Spiropyran as a reusable chemosensor for selective colorimetric detection of aromatic thiols

Article abstract of DOI:10.1039/c3cp55478c

Design of optical molecular probes for selective detection of aromatic thiols has attracted much attention. Although several types of probes have been proposed, all of them exhibit colorimetric or fluorometric response via irreversible reaction with aromatic thiols and cannot be reused. Here we report that a spiropyran dye is the first example of a reusable chemosensor for aromatic thiols. A colorless spiropyran dye (1) dissolved in aqueous media containing aromatic thiols is selectively isomerized to the colored merocyanine form in the dark. In contrast, visible light irradiation of the merocyanine form promotes successful reversion to the colorless spirocyclic form. Kinetic absorption analysis and ab initio calculations of the transition states revealed that this colorimetric response in the dark is ascribed to the decrease in activation energy for isomerization via the nucleophilic interaction between the aromatic thiol and the olefinic carbon of the dye.

Full text of DOI:10.1039/c3cp55478c

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PAPER
Yasuhiro Shiraishi et al.
Spiropyran as a reusable chemosensor for selective colorimetric
detection of aromatic thiols

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Spiropyran as a reusable chemosensor for
selective colorimetric detection of aromatic
thiols†
Cite this: Phys.Chem.Chem.Phys.,
2014, 16, 12137
Yasuhiro Shiraishi,* Kohei Yamamoto, Shigehiro Sumiya and Takayuki Hirai
Design of optical molecular probes for selective detection of aromatic thiols has attracted much attention.
Although several types of probes have been proposed, all of them exhibit colorimetric or fluorometric
response via irreversible reaction with aromatic thiols and cannot be reused. Here we report that a
spiropyran dye is the first example of a reusable chemosensor for aromatic thiols. A colorless spiropyran
dye (1) dissolved in aqueous media containing aromatic thiols is selectively isomerized to the colored
merocyanine form in the dark. In contrast, visible light irradiation of the merocyanine form promotes
successful reversion to the colorless spirocyclic form. Kinetic absorption analysis and ab initio calculations
of the transition states revealed that this colorimetric response in the dark is ascribed to the decrease in
activation energy for isomerization via the nucleophilic interaction between the aromatic thiol and the
olefinic carbon of the dye.
Received 28th December 2013,
Accepted 19th February 2014
DOI: 10.1039/c3cp55478c
www.rsc.org/pccp
that facilitate selective detection of aromatic thiols based on a
reversible interaction is therefore necessary.
Introduction
Thiols are a very important class of molecules in biological
Spiropyran dyes belong to a class of organic photochromes,
systems. Aliphatic thiols are a part of several biological molecules which undergo ring opening/closing isomerization between the
such as cysteine, homocysteine, and glutathione, which are colorless spirocyclic (SP) form and the colored merocyanine
associated with a variety of biologically important functions.1–3 (MC) form upon UV/visible light irradiation.14–16 In the dark,
In contrast, aromatic thiols, which are versatile intermediates for SP - MC isomerization does not occur in common organic
organic synthesis, are very toxic to the human body. Exposure to solvents because the ground state energy of the MC form is
them induces serious damage to the central nervous system and higher than that of the SP form. In contrast, hydrogen-bonding
other related pathologies including increased respiration, muscular solvents such as water and alcohols promote spontaneous
weakness, paralysis, and coma.4–7 The design of optical molecular SP - MC isomerization because stabilization of the MC form
probes, which facilitate selective detection of aromatic thiols by by a hydrogen-bonding interaction lowers the ground state energy
simple spectroanalysis in the environmental and biological samples, of the MC form,17–19 although the isomerization is very slow.
is therefore currently the focus of attention.
Here we report that a spiropyran dye is the first example
A number of optical molecular probes for thiols have been of a reusable chemosensor for aromatic thiols. As shown in
proposed so far; however, there are only a few reports of probes Scheme 1, aromatic thiols, when added to an aqueous solution
that can discriminate aromatic thiols from aliphatic ones.8–13 with a neutral pH containing a simple spiropyran dye (1),
All of the probes are based on nucleophilic reaction of sulfon- selectively enhance SP - MC isomerization, thus facilitating
amide, sulfonate, or the ether group with a thiolate anion of selective colorimetric detection of aromatic thiols. Kinetic
aromatic thiols, resulting in colorimetric or fluorometric absorption analysis and ab initio calculations revealed that
response. All of these reactions, however, occur irreversibly;
these probes cannot be reused. Creation of molecular probes
Research Center for Solar Energy Chemistry, and Division of Chemical Engineering,
Graduate School of Engineering Science, Osaka University, Toyonaka 560-8531,
Japan. E-mail: shiraish@cheng.es.osaka-u.ac.jp; Fax: +81 6 6850 6273;
Tel: +81 6 6850 6271
† Electronic supplementary information (ESI) available: Details of equilibrium
Scheme 1 SP - MC isomerization of a spiropyran dye (1) promoted by
aromatic thiols in aqueous media in the dark.
and kinetic analysis and supplementary data (Tables S1 and S2, Fig. S1–S7, and
Cartesian coordinates). See DOI: 10.1039/c3cp55478c
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the SP - MC isomerization enhanced by aromatic thiols is due
to the decrease in activation energy for isomerization, via the
nucleophilic interaction between the aromatic thiol and the
olefinic carbon of the spiropyran molecule.
Results and discussion
Fig. 1b shows the time-dependent change in absorption spectra
of 120 (30 mM), when stirred in a water–MeCN (7/3 v/v) mixture
(HEPES 0.1 M, pH 7.0) at 25 1C in the dark. At time zero, almost
no absorption appears at l 4 450 nm, indicating that 1 exists
in SP form. As time advances, a distinctive band assigned to the
MC form appears at 512 nm, although its increase is very slow.
In contrast, as shown in Fig. 1a, 30 equiv. of benzenethiol (2a),
Fig. 2 Absorbance at 512 nm of a buffered water–MeCN mixture (7/3 v/v;
HEPES 0.1 M, pH 7.0) containing 1 (30 mM), measured after stirring with
respective nucleophiles (30 equiv.) for 60 min at 25 1C in the dark.
when added to the solution containing 1, significantly enhance
the SP - MC isomerization. As shown in Fig. 2, substituted
benzenethiols (2b–d) also accelerate the isomerization, whereas
other nucleophiles such as aliphatic thiols (3a–d), aniline,
phenol, IÀ, CNÀ, AcOÀ, NO3À, and H2PO4À do not. In addition,
as shown in Fig. S1 (ESI†), the SP - MC isomerization of 1
enhanced by aromatic thiol is unaffected by other nucleophiles.
These data clearly suggest that 1 allows selective colorimetric
detection of aromatic thiols in aqueous media.
After stirring the solution containing 1 and 2a, the solution
was acidified (pH 3) and extracted with CHCl3. GC analysis of
the extract revealed that all of 2a added remained unchanged.
In addition, as shown in Fig. S2 (ESI†), the solution after
stirring with 1 and 2a, when irradiated with monochromatic
light at 550 nm, leads to successful reversion to the SP form.
Furthermore, repeated stirring under the dark and 550 nm light
irradiation conditions shows reversible formation of the MC
and SP forms. These data suggest that 1 reversibly interacts
with aromatic thiol and undergoes rapid SP - MC isomerization.
Fig. 1c shows the effect of the amount of 2a added on the rate of
isomerization of 1. The isomerization rate increases with an
increase in the amount of 2a. These data suggest that aromatic
thiols catalytically accelerate the SP - MC isomerization of 1.
Fig. 1d shows the change in the ratio of absorbance (A/A0) of 1 at
512 nm with the concentration of 2a, where A0 is the absorbance
in the absence of 2a and the data were obtained after stirring each
respective solution for 60 min. A strictly linear relationship is
observed at o240 mM 2a. The 2a concentration at A/A0 4 1.5 is
40 mM. This indicates that the present colorimetric system allows
quantitative detection of 2a in the range of 40–240 mM, although
the system requires a relatively long time for sensing (1 h).
Aromatic thiols do not affect the equilibrium of the SP and
MC forms. This is confirmed by the standard enthalpy (DrH)
and entropy (DrS) for SP - MC isomerization of 1, determined
by the equilibrium absorption analysis with different amounts of
2a at different temperatures (Fig. S3, ESI†).17 Table 1 summarizes
the data obtained at 25 1C. The equilibrium constants (Keq) for the
SP and MC forms are ca. 3 regardless of the absence and presence
of 2a. In addition, van’t Hoff plots of the equilibrium data (Fig. S4,
ESI†) revealed that the DrH values for all of the systems are similar
Fig. 1 Time-dependent change in the absorption spectra of 1 (30 mM)
measured in the (a) presence and (b) absence of 30 equiv. of benzenethiol
(2a) in a buffered water–MeCN mixture (7/3 v/v; HEPES 0.1 M, pH 7.0) in
the dark at 25 1C. (c) Change in absorbance at 512 nm monitored after
addition of different amounts of 2a relative to that of 1 (equiv.). (d) Change
in the ratio of absorbance (A/A0) at 512 nm with the concentration of 2a,
where A0 is the absorbance in the absence of 2a. All of the data were
obtained after stirring each respective solution for 60 min in the presence
of the required amount of 2a.
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Table 1 Equilibrium and kinetic parameters for SP - MC isomerization of 1 in a water–MeCN mixture (7/3 v/v; HEPES 0.1 M, pH 7.0), determined in the
dark at 25 1C with different amounts of 2aa
2ab/equiv.
Keq
DrH/kJ molÀ1
DrS/J KÀ1 molÀ1
kSP-MC/10À4 sÀ1
DHa/kJ molÀ1
DSa/J KÀ1 molÀ1
0
1
2
10
30
2.95
3.12
3.09
3.07
3.05
À6.61 Æ 0.34
À6.61 Æ 0.79
À6.74 Æ 0.54
À6.73 Æ 0.30
À6.68 Æ 0.40
À13.2 Æ 1.0
À12.8 Æ 2.6
À13.2 Æ 1.8
À13.2 Æ 1.0
À13.1 Æ 1.3
0.52
0.62
0.72
3.92
7.38
107.7 Æ 2.9
87.0 Æ 3.6
74.8 Æ 1.3
45.6 Æ 3.5
44.0 Æ 4.7
32.5 Æ 9.1
À33.6 Æ 12.1
À73.3 Æ 4.6
À157.0 Æ 11.9
À157.6 Æ 15.7
a
The detailed procedures for equilibrium and kinetic absorption analysis are described in ESI. The data obtained at different temperatures are
b
summarized in Table S1 (ESI). The amount of 2a added relative to that of 1.
MC form with a TTC structure via the TS3 transition state.
During the isomerization process, the reaction (ii) is the rate-
determining step.22,23 This is confirmed by ab initio calcula-
tions based on the density functional theory (DFT) within the
Gaussian 09 program. Geometry optimizations of the SP form
of 1, intermediates, and the MC form of 1 were performed using
the B3LYP function with the 6-31G* basis set, where the
polarizable continuum model (PCM) was employed with water
as a solvent.24 The transition states were optimized using QST2
and QST3 methods, where the nature of stationary points was
checked by means of frequency calculations, and the transition
Fig. 3 Standard enthalpy (DrH) and activation enthalpy (DHa) for SP - MC
isomerization of 1 in the absence and presence of 30 equiv. of 2a.
states were verified by the intrinsic reaction coordinate (IRC)
calculations.25 Fig. 4 (black) summarizes the optimized structures
of intermediates and transition states on the ground state potential
(ca. À6.6 kJ molÀ1). These data suggest that, as shown in Fig. 3,
surface, along with the relative energies with respect to the SP form
of 1. Comparison of the transition energies for TS1, TS2, and TS3
clearly revealed that TS2 (step ii) exhibits the highest transition
energy (99.8 kJ molÀ1) and is indeed the rate-determining step for
the ground state energy of the MC form scarcely changes even
in the presence of 2a. Furthermore, the DrS values for all of the
systems show similar negative values (ca. À13 J KÀ1 molÀ1) due
to the rearrangement of the solvent molecules associated with
the SP - MC isomerization.21 These findings clearly suggest
that aromatic thiols do not affect the equilibrium of the SP and
MC forms.
SP - MC isomerization of 1. This suggests that, as reported,22,23
a
high rotational barrier around the C2QC3 bond results in very high
transition energy. As a result of this, the compound 1 undergoes
very slow SP - MC isomerization (Fig. 1).
Aromatic thiols exist as thiolate anions at a neutral pH.26
They have a strong nucleophilic character27 and, hence,
undergo nucleophilic addition to the electron-deficient carbon
atom for several types of molecules such as a,b-unsaturated
ketones,13,28 fullerenes,29 and imines.30 In the present case, as
shown in Scheme 2, the thiolate anion undergoes nucleophilic
addition to the electron-deficient C3 atom of the ring-opened
CCC intermediate of 1. This transfers the double bond and
increases the rotational mobility, resulting in a decrease in
activation energy for isomerization. The isomerization process
of 1 enhanced by aromatic thiolate (2a) involves following
reactions (Scheme 2): firstly, (iv) nucleophilic addition of 2a
to the C3 atom of the CCC intermediate produces the CCC-2a
intermediate via the TS4 transition state. According to this
reaction, the C1–C2QC3 bond becomes C1QC2–C3. Secondly, (v)
cis - trans isomerization around the single C2–C3 and C3–C4
bonds produces the CTT-2a intermediate via the TS5 transition
state. Finally, (vi) elimination of the 2a moiety followed by
cis - trans rotation of the resulting single C1–C2 and C3–C4
bonds produces the MC form with a TTC structure via the TS6
transition state.
The SP - MC isomerization of 1 accelerated by aromatic
thiols is due to the decrease in activation energy. This is
confirmed by the kinetic absorption analysis18 for SP - MC
isomerization of 1, performed with different amounts of 2a at
different temperatures (Fig. S5, ESI†). As shown in Table 1, the
rate of SP - MC isomerization (kSP-MC) increases with an
increase in the amount of 2a added. The activation enthalpy
(DHa) determined by the Arrhenius plot decreases with an
increase in the amount of 2a (Fig. S6, ESI†). This indicates that,
as shown in Fig. 3, the interaction between 1 and 2a indeed
decreases the activation energy for isomerization of 1, resulting
in rapid SP - MC isomerization (Fig. 1).
As shown in Scheme 2, thermal SP - MC isomerization of a
spiropyran dye involves three step reactions via the CCC and
CTC intermediates,22,23 where C and T denote cis and trans
forms, respectively, for the dihedral angles of N–C1–C2–C3,
C1–C2–C3–C4, and C2–C3–C4–C5 moieties (Table S2, ESI†). The
isomerization of 1 proceeds as follows: (i) the spiro C1–O bond
cleavage of the SP form produces the CCC intermediate via the
TS1 transition state; (ii) cis - trans isomerization around the
C2QC3 bond of the intermediate produces the CTC intermediate
via the TS2 transition state; and, (iii) cis - trans isomerization
around the C1–C2 bond of CTC results in the formation of
The above mechanism is confirmed by DFT calculations. Fig. 4
(green) shows the potential energy surface for the SP - MC
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