On-surface Synthesis of a Chiral Graphene Nanoribbon with Mixed Edge Structure

Article abstract of DOI:10.1002/asia.202001008

Chiral graphene nanoribbons represent an important class of graphene nanomaterials with varying combinations of armchair and zigzag edges conferring them unique structure-dependent electronic properties. Here, we describe the on-surface synthesis of an un

Full text of DOI:10.1002/asia.202001008

CHEMISTRY
AN ASIAN JOURNAL
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Accepted Article
Title: On-surface Synthesis of a Chiral Graphene Nanoribbon with
Mixed Edge Structure
Authors: Ashok Keerthi, Carlos Sánchez-Sánchez, Okan Deniz, Pascal
Ruffieux, Dieter Schollmeyer, Xinliang Feng, Akimitsu Narita,
Roman Fasel, and Klaus Müllen
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10.1002/asia.202001008
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On-surface Synthesis of a Chiral Graphene Nanoribbon with
Mixed Edge Structure
Ashok Keerthi, [a]† Carlos Sánchez-Sánchez, [b,c]† Okan Deniz,[b] Pascal Ruffieux,[b] Dieter
Schollmeyer,[d] Xinliang Feng,[e] Akimitsu Narita,[f] Roman Fasel,*[b,g] Klaus Müllen*[d,f]
[a]
[b]
Dr. A. Keerthi
Department of Chemistry
The University of Manchester
Oxford road, Manchester M13 9PL, UK
Dr. C. Sánchez-Sánchez; Dr. O.Deniz; Dr. P. Ruffieux; Prof. R. Fasel
Empa
Swiss Federal Laboratories for Materials Science and Technology
8600 Dübendorf, Switzerland
E-mail: roman.fasel@empa.ch
[c]
[d]
[e]
[f]
Dr. C. Sánchez-Sánchez
ESISNA Group, Materials Science Factory
Institute of Materials Science of Madrid (ICMM-CSIC)
Sor Juana Inés de la Cruz 3, 28049 Madrid, Spain
Dr. D. Schollmeyer; Prof. Dr. K. Müllen
Department of Chemistry
Johannes Gutenberg-University
55099 Mainz, Germany
Prof. Dr. X. Feng
Center for Advancing Electronics Dresden (cfaed) & Faculty of Chemistry and Food Chemistry
Technische Universität Dresden
01062 Dresden, Germany
Dr. A. Narita, Prof. Dr. K. Müllen
Max Planck Institute for Polymer Research
Ackermannweg 10,
55128 Mainz, Germany
E-mail: muellen@mpip-mainz-mpg.de
Prof. Dr. R. Fasel
[g]
Department of Chemistry and Biochemistry
University of Bern
3012 Bern, Switzerland
† These authors contributed equally
Supporting information for this article is given via a link at the end of the document
Abstract: Chiral graphene nanoribbons represent an important class
of graphene nanomaterials with varying combinations of armchair
and zigzag edges conferring them unique structure-dependent
electronic properties. Here, we describe the on-surface synthesis of
making armchair-edge GNRs (AGNRs) semiconducting with
width-dependent bandgaps and zigzag-edge GNRs (ZGNRs)
potentially magnetic thanks to their edge-localized states that
can be spin polarized.[2a, 3] Chiral GNRs (ch-GNRs) combining
armchair and zigzag edges are characterized by a chiral vector
(n,m) which connects crystallographically equivalent sites along
the edge and defines the edge structure of the GNR.[4] These
ch-GNRs are theoretically predicted to present electronic band
structures and magnetic properties dependent on the chiral
vector.[2d, 5] In addition to armchair and zigzag edge structures,
one can also consider other geometries like cove and fjord
edges[6], which result in non-planarity due to the repulsion
between neighbouring hydrogen atoms.[7] Because structural
control at the single-atom level is essential to obtain GNRs with
desired properties, conventional top-down nanofabrication
methods (e.g., patterning of graphene or unzipping of carbon
nanotubes) fail to furnish GNRs with defined and singly
hydrogenated edges.[2e, 8] In contrast, bottom-up synthesis can
provide GNRs with atomically precise widths, edge structures
and chemical functionalization, thus attracting increasing
attention over the past decade.[1a, f, 9]
an unprecedented cove-edge chiral GNR with
a benzo-fused
backbone on Au(111) surface using 2,6-dibromo-1,5-
a
diphenylnaphthalene as precursor. The initial precursor self-
assembly and the formation of the chiral GNRs upon annealing are
revealed, along with
a relatively small electronic bandgap of
approximately 1.6 eV, by scanning tunnelling microscopy and
spectroscopy.
Graphene nanoribbons (GNRs), ribbon-shaped cut-outs of
graphene with nanometre-scale widths, have attracted much
attention due to their unique (opto)electronic properties, and
have been recognized as
a promising material for next-
generation electronics.[1] Their properties, such as chemical
stability, optical absorption and emission, charge-carrier mobility,
and spin state, heavily depend on their geometrical features
f, j, 2]
such as edge structure, width, and length.[1e,
The edge
structure is the most critical factor for the properties of GNRs,
1
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Scheme 1. Synthesis of 2,6-dibromo-1,5-diphenylnaphthalene (NAP 7) as the precursor used in UHV-deposition and subsequent on-surface synthesis of ch-GNR
9. Reagents and conditions: (a) N-bromosuccinamide, chloroform, room temperature, 10 h, 98%; (b) phenylboronic acid, K2CO3, toluene, Pd(PPh3)4, 90 °C, 12 h,
90%; (c) BBr3, dichloromethane, room temperature, 12 h, 95%; (d) trifluoromethanesulfonic anhydride, triethylamine, dichloromethane, 50 °C, 10 h, 87%; (e)
Pinacolborane, Pd(dppf)Cl2, dioxane, reflux, 16 h, 61%; (f) CuBr2, dioxane/methanol/water, reflux, 24 h, 73%; (g) Au(111), UHV chamber, 470 K, 20 min; (f)
Au(111), UHV chamber, 570 °C, 90 min.
The bottom-up synthesis can be carried out using tailor-made
molecular precursors, typically through two methods, namely i)
in solution, by the methods of organic and polymer chemistry[1f, k,
2f, 10], and ii) on surfaces, by using tools and strategies of surface
science and on-surface synthesis.[1e, f, j, 2g-i, 11] In particular, the
on-surface protocol allows for the in-situ visualization of the
resulting GNRs by high-resolution scanning probe microscopies,
(UHV) conditions on a Au(111) surface. Characterization by
means of scanning tunneling microscopy and spectroscopy
(STM and STS) shows the non-planar structure of ch-GNR 9
and reveals a bandgap of ~1.6 eV.
For the synthesis of NAP 7, 2,6-dimethoxy naphthalene (1) was
selectively brominated with N-bromosuccinimide to give 1,5-
dibromo-2,6-dimethoxynaphthalene (2)[16] in 98% yield, followed
by a Suzuki coupling reaction with phenylboronic acid to
produce 2,6-dimethoxy-1,5-diphenylnaphthalene (3) in 90% yield
(Scheme 1). Demethylation of 3 was achieved by treatment with
boron tribromide (BBr3) to provide 2,6-dihydroxy-1,5-
diphenylnaphthalene (4) in 95% yield, which was subsequently
reacted with trifluoromethanesulfonic anhydride to afford
bistriflate 5 in 87% yield. Miyaura borylation of 5 gave bisboronic
ester 6 in 61% yield along with mono-substituted byproduct
(~30% yield). Treatment of 6 with CuBr2 in a mixture of diaoxane
and methanol under reflux allowed to convert the boronic ester
to the bromide, providing NAP 7 in 73% yield (see ESI for
reaction conditions and characterization details).
while unambiguous structural elucidation of solution-synthesized
e, 2h, 11-12]
GNRs remains challenging.[1a,
Through on-surface
synthesis, AGNRs with varying widths have been achieved,
exhibiting distinct bandgaps in agreement with theoretical
predictions.[1e, i, 13] Formation of a ZGNR with perfect zigzag edge
structure was also unambiguously demonstrated, along with the
presence of edge-localized electronic states.[11a] A cove-edge
GNR was prepared by using a dibromobichrysene precursor,
demonstrating its non-planar structure on a surface.[7a] In
regards to ch-GNRs, chiral (3,1)-GNRs were synthesized using
bianthryl-based precursors,[14] and a heteroatom-doped chiral
(4,1)-GNR, having oxygen-boron-oxygen segments on the
zigzag edge, was also reported.[15] However, there are still
scarce examples of ch-GNRs and cove-edge GNRs reported to
date. Here, we describe the on-surface bottom-up synthesis of
unique ch-GNR 9 that can be considered as a benzo-fused
chiral (2,1)-GNR having mixed armchair and cove edges. The
2,6-dibromo-1,5-diphenylnaphthalene (NAP 7) molecule serves
as the precursor to ch-GNR 9 growth under ultrahigh vacuum
Prior to the on-surface synthesis, NAP 7 was recrystallized twice
from a mixture of dichloromethane (DCM) and methanol to yield
colourless needle-shape crystals, which also allowed for the
single-crystal X-ray analysis (Figure 1). The NAP molecules
were packed in a P-1 (triclinic) space group and the unit cell
contained two independent molecules with Ci symmetry (Figure
2
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1b and c). This dyad displayed partial π-π interaction (3.4 Å)
between the two naphthalene units. Hydrogen atoms at the
ortho-position of each phenyl ring were participating in C-H–π
(2.96 Å) interactions with the other naphthalene unit in the dyad.
Intermolecular C-H···Br interactions (3.01Å) were also observed
between NAP molecules in neighbouring dyads.
originated by the proximity of monomers of distinct surface
chirality.
a)
b)
For the on-surface synthesis of 9, NAP 7 was thermally
deposited onto a Au(111) substrate held at room temperature
under UHV conditions. As-deposited NAP molecules self–
assembled into homochiral islands, presumably stabilized by
intermolecular Br···H-C hydrogen bonds (Figure 2a and 2b).[17]
Subsequent annealing of the gold substrate at 470 K for 20
minutes led to the formation of bunches of long chains, as
revealed by STM analysis (Figure 2c and d). The presence of
faint lines on the herringbone reconstruction indicated the
debromination of NAP molecules already at this temperature
(see Figure S1 in ESI). However, measured intermolecular
distances (1.1 nm) in these chains were larger than the value of
0.6 nm for the corresponding polymers expected after the aryl–
aryl coupling (see superimposed scaled chemical structures in
Figure 2d), suggesting the formation of a metal-coordinated
intermediate 8. Interestingly, several bright protrusions are
observed in Figure 2d specifically where there is a change in the
stacking symmetry along the supramolecular chains. Given the
large distance between monomers in this phase, these
protrusions are attributed to the presence of Br atoms
underneath specific sites of the polymer, as reported for other
intermediate states of GNRs[18], maybe related to stacking faults
c)
Figure 1. Single-crystal X-ray analysis of NAP 7. a) Front view; b) Dyad of
NAP; c) Crystal packing in crystallographic a-axis.
Figure 2. STM images monitoring the on-surface reactions of NAP 7 to form ch-GNR 9: a) STM image of the as-deposited NAP 7 molecules at room temperature.
Greenish shadowing highlights molecules in second homochiral domain. STM parameters: (20 nm × 20 nm) 0.03 nA, -1.5 V, 77 K. b) Zoom (6.5 nm × 6.5 nm) of
STM image in panel a) showing the molecular orientation in the two domains. c) STM image after annealing the sample at 470 K. STM parameters: (150 nm ×
150 nm) 0.04 nA, 1.0 V, 77 K. d) High-resolution STM image of the chains in panel c), with scaled chemical structures superimposed. STM parameters: (12 nm ×
12 nm) 0.04 nA, -1.2 V, 77 K. e) STM image after annealing at 570 K. STM parameters: (140 nm × 140 nm) 0.03 nA, -1.55 V, 4.8 K. f) High-resolution STM
images of a homochiral segment of ch-GNR 9 corresponding to 10 NAP units. STM parameters: (7.7 nm × 2.3 nm) 0.04 nA, -1.2 V, 4.8 K.
3
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Further annealing the substrate at 570 K for 90 minutes induced
the aryl–aryl coupling and the cyclodehydrogenation between
the phenyl and naphthalene rings, yielding 9, as demonstrated
by STM (Figure 2e and f). This reaction sequence is common for
most of the previous on-surface syntheses of GNRs and
considered to proceed through 1) homolytic cleavage of carbon–
halogen bonds to generate diradical species stabilized by the
metal surface; 2) radical polymerization of the diradical species
that are mobile on the surface; and 3) oxidative
energies and the assignment of the corresponding states, we
have recorded differential-conductance maps at these energies.
The spatial distribution of these states indeed corresponds to the
onset of states located on the GNR (see Figure S2 for the
complete set of dI/dV maps).
In conclusion, NAP
7 was synthesized as the precursor
monomer, which allowed for the fabrication of unprecedented
benzo-fused chiral (2,1) GNR 9 having cove edges, on a Au
(111) surface under UHV conditions. Characterization by STM
evidenced the formation of 9 with a non-planar structure
originating from the cove edge, and an STS analysis revealed its
relatively small bandgap of ~1.6 eV. The obtained GNRs
featured connections of segments with different chirality with
defective structures at the junctions, due to the lack of
enantiomeric selectivity along the ribbons during the
polymerization and cyclodehydrogenation. This work expands
the structural variety of GNRs, providing an insight into the non-
planarity induced by the cove edge geometry and formation of
chiral GNRs. Further efforts to control the chirality of GNRs in
the on-surface synthesis are ongoing in our laboratories.
e, 12a]
cyclodehydrogenation catalysed by the metal surface.[1a,
These ribbons are composed of relatively short segments of 9
connected through kinks. We attribute these kinks to defects
caused by
a lack of enantiomeric selectivity during the
polymerization step. The high resolution STM image in Figure 2f
shows a defect-free, homochiral segment of the obtained GNR
corresponding to 10 units of NAP (see superimposed scaled
chemical structure), thus corroborating the formation of ch-GNR
9 and elucidating its non-planar structure induced by the
presence of cove edges. Similar bright protrusions were
reported for cove-edge GNRs in ref.[7a] and attributed to steric
repulsion between protons of benzene rings at the cove region.
Crystallographic data: Deposition number 2021698 contains the
supplementary crystallographic data for compound NAP 7. This data are
provided free of charge by the joint Cambridge Crystallographic Data Centre
and Fachinformationszentrum Karlsruhe Access Structures service. Please
see the Supporting Information for more details.
Acknowledgements
We are thankful for the financial support from the Max Planck
Society. AK acknowledges Ramsay Memorial Fellowship, the
Royal Society - International Exchanges award (IES\R1\201028)
and also research funding from university of Manchester. CSS
acknowledges financial support from Spanish MINECO
(MAT2017-85089-C2-1-R) and from MCIU for the “Ramón y
Cajal” contract (RYC2018-024364-I). RF acknowledges funding
from Swiss National Science Foundation (200020_182015).
Keywords: graphene nanoribbon • chiral edge • on-surface
synthesis • polycyclic aromatic hydrocarbon • scanning tunneling
microscopy and spectroscopy
Figure 3. Electronic characterization of the ch-GNR 9 by STS. a) Single-point
differential conductance (dI/dV) spectra measured at the GNR edge (blue and
red) and on the Au(111) substrate. Dashed black lines indicate the onsets of
the VB and CB of the GNRs. Inset: STM image showing the GNR where the
spectra have been acquired (blue and red crosses). STM parameters: (7.7 nm
× 7.7 nm) 0.8 nA, -1.5 V. b, c) Differential-conductance maps of occupied and
unoccupied states taken at a sample bias of −0.5ꢀV and 1.1ꢀV, respectively.
dI/dV map parameters: (3.2 nm × 4.0 nm) 0.2 nA.
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10.1002/asia.202001008
Chemistry - An Asian Journal
COMMUNICATION
Entry for the Table of Contents
Tailoring the edge structure of graphene nanoribbons: The design and synthesis of chiral graphene nanoribbons with mixture of zig-
zag, armchair and cove type edges. Our work expands the structural variety of GNRs, providing an insight into the non-planarity
induced by the cove edge geometry and formation of chiral GNRs.
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