Photochromism of Crown Ethers with Incorporated Azobenzene

J. Phys. Chem. B 2005, 109, 93-101

93

Photochromism of Crown Ethers with Incorporated Azobenzene Moiety Krzysztof Janus* and Juliusz Sworakowski Institute of Physical and Theoretical Chemistry, Wrocław UniVersity of Technology (Politechnika Wrocławska), Wyb. Wyspian´ skiego 27, 50-370 Wrocław, Poland ReceiVed: April 16, 2004; In Final Form: October 25, 2004

The kinetics of the thermal cis-trans isomerization was determined in a series of crown ethers containing the azobenzene moiety incorporated into the crowns of various sizes (10- to 19-membered crowns), dissolved in liquid isooctane and in polymer matrixes (PMMA and polystyrene). The kinetic parameters (activation energies and pre-exponential factors) were determined from temporal evolution of the absorbance measured at one of the maxima due to the π-π* transition of the trans forms of the molecules, using both isothermal and nonisothermal procedures. Molecular structures and electronic spectra of both trans and cis forms were calculated employing an ab initio method with 3-21G* basis set and semiempirical ZINDO-S, respectively. A comparison of the quantum-chemical calculations with the experimental results shows that one may exclude the splitting of the UV absorption bands of the trans forms of the molecules under study that resulted from a coexistence of several conformations of these molecules. A linear relationship was found between the activation energy and logarithm of the pre-exponential factor of the thermally driven cis-trans isomerization: all molecules follow a common dependence, except for the smallest (10-membered) crown ether.

Introduction Photochromic systems have recently been extensively studied because of their current and emerging applications in optical information storage and processing (e.g., refs 1-4). A rational design of novel photochromic systems requires that the reaction mechanism be known. This includes, among other features, a good understanding of the reaction kinetics. Among photochromic systems, azobenzene derivatives have occupied a prominent position: representatives of this family of molecules, dissolved in common solvents, liquid-crystalline matrixes, or polymer matrixes or else chemically attached to polymer main chains, have often been employed as active elements of molecular photonic devices. The photochromic reaction in azobenzenes is the trans-cis isomerization resulting in reversible changes of absorption spectra in the near-UV and visible regions (see Figure 1). The reaction is a space-demanding process, hence, one may expect it to depend on the environment of the isomerizing molecules, that is, on the nature of the matrix. The reaction has been studied by several groups (e.g., see the references collected in refs 5-7), and the kinetic parameters of both light-driven trans-cis and cis-trans photoisomerizations and of thermally driven cis-trans isomerization have been reasonably well-known. In particular, the activation energy of the thermal reaction in parent azobenzene and in its derivatives substituted with groups of a modest polarity (compounds of the azobenzene type, according to the classification of Rau5,7) was found8 to range between 85 and 100 kJ/mol, and the preexponential factor ranged between 1012 and 1013 s-1. In our previous papers,9,10 we reported on results of measurements of the kinetics of the photochromic reaction in a family of crown ethers containing the azobenzene moiety incorporated into the crown (azobenzene crown ethers, hereafter referred to as ACEs). The crown ethers, since their synthesis first reported * To whom correspondence should be addressed. E-mail: krzysztof.janus@ pwr.wroc.pl.

by Pedersen,11 have been extensively studied because of their ability of complexing metal ions.12 The azobenzene moiety incorporated into the crowns was used to change the size of the crowns and hence to modify the complexing properties of the molecules.13,14 The object of our studies was chosen because it was expected that, because of many degrees of conformational freedom of the molecules resulting from their shapes, the kinetics of their isomerization would be rather insensitive to the influence of the matrixes. On the other hand, however, the crown was also expected to impose some constraints on the freedom of reorientation and hence to modify the kinetics of the photochromic reaction. Therefore, one could expect a dependence of the shape of the molecule and, in particular, of the isomerization kinetics on the size of the crown. A family of crown ethers differing in the size of the crown was studied (see Figure 1a). The kinetics of the isomerization did exhibit some dependence on the crown size and on the nature of solvent, as will be shown later. Moreover, our measurements9,10,15 showed that the spectra of all molecules under study exhibit a characteristic splitting of the principal band of the trans form. The splitting, absent in the parent azobenzene, was observed, for example, in o-dimethoxyazobenzene.14 In the case of ACEs, a possible explanation of the splitting could be sought in the presence of well-defined conformers, close in their potential energies but differing in the shapes of the crowns. One might expect that these differences could, in turn, affect the geometry of the trans-azobenzene moiety modifying the energies of electronic transitions. The aim of this paper is to present results of systematic measurements of the kinetics of the thermally driven cis-trans isomerization performed on the series of ACEs. These results were derived from the temporal evolution of absorption spectra in the UV-vis spectral region. The experiment has been supplemented with quantum-chemical calculations aiming at reproducing the details of the electronic spectra of these

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94 J. Phys. Chem. B, Vol. 109, No. 1, 2005

Figure 1. (a) Structures of the crown ethers under study and scheme of their photochromic reaction and (b) absorption spectra of the crown ethers in cast PMMA matrix: full lines, trans isomers; dashed lines, photostationary states containing ∼85% of the cis isomer. The spectra have been normalized to their most intense absorption peaks. The spectra of parent azobenzene are shown for comparison.

materials and, in particular, the splitting of the main absorption band of the trans form. Experiment As mentioned above, the kinetic experiments consisted in measuring the temporal evolution of the optical absorption during the thermally driven cis-trans isomerization. Four crown ethers differing in the size of the crown were studied, their sizes ranging from 10 to 19 members (hereafter referred to as O2N2 through O5N2, cf. Figure 1a). The materials were synthesized at the Department of Chemical Technology, Gdan´sk University of Technology by Dr. E. Luboch (O2N2 through O4N2) and Dr. A. Skwierawska (O5N2). Details of the syntheses and purification can be found in refs 16-21. The reaction was monitored in conventional solutions (usually in isooctane) and in polymer matrixes (polystyrene, PMMA) fabricated in various ways. The concentrations of the dyes in the liquid solutions were of the order of 10-5 mol/dm3; the concentrations of solutions in polymer matrixes were of the order of 10-2 mol/dm3. Most polymer films were prepared by casting solutions of a polymer and an ACE on a horizontally positioned glass plate. After drying (at least 24 h at room temperature in the atmosphere rich in solvent vapors allowing for a very slow

Janus and Sworakowski and nearly uniform drying of the sample bulk), the films were removed from the glass using an ultrasonic bath and left in the ambient atmosphere in order to remove remaining solvent. In addition to solvent-cast films, samples referred to as “polymerized” ones were also investigated. They were fabricated by polymerizing solutions of the crown ether in MMA. In some cases, PMMA (up to 25 wt % of the total mass) was additionally dissolved in the solution obtained in such a way in order to increase its viscosity. The solutions were placed between two parallel glass plates; the distances between the plates (and hence the sample thicknesses) were fixed with polyethylene spacers. A small amount of benzoyl peroxide was used as the polymerization initiator. The polymerization lasted for 24 h at 363 K. The samples, prepared in such a way, were expected to be free of traces of solvent. The concentrations of ACEs in the “polymerized” samples were of the order of 10-3-10-2 mol/ dm3. Prior to the experiments, the samples were UV irradiated with a 200 W high-pressure mercury lamp equipped with a water filter and a set of color filters transmitting the 365 nm spectral line only. The irradiation was continued until a photostationary state was reached. A comparison of the spectra of the crown ethers studied in this work with similar spectra of alkylsubstituted ACEs,22 where the concentrations of both forms were determined, allows us to estimate the fraction of the cis isomer to amount to ∼85-90%. We note, however, that the kinetics of a first-order process does not depend on the concentration of the reactant. Thus, a more exact determination of the concentrations was a matter of a secondary importance. The cis-trans reaction occurring in the dark was monitored by measuring temporal changes of the absorbance of a sample in the UV region associated with the absorption of the trans isomer. In most cases, the measurements were performed at ca. 350 nm, that is, at the low-energy component of the doublet; occasionally, however, we also measured the evolution of the higher energy component (cf., e.g., Figure 8). The spectra and their evolution at a predetermined wavelength were measured with a Perkin-Elmer Lambda 20 spectrophotometer equipped with a Peltier thermostated sample holder. Analysis of Experimental Results. One of the principal aims of the research reported in this paper was to analyze details of the kinetic behavior of ACEs under study. Thus, use of reliable methods of analysis of experimental data was essential, particularly in the case of distributions of rate constants. This problem was considered in our earlier papers;9,15 here, we shall only give basic equations used in the analysis of the results. It is straightforward to demonstrate that, in the case of a firstorder process controlled by a discrete rate constant, the concentration of the reactant (in our case, of the cis isomer) follows the equation

n ) exp(-kt) no

(1)

where n and no are the initial and momentary concentrations of the reactant and k is the rate constant. It is equally straightforward to show that

A - A∞ n ) no Ao - A∞

(2)

where A is the momentary absorbance, measured at a preselected wavelength, and Ao and A∞ are absorbances at t ) 0 and t f ∞, respectively. Thus, a plot of ln |A - A∞| versus t should yield a straight line, and its slope should amount to -k. To avoid

Photochromism of Crown Ethers

J. Phys. Chem. B, Vol. 109, No. 1, 2005 95

Figure 2. Molecular structures of cis and trans isomers of the crown ethers under study. The lowest-energy cis isomer and two lowest-energy trans isomers are shown for each molecule.

errors which can be made upon setting wrong values of A∞, one may use a semilog plot of ln |dA/dt| versus t. In the case of processes controlled by distributions of rate constants, reliable results can be obtained from either t×|A A∞| versus ln t or dA/d(ln t) versus ln t plots. To within small correction factors, times at maxima of these dependencies are equal to the inverse of the dominant rate constants, and their widths are related to the widths of distributions of the rate constants.15,23 The kinetic measurements were usually carried out at several (constant) temperatures; the activation energies (Ea) and preexponential factors (ν) were then determined from the Arrhenius equation

( )

Ea k ) ν exp RT

(3)

where R stands for the molar gas constant. In some cases, the isothermal measurements were supplemented with nonisothermal runs, performed following the procedure described in detail in ref 9. In this case, the activation energies were obtained from the slopes of the dependencies

ln

( ) (

)

1 dn dA ) ln n dt (A - A∞) dt

versus 1/T and/or

(

ln T - 2 ln

( )) no n

versus 1/T (cf. refs 9, 15). In all cases, the final stage of the analysis consisted in verifying the validity of the results obtained: the experimental curves were directly compared with those calculated using the sets of temperature-independent

Figure 3. Evolution of the absorption spectrum of O3N2 (solution in isooctane) upon irradiation with 365 nm light: open dots, starting solution containing trans isomer; closed dots, photostationary state. Numbers represent time of irradiation (in minutes). Inset shows the enlarged region of the n-π* transition.

parameters obtained using the methods described above. The parameters were adjusted until a satisfactory agreement was attained. Quantum-Chemical Calculations. The quantum-chemical calculations reported in this section aimed at establishing the equilibrium geometries of ACEs under study in their trans and cis forms and the energies of the spectral transitions in both forms. The presence of the doublet in the UV band and some features of the kinetics prompted us to check whether these features are due to a possible presence of various modifications of the trans molecules, which differ slightly in the energies of their π-π* transitions. The geometry optimization was carried out at the density functional theory (DFT) level using Becke’s three-parameter exchange functional combined with the LYP correlation functional (B3LYP).24 The standard 3-21G* basis set was used.25 All optimizations were performed using the Gaussian 2003


Janus and Sworakowski

Figure 4. Absorption spectra of ACEs compared with calculated electronic transitions. The upper and left axes refer to the experimental spectra; the lower and right ones refer to the calculated ones. The full vertical sections are the oscillator strengths in the lowest-energy isomers; the dashed lines are the oscillator strengths in the second-lowest energy trans isomers.

program package26 run at the Wrocław Center for Networking and Supercomputing. The UV-vis spectra were calculated using

the semiempirical ZINDO-S single-excited configuration interaction (CI) method included in the HyperChem package.



Figure 5. Nondispersive thermal cis-trans isomerization: O5N2 in cast PMMA matrix. Time dependencies of the absorption at 350 nm (a), concentration of the cis isomer (b), and time derivative of concentration of the cis isomer (c) are shown. Temperatures at which measurements were performed are shown in the graphs. (d) Graph shows the temperature dependence of the rate constants plotted in the Arrhenius coordinates. The dashed lines in (b)-(d) are the fits to appropriate equations.

Figure 6. Reaction controlled by a distribution of rate constants: O5N2 in polystyrene matrix. Time dependencies of the absorption at 350 nm (a) and time derivatives of the concentrations of the cis isomers (b) are shown. The dominant rate constants are determined from the positions of maxima of the curves shown in (c). Temperatures at which measurements were performed are shown in the graphs. (d) Graph shows the temperature dependence of the rate constants plotted in the Arrhenius coordinates. The dashed lines in (b) and (d) are the fits to appropriate equations.

The reliability of the method was verified on azobenzene and 2,2′-dimethoxyazobenzene for which calculated geometries were compared with experimental structures. A particular attention

was paid to attain global minima of the potential energies of the molecules, which was not an easy task due to the flexibility of the crowns.



Results Molecular Structures and Electronic Spectra. The calculated structures of the lowest-energy conformers of all molecules under study are shown in Figure 2. We note, however, that, for the trans forms of all ACEs, a few almost isoenergetic structures have been found which differ in the angle between the phenyl rings. To visualize the differences, we also show the conformations of the second-lowest energies of the trans-ACEs under study. These differences, however, were found to have only a minor influence on the calculated transition frequencies, as will be shown below. Figure 1b presents absorption spectra of the crown ethers. The spectra of the trans isomers contain broad π f π* absorption bands with a characteristic splitting, peaking at ∼300 and 350 nm. A weaker band in the visible region due to the n f π* transition is also observed. The size of a crown has a distinct influence on the spectra: the relative intensity of the lower energy component of the UV doublet increases with the size of the crown. Moreover, the relative intensity of the absorption peak in the visible region increases with the crown size. Upon UV irradiation, the intensity of the UV band decreases, while the n f π* absorption peak is slightly shifted to shorter wavelengths, and its intensity increases (cf. Figure 3). The calculated transition energies compared with the experimental spectra are presented in Figure 4. With one exception, all calculated spectra are red-shifted by 2200 ( 100 cm-1 with respect to the experiment, but otherwise, they correctly reproduce the shape of experimental spectra including the n f π* transitions and the splitting of absorption peak in UV region. The calculated spectrum of cis-O3N2 reproduces the experimental spectrum without any shift. To examine the effect of a possible conformational change, for the trans forms of all ACEs we performed calculations not only of the spectra of the lowestenergy conformers but also of the second-lowest ones. The results are shown in Figure 4; it follows from the figure that neither transition frequencies nor oscillator strengths exhibit a strong dependence on the conformation. This is also true for other conformers found in our calculations. Isothermal Kinetics. Upon exposure of the samples to the UV radiation, trans-cis photoisomerization takes place resulting in an evolution of the spectra. An example of the temporal evolution of the spectrum of O3N2 is shown in Figure 3. As was mentioned in the preceding section, changes of the reactant concentrations during the thermally driven cis-trans isomerization were calculated from the temporal changes of the absorbance at the maximum of the long-wavelength component of the π-π* doublet of the trans form, that is, at ∼350 nm. Exemplary results, obtained on O5N2, are shown in Figures 5 and 6. Similar results for O4N2 and O2N2 have been given in our earlier publications.9,10 The Arrhenius plots for all compounds under study dissolved in PMMA matrix are shown in Figure 7, and results obtained from isothermal experiments are collected in Table 1. To get insight into the origin of the splitting of the UV band, we carried out simultaneous measurements of the absorbances at the absorption maxima of the components of the doublet in O4N2 (i.e., at 305 and 352 nm) during the thermally driven cis-trans isomerization. The results, shown in Figure 8a, demonstrate that the relative changes of the intensities are identical; that is, the changes are associated with the same process. We have also measured temperature dependencies of the absorbances of the trans-O4N2 at these two wavelengths. The results are shown in Figure 8b; it is clearly seen that the

Figure 7. A comparison of the rate constants of the cis-trans isomerization of all studied crown ethers and of parent azobenzene in PMMA matrix.

TABLE 1: Parameters of the Arrhenius Equation Determined from the Measurements Performed in the Isothermal and Nonisothermal Regimea matrix isooctane Ea (kJ/mol) ν (s-1) σ (kJ/mol) Ea (kJ/mol) ν (s-1) σ (kJ/mol)

cast PMMA

polymerized PMMA

polystyrene

O2N2 Molecule, Isothermal Regime 150 1.6 × 1016 O2N2 Molecule, Nonisothermal Regime 112 4.1 × 1010

Ea (kJ/mol) ν (s-1) σ (kJ/mol)

O3N2 Molecule, Isothermal Regime 125 94 92 4.6 × 109 3 × 1010 5 × 1014 2.8

89 8 × 109 2.5

Ea (kJ/mol) ν (s-1) σ (kJ/mol)

O4N2 Molecule, Isothermal Regime 97 92 89 8 × 109 4 × 109 1 × 1011 2

91 1.1 × 1010 2.5

O4N2 Molecule, Nonisothermal Regime 73 9.2 × 106

57 3.4 × 104

O5N2 Molecule, Isothermal Regime 107 94 93 1.7 × 1012 1.5 × 1010 1.1 × 1010 2.2

88 3.3 × 109 3

Ea (kJ/mol) ν (s-1) σ (kJ/mol) Ea (kJ/mol) ν (s-1) σ (kJ/mol)

a Nonexponential decays observed in some matrixes were interpreted as due to distribution of rate constants resulting from a Gaussian distribution of the activation energies. For these matrixes, the distribution parameter σ is also given.

intensities are linearly interdependent, as both increase upon decreasing temperature. Nonisothermal Kinetics. In O4N2 and O2N2, the isothermal measurements were supplemented with those employing the nonisothermal technique described in detail in ref 9. Typical results obtained for O4N2 are shown in Figure 9. We note that the activation energies obtained from these measurements are significantly lower than those obtained from isothermal measurements. This discrepancy cannot be attributed to experimental errors, since numerical simulations9 demonstrate that the method should yield exact results with errors lower than 1%. A discussion of this feature will be given in the following section. The results obtained from the experiments performed in the nonisothermal regime are collected in Table 1.



Figure 8. Simultaneous measurements of the evolution of absorbances at both maxima of the UV absorption band of the trans isomer of O4N2 in cast PMMA matrix: (a) Cis-trans dark isomerization at 353 K; (b) a heating-cooling cycle of the trans isomer.

Figure 9. Nonisothermal experiment performed on O4N2 in cast PMMA matrix: (a) the evolution of absorbance and (b) a temperature dependence of (|dn/dt|/n) plotted in Arrhenius coordinates, compared with the results of isothermal experiments performed on the same system (squares).

Discussion and Conclusions Our experiments provide convincing evidence to rule out the possibility that the splitting in the spectrum is associated with a coexistence of a few forms of trans-ACEs. The intensities of the two components of the doublet increase simultaneously upon decreasing temperature, whereas in the case of the equilibrium between various forms of trans-ACEs, one would expect an intensity increase at one frequency accompanied by a decrease at the other. The evolution of the intensities during the cistrans isomerization leads to a similar conclusion. In other words, our results indicate that conformers of trans-ACEs, even if they exist in our samples, do not significantly affect the spectra in the near-UV region. On the other hand, the quantum-chemical calculations point to a possible existence of several nearly isoenergetic conformations of trans-ACEs, differing in the geometries of the crowns and exhibiting deviations from the planarity of the azobenzene moiety characteristic of parent trans-azobenzene. Despite the different geometries, however, differences between calculated transition frequencies of the isomers are insignificant (cf. Figure 4). This result leads us to an important conclusion: because of the flexibility of the crowns, the shapes of the molecules under study may be influenced by external factors (e.g., by stresses imposed by matrixes), but the resulting changes do not affect the spectra (or affect them to a minor degree). Moreover, a comparison of the experimental spectra and the calculated frequencies, shown in Figure 4, proves that the low-energy band is due to a single transition, whereas the high-energy component is a superposition of several closely lying weaker transitions.

We notice, however, that a reasonable agreement between the experimental spectra is attained only after shifting the spectra with respect to one another. The shift amounts to ∼2200 ( 100 cm-1 for all molecules except for cis-O3N2; in the latter system, it amounts to 0 cm-1. We cannot offer a convincing explanation for these differences. A comparison of the kinetic parameters characterizing the thermal isomerization reveals that the dominant activation energies of the cis-trans isomerization, determined from the isothermal experiments, are almost independent of the polymer matrix. It is, however, interesting to note that these energies are systematically lower than those determined from similar experiments performed on solutions in a nonviscous solvent (isooctane) (see Table 1). Such behavior can be rationalized by admitting that the initial structures of the reactants (cis-ACEs) may differ in the polymer matrixes and in the liquid solution. One should remember that as-prepared samples contain the ACEs in their trans forms. The cis forms, which are reactants in the thermally driven isomerization, are then formed upon irradiation. One may speculate that the photoisomerization in the liquid solution results in production of fully relaxed cis isomers, whereas in more rigid polymer matrixes, different geometries may be preferred, characterized by lower isomerization barriers. It is interesting to note that in all ACEs under study, except for O2N2, a “compensation rule” has been found: to within experimental accuracy, the logarithm of the pre-exponential factor is linearly dependent on the activation energy, irrespective of the size of the crown and the matrix used (cf. Figure 10a).



Figure 10. The “compensation rule” (a) and the dependence of ∆Gq on the activation energy (b) for the crown ethers under study: crosses, parent azobenzene; circles, O2N2; triangles, O3N2; squares, O4N2; asterisks, O5N2.

The results given in Table 1 may be used to calculate the parameters characterizing the process within the framework of the transition state theory, namely, the activation enthalpy (∆Hq), the activation entropy (∆Sq), and the Gibbs free energy of activation (∆Gq). For a monomolecular reaction, these parameters are related to the experimental ones by the following equations:27

∆H ) Ea q

∆Sq ) R ln

( ) hν kBT

∆Gq ) ∆Hq - T∆Sq

(4) (5) (6)

where h and kB stand for the Planck constant and Boltzmann constant, respectively. Obviously, the compensation rule mentioned above translates into a linear interdependence between ∆Hq and ∆Sq. Taking 350 K as the average temperature of our experiments, we calculated ∆Gq for the measurements performed on all ACEs in all matrixes. The results are shown in Figure 10b. To within an experimental accuracy, ∆Gq seems to be independent of the matrix: for O3N2, O4N2, and O5N2 in all matrixes employed, ∆Gq ) 111 ( 4 kJ/mol, the same as the value determined for parent azobenzene. This finding points to a common path of the thermal isomerization in all cases under study. A notable exception is O2N2 where ∆Gq ) 128 kJ/mol. The latter result can be rationalized by taking into account the size of the crown, acting as a “molecular string” and making the molecule more rigid, and hence probably inhibiting a path common to all other molecules. As a result, the lifetime of the metastable cis isomer of O2N2 is of the order of 101-102 years

at ambient temperature. Following the arguments put forward by Rau et al.,28,29 one may speculate that the size of the crown in O2N2 prevents rotation, allowing for inversion only, whereas both mechanisms are allowed (at least in principle) in the case of larger crowns. Finally, we comment on the difference between results obtained from the isothermal and nonisothermal measurements. The experiments performed in the nonisothermal regime yielded the activation energies that were significantly lower than those obtained from isothermal measurements. The apparent discrepancy can be explained by the existence, in our samples, of two processes with two different activation energies30 or by a sufficiently broad distribution of the activation energies.31 Numerical simulations, described in ref 30, have demonstrated that it is possible to reproduce both the isothermal and nonisothermal results assuming a coexistence of two processes: a “slow” one, characterized by the activation energy and the frequency factor close to those obtained in isothermal experiments, and a “fast” one with Ea ≈ 70 kJ/mol (e.g., for O2N2 in cast PMMA sample, Ea1 ) 92 kJ/mol, ν1 ) 7.94 × 109 s-1 and Ea2 ) 70 kJ/mol, ν2 ) 1.65 × 107 s-1). We note that the nonisothermal measurements covered the temperature range inaccessible to the experiments carried out in the isothermal regime; the former ones started around room temperature, whereas the latter ones covered the temperature range 330-370 K. The “fast” species (possibly a small amount of a metastable intermediate conformer stabilized by the matrix) react too fast above 330 K and are thus invisible in the isothermal experiments. A similar behavior has already been described in the literature.32,33 A comparison of the isothermal and nonisothermal results, obtained on the same sample, is shown in Figure 9. It is clearly seen that the effective rate constants do not differ much, hence, lowering of the activation energy is compensated by a corresponding decrease of the frequency factor. Interestingly, the values of ∆Gq determined from nonisothermal measurements are equal to those found from isothermal experiments, amounting to 112 kJ/mol for O4N2 and 128 kJ/mol for O2N2. Acknowledgment. The authors thank Drs. E. Luboch and E. Skwierawska (Gdan´sk University of Technology) for the gift of the materials used in the present study, Professor J. F. Biernat (Gdan´sk University of Technology) for helpful comments, and Professor J. Lipin´ski and Dr. T. Misiaszek (Wrocław University of Technology) for their help in performing quantum-chemical calculations. The work was supported by the Polish State Committee for Scientific Research (Grant No. 4 T09A 132 22). References and Notes (1) Brown, G. H., Ed. Photochromism; Wiley-Interscience: New York, 1971. (2) Dorion, G. H.; Wiebe, A. F. Photochromism. Optical and Photographic Applications; Focal Press: London, 1970. (3) Du¨rr, H., Bouas-Laurent, H., Eds. Photochromism. Molecules and Systems; Elsevier: Amsterdam, 1990. (4) Crano, C., Guglielmetti, R. J., Eds. Organic Photochromic and Thermochromic Compounds; Plenum Publishing: New York, 1999. (5) Rau, H. Angew. Chem., Int. Ed. Engl. 1973, 12, 224. (6) Ross, D. L.; Blanc, J. In Photochromism; Brown, G. H., Ed.; WileyInterscience: New York, 1971; Chapter 5. (7) Rau, H. In Photochromism. Molecules and Systems; Du¨rr, H., Bouas-Laurent, H., Eds.; Elsevier: Amsterdam, 1990; Chapter 4. (8) Talaty, E. R.; Fargo, J. C. Chem. Commun. (London) 1967, 65. (9) Janus, K.; Koshets, I. A.; Sworakowski, J.; Nesˇpurek, S. J. Mater. Chem. 2002, 12, 1657. (10) Janus, K.; Sworakowski, J.; Luboch, E. Chem. Phys. 2002, 285, 47. (11) Pedersen, C. J. J. Am. Chem. Soc. 1967, 89, 7017.

Photochromism of Crown Ethers (12) Gokel, G. W. Crown Ethers and Cryptands; Royal Society of Chemistry: Cambridge, U.K., 1991. (13) Shinkai, S.; Nakaji, T.; Nishida, Y.; Ogawa, T.; Manabe, O. J. Am. Chem. Soc. 1980, 102, 1980. (14) Tahara, R.; Morozumi, T.; Nakamura, H.; Shimomura, M. J. Phys. Chem. B 1997, 101, 7736. (15) Sworakowski, J.; Janus, K.; Nesˇpurek, S. IEEE Trans. Dielectr. Electr. Insul. 2001, 8, 543. (16) Luboch, E.; Kravtsov, V. Ch.; Konitz, A. J. Supramol. Chem. 2001, 1, 101. (17) Luboch, E.; Wagner-Wysiecka, E.; Kravtsov, V. C.; Kessler, V. Pol. J. Chem. 2003, 77, 189. (18) Biernat, J. F.; Luboch, E.; Cygan, A.; Simonov, Y. A.; Dvorkin, A. A.; Muszalska, E.; Bilewicz, R. Tetrahedron 1992, 48, 4399. (19) Skwierawska, A.; Luboch, E.; Biernat, J. F.; Kravtsov, V. Ch.; Simonov, Yu. A.; Dvorkin, A. A.; Bel’skii, V. K. J. Inclusion Phenom. 1998, 31, 71. (20) Luboch, E.; Biernat, J. F.; Simonov, Y. A.; Dvorkin, A. A. Tetrahedron 1998, 54, 4977. (21) Skwierawska, A. Ph.D. Thesis, Gdan´sk University of Technology, Gdan´sk, Poland, 1998. (22) Zawisza, I.; Bilewicz, R.; Janus, K.; Sworakowski, J.; Luboch, E.; Biernat, J. F. Mater. Sci. Eng., C 2002, 22, 91. (23) Sworakowski, J.; Nesˇpurek, S. Chem. Phys. Lett. 1998, 298, 21. (24) Becke, A. D. J. Chem. Phys. 1993, 98, 5648. (25) Binkley, J. S.; Pople, J. A.; Hehre, W. J. J. Am. Chem. Soc. 1980, 102, 939. (26) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Montgomery, J. A., Jr.; Vreven, T.; Kudin, K.

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