J. Phys. Chem. B 2000, 104, 12179-12183
12179
Effect of Gel-Trapping on Spectral Properties and Relaxation Dynamics of Some Spiro-Oxazines F. Ortica and G. Favaro* Dipartimento di Chimica, UniVersita` di Perugia, 06123 Perugia, Italy ReceiVed: June 5, 2000; In Final Form: September 27, 2000
The photochromism of three spiro-oxazines was investigated in a gel network (AOT/isooctane/animal gelatin/ water), which is transparent to the visible and near UV light, as well as in a microemulsion (AOT/toluene/ water) and in the hydrocarbon component of the microheterogeneous gelled system. The spectral shifts of the photomerocyanine color band, which were determined in different media, provided information on the location of the molecules in these microheterogeneous systems. By a photokinetic study of the color-forming and color-fading reactions, the photocoloration quantum yields and bleaching rate constants were determined. By comparing the results obtained in a gel network with those determined in a homogeneous solution, it was observed that for two of the compounds investigated the photocolorability markedly decreases in the gel as a consequence of hastening of the bleaching processes. For the third molecule (an anthraquinone derivative), the colorability greatly increases in a gel network. The different behaviors are explained in terms of the ground-state polarity properties of the photomerocyanines and the consequent solute/solvent interaction effects on the dynamics of thermal bleaching.
Introduction Entrapping photochromic molecules in sol-gel materials may provide a route to various applications such as coatings for glasses, optical memory systems and waveguides. A number of spiropyrans and spirooxazines have been studied in different sol-gel matrices, such as polymers formed from alkoxy silane or alumino alkoxy silane monomers.1-3 These media, doped with the photochromic molecules, provide a viscous sol that converts into a solid phase that is suitably shaped (film, rod) for the various applications. In this paper, we studied the photokinetic behavior of three spiroindolino-oxazines trapped in an atypical gel, formed by a hydrophilic polypeptide (animal gelatin) solubilized in a waterin-oil microemulsion.4 In contrast to those mentioned above, this gel is a “soft” solid which looks like aqueous gelatin solutions. The molecules studied were a naphtho-spiro-indoline-oxazine (1), a derivative of 1 substituted with piperidine in the naphtho moiety (2) and an anthraquinone derivative (3). The photochromic behavior of these three molecules was previously investigated in organic solvents5-11 and 1 was also studied in micellar media.12 The photochromism of these molecules is due to cleavage of the C-O spiro-bond upon UV irradiation; the closed spiro-oxazine (SO) is colorless, while the open photomerocyanine (PM) absorbs in the visible region. The reaction is thermoreversible; in the dark, at room temperature, the photoproduced merocyanine reconverts to the starting molecule within a few seconds or minutes (Scheme 1). These compounds, which are completely insoluble in water, can be solubilized in micellar aqueous solution, microemulsions and aqueous gel. In the microheterogeneous systems the chromatic properties as well as the photokinetic behavior change. In this work, the effects of gel trapping on the absorption spectrum of the colored photomerocyanine and the kinetics of
the color-forming and color-bleaching reactions were investigated and compared with results obtained in homogeneous solutions and other microheterogeneous media. Experimental Section Materials. The molecules studied (SOs), supplied by the Great Lakes Chemical Italia S.r.l. for previous works,5-7 were used without further purification. The components of gel, isooctane (Fluka), aerosol-OT (sodium bis-2-ethyl-hexyl-sulfosuccinate), AOT (Fluka), and animal gelatin (porcine skin, SCHEME 1
* Corresponding author. Fax number: +39-075 585 5598. E-mail:
[email protected].
10.1021/jp002026i CCC: $19.00 © 2000 American Chemical Society Published on Web 11/30/2000
12180 J. Phys. Chem. B, Vol. 104, No. 51, 2000
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TABLE 1: Absorption Maxima (λ, nm) in the Visible Region of 1, 2 and 3 PMs in Gel and Microemulsions (AOT/Toluene/Water), Compared with Homogeneous Solutions (Isooctane, Iso-OT; Ethanol, EtOH, and Methylcyclohexane, MeCH) 1 2 3 a
GEL
AOT/toluene/water
iso-OT
EtOHa
MeCHa
613 600 611
600b
553 538 603
613 588 610
585 566 605
618
Taken from ref 6. b Taken from ref 8.
Sigma) were used as supplied. Water was deionized and twice distilled. To prepare the gelled microemulsion doped with the photochromic compound, the SO was first dissolved into 90 mL of isooctane, then the surfactant (AOT, 4.44 g) and water (10 mL) were added. The gelatin (3-4 g) was added to the microheterogeneous solution so obtained. After stirring to attain macroscopic homogeneity, the system was kept at rest for a few hours to allow the nucleation step. The stability of the gelled microemulsion is very critical: it was transparent and solid in the 0-20 °C range; above this temperature range, it became fluid and below, it became turbid.4 However, the transitions were perfectly reversible if the appropriate conditions were restored. We took advantage of this property by gently warming the system, before pouring it into the spectrophotometric cell, and then re-cooling it to the appropriate temperature. Equipment and Measurement Conditions. Absorption spectra were recorded on a Perkin-Elmer Lambda 16 or a diode array Hewlett-Packard 8453 spectrophotometer. A 250 W medium-pressure mercury lamp, filtered by an interference filter (366 nm) or a 150 W Xe lamp coupled with a Jobin-Yvon H10 UV monochromator were used to produce the colored photomerocyanine form (PM). The concentrations of the starting SOs were on the order of 1 × 10-4 mol dm-3, corresponding to absorbances in the 0.31.0 range at the irradiation wavelength. Since the gel was not completely transparent at the irradiation wavelengths, the absorbance of the sample cell, containing the photochromic molecule in gel, was measured vs a cell containing only the gel, to avoid any interference from the background absorbance in the spectrophotometric determinations. The irradiation of the sample (1 cm path cell, 1 cm3 of solution) was carried out in the spectrophotometer holder at a right angle to the analysis light. The increase of the PM absorbance, under stationary irradiation and constant temperature (283 K), was followed at the absorption maximum wavelength (530-620 nm), where the excitation light did not disturb the spectrophotometric measurement, up to photostationary state attainment. The intensity of the irradiating monochromatic light (typically, 10-7 Einstein dm-3 s-1) was determined using potassium ferrioxalate actinometry. An ethanol-thermostat Haake F3 or an Oxford Instrument cryostat were used to control the temperature. The kinetics of the ring-closure reaction was followed from the fading of the colored form at the wavelength of maximum absorbance, after having discontinued the irradiating source. The fading processes followed first-order kinetics; the rate constants were obtained from linear ln A vs time plots (correlation coefficient 0.99); the reproducibility over different solutions was within 10%. For the interpolation with the appropriate functions of the experimental absorbance/time data sets of the color-forming and color-bleaching processes, the computerized graphical software programs were used.
Figure 1. Absorption spectra of 1 and 3 in gel (;) and isooctane (- - -).
Figure 2. Color-forming kinetics of 1 and 3 in gel (;) and isooctane (- - -).
Results Absorption Spectra. The absorption spectra of the photomerocyanines derived from the photochromes under study were taken in gel, microemulsion and in isooctane (the hydrophobic component of the gel network) solution, after UV irradiation up to photostationary state attainment. In Table 1, the observed absorption maxima are compared with those in ethanol and methylcyclohexane. The spectra of the colored forms of 1 and 3 in a homogeneous solution (isooctane) and in a gel are shown in Figure 1. As can be seen, in gel, the color band shifts to the red compared to the iso-octane solution. The
Photochromism of Spiro-Oxazines
J. Phys. Chem. B, Vol. 104, No. 51, 2000 12181 determined by a fit procedure. This equation corresponds to the integration of eq 1,13 assuming F to be time-independent, with
a ) PMΦI0FA′SO/(ΦI0FSO + k∆)
(3)
R ) ΦI0FSO + k∆
(4)
and
Since I0, F, SO, A′SO and k∆ can be experimentally determined, the Φ and PM values can be obtained from the fitting parameters, a and R, by means of the following equations: Figure 3. Example of PM determination based on eq 1 for 1 in gel.
PMΦ ) Ra/I0FA′SO
(5)
red shift is notable for 1 and 2 (not shown in the figure), but small for 3. A change in shape of the absorption band was also observed. It has already been reported that the absorption of 3 is much less sensitive to environment than 1 and 2 in a homogeneous solution.5,6 Photokinetics. The photokinetics of the ring-opening reactions were studied in isooctane, microemulsion and gel. Temperature (283 K) was rigorously controlled, to keep the rate parameter of the thermal back-reaction, k∆, constant. The time dependence of absorbance during irradiation of 1 and 3 in gel and iso-octane is shown in Figure 2. It can be observed that the absorbance at the photostationary state is much higher in gel than in isooctane for 3 but it is lower for 1. In 2, the photocoloration in gel is so weak that it is hardly detectable, and completely undetectable in a microemulsion. To determine the quantum yield of the forward photochemical reaction and the molar absorption coefficient of the metastable PM form, the rate equation of the color-forming process was set up (eq 1) in terms of the time dependence of the PM absorbance (APM) at the analysis wavelength (maximum of the color band). The quantum yield of the reverse photoreaction was assumed to be negligible.11
Φ ) (R - k∆)/I0FSO
(6)
dAPM/dt ) PMΦI FA′SO - k∆APM 0
(1)
In eq 1, PM (dm3 mol-1 cm-1) is the molar absorption coefficient of the PM, Φ is the quantum yield of the photoreaction, I0 is the intensity of the monochromatic excitation light, λexc ) 366 nm, per time unit (Einstein dm-3 s-1), A′SO is the SO absorbance at λexc and F ) [1 - exp(-2.3A′)]/A′ represents the photokinetic factor13-15 (A′ is the total absorbance at λexc), which accounts for the variation of the absorbed fraction of the total incident light, I ) I0[1 - exp(-2.3A′)], during the reaction course. The photokinetic factor is a rigorously time-independent parameter only if irradiation is carried out at, or near at, an isosbestic point of the absorption spectra of the colorless and colored forms. Under our experimental conditions (283 K, λexc ) 366 nm), since the transformation percentage at the photostationary state was very small for 1 and 2, the absorbed light did not appreciably change and F could be considered timeindependent. For 3, the conversion percentage at the photostationary state was higher; however, irradiation was carried out at a wavelength close to an isosbestic point. Three independent approaches were followed to determine Φ and PM. (i) Formally, during the color-forming process, the experimental trend of APM vs time is described by a monoexponential function, eq 2, where a and R are coefficients which were
APM ) a(1 - e-Rt)
(2)
However, only the product PMΦ can be obtained with reasonable accuracy from eq 5, since (R - k∆), in eq 6, has often a very small value obtained as the difference between similar quantities. (ii) An alternative approach to determine the PMΦ value is based on the differential eq 1. By plotting dAPM/dt vs APM, a straight line is obtained. Extrapolation of the color-forming rate at zero time (that is at APM ) 0) provides the PMΦ value by means of
PMΦ ) (dAPM/dt)t)0/I0FA′SO
(7)
An example of treatment of the experimental data is shown for 1 in gel in Figure 3. (iii) The PMΦ value can also be determined using data obtained from the photostationary state, namely, when dAPM/dt ) 0 (eq 8) and the PM absorbance reaches a limiting value (A∞PM). Superscripts ∞ refer to photostationary conditions.
PMΦI0F∞A′∞SO ) k∆A∞PM
(8)
The limiting value, A∞PM, that is, the horizontal asymptote in the graphs of Figure 2, is a measure of the photostationary colorability of the photochrome under fixed conditions (temperature, solvent, concentration, excitation light intensity and wavelength). All of the kinetic methods described were used to determine and compare PMΦ values for the molecules 1 and 2. The most critical point to obtain Φ is using a reliable PM value.6,7,9,11 Comparing the absorption spectral shifts of the PMs derived from 1 and 2 in the microheterogeneous systems and in isooctane solution with those in different nonpolar and polar solvents, an acceptable approximation was using the PM values determined in ethanol for gelled systems, and those determined in methylcyclohexane for isooctane solutions.6 Thus, to calculate Φ, PM ) 61000 and 38000 dm3 mol-1 cm-1 and PM ) 87000 and 51000 dm3 mol-1 cm-1 were used in gel and in isooctane, for 1 and 2, respectively. For 3, the solvent spectral shifts of the PM form are smaller and sometimes in a direction opposite to that expected based on the usual positive solvatochromism of this class of molecules.5 However, in this case, it was possible to determine the PM value by using the integration method, (i), since the difference between the color-forming (R) and the color-bleaching (k∆) rate parameters (eq 6) was large enough to give a reliable result. The bleaching rate coefficient, k∆, was easily measured from first-order kinetic treatment of the decay curve of the metastable
12182 J. Phys. Chem. B, Vol. 104, No. 51, 2000
Ortica and Favaro
Figure 5. Solvatochromic effect for the colored form of 3: energy of the absorption maximum (cm-1) vs the ET(30) Dimroth parameter. (Dimroth, K.; Reichardt, C.; Siepmann, T.; Bohlmann, F. Liebigs. Ann. Chem. 1963, 661, 1).
Figure 4. Color-bleaching kinetics of 1 and 3 in gel (;) and isooctane (- - -).
TABLE 2: Photokinetic Parameters (k∆ and Φ) of 1, 2 and 3 at 283 K Obtained in Gel, AOT Microemulsion and Isooctane, Compared with Those in EtOH and MeCHa 1 Gel AOT iso-OT EtOHc MeCHc b
k∆/s-1 0.15 0.16b 0.0595 0.041 0.052
2
3
F
k∆/s-1
F
k∆/s-1
0.17 0.46b 0.40 0.32 0.41
1.28
F
0.23
0.0023 0.0081 0.06 0.00085 0.028
1.2 0.76 0.86 0.16 0.73
0.012 0.24 0.06
0.52 0.65 0.55
a k values taken from the literature were recalculated at 283 K. ∆ Taken from ref 8. c Taken from ref 6.
PM (Figure 4), obtained after having switched off the irradiating source (eq 9).
APM ) APM∞ e-k∆t
(9)
The parameters obtained from the photokinetic study in different homogeneous and microheterogeneous media at 283 K, are reported in Table 2. Discussion The two microheterogeneous media, microemulsion and gel, where the three photochromes were investigated, contain hydrophobic and hydrophilic sites. In the pure water/AOT/ toluene microemulsions, charges originating from AOT, which is a strong electrolyte, form a semi-diffuse double layer at the oil/water interface. The transparent gelled microemulsion, prepared by following a literature method,4 has a network of nanodroplets interconnected via tropocollagen-like helices; the hydrophilic gelatin is confined to the aqueous core of the nanometer-sized droplets, at the water/oil interface of the nanodroplet aggregates and in the interconnections among the nanodroplets. The closed SO forms are essentially nonpolar and are therefore expected to dissolve in the less polar microenvironment, that is, at the droplet surface (or the fractal nanodroplet
clusters). The open PM form, which is more polar, once formed under UV irradiation, may migrate to a polar environment, such as the water pool in microemulsion and the tropocollagen-like helices of the solubilized gelatin in gel. While the absorption spectra of the closed SOs are scarcely affected by environment, those of the open PMs generally shift to the red with increasing solvent polarity (positive solvatochromism).5 The spectral location of the color band should give information on the nature of the solubilization site of the PMs. The spectra of the colored forms of 1 and 2 in gel are red-shifted by about 60 nm with respect to their position in the hydrocarbon solvent and by a few nanometers compared to EtOH (Table 1). This suggests that the open forms are solubilized at a site which has a greater polarity than ethanol. This means that the PM hydrophilic form, produced in gel under UV irradiation, migrates from the droplets surfaces, where the SO hydrophobic closed form was originally located, to the water-rich helices. The λmax shift observed between isooctane solution and gel is much less marked in 3. This is in agreement with the findings from our previous studies on the solvatochromism of these molecules,5-7 where the redshift due to solvent polarity was minimal for 3 (135 cm-1 from MeCH to EtOH), compared with other photochromic compounds. In some solvents, the solvatochromism of 3 even becomes negative; see Figure 5. Such behavior was attributed to a larger contribution of zwitterionic forms to the resonance hybrid describing the ground-state photomerocyanine. In AOT microemulsions, the photocoloration of 1 was very low, that of 2 could not be detected at the experimental temperature (283 K), while 3 exhibited fairly intense photocoloration. The color-band was located at 618 nm, the maximum red-shift compared to the other media. Considering the dynamics of thermal relaxation of PM in organic solvents (Table 2), it can be observed that the polarity effect on the bleaching rate parameter for 1 was substantially insignificant; for 2, k∆ increased as the polarity of the medium increased, whereas for 3 k∆ decreased as the polarity of the medium increased. Also the effect of the microheterogeneous media (AOT microemulsion and gel) on k∆ is different in the three molecules: k∆ increases for 1 and to a greater extent for 2, while it markedly decreases for 3. These behaviors have dramatic consequences on photocolorability. In fact, when photocoloration is carried out under steady irradiation, as in most practical applications (glasses, containers for photosensitive products, coating materials), the slower the bleaching rate (k∆) and the larger PM and Φ are, the greater the maximum color
Photochromism of Spiro-Oxazines intensity that is attainable at the photostationary state. From an inspection of the data in Table 2, it can be seen that Φ is almost halved in gel compared to isooctane for 1 and 2, while it increases for 3. However, the Φ values are less affected by the medium than the bleaching rate parameters. Therefore, the colorability is mainly determined by the relaxation dynamics; it decreases in gel compared with a homogeneous solution for 1 and 2 and increases greatly for 3. The relaxation dynamic is dominated by the activation energy which is controlled by the solute-solvent interactions in the ground state of PM and in the transition state to the closed SO form. If the PM ground state is polar, as in the case of 3, it is stabilized by solvent polarity more than the less polar transition state, leading to increasing in the activation energy and slowing down of the thermal reaction. That is, for PMs having a polar ground state, the bleaching rate decreases with increased solvent polarity, as expected for a process in which charge is dissipated on going from the ground state to the transition state.16 For molecules having a more quinoid ground state (as 1 and especially 2), the transition state is more stabilized by solvent polarity than the ground state and the activation energy decreases. Positive solvatochromism, that is, red-shift of the maximum of the absorption band from nonpolar to polar solvents, occurs when the excited state (π, π*) dipole moment of the PM is higher than the ground-state dipole moment. The larger this difference, the greater the spectral shift and more the ground-state weakly polar molecule should approach the configuration of the quinoid form. According to expectation, the positive solvatochromism is disfavored by substituents which increase the contribution of zwitterionic forms to the resonance hybrid describing the ground-state photomerocyanine, that is, electron-attracting groups in the oxazine moiety.7 Other factors, such as the difference in the molecular volume between the PM and the transition state (the volume of the spiro form is greater than that of the PM)17 should not be important since the effect is in the same direction for all three molecules. Concluding Remarks In this paper a method was developed to study the photochromism of molecules embedded in a transparent gel network. The spiro-oxazines investigated, which are practically insoluble in water, can be solubilized in an aqueous-gel medium due to the presence of hydrophobic structures. The photoproduced PMs are preferentially located in the hydrophilic sites. The results obtained show that trapping of photochromic molecules in a gel network may change the spectrum of the colored form and cause dramatic effects on its relaxation dynamics which markedly affects the photocolorability. For two of the molecules investigated (1 and 2), the photocolorability decreases in a gel, compared to homogeneous solutions, mainly due to a hastening of the bleaching process and, to a lesser extent, to a decreased quantum yield. For the third molecule (3), the opposite effect was observed; the photocolorability increases and the bleaching rate decreases in a gel network. The
J. Phys. Chem. B, Vol. 104, No. 51, 2000 12183 different photochromic responses of these molecules when entrapped in microheterogeneous systems, depend on the different polar character of the photomerocyanine. Since this property also governs the solvatochromic shifts, the behavior of a photochromic molecule in a gel, where high polarity sites are present, can be predicted by looking at the spectral shift of the color band in organic solvents of different polarities. Compounds showing highly positive solvatochromism would be expected to have less photocolorability when entrapped in the quasi-rigid gel network. In contrast, molecules where the polarity of the PM ground state is similar or higher than that of the excited state should exhibit a fairly intense photocoloration in the gel network. The parallel behavior of the solvatochromism of PM (which involves the energy difference between ground and excited states) and the solvent effect on the relaxation dynamics to the closed form (related to the activation energy of the bleaching process) suggests that the transition state is characterized by a structure which closely resembles that of the PM excited state. Acknowledgment. This research was funded by the “Ministero per l’Universita` e la Ricerca Scientifica e Tecnologica” (Rome) and the University of Perugia in the framework of the “Programmi di Ricerca di Interesse Nazionale” (project: Mechanism of Photoinduced Processes in Homogeneous Media and in Complex Matrixes). A grant from the Italian Consiglio Nazionale delle Ricerche is also acknowledged. The authors thank Prof. V. Malatesta, Great Lakes Chemical Italia, for the samples of photochromic molecules. References and Notes (1) Biteau, J.; Chaput, F.; Boilot, J.-P. J. Phys. Chem. 1996, 100, 9024; Mol. Cryst. Liq. Cryst. 1997, 297, 49. (2) Levy, D. Mol. Cryst. Liq. Cryst. 1997, 297, 31. (3) Sun, X.; Fan M.; Knobbe, E. T. Mol. Cryst. Liq. Cryst. 1997, 297, 57. (4) Quellet, C.; Eicke, H. F.; Sager, W. J. Phys. Chem. 1991, 95, 5642. (5) Favaro, G.; Masetti, F.; Mazzucato, U.; Ottavi, G.; Allegrini, P.; Malatesta, V. J. Chem. Soc., Faraday Trans. 1994, 90, 333. (6) Favaro, G.; Malatesta, V.; Mazzucato, U.; Ottavi, G.; Romani, A. J. Photochem. Photobiol. A: Chem. 1995, 87, 235. (7) Favaro, G.; Malatesta, V.; Mazzucato, U.; Miliani, C.; Ottavi, G. Proc. Indian Acad. Sci. 1995, 107, 659. (8) Chu,N. Y. C. Can. J. Chem. 1983, 61, 300. (9) Kellmann, A.; Tfibel, F.; Dubest, R.; Levoir, P.; Aubard, J.; Pottier, E.; Guglielmetti, R. J. Photochem. Photobiol., A: Chem. 1989, 49, 63. (10) Grummt, U. W.; Reichenbacher, M.; Paetzold, R. Tetrahedron Lett. 1981, 22, 3945. (11) Wilkinson, F.; Hobley, J.; Naftaly, M. J. Chem. Soc., Faraday Trans. 1992, 88, 1511. (12) Favaro, G.; Ortica, F.; Malatesta, V. J. Chem. Soc., Faraday Trans. 1995, 91, 4099. (13) Ottavi, G.; Favaro, G.; Malatesta, V. J. Photochem. Photobiol. A: Chem. 1998, 115, 123. (14) Borderie, B.; Lavabre, D.; Micheau, J. C.; Laplante, J. P. J. Phys. Chem. 1992, 96, 2953. (15) Gauglitz, G. In Photochromism; Du¨rr, H., Bouas-Laurent, H., Eds.; Elsevier: Amsterdam, 1990; p 15. (16) Keum, R. S.; Hur, M. S.; Kazmaier, P. M.; Buncel, E. Can. J. Chem. 1991, 69, 1940. (17) Wilson, P. G.; Drickamer, H. G. J. Chem Phys. 1975, 63, 3649.
