Synthesis and characterization of novel fluorescent BOPIM dyes with large Stokes shift

Article abstract of DOI:10.1016/j.jfluchem.2011.06.018

A novel BOPIM (boron 2-(2-pyridyl)imidazole complex) dye 1 was facilely synthesized by treatment of previously reported 2-(2′-pyridyl)imidazole with BF₃·Et₂O under basic condition. The bromination of BOPIM dye 1 by NBS gives an unexpected product 2-(2′-pyridyl)-4,5-dibromoimidazole (L2) with no BF₂ group. The desired brominated boron complex 2 was obtained by treating L2 with BF ₃·Et₂O. The photophysical properties of these two compounds are thoroughly studied in various solvents. Compound 1 formed aggregates in non-polar solvents, inducing abnormal emission in long-wavelength region. Both 1 and 2 show moderate fluorescent intensity and comparatively large Stokes shift, especially for compound 2 (fluorescent quantum yield is more than 0.30, and Stokes shift is over 70 nm in all adopted solvents) due to its p, π-conjugated effect, which makes BOPIM a valuable building block for synthesis of multi-functional materials.

Full text of DOI:10.1016/j.jfluchem.2011.06.018

Journal of Fluorine Chemistry 132 (2011) 612–616
Contents lists available at ScienceDirect
Journal of Fluorine Chemistry
journal homepage: www.elsevier.com/locate/fluor
Synthesis and characterization of novel fluorescent BOPIM dyes with
large Stokes shift
b
Miaofu Mao a, Shuzhang Xiao a, , Tao Yi , Kun Zou
*
a,**
a Hubei Key Laboratory of Natural Products Research and Development, College of Chemistry and Life Science, China Three Gorges University, Hubei,
Yichang 443002, PR China
b Department of Chemistry, Fudan University, Shanghai 200433, PR China
A R T I C L E I N F O
A B S T R A C T
Article history:
A novel BOPIM (boron 2-(2-pyridyl)imidazole complex) dye 1 was facilely synthesized by treatment of
previously reported 2-(20-pyridyl)imidazole with BF3ꢁEt2O under basic condition. The bromination of
BOPIM dye 1 by NBS gives an unexpected product 2-(20-pyridyl)-4,5-dibromoimidazole (L2) with no BF2
group. The desired brominated boron complex 2 was obtained by treating L2 with BF3ꢁEt2O. The
photophysical properties of these two compounds are thoroughly studied in various solvents.
Compound 1 formed aggregates in non-polar solvents, inducing abnormal emission in long-wavelength
region. Both 1 and 2 show moderate fluorescent intensity and comparatively large Stokes shift, especially
for compound 2 (fluorescent quantum yield is more than 0.30, and Stokes shift is over 70 nm in all
Received 27 April 2011
Received in revised form 15 June 2011
Accepted 16 June 2011
Available online 23 June 2011
Keywords:
Fluorescence
BOPIM
adopted solvents) due to its p,
p-conjugated effect, which makes BOPIM a valuable building block for
Large Stokes shift
synthesis of multi-functional materials.
ß 2011 Elsevier B.V. All rights reserved.
1. Introduction
A few years ago, Yeh et al. synthesized a new class highly
fluorescent derivative of modified BODIPY dye, starting from 2-(2-
pyridyl)naphtha[b]imidazole as ligand [25]. Some other solid-
emissive BODIPY dyes with modified structures are also synthe-
sized and thoroughly studied [26–31]. These dyes show moderate
fluorescent quantum yield, high stability, especially large Stokes’s
shift (normally larger than 90 nm) compared to typical BODIPY
dyes, which make them good candidate for light-emitting
materials and fluorescent probes. However, it’s difficult to further
modify these chromophores. In order to obtain fluorescent dyes
which can be easily functionalized, 2-(2-pyridyl)imidazole was
chosen as the ligand to chelate BF2, since imidazole contains two
reactive protons. Here we report the synthetically versatile
chromophore – fluorescent boron 2-(20-pyridyl)imidazole complex
1 with previously reported 2-(2-pyridyl)imidazole as ligand [32],
and the dibromo derivative 2. Also, the photophysical properties of
compounds 1 and 2 are thoroughly studied in various organic
solvents.
The development of highly emissive dyes has been under
widely research recently for their various applications, especially
those dyes with 4-bora-4,4-difluoro-pyrromethene (BODIPY)
units due to their exceptional spectral properties (high fluores-
cent quantum yield and absorption coefficient), photo-stability
and chemical stability. Many applications have been found for
BODIPY dyes, such as light-emitting devices [1–7], photosensi-
tizer [8], sensors [9–17], fluorescent probes for bio-imaging study
[18–21], and molecular switches [22]. An ideal dye should
comprise high fluorescent quantum yield, high absorption
coefficient, large Stokes shift, high stability, and facile availability.
BODIPY dyes meet most of these acquisitions, however, the Stokes
shifts for most BODIPY dyes are usually in the range of 400–
600 cmꢀ1, which is quite small and results in undesirable
quenching of the emission due to the reabsorption by itself. To
enlarge BODIPY dyes’ Stokes shift, it’s important to make the
electronic structure of the excited state quite different with that of
the ground state [23,24].
2. Results and discussion
The ligand L1 was synthesized with 45% yield after purifica-
tion, starting from 2-pyridinecarbox-aldehyde and aqueous
glyoxal solution [32]. After treating the ligand with BF3ꢁEt2O
using Et3N as base, target product 1 was obtained as a pale yellow
* Corresponding author. Tel.: +86 717 639 7478; fax: +86 717 639 7478.
** Corresponding author.
E-mail address: shuzhangxiao@gmail.com (S. Xiao).
0022-1139/$ – see front matter ß 2011 Elsevier B.V. All rights reserved.
doi:10.1016/j.jfluchem.2011.06.018

M. Mao et al. / Journal of Fluorine Chemistry 132 (2011) 612–616
613
solid (Scheme 1), with moderate yield (18%) after purification by
column chromatography.
Compound 1 has two reactive protons on imidazole, which can
be further substituted for synthesis of multi-functional materials.
With treatment by NBS in chloroform under acidic condition,
compound 1 was obviously brominated, which could be followed
by TLC and also 1H NMR, since 1H NMR can provide clear evidence
for the disappearance of protons on imidazole. After the addition
of 2.5 equivalents NBS, the 1H NMR signal referred to protons on
imidazole disappears completely, indicating the completion of
bromination reaction. However, the Ms analysis gives species of
the corresponding ligand with no BF2 complex (m/z: 300). And 19
F
NMR gives no signal of any resonance, further proving the absence
of BF2 group. Although BODIPY dyes normally exhibit high
chemical- and photo-stability, decomposition also happens in
certain chemical conditions, especially under attack of radicals in
this case [33,34]. Ligand L2 could also be synthesized through
bromination of L1 by bromine with excellent yield (82%),
following previously reported procedure [35]. And the desired
product 2 was obtained by treating L2 with BF3ꢁEt2O under basic
condition.
1H NMR of these compounds show different resonance patterns
as shown in Fig. 1. The ligands L1 and L2 exhibit similar
resonances, except the proton signals on imidazole. Compared to
ligand L1, 1H NMR of 1 only shows one resonance for the two
protons on imidazole (located at 7.18 ppm), indicating that the
chelation with BF2 makes the chemical environment of the two
protons similar with each other. And for compound 2, the intense
Fig. 1. 1H NMR spectra (§ CDCl3; * protons on imidazole of L1; * protons on
imidazole of 1).
BODIPYs, compound 2 shows solvent-dependent properties.
With increasing of the solvent polarity, absorptive band blue-
shifts evidently, and the main absorption was located at 487 nm
in methanol.
p,
p-conjugated effect and chelation with boron largely changes
the electronic density of protons on C3, C4 of pyridine ring, so that
it results in up-field shift for H3, and down-field shift for H4 on
pyridine ring.
Unlike UV–vis absorption, the fluorescent emission of
compound 1 is strongly solvent-dependent as normal BODIPYs
[36,37]. The solvent dependence of the long wavelength
emission is referenced to an excited state with a large dipole
[23]. In polar solvent such as methanol, the maximum emission
band is located at 362 nm, however, a broad band around
500 nm clearly rises up. With solvent polarity decreasing, the
obvious blue-shifts of emission were observed. In mid-polarity
solvents, the main fluorescence bands are of similar patterns,
shifting from 352 nm in dioxane to 359 nm in acetonitrile. As the
polarity further decreasing, another sequences of emission bands
show up in visible region (527, 560 nm) in hexane and
cyclohexane, due to the formation of aggregates. Concentra-
tion-dependent fluorescent emission in hexane was measured to
verify the aggregation-induced fluorescence difference. When
The photophysical properties of compound 1 were studied in
various organic solvents. Typical BODIPY dyes exhibit solvent-
dependent properties, but it’s not evident for the prepared
compound 1 from UV–vis absorption spectra. As depicted in
Fig. 2, the absorption spectra are of similar shape with
maximum absorption band around 300 nm, attributed to S0–
S1 transition. The absorption spectra are only barely affected by
solvent polarity, with the maximum being slightly shifted
hypsochromically (ꢂ5 nm), which is consistent with the general
behavior of non-pyrrole-based BODIPY chromophores [25]. For
compound 2, obvious red-shift was observed compared to 1,
due to the p,
p-conjugated effect which enhances the p system
on a large scale. The longest absorptive band was observed at
393 nm in hexane (starting from 446 nm). Similar with typical
the concentration is lower than 1.0 ꢃ 10ꢀ6 Mꢀ1
, the main
emission is centered at 337 nm, and the emission bands in
long-wavelength region are hardly observed. However, obvious
red-shift occurs with concentration increasing. When the
concentration is higher than 1.0 ꢃ 10ꢀ4 Mꢀ1, this sequence of
emission red-shifts to 342 nm. And the fluorescence around
540 nm becomes much more intense. The fluorescence ratio
between 540 nm region and 337 nm region increases from 5%
(1.0 ꢃ 10ꢀ6 Mꢀ1) to 30% (1.0 ꢃ 10ꢀ4 Mꢀ1). The effort to measure
fluorescent spectrum in more concentrated solution failed due to
the poor solubility in hexane. For compound 2, however, no
aggregate was observed in all the solvents studied, as showing in
concentration-dependent fluorescence (Fig. 2F). There is no
obvious emission shift with concentration changing. However,
the emissive band red-shifts with decreasing of the solvent
polarity. The longest emission was observed at 493 nm in
dioxane, and the shortest emission band was 463 nm in
cyclohexane.
Photophysical data of the boron chelates are shown in Table 1.
Compound 1 shows moderate fluorescence quantum yield in mid-
Scheme 1. Synthesis of the BF2 chelate 1 and dibromo derivative 2.

614
M. Mao et al. / Journal of Fluorine Chemistry 132 (2011) 612–616
Fig. 2. (A) Absorption spectra of compound 1, (B) absorption spectra of compound 2, (C) fluorescent spectra of 1, (D) fluorescent spectra of 2 in different solvents
(1.0 ꢃ 10ꢀ5 Mꢀ1); (E) concentration-dependent fluorescent spectra of 1, (F) concentration-dependent fluorescent spectra of 2 in hexane (Ex: 365 nm).
polar solvents and the highest (0.44) in THF, but poor fluorescence
in extremely high polar solvent (methanol) and non-polar solvents
(hexane, cyclohexane). Plus, compound 1 exhibits comparatively
large Stokes’s shift (over 49 nm) in all solvents, which is much
larger than typical BODIPY dyes. However, compound 2 shows
excellent fluorescent quantum yields in all the adopted solvents,
indicating the enlarged p system highly increased the fluorescent
properties. The Stokes shifts turn over 70 nm in all solvents
studied, and normally around 120 nm in most solvents. The largest
Stokes shift was 136 nm observed in acetonitrile. The large Stokes
shift alleviates self-absorption remarkably and prohibits the
decreasing of fluorescent intensity.

M. Mao et al. / Journal of Fluorine Chemistry 132 (2011) 612–616
615
Table 1
Photophysical properties of compounds 1 and 2 in different solvents.
a
Solvent
labs nm (Mꢀ1 cmꢀ1
)
lem (nm)
Dn (nm)b
FF (%)c
1
Hexane
276 (21,970) 297 (25,700)
277 (20,430) 298 (24,950)
300 (24,860)
337, 527, 560
–
ꢂ3
ꢂ3
14
44
28
28
3
Cyclohexane
Chloroform
THF
338, 528, 560
356
–
56
52
49
58
66
71
71
95
118
127
136
130
298 (25,640)
353
Dioxane
297 (29,940)
352
Acetonitrile
Methanol
Hexane
295 (24,730)
359
293 (24,080)
362
2
393 (20,318)
464
45
47
41
32
35
32
31
Cyclohexane
Chloroform
THF
392 (19,995)
463
374 (22,702)
469
366 (20,843) 344 (21,790)
366 (21,439)
484
Dioxane
493
Acetonitrile
Methanol
356 (20,383)
492
357 (21,632)
487
a
b
c
Wavelength (absorption coefficient).
Stokes’ shift.
Fluorescence quantum yield (determined using anthracene as reference, error: ꢄ 5%).
3. Conclusion
4.3. Treatment of 1 by NBS
A new class BOPIM derivatives were synthesized and their
photophysical properties are studied in various organic solvents.
These dyes show hypsochromically solvent-dependent properties,
with moderate fluorescent quantum yield and large Stokes shift
compared to typical BODIPY dyes. Especially for brominated
BOPIM 2, its excellent photophysical properties and facile
functionalization make it a valuable building block for synthesis
of multi-functional materials.
Boron 2-(20-pyridyl)imidazole complex 1 (41.6 mg, 0.21 mmol)
was dissolved in a mixture of chloroform and acetic acid (8 mL, 1:1
v/v), followed by addition of NBS (95.4 mg, 0.54 mmol) in
chloroform (2 mL). The reaction mixture was stirred at room
temperature overnight, then washed with aqueous Na2S2O3,
K2CO3, and then water. The organic phase was collected and dried
over anhydrous Na2SO4. After removal of the solvent, the crude
product was subjected to a silica plug (chloroform) to provide a
pale yellow solid (88%). 1H NMR (CDCl3, 300 MHz):
d 8.51 (d,
J = 5.10 Hz, 1H), 8.18 (d, J = 7.80 Hz, 1H), 7.86 (m, 1H), 7.34 (m, 1H).
4. Experimental
13C NMR (CDCl3, 75 MHz):
d 148.6, 138.0, 124.0, 120.4. ESI-Ms: m/
z: 300 [M]+.
4.1. General
4.4. Synthetic procedure of Boron 2-(20-pyridyl)-4,5-
All starting materials were obtained from commercial suppliers
and used as received. Moisture sensitive reactions were performed
under an atmosphere of nitrogen, and the solvents were treated
according to standard methods. 1H NMR and 13C NMR were
recorded on Bruker 400 NMR or Varian 300 Mercury spectro-
meters. Chemical shifts are reported in ppm with CDCl3 as
reference (7.26 ppm for 1H NMR, and 77.0 ppm for 13C NMR). MS
data were recorded on a Waters Quattro Micro API LC-MS
spectrometer (Waters, USA) or Applied Biosystems Voyager-DE
STR mass spectrometer. UV–vis and fluorescent spectra were
obtained on Hitachi U-3010 and F-4500, respectively. The
fluorescent quantum yield is calculated using anthracene as
reference.
dibromoimidazole complex 2
In a stirred mixture of compound L2 (2.4 g, 8 mmol) and Et3N
(5 mL) in anhydrous CH2Cl2 (25 mL), BF3ꢁEt2O (5.6 mL, 44 mmol)
was added dropwise at 0 8C. After the addition of BF3ꢁEt2O, the
reaction mixture was allowed to warm to room temperature and
stir at room temperature overnight. The organic phase was washed
with water several times, dried on Na2SO4, and evaporated in
vacuo. Then the obtained crude residue was subjected to column
chromatography on a silica gel column to provide a yellow solid
(38%). 1H NMR (CDCl3, 400 MHz):
J = 7.80 Hz, 1H), 7.92 (d, J = 8.00 Hz, 1H), 7.58 (t, J = 6.60 Hz, 1H).
d 8.43 (d, J = 5.60 Hz, 1H), 8.23 (t,
13C NMR (CDCl3, 100 MHz):
d 145.4, 141.8, 123.9, 117.7. HRMs
calcd for C8H5BBr2F2N3 [M+H]+ 349.8911, found 349.8915.
4.2. Synthetic procedure of Boron 2-(20-pyridyl)imidazole complex 1
Acknowledgments
In a stirred mixture of 2-(2-pyridyl)imidazole [9] (2.0 g,
13.8 mmol) and Et3N (8 mL) in anhydrous CH2Cl2 (30 mL),
BF3ꢁEt2O (9.0 mL, 70 mmol) was added dropwise at 0 8C. After
the addition of BF3ꢁEt2O, the reaction mixture was allowed to
warm to room temperature and stir at room temperature
overnight. The organic phase was washed with water several
times, dried on Na2SO4, and evaporated in vacuo. Then the
obtained crude residue was subjected to column chromatogra-
phy on a silica gel column (hexane:EtOAc = 1:1) to provide a pale
We are grateful for the financial support from National Natural
Science Foundation of China (21002059), and the MS measurement
by Department of Macromolecular Science in Fudan University.
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