BN-Embedded Polycyclic Aromatic Hydrocarbon Oligomers: Synthesis, Aromaticity, and Reactivity

Article abstract of DOI:10.1002/anie.202000556

BN-embedded oligomers with different pairs of BN units were synthesized by electrophilic borylation. Up to four pairs of BN units were incorporated in the large polycyclic aromatic hydrocarbons (PAHs). Their geometric, photophysical, electrochemical, and

Full text of DOI:10.1002/anie.202000556

A Journal of the Gesellschaft Deutscher Chemiker
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Accepted Article
Title: BN-Embedded Polycyclic Aromatic Hydrocarbon Oligomers:
Synthesis, Aromaticity, and Reactivity
Authors: Yijing Chen, Weinan Chen, Yanjun Qiao, Xuefeng Lu, and
Gang Zhou
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To be cited as: Angew. Chem. Int. Ed. 10.1002/anie.202000556
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Angewandte Chemie International Edition
10.1002/anie.202000556
RESEARCH ARTICLE
BN-Embedded Polycyclic Aromatic Hydrocarbon Oligomers:
Synthesis, Aromaticity, and Reactivity
[a]
[a]
[b]
[b]
[a]
Yijing Chen, Weinan Chen, Yanjun Qiao, Xuefeng Lu, and Gang Zhou*
[
a]
Dr. Y. Chen, W. Chen, Prof. Dr. G. Zhou
Lab of Advanced Materials, State Key Laboratory of Molecular Engineering of Polymers
Fudan University
Shanghai 200438, P. R. China
E-mail: zhougang@fudan.edu.cn
Dr. Y. Qiao, Prof. Dr. X. Lu
[
b]
Department of Materials Science
Fudan University
Shanghai 200438, P. R. China
Supporting information for this article is given via a link at the end of the document.
7
Abstract: A series of BN-embedded oligomers with different pairs of
BN units have been successfully synthesized through electrophilic
borylation processes. Organolithium intermediate is avoided in this
approach and thus multiple pairs of BN units up to four have been
facilely incorporated in the large polycyclic aromatic hydrocarbons
fundamental understanding of the aromaticity. Moreover, some
BN-embedded PAHs exhibit distinct electronic and optical
properties, making them promising candidates for various
8
optoelectronic applications. Up to date, the azaborine chemistry
9
has been well established by several groups In general, most
(
PAHs). Their geometric, photophysical, electrochemical, and Lewis
nucleophilic borylation requires a strong base, such as
acidic properties have been systematically investigated by X-ray
crystallography, optical spectroscopy, and cyclic voltammetry
measurements. It is found that the B-N bonds show delocalized
double-bond characteristics and the conjugation can be extended
through the trans-orientated aromatic azaborine units. Further
theoretical calculations reveal the relatively lower aromaticities for
the inner azaborine rings in the BN-embedded PAH oligomers.
Moreover, the frontier orbitals of the longer oligomers are
delocalized over the inner aromatic rings with negligible weights on
the two terminal sides. Consequently, the inner moieties of the BN-
embedded PAH oligomers are more active than the outer parts. This
assumption is further confirmed by a simple oxidation reaction and
the product is verified by single crystal X-ray analysis. Most
interestingly, the oxidation has significant effects on the aromaticities
and the intramolecular charge transfer interactions in these BN-
embedded PAHs. Overall, this work may provide a useful bottom-up
strategy to construct BN-containing PAHs towards BN-embedded
graphene nanoribbons.
butyllithium.10 However, the basic character of organolithium
reagents may cause undesired side reactions, limiting the range
of structural modifications. Moreover, the limited solubility of
intermediate aryllithium in organic solvents makes it more
difficult to access PAHs with multiple BN units.
Conjugated oligomers, which possess distinct electronic
properties and facilitate the understanding of the fundamental
structure-property relationship, are particularly desired for both
chemistry and material scientists.11 However, little research has
focused on the conjugated oligomers with multiple BN units
probably due to the lack of a suitable methodology.12 Although
significant progress has been achieved in azaborine chemistry,
extending the BN-doped heteroaromatics to much larger π-
conjugated systems and the construction of BN-embedded PAH
oligomers still remain challenging.
Herein, we report a strategy to construct BN-embedded PAH
oligomers BN1-BN4 (Figure 1) by electrophilic borylation
approach which avoids the use of organolithium intermediates.
Therefore, large polycyclic aromatic hydrocarbons with multiple
pairs of BN units can be facilely realized. The embedded BN
Introduction
R
R R
R
R
R
R
R
S
S
S
S
S
S
S
S
Polycyclic aromatic hydrocarbons (PAHs) have attracted
tremendous interest because of their extensive applications in
optoelectronic devices, such as light-emitting diodes (LEDs),1
B
N
B
N
B
N
B
N
N
B
N
B
field-effect transistors (FETs),2 and organic photovoltaics
S
S
S
S
3
(
OPVs). The development of heteroatom-substituted PAHs has
BN1
BN2
R
BN3
R
R
R
R
been a hot topic in the past decade since they are useful and
indispensable elements to acquire novel materials with unique
R
R
R
4
properties and tailored functions. Among a variety of possible
S
S
S
S
B
N
B
dopants, boron and nitrogen have received particular attention
N
5
due to their distinct electronic and optical characteristics. One
N
B
N
R = n-C6H13
popular strategy for doping both boron and nitrogen into PAHs is
substituting one or more C=C units by B−N units, which can be
considered as zwitterionic double bonds in the neutral state.6
B
S
S
S
S
R
R
R
R
The replacement of
a
C=C bond by an isosteric and
BN4
isoelectronic B−N bond not only expands the structural diversity
of organic conjugated materials but also enriches the
Figure 1. Chemical Structures of BN-Embedded PAH Oligomers BN1-BN4.
1
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Angewandte Chemie International Edition
10.1002/anie.202000556
RESEARCH ARTICLE
units are in trans-orientation in these PAH oligomers. Benzene
and thiophene rings are fused on the 1,2-azaborine rings. Their
geometric, photophysical, electrochemical, and Lewis acidic
properties have been systematically investigated by X-ray
crystallography, optical spectroscopy, and cyclic voltammetry
measurements. Theoretical calculations were also performed to
gain further insight into the aromaticity properties and molecular
orbitals of the BN-embedded PAH oligomers. It is found that the
B-N bonds show delocalized double-bond characteristics, but
weaker than C=C bonds. Moreover, the molecular orbitals tend
to delocalize on the conjugated skeletons through C=C bonds
instead of azaborine rings. Consequently, the inner moieties of
the BN-embedded PAH oligomers are more active than the two
terminal parts. This assumption was further confirmed by a
simple oxidation reaction and the product was verified by single
crystal X-ray analysis. Interestingly, unlike the BN-embedded
oligomers, remarkable intramolecular charge transfer (ICT)13
interactions can be found in the oxidized products.
dibromo-N,N′-bis(2-bromophenyl)benzene-1,4-diamine
respectively, via Buchwald coupling reaction. Then precursors
N1 and N2 were produced by Suzuki coupling of 2 or 3 and 2-(5-
hexylthienyl)-4,4,5,5-tetramethyl-1,3,2-dioxaborolane.
(3),
For
precursors N3 and N4, to avoid the side reactions and simplify
the synthesis, partial pendant hexylthienyl groups were
incorporated initially. As shown in Scheme 2, the synthetic route
for precursors N3 and N4 both began with 2,5-bis(5-
hexylthienyl)-1,4-phenylenediamine (4) which was prepared
according to previously reported literature.17 After two
asymmetric Buchwald coupling with 2-(2-bromophenyl)-5-
hexylthiophene and 2,5-dibromo-N-(2-(5-hexylthienyl)phenyl)-4-
iodoaniline, the corresponding trimer backbone (6) was provided
and further converted to precursor N3 by attaching two pendant
hexylthienyl groups. Similarly, the skeleton of the tetramer 7 was
afforded by
compound
a
symmetric Buchwald coupling reaction of
and 2.2 equiv. of 2,5-dibromo-N-(2-(5-
4
hexylthienyl)phenyl)-4-iodoaniline.
Then,
the
pendant
hexylthienyl groups were attached on the backbone of the
tetramer via Suzuki coupling reaction. Finally, the oligoaniline
Results and Discussion
Synthesis and Structure Characterization. The synthetic
approach to BN-embedded PAHs BN1-BN4 involved
a
construction of oligoaniline precursors N1-N4 and ended by an
14
electrophilic borylation in the presence of boron bromide and
triethylamine (Scheme 1). To prepare the oligomeric precursors,
15
16
palladium-catalyzed Buchwald and Suzuki coupling reactions
are alternately conducted to construct the oligoaniline
backbones. As shown in Scheme 2, the synthesis of precursors
N1 and N2 both started from 2-bromoaniline (1), which was
directly converted to bis(2-bromophenyl)amine (2) and 2,5-
Scheme 1. Borylation of Oligoanilines into BN-Embedded PAH Oligomers.
R
R
Br
H
N
Br
R
BPin
Br
S
S
S
BBr3 / Et N
3
H
N
I
BN1
o-DCB / 180 o
C
Pd(PPh3)2Cl2 / K2CO3
o
Toluene / H2O / 85
72%
C
74%
9
1%
Pd2(dba)3 / t-BuONa
Toluene / 100 o
6%
2
Br
N1
NH2
R
R
C
S
S
7
Br
H
N
Br
H
N
1
Br
R
S
BPin
BBr3 / Et3N
o-DCB / 180 oC
51%
I
BN2
Pd(PPh3)2Cl2 / K2CO3
o
N
H
N
H
I
Toluene / H2O / 85
C
Br
Br
89%
S
S
Br
3
R
R
R
N2
S
R
R
R
Br
Br
H
N
R
R
R
R
R
R
R
S
S
S
S
S
S
S
S
S
S
H
H
N
Br
Br
H
H
H
N
I
R
Br
N
N
R
BPin
N
S
BBr3 / Et N
3
BN3
S
Pd (dba) / t-BuONa
Pd(PPh ) Cl
o-DCB
NH2
2
3
N
H
3
2
2
N
H
o
6
5%
Pd2(dba)3 / t-BuONa
Toluene / 100 o
8%
Toluene / 100
C
180 o
K2CO3 / Toluene / H2O
85 oC / 69%
C
H2N
42%
S
S
S
S
42%
5
C
NH2
R
R
R
N3
R
6
4
R
R
R
R
R
R
R
S
S
S
S
S
S
S
S
R
H
Br
Br
H
N
Br
H
N
Br
H
N
H
N
4
N
R
BPin
3
BBr3 / Et N
S
BN4
I
N
H
N
H
Pd(PPh ) Cl / K CO
3
Toluene / H2O / 85 oC
0%
N
H
N
H
o-DCB / 180 o
C
3
2
2
2
Br
Br
31%
S
S
S
S
S
S
5
R = n-C6H13
R
R
R
R
R
R
7
N4
Scheme 2. Synthetic Routes for BN-Embedded PAH Oligomers BN1-BN4.
2
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Angewandte Chemie International Edition
10.1002/anie.202000556
RESEARCH ARTICLE
precursors N1-N4 were converted to PAH oligomers BN1-BN4
by refluxing with boron bromide and triethylamine in o-
dichlorobenzene. The isolated yields of the oligomers decreased
from 74% to 31% with the increasing number of BN pairs
obviously due to the enhanced borylation sites. The target
compounds were characterized by H NMR, C NMR, and
HRMS spectroscopies, and were found to be consistent with the
proposed structures. In addition, the MALDI-TOF mass spectra
(a)
(b)
1
13
(c)
(d)
(
Figure 2) exhibited single intense signals corresponding to the
P-helix
M-helix
calculated masses of the BN-embedded PAH oligomers.
To verify the successful doping of BN units into the PAHs and
investigate the molecular structural features, single crystal X-ray
diffraction analysis was performed for BN1 and BN2. The single
crystal of BN1 was obtained as white lamellar crystal by
diffusion of hexane into its chloroform solution and slow
evaporation at room temperature. Single crystal X-ray analysis
dπ-π = 3.51 Å
(
Figure 3, Table S1) demonstrates that the molecules of BN1
are aligned in a triclinic unit cell. The length of the B-N bond is
.4555(18) Å (Figure 3b), which is shorter than the value for a B-
Figure 3. (a) ORTEP drawing of BN1 with thermal ellipsoids shown at 50%
probability. (b) Selected bond lengths for BN1. (c) P- and M-helical structures
of BN1. (d) Crystal packing of BN1 (Hydrogen atoms are omitted for clarity).
1
18
N single bond (1.58 Å), but longer than a localized B=N double
bond (1.40 Å).19 These data indicate the presence of the
delocalized BN double bond character and the effective
conjugation extension through the BN bond. On the other hand,
the bond length of the adjacent N1-C1, N1-C17, B1-C8, and B1-
C24 is 1.4271(16), 1.4285(17) 1.530(2), and 1.5306(19) Å,
respectively, suggesting the characteristics of single bonds.
While C1-C6 (1.4172(19) Å), C7-C8 (1.3786(18) Å), C17-C22
which agrees with the fact that BN1 possesses an extended π-
conjugated backbone through the two azaborine rings.
Unfortunately, the single crystal of BN2 was not successfully
obtained probably owing to the interruption of the packing
structures by the four hexyl chains. To provide further insight
into the chemical structures of the BN-embedded oligomers, the
hexyl substituents in BN2 were shortened to methyl groups and
analogue Me-BN2 was synthesized similarly (Scheme S2). The
single crystal suitable for X-ray crystallography was grown by
slow evaporation of acetone into the solution of Me-BN2 in
carbon disulfide. As shown in Figure 4b, the bond lengths of the
two B-N bonds were 1.447(3) and 1.454(3) Å, displaying as
delocalized double bonds, similar to that in BN1. Moreover, the
bond lengths of C10-C11 (1.436(3) Å) and C33-C34 (1.434(3)Å)
in the outer azaborine rings are slightly shorter than those of C4-
C5 (1.443(3) Å) and C23-C25 (1.442(3) Å) in the inner azaborine
rings, indicating the different charge delocalization effects and
aromaticities between the outer and inner azaborine rings.
Furthermore, Me-BN2 has four cove regions. Two types of
distortions can be found due to the different steric hindrances of
hydrogen atoms on the benzene and thiophene rings. The
dihedral angle between the benzene rings fused on the
azaborine units are 33° and 36°, respectively, while the dihedral
angles between the two thiophene rings are 8° and 17°,
respectively (Figure 4b). Additionally, P- and M-helical structures
(1.4148(18) Å), and C23-C24 (1.3801(18) Å) bonds in the
azaborine rings all exhibit as delocalized double bonds, which
indicates the aromaticities of the two azaborine rings and is
further supported by nucleus-independent chemical shift (NICS)
20
calculations. Moreover, as shown in Figure 3c, the two
azaborine rings in BN1 are slightly twisted due to the steric
repulsion between the hydrogen atoms at the ortho positions of
the heteroatoms. The dihedral angle between the two thiophene
rings fused on the azaborine units is measured to be 23°. While
the dihedral angle between the two benzene rings is 40°. It is
interesting to note that two different conformations can be found
in the same crystal. Each space unit contains a couple of P- and
M-helical structures with BN dipole moments opposite to each
other (Figure 3c). Furthermore, the molecules are arranged in an
offset head-to-tail stacking array along the c axis (Figure 3d).
The π-π distance between P-helix and M-helix is around 3.51 Å,
(
Figure 4c) can be also found in the single crystal of Me-BN2.
Unlike BN1, the P- and M-helical structures in Me-BN2 form
separated arrays. The π-π distances in the two arrays are both
3
.69 Å (Figure 4d), which is slightly higher than that in BN1 and
owing to the more twisted structures of Me-BN2.
Photophysical Properties. The photophysical properties of
BN-embedded PAH oligomers BN1-BN4 in THF solutions (ca.
1 -
0
6
M) were investigated by UV−vis absorption and
photoluminescence (PL) spectroscopies (Figure 5 and Table1).
All the PAHs exhibit two distinct absorption bands. The intense
absorption band in the high energy region typically originates
from the S
0
→S
2
transition and the rather weak shoulder in low
→S transition. As
energy region can be assigned to the S
0
1
shown in Figure 5, the maximum absorption wavelength of BN1
locates at 357 nm. Upon increasing the number of the BN units,
the absorption maximum gradually shifts to 415, 419, and 420
Figure 2. Maldi-Tof mass spectra with expanded views of the molecular-ion
regions of BN-embedded PAH oligomers BN1-BN4.
3
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Angewandte Chemie International Edition
10.1002/anie.202000556
RESEARCH ARTICLE
(
a)
(b)
1.0
0.5
0.0
1.0
BN1
BN2
BN3
BN4
0.5
0.0
(
c)
(d)
P-helix
M-helix
300
400
500
600
Wavelength (nm)
dπ-π = 3.69 Å
Figure 5. UV-vis absorption (dashed line) and PL spectra (solid line) of the
BN-embedded PAH oligomers BN1-BN4 in THF solutions (10 M).
-6
Figure 4. (a) ORTEP drawing of Me-BN2 with thermal ellipsoids shown at
0% probability. (b) Selected bond lengths for Me-BN2. (c) P- and M-helical
structures of Me-BN2. (d) Crystal packing of Me-BN2 (Hydrogen atoms are
comparison to those in THF solutions, which is attributed to the
π-π stacking interactions among the PAH molecules in solid
states. Under the sameꢀconditions, the PL maxima of BN1-BN4
locates at 421, 468, 496, and 504 nm, respectively (Figure S3b).
A distinct bathochromic shift of 33, 46, 67, and 71 nm can be
observed in comparison to those in THF solutions, which are
much larger than those in corresponding absorption spectra and
probably owing to the formation of excimers upon excitation.
Coordinations with Fluoride Ions. It is well known that
three-coordinate boron contains an empty orbital, which can be
5
omitted for clarity).
nm for BN2, BN3, and BN4, respectivly. The bathochromic shift
among the PAH oligomers stepwise reduced from 58 to 4 nm
and further to 1 nm. This is obviously due to the extension of the
effective conjugation length, which is a well-known behavior in
21
conjugated oligomers. Moreover, with the increasing oligomer
length, the absorption shoulder in low energyꢀregion gradually
increases. Originally, the S →S
0 1
transition is symmetry forbidden. occupied by a Lewis base.23 To study their Lewis acidic
However, upon extending the PAH framework, the molecular
symmetry is reduced, thus increasing the oscillator strength of
properties, the BN-embedded PAH oligomers were titrated with
fluoride ions by monitoring their absorption and PL spectra.
Upon addition of tetra-n-butylammonium fluoride (TBAF) to the
solutions of the BN oligomers in THF (ca. 10-6 M), distinct
bathochromic shifts can be observed for both absorption and PL
maxima of the BN oligomers. As shown in Figure S4, the
maximum absorption wavelengths of BN1-BN4 shift from 357,
415, 419, and 420 nm to 385, 435, 461, and 480 nm,
respectively, after adding excess fluoride ions. Similarly,
stepwise addition of TBAF to the oligomer solutions
bathochromically shift their PL maxima from 388, 422, 429, and
433 nm to 438, 490, 510, and 530 nm by 50, 68, 81, and 97 nm,
respectively (Figure S5). All the absorption and PL changes
indicate the appearance of the F-BN complexes and originate
from the occupation of the empty orbitals of the boron atoms by
fluoride ions. It should be noted that the bathochromic shifts for
the longer oligomers are more remarkable than the shorter ones
probably owing to the more coordination sites in the longer
oligomers. Therefore, more significant fluorescence changes
can be found for longer oligomers by naked eyes upon addition
of TBAF. As shown in Figure 6, no obvious color change can be
observed for BN1, while the fluorescence of the BN2, BN3, and
BN4 change from blue to bluish green, green, and pale yellow.
Moreover, the emission colors of all oligomers can be recovered
to their original states after adding one drop of water due to the
decomposition of the fluoride coordinated boron anions.
the
corresponding PL spectra of BN1-BN4 are peaked at 388, 422,
29, and 433 nm with a vibronic shoulder around 411, 445, 454,
and 457 nm, respectively, which can be assigned to the 0-0 and
-1 singlet transitions and indicates that the effective conjugation
S
0
→S
1
transition.22 Under the same conditions, the
4
0
length is not saturated in the BN-embedded PAH tetramer. With
the number of BN units increasing, the intensity of the shoulder
peak increases stepwisely, which suggests that the 0-1 singlet
transition is activated in a longer conjugation system due to the
enhanced electron transition rate. Moreover, the bathochromic
shifts in the PL spectra among the PAH oligomers are 34, 7, and
4
nm, respectively, which are different from those in
corresponding absorption spectra. This suggests that the
backbone geometry changes significantly between the ground
state and the vibronically relaxed excited state.
It is well known that the presence of arylamine donors and
arylborane acceptors result in remarkable charge transfer (CT)
characteristics. To further investigate the CT interactions in the
BN-embedded PAHs, the solvatochromic effects on the
absorption and PL features were investigated (Figures S1 and
S2). The four PAHs exhibit relatively weak solvatochromism
(λmax shift < 13 nm when solvents ranging from hexane to DMF).
This indicates that the CT interactions in the BN-embedded PAH
oligomers are rather poor, which may be a result of the
delocalized BN double-bond characteristics of the oligomers.
In contrast to the absorption spectra in THF solutions, the
films of BN1-BN4 display maximum absorption wavelengths at
To gain insight into the electrochemical behaviors of the BN-
embedded PAH oligomers BN1-BN4 and their fluoride
coordinated complexes, cyclic voltammetry (CV) measurements
were conducted in anhydrous DCM. As depicted in Table 1 and
Figure S6, all the oligomers exhibit irreversible oxidation
366, 420, 425, and 427 nm, respectively (Figure S3a). A
bathochromic shift of 9, 5, 6, and 7 nm can be observed in
4
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Angewandte Chemie International Edition
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RESEARCH ARTICLE
Table 1. Photophysical Data of BN-embedded PAHs BN1-BN4.
Comp.
λ
nm)
abs
ε
λ
abs
λ
PL
λ
PL
Stokes shift
(cm )
E
(eV)
g
E
(V)
onset
E
HOMO
E
(eV)
LUMO
[e]
[
a]
4
-1
-1
[b]
[a]
[b]
-1
[c]
(eV)[d]
-5.38
-5.28
-5.24
-5.22
(
(10 M cm )
(nm)
(nm)
(nm)
BN1
BN2
BN3
BN4
357
415
419
420
0.8
1.3
3.0
4.4
366
420
425
427
388
422
429
433
421
468
496
504
689
400
556
715
3.22
2.81
2.71
2.66
0.58
0.48
0.44
0.42
-2.16
-2.47
-2.53
-2.56
[
a] Absorption (λabs) and PL (λPL) maxima were measured in THF solutions (10-6 M). [b] Absorption (λabs) and PL (λPL) maxima were measured on quartz
substrates. [c] E is optical band gap. [d] Calculated from the onset of the first oxidation potential. EHOMO = −(Eonset + 4.80) eV. [e] ELUMO = E
g
g
+ EHOMO.
ones. One plausible explanation is that the benzene ring
neighboured to the terminal azaborine unit is much easier to
resonate as compared with the one fixed between two inner
azaborine rings. Consequently, the double bonds between the
terminal azaborine unit and the neighbouring benzene ring may
have larger bond resonance energy than the corresponding one
in the inner azaborine rings, which thus contributes to the higher
aromaticities for the two terminal azaborine rings in comparison
to the other inner ones.26 Although the aromaticities of the
azaborine rings in these BN-embedded PAHs are relatively
lower than the corresponding aromatic rings in the related
isoelectronic carbon analogs (Figure 7), on the other hand, only
negligible changes can be observed for the NICS(1) values of
the benzene and thiophene rings neighboured to the azaborine
ring as compared with those for the C=C analogues, suggesting
that the borylation has little influence on the aromaticities of the
surrounded aromatic rings.
Figure 6. Emission images of the BN-embedded PAH oligomers in THF
solutions upon addition of Bu NF.
4
processes, and the oxidation onsets locate at 0.58, 0.48, 0.44,
+
and 0.42 V (vs. Fc /Fc) for BN1, BN2, BN3, and BN4,
respectively. Correspondingly, the highest occupied molecular
24
orbital (HOMO) energy level gradually lifts up from -5.38 eV for
BN1 to -5.22 eV for BN4 as the number of BN units increases.
While the LUMOs of the oligomers are -2.16, -2.47, -2.53 and -
Density functional theory (DFT) calculations27 were also
performed at the B3LYP/6-31G(d) level to learn the electronic
properties of the BN-embedded PAH oligomers. As shown in
Figure 8, the LUMO of BN1 delocalizes over the whole aromatic
skeleton, while the HOMO has no weight at the boron atom.
Unlike previously reported heteroatom-substituted PAHs
containing separated boron and nitrogen where the charge can
facilely transfer from the nitrogen center to the boron center,28
only slight charge separation can be found in BN1, which results
in the weak charge transfer interactions in azaborine containing
compounds.29 Similarly, with the BN-conjugation extension,
oligomers BN2, BN3, and BN4 demonstrate delocalized
HOMOs but without any distribution on the boron atoms.
Moreover, both HOMOs and LUMOs of oligomers BN2, BN3,
and BN4 are partially delocalized over the inner aromatic rings
with ignorable weights on the two terminal sides. These results
suggest that the inner aromatic moieties are more reactive than
the two terminal parts. Most interestingly, oligomers BN2, BN3,
and BN4 display nodal-shaped HOMOs with boundaries at the
BN units. This suggests that the conjugation extension in the
vertical backbone is more pronounced on the effective
conjugation length than the other horizontal one. The calculated
energies, oscillator strength, and compositions of major
electronic transitions are shown in Tables S5–S8 and the
2
.56 eV, respectively. Consequently, with the π-conjugation
extension in the BN-embedded oligomers, the HOMO levels
gradually increase while the LUMO level dramatically decreases,
resulting in stepwisely reduced band gap. This suggests the
conjugation extension through the azaborine rings and the
charge delocalization in the BN-embedded PAH oligomers.
When the boron in the azaborine is coordinated by fluoride ion,
the onsets of the first oxidation potentials of BN1-BN4 locate at -
0.06, -0.13, -0.32, and -0.37 V (Figure S7). A significant negative
shift can be found for all the oligomers, which is attributed to the
anionic four-coordinate boronate converted from an electron
acceptor into a strong electron donor. Moreover, a more
significant shift of the first oxidation potential can be observed
for the longer oligomers. This originates from the more
coordination sites in the longer oligomers, resulting in more
negative charge after coordinating fluoride ions.
Theoretical Approaches. To further understand the
aromaticities of the BN-embedded PAH oligomers, NICS
calculations were carried out. As shown in Figure 7, the two
azaborine rings in BN1 show a NICS(1) value of −4.1 ppm,
which is higher than that for the reported BN-fused
dibenzo[g,p]chrysene (-2.9 ppm). These results suggested that
the incorporation of thiophene ring instead of benzene ring has
indeed successfully improved the aromaticity of the azaborine
25
7a
ring. Most interestingly, with the extension of the oligomer length, calculated HOMO–LUMO levels for the oligomers are consistent
the NICS(1) values of the azaborine rings exhibit two different
sets of values in each oligomer. The NICS(1) values of the two
terminal azaborine rings are ranged between -4.0 ~ -4.2 ppm,
while -3.5 ~ -3.7 ppm for the other inner azaborine rings
embedded in the molecules. This indicates that the aromaticities
of the terminal azaborine rings are higher than the other inner
with those experimentally determined ones.
Oxidations of the BN-embedded PAHs. In addition to the
coordinations with fluoride ions, the as-prepared BN-embedded
PAH oligomers can be oxidized since the sulfur in thiophene
rings can be easily oxidized into sulfoxide and further to sulfone.
As shown in Scheme 3, the oxidation reaction was performed
5
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RESEARCH ARTICLE
Figure 7. Calculated NICS(1) values (in ppm) of BN-embedded PAH oligomers BN1-BN4 and corresponding isoelectronic carbon analogs CC1-CC4.
Figure 8. Calculated frontier orbitals of BN-embedded PAH oligomers BN1-BN4.
by stirring the mixture of BN1 and meta-chloroperoxybenzoic
acid (m-CPBA) in DCM and water at room temperature. A
sulfone compound BN1-O was obtained as the major product.
2
The successful oxidation of only one thiophene ring was
verified by its asymmetric aromatic structure in the low-field
1
region of the H NMR spectrum (Figure S44). The oxidation of
sulfur into sulfone instead of sulfoxide was proved by HRMS
spectroscopy. Moreover, the delocalization of the HOMOs and
LUMOs in the inner parts of the BN-embedded PAH oligomers
suggest that the inner parts are more active than the outer parts
although less steric hindrance may be found for the two
terminal sides of the oligomers. To verify this assumption, BN2
was also chosen to carry out the oxidation reaction since two
environmentally different thiophene rings can be found in the
skeleton of BN2, i.e., two thiophene rings in the outer part and
two in the inner part. As shown in Scheme 3, under the same
condition, a sulfone compound BN2-O
2
was obtained as the
Scheme 3. Oxidation of the BN-embedded PAHs.
major product. The relatively low isolated yield for the oxidized
product is due to the extremely close polarities of the other
mono-oxidized by-products. Moreover, the mono-oxidized
products can be further oxidized into multiple oxidized species
by simply feeding more m-CPBA. However, no selectivity can
6
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Angewandte Chemie International Edition
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RESEARCH ARTICLE
be found for the second oxidation due to the dramatically
changed frontier orbitals of the mono-oxidized products.
To further confirm the oxidation at the inner thiophene rings
and exclude the formation of two sulfoxide units, single crystal
X-ray diffraction analysis was performed. Similar to Me-BN2,
1.346(6) Å. Thus, the shortened bond may significantly affect
the aromaticities of the aromatic rings. As shown in Figure 10,
the calculated NICS(1) value of the oxidized thiophene ring
changes from -9.4 to -0.9 ppm, obviously due to the loss of two
π
electrons on sulfur atom. Moreover, the neighboring
methyl substituted analogue Me-BN2-O
under the same condition and the single crystal was obtained
by slow solvent diffusion method. As shown in Figure 9, the
2
was synthesized
azaborine ring becomes more aromatic with the NICS(1) value
reducing from -3.7 to -4.6 ppm. This suggests that the oxidation
has a strong influence on the aromaticities of the surrounded
aromatic rings. Furthermore, although the oxidized product Me-
backbone of Me-BN2-O
to Me-BN2. However, upon oxidation, the molecules alignments
changes from monoclinic system for Me-BN2 to an
orthorhombic system for Me-BN2-O . Moreover, the length of
2
adopts a twisted conformation similar
2
BN2-O exhibits an asymmetric structure, two types of cove
a
regions with similar distortion angels can be observed in
comparison to Me-BN2 (Figure 9b). In addition, both P- and M-
helical structures can be found in the single crystal of Me-BN2-
2
the B-N bond in the azaborine ring fused on the thiophene
dioxide ring slightly changes from 1.454(3) to 1.456(3) Å,
confirming their essentially double-bond character. In
comparison to the negligible change of the B-N bond, however,
the length of the C-C bond shared by the thiophene dioxide and
the neighboring azaborine rings decreases from 1.379(3) to
2
O (Figure 9c). However, unlike Me-BN2, the P- or M-helical
structures are arranged in an offset face-to-face stacking array
with a π-π distance of 3.65 Å (Figure 9d). All these single
crystal structure date further confirm our assumption that the
inner parts of the BN-embedded PAH oligomers are more
active than the outer parts.
(
a)
(b)
Furthermore, the oxidation product BN2-O
absorption and PL maxima at 455 and 522 nm in THF solution
Figures S8 and S9). Unlike BN1 and BN2 whose
solvatochromic effects are very weak, significant solvent-
dependent PL spectra can be found for both BN1-O and BN2-
. A bathochromic shift of 78 and 56 nm of the maximum PL
wavelength can be observed for BN1-O and BN2-O
respectively, when the solvent polarity increased from hexane
462 nm for BN1-O and 492 nm for BN2-O ) to N,N-dimethyl-
formamide (540 nm for BN1-O and 548 nm for BN2-O ). Such
2
displays
(
2
O
2
2
2
,
(
2
2
(
c)
(d)
2
2
a bathochromic shift is characteristic of an efficient charge
transfer from the electron-donating moiety to the electron-
withdrawing moiety. As shown in Figure 10, significant
separation of the HOMO and LUMO can be found for both
P-helix
M-helix
BN1-O
2
2
and BN2-O , which indicates the notable ICT
interactions in the oxidized products. Therefore, these results
suggest that the oxidation has a significant effect on the ICT
interactions in these BN-embedded PAH oligomers.
dπ-π = 3.65 Å
Figure 9. (a) ORTEP drawing of Me-BN2-O
2
with thermal ellipsoids shown at
5
0% probability. (b) Selected bond lengths for Me-BN2-O
helical structures of Me-BN2-O (d) Crystal packing of Me-BN2-O
Hydrogen atoms are omitted for clarity).
2
. (c) P- and M-
2
.
2
Conclusion
(
In summary, we report the straightforward synthesis of four
BN-embedded oligomers via electrophilic borylation processes.
Within this method, multiple pairs of BN units up to four have
been successfully incorporated in the large polycyclic aromatic
hydrocarbons. Single crystal X-ray analysis indicates the
delocalized BN double bond characteristics in these PAH
oligomers. The photophysical, electrochemical, and theoretical
approaches further reveal that the conjugation can be extended
through the azaborine rings. Moreover, the delocalization of
frontier orbitals and the relatively lower aromaticitis in the inner
moieties of the BN-embedded PAH oligomers suggest that the
inner parts are more active than the outer parts, which is further
verified by a simple oxidation reaction. Most interestingly, the
oxidation has significant effects on the aromaticities and the
ICT interactions in these BN-embedded PAHs. Overall, this
work may help to understand the intriguing electronic and
optical properties of the BN-containing aromatics and provide a
pathway for designing BN-embedded PAHs for further
optoelectronic applications.
Figure 10. Calculated NICS(1) values (in ppm, a and c) and frontier orbitals
b and d) of BN1-O and Me-BN2-O
(
2
2
.
7
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Angewandte Chemie International Edition
10.1002/anie.202000556
RESEARCH ARTICLE
Zhang, K. S. Schellhammer, P. Machata, F. Ortmann, G. Cuniberti, Y.
Fu, J. Hunger, R. Tang, A. A. Popov, B. Berger, K. Müllen, X. Feng, J.
Am. Chem. Soc. 2016, 138, 11606−11615. e) S. Tsuchiya, H. Saito, K.
Nogi, H. Yorimitsu, Org. Lett. 2019, 21, 3855−3860.
Acknowledgements
This work was financially supported by the National Natural
Science Foundation of China (21674023, 51722301, 51903052,
and 21733003), Shanghai Pujiang Project (19PJ1400700), and
National Key R&D Program of China (2018YFA0209401).
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Keywords: aromaticity • azaborine • heterocycle • oligomer •
polycyclic aromatic hydrocarbon
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Entry for the Table of Contents
Polycyclic aromatic hydrocarbons (PAHs) containing multiple pairs of BN units have been designed and successfully synthesized
through electrophilic borylation processes. The results suggest the delocalized BN double bond characteristics and π-conjugation
extension in these PAH oligomers. Moreover, the inner moieties of the oligomers are more active than the outer ones as revealed by
both theoretical and experimental approaches.
9
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