Photoinduced Formation of Superhydrophobic Surface on WhichContact Angle of a Water Droplet Exceeds 170° by ReversibleTopographical Changes on a Diarylethene Microcrystalline SurfaceNaoki Nishikawa,† Hiroyuki Kiyohara,† Shingo Sakiyama,† Seiji Yamazoe,†,○ Hiroyuki Mayama,‡,#

Tsuyoshi Tsujioka,§ Yuko Kojima,∥ Satoshi Yokojima,⊥,⊗ Shinichiro Nakamura,⊗ and Kingo Uchida*,†

†Department of Materials Chemistry, Faculty of Science and Technology, Ryukoku University, Seta, Otsu 520-2194, Japan‡Research Institute for Electronic Science, Hokkaido University, N21, W10 Kita-ku, Sapporo 001-0021, Japan§Department of Arts and Sciences, Faculty of Education, Osaka Kyoiku University, 4-698-1 Asahigaoka, Kashiwara, Osaka 582-8582,Japan∥Mitsubishi Chemical Group Science and Technology Research Center, Inc., 1000 Kamoshida, Yokohama 227-8502, Japan⊥School of Pharmacy, Tokyo University of Pharmacy and Life Sciences, 1432-1 Horinouchi, Hachioji, Tokyo 192-0392, Japan⊗RIKEN Research Cluster for Innovation, Nakamura Laboratory, 2-1 Hirosawa, Wako, Saitama 351-0198, Japan

*S Supporting Information

ABSTRACT: A superhydrophobic surface on which thecontact angle of a water droplet exceeds 170° was reversiblyproduced by alternate irradiation with UV and visible light.Superhydrophobicity is due to the formation of denselygenerated submicrometer sized needle-shaped crystals (lessthan 0.2−0.3 μm diameter and 2.2−2.5 μm long) at 30 °C,which is much lower than the eutectic temperature of eitherisomers of the diarylethene. Below the eutectic temperature,the generated crystals were much smaller than those generatedabove the eutectic temperature. These smaller crystals moreeffectively enhanced the superhydrophobicity.

■ INTRODUCTION

Surface roughness profoundly influences the wetting propertiesof a material, especially for the wetting of superhydrophobicsurfaces. Studies of the superhydrophobic surfaces of variousmaterials, for example, even such natural materials as lotus andtaro leaves, have revealed that fascinating functions weremanifested by combining micro- and nanoscaled hierarchicalstructures with low-surface-energy materials.1−10 To date,superhydrophobic coatings promise a wide range of applica-tions such as self-cleaning surfaces, corrosion-resistant,antiadhesive, and drag-reducing coatings.11−14 The robustnessof superhydrophobicity is a fundamental requisite for theapplications of water-repellant materials. Recently, super-hydrophobic materials are well designed, and contact angles(CAs) larger than 170° have been reported.15−18 Typically,fabrication of superhydrophobic materials with a water CA of178° was performed using a perpendicular nanopin film,15 andCA of 172° was performed on densely packed polyacrylonitrile(PAN) nanofibers made by template synthesis using an anodicaluminum oxide membrane.16

Recently, photocontrollable systems with surface super-hydrophobic properties were reported. Such systems wereprepared by the introduction of photoinduced polaritychangeable materials on rough surfaces.19−21 In such systems,

photochromic molecules are also introduced whose polaritychanged accompanied with photoisomerization. Photochrom-ism is defined as the reversible transformation of a singlemolecule between two states having different absorptionspectra. Hence, photochromic molecules often hold consid-erable potential toward applications as molecular switches andcontrol elements in molecular devices.22,23 Diarylethenes areamong the most promising photochromic compounds,24−28 notonly as memory materials but also as switching units formolecular devices and in supramolecular systems.29,30 Recentlythe surface properties, i.e., surface topographical changesaccompanied with wettability changes31−38 and metal deposi-tion capability,39−41 were controlled by light. Such photo-response was not considered in past photochromic systems inwhich the reversible formation of steps and valleys on adiarylethene single crystal surface was observed.42 In theirstudy, photoirradiation was carried out below the eutectictemperature of open- and closed-ring isomers of diarylethene 1.In contrast, we found the formation of needle-shaped crystals of3c on the microcrystalline surface of 3o by UV irradiation

Received: November 5, 2012Revised: November 29, 2012Published: December 2, 2012

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followed by storage at 30 °C, which is the eutectic temperatureof 3o and 3c.31 Herein we report a new system of amicrocrystalline surface of diarylethene 4o, which forms smallerfibrils below the eutectic temperature and generates asuperwater-repellant surface whose CA of a water dropletexceeds 170°.

■ MATERIALS AND METHODSPreparation of Film and Characterization. Diarylethene 4o was

prepared according to a previous paper.43 The film was prepared bycoating a chloroform solution containing 4o (300 mg/mL) on theslide glass substrate (10 mm × 10 mm). The substrate was stored atroom temperature to evaporate the solvent and placed in a desiccator;the residual solvent was removed under 58 mmHg for 30 min forscanning electron microscopy (SEM) observation. Residual chloro-form was not observed in 1H NMR in C6D6. The film thickness wasapproximately 20 μm by laser microscope (KEYENCE VK-8550) afterscratching the surface. A scanning electron microscope (KEYENCEVE-8800) and an optical microscope (Leica DMLP) were used tostudy the surface microstructure. Photoirradiation (visible light, λ >500 nm) was carried out using a Ushio 500-W xenon lamp with acutoff filter (Toshiba color filter Y-50), and UV irradiation was carriedout with a Spectroline Hand-Held UV lamp, E-series (λ = 313 nm, 8W). Photoirradiation experiments at the eutectic temperature werecarried out on a Mettler Toledo FP90 to which a FP82HT hot stagewas attached. The static CA and SA were measured on an opticalcontact angle meter (Kyowa Interface Science Co., Ltd., Drop Master500) with capillary o.d. 30 μm (straight) at ambient temperature.Deionized water (1.5 μL) was dropped carefully onto the surface. Anaverage CA value was obtained by measuring the samples at eightdifferent positions. An atomic force microscope (AFM, ShimadzuSPM-9600) was used for measuring the force curve on the singlecrystalline surfaces.Differential Scanning Calorimetry (DSC) Measurement of

Different Contents of Mixtures of 4o and 4c. A phase diagramwas prepared according to the DSC measurements of the mixtures of4o and 4c with different components. The 4o and 4c crystals withdifferent ratios were measured and ground in an agate mortar andpestle for 1 h. In total 2 mg of the mixture was used for the DSC

measurements. The heating and cooling rates were 5 °C/min and weremeasured between temperature ranges of 20−220 °C.

Crystal Data of 4o and 4c. The intensity data of the 4o and 4ccrystals were collected by ω scan on a Bruker SMART APEX CCDdiffractometer with graphite-monochromatized Mo Kα radiation (λ =0.710 73 Å) at 93 K. The structure was solved by direct methods usingthe program SHELXS9744 and refined by full-matrix least-squaresagainst F2 of the observed reflections with SHELXL97.45 All non-hydrogen atoms were refined anisotropically. Hydrogen atoms werelocated at ideal positions and refined in isotropic approximation.

The following are the crystal data for the plate-shaped crystals of 4o.(C29H30F6S4Si2) crystal system, monoclinic; space group, P21/c; a =15.8171(11) Å; b = 17.8379(12) Å; c = 11.9561(8) Å; α = 90°; β =102.1840(10)°; γ = 90°; V = 3297.36(3) Å3; Z = 4; Dcalcd = 1.633 Mgm−3; R((I) > 2σ(I)) = R1 = 0.0796; wR2 = 0.2048, T = 103(2) K;CCDC 626732.

The following are the crystal data for the rod-shaped crystals of 4c.(C29H30F6S4Si2) crystal system, monoclinic; space group, P2/c; a =25.378(3) Å; b = 6.1561(8) Å; c = 23.542(3) Å; α = 90°; β =116.779(2)°; γ = 90°; V = 3283.5(3) Å3; Z = 4; Dcalcd = 1.623 Mg m−3;R((I) > 2σ(I)) = R1 = 0.0626; wR2 = 0.1681; T = 103(2) K; CCDC626731. These data can be obtained free from The CambridgeCrystallographic Data Center at www.ccdc.cam.ac.uk/data_request/cif.

Surface Structure Observations and Fractal Analysis. For thefractal analysis, the sample on a cover glass was set on the electronmicroscope stage with conductive carbon tape and the Au−Pd alloywas evaporated onto the sample surface. The cross-section wasobserved for the sample that was cracked with the cover glass and setperpendicularly on the stage.

The fractal dimension of the cross-section of the rough solidsurfaces was calculated from the trace curves of the surfaces by thebox-counting method. A two-dimensional space containing the abovetrace curve was divided by identical boxes of side size r like a piece ofcross section paper. The number of boxes containing trace curve N(r)was counted, and then side size r was changed. The number of boxeswas counted again for new side size r, and the above process wasrepeated. On the basis of the box-counting method, the fractaldimension can be calculated from the following relationship:

∝ −N r r( ) D (1)

Scheme 1. Molecular Structures of Diarylethenes

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where D is the fractal dimension. Dimension D in eq 1 is the fractaldimension of the cross-section, and the dimension of surface Ds isapproximately obtained by Ds = D + 1.

■ RESULTS AND DISCUSSIONDiarylethene 4o, which was synthesized according to a previouspaper,43 underwent cyclization and cycloreversion reactions inthe crystalline state as well as in the solutions, because thedistance between the reactive carbon atoms of 4o in thecrystalline state was 3.634 Å. The distance must be less than 4Å for the cyclization reaction to proceed upon UV irradiation inthe crystalline state.46

The phase diagrams of the open-ring isomer 4o (cubicshaped-crystals; mp, 162 °C) and the closed-ring isomer 4c(needle-shaped crystals; 202 °C) were obtained by measuringthe DSC curves at different component ratios of the 4o and 4cmixtures (Figure 1). Photoinduced topographical changes were

observed at 141 °C of the eutectic temperature. After UVirradiation (5 min at room temperature) to the microcrystallinefilm of 4o, the film was stored at 141 °C in the dark and thesurface was monitored by SEM and CA measurements. Afterbeing stored at 20 min, rod-shaped crystals appeared on thesurface (Figure 2a,b) and the CA reached 138°. The crystalswere 1.5 μm in diameter and 20 μm long on average. Theirshapes agreed well with those estimated from single crystalanalysis (see the Supporting Information, Figure S1). In thiscase, they lie on the surface. Therefore the CA was notenhanced, and superhydrophobicity was not achieved. The CAgradually decreased (Figure 2d) with prolonged heating due tothe melting of the edge of the crystals. At high temperature, thethermal cycloreversion reaction from 4c to 4o proceeded, andthe crystal started to melt by forming an eutectic mixture. Thehalf-life period of 4c at 141 °C due to the thermal recovery to4o was 55 min in the decalin solution.Next we monitored the crystal growth of 4c at 135 °C, which

is lower than the eutectic temperature. UV irradiation was alsocarried out for 5 min at room temperature followed by storageof the film at 135 °C. Then needle-shaped crystals of 4cappeared on the surface whose diameters and lengths were lessthan 1 and 10 μm, and superhydrophobicity was observed.After 15 min, the surface was covered with needle-shapedcrystals of 4c (Figure 3a,b), and the CA of a water droplet was160 ± 0.9° (Figure 3c). When the surface was tilted, the droplet

was pinned on the surface, showing a petal effect.47 The CAprofile of a water droplet depends on storage periods at 135 °C(Figure 3d). The CA decrease after 15 min is attributed to theenlargement of the needle-shaped crystals by Ostwald-ripening.34

Such surface topographical changes due to the crystal growthof each isomer through the melted eutectic mixture werealready reported for the diarylethene derivatives of 1 and 3 onlyabove the eutectic temperature.31−35 No topographical changeswere observed for these derivatives below the eutectictemperatures. In contrast, photoinduced step and valleyformation were reported for single crystals of 1,2-bis(2,4-dimethyl-5-phenylthien-3-yl)perfluorocyclopentene (2).42 Theobserved changes were proceeded in the crystalline state atroom temperature, which is less than the eutectic temperature,and those are attributed to the changes in the molecular volumebetween 2o and 2c.42

To clarify the differences of the systems, we measured theforce curve using an atomic force microscope (AFM) toestimate the surface hardness. The force curves obtained byAFM at 22 °C on the surfaces before and after UV irradiationof the single crystalline surfaces of 2o and 4o are shown inFigure 4. The speed of the approach and the release of thecantilever were both 100 nm/s. The spring constant was 42 N/m. For a single crystalline surface of 2o, no hysteresis betweenthe approach and release processes was observed. This isattributed to the surface hardness. In contrast, hysteresis wasobserved for the surface of a single crystal of 4o, indicating thatthe surface was softened after UV light irradiation. This is dueto the loss of the degree of crystallization of the 4o film, whichis ascertained by weakened X-ray diffraction (XRD) peaks of 4oupon UV irradiation to the deposited film by vacuumevaporation (Figure S2 in the Supporting Information).The results inspired us to generate needle-shaped micro-

crystals of 4c at even a lower temperature, at room temperature.The UV-irradiated microcrystalline surface was also stored inthe dark at 30 °C, and CA changes were monitored. The resultsare summarized in Figure 5. The CA dramatically increased andreached 154° after 4 h even at 30 °C. At a time 12 h later, itreached 162°, which is almost the same CA as a water dropleton a lotus leaf. After prolonged storage at this temperature, theCA gradually increased and finally reached 172° (Figure 5d).The surface shows a lotus effect. The sliding angle (SA) of awater droplet was less than 1°.The SEM images of the surface are shown in Figure 5a,b.

The surface is covered with needle-shaped crystals whosediameters and lengths are around 0.20−0.35 μm and 2.2−2.5μm, respectively. The size is much smaller than that observedon the surface stored at 135 °C, as shown in Figure 3, (0.46−0.71 μm width and 2.1−3.2 μm long), and the 3c fibrils(around 1−2 μm diameters and 10 μm long) whose CA and SAwere 162.9° and less than 2°, respectively.31,34 Theextraordinary high CA of the film of 4 is attributed to thefine structures of the surface.48,49 The fractal analysis of thesurfaces, which were stored for 9 days at 30 °C in the dark, wascarried out by box counting for the cross-sectional trace curvesof the surface’s cross-section (Figure S3 in the SupportingInformation). Figure 6 shows the result. The slope of log N(r)vs log r plot was −1.4, so the fractal dimension of cross-sectionDcross was determined to be 1.4. Fractal dimension D of thesurface was evaluated as D = Dcross + 1 = 2.4. Upper limit scaleL and lower limit scale l of the fractal behavior could bedetermined from the log N(r) vs log r plot in Figure 6. The

Figure 1. Phase diagram of open- and closed-ring isomers ofdiarylethene 4 by DSC.

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values for L and l were found to be 10 and 0.3 μm, respectively,which correspond to the length and diameters of the needle-shaped crystals. The formation of such fine structures can beexplained by considering the temperature dependence of thepopulation of the nuclear formation of the crystals and the rateof the crystal growths. At low temperature, the population ofthe nuclear formation is larger than that of the highertemperature, while the rate is much faster at higher temper-ature. Therefore small crystal formation is observed at lowertemperature.Next we measured the activation energy to form the needle-

shaped crystals of 4c below the eutectic temperature. Theactivation energy to form the lying crystals above the eutectictemperature was not measured because dramatic CA changeswere not observed. We followed the method reported by Tsujiiet al. The activation energy to form a fractal surface of analkylketene dimer or a triglyceride was obtained by measuringthe periods required for the CA to reach 150° at differentstorage temperatures, followed by application of the Arrheniusequation.50 In our system, the CAs of a water droplet increasedwith the growth of the needle-shaped crystals, and CA profilesat five different temperatures were obtained (Figure 7). Atlower temperatures, the CA increased slowly indicating slowercrystal growth. The obtained activation energy was 58 kJ/mol,which is much lower than that of diarylethene 3 (143 kJ/mol).36 Such low activation energy probably reflects the lowcrystallinity of 4, since the molecular structure has bulky 5-trimethylsilylthienyl groups. We would like to point out thatsuch low activation energy in bulky molecules is essentiallycorrelated to the easiness of phase transformation of crystallinestructure or change in molecular packing. Actually, we havefound that bulky molecules such as triacylglycerides, which havethree long alkyl chains on a triglycerol chain, essentially have

several metastable states in the crystalline phase.51−53 Also, wefound that the formation of the rough surface with excesssurface area can be achieved when the deformation energy froma crystalline phase to another one in the phase transformationbecomes equal to the excess surface energy.54 Roughlyspeaking, this is described as

ε γ=E V n S(1/2) 2(2)

where E and ε are the Young modulus and deformation ofelastic body (now, a crystalline phase), respectively, V and S arethe volume of a minimum surface structure (a needle or a flake)and the surface area of a newly formed surface, respectively, n isthe number of new surfaces and γ is the surface energy density(surface tension). Here, it should be noted that E and ε reflectthe interaction between molecules and the change in themolecular packing, respectively, while γ is the molecularstructure which appears in the gas−solid interface. Thereforethe fibril formation of 4c could proceed below the eutectictemperature. Above it, the surface was so soft that needle-shaped crystals could not keep standing during their growingprocess.All the rough surfaces, which were covered with needle-

shaped crystals of 4c (Figures 2, 3, and 5), reverted to anoriginal surface covered with cubic crystals of 4o by visible lightirradiation for 4 h followed by 11 h of storage at 141 °C in thedark (Figure S4 in the Supporting Information). The reversibleformation of the needle-shaped crystals were monitored byXRD. The reversible intensity changes of a diffraction peak at16.9° (2θ) for the standing-needle-shaped 4c crystals,attributed to 012 diffraction of 4c, was observed (Figure S5in the Supporting Information) accompanied with topo-graphical changes (Figure S6 in the Supporting Information),while a diffraction peak at 17.1° (2θ), attributed to 40-4

Figure 2. SEM images of the microcrystalline surface of 4 stored at 141 °C for 20 min after UV irradiation (a, b) and water droplet on the surface(c). CA profile after UV irradiation by maintaining the eutectic temperature (d).

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diffraction of 4c, was observed for the lying 4c crystals (FiguresS7 and S8 in the Supporting Information). These diffractionpeaks (012 and 40-4) showed the orientation directions of thestanding needle-shaped 4c and the lying 4c crystals. To themelted film surface by visible light irradiation to themicrocrystalline surface having lying 4c crystals, UV light was

irradiated for 5 min and stored in the dark at 30 °C for 9 daysto obtain the film with standing needle-shaped crystals. TheSEM image of the surface (Figure S9a in the SupportingInformation) was similar to that in Figure 5a,b. The XRD peakat 16.9° was observed for the film having standing 4c crystals,reproducibly (Figure S9b in the Supporting Information).

Figure 3. SEM images of microcrystalline surface of 4 after storage at 135 °C for 15 min after UV irradiation (a, b), water droplet on surface (c), andCA profile of the surface of storing periods at the eutectic temperature (d).

Figure 4. Force curves obtained by AFM on single crystalline surfaces of 4o ((a) before and (b) after UV irradiation) and 2o ((c) before and (d)after UV irradiation) at 22 °C. Approach and release processes are depicted as pink and blue lines.

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■ CONCLUSIONIn conclusion, a superhydrophobic surface whose CA is largerthan 170° and whose SA is less than 1° was formed on themicrocrystalline surface of diarylethene 4. Such extraordinaryhydrophobic character is attributed to the surface structure,where submicroscale needle-shaped crystals are standing in adensely packed situation. A rough surface was produced belowthe eutectic temperature of the 4o and 4c mixture, indicatingthat the growth mechanism of the crystal is different from ourprevious results.31−38 This difference is considered due to the

bulky substituent of the diarylethene, which induces lowcrystallinity.55−58

■ ASSOCIATED CONTENT*S Supporting InformationEstimated crystal shapes from the crystal units of 4o and 4c,SEM image of a surface covered with cubic crystals of 4o,reversible intensity changes of a diffraction peak around 17°(2θ). This material is available free of charge via the Internet athttp://pubs.acs.org.

■ AUTHOR INFORMATIONCorresponding Author*Phone: +81-77-543-7462. Fax: +81-77-543-7483. E-mail:uchida@rins.ryukoku.ac.jp.

Figure 5. SEM images of the microcrystalline surface of 4 after 5 min of UV irradiation followed by storage at 30 °C in the dark for 9 days (a, b),water droplet on the surface (c), and CA profile of surface of storing periods at 30 °C (d).

Figure 6. Plots of log N(r) versus log r for cross-sectional trace curvesof the microcrystalline surface of 4 after 5 min of UV irradiationfollowed by storage at 30 °C in the dark for 9 days.

Figure 7. CA profiles of the water droplet on the microcrystallinesurface of 4 at each temperature during storage after UV irradiation.

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Present Addresses○Department of Chemistry, School of Science, the Universityof Tokyo, Hongo 7-3-1, Bunkyo-ku, Tokyo, 221-0033, Japan.#Department of Chemistry, Asahikawa Medical University, 2-1-1-1 Midorigaoka-higashi, Asahikawa, Hokkaido 078-8510,Japan.NotesThe authors declare no competing financial interest.

■ ACKNOWLEDGMENTSThis study was supported by Ryukoku University Science andTechnology Fund, Izumi Science and Technology Foundation,and Grants-in-Aids for Scientific Research on Priority Area‘‘New Frontiers in Photochromism (Grant No. 471)’’ from theMinistry of Education, Culture, Sports, Science, and Technol-ogy (MEXT), Japan.

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