Photochromism of a Spiropyran and a Diarylethene in Bile Salt

Article pubs.acs.org/Langmuir

Photochromism of a Spiropyran and a Diarylethene in Bile Salt Aggregates in Aqueous Solution Cerize S. Santos,† Allyson C. Miller,† Tamara C. S. Pace,† Kentaro Morimitsu,*,‡ and Cornelia Bohne*,† †

Department of Chemistry, University of Victoria, P.O. Box 3065, Victoria, BC V8W 3V6, Canada Xerox Research Centre of Canada, 2660 Speakman Drive, Mississauga, ON L5K 2L1, Canada

‡

S Supporting Information *

ABSTRACT: Bile salt aggregates incorporate aqueous-insoluble photochromic compounds. The photochromism of a spiropyran (1, 1′,3′,3′-trimethyl-6-nitrospiro[2H-1]-benzopyran-2,2′-indoline) and a diarylethene derivative (2, 1,2-bis(2,4dimethyl-5-phenyl-3-thienyl)-3,3,4,4,5,5-hexafluoro-1-cyclopentene) was quantified in different bile salt aggregates. These aggregates act as efficient hosts to solubilize aqueous insoluble photochromic compounds where either both isomers are nonpolar, for example, 2, or compounds where one isomer is hydrophobic and the other is more polar, for example, 1. Methodology was developed to determine molar absorptivity coefficients for solutions containing both isomers and to determine the photoconversion quantum yields under continuous irradiation. The methods were validated by determining parameters in homogeneous solution, which were the same as previously reported. In the case of the colored isomer of 1, the molar extinction coefficient in ethanol at 537 nm ((3.68 ± 0.03) × 104 cm−1 M−1) was determined with higher precision. The quantum yields for the photoconversion between the isomers of 2 were shown to be the same in cyclohexane and in the aggregates of sodium cholate (NaCh), deoxycholate (NaDC), and taurocholate (NaTC), showing that bile salt aggregates are not sufficiently rigid to affect the equilibrium between the two possible conformers of the colorless form. In contrast, for 1 the quantum yields for the conversion from the colorless to the colored isomer were higher in bile salts than in ethanol, and the quantum yield was highest in the more hydrophobic aggregates of NaDC, followed by NaCh and then NaTC. The structure of the bile salt had no effect on the quantum yield for the conversion of the colored to the colorless isomer of 1, but these values were higher than in ethanol. For all three bile salts, the absorption maximum for the colored form of 1 suggested that this isomer was located in an environment that is more polar than ethanol.

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INTRODUCTION

colorless open (2a) and colored closed (2b) forms is only induced photochemically.1,3 The spiropyran and diarylethene backbones are not intrinsically soluble in aqueous solution. Limited soluble spiropyrans have been synthesized either through peptide9 or aminoalkyl10 substitution on the indoline nitrogen or through replacement of the nitro group with a carboxylate9,11 or a pyridinium moiety.12 Aqueous solubility for diarylethene derivatives has been achieved through introduction of cationic ammonium,13 pyridinium groups,14 or bulky water-soluble moieties.15−17 A universal approach that easily allows for solubilization of a broad selection of photochromic molecules, without the need for synthetic modification, is the use of water-soluble supramolecular host systems to bind hydrophobic photochromic guests, affording host−guest complexes in aqueous solution. Most work in this area has focused on inclusion complexes where the photochromic molecule, or an appendant moiety, is included in a cyclodextrin,13,18−22 cavitand,23 or

The changes in the optical and other physicochemical properties of photochromic molecules induced by the reversible switching between isomeric forms upon irradiation have been employed in a number of applications, including sensing, imaging, data and signal processing, photocontrol of biological systems, modulation of energy and electron transfer, and incorporation into organic electronic materials.1−8 Development of many applications, especially those involving biological systems, requires use of aqueous media, and strategies to readily solubilize photochromic molecules in water are therefore required. Spiropyrans and diarylethenes are two commonly studied families of photochromic molecules. Generally, a spiropyran (1, 1′,3′,3′-trimethyl-6-nitrospiro[2H-1]-benzopyran-2,2′-indoline, Scheme 1) can be reversibly switched between the colorless spiro form (1a) and the colored merocyanine form (1b) either thermally or photochemically.2 In contrast, a typical diarylethene derivative (2, 1,2-bis(2,4-dimethyl-5-phenyl-3-thienyl)3,3,4,4,5,5-hexafluoro-1-cyclopentene, Scheme 1) exhibits no thermal isomerization, and reversible switching between the © 2014 American Chemical Society

Received: August 7, 2014 Revised: September 4, 2014 Published: September 9, 2014 11319

dx.doi.org/10.1021/la503164e | Langmuir 2014, 30, 11319−11328

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cosolvent.38 Thus, bile salt aggregates are good candidates for the solubilization of photochromic molecules with varying polarities, and the inherent adaptability of these host systems may be ideal for controlling properties of the photochromic system, such as position of the photostationary state, or rates of thermal or photoconversion reactions between the isomers. We have previously shown in a preliminary communication that NaCh is an excellent host system for the solubilization of both 1 and 2 and that the efficiencies of photochromic conversion were only slightly decreased in comparison to those in ethanol.39 In addition, the rate constant for the thermal reequilibration of 1 was affected by increasing the concentration of NaCl, in line with expected changes in the polarity of the guest’s binding environment. In this work, the photochromism of 1 and 2 in bile salt aggregates was comprehensively quantified, and the effect of bile salt structure on the photochromism observed in 1 and 2 was established.

Scheme 1. Photochromic Reactions of 1 and 2 and Structures of NaCh, NaDC, and NaTC

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EXPERIMENTAL METHODS

The purity of the materials and their sources, the equipment used, sample preparation methods, and the continuous irradiation setup (Supporting Information Figure S1) are described in the Supporting Information. Specific methods are described below. The complexes between 1 or 2 and NaCh were stable for at least 24 h for concentrations of 1 below 120 μM and 2 below 300 μM, when monitored by UV−vis spectroscopy.39 The current studies were performed with a maximum concentration of 50 μM. HPLC Methods. For experiments with 1, solutions were kept in an ice bath for at least 15 min prior to irradiation, exposed to ambient light to obtain a solution containing only 1a, then irradiated at 338 nm for 2 min. Absorption spectra were obtained before and after irradiation, then the sample was transferred to an HPLC vial and kept in the dark in an ice bath, and an HPLC chromatogram was obtained as soon as possible. For the HPLC chromatogram of 1, the absorption was detected at 318 and 517 nm, and a mixture of methanol and water (80:20 v/v) was used as the eluent with a flow rate of 1.2 mL/min. Solutions of 2 were exposed to ambient light to obtain solutions containing only 2a, then irradiated at 285 nm (hexane) or 287 nm (bile salts) for 2 min. Absorption spectra were obtained before and after irradiation, then the sample was transferred to an HPLC vial and kept in the dark, and an HPLC chromatogram was obtained. For the HPLC chromatogram of 2, the absorption was detected at 269 and 562 nm, and a mixture of methanol and acetonitrile (70:30 v/v) was used as the eluent with a flow rate of 1.5 mL/min. Quantum Yield Determination. Samples of 1 or 2 were irradiated continuously at the isosbestic point (for 1, 298 nm in ethanol, 299 nm in NaCh and NaTC, and 302 nm in NaDC; for 2, 287 nm in cyclohexane and 292 nm in NaCh and NaDC). Absorption was collected simultaneously at several wavelengths, including the absorption maxima for 1b or 2b in the visible region. During irradiation the samples were stirred continuously, and the temperature of the samples was kept constant at 15 °C to minimize evaporation of organic solvent and minimize the thermal reaction for 1b. Experiments in NaDC were carried out at 25 °C to avoid gel formation by this bile salt. The obtained kinetic curves were numerically fit using the software Scientist 3.0 (Micromath). The potassium ferrioxalate actinometer was prepared40 and was used to determine the light flux incident on the sample. The variation in the light flux was typically smaller than 5% when measured before and after collecting a kinetic trace, and the average value was used for the fits. Thermal Relaxation Kinetics. Bile salt solutions containing 1 were irradiated at 338 nm for 10 min at the temperature of the subsequent kinetic experiment to obtain a solution containing a significant amount of 1b. Immediately following irradiation, the solutions were placed in the spectrophotometer, and the absorption at 517 nm was collected at 1 min intervals until the absorbance did not decrease significantly. Because of experimental limitations, no kinetics

curcubituril.24,25 As these hosts have a defined shape and size, the cavity typically incorporates a single guest molecule through specific interactions; however, these host−guest systems tend to have limited solubility in water. Conversely, the structure of a micelle is dynamic, and multiple guests of varying shapes and sizes can easily be incorporated nonspecifically in the hydrophobic interior of micelles. Solubilization of 126 and other spiropyran derivatives27 was achieved in cationic or nonionic micellar solutions. The small increases observed in both thermo- and photocolorability in the micelles followed the trend established in homogeneous solvents due to polarity changes in the vicinity of the photochromic molecule.27 Bile salt aggregates combine the solubilization properties of micelles with specific host−guest interactions. The hydrophobic convex surfaces of the bile salt molecules interact to form small primary aggregates, which are incorporated into larger secondary aggregates at higher bile salt concentrations. Therefore, bile salt aggregates continuously increase in size with the increase in the concentration of bile salt monomer or the increase in ionic strength.28−30 Primary aggregates provide a restricted environment for bound guests, and the properties of these guest−aggregate systems are somewhat dependent on the size and shape of the guest.31−33 Secondary aggregates provide a more micelle-like environment for bound guests, from which the guest binding dynamics is faster than when bound to primary aggregates.34 Bile salt aggregates can be viewed as adaptable hosts because the guest−aggregate structure depends on the structure of the guest and the concentrations of the bile salt and other salts. Therefore, the size of these structures is yet unknown. Modification to the structure of the bile salt (sodium cholate (NaCh), sodium deoxycholate (NaDC), or sodium taurocholate (NaTC), Scheme 1), either in the headgroup or in the number of hydroxyl groups on the concave face, causes changes in the structure of the guest−aggregate system.32,35 The properties of these systems can also be easily modulated by binding cations,36 changing the ionic strength,37 or adding a 11320


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were monitored for more than 30 h. The absorbance of a blank reference solution, containing all components except 1, was obtained concurrently in the same measurement. The data from the blank were subtracted from the experimental data to obtain the kinetic curves, which were fitted to a sum of exponential functions.

absorptivities for 2b were calculated by dividing the spectrum for 2b by the concentration of 2b (Figure 1, Table 1).

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RESULTS When using absorption techniques to monitor a photochromic reaction, knowledge of the absorption spectra and molar absorptivity coefficients (ε) for both isomers is essential. Solutions of 1 and 2 can be converted to 100% 1a and 2a by irradiation with visible light. The isomers 1b and 2b are similarly obtained by irradiation with UV light; however, a photostationary state is reached where the final concentrations of a and b are unknown. Methods to determine the values of ε for thermally stable isomers, such as 2b, have usually involved the isolation of the isomer by chromatographic separation, concentration of the solution, and measurement of the absorption spectra.41,42 These methods require appreciable amounts of isomer to be isolated and are not suitable for the determination of ε values for thermally unstable isomers, such as 1b, where after isolation the thermal back reaction will lead to formation of 1a. These methods are also not generally applicable to the determination of ε values in host−guest systems that exist in equilibrium in solution. Determination of molar absorptivities from absorption spectra of mixtures at the photostationary state is possible by measuring the spectra at different irradiation wavelengths.43 This method assumes that the quantum yields for photoconversion are the same at different irradiation wavelengths, and it could not be applied in the current study because for 2 and related derivatives the quantum yields show a wavelength dependence.44,45 For mixtures of both photochromic isomers, an overall absorption spectrum is obtained. If the spectrum of one isomer and the ratio of concentrations are known, then the spectrum of the other isomer can be calculated. We describe here an analytical HPLC method to readily carry out such a determination in various solvents, including host−guest solutions. For solutions containing a mixture of 2a and 2b in 80 mM NaCh/0.2 M NaCl, two peaks were observed in the HPLC chromatogram at a detection wavelength of 269 nm (Supporting Information Figure S2); the absorption spectrum of the peak at 4.3 min was consistent with the spectrum of 2a, whereas the absorption spectrum of the peak at 5.4 min showed absorption in the visible region consistent with the absorption of 2b. At a detection wavelength of 562 nm, only the 5.4 min peak was observed, consistent with the assignment of this peak to 2b. Other peaks observed at short retention times were also observed in the chromatogram of the blank and are due to impurities in the bile salt, which have been observed in previous photophysical studies.37 Solutions of known concentrations of 2a in 80 mM NaCh/ 0.2 M NaCl were used to construct an HPLC response curve (Supporting Information Figure S3). A sample of 2 was irradiated at 285 nm, and an absorption spectrum of the resulting mixture of 2a and 2b was measured. The HPLC chromatogram was obtained for this mixture, and the response curve was used to determine the concentration of 2a. The corresponding spectrum for 2a was calculated and was subtracted from the spectrum for the mixture leading to the spectrum of 2b. The concentration of 2b was known because the total concentration of 2 was known, and the molar

Figure 1. Molar absorptivities of 2a (dashed lines) and 2b (solid lines) in hexane (black), NaCh (80 mM, 0.2 M NaCl, blue), and NaDC (80 mM, 0.2 M NaCl, red).

Table 1. Molar Absorptivity (ε) of 1b and 2b at the Absorption Maximum in the Visible Regiona,b ε/104 cm−1 M−1 medium

1b

ethanol hexane 80 mM NaCh/0.2 M NaCl 80 mM NaDC/0.2 M NaCl 80 mM NaTC/0.2 M NaCl

3.68 c 2.84 2.77 2.76

± 0.03 (2) ± 0.06 (2) ± 0.05 (2) ± 0.06 (2)

2b c 1.11 ± 0.03 (2) 1.09 ± 0.04 (2) 1.1 ± 0.1 (2) d

Values of ε determined at 537 nm in ethanol and 517 nm in bile salts for 1b, and at 562 nm in hexane and 577 nm in bile salts for 2b. bThe numbers in parentheses correspond to the number of independent experiments. The errors are average deviations. cNot determined. d Not determined because of solubility limitations as solutions in the presence of 2 were turbid. a

The same procedure was used to determine the ε values for 2b in NaDC and hexane (Figure 1, Table 1). The absorption spectra of 2a were similar in different solvents, whereas a small bathochromic shift was observed for 2b in the bile salt solutions as compared to hexane. At 562 nm the molar absorptivity for 2b in hexane was determined to be (1.11 ± 0.03) × 104 cm−1 M−1, which is in good agreement with the reported value of 1.1 × 104 cm−1 M−1.41 It should be noted that for solutions of 2 in hexane, the peaks for both 2a and 2b in the HPLC chromatogram exhibited a shoulder at longer times (Supporting Information Figure S4). The main peak and the shoulder were found to belong to the same compound based on the absorption spectra (Supporting Information Figure S5), and thus the areas of both the main peak and the shoulder were used in the molar absorptivity calculations. A similar strategy was used to determine ε values for 1b, with the addition of keeping the solutions in an ice bath following irradiation to minimize thermal reversion prior to measurement of the HPLC chromatogram. For solutions containing a mixture of 1a and 1b, two peaks were observed in the HPLC chromatogram at a detection wavelength of 338 nm (Supporting Information Figure S6); the absorption spectrum of the broad peak at 3.1 min showed absorption in the visible region, consistent with the absorption of 1b, whereas the absorption spectrum of the sharp peak at 12.8 min was 11321


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Quantum yields for the interconversion of isomers a and b in a photochromic system can be determined by following the kinetics of the photochemical reaction under continuous irradiation, where rate equations (eqs 1−4) are defined for each photochemical (Φab, Φba) or thermal conversion (kab, kba).

consistent with the spectrum of 1a. For the detection at 517 nm (Supporting Information Figure S6), only the broad 3.1 min peak was observed, consistent with the assignment of this peak to 1b. The position (2.8−3.2 min) and shape of the peak for 1b were found to vary in different experiments. It is possible that a partial conversion of 1b to 1a inside the column leads to the broadening of this peak. This is not problematic as all quantitative information was obtained from the 1a peak; the shape and position of the 1a peak did not vary, and the 1a and 1b peaks never overlapped. Solutions of known concentrations of 1a in 80 mM NaTC/ 0.2 M NaCl were used to construct an HPLC response curve for 1a (Supporting Information Figure S7), and the ε values for 1b in 80 mM NaTC/0.2 M NaCl were calculated as described above (Figure 2, Table 1). Molar absorptivities were also

v1 = Io(1 − 10−εa[a])Φab

(1)

v2 = kab[a]

(2)

v3 = Io(1 − 10−εb[b])Φ ba

(3)

v4 = k ba[b]

(4)

For 1, both thermal and photochemical processes occur, and the differential equation that describes the change in the concentration of 1a when the system is continuously irradiated is given by the sum of the rate equations for the individual processes (eq 5). d[a] = −Io(1 − 10−εa[a])Φab − kab[a] + Io(1 − 10−εb[b]) dt Φ ba + k ba[b]

(5)

where Io is the light flux incident on the sample, [a] and [b] are the concentrations of 1a and 1b, εa and εb are the molar absorptivities of 1a and 1b at the wavelength of irradiation, Φab and Φba are the quantum yields for conversion of 1a to 1b and 1b to 1a, respectively, and kab and kba are the rate constants for the thermal conversion of 1a to 1b and 1b to 1a, respectively. As 2 is thermally stable, the rate equations for thermal conversion can be excluded, and the differential equation that describes the change in the concentration of 2a when the system is continuously irradiated can be simplified to eq 6.

Figure 2. Molar absorptivities of 1a (dashed lines) and 1b (solid lines) in ethanol (black), NaCh (80 mM, 0.2 M NaCl, blue, overlapped with green trace for 1a), NaDC (80 mM, 0.2 M NaCl, red), and NaTC (80 mM, 0.2 M NaCl, green, overlapped with red trace for 1b).

d[a] = −Io(1 − 10−εa[a])Φab + Io(1 − 10−εb[b])Φ ba dt

(6)

The photochemical reactions can be monitored at a wavelength in the visible region where only the b isomer absorbs (eq 7).

determined for 1b in NaCh, NaDC, and ethanol. The absorption spectra of 1a were similar in all solvents, whereas the absorption spectra of 1b were very similar in bile salt solutions, but a large bathochromic shift was observed in ethanol. At 537 nm the molar absorptivity for 1b in ethanol was determined to be (3.68 ± 0.03) × 104 cm−1 M−1, which is similar to the estimated value reported of 3.5 × 104 cm−1 M−1.46,47 This literature value is an estimate based on the mean value of ε assuming that a solution of 1 contains 51−100% 1b at the photostationary state.47 This value, determined for ethanol, has been assumed to be valid in polar solvents in general.46,48−51 To determine the extent of thermal reaction during the time of the HPLC experiment, a solution of 1 in 80 mM NaCh/0.2 M NaCl was irradiated and then kept covered in an ice bath. The absorption spectrum was collected immediately after irradiation, and then the HPLC chromatogram was obtained. A second absorption spectrum and HPLC chromatogram were then obtained for the same solution. The two absorption spectra obtained immediately and 13 min after irradiation were very similar (Supporting Information Figure S8), and the peak areas for 1a, obtained from the HPLC chromatograms 6 and 22 min after irradiation, were not significantly different (114.87 and 114.97 mAU, Supporting Information Figure S9). From these results, we can assume that when the solution is kept in an ice bath following irradiation, no significant thermal conversion occurs before the HPLC chromatogram is obtained.

Abvis = εbvis[b]

(7)

vis where Avis b is the absorbance of isomer b at time t and εb is the molar absorptivity coefficient of isomer b at the monitoring wavelength. The concentration of b at time t, [b], can be expressed as the difference between the total concentration of photochromic compound, [a]o, which is a known and constant quantity, and the concentration of a at time t, [a] (eq 8).

[b] = [a]o − [a]

(8)

The strategy employed in this investigation to determine the quantum yields used a numerical analysis of the above kinetic equations in Scientist 3.0 (see models employed in the Supporting Information). A custom-built spectrometer was used to continuously measure the sample absorption during constant irradiation at a constant temperature. During the irradiation of 2, the absorbance remained constant at the isosbestic point between 2a and 2b (287 nm) and at a wavelength where neither isomer absorbed (800 nm), and the formation of 2b was observed at 562 nm (Figure 3). With this setup, noise with a periodic oscillation was observed at all detection wavelengths. The example shown in Figure 3 shows a worst-case scenario for these signal fluctuations. To ensure that this noise would not affect the fitting of the kinetic traces, the 11322


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isosbestic point were found to be, within error, the same in all of the conditions studied. Table 2. Quantum Yields for the Photoisomerization of 2 Irradiated at the Isosbestic Pointa Φab

medium cyclohexane (2) 80 mM NaCh/0.2 M NaCl (2) 80 mM NaCh/1 M NaCl (2) 80 mM NaDC/0.2 M NaCl (2)

0.39 0.39 0.36 0.36

± ± ± ±

Φba 0.02 0.02 0.03 0.03

0.07 0.06 0.07 0.05

± ± ± ±

0.02 0.02 0.02 0.02

a

The numbers in parentheses correspond to the number of independent experiments, ε287 = (2.02 ± 0.05) × 104 cm−1 M−1 determined in hexane was used for the experiments in cyclohexane, ε292 = (1.6 ± 0.1) × 104 cm−1 M−1 in NaCh, and ε292 = (1.9 ± 0.2) × 104 cm−1 M−1 in NaDC. T = 15 °C, except for NaDC where T = 25 °C.

The same method was used for the determination of Φab and Φba for the isomerization of 1, with the additional consideration of the thermal reactions of 1 (Figure 4). The rates of reaction Figure 3. Top: Absorbance during irradiation at 287 nm of 2 in cyclohexane at 15 °C at 287 nm (a, isosbestic point; the dashed red line is the average value), 562 nm (b, absorption maximum of 2b), and 800 nm (c, baseline). Bottom: Absorbance at 577 nm for 2 in 80 mM NaCh/0.2 M NaCl during irradiation at 292 nm at 15 °C. Experimental data (d, black), numerical fit (d, red), and residuals between the experimental data and the fit (e; the dashed red line indicates zero).

kinetics were followed for at least one cycle of oscillation after the photostationary state was reached. The resulting residuals for fits of the kinetic traces displayed the same periodic oscillation, indicating that the noise was not included in the fit (see the Supporting Information for details). Solutions of 2 were irradiated at the isosbestic point, and absorption was monitored at the maximum of the absorption peak in the visible region for 2b. For the numerical fit of this kinetic data, the molar absorptivity coefficient, εa, was determined from the absorption spectra of a solution of 2a only. As the irradiation was performed at the isosbestic point, the value of εb was equal to the value of εa. As described above, the molar absorptivity coefficients of 2b at other wavelengths are not trivial to determine, and initially the curves were fit without fixing εvis b . This resulted in under-determination of the parameters εvis b , Φab, and Φba. The same under-determination was observed when the reaction was monitored at several wavelengths and the curves were fit simultaneously. In subsequent fits (Figure 3), εbvis was fixed to the value determined as described above. For 2 in cyclohexane, the εvis b values for hexane were used because the absorption of 2 at 287 nm in hexane and cyclohexane was the same within 1%. The resulting quantum yields were determined with a small standard deviation, and the same values were found irrespective of the initial guess values. The parameter found to have the greatest effect on the determination of the quantum yields was εvis b . The fitting was therefore carried out with both the minimum and the maximum values for εvis b , as defined by the experimental error for this parameter, and the resulting quantum yield varied up to ±0.02. Therefore, an uncertainty of ±0.02 was assumed for all quantum yield values except where a larger error was observed for independent experiments (Table 2). The values of both Φab and Φba for isomerization of 2 by irradiation at the

Figure 4. Absorbance at 517 nm for 1 in 80 mM NaCh/0.2 M NaCl during irradiation at 299 nm at 15 °C. Experimental data (a, black), numerical fit (a, red), and residuals between the experimental data and the fit (b, where the dashed red line indicates zero).

for the thermal reactions of 1 were determined in independent kinetic studies (see below), and these parameters were fixed when fitting the kinetic curves for the photoconversion. The resulting quantum yields were determined with a small standard deviation, and the same values were found independent of the initial guess values. The fitting was performed considering the experimental error of εvis b , kab, and kba. The quantum yields from these fits were assumed to have an error of ±0.01 unless a larger error was obtained for independent experiments (Table 3). The quantum yields for 1 were affected by the medium. Both Φab and Φba were smaller in ethanol than in bile salts, and the value of Φab was also seen to depend on the structure of the bile salt. The relaxation rate constant for the thermal equilibration between 1a and 1b can be obtained by monitoring the relaxation of the system after the formation of an excess of 1a or 1b; excess 1a or 1b can be formed by irradiation with visible or UV light, respectively. The kinetics monitored at the absorption maxima for 1b show a fast growth followed by a slow decay when 1a is in excess and fast decay followed by a slow decay when 1b is in excess. The kinetic curves were fit to a sum of two exponentials (eq 9), where A is the absorbance, ai are pre-exponential factors, and ki are first-order rate constants. 11323


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Table 3. Quantum Yields for the Photoisomerization of 1 Irradiated at the Isosbestic Pointa Φab

medium ethanol (2) 80 mM NaCh/0.2 M NaCl (2) 80 mM NaCh/1.0 M NaCl (2) 80 mM NaDC/0.2 M NaCl (2) 80 mM NaTC/0.2 M NaCl (2)

0.15 0.30 0.32 0.51 0.26

± ± ± ± ±

K=

Φba 0.01 0.01 0.01 0.03 0.01

0.04 0.14 0.15 0.15 0.15

± ± ± ± ±

a

The numbers in parentheses correspond to the number of independent experiments, ε298 = (8.57 ± 0.07) × 103 cm−1 M−1 in ethanol, ε299 = (7.3 ± 0.3) × 103 cm−1 M−1 in NaCh, ε302 = (7.6 ± 0.1) × 103 cm−1 M−1 in NaDC, and ε299 = (7.3 ± 0.2) × 103 cm−1 M−1 in NaTC. T = 15 °C, except for NaDC where T = 25 °C.

k ba =

(9)

The fast component (k1) is attributed to the equilibration of 1, whereas the slow component (k2) is attributed to the decomposition of 1 through hydrolysis, similar to that previously observed for water-soluble spiropyrans.9 The decomposition rate constants reported for a water-soluble spiropyran ((2−4) × 10−3 min−1 at pH 7−8)9 are significantly higher than those previously reported for 1 in NaCh aggregates ((3−5) × 10−5 min−1),39 suggesting that the aggregates afford 1 protection from hydrolysis. We have previously reported the relaxation rate constants for the thermal equilibration between 1a and 1b in aqueous solution containing 80 mM NaCh and 0.2 or 1 M NaCl.39 In this previous work, the same relaxation rate constants were obtained from the kinetics for the conversion of excess 1a to 1b or the conversion of excess 1b to 1a, as is expected for relaxation kinetics. All data reported in this current study were measured only for the relaxation of 1b to 1a. A fit to the sum of two exponentials was required to get fits with low and random residuals, and reproducible values for k1. However, as the values obtained for k2 ((1−8) × 10−5 min−1 at 25 °C) were 2 orders of magnitude smaller than those for k1, there was not sufficient data on the time scales monitored for the determination of precise values of k2. No discernible trend in k2 was obtainable, and the individual values for k2 are therefore not presented. The relaxation rate constant, k1, corresponds to the sum of the rate constants for the thermal conversion of 1a to 1b (kab) and 1b to 1a (kba) (eq 10). k1 = kab + k ba

(11)

The absorption of 1b at equilibrium in the absence of decomposition is equal to the difference between the absorption of the solution at time zero (At=0) and the preexponential factor, a1 (eq 9, where At=0 = a1 + a2). From this difference, the concentration of 1b at equilibrium is determined. From the concentration of 1b and the total concentration of 1, the concentration of 1a is obtained, and the equilibrium constant can be calculated using eq 11. Equations 10 and 11 can be combined to determine kba (eq 12), and kab can then be calculated using eq 10.

0.01 0.01 0.02 0.02 0.01

A = a1 e−k1t + a 2 e−k 2t

k [1b] = ab [1a] k ba

k1 1+K

(12)

Relaxation rate constants, k1, were obtained for 1 in various bile salt solutions (Supporting Information Figure S10). The value of k1 varied for the different bile salts (NaDC > NaCh > NaTC), whereas the opposite trend was observed for the equilibrium constants (Table 4). The k1 value in NaCh and 0.2 M NaCl was the same as previously reported, whereas this value at the higher NaCl concentration of 1.0 M was higher than the previous value of (8.74 ± 0.06) × 10−3 min−1.39 This discrepancy could be due to the method used for sample preparation; in the previous work, 1 and 2 were injected after the temperature annealing cycle for the bile salts, whereas in the current work, the samples with the guests were heated, which ensured equilibration of the host−guest system. Rate constants calculated for the thermal conversion between 1a and 1b (kab) and 1b and 1a (kba) were used in the determination of the quantum yields for photoconversion (see above). It is important to note that because the reaction is unimolecular, the initial absorbance at time zero is not significant, and the initial absorbance value is determined solely by the duration of the irradiation to form an excess of 1b.

■

DISCUSSION Supramolecular systems frequently impart properties to the system that are not achievable by the individual, isolated building blocks. In this study, bile salt aggregates were used as hosts in aqueous solution for insoluble photochromic compounds, and the desired property for the supramolecular system was to achieve photochromism in a solvent incompatible with the isolated photochromic molecules. Supramolecular systems are in equilibrium, and therefore characterization of the parameters that describe the photochromism of these systems cannot be studied by isolating individual molecules. This situation led to the requirement for the development of suitable methodology, which will be discussed first.

(10)

The equilibrium constant, K, for the thermal equilibrium between 1a and 1b can be described either as a ratio of rate constants or as a ratio of the concentrations of 1a and 1b (eq 11).

Table 4. Relaxation Rate Constant, k1, Equilibrium Constants, K, and Rate Constants, kab and kba, for the Thermal Equilibrium between 1a and 1b in Aqueous Solutions Containing Various Bile Salts (80 mM) and 0.2 or 1.0 M NaCl at 25 °Ca bile salt

[NaCl]/M

NaCh (3) NaCh (3) NaDC (2) NaTC (2)

0.2 1.0 0.2 0.2

k1/10−3 min−1 5.6 11.2 6.26 4.34

± ± ± ±

K/10−2

0.1 0.1 0.06 0.01

10 2.2 2.9 12.5

± ± ± ±

1 0.4 0.1 0.2

kab/10−3 min−1 0.51 0.24 0.177 0.48

± ± ± ±

0.06 0.04 0.005 0.02

kba/10−3 min−1 5.1 10.9 6.08 3.85

± ± ± ±

0.2 0.1 0.06 0.01

a

The numbers in parentheses correspond to the number of independent experiments. The errors are standard deviations when the number of independent experiments is equal to or larger than three or average deviations when only two independent experiments were performed. 11324


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(0.17, 25 °C), which was determined from transient absorption measurements.48,57 Values of Φba are not reported using this previous method due to experimental limitations; no evidence was observed for the 1b to 1a reaction on the time scale of the fast transient absorption measurements ( NaCh > NaTC), but all were lower than k1 reported in ethanol (2.3 × 10−2 min−1),46 consistent with 1 in bile salts being located in an environment somewhat more polar than ethanol. The opposite trend was observed for the equilibrium constants in these three bile salts (NaTC > NaCh > NaDC). In homogeneous solution, the equilibrium between 1a and 1b shifts toward the more polar 1b as the solvent polarity is increased, resulting in an increase in K. NaDC, which has only two hydroxyl groups on the bile salt backbone, is known to be more hydrophobic and to form more rigid aggregates that better protect molecules from the aqueous solution than NaCh or NaTC.32,35 The I/III ratio in the pyrene fluorescence spectrum is known to be sensitive to the polarity of the environment in which pyrene is located, and this ratio increases with solvent polarity. Studies of pyrene in various bile salt solutions found a higher pyrene I/III ratio in NaCh than in NaDC,60 and an even higher I/III ratio was observed for pyrene in NaTC aggregates.61 Thus, the trends observed in k1 and K, which indicated that 1 experienced the most polar environment in NaTC and the least polar environment in NaDC, are consistent with the trends observed for other guests included in bile salt aggregates. It is important to note that the comparison with the pyrene studies is made with respect to the polarity changes; no information is available on whether the binding site for the isomers of 1 is the same as that for pyrene. A similar dependence on bile salt structure was observed for the values of Φab obtained for 1 (NaDC > NaCh > NaTC), whereas Φba for 1 did not vary with bile salt structure. The values of Φab for 1 are known to decrease with increasing solvent polarity; however, some variation in this trend occurs because the photoisomerization can occur by a singlet or a triplet pathway.62 Both Φab and Φba for 1 were larger in bile salt solutions than in ethanol, and this trend does not correlate with the expected changes with polarity of different solvents. This result could be due to a different partitioning between the singlet and triplet pathways in the bile salt than in homogeneous solution. Alternatively, the trend could also be influenced by a relocation of the less polar isomer 1a from a hydrophobic binding site in the aggregate to a more polar binding site when 1b is formed. This relocation dynamics could play a role in the continuous irradiation experiments used to determine the quantum yield for the photoconversion of 1a to 1b. Finally, it is important to note that the studies here reported were performed at one concentration of bile salt. Bile salts are known to form two types of aggregates, small hydrophobic primary aggregates and larger secondary aggregates, and the size of the aggregate continuously increases as the concentration of bile salt is raised.28−30 It is possible that changes in the aggregate size will have an effect on photochromic properties, and such an effect will be investigated in future.

photochromic parameters. Molar absorptivities were obtained from HPLC experiments, and, in the case of 1b, a high precision value of (3.68 ± 0.03) × 104 cm−1 M−1 was determined at 537 nm in ethanol. Quantum yields for the photoconversion between the isomers were obtained from kinetic experiments using continuous irradiation. The structure of the bile salt did not alter either quantum yield for 2, suggesting that changes in the structure of the aggregates for the different bile salts were not sufficient to change the equilibrium between the reactive and nonreactive conformers of 2. In the case of 1, the structure of the bile salt aggregate influenced the quantum yield for the reaction of 1a to 1b, whereas the quantum yield for the reverse reaction from 1b to 1a was not affected. The position of the absorption maxima indicated that 1b was located in an environment that was more polar than ethanol, and the relaxation kinetic studies suggested that the polarity for 1b was highest in the aggregates of NaTC, followed by NaCh and NaDC. These experiments showed that bile salt aggregates can be used to develop photochromic systems in aqueous solutions that incorporate photochromic molecules where both isomers are hydrophobic or photochromic compounds where the polarity of the isomers is significantly different.

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ASSOCIATED CONTENT

S Supporting Information *

Details for experimental procedures, description of continuous irradiation system, HPLC chromatograms and response curves, numerical models for quantum yield determination, and relaxation kinetics for 1b. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank the Xerox Foundation for a University Affairs Committee grant and the Natural Sciences and Engineering Research Council of Canada (NSERC) for a Collaborative Research and Development grant. We thank Luis Netter for his assistance in the development of the irradiation system.

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REFERENCES

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CONCLUSIONS The photochromism of 1 and 2 was quantified in aqueous solutions of bile salt aggregates with different structures. The incorporation of 1 and 2 into a supramolecular system required the use of methods that did not rely on the isolation of the individual isomers for determination of spectroscopic and 11326


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Photochromism of a Spiropyran and a Diarylethene in Bile Salt

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