Synthesis, crystal structure and binding properties of a new fluorescent molecular clip based on 2,5-diphenylfuran

Article abstract of DOI:10.3184/174751913X13626742215922

A new fluorescent molecular clip derived from 2,5-diphenyl-furan with two benzo[d]-thiazole-2-thio-units as side walls has been synthesised and its structure and conformation confirmed by an X-ray structure. Fluorescence spectroscopic studies show that it can selectively bind Fe³⁺.

Full text of DOI:10.3184/174751913X13626742215922

210 RESEARCH PAPER
APRIL, 210–212
JOURNAL OF CHEMICAL RESEARCH 2013
Synthesis, crystal structure and binding properties of a new fluorescent
molecular clip based on 2,5-diphenylfuran
Hu Sheng-Li,a* Xu Cai-Hua,a Zhang Shu-Shu,a Sun Wen-Binga and Gao Qingb
aHubei Key Laboratory of Pollutant Analysis and ReuseTechnology,Department of Chemical and Enviromental Engineering, Hubei Normal
University, Huangshi 435002, P. R. China
bFaculty of Materials Science and Engineering, Hubei University, Wuhan 430062, P. R. China
A new fluorescent molecular clip derived from 2,5-diphenyl-furan with two benzo[d]-thiazole-2-thio-units as side
walls has been synthesised and its structure and conformation confirmed by an X-ray structure. Fluorescence
spectroscopic studies show that it can selectively bind Fe3+.
Keywords: 2,5-diphenylfuran, molecular clip, X-ray structure, fluorescence, Fe3+
The design and synthesis of chemosensors for heavy and tran-
sition metal ions (HTM) are of prime importance for medical,
environmental, and biological applications.1 Iron is an essen-
tial element for nearly all living systems. It is a catalyst in
some important biological processes such as oxygen metabo-
lism and synthesis of DNA and RNA.2,3 Abnormal levels of
iron could be related to some serious diseases, such as Hun-
tington’s, Parkinson’s and Alzheimer’s diseases.4 Therefore, it
is necessary to develop accurate and rapid detection methods
to detect selectively and quantify iron in environmental and
biological samples. However, examples of Fe3+-selective
fluorescent probes are rare.5–12 Therefore, designing a highly
selective sensor for Fe3+ is still a challenge. Here, we report a
novel and selective fluorescent chemosensor for Fe3+.
is 45.27°. The bis-thiazole ligand, flanked by the furan unit,
have great potential to bind host molecule.
The binding properties of molecule 1 were investigated
by UV-Vis absorption spectroscopy and fluorescence
measurements. Changes of the UV-Vis absorption spectra and
We designed a PET1 sensor 1 (Scheme 1), in which two
benzo[d]-thiazole-2-thio-moieties are linked to a 2,5-diphe-
nyl-furan fluorophore, hoping that this molecule would adopt
a semirigid clip-shaped conformation, which might be able to
bind selectively with a metal ion. The primary test showed that
1 possesses highly selective fluorescence quenching toward
Fe3+ in a aqueous DMF solution.
The synthesis of chemosensor 1 is shown in Scheme 1. The
IR, NMR and MS spectra of the product are in good agreement
with the title compound formulation. In addition, its structure
and conformation in the solid state were confirmed by single
crystal X-ray diffraction, as shown in Fig. 1.
Crystals of 1 were obtained by slow evaporation from
chloroform-methanol solution (20:1 v/v). In compound 1, the
two substituted C5–C10 and C11–C16 phenyl rings are twisted
away form the furan ring, making dihedral angels of 20.96°
and 30.51° with the furan ring. Two benzothiazole rings are
fused to the furan unit to form a semirigid clip-shaped side
wall. The dihedral angle between the two benzothiazole rings
Fig. 1 The crystal structure of 1.
Scheme 1 The synthesis of molecular clip 1.
* Correspondent. E-mail: hushengli168@126.com

JOURNAL OF CHEMICAL RESEARCH 2013 211
Fig. 2 Absorption spectra of 1 (1.0×10−5 M) in DMF/ water
(98:2, v/v) containing BR buffer (10 mM, pH = 7.4) solution in
the presence of various metal ions (5×10−5 M).
Fig. 3 Fluorescence emission changes of 1 (1×10−5 M) in DMF/
water (98:2, v/v) containing BR buffer (10 mM, pH= 7.4) in the
presence of 10×10−5 M solution of various metal ions (excitation
at 305 nm).
fluorescence properties of 1×10−5 M solutions of 1 in DMF/
water (98:2, v/v) caused by various metal ions (K+, Na+, Mg2+,
Hg2+, Cd2+, Fe3+, Zn2+, Co2+, Ni2+, Cu2+, Pb2+, Cr3+, and Mn2+)
(as their Cl− or NO3− salts) are shown in Figs 2 and 3, respec-
tively. The results show that Fe3+ produced significant enhance-
ment in the absorption spectra and quenching in the fluorescent
emission of 1, the other tested metals only show relatively
insignificant changes. So it can be concluded that 1 has a high
selectivity for recognition of Fe3+.
The sensitivity of the fluorescence emission response of 1
towards Fe3+ was also examined under the same conditions
with various Fe3+ concentrations (Fig. 4). The fluorescence
intensity (λem = 351 nm) of 1 was decreased continually upon
addition of Fe3+ with no significant change in the position of
the emission maxima.When the concentration of Fe3+ increased
to 20 equiv., the intensity of 1 was reduced to 24% of the initial
one.
For a homogeneous medium that has only a single compo-
nent exponential decay, the concentration of the quencher can
be calculated using the Stern–Volmer equation.13 Interestingly,
the plots of F0/F versus Fe3+ ion concentration did not fit a
conventional linear Stern–Volmer equation (insert of Fig. 4).
The steep upward curvature may suggest a more complex
quenching model which may involve both dynamic and static
quenching simultaneously.14 The data, however, could be fitted
to the following empirical equation:15
Fig. 4 Fluorescence emission spectra (excitation at 305 nm)
of 1 (1×10−5 M) in DMF/water (98:2, v/v) containing BR buffer
(10 mM, pH = 7.4) in the presence of [FeCl3(H2O)6]. The con-
centrations of Fe3+: 0, 2, 4, 6, 8, 10, 12, 14, 16, 18 and 20×10−5 M.
Inset: Stern–Volmer plot of the emission data.
log[(F0/F)] = Ksv[Q] + C
The calibration plot of log [(F0/F)] versus [Q] shows a good
linear relationship (R=0.99721) for Fe3+ ion concentration
(Fig. 5). From the slope and intercept of the calibration plot,
the KSV and C could be evaluated as 3.3×103 M−1. The limit of
detection of 1 is found to be 1.8×10−6 M, which was calculated
using 3δ/S,16 where δ was the standard deviation of the blank
signal, and S was the slope of the linear calibration plot. To
determine the stoichiometry of compound 1 and Fe3+ ion in the
complex, Job’s method17 was employed using the emission
changes at 351 nm as a function of the molar fraction of Fe3+.
A minimum emission was observed at 0.5 mole fraction
(Fig. 6), indicating that Fe3+ ion forms a 1:1 complex with the
sensing compound 1.
Based on the fluorimetric titration experiments, the X-ray
crystal structure of l, and a recent report,12 a pocket model
seems to be reasonable for the binding site of 1 with Fe3+. In
the structure of 1, there is a nice binding pocket composed
of two benzylic sulfur atoms and two nitrogen atoms on the
Fig. 5 Modified Stern–Volmer relationship between 1 and Fe3+.

212 JOURNAL OF CHEMICAL RESEARCH 2013
Scheme 2 Proposed binding model of sensor 1 with Fe3+.
reflections with I>2σ(I) were used in the structure determination
and refinements. The structure was solved by direct methods with
the SHELXS-97 program and expanded by Fourier techniques. The
non-hydrogen atoms were refined anisotropically and the hydrogen
atoms were determined by theoretical calculation. A full-matrix,
least-squares refinement gave the final R = 0.0672, wR = 0.1697.
All calculations were performed on a PC with the SHELXS-97
program. Crystal data: C32H22N2OS4, M = 578.76, Monoclinic, space
group P2(1)/n, a = 11.3848(19), b = 5.3120(9), c = 46.117(8) Å,
α = γ = 90°, β = 96.154(3)°, V = 2772.9(8) Å3, Z = 4, Dc = 1.386 g cm−3,
µ = 0.372 mm−1. The details of the crystal data have been deposited
with the Cambridge Crystallographic Data Centre as Supplementary
Publication No. CCDC 890177.
Binding studies: A stock solution of compound 1 was prepared by
dissolution in DMF/water (98:2, v/v) containing Britton–Robison
(BR) buffer (10 mM, pH = 7.4) (1.0×10−5 M). The solutions of metal
ions were prepared from Pb(NO3)2 and the chlorides of K+, Na+, Mg2+,
Hg2+, Cd2+, Fe3+, Cu2+, Zn2+, Co2+, Ni2+, Pb2+, Cr3+, and Mn2+, respec-
tively, and were dissolved in water (3.0×10−3 M). Fluorescence titra-
tion was performed on 3 mL solution of compound 1 in a quartz cell
of 1 cm optical path length, by adding different stock solutions of cations
into the quartz cell portionwise using a microsyringe each time.
Fig 6 Job’s plot of a 1:1 complex of 1 (1×10−5 M) with Fe3+.
Total [1] + [Fe3+] = 1×10−5 M.
benzothiazole moieties, which is probably a suitable binding
site for Fe3+ (Scheme 2). The capture of Fe3+ causes the elec-
tron or energy transfer from the excited benzo[d]-thiazole-2-
thio unit to 2,5-diphenylfuran and Fe3+, thus resulting in the
fluorescence quenching of 2,5-diphenyl-furan.
In conclusion, a new fluorescent molecule clip 1 derived
from 2,5-diphenylfuran and benzo[d]-thiazole-2-thio has
been designed and synthesised. Its binding properties, investi-
gated by fluorescence spectroscopy, showed that it can selec-
tively bind Fe3+ in aqueous DMF solution with fluorescence
quenching.
We thank the Hubei Province Education Ministry Foundation
of China (No. D20112507) and the Science Technology
Foundation for Creative Research Group of HBDE(2013) for
financial support.
Experimental
All reagents obtained from commercial sources were of AR grade.
Melting points were determined with a XT4A micromelting point
apparatus and were uncorrected. 1H NMR spectra were recorded on a
Bruker 300 spectrometer with TMS as internal reference and CDCl3
as solvent. IR data were recorded on a Perkin−Elmer PE−983 IR
spectrometer as KBr pellets with absorption in cm−1. MS were obtained
with a Finnigan Trace MS instrument using the EI method. UV spec-
tra were measured on a SP-1900 spectrophotometer. Fluorescence
spectra were determined on a Hitachi F-4500 instrument.
Received 30 December 2012; accepted 1 February 2013
Paper 1201702 doi: 10.3184/174751913X13626742215922
Published online: 19 April 2013
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