9024
J. Phys. Chem. 1996, 100, 9024-9031
Photochromism of Spirooxazine-Doped Gels John Biteau, Fre´ de´ ric Chaput, and Jean-Pierre Boilot* Groupe de Chimie du Solide, Laboratoire de Physique de la Matie` re Condense´ e URA CNRS 1254 D, Ecole Polytechnique, 91128 Palaiseau, France ReceiVed: December 6, 1995; In Final Form: March 1, 1996X
Spirooxazine photochromic dyes were trapped in gel thin films. Organically modified doped gels showed normal photochromism. The kinetics of the photochromism in these gels was quantified using both Gaussian and simple biexponential models in order to take into account the inhomogeneous distribution of free volume in the gel matrix and to compare the photodynamics with that in organic polymers. In organic-inorganic hybrid matrices, a competition was observed between normal and reverse photochromisms because of the presence of two chemical environments for the dye molecules. The kinetics of photodegradation for dyedoped gels was deduced from the recording of the excitation/emission spectra during UV light irradiation.
Introduction
SCHEME 1
The sol-gel process is now well known as a synthetic route to trap organic molecules in a solid matrix. The polymerization process was usually initiated by adding water to a solution of alkoxide in ethanol. Concerning silicon alkoxide precursors (such as the tetraethoxysilane Si(OC2H5)4 or TEOS), the chemical conditions were generally carefully defined in such a way that nearly complete hydrolysis occurred in a few minutes. This allowed us to realize separately condensation with the formation of siloxane bridges [tSisOsSit] between the hydrolyzed species [tSisOH]. Gelation was considered as the percolation of clusters resulting from the aggregation phase of small organized units. Gels turned into dried materials known as xerogels after conventional drying in air at low temperature (20-60 °C). The preparation of doped xerogels generally consisted of adding a solution of the desired dopant to the initial polymerizing system. Numerous materials and their applications have been proposed, such as sensors, optical limiters, information recording materials, and dye-laser materials.1-5 Concerning photochromic materials, spiropyran photochromic molecules were embedded in sol-gel matrices both to follow the sol-gel process and to prepare photochromic materials. Two types of photochromic behavior were observed by Levy et al.6,7 for spiropyrans trapped in organically modified sol-gel matrices. The normal photochromism of spiropyrans, i.e., 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, i.e., colored in the dark and fading through the action of light. The authors assumed that the photochromic behavior was related 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.8 For the latter precursor (or the pure TEOS precursor9), the merocyanine zwitterionic form was stabilized by strong hydrogen bonds to the silanols of the cage and photochromism was reversed. However, by use of the TEOSPDMS mixture, the photochromism remained normal if the X
Abstract published in AdVance ACS Abstracts, May 1, 1996.
S0022-3654(95)03607-0 CCC: $12.00
spiropyran was covalently attached to the gel network, preventing the diffusion of molecules to the more polar silica zones.10 The photochromism of spirooxazine molecules is due to photocleavage of the C(spiro)-O bond under irradiation to give an open merocyanine structure that absorbs in the visible (Scheme 1). However, spirooxazines have much greater photostability than spiropyrans. Spirooxazine photochromic molecules were entrapped in organic-inorganic composites. Materials presented normal photochromism, and compared to the dye in ethanol, the fading rate was similar while the photostability was improved.11 In this paper, we first present the kinetics of normal photochromism of spirooxazines in gel thin films obtained by polymerizing organically modified precursors such as CH3Si(OC2H5)3 (MTEOS). In the second part, we study the competition between normal and reverse photochromism as a function of the sol-gel precursor composition for CH2dCHSi(OC2H5)3 (VTEOS) and TEOS mixtures. Finally we show that fluorescence measurements appear as a powerful method for detecting and quantifying the photodegradation of the photochromic molecules. Experimental Section 1. Preparation of Photochromic Materials. Doped xerogels were prepared by hydrolysis-condensation of different organically modified alkoxide precursors under acid-catalyzed conditions with acetone as the common solvent. Matrices were hereafter noted as the molecular precursors CH3Si(OC2H5)3 (MTEOS), CH2dCHSi(OC2H5)3 (VTEOS), C6H5Si(OC2H5)3 (PhiTEOS), or Si(OC2H5)4 (TEOS). In the starting mixture, the alkoxide:water (pH ) 2.5):acetone molar ratios were, respectively, 1:3:3 and 1:5:3. After hydrolysis for 1 h at room temperature, a small amount of amine (10-3 mol/L) was added both to partially neutralize the protonic species, avoiding subsequent chemical degradation of the dyes, and to catalyze condensation reactions. An acetonic solution of a spirooxazine was then added, and the sol was exposed to © 1996 American Chemical Society
Spirooxazine-Doped Gels
J. Phys. Chem., Vol. 100, No. 21, 1996 9025
SCHEME 2
ambient air with magnetic stirring until the dye concentration reached 10-2 mol/L. Coatings of different thickness were prepared by spreading the viscous sol on glass slide substrates, using the spin-coating technique (the angular velocity of the spinner was 1000 rpm). The coatings were heated to 60 °C and maintained at this temperature for 3 days. The photochromic molecules employed in this study were two spirooxazine derivatives (Scheme 2): the 5-methoxy-3,3dimethyl-1-n-propylspiro[indoline-pyridobenzoxazine] noted SO1 and the 1,3,3-trimethylspiro[indoline-naphthoxazine] noted SO2. 2. Setup for Kinetics Measurements of Normal Photochromism. A schematic representation of the basic setup is illustrated in Figure 1. Either visible or UV light from a 150 W Xe lamp was selected by using filters. For samples exhibiting normal photochromism, visible light was first used for 10 min to completely decolor the sample (12.3 mW/cm2 on the surface of the sample). UV was then used for 30 min to activate the photocoloration in the photochromic sample (1.66 mW/cm2 on the surface of the sample). Finally, the light beam was turned off to follow for 30 min the dark decoloration. The absorption spectra and kinetics of photocoloration and dark fading were recorded on a Zeiss visible spectrophotometer. The measurements were made from -10 to 35 °C. 3. General. The absorption spectra were recorded on a Shimadzu UV-vis 160A spectrophotometer. Samples for the study of photostability were irradiated at 25 °C at different times with UV light produced from a Vilber-Lourmat 215 BLB tube (2.3 mW/cm2 on the surface of the sample). The emission measurements were made on a Hitachi fluorescence spectrophotometer F4500. Tridimensional spectra formed from isointensity lines were collected by recording successive fluorescence spectra in the 250-450 nm excitation wavelength range. Measurements on photoproducts were made on deaerated ethanolic solutions. Results and Discussion 1. Normal Photochromism for Organically Modified Matrices. Thus prepared, SO1-doped organically modified gels showed a clear green color. The corresponding low intensity absorption band in the visible region at room temperature denoted that a thermal equilibrium was established between the blue open and the yellowish closed form. The absorption intensity increased as the temperature increased, indicating that these materials exhibited thermochromic properties. For SO2-doped gels, the band was hardly detectable at room temperature. This reflects the higher degree of stabilization of the merocyanine form of SO1, achieved by substituting a carbon atom in the naphtho group by the nitrogen atom and mainly by adding the OR group on the indoline side of the spirooxazine.
Figure 1. Schematic diagram of the apparatus used for the kinetics measurements.
Figure 2. Absorption spectra for SO1-doped VTEOS gels before and after UV irradiation.
TABLE 1: Values of the Absorption Maximum for SO1-Photomerocyanine in Different Environments matrix or solvent
λmax (nm)
matrix or solvent
λmax (nm)
acetone MTEOS
615 618
VTEOS PhiTEOS
620 623
SCHEME 3
As previously observed by Levy et al.7 for spiropyrans, spirooxazines trapped in organically modified matrices exhibit normal photochromism. Figure 2 shows the absorption spectra for SO1-doped VTEOS gels before and after UV irradiation. The spectrum of the photomerocyanine presents two contributions: 580 nm (shoulder) and 620 nm, the most intense peak. The pronounced red shift in λmax for the blue open form upon transition from acetone to VTEOS should be noted. In fact, for organically modified gel matrices, compared with acetone, the red shift increases in the sequence MTEOS < VTEOS < PhiTEOS (Table 1). The photomerocyanine is generally described as an equilibrium between the quinoid and zwitterion structures (Scheme 3). In solution, it is well known that photomerocyanine forms of spirooxazines are characterized by a positive solvatochromism corresponding to the increase of the red shift when the polarity of the solvent increases.12 The positive solvatochromism is characteristic of compounds having
9026 J. Phys. Chem., Vol. 100, No. 21, 1996
Biteau et al.
Figure 3. Photocoloration (triangles) and thermal bleaching (crosses) in the dark at 20 °C for SO1-doped MTEOS gel. Solid lines correspond to biexponential fits. Figure 5. Distribution of kinetic constants deduced from the fit shown in Figure 4. The parameters of the distribution are kav ) 0.159 min-1 and γ ) 2.16 (the pre-exponential factor of the Gaussian function was taken to be one). Vertical lines represent the average constants obtained by using the biexponential model (k1 ) 0.69 min-1, k2 ) 0.084 min-1). Heights represent pre-exponential factors (A1/A2 ) 0.69) whose sum was normalized to one.
Figure 4. Gaussian and biexponential fits for the thermal bleaching in the dark at 20 °C for SO1-doped MTEOS gel.
a weakly polar ground state, implying a small charge delocalization. Therefore, in solution, the structure of the photomerocyanine form of spirooxazines is much more quinoidal than zwitterionic. Assuming that the structure of the photomerocyanine is mainly quinoidal in gel matrices, the polarity of the gel matrix should increase in the same sequence as the sequence for the red shift. In fact, these results are well supported by 29Si NMR data. Calculations of the paramagnetic shielding of the 29Si nucleus in tetracoordinated compounds show an approximately linear correlation between the relative screening constant and the net charge δ+ of the silicon in the -40 to -120 ppm chemical shift range.13 Increasing net charges of the silicon and the polarity of the gel matrix induce high-field shifts in the following sequence: MTEOS < VTEOS < PhiTEOS < TEOS.14 2. Analysis of Kinetics of Normal Photochromism. Figure 3 presents a coloration-decoloration cycle characteristic of the normal photochromism for spirooxazines trapped in organically modified gel matrices. The kinetics of the thermal back or ring closure reaction was studied following the fading of color at the maximum absorption. As is the case in polymers, the kinetics of thermal unimolecular reactions of spirooxazines from metastable species to stable ones did not proceed as first-order kinetics. In fact, it was generally accepted that the common observation of nonexponential isomerization kinetics in polymers indicates site-specific matrix effects by imposing a distribution of localized barriers to the steric requirements of the reaction.15-17 Attribution of the deviation from first-order kinetics to the conformational statistics of the matrix was confirmed by the approach to a single
exponential decay pattern as the temperature was raised beyond the glass transition. The results can be analyzed by using the Gaussian model developed by Albery et al.,18 which has been applied to porous silica by Samuel and co-workers.19 The basic assumption of the model is that the distribution of rate constants, k, is due to a normal distribution of free energies leading to a Gaussian. The distribution of energies leads to a distribution of ln(k) around some ln(kav.), where kav is the average constant of the reaction. The two fitting parameters are kav and γ, the dispersion of the Gaussian in ln(k) that gives a quantitative estimation of the extent of heterogeneity of the matrix. Figure 4 indicates the excellent agreement between this model and the experimental thermal decoloration of gels. The Gaussian distribution of ln(k) corresponding to the best fit is shown in Figure 5. However, kinetic data concerning the thermal back reaction of spirooxazine in polymers had been previously analyzed as biexponential processes.20 In this case, the thermal ring closure of the photomerocyanine was described by the following equation:
A(t) ) A1 e-k1t + A2 e-k2t + Ath
(1)
where A(t) was the optical density at λmax and A1 and A2 were contributions to the initial optical density A0. Ath reflected the thermal equilibrium between the two forms. In this simple model, two types of photomerocyanine molecules are decolorized with different rate constants. 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. Although the Gaussian model was a better fit for thermal decoloration than the biexponential model (Figure 4), we essentially used the latter, which allowed for a quantitative comparison between the kinetics in doped gels and previously reported data in polymers. Table 2 presents constant values deduced from the fit of the experimental data for different samples by using biexponential processes. For SO1-doped materials the kinetics of fading was analyzed at different temperatures, with a constant value for the A1/A2 ratio, in order to deduce the mean activation energies. The values deduced by using Arrhenius plots were about 60 and 80 kJ/mol for the fast and the slow process, respectively.
Spirooxazine-Doped Gels
J. Phys. Chem., Vol. 100, No. 21, 1996 9027
TABLE 2: Kinetics Parameters for the Thermal Decoloration at 20 °C of the Spirooxazines in Gel and Polymeric Films dye
matrixb
SO1 SO1 SO1 SO1 SO1 SO2 SO2 SO2 SO2
MTEOS MTEOS MTEOS VTEOS PUa (ref 24) MTEOS PMMA (ref 20) PVB (ref 20) PUa (ref 24)
initial k1 k2 mixture (min-1) (min-1) SiH3A3 SiH3A3 SiH5A3 SiH3A3 SiH3A3
0.69 0.67 0.63 0.49 0.81 2.8 5.1 3.2 2.25
0.084 0.079 0.071 0.067 0.097 0.19 0.47 0.31 0.17
A1/A2
Ath
0.69 0.66 0.67 0.61 0.42 1.37
0.037 0.030 0.033 0.037 7.10-4
0.96 0.95
a 25 °C. b Polyvinylbutyral, PVB; polymethylmethacrylate, PMMA; polyurethane, PU.
TABLE 3: Kinetic Parameters for the Photocoloration at 20 °C of the Spirooxazines in Gel Films dye
matrix
initial mixture
k′1 (min-1)
k′2 (min-1)
SO1 SO1 SO1 SO2
MTEOS MTEOS MTEOS MTEOS
SiH3A3 SiH3A3 SiH5A3 SiH3A3
1.19 1.23 1.17 3.20
0.42 0.38 0.41 0.63
An intermediate value of 70 kJ/mol was also deduced from the thermal evolution of kav, by using the Gaussian model. The kinetic parameters of the thermal fading for spirooxazinedoped gels appear similar to the ones observed in rigid organic polymers such as polyurethane. The dynamics is clearly slowed down in comparison with less reticulated polymers such as PMMA, showing the importance of the matrix rigidity on the thermal ring process. In solids the trans-cis isomerization of the photomerocyanine is known as the limiting step for the kinetics of thermal ring closure. Obviously, as observed in rigid matrices, the shrinking cage around the trapped molecules renders the trans-cis isomerization of the merocyanine more difficult. Finally, as in solutions and in polymers, the rate constants are lower for SO1 than for SO2. This confirms the higher stability of the photomerocyanine form of SO1 compared to SO2. The biexponential model was also used for the photocoloration in order to take into account the effects of the inhomogeneous distribution of free volume in the matrix. Assuming the presence of two mean forms of the closed spirooxazine differing in the quantum yields of the photocoloration, the kinetics was given by eq 2, which well described the experimental data (Figure 3):
A(t) ) R1[1 - e-β1t] + R2[1 - e-β2t]
(2)
From values for thermal bleaching (k1 and k2 in eq 1), the apparent rate constants k′1 and k′2 and the relative contributions of the two forms to the photocoloration were deduced by using the relations β1 ) k1 + k′1, β2 ) k2 + k′2, R1 ) A′1/(1+ k1/k′1), R2 ) A′2/(1+ k2/k′2), and A′2/A′1 ) A2/A1. It is obvious that the quantum yields could not be deduced from our study, and the values of the constants were only charateristic of photocoloration under our experimental conditions. Table 3 shows constant values deduced from the fit of experimental data for different samples. Similar to the thermal ring reaction, the kinetics of photocoloration appears faster for SO2-doped gels than for SO1 ones. This could result from steric effects due to the greater size for the closed form of the SO1 spirooxazines.
Figure 6. Photochromic cycles at 31 °C for SO1-doped gels prepared from different VTEOS-TEOS mixtures (at 25 °C for the pure VTEOS).
Figure 7. Evolution in the maximum of absorption of the photomerocyanine form for SO1-doped gels as a function of the inorganic TEOS part in the matrix.
3. Competition between Normal and Reverse Photochromism in VTEOS-TEOS Gel Mixtures. The previous results on the evolution of the matrix polarity being taken into account, photochromism was studied in SO1-doped matrices prepared from different VTEOS-TEOS mixtures (Figure 6). In this case, the photochromism cycle was performed without the first step of complete photodecoloration. The sample was irradiated for 15 min by a visible-UV light mixture, and the thermal back reaction was then studied for 20 min. As expected, the increase of the pure inorganic TEOS component leads to an increase of the polarity of the matrix. Therefore, as previously observed for spiropyrans in the pure TEOS, a reverse photochromism was noted for the 33% VTEOS-66% TEOS matrix. The sample was bleached through the action of the light and was colored in the dark. This indicates that for the more inorganic and polar matrix, the open photomerocyanine is more stable than the SO1 closed form. An important result, displayed because of the visible-UV mixed light irradiation, was that the composition from 83% VTEOS-17% TEOS to 50% VTEOS-50% TEOS exhibited the superimposition of the two photochromic behaviors: the
9028 J. Phys. Chem., Vol. 100, No. 21, 1996
Biteau et al.
Figure 8. Schematic representation for the two types of chemical environments for photochromic molecules in mixed VTEOS-TEOS matrices: (a) apolar; (b) polar.
SCHEME 4
normal photochromism for the pure VTEOS matrix and the reverse one for the more polar 33% VTEOS-66% TEOS gel (Figure 6). Another point of interest was the blue shift observed in λmax of the photomerocyanine when the polarity of the matrix increased (Figure 7). This showed that the equilibrium between the two structures of the merocyanine was progressively transferred from the quinoid structure to the zwitterionic structure when the inorganic TEOS part increased in the SO1doped matrices. The zwitterionic structure was stabilized by strong hydrogen bonds to the silanols of the silica cage, lowering the ground state of the merocyanine and increasing the gap to the first excited state. All these results confirm the assumption of Levy et al.7 concerning the presence of two chemical environments for the dye molecules in mixed matrices (Figure 8). A first part of spirooxazine molecules was surrounded by apolar vinyl groups, which do not stabilize the merocyanine form. As in organically modified matrices, the photochromism is normal with a stable closed form and a quinoid structure for the open form. For the second part of molecules, the merocyanine zwitterionic form is stabilized by hydrogen bond type interactions with the inorganic silica cage. As in pure inorganic TEOS matrices, the photochromism is reversed with a stable open zwitterionic form. Concerning thermal back reactions, it can be noted, from the curve performed for the 83% VTEOS-17% TEOS sample (Figure 6), that the kinetics of ring closure corresponding to normal photochromism for molecules in an apolar environment was faster than the merocyanine formation for reverse photochromism in polar zones. The competition between the two photochromisms is schematized in Figure 9. 4. Photodegradation. Contrary to spiropyrans, spirooxazines in solution are rather stable toward ultraviolet light irradiation. Spirooxazines have a high quantum yield of photocoloration and usually do not show fluorescence or phosphorescence at room temperature. During the process of photocoloration a spirooxazine passes through an excited singlet
Figure 9. Schematic representation for the superimposition of normal and reverse photochromisms.
state and not a triplet state. This pathway thus explains the absence of fluorescence and the very good photostability.12-21 Nevertheless, a long exposure of UV irradiation generates the progressive photodecomposition of photochromic molecules. The photodegradation of SO2 and SO1 has been studied in terms of mechanism and photoproducts by G. Baillet et al.22-24 Some photoproducts obtained for a solution of SO2 in toluene are presented in Scheme 4. As previously reported, in solution the excited single states of spirooxazines were found to be nonfluorescent in organically modified gels, and SO-doped xerogels showed a decrease of the photochromic response under continuous UV irradiation. However, an important point was that the decrease of photochromism during irradiation led to the appearance of a weak fluorescence. Light emission could be then attributed to the molecules produced by the photodegradation of the photochromic spirooxazines in the gel matrix. Three of the SO2 photoproducts, supplied by G. Baillet, were dissolved in ethanol for excitation/fluorescence measurements. All these molecules presented specific emission bands shown in Figure 10. The different wavelengths of excitation and fluorescence are summarized in Table 4 for both photoproducts in solution and photodegraded SO2-doped MTEOS gels. Assuming a normal small shift of the excitation/fluorescence bands of molecules by changing from the ethanolic solution to the gel matrix, the spectrum of the photodegraded SO2-doped thin films (Figure 10) can be clearly seen as a composition of the spectra of photoproducts in solution. The spectrum of the
Spirooxazine-Doped Gels
J. Phys. Chem., Vol. 100, No. 21, 1996 9029
Figure 10. Excitation/fluorescence spectra for three of the photoproducts shown in Scheme 4 and for the photodegraded SO2-doped MTEOS film.
TABLE 4: Wavelengths of Excitation/Fluorescence Bands for Synthetic Photoproducts in Ethanol and for Photodegraded SO-Doped MTEOS Thin Films photoproduct or dye trimethyloxindole dimethyloxindole naphthoxazol photodegraded SO2-MTEOS
photodegraded SO1-MTEOS
excitation fluorescence relative band (nm) band (nm) intensitya 350 310 450 315 340 325 300 370 330 320 285 370 335
435 360 555 385 430 335 335 430 350 350 350 440 410
53 100 6 100 1 100 80 13 86 78 100 53 100
a For each spectrum, the intensity of the main band was normalized to 100.
Figure 11. Excitation/fluorescence spectra for the photodegraded SO1doped MTEOS film.
photodegraded film then appears from both the fluorescence of the naphthoxazol and the fluorescence of the trimethyloxindole. However, the two other photoproducts could be present in gels because, on the one hand, we do not know the emission spectrum of the tetrahydrodioxodimethylquinoline and, on the other hand, the characteristic band of the dimethyloxindole could be masked by the intense bands of the naphthoxazol. The excitation/fluorescence spectrum of a partially photodegraded SO1-doped thin film, in Figure 11 and Table 4, presented only two characteristic bands. The band at 370/440 nm can be due to the methoxy-1-n-propyl-3,3-dimethyloxindole photoproduct,23 which probably has a characteristic response
similar to that of the trimethyloxindole. Although another contribution of a new photoproduct cannot be thoroughly eliminated, at least two solutions can be proposed for the assignment of the other band at 335/410 nm as deduced from the SO1 degradation: (1) pyridobenzoxazole, which is probably generated and has bands shifted according to those of the naphthoxazol because of the substitution of a carbon by a nitrogen atom on the naphthyl cycle; (2) methoxy-1-n-propyl3,3-dimethyloxindole, as it exists for the trimethyloxindole at 310/360 nm. After the photodegradation of photochromic molecules, like these spirooxazines, could be detected via the fluorescence of
9030 J. Phys. Chem., Vol. 100, No. 21, 1996
Biteau et al. respectively, showing a higher photostability for SO1 spirooxazine molecules in gel matrices. Conclusion
Figure 12. Kinetics of photodegradation deduced from the evolution of the excitation/emission band intensities at 285/350 nm and at 335/ 410 nm for SO2- and SO1-doped gels, respectively. Solid lines correspond to best fits obtained by using the eq 4.
photoproducts was shown, a second problem was to quantitatively follow the photodegradation of doped gels during continuous UV irradiation. In fact, the formation of several heavy photoproducts in solution has been previously shown,23 which probably makes it impossible to quantitatively analyze the photodegradation by fluorescence of only one of the produced molecules. Nevertheless, supposing that the photoproducts are generated in constant proportions all along the photodegradation of merocyanines, we can deduce the kinetics of the relative photodegradation under UV irradiation from the time evolution of the fluorescence intensity for one band. Figure 12 shows the evolution of the band intensities at 285/ 350 nm for the photodegraded SO2-doped gels and at 335/410 nm for the photodegraded SO1-doped gels. Both curves present a maximum of the fluorescence intensity, suggesting the presence of two consecutive reactions. Fluorescent molecules are photoproduced in a first reaction from the spirooxazine molecules and then are photodegraded into nonfluorescent ones during a second step. By use of a simple model with two firstorder consecutive reactions set, A (spirooxazines) f B (fluorescent molecules) f C (nonfluorescent molecules), the concentration of B fluorescent species is given by the following relation:
[B]t ) [A]0
{
kf (e-kft-e-kdt) kd - kf
}
(3)
where [A]0 is the initial concentration of spirooxazine molecules in the gel film and kf and kd are the apparent rate constants for the formation and the degradation reaction of fluorescent molecules, respectively. The fluorescence intensity is proportional to the product of [B] by an absorption term, dependent on time, which is essentially due to the two forms of the photochromic molecules. For a thin film, the intensity is then given by
I ∝ [B]τ(1 - K[A]τ)
(4)
where K is a constant corresponding to the film thickness multiplied by the extinction coefficient. Equation 4 adequately described the experimental data (Figure 12). Under our experimental conditions, the half-lives (t1/2 ) [A]0/2) are 470 and 1570 min for SO2- and SO1-doped gels,
To sum up, photochromic thin films were prepared by using two SO1 and SO2 spirooxazines and different sol-gel precursors. Organically modified doped gels showed normal photochromism with a quinoid structure for the metastable photomerocyanine. As in polymers, the photodynamics did not proceed with firstorder kinetics because of the inhomogeneous distribution of free volume in the gel matrix. The agreement between the results from the Gaussian model and the experimental thermal decay of the coloration was remarkable. However, for both the formation of the photomerocyanine and the thermal ring closure, a simple biexponential model allowed for the comparison of photodynamics in gels with previous data in polymers. Concerning the dark decoloration, the kinetics was similar to the one observed in rigid organic polymers such as polyurethanes. In VTEOS-TEOS mixed matrices, the assumption of Levy et al.7 concerning the presence of two chemical environments for the dye molecules was demonstrated. For the first part of the spirooxazine molecules, surrounded by vinyl groups, the photochromism was normal with a stable closed form and a quinoid structure for the metastable merocyanine. For the second part of the molecules, surrounded by hydroxyl groups, the photochromism was reversed with a stable open zwitterionic form. Finally, in contrast with spirooxazine dyes, excited states of photoproducts were found to be fluorescent. This allowed us both to detect and to follow the kinetics of photodegradation of spirooxazines in solid media by recording excitation/emission spectra during UV light irradiation. However, further experiments are required, implying the synthesis of photoproductsdoped gels with a known concentration of doping molecules, to thoroughly analyze the photodegradation of the reference molecule SO2 using fluorescence data. Acknowledgment. We thank PPG Industries for supplying SO1 molecules. Aid in analysis and discussions with G. Baillet, P. Tardieu, and J.-P. Cano, all of Essilor International, are gratefully acknowledged. References and Notes (1) Avnir, D.; Levy, D.; Reisfeld, R. J. Phys. Chem. 1984, 88, 5956. (2) Canva, M.; Le Saux, G.; Georges, P.; Brun, A.; Chaput, F.; Boilot, J.-P., Optics Letters 1992, 17, 218. (3) Bentivegna, F.; Canva, M.; Georges, P.; Brun, A.; Chaput, F.; Malier, L.; Boilot, J.-P. Appl. Phys. Lett. 1993, 62 (15), 1721. (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.; Georges, P.; Perelgritz, J.-F.; Brun, A.; Chaput, F.; Boilot, J.-P., Appl. Opt. 1995, 34, 428. (6) Levy, D.; Avnir, D. J. Phys. Chem. 1988, 92, 4734. (7) Levy, D.; Einhorm, S.; Avnir, D. J. Non-Cryst. Solids 1989, 113, 137. (8) Preston, P.; Pouxviel, J.-C.; Novinson, T.; Kaska, W.; Dunn, B.; Zink, J. I. J. Phys. Chem. 1990, 94, 4167. (9) Matsui, K.; Morohoshi, T.; Yoshida, S. Proc. MRS Int. Meet. AdV. Mater. 1989, 12, 203. (10) Nakao, R.; Abe, Y.; Horii, T.; Kitao, T. Proc. Int. Symp. Chem. Funct. Dyes, 2nd 1992, 388. Nakao, R.; Ueda, N.; Abe, Y.; Horii, T.; Inoue, H. in Polym. AdV. Technol. 1993, 5, 240. (11) Hou, L.; Hoffmann, B.; Menning, M.; Schmidt, H. J. Sol-Gel Sci. Technol. 1994, 2, 635. (12) Kellmann, A.; Tfibel, F.; Dubest, R.; Levoir, P.; Aubard, J.; Pottier, E.; Guglielmetti, R. J. Photochem. Photobiol. A 1989, 49, 63.
Spirooxazine-Doped Gels (13) Engelhardt, G.; Michel, D. In High Resolution Solid State NMR of Silicates and Zeolites; Wiley: New York, 1987. (14) Malier, L.; Devreux, F.; Chaput, F.; Boilot, J.-P. in Chemical Processing of AdVanced Materials; Hench, L., West, J. K., Eds.; Wiley: New York, 1992; Vol. 6, p 59. (15) Kryszewski, M.; Nadolski, B.; North, A. M.; Pethrick, R. A. J. Chem. Soc., Faraday Trans 2 1980, 76, 351. (16) Richert, R. Chem. Phys. Lett. 1985, 118, 5, 534. (17) Munakata, Y.; Tsutui, T.; Saito, S. Polym. J. 1990, 22, 9, 843. (18) Albery, W. J.; Bartlett, P. N.; Wilde, C. P.; Darwent, J. R. J. Am. Chem. Soc. 1985, 107, 1854. (19) Samuel, J.; Ottolenghi, M.; Avnir D. J. Phys. Chem. 1992, 96, 6398
J. Phys. Chem., Vol. 100, No. 21, 1996 9031 (20) Marevtsev, V. S.; Kol’tsova, L. S.; Lyubimov, A. V.; Cherkashin, M. I. IzV. Akad. Nauk SSSR, Ser. Khim. 1988, 10, 2259. (21) Chu, N. Y. C., in Photochromism Molecules and Systems; Du¨rr, H., Bouas-Laurent, H., Eds.; Elsevier: Amsterdam, 1990; p 493. (22) Baillet, G.; Giusti, G.; Guglielmetti, R. Proc. Int. Symp. Chem. Funct. Dyes, 2nd 1992, 417. (23) Baillet, G., Thesis, University of Aix-Marseille, France, 1994. (24) Baillet, G.; Giusti, G.; Guglielmetti, R. Bull. Chem. Soc. Jpn. 1995, 68, 1220.
JP953607O
