Direct access to 4,8-functionalized benzo[1,2-b:4,5-b′]dithiophenes with deep low-lying HOMO levels and high mobilities

Article abstract of DOI:10.1039/c4ta01226g

A general methodology has been proposed for the straightforward access to 4,8-functionalized benzo[1,2-b:4,5-b′]dithiophenes (BDTs) via Pd mediated coupling reactions including Suzuki-Sonogashira coupling and carbon-sulfur bond formation reactions. This versatile platform can be used to construct a library of BDT core centred conjugated systems, featuring large fused-ring structure and good charge mobility, where a hole mobility of 0.061 cm² V ^-1 s^-1 is demonstrated. With the energy level fine-tuned with functionalization, the charge transporting BDTs show great potential for donor-acceptor polymers. This journal is the Partner Organisations 2014.

Full text of DOI:10.1039/c4ta01226g

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Direct access to 4,8-functionalized benzo[1,2-
b:4,5-b0]dithiophenes with deep low-lying HOMO
levels and high mobilities†
Cite this: J. Mater. Chem. A, 2014, 2,
13580
Enwei Zhu,a Guidong Ge,a Jingkun Shu,b Mingdong Yi,b Linyi Bian,a Jiefeng Hai,a
a
a
a
a
*
Jiangsheng Yu, Yun Liu, Jie Zhou and Weihua Tang
A general methodology has been proposed for the straightforward access to 4,8-functionalized benzo[1,2-
b:4,5-b0]dithiophenes (BDTs) via Pd mediated coupling reactions including Suzuki–Sonogashira coupling
and carbon–sulfur bond formation reactions. This versatile platform can be used to construct a library of
BDT core centred conjugated systems, featuring large fused-ring structure and good charge mobility,
where a hole mobility of 0.061 cm2 Vꢀ1 sꢀ1 is demonstrated. With the energy level fine-tuned with
functionalization, the charge transporting BDTs show great potential for donor–acceptor polymers.
Received 12th March 2014
Accepted 15th June 2014
DOI: 10.1039/c4ta01226g
www.rsc.org/MaterialsA
To date, benzo[1,2-b:4,5-b0]dithiophene (BDT) has been proved building blocks, and a large spectrum of BDT derived donors
to be one of the most important fused-ring structured building have been developed since then by introducing various
blocks in the design of conjugated polymer donors for bulk substituents including alkyl, alkylthienyl, and alkyl phenyl-
heterojunction (BHJ) solar cells due to its high mobility and ethynyl to the benzene core of BDT.3b,10 Particularly, the ethy-
lower highest occupied molecular orbital (HOMO) level.1–4 As a nylene and thienyl-substituted BDT derived linear or two-
donating moiety, BDT has been chosen for the following merits: dimensional (2D) small molecules and conjugated polymers
(a) the fused BDT ring structure allows the incorporation of exhibited promising performance in BHJ solar cells. For
substituents on the central benzene core, while also maintain- example, small molecules and polymer based ethynylene-con-
ing the planarity of the two thiophene units; (b) its structural taining benzo[1,2-b:4,5-b0]dithiophene derivatives displayed low
symmetry and the rigid fused aromatic system could enhance HOMO levels and an excellent hole mobility up to 1.1–1.2 cm2
the electron delocalization and interchain interactions to Vꢀ1 sꢀ1 (on/off current ratio ꢁ 107) in eld effect transistors
improve the charge mobility and eliminate the need to control (FETs).2b,d The blend of the 4,8-triisopropylsilylethynyl func-
the regioregularity during the polymerization process;5 (c) it tionalized BDT polymer and PC71BM in 1 : 2 weight ratio
would maintain a low HOMO energy level of the resulting delivered a power conversion efficiency (PCE) of 6% in BHJ
polymers as demonstrated by donor-strong and acceptor-weak
polymers.6 Kobmehl et al. rst synthesized 4,8-dimethoxybenzo
[1,2-b:4,5-b0]dithiophene with benzo[1,2-b:4,5-b0]dithiophene-
4,8-dione as the precursor using a two-step reaction protocol
(Scheme 1).7 Several extended methods were then explored for
the synthesis of alkyl, alkenyl and aromatic ring substituted
BDT derivatives.8,9
The BDT derived polymers were rst developed as donors for
BHJ solar cells by Hou et al. in 2008.9 In the meantime, many
research groups showed strong interest in these fascinating
aKey Laboratory of So Chemistry and Functional Materials, Nanjing University of
Science and Technology, Nanjing 210094, China. E-mail: whtang@njust.edu.cn;
Fax: +86 25 8431 7311; Tel: +86 25 8431 7311
bKey Laboratory of Organic Electronics and Information Displays, Institute of Advanced
Materials, Nanjing University of Posts and Telecommunications, Nanjing 210046,
China. E-mail: iammdyi@njupt.edu.cn
† Electronic supplementary information (ESI) available: Details of synthesis and
characterization data as well as both 1H and 13C NMR spectra of all products.
See DOI: 10.1039/c4ta01226g
Scheme 1 Synthetic routes for BDT derivatives in the literature.
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cells.11 A PCE of 8.13% was even achieved from 2D small can be successfully extended for the synthesis of conjugated
molecules with a bithienyl-substituted BDT donor, which per- polymers. This effective strategy provides a versatile platform to
formed much better than its thiophene-substituted BDT construct 2D conjugated systems centred on the BDT core,
analogues.12 The open circuit voltage (Voc) is directly propor- featuring large fused-ring structure and good planarity. The new
tional to the HOMO level of the electron donor.13 The lower the BDT derivatives developed with this approach, we believe, will
HOMO level of the donor, the higher the Voc. Thus, the high nd great potential in the design of new molecular/polymeric
mobility and low HOMO level are prerequisites to achieve high- semiconductors for organic electronics.
performance solar cells.6a,13,14
As shown in Scheme 2, 4,8-dihydrobenzo[1,2-b:4,5-b0]
To achieve air-stable and high charge-transporting donor dithiophene-4,8-dione 1 was readily synthesized according to
materials, the design and preparation of 2D BDTs by attaching the reported procedure with thiophene-3-carboxylic acid as the
conjugated substituents onto the central benzene core to ne- starting material.9 A reduction of 1 with sodium borohydride in
tune the energy levels and facilitate regular p–p stacking of ethanol afforded 2 in 99% isolated yield under heating at
conjugated molecules and polymer backbones is considered to 85 ꢂC.18 Ensuing treatment of 2 with Tf2O and pyridine in
be a critical issue in the development of high-performance solar CH2Cl2 provided the key intermediate, 4,8-bis(triuoro
cells.12,15 However, the previous methods have certain draw- methanesulfonyloxy)benzo[1,2-b:4,5-b0]dithiophene 3, in 85%
backs: (i) the synthesis of BDT derivatives with the n-BuLi/SnCl2 yield. It should be noted that reaction with DMF as the solvent
method has a limited scope for the substituents that may be instead of CH2Cl2 was not successful. The structure of the as-
introduced;9 (ii) the use of the Grignard method16 for the obtained 3 was conrmed by the 13C NMR spectrum (Fig. S4†).
preparation of acetylene-substituted BDTs instead of lithiation The coupling constant of the characteristic peak (d
¼
only work for limited phenyl substituents with low yields; (iii) 118.6 ppm) corresponding to triuoromethanesulfonyloxy
larger fused aromatic substituents like thio[3,4-b0]thiophene groups was shown to be 319 Hz, which was in good agreement
could not be accessed with any methods in the literature, with the literature.17b With the key precursor triate 3 at hand,
probably due to steric hindrance.
Suzuki cross-coupling between 3 and aryl boronic acids or esters
In conjunction with the development of new donors for solar (Table 1, entries 1–10) proceeded smoothly in the presence of
cells, we are keen to search for more general synthesis Pd(PPh3)4 and Na2CO3 to afford a new library of 4,8-diaryl-BDT
approaches to expand novel p-extended BDT-based donor monomers in 50–90% yields. The scope of thiophene analogues
materials. Herein we report a versatile method (Scheme 2) for was extended from a single thiophene unit to fused thio[3,4-b0]-
the synthesis of a library of amazing compounds using 4,8-tri- thiophene and bithiophene. The coupling reaction with 2-
ated BDT as the precursor for the rst time. Aryl triates have thienylboronic acid afforded 4 with a yield of 80% (entry 1),
frequently been used as electrophiles in coupling reactions.17 By while the couplings with 5-(trimethylsilyl)-2-(4,4,5,5-tetra-
introducing triuoromethanesulfonyloxy groups on 4,8-posi- methyl-1,3,2-dioxaborolan-2-yl)-thiophene,14 5-(octyl)-2-(4,4,5,5-
tions of BDT as leaving groups, a library of 4,8-functionalized tetramethyl-1,3,2-dioxaborolan-2-yl)-thiophene and 5-(dodecyl-
BDT derivatives have been successfully prepared via Pd medi- thio)-2-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)-thiophene19a
ated coupling reactions including Suzuki and Sonogashira were explored to prepare 5–7 (entries 2–4) with ꢁ70% yields.
coupling. Importantly, the straightforward access to dialkylthio
To the best of our knowledge, the larger the conjugated ring
BDTs can be easily achieved with high yields ($90%) through directly linked to the core in the cross-coupling, the more
one-step palladium-catalyzed coupling reaction. Dialkylthio difficult it is to achieve it. Hence, the coupling with fused thio
substituted BDTs were reported to be synthesized with a two- [3,4-b0]thiophene (TT)19 and bithiophene gave 8 and 9 (entries
step protocol, where a Newman–Kwart rearrangement under 5–6) in relatively low yields (55% and 60%). It should be noted
245 ꢂC heating is the key step.18 Excitingly, the Suzuki coupling that 8 was rst prepared with BDT triate, since the incorpo-
ration of a TT ring using n-BuLi was found to be inaccessible.
The results indicated the applicability of this protocol for the
incorporation of relatively large substituents to 4,8-positions of
BDTs. We extended this approach to other aromatic rings. For
instance, the coupling with 2-furanboronic acid afforded 10
(entry 7) in 81% yield. And the synthesis of BDTs with phenyl or
its analogues with uorine and triuoromethyl substituents
(11–13) was successfully achieved with a yield of 90, 81 and 82%
(entries 8–10), respectively. It should be noted that aryl boronic
acids were more reactive than the corresponding Bpin esters in
Suzuki cross-coupling reactions and the electron-withdrawing
strength of substituents had a positive effect on the yield of
couplings with phenyl analogues.
The BDT triate was also explored for Sonogashira cross-
coupling reactions with acetylene substituents (Scheme 3). The
reaction proceeded smoothly with 10 mol% [PdCl2(PPh3)2] and
20 mol% CuI as catalysts in the DMF–Et3N (2 : 1, v/v) solvent
Scheme 2 Access to 4,8-difunctionalized BDT monomers/polymers
via BDT triflate involved Suzuki coupling.
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Table 1 Coupling reactions between BDT triflate 3 and different
condition of excess base, however, the coupling with trime-
thylsilylacetylene only gave 14 as the product with a yield of
55%.
coupling reagentsa
Entry
Coupling reagent
Product
T (oC)
85
Yield (%)
We were pleased to note that our protocol can also be used
for the preparation of BDT thiols from the alkyl/aryl thiol
substrates with BDT triate (entries 14–16). The BDT suldes
17–19 (Scheme 3) were successfully prepared via transition-
metal-catalyzed cross-coupling transformation using Pd2(dba)3
and the Xantphos phosphine ligand without formation of their
disulde compounds.20 Octylmercaptan, dodecylmercaptan
and benzylmercaptan were S-arylated in excellent yields of 90%,
91% and 90%, respectively.
1
2
4
5
80
70
85
3
4
6
7
85
85
68
69
We are also interested in the developing protocols that allow
the preparation of conjugated polymers from BDT triate 3 and
aryl boronic ester. Fluorene is the most frequently used moiety
for organic electronics.14,21 Hence, a Suzuki cross-coupling
between BDT triate 3 and uorene diboronic ester was
explored in a biphasic mixture of aqueous K2CO3 and toluene
with Pd(PPh3)4 as the catalyst and tetrabutylammonium
bromide as the phase transfer catalyst (Scheme 3). The polymer
P1 was obtained with good solubility in organic solvents
(ca. CHCl3, THF, and dichlorobenzene) and a high molecular
weight (Mw ¼ 21.4 kDa, Mw/Mn ¼ 1.81), a value comparable to
those of polyuorenes prepared with arylbromide.19b
5
8
85
55
6
7
9
85
85
60
81
10
8
9
11
12
85
85
90
81
In order to evaluate the applicability of the as-prepared BDT
monomers as donating units in the development of donor–
acceptor (D–A) alternating narrow bandgap polymers for solar
cells, the optical and electrochemical properties of the as-
prepared BDT monomers (4–19) were further studied. The UV-
vis absorption spectra of monomers 4–19 in both CHCl3 solu-
tions and thin lms are shown in Fig. S39.† All spectra feature
two distinct absorption peaks, with the one at 300–450 nm
assigned to the delocalized p–p* transition between the BDT
core and the attached side chains and the other beyond 300 nm
corresponding to the incorporated side chains. For electron-
rich thiophene analogues (4–9), a relatively large red-shi
absorption was observed for the lms compared to the solu-
tions. Interestingly, in the case of 4 and 9, by changing the
single thiophene unit to the bithiophene unit vertical to the
core of BDT, the low-energy absorbance increased in intensity
and a larger red-shi was observed. The reason was that the
resonance structure in BDT increased the double-bond char-
acter of the single bonds in 9 and this consequent reduction of
the bond-length alternation effectively modied the absorption
behaviours of BDT.22 When comparing the UV-vis spectra of 8
with that of 4, the larger planar structure of the thio[3,4-b0]-
thiophene unit was the so-called “prequinoid” monomer and
the quinoid resonance form is lower in energy than the single
thiophene form. The stabilized quinoid form effectively reduces
the bandgap of related conjugated systems.3a,21,23 As expected,
the thio[3,4-b0]thiophene substituted BDT monomer 8 exhibited
red shied absorption than 4 in both solution (15 nm) and the
lm (nearly 20 nm). By attaching trimethylsilyl or octyl thio-
phene, the resulting BDT monomers 5 and 6 exhibited similar
absorption spectra with 4, indicating that alkyl chains had a
negligible effect on the electronic structures of BDT.24
10
11
12
13
13
14
15
16
85
82
55
90
88
100
100
100
14
15
16
17
18
19
Reux
Reux
Reux
90
91
90
a
Conditions: Pd[PPh3]4 (5 mol%), THF–Na2CO3 (aq. 1 M) (4/1, v/v), 85
ꢂC, 16 h for entries 1–10; Pd(PPh3)2Cl2 (10 mol%), CuI (20 mol%),
DMF–Et3N (2/1, v/v), 100 ꢂC, 16 h for entries 11–13; Pd2(dba)3 (2.5%),
Xantphos (5%), i-Pr2NEt (2.3 equiv.), toluene, reux, 12 h for entries
14–16.
Scheme 3 Access to 4,8-difunctionalized BDT monomers via BDT
triflate involved Sonogashira coupling and carbon–sulfur formation.
mixture under heating at 100 ꢂC for 16 h. The coupling between
3 and 2.5 equiv. of 1-octyne, or 4-methyl-phenyl-acetylene gave
the corresponding products 15 and 16 (entries 12 and 13) with a
yield of 80 and 88%, respectively. Due to desilylation under the
13582 | J. Mater. Chem. A, 2014, 2, 13580–13586
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For 10, the lm with an onset absorption (ledge) of 490 nm HOMO energy levels of 17–19 were calculated to be ꢀ5.41, ꢀ5.41
displayed 55 nm bathochromic shi than its solution, indi- and ꢀ5.42 eV, respectively, much deeper than that of BDT-O-
cating an increased degree of BDT aggregation in the solid state.
C
12H25 (ꢀ5.18 eV). This can be explained with the poorer elec-
Compared with monomer 11, it was found that the anchoring of tron-donating ability of the sulfur atom than the oxygen atom
uorine or triuoromethyl groups at 4,8-positions of the BDT due to the poorer overlap of its orbitals with the p-system of the
core for 12 and 13 had no distinct effect on the electronic BDT backbone.
structures. The phenylacetylenated 16 exhibited the widest
According to the electrochemical studies discussed above,
absorption band, with a ledge extended to 500 nm. And a the electron-deciency of the screened substituents can be
remarkable red-shied absorption (ꢁ80 nm) was observed in arranged by the order: thiophene analogues < ethynylene
the lm when comparing with its dilute solution, suggesting J- analogues < phenyl analogues. The poorer the aryl substituent,
aggregation as the dominant packing mode in the solid state.25 the lower the HOMO level. Amazingly, most of BDT monomers
Monomers 17, 18 and 19 exhibited about 5 nm red-shied exhibit a HOMO level close to the HOMO of “ideal” polymers for
absorption than the reference 4,8-didodeoxyl BDT (BDT-O- high-efficiency solar cells (ca. ꢀ5.4 eV).13 Though there are
C
12H25). This indicates that stronger p–p stacking between BDT arguments on the inheritance of the HOMO level of D–A poly-
and the diarylthio or dialkylthio substituent occurs than that in mers from the electron-rich monomers,28 the closeness of the
dialkoxy substituted BDT analogues. HOMO level of BDT monomers to that of the “ideal” polymer
The optical and electrochemical data of the BDT monomers donors in a necessarily desirable situation will help in nding
are summarized in Table S1.† The alignment of HOMO and matchable electron-decient monomers to obtain new D–A
LUMO energy levels of BDT monomers are further plotted in polymers for high-performance solar cells.
Fig. 1. The HOMO energy level of BDT monomers is found to be
In order to demonstrate the electronic structures as well as
dependent upon the electron-donating substituents on the BDT molecular energy levels of the BDT monomers, quantum
core. For monomers with relatively electron-decient phenyl chemistry calculations were performed by using the density
analogues, the HOMO energy levels were estimated to be ꢀ5.55, functional theory (DFT) on the B3LYP/6-31G level.26 As shown in
ꢀ5.56 and ꢀ5.58 eV, respectively, for 11–13. Phenyl substituent Fig. 2, these four units show planar structures and the p-elec-
bearing six p-electrons on the BDT core introduced a relatively trons are well delocalized along their backbones, however, the
larger perturbation to the conjugated backbone, further HOMO and LUMO levels are different. The bandgaps of 5, 8 and
lowering the LUMO level. The HOMO energy levels of ethyny- 9 are 3.75 eV, 3.58 eV and 3.37 eV, respectively, indicating that
lene-containing BDTs (14–16) were estimated to be ꢀ5.47, the bandgap becomes smaller with increase of the conjugation
ꢀ5.48 and ꢀ5.51 eV, respectively, which indicated that the length of substituents. The HOMO level of 17 is ꢀ5.61 eV,
introduction of triple bonds led to an increased conjugation implying that the introduction of sulfur as the heteroatom
length and good planarity to give the low HOMO energy level.
affects the molecular energy levels of fused aromatic systems
For electron-rich thiophene analogues, the HOMO energy distinctly.27 The results from DFT calculations clearly show the
levels estimated for 4–9 were ꢀ5.41, ꢀ5.45, ꢀ5.43, ꢀ5.42, ꢀ5.43 similarity/difference of the energy levels of the frontier orbitals
and ꢀ5.38 eV, respectively, indicating that the incorporated 2- as well as the molecular electronic structures among 5, 8, 9 and
functionalized thiophene had a negligible effect on the HOMO 17.
energy level when comparing 5–7 with 4. The thio[3,4-b0]thio-
The ordered-structure and crystallization features of BDT
phene substituted 8, with the so-called “pre-quinoid” unit, monomers were investigated by lm X-ray diffraction (XRD) and
showed a low HOMO energy level due to the enforced planarity atomic force microscopy (AFM). As depicted in Fig. 3a–b, the
and more effective p-electron delocalization than single thio- lms of 5 and 17 displayed a series of equidistant peaks in XRD
phene.24 Monomer 9 showed the highest HOMO level among all
thiophene analogues (4–9), due to the existence of more p-
electrons with two 5-membered rings in comparison to other
thiophene substituents. The HOMO level of furan-substituted
10 was calculated to be ꢀ5.42 eV, similar to that of 4. The
Fig. 1 The alignment of HOMO and LUMO energy levels of BDT Fig. 2 Calculated HOMOs and LUMOs of 5, 8, 9 and 17 with the
monomers.
TDD-FT B3LYP/6–31(d) level.
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Fig. 4 The output (left) and transfer (right) curves of 5 and 17 FETs.
to 0.045 cm2 Vꢀ1 sꢀ1 with the highest mobility of 0.06 cm2 Vꢀ1 sꢀ1
and an on/off current ratio of 4 ꢃ 105. The vacuum-deposited
device on the OTS-treated Si/SiO2 substrate of 17 also demon-
strated an average mobility of 0.015 cm2 Vꢀ1 sꢀ1. The highest
mobility was 0.025 cm2 Vꢀ1 sꢀ1, with an on/off current ratio of
2.0 ꢃ 106.
Fig. 3 X-ray reflection diagrams of 5 (a) and 17 (b) thin films on the
OTS-treated Si/SiO2 substrate at 25 ꢂC. AFM images of 5 (c and d) films
on the OTS-treated Si/SiO2 substrate and 17 (e and f) films on the glass
substrate.
It was amazing that negligible variation in structures of
monomers led to prodigious differences in the XRD, AFM and
FET devices. The correlation study of structure–property is still
under progress and the optimized FET data will be reported in
due course. The as-prepared BDTs exhibited low HOMO levels
close to that of an “ideal” polymer as well as high hole mobil-
ities. According to the strategy of weak donor–strong acceptor to
design ideal polymers for organic solar cells,28a if suitable strong
acceptors, such as diketopyrrolopyrrole,29 benzooxadiazole30
and naphtho[1,2-c:5,6-c]bisthiadiazole,31,32 would be selected to
construct alternating low bandgap copolymers via internal
charge transfer with as-developed BDT monomers, we believe,
new D–A polymers with suitable energy level alignment and
high hole mobilities would be developed to achieve high-
performance solar cells with optimized device engineering.
In summary, we have presented here a versatile method for
the 4,8-functionalization of benzo[1,2-b:4,5-b0]dithiophene via
spectra,24 indicating highly crystalline property of the thin lms.
The d-spacing of 1.73 nm for 5 and 1.74 nm for 17 corresponds
exactly to the strong primary diffraction peak at 5.11ꢂ and 5.08ꢂ,
respectively, suggesting that the molecules oriented normal to
the substrate surface with a certain extent of tilt.26 Interestingly,
no peaks were observed for the other BDT monomers, indi-
cating the poor crystalline nature of their lms. As illustrated in
Fig. 2c–f, AFM images of 5 and 17 also showed a ordered thin-
lm morphology. Both lms of 5 and 17 exhibited terrace
structures, which are generally favorable for high charge
transport in FETs.19a
Bottom-gate top-contacted transistors were further fabri-
cated to investigate the charge transport properties. Among all
BDT monomers, only 5 and 17 exhibited p-channel perfor-
mance with the mobility calculated from the saturation regime,
which agreed well with the results of the XRD and AFM. Fig. 4
shows the typical current-voltage characteristics of 5 and 17
devices made by vacuum deposition at room temperature on the
octyltrichlorosilane (OTS)-treated Si/SiO2 substrate.
Table 2 The FET data for monomers 5, 8, 9, 16 and 17
Monomers
mc (cm2 Vꢀ1 sꢀ1
)
On/off ratio
Vth (V)
5a
0.013 (0.016)
0.045 (0.061)
0.013 (0.015)
0.012 (0.014)
0.009 (0.011)
0.007 (0.009)
0.016 (0.025)
105 to 106
104 to 105
105 to 106
105 to 106
105 to 106
107 to 108
104 to 106
ꢀ25.60
ꢀ37.01
ꢀ26.93
ꢀ27.16
ꢀ36.45
ꢀ41.25
ꢀ40.68
The representative FET results for monomers 5, 8, 9, 16 and
17 are summarized Table 2 (with the FET data for all monomers
listed in Table S2†). The vacuum-deposited device of 5 based on
polymethyl methacrylate (PMMA) spin-coated on the Si/SiO2
substrate showed the relatively good average mobility, i.e., the
thin lm transistors demonstrated a maximum hole mobility of
0.025 cm2 Vꢀ1 sꢀ1 and an on/off current ratio of 1.2 ꢃ 105
(Fig. S40†). When adopting the OTS-treated Si/SiO2 substrate,
the device of 5 achieved a much improved average mobility up
5b
8a
9a
12a
16a
17b
a
b
PMMA-modied Si/SiO2 substrate. OTS-treated Si/SiO2 substrate.
The average mobility and the maximum in parenthesis out of six
c
samples.
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palladium-catalyzed Suzuki and Sonogashira couplings as well
as carbon–sulfur bond formation reaction. Using this method-
ology, a library of 4,8-functionalized benzo[1,2-b:4,5-b0]dithio-
phene monomers have been developed with aryl, ethynyl,
arylethynyl, alkylthio and arylthio substituents. The absorption
and energy levels of BDT monomers can be ne-tuned by
adjusting the donating strength of the incorporated substitu-
ents. The as-prepared BDT monomers exhibit a bandgap
ranging from 2.48 to 3.34 eV and a relatively low HOMO energy
level. The high crystalline nature and hole mobility of BDT
monomers demonstrated for 5 and 17 shows that manipulating
the functionalization of BDT core can result in prodigious
differences in charge transport properties. High FET mobilities
of 0.061 cm2 Vꢀ1 sꢀ1 and 0.025 cm2 Vꢀ1 sꢀ1 were readily ach-
ieved for 5 and 17. The as-developed BDT monomers may be
promising donating units for constructing D–A narrow bandgap
polymers for high-performance solar cells and p-type FETs if
high mobility co-monomers are used and the resulted polymers
have highly ordered structures.
4 (a) Y. Huang, X. Guo, F. Liu, L. J. Huo, Y. N. Chen,
T. P. Russell, C. C. Han, Y. Li and J. Hou, Adv. Mater., 2012,
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Acknowledgements
This work is supported by the National Natural Science Foun-
dation of China (grant no. 21074055), the Program for New
Century Excellent Talents in University (NCET-12-0633), The
Jiangsu Province Natural Science Fund for Distinguished Young
scholars (BK20130032), the Doctoral Fund of Ministry of
Education of China (no. 20103219120008), and the Funda-
mental Research Funds for the Central Universities
(30920130111006). The authors thank Dr Baojing Zhou for the
theoretical calculation.
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13586 | J. Mater. Chem. A, 2014, 2, 13580–13586
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