Entropy-Driven Thermal Isomerization of Spiropyran in Viscous Media

ARTICLE pubs.acs.org/JPCA

Entropy-Driven Thermal Isomerization of Spiropyran in Viscous Media Yasuhiro Shiraishi,* Takuya Inoue, Shigehiro Sumiya, and Takayuki Hirai Research Center for Solar Energy Chemistry, and Division of Chemical Engineering, Graduate School of Engineering Science, Osaka University, Toyonaka 560-8531, Japan

bS Supporting Information ABSTRACT: Effects of solvent viscosity on the thermal isomerization properties of a spiropyran derivative have been studied in glycerol, ethylene glycol, 1,4-butanediol, and ionic liquid solutions. Thermal isomerization of the colorless spirocyclic (SP) form to the colored merocyanine (MC) form is enhanced with an increase in the concentrations of viscous solvents in solution. Equilibrium absorption analysis revealed that the enhanced SP f MC isomerization in viscous media is due to the strong solventsolvent interaction, which suppresses the ordering of solvent molecules around the MC form. This results in a positive entropy change for isomerization and, hence, promotes entropy-driven isomerization. Kinetic absorption analysis revealed that the solvent viscosity scarcely affects the thermal activation process for isomerization, where the activation enthalpy and entropy parameters are solely affected by the solvent polarity.

1. INTRODUCTION Spiropyran derivatives belong to a class of organic photochromes1 that have been studied extensively in application to various functional materials2 such as optical switches,3 memories,4 and sensors.5 These dyes are converted to the colored merocyanine (MC) form upon irradiation of UV light but revert to the colorless spirocyclic (SP) form upon irradiation of visible light (Scheme 1a).6 In common organic solvents, the ground state energy of the MC form is higher than that of the SP form (Figure 1a). Thermal SP f MC isomerization therefore does not occur, although the MC form is thermally reverted to the SP form. A few reports, however, revealed that thermal SP f MC isomerization occurs in some solvents such as water7 and fluoroalcohols8 (Scheme 1b). This is because, as shown in Figure 1b, the hydrogen bonding interaction between the MC form and the solvent molecules leads to a decrease in the ground state energy of the MC form lower than that of the SP form.7a The Gibbs free energy change (ΔrG) for SP f MC isomerization of a spiropyran derivative can be expressed with a temperature, standard enthalpy (ΔrH), and entropy (ΔrS) change.9 Δr G ¼ Δr H TΔr S

ð1Þ

In hydrogen bonding solvents (Figure 1b), the lowered ground state energy of the MC form promotes ΔrH decrease. This leads to ΔrG decrease and promotes thermal SP f MC isomerization, suggesting that the isomerization in these solvents is an enthalpy-driven process. The MC form of spiropyran derivatives has a larger rotational freedom than the SP form; therefore, the entropy change for SP f MC isomerization is intrinsically positive.10 The MC form, however, has a larger dipole moment than the SP form.11 The formation of MC form therefore promotes an ordering of solvent molecules around the r 2011 American Chemical Society

Scheme 1. Photochromic and Thermochromic Isomerization of a Spiropyran Derivative

MC form.12 This promotes a negative entropy change, and the thermal SP f MC isomerization usually results in ΔrS decrease.13 It is well-known that, in viscous media, the solvent ordering around the solute molecule is suppressed because of strong solventsolvent interaction.14 This suggests that thermal SP f MC isomerization, if performed in viscous media, would suppress the solvent ordering around the MC form. This might result in ΔrS increase and promote entropy-driven isomerization. Received: February 3, 2011 Revised: July 16, 2011 Published: July 18, 2011 9083

dx.doi.org/10.1021/jp201137d | J. Phys. Chem. A 2011, 115, 9083–9090

The Journal of Physical Chemistry A

Figure 1. Energy diagrams for thermal isomerization of a spiropyran derivative.

Herein, we report that the entropy-driven SP f MC isomerization indeed occurs in viscous media. Effects of solvent viscosity on the thermal isomerization of a spiropyran derivative, 1 (Scheme 1), have been studied in glycerol, ethylene glycol, 1,4-butanediol, and an ionic liquid solution. Thermal isomerization of 1 is enhanced with an increase in the concentrations of viscous solvents. Equilibrium absorption analysis clearly revealed that the isomerization enhancement is triggered by the ΔrS increase. Kinetic absorption analysis was also carried out to clarify the effect of solvent viscosity on the thermal activation process for isomerization.

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Figure 2. Time-dependent change in absorption spectra of 1 (50 μM) measured at 50 °C in (a) glycerol/MeOH (5/5 v/v) mixture and (b) MeOH in the dark.

2. EXPERIMENTAL SECTION 2.1. Materials. All of the reagents used were supplied from Wako, Aldrich, and Tokyo Kasei and used without further purification. Water was purified by a Milli-Q system. An ionic liquid, 1-butyl-3methylimidazolium tetrafluoroborate ([BMIM][BF4]), was mixed with CH2Cl2 and stirred with activated carbon under N2. After removal of activated carbon by filtration and CH2Cl2 by evaporation, the resulting ionic liquid was used for measurement. Spiropyran derivative, 20 ,30 ,30 -trimethyl-6-nitro-30 -H-spiro[chromene-2,20 indol]-10 -yl, 1, was synthesized according to procedure described previously.5b 2.2. Analysis. Absorption spectra were measured using a 10 mm path length quartz cell on an UVvis photodiode-array spectrophotometer (Shimadzu; Multispec-1500) equipped with a temperature controller (Shimadzu; S-1700).15 The measurements in ionic liquid solutions were carried out under N2, and those in other solutions were performed in an aerated condition. UV or visible light irradiation was carried out with a Xenon lamp (300 W; Asahi Spectra Co. Ltd.; MAX-302) equipped with a 280 nm band-pass filter (LX280; light intensity, 36.2 W m2) or a 550 nm band-pass filter (MX550; light intensity, 0.16 W m2), where the light intensities were measured with a spectroradiometer (USR-40, Ushio Inc.).16 Viscosity of the solution was measured using a TOKI SANGYO TVE-22 L viscometer.17 2.3. Determination of Molar Extinction Coefficient. Molar extinction coefficients of the MC form (εMC) in respective solvents were determined according to literature procedure,18 which include (20% errors. The measurements were carried out, as follows: in a typical analysis in MeOH (see Supporting Information, Figure S1), the solution containing 1 (50 μM) was stirred at 25 °C for 5 min under 550 nm irradiation, and the MC absorbance of solution was set at almost zero. The solution was then irradiated with a 280 nm light until the MC absorbance increase is saturated (ca. 30 min). After irradiation, the absorbance at 269 nm was decreased to 37% of its original value.

Figure 3. (A) Time-dependent change in MC absorbance of 1 (50 μM) measured in different solvents at 50 °C in the dark. The lines are just the guide for the eyes. The spectra for the samples c and j are shown in Figure 2, and those for the other samples are summarized in Supporting Information (Figures S2S11). (B) Photographs of the solution containing 1.

The MC concentration therefore must lie between 63 and 100% of the prepared concentration of 1 (50 μM), thus, providing an εMC value between 4.64 and 2.91 104 M1 cm1. The mean value, (3.77 ( 0.86) 104 M1 cm1, was employed, where other parameters such as Keq, ΔrH, ΔrS, and ΔrG derived from εMC include the errors accordingly. 2.4. Kinetic Measurement. Kinetic absorption analysis was carried out as follows: the temperature of solution was adjusted with magnetic stirring using a temperature controller. A 550 nm light was irradiated to the solution for 5 min, and the MC absorbance of solution was set at almost zero. The light was turned off, and the kinetic measurement was carried out in the dark at the constant temperature.

3. RESULTS AND DISCUSSION 3.1. Thermal Isomerization in Viscous Media. Figure 2a shows the time-dependent change in absorption spectra of 1 9084

dx.doi.org/10.1021/jp201137d |J. Phys. Chem. A 2011, 115, 9083–9090

9.16 ( 0.72 1.89 ( 2.53

0.09 ( 2.72 7.55 ( 0.74 0.060 ( 0.019 7.52 ( 0.41

0.033 ( 0.010 9.77 ( 0.10 53.0 28.4

53.2 22.2 3.53 ( 0.81 537

539 [BMIM][BF4]/ MeOH (8/2 v/v) l

k 1,4-butanediol/MeOH (8/2 v/v)

3.50l ( 0.80

56.1

55.2 54.3

1.8

1.4 0.4

2.50 ( 0.57 510

518 527 water/MeOH (2/8 v/v) MeOH

h water/MeOH (5/5 v/v)

I j

3.52 ( 0.80 3.77 ( 0.86

54.5 1.3 325 g ethylene glycol/MeOH (2/8 v/v)

3.70 ( 0.84

54.5

54.5

1.9

3.3

3.58 ( 0.82

ethylene glycol/MeOH (5/5 v/v)

3.62 ( 0.83

525

525

e glycerol/MeOH (2/8 v/v)

f

54.8

54.6

15.1

3.3

3.20 ( 0.73

3.43 ( 0.78

522

524

c glycerol/MeOH (5/5 v/v)

d glycerol/MeOH (3/7 v/v)

55.0 54.9 76.8 39.4 2.87 ( 0.65 2.97 ( 0.68 520 521 a glycerol/MeOH (8/2 v/v) b glycerol/MeOH (7/3 v/v)

(10 M (nm)

Maximum wavelength of the MC absorption at 25 °C. b Molar extinction coefficient determined at 25 °C. c Solvent viscosity measured at 25 °C. d Solvent polarity parameter (= 28591/λMC).29 e Calculated from the equilibrium absorption data in Figure S12. f Determined by the van’t Hoff plots (Figure S14) using the equation lnKeq = ΔrH/RT + ΔrS/R.9 g Calculated using eq 1. h Determined from the kinetic absorption data (Figure S13). i Determined using eq 8. j Determined by the Arrhenius plots (Figure S15) using the equation ln k = ln A Ea/RT, where A is the frequency factor and Ea is the activation energy (= ΔH‡ + RT).24 k Determined by the Arrhenius plots (Figure S15) using the equation, ΔS‡ = R [ln A 1 ln(kBT/h)], where kB and h are the Boltzmann’s constant and the Plank’s constant, respectively.24 l From ref 12. a

26.1 ( 2.2 24.2 ( 2.0

38.9 ( 2.2 110.3 ( 5.4

108.8 ( 4.1 94.2 ( 12.7 3.78 ( 1.12 2.19 ( 0.64

10.2 ( 3.0 1.77

2.85 8.04 12.0 ( 2.70 5.04 ( 0.82 14.6 ( 2.30 9.61 ( 0.72

1.366 ( 1.605 3.27 ( 0.91 7.57 ( 1.54 0.84 ( 2.09

0.153 ( 0.055 1.15 ( 0.22 0.028 ( 0.009 4.88 ( 0.95

4.6 ( 3.0

3.5 ( 2.3 103.3 ( 5.6 1.94 ( 0.57 4.71 8.46 ( 0.73 11.7 ( 2.4 0.043 ( 0.013 4.68 ( 0.39

6.4 ( 2.4 99.2 ( 10.5

102.3 ( 2.1 3.31 ( 0.96 3.31

2.61 ( 0.77 4.39 7.41 ( 0.75

4.48 ( 2.89 5.96 ( 0.78

10.0 ( 2.6 0.063 ( 0.020 4.18 ( 1.16

0.109 ( 0.037 4.52 ( 0.52

15.3 ( 2.1

2.7 ( 2.3 101.6 ( 1.4

105.9 ( 10.7 2.91 ( 0.85

3.41 ( 1.01 2.14

3.34 7.82 ( 2.73 6.31 ( 0.77

4.35 ( 3.18 4.51 ( 0.85 0.186 ( 0.069 3.12 ( 1.00

0.096 ( 0.032 3.77 ( 0.79

26.1 ( 2.2 22.9 ( 2.3 108.3 ( 4.3 107.7 ( 2.1 4.69 ( 1.39 3.96 ( 1.17 1.26 1.59 1.41 ( 1.21 2.97 ( 0.97 3.49 ( 5.41 0.35 ( 4.04

(J K (kJ mol )

(50 °C) cm ) (cP) (kcal mol )

0.591 ( 0.337 2.54 ( 0.37 0.331 ( 0.144 3.09 ( 0.30

J K1 mol1) kJ mol1) 104 s1) 103 s1) kJ mol1) mol )

ΔrGg (50 °C; kobsdh (50 °C; kSPfMCi (50 °C; ΔH‡j (298 K; ΔS‡k (298 K;

1

ΔrSf

1 1

ΔrHf Keqe

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1

ETd ηc

1 1

εMCb

4

λMCa

Table 1. Equilibrium and Kinetic Absorption Data for Thermal SP f MC Isomerization of 1 in Different Solvents


(50 μM) measured in a glycerol/MeOH (5/5 v/v) mixture at 50 °C in the dark. At time zero, the solution shows almost no absorption in the visible region (>450 nm), indicating that 1 exists as a SP form. As the time advances, a distinctive absorption band assigned to the MC form appears at 440640 nm with a maximum absorption at 522 nm. A clear isosbestic point at 298 nm indicates that the transformation of SP form solely produces the MC form without decomposition. Figure 2b shows the spectra of 1 measured in pure MeOH at 50 °C. Only a weak MC absorption appears even after stirring for 80 min. It is noted that no MC absorption appears in other common organic solvents such as MeCN, CHCl3, benzene, and THF. These indicate that thermal SP f MC isomerization is enhanced by the addition of glycerol. Figure 3 shows the time-dependent change in MC absorbance of 1 in different solvents at 50 °C. The MC absorption scarcely increases in MeOH (j), but significantly increases in glycerol/MeOH mixtures (a, c) and an ethylene glycol/MeOH mixture (f). These imply that the addition of viscous solvents promotes thermal SP f MC isomerization. 3.2. Effect of Hydrogen Bonding Interaction. The hydrogen bonding interaction between the MC form and solvent molecules strongly affects the thermal SP f MC isomerization.7,8 Figure 3h,i show the change in MC absorbance of 1 in water/ MeOH (5/5, 2/8 v/v) mixtures at 50 °C. The MC absorbance increases with time, indicating that thermal isomerization is promoted in aqueous media.7 The MC form, stabilized by a hydrogen bonding interaction, shows a blue shift of absorption spectrum;19 a shorter wavelength absorption represents the lower ground state energy of MC form. Table 1 summarizes the maximum wavelengths of MC band (λMC) of 1 in different solvents. λMC in pure MeOH is 527 nm (j) and decreases with an increase in water content (h, i), indicating that the MC form is indeed stabilized by a hydrogen bonding interaction. The λMC values in glycerol/MeOH (8/2, 5/5 v/v) mixtures are 520 nm (a) and 522 nm (c), respectively, which are longer than that obtained in a water/MeOH (2/8 v/v) mixture (518 nm, i). This suggests that the MC forms in these viscous solutions are less stabilized than that in the aqueous solution. However, as shown in Figure 3a,c,i, the increase in MC absorbance in the glycerol/ MeOH mixtures is much larger. This indicates that the SP f MC isomerization is enhanced in glycerol solution even though the MC form is less stabilized. 3.3. Equilibrium Analysis. Equilibrium absorption analysis for SP f MC isomerization of 1 was carried out to clarify the thermodynamic equilibrium constants between the SP and MC forms (Keq). The solutions containing different concentrations of 1 were stirred at different temperatures in the dark for 4 h,7a where the absorption spectra for all solutions attained the equilibria. Table 1 summarizes the Keq values at 50 °C in respective solvents, and the standard enthalpy (ΔrH) and entropy (ΔrS) determined by the van’t Hoff plots9 of the equilibrium data (Figure S14, Supporting Information). The Keq values of 1 in water/MeOH mixtures (h, i) are much higher than that in MeOH (j), indicating that SP f MC isomerization is enhanced by water addition.7 The ΔrH values in water/MeOH mixtures are much lower than that in MeOH, suggesting that water indeed lowers the ground state energy of MC form by a hydrogen bonding interaction (Figure 1b).7a It must be noted that the increase in water content also leads to ΔrS increase. These suggest that, as expressed by eq 1, the isomerization enhancement in aqueous media is contributed by both enthalpy (ΔrH decrease) and entropy (ΔrS increase) effects. 9085



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Figure 4. (Top) ΔrG and the enthalpy (ΔrH) and entropy terms (TΔrS) during thermal SP f MC isomerization of 1 at 50 °C in (a) water/MeOH, (b) glycerol/MeOH, and (c) ethylene glycol/MeOH mixtures with different compositions. (Bottom) Percent contribution of the enthalpy and entropy terms to ΔrG decrease, determined using eqs 2 and 3.

Figure 4a (top) shows the change in ΔrG and the enthalpy (ΔrH) and entropy terms (TΔrS) in eq 1, with an addition of water to MeOH. ΔrG and both terms decrease with an increase in water content. To clarify the contribution of the respective terms to ΔrG decrease, the percent contribution of respective terms were defined as follows: %contributionenthalpy ¼

Δr HMeOH Δr H 100 Δr GMeOH Δr G

ð2Þ

%contributionentropy ¼

TðΔr SMeOH Δr SÞ 100 Δr GMeOH Δr G

ð3Þ

where ΔrHMeOH, ΔrSMeOH, and ΔrGMeOH are the thermodynamic parameters obtained in pure MeOH (Table 1,j). Figure 4a (bottom) summarizes the contribution of the enthalpy and entropy terms to a ΔrG decrease in the water/MeOH mixtures. The contribution of the enthalpy term is >70% and is much larger than that of the entropy term. This suggests that the SP f MC isomerization in aqueous media is an enthalpy-driven process, promoted by the stabilization of MC form (ΔrH decrease) via a hydrogen bonding interaction. As shown in Table 1,ag, the addition of glycerol or ethylene glycol to MeOH also promotes a Keq increase. Figure 4b,c (top) shows the change in ΔrG and the enthalpy and entropy terms in glycerol/MeOH and ethylene glycol/MeOH mixtures. In both mixtures, ΔrG decreases with an increase in the amount of glycerol or ethylene glycol along with a decrease in both terms, as is the case for aqueous media (Figure 4a). The decrease in the entropy term is, however, much larger than the enthalpy term. As shown in Figure 4b,c (bottom), the contribution of the entropy term to the ΔrG decrease is about 70%, and that of the enthalpy term is much lower. This clearly suggests that the SP f MC isomerization in these viscous media is an entropy-driven process. The change in entropy (ΔrS) during the SP f MC isomerization process can be expressed as the sum of two contributions.20 Δr S ¼ Δr Sintr þ Δr Ssolv

ð4Þ

Scheme 2. Schematic Representation of Solvent Ordering During SP f MC Isomerizationa

a

The arrows denote the dipole moment of solvent molecules.

where ΔrSintr is the intrinsic entropy change for the spiropyran molecule during isomerization, and ΔrSsolv is due to the rearrangement of solvent molecules associated with the isomerization. The MC form of spiropyran derivatives has a larger rotational mobility than the rigid SP form, thus, showing a positive ΔrSintr.10 The MC form has a larger dipole moment than the SP form.11 The formation of the MC form therefore promotes an ordering of solvent molecules around the MC form, as schematically shown in Scheme 2a, and results in a negative ΔrSsolv.12 This solvent ordering effect offsets the ΔrSintr gain and promotes a net decrease in ΔrS in common solvents.13 In contrast, in viscous media, the solvent molecules are associated strongly with each other (solventsolvent interaction),14 as shown in Scheme 2b. This interaction probably suppresses the 9086



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Scheme 3. Thermal Activation Process for SP f MC Isomerization

Figure 5. Relationship between viscosity and ΔrS for SP f MC isomerization of 1 measured in different solutions (a-l). The solvents, a-l, correspond to those listed in Table 1.

solvent ordering around the MC form. This thus suppresses a ΔrSsolv decrease and promotes a net increase in ΔrS. Figure 5 shows the relationship between the viscosity and ΔrS for SP f MC isomerization of 1 measured in different solvents. ΔrS increases with an increase in viscosity regardless of the media. This indicates that solvent viscosity is the crucial factor for ΔrS increase and promotes entropy-driven isomerization. Thermal SP f MC isomerization is also enhanced in other viscous solvents. As shown in Table 1,k, SP f MC isomerization is enhanced in a 1,4-butanediol/MeOH (8/2 v/v) mixture. Both ΔrH and ΔrS values obtained in the mixture are higher than those in pure MeOH. The ΔrH increase is because the hydrogen bonding interaction between the MC form and solvent molecules is weaker due to the lower hydrogen bonding acidity of 1,4-butanediol (KamletTaft parameter, R = 0.63) than MeOH (R = 0.98)21 and the MC form is less stabilized. The results indicate that entropy-driven isomerization also occurs in the solution and the hydrogen bonding interaction scarcely affects isomerization. It is well-known that an ionic liquid shows a high viscosity due to the coulomb interaction,22 although the viscosity of glycerol, ethylene glycol, and 1,4-butanediol arises from the hydrogen bonding interaction. As shown in Table 1,l, a mixture of an ionic liquid (1-butyl-3-methylimidazolium tetrafluoroborate: [BMIM][BF4]) and MeOH (8/2 v/v) also enhances isomerization. In that, both ΔrH and ΔrS are higher than those obtained in MeOH. Some literatures report that the MC form is stabilized in ionic liquids via the interaction between the phenolate oxygen of MC form and the cationic part of ionic liquids.22 The ΔrH increase indicates that the MC form is less stabilized. This is because the MCcation interaction is weaker than the MCMeOH hydrogen bonding interaction due to the lower hydrogen bonding acidity of [BMIM][BF4] (R = 0.52) than MeOH (R = 0.98).21 These indicate that the SP f MC isomerization in the ionic liquid solution is also the entropydriven process. As shown in Figure 5, the data for ΔrS and the solvent viscosity during SP f MC isomerization in the 1,4butanediol (k) and ionic liquid (l) solutions have a relationship similar to those obtained in water, glycerol, and ethylene glycol solutions. This suggests that solvent viscosity is the crucial factor for ΔrS increase and promotes entropy-driven isomerization. 3.4. Kinetic Analysis. Solvent viscosity scarcely affects the thermal activation process for SP f MC isomerization. This is confirmed by the activation enthalpy (ΔH‡) and activation entropy (ΔS‡) for isomerization determined by the kinetic absorption analysis. The apparent rate constant, kobsd, for isomerization is expressed by the sum of the forward, kSPfMC,

and backward rate constant, kMCfSP.23 kSP f MC

SP s rf MC

ð5Þ

kMC f SP

kobsd ¼ kSP f MC þ kMC f SP

ð6Þ

The equilibrium constant, Keq, is expressed as follows: Keq ¼

kSP f MC kMC f SP

ð7Þ

The forward rate constant, kSPfMC, is therefore expressed as follows: kSP f MC ¼

Keq kobsd 1 þ Keq

ð8Þ

The time-dependent change in absorption spectra of 1 in respective solvents was measured in the dark at different temperatures. Table 1 summarizes the kSPfMC values obtained at 50 °C, and the ΔH‡ and ΔS‡ values determined by the Arrhenius plots24 of the kSPfMC data (Figure S15, Supporting Information). The kSPfMC values of 1 in glycerol/MeOH mixtures (ae) are larger than that in MeOH (j), indicating that the isomerization is accelerated by the addition of glycerol. The ΔH‡ values of 1 in glycerol/MeOH mixtures (ae) are larger than that in MeOH (j). It is well-known that the SP f MC isomerization of spiropyran derivatives in more polar media show larger ΔH‡.12 The isomerization proceeds via two-step reactions, as shown in Scheme 3;25 the first is the cleavage of CO bond of the SP form and the second is the rotation of the species (cistrans isomerization), where the rotation is the ratedetermining step.26 The MC form is a resonance hybrid of zwitterionic and quinoidal forms (Scheme 3), and the zwitterionic form is dominant in more polar media.27 The zwitterionic form has a larger energy barrier for rotation due to the essential double bond nature of the central bond.28 This thus results in larger ΔH‡ in more polar media. Figure 6A shows the relationship between ΔH‡ and the solvent polarity parameter (ET).29 ΔH‡ indeed increases with an increase in polarity regardless of 9087



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Figure 6. Plots of (A) ΔH‡ vs ET, (B) ΔH‡ vs η, (C) ΔS‡ vs ET, and (D) ΔS‡ vs η for SP f MC isomerization of 1 in different solvents (aj) at 25 °C. The solvents, aj, correspond to that in Table 1. The detailed kinetic data and solvent properties are summarized in Table 1.

Figure 7. Relationship between ΔH‡ and ΔS‡ for thermal SP f MC isomerization of 1 in different solvents (aj). The solvents, aj, correspond to those in Table 1.

the media, indicating that ΔH‡ strongly depends on the solvent polarity. As shown in Figure 6B, the plots of ΔH‡ versus the solvent viscosity do not provide a clear relationship. These indicate that the viscosity scarcely affects the rotational motion of transition states, leaving the solvent polarity as the dominant factor for ΔH‡. As reported,12 ΔS‡ for thermal SP f MC isomerization also increases with an increase in polarity of the media. As shown in Figure 6C, the ΔS‡ values in respective solvents increase with an increase in polarity, as does ΔH‡ (Figure 6A). In contrast, there is no clear relationship between ΔS‡ and the solvent viscosity (Figure 6D), implying that solvent polarity is also the dominant factor for ΔS‡. In polar media, as shown in Scheme 3, the CO cleaved cis-intermediate has a rigid zwitterionic character and, hence, has low intrinsic entropy.30 The rotation of the cisintermediate, therefore, promotes larger entropy increase and results in larger ΔS‡ in more polar media. Figure 7 shows the relationship between ΔH‡ and ΔS‡ obtained in different

Figure 8. (a) Time-dependent change in absorption spectra of 1 (50 μM) measured in a glycerol/MeOH (8/2 v/v) mixture under irradiation of 550 nm light (intensity, 0.16 W m2) at 50 °C, where the measurement was started after the solution attained the equilibrium by 60 min stirring in the dark. (b) Change in MC absorbance of 1 during the repeated stirring in the dark or visible light irradiation condition at 50 °C.

solvents. A strictly linear relationship indicates that both parameters are strongly affected by a single factor.31 These results 9088


The Journal of Physical Chemistry A clearly indicate the thermal activation process for isomerization is affected solely by the polarity of media, whereas the viscosity scarcely affect. The equilibrium and kinetic analysis revealed that the enhanced thermal isomerization of 1 in viscous media is due to the suppression of solvent ordering around the MC form (Scheme 2). This promotes ΔrS increase and allows entropydriven isomerization. It is noted that the MC form in viscous media is successfully reverted to the corresponding SP form by irradiation of visible light, as usually observed in common solvents.32 Figure 8a shows the time-dependent change in absorption spectra of 1 in a glycerol/MeOH (8/2 v/v) mixture under irradiation of 550 nm light at 50 °C, where the measurement was started after the solution attained absorption equilibrium by 60 min stirring in the dark condition. The MC absorbance decreases with the photoirradiation time, leaving the spectrum of SP form. This indicates that, even in viscous media, photoexcitation of the MC form promotes reversion to the SP form. Figure 8b shows the change in MC absorbance of 1, when the solution was stirred in the dark or under visible light irradiation sequentially at 50 °C. Visible light irradiation leads to a decrease in MC absorbance, but sequential stirring in the dark successfully regenerates the MC absorption. The change in MC absorbance is unchanged during the repeated three cycles, suggesting that thermal SP f MC isomerization and the photochemical MC f SP isomerization is repeatable without decomposition.

4. CONCLUSION Thermal SP f MC isomerization of a spiropyran derivative is enhanced with an increase in the concentrations of viscous solvent in MeOH. The isomerization in viscous media is an entropy-driven process, which is due to the strong solvent solvent interaction. This suppresses the ordering of solvent molecules around the MC form and promotes ΔrS increase, leading to ΔrG decrease. In contrast, the increased solvent viscosity scarcely affects the thermal activation process for isomerization, where the activation enthalpy and entropy parameters are solely controlled by the solvent polarity. The MC form formed in viscous media is successfully reverted to the SP form upon irradiation of visible light. The thermal isomerization property driven by viscosity of the media has a potential for creation of new optical materials, when combined with viscositysensitive systems such as polymers and gels. The work along these lines is currently in progress. ’ ASSOCIATED CONTENT

bS

Supporting Information. Additional data presented in Figures S1S15. This material is available free of charge via the Internet at http://pubs.acs.org.

’ AUTHOR INFORMATION Corresponding Author

*Tel.: +81-6-6850-6271. Fax: +81-6-6850-6273. E-mail: shiraish@ cheng.es.osaka-u.ac.jp.

’ ACKNOWLEDGMENT This work was supported by the Grant-in-Aid for Scientific Research (No. 23656503) from the Ministry of Education, Culture, Sports, Science and Technology, Japan (MEXT).

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S.S. thanks the Japan Society for the Promotion of Science (JSPS) Research Fellowships for Young Scientists and the Global COE Program “Global Education and Research Center for Bio-Environmental Chemistry” of Osaka University. We thank the Division of Chemical Engineering for the LendLease Laboratory System.

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