A novel three-coordinate organoboron derivative: synthesis, photophysical property and ion recognization

Article abstract of DOI:10.1016/j.tetlet.2007.07.127

A new push-pull type compound trans-4-dimesitylboryl-4′-(1,4,7,10-tetraoxa-13-aza-cyclopentadecyl) stilbene (DBTS) with aza-15-crown-5 as a donor and a three-coordinate organoboron as an acceptor was designed and synthesized. DBTS shows intense fluorescence in a wide spectra range in different solvents. Furthermore, DBTS can recognize fluoride anions with a high selectivity for the special Lewis acid-base interaction between a trivalent boron atom and a fluoride anion. As a typical alkaline earth cations receptor, DBTS also showed spectral changes upon their addition.

Full text of DOI:10.1016/j.tetlet.2007.07.127

Tetrahedron Letters 48 (2007) 6633–6636
A novel three-coordinate organoboron derivative: synthesis,
photophysical property and ion recognization
Zhiguo Zhou, Fuyou Li,* Tao Yi* and Chunhui Huang*
Department of Chemistry and Lab of Advanced Materials, Fudan University, Shanghai, PR China
Received 5 June 2007; revised 18 July 2007; accepted 20 July 2007
Available online 27 July 2007
Abstract—A new push–pull type compound trans-4-dimesitylboryl-40-(1,4,7,10-tetraoxa-13-aza-cyclopentadecyl) stilbene (DBTS)
with aza-15-crown-5 as a donor and a three-coordinate organoboron as an acceptor was designed and synthesized. DBTS shows
intense fluorescence in a wide spectra range in different solvents. Furthermore, DBTS can recognize fluoride anions with a high selec-
tivity for the special Lewis acid–base interaction between a trivalent boron atom and a fluoride anion. As a typical alkaline earth
cations receptor, DBTS also showed spectral changes upon their addition.
ꢀ 2007 Published by Elsevier Ltd.
1. Introduction
also reported the colorimetric and two-photon fluores-
cent sensors for fluoride anions derived from three-coor-
dinate organoborons.9a Herein, as an extension of our
previous work on organoboron-based chemosensor, we
designed and synthesized a new bis-functional derivative
DBTS (see Scheme 1) to recognize alkaline earth cations
and fluoride anions independently by a colorimetric
method.
Three-coordinate organoborons containing p-conjugate
systems have recently received tremendous attention
due to their intriguing electronic and photophysical
properties as a result of overlap between the vacant p-
orbital of the boron atom and the conjugate system.1
The extensive studies on them have demonstrated their
potential uses for various photonic and optoelectronic
applications such as nonlinear optical properties,2 two-
photon fluorescence,3 organic light-emitting diodes
(OLEDs).4 In particular, several research groups have
reported that organoborons derivatives could be used
as effective fluorogenic, colorimetric and ratiometric
chemosensors for fluoride anions.5–7 Recently, colori-
metric sensors are popular due to their capability to
detect analytes by the naked eyes without resorting to
the use of any expensive instruments.8 Our group has
Compound DBTS (trans-4-dimesitylboryl-40-(1,4,7,10-
tetraoxa-13-aza-cyclopentadecyl)stilbene) was synthe-
sized as described in Scheme 2. The precursor 1 was
prepared by Witting reaction. Substitution of the 40-bro-
mine atom on the precursor 1 by a dimesitylboron group
in the presence of n-butyllithium at low temperature
afforded 40% yields of DBTS. The structure of DBTS
1
was characterized by H NMR, 13C NMR, mass spec-
troscopy and elemental analysis.10
Mes
O
M2+
CH3CN
Mes
Mes
B
O
O
O
Mes
M2+
F-
N
B
O
O
O
O
Mes
B
N
F
O
O
O
N
THF
O
Mes
DBTS
M2+=Mg2+, Ca2+, Ba2+
Scheme 1. The concept of a bis-functional chemosensor.
Keywords: Organoboron; Fluoride anion; Alkaline earth cation; Aza-15-crown-5.
*
Corresponding authors. Tel.: +86 21 55664185 (C.H.); e-mail: chhuang@fudan.edu.cn
0040-4039/$ - see front matter ꢀ 2007 Published by Elsevier Ltd.
doi:10.1016/j.tetlet.2007.07.127

6634
Z. Zhou et al. / Tetrahedron Letters 48 (2007) 6633–6636
O
O
O
O
O
a
Br
O
CHO
Br
N
BrPh3PH2C
N
O
O
3
2
1
Mes
O
O
B
Mes
b
O
N
O
DBTS
Scheme 2. Reagents and conditions: (a) t-Bu4OK, anhydrous THF, room temperature, 4 h, 60%; (b) FB(Mes)2, n-BuLi, anhydrous THF, ꢀ78 ꢁC,
12 h, 40%.
2. Experiment
(kem) showed remarkable bathochromic shifts with
increasing polarity of the solvent. For example, kem
was centered at 490 nm in xylene and red-shifted to
577 nm in acetonitrile. Furthermore, the fluorescence
quantum yields and fluorescent lifetimes were also
increased with increasing polarity of the solvent. These
results suggest that DBTS has a fairly large excited-state
dipole moment, indicating the strong intramolecular
charge transfer process (ICT) between the dimesityl-
boron group (as an electron acceptor) and the aza-crown
group (as an electron donor).9a
The photophysical properties of DBTS are investigated
in detail and the data are shown in Table 1. As shown
in Figure 1a, the maximum absorption peak (kabs) of
DBTS in acetonitrile is very similar to that in xylene
(see Table 1). However, the emission of DBTS was sen-
sitive to different solvents. Its maximum emission peak
Table 1. Summary of photophysical properties in different solvents
kabs (nm)
kem (nm)
Dmst (cmꢀ1
)
u
s (ns)
A great deal of efforts has been devoted to the design
and synthesis of fluoride anion chemosensors which
are highly relevant to health and environmental issues.11
To explore the application of DBTS in sensing fluoride
anions, the UV–vis absorption and fluorescence spectra
were investigated in the presence of anions. When n-
Bu4NF (TBAF, a fluoride source) was added to the
THF solution of DBTS, an obvious color change from
green-yellow to colorless occurred under ambient light.
The characteristic absorption band of DBTS centered
at 410 nm completely disappeared, and a new band cen-
tered at 330 nm arose (see Fig. 2a), corresponding to a
greater than 90 nm blue-shift. The binding constant
(Ka) was determined from the absorption–titration
curves to be 3.1 · 105 Mꢀ1 in THF solution, which is
comparable to that of analogous compounds previously
reported.9a As shown in Figure 2b, the intense emission
band of DBTS at 540 nm (kex = 410 nm) progressively
quenched, when fluoride anions were added. These
findings indicate that the strong B–F interaction
results in sp3 hybridization of the boron atom and sub-
sequently interrupts the extended p–p conjugation,
thereby causes a dramatic change in the photophysical
property.12
Xylene
THF
Acetone
MeCN
408
410
410
406
490
540
565
577
4042
5931
6751
7300
0.48
0.61
0.66
0.75
0.74
1.31
1.52
4.04
1.2
1.0
0.8
0.6
0.4
0.2
0.0
a
Xylene
THF
MeCN
Acetone
300
360
420
480
540
600
Wavelength /nm
b
1.0
0.8
0.6
0.4
0.2
0.0
Xylene
THF
MeCN
Acetone
For an excellent chemosensor, high selectivity is a mat-
ter of necessity. Using the ratio of absorbance at 330
and 410 nm (A330 nm/A410 nm) as an output signal, the
responses of DBTS to 5.0 equiv of different anions
are shown in Figure 3. Compared with the special
blue-shift of DBTS with fluoride anions, no obvious
special change was observed upon addition of other
halide anions, AcOꢀ, NO2 and H2PO ꢀ. Similar selec-
ꢀ
4
tivity was also confirmed in the corresponding
450 500 550 600 650 700 750
Wavelength / nm
fluorescence spectra (see Supplementary data). These
results imply that DBTS can detect fluoride anions with
very high selectivity. The highly selective recognition
of DBTS for fluoride anions can be probably attrib-
Figure 1. Normalized absorption (a) and fluorescence spectra (b) of
DBTS in different solvents (kex = 410 nm).

Z. Zhou et al. / Tetrahedron Letters 48 (2007) 6633–6636
6635
0.30
0.25
0.20
0.15
0.10
0.05
0.00
0.18
0.12
0.06
0.00
a
b
a 0.30
F-
0.24
410 nm
0.16
0.08
0.00
0
0.24
0.18
0.12
0.06
Mg2+
2 eq
0.0 0.5 1.0 1.5 2.0
[Mg2+] / [DBTS]
0
1
2
3
[F– ] / [DBTS]
350
400
450
500
550
300
400
500
600
Wavelength / nm
Wavelength / nm
750
600
450
300
150
600
500
400
300
200
100
b
600
450
300
150
600
450
Mg2+
F-
580 nm
300
150
0
540 nm
0
0
0.0 0.5 1.0 1.5 2.0
[Mg2+] / [DBTS]
1
2
3
4
[F– ] / [DBTS]
0
0
450 500 550 600 650 700 750
450 500 550 600 650 700 750
Wavelength / nm
Wavelength / nm
Figure 2. Absorption (a) and emission (b) spectra (kex = 410 nm) of
DBTS (1.0 · 10ꢀ5 M) in THF solution upon addition of different
equivalents of fluoride anions.
Figure 4. Absorption and emission spectra (kex = 410 nm) of DBTS
(1.0 · 10ꢀ5 M) in CH3CN solution upon addition of different equiv-
alent of Mg2+ ions.
CH3CN solutions of DBTS, the characteristic absorp-
tion band of DBTS centered at 410 nm completely dis-
appeared, and the band centered at 330 nm arose. The
blue-shift of the absorption spectrum of DBTS in the
presence of alkaline earth cations was due to the interac-
tion of the nitrogen atom of aza-crown with alkaline
earth cations which weaken the electron-donating
ability of the aza-crown group in the chromophoric
p-system.12 Furthermore, a quenching change in the
fluorescent emission was also observed at 540 nm for
DBTS upon addition of magnesium ions (kex = 410 nm).
Similar phenomena were observed upon addition of cal-
cium and barium ions (see Supplementary data). The
facts indicate that these alkaline earth cations may be
attributed to the reduced intramolecular charge transfer
process (ICT) upon complexation. The binding con-
26
25
20
15
10
5
1
1
1.4 1.4
1.4 1.4
NO3–
Cl– Br–
1.4
I–
0
–ClO4–
F–
Ac–
H2PO4
Figure 3. A comparison of the ratio of absorbance at 330 nm and
410 nm of DBTS in THF solution (10 lM) in the presence of 5 equiv of
anions.
stants (Ka) of DBTS in CH3CN solution for Mg2+
,
uted to two factors. The specific Lewis acid–base inter-
action is strong between trivalent boron atom and fluo-
ride anions. Furthermore, the special steric structure of
dimesityl group may make some unique contributions.
Ca2+ and Ba2+ are determined from the absorption–
titration curves to be 3.5 · 105, 8.6 · 105 and 9.1 ·
105 Mꢀ1, respectively.
Alkaline earth cations are the most abundant divalent
cations in living cells and play vital roles in many cellu-
lar processes. With the presence of aza-15-crown-5,
DBTS may also recognize alkaline earth cations. Herein,
the binding events of DBTS were carried out using the
absorption and fluorescent technology. The spectra were
slightly or not at all affected by alkali metal ions,
whereas the great spectral changes for DBTS were
observed upon addition of alkaline earth ions. As shown
in Figure 4a, when magnesium ions were added into the
3. Conclusion
In summary, we have demonstrated a bisfunctional
organic borane DBTS by introducing both aza-15-
crown-5 and dimesitylboryl groups. Its absorption prop-
erty was blue-shifted and its fluorescence was suppressed
upon complexation with fluoride anions and alkaline
earth cations individually. The result provides a useful
design strategy for the synthesis of new chemosensors
for further applications.

6636
Z. Zhou et al. / Tetrahedron Letters 48 (2007) 6633–6636
5. (a) Yamaguchi, S.; Akiyama, S.; Tamao, K. J. Am. Chem.
Acknowledgements
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The authors thank NSFC of China (20490210, 20501006
and 20571016), National High Technology Program
of China (2006AAZ318), Huo Yingdong Education
Foundation (104012) and Shanghai Sci. Tech. Comm.
(05DJ14004 and 06QH14002) for financial support.
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Supplementary data
Supplementary data associated with this article can be
found, in the online version, at doi:10.1016/j.tetlet.
2007.07.127.
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