Photochromism and Thermochromism of some Spirooxazines and

Mar 12, 2010 - Michel Frigoli , Francois Maurel , Jérôme Berthet , Stéphanie Delbaere , Jérôme Marrot , and Maria. M. Oliveira. Organic Letters 2...
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J. Phys. Chem. C 2010, 114, 6123–6131

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Photochromism and Thermochromism of some Spirooxazines and Naphthopyrans in the Solid State and in Polymeric Film Maria Rosaria di Nunzio, Pier Luigi Gentili, Aldo Romani, and Gianna Favaro* UniVersita` di Perugia, Dipartimento di Chimica, 06123 Perugia, Italy ReceiVed: NoVember 19, 2009; ReVised Manuscript ReceiVed: February 24, 2010

In this work the photochromic behaviors of four Reversacol compounds from James Robinson Ltd., previously investigated in solution, have been studied in a microcrystalline solid phase and embedded in a poly(methyl methacrylate) matrix (PMMA). The compounds studied belong to the classes of spirooxazines and naphthopyrans. In solution, they showed photocoloration and thermocoloration. Embedded into polymeric films, as well as in the microcrystalline phase, they have maintained their photochromic and thermochromic behavior. The photocoloration quantum yields in PMMA were determined and found to be fairly high. Compared to solution, the kinetic and thermodynamic activation parameters of thermal bleaching significantly changed. The solid environment has the effect of slowing down the bleaching rates in films as well in the pure crystalline phase and introduces multiexponential terms in the photocoloration and thermal bleaching kinetics. Activation enthalpy and entropy decrease, compared to solution, leaving the activation free energy substantially unchanged. Thermochromism was characterized through the equilibrium constant and the thermodynamic standard parameters, ∆H0, ∆S0, and ∆G0, of the thermal reactions. Time-dependent processes were also analyzed according to the maximum entropy method (MEM). Based on the data from the leastsquares and MEM treatments, the occurrence of multiexponential kinetics in polymers was attributed to the effects of the inhomogeneous distribution of free volume in the matrix. CHART 1

1. Introduction 1,2

3-6

Spirooxazines and chromenes (benzo- and naphthopyrans) belong to classes of photochromic compounds that have been widely studied for a long time and are object of continuous interest up to now. These molecules change from colorless to colored upon UV irradiation that induces cleavage of the carbon-oxygen bond of oxazine or pyran cycles, originating colored merocyanine open structures. The photoreaction is thermally and photochemically reversible.

The compounds here investigated are two spirooxazines (SO-1 and SO-2) and two naphthopyrans (NP-1 and NP-2), Chart 1. They have been previously studied in solution where they exhibited interesting properties for potential applications as photochromic thermoreversible materials as well as efficient thermochromic substances.7,8 In the closed form, their absorption spectra allow activation even by solar light; the photoproduced colored merocyanines (PMs) have absorption spectra that extend over a wide range of the visible radiation, exhibiting neutral hues very promising for applications. The photocolorability in solution is fairly high due to significant quantum efficiency of the photochemical process (>20%) and high values of the absorption coefficients of the PMs. An indispensable step for useful applications of photochromic materials is to pass from the solution to the solid state. Inclusion of photochromes in solid media may provide a route to optically switchable materials.9,10 In a solid matrix, the photochromic guest and the surrounding microenvironment exert a reciprocal influence that may affect the relaxation channels of the * Corresponding author. Tel.: +39 075 5855573. Fax: +39 075 5855598. E-mail: [email protected].

electronically excited states, the time scale of the photochemical and thermal processes, and the chromatic properties of the systems. Spirooxazines and spiropyrans in microcrystalline solid phase have been for a long time reported not to show coloration upon UV irradiation. Nevertheless, Masuhara et al. have found that colored merocyanines can be produced by intense laser excitation.11,12 Also, steady UV irradiation resulted in photocoloration of a spirooxazine in powder.13 Lately, (PMMA)/ spirooxazine microspheres were prepared and their thermoreversible photochromic reaction was investigated.14 These new

10.1021/jp9109833  2010 American Chemical Society Published on Web 03/12/2010

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CHART 2

materials could find an application as sensors. The matrix effect on the photochromic reaction of a spirooxazine has been recently investigated, finding that the photocoloration rate of SP is faster in PMMA than in other matrices.15 The aim of this paper was to investigate the photochromic and thermochromic behaviors of the four photochromic compounds in pure solid phase and in poly(methyl methacrylate) (PMMA) films by determining their spectral and dynamic properties and to study the effect of the environment on these properties. PMMA is a clear and colorless thermoplastic material, with Tg that can range from 100 to 122 °C, depending on the molecular weight and polydispersity. It has a good scratch and UV resistance and low permeability to oxygen. Because of these properties it is used extensively for optical applications, such as lenses, light covers, and glazing. The repeat unit of PMMA is shown in Chart 2. The carboxy function is the site of interaction of the dipolar PM with the polymer matrix.14 This study was carried out under continuous irradiation; the kinetics of photocoloration and thermal bleaching were also analyzed using the maximum entropy method (MEM).16-18 2. Experimental Section 2.1. Materials. The four compounds studied (Chart 1) were purchased from James Robinson Ltd. and checked for purity (>98%) by HPLC. They are 1,3-dihydro-3,3-dimethyl-1-isobutyl-6′-(2,3-dihydro-1H-indol-1-yl)spiro[2H-indole-2,3′-3H-naphtho[2,1-b][1,4]oxazine] (SO-1), 1,3-dihydro-3,3-dimethyl-1neopentyl-6′-(4′′-N,N-diethylanilino)spiro [2H-indole-2,2′-3Hnaphtho[1,2-b][1,4] oxazine] (SO-2), 2-(4′-piperidinophenyl)2-phenyl-5-carbomethoxy-9-dimethylamino-2H-naphtho[1,2b]pyran (NP-1), and 2-(4′-dimethylaminophenyl)-2-(4′′methoxyphenyl)-5-hydroxymethyl-9-pyrrolidino-2H-naphtho[1,2b] pyran (NP-2). All compounds are in the microcrystalline phase, as controlled by optical microscopy. The polymer used was poly(methyl methacrylate) (PMMA) from Sigma-Aldrich: Mn ) 46000, Mw ) 97000, and Tg ) 105 °C. For the preparation of the films, 0.2 g of PMMA were dissolved in ethylacetate, chloroform, 1,2-dichloroethane or dichloromethane. Then, a weighted amount of photochrome was added to the polymer solution and stirred to obtain a homogeneous fluid; it was poured over a smooth surface by multiple spreading-drying cycles and left to dry. The film formed was peeled-off from the surface and stored in the dark. Its thickness, measured with a micrometer, was about 200 µm. Because the dye content in the film was e0.15%, we can assume that Tg was virtually identical in film as in the pure polymer. The best results, in terms of smooth and homogeneous film surface, were obtained using 1,2-dichloroethane. This solvent (bp ) 84 °C) showed an ideal evaporation rate from the polymer, precluding the formation of bubbles within the films. Kept in the dark for six months, the films did not show any changes in their thickness and UV-vis absorption spectra. 2.2. Photokinetic Measurements. The absorption spectra were recorded using a HP 8453 diode-array spectrophotometer or a double beam Lambda 800 UV-vis spectrophotometer. Details on measurements carried out in solution have been reported in a previous work.7

Figure 1. Absorption spectrum of NP-2 in MeCN solution (purple) and embedded in PMMA film (light gray) at room temperature. Inset: zoom on the longer wavelength region.

Figure 2. Absorption spectrum of pure solid SO-2: (1) before irradiation, (2) after 20 min UV irradiation, and (3) after 90 min in the dark. Top inset: microscope image (40×) reveals two different kinds of packing in the SO-2 crystalline phase: bundled fibrous assemblies (a few micrometers in length) and more compact crystalline structures. Bottom inset: kinetics of thermal bleaching for SO-2 in solid microcrystalline powder at room temperature. The light gray trace represents triexponential fitting function

For the photocoloration of powders, a Mineral Light source was used that essentially emits the 254 nm Hg line. The absorption spectra from the powders, packed into a spectrophotometric cell (0.1 cm path length), were collected in a reflectance mode using a Varian 4000 UV-vis spectrophotometer equipped with an integrating sphere made of barium sulfate. The total reflectance spectra, containing both the diffuse and specular components, were converted into Kubelka-Munk values (K/S).19-21 The kinetics of the thermal ring-closure reaction were spectrophotometrically recorded following the color bleaching of the irradiated sample at the maximum absorbance wavelength of PM, immediately after switching off the irradiating source. For the photocoloration of the photochromic films, a 125W Xe lamp, coupled with a Jobin-Yvon H10 UV monochromator and a fiber-optic system, was used, keeping the sample in the spectrophotometer holder at an acute angle with respect to the excitation beam, while the monitoring beam was at 90° with respect to the film surface. The photocoloration-thermal bleaching cycles were carried out using the same spectrophotometer/irradiating-lamp/monochromator setup. For the temperature control, a SPECAC variable temperature cell with silica fused windows was used. The irradiation was performed with a fiber optic focused on the sample through a mobile mirror. When this setup was used, no thermal radiation could reach the sample. The quantum yields were determined by treating the experimental time/absorbance data according to the initial

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TABLE 1: Comparison of the Bleaching Kinetic Constants at Room Temperature Determined in Powder with those in a Fluid MeCN Solution at 298 K MeCN solutiona 298

cmpd

k∆

SO-1 SO-2 NP-1 NP-2 a

-1

(s )

1.35 0.51 0.11 0.056

powder (298 K) -1

k∆1 (s )

-1

-1

k∆2 (s )

k∆3 (s )

0.0065 0.0042 0.0066 0.0027

0.00098 0.00062 0.00123 0.00059

0.000074 0.000017 0.00021 0.000086

Data taken from ref 7.

rate method. The ferrioxalate actinometer (0.1 cm3 solution, 0.1 cm path cell) and the polymer film were positioned using the same geometrical arrangement with respect to the irradiating source. The actinometer response (Iact) was related to the absorbance of the photochromic film at λexc (Afilm). Thus, the light intensity absorbed by the sample (Ifilm) could be determined, eq 1, where Aact is the absorbance of the actinometer.

Ifilm)Iact(1 - 10-Afilm)/(1 - 10-Aact)

(1)

Due to various sources of error, such as the imperfect homogeneity of the film over the 1 cm2 irradiated surface, uncertainty on the thickness of the film, and reproducibility of the position of the chemical actinometer, the yields are affected by a consistent error, estimated approximately to be (20%. 2.3. Analysis of Kinetic Data Sets. The kinetics of photocoloration and thermal bleaching were analyzed through two methods: the nonlinear least-squares method, by using the Origin Professional Software, and the Maximum Entropy Method (MEM), by using the MemExp Software available online.17,18 The kinetics were fitted by the relationship given in eq 2 using MEM, where g(log τ) and h(log τ) are the lifetime distributions that correspond to decay and rise kinetics, respectively, and the polynomial accounts for the baseline.

A(t) ) D0

∫-∞+∞ (g(log τ) -

h(log τ))e-t/τ d (log τ) +

3

∑ (bk - ck)(t/tmax)k

(2)

k)0

The fit procedure entails the maximization of the function Q defined in eq 3:

Q ) S - λχ2 - RI

(3)

where S is entropy, I is a normalization factor, and λ and R are Lagrange multipliers. In the definition of χ2, the standard errors in the measured data were assumed to be Gaussian type. The

choice of the best-fit function was made by considering the magnitude of χ2 and of the correlation length of the residuals. 3. Results 3.1. General Spectral Features. Absorption spectra of the four compounds were recorded in solid media at room temperature before and under UV irradiation up to photostationary state attainment. Then the kinetics of thermal bleaching were determined. For the films, also the quantum yields of the photochemical opening reactions and the thermodynamic activation parameters of the decoloration thermal processes were evaluated. The spectra recorded in powder and PMMA film at room temperature were compared with those in MeCN solution. The comparison showed a good correspondence of the spectra at short wavelengths but an appreciable increase of absorbance in polymer in the visible region due to thermal coloration (Figure 1). In powders, thermocoloration was even more marked. 3.2. Photochromism. 3.2.1. Photochromism in Powders. Considerable amounts of thermally equilibrated PM were spectrophotometrically detectable in powdery samples of the four compounds at room temperature even before irradiation. When excited with UV radiation (254 nm) all samples intensified their color and slowly bleached in the dark (Figure 2). The bleaching substantially restored the initial conditions after several hours. No photobleaching was observed. The bleaching kinetics, determined from the absorption-time data sets, were fitted to multiexponential decay functions, eq 4, where k∆i are bleaching rate constants, A0i are the relative weights of each decay contribution, and R accounts for the residue.

A)

∑ Ai0 × exp(-k∆i × t) + R

(4)

In general, the triexponential fit was satisfactory to describe the kinetic behavior, as depicted in the bottom inset of Figure 2 for SO-2. The k∆i values determined for the powdery photochromic samples are compared with the results in MeCN solution in Table 1. It can be observed that the k∆1 constants are markedly slowed down compared to the values in MeCN solution. A progressive reduction of order of magnitude is observed for k∆2 and k∆3. 3.2.2. Photochromism in PMMA. When exposed to UV irradiation, PMMA films, doped with any of the photochromes under investigation, became colored due to photoproduction of the PMs. The colored films slowly bleached in the dark. The maxima of the color bands were at wavelengths close to those in MeCN solution, as reported in Table 2, indicating that the interaction of the substrate with the polar polymer matrix is quite similar to that with the polar organic solvent. This finding allowed the reasonable assumption that molar absorption coefficients are also similar.

TABLE 2: Comparison of λmax of the Colored PMs and Photochemistry Quantum Yields (ΦPC) in MeCN Solution and PMMA Films at 295 K Determined at the Same Excitation Wavelengths MeCN solutiona cmpd (λexc/nm) SO-1 SO-2 NP-1 NP-2 a

(355) (301) (320) (324)

Data taken from ref 7.

λmax

PM

(nm)

601 629 496, 584 492, 604

PMMA film ΦPC 0.24 0.21 0.59 0.71

λmax

PM

(nm)

599 633 498, 578 488, 598

lfilm (µm; thickness)

ΦPC

180 170 160 200

0.4 ( 0.1 0.6 ( 0.1 0.8 ( 0.1 1.1 ( 0.3

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Figure 3. Spectral changes observed upon UV irradiation of the photochromic PMMA films doped with the four compounds. Insets: kinetics of photocoloration and thermal bleaching at 295 K; the continuous lines represent the multiexponential fit functions: (A) SO-1: λexc ) 355 nm; (B) SO-2: λexc ) 301 nm; (C) NP-1: λexc ) 320 nm; (D) NP-2: λexc ) 324 nm.

Figure 4. Arrhenius plots of thermal bleaching components in PMMA polymer film of the PM derived from SO-2 (plots for the other compounds are reported as Supporting Information).

The kinetics of photocoloration in PMMA were well described by polyexponential functions, eq 5, were Ri behave as rate coefficients that depend on the light intensity absorbed, the reaction quantum yield, and the bleaching rate. Similarly, the kinetics of thermal bleaching followed eq 4.

A)

∑ Ai0 × [1 - exp(-Ri × t)]

(5)

Both photocoloration and bleaching kinetics were well described by 3-4 rate components. The spectral evolutions of the photochromic PMMA films of the four compounds upon UV irradiation and the kinetics of photocoloration and thermal bleaching are depicted in Figure 3. No photobleaching was observed. To determine the photocoloration quantum yield, ΦPC, the initial rate method was used, eq 6, where εPM is the molar

Figure 5. Eyring plots of the thermal bleaching of the PMs derived from the four compounds in PMMA polymer film (9, first component; 2, second component; b, third component; [, fourth component).

absorption coefficient of the PM, Ifilm is the intensity of light absorbed by the sample, and lfilm is the thickness of the polymer film.

ΦPC )

(dAPM /dt)tf0 εPM×Ifilm×lfilm

(6)

This method allows the yield of the fastest process that dominates the initial steps to be estimated. Considering the spectral similarity of PM absorption spectra in MeCN and in PMMA, the εPM values in polymer film were approximated to those determined in organic solvent7 and used for the calculation of ΦPC. Spectral and photochemical results in films and in solution are compared in Table 2.

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TABLE 3: Comparison of the Bleaching Kinetic Parameters (k∆i and Eai) Determined in PMMA with those Determined in a MeCN Solution at 298 K MeCNa

PMMA

cmpd

k∆ (s-1)

Ea (kJ mol-1)

k∆1 (s-1)

Ea1 (kJ mol-1)

k∆2 (s-1)

Ea2 (kJ mol-1)

k∆3 (s-1)

Ea3 (kJ mol-1)

k∆4 (s-1)

Ea4 (kJ mol-1)

SO-1 SO-2 NP-1 NP-2

1.35 0.51 0.11 0.056

51 77 66.5 61

0.078 0.017 0.032 0.020

21 29 27 35

0.018 0.0033 0.0068 0.0053

33 22 21 28

0.0026 0.00047 0.0012 0.00095

50 24 23 31

0.00057

40

0.00014 0.0001

26 27

a

Data taken from ref 7.

TABLE 4: Activation Thermodynamic Parameters of the Bleaching Process Determined in PMMA (Fastest Kinetic Component) at 298 K Compared with those Obtained in a Fluid MeCN Solutiona MeCN solutionb

PMMA films -1

*

-1

*

cmpd

∆H1 (kJ mol )

∆G1 (kJ mol )

SO-1 SO-2 NP-1 NP-2

19 26 24 33

79 83 82 83

a

*

∆S1 (J mol

-1

-200 -190 -190 -170

-1

K )

*

-1

∆H (kJ mol )

∆G* (kJ mol-1)

∆S* (J mol-1 K-1)

48.5 74.5 64.0 58.7

72.2 74.7 78.5 80.1

-80 -0.7 -49 -72

Values for all kinetic components are reported as Supporting Information. b Data taken from ref 7.

Measurements of thermal bleaching were carried out as a function of the temperature in the 290-360 K range. The activation energies and frequency factors of the individual kinetic components (three for SO-2 and four for SO-1, NP-1, and NP-2) were obtained by Arrhenius treatment. An example of Arrhenius plots is shown in Figure 4 for the PM derived from SO-2. The kinetic parameters (rate constants and activation energies) for the multistep thermal bleaching of the four compounds are reported in Table 3. By using the Eyring equation (eq 7), the thermodynamic activation parameters of the thermal back reactions, ∆H*, ∆S*, and ∆G*, were evaluated for all individual steps. The plots obtained are shown in Figure 5 and parameters of the first (fastest) kinetic process for each molecule are compared in Table 4 with results previously obtained in MeCN solution.7

ln(k∆ /T) ) ln(kB /h) - ∆H* /RT + ∆S* /R *

Figure 6. Rise (A) and decay (B) times of the absorbance of PM originating from SO-1 upon UV-irradiation in MeCN, at 263 K, interpolated with monoexponential functions (bottom) and treated according to MEM (top). In the top A and B graphs, the ordinate fj/∑j fj × 100 represents the percentage weight of the jth component; red arrows indicate time parameters resulting from monoexponential fit.

(7)

From the table and plots it can be seen that ∆H , which is lower in film than in solution, does not vary much from step to step (the representative trends are almost parallel), whereas there is a very significant decrease of ∆S* in PMMA film (∆S* = -200 J mol-1 K-1) compared to solution and a further decrease is observed from step to step. Time-dependent parameters, both in MeCN solution and PMMA film, were treated according to the Maximum Entropy Method. This method is useful to analyze data that are described by a large number of exponential phases. Shaver and McGown have demonstrated the potential of MEM in recovering fluorescence lifetime distribution from frequency domain lifetime data.22 There are several examples of studies on lifetime analysis utilizing MEM that have appeared in the literature.23,24 Recently, the MEM has been proven to be a valuable tool in describing polyexponential fluorescence decays in samples consisting of an ensemble of macromolecular conformers in solution25,26 and in microcrystalline compounds.27 Therefore, even though other mathematical treatments might be proposed (as, e.g., a stretched exponential function28), the MEM has the value to well visualize the situation in solid, where the initial nonrandom distribution of molecules reflects on the excited state formation and relaxation rates. For the systems under investigation, photocoloration and thermal bleaching kinetics in solution were described

by monoexponential functions;7 in this case, the MEM treatment led to a unique and narrow time distribution, in excellent agreement with the experimental determinations (see, e.g., Figure 6 for SO-1). In contrast, for PMMA films, and in accordance with the experimental multiexponential behaviors, several time distributions appeared, extending over a very long period, for both photocoloration and bleaching. An example of such behavior is shown in Figure 7. For one of the molecules investigated, NP-2, tests of reversibility were carried out that apparently revealed a quite good reproducibility at least after four cycles, as illustrated in Figure 8. However, as can be seen from the figure, after each decoloration process a residual absorbance remained whose relative amount progressively decreased (bottom sketch in Figure 8). The kinetics of coloration and decoloration steps were both satisfactorily fitted by biexponential functions, because long components (see above) could not be detected over the relatively short times of observation. Numerical results are reported in Table 5. From data reported in Table 5, it results that, at the macroscopic level, the kinetics of on-off processes are well reproducible from cycle to cycle. Data of photocoloration and thermal relaxation treated according to the MEM model, are shown in Figure 9. For the photocoloration kinetics (Figure 9A)

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Figure 7. Rise (A) and decay (B) times of the absorbance of PM originating from NP-1 in PMMA at room temperature treated according to MEM. The ordinate fj/∑j fj × 100 represents the percentage weight of the jth component; red arrows indicate time parameters resulting from nonlinear least-squares fit method.

K)

APM /εPM [PM] ) [colorless] c0 - APM /εPM

(9)

From the foregoing, a complete view of the thermodynamics (Table 6) and kinetics (Table 7) of the forward thermal reaction in PMMA can be obtained and compared with values previously determined in MeCN solution.7 Compensation between decrease of both ∆H° and ∆S° leads to practically unchanged ∆G° and K. 4. Discussion Figure 8. Consecutive photocoloration (λexc ) 326 nm) and thermal bleaching cycles for NP-2 in PMMA at room temperature (294 K).

it can be observed that there is a narrow time distribution around 63 s that corresponds to the short rise-time component, ∼67 s, and a broader time distribution centered around ∼360 s corresponding to the longer lifetime component, ∼400 s. For the bleaching process (Figure 9B), the lifetime distributions change more randomly at each cycle. 3.3. Thermochromism. Thermally equilibrated PMs were also present in polymer PMMA film and in powders, as previously observed in solution.7 By applying the van’t Hoff equation, expressed in terms of PM absorbance, eq 8, to the thermochromic reaction in PMMA, the reaction enthalpy could be determined.

d ln APM d ln K ∆Ho ) ) R d(T-1) d(T-1)

(8)

This application is shown in Figure 10, from where the reaction enthalpy can be obtained for three of the compounds under study. For NP-1, thermochromism, even if recognizable, was so weak that quantitative determinations were not reliably feasible. Combining these data with the activation energies of the fastest thermal bleaching processes in PMMA (Table 3), the activation energy for the forward ground state reaction of thermal breakage of the C-O bond can be determined, Ea′ ) ∆H° + Ea. The equilibrium constants, K, can be calculated from the PM absorbance by using the approximate relationship of eq 9, wherefrom also the rate constant, k∆′ ) k∆ × K, of the forward reaction can be obtained.

The results described above show that the four compounds investigated maintain their photochromic properties in crystalline solid phase as well as in PMMA films. The photochromic process is characterized by polyexponential kinetic behavior for both photocoloration and thermal back reaction. This finding (most frequently reported in the literature for bleaching) is common to several studies on photochromic compounds carried out in organized systems such as gels,29,30 liquid crystals,31 membranes,32-34 and polymer films.35-37 Different interpretations were proposed to explain such behaviors: (i) change in bleaching rates of different PM isomers;29,38,39 (ii) nonhomogeneous distribution of the free volume in the polymer;35 and (iii) differences in porosity or in chemical composition in matrices.40 Concentration effects on the spectral shape of absorption and thermal bleaching kinetics of the PMs were interpreted based on the well-known propensity of PMs to self-aggregate.37,41 The most convincing model proposed to explain the nonexponential time-dependence of concentration in matrices is that put forward by Richert and Ba¨ssler in 1985.36 They introduced the concept of dispersiVe reaction, based on the principle that there exists a statistical distribution of the thermodynamic activation parameters of the reaction, ∆H* and ∆S*. These parameters depend on the variable local geometry of the reaction site and, therefore, are governed by a Gaussian distribution, thereby the concentration of PM as a function of time is expressed by a convolution of first order decay functions, each reflecting the monomolecular character of the elementary reaction. The results can be analyzed by using the Gaussian model developed by Albery et al.,42 which was applied to porous silica.43 The hypothesis that specific sites in the matrix offer different and localized barriers to the steric requirements of the reaction was supported by the approach to a single exponential decay pattern as the temperature was raised beyond the glass transition.44-46 A model that takes into account possible environmental changes during relaxation was applied to structurally related spiro compounds by Levitus and Aramendia.47

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Figure 9. Rise (A) and decay (B) times of the PMs originating from NP-2 in the four cycles illustrated in Figure 8, treated according to MEM. The numbering (1-4) indicates successive cycles. The ordinate fj/∑j fj × 100 represents the percentage weight of the jth component; red arrows indicate time parameters resulting from nonlinear least-squares fit method.

TABLE 5: Kinetic Parameters of the Color-Forming and Color-Bleaching Processes Determined in Four Consecutive On-Off Cycles of NP-2 at 294 K light on

Figure 10. Absorbance/temperature data for SO-1, SO-2, and NP-2 in PMMA, treated according to the van’t Hoff equation. Inset: an example of thermochromic spectral evolution in PMMA from 289 to 363 K ([SO-2] ) 0.1 mol dm-3, thickness ) 120 µm).

In this work we applied both the nonlinear least-squares and the maximum entropy methods to the time-dependent processes, with the aim to gain more insights at the molecular level into the reciprocal influence of the probe and environment and consequences on the photochromic and thermochromic properties of the compounds investigated. 4.1. Photocoloration. The ring-opening of SOs and NPs involves a great change of size and dipole moment of the molecules since it implies cleavage of a C-O bond and mutual rotation of the molecular subunits around the C-C bond of oxazine or pyran. Therefore, an important effect of the surrounding environment is expected. In a fluid MeCN solution, photocoloration was clearly monoexponential for the four compounds examined;7 wheareas, in solid media, multiexponential fit was required to describe all kinetics. Rate laws are no more simple in solid because of a nonrandom initial distribution of molecular species; depending on the nature of this distribution, different rate laws hold because of a continuous of barrier heights. In accordance, in solution, MEM treatment of the timedependent data indicated a narrow time distribution around a single value that well corresponded to the rise time, resulting from exponential fit (see, e.g., Figure 6A). In contrast, multiple time distributions were found in PMMA (see, e.g., Figure 7A). Because the photocoloration kinetics depend on both the quantum yield of the forward photoreaction and the rate of the backward thermal bleaching, applying multiexponential models to photocoloration implies the hypothesis that various forms of closed molecules exist in polymer matrix which differ either in the quantum yield of the photoreaction or in the thermal

light off

cycle

R1 (s-1)

τ1(s)

R2 (s-1)

τ2(s)

k∆1 (s-1)

τ1(s)

k∆2 (s-1)

τ2(s)

1 2 3 4

0.014 0.015 0.014 0.017

71 67 71 59

0.0023 0.0027 0.00225 0.0029

435 370 445 345

0.0058 0.0059 0.0067 0.0062

172 170 150 161

0.0012 0.0010 0.0010 0.0009

833 1000 1000 1110

bleaching rate or in both, depending on the site they occupy and the free volume therein available. Finding multiple rise times that describe the photocoloration kinetics in PMMA indicates that there are different SO conformations because of inhomogeneous microscopic environment. The number of fitting parameters necessary to describe the experimental kinetics gives an estimation of the extent of heterogeneity of the matrix. The MEM model indicates that rise times are concentrated around preferential values (see e.g., Figure 7A), and their spreading around these values strongly depends on the molecular structure, but also on the specimen analyzed and the temperature. The experimental quantum yields determined in PMMA, Table 2, refer to the fastest step of photocoloration and are apparently larger than those determined in MeCN solution, but, because of the numerous sources of uncertainty (homogeneity and thickness of the film, actinometry, and assumptions on PM molar absorption coefficient), they can be very cautiously considered to be approximately of the same order of magnitude; also, the relative values for the different molecules are close to those found in MeCN (Table 2). 4.2. Thermal Bleaching. In homogeneous solution thermal bleaching was also found to be monoexponential for the four molecules. In accordance, a narrow time distribution around a single value, corresponding to the decay time resulting from exponential fit, was developed by MEM treatment (Figure 6B). In the crystalline phase, polyexponential bleaching kinetics (Figure 2, bottom inset, and Table 1) can be related to different crystalline aggregates, whose existence is also suggested by the microscope image in the inset of Figure 2. In PMMA, polyexponential least-square behaviors (Table 3), corresponding to multiple time distributions revealed by MEM, were found (Figure 7B). Depending on local surrounding where it was produced, each PM molecule has its own bleaching rate constant, leading to a continuous rate distribution which is well described by three to four exponential functions. At the macroscopic level all bleaching processes are slowed down in PMMA compared to solution by approximately 1 order of

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TABLE 6: Thermodynamic Parameters of the Thermocoloration in PMMA Film, Compared with those Previously Determined in MeCN Solution at 298 K MeCNa

PMMA

cmpd K (× 103) ∆H° (kJ mol-1) ∆G° (kJ mol-1) ∆S° (J mol-1 K-1) K (× 10-3) ∆H° (kJ mol-1) ∆G° (kJ mol-1) ∆S° (J mol-1 K-1) SO-1 SO-2 NP-1 NP-2 a

1.5 3

5.5 5.6

16.2 14.5

-36 -30

6

6.1

12.8

-22

12.3 15.7 18.5 13.4

16.1 14.5 18.9 13.5

-12 4 -1 ∼0

Data taken from ref 7.

TABLE 7: Kinetic Parameters of the Thermocoloration in PMMA Film Compared with those Previously Determined in MeCN Solution MeCNa

PMMA

cmpd k′∆,298 K (s-1) Ea′ (kJ mol-1) k′∆,298 K (s-1) Ea′ (kJ mol-1) SO-1 SO-2 NP-1 NP-2 a

2 3 0.5 5

0.00012 0.00005

27 34.5

0.00012

41.5

0.0021 0.0017 0.000051 0.00028

63.2 92.7 85.1 74.7

Data taken from ref 7.

magnitude for the shortest components (Table 3). Dissipation of irradiating energy into the polymer matrix locally changes its structure; this reflects on a scarce reproducibility of kinetics at the molecular level, as proved by the MEM treatment of bleaching kinetics for subsequent on-off cycles (see Figure 9B). The above interpretation is consistent with the multilocal minima model proposed by Irie et al.48 to explain the environmental effect at the single molecule level for some diarylethenes in various polymer matrices. This model is based on the concept that the polymer around the molecule behaves as a steric hindrance creating barriers and local minima on the potential energy surfaces of both the ground and excited states. All kinetic components of the thermal decoloration, determined in the appropriate temperature range for each compound, were satisfactorily fitted to Arrhenius and Eyring relationships (see Figures 4 and 5), which allowed kinetic (rate constants and activation energies) and thermodynamic activation parameters (∆H*, ∆S*, and ∆G*) of the thermal reactions to be determined (Tables 3 and 4). These parameters are significantly altered in PMMA compared with solution. The polymer matrix has the effect of reducing both activation energy and preexponential factor of thermal bleaching. The activation energy referred to the fastest process diminishes by about 40-60% compared to MeCN; it does not change much from one to other kinetic component within each molecule (Table 3). The significant decrease of rate compared to solution is mainly due to the marked decrease of the pre-exponential factor of the bleaching kinetics that leads to exceptionally highly negative activation entropies. They are on the order of ∼-200 J mol-1 K-1 for the fastest kinetic component and further decrease for longer kinetic contributions. The matrix effect on activation entropy is related to the decreased torsional freedom in the transition state. This negative contribution to activation entropy by far exceeds positive contributions coming from the randomization of the environmental molecules organized around the more polar ground state open form. In solid media it could happen that intermediates that are short-lived in solution, such as PMs in a nonplanar cisoid geometry, can be trapped in singular sites without evolving to more stable isomers. Such structures were recognized in solution by using ultrafast spectroscopic methods.49 Molecules trapped in a solid matrix in cisoid-cis geometry may well have a low

activation energy to the closure process and a highly negative activation entropy. In solution the closeness of times of the color-forming (photochemical) and color-bleaching (thermal) processes indicates that the photobehavior is dominated by the rate of the back process, whereas in solid phase the multiplicity of rate parameters, needed to describe both processes, and their marked differences, make impossible any correlation. 4.3. Thermocoloration. The presence of the colored form at room temperature is more evident in both non irradiated powders and films than in solution, as illustrated in the example of Figure 1 and can also be appreciated visually. This could be tentatively explained considering the higher concentration of substrate in polymer and the packaging of the molecules in powders. Compared with thermochromic data obtained in solution, the enthalpy difference between the colored and colorless forms, ∆H°, is reduced by more than a half; this means that the equilibrium is less sensitive to the temperature in polymer than in solution. The reaction is no more isoentropic as found in solution. Moderately negative entropy values may be due to a more constrained conformation of PM due to interplay with the polymer environment. As outlined above, the structure of PMMA favors interaction with the dipolar PM. However, at room temperature, the equilibrium constant and, therefore, the reaction standard free energy do not change appreciably due to compensating effects of decrease of both reaction enthalpy and entropy. 5. Summary and Conclusions Four photochromic compounds belonging to the spirooxazine and naphthopyran families were studied in microcrystalline phase and entrapped into PMMA films. Colorless transparent films of less than 200 µm thickness were prepared that became colored upon exposure to UV light showing absorption in the visible spectral range. Photocoloration quantum yields and kinetic parameters of thermal bleaching in PMMA were determined. The effect of the matrix on the activation parameters, implies that the reacting molecule moves over a lower energy surface in PMMA compared to MeCN solution. The interplay of the photochromic molecules with the embedding matrix was found to alter photocoloration and thermal bleaching kinetics compared with the behavior of these molecules in solution. The kinetics of bleaching were slowed down and instead of one single process, a wide distribution of rate parameters was observed. This multiplicity of kinetics can be referred to the possibility that molecules exist in different crystalline forms in powders and in distinct cages in polymer, which impose specific conformations with maximum probability for those corresponding to the lifetimes experimentally determined. Application of MEM treatment to these systems has demonstrated the power of the method in analyzing polyexponential kinetics, showing the complex behavior of molecular systems embedded in a microheterogeneous surrounding in both pho-

Photo- and Thermochromism in Solid State and Film toproduction and thermal relaxation of excited states. It highlights that there is only one molecular conformation of high probability in the homogeneous medium whereas a wide range of possibilities are created in a microheterogeneous phase. Acknowledgment. This research was funded by the Italian “Ministero per l’Universita` e la Ricerca Scientifica e Tecnologica” and the University of Perugia in the framework of a PRIN-2006 Project (“Photophysics and Photochemistry of Chromogenic Compounds for Technological Applications”). Supporting Information Available: “Arrhenius Plots of the Thermal Bleaching in PMMA Film”; “Thermodynamic Parameters of the Bleaching in PMMA”. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Guglielmetti, R. J. In Photochromism of Organic Compounds; Du¨rr, H., Bouas- Laurent, H., Eds.; Elsevier: The Netherlands, 1990; pp 314466. (2) Lokshin, V.; Samat, A.; Metelitsa, A. V. Russ. Chem. ReV. 2002, 71, 893–916. (3) Becker, R. S.; E.Dolan, E.; Balke, D. E. J. Chem. Phys. 1969, 50, 239–245. (4) Tyer, N. W.; Becker, R. S. J. Am. Chem. Soc. 1970, 92, 1289– 1294. (5) Lenoble, C.; Becker, R. S. J. Photochem. 1986, 33, 187–197. (6) Van Gemert, B. In Organic Photochromic and Thermochromic Compounds; Crano, J. C., Guglielmetti, R. J., Eds.; Kluwer Academic/ Plenum Publishers: New York, 1999; Vol. 1, pp 111-140. (7) di Nunzio, M. R.; Gentili, P. L.; Romani, A.; Favaro, G. ChemPhysChem 2008, 9, 768–775. (8) di Nunzio, M. R.; Romani, A.; Favaro, G. J. Phys. Chem. A 2009, 113, 9424–9433. (9) Crano, J. C.; Kwak, W. S.; Welch, C. N. In Applied Photochromic Polymer Systems; McArdle, C. B., Ed.; Chapman and Hall: New York, 1992. (10) Berkovic, G.; Krongauz, V.; Weiss, V. Chem. ReV. 2000, 100, 1741–1754. (11) Asahi, T.; Suzuki, M.; Masuhara, H. J. Phys. Chem. A 2002, 106, 2335–2340. (12) Asahi, T.; Masuhara, H. Phys. Chem. Chem. Phys. 2002, 4, 185– 192. (13) Gentili, P. L.; Nocchetti, M.; Miliani, C.; Favaro, G. New J. Chem. 2004, 28, 379–386. (14) Lee, E.; Choi, M. S.; Han, Y. A.; Cho, H.; Kim, S. H.; Ji, B. C. Fibers Polym. 2008, 9, 134–139. (15) Lin, J. S. Eur. Polym. J. 2003, 39, 1693–1700. (16) Brochon, J. C. Methods Enzymol., Part B 1994, 240, 262–311.

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