The synthesis and 1O2 photosensitization of halogenated asymmetric aniline-based squaraines

Article abstract of DOI:10.1039/c0nj00949k

The halogenated (brominated or iodinated) squaraines have promising potential in the application of photodynamic therapy (PDT). Though aniline-based squaraines are the most classical squaraine dyes, no halogenated derivatives with the halogen atoms directly incorporated in the conjugation skeleton of the squaraine chromophore have been reported so far. In this work, a stepwise synthetic approach, i.e. by way of the halogenated semi-squaraines, was applied successfully to prepare halogenated asymmetric aniline-based squaraines. Such a structure makes iodine atoms be able to exert significant heavy atom effect on the intersystem crossing (ISC) efficiency of the squaraine dye. As a result, the iodinated asymmetric squaraine (SQ-OH-I) exhibits a singlet oxygen ( ¹O₂) quantum yield as high as 0.54. In contrast, the ¹O₂ quantum yield of the non-halogenated squaraine analogue (SQ-OH) is only 0.02. The Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2011.

Full text of DOI:10.1039/c0nj00949k

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PAPER
1
The synthesis and O2 photosensitization of halogenated asymmetric
aniline-based squarainesw
Chao Luo, Qianxiong Zhou, Guoyu Jiang, Liqing He, Baowen Zhang and
Xuesong Wang*
Received (in Montpellier, France) 1st December 2010, Accepted 8th March 2011
DOI: 10.1039/c0nj00949k
The halogenated (brominated or iodinated) squaraines have promising potential in the application
of photodynamic therapy (PDT). Though aniline-based squaraines are the most classical
squaraine dyes, no halogenated derivatives with the halogen atoms directly incorporated in the
conjugation skeleton of the squaraine chromophore have been reported so far. In this work,
a stepwise synthetic approach, i.e. by way of the halogenated semi-squaraines, was applied
successfully to prepare halogenated asymmetric aniline-based squaraines. Such a structure makes
iodine atoms be able to exert significant heavy atom effect on the intersystem crossing (ISC)
efficiency of the squaraine dye. As a result, the iodinated asymmetric squaraine (SQ–OH–I)
1
exhibits a singlet oxygen (1O2) quantum yield as high as 0.54. In contrast, the O2 quantum yield
of the non-halogenated squaraine analogue (SQ–OH) is only 0.02.
red to near-infrared (NIR) region.4 The intense absorption of
squaraines in the NIR region makes them suitable for PDT
Introduction
Photodynamic therapy (PDT) is a noninvasive method for the
application, however, the low intersystem crossing (ISC)
efficiency severely hampers the 1O2 generation ability and thus
treatment of a variety of cancers by the interactions of three
elements: light, a photosensitizer, and oxygen. When exposed
the photodynamic activity of squaraines. In order to increase
to proper wavelengths of light, the photosensitizer can be
the ISC efficiencies of squaraines, Ramaiah and co-workers
activated and undergo electron/energy transfer through its
first introduced heavy atoms (Br, I) into the phloroglucinol-
triplet excited state to generate cytotoxic reactive oxygen
based squaraines (SQ1 and SQ2 shown in Scheme 1) over a
species (ROS), mainly singlet oxygen (1O2).1 Although several
decade ago.5 The heavy atom effect from Br or I atoms6
endows SQ1 and SQ2 good 1O2 sensitization abilities, but
the absorption bands of SQ1 and SQ2 are not mainly within
porphyrin photosensitizers, e.g. Photofrin, are approved for
clinical treatment of certain types of cancers, they are far from
being ideal photosensitizers.2 One of the main drawbacks of
the phototherapeutic window. Since then, many heavy atom-
these photosensitizers is the relatively weak absorbance in the
containing squaraines, such as benzothiazole, benzoselenazole,
phototherapeutic window of 600–900 nm. Therefore, great
quinoline, pyrrole, and quinaldine-based squaraines, were
efforts have been made to search for new and more effective
synthesized for PDT applications, with absorption bands fully
falling in the phototherapeutic window.7 Interestingly and
photosensitizers, including porphyrin photosensitizers such
as chlorin and bacteriochlorin, and nonporphyrin photo-
surprisingly, though aniline-based squaraines are the most
sensitizers such as BODIPY, cyanine dyes, and squaraines.3
Squaraines are versatile organic dyes which have been
extensively used in photoconductivity, optical data storage,
solar cells, low-gap polymers, nonlinear optics, and chemo-
sensors, due to the unique photophysical properties, parti-
cularly sharp and intense absorption and fluorescence in the
Key Laboratory of Photochemical Conversion and Optoelectronic
Materials, Technical Institute of Physics and Chemistry,
Chinese Academy of Sciences, Beijing 100190, P. R. China.
E-mail: xswang@mail.ipc.ac.cn, g203@mail.ipc.ac.cn;
Tel: +86 10-82543592
w Electronic supplementary information (ESI) available: The absorption
and fluorescence spectra of SQ–OH and SQ–OH–Br, intensity changes
of DPBF fluorescence with the illumination time of squaraines. See
DOI: 10.1039/c0nj00949k
Scheme 1 The chemical structures of the examined squaraines and
the related squaraines.
c
1128 New J. Chem., 2011, 35, 1128–1132
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classical squaraines and exhibit intense absorption bands
totally in the phototherapeutic window,8 no heavy atom-
containing derivatives can be found until 2007 when Smith
and co-workers reported the first iodinated aniline-based
squaraine (SQ3 shown in Scheme 1).9 Nevertheless, the iodine
atoms are attached to the side chains of the two aniline
moieties rather than on the squaraine conjugation skeleton,
limiting the heavy atom effect that the iodine atoms should
have. In this work, we present a new strategy to synthesize
asymmetric aniline-based squaraines, SQ–OH–Br and
SQ–OH–I shown in Scheme 1, with a Br or I atom incorporated
into one of the two conjugation branches. Such a strategy may
also allow for the independent structure optimization of the
non-halogenated branch to tune absorption wavelength, water
solubility, biocompatibility, as well as targeting property for
application in PDT.
Scheme 3 The synthetic routes for aniline-based semi-squaraines.
can give the desired asymmetric squaraines in good yields
(45% for SQ–OH), reflecting again the lower nucleophilic
activities of 1a and 1b with respect to their non-halogenated
analogues.
Finally, we used 3a or 3b as starting material to prepare
halogenated asymmetric squaraines. Interestingly, N,N-
diethylaniline cannot react with 3a or 3b whereas SQ–OH–Br
and SQ–OH–I were obtained in moderate yields (30% for
SQ–OH–Br and 25% for SQ–OH–I) by condensation reactions
between 3-hydroxy-N,N-diethylaniline and 3a or 3b (Scheme 4).
Compared to the fact that 3c is able to react with both
N,N-diethylaniline and 3-hydroxy-N,N-diethylaniline to yield
the corresponding squaraines, one may reach the conclusion
that the electrophilic activities of 3a and 3b are lower than that
of 3c. However, due to the electron-withdrawing character of
Br or I atoms, it is expected that 3a and 3b should have
stronger electrophilicity than 3c. Ramaiah and coworkers also
found that the bromination or iodination may improve the
electrophilicity of quinaldine-based semi-squaraines.7e There-
fore, there must be some factors other than the electronic
induction effect of Br or I atoms to influence the electrophilic
reactivity of 3a and 3b, and we tentatively attribute it to the
steric effect. A Br or I atom is at the 3-position in 1a and 1b,
which leads to the close proximity between the two large atoms
and the cyclobutene ring in 3a and 3b. Gaussian calculation
reveals that the optimized conformation of 3a or 3b does not
adopt a coplanar strategy between the halogenated aniline
Results and discussion
Synthesis of the halogenated aniline-based squaraines
To synthesize halogenated aniline-based squaraines, the most
straightforward strategy is to carry out condensation reactions
between 1 equiv. of squaric acid and 2 equiv. of halogenated
aniline derivatives. As shown in Scheme 2, squaric acid and
3-bromo-N,N-dimethyl aniline (or 3-iodo-N,N-dimethyl aniline)
were refluxed either in toluene/n-butanol mixed solvent or
in isopropanol/tri-n-butyl orthoformate mixed solvent.10
Though tried many times at varied reaction temperatures
and reaction durations, neither of the two procedures can
lead to the production of the target symmetric halogenated
squaraines. Because the production of aniline-based
squaraines highly depends on the nucleophilic attack of the
aniline derivatives to the electron-deficient cyclobutene ring of
squaric acid, the failure of the condensation reactions most
likely results from the reduced nucleophilicity of 1a and 1b due
to the presence of the electron-withdrawing atom of Br or I.
Then, we attempted to synthesize halogenated aniline-based
squaraines in a stepwise manner, i.e. through the semi-
squaraine intermediates. Using reported methods as shown
in Scheme 3,11 five aniline-based semi-squaraines (3a–e) were
all synthesized successfully, including brominated and
iodinated semi-squaraines 3a and 3b, presumably resulting
from the stronger reactivity of 3,4-dichlorocyclobut-3-ene-
1,2-dione than that of squaric acid.
With the five semi-squaraines (3a–e) in hand, we tried our
best to prepare halogenated aniline-based squaraines. The
reactions of 3d or 3e with 1a or 1b did not yield the desired
halogenated asymmetric aniline-based squaraines either
(Scheme 4). In contrast, the condensation reactions between
3c and N,N-diethylaniline or 3-hydroxy-N,N-diethylaniline
Scheme 2 The unsuccessful trials for the synthesis of symmetric
Scheme 4 The synthesis of squaraines starting from the semi-squaraines
3a–e.
halogenated aniline-based squaraines.
c
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absorption and emission spectra of SQ–OH and SQ–OH–Br).
It is clear that the sharp and intense absorption bands of both
SQ–OH–Br and SQ–OH–I are mainly located in the photo-
therapeutic window, with the absorption maximum at 632 nm
and 635 nm, respectively. As the result of the heavy atom
effect, the fluorescence quantum yields of SQ–OH–Br and
SQ–OH–I are lower than that of SQ–OH, particularly in the
case of SQ–OH–I, suggesting that SQ–OH–I might have a
Fig. 1 The optimized conformations of semi-squaraines 3a (middle),
3b (right) and 3c (left).
1
high efficiency of intersystem crossing (ISC) and a large O2
moiety and the cyclobutene ring, having a dihedral angle of
22.121 for 3a and 31.591 for 3b, while the optimized conformation
of 3c is in a coplanar manner (a dihedral angle of only 0.161),
Fig. 1. The non-coplanar conformation may either decrease
the electrophilic activity of 3a and 3b or destabilize the
reaction transition state, resulting in no reaction between
N,N-diethylaniline and 3a or 3b. Due to the lower nucleo-
philicity of 1a and 1b than N,N-diethylaniline, it is not strange
that 3a and 3b cannot react with 1a and 1b to gain the
corresponding squaraines. Benefited from the higher nucleo-
philicity of 3-hydroxy-N,N-diethylaniline, which in turn
results from the electron-donating property of the hydroxy
substituent, two halogenated asymmetric aniline-based squar-
aines, SQ–OH–Br and SQ–OH–I, are obtained successfully.
quantum yield, prerequisite properties for PDT application.
The absorption and fluorescence properties of the examined
squaraines in THF are very similar to those in CHCl3 (Fig. S3
and Table S1, ESIw). However, some differences can be found
in CH3CN and DMSO. Fig. S5(a), ESIw, shows the absorption
spectra of SQ–OH–I in CHCl3, THF, CH3CN and DMSO.
Though the concentrations of SQ–OH–I were all kept at 3 mM,
the spectra in CH3CN and DMSO are markedly broadened
than those in CHCl3 and THF, suggesting the aggregation of
SQ–OH–I in CH3CN and DMSO. Meanwhile, the fluores-
cence intensities of SQ–OH–I in CH3CN and DMSO are
significantly lower than those in CHCl3 and THF, also
suggesting the aggregation of SQ–OH–I in CH3CN and
DMSO. The aggregation of SQ–OH–I may result from its
decreased solubility in CH3CN and DMSO. SQ–OH and
SQ–OH–Br display similar behaviors in CH3CN and DMSO
as SQ–OH–I. The insolubility of the three squaraines limits the
property study in aqueous solutions.
Photophysical properties of the halogenated aniline-based
squaraines
The basic photophysical properties of SQ–OH, SQ–OH–Br,
and SQ–OH–I, such as absorption maxima, fluorescence
maxima, and fluorescence quantum yields in chloroform, are
presented in Table 1, and Fig. 2 shows the absorption and
fluorescence spectra of SQ–OH–I (see Fig. S1, ESIw, for the
In our experiments, we used 1,3-diphenylisobenzofuran
12
(DPBF) as the chemical trap of 1O2 and methylene blue
(MB, F(1O2) = 0.51 in CHCl3)13 as the reference to determine
the 1O2 quantum yields of the examined squaraines. The
consumption of DPBF was monitored by its fluorescence
intensity decrease as shown in Fig. S2, ESI.w Consistent with
the trend of the fluorescence quantum yields, the 1O2 quantum
yields are in the order of SQ–OH o SQ–OH–Br { SQ–OH–I
Table 1 The photophysical properties of SQ–OH, SQ–OH–Br, and
SQ–OH–I in CHCl3
1
(Table 1). The O2 quantum yield of SQ–OH–I is as high as
lmabax/nm e/Â 105 MÀ1 cmÀ1 lmemax/nm Ffa F(1O2)b
0.54, 27-fold higher than that of SQ–OH, fully demonstrating
1
the key role of the heavy atom I in tuning the ISC and O2
SQ–OH
SQ–OH–Br 632
628
3.19
1.20
1.66
656
664
668
0.89 0.02
0.81 0.08
0.35 0.54
efficiencies. The relative 1O2 quantum yields in THF are also in
the order of SQ–OH o SQ–OH–Br { SQ–OH–I, Fig. S4 and
Table S1, ESI.w The 1O2 quantum yields of SQ–OH–I are
too low to be accurately measured in CH3CN and DMSO,
probably due to the aggregation of SQ–OH–I in the both
solvents.
SQ–OH–I
635
a
Fluorescence quantum yield measured using bis[4-(dimethylamino)-
phenyl]squaraine (0.45 in CH2Cl2) as a reference.8 b 1O2 quantum
yield measured via the chemical trap method using MB (0.51 in CHCl3)
as a reference.13
To further confirm the 1O2 sensitization ability of SQ–OH–I,
we performed spin-trapping EPR experiments using 2,2,6,6-
tetramethyl-4-piperidone (TEMP) as a spin-trapping agent of
1O2.14 Upon irradiation (532 nm laser) of the air-saturated
chloroform solution of SQ–OH–I and TEMP, a three line
EPR signal with equal intensity and a hyperfine coupling
constant of aN = 16.0 G was observed (Fig. 3), which is
attributable to the adduct product of TEMP and 1O2
(TEMPO).15 The irradiation, air, and SQ–OH–I are all
necessary for the generation of the TEMPO signal. The
quenching of the EPR signal by NaN3 or 1,4-diazabicyclo-
[2,2,2]octane (DABCO), the well known 1O2 scavengers,
further supports the assignment of the signal. In contrast, in
the case of SQ–OH, the TEMPO signal can be neglected upon
1
Fig. 2 The absorption and fluorescence spectra of SQ–OH–I in CHCl3.
irradiation, in line with its poor O2 quantum yield.
c
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the EPR spectrometer with a Nd:YAG laser at 532 nm (5–6 ns of
pulse width, 10 Hz of repetition frequency, 30 mJ per pulse energy).
Gaussian calculations were performed with the Gaussian 03
(G03) program package employing the DFT method with
Becke’s three-parameter hybrid functional and Lee–Yang–Parr’s
gradient corrected correlation functional (B3LYP). The
LanL2DZ basis set and effective core potential were used for
the I atom, and the 6-31 G* basis set was applied for H, C, N,
O and Br. The ground-state geometries of the compounds
were optimized in toluene using the conductive polarizable
continuum model (CPCM), and frequency calculations were
also performed to verify that the optimized structure is at an
energy minimum.
1
The reaction of O2 with DPBF12 was adopted to measure
the 1O2 quantum yields of the examined squaraines using MB
as the reference, whose 1O2 quantum yield was measured to be
0.51 in chloroform.13 A series of 2 mL of air-saturated chloro-
form solutions containing DPBF and squaraines, in which the
absorbance at 630 nm originating from the absorption of the
examined squaraines was adjusted to be the same (OD = 0.15),
were separately charged into an open 1 cm path fluorescence
cuvette and illuminated with the light of 630 nm (obtained
from a Hitachi F-4500 fluorescence spectrophotometer).
The consumption of DPBF was followed by monitoring its
fluorescence intensity decrease at the emission maximum
(lex = 405 nm, lem = 479 nm).
Fig. 3 The EPR spectra of air-saturated solution of SQ–OH–I (0.1 mM)
in CHCl3 in the presence of TEMP (50 mM) before and after laser
irradiation at 532 nm.
Conclusions
In summary, by way of the halogenated semi-squaraines, we
for the first time synthesized halogenated aniline-based squar-
aines, SQ–OH–Br and SQ–OH–I, with Br or I atoms directly
incorporated in the conjugation skeleton of the squaraine
chromophore. Such a structure makes iodine atoms be able
to exert significant heavy atom effect on the ISC efficiency of
the squaraine dye. As a result, SQ–OH–I exhibits a much
improved 1O2 yield, as high as 0.54. Further structure
modification is in progress to render our halogenated
asymmetric squaraines more ideal characters for PDT
application, such as water solubility and targeting property.
Synthesis
Synthesis of 2a–e17. Aluminium chloride was dissolved in
dichloromethane under reflux. 3,4-Dichlorocyclobut-3-ene-
1,2-dione and corresponding aniline derivatives (1a–e) were
dissolved in dichloromethane, and added dropwise into the
refluxed solution over a period of 10 min. After 1.5 h, the
resulting solution was poured into ice water in which two
drops of concentrated hydrochloric acid were added. Then, the
aqueous solution was extracted with dichloromethane for
three times. The organic phase was washed with water, dried
over MgSO4, and concentrated to dryness. The crude product
was purified by column chromatography on silica gel using
n-hexane/ethyl acetate (3 : 1 in volume ratio) as eluent. The
desired product was obtained as yellow solid via recrystalliza-
tion from n-hexane/ethyl acetate (10 : 1 in volume ratio). 2a–e
have similar absorption spectra of typical semi-squaraines. 2a,
Experimental section
General
Squaric acid, 3-hydroxy-N,N-diethylaniline, 3-bromo-N,N-di-
methylaniline, 3-iodoaniline, tri-n-butyl-orthoformate, and
sodium cyanoborohydride were purchased from Alfa Aesar
and used without further purification. 1,3-Diphenylisobenzo-
furan (DPBF), sodium azide (NaN3), 2,2,6,6-tetramethyl-4-
piperidone (TEMP), and 1,4-diazabicyclo[2,2,2] octane (DABCO)
were purchased from Sigma-Aldrich and used without further
purification. Other materials, such as N,N-dimethylaniline,
N,N-diethylaniline, aluminium chloride, thionyl chloride,
and solvents were purchased from Beijing Chemical Plant
and used as received. 3-Iodo-N,N-dimethylaniline and 3,4-
dichlorocyclobut-3-ene-1,2-dione were synthesized following
the literature methods.16,17
1H NMR spectra were obtained on a Bruker DMX-400 MHz
spectrophotometer. High resolution mass spectra were
performed on a Bruker APEX IV FT-MS. UV/vis spectra
were recorded on an UV-2450 spectrophotometer. Fluores-
cence spectra were run on a F-4500 spectrophotometer and
the fluorescence quantum yields were measured using bis-
[4-(dimethylamino)phenyl]squaraine (0.45 in CH2Cl2) as a
reference.8 The EPR spectra were recorded at room tempera-
ture on a Bruker-ESP-300E spectrometer at 9.8 GHz, X-band
with 100 Hz field modulation. Samples were injected quanti-
tatively into quartz capillaries and illuminated in the cavity of
1
2b and 2d were also characterized by H NMR.
Synthesis of 3a–e. 2a–e were refluxed in the aqueous solution
of 18% HCl for 4 h. After the resulting solution cooled to
room temperature, the aqueous solution of NaHCO3 was
added until a large amount of precipitation was generated.
The precipitation was filtered and dried overnight. The
product was directly used to synthesize the corresponding
squaraines without further purification.
Synthesis of SQ–OH, SQ–OH–Br, and SQ–OH–I. The
target squaraine dyes were synthesized by the condensation
of semi-squaraine derivatives 3a–c and 3-hydroxy-N,N-diethyl-
aniline with azeotropical removal of water in n-butanol/
toluene (1 : 1 v/v) under reflux. After the resulting solution
cooled to room temperature, the precipitation was filtered,
c
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washed with ether and methanol, and dried overnight.
The product was collected as blue powder.
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3-Chloro-(2-bromo-4-(dimethylamino)phenyl)cyclobut-3-ene-
1,2-dione (2a). Yield, 10%; 1H NMR (400 MHz, CDCl3),
d = 7.85 (d, 1H, J = 9.04 Hz), 7.01 (s, 1H), 6.68 (d, 1H,
J = 6.4 Hz), 3.10 (s, 6H).
3-Chloro-4-(2-iodo-4-(dimethylamino)phenyl)cyclobut-3-ene-
1,2-dione (2b). Yield, 10%; 1H NMR (400 MHz, CDCl3),
d = 7.57 (d, 1H, J = 8.88 Hz), 7.33 (s, 1H), 6.74 (d, 1H,
J = 6.4 Hz), 3.08 (s, 6H).
3-Chloro-4-(4-(diethylamino)phenyl)cyclobut-3-ene-1,2-dione (2d).
1
Yield, 25%; H NMR (400 MHz, CDCl3), d = 8.16 (d, 2H,
J = 9.16 Hz), 6.90 (d, 2H, J = 8.84 Hz), 3.50 (q, 4H, J =
7.13 Hz), 1.26 (t, 6H, J = 7.12 Hz).
(2-Hydroxy-4-diethylaminio-phenyl)-(4-dimethylamino-phenyl)
squaraine (SQ–OH). Yield, 45%; 1H NMR (400 MHz,
CDCl3), d = 8.20 (d, 2H, J = 9.00 Hz), 8.09 (d, 1H, J =
9.28 Hz), 6.79 (d, 2H, J = 8.72 Hz), 6.38 (d, 1H, J = 8.04 Hz),
6.12 (s, 1H), 3.50 (q, 4H, J = 6.95 Hz), 3.15 (s, 6H), 1.27
(t, 6H, J = 7.14 Hz); HRMS (ESI), calcd for (C22H24N2O3 + H+)
365.18597, found 365.18651. Anal. calcd for C22H24N2O3:
C, 72.49; H, 6.64; N, 7.69; found: C, 72.41; H, 6.54; N, 7.45%.
(2-Hydroxy-4-diethylaminio-phenyl)-(2-bromo-4 dimethylamino-
phenyl) squaraine (SQ–OH–Br). Yield, 30%; 1H NMR
(400 MHz, CDCl3), d = 8.29 (d, 1H, J = 8.96 Hz), 8.17
(d, 1H, J = 9.44 Hz), 7.37 (s, 1H), 6.74 (d, 1H, J = 9.04 Hz),
6.41 (dd, 1H, J = 7.08, 2.36 Hz), 6.12 (d, 1H, J = 2.4 Hz), 3.53
(q, 4H, J = 7.17 Hz), 3.08 (s, 6H), 1.30 (t, 6H, J = 7.18 Hz);
HRMS (ESI), calcd for (C22H23N2O3Br + H+) 443.09648,
found 443.09712. Anal. calcd for C22H23N2O3Br: C, 59.72; H,
5.24; N, 6.33; found: C, 59.65; H, 5.12; N, 6.16%.
(2-Hydroxy-4-diethylaminio-phenyl)-(2-iodo-4-dimethylamino-
phenyl) squaraine (SQ–OH–I). Yield, 25%; 1H NMR (400 MHz,
CDCl3), d = 8.27 (d, 1H, J = 8.96 Hz), 8.17 (d, 1H, J =
9.44 Hz), 7.41 (s, 1H), 6.80 (m, 1H), 6.42 (d, 1H, J = 9.48 Hz),
6.12 (d, 1H, J = 2.28 Hz), 3.53 (q, 4H, J = 7.20 Hz), 3.08
(s, 6H), 1.30 (t, 6H, J = 7.16 Hz); HRMS (ESI), calcd for
(C22H23N2O3I + H+) 491.08262, found 491.08365; Anal.
calcd for C22H23N2O3I: C, 53.87; H, 4.73; N, 5.71; found: C,
53.81; H, 4.58; N, 5.50%.
12 R. H. Young, K. Wehrly and R. L. Martin, J. Am. Chem. Soc.,
1971, 93, 5774.
Acknowledgements
13 (a) B. Gao, Y. Lei, B. Cao, P. Chen, Y. Xiong and G. Tang,
Optoelectron. Technol. Inf., 2005, 18, 33; (b) D. Severino,
H. C. Junqueira, M. Gugliotti, D. S. Gabrielli and
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14 Y. Lion, M. Delmelle and A. V. Vorst, Nature, 1976, 263, 442.
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1991, 87, 2943.
We greatly appreciate the financial support from NNSFC
(20772133, 20873170) and CAS (KJCX2.YW.H08, KGCX2-
YW-389).
Notes and references
1 J. P. Celli, B. Q. Spring, I. Rizvi, C. L. Evans, K. S. Samkoe,
S. Verma, B. W. Pogue and T. Hasan, Chem. Rev., 2010, 110,
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16 R. F. Borch and A. I. Hassid, J. Org. Chem., 1972, 37, 1673.
17 R. C. De Selms, C. J. Fox and R. C. Riordan, Tetrahedron Lett.,
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c
1132 New J. Chem., 2011, 35, 1128–1132
This journal is The Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2011