A simple complex: ‘on–off–on’ colorimetric and ratiometric fluorescence response towards fluoride ions and its solid state optical properties

Article abstract of DOI:10.1016/j.tetlet.2016.02.076

A simple 1,1′-bi-2-naphthol (1,1′-BINOL) boron complex has been rationally designed and synthesized, the structure of which was confirmed by single crystal X-ray diffraction analysis. Furthermore, the complex exhibited turn-on fluorescence for fluoride ions with high selectivity and sensitivity. The recognition mechanism for promoting was determined by ¹H, ¹⁹F and ¹¹B NMR titrations, which indicated that both the photo-induced electron-transfer (PET) and the intramolecular charge transfer (ICT) effects were functionalized. In addition, the complex showed strong solid-state fluorescence, and the emission spectrum and quantum efficiency (ΦF) of the solid powders were also measured.

Full text of DOI:10.1016/j.tetlet.2016.02.076

Tetrahedron Letters xxx (2016) xxx–xxx
Contents lists available at ScienceDirect
Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
A simple complex: ‘on–off–on’ colorimetric and ratiometric
fluorescence response towards fluoride ions and its solid state optical
properties
Shengying Wu a,b, Zhijun Chen a, Kewei Zhang a, Gang Hong a, Guosheng Zhao b, Limin Wang a,c,
⇑
a Shanghai Key Laboratory of Functional Materials Chemistry, East China University of Science and Technology, Meilong Road, 130, Shanghai 200237, China
b Zhejiang Runtu Co. Ltd, Daoxu Town, Shangyu City, Zhejiang, China
c Key Laboratory of Organofluorine Chemistry, Shanghai Institute of Organic Chemistry, The Chinese Academy of Sciences, 345 Lingling Road, Shanghai 200032, China
a r t i c l e i n f o
a b s t r a c t
A simple 1,10-bi-2-naphthol (1,10-BINOL) boron complex has been rationally designed and synthesized,
the structure of which was confirmed by single crystal X-ray diffraction analysis. Furthermore, the com-
plex exhibited turn-on fluorescence for fluoride ions with high selectivity and sensitivity. The recognition
mechanism for promoting was determined by 1H, 19F and 11B NMR titrations, which indicated that both
the photo-induced electron-transfer (PET) and the intramolecular charge transfer (ICT) effects were func-
tionalized. In addition, the complex showed strong solid-state fluorescence, and the emission spectrum
Article history:
Received 25 January 2016
Accepted 19 February 2016
Available online xxxx
Keywords:
BINOL–boron complex
Fluoride ion probe
Colorimetric
and quantum efficiency (
UF) of the solid powders were also measured.
Ó 2016 Elsevier Ltd. All rights reserved.
Ratiometric
Solid-state optical properties
Introduction
structures furnishing both the good optical properties and the high
19
selectivities for anions
.
To date, fluorophores such as BODIPY, incorporating a four-
coordinate boron bound to a
-conjugated chelate1–5 have been
applied widely ranging from biological sensing and imaging to
the search for new electroluminescent devices.6–8 Although many
BODIPY dyes exhibit high fluorescence quantum yields in dilute
1,10-bi-2-Naphthol (BINOL) is a commercially available material
that can be synthesized and functionalized in a large quantity. The
outstanding C2 chiral properties make BINOL an excellent ligand
for asymmetric catalysis20 as well as a chiral induction core for
probes.21 Although many probes based on BINOL framework have
p
solutions (
U
F > 0.5), they are scarcely emitted in the solid state
been developed in the form of small molecules21 or cations22
,
as a result of emission quenching upon aggregation. Therefore,
the usage of BODIPY dyes in a practical level still remains a major
challenge.9–11 Of the many exceptional optical molecules, fluorine–
boron complexes, Boranils, as the analogues of BODIPY, have been
reported to have achieved a high solid state fluorescence12 or as
examples about the use of BINOL for sensing anions are rare.23 In
addition, the large dihedral angle and twisted conformation of
the binaphthyl unit yields a stable amorphous phase that can
increase the luminescence intensity.24 To the best of our
knowledge, rare studies of Boranil complexes based on BINOL
framework25 as sensors have been reported.
the receptor for various cations13
, , and also as a
anions14
fluorescent label for various bioactive molecules.15 To improve
the molecular diversity within the boron-based dye family,
As fluoride anion is a strong hydrogen bond receptor and has a
high affinity for silicon atom and boron atom, these unique
N^O16
p-conjugated chelating fragments coordinated to boron-
containing entities for examples, BF2, B(Ar)2, B(OAr)2, and
B(ArF5)2, have been investigated (Fig. 1).17 However, most of these
N^O bidentate
p-conjugated Boranil complexes are based on
N
O
N
N
O
N
N
O
B
B
O
B
2-(2-hydroxyphenyl)benzoxazole (HBO) scaffold or its ana-
logues.18 Consequently, there has been an increasing interest in
the development of novel boron complexes with well-designed
B
O
O
6N^O
5N^O
N^N^O^O
O^N^O
⇑
Corresponding author. Tel./fax: +86 021 6425 3881.
E-mail address: wanglimin@ecust.edu.cn (L. Wang).
Figure 1. Various chelating modes of B(III) complexes.
http://dx.doi.org/10.1016/j.tetlet.2016.02.076
0040-4039/Ó 2016 Elsevier Ltd. All rights reserved.
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S. Wu et al. / Tetrahedron Letters xxx (2016) xxx–xxx
chemical properties have been employed in the design and devel-
opment of many fluorescent sensors of fluoride anions.26 However,
many fluorescent probes of fluoride ions are based on fluorescence
measurement at a single wavelength, which may be influenced by
variations in the sample environment. To increase the selectivity
and sensitivity, ratiometric fluorescent sensors were observed to
be undergoing some changes in the ratio of the intensities of the
absorption or the emission at two wavelengths.
Thus, ratiometric fluorescent sensors can provide a built-in cor-
rection for environmental effects.27 We envisioned that a molecule
possessing both hydrogen bond and boron core centre emitting
dual peaks could be used as a colorimetric and ratiometric fluoride
ions probe. Herein, we synthesized a new BINOL–boron complex
(Scheme 1), which can be used for fluoride ions probe, the complex
showed a colorimetric and ratiometric fluorescence response for
Fꢀ. It exhibited a high selectivity for Fꢀ over other anions in THF.
silica gel column chromatography, giving the yellow solid with
92% yield.
The X-ray structure of FBB
The molecular structure of FBB was confirmed by single crystal
X-ray diffraction analysis29 (Fig. 2), and carbon and hydrogen
atoms are omitted for clarity. In the structure, a new six-mem-
bered heterocyclic ring has been formed, and the N^O fragment
appears quasi-planar except for the B atom with a slightly out of
the planarity [C–O(1)–B(1)–N(1): 12.7(8)] and [C–N(1)–B(1)–O
(1): ꢀ7.1(9)] (Table S1). Meanwhile, the B atom has a slightly dis-
torted tetrahedral geometry with angles ranging from 105.0(6) to
111.2(6) (Fig. 2b and Table S1). The two fluorine chelate units are
distorted from the planarity. The B(1)ꢁ ꢁ ꢁN(1) distance (1.584
(7) Å) is similar with the sum of the Van der Waals radii
(1.580 Å) for the corresponding atoms, which indicates the forma-
tion of covalent-like bond between boron and nitrogen resulting in
the stability of FBB.
The detection limit was calculated to be 0.12
also excited strong solid-state fluorescence, and the emission spec-
trum and quantum efficiency ( F) of the solid powders were mea-
lM. Meanwhile, it
U
sured, which indicated that it or its derivative have great potential
application prospects in OLEDs.
Colorimetric and ratiometric fluorescent probe of fluoride ions
Initially, a THF solution of probe FBB was treated with various
Results and discussion
tetrabutyl ammonium salts, such as Fꢀ, Clꢀ, Brꢀ, Iꢀ, HSO4
,
ꢀ
Synthesis of 3-butyl-2,2-difluoro-10-(2-hydroxynaphthalen-1-
yl)-2H-naphtho[2,3-e][1,3,2]- oxazaborinin-3-ium-2-uide (FBB)
FBB (3-butyl-2,2-difluoro-10-(2-hydroxynaphthalen-1-yl)-2H-
naphtho[2,3-e][1,3,2]oxazaborinin-3-ium-2-uide) was synthesized
from BINOL in five steps by the procedure depicted in Scheme 1.
The aldehyde 1 was prepared according to our previous Letters20,22
and the literature procedures.28 The red precipitate imine was
obtained by treating 1 with n-butylamine in refluxing ethanol.
And then the complexation was feasible with BF3ꢁEt2O in basic con-
ditions (diisopropylethylamine, DIPEA) to quench the nascent HF
under an inert atmosphere of argon. The mixture was purified by
O
N
B
H
F
1)
, EtOH, reflux
NH2
O
F
OH
OH
OH
2) BF3gOEt2, DIPEA, DCE, 40 o
C
Figure 3. UV–vis absorption spectra of probe FBB solution (5 ꢂ 10ꢀ5 M) in the
absence and presence of 5.0 equiv of various anions (Fꢀ, Clꢀ, Brꢀ, Iꢀ, HSO4ꢀ, H2PO4
,
ꢀ
1
FBB
Scheme 1. The synthetic route of probe FBB.
OAcꢀ, NO3ꢀ, CNꢀ) in THF. Inset: a photograph showing the colour change of FBB
(1 ꢂ 10ꢀ4 M) under ambient light in THF without or with addition of 5.0 equiv of
TBAF.
Figure 2. (a) The X-ray crystal structure of FBB showing the partial atom-labelling scheme. Carbon and hydrogen atoms are omitted for clarity. (b) Perpendicular view to
highlight the slightly distortion at the B atom.
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S. Wu et al. / Tetrahedron Letters xxx (2016) xxx–xxx
3
Subsequently, fluorescence titration of FBB by increasing
amounts of Fꢀ ions was examined (Fig. 6). Upon excitation at
320 nm, the free sensor showed an intense emission at 539 nm,
however, the addition of the Fꢀ led to a drastic decrease at
539 nm and simultaneous appearance of a new emission band at
435 nm.
Inset in Figure 5 shows a near-linear correlation of FBB between
intensity ratios of absorbance at 363 nm to those at 422 nm
(A363/A422) vs concentrations of Fꢀ in the range from 0 to 150
lM
(0–3.0 equiv) in THF. The linear equation was found to be
y = 0.70 ꢂ 105x + 0.73 (R2 = 0.9984) (Fig. S1), where
y is the
absorbance at 363 nm measured at a given Fꢀ concentration and
x represents the concentration (10ꢀ5 mol/L) of F ꢀ. According to
IUPAC, the detection limit was determined from three times the
standard deviation of the blank signal (3d) as 0.12 lM. This also
demonstrated the potential utility of probe FBB for calibrating
and determining fluoride ion concentration in THF. Furthermore,
a
corresponding correlation between emission ratiometric
Figure 4. Fluorescent spectra of probe FBB solution (5 ꢂ 10ꢀ5 M) in the absence
and ꢀpresence of 5.0 equiv of various anions (Fꢀ, Clꢀ, Brꢀ, Iꢀ, HSO4ꢀ, H2PO4ꢀ, OAcꢀ,
response of FBB at 539 nm and 435 nm (I435/I539) and fluoride
ion concentration in THF (Fig. 6, inset). Thus, FBB could also serve
as a ratiometric fluorescent probe for Fꢀ. As a result, the probe pro-
vided a ‘on–off–on’ colorimetric (A363/A422) and ratiometric fluo-
rescent response (I435/I539) to Fꢀ.
NO3
, a photograph showing the colour change of FBB
CNꢀ) in THF. Inset:
(1 ꢂ 10ꢀ4 M) under a UV lamp (at 365 nm) in THF without or with addition of
5.0 equiv of TBAF.
On the basis of the above experiments, we hypothesized that
the affinity of Fꢀ to boron centre led to the cleavage of B and N,
and the phenolic O–H and benzylic N engaged in strong hydrogen
bonds with fluoride. The change promoted both the photo-induced
electron-transfer (PET) and the intramolecular charge transfer
(ICT) effects30, as shown in Scheme 2. To confirm this assumption,
the titrations of 1H NMR (Fig. 7a), 19F NMR (Fig. 7b) and 11B NMR
(Fig. S2) were adapted. From the 1H NMR titration spectra, the Ha
shifted to the low field due to the deshielding effect induced by
the combination of Fꢀ and Ha. The peak position of Hb in 1H NMR
illustrated by the addition of Fꢀ, the first cleavage of B–N led to
the slight movement of Hb’s chemical shift to the upfield. When
the Fꢀ concentration became higher, the deshielding effect was
created due to the strong affinity between B and Fꢀ and the pheno-
lic O–H and benzylic N engaging in strong hydrogen bonds with
fluoride, thus, the position of Hb moved to the low field in the spec-
trum. The intramolecular hydrogen bond effect caused the chemi-
cal shift of Hc changing to the downfield. In the 19F NMR, the three
group of peaks of different fluorine ions found in the spectra with
the addition of Fꢀ could illustrate that the Fꢀ from the TBAF formed
the complex with the probe molecules.31 In the process of the 19F
NMR titration, the distance of Fa and Fb in the chemical shift was
Figure 5. UV–vis absorption spectra of FBB (5 ꢂ 10ꢀ5 M) in THF upon addition of
0–5.0 equiv of TBAF. Inset: plot of the absorbance ratio of FBB between 363 nm and
422 nm (A363 nm/A422 nm) versus concentration of Fꢀ in THF.
H2PO4ꢀ, OAcꢀ, NO3ꢀ, CNꢀ, to investigate the selectivity (Figs. 3 and
4). Free FBB displayed a maximum absorption at 422 nm. nBu4NF
(TBAF) as a fluoride source was added to a THF solution of the
probe FBB, the peak at 422 nm was decreased along with a new
peak at 363 nm appeared. However, the addition of other anions
caused little changes in the absorption (Fig. 3). The similar phe-
nomena occurred on the fluorescence with the addition of anions.
Upon addition of 5.0 equiv of Fꢀ ions to FBB solution, the emission
at 539 nm was decreased, accompanied with the appearance of a
strong emission band at 435 nm and a remarkable hypochromatic
shift of 104 nm, while other anions induced little quenching effect
at 539 nm without any new bank emerged (Fig. 4). Thus, FBB has
good sensitivity and selectivity for Fꢀ ions.
To get insight into the mode of probe FBB with the fluoride ions,
UV–vis absorption and fluorescence emission spectra were investi-
gated. In the absence of anions, the maximum absorption wave-
length of FBB was at about 422 nm. Upon progressive addition of
TBAF, the peak at 422 nm decreased gradually, and a large blue
shift of the maximum could be observed and increased,
meanwhile, a new band at 363 nm was developed with a clear
isosbestic point at 396 nm, which indicated the formation of the
new compound (Fig. 5).
Figure 6. Emission spectra of FBB (5 ꢂ 10ꢀ5 M) in THF (kex = 320 nm) upon the
addition of 0–5.0 equiv of TBAF. Inset: plot of the emission intensity ratio of FBB
between 435 nm and 539 nm (I435 nm/I539 nm) versus concentration of Fꢀ in THF.
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S. Wu et al. / Tetrahedron Letters xxx (2016) xxx–xxx
nearer and then farther, which proved that the steric and electronic
exclusion between Fa and Fb was from strong to weak, and then
became strong due to the addition of Fꢀ.
The 11B NMR signal of compound FBB in CDCl3 showed a triplet
peak at 0.22 ppm when boron trifluoride diethyl etherate was used
as an external reference (Fig. S2). Upon the addition of excess
Hb
N
Hb
Hb
N
B
N
F-
F-
Fa
Fb
Fa
Fb
Fa
Fb
Fc
B
B
O
O
O
O
O
Ha
Fc
O
Ha
Ha
Hc
Hc
Hc
Strong Fluorescent
Weak Fluorescent
Weak Fluorescent
Scheme 2. Proposed binding mode of FBB and Fꢀ.
Figure 7. (a) Partial 1H NMR and (b) 19F NMR titration spectra of probe FBB in CDCl3 in the presence of 0 to excess equivalent of TBAF (from bottom to up in each spectrum):
the equivalent of TBAF in (a) and (b) is 0, 0.2, 0.5, 0.7, 1.0, 1.5, 2.0, 3.0 and excess.
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S. Wu et al. / Tetrahedron Letters xxx (2016) xxx–xxx
5
Conclusion
In summary, a simple BINOL–boron complex FBB was designed
and synthesized, which was confirmed by single crystal X-ray
diffraction analysis. It can be used as a novel colorimetric and
ratiometric fluorescent probe for Fꢀ. The probe was highly selective
for Fꢀ over other anions. The detection limit was calculated to be
0.12 lM. In the meantime, the solid-state optical properties have
been disclosed. Further efforts in application of FBB and its deriva-
tives are currently underway in our laboratory.
Acknowledgements
The work was supported by the National Nature Science Foun-
dation of China (NSFC, 21272069, 20672035), the Fundamental
Research Funds for the Central Universities and Key Laboratory
of Organofluorine Chemistry, Shanghai Institute of Organic Chem-
istry, Chinese Academy of Sciences.
Figure 8. Fluorescent spectrum of FBB in the solid-state. Inset: the solid-state
fluorescence of the crystal.
Supplementary data
Fꢀ, two new peaks emerged accompanied with the original signal’s
disappearance. The new broad peak at 3.16 ppm showed that an
sp2-hybridized boronate complex was generated, which supported
the cleavage of B–N bond indirectly. The chemical shift as well as a
clear quartet at 0.07 ppm supported the formation of a ternary
complex and an sp3-hybridized boronate complex.32,33 When the
BF2 group in the probe was finally converted to a trifluoroborate,
the phenolic proton formed strong hydrogen bonds not only with
the BF3 unit but also with the benzylamine nitrogen, thereby
blocking the PET quenching pathway and switching on fluores-
cence emission. In another case, the cleavage of B–N bond pro-
moted the ICT mechanism. These facts supported the mechanism
proposed above (Scheme 2).
Supplementary data (general information and copies of NMR,
FT-IR, etc.) associated with this article can be found, in the online
version, at http://dx.doi.org/10.1016/j.tetlet.2016.02.076.
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U
U
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