Solid-emissive boron-fluorine derivatives with large Stokes shift

Article abstract of DOI:10.1016/j.tet.2012.04.052

Novel BOPIM (boron 2-(2′-pyridyl)imidazole complex) derivatives with large Stokes shift were efficiently synthesized through two-step reactions, starting from commercially available 2-pyridinecarboxaldehyde and α-diketone. The dyes exhibit high fluorescen

Full text of DOI:10.1016/j.tet.2012.04.052

Tetrahedron 68 (2012) 5037e5041
Contents lists available at SciVerse ScienceDirect
Tetrahedron
journal homepage: www.elsevier.com/locate/tet
Solid-emissive boronefluorine derivatives with large Stokes shift
,
b
c
a
a
a
Miaofu Mao a, Shuzhang Xiao a , Jianfeng Li , Ying Zou , Ronghua Zhang , Jiarong Pan , Feijun Dan ,
*
,
Kun Zou a, Tao Yi c
*
a College of Chemistry and Life Science, China Three Gorges University, Hubei Yichang 443002, PR China
b Graduate University of Chinese Academy of Sciences, Beijing 100049, PR China
c Department of Chemistry & Laboratory of Advanced Materials, 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
Novel BOPIM (boron 2-(20-pyridyl)imidazole complex) derivatives with large Stokes shift were efficiently
Article history:
Received 23 February 2012
Received in revised form 3 April 2012
Accepted 12 April 2012
synthesized through two-step reactions, starting from commercially available 2-pyridinecarboxaldehyde
and
intermolecular non-planar interactions. According to X-ray single crystal measurements, the non-
covalent interactions (such as CeH/FeB, etc.) play important role in inhibiting planar stacking,
and the existence of terminal phenyl rings increases the electronic density of system, which facilitates
the charge transfer from the electron-donating system to the electron-accepting boron moiety. DFT
a-diketone. The dyes exhibit high fluorescent intensity in solution and also in solid state due to the
Available online 21 April 2012
pep
p
Keywords:
BOPIM
Fluorescence
Solid emission
DFT
p
calculation based on X-ray crystallographic analysis was carried out for compound 2, giving consistent
results with photophysical measurements.
Ó 2012 Elsevier Ltd. All rights reserved.
1. Introduction
of BODIPYs in photoelectric conversion and electroluminescence
materials are rare.6 In order to obtain solid-emissive BODIPYs with
Fluorescent dyes continue to be developed because of their
utilities in various research fields, and the many important appli-
cations in advanced science and technology.1 In recent years there
has been increased investigation of organic boronefluorine com-
plexes as fluorescent dyes, especially BODIPY dyes. These com-
pounds exhibit a large number of advantages over other dyes, such
as high fluorescence quantum yield and absorption coefficient,
excellent chemical- and photochemical-stability, long excited-state
lifetimes, a large two-photon cross-section for multiphoton exci-
tation, good solubility, and narrow emission spectra, which may
result in attractively high color purity. However, typical BODIPY
dyes have two serious disadvantages: propensity to self-quenching
and small Stokes shift.1c,2 The high structural planarity allows them
to pack closely in solid state or similar aggregates, resulting in
concentration-induced fluorescence quenching; whereas, the small
Stokes shift allows reabsorption of the emission, resulting in low-
ered fluorescence intensity. Dyes with solid-emissive properties
and large Stokes shifts are actually very important for practical
applications, such as photoelectric conversion,3 OLED,4 and mo-
lecular switches.5 But only a limited number of organic solid-
emissive BODIPYs have been reported, and practical applications
large Stokes shift, some efforts have been made to decrease the
energy lost via non-radiative decay processes.7 One promising
strategy is to introduce bulky groups into the BODIPY core structure
to hinder close stacking of the chromophore.8 There is also evi-
dence that rigidification of the chromophore can enlarge the Stokes
shift.9 However, further work is needed to produce high perfor-
mance solid-emissive BODIPY dyes.
Recently, it was reported that the molecules with intermolecular
interactions (such as CeH/FeB, CeH/O, C/C, and C/N) can emit
fluorescence in the solid state, even though those molecules have
close lattice packing and no rigid side chains.10 Some BODIPY de-
rivatives can even crystallize in different forms to emit different
fluorescence.10c,11 In these cases, the strategy of molecular design is
to use modified structures with non-pyrrole-based ligands to al-
leviate stacking-induced fluorescence quenching, and facilitate an
efficient intramolecular charge transfer (ICT) process that increases
the Stoke shift. With these factors in mind, we have designed a new
class of boron 2-(20-pyridyl)imidazole complexes (BOPIMs), and
found that they exhibited a large Stokes shift.12 However, termi-
nally attached groups would affect the electronic environment of
the chromophore core, which may affect ICT process and induce
photophysical change of BOPIM dyes. In this report, we extend our
work, by describing the synthesis, X-ray single structures, and
photophysical properties of two BOPIM dyes with a different
number of phenyl rings as electron-donating groups.
* Corresponding authors. E-mail addresses: shuzhangxiao@gmail.com (S. Xiao),
yitao@fudan.edu.cn (T. Yi).
0040-4020/$ e see front matter Ó 2012 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tet.2012.04.052

5038
M. Mao et al. / Tetrahedron 68 (2012) 5037e5041
2. Results and discussion
for compound 2 in hexane referenced to S0eS1 transition, which
indicates that the introduction of more electron-donating groups
can result in red-shift of the absorption. With increasing of the
solvent polarity, absorption bands significantly blue-shift.
2.1. Design and synthesis
2-(20-Pyridyl)imidazole was adopted to coordinate with boron-
fluoride to form the BOPIM chromophore. In order to inhibit in-
The emission properties are more complicated since environ-
ment polarity is only one of the influencing factors. For compound
1, the emission is solvent-dependent. The longest wavelength of
emissive bands is observed at 514 nm in acetonitrile, while the
shortest emission wavelength is observed in cyclohexane (486 nm).
Quite different behavior was observed for compound 2, although
there is no big structural difference between these two dyes. The
longest wavelength of emissive bands is at 537 nm in THF, while the
shortest emission wavelength is at 498 nm in acetonitrile. This
means that the phenyl rings at the end of imidazole contribute a lot
for the fluorescence of these dyes, which is also proved by com-
parison of these two dyes to BOPIMs with no phenyl rings.12 The
effect of terminal groups on their photophysical properties can be
termolecular planar
pep interactions, which would result in ag-
gregate formation, phenyl rings were introduced at the end as
bulky steric groups and also act as electron-donating groups. On the
one side, the steric groups can prohibit the free rotation of phenyl
ring around the
electron-rich phenyl rings extend the
d
bonds to reduce the energy lost. On the other side,
system, which would fa-
p
cilitate the charge-transfer transition. These two compounds were
synthesized by standard method starting from commercially
available 2-pyridinecarboxaldehyde and a-diketone, according to
previously reported procedure as listed in Scheme 1.13 After treat-
ment with BF3$Et2O under basic condition, compounds 1 and 2
were obtained as yellow solids with moderate yield (25% and 32%,
respectively) after purification by column chromatography. These
two compounds have been characterized thoroughly by 1H NMR,
13C NMR, HRMS, and X-ray single crystallographic analysis.
explained by ICT mechanism. Photoexcitation of a ‘pushepull’
p
electron system will generate an electric field with the positive pole
at electron donor region and negative pole at acceptor. BOPIM 1 has
one phenyl ring as electron donor, which can rotate freely along the
CeC
d bond. In polar solvent, the emission is mainly from twisted
2.2. Single X-ray crystallography
intramolecular charge transfer. In contrast, the presence of two
twisted phenyl rings in 2 prohibits their rotation due to the steric
effect, resulting in a different solvent-dependent emission charac-
ter with that of 1.
X-ray single crystal measurement for compound 1 indicates that
the phenyl ring at the end of imidazole ring can rotate freely around
the CeC
d
bond, even under low temperature (liquid nitrogen). It’s
It’s observed that the fluorescence quantum yields in polar
solvent are significantly lower than those in apolar solvents for
both dyes. Since there is no sign of aggregate formation as revealed
by concentration-dependent fluorescent spectra (Fig. 2C, F), the low
quantum yield in polar solvent is mainly due to the strong intra-
molecular charge transfer characteristics. And for compound 2,
aggregate-induced fluorescent quenching is much less than that of
compound 1 as expected, indicating that the steric effect plays
important role to prevent aggregation. These two dyes also exhibit
less intense fluorescence in polar solvents due to ICTcharacteristics,
which results in red-shift of the emission in polar solvent. The
sensitive photophysical response to media, along with the high
fluorescence yield will make BOPIM dyes suitable sensors for
communicating local environmental properties. On the other hand,
large Stokes shifts (>75 nm) in all the studied solvents were ob-
served for synthesized BOPIMs, which would induce suppression of
Forster-type energy transfer, thus helps to alleviate concentration-
induced emission quenching. Especially in polar solvents, the ob-
served Stokes shifts are more than 120 nm. Such big Stokes shifts
were rarely reported for boron chelates till now.10,14 Since fluo-
rescent probes are regularly applied in biological environment,
BOPIMs with large Stokes shifts in polar condition are of popular
interest for bio-imaging study.
not surprising since there is no much steric hindrance for this phenyl
ring. However, intermolecular non-covalent interactions
ꢀ
ꢀ
(C1eH1/N3: 2.470 A, C7eH7/B1: 2.706 A) are apparent to pro-
vide a rigid structure in crystal state (Fig. 1). These non-covalent
interactions of BOPIMs can be more clearly elaborated in the crys-
tal structure of compound 2. Moreover, there is almost no overlap
between intermolecular chromophore cores. The hindrance of in-
termolecular overlap of the chromophores is mainly due to the
twisted benzene rings on the end of the chromophore. Meanwhile,
the FeBeF groupacts as a bifurcated acceptor with an angle of 111.7ꢀ,
and F1 atom forms CeH/FeBꢀcontact with the neighboring donor
ꢀ
molecule (CeH/F: 3.24 A, 151 ). However, F2 interacts with C2, C3
ꢀ
on pyridine ring to form triangle structure with 3.11 and 3.15 A, re-
spectively. Due to these non-covalent interactions, aggregate for-
mation through planar pep stacking is inhibited.
2.3. Photophysical properties
The absorption and emission data of compounds 1 and 2 in
various solvents were listed in Table 1. Compared to typical BODI-
PYs with insensitivity to environment, these two BOPIM dyes ex-
hibit significant solvent-dependent properties, known as
‘solvatochromism’. As revealed from UVevis absorption spectra,
both of these two dyes show multiple absorptive bands in all or-
ganic solvents and the absorption bands shift significantly to sol-
vent polarity (Fig. 2A, D). In non-polar solvents such as hexane, the
dyes show the longest absorptive wavelength. The center of the
absorption band is located at 412 nm for compound 1 and 427 nm
It’s interesting to note that these two compounds are highly
fluorescent in solid state and in crystals. In the solid film prepared
by evaporation of a concentrated THF solution, the S0eS1 transition
band in absorption turns slightly broader. The intense emission
peaks were observed at 513 nm for 1, and 524 nm for 2. However,
the emissions become narrower compared to those in chloroform
Scheme 1. Synthesis of BOPIM dyes 1 and 2.

M. Mao et al. / Tetrahedron 68 (2012) 5037e5041
5039
absolute fluorescent quantum yield of 1 and 2 in solid film is esti-
mated to be 0.12 and 0.18 by integrating sphere method, which
makes them ideal solid-emitting materials for various applications.
The lower fluorescent quantum yield of 1 compared to 2 is mainly
due to the fast rotation of the phenyl ring and less electron density.
The solid-emissive property is the result of the abnormal non-
covalent intermolecular interactions, which inhibits the
concentration-induced quenching, as proved by X-ray single crystal
analysis and fluorescence measurement.
2.4. DFT calculation
To better understand the correlation of spectra and electron
properties, molecular orbital calculations for compound 2 were
carried out on the density functional theory (DFT) level employing
the Gaussian 03 programs. Geometric parameters from X-ray dif-
fraction analysis were used for the calculation (Fig. 5). It indicated
that the HOMO distribution was donated by all atomic orbitals in
the aromatic rings, while LUMO was mainly contributed by the part
of BOPIM chromophore. Therefore, efficient intramolecular charge
transfer (ICT) process could be a key factor for large Stokes shift and
solvent-reliant photophysical properties.15
Fig. 1. Single crystal structures, intermolecular interactions of compounds 1 and 2.
Table 1
Photophysical properties of BOPIMs in various solvents
Dye
Solvent
UV
lem (nm)
n
(nm)
FF
3. Conclusions
labs
3
1
Hexane
Cyclohexane
THF
327
412
329
412
312
394
369
312
394
298
384
297
382
393
24,580
26,500
24,120
26,550
22,270
27,350
24,220
23,080
27,140
20,170
25,300
20,550
28,050
w
487
486
509
75
74
0.32
0.33
0.22
In conclusion, we synthesized two novel BOPIM derivatives,
starting from simple ligands reported previously. The dyes exhibit
high fluorescent intensity and large Stokes shift in all organic sol-
vents studied. Due to the intermolecular non-planar interactions,
which inhibit concentration-induced quenching, they exhibit
highly solid-emissive property. And introduction of bulky steric
groups is essential for prohibition of aggregate formation. Solid-
emissive BOPIMs with large Stokes shift will make them suitable
candidates for application of OLED, memory, fluorescent
probes, etc.
115
Chloroform
Dioxane
501
509
132
115
0.24
0.24
Acetonitrile
Methanol
Solid
514
503
513
130
121
120
0.21
0.17
0.12
4. Experimental section
4.1. General
2
Hexane
278
338
427
277
339
429
310
404
268
388
273
387
363
264
291
391
402
15,630
14,880
23,480
14,910
14,410
24,390
14,110
17,200
20,370
35,560
16,580
50,390
28,610
16,380
18,690
19,900
w
515
517
88
88
0.30
0.34
Cyclohexane
All starting materials were obtained from commercial supplies
and used as received. 1H NMR and 13C NMR spectra were recorded
on Varian or Bruker instrument, using CDCl3 or DMSO-d6 as sol-
vents. Chemical shifts are reported in parts per million with CDCl3
or TMS (when DMSO-d6 is adopted as solvent) as reference (in
CDCl3: 7.26 ppm for 1H NMR, and 77.0 ppm for 13C NMR). The
fluorescent quantum yield is calculated using anthracene as refer-
ence in solution, integrating sphere method in solid state. UVevis
and fluorescent measurements were carried out on Hitachi U-3010
and F-4500, respectively. MS data were recorded on a Waters
Quattro Micro API MS spectrometer.
THF
537
528
510
130
140
123
0.17
0.30
0.26
Chloroform
Dioxane
Acetonitrile
Methanol
498
534
135
143
0.19
0.10
Solid
524
122
0.18
4.2. Ligand L1
solution (Fig. 3), which is quite unusual and exciting because typical
dyes exhibit spectral broadening in solid state. It needs to note that
there appears two emissive peaks for compound 1 in solid state
(Fig. 3A), due to a minimal aggregate formation, while this phe-
nomenon is not observed at all for BOPIM 2, indicating that the
existence of two steric bulky groups is essential for efficient ag-
gregate prohibition. Because of the aggregate prohibition, both
dyes emit high fluorescence in solid state, and also in crystal state.
As can be seen from Fig. 4, intense fluorescence was observed in
amorphous powder and also in a single crystal under 365 nm light
irradiation for both dyes. In fact, the fluorescent intensity is so high
that it even can be visibly seen under day-light excitation. The
The synthesis was performed according to previously reported
procedure.13a 1H NMR (DMSO-d6, 300 MHz):
d
8.61 (d, J¼3.90 Hz,
1H), 8.12 (d, J¼7.80 Hz, 1H), 7.90 (m, 3H), 7.75 (s, 1H), 7.38 (t,
J¼7.50 Hz, 3H), 7.22 (t, J¼7.20 Hz, 1H).
4.3. Ligand L2
The synthesis was performed according to previously reported
procedure.13b 1H NMR (CDCl3, 300 MHz):
8.28 (d, J¼8.00 Hz, 1H), 7.76 (t, J¼8.80 Hz, 1H), 7.66 (d, J¼7.00 Hz,
2H), 7.45 (d, J¼6.80 Hz, 2H), 7.32 (m, 6H).
d
8.51 (d, J¼5.00 Hz, 1H),

5040
M. Mao et al. / Tetrahedron 68 (2012) 5037e5041
Fig. 2. Absorption, and fluorescent spectra of compound 1 (A, B) and compound 2 (D, E) in various organic solvents (1.0ꢁ10ꢂ5 M); concentration-dependent fluorescence in CHCl3
for compound 1 (C) and compound 2 (F) (Ex: 365 nm).
Fig. 4. Bright field images and fluorescent images (under 365 nm light) of compounds
1 (A) and 2 (B) in solid powder and in crystal state.
Fig. 5. Diagrams showing the HOMO and LUMO levels of compound 2.
4.4. Synthesis of BOPIMs
Ligand (2.18 mmol) was dissolved in anhydrous dichloro-
methane (20 mL), followed by addition of triethylamine (4.40 mL)
under N2. This mixture was cooled by ice, and then BF3$Et2O
(3.50 mL, 27.4 mmol) was added dropwise. Then the reaction
mixture was allowed to stir at room temperature overnight. After
quenching with water, the organic phase was separated and the
aqueous phase was extracted with dichloromethane. The combined
organic phases were washed with water, dried on Na2SO4, and
evaporated in vacuo. The crude residue was subjected to a silica gel
column to afford target products as yellow solid.
Fig. 3. Normalized absorption and fluorescent spectra of 1 and 2 in chloroform solu-
tion and in solid state (Ex: 365 nm).

M. Mao et al. / Tetrahedron 68 (2012) 5037e5041
5041
4.4.1. BOPIM 1. Yellow solid (28%). 1H NMR (CDCl3, 300 MHz):
8.46 (d, J¼4.20 Hz, 1H), 8.18 (t, J¼7.50 Hz, 1H), 8.00 (d, J¼7.50 Hz,
L.; Jiang, K. J.; Shao, K. F.; Yang, L. M. Chem. Commun. 2006, 2792e2794; (g)
Zhang, Y. Q.; Wang, J. X.; Ji, Z. Y.; Hu, W. P.; Jiang, L.; Song, Y. L.; Zhu, D. B. J.
Mater. Chem. 2007, 17, 90e94.
d
1H), 7.82 (d, J¼6.90 Hz, 2H), 7.59 (s, 1H), 7.47 (m, 3H), 7.33 (t,
4. (a) Schi, J.; Tang, C. W. Appl. Phys. Lett. 1997, 70, 1665e1667; (b) Kraft, A.;
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J¼6.90 Hz, 1H). 13C NMR (CDCl3, 75 MHz)
d 144.64, 141.23, 133.38,
129.00, 128.02, 125.79, 122.76, 117.59. HRMS calcd for C14H11BF2N3
[MþH]þ 270.1014, found 269.1050.
4.4.2. BOPIM 2. Yellow solid (32%). 1H NMR (CDCl3, 300 MHz)
d
8.43 (d, J¼5.40 Hz, 1H), 8.16 (m, 1H), 8.09 (d, J¼7.80 Hz, 1H), 7.60
(m, 4H), 7.46 (m, 1H), 7.36 (m, 6H). 13C NMR (CDCl3, 75 MHz)
d
144.68, 141.26, 134.56, 130.66, 128.68, 128.59, 128.34, 128.14,
127.31, 122.78, 117.73. HRMS calcd for C20H15BF2N3 [MþH]þ
346.1325, found 346.1315.
4.5. X-ray structures
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Single crystals suitable for X-ray analysis were obtained by slow
evaporation of mixed organic solvents. These data (CCDC-842917
for 1, and CCDC-804488 for 2) can be obtained free of charge from
the Cambridge Crystallographic Data Centre via www.ccdc.cam.a-
c.uk/data_request/cif.
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Acknowledgements
We are grateful for the financial support from National Natural
Science Foundation of China (21002059, 91022021), and China
Three Gorges University (KJ2011B004, 2011PY053).
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Supplementary data
1H NMR, 13C NMR spectra of intermediates and BOPIM dyes, X-
ray crystallography of the complexes. Supplementary data associ-
ated with this article can be found in the online version, at
doi:10.1016/j.tet.2012.04.052.
12. (a) Mao, M. F.; Zhang, R. H.; Xiao, S. Z.; Zou, K. J. Coord. Chem. 2011, 64,
3303e3310; (b) Mao, M. F.; Xiao, S. Z.; Yi, T.; Zou, K. J. Fluorine Chem. 2011, 132,
612e616.
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