Rapid colorimetric sensing of cyanide anion in aqueous media with a spiropyran derivative containing a dinitrophenolate moiety

Article abstract of DOI:10.1016/j.tetlet.2011.01.110

A spiropyran derivative containing a dinitrophenolate moiety (2: 1′,3′,3′-trimethyl-6,8-dinitro-spiro-[2H-1-benzopyran-2, 2′-indoline]) behaves as a receptor for selective detection of cyanide anion (CN^-) in aqueous media. Compound 2, when dissolved in aqueous media, spontaneously produces the spirocycle-opened merocyanine (MC) form even in dark condition. The absorption band of the MC form decreases selectively upon addition of CN^-, via a nucleophilic addition of CN^- to the spirocarbon of the MC form. The nucleophilic addition occurs very rapidly (within 1 min) and enables rapid and selective quantification of very low levels of CN^- (>0.8 μM) by an absorption analysis.

Full text of DOI:10.1016/j.tetlet.2011.01.110

Tetrahedron Letters 52 (2011) 1515–1519
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Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
Rapid colorimetric sensing of cyanide anion in aqueous media with
a spiropyran derivative containing a dinitrophenolate moiety
⇑
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
A spiropyran derivative containing a dinitrophenolate moiety (2: 10,30,30-trimethyl-6,8-dinitro-spiro-[2H-
1-benzopyran-2,20-indoline]) behaves as a receptor for selective detection of cyanide anion (CNꢀ) in
aqueous media. Compound 2, when dissolved in aqueous media, spontaneously produces the spirocy-
cle-opened merocyanine (MC) form even in dark condition. The absorption band of the MC form
decreases selectively upon addition of CNꢀ, via a nucleophilic addition of CNꢀ to the spirocarbon of
the MC form. The nucleophilic addition occurs very rapidly (within 1 min) and enables rapid and selective
Article history:
Received 17 December 2010
Revised 21 January 2011
Accepted 26 January 2011
Available online 1 February 2011
Keywords:
Spiropyran
Receptor
Cyanide
quantification of very low levels of CNꢀ (>0.8
l
M) by an absorption analysis.
Ó 2011 Elsevier Ltd. All rights reserved.
Water
Colorimetric
Cyanide anion (CNꢀ) is extremely toxic to living organisms;1 it
strongly binds the active site of cytochrome C and inhibits the
electron-transport chain in mitochondria, causing a decreased oxi-
dative metabolism and oxygen utilization.2 The maximum permis-
analysis. The MC form of 1 has a ground state energy higher than
that of the SP form. Therefore, in the current method, UV irradia-
tion is necessary to generate the MC form for reaction with CNꢀ.
For simple CNꢀ analysis, it is thus desirable to omit the UV irradi-
ation from the procedure.
The stability of the MC form depends on the functional groups
attached on the spiropyran platform.9 Hobley et al.10 have reported
that the MC form of the spiropyran derivative containing a dinitro-
phenolate moiety (2: 10,30,30-trimethyl-6,8-dinitro-spiro-[2H-1-
benzopyran-2,20-indoline], Scheme 2) is more stable than the
corresponding SP form. Compound 2, when dissolved in polar sol-
vents, such as chloroform, DMSO, or acetone, is spontaneously
isomerized to the MC form even without UV irradiation.
sive level of cyanide in drinking water is, therefore, set at 1.9 lM
by the World Health Organization (WHO).3 There is thus a strong
need for CNꢀ-selective receptors that rapidly detect CNꢀ in aque-
ous media by simple spectroanalysis. Various colorimetric or fluo-
rometric CNꢀ-selective receptors have been proposed so far. Many
of these receptors, however, display relatively poor selectivity4 and
a high detection limit (>1.9 l
M).5 CNꢀ receptors with high selectiv-
ity and sensitivity have also been proposed; however, many of
these require long assay time for sensing (>10 min). To the best
of our knowledge, there are only five reports of CNꢀ receptors with
high selectivity, high sensitivity, and short assay time (<5 min).6
Earlier, we reported that a simple spiropyran derivative (1:
10,30,30-trimethyl-6-nitro-spiro-[2H-1-benzopyran-2,20-indoline]),
a photochromic dye,7 behaves as a selective and sensitive CNꢀ
receptor in aqueous media under UV irradiation (Scheme 1).8 The
colorless spirocyclic (SP) form of 1 is converted into the colored
merocyanine (MC) form upon UV irradiation. The nucleophilic
addition of CNꢀ to the positively-charged spirocarbon of MC form
leads to a formation of the 1ꢀCNꢀ adduct, which shows a distinc-
tive absorption band at 350–500 nm. This thus allows quantifica-
NO2
UV
N
O
NO2
N
O
1
1
(SP)
(MC)
-
CN
O
tion of very low levels of CNꢀ (>1.7
lM) by an absorption
NO2
N
CN
-
1-CN
⇑
Corresponding author. Tel.: +81 6 6850 6271; fax: +81 6 6850 6273.
E-mail address: shiraish@cheng.es.osaka-u.ac.jp (Y. Shiraishi).
Scheme 1. Structure change of 1 in the presence of CNꢀ.
0040-4039/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2011.01.110

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Y. Shiraishi et al. / Tetrahedron Letters 52 (2011) 1515–1519
Table 1
NO2
Equilibrium data between the SP and MC forms of 1 or 2, and kinetic data for reaction
of the MC forms with CNꢀ measured in a water/MeCN mixture (5:5 v/v; CHES
100 mM, pH 9.3)a
N
O
NO2
N
O
NO2
(slow)
H– (298 K)
(kJ molꢀ1
T
(°C)
Kequiv
kCN
b
O2N
eMC
D
Hisom
D
(Mꢀ1 cmꢀ1
)
(kJ molꢀ1
)
)
(10ꢀ3 sꢀ1
)
-
2
2
CN
(MC)
(SP)
1
2
2.86 ꢁ 104
3.31 0.45
22.6 6.0
15
25
40
50
60
5
15
25
40
50
60
1.42c
1.83c
3.24c
0.48
0.52
0.54
0.56
NO2
O
2.89 ꢁ 104
ꢀ7.03 1.44
13.7 0.5
41.3
53.6
66.9
NO2
N
CN
-
2.58
2.18
2.02
1.92
2-CN
Scheme 2. Structure change of 2 in the presence of CNꢀ.
a
The equilibrium and kinetic absorption data are summarized in Figures S4–S7
(Supplementary data).
In the present work, compound 2 was employed as a receptor
for colorimetric CNꢀ sensing (Scheme 2). The receptor dissolved
in aqueous media produces the MC form with high concentration
even in the dark condition. In addition, the nucleophilic CNꢀ addi-
tion to the spirocarbon of the MC form occurs very rapidly (within
1 min), thus enabling rapid determination of very low levels of CNꢀ
b
Molar extinction coefficient for the MC forms determined according to proce-
dure in Ref. 16.
c
Measured under irradiation of UV light (334 nm; light intensity, 1.10 W mꢀ2).
(0.8 lM) without UV irradiation.
The receptor 2 was obtained by N-methylation of 2,3,3-tri-
methyl-3H-indole with CH3I followed by condensation with 3,5-
dinitrosalicylaldehyde in 64% yield (see Synthesis, Supplementary
data). The purity of 2 was fully confirmed by 1H, 13C NMR, and
EI-MS analyses (Figs. S1–S3, Supplementary data).10a
Figure 1 shows the absorption spectra of 2 (20 lM) measured in
a buffered water/MeCN mixture (2:98 v/v; CHES 50 mM, pH 9.3) in
the dark condition. As shown by the red line, without anion, 2
shows a distinctive absorption band centered at 523 nm, assigned
to the MC form.10 This suggests that 2 produces the MC form even
in the dark condition. This spontaneous MC formation is due to the
lower ground state energy of the MC form than that of the corre-
sponding SP form. This is confirmed by the equilibrium constants
between the SP and MC forms (Kequiv) and the standard enthalpy
for isomerization (
(Table 1).11 In the case of compound 1 (Scheme 1), Kequiv values are
<1 and
Hisom is positive (3.3 kJ molꢀ1). This indicates that, as
DHisom), determined by the equilibrium analysis
D
shown in Figure 2a, the SP?MC isomerization of 1 is not favored
thermodynamically and the MC form is less stable than the SP
form, as is usually observed for the related spiropyran deriva-
tives.12 In contrast, Kequiv values of 2 are >1 and
DHisom is negative
(ꢀ7.0 kJ molꢀ1). This clearly indicates that, as shown in Figure 2b,
Figure 2. Energy diagrams for equilibrium between the SP and MC forms and the
0.5
reaction with CNꢀ for (a) 1 and (b) 2. The
D
Hisom and
D
H– data were obtained in a
water/MeCN mixture (5:5 v/v; CHES 100 mM, pH 9.3).
none
-
-
- -
0.4
0.3
0.2
0.1
0
F , Cl , Br , I ,
-
-
-
-
CN
AcO , H2PO4 , HSO4
,
-
-
-
ClO4 , NO3 , SCN ,
the ground state energy of the MC form of 2 is lower than that of
the SP form and, hence, results in spontaneous MC formation even
in the dark condition.
-
2-
S2 , SO3
Each respective anion (3 equiv) was added to the solution con-
taining 2 and the change in absorption spectra was monitored. As
shown in Figure 1, most of the anions show spectra similar to that
of the MC form of 2. In contrast, addition of CNꢀ leads to a decrease
in the MC band, indicating that 2 associates selectively with CNꢀ. It
must be noted that the CNꢀ-induced spectral change is unaffected
by other anions (Fig. S8, Supplementary data), suggesting that 2
detects CNꢀ selectively even in the presence of other anions. An-
other notable feature of 2 is the rapid reaction with CNꢀ. As shown
in Figure 3, the CNꢀ-induced spectral change of 2 terminates with-
in 1 min, while the probe 1 requires 25 min under UV irradiation
300
400
500
600
Wavelength / nm
Figure 1. Absorption spectra of 2 (20 lM) measured (red) without and (black) with
3 equiv of each respective anion as n-Bu4N+ (CNꢀ, Fꢀ, Brꢀ, Iꢀ, AcOꢀ, H2POꢀ4 , HSOꢀ4
,
ClO4ꢀ, NO3ꢀ and SCNꢀ) and Na+ (S2ꢀ and SO23ꢀ) salts in a buffered water/MeCN
mixture (2:98 v/v; CHES 50 mM, pH 9.3) in the dark condition. The red spectrum
was obtained 1 min after dissolution of 2 to the solution. The other spectra were
obtained after stirring the solution containing 2 and each anion for 10 min.

Y. Shiraishi et al. / Tetrahedron Letters 52 (2011) 1515–1519
1517
0.25
0.2
0.5
0.4
0.3
0.2
0.1
0
0.8 μM
A
0.2
0.5
0.4
0.3
0.2
0.1
0
0.15
0
1
2
0
2
4
6
Time / min
A
0.1
0
12 μM
0
10
[CN–] /
20
30
μM
Figure 5. Change in the MC absorbance of 2 (10 l
M) upon CNꢀ addition. The
titration was performed in a water/MeCN (8:2 v/v; CHES 100 mM, pH 9.3) in the
300
400
500
600
Wavelength / nm
dark at 25 °C.
Figure 3. Time-dependent change in the absorption spectra of 2 (20
lM) measured
in a water/MeCN mixture (5:5 v/v; CHES 100 mM, pH 9.3) in the dark after addition
of 30 equiv of CNꢀ. (inset) Change in 501 nm absorbance.
Ha
NO2
Hb
Hd
Hh
Hd
NO2
He
Hi
Hf
Hc
Hb
O
Hf
Hi
-
CN
Hc
(Fig. S9, Supplementary data). This indicates that the probe 2 en-
ables rapid CNꢀ detection.
N
O
NO2
NO2
N
He
CN
Hh
Ha
Figure 4 shows the absorption titration result of 2 with CNꢀ in
the dark condition. The isosbestic point at 440 nm suggests that
the reaction of 2 with CNꢀ produces a single component. Nonlinear
fitting of the data (inset) reveals that 2 reacts with CNꢀ in a 1:1 stoi-
chiometry, as does 1.8 The 1:1 reaction is confirmed by ESI-MS anal-
ysis; a MeCN solution of 2 with CNꢀ shows a peak at m/z 393.1,
assigned to [2+CNꢀ] ion (Fig. S10, Supplementary data). The associ-
i)
N-CH3
Hb
Hd,Hh
Hc,Hf
Ha
Hi He
ation constant for this reaction is determined to be 4.0 ꢁ 105 Mꢀ1
,
ii)
which is much larger than that for 1 (9.8 ꢁ 103 Mꢀ1) obtained under
UV irradiation.8 Figure 5 shows the change in absorbance of the MC
band of 2 upon CNꢀ addition. A linear relationship in the range of
Hh
Hi
Hc,Hd
Hb
He
Ha
0.8–12
tration range. The detection limit, 0.8
lines of drinking water (1.9 M) and that of 1 (1.7
that 2 allows sensitive CNꢀ detection.
l
M CNꢀ indicates that 2 enables CNꢀ sensing in this concen-
Hf
lM, is below the WHO guide-
l
l
M), suggesting
4 3
9
8
7
δ / ppm
The CNꢀ-induced absorption change of 2 is due to the nucleo-
philic addition of CNꢀ to the spirocarbon (formation of the 2–
CNꢀ adduct), as is the case for 1 (Schemes 1 and 2).8 This is con-
firmed by 1H NMR spectra of 2 obtained without and with CNꢀ.
As shown in Figure 6i, 2 dissolved in DMSO-d6 shows distinctive
Figure 6. 1H NMR chart of 2 (10 mM) measured in DMSO-d6 (i) without and (ii)
with 1 equiv of CNꢀ in the dark.
chemical shifts assigned to the MC form,10a indicating that 2
mainly exists as the MC form. The CNꢀ addition to the solution
leads to a decrease in chemical shift for all protons (Fig. 6ii). The
olefinic He and Hi protons of both 2 and the adduct show a charac-
teristic trans J-coupling of 16 Hz, indicating that the adduct has a
spirocycle-opened structure, as does 2.13 As shown in Figure 6i,
N-methyl protons of 2 appear at 3.97 ppm; however, CNꢀ addition
leads to an upfield shift (ꢀ1.28 ppm). This is because the CNꢀ addi-
tion to the spirocarbon suppresses the quaternarization of the
nitrogen atom in the indole ring.14 These data clearly indicate that
the reaction of 2 with CNꢀ produces the spirocycle-opened 2–CNꢀ
adduct (Scheme 2).
0.4
0.3
0.2
A
0.4
0.1
0 eq.
0.3
0.2
0.1
0
0
0
5
10
[CN–]/[2]
The blue-shifted absorption of the 2–CNꢀ adduct compared to
the MC form of 2 (Fig. 1) is due to the localization of
p-electrons
on the dinitrophenolate moiety. This is confirmed by ab initio cal-
culation on the time-dependent density functional theory (TDDFT)
using a B3LYP/6-31+G⁄ basis set within the GAUSSIAN 03 program.15
The singlet electronic excitation of 2(MC) and 2–CNꢀ are mainly
contributed by HOMO?LUMO (S0?S1) and HOMO-1?LUMO+1
(S0?S4) transitions, respectively (Table S1, Supplementary data).
Figure 7 shows the respective orbitals for main transitions. The
transition energy of 2(MC) is 2.86 eV, whereas that of 2–CNꢀ is
10 eq.
300
400
500
600
Wavelength / nm
Figure 4. Absorption titration of 2 (20 l
M) with CNꢀ in a water/MeCN mixture (5:5
v/v; CHES 100 mM, pH 9.3) in the dark at 25 °C. (inset) Change in MC absorbance,
where the line is the nonlinear fitting curve obtained assuming a 1:1 association
between 2 and CNꢀ (see Supplementary data).

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Y. Shiraishi et al. / Tetrahedron Letters 52 (2011) 1515–1519
Figure 7. Energy diagrams of the HOMO and LUMO orbitals for 2(MC) and the HOMO-1 and LUMO+1 orbitals for 2–CNꢀ, calculated at the DFT level using a B3LYP/6-31+G⁄
basis set.
3.48 eV, suggesting that 2–CNꢀ has larger transition energy. This is
Acknowledgments
explained by the electronic configuration of respective orbitals. The
p
-electrons of 2(MC) are distributed over the entire molecule. In
This work was supported by Grant-in-Aids for Scientific Re-
search (No. 21760619) from the Ministry of Education, Culture,
Sports, Science and Technology, Japan (MEXT).
contrast,
p
-electrons of 2–CNꢀ are localized on the dinitropheno-
late moiety. This is due to the decrease in electron density of the
indole moiety upon CNꢀ addition. The S0?S1 transition energy
for 2(MC) is 500 nm (2.48 eV, Table S1, Supplementarydata), which
is similar to kmax of the 2(MC) spectrum (523 nm, Fig. 1). In con-
trast, the S0?S4 transition energy for 2–CNꢀ (401 nm, 3.09 eV) is
much lower than that of 2(MC) and is similar to kmax of the 2–
CNꢀ spectrum (396 nm). These closely matched results indicate
Supplementary data
Supplementary data (experimental details, Figures S1–S11, Ta-
ble S1, and Cartesian coordinates for compounds) associated with
this article can be found, in the online version, at doi:10.1016/
j.tetlet.2011.01.110.
that the CNꢀ addition to 2 leads to a localization of the
p-electrons
on the dinitrophenolate moiety, thus resulting in a blue-shift of the
absorption spectrum.
References and notes
The probe 2 reacts with CNꢀ very rapidly; as shown in Figure 3,
the reaction terminates within 1 min. As summarized in Table 1,
1. (a) Koenig, R. Science 2000, 287, 1737–1738; (b) Baud, F. Hum. Exp. Toxicol.
2007, 26, 191–201.
2. Camerino, P. W.; King, T. E. J. Biol. Chem. 1966, 241, 970–979.
3. Guidelines for Drinking-Water Quality; World Health Organization: Geneva,
1996.
kinetic absorption analysis8 reveals that the rate constant (kCN
)
for the reaction of
2
with CNꢀ (30 equiv) in the dark is
6.69 ꢁ 10ꢀ2 sꢀ1 (25 °C). In constant, kCN for the reaction of 1 with
CNꢀ obtained under UV irradiation is 1.83 ꢁ 10ꢀ3 sꢀ1 (25 °C). This
suggests that the nucleophilic CNꢀ addition to 2 occurs much faster
than that to 1.
4. Colorimetric receptors: (a) Lou, X.; Zhang, L.; Qin, J.; Li, Z. Chem. Commun. 2008,
5848–5850; Fluorometric receptors: (b) Kwon, S. K.; Kou, S.; Kim, H. N.; Chen,
X.; Hwang, H.; Nam, S.-W.; Kim, S. H.; Swamy, K. M. K.; Park, S.; Yoon, J.
Tetrahedron Lett. 2008, 49, 4102–4105; (c) Jamkratoke, M.; Ruangpornvisuti, V.;
Tumcharern, G.; Tuntulani, T.; Tomapatanaget, B. J. Org. Chem. 2009, 74, 3919–
3922; (d) Lin, Y.-C.; Chen, C.-T. Org. Lett. 2009, 11, 4858–4861.
5. Colorimetric receptors: (a) García, F.; García, J. M.; García-Acosta, B.; Martínez-
Máñez, R.; Sancenón, F.; Soto, J. Chem. Commun. 2005, 2790–2792; (b) Ros-Lis, J.
V.; Martínez-Máñez, R.; Soto, J. Chem. Commun. 2005, 5260–5262; Fluorometric
receptors: (c) Hudnall, T. W.; Gabbaï, F. P. J. Am. Chem. Soc. 2007, 129, 11978–
11986; (d) Jo, J.; Lee, D. J. Am. Chem. Soc. 2009, 131, 16283–16291.
6. Colorimetric receptors: (a) Tomasulo, M.; Raymo, F. M. Org. Lett. 2005, 7, 4633–
4636; (b) Tomasulo, M.; Sortino, S.; White, A. J. P.; Raymo, F. M. J. Org. Chem.
2006, 71, 744–753; (c) Männel-Croisé, C.; Zelder, F. Inorg. Chem. 2009, 48,
1272–1274; Fluorometric receptors: (d) Sun, Y.; Liu, Y.; Guo, W. Sens. Actuators,
B 2009, 143, 171–176; (e) Sun, Y.; Liu, Y.; Chen, M.; Guo, W. Talanta 2009, 80,
996–1000.
7. (a) Minkin, V. I. Chem. Rev. 2004, 104, 2751–2776; (b) Raymo, F. M.; Tomasulo,
M. Chem. Soc. Rev. 2005, 34, 327–336.
8. Shiraishi, Y.; Adachi, K.; Itoh, M.; Hirai, T. Org. Lett. 2009, 11, 3482–3485.
9. (a) Sunamoto, J.; Iwamoto, K.; Akutagawa, M.; Nagase, M.; Kondo, H. J. Am.
Chem. Soc. 1982, 104, 4904–4907; (b) Shimizu, I.; Kokado, H.; Inoue, E. Bull.
Chem. Soc. Jpn 1969, 42, 1730–1734.
10. (a) Hobley, J.; Malatesta, V.; Millini, R.; Montanari, L.; Parker, W. O. N., Jr. Phys.
Chem. Chem. Phys. 1999, 1, 3259–3267; (b) Hobley, J.; Malatesta, V.; Giroldini,
W.; Stringo, W. Phys. Chem. Chem. Phys. 2000, 2, 53–56; (c) Hobley, J.; Pfeifer-
Fukumura, U.; Bletz, M.; Asahi, T.; Masuhara, H.; Fukumura, H. J. Phys. Chem. A
2002, 106, 2265–2270.
The enhanced CNꢀ addition to 2 is due to the formation of a lar-
ger amount of MC form and the strong positive charge of the spi-
rocarbon. The equilibrium mol fraction of the MC form of 2 at
25 °C in the dark is determined by a molar absorption coefficient
(Table 1)16 to be 54%, which is much larger than that of 1 obtained
under UV irradiation (24%). This indicates that larger amount of MC
form of 2 is produced even without UV irradiation, which might be
one of the reasons for rapid reaction with CNꢀ. As shown in Figure
2, the activation enthalpy,
D
H–, for the reaction of 2 with CNꢀ is
13.7 kJ molꢀ1, which is much lower than that of 1 with CNꢀ
(22.6 kJ molꢀ1). This suggests that the nucleophilic addition of
CNꢀ to 2 is energetically more favorable. The Mulliken atomic
charge on the spirocarbon of 2(MC), as determined by the DFT cal-
culation, is 0.804, which is much higher than that of 1(MC) (0.756).
This is due to the strong withdrawal of p-electrons by the two nitro
groups. The strong positive charge of the spirocarbon of 2(MC),
therefore, probably lowers the activation energy for nucleophilic
CNꢀ addition.17 The above results suggest that the larger amount
of MC form and the strong positive charge of the spirocarbon of
2 enable the rapid reaction with CNꢀ.
In conclusion, we found that the spiropyran derivative, 2, con-
taining a dinitrophenolate moiety enables rapid, selective, and sen-
sitive CNꢀ detection in aqueous media18 without UV irradiation.
This is achieved by the stabilized MC form and the strong positive
charge of the spirocarbon of 2. The results presented here may con-
tribute to the design and development of more efficient and more
useful CNꢀ receptors based on the spiropyran platform.
11. Shiraishi, Y.; Itoh, M.; Hirai, T. Phys. Chem. Chem. Phys. 2010, 12, 13737–13745.
12. Flannery, J. B., Jr. J. Am. Chem. Soc. 1968, 90, 5660–5671.
13. Malatesta, V.; Neri, C.; Wis, M. L.; Montanari, L.; Millini, R. J. Am. Chem. Soc.
1997, 119, 3451–3455.
14. (a) Guo, X.; Zhou, Y.; Zhang, D.; Yin, B.; Liu, Z.; Liu, C.; Lu, Z.; Huang, Y.; Zhu, D. J.
Org. Chem. 2004, 69, 8924–8931; (b) Yagi, S.; Nakamura, S.; Watanabe, D.;
Nakazumi, H. Dyes Pigm. 2009, 80, 98–105.
15. (a) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.;
Cheeseman, J. R., Jr.; Montgomery, J. A.; Vreven, T.; Kudin, K. N.; Burant, J. C.;
Millam, J. M.; Iyengar, S. S.; Tomasi, J.; Barone, V.; Mennucci, B.; Cossi, M.;
Scalmani, G.; Rega, N.; Petersson, G. A.; Nakatsuji, H.; Hada, M.; Ehara, M.;

Y. Shiraishi et al. / Tetrahedron Letters 52 (2011) 1515–1519
1519
Toyota, K.; Fukuda, R.; Hasegawa, J.; Ishida, M.; Nakajima, T.; Honda, Y.; Kitao,
II; Keith, T.; Millam, J.; Eppinnett, K.; Hovell, W. L.; Gilliland, R. GAUSSVIEW
Version 3.09; Semichem: Shawnee Mission, KS, 2003.
16. Heller, C. A.; Fine, D. A.; Henry, R. A. J. Phys. Chem. 1961, 65, 1908–1909.
,
O.; Nakai, H.; Klene, M.; Li, X.; Knox, J. E.; Hratchian, H. P.; Cross, J. B.; Bakken,
V.; Adamo, C.; Jaramillo, J.; Gomperts, R.; Stratmann, R. E.; Yazyev, O.; Austin, A.
J.; Cammi, R.; Pomelli, C.; Ochterski, J. W.; Ayala, P. Y.; Morokuma, K.; Voth, G.
A.; Salvador, P.; Dannenberg, J. J.; Zakrzewski, V. G.; Dapprich, S.; Daniels, A. D.;
Strain, M. C.; Farkas, O.; Malick, D. K.; Rabuck, A. D.; Raghavachari, K.;
Foresman, J. B.; Ortiz, J. V.; Cui, Q.; Baboul, A. G.; Clifford, S.; Cioslowski, J.;
Stefanov, B. B.; Liu, G.; Liashenko, A.; Piskorz, P.; Komaromi, I.; Martin, R. L.;
Fox, D. J.; Keith, T.; Al-Laham, M. A.; Peng, C. Y.; Nanayakkara, A.; Challacombe,
M.; Gill, P. M. W.; Johnson, B.; Chen, W.; Wong, M. W.; Gonzalez, C.; Pople, J. A.
GAUSSIAN 03, Revision B.05; Gaussian: Wallingford CT, 2004; (b) Dennington, R.,
17. (a) Banks, H. D. J. Org. Chem. 2003, 68, 2639–2644; (b) Kuznetsov, M. L. J. Mol.
Struct. 2004, 674, 33–42.
18. The pH of solution affects the sensitivity of 2 to CNꢀ; a basic pH is appropriate
for sensitive CNꢀ sensing and the sensitivity decreases with a pH decrease. The
association constant between
2
and CNꢀ obtained at pH 7.5 (HEPES) is
1.5 ꢁ 104 Mꢀ1 (Fig. S11, Supplementary data), which is much smaller than that
obtained at pH 9.3 (4.0 ꢁ 105 Mꢀ1). This is because, at lower pH range, the
protonation of CNꢀ (pKa = 9.2) suppresses the nucleophilic addition of CNꢀ to 2.