A highly emissive cruciform triarylborane as a ratiometric and solid state fluorescence sensor for fluoride ions

Article abstract of DOI:10.1016/j.tetlet.2011.05.076

We disclosed a novel cruciform tri-coordinate organoboron compound, 2′,5′-bis{[(4-dimesitylboryl)phenyl]ethynyl}-1′, 4′-bis[(4-N,N-diphenylamino)phenyl]-[1,1′:4′,1′] terphenyls, which displays a characteristic intramolecular charge transfer transition and

Full text of DOI:10.1016/j.tetlet.2011.05.076

Tetrahedron Letters 52 (2011) 3832–3835
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Tetrahedron Letters
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A highly emissive cruciform triarylborane as a ratiometric and solid state
fluorescence sensor for fluoride ions
⇑
Yi-Hong Zhao, Hong Pan, Guang-Liang Fu, Ji-Mao Lin, Cui-Hua Zhao
School of Chemistry and Chemical Engineering, Shandong University, Jinan 250100, People’s Republic of China
a r t i c l e i n f o
a b s t r a c t
Article history:
We disclosed
ryl)phenyl]ethynyl}-10,40-bis[(4-N,N-diphenylamino)phenyl]-[1,10:40,10]terphenyls, which displays
characteristic intramolecular charge transfer transition and is highly emissive both in solutions and solid
state. The complexation with fluoride ions induces a large blue shift in fluorescence, enabling ratiometric
fluorescence sensing of fluoride. In addition, its prompt response to fluoride ions was also observed even
in the solid state.
a
novel cruciform tri-coordinate organoboron compound, 20,50-bis{[(4-dimesitylbo-
Received 29 January 2011
Revised 3 March 2011
Accepted 17 May 2011
Available online 23 May 2011
a
Keywords:
Ó 2011 Elsevier Ltd. All rights reserved.
Cruciform triarylborane
Highly emissive
Ratiometric
Solid state
Fluoride sensing
The selective sensing of fluoride ion is of great interest owing to
its important roles in a wide range of biological, medical, and
chemical processes and applications, such as dental care,1 treat-
ment of osteoporosis,2 fluorination of water supplies,3 and even
in chemical and nuclear warfare agents.4 The design principles of
fluoride probes are typically based on the hydrogen bonding for-
mation between the active N–H group of organic chromophores
and fluoride ions.5 Recently, specific Lewis acid/base interactions
have been adopted as a new efficient approach for fluoride sensing.
One intriguing example is the complexation of tri-coordinate
boron compounds with fluoride anions, which would disrupt the
NPh2
Mes2B
BMes2
Ph2N
1
p –p⁄ conjugation between the boron center and the attached
Figure 1. Structure of cruciform triarylborane 1.
p
p-conjugated system and thus lead to a significant change in the
UV/Vis absorption, fluorescence, and two-photon excited fluores-
cence.6–12 In addition, these organoboron compounds possess high
selectivity for fluoride binding over other anions, such as chloride,
bromide, and iodide, owing to the steric hindrance of bulky substit-
uents on the boron center, which not only ensures stability to
water including atmospheric moisture but also prevents the com-
plexation of the boron center with other large Lewis bases. Hereby,
tri-coordinate conjugated organoboron compounds have attracted
increased attention as efficient fluoride ion sensors. Among the
various analytical methods, fluorescent sensing is highly desirable
because of its high sensitivity. Most of the reported fluorescent
fluoride sensors of organoboron compounds are based either on
fluorescent quenching7b or on fluorescence enhancement,6b,11a,c
few on ratiometric fluorescence, where the detection sensitivity
may be increased by measuring the ratio changes of the fluores-
cence intensities at two different wavelengths. To develop ratio-
metric response probes, one seductive strategy is to design
cruciform chromophores possessing spatially separated frontier
molecular orbitals, for which the analyte binding would affect
either HOMO or LUMO to a different degree and thus induce a
significant change in emission and absorption spectra.13 Herein,
we designed
a new cruciform tri-coordinate organoboron
compound, 20,50-bis{[(4-dimesitylboryl)phenyl]ethynyl}-10,40-bis-
[(4-N,N-diphenylamino)phenyl]-[1,10:40,10]terphenyls 1 (Fig. 1), in
which the electron-withdrawing [(4-dimesitylboryl)phenyl]ethy-
nyl branch and the electron-donating 4-(N,N-diphenylamino)-
phenyl branch are connected perpendicularly with via a central
benzene core. We envisioned that the HOMO would be delocalized
over the electron-donating 4-(N,N-diphenylamino)phenyl arm
⇑
Corresponding author. Tel.: +86 531 88364464; fax: +86 531 88564464.
E-mail address: chzhao@sdu.edu.cn (C.-H. Zhao).
0040-4039/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2011.05.076

Y.-H. Zhao et al. / Tetrahedron Letters 52 (2011) 3832–3835
3833
Table 1
Photophysical properties of 1
while the LUMO would be delocalized over the electron-withdraw-
ing [(4-dimesitylboryl)phenyl]ethynyl axis and 1 would display a
characteristic intramolecular charge transfer (CT) emission. The
complexation of 1 with fluoride would increase LUMO level inde-
pendently and interrupt the intramolecular CT transition, resulting
in a remarkable blue shift in fluorescence. In fact, we found that 1
was highly emissive in solutions and could retain high fluorescence
efficiency with large blue shift in fluorescence spectra after binding
with fluoride ions, enabling ratiometric fluorescence sensing of
fluoride ions. Moreover, it was observed that this cruciform triar-
ylborane displayed intense emission as well as prompt fluores-
cence response to fluoride ions even in the solid state.
c
kabs/nm (log
e
)
kem/nmb
UF
Cyclohexane
Toluene
THF
352 (4.10), 411 (sha, 3.39)
470
498
540
0.98
0.98
0.43
359 (4.59), 406 (sha, 3.85)
358 (4.85), 418 (sha, 4.12)
a
b
c
Shoulder band.
Excited at the longest absorption maxima.
Calculated using fluorescein as standard.
absorption
fluorescence
The synthetic route to the cruciform triarylborane 1 is shown in
Scheme 1. It was readily prepared in two steps, using 1,4-dibro-
mo-2,5-diethynylbenzene 2 as the starting material.14 Thus, the
Pd(0)-catalyzed Sonogashira coupling reaction of 2 with 1-iodo-4-
dimesitylborylbenzene produced the key precursor 1,4-dibromo-
2,5-bis[(4-dimesitylboryl)phenyl]ethynyl)benzene 3 in 68% yield.
The following Pd(0)-catalyzed Suzuki coupling reaction of 3 with
4-(N,N-diphenylamino)phenylboronic acid afforded the cruciform
triarylborane 1 in 34% yield.15 The obtained compound 1 is very sta-
ble in air and water and can be purified by silica-gel column
chromatography.
cyclohexane
toluene
THF
350
400
450
500
550
600
650
700
Wavelength / nm
Figure 2. UV/Vis absorption and fluorescence spectra of 1.
The photophysical property data of compound 1 are summa-
rized in Table 1 and the UV/Vis absorption and emission spectra
are shown in Figure 2. In cyclohexane, in addition to the intense
absorption below 400 nm, 1 shows a relatively weaker shoulder
band at the longer wavelength (kabs = 411 nm; log e = 3.39). To
clarify the origin of this shoulder band, theoretical calculations
were performed by using time-dependent density functional the-
ory (TD-DFT) at the B3LYP/6-31G(d) level of theory. According to
the calculation results, the present cruciform triarylborane 1
exhibits a disjoint frontier molecular orbital structure, in which
the HOMO and LUMO are mainly delocalized over the electron-
donating 4-(N,N-diphenylamino)phenyl moiety and the electron-
withdrawing
4-[(dimesitylboryl)phenyl]ethynyl
framework,
respectively (Fig. 3). The shoulder absorption band at 411 nm is
assignable to the intramolecular CT transition from the HOMO to
the LUMO. These calculation results clearly suggest a polarized
structure in the excited state. In the fluorescence spectra, 1 dis-
plays extremely strong greenish blue fluorescence at 470 nm with
a quantum yield close to unity. Notably, the fluorescence is highly
dependent on the polarity of the solvent while the absorption
Figure 3. Pictorial drawings of (a) HOMO and (b) LUMO of 1 calculated at the
B3LYP/6-31G(d) level of theory.
shows only trivial solvent dependence. With increasing polarity
of solvent, the fluorescence maximum is red shifted remarkably
from 470 nm in cyclohexane to 498 nm in toluene, and 540 nm
in THF, indicating the intramolecular CT character in the excited
state, which is in good accordance with the theoretical calculation
results.
BMes2
In the course of purification of 1 by flash column, we noticed
that it emitted strong yellowish green fluorescence after evapora-
tion of solvent, similar to that in toluene. This observation
prompted us to investigate its photophysical properties in the solid
state in details (Fig. 4). It was found that in addition to strong emis-
sion in powder form, 1 exhibited remarkably intense yellowish
green emission for thin film, which was prepared by spin-coating
from a solution of 1 in dichloromethane (3 mg/mL). Moreover, a
spin-coated poly(methyl methacrylate) (PMMA) film doped with
1 also shows brilliant bluish green fluorescence. In view of the
absorption and emission spectra, it maintains almost the same
spectra in thin film and PMMA film as those in toluene solution,
indicating formation of neither aggregation in the ground state
nor an excimer in the excited state, presumably owing to the sup-
pressed intermolecular interaction as a result of the nonplanar
structures of central para-terphenyl framework in the cruciform
structure16 and the steric bulkiness of dimesitylboryl groups. The
suppressed intermolecular interaction is supposed to be an impor-
tant factor contributing to the intense solid state fluorescence.
H
Br
Br
I
BMes2
Pd(PPh3)4,CuI
1/3 Et3N/THF r.t.
Br
Br
H
2
Mes2B
3 68%
(HO)2B
NPh2
1 34%
Pd(PPh3)4, NaOH (2M)
toluene, reflux
Scheme 1. Synthesis of cruciform triarylborane 1.

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Y.-H. Zhao et al. / Tetrahedron Letters 52 (2011) 3832–3835
14
12
10
a
b
I
8
I
6
4
2
0
0
5
10
15
F
20
25
30
absorption
fluorescence
(
μΜ)
toluene
PMMA film
thin film
Figure 6. Plot of fluorescence intensity ratios between 442 and 540 nm (I442/I540
)
versus concentration of Fꢀ in THF.
sion intensities at 442 and 540 nm (I442/I540) exhibit a dramatic
change from 0.02:1 to 13.6:1. Such a large change of emission
intensity ratios at two wavelengths is desirable for ratiometric
fluorescent probes, as the sensitivity and the dynamic range of
ratiometric probes are controlled by the emission ratio. Moreover,
a dramatic change of emission was observed from yellow to sky-
blue, thus enabling colorimetric fluoride ion sensing by naked eyes.
In contrast to the remarkable changes in the fluorescence, only
trivial changes were observed in the absorption spectra after addi-
tion of excess fluoride anions (Fig. S-2).
The realization of solid state detection is quite meaningful for
the sensor in its practical application in portable sensing devices.
Since 1 maintains intense fluorescence in the solid state, its fluo-
ride sensing in the solid state was also investigated. We prepared
spin-coated PMMA film doped with the probe compound 1
(0.5 mg of 1 and 500 mg PMMA). The cruciform triarylborane 1
in polymer matrix responded to TBAF solution immediately. Figure
7 shows the patterned film images of two characters of Chinese
abbreviation for Shandong University, which were inscribed by
using a writing brush with a THF solution of TBAF under UV light.
Evident emission color change was observed upon exposure to
fluoride. In addition, the polymer film is stable under ambient light
and temperature over weeks with no obvious change in response.
Thereby it was showed that compound 1 exhibited excellent fluo-
rescence sensing performance even in the solid state, which will be
very useful for the fabrication of sensing devices with fast and con-
venient detection for fluoride ions.
In conclusion, we have designed and synthesized a novel cruci-
form triarylborane 1, in which the electron-withdrawing [(4-dim-
esitylboryl)phenyl]ethynyl branch and the electron-donating
4-(N,N-diphenylamino)phenyl branch are connected perpendicu-
larly with via a central benzene core. This compound displays a
characteristic intramolecular CT transition and is highly emissive
both in solutions and in the solid state. Binding of fluoride ions leads
to the remarkable fluorescence spectra and color changes, enabling
colorimetric and ratiometric fluoride ion sensing. In addition, this
triarylborane also displays prompt response to fluoride ions even
in the solid state.
350
400
450
500
550
600
650
Wavelength / nm
Figure 4. (a) Photographs of
following order: in toluene, powder, thin film and PMMA film. (b) UV/Vis
absorption and fluorescence spectra of 1 in toluene and in the solid state.
1 under irradiation at 365 nm, arranged in the
Considering the characteristic intramolecular CT emission of the
present cruciform triarylborane 1, we envisioned that the co-ordi-
nation of a fluoride ion to the boron center would interrupt the
strong intramolecular CT transition, leading to a significant change
in the fluorescence spectra. On basis of this idea, we investigated
the fluoride sensing ability of 1. The titration experiment of 1 with
fluoride ion was carried out in THF by using n-Bu4NF (TBAF) as the
fluoride source. The fluorescence change of 1 (2.38 lM) upon addi-
tion of TBAF is shown in Figure 5. Although 1 contains two boron
atoms, no stepwise changes were observed. As the concentration
of TBAF increased, the emission at 540 nm decreased gradually,
and a new blue-shifted band appeared at 442 nm. This change be-
came saturated when the concentration of TBAF amounted to
29.0 lM. The presence of a clear isobestic point might suggest that
only one new species was formed during the titration process. The
molar ratio analysis showed that the spectral change was attrib-
uted to the formation of a 1:2 complex of 1 with two equivalents
of Fꢀ ions (see the Supplementary data). The binding constant
was determined to be 3.27 ꢁ 1010 Mꢀ2, which is comparable to
those of other tri-coordinate organoboron compounds.6–12 Nota-
bly, the complexation with fluoride ions did not cause any decrease
in the fluorescence efficiency (UF = 0.43 for 1; UF = 0.83 for 1ꢂF2ꢀ
)
2
and the difference at the two wavelengths is very large, ca.
100 nm. This situation provides the opportunity for a ratiometric
fluorescence sensing. Figure 6 shows a correlation between inten-
sity ratios of fluorescence intensity at 442 nm with those at
540 nm (I442/I540) versus fluoride concentration. The ratios of emis-
1
F
n-Bu4NF / μM
0
33.6
650
400
450
500
550
600
700
Wavelength / nm
Figure 7. Photographs of (a) PMMA film dope with 1 under irradiation at 365 nm
and (b) the patterned film images of two characters of the Chinese abbreviation for
Shandong University inscribed by using a writing brush with fluoride ion under
irradiation at 365 nm.
Figure 5. Fluorescence spectra change of 1 (2.38
lM in THF) upon addition of TBAF
(kex = 370 nm). Inset: emission color change upon addition of TBAF.

Y.-H. Zhao et al. / Tetrahedron Letters 52 (2011) 3832–3835
3835
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Acknowledgments
We are grateful for financial support from the National Nature
Science Foundation of China (Grant Nos. 21072117, 20802041),
Nature Science Foundation of Shandong Province (No.
Q2008B02), Science Foundation of Ministry of Education of China
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and NMR spectra for all new compounds, theoretical calculations
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of titration of 1 with fluoride ions) associated with this article
can be found, in the online version, at doi:10.1016/
j.tetlet.2011.05.076.
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bis{[(4-dimesitylboryl)phenyl]ethynyl}benzene
3 (93.2 mg, 0.1 mmol), 4-
(diphenylamino)phenyl boronic acid (60.0 mg, 0.21 mmol), Pd(PPh3)4
(11.6 mg, 0.01 mmol) were added degassed toluene (10 ml) and NaOH (2M,
0.2 ml) successively under a stream of nitrogen. The reaction mixture was
refluxed overnight. The mixture was cooled to room temperature and then
extracted with CH2Cl2. The organic layer was dried over anhydrous Na2SO4,
filtered, and concentrated under reduced pressure. The resulting mixture was
subjected to a silica gel column chromatography (petroleum ether/CH2Cl2 3:1,
Rf = 0.18) to afford 35 mg (0.034 mmol) of 1 in 34% yield as yellowish green
solids: mp >300 °C; 1H NMR (CDCl3, 300 MHz): d 2.00 (s, 24H), 2.33 (s, 12H),
6.84 (s, 8H), 6.97 (t, J = 6.9 Hz, 4H), 7.19 (m, 20H), 7.30 (d, J = 7.8 Hz, 4H), 7.43
(d, J = 7.8 Hz, 4H), 7.59 (d, J = 8.7 Hz, 4H), 7.71 (s, 2H); 13C NMR (CDCl3,
100 MHz): d 21.2, 23.4, 91.9, 94.4, 121.6, 123.0, 124.5, 126.6, 128.3, 129.3,
130.3, 130.8, 133.3, 133.4, 136.1, 138.9, 140.8, 141.6, 142.2, 147.6, 147.7;
HRMS (FTMS): (M+) 1258.6734; calcd for C94H82N2B2: 1258.6731.
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