A versatile strategy to directly synthesize 4,8-functionalized benzo[1,2-b:4,5-b′]difurans for organic electronics

Article abstract of DOI:10.1039/c4ta06140c

A direct synthesis of 4,8-functionalized benzo[1,2-b:4,5-b′]difurans (BDFs) is developed. By fine-tuning the energy levels with different 4,8-functionalities or incorporating with electron-accepting units, BDFs show great potential as organic electronic materials, as demonstrated by 4.61% power conversion efficiency for polymer solar cells.

Full text of DOI:10.1039/c4ta06140c

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DOI: 10.1039/C4TA06140C
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ARTICLE TYPE
Versatile strategy to direct access 4,8ꢀfunctionalized benzo[1,2ꢀb:4,5ꢀ
b’]difurans for organic electronics
Linyi Bian, Jiefeng Hai, Enwei Zhu, Jiangsheng Yu, Yun Liu, Jie Zhou, Guidong Ge and Weihua Tang*
Received (in XXX, XXX) Xth XXXXXXXXX 20XX, Accepted Xth XXXXXXXXX 20XX
5
DOI: 10.1039/b000000x
The direct access to 4,8ꢀfunctionalized benzo[1,2ꢀb:4,5ꢀ
for substrates and always accompanied by low yields; ii) larger
b′]difurans (BDFs) is developed. By fineꢀtuning the energy 50 fused aromatic substituents could hardly be incorporated onto the
levels with different 4,8ꢀfunctionalities or incorporating with
electronꢀaccepting units, BDFs show great potential as
organic electronic materials, as demonstrated with 4.61%
power conversion efficiency for polymer solar cells.
benzene core due to steric hindrance.
In light of the everꢀincreasing interest in developing 2ꢀ
dimensional (2D) aromatic fused ring structure to fine tune the
energy levels and facilitate regular πꢀπ stacking of conjugated
molecules and polymer backbones, our group have recently
developed an efficient synthetic route for the direct access to 4,8ꢀ
functionalized BDT monomers by using 4,8ꢀtriflated BDT as the
1
0
5
6
6
5
0
5
Organic semiconductors are gaining considerable interest because
1ꢀ4
of their flexibility and easy structural modifications. As one of
the most important fusedꢀring structures, benzo[1,2ꢀb:4,5ꢀ
b']dithiophene (BDT) is most commonly explored as πꢀelectron
donors in organic electronics such as organic field effect
transistors, organic lightꢀemitting diodes as well as dyeꢀsensitized
17
key prescusor.
However, the ringꢀclosure formation of
precursors BDT/BDFꢀ4,8ꢀdione need undergo harsh reactions
conditions with low yields.
1
2
2
3
3
4
4
5
0
5
0
5
0
5
In this work, we have herein present a general method for the
direct access of 4,8ꢀfunctionalized BDF building blocks under
mild reaction conditions (Scheme 1). By adopting the acidꢀ
catalysed furan arylation, 4,8ꢀdibromobenzo[1,2ꢀb:4,5ꢀb′]difuran,
demonstrates its versatility for 4,8ꢀfunctionalization of BDF to
manipulate the energy levels of the resulted BDF monomers. This
paved the way for further bandgap engineering of BDFꢀbased Dꢀ
A copolymers.
5
,6
and bulk heterojunction (BHJ) solar cells. For example, BDTꢀ
based singleꢀlayer BHJ solar cells achieved a landmark power
6
f
conversion efficiency of 9.2%. The BDTꢀbased small molecule
2
ꢀ1 ꢀ1
exhibited a hole mobility up to 0.81 cm V s in single crystal
5
transistor.
In comparison to thiopheneꢀbased BDT, benzo[1,2ꢀb:4,5ꢀ
b′]difuran (BDF) has attracted much less attention for organic
electronics. However, BDF counterparts can harvest deeper
HOMO and LUMO levels due to the smaller diameter but higher
electronegativity of oxygen atom has than sulfur atom. BDFꢀ
based donorꢀacceptor (DꢀA) type copolymers have shown tighter
packing and smaller reorganization energy than their BDT
counterparts, which could lead to higher charge mobility and
smaller energy bandgap in coupling with higher HOMO energy
7
levels. Furthermore, furan based materials also show improved
solubilities, weaker steric hindrance, biodegradable and
8,9
biorenewable properties. Several BDF DꢀA copolymers were
reported to deliver PCEs of 4~6% in BHJ solar cells when used
8
ꢀ12
as donors.
New interests in constructing structural hybrid of
BDF and BDT, i.e., thieno[2,3ꢀf]benzofuran (TBF), even emerge
1
3
for their promising photovoltaic properties. BDF unit has thus
been regarded as an attractive building block in constructing DꢀA
conjugated systems for various optoelectronic applications.
To date, the 4,8ꢀfunctionalization of BDF has been explored by
adopting the similar approaches as for BDT with its 4,8ꢀdione as
14,15
the precursor.
Starting from benzo[1,2ꢀb:4,5ꢀb']difuranꢀ4,8ꢀ 70 Scheme 1. Synthetic route to 4,8ꢀfunctionalized BDF monomers with 4,8ꢀ
dibromobenzo[1,2ꢀb:4,5ꢀb′]difuran as a versatile synthon.
dione, BDF monomers with alkyl, alkoxy, and aromatic ring to
substituted onto the benzene core were developed as donor units
1
7ꢀ21
A threeꢀstep protocol starting from hydroquinone facilely
afforded the key synthon, 4,8ꢀdibromobenzo[1,2ꢀb:4,5ꢀb′]difuran
(dibromo BDF), with an overall yield of 32% (see Electronic
for DꢀA polymers.
However, the reported methods have
certain drawbacks: i) the methods employing either nꢀ
15
16
BuLi/SnCl2 and Grignard reagents have quite limited options
This journal is © The Royal Society of Chemistry [year]
[journal], [year], [vol], 00–00 | 1

Journal of Materials Chemistry A
Page 2 of 5
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DOI: 10.1039/C4TA06140C
Supplementary Information (ESI)). As illustrated, 1,4ꢀdibromoꢀ
,5ꢀbis(2,2ꢀdiethoxyethoxy)benzene 2 was synthesized using
Williamsonꢀetherification between 2,5ꢀdibromobenzeneꢀ1,4ꢀdiol
and 2ꢀbromoꢀ1,1ꢀdiethoxyethane with an overall yield beyond
80%. A further cyclocondensation of 2 using polyphosphoric acid
PPA) as acid catalyzed agent afforded 4,8ꢀdibromobenzo[1,2ꢀ
range of 2.58~3.50 eV (Table 1).
2
1
5
0
5
0
5
0
5
0
5
0
5
(
b:4,5ꢀb′]difuran 3 with 60% yield. The replacement of PPA with
Amberlystꢀ15 resin to increase the yield didn’t work as expected,
18
probably due to the slow reaction kinetics and the aromaticity.
1
1
2
2
3
3
4
4
5
5
With dibromo BDF 3 at hand, a wide variety of 4,8ꢀ
functionalized BDF monomers could thus be readily be prepared
using palladiumꢀcatalyzed Stille and Sonogashira coupling as
well carbonꢀsulfur formation reactions at high yields (≥ 86%).
Dialkylthio substituted BDFs were prepared via straightforward
carbonꢀsulfur bond formation reaction, which was previously
60
Fig. 1 (a) Normalized absorption spectra of BDF monomers in CHCl
solutions, (b) cyclic voltammogram of BDF monomers in solutions.
3
When comparing the λmax values of three TIPSꢀethynylꢀ
substituted benzodichalcogenophenes, we can find that BDFꢀ
afforded via highꢀtemperature NewmanꢀKwart rearrangement 65 TIPS exhibits more blueꢀshifted λmax (351 nm) than TBFꢀTIPS
1
9
13
29
reaction using irritant reagents.
(366 nm) and BDTꢀTIPS (384 nm). This indicated that the
lower energy transition was significantly shifted with the
increasing number of heavier chalcogen atoms, probably due to
the electronegativity of the heteroatom (O, 3.5; S, 2.5) playing a
To demonstrate the versatility of this approach, both electron
donating (alkylthio, aromatic ring) and accepting units
(triisopropylsilylethynyl) were explored for the 4,8ꢀ
1
3
functionalizatin of BDF. The Still crossꢀcoupling between 70 fundamental role in the absorption maxima location. Compared
17
dibromo BDF and 2ꢀdodecylꢀ5ꢀtributyltinthiophene or 2ꢀ
the absorption peak of BDFꢀS with that of BDTꢀS, obvious redꢀ
shift from 308 nm to 355 nm were found for their chloroform
solutions. Similar phenomenon was also found for the optical
2
0
dodecylthioꢀ5ꢀtributyltinthiophene
proceeded efficiently to
afford new BDF monomers 6 (BDFT) and 7 (BDFTꢀS) in 90%
and 87% yield, respectively. The dibromo BDF was also explored
for Sonogashira coupling with (triisopropylsilyl)acetylene in the 75 incorporated into the benzodichalcogenophenes, the more redꢀ
17
spectra of BDFT and BDTT films. The more S atoms were
presence of bis(triphenylphosphine) palladium(II) dichloride and
cuprous iodide as catalyst to afford triisopropylsilyl ethynylꢀ
substituted 8 (BDFꢀTIPS) with a yield of 90%. The dialkylthio 9
shift absorption.
Table 1 Optical and electrochemical properties of BDF monomers
2
0
UVꢀvis
CV
(BDFꢀS) was easily prepared via C–S crossꢀcoupling under the
catalysis of Pd (dba) and DPPF (1,1'ꢀ
bis(diphenylphosphino)ferrocene) ligand with 95% yield.
opt
ox
λ
max
λ
onset
λ
max
λ
onset
E
g
E
onset
HOMO LUMO
2
3
a
a
b
b
c
d
e
(
nm)
371
382
3
308
6
7
7
6
9
8
7
9
5
(nm) (nm)
(nm) (eV)
419 2.96
480 2.58
369 3.36
354 3.50
973 1.27
(V)
(eV)
(eV)
BDFT
414
474
367
351
905
375
388
351
312
709
1.36
1.42
1.35
1.28
0.71
ꢀ5.76 ꢀ2.80
ꢀ5.82 ꢀ3.24
ꢀ5.75 ꢀ2.39
ꢀ5.68 ꢀ2.18
ꢀ5.11 ꢀ3.84
The absorption spectra of BDF monomers in chloroform
solution are plotted in Figure 1 (thin films shown in Fig. S1). In
solution, four BDF monomers BDFT, BDFTꢀS, BDFꢀTIPS and
BDFꢀS showed an absorption peak (λmax) at 371 nm, 382 nm, 348
nm and 308 nm, respectively (Table 1). BDFꢀTIPS and BDFꢀS
displayed two shoulder peaks at 331 nm and 321 nm. In solid
state, BDF monomers displayed similar absorption profiles but
extended absorption, indicating the increased intermolecular
interactions and aggregation. The redꢀshifted absorption spectra
followed the order of BDFꢀS<BDFꢀTIPS<BDFT<BDFTꢀS.
BDFTꢀS
BDFꢀTIPS
BDFꢀS
48
93
12
01
96
02
51
95
45
59
P1
P2
P3
P4
P5
P6
P7
P8
951
906
929
ꢀ
744
708
715
945
947
853
981
570
b
1016 1.22
942 1.32
970 1.28
0.69
0.78
0.75
0.47
0.60
0.76
0.62
0.91
ꢀ5.09 ꢀ3.87
ꢀ5.18 ꢀ3.86
ꢀ5.15 ꢀ3.87
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ4.87
ꢀ5.00
ꢀ5.16
ꢀ5.02
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
A close look at the optical results of BDFꢀTIPS and BDFꢀS,
one would find that the introduction of a TIPSꢀethynyl substituent
to BDF core instead of alkthio leads to a ~40 nm redꢀshifted
absorption spectra, mainly due to the expanded conjugated
system and the electronꢀwithdrawing effect of TIPSꢀethynyl
group. In comparison to BDFꢀS and BDFꢀTIPS, BDFTꢀS and
BDFT show more redꢀshifted absorption, suggesting the
enhanced electron delocalization over 2D conjugated BDF
moieties when substituted with aromatic functionalities. BDFTꢀS
exhibited ~11 nm redꢀshifted absorption than BDFT when
substituted BDF with alkylthiothienyl instead of alkylthienyl
group. As known, the longer wavelength domain corresponds to
the πꢀπ* transition, dominated by the electron excitation from
HOMO to LUMO. Based on onset of the lowest energy
666
730 1.70
ꢀ5.31 ꢀ3.61
P9
a
c
opt
d
absorption in solution, absorption in film, E
g
= 1240/λonset, HOMO
ox
e
opt
=
g
ꢀe(Eonset + 4.4) (eV), LUMO = HOMO + E .
80
Electrochemical properties of the asꢀprepared BDF monomers
were investigated by cyclic voltammetry (Fig. 1b). All BDF
monomers underwent an irreversible oxidation, similar to TBF
1
3,17
and BDT monomers.
(
The onset potential for oxidation
ox
Eonset ) was observed to be 1.36, 1.42, 1.35 and 1.28 V for
85
BDFT, BDFTꢀS, BDFꢀTIPS and BDFꢀS, respectively. From
ox
Eonset , the HOMO levels of derivatives were thus calculated
(Table 1). The LUMO levels of BDF monomers were further
determined in the range of ꢀ5.82 ~ ꢀ5.68 eV with their
opt
2
1
corresponding Eg and HOMO energy levels. A close look at the
absorption bands (λonset) of the films, the corresponding optical
opt
90 data for BDFT and BDFTꢀS in Table 1, the 4,8ꢀfunctionalization
of BDF core with alkylthienyl and alkylthiothienyl substituent
bandgap (Eg ) of BDF monomers was estimated to be in the
2
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DOI: 10.1039/C4TA06140C
opt
proved to be more effective in lowering both LUMO and HOMO
energy levels, due to the incorporated aromatic moieties
enhanced the planarity to induce more effective πꢀelectron
delocalized conjugated systems. Compared with BDFT,
nm in films (Fig. S3), with Eg calculated to be range from 1.22
eV to 1.32 eV. BTQxꢀbased polymers P5ꢀP8 showed a typical
twoꢀabsorptionꢀband profile (Fig. S4), i.e., one at 300~600 nm
and the other extending from 600 nm to the nearꢀinfrared region
alkylthioꢀsubstituted BDFTꢀS maintained even lower energy 45 (>1100 nm), which endowed extremely narrow Eg values (≤
opt
5
levels and wider absorption region, due to the electronꢀdonating
effect of alkylthio group and enlarged conjugation.
Compared with BDFꢀS, BDFꢀTIPS displayed both lower
LUMO and HOMO energy levels, which indicated increased
1.13 eV) for P5ꢀP8. TTzꢀbased P9 exhibited one intensified
absorption band at 400~650 nm. All polymers exhibited HOMO
levels ranging from ꢀ4.87 to ꢀ5.31 eV and LUMO levels from ꢀ
3.61 to ꢀ3.87 eV. As shown, the bandgaps as well and molecular
1
1
2
2
0
5
0
5
conjugation length and planarity were induced with the 50 electronic energy levels of BDFꢀcored polymers are readily tuned
introduced
triple bonds. For
TIPSꢀethynylꢀsubstituted
by copolymerizing with accepting units with different electronꢀ
withdrawing capability, with TTz<DPP<BTQx ranked in our case.
benzodichalcogenophene anologues, the HOMO level increased
in the order of BDF (ꢀ5.75 eV) < TBF (ꢀ5.70 eV)<BDT (ꢀ5.58
eV), as a result of higher electronegativity of oxygen atom.
10,13
Deeper HOMO level were also observed for BDF monomers,
when comparing dialkylthio substituted BDFꢀS (ꢀ5.68 eV) with
17
BDTꢀS (ꢀ5.41 eV) and BDFT (ꢀ5.76 eV) with BDTT (ꢀ5.42).
To explore these energy level finelyꢀtuned BDF monomers for
optoelectronic applications, BDFꢀbased DꢀA copolymers were
4
,22
prepared by alternating with diketopyrrolopyrrole (DPP),
2
3
thiadiazolo[3,4ꢀg]quinoxaline (BTQx)
and thiazolo[5,4ꢀ
24
d]thiazole (TTz) via Still coupling reaction (Scheme 2). DPP
and TTz were selected due to their moderate electroꢀwithdrawing
ability to build narrow bandgap conjugated materials (ca, 1.5~2.0
eV), while stronger accepting BTQx unit have been explored for
organic semiconductors with extremely low bandgaps (ca.
2
5
1
.0~1.5 eV) for near infrared optoelectronics.
C
8
H
17
N
O
Br
Br
S
S
C
8
H
H
17
O
N
N
O
O
O
Sn
Sn
C
8
H
17
S
S
O
Pd2(dba)3, P(o-tol)3
toluene, reflux
O
n
O
N
C
8
17
10-13
S
P1-P4
N
N
N
Br
n-BuLi
Me SnCl
78o
S
3
Br
S
N
S
-
C
N
N
S
12
C H
25
O
OC12
H
25
R
O
C
12
H
25
O
OC12
H
25
S
n
O
O
N
N
Pd
2 3 3
(dba) , P(o-tol)
Fig. 2 Normalized absorption spectra of (a) P1ꢀP4, (c) P5ꢀP8 and (e) P9
O
toluene, reflux
C
12
H
25
O
OC12
H
25
55
3
in CHCl solutions. CV traces of (b) P1ꢀP4, (d) P5ꢀP8 and (f) P9.
R
6-9
C
12
H
25
O
OC12
H
25
P5-P8
C
12
H
25
The photovoltaic potential of BDF polymers were explored
using the conventional singleꢀlayer device with a configuration of
ITO/PEDOT:PSS/P9:PCBM (1:2, w/w)/LiF/Al (Table 2 and Fig.
C
12
H
25
S
C12H25
Si
=
S
S
S
6
, 10, P1, P5
7, 11, P2, P6
8, 12, P3, P7
9, 13, P4, P8
OC20
H
41
Si
3
). Blend films of both P9:PC BM and P9:PC BM spinꢀcoated
61 71
S
N
S
S
Br
Br
C
20
S
H
41
O
S
N
60 from the corresponding chlorobenzene solutions were acted as the
photoactive layers. The preliminary study has revealed that
P9:PC BM device delivered power conversion efficiencies
N
S
O
S
20
41
C H O
12
S
O
N
n
Pd (dba) , P(o-tol)
2
3
3
, toluene, reflux
OC20H
41
6
1
Si
2
P9
(PCEs) up to 2.29% under AM1.5 illumination (100 mW/cm ).
And P9:PC BM device exhibited a maximum PCE of 2.63%.
The devices were further optimized by spinꢀcoating P9:PC BM
71
71
Scheme 2 Synthetic route to BDFꢀbased DꢀA polymers.
With BDF tin reagents 10ꢀ13 (Scheme 2) in hand, structurally
wellꢀdefined alternating polymers P1ꢀP9 were synthesized for
BHJ solar cell application. All polymers showed excellent
solubility in common organic solvents and exhibited relatively
6
7
7
5
0
5
solution with the addition of 5% DIO. The best devices delivered
3
3
4
0
5
0
2
a PCE of 4.61%, with a Voc of 0.80 V, a J of 9.81 mA/cm and a
sc
FF of 58.7%. The photovoltaic performances of BDT counterpart
polymer solar cells delivered a highest PCE of 4.33%, with a J
sc
2
26
of 9.77 mA/cm , a V of 0.89 V, a FF of 49.8%. The higher FF
oc
high molecular weights (M = 9.6~41.7 KDa) (Table S1). The
n
may attribute to the smaller diameter but higher electronegativity
of oxygen atom. Our model BDF polymers are very attractive for
photovoltaic applications. The relatively lower Jsc and FF, we
believe, can be optimized with device engineering. The
characterization of photovoltaic properties for polymers P1ꢀP8
with stronger accepting units (BTQx and DPP) is in progress and
thermogravimetric analysis (TGA) revealed the excellent thermal
stability of polymers P1ꢀP9, with T (temperature correspond to 5%
d
o
weight loss) ranging from 320°C to 423 C (Fig. S2).
The BDF polymers P1ꢀP9 exhibited strongly acceptorꢀ
dependent photophysical and electronic properties (Table 1 and
Fig. 2). DPPꢀbased P1ꢀP4 exhibited strong absorption in 550~850
This journal is © The Royal Society of Chemistry [year]
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Page 4 of 5
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DOI: 10.1039/C4TA06140C
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4
4
5
5
6
6
7
7
8
8
9
9
0
5
0
5
0
5
0
5
0
5
0
5
3
2
Device
P9:PC61BM
V
OC (V)
J
SC (mA/cm )
6.12
FF(%) PCE (%)
0.83
45.4
43.8
58.7
2.29
2.63
4.61
P9:PC71BM
0.80
7.51
P9:PC71BM (with 5% DIO)
0.80
9.81
2
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5
Fig. 3 JꢀV characteristics of ITO/PEDOT:PSS/P9:PCBM (1:2,
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1
(a) X. Chen, B. Liu, Y. Zou, L. Xiao, X. Guo, Y. He, Y Li. J. Mater.
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2
w/w)/LiF/Al devices under the irradiation of AM 1.5 G (100 mW cm ).
In conclusion, we present a versatile synthetic strategy for 4,8ꢀ
functionalized BDF building blocks using 4,8ꢀdibromobenzo[1,2ꢀ
b:4,5ꢀb′]difuran as a versatile synthon. A platform is provided for
feasible lateꢀstage BDF derivatization. The chalcogenꢀproperty
correlation study reveals a narrowing bandgap and lowing energy
levels on going from sulfur to oxygen. BDFꢀbased polymers with
fineꢀtuned absorption and energy levels exhibit promising
potential for solar cells with a PCE of 4.61% demonstrated in
initial testing. As a consequence, the present work provides an
insight on the structureꢀproperty relationship of BDF based
conjugated materials and opens an avenue to develop new library
of extended πꢀconjugated BDF molecules/polymers with high
structural diversity for optoelectronic applications.
This work is supported by the National Natural Science
Foundation of China (Grant No. 21074055), Program for New
Century Excellent Talents in University (NCETꢀ12ꢀ0633),
Jiangsu Province Natural Science Fund for Distinguished Young
Scholars (BK20130032), Doctoral Fund of Ministry of Education
of China (No. 20103219120008), and the Fundamental Research
Funds for the Central Universities (30920130111006).
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Nanjing 210094, China. Fax: +86 25 8431 7311; Tel: +86 25 8431 7311;
Eꢀmail: whtang@mail.njust.edu.cn
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DOI: 10.1039/C4TA06140C
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Q. Shi, H. Fan, Y. Liu, W. Hu, Y. Li, X. Zhan. Macromolecules
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Table of content
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Versatile strategy to direct access 4,8ꢀfunctionalized benzo[1,2ꢀb:4,5ꢀb’]difurans for organic
electronics
Linyi Bian, Jiefeng Hai, Enwei Zhu, Jiangsheng Yu, Yun Liu, Jie Zhou, Guidong Ge and Weihua Tang
1
0
Direct access to 4,8ꢀfunctionalized benzo[1,2ꢀb:4,5ꢀb′]difurans was developed for organic electronics, with 4.61% power conversion
efficiency for polymer solar cells.
1
5
This journal is © The Royal Society of Chemistry [year]
Journal Name, [year], [vol], 00–00 | 5