Dansyl-styrylquinoline conjugate as divalent iron sensor

Article abstract of DOI:10.1016/j.tetlet.2010.10.043

Synthesis and photophysical studies of a simple and efficient dansyl-styrylquinoline conjugate (DansSQ) which shows excellent selectivity towards Fe²⁺ over other transition metal ions including Fe ³⁺ is described. The fluorescent emi

Full text of DOI:10.1016/j.tetlet.2010.10.043

Tetrahedron Letters 51 (2010) 6626–6629
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Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
Dansyl-styrylquinoline conjugate as divalent iron sensor
⇑
⇑
L. Praveen, M. L. P. Reddy , R. Luxmi Varma
Chemical Sciences and Technology Division, National Institute for Interdisciplinary Science and Technology (CSIR), Trivandrum 695 019, India
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 7 July 2010
Revised 7 October 2010
Accepted 11 October 2010
Available online 15 October 2010
Synthesis and photophysical studies of a simple and efficient dansyl-styrylquinoline conjugate (DansSQ)
which shows excellent selectivity towards Fe2+ over other transition metal ions including Fe3+ is
described. The fluorescent emission of DansSQ at 450 nm was enhanced upon addition of Fe2+ in the pres-
ence of common interfering metal ions in background. The studies suggest that DansSQ could be used as
fluorescent sensor for Fe2+
.
Ó 2010 Elsevier Ltd. All rights reserved.
Keywords:
8-Hydroxyquinoline
Dansyl chloride
Selective recognition Fe2+ sensor
Selective detection of metal ions is of fundamental importance
in the broad areas of chemistry and biology.1 Significant advances
in the field of fluorescence spectroscopy in this regard has been
made particularly due to its several advantages like easy detection,
high sensitivity and tunability.2 This has been made possible by the
progress made in the design and development of fluoroionophores
consisting of small molecules with inherent properties of selective
binding and sensing of physiologically and environmentally rele-
vant cations.3 Iron, present in both Fe(II) and Fe(III) oxidation
states, plays an important role in biology and is a vital element
for the survival of all living organisms.4 Its presence is essential
for a number of vital cell functions like oxygen metabolism,5 elec-
tron transfer processes,6 and formation of DNA and RNA to men-
tion a few.7
Compared to numerous reports available in literature that have
explored the fluorescence sensing of main group and transition
metal ions such as K+, Na+, Li+, Mg2+, Ca2+, Zn2+, Cu2+, Hg2+, etc., spe-
cific examples of fluorescent sensors for the detection of iron ions
are less. Further, examples of fluorophores that show enhanced
fluorescence in the presence of Fe ions are even more scarce due
to paramagnetism of Fe(II) and Fe(III) ions causing quenching of
the fluorescence.8 Successful fluorescent Fe(III) sensor designs are
based on complexation by covalently linked fluorescent dyes, ion-
izable chelates, and macrocycles with polydentate binding sites.9 A
typical example of such a sensor molecule consists of a mixed do-
nor atom (N, S, O) macrocycle as receptor unit and 1,3,5-triarylpy-
razoline or boron dipyrromethene as signaling units reported by
Bricks et al.10 A rhodamine 6G based sensor molecule was also
shown to have enhanced fluorescence behavior selective to
Fe(III).9a Among iron sensors, it is noteworthy to mention the
highly selective and sensitive inorganic/organic hybrid polymer
system which exhibited turn on fluorescence for Fe2+ ions.11
To be of considerable practical application, organic receptors
that evoke optical or electrochemical response on binding with a
cation/anion should not only have the inherent property of selec-
tive sensing of the analyte among a large number of species that
co-exist but also be easily accessible by simple synthetic routes.
8-Hydroxyquinoline is an efficient chelator for cations, and one
of the best reagents used as complexing agent after EDTA.12 It
has been employed for OLED materials,13 sensitizing lanthanides,14
solvent extraction,15 etc. 8-Hydroxy quinoline based fluorescent
chemosensors have received increasing interest in recent years
by virtue of their high fluorescent properties and ease of
modification.16
Peng et al. have reported a series of 5-dialkylaminomethyl-8-
hydroxyquinoline dansylates which exhibited selective quenching
of fluorescence in the presence of Fe3+. Hydroxyquinoline dansy-
lates as such are highly fluorescent molecules like alkoxy quino-
lines and upon addition of metal ions the fluorescence is
quenched due to an efficient PET process.17 We in our laboratory
have shown that 8-methoxyquinoline with 4-nitrostyryl substitu-
ent at the 2-position is a non-fluorescent molecule due to an effi-
cient intramolecular charge transfer process (ICT) upon
excitation. With the idea of designing a fluorescent ‘turn on sensor’
for iron ion, we decided to synthesize a dansyl-styrylquinoline con-
jugate, since sulphoxide oxygen is known to have an affinity to-
wards iron ions.18 The results of our investigations are the
subject matter of this letter.
⇑
Corresponding authors. Tel.: +91 4712515275; fax: +91 4712491712 (R.L.V.).
E-mail addresses: mlpreddy55@gmail.com (M.L.P. Reddy), lux_varma@rediff
The synthesis of the chemosensor dansyl-styrylquinoline
(DansSQ) was carried out as outlined in Scheme 1. The quinoline
mail.com (R.L. Varma).
0040-4039/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2010.10.043

L. Praveen et al. / Tetrahedron Letters 51 (2010) 6626–6629
6627
1.4x106
1.2x106
1.0x106
8.0x105
6.0x105
4.0x105
2.0x105
0.0
400
450
500
550
600
650
Wavelength in nm
Figure 2. Fluorescence spectra (kexc = 352 nm) of DansSQ (6 Â 10À6 M) in acetoni-
trile with increasing amount of Fe2+ (2.4 Â 10À4 M) ACN–H2O (9:1, v/v).
Scheme 1. Synthesis of receptor DansSQ.
carbaldehyde 1 was synthesized as reported.19 Wittig-Horner reac-
tion of 1 with diethyl 4-nitrobenzyl phosphonate in the presence of
NaH yielded 4-nitrostyryl quinoline-8-ol 3. Subsequent reaction of
3 with dansyl chloride 4 in dry CHCl3 with Et3N as base yielded the
product in 35% yield. The product DansSQ was characterized by
analytical and spectral techniques.20 The UV–vis absorption spec-
trum of the compound showed two broad bands at 352 nm
gen to nitrostyryl group upon excitation.21 On binding with Fe2+
,
the ICT process is disrupted thereby regaining the fluorescence.
The origin of fluorescence in DansSQ is due to quinoline core and
not due to dansyl chromophore was further proved by recording
excitation spectrum and comparing with literature data.22 The
quantum yield (UF) of the Fe2+–DansSQ complex was found to be
0.075.23 Cyclic voltametric studies indicate that no electron trans-
fer takes place from Fe2+ to the ligand in the complex, as there is no
changes were recorded in the cyclic voltammogram of the ligand
(
e
= 1.8 Â 104 MÀ1 cmÀ1) and 256 nm (
e
= 2.12 Â 104 MÀ1 cmÀ1),
*
corresponding to the
p–p
transitions of the quinoline core. To ob-
tain insight into the binding properties of DansSQ towards various
metal ions, we initiated our studies with Fe3+ and Fe2+ ions fol-
lowed by metal ions like Mn2+, Co2+, Ni2+, Cu2+, Zn2+, Hg2+, Ag+,
Pb2+, Cd2+, Na+, K+, Ca2+ and Al3+. Among all the metal ions tested,
only Fe2+ elicited marked changes in the absorption spectrum of
DansSQ.
upon addition of Fe2+
.
1:1 stoichiometry was confirmed for Fe2+ and DansSQ by Job’s
plot (Fig. 3). The stoichiometry of Fe2+/DansSQ complex has also
been confirmed by mass spectrometric determination. The FAB
mass spectrum of this complex showed signals of m/z 779.30,
which can be assigned as signals for [Fe(ClO4)2+DansSQ+1]+. Asso-
ciation constant (Ka) calculated by Benesi-Hildebrand method was
found to be 2.837 Â 103 MÀ1. The detection limit was calculated as
three times the standard deviation of the background noise and
was found to be 3.6 Â 10À6 M in ACN–H2O (9:1, v/v).24
Upon addition of Fe2+ (0–5.0 equiv) the absorption band at
256 nm decreased in intensity and the band at 352 nm underwent
a bathochromic shift of 22 nm which tailed out up to 450 nm as
shown in Figure 1. Appearance of a new absorption band at
302 nm, which increased in intensity upon addition of Fe2+ ions
and well defined isosbetic points at 272 nm, 319 nm and 352 nm
indicate formation of an active Fe2+–DansSQ complex.
The fluorescence spectrum of DansSQ (kexc = 352) in acetonitrile
(ACN) as well as ACN/H2O (9:1, v/v) showed highly structured but
very weak emission band centered at 450 nm. Upon addition of
increasing amounts of Fe2+ to a solution of DanSQ, the fluorescent
intensity was remarkably enhanced (15-fold) as shown in Figure 2.
In acetonitrile and partial acetonitrile solutions, the low quantum
yield (UF) of DansSQ is attributed by the ICT from quinoline nitro-
When 1,10-phenanthroline was added to the solution of Dans-
SQ–Fe2+ solution, an immediate reversal of the absorbance peak
at 374 nm in the UV–vis spectrum was observed with concomitant
formation of a new peak at 510 nm which corresponded to the for-
mation of the orange-red Fe2+–1,10-phenanthroline complex.25
This experiment brings forth the reversibility of DansSQ as Fe2+
sensor (Fig. 4). Similar observations were noticed during the fluo-
rescence studies, when 1,10-phenanthroline was added to Dans-
SQ–Fe2+ complex. Fluorescence response of DansSQ on the
4.5
4.0
3.5
3.0
2.5
2.0
1.5
1.0
0.5
0.25
0.20
0.15
0.10
0.05
0.00
0.0
0.2
0.4
0.6
0.8
1.0
250
300
350
400
450
Wavelength in nm
Mole fraction of Fe(II)
Figure 1. Changes in the absorption spectra of the DansSQ (6 Â 10À6 M) with the
addition of Fe2+ (2.4 Â 10À4 M) in ACN–H2O (9:1, v/v).
Figure 3. Job’s plot of DansSQ (6 Â 10À6 M) and Fe2+ (6 Â 10À6 M), where fluores-
cent intensity of DansSQ at 450 nm was plotted against the mole fraction of Fe2+
.

6628
L. Praveen et al. / Tetrahedron Letters 51 (2010) 6626–6629
1.5
1.0
0.5
0.0
16
14
12
10
8
6
4
2
0
Na+ Ca+ Ag+ Al3+Cr3+ Mn2+Pb2+Fe2+Cu2+Zn2+Cd2+Hg2+
300
400
500
600
Wavelength in nm
Metal ions
Figure 4. Changes in absorption spectra of Fe2+–DansSQ complex by the addition of
10 equiv of 1,10-phenanthroline (8 Â 10À4 M).
Figure 6. Competitive selectivity of the receptor DansSQ towards Fe2+ in the
presence of other metals (50 equiv).
addition of other metal ions was also investigated. As shown in Fig-
ure 5 only the addition of 5 equiv of Cr3+ showed a threefold
enhancement in fluorescent intensity.
1.2
1.0
The fluorescence response of DansSQ towards Fe2+ in the pres-
ence of 50 equiv each of alkali, alkaline earth and transition metal
ions mentioned above were also investigated and it was found that
there was no interference of any of these metal ions at the above
concentrations (Fig. 6). However, addition of more than 200 equiv
of Cu2+, Hg2+, Fe3+ or Cr3+ was found to cause significant fluorescent
enhancement to DansSQ, hampering the selectivity.
DansSQ
DansSQ+ Fe2+
0.8
0.6
0.4
0.2
0.0
-0.2
1622
786
1046
622
1177
1112
Further evidences for metal complexation were obtained from
IR spectral studies. In the FTIR spectrum, the medium sized band
at 1177 cmÀ1 corresponding to symmetric stretching of S@O bond
became fairly weak after coordination with Fe2+ (Fig. 7). A broad
peak with three shoulders was observed between 1190 cmÀ1 and
1030 cmÀ1 in the Fe2+ bound DansSQ complex. Also characteristic
stretching of ClOÀ4 1 visible at 622 cmÀ1 further supports the co-
ordination event.26 It can be anticipated that selectivity of DansSQ
towards Fe2+ arises from the S@O group and the quinoline nitrogen
in the vicinity creating in a congenial environment for the com-
plexation event.
In conclusion, we have devised a new fluorogenic chemosensor,
by conjugating the well known 8-hydroxyquinoline and dansyl
group. The prepared ionophore showed selective and sensitive
OFF–ON type fluorescent behaviour and gives a 15-fold fluores-
cence enhancement upon addition of Fe2+. The chemosensor shows
a detection limit which is sufficiently low to allow fluorogenic
detection of submillimolar concentrations of Fe2+. The selective
1596
1517
1339
1077
1200
Wavelength in cm
1800
1500
900
600
-1
Figure 7. IR spectrum of DansSQ and solid complex of DansSQ–Fe2+ in the region
2000–600 cmÀ1
.
detection of the Fe2+ over other bio relevant cations and the possi-
bility of its direct quantification using an unsophisticated fluores-
cent sensor DansSQ are especially significant. Further studies
designed to increase the fluorescent intensity and water solubility
are in progress.
Acknowledgements
The authors thank the Council of Scientific and Industrial Re-
search, New Delhi for financial assistance (NWP 0010). L.P. thanks
CSIR for SRF. We are grateful to Dr. D. Ramaiah for useful discus-
sions. Thanks are also due to Mr. B. Adarsh, Mr. Preethanuj, Ms.
Ambili Raj and Ms. S. Viji for NMR, IR and mass spectral data.
16
14
12
10
8
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