Functionalization of boron diiminates with unique optical properties: Multicolor tuning of crystallization-induced emission and introduction into the main chain of conjugated polymers

Article abstract of DOI:10.1021/ja510985v

In this article, we report the unique optical characteristics of boron diiminates in the solid states. We synthesized the boron diiminates exhibiting aggregation-induced emission (AIE). From the series of optical measurements, it was revealed that the optical properties in the solid state should be originated from the suppression of the molecular motions of the boron diiminate units. The emission colors were modulated by the substitution effects (λ_PL,crystal = 448-602 nm, λ_PL,amorphous = 478-645 nm). Strong phosphorescence was observed from some boron diiminates deriving from the effects of two imine groups. Notably, we found some of boron diiminates showed crystallization-induced emission (CIE) properties derived from the packing differences from crystalline to amorphous states. The 15-fold emission enhancement was observed by the crystallization (φ_PL,crystal = 0.59, φ_PL,amorphous = 0.04). Next, we conjugated boron diiminates with fluorene. The synthesized polymers showed good solubility in the common solvents, film formability, and thermal stability. In addition, because of the expansion of main-chain conjugation, the peak shifts to longer wavelength regions were observed in the absorption/emission spectra of the polymers comparing to those of the corresponding boron diiminate monomers (λ_abs = 374-407 nm, λ_PL = 509-628 nm). Furthermore, the absorption and the emission intensities were enhanced via the light-harvesting effect by the conjugation with fluorene. Finally, we also demonstrated the dynamic reversible alterations of the optical properties of the polymer thin films by exposing to acidic or basic vapors.

Full text of DOI:10.1021/ja510985v

Article
pubs.acs.org/JACS
Functionalization of Boron Diiminates with Unique Optical
Properties: Multicolor Tuning of Crystallization-Induced Emission
and Introduction into the Main Chain of Conjugated Polymers
Ryousuke Yoshii, Amane Hirose, Kazuo Tanaka,* and Yoshiki Chujo*
Department of Polymer Chemistry, Graduate School of Engineering, Kyoto University, Katsura, Nishikyo-ku, Kyoto 615-8510, Japan
*
S Supporting Information
ABSTRACT: In this article, we report the unique optical characteristics of
boron diiminates in the solid states. We synthesized the boron diiminates
exhibiting aggregation-induced emission (AIE). From the series of optical
measurements, it was revealed that the optical properties in the solid state
should be originated from the suppression of the molecular motions of the
boron diiminate units. The emission colors were modulated by the
substitution effects (λPL,crystal = 448−602 nm, λPL,amorphous = 478−645 nm).
Strong phosphorescence was observed from some boron diiminates
deriving from the effects of two imine groups. Notably, we found some of
boron diiminates showed crystallization-induced emission (CIE) properties
derived from the packing differences from crystalline to amorphous states.
The 15-fold emission enhancement was observed by the crystallization
(
Φ
= 0.59, Φ
= 0.04). Next, we conjugated boron
PL,crystal
PL,amorphous
diiminates with fluorene. The synthesized polymers showed good solubility in the common solvents, film formability, and
thermal stability. In addition, because of the expansion of main-chain conjugation, the peak shifts to longer wavelength regions
were observed in the absorption/emission spectra of the polymers comparing to those of the corresponding boron diiminate
monomers (λabs = 374−407 nm, λ = 509−628 nm). Furthermore, the absorption and the emission intensities were enhanced
PL
via the light-harvesting effect by the conjugation with fluorene. Finally, we also demonstrated the dynamic reversible alterations
of the optical properties of the polymer thin films by exposing to acidic or basic vapors.
INTRODUCTION
solution states but significantly emissive in the aggregate
■
6
states. Boron ketoiminates were also found as AIE-active
Organoboron dyes and polymers are a versatile platform for
fabricating opto and/or electric devices because of their
superior properties such as light absorption, emission, high
7
molecules. In particular, we have recently presented that ACQ-
active organoboron dyes composed of the β-diketonate
structure can be transformed to the AIE-active dyes by
replacing one of the oxygen atoms in the β-diketonate ligand
1
stability, and the electron acceptability. For example, Jak
̈
le et
al. and Yamaguchi et al. have presented that organoboron dyes
and organoboron-containing polymers show the multiple
functions such as low-lying LUMOs, strong emissions, and
high-electron-transport properties, respectively. Fraser et al.
have reported the versatile properties of boron diketonates
involving prominent fluorescence, room temperature phosphor-
escence, and reversible mechanochromic fluorescence between
7a
to nitrogen to form the ketoiminate structure. In the solid
state, the molecular motions at the boron-chelating ring, which
induced the emission quenching, should be suppressed.
Thereby, significant emission was observed only from the
aggregate state of the boron ketoiminate dyes. We also reported
that the AIE behavior of boron ketoiminates can maintain in
the main chain of the polymer. As a result, we obtained the8
AIE-active polymers composed of the boron ketoiminate units.
Furthermore, their photoluminescence (PL) can be tuned by
2
,3
4
the solid and the melt states. So far, various smart optical
materials have been developed based on organoboron
complexes. Although most of the boron complexes exhibited
strong emissions in the solution states, their emissions in the
solid states were usually spoiled by intermolecular π−π stacking
interaction, called as aggregation-caused quenching (ACQ)
9
modulating the substituents on the nitrogen atom. These
observations suggest that the boron ketoiminate derivatives
should be a prominent solid-state emissive material and a useful
building block for constructing highly emissive conjugated
polymers.
Toward the next generations of AIE-active dyes and
polymers with further functionality, we focused on the
5
effects. The applications of organoboron dyes such as for
advanced light-emitting devices or sensor materials are still
limited by the ACQ effects.
Aggregation-induced emission (AIE) is a key phenomenon
for overcoming the ACQ effect and obtaining highly efficient
light-emitting solid materials. Tang et al. have reported a series
of π-conjugated molecules that were nonemissive in the
Received: October 25, 2014
©
XXXX American Chemical Society
A
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Chart 1. Boron Diiminate Derivatives
Scheme 1. Synthetic Routes for Compounds (a) 1 and (b) 3 and (c) the Boron Diiminates Derivatives
diiminate structure (Chart 1). Another functional group is
acceptable at the nitrogen atom. Therefore, it can be expected
that multiple functions are expressed due to the combination of
those substituents as well as each intrinsic property. Indeed, we
observed that the simple compounds of boron diiminates
10
showed AIE behaviors similarly to the previous complexes. In
addition, the crystallization-induced emission enhancement
(CIEE) property originated from the difference of the packing
B
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Scheme 2. Synthetic Routes for the Boron Diiminate-Containing Polymers
1
0
structure in the solid states was eventually obtained. Thus,
boron diiminates are potential units for receiving unique optical
properties. However, the synthesis and the evaluation of the
electronic structures of boron diiminate derivatives have been
with [9,9-bis(2-ethylhexyl)-9H-fluorene-2,7-diyl]bisboronic
acid and diiodo-substituted boron diiminates which were
prepared according to the same reactions as other boron
diiminates in the toluene water solvent mixture in the presence
of 2-dicyclohexylphosphino-2′,6′-dimethoxybiphenyl (S-Phos)
and tris(dibenzylideneacetone) dipalladium (Pd (dba) ,
Scheme 2). To improve the solubility of the products in
common organic solvents, their numbers of repeating unit were
limited to 20 by controlling the feed ratio of monomers. The
terminal units of the synthesized polymers were capped with p-
xylene units. While the synthesized polymers showed good
solubility in nonpolar solvents such as THF, o-xylene, toluene,
chloroform, and dichloromethane, they hardly dissolved in
polar solvents such as MeOH, EtOH, AcOH, and acetonitrile.
In addition, the homogeneous amorphous films were
successfully prepared from the synthesized polymers by casting
(Figure S4). The size exclusion chromatography (SEC) in
11
studied only in a few reports. Especially, the effects on the
conjugation with other functional units on the boron diiminates
have been still unclear yet. The detailed study and the
incorporation into the conjugation system should be required
to clarify the intrinsic properties of boron diiminates and to
apply for the optical materials.
Herein, we report the unique optical properties of the solid
materials based on the boron diiminate-containing conjugated
polymers. Initially, we prepared a series of boron diiminates and
studied systematically the relationship between the substituents
and their electronic structures. From the results of the optical
measurements, it was found that all of the synthesized boron
diiminates are AIE- and CIEE-active, and the emission colors of
the crystallization-induced emission (CIE) can be drastically
modulated from blue to orange by the alteration of the
substituents. The significant enhancement (approximately 15-
times) of the emission quantum yield was obtained by the
crystallization. Next, based on these data, we synthesized the
copolymers with the fluorene unit as a comonomer to offer the
further functionalities of optical characteristics of boron
diiminates through the main-chain conjugation. Accordingly,
boron diiminates-containing polymers showed strong absorp-
tion, higher emission efficiency, and red-shifted absorption/
emission bands compared to the corresponding boron
diiminates owing to the conjugation with fluorene. Moreover,
the synthesized polymers possess material properties such as
good solubility in common solvents, film-formability to form
homogeneous films, and thermal stability. Finally, we also
demonstrate the dynamic regulations of the optical properties
of the polymer film based on the modulation of the
electronegativity of the substituents using the acid−base
treatments.
2
3
CHCl with the polystyrene standards revealed the number-
3
average molecular weight (M ) and the molecular weight
n
distribution (M /M ) of 26,000 and 3.3 for P-H(Me), 13,200
w
n
and 2.9 for P-H(Ph), 17,000 and 3.1 for P-OMe, 19,400 and
3.6 for P-NO , and 17,900 and 3.2 for P-NMe , respectively
2
2
(Table 1). The chemical structures of the polymers were
12
a
Table 1. Physical Properties of Boron Fluorene Polymers
b
c
compound
Mn
Mw
M /Mn
w
n
yield (%)
P-H(Me)
P-H(Ph)
P-OMe
26,000
13,200
17,000
19,400
17,900
85,800
38,300
52,700
69,800
3.3
2.9
3.1
3.6
3.2
35
16
20
23
21
86
83
92
92
97
P-NO2
P-NMe2
57.300
a
Estimated by SEC with the polystyrene standards in CHCl .
3
b
Average number of repeating units calculated from M and molecular
n
c
weights of repeating units. Isolated yields after reprecipitation.
confirmed by H, 13C and 11B NMR spectroscopies. All the
purified compounds gave satisfactory spectroscopic data
corresponding to their expected molecular structures (Figure
S1−S3). The thermal properties of the synthesized compounds
were investigated by thermogravimetric analysis (TGA) and
differential scanning calorimetry (DSC). While the 5 wt %
weight loss temperatures (T ) of boron diiminates attributable
1
RESULTS AND DISCUSSION
■
4
Synthesis and Characterization. The diimine ligands
a−e with various substituents were synthesized via the
reaction of N-phenylimine 1 and imidoyl chloride 3 with the
corresponding substituents. The desired boron diiminates were
prepared from the compounds 4a−e by the boron-complex-
ation using BF ·OEt (Scheme 1). Next, we prepared the
5d
to the vaporization or the decomposition were observed at
266−298 °C, the polymers showed higher thermal stabilities
(T = 365−414 °C, Figure S5). In DSC measurements, the
3
2
copolymers with fluorene. The polymerization was accom-
plished by the palladium-catalyzed Suzuki−Miyaura coupling
5d
C
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−5
Figure 1. UV−vis absorption spectra of boron diiminates in (a) THF (1.0 × 10 M). PL spectra of the synthesized boron diiminates in (b) the
crystalline states and in (c) the amorphous states upon the excitation at each absorption maximum. (d) Photographs of boron diiminates in
crystalline and amorphous states under UV irradiation.
boron diiminates showed strong endothermic peaks as melting
points around 173−261 °C. In contrast, the synthesized
polymers hardly exhibited significant thermal transition peaks
in the measurement temperature range from 0 to 300 °C
Table 2. UV−vis Absorption and Electrochemical Properties
of Boron Diiminates
λabs
Eg
[eV]
E
HOMO
e
LUMO
red.
[V]
a
b
c
,
d
f
compound
[nm]
[eV]
[eV]
(
Figure S6 and Table S1). These observations strongly suggest
M-H(Me)
M-H(Ph)
M-OMe
M-NO2
355
376
377
391
365
3.12
2.91
2.87
2.83
2.54
−2.18
−1.95
−1.98
−1.62
−2.02
−5.75
−5.77
−5.70
−6.02
−2.63
−2.86
−2.83
−3.19
−2.79
that the copolymerization with the fluorene unit is an effective
approach to improve the thermal stability and film formability
of boron diiminates.
Optical and Electrochemical Properties of Boron
Diiminates. The optical properties of the synthesized boron
diiminates were investigated by a UV−vis absorption and PL
spectroscopies (Figure 1). In UV−vis absorption spectra of the
M-NMe2
−5.33
−5
a
b
UV−vis spectra were measured in THF (1.0 × 10 M). The optical
band gap estimated from the onset wavelength of the UV−vis spectra
in THF. CV was carried out in THF with 0.1 M Bu NPF as
c
4
6
−5
d
boron diiminates in THF (1 × 10 M, Figure 1a, Tables 2 and
), they showed the strong absorption attributable to π → π*
supporting electrolyte.
wave (Figures S7 and S8). Calculated from LUMO and optical band
E
is the onset potential of first reduction
red
e
3
f
gap (E ), HOMO = LUMO − E (eV). Calculated from the empirical
transition at λabs = 355−391 nm. The values of the optical band
gaps (E ) which were estimated from the onset wavelength of
g
g
1
3
formula, LUMO = −E − 4.80 (eV).
red
g
the UV−vis spectra were in the order of M-H(Me) > M-H(Ph)
>
M-OMe > M-NO > M-NMe . M-NMe showed broader
2 2 2
corresponding compounds. All compounds showed low-lying
LUMO energy levels in the region from −2.63 to −3.19 eV. It
is suggested that boron diiminates work as an electron acceptor
and red-shifted absorption than those of the others. It is
implied that donor−acceptor interaction could be formed
between the electron-donating amino unit and the electron-
in the conjugation system. The chelation with a boron atom
8
accepting boron chelating ring in M-NMe2.
should induce the lowering effect. Especially, M-NO exhibited
2
The electrochemical properties of the synthesized boron
the lowest-lying LUMO level than those of the other boron
diiminates. This result can be explained by the effect of the
nitro unit with the strong electron-accepting ability.
To theoretically support the optical and electrochemical
behaviors of boron diiminates, density functional theory (DFT)
calculations were carried out (Figure 2). The order of the
diiminates are summarized in Table 2. Their cyclic voltammo-
grams (CV) are shown in Figure S7. Boron diiminates showed
two or three reduction peaks. Their LUMO energy levels were
estimated from the onsets of the first reduction waves by the
1
3
empirical formula. Their HOMO energy levels were
calculated from LUMO energy levels and Eg of the
HOMO−LUMO band gaps (E ) and each energy level
g
D
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a
Table 3. Optical Properties of Boron Diiminates
the substituent binding to carbon atoms on the imine units
should more effectively influence the LUMO than HOMO. In
fact, although the HOMO of M-H(Ph) was hardly changed by
the substitution of the phenyl unit in comparison with M-
H(Me), the LUMO was largely reduced by the substitution of
benzene (Table 2 and Figure 2). These data indicate the
HOMO and LUMO energy levels of boron diiminates can be
efficiently modulated by the type of the substituents and the
4
λabs
ε × 10
λPL,cr
λPL,am
e
b
[M−1 cm−1]
c
d
d
,
f
,
ef
compound [nm]
[nm]
[nm]
ΦPL,cr
ΦPL,am
M-H(Me)
M-H(Ph)
M-OMe
M-NO2
355
376
377
391
365
2.26
3.27
2.55
2.79
2.19
448
473
470
509
602
478
547
564
568
645
0.11
0.23
0.59
0.04
0.08
0.02
0.02
0.04
0.01
0.01
M-NMe2
substitution positions.
a
b
−5
c
Excited at λ . Measured in THF (1 × 10 M). Molar absorption
coefficients of the absorption maxima at longer wavelength region.
Measured in crystalline states. Measured in amorphous states.
The PL spectra of synthesized boron diiminates (1 × 10−5
abs
d
e
M, Table 3) showed slight luminescence in THF (Φ < 0.01). In
contrast, crystallization led to large increases in their emission
intensities. These results clearly indicate that the synthesized
boron diiminates are AIE-active molecules. Next, the depend-
encies of the optical properties of the boron diiminates on
temperature were examined (Figures S9−S13). In 2-methylte-
trahydrofuran (2Me-THF), the emission intensities were much
higher at low temperature (77 K) than those at room
temperature (298 K). Since molecular motions such as torsion
and vibration should be greatly suppressed under low
f
Determined using integrated sphere method.
estimated from the DFT calculations showed similar trends to
the experimental data estimated from the UV−vis absorption
spectra and CV measurements. Furthermore, the existence of
the intramolecular donor−acceptor interaction in M-NMe was
2
proposed by the results from DFT calculations and CV
measurements. These data mean that the substitution of an
electron-donating amino unit led to the increase in the HOMO
and the localization of the HOMO on the amino-substituted
benzene. The LUMOs of the other boron diiminates (M-
6c
temperature environments, the emission enhancement was
observed at the low temperature. Furthermore, some boron
diiminates showed dual emission bands attributable to
fluorescence and phosphorescence at low temperature (Figures
S9−13). According to the El-Sayed rule, when a change in spin
multiplicity is accompanied by a change in electron
configuration, the spin−orbit coupling is very large, and
H(Me), M-H(Ph), M-NO ) were delocalized through whole
2
units. Meanwhile, the HOMOs were localized on a boron
chelating ring and two phenyl units binding to nitrogen atoms.
Thus, it is suggested that the energy levels of HOMO and
LUMO should be strongly affected by the substituent on two
phenyl units binding to the nitrogen atoms. On the other hand,
15
intersystem crossing is very rapid. In addition, Fraser et al.
have reported that boron diketonate derivatives including two
14
Figure 2. Structures and molecular orbital diagrams for the LUMO and HOMO of boron diiminates ((B3LYP/6-31G (d)//B3LYP/6-31G (d)).
E
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Figure 3. Molecular structures of (a) M-H(Me), (b) M-H(Ph), (c) M-OMe, (d) M-NO , and (e) M-NMe . Thermal ellipsoids are scaled to the
2
2
5
0% probability level.
4a−c
carbonyl groups showed phosphorescence.
Therefore, in
crystalline samples (Figure 1b,c and Table 3). Notably, the
this case, the existence of the lone-pair electrons on the two
imine groups could be responsible for the strong phosphor-
escence. These observations suggest applicability of boron
diiminates as phosphorescent materials and triplet sensitizers.
Next, the relationship between their morphologies and the
emission properties in the solid state was investigated. Two
types of solid samples were prepared. The crystal of boron
diiminate was grown from MeOH. The amorphous sample was
prepared by rapidly cooling the melted compound in a
crystallization of M-OMe induced approximately 15-fold larger
emission efficiencies from the amorphous state (Φ
=
PL,crystal
0.59, Φ
= 0.04). It is assumed that the formation of
PL,amorphous
excimers via π−π stacking of phenyl units, which often causes
red-shifts of PL spectra and quenching, could be absent at the
6e,f
crystalline state. From these results, it is clearly shown that
the synthesized boron diiminates are CIEE-active. Moreover,
the colors of the CIE can be tuned from blue to red by the
substituents.
6e,f
refrigerator at −20 °C. To examine the orientation of both
samples in the solid state, we performed the powder X-ray
diffraction (XRD). The XRD results of the crystal samples
exhibited sharp and intense reflection patterns (Figure S4).
These data indicate that the crystal samples involve ordered
structures. In contrast, less clear reflections were observed in
the amorphous samples. It was confirmed that the rapidly
cooled samples should form amorphous states. In the PL
X−Ray Crystal Structures of Boron Diiminates. The
molecular conformations in the single crystals were investigated
by X-ray analysis (Figures 3 and S14−S18 and Tables S2−S6).
It was revealed that the lengths of C1−C2 and C2−C3 bonds
in boron diiminates were hardly changed by the substituents.
This fact suggests that the π-electrons on the boron-chelating
ring could be delocalized in the diimine units regardless of the
substituents. In the packing structures of boron diiminates,
small degrees of π−π interactions were observed between their
phenyl rings. Since the boron diiminates showed shorter
intermolecular CH···F distances (2.31−2.65 Å) than that of the
sum of van der Waals radii (2.67 Å), the molecular
conformation in the crystal could be stabilized and locked by
spectra, the peak positions of the PL maxima (λ ) showed
PL
good correlation with the order of their E values. Strong
g
emission was observed from the crystalline samples. On the
other hand, the amorphous samples showed weaker emission in
longer wavelength regions than those from the corresponding
F
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Figure 4. UV−vis absorption spectra of the synthesized polymers in (a) THF (1.0 × 10− M) and (b) the thin-film state. (c) PL spectra of the
synthesized polymers in the thin-film state upon excitation at each absorption maximum. (d) Photographs of the synthesized polymers in the film
state under the UV irradiation.
5
the multiple CH···F hydrogen bonds (Figures S14−S18). It is
proposed that these interactions observed in the crystals could
induce the blue-shifted emissions and the larger quantum yields
of the crystal samples comparing to the amorphous samples.
Optical Properties of Boron Diiminate-Containing
Polymers. The UV−vis absorption spectra of the synthesized
Table 4. UV−vis Absorption and Electrochemical Properties
of the Polymers
λabs
[nm]
Eg
[eV]
E
HOMO
e
LUMO
red
[V]
a
b
c
,
d
f
compound
[eV]
[eV]
P-H(Me)
P-H(Ph)
P-OMe
P-NO2
374
399
396
407
392
2.99
2.73
2.71
2.64
2.53
−1.41
−1.48
−1.94
−1.51
−1.97
−6.34
−6.06
−5.58
−5.94
−5.37
−3.40
−3.33
−2.87
−3.30
−2.84
−5
polymers were recorded in THF (1 × 10 M, Figure 4a) and
in thin-film states (Figure 4b). In the solution states, they
showed red-shifted and stronger absorption bands assigned as
π−π* transitions comparing to the corresponding boron
P-NMe2
a
UV−vis spectra of CM, NM, CP, and NP were measured in THF
−
5
b
diiminates (M-H(Me), M-H(Ph), M-OMe, M-NO , M-
(1.0 × 10 M). The optical band gap estimated from the onset
wavelength of the UV−vis spectra in THF. CV was carried out in
THF with 0.1 M Bu NPF as supporting electrolyte. E is the onset
2
c
NMe ). These results indicate that the π-conjugated systems
2
d
were effectively elongated through the polymer main chains. In
the case of thin-film sates, the polymers showed broader
absorption than those in THF. It is suggested that interstrand
interaction should be enhanced in the thin-film states.
4
6
red
e
potential of first reduction wave. Calculated from LUMO and optical
band gap (E ) of the synthesized compounds, HOMO = LUMO − E
g
g
f
(eV). Calculated from the empirical formula, LUMO = −E − 4.80
(eV).
red
13
The electrochemical properties of the polymers are
summarized in Table 4. Their CVs are shown in Figure S8.
The polymers showed clear reduction peaks, and most of the
polymers exhibited lower-lying LUMO energy levels comparing
to the corresponding boron diiminates. These data mean that
the expansion of main-chain conjugation strongly influences the
LUMO levels as the DFT calculations proposed (Figure 2).
Moreover, it is proposed that the boron diiminate could work
as an electron-accepting unit in the polymer since the fluorene
unit has little electron acceptability. HOMO and LUMO of P-
NMe exhibited almost same value as those of M-NMe . This
the amino unit and the boron chelating ring than from the
main-chain conjugation.
PL spectra of the synthesized polymers were measured in
−5
THF (1 × 10 M) and the thin-film states upon the excitation
at each absorption maximum (Figure 4c and Table 5). As the
typical AIE behavior, the thin film showed large increases in the
emission efficiencies, while they showed weak luminescence in
the solution states (Φ < 0.01). In addition, the peak positions
in the emission band (λ ) showed good correlation with the
PL
2
2
order of their E values. Moreover, the emission colors were
g
fact indicates that the HOMO and LUMO in P-NMe have
varied from green to red by changing the substituents at the
boron diiminate unit. The polymer formed an amorphous state
2
large contributions from the donor−acceptor system between
G
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Table 5. Optical Properties of Boron Fluorene Polymers
acceptor interaction between the amino group and the boron
chelating ring could be suppressed by the protonation of the
amino group (Figure 5c). As a result, the color was changed
from orange to yellow. In the PL spectra, the emission of P-
4
λabs,THF
[nm]
λabs,film
[nm]
ε × 10
λ
PL,film
[nm]
[M cm−1]
−
1
a
b
,
c
, ,
bcd
compounds
ΦPL,film
P-H(Me)
P-H(Ph)
P-OMe
P-NO2
374
399
396
407
392
370
396
393
405
391
6.34
5.99
6.07
5.80
6.06
509
542
542
575
628
0.11
0.07
0.04
0.05
0.02
NMe film also shifted to short-wavelength region after the
2
exposure with TFA to induce the protonation; emission color
was changed from red to yellow. Furthermore, the optical
properties of the TFA-exposed film were converted back into
P-NMe2
the parent state again by exposing to NEt vapor. These data
a
3
Molar absorption coefficients of the absorption maxima at longer
b
c
clearly show that the optical properties can be tuned reversibly.
The boron diiminate-containing polymers could find applica-
tion as various kinds of film-type optical sensors such as pH-,
redox- or temperature-sensors.
wavelength region. Excited at λ
. Measured in thin-film states.
abs,film
d
Determined using integrated sphere method.
(Figure S4). Furthermore, all films of the polymers exhibited
higher emission efficiencies (Φ = 0.02−0.11) relative to the
PL
CONCLUSION
■
corresponding amorphous samples of boron diiminates. These
results show that the conjugation with fluorene contributes to
the improvement of the molar absorption coefficient and the
emission intensity.
We present here the unique properties of boron diiminates and
demonstrate various functionalization such as the emission
color tuning and the effect by the conjugation of boron
diiminates with fluorene. The synthesized boron diiminates
exhibited AIE and CIEE properties. The emission colors were
efficiently modulated by introduction of substituents. Espe-
cially, the introduction of strong electron-donating substituents
such as dimethylamino group induced large red-shifted
emission. It was revealed that the incorporation into the
conjugation system significantly influenced the electronic
structures of boron diiminates. In this study, the beneficial
alteration of optical properties such as the increases of the
absorption and emission properties was obtained by employing
the polymerization. Finally, the dynamic reversible regulation of
the optical properties of the polymer thin film was also
accomplished by modulating the electron-donating and -with-
drawing capability in the substituents using acid−base
Finally, the dynamic modulation of the optical properties was
performed by acid and base sensing experiments using
trifluoroacetic acid (TFA) and triethylamine (NEt ), respec-
3
tively. The film of P-NMe showed red-shifted absorption and
2
PL spectra in comparison with the others because of the strong
donor−acceptor interaction (Figure 5). On the other hand,
when the P-NMe2 film was placed in a small container
saturated with TFA vapors, the fumed film showed blue-shifted
absorption and emission spectra than those of the parent
samples. In the absorption spectra, while the absorption around
5
00 nm decreased, the absorption around 400 nm increased
after the exposure with TFA. These data indicate that the
optical properties of boron diiminates-containing polymers can
be dynamically regulated in the film states. The strong donor−
Figure 5. (a) UV−vis absorption and (b) PL spectra of P-NMe before and after exposure to TFA and NEt vapor in the thin-film state upon
2
3
excitation at 391 nm. (c) Expected mechanism of the pH-responsive behavior and a photograph of pH sensing.
H
dx.doi.org/10.1021/ja510985v | J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

Journal of the American Chemical Society
Article
interactions. These findings specifically propose that boron
diiminates are brilliant organic materials with high functionality
involving AIE, CIE, and phosphorescence emissive properties.
Such characteristics of boron diiminates might be useful/
utilized for developing advanced organic opto and/or electric
devices or smart materials such as conjugated polymers, liquid
crystal materials, covalent organic framework, and metal organic
frameworks, and so on.
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ASSOCIATED CONTENT
Supporting Information
General, materials, computational details, NMR spectra, CV
pots, crystallographic data, UV−vis absorption, and PL data.
This material is available free of charge via the Internet at
http://pubs.acs.org.
■
*
S
(
8) Yoshii, R.; Tanaka, K.; Chujo, Y. Macromolecules 2014, 47, 2268−
278.
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(
AUTHOR INFORMATION
Corresponding Authors
chujo@chujo.synchem.kyoto-u.ac.jp
kazuoppa123@yahoo.co.jp
Notes
■
(
2
(
(
The authors declare no competing financial interest.
Polym. Chem. 1993, 31, 2465−2471. (b) Scherf, U.; List, E. J. W. Adv.
Mater. 2002, 14, 477−487. (c) Yang, R.; Tian, R.; Yan, J.; Zhang, Y.;
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ACKNOWLEDGMENTS
■
3
2
8, 244−253. (d) Lim, E.; Jung, B.-J.; Shim, H.-K. Macromolecules
This work was partially supported by the SEI Group CSR
Foundation (for K.T.), “the Adaptable and Seamless Technol-
ogy Transfer Program” through target-driven R&D, Japan
Science 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.
003, 36, 4288−4293. (e) Yeo, H.; Tanaka, K.; Chujo, Y.
Macromolecules 2013, 46, 2599−2605. (f) Yoshii, R.; Nagai, A.;
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Am. Chem. Soc. 2008, 130, 12828−12833. (b) Morisaki, Y.; Ueno, S.;
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