Helically Coiled Graphene Nanoribbons

Article abstract of DOI:10.1002/anie.201611834

Graphene is a zero-gap, semiconducting 2D material that exhibits outstanding charge-transport properties. One way to open a band gap and make graphene useful as a semiconducting material is to confine the electron delocalization in one dimension through t

Full text of DOI:10.1002/anie.201611834

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Communications
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International Edition: DOI: 10.1002/anie.201611834
German Edition: DOI: 10.1002/ange.201611834
Graphene Nanoribbons Very Important Paper
Helically Coiled Graphene Nanoribbons
Maxime Daigle, Dandan Miao, Andrea Lucotti, Matteo Tommasini, and Jean-FranÅois Morin*
Abstract: Graphene is a zero-gap, semiconducting 2D material
that exhibits outstanding charge-transport properties. One way
to open a band gap and make graphene useful as a semi-
conducting material is to confine the electron delocalization in
one dimension through the preparation of graphene nano-
ribbons (GNR). Although several methods have been reported
so far, solution-phase, bottom-up synthesis is the most promis-
ing in terms of structural precision and large-scale production.
Herein, we report the synthesis of a well-defined, helically
of GNRs with control over edge structure and precise
heteroatom doping.[7] Nevertheless, the Scholl reaction pres-
ents drawbacks that limit its usefulness for the synthesis of
more complex GNRs architectures.[8] One of the most
detrimental features is poor regioselectivity that can lead to
structural defects that affect the properties of the GNRs.[9]
Recently, we reported the regioselective, solution-phase
synthesis of nanographenes and GNR fragments using the
photochemical cyclodehydrochlorination (CDHC) reaction
on chlorine-containing polyphenylene precursors.[10] Since
CDHC takes place in metal-free, milder reaction conditions
than the Scholl reaction, it proceeds selectively and without
the formation of side-products. Moreover, the CDHC reac-
tion is compatible with different heterocycles and provides
better control over the edge configuration of nanographenes
than the Scholl reaction.
coiled GNR from
a polychlorinated poly(m-phenylene)
through a regioselective photochemical cyclodehydrochlorina-
tion (CDHC) reaction. The structure of the helical GNR was
confirmed by 1H NMR, FT-IR, XPS, TEM, and Raman
spectroscopy. This Riemann surface-like GNR has a band
gap of 2.15 eV and is highly emissive in the visible region, both
in solution and the solid state.
Herein, we successfully employ the CDHC reaction for
the regioselective synthesis of the first polyhelicene-like GNR
(helical graphene nanoribbon (HGNR); Figure 1). With
G
raphene can arguably be considered as one of the most
interesting and versatile materials for practical applications as
it exhibits outstanding charge transport properties, very high
specific surface area, high thermal conductivity, and unrivaled
mechanical strength.[1] In the form of a single-layer two-
dimensional sheet however, graphene is a zero band gap
semiconductor, or semimetal, which limits its use as an active
component in traditional electronic devices.[2] To open the
band gap, different strategies of quantum confinement have
been developed.[3] In terms of atomic precision, the formation
of graphene nanoribbons (GNRs) that restrict electron
delocalization to one dimension is probably the most
promising.[4] In fact, the band gap of GNRs can be precisely
tuned by varying the width and edge configuration, providing
GNRs with a wide range of optoelectronic properties.[5]
A promising approach to well-defined GNRs was pio-
neered by the Mꢀllen group and consists of using standard
solution-phase multistep synthesis.[6] The key step of their
strategy is to submit a carefully designed polyphenylene
precursor to a cyclodehydrogenation reaction (Scholl reac-
tion) to obtain soluble and dispersible GNRs of different
widths and edge configurations. Unlike other approaches, this
bottom-up strategy allows for the synthesis of gram quantities
[*] M. Daigle, D. Miao, Prof. J.-F. Morin
Dꢀpartement de chimie and Centre de Recherche sur les Matꢀriaux
Avancꢀs (CERMA)
Universitꢀ Laval
1045 Ave de la Mꢀdecine, Quꢀbec, G1V 0A6 (Canada)
E-mail: jean-francois.morin@chm.ulaval.ca
A. Lucotti, M. Tommasini
Dipartimento di Chimica, Materiali e Ingegneria Chimica “G. Natta”
Politecnico di Milano
Piazza Leonardo da Vinci, 32, 20133 Milano (Italy)
Supporting information for this article can be found under:
http://dx.doi.org/10.1002/anie.201611834.
Figure 1. Synthesis of the helical graphene nanoribbon (HGNR), see
text for details.
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appropriate placement of chlorine atoms on a poly(m-phen-
ylene) precursor, it is possible to force the formation of
a conjugated helical structure resembling that of helicenes.
This structure can be seen as the first example of a synthetic
graphenic Riemann surface. Theoretical calculations suggest
that such material could have outstanding inductor perfor-
mance and thus be used as nanosolenoid.[11] The HGNR
structure is characterized with a variety of techniques,
The H NMR analysis of P1 and HGNR in CDCl3 were
conducted and the results are shown in Figure 2. As expected,
1
including H NMR, FTIR, XPS, TEM and Raman spectros-
copy.
The syntheses of HGNR and its polymeric precursor P1
are shown in Figure 1. Starting from 4-tetradecylaniline,
bromination at the 2 and 6 positions was performed using
bromine, a subsequent one-pot diazotization/iodination reac-
tion yielded compound 2.[12] Then, a selective Suzuki–
Miyaura coupling using PEPPSI-iPr as the catalyst provides
compound 3,[13] which was subjected to a double borylation
reaction in optimized conditions[14] to give monomer 4 in
moderate yield. Finally, a Suzuki–Miyaura polymerization
with 2,3-dichloro-1,4-diiodobenzene using Pd2dba3·CHCl3 as
the catalyst and SPhos as the ligand provided P1 in 92% yield.
This particular catalyst/ligand system proved to be efficient
for the synthesis of poly(m-phenylene)s with few defects.[15]
For the synthesis of HGNR, P1 was dissolved in degassed
decalin to a concentration of 0.002m and the solution was
irradiated using low-pressure mercury lamps (lem = 254 nm,
16 ꢁ 7.2 W) for 48 h under argon flow. Based on our previous
study, this reaction time is sufficient to allow completion of
the CDHC reaction.[10] It is worth mentioning that Mallory-
type reactions that would lead to a planarization of the fjord
region on the inner edge of the HGNR did not occur under
these conditions since no oxidant was used.[10] Fortunately,
photocyclization occurred as the solution went from colorless,
blue fluorescent to orange, green–yellow fluorescent, which is
indicative of significant structural changes in the p-conju-
gated backbone. After usual polymer treatment (see Support-
ing Information), an orange solid was obtained. Although
HGNR exhibits relatively good solubility in organic solvents,
complete drying makes it difficult to dissolve again, probably
due to strong intermolecular interactions.
Size-exclusion chromatography (SEC) analysis was per-
formed on P1 and HGNR using polystyrene standards and
CHCl3 as the eluent. P1 exhibits a unimodal molecular weight
distribution with a Mn value of 16000 gmolÀ1, corresponding
to a degree of polymerization (DP) of 32 units, and a polydis-
persity index (PDI) of 2.0 (see Figure S11). As expected,
HGNR exhibits a slightly lower Mn value (15200 gmolÀ1,
PDI = 1.9) than P1 due to the loss of HCl molecules and the
formation of a compact helical p-conjugated structure with
a lower hydrodynamic radius compared to the linear pre-
cursor P1. The decrease of the Mn value upon covalent
immobilization of a helical structure has been reported
previously for poly(m-phenyleneethynylene) foldamers.[16]
Considering that one helical pitch consists of six monomeric
units, a DP value of 32 represents approximately 5 helical
pitches in average. The decrease in the Mn value also suggests
that no intermolecular cross-linking reaction occurs upon
irradiation of P1.
Figure 2. 1H NMR spectrum of a) P1 in CDCl3 at 258C and b) HGNR
in CDCl3 at 608C. Peaks marked with an asterisk are attributed to
residual solvent.
the spectrum of P1 exhibits rather broadened signals com-
pared to its monomeric analogues. The ensemble of aromatic
protons produce an unresolved broad peak centered at d =
6.8 ppm. After irradiation to produce HGNR, the spectrum
flattens, especially in the aromatic region, and the peaks are
shifted downfield as a result of the formation of a rigid
structure. The presence of very broad peaks in the aromatic
region can also be attributed to the coexistence of different
conformations with different symmetries owing to the pres-
ence of a contorted region (fjord region) on the inner edge of
the HGNR. This line broadening behavior was observed and
studied in case of [10]cloverphene[17] and other hexabenzo-
triphenylene motifs.[18] As shown in Figure 2b, a very broad
signal centered at d = 8.6 appears, and the broad signal of P1
originally centered at d = 6.8 ppm shifts downfield to 7.2 ppm.
The resonance at d = 8.6 ppm can be ascribed to the outer
edge protons Ha of the so-called bay region, whereas that at
7.3 ppm is attributed to the outer protons Hb and inner
protons Hc and Hd. Many attempts to obtain a MALDI-TOF
MS spectrum of HGNR failed to produce meaningful results.
Thus, X-ray photoelectron spectroscopy (XPS) analysis was
performed to assess the disappearance of the chlorine atoms
and the formation of a graphenic structure after irradiation.
As expected, P1 exhibits a peak at 202 eV, corresponding to
the Cl2p band (Figure S19). Interestingly, the XPS spectrum
of HGNR shows no trace of this peak, meaning that the
CDHC reaction is complete and that no chlorine-containing
side-product has been formed during the reaction (Fig-
ure S20).
Figure 3 shows the FTIR spectra of P1 and HGNR. The
success of the CDHC reaction can be assessed with the band
at 4054 cmÀ1, which is associated with the free rotation of the
phenyl group.[19] While this band can clearly be seen in the
spectrum of P1, it is essentially absent in that of HGNR,
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similar, in agreement with UV/Vis data (see below). The
experimental data of Figure 3c show structured G and D
lines. Our DFT model of the Raman response allows
assigning the main two G lines to collective ring stretching
vibrations that occur either along the helix pattern (Ghelix) or
radially (Gradial). The helical coil of the ribbon induces this
split of the G line, at difference from planar GNRs, which
display a single G line.[20a,b,g,h] Similar to recently investigated
GNRs, graphene molecules and helicenes,[20] the strongest
feature in the D region is assigned to the expected vibrational
pattern found in graphene structures.[21] The D and G modes
of HGNR imply collective ring vibrations (marked by
asterisks in Figure 3c) forming typical patterns, which closely
match those theoretically predicted for graphene (see also
Supporting Information).
Optical properties of HGNR were determined by UV/Vis
and fluorescence spectroscopy and the results are shown in
Figure 4. The UV/Vis spectrum of HGNR (Figure 4a) reveals
absorption between 400 and 575 nm, with three vibronic
bands at 409, 435, and 455 nm, indicating that a rigid, well-
defined p-conjugated structure was formed upon irradiation.
In contrast, the UV/Vis spectrum of P1 showed a featureless
absorption band centered below 300 nm, which is indicative
of a non-rigid, less conjugated structure. The solid-state
absorption spectrum of HGNR exhibits a band that is slightly
red-shifted (7 nm) compared to HGNR in solution. The small
red shift in the lmax value suggests that HGNR possesses
Figure 3. a),b) FTIR spectrum of P1 (black lines) and HGNR (red lines)
Blue bars highlight spectral differences, see text for details. c) FT-
Raman spectrum of HGNR in the solid state and CHCl3 solution
(10 mgmLÀ1). Nuclear displacement patterns are also shown for
characteristic G and D modes (see text for clarification. Just a section
of the helical model is displayed): red arrows represent displacement
vectors; CC bonds are represented as green and blue lines of different
thicknesses according to their relative stretching (shrinking). The gray
shaded arrows in the Ghelix represent the direction of ring stretching
along the helical structure. Selected rings are marked by asterisks to
assist the reader in recognizing the characteristic ring vibration
patterns associated to G and D modes (see Ref. [21] and Supporting
Information for details).
indicating that the CDHC reaction went to near completion,
1
as indicated by H NMR spectroscopy. Moreover, the inten-
sity of the bands at 3085, 3059, and 3027 cmÀ1 (see Fig-
À
ure S16), attributed to C H aromatic stretching modes,
À
decreases as fewer aromatic C H groups are present in
HGNR (Figure S16). The intensity of the bands associated
À
with C H stretching of alkyl chains remains constant (2920
and 2851 cmÀ1) upon irradiation, indicating the integrity of
the alkyl substituent (see Figure S17). Furthermore, bands
=
associated with C C stretching of chlorinated aromatic
molecules at 1270 cmÀ1 and 1312 cmÀ1 completely vanish
after irradiation (see Figure 3b). This observation, along with
the decrease in intensity of bands that are associated with
À1
À
aromatic C H out-of-plane located at 912 and 698 cm ,
strongly suggest the loss of chlorine atoms and formation of
À
HGNR (see Figure S18). The remaining C H bonds of the
aromatic backbone of the HGNR are responsible for the
residual signal.
Figure 4. a) UV/Vis absorption spectrum of P1 (green line), HGNR in
THF (red line), and HGNR in thin film (blue line). Inset: magnification
of the 400 to 600 nm region of the spectrum in solution and a photo
of a dilute solution of HGNR in CHCl3 under ambient light. b) Photo-
luminescence spectrum of HGNR in THF (red line, lex =460 nm) and
HGNR in thin film (blue line, lex =460 nm). Inset: dilute solution (left)
and drop-cast film (right) of HGNR in CHCl3 under UV (365 nm) light.
Raman spectroscopy of HGNR was carried out both in
solution and the solid state to reveal the expected character-
istic G and D peaks typical of nanostructured graphenes and
nanoribbons. The solution and solid-state spectra are very
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similar conformation in both solution and solid state, which is
Acknowledgements
expected for a rigid p-conjugated backbone. Interestingly,
a shoulder at 535 nm appeared in both the solution and solid-
state spectra, which can be attributed to intramolecular p-
stacking of the helical structure.[22] The optical band gap of
HGNR measured at the absorption band onset is 2.15 eV,
which is one of the highest measured to date for a GNR.
Fluorescence spectroscopy experiments were also per-
formed in both solution and the solid state (Figure 4b). In
solution, HGNR exhibits three vibronic emission bands
located at 473, 500 and 538 nm. In thin film, the emission
band is red-shifted by 69 nm at 607 nm, with a shoulder in the
higher energy region (569 nm), close to the emission mea-
sured in solution. Although it is difficult in our case to gather
direct experimental evidence of it, we hypothesize that this
behavior can be attributed to an intramolecular excimer-like
emission, as it is very similar to what has been observed in the
case of foldamers based on conjugated units.[22,23]
This research was supported by the NSERC through a Dis-
covery Grant. M.D. thanks the FRQ-NT for a PhD scholar-
ship. We would like to thank Pascale Chevallier (XPS) and
Rodica Plesu (SEC) for their help in polymer character-
ization.
Conflict of interest
The authors declare no conflict of interest.
Keywords: carbon materials · graphene nanoribbons ·
helical polymer · helicenes · photochemistry
Transmission electron microscopy (TEM) was performed
to study the morphological features and structural parameters
of HGNR. The results are shown in Figure 5. When a dis-
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a copper grid, large aggregates of few microns were observed
(Figures S24). As shown on Figure 5a, individual 1D, rigid
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HGNR (54.9 ꢂ). Interestingly, a slight change in the solvent
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In conclusion, we have prepared the first helical graphene
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Manuscript received: December 5, 2016
Final Article published: && &&, &&&&
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Communications
Graphene Nanoribbons
M. Daigle, D. Miao, A. Lucotti,
M. Tommasini,
J.-F. Morin*
&&&& — &&&&
Helically Coiled Graphene Nanoribbons
Made into ribbons: Helicene-like gra-
phene nanoribbons (HGNR) have been
prepared through a regioselective photo-
chemical cyclodehydrochlorination
(CDHC) reaction from a polychlorinated
polyphenylene precursor.
6
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