A simple approach to aggregation-induced emission in difluoroboron dibenzoylmethane derivatives

Article abstract of DOI:10.1016/j.tetlet.2013.05.099

The first example of an AIE active BF₂dbm derivative with a single dimethylamino group connecting to the phenyl ring was revealed. It also showed capability of sensing HCl gas both in solution and in the solid state.

Full text of DOI:10.1016/j.tetlet.2013.05.099

Tetrahedron Letters 54 (2013) 4167–4170
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Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
A simple approach to aggregation-induced emission
in difluoroboron dibenzoylmethane derivatives
⇑
Junyi Hu, Zhen He, Zhi Wang, Xiaoyu Li, Jingsong You, Ge Gao
Key Laboratory of Green Chemistry and Technology of Ministry of Education, College of Chemistry, Sichuan University, 29 Wangjiang Road, Chengdu 610064, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
The first example of an AIE active BF2dbm derivative with a single dimethylamino group connecting to
Received 28 February 2013
Revised 12 May 2013
Accepted 23 May 2013
Available online 4 June 2013
the phenyl ring was revealed. It also showed capability of sensing HCl gas both in solution and in the solid
state.
Ó 2013 Elsevier Ltd. All rights reserved.
Keywords:
Difluoroboron dibenzoylmethane
Twisted intramolecular charge transfer
Aggregation-induced emission
Sensor
Difluoroboron compounds with intense emission are useful as
bioprobes, multifunctional materials, and optoelectronic materi-
als.1 Of particular interest are boron dipyrromethene (BODIPY)
dyes,2 which have prominent properties such as sharp fluorescence
peaks, high quantum yields, and high photostability. These dyes
usually exhibit high fluorescence in dilute solutions but the fluo-
rescence diminishes in the solid state due to aggregation-caused
quenching (ACQ). In 2001, Tang group discovered that hexaphenyl-
silole (HPS) is non-luminescent in solution but emissive upon
aggregation, which is termed as ‘aggregation-induced emission’
(AIE).3 This ‘abnormal’ phenomenon attracts much attention and
a lot of work on new AIE luminogens has been reported.4 However,
only very few difluoroboron dyes such as BODIPY derivatives,5 BF2-
hydrazone,6 and BF2-Thiazole7 have been reported to exhibit AIE
effect.
Difluoroboron dibenzoylmethane derivatives (BF2dbms), are
another type of important difluoroboron dyes structurally related
to BODIPYs.8 These dyes possess large molar extinction coeffi-
cients, two-photon absorption cross sections, high emission quan-
tum yields and high sensitivity toward surrounding medium.9 But
AIE effect of these dyes has never been disclosed thus far.
The BF2dbm fluorogenic core 1 is a conjugated aromatic system
with an electron-accepting center at the difluoroboron moiety. We
envisage that connecting a simple electron-donating dimethyl-
amino group with the phenyl ring through a C–N single bond
BF2dbms with AIE effect.4 Herein, we want to report the first exam-
ple of AIE active BF2dbms by this design. In addition, these dyes can
be used as the colorimetric and fluorescent sensors for HCl gas
both in solution and in the solid state.
All of the boron dibenzoylmethane derivatives 1–6 (Scheme 1)
were synthesized in moderate yields by the classic Claisen conden-
sation of the corresponding acetophenones with benzoates fol-
lowed by the coordination with BF3/Et2O or (C6F5)3B. The optical
properties of 1–6 are summarized in Table 1. The C2 symmetric
unsubstituted BF2dbm 1, as the fluorogenic core, showed the
absorption at 364 nm and the emission at 414 nm in THF with a
weak blue fluorescence of the quantum yield of 0.11, and no signif-
icant variation was observed in different solvents. The titration of
water into the THF solution of 1 resulted in small variation of the
emission (Fig. S1) and invisibility of the Tyndall phenomenon
(Fig. S2),7,11 suggesting no AIE effect.
When a dimethylamino group was introduced into the para po-
sition of one of the phenyl rings, the BF2dbm derivative 2 exhibited
the absorption at 454 nm and the emission at 530 nm with a large
red shifts of 90 nm and 116 nm compared with 1, respectively,
R1 R1
B
1: R1 = F,
2: R1 = F,
3: R1 = F,
R2 = R3 = H
O
O
R2 = -NMe2, R3 = H
R2 = R3 = -NMe2
would form a D–p–A molecular system with twisted intramolecu-
lar charge transfer (TICT) property,10 which might generate
4: R1 = C6F5, R2 = -NMe2, R3 = H
5: R1 = F,
6: R1 = F,
R2 = -NHMe, R3 = H
R2 = -OEt, R3 = H
R2
R3
⇑
Corresponding author. Tel.: +86 2885418195.
E-mail address: gg2b@scu.edu.cn (G. Gao).
Scheme 1. The molecular structures of compounds 1–6.
0040-4039/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.tetlet.2013.05.099

4168
J. Hu et al. / Tetrahedron Letters 54 (2013) 4167–4170
Table 1
Photophysical properties of 1–6 in solution (THF) and film
a,b
a
Compounds
kab (nm)
kem (nm)
Filmc,e
(UF)
Solution (
e
)
Solutionb,c,d
(
UF)
1
2
3
4
5
6
364 nm
(16,480)
454 nm
(33,060)
481 nm
(29,680)
478 nm
(36,490)
445 nm
(35,530)
363 nm
(20,460)
414 nm (0.11)
530 nm (0.02)
512 nm (0.69)
544 nm (<0.01)
500 nm (0.05)
431 nm (0.75)
439 nm (0.15)
532 nm (0.75)f
545 nm (0.62)
534 nm (0.80)
518 nm (0.63)
509 nm (0.36)
a
b
c
d
e
f
1 nm.
Measured at a concentration of 1 Â 10À5 M.
Figure 1. The fluorescent emission spectra of 2 in the MeCN/water mixtures.
Measured at a concentration of 1 Â 10À4 M.
0.03.
Excited at each kab
Excited at each kex
The quantum yield of 2 in the solid state is 0.06.
.
.
cence intensity of 2 in MeCN with the maximum 70% fw than with-
out water, indicating the AIE occurred. After the fw continuously
increased over 70%, the fluorescence intensity just decreased. AIEs
were also observed in other polar solvents such as THF, acetone,
and DMSO (Fig. S4). The photographs of 2 in the MeCN/water
and DMSO/water mixtures taken under the UV irradiation were
shown in Figures S5 and S6. The Tyndall phenomenon of 2 in the
DMSO/water solutions demonstrated the nano-sized aggregates
of 2 were formed (Fig. S7).7,11
The BF2dbm derivative 2 in the solid state emitted weak red
fluorescence (kem = 641 nm and UF = 0.06), indicating the existence
of nonradiative decay caused by the intermolecular interactions in
the solid state. In order to better understand the molecular packing
of 2 in the solid state, the single crystals were obtained by slow
evaporation of the THF solution of 2 and subjected to the X-ray dif-
fraction analysis (Fig. 2). It showed that the molecule is almost pla-
nar but the BF2 moiety bends off the skeleton plane 26.4° (the
dihedral angle between the B1–O1–O2 plane and the O1–C10–
C4–O2 plane). Two molecules pack head-to-tail each other to form
a dimer by two hydrogen bonds. Each one is 2.639 Å and formed
between a F atom of one molecule and a methyl hydrogen atom
of the other molecule. The two dimethylaminophenyl rings are
partially overlapped with a centroid distance of 3.647 Å, indicating
which suggested a typical D–p–A conjugated structure with ICT
process from the electron-donating dimethylamino group to the
electron-accepting difluoroboron moiety.12 The quantum yield of
2 in THF dramatically decreased to only 0.02. In addition, the fluo-
rescence behavior of 2 varied in different solvents: Upon excited by
UV light, the solution of 2 emitted strong yellow–green fluores-
cence in a nonpolar solvent such as toluene. Yellow-colored and
weakened fluorescence was observed in relatively polar solvents
such as chloroform and THF. In highly polar media such as acetone,
acetonitrile (MeCN), DMF, and DMSO, the emission became so
weak that it could hardly be seen with naked eyes (Fig. S3). This
bathochromic shift in color and weakened intensity with the in-
crease of solvent polarity indicated that a twisted intramolecular
charge transfer (TICT) process might be involved.
The temperature effect was then investigated to verify the TICT
process in the BF2dbm derivative 2. When elevating the tempera-
ture of the THF solution of 2 from À10 to 60 °C, the fluorescence
intensity gradually increased, accompanied by a small blue-shift
of the maximum emission from 536 nm to 526 nm (Fig. S3d). This
enhanced fluorescence with mounting temperature was consistent
with the typical TICT process.5 Therefore, the BF2dbm derivative 2
is TICT active.
the
p–p stacking between these two molecules. The dimers pack
into columns by another two hydrogen bonds of 2.495 Å, formed
between the other F atom and the other methyl hydrogen atom.
Notably, this F atom also forms three hydrogen bonds with two
phenyl hydrogen atoms (2.430 and 2.432 Å) and a methylene
hydrogen atom (2.517 Å) of another molecule in the adjacent col-
umn, which makes packing columns staggered (Fig. S12). These
In a nonpolar solvent such as toluene, 2 stayed in the locally ex-
cited (LE) state with good conjugation of the donor (D) and the
acceptor (A) to emit strong fluorescence. When dissolved in a polar
solvent such as MeCN, It rotated to the TICT state, which resulted in
elevation of the HOMO level and the narrowed band gap, leading to
red shift of the emission. The weakened emission could be inter-
preted by the increased nonradiative decay processes in the TICT
state.13 The AIE phenomenon was expected when a poor solvent
such as water was added to induce the aggregation of 2 because
the restriction of intramolecular rotation (RIR) of a TICT active mol-
ecule in the aggregates was considered as the main reason for the
AIE effect.4
The fluorescence properties of 2 were then investigated in the
MeCN and water mixtures of various ratios (Fig. 1). The MeCN solu-
tion of 2 emitted weak yellow fluorescence under UV irradiation,
which was quenched when a small amount of water was added
into the solution. In pace with the increasing water fraction (fw),
its emission weakened gradually with the fw between 10% and
50%. When the fw exceeded 50%, the original emission peak at
550 nm completely vanished, and a new peak at 622 nm appeared,
which was attributed to the solid-state fluorescence of BF2dbm
derivatives.14 There was about 16-fold enhancement of the fluores-
multiple hydrogen bonding and
p–p stacking may be responsible
for the red shift and weakened emission of 2 in the solid state.
As we mentioned before, 2 is a TICT active molecule and its
weak emission in THF is due to the nonradiative de-excitation
caused by the free rotation of the phenyl ring and the dimethyl-
amino group. Restricting its free rotation and preventing the multi-
ple intermolecular interactions at the same time could regenerate
its emission in the solid state. So we prepared a poly(methylmeth-
acrylate) (PMMA) film doped with 0.1% 2. The film exhibited strong
yellow-colored fluorescence with a quantum yield of 0.75, demon-
strating the effects of both RIR and the isolation of the molecules.15
Interestingly, BF2dbm derivatives 3, which has two electron-
donating dimethylamino groups on both sides of the phenyl rings
to recover the C2 symmetry, showed much higher quantum yield
than 2 in THF solution (UF = 0.69). The enhanced emission of 2
was attributed to the ICT process compared with 1. The solvent ef-

J. Hu et al. / Tetrahedron Letters 54 (2013) 4167–4170
4169
Figure 3. The pictures of 2 (left) and 2–H+ (right) on a silica support under ambient
light (top) and UV light (bottom).
lid state. For example, a TLC plate dipped with 2 exhibited a red
color under ambient light and emitted red fluorescence under UV
light. When exposed to HCl gas, the red color of the plate faded
and emitted a blue fluorescence, which was attributed to the
shut-down of the ICT pathway. The plate regained its original state
quickly after exposed to NH3 gas (Fig. 3).17 The derivatives 3 and 4
showed the same ability as 2 in sensing HCl gas in the solid state
(Fig. S14). This simple acid and base sensing experiment demon-
strated the potential applications of the BF2dbm derivatives as so-
lid-state sensors.18
It is worth noting, that the fluorescent sensing behavior of 2 to-
ward HCl gas in solution was dichotomous depending on the polar-
ity of the solvents used. The fluorescence of 2 was quenched in the
nonpolar solvents such as toluene and chloroform while it was
turned on in the polar solvents such as THF, acetone, and MeCN
with addition of HCl gas (Figs. S15 and S17a–e). This difference
could be interpreted using LE and TICT mechanism: In nonpolar
solvents, 2 stays in the LE state and emits strong fluorescence
through ICT. The protonation of the amino group interrupts the
ICT process and quenches the fluorescence. In polar solvents, 2
stays in the TICT state and emits weak fluorescence. The proton-
ation of the amino group blocks the TICT process and recovers
the fluorescence. Unlike 2, derivative 3 has no TICT state, therefore
fluorescence quenching and blue shifts were always observed in
different solvents (Figs. S16 and S17f–h).
In conclusion, a series of boron dibenzoylmethane derivatives
1–6 were synthesized and their photophysical properties were
investigated. It showed that the BF2dbm 2 with a single dimethyl-
amino group connecting to the phenyl ring is AIE active, which rep-
resents the first AIE active BF2dbm dye, while the BF2dbm 3 with
two dimethylamino groups connecting to the two phenyl rings is
not. The X-ray diffraction analysis revealed that multiple hydrogen
Figure 2. (a) The crystal structure of 2; (b) a wire drawing of the crystal packing of
2 (the interactions and H–F bonds are shown as dashed lines).
p–p
fect experiment showed that with an increasing solvent polarity,
the fluorescence of 3 was just red-shifted without much change
of the intensity. This indicated that 3 does not have a TICT state,
which was confirmed by the gradual increase of the emission when
lowering the temperature (Fig. S8). The AIE phenomenon was
therefore not observed.
By replacing the BF2 group in the derivative 2 into the B(C6F5)2
group, the derivative 4 gave the quantum yields of less than 0.01
and 0.8 in THF and in the PMMA film, respectively. Meanwhile, a
22-fold increase of the fluorescent intensity of 4 was observed in
the MeCN solution when the water fraction reached 50% (Figs. S9
and S10). The enhanced AIE effect was due to the reinforced TICT
in the molecule 4 caused by the larger electron-accepting ability
of the B(C6F5)2 group and the rotation of the C6F5 rings.16
To further address the role of the dimethylamino group in 2 for
AIE, analogues 5 and 6 with the methylamino group and the ethoxy
group were synthesized, respectively. Compared with 2, 5 has a N–
H as the hydrogen bond donor. The water titration induced
quenching of the fluorescence of 5 in the THF solution. When fw
reached 50%, a small increase with red shift was observed. The
fluorescence quenched again when fw kept going higher
(Fig. S11a). This fluorescence variation suggested that the aggrega-
tion of 5 caused by water addition was different from that of 2 and
the N–H was probably involved in multiple hydrogen bonding
interactions to open the nonradiative pathway. On the other hand,
6 has an oxygen atom as the hydrogen bond acceptor. The water
titration resulted in constant quenching of the fluorescence of 6
in the THF solution (Fig. S11b). Both 5 and 6 have no AIE effect.
The boron dibenzoylmethane derivatives 2–4 containing amino
group are capable of sensing HCl gas both in solution and in the so-
bonding and p–p stacking in 2 are responsible for its red but weak
fluorescence in the solid state. Finally, the dyes 2–4 are capable of
sensing HCl gas by reversible changes of color and fluorescence,
which may potentially serve as simple colorimetric and fluorescent
acid sensors both in solution and in the solid state.
Acknowledgments
We thank the financial support from the NSFC (Nos. 21172159,
21025205 and 21021001) and the SRF for ROCS, SEM (No
3520111568-8-2). We also thank the Centre of Testing & Analysis,
Sichuan University for NMR measurements and X-ray analysis.
Supplementary data
Supplementary data (these data include copies of 1H and 13C
NMR spectra, photophysical data and crystallographic data of com-
pounds described in this Letter. The crystallographic data for the
compound 2 have been deposited with the Cambridge Crystallo-

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