Colorimetric response of spiropyran derivative for anions in aqueous or organic media

Article abstract of DOI:10.1016/j.tet.2010.12.021

We previously found that a simple spiropyran derivative (1:1′,3′,3′-trimethyl-6-nitro-spiro-[2H-1-benzopyran-2, 2′-indoline]) behaves as a selective and sensitive cyanide anion (CN ^-) receptor in aqueous media under UV irradiation¹³.

Full text of DOI:10.1016/j.tet.2010.12.021

Tetrahedron 67 (2011) 891e897
Contents lists available at ScienceDirect
Tetrahedron
journal homepage: www.elsevier.com/locate/tet
Colorimetric response of spiropyran derivative for anions in aqueous
or organic media
*
Yasuhiro Shiraishi , Masataka Itoh, Takayuki Hirai
Research Center for Solar Energy Chemistry, and Division of Chemical Engineering, Graduate School of Engineering Science, Osaka University,
1
-3 Machikaneyama-cho, Toyonaka 560-8531, Japan
a r t i c l e i n f o
a b s t r a c t
0 0 0
We previously found that a simple spiropyran derivative (1:1 ,3 ,3 -trimethyl-6-nitro-spiro-[2H-1-benzopyran-
Article history:
Received 3 November 2010
Received in revised form 6 December 2010
Accepted 7 December 2010
Available online 17 December 2010
0
ꢀ
,2 -indoline]) behaves as a selective and sensitive cyanide anion (CN ) receptor in aqueous media under UV
2
13
ꢀ
irradiation . The receptor, when irradiated by UV light in a water/MeCN mixture, creates a CN -selective
absorption band via a nucleophilic addition of CN to 1 (formation of the 1eCN species) and allows quanti-
ꢀ
ꢀ
ꢀ
tative determination of very low levels of CN . In the present work, effects of pH and water content on the
response of 1 to anions were studied to clarify the detailed properties of 1. In aqueous media,1 reactsselectively
with CN regardless of pH and water content, but the reaction is suppressed by a decrease in pH and an increase
Keywords:
Spiropyran
Receptor
Optical sensing
Anions
ꢀ
ꢀ
ꢀ
in water content due to the protonation of CN . In contrast, in pure MeCN, addition of F also creates a new
ꢀ
ꢀ
absorption band, as does CN . This is promoted via a nucleophilic interaction between 1 and F in a 1:2 stoi-
chiometry (formation of the 1e2F species). The 1eCN and 1e2F species have different photochemical
properties; the 1eCN species is stable upon UV irradiation, while the UV irradiation of the 1e2F species leads
to a decomposition of the spiropyran platform.
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
Water
Ó 2010 Elsevier Ltd. All rights reserved.
1. Introduction
UV light in a water/MeCN (50/50 v/v) mixture, is converted to the
colored merocyanine (MC) form with an absorption band at
ꢀ
Anions play important roles in the areas of biological and envi-
ronmental chemistry. The design of receptors that selectively detect
the targeted anions has therefore attracted a great deal of attention.
450e550 nm. The nucleophilic addition of CN to the positively-
charged spirocarbon of the MC form of 1 leads to a formation of the
1
ꢀ
1eCN species with a blue-shifted absorption at 350e500 nm. This
thus enables quantification of very low levels of CN (>1.7
ꢀ
Various receptors have been proposed for selective detection of
mM) by
ꢀ
2
ꢀ
3
ꢀ 4
anions, such as fluoride (F ), chloride (Cl ), acetate (AcO ),
absorption analysis.
ꢀ
),5 and hydrogensulfate (HSO
ꢀ
6
dihydrogenphosphate (H
2
PO
4
4
). In
The purpose of the present work is to clarify the detailed
properties of the receptor 1 for anions. The effects of pH and water
content of the solution on the response of 1 to anions have been
studied. In aqueous media, 1 reacts selectively with CN regardless
of pH and water content, but the reaction is suppressed by a de-
crease in pH and an increase in water content. In contrast, in pure
MeCN, addition of F to 1 also creates a blue-shifted absorption as
does CN . This is promoted via a nucleophilic interaction between 1
and F in a 1:2 stoichiometry (formation of the 1e2F species)
(Scheme 1). We also describe here the different properties of the
ꢀ
particular, the cyanide anion (CN ) receptors have attracted much
attention because CN is extremely toxic to living organisms. The
ꢀ
7
ꢀ
maximum permissive level of cyanide in drinking water is therefore
8
set at 1.9
m
M by the World Health Organization (WHO). Several
ꢀ
kinds of CN receptors have been proposed, but many of these rely
on a hydrogen-bonding motif and act only in organic media. To
overcome this limitation, reaction-based CN receptors have been
however, many of these display relatively poor se-
M) in aqueous media.
There are only a few reports of CN receptors with high selectivity
9
ꢀ
ꢀ
ꢀ
10e12
ꢀ
ꢀ
proposed;
lectivity10 and a high detection limit (>1.9
m
11
ꢀ
12
ꢀ
ꢀ
1eCN and 1e2F species upon UV irradiation.
and sensitivity in aqueous media.
Earlier, we reported that
a
simple spiropyran derivative
1:1 ,3 ,3 -trimethyl-6-nitro-spiro-[2H-1-benzopyran-2,2 -indoline]),
2. Result and discussion
0
0
0
0
(
a photochromic compound, behaves as a highly selective and sen-
2.1. Effect of pH
ꢀ
sitive CN receptor in aqueous media under UV irradiation (Scheme
13
1
). The colorless spirocyclic (SP) form of 1, when irradiated by
The change in absorption spectra of 1 in aqueous media with
basic pH has already been described in the previous work, and
a brief description is made here. Fig.1a shows the absorption spectra
13
*
Corresponding author. Tel.: þ81 6 6850 6271; fax: þ81 6 6850 6273; e-mail
address: shiraish@cheng.es.osaka-u.ac.jp (Y. Shiraishi).
of 1 (20
mM) measured in a buffered water/MeCN (50/50 v/v)
0
040-4020/$ e see front matter Ó 2010 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tet.2010.12.021

892
Y. Shiraishi et al. / Tetrahedron 67 (2011) 891e897
ꢀ
ꢀ
Scheme 1. Structure and color change of 1 in the presence of CN or F under UV irradiation.
(a) pH 9.3, water/MeCN = 50/50 v/v
(b) pH 7.5, water/MeCN = 50/50 v/v
(c) pH 9.3, water/MeCN = 20/80 v/v
(d) water/MeCN = 0/100 v/v
0
0
0
.3
.2
.1
0
0.3
none (UV)
1
0.5
0
none (UV)
other anions
–
other anions
CN
–
0
.3
CN
–
0.2
CN
–
none (UV)
none (UV)
–
CN
0.2
none
dark)
F
other anions
other anions
(
0.1
0
0
.1
0
none
dark)
none
dark)
none
dark)
(
(
(
300
400
500
600
300
400
500
600
300
400
500
600
300
400
500
600
Wavelength / nm
Wavelength / nm
Wavelength / nm
Wavelength / nm
ꢃ
Fig. 1. Absorption spectra of 1 (20
m
M) measured in buffered water/MeCN mixtures with different pH and compositions at 25 C. (a) pH 9.3 (CHES 100 mM), 50/50 v/v, (b) pH 7.5
(
HEPES 100 mM), 50/50 v/v, (c) pH 9.3, 20/80 v/v, and (d) 0/100 v/v. The blue spectra were obtained without anion in the dark. The red spectra were obtained without anion under
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ ꢀ ꢀ ꢀ ꢀ ꢀ
2 4 4 4 3
PO , HSO , ClO , NO , SCN , or CN ) under UV irradiation for
UV irradiation (334 nm, 5 min). The black spectra were obtained with each respective anion (F , Cl , Br , I , AcO , H
0 min, where 50 equiv of anion was added to the samples a, c, and d, and 500 equiv of anion was added to the sample b.
3
0.3
mixture with pH 9.3 in the presence of 50 equiv of respective anion
13
0.2
as an n-tetrabutylammonium salt. As shown by blue line, without
anion in the dark,1 shows almost no absorption in the visible region,
indicating that 1 exists as an SP form. As shown by red line, UV ir-
radiation (334 nm) of the sample leads to a generation of the ab-
sorption band at 450e650 nm, assigned to the MC form. Addition of
(a)
0
.3
0.1
0
0.2
0
10 20 30 40 50
ꢀ
–
CN to the solution under UV irradiation leads to a decrease in the
[CN ]/[1]
MC band along with an appearance of new absorption band at
ꢀ
ꢀ
3
50e500 nm, assigned tothe 1eCN species (Scheme 1). In contrast,
0.1
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ ꢀ
ꢀ
4
addition of other anion (F , Cl , Br , I , AcO , H
2
PO
ꢀ
3
, or SCN ) does not show any spectral change. Fig. 1b shows the
4 4
, HSO , ClO ,
ꢀ
NO
change in absorption spectra of 1 measured in a buffered water/
MeCN (50/50 v/v) mixture at pH 7.5. The spectral change upon UV
0
3
00
400
500
600
700
irradiation and addition of anions is similar to that observed at pH
ꢀ
Wavelength / nm
9
.3 (Fig.1a). This suggests that the receptor 1 selectively detects CN
even at neutral pH.
ꢀ
0.3
0.2
0.1
Fig. 2a shows the result of absorption titration of 1 with CN in
a water/MeCN (50/50 v/v) mixture at pH 9.3 under UV irradiation.
(b)
ꢀ
The CN addition leads to a continuous decrease in the 519 nm MC
0
.3
.2
ꢀ
band, along with an increase in the 421 nm 1eCN band. The ab-
sorbance increase is saturated upon addition of 30 equiv of CN .
The receptor 1 associates with CN in a 1:1 stoichiometry. This is
ꢀ
ꢀ
0
0
0
confirmed by a non-linear fitting of the titration data. As described
previously,13 when assuming a 1:1 association between 1 and CN ,
the relationship between the absorbance of the MC form and the
1000
2000
ꢀ
–
[CN ]/[1]
ꢀ
0.1
CN concentration is expressed as follows:
rffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
h
i
2
ꢀ
C ꢀ C ꢀ 4½1ꢁ CN
0
0
A ¼ A þ A
max ꢀ A0
ꢂ
0
(1)
(2)
0
300
400
500
600
700
½
1ꢁ0
2
Wavelength / nm
ꢀ
Fig. 2. Absorption titration of 1 (20
mixture with (a) pH 9.3 and (b) pH 7.5 under UV irradiation at 25 C. (inset) Change in
MC band absorbance, where the lines are the non-linear fitting curves obtained as-
suming a 1:1 association with 1 and CN .
m
M) with CN in a buffered water/MeCN (50/50 v/v)
ꢃ
h
ꢀi
1
C ¼ ½1ꢁ þ CN 0þK
0
ꢀ
ass

Y. Shiraishi et al. / Tetrahedron 67 (2011) 891e897
893
ꢀ
ꢀ
ꢀ
ꢀ
where [1]
respectively, A
0
and [CN ]
0
are the initial concentrations of 1 and CN ,
CN and F in MeCN. The 427 nm band is blue-shifted as compared
ꢀ
0
is the absorbance of 1 obtained without anion, A is
the absorbance of 1 obtained with CN , Amax is the absorbance of 1
obtained with excess amount of CN , and Kass is the association
constant. As shown by the line in inset of Fig. 2a, the titration data
are fully explained with the calculated curve, indicating that 1 in-
deed associates with CN in a 1:1 stoichiometry. The association
constant, Kass, is determined to be 9.8ꢂ10 M (Table 1).
to the 1eCN band (443 nm), indicating that the interaction of 1
with F produces the species with a different electronic state.
Effect of water content on the association constant between 1
and CN was studied. As shown in Table 1, Kass in pure MeCN is
ꢀ
ꢀ
ꢀ
ꢀ
6
ꢀ1
, but the value decreases with an increase in water
5.5ꢂ10
M
ꢀ
4
3
content: 2.5ꢂ10 (10/90 v/v water/MeCN)>9.8ꢂ10 (50/50 v/v)>
3
ꢀ1
3
ꢀ
6.0ꢂ10 (70/30 v/v). This indicates that the reaction of 1 with CN is
Table 1
ꢀ
ꢀ
The first-order rate constant (k), activation energy (E
a
), and association constant (Kass) for interaction between 1 and CN or F in a water/MeCN mixture
ꢃ
ꢀ
1 a
ꢀ1
k/s
Anion
Water/MeCN/v/v
70/30
pH
9.3
E
a
/kJ mol
T/ C
K
ass
ꢀ
ꢀ4 c
ꢀ4 c
CN
33.1ꢄ14.5
15
5
0
15
5
0
25
15
5
0
15
5
6.66ꢂ10
8.50ꢂ10
1.97ꢂ10
1.70ꢂ10
2.44ꢂ10
3
3
ꢀ1 b
ꢀ1
2
4
(6.00ꢄ0.50)ꢂ10
M
ꢀ
ꢀ
3 c
3 c
50/50
9.3
26.9ꢄ1.1
ꢀ3 c
3 c
2
4
(9.80ꢄ0.20)ꢂ10
80.0ꢄ0.7 Mꢀ
M
4.17ꢂ10ꢀ
1
7.5
3.74ꢂ10ꢀ
3 c
1
0
0/90
/100
9.3
9.4ꢄ4.5
4
ꢀ1 b
ꢀ3 c
2
4
(2.50ꢄ0.30)ꢂ10
M
4.57ꢂ10
5.15ꢂ10
ꢀ
3 c
>2.87ꢂ10ꢀ
2 d
6
8
ꢀ1 b
ꢀ2
2
(5.50ꢄ0.50)ꢂ10
(1.83ꢄ0.11)ꢂ10
M
ꢀ
ꢀ2 e
ꢀ2 e
2 e
F
0/100
25.6ꢄ10.6
15
5
0
1.43ꢂ10
2.40ꢂ10
2
4
M
3.40ꢂ10ꢀ
a
The Arrhenius plot of the kinetic data are summarized in Fig. S4 (Supplementary data).
b
c
The absorption titration data for these samples are summarized in Fig. S1 (Supplementary data), where the non-linear fitting results are also shown.
The kinetic absorption data are summarized in Fig. S2 (Supplementary data).
d
e
ꢀ
The reaction of 1 with CN terminates within 1 min, and the accurate rate constant cannot be determined.
The kinetic absorption data are summarized in Fig. S3 (Supplementary data).
Fig. 2b shows the absorption titration results obtained at pH 7.5.
suppressed by an increase in water content. This is because the
protonation of CN occurs more significantly in a solution with
a larger water content.
ꢀ
The spectral change is similar to that obtained at pH 9.3 (Fig. 2a),
ꢀ
but the change is saturated upon addition of 2000 equiv of CN . As
shown in inset, the titration data are well explained by a calculated
Effectof watercontentonthe activation energy, E
a
, of thereaction
ꢀ
ꢀ
line, indicating that 1 associates with CN in a 1:1 stoichiometry
between 1 and CN was studied based on the kinetic absorption
ꢀ
1
13
even at neutral pH. The Kass value is 80 M , which is much lower
measurement. As described previously, the first-order rate con-
3
ꢀ1
ꢀ1
than that obtained at pH 9.3 (9.8ꢂ10 M ). It is well known that
stant for reaction, k (s ), can be expressed with the absorbance of
ꢀ
11b
ꢀ
CN is protonated in acidic or neutral pH, as follows.
the 1eCN species, as follows:
þ
ꢀ
H
þ CN %HCN ðpK
a
¼ 9:2Þ
ꢀ
(3)
ꢀ
ꢁ
A
N
ꢀ A
t
ln
¼ kt
(4)
The protonation of CN at neutral pH therefore probably sup-
presses the reaction between 1 and CN , resulting in low associa-
tion constant. The results indicate that the pH of solution does not
affect the CN selectivity of 1, but strongly affects the association
AN ꢀ A
0
ꢀ
ꢀ
A
0
, A
t
, and A
N
are the absorbance of the 1eCN species at time 0,
ꢀ
t, and infinity, respectively. The measurement was carried out with
0 equiv of CN to 1. UV light was irradiated to the solution con-
ꢀ
5
strength.
ꢀ
taining 1 for 5 min. CN was added to the solution, and the spectral
measurement was started with continued UV irradiation. The ki-
netic absorption data obtained at different temperature are shown
in Fig. S2 (Supplementary data), and the rate constants are sum-
marized in Table 1. The rate constant increases with a decrease in
water content of the solution, indicating that the protonation of
2
.2. Effect of water content
Fig. 1c and d show the absorption spectra of 1 measured in a 20/
0 v/v water/MeCN mixture (pH 9.3) and pure MeCN, respectively.
8
ꢀ
CN affects strongly the reaction with 1. In pure MeCN, the ab-
The absorption spectra obtained without anion (shown by blue and
red line) are similar to that obtained in a 50/50 v/v mixture (Fig. 1a).
It is noted that the MC absorption band generated by UV irradiation
red spectra) blue-shifts with an increase in water content; 0% so-
lution shows a band at 560 nm, while 20 and 50% solutions show at
28 and 519 nm, respectively. This is because the MC form is sta-
bilized by a hydrogen bonding interaction with water molecules.
In a 20/80 v/v water/MeCN solution (Fig. 1c), addition of anions
show absorption change similar to that obtained in a 50/50 v/v
ꢀ
sorption change after CN addition terminates within 1 min, and
the rate constant cannot be determined. The activation energy (E
of the interaction is determined with the Arrhenius equation, as
follows:
a
)
(
13
5
ln k ¼ ln A ꢀ E
a
(5)
14
RT
Fig. S4 (Supplementary data) shows the Arrhenius plot of the
kinetic data. As summarized in Table 1, E values increase with an
ꢀ
mixture (Fig. 1a), indicating that 1 selectively detects CN regard-
a
less of water content. Absorption behavior in pure MeCN is, how-
ever, different. As shown in Fig. 1d, addition of F to 1 in MeCN also
increase in water content of the solution. This again suggests that
the protonation of CN occurs more significantly in the solution
ꢀ
ꢀ
leads to an appearance of blue-shifted absorption at 427 nm, as
does CN . This suggests that the MC form of 1 associates with both
with larger water content and suppresses the reaction of 1 with
CN .
ꢀ
ꢀ

894
Y. Shiraishi et al. / Tetrahedron 67 (2011) 891e897
L
2
.3. Response of 1 with F in MeCN
0.667
0
0
.4
.2
0
As shown in Fig. 1d, 1 dissolved in pure MeCN shows a blue-
ꢀ
ꢀ
shifted absorption upon addition of F as well as CN . Fig. 3 shows
ꢀ
the result of absorption titration of 1 with F in MeCN under UV
ꢀ
irradiation. The stepwise F addition leads to a continuous decrease
in the 560 nm MC band, along with an appearance of 427 nm ab-
sorption band. The isosbestic points at 397 and 472 nm suggest that
ꢀ
the interaction of 1 with F produces a single component, as is the
ꢀ
ꢀ
case for CN . However, the absorption titration result of 1 with F
cannot be explained by a non-linear fitting obtained assuming a 1:1
0
.0
0.5
X
1.0
ꢀ
association between 1 and F .
ꢀ
ꢀ
ꢀ
Fig. 5. Job’s plot of 1 with F (X¼[F ]/([F ]þ[1])) in MeCN. The total concentration of 1
ꢀ
and F is 100
mM.
0
0
.4
.2
0
1
.5
0
determined at different temperature are summarized in Table 1. E
for this interaction is determined by the Arrhenius plot analysis to
a
ꢀ
1
ꢀ
be 25.6 kJ mol . In contrast, E
is<9.4 kJ mol , indicating that the interaction of 1 with F is much
a
for reaction between 1 and CN
0
10
20
30
–
ꢀ1
ꢀ
[
F ]/[1]
0
weaker.
L
L
2
.4. Structure of 1eCN and 1e2F species
ꢀ
The structure of the 1e2F species must be clarified. Figs. 6 and
3
00
400
500
Wavelenght / nm
600
700
1
7 show the change in H NMR spectra of 1 obtained by UV irradi-
ꢀ
ꢀ
ation in the presence of CN and F for 10 min. Upon addition of
ꢃ
ꢀ
Fig. 3. Absorption titration of 1 (20
inset) Change in absorbance at 427 nm.
m
M) with F in MeCN under UV irradiation at 25 C.
ꢀ
ꢀ
both CN and F , large decrease in chemical shift (ca. ꢀ0.7 ppm) is
observed for the H proton in the ortho position relative to the
phenolate oxygen of 1. This indicates that the interaction of 1 with
(
g
ꢀ
Compound 1 associates with F in a 1:2 stoichiometry. This is
15
ꢀ
ꢀ
confirmed by the BenesieHildebrand analysis of the absorption
titration data. When assuming a 1:n stoichiometry for association
between 1 and F , the association constant, Kass, is given by the
both CN and F under UV irradiation produce a spirocycle-opened
16,12e
17
species.
As reported, the N-methyl protons of the MC form
ꢀ
shift downfield as compared to the corresponding SP form due to
following equation:
the quaternarization of nitrogen atom. However, in the present
ꢀ
ꢀ
case, addition of CN or F leads to an upfield shift of the N-methyl
A0
A0 ꢀ A
1
protons. This is because the interaction between the spirocarbon
¼
h
i þ 1
(6)
n
ꢀ
ꢀ
ꢀ
with CN or F suppresses the quaternarization of the nitrogen
K
ass
F
ꢀ
13
ꢀ
0
atom. These results indicate that, as is the case for CN ,
F in-
teracts with the spirocarbon of the MC form of 1 and produces
A
0
is the absorbance of 1 without anion, A is the absorbance of 1
ꢀ
ꢀ
ꢀ
a spirocycle-opened species.
obtained with F , and [F ]
shown in Fig. 4, the plot of A
relationship, indicating that 1 associates with F in a 1:2 stoichi-
0
is the initial concentrations of F . As
ꢀ
ꢀ
ꢀ
2
To clarify the geometry of the 1e2F species, ab initio calcula-
0
/(A
0
ꢀA) against 1/[F ]
0
shows a linear
tions were performed at the DFT level within the Gaussian 03
18
ꢀ
program. Fig. 8 shows the optimized geometry of the MC form of
ometry (formation of 1e2F species). Kass is determined from the
ꢀ
ꢀ
ꢀ
8
ꢀ2
1, 1eCN , and 1e2F species, respectively. In the case of 1eCN
species, the distance between the spirocarbon of 1 and the C atom
slope to be 1.83ꢂ10 M . Further confirmation of the 1:2 associ-
ation is made by the Job’s plot analysis; as shown in Fig. 5, the plot
ꢀ
of CN is 1.50 A, which is similar to the CeC covalent bond distance
ꢀ
ꢀ
ꢀ
shows a maximum absorption at X¼[F ]/([F ]þ[1])¼0.667. Kinetic
ꢀ
19
ꢀ
ꢀ
(1.53 A). This suggests that CN associates strongly with the spi-
absorption measurement for interaction between 1 and F was
ꢀ
ꢀ
rocarbon. In the case of 1e2F species, two F atoms are located at
carried out (Fig. S3, Supplementary data). The rate constants
the top and bottom side of the spirocarbon of the indole plane. The
ꢀ
ꢀ
1
1
.4
.2
1
average distance between the spirocarbon and F is 2.36 A, which is
ꢀ
much larger than the CeF covalent bonding distance (1.39 A) but
smaller than the sum of the van der Waals radii of C and F
3.54 A). This suggest that the interaction between the spi-
rocarbon and F is weaker than that of CN . This weak interaction is
probably the reason for almost no response of 1 against F in
aqueous media and the different absorption spectrum of the 1e2F
ꢀ
ꢀ
20
(
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
species from the 1eCN species (Fig. 1).
19
F NMR analysis was carried out to further clarify the structure
of 1e2F species. Fig. 9 shows the NMR chart of a CDCl
ꢀ
3
solution
0
.8
containing 1 and n-tetrabutylammonium fluoride (5 equiv) after
UV irradiation. A new singlet signal appears at ꢀ129.8 ppm in ad-
0
2
/[F ]
4
6
–
2
0
–7
–2
1
10 / M
ꢀ
21
ꢀ
dition to the signals for free F (ꢀ82.0 ppm) and FHF dimer
2
2
ꢀ
(
ꢀ152.5 ppm) formed by the reaction of F with water. The ap-
Fig. 4. BenesieHildebrand plot of the absorption titration data for association of 1 with
ꢀ
ꢀ
ꢀ
pearance of this singlet signal indicates that two F in the 1e2F
species are chemically equivalent. This supports the symmetrical
F
using Eq. (6), assuming a 1:2 stoichiometry (n¼2). The plots assuming 1:1 and 1:3
stoichiometries are shown in Fig. S5 (Supplementary data).

Y. Shiraishi et al. / Tetrahedron 67 (2011) 891e897
895
ꢃ
Fig. 6. 1H NMR spectra (270 MHz, 30 C, CD
CN) of 1 (5 mM) measured (a) in the dark, (b) with 10 equiv of CN and UV irradiation for 10 min, and (c) after UV irradiation of the
ꢀ
3
sample (b) for 1 h.
ꢃ
Fig. 7. 1H NMR spectra (270 MHz, 30 C, CD
CN) of 1 (5 mM) measured (a) in the dark, (b) with 10 equiv of F and UV irradiation for 10 min, and (c) after UV irradiation of the
ꢀ
3
1
sample (b) for 1 h H NMR spectra of (d) 1,3,3-trimethyl-2-methyleneindoline, 2, and (e) 2-formyl-4-nitro-phenol anion, 3.
ꢀ
ꢀ
Fig. 8. Top and side views of calculated structures of (a) the MC form of 1 (B3LYP/6-31þG
*
), (b) 1eCN species (B3LYP/6-31þG
*
), and (c) 1e2F species (B3LYP/6-31G**).
ꢀ
ꢀ
ꢀ
geometry of two F in the calculated 1e2F structure (Fig. 8c). This
also suggests that the spirocarbon of the MC form of 1 simulta-
probably due to the weak nucleophilicity of F as compared to
ꢀ
23
ꢀ
CN ; two F are required for stabilization of the positive charge of
indole moiety. The weaker association with F than CN is sup-
ported by the higher activation energy (Table 1) and the longer
distance between the spirocarbon and F (Fig. 8). Further confir-
mation is made by H NMR analysis. As reported, the olefinic H
proton of the MC form is magnetically deshielded due to the ring
ꢀ
ꢀ
ꢀ
neously associates with two F . The isosbestic points observed in
the absorption titration data with F (Fig. 3) is therefore due to the
simultaneous interaction between 1 and two F .
ꢀ
ꢀ
ꢀ
ꢀ
1
24
1
associates with CN in a 1:1 stoichiometry, but associates with
in a 1:2 stoichiometry. The 1:2 association between 1 and F is
i
ꢀ
ꢀ
F

896
Y. Shiraishi et al. / Tetrahedron 67 (2011) 891e897
1
irradiation time (427/434 nm). Figs. 6c and 7c show the H NMR
ꢀ
ꢀ
spectra of the 1eCN and 1e2F species after UV irradiation for 1 h.
ꢀ
As shown in Fig. 6c, the 1eCN spectrum is similar to that obtained
before irradiation (Fig. 6b), indicating that the 1eCN species is
ꢀ
stable against UV irradiation. In contrast, UV irradiation of the
ꢀ
1
e2F species shows a spectrum (Fig. 7c) completely different from
the spectrum obtained before irradiation (Fig. 7b). This clearly
ꢀ
suggests that the 1e2F species is decomposed by UV irradiation.
ꢀ
Although the decomposition product of the 1e2F species can-
not be identified at the present stage, some of the chemical shifts of
1
the products can be assigned. Fig. 7d and e show the H NMR spectra
Fig. 9. 9F NMR spectrum (376 MHz, 30 ꢃC, CDCl
) of 1 (30 mM) measured with 5 equiv
1
3
of 1,3,3-trimethyl-2-methyleneindoline (2) and 2-formyl-4-nitro-
phenol anion (3), respectively, which are the precursors used for the
ꢀ
of F and UV irradiation for 10 min.
13
0
0
,
synthesis of 1. The chemical shifts of the aromatic protons (H
H , and H
f h
c
, H
0
) for 2 at 6.90e7.25 ppm are also observed in the spec-
d
0
current effect by the adjacent indole moiety and shifts downfield as
compared to the SP form. However, as shown in Fig. 6, the H
of 1 shifts upfield upon addition of CN and UV irradiation. This is
because the interaction between the spirocarbon and CN leads to
trum of the decomposition product (Fig. 7c). In addition, as shown in
i
proton
0
ꢀ
Fig. 7e, distinctive three signals for 3 (doublet H
a
, 8.35 ppm; doublet
0
0
ꢀ
of doublets H
b
, 7.96 ppm; doublet H
g
, 6.44 ppm) are also observed
in the spectrum of the decomposition product (Fig. 7c). These data
a decrease in electron density of the indole moiety and suppresses
ꢀ
the ring current effect.13 In the case of F (Fig. 7), the H
ꢀ
imply that UV irradiation of the 1e2F species leads to a dissociation
i
proton of 1
of the C]C bond of the MC form, producing the fragments con-
taining the indole or nitroaromatic moiety. This C]C bond cleavage
promoted by UV irradiation is probably due to the high electron
negativity of the adjacent F atom, as is observed for related fluori-
still shifts downfield. This is probably because the decrease in
electron density of the indole moiety is lower than that of CN due
to the weak association between the spirocarbon and F .
ꢀ
ꢀ
2
5
nated compounds. It must be noted that, as shown in the Fig. 7c,
the decomposition product shows a chemical shift at lower mag-
netic field (10.21 ppm), which is probably assigned to the aldehyde
L
L
2
.5. Photostability of 1eCN and 1e2F species
ꢀ
ꢀ
The stability of the 1eCN and 1e2F species against UV irra-
diation must be noted. Fig. 10 shows the change in absorption
spectra of the 1eCN and 1e2F species in pure MeCN during UV
irradiation. The spectrum of the 1eCN species scarcely changes
even after UV irradiation for 1 h (Fig. 10a). However, as shown in
ꢀ
proton as observed for compound 3. This implies that the F -pro-
moted cleavage of the C]C bond followed by oxidation with mo-
ꢀ
ꢀ
lecular oxygen is involved in the decomposition pathway of the
ꢀ
ꢀ
1
e2F species.
ꢀ
Fig. 10b, the spectrum of the 1e2F species red shifts with the
3. Conclusion
The detailed properties of spiropyran derivative, 1, for anions in
(a)
0
0
0
.6
.4
.2
0
aqueous or organic media have been studied under UV irradiation.
ꢀ
In aqueous media, 1 selectively interacts with CN (formation of
1
0
70 min
ꢀ
1
eCN species) regardless of pH and water content of the solution.
The reaction is, however, suppressed by a decrease in pH and an
ꢀ
increase in water content due to the protonation of CN . In contrast,
ꢀ
in pure MeCN, addition of F also creates a blue-shifted absorption
ꢀ
band as well as CN . This is due to the nucleophilic interaction
ꢀ
ꢀ
between 1 and F in a 1:2 stoichiometry (formation of 1e2F
ꢀ
species). The 1eCN species is robust against UV irradiation. In
contrast, UV irradiation of the 1e2F species leads to a fragmenta-
tion of the MC form of 1, probably due to the F -promoted C]C
ꢀ
ꢀ
4
00
450
500
Wavelength / nm
bond cleavage.
4
4
. Experimental section
(
b)
0
0
0
.6
.4
.2
0
10
70 min
.1. Materials
All of the reagents used were supplied from Wako, Aldrich, and
Tokyo Kasei and used as received. Water was purified by the Milli Q
system. The receptor 1 was synthesized according to the procedure
13
described previously.
.2. Analysis
Absorption spectra were measured with magnetic stirring using
4
4
00
450
500
Wavelength / nm
a 10 mm path-length quart cell on a UV-visible photodiode-array
spectrophotometer (Shimadzu; Multispec-1500) equipped with
a temperature controller (Shimadzu; S-1700). UV light was irra-
diated with a Xenon lamp (300 W; Asahi Spectra Co. Ltd.; Max-302)
equipped with 334 nm band-pass filter (LX334; light intensity,
ꢀ
ꢀ
Fig. 10. Time-dependent change in absorption spectra of (a) 1eCN and (b) 1e2F
species measured in MeCN under UV irradiation (334 nm). The measurements were
carried out as follows: 1 (20 mM) and CN or F (50 equiv) were stirred in MeCN under
UV irradiation for 10 min to generate the respective species, and measurements were
started with continued UV irradiation.
26
ꢀ ꢀ
ꢀ2 27 1
1.10 W m ).
H NMR spectra were obtained by a JEOL JNM-

Y. Shiraishi et al. / Tetrahedron 67 (2011) 891e897
897
GSX270 Excalibur using TMS as an internal standard. 19F NMR
spectra were obtained by a Bruker DPX-400 using CF
12879e12885; (c) Hong, S.-J.; Yoo, J.; Kim, S.-H.; Kim, J. S.; Yoon, J.; Lee, C.-H.
Chem. Commun. 2009, 189e191; (d) Park, I. S.; Jung, Y.-S.; Lee, K.-J.; Kim, J.-M.
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3 6 5
C H
2
8
(
ꢀ63.9 ppm) as an external standard in CDCl
3
.
1
4
.3. Computational details
4
2
858e4861; (d) Agou, T.; Sekine, M.; Kobayashi, J.; Kawashima, T. Chem.dEur. J.
009, 15, 5056e5062.
Preliminary geometry optimizations were performed using the
ꢁ~
1
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tight convergence criteria at the DFT level with the Gaussian 03
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1
3
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Acknowledgements
7
1
9
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 Prof. Y.
(
(
i) Lou, X.; Qiang, L.; Qin, J.; Li, Z. ACS Appl. Mater. Interfaces 2009, 1, 2529e2535;
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Das, A. Org. Lett. 2010, 12, 3406e3409.
19
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Tobe and Dr. K. Tahara (Osaka University) for F NMR analysis.
1
1
1
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b
1
1
Figs S1eS5, and Cartesian coordinates for the MC form of
ꢀ
ꢀ
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1
, 1eCN , and 1e2F species. Supplementary data related to this
article can be found online at doi:10.1016/j.tet.2010.12.021. These
data include MOL files and InChiKeys of the most important com-
pounds described in this article.
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