Synthesis of Ring-Locked Tetracyclic Dithienocyclopentapyrans and Dibenzocyclopentapyran via 1,5-Hydride Shift and Copper-Catalyzed C-O Bond Formation for Nonfullerene Acceptors

Article abstract of DOI:10.1021/acs.orglett.1c00110

We discovered a unique synthetic route to construct 2H-pyran-containing tetracyclic dithienocyclopentapyran (DTCP) and dibenzocyclopentapyran (DBCP) architectures. The synthesis involves an acid-induced dehydration cyclization followed by a [1,5] hydride-shift isomerization to form a cyclopentanone moiety which was converted to the pyran-embedded tetracyclic products by a CuI-catalyzed intramolecular C-O bond formation in good yield. DTCP was used as a building block to prepare an acceptor-donor-acceptor (A-D-A) type n-type material DTCP-BC leading to a solar cell efficiency of 9.32%.

Full text of DOI:10.1021/acs.orglett.1c00110

pubs.acs.org/OrgLett
Letter
Synthesis of Ring-Locked Tetracyclic Dithienocyclopentapyrans and
Dibenzocyclopentapyran via 1,5-Hydride Shift and Copper-
Catalyzed C−O Bond Formation for Nonfullerene Acceptors
Tze-Gang Hsu, Ching-Li Huang, Wen-Ching Yin, Fong-Yi Cao, Chia-Wei Wang, Santosh K. Sahoo,
Shao-Ling Chang, Hsiao-Chieh Chou, and Yen-Ju Cheng*
Cite This: Org. Lett. 2021, 23, 1692−1697
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ABSTRACT: We discovered a unique synthetic route to construct 2H-pyran-containing tetracyclic dithienocyclopentapyran
(DTCP) and dibenzocyclopentapyran (DBCP) architectures. The synthesis involves an acid-induced dehydration cyclization
followed by a [1,5] hydride-shift isomerization to form a cyclopentanone moiety which was converted to the pyran-embedded
tetracyclic products by a CuI-catalyzed intramolecular C−O bond formation in good yield. DTCP was used as a building block to
prepare an acceptor−donor−acceptor (A−D−A) type n-type material DTCP-BC leading to a solar cell efficiency of 9.32%.
sistors.36−40 It is of substantial interest to implement the
lanarization and rigidification of a conjugated system have
Pbeen proven to extend the conjugation with red-shifted chemical planarization strategy in the TVT and PVP systems.
However, to our surprise, the carbon-bridge-ring-locked
dithienodihydropentalene (DTDP) has never been reported
and investigated presumably due to the challenging synthesis
(Scheme 1). Herein, we disclosed a unique synthetic route to
construct tetracyclic dithienocyclopentapyran (DTCP) and
dibenzocyclopentapyran (DBCP) architectures (Scheme 1)
where the TVT and PVP are covalently ring-locked by a
carbon and a carbon−oxygen bridge to form an endocyclic
olefin embedded between a CP and a pyran ring. The
diformylated DTCP moiety was coupled with 2-(3-oxo-2,3-
dihydro-1H-benzo[b]cyclopenta[d]thiophen-1-ylidene)-
malononitrile denoted as BC to form a low band gap n-type
A−D−A type DTCP-BC material for organic photovoltaics.41
The synthetic route is depicted in Scheme 2. In the presence
of titanium tetrachloride and a reducing agent of zinc powder,
3-bromothiophene-2-carbaldehyde 1a underwent McMurry
coupling reaction to afford the dimerized alkene (E)-1,2-
absorption and minimize the structural fluctuation to improve
charge mobility.1−8 Introduction of a bridge element to
covalently ring-lock the neighboring aryl or heteroaryl groups
connected by a single bond has led to a variety of fascinating
multifused ladder-type structures. As such, a single-element
bridge such as carbon, silicon, nitrogen ends up forming the
corresponding embedded five-membered cyclopentadiene
(CP),9−11 silole,12−19 and pyrrole20−22 rings. The carbon-
bridged ladder-type structures are particularly superior donor
(D) building blocks to make n-type acceptor−donor−acceptor
(A−D−A) type nonfullerene acceptors (NFA) for achieving
high-performance organic photovoltaics.23−29 More recently,
utilizing a two-element carbon−oxygen bridge leading to an
embedded six-membered 2H-pyran ring has also attracted
much interest considering that the electron-donating enolic
oxygen can further fine-tune steric and electronic proper-
ties.28−34 Incorporation of a compact trans-1,2-vinylene
(CHCH) linkage in the conjugated backbone can effectively
narrow the optical band gap and improve charge mobility due
to the facile π-delocalization and enhanced coplanarity.35 As a
result, thiophene−vinylene−thiophene (TVT) and phenyl-
ene−vinylene−phenylene (PVP, stilbene) and their derivatives
have been widely used as organic semiconducting materials for
organic light-emitting diodes and organic field-effect tran-
Received: January 12, 2021
Published: February 23, 2021
https://dx.doi.org/10.1021/acs.orglett.1c00110
© 2021 American Chemical Society
Org. Lett. 2021, 23, 1692−1697
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which was then swiftly isomerized to 5a-C1. The isomerization
involves the generation of a triaryl carbocation upon cleavage
of the O−C bond in 4a-C1 followed by an intramolecular
[1,5]-hydride shift from the hydroxyl carbon to the adjacent
triaryl carbocation to simultaneously form the ketone group in
5a-C1.42−44 The key intermediate 4a-C1 can be observed and
isolated when 3a-C1 was treated with HCl in acetic acid. 4a-C1
can be indeed converted to 5a-C1 under the identical
conditions, i.e. methanesulfonic acid in toluene, which verified
the proposed mechanism. Despite the fact that the acid-
induced cyclization of 3a-C1 resulted in the unexpected cyclic
ketone in 5a-C1, we proposed that 5a-C1 could further conduct
an intramolecular dehydrogenative cyclization to form a new
C−O single bond via a metal-catalyzed methodology, leading
to the tetracyclic DTCP (Table 1).
Scheme 1. Chemical Planarization of TVT and PVP
Skeletons To Form Dithienocyclopentapyran (DTCP) and
Dibenzocyclopentapyran (DBCP)
Scheme 2. Synthetic Route for DTCP, DTCP-C6, and
DBCP
Table 1. Optimization of Metal-Catalyzed Cyclization
a
Reaction
b
c
d
entry
catalyst
base
solvent
yield (%)
1
2
3
4
5
6
7
8
Pd(PPh3)2Cl2
Pd(PPh3)2Cl2
Pd(PPh3)2Cl2
Pd(OAc)2
CuI
CuI with TEMPO
CuCl
CuO
DBU
THF
THF
THF
THF
THF
THF
THF
THF
30
NaOEt
NaOAc
DBU
DBU
DBU
none
none
25
80
66
e
DBU
DBU
75
none
a
b
Reactions were completed within 15 min. Catalytic amount (10
c
d
mol %). Stoichiometric amount. Isolated yield after silica gel
chromatography. 1 equiv of TEMPO was added.
e
The screening of optimal reaction conditions is shown in
Table 1. At first, we found that palladium catalyst Pd-
(PPh3)2Cl2 or Pd(OAc)2 combined with 1,8-diazabicyclo-
[5.4.0]undec-7-ene (DBU) as the base successfully carried out
the reaction to form DTCP in 30% and 25% yield (entry 1 and
4), respectively. The structure of DTCP was unambiguously
confirmed by the single crystal X-ray diffraction shown in
Figure 1. Using bases such as NaOEt and NaOAc resulted in
failure of the reaction (entry 2 and 3), indicating that DBU is a
key component. Most importantly, we found that copper(I)
iodide and copper(I) chloride with DBU were able to catalyze
the reaction at room temperature in a higher yield of 80% and
75%, respectively, in just 15 min (entry 5 and 7). However,
copper(II) oxide failed to yield the product (entry 8). This
bis(3-bromothiophen-2-yl)ethene 2a. Lithiation of 2a by n-
butyllithium to react with 4, 4′-dimethylbenzophenone
afforded the bis(triaryl) alcohol 3a-C1. Initially, we envisaged
that 3a-C1 can carry out acid-induced double cyclization to
yield DTDP. Nevertheless, when 3a-C1 was treated with
methanesulfonic acid, an unexpected product, monocyclic
ketone 5a-C1, was obtained and its structure was confirmed by
1H and 13C NMR spectroscopy and mass spectrometry. The
plausible mechanism is depicted in Scheme 3. In the presence
of methanesulfonic acid, 3a-C1 first underwent a tandem
cyclization to a form a bicyclic 5−6 ring intermediate 4a-C1
Scheme 3. Proposed Mechanism of the Acid-Induced
Cyclization of 3a-C1
Figure 1. X-ray single crystal structure of DBCP, DTCP, and
compound 5b (50% probability for thermal ellipsoids).
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reaction is still valid in the presence of a radical scavenger
(2,2,6,6-tetramethylpiperidin-1-yl)oxyl (TEMPO), suggesting
that this reaction does not proceed via radical mechanism
(entry 6). The proposed mechanism of catalytic cycle for the
Cu(I)-catalyzed ring closure is thus depicted in Scheme 4.
Scheme 5. Synthesis of n-Type Material DTCP-BC
Scheme 4. Proposed Mechanism of the Copper-Catalyzed
Cyclization Reaction
Figure 2. (a) Normalized absorption spectra of DTCP-BC in
dichloromethane and thin film state, (b) energy levels of PBDB-T
polymer and DTCP-BC.
Table 2. Summary of the Intrinsic Properties of DTCP-BC
Deprotonation of 5a-C1 by DBU forms the enolate ion to yield
the copper-coordinated species A. The sp3-CH bond is
activated and deprotonated to form complex B which abstracts
a proton to yield complex C. This is the putative active species
that undergoes reductive elimination to afford the pyran-ring in
DTCP. The resulting copper(I) hydride reacts with the
protonated base to produce hydrogen and regenerate the
active copper(I) species.45,46
λmax (nm)
λonset
Egopt
HOMO LUMO
Egele
a
b
c
c
c
NFA
CH2Cl2 Film (nm)
(eV)
(eV)
(eV)
(eV)
DTCP-
BC
738 796 861
1.44
−5.34
−3.90
1.44
a
b
c
Calculated in the solid state. Egopt = 1240/λonset. Determined by
cyclic voltammetry.
In a similar pathway through dimerization and nucleophilic
addition, 1-bromo-2-benzaldehyde 1b can lead to 3b which
underwent the acid-induced cyclization followed by the [1,5]
hydride shift to form 5b (Scheme 2). The structure of 5b was
confirmed by single crystal X-ray diffraction unambiguously
(Figure 1). Finally, the copper-catalyzed intramolecular C−O
bond formation in 5b yielded the corresponding DBCP in 66%
yield. On the other hand, DTCP-C6 using the 4-hexylphenyl
group as the side chain can be also synthesized by using 3a-C6
as the starting material.
The absorption spectra of DTCP and DBCP are shown in
Figure S1. DTCP showed a significant bathochromic shift of
the absorption onset (λonset = 446 nm) compared to DBCP
(λonset = 387 nm). The larger Egopt of DBCP is caused by the
higher aromaticity of the benzene rings. Similar results are
observed in our previous works.47,48 The ring-locked molecules
exhibited greater red-shifted absorption than the nonfused
TVT and stilbene counterparts due to the enhanced rigidity
and the oxygen atom incorporation.
DTCP-C6 was further formylated by POCl3/DMF Vilsme-
ier−Haack reaction to afford DTCP-CHO (Scheme 5). Aldol
condensation of DTCP-CHO with BC41 formed a new A−D−
A type NFA DTCP-BC. DTCP-BC with four 4-hexylphenyl
side chains exhibited adequate solubility in common organic
solvents. DTCP-BC showed a high thermal decomposition
temperature (Td) of 346 °C determined by thermogravimetric
analysis (TGA). The absorption spectrum of DTCP-BC in
solution state and thin film are shown in Figure 2a. The optical
properties are listed in Table 2. DTCP-BC exhibited a
maximum absorption peak at 738 nm in CH2Cl2 solution
and a broad absorption band extending to near-infrared region
from 500 to 900 nm in thin film which is beneficial to
obtaining high short circuit current (Jsc) in photovoltaic
devices.
The oxidation and reduction potential of DTCP-BC were
measured by cyclic voltammetry (CV). The HOMO and
LUMO level were determined to be −5.34/−3.90 eV. The
energy level diagram of DTCP-BC and polymer donor PBDB-
T is depicted in Figure 2b. It should be emphasized that
DTCP-BC with a relatively short tetracyclic DTCP unit can
readily possess a very small band gap of 1.44 eV. The
endocyclic double bond promotes π-delocalization to facilitate
intramolecular charge transfer (ICT), while the enolic oxygen
enhances the electron-donating strength of the central DTCP
to upshift the HOMO energy level. Both factors dramatically
narrow the optical band gap of DTCP-BC.
The inverted solar cell devices using the ITO/ZnO/active
layer/MoO3/Ag configuration were fabricated. PBDB-T was
chosen as the p-type polymer in the active layer.49 The J−V
curve and the corresponding external quantum efficiency
(EQE) spectrum are depicted in Figure 3. The PBDB-
T:DTCP-BC (1:1 in wt %) device delivered a moderate
efficiency of 9.32% with an open circuit voltage (Voc) of 0.78
V, a high Jsc of 20.96 mA/cm2, and an FF (fill factor) of
56.99%. The Jsc is ascribed to the wide absorption spectrum of
DTCP-BC with two strong absorption bands in the longer
(796 nm) and shorter (325 nm) wavelength regions.
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Yang Ming Chiao Tung University, Hsinchu 30010, Taiwan;
Department of Applied Chemistry, National Chiao Tung
University, Hsinchu 30010, Taiwan
Ching-Li Huang − Department of Applied Chemistry and
Center for Emergent Functional Matter Science, National
Yang Ming Chiao Tung University, Hsinchu 30010, Taiwan;
Department of Applied Chemistry, National Chiao Tung
University, Hsinchu 30010, Taiwan
Wen-Ching Yin − Department of Applied Chemistry and
Center for Emergent Functional Matter Science, National
Yang Ming Chiao Tung University, Hsinchu 30010, Taiwan;
Department of Applied Chemistry, National Chiao Tung
University, Hsinchu 30010, Taiwan
Fong-Yi Cao − Department of Applied Chemistry and Center
for Emergent Functional Matter Science, National Yang
Ming Chiao Tung University, Hsinchu 30010, Taiwan;
Department of Applied Chemistry, National Chiao Tung
University, Hsinchu 30010, Taiwan
Chia-Wei Wang − Department of Applied Chemistry and
Center for Emergent Functional Matter Science, National
Yang Ming Chiao Tung University, Hsinchu 30010, Taiwan;
Department of Applied Chemistry, National Chiao Tung
University, Hsinchu 30010, Taiwan
Santosh K. Sahoo − Department of Applied Chemistry and
Center for Emergent Functional Matter Science, National
Yang Ming Chiao Tung University, Hsinchu 30010, Taiwan;
Department of Applied Chemistry, National Chiao Tung
University, Hsinchu 30010, Taiwan
Shao-Ling Chang − Department of Applied Chemistry and
Center for Emergent Functional Matter Science, National
Yang Ming Chiao Tung University, Hsinchu 30010, Taiwan;
Department of Applied Chemistry, National Chiao Tung
University, Hsinchu 30010, Taiwan
Hsiao-Chieh Chou − Department of Applied Chemistry and
Center for Emergent Functional Matter Science, National
Yang Ming Chiao Tung University, Hsinchu 30010, Taiwan;
Department of Applied Chemistry, National Chiao Tung
University, Hsinchu 30010, Taiwan
Figure 3. (a) J−V curve and (b) EQE spectrum of the PBDB-
T:DTCP-BC (1:1 in wt %) device.
In summary, new multifused tetracyclic dithienocyclo-
pentapyran (DTCP) and dibenzocyclopentapyran (DBCP)
architectures were developed. The symmetrical TVT- or
stilbene-based skeleton 3 substituted with bis-triaryl alcohols
underwent an acid-catalyzed ring closure followed by a [1,5]
hydride shift to form a cyclopentanone moiety in 5. CuI/DBU-
catalyzed intramolecular C−O bond formation in 5 efficiently
furnished the central bicyclic CP-pyran ring in the tetracyclic
products in good yield. The ring-locked molecules showed
significant red-shifted absorption spectra as a result of the
enhanced rigidity and oxygen incorporation. DTCP was used
as a donor to prepare a A−D−A type nonfullerene acceptor
DTCP-BC showing a low optical band gap of 1.44 eV. The
DTCP-BC-based solar cell device exhibited an efficiency of
9.32%. This research disclosed a facile and general protocol to
access a class of unprecedented CP-pyran ring-locked 1,2-
diarylvinylene derivatives. These new π-conjugated frameworks
can serve as promising building blocks for making high-
performance organic semiconducting materials. Further ma-
terial derivatization and device optimization are currently
underway in our laboratory.
ASSOCIATED CONTENT
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The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/acs.orglett.1c00110.
Complete contact information is available at:
https://pubs.acs.org/10.1021/acs.orglett.1c00110
Detailed synthetic and device fabrication procedures,
single-crystal X-ray crystallographic data, photophysical
properties, and NMR spectra (PDF)
Notes
Accession Codes
The authors declare no competing financial interest.
CCDC 2009366−2009368 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.
ACKNOWLEDGMENTS
■
This work is supported by Ministry of Science and
Technology, Taiwan (Grant Nos. MOST107-3017-F-009-003
and 107-2628-M-009-003-MY3) and Ministry of Education,
Taiwan (SPROUT Project-Center for Emergent Functional
Matter Science of National Yang Ming Chiao Tung
University).
AUTHOR INFORMATION
■
Corresponding Author
Yen-Ju Cheng − Department of Applied Chemistry and Center
for Emergent Functional Matter Science, National Yang Ming
Chiao Tung University, Hsinchu 30010, Taiwan;
Department of Applied Chemistry, National Chiao Tung
University, Hsinchu 30010, Taiwan; orcid.org/0000-
0003-0780-4557; Email: yjcheng@nctu.edu.tw
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