Cationic Nitrogen-Doped Helical Nanographenes

Article abstract of DOI:10.1002/anie.201707714

Herein, we report the design and synthesis of a series of novel cationic nitrogen-doped nanographenes (CNDNs) with nonplanar geometry and axial chirality. Single-crystal X-ray analysis reveals helical and cove-edged structures. Compared to their all-carbon analogues, the frontier orbitals of the CNDNs are energetically lower lying, with a reduced optical energy gap and greater electron-accepting behavior. Cyclic voltammetry shows all the derivatives to undergo quasireversible reductions. In situ spectroelectrochemical studies prove that, depending on the number of nitrogen dopants, either neutral radicals (one nitrogen dopant) or radical cations (two nitrogen dopants) are formed upon reduction. The concept of cationic nitrogen doping and introducing helicity into nanographenes paves the way for the design and synthesis of expanded nanographenes or even graphene nanoribbons with cationic nitrogen dopants.

Full text of DOI:10.1002/anie.201707714

A Journal of the Gesellschaft Deutscher Chemiker
Angewandte
Chemie
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Accepted Article
Title: Cationic Nitrogen Doped Helical Nanographenes
Authors: Kun Xu, Xinliang Feng, Reinhard berger, Alexey A Popov, Jan
J Weigand, Ilka Vincon, Peter Machata, Felix Hennersdorf,
Youjia Zhou, and Yubin Fu
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To be cited as: Angew. Chem. Int. Ed. 10.1002/anie.201707714
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10.1002/anie.201707714
Angewandte Chemie International Edition
DOI: 10.1002/anie.200((will be filled in by the editorial staff))
Charged Nanographene
Cationic Nitrogen Doped Helical Nanographenes**
Kun Xu, Yubin Fu, Youjia Zhou, Felix Hennersdorf, Peter Machata, Ilka Vincon, Jan J. Weigand,
Alexey A. Popov, Reinhard Berger and Xinliang Feng*
Dedicated to Professor Klaus Müllen on the occasion of his 70th birthday
Abstract: Herein, we report on the design and synthesis of a series of
novel cationic nitrogen doped nanographenes (CNDN) with non-
planar, axial chiral geometries. Single-crystal X-ray analysis reveals
helical and cove-edged structures. Compared to their all-carbon
analogues, the CNDN exhibit energetically lower lying frontier
orbitals with a reduced optical energy gap and an electron accepting
behavior. All derivatives show quasi reversible reductions in cyclic
voltammetry. Depending on the number of nitrogen dopant, in situ
spectroelectrochemistry proves the formation of neutral radicals (one
nitrogen dopant) or radical cations (two nitrogen dopants) upon
reduction. The concept of cationic nitrogen doping and introducing
helicity into nanographenes pave the way for the design and synthesis
of expanded nanographenes or even graphene nanoribbons
containing cationic nitrogen doping.
nanographenes, cationic nitrogen doped nanographenes (CNDN)
remain less available, mainly due to the synthetic difficulties.
Pioneering work in the synthesis of CNDN has been carried out with
planar structures (Figure 1, left),[5a-f] which revealed interesting
optoelectronic properties as well as supramolecular behavior in
solution, at the liquid-solid interface and in solid state.[5a-d] As our
continuous efforts, we envisioned that the incorporation of helicity
into the CNDN would provide unique graphene nanostructures with
interfered π-electron delocalization and π-π interactions, which in
turn influence their physical properties.[6-8] In addition, helicity
provides nanographenes with chiral center, which renders the helical
nanographenes as potential chiral material for applications such as
organic spin filter.[9] To realize our hypothesis, herein we designed
and synthesized a class of CNDN featured with helical[7] and cove-
edge[8] structures exemplified for mono charged aza[5]helicene and
doubly charged aza[4]helicenes in Figure 1.[10] The obtained charged
nanographenes have energetically low lying frontier molecular
orbitals with a pronounced stabilization of the lowest unoccupied
molecular orbital (LUMO) causing a higher electron affinity and a
reduced optical energy gap than their all-carbon analogies. Moreover,
electrochemical and spectroelectrochemistry studies prove in situ
formation of nitrogen containing neutral and cationic radicals
depending on the number of nitrogen atoms. For the cationic double
aza[4]helicenes, viologen-like properties and stable redox properties
are revealed.
Nitrogen doped graphenes are fascinating materials due to their
potential catalytic, optical and electronic applications[1] such as fuel
cells,[1b] solar cells,[1c] sensors[1d] and transistors.[1e] Various top-down
methods toward the synthesis of nitrogen doped graphene have been
developed in the last decade.[1a] In particular, diverse types of nitrogen
dopant have been found and classified as pyridinic, pyrrolic and
graphitic ones, among which graphitic nitrogen is considered as one
of the most active sites for the electrocatalysis.[2] From a molecular
point of view, the graphitic-type nitrogen results from the
replacement of a quaternary carbon atom with a nitrogen atom. This
species leads to cationic nitrogen dopant, which changes the local
density of states around the Fermi level and plays an important role
in tailoring the electronic structure of the sp2 carbon-frameworks.[2]
However, controlling the nitrogen doping at specific positions of
carbon materials at the molecular level (and with precise content) is a
longstanding synthetic challenge.[1a,3]
Bottom-up organic synthesis offers the possibility to access
nitrogen doped nanographenes with tailor-made structures and
physical properties.[4,5] Although substantial efforts have been made
in the synthesis of pyridinic[4d-j] and pyrrolic[4k-m] nitrogen containing
Figure 1. Cationic nitrogen doped graphene segments. a) planar zigzag edged
nanaographene, b) planar armchair nanaographene, c) nonplanar cove edged
nanographene (doubly charged aza[4]helicenes), d) nonplanar helical
nanographene (mono charged aza[5]helicene)
[]
Dr. K. Xu, Y. Fu, Y. Zhou, Dr. R. Berger, Prof. Dr. X. Feng
Center for Advancing Electronics Dresden (cfaed) & Department of
Chemistry and Food Chemistry, Technische Universität Dresden
01062 Dresden (Germany)
We commenced our study with the synthesis of a series of singly
charged nitrogen doped helical nanographenes. Via a rhodium
catalyzed annulation process,[11] the desired charged aza[5]helicene
trifluoracetate salt 1a was obtained with 43% isolated yield (Scheme
1). Although in the literature reported procedure, oxygen was used as
the oxidant for annulation reaction and excellent yields were obtained,
in our case, the reaction was very sluggish and thus we optimized the
conditions by using excess of silver trifluoroacetate. To avoid the
possible aggregation of the helical structure in solution, an alkyne
with bulky tert-butyl group, 1,2-bis(4-(tert-butyl)phenyl)ethyne was
used as the annulation partner. With this conditions, we then extended
to higher helical systems and larger conjugation. The annulation
precursors (2a-e) were synthesized via Suzuki coupling according to
E-mail: xinliang.feng@tu-dresden.de
F. Hennersdorf, Prof. Dr. J. J. Weigand
Chair of Inorganic Molecular Chemistry, Technische Universität
Dresden, 01062 Dresden (Germany)
Dr. P. Machata, I. Vincon, Dr. A. A. Popov
Leibniz-Institute for Solid State and Materials Research, 01069
Dresden (Germany)
((Ac Supporting information for this article is available on the WWW
under http://www.angewandte.org or from the author.
1
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10.1002/anie.201707714
Angewandte Chemie International Edition
literature reported procedures (Supporting Information). To our
delight, under similar reaction conditions, the corresponding racemic
charged aza-[6] and [7] helicenes[12] (1b+-c+) were achieved in
moderate yields (46% to 70%) after purification as their
trifluoracetate salts (1b-1c). Charged aza-[5] and [6] helicenes with
incorporated pyrene moiety (1d+ and 1e+) afforded slightly better
yields than those of 1a+ and 1b+ (Scheme 1). Considering the intrinsic
challenge for the formation of the five-membered cyclometalated
species with sterically hindered aromatic helical backbones[13] during
the rhodium catalyzed C-H activation step according to the
mechanism proposed by Huang and coworkers,[11] the yields of
charged aza-[6] and [7] helicenes (1b+-c+ and 1e+) are remarkable. To
the best of our knowledge, the charged aza[7]helicene 1c+ is the
highest helicene with fully conjugated six-membered rings and
cationic nitrogen doping.[13d-e]
map). The packing diagrams of 1b+, 1d+ and 1e+ show that the (P)
and (M) isomers are arranged in an alternating fashion, while counter
anions are distributed around the charged nitrogen center. More
detailed crystal data were described in Supporting Information (S6).
Scheme 2. Synthesis of cationic nitrogen doped cove edged nanographenes
3a2+-c2+. (a) 2-bromopyridine (3.0 equiv), Pd(PPh3)4 (6 mol%), Na2CO3 (3.0
equiv), toluene/MeOH/H2O (2/1/1), 90 oC, 15 hours. (b) 1,2-bis(4-(tert-
butyl)phenyl)ethyne (2.0 equiv), [Cp*RhCl2]2 (20 mol%), AgOTf (4.0 equiv),
MeOH (0.05 M), 120 oC, 24 hours. (c) 3a2+-c2+ are obtained as triflate salts (3a-
c). Naph = naphthalene-1,5-diyl, anth = anthracene-1,5-diyl, pyre = pyrene-1,6-
diyl.
Compound 3a2+ clearly show the nonplanar configuration (Figure
2b); chloride was observed as the counter anion, which might result
from the purification or crystal growth process using chlorinated
solvents. In the solid state, only (M,M) and (P,P) of 3a2+ were
observed (Figure 2b), which suggested the higher stability of the
concave type configuration than the (M,P)-isomer. The interplanar
angles between the two terminal rings for cove edges are 38.50o and
39.26o, which are significantly larger than the reported
[4]carbohelicene (24.87o).[16] The dihedral angles for both coves C4-
C5-C6-C7 (17.71o) with C5-C6-C7-C8 (28.21o), and C16-C17-C18-
C19 (17.84o) with C17-C18-C19-C20 (29.27o) are slightly different.
The central naphthalene rings are distorted with torsion angles of
17.16o (C6-C7-C19-C20), 17.37o (C22-C6-C7-C19), 17.25o (C8-C7-
C19-C18), and 19.30o (C7-C19-C18-C10). The high degree of
distortion of the molecule results in the concave structure of 3a2+ with
width of 11.179 Å, and depth of 1.615 Å (Figure 2b). Such type of
concave structure may allow 3a2+ to be a potential ‘bucky catcher’ for
fullerenes such as C60.[17] Compared to 3a2+, compound 3b2+ has
interplanar angles of 17.66° and 17.74°, with width of 12.683Å and
depth of 1.126Å for the concave core, which indicates a less distorted
structure due to the extended π-spacer of the anthracene moiety.
Notably, unlike 3a2+ and 3b2+, compound 3c2+ was only observed in
its (P,M)/(M,P) form in the solid state, which may be due to the more
rigid pyrene spacer.
Scheme 1. Synthesis of cationic nitrogen doped helicenes 1a+-e+. (a) Isoquinoline
derivative 2a-e (1.0 equiv), 1,2-bis(4-(tert-butyl)phenyl)ethyne (1.0 equiv), [Cp*RhCl2]2
(20 mol%), AgO2CCF3 (2.0 equiv), MeOH (0.05 M), 120 oC, 20 hours. (b) The helicenes
are obtained as trifluoroacetate salts (1a-e) in their racemic form.
Having had the success for the synthesis of monocharged nitrogen
doped monohelicenes (1a+-e+), we then sought to the synthesis of
larger CNDN with cove edge structures (namely, two cationic double
aza[4]helicenes, 3a2+-c2+). The corresponding precursors (4a-4c)
were synthesized in one step via Suzuki coupling of 2-bromopyridine
and easily available pinacol esters (5a-c). To our delight, compounds
3a2+-c2+ were obtained in good to excellent yields in both Suzuki
coupling (4a-c, 83-96%) and annulation (3a2+-c2+, 87-95%) steps
(Supporting Information). To showcase the variability of the counter
anion, for the synthesis of 3a2+-c2+, silver triflate was employed to
introduce triflate as the counter anion (3a-c).[14a]
Single crystals of monohelicenes 1b+, 1d+ and 1e+, as well as
doubly charged aza-[4]helicenes 3a2+-c2+, were successfully grown
via slow evaporation of their dichloromethane/hexane solutions.[14b]
As revealed in the crystal structures of 1b+, 1d+ and 1e+, the
corresponding (P) and (M) isomers were observed, their interplanar
angles are 41.28o, 44.59o and 45.48o, respectively (Figure 2, a).
Remarkably enough, the interplanar angle of charged aza[6]helicene
1b+ (41.28o) is much smaller than the all-carbon [6]helicene
(59.57o),[15] indicating a relatively less twisted configuration, possibly
due to an attractive force stemmed from the unequal charge
distribution (see Supporting Information for electrostatic potential
2
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10.1002/anie.201707714
Angewandte Chemie International Edition
Figure 2. a) Molecular structure of [P]-1b+ (left), [P]-1d+ (middle), [P]-1e+ (right)
(4-(tert-butyl)phenyl, H and anions are omitted for clarity). b) Molecular structure
of [P,P]-3a2+ (left), [P,P]-3b2+ (middle), [P,M]-3c2+ (right) (top view and side
view). Displacement ellipsoids are drawn at the 50% probability level.
To get further insight into the conformation of 3a2+ with doubly
charged aza[4]helicene structure, DFT calculations at the B3LYP/6-
31G+(d) level were performed by comparing the energy level of (P,P),
(M,M) and (M,P) conformations (Figure 3). The optimized energy of
meso-(M,P) is 2.4 kcal/mol higher than that of the (P,P) and (M,M)
isomers. The isomerization from [P,P]-3a2+ or [M,M]-3a2+ to [M,P]-
3a2+ proceeds via the transition state [TS]-3a2 which has energy
+
barrier of +8.6 kcal/mol. The higher thermodynamic stability of (P,P)
or (M,M) isomers can explain why the (M,P) isomer was not
identified in the crystal structures. Interestingly, the pristine (3a’
without nitrogen doping) and the corresponding carbohelicene (3a’’
without substituents and nitrogen doping) of 3a2+ also favor their (P,P)
and (M,M) isomers based on the calculated results. On the other hand,
the isomerization energy of [P]-1b+ to [M]-1b+ is 35.5 kcal/mol,
similar to the pristine 1b’ (36.4 kcal/mol, 1b+ without substituents)
and full carbon hexahelicene 1b’’ (37.2 kcal/mol). A detailed
discussion can be found in S7 in the Supporting Information.
Figure 4. a) UV/Vis spectra in 1.0 x 10-5 M CH3CN (top left). b) Fluorescence
spectra in 1.0x10-7 M CH3CN (top right, excitation wavelengths are given in
parenthesis). c) Cyclic voltammetry in a solution of Bu4NPF6 (0.1 M) in DCM at
room temperature, using Ag wire as reference electrode and
ferrocene/ferrocenium (Fc/Fc+) as external standard, with scan rate of 0.05 V/s
for 1a-e (CF3CO2 salts of 1a+-e+, bottom left), 1.0 V/s for 3a-c (OTf salts of 3a2+
c2+ bottom right).
-
The electrochemical behavior of CNDN were investigated by
cyclic voltammetry in dichloromethane solution. The reduction of
charged azahelicenes 1a+-e+ provided one quasi reversible peak in a
potential range of -0.86 V to -0.69 V vs Ag/Ag+. Notably, compounds
3a2+-c2+ exhibited two quasi reversible reduction peaks, indicating the
formation of radical cations (first reduction) and neutral species
(second reduction). This result suggests their potential to be used as
viologen type materials.[19] Using ferrocene as an external standard,
energy levels of the LUMO of monohelicenes are determined to be -
3.25 (1a+), -3.29 (1b+) and -3.42 eV (1c+). For the cove edge nitrogen
doped nanographenes, energy levels of the LUMO are derived to be
-3.59 (3a2+), -3.41 (3b2+) and -3.60 eV (3c2+). Based on the LUMO
and optical energy gap values, the corresponding HOMOs -6.30
(3a2+), -6.10 (3b2+) and -6.00 eV (3c2+) are derived respectively. All
optoelectronic data of 1a+-e+ and 3a2+-c2+ as well as a comparison
with their pristine carbon analogues are provided in Table 7 in the
Supporting Information.
To evaluate the role of cationic nitrogen doping, DFT calculations
of 1b+ and 3a2+, their pristine structures (1b’ and 3a’) as well as their
all carbon analogies (1b’’ and 3a’’) were performed (Supporting
Information, Table 7). The calculated LUMO values (EthLUMO) are in
good agreement with experimental LUMOs (ELUMO), while
calculated HOMOs (EthHOMO) differ the HOMOs (EHOMO) derived
from optical energy gap (Eg) and ELOMO in the range of 0.63-0.94 eV
due to polarization effects. In comparison with their all-carbon
analogies, their HOMO values are lowered by 0.8 eV (1b+ vs 1b’’)
and 1.16 eV (3a2+ vs 3a’’), and LUMO values are lowered by 1.4 eV
(1b+ vs 1b’’) and 1.87 eV (3a2+ vs 3a’’). The strong effect on the
LUMO levels renders them with electron accepting properties.
Noteworthy, substituents (4-(tert-butyl)phenyl) could also lower the
HOMO and LUMO values (1b+ vs 1b’ and 3a2+ vs 3a’), which offers
the possibility to finely tune the electronic nature via modifying the
substituents of CNDN.
Figure 3. Isomerization process of 3a2+ from (P,P)-isomer to (M,P)-isomer. The
relative Gibbs free energy (kcal/mol) was calculated at the B3LYP/6-31G+(d)
level.
The photophysical properties of 1a+-e+ and 3a2+-c2+ were studied
in CH3CN solutions by UV-vis and photoluminescence spectroscopy
(Figure 4). The absorption spectra of 3a2+, 3b2+ and 3c2+ exhibited
maximum absorption peak at 335, 360 and 373 nm, respectively. This
result suggests significant red-shift as the π-conjugation extends. The
same trend was also observed for the helicenes 1a+-1e+ ranging from
439 nm (1a+) to 498 nm (1e+). Based on the absorption onsets, the
optical energy gaps for 3a2+, 3b2+ and 3c2+ are estimated to be 2.81,
2.69 and 2.33 eV, respectively. According to TD-DFT calculations
(3a2+-c2+) (Supporting Information), the low-energy absorption bands
from 400-550 nm are assignable to the HOMO→LUMO transitions.
For cationic nitrogen doped monohelicenes, the optical energy gaps
are from 2.68 eV (1a+) to 2.20 eV (1e+), suggesting that a relatively
lower energy gap could be obtained by extending the conjugation
system. All of the obtained compounds showed fluorescence in
CH3CN solution, with maximum emission peak ranging from 447
(3a2+) to 600 nm (1e+). In comparison with literature reported
maximum absorptions for [5], [6] and [7]carbohelicenes (395, 413
and 425 nm, respectively),[18] the maximum absorptions of cationic
nitrogen doped [5], [6] and [7]helicenes (439, 448 and 469 nm for 1a+,
1b+ and 1c+, respectively) show bathochromic shift of 35 to 44 nm,
implying that the cationic nitrogen doping could lower the optical
energy gap of the helicene system.
3
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10.1002/anie.201707714
Angewandte Chemie International Edition
Figure 6. a) DFT-computed spin density distribution and main hyperfine
coupling constants (in Gauss; black – protons, blue - nitrogen) in the radical
form of 1c+ ([1c]˙, top left). b) DFT-computed spin density distribution and main
hyperfine coupling constants (in Gauss; black – protons, blue - nitrogen) in the
radical cation form of 3c2+ ([3c]˙+, top right). c) Chemical structures of in situ
generated neutral radical [1c]˙, radical cation [3c]˙+ upon first reduction of 3c2+
and neutral species [3c]¨ upon second reduction of 3c2+. R = 4-(tert-
butyl)phenyl.
To conclude, we have designed and synthesized a novel class of
CNDN with cove edges and helical structures. These molecules
exhibit unique nonplanar configurations with axial chirality. The
lower lying HOMOs/LUMOs and energy gaps, as well as quasi
reversible electrochemical reduction allow the in situ characterization
of their reduced radical and cationic radical species. These results
imply their potential as (opto)electronic materials. The concept of
cationic nitrogen doping and introducing helicity into nanographenes
holds promise for our ongoing design and synthesis of CNDN with
expanded π-systems, multiple doping centers, novel nonplanar
structures and even graphene nanoribbons.
Figure 5. a) UV/vis/NIR spectra measured during electrochemical reduction of
1c+ (top left). b) Experimental and simulated EPR spectra of the reduced form
of 1c+ (top right). c) UV/vis/NIR spectra measured during the first redox step
(radical cation) of 3c2+ (bottom left). d) Experimental and simulated EPR spectra
of the radical cation form of 3c2+ (bottom right).
Inspired by these cyclic voltammetry and DFT calculation results,
detailed spectroscopic characterizations by in situ EPR (electron
paramagnetic resonance) and UV/vis/NIR absorption spectroscopy
were performed for compounds 1c+ and 3c2+ as representative
examples. Figure 5 shows the spectra measured during cyclic
voltammetry of 1c+ at its reduction step. The formation of the neutral
radical [1c]˙ is accompanied by depletion of the absorption of 1c+ at
469 nm and appearance and growth of new absorption bands at 513,
692, and 817 nm (the latter is broad and extended to ca 1000 nm).
Weak absorption is also found at 1450 nm (Figure 4a). EPR
measurements (performed simultaneously with optical spectroscopy)
confirm the formation of the stable helical radical.[20] The spin density
in the radical is distributed over the azahelicene backbone and does
not extend to the substituents (4-(tert-butyl)phenyl). The hyperfine
structure of the experimental EPR spectrum is reproduced well with
the coupling constants of four protons and the nitrogen, all near or
exceeding 2 G (Figure 6 and Supporting Information). Even more, the
in situ generated radical cation [3c]˙+ upon one electron reduction
(first reduction of 3c2+) was clearly proved by in situ electrochemistry
studies. As depicted in Figure 5c, the radical cation shows intense
absorption bands peaked at 766, 1200 and 1410 nm. EPR spectrum
(Figure 5d) further proved the presence of radical cation [3c]˙+. The
two electron reduced neutral species [3c]¨ (second reduction of 3c2+),
although highly unstable, was also observed via in situ UV/vis/NIR
(Supporting Information).
Acknowledgements
This research was supported financially by ERC grants on
2DMATER, the EC under Graphene Flagship (No. CNECT-ICT-
604391), the Center for Advancing Electronics Dresden (cfaed), the
European Social Fund, and the Federal State of Saxony (ESF-Project
“GRAPHD”, TU Dresden). We thank Ji Ma, Shunqi Xu, Dr.
Ajayakumar Murugan Rathamony, Dr. Junzhi Liu, Dr. Jian Zhang,
Dr. Stavroula Sfaelou and Dr. Xiaodong Zhuang for helpful
discussions. JJW thanks the DFG for funding a diffractometer
(INST269/618-1).
Conflict of interest
The authors declare no conflict of interest.
Keywords: Nanographene · Nitrogen doping · Polycyclic aromatic
hydrocarbon · Helicene · Graphene
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Received: ((will be filled in by the editorial staff))
Published online on ((will be filled in by the editorial staff))
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5
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10.1002/anie.201707714
Angewandte Chemie International Edition
Charged Nanographene
Kun Xu, Yubin Fu, Youjia Zhou, Felix
Hennersdorf, Peter Machata, Ilka
Vincon, Jan J. Weigand, Alexey A.
Popov, Reinhard Berger and Xinliang
Feng* __________ Page – Page
Cationic Nitrogen Doped Helical
Nanographenes
Nitrogen Plus: Engineering of nanographenes via a new bottom-up synthetic
method towards cationic nitrogen doped nanographenes with nonplanar helical
structures is described. The protocol provides easy access to novel types of cationic
nitrogen doped nanographenes with interesting properties, which holds the potential
as optical and electronic materials.
6
This article is protected by copyright. All rights reserved.