Highly emissive boron ketoiminate derivatives as a new class of aggregation-induced emission fluorophores

Article abstract of DOI:10.1002/chem.201203703

A series of boron ketoiminate derivatives that exhibited clear aggregation-induced emission (AIE) characteristics (in THF: Φ _PL≤0.01; in the solid state: Φ_PL=0.30-0.76) were prepared by the reactions of 1,3-enaminoketone derivatives with boron trifluoride-diethyl etherate. The structures and optical properties were investigated by UV-visible spectroscopy, photoluminescent (PL) spectroscopy, and X-ray single-crystal measurements. These results indicate that the AIE characteristics were derived from molecular motions of the boron-chelating rings with a boron-nitrogen (B-N) bond. Furthermore, the optical properties were controllable by steric hindrance of the substituted groups on the nitrogen atom. Copyright

Full text of DOI:10.1002/chem.201203703

DOI: 10.1002/chem.201203703
Highly Emissive Boron Ketoiminate Derivatives as a New Class of
Aggregation-Induced Emission Fluorophores
Ryousuke Yoshii,[a] Atsushi Nagai,[b] Kazuo Tanaka,[a] and Yoshiki Chujo*[a]
Abstract: A series of boron ketoimi-
nate derivatives that exhibited clear ag-
gregation-induced emission (AIE)
characteristics (in THF: FPL ꢀ0.01; in
the solid state: FPL =0.30–0.76) were
prepared by the reactions of 1,3-enami-
noketone derivatives with boron tri-
fluoride–diethyl etherate. The struc-
tures and optical properties were inves-
tigated by UV-visible spectroscopy,
photoluminescent (PL) spectroscopy,
and X-ray single-crystal measurements.
These results indicate that the AIE
characteristics were derived from mo-
lecular motions of the boron-chelating
ꢁ
ꢁ
rings with a boron nitrogen (B N)
bond. Furthermore, the optical proper-
ties were controllable by steric hin-
drance of the substituted groups on the
nitrogen atom.
Keywords: aggregation
· boron ·
fluorophores · ketoiminates · lumi-
nescence
Introduction
duced to efficiently emit fluorescence when molecular ag-
gregates occur in poor solvents or in the solid state. This be-
havior is exactly the opposite of the common ACQ effect of
conventional chromophores. Only a limited number of or-
ganic compounds with AIE or AIEE properties have been
recently reported.[4] Therefore, the exploration of new AIE
(or AIEE) chromophores is still of great interest to realize
highly emissive solid materials.
Fluorescent organic materials have received a great deal of
attention on account of their potential applications as organ-
ic light-emitting diodes (OLEDs) due to their low manufac-
turing cost and tunable electronic properties.[1] The enhance-
ment of the emission of these materials in the solid state is
especially important for such applications because these ma-
terials are commonly used as thin films in the devices. Al-
though fluorescent organic materials usually exhibit strong
fluorescence in the dilute solution states, most of their emis-
sions in the solid states or in the highly concentrated solu-
tion states are quenched by chromophore aggregation.
These quenching processes are known as aggregation-caused
quenching (ACQ) on account of the formation of delocal-
ized excitons or excimers.[2] However, the aggregation can
positively restrict the intramolecular rotations that often
cause nonirradiative decay. Therefore, several molecules can
exhibit strong emission when a molecular aggregate occurs
in poor solvents or in the solid state. This property is called
aggregation-induced emission (AIE) or aggregation-induced
emission enhancement (AIEE). AIE or AIEE materials
were first reported by Tang et al. in 2001.[3] p-conjugated
molecules that are nonemissive in the solution state are in-
Boron diketonate derivatives that involve boron ketoimi-
nate are an important class of organoboron dyes because
they possess prominent photoproperties such as large molar
absorption coefficients and high fluorescence quantum
yields.[5] Fraser et al. have reported that boron diketonate
derivatives exhibit prominent fluorescence, room-tempera-
ture phosphorescence, and further intriguing reversible me-
chanochromic fluorescence between the solid and the melt
states.[6a–c] Previously, we have also embarked on a keynote
aimed at preparing highly luminescent materials that are
based on boron diketonate derivatives.[7] Although these
boron diketonate derivatives exhibited strong emissions in
the solution states, their emissions in the solid states were
usually quenched by means of ACQ effects that resulted
from intermolecular p–p stacking interactions. Boron ketoi-
minate is an analogue to boron diketonate, which has an
asymmetric structure on the border of a boron atom. Recent
investigations of boron ketoiminates have been carried out
in the realm of structural chemistry[8] and reaction chemis-
try.[9] In particular, from the recent work, it was implied that
boron ketoimine could be a promising structure for receiv-
ing AIE or AIEE properties. Gardinier et al. have reported
[a] R. Yoshii, Dr. K. Tanaka, Prof. Y. Chujo
Department of Polymer Chemistry
Kyoto University, Katsura, Nishikyo-ku
Kyoto 615-8510 (Japan)
Fax : (+81)75-383-2605
E-mail: chujo@chujo.synchem.kyoto-u.ac.jp
the synthesis of boron ketoACTHNURGTNE(NUG aryl)iminates and their optical
behaviors.[10] The emissions of these boron compounds were
strong in the solid states, and they became weak in the solu-
tion states. This phenomenon compelled us to explore the
potential of new AIE-active materials. However, informa-
tion on their AIE properties such as emission quantum
[b] Dr. A. Nagai
Department of Materials Molecular Science
Institutes for Molecular Science, Higashiyama
Okazaki, Aichi, 444-8787 (Japan)
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.201203703.
4506
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Chem. Eur. J. 2013, 19, 4506 – 4512

FULL PAPER
31.51 ppm; HRMS (ESI): m/z calcd: 298.1443 for [M+H]+; found:
298.1442.
yields in solution and the solid state has not yet been deter-
mined. There is still much room to understand these proper-
ties. We describe herein the synthesis and the photoproper-
ties of boron ketoiminate derivatives that have hydrogen or
alkyl substituents on the nitrogen atom, and the verification
of their AIE properties. They exhibited efficient AIE char-
Synthesis of 1c: Compound 1c was prepared from 2’ and n-isopropyla-
mine according to the method for 1b (transparent solid, 59%). 1H NMR
ꢁ
(CDCl3): d=11.20 (d, J=9.28 Hz, 1H; NHCH
G
ꢁ
8.80 Hz, 2H; Ar H), 7.36 (d, J=8.80 Hz, 2H; Ar H), 6.96 (d, J=
ꢁ
ꢁ
8.80 Hz, 2H; Ar H), 6.89 (d, J=8.80 Hz, 2H; Ar H), 5.65 (s, 1H; CH=
C<), 3.86 (s, 3H; Ar OCH3), 3.84 (s, 3H; Ar OCH3), 3.67 (m, 1H;
ꢁ
ꢁ
acteristics (in THF: FPL ꢀ0.01; in the solid state: FPL
=
ꢁ
NHCH
(CH3)2), 1.20 ppm (d, J=6.60 Hz, 6H; NHCH
0.30–0.76) that resulted from molecular motions of boron-
(CDCl3): d=187.04, 165.26, 161.57, 160.27, 133.10, 128.91, 128.76, 128.67,
113.81, 113.32, 92.91, 55.38, 55.33, 46.10, 24.33 ppm; HRMS (ESI): m/z
calcd: 326.1756 [M+H]+; found: 326.1736.
ꢁ
ꢁ
chelating rings with a boron nitrogen (B N) bond. Addi-
tionally, we demonstrate that the optical properties can be
tuned by steric hindrance of the substituted groups on the
nitrogen atom.
Synthesis of 2a: BF3·Et2O (0.87 mL, 7.1 mmol) was added to a solution
of 1a (1.0 g, 3.5 mmol) in CH2Cl2 (50 mL). The reaction mixture was stir-
red under a nitrogen atmosphere at reflux temperature for 24 h and then
poured into water (100 mL). The organic layer was washed with water
(100 mLꢁ2). The combined organic layer was dried over magnesium sul-
fate and was concentrated by a rotary evaporator. The obtained residue
was purified by silica gel chromatography (hexane/AcOEt 3:1). The
product was recrystallized from isopropanol/acetone (3:1 v/v) to give
pure 2a as a pale yellow crystal (0.27 g, 23%). 1H NMR (CDCl3): d=
Experimental Section
Instrumentation: 1H (400 MHz), 13C (100 MHz), and 11B (128 MHz)
ꢁ
ꢁ
8.01 (d, J=9.04 Hz, 2H; Ar H), 7.70 (d, J=8.80 Hz, 2H; Ar H), 7.13
NMR spectra were recorded using JEOL JNM-EX400 spectrometers. In
ꢁ
ꢁ
ꢁ
(brs, 1H; NHBF2 ), 7.05 (d, J=9.04 Hz, 2H; Ar H), 6.97 (d, J=
1
the H and 13C NMR spectra tetramethylsilane (TMS) was used as an in-
ꢁ
ꢁ
ꢁ
9.04 Hz, 2H; Ar H), 6.44 (s, 1H; CH=C<), 3.91 (s, 3H; Ar OCH3),
ternal standard in CDCl3, and 11B NMR spectra were referenced exter-
nally to BF3·OEt2 (sealed capillary). UV-visible absorption spectra were
recorded using a Shimadzu UV-3600 spectrophotometer. Fluorescence
emission spectra and absolute quantum yields were recorded using a
Horiba Jobin–Yvon Fluoromax-4P spectrofluorometer. X-ray crystallo-
graphic analysis was carried out using a Rigaku R-AXIS RAPID-F
graphite-monochromated MoKa radiation diffractometer with an imaging
plate. A symmetry-related absorption correction was carried out by using
the program ABSCOR.[11] The analysis was carried out with direct meth-
ods (SHELX97[12] or SIR92)[13] using Yadokari-XG.[14] The program
ORTEP35 was used to generate the X-ray structural diagram.[15]
3.89 ppm (s, 3H; Ar OCH3); 13C NMR (CDCl3): d=172.99, 168.43,
ꢁ
163.35, 163.15, 129.56, 128.41, 126.11, 125.92, 114.81, 113.97, 90.32, 55.59,
55.46 ppm; 11B NMR (CDCl3): d=0.68 ppm; HRMS (ESI): m/z calcd:
354.1089 [M+Na]+; found: 354.1091; elemental analysis calcd (%) for
C17H16BF2NO3: C 61.66, H 4.87, N 4.23; found: C 61.75, H 4.63, N 4.25.
Synthesis of 2b: Compound 2b was prepared from 1b (0.7 g, 2.2 mmol)
according to the method for 2a. The product was recrystallized from
CH2Cl2/hexane (1:3 v/v) to give pure 2b as a pale yellow crystal (80 mg,
1
ꢁ
11%). H NMR (CDCl3): d=7.90 (d, J=9.02 Hz, 2H; Ar H), 7.37 (d,
ꢁ
ꢁ
J=9.02 Hz, 2H; Ar H), 7.03 (d, J=8.77 Hz, 2H; Ar H), 6.92 (d, J=
9.02 Hz, 2H; Ar H), 6.02 (s, 1H; CH=C<), 3.89 (s, 3H; Ar OCH3),
ꢁ
ꢁ
ꢁ
Materials: Difluoroboron b-diketonate derivative (2’) was synthesized
from 1,3-bis(4-methoxyphenyl)-1,3-propanedione according to the report-
ed method.[5c] 1,3-Bis(4-methoxyphenyl)-1,3-propanedione (Tokyo Kasei
Kogyo, Co.) and boron trifluoride diethyl etherate (BF3·OEt2, Aldrich
Chemical, Co.) were used as received.
3.86 (s, 3H; Ar OCH3), 3.23 ppm (s, 3H; NCH3BF2 ); 13C NMR
(CDCl3): d=170.80, 168.96, 162.90, 161.30, 129.22, 129.05, 126.56, 125.74,
114.29, 113.96, 94.96, 55.47, 55.44, 34.53 ppm; 11B NMR (CDCl3): d=
0.98 ppm; HRMS (ESI): m/z calcd: 346.1426 [M+H]+; found: 346.1429;
elemental analysis calcd (%) for C18H18BF2NO3: C 62.64, H 5.26, N 4.06;
found: C 62.51, H 5.32, N 4.07.
ꢁ
ꢁ
ꢁ
Synthesis of 1a: Ammonium formate (5.5 g, 88 mmol) was added to a
solution of 1,3-bis(4-methoxyphenyl)-1,3-propanedione (5.0 g, 18 mmol)
in ethanol (100 mL). The reaction mixture was stirred at reflux for 24 h,
and the solvent was removed by a rotary evaporator. The obtained resi-
due was dissolved in AcOEt (100 mL) and washed with water (150 mLꢁ
3). The organic layer was dried over magnesium sulfate and concentrated
by a rotary evaporator. The crude product was purified by silica gel chro-
Synthesis of 2c: Compound 2c was prepared from 1c (1.0 g, 3.1 mmol)
according to the method for 2a. The product was recrystallized from
CH2Cl2/hexane (1:5 v/v) to give pure 2c as a transparent crystal (0.25 g,
1
ꢁ
22%). H NMR (CDCl3): d=7.89 (d, J=8.80 Hz, 2H; Ar H), 7.28 (d,
ꢁ
ꢁ
J=8.55 Hz, 2H; Ar H), 7.02 (d, J=8.55 Hz, 2H; Ar H), 6.91 (d, J=
8.80 Hz, 2H; Ar H), 5.94 (s, 1H; CH=C<), 4.13 (brs, 1H; NCH-
ꢁ
ꢁ
ꢁ
matography (CH2Cl2) to give 1a as a pale yellow solid (3.11 g, 62%).
ꢁ
ꢁ
AHCTNUERTGGUN(NN CH3)2BF2), 3.88 (s, 3H; Ar OCH3), 3.85 (s, 3H; Ar OCH3), 1.39 ppm
1
ꢁ
H NMR (CDCl3): d=10.39 (brs, 1H; NH2), 7.93 (d, J=8.77 Hz, 2H;
(d, J=6.53 Hz, 6H; NCHACTHNUTRGNEUNG
(CH3)2BF2); 13C NMR (CDCl3): d=170.55,
ꢁ
ꢁ
ꢁ
ꢁ
Ar H), 7.58 (d, J=8.77 Hz, 2H; Ar H), 6.96 (d, J=8.53 Hz, 2H; Ar
168.75, 162.82, 160.67, 129.24, 127.96, 125.66, 125.63, 114.33, 113.90, 96.17,
55.46, 55.45, 52.32, 22.18 ppm; 11B NMR (CDCl3): d=1.08 ppm (t, J=
25.04 Hz); HRMS (ESI): m/z calcd: 396.1559 [M+Na]+; found: 396.1543;
elemental analysis calcd (%) for C20H22BF2NO3: C 64.37, H 5.94, N 3.75;
found: C 64.38, H 5.90, N 3.67.
ꢁ
ꢁ
H), 6.93 (d, J=8.77 Hz, 2H; Ar H), 6.09 (s, 1H; CH=C<), 5.38 (brs,
1H; NH2), 3.85 ppm (s, 6H; Ar OCH3); 13C NMR (CDCl3): d=188.73,
161.92, 161.80, 161.44, 133.16, 129.81, 128.98, 127.65, 114.23, 113.38, 90.78,
55.41, 55.33 ppm; HRMS (ESI): m/z calcd: 284.1287 [M+H]+; found:
284.1290.
ꢁ
ꢁ
CCDC-905565 (2’), 905566 (2a), 905567 (2b), and 905568 (2c) contain
the supplementary 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.
Synthesis of 1b: Methylamine (2m solution in THF, 4.5 mL, 9.0 mmol)
was added to a solution of 2’ (1.0 g, 3.0 mmol) in acetonitrile (150 mL).
After the reaction mixture was stirred at room temperature for 72 h, the
solvent was removed by a rotary evaporator. The obtained residue was
dissolved in CH2Cl2 (100 mL) and washed with water (150 mLꢁ3). The
organic layer was dried over magnesium sulfate and concentrated by a
rotary evaporator. The crude product was purified by silica gel chroma-
tography (hexane/AcOEt 3:1) to give 1b as a pale yellow solid (0.39 g,
Results and Discussion
44%). 1H NMR (CDCl3): d=11.25 (brs, 1H; NHCH3), 7.87 (d, J=
ꢁ
ꢁ
ꢁ
8.80 Hz, 2H; Ar H), 7.37 (d, J=8.55 Hz, 2H; Ar H), 6.97 (d, J=
Synthesis and characterization: The boron ketoiminate de-
rivatives 2a–c and boron diketonate derivative 2’ as a coun-
terpart were prepared by the reactions of the 1,3-enamino-
ketone derivatives (1a–c) or 1,3-diketone derivative with
ꢁ
ꢁ
ꢁ
8.55 Hz, 2H; Ar H), 6.90 (d, J=8.80 Hz, 2H; Ar H), 6.73 (s, 1H; CH=
ꢁ
ꢁ
C<), 3.86 (s, 3H; Ar OCH3), 3.84 (s, 3H; Ar OCH3), 2.93 ppm (d, J=
5.38 Hz, 3H; NHCH3); 13C NMR (CDCl3): d=187.13, 166.97, 161.54,
ꢁ
160.40, 133.08, 129.21, 128.71, 127.78, 113.81, 113.29, 92.82, 55.35, 55.30,
Chem. Eur. J. 2013, 19, 4506 – 4512
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4507

Y. Chujo et al.
Scheme 1. Synthetic route to boron ketoiminate 2a–c.
boron trifluoride–diethyl etherate according to Scheme 1.
are more disturbed than that of 2’ (i.e., HOMOs of 2a–c are
localized on oxygen atom side, unlike those of 2’, which ex-
pands throughout the three six-membered rings (Figure 2)).
Additionally, in the mixed solvent of THF/H2O (1:9 v/v),
c=5.0ꢁ10ꢁ5 m), the UV-visible absorption spectra of 2’ and
2a–c showed a leveled-off tail in the visible region and a de-
crease in the absorbance, which resulted from the forma-
tions of the aggregates of 2’ and 2a–c (see Figure 4a below
and Figure S1 in the Supporting Information).[17]
1
The structures were characterized by H, 13C, and 11B NMR
spectroscopy and elemental analysis (see the Supporting In-
formation). The methoxy groups were introduced into 2a–c
and 2’ to enhance their luminescence properties.[6b]
UV-visible absorption and photoluminescence (PL): The op-
tical properties of 2a–c and 2’ were investigated by UV-visi-
ble absorption spectroscopy in THF (c=1ꢁ10ꢁ5 m, Figure 1).
Figure 3 shows the photoluminescence (PL) spectra of
2a–c and 2’. The boron ketoiminates (2a–c) showed very
weak luminescence with emission maxima around 425 nm
Figure 1. UV-visible absorption spectra of 2a–c and 2’ in THF (1ꢁ
10ꢁ5 m).
Whereas 2’ showed strong absorption that corresponded to
the p!p* transition at labs =408 nm, 2a–c (labs =360–
365 nm) showed weaker absorptions and blueshifted spectra
than that of 2’. The absorption behaviors imply a lower
degree of p conjugation and smaller overlap between
HOMOs and LUMOs in 2a–c on account of steric hin-
drance of the substituent on the nitrogen atom and different
electronegativities between the oxygen and nitrogen atom.
This assumption can be supported by the results of DFT
calculations. The expansions of the p conjugation in 2a–c
Figure 2. Structures and molecular orbital diagrams for the LUMO and
HOMO of a) 2’, b) 2a, c) 2b, and d) 2c (B3LYP/6-31G(d)//B3LYP/6-
31G(d)).[16]
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Boron Ketoiminate Derivatives
FULL PAPER
0.36 nm). It presumably results from easier formation of a
delocalized exciton or excimer of 2’, which is the main cause
of the ACQ effect, than that of 2a–c due to the planar struc-
ture and the effectively expanded HOMO and LUMO of 2’
(Figure 2a). On the contrary, the boron ketoiminate deriva-
tives showed much higher quantum yields in the solid states
(lPL =0.30–0.76 nm) than those in THF (lPL ꢀ0.01 nm). Fur-
thermore, the quantum yields of boron ketoiminate deriva-
tives decreased as the steric hindrance of the substituent on
the nitrogen atom in the solid state increased. The trend of
their quantum yields is discussed below.
The AIE properties of 2a–c were confirmed by measure-
ment of the dependence of the optical properties on solvent
compositions of the THF/H2O mixture (1ꢁ10ꢁ4 m). The UV-
visible absorption spectra of 2a changed less upon addition
of H2O up to 80 vol%. The spectra gradually showed a lev-
eled-off tail in the visible region and the absorbance de-
creased as the content of H2O increased above 90 vol%. In
the solution with 99 vol% of water content, the shape of ab-
sorption bands was changed. In addition, white turbidity ap-
peared in the sample (when the content of H2O was
ꢂ90 vol%; Figure 4a). Furthermore, 2b and 2c also showed
almost same trends (Figures S3 and S4 (2b and 2c) in the
Supporting Information). These data suggest that the aggre-
gation could occur up to a water content of 90 vol%.
Although the PL spectra of 2’ exhibited a drastic redshift
and a decrease in the emission intensity as aggregates
formed (H2Oꢂ90 vol%; Figure S2 in the Supporting Infor-
mation), the PL spectra of 2a–c exhibited drastic redshifts
and an increase in the emission intensities as the aggregates
formed (H2Oꢂ90 vol%; Figure 4b (2a), Figures S3 and S4
(2b and 2c) in the Supporting Information). Figure 4c shows
the dependence of the relative quantum yields (FPL), which
were determined from the above optical properties on sol-
vent compositions by a relative method using 2,2-dimethy-
lanthracene as a reference,[18] on solvent compositions of the
THF/H2O mixture (1ꢁ10ꢁ4 m). The FPL values at low H2O
content (<90 vol%) were very low and hardly changed
(FPL ꢀ0.01). The FPL values significantly increased as the
water content increased over 90 vol%. The absolute FPL
value in the solid state was higher than the relative FPL
values in the aggregate state (H2Oꢂ99 vol%) since, as com-
monly observed, the absorptions of the relative FPL values
were overestimated due to the inclusion of strong light-scat-
tering of the aggregates and absorption of weakly emissive
2a in the solution state.[3e] These dependencies of
Figure 3. PL spectra of 2a–c and 2’ (5.0ꢁ10ꢁ5 m) in THF (solid line) and
THF/H2O (1:9) mixed solvent (dashed line) upon excitation at each ab-
sorption maximum.
upon excitation with each absorption maximum in THF (c=
5.0ꢁ10ꢁ5 m). Notably, the aggregation states of 2a–c in the
mixed solvent of THF/H2O (1:9 v/v) led to large increases in
their emission intensities and shifts of their emission spectra
to longer wavelength region derived from intermolecular in-
teractions in aggregation states. These results clearly indi-
cate that 2a–c are AIE-active molecules. Meanwhile, boron
diketonate 2’, which has almost the same structure as boron
ketoiminate except for replacement of the nitrogen atom
with an oxygen atom on the chelating unit, exhibited normal
PL properties (i.e., the strong blue emission of 2’ in THF
was almost lost through ACQ in the aggregation state).
These results suggest that the six-membered ring including
ꢁ
the B N bond should play an important role in developing
the AIE properties. In other words, the quenching of 2a–c
in the solution states would be less affected by rotational or
vibrational motions derived from the p-methoxyphenyl
units.
The optical properties of 2a–c and 2’ are summarized in
Table 1. Compared to the PL spectrum and the absolute
quantum yield of 2’ in THF (lPL =429 nm, lPL =0.91 nm),
which were measured by the integrated sphere method, the
solid sample of 2’ exhibited strongly redshifted PL spectra
and a decrease in the quantum yield (lPL =488 nm, lPL
=
the optical properties on the water content are vi-
sualized in the photographs in Figure 4d. In good
solvents (content of H2O=0 and 50 vol%), 2a
completely dissolved and displayed weak blue
emissions. However, when 2a was in the poor sol-
vent (content of H2O 99 vol%), the solution
showed turbidity and strong light-blue emission.
These results strongly indicate that the boron ke-
toiminate derivatives are apparent AIE-active ma-
terials.
Table 1. Optical properties of 2a–c and 2’.
In THF (1ꢁ10ꢁ5 m)[a]
In the solid state[a]
[b]
[c]
[d]
[d,e]
labs [nm]
lPL [nm]
FPL
lPL(agg) [nm]
lPL(solid) [nm]
FPL(solid)
2a
2b
2c
2’
365
363
360
408
426
424
425
429
0.01
<0.01
<0.01
0.91
453
439
431
479
471
470
443
488
0.76
0.42
0.30
0.36
[a] Excited at labs. [b] Determined using the integrated sphere method. [c] Measured in
THF/H2O 1:9 mixed solvent (5.0ꢁ10ꢁ5 m). [d] The solid samples were prepared to dry
the concentrated solutions of 2a–c and 2’ in CH2Cl2 on quartz plates at 408C. [e] De-
termined using the integrated sphere method.
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4509

Y. Chujo et al.
Figure 4. Dependence of a) UV-visible spectra, b) PL spectra, and c) quantum yields of 2a on solvent compositions of the THF/H2O mixture (1ꢁ10ꢁ4 m)
upon excitation at 365 nm. d) Photograph of 2a with solvent compositions of the THF/H2O mixture.
To prove that inhibition of the molecular motion is re-
sponsible for the emission from boron ketoiminate com-
plexes, we investigated the dependences of the optical prop-
erties of 2a on solvent viscosity and temperature (Figure 5).
The UV-visible absorption spectra of 2a were almost the
same between low-viscosity and high-viscosity solvents
(ethanol and ethanol/glycerol (1:4 v/v) mixed solvent). How-
ever, the PL intensity of 2a dramatically increased in highly
viscous solvent (ethanol/glycerol, 1:4 v/v) relative to that in
less-viscous solvent (ethanol; Figure 5a). Moreover, the PL
intensity of 2a in 2-methyltetrahydrofuran (2Me·THF) was
much higher at low temperature (77 K) than that at room
temperature (298 K). The PL spectrum at low temperature
includes phosphorescence with the peak around 500 nm and
strong vibrational structure that results in strong prevention
of nonradiative deactivation and molecular motion (Fig-
ure S8 in the Supporting Information). The PL maxima of
2a around 425 nm in the ethanol/glycerol mixed solvents or
2Me·THF were hardly influenced by the solvent viscosity or
temperatures. Furthermore, 2b and 2c also showed almost
the same trends (Figures S5–S7 in the Supporting Informa-
tion). These data imply that the enhanced emission of 2a–c
could not originate from the environmental changes such as
polarity in aggregation states and formation of an excimer.
The PL intensities increased as solution viscosity increased
or temperature decreased without formation of the aggre-
gates. It is well known that high viscosity or a low-tempera-
ture environment suppresses molecular motions such as tor-
sion and vibration.[3c,f] Therefore, these results strongly sug-
gest that the low emissions of boron ketoiminate derivatives
in the solution states were derived from quenching through
the molecular motions of the boron-chelating ring.
X-ray crystal structures of 2a–c: The X-ray crystal structures
of 2a–c and 2’ are shown in Figure 6. The results clearly re-
vealed that the structures of 2a–c have a larger contribution
from the enolimine skeleton than that from the enaminoke-
ꢁ
tone one. Specifically, they have N1 C2 (1.317 (2a), 1.319
ꢁ
(2b), 1.324 ꢂ (2c)) and C1 C10 (1.366 (2a), 1.371 (2b),
1.371 ꢂ (2c)) bonds with partial double-bond character, and
ꢁ
ꢁ
O1 C10 (1.328 (2a), 1.324 (2b), 1.316 ꢂ (2c)) and C1 C2
(1.418 (2a), 1.425 (2b), 1.417 ꢂ (2c)) bonds with partial
4510
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Chem. Eur. J. 2013, 19, 4506 – 4512

Boron Ketoiminate Derivatives
FULL PAPER
Figure 5. a) UV-visible absorption and PL spectra of 2a in ethanol
(dashed line, 1.0ꢁ10ꢁ5 m) and ethanol/glycerol 1:4 (v/v) mixed solvents
(solid line, 1.0ꢁ10ꢁ5 m) upon excitation at 365 nm. b) PL spectra of 2a in
2Me·THF (1.0ꢁ10ꢁ5 m) at 77 K (solid line) and at 298 K (dashed line)
upon excitation at 365 nm.
Figure 6. ORTEP drawings of a) 2’, b) 2a, c) 2b, and d) 2c, and crystal
packing of top view and front view.
chelating ring (5.0 ꢂ) in the synthesized boron ketoiminate
derivatives since the intermolecular p–p stacking was sup-
pressed by the poor planarity of 2c. Although a high degree
intermolecular p–p stacking usually produces a strong ACQ
effect, the trend of FPL of 2a–c is completely the opposite.
That is, the FPL values of 2a–c increase as the intermolecu-
lar p–p stacking increases. The result implies that, in the
case of boron ketoiminate, the increase in FPL derived from
the strong restriction of boron-chelating rings by the p–p
stacking overcomes the decrease in FPL derived from the
ACQ effects of the p–p stacking.
These results from X-ray analysis also clearly correspond
to their optical properties. The emission maxima in the PL
spectra from the solutions with 2a–c were observed at simi-
lar positions (lPL =424–429 nm). In contrast, the largely red-
shifted emission maxima were detected in the solid samples
single-bond character, which agree with the results of the
previous report.[8] Compared to 2’, the BF2 units on 2a–c are
ꢁ
ꢁ
located slightly off-center to the O N line, and the B1 N1
bonds (1.523 (2a), 1.541 (2b), 1.559 ꢂ (2c)) are longer than
ꢁ
the B1 O1 bonds (1.470 (2a), 1.468 (2b), 1.476 ꢂ (2c)).
These results mean that the boron-chelating rings on 2a–c
should be more physically active against vibrational or tor-
sional motions than that on 2’ because of the unsymmetrical
structures and the distortions of the boron-chelating rings.
Although the molecular structure of 2’ has high planarity
(dihedral angle of 2’: O1-C2-C3-C8=2.48), 2a–c showed less
planarity than that of 2’ (dihedral angle 2a: N1-C2-C3-C8=
20.78, 2b: N1-C2-C3-C8=47.18, 2c: N1-C2-C3-C8=75.98).
Furthermore, the planarity of 2a–c decreased as the steric
hindrance of the substituent on the nitrogen atom increased
(Figure 6b,c).
of the complexes except 2c (2a: lPL =471 nm; 2b: lPL
=
470 nm; 2c: lPL =443 nm). In addition, in the solid states, 2a
and 2b showed higher quantum yields than that of 2c.
These data suggest that the molecular motions of the boron-
chelating rings in 2a and 2b should be strongly suppressed
by intermolecular stacking between each boron-chelating
ring. In particular, the highly planar structure of 2a is favor-
able to the stacking formation at the boron-chelating rings,
thus leading to the larger quantum yields of the boron ketoi-
Crystal-packing structures of 2a–c and 2’ are also descri-
bed in Figure 6. The packing structures of 2’, 2a, and 2b
have short intermolecular distances of approximately 3.5 ꢂ
between each boron-chelating ring. The distances indicate
an effective intermolecular p–p stacking,[19] which usually
causes redshifts of PL spectra and quenching. Compound 2c
has the longest intermolecular distance between each boron-
Chem. Eur. J. 2013, 19, 4506 – 4512
ꢀ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
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Conclusion
A series of boron ketoiminate derivatives were prepared by
the reactions of the 1,3-enaminoketone derivatives with a
boron trifluoride–diethyl etherate. They exhibited clear AIE
properties (in THF: FPL ꢀ0.01; in the solid state: FPL
=
0.30–0.76), which resulted from rotational or vibrational mo-
tions of the boron-chelating rings of the boron ketoiminate
derivatives. Furthermore, the optical properties can be suc-
cessfully controlled by the steric hindrance of the substituted
groups on the nitrogen atom. To the best of our knowledge,
this report is the first to specify that four-coordinated boron
complexes have an inherently strong effect on the genera-
tion of AIE properties. The boron ketoiminate units can be
applied as a new building block of various AIE-active mate-
rials. Additionally, the boron ketoiminate derivatives with
prominent AIE properties have significant potential to be
efficient sensory and organic light-emitting diodes (OLED)
materials. We are currently attempting to synthesize novel
conjugated polymers with expanded p conjugation through
boron ketoiminate units.
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Acknowledgements
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Received: October 17, 2012
Published online: January 30, 2013
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Chem. Eur. J. 2013, 19, 4506 – 4512