Regioselective synthesis of nanographenes by photochemical cyclodehydrochlorination

Article abstract of DOI:10.1002/anie.201509130

Novel nanographenes were prepared by a photochemical cyclodehydrochlorination (CDHC) reaction. Chlorinated precursors were irradiated in acetone in the presence of a base or in pure benzene and underwent multiple (up to four) regioselective cyclization reactions to provide rigid π-conjugated molecules. Pure compounds were recovered in good yields by simple filtration at the end of the reaction. The CDHC reaction showed compatibility with both electron-poor and electron-rich substrates, thus allowing the synthesis of pyridine- and thiophene-fused nanographenes. It also enabled the synthesis of sterically hindered contorted π-conjugated molecules without causing full aromatization. A kinetic study showed that the CDHC reaction under the conditions used is a very fast process, and some reactions are completed within minutes. The CDHC reaction thus shows great potential as an alternative to other reactions involving harsher conditions for the preparation of nanographenes.

Full text of DOI:10.1002/anie.201509130

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International Edition: DOI: 10.1002/anie.201509130
German Edition: DOI: 10.1002/ange.201509130
Carbon Materials
Regioselective Synthesis of Nanographenes by Photochemical
Cyclodehydrochlorination
Maxime Daigle, Audrey Picard-Lafond, Eliane Soligo, and Jean-FranÅois Morin*
[
5]
Abstract: Novel nanographenes were prepared by a photo-
chemical cyclodehydrochlorination (CDHC) reaction. Chlori-
nated precursors were irradiated in acetone in the presence of
a base or in pure benzene and underwent multiple (up to four)
regioselective cyclization reactions to provide rigid p-conju-
gated molecules. Pure compounds were recovered in good
yields by simple filtration at the end of the reaction. The CDHC
reaction showed compatibility with both electron-poor and
electron-rich substrates, thus allowing the synthesis of pyridine-
and thiophene-fused nanographenes. It also enabled the syn-
thesis of sterically hindered contorted p-conjugated molecules
without causing full aromatization. A kinetic study showed that
the CDHC reaction under the conditions used is a very fast
process, and some reactions are completed within minutes. The
CDHC reaction thus shows great potential as an alternative to
other reactions involving harsher conditions for the prepara-
tion of nanographenes.
nanographenes and GNRs that can be prepared. One of its
most detrimental features is its poor regioselectivity, which
results in structural defaults that affect the properties of the
resulting materials. Other disadvantages include the forma-
tion of by-products by undesired rearrangements and the use
[5d]
of a metal-based catalyst.
To overcome these problems,
other techniques, such as palladium-catalyzed direct CÀH
[
6]
[7]
arylation, Al O -mediated HF elimination, Yamamoto
2
3
[
8]
cyclotrimerization, palladium-catalyzed [2+2+2] cycloaddi-
[
9]
tion, sequential IÀCl-induced cyclization and Mizoroki–
[
10]
[11]
Heck coupling, and the Katz-modified Mallory reaction,
have been used for the cycloaromatization of the polyphen-
ylene precursor. However, these methods may suffer from
modest yields, lead to a significant amount of side products, or
require a high metal-catalyst loading.
Herein we report a new solution-phase strategy for the
preparation of novel nanographenes and GNR fragments by
the photochemical cyclodehydrochlorination (CDHC) of
various aryl chlorides (Scheme 1). The optical properties of
the new nanographenes are also reported, along with the
results of DFT and TD-DFT calculations.
B
ottom-up, solution-phase synthesis is arguably one of the
most promising strategies for obtaining well-defined nano-
graphenes and graphene nanoribbons (GNRs) with tunable
[
1]
electronic and optical properties. Unlike well-known phys-
ical top-down methods in which
GNRs are obtained by “unzipping”
[
2]
carbon nanotubes
or cutting
[3]
stripes from graphene sheets,
bottom-up approaches enable pre-
cise control of the widths and edge
configurations of nanographenes
and GNRs. In their seminal studies,
the Müllen group showed that it is
possible to obtain nanographenes
and GNRs with different widths
and edge configurations through
multiple Lewis acid catalyzed cyclo-
dehydrogenation reactions, known
[4]
as the Scholl reaction, on poly-
[
1]
phenylene precursors. Although
the usefulness of the Scholl reaction
is widely acknowledged, it pos-
sesses some serious drawbacks that
Scheme 1. Different reaction paths for the CDHC (X=Cl) and Scholl (X=H) reactions. Newly formed
limit the structural diversity of the bonds are shown in red.
[
*] M. Daigle, A. Picard-Lafond, E. Soligo, Prof. J.-F. Morin
DØpartement de chimie and Centre de Recherche sur les MatØriaux
AvancØs (CERMA), UniversitØ Laval
The first reports of the formation of simple polycyclic
aromatic hydrocarbons (PAHs) by the use of the CDHC
reaction appeared in the early 1970s. Surprisingly, very few
articles have mentioned the use of this reaction for the
synthesis of PAHs, as the Scholl reaction is seen as the “go-to”
reaction for the synthesis of such compounds. Recently,
[12]
1
045 Ave de la MØdecine, QuØbec, G1V 0A6 (Canada)
E-mail: jean-francois.morin@chm.ulaval.ca
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201509130.
2042
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[a]
Hartley and co-workers used this reaction to prepare simple
Table 1: Synthesis of nanographenes 1–7 by the CDHC reaction.
[13,14]
PAHs with liquid-crystal properties,
whereas Schnapper-
elle and Bach employed it to prepare phenanthro[9,10-
[
15]
c]thiophenes. A similar photochemical cyclodehydrofluori-
nation reaction of aryl fluorides was also reported recently for
the preparation of triphenylene and phenanthrene deriva-
tives, but good yields were observed for the cyclization of
[16]
polyfluorinated precursors only.
The CDHC reaction is very attractive for the synthesis of
nanographenes for several reasons: 1) It is regioselective,
meaning that new well-defined architectures that were not
accessible with the Scholl reaction could potentially be
prepared; 2) it provides better control over the edge config-
urations of the nanographenes; 3) it proceeds without the use
of a metal catalyst, which is particularly important if
electronic applications are targeted; 4) it proceeds cleanly
without rearrangement or the formation of side products
under appropriate conditions; 5) chlorine atoms can be
introduced into polyphenylene precursors by careful design
of the monomers; and 6) it is conducted under very mild
conditions, thus enabling the introduction of different func-
tional groups onto the nanographenes to modulate their
properties. Scheme 1 highlights the difference in regioselec-
tivity between the CDHC reaction and the oxidative cyclo-
dehydrogenation (Scholl) reaction.
To investigate the usefulness of the CDHC reaction for
the preparation of nanographenes, we undertook the syn-
thesis of compounds 1–7 from their chlorinated precursors
(
Table 1; see the Supporting Information for synthetic
details). The triphenylene 1 was prepared as a model com-
pound, whereas the small nanographenes 2 and 3 were
prepared to demonstrate the regioselectivity of the photo-
chemical CDHC reaction. Nanographenes 4 and 5 were
synthesized to assess the usefulness of the CDHC reaction in
multiple cyclization processes and to evaluate its efficiency
for the preparation of large nanographenes. Finally, molecules
[
0
4
a] Reaction conditions for the synthesis of nanographenes 1, 2, 3, and 7:
.01m, acetone, 0.1m aqueous Na CO , 16”7.2 W lamps @300 nm,
2 3
8 h, room temperature. For compounds 4–6, a 450 W medium-pressure
mercury lamp was used instead, and the concentration of the reaction
mixture was 5”10 m. [b] The yield per cyclization reaction is given in
parentheses.
À5
6
and 7 were targeted to show the versatility of the CDHC
reaction for the preparation of nanographenes fused with
either electron-rich (thiophene, 6) or electron-poor (pyridine,
reported for the formation of indole derivatives through
[18]
cyclodehydrohalogenation, was used as an alternative to
benzene. We hypothesized that the sensitization effect of
acetone as well as the presence of a base to quench the HCl
formed during this reaction could be beneficial for this
reaction. The solution was irradiated at room temperature
with a medium-pressure 450 W Hg lamp for 2–5 h, depending
on the substrate. Even under the conditions of relatively high
dilution used, the desired products gradually precipitated
from the solution, thus allowing the recovery of nanogra-
phenes 1–6 through simple filtration. In the case of compound
7, a standard workup and column chromatography were
necessary to isolate the product, as no precipitation was
observed during the course of the reaction. In all other cases,
simple filtration was enough to obtain the nanographenes in
their pure form. A photoreactor equipped with 16 300 nm UV
lamps (7.2 W) could also be used successfully for the CDHC
reaction of our chlorinated precursors. When the photo-
reactor was used, the yields of formation for nanographenes
1–7 were very similar to those observed with the medium-
pressure Hg lamp, although the reactions were generally
slower. The all-benzene products 1–5 of the CDHC reaction
7
) moieties.
The common building blocks for all these molecules are
dichlorobenzene derivatives that are either commercially
available (2,3-dichloroaniline) or can be synthesized on
a gram scale in a few straightforward synthetic steps. 1,2-
Dichloro-3,6-diiodobenzene was prepared from commercially
available 2,3-dichloroaniline according to a reported proce-
[
17]
dure,
whereas 1,4-diiodo-2,5-dichlorobenzene was pre-
pared in two steps from 2,5-dichloroaniline (see the Support-
ing Information). 1,2-Dichloro-3-(4,4,5,5-tetramethyl-1,3,2-
dioxaborolane)benzene was prepared in one step from the
corresponding commercially available iodo derivative. Awide
variety of nanographene precursors can be prepared from
these building blocks by Suzuki–Miyaura coupling. All nano-
graphenes are presented with the corresponding chlorinated
precursor in Table 1.
In a typical photochemical CDHC reaction, a solution of
À5
the chlorinated precursor at a concentration of 5 ” 10 m in
acetone and aqueous Na CO3 (0.1m, 1 equiv per CDHC
reaction) was prepared. This solvent system, which was
2
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were isolated in relatively high yields of 79–95%. Unfortu-
nately, the lack of solubility of these nanographenes did not
enable us to perform HPLC experiments to determine the
precise conversion of the precursors into the desired products.
Nonetheless, monitoring of the reactions by thin-layer
chromatography suggested that the chlorinated precursors
reacted quickly.
We conducted a kinetic study of the CDHC reaction by
monitoring the fluorescence of 5 as a function of the
irradiation time of the corresponding chlorinated precursor.
The experiment performed in an acetone/Na CO mixture at
2
3
3
00 nm suggests that the CDHC reaction behaves like a zero-
order process at the beginning of the reaction, as the emission
intensity increased linearly with irradiation time from t = 0 to
À1
t = 50 s with a slope (k value) of 21.4 s (Figure 1a; see also
[19]
the Supporting Information). After only 1 min, a significant
decrease in the emission intensity was observed. This change
can be attributed to the rapid precipitation of 5 and,
hypothetically, to some degradation due to the light-induced
[20]
formation of acetone radicals.
Interestingly, the CDHC
reaction in the same solvent system was much faster at 300 nm
À1
than at 254 nm (k = 4.6 s ). When benzene was used as the
solvent at 300 nm, the reaction proceeded at a slower rate
À1
with k = 12.2 s (Figure 1b), although a higher yield was
observed (96% isolated, 99% per CDHC reaction). Unlike
that in acetone, the reaction in benzene did not lead to
a decrease in the emission intensity, as compound 5 remained
soluble. Also, degradation induced by benzene by-products is
presumably less likely, since benzene is stable upon UV
[
21]
irradiation.
Nonetheless, we believe that side reactions
involving acetone are negligible, since the desired nano-
graphenes precipitated out of the solution quite rapidly, and
the yields were generally good. None of the kinetic experi-
ments showed distinct emission bands that could be associ-
ated with reaction intermediates. This result suggests that
once a first CDHC reaction had occurred, the second was very
fast and happened almost instantly in the time frame of our
experiments.
To understand the role of the solvent in the CDHC
reaction, we performed the synthesis of compound 2 in pure
acetonitrile (without a base), which is known as a transparent,
nonsensitizing solvent (see Table S1 in the Supporting Infor-
mation). Interestingly, the yield was similar to that observed
in pure acetone (87 versus 92%; see Table S1), thus indicating
that the presence of a photosensitizer might not be necessary
for the CDHC reaction. However, the formation of a small
amount of polar by-products was observed when acetonitrile
was used. Furthermore, the yield observed in pure acetone
(
92%) is comparable to that observed in the presence of
Na CO (85%), thus indicating that the presence of a base is
2
3
neither necessary for nor detrimental to the success of the
reaction. This feature is a significant advantage, since a base
could be used to quench the HCl produced during the CDHC
reaction of compounds bearing acid-sensitive moieties. Tri-
ethylamine was also tested as a base, but the yield was much
lower (62%; see Table S1) as several by-products were
formed, as observed by TLC analysis.
Figure 1. Photoluminescence kinetic study for the formation of nano-
graphene 5 under irradiation at 300 nm in a) an acetone/Na CO3
2
mixture and b) pure benzene. c,d) Plots showing the variation of
emission intensity at 446 nm as function of time. The k values in (c,d)
indicate the slope of the linear region of the plot for the reaction
conducted in acetone/Na CO and pure benzene, respectively.
Previous mechanistic studies on chloro derivatives sug-
gested that the CDHC reaction proceeds through a photo-
2
3
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chemical conrotatory (4n+2)p electrocyclization, followed by
[12b,15,22]
the elimination of HCl.
The other plausible mecha-
nism involves the photolysis of the CÀCl bond to generate
a phenyl radical intermediate, as observed for the corre-
[23]
sponding bromo and iodo derivatives.
However, such
a mechanism would also involve intermolecular side reactions
with the solvent (e.g. hydrogen abstraction), which was not
observed in any of our reactions. The greater strength of the
CÀCl bond as compared to CÀBr and CÀI could also explain
[24]
the nonradical mechanism of the CDHC reaction. The very
low amount of side products also points towards a non-
radical-based mechanism.
As well as being a powerful tool to prepare planar
nanographenes, such as 4 and 5, the CDHC reaction is very
efficient at forming sterically hindered, contorted nanogra-
phenes, such as 2, which was prepared in 85% yield through
two CDHC reactions (92% per CDHC reaction). Contorted
polycyclic aromatic compounds are particularly interesting
for organic electronics and supramolecular chemistry, since
their twisted conformation can yield concave p surfaces and
chiral materials, besides increasing their solubility in organic
[
25]
solvents.
The synthesis of nanographene 2 also demon-
strates that the photochemical CDHC reaction enables
improved control over the edge configurations of nano-
graphenes as compared to that observed with the Scholl
reaction, which converted the same precursor (without the
chlorine atoms) exclusively into the completely cyclized
[
1a]
compound. The synthesis of nanographene 3, a structural
isomer of 2, demonstrates that the CDHC reaction is
regioselective and that a simple change in the position of
the chlorine atoms on the precursor can lead to different
isomers.
Figure 2. a) UV/Vis absorption spectra of nanographenes 4–6 in
chloroform. The inset is a zoom of the 400–440 nm region of the
spectrum of 5. b) Photoluminescence spectra of nanographenes 4–6 in
chloroform (solid lines) and in the solid state.
The yield of the thiophene-containing compound 6 was
much lower at 21%, which corresponds to 68% per CDHC
reaction. This lower yield can be attributed to the various
rearrangement reactions that can occur upon the irradiation
[26]
of phenyl-appended thiophene derivatives. We could not
determine the exact nature of the side products, as they were
not isolated. Although the efficiency of this reaction needs to
be further optimized, the result demonstrates that substituted
thiophenes are compatible with the photochemical cyclization
process, which shows the versatility of the CDHC reaction.
Likewise, compound 7 with an electron-poor fused pyridine
unit was prepared in 76% yield.
To further study the electronic properties of nanogra-
phenes 4–6, we performed DFT and time-dependent DFT
(TD-DFT) calculations at the commonly used B3LYP/6-
311G(d,p) level of theory (see Figure S4 in the Supporting
[29]
Information). The optimized structures of nanographenes
4–6 show a highly planar conformation with HOMO and
LUMO frontier orbitals delocalized over the entire molecule.
For compound 4, the calculated electronic transitions are in
good agreement with those observed experimentally (see the
Supporting Information). The HOMO–LUMO transition
occurs at 404 nm (oscillator strength, f = 0.68) versus 406 nm
in the experimental spectrum, whereas the second-lowest
energy transition, corresponding to a combination of the
HOMOÀ1 to LUMO (54%) and HOMO to LUMO+2
transitions (43%), occurs at 380 nm (f = 0.01). For compound
6, the TD-DFT calculations are in good agreement with the
experimental data for the lowest-energy transition, with
a value of 431 nm (f = 0.71), whereas slight discrepancies
appeared at higher energy, as the second-lowest energy
transition appeared at 390 nm (versus 405 nm in the absorp-
tion spectrum).
The optical properties of nanographenes 1–6 were studied
by UV/Vis absorption and emission spectroscopy, and the
spectra of nanographenes 4–6 are shown in Figure 2a. As
usually observed for very rigid p-conjugated molecules, the
UV/Vis spectra of nanographenes 4–6 exhibit a highly defined
vibronic structure, within which the b-bands are the most
[
27]
intense. For nanographene 5, a set of three bands between
4
05 and 430 nm, associated with the a-bands in the Clar
[28]
nomenclature,
3
were found with e values below
À1
À1
000m cm (Figure 2, inset). As expected, the absorption
spectrum of nanographene 6, containing two electron-rich
fused thiophene units, is red-shifted (25 nm) relative to that of
its phenyl analogue 4, thus resulting in a lower optical band
gap (2.82 eV) as compared to that of compound 4 (3.00 eV).
In conclusion, we have prepared several nanographenes
by a photochemical cyclodehydrochlorination (CDHC) reac-
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tion. The reaction is efficient, even when multiple cyclization
steps are necessary to provide the desired product. Unlike
other aromatization methods, photochemical CDHC is com-
patible with both electron-rich and electron-poor moieties.
Most importantly, this reaction is regioselective and allows
the preparation of nanographenes with different edge con-
figurations, including sterically demanding contorted PAHs.
Our attention is now focused on the use of this reaction to
prepare soluble graphene nanoribbons with well-defined edge
configurations.
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Angewandte
Communications
Chemie
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Received: September 29, 2015
Published online: December 22, 2015
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