Thermal isomerization of spiropyran to merocyanine in aqueous media and its application to colorimetric temperature indication

Article abstract of DOI:10.1039/c0cp00140f

Thermally-induced isomerization of spiropyran derivatives in aqueous media has been studied. The colorless spirocyclic (SP) forms of spiropyran derivatives are isomerized to colored merocyanine (MC) forms even in dark conditions at elevated temperature. Equilibrium, kinetic, and deuterium experiments reveal that the thermal SP → MC isomerization is due to the stabilization of MC form by a hydrogen bonding interaction with water molecules. This leads to the ground state energy of the MC form decreasing to lower than that of the SP form, resulting in SP → MC isomerization. The thermal isomerization property is applicable to a rough indication of solution temperature. The spiropyran derivatives, when dissolved in aqueous media under irradiation of visible light with an appropriate light intensity, demonstrate an increase in MC absorbance with a rise in temperature. The absorption response occurs reversibly regardless of the heating/cooling sequence. The spiropyran derivatives therefore have a potential for colorimetric temperature indication.

Full text of DOI:10.1039/c0cp00140f

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PAPER
www.rsc.org/pccp | Physical Chemistry Chemical Physics
Thermal isomerization of spiropyran to merocyanine in aqueous media
and its application to colorimetric temperature indicationw
Yasuhiro Shiraishi,* Masataka Itoh and Takayuki Hirai
Received 6th April 2010, Accepted 28th July 2010
DOI: 10.1039/c0cp00140f
Thermally-induced isomerization of spiropyran derivatives in aqueous media has been studied.
The colorless spirocyclic (SP) forms of spiropyran derivatives are isomerized to colored
merocyanine (MC) forms even in dark conditions at elevated temperature. Equilibrium, kinetic,
and deuterium experiments reveal that the thermal SP - MC isomerization is due to the
stabilization of MC form by a hydrogen bonding interaction with water molecules. This leads to
the ground state energy of the MC form decreasing to lower than that of the SP form, resulting
in SP - MC isomerization. The thermal isomerization property is applicable to a rough
indication of solution temperature. The spiropyran derivatives, when dissolved in aqueous media
under irradiation of visible light with an appropriate light intensity, demonstrate an increase in
MC absorbance with a rise in temperature. The absorption response occurs reversibly regardless
of the heating/cooling sequence. The spiropyran derivatives therefore have a potential for
colorimetric temperature indication.
1
. Introduction
converted to the colored merocyanine (MC) form upon
irradiation with UV light, but revert to the colorless spirocyclic
7
The design of molecular systems that perform as an optical
chemosensor is an area of active research, since they allow
(
SP) form upon irradiation with visible light (Scheme 1a).
The SP form has a ground state energy lower than the MC
form; therefore, the MC form also reverts thermally to the SP
1
prompt detection of analytes by simple optical measurements.
Of particular interest is the design of temperature sensors.
8
form. Various functional materials, such as optical switches,
9
2
3
Various colorimetric or fluorometric thermometers have
been proposed. These thermometers enable ‘‘remote’’ sensing
of medium temperature, where no electric wires are required to
connect them. These optical thermometers, therefore, have
advantages over traditional ones in applications where
electromagnetic noise is strong and sparks could be hazardous.
Colorimetric thermometers are more advantageous than the
fluorometric ones because of the simplicity of temperature
sensing. Molecule-based ‘‘small’’ thermometers are more
favorable because they allow monitoring of local temperature
within condensed matters and living organisms. The early
reported colorimetric thermometers are, however, based on
10
11
memories, and sensors, have been proposed based on the
photo- and thermoresponsive isomerization properties.
These properties, however, have been studied mainly in
organic media.
Some reports describe that the SP forms of spiropyran
derivatives with a carboxy, carboxylate, or sulfoxy moiety,
when dissolved in aqueous media, are thermally isomerized to
12
the MC forms even in dark conditions (Scheme 1b).
However, the detailed mechanism of the thermal SP - MC
isomerization is still unclear. In the present work, the detailed
isomerization mechanisms of methyl- and ethanol-substituted
simple spiropyran derivatives, 1 and 2 (Scheme 2), have been
studied in aqueous media based on the equilibrium, kinetic,
2
solids, polymers, or organic aggregates. Thermochromic
4
compounds such as salicyl-Schiff bases and overcrowded
5
ethylenes, which show a thermoresponsive transformation
of chemical species with different absorption spectra, are one
of the potential colorimetric molecular thermometers.
However, the former are usually used in a solid state, and
the latter require organic solvents. Exploration of new thermo-
chromic compounds are necessary for the development of
efficient and useful colorimetric molecular thermometers.
Spiropyran derivatives belong to a class of organic photo-
6
chromes that have been studied extensively. These dyes are
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
w Electronic supplementary information (ESI) available: Supplementary
data (Fig. S1–S13). See DOI: 10.1039/c0cp00140f
Scheme
1
Photoresponsive and thermoresponsive isomerization
between the SP and MC forms of spiropyran derivatives.
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evaporation. The residue and 2-hydroxy-5-nitrobenzaldehyde
(
1.1 g, 6.0 mmol) were refluxed in EtOH (10 mL) for 3 h under
N and stirred at room temperature for 12 h. The resultant was
2
recovered by filtration, washed with EtOH, and dried in
1
vacuo, affording 2 as an orange solid (1.8 g, 68%). H NMR
Scheme 2 Structures of spiropyran derivatives, 1 and 2.
(270 MHz, CDCl , TMS): d (ppm) = 1.19, 1.29 (s, 6H,
3
–
3 2 3
C(CH ) ), 2.74 (s, 3H, –NCH ), 5.85 (d, 1H, –CCHCH–),
and deuterium experiments. We clarified that the thermal
SP - MC isomerization is due to the stabilization of MC
form via a hydrogen bonding interaction with water molecules.
In addition, we found that this thermal isomerization property
has a potential as a colorimetric thermometer for rough
determination of solution temperature.
6.55 (d, 1H, ArH), 6.76 (d, 1H, ArH), 6.85–6.93 (m, 2H,
–CCHCH– and ArH), 7.08 (d, 1H, ArH), 7.20 (t, 1H, ArH),
1
3
3
7.99–8.02 (m, 2H, ArH). C NMR (68 MHz, CDCl , TMS):
d (ppm) = 159.56, 147.46, 140.79, 135.90, 128.05, 127.63,
125.64, 122.46, 121.47, 121.36, 119.58, 118.51, 115.29, 106.88,
106.23, 52.23, 28.81, 25.88, 19.94. FAB-MS: Calcd for
+
1
13
C H N O 322.4, found m/z 323.3 (M+H ). H, C NMR,
1
9
18
2
3
and FAB-MS spectra of 2 are shown in Fig. S4–S6 (ESIw).
2
. Experimental
2
.2 Analysis
Absorption spectra were measured with magnetic stirring using a
0 mm path length quartz cell on an UV-visible spectrophoto-
meter (Shimadzu; Multispec-1500) equipped with a temperature
2
.1 Matelials
All reagents were supplied from Wako, Aldrich, and Tokyo
Kasei and used as received. Water was purified by a Milli-Q
1
0
0
0
0
system. Spiropyran derivatives, 2-(3 ,3 -dimethyl-6-nitro-3 -H-
0
2a
0
0
0
controller (Shimadzu; S-1700). UV light was irradiated by a
Xenon lamp (300 W; Asahi Spectra Co. Ltd.; MAX-302)
equipped with a 334 nm band-pass filter (LX334; light intensity,
spiro[chromene-2,2 -indol]-1 -yl)-ethanol (1) and 2 ,3 ,3 -tri-
0
0
0
methyl-6-nitro-3 -H-spiro[chromene-2,2 -indol]-1 -yl (2), were
1
3
prepared according to literature procedure, as follows:
ꢁ2 11b
1.10 W m ). Visible light was irradiated with a fluorescent
Synthesis of 1. 2,3,3,-trimethyl-3H-indole (1.3 g, 8.1 mmol)
lamp (lmax = 439 nm). The light intensity was measured with a
spectroradiometer (USR-40, Ushio Inc.). The light intensity was
14
and 2-bromoethanol (1.2 g, 9.8 mmol) were refluxed in MeCN
(
2
15 mL) for 24 h under N and concentrated by evaporation.
tuned by changing the distance between the lamp and the
1
13
The residue was added to n-hexane (15 mL) and dispersed well
by ultrasonication. The insoluble solid was recovered by
filtration. The solid and KOH (0.46 g, 8.0 mmol) were added
to water and stirred at room temperature for 10 min.
The solution was extracted with diethyl ether (3 ꢀ10 mL).
The combined organic layer was dried over Na SO and
sample. H and C NMR charts were recorded with a JEOL
JNMGSX270 Excalibur JEOL JNM-AL400 using TMS as
standard. FAB-MS charts were obtained by a JEOL JMS 700
Mass Spectrometer.
2
4
3. Results and discussion
concentrated by evaporation. The residue and 2-hydroxy-5-
nitrobenzaldehyde (1.1 g, 6.0 mmol) were refluxed in EtOH
3.1 SP - MC isomerization in aqueous media
(
10 mL) for 3 h under N and stirred at room temperature for 12 h.
2
Fig. 1a shows the time-dependent change in absorption spectra
of an ethanol-substituted spiropyran derivative, 1 (50 mM), in a
water–MeOH (1: 1 v/v) mixture at 45 1C in the dark condition.
At time zero, almost no absorption appears at >450 nm,
The resultant was recovered by filtration, washed with EtOH,
and dried in vacuo, affording 1 as a purple solid (1.4 g, 50%).
1
H NMR (400 MHz, CDCl
s, 6H, –C(CH ), 3.31–3.49 (m, 2H, –NCH
CH ), 5.88 (d, 1H, –CCHCH–), 6.66 (d, 1H, ArH),
3
, TMS): d (ppm) = 1.19, 1.29
(
3
)
2
2
CH ), 3.71–3.81
2
(m, 2H, –NCH
2
2
6
.76 (d, 1H, ArH), 6.89–6.91 (m, 2H, –CCHCH– and ArH),
.09 (d, 1H, ArH), 7.18 (t, 1H, ArH), 7.99–8.02 (m, 2H, ArH).
7
1
3
C NMR (100 MHz, CDCl , TMS): d (ppm) = 159.11, 146.8,
3
1
1
2
41.06, 135.68, 128.1, 127.7, 125.82, 122.61, 121.81, 121.75,
19.86, 118.42, 115.39, 106.79, 106.62, 60.83, 52.79, 46.09,
5.9, 20.03. FAB-MS: Calcd for C H N O 352.4, found
2
0
20
2
4
+
1
13
m/z 353.2 (M+H ). H, C NMR, and FAB-MS charts of
1
are shown in Fig. S1–S3 (ESIw).
Synthesis of 2. 2,3,3,-trimethyl-3H-indole (1.3 g, 8.2 mmol)
and CH
3
I (1.4 g, 10 mmol) were refluxed in MeCN (15 mL) for
and concentrated by evaporation. The residue
3
h under N
2
was added to n-hexane (15 mL) and dispersed well by ultra-
sonication. The insoluble solid was recovered by filtration. The
solid and KOH (0.46 g, 8.0 mmol) were added to water and
stirred at room temperature for 10 min. The solution was
extracted with diethyl ether (3 ꢀ10 mL). The combined
organic layer was dried over Na SO and concentrated by
Fig. 1 Time-dependent change in absorption spectra of 1 (50 mM)
measured at 45 1C in (a) water–MeOH (1 : 1 v/v) and (b) MeCN during
50 min stirring in the dark.
2
4
1
3738 Phys. Chem. Chem. Phys., 2010, 12, 13737–13745
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15
indicating that 1 exists as a SP form. However, as time
advances, a distinctive absorption band, assigned to the MC
form of 1, appears at 440–640 nm with a maximum absorption
at 515 nm. A clear isosbestic point at 301 nm indicates that the
transformation of SP form of 1 produces the MC form solely,
without decomposition. The spectral change almost stops after
5
0 min stirring. As shown in the table of contents image, the
solution color changes from colorless to pink according to this
spectral change. Fig. 1b shows absorption spectra of 1 measured
in MeCN in the dark. No MC absorption band appears even
after stirring the solution for 50 min. Also in other organic
3
solvents, such as CHCl , benzene, and THF, MC absorption
Fig. 3 Time-dependent change in absorbance of the MC band (AMC
)
does not appear. This indicates that the SP - MC isomerization
indeed takes place in aqueous media even in the dark, as
of 1 (50 mM) measured in (a) D
2
O–MeOH (1 : 1 v/v), (b) water–MeOH
1
2
(1 : 1 v/v), (c) water–MeOH (1/4 v/v), (d) MeOH, and (e) MeCN in the
dark at 45 1C. The spectra for the samples (b) and (e) are shown in
Fig. 1, and the spectra for the samples (a), (c), and (d) are shown in
Fig. S8 (ESIw).
observed in earlier reports. Fig. 2 shows the time-dependent
change in absorbance of the 515 nm MC band of 1 in a
water–MeOH (1 : 1 v/v) mixture at different temperature.
At 25 1C, the absorbance increase is very slow, but becomes
much faster with a rise in temperature. This indicates that the
SP - MC isomerization in aqueous media is accelerated at
elevated temperature.
1
7
isomerization. Similarly, some organic solvents or inorganic
1
materials such as fluoroalcohols, ionic liquids, and silica
8
19
20
stabilize the MC form via a hydrogen bonding interaction and
also decelerate the MC - SP isomerization. The hydrogen
bonding donor abilities of water, MeOH, and MeCN are 1.17,
3
.2 Hydrogen bonding interaction
2
1
The thermal SP - MC isomerization of 1 in aqueous media
involves a hydrogen bonding interaction between 1 and water
molecules. Fig. 3 shows the time-dependent change in MC
absorbance of 1 in different solvents at 45 1C. In MeCN (e), no
absorbance increase is observed, as is also shown in Fig. 1b. In
contrast, 1 in MeOH (d) shows an increase in MC absorbance,
although only to a weak level. Addition of water to MeOH
0.93, and 0.19, respectively. The order agrees with the
tendency of thermal SP - MC isomerization (Fig. 3). These
data imply that the hydrogen bonding interaction between the
MC form and water molecules leads to a thermal SP - MC
isomerization.
The above mentioned hydrogen bonding mechanism is
confirmed by absorption analysis with D O. Fig. 3a shows
2
(
b and c), however, leads to a significant absorbance increase.
the time-dependent change in MC absorbance of 1 in a
The increase is accelerated with an increase in the water
content, suggesting that water indeed promotes the SP - MC
isomerization of 1.
D O–MeOH (1 : 1 v/v) mixture. The MC absorbance is much
2
2
2
larger than that in water–MeOH (Fig. 3b). As reported,
a
hydrogen bonding interaction between D O and solute molecules
2
It is well known that the ground state energy of the MC
form of spiropyran derivatives is higher than that of the SP
is stronger than that for water. The larger MC absorbance in
the D O system indicates that the hydrogen bonding inter-
2
1
6
form; therefore, thermal SP - MC isomerization does not
occur in organic solvents, although conventional thermal
MC - SP isomerization occurs. Several reports state that
the MC form is strongly stabilized by an association with
metal cations and, hence, decelerates the thermal MC - SP
action between the MC form of 1 and water molecules indeed
promotes thermal SP - MC isomerization. Some reports
suggest that a solvent polarity also affects the stabilization of
1
2a,b,23
MC form,
and highly polar solvents better stabilize the
MC form. The Reichardt’s solvent polarity parameter
N
(
(
E
E
T
2
) of D O (0.99) is, however, similar to that of water
24
= 1.00). The larger MC absorbance in the D O system
2
N
T
indicates that the SP - MC isomerization is triggered by a
hydrogen bonding interaction between the MC form and
solvent molecules.
3
.3 Equilibrium analysis
The thermal SP - MC isomerization of 1 is triggered by a
decrease in the ground state energy of the MC form to lower
1
2a
than that of SP form, due to the stabilization of MC form
via a hydrogen bonding interaction. As a result of this, the
equilibrium between the SP and MC forms shifts to MC
formation and promotes thermal SP - MC isomerization.
The decrease in ground state energy of the MC form is
confirmed by the thermodynamic equilibrium constants
Fig. 2 Time-dependent change in 515 nm absorbance of 1 (50 mM)
measured in a water–MeOH (1 : 1 v/v) mixture at different temperature
in the dark. The change in absorption spectra measured at 25, 40, 50,
55, and 60 1C are summarized in Fig. S7 (ESIw).
between the SP and MC forms, K , and the standard enthalpy,
eq
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Table 1 Molar extinction coefficient (eMC) of the MC form for
spiropyran derivatives (1,2) in different solvents
a
ꢁ
1
ꢁ1
b
Solvents
O–MeOH (1 : 1 v/v)
Water–MeOH (1 : 1 v/v)
Water–MeOH (1 : 1 v/v)
MeOH
T/1C
e
MC/M cm
lmax/nm
4
1
D
2
25
25
60
25
25
25
2.40 ꢀ10
2.54 ꢀ10
2.59 ꢀ10
3.40 ꢀ10
3.99 ꢀ10
2.49 ꢀ10
515
515
515
529
563
512
4
4
4
4
4
MeCN
D O–MeOH (1 : 1 v/v)
2
2
a
The values were determined according to literature procedure
b
ref. 26). Maximum absorption wavelength of the MC band.
(
The Keq values of 1 in a water–MeOH (1 : 1 v/v) mixture are
1, indicating that a larger amount of MC form than SP form
exists at these temperatures (>40 1C). D H of SP - MC
>
r
ꢁ
1
isomerization shows a negative value (ꢁ6.6 kJ mol ). This
indicates that the ground state energy of the MC form is lower
than that of the SP form, and the SP - MC isomerization
occurs exothermically. In contrast, Keq values in MeOH
ꢁ
2
ꢁ3
(
o5 ꢀ10 ) and MeCN (o2 ꢀ10 ) are much lower than
that in aqueous media. In addition, D H values in MeOH
3.3 kJ mol ) and MeCN (7.7 kJ mol ) are positive, indicating
r
ꢁ
1
ꢁ1
(
that SP - MC isomerization is not favored thermo-
dynamically. This suggests that the MC form is strongly
stabilized in aqueous media, while less stabilized in organic
solvents. The energy diagrams for isomerization of 1 in the
respective solvents are summarized in Fig. 6a–d.
Fig. 4 Plots of C
absorption experiments (4 h) at different temperature in
a) D O–MeOH (1 : 1 v/v), (b) water–MeOH (1 : 1 v/v), (c) MeOH, and
T
vs. AMC data for 1 obtained by the equilibrium
(
2
(d) MeCN in the dark. The numbers in the figures denote temperature.
As shown in Fig. 3, the SP - MC isomerization in a
D
2
O–MeOH (1 : 1 v/v) mixture is more enhanced than that
in water–MeOH (1 : 1 v/v). The Keq values in the D O system
at each temperature are higher than that in the water system
Table 2), meaning that, in the D O system, the thermal
D
SP and MC concentrations are expressed as follows:
r
H, for isomerization. The relationship between Keq and the
2
2
5
(
2
½
MCꢂ
eq
Keq ¼
ð1Þ
ð2Þ
equilibrium between the SP and MC forms shifts more to
½
SPꢂ
eq
the MC formation. In addition, D H in the D O system
r
2
ꢁ
1
(
ꢁ12.5 kJ mol ) is more negative than that in the water
ꢀ
ꢁ
ꢁ1
system (ꢁ6.6 kJ mol ), indicating that the MC form is more
AMC
eMC
1
CT ¼½SPꢂeq þ½MCꢂeq
¼
1 þ
stabilized by stronger hydrogen bonding interaction with D O.
2
Keq
These suggest that the decrease in ground state energy of the
MC form is due to the hydrogen bonding interaction and this
promotes thermal SP - MC isomerization in aqueous media.
where [MC]eq and [SP]eq are the equilibrium concentrations of
MC and SP forms, C is the total concentration of MC and SP
T
forms, and AMC and eMC are the absorbance and molar
extinction coefficient of MC form. Keq is determined by the
equilibrium absorption experiments: the solutions containing
3.4 Kinetic analysis
Kinetic analysis of the thermal SP - MC isomerization was
carried out. The apparent rate constant, kobsd, for isomerization
is expressed by the sum of the forward, kSP-MC, and
different concentrations of 1 (C
dark at different temperature, and the MC absorbance (AMC
was plotted against C (Fig. 4). It must be noted that the
T
) were stirred for 4 h in the
)
T
2
8
backward rate constant, kMC-SP
.
solutions used in this experiment (Fig. 4) attain the absorption
equilibrium by 4 h stirring. For all of the solutions, a linear
relationship is observed between C and AMC. The eMC values
kSP!MC
T
2
6
SP ꢁ!MC
ꢁ
ð4Þ
of 1 are determined according to a literature procedure
Table 1). The Keq values in the respective solvents are therefore
determined with eMC and the slope of Fig. 4, as summarized in
Table 2. The standard enthalpy (D H) for isomerization is
k
MC!SP
(
k
obsd = kSP-MC + kMC-SP
(5)
r
2
7
determined using the Van’t Hoff equation (eqn (3)). Fig. 5
shows the Van’t Hoff plots of the equilibrium absorption data,
The equilibrium constant, Keq (eqn (1)), is therefore rewritten
as follows.
r
and the obtained D H values are summarized in Table 2.
d ln K
DrH
R
k
SP!MC
MC!SP
¼
ꢁ
ð3Þ
Keq
¼
ð6Þ
dð1=TÞ
k
1
3740 Phys. Chem. Chem. Phys., 2010, 12, 13737–13745
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Table 2 Equilibrium and kinetic absorption data for spiropyran derivatives (1,2) in different solvents
z
z
ꢁ1
d
D
r
H/
kJ mol
k
obsd
10
/
s
k
10
SP-MC
/
k
MC-SP
10
/
DH (298 K)/
kJ mol
DS (298 K) /
J K mol
ꢁ1
ꢁ4 ꢁ1
ꢁ4 ꢁ1
ꢁ4 ꢁ1
ꢁ1
ꢁ1
Solvents
O–MeOH
T/1C
K
eq
s
s
1
D
2
40
1.89
ꢁ12.5 ꢃ1.82
5.58
3.66
1.93
108.4 ꢃ13.6
35.0 ꢃ20.9
(SP - MC)
(1 : 1 v/v)
b
50
55
60
1.65
1.50
1.43
1.20
19.4
37.1
84.3
5.91
12.1
22.2
49.7
3.24
7.33
14.9
34.6
2.68
(SP - MC)
Water–MeOH
1 : 1 v/v)
40
ꢁ6.64 ꢃ1.88
107.7 ꢃ9.1
32.1 ꢃ14.1
(SP - MC)
(
b
50
55
60
1.10
1.05
1.04
4.60 ꢀ10
4.77 ꢀ10
4.87 ꢀ10
4.96 ꢀ10
22.2
39.4
84.3
11.7
20.2
43.1
10.6
19.2
41.2
13.1
49.9
84.9
(SP - MC)
ꢁ
ꢁ
ꢁ
ꢁ
2
2
2
2
a
a
a
MeOH
MeCN
40
3.30 ꢃ0.18
7.70 ꢃ0.11
106.6 ꢃ5.5
40.3 ꢃ8.5
c
50
55
60
(MC - SP)
(MC - SP)
a
166
a
a
a
a
10
6.63
13.8
24.3
90.6
90.1 ꢃ5.5
13.0 ꢃ9.4
c
15
20
30
35
45
50
55
(MC - SP)
(MC - SP)
ꢁ3
ꢁ3
ꢁ3
ꢁ3
1.62 ꢀ10
1.78 ꢀ10
1.86 ꢀ10
1.95 ꢀ10
1.21
2
D
(
2
O–MeOH
1 : 1 v/v)
40
ꢁ5.03 ꢃ0.80
3.97
2.17
1.79
111.6 ꢃ4.9
41.5 ꢃ7.6
b
50
55
60
1.16
1.11
1.08
16.9
31.5
58.1
9.08
16.6
30.2
7.86
15.0
27.9
(SP - MC)
(SP - MC)
a
b
Determined from eqn (10), where kobsd is nearly equal to kMC-SP because the Keq values are significantly low (o 0.05). Determined by the plots
c
d
z
of kSP-MC with 1/T (Fig. 8). Determined by the plots of kMC-SP with 1/T (Fig. 8). DS = R(lnAꢁ1ꢁlnk
B
T/h), where A is the frequency factor,
and k and h are the Boltzmann’s constant and the Planck’s constant, respectively (ref. 30). The lnA values were determined from the intercept of
B
the Arrhenius plot (Fig. 8).
The relationship between the MC absorbance and the rate
constants for respective SP - MC and MC - SP isomerizations
2
5,29
are expressed as follows:
ꢂ
ꢃ
ꢃ
A1ꢁAt
A1ꢁA0
ln
ln
¼ꢁkobsdt
¼ꢁkobsdt
ð9Þ
ꢂ
At ꢁA1
A0 ꢁA1
ð10Þ
0 t N
where A , A , and A are the MC absorbance at time 0, t,
and infinity, respectively. Fig. 7a and b show the plots of
ln[(A
N
ꢁA
t
)/(A
N
ꢁA
0
)] vs. time (eqn (9)) during the SP - MC
in O–MeOH and water–MeOH
isomerization of
1
D
2
mixtures, respectively. In both cases, a linear relationship is
observed at the respective temperature. The rate constants,
kSP-MC and kMC-SP, are therefore determined using eqn (7)
and (8), as shown in Table 2. In MeOH and MeCN, kSP-MC
cannot be determined because the Keq values are significantly
small (o0.05). In contrast, kMC-SP can be determined by the
Fig. 5 Van’t Hoff plots of the Keq data for 1 obtained in (a)
D
2
O–MeOH (1 : 1 v/v), (b) water–MeOH (1 : 1 v/v), (c) MeOH, and
(d) MeCN and for 2 obtained in (e) D O–MeOH (1 : 1 v/v).
2
plots of ln[(A
t
ꢁA
N
)/(A
0
ꢁA
N
)] vs. time (eqn (10)) during the
The respective rate constants are therefore expressed with Keq
.
MC - SP isomerization of 1 (Fig. 7c and d), where the MC
form was preliminarily produced by irradiation of UV light
(334 nm) of the solution. As shown in Table 2, kMC-SP values
Keq
kSP!MC ¼
kMC!SP ¼
kobsd
ð7Þ
ð8Þ
1
1
þKeq
in
2
a D O–MeOH mixture are lower than that in a
water–MeOH mixture and are much lower than the values
in MeOH and MeCN. This indicates that the MC - SP
isomerization is significantly suppressed in aqueous media.
1
kobsd
þKeq
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Fig. 6 Energy diagrams for isomerizations between the SP and MC forms of spiropyran derivatives (1,2) in different solvents.
z
where A is frequency factor. The activation enthalpy, DH , for
isomerization is expressed as follows:
1
6b
z
DH = E
a
ꢁRT
(12)
z
As shown in Table 2, DH (298 K) value of SP - MC
ꢁ
1
isomerization of 1 in a water–MeOH mixture is 107.7 kJ mol
ꢁ
,
1
which is similar to that in D O–MeOH (108.4 kJ mol ). As
2
3
1
reported, the thermal SP - MC isomerization proceeds via
a two-step reaction; the first step is the cleavage of C–O bond
of the SP form and the second step is the rotation (cis–trans
isomerization) of the species, where the second reaction is the
rate-determining step. Some reports describe that the transition
state in the thermal isomerization has an ionic and polar
2
5,32
z
character.
2
The similar DH values in the D O and water
systems, however, suggest that the transition state scarcely
associates with water molecules via a hydrogen bonding inter-
action and, hence, shows similar energy for rotation. This is
probably because the transition state is less polar than the
3
3
corresponding MC form. These indicate that the MC form is
strongly stabilized by hydrogen bonding interaction, while the
transition state is not.
z
As shown in Table 2, the activation entropy (DS , 298 K)
of the SP - MC isomerization of 1 in D O–MeOH
2
Fig.
a) D
d) MeCN in the dark at different temperature. (a) and (b) show the data
7
Plot of kinetic absorption data of
1
obtained in
ꢁ1
ꢁ1
(
35.0 J K
mol ) is similar to that in water–MeOH
(
2
O–MeOH (1 : 1 v/v), (b) water–MeOH (1 : 1 v/v), (c) MeOH, and
ꢁ1
ꢁ1
(
32.1 J K mol ). This again supports the no hydrogen
(
bonding interaction between the transition state and solvent
for SP - MC isomerization obtained with the SP form as an initial state
using eqn (9). (c) and (d) show the data for MC - SP isomerization
obtained with the MC form as an initial state using eqn (10), where the
MC form was preliminarily generated by UV irradiation (334 nm) for
15 min. The change in absorption spectra of the sample (b) is shown in
Fig. 1a and Fig. S7 (ESIw), and the spectra of the samples (a), (c), and (d)
are shown in Fig. S9–S11 (ESIw).
This is because highly polarized MC form is stabilized by a
1
8–20
hydrogen bonding interaction,
which is consistent with
the equilibrium data. In addition, kSP-MC values in a
D O–MeOH mixture are higher than that in a water–MeOH
2
mixture, suggesting that the hydrogen bonding interaction
accelerates the SP - MC isomerization, while suppressing
the MC - SP isomerization.
a
The activation energy, E , for isomerization is determined
3
by the Arrhenius plots (Fig. 8), using the following equation.
0
Fig. 8 Arrhenius plot of the kinetic absorption data for 1 obtained in
(
a) D
2
O–MeOH (1 : 1 v/v), (b) water–MeOH (1 : 1 v/v), (c) MeOH, and
O–MeOH (1 : 1 v/v) in
(
d) MeCN and the data for 2 obtained in (e) D
2
Ea
the dark. (a), (b), and (e) show the data for kSP-MC, whereas (c) and
(d) show the data for kMC-SP
ln k ¼ln A ꢁ
ð11Þ
RT
.
1
3742 Phys. Chem. Chem. Phys., 2010, 12, 13737–13745
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3
4
molecules. The above findings clearly suggest that the
thermal SP - MC isomerization in aqueous media is triggered
by a hydrogen bonding interaction of the MC form,
decreasing the ground state energy of the MC form lower
than that of the SP form.
3
.5 Effect of spiropyran structure
To clarify how the MC form associates with water molecules,
the equilibrium and kinetic analysis of a N-methylated spiro-
pyran, 2 (Scheme 1), was carried out. As shown in Fig. S12
(
2
ESIw), 2 dissolved in a D O–MeOH (1 : 1 v/v) mixture shows
a SP - MC isomerization even in the dark, as does 1. As
shown in Table 2, the Keq values of 2 are > 1, indicating that
SP - MC isomerization also occurs exothermically. The Keq
values are, however, much smaller than that of 1. The D H
r
value of SP - MC isomerization of 2 in D O–MeOH is
2
ꢁ
ꢁ
1
ꢁ
5.0 kJ mol , which is more positive than that of 1
1
(
ꢁ12.5 kJ mol ). This suggests that the MC form of 2 is less
18,19
stabilized than that of 1. As reported,
hydrogen bonding
interaction of the MC form occurs on the o-nitrophenolate
moiety. 1 has an acidic ethanol group on the indole moiety,
while 2 has a methyl group. The hydrogen bonding interaction
between the ethanol group and water molecule may further
stabilize the MC form of 1. This probably results in a lower
ground state energy of the MC form of 1 than that of 2.
Fig. 9 (a) Time-dependent change in absorption spectra of 1 (50 mM)
in a water–MeOH (1 : 1 v/v) mixture under visible light irradiation
ꢁ2
(light intensity at 380–500 nm: 119 mW m ) at 60 1C, where the
measurement was started after the solution attains the absorption
equilibrium by 10 min stirring in the dark. (b) Change in MC
absorbance of 1 during the repeated stirring in the dark or under
visible light irradiation at 60 1C.
z
As shown in Table 2, DH of SP - MC isomerization of 2
in D O–MeOH is 111.6 kJ mol , which is similar to that of 1
ꢁ
1
2
ꢁ
1
108.4 kJ mol ). This indicates that the transition state of 2 is
(
not affected by hydrogen bonding interaction, as is the case for 1.
ꢁ1
z
ꢁ1
In addition, DS (298 K) of 2 in D
2
O–MeOH (41.5 J K mol
)
ꢁ
1
ꢁ1
suggesting that the SP - MC and MC - SP isomerizations
are repeatable without decomposition of 1.
is similar to that of 1 (35.0 J K mol ). This also supports the
no hydrogen bonding interaction of the transition state of 2, as is
the case for 1. These findings suggest that stronger hydrogen
bonding interaction of the MC form of spiropyran derivatives
leads to a decrease in the ground state energy of the MC form,
resulting in acceleration of the SP - MC isomerization.
3.7 Application to temperature indication
The thermal SP - MC isomerization property of the
spiropyran derivative has potential for colorimetric temperature
indication. For practical applications, rapid temperature
2a,3c
3
.6 Reversion by visible light
indication is necessary.
However, as shown in Fig. 2,
The MC form of spiropyran derivatives stabilized in aqueous
media is successfully reverted to the SP form by visible light
irradiation. Fig. 9a shows the time-dependent change in
absorption spectra of 1 in a water–MeOH mixture under
the absorption equilibrium for SP - MC isomerization in
the dark requires a relatively long time, especially at lower
temperatures; for example, 90 min stirring is required at 40 1C,
while 5 min stirring is enough at 60 1C. This problem is solved
by irradiation of visible light with an appropriate intensity,
which shortens the time for saturation of the MC absorbance.
Fig. 10 shows the time-dependent change in MC absorbance
of 1 in a water–MeOH (1 : 1 v/v) mixture under irradiation of
ꢁ2
irradiation of visible light (119 mW m ) at 60 1C, where the
measurement was started after the solution attained the
absorption equilibrium by stirring for 10 min in the dark.
The MC absorption decreases with time and disappears
completely after 2 min irradiation, leaving the absorption of
SP form (Fig. 1a). This indicates that, even in aqueous media,
ꢁ2
visible light with the intensity 39.7 mW m (380–500 nm).
The increase in MC absorbance saturates within 5 min even at
low temperature. Upon irradiation of visible light, photo-
35
visible light irradiation promotes the MC - SP isomerization,
9
7
as also in organic media; in other words, photoexcitation of
MC form allows successful reversion to the SP form even
though the MC form is stabilized by a hydrogen bonding
interaction. Fig. 9b shows the change in MC absorbance of 1
when the solution was stirred in the dark or under irradiation
of visible light sequentially. Visible light irradiation leads to a
decrease in the MC absorption, but sequential stirring in the
dark successfully regenerates the MC absorption. The change
in MC absorbance is similar during the repeated three cycles,
chemical MC - SP isomerization occurs competitively with
the thermal SP - MC isomerization. This therefore shortens
the time for saturation of MC absorbance.
Fig. 11A(c) shows the temperature-dependent change in MC
absorbance of 1 in a water–MeOH mixture under irradiation of
ꢁ2
visible light (39.7 mW m ). At low temperature, thermal
SP - MC isomerization is slow while the MC - SP
isomerization is accelerated by visible light irradiation, thus
resulting in low MC absorbance. A rise in temperature,
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temperature. It must be noted that the practical application
of the present system to real sample analysis involves several
problems. This system is sensitive to the intensity of visible
light. Fig. 11A shows the temperature–absorbance profiles
obtained under irradiation of visible light with different
intensity. The irradiation of weaker light provides a profile with
a larger MC absorbance because the photochemical MC - SP
isomerization is slow. This suggests that accurate temperature
indication requires continuous irradiation of visible light with
an appropriate intensity. In addition, the present system is
affected by a solvent composition (Fig. 3); the MC absorbance
depends strongly on the water amount. This is one of the
biggest problems for application to biological and environmental
samples. Furthermore, impurity cations and anions and the
Fig. 10 Time-dependent change in MC absorbance of 1 (50 mM) in a
water–MeOH (1 : 1 v/v) mixture at different temperature under visible
ꢁ2
light irradiation with the intensity 39.7 mW m (380–500 nm).
3
6
pH of the samples may also affect the accuracy of temperature
however, accelerates the thermal SP - MC isomerization and,
hence, leads to a MC absorbance increase. This thus provides
a temperature–absorbance profile showing a MC absorbance
increase with a rise in temperature. It is notable that the
present system enables reversible indication of solution
temperature regardless of heating/cooling sequence. As shown
in Fig. 11A(c) (closed symbols), the temperature-absorbance
profile obtained during a cooling sequence is almost the same
as that obtained during a heating sequence (open symbols). In
addition, as shown in Fig. 11B, the repeated change in
temperature between 10 and 60 1C shows the similar MC
absorbance change. These indicate that the present system
allows reversible temperature indication.
sensing, as is the case for early reported colorimetric
2,4,5
thermometers and thermochromic compounds.
There are,
therefore, many issues that must be addressed for application to
real sample analysis at the present stage. Nevertheless, the
basic concept presented here using spiropyran derivatives may
contribute to the development of an efficient and useful
colorimetric molecular thermometer.
4. Conclusions
The thermal SP - MC isomerization properties of spiropyran
derivatives have been studied in aqueous media. The colorless
SP forms are transformed to the colored MC form in the dark
at elevated temperature. The thermal SP - MC isomerization
is due to the decrease in ground state energy of the MC form
via the stabilization of the MC form by a hydrogen bonding
interaction. A rough but rapid and reversible detection of
solution temperature is achieved using a spiropyran derivative
under visible light irradiation with an appropriate light
intensity. Although there are many issues that must be addressed
for application to a temperature sensing of real samples, the
basic concept and the results presented here may contribute to
the development of efficient and useful molecule-based
temperature indicators.
The above results suggest that spiropyran derivatives have a
potential for rough but rapid indication of solution
Acknowledgements
This work was supported by the Grant-in-Aid for Scientific
Research (No. 21760619) from the Ministry of Education,
Culture, Sports, Science and Technology, Japan (MEXT). We
thank Mr. Ryo Miyamoto for experimental assistance.
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