Controlling Energy Gaps of π-Conjugated Polymers by Multi-Fluorinated Boron-Fused Azobenzene Acceptors for Highly Efficient Near-Infrared Emission

Article abstract of DOI:10.1002/asia.202100037

We demonstrate that multi-fluorinated boron-fused azobenzene (BAz) complexes can work as a strong electron acceptor in electron donor-acceptor (D-A) type π-conjugated polymers. Position-dependent substitution effects were revealed, and the energy level of

Full text of DOI:10.1002/asia.202100037

DOI: 10.1002/asia.202100037
Full Paper
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Controlling Energy Gaps of π-Conjugated Polymers by Multi-
Fluorinated Boron-Fused Azobenzene Acceptors for Highly
Efficient Near-Infrared Emission
[a]
Masayuki Gon, Junko Wakabayashi, Masashi Nakamura, Kazuo Tanaka,* and Yoshiki Chujo
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Abstract: We demonstrate that multi-fluorinated boron-fused
azobenzene (BAz) complexes can work as a strong electron
acceptor in electron donor-acceptor (D-A) type π-conjugated
polymers. Position-dependent substitution effects were re-
vealed, and the energy level of the lowest unoccupied
molecular orbital (LUMO) was critically decreased by fluorina-
tion. As a result, the obtained polymers showed near-infrared
of the BAz units, deeper NIR emission (λ =852980 nm) was
detected in film state. Clear solvent effects prove that the NIR
PL
emission is from a charge transfer state originating from a
strong D-A interaction. The effects of fluorination on the
frontier orbitals are well understandable and predictable by
theoretical calculation with density functional theory. This
study demonstrates the effectiveness of fluorination to the
BAz units for producing a strong electron-accepting unit
through fine-tuning of energy gaps, which can be the
promising strategy for designing NIR absorptive and emissive
materials.
(
NIR) emission (λ =758–847 nm) with high absolute photo-
PL
luminescence quantum yield (Φ =7–23%) originating from
PL
low-lying LUMO energy levels of the BAz moieties (À 3.94 to
À 4.25 eV). Owing to inherent solid-state emissive properties
1
. Introduction
Narrow energy gaps of the D-A type π-conjugated polymers
can be an origin of absorption and emission in the near-infrared
(NIR) region and those were significant for effective use of sun
Growing demands for high performance and sophisticated
materials require finely predictable and controllable design
strategies for constructing functional organic compounds. As
one of the candidates for designable organic materials, π-
conjugated polymers have attracted much attention owing to
unique characteristics, such as intense luminescence, high
electrical conductivities and good film-formability. The energy
gaps, which are defined as a width of the energy levels
between a highest occupied molecular orbital (HOMO) and a
lowest unoccupied molecular orbital (LUMO), are critical factors
for determining the material properties. In particular, electron
donor-acceptor (D-A) type π-conjugated polymers composed of
an alternating array of electron-donor and acceptor moieties
have actively studied by feature of easily controllable energy
gaps with selecting a donor and an acceptor having desired
HOMO and LUMO energy levels as co-monomers, respectively.
From these utilities, D-A type π-conjugated polymers have been
[6]
light for organic film solar cells and bioimaging, optical
[7]
communication. In order to construct stable π-conjugated
polymers having the narrow energy gaps, reduction of a LUMO
energy level of an acceptor unit is required because elevation
of a HOMO energy level has a risk of oxidation by air under
[1]
[8]
ambient conditions. One of the strategies for lowering the
LUMO energy level is the introduction of fluorine substituents
[9]
to the acceptor. A fluorine atom intrinsically has high electron
negativity and is inactive to chemical substances. Fluorination
enhances electron-accepting ability of the organic compounds
with good chemical stability.
[
2]
Recently, we proposed the concept of “element-blocks”,
which are structural functional units consisting of various
[
10]
groups of elements, to create advanced materials. According
to the concept, we revealed that boron-fused azobenzene/
azomethine (BAz/BAm) complexes, which are “element-blocks”
constructed mainly by B, N, O, C and H, worked as a strong
acceptor in D-A type π-conjugated polymers with highly
efficient emissions including NIR region both in solution and
[3]
often applied to organic light-emitting diodes (OLEDs),
organic field effect transistors (OFETs) and organic photo-
voltaics (OPVs). Hence, the development of novel compounds
showing electron-donating and accepting abilities is still
necessary for preparing D-A type π-conjugated polymers.
[
4]
[5]
[11]
film states. The film-state emission is valuable because the
emission is generally spoiled in a condensed state due to loss of
excitation energy via non-radiative pathway mainly by inter-
[12]
molecular π–π interaction. One of the reasons of the film-
state emission could be attributable to the fact that the BAz/
BAm complexes potentially enhanced their emission in solid or
crystalline states although the emission is critically quenched in
[a] Dr. M. Gon, J. Wakabayashi, M. Nakamura, Prof. Dr. K. Tanaka,
Prof. Dr. Y. Chujo
Department of Polymer Chemistry, Graduate School of Engineering
Kyoto University Katsura, Nishikyo-ku, Kyoto 615-8510 (Japan)
E-mail: tanaka@poly.synchem.kyoto-u.ac.jp
[13]
the diluted solution state. These phenomena are known as
[14]
aggregation-induced emission (AIE) or crystallization-induced
Supporting information for this article is available on the WWW under
https://doi.org/10.1002/asia.202100037
This manuscript is part of a special collection dedicated to Early Career
Researchers.
[15]
emission enhancement (CIEE) properties.
Herein, to reinforce electron-accepting ability of the BAz
skeleton, we synthesized fluorinated BAz complexes and
Chem Asian J. 2021, 16, 696–703
696
© 2021 Wiley-VCH GmbH

Full Paper
corresponding D-A type π-conjugated polymers with bithio-
phene units as a donor. Azobenzene scaffolds were chosen by
their inherent higher electron-accepting abilities than azome-
thine ones to prepare strong acceptors aiming to deeper NIR
absorption and emission. As a result, the LUMO energy levels,
which are one of the indexes evaluating the degree of accept-
ors, were lowered depending on the number of introduced
fluorine atoms with the BAz complexes. The obtained D-A type
π-conjugated polymers showed highly-efficient NIR emission
mers with
a
bithiophene co-monomer were synthesized
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(Scheme 2 and Table 1). Migita-Kosugi-Stille cross-coupling
polymerizations with BAz-3F, BAz-5F or BAz-35F and 5,5’-bis
(trimethylstannyl)-3,3’-didodecyl-2,2’-bithiophene (BT) were
[
17]
[16]
executed in a catalytic system involving Pd (dba)3 (dba=
2
dibenzylideneacetone) and 2-dicyclohexylphosphino-2’,4’,6’-trii-
sopropylbiphenyl (XPhos) to afford target copolymers, P-BAz-
3F, P-BAz-5F or P-BAz-35F, respectively. The synthetic results
and polymer data are listed in Table 1. In the case of P-BAz-5F
and P-BAz-35F, high molecular weight polymers were fractio-
nated by high performance liquid chromatography (HPLC) in
chloroform as an eluent. In this research, the synthetic polymer
data of P-BAz were cited or recollected as comparison.
synthesized compounds showed good solubility in common
organic solvents such as toluene, chloroform, dichloromethane
(
λPL =758~847 nm) in diluted solution and moderate NIR
1
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emission (λ =852~980 nm) in film state. The solvent effects
PL
evaluated by Lippert-Mataga plots proved that the polymers
formed clear D-A interactions. Moreover, the experimental data
were good agreement with the results of theoretical calcula-
tions by density functional theory (DFT), and therefore the
properties were predictable. We proposed that the high
designability of D-A type π-conjugated polymers by fluorination
with BAz complexes was good example for constructing desired
functional materials having controllable energy gaps.
[
11b,d,e]
All
1
13
11
and tetrahydrofuran, and can be characterized by H, C,
B
NMR, MS spectra (see Supporting Information). From those
characterization data, we concluded that the samples have the
objective structures and enough purity for further analyses.
2
.1 Synthesis
2.2 Optical measurements
Scheme 1 shows the syntheses of three types of fluorinated BAz
complexes modified with the fluorine atoms at 3,3’ positions
To investigate the position-dependent substituent effect of
fluorine atoms on the electronic situation in the ground state,
UV-vis absorption measurements were executed in the diluted
(BAz-3F), 5,5’ positions (BAz-5F) and both 3,3’ and 5,5’ positions
À 5
(
BAz-35F). The pristine BAz complex, BAz, was prepared
chloroform solutions (1.0×10 M) (Figures 1 and S1, Table 2).
[11b]
according to our previous report.
The fluorinated BAz
complexes, BAz-3F, BAz-5F, and BAz-35F, were isolated in high
yields by the condensation reaction at 100°C in toluene
Table 1. Molecular weights and isolated yields of the synthesized
polymers.
between the corresponding azobenzene tridentate ligands, Az-
1
2
a
a
w
R
R
M
n
/kDa
M
/kDa
w
M /M
n
yield /%
3
F-OH, Az-5F-OH and Az-35F-OH, and boron trifluoride diethyl
etherate (BF ·Et O), respectively. Next, by using the obtained
b
3
2
P-BAz-F
H
F
H
H
H
F
21.4
13.8
24.5
13.4
55.6
38.8
48.5
20.8
2.5
3.2
2.0
1.6
85
54
54
44
BAz complexes as monomers, D-A type π-conjugated copoly-
P-BAz-3F
P-BAz-5F
P-BAz-35F
c
c
F
F
[
a] Determined by a gel permeation chromatography (GPC) with
polystyrene standards. [b] ref 11b. [c] After fractionated by HPLC.
Table 2. Optical data of the BAz monomers and polymers in solution and
solid or film states.
solution
solid or film
d d
a
b
b,c
c,d
λ
abs
/
λ
PL
/
Φ
PL
λ
abs
/
λ
PL
/
Φ
PL
/
nm
nm
/%
nm
nm
%
Scheme 1. Syntheses of monomers, BAz, BAz-3F, BAz-5F and BAz-35F with
the name of substitution positions.
BAz
486
480
513
510
631
643
666
670
621
644
654
688
758
794
821
847
<1
<1
<1
<1
23
8
–
–
–
–
660
682
698
717
645
613
637
644
852
926
916
980
7
2
6
2
4
<1
<1
<1
BAz-3F
BAz-5F
BAz-35F
P-BAz
P-BAz-3F
P-BAz-5F
P-BAz-35F
11
7
À 5
À 5
[a] 1.0×10 M for monomers and 1.0×10 M per repeating unit for
À 5
À 5
3
polymers in CHCl . [b] 1.0×10 M for monomers and 1.0×10 M per
repeating unit for polymers in toluene, excited at absorption maxima for
PL. [c] Absolute PL quantum yield excited at absorption maxima. [d] Solid
state for monomers and spin-coated film on the quartz substrate
(
0.9 cm×5 cm) prepared from chloroform solution (0.10 mL, 1000 rpm,
concentration: 1.0 mg/0.30 mL) for polymers; excited at absorption
maxima for PL.
Scheme 2. General procedure for polymer synthesis.
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Figure 1. UV-vis absorption spectra of (A) BAz monomers, (B) polymers in chloroform (1.0×10 M for monomers, 1.0×10 M per repeating unit for polymers)
and (C) polymer films.
Interestingly, it was found that the substituent effects worked
independently. By substitution at 3,3’ positions, the middle
transition bands at around 400 nm raised and the energy gaps
were preserved (Figures 1A and S1 A). On the other hand, the
bathochromic shifts of absorption spectra were observed by
substitution at 5,5’ positions with retaining almost the same
spectra shape. Furthermore, the two different effects had
additive property and BAz-35F showed both substitution
effects at 3,3’ and 5,5’ positions. As a result, BAz and BAz-3F or
BAz-5F and BAz-35F had the same energy gap, respectively,
and the bathochromic shifts of absorption spectra were
detected in BAz-5F and BAz-35F compered to BAz and BAz-3F
motions are effectively restricted and therefore the emission
should be recovered. These situations were often shown in the
series of tetracoordinated boron with the azobenzene/azome-
thine-based tridentate ligands and those compounds also
[
11,13]
exhibited AIE or CIEE properties.
Contrary to the monomers with environment-sensitive
luminescent properties, intense NIR emission was observed
from the polymers even in the diluted solution (Figures 2C, 2D
and Table 1). The order of the maximum PL wavelength (λPL)
was the same with that of the λabs. The λPL and ΦPL were 758~
847 nm and 7~23%, respectively, and it was shown that the
BAz scaffold should be good candidate for NIR-emissive
“element-blocks”. Since the excitation-driven molecular motions
of the monomeric units would be inhibited by incorporating
(Figures 1A and S1 A).
In the D-A type π-conjugated polymers, the absorption
[11a,b,18]
bands were located at the longer wavelength regions than
those of the corresponding monomers (Figures 1A and 1B).
Contrary to the monomers, the value of absorption maximum
wavelength (λabs) was larger in the order of P-BAz-35F, P-BAz-
into the polymer chains,
induced in diluted solution. In the films, the BAz polymers also
showed emission in the NIR region (Figure 2D and Table 1). P-
the intense emission should be
BAz showed intense emission even in the film (λ =858 nm,
PL
[11b,e]
5
F, P-BAz-3F and P-BAz. Considering that electron-accepting
Φ =4%).
Although the other polymers exhibited weak
PL
ability of a compound is enhanced by fluorination, the order
should be correlated to the strongness of D-A interaction
between BAz and bithiophene units. In other words, the ability
of BAz acceptors was able to be reinforced by fluorination.
Moreover, homogeneous thin films were easily prepared by the
commodity spin-coated method because of good film-form-
ability of the synthesized polymers. The slight bathochromic
shifts and broader absorption bands were observed in the film
samples compared to those in the diluted solutions (Figures 1B
and 1 C, Table 1). Owing to the strong D-A interaction leading
to a narrow energy gap, the absorption maximum reached a
NIR region.
emission (Φ <1%), however the λ s were detectable at 916~
PL
PL
980 nm. Therefore, it can be said that BAz unit has potential to
targeting to fabricate functional materials showing deeper NIR
emission.
2.3 Solvent effects
To obtain more insight about the D-A system originating from
strong electron-accepting ability of fluorinated BAz units, we
investigated solvent effects on UV-vis-NIR absorption and PL
spectra. All the spectroscopic data are shown in Figures S2 and
S3, Tables S1 and S2. From the Lippert-Mataga plot of BAz
monomers in three types of solvents, toluene (Δf=0.013, Δf:
orientation polarizability), chloroform (Δf=0.15) and dichloro-
methane (Δf=0.22), there were almost no correlations between
To investigate electronic situations of the BAz complexes in
the excited state, the photoluminescence (PL) properties were
À 5
evaluated in toluene (1.0×10 M) and in crystal (Figure 2 and
Table 1). It was clearly demonstrated that AIE or CIEE properties
were obtained from the complexes. The emission intensity was
2
Δf and Stokes shift (v) (R <0.50, R: correlation coefficient)
obviously very weak in solution (Φ <1%), whereas intense
(Figure 3A). This means that the emission of BAz monomer is
not from a charge transfer (CT) state but from a localized
electronic (LE) state. On the other hand, BAz polymers had
good correlations (R >0.90) between Δf and v by using five
types of the solvent including benzene (Δf=0.003) and
chlorobenzene (Δf=0.14) (Figure 3B). Although we also eval-
PL
emission was monitored in crystal (Φ =2~7%) (Figure 2A and
PL
2
B). Our previous work proposed that the emission quenching
2
in diluted solution should be caused by excitation-driven
bending motions which promote non-radiative excitation
decay.
[11a,b,13a,18]
In solid or crystal packing, those molecular
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Figure 2. PL spectra of BAz monomers (A) in chloroform (1.0×10 M) and (B) in solid state, BAz polymers (C) in chloroform (1.0×10 M per repeating unit)
and (D) in film state with the excitation light at each absorption maximum.
À 5
À 5
Figure 3. Lippert-Mataga plots of (A) BAz monomers and (B) polymers in the diluted solutions (1.0×10 M for monomers and 1.0×10 M per repeating unit
for polymers) at room temperature. The formulas of approximate straight lines are described in the same figures. Plotted data are listed in Tables S1 and S2.
uated the solvent effect in tetrahydrofuran (Δf=0.21), the
emission band was observed in a position separated from
expectation (Figure S4). This might be because nucleophilicity
of THF originating from lone pairs of oxygen affected the boron
atom in addition to the solvent effect. The slopes of the
approximation straight lines in Lippert-Mataga plots were
almost the same, while those tended to be decreased by
fluorination at 5,5’ positions.
During the investigation of the solvent effects, we found
generation of the unique absorption band from 700 to 900 nm
only from P-BAz-3F in benzene/chloroform, toluene/chloroform
or dichloromethane/chloroform=99/1 v/v mixed solutions (Fig-
ure S3). From the variable temperature (VT) UV-vis-NIR absorp-
tion measurements in toluene/chloroform=99/1 v/v solution,
the band gradually decreased by increasing temperature and
subsequently disappeared at 50°C (Figure S5A). Subsequently,
the band was recovered by a cooling process and finally the
original spectrum was reproduced at 10°C (Figure S5B). Inter-
estingly, the hysteresis was observed by holding each temper-
ature for 10 minutes per 10°C (Figure S5C). The disappearance
and reproduction of the band slowly proceeded by monitoring
at 40°C under heating and cooling processes, respectively
(Figure S5D). High reversibility was obtained at least five times
(Figure S5E) and the hysteresis behavior was reproducible.
Furthermore, weak NIR emission at around 950 nm was
detectable by excitation at the absorption band (Figure S5F). In
the case of the other polymers, P-BAz, P-BAz-5F and P-BAz-
35F, there was no unusual temperature dependency regardless
of the presence or absence of fluorine substituents (Fig-
[19]
ure S6). The generation of the band in longer wavelength
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À 3
À 3
Figure 4. Cyclic voltammograms of (A) BAz monomers and (B) BAz polymers in dichloromethane (1.0×10 M for monomers, 1.0×10 M per repeating unit
À 1
for polymers) containing nBu
4
NPF
6
(0.10 M) at room temperature with a scan rate of 0.1 Vs (negative scan). The black arrows denote sweep directions
(negative scan).
Table 3. Energy levels of molecular orbitals of BAz monomers and
region could be derived from intra- or intermolecular π–π
interaction of the polymer chains of P-BAz-3F in solution
although significant aggregation was not suggested from a
polymers.
a
b
red c
onset
d
d
λ
nm
abs,edge
/
E
eV
g,opt
/
E
/
E
eV
LUMO
/
HOMO
E /
eV
V
[20]
dynamic light scattering (DLS) data. According to the crystal
packing structures of BAz and BAz-3F, it was suggested that
the fluorine atom at 3’ position is responsible for intermolecular
interaction between two molecular fractions of BAz-3F (Figur-
es S7~S9, Tables S3 and S4). In particular, there were three
short contacts in BAz-3F and they were shorter than those of
BAz, respectively (Figure S9). As a result of the strong binding,
the molecules of BAz-3F were able to have almost planar
conformation in the condensed state. Relatively-planar con-
formation might be favorable for the formation of assembly
followed by the generation of the new absorption band in
solution.
BAz
546
546
579
579
722
743
769
787
2.27
2.27
2.14
2.14
1.72
1.67
1.61
1.58
À 0.80
À 0.61
À 0.63
À 0.46
À 0.86
À 0.71
À 0.72
À 0.55
À 4.00
À 4.19
À 4.17
À 4.34
À 3.94
À 4.09
À 4.08
À 4.25
À 6.27
À 6.46
À 6.31
À 6.48
À 5.66
À 5.76
À 5.69
À 5.83
BAz-3F
BAz-5F
BAz-35F
P-BAz
P-BAz-3F
P-BAz-5F
P-BAz-35F
À 5
À 5
[
a] In chloroform (1.0×10 M for monomers, 1.0×10 M per repeating
unit for polymers), estimated from Figure S1. [b] E =1240/λabs,edge. [c] In
dichloromethane (1.0×10 M for monomers, 1.0×10 M per repeating
g,opt
À 3
À 3
unit for polymers) containing nBu NPF (0.10 M) at room temperature with
4
6
À 1
red
[21]
a scan rate of 0.1 Vs (negative scan). [d] ELUMO =À (4.8À Eonset ) (eV),
EHOMO =ELUMOÀ Eg,opt
.
2
.3 Cyclic voltammetry
accepting ability of BAz unit in the polymer was enhanced up
to À 4.25 eV in the LUMO energy level of P-BAz-35F. Here,
although the oxidation peaks were detectable in the voltammo-
grams of BAz polymers, the same estimation method as the
monomers was adopted because the values should be
compared with the same criteria and the substituent effect
should be prominently reflected on the LUMO-localized accept-
or moiety. The HOMO energy levels evaluated from the
oxidation peaks in the voltammograms are shown in Figure S10
and Table S5. In summary, it was confirmed that energy levels
of frontier orbitals can be tuned by fluorination at adequate
positions.
To obtain further information on the substituent effect of
fluorine atoms on electronic properties, we estimated molecular
orbital (MO) energy levels with a cyclic voltammetry (CV). The
results are shown in Figure 4 and Table 3. The LUMO energy
[21]
levels were calculated from the onset of the reduction curve.
In both of BAz monomers and polymers, it was found that
LUMO energy levels were critically lowered depending on the
number of fluorine atoms. In BAz monomers, the HOMO energy
levels were not able to be estimated from the cyclic voltammo-
grams because the oxidation peaks were out of the potential
window. Therefore, we calculated the HOMO energy levels
using optical energy gaps (Eg,opt) obtained from the edge of the
UV-vis-NIR absorption spectra (Figure S1). It was shown that
fluorination at 3,3’ positions, both HOMO and LUMO energy
levels were lowered, whereas at 5,5’ positions, only LUMO
energy levels were reduced. The similar effect was also
monitored in the polymers. The effects had additivity property
as shown in the absorption spectra. Accordingly, the electron-
2.4 Theoretical calculation
To support experimental results, we carried out theoretical
calculation with density functional theory (DFT) and time
dependent DFT (TD-DFT) at B3LYP/6-311G(d,p) level. The results
are summarized in Figures 5, 6, S11, S12 and Tables S6, S7. In
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Figure 5. (A) Energy diagrams of BAz monomers (solid line: from DFT calculation, dotted line: from CV and UV-vis absorption spectra, Table 3), (B) HOMO and
LUMO of BAz and BAz-35F obtained from DFT calculations (isovalue=0.03). Hydrogens were omitted for clarity.
monomers, variation tendency of calculated HOMO and LUMO
energy levels was precisely matched to the experimental results
increase of S !S transition without any disturbance in HOMO-
0
3
LUMO energy gaps.
(
Figure 5A). In LUMO, there were little orbital interaction of
To estimate influence of the substituent effect on the
electronic structures in polymers, the model compounds having
BT units in both sides of the BAz unit were used for calculations
(Figures 6, S12 and Table S7). The calculation with model
compounds well helped interpreting the situation of the
polymers. Contrary to the monomers, BT units worked as
donors and the standard HOMO energy levels were dramatically
raised (Figure 6A). In polymers, the orbital interaction of fluorine
atoms was still effective as shown in the monomers (Figure 6B
and S12) and slight elevation of the HOMO energy levels was
detected by fluorination at 5,5’ positions (Figure 6A). However,
the effect was weakened probably because of large delocaliza-
tion of HOMO through the BT units (Figure 6B and S12).
Meanwhile, relatively localized LUMO was strongly affected by
reduction of energy levels through the inductive effect of
fluorine atoms. From those effects, the energy gaps were
narrower in the order of P-BAz-35F, P-BAz-5F, P-BAz-3F, P-BAz
(Figure 6A and Table S7). It can be said that the optical
properties of the BAz polymers can be finely tuned by selecting
the substituent positions.
fluorine atoms both at 3,3’ and 5,5’ positions, and the only
inductive effect of them influenced on energy levels (Figures 5B
and S11). Thereby, the step-by-step reduction of LUMO energy
levels can be accomplished as increasing the number of fluorine
atoms. On the other hand, in HOMO, the orbital interaction of
fluorine atoms existed only at 5,5’ positions and electronic
donation from the lone pair of fluorine atoms occurred in
addition to inductive effect of them (Figure 5B). Hence,
elevation of HOMO energy levels was detected in BAz-5F and
BAz-35F (Figure 5A). Consequently, the bathochromic shifts of
absorption spectra should be caused by the elevation of HOMO
energy levels (Figure 1A and Table S5). We previously reported
the similar effect by using bromine atoms as a unique
1
1d
substitution effect at 5,5’ positions of BAz derivatives. It was
noted the orbital interaction at 3,3’ positions was observable in
HOMO-2, which led to enhancement of S !S transition (Fig-
0
3
ure S11 and Table S6). As a result, rise of the middle absorption
band at around 400 nm (Figure 1A) was attributed to the
Figure 6. (A) Energy diagrams of model compounds of BAz polymers (solid line: from DFT calculation of model compounds, dotted line: from CV and UV-vis-
NIR absorption spectra of BAz polymers, Table 3) and (B) chemical structures of model compounds, and HOMO and LUMO of BAz-BT and BAz-BT-35F with
DFT calculations (isovalue=0.025). Hydrogens were omitted for clarity.
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3
. Conclusion
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Electronic properties of multi-fluorinated BAz complexes were
evaluated. Distinct and different substitution effects at 3,3’ and
5
,5’ positions were revealed. Reduction of LUMO energy level
was caused by fluorination both at 3,3’ and 5,5’ positions,
whereas elevation of HOMO energy level was brought about
only by fluorination at 5,5’ positions. The prepared BAz
complexes hold potential to solid-state emission with CIEE
properties. Owing to strong D-A interaction of π-conjugated
polymers with fluorinated BAz and BT units, we obtained highly
efficient NIR emission. The energy gaps of the π-conjugated
polymers were finely tuned depending on fluorinated positions
and numbers; as a result, the maximum emission wavelength
reached 847 nm in the diluted solution and 980 nm in the film.
Additionally, the solvent effect originating from electronic
transition attributed to a CT state was clearly shown in the
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observed in P-BAz-3F probably because of interaction between
additional fluorine and nitrogen atoms. Furthermore, theoretical
calculation well supported the interpretation of the effects
gifted by fluorination. Through fluorinated BAz compounds, it
was revealed that the influence of fluorination was systemati-
cally understandable and the fluorination was effective in
controlling energy gaps of π-conjugated polymers. The study
disclosed high potentiality of BAz compounds as a strong
electron acceptor.
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This work was partially supported by the Mizuho Foundation for
the Promotion of Sciences, Japan (for M.G.) and a Grant-in-Aid for
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