Photochromism in cucurbit[8]uril cavity: Inhibition of hydrolysis and modification of the rate of merocyanine-spiropyran transformations

Article abstract of DOI:10.1021/jp207708x

The effect of inclusion complex formation on the photochromic behavior of a spirobenzopyran dye and its merocyanine isomer was studied in aqueous solution using cucurbit[8]uril (CB8) as a host. The merocyanine (MC) and protonated merocyanine (MCH⁺) were the most stable forms both in water and inside the cavity of CB8. The equilibrium constant of 1:1 complexation with CB8 was found to be 1.7 × 10⁵ and 2.0 × 10⁶ M ^-1 for the former and latter species, respectively. The encapsulation led to significant change in the rate of the photoinduced and thermal photochromic transformations and hindered the hydrolysis of MC. The effect of CB8 on the reaction kinetics strongly altered with pH. The transition from the spiropyran form to trans-MC in a thermal reaction had 33 kJ mol^-1 lower activation energy and more than 5 orders of magnitude smaller Arrhenius pre-exponential factor in CB8 than in water.

Full text of DOI:10.1021/jp207708x

ARTICLE
pubs.acs.org/JPCB
Photochromism in Cucurbit[8]uril Cavity: Inhibition of
Hydrolysis and Modification of the Rate of
MerocyanineꢀSpiropyran Transformations
Zsombor Miskolczy and L ꢀa szl ꢀo Bicz ꢀo k*
Chemical Research Center, Hungarian Academy of Sciences, P.O. Box 17, 1525 Budapest, Hungary
S Supporting Information
b
ABSTRACT: The effect of inclusion complex formation on the photochromic behavior of
a spirobenzopyran dye and its merocyanine isomer was studied in aqueous solution using
cucurbit[8]uril (CB8) as a host. The merocyanine (MC) and protonated merocyanine
+
(
MCH ) were the most stable forms both in water and inside the cavity of CB8. The
5
equilibrium constant of 1:1 complexation with CB8 was found to be 1.7 ꢁ 10 and 2.0 ꢁ
6
ꢀ1
1
0 M for the former and latter species, respectively. The encapsulation led to signi-
ficant change in the rate of the photoinduced and thermal photochromic transformations
and hindered the hydrolysis of MC. The effect of CB8 on the reaction kinetics strongly
altered with pH. The transition from the spiropyran form to trans-MC in a thermal
ꢀ
1
reaction had 33 kJ mol lower activation energy and more than 5 orders of magnitude
smaller Arrhenius pre-exponential factor in CB8 than in water.
1
. INTRODUCTION
We have shown that the inclusion in cucurbit[7]uril is a
37,38
powerful tool to modify photophysical properties,
40
promote
Organic photochromic compounds have been receiving con-
39
tautomerization,2 impede nucleophilic addition, and inhibit
siderable attention because of their versatile applications in optical
information storage, logic gates, molecular machines, ophthalmic
lenses, photoresponsive materials, and ion sensors. The revers-
ible transformation between two isomers of these molecules has a
great potential to be used in neurobiology, biomedical imaging,
photocontrol of protein activity, and transport through biological
membranes. Switching of fluorophores by light-induced structur-
al change of a photochrome can offer the opportunity to achieve
subdiffraction-limit resolution in the imaging of cellular struc-
tures.
moieties are of substantial interest in a broad range of biomedical
9
photooxidation. The objective of the present work is to reveal
1
ꢀ6
how the encapsulation in cucurbit[8]uril (CB8) influences the
kinetics of the color change and the stability of a photochromic
0 0
dye (Scheme 1), N-(2-hydroxyethyl)-3 ,3 -dimethyl-6-nitrospiro
3
7
0
2H-1-benzopyran-2,2 -indoline] (SP). This type of dye has
[
8
usually been studied in organic solvents or organic solventꢀ
water mixtures, but very little information is available on their
reactions in aqueous medium. Shiraishi and co-workers recently
showed that SP isomerizes to colored merocyanine (MC) in a
thermally induced process in methanol:water 1:1 v/v mixture
because the highly polarized MC form is stabilized by hydrogen-
9ꢀ11
Biomacromolecules functionalized by photoresponsive
1
2
fields.
41
bonding interactions. Upon irradiation with visible light, MC is
converted into SP, even in aqueous medium. Stafforst and Hilvert
found a fast hydrolysis of peptide-substituted MC yielding
Fischer’s base and 4-nitrosalicylaldehyde under physiological
The majority of the novel applications require systems with
aqueous solubility, which can be achieved by modification of
photochromes with hydrophilic functional groups. This approach
usually involves tedious multistep covalent syntheses and a time-
consuming purification procedure.
strategy is the solubilization by self-assembly with surfactants,
vesicles, bile salt aggregates, or cyclodextrins.
mic compounds form inclusion complex with cyclodextrins, but the
binding constants are only about (2ꢀ3) ꢁ 10 M . Cucurbiturils, a
family of pumpkin-shaped cavitands comprised of glycoluril units
linked by a pair of methylene groups, usually show a stronger binding
1
3
13
14,15
conditions (Scheme 1). Since the decomposition seriously
limits the utility of this photochrome in water, it is important
to examine whether the inclusion in CB8 cavity hinders hydro-
lysis, alters photochromic behavior, or enhances solubility
in water.
An alternative, more efficient
1
6
17
18
17,19ꢀ23
Photochro-
3
ꢀ1
2
. EXPERIMENTAL SECTION
24ꢀ26
propensity.
Because of the considerable negative charge density
SP (TCI) was used without further purification. High-purity
cucurbit[8]uril, kindly provided by Dr. Anthony I. Day, was dried
in high vacuum for several days prior to use. The pH of the
of the ureido carbonyl groups and the inner surface of the hydro-
phobic cavity, these macrocycles preferentially bind cationic com-
pounds and serve as versatile receptors for redox active substrates.
The confinement in cucurbiturils enhances photostability,
itates stereoselective photodimerization,
the intermolecular forces responsible for the aggregation.
27
28
2
9,30
facil-
Received: August 11, 2011
31ꢀ34
and effectively disrupts
Revised:
September 16, 2011
3
5,36
Published: September 20, 2011
r 2011 American Chemical Society
12577
dx.doi.org/10.1021/jp207708x
|J. Phys. Chem. B 2011, 115, 12577–12583

The Journal of Physical Chemistry B
ARTICLE
+
solutions, adjusted with HCl or KOH, was measured with
Consort C832 equipment. The glass electrode was calibrated
at pH 4, 7, and 10 with buffer standards. The UVꢀvisible
absorption spectra were recorded on a Unicam UV 500 or a
Hewlett-Packard spectrophotometer. Corrected fluorescence
spectra were obtained on a Jobin-Yvon Fluoromax-P photon-
counting spectrofluorometer. 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 thermo-
stat. The experimental data were analyzed by the ORIGINPRO8
software.
of SP to MCH occurred (Scheme 2), and the product became
+
much more soluble in water due to its ionic character. MCH is
thermodynamically more stable than SP because of the more
efficient stabilization of the cationic species by hydrogen
41
bonding. The increase of the polarity in a series of organic
solvents was also found to promote the formation of the mero-
4
2
cyanine form.
The inset in Figure 1 shows the variation of the absorption
spectrum upon addition of KOH. The gradual disappearance of
the MCH absorption, which is accompanied by the rise of a
band with a maximum at 505 nm, is attributed to the formation of
MC via the deprotonation of the phenolic OH-group of MCH .
+
+
In the presence of CB8, a higher pH was needed to bring about
spectral changes. The negative logarithm of the equilibrium
3. RESULTS AND DISCUSSION
+
constant of the proton dissociation from MCH (pK ) was
a
calculated by nonlinear least-squares fit of the Boltzmann func-
tion (eq 1) to the experimental data.
The Effect of pH Variation. Despite the hydroxyethyl moiety,
which was introduced to enhance hydrophilicity, SP crystals
could not be dissolved in water. However, after storing in the
dark for 2 days in 4.3 mM HCl solution, SP was dissolved,
resulting in a yellow solution, the absorption spectrum of which
A0 ꢀ A∞
A ¼ 1
þ A∞
ð1Þ
þ exp½ðpH ꢀ pKaÞ=Pꢂ
+
corresponded to that of the protonated merocyanine (MCH )
14
where P denotes a fitting parameter, and A
0
and A
∞
are the
form of similar compounds. Acid-promoted thermal transformation
absorbances at low and high pH, respectively. The analysis of the
Scheme 1
pH dependence of the absorbances in Figure 1 provided pK =
a
4
1
.52 in water and 5.60 in CB8 cavity. The acidity diminution of
.08 pH unit upon confinement in CB8 originated from the
+
preferential binding of MCH cation promoted by ionꢀdipole
+
Figure 1. Effect of pH on the normalized absorbance of 6.6 μM MCH
solution in the presence of 77.5 μM CB8 (2) and in water (9). The
inset displays the absorption spectra in water at pH 2.3, 3.6, 4.0, 4.2, 4.9,
and 7.8.
Scheme 2. Reactions in Water
1
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The Journal of Physical Chemistry B
ARTICLE
Figure 2. Variation of absorption and fluorescence in aqueous solution upon addition of CB8. (A) [MC] = 11.8 μM; [CB8] = 0, 2.6, 6.6, 19.9 μM; pH
.1; optical path, 1 cm; inset, absorbance at 531 (2) and 372 nm (9). (B) [MC] = 10.8 μM; [CB8] = 0 and 20 μM; pH 8.1; excitation at 450 nm; inset,
8
+
fluorescence intensity at 580 nm. (C) [MCH ] = 2.8 μM; [CB8] = 0, 1.6, 2.8, and 13.3 μM; pH 3.1; optical path, 5 cm; inset, absorbance at 405 nm. (D)
+
[MCH ] = 24.3 μM; [CB8] = 0, 4.6, 7.7, 12.4, 17.8, and 137 μM; pH 2.3; excitation at 350 nm; inset, fluorescence intensity at 632 nm. The lines in the
insets represent the fitted function.
interactions with the carbonyl groups on the CB8 portal. The
decrease of the positive charge density of the guest upon depro-
tonation weakened the interaction with the partial negative charge
of the carbonyl-fringed portal, leading to smaller stability for the
MCꢀCB8 complex. Above pH 8, MC absorption irreversibly
decreased due to hydrolysis (Scheme 2). As seen in Figure 1,
enhanced resistance against decomposition was found in the
presence of 77.5 μM CB8.
in Figure 2B was fitted with an analogous function. For the
5
equilibrium constant of MC confinement in CB8, K = 1.7 ꢁ 10
ꢀ
1
M
was calculated by the global analysis of the absorption and
fluorescence titrations. When the experiments were performed
+
with MCH solutions at pH 3.1, the intensity of the first absorp-
tion band decreased with growing CB8 concentration and fluo-
rescence quenching accompanied by a blue-shift of the fluorescence
maximum occurred (Figure 2C,D). The analysis of the absor-
6
ꢀ1
Determination of the Equilibrium Constants for the Bind-
bances provided 2.0 ꢁ 10 M for the equilibrium constant of
+
ing to CB8. The results of spectrophotometric and fluorescence
MCH encapsulation in CB8. Unfortunately, the structure of the
+
spectroscopic studies confirmed the encapsulation of MCH and
complexes could not be determined by NMR spectroscopy
because of the low solubility.
MC in CB8. Titration with CB8 at pH 8.1 brought about slight
hypsochromic displacement in the first and a hypochromic effect
in the second absorption bands of MC (Figure 2A). A con-
comitant small rise of the fluorescence intensity and a blue-shift
of the fluorescence maximum were also observed (Figure 2B).
These changes were evidence of MC binding to CB8. The results
could be described well by assuming complex formation with 1:1
stoichiometry. The following function gave the relationship
The fluorescence maximum of the MCꢀCB8 complex had
about 10 nm blue-shift compared to that of MC (Figure 2B) due
to the low polarity of CB8 cavity. Interestingly, the fluorescence
+
spectrum of MCH solution (Figure 2D) corresponded to that of
+
MC. This implies that MCH rapidly loses a proton in the
singlet-excited state because of the acidity enhancement upon
light absorption. Such behavior has been observed for 2- and
44
+
between the absorbance at a particular wavelength (A ) and
3-cyanophenols. When MCH is excited, the increase of CB8
concentration causes fluorescence quenching. The less polar
microenvironment in CB8 decelerates the deprotonation of the
λ
4
3
the total concentration of the host compound ([CB8] ):
0
Aλ ¼ A0
+
complexed MCH in the excited state, allowing more efficient
8
<
competition for radiationless deactivation. As a result, less
singlet-excited MCꢀCB8 complex is produced. Consequently,
significantly weaker fluorescence is emitted in the presence of
CB8 than in water.
A ꢀ A
½CB8ꢂ0
1
∞
0
þ
1 þ
þ
2
:
½Dyeꢂ0
K½Dyeꢂ0
9
2
3
5
!
1=2
+
2
>
=
When the titrations with CB8 are repeated at 24.3 μM MCH
½
CB8ꢂ
1
½CB8ꢂ0
0
4
concentration, the absorbance and fluorescence intensity changes
ꢀ
1 þ
þ
ꢀ 4
ð1Þ
>
;
+
½
Dyeꢂ0
K½Dyeꢂ0
½Dyeꢂ0
level off at equimolar MCH :CB8 solution (Figure 2D), corro-
borating the 1:1 stoichiometry of binding under our experimental
conditions.
Inhibition of Hydrolysis by CB8. The effect of the inclusion
in CB8 on the kinetics of MC hydrolysis was studied at pH 8.5 by
where K represents the binding constant, and A and A denote
∞
0
the absorbance of the fully complexed and free dye, whose initial
concentration is[Dye] . The fluorescence intensitydata presented
0
1
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The Journal of Physical Chemistry B
ARTICLE
Figure 3. Reciprocal MC concentration as a function of reaction time at
pH 8.5 in the case of 0 (9), 0.77 (2), and 1.42 (1) CB8:MC molar
ratios. The lines show the fitted functions.
Figure 5. (A) Relative absorbance variation of 17 μM MC at 505 nm
upon irradiation with 505 nm light in water at pH 7.2 (9) and in the
presence of 20.5 μM CB8 at pH 8.5 (2). Inset: absorption spectrum of
MC and SP in water. (B) Rise of absorbance at 505 nm due to MC
formation from 13 μM SP in the dark in water at pH 7.2 (9) and in
20.5 μM CB8 solution at pH 8.3 (2).
photochemical processes (Figure 5). Irradiation of 17 μM MC
solution at pH 7.2 with 505 nm light brought about a gradual
disappearance of the MC absorption. Complete photoisomeriza-
tion to colorless SP was achieved and the characteristic orange-
red color of MC solution totally vanished (inset in Figure 5A).
The photoinduced reaction proceeded via two main steps. The
conformational transition to cis-MC was followed by ring
Figure 4. Energy-minimized structure of trans-MCꢀCB8 complex.
the detection of the absorption at 505 nm. MC concentration was
calculated using a molar absorption coefficient of ε(505nm) =
ꢀ
1
ꢀ1
+
ꢀ1
ꢀ1
1
8 100 M cm . For MCH , ε(405nm) = 18 400 M cm
45,46
was obtained. As shown in Figure 3, linear correlations appear
between the reciprocal MC concentration and the reaction time.
This indicates that the disappearance of MC follows second-
order kinetics. For the apparent rate constants, k = 1.29, 0.52, and
closure.
For the closely related N-methyl derivative, G €o rner
demonstrated that the transꢀcis photoisomerization occurs
predominantly from the singlet-excited state via an intermediate
47
possessing perpendicular conformation+. A large range of iso-
ꢀ
1 ꢀ1
48
0
.21 M
and 1.42 CB8:MC molar ratios, respectively. The more than
-fold decrease in k values close to 1:1 molar ratio implies strong
s
are obtained from the slopes in the case of 0, 0.77,
mers is possible for the MC and MCH forms. The trans and
cis species in Scheme 2 denote the distributions of different
isomers with dominating trans or cis characters. The two pre-
dominant ground-state isomers probably have transꢀtransꢀcis
and transꢀtransꢀtrans conformations, as suggested for a closely
6
binding of MC to CB8. Inclusion complex formation inhibits the
base-catalyzed hydrolysis of MC. The acceleration of the decom-
ꢀ
49
position with growing pH reflects the important role of OH in
related N-methyl derivative. The structure of these isomers is
the process. Quantum chemical calculations with RM1 semiem-
pirical method using HyperChem 8.0 program give the energy-
minimized structure shown in Figure 4. A similar result is
shown in the Supporting Information. Addition of CB8 in 1.6-
fold excess barely modified the shape of the spectra but sig-
nificantly decelerated the photoinduced SP formation. Under
identical irradiation conditions, the plot of the relative absor-
bance variation as a function of time showed about 3-fold slower
reaction rate for the MCꢀCB8 complex compared to that found
for the free MC. The strong hostꢀguest interactions stabilized
trans-MCꢀCB8 and sterically hindered the isomerization pre-
ceding the formation of the closed SP form.
+
obtained for MCH ꢀCB8 complex. The dimethylindoline moi-
ety is embedded in the cavity of CB8, whereas the alkene moiety
is located in the vicinity of the carbonyl-rimmed portal. The
electrostatic repulsion with the high electron density of the
carbonyl oxygens of the macrocycle hinders the approach of
anions to the alkene moiety. Therefore, the inclusion complex
ꢀ
formation protects the double bond against the OH -catalyzed
The kinetics of the back reaction was affected by CB8 in the
opposite way (Figure 5B). SP reverted to trans-MC in the dark
6.4 times more rapidly inside CB8 macrocycle than in water. The
hydrolysis.
Effect of CB8 on the Kinetics of Photochromic Reactions.
The inclusion in CB8 altered the rate of both the thermal and
ꢀ
4
process followed first-order kinetics with rate constants of 4.9 ꢁ 10
1
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The Journal of Physical Chemistry B
ARTICLE
Figure 6. (A) Relative absorbance change in the course of photolysis
with 410 nm light at pH 2.35 at 0 (9) and 9.8 (2) CB8:MCH molar
Figure 7. Arrhenius plots of the rate constants of the thermal back
reactions in the presence (2) and absence (9) of CB8 at (A) pH 2.7 and
(B) 7.7.
+
ratios. Inset: absorption spectrum of photoproducts in water (thin line+)
and in 126 μM CB8 solution (heavy line). (B) Back formation of MCH
+
in the dark at 0 (9) and 3.6 (2) CB8:MCH molar ratios (pH 2.35).
Table 1. Arrhenius Parameters of the Thermal Back
Reactions
ꢀ
5
ꢀ1
and 7.6 ꢁ 10
s
in the presence of 21 μM CB8 and in water,
+
respectively. The faster reaction in CB8 is probably due to the less
polar microenvironment in the CB8 cavity. The acceleration of the
SP f trans-MC conversion with decreasing solvent polarity has been
SP f trans-MC at pH 7.7 product f trans-MCH at pH 2.7
ꢀ1
ꢀ1
ꢀ1
ꢀ1
A
E /kJ mol
A/s
E
A
/kJ mol
A/s
50
observed for other spirobenzopyrans.
14
9
12
11
in water
in CB8
107
74
5.8 ꢁ 10
96
92
6.3 ꢁ 10
2.2 ꢁ 10
+
When MCH is photolyzed with 410 nm light at pH 2.35, the
product has significantly different absorption spectrum in the
presence of CB8 and in water (inset of Figure 6A), indicating that
3.4 ꢁ 10
+
the initially formed cis-MCH cannot react further in a ring
energies (E ) and pre-exponential factors (A). It is important to
A
closure process within the host. The spectrum of the photo-
product in water corresponds to that of SP, but the large stability
note that E and A values are empirical parameters, which are
A
related only indirectly to the enthalpy and entropy of activation
for the elementary processes. Since all of the photochromic
transformations are multistep reactions, it is not justified to
employ the EyringꢀPolanyi equation of the transition state
+
of the cis-MCH ꢀCB8 complex prevents deprotonation of the
dye inside the host. As a consequence, SPꢀCB8 cannot be
formed because the closure of the spiro ring requires deprotona-
24
+
tion. Since CB8 preferentially binds cationic species, the loss of
theory. At pH 2.7, the cisꢀtrans isomerization of MCH in
+
+
proton is not favored for MCH ꢀCB8, and the photoirradiation
CB8 macrocycle and the transformation of SP to trans-MCH in
+
ꢀ1
results in cis-MCH ꢀCB8 product. In water, the initially formed
water have similar activation energies, 92 and 96 kJ mol for the
+
cis-MCH can lose a proton, allowing spiro ring formation. Since
former and latter processes, respectively. This may indicate that
the opening of the spiro ring has small activation energy and the
cisꢀtrans isomerization is the rate-determining step in water.
The almost 30-fold smaller A factor of the cisꢀtrans isomeriza-
SP is produced via three steps, its formation in water is slower
+
than the simple isomerization to cis-MCH in CB8.
+
Both cis-MCH ꢀCB8 and SP are formed in reversible pro-
+
+
cesses. In the dark, they are converted back to the most stable
tion of MCH in CB8 makes the back formation of trans-MCH
+
MCH conformation in first-order reactions, whose rate
significantly slower in CB8 than in water.
At pH 7.7, the transition from SP to trans-MC is faster in the
macrocycle below 339 K. Above this temperature the reaction is
ꢀ
5
ꢀ5 ꢀ1
constants are 1.4 ꢁ 10 and 8.3 ꢁ 10
s
for the complexed
and free dyes, respectively (Figure 6B).
Temperature Dependence of the Photochromic Reac-
tions. To understand why the confinement in CB8 accelerates
the thermal back reaction in alkaline medium but diminishes its
rate in acidic solution, temperature-dependent measurements
were performed. The Arrhenius plots of the rate constants are
presented in Figure 7, and Table 1 summarizes the activation
more rapid in water. The interactions with CB8 decrease E to
A
1
ꢀ
ꢀ1
74 kJ mol from the 107 kJ mol value in water, and bring about
more than 5 orders of magnitude A factor diminution (Table 1).
Such a big A factor difference implies that the transition states
have less degrees of freedom in CB8 in the case of all elementary
reaction steps. The less polar microenvironment in CB8 may also
1
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The Journal of Physical Chemistry B
ARTICLE
the development of a new generation of photoresponsive materials
with unique properties. The photochromic behavior of the encap-
sulated dye can be adjusted by the variation of pH. The confinement
in CB8 leads to the most substantial decrease in the Arrhenius
parameters of the SP f trans-MC process, but the photoinduced
isomerizations do not show temperature dependence.
’
ASSOCIATED CONTENT
S
Supporting Information. The structure of the transꢀtransꢀ
b
cis and transꢀtransꢀtrans isomers are shown. This information is
available free of charge via the Internet at http://pubs.acs.org.
’
AUTHOR INFORMATION
Corresponding Author
*
Fax: +36-1-438-1143. E-mail: biczok@chemres.hu.
’
ACKNOWLEDGMENT
The authors very much appreciate the support of this work by
the Hungarian Scientific Research Fund (OTKA, Grant K75015).
Figure 8. Effect of temperature variation on the photochemical deco-
loration kinetics in CB8 cavity for 15.7 μM trans-MCH at 410 nm, pH
’
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+
(
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(
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ꢀ
1
14 ꢀ1
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s
was found in
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A
ꢀ1
10 ꢀ1
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(
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2
trans-MC induced by excitation in the lowest-energy absorption
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alike. As representative examples, Figure 8 displays the absor-
bance diminutions during irradiation in CB8 at pH 2.3 and 7.7. It
is apparent that the rate of photodecoloration does not vary with
temperature at low conversion. The effect at longer irradiation
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higher temperatures (vide supra). The photoisomerization prob-
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(
(
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4
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(
1
(
(
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4
. CONCLUSIONS
The encapsulation in a macrocycle affects the photochromic
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behavior of a spirobenzopyran in a totally different manner for
CB8 than previously found for cyclodextrins. In the latter host,
SP is preferentially incorporated, whereas CB8 produces the
most stable complex with MCH form. Inclusion complex
formation with CB8 has three advantageous effects, which help
to solve the problems limiting the applications of spirobenzopyr-
ans in aqueous solution. The encapsulation not only improves
the solubility and stability of the dye but also promotes the tuning
of its photochromic behavior. These benefits and the remarkably
strong binding of spirobenzopyran to cucurbit[8]uril may open
up new possibilities in a wide variety of applications. Cucurbituril-
type molecular containers have a great potential to be used for
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