Supramolecular assembly of a new squaraine and β-cyclodextrin for detection of thiol-containing amino acids in water

Article abstract of DOI:10.1080/10610278.2011.603730

A new unsymmetrical aniline-based squaraine (SQ2) bearing binding unit of Hg²⁺ ion was designed and synthesised. SQ2 can form 1:2 inclusion complex with β-cyclodextrin, and the resulting complex, which undergoes absorption and fluorescence blea

Full text of DOI:10.1080/10610278.2011.603730

This article was downloaded by: [McGill University Library]
On: 19 November 2014, At: 18:52
Publisher: Taylor & Francis
Informa Ltd Registered in England and Wales Registered Number: 1072954 Registered office: Mortimer House,
37-41 Mortimer Street, London W1T 3JH, UK
Supramolecular Chemistry
Publication details, including instructions for authors and subscription information:
http://www.tandfonline.com/loi/gsch20
Supramolecular assembly of a new squaraine and β-
cyclodextrin for detection of thiol-containing amino
acids in water
Chao Luo a , Qianxiong Zhou a , Wanhua Lei a , Jianguang Wang a , Baowen Zhang a &
Xuesong Wang a
a Key Laboratory of Photochemical Conversion and Optoelectronic Materials, Technical
Institute of Physics and Chemistry, Chinese Academy of Sciences , Beijing , 100190 , P.R.
China
Published online: 31 Aug 2011.
To cite this article: Chao Luo , Qianxiong Zhou , Wanhua Lei , Jianguang Wang , Baowen Zhang & Xuesong Wang (2011)
Supramolecular assembly of a new squaraine and β-cyclodextrin for detection of thiol-containing amino acids in water,
Supramolecular Chemistry, 23:9, 657-662, DOI: 10.1080/10610278.2011.603730
To link to this article: http://dx.doi.org/10.1080/10610278.2011.603730
PLEASE SCROLL DOWN FOR ARTICLE
Taylor & Francis makes every effort to ensure the accuracy of all the information (the “Content”) contained
in the publications on our platform. However, Taylor & Francis, our agents, and our licensors make no
representations or warranties whatsoever as to the accuracy, completeness, or suitability for any purpose of the
Content. Any opinions and views expressed in this publication are the opinions and views of the authors, and
are not the views of or endorsed by Taylor & Francis. The accuracy of the Content should not be relied upon and
should be independently verified with primary sources of information. Taylor and Francis shall not be liable for
any losses, actions, claims, proceedings, demands, costs, expenses, damages, and other liabilities whatsoever
or howsoever caused arising directly or indirectly in connection with, in relation to or arising out of the use of
the Content.
This article may be used for research, teaching, and private study purposes. Any substantial or systematic
reproduction, redistribution, reselling, loan, sub-licensing, systematic supply, or distribution in any
form to anyone is expressly forbidden. Terms & Conditions of access and use can be found at http://
www.tandfonline.com/page/terms-and-conditions

Supramolecular Chemistry
Vol. 23, No. 9, September 2011, 657–662
Supramolecular assembly of a new squaraine and b-cyclodextrin for detection of thiol-containing
amino acids in water
Chao Luo, Qianxiong Zhou, Wanhua Lei, Jianguang Wang, Baowen Zhang and Xuesong Wang*
Key Laboratory of Photochemical Conversion and Optoelectronic Materials, Technical Institute of Physics and Chemistry, Chinese
Academy of Sciences, Beijing 100190, P.R. China
(Received 5 April 2011; final version received 5 July 2011)
A new unsymmetrical aniline-based squaraine (SQ2) bearing binding unit of Hg2þ ion was designed and synthesised. SQ2
can form 1:2 inclusion complex with b-cyclodextrin, and the resulting complex, which undergoes absorption and
fluorescence bleaching upon binding Hg2þ, can serve as a turn-on colorimetric or fluorescent chemosensor in organic
solvent-free aqueous solution for thiol-containing amino acids with high selectivity and tunable measuring range.
Keywords: squaraine; b-cyclodextrin; thiol-containing amino acids; molecular recognition
1. Introduction
sequester Hg2þ and restore the absorption and fluor-
escence of SQ1. In this way, the sensing of the thiol-
containing amino acids becomes a ‘turn-on’ manner. More
importantly, the measuring range of the thiol-containing
amino acids may be easily controlled by adjusting the
excess amount of Hg2þ, because the free Hg2þ will be
consumed first by the thiol-containing amino acids.
Thiol-containing amino acids which are known to be the
key constituents of many proteins and enzymes such as
cysteine or intermediate metabolites such as homocysteine
play essential roles in the biological systems (1, 2). They are
important indicators of a large variety of diseases such as
Alzheimer’s and cardiovascular diseases (1, 2). For instance,
the normal concentration of total cysteine (tCys) in plasma is
in the range of 250–275 mM. Either over 300 mM or less
than 225 mM of tCys hints a high risk of vascular diseases
(2). Thus, it is of great value to develop rapid, sensitive and
selective methods to quantitatively detect the thiol-contain-
ing amino acids. In this regard, both colorimetric and
fluorescent sensors are highly pursued by virtue of their
sensitive responses, inexpensive instrumentation, simple
proceduresand evennaked-eye recognition. So far, a number
of sensitive and selective chromogenic or fluorogenic probes
have been ingeniously developed for sensing thiol-contain-
ing amino acids (3–17). However, few of them have a wide
measuring range spanning from several millimolars to
several hundred millimolars, the concentration regime of
thiol-containing amino acids in vivo (18–22).
However, the SQ1- and Hg2þ-based chemosensor
ensemble suffers from two drawbacks, one is the poor
water solubility of SQ1 which limits the system to be
utilised only in organic solvent-containing media (e.g.
acetonitrile/water in 2:1 volume ratio), and the other is its
low selectivity which leads to responses not only to thiol-
containing amino acids but also to histidine. It has been
reported that some squaraine derivatives can form host–
guest inclusion complexes with b-cyclodextrin, by which
the aggregation tendency of squaraines is restricted
greatly, and meanwhile the water solubility of squaraines
is enhanced remarkably (24–26). This prompted us to use
b-cyclodextrin to improve our SQ1- and Hg2þ-based
chemosensor system. Nevertheless, the two terminals of
SQ1 are so large in size that the b-cyclodextrin cannot
incorporate SQ1 into its cavity. Therefore, we designed
and synthesised a new unsymmetrical squaraine SQ2
(Scheme 1) on the basis of SQ1, one side of which is
replaced by N,N-diethylaniline that is small enough to be
encapsulated into b-cyclodextrin. It was found that the
SQ2–Hg2þ–b-cyclodextrin ensemble can discriminate
thiol-containing amino acids from other amino acids
including histidine in organic solvent-free aqueous
solution, and the dynamic range still remains tunable as
Very recently, we have presented an intriguing
strategy to modulate the concentration window of thiol-
containing amino acids that a squaraine-based chemosen-
sor can respond (23). We designed and synthesised a
squaraine derivative (SQ1, Scheme 1) bearing sulphur-
containing binding units of Hg2þ ion at the two terminals
of the p-conjugation scaffold. Chelation of Hg2þ disrupts
the p conjugation and leads to absorption and fluorescence
turn-off of SQ1. The thiol-containing amino acids can
the case of SQ1–Hg2þ
.
*Corresponding author. Email: xswang@mail.ipc.ac.cn; g203@mail.ipc.ac.cn
ISSN 1061-0278 print/ISSN 1029-0478 online
q 2011 Taylor & Francis
http://dx.doi.org/10.1080/10610278.2011.603730
http://www.tandfonline.com

658
C. Luo et al.
O–
S
S
S
S
N+
N
O
SQ1
HO
O
OH
O
Cl
Cl
CH2Cl2,
AlCl3, reflux
Cl
O
SOCl2, DMF
N
O
N
Benzene
reflux 4h
Yield, 42%
Yield, 30%
O
O
O–
OH
S
18% HCl/reflux
Yield, 75%
Toluene/n-butanol
N+
N
O
N
Yield, 30%
S
S
O
SQ2
O
N
S
1
2
Scheme 1. The chemical structure of SQ1 and the synthetic route of SQ2.
2. Experimental section
(compound 1) and N,N-bis(2-(ethylthio)ethyl) aniline
(compound 2) were synthesised according to the literature
procedures (27, 28). SQ2 was then obtained by the
condensation of compounds 1 and 2 in toluene/n-butanol
(1:1, v/v) under reflux, and characterised by 1H, 13C NMR
and high-resolution MS.
2.1 General
Squaric acid, N-phenyldiethanolamine and ethanethiol
were purchased from Alfa Aesar, Tianjin, China and used
without any further purification. Other materials, such as
p-toluenesulphonyl chloride, N,N-diethylaniline, aluminium
chloride, thionyl chloride and solvents were purchased from
Beijing Chemical Plant, Beijing, China and used as received.
1H and 13C NMR spectra were obtained on a Bruker
DMX-400 MHz spectrophotometer. High-resolution mass
spectra were carried out on a Bruker APEX IV FT_MS.
UV/vis spectra were recorded on a Shimadzu UV-2450
spectrophotometer. Fluorescence spectra were run on a
Hitachi F-4500 spectrophotometer.
As shown in Figure 1, SQ2 shows remarkable
aggregation tendency in the aqueous solution, evidenced
by a very broad absorption band extending from 480 to
780 nm and a negligible fluorescence emission (also see
Figure S1, available online). Upon the addition of b-
cyclodextrin, the absorption band of SQ2 is narrowed and
intensified gradually and finally appears as a sharp peak
centred at 648 nm, a typical absorption character for
classical squaraine dyes in the monomeric form (29).
Additionally, the fluorescence emission intensity of SQ2 is
increased gradually with the addition of b-cyclodextrin, in
good agreement with the transformation of a squaraine dye
from the aggregated form to the monomeric form (30, 31).
Such transformation is believed to be the result of the
host–guest inclusion interaction between SQ2 and b-
cyclodextrin. Such an inclusion dissociates the aggregate,
protects the fluorophore from water quenching and restricts
rotations of groups, all disfavouring the non-radiative decay
and, therefore, favouring the fluoresence enhancement. The
fluorescence quantum yields of SQ2 in different solvents
were measured and presented in Table S1 (available
online). SQ2 is not emissive in water, but in the presence of
b-CD the fluorescence quantum yield is as high as 0.28.
The inclusion complex is consistent with 1:2
stoichiometry between SQ2 and b-cyclodextrin, and the
association constant was calculated to be 1.13 £ 105 M22
using Benesi–Hildebrand method (Figure S2, available
online) (32, 33). The low association constant between
SQ-2 and b-cyclodextrin prevents the measurement of the
2.2 Synthesis of SQ2
Compounds 1 and 2 (Scheme 1) were synthesised
according to the reported literatures (27, 28). SQ2 was
then synthesised by the condensation of compounds 1 and
2 with azeotropic removal of water in n-butanol/toluene
1
(1:1, v/v) under reflux. Melting point: 2008C (dec); H
NMR (400 MHz, CDCl3): d 8.42 (t, 4H, J ¼ 9.78 Hz), 6.82
(d, 2H, J ¼ 9.00 Hz), 6.76 (d, 2H, J ¼ 9.00 Hz), 3.70 (t,
4H, J ¼ 7.54 Hz), 3.54 (q, 4H, J ¼ 7.12 Hz), 2.79 (t, 4H,
J ¼ 7.56 Hz), 2.62 (q, 4H, J ¼ 7.40 Hz), 1.29 (m, 12H);
13C NMR (CDCl3, 400 MHz): d 190.7, 187.4, 183.3,
153.9, 152.0, 134.2, 133.0, 120.7, 119.7, 112.5, 112.2,
51.8, 45.5, 29.1, 26.6, 15.0, 12.9; HR-MS (ESI): calcd for
(C28H36N2O2S2 þ Hþ) 497.2291, found 497.2284.
3. Results and discussion
Scheme 1 shows the method that we used to synthesise
SQ2. The N,N-diethylaniline-based semi-squaraine

Supramolecular Chemistry
659
Figure 1. The absorption (a) and fluorescence (b) spectra of
SQ2 (10 mM) in the aqueous solution in the presence of varied
concentrations of b-cyclodextrin.
Figure 2. The absorption (a) and fluorescence (b) spectra of
SQ2 (5 mM) and b-cyclodextrin (15 mM) in the aqueous solution
in the presence of varied concentrations of Hg2þ ion.
stoichiometric ratio by Job’s plot. When the total
concentration of SQ2 and b-cyclodextrin is as low as
1025 M, the interaction between them is too weak to be
measured. When the total concentration of them is as high
as 1024 M, the poor solubility of SQ2 will lead to its
precipitation. The low association constant also hinders
the stoichiometry measurement by MALDI-TOF method.
In contrast, there are no detectable absorption and
fluorescence emission responses of SQ1 towards b-
cyclodextrin, suggesting that the incorporation of SQ2 in
b-cyclodextrin is a threading process through the N,N-
diethylaniline side, which is much smaller in size than the
other side of SQ2. The optimal structure of bis[4-
(diethylamino)phenyl]squaraine is obtained by Gaussian
calculations as shown in Scheme S1 (available online).
The distance between the two oxygen atoms in the
because the (ethylthio)ethyl side chains are expected to be
out of the cavity of b-cyclodextrin.
The inclusion interaction between SQ2 and
b-cyclodextrin is further confirmed by circular dichroism
(CD) spectra (Figure S3, available online). SQ2 does not
have any chiral centre and therefore CD signal. However,
a sharp and intense CD band emerges at 650 nm in the
presence of b-cyclodextrin, attributable to the induced CD
signal of SQ2. This induced CD signal obviously results
from the inclusion of SQ2 within the chiral cavity of b-
cyclodextrin.
Figure 2 shows the absorption and fluorescence
variations of the aqueous solution of SQ2 and b-
cyclodextrin upon the addition of Hg2þ ion. When the
concentration of Hg2þ reached 40 mM, both the visible
absorption and the fluorescence emission were nearly
bleached. Similar Hg2þ responses were also observed for
SQ1 in acetonitrile/water (2:1) solution (23), attributable
to the chelation of Hg2þ to the terminal binding unit,
which diminishes the electron-donating ability of nitrogen
atom and switches off the visible absorption and emission
of the squaraine chromophore. The binding stoichiometry
˚
cyclobutene ring is 4.565 A, whereas the cavity diameter
˚
of b-cyclodextrin is 6.0–6.5 A (34), supporting the
threading interaction mode. Such an assembly manner
not only makes SQ2 to be dissolved in organic solvent-free
aqueous solution in its monomeric form, but also makes
the inclusion complex to keep Hg2þ ion binding ability

660
C. Luo et al.
work, SQ1–Hg2þ in acetonitrile/water (2:1, v/v) can
response histidine as efficiently as thiol-containing amino
acids. However, for SQ2–Hg2þ–b-cyclodextrin in water,
the sensitivity to histidine is greatly reduced, consequently
the improved selectivity of SQ2–Hg2þ–b-cyclodextrin
allows for the discrimination of thiol-containing amino
acids from histidine and other amino acids.
The selectivity difference between SQ2–Hg2þ–b-
cyclodextrin in water and SQ1–Hg2þ in acetonitrile/water
(2:1, v/v) may originate either from the presence of b-
cyclodextrin or from the change of solvent. To clarify the
reason, we first investigated the interaction between
histidine and b-cyclodextrin which may impede the
chelation of Hg2þ and histidine. However, the addition of
b-cyclodextrin does not lead to any absorption spectrum
changes of histidine, suggesting negligible interactions
between them. Then, we studied the influence of solvent
on the amino acid sensing behaviour of SQ2–Hg2þ. As
shown in Figure 4 and Figure S6 (available online), we
measured the absorption recovery of SQ2–Hg2þ system
caused by cysteine and histidine in the mixture of water
and THF. When the volume ratio of water to THF is 5:5,
both cysteine and histidine can efficiently restore the
absorption of SQ2. With the increase in the volume ratio
between water and THF, the absorption restoring
capability of histidine declines remarkably, while that of
cysteine decreases only a little bit. As a result, at the
volume ratio of 8:2 for water and THF, the response of
SQ2–Hg2þ is still significant to cysteine, however,
negligible to histidine. These results demonstrate that the
improved selectivity of SQ2–Hg2þ–b-cyclodextrin in
water than SQ1–Hg2þ in acetonitrile/water (2:1, v/v)
primarily results from the solvent variation.
Figure 3. Absorbance at 648 nm (a) and fluorescence intensity
at 675 nm (b) of the water solutions of SQ2 (5 mM), b-
cyclodextrin (15 mM) and Hg2þ (80 mM) upon the addition of
various amino acids (80 mM).
In our previous work, we have demonstrated that the
SQ1–Hg2þ in acetonitrile/water can detect cysteine with a
tunable measuring range by simply changing the
of the inclusion complex and Hg2þ is consistent with a 1:1
mode, and the association constant was calculated to be
3.3 £ 103 M21 using Benesi–Hildebrand method (Figure
S4, available online). Several Hg2þ sensors based on
squaraine dyes have also been reported recently (35–37).
Then, we examined the thiol-containing amino acid
sensing ability of SQ2–Hg2þ–b-cyclodextrin ensemble in
organic solvent-free aqueous solution. Figure 3 shows the
responses of SQ2 (5 mM)–Hg2þ (80 mM)–b-cyclodextrin
(15 mM) ensemble to various amino acids. Among the
examined 23 amino acids and short peptide, only cysteine,
homocysteine and GSH (glutathione) which contain thiol
group can restore the visible absorption and fluorescence
of SQ2 (Figure S5, available online, shows the case of
cysteine). The fluorescence intensity was enhanced by
about sevenfold in the presence of thiol-containing amino
acids, corresponding to a recovery of 85% with respect to
the fluorescence intensity before Hg2þ addition, whereas
the colour of the solution altered from colourless to light
green, discernible easily by naked eye. In our previous
Figure 4. The absorbance values at 648 nm of SQ2 (5 mM)–
Hg2þ (200 mM) solution in water/THF of varied volume ratio in
the presence of either cysteine (200 mM) or histidine (200 mM).

Supramolecular Chemistry
661
solvent-free aqueous solution in its monomeric form. As a
result, SQ2–Hg2þ–b-cyclodextrin ensemble can sense
thiol-containing amino acids not only in the aqueous
solution but also with improved selectivity, allowing for
discrimination of thiol-containing amino acids from all
other amino acids including histidine. Furthermore, the
measuring range of thiol-containing amino acids may be
easily modulated by changing the concentration of Hg2þ
ion. Further investigation to find biological benign metal
ion for the substitution of Hg2þ is currently underway.
Acknowledgements
We greatly appreciate the financial support from NNSFC
(20873170, 20772133) and CAS (KJCX2-YW-H08, KGCX2-
YW-389).
References
(1) Seshadri, S.; Beiser, A.; Selhub, J.; Jacques, P.F.;
Rosenberg, I.H.; D’Agostino, R.B.; Wilson, P.W.F. New
Engl. J. Med. 2002, 346, 476–483.
(2) El-Khairy, L.; Ueland, P.M.; Refsum, H.; Graham, I.M.;
Vollset, S.E. Circulation 2001, 103, 2544–2549.
(3) Chen, X.; Zhou, Y.; Peng, X.; Yoon, J. Chem. Soc. Rev.
2010, 39, 2120–2135.
(4) Ruan, Y.B.; Li, A.-F.; Zhao, J.-S.; Shen, J.-S.; Jiang, Y.-B.
Chem. Commun. 2010, 46, 4938–4940.
(5) Chen, X.; Ko, S.-K.; Kim, M.J.; Shin, I.; Yoon, J. Chem.
Commun. 2010, 46, 2751–2753.
(6) Jiang, W.; Cao, Y.; Liu, Y.; Wang, W. Chem. Commun.
2010, 46, 1944–1946.
(7) Lin, W.; Long, L.; Tan, W. Chem. Commun. 2010, 46,
1503–1505.
Figure 5. Absorbance at 648 nm (a) and fluorescence intensity
at 675 nm (b) of aqueous solutions of SQ2 (5 mM), b-
cyclodextrin (15 mM) and Hg2þ in the presence of varied
concentrations of cysteine. The concentrations of Hg2þ are
50 mM (A), 60 mM (B), 70 mM (C) and 80 mM (D), respectively.
(8) Xiong, L.; Zhao, Q.; Chen, H.; Wu, Y.; Dong, Z.; Zhou, Z.;
Li, F. Inorg. Chem. 2010, 49, 6402–6408.
(9) Durocher, S.; Rezaee, A.; Hamm, C.; Rangan, C.; Mittler,
S.; Mutus, B. J. Am. Chem. Soc. 2009, 131, 2475–2477.
(10) Yi, L.; Li, H.; Sun, L.; Liu, L.; Zhang, C.; Xi, Z. Angew.
Chem. Int. Ed. 2009, 48, 4034–4037.
(11) Lin, W.; Yuan, L.; Cao, Z.; Feng, Y.; Long, L. Chem. Eur. J.
2009, 15, 5096–5103.
(12) Li, H.; Fan, J.; Wang, J.; Tian, M.; Du, J.; Sun, S.; Sun, P.;
Peng, X. Chem. Commun. 2009, 45, 5904–5906.
(13) Zhang, X.; Ren, X.; Xu, Q.; Loh, K.P.; Chen, Z. Org. Lett.
2009, 11, 1257–1260.
(14) Sreejith, S.; Divya, K.P.; Ajayaghosh, A. Angew Chem. Int.
Ed. 2008, 47, 7883–7887.
(15) Jiang, W.; Fu, Q.; Fan, H.; Ho, J.; Wang, W. Angew Chem.
Int. Ed. 2007, 46, 8445–8448.
(16) Tang, B.; Xing, Y.; Li, P.; Zhang, N.; Yu, F.; Yang, G.
J. Am. Chem. Soc. 2007, 129, 11666–11667.
concentration of Hg2þ. Such an interesting feature still
remains for SQ2–Hg2þ–b-cyclodextrin in water.
As shown in Figure 5, when the concentration of Hg2þ
is 50 mM, the absorption and fluorescence turn-on of the
system begins at 32 mM of cysteine and completes at
48 mM of cysteine. The linear detection range is from 36 to
48 mM as shown in Figure S7 (available online)
(correlation coefficient ¼ 0.996). In the case of 60 mM of
Hg2þ, the ‘turn-on’ begins at 40 mM and completes at
56 mM. In a word, with the increase in the concentration of
Hg2þ, the response window of SQ2–Hg2þ–b-cyclodex-
trin shifts to higher concentration level of cysteine. By
changing the concentration of Hg2þ to 300 mM, the
cysteine in the range of 275–290 mM, the concentrations
out of the normal level (250–275 mM) of tCys in plasma,
may be detected (Figure S8, available online).
(17) Rusin, O.; Luce, N.N.St.; Agbaria, R.A.; Escobedo, J.O.;
Jiang, S.; Warner, I.M.; Dawan, F.B.; Lian, K.; Strongin,
R.M. J. Am. Chem. Soc. 2004, 126, 438–439.
(18) Liu, J.; Bao, C.; Zhong, X.; Zhao, C.; Zhu, L. Chem.
Commun. 2010, 46, 2971–2973.
(19) Shiu, H.-Y.; Chong, H.-C.; Leung, Y.-C.; Wong, M.-K.;
Che, C.-M. Chem. Eur. J. 2010, 16, 3308–3313.
(20) Lin, W.; Long, L.; Yuan, L.; Cao, Z.; Chen, B.; Tan, W.
Org. Lett. 2008, 10, 5577–5580.
4. Conclusions
In summary, the host–guest interactions between SQ2 and
b-cyclodextrin are utilised to dissolve SQ2 in organic

662
C. Luo et al.
(21) Zhang, M.; Yu, M.; Li, F.; Zhu, M.; Li, M.; Gao, Y.; Li, L.;
Liu, Z.; Zhang, J.; Zhang, D.; Yi, T.; Huang, C. J. Am Chem.
Soc. 2007, 129, 10322–10323.
(22) Chen, H.; Zhao, Q.; Wu, Y.; Li, F.; Yang, H.; Yi, T.; Huang,
C. Inorg. Chem. 2007, 46, 11075–11081.
(29) Law, K.Y. J. Phys. Chem. 1987, 91, 5184–5193.
(30) Das, S.; Thomas, K.G.; Thomas, K.J.; Madhaven, V.
J. Phys. Chem. 1996, 100, 17310–17315.
(31) Chen, H.; Farahat, M.S.; Law, K.Y.; Whitten, D.G. J. Am.
Chem. Soc. 1996, 118, 2584–2594.
(23) Luo, C.; Zhou, Q.; Zhang, B.; Wang, X. New J. Chem. 2011,
35, 45–48.
(32) Wang, W.; Fu, A.; You, J.; Gao, G.; Lan, J.; Chen, L.
Tetrahedron 2010, 66, 3695–3701.
(24) Arun, K.T.; Epe, B.; Ramaiah, D. J. Phys. Chem. B 2002,
106, 11622–11627.
(33) Purkayastha, P. J. Photochem. Photobiol. A 2010, 212,
43–48.
(25) Das, S.; Thomas, K.G.; George, M.G.; Kamat, P.V. J. Chem.
Soc. Faraday Trans. 1992, 88, 3419–3422.
(26) Arun, K.T.; Jayaram, D.T.; Avirah, R.R.; Ramaiah, D.
J. Phys. Chem. B 2011, 115, 7122–7128.
(27) Keil, D.; Hartmann, H. Dyes Pigments 2001, 49, 161–179.
(28) De Selms, R.C.; Fox, C.J.; Riordan, R.C. Tetrahedron Lett.
1970, 11, 781–782.
(34) Szejtli, J. Chem. Rev. 1998, 98, 1743–1754.
(35) Avirah, R.R.; Jyothish, K.; Ramaiah, D. Org. Lett. 2007, 9,
121–124.
(36) Hewage, H.S.; Anslyn, E.V. J. Am Chem. Soc. 2009, 131,
13099–13106.
(37) Chen, C.; Wang, R.; Guo, L.; Fu, N.; Dong, H.; Yuan, Y.
Org. Lett. 2011, 13, 1162–1165.