B2N2-Dibenzo[a,e]pentalenes: Effect of the BN Orientation Pattern on Antiaromaticity and Optoelectronic Properties

Article abstract of DOI:10.1021/jacs.5b05056

Two BN units were embedded in dibenzo[a,e]pentalene with different orientation patterns, which significantly modulated its antiaromaticity and optoelectronic properties. Importantly, the vital role of the BN orientation in conjugated molecules with more t

Full text of DOI:10.1021/jacs.5b05056

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Communication
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BN-Dibenzo[a,e]pentalenes: Effect of the BN Orientation
Pattern on Antiaromaticity and Optoelectronic Properties
Xiao-Ye Wang, Akimitsu Narita, Xinliang Feng, and Klaus Müllen
J. Am. Chem. Soc., Just Accepted Manuscript • Publication Date (Web): 05 Jun 2015
Downloaded from http://pubs.acs.org on June 7, 2015
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B2N2-Dibenzo[a,e]pentalenes: Effect of the BN Orientation Pattern
on Antiaromaticity and Optoelectronic Properties
Xiao-Ye Wang,† Akimitsu Narita,† Xinliang Feng,*,‡ Klaus Müllen*,†
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† Max Planck Institute for Polymer Research, Ackermannweg 10, 55128 Mainz, Germany
‡ Center for Advancing Electronics Dresden (cfaed) & Department of Chemistry and Food Chemistry, Technische Universi-
tät Dresden, 01062 Dresden, Germany
Supporting Information Placeholder
incorporated DBPs with different orientation patterns of BN units
(Chart 1, anti-orientation: termed as BNNB-DBP; syn-orientation:
termed as BNBN-DBP). X-ray single-crystal analysis revealed
their planar structures, and theoretical, spectroscopic, and electro-
chemical studies demonstrated that the antiaromaticity and optoe-
lectronic properties have been significantly modulated by the
different BN substitution patterns.
ABSTRACT: Two BN units were embedded in diben-
zo[a,e]pentalene with different orientation patterns, which signifi-
cantly modulated its antiaromaticity and optoelectronic properties.
Importantly, the vital role of BN orientation in conjugated mole-
cules with more than one BN unit was demonstrated for the first
time. This work indicates a large potential of the BN/CC isoster-
ism for the development of new antiaromatic systems and high-
lights the importance of precise control of the BN substitution
patterns in conjugated materials.
Chart 1. DBP and its BN-substituted derivatives.
Aromaticity is one of the most important concepts in chemistry.
Over decades, aromatic compounds have always been the majori-
ty of organic conjugated molecules, whereas antiaromatic ones,
which exhibit completely different properties, have been relative-
ly underexplored.1 Recently, the fundamental interest in BN-
substituted benzene, in which a C=C bond is replaced with its
isoelectronic B–N bond, has triggered growing research efforts in
azaborine chemistry.2 The BN/CC isosterism not only expands the
structural diversity of organic compounds and enriches the fun-
damental understanding of aromaticity,3 but also provides promis-
ing materials for various applications, for example, in hydrogen
storage,4 biomedicine,5 and organic electronics.6 A number of
BN-substituted aromatic systems have already been reported,
including BN-substituted benzene,7 naphthalene,8 indole,9 phe-
nanthrene,10 anthracene,11 triphenylene,12 pyrene,13 coronene,14
and other polycyclic aromatic compounds.15 However, the appli-
cation of this BN substitution strategy in antiaromatic systems has
been extremely limited.2 On the other hand, in conjugated systems
incorporating more than one BN unit, there is a fundamental
question about the effect of the BN orientation patterns on their
chemical and physical properties. Nevertheless, to the best of our
knowledge, there has been no investigation on this topic up to
now, presumably due to the great challenge of precisely manipu-
lating the BN orientation in the same conjugated skeleton.
The synthetic routes to BNNB-DBP and BNBN-DBP are illus-
trated in Scheme 1. First, 1-bromo-2-nitrobenzene (1) was con-
verted to 2,2’-dibromohydrazobenzene (2) under a reductive
condition. Then t-BuLi was used to perform deprotonations and
lithium-halogen exchanges simultaneously at 0 °C to generate a
tetralithiated intermediate, which was subsequently reacted with
dimethoxy mesitylborane to obtain BNNB-DBP in 68% yield in
one pot. Compared to the electrophilic borylation method used by
Cui et al.,18 this approach is not affected by the electron density of
the reactants and can also avoid high reaction temperatures, which
make it applicable to a broader scope of substrates. The target
compound was stable enough to be purified by column chroma-
tography on silica gel and recrystallization from CH2Cl2/hexane to
give colorless crystals. However, when dimethoxy phenylborane
was used in the final step, only a complex mixture was obtained,
indicating the necessity of bulky protecting groups on the boron
centers in this nucleophilic substitution step.
The skeleton of the other isomer BNBN-DBP was constructed
in a stepwise manner (Scheme 1). First, N-mesityl-benzene-1,2-
diamine (3) was condensed with (2-bromophenyl)boronic acid in
refluxing o-xylene to afford precursor 4 in 56% yield. Then lithia-
tion of 4 was performed with t-BuLi at –78 °C to avoid decompo-
sition of the diazaboroline ring. Subsequent nucleophilic substitu-
tion reactions of the dilithiated species with dimethoxy mesity-
lborane afforded BNBN-DBP in 70 % yield. The solvents played
an important role in this step, probably due to the different solu-
bility of LiOMe which was generated after the ring closure. When
Dibenzo[a,e]pentalene (DBP) is a ladder-type polycyclic hy-
drocarbon which possesses an antiaromatic character with a 4nπ-
electron periphery.16 Since the first synthesis in 1912, DBPs have
attracted considerable research interest regarding the development
of convenient synthetic methods and the exploration of their
applications in materials science.17 Herein, we chose the DBP
skeleton to investigate the effect of BN substitution on the proper-
ties of such antiaromatic systems.18 We synthesized two B2N2-
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THF was first used, BNBN-DBP could not be obtained, presuma-
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bly due to the ring opening of the product, which was caused by
the nucleophilic attack of the solubilized LiOMe at the unprotect-
ed boron, namely the one not connected to the mesityl group
(Scheme S1). Nevertheless, by using diethyl ether or dibutyl ether
as the solvent, LiOMe could be mostly precipitated from the
reaction mixture, thus keeping the majority of the product intact.
BNBN-DBP was less stable than BNNB-DBP due to the presence
of one unprotected boron, and thus purification by column chro-
matography on silica gel or aluminum oxide was unsuccessful,
leading to severe hydrolysis. However, BNBN-DBP could be
purified by careful crystallization from hexane to give a crystal-
line solid. The solution of BNBN-DBP is sensitive to water, so all
the characterizations were performed in freshly dried solvents. In
contrast, the solid of BNBN-DBP is stable and can be stored
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Figure 1. Single-crystal structures of (a) compound 2, (b) com-
pound 4, (c) BNNB-DBP, and (d) BNBN-DBP with thermal
ellipsoids shown at 50% probability. Hydrogen atoms are omitted
for clarity. B and N atoms in (d) BNBN-DBP are not crystallo-
graphically distinguishable owing to disorder. Colors: gray, car-
bon; red, boron; blue, nitrogen; orange, bromine.
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under ambient conditions, showing no change in the H NMR
spectra for at least 2 weeks.
Scheme 1. Synthetic Routes to BNNB-DBP and BNBN-
DBP.
To understand how the BN substitution and the orientation pat-
terns of BN units influence the antiaromaticity of DPB, we per-
formed nucleus-independent chemical shift (NICS) calculations20
at the B3LYP/6-311+G(2d,p) level. As illustrated in Figure 2, the
parent DBP has a highly antiaromatic pentalene core with a large
positive NICS(1)zz value of 23.7 ppm and moderately aromatic
fused benzene rings. Differently, in BNNB-DBP, the fused ben-
zene rings exhibit high aromaticity, whereas the BNNB-
substituted pentalene core is almost nonaromatic. In contrast, the
asymmetric BNBN-DBP has different NICS(1)zz values for each
ring. The fused benzene rings are aromatic, with the one on the N
side much stronger. Notably, the BNBC2 ring shows a highly
antiaromatic feature (18.7 ppm), whereas the NBNC2 ring dis-
plays weak aromaticity (–2.6 ppm).
The chemical structures of BNNB-DBP and BNBN-DBP were
fully characterized by 1H, 13C, and 11B NMR spectroscopy as well
as high-resolution mass spectrometry. Notably, only one boron
signal was observed at 37.3 ppm for BNNB-DBP, whereas two
signals at 50.4 and 38.3 ppm were recorded for BNBN-DBP due
to the presence of two kinds of boron atoms. The thermal stability
of these two compounds was examined by thermogravimetric
analysis (TGA), which revealed high decomposition temperatures
of 288 °C for BNNB-DBP and 257 °C for BNBN-DBP (Figure
S1).
These differences may partly relate to the lone pairs of N atoms
and the vacant p orbitals of B atoms. The BNNC2 ring is in prin-
ciple aromatic with six π-electrons in total (Figure S4). However,
when it is incorporated in the fused system of BNNB-DBP, the
lone pair of N atom is attracted by the extra B atom, which reduc-
es the effective diatropic ring current and leads to lowered aroma-
ticity. Similarly, the (anti)aromatic properties of BNBN-DBP can
also be rationalized by the presence of six π-electrons in the
NBNC2 ring and four π-electrons in the BNBC2 ring. The aroma-
ticity and antiaromaticity are both reduced due to the extra B or N
atoms connected to each ring in BNBN-DBP. In addition to the
effect of the heteroatoms, the global paratropic ring current from
these 4nπ-electron systems further reduces the aromaticity but
enhances the antiaromaticity of individual rings.21 These results
indicate that although DBP and its BN-embedded derivatives
possess the same 4nπ-electrons in total, the (anti)aromaticity of
each ring is significantly modified by the BN substitution, and
different BN orientation patterns lead to completely different
electronic structures.
BNNB-DBP and BNBN-DBP as well as their corresponding
precursors were successfully characterized by single crystal X-ray
diffraction. The 6-5-5-6 fused ring structures of BNNB-DBP and
BNBN-DBP can be clearly recognized in Figure 1 as fully planar
skeletons. The mesityl substituents are almost perpendicular to the
conjugated plane with twisting angles of about 70 – 80°. The three
bonds connected with the boron atoms (one B−N bond and two
B−C bonds) are in the same plane with the sum of the three bond
angles of about 360°. As shown in Figure 1d, B and N atoms in
BNBN-DBP could not be crystallographically distinguished. This
phenomenon was caused by the chemically asymmetric BNBN
sequence (compared to the centrosymmetric BNNB sequence) in
a geometrically centrosymmetric skeleton. The molecules packed
in the crystal without any preferred orientation of the embedded
BNBN sequence, thus leading to a disordered packing structure.
In contrast, BNNB-DBP does not have this problem, and provides
more detailed structural information. The B−N bond lengths in
BNNB-DBP are in the range of 1.40 – 1.44 Å, indicative of local-
ized BN double bond character.19 The N−N bond length is around
1.42 Å, a typical value for N−N single bond (1.40 Å in compound
2). These structural features indicate that the bond length altera-
tion in BNNB-DBP is similar to that in DBP.
Figure 2. NICS(1)zz values (ppm) of DBP, BNNB-DBP, and
BNBN-DBP calculated at the GIAO-B3LYP/6-311+G(2d,p) level.
The photophysical and electrochemical properties of BNNB-
DBP and BNBN-DBP were investigated by UV–Vis absorption
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and photoluminescence spectroscopies as well as cyclic voltam-
3). These results indicate that different BN substitution patterns
lead to significantly distinct electronic structures, which deter-
mine their varying photophysical properties.
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metry (CV). As displayed in Figure 3, BNNB-DBP displays two
main absorption bands peaking at 268 and 342 nm and an absorp-
tion onset of 375 nm, which corresponds to an optical bandgap of
3.31 eV. On the other hand, BNBN-DBP shows a red-shifted
absorption maximum at 279 nm and a very weak absorption be-
tween 320 and 400 nm (Figure 3, inset). The optical bandgap
deduced from the low-energy absorption onset is 3.10 eV (Figure
S3), which is smaller than that of BNNB-DBP. Overall, both
compounds revealed blue-shifted absorption onsets, that is, larger
optical bandgaps, relative to that of DBP.17d The photolumines-
cence spectroscopic analysis indicated that the antiaromatic
BNBN-DBP is non-emissive, which is similar to the case of
DBPs.17 In contrast, BNNB-DBP featuring a nonaromatic pen-
talene core exhibited deep blue fluorescence with an emission
maximum at 403 nm and a quantum yield of 18%, which is quite
unusual in pentalene derivatives. The energy levels of these two
compounds were estimated by CV experiments (Figure S2).
BNNB-DBP possesses a lower HOMO level of −5.93 eV com-
pared to that of BNBN-DBP (−5.62 eV), whereas their LUMO
levels are quite similar (−2.67 eV for BNNB-DBP and −2.60 eV
for BNBN-DBP). The electrochemical bandgaps of BNNB-DBP
and BNBN-DBP are 3.26 and 3.02 eV, respectively, which agreed
well with the optical bandgaps.
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Figure 4. Frontier molecular orbitals and their energy levels (unit:
eV) of DBP, BNNB-DBP, and BNBN-DBP calculated at the
B3LYP/6-311G(d,p) level.
In summary, two BN-embedded dibenzo[a,e]pentalenes,
BNNB-DBP and BNBN-DBP were synthesized through multiple
lithiations (on carbon and nitrogen at the same time) followed by
nucleophilic substitutions. This strategy can be a promising
method for the synthesis of BN-embedded conjugated molecules.
Moreover, we have for the first time revealed that different BN
orientation patterns significantly influenced the antiaromaticity
and the optical and electronic properties. These results indicate
that precise control of the BN orientation pattern is of great im-
portance to effectively modulate the chemical and physical prop-
erties of conjugated molecules embedded with more than one BN
unit. Further exploration of the BN substitution strategy in anti-
aromatic systems can open up new possibilities to extend the
scope of antiaromatic molecules and deepen the fundamental
understanding of antiaromaticity.
Figure 3. The absorption spectra of BNNB-DBP and BNBN-DBP
as well as the emission spectrum of BNNB-DBP (excitation
wavelength: 342 nm) in CH2Cl2. Inset shows an enlarged region
of the absorption spectrum of BNBN-DBP.
To gain more insights into the electronic properties of BNNB-
DBP and BNBN-DBP compared to their carbon analogue DBP,
we performed density functional theory (DFT) calculations at the
B3LYP/6-311G(d,p) level. As depicted in Figure 4, BN substitu-
tion resulted in completely different electronic structures. First,
BNNB-DBP and BNBN-DBP both exhibited larger bandgaps
than the parent DBP. BNBN-DBP had the same LUMO level as
that of BNNB-DBP, but a much higher HOMO level and thus a
lower bandgap, which is consistent with the experimental results
as discussed above. Second, as indicated previously,17f the
HOMOàLUMO transition of DBP is symmetry-forbidden.
BNBN-DBP maintains the similar character, although the orbital
distributions are polarized, with larger HOMO coefficient on the
N side and severely localized LUMO on the B side (Figure 4).
According to time-dependent DFT (TD-DFT) calculations, the
lowest-energy absorption of BNBN-DBP can be assigned to the
symmetry-forbidden HOMOàLUMO transition, which exhibited
an extremely low oscillator strength (f) of 0.0001 (Figure S6). In
contrast, TD-DFT calculations on BNNB-DBP indicated that its
HOMO possessed a different symmetry compared to that of DBP,
while their LUMO symmetries were identical (Figure 4). There-
fore, the HOMOàLUMO transition (f: 0.2823) of BNNB-DBP is
allowed, corresponding to the absorption band at 342 nm (Figure
ASSOCIATED CONTENT
Supporting Information
Experimental details, synthesis, characterizations, single crystal
data (CIF), computational studies, and NMR spectra. This materi-
al is available free of charge via the Internet at http://pubs.acs.org.”
AUTHOR INFORMATION
Corresponding Author
xinliang.feng@tu-dresden.de
muellen@mpip-mainz.mpg.de
Notes
The authors declare no competing financial interests.
ACKNOWLEDGMENT
The authors sincerely thank Wen Zhang for high-resolution
MALDI-TOF MS measurements and Dr. Dieter Schollmeyer
(Institute for Organic Chemistry, Johannes Gutenberg University
Mainz) for single-crystal X-ray structural analysis. We are grate-
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Harhausen, M.; Liedtke, R.; Bussmann, K.; Fukazawa, A.; Yamaguchi, S.;
ful for the financial support from the European Research Council
grant on NANOGRAPH, DFG Priority Program SPP 1459, Gra-
phene Flagship (No. CNECT-ICT-604391), and European Union
Projects UPGRADE and MoQuaS.
Petersen, J. L.; Daniliuc, C. G.; Fröhlich, R.; Kehr, G.; Erker, G. Angew.
Chem. Int. Ed. 2013, 52, 5992. (d) Maekawa, T.; Segawa, Y.; Itami, K.
Chem. Sci. 2013, 4, 2369. (e) Levi, Z. U.; Tilley, T. D. J. Am. Chem. Soc.
2009, 131, 2796. (f) Kawase, T.; Fujiwara, T.; Kitamura, C.; Konishi, A.;
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Miyazaki, E.; Takimiya, K. Angew. Chem. Int. Ed. 2010, 49, 7728.
(18) During the preparation of this manuscript, Cui et al. reported an
elegant synthesis of BNNB-substituted benzopentalenes by electrophilic
borylation from hydrazones. See: Ma, C.; Zhang, J.; Li, J.; Cui, C. Chem.
Commun. 2015, 51, 5732.
(19) Abbey, E. R.; Zakharov, L. N.; Liu, S. Y. J. Am. Chem. Soc. 2008,
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(20) Chen, Z.; Wannere, C. S.; Corminboeuf, C.; Puchta, R.; Schleyer,
P. v. R. Chem. Rev. 2005, 105, 3842.
(21) Cao, J.; London, G.; Dumele, O.; von Wantoch Rekowski, M.;
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