Synthesis, Characterization, and Properties of Bis-BN Ullazines

Article abstract of DOI:10.1021/acs.orglett.8b00554

A series of bis-BN ullazine derivatives, including the parent species, were synthesized in a small number of steps from commercially available materials. X-ray crystallographic analysis revealed that bis-BN ullazines have rigid and planar frameworks. Most of the bis-BN ullazines are stable toward air and moisture. In addition, the absorption and emission bands of these ullazines are blue-shifted, compared to those of their carbonaceous ullazine analogs.

Full text of DOI:10.1021/acs.orglett.8b00554

Letter
pubs.acs.org/OrgLett
Cite This: Org. Lett. XXXX, XXX, XXX−XXX
Synthesis, Characterization, and Properties of Bis-BN Ullazines
Chenglong Li,† Yuming Liu,† Zhe Sun,† Jinyun Zhang,† Meiyan Liu,† Chen Zhang,† Qian Zhang,†
Hongjuan Wang,§ and Xuguang Liu*,†,‡
†Tianjin Key Laboratory of Organic Solar Cells and Photochemical Conversion, School of Chemistry and Chemical Engineering,
Tianjin University of Technology, Tianjin 300384, People’s Republic of China
‡Key Laboratory of Synthetic and Self-Assembly Chemistry for Organic Functional Molecules, Shanghai Institute of Organic
Chemistry, Chinese Academy of Sciences, 345 Lingling Road, Shanghai 200032, People’s Republic of China
§Institute of New Energy Materials & Low Carbon Technology, School of Material Science & Engineering, Tianjin University of
Technology, Tianjin 300384, People’s Republic of China
S
* Supporting Information
ABSTRACT: A series of bis-BN ullazine derivatives, including the parent species, were synthesized in a small number of steps
from commercially available materials. X-ray crystallographic analysis revealed that bis-BN ullazines have rigid and planar
frameworks. Most of the bis-BN ullazines are stable toward air and moisture. In addition, the absorption and emission bands of
these ullazines are blue-shifted, compared to those of their carbonaceous ullazine analogs.
reaction in which two boron atoms are bonded to a central
pyrrole. Structures with multiple boron atoms attached to a
simple aromatic ring has been reported in the literature.15 In
addition, BN-aromatics based on pyrrole are rare.16 We
envisioned that replacing CC units with B−N units will
cause this new class of BN-heterocycles to have interesting
photophysical properties (Figure 1).
olycyclic aromatic hydrocarbons (PAHs) have captured
Pthe attention of the scientific community. Notably, the
formal replacement of a CC bond with an isoelectronic and
isostructural B−N bond can generate novel PAHs with
different properties.1 These unique characteristics of BN-
aromatics make their application in materials chemistry quite
interesting. The synthesis and properties of a variety of BN-
aromatics, including BN-naphthalenes,2 BN-phenanthrenes,3
BN-anthracenes,4 BN-pyrene,5 BN-helicenes,6 BN-indene,7
BN-coronenes and nanographene,8 and higher BN-doped
PAHs,9,10 have been well documented. BN-containing PAHs
have been explored as promising components for organic light-
emitting diodes (OLEDs),9 organic field-effect transistors
(OFET),10a,b and organic photovoltaics (OPVs).10c BN-
aromatics are predicted to be uniquely able to serve as singlet
fission materials.11
Figure 1. Ullazine and bis-BN ullazine.
Ullazine, which is isoelectronic with pyrene, is a type of PAH
that contains a conjugated 16π-system with one nitrogen atom
in the center. Balli and Zeller synthesized the first ullazine in
1983.12 In the past two decades, several synthetic protocols
have been reported, and most of them are Lewis acid catalyzed
or transition-metal-catalyzed annulation reactions.13 Ullazines
are of great interest in the field of optoelectronics and,
BN-pyrrolo[1,2-a]quinoline, which can be considered “half”
of the bis-BN ullazine, was synthesized first. The starting
material, 1-(2-nitrophenyl)-1H-pyrrole (1), was prepared
through a Clauson−Kaas reaction between 2-nitroaniline and
2,5-dimethoxytetrahydrofuran.17 Subsequently, reduction of
compound 1 using NaBH4 with BiCl3 gave 2-(1H-pyrrol-1-
yl)aniline (2) in good yield. Gratifyingly, electrophilic
borylation of 2 with PhBCl2 produced BN-pyrrolo[1,2-
a]quinoline (3b) in moderate yield (Method B, Scheme 1).
14
particularly, in dye-sensitized solar cells (DSCs).13a,b
and
Surprisingly, BN isosteres of ullazines remain completely
unexplored; thus, the present study describes, to the best of
our knowledge, the first synthesis of BN ullazines, including the
parent compound. Our synthesis is based on a simple, highly
efficient, dual Friedel−Crafts-type electrophilic borylation
Received: February 22, 2018
© XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.8b00554
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
ppm range, which suggests the compound is aromatic. The
chemical shift of the B−H proton in 6d is at approximately 5.21
ppm (doublet), which is similar to the peaks reported for other
unsubstituted BN-aromatics bearing B−H bonds.2a,19 The
nucleus-independent chemical shift (NICS) values are
important magnetic criteria of aromaticity.20 The NICS (1)
value of the sub-BN ring in 6d (−1.35) is larger than that of the
sub-BN ring in carbonaceous ullazine 6d′ (−3.24), which
indicates that 6d is less aromatic than 6d′ (Figure 2). Density
Scheme 1. Synthetic Route to “Half” of the Bis-BN Ullazines
(BN-pyrrolo[1,2-a]quinolines) 3 and Bis-BN Ullazines 6
Figure 2. NICS (1) values, frontier orbital maps and energy levels of
bis-BN ullazine 6d and carbonaceous ullazine 6d′.
functional theory (DFT) calculations revealed that there is
significant HOMO density on the peripheral nitrogen atom, but
none on the boron atom. The calculations also showed
significant LUMO density on the boron atom and that the
LUMO is less delocalized on the benzene ring and partially
resides on the pyrrole ring of bis-BN ullazine 6d. The position
of the LUMO of bis-BN ullazine 6d is different from its
isoelectronic carbonaceous ullazine 6d′ for which the LUMO
primarily resides on the central ring, less on the pyrrole ring
and none on the bottom half of the molecule. The replacement
of a CC unit with the B−N in ullazine 6d′ resulted in a lower
HOMO level and a slightly higher LUMO level. A similar trend
was observed for 3d and its all-carbon analog 3d′ (Figure S1).
We were able to obtain crystals of bis-BN ullazines 6a−6d
suitable for X-ray analysis by slow diffusion of hexane into a
solution of 6 in dichloromethane. The structures of bis-BN
ullazines 6a−6d are shown in Figure 3, and selected bond
distances are presented in Table S10. All four bis-BN ullazines
6a−6d had perfectly flat structures. The dihedral angles
between the top pyrrole ring and the bottom benzene ring
are very small (1.1°−6.7°, Table S10). Parental bis-BN ullazine
6d showed the smallest dihedral angle (1.1°), which indicates it
had the highest planarity. The aryl substituents on boron
(mesityl in 6a; phenyl in 6b) are out of the plane with dihedral
angles of approximately 80° for 6a and 40° for 6b, which limits
their conjugation with the polycyclic ullazine core. X-ray
structures of “half” bis-BN ullazines 3b−3d showed planar
frameworks as well (see Figure S7, Table S11).
The B-Mes and B-Me analogs (3a and 3c) could be prepared
via the corresponding two-step sequence: first, borylative
cyclization of 2 with BCl3 and, then, nucleophilic substitution
of the B−Cl moiety by the appropriate Grignard reagent
(Method A, Scheme 1). Alternatively, parental BN-pyrrolo[1,2-
a]quinoline 3d could be produced by treating the initial
cyclized intermediate with LiAlH4 instead of a Grignard reagent
(Method C, Scheme 1). We noticed that BN-pyrrolo[1,2-
a]quinolines 3a−3c can be purified by chromatography on
silica gel, but the parental analog 3d gradually decomposed
during chromatography. Pure 3d was obtained by recrystalliza-
tion. X-ray structures of 3b, 3c, and 3d were obtained (Figure
S7 and Table S11).
The successful synthesis of BN-pyrrolo[1,2-a]quinoline 3
encouraged us to synthesize bis-BN ullazines. The synthesis of
bis-BN ullazine 6 began with the Clauson−Kaas reaction
between 2,5-dimethoxytetrahydrofuran and commercially avail-
able 2,6-dinitroaniline followed by the reduction of the NO2
group to afford 2-(1H-pyrrol-1-yl)benzene-1,3-diamine 5a,18
which can be alkylated to give compound 5b in good yield. Bis
BN-ullazines 6a−6g were obtained by 2-fold borylative
cyclizations of 5a or 5b in good yields in the absence of a
catalyst (Method B, Scheme 1). Notably, the yield of the
borylation reaction in the presence of aluminum chloride in
Dewar’s synthesis of BN-pyrene was only 21%.15 This reactivity
difference is probably due to the more electron-rich nature of
the pyrrole ring.
The B−N bonds in bis-BN ullazines 6a and 6b are much
shorter than typical B−N single bonds (1.58 Å) and slightly
longer than a localized BN double bond (1.403(2) Å) (Table
S10), confirming that these bonds have noticeable double-bond
character. The B−N bonds in bis-BN ullazines 6c and 6d are
comparable to localized BN double bonds (Table S10),
which indicates they have very strong double-bond character
(Table S10). Furthermore, the inner ring B−C bonds (1.45−
1.54 Å) are shorter than the B−C single-bond covalent radius
of 1.6 Å. The B−C bonds of 6c (∼1.49 Å) and 6d (∼1.51 Å)
We found parental bis-BN ullazine 6d was extremely unstable
to chromatography on silica gel, and it slowly decomposed
during recrystallization. Similar reports have shown that
substitution at the 3 or 9 position of carbonaceous ullazine
can improve its stability.12b The 1H NMR chemical shifts of the
N−H protons in bis-BN ullazines 6a−6d are in the 6.94−7.40
B
DOI: 10.1021/acs.orglett.8b00554
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Letter
Figure 5. Normalized UV−vis absorption (left) and emission spectra
(right) of 6c in different solvents.
The low energy band of bis-ullazine 6c (309 nm) was blue-
shifted, compared to the low energy bands of the other three
bis-BN ullazines (6a, 318 nm; 6b, 331 nm; 6d, 316 nm). The
parental bis-BN ullazine 6d showed an absorption maxima of
316 nm in CH2Cl2 (λabs = 315 nm in MeOH, Figure S11),
which is substantially blue-shifted, compared to its carbona-
ceous ullazine analog 6d′ (λabs = 374 nm in EtOH).12a
Similarly, the absorption band of 6b (λabs = 331 nm) is also
Figure 3. Solid state structures of bis-BN ullazines 6a−6d with views
parallel and perpendicular to the polycyclic planes. Thermal ellipsoids
are set at the 35% probability level. H atoms have been omitted for
clarity.
blue-shifted, compared to its carbonaceous ullazine 6b′ (λabs
=
392 nm) (Figure S10 and Table S35).
The emission spectra of bis-BN ullazines 6a−6d are shown
in Figure 4. Emission maxima of 6 decrease in the order 6b >
6d ≈ 6a > 6c. This trend is consistent with that observed in
their absorption spectra. The red shift of the emission band of
6b (408 nm) relative to that of 6a (382 nm) indicated the B-Ph
was in better conjugation with the ullazine scaffold. The
emission band of 6b (λem = 408 nm in CH2Cl2) was blue-
are significantly short. The short N−C bonds (1.38−1.42 Å) in
bis-BN ullazines 6a−6d are similar to those reported in the
literature for other BN-aromatics.2−8
The UV/vis absorption spectra of bis-BN ullazines 6a−6d in
CH2Cl2 are presented in Figure 4, and their absorption band
shifted relative to that of its carbonaceous ullazine 6b′ (λem
=
518 nm) as well (Figure S10 and Table S35). The emission
band of 6c, bearing a methyl substituent on the boron atom,
was blue-shifted relative to that of parental bis-BN ullazine 6d.
However, the solution state fluorescence quantum yields of bis-
BN ullazines 6a−6d are moderate (0.10−0.38). The small
Stokes shifts of bis-BN ullazines 6a−6d (103 cm−1, Table 1)
indicate that the structural relaxations are small presumably due
to the structural rigidities of 6. The absorption and emission
spectra of BN-pyrrolo[1,2-a]quinoline 3 were also recorded
(Figure S8 and Table S33).
Figure 4. Normalized absorption (left) and emission spectra (right) of
6a−6d in CH2Cl2 at a concentration of 10−5 M.
Unlike the limited solvent effect observed for the absorption
spectra, we found a clear positive solvatochromic effect in the
emission spectra of bis-BN ullazines 6 (Figure 5 for 6c, Figure
S11 for the other compounds). Bis-BN ullazine 6c showed a
vibronic structure in low-polarity solvents with a vibronic
spacing of approximately 1300 cm−1, which corresponds to the
breathing mode of the aromatic ring. Therefore, the emitting
state of 6c in low-polarity solvents is dominated by the π,π*
state. In contrast, the emission band of 6c becomes
structureless and red-shifted in more polar solvents, which
might be caused by solvent-induced vibrational relaxation.21
Similar phenomena were also found in other bis-BN ullazines
(Figure S11). The alkyl substituents on the nitrogen atoms of
bis-BN ullazines 6e−6g did not substantially affect the
photophysical properties of these compounds (Figures S9
and S12 and Table S34).
The electrochemical properties of bis-BN ullazines 6 were
investigated using cyclic voltammetry (Figure S13 and Table
S36) and differential pulse voltammetry (Figure S14). Bis-BN
ullazines 6 exhibited irreversible oxidation and reduction waves.
The HOMO levels of bis-BN ullazines 6a−6d calculated from
their oxidation onset potentials are comparable (6a, −4.83 eV;
6b, −5.24 eV; 6c, −4.82 eV, 6d, −4.87 eV). The LUMO energy
Table 1. Photophysical Properties of 6a−6d
opt
λabs
λonset
(nm)
λem
(nm)
Stokes shift
EG
a
b
compd (nm)
Φpl
(103 cm−1
)
(eV)
6a
6b
6c
6d
318
331
309
316
354
376
341
360
382
408
369
0.18
0.14
0.10
0.38
1.22
2.70
5.26
5.26
3.28
3.04
3.64
3.44
385
a
b
Stokes shift = 1/λabs − 1/λem. Optical band gap EGopt = 1240/λonset
.
maxima are presented in Table 1. Bis-BN ullazines 6a−6d
showed absorption bands below 380 nm, which can be
attributed to the π,π* transitions of the aromatic core. This
assignment was further supported by TDDFT calculations
(Tables S5−S8 and Figures S2−S5). The observation of a
minor solvent effect also supports this assignment (Figure 5 for
6c, Figure S11 for other compounds). The absorption onset of
bis-BN ullazine 6b showed a bathochromic shift (376 nm)
relative to 6a (354 nm), indicating that the B-Ph moiety in 6b
was in better conjugation with the main plane than the B-Mes
moiety in 6a, which was consistent with their X-ray structures.
C
DOI: 10.1021/acs.orglett.8b00554
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Letter
levels were calculated from the HOMO energy levels and the
optical band gaps (6a, −1.55 eV; 6b, −2.20 eV; 6c, −1.19 eV,
6d, −1.43 eV) (Table S36).
Team of Colleges and Universities in Tianjin (TD13-5020).
We also thank Dr. Jiongpeng Zhao and Dr. Shengyun Liao at
Tianjin University of Technology for their assistance with the
X-ray crystallographic analysis.
Tricoordinate boron species are commonly used to detect
fluoride ions due to the Lewis acidity of boron.22 However, we
found that the addition of excess tetrabutylammonium fluoride
(TBAF) to solutions of bis-BN ullazines 6 caused slight
changes, except for 6d, in their absorption and emission spectra
(Figure S15). These phenomena are in stark contrast to the
shifts observed for other BN-aromatics.23 However, we found
the boron signal of bis-BN ullazines 6 was diminished after
adding an excess of TBAF (Figure S16). It should be noted that
the 11B NMR was recorded at a much higher concentration of
bis-BN ullazines 6 than the photophysical studies.
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Corresponding Author
*E-mail: xuguangliu@tjut.edu.cn.
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Xuguang Liu: 0000-0002-7859-3139
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We gratefully thank the financial support from the National
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DOI: 10.1021/acs.orglett.8b00554
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