Bottom-Up Synthesis of Nitrogen-Doped Polycyclic Aromatic Hydrocarbons

Article abstract of DOI:10.1055/s-0039-1690767

Bottom-up organic synthesis serves as an efficient method to provide atomically precise heteroatom-doped polycyclic aromatic hydrocarbons (PAHs) with not only well-defined size and edge structures but also specific concentrations and positions of the heteroatoms. We provide a plenary account of the preparation of nitrogen-doped PAHs (N-PAHs) through 1,3-dipolar cycloaddition between different dipolarophiles, as well as pyrazine-type N-doped diaza-hexa-peri-hexabenzocoronene (diaza-HBC). Additionally, we present the synthesis of a class of helical N-charged PAHs, including one charged aza[5]helicene and two charged aza[4]helicenes. Moreover, the bottom-up organic synthesis strategy is further extended to the construction of novel nitrogen-boron-nitrogen (NBN)-containing PAHs. Finally, we discuss the synthesis of four-coordinate boron chromophores containing 6,12,18-tris(alkyl amine)-5,11,17-triazatrinaphthylene derivative ligands. 1 Introduction 2 Nitrogen-Doped PAHs Based on Dibenzo-9a-azaphenalene (DBAP) 3 Cationic Nitrogen-Doped Helical PAHs 4 Nitrogen-Boron-Nitrogen-Doped PAHs 5 Conclusion and Outlook.

Full text of DOI:10.1055/s-0039-1690767

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Bottom-Up Synthesis of Nitrogen-Doped Polycyclic Aromatic
Hydrocarbons
Junzhi Liu*a,b
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Xinliang Feng*b
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a Department of Chemistry and State Key Laboratory of Synthetic
Chemistry, The University of Hong Kong, Pokfulam Road, Hong
Kong, P. R. of China
juliu@hku.hk
b Center for Advancing Electronics Dresden (cfaed) & Faculty of
Chemistry and Food Chemistry, Technische Universität Dresden,
01062 Dresden, Germany
xinliang.feng@tu-dresden.de
Received: 17.10.2019
Accepted after revision: 25.11.2019
Published online: 10.12.2019
polyphenylene precursors with three-dimensional dendrit-
ic or hyperbranched structures.7,8 By utilizing such a syn-
thetic strategy, different types of large PAHs have been suc-
cessfully achieved with various sizes, edge peripheries, and
topological structures in the last two decades.9 In particu-
lar, hexa-peri-hexabenzocoronene (HBC, with 42 carbon at-
oms) is a typical case of the most widely investigated
PAH.10–15 Superficially, people may consider that PAHs rep-
resent a solitary class of similar molecular structures con-
taining only sp2 carbons. However, the chemical reactivity
and physical properties of PAHs are absolutely different de-
pending on their size and topology, as well as their periph-
eral substituents.16–21 There is no doubt that the electronic
properties of PAHs, e.g., the energy gaps, are of great impor-
tance for the prospective applications of PAHs in organic or
graphene-based nanoelectronics.22 In addition to the size
and edge structures, the introduction of heteroatoms into
the lattice of sp2-carbon frameworks is an effective method
for tailoring the intrinsic physical and chemical properties,
such as the energy gap, optical properties, and redox be-
havior, of PAHs.23–25,26,27 The initial investigations of hetero-
atom-doped PAHs date back to studies on natural dyes. For
instance, in 1907, R. Scholl elucidated the structure of fla-
vanthrone.28 During the past two decades, research on large
polycyclic heteroaromatics has advanced with the develop-
ment of graphene and its application in molecular and or-
ganic electronics.29–31 Bottom-up organic synthesis can af-
ford atomically precise heteroatom-doped PAHs (N-PAHs)
with different sizes and topologies, as well as different con-
centrations and positions of heteroatoms.29 Furthermore,
the introduction of heteroatoms into PAHs can promote the
development of stable cationic aromatics, spin-polarized
organic compounds, and special redox and supramolecular
self-assembly behavior.21,32–34 Since the introduction of het-
eroatom-doped PAHs, nitrogen (N) has always been the key
‘dopant’ atom because of the feasibility of the synthetic
DOI: 10.1055/s-0039-1690767; Art ID: st-2019-a0552-a
Abstract Bottom-up organic synthesis serves as an efficient method
to provide atomically precise heteroatom-doped polycyclic aromatic
hydrocarbons (PAHs) with not only well-defined size and edge struc-
tures but also specific concentrations and positions of the heteroatoms.
We provide a plenary account of the preparation of nitrogen-doped
PAHs (N-PAHs) through 1,3-dipolar cycloaddition between different di-
polarophiles, as well as pyrazine-type N-doped diaza-hexa-peri-hex-
abenzocoronene (diaza-HBC). Additionally, we present the synthesis of
a class of helical N-charged PAHs, including one charged aza[5]helicene
and two charged aza[4]helicenes. Moreover, the bottom-up organic
synthesis strategy is further extended to the construction of novel ni-
trogen-boron-nitrogen (NBN)-containing PAHs. Finally, we discuss the
synthesis of four-coordinate boron chromophores containing 6,12,18-
tris(alkyl amine)-5,11,17-triazatrinaphthylene derivative ligands.
1
2
Introduction
Nitrogen-Doped PAHs Based on Dibenzo-9a-azaphenalene
(DBAP)
3
4
5
Cationic Nitrogen-Doped Helical PAHs
Nitrogen–Boron–Nitrogen-Doped PAHs
Conclusion and Outlook
Key words polycyclic aromatic hydrocarbons, nanographenes, het-
eroatoms, nitrogen doping, boron doping
1 Introduction
Polycyclic aromatic hydrocarbons (PAHs), namely,
nanographenes (NGs), can be considered as segments of
graphene with sp2-carbon frameworks.1,2 The chemical
synthesis of PAHs was pioneered by R. Scholl and E. Clar.3,4
Their systematic studies and applications in organic elec-
tronics have attracted scientists for decades, especially with
the seminal contribution from K. Müllen.5,6 The classic syn-
thesis of -extended PAHs is based on intramolecular cyclo-
dehydrogenation (Scholl reaction), i.e., planarization of the
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J. Liu et al.
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methods and the chemical stability of the N-doped PAHs.29
In particular, because of their prospective optical, electron-
ic, and magnetic properties, N-containing PAHs have re-
ceived much attention for promising applications in elec-
trocatalysts, organic solar cells, sensors, and field-effect
transistors.35–39 For instance, using electronegative nitrogen
atoms to replace C–H units can promote the electron injec-
tion and charge-transport properties of PAHs.40,41 More im-
portantly, such nitrogen atoms can influence the electronic
feature while still maintaining the molecular skeleton of
PAHs, and the overall electron-accepting properties can be
increased due to the presence of imino nitrogens relative to
those of pure hydrocarbons or other heteroatom-doped
PAHs. For instance, triphenylenes exhibit p-type charge car-
rier transport, while hexaazatriphenylenes, a type of N-
doped triphenylenes, are n-type semiconductors.42 More-
over, N-PAHs have a significant tendency to pack into 1D
column supermolecular structures, which can serve as a fa-
vorable charge carrier transport pathway.43,44 The N atom
can be positioned at the edge, leading to pyridine-type
nanographenes (such as 1, Figure 1), or at the inner plane of
sp2-carbon frameworks, resulting in ‘graphitic’-type N-
doped molecules (such as 2). For externally N-derived mol-
ecules, arylation, alkylation, or protonation at the N atom
can result in a positive charge (such as 3). Furthermore, a
positive charge on the nitrogen atoms can also be achieved
by doping at a ring junction (such as 4). The chemical fea-
tures of N-dopants, i.e., pyridinic, pyrrolic, and ‘graphitic’ N
atoms, are consistent with those of various types of N-
doped 6-aromatic rings instead of benzene rings. Among
these, graphitic-type nitrogen is known as one of the most
active sites for carbon-based electrocatalysis.45–49 In this Ac-
count, we will mainly describe our recent endeavors in the
development of nitrogen-doped PAHs in the past few years.
Biographical Sketches
Dr. Junzhi Liu received his
Bachelor’s degree in materials
chemistry in 2008 and Master’s
degree in materials science in
2011. Then he joined the Max
Planck Institute for Polymer Re-
search in Prof. Klaus Müllen’s
group for PhD thesis and ob-
tained his PhD degree in syn-
thetic chemistry in January
2016. In July 2015, he joined
Prof. Xinliang Feng’s group as a
postdoc at Technische Universi-
tät Dresden (TU Dresden). Since
July 2017, he was a research
group leader at the Chair for
Molecular Functional Materials
at TU Dresden. In August 2019,
Dr. Liu started his independent
research group at the Depart-
ment of Chemistry at The Uni-
versity of Hong Kong. His
current scientific interests in-
clude bottom-up organic syn-
thesis of topological carbon
nanostructures and their appli-
cations for organic electronics.
He is an Early Career Advisory
Board for Chemistry – An Asian
Journal (2020). Dr. Liu has been
awarded several prizes such as
International Union of Pure and
Applied Chemistry (IUPAC) In-
ternational Award for Young
Chemists - Honorable Mention
Award (2017), ‘Chinese Govern-
ment Award for Outstanding
Self-financed Students Abroad’
in 2015, ‘Marie-Curie’ fellow-
ship in 2011–2014.
Prof. Feng received his Bache-
lor’s degree in analytic chemis-
try in 2001 and Master’s degree
in organic chemistry in 2004.
Then he joined the Max Planck
Institute for Polymer Research
for PhD thesis, where he ob-
tained his PhD degree in April
2008. In December 2007, he
was appointed as a group leader
at the Max-Planck Institute for
Polymer Research, and in 2012
Universität Dresden. His current
scientific interests include or-
ganic synthetic methodology,
organic synthesis and supramo-
lecular chemistry of -conjugat-
ed system, bottom-up synthesis
and top-down fabrication of
graphene and graphene na-
noribbons, 2D polymers and su-
pramolecular polymers, 2D
carbon-rich conjugated poly-
mers for optoelectronic applica-
tions, energy storage and
conversion, new energy devices
and technologies. Prof. Feng has
been awarded several presti-
gious prizes such as IUPAC Prize
for Young Chemists (2009), Eu-
ropean Research Council (ERC)
Starting Grant Award (2012),
Journal of Materials Chemistry
Lectureship Award (2013), Fel-
low of the Royal Society of
Chemistry (FRSC, 2014), Highly
Cited Researcher (Thomson Re-
uters,
Young Innovator Award (2017),
Hamburg Science Award
2014–2019),
Small
(2017), EU-40 Materials Prize
(2018), and Member of the Eu-
ropean Academy of Sciences
(2019).
he became
a
distinguished
group leader. Since August
2014, Prof. Feng was the head
of the Chair of Molecular Func-
tional Materials at Technische
© 2019. Thieme. All rights reserved. Synlett 2019, 30, A–L

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–
–
+
N
+
N
N
N
N
N
AMY 1
1a
1b
1c
+
1
3
2
4
Figure 2 The isoelectronic structure of AMY 1 with two ionic struc-
tures (1a and 1b) and one diradical structure (1c)
N
+
R
+
N
Suzuki coupling of 2,6-dibromo-4-(tert-butyl) aniline (2)
and 1-hydroxy-3H-2,1-benzoxaborole. Second, precursor 4
was generated using HCl-induced cyclization of 3 under mi-
crowave conditions. Then, the key compound PAMY was
achieved by treatment of 4 with a base. We found that di-
methyl-8-(tert-butyl)-2a,13b-dihydrobenzo[7,8]indoliz-
ino[6,5,4,3-def]phenanthridine-1,2-dicarboxylate (5) could
be directly formed via a selective 1,3-dipolar cycloaddition
Figure 1 PAHs with different types of nitrogen atoms
2 Nitrogen-Doped PAHs Based on Dibenzo-
9a-azaphenalene (DBAP)
Azomethine ylides (AMY 1, Figure 2) are classic cases of
1,3-dipolar molecules.50,51 Because the negative charge of
the allyl anion can be equally distributed on the two adja-
cent carbon atoms, AMY 1 features isoelectronic properties
(see Figure 2). In addition to exhibiting two ionic structures
(1a and 1b), AMY 1 also exhibited diradical (1c) character
in the ground state (Figure 2).52–55 Moreover, the diradical
character of AMYs has been quantified by theoretical calcu-
lations, and their chemical reactivity has also been correlat-
ed to the 1,3-dipolar cycloaddition.56,57 Normally, AMYs are
extremely reactive; thus far, only a few cases of stable
AMYs have been achieved.58–63 The extreme reactivity of
AMYs enables the facile synthesis of heterocycles contain-
ing five-membered rings, and different kinds of approach-
es, including desilylation and deprotonation of derivative
precursors, have been explored.64 Recently, the fabrication
of AMY conjugated with aromatic rings (PAMY) was first
achieved by our group,65,66 providing an unprecedented
building block for the construction of PAHs containing ni-
trogen atoms (N-PAHs, see Scheme 1).
by
treating
2-(tert-butyl)-8H-isoquinolino[4,3,2-
de]phenanthridin-9-ium chloride (4a) with triethyl amine
and dimethoxy acetylene dicarboxylate (7a, DMAD, see Fig-
ure 3). The resultant 5 with polycyclic aromatics strongly
indicates that such 1,3-dipolar cycloaddition can be further
developed to synthesize an extended N-PAH. Subsequently,
8-(tert-butyl)-benzo[7,8]indolizino[6,5,4,3-def]phenanthri-
dine-1,2-dicarboxylate (6) with a planar -extended geom-
etry was achieved in 82% yield through the oxidative dehy-
drogenation of 5 using 2,3-dichloro-5,6-dicyano-1,4-ben-
zoquinone (DDQ) as the oxidant. By this means,
unprecedented nitrogen-containing N-PAHs 12–16 could be
accessed using different dipolarophiles (Figure 3), such as
tilted (8), pentacyclic (9), and hexacyclic (naphthalene-1,4-
dione 10 and benzoquinone 11) ethylenes.
Compound 11 can be regarded as a ‘double-dipolaro-
phile’ and offers a two-sided 1,3-dipolar cycloaddition reac-
tion with the PAMY precursor. Accordingly, -extended N-
PAHs 17a and 17b (Figure 3) were obtained as red powders.
Importantly, compound 17 with 48 -electrons includes
two pyrrolic-type N atoms in the inner planar skeleton of
PAH, in which the electrons are delocalized throughout the
The synthesis of PAMY involves the following three
steps (Scheme 1). First, compound 3 was obtained through
X–
OH OH
–
+
+
N
NH2
N
NH2
Br
Br
a
b
c
X = Cl, BF4
R
R
R
R
4a R = tBu, X = Cl
2a R = tBu
2b R = decyl
2c R = OCH3
3a R = tBu
3b R = decyl
3c R = OCH3
PAMYa R = tBu
PAMYb R = decyl
PAMYc R = OCH3
4b R = tBu, X = BF4
4c R = decyl, X = BF4
4d R = OCH3, X = BF4
d
DMAD
H3COOC
COOCH3
H3COOC
COOCH3
N
N
e
tBu
tBu
6
5
Scheme 1 Synthetic route to 6 based on PAMY. Reagents and conditions: (a) Pd(PPh3)4, 1-hydroxy-3H-2,1-benzoxaborole, K2CO3, ethanol, toluene; (b)
microwave, HCl in dioxane, O2; (c) triethyl amine (Et3N), dichloromethane; (d) DMAD, Et3N, dichloromethane; (e) DDQ, toluene.
© 2019. Thieme. All rights reserved. Synlett 2019, 30, A–L

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O
O
O
R
R
R1
R2
O
COOCH3
BF4
N
EWG
N
=
COOCH3
O
dipolarophiles
O
N
N
N
N
b
a
7a R1 = R2 = COOCH3
7b R1 = R2 = C6F6
7c R1 = R2 = C6H6
7d R1 = C6H6, R2 = H
8
9
10
11
4d R = OCH3
4e R = CN
4f R = H
R
R
c
F
F
R
R
+18.9
(+13.7)
F
O
F
18a R = OCH3
18b R = CN
18c R = H
COOCH3
19 R = OCH3
F
R
N
R
N
R
N
N
F
COOCH3
F
O
F
F
F
N
N
14a R = tBu
14b R = (CH2)9CH3
14c R = OCH3
12 R = OCH3
13 R = tBu
O
O
O
N
O
R
+33.5
(+24.5)
20 R = H
R
N
R
N
R
N
R
Scheme 2 In-solution synthesis of diaza-HBP 19 and surface-assisted
synthesis of N-doped diaza-HBC 20. Reagents and conditions: (a) Tribu-
tylamine, dimethyl sulfoxide (DMSO), 190 °C; (b) DDQ, C2D2Cl4, 100 °C;
(c) on the Ag(111) surface in vacuo. Blue: nucleus-independent chemi-
cal shifts (NICS) values (unit: ppm) from calculations; in parentheses:
NICS(1) value.
O
O
15a R = tBu
15a R = (CH2)9CH3
16a R = tBu
16a R = (CH2)9CH3
17a R = tBu
17a R = (CH2)9CH3
Figure 3 Synthesis of N-PAHs 12–17 with different dipolarophiles 7–
11 using the 1,3-dipolar cycloaddition
different ultrahigh vacuum (UHV) conditions on Ag(100),
Ag(111), and Cu(111), nor was the incorporation of boron
nitride (BN) on Cu(111), indicating that the homocoupling
of 4f and dehydrogenation of 20 occurred simultaneously at
this deposition temperature. Finally, the structure of 20 was
confirmed by high-resolution STM (Figure 4, b) and FM-
AFM (Figure 4, c and d). Interestingly, a slightly distorted
molecular skeleton. In general, from a synthetic point of
view, the preparation of such interior N-doped PAHs is
more challenging than the synthesis of PAHs in which ni-
trogen is doped at the corresponding peripheries. Thus, this
efficient synthetic route paves the way to develop novel N-
PAHs containing central nitrogen atoms.67
As mentioned above, compound 5 can be achieved
through the [3+2] cycloaddition of PAMY and DMAD. Inter-
estingly, dimerized compound 18 was also generated in the
solution of PAMY without DMAD in only 3% yield (Scheme
2). Actually, for other AMYs, such dimerization has also
been observed. Importantly, the yield of 18 (51%) can be
improved by heating the solutions of precursor 4 in a high-
boiling base, such as tributylamine (Scheme 2). Oxidation
of 18a with excess DDQ in dry C2D2Cl4 afforded pyrazine-in-
corporated hexabenzoperylene (HBP) derivative 19. Diaza-
HBP 19 was highly unstable, probably as a result of its an-
tiaromatic pyrazine-type core.68
Notably, through the on-surface coupling of dibenzo-
9a-azaphenalene (DBAP) salt 4f, N-doped diaza-HBC 20
was achieved by the subsequent thermally induced cyclo-
dehydrogenation (Scheme 2).68 The chemical structure of
diaza-HBC 20 was clearly elucidated by high-resolution
scanning tunneling microscopy (STM) and frequency-mod-
ulated atomic force microscopy (FM-AFM; Figure 4). First,
compound 4f was deposited on Ag(111) through molecular
beam evaporation. Then, a small amount of 20 was ob-
served upon annealing at 270 °C (Figure 4, a). In contrast to
the results of solution synthesis (Scheme 2), intermediate
18c was not detected after annealing the deposited 4f in
Figure 4 Surface-assisted coupling of 4f provides diaza-HBC 20 on
Ag(111). (a) Overview scanning tunneling microscopy (STM) image on
Ag(111); Vs = 30 mV, It = 10 pA; (b) constant-height STM image of dia-
za-HBC 20 with partially superposed molecular model; Vs = 5 mV; (c)
frequency-modulated atomic force microscopy (FM-AFM) image of 20;
(d) Laplace-filtered FM-AFM image of diaza-HBC 20; apparent C=C dis-
tance of 1.0 Å (black), and N–C distance of 1.8 Å (blue).
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E
J. Liu et al.
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central pyrazine-ring in 20 is observed in the FM-AFM re-
sults on Ag(111) (Figure 4, d). Furthermore, nucleus-inde-
pendent chemical shift (NICS) calculations were performed
for 19 and 20, in which the central pyrazine cores possess
high positive values, revealing their antiaromaticity
(Scheme 2). These results demonstrate the antiaromatic
feature of pyrazine-containing PAHs and offer the possibility
of on-surface synthesis of N-doped graphene nanostructures.
Apart from benzene, phenalene is the smallest
graphene molecule with full zigzag edge structures.69,70
Therefore, the zigzag-edged nanographenes or graphene
nanoribbons (GNRs) can be realized by repeating this phe-
nalene unit. This suggests that the N-doped zigzag GNRs
can be synthesized by extending the corresponding com-
pound 4 with zigzag edges, which is exemplified by the
next-generation 24-1,66,71 a fused dimer with two PAMY
units. The synthetic route towards 24-1 is described in
Scheme 3. First, one-sided Suzuki coupling was carried out
between 1-hydroxy-3H-2,1-benzoxaborole and 2,6-dibro-
mo-4-(tert-butyl) aniline (1) to provide 21. Then, twofold
Suzuki coupling of l,5-dibora-2,6-dioxa-sym-hydrinda-
cene-l,5-diol and 21 resulted in the synthesis of 22.
Subsequently, HCl-induced cyclization of 22 under mi-
crowave conditions afforded compound 23. Finally, under
inert conditions, the dimer 24-1 was generated through the
deprotonation of 23 using anhydrous triethylamine. Com-
pared to that of the monomer PAMY a (max = 580 nm;
Scheme 1), the maximum UV/Vis absorption of dimer 24-1
was observed at  = 751 nm. This large redshift of the
UV/Vis absorption of dimer 24-1 is due to the extended -
conjugation in the two resonance structures 24-1 and 24-2
(Scheme 3), as well as by an additional quinoid resonance
structure in 24-3. In addition, this dimeric PAMY 24 can
also be considered a potential building block for the synthe-
sis of large N-doped PAHs or corresponding N-doped GNRs
through 1,3-dipolar cycloaddition reactions or surface-as-
sisted homocoupling. In addition to the aforementioned
work reported by our group, Ito and Nozaki et al. presented
the synthesis of the same isoquinolino[4,3,2-de]phenan-
thridine (PAMY) core, which was used for 1,3-dipolar cy-
cloadditions with different kinds of alkynes and alkenes to
synthesize the corresponding 2,5-dihydropyrroles, pyrroli-
dines, and pyrroles.72 Moreover, this PAMY was also utilized
as a model segment to generate benzene-fused azacoran-
nulene,73 pyrrole-fused corannulene,74 and a corannu-
lene/azacorannulene hybrid nitrogen-containing bucky-
bowl.75
3 Cationic Nitrogen-Doped Helical PAHs
From a molecular perspective, graphitic-type N doping
can be defined as the substitution of one quaternary carbon
atom by an N atom. Therefore, this N-doped motif results in
a cationic nitrogen species, which alters the electron densi-
ty at the Fermi level and has a tremendous influence on tai-
loring the optical and electronic structures of graphene.76–84
Cationic N-doped nanographenes (CNDNs) with planar
structures have been synthesized as mentioned in section
2, such as PAMY, 4 and 23, which possess interesting chem-
ical and physical properties. However, CNDNs possessing a
cove
+
N
N
N
+
+
helical
+
N
N
+
Figure 5 Cove-edged nonplanar and helical N-charged PAHs
R
R
OH
NH2
OH OH
NH2
–
BF4
+
N
NH2
Br
Br
a
b
c
Br
NH2
N
+
–
BF4
R
OH OH
R
1
R
R
21
22
23
R = tBu
d
R
N
R
N
R
–
+
N
N
N
N
+
–
24-3
24-2
24-1
R
R
R
Scheme 3 Synthetic route towards the next higher homologue 23 and PAMY dimer 24-1. Reagents and conditions: (a) Pd(PPh3)4, 1-hydroxy-3H-2,1-
benzoxaborole, K2CO3, ethanol, toluene; (b) Pd(PPh3)4, l,5-dibora-2,6-dioxa-sym-hydrindacene-l,5-diol, K2CO3, ethanol, toluene; (c) 1) HCl, dioxane, O2,
microwave; 2) trityl BF4, toluene, acetonitrile; (d) triethylamine. Resonance structures of 24-1, 24-2, and 24-3, in which the Clar sextets are highlighted
in gray.
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nonplanar geometry, such as helicenes, offer extraordinary
graphene nanostructures with unique – interactions and
-electron delocalization. In turn, this helical geometry
would also affect their physical and chemical properties.85–
First, the synthesis of helical N-charged nanographenes
27a+–e+ is described in Scheme 4. The desired aza[5]helice-
nium salt 27a+–c+, in which trifluoracetate was incorporat-
ed as an anion by using an excess of silver trifluoroacetate,
was achieved by a rhodium-induced annulation process.98,99
To enhance the solubility of the helical N-charged nanog-
raphenes in solution, 1,2-bis[4-(tert-butyl)phenyl]ethyne
with a bulky group was employed as the annulation unit. By
means of a similar synthetic strategy, a higher-order heli-
cene and larger -conjugation system was also achieved.
Accordingly, the racemic N-doped aza[6]helicenium (27d+)
and aza[7]helicenium (27e+) cations were synthesized as
their trifluoroacetate salts in yields of 46% and 70%, respec-
tively. In addition, the successful achievement of singly N-
charged helical nanographenes (27a+–e+) led to the synthe-
sis and investigation of larger helical CNDNs, such as those
with cove-edged peripheries. Finally, doubly charged N-
doped aza[4]helicenium cations 30a2+–c2+ were realized
through this bottom-up organic synthesis (Scheme 5). The
related precursors 29a–c were obtained by Suzuki coupling
of pinacol esters 28a–c and 2-bromopyridine in one step.
N-Charged 30a2+–c2+ were synthesized through rhodium-
induced annulation as either triflate or trifluoroacetate
salts in good yields (83–96%). Moreover, to demonstrate the
feasibility of counterions in the synthesis, silver triflate was
utilized to present triflate as an anion for 30c2+[OTf–]2.
Compared with all-carbon PAHs, the above N-charged
helical PAHs possess energetically low-lying frontier molec-
ular orbitals with a significant stabilization of the lowest
unoccupied molecular orbital (LUMO), which leads to a nar-
rower optical energy gap and a higher electron affinity.
Moreover, electrochemical or spectroelectrochemical inves-
93
For instance, PAHs with chiral centers can be obtained
through helicity, which renders helical PAHs promising chi-
ral materials in organic spin filters.94–96 Recently, we
demonstrated the fabrication of a novel class of helical
CNDNs, such as singly charged aza[5]helicene and cove-
edged aza[4]helicenes with two charges (Figure 5).97
tBu
Ar
Ar
Ar
Ar
a
–
tBu
tBu
CF3CO2
+
+
N
N
26
tBu
25a–e
27a–e
tBu
CF3CO2
tBu
tBu
tBu
–
–
+
N
–
CF3CO2
CF3CO2
+
+
N
N
tBu
tBu
27a
27c
27b
tBu
tBu
–
–
+
N
CF3CO2
+
N
CF3CO2
tBu
tBu
27d
27e
Scheme 4 Synthesis of N-charged helicenes 27a+–e+. Reagents and con-
ditions: (a) [Cp*RhCl2]2, AgO2CCF3, MeOH, 120 °C, 20 h; Cp* = C5Me5.
tBu
tBu
tBu
tBu
a
+
Bpin
N
N
Bpin
N
N
+
Ar
Ar
Ar
b
28a–c
29a–c
30a CF3CO2
30b CF3CO2
30c OTf
tBu
tBu
tBu
tBu
tBu
tBu
tBu
tBu
–
N
CF3CO2
–
+
N
+
+
N
N
CF3CO2
N
+
OTf –
–
+
OTf –
+
N
CF3CO2
–
CF3CO2
tBu
tBu
30a
30b
30c
tBu
tBu
tBu
tBu
Scheme 5 Synthesis of double N-charged nanographenes 30a2+–c2+ with cove-edges. Reagents and conditions: (a) Pd(PPh3)4, 2-bromopyridine, Na2CO3,
toluene/H2O/MeOH, 90 °C, 15 h; (b) [Cp*RhCl2]2, 1,2-bis[4-(tert-butyl)phenyl]ethyne, AgO2CCF3 (30a2+–b2+) or AgOTf (30 c2+), MeOH, 120 °C, 24 h.
© 2019. Thieme. All rights reserved. Synlett 2019, 30, A–L

G
J. Liu et al.
Account
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tigations confirmed the in situ generation of N-containing
neutral or cationic radicals, depending on the number of N
atoms. The N-doped aza[4]helicenes with two charges
showed stable redox behavior and viologen-like character-
istics. Figure 6 (a) displays the UV/Vis/NIR spectra of
havior of the electronegativities of nitrogen and boron at-
oms in a BN motif results in BN-doped nanographenes with
unique polarities.108–112 Therefore, alteration of the loca-
tions of nitrogen and boron atoms in BN-doped PAHs
strongly influences their dipole moments. Recently, we pre-
sented an efficient synthetic method for the fabrication of
various ladder-type heteroacenes containing BN units
based on the cyclization of aromatic borylation.113–117 In ad-
dition to the introduction of BN heteroatoms into PAHs,
nanographenes containing nitrogen–boron–nitrogen (NBN)
units have also attracted great interest due to their prospec-
tive applications in molecular optoelectronic devices.118–125
Importantly, the NBN unit in PAHs can be selectively oxi-
dized into the corresponding radical cation, which is the
isoelectronic structure of its pristine carbon frameworks
with open-shell character (Scheme 6, a).
–
27e+[CF3CO2 ], which were measured at the reduction step
during cyclic voltammetry. Generation of the N-containing
neutral radical [27e]• is followed by loss of the peak at  =
–
469 nm for 27e+[CF3CO2 ], and the new bands appear at  =
513, 692, and 817 nm (Figure 6, a). A weak NIR absorption
was also observed at  = 1450 nm (Figure 6, a insert). More-
over, the in situ EPR measurements verify the generation of
the helical radical (Figure 6, b). According to the theoretical
calculation, the spin density of this N-containing helical
radical is distributed on the skeleton of the azahelicene core
(Figure 6, c). Moreover, the hyperfine EPR spectrum could
be reproduced very well with the coupling constants of the
nitrogen atom and four protons (Figure 6, b). The helical
structure of the in situ formed N-containing neutral radical
[27e]• is described in Figure 6 (d).
a)
•+
•+
H
N
H
H
H
H
N
isoelectronic
isoelectronic
N
[O]
N
N
B
B
b)
H
H
N
TMS
TMS
N
Br
Br
B
Br
Br
b
a
c
NH2
NH2
31
32
33
34
c)
•+
H
H
N
N
Cu(OTf)2
B
34
MeCN
92%
B
N
N
H
H
•+
34-2
Cu2+
H
N
H
N
hydrazine hydrate
92% in 2 steps
B
B
N
H
N
H
Figure 6 (a) UV/Vis/NIR spectra of 27e+ performed at the electro-
chemical reduction. (b) Simulated and experimental EPR spectra during
the electrochemical reduction of 27e+. c) Spin density distribution and
main hyperfine coupling constants of neutral radicals [27e]• from the
calculation (in Gauss; H black, N gray). (d) The proposed chemical struc-
tures of the in situ produced [27e]•.
34-2
Scheme 6 a) The oxidation process of NBN unit and its radical cation
as well as its isoelectronic structure. b) Synthesis of NBN-edged PAH 34:
(a) THF, LDA, TMSCl, –78 °C; (b) Pd(PPh3)4, NaHCO3, DME, 90 °C; (c)
BCl3, Et3N, o-DCB, 180 °C. c) Plausible oxidation process of 34.
We demonstrated the efficient synthesis of the first case
of NBN-doped 1,9-diaza-9a-boraphenalenes with zigzag
edges, namely NBN-dibenzophenalenes (NBN-DBPs, 34,
Scheme 6, b).118 Based on a three-step method, the synthe-
sis of 34 is described in Scheme 6 (b). As a key point of this
synthetic route, a trimethylsilyl (TMS) group was adopted
as a guiding unit to easily generate the N–B–N bond at the
periphery of PAHs by electrophilic-induced aromatic bory-
lation. First, compound 32 was synthesized through selec-
tive lithiation of 31 using lithium diisopropylamide (LDA),
followed by quenching the reactions by trimethylsilyl chlo-
ride (TMSCl). Subsequently, compound 33 was obtained
4 Nitrogen–Boron–Nitrogen-Doped PAHs
The chemical and thermal stabilities of polyacenes can
be dramatically improved by the introduction of heteroat-
oms into their carbon skeletons; accordingly, the molecular
energy levels (such as HOMO and LUMO) of the resulting
heteroacenes can be essentially tailored.100–103 In this re-
spect, replacement of one or more sp2 carbon–carbon (C=C)
units with isoelectronic boron–nitrogen (BN) bonds pro-
vides BN-doped PAHs with outstanding alteration in their
electronic structures.104–107 In particular, the different be-
© 2019. Thieme. All rights reserved. Synlett 2019, 30, A–L

H
J. Liu et al.
Account
Synlett
through the Suzuki coupling of 2-aminoarylboronic acid es-
ters and 32. Finally, NBN-doped PAHs 34 were achieved in
90% yield through electrophilic aromatic borylation in the
treatment of BCl3 and triethylamine at 180 °C. Following a
similar synthetic strategy, -extended compound 39 was
synthesized by further modification of the carbon frame-
work of 34 (Scheme 7), which can serve as the next-genera-
tion homologue NBN-edged dibenzoheptazethrene (denot-
ed as NBN-DBHZ). This efficient protocol offers the oppor-
tunity for the in-solution synthesis of NBN-edged PAHs or
GNRs with a stable zigzag periphery.
O
Br
Br
a
b
Br
B
O
NH
NH
c
C6H13
C6H13
31
35
36
Br
NHC6H13
Figure 7 a) UV/Vis/NIR absorption spectra of compound 34 upon ad-
dition of different concentrations of Cu(OTf)2 in acetonitrile (10–5 M,
Cu(Otf)2: 0, 0.5, 1.0, 1.5, 2.0, 2.5 equiv); b) EPR results of the reaction
mixture of 34 + Cu(Otf)2 in acetonitrile (10–3 M); c) cyclic voltammetry
(scan rate: 100 mV/s) of compound 34 in acetonitrile (n-Bu4NPF6, 10–1 M).
Br
37
NHC6H13
C6H13HN
C6H13 C6H13
N
N
C6H13HN
d
We recently reported an efficient approach towards the
construction of NBN-doped PAHs 43 and 46 containing
pentagonal and heptagonal rings in addition to hexagonal
rings (Scheme 8).126 First, N2-(2-bromophenyl)-2′-(trimeth-
ylsilyl)-[1,1′:3′,1′′-terphenyl]-2,2′′-diamine (41) was syn-
thesized by treatment of 40 with 1 equivalent of 1-bromo-
2-iodobenzene. Then, 9-(2-bromophenyl)-8H,9H-8,9-dia-
za-8a-borabenzo[fg]tetracene (42) was obtained by heating
a solution of 41 with BCl3 and excess triethylamine in o-di-
chlorobenzene (o-DCB) at 180 °C. Afterwards, 4b,15b-dia-
za-4b1-borabenzo[fg]indeno[1,2,3-op]tetracene (43) was
B
B
N
N
NHC6H13
C6H13 C6H13
NHC6H13
38
39
Scheme 7 Synthetic route towards NBN-edged PAH 39. Reagents and
conditions: (a) Pd(PPh3)4, CsCO3, toluene/ethanol, 90 °C; (b) (Bpin)2,
KOAc, DMSO, 100 °C; (c) Pd(PPh3)4, CsCO3, toluene/ethanol, 90 °C; (d)
BCl3, Et3N, o-DBC, 180 °C.
Interestingly, compound 34 was quantitatively trans-
formed into its -dimer 34-2 upon chemical oxidation in
the treatment of Cu(OTf)2 (Scheme 6, c). From the UV/Vis
spectrum, the maximum absorption peak of 34 is at ap-
proximately  = 351 nm, which is associated with two ab-
sorption bands in the range of 250–300 and 300–400 nm.
Compared to that of monomer 34, the maximum absorp-
tion peak of the NBN-edged 39 is shifted to  = 435 nm. This
redshift of 39 is due to the extended -conjugation. It is
worth noting that compound 39 displays split emission
bands and exhibits a much better fluorescence quantum
yield (ΦPL = 0.83) than monomer 34 (ΦPL = 0.24). Titration of
compound 34 with Cu(OTf)2 led to the generation of radical
cations, which could be detected by UV/Vis/NIR spectrosco-
py measurements (Figure 7, a). By addition of Cu(OTf)2, a
new absorption band of 34 gradually evolved in the visi-
ble/NIR regions at  = 404, 620, and 1150 nm, accompany-
ing decreased peaks at  = 336 and 352 nm. Additionally,
the EPR analysis demonstrated a significant signal at g =
2.0033 (Figure 7, b). This oxidation behavior of 34 clearly
revealed the regioselective activity of this kind of NBN-
doped PAH with zigzag edges.
TMS
TMS
Br
H
a
b
N
N
B
NH
Br
NH2
NH2
NH2
41
40
42
a
c
TMS
NH
Br
HN
Br
N
N
44
B
b
43
Br
Br
d
N
N
N
N
B
B
45
46
Scheme 8 Synthesis of NBN-doped PAH 43 and 46. Reagents and condi-
tions: (a) 1-bromo-2-iodobenzene, DPEphos, Pd(OAc)2, NaOtBu, tolu-
ene; (b) BCl3, o-DCB, Et3N; (c) DPEphos, Pd(OAc)2, NaOtBu, toluene; (d)
Ni(COD)2, 4,4′-bipyridine, COD, DMF/toluene.
© 2019. Thieme. All rights reserved. Synlett 2019, 30, A–L

I
J. Liu et al.
Account
Synlett
synthesized through a Buchwald–Hartwig reaction of 42 in
70% yield. Following a similar synthetic method, 8,9-bis(2-
bromophenyl)-8H,9H-8,9-diaza-8a-borabenzo-[fg]tetra-
cene (45) could be obtained from the same starting materi-
al 40 (Scheme 8). Finally, the Yamamoto intramolecular re-
action of 45 afforded 4b,15b-diaza-4b1-boraben-
zo[fg]dibenzo-[4,5:6,7]cycl-ohepta[1,2,3-op]tetracene (46)
in 78% yield. Introduction of five- and seven-membered
rings into NBN-doped PAHs can not only tune their optical
and electronic properties but also alter the geometry of the
molecules. For instance, 43 containing one five-membered
ring displays a planar structure; however, compound 46
possesses a double hetero[4]helicene geometry. Interest-
ingly, pentagon-embedded 43 and heptagon-embedded 46
exhibit higher stability under electrochemical oxidative
COOCH3
NH
COOH
NH
COOCH3
NH2
Br
b
a
HN
N
H
HN
N
H
Br
Br
H3COOC
COOCH3 HOOC
COOH
47
48
49
F
F
c
R
F
B
N
N
R
N
F
B
N
B
N
F
e
N
R
F
R
NH
N
N
Cl
N
N
R
52a R = C6H13
52b R = C12H25
d
N
Cl
H
N
N
Ph Ph
B
conditions
than
previously
reported
BN-doped
HN
Cl
R
f
PAHs.113,114,117 Moreover, on the basis of DFT calculations
and single-crystal X-ray studies, these nonhexagonal ring-
embedded 43 and 46 display unique global-aromatic fea-
tures with antiaromatic NBN rings, indicating that the -
electrons are delocalized over the whole skeleton. The sin-
gle-crystal OFETs of 43 and 46 deliver a p-type mobility of
0.05 cm2V–1s–1 and 0.01 cm2V–1s–1, respectively, indicating
the potential application in organic electronics for such
NBN-embedded -conjugated compounds.
R
N
N
R
51a R = C6H13
51b R = C12H25
50
N
B
Ph
Ph
N
B
Ph
N
Ph
N
R
53a R = C6H13
53b R = C12H25
Scheme 9 Synthesis of triple B,N-coordinated chromophores. Reagents
and conditions: (a) Pd(OAc)2, Cs2CO3, toluene, reflux; (b) 5% NaOH, ace-
tone, reflux; (c) POCl3, reflux; (d) R–NH2, reflux; (e) Et3N, BF3·OEt2, 90 °C,
toluene; (f) B(Ph)3, toluene, 120 °C.
In contrast, the introduction of four-coordinate boron
units into various sp2-carbon skeletons leads to the synthe-
sis of boron-containing chromophores, which have
achieved significant progress.127–131 In this Account, both
the weak B,N-coordinated bond and the strong B,N-cova-
lent bond are considered NBN systems. Interestingly, this
four-coordinate NBN system not only possesses a shape-
persistent molecular skeleton but also has a great influence
on controlling molecular packing and optical and electronic
properties.132,133 Moreover, the position and number of the
B,N-coordinated unit in -expanded PAHs also affect the
geometric and electronic structures.134–136 Recently, we pre-
sented the synthesis of unique chromophores with triple
B,N-coordinated units (Scheme 9).137 Through a typical
three-step reaction followed by the Buchwald–Hartwig re-
action, hydrolysis, and cyclization, the key building block of
6,12,18-trichloro-5,11,17-triazatrinaphthylene (50) was
obtained from 1,3,5-tribromobenzene. Then, 6,12,18-
tris(hexylamine)-5,11,17-triazatrinaphthylene 51a and
6,12,18-tris(dodecylamine)-5,11,17-triazatrinaphthylene
51b were synthesized by the reaction of 50 with different
kinds of alkylamines at 135 °C. Compounds 52a and 52b
were obtained with yields of 69% and 66%, respectively,
upon treatment of ligands 51a and 51b with boron trifluo-
ride etherate (BF3·OEt2) at 80 °C in toluene. 53a and 53b
were obtained with yields of 60% and 64%, respectively, by
using triphenylboron under similar reaction conditions.
Interestingly, the lengths of the covalent and coordina-
tion B–N bonds in 52 and 53 are similar (1.53 Å and 1.56 Å
for 52a; 1.56 Å and 1.61 Å for 53a), indicating that the elec-
trons in 52 and 53 are likely delocalized over the N–B–N
system. However, the lengths of the B–N bonds in 52 and 53
are longer than those of the aforementioned NBN-embed-
ded PAHs. For instance, the B–N bond lengths in 34 are ap-
proximately 1.42–1.44 Å. Notably, in comparison to the
maximum absorption of 52a (max = 428 nm), the maxi-
mum absorption of 53a (max = 454 nm) is exceedingly ba-
thochromically shifted by 26 nm, indicating that the optical
properties of the B,N-coordinated system can be influenced
by the substituents on the boron centers. This bathochro-
mic shift for 53a can be caused by the electron-donating ef-
fect of the phenyl groups. Moreover, among the B,N-coordi-
nated chromophores, 52a and 53a display the highest
Stokes shifts of 106 nm and 157 nm, respectively. In addi-
tion, upon irradiation with white and UV light, 52a can be-
come cross-linked both in the solid state and in solution
and is thus employed as a negative photoresist.138
5 Conclusion and Outlook
In recent decades, the synthesis of PAHs containing het-
eroatoms has received increasing attention because of their
promising electronic and magnetic properties, with prom-
ising applications in organic electronics, sensing, and met-
al-free catalysis. The bottom-up organic synthetic method
provides the opportunity to access the precise control of
different heteroatoms and doping positions at the molecu-
© 2019. Thieme. All rights reserved. Synlett 2019, 30, A–L

J
J. Liu et al.
Account
Synlett
lar level. In the past decade, different groups have explored
the synthesis and device applications of N- or NBN-embed-
ded PAHs.72,111,139–143 However, efficient synthetic protocols
are highly demanded to enhance the chemical diversity of
such heteroatom-doped PAHs. In this Account, we have
highlighted our recent efforts towards efficient synthesis.
For instance, we have demonstrated synthetic access to
PAMYs, which can serve as flexible building blocks for the
fabrication of large N-containing nanographenes. Moreover,
the solution chemistry of PAMYs can be employed for the
surface-assisted synthesis, yielding pyrazine-type diaza-
HBC through on-surface homocoupling and concomitant
cyclodehydrogenation. Instead of solely N-doped NGs, re-
placement of one or more carbon–carbon (C=C) bonds with
isoelectronic B–N units provides BN-doped PAHs with ex-
ceptional variation in their electronic structures. The feasi-
ble synthesis of NBN-embedded PAHs containing pentago-
nal and heptagonal rings has also been achieved using met-
al-catalyzed cyclization. The successful synthesis of
heteroatom-doped nanographenes at the molecular level
provides substantial opportunities not only for the investi-
gation of their electronic structures but also for their appli-
cation in organic electronics, such as in OLEDs and OFETs.
Moreover, the exploration of heteroatom-doped, structural-
ly defined GNRs144,145 is a future goal in this area.
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Funding Information
This work was financially supported by the European Union's Horizon
2020 research and innovation programme (Grant No. 785219), EU
Graphene Flagship, Deutsche Forschungsgemeinschaft and National
Natural Science Foundation of China (DFG-NSFC Joint Sino-German
Research Project, EnhanceNano), Center for Advancing Electronics
Dresden (cfaed), European Social Fund and the Federal State of Saxony
(ESF-Project ‘GRAPHD’, TU Dresden). J. Liu is grateful for the startup
funding from the University of Hong Kong.
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