trans-6Az10-PVA monolayer has revealed that the film is composed of solidlike multidomains and that the compression procedure gathers these floating domains.20 The

rise in ∆V undoubtedly stems from the increase in numbers of Az unit per unit area. The maximum of ∆V (220 mV) was observed at the midpoint of the slope of the pressure isotherm. This profile is similar to those observed by Ahluwalia et al.33 for Az-containing poly(L-lysine) monolayers. The final value of ∆V around the collapse was ca. 200 mV in the trans-6Az10PVA monolayer. In the case of the UV-irradiated 6Az10-PVA monolayer, the surface potential profile was completely different in shape. The largest ∆V value (ca. 400 mV) was observed at the most expanded regions before compression, exceeding 1.0 nm2 per Az unit in the present experimental procedure. Upon compression, the surface potential decreased monotonically until the monolayer collapsed at 0.35 nm2 per Az unit, which coincided well with the inflection point in the pressure-area isotherm. Further compression after the collapse gave an almost constant value of ∆V (200-220 mV). In all area regions, the ∆V values for the cis-Az monolayer were larger than those for the transAz monolayer. This can be attributed to the increased dipole moment of the cis isomer of the Az moiety. Despite the fact that the lateral density of the Az unit was increased upon compression, ∆V decreased until the film collapse. This is strongly indicative of an occurrence of compression-induced orientational changes of Az unit from a laid (lower pressures) to a perpendicular state to the surface plane (higher pressures). This situation is schematically illustrated in Figure 3. We believe that surface potential data obtained here confirm the previously supposed mechanism in the photoinduced film deformation illustrated in Figure 1.17-19 The Az moiety in the cis form should be contacting with the water surface at lower pressures where photomechanical responses have been usually observed in our study. The surface potential of the UVirradiated 6Az10-PVA monolayer (b) around 0.4 nm2 per Az unit (ca. 200 mV) was in good agreement with that observed in the compressed trans-Az monolayer in the same area regime (a). This further implies the perpendicular orientation of the cis-Az side chain in compressed states. According to this model, the orientational and positional change of the Az moiety should lead to thickness changes of the monolayer. Quantitatively this is in agreement with the changes in the light reflectivity in the BAM observation.20 Direct thickness evaluations can be made by atomic force microscopy.34 Thickness of the monolayered 6Az10-PVA film on freshly cleaved mica was ca. 2.5 and ca. 1.1 nm, in the trans and UVirradiated cis form, respectively. Details of AFM study will be reported elsewhere in due course. The features of the surface potential-area curve of UVirradiated 6Az10-PVA differs largely in shape from that of UVirradiated Az-pendant poly(L-lysine) reported by Ahluwalia et al.33 The UV-irradiated poly(L-lysine) derivative shows en-

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Figure 4. Changes in the surface potential of the 6Az10-PVA monolayer during the intermittent UV (365 nm) light irradiation (0.4 mW cm-2) starting from the trans state at 20 ( 1 °C. Between the UV light irradiation periods, the monolayer was kept in the dark.

hancement of surface potential upon compression similarly to that for the trans-Az monolayers, which opposes to our results shown in Figure 2b. It is thus implied that the orienting behaviors of the Az side groups are different between these two monolayers, as already mentioned in their paper.33 This may in turn lead to a discrete photomechanical response. In fact, according to the photomechanical data of Malcolm and Pieroni,14 the area expansion and contraction behavior of the poly(L-lysine) monolayer takes place in an opposite way to that of the 6Az10PVA monolayer. The 6Az10-PVA monolayer behaves in a similar fashion as Menzel’s materials do.15,19 The surface potential of the 6Az10-PVA monolayer was also measured on the process of UV illumination at 2 mN m-1. Figure 4 shows the changes in the surface potential with intermittent UV light irradiation starting from the trans-Az state. In this procedure, the area increased with UV light illumination. As indicated, the surface potential increased with UV light irradiation up to values above 400 mV as expected from the data of Figure 2 at lower pressures. The UV irradiation constantly induced increases in ∆V, but the dark adaptation between the illumination periods showed some characteristic features. At an earlier stage of dark adaptation, i.e., two cycles in this experiment, ∆V showed some recoveries; however, it remained constant at later stages. This may be correlated to the nonlinear response in the expansion process as will be discussed later. In summary of this section, area estimations from the Langmuir film balance experiments,17 variations in surface potential and changes in film thickness all support the molecular motional model of 6Az10-PVA illustrated in Figure 1. 3.2. Brewster Angle Microscopy. Our earlier BAM study20 revealed the morphological and rheological features of the 6Az10-PVA monolayer at the air-water interface. The trans6Az10-PVA monolayer before compression is composed of iceberglike domains having clear and solid boundaries. In contrast, the cis-6Az10-PVA monolayer is highly fluid and homogeneous at all areas and the boundaries are obscure. In the trans form, small pieces of solidlike domains are floating, and they can be separated to obtain an isolated monolayer. We anticipated that, if the photomechanical response for one isolated monolayer is evaluated by BAM, this provides the inherent mechanical behavior at zero pressures. Using the ordinary Langmuir film balance, however, it was difficult to follow a target piece of the monolayer because an isolated photoresponsive monolayer was highly mobile in the longitudinal direction

Seki et al.

Figure 5. A setup for BAM observation of a single domained pressure free monolayer. Light irradiation is carried out from the bottom of the small trough. Selection of UV (365 nm) or visible (436 nm) light is achieved through an optical filter set below. Since a photoresponsive pressure-free monolayer is highly mobile, the X-Y position is controlled by hand so that the objective single domain monolayer can be followed within the microscope field.

under illumination. A target piece rapidly escaped out of the microscopic field. In this context, it was necessary to introduce a newly designed setup with which one can chase a moving monolayer. The configuration of our setup is schematically shown in Figure 5. A minitrough was placed on an X-Y stage. One can pursue a mobile target monolayer by adjusting the X-Y stage simply by hand. Using this apparatus, it was found that a small island of the 6Az10-PVA monolayer in a diameter of a few hundred micrometers readily moves nearly 1 cm in the two dimensions of the water surface upon irradiation. In this home-built apparatus, light illumination is performed through the trough bottom made of a quartz plate. Great advantages of this configuration are that (i) the light intensity is accurately adjusted and calibrated and, (ii) if necessary (not undertaken in this work), polarized light irradiation can be readily performed. Figure 6 shows the typical BAM images of an isolated domain of the 6Az10-PVA monolayer. In this examination, a very small amount of 6Az10-PVA chloroform solution was spread in the minitrough so that the bare water surface occupied the majority area of the trough. In the figure, four selected snapshots are displayed out of a series of continuously recorded images in a videotape. As shown here, an isolated domain of trans-6Az10PVA monolayer (a, before irradiation) continuously became larger on UV light illumination (b (100 s) f c (300 s) f d(500 s); irradiation power, 0.26 mW cm-2). Concomitantly the light reflectivity from the monolayer continuously decreased to give poorer contrast to the water surface. This can be ascribed to a reduction of the film thickness. By tracing the contour of this island, the area was estimated by a personal computer using the NIH image program. The areas obtained in this manner were plotted in Figure 7 as a function of irradiation time. This figure contains two sets of data obtained with two different illumination powers, 0.50 (close circles) and 0.26 mW cm-2 (open circles). As shown here, the time constant of film expansion process is in proportion to the illuminated light intensity. The time period required for the full expansion was almost twice longer at 0.26 mW cm-2 (ca. 500 s) than that at 0.5 mW cm-2 (ca. 250 s). Data shown in Figure 7 manifest two important issues. First, the final area of the film after expansion is 4-5 times larger than the initial state. This is even larger than our former

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Figure 6. An example of the series of BAM images exhibiting the UV (365 nm) light induced area changes of the trans-6Az10-PVA single domain monolayer. Images were taken after irradiation for 0 (a), 100 (b), 300 (c), and 500 s (d) with a light intensity of 0.26 mW cm-2. The diameter of the microscope field was 0.6 mm. The time course of area changes for this island is indicated by open circles in Figure 7.

Figure 7. Time course of area expansion for two examples of single domain pressure-free 6Az10-PVA monolayers. The light intensities of UV (365 nm) light were 0.50 (closed circles) and 0.26 mW cm-2 (open circles).

evaluation obtained for the macroscopic area change for this monolayer monitored at 2 mN m-1 (ca. 3 times).17,18 The magnitude of the deformation obtained at zero pressure should be reflecting the inherent behavior of the 6Az10-PVA monolayer. Thus, area monitoring under an applied pressure is prone to underestimation of the expansion degree. Second, upon continuous illumination, an induction period is clearly observed before the film exhibits expansion. As will be shown in section 3.3, the photoisomerization process from the trans to cis form immediately starts on UV light illumination. Therefore, the expansion process is obviously a nonlinear function with respect

to the photoisomerization process. The existence of induction period is presumably the consequence of self-assembling interactions among the trans-Az side chains through the van der Waals force among hydrocarbons and π-stacking forces among aromatics. Once the content of cis isomer exceeds a criterion level (see section 3.3), this aggregation force is overcome by the expanding motion. The phenomenon may be related to the self-contracting behavior as will be stated just below. The length of induction time depended on the light intensity irradiated and was reduced to almost half when the light power was increased from 0.26 to 0.50 mW cm-2. Next we undertook a BAM observation for the reverse process under visible light illumination. Figure 8 shows the changes in the BAM image of this monolayer starting from a fully expanded film after UV light irradiation (a) and snapshots of the film morphology on the course of visible light irradiation (b (44 s) f c (90 s) f d (300 s) from onset of 436 nm irradiation of 0.41 mW cm-2). For the fully dilated film, the contour was highly winded (a). After onset of visible light illumination, shrinkage of the film immediately occurred. At an early stage of contraction, the contour became more straight, maintaining the homogeneity of the film (b). Further contraction caused a heterogeneous structure; bright parts were accumulated in the center which were surrounded by darker sparse regions (c and d). The darker parts seem to comprised of film substance that is left behind on the process of shrinkage motion. It is to be stressed here that the 6Az10-PVA monolayer exhibits a “selfcontraction” without applying any surface pressures. This in turn shows that the contraction behavior under visible light

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Figure 8. An example of the series of BAM images exhibiting visible (436 nm) light induced area changes of the cis-6Az10-PVA single domain monolayer. Irradiation was achieved for 0 (a), 44, (b), 90 (c), and 300 s (d) with a light intensity of 0.4 mW cm-2. The diameter of the microscope field was 0.6 mm.

illumination itself is not an artifact arising from the applied pressure in the Langmuir trough experiment. We assume that this contracting motion stems from attracting forces working among the trans-Az side chains as stated in the above paragraph. This observation as well as the existence of induction period on the expansion process can be ascribed to the consequences of cooperative motions of side chains aligned in two dimensions. In this sense, the photomechanical response of the 6Az10-PVA monolayer resembles thermotropic processes of melting or freezing of molecules. In the photochemical processes, the transition occurs isothermally.20, 35, 36 3.3. UV-Visible Absorption Spectroscopy. To our knowledge no quantitative data are supplied so far on the time correlation between the photoisomerization proceeding and the assorted film deformation. This prompted us to undertake a UV-visible absorption spectroscopic study under precisely controlled light irradiation, which would be therewith compared to the behavior of film deformation described in section 3.2. Figure 9 depicts the UV-visible absorption spectral data of the 6Az10-PVA monolayer at a fixed area per Az unit of 0.3 nm2 under UV light irradiation. The light power applied in this experiment was 0.7 mW cm-2. The figure involved the spectral changes (a) and the time profiles of the peak position of the long axis π-π* band (b) and absorbance at 360 nm (c). As indicated in these figures, the trans-to-cis photoisomerization proceeded immediately after onset of UV light irradiation. The π-π* band peaked around 320 nm and decreased immediately, and the change ceased within 200-300 s. The peak position of the trans Az showed a hypsochromic shift from that obtained in chloroform solution (352 nm), showing an involvement of

Figure 9. UV-visible absorption spectral changes of the 6Az10-PVA monolayer upon UV (365 nm) light irradiation (0.7 mW cm-2) starting from the trans-Az state (a). The area was fixed at 0.30 nm2 per Az unit. The arrow shows the wavelength of absorbance monitoring. The smaller figures below, (b) and (c), show the changes in the peak position of the π-π* absorption band and absorbance at 350 nm with irradiation time, respectively.

H-type aggregation. The profiles of the spectral shift (b) and absorbance decrease (c) coincided well with each other, suggesting that a simple photoisomerization process is involved. The same experiments were undertaken, changing the area to 0.4 and 0.5 nm2 per Az unit at the identical irradiation power. The surface pressures corresponding to the areas of 0.3, 0.4, and 0.5 nm2 per Az unit were 30, 5, and 0-1 mN m-1 (see the pressure area isotherm of Figure 2a). Measurements at large

Inherent Photomechanical Response in Monolayers areas exceeding 0.5 nm2 per Az unit led to scattered results due to heterogeneous distribution of monolayer domains on the water surface.20 This is well understood also from the surface potential curve of Figure 2a which exhibits a sudden reduction of ∆V at areas beyond 0.5 nm2. From the kinetic data for the three areas, the reaction proceeded essentially at the same rate regardless of the packing density. The time required to reach the photostationary state was 220 ( 30 s. The band peak position of the trans-Az for 0.5 nm2 monolayer was less shifted (335 nm), but this difference in aggregation degree did not influence significantly the photoreaction kinetics. The coincidence of the spectral kinetic data at all areas rationalizes an assumption that the photoreaction kinetics obtained here can be compared with the photomechanical response of the isolated monolayer described in section 3.2. The spectral kinetic data taken with the light intensity of 0.7 mW cm-2 can be favorably compared to the BAM observation at 0.5 mW cm-2 in Figure 7 (closed cycles). Since the time constant of the mechanical response is proportional to the irradiation power (see section 3.2), proper comparisons can be made by taking into account a correction factor of the light intensity. At an intensity of 0.5 mW cm-2, the film expansion ceased in 250-300 s, and this is in good agreement with the time to reach the photostationary state, around 200 s at 0.7 mW cm-2 (40% excess intensity). A more important outcome by the comparison is that the criterion level of cis isomer content for initiating expansion can be determined to be ca. 40%, taking into account the correction factor of light intensity (see section 3.2). Once the film expansion commenced, it caught up with the photoreaction rate and consequently ceased at almost the same period as the photoreaction reached the photoequilibrium. Requirement of the criterion content of the cis isomer estimated here may be correlated to the results of the surface potential measurement under intermittent UV light irradiation (Figure 4). At earlier stages, the intermitted dark adaptation gave rise to some recoveries, and at later stages the surface potential at the dark period remained constant. The borderline level of the surface potential change that separates these features is also approximately 40% of the total. This probably shows that, below 40% content of cis isomer, the contribution of selfcontracting motion of the trans-Az side chains surpasses the expansion, and the photoisomerized cis unit is tending to be set aside perpendicular to the water surface. 3.4. Macroscopic Evaluations Using a Film Balance. Although we have reported some preliminary results on the mechanical response of the 6Az10-PVA monolayer at 2 mN m-1,17,18 the above observations prompted us to reexamine the macroscopic behavior under controlled light irradiation. Our previous exploration with intermitted UV light irradiation revealed that the film expansion immediately is halted on pausing the irradiation. On the basis of this behavior, we previously assumed that the area at the intermediate state is settled by the ratio of trans/cis isomers involved in the monolayer.17 Results presented in section 3.3 show this earlier interpretation to be quite dubious. The photoinduced area changes monitored at 2 mN m-1 together with the content of Az isomers obtained in the simultaneous spectral measurement under controlled irradiation are displayed in Figure 10. The ordinate on the left-hand side represents the normalized relative area in which the initial area in the trans form is defined as unity. The content of Az isomer was estimated by absorbance changes at 360 nm.25 This evaluation should involve (10% errors in our experiments due

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Figure 10. Simultaneous evaluation of irradiation-induced macroscopic area (open circles) and content of trans-Az unit (open triangles) by UV-visible spectroscopy at 2 mN m-1 and 20 ( 1 °C. Irradiation was performed with UV (a) and visible (b) light with intensities of 0.2 and 0.3 mW cm-2, respectively.

to spectral noise; however, this does not affect essentially the following arguments. For the film expansion process on irradiation of UV light (0.2 mW cm-2), the discrepancy of time constants between the photochemical process and area changes became more manifest (Figure 10a). The film exhibited a slight contraction at an early stage followed by a continuous slower expansion. It was rather surprising to us that the expansion commenced after the photoreaction had nearly reached the photostationary state after 400 s. Despite this, the dilatational motion exactly stopped with a halt of irradiation even after 600 s (data not shown). These data are absolutely contradictory to our previous assumption that the area expansion is coupled with the photoisomerization process.17 At present, we explain the above features as follows. The initial minor contraction is due to fluidization of the monolayer accompanied by the increase of cis isomers.20 The applied pressure, 2 mN m-1 in this case, gives rise to an apparent contraction of the monolayer as an artifact. The highly delayed expansion process and necessity of continuous light illumination even after reaching the photoequilibrium state clearly show that an excess energy is required for the film to exhibit the expansion in addition to that used for photoreaction proceeding. According to the molecular model in Figure 1, on the expansion process, the photoisomerized cis-Az unit needs to find a place to settle down on the water surface. In this process, a cis-isomerized Az side chain needs to push aside the surrounding side chains constrained with the applied surface pressure, and the light energy is seemingly used for work. It seems most likely that the local heating effect supplied by the nonirradiative process from the photoexcited Az units enhances the molecular motion which exerts the dilatational movement. It is however difficult to detect a “hot” monolayer since such heating should be localized in the vicinity of the water surface. In any event, it is clear that the initial slight contraction and the highly delayed initiation of expansion are resulting from

5320 J. Phys. Chem. B, Vol. 102, No. 27, 1998 artifacts stemming from the applied surface pressure. In the previous work,17 we did not notice the initial contraction. This can be attributed to a large difference in the irradiation power. The present experiment was undertaken with a properly attenuated light from a 150 W Hg-Xe lamp, which allows simultaneous spectroscopic measurements. On the other hand, the earlier experiments were carried out with a high-power Hg lamp (500 W super-high-pressure Hg lamp, the light power unmeasured). Under this high-power irradiation, the initial contraction process was probably hidden within the time constant of the mechanical servo systems of the film balance equipment. In contrast to the expansion process, the contraction process induced by visible light illumination proceeded concurrently with the photoreaction (Figure 10b). The contraction started immediately after onset of irradiation (0.3 mW cm-2) and ceased as the cis-to-trans photoreaction terminated in 250-300 s. On this reverse process, the conversion from the cis to trans isomer can induce immediate detachment of Az unit from the surface to orient toward the air. Since there are free spaces on the air side, Az side chains can show instantaneous motion toward the air without constraints. This motion may be in part assisted by the surface pressure applied. As a summary of this section, the following are proposed. First, the expansion and contraction motions are unsymmetrical processes in terms of the photoreaction progress. Second, the film deformation behavior involve a great deal of artifacts originating from the applied pressure which is unavoidable for macroscopic area monitoring. This emphasizes again the validity of the BAM observation which allows the evaluation of inherent photomechanical response in monolayers. 4. Conclusion In the present work, photomechanical effects observed in the monolayer of an azobenzene containing amphiphilic polymer (6Az10-PVA) at the air-water interface have been investigated in detail. The following are concluded and proposed. (1) Reversible contact and detachment motions of the azobenzene moiety with the water surface is supposed to be involved in the photoinduced area changes of the 6Az10-PVA monolayer at low pressures. Surface potential measurements provide strong support for this model. (2) Inherent photomechanical behavior of the 6Az10-PVA monolayer can be properly evaluated at zero pressure by following an isolated domain by means of in situ Brewster angle microscopic observations combined with a mobile minitrough. The expansion process is found to show a nonlinear response with photoreaction, which is characterized by the existence of an induction period. (3) Self-contraction of the 6Az10-PVA monolayer has been observed by BAM on visible light illumination at zero pressure. This confirms a fact that the contracting motion of 6Az10-PVA motion itself is not an artifact from an applied pressure. (4) Photochemical process of the trans-to-cis isomerization starts immediately after onset of UV light irradiation. In comparison with the BAM observation, the expansion process upon UV light irradiation at zero pressure is found to start at the cis conversions beyond ca. 40%. This may be also correlated to the characteristic behavior of surface potential changes on UV irradiation. The expanding motion ceases nearly at the stages of the photoequilibrium. (5) Macroscopic photomechanical responses evaluated by the Langmuir film balance with an applied pressure involves large distortions in mechanical responses in the expansion process. At 2 mN m-1 the expansion of 6Az10-PVA is initiated after

Seki et al. almost reaching to a photoequilibrium state on UV light irradiation. Despite this fact, the expanding motion was justly halted on pausing of irradiation, suggesting a requirement of an additional light energy for expansion. The contraction of the 6Az10-PVA monolayer, in contrast, occurs in concurrently with the cis-to-trans photoisomerization course. In this way, the expansion and contraction mechanical processes are unsymmetrical in terms of light energy required and molecular mechanisms involved. It is strongly suggested that proper evaluations of mechanical responses in photochromic monolayers should be undertaken by microscopic observations at zero pressure together with corresponding spectroscopic analysis. This work revealed the inherent and cooperative nature of the photoresponding properties in these procedures. We assume that the procedures proposed here are applicable to a wide variety of photochromic monolayer systems. Precise understandings of mechanical response of polymer assemblies at a molecular level should be of particular help for designing and constructing new photoresponsive materials. Acknowledgment. We thank Prof. H. Nakahara at Saitama University for taking the convenience of surface potential measurement, Prof. C. Hirose for his help in fabrication of a mobile minitrough, and J. Kojima for AFM measurements. This work was supported in part by the Grant-in Aid for Scientific Research on Priority Areas, “New Polymers and Their NanoOrganized Systems” (No. 277/09232220), from the Ministry of Education, Science Sports and Culture of Japan. Supporting Information Available: Summary of the photomechanical effects observed in the monolayer of 6Az10-PVA at the air-water interface and figures of the molecular structure of 6Az10-PVA and the time course of area expansion of 6Az10PVA at zero pressure (1 page). Ordering information is given on any current masthead page. References and Notes (1) Smets, G. AdV. Polym. Sci. 1983, 50, 18 and references therein. (2) Irie, M. AdV. Polym. Sci. 1990, 94, 27 and references therein. (3) Kumar, G. S.; Neckers, D. C. Chem. ReV. 1989, 89, 1915 and references therein. (4) Kinoshita, T. Prog. Polym. Sci. 1995, 20, 527 and references therein. (5) Ueno, A.; Takahashi, K.; Anzai, J.; Osa, T. J. Am. Chem. Soc. 1981, 103, 6410. (6) Fissi, A.; Pieroni, O.; Ciardelli, F. Biopolymers 1987, 26, 1993. (7) Geue, Th.; Zeigler, A.; Stumpe. Macromolecules 1997, 30, 5729. (8) Blair, H. S.; Pogue, H. I. Polymer 1979, 20, 99. (9) Blair, H. S.; Pogue, H. I.; Riordan, J. E. Polymer 1980, 21, 1195. (10) Blair, H. S.; McArdle, B. Polymer 1984, 25, 1347. (11) Gruder, H.; Vilanove, R.; Rondelez, F. Phys. ReV. Lett. 1980, 44, 590. (12) Vilanove, R.; Hervet, H.; Gruder, H.; Rondelez, F. Macromolecules 1983, 16, 825. (13) Panaiotov, I.; Taneva, S.; Bois, A.; Rondelez, F. Macromolecules 1991, 24, 4250. (14) Malcolm, B. R.; Pieroni, O. Biopolymers 1990, 29, 1121. (15) Menzel, H. Macromol. Chem. Phys. 1994, 195, 3747. (16) Higuchi, M.; Minoura, M.; Kinoshita, T. Colloid Polym. Sci. 1995, 273, 1022. (17) Seki, T.; Tamaki, T. Chem. Lett. 1993, 1739. (18) Seki, T.; Fukuda, R.; Yokoi, M.; Tamaki, T.; Ichimura, K. Bull. Chem. Soc. Jpn. 1996, 69, 2375. (19) Seki, T.; Sekizawa, H.; Fukuda, R.; Tamaki, T.; Yokoi, M.; Ichimura K. Polym. J. 1996, 28, 613. (20) Seki, T.; Sekizawa, H.; Ichimura, K. Polym. Commun. 1997, 38, 725. (21) Ho¨nig, D.; Mo¨bius, D. J. Phys. Chem. 1991, 95, 4590. (22) He´non, S.; Meunier, J. ReV. Sci. Instrum. 1991, 62, 936. (23) Lo¨sche, M.; Sackmann, E.; Mo¨hwald, H. Ber. Bunsen-Ges. Phys. Chem. 1983, 87, 848. (24) Tieke, B.; Weiss, K. J. Colloid Interface Sci. 1984, 101, 129.

Inherent Photomechanical Response in Monolayers (25) Maack, J.; Ahuja, R. C.; Tachibana, H. J. Phys. Chem. 1995, 99, 9210. (26) Tabe, Y.; Yokoyama, H. Langmuir 1995, 11, 4609. (27) Yoneyama, M.; Fujii, A.; Maeda, S.; Murayama, T. J. Phys. Chem. 1992, 96, 8982. (28) Karthaus, O.; Shimomura, M.; Hiroki, M.; Tahara, R.; Nakamura, H. J. Am. Chem. Soc. 1996, 118, 9174. (29) Bullock, D. J. W.; Cumper, C. W. N.; Vogel, A. I. J. Chem. Soc. 1965, 5316. (30) Seki, T.; Sakuragi, M.; Kawanishi, Y.; Suzuki, Y.; Tamaki, T.; Fukuda, R.; Ichimura, K. Langmuir 1993, 9, 211. (31) For example: Ulman, A. Introduction to Ultrathin Organic Films; Academic Press: Boston, 1991.

J. Phys. Chem. B, Vol. 102, No. 27, 1998 5321 (32) Oliveira, O. N.; Bonardi, C. Langmuir 1997, 13, 5920. (33) Ahluwalia, A.; Piolanti, R.; De Rossi, D. Langmuir 1997, 13, 5909. (34) A preliminary account on AFM study of the 6Az10-PVA monolayer has already appeared: Seki, T.; Tanaka, K.; Ichimura, K. Macromolecules 1997, 30, 6401. In this paper the reduction of the film thickness due to UV light illumination is reported to be only 13% of the trans-Az film. The discrepancy can be ascribed to the difference in the experimental procedure of AFM measurements. We have newly succeeded in carrying out in situ observations at an identical position with much shorter time for measurement. We believe that the thickness values stated in the text of this paper is more creditable. (35) Leier, C.; Pelzl, G. J. Prakt. Chem. 1979, 321, 197. (36) Ikeda, T.; Tsutsumi, O. Science 1995, 268, 1873.