Fluorescent sensor of fluorene derivatives having phosphonic acid as a fluorogenic ionophore: Synthesis and static quenched properties for Fe(III)

Article abstract of DOI:10.1016/j.tetlet.2014.05.105

A series of novel fluorene derivatives containing -PO(OH)₂ (1, 2), -HS (3), and -N⁺ (CH₃)₃ (4) were synthesized and only phosphonic-functionalized fluorene derivatives can detect Fe³⁺ with high selectivity over other metal ions. The fluorescence quenching of 1 and 2 with titration Fe³⁺ in water-DMF solution fits the Perrin model of static quenching. Spectral analysis showed that Fe³⁺ bound with fluorene-based chemosensor through the O-atoms at the phosphonic group to form nonfluorescent complexes. The binding ratio was estimated by the Job plot and the trace levels of iron in samples were successfully monitored.

Full text of DOI:10.1016/j.tetlet.2014.05.105

Accepted Manuscript
Fluorescent sensor of fluorene derivatives having phosphonic acid as a fluoro-
genic ionophore: Synthesis and Static quenched Properties for Fe(III)
Chang Yi, Bo Song, Wenwen Tian, Xia Cui, Qi Qi, Jiang Wei, Zhengjian Qi,
Yueming Sun
PII:
S0040-4039(14)00936-8
DOI:
http://dx.doi.org/10.1016/j.tetlet.2014.05.105
TETL 44696
Reference:
Tetrahedron Letters
To appear in:
Received Date:
Revised Date:
Accepted Date:
7 February 2014
19 May 2014
26 May 2014
Please cite this article as: Yi, C., Song, B., Tian, W., Cui, X., Qi, Q., Wei, J., Qi, Z., Sun, Y., Fluorescent sensor of
fluorene derivatives having phosphonic acid as a fluorogenic ionophore: Synthesis and Static quenched Properties
for Fe(III), Tetrahedron Letters (2014), doi: http://dx.doi.org/10.1016/j.tetlet.2014.05.105
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Fluorescent sensor of fluorene derivatives having phosphonic acid as a
fluorogenic ionophore: Synthesis and Static quenched Properties for
Fe(III)
Chang Yi a,b, Bo Song a, Wenwen Tian a, Xia Cui a, Qi Qi a, Jiang Wei a, Zhengjian Qi a, Yueming Suna,*
*Corresponding author. Tel.: +86 25 52090621; fax: +86 25 83792637.
E-mail address: sun@seu.edu.cn (Y. Sun).

1
Tetrahedron
jour nal homepage: w ww.elsevier. com
Letters
Fluorescent sensor of fluorene derivatives having phosphonic acid as a
fluorogenic ionophore: Synthesis and Static quenched Properties for Fe(III)
a
1 Chang Yi a,b, Bo Song a, Wenwen Tian a, Xia Cui a, Qi Qi a, Jiang Wei a, Zhengjian Qi , Yueming Sun a,
*
2
3
4
a School of Chemistry and Chemical Engineering, Southeast University, Nanjing, 211189, P. R China.
b Department of Chemical Engineering, Zhenjiang College, Zhenjiang, Jiangsu 212003, P. R China
5
AR TIC LE INFO
ABSTRACT
A series of novel fluorene derivatives containing -PO(OH) 2 (1, 2), -HS (3) and -N+
(CH3)3 (4) were synthesized and only phosphonic -functionalized fluorene derivatives
can detect Fe3+ with high selectivity over other metal ions. The fluorescence
quenching of 1 and 2 with titration Fe3+ in water-DMF solution fits the Perrin model
of static quenching. Spectral analysis showed that Fe3+ bound with fluorene -based
chemosensor through the O-atoms at the phosphonic group to form nonfluorescent
complexes. The binding ratio was estimated by the Job plot and the trace levels of iron
in samples were successfully monitored.
Article history:
Received
Received in revised form
Accepted
Available online
Keywords:
Fluorescence
Sensor
Iron
Static quenching
Fluorene
.
6
7
Scheme 1. Synthesis of chemosensor 1, 2, 3 and 4.
8
Iron is an essential element present in the biosystem and
both its deficiency and overload can induce various disorders.1, 2
According to the U.S. Environmental Protection Agency (EPA),
the maximum content of iron in drinking water is ∼5.357 µM.
For that reason, the detection of trace irons becomes very
important in the chemical research. Therefore, highly sensitive
and selective Fe3+ fluorescent sensors are greatly desirable. A
fluorescent sensor is generally constituted by a fluorophore
recent years, several fluorescent sensors have been developed
for the detection of transition and heavy metal ions, such as
naphthalene fluorophore containing -PO(OH)2 for detecting
Cu2+,5 thiourea-appended naphthalimides for detecting Zn2+ and
Hg2+,6 aromatic imino diacetate for detecting Cd2+ and F-.7,8
Meanwhile, various sensors for Fe3+ have been reported.9-12
However, to the best of our knowledge , the static quenching
mechanism for fluorene-based chemosensor with Fe3+ is still
unexplored. Moreover, using the Perrin model as external
standard calibration curve to monitor trace levels of iron ions is
rarely reported.
(signaling moiety)，which is covalently linked, through an
appropriate spacer, to a ionophore (recognition moiety).3,4
The recognition of the target species by the recognition
moiety as a result of a selective fluorophore-ionophore
interaction between the two is converted into an
enhancement or quenching of the fluorophore emission. In
We have recently reported two phosphonic acid-
functionalized fluorene derivatives 9,9-Bis (3'- phosphonic acid
propyl)-2,7-diphenylfluorene and 2,7-diphenylfluorene -9-
*Corresponding author. Tel.: +86 25 52090621; fax: +86 25 83792637.
E-mail address: sun@seu.edu.cn (Y. Sun).

2
ylphosphonic acid.13 In order to further extend this work, other
new fluorescent materials with various functional groups
(compounds 1, 2, 3, 4) have been constructed. Compared to our
previously reported molecules, the sensitivity of compound 1
and 2 for iron ions has been enhanced through changing the
position of phosphonic acid and the conjugated length of
fluorophore, respectively. The fluorescence of compound 3 and
4 has been hardly quenched with titration iron ions by replacing
the phosphonic acid to -HS and -N+(CH3)3, respectively.
Meanwhile, we investigated their ability to function as a
fluorescent sensor for Fe3+ in semi-aqueous solution and
discussed the quenching mechanism by the fluorescence
lifetime measurement. We also successfully monitored trace
levels of iron (III) with them in samples.
The synthesis routes of the monomer and fluorescent
materials 1, 2, 3, 4 are presented in Scheme 1. The boronic acid
derivative was coupled with two equiv of monomer 5 under
palladium-catalyzed Suzuki conditions to afford compound 12,
subsequently the reaction of 12 with bromotrimethylsilane and
methanol afforded fluorescent material 1 in a yield of 30%
without further purification. The monomer 9 was also subjected
to Suzuki coupling with 11 (made by metal-halogen exchange
and nucleophilic substitution of 10) to produce fluorescent
material 2. Formation of 3 and 4 from 14 was achieved by using
Thiourea and trimethylamine respectively. Structures of
compounds 1, 2, 3 and 4 were characterized by FT-IR, 1H
NMR, 13C NMR, 31P NMR and HRMS (data is shown in the
Supporting information Figs. S1–S13).
The influence of iron (III) on the fluorescence response of 1,
2, 3 and 4 in water-DMF (5/5, v/v) solution was studied at
pH=6-7. The emission intensity of 1 and 2 at 386 and 397 nm
monotonically decreases with titration Fe3+ and finally the
fluorescence disappears when the concentration of Fe3+ achieves
16 µM (Fig. 1a and 1b). Such high quenching phenomenon is
probably due to photoinduced electron transfer (PET) or energy
transfer.14,15 For 3 and 4, the fluorescence is hardly quenched
with titration Fe3+ (Fig. S14). These phenomenons suggest that
the phosphonic acid-functionalized fluorene derivatives are
sensitive to iron (III), whereas the trialkylamino-functionalized
and mercapto-functionalized fluorene derivatives are
insensitive. Therefore, it indicates that the metal binding sites
for fluorene derivatives are the phosphonic acids.16, 17
Figure 1. Fluorescence spectrum changes of compound 1 (c=10
µM) (a) and compound 2 (c=10 µM) (b) in water-DMF (5/5, v/v) at
increasing concentrations of Fe3+ solution with 330 nm excitation.
Insert: Variation of F0 /F of emission as a function of Fe3+. (where
F0 and F refer to the emission luminance intensity for compounds
in the absence and presence of Fe3+).
As seen in figure 2a, under lower concentration, the observed
fluorescence quenching, F0/F, of these compounds is not linear
with quencher (Fe3+) concentration and thus fails to obey the
Stern-Volmer model of dynamic quenching. However, the ln
F0/F values are linear with quencher concentration, in agreement
with the Perrin model of static quenching (Fig. 2b). The Perrin
linear equations of 1 and 2 are showed in Table S1. The
fluorescence quenching efficiency values (kSV) of them, in the
Perrin equation are K≈1.992×105 M-1 and 2.594×105 M-1
respectively. It suggests that all the added quencher (Fe3+) is
bound to the ligands. This agreement with the Perrin model
further supports that fluorescence quenching for systems of this
type occurs intramolecularly. 18,19

3
Figure 2. Plots for dynamic quenching (a) and static quenching (b)
of the compound 1 and 2 by Fe3+. The conditions were as described
in the legend of Figure 1.
Figure 3. Fluorescence decay of 1 at λem = 386 nm (a) and 2 at λem
= 397 nm (b), obtained by irradiating (λexc = 330 nm) in water-
DMF (5/5, v/v) solutions of different Fe3+ concentrations: 0 µM
(red line) and 6 µM (blue line). Additionally, the corresponding
fitted curves (black line) and the instrumental response function
(IRF, gray line) are shown. The decay time τ is summarized in
Table S2. Insets: Photoluminescence spectra recorded with 0 µM
and 6 µM Fe3+ (λex=330 nm).
In order to investigate the possible static quenching of Fe3+
for chemosensor 1 and 2, their fluorescence lifetime was
recorded in water-DMF (5/5, v/v) solutions containing 0µM and
6µM Fe3+. The fluorescence decay time profiles of these
chemosensors with (blue line) or without (red line) 6µM Fe3+
were shown in Figure 3 together with the instrumental response
function (IRF, gray line). The corresponding curves (black line)
In order to confirm the fact that the quenching effect of
chemosensor 1 and 2 is attributed to the formation of complexes
between iron (III) and phosphonic groups, the complexes were
prepared by stirring the FeCl3 in C2H5OH with chemosensor 1
or 2 for 10 mins, and then characterized by UV and FTIR.
Figure S15 shows the UV spectra of compound 1, complex 1-
Fe, 2 and complex 2-Fe in water-DMF solutions. The
absorption onset of 1 was blue-shifted by 40 nm after
coordinating with iron (Fig. S15 a), and the absorption onset of
2 was blue-shifted by 12 nm (Fig. S15 b). The changes in the
absorption spectra indicated that the new complexes were
formed by the strong association interaction between the
analytes (Fe3+) and the receptors (phosphonic groups). The
FTIR spectra of them were shown in figure 4. The FT-IR
spectrum of 1 exhibited stretching peaks originating from (P=O)
and (P–OH) at 1110 cm−1 and 995 cm−1. The absorption signals
at 1110 cm−1 and 990 cm−1 showed stretching frequency of
(P=O) and (P–OH) of 2. However, the characteristic absorption
peaks of P=O, P–OH all disappeared and overlapped to form a
new broad band (P–OFe stretching band) at 1034 cm−1 for
complex 1–Fe and 2–Fe. It provided insight into the
coordination mode of ligand to iron. These results were similar
to those published by Chung et al,9 who studied Fe3+-binding
with pyrene-based chemosensor .
were
equation20
fitted
according
to
. The contribution of
n












t
Ι (t) =
Β ⋅exp
−
⊗ IRF t + A
( )
∑
fl
i
τi
i=1
background fluorescence and stray light is included in the
additive parameter A. The time constant τi as well as the
n≤3
amplitude-weighted average decay time
is given in
i
< τ >= W ⋅τ
∑
i
i=1
Table S2. Static quenching refers to the formation of a ground-
state complex between a fluorophore and a quencher. It means
that the fluorescence decay intensity of the fluorophore
decreases without altering the excited state lifetime. So static
quenching is observed because that the fluorophore–quencher
interaction does not change the decay time < τ >. Therefore, the
fluorescence lifetime measurement is the most common method
to identify the quenching mechanism. 21, 22 As shown in Figure
3, the decay of them is all monoexponential. The florescence
intensity of 1 and 2 is all quenched with addition of 6 µM Fe3+
(insert of Fig. 3a, 3b), while their fluorescence lifetime does not
change. It indicates that the quenching is not initiated by
dynamic quenching, but probably by static quenching resulting
from the formation of 1-Fe and 2-Fe complexes.

4
Figure. 4 FT-IR spectra of compound 1 (a), complex 1–Fe (b),
compound 2 (c) and complex 2–Fe (d).
Furthermore, the images of these chemosensors were
obtained under ultraviolet radiation. The chemosensor 1 and 2
emit blue light (Fig. 5a, 5c), whereas complete fluorescence
quenching takes place for the 1–Fe and 2–Fe (Fig. 5b, 5d).
Figure 6. Job's plot for determining the binding stoichiometry in
DMF/H2O (v/v = 1/1) solution, showing a 1:1 stoichiometry
between 1 and Fe3+ (a), 2 and Fe3+ (b). The variation of the Job’s
function (FJob) at 386 nm (a) or at 397 nm (b) was measured as a
function of the molar ratio X ([Fe3+]/ ([Fe3+] + [chemosensor])),
respectively. Insets: Photoluminescence spectra recorded with
various mole fraction of Fe3+ (X) at a constant total concentration of
10 µM (λex=330 nm).
Figure 5. Fluorescence photographs of the chemosensor 1 (10 µM)
(a), complex 1–Fe (10 µM) (b), chemosensor 2 (10 µM) (c) and
complex 2–Fe (10 µM) (d) in water/DMF (5/5, v/v) solutions under
a 365 nm UV lamp.
Iron sensing ability of chemosensors was investigated in
water–DMF (5/5) (v/v) at different pH using HCl and Tris. It
could be seen that the optimal pH was at the range from 5.0 to
7.0 (Fig. S17). The value of pH 7 was adjusted throughout the
experiment. At the same time, the selectivity of compound 1
and 2 for Fe3+ over other metal ions (Fe2+, Na+, K+, Mg2+,Ca2+,
Ni2+, Mn2+, Co2+, Cu2+ , Cd2+, Zn2+ and Ag+) was investigated by
the competition experiments (Fig. 7). While only Fe3+ induced
dramatic decrease of fluorescence intensity, all competitive ions
had no obvious interference with the detection of Fe3+ . These
observations indicate that Fe3+ is the only metal ion that readily
binds with fluorene-based chemosensors (1 and 2), causing
static fluorescence quenching and forming complexes.9, 27, 28 So
they can serve as potential “On–Off” fluorescent sensors to
selectively detect Fe3+ in water-DMF solution at pH 7.0.
To elicit the interactions between Fe3+ and chemosensor 1, 2
respectively, the binding stoichiometry was determined by Job
plot.23, 24 The Job’s function FJob is calculated according to the
equation FJob= (1-X) F0-F. The plot of the luminescence versus
the mole fraction of the added Fe3+ (Fig. 6) shows two parts that
can be fitted by straight lines. The lines intersect at X equals to
0.5. These results reveal the formation of a 1:1 complex
formulated as 1- Fe3+ and 2- Fe3+, respectively. So the possible
coordination mode is shown in Scheme S1. According to the
Benesi-Hilderbrand equation 1/( F0-F) = 1/{Ka * (F0- Fmin) *
[Fe3+]} + 1/( F0- Fmin), (where Fmin the minimum fluorescence
intensity at 386 nm in the presence of Fe3+),25,26 the binding
constant (Ka) is calculated to be 5.67×104 M−1 and 9.28×103 M−1
respectively (Fig. S16).

5
was a static quenching process, which was also demonstrated by
the unchanging fluorescence lifetime and the new formation of
broad bands (P-OFe stretching bands). Meanwhile, the binding
ratios of 1-Fe and 2-Fe complexes were both determined to be
1:1. Moreover, an external standard calibration curve of
fluorescence quenching was applied successfully to the
determination of iron (III) concentration in samples. Further
experiments will be in progress to detect Fe3+ in biological
systems using these sensors.
Acknowledgements
We are grateful for the grants from the National Basic Research
Program China (2013CB932902), National Natural Science
Foundation
of
China
(21173042)
and the science and technology support program (industry) proj
ect of Jiangsu province (BE 2013118, BE2012810).
Supplementary data
Supplementary data (synthetic and experimental details)
associated with this article can be found, in the online version,
at
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External standard
samplea
ICP-OES(µM)
Calibration(µM)
Compound 1
6.43
20.58
47.43
Compound 2
7. 95
21.36
49.95
M1
M2
M3
7.84
22.0
48.13
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