Interactions between Spiropyrans and Room-Temperature Ionic

May 29, 2008 - Interactions between Spiropyrans and Room-Temperature Ionic Liquids: Photochromism and Solvatochromism. Yusong Wu*, Takashi Sasaki, Kon...
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J. Phys. Chem. B 2008, 112, 7530–7536

Interactions between Spiropyrans and Room-Temperature Ionic Liquids: Photochromism and Solvatochromism Yusong Wu,*,† Takashi Sasaki,‡ Kono Kazushi,‡ Toshihiro Seo,‡ and Kensuke Sakurai†,‡ Venture Business Laboratory, and Department of Materials Science and Engineering, Graduate School of Engineering, UniVersity of Fukui, Bunkyo 3-9-1, Fukui, 910-8507, Japan ReceiVed: February 1, 2008; ReVised Manuscript ReceiVed: April 8, 2008

A series of imidazolium-based room-temperature ionic liquids (RTILs) containing anions from organic carboxylic acids were prepared. A set of dye probes, including Reichardt’s dye (30), 4-nitrioaniline, and N,N-diethyl-4-nitroaniline, were used to determine the ET(30) scales and the Kamlet-Taft parameters (π*, R, and β) of the RTILs. On the basis of the polarity properties, these RTILs were categorized into three groups: group A with β > 0.9, R < 0.9; group B with β < 0.9, R < 0.9; and group C with β < 0.9, R > 0.9. Interactions of these RTILs with four photochromic spiropyran derivatives (SP-I, SP-II, SP-III, and SP-IV) were investigated. It was found that the spiropyrans could present photochromism (positive or negative) or not, depending mainly on the polarity properties of the RTILs and also on the structure itself. A new spectroscopic method based on the molecular transition energy of the spiropyran probes (ESP) was proposed to determine the polarity of those protic or fluorine-containing RTILs, which were failed with the Reichardt’s dye (30) probe. SCHEME 1

Introduction Science and technologies in relation to ionic liquids, especially those room-temperature ionic liquids (RTILs), have achieved burgeoning progress in recent years, and it has been generally expected that RTILs may be used as substitution solvents for volatile organic solvents due to many desirable properties, novel electrolyte materials for energies devices, recyclable catalysts, etc.1–3 Study on the polarity of RTILs is a subject of great interest to many researchers.4–11 Recent studies have shown that the individual interactions between the RTILs and the solvates had direct effects on solvating abilities, reaction products, production ratios, kinetics, and enzyme reactivities.2,12–17 The methods often utilized to estimate the polarity of RTILs mainly include a spectroscopic method based on solvatochromic probes such as a single Reichardt’s betaine dye to determine the ET(30) scale18 and a set of dye probes to give the separate parameters (R, β, and π*) developed by Kamlet and Taft and co-workers,19 as well as a chromatographic method based on the multiple solvation model of Abraham et al.20 By means of the multiple parameters equations, it has also been shown that the imidazolium-based RTILs have a hydrogen-bond acidity (R) comparable to or lower than that of aniline, being determined mainly by the cation, and a hydrogen-bond basicity (β) ranging from acetonitrile to lower values governed by the nature of the counteranion. Besides, RTILs are characterized by very similar values of dipolarity/polarizability (π*) that are higher than those of most molecular solvents. The main purpose of the present research is to study the interactions of spiropyran compounds in RTILs. Spiropyrans belong to a series of photochromic compounds that can be potentially applied in information-recording systems, such as optical memory and switching devices,21 and have been the most * Corresponding author. E-mail: [email protected]. Fax and Phone: +81-776-27-8618. † Venture Business Laboratory. ‡ Department of Materials Science and Engineering.

extensively studied for several decades.22–26 Once exposed to UV light, the colorless spiro form (SP) converts to the ringopened, colored merocyanine form (MC) whose color (or maximum absorbance wavelength, λmax) is remarkably dependent on the polarity of the solvent, whereas the reverse process is induced by visible light or heat (see Scheme 1). Accordingly, studying such inherent interconversion phenomenon (i.e., SP h MC) in various solvents may give us important information about the solvent properties of the latter. RTILs are among the most complex solvents given their multifunctional groups and structural designability. Thus, they are capable of most types of intermolecular solute-solvent interactions, including nonspecific and specific. Therefore, it would be interesting to study the photochromic behavior of spiropyran compounds in RTILs, through which a better understanding of the properties of RTILs and a control of the photochromism of the spiropyrans can be expected. In this paper, photochromic behavior of four spiropyran compounds with different substituents (SP-I, SP-II, SP-III, and SP-IV, see Scheme 2) in a range of RTILs having different cation and anion structures (see Scheme 3) are reported. Considering the increasing importance of RTILs having nonhalogenated, carboxylate anions due to their different solubilizing and catalytic activity,2,3,27–30 we are especially interested in preparing and characterizing their properties. We also propose a new solvatochromic method to determine the polarity of RTILs based on spiropyran probes and compare the results obtained by using the Reichardt’s betaine (30) dye probe.

10.1021/jp800957c CCC: $40.75  2008 American Chemical Society Published on Web 05/29/2008

Interactions between Spiropyrans and RTILs

J. Phys. Chem. B, Vol. 112, No. 25, 2008 7531

SCHEME 2

RTILs) or 1.0 cm cell (for others). Photochromism studies were performed by irradiating the spiropyran solutions using a UV lamp (365 nm, 16 W). The distance between the quartz cell and the lamp was 10 cm, and the irradiation time was 3 min or more. The spectra were recorded immediately after irradiation. Results and Discussion

Experimental Section Chemicals and Reagents. Reichardt’s betaine dye (30) was purchased from Sigma-Aldrich. N,N-Diethyl-4-nitroaniline was received from Finton laboratory. 4-Nitrioaniline was from Wako and used without further purification. All the ionic liquids used in this study were prepared in our laboratory by referencing reported methods. The imidazolium carboxylate salts (1-11) were prepared following the same methodology as used to prepare a series of RTILs containing amino acid anions.30 1-Hydroxypropyl-3-methylimidazolium bis(trifluromethylsulfonyl)imide (12) and 1-hydroxyethyl-3-methylimidazolium bis(trifluromethylsulfonyl)imide (13) were prepared according to a method described in literature.31 The structures of all the prepared compounds were well-characterized by 1H NMR. Spiropyran compounds were synthesized according to a general method24 by reacting of 2,3,3-trimethylindolenine with 1-iodobutane, 1-iodooctane, β-iodopropionic acid, and γ-sultone, respectively, followed by condensation with 5-nitrosalicaldehyde in the presence of piperidine and were fully characterized. All the other organic compounds were purchased from Kanto Chem Co. Ltd. and were used as received. Analytical Methods. For the measurements of ET(30) scales and Kamlet-Taft parameters, the appropriate amount of the individual dye stock solution, that is, Reichardt’s dye 30 (A) with a concentration of 5.0 × 10-4 mol/L, N,N-diethyl-4nitroaniline (B) with a concentration of 1.0 × 10-3 mol/L, and 4-nitrioaniline (C) with a concentration of 1.0 × 10-3 mol/L in methanol, was added to the ionic liquid using a micropipette. The residual methanol was then carefully removed by vacuum drying at 40 °C for 48 h. The dye/ionic liquid solutions were put into quartz cells with 1.0 mm light-path length. The cells were sealed with a silicon cork and placed at the holder fixed to the light-path. The absorption spectra were recorded with a Hitachi U-3000 spectrometer at room temperature (25 °C). All the recorded absorbencies were less than 0.2. The ET(30) scales and the Kamlet-Taft parameters were then calculated using the following equations:19

ET(30) ) 28 591/λmax(A)

(1)

π/ ) 0.314(27.52 - νB)

(2) /

R ) 0.0649ET(30) - 2.03 - 0.72π

(3)

β ) (1.035νB + 2.64 - νC) ⁄ 2.80

(4)

where λmax(dye) is the wavelength corresponding to maximum absorption and νdye ) 1/λmax(dye) × 10-4 kK. For the absorption measurements of spiropyrans in the molecular solvents and the ionic liquids, weighed spiropyran dyes were added directly to the solvents so as to obtain dye concentrations of around 1.0 × 10-4 mol/L (for water, ethylene glycol, and RTILs 12 and 13) and 1.0 × 10-3 mol/L (for others). The solutions were stored in the dark overnight in order to achieve equilibriums between the uncolored spiropyran forms and the colored merocyanine forms. Absorption measurements were carried out using a 1.0 mm quartz cell (for water and

Preparation and Polarity Scales of RTILs. The imidazolium carboxylate salts were prepared by changing the halide anions (Cl- or Br-) of l-butyl-3-methylimidazolium chloride (BminCl) or 1-hydroxypropyl-3-methylimidazolium bromide (HOpminBr) to OH- through an anion-exchange resin, followed by neutralizing the obtained [Bmin][OH] and [HOpmin][OH] with different mono- and diprotic acids. Neutralizing the [Bmin][OH] with equimolar monoprotic acids and diprotic organic carboxylate acids resulted in a series of carboxylate salts 1-4 and a series of hydrogen carboxylate salts 8-10, respectively, whereas neutralizing the [Bmin][OH] with half-equimolar diprotic carboxylate acids led to a series of dicarboxylate salts 5-7 (see Table 1). The neutralization of an aqueous [HOpmin][OH] solution with equimolar acetic acid produced 11. All the compounds are transparent and colorless liquids at room temperature. The compounds 5-7 and 9 are apparently more viscous than others. The reason may be due to more compact charge distribution in the anions, which leads to stronger Coulomb interactions between the anions and the countercations (for 5-7), or a relatively larger anion (for 9). Except that 6 and 7 are immiscible with a low-polarity solvent of chloroform, these imidazolium carboxylates are miscible with water, acetonitrile, and chloroform. The ET(30) scales and the Kamlet-Taft parameters (R, β and π*) determined by using a set of dyes, including Reichardt’s dye (30), 4-nitrioaniline, and N,N-diethyl-4-nitroaniline, are listed in Table 1. The determinations of the ET(30) scales and R values of 8-10 and 12 by using the Reichardt’s dye as a solvatochromic probe failed because the betaine dye faded to colorless immediately after being added to these RTILs; thus, no visible absorption band could be detected in our experiments. Similar phenomena have also been observed when using the Reichardt’s dye (30) as a solvatochromic probe for determining the polarity scales of some RTILs containing dissociable protons or a [PF6]- anion.5,32 The reason has been ascribed to the protonation of the basic phenolic oxygen in Reichardt’s dye 30 under acidic conditions resulting from dissociation of active protons or potential formation of hydrogen halides via disproportionation of the anions. As a result, in the present study, the ET(30) scales and R values for 8-10 and 12 were indirectly determined by a spiropyran dye probe according to a standardized linear line obtained from 21 ionic and nonionic solvents (details will be discussed in the next section). For comparison, the reported33 polarity scales for those RTILs containing anions from strong acid (coded as 14-17) are also listed in Table 1. As can be seen from Table 1, all the RTILs tested in this study have similar π* (dipolar/polarizability) values. In addition, with the ET(30) scale, the carboxylate-based RTILs (1-11) are less different from the other ionic liquids containing anions originating from strong acids (14-17). The most remarkable differences between the carboxylate-based RTILs and those ionic liquids containing strong acid anions can be observed from their hydrogen-bond acidity (R) and hydrogen-bond basicity (β) values. In general, the carboxylate-based RTILs are characterized by distinctly higher β values but slightly lower R values, in comparison with the others. The carboxylate anions are the conjugate bases of weak acids. Thus, they are supposed be

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Wu et al.

SCHEME 3

TABLE 1: ET(30) Scales and Kamlet-Taft Parameters of RTILs Reichardt’s dye

Kamlet-Taft parameters

code

solvent

ET (30) (kcal mol-1)

π*

R

β

1 2 3 4 5 6 7 8 9 10 11 12 13 14c 15c 16c 17c

[Bmin][acetate] [Bmin][propionate] [Bmin][butyrate] [Bmin][glycolate] [Bmin]2[malate] [Bmin]2[succinate] [Bmin]2[maleate] [Bmin][H-malonate] [Bmin][H-succinate] [Bmin][H-maleate] [HOpmin][acetate] [HOpmin][N(Tf)2] [HOemin][N(Tf)2] [Bmin][BF4] [Bmin][PF6] [Bmin][TfO] [Bmin][N(Tf)2]

50.5 49.1 49.3 50.5 49.8 49.4 48.8 48.6a 48.2a 47.6a 51.1 56.8a 60.8 (52.4) (51.1) (51.1) (50.8)

1.04 0.94 0.92 1.12 1.10 1.09 1.11 1.03 1.07 1.08 1.08 1.06 1.08 (1.05) (1.03) (1.00) (0.98)

0.43 0.48 0.51 0.44 0.41 0.39 0.34 0.38b 0.36b 0.32b 0.51 0.91b 1.14 (0.63) (0.63) (0.62) (0.62)

1.05 1.16 1.23 0.87 1.00 1.08 1.02 0.71 0.82 0.62 0.99 0.24 0.28 (0.38) (0.21) (0.46) (0.24)

a Data of the ET(30) values were derived from the analogous ESP values, obtained by means of the solvatochromic visible absorption band of SP-I (see the text) using a linear relationship of ET(30) ) 3.424ESP - 128.421. b Indirect R values recalculated from the ET(30) and π* values according to eq 3. c Data from ref 7.

stronger hydrogen-bond acceptors than anions originating from strong acids and contributed to the large β values. The results also show that β values of the RTILs are dominated by the anion parts and are dependent on the structures of the carboxylates: it increases with the increasing of the length of the alkyl chains in the anions (as the cases of 1-3) as the results of the electron-donating effect of the fatty groups, whereas it decreases as a results of introducing an electronwithdrawing group, such as a hydroxyl group (as the cases of 4 and 5) or a carboxyl group (as the cases of 8-10).

It is rather interesting to notice that, when an RTIL is constructed by an carboxylate anion, introduction of a hydroxyl group to either the anion (as the cases of 4 and 5) or the cation (as the case of 11) does not lead to notable change of the R values, although a hydroxyl group can be considered as a strong hydrogen-bond donor. In contrast, when an RTIL is constructed by an anion originating from strong acids, introducing a hydroxyl group to the cation results in a distinct increase of the R value (as the cases of 12 and 13). The results suggest that one may not expect an ionic liquid with exceptional large R value and β value at the same time. In other words, in order to obtain an RTIL with exceedingly high hydrogen-bond acidity, one has to sacrifice the corresponding hydrogen-bond basicity by selecting anions originating from strong acids, for example, (Tf)2N-, BF4-, PF6-, and so on. On the basis of a plot of the two dominant polarity scales, hydrogen-bond acidity (R) and hydrogen-bond basicity (β), we can categorize the RTILs into three groups as shown in Figure 1. The RTILs belonging to group A (1-3, 5-7, and 11) are characterized by very similar high β values (β > 0.9) but relatively lower R values (R < 0.9). 4 and 8-10 are categorized as group B, which are characterized by medium or relatively low R and β values similar to those of halogen-containing RTILs and most molecular solvents. RTILs 12, 13, and a molecular solvent, water, are classified as group C because they have very similar high R value (R > 0.9) but very low β value (β < 0.9). We categorize the RTILs in this way because this classification is consistent with the interactions of the RTILs to the spiropyrans as will be discussed below. Photochromic Behavior of Spiropyrans in RTILs. Group A. All the four spiropyran compounds, regardless of their substituted groups, appeared as yellow or pale yellow when being added to the RTILs belonging to this group. Figure 2 depicts the UV spectra of SP-I in the seven RTILs belonging to group A. SP-I gave rise to a strong absorption band with λmax ) 420 to ∼450 nm corresponding to the observed colors of the solutions, as well as a weak absorption band at around

Interactions between Spiropyrans and RTILs

J. Phys. Chem. B, Vol. 112, No. 25, 2008 7533 SCHEME 4

Figure 1. Plot illustrating the grouping of the RTILs and water upon their hydrogen-bond basicity (β) and hydrogen-bond acidity (R) characteristics. The structures the RTILs are shown in Table 1.

Figure 2. Absorption spectra of SP-I in the RTILs with strong hydrogen-bond acceptability (group A).

λmax ) 350 to ∼360 due to uncolored forms of the spiropyrans. When the solutions were placed in the dark and irradiated with a UV lamp (λ ) 365 nm) for 3 min or more, the color, and thus the corresponding absorption position and the intensity, remained indeed unchanged and exhibited no thermal decay over a 2 day monitoring period. Consequently, the spiropyran compounds showed no evident photochromism in these RTILs. It has been known that spiropyrans can present as eight (four trans and four cis) conformers. The trans-merocyanine forms are considered to be more stable and are generally observed at λmax longer than 500 nm depending on the solvent and structure, whereas the cis-merocyanine forms, with λmax blue-shifted, are relatively unstable and could be observed when being stabilized by forming complexes with some two-valent metal ions.25 Therefore, our results suggest that the spiropyrans may not present as fully trans-MC forms, but most likely form cismerocyanine complexes stabilized by the RTILs belonging to group A. Since these RTILs are characterized by the largest hydrogen-bond accepting properties, the anions and cations may strongly interact through forming hydrogen bonds. In addition, the carboxylate anions have strong coordinating ability, and they

may interact with the positively charged N-indolino group of spiropyrans. As a result, the ionic liquids, with the anions and cations strongly hydrogen-bonded, play as bridges that bind the negatively charged phenolic oxygen and positively charged indolino groups of spiropyrans in cis-merocyanine forms. One of the possible structures is presented in Scheme 4. Group B. The spiropyrans gave rise to pale violet to violet (λmax ) 530 to ∼560 nm) colors when being dissolved in the RTILs belonging to this group. Figure 3 displays the UV absorption spectra of SP-I in 9 obtained before and after UV exposure. The absorption band centered at λmax ) 554.5 nm is assigned to the corresponding colored trans-merocyanine form. The intensity of this absorption became significantly increased after being irradiated by a UV light, indicating a switch of the colorless spiropyran form to the colored merocyanine form (SP h MC). When the solution was stored in the dark, a gradual decay of the absorption due to the colored MC form was observed, suggesting a conversion of the merocyanine form to the spiropyran form (MC f SP) (see Figure 4). Eventually, SP-I displayed normal positive photochromism in this solvent. The other three spiropyrans, that is, SP-II, SP-III, and SP-IV, showed similar positive photochromism in other RTILs of this group, but with their λmax of the merocyanine forms shifting (see Table 2). Group C. The four spiropyrans appeared as violet-red in the RTIL 12 and 13 with absorption being comparable to that in water but apparently larger than those in the RTILs belonging to group B. Interestingly, they displayed different photochromic behavior depending on the structures in this solvent with the largest ET(30) and R values. In RTIL 13, SP-I gave rise to an absorption band with λmax ) 524.0 nm due to formation of the colored MC form. When exposed to the UV light, this absorption

Figure 3. Absorption spectra of a solution of SP-I in [Bmin][Hsuccinate] before (a) and after (b) continuous UV irradiation (λ ) 365 nm, 16 W, 3 min), showing positive photochromism.

7534 J. Phys. Chem. B, Vol. 112, No. 25, 2008

Figure 4. Overlay spectra illustrating the thermal decay of a solution of SP-I in [Bmin][H-succinate] after exposure to UV irradiation (scanning at 4 min intervals).

band suffered a marked decrease indicating a conversion of MC f SP (Figure 5). The absorption intensity of this band was found to restore in the dark gradually (see Figure 6). Thus, SP-I shows negative photochromism in this solvent. Similar negative photochromic behavior was also observed for the SP-II in 13. In contrast, for SP-III and SP-IV, a positive photochromism similar to those of found in the RTILs belonging to group B was observed in both cases. Solvatochromic Properties of Spiropyrans. The results (see Table 2) also show that there are strong relationships between the λmax of the merocyanines derived from the spiropyran compounds and the polarity properties of the RTILs. Thus, we consider that the interaction of spiropyrans and RTILs can be used to give rise to a polarity scale for the latter. Similar to the expression for the empirical ET(30) scale,18 we define a new polarity scale, ESP, based on the molecular transition energy of a spiropyran probe:

ESP (kcal · mol-1) ) hcνmaxNA ) 28 591/λmax (nm) (5) The ESP and ET(30) scales of the RTILs determined in this study are summarized in Table 2. For comparison, estimated data for 20 molecular solvents are also given in the table. It is suggested from the results shown in Table 2 that all the four spiropyran compounds display similar negative solvatochromism as the Reichardt’s dye probe in ether the molecular or the RTIL solvents, that is, as the solvent polarity increased, hypsochromic shifts of the corresponding trans-merocyanine forms are observed. A plot of the ET(30) scales versus the ESP scales obtained in 20 molecular solvents and two RTILs determined in this study (for data, see Table 2) is shown in Figure 7. It can be seen clearly that by using SP-I, SP-II, and SP-III as solvatochromic probes, the ET(30) scales and the ESP scales follow a similar trend in the full polarity range and have good linearity. However, with the SP-IV probe, the distinct hypsochromic shift is observed with increasing solvent polarity only in the highly polar region (ET(30) > 43.8), whereas no hypsochromic shift is observed in the weakly polar region (ET(30) < 43.8), and the determined ESP scales vary in a lesser range around a value of 52.0. As a result, the SP-IV seems to be insensitive to the solvent polarity for those weakly polar solvents. Evidently, the highly ionized and polar groups (SO3-) appended on the SP-IV may

Wu et al. greatly increase the polarity of the microenvironment of zwitterionic forms of the merocyanine (see Scheme 1). In the solvents with lower polarity, the microenvironments of the merocyanines are assumed to be dominated by the polar ionic SO3- side group on the structure of SP-IV. It can also be seen from Table 2 that the determined ESP scales (or λmax due to MC forms) for SP-III and SP-IV were generally found to red-shift to longer wavelengths as compared with those of SP-I and SP-II when being dissolved in the same ionic liquid solvents. In contrast to SP-I and SP-II which contain less polar alkyl groups, SP-III and SP-IV are appended by polar groups (-COOH and -SO3H). In RTILs, the electrostatic interactions between the zwitterionic merocyanines and the polar ionic solvents become the dominant ones in controlling the microenvironments of the merocyanine structures. Such interactions may be partially counteracted by the interactions between the polar SO3H and COOH side groups and the surrounding ionic liquid solvent molecules. This effect may result in less polar microenvironments for the zwitterionic forms of the merocyanines, which finally lead to small shifts of the absorption bands to longer wavelengths. The above results suggest that it may be reliable to use the spiropyran compounds as solvatochromic probes to determine the polarity (with the corresponding ESP scales) of RTILs, although small differences may be induced depending on the structures of the probes. It must be pointed out that although equilibriums existed between the spiro forms and merocyanine forms, the colored merocyanine forms were generally found to be present in the RTILs in large enough quantity without any UV irradiation, which rendered the measurements quite convenient indeed. In addition, the spiropyran compounds are obviously advantageous over the Reichardt’s betaine dye 30 in determining the polarity of the ionic liquids containing dissociable protons and those fluorine-containing RTILs. Therefore, the solvatochromic spiropyran dyes may be used as probes to deduce the corresponding ET(30) scale on the basis of the linear correlation between the ESP and the ET(30) scales and eventually estimate the polarity of those protic or fluorine-containing RTILs, which are often reported to be failed with the widely used Reichardt’s betaine dye probe.5,32 However, the spiropyran compounds can not be used to determine the polarity of the ionic liquids with strong hydrogen-bond acceptability (with β > 0.9), such as the RTILs belonging to group A in the present study, due to forming complexes with the solvents and showing no solvato- and photochromism. Finally, it is interesting to notice that there exists a strong relationship between the photochromic behavior of the spiropyran compounds and the corresponding polarity properties of the environments, that is, whether a spiropyran displays positive or negative photochromism depends on the ESP scales of the solvents (Table 2). In general terms, the spiropyrans show positive photochromism with the ESP scales less than about 53.9, whereas they show negative ones with the ESP scales larger than 53.9. Conclusions RTILs from simple but readily obtainable organic carboxylic acids are realized to become a new and important class of RTILs. We prepared a family of RTILs (1-11) containing carboxylate anions with various structures. A set of dye molecules were used to determine the ET(30) scales and the Kamlet-Taft parameters (π*, R, and β) of the carboxylate-based RTILs and two RTIL with halogen-containing anions (12 and 13). On the basis of the polarity properties, these RTILs were categorized into three groups (A, B, and C).

Interactions between Spiropyrans and RTILs

J. Phys. Chem. B, Vol. 112, No. 25, 2008 7535

TABLE 2: ET(30) and Esp Scales of the RTILs (Groups B and C) and Some Molecular Solvents, as Well as the Photochromism of Spiropyrans ESP (kcal mol-1) (λmax (nm)) solventa 10 9 8 4 12 13 water ethylene glycol methanol ethanol benzyl alcohol n-butyl alcohol isopropyl alcohol acetonitrile dimethyl sulfoxide N,N-dimethylformamide acetone methylene chloride pyridine chloroform tetrahydrofuran 1,4-dioxane diethyl ether benzene toluene n-hexane

ET(30) (kcal mol-1) c

47.6 48.2c 48.6c 50.5d 56.8c 60.8d 63.1e 56.3e 55.5e 51.9e 50.8e 50.2e 48.6e 46.0e 45.0e 43.8e 42.2e 41.1e 40.2e 39.1e 37.4e 36.0e 34.6e 34.5e 33.9e 30.9e

I 51.4 51.6 51.7 51.7 54.1 54.6 n.d.f 54.4 53.8 52.6 52.2 52.2 52.0 51.0 50.9 50.5 50.2 49.5 49.4 49.2 48.7 48.1 47.4 47.1 47.1 46.3

(556.0) (554.5) (552.5) (553.0) (528.5) (524.0) (526.0) (531.0) (543.5) (547.5) (547.5) (550.0) (560.5) (562.0) (566.0) (569.0) (578.0) (579.0) (580.5) (587.0) (594.0) (602.5) (607.5) (606.5) (617.5)

II 51.4 51.6 51.8 51.6 53.9 54.4 n.d. 54.3 53.9 52.6 52.2 52.1 52.0 51.1 50.8 50.5 50.2 49.4 49.3 49.1 48.5 47.9 47.5 47.1 47.1 46.2

photochromismb

III

(556.5) (554.5) (552.0) (554.0) (530.0) (526.0) (526.5) (530.5) (543.5) (548.0) (548.5) (550.0) (559.5) (563.0) (566.0) (569.5) (579.0) (579.5) (582.0) (589.0) (597.0) (602.0) (607.0) (607.5) (619.5)

51.1 51.4 51.6 51.4 53.7 53.5 n.d. 54.0 53.6 52.3 51.8 52.0 51.6 50.7 50.7 50.1 49.7 52.5 49.0 49.5 48.5 47.4 47.4 n.d. n.d. n.d.

(559.5) (555.5) (554.0) (556.5) (532.5) (534.0) (529.5) (534.0) (547.0) (552.0) (550.0) (554.5) (563.5) (564.0) (571.0) (575.0) (544.5) (583.0) (578.0) (589.5) (602.5) (602.5)

IV 51.3 51.5 51.6 51.4 53.0 53.8 56.5 53.9 53.5 52.4 51.8 51.9 51.8 51.8 50.8 50.6 53.1 52.5 50.6 51.7 52.4 52.2 n.d. n.d. n.d. n.d.

(557.0) (555.0) (554.5) (556.0) (539.0) (531.5) (506.0) (530.5) (534.5) (546.0) (552.5) (551.0) (552.0) (552.0) (563.0) (565.0) (538.5) (545.0) (565.5) (553.0) (545.5) (548.0)

I

II

III

IV

P P P P N N n.d. N P P P P P P P P P P P P P P P P P P

P P P P N N n.d. N P P P P P P P P P P P P P P P P P P

P P P P P P n.d. N P P P P P P P P P P P P P P P n.d. n.d. n.d.

P P P P P P N P P P P P P P P P P P P P P P n.d. n.d. n.d. n.d.

a Arranged according to the ET(30) values for the ionic and nonionic solvents, respectively. b P, positive photochromism; N, negative photochromism. c Data of the ET(30) values of were derived from the analogous ESP values, obtained by means of the solvatochromic visible absorption band of SP-I. d Determined ET(30) values using the solvatochromic Reichardt’s dye 30 probe. e Data from ref 33. f n.d.: no data can be given due to the insoluble of the spiropyran compounds in the solvents.

Figure 5. Absorption spectra of a solution of SP-I in [HOemin][Tf2N] before (a) and after (b) continuous UV irradiation (λ ) 365 nm, 16 W, 3 min), showing negative photochromism.

Figure 6. Overlay spectra illustrating the restoration of the absorption band due to the MC form observed for SP-I in [HOemin][Tf2N] after exposure to UV irradiation (scanning at 20 min intervals).

These RTILs interact with the four photochromic spiropyran derivatives (SP-I, SP-II, SP-III, and SP-IV) prepared in the present work quite differently. In general terms, the spiropyrans, regardless of the structures of the substituted groups, formed complexes with the RTILs characterized by strong hydrogenbond acceptable properties (group A) and did not display apparent photochromism. In contrast, the spiropyrans showed distinct positive photochromism irrespective of the structures in the RTILs with relatively low or medium R and β values (group B). In the RTIL with strong hydrogen-bond acidity

(group C), the spiropyrans may present positive or negative photochromism depending on the structures of the substituted side groups. Our results suggest that the spiropyrans may be useful solvatochromic probes for determining the polarity of RTILs, and it excels in determining the polarity of those RTILs containing associable protons and some fluorine-containing RTILs, which have been incapable with the presently accepted Reichardt’s betaine (30) dye. However, the spiropyran probes are invalid in determining the polarity scales of the RTILs with strong hydrogen-bond basicity.

7536 J. Phys. Chem. B, Vol. 112, No. 25, 2008

Figure 7. Plot of the ESP scales vs the ET(30) scales of 20 molecular solvents and 2 ionic liquid solvents (for data, see Table 2). The ESP scales were determined using the spiropyrans prepared in this work as probes, and the ET(30) scales are reported in ref 33. The results of multiple correlation analysis for the four spiropyran dyes are the following: SP-I, ET(30) ) 3.424ESP - 128.421 (r ) 0.99200, s ) 0.3179, n ) 21); SP-II, ET(30) ) 3.423ESP - 128.193 (r ) 0.99120, s ) 0.3337, n ) 21); SP-III, ET(30) ) 4.077ESP - 161.311 (r ) 0.90920, s ) 0.8641, n ) 18); SP-IV, ET(30) ) 3.901ESP - 153.012 (r ) 0.93515, s ) 0.6156, n ) 12, for ET(30) > 43.8).

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