Efficient synthesis of solid-emissive Boron-Fluorine derivatives

Article abstract of DOI:10.1016/j.tetlet.2013.05.116

A highly solid-emissive Boron-Fluorine scaffold with two conjugated alkyne groups (BOPIM-S) is facilely synthesized, which shows enhanced fluorescent intensity compared to the BOPIM dye without alkyne units. According to X-ray single crystal analysis, the terminal alkyne groups interact with atoms B, F, C and N on neighbouring molecules, producing a rigid rectangular structure, which helps to avoid energy loss via non-irradiative decay. Furthermore, BOPIM-S can undergo copper catalysed alkyne-azide-cycloaddition (CuAAC) chemistry with aromatic azides to produce conjugated molecules with high yields. According to absorption and fluorescent measurements, all these compounds synthesized by CuAAC emit large Stokes shift (over 100 nm) both in solution and in solid state. But the electronic environments of terminal aromatic rings slightly affect their photophysical properties.

Full text of DOI:10.1016/j.tetlet.2013.05.116

Tetrahedron Letters xxx (2013) xxx–xxx
Contents lists available at SciVerse ScienceDirect
Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
Efficient synthesis of solid-emissive Boron–Fluorine derivatives
⇑
Xiaohong Chen, Shuzhang Xiao , Sa Wang, Qiong Cao, Kun Zou, Nianyu Huang, Zhangshuang Deng
Hubei Key Laboratory of Natural Products Research and Development, College of Chemistry and Life Science, China Three Gorges University, Hubei Yichang 443002, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
A highly solid-emissive Boron–Fluorine scaffold with two conjugated alkyne groups (BOPIM-S) is facilely
synthesized, which shows enhanced fluorescent intensity compared to the BOPIM dye without alkyne
units. According to X-ray single crystal analysis, the terminal alkyne groups interact with atoms B, F, C
and N on neighbouring molecules, producing a rigid rectangular structure, which helps to avoid energy
loss via non-irradiative decay. Furthermore, BOPIM-S can undergo copper catalysed alkyne–azide-cyclo-
addition (CuAAC) chemistry with aromatic azides to produce conjugated molecules with high yields.
According to absorption and fluorescent measurements, all these compounds synthesized by CuAAC emit
large Stokes shift (over 100 nm) both in solution and in solid state. But the electronic environments of
terminal aromatic rings slightly affect their photophysical properties.
Received 28 February 2013
Revised 21 May 2013
Accepted 27 May 2013
Available online xxxx
Keywords:
Fluorescence
Solid-emission
Boron–Fluorine
CuAAC
Ó 2013 Elsevier Ltd. All rights reserved.
Interest in solid-emissive dyes is still ongoing, as they have a
wide range of potential applications in optoelectronic devices, sen-
sors and light harvesting system.1 Although a large number of
chromophores exhibit excellent fluorescent property in dilute
solutions, examples of solid-emissive dyes are limited because
most of them tend to aggregate in concentrated solution or in solid
state. A lot of factors may cause severe concentration-induced fluo-
rescence quenching, such as torsional induced non-radiative deac-
tivation, presence of traps or dopants and most importantly plane-
to-plane stacking-induced intermolecular coupling of electronic
transition dipole moments.2 To avoid these limitations, some strat-
egies have been proved to be effective, such as introducing bulky
steric groups,3 constructing non-covalent bonds to promote non-
parallel packing etc. Among all the chromophores developed till
now, we are most interested in Boron–Fluorine derivatives, be-
cause of their large number of advantages over other dyes, namely
high fluorescence quantum yield and large absorption coefficient,
excellent chemical- and photochemical-stability, long excited-
state lifetimes, a large two-photo cross-section, good solubility
and narrow emissive band. In order to solve the problem of con-
centration-induced fluorescence quenching, heterocyclic rings
can be adopted instead of pyrrole to construct intermolecular
non-covalent bonds, which would result in non-parallel packing
in solid state.4 Previously, we developed such a class of solid-emis-
sive Boron–Fluorine dyes (BOPIM), using 2-(20-pyridyl) imidazole
derivatives as ligands.5 This class of dyes exhibit high fluorescent
intensity in solution, amorphous solid film and crystal state. And
the dyes’ Stokes shifts are highly enlarged compared to typical
BODIPY dyes (normally over 100 nm), due to the intramolecular
charge transfer (ICT) from electron-rich groups to electron-defi-
cient boron centre. We also found that the terminal groups conju-
gated to the imidazoles play important roles in determining
BOPIMs’ photophysical properties. For example, when terminal
groups are halogen atoms or methoxyl moieties which can easily
form intermolecular non-covalent bonds, they show comparatively
high fluorescence quantum yields both in solution and in solid sta-
te.5b–d If the terminal groups are fluorescence quenching groups
(like nitro), or torsional moieties (N,N0-dimethylamino) with TICT
characteristic, very low or no fluorescence were observed in solu-
tion or amorphous powder.5e BOPIM dyes’ emission wavelength
is also tunable by conjugating electron donors or acceptors to the
BOPIM skeleton.5c But all the BOPIM dyes reported so far are syn-
thesized from original starting materials. Therefore, it is necessary
to develop a more straightforward method to synthesize solid-
emissive BOPIM dyes from a simple intermediate under mild reac-
tion conditions. And conjugated structures are preferred, because
their photophysical properties are highly related to conjugation.
Based on the above considerations, we describe here the synthesis
and characterizations of a novel solid-emissive BOPIM scaffold
with conjugated alkyne groups (BOPIM-S), and use it as a core
scaffold to react with different aromatic azides to provide various
conjugated structures (see Scheme 1).
Synthesis of the BOPIM core scaffold is straightforward. A mul-
ticomponent one-pot reaction with 4-ethynyl-benzaldehyde and
2-cyanopyridine as starting materials gives target ligand L in high
yield (75%). By treating the ligand with BF3ꢀOEt2 under basic condi-
tions, the intermediate BOPIM-S was obtained as a yellow powder
with 35% yield after purification by column chromatography. It is
well known that non-conjugated alkyne groups can undergo
⇑
Corresponding author. Tel./fax: +86 717 6397478.
E-mail address: shuzhangxiao@gmail.com (S. Xiao).
0040-4039/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.tetlet.2013.05.116
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X. Chen et al. / Tetrahedron Letters xxx (2013) xxx–xxx
Scheme 1. Synthetic route of BOPIM derivatives.
copper catalysed azide–alkyne-cycloaddition (CuAAC) reaction
with aliphatic azides efficiently under mild reaction conditions.
However, alkyne moieties conjugated with aromatic rings can also
react with aliphatic azides, and even aromatic azides to produce
triazole derivatives with high yield, which has been adopted to
synthesize highly conjugated polymers.6 Three aromatic azides
with different electronic environments were synthesized according
to previously reported procedure.7 By reaction with BOPIM-S in a
mixture of water/t-butanol at 60° under N2 atmosphere, target
conjugated structures were obtained with high yield (over 80%)
after purification by column chromatography.
shift, which would result in self-absorption of its fluorescence,
however, BOPIM dyes usually exhibit large Stokes shift due to
the efficient ICT process. The Stokes shifts of BOPIM-S in all the
studied solvents are found to be over 100 nm and there is negligi-
ble spectral overlap between its absorption and fluorescence
(Fig. 2A), which would suppress Forster-type energy transfer and
enhance emission intensity. It is worth to note that it shows high
fluorescence quantum yield in all studied solvents. The lowest
quantum yield is found to be 0.25 in methanol. For a comparison,
the BOPIM dye with no alkyne groups emits at 534 nm with quan-
tum yield 0.10 in methanol.5a In chloroform, the value reaches 0.68
for BOPIM-S. Most importantly, intense emission was observed in
solid powder and also crystals. And it is exciting that the fluores-
cent band in amorphous solid film is almost identical with that
in dilute THF solution, indicating there is almost no aggregate for-
mation in solid state. To verify this hypothesis, concentration-
dependent fluorescent analysis was conducted in chloroform solu-
tions. As shown in Figure 2B, its emission wavelength is indepen-
dent of concentration, further proving that it can resist aggregate
formation in concentrated solution or even solid state. This phe-
nomenon has been observed for other BOPIM dyes, however, its
high fluorescence quantum yield is unusual. The terminal groups
conjugated to the imidazoles should be a key factor for its high
fluorescence quantum yield and aggregate-resistance ability.
When terminal groups are halogen atoms or methoxyl moieties
which can easily form intermolecular non-covalent bonds, they
can make the structure more rigid and less flexible, thus alleviating
energy loss through bond rotation. Alkyne is also a well-known
group that can form intermolecular non-covalent interactions,
which has been proved by X-ray single crystal analysis discussed
above. Therefore, it exhibits excellent fluorescent properties both
in solution and in solid state.
In order to elucidate the packing mode of BOPIM-S in solid
state, X-ray single crystal diffraction measurement was performed.
As shown in Figure 1, molecules were connected by intermolecular
non-covalent bonds to produce a rigid network with limited
p–p
interactions. From the side view, the Boron–Fluorine chromophore
core is almost in the same plane with 4-substituted phenyl ring,
however, 5-phenyl ring is twisted and nearly perpendicular to this
plane. This phenyl ring acts as an obstacle to enlarge the distance
between two neighbouring molecules. This packing mode makes
the chromophore core only partially overlap with the neighbouring
4-phenyl ring, but the distance between the two planes is over
3.30 Å due to the existence of twisted 5-phenyl ring. And the inter-
molecular contacts C6. . .C11 (3.39 Å) and C7. . .C10 (3.31 Å) further
rigidifies this structure. From the top view, all the molecules are
lined almost planar. Interestingly, the chromophore core interacts
with neighbouring alkyne groups to produce a tetragonal structure
by contacts B1. . .H16 (3.04 Å), F2. . .H16 (2.49 Å) and N2. . .H24
(2.38 Å). In this packing mode, the closed distance of the intermo-
lecular aromatic rings was found to be 13.88 Å.
Physical properties of BOPIM-S were measured in solution, and
also solid film. All BOPIM dyes reported previously show solvent-
dependent absorption and fluorescence, and it is also the case for
BOPIM-S. In chloroform its maximum absorption is centred at
414 nm, showing a broad absorption band. With solvent polarity
increasing, its absorption blue-shifts to high energy wavelength
(Table 1). However, different from other BOPIM dyes, the emissive
band of BOPIM-S shifts slightly with solvent polarity changing,
located around 525 nm. Typical BODIPY dyes show small Stokes
The terminal groups conjugated to the chromophore core could
severely affect the dyes’ photophysical properties. Electron-donat-
ing groups facilitates intramolecular charge transfer (ICT) process
from electron-rich groups to electron-accepting BOPIM skeleton,
thus red-shifts the absorption and emission. For the BOPIM dyes
synthesized by ‘click’ chemistry, although the terminal dimethyl-
amino, hydrogen and ester are conjugated to the chromophore
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X. Chen et al. / Tetrahedron Letters xxx (2013) xxx–xxx
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Figure 1. Crystal packing of BOPIM-S (A) side view; (B) top view.
Table 1
Photophysical data of BOPIM dyes in various solvents
Dyes
solvent
UV
Fluorescence
(nm)
kabs (nm)
e
(ꢁ104)
kem (nm)
m
UF
BOPIM-S
CHCl3
THF
Dioxane
CH3OH
Solid
414
403
405
386
406
1.39
1.47
0.65
1.38
—
520
527
525
526
526
106
124
120
140
120
0.68
0.50
0.59
0.25
0.31
BOPIM-1
BOPIM-2
BOPIM-3
CHCl3
THF
Dioxane
CH3OH
Solid
427
415
417
391
412
1.15
1.28
1.31
1.24
—
537
544
543
531
544
110
129
126
140
132
0.39
0.22
0.24
0.01
0.01
CHCl3
THF
Dioxane
CH3OH
Solid
423
413
417
393
412
1.40
1.35
1.18
1.35
—
533
541
539
535
540
110
128
122
142
128
0.91
0.60
0.31
0.30
0.28
CHCl3
THF
Dioxane
CH3OH
Solid
419
409
398
387
426
1.52
1.27
1.39
1.56
—
534
542
547
538
531
115
133
149
151
105
0.43
0.40
0.15
0.23
0.26
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X. Chen et al. / Tetrahedron Letters xxx (2013) xxx–xxx
Figure 2. (A) Absorption and fluorescent spectra of BOPIM-S in solution (THF,
10ꢂ5 M) and in solid film; (B) concentration-dependent fluorescent spectra in
chloroform.
core, they scarcely affect the dyes’ absorption and fluorescence. As
shown in Figure 3, their absorption and fluorescence are almost in
the same pattern in dilute solutions. The maximum absorption
band is located around 410 nm in mid-polar solvents, and their
fluorescence is around 540 nm. Similar to BOPIM-S, their absorp-
tion bands blue-shift with solvent polarity increasing, but their
fluorescence bands are almost independent of solvent polarity.
Compared to BOPIM-S, their absorption and fluorescence shift to
longer wavelengths because of the enhanced conjugation degree.
However, it seems that the chromophore is mostly affected by tri-
azole ring, and the electron donor (dimethylamino) and acceptor
(ester) do not induce obvious differences. This is inconsistent with
our previous observations,5c and may be explained by the poor
overlap of the
p electrons on triazoles and terminal phenyl rings.
DFT calculations were performed for these compounds, suggesting
that the HOMO distributions primarily reside on the atomic orbi-
tals from triazole to Boron–Fluorine centre, while LUMO was
mainly contributed by BOPIM chromophore (Fig. S1). Therefore,
the terminal aromatic phenyl rings with functional groups do not
severely affect their photophysical properties. The fluorescence
quantum yield of BOPIM-1 is found to be the lowest among the
three, which should be due to energy loss via fast rotation of
dimethylamino groups. BOPIM-2 and BOPIM-3 exhibit intense
emission in solid state (Fig. 3D), mainly due to that aggregate for-
mation is inhibited due to their non-parallel packing mode. But
they show slight fluorescence difference in solid state, which
may be aroused by the steric effect and supramolecular interac-
Figure 3. (A) Absorption and (B) fluorescent spectra of synthesized BOPIM dyes in
solution (THF, 10ꢂ5 M); (C) absorption and (D) fluorescent spectra of synthesized
BOPIM dyes in solid film.
tions of the terminal moieties. And their Stokes shifts are all over
110 nm in solution and also in solid state.
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X. Chen et al. / Tetrahedron Letters xxx (2013) xxx–xxx
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In conclusion, we designed and synthesized a highly solid-
emissive BOPIM scaffold core with conjugated alkyne groups. The
synthesized BOPIM-S can undergo CuAAC with aromatic azides
efficiently to provide conjugated solid-emissive dyes with different
functional groups.
Acknowledgments
We are grateful for the financial support from the National Nat-
ural Science Foundation of China (21002059), Startup Foundation
from China Three Gorges University (KJ2010B035, KJ2011B004)
and SRF for ROCS, SEM.
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