Effect of extended conjugation on the optoelectronic properties of benzo[1,2-d:4,5-d′]bisoxazole polymers

Article abstract of DOI:10.1071/CH13528

Four copolymers comprising benzo[1,2-d:4,5-d']bisoxazole (BBO) and benzo[1,2-b:4,5-b′]dithiophene (BDT) bearing phenylethynyl substituents on either the BBO, BDT moieties or both units were synthesised and the influence of two-dimensional conjugation on their optoelectronic properties investigated. Extending conjugation along the BBO resulted in a 0.5eV decrease in the LUMO level, whereas the HOMO level was raised by 0.2eV. Extending conjugation across the BDT moiety in also resulted in a 0.5eV decrease in the LUMO level, however, the effect was negligible on the HOMO level. Thus, cross-conjugation can be used to independently tune the LUMO level within these systems. CSIRO 2014.

Full text of DOI:10.1071/CH13528

RESEARCH FRONT
CSIRO PUBLISHING
Aust. J. Chem. 2014, 67, 711–721
Full Paper
http://dx.doi.org/10.1071/CH13528
Effect of Extended Conjugation on the Optoelectronic
Properties of Benzo[1,2-d:4,5-d9]bisoxazole Polymers
A
B
B
Brian C. Tlach, Aime´e L. Tomlinson, Kiley D. Morgan,
B
A
A C
,
Christopher R. Collins, Michael D. Zenner, and Malika Jeffries-EL
A
Department of Chemistry, Iowa State University, Ames IA 50010, USA.
B
Department of Chemistry and Biochemistry, University of North Georgia,
GA 30597, USA.
C
Corresponding author. Email: malikaj@iastate.edu
Four copolymers comprising benzo[1,2-d:4,5-d’]bisoxazole (BBO) and benzo[1,2-b:4,5-b0]dithiophene (BDT) bearing
phenylethynyl substituents on either the BBO, BDT moieties or both units were synthesised and the influence of two-
dimensional conjugation on their optoelectronic properties investigated. Extending conjugation along the BBO resulted in
a 0.5 eV decrease in the LUMO level, whereas the HOMO level was raised by 0.2 eV. Extending conjugation across the
BDT moiety in also resulted in a 0.5 eV decrease in the LUMO level, however, the effect was negligible on the HOMO
level. Thus, cross-conjugation can be used to independently tune the LUMO level within these systems.
Manuscript received: 1 October 2013.
Manuscript accepted: 4 December 2013.
Published online: 30 January 2014.
substituents and their location on the central molecule.[7]
Although there have been many reports on the synthesis of
cruciform small molecules,[7,8] reports on the effect of extended
conjugation in polymeric systems are limited, whereby many of
them are based on copolymers of benzo[1,2-b:4,5-b0]dithio-
phene (BDT) derivatives featuring extended conjugation.[9]
BDT is an electron-rich molecule that has been widely investi-
gated for the synthesis of conjugated polymers because it has a
planar, conjugated structure that facilitates p–p stacking and
promotes charge carrier mobility.[10] As a result, power conver-
sion efficiencies of polymer solar cells, involving BDT copoly-
mers, have exceeded 9 %.[10c,11] By replacing the electron-rich
alkoxy groups with thien-2-yl, aryl, alkynyl, and phenylethynyl
groups on the 4- and 8-positions of the BDT moiety, the HOMO
level can be lowered.
Similarly, benzo[1,2-d:4,5-d0]bisoxazole BBOs are electron
deficient moieties that have been incorporated in polymers with
exceptional thermal and chemical stability,[12] efficient electron
transport, and blue emission.[13] [14] Similarly to BDT, BBO can
be substituted at the 4- and 8-positions with aryl, alkynyl, and
phenylethynyl groups.[8a,15] It has been demonstrated that the
HOMO and LUMO levels of BBO in small molecules could
readily be tuned by substitution[7d,16] and that the resulting effect
on the energy levels is dependent on the nature and location of
the substituent.[8a] However, the effect of extended conjugation
on the HOMO, LUMO, and the band gap of BBO-based
polymers is yet to be explored. Herein, we synthesised four
new polymers using a combination of one- and two-dimensional
BDT and BBO monomers. The effect of the extended conjuga-
tion on the optoelectronic properties was evaluated using cyclic
voltammetry, UV–Vis spectroscopy, and density functional
theory (DFT).
Introduction
In recent years there has been a large amount of research
devoted to the development of organic semiconductors OSCs as
replacements for conventional inorganic materials.[1] These
materials have been applied in several areas such as field effect
transistors,[2] organic light-emitting diodes,[3] and organic
photovoltaic cells.[4] OSCs offer several advantages over
inorganic semiconductors, including ease of preparation in
solution,[5] which can reduce fabrication costs and facilitate
tuning of the optoelectronic properties through chemical syn-
thesis. Generally, the introduction of flexible side chains in the
conjugated polymer backbone not only improves the solubility
and film-forming characteristics, but also influences the
nanoscale morphology, and the electronic, optical, and physical
properties.[6] Varying the electron-donating or electron-
accepting strength of the arenes along the polymer backbone
can further modify the optical and electronic properties of these
materials. Thus, in principle, OSCs featuring highest occupied
molecular orbitals (HOMOs), lowest unoccupied molecular
orbitals (LUMOs), and band gaps that are tailored to specific
applications can be synthesised. Unfortunately, in practice, this
is often difficult to accomplish because of the complex inter-
play between the structure of the material and its properties,
resulting from the extensive delocalisation of electrons within
the OSCs. Therefore, synthetic strategies that facilitate the
independent tuning of the HOMO and LUMO levels are highly
sought after.
To this effect, 2-dimensional cross-shaped ‘cruciform’ mole-
cules that feature two conjugation axes are particularly promis-
ing. These compounds have spatially segregated frontier
molecular orbitals (FMOs) that facilitate the individual modifi-
cation of either the LUMO or the HOMO by changing the
Journal compilation Ó CSIRO 2014
www.publish.csiro.au/journals/ajc

712
B. C. Tlach et al.
Results and Discussion
steric hindrance exerted by the 2-ethylhexyl chain adjacent the
trimethoxymethyl moiety. BBO 2 was then easily brominated in
69 % yield to give one-dimensional BBO monomer 3.
Synthesis of BBO monomers
Previously, we reported the synthesis of 2,6-(dithien-2-yl)
BBOs via the Lewis-acid catalysed condensation of diamino
diols and aryl orthoesters.[17] A similar approach was herein
used for the synthesis of the BBO monomer. Orthoester
1 (Scheme 1) was first synthesised from 2-bromo-3-(2-
ethylhexyl)thiophene[18] in 54 % yield using the method first
reported by Tschitschibabin.[17,19] This alkyl substituent was
used to increase the solubility of the BBO monomers and
facilitate bromination at the 5-position. Furthermore, location of
the alkyl chain at the 3-position instead of the 4-position on
thiophene is expected to decrease steric interactions between the
monomer units in the polymer and steric hindrance during
polymerisation. The reaction between 1 and 2,5-diamino-1,4-
hydroquinone bishydrochloride (DAHQ)[20] afforded BBO 2 in
55 % yield (Scheme 2). This yield was lower than our previous
reports using similar compounds, and was likely because of
A similar approach was used to synthesise the two-dimen-
sional BBO monomer and is outlined in Scheme 3. The conden-
sation reaction between 1 and 3,6-diamino-2,5-dibromo-1,4-
hydroquinone[21] (Br-DAHQ) afforded BBO 4 in 19 % yield.
The lower yield of the reaction compared with that of 2 is likely
because of the reduced nucleophilicity of Br-DAHQ. The two-
dimensional BBO intermediate 5 was obtained from the Sono-
gashira coupling between 4 and 3,5-di-(2-ethylhexyloxy)-1-
ethynylbenzene 6 in 91 % yield (Scheme 4). The alkyne was
easily synthesised from 3,5-di-(2-ethylhexyloxy)benzalde-
hyde[22] via the Corey–Fuchs reaction. This alkyne was used
C4H9
C4H9
C2H5
C2H5
O
O
O
O
1. CBr4, PPh3
CH2CI2
CHO
C4H9
C4H9
2. nBuLi, THF
3. H2O
C2H5
C2H5
6 (81 %)
1. iPrMgCI, Et2O
2. C(OEt)4
C2H5
C2H5
Br
C(OEt)3
1 (54 %)
S
S
C4H9
C4H9
Scheme 4. Synthesis of substituted alkyne 6.
Scheme 1. Synthesis of substituted orthoester 1.
C4H9
C4H9
C2H5
C2H5
Br
H2N
OH
· 2 HCI
NH2
S
S
O
N
O
N
O
O
1
NBS
CHCI3
AcOH
Yb(OTf)3
pyridine
THF/DMSO
N
S
N
S
HO
Br
2 (55 %)
3 (69 %)
C2H5
C2H5
C4H9
C4H9
Scheme 2. Synthesis of linear BBO 3.
C4H9
C2H5
Br
Br
Br
H2N
OH
N
O
O
N
S
1
6
Yb(OTf)3
THF/DMA
Pd(PPh3)2CI2, Cul
iPr2NH, THF
S
NH2
C4H9
HO
C2H5
Br
C2H5
4 (19 %)
C4H9
C4H9
O
Ar ꢀ
Ar
Ar
C4H9
O
C2H5
C2H5
1. nBuLi, THF
TMEDA
Br
C2H5
S
O
N
S
N
O
O
N
N
O
C4H9
2. CBr4
S
S
Br
C2H5
C2H5
C4H9
C4H9
Ar
Ar
7 (62 %)
5 (91 %)
Scheme 3. Synthesis of two-dimensional BBO monomer 7.

Benzo[1,2-d:4,5-d0]bisoxazole Polymers
713
because it enabled the introduction of long-branched alkyl
chains for solubility purposes, whereas the meta-substitution
pattern prevented electron-donation from the oxygen atoms to
the BBO. Because electrophilic brominating reagents could not
be used on BBO 5, a double lithium–hydrogen exchange, using
n-butyl lithium and N,N-N0,N0-tetramethylethylenediamine
(TMEDA) was performed, and quenching the resulting anion
with carbon tetrabromide produced the target two-dimensional
BBO monomer 7 in 62 % yield.
stannylation of 8 and 9 afforded monomers 10 and 11 in yields of
66 % and 86 %, respectively. The identity of all the compounds
was confirmed by 1H NMR, 13C NMR, and high-resolution mass
spectroscopy.
Synthesis of Copolymers
The copolymers were synthesised using a Stille cross-coupling
catalysed by Pd(PPh3)4 in a solution of toluene and DMF as
shown in Scheme 6. The isolated polymers P1–P4 were
obtained in 43 %–80 % yield and the structures are shown in
Fig. 1. The polymers were characterised by gel permeation
chromatography (GPC), UV–Vis spectroscopy, and cyclic vol-
tametry. 1H NMR spectra of the four polymers were consistent
with the proposed structures. The molecular weights of P1–P4
determined by GPC are shown in Table 1. The polymers showed
good solubility in chlorinated solvents and toluene, and had
moderate-to-high molecular weights that vary significantly. P2
displayed the highest number average molecular weight (Mn)
i.e. 39000 Da and P1 featured the lowest Mn i.e. 7000 Da. The
low molecular weight of P1 was due to the reduced number of
Synthesis of BDT Monomers
The synthesis of both the one-dimensional and two-dimensional
BDT monomers is shown in Scheme 5. The one-dimensional
intermediate 8 was synthesised from benzo[1,2-b:4,5-b0]
dithiophene-4,8-dione[23] according to the reported proce-
dure.[24] The two-dimensional BDT system was prepared by
nucleophilic addition of lithiated 6 (via n-butyl lithium) to benzo
[1,2-b:4,5-b0]dithiophene-4,8-dione. This intermediate was
aromatised in situ to afford 9 in 49 % yield. Optimal results were
obtained using a slight excess amount of the alkyne. Subsequent
C4H9
O
C2H5
C2H5
C4H9
O
1. Zn, EtOH, NaOH (aq)
1. nBuLi, THF
2. Me3SnCI
S
S
Sn
Sn
C4H9
S
S
C4H9
O
2. TBAB,
O
O
C2H5
C2H5
Br
O
O
C2H5
C4H9
C2H5
C4H9
S
8 (38 %)
10 (66 %)
Ar ꢀ
S
Ar
Ar
O
C2H5
Sn
C4H9
1. nBuLi, THF
2. Me3SnCI
S
S
1. Li
Ar, THF
Sn
S
S
2. SnCI2, HCI (aq)
Ar
Ar
9 (49 %)
11 (86 %)
Scheme 5. Synthesis of BDT donors.
Ph
S
R2
S
C4H9
n
C4H9
C2H5
C2H5
R1
R1
R1
Br
R2
S
S
N
O
N
N
O
O
N
Pd(PPh3)4
toluene/DMF
O
S
S
Ph
Br
R1
C2H5
C2H5
ꢁ
C4H9
Sn
C4H9
P1: R1 ꢀ H, R2 ꢀ 2-ethylhexyloxy (43 %)
R2
P2: R1 ꢀ 3,5-di-(2-ethylhexyloxy)phenyl R2 ꢀ 2-ethylhexyloxy (44 %)
P3: R1 ꢀ H, R2 ꢀ 3,5-di-(2-ethylhexyloxy)phenyl (80 %)
P4: R1 ꢀ R2 ꢀ 3,5-di-(2-ethylhexyloxy)phenyl (69 %)
S
Sn
S
R2
Scheme 6. Synthesis of polymers P1–P4.

714
B. C. Tlach et al.
C4H9
side chains and prominent rigid rod-like structure when com-
pared with those of the remaining polymers, allowing for
increased p–p stacking. In the case of P4, the large size of the
two-dimensional monomers likely contributed to its reduced
Mn. The general increase in Mn for the cross-conjugated poly-
mers P2–P4 was likely because of the increased steric bulk
of the phenylacetylene side chains and increased side-chain
density that disrupted p–p stacking.
C2H5
Ph
S
O
n
C4H9
C2H5
Ph
O
S
N
O
S
O
N
C4H9
C2H5
S
P1
C4H9
The thermal properties of the polymers were studied using
differential scanning calorimetry (DSC) and thermogravimetric
analysis (TGA). Based on the DSC analysis, none of the
polymers displayed transitions below 3008C (Fig. S29), indicat-
ing the amorphous nature of the polymers. The polymers all
showed excellent thermal stability: a 5 wt-% mass loss was
observed at varying temperatures between 3598C (P1) and
4128C (P3) as determined by TGA in air (Fig. S30).
C2H5
C2H5
C4H9
O
Ph
S
Ar
n
C4H9
C2H5
Ph
O
O
N
N
O
S
S
C4H9
S
C2H5
Spectroscopic and Electronic Characterisation
P2
C4H9
Ar
The normalised absorption spectra of P1–P4 both as dilute
CHCl3 solutions and thin films are shown in Figs 2 and 3,
respectively, and the optical data are summarised in Table 2. In
comparison with the solution-based spectrum, the thin film-
based spectrum of the unsubstituted polymer, P1, exhibits a
C2H5
Ar
Ph
S
n
C4H9
C2H5
S
N
S
O
Ar
O
1.0
S
N
P1
P2
P3
Ph
P3
C4H9
0.8
0.6
0.4
0.2
0
P4
C2H5
Ar
Ph
S
Ar
n
C4H9
C2H5
Ph
S
N
S
O
N
S
O
Ar
P4
C2H5
C2H5
C4H9
C4H9
C2H5
O
O
300
350
400
450
500
550
600
650
Ar
Wavelength [nm]
Ar ꢀ
C4H9
Fig. 2. Solution UV–Vis spectra of P1–P4 in chloroform.
Fig. 1. Structures of polymers P1–P4.
1.0
0.8
0.6
0.4
0.2
0
P1
P2
P3
P4
Table 1. Molecular weight data for polymers P1–P4
Data given are relative to that obtained for polystyrene standard in chloro-
form at 508C
MAn [kDa]
PDIB
DPCn
TDd [8C]
P1
P2
P3
P4
7.02
39.8
28.9
19.2
1.17
2.31
1.44
1.55
7
23
20
9
359
372
412
402
AMn: number average molecular weight.
BPDI: Mw/Mn where Mw is the molecular weight.
cDPn: Degree of polymerisation based on Mn.
300
350
400
450
500
550
600
650
Wavelength [nm]
DTd: Temperature at which a 5 wt-% loss was observed, as determined by
Fig. 3. Thin film UV–Vis spectra of P1–P4.
TGA in air.

Benzo[1,2-d:4,5-d0]bisoxazole Polymers
715
slight broadening of the absorption band and a slight red shift in
the absorption maximum. In contrast, the substituted polymers
P2, P3, and P4 displayed similar solution- and film-based
spectra, indicating that p-stacking in the polymers in the solid
state is not efficient, likely because of the bulky side chains. In
both solution and film forms, all four polymers exhibit a single
absorption band, with two peaks corresponding to the p–p*
transition. However, all spectra lack the presence of the low-
energy band characteristic of intramolecular charge transfer
(ICT) between the electron-donating and electron-accepting
units within the polymer backbone.[25]
The electrochemical properties of the polymers were inves-
tigated using cyclic voltammetry and the profiles are shown in
Fig. S23 (Supplementary Material). All four polymers exhibited
measurable and reproducible oxidation and reduction processes.
The HOMO and LUMO levels were estimated from the onset of
the oxidation and reduction processes, respectively, using the
absolute energy level of ferrocene/ferrocenium (Fc/Fcþ) as
4.8 eV under vacuum; the results are summarised in Table 3.[26]
The unsubstituted polymer P1 has a HOMO level of ꢀ5.6 eV
and a LUMO level of ꢀ2.8 eV, resulting in an electrochemical
band gap of 2.8 eV. Extension of conjugation across the BBO
moiety affords P2 with a slightly raised HOMO level at
ꢀ5.4 eV, a lower LUMO level of ꢀ3.2 eV, and a narrower band
gap of 2.2 eV. Likewise, extension of conjugation across the
BDT moiety affords P3 with an unchanged HOMO level of
ꢀ5.6 eV, a lower LUMO level of ꢀ3.3 eV, and an optical band
gap of 2.3 eV. Interestingly, P4 that has extended conjugation
across both the BBO and BDT moieties features the same
HOMO and LUMO levels as those present in P3.
Table 2. Optical properties of BBO polymers
lsmoalnx [nm]
lfmilamx [nm]
Eogpt [eV]A
P1
P2
P3
P4
489, 525
504, 543
500, 538
492, 525
493, 531
506, 539
506, 545
504, 542
2.3
2.2
2.2
2.2
ACalculated from the intersection between the thin film-based absorption
and emission spectra (Fig. S25).
To further elucidate the influence of extended conjugation on
the optical and electronic properties of these polymers, ground-
state geometry optimisations (Fig. 4) were performed by DFT,
employing a B3 LYP[27] functional, a 6–31G* basis, and the
Gaussian 09 software package.[28] Frontier molecular orbital
(FMO) diagrams and electrostatic potential maps, as shown in
Fig. 5, were generated from the DFT data. In addition, a time-
dependent DFT routine with the aforementioned functional and
basis set was used to generate the excited states and the results
are summarised in Table 3. According to DFT, the band gaps of
polymers P1–P4 increased in the order of P2 , P3 , P4 , P1,
although the difference between the values was only 0.08 eV.
The calculated HOMO energy values of the polymers decreased
in the order of P1 . P3 . P4 . P2; the difference between P3
and P4 was negligible. All of these observations agreed with the
experimental data.
Table 3. Experimental and theoretical comparison between the elec-
tronic properties of BBO polymers
HOMOA [eV]
LUMOB [eV]
ECg [eV]
Polymer Experiment Theory Experiment Theory Experiment Theory
P1
P2
P3
P4
ꢀ5.6
ꢀ5.4
ꢀ5.6
ꢀ5.6
ꢀ4.98
ꢀ4.74
ꢀ4.86
ꢀ4.83
ꢀ2.8
ꢀ3.2
ꢀ3.3
ꢀ3.3
ꢀ2.68
ꢀ2.58
ꢀ2.63
ꢀ2.55
2.8/2.3
2.2/2.2
2.3/2.2
2.3/2.2
1.94
1.86
1.88
1.93
ACalculated from the oxidation onset using ꢀ(4.8 þEOonXset).
BCalculated from the reduction onset using ꢀ(4.8 þEroendset).
CThe data are displayed as EgEC/Egopt where EgEC LUMO ꢀ HOMO and Eogpt
was calculated from the intersection between the thin film-based absorption
and emission spectra.
P1
P2
P3
P4
Fig. 4. Optimised geometries for model dimers of P1–P4.

716
B. C. Tlach et al.
Electrostatic potential map
HOMO
LUMO
P1
P2
P3
P4
Fig. 5. Electrostatic potential maps and FMO diagrams of P1–P4.
Copolymers that comprise alternating electron-donating and
electron-accepting moieties have energy levels that are easily
tuned by changing the strength of the donor and acceptor units.
In these systems, the LUMO level is primarily related to the
acceptor unit, whereas the HOMO level is largely determined by
the donor unit.[29] Because the phenylethynyl BBO is a stronger
acceptor than the unsubstituted BBO, extension of the conjuga-
tion with an electron-accepting phenylethynyl substituent across
BBO is expected to lower the LUMO. Accordingly, P2 had
a lower LUMO (by 0.5 eV) than P1 and P3, and it also had a
slightly higher HOMO level (by 0.2 eV). This effect is likely a
combination of the electron-accepting alkyne and the extended
conjugation across BBO.[15] On the other hand, because pheny-
lethynyl BDT is a weaker donor than unsubstituted BDT,
extension of the conjugation along BDT is expected to lower
the HOMO level. Hence, P3 is expected to have a lower HOMO
level than P1 and P2, however, this was not observed. We
believe that this is a result of twisting along the polymer
backbone in P3, as demonstrated by the optimised geometry
model dimers in this system. As seen in Fig. 4, the unsubstituted
polymer P1 is planar unlike the other polymers. Likewise, P4
that has substituents on both BDT and BBO has the same
HOMO, LUMO, and band gap values as those of P3 despite
the presence of the second substituent. The optimised geometry
for model dimers in this system also revealed significant
twisting. Thus, the resulting energy levels are not merely a
function of the increased electron density around the BBO and
BDT moieties; steric effects also play a major role. The latter
reduces the planarity of the system and decreases the delocalisa-
tion of electron density along the polymer backbone.
ICT is not occurring in these systems owing to the delocalisation
of the FMOs.
Conclusions
To experimentally and theoretically evaluate the use of cross-
conjugation for selectively modifying FMOs within conjugated
polymers, three new two-dimensional polymers were synthe-
sised and compared with the newly synthesised one-
dimensional polymer. The UV–Vis spectra of all polymers were
similar, indicating that there were only minor differences
between the optical properties of the polymers. Additionally, the
comparable solution- and film-based spectra of P1–P4 indicated
that there was little-to-no aggregation in the solid state, most
likely because of steric effects introduced by the bulky side
chains. However, the spectra of P1 indicated that ICT was not
occurring within the polymer backbone, owing to the relative
strength and weakness of the BDT donor and BBO acceptor,
respectively. The extended conjugation of P2–P4 failed to
increase ICT, as indicated by UV–Vis analysis and the lack of
localisation of the electron density in the FMOs. The absence of
donor–acceptor behaviour in the cross-conjugated polymers is
partially a result of the steric twist along the polymer backbone,
owing to the large side chains, along with the relative strength
and weakness of the BDT donor and BBO acceptor, respec-
tively. Nonetheless, the relatively constant HOMO level and
modulation of the LUMO realised by structural modifications
indicate that cross-conjugation is a promising approach for
selectively tuning the LUMO level of OSCs. However, to further
evaluate this approach, polymer design needs to be improved by
using appropriate substituents to decrease steric interactions and
realise stronger electron-rich monomers to promote ICT. Future
work will focus on these alterations using experimental and
theoretical methods.
The FMO diagrams and electrostatic potential maps of all
polymers indicate that the electron density is uniformly distrib-
uted along the polymer backbone. This further supports the
experimental and theoretical findings that demonstrate that
there is very little difference between the electronic properties
of these polymers. Furthermore, the negligible difference
between P3 and P4 can be attributed to the partial localisation
of LUMO on the BDT moiety when compared with those of P1
and P2 that have complete delocalisation of the LUMO. These
diagrams are consistent with the UV–Vis findings that show that
Experimental
Characterisation
Nuclear magnetic resonance (NMR) experiments were carried
out in CDCl3 at 400 MHz (1H) or 100 MHz (13C) on a Varian
MR-400. 1H NMR spectra are internally referenced to the

Benzo[1,2-d:4,5-d0]bisoxazole Polymers
717
residually protonated solvent peak (7.26 ppm) and 13C NMR
spectra are referenced to the central carbon peak (77.16 ppm) of
CDCl3. In all spectra, chemical shifts are given in d relative to
tetramethylsilane. Coupling constants are reported in Hz. High-
resolution mass spectra were recorded on a double-focusing
magnetic sector mass spectrometer using electrospray ioniza-
tion (ESI) or atmospheric pressure chemical ionization (APCI).
Melting points were obtained on a melting point apparatus with
a 2608C-upper limit and were uncorrected. Gel permeation
chromatography (GPC) measurements were performed on a
separation module equipped with three 5-mm I-gel columns
connected in series (guard, high molecular weight, medium
molecular weight, and low molecular weight) with a UV–Vis
detector. Analyses were performed at 508C using CHCl3 as the
eluent at a flow rate of 1.0 mL minꢀ1 with calibration based on
polystyrene standards. Electrochemistry was performed on a
eDAQ e-Corder 410 using 0.01 M AgNO3 in acetonitrile as the
reference electrode, platinum wire as the counter electrode, and
platinum button electrode as the working electrode at a scanning
rate of 50 mV sꢀ1. All measurements were taken under argon
atmosphere in deoxygenated acetonitrile with 0.1 M tetra-
butylammonium hexafluorophosphate as the electrolyte. The
potentials measured versus Agþ were externally referenced to
Fc/Fcþ (ꢀ4.8 eV versus vacuum). Thermogravimetric analysis
(TGA) measurements were performed over a temperature range
of 25–8008C at a heating rate of 208C minꢀ1 under ambient
atmosphere. Differential scanning calorimetry (DSC) was per-
formed using an initial scan at a heating rate of 108C minꢀ1
followed by a second scan to evaluate transitions between 0 and
3008C under nitrogen. All sample films that were drop-cast from
turnings and heated under vacuum at 908C for 1 h. The flask was
back-filled with argon and a few crystals of iodine were added
and allowed to sublime for 20 min. The flask was cooled to room
temperature and filled up with 35 mL dry diethyl ether (Et2O).
Then, 4.6 mL (50 mmol) of 2-chloropropane in 10 mL dry Et2O
was added dropwise using a funnel to maintain gentle reflux, and
refluxed for an additional 1 h. A solution of 6.88 g (20.0 mmol)
2-bromo-3-(2-ethylhexyl)thiophene in 5 mL dry Et2O was
added dropwise and the solution was refluxed for 24 h. The
solution was cooled to room temperature prior to dropwise
addition of 5.8 g (30 mmol) tetraethylorthocarbonate, and the
reaction mixture was heated to reflux for 12 h. The reaction was
then cooled to room temperature and poured in a cold saturated
aqueous NH4Cl solution and the layers were separated. The
aqueous layer was extracted with Et2O and the combined
organic layers were washed with H2O and brine, and dried over
Na2SO4. The solution was filtered and the solvent removed
under vacuum. Low-boiling impurities were removed by Kugel-
rohr distillation to yield a yellow oil that was used without
further purification (3.71 g, 54 %). dH (CDCl3) 7.17 (d, J 4, 1H),
6.89 (d, J 4, 1H), 3.42 (q, J 8, 6 H), 2.70 (dd, J 4, 2H), 1.71 (m, J
8, 1H), 1.28–1.21 (comp, 8H), 1.21 (t, J 8, 9H), 0.87 (t, J 8, 6H).
dC (CDCl3) 140.5, 134.6, 129.4, 124.4, 113.5, 57.9, 39.5, 32.9,
32.7, 29.1, 26.0, 23.3, 15.0, 14.3, 11.0.
2,6-Bis(3-(2-ethylhexyl)thien-2-yl)benzo[1,2-d:4,5-d0]
bisoxazole (2)
A dry Schlenk flask was filled with 1.03 g (3.00 mmol) 1 and
3 mL dry THF and the solution was deoxygenated for 20 min.
Concurrently, a dry pear-shaped flask was filled with 1 mL
DMSO and 180 mg (2.25 mmol) pyridine, and the solution was
deoxygenated for 20 min. Then, 213 mg (1.00 mmol) DAHQ
was added to the pear-shaped flask and allowed to dissolve. The
Schlenk flask was kept under argon and filled with 31 mg
(0.05 mmol) ytterbium triflate Yb(OTf)3, and heated to 608C.
The DAHQ solution was added dropwise and 1 mL THF was
added after 2 h; stirring continued at 608C overnight. The warm
mixture was diluted with a small volume of CHCl3 to dissolve
the solids and the mixture was precipitated in 150 mL methanol
at ꢀ788C. The precipitate was filtered and rinsed with methanol
to yield a white powder (300 mg, 55 %), mp 102–1048C. dH
(CDCl3) 7.81 (s, 2H), 7.46 (d, J 8, 2H), 7.02 (d, J 4, 2H), 3.15
(d, J 8, 4H), 1.75 (m, 2H), 1.39–1.29 (comp, 16H), 0.93 (t, J 8,
6H), 0.86–0.95 (comp, 12H). dC (CDCl3) 160.9, 148.2, 140.1,
131.9, 128.9, 123.9, 100.5, 40.9, 34.2, 33.0, 29.0, 26.1, 23.2,
14.3, 11.1. m/z 549.2604. HRMS (ESI) Anal. Calc. for
C32H41N2O2S2 549.2604. Found 549.2602.
1,2-dichlorobenzene solutions (oDCB, 2 mg mLꢀ1
) were
annealed at 808C under vacuum for 6 h before analysis. UV–Vis
spectroscopy was performed on a Varian Cary Eclipse spec-
trometer on dilute chloroform solutions or thin films. Thin films
were spin-coated from mixtures of chloroform and oDCB
(5 mg mLꢀ1) onto 25 ꢁ 25 ꢁ 1 mm glass slides at 74 g on a
Headway Research, Inc. PWM32 spin-coater and annealed at
808C under vacuum for 6 h before analysis.
Materials
Tetrabutylammonium hexafluorophospate was purchased
from Oakwood Chemical and was recrystallised from methanol
before use. All other chemicals were purchased from commercial
sources and used without further purification. 2,5-Diamino-
1,4-hydroquinone bishydrochloride (DAHQ),[20] 3,6-diamino-
2,5-dibromo-1,4-hydroquinone (Br-DAHQ),[15] 2-bromo-3-
(2-ethylhexyl)thiophene (7),[18] 3,5-bis(2-ethylhexyloxy)benz-
aldehyde,[22] benzo[1,2-b:4,5-b0]dithiophene-4,8-dione,[23] and
2,6-bis(trimethylstannyl)-4,8-bis(2-ethylhexyloxy)benzo[1,2-b:
4,5-b0]dithiophene (8)[24] were synthesised according to litera-
ture procedures.
2,6-Bis(3-(2-ethylhexyl)-5-bromothien-2-yl)benzo[1,2-
d:4,5-d0]bisoxazole (3)
A small flask, protected from light, was filled with 560 mg
(1.02 mmol) 2 and dissolved in 35 mL CHCl3/acetic acid (6 : 1).
Upon warming the solution to 408C, 540 mg (3.06 mmol)
N-bromosuccinimide (NBS) was added portion-wise followed
by two drops of HBr. The reaction was stirred for three days at
408C, then poured in 300 mL cold methanol. The precipitate was
filtered, rinsed with methanol, and the crude product was
recrystallised from ethyl acetate to yield yellow crystals
(500 mg, 69 %), mp 157–1598C. dH (CDCl3) d (d, J 8, 2H),
6.98 (s, 2H), 3.10 (d, J 8, 4H), 1.71 (m, 2H), 1.31–1.37 (comp,
16H), 0.92 (t, J 8, 6H), 0.89 (t, J 8, 6H). dC (CDCl3) 159.7, 148.1,
147.7, 140.1, 135.6, 125.4, 117.0, 100.6, 40.8, 34.2, 32.9, 29.0,
General Synthetic Details
Unless otherwise noted, all reactions were carried out in oven-
dried glassware, under argon atmosphere, with stirring using
standard Schlenk techniques. Column chromatography was car-
ried out using silica gel (35–70mm) unless otherwise specified.
Synthesis of 3-(2-ethylhexyl)-2-(triethoxymethyl)
thiophene (1)
A dry two-neck flask equipped with an addition funnel and
reflux condenser was filled with 1.22 g (50 mmol) magnesium

718
B. C. Tlach et al.
26.0, 23.2, 14.3, 11.0. m/z 707.0794. HRMS (ESI) Anal. Calc.
for C32H39Br2N2O2S2 707.0794. Found 707.0798.
40 mL dry THF, then cooled to ꢀ788C in a dry ice/acetone bath.
Then, 10.3 mL n-butyl lithium (nBuLi; 2.5 M hexane solution)
was added dropwise and the reaction was allowed to warm to
room temperature overnight. The reaction was quenched with
saturated NH4Cl (aq.) and extracted with hexane. The combined
organic layers were washed with H2O and brine, and dried over
Na2SO4. The solution was filtered and concentrated under
vacuum. The product was purified by column chromatography
using hexane as eluent to yield a yellow oil (2.90 g, 81 %). dH
(CDCl3) 6.63 (s, 2H), 6.47 (s, 1H), 3.80 (d, J 8, 4H), 3.01 (s, 1H),
1.70 (m, 2H), 1.52–1.31 (comp, 16H), 0.93–0.89 (comp, 12H).
dC (CDCl3) 160.4, 123.2, 110.4, 103.3, 84.0, 78.5, 39.5, 30.6,
29.2, 24.0, 23.2, 14.2, 11.2. m/z 359.2945. HRMS (ESI) Anal.
Calc. for C24H39O2 359.2945. Found 359.2948.
4,8-Dibromo-2,6-bis(3-(2-ethylhexyl)thien-2-yl)benzo
[1,2-d:4,5-d0]bisoxazole (4)
A dry Schlenk flask was filled with 3.95 g (11.5 mmol) 1,
4 mL dry THF, and 4 mL dry N,N-dimethylacetamide (DMA),
and the solution was deoxygenated for 20 min. The solution was
warmed to 558C under argon and 119 mg (0.19 mmol) Yb(OTf)3
was added. Then, 1.15 g (3.85 mmol) of freshly prepared Br-
DAHQ was added portion-wise over 20 min followed by 3 mL
THF after 2 h. Stirring was continued at 558C overnight and
the warm mixture was poured in 200 mL cold methanol. The
precipitate was filtered, rinsed with methanol, and the crude
product was recrystallised from ethyl acetate to yield off-white
crystals (485 mg, 19 %), mp 205–2078C. dH (CDCl3) 7.51 (d, J 8,
2H), 7.03 (d, J 4, 2H), 3.20 (d, J 8, 4H), 1.79 (m, 2H), 1.43–1.29
(comp, 16H), 0.92 (t, J 8, 3H), 0.87 (t, J 8, 3H). dC (CDCl3)
161.0, 148.2, 146.5, 139.4, 132.1, 129.9, 122.9, 91.2, 40.7, 34.5,
32.6, 28.8, 25.7, 23.3, 14.3, 10.8. m/z 707.0794. HRMS (ESI)
Anal. Calc. for C32H39Br2N2O2S2 707.0794. Found 707.0798.
4,8-Bis(3,5-di-(2-ethylhexyloxy)phenylethynyl)-2,6-bis
(5-bromo-3-(2-ethylhexyl)thien-2-yl)benzo[1,2-d:4,5-
d0]bisoxazole (7)
In a dry Schlenk flask, 1.02 g (0.81 mmol) 5 was dissolved in
20 mL dry THF and the solution was cooled to ꢀ788C in a dry
ice/acetone bath. Then, 1.0 mL nBuLi (2.5 M hexane) was added
dropwise over 10 min and stirred for 90 min at ꢀ788C. Then,
940 mg (2.84 mmol) CBr4 was introduced and the reaction was
stirred for 3 h at ꢀ788C and allowed to warm to room tempera-
ture overnight. The reaction was diluted with Et2O, quenched
with saturated NH4Cl (aq.), and the layers were separated. The
aqueous layer was extracted with Et2O, and the combined
organic layers were washed with H2O and brine, and dried over
Na2SO4. The solution was filtered and concentrated under
vacuum, and the product was purified by column chromatogra-
phy using hexane/CH2Cl2 eluent at varying ratios from 95 : 5 to
70 : 30 to yield a sticky red oil (0.71 g, 62 %). dH (CDCl3) 7.00 (s,
2H), 6.84 (d, J 4, 2H), 6.54 (t, J 4, 4H), 3.89 (d, J 8, 8H), 3.18 (dq,
Jd 40, Jq 8, 4H), 1.75 (comp, 6H), 1.55–1.17 (comp, 48H), 0.97–
0.91 (comp, 30H), 0.78 (t, J 8, 6H). dC (CDCl3) 160.5, 148.5,
148.3, 140.6, 134.6, 125.0, 123.8, 117.6, 110.4, 103.6, 100.7,
98.3, 79.1, 70.9, 40.8, 39.6, 34.5, 32.5, 30.7, 29.3, 28.7, 26.0,
24.1, 23.2, 23.1, 14.3, 14.26, 14.17, 11.3, 10.9. m/z 1417.6245.
HRMS (ESI) Anal. Calc. for C80H111Br2N2O6S2 1417.6245.
Found 1417.6204.
4,8-Bis(3,5-di-(2-ethylhexyloxy)phenylethynyl)-2,6-bis
(3-(2-ethylhexyl)thien-2-yl)benzo[1,2-d:4,5-d0]
bisoxazole (5)
A dry two-neck round-bottom flask equipped with a reflux
condenser was filled with 887 mg (2.48 mmol) 6, 1.5 mL
(11.0 mmol) diisopropylamine (iPr2NH), and 12 mL dry THF,
and the solution was deoxygenated for 30 min. The flask was
filled with 775 mg (1.10 mmol) 4, 10.5 mg (0.055 mmol) CuI,
and 38.6 mg (0.055 mmol) bis(chloro)bis(triphenylphosphine)
palladium(II) PdCl2(PPh3)2, and the mixture was further deoxy-
genated for 10 min, and heated to reflux for 48 h. The mixture
was allowed to cool to room temperature, filtered through a
small pad of celite, rinsed with CH2Cl2, and concentrated under
vacuum. The crude product was purified by column chromatog-
raphy using hexane/CH2Cl2 eluent at varying ratios from 90 : 10
to 70 : 30 to yield a viscous red oil (1.26 g, 91 %). dH (CDCl3)
7.49 (d, J 8, 2H), 7.03 (d, J 4, 2H), 6.86 (d, J 4, 4H), 6.54 (s, 4H),
6.54 (t, J 4, 2H), 3.89 (d, J 8, 8H), 3.23 (dq, Jd 44, Jq 8, 4H), 1.75
(m, 6H), 1.54–1.15 (comp, 48H), 0.97–0.91 (comp, 30H), 0.77
(t, J 8, 6H). dC (CDCl3) 161.1, 160.5, 148.4, 148.5, 140.7, 131.9,
129.4, 124.0, 123.5, 110.4, 103.5, 100.4, 98.2, 79.4, 70.9, 40.8,
39.6, 34.4, 32.5, 30.7, 29.3, 28.7, 26.1, 24.1, 23.2, 23.1, 14.3,
14.2, 11.3, 11.0. m/z 1261.8035. HRMS (ESI) Anal. Calc. for
C80H113N2O6S2 1261.8035. Found 1261.8024.
4,8-Bis(3,5-di-(2-ethylhexyloxy)phenylethynyl)benzo
[1,2-b:4,5-b’]dithiophene (9)
A dry two-neck round-bottom flask equipped with a reflux
condenser was filled with 700 mg (1.95 mmol) 6 and 5.5 mL dry
n
THF. The flask was cooled to 08C and 0.82 mL BuLi (2.5 M
hexane) was added dropwise. The reaction was stirred at 08C for
1 h, warmed to room temperature, and 187 mg (0.85 mmol)
benzo[1,2-b:4,5-b0]dithiophene-4,8-dione was introduced. The
reaction was heated to reflux for 18 h and then allowed to cool to
3,5-Di-(2-ethylhexyloxy)ethynylbenzene (6)
In a round-bottom flask, 10.8 g (41.0 mmol) triphenylpho-
sphine (PPh3) was dissolved in 30 mL CH2Cl2, cooled to 08C,
and 6.80 g (20.5 mmol) CBr4 was added to the solution. Then,
3.63 g (10.0 mmol) 1,3-di-(2-ethylhexyloxy)benzaldehyde was
dissolved in 20 mL CH2Cl2 and the resulting solution was added
drop-wise with a funnel to the reaction mixture. The reaction
was stirred for 1 h at 08C and then warmed to room temperature
over 2 h. The solid was filtered, rinsed with hexane, and the
filtrate was concentrated under vacuum. The crude mixture was
suspended in hexane, loaded on a pad of silica gel, and the
product was eluted with hexane. The eluted product was
concentrated and dried under vacuum with stirring. The flask
was back-filled with argon, and the resulting oil was dissolved in
room temperature.
A solution of 770 mg (3.40 mmol)
SnCl2 ꢂ 2H2O dissolved in 3.4 mL HCl (3 M) was added to the
reaction mixture followed by addition of 5 mL THF. The
resulting mixture was heated to reflux for 6 h and then allowed
to cool to room temperature. The mixture was extracted with
Et2O and the combined organic layers were washed with H2O
and brine, and dried over Na2SO4. The solution was filtered and
concentrated under vacuum. The crude product was purified by
column chromatography (basic alumina) using hexane/CH2Cl2
eluent at varying concentration ratios from 95 : 5 to 75 : 25 to
yield a viscous yellow oil (380 mg, 49 %). dH (CDCl3) 7.73 (d, J
4, 2H), 7.60 (d, J 4, 2H), 6.82 (s, 4H), 6.53 (s, 2H), 3.90 (d, J 8,

Benzo[1,2-d:4,5-d0]bisoxazole Polymers
719
8H), 1.75 (m, 4H), 1.56–1.34 (comp, 32H), 0.98–0.93 (comp,
24H). dC (CDCl3) 160.6, 140.5, 138.5, 128.3, 124.0, 123.4,
112.1, 110.1, 103.2, 99.6, 85.1, 70.8, 39.6, 30.7, 29.3, 24.1, 23.2,
14.3, 11.3. m/z 903.5414. HRMS (ESI) Anal. Calc. for
C58H78O4S2 903.5414. Found 903.5404.
Poly[(4,8-bis(2-ethylhexyloxy)benzo[1,2-b:4,5-b’]
dithiophene)-2,6-diyl-alt-(4,8-bis(3,5-di-(2-
ethylhexyloxy)phenylethynyl)-2,6-bis((2-ethylhexyl)
thien-2-yl)benzo[1,2-d:4,5-d0]bisoxazole)] (P2)
P2 was synthesised from 276 mg (0.20 mmol) 7 and
150.3 mg (0.20 mmol) 10 to yield a dark red solid (146 mg,
44 %). dH (CDCl3) 7.67 (2H), 6.97–6.90 (8H), 6.57 (4H), 4.25
(4H), 3.91 (8H), 3.26 (4H), 1.78–0.82 (120H).
2,6-Bis(trimethylstannyl)-4,8-bis(3,5-di-(2-
ethylhexyloxy)phenylethynyl)benzo[1,2-b:4,5-b’]
dithiophene (11)
Poly[(4,8-bis-(3,5-di-(2-ethylhexyloxy)phenylethynyl)
benzo[1,2-b:4,5-b’]dithiophene)-2,6-diyl-alt)-(2,6-bis
((2-ethylhexyl)thien-2-yl)benzo[1,2-d:4,5-d0]
bisoxazole)] (P3)
In a dry Schlenk flask, 1.41 g (1.56 mmol) 9 was dissolved in
40 mL dry THF and the solution was cooled to ꢀ788C in a dry
ice/acetone bath. Then, 1.75 mL nBuLi (2.5 M hexane) was
added dropwise and the reaction was stirred for 90 min at
ꢀ788C. Then, 4.5 mL of trimethyltin chloride (1.0 M in THF)
was added and the reaction allowed to warm to room tempera-
ture overnight. The reaction mixture was diluted in hexane,
quenched with H2O, and the layers were separated. The organic
layer was washed with H2O and brine, and dried over Na2SO4.
The solution was filtered and concentrated under vacuum, and
the crude product was dissolved in a minimal volume of warm
CHCl3. The solution was precipitated in 400 mL cold methanol
and the precipitate was filtered and rinsed with methanol to yield
a yellow powder (1.64 g, 86 %), mp 101–1038C. dH (CDCl3)
7.73 (s, 2H), 6.84 (d, J 4, 4H), 6.53 (d, J 4, 2H), 3.90 (d, J 8, 8H),
1.75 (m, 4H), 1.57–1.34 (comp, 32H), 0.97–0.90 (comp, 24H),
0.49 (s, 18H). dC (CDCl3) 160.5, 144.8, 143.7, 139.3, 131.0,
124.3, 110.23, 110.20, 103.1, 98.9, 85.8, 70.9, 39.6, 30.7, 29.3,
24.1, 23.2, 14.3, 11.3, ꢀ8.1. m/z 1231.4710. HRMS (ESI) Anal.
Calc. for C64H95O4S2Sn2 1231.4710. Found 1231.4698.
P3 was synthesised from 177 mg (0.25 mmol) 3 and 307 mg
(0.25 mmol) 11 to yield a dark red solid (289 mg, 80 %). dH
(CDCl3) 7.82 (2H), 7.38 (2H), 7.00 (2H), 6.90 (4H), 6.65 (2H),
4.04 (8H), 3.19 (2H), 1.83–0.97 (90H).
Poly[(4,8-bis(3,5-di(2-ethylhexyloxy)phenylethynyl)
benzo[1,2-b:4,5-b’]dithiophene)-2,6-diyl-alt-(4,8-bis
(3,5-di-(2-ethylhexyloxy)phenylethynyl)-2,6-bis
((2-ethylhexyl)thien-2-yl)benzo[1,2-d:4,5-d0]
bisoxazole)] (P4)
P4 was synthesised from 240 mg (0.17 mmol) 7 and 207 mg
(0.17 mmol) 11 to yield a red solid (252 mg, 69 %). dH (CDCl3)
7.87 (2H), 7.35 (2H), 6.87 (8H), 6.53 (d, J 8, 4H), 3.90 (q, J 8,
16H), 3.23–3.33 (4H), 1.87 (2H), 1.72 (8H), 1.52–1.21 (80H),
0.97–0.79 (60H).
Computational Details
Synthesis of Polymers P1–P4
All computations were performed using Gaussian 09 through
the National Science Foundation Extreme Science and Engi-
neering Discovery Environment and San Diego Supercomputer
Center Trestles Cluster. Excited states were generated through
time-dependent density functional theory and applied to the
optimised ground state of each polymer subunits, n ¼ 1, 2, 3,
and 4. The long chain limits for the HOMO, LUMO, and band
gap were generated from the excited computations and fitted
using the Kuhn expression.[30]
A dry two-neck flask equipped with a reflux condenser under
argon was filled with stannane 3 or 7 and bromide 10 or 11 in
equimolar amounts (outlined below). A solution of dry toluene
and DMF (8 : 1) was then added and the mixture was thoroughly
deoxygenated for 30 min. Pd(PPh3)4 (5 mol-%) was introduced
and the mixture was deoxygenated for an additional 10 min and
heated to reflux for 48 h. One drop of trimethyl(phenyl)tin and
1 mL toluene were then added and reflux was continued for 6 h.
Two drops of iodobenzene were added, followed by refluxing
for 12 h to complete the polymer end-capping process. The
warm polymer solution was precipitated in methanol and fil-
tered through a cellulose thimble. The polymer was placed in a
Soxhlet extractor and washed with methanol, acetone, hexane,
and CHCl3. The solution of polymer in chloroform was cooled
to 508C, stirred with Silicycle DMTÒ for 8 h to remove metal
impurities, and the crude polymer was concentrated to ,5 mL
under vacuum. The crude polymer solution was filtered through
a small pad of silica gel using CHCl3 eluent and the solution was
concentrated to ,5 mL under vacuum. The polymer solution
was re-precipitated in cold methanol, and the solid was filtered,
rinsed with methanol, and dried under vacuum to yield polymers
P1–P4.
Supplementary Material
NMR spectra for all new compounds, thermal data, additional
UV-Vis absorption and emission spectra for the polymers are
available on the Journal’s website.
Acknowledgements
The authors acknowledge the Petroleum Research Fund for financial support
and 3M Foundation and the National Science Foundation (DMR-0846607)
for partial support. The authors thank Kamel Harrata and the Mass Spec-
troscopy Laboratory Iowa State University (ISU) for analysis of our com-
pounds and the National Science Foundation Extreme Science and
Engineering Discovery Environment (TG-CHE-120081) for providing the
resources for all the computational work included in this study.
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