Photochromism of a merocyanine dye bound to sulfonatocalixarenes: Effect of ph and the size of macrocycle on the kinetics

Article abstract of DOI:10.1021/jp310167j

The effect of 1:1 complex formation on the photochromic behavior of the merocyanine isomer of a nitro-substituted spirobenzopyran dye was studied in aqueous solution using 4-sulfonatocalixarene (SCXn) cavitands possessing four or eight phenol units. The binding constants were independent of the size of the macrocycle, and about 7-fold more stable associates were produced at pH 2.3 than in slightly alkaline solution. The complexation with SCXn diminished the acidity of protonated merocyanine (trans-MCH⁺) and precluded its photoinitiated transition to spirobenzopyran form, but did not affect the reactions in basic media. Upon exposure to light, the complexed trans-MCH ⁺ was converted to cis isomer. The association with 4-sulfonatocalix[4]arene slowed down the thermal back reaction in the dark to a larger extent than the confinement to 4-sulfonatocalix[8]arene. Both the activation energy and the Arrhenius A factor were significantly larger when the smaller, more rigid macrocycle served as a host.

Full text of DOI:10.1021/jp310167j

Article
pubs.acs.org/JPCB
Photochromism of a Merocyanine Dye Bound to
Sulfonatocalixarenes: Effect of pH and the Size of Macrocycle on the
Kinetics
Zsombor Miskolczy and Laszlo Biczok*
́
́
́
Institute of Molecular Pharmacology, Research Centre for Natural Sciences, Hungarian Academy of Sciences, P.O. Box 17, 1525
Budapest, Hungary
ABSTRACT: The effect of 1:1 complex formation on the photochromic behavior of
the merocyanine isomer of a nitro-substituted spirobenzopyran dye was studied in
aqueous solution using 4-sulfonatocalixarene (SCXn) cavitands possessing four or
eight phenol units. The binding constants were independent of the size of the
macrocycle, and about 7-fold more stable associates were produced at pH 2.3 than in
slightly alkaline solution. The complexation with SCXn diminished the acidity of
protonated merocyanine (trans-MCH+) and precluded its photoinitiated transition to
spirobenzopyran form, but did not affect the reactions in basic media. Upon exposure
to light, the complexed trans-MCH+ was converted to cis isomer. The association
with 4-sulfonatocalix[4]arene slowed down the thermal back reaction in the dark to a
larger extent than the confinement to 4-sulfonatocalix[8]arene. Both the activation
energy and the Arrhenius A factor were significantly larger when the smaller, more rigid macrocycle served as a host.
solution,27 but complexation of the merocyanine form took
1. INTRODUCTION
place with γ-cyclodextrins in water−dimethy sulfoxide
The reversible interconversion between spirobenzopyrans and
mixtures.9 We have shown that both merocyanine and its
merocyanines has been extensively studied due to its numerous
protonated form produce a very stable inclusion complex with
applications.1−5 The equilibrium between the two photo-
cucurbit[8]uril cavitand leading to enhanced chemical stability,
chromic forms is strongly solvent- and substituent-depend-
improved solubility, and significantly altered photochromic
ent.6,7 Introduction of a nitro group in the 6-position of the
behavior.28 Using the smaller cucurbit[7]uril macrocycle, a
benzopyran moiety shifts the equilibrium from the spiro toward
the merocyanine isomer. Because of its large dipole moment,8,9
preferential binding of the protonated merocyanine was
observed, and the photoinduced transformation of this species
the colored planar merocyanine form is stabilized by hydrogen
to the spiropyran isomer was selectively accelerated.29 As a
bonding and dipole−dipole interactions in polar solvent,10
whereas the closed colorless spiro form is usually thermody-
continuation of these studies, now, we extend the investigations
to 4-sulfonatocalixarenes (SCXn), the highly water-soluble
namically more stable in apolar medium. Spiropyrans have been
used for optical sensing of metal ions,11−13 anions,14,15 and
macrocycles that are versatile building blocks in supramolecular
chemistry.30 Their π-electron-rich, flexible cavity composed of n
amino acids.16 The selective association between the
4-hydroxy-benzenesulfonate units linked by methylene groups
membrane-bound merocyanine form with zwitterionic amino
can encapsulate a wide variety of organic compounds and
acid permits of the photocontrolled transfer of amino acid from
cations.31−34 We focus on the confinement of the photo-
the aqueous phase into the liposomal bilayer.17 The entirely
chromic forms of N-(2-hydroxyethyl)-3′,3′-dimethyl-6-nitro-
different binding properties of the two photochromic forms
were exploited to control the association with DNA by light.18
Reversible photoswitchable aptamer recognition was achieved
by the selective binding of spiropyran to RNA oligonucleo-
tide,19 and photocontrol of enzyme activity was accom-
plished.20 A spiropyran derivative was developed that had no
significant effect on the cellular survival, but the intracellular
photoisomerization to the merocyanine form induced a
dramatic toxic response.21
The low solubility of the majority of photochromic
compounds in water was circumvented by confinement in
self-assembled systems, such as micelles,22,23 vesicles,24 and bile
salt aggregates.25 Cationic derivatives were synthesized to study
the effects of inclusion in cucurbit[7]uril.26 Spiropyrans showed
little tendency for encapsulation in cyclodextrins in aqueous
spiro[2H-1-benzopyran-2,2′-indoline], which can be applied in
molecule-based logic circuits, optical signal switches, and
molecular machines.35−38 Our main goal is to reveal how the
cavity size and pH influence the kinetics of the merocyanine−
spiropyran interconversion in flexible anionic SCXn cavitands
and the stability of the complexes. The studied reactions and
the formula of the employed calixarenes are shown in Scheme
1. Several merocyanine isomers with dominating trans or cis
characters exist in solution.39,40 Scheme 1 displays the most
probable trans and cis conformers participating in the
Received: October 15, 2012
Revised: November 26, 2012
Published: December 27, 2012
© 2012 American Chemical Society
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Article
Scheme 1. Studied Reactions and Formula of 4-Sulfonatocalix[n]arenes
photochromic reactions, as suggested by quantum chemical
calculations.41
2. EXPERIMENTAL SECTION
N-(2-Hydroxyethyl)-3′,3′-dimethyl-6-nitro-spiro[2H-1-benzo-
pyran-2,2′-indoline] (SP) (TCI) was used without further
purification. Aqueous solutions of MCH+ and MC were
prepared from SP as described previously.28 4-
Sulfonatocalix[4]arene (SCX4) (Fluka) and 4-
sulfonatocalix[8]arene (SCX8) (Acros Organics) held 1:9 and
1:21 stoichiometric amounts of water in their crystal
structures.42 The pH of the 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 UV−vis 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 spectrofluorometer. The samples were photolyzed
using a 150 W xenon lamp and monochromator in a 1 cm × 0.4
cm quartz cell. The whole solutions were exposed to light. The
temperature was controlled with a Julabo thermostat. The
experimental data were analyzed by the ORIGINPRO8
software.
Figure 1. Variation of the normalized absorbance of 0.016 mM trans-
MCH+ solution at 505 nm as a function of pH in the presence of 0.77
■
●
▲
mM SCX4 ( ), 0.79 mM SCX8 ( ), and in water ( ). The lines
represent the results of the nonlinear least-squares fit of eq 1 to the
experimental data. Inset displays the absorption spectra in 0.77 mM
SCX4 solution at pH 2.84 (thick line) and 7.38 (thin line).
A0 − A∞
1 + exp[(pH − pKa)/P]
A =
+ A∞
(1)
where pKa represents the negative logarithm of the equilibrium
constant of the proton dissociation from trans-MCH+, P = 1/ln
10, whereas A0 and A∞ are the absorbances at low and high
pHs, respectively. A marked shift of the titration curve toward
less acidic range was observed in the presence of SCX4, while
the bulkier SCX8 macrocycle brought about even larger
displacement. The pKa values were found to be 4.52, 5.35,
and 6.02 in water, SCX4, and SCX8 complex, respectively. The
estimated uncertainty of the calculated pKa values is 0.04. The
interaction with the anionic hosts impeded the proton loss of
trans-MCH+ because this cationic form had larger binding
affinity than the uncharged trans-MC. The larger negative
charge of SCX8 hindered the proton removal more efficiently
bringing about thereby more significant pKa enhancement than
the complexation with SCX4.
3. RESULTS AND DISCUSSION
3.1. Effect of SCXn on the Acidity of trans-MCH+. In the
ground state, trans-MCH+ is the thermodynamically most
stable photochromic form in acidic aqueous solution below pH
3 due to its significant stabilization by interaction with solvent
molecules.10 Gradual addition of KOH leads to color change
from yellow to orange-red indicating that the deprotonation of
the phenolic OH group produces trans-MC. In this species, the
lone electron pair of the negatively charged oxygen participates
in the extended conjugation in the ground state but the dipole
moment substantially diminishes in the first singlet-excited
state.9 Figure 1 displays the emergence of trans-MC absorbance
at 505 nm as a function of pH. As a representative example, the
inset shows the absorption spectra in the presence of 0.77 mM
SCX4 at pH 2.84 and 7.38. The experimental results were
analyzed by nonlinear least-squares fit of the following function:
It is worth noting that the interaction with SCXn did not
alter the kinetics of trans-MC decomposition, the process
taking place above pH 8.29,43 This is in sharp contrast to the
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Article
behavior of the dye in cucurbit[8]uril cavity, where the
nucleophilic addition of OH− to trans-MC is inhibited by the
considerable negative charge density of the carbonyl-rimmed
portal of the host.28 The more open structure of SCXn cannot
provide protection against the attack of OH− to the 2-position
of the heterocyclic ring of trans-MC.
3.2. Determination of the Binding Constants. The
sulfonic acid moieties of SCXn are completely dissociated
above pH 0.4. The pKa’s of the gradual deprotonation of the
phenolic OH groups are reported to be 3.44, 4.26, 7.78, and
10.3 for SCX8, whereas they are 3.28 and 11.5 for SCX4 in the
presence of 0.1 M NaCl.44 Somewhat larger pKa values were
found at lower ionic strength.44 We examined the complexation
of trans-MCH+ at pH 2.3 to ensure that only the sulfonic acid
moieties of the hosts lose protons. The studies with trans-MC
were performed at pH 7.6−8.0, where only one OH at the
lower rim of SCX4 was deprotonated and the majority of SCX8
macrocycles had two phenolate groups.
transfer from SCXn to the singlet-excited guest usually results
in efficient quenching. Such a process cannot occur in excited
trans-MCH+−SCX8, because (i) electron transfer cannot
compete with the fast photoinduced deprotonation generating
singlet-excited trans-MC−SCX8 and (ii) the dye in the latter
species is a weak electron acceptor due to its uncharged
character and low excited-state energy.
The change of the fluorescence intensity (I) with the total
SCX8 concentration ([SCX8]0) could be described well by the
function derived for 1:1 binding with an equilibrium constant
K:47
⎧
⎪
⎨
I∞ − I0
[SCX8]0
[dye]0
1
I = I0 +
1 +
+
2
K[dye]0
⎪
⎩
1/2
⎫
⎪
⎬
2
⎡
⎤
⎛
⎞
[SCX8]0
[dye]0
[SCX8]0
[dye]0
1
⎢
⎥
−
1 +
+
− 4
⎜
⎝
⎟
⎠
⎢
⎥
K[dye]0
⎪
⎭
⎣
⎦
Figure 2 shows the absorbance and fluorescence intensity
variations as a function of SCX8 concentration at pH 2.3,
(2)
where [dye]0 is the initial dye concentration and I∞ and I0
stand for the fluorescence intensities of the complexed and free
dyes. The global nonlinear least-squares analysis of the
fluorescence data provided K = (2.2 0.3) × 104 M−1. Similar
treatment of the absorbance data in the 300−500 nm range
resulted in K = (2.5 0.5) × 104 M−1. Considering the slight
absorbance alteration upon trans-MCH+−SCX8 formation
(Figure 2A), the agreement between the K values derived
from fluorescence and spectrophotometric titrations are fair,
and the fluorescence titration is deemed more accurate method.
The nonionic trans-MC had considerably lower binding
affinity to SCX8 as seen from the association constants listed in
Table 1. The absorption spectrum of trans-MC at pH 7.6 did
Table 1. Equilibrium Constants of Complex Formation of
the Photochromic Species with SCXn Derived from
Fluorescence Titrations
K/103 M−1
SCX8
SCX4
MCH+
22 0.3
25 0.5
3.2 0.4
25 0.3
a
MC
3.5 0.4
Figure 2. (A) Absorbance change of 0.012 mM trans-MCH+ at 390
nm and (B) fluorescence enhancement in 0.0065 mM trans-MCH+
aqueous solution at 632 nm at pH 2.3 upon addition of SCX8. The
lines display the results of the global nonlinear least-squares analysis
(see text). Insets: (A) absorption spectra in the presence of 0 and 0.20
mM SCX8; (B) fluorescence spectra in the presence of 0 and 0.75 mM
SCX8 (excitation at 428 nm).
a
From absorbance change.
not alter even in the presence of 1 mM SCX8. However, the
fluorescence intensity slightly diminished owing to trans-MC−
SCX8 complex formation (Figure 3). Dynamic fluorescence
quenching can be excluded because the fluorescence lifetime of
trans-MC is shorter than 100 ps in water.29
whereas the insets depict the spectra of the uncomplexed and
bound trans-MCH+. The slight hypochromicity and the
bathochromic shift of the absorption maximum from 406 to
410 nm are evidence for complex formation. The considerable
displacement of the fluorescence maximum relative to the
absorption band is attributed to the rapid proton release from
trans-MCH+ in the singlet-excited state since the spectrum
corresponds to that found upon excitation of trans-MC (vide
infra). Complex formation with SCX8 produces negligible
change in the shape and position of the fluorescence band, but
causes 36% fluorescence intensity enhancement. Fluorescence
increase upon inclusion in SCXn is rare45,46 because electron
When the smaller SCX4 served as a host, the binding of
neither trans-MCH+ nor trans-MC caused absorbance change,
but the intensity of the fluorescence band decreased (Figure 4).
The similar fluorescence spectra at pH 2.3 and 7.6 demonstrate
the rapid photoinduced deprotonation in trans-MCH+−SCX4.
The binding constants (K) derived from the nonlinear least-
squares fit of eq 2 to the experimental data are given in Table 1.
Surprisingly, the K values are practically independent of the
cavity size of the host. This suggests that the photochromic dye
is not embedded deeply in the calixarene macrocycle. The
larger stability of trans-MCH+−SCXn complexes compared to
that of trans-MC−SCXn associates indicates that electrostatic
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Figure 5. (A) Relative absorbance variation of 0.017 mM trans-MCH+
during irradiation with 410 nm light in the absence of additive ( ) and
Figure 3. Fluorescence intensity diminution of 0.0065 mM trans-MC
at 632 nm as a function of SCX8 concentration (excitation at 470 nm,
pH 7.6). The line shows the result of the nonlinear least-squares fit of
eq 2 to the experimental data. Inset: fluorescence spectra in the
presence of 0 (thin line) and 0.77 mM SCX8 (thick line).
■
▲
●
in the presence of 0.81 mM SCX8 ( ) and 0.85 mM SCX4 ( ) at pH
2.3. The lines were obtained by linear least-squares fit. Inset:
absorption spectra at pH 2.3 before (1) and after photolysis (2) of
0.050 mM trans-MCH+ and 0.37 mM SCX8 solution (optical path 4
mm). SP absorption spectrum in water (3).
that the photoproduct is not SP. The shape and maximum of
the absorption band of the photoproduct are analogous to
those of the cis-MCH+−cucurbit[8]uril inclusion complex.28
Thus, we can conclude that trans-MCH+−SCXn undergoes
photoisomerization leading to cis-MCH+−SCXn. The inter-
action with SCXn blocks the deprotonation of cis-MCH+ just
like the inclusion in the cucurbit[8]uril cavity. Since the loss of
proton is indispensable for the closure of the spiro ring
(Scheme 1), the complexed cis-MCH+ cannot be transformed
into SP. The trans−cis photoisomerization was followed by
absorbance measurements at 406 nm. The initial reaction rate
was independent of temperature, but markedly differed for
SCX4 and SCX8 complexes. As seen in Figure 5, the
association with the smaller macrocycle accelerated the
photoisomerization by ca. 44%, whereas the reactions of
SCX8-bound and free trans-MCH+ had practically the same
kinetics. For the closely related N-methyl derivative, Gorner
̈
demonstrated that the trans−cis photoisomerization takes place
predominantly from the singlet-excited state via an intermediate
possessing perpendicular conformation.48
The thermal back isomerization from cis- to trans-MCH+ in
SCXn complex followed first-order kinetics (inset to Figure 6).
The rise of absorbance (A) at 410 nm was analyzed with the
relationship:
Figure 4. Effect of association with SCX4 on the fluorescence intensity
alteration at 632 nm in photochromic dye solutions at pH 2.3 (λexcitation
= 428 nm) (A) and pH 7.8 (λexcitation = 502 nm) (B). The lines give the
results of the nonlinear least-squares fit of eq 2 to the plotted
experimental data. Insets: fluorescence spectra for uncomplexed dye
(thin line) and SCX4 complex (thick line) at pH 2.3 (A) and 7.8 (B).
A = A∞ − (A∞ − A0)e−kt
(3)
where t denotes the reaction time and A0 and A∞ are the initial
and final absorbances. Nonlinear least-squares fit of the
experimental results at 298 K gave k = (1.8
forces between host and guest significantly contribute to the
binding affinity. The low solubility of the dye thwarted the
determination of the structure of the complexes by NMR
spectroscopy.
3.3. Photochromic Reactions of SCXn Complexes at
pH 2.3. The yellow color of trans-MCH+−SCXn solutions
gradually fades during irradiation with 410 nm light. As a
representative example, the inset to Figure 5 presents the
absorption spectrum before and after photolysis in the case of
SCX8 host. Analogous spectra are recorded using SCX4. For
the sake of comparison, the absorption of the spiropyran (SP)
form, the species produced from free trans-MCH+ upon
exposure to light, is also displayed. The spectra clearly show
0.1) × 10−5
and (9.8 0.5) × 10−6 s−1 for the rate constant of the back
reaction from cis-MCH+−SCX8 and cis-MCH+−SCX4, respec-
tively. To gain more insight into the effects controlling the
kinetics of isomerization, temperature-dependent measure-
ments were carried out. Figure 6 presents the Arrhenius plots
of the rate constant of cis−trans isomerization in SCXn
complex and, for the sake of comparison, the rate constant of
the transformation of free SP to trans-MCH+. The calculated
pre-exponential factors (A) and activation energies (EA),
obtained by nonlinear least-squares fit, are summarized in
Table 2. Since the back reactions to trans-MCH+ are not
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Figure 6. Arrhenius plot of the rate constants of the thermal back
●
▲
reactions in the presence of 0.85 mM SCX4 ( ), 0.81 mM SCX8 ( ),
■
and in the absence of additive ( ) at pH 2.3. Inset: representative
example of the absorbance rise at 410 nm due to back formation of
trans-MCH+−SCX8 in the dark at 298 K and the fitted line according
to eq 3 (pH 2.3).
Figure 7. (A) Relative absorbance variation of 0.013 mM trans-MC at
506 nm during irradiation light above 480 nm in the absence of
Table 2. Arrhenius Parameters of the Thermal Back
Reactions from the Photoproduct to trans-MCH+ at pH 2.3
■
●
▲
additive ( ) and in the presence of 0.83 mM SCX8 ( ) and 0.78 mM
SCX4 ( ) at pH 8.0. Inset: absorption spectra at pH 8.0 before (thin
photoproduct
EA/kJ mol−1
A/s−1
line) and after (thick line) photolysis in the presence of SCX8. (B)
Arrhenius plot of the rate constants of SP transformation to trans-MC
uncomplexed SP
cis-MCH+−SCX8
cis-MCH+−SCX4
96
116
124
2
2
2
(6 3) × 1012
(4 2) × 1015
(6 2) × 1016
●
▲
in the dark in 0.87 mM SCX4 ( ) and 0.82 mM SCX8 ( ) solutions
■
and in the absence of additive ( ) at pH 8.0.
variation in the course of photoreaction at 506 nm. The gradual
disappearance of MC has the same kinetics in water and in
SCXn complex. The initial rate of the reactions does not vary
with temperature.
elementary processes, A and EA should be considered as
empirical parameters. Despite the fact that the opening of the
spiro ring precedes the isomerization in the absence of SCXn,
lowest EA is found in neat water. Quantum chemical
calculations demonstrated that the cleavage of the spiro ring
in water requires much lower activation energy than cis−trans
isomerization.41 The binding to SCX8 augments EA by 20 kJ
mol−1, whereas a further 8 kJ mol−1 EA increase takes place for
the SCX4 complex. The association with macrocycles impedes
cis−trans isomerization in the ground state due to steric
reasons. The more rigid SCX4 represents larger steric
hindrance than the conformationally mobile SCX8, which can
easily adapt itself to the geometrical features of the guest. Thus,
more energy is required to initiate conformation change when
cis-MCH+ is bound to SCX4. The Arrhenius A factor exhibits
parallel increase with EA. The large A factors of the cis-MCH+−
SCXn → trans-MCH+−SCXn reactions imply substantial
positive activation entropy. The isomerization goes through a
transition state in which the components are looser bound than
in the initial complex. The reaction of SCX4 complex possesses
considerably larger A factor than that of SCX8 because a more
substantial increase in the degrees of freedom is feasible upon
passage to the transition state when the process starts from the
complex of the more rigid SCX4 macrocycle.
The thermal back formation of trans-MC from SP is a first-
order process with cis-MC intermediate. The Arrhenius plot of
the rate constants determined at various temperatures is
presented in Figure 7B. In contrast to the findings at pH 2.3,
negligible difference is obtained among the Arrhenius
parameters of SP → trans-MC transformation in water and in
SCXn complexes. The EA = 107 9 kJ mol−1 and A = (5.8
1.7) × 1014 s−1 values of this process are significantly smaller
than the corresponding parameters for the cis-MCH+−SCXn →
trans-MCH+−SCXn isomerization listed in Table 2. The
difference indicates that the looser binding of the uncharged
SP and cis-MC to SCXn hinders the structural alteration to a
lesser extent and, consequently, less activation energy is needed
for the isomerization. Moreover, the weaker host−guest
interactions restrict the movement of the constituents to a
lesser extent. Therefore, less entropy gain is attained in the
transition states when the structural rearrangement starts from
the looser-bound SP−SCXn complex compared to the back
isomerization of the stronger-bound cis-MCH+−SCXn.
4. CONCLUSIONS
3.4. Photochromic Reactions of SCXn Complexes at
pH 8.0. When trans-MC−SCXn is exposed to light above 480
nm at pH 8.0, the orange-red color of the solution vanishes.
The absorption spectrum of the photoproduct agrees with that
obtained in the photolysis of free trans-MC, which is known to
generate SP. The weak binding of the dye to SCXn (vide supra)
does not prevent the closure of the spiro ring after trans−cis
photoisomerization. Figure 7A displays the spectra before and
after light exposure together with the relative absorbance
The binding to the highly negative flexible SCXn cavitands
changes the photochromic characteristics completely differently
from that observed previously for cucurbiturils. In the case of
the latter hosts, the equilibrium constant of 1:1 complex
formation of trans-MCH+ significantly increases with growing
cavity size, but the strength of the association with SCXn is
found to be independent of the number of phenol units in the
macrocycle. This suggests that trans-MCH+ is not included
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the stabilization of trans-MCH+−SCXn. The considerably
lower binding affinity of uncharged trans-MC indicates the
importance of electrostatic forces in trans-MCH+−SCXn
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AUTHOR INFORMATION
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(31) Guo, D.-S.; Wang, K.; Liu, Y. J. Inclusion Phenom. Macrocyclic
Chem. 2008, 62, 1−21.
Corresponding Author
*Fax: +36-1-438-1143. E-mail: biczok.laszlo@ttk.mta.hu.
(32) Megyesi, M.; Biczok
(33) Miskolczy, Z.; Biczok
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Notes
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The authors declare no competing financial interest.
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(35) Raymo, F. M.; Giordani, S. J. Am. Chem. Soc. 2001, 123, 4651−
ACKNOWLEDGMENTS
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4652.
The authors very much appreciate the support of this work by
the Hungarian Scientific Research Fund (OTKA, Grant
K104201).
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