Influence of the Solvent on the Thermal Back Reaction of One

Article pubs.acs.org/jchemeduc

Influence of the Solvent on the Thermal Back Reaction of One Spiropyran Jonathan Piard* Chemistry Department and Laboratoire de Photophysique et Photochimie Supramoléculaires et Macromoléculaires (PPSM, CNRS UMR 8531), ENS Cachan, 94235 Cachan Cedex, France S Supporting Information *

ABSTRACT: The solvent influence on the absorption spectra and the kinetics of the back reaction of the 1′,3′-dihydro-1′,3′,3′-trimethyl-6-nitrospiro(2H-1-benzopyran2,2′-2H-indole) (6-NO2-BIPS) has been investigated by means of temperaturecontrolled, UV−visible spectroscopic measurements. The back reaction process was proved to follow first-order kinetics for five different solvents (toluene, THF, acetonitrile, DMSO, and ethanol) at several temperatures. Kinetics parameters (Ea, A, τ, ΔH#, ΔS#, and ΔG#) were extracted from absorbance decay profiles for all solvents, and a simple model based on solvation mechanism was proposed in good agreement with experimental results. Such a photochromic compound also exhibits strong solvatochromism in its merocyanine form. All these experiments were performed to illustrate solvent effects on kinetic processes and optical properties for students with an undergraduate physical chemistry theoretical background. All the experiments described could be performed in a physical chemistry undergraduate laboratory. KEYWORDS: First-Year Undergraduate/General, Second-Year Undergraduate, Physical Chemistry, Hands-On Learning/Manipulatives, Photochemistry, UV-Vis Spectroscopy

■

INTRODUCTION Photochromism is a reversible transformation of a molecular entity between two forms, A and B, each form having different absorption spectra, induced in one or both directions by absorption of electromagnetic radiation.1 The thermodynamically stable form A is transformed by irradiation into the commonly colored form B. From this point, the back reaction to regenerate the form A can occur thermally (T-type photochromism) or photochemically (P-type photochromism).2−4 Among T-type photochromic compounds, the family of spiropyrans has attracted much attention since the 1960s due to their potential technological applications as photochromic glasses5 and more recently as data storage devices6−9 or for bioimaging.10,11 For such a family, light irradiation induces a photochemical ring-opening reaction (a carbon−oxygen bond is broken) to yield to an isomeric colored merocyanine form (MC isomer). Through photochromic reaction, not only the absorption but also other physical and chemical properties, such as fluorescence,12−14 refractive index,15−18 and oxidation/ reduction potentials,19−21 can reversibly change. The syntheses of spiropyrans,5,22 as well as their thermochromism23 and its potential applications,24,25 have already been reported in this Journal. T-type photochromism provides an interesting and visually appealing way to follow the kinetics of a first-order reaction since the thermal back reaction is accompanied by a decoloration of the solution. Thus, the kinetics rate constant can be easily extracted from UV−visible analysis by means of a spectrometer. In addition, the temperature factor and solvent © XXXX American Chemical Society and Division of Chemical Education, Inc.

effect on kinetics can easily be understandable. Moreover, by considering the changes in the electric properties that occur during photochromism, students are able to establish simple schemes in order to interpret experimental results. The present article describes a series of chemical kinetics experiments in order to evaluate the solvent effect in the kinetics of the back reaction and absorption spectra of one spiropyran: 1′,3′-dihydro-1′,3′,3′-trimethyl-6-nitrospiro(2H-1benzopyran-2,2′-2H-indole) (also known as 6-NO2-BIPS, Figure 1). We show how students can determine kinetics parameters for the backward process (relaxation times and activation energy) and observed absorption change with the solvent: solvatochromism. Such investigation is based on an undergraduate physical chemistry theoretical background. This work can be seen as a complement to the previous investigation

Figure 1. Structure and photochromic reaction of 6-NO2-BIPS. Atoms involved in the photochromic reaction are displayed in red.

A

dx.doi.org/10.1021/ed4005003 | J. Chem. Educ. XXXX, XXX, XXX−XXX

Journal of Chemical Education

Article

on photochromism and photochemical kinetics for undergraduate physical chemistry laboratory courses published by Hernán E. Prypsztejn and R. Martiń Negri.26 6-NO2-BIPS, a colorless spiropyran, referred to as the normal form (N isomer), undergoes a photochemical ring-opening reaction under ultraviolet irradiation to yield an isomeric colored merocyanine form (MC isomer) (Figure 1). It has been proposed that the colored form (MC) is a zwitterion.2,3 The N isomer is commonly thermodynamically the more stable isomer and absorbs in the UV region, whereas MC absorbs in both the UV and the visible regions with a strong and characteristic absorption band between 500 and 600 nm. When irradiating a solution of 6-NO2-BIPS with UV-light, formation of the MC isomer is induced. The increase of the concentration of the MC isomer in the solution is accompanied by an increase of color intensity and absorbance in the visible region of absorption spectrum. After the UV irradiation is stopped, MC returns spontaneously to N leading to the decoloration of the solution. The kinetics of the back reaction process can easily be characterized by measuring the visible absorption of MC as a function of time. For such experiments, a UV−vis spectrophotometer with a temperature-controlled sample compartment must be used.

■

Figure 2. Experimental setup during the UV irradiation of the sample (side view). The covers of the spectrometer compartment housing and the sample cell are temporarily removed. They are replaced when irradiation is stopped and before starting experiments.

housing and sample cell (except for very quick reaction) were replaced as quickly as possible before starting the experiments. Kinetics Measurements

UV−visible absorption spectra were recorded with a double beam Varian Cary 5000 spectrometer with a temperaturecontrolled (Peltier effect) sample holder. The temperature of the liquids in the quartz cells was controlled by a thermocouple for each temperature. The theoretical temperature range is 0− 100 °C but limited by the melting and boiling points of the chosen solvents. One centimeter path length quartz Suprasil cuvettes from Hellma were used for the absorption measurements. Plastic cuvettes can be used when the absorption of the UV band was not needed as for kinetics measurements. The absorbance decay was followed at λmax (MC) using a Varian software program. This maximum was previously determined by recording the absorption spectrum of a solution irradiated several minutes at 365 nm at 25 °C (Figure 3).

MATERIALS AND METHODS

Reagent and Solvent

1′,3′-Dihydro-1′,3′,3′-trimethyl-6-nitrospiro(2H-1-benzopyran2,2′-2H-indole) from Sigma-Aldrich was used as supplied. The solvents were spectrometric grade from Aldrich, Sigma or Merck. Solvents used for this study were chosen to cover a wide range of electric properties (dielectric constant and dipole moment). Sample Preparation

Solutions were prepared by taking 3.2 mg of 6-NO2-BIPS and dissolving it in approximately 20 mL of solvent to give a 5.0 × 10−4 mol L−1 solution. The concentration was chosen such that initial absorbance at λmax of the UV absorption peak was below 4 at room temperature and to avoid the dimerization process. Nevertheless, if the common UV−visible spectrometer usually applied for undergraduate experiments is used, the value must not exceed 1. No volumetric glassware is required. Solutions can be stored at room temperature in the dark for a week. UV-Irradiation Conditions

The photochromic reaction from N to MC was induced by a continuous irradiation. The sample was directly irradiated in the spectrophotometer by using a mercury/xenon (Hg/Xe) lamp (Hamamatsu, LC6 Lightingcure, 200 W) equipped with a light guide (supplied by Hamamatsu) and condenser lenses. A filtered beam at 365 nm was obtained by means of a narrow band interference filter from Semrock (Hg01). Irradiation power at 365 nm was evaluated to be 130 mW·cm−2. External light was manually blocked off when desired by an internal filter placed at the entrance of the light guide. Nevertheless, a common UV-irradiation lamp or a photographic flash can be used for the same purpose. The covers of the spectrometer compartment housing and the sample cell were temporarily removed during the irradiation procedure. A schematic experimental setup during this procedure is represented in Figure 2. Light gets through the sample cell from the top. When irradiation was stopped (manually), the irradiation system was removed and both covers of the spectrometer compartment

Figure 3. Absorption spectra of 6-NO2-BIPS in various solvents at 25 °C (toluene (a), light blue; THF (b), dark blue; DMSO (c), dark purple; acetonitrile (ACN) (d), purple; ethanol (e), magenta) (top) before irradiation (N isomer) and (bottom) after irradiation at 365 nm (photostationary state: N + MC isomer). Inset: zoom of the absorption of N in the UV region. Absorption spectra are normalized at 1 at the maximum of the UV band for N and visible band for MC. B



Article

Such variation of optical properties with solvent polarity is referred as solvatochromism.

Irradiation was stopped when colors remained stable for 1 min. Two minutes of irradiation is usually sufficient to generate a sufficient amount of MC isomer when irradiation power is 130 mW·cm−2. The temperature was controlled to be constant during all the kinetic measurements.

■

Kinetic Measurements

It was formerly proved in this Journal that the back reaction of the 6-NO2-BIPS follows first-order kinetics in ethanol solution.26 Since the visible band can be fully attributed to the MC isomer, the decay of the absorbance A(t) at λmax, after irradiation at 365 nm is stopped, follows the equation:

RESULTS AND DISCUSSION

UV−Visible Properties

The forward reaction from N to MC via excited states is induced by a UV irradiation (365 nm) of N. No attempt to follow the forward reaction was performed. Irradiation is stopped when colors remain stable during 1 min. The spectral changes in various solvents at 25 °C (toluene to ethanol) before (N) and after (photostationary state: N + MC) irradiation at 365 nm are shown in Figure 3. For a better comparison, spectra are normalized at 1 at λmax in the UV region for N and visible region for irradiated solutions. The color changes induced by photochromic reaction of 6-NO2BIPS upon irradiation at 365 nm for the various solvents are shown in Figure 4. Irradiation at 365 nm leads to a solution

A(t ) = (A 0 − A i ) e−(t / τ) + A i = (A 0 − A i ) e−kt + A i (1)

where τ and k represent, respectively, the time and rate constants of the decoloration process, A0 is the absorbance of MC isomer at the initial time zero (just when irradiation is stopped), and Ai is the absorbance of the solution before irradiation (or at long times after the UV irradiation when thermal back reaction is complete), measured at λmax of the visible band of MC. The time evolution of the absorbance at λmax (MC) in acetonitrile at 25 °C is given in Figure 5 as an example. As a result of eq 1, first-order kinetics leads to a linear behavior of ln(A(t) − Ai) with time. As shown in Figure 6, linear time evolution is confirmed for all the solvents (from toluene to ethanol) in accordance with a first-order decay profile. Similar behavior was observed in all solvents at all temperatures assayed (see the Supporting Information). Nevertheless, a slight deviation can be observed for THF (Figure 6, curve b) and acetonitrile (Figure 6, curve d) and can be attributed to several phenomena (possible parallel reactions of other isomeric structures, simple approach to the thermal equilibrium concentrations or photodegradation of the N isomer). The values of the time constant of the thermal back reaction, τ, obtained at 25 °C for all solvents are reported in Table 1 and will be discussed later. The activation energy (Ea) in each solvent can be determined from the slope of ln(τ) as a function of 1/T according to the Arrhenius equation: k=

1 = A e−(Ea / RT ) τ

(2)

where k is the rate constant, τ is the time constant of the back reaction process, T is the temperature in kelvin, A is the preexponential factor in reciprocal time units, and R is the universal gas constant. As shown in Figure 7, for all solvents, the data are in a good accordance with a linear behavior. The Arrhenius equation can be applied safely. Then the activation enthalpy and entropy, ΔH# and ΔS#, can be calculated using the activated complex theory for unimolecular reactions in solution:26 Figure 4. Color changes of 6-NO2-BIPS upon UV irradiation cycles in different solvents (from toluene to ethanol).

ΔH # = Ea − RT

(3)

and ⎛ hA ⎞ ⎟ ΔS # = R ln⎜ ⎝ ekT ⎠

composed of a mixture of N and MC, the photostationary state. In all solvents, N isomer solutions are colorless with a maximum absorption in the UV region localized around 340 nm, slightly sensitive to solvent variation: from 335 nm in toluene to 350 nm in DMSO (inset, Figure 3). On the contrary, the MC isomer exhibits not only a UV band but also a pronounced absorption band in the visible region with a maximum position strongly dependent on solvent: from 536 nm in ethanol to 603 nm in toluene (Figure 3 and Table 1).

(4)

where A represents the pre-exponential factor, R the gas constant, h the Planck constant, and k the Boltzmann constant. The activation free energy, ΔG# is then deduced by using the following equation: ΔG # = ΔH # − T ΔS # C

(5)



Article

Table 1. Comparison of Experimental Results by Solvent

a Dielectric constant (ε).28,29 bDipolar moment (μ).28,29 cTime constant of the back reaction process at 25 °C (τ(25 °C)). dMaximum wavelength in the visible region for MC (λmax (MC)). eActivation energy (Ea). fLogarithm of the pre-exponential factor (Ln(A)). gActivation enthalpy at 25 °C (ΔH#(25 °C)). hActivation entropy at 25 °C (ΔS#(25 °C)). iActivation Gibbs free energy at 25 °C (ΔG#(25 °C). jAbsorbance at λmax in the visible for MC isomer at 25 °C (A(λmax (MC))). kThermal equilibrium constant at 25 °C (Keq (25 °C). lGibbs free energy of the reaction N → MC at 25 °C (ΔrG0(25 °C)).

Figure 6. Ln(A(t) − Ai) versus time in various solvents at 25 °C: (a) light blue, toluene; (b) dark blue, THF; (c) dark purple, DMSO; (d) purple, ACN; (e) magenta, ethanol. Absorbance was followed at the absorption maximum wavelength of MC isomer (see Table 1). The Ai values used for each solvent are detailed in the Supporting Information.

Figure 5. Evolution of the absorbance at 556 nm (λmax, MC) with time in acetonitrile at 25 °C.

As for τ (25 °C), the values of Ea, ln(A), ΔH# (25 °C), ΔS# (25 °C), and ΔG# (25 °C) obtained in each solvent are reported in Table 1 and discussed later.

requires knowledge of the molar extinction coefficient ε of both the N and MC isomer. These values are unfortunately not known for 6-NO2-BIPS but are generally in the range (3.5−6.0) × 104 L·mol−1·cm−1 for spiropyrans derivatives.27 Hence,

Equilibrium Measurements

In the absence of UV radiation, a thermal equilibrium between N and MC is established in all solvents. Calculation of Keq D



■

according to former study,31 the value 3.5 × 104 L·mol−1·cm−1 in ethanol was used for polar solvents (ACN, DMSO) and 5.2 × 104 L·mol−1·cm−1 for nonpolar solvents (toluene, THF) for ε (λmax (MC)). The value of the thermal equilibrium constant (Keq) and ΔrG° can be estimated by measuring the absorbance in the visible region at 25 °C (Figure 3) as follows: [MC]eq [N]eq

=

A(λmax (MC)) ε(λmax (MC))l[N]0 − A(λmax (MC)) (6)

and ΔrG° = −RT ln(Keq)

DISCUSSION

From the results described above and reported in Table 1, it appears that the spectral position of the visible absorbance band of the MC isomer, the color of the MC isomer solution, and the kinetic characteristics of the back-reaction process (τ, Ea, A, ΔH#(25 °C), ΔS# (25 °C), and ΔG# (25 °C)) as well as thermal equilibrium between N and MC (ΔrG0 (25 °C)) are strongly influenced by solvent properties. Indeed, an MC isomer solution is light blue in toluene, dark blue in THF, light purple in ACN, dark purple in DMSO, and magenta in ethanol (Figure 4). Moreover, as mentioned in Table 1, the polarity of the solvent increases from toluene to THF and from THF to ACN. As a consequence, the hypsochromic shift (blue shift) observed with increasing solvent polarity reveals a lower dipole moment in the excited state than in the ground state of MC and a so-called negative solvatochromism. The effect of solvation stabilization on the energy of the ground and excited states of a different merocyanine dye has been reported in this Journal.30 As detailed in Table 1, the relaxation time of the back reaction and the activation energy increases while ΔrG0 decreases with the polarity of the solvent (ε, μ). From these results, both a potential energy diagram (Figure 8a) and a Gibbs free energy diagram (Figure 8b) can be constructed for all solvents at 25 °C. Figure 8 represents the comparison of such diagrams for two solvents with S1 more polar than S2. These diagrams are particularly well representative when S1 and S2 present important polarity differences: toluene and DMSO for instance. In four of the solvents (THF, ACN, DMSO, and toluene) the results can be explained by exploring solvents effect on both the transition state (referred as TS) and the MC isomer. Indeed, due to its zwitterionic character, the polarity of the MC isomer appears to be higher than that of transition state (TS).31 Since polar solvent preferentially solvate and thus stabilize polar species, the MC isomer is expected to be more stabilized in comparison to TS. This leads to an increase of the activation barrier since Ea (Figure 8a) and ΔG# (Figure 8b) with an increase in solvent polarity.

Figure 7. Ln τ as a function of 1000/T in various solvents: (a) toluene, light blue; (b) THF, dark blue; (c) DMSO, dark purple; (d) ACN, purple; (e) ethanol, magenta. Dashed lines correspond to the best linear fit obtained.

Keq =

Article

(7)

where [N]0 is equal to the initial concentration (5.0 × 10−4 mol·L−1) and l is the path length (1 cm). Values for A(λmax (MC)) were determined at the end of the kinetics at 25 °C.

Figure 8. (a) Potential energy diagram versus reaction coordinate; (b) Gibbs free energy diagram of 6-NO2-BIPS thermal back reaction for two different solvents (S1 and S2). S1 is assumed to be more polar than S2. TS is the transition state. E



Article

In the case of ethanol as solvent, the values of τ and Ea are exceptionally high. This observation can be interpreted by taking into account the high protic character of ethanol. Hydrogen bonding can be present between ethanol and the oxygen atom of MC. As a consequence, the MC isomer is dramatically stabilized in ethanol in comparison with ACN for instance. It is important to note that the results in ethanol (ΔH#(25 °C) = 107.7 ± 2.5 kJ mol−1; ΔS#(25 °C) = 54.4 ± 10 J·K−1· mol−1, Ea = 110.2 ± 2.5 kJ mol−1, ΔrG0(25 °C) = 11.8 ± 0.26 kJ mol−1) presented here are in excellent agreement with those reported previously in this Journal26 (ΔH#(25 °C) = 110 ± 2 kJ mol−1; (ΔS#(25 °C) = 61 ± 7 J·K−1·mol−1 and Ea = 112.2 ± 2 kJ mol−1) and former study31 ΔrG0(25 °C) = 11.7 ± 0.83 kJ mol−1). Experiments in cyclohexane, a nonpolar solvent, were also performed. However, the highly polar MC isomer quickly precipitates. No measurement could be performed.

Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS These experiments have been performed as part of a project initiated by undergraduate chemistry students at the Lycée Sainte Geneviève, Versailles, France. I am grateful to Amélie Nicolay, Célia Souque and Laure Vuaille for their help, their assistance, and their valuable discussions. I would like to thank Keitaro Nakatani for his useful advice and remarks. The Laboratoire de Photophysique et Photochimie Supramoléculaires et Macromoléculaires (PPSM, CNRS UMR 8531) and ENS Cachan are also acknowledged for their financial support. The author would like to thank the reviewers for their comments that helped improve the manuscript.

■

■

CONCLUSION T-type photochromic compounds, in particular 6-NO2-BIPS, were shown to be very appropriate to investigate solvation effects both on kinetic processes and on spectroscopic properties. The back reaction isomerization from the MC to N spontaneously occurs following first-order kinetics in all solvents and at several temperatures. Kinetic parameters such as relaxation time or activation energy can easily be extracted by using simple equations. Photochromism provides visually appealing experiments thanks to color changes of the solution: coloration upon UV irradiation followed by a thermal decoloration when irradiation is stopped. The back reaction rate constant can be qualitatively and quantitatively compared for various solvents by following the decoloration of the solution by eye (see the video in the Supporting Information) or by means of a spectrometer. The students are also allowed to evaluate solvatochromism of the MC isomer through its color change with solvent. All the experiments described here are easy to implement and can readily be done using apparatus available in a laboratory dedicated to undergraduate courses. A temperature-controlled sample holder is moreover needed in order to obtain quantitative results. Indeed, the chemical reagents are commercially available and UV irradiation by means of an irradiation lamp can be replaced by a commercial photographic flash. Such experiments take approximately 1−2 h for each solvent and should be done by groups of students to be more time-efficient: one group for each solvent, for instance. This work can be seen as a good complement to the previous investigation on photochromism and photochemical kinetics for undergraduate physical chemistry laboratory courses published in this Journal in 2001.26

■

ASSOCIATED CONTENT

S Supporting Information *

Details of the evolution of absorbance and ln(A(t) − Ai) as a function of time at all temperatures for all solvents; errors in kinetic parameters calculated; a movie illustrating the influence of the solvent on the decoloration rate constant (.zip). This material is available via the Internet at http://pubs.acs.org.

■

REFERENCES

(1) Braslavsky, S. E. Glossary of Terms Used in Photochemistry. Pure Appl. Chem. 2007, 79 (3), 293−465. (2) Brown, G. H. Photochromism; Wiley-Interscience: New York, 1971; p 853. (3) Dürr, H.; Bouas-Laurent, H. Photochromism, Molecules, and Systems; Elsevier: Amsterdam, 1990. (4) Feringa, B. L.; Delden, R. A.; Wiel, M. K. J., Chiroptical Molecular Switches. In Molecular Switches; Feringa, B. L., Ed.; WileyVCH: Darmstadt, 2001. (5) Osterby, B.; McKelvey, R. D.; Hill, L. Photochromic Sunglasses: A Patent-Based Advanced Organic Synthesis Project and Demonstration. J. Chem. Educ. 1991, 68, 424−425. (6) Dvornikov, A. S.; Walker, E. P.; Rentzepis, P. M. Two-Photon Three-Dimensional Optical Storage Memory. J. Phys. Chem. A 2009, 113 (49), 13633−13644. (7) Parthenopoulos, D. A.; Rentzepis, P. M. 3-Dimensional Optical Storage Memory. Science 1989, 245 (4920), 843−845. (8) Raymo, F. M.; Giordani, S.; White, A. J. P.; Williams, D. J. Digital Processing with a Three-State Molecular Switch. J. Org. Chem. 2003, 68 (11), 4158−4169. (9) Raymo, F. M.; Tomasulo, M. Electron and Energy Transfer Modulation with Photochromic Switches. Chem. Soc. Rev. 2005, 34 (4), 327−336. (10) Zhu, L. Y.; Wu, W. W.; Zhu, M. Q.; Han, J. J.; Hurst, J. K.; Li, A. D. Q. Reversibly Photoswitchable Dual-Color Fluorescent Nanoparticles As New Tools for Live-Cell Imaging. J. Am. Chem. Soc. 2007, 129 (12), 3524−3526. (11) Zhu, M. Q.; Zhang, G. F.; Li, C.; Aldred, M. P.; Chang, E.; Drezek, R. A.; Li, A. D. Q. Reversible Two-Photon Photoswitching and Two-Photon Imaging of Immunofunctionalized Nanoparticles Targeted to Cancer Cells. J. Am. Chem. Soc. 2011, 133 (2), 365−372. (12) Irie, M.; Fukaminato, T.; Sasaki, T.; Tamai, N.; Kawai, T. Organic Chemistry: A Digital Fluorescent Molecular Photoswitch. Nature 2002, 420 (6917), 759−760. (13) Kawai, T.; Sasaki, T.; Irie, M. A Photoresponsive Laser Dye Containing Photochromic Dithienylethene Units. Chem. Commun. 2001, No. 8, 711−712. (14) Norsten, T. B.; Peters, A.; McDonald, R.; Wang, M. T.; Branda, N. R. Reversible [7]-Thiahelicene Formation Using a 1,2-Dithienylcyclopentene Photochrome. J. Am. Chem. Soc. 2001, 123 (30), 7447− 7448. (15) Chauvin, J.; Kawai, T.; Irie, M. Refractive Index Change of an Amorphous Bisbenzothienylethene. Jpn. J. Appl. Phys., Part 1 2001, 40 (4A), 2518−2522. (16) Kawai, T.; Fukuda, N.; Groschl, D.; Kobatake, S.; Irie, M. Refractive Index Change of Dithienylethene in Bulk Amorphous Solid Phase. Jpn. J. Appl. Phys., Part 2 1999, 38 (10B), L1194−L1196. (17) Kim, E.; Choi, Y. K.; Lee, M. H. Photoinduced Refractive Index Change of a Photochromic Diarylethene Polymer. Macromolecules 1999, 32 (15), 4855−4860.

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. F



Article

(18) Kim, M. S.; Maruyama, H.; Kawai, T.; Irie, M. Refractive Index Changes of Amorphous Diarylethenes Containing 2,4-Diphenylphenyl Substituents. Chem. Mater. 2003, 15 (24), 4539−4543. (19) Gilat, S. L.; Kawai, S. H.; Lehn, J. M. Light-Triggered Molecular DevicesPhotochemical Switching of Optical and Electrochemical Properties in Molecular Wire Type Diarylethene Species. Chem.Eur. J. 1995, 1 (5), 275−284. (20) Kawai, T.; Kunitake, T.; Irie, M. Novel Photochromic Conducting Polymer Having Diarylethene Derivative in the Main Chain. Chem. Lett. 1999, 9, 905−906. (21) Matsuda, K.; Irie, M. A Diarylethene with Two Nitronyl Nitroxides: Photoswitching of Intramolecular Magnetic Interaction. J. Am. Chem. Soc. 2000, 122 (30), 7195−7201. (22) Guglielmetti, R.; Meyer, R.; Dupuy, C. J. Synthesis of a Photochromic Benzothiazolinic Spiropyran. J. Chem. Educ. 1973, 50, 413−415. (23) Byrne, J. P. Thermodynamic Data from the Thermochromic Effect. J. Chem. Educ. 1978, 55, 267−268. (24) Chambers, K. W.; Smith, I. M. An Inexpensive Flash Photolysis Apparatus and Demonstration Experiment. J. Chem. Educ. 1974, 51, 354−356. (25) Jacques, P. Synergistic Effects in Binary Solvent Mixtures. A Simple Approach to a Special Case of Nonideal Binary Mixture Behavior. J. Chem. Educ. 1991, 68, 347−348. (26) Prypsztejn, H. R.; Martín Negri, R. An Experiment on Photochromism and Kinetics for the Undergraduate Laboratory. J. Chem. Educ. 2001, 78, 645−648. (27) Calvert, J. G.; Pitts, J. N., Jr. Photochemistry; Wiley and Sons: New York, 1966; p 487. (28) Sigma Aldrich Web Page. http://www.sigmaaldrich.com (accessed Aug 2014). (29) Acros Organics Web Page. http://www.acros.com (accessed Aug 2014). (30) Shah, S. S.; Henscheid, L. G. Absorption Spectroscopy in Homogeneous and Micellar solutions. J. Chem. Educ. 1983, 60, 685− 687. (31) Flannery, J. B. J. Photo- and Thermochromic Transients from Substituted 1′,3′,3′-Trimethylindolinobenzospiropyrans. J. Am. Chem. Soc. 1968, 90, 5660−5671.

G


Influence of the Solvent on the Thermal Back Reaction of One

Recommend Documents