© Copyright 1998 American Chemical Society
SEPTEMBER 1, 1998 VOLUME 14, NUMBER 18
Letters Controlling Microdroplet Formation by Light G. Mo¨ller, M. Harke, and H. Motschmann* Max-Planck Institute of Colloids and Interfaces, Rudower Chaussee 5, D-12489 Berlin, Germany
D. Prescher Institut fu¨ r Du¨ nnschichttechnologie und Mikrosensorik e.V, Kantstrasse 55, D-14513 Teltow, Germany Received April 8, 1998. In Final Form: July 2, 1998 A surface with photocontrollable wetting behavior is introduced. A monolayer of a polymeric material containing 4′-[trifluormethoxy-4,4′-dibenzoazo] dyes in the side chains has been transferred on quartz slides and silicon wafer. The azobenzene chromophore possesses two distinct isomers, cis and trans. Transition between these states can be triggered by illumination with light of two different wavelengths. It will be demonstrated that with the use of light and a mask, fine cis-trans patterns on the order of micrometers can be written in the monolayer. The corresponding interface exhibits different wetting behavior. This is visualized by a surface decoration with water droplets. The formation of water microdroplets on the patterned monolayer can be controlled by light. Writing and erasing of patterns is completely reversible. The system has potential for studying wetting behavior on microstructured surfaces.
1. Introduction Azobenzene chromophores are unique systems since the photoisomerization of this unit is widely reversible with little degradation occurring after many switching cycles. The system has been studied by many groups, and a recent review can be found in ref 1. Several optical storage devices based on these molecules have been proposed. The drastic change in the shape of the molecule due to the photoisomerization induces changes in the alignment of liquid crystals within the mesoscopic phases.1 This has been successfully employed to write holographic patterns in mesoscopic phases with remarkably high storage densities.2 Another approach utilizes the well-established feature of liquid crystals, which is that their orientation is strongly dependent on the properties of the surface. It (1) Anderle, K.; Wendorff, J. H. Mol. Cryst. Liquid Cryst. 1995, 77, 481. (2) Wei, X.; Yan, X.; Zhu, D. R.; Lin, W. Z.; Mo, D.; Liang, Z. Chin. J.Phys. 1996, 34, 24.
was demonstrated that a single Langmuir-Blodgett (LB) monolayer containing azobenzene chromophores can act as a command surface capable of controlling the orientation of several micrometers of the liquid crystal. The orientation within the nematic phase is determined by the ratio of cis and trans molecules at the interface.3 The present contribution demonstrates that a monolayer of polymer with azobenzene dyes in the side chains possesses a photocontrollable wetting behavior. Cis and trans states differ in their dipole moment, which in turn has also an impact on the corresponding wetting behavior. This can be further enhanced by a proper substitutent attached to the azobenzene chromophore. For instance the short fluorinated chain used in our study leads to very pronounced differences in the wettability. It is demonstrated that rather fine cis/trans patterns can be written into the monolayer. The pattern can be visualized by the formation (3) Knobloch, H.; Orendi, H.; Bu¨chel, M.; Seki, T.; Ito, S.; Knoll, W. J. Appl. Opt. 1995, 77, 481.
S0743-7463(98)00400-4 CCC: $15.00 © 1998 American Chemical Society Published on Web 08/06/1998
4956 Langmuir, Vol. 14, No. 18, 1998
Letters
Figure 1. Amphiphilic polymer with azobenzene chromophore in the side chains.
of microdroplets. The photoinduced changes are reversible, and the prevailing microdroplet pattern can easily be erased and restored with light of the appropriate wavelength. Recently, there is an increasing interest in understanding the structure adopted by liquids on surfaces with patches of different wettability.4 Our model system could serve as an experimental tool to assess theoretical predictions. The wettability of these surfaces can be reversibly changed with light, and the analysis is not further complicated by thickness variations within the surface. The range of the modulations can be varied from several hundreths of a micrometer up to the diffraction limit of an optical microscope and can be further improved by optical near field microscopy.7 Besides azobenzene, other systems, for example, spyropyran/merocyanin (12.7 D)5 also possess attractive features for these studies. 2. Materials and Methods Materials. The structure of the material used in this study is given in Figure 1. The material comprises four structural elements: (1) a polymeric backbone with hydroxy groups as an hydrophilic element (2) the photoactive azobenzene chromophore in the side chains of the polymer; (3) a decoupling from the polymeric backbone by a spacer consisting of five CH2 units; (4) a fluorinated tail to ensure sufficient hydrophobicity in order to process the material by means of the Langmuir-Blodgett technique. The ratio between A and B was determined by the fluorene content of an elemental analysis and is 1:9. The molecular weight of the polymer is 20 800 g/mol and was determined by size exclusion chromatography (Knauer HPLC-pump-64) using polystyrene standards. The synthesis of the monomers is described in detail in ref 6. The copolymerization of both monomers was carried out with 2,2-azoisobutyronitrile (AIBN) as initiator at 70 °C in dimethylformamide (DMF). Sample Preparation. Monolayer film formations were carried out from the pure water subphase with no deliberately added solutes (Milli-Q). The films were prepared on a commercial Langmuir trough purchased from Lauda Instruments. The polymer was dissolved in chloroform and switched to the cis state before being spread. Monolayers were transferred in an upstroke in a vertical arrangement onto hydrophilic surfaces of glass or silicon wafers. The pressure was kept constant at 13 mN/m during transfer, and the transfer ratio was approximately 0.9. The solid support was dipped before spreading the surface active solution. Optical Characterization. A high-pressure mercury lamp (Mu¨ller Elektronik LAX1450) in combination with (4) Dietrich, S. J. Phys.: Condens. Matter 1996, 8, 9127. (5) Lapienis, D.; Kryszewsky, M.; Nadolski, B. J. Chem. Soc., Faraday Trans. 2 1979, 75, 312. (6) Prescher, D; Thiele, T.; Ruhmann, R.; Schulz, G. J. Fluorine Chem. 1995, 74, 185. (7) Pohl, D. W.; Denk, W.; Lenz, M. Appl. Phys. Lett. 1984, 7, 44.
Figure 2. UV-vis spectra of a monolayer on quartz measured in transmission. The dotted line refers to the trans state. The monolayer can be switched to the cis isomer by illuminating the sample with UV light (350 nm). The increased number density of the cis isomer enhances the intensity of the n-π band at 440 nm.
an appropriate combination of filters (Schott color filters BG12 and GG420 yields 450 nm) was used for the cis w transition; the filter combination UG11 and WG360 yielded 360 nm radiation and was used for the transition of trans w cis and were used for the sample illumination. UV-vis spectra were recorded on a UV-vis spectrometer (Carry 4E), which is capable of recording spectra of a single monolayer. The wetting structures were observed with a microscope (Nicon). Electron microcoscopic grids (Plano) served as a mask to write patterns on a micrometer scale in the samples. As an alternative approach, masks were inserted in the microscope and focused onto the surface. Care was taken to ensure that the observed wetting patterns are not caused by a thermal or photoablation process. 3. Results and Discussion An important observation was that a photoisomerization within the LB film can only be observed if the Langmuir film has been switched to the cis state before the transfer. We attribute this to a lack of free volume within the monolayer. According to the isotherms the packing density of the chromophore in the trans state is higher and more regular compared to the corresponding cis state. The photochromic transition is accompanied by rearrangement processes that require free volume within the monolayer. This observation suggests a higher quantum efficiency of the process if there is some degree of disorder within the monolayer. The photoisomerization can be observed in the UV-vis spectra. Figure 2 shows the corresponding transmission spectra of a monolayer on a quartz slide. The solid line corresponds to the trans isomer. Illumination of the sample with UV light (350 nm) leads to the trans-cis photoisomerization as indicated by an increase of the n-π* band and accompanying decrease of the intensity of the π-π* band (dashed line). This process can be reversed if the sample is exposed to blue light (450 nm). No signs of degradation were observed even after hundreds of cycles. The trans state of the azobenzene chromophore belongs to the unpolar point group C2h, whereas the cis isomer possesses C2 symmetry with the C2 axis as the polar axis. The magnitude of the dipole moment of the unsubstituted azobenzene is about 3.5 D according to semiempirical quantum chemical calculations (AM1, Hyperchem). The difference in the polarity between both isomers is further enhanced by the CF3 substituent. Thus, it is possible to deliberately change and control the dipole pattern of the surface with light. In addition if the chromophore is in the trans state, the corresponding interface is terminated by the hydrophobic
Letters
Langmuir, Vol. 14, No. 18, 1998 4957
in a humid atmosphere. The droplets accumulate preferentially on the polar cis region. Figure 3 shows an image of the droplet pattern on the surface as observed by a conventional optical microscope. A single monolayer is sufficient to modify macroscopic quantities. As a control experiment we used an unmodified quartz and silicon wafer and proceeded in exactly the same manner. No pattern was observed on both control surfaces, and hence artifacts eventually caused by the mask or thermal effects can be ruled out. All cis/trans patterns can be completely erased by an illumination of the sample with blue light (455 nm), and subsequently a new water droplet pattern can be formed by illumination with 350 nm radiation. The image presented in Figure 3 was taken after four complete writing and erasing cycles. The azobenzene surface thus acts as a rewriteable wetting pattern transducer.
Figure 3. A cis-trans pattern written in the monolayer by illumination with UV light and a mask. Their corresponding interfaces differ in wetting behavior. Water microdroplets are formed on the illuminated part by cooling the sample in a humid athmosphere and can be observed with an optical microscope. The width of the bars is 2 µm and the mesh size is 10 µm. Examples of two subsequent patterns with different magnification are presented.
CF3 group. Due to geometrical rearrangements with the photoisomerization, the CF3 group is buried in the cis state within the monolayer. Both corresponding surfaces, cis and trans, exhibit a different wetting behavior. To demonstrate this, we illuminated the sample with UV light at a wavelength of 350 nm using an electron microscopic grid as a mask. The grid consists of copper bars with a width of 2 µm and a mesh size of 10 µm. The photoisomerization trans w cis occurs only within the illuminated parts. As a result a cis/trans pattern is written into the monolayer. The different wetting behavior of both interfaces can be visualized by the formation of water microdroplets. Droplets are formed by cooling the sample
4. Conclusion It has been demonstrated that a monolayer containing azobenzene groups possesses a photocontrollable wetting behavior. This was illustrated by the water microdroplet formation on cis/trans pattern generated by light and a mask within a LB monolayer. The droplet pattern can be formed and subsequently erased by light, and all changes are completely reversible. The difference between cis and trans surfaces can still be further optimized; important criteria are the choice of the substituent and an optimization of the packing density of the azo dye within the monolayer. The monolayer is acting as a smart surface with photocontrollable properties. These features might be utilized for a printing technique. Spraying dye solutions onto the surface and evaporation leads to a dye pattern on the monolayer that can then be transferred onto the printing medium. In addition these smart surfaces can be of basic interest for fundamental wetting studies on microstructured surfaces allowing switching between wetting and nonwetting surfaces. Acknowledgment. The authors thank Professor Mo¨hwald for stimulating discussions. LA980400O
