Fluorescent detection of anions by dibenzophenazine-based sensors

Article abstract of DOI:10.1016/j.tetlet.2011.11.121

Binding studies of two sulfonamide-functionalized dibenzophenazine-based sensors are reported. Spectroscopic studies showed that both sensors are effective fluorescent turn-on sensors for several anions. Both sensors showed responses to acetate, benzoate,

Full text of DOI:10.1016/j.tetlet.2011.11.121

Tetrahedron Letters 53 (2012) 661–665
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Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
Fluorescent detection of anions by dibenzophenazine-based sensors
Ala’a O. El-Ballouli a, Yadong Zhang b, Stephen Barlow b, Seth R. Marder b, Mohammad H. Al-Sayah c,
,
⇑
Bilal R. Kaafarani a,
⇑
a Department of Chemistry, American University of Beirut, Beirut 1107 2020, Lebanon
b School of Chemistry and Biochemistry and Center for Organic Photonics and Electronics, Georgia Institute of Technology, Atlanta, GA 30332 0400, USA
c Department of Biology, Chemistry and Environmental Sciences, American University of Sharjah, PO Box 26666, Sharjah, United Arab Emirates
a r t i c l e i n f o
a b s t r a c t
Article history:
Binding studies of two sulfonamide-functionalized dibenzophenazine-based sensors are reported. Spec-
troscopic studies showed that both sensors are effective fluorescent turn-on sensors for several anions.
Both sensors showed responses to acetate, benzoate, cyanide, and fluoride ions. NMR titrations confirmed
the mode of binding of the sensors to be through H-bonding to the sulfonamide groups.
Ó 2011 Elsevier Ltd. All rights reserved.
Received 3 August 2011
Revised 13 November 2011
Accepted 25 November 2011
Available online 2 December 2011
Keywords:
Dibenzo[a,c]phenazine
Fluorescent sensors
Anion detection
Chemical sensors
Spectroscopic titrations
NMR titrations
The design of anion sensors is a fast emerging research field
due to the importance of detecting anions in medicine, chemical
industry, and the environment.1–7 Chloride and nitrate ions, for
example, are tracers of water pollution8 while cyanide, a highly
toxic anion, is a dangerous environmental contaminant associated
with leakage, from electroplating and herbicide industries.9,10
Therefore, the development of fast, easy, and sensitive detection
techniques for the continuous monitoring of anions is important.
Such diagnostic tools can be developed through the utility of fluo-
rescent sensors that can be tailored to have high selectivity or
broad detection of ions.6,9 While the development of ion sensors
that work efficiently in aqueous media is still challenging, non-
aqueous systems can serve as useful model systems and may still
have niche applications. The design of an effective fluorescent
sensor relies mainly on an active recognition site for the anion,
a fluorophore capable of exhibiting a high quantum yield, and
an efficient signaling mechanism whereby anion binding affects
the fluorophore. Typically, for an effective fluorescent sensor, it
is desired that the binding of an anion to the active site triggers
electronic changes in the fluorophore leading to a ‘turn ON’ of
the fluorescence.
binding site to the fluorophore take place through a ‘push–pull’
mechanism, with binding affecting the donor/acceptor strength of
the binding substituent12,13 and the resultant change in electronic
structure leading to either emission enhancement or quenching. A
variety of neutral receptors have been reported for selective anion
recognition based on H-bonding owing to the strength and selectiv-
ity of this interaction.14,15 This has been conventionally achieved by
employing N–H fragments of amides, ureas, thioureas, and pyrroles,
which can bind to anions in a reversible manner and, therefore,
allowing continuous monitoring.5,6
In our ongoing efforts to develop fluorescent sensors, we have
reported sensors for cations16 and anions17 employing quinoxali-
nophenanthrophenazine (TQPP) and phenanthrophenazine (DPP)
as fluorophores using crown ethers and sulfonamide groups as
binding sites, respectively. In this Letter, we introduce two new
fluorescent sensors 1 and 2 for anions using a dibenzo[a,c]phena-
zine fluorescent core functionalized with sulfonamide groups as
the H-bonding site for anions. As shown in Scheme 1, both sensor
molecules are functionalized with solubilizing dodecyl groups,
while the structure of 2 differs from that of 1 by the presence of io-
dine substituents, the electronic effects of which are expected to
result in different emission responses.
Intramolecular charge transfer (ICT) is one of the various signal-
ing mechanisms11–13 by which the electronic transitions from the
Compound 2 has previously been reported in the literature as
an intermediate in the synthesis of a ‘nano-truck’18 and was syn-
thesized from the diiododiketone 4 and the ditosylated tetraamine
5 following the previously reported procedure (Scheme 1). Dike-
tone 3 is the immediate precursor to 418 and was also converted
⇑
Corresponding authors. Tel.: +961 3 151451; fax: +961 1 365217 (B.R.K.).
E-mail addresses: malsayah@aus.edu (M.H. Al-Sayah), bilal.kaafarani@aub.edu.lb
(B.R. Kaafarani).
0040-4039/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2011.11.121

662
A. O. El-Ballouli et al. / Tetrahedron Letters 53 (2012) 661–665
Ha
Hb
O
S
O
S
NH HN
O
O
Hc
O
S
O
O
S
O
O
O
NH HN
EtOH / AcOH
reflux
+
N
N
R
R
C12H25
C12H25
H2N
NH2
R
R
3, R = H
4, R = I
5
C12H25
C12H25
1
, R = H, 68 %
2, R = I, 92 %
Scheme 1. Synthesis of sensors 1 and 2.
into the corresponding dibenzo[a,c]phenazine 1, which is a new
compound, by the reaction with 5.
and 420 nm (Fig. 1). The addition of anion solutions to the sensors
resulted in similar spectral changes of the sensors with the same
anions. For example, the addition of acetate, cyanide, and fluoride
Absorption, emission, and NMR spectroscopic studies were con-
ducted on sensors 1 and 2 in the presence of anion solutions at dif-
ferent concentrations to evaluate the efficacy with which they
detect various anions and the effect of the iodo substituents on
the output signal. The absorption spectra of the two sensors were
similar, exhibiting characteristic peaks at 280 nm, 325 nm, 400 nm,
anions (1.0 mM in CHCl3) to a solution (5.0 lM in CHCl3) of the
sensors resulted in major changes in the UV spectra (Fig. 1) includ-
ing enhancement of absorbance at ꢀ310 nm and ꢀ455 nm with
concurrent reduction in absorbance at 280 nm, 400 nm, and
420 nm. Isosbestic points at 300 nm, 365 nm, and 425 nm indicate
a clean transformation of the free sensor into the sensor-anion
complex. On the other hand, the addition of chloride, bromide, io-
dide, nitrate, and perchlorate anions had no significant effect on
the absorption spectra of either sensor.
0.7
0.14
A
0.12
These spectral changes were attributed to strong hydrogen-
bonding interaction between the N–H bonds of the sulfonamide
groups and the anions in line with the similar changes seen for
other sulfonamide-based systems.19 Proton-transfer between the
sulfonamide and the anions was considered unlikely due to the
low polarity of the solvent (previously reported related sulfon-
amide sensors were reported to form hydrogen bonds to anions
in acetonitrile, but to undergo proton transfer to the anions in
the more polar DMSO19). This reasoning was supported by the re-
sults obtained from the titration of the sensors 1 and 2 with TBA
hydroxide, where the anion is more basic. Fig. 2 depicts the
changes in the absorbance of 1 and 2 at 455 nm and 460 nm,
respectively, as a function of the concentration of various anions,
including the hydroxide anion. Both sensors exhibited ꢀ10 times
enhancement in the absorbance at these wavelengths after the
addition of ꢀ1 M equiv of carboxylate, cyanide, or fluoride anions.
On the other hand, the addition of the hydroxide anion gives qual-
itatively different changes, resulting in a ꢀ5 times increase in the
absorbance of sensor 1 after more than two molar equivalents of
the anion were added. Sensor 2 showed much less enhancement
in its absorbance at 460 nm even after addition of more than five
molar equivalents of the anion. These results indicated different
modes of interaction for the hydroxide anion and the other anions
with the sensors. This further favors the hydrogen-bonding inter-
action mode of the anions with sulfonamides rather than proton-
transfer (vide infra NMR studies).
0.10
0.6
0.08
0.06
0.5
0.4
0.3
0.2
0.1
0.0
0.04
0.02
0.00
0.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1.0
[1]/([1]+ [b enzoate])
250
300
350
400
450
500
550
Wavelength (nm)
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0.0
0.10
0.08
0.06
0.04
0.02
0.00
B
0.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1.0
[2]/([2 ]+ [cya nide])
The binding isotherms of acetate, benzoate, cyanide, and fluo-
ride anions were generated from the change in absorption of the
sensors (Fig. 3). To calculate the binding constants of the sensors
to these anions (Table 1), the binding isotherms were fitted to a
1:1 binding model as suggested by the Job plots. The results ob-
tained were in accordance with previously observed and reported
results on similar systems.17,19 Meanwhile, the spectral alterations
effected by the addition of chloride, bromide, iodide, nitrate, and
perchlorate anions were too insignificant to generate any meaning-
ful binding isotherm.
250
300
350
400
450
500
550
Wavelength (nm)
Figure 1. UV–vis spectra of 1 (A) and 2 (B) (5 lM) upon titrating with solutions
(1.0 mM) of TBAOBn and TBACN, respectively, in CHCl3. The insets are the Job plots
for each anion.
The binding of anions to 1 and 2 resulted in drastic changes in
their emission spectra, but to different extents for each sensor.

A. O. El-Ballouli et al. / Tetrahedron Letters 53 (2012) 661–665
663
Table 1
0.30
0.25
0.20
0.15
0.10
0.05
0.00
A
Binding constants of sensors
1
and
2
with several
anionsa
F_
Sensor
Anion
1
logK
2
logK
Cl _
OAcÀ
OBzÀ
CNÀ
FÀ
5.93
6.73
6.01
5.72
5.94
6.43
7.73
5.95
Br _
I _
NO3_
ClO4_
OAc _
OBn _
CN_
OH_
a
Ka values were calculated from changes in the
absorbance of the sensors at 455 nm for 1 and 460 nm
for 2. See Supplementary data for more details.
the emission at 550 nm. Meanwhile, the addition of the same ions
(acetate, cyanide, or fluoride) to 2 resulted in a 700–800-fold
enhancement of its emission at 555 nm (again with kex = 350 nm)
(Fig. 4). These differences can partly be attributed to the greater in-
crease in the absorbance at 350 nm (the excitation wavelength)
seen for 2 on binding (Fig. 1), but also depend on the relative quan-
tum yields of bound and unbound forms of each sensor. The bath-
ochromic shift of both absorption and emission seen on binding is
0
1
2
3
4
5
6
[anion]/[1]
0.25
0.20
0.15
0.10
0.05
0.00
B
F_
Cl _
consistent with binding leading to an increase in the
p donor
Br _
I _
strength of the sulfonamides and so with the lowest energy states
of the bound species having significant ICT character between the
sulfonamide donors and the electron-poor heterocyclic acceptor.
The slightly longer wavelength emission found for 2 is consistent
with the expected weak electron-withdrawing effect of the iodine
lowering the energy of such an ICT state. Finally, the addition of
chloride, bromide, iodide, nitrate, and perchlorate anions had
insignificant effect on the emission of the sensors (Fig. 4).
NO3_
ClO4_
OAc _
OBn _
CN_
OH_
NMR titrations of 1 and 2 further supported the trend ob-
served for anion binding in the photophysical studies. The chem-
ical shifts of the N–H protons of the sulfonamide groups which
are associated with the binding process disappeared from the
proton NMR spectrum upon the first addition of anion solution
due to chemical exchange with the residual water in the solvent.
Thus, the chemical shifts of the neighboring C–H protons, Hc and
Hb (Scheme 1), were monitored as aliquots of anion solution were
added into solutions of the sensors in 2% DMSO-d6 in CDCl3. Fig. 5
shows partial proton NMR spectra of 1 where the signals of the
aromatic protons are shown as the concentration of the fluoride
anion was increased. The signals of Hc and Ha shifted upfield from
8.03 and 7.27 ppm to 7.83 and 7.17 ppm, respectively. This
0
1
2
3
4
5
6
[anion]/[2]
Figure 2. The change in the absorbance of 1 (A) and 2 (B) (5.0 lM) at 455 nm and
460 nm, respectively, upon titrating with different anion solutions (1.0 mM) in
CHCl3.
Sensor 1 shows an emission maximum at 438 nm upon excitation
at 350 nm in CHCl3; the addition of acetate (Fig. 3), cyanide, or
fluoride anions resulted in a substantial bathochromic shift of the
emission band to 550 nm. Accordingly, the emission at 438 nm is
quenched and there is a large (140–160 times) enhancement of
1000
800
1000
800
600
400
200
0
600
438 nm
550 nm
400
200
0
0
1
2
3
4
5
6
[acetate]/[1]
400
450
500
550
600
650
Wavelength (nm)
Figure 3. Emission spectra of 1 (5.0
lM, CHCl3) upon the addition of acetate anion solution. The inset depicts the change in the emission intensity at 438 nm and 550 nm as a
function of molar equivalence of the anion relative to 1.

664
A. O. El-Ballouli et al. / Tetrahedron Letters 53 (2012) 661–665
900
800
700
600
500
400
300
200
100
0
1200
1000
800
600
400
200
0
F_
Cl _
Br _
_
I
_
NO3
_
ClO4
OAc
OBn
CN_
_
_
0
1
2
3
4
5
6
[anion]/[2]
450
500
550
600
650
Wavelength (nm)
Figure 4. Emission spectra of 2 (5.0 lM, CHCl3) upon the addition of the fluoride anion solution. The inset depicts the relative change in the emission of 2 (5.0 lM) at 555 nm
upon titrating with different anion solutions (1.0 mM) in CHCl3.
Figure 5. Partial 1H NMR spectra of 1 (1.0 mM) upon titrating with TBAF solution (10 mM) in 2% DMSO-d6 in CDCl3.
upfield shift can be attributed to the expected increase in the
electronic density on the neighboring sulfonamide groups caused
by H-bonding of the N–H protons to the anions. Meanwhile, the
signal of Hb shifted downfield from 7.82 ppm to 7.85 ppm, pre-
sumably due to a through-space effect of the anion bound in close
proximity to these protons. Effects on the chemical shift of other
signals were only minor.
The effects on the NMR spectra of titration with hydroxide an-
ions were insignificant compared to those of acetate, benzoate,
cyanide, or fluoride anions. Fig. 6A depicts the change in the chem-
ical shift of the Hc protons of 1 and 2 upon titrating with acetate or
hydroxide anions. The chemical shift of Hc decreased (ꢀ0.2 ppm)
upon the addition of acetate anions and it reached a constant value
once one molar equivalent of the anion had been added indicating
strong binding with 1:1 stoichiometry (Fig. 6B). Meanwhile, the
addition of more than five molar equivalents of hydroxide anions
shifted the signals of the Hc protons by less than 0.04 ppm. These
results further support our interpretation that the interaction of
the anions with the sensor is mainly due to hydrogen-bonding
rather than proton-transfer to the anions. Both sensors exhibited
similar trends in the changes of chemical shifts of the aromatic
protons (Hc and Hb) upon the addition of acetate, benzoate, cya-
nide, or fluoride anions.
In summary, two dibenzophenazine-based sulfonamide mole-
cules have been synthesized and have been shown to be effective
fluorescent turn-on sensors for several anions in chloroform. Both
sensors showed responses to acetate, benzoate, cyanide, and fluo-
ride ions. In particular, sensor 2 exhibited the strongest changes in

A. O. El-Ballouli et al. / Tetrahedron Letters 53 (2012) 661–665
665
0.12
A
B
0.00
-0.04
-0.08
-0.12
-0.16
-0.20
1
2
0.10
0.08
δ
1 + TBAOAc
δ
0.06
2 + TBAOAc
1 + TBAOH
2 + TBAOH
0.04
0.02
0.00
0.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1.0
0
1
2
3
4
5
[Acetate]/([Sensor]+[Acetate])
[anion]/[sensor]
Figure 6. (A) The change in the chemical shifts of Hc for 1 and 2 (1 mM) upon titrating with TBAOAc and TBAOH solutions (10 mM) in 2% DMSO-d6/CDCl3; (B) Job plots of the
acetate NMR titration of 1 and 2.
its emission spectra upon the addition of the anions at different
concentrations. NMR titrations suggested that the anions bind to
the sensors through H-bonding to the sulfonamide groups.
Sensor 2 was synthesized as previously described from the
diketone 4 and ditosylated tetraamine 5 as shown in Scheme 1.18
Sensor 1 is a new compound and was obtained in 68% yield in
the same way, but using diketone 318 in place of 4.
of Sharjah (Grant #: FRG10-02), and by the Petroleum Research
Foundation of the American Chemical Society (Grant #: 47343-
B10). Work at Georgia Tech was supported by Solvay S.A. and the
Science and Technology Center Program of the National Science
Foundation (DMR-0120967).
Supplementary data
Data for 1: 1H NMR (400 MHz, CDCl3): d 9.03 (d, J = 8.4 Hz, 2H),
8.23 (s, 2H), 7.79 (s, 2H), 7.70 (d, J = 8.4 Hz, 4H), 7.47 (d, J = 8.4 Hz,
2H), 7.30 (s, 2H), 7.21 (d, J = 8.4 Hz, 4H), 2.85 (t, J = 7.6 Hz, 4 H), 2.34
(s, 6 H), 1.75 (m, 4 H), 1.39 (m, 4 H), 1.23 (m, 32H), 0.85 (t, J = 6.8
Hz, 6H) ppm. 13C{1H} NMR (100 MHz, CDCl3): d 145.67, 144.52,
142.72, 139.93, 135.00, 131.97, 131.87, 129.83, 128.40, 127.71,
127.61, 126.17, 123.55, 122.31, 32.60, 31.89, 31.62, 29.67, 29.63,
29.58, 29.48, 29.34, 22.66, 21.58, 14.10 (one alkyl resonance not
observed, presumably due to overlap). HRMS (MALDI): Calcd for
Supplementary data (calculations of binding constants, changes
in the absorption of 1 and 2 upon the addition of different anions,
1H NMR spectra of 2 upon titrating with TBAF, and the change in
the chemical shift of Hc in 1 and 2 upon titration with different an-
ions) associated with this article can be found, in the online ver-
sion, at doi:10.1016/j.tetlet.2011.11.121.
References and notes
C
C
58H75N4O4S2 (MH+): 955.5230. Found: 955.2223. Anal. Calcd for
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7.97; N, 5.91.
Spectroscopic titration: A solution of the sensor (5 mM, 2 mL)
in CHCl3 placed into a 1 cm. 1 cm cuvette was titrated with a solu-
tion of a tetra-n-butylammonium salt of the anion (1.0 mM, CHCl3)
that contained the sensor (5 mM). Aliquots of the anion solution
were added to the cuvette via a syringe until a total of 10 or more
equivalents of the anion had been added (the number of additions
was around 20 with an increase in the amount of anion solution
added). The UV–vis spectrum and emission spectrum (lex= 350
nm) were recorded after each addition.
1H NMR titration: A solution of the sensor (1 mM, 600 mL) in 2%
DMSO-d6/CDCl3 placed in an NMR tube was titrated with a solution
of the anion (10 mM). Aliquots of the anion solution were added to
the NMR tube via a syringe until a total of 10 equiv of the anion
had been added (the number of additions was around 15 with an in-
crease in the amount of anion solution added). A 1H NMR spectrum
was collected after each addition and the chemical shifts of the aro-
matic protons were recorded. The collected data was analyzed using
a non-linearleast-squares regression program to fit the data to a the-
oretical model with a 1:1 binding stoichiometry.
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Acknowledgments
This work was supported by the University Research Board
(URB) of the American University of Beirut, American University