Characterization of the Spironaphthooxazine Doped Photochromic

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J. Phys. Chem. C 2008, 112, 1140-1145

Characterization of the Spironaphthooxazine Doped Photochromic Glass: The Effect of Matrix Polarity and Pore Size Chang Woo Kim,† Sun Wha Oh,‡ Young Hwan Kim,† Hyun Gil Cha,† and Young Soo Kang*,† Department of Chemistry, Pukyong National UniVersity, Busan 608-737, Korea, and Basic Science Research Institute, Pukyong National UniVersity, 599-1, Daeyeon 3-dong, Nam-gu, Busan 608-737, Korea ReceiVed: May 10, 2007; In Final Form: October 2, 2007

The photochromic gel thin films on the glass substrate were prepared by doping photochromic dye, spironaphthooxazine, into the phenyltriethoxysilane (PhiTEOS), vinyltriethoxysilane (VTEOS), methyltriethoxysilane (MTEOS), and tetraethylorthosilicate (TEOS) gels. The λmax of the spironaphthooxazine doped into the gels was correlatively studied with the polarity of the microenvironments of the gels. The kinetics of the normal photochromism of decoloration among PhiTEOS, VTEOS, and MTEOS were studied by fitting biexponential constants of the equation. This was carried out by obtaining the decay curves of optical absorbance versus delay time after irradiation of wavelength selected UV-vis light. The decreasing order of the kinetic constant of the decoloration of spironaphthooxazine was observed as PhiTEOS > VTEOS > MTEOS. The results are interpreted by the increasing polarity as PhiTEOS < VTEOS < MTEOS and the decreasing pore size of the gel matrixes of the inverse order.

Introduction Novel recent progress in doping organic compounds into inorganic oxides using the sol-gel process opens a broad range of possibilities for the preparation of optical organic materials, whose properties can be tuned by selecting organic components as dopants with appropriate photoactivities, bioactivities, and chemical activities.1-10 As examples, organic and organometallic photorefractive compounds have been employed to probe the physicochemical structural changes during the sol-gel reactions.11-15 Laser action, photochromism, and nonlinear optical effects were introduced into silica or other metal oxide by doping with appropriate organic dyes. On the other hand, a system is said to be photochromic when it contains a reversible photochemical reaction of a single chemical species. Such a system can be prepared by doping organic photochromic materials in polymer matrixes and inorganic network by sol-gel process.16-18 There are two methods to yield metal oxide glasses doped with organic molecules.1,4 The first consists of putting a thin layer of sol solution followed by drying for a relatively short period, whereas bulk glasses are formed by the second method in which a solgel solution is slowly evaporated at ambient temperature to give monolithic glasses preventing crack formation. Furthermore, the inorganic matrixes such as silica are amorphous and have large isotropic refractive indexes and excellent optical transparency. This enables one to obtain new transparent solid-state materials by the sol-gel process. The porous glass (xerogel) obtained by this process is of particular interest because of its ability to encapsulate organic and organometallic molecules in inorganic matrixes.2,19-21 Brinker et al. have studied the differences in the structure and properties between sol-gel bulk material and sol-gel thin film utilizing gas adsorption on surface acoustic * Address correspondence to this author. Phone: + 82 51 510 6379. Fax: + 82 51 581 8147. E-mail: [email protected]. † Department of Chemistry, Pukyong National University. ‡ Basic Science Research Institute, Pukyong National University.

wave substrate and showed that the structure of films is considerably more compact than that of the bulk xerogels prepared from identical precursors.22,23 Several studies were concerned with the photophysical properties of organic dye molecules in bulk glass and thin films.24-27 However, there have been few systematic investigations to reveal the difference in chemical behavior of the differences between the bulk and the films. Concerning photochromic materials, spiropyran photochromic molecules were embedded in sol-gel matrixes both to follow the sol-gel process and to prepare photochromic materials. Two types of photochromic behavior were observed by Levy et al. and Ichimura et al. for spiropyrans and azobenzenes trapped in organically modified sol-gel matrixes.28,29 The normal photochromism of spiropyrans, that is, colorless samples converted when being irradiated to colored ones, was observed in a matrix prepared by polymerizing C2H5Si(OC2H5)3 (ETEOS) sol-gel precursor. In contrast, when the precursor was a mixture of Si(OC2H5)4 (TEOS) and polydimethylsiloxane (PDMS), the materials exhibit reverse photochromism, that is, colored in the dark and fading through the action of light. This is ascribed to the polarity of the cage within which the spiropyran was trapped. For the former precursor, the cage surface was composed of apolar ethyl groups which did not stabilize the merocyanine form and led to normal photochromism. Normal photochromic materials were also obtained by encapsulating spiropyrans in aluminosilicate gels.30,31 In the present study, the silica precursor sol solutions of spironaphthooxazine were prepared and coated on the glass surface by spin coating. The gelation was treated with heat. The kinetics of the normal photochromisms of decoloration by the polarity and rigidity change by pore size change of microenvironment of the doped spironaphthooxazine in the gels of the phenyltriethoxysilane (PhiTEOS), vinyltriethoxysilane (VTEOS), methyltriethoxysilane (MTEOS), and tetraethylorthosilicate (TEOS) was comparatively investigated with fitting the constants of the biexponential equation. The relative pore sizes of the gel

10.1021/jp073587d CCC: $40.75 © 2008 American Chemical Society Published on Web 01/08/2008

Spironaphthooxazine Doped Photochromic Glass

Figure 1. Structure of spironaphthooxazine.

matrixes was determined indirectly with electron nuclear double resonance (ENDOR) by measuring the linewidths of the proton matrix ENDOR of photoproduced radical cation of N-methylphenothiazine (PC1) in the different gel matrixes. Experimental Section Preparation of Alkoxide Sols. 1,3-dihydro-1,3,3-trimethylspiro[2H]-indole-2,3′-[3H]-naphtho[2,1-b]1,4-oxazine, of which the structure is shown in Figure 1, was obtained from Aldrich Chemical Co. and used without any further purifications. N-mehtylphenothiaizne (99%) was purchased from Aldrich Chemical Co. and used without any further purifications. Phenyltriethoxysilane (99%, PhiTEOS), vinyltriethoxysilane (98%, VTEOS), methyltriethoxysilane (99%, MTEOS), and tetraethylorthosilicate (98%, TEOS) were obtained from Aldrich Chemical Co. and used without any further purifications. Housedistilled water was passed through a four-cartridge Barnstead Nanopure II purification train consisting of macropure treatment, organic free (for removing organics), two ion exchangers, and 0.2 µm hollow-fiber filter for removing particles. Its resistivity was 18.3 MΩcm. Organic solvents, such as dichloromethane, chloroform, methanol, and acetone, were either ACS certified spectroanalyzed or HPLC grade. The acids used in these experiments were reagent grade and used as received from Fisher. A 0.05 M spironaphthooxazine solution was prepared in the mixed solvent of dichloromethane (1 mol) and acetone (1 mol). Sol solution was prepared by hydrolysis of alkoxide mixed with water and acetone. The mixed solution of alkoxide (1 mol) and acetone (0.75 mol) was prepared by stirring for 5 min. This solution was added slowly into the mixed solution of water (1 mol) and acetone (0.187 mol) drop by drop and continuously stirred for 30 min. The resulted solution of sol keeps molar ratio of alkoxide/water/acetone (1:4:1.5). Also the completely hydrolyzed sol solution show a clear solution and spironaphthooxazine solution was added into the clear solution of alkoxide and spironaphthooxazine (v/v%, 2:1) and the solution was stirred for 5 min and stored for 12 h. The molar concentration of spironaphthooxazine in the final sol solution was 0.01 M. All of this preparation was carried out at room temperature. Preparation of Gel Thin Films by Spin Coatings on Glass Slide. Sol solutions for spin coating were filtered with filter paper of 0.2 µm pore size. The filtered sol solution was dropped by three drops with a pipet on the surface of a glass slide in the spin coater. The glass slide was cleaned by successive treatment with a saturated KOH solution of isopropanol for 30 min and with 1.0 M sulfuric acid. The treated slide glass was sonicated in the pure water for 30 min and dried with a heat gun. Then the coated glass slide was spin coated at 1000 rpm for 10 s. The spin coated sol on the substrate was changed to gel by heat treatment at 50 °C for 48 h under the reduced pressure in the vacuum oven. The thickness of the prepared film was determined as 2.8 µm with R-step profiler (Tencor Model 500). The color of the coated substrate showed a light yellow. UV-Vis Experiments. Photoirradiation of the sample was initially carried out at room temperature for 10 min with 300 W Cermax Xe lamp (LX 300 UV) with power supply from

J. Phys. Chem. C, Vol. 112, No. 4, 2008 1141 ILC Technology. This was done to get a complete ring closed spironaphthooxazine sample. The light passed through a 10 cm water filter and glass filter (Corning glass filter 4-97). This passes long wavelength light in the range of λ > 350 nm. The light intensity was measured as 13.85 mW/cm2 with a Molectron Detector and Hewlett-Packard 34401 A multimeter at the sample position. Then the sample was irradiated in the range of 230 nm < λ < 400 nm as a function of irradiation time using glass filter (Corning 7-54). The intensity of light at the sample position was determined as 2.21 mW/cm2. UV-vis spectra were obtained with a Varian Carry UV-vis spectrophotometer. All of these experiments were carried out at room temperature. Electron Nuclear Double Resonance Experiments. A 2.0 mM stock solution of N-methylphenothiazine (PC1) in chloroform was prepared and mixed with each sol solution. Each sol solution was prepared by the same procedure described before. The mixed solution of alkoxide (1mol) and acetone (0.75 mol) was prepared by stirring for 5 min. This solution was added slowly into the mixed solution of water (1 mol) and acetone (0.187 mol) drop by drop and continuously stirred for 30 min. The resulted sol solution keeps a molar ratio of alkoxide/water/ acetone (1:4:1.5). Also the completely hydrolyzed sol solution shows a clear solution, and then the stock solution of Nmethylphenothiazine was added into the clear solution with volume ratio of alkoxide (2) and N-methylphenothiazine (1) solutions. The solution was stirred for 5 min and kept for 12 h more without stirring. The concentration of N-methylphenothiazine in the final sol solution was 0.4 mM. All of this preparation was carried out at room temperature. Preparation of gel thin films by spin coatings of quartz slide was prepared as described before. The photoirradiation of the ENDOR sample was carried out at 77 K for 10 min with 300 W Cermax Xe lamp (LX 300 UV) with a power supply from ILC Technology. This was done to get a photoproduced cation radical of PC1. The light passed through a 10 cm water filter and glass filter (Corning glass filter 4-97). This passes long wavelength light in the range of λ > 350 nm. The light intensity was measured as 13.85 mW/cm2 with a Molectron Detector and Hewlett-Packard 34401 A multimeter at the sample position. Then the sample was irradiated in the range of 230 nm < λ < 400 nm as a function of irradiation time using a glass filter (Corning 7-54). The intensity of light at the sample position was determined as 2.21 mW/cm2. All of these experiments were carried out at 77 K. ESR spectra were recorded at the X-band using a JEOL JESPX1050 FT-ESR spectrometer with 100 kHz field modulation to get the field modulation of the ENDOR experiment. The irradiated sample tube was placed in a quartz ESR Dewar (Wilmad Glass Co.) filled with liquid nitrogen and secured in a TE102 cavity. The loaded Q factor of this cavity was measured as about 1700. The microwave power was maintained at 1.97 mW which is well below the saturation level for irradiated phenothiazine gel samples. The standard spectrometer settings used in these ESR experiments were 0.281 mT modulation field amplitude, 20 mT sweep width, 7 scan accumulations, 56 s scan time constant, 9.501 GHz microwave frequency and 1.25 × 105 receiver gain. ENDOR spectra were recorded with a Bruker 350 ENDOR unit. Radio frequency modulation was performed at constant magnetic field which resulted in a first derivative ENDOR spectrum. Each spectrum was accumulated for 64 scans. Typical experimental conditions were 1.97 mW microwave power and 100 W radio frequency power. A Bruker ER 4111 ENDOR variable temperature nitrogen flow unit was used in measuring the peak-to-peak separation distance of the first derivative signal.

1142 J. Phys. Chem. C, Vol. 112, No. 4, 2008

Kim et al.

Figure 2. UV-vis absorbance spectra of spironaphthooxazines doped in the phenyltriethoxysilane gel before (line) and after (dashed line) irradiation of light in the range of 230 nm < λ < 420 nm for 10 min.

TABLE 1: λmax Values of the Spironaphthooxazine Doped in the Different Matrixes after Photoirradiation of Light in the Range of 230 nm < λ < 420 nm matrixes

PhiTEOS

VTEOS

MTEOS

TEOS

λmax

623

619

617

614

Data Manipulation on Kinetics using Sigmaplot Software. The decay curves of UV-vis absorption intensity at λmax versus the irradiation time were obtained from the data of the optical absorbance after the photoirradiation for 20 min on the PhiTEOS, VTEOS, and MTEOS matrixes was stopped. The data from decay curves were simulated with SigmaPlot software to obtain the pre-exponential constants (Ath, A1, and A2) and kinetics constants (k1 and k2) of the kinetics equation of biexponential equation. This indicates the photochromic kinetics of the spironaphthooxazine doped in the gels.

Figure 3. Optical absorbance spectra of spironaphthooxazine doped in the phenyltriethoxysilane gel by irradiation of light in the wavelength range of 230 nm < λ < 420 nm versus irradiation time for 1, 2, 3, 4, and 5 min.

Figure 4. Schematic drawings of normal and reverse photochromism between the closed and opened form of spironaphthooxazine during the coloration by irradiation of light and decoloration by storing without light.

Results and Discussion The optical absorption spectra of spironaphthooxazines doped in the phenyltriethoxysilane gel before and after irradiation of light in the range of 250 nm < λ < 400 nm for 10 min are shown in Figure 2. The absorption spectra of spironaphthooxazine doped in PhiTEOS gel after UV irradiation present two contributions: 580 nm (shoulder) and 620 nm, the most intense peak. The corresponding low intensity absorption band around 450 nm at room temperature denoted that a thermal equilibrium was established between the blue opened and the colorless closed forms. The λmax values of the opened form of spironaphthooxazine doped in the PhiTEOS, VTMOS, MTEOS, and TEOS are shown in Table 1. The blue shift of λmax value appeared with changing gel matrixes from PhiTEOS to TEOS. The positive solvatochromism is characteristic of compounds having a slightly more polar ground state, implying a less charge delocalization. The normal photochromisms of optical absorbance change of spironaphthooxazine doped in PhiTEOS gel at a wavelength of 623 nm by irradiation of light in the range of 250 nm < λ < 400 nm versus irradiation time are shown in Figure 3. The optical intensity at 623 nm exponentially increased with increasing irradiation time at a given wavelength range of light. This indicates that the oxazine ring was opened by irradiation of light and shows the highest absorption band at 623 nm. The opened and closed structure of spironaphthooxazine by photochromism of spironaphthooxazine by light with different wavelength range is schematically shown in Figure 4. The normal and reverse photochromism of spironaphthooxazine, that is, colorless samples converted when irradiated to colored ones or vice versa, were observed in the different gel matrixes. The exponentially increasing absorbance at 623 nm during

Figure 5. Optical absorbance change of spironaphthooxazine doped in the tetraethoxyorthosilicate gels at λmax by irradiation of light in the range of 230 nm < λ < 420 nm versus irradiation time for 1, 2, 3, 4, and 5 min.

photoirradiation indicates the exponentially increasing proportion of the opened form of spironaphthooxazine by irradiation of light. The exponentially decreasing absorbance at 623 nm of spironaphthoxazine doped in TEOS gel by irradiation of light are shown in Figure 5. This is corresponding to reverse photochromism. The normal and reverse photochromism of spironaphthooxazine doped in the different gels are comparatively shown in Figure 6. In the normal photochromism, photoirradiation of samples for 20 min at room temperature resulted in an exponential increase of absorbance of the opened form of spironaphthooxazine doped in the gel matrixes; thereafter, the intensity of the opened form of spironaphthooxazine was exponentially decreased because of the transformation of opened form to closed form. In the reverse photochromism, photoirradiation resulted in an exponential decrease of the opened form of spironaphthooxazine during photoirradiation, and thereafter, the intensity of the opened form of spironaphthooxazine was exponentially increased. The optical absorbance

Spironaphthooxazine Doped Photochromic Glass

J. Phys. Chem. C, Vol. 112, No. 4, 2008 1143 TABLE 2: Kinetic Constants (abs‚mol-1‚min-1) of Biexponential Equation for the Decoloration of the Opened Form of Spironaphthooxazine Doped at Different Matrixes

Figure 6. Optical absorbance at λmax of spironaphthooxazine doped phenyltriethoxysilane (O), vinyltriethoxysilane (0), methyltriethoxysilane (4), and tetraethoxyorthosilicate (b) gels with irradiation of light in the wavelength range of 230 nm < λ < 420 nm for 20 min and without light for 20 min more.

Figure 7. (a) Decay curves of optical absorbance at λmax of the spironaphthooxazine doped phenyltriethoxysilane (O), vinyltriethoxysilane (0), and methyltriethoxysilane (4) gel during decoloration process after irradiation of light in the range of 230 nm < λ < 420 nm for 20 min. (b) The resonance structures of spironaphthooxazine between quinoidal and zwitterionic structures.

change of spironaphthooxazine around 620 nm versus irradiation time indicates that the different gel matrixes critically control the kinetics and equilibrium of the photochromism of spironaphthooxazine. This is caused by the competition between normal and reverse photochromisms because of the presence of different chemical microenvironment of the matrixes surrounding the spironaphthooxazine molecule. In the normal photochromism, the decay curves of the exponentially decreasing absorbance of the opened form of spironaphthooxazine around 620 nm are shown in Figure 7a. The different slopes of decay curves of the opened form of spironaphthooxazine in the PhiTEOS, VTEOS, and MTEOS are caused by the transformation kinetics from the closed to the opened form of spironaphthooxazine in the different matrixes. This is due to the different polarity of the organically modified gel matrixes. This is already known by the red shift increase in the sequence MTEOS < VTEOS < PhiTEOS gel matrixes as shown in Table 1. The opened form

constant

PhiTEOS

VTEOS

MTEOS

Ath A1 k1 A2 k2

0.0632 0.0183 0.5480 0.0168 0.1009

0.0020 0.0163 0.5295 0.0095 0.0761

0.0342 0.0486 0.5039 0.0292 0.0671

of spironaphthooxazine can be described as resonance structures between the quinoidal and the zwitterionic structures as shown in Figure 7b. The equilibrium between zwitterionic and quinoidal structures of the opened form of spironaphthooxazine can be controlled by the polarity of the gel matrixes. The increased polarity of the gel matrixes should increase the red shift as the same sequence of the polarity. A previous study already reported that the increasing net charge of the silicon and the polarity of the gel matrixes induce high-field shifts as PhiTEOS < TEOS.16,32 The higher polarity of TEOS stabilizes the ionic zwitterionic structure of the spironaphthooxazine more than the quinoidal structure. This is the reason why the reverse photochromism of spironaphthooxazine was observed in the polar TEOS gel matrixes. The kinetics of the thermal back or ring closure reaction (decoloration) was studied following the fading of color at λmax (620 nm) of the opened form of spironaphthooxazine. The kinetics of the photochromism in these gels was quantified using simple biexponential models in order to take into account the inhomogeneous distribution of free volume in the gel matrixes and to compare the photodynamics among them. The kinetics of the thermal unimolecular reactions of spironaphthooxazine from a metastable species to a stable one did not proceed as first-order kinetics. In fact, it was generally accepted that the common observation of non-exponential isomerization kinetics in gel matrixes indicates the site-specific matrix effects by imposing a distribution of localized barriers to the steric requirements of the reaction.33-35 Attribution of the deviation from first-order kinetics to the conformational statistics of the matrixes was confirmed by the approach to a single-exponential decay pattern. The results can be analyzed by using Gaussian model developed by Albery et al., which has been applied to porous silica by Samuel and co-workers.36 The basic assumption of the model is that the distribution of rate constant, k, is due to a normal distribution of free energies leading to a Gaussian. Kinetic data concerning the thermal back reaction of spironaphthooxazine in polymer and gel matrixes has been previously analyzed as a biexponential process.37 In this case, the thermal ring closure of the spironaphthooxazine can be described by the following equation:

A(t) ) A1e-k1t + A2e-k2t + Ath where A(t) was the optical density of spironaphthooxazine at 623 nm and A1 and A2 were contributed to the initial optical density A0. Ath reflected the thermal equilibrium between the closed and the opened forms of spironaphthooxazine. In this simple model, the spironaphthooxazine molecule in PhiTEOS, VTEOS, and MTEOS matrixes was thermally decolorized with different rate constants as shown in Table 2 and Figure 7a. From the viewpoint of a uniform distribution of free volume, the separated constants k1 and k2 should be understood as empirical mean values between the fast and the slow constants. The constants from the fitting of the experimental data for different samples by using biexponential processes were

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Kim et al. MTEOS > TEOS can be explained by the increasing polarity of the gel matrixes as PhiTEOS < VTEOS < MTEOS < TEOS along the inverse order of the pore size of the gel matrixes. The higher polarity of the gel matrixes suppresses the kinetics of the decoloration by the higher stability of the zwitterionic structure of the opened spironaphthooxazine in the gel matrixes. This is also matched with the larger rigidity of the gel matrixes by smaller pore size of the gel matrixes. Conclusion

Figure 8. Proton matrix ENDOR signals of PC1 in the gel matrixes of MTEOS, VTEOS, and PhiTEOS

analyzed to deduce the mean activation energy for the equilibrium. The constants at different gel matrixes are shown in Table 2. The density constants (A1, A2, and Ath) indicate the contribution of each thermal decoloration reaction of spironaphthooxazine in the gel matrixes. The kinetic constants (k1 and k2) indicate the stability of the opened form of spironaphthooxazine in the gel matrixes. The lowest kinetic constant indicates that the opened form of spironaphthooxazine is more stabilized in the polar matrix of the gel. The kinetic parameters of the thermal fading for spironaphthooxazine doped gels are also dependent on the pore size of the matrixes. The dynamics clearly slowed down in the less restricted matrixes which have larger pore size. The similar results are shown in the organic polymer matrixes of polyethylene oxide and polypropylene oxide with photoinduced charge separated phenothiazines.37 The relative pore size of the polymer matrixes were investigated with electron nuclear double resonance (ENDOR) by determining ENDOR line widths. The pore size of the polymer matrixes gives a clue on the importance of the matrix rigidity on the thermal processes. In solids, the transcis isomerization of the opened form of spironaphthooxazine is known as the limiting step for the kinetics of thermal ring closure.38 Obviously, as observed in rigid matrixes, the shrinking cage around the trapped molecules renders the trans-cis isomerization of the spironaphthooxazine. The relative pore size of the matrixes of PhiTEOS, VTEOS, and MTEOS can be determined with ENDOR by measuring proton matrix ENDOR line widths of the N-methylphenothiazine radicals produced by photoirradiation. The results are shown in Figure 8. The results on the proton matrix ENDOR showed that the pore size of the matrixes shows a decreasing order PhiTEOS > VETOS > MTEOS. The larger pore size of the gel matrixes results in the less rigidity of the spironaphthooxazine. The less rigidity gives a faster kinetic constant of the decoloration of the spironaphthooxazine. This is another possible explanation for the increasing kinetic constant of the decoloration of spironaphthooxazine as MTEOS < VTEOS < PhiTEOS. Conclusively, the decreasing kinetic constant of the decoloration of spironaphthooxazine as PhiTEOS > VTEOS >

The photochromic glass was prepared by doping the spironaphthooxazine dye molecule in PhiTEOS, VTEOS, MTEOS, and TEOS gel matrixes with the spin coating of them on the glass surface. The normal and reverse photochromisms of it were critically affected by the microenvironment such as polarity and pore size of the gel matrixes. PhiTEOS, VTEOS, and MTEOS showed the normal photochromism and TEOS showed a reverse photochromism. The polarity of the gel matrixes was investigated with the red shift of the λmax of the opened form of the spironaphthooxazine molecule as a decreasing order PhiTEOS < VTEOS < MTEOS < TEOS. The higher polarity of the TEOS gel matrix can stabilize the zwitterionic structure of the opened form of the spironaphthooxazine. This leads to the reverse photochromism of the spironaphthooxazine in the TEOS gel matrix. The determined kinetic constant among PhiTEOS, VTEOS, and MTEOS showed a decreasing order PhiTEOS > VETOS > MTEOS. This is interpreted as the polar microenvironment of the gel matrix suppressing the decoloration reaction of the opened form of spironaphthooxazine because the zwitterionic structure of the opened form of the spironaphthooxazine can be stabilized more in the polar gel matrix. This results in the greater activation energy for the decoloraion reaction and pore size of the gel. Acknowledgment. We would like to thank the financial support by the Nano R & D Program (Korea Science & Engineering Foundation, Grant 2007-02628) and the Brain Korea 21 project in 2007. References and Notes (1) Jones, R.W. Fundamental Principles of Sol-Gel Technology; Institute of Metals: London, 1989. (2) Avnir, D.; Levy, D.; Reisfeld, R. J. Phys. Chem. 1984, 88, 5956. (3) Canva, M.; Le Saux, G.; George, P.; Brun, A.; Chaput, F.; Boilot, J.-P. Opt. Lett. 1992, 17, 218. (4) Collino, R.; Therasse, J.; Binder, P.; Chaput, F.; Boilot, J.-P.; Levy, Y. J. Sol-Gel Sci. Technol. 1994, 2, 823. (5) Canva, M.; Geprege, P.; Perelgritz, J.-F.; Brun, A.; Chaput, F.; Boilot, J.-P. Appl. Opt. 1995, 34, 428. (6) Avnir, D. Acc. Chem. Res. 1995, 28, 328. (7) Zink, J. I.; Dun, B. In Sol-Gel Optics: Processing and Application; Klein, L.C., Ed.; Kluwer Academic Publishers: Norwell, MA, 1994; p 303. (8) Avnir, D.; Braun, S.; Lev, O. D.; Ottolenghi, M. In Sol-Gel Optics: Processing and Applications; Klein, L.C., Ed.; Kluwer Academic Publishers: Norwell, MA, 1994; p 539. (9) Reisfeld, R. J. Non-Cryst. Solids 1990, 121, 254. (10) Dave, B. C.; Dunn, B.; Valentine, J. S.; Zink, J. I.; Allick, T. H.; Chandra, S.; Hutchison, J. A. Mater. Res. Soc. Symp. Proc. 1994, 329, 269. (11) Fuji, T.; Mabuchi, T.; Kitamura, H.; Kawauchi, O.; Negishi, N.; Anpo, M. Bull. Chem. Soc. Jpn. 1992, 65, 720. (12) Matsui, K.; Nakanaka, T.; Morisaki, H. J. Phys. Chem. 1991, 95, 976. (13) Matsui, K.; Matsuzuka, T.; Fujita, H. J. Phys. Chem. 1989, 93, 4991. (14) Dunn, B.; Knobbe, E.; Mckiernan, J. M.; Pouxviel, J. C.; Zink, J. I. Mater. Res. Soc. Symp. Proc. 1988, 121, 331. (15) Avnir, D.; Levy, D. J. Phys. Chem. 1988, 92, 4734. (16) Kang, S. H.; Shin, H. D.; Oh, C. H.; Choi, D. H.; Park, K. H. Bull. Korean Chem. Soc. 2002, 23, 957. (17) Choi, D. H.; Ban, S. Y.; Kim, J. H. Bull. Korean Chem. Soc. 2002, 24, 441.

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