Bottom-up Assembly of Nanoporous Graphene with Emergent Electronic States

Article abstract of DOI:10.1021/jacs.0c05235

The incorporation of nanoscale pores into a sheet of graphene allows it to switch from an impermeable semimetal to a semiconducting nanosieve. Nanoporous graphenes are desirable for applications ranging from high-performance semiconductor device channels to atomically thin molecular sieve membranes, and their performance is highly dependent on the periodicity and reproducibility of pores at the atomic level. Achieving precise nanopore topologies in graphene using top-down lithographic approaches has proven to be challenging due to poor structural control at the atomic level. Alternatively, atomically precise nanometer-sized pores can be fabricated via lateral fusion of bottom-up synthesized graphene nanoribbons. This technique, however, typically requires an additional high temperature cross-coupling step following the nanoribbon formation that inherently yields poor lateral conjugation, resulting in 2D materials that are weakly connected both mechanically and electronically. Here, we demonstrate a novel bottom-up approach for forming fully conjugated nanoporous graphene through a single, mild annealing step following the initial polymer formation. We find emergent interface-localized electronic states within the bulk band gap of the graphene nanoribbon that hybridize to yield a dispersive two-dimensional low-energy band of states. We show that this low-energy band can be rationalized in terms of edge states of the constituent single-strand nanoribbons. The localization of these 2D states around pores makes this material particularly attractive for applications requiring electronically sensitive molecular sieves.

Full text of DOI:10.1021/jacs.0c05235

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Article
Bottom-up Assembly of Nanoporous
Graphene with Emergent Electronic States
Peter H. Jacobse, Ryan D. McCurdy, Jingwei Jiang, Daniel J. Rizzo, Gregory Veber,
Paul Butler, Rafal Zuzak, Steven G. Louie, Felix R. Fischer, and Michael F. Crommie
J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.0c05235 • Publication Date (Web): 09 Jul 2020
Downloaded from pubs.acs.org on July 9, 2020
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Bottom-up Assembly of Nanoporous Graphene with Emergent
Electronic States
Peter H. Jacobse,†,‡ Ryan D. McCurdy,‡,‡ Jingwei Jiang,†,‡ Daniel J. Rizzo,† Gregory Veber,‡ Paul
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Butler,† Rafał Zuzak,†,| Steven G. Louie,*,†,§ Felix R. Fischer,*,‡,§,¶ Michael F. Crommie*,†,§,¶
†Department of Physics, University of California, Berkeley, CA 94720, U.S.A.
‡Department of Chemistry, University of California, Berkeley, CA 94720, U.S.A.
§Materials Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, CA 94720, U.S.A.
¶Kavli Energy NanoSciences Institute at the University of California Berkeley and the Lawrence Berkeley National
Laboratory, Berkeley, California 94720, U.S.A.
|Center for Nanometer-Scale Science and Advanced Materials, NANOSAM, Faculty of Physics, Astronomy and Applied
Computer Science, Jagiellonian University, PL 30-348 Kraków, Poland
ABSTRACT: The incorporation of nanoscale pores into a sheet of graphene allows it to switch from an impermeable
semimetal to a semiconducting nano-sieve. Nanoporous graphenes are desirable for applications ranging from high-
performance semiconductor device channels to atomically-thin molecular sieve membranes, and their performance is highly
dependent on the periodicity and reproducibility of pores at the atomic level. Achieving precise nanopore topologies in
graphene using top-down lithographic approaches has proven to be challenging due to poor structural control at the atomic
level. Alternatively, atomically-precise nanometer-sized pores can be fabricated via lateral fusion of bottom-up synthesized
graphene nanoribbons. This technique, however, typically requires an additional high temperature cross-coupling step
following the nanoribbon formation that inherently yields poor lateral conjugation, resulting in 2D materials that are weakly
connected both mechanically and electronically. Here we demonstrate a novel bottom-up approach for forming fully
conjugated nanoporous graphene through a single, mild annealing step following the initial polymer formation. We find
emergent interface-localized electronic states within the bulk band gap of the graphene nanoribbon that hybridize to yield a
dispersive two-dimensional low-energy band of states. We show that this low-energy band can be rationalized in terms of
edge states of the constituent single-strand nanoribbons. The localization of these 2D states around pores makes this material
particularly attractive for applications requiring electronically sensitive molecular sieves.
coupling upon lateral extension results in, at most, a modest
decrease in the overall band gap compared to that of the
constituent one-dimensional (1D) ribbons. In addition, the
linkages between ribbons up to now are unable to host low-
energy frontier states around the pores as required for
possible electronic sieving applications.
INTRODUCTION
Nanoporous graphene (NPG) is unique in that it exhibits
both electronic functionality as a tunable semiconductor
and mechanical functionality as a tunable molecular filter
membrane. Combining these properties into a single
atomically-thin, mechanically robust platform makes NPG
an excellent candidate for electronically active nanosieve
applications such as sequencing, sensing, ion transport, gas
separation, and water purification.1–10 The utility of this
material, however, hinges on the ability to induce periodic,
atomically-precise nanopores and to tailor their precise
dimensions and electronic properties. Top-down methods
have so far been limited in this regard because they typically
produce random, imprecise pores within a material that
remains semimetallic.1,3,5 By contrast, bottom-up
approaches offer atomic precision in NPG synthesis through
rational design of molecular precursors.11,12 This has been
demonstrated recently through cross-dehydrogenative
coupling of bottom-up synthesized graphene nanoribbons
(GNRs) featuring periodic bay regions along the edges.11–14
A disadvantage of the few reported bottom-up synthesized
NPGs fabricated in this way is that the constituent
nanoribbons are wide gap semiconductors. Electronic
Here we present a surface-mediated approach to creating
NPG by utilizing a novel crosslinking handle that yields a
fully conjugated linkage between constituent nanoribbons
and results in a new low-energy band of extended states but
with much of their wavefunctions localized along the
periphery of the pores. We have designed two molecular
precursors (1 and 2 in Scheme 1) functionalized with
thermally labile methyl and methylene groups that serve as
crosslinking handles. The respective C–H bond dissociation
energies (BDE) are similar to that of the Caryl–H bond
cleaved
during
the
thermally
induced
cyclodehydrogenation of the GNR backbone.15 The
geometry of these chevron-type precursors defines the pore
shape and size whereas the reactivity of the crosslinking
handles allows for the formation of benzene-fused linkages
and facilitates two-dimensional extension of the structure.
Our approach is high-yielding and selectively produces
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rubicene-type interfaces (highlighted in red in Scheme 1) by
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comparison to the hot substrate deposition experiments.
High quality samples of C-NPG are achieved by depositing
0.75 monolayer of 2 onto Au(111) held at T1 = 180 C, as
shown in Figure 1C. Areas larger than 20 nm  20 nm (white
square in Figure 1C) and as wide as 40 nm in the lateral
crosslink direction are observed. We attribute the high yield
of the crosslinking reaction at T2 = 400 C to the similar
activation energies required for homolytic C–H bond
dissociation of the fluorenyl methylene groups and the
barrier associated with cyclodehydrogenation of the GNR
backbone.15 We observe no significant difference between
the two molecular precursors with regard to the ultimate
product, although 2 shows a higher tendency to pre-
organize in the polymer phase, possibly due to π-stacking
interactions between the fluorenyl groups of neighboring
units as shown in Figure S1. Although only Au(111) was
used in the current work, in light of the successful synthesis
of chevron-type GNRs on Cu(111), it is not inconceivable
that other coinage metal surfaces could be used in the future
for the synthesis of C-NPG.16
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laterally fusing benzene rings through 5-membered rings
along either side of an extended acene. The resulting NPG
features interface-localized frontier electronic states that
emerge upon formation of these interfaces. The emergence
of these states can be understood as linear combinations of
molecular orbitals localized on the 5-membered rings lining
the edges of fluorene-chevron GNRs that couple into a two-
dimensional (2D) superlattice geometry. We have
experimentally characterized these electronic features
using STS for both the final fused 2D nanopore states as well
as isolated 5-membered ring-localized states in 1D
fluorene-chevron GNRs. The novel crosslinking handle
utilized here represents progress toward creating
electronically useful NPG, thus opening the door to
exploring its potential in nanosieving and semiconductor
device applications.
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Scheme 1. Schematic representation of the bottom-up
synthesis of chevron-type nanoporous graphene (C-
NPG) from molecular precursors 1 and 2 (anti
configuration).
RESULTS AND DISCUSSION
The structures of molecular precursors
1 and 2
functionalized, respectively, with methyl and methylene
crosslinking handles are depicted in Scheme 1. Deposition
of precursors 1 or 2 onto a Au(111) surface held at T1 = 180
C and subsequent annealing to T2 = 400 C yields two-
dimensionally extended chevron-type nanoporous
graphene (C-NPG). Figure 1A shows that for an initial 0.25
monolayer coverage of 1 a vast majority of the starting
material forms crosslinked C-NPG with some areas reaching
7 ribbons in width. Depositing 0.75 monolayer coverage of
2 onto a Au(111) surface held at 22 C followed by
annealing to T2 = 400 C results in a denser coverage of
smaller C-NPG patches as shown in Figure 1B. Here close to
100% of structures on the surface are crosslinked, with
several interconnected patches of roughly 10 nm  10 nm in
size. Long-range order however is diminished in
Figure 1. Synthesis of C-NPG. (A) STM topographic image of
0.25 monolayer coverage of C-NPG on Au(111) after deposition
of 2 onto Au(111) held at T1 = 180 C and subsequent annealing
to T2 = 400 C (V = 1.2 V, I = 50 pA). (B) STM topographic image
of 0.75 monolayer coverage of C-NPG on Au(111) after
deposition of 1 onto Au(111) held at 24 C and subsequent
annealing to T2 = 400 C (V = –1.2 V, I = 50 pA). (C) STM
topographic image of 0.75 monolayer coverage of C-NPG on
Au(111) after deposition of 2 onto Au(111) held at T1 = 180 C
and subsequent annealing to T2 = 400 C (V = 1.2 V, I = 50 pA).
(D) Bond-resolved STM image of a region containing three
fused ribbons (V = –50 mV, I = 200 pA, Vosc = 15 mV, f = 620 Hz).
(E) High-resolution bond-resolved STM image of the region
indicated in (D) (V = 20 mV, I = 250 pA, Vosc = 10 mV, f = 620
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Hz). (F) Schematic representation of the rubicene-type
interface imaged in (D,E).
fusion product leading to C-NPG. The interface originates
from a cross-dehydrogenative coupling that leads to the
formation of two new C–C bonds that define the central
benzene ring of the rubicene core (highlighted in red in
Figure 1F). Another motif that emerges from the lateral
fusion is a nonacene core where the newly created benzene
ring represents the central ring (highlighted in blue in
Figure 1F).20 The fusion process ensures that carbon atoms
at the apices of the five-membered rings adopt a trigonal
planar conformation, thus contributing a singly occupied p-
orbital to the aromatic framework.
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To resolve the atomic structure of this C-NPG, we
performed bond-resolved scanning tunneling microscopy
(BRSTM) using a carbon monoxide-passivated tip.17–19
Figure 1D shows the BRSTM image of three interconnected
ribbons over a 4 nm  4 nm area. The ribbons are fused by
rubicene-type linkages giving rise to a nanoporous
structure containing parallelogram-shaped pores that are
~0.5 nm wide in the short direction and ~1 nm wide in the
long direction. The orientation of the pores shown in Figure
1D alternates, resulting in a herringbone pattern with a unit
cell that has screw symmetry in the ribbon axis. In the
following discussion this lateral fusion pattern will be
referred to as anti. An alternative coupling yielding aligned
pores with inversion symmetry points in the ribbon axis
will be referred to as syn (Figure S2). A higher-resolution
BRSTM image around one of the interfaces in the anti
configuration (Figure 1E) provides unambiguous
assignment of the covalent bond structure of the lateral
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Having established the chemical structure of the C-NPG,
we shift our focus to its electronic structure. Since the C-
NPG is laterally fused through an extended acene, the
electronic coupling between neighboring nanoribbons can
be expected to be large. In addition, rubicene is known to be
an electron acceptor and so it is reasonable to expect C-NPG
to exhibit accessible frontier states.21 We have used
scanning tunneling spectroscopy (STS) to verify these
hypotheses, as shown below.
Figure 2. Electronic structure of the C-NPG. (A) STS dI/dV spectrum recorded on the rubicene-interface of C-NPG. The inset shows
a constant current STM topograph (V = 0.7 V, I = 50 pA) with the point spectroscopy location marked by a red cross (same region of
C-NPG as shown in Figure 1D). (B) Differential conductance maps of valence band (VB) (–0.90 V), in-gap band (IGB) (0.72 V), and
conduction band (CB) (1.60 V). (C) DFT simulated LDOS maps of the structure shown in the inset of (A). The maps are calculated by
integration over an energy window of 10 meV from the band edge. (D) DFT simulated density of states.
Figure 2A shows the energy-dependent local density of
states (LDOS) recorded by measuring the differential
conductance (dI/dV) of the STM tunnel junction while
holding the tip above the interface region of the C-NPG
shown in the inset (position indicated by red cross). Valence
band (VB) and conduction band (CB) onsets are observed at
–0.9 V and 1.6 V, respectively, reflecting a bulk GNR band
gap of 2.7 eV (similar to previously measured values for
chevron GNRs on Au(111)).17,22–24 A pronounced resonance,
however, is observed at 0.72 V that has a distinct shoulder
at higher bias. Because this resonance lies inside the bulk
chevron GNR energy gap, we herein refer to this new
feature as the in-gap band (IGB). Figure 2B shows energy-
resolved dI/dV maps corresponding to the LDOS of the CB
onset, the 0.72 V resonance feature, and the VB onset (top
to bottom). We observe that the electronic wavefunctions
corresponding to the band onsets are delocalized over the
entire structure (i.e., all interfaces and edges light up)
whereas the 0.72 V feature is localized exclusively at the
pore interfaces.
The resonances observed in the dI/dV point spectra and
the patterns observed in dI/dV mapping can be reasonably
reproduced by ab initio electronic structure calculations.
Figure 2D shows the C-NPG density of states (DOS)
calculated using density functional theory (DFT) while
Figure 2C shows the calculated LDOS maps corresponding
to the CB onset, the IGB, and the VB onset for three fused
ribbons having the same geometry as seen experimentally.
The theoretical bulk CB and VB band edge states extend
throughout the C-NPG structure whereas the calculated IGB
localizes at the pore interfaces with small wavefunction
overlap with neighboring pores, in good agreement with
experiment. The peak shape of the IGB, including the
distinct high-energy shoulder, is also accurately reproduced
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by the calculation. These results demonstrate that C-NPG is
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onto the fluorenyl groups highlighted in red in Figure 3A).
The 1D GNR band structure shows bulk VB and CB band
edges as well as an IGB. The projected LDOS (Figure 3C)
indicates that the IGB arises from atoms lining the convex
edge of fluorenyl-chevron GNRs. A significant difference
between this isolated GNR and C-NPG is that the isolated
GNR is metallic due to the in-gap band straddling the Fermi
level.33
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a new 2D electronic material with low-energy electronic
states deep within the bulk band gap of the constituent
GNRs that are primarily localized on the rubicene interfaces
adjacent to pores. Furthermore, even though a defect is
clearly present in the bottom-left corner, the electronic
structure remains unaffected, with the IGB uniformly
distributed over the interfaces, in good accordance with
theory.
The semiconducting band structure of 2D C-NPG (shown
in Figure 3E), by contrast, shows a higher energy IGB that
lies ~0.5 eV above the Fermi level. Close inspection shows
another difference between the 2D C-NPG DOS (Figure 3F)
and the isolated 1D GNR DOS (Figure 3C): the localized
interface states for C-NPG appear at two energies, not just
one. One band of states is the familiar in-gap band (IGB)
above the Fermi energy, but the other occurs much lower in
energy and appears below the VB edge. Formation of C-NPG
is thus seen to cause the edge states of isolated GNRs
(Figure 3B, C) to split in order to yield two sets of C-NPG
interface bands. Here, fusion of each fluorenyl pair shared
between adjacent GNRs into a single rubicene moiety
causes the fluorenyl orbitals to hybridize and split
energetically into bonding/anti-bonding pairs. The anti-
bonding states combine to form the C-NPG IGB at higher
energy while the bonding states form the lower energy C-
NPG band. This picture is supported by analysis of the wave
functions, where the bonding (antibonding) 2D interface
states can be recognized as an antisymmetric (symmetric)
combination of 1D edge states (see Figure S3). Edge
hybridization thus causes what would otherwise be isolated
1D metallic GNRs to evolve (at least conceptually) into a
semiconducting 2D nanopore mesh.
Since the interfaces define a superlattice, it is expected
that the electronic states localized on them should give rise
to 2D dispersing features in the C-NPG band structure.25–32
It is useful to analyze this interface behavior in more depth
in order to gain insight into how to tune the electronic
structure of NPG. This is not only relevant to the specific C-
NPG synthesized here, but in general to other possible C-
NPG varieties that may be designed through functional
analogs of monomers 1 and 2.
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We start by considering a hypothetical isolated fluorenyl-
chevron GNR exhibiting evenly spaced sp2-hybridized -
radicals at the apices of fluorenyl groups lining both edges
of the ribbon (Figure 3A). This structure was chosen as a C-
NPG building block because the fluorenyl groups along the
edges (highlighted in red) constitute one half of the
rubicene interface structure found in C-NPG. Furthermore,
this structure represents the GNR expected to arise from
precursors 1 and 2 (other possible structures, e.g.
fluorenylidene edges (Figure 4C), give similar results).
Figure 3B shows the DFT calculated band structure of this
GNR while Figure 3C shows the local (red) and total (black)
DOS (the LDOS here is obtained by projecting band states
Figure 3. Electronic structure of the 1D fluorenyl-chevron GNR and 2D C-NPG. (A) Structure of an isolated fluorenyl-chevron GNR
with the edge fluorenyl groups highlighted in red and the quasi 1D unit cell indicated by a rectangle. (B) Electronic dispersion of
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fluorenyl-chevron GNR. The black bands are predicted by DFT calculations while the red, dashed bands are those of an effective
tight-binding model (with parameters fit to DFT results) described in the text. (C) Local (red) and total (black) density of states (200
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k-points in the Brillouin zone) of fluorene-chevron GNR. (The local density of states is obtained by projecting states onto the -
orbitals of the fluorenyl groups.) (D) Structure of the C-NPG anti configuration with the interface (rubicene) groups highlighted in
red and the 2D unit cell indicated by a rectangle. (E) Electronic dispersion of C-NPG. The black bands are the DFT result while the
red, dashed bands are those of an effective tight-binding model (with parameter fit to DFT results) described in the text. (F) Local
(red) and total (black) density of states (20 by 46 k-points) of C-NPG. (The local density of states is obtained by projection onto the
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-orbitals in the pentagon region.) (G) Effective tight-binding model using basis states that represent the fluorenyl groups (red
circles) coupled via nearest neighbor electronic hopping parameters t1 (black) and t2 (blue), and next-nearest-neighbor hopping
parameters t3 and t4 (green and red, respectively).
These ab initio DFT electronic structure results can be
more intuitively understood using an effective tight-binding
model. Here we consider the edge orbitals localized on the
fluorenyl groups as effective basis states that link up to form
a 1D electronic network for the isolated GNR (Figure 3A)
and a 2D electronic network for the C-NPG (as shown by the
red circles in Figure 3G). In the 1D case, the electronic
dispersion can be found by assuming that an electron placed
on one edge orbital has an amplitude t2 to hop along the
ribbon to an adjacent orbital. In the 2D case, the edge orbital
network has an even stronger coupling t1 between edge
orbitals within a single rubicene moiety that is responsible
for splitting them into bonding and anti-bonding interface
states (weaker next-nearest-neighbor coupling is described
by hopping amplitudes t3 and t4). The 1D GNR thus has two
basis states per unit cell with one hopping parameter t2, and
so results in two bands that fit well to the GNR DFT band
structure for t2 = –65 meV, as shown by the two dashed lines
in Figure 3B. The 2D C-NPG, on the other hand, has four
basis states per unit cell which result in four bands as shown
by the dashed lines in Figure 3E. These bands fit well to the
interface bands of the 2D C-NPG DFT band structure after
optimization of the hopping amplitudes (t1 = 645 meV, t2 =
–23 meV, t3 = –22 meV, t4 = –20 meV). These interface states
form a distorted honeycomb lattice with distinct hopping
parameters compared to a normal graphene lattice. The C-
NPG bands are split into an upper pair that lie in the bulk
gap and a lower pair that are resonant with the valence
band complex. These are precisely the bands that arise from
hybridization-induced splitting of orbitals or modes
localized in the fluorenyl edge groups of the isolated GNR in
Figure 3A–C. The electronic behavior of the 2D C-NPG pore
states is thus encoded in the edge-states engineered for the
constituent 1D building block GNRs through rational design
of the molecular precursor.
segments) but is highly reproducible. The apex of the edge-
structure in Figure 4C extends further than the edge-
structure in Figure 4B and also appears to be pulled closer
to the surface (i.e., it has darker contrast in the BRSTM
image).
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A remaining question is whether the C-NPG GNR building
block, the fluorenyl-chevron GNR shown in Figure 3A, can
actually exist in a laboratory rather than simply being a
useful construct for understanding C-NPG electronic
structure. The answer here is mixed. We do observe
isolated, unlinked GNRs in lower-coverage samples, but
their structure is not identical to Figure 3A. A typical
example is shown in Figure 4A which shows a BRSTM image
of an isolated GNR derived from precursor 1 (a standard
STM topograph of this GNR can be seen in Figure S4B).
Close-up images of the edges of this GNR in Figures 4B and
4C show two different edge-structures (other types are also
possible, see Figure S4). The edge-structure shown in
Figure 4B features a hydrogen-saturated five-membered
ring within a fluorene group and is by far the dominant edge
structure (80% of edge segments). The edge-structure in
Figure 4C is seen much less frequently (5% percent of edge
Figure 4. Edge electronic structure of isolated fluorene-
chevron GNR. (A) Bond-resolved STM image of an isolated
fluorene-chevron GNR using CO-tip (V = –50 mV, I = 180 pA, Vosc
= 20 mV). (B) Bond-resolved STM image of the segment
indicated in (A) containing a methylene edge group (V = 15 mV,
I = 200 pA, Vosc = 10 mV). (C) Bond-resolved STM image of the
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neighboring segment indicated in (A) (V = 20 mV, I = 250 pA,
within this 2D pore network exhibit rubicene-type linkers
that arise from the lateral fusion of partially
dehydrogenated fluorene-chevron GNRs. The resulting C-
NPG exhibits a novel in-gap band localized on the rubicene
interfaces around the pores. These features are reproduced
by both DFT and coarse-grained tight-binding calculations
which show that the electronic structure of this new 2D
material may be understood in terms of the edge modes of
1D GNR building blocks. The fully-linked 2D behavior is also
consistent with the experimental properties of isolated
GNRs where sp2-hybridized -radicals exist due to spurious
dehydrogenation of the fluorenyl groups lining the edge.
The cross-linking handles can be incorporated in a wide
variety of molecular precursors, thus paving the way
towards nanoporous graphenes with variable pore density
and morphology. The control and incorporation of π-radical
states into 2D porous graphene topologies opens exciting
new possibilities for exploring tunable pore networks in the
future.
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Vosc = 10 mV). Possible chemical structures are depicted below.
(D) Scanning tunneling spectroscopy of an isolated fluorene-
chevron GNR (Vosc = 4 mV, f = 620 Hz). The red and blue spectra
are recorded at the positions indicated in the inset (Vosc = 20
mV, f = 620 Hz). The inset shows a constant height dI/dV scan
of the GNR obtained at V = –0.7 V for the same nanoribbon
shown in panel (A). Location of the radical state is marked by
an arrow (same edge element as shown in (C)).
STM spectroscopy helps us to better understand the
nature of these different GNR edge-structures. Figure 4D
shows a dI/dV spectrum (blue curve) obtained with the
STM tip held over the dominant edge-structure shown in
Figure 4B. Valence and conduction band peaks are clearly
visible and result in a bandgap of 2.4 eV, consistent with
previous spectroscopy of conventional chevron GNRs
lacking the five membered ring of the fluorenyl group.17,22,23
No signs of in-gap states are seen for this edge-structure.
Spectroscopy performed on edge-structures like that
shown in Figure 4C, on the other hand, are different as
shown by the red curve in Figure 4D. Here the VB and CB
features are seen as before, but a new in-gap feature also
arises, a broad peak centered at ~ –0.4 V that is not seen for
the more common edge-structure of Figure 4B. The spatial
extent of this new electronic feature can be seen in the
dI/dV image of Figure 4D (inset) obtained at –0.7 V. Here,
edge groups having an appearance like the structure in
Figure 4B are featureless while an edge group equivalent to
that in Figure 4C (arrow) has a prominent protrusion at the
apex that lights up at this energy.
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EXPERIMENTAL SECTION
Materials and General Methods. Unless otherwise stated, all
manipulations of air- and/or moisture-sensitive compounds were
carried out in oven-dried glassware under an atmosphere of N2. All
solvents and reagents were purchased from Alfa Aesar, Spectrum
Chemicals, Acros Organics, TCI America, and Sigma-Aldrich and
were used as received unless otherwise noted. Organic solvents
were dried by passing through a column of alumina and degassed
by vigorous bubbling of N2 or Ar through the solvent for 20 min.
Flash column chromatography was performed on SiliCycle silica
gel (particle size 40–63 μm). Thin layer chromatography was
performed using SiliCycle silica gel 60 Å F-254 precoated plates
This behavior is explained by the presence of methylene
(CH2) groups at the common apex structure shown in Figure
4B. Here hydrogen passivation removes the fluorenyl
radical state by re-hybridization of the apical carbon atom
into the tetrahedral methylene group in fluorene, thus
excluding it from interaction with the -system.15,34–36 This
explains why spectroscopy of the fluorene edge-structure
shows the same behavior as conventional chevron GNRs
which lack the apical carbon atom of fluorene-chevron
GNRs. The edge-structure of Figure 4C, on the other hand, is
consistent with an sp2-hybridized π-radical obtained from
spurious dehydrogenation of a fluorene group (e.g., a
fluorenyl radical or fluorenylidene as depicted in Figure
4C). This is consistent with on-surface dehydrogenation in
cyclopentadiene moieties which has been shown to
compete with methylene-directed fusion and to result in
highly reactive carbene or methyne groups.15,36
Stabilization of these intermediate radical species by the
surface explains the dark contrast in the BRSTM image, as
well as the broadness of the in-gap resonance (due to strong
coupling with the underlying Au(111) surface).37 The edge
motif of Figure 4C thus exhibits the type of radical behavior
that can be viewed as the basis of C-NPG electronic
structure.
1
(0.25 mm thick) and visualized by UV absorption. All H and 13C
{1H} NMR spectra were recorded on Bruker AV-400 and AV-600
MHz spectrometers, and are referenced to residual solvent peaks
(CDCl3 1H NMR = 7.26 ppm, 13C NMR = 77.16 ppm; C6D6 1H NMR δ
= 7.16 ppm, 13C NMR δ = 128.06 ppm; CD2Cl2 1H NMR = 5.32 ppm,
13C NMR = 53.48 ppm). ESI mass spectrometry was performed on
a
Finnigan LTQFT (Thermo) spectrometer. MALDI mass
spectrometry was performed on a Voyager-DE PRO (Applied
Biosystems Voyager System 6322) in positive mode using a matrix
of dithranol.
6,11-dibromo-2-(6-methyl-[1,1'-biphenyl]-2-yl)-1,4-
diphenyltriphenylene (1) A 5 mL Schlenk tube was charged under
N2
with
5,10-dibromo-1,3-diphenyl-2H-
cyclopenta[l]phenanthren-2-one (0.184 g, 0.34 mmol) and 2-
ethynyl-6-methyl-1,1'-biphenyl (0.065 g, 0.34 mmol) in degassed
o-xylene (2 mL). The reaction mixture was sealed under N2 and
stirred for 18 h at 145 C. The reaction mixture was cooled to 24 C
and concentrated on a rotary evaporator. Column chromatography
(SiO2; 3:1 hexane/CH2Cl2) yielded 1 (0.186 g, 0.26 mmol, 78%) as
a colorless crystalline solid. 1H NMR (600 MHz, CD2Cl2) δ = 8.19 (dd,
J = 14.5, 8.7 Hz, 2H), 7.69 (d, J = 2.0 Hz, 1H), 7.55–7.39 (m, 9H),
7.31–7.19 (m, 6H), 7.11–7.03 (m, 2H), 7.03–6.94 (m, 3H), 6.41 (s,
1H), 6.34 (s, 1H), 1.99 (s, 3H) ppm; 13C {1H} NMR (151 MHz, CDCl3)
δ = 143.7, 141.5, 141.0, 140.9, 140.4, 139.3, 137.8, 137.3, 136.4,
134.8, 133.4, 133.1, 132.8, 132.8, 131.9, 131.6, 131.1, 130.6, 130.3,
129.8, 129.7, 129.6, 129.6, 129.5, 129.5, 129.5, 129.5, 129.4, 129.0,
128.6, 128.3, 127.8, 127.6, 127.4, 127.2, 126.7, 126.5, 124.7, 124.6,
120.2, 120.0, 21.4 ppm; HRMS (EI-TOF) m/z: [C43H28Br2]+ calcd.
[C43H28Br2] 702.0558; found 702.0559.
CONCLUSION
Depositing methyl and methylene bearing precursors 1 or
2 onto a Au(111) substrate and annealing to 400 C yields
two-dimensionally extended C-NPG. This new 2D material
emerges due to the reactivity of methyl and methylene
crosslinking handles towards cross-dehydrogenative
coupling at low temperatures. Bond-resolved scanning
tunneling microscopy reveals that internal interfaces
6,11-dibromo-2-(9H-fluoren-4-yl)-1,4-diphenyltriphenylene (2) A
10 mL Schlenk flask was charged with 4-(6,11-dibromo-1,4-
diphenyltriphenylen-2-yl)-9H-fluoren-9-one (71.6 mg, 0.10
mmol), red phosphorous (21.1 mg, 0.68 mmol) and HI (0.28 mL, 57
wt.%) in propionic acid (2 mL). The suspension was heated to 145
°C under N2 for 18 h. The suspension was cooled to 24 °C and
quenched with saturated aqueous K2CO3 solution. The suspension
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was washed three times with CH2Cl2. The combined organic phases
Award 0939514 (DFT calculations), and the National Science
Foundation under grant DMR-1926004 (tight-binding
formulation and calculations). P.H.J. acknowledges fellowship
support from the Dutch Research Council through the Rubicon
Award (019.182EN.18). Computational resources were
provided by the DOE at Lawrence Berkeley National
Laboratory’s NERSC facility and the NSF through XSEDE
resources at NICS. R.Z. acknowledges fellowship support from
the National Science Center, Poland (2017/24/T/ST5/00262).
were washed with saturated aqueous NaCl solution, dried over
MgSO4, and concentrated on a rotary evaporator. Column
chromatography (SiO2; 1:1 CH2Cl2/hexanes) yielded 2 (59.2 mg,
0.084 mmol, 84%) as a colorless solid. 1H NMR (500 MHz,CD2Cl2) δ
= 8.28 (dd, J = 8.8, 3.7 Hz, 2H), 7.88 (d, J = 2.0 Hz, 1H), 7.71 (m, 2H),
7.58 (dd, J = 8.7, 2.0 Hz, 1H), 7.53–7.41 (m, 8H), 7.20–7.10 (m, 5H),
7.02–6.96 (m, 4H), 6.88 (d, J = 2.0 Hz, 1H), 3.91 (s, 2H) ppm; 13C
{1H} NMR (151 MHz, CDCl3) δ = 143.6, 143.4, 143.2, 141.6, 140.9,
140.3, 139.4, 139.0, 137.6, 136.0, 133.0, 132.7, 132.7, 132.4, 131.7,
131.4, 131.1, 130.7, 129.8, 129.6, 129.5, 129.5, 129.3, 129.2, 129.1,
128.4, 128.2, 127.6, 127.0, 126.5, 126.3, 125.7, 124.7, 124.6, 124.5,
123.4, 122.6, 120.1, 120.0, 36.8 ppm; HRMS (ESI-TOF) m/z:
[C43H25Br2]– calcd. [C43H25Br2] 699.0328; found 699.0325.
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ASSOCIATED CONTENT
Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website.
Figures S1–S4, methods and instrumentation,
synthetic procedures for 1 and 2, Schemes S1–S2, and
NMR spectra (Figures S5–S12).
AUTHOR INFORMATION
Corresponding Author
* sglouie@berkeley.edu; crommie@berkeley.edu;
ffischer@berkeley.edu
Author Contributions
All authors have given approval to the final version of the
manuscript
‡These authors contributed equally.
ACKNOWLEDGMENT
Research supported by the Office of Naval Research Program
N00014-19-1-2503 (design and synthesis of molecular
precursors), the Office of Naval Research MURI Program
N00014-16-1-2921 (STS measurements and analyses), the US
Department of Energy (DOE), Office of Science, Basic Energy
Sciences (BES) under the Nanomachine Program award
number DE-AC02-05CH11231 (surface growth, image
analysis), and the Center for Energy Efficient Electronics NSF
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Wu, M.; Rizzo, D. J.; Rodgers, G. F.; Cloke, R. R.; Durr, R. A.; Sakai, Y.;
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