Fjord-Edge Graphene Nanoribbons with Site-Specific Nitrogen Substitution

Article abstract of DOI:10.1021/jacs.0c07657

The synthesis of graphene nanoribbons (GNRs) that contain site-specifically substituted backbone heteroatoms is one of the essential goals that must be achieved in order to control the electronic properties of these next generation organic materials. We h

Full text of DOI:10.1021/jacs.0c07657

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Article
Fjord-edge Graphene Nanoribbons with Site-Specific Nitrogen Substitution
Yolanda L. Li, Chih-Te Zee, Janice B. Lin, Victoria M. Basile, Mit Muni, Maria D. Flores, Julen
Munarriz, Richard B. Kaner, Anastassia N. Alexandrova, K. N. Houk, Sarah H. Tolbert, and Yves Rubin
J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.0c07657 • Publication Date (Web): 07 Sep 2020
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!,.0&5'&)') 0#/*'-')!#-.0+$$.-1)4+2*)"+2'5"/'%+(+%)!+20.)'-))
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Yolanda L. Li, Chih-Te Zee, Janice B. Lin, Victoria M. Basile, Mit Muni, Maria D. Flores, Julen Munárriz, Rich-
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ard B. Kaner, Anastassia N. Alexandrova, K. N. Houk, Sarah H. Tolbert, and Yves Rubin*
Department of Chemistry and Biochemistry, University of California, Los Angeles, 607 Charles E. Young Dr.
East, Los Angeles, California 90095–1567, United States
ABSTRACT: The synthesis of graphene nanoribbons (GNRs) that contain site-specifically substituted backbone heteroatoms is one of the
essential goals that must be achieved in order to control the electronic properties of these next generation organic materials. We have exploit-
ed our recently reported solid-state topochemical polymerization/cyclization-aromatization strategy to convert the simple 1,4-bis(3-
pyridyl)butadiynes 3a,b into the fjord-edge nitrogen-doped graphene nanoribbon structures 1a,b (fjord-edge N2[8]GNRs). Structural as-
13
signments are confirmed by CP/MAS C NMR, Raman, and XPS spectroscopy. The fjord-edge N2[8]GNRs 1a,b are promising precursors
for the novel backbone nitrogen-substituted N2[8]AGNRs 2a,b. Geometry and band calculations on N2[8]AGNR 2c indicate that this class of
nanoribbons should have unusual bonding topologies and metallicities.
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Graphene nanoribbons (GNRs) are expected to usher in the
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ultimate nanosizing of electronics1 2 3 4 and sensors5 6 for next gen-
eration devices.The electronic properties of GNRs can be exquis-
itely tuned by modification of their width, backbone, and edge
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structure.1 7 8 9 10 In the last decade, both on-surface and in-solution
bottom-up syntheses have achieved precise structural control
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over these benchmarks.11 12 13 14,15 Early bottom-up syntheses have
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focused on GNRs with armchair16 17 18 19 20 or zigzag21 edges. More
recently, intricate edge or interior configurations, such as chev-
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ron,11 22 23 24 cove,25 26 27 fjord28 or holey,29 30 31 have been obtained.
These novel topologies significantly alter the electronic or mag-
netic properties of GNRs, as do atomically precise32 substitutions
($"@FA;6F= 4DC36;F>7B &
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of carbons with heteroatoms such as boron,33 34 sulfur,35 36 or ni-
trogen.30 37 38 Crucially, site-specific doping at the GNR backbone
produces a dramatic alteration of its electronics, making such
structures the most desirable targets for synthesis.32 39 40 41 Nitrogen
doped GNRs are of particular interest as they produce p-doped
materials.24 37 38 42 43 44 45
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Herein, we describe the synthesis of the first eight-atom wide,
fjord-edge nitrogen-doped graphene nanoribbons 1a,b (fjord-
edge N2[8]GNR; Figure 1). Fjord-edge N2[8]GNRs 1a,b were
obtained in a facile two-step conversion starting from dipyridyl
diynes 3a,b. Photochemically-induced topochemical polymeriza-
tion in the crystalline state afforded polydiacetylenes (PDAs)
4a,b, which were thermally converted to GNRs 1a,b with no loss
of the sidechains. The Hopf cyclization and ensuing aromatiza-
tion from PDAs 4a,b to GNRs 1a,b were monitored by cross-
polarization magic angle spinning (CP/MAS) solid state 13C
NMR. X-ray photoelectron spectroscopy (XPS) revealed both
the pyridinic and amide bonding states of the nitrogen atoms.
Raman spectroscopy further confirmed the structural integrity of
the fjord-edge N2[8]GNRs 1a,b.
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Figure 1. Synthesis of fjord-edge nitrogen-doped graphene nanorib-
bons (fjord-edge N2[8]GNRs) 1a,b).
"
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ly) as fibrous powders after filtration. The low polymerization
!!'"(*$)(!-%!!!#( *((#&%!!
yield for 3b appears to be inherent to this derivative, since repeat-
ed attempts to increase yields by using nanocrystalline material
could not raise the conversion yield for this substrate.
Diyne monomer synthesis: The topochemical polymerization
of diynes requires suitable packing of the monomers in the crystal
to trigger subsequent chain reactions.46,47,48 Here, the dipyridyl
diyne units of 3a,b,d (Scheme 1) needed to have their diyne 1,4-
carbons within van-der-Waals contact distance (~3.5 Å) to pro-
mote the facile formation of intermolecular bonds, a process that
,
often occurs under ambient light.49 50 Although we synthesized
9
several isomeric dipyridyl diyne systems,51 only one series, based
on 3-amino-5-alkynylpyridine, gave the polymerizable diynes
3a,b. Accordingly, 3-amino-5-bromopyridine was coupled with
trimethylsilylacetylene under Sonogashira conditions, followed
by acylation of amine 5 with the corresponding acid chlorides
(Scheme 1, R = i-Pr, n-Hex, Me). Removal of the trimethylsilyl
protecting group gave alkynyl amides 6a,b,d in good to excellent
yields. Oxidative coupling under the Hay conditions afforded
diyne amides 3a,b,d in good to high yields.
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=7<
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Figure 2. a) Crystal packing structure for diyne 3a displaying the
short C1-C4’ distance directed by the C=O···H–N hydrogen-
bonded network. b) View of 3a down the H-bonding axis.
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Conversion of PDAs 4a,b to GNRs 1a,b: Thermal conversion
experiments were carried out in separate runs on PDAs 4a,b un-
der increasingly higher temperatures in argon.51 This transfor-
Scheme 1. Synthesis of the 1,4-bis(3-pyridyl)butadiynes 3a,b,d.
13
mation could be conveniently monitored by CP/MAS C solid
X-ray structure: Crude diyne 3a afforded single crystals after
slow evaporation from methanol (Figures S28a,b).51 X-ray diffrac-
tion at the Brookhaven Synchroton X-ray source (Figure S29)
afforded a 1.0 Å resolution crystal structure of diyne 3a (Figure 2,
Table S1).51
state NMR, focusing on the four distinct carbon signal ranges
corresponding to the four functional groups of interest (Figure
3b,c): amide carbonyls (160–170 ppm), aromatic carbons (110–
150 ppm), alkynyl carbons (~100 ppm), and amide sidechains
(10–40 ppm). As the PDAs 4a,b were heated under increasingly
higher temperatures (1h each per run), the distinct 13C NMR
signals tracked an initial Hopf cyclization, as evidenced by the
disappearance of the alkyne peak at temperatures between 300
and 350 °C (Figure 3b,c). Thus, the Hopf cyclization reactions of
PDAs 4a,b occur more readily, ~100 ˚C lower than for our phe-
nyl analogs.50 The Hopf cyclization step is followed by further
aromatization reactions that form fjord-edge N2[8]GNRs 1a and
1b at temperatures between 350 and 400 °C (Figure 3b,c), as
revealed by the changes in the overall envelope for the aromatic
signals between 110–150 ppm, which adopt an underlying inten-
sity ratio of 1:2:1 for both 1a and 1b (Figure 3b,c, Table 1a).
Curve fitting of the experimental spectrum of 1b in the 110–150
ppm range with seven Gaussian curves of equal intensity and
The crystal packing geometry for molecules of diyne 3a vali-
dates the desired short, non-bonded C1–C4’ distance of 3.45 Å
(Figure 2a). The hydrogen bonds between the carbonyl oxygens
and amide hydrogens have an optimal distance of 2.00 Å, guiding
the assembly of diyne units in 3a along the unit cell vector a. The
relative strength of these intermolecular interactions is reflected
in the crystal morphology and powder diffraction (Figures S28
and S30). To accommodate the H-bonding motif, the polymer
growth axis exhibits a horizontal offset between each molecule,
organizing the diynes into an optimal arrangement for topochem-
ical polymerization (Figure 2b). While we were not able to obtain
the single crystal structure of 3b, its powder diffraction displayed
a similar packing arrangement to 3a (Figure S31).
13
width, representing the expected number of aromatic C signals
Topochemical polymerization of diynes 3a,b: Both dipyridyl
diynes 3a,b quickly polymerized to dipyridyl PDAs 4a,b when
subjected to UV light, as well as under ambient light, while diyne
3c was unreactive. The polymerizations were carried out by irra-
diation of finely pulverized dispersions of the crystals in hexanes
using a medium pressure Hanovia lamp (Pyrex filter), typically
for 12 h, producing deep purple/black material. Dissolution of
unreacted monomer from the polymerized crystals gave the near-
ly insoluble pristine polydiacetylenes 4a,b (18 and 4%, respective-
for fjord-edge N2[8]GNR 1b, affords the fitted peaks in Table 1.
These values compare rather well with the calculated values for
model compound 1e (Table 1b, top). Furthermore, the clusters
of peaks for the aromatic carbons for each of the alternate possible
model structures, i.e. 7c and 8c (Table 1b, middle and bottom),
which are the structural alternatives in the conversion of polydi-
acetylenes 4a,b to fjord-edge N2[8]GNRs 1a,b (see discussion
below and Figure 5), do not fit the experimental curve as well. In
particular, structure 7c has its A,B peaks clustered around 144
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edge structure of 1a,b imparts strong steric repulsion between the
ures 3 and 4). In fact, cyclization appears to favor the 6-position,
2
3
4
5
6
7
8
9
pyridine nitrogen lone pairs and the C– H bonds of adjacent di-
azachrysene units. Thus, the two possible key conformations, all-
zigzag (alternating up-down pyridyl units) and helical (all pyridyl
units staggered in a non-alternating fashion) were calculated at
the semiempirical (PM3) and DFT (B3LYP) levels (see Section
S4.8).51 The differences in energy (35.9 and 60.2 kcal·mol–1 for
PM3 and B3LYP, respectively) between the two conformers is
very high, thus it is likely that only the zigzag conformation exists
in the fjord-edge N2[8]GNRs 1a,b as shown in Figure 1.
which produces the lesser strained, internally doped fjord-edge
N2[8]GNRs 1a,b with their amide sidechains pointing away from
the series of fused “diazachrysene” units. The transition state
calculations reported below lend further strong support for the
fact that the Hopf cyclization occurs at the 6-pyridyl positions to
yield fjord-edge N2[8]GNRs 1a,b.
To further understand the energetics and pathway of the Hopf
cyclization of PDAs 4a,b, we base our theoretical considerations
on previous results by Prall et al.67 and our own work.50 The Hopf
cyclization mechanism has been calculated to proceed through an
initial 6π-electrocyclization, followed by two consecutive [1,2]-H
shifts, with the first H-shift as the rate-determining step.67 As dis-
cussed above (Figure 5), the enediyne units of PDAs 4a,b can
undergo cyclization at either the 6-position (para to the amide
group) or the 4-position (ortho to the amide group), the latter of
which is intuitively unfavorable due to the large steric bulk of the
amide group, compared to only H in 1a,b (Figure 5).
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There are two possible Hopf cyclization pathways for PDAs
4a,b (Figure 5), which can afford fjord-edge GNRs with two dif-
ferent topologies. The internal-like topology (1a,b) has the nitro-
gen atoms opposite to C–H bonds of the next “diazachrysene”
units, while the edge-like topology has them at the edges of the
fjord-edge nanoribbon 7a,b, or the ensuing N2[8]AGNR 8, since
it can be expected that the amide sidechains should be easily lost
from structure 7a,b under our heating conditions. Unlike our
previous work on [8]AGNR,50 which forms the same structure
regardless of the initial cyclization pathway at the 2 or 4-positions
of the PDAs’ m-amidophenyl rings, cyclization at either the simi-
larly related 4 or 6-positions of the pyridyl rings in PDAs 4a,b
could give two different fjord-edge GNRs, or a statistical mixture
alternating both pathways along the nanoribbon length. However,
cyclization at the 4-position should be strongly disfavored owing
to the severe steric clash between the amide groups and adjacent
pyridyl units during Hopf cyclization (structures 7a,b, Figure 5).
Furthermore, aromatization to an edge-doped armchair GNR
should easily ensue if the edge-like pathway is followed, resulting
in total loss of the sidechains. This is not the case, based on our
Using density functional theory (DFT), we computed the ge-
ometries of the model system 9 (Figure 6), the transition states
for the initial 6π electrocyclizations (10 and 10’), the strained
allene intermediates 11 and 11’, and the transition states for the
1,2-shifts (12 and 12’). These structures were optimized in the
gas-phase using B3LYP/6-31G(d), and single-point energy calcu-
lations were performed using M06-2X/6-311+G(d,p) with
B3LYP frequencies to obtain free energy values. The potential
energy surfaces for the two cyclization pathways are shown in
Figure 6.
The energetic trends for this bispyridyl system are similar to
the all-carbon system previously studied by us.50 As expected, the
barriers for cyclization at the 4-position are higher than at the 6-
13
experimental data (CP/MAS solid-state C NMR and XPS, Fig-
!
Figure 6. Free energies (in kcal·mol-1) of the intermediates and transition states for both the favored (left) and disfavored (right) Hopf cyclization
pathways relative to starting structure 9.
!
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!!)/.%+,!&*#+,)).&+*!
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2
This work was supported by grants from the National Science Foun-
dation to Y.R. (NSF-CHE-1808836), to K.N.H. (NSF-DMR-
1335645 and NSF-CHE-1254897), to A.N.A. (NSF-CHE-1351968),
to S.H.T. and Y.R. (NSF-CHE-1608957) and NSF-MRI-1532232.
This work used computational and storage services associated with
the Hoffman2 cluster provided by the UCLA Institute for Digital
Research and Education’s Research Technology Group, the NSF-
supported Extreme Science and Engineering Discovery Environment
(XSEDE) (NSF-OCI–1054575 and TG-CHE160054), and high-
performance research computing resources by Texas A&M Universi-
ty (http://hprc.tamu.edu). M.D.F. is funded by a Ruth L. Kirsch-
stein National Research Service Award GM007185 and a National
Science Foundation Graduate Research Fellowship. C.-T.Z. is funded
by a George Gregory Fellowship. R.B.K. thanks the Dr. Myung Ki
Hong Chair in Materials Innovation. Data collection at the Argonne
National Lab was funded by DOE grant no. DE-FC02-02ER63421.
We thank Drs. Duilio Cascio and Michael R. Sawaya for organizing
the single crystal data collection at the Advanced Photon Source of
Argonne National Laboratory, and Prof. Jose A. Rodriguez for advice
and assistance with the crystal data collection and structure analysis.
3
4
(**#+)('"&'$ A-,%(*
Yves Rubin – Department of Chemistry and Biochemistry, Uni-
versity of California, Los Angeles, 607 Charles E. Young Dr.
East, Los Angeles, California 90095–1567, United States;
orcid.org/!0000-0003-0187-9689.
5
6
7
8
Email: rubin@chem.ucla.edu
9
Yolanda L. Li – 0000-0002-2180-3086
Chih-Te Zee – 0000-0002-6630-706x
Janice B Lin – 0000-0002-9333-5760
Maria D. Flores – 0000-0002-4483-078x
K. N. Houk – 0000-0002-8387-5261
Richard B. Kaner – 0000-0003-0345-4924
Sarah H. Tolbert – 0000-0001-9969-1582
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!(,#+
The authors declare no competing financial interest.
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