J. Phys. Chem. B 2009, 113, 5769–5776
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Preparation of Silica Coatings Heavily Doped with Spiropyran Using Perhydropolysilazane as the Silica Source and Their Photochromic Properties Akihiro Yamano Department of Integrated Engineering, Graduate School of Engineering, Kansai UniVersity, 3-3-35 Yamate-cho, Suita, Osaka 564-8680, Japan
Hiromitsu Kozuka* Department of Chemistry and Materials Engineering, Faculty of Chemistry, Materials and Bioengineering, Kansai UniVersity, 3-3-35 Yamate-cho, Suita, Osaka 564-8680, Japan ReceiVed: NoVember 07, 2008; ReVised Manuscript ReceiVed: February 17, 2009
Silica coatings doped with spiropyran (SP) were prepared using xylene solutions of perhydropolysilazane (PHPS) as the silica source, where the SP-doped PHPS coatings were prepared by spin-coating and the PHPSto-silica conversion was achieved by exposing the coatings to the vapor from aqueous ammonia at room temperature. The films could be heavily doped with SP at SP/(SP + PHPS) mass ratio as high as 0.2. The as-deposited SP-doped PHPS films were transparent and light-yellow, which turned to red as the PHPS-tosilica conversion proceeded, where the absorbance at 500 nm increased. When the films were stored in air in the dark for 73 h after the exposure treatment, the absorbance at 500 nm further increased, where the film turned from red to dark red. The SP-doped silica coatings thus obtained showed reversible photochromic reaction, where the absorbance at 500 nm decreased and increased when the films were irradiated with visible and ultraviolet light, respectively. The pencil hardness of the films was higher than 9H at a load of 1 kg, while significant amount of SP was leached out when the films were soaked in xylene. 1. Introduction Spiropyran (SP) is a photochromic compound, which transforms reversibly between colorless spiro- and colored merocyanine-forms by irradiation with ultraviolet or visible light1-3 as shown in Figure 1a. SP-doped materials have been expected to be used as photosensitive materials,4 optical switches,4-6 and optical memories.4,5 Inorganic materials have properties superior to organic materials such as mechanical, thermal, and chemical stabilities and hence have great advantages as matrixes for organic molecules such as SP. The sol-gel technique has attracted much attention because it allows inorganic matrixes to be prepared at relatively low temperatures, which avoids the thermal decomposition of organic molecules. Several attempts have been made to dope inorganic matrixes with SP molecules via the sol-gel method as listed in Table 1.7-22 The inorganic matrixes prepared so far include silica gels,7,9,15 aluminosilicate gels,8,9 organosilicate gels,10-12,14 and organic polymer-silica hybrids,16 in the form of bulk7-10 and thin films.11,12,14-16 As far as thin films are concerned, high concentrations of colorants in thin films are required in order to visualize the change in color caused by photochromic reaction. However, SP has a low solubility in sols, where the amount of SP in sols is normally limited to 0.1-0.8 mass% of alkoxides23 (Table 1). In order to overcome this problem, Nakazumi et al.11 incorporated superfine SP particles 3-8 nm in size in high content (10 mass%) in silica gel coatings. Spagnoli et al.14 also embedded 100-500 nm SP nanocrystals in silica gel films, where the SP/(SP + alkoxide) mass ratio was 0.1. Mo et al.16 prepared silica-polymethylmethacryrate (PMMA) hybrid gel films doped with SP using a 10 mass% * To whom correspondence should be addressed.
solution containing TEOS. However, the SP/(SP + alkoxide) mass ratio is unclear, and the films may not be optically transparent, judging from the background of the optical absorption spectra. Herrero et al.10 prepared organosilicate gels doped with SP from TEOS and 3-glycidoxypropyltrimethoxysilane (Si(OCH3)3(C6H11O2), GLYMO) solutions, where the SP/(SP + alkoxide) mass ratio was about 0.11; the high content of SP may be allowed because of the hydrophobic nature of GLYMOderived matrix. In addition to the demands of the heavy doping, heattreatment is generally needed to densify and harden the sol-gelderived films. Nakazumi et al.11 heated the gel films at 100 °C for 15 min, and Tagaya et al.12 heated the films at 80 °C for 20 min. Previously, the authors’ group24-26 found a new route to obtain silica coatings from perhydropolysilazane (PHPS), where PHPS is an inorganic polymer consisting of Si-N bonds with Si-H terminal groups as shown in Figure 1b.27 PHPS coatings were prepared from xylene solutions of PHPS by spin-coating, and the coatings thus obtained were found to be converted into silica coatings via exposure to the vapor from aqueous ammonia at room temperature. Bauer et al.28 later reported that the PHPSto-silica conversion needs the moisture-containing atmosphere as well as ammonia or amine as catalysts. The present authors’ group also demonstrated that the PHPS-derived silica coatings have higher hardness and higher durability in hot water than that of the alkoxide-derived silica gel films.24-26 Because PHPS and xylene are hydrophobic, the hydrophobic SP molecules are expected to be incorporated in PHPS-derived coatings in high contents. It is also expected that such PHPSderived silica coatings heavily doped with SP could have high hardness without heat-treatment. The present study aims at the
10.1021/jp809831c CCC: $40.75 2009 American Chemical Society Published on Web 04/06/2009
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Figure 1. Transformation of (a) a spiropyran (1,3,3-trimethylindolino-6′-nitrobenzopyrylospiran) molecule from spiro- to merocyanine-forms and vice versa by irradiation with ultraviolet or visible light, and of (b) perhydropolysilazane to silica via exposure to the vapor from aqueous ammonia.
TABLE 1: Gels Doped with SP Reported in Literature authors Levy et al.7 Preston et al.8 Nogami et al.9
materials
Herrero et al.10
silica aluminosilicate aluminosilicate silica organosilicate
Nakazumi et al.11
organosilicate
Tagaya et al.12
organosilicate
Bae et al.13
fluorinated mesoporous organosilicate
Spagnoli et al.14
organosilicate
Leaustic et al.15
silica
Mo et al.16
silica-polymethylmethacrylate hybrid
SP/(SP + alkoxide) mass ratio
source Bulk Gels Si(OCH3)4 (C4H9O)2Al-O-Si(OC2H5)3 Al(OC4H9)3, Si(OC2H5)4 Si(OCH3)4 Si(OC2H5)4, Si(OCH3)3(C6H11O2) (3-glycidoxypropyltrimethoxysilane) Thin Films (CH3O)3Si-[OSi(OCH3)2]nOCH3(CH3O)3Si-[OSi(OCH3)2]nOCH3 (n ) 1-4) C2H5Si(OC2H5)3
3.3 × 10-5 ca. 2.0 × 10-4 1.4 × 10-4 2.1 × 10-4 ca. 0.11
0.1 4 × 10-2
heptadecafluorodecyl)trimethoxysilane, Si(OCH3)4 Si(OCH3)4, CH3(OCH3)3 Si(OCH3)4
films were placed in 1.5 × 10-3 M SP solution
Si(OC2H5)4
10% solution
preparation of silica coatings heavily doped with SP molecules using PHPS as the silica source, where the hardness, chemical durability, and photochromic properties are also evaluated. It is reported that the SP molecules are converted into merocyanine (MC) forms in the polar environment.1,29-38 When the PHPS-to-silica conversion occurs, the polarity of the environment of SP molecules increases. Therefore, SP molecules are expected to be transformed into MC forms during the PHPSto-silica conversion, which was also studied and examined in the present work.
remarks
ca. 0.1 4.4 × 10-2
superfine SP particles (3-8 nm) heated at 100 °C for 15 min sulfonated spiropyran (SP-SO3-) heated at 80 °C for 20 min SP incorporated by impregnation SP nanocrystals (100-500 nm) cationic SP
2. Experimental Section 1. Preparation. A xylene solution of PHPS (20%, NN11020, AZ Electronic Materials, Tokyo, Japan) was used as the silica source, and 1,3,3-trimethylindolino-6′-nitrobenzopyrylospiran (Tokyo Kasei Kogyo, Tokyo, Japan, Figure 1a) was used as the photochromic dye. A solution of SP/(SP + PHPS) mass ratio of 0.2 was prepared by the following procedure. Yellow SP powders were added to the xylene solution of PHPS, followed by stirring for 1 h in a glass bottle filled with N2 gas, resulting in a transparent yellow solution, which served as the
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Figure 2. Configuration of the sample for exposure to the vapor from the aqueous ammonia at room temperature.
coating solution. SP-doped films were deposited on silica glass and Si(100) substrates (20 mm × 40 mm) by spin-coating at 1500 rpm. Immediately after deposition, the SP-doped films were exposed to the vapor from aqueous ammonia at room temperature for 0-24 h in the manner illustrated in Figure 2; the as-deposited films were kept standing vertically over 20 g of 10% aqueous ammonia using a glass support, where the lid of the bottle had three holes 5 mm in diameter so that the condensation of aqueous ammonia was avoided on the surface of the sample. Finally, SP-doped films about 0.7 µm in thickness were obtained. For comparison, SP-doped films were prepared also from an alkoxide solution by the following procedure. Tetramethylorthosilicate, (Si(OCH3)4, TMOS, Tokyo Kasei Kogyo, Tokyo, Japan), ethanol (C2H5OH, Wako Pure Chemical Industries, Osaka, Japan), 1 M nitric acid (HNO3, Wako Pure Chemical Industries), and ion-exchanged water were used as the raw materials. First, a solution of mole ratio TMOS:H2O:CH3OH: HNO3 ) 1:2:5:0.01 was prepared by adding a solution of water, nitric acid, and half of the prescribed amount of ethanol to a solution consisting of TMOS and the other half of the prescribed amount of ethanol under magnetic stirring. The SP powders were added in the solution, followed by stirring for 90 min, resulting in a solution of SP/(SP + TMOS) mass ratio of 0.009 which was served as the coating solution. Silica gel films doped with SP were deposited on silica glass and Si(100) substrates (20 mm × 40 mm) by spin-coating at 1500 rpm. The gel films were heated at 100 °C for 10 min in an electric furnace (Model KDF005S, Denken Co., Kyoto, Japan), resulting in SP-doped films about 0.8 µm in thickness. 2. Measurement and Observation. The thickness of the coatings was measured using a contact probe surface profilometer (SE-3400, Kosaka Laboratory, Tokyo, Japan). A part of the films was scraped off with a surgical knife just after spincoating, and the level difference thus created on the substrates was measured by the profilometer. The ultraviolet-visible (UV-vis) and infrared (IR) absorption spectra were measured on the coatings deposited on silica glass and Si(100) substrates, respectively. A spectrophotometer (Model V-570, Jasco, Tokyo, Japan) and a Fourier transform infrared spectrometer (Model FT/IR-410, Jasco, Tokyo, Japan) were used for the UV-vis and IR absorption spectra measurement, where bare silica glass and Si(100) substrates were used as the reference, respectively. X-ray diffraction (XRD) measurement was conducted on the coatings by ordinary 2θ/θ mode using an X-ray diffractometer (Model Rint 2550V, Rigaku, Tokyo, Japan) with Cu kR radiation operated at 40 kV and 300 mA. The microstructure of the films was observed using a field emission scanning electron microscope (FE-SEM) (Model JSM-
Figure 3. Dependence of the thickness of the PHPS-derived, SP-doped films on the time of the exposure treatment.
6500F, JEOL, Tokyo, Japan), where osmium was deposited by an osmium coater (Model Neoc-ST, Meiwafosis. Co., Osaka, Japan). The pencil hardness was measured on the coatings using a pencil hardness tester (Model 553-M1, Yasuda Seiki, Hyogo, Japan), where a 1 kg load was applied on the sample. The hardness was defined as the highest pencil grade that did not delaminate the films from the substrate completely. The chemical durability in xylene was evaluated for coatings deposited on silica glass substrates, where the samples were kept standing vertically in 110 g of xylene in a glass bottle at room temperature for 24 h. The UV-vis absorption spectra and the thickness were measured before and after the soaking. The photochromic behavior was evaluated for the coatings deposited on silica glass substrates by irradiating the samples with white light or UV light. The irradiation of white light was conducted using a 100 W halogen lamp (Model JCR12V100WB, Ushio, Tokyo, Japan) with a light guide power supply for microscope (Model LG-PS2, Olympus, Tokyo, Japan). The halogen lamp was equipped with a dichroic ellipsoidal mirror that transmits IR light and reflects vis light. The irradiation of UV light was conducted using UV irradiating equipment (Model EF-280C/J, Spectronics, New York, NY) that radiates 245 nm UV light from two 8 W UV tubes (Model BLE-8T254, Spectronics). UV-vis absorption spectra were measured after the irradiation of vis or UV light. 3. Results 1. Solubility of SP in the PHPS and TMOS Solutions. The SP powders dissolved easily in the xylene solution of PHPS even at an SP/(SP + PHPS) mass ratio of 0.2, resulting in a transparent yellow solution. On the other hand, the SP powders were soluble and insoluble in the TMOS solution at SP/ (SP+TMOS) mass ratios of 0.00955 and 0.0123, respectively. 2. UV-vis and IR Absorption Spectra of the Films. SPdoped PHPS films were exposed to the vapor from the aqueous ammonia for various periods of time. The thickness of the coatings was almost constant at about 0.7 µm during the exposure treatment as seen in Figure 3. Figure 4 shows the IR absorption spectra of the coatings measured during the exposure treatment, where reduction was observed in the absorption peaks assigned to the Si-H stretching (2160-2175, 2240 cm-1), the N-H bending in Si-NH-Si (1182 cm-1), the Si-H deformation (950 cm-1), and the Si-H wagging (830 cm-1) vibrations. On the other hand, the bands assigned to the Si-O-Si asymmetric stretching TO mode (1075 cm-1) and the Si-O-Si
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Figure 6. UV-vis absorption spectra of the SP films prepared from the xylene solution of SP, measured before and after the exposure to the vapor from the aqueous ammonia for 18 h.
Figure 4. IR absorption spectra of the PHPS-derived, SP-doped films exposed to the vapor from the aqueous ammonia for various periods of time. The films were deposited on Si(100) substrates.
Figure 7. Change in the UV-vis absorption spectra of the PHPSderived, SP-doped films during storage in air in the dark. The films were subjected to the exposure treatment for 18 h, followed by storage in air in the dark for 73 h.
Figure 5. UV-vis absorption spectra of the PHPS-derived, SP-doped films exposed to the vapor from the aqueous ammonia for various periods of time. The films were deposited on silica glass substrates.
rocking mode (450 cm-1) emerged. Figure 5 shows the UV-vis absorption spectra measured during the exposure treatment. Absorption peaks were initially observed at about 270 and 350 nm, where the films were transparent and light-yellow. The absorbance at 270 nm decreased and that at 500 nm increased with increasing exposure time, where the color of the films turned from light-yellow to red. SP films were prepared from the xylene solution of SP and exposed to the vapor from the aqueous ammonia for 18 h. The as-deposited SP film had the absorption peaks at 210 and 280 nm as seen in Figure 6. When the film was exposed to the vapor, no increase in the absorbance at 500 nm was observed unlike in the case of the PHPS-derived films.
SP-doped PHPS films were subjected to the exposure treatment for 18 h and then stored in air in the dark for 73 h. Figure 7 shows the change in the UV-vis absorption spectra of the films observed during storage in air in the dark. The asexposed film was red and had absorption peaks at 270 and 350 nm. During storage in air in the dark, the absorbance at 500 nm increased, where the film turned from red to dark red. TMOS-derived, SP-doped films were deposited on silica glass substrates and heated at 100 °C for 10 min, followed by storage in air in the dark for 72 h. The heat-treated film was lightyellow and had an absorption peak at about 310 nm as seen in Figure 8. When stored in the dark, the film became slightly intense in color, and the absorbance at 320-600 nm increased (Figure 8). 3. XRD Patterns of the Films. Figure 9 shows the XRD patterns of the PHPS-derived, SP-doped film, the SP film, and the SP powders. Both films were deposited on silica glass substrates. The PHPS-derived, SP-doped film was obtained via the exposure treatment for 18 h, followed by storage in air in darkness for 181 days. The SP film was prepared from the xylene solution of SP and stored in air in the dark for 1 day. The SP powders were the SP reagent ground in a mortar. A halo peak was observed at 2θ ) 21° in both films, which is due to the silica glass substrates. The SP film and the SP powders had a sharp diffraction peak at 2θ ) 16.5°, which was not observed in the PHPS-derived, SP-doped film.
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Figure 8. Change in the UV-vis absorption spectra of the TMOSderived, SP-doped films during storage in air in the dark. The films were heated at 100 °C for 10 min, followed by storage in air in the dark for 72 h.
Figure 9. XRD patterns of the PHPS-derived, SP-doped film and the SP film deposited on silica glass substrates. The PHPS-derived, SPdoped film was exposed to the vapor from the aqueous ammonia for 18 h, followed by storage in air for 181 days. The SP film was prepared from the xylene solution of SP, followed by storage in air for 1 day. The SP powders were the SP reagent ground in a mortar.
4. Microstructure of the Films. Figure 10 shows the SEM pictures of the surface and cross-section of the PHPS-derived, SP-doped film that was stored in air in the dark for 73 h after being subjected to the exposure treatment for 18 h. The film was dense and composed of spherical grains about 20 nm in diameter. 5. Pencil Hardness of the Films. Table 2 shows the pencil hardness of the films derived from the PHPS and TMOS solutions. The pencil hardness of the as-deposited, TMOSderived films was lower than 6B, but the hardness increased
Figure 10. SEM pictures of the surface and cross-section of the PHPSderived, SP-doped film that was subjected to the exposure treatment for 18 h, followed by storage in air in darkness for 73 h.
over 9H when the films were stored in air for longer time or heated at 100 °C. The PHPS-derived films, on the other hand, had the hardness over 9H even immediately after the exposure treatment. 6. Dissolution of SP Molecules from the Films in Xylene. PHPS-derived films were exposed to the vapor from the aqueous ammonia for 18 h, followed by storage in air in the dark for 73 h. TMOS-derived films were heated at 100 °C for 10 min, followed by storage in air in darkness for 1 h. These films deposited on silica glass substrates were soaked in xylene at room temperature for 24 h, in order to evaluate the degree of the dissolution of SP molecules from the films.The reduction in thickness occurring during the soaking was 0.95 and 4.7%
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TABLE 2: Pencil Hardness of the Films Derived from the PHPS and TMOS Solutions time silica suspending over heating at substrate 100 °C source NH3 aq. PHPS PHPS TMOS TMOS TMOS TMOS TMOS TMOS TMOS TMOS TMOS TMOS
SiO2 Si(100) SiO2 SiO2 SiO2 SiO2 Si(100) Si(100) Si(100) Si(100) Si(100) Si(100)
18 h 18 h -
10 min 10 min 10 min 10 min 10 min
storage in air
pencil hardness
