Synthesis and characterization of indolocarbazole-quinoxalines with flat rigid structure for sensing fluoride and acetate anions

Article abstract of DOI:10.1039/b801447g

A new series of indolocarbazole-quinoxalines (ICQ, receptors 6 and 7) are prepared and characterized for effective fluoride and acetate anion sensing. The new indole-based system has a highly flat rigid structure with a large π system, and exhibits high binding affinity and sensitivity for acetate and fluoride anions. Receptors 6 and 7 give abundant and unique spectral features in dimethyl sulfoxide (DMSO). Both fluoride and acetate anions cause a bathochromic shift of the absorption peaks of receptor 7 in DMSO, whereas only fluoride anion results in a remarkable shift of the absorption peak of receptor 6 in DMSO. Receptors 6 and 7 can also operate as efficient colorimetric sensors for naked-eye detection of fluoride and acetate anions, and their combined use also offers a simple way for distinguishing these two anions by the naked-eye. The analysis of a Job's plot for the binding of receptor 7 and F^-, single crystal structures of 7?TBACl and 7?TBACH₃COO confirm 1: 1 binding stoichiometry. Notably, the ICQ system offers novel and excellent receptors for acetate anion both in solution and in crystalline solid through the formation of two hydrogen bonds. The Royal Society of Chemistry.

Full text of DOI:10.1039/b801447g

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www.rsc.org/obc | Organic & Biomolecular Chemistry
Synthesis and characterization of indolocarbazole-quinoxalines with flat rigid
structure for sensing fluoride and acetate anions†
Ting Wang, Yu Bai, Liang Ma and Xiu-Ping Yan*
Received 25th January 2008, Accepted 29th February 2008
First published as an Advance Article on the web 28th March 2008
DOI: 10.1039/b801447g
A new series of indolocarbazole-quinoxalines (ICQ, receptors 6 and 7) are prepared and characterized
for effective fluoride and acetate anion sensing. The new indole-based system has a highly flat rigid
structure with a large p system, and exhibits high binding affinity and sensitivity for acetate and
fluoride anions. Receptors 6 and 7 give abundant and unique spectral features in dimethyl sulfoxide
(DMSO). Both fluoride and acetate anions cause a bathochromic shift of the absorption peaks of
receptor 7 in DMSO, whereas only fluoride anion results in a remarkable shift of the absorption peak of
receptor 6 in DMSO. Receptors 6 and 7 can also operate as efficient colorimetric sensors for naked-eye
detection of fluoride and acetate anions, and their combined use also offers a simple way for
distinguishing these two anions by the naked-eye. The analysis of a Job’s plot for the binding of
receptor 7 and F−, single crystal structures of 7·TBACl and 7·TBACH3COO confirm 1 : 1 binding
stoichiometry. Notably, the ICQ system offers novel and excellent receptors for acetate anion both in
solution and in crystalline solid through the formation of two hydrogen bonds.
also offers a simple way for distinguishing these two anions by
Introduction
the naked-eye. Notably, the ICQ system offers novel and excellent
Anions play a fundamental and important role in a wide range
receptors for acetate anions both in solution and in crystalline
of chemical, biological, medical and environmental processes.
solid through the formation of two hydrogen bonds.
Increasing efforts have been made to employ a variety of motifs
to design efficient anion sensors, including those based on amide,
phenol, urea and pyrrole units.1,2 As an example, the urea, thiourea
and guanidinium motifs have been demonstrated to be good hy-
drogen bond donors and excellent receptors for carboxylates such
Results and discussion
ꢀ
Receptors 6 and 7 relying on an a,a -connection of two indole
motifs of DIQ were synthesized according to Scheme 1. 2,3-
Diindol-3ꢀ-yl diketone (3) was prepared from indole (1) and 2-(3-
as the acetate anion, a particularly common and important func-
tional group in biological and synthetic organic molecules, and
indolyl)-2-oxoacetyl chloride (2) in a mixture of dichloroethane
and heptane in the presence of AlCl3 but without the need for
an indole Grignard agent.11 The synthesis of DIQ (4 and 5) was
adapted from previously reported procedures.7,12 Intramolecular
crossed-dimerization of the two indole motifs was achieved
have attracted immense attention.3 The rational design of efficient
receptors for the Y-shaped acetate anion using other novel motifs
compared with those motifs used widely is relatively difficult.
Recently, several groups4–10 have prepared a few indole-based
receptors such as indolocarbazoles and diindolylquinoxalines
(DIQ, receptors 4 and 5) as potential hydrogen bond donors for
anions, and demonstrated their promising prospects for binding
anions. However, to the best of our knowledge, few indole-
based receptors for the acetate anion, especially structurally
characterized receptor–acetate complexes with rational designs,
have been reported so far.
Herein, we report a new series of indolocarbazole-quinoxalines
(ICQ, receptors 6 and 7) for effective acetate and fluoride anion
sensing. The new indole-based system prepared has a highly flat
rigid structure with a large p system, and exhibits high binding
affinity and sensitivity for acetate and fluoride anions. Receptors
6 and 7 can operate as efficient colorimetric sensors for naked-eye
detection of acetate and fluoride anions, and their combined use
Research Center for Analytical Sciences, College of Chemistry, Nankai
University, 94 Weijin Road, Tianjin, 300071, China. E-mail: xpyan@nankai.
edu.cn; Fax: (+86)22-23506075
† Electronic supplementary information (ESI) available: Details of spectral
data of receptors 6 and 7 and crystallographic data for CCDC reference
numbers 657906 and 657907. See DOI: 10.1039/b801447g
Scheme 1 Synthesis of receptors 6 and 7.
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through acid-promoted dimerization.13 With the aid of dichlorod-
icyanoquinone (DDQ), DIQ in boiling trifluoroacetic acid turned
into final receptors 6 and 7 in quantitative yields. The product was
recrystallized for further investigation.
Receptors 6 and 7 give five UV–vis absorption peaks with a
big molar absorption coefficient in dimethyl sulfoxide (DMSO)
(Fig. 1 and 2). The UV–vis absorption titration shows the spectral
change of receptors 6 and 7 upon addition of anions. Addition of
CH3COO− and F− to receptor 7 in DMSO caused a bathochromic
shift of the absorption peaks from 372 nm to 390 nm (Dkmax
=
18 nm) for acetate anion, to 404 nm (Dkmax = 32 nm) for fluoride
anion and from 451 nm to 560 nm (Dkmax = 109 nm) for both anions
(Fig. 1). Notably, complex formation can be visually perceived
through a distinct color change from bright yellow to gray. How-
ever, only F− resulted in a remarkable decrease of the absorption
peak at 420 nm and a new absorption peak at 460 nm for receptor
6 in DMSO, and also a clear color change from yellow to orange.
Interestingly, addition of CH3COO− and F− to receptor 6 in a less
polar solvent, acetonitrile, slightly changed the absorption peaks
(Dkmax = 7 nm) (Fig. 2). The corresponding color change is difficult
to discriminate by the naked eye. The phenomena are attributed
to a solvent effect which plays a crucial role in controlling anion
binding strength and selectivity.1b The color changes of receptors 6
and 7 upon addition of various anions indicate that the combined
use of these receptors can provide an easy way to distinguish
acetate and fluoride anions by the naked eye on the basis of the
different color changes of receptors 6 and 7 (Fig. 3).
Fig. 2 UV–vis spectral changes of receptor 6 (1.5 × 10−5 M) observed
upon addition of fluoride and acetate anions (10 equiv.). (Inset) The
titration with F− in DMSO (0 to 28 equiv.). 1: Receptor 6 in DMSO.
2: Receptor 6 in acetonitrile. 3: Receptor 6 + 10 equiv. F− in DMSO. 4:
Receptor 6 + 10 equiv. F− in acetonitrile.
Fig. 3 Color changes of receptors 6 and 7 upon addition of anions. [6] =
[7] = 3 × 10−5 M in DMSO; 6 + 10 equiv. anion (top), 7 + 5 equiv. anion
−
(bottom). From left to right: none, F−, Cl−, Br−, I−, HSO4−, H2PO4
NO3−, CH3COO−.
,
& 7 although only slight structural modification was made from
ꢀ
receptors 4 and 5 to receptors 6 and 7 through the a,a -connecting
of two indole motifs in receptors 4 and 5.
Association constants determined from the UV–vis absorption
titration or fluorescence titration are summarized in Table 1.
The titration curves with F−, Cl− and H2PO4 (Figures S2 and
−
Table 1 Association constants (Ka/M−1)of some indole-based receptors
with various anions at room temperature
Fig. 1 UV–vis spectral changes of receptor 7 (1.5 × 10−5 M) observed
upon addition of fluoride anion (10 equiv.). (Inset) The titration with F−
in DMSO (0 to 28 equiv.). 1: Receptor 7 in DMSO. 2: Receptor 7 + 10
equiv. CH3COO− in DMSO. 3: Receptor 7 + 10 equiv. F− in DMSO.
4a
5a
6b
7b
F−
2100
170
—
—
6800
80
—
470
—
—
20000
250
—
15531
514
152535
821
Cl−
Br−
I−
<100c
<100c
9933
<100c
<100c
78845
<100c
<100c
24452d
In contrast, receptors 4 and 5 give less UV–vis absorption peaks
with a smaller molar absorption coefficient in DMSO. Addition of
fluoride and acetate anions to receptors 4 and 5 in DMSO did not
cause significant changes of their spectra and color (Figure S1,
Supporting Information). However, fluoride and acetate anions
resulted in remarkable spectral and color changes of receptors 6
and 7 in DMSO as these receptors possess a large p system of the
flat rigid structure and offer high binding affinity for the anions in
DMSO. The above results show significant differences in binding
and optical properties between receptors 4 & 5 and receptors 6
−
−
H2PO4
HSO4
<100c
<100c
20602d
−
NO3
—
—
CH3COO−
—
a Values from Ref. 7 and determined in dichloromethane. b Determined
by UV–vis spectroscopic titration in DMSO with errors ranging from 3%
(6 + CH3COO−) to 14% (7 + F−). Anions were used in the form of their
tetrabutylammonium (TBA) salts. c Estimated, clear binding profiles were
not observed. d Determined by the fluorescence titration.
1752 | Org. Biomol. Chem., 2008, 6, 1751–1755
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S3, Supporting Information) reveal several isosbestic points as
expected for a 1 : 1 binding stoichiometry. A Job’s plot for the
binding of receptor 7 with F− (Figure S4, Supporting Information)
and the single crystal X-ray diffraction analysis of the complex
formed between receptor 7 and TBACl (Fig. 4)14 confirm their
1 : 1 binding stoichiometry. In the single crystal structure of the
7·TBACl complex the chloride anion is located between the middle
of two indole NH groups in the same plane and stabilized by two
anions with suitable size, shape and basicity. For instance, receptor
7 shows high selectivity for fluoride anion (Ka(F)/Ka(Cl) = 185,
Ka(F)/Ka(HSO4) > 1500, Ka(F)/Ka(NO3) > 1500).
The flat rigid structure with a large p system also gives the ability
of ICQ to operate as a fluorescent sensor for anions. Significant
quenching of emission with no change in the structure of the
emission bands of receptor 6 was observed upon addition of
CH3COO− (Fig. 5). A similar result was also observed for F−
and H2PO4−. The quenching of emission by these anions indicates
that the receptor–anion complexes participate in photoinduced
electron transfer (PET) quenching phenomena.3h Significant fluo-
rescence quenching also occurred upon addition of these anions
to receptor 7. A typical such change for CH3COO− is shown in
Figure S6 (Supporting Information).
˚
˚
hydrogen bonds with N–H · · · Cl distances of 2.262 A and 2.307 A,
respectively. Furthermore, the planes of the receptor molecule
are partly overlapped as consequence of p–p interactions with
˚
a distance of 3.41 A.
Fig. 5 Fluorescence quenching of receptor 6 (1.5 × 10−5 M) upon titration
Fig. 4 Single-crystal X-ray diffraction structure of 7·TBACl. Thermal
ellipsoids are scaled to the 30% probability level. The TBA counter-ions
and most hydrogen atoms have been omitted for clarity. Dashed lines
indicate the hydrogen-bond interactions.
with CH3COO− in DMSO.
It is worth mentioning that the shape of the indole NH groups
of the flat molecule is similar to that of urea and thiourea.
Therefore, the ICQ system is a particularly excellent receptor for
Y-shaped anions such as carboxylates through the formation of
two hydrogen bonds for 1 : 1 binding stoichiometry like urea
binding these anions.3e The proposed conformation (Scheme 2)
is confirmed by single crystal X-ray diffraction analysis of the
complex formed between receptor 7 and TBACH3COO (Fig. 6).15
The N–H · · · O distances of the two hydrogen bonds between the
A further insight into Table 1 reveals that the highest affinity of
receptor 6 is displayed for CH3COO− and F−, receptor 7 for F−,
followed by H2PO4− >> Cl− > HSO4 ∼NO3−. Due to the increase
−
in the acidity of indole NHs caused by the electron withdrawing
nitro group, nitro-bearing receptor 7 shows much higher binding
affinity than receptor 6. Compared with receptors 4 and 5 whose
binding constants were determined in dichloromethane, receptors
6 and 7 give much higher binding affinity for the anions even
in the more polar solvent DMSO. We ascribe such character to
˚
˚
O (acetate anion) and the NH (receptor 7) (1.799 A and 1.828 A)
are similar to those between the O (acetate anion) and the NH
3f
(urea) (1.797 A and 1.852 A). 1H NMR titration in DMSO-d6
shows that the addition of acetate anion resulted in a significant
downfield shift and disappearance of the two split indole NHs
as a result of the formation of two hydrogen bonds (Figure S7,
Supporting Information).
˚
˚
ꢀ
the flat rigid structure formed by the a,a -connection of the two
indole motifs. The larger p system (receptors 6 and 7 cf. receptors
4 and 5) along with the electron withdrawing of the nitro group
(receptor 7 cf. receptor 5) increases the acidity of indole NHs,
making receptors 6 and 7 bind electron-rich anions more favorably
than receptors 4 and 5. On the other hand, the rigid plane without
any flexible bonds of receptors 6 and 7 gives a fixed cavity for
binding anions without bond rotation.
Conclusions
However, receptors 4 and 5 based on b-connectivity linking
the two indole motifs to the quinoxaline core provide a more open
and less rigid cavity to favor binding the relatively large dihydrogen
phosphate anion. Receptors 6 and 7, further modified through the
a,a -connection of two indole motifs in receptors 4 and 5, possess
rigid structure and also offer considerable selectivity for special
We have successfully developed a novel series of indole-based
receptors 6 and 7 that are particularly efficient for sensing acetate
and fluoride anions. Combined use of receptors 6 and 7 on the
basis of their color change upon addition of acetate and fluoride
anions also offers a simple way for distinguishing these two anions
by the naked-eye. This new ICQ system with a large p system gives
ꢀ
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300, respectively. High resolution mass spectra (HRMS) were
determined on an IonSpec 7.0T FT-ICR mass spectrometer.
UV–vis absorption spectra were measured with a Shimadzu
UV-3600 UV-Vis spectrophotometer. Fluorescence spectra were
recorded at room temperature on a Hitachi FL-4500 fluorescence
spectrometer.
Synthesis of receptor 6
2,3-Di(1H-indol-3-yl)quinoxaline (4, 302 mg, 0.84 mmol) and
dichlorodicyanoquinone (DDQ, 230 mg, 1.01 mmol) were dis-
solved in neat trifluoroacetic acid (30 mL) and the resultant
solution was heated at reflux for 4 hours until the reaction
was complete (TLC). The residual trifluoroacetic acid was then
removed by vacuum distillation. The semi-solid was washed
with brine, saturated aq. NaHCO3 and brine. The product was
recrystallized from ethyl acetate to give 6 (101 mg, 34%) as
orange–red needle-shaped crystals. 1H NMR (400 MHz, DMSO-
d6, 298 K, TMS,) d = 7.45–7.52 (m, 4H; ArH), 7.88–7.93 (m,
4H; ArH), 8.41 (dd, J = 3.43 Hz, 6.46 Hz, 2H; ArH), 9.11 (dd,
J = 1.83 Hz, 6.67 Hz, 2H; ArH), 12.00 (s, 2H; NH); 13C-NMR
(75 MHz, DMSO-d6, 298 K, TMS) d = 112.0, 112.5, 121.3, 122.4,
124.2, 124.8, 128.3, 128.8, 129.9, 137.9,139.7, 140.2; HRMS (ESI)
C24H15N4: calcd. 359.1297; found m/z 359.1282 [M + H+]; UV–
Vis (dimethyl sulfoxide): kmax(e) = 272 (47017), 283 (52917), 307
(44417), 320 (50050), 420 (22900).
Fig. 6 Single-crystal X-ray diffraction structure of 7·TBACH3COO·
CHCl3·1/2H2O. Thermal ellipsoids are scaled to the 30% probability
level. The TBA counter-ions, most hydrogen atoms and solvent molecule
(CHCl3) have been omitted for clarity. Dashed lines indicate the hy-
drogen-bond interactions. The water molecule is disordered over two
positions.
Synthesis of receptor 7
2,3-Di(1H-indol-3-yl)-6-nitroquinoxaline (5, 300 mg, 0.74 mmol)
and dichlorodicyanoquinone (DDQ, 201 mg, 0.89 mmol) were
dissolved in neat trifluoroacetic acid (30 mL) and the resultant
solution was heated at reflux for 4 hours until the reaction
was complete (TLC). The residual trifluoroacetic acid was then
removed by vacuum distillation. The semi-solid was washed
with brine, saturated aq. NaHCO3 and brine. The product was
recrystallized from N,N-dimethylformamide to give 7 (150 mg,
50%) as red needle-shaped crystals. 1H NMR (400 MHz, DMSO-
d6, 298 K, TMS) d = 7.40–7.50 (m, 4H; ArH), 7.84 (t, J = 8.41 Hz,
8.41 Hz, 2H; ArH), 8.36 (d, J = 9.19 Hz, 1H; ArH), 8.43 (dd, J =
2.05 Hz, 9.14 Hz, 1H; ArH), 8.93 (t, J = 8.60 Hz, 8.60 Hz, 2H;
ArH), 9.00 (d, J = 1.88 Hz, 1H; ArH), 11.98 (s, 1H; NH), 12.05 (s,
1H; NH); HRMS (ESI) C24H14N5O2: calcd. 404.1147; found m/z
404.1138 [M + H+]; UV–Vis (dimethyl sulfoxide): kmax(e) = 275
(39714), 305 (38733), 318 (39152), 371 (20390), 451 (21800). The
structure was also confirmed by the single crystal X-ray diffraction
analysis.
Scheme 2 a) The formation of two hydrogen bonds between acetate anion
and the urea (X = O) or thiourea (X = S) receptor. R1 and R2 are functional
groups. b) Proposed conformation of ICQ system (R3 = H, receptor 6; R3 =
NO2, receptor 7) Binding with acetate anion through the formation of two
hydrogen bonds for 1 : 1 binding stoichiometry.
a unique spectral feature, being promising as the basis for new
dyes or electron-donor–receptor materials.
Experimental
General
X-Ray crystallography
All reagents used were of at least analytical grade. All solvents
were purified by standard procedures. Tetra-n-butylammonium
fluoride trihydrate, tetra-n-butylammonium iodide, tetra-n-
butylammonium nitrate, tetra-n-butylammonium hydrogen sul-
fate, tetra-n-butylammonium acetate, tetra-n-butylammonium
dihydrogenphosphate were purchased from Alfa Aesar China
(Tianjin) LTD, tetra-n-butylammonium chloride and tetra-n-
butylammonium bromide were purchased from Guangfu Fine
Chemical Research Institute (Tianjin, China) and used without
further purification. 1H NMR and 13C NMR spectra were
recorded on a Varian UNITY PLUS-400 and a Bruker Avance
Single crystals of 7·TBACl and 7·TBACH3COO were obtained
by slow diffusion of n-hexane into a chloroform solution of
receptor 7 and the corresponding anion at room temperature.
The X-ray single crystal diffraction data for 7·TBACl and
7·TBACH3COO·CHCl3·1/2H2O were collected on a Rigaku Mi-
˚
croMAX007withMo-Karadiation (k = 0.71073 A) at 113 K 2 K
in the x–2h scanning mode. The structures were solved by direct
methods using the SHELXS-97 program and refined by full-matrix
least-squares techniques (SHELXL-97) on F2.16,17 Anisotropic
thermal parameters were assigned to all non-hydrogen atoms.
1754 | Org. Biomol. Chem., 2008, 6, 1751–1755
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The organic hydrogen atoms were generated geometrically. Details
of crystal data, data collections, and structure refinements are
summarized in Tables S1 and S2 in the Supporting Information.
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46, 6427.
4 D. Curiel, A. Cowley and P. D. Beer, Chem. Commun., 2005, 236.
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Angew. Chem., Int. Ed., 2005, 44, 7926; (b) K.-J. Chang, M. K. Chae,
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Acknowledgements
This work was supported by the National Basic Research Program
of China (2006CB705703) and the National Natural Science
Foundation of China (No. 20775037). We thank Dr Yi-Zhou Zhu
for the gift of DDQ and Yuan-Wei Zhang for help of with single
crystal growth.
Notes and references
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1 (35 Years of Synthetic Anion Receptor Chemistry 1968–2003. See
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L. Fabbrizzi and M. Licchelli, Acc. Chem. Res., 2006, 39, 343; (h) P. A.
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¯
14 Crystal data for 7·TBACl: C40H49ClN6O2, triclinic, space group P◦1, a =
◦
˚
˚
10.0553(19) A, a = 64.024(7) , b = 13.715(2) A, b = 80.245(12) , c =
◦
3
˚
˚
15.384(3) A, c = 70.714(10) , V = 1799.5(6) A , Z = 2; Mr = 681.30,
qcalcd = 1.257 Mg m−3, l(Mo-Ka) = 0.150 mm−1, final R1 = 0.0609 for
6329 reflection of I > 2r(I), R1 = 0.0756, wR2 = 0.1702 for all 18561
reflections. CCDC-657906†.
3 (a) P. J. Smith, M. V. Reddington and C. S. Wilcox, Tetrahedron Lett.,
1992, 33, 6085; (b) E. Fan, S. A. Van Arman, S. Kincaid and A. D.
Hamilton, J. Am. Chem. Soc., 1993, 115, 369; (c) R. Prohens, S. Toma`s,
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42, 4933; (e) R. J. Fitzmaurice, G. M. Kyne, D. Douheret and J. D.
Kilburn, J. Chem. Soc., Perkin Trans. 1, 2002, 41, 48; (f) M. Boiocchi,
L. D. Boca, D. Esteban-Go´mez, L. Fabbrizzi, M. Licchelli and E.
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Gunnlaugsson, M. Glynn, G. M. Tocci, P. E. Kruger and F. M. Pfeffer,
15 Crystal data for 7·TBACH3COO·CHCl3·1/2H2O: C◦43H54Cl3N6O4.5
,
¯
˚
triclinic, space group P1, a = 12.077(2) A, a = 108.◦73(3) , b = 13.878(3)
◦
3
˚
˚
˚
A, b = 107.18(3) , c = 14.279(3) A, c = 95.52(3) , V = 2116.4(7) A ,
Z = 2, Mr = 833.27, qcalcd = 1.308 Mg m−3, l(Mo-Ka) = 0.267 mm−1
,
final R1 = 0.0537 for 7406 reflection of I > 2r(I), R1 = 0.0658, wR2 =
0.1181 for all 13051 reflections. CCDC-657907†.
16 G. M. Sheldrick, SHELXS-97, Program for X-ray Crystal Structure
Solution, University of Go¨ttingen, Go¨ttingen, Germany, 1997.
17 G. M. Sheldrick, SHELXL-97, Program for X-ray Crystal Structure
Refinement, University of Go¨ttingen, Go¨ttingen, Germany, 1997.
This journal is
The Royal Society of Chemistry 2008
Org. Biomol. Chem., 2008, 6, 1751–1755 | 1755
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