Benzo- And Thieno-Annulated Tetracenes: A One-Pot Synthesis via Cross-Dehydrogenative Annulation

Article abstract of DOI:10.1021/acs.orglett.0c01244

A facile method for the direct cross-annulation of unfunctionalized tetracene is reported. The one-pot oxidative cross-dehydrogenative coupling (CDC) between tetracene and aromatic compounds, such as benzene or 2-methylthiophene, furnished annulated products with an extended π-network. Moreover, relative to the benzo-annulated tetracenes, thieno-annulated tetracenes exhibited notably improved photooxidative stability. This behavior stands in sharp contrast with that of tetracene and its derivatives, such as rubrene, which readily engage in photoinduced oxidation reactions.

Full text of DOI:10.1021/acs.orglett.0c01244

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Letter
Benzo- and Thieno-Annulated Tetracenes: A One-Pot Synthesis via
Cross-Dehydrogenative Annulation
Michihisa Murata,* Masahiro Togo, Daisuke Mishima, Ai Harada, and Masahiro Muraoka
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ABSTRACT: A facile method for the direct cross-annulation of
unfunctionalized tetracene is reported. The one-pot oxidative cross-
dehydrogenative coupling (CDC) between tetracene and aromatic
compounds, such as benzene or 2-methylthiophene, furnished annulated
products with an extended π-network. Moreover, relative to the benzo-
annulated tetracenes, thieno-annulated tetracenes exhibited notably
improved photooxidative stability. This behavior stands in sharp contrast
with that of tetracene and its derivatives, such as rubrene, which readily
engage in photoinduced oxidation reactions.
ligoacenes that consist of linearly fused benzene rings
Ohave attracted substantial interest due to their unusual
optoelectronic properties. For example, oligoacenes possess a
singlet biradical ground state, a small HOMO−LUMO gap,
and a high charge-carrier mobility, and can undergo singlet
exciton fission.1 However, larger oligoacenes, such as tetracene
and pentacene, suffer from significant practical drawbacks
associated with, for example, a high susceptibility to engage in
Figure 1. Structures of tetracene, rubrene, and bisindeno-fused
tetracene 1.
photochemical oxidation and dimerization reactions as well as
limited solubility.2 For example, the representative tetracene
derivative rubrene shows an exceptionally high charge-carrier
mobility but simultaneously shows a high propensity to engage
in rapid photooxidative degradation.3 Whereas a number of
oligoacene derivatives have been intensively studied,4 the
development of novel synthetic strategies for the versatile
functionalization of acene derivatives and the improvement of
their photooxidative stability remain of paramount importance
for the continued progress of acene-based materials chemistry.
Recently, efficient methods to extend the π-network of
oligoacenes using intramolecular Scholl cyclizations have been
reported.5 The Scholl cyclization is one of the most effective
and versatile C−C bond-forming reactions for the generation
of polycyclic aromatic hydrocarbons.6 For example, the two-
fold intramolecular Scholl cyclization of 5,11-diaryltetracenes
furnishes dibenzo- or dithieno-annulated structures. These
electron-deficient polyaromatic compounds, such as 1, exhibit
a narrow HOMO−LUMO gap (Figure 1).5a,d Chi and
coworkers reported that bisindeno-annulated pentacene
derivatives,5b obtained via the two-fold Scholl cyclization of
6,13-diarylpentacenes, exhibit an enhanced stability toward
photooxidation, which is even higher than that of 6,13-
bis(triisopropylsilylethynyl)pentacene (TIPS-pentacene).1a
Although the annulation of indeno-type structures is
effective for the generation of semiconductor materials with
enhanced photooxidative stability, most synthetic pathways
used for the π-extension of acenes require a transition-metal-
catalyzed cross-coupling or a multistep process.6,7 Herein we
report a simple and powerful route to an indeno-annulated
structure from tetracene. The use of a one-pot cross-
dehydrogenative annulation (CDA) enables the synthesis of
a series of π-extended tetracenes. Moreover, systematic studies
clarified that the introduction of a thieno-annulated structure
significantly enhances the photooxidative stability of the
tetracene core.
The first route that we examined for the synthesis of indeno-
annulated tetracene was the oxidative cross-dehydrogenative
Received: April 7, 2020
https://dx.doi.org/10.1021/acs.orglett.0c01244
© XXXX American Chemical Society
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coupling (CDC) between unfunctionalized tetracene and
benzene. Even though the formation of C−C bonds in CDC
reactions tends to suffer from low selectivity, this issue can
partly be addressed by the judicious choice of oxidants or
techniques, for example, hypervalent iodine reagents8 or
“radical cation pools”.9 It should also be noted here that
single-step cross-annulation reactions have been developed and
extensively studied by Itami and coworkers.10 On the contrary,
tetracene, which is an electron-rich relative of simple arenes or
heteroarenes, is a suitable substrate for direct CDA reactions.
When tetracene was subjected to a one-pot treatment at room
temperature with unfunctionalized benzene using a p-
chloranil/TfOH system (typical conditions for the Scholl
reaction), indenotetracene 2a (36%) was obtained together
with 3a (19%), which contains an additional phenyl group
(Scheme 1). The yield of 3a improved to 49% upon increasing
Figure 2. Single-crystal X-ray diffraction structure of 2a. (a)
Molecular structure with thermal ellipsoids at 50% probability. Only
one of the two crystallographically independent molecules is shown,
and hydrogen atoms are omitted for clarity. (b) Herringbone
structure of 2a with the van der Waals surface. Selected bond lengths
(Å) for 2a: a, 1.410(2); b, 1.451(2); c, 1.362(2); d, 1.358(2); e,
1.4326(18); f, 1.472(2); g, 1.470(2).
the range of typical π-stacked structures (3.4 to 3.7 Å).13 This
result stands in sharp contrast with tetracene, which arranges in
a significantly slipped fashion.1c In the crystalline states, 2a and
disordered 2d form a packing structure through two effective
interactions, that is, π−π and C−H/π interactions (for details,
see the Supporting Information), insight that will be useful for
the design of molecular arrangements in the solid state.
The absorption spectra of 2a and 2e were recorded (Figure
3), and the electronic properties were examined. The
Scheme 1. One-Pot Oxidative Cross-Dehydrogenative
Annulation (CDA) between Unfunctionalized π-Systems
Figure 3. UV−vis absorption spectra of benzo- and thieno-annulated
tetracenes 2a and 2e in toluene.
absorption spectrum of 2a showed its longest-wavelength
absorption at λmax = 566 nm (log ε = 3.76). Whereas 2e
showed an absorption band in a region similar to that of 2a,
the absorption band of 2e also had a tail that extended to 720
nm. Time-dependent (TD) DFT calculations at the CAM-
B3LYP/6-31G*//B3LYP/6-31G* level of theory suggested
that the longest wavelength absorption band of 2a has an
oscillator strength of f = 0.17 and arises from a π−π* transition
with a major contribution from the HOMO → LUMO
transition. In contrast, the longest-wavelength absorption of 2e
has a much smaller oscillator strength (f = 0.029) and likely
stems from a π−π* transition with a major contribution from
the HOMO−1 → LUMO transition.
During our study, we noticed that the addition of an indeno-
annulated moiety to tetracene induces markedly improved
stability toward photooxidation (Figure 4). Thus the photo-
oxidation behavior of 2a and 2e under ambient conditions was
studied by monitoring the decay of each absorption band
compared with the behavior of tetracene and rubrene. The
results indicate that indeno-annulation has a notable effect on
the stability toward photooxidation. In contrast, rubrene,
the amount of benzene (5 equiv) and p-chloranil (2 equiv),
whereas 1 was not generated. t-Butylbenzene, anisole,
fluorobenzene, and 2-methylthiophene could also be used
instead of benzene to give 2b−e. Uncyclized products, that is,
5-aryltetracenes, were not observed in these reactions, which is
indicative of facile intramolecular cyclizations. The rapid
cyclization relative to the initial C−C bond formation was,
assuming a dicationic mechanism5f for the Scholl cyclization
(for details, see the Supporting Information), supported by
density functional theory (DFT) calculations.11 We did not
observe the formation of an indeno-annulated product when
anthracene was used instead of tetracene.
The structure of 2a was unambiguously determined by
single-crystal X-ray diffraction analysis (Figure 2). Within the
crystals, molecules of 2a adopt a herringbone packing motif,
similar to that of the parent tetracene12 with predominant
edge-to-face interactions between the C−H bonds and the π-
surface of adjacent molecules. Moreover, because of this
extended π-network, 2a forms π-stacked columns with
intermolecular distances of 3.29 to 3.39 Å, which are within
B
https://dx.doi.org/10.1021/acs.orglett.0c01244
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Scheme 2. Proposed Pathways for the Photooxygenation of
Indeno-Annulated Tetracene 2a with Calculated Barriers
(ΔG‡ in kcal/mol)
Figure 4. Decay profiles of the absorption maximum of 2a, 2e,
tetracene, and rubrene upon photoirradiation with artificial light (32
W fluorescent lightning strips) in a toluene solution (1.0 × 10−4 M)
under atmospheric conditions.
In summary, we have demonstrated a one-pot direct
annulation based on an oxidative CDA between unfunction-
alized tetracene and simple aromatic compounds such as
benzene and 2-methylthiophene. A single-crystal X-ray
diffraction analysis of indeno-annulated tetracene (2a) revealed
that the crystal packing involves effective π−π and C−H/π
interactions. Moreover, this synthetic strategy allowed us to
carry out systematic studies on the photooxidative stability of
the extended π-network of tetracenes. We discovered that the
introduction of a thieno-annulated structure significantly
suppresses the photochemical degradation relative to indeno-
annulated structures. Further studies on (hetero)arene-
annulated acenes synthesized via CDAs for applications in
semiconducting materials are currently in progress in our
laboratory, and the results will be reported in due course.
which contains four additional phenyl groups relative to
tetracene, undergoes very rapid degradation in comparison
with tetracene. Regardless of whether the derivative contains
an electron-donating or -withdrawing group, slightly improved
photooxidative stability was observed for 2b−d. (For details,
see the Supporting Information.) Moreover, we found that the
stability is further enhanced by the addition of a thiophene ring
to the structure. The degradation of 2e in a dilute toluene
solution was negligible under atmospheric conditions in the
presence of the artificial light (32 W fluorescent lighting
strips).
Even though the photooxidation of acenes and their
substituted derivatives has been extensively studied, the
ASSOCIATED CONTENT
* Supporting Information
underlying mechanism remains a matter of debate.4,14
A
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lowered LUMO level plays an important role for the
stabilization of TIPS-pentacene by suppressing the generation
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/acs.orglett.0c01244.
of the radical anion O2 via an electron transfer.14a Cyclic
•−
voltammograms of 2a and 2e in CH2Cl2 each show one
reversible reduction wave (E1/2 = −1.58 V and −1.59 V vs Fc/
Fc+, respectively), indicating that the LUMO levels of 2a and
2e are comparable. (For details, see the Supporting
Information.) On the contrary, 9,10-diarylanthracenes are
thought to react with singlet oxygen (1O2) via a stepwise
pathway that involves a zwitterionic intermediate.4c,14 This
stepwise pathway proceeds faster than the concerted [4 + 2]
cycloaddition with 1O2.4c Rubrene is considered to be oxidized
rapidly, which is, at least in part, due to the stabilization of its
polar intermediate by its phenyl groups. However, on the basis
of our DFT calculations at the M06-2X/6-31G* level of
theory, the barrier for the generation of the polar intermediate
4 (ΔG‡ = 23.8 kcal/mol) is likely to be significantly higher
than the barrier for the concerted pathway to give 6a via
transition state 5 (ΔG‡ = 10.1 kcal/mol) (Scheme 2).
Reflecting its rigid annulated skeleton, the barrier for the
concerted pathway of 2a is higher than that of tetracene and
rubrene (ΔG‡ = 8.9 and 6.1 kcal/mol, respectively) and
therefore corroborates the experimental data. The high
photooxidative stability imparted by the thieno-annulated
framework of 2e is not completely explained by the concerted
mechanism, that is, ΔG‡ = 10.0 kcal/mol for the photo-
oxidation of 2e. This suggests that the thieno-annulated
framework might additionally induce a rapid nonradiative
decay3c that suppresses the photochemical generation of the
Experimental details, characterization data for all new
compounds, crystallographic data for 2a and 2d, and the
details of the computational studies (PDF)
Accession Codes
CCDC 1995329−1995330 contain the supplementary crys-
tallographic data for this paper. These data can be obtained
free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by
emailing data_request@ccdc.cam.ac.uk, or by contacting The
Cambridge Crystallographic Data Centre, 12 Union Road,
Cambridge CB2 1EZ, UK; fax: +44 1223 336033.
AUTHOR INFORMATION
Corresponding Author
■
Michihisa Murata − Department of Applied Chemistry, Faculty
of Engineering, Osaka Institute of Technology, Osaka 535-8585,
Japan; JST-PRESTO, Kawaguchi, Saitama 332-0012, Japan;
orcid.org/0000-0002-5479-2993;
Email: michihisa.murata@oit.ac.jp
Authors
Masahiro Togo − Graduate School of Engineering, Osaka
Institute of Technology, Osaka 535-8585, Japan
Daisuke Mishima − Graduate School of Engineering, Osaka
Institute of Technology, Osaka 535-8585, Japan
Ai Harada − Graduate School of Engineering, Osaka Institute of
Technology, Osaka 535-8585, Japan
1
•−
reactive species O2 and O2
.
C
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Letter
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The authors declare no competing financial interest.
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This research was partially funded by the Asahi Glass
Foundation, JSPS KAKENHI grant 16H04111, and start-up
funds from the Osaka Institute of Technology. The
synchrotron radiation experiments for 2a and 2d were
performed at the BL02B1 beamline of SPring-8 with the
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2018B1179, 2019A1677, 2019A1057, and 2019B1129). We
are grateful to Prof. T. Sasamori (Nagoya City University) for
his kind support and comments. This study was carried out
using NMR and MS spectrometers at the Joint Usage/
Research Center (JURC) at the ICR (Kyoto University).
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