A Highly Efficient Near-Infrared-Emissive Copolymer with a N=N Double-Bond π-Conjugated System Based on a Fused Azobenzene–Boron Complex

Article abstract of DOI:10.1002/anie.201803013

Fused azobenzene–boron complexes (BAzs) show highly efficient near-infrared (NIR) emission from the nitrogen–nitrogen double bond (N=N) containing π-conjugated copolymer. Optical measurements showed that BAz worked as a strong electron acceptor because of the intrinsic electron deficiency of the N=N double bond and the boron–nitrogen (B?N) coordination which dramatically lowered the energy of the lowest unoccupied molecular orbital (LUMO) of the azobenzene ligand. The simple donor–acceptor (D–A) type copolymer of bithiophene (BT) and BAz exhibited intense photoluminescence (PL) in the NIR region both in the dilute solution (λ_PL=751 nm, Φ_PL=0.25) and in the film (λ_PL=821 nm, Φ_PL=0.038). The BAz monomer showed slight PL in the dilute solution, and aggregation-induced emission (AIE) was detected. We proposed that N=N double bonds should be attractive and functional building blocks for designing π-conjugated materials.

Full text of DOI:10.1002/anie.201803013

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Angewandte
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Accepted Article
Title: A Highly-Efficient Near-Infrared-Emissive Copolymer with
N=N Double-Bond π-Conjugated System Based on a Fused
Azobenzene-Boron Complex
Authors: Masayuki Gon, Kazuo Tanaka, and Yoshiki Chujo
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To be cited as: Angew. Chem. Int. Ed. 10.1002/anie.201803013
Angew. Chem. 10.1002/ange.201803013
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A Highly-Efficient Near-Infrared-Emissive Copolymer with N=N
Double-Bond π-Conjugated System Based on a Fused
Azobenzene–Boron Complex
Masayuki Gon, Kazuo Tanaka* and Yoshiki Chujo*
8
Abstract: The fused azobenzene–boron complexes (BAzs) were
prepared, and highly-efficient near-infrared (NIR) emission was
observed from the nitrogen–nitrogen double bond (N=N) containing
π-conjugated copolymer. From the optical measurements, it was
found that BAz worked as a strong electron-acceptor originating from
intrinsic electron-deficiency of the N=N double bond and the boron–
nitrogen (B–N) coordination which dramatically lowered the energy
level of the lowest unoccupied molecular orbital (LUMO) of the
azobenzene ligand. The simple donor–acceptor (D–A) type
copolymer with bithiophene (BT) and BAz exhibited intense
photoluminescence (PL) in the NIR region both in the dilute solution
double bond. So far, several π-conjugated polymers containing
N=N double bonds in the main chain have been reported, and
their photoisomerization, electrochemical and photovoltaic
9
properties were investigated, while there were, to the best of our
knowledge, no report about photoluminescent properties. It is well
recognized that excited deactivation of azobenzene would rapidly
proceed via the non-radiative photoisomerization processes.10
Therefore, the azobenzene is considered to be not suitable for the
application to light-emitting materials.11 On the other hand,
Kawashima and coworkers proposed that azobenzene was a
potential building block as a highly-efficient emitter if the
photoisomerization was restricted and the electronic structures
were varied, for example, by formation of the B–N coordination.12
Additionally, it was proposed that the energy level of the LUMO of
the π-conjugated molecule involving the N=N double bond was
lower than that of the C=C analog.13 Furthermore, the B–N
coordination in the π-conjugated system effectively lowered the
LUMO level of the π-conjugated unit.14 That is, the N=N double
bond has a potential to work as a strong electron-acceptor in the
π-conjugated system as well as an emitter. Thus, it can be
expected that the N=N double bond could be a new element-block
for realizing narrow band-gap π-conjugated polymers.
(λPL = 751 nm, ΦPL = 0.25) and in the film (λPL = 821 nm, ΦPL = 0.038).
Moreover, BAz monomer showed slight PL in the diluted solution,
meanwhile aggregation-induced emission (AIE) was detected. We
proposed that N=N double bonds should be attractive and functional
building blocks for designing π-conjugated materials.
In designing π-conjugated polymers, electronic functions of the
1
monomer unit are a striking factor in device property. Thus,
introduction of unique π-conjugated units into polymer main
chains and comprehension of their electronic properties are
essential for realizing superior properties from the products. From
this stand point, the concept of “element-blocks”, which are
structural functional units consisting of various groups of element,
is valid for creating functional polymers.2
Herein, we report multi functions of the azobenzene complex
with the fused structure by boron coordination. Although the
synthesis of BAz analogs was already achieved by Hohaus in
980s,15 the optical properties were still vailed. This is the first
1
By the combination of carbon–carbon double bonds (C=C) and
construction of conjugated system, polymeric materials having
efficient PL properties can be obtained such as poly(para-
phenylenevinylene) (PPV) which was used for firstly fabricating
an organic light-emitting diode (OLED).3,4 On the other hand, in
comparison to conventional π-conjugated system with C=C
double bonds, the number of π-conjugated polymers including
heteroatom–heteroatom double bonds (X=X) is limited.5,6 Due to
their intrinsically high reactivity and difficulty in handling, it is still
challenging to obtain robust materials.7 A N=N double bond
shows relatively higher stability among the other X=X double
bonds. As a representative example, azobenzene has been
widely used as a key component in stimuli-responsive materials
based on the cis–trans photoisomerization derived from the N=N
example, to the best of our knowledge, to offer unique optical
performances of the N=N double bond in the expanded π-
conjugated system.
Scheme 1 shows the synthesis of the BAzs and the pristine
azobenzene compounds. BAz-H was easily synthesized by boron
complexation with the commercially-available compound, 2,2'-
dihydroxyazobenzene (1). The dibrominated monomer BAz-Br
was prepared from 4-bromo-2-methoxyaniline (2). The ligand 3
was obtained via the oxidative coupling reaction with
manganese(IV) oxide (MnO
from the methoxy groups to hydroxyl one with boron tribromide
BBr ). After boron complexation, BAz-Br was obtained. The
pristine azobenzene Az-H was also synthesized by the oxidative
coupling reaction of o-anisidine (4) with MnO
2
) and followed by the conversion
(
3
2
.
Following Scheme 2, the copolymers with BT were synthesized.
The Migita–Kosugi–Stille coupling polymerization16 of BAz-Br
was carried out with 5,5'-bis(trimethylstannyl)-3,3'-didodecyl-2,2'-
bithiophene (5) in the catalytic system of Pd (dba) (dba =
2 3
dibenzylideneacetone) using 2-dicyclohexylphosphino-2',4',6'-
[]
M. Gon, K. Tanaka, 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 or
chujo@poly.synchem.kyoto-u.ac.jp
Supporting information for this article is given via a link at the end of
the document.
triisopropylbiphenyl (XPhos) as a phosphine ligand to obtain P-
BAz as a deep red purple solid in 85% isolated yield (M
n
= 21,400,
M
w
= 55,600, M /M = 2.59). The pristine azobenzene copolymer
w
n
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COMMUNICATION
Figure 1. ORTEP drawings of BAz-H (50% probability for thermal ellipsoids). A
minor conformation of BAz-H is omitted to clarify. All crystallographic data are
shown in Supporting Information.
Table 1. Spectroscopic data of pristine and fused azobenzenes
[
a]
[a]
PL
[a,b]
PL
τ[a,c]
/ns
[d]
[d]
λ
/
abs
λ
Φ
k
r
k
/s
nr
–1
–1
nm
/nm
/s
Az-H
P-Az
BAz-H
372
458
479
–
–
–
–
–
–
–
–
–
–
–
–
Scheme 1. Synthesis of azobenzene and BAz derivatives
617
(603)
<0.001
(0.023)
<0.10
(1.09)
7
8
(
Aggre-
(2.1×10 )
(9.0×10 )
[
e]
gation)
>5.0×107
2.3×108
3.7×108
>9.9×10
5.8×10
1.1×10
9
BAz-M1
BAz-M2
P-BAz
558
601
632
643
690
751
(821)
0.005
0.038
0.25
<0.10
0.17
0.68
9
9
[
f]
(Film)
(661)
(0.038)
[
a]
In toluene (1.0×10−5 M); excited at λabs for PL. [b] Absolute PL quantum
[
c]
Emission lifetime at λPL. [d]
efficiency excited at λabs
Φ
.
k = ΦPL/τ, knr = (1 −
r
PL)/τ. [ In 1,4-dioxane/H
e]
O = 1/99 (1.0×10 M). [f] Spin-coated film on the
−4
2
quartz substrate (1 cm×5 cm) prepared from chloroform solution (0.10 mL,
000 rpm, concentration: 1.0 mg / 0.30 mL).
1
of the fused five and six membered ring system. The structure has
in common with the double bond-included tridentate ligands such
as 2,2'-dihydroxyazomethine.1
7,18
The azobenzene scaffold of
BAz-H was slightly bent due to the tetrahedral structure of the
boron atom. The torsion angle (φ) of the C(1)–N(1)–N(2)–C(7)
was 165°. The bond length of the N=N double bond was 1.278 Å,
and the fluorine atom projected from the azobenzene surface.
Because of the asymmetric structure, the boron atom became
chirality center. Indeed, BAz-H formed racemic crystal (Figure
S32). The enantiomers were able to be separated with chiral liquid
chromatography. Although the details of the chiroptical properties
were still under investigation, the conditions of the optical
resolution, circularly dichroism (CD) spectra and simulated CD
spectra are shown in Figure S33.
Scheme 2. Synthesis of copolymers and model compounds
To examine electronic structures in the ground state, the
absorption properties of the compounds were evaluated (Table 1).
Figures 2 and 3 show the UV–vis absorption spectra in toluene
(1.0×10−5 M and 1.0×10−5 M per repeating unit for the copolymers).
Az-H presented the typical spectra composed of the strong
absorption bands assigned to the permitted π–π* transition in the
shorter wavelength region (ca. 350−400 nm) and the weak
absorption bands originating from the forbidden n–π* transition in
the longer wavelength region (ca. 400−500 nm). Meanwhile, it
should be noted that BAz-H exhibited the strong absorption band
attributed to the π–π* transition in the longer wavelength region
(479 nm). This indicates that the B–N coordination transformed
n
P-Az was prepared as a red solid in 79% isolated yield (M =
w w n
20,900, M = 52,100, M /M = 2.50) with the same reaction
condition for P-BAz. Model compounds BAz-M1 and BAz-M2
were also prepared with the same protocol for preparing the
copolymers using 5-trimethylstannyl-3-dodecylthiophene (6) and
3,3'-didodecyl-5-trimethylstannyl-2,2'-bithiophene
(8),
respectively. The structures of all new compounds were
confirmed by H, 13C and 11B NMR spectroscopy, high-resolution
mass spectrometry (HRMS), and elemental analyses. The film
samples were prepared via the spin-coating on the quartz
substrate with the chloroform solutions.
1
The structure of BAz-H was clarified by a single crystal X-ray
analysis. Figure 1 shows ORTEP drawings of BAz-H. The 2,2'-
dihydroxyazobenzene scaffold worked as a tridentate ligand and
was fixed by the four-coordinated boron atom with the formation
0 1
the S –S transition from the forbidden n–π* transition to the
permitted π–π* one. This drastic change would play a positive
role in inducing intense photoluminescence (PL) property from
azobenzene.12
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Figures 3B and S34 show the PL spectra in toluene (1.0×10−5
M). Emission band of Az-H was not observed and that was in
common with conventional azobenzene derivatives which hardly
showed emission due to the fast non-radiative deactivation
process. From BAz-H, weak PL was detected (Tables 1 and S2,
λ
PL = 617 nm, ΦPL < 0.001). Those data were different from the
Kawashima and coworkers’ reports on the 2-borylazobenzene
derivatives having intense emission in the diluted solution.12 In
contrast, it should be noted that the emission intensity was
enhanced by forming aggregates or suppressing the molecular
motion in the polymer matrix. (Tables 1 and S2, Figure S35),
indicating that the BAz-H has AIE property.19 This fact involves
two significant issues. First, as we presumed according to the
Kawashima’s report, the fused azobenzene has PL properties.
However, emission annihilation occurred even in the solution
state. Second, the AIE behavior of BAz-H is unusual according to
the photochemical mechanism (restriction of intramolecular
rotation, RIR) in conventional AIE-active molecules.20 It is
suggested that emission annihilation should be generally induced
by molecular motions and vibration at the substituents in the
solution state.21 However, BAz-H has rigid structure fixed by
boron complexation. Additionally, the wavelengths of PL peak
tops were blue-shifted after aggregation formation. These results
suggest that the AIE property was originated not from
conventional RIR mechanism but from suppression of structural
relaxation of the π-conjugated complex in the excited state. We
have recently reported that the fused boron–azomethine complex
Figure 2. UV–vis absorption spectra of (A) Az-H and BAz-H in toluene
(
1.0×10−5 M); (B) P-Az and P-BAz in toluene (1.0×10−5 M per repeating units).
also showed the AIE property in the absence of movable
Figure 3. (A) Normalized UV–vis absorption spectra and (B) PL spectra of BAz-
H, BAz-M1, BAz-M2 and P-BAz in toluene (1.0×10−5 M, per repeating units for
P-BAz) and the film of P-BAz prepared on the quartz substrate. (C,D) Photos
of BAz-H, BAz-M1, BAz-M2, P-BAz and film of P-BAz under room light and
excited by white LED.
substituents.17
Next, emission properties of the polymers were examined.
Surprisingly, the intense emission band was obtained from P-BAz
in the NIR region (λPL = 751 nm, ΦPL = 0.25 in toluene) although
the monomer showed emission annihilation (Table 1 and Figure
3
B). By incorporating the BAz unit into the polymer main chain,
In both copolymers P-BAz and P-Az, the wavelengths of the
peak tops were significantly red-shifted compared with those of
the monomers. This means that electron delocalization through
the main chain including the N=N double bonds should be
extended. In the case of the pristine azobenzene-containing
polymers, it was reported that the absorption bands attributable to
n–π* and π–π* transitions were observed in the similar region.9
Correspondingly, the absorption band of P-Az with the peak at
molecular motions would be effectively suppressed, followed by
emission in the solution. It should be emphasized that the film of
P-BAz also exhibited efficient NIR emission (λPL = 821 nm, ΦPL
=
0
.038). These ΦPL values were relatively-high in the NIR-emissive
organic materials, especially in the polymers.23 To gather further
22
information on the effect of polymerization on electronic
properties of BAzs, photochemical mechanism was investigated.
It was found from the comparison of optical properties between
the model compounds and polymers that the values of λPLs and
j
458 nm was detected in the similar area of that originating from
n–π* transition of the monomeric unit Az-H. Meanwhile, the
significant absorption band with the peak at 632 nm appeared
from P-BAz in distinctly different wavelength region from those of
the π–π* transition of BAz-H. These data strongly support
extension of electronic delocalization through the polymer main
chains in P-BAz. In UV–vis absorption spectra of the model
compounds BAz-M1 and BAz-M2, which were mono and
bithiophene-substituted BAzs, respectively, the red-shifted
absorption bands were also observed, indicating that robust
electronic interaction should exist between BAz and thiophene
units. P-BAz showed good film-formability, and the homogeneous
film was able to be obtained. As shown in Figure 3A, the peak-top
wavelength of the film sample was slightly red-shifted (661 nm) in
comparison with that in the diluted solution in toluene (632 nm)
and those of the model compounds (558 nm for BAz-M1, 601 nm
for BAz-M2).
Φ
PLs increased (Table 1). It is likely that extension of π-
conjugated system through the polymer main chain contributed to
narrowing the energy gap in the S −S transition. According to the
0
1
solvatochromism of emission bands and subsequently
Lippert−Mataga plots, it was revealed that the emission bands of
P-BAz, BAz-M1 and BAz-M2 should be from the charge transfer
(
CT) state (Figures S37 and S39, Table S4). From the calculation
of the kinetic parameters with a PL lifetime measurement (Table
and Figure S36), it was shown that increase in the radiative rate
constants (k s) and decrease in the non-radiative rate constants
nrs) were simultaneously induced by polymerization. Because
1
r
(
k
structural relaxation would be disturbed in the polymer main chain,
enhancement to the radiation process should be induced.
Additionally, using the fluorene unit as the different comonomer,
the similar behavior was observed with polymerization, and it was
found that BT unit was more effective in enhanmcement of the PL
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Figure 4. The results of quantum calculation. (A) Energy diagram, selected MOs and (B) oscillator strength (f) of selected transition bands of azobenzene, Az-H
and BAz-H obtained with DFT and TD-DFT calculations at the TD-B3LYP/6-311G(d,p)//B3LYP/6-311G(d,p) level (isovalue = 0.03). (C) Optimized structures both
in the ground and excited states of BAz-H with DFT and TD-DFT calculations at the B3LYP/6-311G(d,p) and TD-B3LYP/6-311+G(d,p) levels, respectively.
performance than fluorene one (Figures S38 and S39, Tables S3
and S4). On the other hand, no emission was obtained from the
pristine azobenzene-containing polymer P-Az, indicating that the
B–N coordination and formation of the fused structure are
necessary for presenting luminescence.
BAz-M1 and BAz-M2, where dodecyl groups were changed to
methyl groups for simplification in the caluculation, clearly showed
the extension of π-conjugation in the ground state (Figure S42).
Increase in the oscillator strengths and decrease in the enegy
0 1
gaps of S –S transitions of the model compounds compared with
LUMO levels of the azobenzene moieties were estimated from
the onset potentials with electrochemical analyses. Cyclic
those of the monomer BAz-H (Table S6) were in good agreement
with the results of the UV–vis absorption spectra measurements
(Figure S37). Those should lead to enhamcement of PL
−
3
voltammetry (CV) was carried out in dichloromethane (1.0×10
of samples and 0.1 of tetrabutylammonium
hexafluorophosphate (NBu PF )). The results are shown in Figure
M
M
r
performances with increase in k and decrease in knr.
4
6
To evaluate validity of the plausible mechanism of AIE in BAz-
H via large structural relaxation in the excited state, optimized
structures of BAz-H both in the ground and excited states were
compared by (TD-)DFT calculations (Figure 4C). The optimized
structure in the ground state had the four-coordinated boron atom
and the slightly-bent azobenzene moiety (φ(C–N=N–C) = 165°) which
showed good agreement with the crystallographic data (Figure 1).
On the other hand, the calculated structure in the excited state
was obviously bent (φ(C–N=N–C) = 141°). Increase in the length of
the N=N double bond (1.271 Å for the ground state and 1.370 Å
for the excited state) was detected. It was implied that extension
of the bond length could cause the bending of BAz-H by excitation.
Based on these structural changes, the AIE behavior of BAz-H
can be clearly explained. As proposed in the optical
measurements, emission annihilation should occur during
structural alteration in the solution after excitation, meanwhile,
since there is hardly room to show structural relaxation in the
condensed state, emission can be recovered.17
S40 and Table S5. It was shown that the Az-H possesses the
similar LUMO level (−3.00 eV) with that of the conventional
electron-acceptor, benzothiadiazole (−2.86 eV). The LUMO levels
of BAz-H (−3.87 eV) and P-BAz (−3.98 eV) were lower by
approximately 0.8 eV than that of Az-H and P-Az (−3.20 eV)
indicating that the azobenzene derivatives intrinsically worked as
a strong electron-acceptor and the boron complexation is
responsible for further enhancement of electron-accepting ability.
Thus, the D–A type copolymer composed of BT and BAz can
exhibit the narrow band-gap property with the NIR emission.
The electronic states of the azobenzene derivatives were
supported by quantum calculation. Density functional theory
(
DFT) and time-dependent DFT (TD-DFT) were performed, and
the resulting molecular orbitals (MOs) were examined (Figures 4A
and S41). The HOMO−1s, HOMOs and LUMOs of azobenzene
were assigned to π, n and π* orbitals, respectively. In Az-H, the
HOMO−1 and HOMO consisted of the mixture of the n and π
orbitals derived from azobenzene because the optimized
structure was not planar due to the methoxy groups. The S –S
0 1
transitions were assigned to the forbidden transitions (f = 0.0000
for azobenzene and f = 0.0305 for Az-H, f: oscillator strength),
Finally, the photostability was evaluated as the change of the
UV–vis absorption spectra with irradiation by transilluminator (365
−
2
nm, 6,500 μW cm ) and white room light (the spectrum is shown
in Figure S47) in the diluted solution (1.0×10−5 M in chloroform).
The results are summarized in Figures S43, S44 and S45. The
pristine azobenzene moieties, Az-H and P-Az, rapidly showed the
typical spectral changes caused by cis–trans photoisomerization.
In contrast, the absorption spectrum of BAz-H was slightly altered
by UV irradiation. The spectral changes reached a plateau after 2
h for BAz-H (Figure S46). The obtained spectrum was completely
identified with that of the corresponding ligand 1, indicating that
the spectral changes were caused not by cis–trans
photoisomerization but by boron elimination. Meanwhile, the
spectrum of P-BAz was preserved even after irradiation with UV
and the white room light (Figure S44B). The photostability of the
0 2
and the S –S transitions were permitted ones (f = 0.7705 for
azobenzene and f = 0.5018 for Az-H) (Figure 4B). Conversely, in
BAz-H, the energy level of the MO derived from n orbital was
lowered after the boron complexation (HOMO−3), and instead of
the n orbital, the π orbital became the HOMO. Thereby, the S
0
–
S
1
transition was the permitted transition (f = 0.3118). These
results are similar with those from the Kawashima’s reports on 2-
borylazobenzene derivatives.12 Although the HOMO and LUMO
levels of azobenzene were lifted up by modification with the
methoxy groups, the LUMO level was strikingly lowered by the
boron complexation. The results of the TD-DFT calculation of
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model compounds BAz-M1 and BAz-M2 also increased. These
results indicate that the extension of π-conjugation contributed to
improving photostability. Considering the fact that P-BAz showed
more efficient emission property than BAz-H, it was reasonable
that the bending was restricted by extension of π-conjugation and
increasing the rigidity of the polymer main chain.
In conclusion, fused azobenzene–boron complexes and their
copolymers were successfully synthesized. By the B–N
0 1
coordination, the forbidden S –S transition in BAz-H was
transformed to the permitted transition. It was suggested that
elongation of the N=N double bond occurred by photoexcitation,
resulting in distortion of molecular frameworks. Based on this
structural relaxation in the excited state, AIE behaviors were
obtained. Owing to the strong electron-accepting ability of BAzs
and extension of the π-conjugation by copolymerizaition, the D–
A type copolymer P-BAz exhibited highly-efficient NIR emission
in both of the solution and the film states. The extension of the π-
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r
conjugation effectively increased k and decreased knr both of
which were essential for obtaining good PL performances. In
addition, P-BAz had higher photostability than the other
azobenzene derivatives. Those unique and useful properties
were attributed to the N=N double bond including fused structure
with B–N coordination. This concept shuold be applicable for
fabricating advanced optically-functional materials utilizing the
heteroatom-coordinated azobenzene as a key element-block.
1
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Angewandte Chemie International Edition
10.1002/anie.201803013
COMMUNICATION
Entry for the Table of Contents
COMMUNICATION
Masayuki Gon, Kazuo Tanaka*, Yoshiki
Chujo*
Page No. – Page No.
A Highly-Efficient Near-Infrared-
Emissive Copolymer with N=N
Double-Bond π-Conjugated System
Based on a Fused Azobenzene–
Boron Complex
N=N double bond: A nitrogen–nitrogen double bond (N=N) containing π-
conjugated copolymer showed a highly-efficient near-infrared (NIR) emission. The
N=N double bond should be a new element-block for realizing narrow band-gap π-
conjugated polymers.
This article is protected by copyright. All rights reserved.