Dipyrrolyl quinoxalines with extended chromophores are efficient fluorimetric sensors for pyrophosphate

Article abstract of DOI:10.1039/b301362f

2,3-Di(pyrrole-2-yl)quinoxaline anion sensors with extended chromophores display dramatic enhancement in fluorescence intensity (sensing output), increased affinity for inorganic anions as well as strong selectivity for pyrophosphate over phosphate anion.

Full text of DOI:10.1039/b301362f

Dipyrrolyl quinoxalines with extended chromophores are efficient
fluorimetric sensors for pyrophosphate†
Dmitry Aldakov and Pavel Anzenbacher, Jr*
Department of Chemistry, Center for Photochemical Sciences, Bowling Green State University, Bowling
Green, Ohio, 43402, USA. E-mail: pavel@bgnet.bgsu.edu; Fax: 419-372-9809; Tel: 419-372-2080
Received (in Columbia, MO, USA) 3rd February 2003, Accepted 17th April 2003
First published as an Advance Article on the web 14th May 2003
2,3-Di(pyrrole-2-yl)quinoxaline anion sensors with extended
chromophores display dramatic enhancement in fluores-
cence intensity (sensing output), increased affinity for
inorganic anions as well as strong selectivity for pyr-
ophosphate over phosphate anion.
conjugated chromophore, and tune the emission wavelength of
the sensor through substituent effects. This electronic tuning of
absorption and emission properties is considered particularly
useful as it may be used to avoid potential interference from
fluorescent impurities present in the anion samples.
The synthesis of sensors S1–S4 is accomplished by con-
densation of 3,6-dibromo-1,2-phenylenediamine (1)7 with
1,2-dipyrrolyl-2A-ylethanedione (2)6a,b to give 3. The Stille
coupling8 of 3 with stannylated arenes9 yields sensors S1–S4 in
reasonably high yields, 22, 38, 41 and 48%, respectively (Fig.
2).
Inorganic anions such as chloride, phosphate, pyrophosphate,
cyanide or fluoride play an important role in energy transduc-
tion in organisms, and control metabolic processes via partici-
pation in enzymatic reactions.1 Inorganic anions are also used in
industry, for example as agricultural fertilizers and raw
materials in numerous industrial processes.2
There are several reasons why reliable sensing of anions is a
challenging area of research.3 Compared to cations, anions are
larger, display a wide range of geometries and significant
charge delocalization.4 All these factors result in a low surface
charge concentration and make binding of anions less effective.
It is not surprising that sensitive and selective sensors for anions
are still rare.4 In general, the performance of chemosensors can
be improved either by (i) increasing its selectivity through
preorganization of the receptor, or (ii) through the improved
signal transduction (read-out) that leads to a reliable substrate-
specific response. Given the sensitivity of fluorescence as a
method, we believe that the latter can be achieved by using
sensors with a strong fluorescent (instead of or in addition to
colorimetric) response.5,6 Although both strategies for sensor
development are equally useful, a majority of the research
efforts are focused on the improvements of the receptor
moieties. Our results suggest that the improvements in a signal
transduction, a road significantly less travelled in the anion
sensing field, may result in a dramatic improvement of the
overall sensor performance.
Fig. 2 Synthesis of sensors S1–S4.
As expected, the presence of 5,8-aryl substituents in sensors
S1–S4 results in both a dramatic increase of emission intensity
and shifted emission maximum compared to the parent DPQ (lf
= 490 nm, Ff = 0.155). Thus S1 displays the emission
maximum at 500 nm (Ff = 0.199)‡, S2 – 502 nm (Ff = 0.237),
S3 – 590 nm (Ff = 0.222), and S4 – 610 nm (Ff = 0.256). Both
properties may be easily observed by the naked eye (Fig. 3).
In order to illustrate this point, we have selected 2,3-di-
(pyrrole-2-yl)quinoxaline (DPQ),6 a compound that binds
anions via hydrogen bonding to the pyrrole NH-moieties. DPQ
was recently recognized as a potential colorimetric and
fluorescent sensor for anions.6a Previous efforts were oriented
towards improving the receptor via tuning the electronic
properties, cryptate effect, etc.6 In this study we focus on
improving the signal transduction process in the DPQ-derived
anion sensors (Fig. 1).
Fig. 3 The strong fluorescence emission can be observed by the naked eye.
A photograph is taken upon illumination with a UV-lamp (360 nm). The
solutions of sensors were 1.0 µM in CH2Cl2.
The qualitative estimation of the anion binding affinity of
sensors S1–S4 was performed visually (Fig. 4). The observed
changes in fluorescence intensity in the sample without any
anion and in the presence of the tested anion allow for the
prediction of the sensor-anion binding affinities. Sensors S1–S4
responded to the presence of fluoride (hexahydrate) and
pyrophosphate (dihydrate) anions through a dramatic decrease
in the sample emission. Such quenching is not observed with
chloride (dihydrate), bromide, dihydrogenphosphate (dihy-
drate) or hydrogensulfate (dihydrate).§ The fact that the
presence of water solvates does not interfere with the sensing
process is encouraging considering the demand for sensors
capable of anion sensing in the presence of water. Sensors S1–
S4 are capable of anion sensing in acetonitrile ( < 0.3% of water)
and DMSO ( < 0.5% of water). Dichloromethane for titration
experiments was selected to generate data comparable with the
rest of the literature on DPQ-related sensors.6
Fig. 1 Structures of 2,3-di(pyrrole-2A-yl)quinoxaline (DPQ), 5,8-diphenyl-
2,3-di(2-pyrrolyl)quinoxaline (S1), 5,8-di(4-methoxyphenyl)-2,3-di(2-pyr-
rolyl)quinoxaline (S2), 5,8-di(EDOT)-2,3-di(2-pyrrolyl)-quinoxaline (S3)
and 5,8-di(4,4A-N,NA-dimethylaminophenyl)-2,3-di(2-pyrrolyl)quinoxaline
(S4).
In order to enhance and tune the fluorescence output from the
DPQ sensor, aryl moieties were attached to the 5- and
8-positions of the quinoxaline ring. The 5,8-aryl substituents
serve to enhance the sensor emissivity by expanding the
† Electronic supplementary information (ESI) available: experimental data.
See http://www.rsc.org/suppdata/cc/b3/b301362f/
1394
CHEM. COMMUN., 2003, 1394–1395
This journal is © The Royal Society of Chemistry 2003

View Article Online
the pyrophosphate may be capable of effective ditopic hydrogen
bonding to the two pyrrole moieties of the sensor. Such ditopic
binding could account for the strong complexation of pyr-
ophosphate. This feature is a subject of continuing studies.
Figure 6 shows schematic representation of the complex
(energy minimized using MM+, HyperChem 5.1).
Fig. 4 The photograph under UV light of the 1.0 µM in CH2Cl2 solutions of
the sensor S1 (1) in the presence of the following anions (100 eq.): fluoride
(2), chloride (3), pyrophosphate (4), and phosphate (5).
Further insight and quantitative evaluation of the sensing
capability of sensors S1–S4 was obtained from UV-vis and
fluorescence quenching experiments. Representative titration
curves are shown in Fig. 5. In general, the addition of fluoride
and pyrophosphate anions to the solution of sensors S1–S4 in all
cases results in the decrease of the absorption intensity in the
400–450 nm region together with the appearance of a strong
band centered at 500–550 nm, which is responsible for the red
color of the solution. The addition of fluoride and pyr-
ophosphate resulted in a significant quenching ( > 95%) of the
fluorescence emission intensity.
Fig. 6 Proposed structure of the pyrophosphate–DPQ complex (A: side
view, B: front view).
In conclusion, we demonstrated that the chromophore
extension in simple fluorescence-based anion sensors such as
DPQ may result in dramatic improvement of sensor perform-
ance as judged by the enhanced fluorescence output and
improved affinity for anionic substrates. The fact that simple
sensors such as DPQ can selectively bind pyrophosphate is
scientifically noteworthy since such sensors may serve as a
departing platform for the development of sensors for other
biologically important anions. Efforts toward the synthesis of
materials capable of anion sensing in the presence of water are
under way.
Financial support from Kraft Foods, the Petroleum Research
Fund (ACS-PRF #38110-G), NSF (NER 0304320) and BGSU
to P.A. and McMaster Fellowship to D.A. is gratefully
acknowledged.
Notes and references
‡ Fluorescent quantum yields (Ff) referenced to quinine sulfate in 0.1 N
H2SO4 (Ff = 0.545)11 as a standard.
§ The anions used were in the form of tetrabutylammonium salts (TBA)
hydrates. The degree of hydration was determined from elemental analyses
of the respective salts.
Fig. 5 Left panel: changes in UV-vis spectra of S1 (10 µM in CH2Cl2) upon
addition of pyrophosphate TBA salt (0–1.0 mM). Right panel: decrease in
fluorescence emission intensity of S1 (2 µM in CH2Cl2) upon the addition
of pyrophosphate (0–86 µM); (lexc = 446 nm) Inset: linearized binding
isotherm used for a binding constant determination.
The emission quenching data was used to calculate the
respective binding constants summarized in Table 1. One can
clearly see that DPQ-sensors with extended chromophores,
namely S1 and to a lesser extent also S2–S4, show substantially
increased affinity (sensitivity) for fluoride and pyrophosphate
compared to the parent DPQ sensor. More importantly, all
sensors S1–S4 show a reversed fluoride/pyrophosphate se-
lectivity (KPP ≈ 2KF) compared to DPQ (KPP 4 KF) (Table
1).
1 L. Stryer in Biochemistry, 4th Ed, W. H. Freeman & Co., New York,
1995.
2 (a) Riegel’s Handbook of Industrial Chemistry, Ninth Edition, ed. J. A.
Kent, Van Nostrad Reinhold-International Thomson Publishing, New
York, 1992; (b) Ullmann’s Encyclopedia of Industrial Chemistry, 6th
Edition, Eds. J. E. Bailey; M. Bohnet; C. J. Brinker; B. Cornils; T.
Evans; H. Greim; L. L. Hegedus; J. Heitbaum; W. Keim; A. Kleemann;
G. Kreysa; J. Löliger; J. L. McGuire; A. Mitsutani; L. Plass; G.
Stephanopoulos; D. Werner; P. Woditsch; N. Yoda; 1999 Electronic
Release, Wiley VCH, New York, 1999.
Table 1 Anion binding constants (Ka)a (M21) obtained from titrations of the
sensors with anions
3 Comprehensive Supramolecular Chemistry, Eds. J. L. Atwood; J. E.
Davies; D. D. MacNicol; F. Vögtle; D. N. Reinhoudt; J.-M. Lehn;
Pergamon-Elsevier Science, Oxford, 1996.
Anionb
4 (a) P. D. Beer and P. A. Gale, Angew. Chem. Int. Ed., 2001, 40, 486; (b)
J. L. Atwood and J. W. Steed, Supramol. Chem. Anions, 1997, 147.
5 (a) A. P. de Silva, H. Q. N. Gunaratne, T. Gunnlaugsson, A. J. Huxley,
C. P. McCoy, J. T. Rademacher and T. E. Rice, Chem. Rev., 1997, 97,
1515; (b) S. C. Burdette, G. K. Walkup, B. Spingler, R. Y. Tsien and S.
J. Lippard, J. Am. Chem. Soc., 2001, 123, 7831.
6 (a) C. B. Black, B. Andrioletti, A. C. Try, C. Ruiperez and J. L. Sessler,
J. Am. Chem. Soc., 1999, 121, 10438; (b) P. Anzenbacher Jr., A. C. Try,
H. Miyaji, K. Jursíková, V. M. Lynch, M. Marquez and J. L. Sessler, J.
Am. Chem. Soc., 2000, 122, 10268; (c) J. L. Sessler, H. Maeda, T.
Mizuno, V. M. Lynch and H. Furuta, Chem. Commun., 2002, 862; (d) J.
L. Sessler, H. Maeda, T. Mizuno, V. M. Lynch and H. Furuta, J. Am.
Chem. Soc., 2002, 124, 13474.
7 Y. Tsubata, T. Suzuki and T. Myashi, J. Org. Chem., 1992, 57, 6749.
8 J. K. Stille, Angew. Chem. Int. Ed. Engl., 1986, 25, 508.
9 All aryl-tributyltins except phenyl were synthesized according to
procedure from: S. S. Zhu and T. M. Swager, J. Am. Chem. Soc., 1997,
119, 12568.
10 K. A. Connors, Binding Constants, John Wiley and Sons, New York,
1987.
11 J. R. Lakowicz, Principles of fluorescence spectroscopy, 2nd ed.,
Plenum, New York, 1999.
32
2
F2
Cl2
HP2O7
H2PO4
Molar Ratio Water-Anion 6:1
2:1
2:1
2:1
DPQ6a
S1c
S2
S3
S4
18200
< 50
< 100
< 100
< 100
< 50
14300
93700
58900
57300
39000
< 100
< 200
< 100
< 100
< 50
51300
24700
25600
27500
a All errors are ± 15%. All binding constants are reported as the average of
3 trials. b Anions were used as TBA salts. c Ka were determined by
fluorescence quenching in CH2Cl2. Fitting was carried out according to
Connors.10 The 1:1 stoichiometry of the complexes was determined from
Job-plots3 carried out at different sensor concentrations.
While the strong affinity for fluoride is not unexpected given
its small size and resulting high surface charge density, the high
binding constants for the pyrophosphate anion are particularly
interesting in light of a potential application in the sensing of
nucleotide di- and triphosphates. Such remarkable selectivity
may be explained by a different binding mode. We propose that
CHEM. COMMUN., 2003, 1394–1395
1395