Boron diiminate with aggregation-induced emission and crystallization- induced emission-enhancement characteristics

Article abstract of DOI:10.1002/chem.201402946

Organoboron complexes having aggregation- and crystalization-induced emission properties are presented. The series of boron diiminates were synthesized and the emission behaviors of the synthesized compounds were investigated. Finally, it was found that s

Full text of DOI:10.1002/chem.201402946

DOI: 10.1002/chem.201402946
Communication
&
Sensors
Boron Diiminate with Aggregation-Induced Emission and
Crystallization-Induced Emission-Enhancement Characteristics
Ryousuke Yoshii, Amane Hirose, Kazuo Tanaka, and Yoshiki Chujo*[a]
the other hand, we have reported that boron ketoiminates
Abstract: Organoboron complexes having aggregation-
were AIE-active materials, and their AIE properties were strong-
and crystalization-induced emission properties are pre-
ly affected by the boron-chelating ring, which includes
sented. The series of boron diiminates were synthesized
a boron-nitrogen (BꢀN) bond.[10]
and the emission behaviors of the synthesized com-
pounds were investigated. Finally, it was found that signif-
icant enhancement of the emission depended on the crys-
tallinity in the solid states.
Solid-state fluorescent organic dyes have attracted a great deal
Scheme 1. Boron diketonate derivatives.
of attention because of their fundamental importance and ap-
plicability in various modern technologies, such as organic
light-emitting diodes,[1] photodynamic therapy,[2] and fluores-
Herein, we focus on boron diiminate derivatives, which are
cent sensors.[3] Although the emission of fluorescent organic
an analogue to boron diketonate, as a new class of AIE- and
dyes is usually quenched in solid state by aggregation-caused
quenching (ACQ), effect due to the formation of delocalized
excitons or excimers,[4] several molecules, which exhibit aggre-
gation-induced emission (AIE) property were first reported by
Tang and co-workers in 2001.[5] In addition, Tang et al. have
also presented some fluorescent organic dyes with crystalliza-
tion-induced emission-enhancement (CIEE) characteristics:[6]
the strong emission of p-conjugated system can be induced
efficiently only in the crystal states. This behavior is exactly op-
posite to the common ACQ effect of usual chromophores.
Thus, CIEE-active molecules are promised to be multistimuli-re-
sponsive materials, such as thermal and vapor sensors. Howev-
er, only limited number of CIEE-active organic compounds has
been discovered to date. Therefore, an exploration in new CIEE
chromophores is still of great interest.
CIEE-active organoboron complexes, because boron diiminates
have a boron-chelating ring containing two BꢀN bonds
(Scheme 1). Furthermore, comparing to boron diketonates and
boron ketoiminates, there is room in boron diiminates for func-
tionalization deriving from their two nitrogen atoms, which
can bond various substituents. Because the regulation of inter-
molecular interactions in solid state is very important to devel-
op a CIEE property, boron diiminates with high functionality
have a high potential as CIEE-active materials, although only
a few examples of the synthesis and the optical properties of
boron diiminate have been reported.[11]
Herein, we describe the efficient synthesis, the photoproper-
ties of boron diiminate derivatives, and the verification of their
AIE properties. They exhibited not only AIE characteristics but
also CIEE characteristics (in THF: quantum yield of photolumi-
nescence FPL <0.01, in the crystal states: FPL =0.11–0.23, in
the amorphous states: FPL =0.02), resulting from suppression
of molecular motions involving boron-chelating rings and dif-
ference of the packing structure in the solid states. In addition,
we demonstrate the reversible regulations of optical properties
by simple external stimuli, such as fuming/heating or heating/
cooling cycles.
Boron diketonate derivatives are an important class of orga-
noboron dyes due to their prominent photoproperties.[7] Fraser
and co-workers have found prominent fluorescence and room-
temperature phosphorescence from boron diketonates[8a,b] and
further intriguing reversible mechanochromic fluorescence
from boron avobenzone between solid and melt states.[8c] Pre-
viously, we have also embarked on a keynote aimed at prepar-
ing high-luminescent materials based on boron diketonate de-
rivatives.[9] However, in the solid state, the emissions of those
boron diketonate moieties usually decreased through the ACQ
effect caused by intermolecular p–p stacking interaction. On
To synthesize 1,3-diaryldiimine derivatives, the reaction of N-
phenylimine
1 and imidoyl chloride 2 was employed
(Scheme 2). As a result, the diimine derivatives were successful-
ly obtained. Finally, the desired boron diiminates (4a and b)
were prepared by boron complexation by using BF3·OEt2. It
was proposed that this synthetic strategy should be valid for
the preparation of boron diiminates with bulky groups at the
nitrogen atoms.
[a] R. Yoshii, A. Hirose, K. Tanaka, Prof. Y. Chujo
Department of Polymer Chemistry, Kyoto University
Katsura, Nishikyo-ku, Kyoto 615-8510, (Japan)
E-mail: chujo@chujo.synchem.kyoto-u.ac.jp
The optical properties of 4a and b were investigated by
UV/Vis absorption spectroscopy in acetonitrile (concentration
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/chem.201402946.
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Scheme 2. Synthetic route of boron diiminate 4.
(c)=1ꢁ10ꢀ5 m, Figure S4 in the Supporting Information). Com-
pound 4a showed strong absorption corresponding to p !p*
transition in the longer wavelength region (labs =376 nm in
THF and 368 nm in acetonitrile) than that of 4b. The absorp-
tion behavior implies that 4a has a higher degree of p conju-
gation than that of 4b due to the substituted four phenyl
units on 4a. The AIE properties of the boron diiminates were
evaluated by the measurements of the dependence of the op-
tical properties on solvent compositions with acetonitrile/H2O
mixture system (c 5ꢁ10ꢀ5 m). Slight changes in the UV/Vis ab-
sorption spectra of 4a and b were observed upon addition of
H2O up to 80 vol%. In the spectra, level-off tail gradually ap-
peared in the visible region, and the decrease in the absorb-
ance was obtained as increasing the content of H2O above
80 vol%. In addition, the white turbidity appeared in the
sample (the content of H2Oꢁ80 vol%). These data suggest
that the aggregation should occur up to 80 vol% of the water
content. The peak shifts to the long-wavelength region were
observed from both samples by increasing water contents. The
strong electronic interaction through p stacking in the aggre-
gates should be responsible for these alterations.
Figure 1. Dependence of a) PL spectra and b) emission-intensity ratio of 4a
on solvent compositions of the acetonitrile/H2O mixture (c=5ꢁ10ꢀ5 m)
upon excitation at the peak position in the absorption spectra.
The PL spectra of 4a and b exhibited drastic increases in the
emission intensities as the formation of aggregates (H2Oꢁ80
vol%, Figure 1a and Figure S5 in the Supporting Information).
Figure 1b shows the dependency of the emission-intensity
ratio (I/I0) of 4a on solvent compositions with the acetonitrile/
H2O mixture system (c=5ꢁ10ꢀ5 m). The I/I0 values in the low
proportion of H2O (<80 vol%) were suppressed at the low
level (I/I0 ꢂ1). The I/I0 values significantly increased by much in-
creasing the water content over 80 vol% (4a: I99/I0 152, 4b: I99/
I0 52). The optical properties of 4a and b are summarized in
Table 1. The peak positions of the PL maxima (lPL) showed
good correlations with the order of their absorption maxima.
That is, the PL maxima of 4a exhibited larger bathochromic
shifts than that of 4b. These data indicate efficient extension
of p conjugation in 4a. The boron diiminate derivatives
showed much higher quantum yields in the crystal states (4a:
FPL 0.23, 4b: FPL 0.11) than those in THF (FPL <0.01). These re-
sults strongly indicate that the boron diiminate derivatives are
apparent AIE-active materials. Peak shifts to the long-wave-
length region were also observed by adding water similarly as
Table 1. Optical properties of 4a and b.[a,b]
[c]
[d]
FPL FPL
[e]
labs.(THF) [nm][c] lPL [nm][c] lPL [nm][d] lPL [nm][e] FPL
4a 376
4b 355
–
–
473
448
547
478
<0.01 0.23 0.02
<0.01 0.11 0.02
[a] Excited at labs. [b] Determined by using integrated sphere method.
[c] Measured in THF (c=1ꢁ10ꢀ5 m). [d] Measured in crystalline states.
[e] Measured in amorphous states.
shown in the absorption spectra. Strong p stacking should
induce these alterations.
To ensure the contribution of the boron diiminate rings to
the AIE properties, the PL properties of 4a were compared
with 1,2,4,5-tetraphenylbenzene, which has similar structure to
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4a (Figure S6 in the Supporting Information). Although the PL
intensity of 4a increased 152-fold as the formation of aggre-
gates (H2Oꢁ99 vol%), the PL intensity of 1,2,4,5-tetraphenyl-
benzene was hardly changed by the formation of aggregates
(H2Oꢁ99 vol%, I99/I0 ꢂ1). It is implied that the boron-chelating
ring in the boron diiminate plays an important role in the AIE
property.
points (4a: Tm 2148C, 4b: Tm 1738C). The amorphous samples
showed glass-transition points and strong endothermic peaks
as melting points (4a: Tg 688C, 4b: Tg 388C) and exothermic
peaks assigned to a crystallization point (4a: Tc 1038C, 4b: Tc
1038C), respectively. These results show that the crystal and
amorphous samples, as expected, form crystal and amorphous
states, respectively.
Next, the dependencies of the optical properties of the
boron diiminates on solvent viscosity and temperature were
examined (Figures S7 and S8 in the Supporting Information).
The PL intensities of boron diiminates dramatically increased in
the highly viscous solvent (ethanol/glycerol 1:4) than that in
the lower-viscous solvent (ethanol). Moreover, in 2-methyltetra-
hydrofuran (2Me-THF), the PL intensity values were much
higher at low temperature (77 K) than that at room tempera-
ture (298 K). These data suggest that the enhanced emission
should be less influenced by the environmental changes, such
as polarity in aggregation states and formation of excimer. Be-
cause high viscosity or low-temperature environments can
suppress molecular motions, such as torsion and vibration,[5c,f]
the emission enhancement in the solid states would be caused
by the prevention of the molecular motions, especially in the
boron-chelating ring.
Their PL properties of each sample were examined (Fig-
ure 2c and d). In the PL spectra, a strong emission was ob-
served from the crystal samples of 4a and b (4a: lPL 473 nm,
FPL 0.23, 4b: lPL 448 nm, FPL 0.11). On the other hand, the
amorphous samples showed weaker emission in longer wave-
length regions (4a: lPL 547 nm, FPL 0.02, 4b: lPL 478 nm, FPL
0.02) than those from the crystal samples. From these results,
it is clearly indicated that the boron diiminates are not only
AIE active, but also CIEE active. These data suggest that the
amorphous samples could form excimers through p–p stack-
ing of phenyl units, which often cause redshifts of PL spectra
and quenching.[6]
To explain the difference of the PL behaviors between crystal
and amorphous states, the molecular conformations in the
single crystals were investigated by X-ray analysis (Figures S9
and S10 and Tables S1 and S2 in the Supporting Information;
CCDC-991936 (4a) and CCDC-991937 (4b) contain the supple-
During the sample preparation, it was found that the solid
sample of 4a exhibited uneven
emission pattern under the UV
irradiation. To explore this be-
havior, the relationship between
their morphologies and the
emission properties in the solid
state were investigated. Thereby,
two types of solid samples were
prepared; crystal sample and
amorphous sample. The crystal
sample was grown from MeOH,
and the amorphous sample was
prepared by rapidly quenching
its melt in
a refrigerator at
ꢀ208C.[6a,b] Initially, the powder
X-ray diffraction (XRD) and differ-
ential scanning calorimetry (DSC)
with both samples were per-
formed. The XRD results of the
crystal samples of 4a and b ex-
hibited sharp and intense reflec-
tion patterns (Figure 2a). These
data indicate that these samples
involve regulation structures. In
contrast, less significant reflec-
tions were observed in the
amorphous samples. Hence,
these samples should be amor-
phous. Furthermore, from the
DSC measurements (Figure 2b),
the crystal samples of 4a and
b showed a strong endothermic
Figure 2. a) XRD patterns and b) DSC curves of the boron diiminates in the amorphous states and the crystalline
peaks corresponding to melting states. PL spectra of c) 4a and d) 4b in the amorphous states and the crystalline states.
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mentary crystallographic data for this paper. These data can be
obtained free of charge from The Cambridge Crystallographic
Data Centre via www.ccdc.cam.ac.uk/data_request/cif). In the
packing structures of 4a and b, less p–p interactions were ob-
served between their phenyl rings. The molecular conforma-
tion in the crystal is stabilized and locked by the multiple
CH···F hydrogen bonds, leading to the larger quantum yields of
the crystal samples.
Finally, a reversible control of the PL properties was execut-
ed by using a heating/cooling process. After the amorphous
sample was heated above their crystallization points, the
heated samples showed blueshifted and stronger emission in
comparison with the parent samples resulting in crystallization
(Figure S13 in the Supporting Information). Furthermore, the
crystallized samples were converted into the amorphous sam-
ples according to the same method in the fuming/heating
cycle. The PL properties were also returned to those of the
parent sample after the cooling process. These switching of
4b through fuming/heating and heating/cooling cycles can be
repeated many times (Figure 3).
The results of the time-resolved fluorescence measurement
also support the speculation (Table 2). Although the fluores-
cence decay of the crystal sample of 4a at 470 nm was found
In summary, we have designed the boron diiminate deriva-
tives and revealed that they possess not only AIE, but also
CIEE properties. The behavior can be explained by the molecu-
lar motions of the boron-chelating rings and the phase transi-
Table 2. Results of PL lifetime measurements for 4a and b.
lPL
[nm][a]
t (crystalline state)
[ns]
lPL
[nm][b]
t (amorphous state)
[ns]
t=1.9 (100.0%)
c=1.06
t1 =1.2 (30.9%)
t2 =3.2 (69.1%)
c=1.09
t1 =0.4 (30.3%)
t2 =1.3 (69.7%)
c=0.92
4a 470
538
490
t1 =0.8 (81.2%)
t2 =2.4 (18.8%)
c=1.08
4b 448
[a] Monitored wavelength in the time-resolved fluorescence measure-
ment for crystalline samples. [b] Monitored wavelength in the time-re-
solved fluorescence measurement for amorphous samples.
as a single exponential decay (fluorescence lifetime (t) 1.9 ns),
the amorphous sample exhibited a double-exponential fluores-
cence decay at l=538 nm: t1 =1.2 ns (31%) and t2 =3.2 ns
(69%). It is implied that the excited state of amorphous
sample decays through two pathways. The fast pathway
should be identical to the intrinsic decay in the crystal ones in-
cluding intermolecular energy transfer. The slow dominant
pathway could be assigned to the decay involved excimers
states.[12] Compound 4b also showed similar trend to 4a. That
is, the slow pathway becomes dominant in the amorphous
sample. These results propose that the weaker and redshifted
emission of the amorphous samples is strongly affected by the
formation of excimers.
The repeated switching of their PL properties was performed
by fuming/heating and heating/cooling cycles, because these
processes have a potential to change morphologies of organic
dyes. At the fuming/heating cycle, the amorphous samples of
4a and b coated on quartz cell were placed in a small contain-
er saturated with CH2Cl2 vapors for 0.5 h. The fumed samples
of 4b showed blueshifted and stronger emissions than those
of the parent samples. It was confirmed by the XRD measure-
ments that morphologies were changed from amorphous
states to crystalline states by the vapor-fuming process (Fig-
ure S11 in the Supporting Information). The fumed samples
were heated above the melting points and then quickly cooled
in the freezer (ꢀ208C) to prepare the amorphous sample
again. The PL properties of the heated samples thus exhibited
almost same PL properties to the parent amorphous samples
(Figures S12 and S13 in the Supporting Information).
Figure 3. Repeated switching of the of 4b between amorphous and crystal-
line states by a) fuming-heating and b) heating-cooling cycles upon.
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This study was partially supported by the Shorai Foundation
For Science and Technology (for K.T.) and Technology Agency
(JST), and a Grant-in-Aid for Scientific Research on Innovative
Areas “New Polymeric Materials Based on Element-Blocks (No.
2401;” 24102013) of The Ministry of Education, Culture, Sports,
Science, and Technology, Japan.
Keywords: aggregation
·
boron
·
crystallization-induced
emission enhancement · diimines · sensors
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Published online on June 2, 2014
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