Accessing extended and partially fused hexabenzocoronenes using a benzannulation-cyclodehydrogenation approach

Article abstract of DOI:10.1039/c3sc51212f

A rapid and efficient approach to prepare extended or partially fused hexabenzocoronene derivatives is described. The method is based on the sequential benzannulation and cyclodehydrogenation (Scholl oxidation) of simple diaryl alkynes. The benzannulation reaction proceeds efficiently on highly congested substrates and with complete regioselectivity. Scholl oxidation of the resulting oligo(arylene)s proceeds without rearrangements and provides either fully fused or specific partially fused polycyclic aromatic hydrocarbon products. The partially fused derivatives are a new class of contorted aromatic systems with high solubility, enhanced visible absorption, and reversible redox processes. The efficiency and specificity of the benzannulation and oxidation reactions are promising for accessing new classes of organic semiconductors and carbon nanostructures.

Full text of DOI:10.1039/c3sc51212f

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Accessing extended and partially fused
hexabenzocoronenes using a benzannulation–
cyclodehydrogenation approach†
Cite this: DOI: 10.1039/c3sc51212f
*
Hasan Arslan, Fernando J. Uribe-Romo, Brian J. Smith and William R. Dichtel
A rapid and efficient approach to prepare extended or partially fused hexabenzocoronene derivatives is
described. The method is based on the sequential benzannulation and cyclodehydrogenation (Scholl
oxidation) of simple diaryl alkynes. The benzannulation reaction proceeds efficiently on highly
congested substrates and with complete regioselectivity. Scholl oxidation of the resulting oligo(arylene)s
proceeds without rearrangements and provides either fully fused or specific partially fused polycyclic
aromatic hydrocarbon products. The partially fused derivatives are a new class of contorted aromatic
systems with high solubility, enhanced visible absorption, and reversible redox processes. The efficiency
and specificity of the benzannulation and oxidation reactions are promising for accessing new classes of
organic semiconductors and carbon nanostructures.
Received 3rd May 2013
Accepted 10th July 2013
DOI: 10.1039/c3sc51212f
www.rsc.org/chemicalscience
Extended polycyclic aromatic hydrocarbons (PAHs),1–4 such as efficiency of this transformation to convert the alkyne functions
the hexabenzocoronenes (HBCs), are functional p-electron of poly(phenylene ethynylene) (PPE) into 2,3-diary-
a
systems whose desirable optical and electronic properties have lnaphthalene moieties.27 The reaction tolerates aryl substitu-
attracted interest for organic thin lm transistor (OTFT)5–7 and ents adjacent to the alkynes, a promising feature for PAH
organic photovoltaic (OPV) devices.8,9 The device properties of synthesis provided that the resulting oligoarylenes can be
specic PAHs depend both on their electronic structure and cyclodehydrogenated efficiently and without rearrangements.
packing behaviour. The nature of these assemblies is inuenced
by molecular shape, the position and identity of solubilizing to
Substituted diarylalkyne derivatives 1a–d are ideal substrates
evaluate the benzannulation–cyclodehydrogenation
groups, molecule–substrate interactions, and processing approach for PAH synthesis (Fig. 1). This strategy provides HBC
¨
methods used to form thin lms. For example, Mullen and derivatives with extended conjugation and complementary
coworkers have made major contributions to both the synthesis substitution patterns relative to existing methods. Here we
of novel HBC derivatives and their solid-state organization.10–12 show that the benzannulation reaction is efficient for several
Other instructive examples of the interplay between HBC HBC precursors and that typical Scholl oxidation conditions do
structure and long-range order that produce emergent proper-
ties include nanotubes derived from amphiphilic HBCs studied
by Aida13–15 and contorted HBCs studied by Nuckolls16–25 and
Miao26 for OTFTs and OPVs.
Efficient and convergent syntheses of HBCs with novel
substitution patterns, extended conjugation, or nonplanar
structures will allow their full potential to be realized. Less
symmetric substitution patterns, such as those of interest for
amphiphilic derivatives, can require laborious syntheses, while
introducing additional fused rings confers lower HOMO–LUMO
gaps and can induce curvature. Here we introduce a new
strategy to access HBC derivatives by benzannulating readily
accessible diarylalkynes. We recently demonstrated the high
Baker Laboratory, Department of Chemistry and Chemical Biology, Cornell University,
Ithaca, NY 14853-1301, USA. E-mail: wdichtel@cornell.edu
† Electronic supplementary information (ESI) available: Experimental procedures
and characterization data for all reported compounds and precursors. See DOI:
10.1039/c3sc51212f
Fig. 1 A series of dialkyne compounds 1a–d provide partially fused HBC deriv-
atives 4a–d and fully fused products 5a and 5d using a two-step benzannulation–
cyclodehydrogenation protocol.
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not induce rearrangements. In some cases, complete cyclo-
dehydrogenation is achieved; however, shorter reaction times or
strategically placed substituents provide partially fused prod-
ucts. When complete oxidation does not occur, the order of
carbon–carbon bond formation is well dened, providing
specic, non-planar coronene derivatives 4a–c that exhibit high
solubility, strong visible absorption, and reversible redox
processes. Collectively, these molecular compounds also
represent important model systems for oxidizing benzannu-
lated PPEs to produce structurally precise graphene nano-
ribbons, and their lack of rearrangements show great promise
in this regard.
We evaluated the generality and efficiency of the benzan-
nulation reaction across several substituted di(arylethynyl)-
benzene derivatives. The di(n-nonyl)terphenyl compound 1a is a
molecular analogue of the repeat unit of our previously reported
PPE system27 that was benzannulated in the presence of
2-(phenylethnyl)benzaldehyde, Cu(OTf)2, and CF3CO2H. Similar
conditions provide the oligoarylene 3a in 98% isolated yield. We
also evaluated dialkynes 1b and 1d, which have four and two
peripheral t-butyl substituents, respectively. The tBu groups
serve three primary functions: (1) they are effective solubilizing
groups for PAHs; (2) they are convenient and sensitive NMR
handles for rapidly diagnosing side reactions and rearrange-
ments; and (3) oxidation of tBu-substituted substrates oen do
not fuse completely because of steric hindrance.28 Such partially
oxidized products offer unique properties associated with their
non-planar structures as well as insight into the sequence of
C–C bond forming steps of fully fused systems. Finally, we
evaluated compound 1c, which has 2-naphthyl groups as central
aryl substituents. This compound provides access to larger
fused aromatic systems and serves as a model for a benzannu-
lated polymer that might provide a graphene nanoribbon with
an armchair edge structure. Dialkynes 1b–d were each ben-
zannulated in similarly high efficiency based on analysis of
their crude reaction mixtures.
We also benzannulated 1a with isotopically labelled benz-
aldehyde 2-13C2 (Fig. 2). The resulting benzannulated product
3a-13C2 contained one isotopic label per reactive site, as deter-
mined by mass spectroscopy, 13C NMR spectroscopy (Fig. 2b),
and a full complement of 2D NMR spectroscopy experiments
(Fig. S24 and S44–S51 in the ESI†). The incorporation of a single
isotopic label at these positions is consistent with Yamamoto's
proposed mechanism29 with the regioselectivity indicated in
Fig. 2c, while the other 13C label is eliminated upon aromati-
zation of the newly formed naphthalene. This regioselectivity
might derive from the formal [4 + 2] cycloaddition occurring in a
stepwise manner, in which the rst new carbon–carbon s bond
is formed between the less hindered alkyne carbon and the
oxonium carbon of the benzopyrilium intermediate. The
regioselectivity of the benzannulation reaction, combined with
isotopic labelling, offer a powerful means to characterize Scholl
oxidation products. These ndings also suggest that non-
symmetric derivatives of 2 will provide PAHs with specically
placed substituents.
Fig. 2 (A) The benzannulation of 1a with 2-13
C2 provides the labelled derivative
3a-13C2, indicating that the reaction is regioselective. (B) Partial 13C NMR of
3a-13C2. (C). Regiochemistry of the benzannulation reaction consistent with this
experiment.
indicated a loss of 12 amu, corresponding to the formation of 6
C–C bonds and complete fusion of the coronene core. Although
1H and 13C NMR spectroscopy of 5a were consistent with the
expected structure, the low solubility and propensity of this
compound to aggregate complicated its denitive structural
characterization. Furthermore, King et al. noted rearrange-
ments in a closely related system that had been misassigned in
the literature for many years (Fig. 3).30 Therefore, we prepared
several additional model compounds that would exhibit
improved solubility to enable unambiguous characterization of
the fused PAHs. These model systems incorporate 40-tBuphenyl
substituents, as well as variations of the central arylene group.
Fig. 3 (A) Rearrangement during the Scholl oxidation observed by King et al.30
Dinonyl derivative 3a was cyclodehydrogenated using FeCl3.
(B) The expected (observed) and possible rearranged (not observed) products for
MALDI-TOF mass spectrum of the reaction mixture aer 1 h the oxidation of 3b.
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In order to investigate the possibility of rearrangements each pair of adjacent naphthalene systems, which we also
during the oxidation, we subjected 3b to the same Scholl attribute to steric interactions associated with the tBu groups.
oxidation conditions. By incorporating the tBu groups on the
We next analyzed the reaction mixtures of various Scholl
central terphenyl and external phenyl groups, 3b does not fuse oxidations as a function of reaction time and gained insight into
completely. MALDI-TOF MS indicated a reduction in mass of the order and relative rates of C–C bond formation within these
8 amu, corresponding to the formation of four of the six benzannulated compounds. We rst characterized the oxida-
expected C–C bonds. The 1H NMR spectrum of 4b provides tion of compound 3d, which retains the two 4-tBu phenyl
unambiguous evidence that 1,2-aryne rearrangements observed substituents on the central aromatic ring but lacks the external
in other systems do not occur (Fig. 4). For example, two singlets tBu groups of compound 3b. Prior to FeCl3 addition, 3d exhibits
corresponding to tBu resonances are observed at 1.46 and 1.25 an isotopically resolved MALDI-TOF mass spectrum corre-
ppm, respectively, which integrate to a 1 : 1 ratio. The analo- sponding to its expected mass (Fig. 5, t ¼ 0). A spectrum of the
gous rearrangement to that reported by King and coworkers reaction mixture recorded 20 min aer adding the oxidant
would instead provide a compound with three tBu resonances exhibits its most intense peak eight mass units lower, corre-
in a 1 : 2 : 1 integration ratio (Fig. 3b). The aromatic region of sponding to the formation of four new C–C bonds. No peaks are
the 1H NMR spectrum of 4b indicates the formation of the non- observed for species of intermediate mass between 3d and this
rearranged but partially fused structure depicted in Fig. 4. partially fused intermediate, and low intensity signals 1–4 amu
Eleven aromatic resonances are observed, rather than nine lower than the main peak correspond to further fused products.
expected for the fully fused compound. A complete assignment At 60 min, these peaks increase in intensity, and the mass
of the spectrum using complementary 2D NMR experiments spectrum indicates essentially complete formation of 5d at
(COSY, ROESY, HSQC and HMBC; see Section D in the ESI†)
indicates that C–C bonds between the tBu functionalized
aromatic rings do not form. We hypothesize that steric
hindrance associated with these groups causes distortions from
planarity that prevent these nal two C–C bonds from forming.
The ROESY spectrum of 4b is consistent with this hypothesis, as
clear cross peaks are observed between Hc and Hb (7 bonds
apart) and Hj and Hk (6 bonds apart) in addition to those
between adjacent hydrogen atoms on the periphery. A derivative
that contains central 2-naphthyl substituents (3c), also partially
fuses under the oxidation conditions. MALDI-TOF MS indicated
the formation of four C–C bonds. The regiochemistry of bond
formation was unambiguously characterized by NMR spec-
troscopy. Interestingly, only one C–C bond was formed between
Fig. 5 MALDI-TOF MS spectra taken of the oxidation of 3d as a function of
reaction time. Four carbon–carbon bonds are formed in fewer than 20 minutes,
Fig. 4 Partial 1H NMR spectrum of the aromatic and tBu regions of 4b.
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while full oxidation occurs after longer reaction times.
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240 min. By analogy to 4b, in which the nal two C–C bonds do observed by NMR spectroscopy. Although both structures are
not form even at extended reaction times, we hypothesized that viable, we predict that 4a adopts the twisted form based on
the four C–C bonds formed rapidly in 4d correspond to those previous work with a similar contorted compound26 and the
present in 4b. This hypothesis is further supported by a similar limited conformational freedom of precursor 3a. In contrast,
study of the oxidation of the di-n-nonyl functionalized fully fused compounds 5a and 5d are nearly planar (2–3ꢀ dihe-
compound 3a. MALDI-TOF MS of the oxidation of 3a at t ¼ 5 dral angle). For 5b and 5c, which were not observed experi-
min also indicated formation of an intermediate oxidation mentally, the internal arene torsion of the optimized structure
product containing the rst four C–C bonds, followed by is 10–11ꢀ, suggesting that the strain of forming the nal
complete oxidation at longer reaction times. We stopped a carbon–carbon bonds precludes complete fusion of these
preparative scale oxidation of 3a aer 5 min and isolated the compounds.
partially oxidized product, which was highly soluble in many
Contorted HBCs have attracted interest for their shape
organic solvents. 1H NMR analysis indicated twelve resolved complementarity to fullerenes and distinct optical and elec-
aromatic resonances consistent with the structure 4a (Sections tronic properties. For example, HBCs are typically yellow
C and D in the ESI†). Interestingly, partial fusion of a related o- compounds, whereas solutions of 4a and 4b are orange and
31
¨
phenylene containing systems was observed by Mullen and those of 4c are red. UV/vis spectroscopy indicated similar
Moore.32 However, a different regiochemistry (order of bond spectra for each compound, with signicant absorption bands
formation) was observed. A comparison of these results in the 450–550 nm range and molar absorptivity $20 000 Mꢁ1
suggests that the course of the Scholl oxidation is highly cmꢁ1 (Fig. 7a). The absorption onsets of each compound, from
sensitive to subtle changes in structure.
which the optical HOMO–LUMO gaps are determined, are ꢂ540
The partially fused systems are nonplanar PAHs, as indicated nm for 4a and 4b and 575 nm for the more extended derivative
by calculating their structures using Density Functional Theory 4c. These onsets correspond to optical HOMO–LUMO gaps of
(DFT). We optimized the geometries of 4a–d and 5a–d (for 4a 2.32 eV for both 4a and 4b and 2.16 eV for 4c (these optical and
and 5a, the n-nonyl chains were truncated to methyl groups). electrochemical HOMO–LUMO gaps are collected in Table S3 in
Each of the partially fused structures 4a–d show signicant the ESI†). For all structures analyzed computationally, both the
distortion from planarity, in which the dihedral torsion angle at HOMO and LUMO are delocalized across the molecule,
the site of nal cyclodehydrogenation is 56ꢀ (Fig. 6 and Fig. S86 consistent with a small Stokes shi (see Table S1 in the ESI†).
in the ESI†). For compound 4a, we considered two possible The DFT calculated HOMO–LUMO gaps trend is consistent with
conformations, namely twisted (Fig. 6) and anti (Fig. S87 in the the experimental values (2.72 eV, 2.76 eV, and 2.60 eV for 4a, 4b,
ESI†). The two structures are energetically similar, with the and 4c, respectively). The lowest energy optical transition of 4a
twisted conformation predicted to be 0.9 kcal molꢁ1 more is inuenced by its nonplanar structure, as evidenced by the
stable in the gas phase, and both structures exhibit large devi- spectrum of the corresponding fully fused derivative 5a, which
ations from planarity. Their barrier to interconversion is high does not absorb strongly in this region (Fig. S84 in the ESI†). A
(>20 kcal molꢁ1), and only a single conformation of 4a was contorted PAH most similar in structure to the aromatic core of
4a–c exhibits similar absorbance and uorescence properties,33
and these effects on the absorbance of nonplanar coronene
derivatives were also noted by Nuckolls and coworkers.23 Each
compound is strongly uorescent, and 4a and 4b have nearly
identical photoemission spectra with lmax at 505 nm (Fig. 7b).
The emission of 4c has a very similar peak shape and lmax of
540 nm. These maximum emission wavelengths correspond to
low Stokes shis (ꢂ25 nm) for each compound that are
consistent with their expected rigid structures. The partially
fused derivatives described here maintain reasonable uores-
cence quantum yield, whereas nonplanarity induced by incor-
porating ve-membered rings into similar systems was
accompanied by dramatic reductions in their quantum yield.23
Electrochemical studies provided additional insight into the
redox processes and energy levels of the partially fused
compounds (Fig. 8). Cyclic voltammetry of 4a–c was performed
in 0.1 M Bu4NPF6 in CH2Cl2 on a Pt button electrode, with
potentials reported relative to the ferrocene/ferrocenium
(Fc/Fc+) redox couple. 4a and 4b exhibit two electrochemically
irreversible but chemically reversible oxidation processes, each
with a rst onset potential of 0.22 V. The corresponding
oxidation processes are also observed for 4c at similar onset
Fig. 6 DFT-optimized structures of partially and fully fused compounds 4a and
potentials of 0.24 V. Differences in the reduction processes of
these compounds are more pronounced. 4a and 4b are reduced
5a, respectively. The n-C9H19 chains were truncated to –CH3 groups in the
calculations.
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Fig. 8 Cyclic voltammetry (0.1 M Bu4NPF6 in CH2Cl2; Pt button electrode) of 4a–c.
chemically irreversible (Fig. S87 in the ESI†). Overall, the similar
oxidation potentials of each compound, along with the shi in
the reduction potential of 4c relative to 4a and 4b suggest that
the extended conjugation of 4c predominantly lowers its LUMO
energy, making it a better electron acceptor.
Conclusions
We have demonstrated a sequential benzannulation–cyclo-
dehydrogenation strategy to access new PAH architectures from
easily prepared arylene ethynylenes. This approach provides
efficient access to both fully fused and partially fused hex-
abenzocoronene derivatives with extended conjugation and
differing substitution patterns than those available from exist-
ing methods. Rearranged products were not observed under
these conditions. The partially fused derivatives are underex-
plored and feature high solubility, increased absorptivity in the
visible spectrum, and reversible redox processes. The high
efficiency of the benzannulation reaction shows great promise
for synthesizing other novel PAH systems. The benzannulated
derivatives explored here are also important models for
accessing graphene nanoribbons by applying the benzannula-
Fig. 7 (A) UV/vis and (B) fluorescence spectra of contorted compounds 4a–c.
tion–cyclodehydrogenation
approach
to
appropriately
at onset potentials of ꢁ1.79 and ꢁ1.80 V, respectively. The
reduction of 4a is chemically reversible, whereas the reduction
of 4b is chemically irreversible and may correspond to a mul-
tielectron reduction process. The reduction of 4c occurs at a less
negative potential of ꢁ1.57 V and is also chemically irreversible.
These measurements give rise to electrochemical HOMO–
LUMO gaps of 2.01, 2.02, and 1.81 eV for 4a–c, respectively,
which correspond reasonably well to those determined opti-
cally. In contrast, CV of a lm of the fully fused derivative
5a-13C2 cast on a Pt button working electrode exhibited no
substituted PPEs. This study conrms the promise of such an
approach in that no rearrangements were observed in the Scholl
oxidation step, although complete fusion in the polymer
systems should be demonstrated carefully. We will next apply
these methods to access new carbon nanostructures and other
aromatic systems.
Acknowledgements
reduction processes at potentials >ꢁ2.2 V. The oxidation onset This work was supported by the Beckman Young Investigator
of these lms is at 0.40 V, 0.18 V more positive than 4a and is Program of the Arnold and Mabel Beckman Foundation, the NSF
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(CHE-1124754), the Doctoral New Investigator Program of the 16 C.-Y. Chiu, B. Kim, A. A. Gorodetsky, W. Sattler, S. Wei,
ACS Petroleum Research Fund (52019-DNI7), the Cottrell Scholar
Program of the Research Corporation for Science Advancement,
A. Sattler, M. Steigerwald and C. Nuckolls, Chem. Sci.,
2011, 2, 1480–1486.
and a Sloan Research Fellowship from the Alfred P. Sloan Foun- 17 Y. S. Cohen, S. Xiao, M. L. Steigerwald, C. Nuckolls and
dation. This work made use of the Cornell Center for Materials C. R. Kagan, Nano Lett., 2006, 6, 2838–2841.
Research Facilities supported by the National Science Foundation 18 A. M. Hiszpanski, S. S. Lee, H. Wang, A. R. Woll, C. Nuckolls
under Award Number DMR-1120296. We acknowledge Dr Ivan
Keresztes for assistance with NMR spectroscopy.
and Y.-L. Loo, ACS Nano, 2013, 7, 294–300.
19 S. J. Kang, S. Ahn, J. B. Kim, C. Schenck, A. M. Hiszpanski,
S. Oh, T. Schiros, Y.-L. Loo and C. Nuckolls, J. Am. Chem.
Soc., 2013, 135, 2207–2212.
20 S. J. Kang, J. B. Kim, C.-Y. Chiu, S. Ahn, T. Schiros, S. S. Lee,
K. G. Yager, M. F. Toney, Y.-L. Loo and C. Nuckolls, Angew.
Chem., Int. Ed., 2012, 51, 8594–8597.
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