A chemodosimetric gelation system showing fluorescence and sol-to-gel transition for fluoride anions in aqueous media

Article abstract of DOI:10.1039/c2nj20878d

We developed a chemodosimetric gelation system which turns into a fluorescent gel in the presence of fluoride anions in aqueous media. The selective cleavage of a silyl ether protecting group on the gelator by a fluoride anion gives rise to an optical change (fluorescence turn-on) and a sol-to-gel transition in aqueous media. The Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2012.

Full text of DOI:10.1039/c2nj20878d

View Article Online / Journal Homepage / Table of Contents for this issue
Dynamic Article Links
NJC
Cite this: New J. Chem., 2012, 36, 1145–1148
www.rsc.org/njc
LETTER
A chemodosimetric gelation system showing fluorescence and sol-to-gel
transition for fluoride anions in aqueous mediaw
Miae Park, Donghak Jang, Soon Young Kim and Jong-In Hong*
Received (in Gainesville, FL, USA) 12th October 2011, Accepted 29th February 2012
DOI: 10.1039/c2nj20878d
We developed a chemodosimetric gelation system which turns into a
fluorescent gel in the presence of fluoride anions in aqueous media.
The selective cleavage of a silyl ether protecting group on the gelator
by a fluoride anion gives rise to an optical change (fluorescence
turn-on) and a sol-to-gel transition in aqueous media.
Fluorides and protons were reported to trigger gel to sol
transition and fluorescence emission after interaction with
bisurea-functionalized naphthalene organogelators.16 In addition,
organogelators made with urea,17 oxalamide,18 or hydrazide19
were also reported to induce gel to sol transitions and optical
changes after the addition of fluoride anions, because the amidic
NHs of these amide-containing organogelators could interact with
the fluoride anions thus disturbing the effective hydrogen-bonding
interactions between the gelators. However, because amidic
NHs in the organogelators could also interact with other
anions, the initiation of gel to sol transitions and optical
changes would not necessarily be selective to fluoride.
Self-assembled supramolecular gels consist of low molecular
weight gelators (LMWGs) and solvent molecules, in which
noncovalent interactions, such as hydrogen bonding, van der
Waals interactions, p–p stacking, electrostatic forces, and
metal coordination, are crucial to forming three-dimensional
networks.1 Compared to gels derived from polymers or inorganic
materials, self-assembled supramolecular gels made from
LMWGs are relatively easy to modify to control their physical
or optoelectronic properties.2 Furthermore, the properties of gels
derived from LMWGs can be tuned using the external stimuli
that arise from reversible noncovalent interactions.3
Taking advantage of the selective cleavage of a Si–O bond
by fluoride,20 we developed chemodosimetric gelation systems,
1 and 2 (Scheme 1) having a silyl ether protecting group and a
fluorescent signalling group,21,22 that can selectively detect fluoride
anions by the formation of fluorescent gels in aqueous media.
A simple silyl ether protection of a 7-hydroxycoumarin gelator
(3) not only quenches its fluorescence emission but also prevents
gelation. However, when the silyl ether protecting group is
selectively cleaved by fluoride anions, the reaction triggers gelation
and turns on the fluorescence emission. The effective aggregation
between molecules of compound 1 or those of compound 2 is
suppressed because the tert-butyldimethylsilyl (TBDMS) and
tert-butyldiphenylsilyl (TBDPS) groups are relatively bulky. There-
fore, removal of a bulky silyl ether protecting group by fluoride
could induce gelation. At the same time, fluorescence emission
increases upon release of phenolate in coumarin.23
Recently, several gelation systems which induce fluorescence
were reported. One can generally develop fluorescent gels by
incorporating p-conjugated chromophores into LMWGs.4
Spectral shift or turn-on of fluorescence was usually observed
upon sol–gel transition, which triggers excimer (or exciplex)
formation,5 intermolecular exciton coupling,6 conformation
restriction,7 and aggregation-induced enhanced emission.8
Tuning of fluorescence emission wavelength through gelation
has been utilized in light-emitting devices.6a,b,7a,9 In addition,
fluorescent gels doped with chromophores in a gel matrix were
applied to light harvesting antennas10 and fluorescent probes.11
In particular, gelation systems which induced sol–gel transition
and subsequently fluorescence change by external stimulus have
been utilized as chemical probes for volatile solvents12 and
information storage devices.13
Compound 1 was prepared by coupling 7-hydroxycoumarin-
4-acetic acid with dodecylamine, after which a silyl ether moiety
was attached by the reaction with tert-butyldimethylsilyl
chloride.24 We also prepared compound 3 without a silyl ether
In recent years, many researchers have shown interest in the
construction of anion-responsive supramolecular gels.14
Among these, fluoride-responsive gels have received a great
deal of attention because fluoride is involved in preventing
dental caries and in medical treatment for osteoporosis.15
Department of Chemistry, College of Natural Sciences,
Seoul National University, Seoul 151-747, Korea.
E-mail: jihong@snu.ac.kr; Fax: +81-2-889-1568; Tel: +81-2-874-5902
w Electronic supplementary information (ESI) available. See DOI:
10.1039/c2nj20878d
Scheme 1 Molecular structure of compounds 1, 2, and 3, and Si–O bond
cleavage by fluoride.
c
This journal is The Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2012
New J. Chem., 2012, 36, 1145–1148 1145

View Article Online
moiety to examine whether or not 3 would form a gel. First, we
tested the gelation behaviour of 3 using varying solvent conditions.
A white opaque gel was formed, but with little fluorescence
emission, in a 1 : 1 methanol/water cosolvent25 after cooling a
hot solution of 3. The critical gelation concentration for 3 was
0.25 wt%, and was determined using the tube inversion
method. We then tested self-assembling behaviour of 1 at
0.6 wt%. 1 was fully dissolved in methanol, but when an
equal amount of water was added to the solution, a turbid
solution formed. Although this turbid solution turned into a
clear solution when heated, the clear solution turned turbid
again after cooling to room temperature. As expected, the
bulky TBDMS group prevented 1 from aggregating in an
orderly way and forming gels. In the presence of 1.0 equivalent of
Fꢀ, however, an opaque gel was formed, similar to that formed
with 3 (Fig. 1(a)). Interestingly, although the turbid solution of 1
and the gel 3 showed little fluorescence emission, upon the
addition of Fꢀ to suspension of 1, the intensity of the fluores-
cence emission increased dramatically in the gel phase (Fig. 1(b)).
Quantum yield of fluorogenic gels derived from 1 + Fꢀ is 0.15
with a lifetime of 33 ns, although the quantum yield is lower than
that of solution phase 7-hydroxycoumarin.26
This sol-to-gel transition induced by Fꢀ occurs as a result of
the effective aggregation of gelators, which is made possible
by the cleavage of a silyl ether group. This cleavage also results
in a large enhancement of fluorescence. Compound 2 behaved
similarly, but was not as soluble as 1 in the same solvent system,
and a higher concentration was necessary for gel formation.
SEM images of the xerogels of 3 and 1 + Fꢀ exhibited
bundles of fibrous structures of different sizes (Fig. 1(c) and
(d)). To examine differences in the self-assembled structures
and fluorescence properties, we performed wide-angle X-ray
scattering (WAXS) and 1H NMR analysis. The 1H NMR
spectra of the xerogels of 1 + Fꢀ indicated that the silyl ether
protecting groups were not completely cleaved by Fꢀ (Fig. 2).
Fig. 2 1H NMR spectra of 1, xerogel of 1 + Fꢀ, and 3 in CD3OD.
Symbols under the bottom spectrum indicate chemical shifts of solvent
residuals and impurity.z: H2O, y: MeOH, and z: diisopropylurea (impurity).
Residual signals representing 1 were still observed even after
heating with Fꢀ.
This partial removal of the silyl ether protecting groups
seemed to influence the fluorescence and gelation behaviours
of 1 + Fꢀ. When 1 was treated with 1.0 equivalent of NaF,
nonfluorogenic 1 (in which a TBDMS group remained intact)
seemed to be intercalated between fluorogenic products
formed by the removal of the silyl ethers on 1. Therefore, the
self-quenching effect, which arises from the complete aggrega-
tion of fluorescent molecules, was attenuated. This assumption
was confirmed after investigating the self-assembled structures
in a mixture of 1 and 3 in an appropriate ratio under the same
gelation conditions. A 1 : 2 mixture of 1 and 3 formed gels that
were fluorescent, but a 1 : 1 mixture of 1 and 3 did not form gels
(Fig. S1, ESIw). In particular, gels derived from a 1 : 2 mixture
of 1 and 3 at pH 9.00 demonstrated a stronger fluorescence
emission than those formed at pH 7.40 because the phenolate
form of gelators would strengthen ICT than the phenol form of
gelators. Furthermore, gels formed after the complete removal
of the TBDMS protecting groups on 1 by Fꢀ showed relatively
weak fluorescence, but the gels of 1 + Fꢀ in which the TBDMS
groups were not completely removed emitted a more intense
fluorescence (Fig. S2, ESIw). Densely stacked coumarin moieties
in gels formed after the complete removal of the TBDMS
protecting groups would strengthen the self-quenching effect
which induces a decrease of fluorescence.
According to WAXS data, gels derived from 3 and 1 + Fꢀ
are similarly self-assembled, leading to lamellar-type structures
(Fig. 3). WAXS spectra revealed that depth and width of a unit
cell are about 25.6 A and 6.8 A, respectively. Considering that
the length of molecule 3 is about 22.5 A27 and the optimal
distance for van der Waals interaction is 3.8 A,28 we could
expect that gels derived from 3 and 1 + Fꢀ are self-assembled
into fibril structures with gelators interlocked with each other
like a zipper (Fig. S4, ESIw). However, in the case of gels
derived from 1 + Fꢀ, interactions among fibrils were disturbed
by projecting TBDMS moieties. Consequently, self-assembled
fibrils in gels from 1 + Fꢀ remained as thin fibrils unlike gels
from 3.
Fig. 1 (a) Visual photograph and (b) fluorescence photograph of 3
(0.6 wt%) (left), 1 + Fꢀ (0.6 wt% of 1, 1.0 equivalent NaF) in a 1 : 1
methanol/10 mM HEPES buffer (pH 7.40) cosolvent before the
gelation process (middle), and after the gelation process (right).
Fluorescence emission was observed upon excitation by UV irradiation
(lex = 365 nm). SEM images of xerogels of (c) 3 and (d) 1 + Fꢀ. Scale
bar = 20 mm.
To confirm the selective response of 1 to Fꢀ, gelation
behaviour was investigated upon the addition of various other
anions. In contrast to fluoride, Clꢀ, Brꢀ, Iꢀ, H2PO4ꢀ, AcOꢀ,
c
1146 New J. Chem., 2012, 36, 1145–1148
This journal is The Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2012

View Article Online
We developed chemodosimetric gelation systems which turn
into fluorescent gels in aqueous solution when their silyl
protecting groups are cleaved by Fꢀ. The above-mentioned
system is highly selective to Fꢀ and exhibits both gelation and
strong fluorescence as optical signals. This selectivity is a result
of the affinity between Fꢀ and silanol derivatives, which
distinguishes this system from the amide-based Fꢀ responsive
gel systems previously reported.
Experimental
Synthesis of 1
Fig. 3 Wide angle X-ray scattering spectra of gels from 3 and 1 + Fꢀ.
To a mixture of 3 (201 mg, 0.52 mmol) and imidazole (114 mg,
1.68 mmol) in 2 mL DMF, tert-butyldimethylchlorosilane
(97 mg, 0.65 mmol) was added and the reaction mixture was
stirred at rt for 6 h. A white turbid solution was formed after
the addition of about 50 mL of water. The addition of ethyl
acetate was followed by washing with a large volume of
water several times to give a solution, which was dried over
anhydrous Na2SO4 and concentrated. After silica-column
chromatography (CH2Cl2 to CH2Cl2 : MeOH = 100 : 1), the
light yellowish solid 1 was obtained (191 mg, 73.4% yield).
1H NMR (300 MHz, CDCl3): d 0.27 (s, 6H), 0.89 (t, J =
6.8 Hz, 3H), 1.00 (s, 9H), 1.23–1.31 (m, 18H), 1.45 (t, J =
6.2 Hz, 2H), 3.24 (q, J = 6.7 Hz, 2H), 3.66 (s, 2H), 5.56
(t, 1H), 6.24 (s, 1H), 6.79 (d, J = 2.4 Hz, 1H), 6.82 (s, 1H),
7.56 (d, J = 9.4 Hz, 1H). 13C NMR (75 MHz, CDCl3): d ꢀ4.4,
14.1, 18.3, 22.7, 25.6, 26.8, 29.2, 29.4, 29.5, 29.6, 29.6, 31.9,
40.0, 41.0, 107.9, 113.0, 113.7, 117.6, 126.0, 149.7, 155.3, 159.7,
161.0, 167.3. HRMS (FAB+): m/z calcd for C29H48NO4Si
[M + H]+: 502.3353, found: 502.3342.
ꢀ
NO3ꢀ, ClO4ꢀ, and N3 did not induce gelation or fluorescence
enhancement using the same solvent conditions. These anions
produced clear solutions upon heating, but only yielded turbid
solutions after cooling. Only fluoride anions induced both gelation
and fluorescence enhancement (Fig. 4). The other anions did not
break the Si–O bond in the coumarin and, therefore, did not lead
to gel formation or fluorescence emission. We also investigated
whether pH influenced gelation behaviours of 1. Although
suspension of 1 in MeOH + 1.5 N aq. HCl (v/v, 1 : 1) was heated
until forming a clear solution and subsequently cooled to room
temperature, the acidic solution of 1 didn’t show any changes such
as gelation. In the case of strong basic condition (1.5 N aq.
NaOH), heated solution of
1 formed greenish solution
without fluorescence. In the case of mild basic condition (1 equiv.
NaHCO3), heated solution of 1 also didn’t form gels, but exhibited
very weak fluorescence emission (Fig. S3, ESIw).
Synthesis of 2
The synthetic procedure for the production of 2 was identical
to that of 1, except that tert-butyldiphenylchlorosilane was
used instead of tert-butyldimethylchlorosilane, to give 2 as a
yellow liquid (333 mg, 77.7% yield). 1H NMR (300 MHz,
CDCl3): d 0.89 (t, J = 6.7 Hz, 3H), 1.12 (s, 9H), 1.24–1.26
(m, 18H), 1.42 (t, J = 6.4 Hz, 2H), 3.21 (q, J = 6.6 Hz, 2H),
3.59 (s, 2H), 5.49 (t, 1H), 6.69 (s, 1H), 6.76 (d, J = 6.4 Hz,
1H), 7.38–7.47 (m, 7H), 7.71 (d, J = 7.1 Hz, 4H). 13C NMR
(125 MHz, CDCl3): d 14.2, 19.5, 22.7, 26.8, 29.2, 29.3, 29.5,
29.6, 29.7, 31.9, 40.0, 40.9, 107.9, 112.8, 113.6,117.4, 125.8,
128.1, 130.4, 131.6, 135.4, 149.5, 155.0, 159.5, 160.9, 167.2.
HRMS (FAB+): m/z calcd for C39H52NO4Si [M + H]+:
626.3666, found: 626.3657.
Synthesis of 3
To a solution of 7-hydroxycoumarin-4-acetic acid (619 mg,
2.81 mmol) and 1.0 equiv. of 1-hydroxybenzotriazole (382 mg,
2.83 mmol) in 20 mL THF, 1.1 equiv. of N,N0-diisopropyl-
carbodiimide (0.48 mL, 3.1 mmol) were added, followed by
the addition of 1.0 equiv. of dodecylamine (530 mg,
2.86 mmol). After the yellowish reaction mixture was stirred
at room temperature overnight, the mixture was concentrated.
The residue was dissolved in ethyl acetate and washed with
1 N HCl and brine. The solution was dried over Na2SO4
and concentrated in vacuo. Purification by silica-column
Fig. 4 (a) Fluorescence intensity (lex = 405 nm) and (b) photographs
and fluorescence photographs (lex = 365 nm) of 1 in the presence of
each anion (0.6 wt% of 1, 1.0 equivalent NaX).
c
This journal is The Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2012
New J. Chem., 2012, 36, 1145–1148 1147

View Article Online
chromatography (CH2Cl2 to CH2Cl2 : MeOH = 50 : 1) was
conducted to give 3 as a white solid (553 mg, 41.2% yield).
1H NMR (300 MHz, DMSO-d6): d 0.85 (t, J = 6.9 Hz, 3H),
1.21–1.23 (m, 18H), 1.38 (t, 2H), 3.04 (q, J = 5.9 Hz, 2H), 3.59
(s, 2H), 6.09 (s, 1H), 6.64 (s, 1H), 6.71 (d, J = 8.6 Hz, 1H),
7.56 (d, J = 8.7 Hz, 1H), 8.16 (t, J = 5.3 Hz, 1H). 13C NMR
(75 MHz, DMSO-d6): d 13.9, 22.1, 23.3, 26.3, 28.7, 28.9, 28.9,
29.0, 31.3, 38.7, 38.9, 39.8, 102.3, 111.4, 111.6, 112.8, 126.6,
151.3, 155.0, 160.2, 161.3, 167.4. HRMS (FAB+): m/z calcd
for C23H34NO4 [M + H]+: 388.2488, found: 388.2488.
S. Y. Park, Chem.–Eur.J., 2010, 16, 7437; (c) J. H. Lee, H. Lee,
S. Seo, J. Jaworski, M. L. Seo, S. Kang, J. Y. Lee and J. H. Jung,
New J. Chem., 2011, 35, 1054.
8 (a) T. H. Kim, M. S. Choi, B.-H. Sohn, S.-Y. Park, W. S. Lyoo and
T. S. Lee, Chem. Commun., 2008, 2364; (b) M. Wang, D. Zhang,
G. Zhang and D. Zhu, Chem. Phys. Lett., 2009, 475, 64;
(c) A. Perez, J. L. Serrano, T. Sierra, A. Ballesteros, D. de Saa
´ ´
and J. Barluenga, J. Am. Chem. Soc., 2011, 133, 8110.
9 (a) C. Vijayakumar, V. K. Praveen and A. Ajayaghosh,
Adv. Mater., 2009, 21, 2059; (b) C. A. Strassert, C.-H. Chien,
M. D. G. Lopez, D. Kourkoulos, D. Hertel, K. Meerholz and
L. De Cola, Angew. Chem., Int. Ed., 2011, 50, 946.
10 A. Ajayaghosh, V. K. Praveen, C. Vijayakumar and S. J. George,
Angew. Chem., Int. Ed., 2007, 46, 6260.
11 (a) D. Bardelang, Md. B. Zaman, I. L. Moudrakovski, S. Pawsey,
J. C. Margeson, D. Wang, X. Wu, J. A. Ripmeester, C. I. Ratcliffe
and K. Yu, Adv. Mater., 2008, 20, 4517; (b) X. Chen, Z. Huang,
S.-Y. Chem, K. Li, X.-Q. Yu and L. Pu, J. Am. Chem. Soc., 2010,
132, 7297.
12 C. Vijayakumar, G. Tobin, W. Schmitt, M.-J. Kim and
M. Takeuchi, Chem. Commun., 2010, 46, 874.
13 S. Srinivasan, P. A. Babu, S. Mahesh and A. Ajayaghosh, J. Am.
Chem. Soc., 2009, 131, 15122.
Acknowledgements
This work was supported by the Basic Science Research
Program through a National Research Foundation (NRF)
grant funded by the Ministry of Education, Science, and
Technology (MEST) of Korea (Grant No. 2012-0000159).
We acknowledge the BK21 fellowship to M.P.
14 (a) M.-O. M. Piepenbrock, G. O. Lloyd, N. Clarke and
J. W. Steed, Chem. Rev., 2010, 110, 1960; (b) H. Maeda,
Chem.–Eur. J., 2008, 14, 11274; (c) Y.-M. Zhang, Q. Lin,
T.-B. Wei, X.-P. Qin and Y. Li, Chem. Commun., 2009, 6074;
(d) M.-O. M. Piepenbrock, N. Clarke, J. A. Foster and J. W. Steed,
Chem. Commun., 2011, 47, 2095.
15 (a) K. L. Kirk, Biochemistry of the Halogens and Inorganic Halides,
Plenum Press, New York, 1991, pp. 58; (b) B. L. Riggs, Bone and
Mineral Research, Annual 2, Elsevier, Amsterdam, 1984,
pp. 366–393.
16 H. Yang, T. Yi, Z. Zhou, J. Wu, M. Xu, F. Li and C. Huang,
Langmuir, 2007, 23, 8224.
17 (a) T. H. Kim, M. S. Choi, B.-H. Sohn, W. S. Lyoo and T. S. Lee,
Chem. Commun., 2008, 2364; (b) M. Teng, G. Kuang, X. Jia,
M. Gao, Y. Li and Y. Wei, J. Mater. Chem., 2009, 19, 5648.
Notes and references
1 (a) P. K. Venula, J. Li and G. John, J. Am. Chem. Soc., 2006,
128, 8932; (b) A. Wada, S. Tamaru, M. Ikeda and I. Hamachi,
J. Am. Chem. Soc., 2009, 131, 5321; (c) V. Jayawama, M. Ali,
T. A. Jowitt, A. E. Miler, A. Saiani, J. E. Gough and R. V. Ulijn,
Adv. Mater., 2006, 18, 611.
2 (a) G. O. Lloyd and J. W. Steed, Nat. Chem., 2009, 1, 437;
(b) M. Yamanaka, T. Nakamura, T. Nakagawa and H. Itagaki,
Tetrahedron Lett., 2007, 48, 8990; (c) R. Varghese, S. J. George and
A. Ajayaghosh, Chem. Commun., 2005, 593; (d) M.-O.
M. Piepenbrock, G. O. Lloyd, N. Clarke and J. W. Steed, Chem.
Commun., 2008, 2644.
3 (a) N. M. Sangeetha and U. Maitra, Chem. Soc. Rev., 2005,
34, 821; (b) I. Hwang, W. S. Jeon, H.-J. Kim, D. Kim, H. Kim,
N. Selvapalam, N. Fujita, S. Shinkai and K. Kim, Angew. Chem.,
Int. Ed., 2007, 46, 210; (c) J. Eastoe, M. Sanchez-Dominguez,
P. Wyatt and R. K. Heenan, Chem. Commun., 2004, 2608;
(d) J. Gao, H. Wang, L. Wang, J. Wang, D. Kong and Z. Yang,
J. Am. Chem. Soc., 2009, 131, 11286; (e) C. Wang, Q. Chen,
F. Sun, D. Zhang, G. Zhang, Y. Huang, R. Zhao and D. Zhu,
J. Am. Chem. Soc., 2010, 132, 3092; (f) W. L. Leong,
S. K. Batabyal, S. Kasapis and J. J. Vittal, Chem.–Eur. J., 2008,
14, 8822.
18 Z. Dzolic, M. Cametti, A. D. Cort, L. Mandolini and M. Zinic,
´ ´
Chem. Commun., 2007, 3535.
19 J.-W. Liu, Y. Yang, C.-F. Chen and J.-T. Ma, Langmuir, 2010,
26, 9040.
20 (a) E. J. Corey and B. B. Snider, J. Am. Chem. Soc., 1972, 94, 2549;
(b) J. H. Clark, Chem. Rev., 1980, 80, 429; (c) E. A. Schmittling and
J. S. Sawyer, Tetrahedron Lett., 1991, 32, 7207.
21 S. Y. Kim and J.-I. Hong, Org. Lett., 2007, 9, 3109.
22 S. Y. Kim, J. Park, M. Koh, S. B. Park and J.-I. Hong, Chem.
Commun., 2009, 4735.
23 (a) N. J. Turro, Modern molecular photochemistry, The Benjamin/
Cummings, Menlo Park, 1978, pp. 170–172; (b) K. Setsukinai,
Y. Urano, K. Kikuchi, T. Higuchi and T. Nagano, J. Chem. Soc.,
Perkin Trans. 2, 2000, 2453; (c) G. Ramakrishna, A. K. Singh,
D. K. Palit and H. N. Ghosh, J. Phys. Chem. B, 2004, 108, 12489.
24 See the experimental section for details on the preparation and
characterization of new compounds.
4 (a) S. S. Babu, K. K. Kartha and A. Ajayaghosh, J. Phys. Chem.
Lett., 2010, 1, 3413; (b) G. S. Vadehra, B. D. Wall, S. R. Diegelmann
and J. D. Tovar, Chem. Commun., 2010, 46, 3947.
5 (a) C. Wang, Z. Wang, D. Q. Zhang and D. B. Zhu, Chem. Phys. Lett.,
2006, 428, 130; (b) S. S. Babu, V. K. Praveen, S. Prasanthkumar and
A. Ajayaghosh, Chem.–Eur. J., 2008, 14, 9577.
6 (a) S. Diring, F. Camerel, B. Donnio, T. Dintzer, S. Toffanin,
R. Capelli, M. Muccini and R. Ziessel, J. Am. Chem. Soc., 2009,
131, 18177; (b) J. Seo, J. W. Chung, E.-H. Jo and S. Y. Park, Chem.
Commun., 2008, 2794; (c) T. Kitahara, N. Fujita and S. Shinkai,
Chem. Lett., 2008, 912.
7 (a) P. Xue, R. Lu, G. Chen, Y. Zhang, H. Nomoto, M. Takafuji
and H. Ihara, Chem.–Eur. J., 2007, 13, 8231; (b) M. K. Nayak,
B.-H. Kim, J. E. Kwon, S. Park, J. Seo, J. W. Chung and
25 10 mM HEPES buffer (pH 7.40) was used.
26 V. K. Lacey, A. R. Parrish, S. Han, Z. Shen, S. P. Briggs, U. Ma
and L. Wang, Angew. Chem., Int. Ed., 2011, 50, 8692.
27 Length of 3 was calculated by SPARTAN’08 (WAVEFUNCTION,
INC.) when molecules were fully stretched.
28 J. M. Berg, J. L. Tymoczko and L. Stryer, Biochemistry, W. H. Freeman
and Company, New York, 6th edn, 2007, pp. 8–11.
c
1148 New J. Chem., 2012, 36, 1145–1148
This journal is The Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2012