Photoresponsive monolayers on water and solid surfaces

PII: S0968-5677 (98) 00034-0

Supramolecular Science 5 (1998) 373—377  1998 Elsevier Science Limited Printed in Great Britain.All rights reserved 0968-5677/98/$19.00

Photoresponsive monolayers on water and solid surfaces Takahiro Seki*, Hidehiko Sekizawa, Keisuke Tanaka, Yoko Matsuzawa and Kunihiro Ichimura Research Laboratory of Resources Utilization, Tokyo Institute of Technology, 4259 Nagatsuta, Midori-ku, Yokohama 226-8503 Japan

Organic photochromic units and molecules can be regarded as light-driven nano molecular machines. Once such molecules are aligned at a surface, the supramolecular organization provides an efficient macroscopic mechanical response in a collective way. Amphiphilic polymers having an azobenzene (Az) side chain are the favorable materials for observation of such effects since they show marked photomechanical response with essentially full reversibility. An in situ Brewster angle microscopic observation showed marked morphological and rheological photoinduced changes in the molecular films. Moreover, we have newly found that the identical photosensitive molecular film transferred on to a solid mica surface shows large morphological changes under highly humid conditions as proven by atomic force microscopy (AFM). It is supposed that the molecular film is driven in the same mechanism both on water and water-adsorbed mica surfaces. These microscopic observations provide new insights of the photomechanical response in photochromic monolayers.  1998 Elsevier Science Limited. All rights reserved. (Keywords: photochromic monolayers; azobenzene; photomechanical effect; Brewster angle microscopy; atomic force microscopy)

INTRODUCTION Photochromic polymer systems frequently show a macrosize mechanical effect. Photomechanical effects in monolayers of photochromic polymers at the air—water interface have also attracted considerable interest due to their characteristic behavior taking place in a two-dimensional state in which the photochromic units are arrayed or aligned—. Due to their molecularly arrayed arrangement, the macroscopic film deformation exactly corresponds to the sum of the individual motion of a large number of existing molecules or polymer units (typically in the order of 10 for film balance experiments). Such a behavior perceived on a macroscopic scale can be regarded as a typical example of the collective and active performance of supramolecules. We have found that monolayers of poly(vinyl alcohol) derivatives bearing an azobenzene side chain 6Azn—PVA (Figure 1) exhibit large photoinduced area changes—. The area changes can be interpreted as a consequence of reversible on/off contact motion with the water surface which is switched by the polarity change between the trans (0.5 D for non-substituted azobenzene) and cis (3.1 D) Az photoisomers (Figure 2). It was found that the longer spacer length leads to the larger film deformation. The expansion factors defined as the final area on UV light irradiation divided by the initial one for

* To whom correspondence should be addressed

6Az1—PVA, 6Az5—PVA, and 6Az10—PVA were ca. 1.5, 2, and 3, respectively, when the area was monitored at a pressure of 2 mN m\. To our knowledge 6Az10—PVA monolayer exhibits the largest film deformation as the monolayer state. Film expansion and contraction processes can be repeated many times with essentially full reproducibility. To date, almost all efforts have been paid to the two extremes in terms of length scale—; the macroscopic behavior at a centimeter level (film area and surface pressure) obtained with a Langmuir film balance, and the photoinduced response at a molecular level obtained by UV-visible absorption spectroscopy and surface potential measurements. In this context, our recent attention has been paid to microscopic observations of photoresponsive monolayers, which cover the ranges between the above scale levels as illustrated in Figure 3. Data obtained from in situ Brewster angle microscopic (BAM) observation of our Az-containing monolayers have already been accumulated to a large extent—. Furthermore, we recently observed the mobile nature of our Az containing polymeric monolayers on a mica surface under highly humid conditions by atomic force microscopy (AFM). On a mica surface the adsorbed water layer should play a crucial role in this phenomenon, and hence the light-driven mechanism for both systems arises probably from the same origin. In this article, we will summarize the results of our recent contributions on microscopic observations of our photoresponsive Az containing monolayers.

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Photoresponsive monolayers: T. Seki et al. IN SI¹º OBSERVATION ON WATER SURFACE BY BAM

Figure 1 Chemical structure of 6Azn—PVA

Figure 2 Schematic representation of photoinduced film deformation of 6Azn—PVA

Figure 3 Tools for observation of photomechanical monolayer systems

Our recent study on BAM observation of Az containing monolayers are briefly summarized in this section. The 6Azn—PVA monolayers were spread on pure water (Milli-Q SP grade) filled in a Lauda FW1 film balance at 20$0.5°C from a chloroform solution (1;10\ mol dm\). BAM observation was carried out with a home-made apparatus or a Nippon Laser Electronics EMM-633 and recorded on a commercially available video system. The diameter of the microscopic field was ca. 0.7 mm. UV (365 nm) and visible (436 nm) light irradiation onto the water surface was performed with a 150 W mercury-xenon lamp (San-ei UVF-202S) guided with a quartz optical fiber. Static morphologies of 6Azn—PVA monolayers were first observed by BAM in correlation with the surface pressure—area curve. A trans-6Az10—PVA monolayer at 0 mN m\ exhibited an iceberg-like domain structure having solid boundaries. When a large domain was damaged mechanically with a needle tip, cracking of the domain was observed. This indicates that trans-6Az10—PVA monolayer is rigid and solid-like. At a surface pressure of 2—3 mN m\, the domains started to fuse with one another and formed a homogeneous monolayer until the collapse pressure. When the monolayer was compressed beyond the collapse point below 0.2 nm per Az unit area, bright stripes of ca. 0.1 mm widths appeared, clearly indicating a collapse of the rigid monolayer. The direction of the stripes was orthogonal to the compression direction. A similar BAM observation of stripes in a collapsed state was reported by Hirano and Fukuda for monolayers composed of polyamic acid tertiary amine salts. A cis-6Az10—PVA monolayer was highly homogeneous in all area regions. Interestingly, even after the film collapse below 0.3 nm, the film maintained homogeneous morphology with an enhanced light reflection. This fact implies that the cis-6Az10—PVA monolayer is fluid and highly amorphous. Direct observation of º» light-induced film deformation Figure 4 shows the direct observation of UV light induced morphological changes of the 6Az5—PVA monolayer (expansion factor, ca. 2 at 2 mN m\) as a typical example. The darker and brighter regions in the BAM images correspond to the bare water surface and domains of the floating monolayer, respectively. The trans-6Az5—PVA monolayer was spread on the water surface at an area of 0.6 nm per Az unit, and irradiated with UV light. Upon UV light irradiation, clear contours of iceberg-like domains (a) of the trans monolayer became obscure (b), and finally a highly fluid cis-isomerized 6Az5—PVA monolayer was observed (c). The photoinduced morphological and rheological changes for 6Az1—PVA and 6Az10—PVA are essentially the same. Due to the differences in the expansion factor, the

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Photoresponsive monolayers: T. Seki et al. present system, a homogeneous film was isothermally obtained by the photochemical process instead of the heating process. Monitoring the area changes of an isolated small domain We further attempted to observe the photoinduced film expansion and contraction for an isolated small domain under zero pressure conditions. For these experiments, a home-made small trough placed on a mobile X—½ stage was used. The light irradiation was performed from the bottom of the trough through a quartz window. The area changes were estimated by a computer image processing. New insights were obtained in this approach as follows. (1) 6Az10—PVA monolayer at zero pressure exhibits 4—5-fold expansion which is even larger than that estimated with a macroscopic film balance experiment (ca. three-fold). (2) The UV light-induced expansion is a non-linear process with respect to the proceeding of the transPcis photoisomerization, indicating an induction time at an early stage. (3) Upon visible light irradiation, the film shows self-contraction without pressure. These experiments should give knowledge on the intrinsic film properties expelled from any distortions from an applied pressure. Details of these new results will be reported elsewhere in due course.

OBSERVATION OF MORPHOLOGIES ON MICA SURFACE BY AFM

Figure 4 In situ observation of photoinduced morphological changes of 6Az5—PVA monolayer induced by locally irradiated UV light. (a) Before irradiation, (b) intermediate state of irradiation, and (c) fully irradiated monolayer. Bright and dark areas correspond to the film and exposed water surfaces, respectively

final state of the BAM images were when the same area per Az unit (0.6 nm) was given. The degree of the exposed bare water surface differed among the three monolayers. These BAM images present the first realtime direct observation of morphological changes in a photoresponsive monolayer. Morphological changes on the cyclic process The morphologies of 6Az10—PVA monolayer were observed on a cycle process (UVPvisible) under irradiation of the whole film area at 2 mN m\ . The initial trans-6Az10—PVA monolayer consisting of irregularly shaped domains became homogeneous after UV light irradiation. The following contraction process on visible light irradiation resulted in a highly homogeneous monolayer. Long chain fatty acid monolayers in the as-spread state mostly show a heterogeneously domained morphology, but they could be converted to more homogeneous films by annealing. It is worth mentioning that, in the

In this section, a highly mobile nature is presented for transferred monolayers of 6Az10—PVA on a mica surface under highly humid conditions, as proven by atomic force microscopy (AFM). Depending on the lateral packing density, the monolayers showed photo-driven large and characteristic two- and three-dimensional (2D and 3D) morphological changes at a sub-micrometer scale. To our knowledge this work presents the first visualization of the photomechanical response of a monolayered photochromic polymer taking place on a solid surface. A chloroform solution containing 6Az10—PVA (1.0;10\ unit mol dm\) was pre-irradiated with 365 nm light, and then spread on to the water surface at 20$0.5°C. Approximately 90% of the Az units were transformed to the cis-isomer. Single layered LB films were prepared by the vertical lifting at two pressures, 13 and 2 mN m\, on to a freshly cleaved mica surface. These conditions rendered molecular films having the average areas per Az unit of ca. 0.4 (Film A) and 1.2 nm (Film B), respectively. The LB monolayers were stored in a highly humid vessel containing a wet cotton at room temperature (relative humidity, ca. 100%) in the dark for four days. This dark adaptation allowed a complete thermal conversion from the cis to trans form of Az unit. AFM measurements were achieved with a SPA300/ SPI3700 system (Seiko Instruments Inc.) in the dynamic force (tapping) mode. All AFM images were recorded under ambient air conditions (ca. 20°C, relative humidity of 30—40%).

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Photoresponsive monolayers: T. Seki et al. Figure 5 shows topographical AFM images of Film B (a lower coverage density film). After dark adaptation for four days, the film exhibited network-shaped morphologies (a), which can be ascribed to a contraction of the film during the process of the cis to trans thermal isomerization. The darker regions in the AFM image should indicate the bare mica surface judging from the flatness within 0.2 nm. After UV light irradiation for 7 h, the network-shaped films exhibited marked swelling nearly maintaining the film thickness (b). The height profiles indicate that the photoinduced expansion was accompanied by a slight decrease in the film thickness from 2.4$0.2 to 2.1$0.2 nm. The swollen morphology was

virtually reverted to the original one after another dark adaptation in the highly humid vessel for four days, and thus, the morphological change was a reversible process. Thus, Film B exhibits a 2D expansion and contraction maintaining the monolayer state. AFM observations of Film A (a higher coverage density film) were performed using the same procedure (Figure 6). The first dark adaptation gave a spongeshaped film having a thickness of 1.9$0.2 nm (a) similarly due to the contraction after deposition as observed for Film B. The diameters of the defects ranged from a few tens to a few hundred nanometers. After UV light irradiation, the film morphology exhibited a complete

Figure 5 Topographical AFM images of the 6Az10—PVA monolayer on mica (Film B, a low coverage film) before (a) and after (b). UV light irradiation (at 365 nm, 4.5 mW cm\ for 7 h). The film was transferred giving an area per Az unit of 1.2 nm. The sample was kept at a relative humidity of ca. 100% during all processes

Figure 6 Topographical AFM images of the 6Az10—PVA monolayer on mica (Film A, a high coverage film) (a) before and (b) after UV light irradiation. The experimental conditions are the same as for Figure 5 except for the lateral film density. The average area per Az unit was 0.4 nm

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Photoresponsive monolayers: T. Seki et al. change. Most of the defects disappeared and instead huge shallow pan-like protrusions of 200—300 nm in width and ca. 10 nm in height appeared (b). A 3D morphological change took place in this case. We assume at present that the light-driven expansion of the monolayer first fills up the defects, and then an excess lateral film pressure gives rise to structuring of such protrusions. The flat gray regions that occupy the major part in the image should correspond to the top of the monolayer. This 3D change was again nearly a reversible process; dark adaptation for four days almost reverted the film morphologies. The marked morphological changes stated above were promoted under highly humid conditions. Essentially no morphological changes were observed in a dry vessel containing silica gels (relative humidity, less than 20%). It can therefore be concluded that, the formation of an adsorbed water layer on mica plays the critical role in providing the dynamic nature of the 6Az10—PVA film. We are now conducting further studies for elucidation of detailed aspects of this phenomenon.

CONCLUSION We have shown that optical (BAM) and scanning probe (AFM) microscopic observation can be powerful tools to obtain new insights in photomechanical phenomena taking place in Az-containing photochromic monolayers. These data are anticipated to bridge the understandings between the macroscopic phenomena and nanometer level molecular events. Accumulation of such fundamental knowledge obtained in simplified two-dimensional films should be of particular help for the design and fabrication of future molecular mechanical devices and polymer-based micro-machines even in the three dimensionality.

ACKNOWLEDGMENTS We thank S. Morino and M. Nakagawa for helpful discussions. This work was in part supported by the Grant-in-Aid for Scientific Research on Priority Areas, ‘‘New Polymers and Their Nano-Organized Systems’’ (No. 277/08246217) from the Ministry of Education, Science, Sports and Culture.

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