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Cite This: Langmuir 2018, 34, 4217−4223
Study on the Mechanism of Diarylethene Crystal Growth by In Situ Microscopy and the Crystal Growth Controlled by an Aluminum Plasmonic Chip Taiga Kadoyama,† Ryo Nishimura,‡ Mana Toma,† Kingo Uchida,‡ and Keiko Tawa*,† †
School of Science and Technology, Kwansei Gakuin University, 2-1 Gakuen, Sanda, 679-1337 Hyogo, Japan Department of Materials Chemistry, Faculty of Science and Technology, Ryukoku University, Seta, Otsu 520-2194, Japan
‡
S Supporting Information *
ABSTRACT: The microcrystalline film of an open-ring isomer (1o) of diarylethene 1 was prepared on an Al plasmonic chip with a grating structure. Photoisomerization from 1o to the closed-ring isomer (1c) and growth of needleshaped crystals in 1c were observed in situ under an uprightinverted microscope. In the center part of the film, crystal growth of needle-shaped-crystal of 1c was observed upon UV irradiation from the top side, but not upon UV irradiation from the bottom side. However, crystallization occurred at the edge of the film upon UV irradiation from the bottom side. It was suggested that crystal growth of 1c required a high mobility of 1c near the film surface. Furthermore, the existence of 1o platform is also found to be required for alignment of 1c molecules by the results under the irradiation from the bottom and top sides. With the Al plasmonic chip, the conversion rate from 1o to 1c was larger inside the grating by the plasmonic enhanced field. Therefore, when the attenuated UV light was irradiated to the film edge with high mobility of 1c from the bottom side, the conversion rate was more than 60%, and the needle-shaped crystals of 1c were observed only inside the grating area. Crystal growth was controlled by the conversion rate of 1c promoted inside the grating. From the above, the larger conversion rate of 1c more than 60%, a high mobility of 1c near the film surface or edge, and the existence of the 1o platform for alignment of 1c molecules, are considered to be required for crystal growth in 1c.
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INTRODUCTION Diarylethenes (DAEs) are organic photochromic molecules, and they have many kinds of derivatives.1,2 In DAEs, absorption spectra depend on the structure, and DAE molecules with fluorescence characteristics are also widely known.3,4 In general, an open-ring isomer converts to the closed-ring isomer upon irradiation with ultraviolet (UV) light, and the closed-ring isomer reverts to the open-ring isomer with visible light. DAEs are expected to be used in applications of optical-memory and optical-switches because of their thermal irreversibility and fatigue resistance.5−15 1,2-Bis(2-methoxy-5-trimethylsilylthien3-yl)perfluorocyclopentene 1o is one of the promising DAEs (Scheme 1a), with an open-ring isomer (1o) and a closed-ring isomer (1c) showing an absorption peak at 600 nm (Scheme 1b).16,17 1o and 1c have cubic-shaped-crystals and needleshaped crystals, respectively, and the eutectic phase of 1o and 1c is controlled by photoirradiation.16,17 The melting temperature of the microcrystalline film composed of 1o and 1c decreases to 30 °C during the conversion from 1o to 1c under UV irradiation,16 and then the molecular alignment of 1c is promoted and microscale needle-shaped crystals of 1c appear. The size of the crystals changes by storage temperature, and a larger crystals generated at higher temperatures.17 The crystalline properties of 1c have been applied to control of © 2018 American Chemical Society
the water repellency and a crystal surface structure was formed such as lotus leaves effect rolling a water droplet and rose petal effect pinning a water droplet.16−19 However, most of the crystal structures have been studied by scanning electron microscopy (SEM) and the mechanism of macroscopic crystallization assisted by photoisomerization is not completely understood at molecular levels. A plasmonic chip has a wavelength-scale periodic structure covered with a thin metal film, and it can provide an enhanced electric field based on the grating-coupled surface plasmon resonance (GC-SPR)20−23 at the metal surface by coupling incident light. The resonance condition of GC-SPR is described as24,25 k spp = k phx + mk g k ph
(m = ±1, 2, ...)
εdεm 2π = k ph sin θ + m · εd + εm Λ
(1)
(2)
where kspp, kg, and kphx indicate the wavenumber vectors of the surface plasmon resonance, grating, and incident light of the x Received: January 19, 2018 Revised: March 12, 2018 Published: March 20, 2018 4217
DOI: 10.1021/acs.langmuir.8b00200 Langmuir 2018, 34, 4217−4223
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Scheme 1. (a) Photoisomerization of Diarylethene 1a; (b) Absorption Spectra of 1o and 1c in Hexane Solution (1.9 × 10−5 mol/L)b
a
Value in parentheses indicates photochromic reaction quantum yields. bOpen-ring isomer 1o (solid line), closed-ring isomer 1c (broken line), and the photostationary state (dotted line) under irradiation with UV light (254 nm). Reproduced from Uchida, K.; Izumi, N.; Sukata, S.; Kojima, Y.; Nakamura, S.; Irie, M. Angew. Chem. Int. Ed. 2006, 45, 6470−6473 with permission.
component, and kph, εd, εm, θ, and Λ indicate wavenumber of incident light, the complex dielectric constants for the dielectric media and metal, the incident angle, and the pitch of the grating, respectively. As found from eq 2, a small resonance angle can be adjusted according to the periodic structure. Under the microscope, the incident light can be coupled with the surface plasmon on the metal grating surface through an objective with a small numerical aperture corresponding to a small illumination angle. Therefore, the enhanced electric field produced by GC-SPR can be fundamentally applied to the microscopic observation. In our laboratory, plasmonic chips coated with thin silver films were used for fluorescence measurements, and were applied to sensitive biosensing26,27 and bioimaging.27−29 In cell imaging by fluorescence microscopy, images of live breast cancer cells were obtained with a fluorescence 10-fold brighter than on a glass substrate.28,29 In this study, an Al plasmonic chip is available to enhance the electric field in the wavelength range of UV light,30 which is required for photoisomerization from 1o to 1c. Therefore, 1o film was prepared on Al plasmonic chip. Under an uprightinverted microscope, epi-illumination light, transmitted light, and confined light by the plasmonic field were used for local photoisomerization of 1o to 1c and observation of their processes, and optical images were in situ taken with an electron multiplying-charge coupled device (EM-CCD) camera under the irradiation for photoisomerization. The process of needle-shaped crystals depending on the direction of UV irradiation (from the bottom or top side) and the film position (center or edge) was studied, and the mechanism of the crystal growth of 1c is discussed. Furthermore, control of photoisomerization and crystallization was studied by the plasmonic chip.
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adhesion layer and Al by radiofrequency (rf)-sputtering. The thickness of the Al layer was 49 ± 6 nm, measured by UV−vis absorption spectroscopy and calibration curve created by our lab (not shown here). The grating pitch and groove depth were evaluated as 320 ± 6 nm and 34 ± 5 nm as measured by atomic force microscopy (AFM) as shown in Figure 1. Surface roughness was 4 nm for the chip inside and outside the grating.
Figure 1. AFM image of an Al plasmonic chip.
Preparation of Diarylethene 1 Films. The synthesis of diarylethene 1 was described in a previous paper.16 Diarylethene 1 was dissolved in ethanol as 0.08 M, and the solution was sonicated for 10 min under heating condition. Five microliters of the solution was dropped on the Al plasmonic chip and the ethanol was evaporated. The cast film on the plasmonic chip was overlaid with a florinated glass substrate in order to make a flat film, and they were incubated for 1 h at 115 °C in the oven. Next, this film was stored at room temperature, and the fluorinated glass plate was removed. All procedures were performed at room temperature in the dark. The film thickness of diarylethene 1 was evaluated as 2.8 ± 1.0 μm by a micrometer. Microspectroscopic Imaging. With an upright-inverted microscope (custom-made, Olympus), the isomerization of diarylethene 1 was in situ observed under the irradiation (Figure 2).31 Optical images and reflection microspectroscopic images were obtained with the EMCCD camera (Luca R, Ander) via a switchable spectrometer (CLP-50, Andar). For microscopic imaging, a halogen lamp and a long-pass filter with a 500 nm-blocking edge were used for observation. Xe and Hg lamps were used for photoisomerization under the irradiation from the bottom and top side, respectively, combined with a bandpass filter for 360−370 nm and a long-pass filter with a 500 nm-blocking edge for UV and visible light, respectively. The experimental conditions
EXPERIMENTAL SECTION
Fabrication of the Plasmonic Chip. UV curable resin (PAK02-A, Toyo Gosei) was dropped on a glass substrate that was modified with silane coupling reagent (KBM-503, Shinetsu Chemical) in advance, and was overlaid with a glass mold with a one-dimensional grating structure. They were exposed to UV light for 80 s. After removing the mold from the glass substrate, the replica was coated with Ti as 4218
DOI: 10.1021/acs.langmuir.8b00200 Langmuir 2018, 34, 4217−4223
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the bottom side for 10 min (1.1 J/cm2). Immediately after UV irradiation, the irradiation area was blackened (Figure 5c). Several hours after UV irradiation, needle-shaped crystals were observed even with UV irradiation from the bottom side (inset of Figure 5d) regardless of whether the area was inside or outside the grating. After several days, needle-shaped crystals grew (Figure 5e). Crystallization was found to be also induced in the film edge by UV irradiation from the bottom. This might be attributed to the fact that the mobility of 1c not only near the surface but also at the edge was higher than that in the center part of the film. However, if the film thickness of 1o was too thin, the crystal was not formed even on the edge (data not shown here). Furthermore, 1o film was exposed to UV light for 5 min from the bottom side through a 100x objective in the setup of type IV described in Table 1. Immediately after UV irradiation, the irradiated area was blackened (Figure 6c). As expected, needleshaped crystals were not observed regardless of whether the area was inside or outside the grating even after 20 h incubation post UV irradiation (Figure 6d). After the incubation, UV light was again irradiated to the film for 3 min from the top side through 40x objective, in which irradiated area from the top (1.6 × 10−3 cm2) was larger than that from the bottom (9.3 × 10−4 cm2). At 7 h after UV irradiation from the top, needleshaped crystals were observed in the boundary region between the irradiation spots from the top and bottom sides (Figure 6g). In the boundary region, 1o isomerized to 1c on the film surface exposed to UV light from the top side, whereas the bottom of the film 1o was considered to remain unexposed to UV light, i.e., this part corresponds to the 1o platform. By contrast, needle-shaped crystals of 1c were not observed in the center part that completely converted to 1c by the UV irradiation from both sides (Figure 6g). According to these results, the existence of a 1o platform is required for alignment of 1c molecules and growth of crystals. Conversion Rate from 1o to 1c and Promotion by the Plasmon Field on the Al Plasmonic Chip. Figure 7 shows optical image of 1o film after exposure to UV light from the bottom for 5 min at 1.7 × 10−3 W/cm2 (= 0.51 J/cm2). The 1c molecules in the black area inside and outside grating were taken out with a syringe and were dissolved in chloroform, individually. The red curves depicted in Figure 8a,b show UV− vis absorption spectra in chloroform solution of 1c inside and outside the grating, respectively. The blue curve in Figure 8a,b correspond to the spectra for chloroform solution of 1o completely reverted from 1c under the yellow light. Finally, the orange curves in Figure 8a,b correspond to the spectra for solution of 1c fully converted from 1o under the UV light. The conversion rate was evaluated from the ratio of absorptions for 1c in the film state (red curves) to that for 1c in the solution (blue curves). The values of conversion rate were summarized in Figure 9 and they became larger with UV irradiation time
Figure 2. Schematic of an upright-inverted microscope. including objective, light source, UV light intensity, and irradiation area are summarized in Table 1.
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RESULTS AND DISCUSSION Mobility of 1c in the Film Surface and Growth of Needle-Shaped Crystals in 1c Film. Crystal growth of 1c assisted with photoisomerization to 1c was studied in the center part of the film on the Al plasmonic chip. The needle-shaped crystals of 1c were found to depend on the UV irradiation direction from the bottom or top of the 1o films for which the individual experimental setup for microscopy was type I and II (see Table 1). The grating structure and flat metal correspond to the upper half side (darker part) and lower half side (brighter part) in Figures 3−6, and 10, respectively. As shown in Figures 3c and 4c, UV light was irradiated onto the 1o film for 5 min, and the irradiated area changed to black. Thus, the changes of monochromatic gradation correspond to the absorption in the visible range, and it was considered that 1o converted to 1c.31 Several hours after UV irradiation, needleshaped crystals of 1c were not observed under UV irradiation from the bottom side regardless of whether the area was inside or outside the grating (Figure 3d), and even several days after UV irradiation. However, needle-shaped crystals of 1c were observed after the UV irradiation from the top side (Figure 4d). As a result, it was considered that needle-shaped crystals of 1c were not formed without mobility of 1c molecules on the film surface. The 1o in the surface may suppress the molecular alignment of 1c facing the substrate in the UV irradiation from the bottom side. The dependence of crystallization in the film on the exposure position was examined in the setup type III described in Table 1. Figure 5b−e shows the results for isomerization and crystallization. UV light was irradiated to the film edge from
Table 1. Experimental Conditions (Objective, Light Source, UV Power and Irradiation Area) inverted side objective I II III IV V
PLFLN100x PLFLN100x PLFLN100x LUCPLFLN20x
light source Xe Xe Xe Xe
upright side
irradiation power/Wcm −3
1.8 × 10 1.8 × 10−3 1.8 × 10−3 1.7 × 10−3
−2
spot area/cm
2
−4
9.3 × 10 9.3 × 10−4 9.3 × 10−4 1.1 × 10−2 4219
objective
light source
irradiation power/Wcm−2
spot area/cm2
LUCPLFLN40x LUCPLFLN40x -
Hg Hg -
2.9 × 10−1 2.9 × 10−1 -
1.6 × 10−3 1.6 × 10−3 -
DOI: 10.1021/acs.langmuir.8b00200 Langmuir 2018, 34, 4217−4223
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Figure 3. Schematic of UV irradiation (a), and optical images of diarylethene 1 film on the Al plasmonic chip: before UV irradiation (b), immediately after 5 min UV irradiation from the bottom side (c), several hours after UV irradiation (d), and several days after UV irradiation (e).
Figure 4. Schematic of UV irradiation (a), and optical images of diarylethene 1 film on the Al plasmonic chip: before UV irradiation (b), immediately after 5 min UV irradiation from the top side (c), and several hours after UV irradiation (d).
Figure 5. Schematic of UV irradiation (a), and optical images of diarylethene 1 film in the case of UV irradiation at the film edge: before UV irradiation (red curve corresponds to the film edge) (b), immediately after 10 min UV irradiation from the bottom side (c), several hours after UV irradiation (d), and several days after UV irradiation (e). Inset of (d): enlarged view for a blue circle area.
Figure 6. Schematic of UV irradiation (UV light was irradiated in order of first and second), and optical images of diarylethene 1 film in the case of UV irradiation at the center area of a film: before UV irradiation (b), immediately after 10 min UV irradiation from the bottom side (c), at 20 h after UV irradiation (d), immediately after 3 min UV irradiation from the top side (e), and at 7 h after UV irradiation from the top side (f). (g) indicates changing contrasts of (f).
Control of Crystallization with the Plasmonic Effect. For control of crystallization with the plasmonic effect, the film edge was exposed to UV light for 5 min from the bottom side through 20x objective (0.51 J/cm2). The exposed area changed to black (Figure 10c) (the setup was type V in Table 1), and the crystal growth of 1c was observed inside the grating 4 h later (Figure 10d). After 23 h, growth of needle-shaped crystals was observed at inside the grating, but crystal growth of 1c had not occurred outside the grating (Figure 10e). This result was
(the setup was type V in Table 1). The difference in conversion rate was observed between inside and outside the grating. The conversion rate from 1o to 1c was accelerated by a factor of 2 because of the plasmonic enhanced field under the irradiation for 1 min, i.e., at 0.10 J/cm2. In this plasmonic chip, the enhanced electric field in the wavelength region of UV light was indicated by an enhanced fluorescent image in this plasmonic chip (see SI, Figures S1 and S2). 4220
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Figure 7. Optical image of 1o film inside and outside grating after exposure to UV light from the bottom side for 5 min at 1.7 × 10−3 W/ cm2 (= 0.51 J/cm2). Black squares correspond to the grating area and a black circle shows the isomerization to 1c within UV irradiation spot. Figure 9. Conversion rate plotted against the irradiation energy of UV light inside and outside grating. The rhombus and square symbols correspond to the value inside grating and outside grating, respectively.
inconsistent with the results of Figure 5. Creation of crystal nucleus depends on the conversion rate. In 1c with a glass temperature less than a room temperature, a melting point (Tm) for mixture of 1o and 1c is close to a room temperature at the 25% of conversion rate to 1c.16 Therefore, 1c molecules are considered to cooperatively align during the conversion proceeded over the lowest Tm, i.e., the conversion rate required for creation of microcrystal nucleus is considered to be at least over 25% with the lowest Tm.16 The exposure dose of the UV light in Figure 10 (0.51 J/cm2) was lower than that in Figure 5 (1.1 J/cm2). The electric field enhanced by the surface plasmon inside the grating promoted conversion rate more than 60% at 0.51 J/cm2 and the conversion rate for outside of grating was less than 60% as shown in Figure 9. As a result, crystal growths was observed only inside grating. Under the exposure dose of 0.10 J/cm2, the conversion rate was less than 60% even inside grating, and the crystals were not observed anywhere after incubation. On the other hand, under an exposure dose of 0.64 J/cm2, the conversion rate was over 60% on both sides, and crystals were also observed in both. In summary, it is
considered to be important for crystallization that the conversion rate of 1c is larger than 60%, the mobility of 1c is high at the film edge or the film surface of 1c on the 1o platform, and the film thickness is not too thin. Crystallization can be controlled by the conversion rate of 1c promoted inside the grating in which the electric field was enhanced with the Al plasmonic chip.
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CONCLUSIONS Growth of needle-shaped crystals in the films of 1o and 1c was observed in situ with an upright−inverted microscope. Crystal growth at the center part of the 1c film occurred after conversion to 1c by irradiation of UV light from the top side, but not from the bottom side. However, at the film edge, needle-shaped crystals were observed after UV irradiation even from the bottom side. The crystallization of 1c was found to depend on the UV irradiation direction (bottom or surface) and the position (center or edge) photoisomerized in the film.
Figure 8. 1c molecules in the black area as shown in Figure 7 inside and outside grating were taken out with a syringe and were dissolved in chloroform, individually: (a) UV−vis spectra inside grating and (b) outside grating. Red curves show spectra in chloroform solution of 1c inside and outside the grating, respectively. The blue curves correspond to the spectra for chloroform solution of 1o completely reverted from 1c under the yellow light. Finally, the yellow curves correspond to the spectra for solution of 1c fully converted from 1o under the UV light. 4221
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Figure 10. Schematic of UV irradiation (a), and optical images of diarylethene 1 film under the UV irradiation at the film edge; before UV irradiation (red curve corresponds to the film edge) (b), immediately after 5 min UV irradiation from the bottom side (c), at 4 h after UV irradiation (d), and at 23 h after UV irradiation (e).
The high mobility of 1c near the film surface or edge and the existence of 1o platform for alignment of 1c molecules are considered to be essential for crystal growth of 1c. On the other hand, conversion rate from 1o to 1c was accelerated by the plasmonic enhanced field by a factor of 2. Under UV irradiation with lower intensity, needle-shaped crystals of 1c were observed only inside the grating at the film edge by irradiation from the bottom side. At the film edge, conversion to 1c inside the grating was higher than that outside the grating because of the plasmonic enhanced field with the Al plasmonic chip. Therefore, crystal growth of 1c was promoted at the film edge within the grating. Crystallization can be controlled by the conversion rate of 1c promoted inside the grating.
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.langmuir.8b00200. Enhanced fluorescence image of fluorescent molecules and surface plasmon resonance angle in the plasmonic chip prepared in this study (PDF)
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
Keiko Tawa: 0000-0002-5736-1187 Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS T.K, M.T., and K.T. thank Toyo Gosei Co., for providing the photocurable resin. This work was supported by JSPS KAKENHI a Grant-in-aid for Scientific Research on Innovative Areas “Photosynergentics” (JP15H01100, JP26107012), Japan.
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