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Abstract

  1. Top of page
  2. Abstract
  3. Introduction
  4. Materials and Methods
  5. Results and Discussion
  6. Conclusions
  7. Acknowledgments
  8. References

The effect of cucurbit[7]uril (CB7) on the spiropyran-merocyanine photochromic interconversion was studied in acidic and alkaline aqueous solutions. The merocyanine (MC) isomer was found to be the thermodynamically most stable form both in water and in the presence of CB7. A preferential binding of the protonated merocyanine (MCH+) to CB7 was observed with an equilibrium constant of 7.4 × 104 m−1, and the complex formation led to significant diminution of acidity of the guest. The photoinduced transformation of MCH+ to the spiropyran isomer was accelerated 2.3-fold upon addition of CB7, whereas the rates of the other photochromic processes were not affected. The partial inclusion of MCH+ in CB7 led to dual fluorescence due to the incomplete deprotonation in the singlet-excited state.


Introduction

  1. Top of page
  2. Abstract
  3. Introduction
  4. Materials and Methods
  5. Results and Discussion
  6. Conclusions
  7. Acknowledgments
  8. References

Photochromism is a light-initiated reversible transformation between two species possessing significantly different absorption spectra. Organic photochromic compounds have diverse applications in optical information storage, ion sensors, ophthalmic lenses, light modulators and photoresponsive materials (1–5). Photoswitchable compounds open new possibilities in subdiffraction-resolution microscopy (6), photocontrol of protein activity and transport through biological membranes (7). Photochromism-based assay has been developed to distinguish target proteins (8), and photoswitched DNA-binding has been accomplished (9). The biology-related utilizations require photochromes with aqueous solubility, which can be achieved by introducing hydrophilic substituents (10–12) or by confinement in self-assembled systems, such as e.g. micelles (13,14), vesicles (15) and bile salt aggregates (16). Inclusion in cyclodextrin macrocycles cannot be used for this purpose because the complexes of these hosts have low solubility in water (17,18). Moreover, when spirobenzopyran substituted with sulfonate group is embedded in β-cyclodextrin, the photoinitiated coloration is precluded (19). Spiropyrans are one of the most frequently studied classes of photochromes. These compounds are interconverted into the corresponding colored merocyanine form by thermally or photochemically stimulated processes. In water and its 1:1 mixture with methanol, the highly polar merocyanine becomes the most stable form due to hydrogen bonding and dipole interactions with the solvent (20,21). We have shown that inclusion in cucurbit[8]uril (CB8) cavity has three beneficial consequences: (1) it enhances the solubility, (2) increases the chemical stability, and (3) permits of the tuning of the photochromic behavior (21). As these effects may considerably alter with the ring size of the host, now, we use the smaller cucurbit[7]uril (CB7), which is composed of 7 glycoluril units linked by a pair of methylene groups, to reveal whether this cavitand is able to influence the interconversion among the spiro (SP), the protonated merocyanine (MCH+) and the merocyanine (MC) forms. The formulas of the investigated substances are given in Scheme 1.

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Figure  Scheme 1. .  The formulas of the studied compounds.

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Materials and Methods

  1. Top of page
  2. Abstract
  3. Introduction
  4. Materials and Methods
  5. Results and Discussion
  6. Conclusions
  7. Acknowledgments
  8. References

N-(2-hydroxyethyl)-3′,3′-dimethyl-6-nitro-spiro[2H-1-benzopyran-2,2′-indoline] (SP; TCI) was used without further purification. High purity cucurbit[7]uril, kindly provided by Dr. Anthony I. Day, was dried in high vacuum for several days prior to use. The pH of the solutions, adjusted with HCl or KOH, was measured with Consort C832 equipment. Glass electrode was calibrated at pH 4, 7 and 10 with buffer standards. The UV–visible absorption spectra were recorded on a Unicam UV 500 (Unicam Instruments, Cambridge, UK) or a Hewlett-Packard 8452A (Hewlett-Packard, Palo Alto, CA) spectrophotometer. Corrected fluorescence spectra were obtained on a Jobin-Yvon Fluoromax-P photoncounting spectrofluorometer (HORIBA Scientific, Edison, NJ). The time-correlated single photon apparatus has been described (22). Photoirradiations were performed using 150 W xenon lamp and monochromator in 1 × 0.4 cm quartz cell. The whole solutions were exposed to light. The temperature of the samples was controlled with a Julabo thermostat. The experimental data were analyzed by the ORIGINPRO8 software (OriginLab Corporation, Northampton, MA). Molecular modeling calculations were carried out with RM1 method using HyperChem 8.0 program (Hypercube Inc., Gainesville, FL).

Results and Discussion

  1. Top of page
  2. Abstract
  3. Introduction
  4. Materials and Methods
  5. Results and Discussion
  6. Conclusions
  7. Acknowledgments
  8. References

Complex formation in acidic solution

Spiro undergoes a thermal acid-catalyzed ring opening leading to MCH+ at pH 2.3. Due to the stabilization by the interaction with water, the latter cationic form becomes the most stable. The inset to Fig. 1 presents the alteration of the absorption spectrum of MCH+ with increasing CB7 concentration. The isosbestic point appearing at 406 nm suggests 1:1 binding. The partial encapsulation brings about a slight bathochromic shift of the first absorption band because the core of the macrocycle provides a microenvironment of low polarity and polarizability (23). Due to the larger dipole moment in the ground state than in the first singlet-excited state (24,25), solvent–solute interactions stabilize the ground state more efficiently resulting in a larger energy gap between the ground and the first singlet-excited states in polar medium. As a representative example, Fig. 1 shows the alteration of MCH+ absorbance at 435 and 325 nm with increasing CB7 concentration. In the case of 1:1 binding, the following function describes the experimental results (26):

  • image(1)

where Aλ represents the absorbance at a particular wavelength, A and A0 denote the absorbance of the fully complexed and free dyes, [CB7]0 and [Dye]0 are the total concentration of the host and guest compounds, respectively. The global fit of Eq. (1) to the experimental data provides K = 7.3 × 104 m−1 for the equilibrium constant of the 1:1 complex formation. This value is more than one order of magnitude smaller than that obtained for MCH+ inclusion in the larger CB8 homolog (K = 2.0 × 106 m−1; 21). The more spacious interior of CB8 probably permits of a deeper embedment resulting in stronger host–guest interactions.

image

Figure 1.  Variation of the absorbance of 11.1 μm MCH+ at 435 (▾) and 325 nm (▪) as a function of CB7 concentration in aqueous solution at pH 2.3. Inset: absorption spectra of MCH+ in the presence of 0, 8.4, 24, 54 and 490 μm CB7.

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As we noted in a previous article (21), the photoexcitation of MCH+ leads to a strongly redshifted fluorescence band corresponding to MC emission. This is attributed to the substantial growth of the acidity of the phenolic OH moiety upon excitation, which results in a rapid loss of proton in the singlet-excited state. As this process is very fast even at pH 2.3, negligible MCH+ fluorescence is detected in water. However, the gradual increase of CB7 concentration diminishes the intensity of the MC fluorescence, whereas a concomitant rise of a new band is observed in the 460–560 nm domain and an isoemissive point appears at 570 nm (inset Fig. 2). The excitation spectra for the two fluorescence bands do not differ and correspond to the absorption spectrum of MCH+ indicating that the dual fluorescence originates from MCH+ excitation. The short-wavelength emission is assigned to MCH+. When MCH+ is bound to CB7, the deprotonation in the singlet-excited state becomes slower, thereby enabling the competition for the fluorescence emission. It is noteworthy that the complexation of MCH+ with the larger CB8 cavitand under analogous conditions causes only intensity diminution in MC emission, but no MCH+ fluorescence emerges (21). This may indicate that the rate of photoinduced deprotonation is decelerated to a much smaller extent upon binding to the larger CB8 host. To confirm this hypothesis, time-resolved fluorescence measurements with time-correlated single-photon counting technique were performed, but the fluorescence decay times for free and complexed MCH+ were shorter than the time-resolution of our apparatus (ca 100 ps). The spectral changes shown in Fig. 2 were fitted with a function analogous to Eq. (1). The global analysis gave K = 7.4 × 104 m−1 for the equilibrium constant of the 1:1 binding of MCH+ to CB7 in excellent agreement with the result of spectrophotometric titration (vide supra). The calculated curves, displayed in Fig. 2 as a solid line, match the experimental data well confirming the 1:1 binding stoichiometry.

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Figure 2.  Fluorescence intensity at 522 (▾) and 630 nm (▪) as a function of CB7 concentration in 20 μm MCH+ aqueous solution at pH 2.3. Inset: fluorescence spectra of 20 μm MCH+ in the presence of 0, 7.3, 23, 53, 85 and 350 μm CB7 at pH 2.3. Excitation is at 405 nm.

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Effect of pH alteration

When the pH of the solution is raised, the absorption of MCH+-CB7 complex gradually vanishes and a markedly redshifted band emerges due to the deprotonation of the phenolic OH moiety (Fig. 3A). Isosbestic points appear at 374 and 449 nm, whereas the maximum of the deprotonated MC absorption is located at 504 nm. The spectral changes closely resemble that found in the absence of CB7. Fig. 3B shows the rise of the absorbance at 505 nm as a function of pH. The sigmoid-shaped titration curves were analyzed by nonlinear least-squares fit of the Boltzmann function (Eq. [2]):

  • image(2)

where P denotes a fitting parameter, A0 and A are the absorbances at low and high pH, respectively. The negative logarithm of the equilibrium constant of the proton dissociation of MCH+ was found to be pKa = 4.52 and pKa′ = 5.73 for the free dye and the CB7 complex, respectively. The acidity diminution on the confinement in CB7 is attributed to the stabilization of the cationic protonated form of the guest by ion–dipole interactions with the carbonyl-laced portals of the macrocycle (27,28). The pKa′ value of MCH+ in CB7, determined in the present study, insignificantly differs from that observed in the larger CB8 (pKa = 5.60) (21). Thus, we can conclude that cavity size of the hosts affects only the binding affinity, but do not influence the acidity of the embedded MCH+.

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Figure 3.  (A) Absorption spectra of 18.8 μm MCH+ in 381 μm CB7 solution at pH 3.8, 4.7, 5.4, 5.6, 5.8, 6.2 and 7.9. (B) Alteration of MC absorbance at 505 nm in the absence (▪) and presence (▴) of 380 μm CB7 as a function of pH. The measurements were performed 3 min after the pH change.

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As pH is raised above 8, the absorption of MC gradually diminishes due to a chemical reaction with OH. The data recorded 3 min after the pH change are shown in Fig. 3B. This time was needed to adjust the pH and to measure the absorption spectrum. In alkaline media, the pH dependence of the absorbance does not alter upon addition of CB7 indicating that this host does not inhibit the decomposition of MC. The acceleration of the reaction with growing pH reflects the important role of OH in the process. The first step of the MC decomposition is probably the nucleophilic addition of OH anion to the two position of the heterocyclic ring of MC (Scheme 2) because the absorption spectrum of the primary product, displayed with thin line in the insert of Fig. 4, closely resembles that reported for the analogous products of CN and F addition (29). The reaction of MC with OH follows second order kinetics. The slope of the linear correlation between the reciprocal MC concentration and the reaction time provides k = 1.29 m−1 s−1 for the rate constant of MC disappearance both in the absence and presence of CB7. The lack of the CB7 effect is in sharp contrast to the significant inhibition of the transformation of MC in alkaline solution by inclusion in CB8 (21). In the case of MC-CB8 complex, the two position of the heterocyclic ring of MC is protected against the nucleophilic addition of OH by the electrostatic repulsion due to the partial negative charge of the carbonyl-rimmed portal of CB8 macrocycle. Consequently, the first step of the hydrolysis, the MC-ΟΗ adduct formation, is significantly slowed down. Such an effect does not occur in the presence of CB7 because MC is not embedded in the relatively small CB7 cavity.

image

Figure  Scheme 2. .  MC hydrolysis in alkaline solution.

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image

Figure 4.  Absorbance at 358 (▪), 390 (•) and 450 nm (▴) as a function of time after addition of 4.5 mm KOH to 6 μm MC in water at 333 K. Inset: Absorption spectrum of reaction products at 3 min (thin line) and 120 h (thick line) after addition of 1 mm KOH to 23.4 μm MC in water at 296 K.

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The initially produced adduct of MC and OH slowly reacts further at room temperature. The thick line in the insert to Fig. 4 gives the absorption spectrum of the final products, which matches the spectrum reported by Stafforst and Hilvert (11) for Fischer’s base and 4-nitrosalicylaldehyde hydrolysis products (Scheme 2). The kinetics of the formation of these products at 333 K is shown in Fig. 4. The absorbance change at various wavelengths fits to exponential function, and 5.7 × 10−4 s−1 is obtained for the rate constant of the first-order transformation of MC-OH adduct to the hydrolysis products at 333 K.

Quantum chemical calculations

To get information on the molecular structure of CB7 complexes, quantum chemical calculations were performed with RM1 semiempirical method using HyperChem 8.0 program (Hypercube Inc., Gainesville, FL). Partial inclusion of MCH+ was found in CB7. In the energy-minimized structure (Fig. 5), the benzene ring of the indoline moiety is located in the vicinity of the carbonyl-laced portal of CB7. The substituents of the indoline ring sterically hinder the deeper penetration in the hydrophobic core of the host. SP and MC remain outside the CB7 macrocycle in the energy-optimized structure in accordance with the lack of change in the absorption spectra of these species upon addition of CB7. The small affinity of SP and MC to CB7 arises from sterical hindrance and the uncharged character of these species. Cucurbiturils preferentially bind cationic species because of the considerable negative charge density of the ureido carbonyl groups and the inner surface of the hydrophobic cavity (30). The calculations also show that the two position of the indoline moiety of MC possesses considerable partial positive charge (δ = 0.214) providing a preferable site for nucleophilic attack by OH.

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Figure 5.  Energy-minimized structure of MCH+-CB7 complex in the ground state calculated by RM1 method. Color codes: CB7 green, for MCH+ oxygen red, nitrogen blue and carbon light blue.

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Effect of CB7 on the kinetics of photochromic transformations

MCH+ is the thermodynamically most stable photochromic form both in water and CB7 cavity at pH 2.35. MCH+-CB7 complex exhibits inverse photochromism. Its yellow solution is bleached by exposure to 410 nm light. The inset in Fig. 6 shows the absorption spectrum before and after photolysis. The free and complexed MCH+ cations give the same photoproduct, which is identified as the spiropyran isomer (SP) on the basis of its characteristic absorption spectrum (31). SP is formed in a multistep process initiated by trans-cis photoisomerization followed by loss of proton and ring closure. The disappearance of MCH+ can be followed selectively by measuring the absorbance at 406 nm. The rates of MCH+ transformation in water and in the presence of 0.63 mm CB7 are compared in Fig. 6. The binding to CB7 results in 2.3-fold reaction rate enhancement. The interactions with the host modify the electron distribution in MCH+, promoting thereby the conversion to SP. The partial inclusion in the nonpolar and extremely nonpolarizable cavity of CB7 (32) lessens the interaction with water, which also contributes to the alteration of the reaction kinetics. A previous study has shown that a microenvironment of low polarity accelerates the photochemical conversion to SP (8).

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Figure 6.  Absorbance diminution at 406 nm during photoirradiation at 410 nm in the presence of 0 (▪) and 630 μm (▴) CB7 at pH 2.3; [MCH+] = 20.5 μm. Inset: absorption spectrum of MCH+-CB7 before (thin line) and after (thick line) photolysis at 410 nm.

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The formation of SP is a reversible process even in the presence of CB7, and this cavitand does not alter the kinetics of the thermal back isomerization. An exponential rise of the absorption band of MCH+ takes place in the dark with a rate constant of 8.3 × 10−5 s−1 at 295 K. Temperature dependent studies show that the Arrhenius parameters of this reaction also do not alter upon addition of 0.38 mm CB7 (Fig. 7). The activation energy and the pre-exponential factor are 96 kJ mol−1 and 6.3 × 1012 s−1, respectively. The rates of the interconversion between MC and SP are also insensitive to the addition of CB7 in accordance with the results of quantum chemical calculations, which showed that MC and SP are not encapsulated in CB7.

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Figure 7.  Arrhenius plot of the rate constants of SP conversion to MCH+ in the dark in the presence of 0 (▪) and 380 μm (▴) CB7 at pH 2.3. [MCH+] = 15.1 μm.

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Conclusions

  1. Top of page
  2. Abstract
  3. Introduction
  4. Materials and Methods
  5. Results and Discussion
  6. Conclusions
  7. Acknowledgments
  8. References

CB7 affects the photochromic behavior an entirely different manner than the larger CB8 homolog. Only MCH+ is embedded in CB7, and this complexation accelerates the rate of the photoinduced transition from MCH+ to SP. The vanishing affinity of CB7 to MC compared with MCH+ arises from the decrease of the positive charge density on the indole ring upon deprotonation due to the contribution of the lone electron pair of phenolate oxygen to the extended conjugated system of MC. Despite the 27-fold smaller equilibrium constant for MCH+ binding to CB7 compared with that of the inclusion in CB8, both hosts decrease the acidity of MCH+ to the same extent. The confinement of MCH+ to CB7 leads to dual fluorescence due to the hindered deprotonation of the phenolic OH moiety in the singlet-excited state. The hydrolysis of MC undergoes two main steps. The initial nucleophilic addition of hydroxyl ion follows second order kinetics, whereas the decomposition of the MC-OH intermediate is a first-order process. Neither of these reactions is affected by CB7 as MC and MC-OH do not form inclusion complex with this cavitand.

Acknowledgments

  1. Top of page
  2. Abstract
  3. Introduction
  4. Materials and Methods
  5. Results and Discussion
  6. Conclusions
  7. Acknowledgments
  8. References

Acknowledgements— We thank the Hungarian Scientific Research Fund (OTKA, grant no. K75015) for their support.

References

  1. Top of page
  2. Abstract
  3. Introduction
  4. Materials and Methods
  5. Results and Discussion
  6. Conclusions
  7. Acknowledgments
  8. References
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