Tuning the optical and electronic properties of 4,8-disubstituted benzobisoxazoles via alkyne substitution

Article abstract of DOI:10.1021/jo201078w

In an effort to design new electron-deficient building blocks for the synthesis of conjugated materials, a series of new trans-benzobisoxazoles bearing halogen or alkynyl substituents at the 4,8-positions was synthesized. Additionally, the impact of these modifications on the optical and electronic properties was investigated. Theoretical calculations predicted that the incorporation of various alkynes can be used to tune the energy levels and band gaps of these small molecules. The targeted 4,8-disubstituted benzobisoxazoles were easily prepared in good yields using a two-step reaction sequence: Lewis acid catalyzed orthoester cyclization followed by Sonogashira cross-coupling. The experimentally determined HOMO values for these 4,8-disubstituted benzobisoxazoles ranged from -4.97 to -6.20 eV and showed reasonable correlation to the theoretically predicted values, with a percent deviation that ranged from 2.4-12.8%. However, the deviation between actual and predicted HOMO values was reduced to less than 3.5% when the theoretical values were extrapolated to the long-chain limit and compared to copolymers containing the 4,8-disubstituted benzobisoxazoles. Collectively, these results indicate that these 4,8-disubstituted trans-benzobisoxazoles can be used for the synthesis of new conjugated materials with electronic properties that are variable and predictable.

Full text of DOI:10.1021/jo201078w

Article
pubs.acs.org/joc
Tuning the Optical and Electronic Properties of 4,8-Disubstituted
Benzobisoxazoles via Alkyne Substitution
Brian C. Tlach,† Aimee L. Tomlinson,‡ Achala Bhuwalka,† and Malika Jeffries-EL*,†
́
†Department of Chemistry, Iowa State University, Ames, Iowa 50010, United States
‡Department of Chemistry, North Georgia College & State University, Dahlonega, Georgia 30597, United States
S
* Supporting Information
ABSTRACT: In an effort to design new electron-deficient building blocks for
the synthesis of conjugated materials, a series of new trans-benzobisoxazoles
bearing halogen or alkynyl substituents at the 4,8-positions was synthesized.
Additionally, the impact of these modifications on the optical and electronic
properties was investigated. Theoretical calculations predicted that the
incorporation of various alkynes can be used to tune the energy levels and
band gaps of these small molecules. The targeted 4,8-disubstituted
benzobisoxazoles were easily prepared in good yields using a two-step reaction
sequence: Lewis acid catalyzed orthoester cyclization followed by Sonogashira
cross-coupling. The experimentally determined HOMO values for these 4,8-
disubstituted benzobisoxazoles ranged from −4.97 to −6.20 eV and showed reasonable correlation to the theoretically predicted
values, with a percent deviation that ranged from 2.4−12.8%. However, the deviation between actual and predicted HOMO
values was reduced to less than 3.5% when the theoretical values were extrapolated to the long-chain limit and compared to
copolymers containing the 4,8-disubstituted benzobisoxazoles. Collectively, these results indicate that these 4,8-disubstituted
trans-benzobisoxazoles can be used for the synthesis of new conjugated materials with electronic properties that are variable and
predictable.
polymers because of their near-planar structure which can
facilitate efficient packing and charge transport.19−22 Recently,
BBOs have been used for the synthesis of donor−acceptor
conjugated polymers; however, because of the relatively weak
electron-accepting nature of the BBO moiety, the resulting
materials have had fairly wide bands gaps (1.9−2.3 eV).23−25
Since BBO has two positions on the central benzene ring (4-
and 8-) available for structural modification, it may be possible
to design new derivatives that are better electron acceptors. In
this paper, we evaluate alkyne substitution as an approach to
increase electron affinity. We investigated the use of alkynes
since, according to the Pauling scale, their electronegativity is
similar to that of the cyano group (3.3), which has been widely
used in the synthesis of electron-deficient building blocks.26
Alkynes have also been utilized to alter the electronic properties
of conjugated materials.27−34 In addition, alkynes offer the
advantage of facile installation via Sonogashira cross-coupling.
In the case of the BBO moiety, the additional structural
variation through substitution facilitates the efficient synthesis
of materials with tunable electronic properties by using shared
synthetic intermediates. Herein, we examine the influence of
alkyne substitution at the central benzene ring on the optical
and electronic properties of 4,8-disubstituted benzo[1,2-d;4,5-
d′]bisoxazoles (BBO)s, using a combination of synthetic and
computational techniques.
INTRODUCTION
■
Conjugated polymers (CPs) are currently being investigated as
replacements for traditional inorganic materials in applications
such as field effect transistors (FET)s,1−5 organic light emitting
diodes (OLED)s,6−8 and photovoltaic cells (PVC)s.9−12 The
development of these organic semiconductors is motivated by
the potential to reduce the cost of device fabrication through
the use of solution-based techniques, and the ability to tune the
energy levels for specific applications through chemical
synthesis.13 Currently, a common approach for tuning the
properties of CPs is through the synthesis of materials
composed of alternating electron-donating and electron-
accepting moieties.14−17 By varying the strength of the donor
and acceptor components in the polymer backbone, the highest
occupied molecular orbital (HOMO), lowest unoccupied
molecular orbital (LUMO), and band gap of the resulting
polymer can be readily manipulated.15,18 Since there are many
known π-systems, a wide range of energy levels can be obtained
by combining different components. However, in practice,
certain combinations of properties such as electron-donating
and hole-transporting materials (p-type) with low-lying LUMO
levels and narrow band gaps or electron-accepting and electron-
transporting materials (n-type) with good solubility are more
difficult to attain. Thus, the design of new electron-rich or
electron-poor building blocks remains a flourishing area of
research.13
In this respect, benzobisoxazoles (BBOs) are promising
electron-deficient heterocycles for the development of new
Received: June 9, 2011
Published: October 4, 2011
© 2011 American Chemical Society
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a
Scheme 1. Synthesis of 2,6-Disubstituted-4,8-dihalogenated BBOs
a
10b was synthesized according to the literature procedure.35
Scheme 2. Synthesis of Substituted Phenylacetylenes
RESULTS AND DISCUSSION
Scheme 3. Synthesis of Bis(phosphonate) Esters by Arbuzov
Reaction
■
Model Compound Synthesis. The synthesis of 4,8-
dihalogenated-2,6-disubstituted BBOs is shown in Scheme 1.
Recently, we reported a simple method for the synthesis of 2,6-
disubstituted BBOs that occurs readily at low temperatures and
is tolerant of a number of functional groups.35 Utilizing this
method and the halogenated diaminohydroquinones 5 and 6
we were able to synthesize several 4,8-dichloro- and 4,8-
dibromo-2,6-disubstituted BBOs in good yields. The lowest
yields were observed when ethyl triethoxyacetate (7e) was used
for the synthesis of 8e and 9e. The reduction in yield is likely
due to the poor nucleophilic site adjacent to the ester group.
Although this reaction proceeded slowly, the moderate yields
obtained (26−41%) are higher than previously reported
values.21 We also modified the synthesis of 8d and 9d by
using THF as the solvent due to the limited solubility of triethyl
orthopropiolate (7d) in DMA or DMSO. In all cases, the
products were easily isolated by precipitation of the reaction
mixture in water, filtration, followed by recrystallization from an
appropriate solvent. Because of the scalable nature of the
reactions, several compounds were prepared in multigram
quantities without a decrease in yield.
Scheme 4. Modification of 4,8-Dibromobenzobisoxazoles by
Sonogashira Cross-Coupling
The synthesis of the 4-alkyl and 4-alkoxyphenylacetylenes is
shown in Scheme 2. The flexible side chains were appended to
improve the solubility of the resulting BBOs. The correspond-
ing 4,8-bisalkynyl BBOs 11b, 13b, 14b, 15b, 15f, and 16f were
obtained in good yields (73% to 88%) with 12b in a lower yield
(49%) by the Sonogashira cross-coupling reaction of 4,8-
dibromo-2,6-dimethyl BBO 8b and various alkynes as shown in
Scheme 4.
benzobisoxazoles via recrystallization or slow solvent evapo-
ration produced either powders or small crystals that were
unsuitable for X-ray crystallography. However, we were able to
obtain suitable crystals of 8c, 9a, 9c, and 10a (see the
crystallographic data in the Supporting Information).
The resultant substituted BBOs were fully characterized by
1H NMR, 13C NMR, and high-resolution mass spectroscopy.
For the most part, all attempts to grow crystals of the
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Scheme 5. Synthesis of Benzobisoxazole Polymers
Polymer Synthesis. The synthesis of the 4,8-disubstituted
poly(arylene vinylene benzobisoxazole)s is shown in Scheme 5.
The bis(phosphonate) esters 8f and 9f were synthesized by the
Arbuzov reaction of the corresponding 2,6-bis(chloromethyl)
BBOs 8c and 9c and triethyl phosphite as shown in Scheme
3.23 The bis(phosphonate) esters 15f and 16f were synthesized
as shown in Scheme 4 by Sonogashira cross-coupling of 8f with
1-decyne and 4-dodecylphenylacetylene, respectively. The
Horner−Wadsworth−Emmons (HWE) polymerization of
BBO monomers 8f, 15f, or 16f and 2,5-didodecyloxytereph-
thaldehyde36 produced polymers P1, P2, and P3 in yields of
56%, 87%, and 55%, respectively. Similarly, the HWE
polymerization of BBO monomers 8f or 9f and 3,4-
didodecylthiophene dicarboxaldehyde37 yielded the corre-
sponding polymers P4 and P5 in yields of 61% and 55%,
respectively. P1, P3, P5, and P6 were polymerized using
potassium tert-butoxide as the base. The base-sensitive
propargyl position on 16f required the use of a LiBr/Et3N
for selective deprotonation of the phosphonate ester to obtain
P2 in 87% yield.38 All of the polymers were soluble in several
organic solvents, and the structures of the polymers were
verified by 1H NMR spectroscopy (see the Supporting
Information) and gel permeation chromatography, which
revealed monomodal molecular weight distributions.
Spectroscopic Characterization of Model Com-
pounds. The UV−vis absorption (Figure 1) and photo-
luminescence (PL) (S50 Supporting Information) properties of
the BBOs in solution have been investigated, and the spectral
data are summarized in Table 1. The absorption and PL spectra
vary depending on the type of substituents present on the 4,8-
position of the BBO core. When excited at their respective λmax
,
BBOs 8b, 9b, and 10b exhibited very weak fluorescence in
solution, whereas all of the other BBOs exhibited strong
fluorescence in solution. The lack of fluorescence for 8b and 9b
is likely due to the heavy-atom effect. The absorbance spectra
of the halogen-substituted BBOs 8b and 9b displayed intense
vibronic coupling with three dominant absorption bands,
whereas the unsubstituted BBO 10b had two major peaks,
each showing strong vibronic coupling. The absorbance spectra
of the ethynyl-substituted BBOs 11b−15b displayed weak
vibronic coupling. All of the substituted BBOs had red-shifted
absorbance spectra due to the extended conjugation of the
system relative to the unsubstituted BBO 10b (abs λmax 209,
285 nm). The phenylethynyl benzobisoxazoles 13b−15b
featured longer conjugation lengths and thus absorbed at the
Figure 1. UV−vis spectra of parent and halogenated BBOs (top) and
alkynyl substituted BBOs (bottom).
longest wavelengths. The introduction of the alkoxy-group onto
the para position of the phenyl substituent resulted in a red
shift in the absorbance spectrum; consequently, 14b absorbed
at a longer wavelength than 15b.
The PL and absorption spectra of the polymers in solution
(Figure 2) and in thin film (S52 Supporting Information) were
also measured. In solution, P1 had a PL maximum at 524 nm,
with absorption maximum at 476 nm. P4 had a PL maximum at
563 nm, with an absorption maximum at 506 nm, and P6 had a
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of the HOMO level.39−41 The UPS HOMO values of the BBOs
ranged from −5.70 to −6.50 eV, and the experimentally
determined band gaps ranged from 3.19 to 4.62 eV. The
LUMO levels ranged from −1.28 to −2.59 eV, and were
calculated by adding the optical band gap to the HOMO values.
The introduction of electron-withdrawing groups into π-
conjugated systems stabilized the LUMO energy and increased
the electron-transporting ability.3 In general, the alkynyl
substituted BBOs had deeper HOMOs and smaller band gaps
than the halogenated BBOs. When a phenyl ring was added
onto the alkynyl substituent, the HOMO level was slightly
decreased, and the LUMO level was slightly increased in
comparison to the unsubstituted alkynyl group. The HOMO
was further raised when electron-donating alkyl or alkoxy
substituents were added to the ring.
Computational Studies. To elucidate the influence of
structural modification on the optical and electronic properties
of the 4,8-disubstituted BBOs, ground-state geometry opti-
mizations were performed utilizing density functional theory
(DFT) employing a B3LYP42 functional, a 6-31G* basis and
the Gaussian 03W software package. DFT/B3LYP/6-31G* is a
reliable method that has been widely used for calculating the
structural and optical properties of many systems.43−46 UV−vis
spectra were simulated utilizing the first 10 excited states and
the time-dependent density functional theory (TDDFT)
routine with the aforementioned functional and basis set (see
Figure 4 and the Supporting Information). In addition, the
HOMO, LUMO, and optical band gaps (the first excited state)
were produced from the TDDFT output. To minimize
computational cost, methyl groups were used at the 2 and 6
positions. Also, all alkoxy side chains were truncated to
methoxy side chains and alkyl side chains omitted. Hence, the
results for structure 15b are not reported here since it would be
a replication of the prediction for structure 13b. To evaluate the
predictive power of the computational method, a comparison of
computed HOMO, LUMO, and optical band gap values to
experimental data was made. These results are summarized in
Table 2.
Table 1. Optical Data of BBO Model Compounds
a
b
(M−1 cm−1
)
λems (nm)
c
λabs (nm)
222*, 274
219*, 274
ε
8b
38900
23600
16200
38700
33600
43600
45600
58600
307, 385
313, 385
308, 401
348
9b
10b
11b
12b
13b
14b
15b
209, 252*, 285
243,304, 317*
242, 315*
248, 350*, 369
226, 259, 362*, 383
229, 355*, 376
362
376
397
383
a
b
All measurements performed in THF. Extinction coefficients based
c
on absorbance at λmax. Performed at λmax. Asterisk (*) denotes λmax
.
PL maximum at 560 nm, with an absorption maximum at 510
nm. When the dialkoxyphenyl comonomer was replaced with
the less aromatic and more electron-rich dialkylthiophene, a red
shift in the absorbance spectra occurred. In all cases, the PL and
absorption spectra were very similar to the analogous
nonhalogenated polymers: poly(2,5-bisdodecyloxyphenylene
vinylene)-alt-benzo[1,2-d;4,5-d′]bisoxazole]-2,6-diyl23 and
poly(3,4-didodecylthiopene vinylene)-alt-benzo[1,2-d;4,5-d′]-
bisoxazole]-2,6-diyl.24 The alkyl-alkyne substituted polymer
P2 had a PL maximum at 576 nm, with absorption maximum at
491 nm, whereas the phenylethynyl substituted P3 had a PL
maximum at 601 nm with an absorption maximum at 520 nm.
The bathochromic shift in the absorbance spectra of these
polymers relative to their halogenated counterparts was a result
of the increased acceptor strength of the substituted BBOs.
The energy levels of the BBO model compounds were
investigated and compared to the theoretical data (see Table
2). The HOMO values obtained for 8b−10b using cyclic
voltammetry had excellent correlation to those predicted by
theory, with a percent error of less than 5%. However, we saw a
large deviation between the electrochemically determined
HOMO values and the predicted values for the other BBOs.
As a whole, the electrochemistry of the BBOs was not well
behaved and all molecules exhibited nonreversible oxidation
cycles. Furthermore, a reduction cycle was not seen for any of
the compounds within the solvent window for the acetonitrile/
Bu4N+BF4− (−2.7 to −3.0 eV versus Ag/AgCl). For this reason,
we evaluated the BBOs using ultraviolet photoelectron
spectroscopy (UPS), which provides an absolute determination
There was a good correlation for BBOs without conjugated
substitutents (8b−10b), such that the difference between the
theoretical and the experimental values for the HOMOs was
less than 0.3 eV. However, the difference between predicted
and measured values was as large as 0.8 eV for the optical band
Figure 2. UV−vis spectra of polymers: in solution and as films spun from THF or chloroform/o-dichlorobenzene solutions (right).
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Table 2. Experimental and Theoretical Comparison of the Electronic Properties of BBO Model Compounds
HOMO (eV)
theory
LUMO (eV)
theory
Egopt (eV)
a
b
expt
% dev
expt
% dev
expt
theory
% dev
c
8b
−6.50
−6.50
−5.90
−5.93
−6.09
−5.92
−6.20
−6.30
−6.04
−5.40
−5.70
−5.30
−5.67
−4.97
−5.54
n/a
4.6
3.1
−2.28
−2.16
−1.28
−2.24
−2.47
−2.65
−1.45
−1.47
−1.01
−1.43
−1.81
−1.97
−1.67
−1.74
−1.54
n/a
36.4
31.9
21.1
36.2
26.7
25.7
37.0
32.6
40.3
n/a
4.22 (294)
4.34 (286)
4.62 (296)
3.69 (336)
3.62 (343)
3.27 (379)
4.42
4.52
4.70
3.84
3.74
3.16
3.76
3.03
3.41
n/a
4.7
4.1
d
d
d
d
d
9b
10b
11b
12b
13b
13b
14b
14b
15b
2.4
1.7
8.9
4.1
6.5
3.3
10.5
4.3
3.4
e
e
18.0
2.9
d
−5.70
12.8
2.9
−2.58
3.12 (398)
9.3
d
−5.78
n/a
−2.59
b
3.19 (386)
n/a
a
c
d
Determined using ultraviolet photoelectron spectroscopy. Calculated from HOMO − Egopt. Measured from the λmax absorbance. Calculated
from lowest energy absorption onset from the intersection of the leading edge tangent with the x-axis. Based on an optimized geometry of 90°.
e
gaps. This was not surprising as DFT methods are known to
underestimate band gaps.47−50 The difference between the
theoretical and the experimental values for the LUMOs was as
large as 0.4 eV. These values possessed the highest amount of
deviation (12−58%) due to the propagated deviations in the
optical band gap and the HOMO values. A larger deviation in
theoretical and experimental HOMO values was observed for
BBOs bearing alkynyl substituents. The deviation was moderate
for structures 11b and 12b (0.53 and 0.39 eV), whereas the
variance in structures 13b and 14b was much higher (0.62 and
0.73 eV). We hypothesized that the optimized geometry could
be the source of error for the phenylethynyl substituted
structures. The minimized geometry predicted a planar
configuration whereas it is likely the actual compounds
possessed rings that were twisted out of plane. The idea of
nonplanarity in phenylethynyl side chains is supported by a
number of studies.51,52 To validate this hypothesis, optimized
geometries and subsequent excited-state calculations were
performed on structures in which the phenyl substituent was
forced perpendicular to the BBO core (see Figure 3). Adapting
respectively. The black bars and solid squares were indicative of
the oscillator strength and the corresponding excited state.
There was a very good agreement between the two data sets
especially when one considers that all theoretical computations
were performed in vacuum and did not account for solvent
stabilization that was present in the experimental sample. This
was further demonstrated in an analysis of the contributions of
the excited states (see Supporting Information).53 In all cases,
the lowest lying state was primarily due to electronic excitations
directly between the HOMO and the LUMO. For 8b, 9b, and
12b, this lowest state was split into two excited states in the
computed spectra, whereas the experimental spectra displayed
one broad peak. To further evaluate the competency of this
method, we generated a correlation plot of the observed and
computed wavelengths (see Figure 4). For this comparison, an
Figure 4. Comparison of the experimental UV−vis spectrum of 12b
with the predicted excited states.
R2 value of 0.87 was found, indicating good correlation
between the experimental and theoretical values.
Figure 3. Representation for the optimized (bottom) and
perpendicular (top) geometries of 13b.
To examine the influence of substitution on the electron
density of the BBO core, the frontier orbitals and electrostatic
potential (ESP) maps for each of the model compounds were
generated and are shown in Figure 6. The electron-withdrawing
nature of the halogens is demonstrated by the frontier orbital
HOMOs for 8b and 9b, which show a significant amount of
electron density that has been drawn away from the BBO
backbone. This effect was further exhibited by the ESP maps in
which the backbone is slightly greener (less electron rich) than
that of the nonsubstituted core 10b. Similarly, the electron-
withdrawing alkynyl substituents also exhibited a reduction in
a twisted geometry reduced the large deviation between the
experimental and theoretical HOMO values for 13b and 14b
from 0.62 to 0.25 eV and from 0.73 to 0.16 eV, respectively.
Strong correlation between the experimental and simulated
UV−vis spectra was also observed. A direct comparison for 12b
is shown in Figure 5, and the remaining structures are shown in
the Supporting Information (Figures S53−S58). The gray and
black lines represented the experimental data and theoretical fit,
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semiconductors. To evaluate the ability of our computational
methods to predict the properties of 4,8-disubstituted BBO
polymers, the geometries of model oligomers (n = 1, 2, 3, and
4) for P1, P2, P3, P4, and P6 were optimized at the density
functional theory B3LYP/6-31G* level. In all cases, the
bisdodecyl substituents were truncated to methyl groups to
reduce computational costs. Unfortunately, P3 was found to be
too large to extrapolate to the long-chain limit. The HOMO,
LUMO and band gaps for these polymers were obtained by
fitting the aforementioned set of oligomers with the Kuhn
expression58,59
Figure 5. Correlation plot of the observed and computed wavelengths.
(1)
the density of the BBO core (see 11b, 12b, 13b, and 14b).
However, the addition of substituents bearing phenyl rings led
to a large reorganization of electron density from the BBO core
(see 13b and 14b). This redistribution was so significant that it
appears that the BBO core was no longer the primary
conjugation axis, which is noteworthy because it suggests that
alkynyl substitution can be used in the synthesis of two-
dimensional π-delocalized polymers. Such cruciforms can
exhibit interesting optical and electronic properties due to
their multiple conjugated pathways.20,22,54−57
where E0 is the transition energy of a formal double bond, N is
the number of double bonds in the oligomer, and k′/k0 is an
adjustable parameter. The results are summarized in Table 3.
Overall, there was excellent correlation between all predicted
and experimental values. The largest deviation for both the
HOMO and LUMO levels were 1.5% and 4.0%, respectively.
This trend was quite an improvement over the small molecule
cases where the deviation was quite a bit larger (12.8% for
HOMO and 40.3% for LUMO). The polymeric band gaps were
found to experience the largest amount of deviation at 10.7%,
which was attributed to DFT overestimation.43,46 As a whole,
these results indicated that this level of theory can be used to
The ability to accurately predict the electronic properties of
polymeric materials is essential for the design new organic
Figure 6. Pictorial representations of the frontier orbitals for compounds 8b−14b (left) and electrostatic potential maps (right). For the frontier
orbitals, the red lobes are positive and the yellow lobes are negative. The charge density within the electrostatic potential maps range from red
(electron rich) to blue (electron poor).
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Table 3. Experimental and Theoretical Comparison of the Electronic Properties of BBO Polymers
HOMO (eV)
theory
LUMO (eV)
theory
Egopt (eV)
expt
% dev
expt
% dev
expt
theory
% dev
a
a
a
a
a
b
b
b
b
b
c
P1
P2
P3
P4
P5
−5.06
−5.05
−5.00
−5.38
−5.38
−5.23
−4.98
n/a
3.4
1.4
−2.91
−3.00
−2.96
−3.30
−3.31
−2.93
−2.78
n/a
0.7
4.0
n/a
2.1
1.5
2.15
2.05
2.04
2.05
2.07
2.03
1.98
n/a
5.6
3.4
n/a
6.3
7.7
c
c
c
c
n/a
0.93
1.5
−5.43
−3.23
−3.26
1.92
−5.46
1.91
a
b
Calculated from LUMO − Egopt
.
Measured in degassed CH3CN using 0.1 M Bu4NPF6; referenced to Fc/Fc+ (HOMO level −4.8 eV below
c
vacuum). Measured from the absorbance at λmax. Calculated by the energy absorption onset from the intersection of the leading edge tangent with
the x-axis.
vis detector. Analyses were performed at 35 °C using THF as the
eluent with the flow rate at 1.0 mL/min. Calibration was based on
polystyrene standards. Fluorescence spectroscopy and UV−visible
spectroscopy were obtained using polymer solutions in THF, and thin
films were spun from THF or CHCl3/o-dichlorobenzene solutions.
The films were made by spin-coating 25 × 25 × 1 mm glass slides,
using 10 mg/mL polymer solutions at a spin rate of 5000 rpm on a
spin-coater. Ultraviolet Photoelectron Spectroscopy measurements
were performed on sample films. X-ray radiation and the detector to
crystal distance of 5.03 cm. X-ray crystal structure data for compounds
10a (CDC 734101), 10b (CCDC 687294), 10c (CCDC 734103),
10d (CCDC 734102), and 9c (CCDC 838477) were deposited with
the Cambridge Crystallographic Data Centre, 12 Union Road,
Cambridge CB2 1EZ, UK. Triethyl orthochloroacetate (7c),60
trimethylsilyl ethyl orthopropiolate (7d),35 ethyl triethoxyacetate
(7e),61 p-bromanil,62 4-bromo-trimethylsilylbenzene,5 2,6-
dimethylbenzo[1,2-d;4,5-d′]bisoxazole (10b),35 2,5-didodecyloxyter-
ephthaldehyde,36 and 3,4-didodecylthiophene dicarboxaldehyde37
were prepared according to literature procedure.35,63 3,6-Diamino-
2,5-dichloro-1,4-benzoquinone was prepared from p-chloranil accord-
ing to literature procedure.62
3,6-Diamino-2,5-dibromo-1,4-benzoquinone (3). A 500 mL,
three-neck round-bottom flask was equipped with an addition funnel
and mechanical stirrer and charged with 42.9 g (100 mmol) of p-
bromanil (1) and 170 mL of 2-methoxyethyl acetate. The slurry was
vigorously stirred while heating to 60 °C. The mixture was removed
from the heat source and the addition funnel charged with 43.3 mL
(300 mmol) of 27% NH4OH and added dropwise over 25 min (the
reaction exothermed to over 100 °C during the addition). The
reaction mixture was allowed to cool to approximately 80 °C, stirred at
this temperature for 3 h, and allowed to cool to room temperature
overnight. The dark red mixture was filtered and the collected
precipitate washed with large portions of distilled water. The collected
solid was placed in a flask containing acetone, stirred, and refiltered.
The precipitate was then washed with acetone and dried in vacuo to
yield a brick-red powder (27.9 g, 94% yield). Because of the limited
solubility of this compound it was used without further purification or
analysis.
accurately predict the HOMO, LUMO, and band gaps for
conjugated polymers possessing BBO moieties.
CONCLUSIONS
■
In summary, we have demonstrated that alkyne substitution at
the central benzene ring of benzo[1,2-d;4,5-d′]bisoxazoles can
be used to modify the optical and electronic properties of these
compounds. We were able to synthesize the target compounds
in good yields by first synthesizing halogenated benzobisox-
azoles and then forming two-dimensional π-systems using
Sonogashira cross-coupling. The predicted HOMO values for
the small molecules matched well with the experimental results,
although as expected, a higher degree of error was seen for the
LUMO and band gap values. In contrast, the predicted energy
levels for polymeric materials exhibited excellent correlation for
all parameters, further exemplifying the usefulness of the
theoretical methods for designing new materials. Although
theoretical calculations predicted that attachment of alkynes
could be used to vary the energy levels of BBOs by almost 1 eV,
smaller changes were observed for the BBO polymers. While it
was our intention to minimize computational requirements by
using small molecules to evaluate the impact of these structural
modifications, this was unfortunately not reasonable as it
appears that most of the electronic tuning in BBO small
molecules was due to a change in the conjugation axis.
Currently, we are designing new BBOs bearing other electron
withdrawing groups to further improve the acceptor strength of
this group and synthesizing new conjugated polymers based on
these alkyne substituted BBOs.
EXPERIMENTAL SECTION
■
General Methods. Unless otherwise noted, reactions were
conducted under ambient atmosphere at 18−26 °C. Nuclear magnetic
resonance (NMR) experiments were carried out in either CDCl3 or
DMSO-d6 at 400 MHz (1H) or 100 MHz (13C). 1H NMR spectra are
internally referenced to the residual protonated solvent peak, and the
13C NMR are referenced to the central carbon peak of the solvent. In
all spectra, chemical shifts are given in δ relative to the solvent.
Coupling constants are reported in hertz (Hz). Cyclic voltammetry
was performed using a potentiostat, scanning at a rate of 50−100 mV/
s. The polymer solutions (2−10 mg/mL) were drop-cast on a
platinum electrode, and Ag/Ag+ was used as the reference electrode.
The reported values are referenced to Fc/Fc+ (−4.8 eV versus
vacuum). All electrochemistry experiments were performed in dry-
degassed CH3CN under argon atmosphere using 0.1 M tetrabuty-
lammonium hexafluorophosphate as the electrolyte. High-resolution
mass spectra were recorded on a double-focusing magnetic sector mass
spectrometer using EI or ESI at 70 eV. Melting points were obtained
using a melting point apparatus, upper temperature limit 260 °C. Gel
permeation chromatography (GPC) measurements were performed
on a separation module equipped with three 5 μm I-gel columns
connected in a series (guard, HMW, MMW, and LMW) with a UV−
3,6-Diamino-2,5-dibromo-1,4-hydroquinone (5). A 100 mL
round-bottom flask equipped with a large stir bar was charged with
3.26 g (11.0 mmol) of 3,6-diamino-2,5-dibromo-1,4-benzoquinone
(3), 33 mL of ethanol, and 7 mL of distilled water. The flask was fitted
with an addition funnel and heated to 55 °C with stirring under argon
atmosphere. The addition funnel was charged with 4.78 g (27.5 mmol)
of Na2S2O4 dissolved in 50 mL of distilled water, the solution added
dropwise, and the mixture stirred for 1 h. The mixture was allowed to
cool to room temperature and the precipitate filtered and washed with
distilled water and cold ethanol. The resulting solid was dried in vacuo
to yield a tan powder (3.07 g, 94% yield). Because of the insolubility
and air sensitivity of this compound it was used immediately without
further purification or analysis.
3,6-Diamino-2,5-dichloro-1,4-hydroquinone (6). Prepared
analogously to 5 from 3,6-diamino-2,5-dichloro-1,4-benzoquinone
(4) (11 mmol) to yield a light tan powder (2.30 g, 90% yield).
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Because of the insolubility and air sensitivity of this compound it was
used immediately without further purification or analysis.
CHCl3) δ 2.75 (6 H, s); 13C NMR (100 MHz, CHCl3) δ 15.1, 104.0,
137.5, 145.5, 166.1; HRMS (ESI) calcd for C10H7N2O2Cl2, 256.9879
(M + H)+, found 256.9881.
Representative Procedure for the Preparation of 4,8-
Dibromobenzobisoxazoles. A dry round-bottom flask was placed
under an argon atmosphere and charged with 0.13 g (0.25 mmol) of
Y(OTf)3, 5 mL of DMA, and 3 equiv of orthoester 7a−e. The mixture
was warmed to 55 °C, and 1.49 g (5.0 mmol) of 3,6-diamino-2,5-
dibromo-1,4-hydroquinone (5) was added portionwise over 30 min
and stirred at 55 °C for 2 h. The mixture was allow to cool room
temperature and diluted with water. The resulting precipitate was
collected by filtration and washed with distilled water and cold ethanol.
4,8-Dibromobenzo[1,2-d;4,5-d′]bisoxazole (8a; X = Br, R =
H). Prepared from 5 (5 mmol) and triethyl orthoformate 7a. Purified
by recrystallization from acetic acid to yield white needles, (0.77 g,
4,8-Dichloro-2,6-bis(chloromethyl)benzo[1,2-d;4,5-d′]-
bisoxazole (9c; X = Cl, R = CH2Cl). Prepared from 6 (20 mmol)
and 7c. Purified by recrystallization from chloroform/ethanol to yield
1
yellow needles (4.69 g, 74% yield): mp 233−234 °C; H NMR (400
MHz, CHCl3) δ 4.84 (4H, s); 13C NMR (100 MHz, CHCl3): δ 23.6,
106.0, 117.6, 138.1, 146.1, 163.4; HRMS (ESI) calcd for
C10H5N2O2Cl4 324.9103 (M + H)+, found 324.9100.
4,8-Chloro-2,6-bis(trimethylsilylethynyl)benzo[1,2-d;4,5-d′]-
bisoxazole (9d; X = Cl, R = C2TMS). Prepared from 6 (3 mmol)
and 7d using THF in place of DMSO and heating to 50 °C. The
solvent was removed in vacuo and the crude product purified by
recrystallization from heptanes to yield pale yellow needles (0.73 g,
58% yield): mp 232−234 °C; 1H NMR (400 MHz, CDCl3) δ 0.34 (18
H, s); 13C NMR (100 MHz, CDCl3) δ 0.2, 90.4, 105.2, 105.3, 138.3,
145.3, 148.6; HRMS (ESI) calcd for C18H19N2O2Si2Si2,421.0357 (M +
H)+, found 421.0350.
1
48% yield): mp > 260 °C; H NMR (400 MHz, DMSO-d6) δ 9.05
(2H, s); 13C NMR (100 MHz, DMSO-d6) δ 92.4, 137.7, 145.7, 156.3;
HRMS (ESI) calcd for C8H3N2O2Br2 316.8556 (M + H)+, found
316.8558.
4,8-Dibromo-2,6-dimethylbenzo[1,2-d;4,5-d′]bisoxazole
(8b; X = Br, R = CH3). Prepared from 5 (20 mmol) and triethyl
orthoacetate 7b. Purified by recrystallization from chloroform/ethanol
4,8-Dichloro-2,6-bis(ethylacetyl)benzo[1,2-d;4,5-d′]-
bisoxazole (9e; X = Cl, R = CO2Et). Prepared from 6 and 7e.
Purified by recrystallization from chloroform/ethanol to yield light
1
to yield white needles (5.53 g, 85% yield): mp > 260 °C; H NMR
(400 MHz, CDCl3): δ 2.74 (6H, s); 13C NMR (100 MHz, CDCl3): δ
15.1, 92.1, 136.7, 146.8, 165.8; HRMS (EI) Calcd for C10H6N2O2Br2,
343.87960 (M+), found 343.88060.
1
yellow needles (0.46 g, 41% yield): mp > 260 °C; H NMR (400
MHz, CDCl3) δ 1.52 (6 H, t, J = 8 Hz), 4.62 (4 H, q, J = 8 Hz); 13
C
NMR (100 MHz, CDCl3) δ 14.4, 64.3, 108.2, 139.2, 146.3, 155.0,
155.4; HRMS (ESI) calcd for C14H11N2O6Cl2 372.9989 (M + H)+,
found 372.9992.
4,8-dibromo-2,6-bis(chloromethyl)benzo[1,2-d;4,5-d′]-
bisoxazole 8c (X = Br, R = CH2Cl). Prepared from 5 (30 mmol) and
triethyl orthochloroacetate 7c. The crude product was heated in
chloroform (500 mL) and filtered. The filtrate was concentrated to
200 mL diluted 1:1 with ethanol to obtain yellow needles (8.33 g, 70%
1-(3,7-Dimethyloctyloxy)-4-ethynylbenzene. 1-Bromo-4-
(3,7-dimethyloctyloxy)benzene. A 250 mL round-bottom flask was
charged with 100 mL of DMSO and 7.0 g (125 mmol) of KOH and
stirred for 1 h at room temperature. Then 17.3 g (100 mmol) of 4-
bromophenol and 27.6 g (125 mmol) of 1-bromo-3,7-dimethyloctane
were added successively, and the reaction mixture stirred at room
temperature overnight. The mixture was poured into 200 mL of water
and extracted with dichloromethane (3×). The combined organic
layers were washed with 2 N HCl (3×) and brine (1×) and dried over
MgSO4. The solution was filtered and the solvent removed in vacuo.
Distillation of the crude product under reduced pressure (185 °C, 260
1
yield): mp > 260 °C; H NMR (400 MHz, CDCl3) δ 4.88 (4 H, s);
13C NMR (100 MHz, CDCl3) δ 36.0, 93.2, 138.3, 147.5, 163.2; HRMS
(ESI) calcd for C10H5N2O2Cl2Br2 412.8092 (M + H)+, found
412.8089.
4,8-Dibromo-2,6-bis(trimethylsilylethynyl)benzo[1,2-d;4,5-
d′]bisoxazole (8d; X = Br, R = C2TMS). Prepared from 5 (3 mmol)
and triethyl orthopropiolate 7d using THF in place of DMA and
heated to 50 °C. The solvent was removed in vacuo and the crude
product purified by recrystallization from heptanes to yield small
yellow needles (0.96 g, 63% yield): mp 245 °C; 1H NMR (400 MHz,
CDCl3): δ 0.34 (18 H, s); 13C NMR (100 MHz, CDCl3): δ 0.7, 90.5,
92.4, 105.2, 139.5, 146.6, 148.3; HRMS (ESI) Calcd for C18H19
O2N2Si2Br2, 508.9346 (M + H)+, found 508.9348.
4,8-Dibromo-2,6-bis(ethyl acetyl)benzo[1,2-d;4,5-d′]-
bisoxazole (8e; X = Br, R = CO2Et). Prepared from 5 (5 mmol)
and ethyl triethoxyacetate 7e. Purified by recrystallization from
chloroform/heptanes to yield pale yellow needles (0.59 g, 26%
yield): mp > 260 °C; 1H NMR (400 MHz, CDCl3) δ 1.54 (6H, t, J = 8
Hz), 4.62 (4H, q, J = 8 Hz); 13C NMR (100 MHz, CDCl3) δ 23.6,
57.0, 93.5, 123.89, 151.1, 154.3, 157.3; HRMS (ESI) calcd for
C14H11N2O6Br2 460.8978 (M + H)+, found 460.8975.
Representative Procedure for the Preparation of of 4,8-
Dichlorobenzobisoxazoles. A dry round-bottom flask was placed
under an argon atmosphere and charged with 0.13 g (0.25 mmol) of
Y(OTf)3, 5 mL of DMSO, and 3 equiv of orthoester 7a−e. The
mixture was warmed to 55 °C, 1.05 g (5.00 mmol) of 3,6-diamino-2,5-
dichloro-1,4-hydroquinone (6) was added portionwise over 30 min,
and the mixture was stirred at 55 °C for 2 h. The mixture was allowed
to cool to room temperature and diluted with water. The resulting
precipitate was collected by filtration and washed with water and cold
ethanol.
4,8-Dichlorobenzo[1,2-d;4,5-d′]bisoxazole (9a; X = Cl, R =
H). Prepared from 6 (5 mmol) and 7a. Purified by recrystallization
from chloroform to yield small white needles (1.56 g, 84% yield): mp
> 260 °C; 1H NMR (400 MHz, DMSO-d6) δ 9.07 (2H, s); 13C NMR
(100 MHz, DMSO-d6) δ 104.4, 136.5, 144.3, 156.6; HRMS (ESI)
calcd for C8H3N2O2Cl2, 228.9566 (M + H)+, found 228.9572.
4,8-Dichlorobenzo-2,6-dimethylbenzo[1,2-d;4,5-d′]-
bisoxazole (9b; X = Cl, R = CH3). Prepared from 6 (20 mmol) and
7b. Purified by recrystallization from chloroform/ethanol to yield
white needles (0.78 g, 79% yield): mp > 260 °C; 1H NMR (400 MHz,
1
mTorr) yielded a clear oil (31.0 g, 95% yield): H NMR (400 MHz,
CDCl3) δ 0.87 (6H, d, J = 8 Hz), 0.96 (3H, d, J = 8 Hz), 1.15−1.87
(10H, overlapping multiplets), 3.96 (2H, sextet, J = 8 Hz), 6.79 (2H,
d, J = 8 Hz), 7.37 (2H, d, J = 12 Hz); 13C NMR (400 MHz, CDCl3)
20.0, 22.8, 28.2, 30.0, 36.3, 37.5, 39.4, 66.7, 112.7, 116.5, 132.3, 158.4;
HRMS (EI) calcd for C16H25OBr 312.1089 (M+), found 312.1084.
1-(3,7-Dimethyloctyloxy)-4-(trimethylsilylethynyl)benzene. A dry
pressure flask was sealed with a septum, equipped with a stir bar, and
purged with argon. The flask was charged with 50 mL of dry/degassed
Et3N and 3.29 g (10.0 mmol) of 1-(3,7-dimethyloctyl)oxy-4-
bromobenzene, 0.35 g (0.30 mmol) of Pd(PPh3)4, and 0.038 g
(0.20 mmol) of CuI followed by 1.28 g (13.0 mmol) of
trimethylsilylacetylene. The flask was sealed with a Teflon cap and
heated to 80 °C for 24 h. The solution was cooled to room
temperature, diluted with water, and extracted with hexanes (4×). The
combined organic extracts were washed with water (2×) and brine
(2×) and dried over MgSO4. The solution was filtered and the solvent
removed in vacuo. The product was purified by column chromatog-
raphy eluting with hexanes/ethyl acetate (98:2) to yield a clear oil
(1.62 g, 47% yield): 1H NMR (400 MHz, CDCl3) δ 0.24 (9H, s), 0.88
(6H, d, J = 8 Hz), 0.94 (3H, d, J = 8 Hz). 1.15−1.89 (10H,
overlapping multiplets), 3.98 (2H, m), 6.82 (2H, d, J = 12 Hz), 7.39
(2H, d, J = 12 Hz); 13C NMR (100 MHz, CDCl3) δ 19.9, 22.9, 24.9,
28.2, 30.0, 36.3, 37.49, 39.45, 66.6, 92.4, 105.5, 114.5, 115.16, 133.6,
159.5.
1-(3,7-Dimethyloctyoxy)-4-ethynylbenzene. A 50 mL round-
bottom flask was charged with 3.04 g (8.80 mmol) of 4-(3,7-
dimethyloctyl)oxy-1-(trimethylsilylethynyl)benzene, 10 mL of
CH2Cl2, and 10 mL of methanol. K2CO3 (0.24 g, 1.76 mmol) was
added and the mixture stirred room temperature overnight. The
solvents were removed in vacuo, and the product was purified by
column chromatography eluting with hexanes/ethyl acetate (99:1) to
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yield an orange oil (2.15 g, 95%): 1H NMR (400 MHz, CDCl3) δ 0.88
(6H, d, J = 8 Hz), 0.94 (3H, d, J = 8 Hz), 1.62−1.85 (10H,
overlapping multiplets), 3.0 (1H, s), 4.0 (2H, m), 6.84 (2H, d, J = 8
Hz), 7.43 (2H, d, J = 12 Hz; 13C NMR (100 MHz, CDCl3) δ19.9,
22.8, 22.9, 24.9, 28.2, 30.0, 36.3, 37.5, 39.4, 66.5, 75.9, 83.7, 114.0,
114.6, 133.7, 159.7; HRMS (ESI) calcd for C18H27O 259.2056 (M +
H)+, found 259.2060.
Trimethylsilylacetylene (0.58 g, 6.5 mmol) was added slowly dropwise
syringe over 10 min and the mixture stirred for 2 h at 0 °C then 24 h at
room temperature. The mixture was poured into saturated NH4Cl and
extracted with hexanes (4×). The combined organic extracts were
washed with 1 N HCl (3×), water (1×), and brine (1×) and dried
over MgSO4. The solution was filtered, and the solvents were removed
in vacuo. The product was filtered through a silica gel plug eluting with
1
4-Dodecy-1-ethynylbenzene. Dodecylboronic Acid. A dry,
three-neck, 500 mL round-bottom flask was fitted with a reflux
condenser, addition funnel, stir bar, and septum and placed under
argon atmosphere. The flask was charged with 8.75 g (360 mmol) of
magnesium turnings followed by a few crystals of iodine. Ether (300
mL) was added, the suspension was refluxed until a clear solution
developed, and the flask was allowed to cool to room temperature.
C12H25Br (74.8 g, 300 mmol) was added dropwise via addition funnel
to maintain a gentle reflux and then heated to reflux for 2 h. A separate
dry, three-neck, 1 L round-bottom flask was equipped with an addition
funnel and mechanical stirrer and charged with 72.7 g (750 mmol) of
B(OMe)3 and 500 mL of diethyl ether. The flask was cooled to 0 °C in
an ice−water bath and the fresh C12H25MgBr solution added via
addition funnel. The viscous mixture was allowed to warm to room
temperature overnight, cooled back to 0 °C, and quenched with 1 N
HCl. The layers were separated, the aqueous layer was extracted with
diethyl ether (3×), and the combined organic layers were washed with
water (2×) and brine (1×) and dried over MgSO4. The solution was
filtered and the solvent removed in vacuo. The crude product was
purified by recrystallization from hexanes to yield white needles (24.2
g, 38% yield): 1H NMR (400 MHz, CDCl3) δ 1.39 (3H, m), 1.26 (9H,
overlapping multiplets), 0.88 (3H, t, J = 8 Hz); 13C NMR (100 MHz,
CDCl3) δ 0.3, 14.4, 22.9, 24.6, 29.6, 29.7, 29.82, 29.86, 29.92, 32.2,
32.6.
hexanes to yield a pale yellow oil (1.48 g, 87% yield): H NMR (400
MHz, CDCl3) δ 0.26 (9H, s), 0.90 (3H, t, J = 8 Hz), 1.27 (18H,
overlapped multiplets), 1.60 (2H, m), 2.60 (2H, t, J = 8 Hz), 7.11 (2H,
d, J = 8 Hz), 7.39 (2H, d, J = 8 Hz); 13C NMR (100 MHz, CDCl3) δ
0.3, 14.4, 23.0, 29.4, 29.6, 29.7, 29.9, 29.9, 31.5, 32.2, 93.4, 105.6,
120.4, 128.5, 132.1, 143.8.
4-Dodecyl-1-ethynylbenzene. A small round-bottom flask was
equipped with a stir bar and charged with 1.48 g (4.36 mmol) of 4-
dodecyl-1-(trimethylsilylethynyl)benzene, 5 mL of CH2Cl2, and 5 mL
of methanol. K2CO3 (0.17 g, 0.87 mmol) was added and the mixture
stirred at room temperature for 4 h then diluted with 10 mL of water
and 15 mL of CH2Cl2. The layers were separated, and the aqueous
layer was extracted with CH2Cl2 (3×). The combined organic layers
were washed with water (1×) and brine (2×) and dried over MgSO4.
The solution was filtered, and the solvents were removed in vacuo.
The crude product was filtered through a silica plug eluting with
1
hexanes to yield a pale yellow oil (1.16 g, 98% yield): H NMR (400
MHz, CDCl3) δ 0.90 (3H, t, J = 8 Hz), 1.31 (18H, overlapped
multiplets), 1.59 (2H, m), 2.60 (2H, t, J = 8 Hz), 3.04 (2H, s), 7.14
(2H, d, J = 8 Hz, 7.41 (2H, d, J = 8 Hz); 13C NMR (100 MHz,
CDCl3) δ 14.4, 22.9, 29.5, 29.6, 29.7, 29.8, 29.9, 29.9, 31.5, 32.2, 36.1,
76.6, 84.1, 119.4, 128.6, 132.2, 144.2.
4,8-Bis(1-hexynyl)-2,6-dimethylbenzo[1,2-d-4,5-d′]-
bisoxazole (11b). A dry pear-bottom flask was placed under argon
atmosphere and charged 0.70 g (2.00 mmol) of 8b, 0.0702 g (0.10
mmol) of Pd(PPh3)2Cl2, 0.019 g (0.10 mmol) of CuI, and 0.026 g
(0.10 mmol) of PPh3 and dissolved in 60 mL of dry/degassed THF.
This solution was added to a degassed solution of 0.49 g of 1-hexyne
(6.00 mmol) and 2.02 g (20.0 mmol) of diisopropylamine in 5 mL of
dry THF under argon atmosphere in a two-neck round-bottom flask
equipped with a reflux condenser. The solution was refluxed for 24 h.
The mixture was allowed to cool to room temperature ,and the volatile
components were removed in vacuo. The crude product was dissolved
in hot hexanes and filtered, and the filtrate was concentrated in vacuo.
Further purification by recrystallization from hexanes resulted in white
4-Dodecyl-1-(trimethylsilyl)benzene. A dry two-neck, round-bot-
tom flask was fitted with a reflux condenser and septum and placed
under argon atmosphere. The flask was charged with 7.71 g (36.6
mmol) of dodecylboronic acid, 0.43 g (0.6 mmol) of Pd(dppf)Cl2,
12.8 g (60.0 mmol) of K3PO4, and 7.08 g (30.0 mmol) of 4-bromo-1-
(trimethylsilyl)benzene. The flask was then charged with 60 mL of
dry/degassed toluene and the mixture heated to 100 °C for 36 h then
allowed to cool to room temperature. The solids were filtered, the
filter cake was washed with hexanes, and the filtrate was concentrated
in vacuo. The crude product was filtered through a silica gel plug
eluting with hexanes. The impurities were removed by careful vacuum
1
1
distillation to yield a dark yellow oil (7.98 g, 83% yield): H NMR
needles (0.62 g, 88% yield): mp 168−170 °C; H NMR (400 MHz,
(400 MHz, CDCl3) δ 0.41 (9H, s), 1.04 (3H, t, J = 8 Hz), 1.45 (18H,
overlapped multiplets), 1.77 (2H, m), 2.74 (2H, t, J = 8 Hz), 7.32 (2H,
d, J = 8 Hz), 7.59 (2H, d, J = 8 Hz); 13C NMR (100 MHz, CDCl3)
0.8, 14.4, 23.0, 29.7, 29.76, 29.86, 29.93, 29.98, 30.0, 36.3, 128.1, 133.5,
137.2, 143.8.
CDCl3) δ 0.98 (6H, t, J = 8 Hz), 1.55 (4H, m), 1.69 (4H, m), 2.63
(4H, t, J = 8 Hz), 2.72 (6H, s); 13C NMR (100 MHz, CDCl3) δ 13.8,
15.0, 20.1, 23.3, 30.9, 70.7, 98.1, 101.7, 139.7, 148.8, 165.3; HRMS
(ESI) calcd for C22H25N2O2 349.1911 (M + H)+, found 349.1913.
4,8-Bis(trimethylsilylethynyl)-2,6-dimethylbenzo[1,2-d-4,5-
d′]bisoxazole (12b). A dry three-neck, round-bottom flask equipped
with a reflux condenser and addition funnel was placed under argon
atmosphere and charged with 3.74 g (10.0 mmol) of 8b, 0.35 g (0.5
mmol) of PdCl2(PPh3)2, 0.095 g (0.5 mmol) of CuI, 0.13 g (0.5
mmol) of PPh3, 10.1 g (100 mmol) of degassed diisopropylamine, and
150 mL of dry/degassed THF. A solution of 2.95 g (30.0 mmol) of
trimethylsilylacetylene diluted in 12 mL of degassed THF was added
dropwise at room temperature and the mixture heated to 50 °C
overnight. The mixture was allowed to cool to room temperature, and
the volatile components were removed in vacuo. The crude product
was dissolved in hot hexanes and filtered, and the filtrate was
concentrated in vacuo. The product was further purified by
recrystallization from ethanol to yield white flakes (1.87 g, 49%
4-Dodecyl-1-iodobenzene. A round-bottom flask was equipped
with a stir bar, charged with 4.78 g (15.0 mmol) of 4-dodecyl-1-
(trimethylsilyl)benzene and 15 mL of CH2Cl2, and cooled to 0 °C in
an ice water bath. A solution of 3.04 g (18.75 mmol) of ICl in 6 mL of
CH2Cl2 was added dropwise and the solution allowed to warm to
room temperature over 1 h. The reaction was diluted with CH2Cl2 and
quenched with saturated NaHSO3. The layers were separated, the
aqueous layer was extracted with CH2Cl2 (3×), and the combined
organic layers were washed with water (1×) and brine (3×) and dried
over MgSO4. The solution was filtered and the solvent removed in
vacuo. The crude product was purified by recrystallization at −20 °C
from ethanol to yield glistening white flakes (4.94 g, 88% yield): mp <
1
35 °C; H NMR (400 MHz, CDCl3) δ 0.88 (3H, t, J = 8 Hz), 1.28
1
(18H, overlapped multiplets), 1.57 (2H, m), 2.53 (2H, t, J = 8 Hz),
6.93 (2H, d, J = 8 Hz), 7.58 (2H, J = 8 Hz); 13C NMR (100 MHz,
CDCl3) δ 14.4, 22.9, 29.4, 29.6, 29.7, 29.8, 29.9, 31.5, 32.2, 35.7, 90.7,
130.8, 137.4, 142.7.
yield): mp > 260 °C; H NMR (400 MHz, CDCl3) δ 0.63 (18H, s),
2.75 (6H, s); 13C NMR (100 MHz, CDCl3) δ 0.21, 15.2, 94.0, 98.3,
106.8, 140.0, 149.1, 165.9; HRMS (ESI) calcd for C20H25N2O2Si2
380.13762 (M + H)+, found 380.13853.
4-Dodecyl-1-(trimethylsilylethynyl)benzene. A dry round-bottom
flask was placed under argon atmosphere and charged with 1.87 g (5.0
mmol) of 4-dodecyl-1-bromobenzene, 0.0702 g (0.1 mmol) of
Pd(PPh3)Cl2, and 0.0095 g (0.05 mmol) of CuI. Dry/degassed Et3N
(35 mL) was added and the flask cooled to 0 °C in an ice−water bath.
4,8-Bis(phenylethynyl)-2,6-dimethylbenzo[1,2-d-4,5-d′]-
bisoxazole (13b). Prepared similarly to 11b from 8b and phenyl-
acetylene. Purified by recrystallization from chloroform/ethanol to
yield small yellow needles (0.62 g, 78% yield): mp > 260 °C; 1H NMR
(400 MHz, CDCl3) δ 2.78 (6H, s), 7.40 (6H, m) 7.22 (4H, m); 13C
8678
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NMR (100 MHz, CDCl3) δ 15.2, 79.5, 98.2, 100.1, 122.6, 128.5, 129.3,
132.3, 148.7, 165.9; HRMS (ESI) calcd for C26H17N2O2, 389.1285 (M
+ H)+, found 389.1287.
4,8-Bis-(4-dodecylphenylethynyl)-2,6-dimethylbenzo[1,2-d-
4,5-d′]bisoxazole Diethylphosphonate Ester (15f). Prepared
similarly to 16f from 8f and 4-(dodecyl)-1-ethynylbenzene to yield a
bright yellow powder (5.15 g, 65% yield): mp 110−111 °C; 1H NMR
(400 MHz, CDCl3) δ 0.89 (6H, t, J = 8 Hz), 1.27−1.35 (36H,
overlapping multiplets), 1.38 (12H, t, J = 8 Hz), 1.63 (4H, m), 2.64
(4H, t, J = 8 Hz), 3.67 (4H, d, J = 28 Hz), 4.25 (8H, m), 7.21 (4H, d, J
= 8 Hz), 7.57 (4H, d, J = 8 Hz); 13C NMR (100 MHz, CDCl3) δ 14.3,
16.6 (d, 3J = 6 Hz), 22.9, 27.86 (d, 1J = 138 Hz), 27.9, 29.5, 29.6, 29.7,
4,8-Bis(4-(3,7-dimethyloctyloxy)phenylethynyl)-2,6-
dimethylbenzo[1,2-d-4,5-d′]bisoxazole (14b). Prepared similarly
to 11b from 8b and 1-(3,7-dimethyloctyloxy)-4-ethynylbenzene. The
product was purified by recrystallization from ethanol to yield yellow
1
needles (0.59 g, 84% yield): mp 163−165 °C; H NMR (400 MHz,
CDCl3) δ 0.88 (12H, d, J = 8 Hz), 0.96 (6H, d, J = 8 Hz), 1.67−1.88
(10H, overlapping multiplets), 2.77 (6H, s), 4.05 (4H, m), 6.91 (4H,
d, J=8 Hz), 7.64 (4H, d, J = 8 Hz); 13C NMR (100 MHz, CDCl3) δ
15.2, 19.9, 22.8, 24.9, 28.2, 30.1, 36.3, 37.5, 39.5, 66.6, 78.4, 98.2,
100.3, 114.5, 114.7, 133.9, 139.5, 148.6, 160.0, 165.6; HRMS (ESI)
calcd for C46H57N2O4,701.4313 (M + H)+, found 701.4315.
2
29.8, 29.8, 29.9, 29.9, 31.4, 32.1, 63.4 (d, J = 6 Hz), 78.9, 99.0, 100.8,
2
119.8, 128.7, 132.1, 139.9, 144.7, 149.1, 160.6 (d, J = 9 Hz); HRMS
(ESI) calcd for C38H59N2O8P2, 733.3741 (M + H)+, found 733.3739.
General Polymerization Procedure (P1, P3, P4, and P5). A
dry Schlenk flask was placed under argon atmosphere and charged
with equimolar amounts of phosphonate ester 8f (P1, P4), 9f (P5), or
15f (P3) and 2,5-didodecyloxyterephthaldehyde (P1, P3) or 3,4-
didodecylthiophene dicarboxaldehyde (P4, P5) dissolved in dry THF
to make a 0.06 M solution of phosphonate ester. The mixture was
stirred at room temperature while adding 2.5 equiv of potassium tert-
butoxide (1.0 M in THF) in one portion. The mixture was stirred at
room temperature for 3 days, the reaction diluted 1.3 times with dry
THF, and the reaction stirred for 2 additional days before the polymer
was precipitated into 200 mL of methanol. The precipitated polymer
was filtered into a cellulose extraction thimble, placed into a Soxhlet
extractor and washed with methanol, hexane, and THF (P1 and P3,
and P5) or CHCl3 (P6). Polymer was recovered from the THF or
CHCl3 extract by evaporation of the solvent.
4,8-Bis(4-dodecylphenylethynyl)-2,6-dimethylbenzo[1,2-d-
4,5-d′]bisoxazole (15b). Prepared similarly to 11b from 8b and 4-
dodecyl-1-ethynylbenzene. The product was purified by recrystalliza-
tion from ethanol to yield small white needles (0.53 g, 73% yield): mp
194−197 °C; 1H NMR (400 MHz, CDCl3) δ 0.88 (6H, d, J = 8 Hz),
1.27−1.31 (36H, overlapped multiplets), 1.63 (4H, m), 2.64 (4H, t, J
= 8 Hz), 2.75 (6H, s), 7.20 (4H, d, J = 8 Hz), 7.62 (4H, d, J = 8 Hz);
13C NMR (100 MHz, CDCl3) 14.4, 15.1, 22.9, 29.5, 29.6, 29.7, 29.8,
29.86, 29.89, 31.4, 32.1, 36.2, 79.0, 98.2, 100.3, 119.8, 128.6, 132.2,
139.6, 144.5, 148.7, 165.7; HRMS (ESI) Ccalcd for C50H65N2O2
725.5041 (M + H)+, found 725.5031.
4,8-Dibromo-2,6-dimethylbenzo[1,2-d-4,5-d′]bisoxazole Di-
ethylphosphonate Ester (8f). A dry pressure flask was equipped
with a stir bar, capped with a septum, and placed under argon
atmosphere. The flask was charged with 2.68 g (6.46 mmol) of 8c and
3.23 g (19.4 mmol) of triethyl phosphate, the flask sealed with a
Teflon cap, and the mixture heated to 150 °C for 6 h. The mixture was
allowed to cool to room temperature, the crude product dissolved in a
minimal amount of CHCl3, and the product precipitated into 5× the
volume of heptanes. The precipitate was collected by filtration and
washed with heptanes to yield a yellow-white powder (3.62 g, 91%
yield): mp 163−165 °C; 1H NMR (400 MHz, CDCl3) δ 1.40 (12H, t,
J = 8 Hz), 3.66 (4H, d, J = 28 Hz), 4.23 (8H, m); 13C NMR (100
MHz, CDCl3) δ 16.5 (d, 3J = 5 Hz), 27.9 (d, 1J = 138 Hz), 63.4 (d, 2J
1
Polymer P1: 0.45 g, 56% yield; H NMR δ 0.88 (−CH3, broad),
1.25−1.96 (−C10H25, broad), 4.13 (−OCH2, broad), 6.9−7.25 (aryl-H
and vinyl protons, broad); UV−vis (THF) λmax = 476 nm; GPC Mn =
3300, Mw = 7900, PDI = 2.4; fluorescence (THF) λem = 524 nm (λexc
= 476 nm).
1
Polymer P3: 0.33 g, 55% yield; H NMR (400 MHz, CDCl3) δ
0.89 (−CH3, t), 1.25−1.20 (−C11H25, broad) 2.46 (aryl −-CH2,
broad), 3.64 (OCH2, broad), 6.93−7.00−7.25 (aryl, vinyl −CH,
broad), 7.25−8.0 (aryl −CH, broad); UV−vis (THF) λmax = 520 nm;
GPC Mn = 5708, Mw = 14120, PDI = 2.5; fluorescence (THF) λem
=
601 nm (λexc = 520 nm).
2
1
= 7 Hz), 92.0, 138.9, 147.2, 160.9 (d, J = 11 Hz); HRMS (EI) calcd
Polymer P4: 0.25 g, 61% yield; H NMR (400 MHz, CDCl3) δ
0.88 (−CH3, t), 1.20−1.45 (−C11H25, broad), 2.77 (−CH2, broad),
6.92−7.00 (vinyl −CH, broad), 8.02−8.04 (vinyl −CH, broad); UV−
vis (THF) λmax = 506 nm; GPC Mn = 4300, Mw = 6000, PDI = 1.4;
fluorescence (THF) λem = 563 nm (λexc = 506 nm).
for C18H20N2O8P2Br2 615.93746 (M+), found 615.93965.
4,8-Dichloro-2,6-dimethylbenzo[1,2-d-4,5-d′]bisoxazole Di-
ethylphosphonate Ester (9f). Prepared similarly to 8f from 9c
and triethyl phosphite to yield an off-white powder (4.79 g, 91%
yield): mp 164−165 °C; 1H NMR (400 MHz, CDCl3) δ 1.38 (12H, t,
J = 8 Hz), 3.67 (4H, d, J = 28 Hz), 4.25 (8H, m); 13C NMR (100
MHz, CDCl3) δ 16.5 (d, 3J = 5 Hz), 29.2 (d, 1J = 138 Hz), 63.4 (d, 2J
1
Polymer P5: 0.20 g, 55% yield; H NMR (400 MHz, CDCl3) δ
0.88 (−CH3, t), 6.93−7.00 (−C11H25, broad), 2.78 (−CH2, broad),
6.92−7.00 (vinyl −CH, broad), 7.98−8.04 (vinyl −CH, broad); UV−
vis (THF) λmax = 510 nm; GPC Mn = 3300, Mw = 7900, PDI = 2.4;
fluorescence (THF) λem = 560 nm (λexc = 495 nm).
2
= 7 Hz), 104.8, 137.7, 145.9, 161.3 (d, J = 11 Hz); HRMS (ESI)
Ccalcd for C18H25N2O8Cl2P2 529.0458 (M + H)+, found 529.0460.
4,8-Bis(decynyl)-2,6-dimethylbenzo[1,2-d-4,5-d′]bisoxazole
Diethylphosphonate eEster (16f). A dry, two-neck round-bottom
flask was equipped with a stir bar and reflux condenser and placed
under argon atmosphere. Dry/degassed THF (140 mL, 9.11 g),
diisopropylamine (90.0 mmol), and 1-decyne (3.73 g, 27.0 mmol)
were added, and the mixture was degassed for 15 min. The flask was
then charged with 5.56 g (9.00 mmol) of 8f, 0.095 g (0.36 mmol) of
PdCl2(PPh3)2, 0.068 g (0.36 mmol) of PPh3, and 0.25 g (0.36 mmol)
of CuI. The solution was degassed for 10 min and heated to reflux for
24 h. The solution was allowed to cool to room temperature, and the
volatile components were removed in vacuo. The crude product was
filtered through a short silica plug eluting with Et2O/CHCl3 (4:1).
The product was further purified by recrystallization from hexanes to
Polymer P2. A dry Schlenk flask was placed under argon
atmosphere and charged with 0.36 g (0.50 mmol) of 16f, 0.25 g
(0.50 mmol) of 2,5-didodecyloxyterephthaldehyde, 0.11 g (1.25
mmol) of LiBr, and 9 mL of dry THF. The mixture was stirred at
room temperature, 0.12 g (2.40 mmol) of Et3N diluted in 1 mL of
THF was added dropwise, the reaction was stirred for 3 days, and the
polymer was precipitated into 150 mL of methanol. The precipitated
polymer was filtered into a cellulose extraction thimble, placed in a
Soxhlet extractor, and washed with methanol, hexane, and THF. The
polymer was recovered from the THF extract by evaporation of the
1
solvent (0.40 g, 87% yield): H NMR (400 MHz, CDCl3) δ 0.92,
(−CH3, broad), 1.35−1.64 (−C11H25, −C6H12, broad) 2.70 (propargyl
−CH2, broad), 3.25 (−OCH2, broad), 6.93−7.00 (−C11H22, 7.04−
7.25 (vinyl,aryl −CH, broad), 7.25−7.75 (aryl −CH, broad); UV−vis
(THF) λmax = 491 nm; GPC Mn = 22683, Mw = 113798, PDI = 5.0;
luorescence (THF) λem = 576 nm (λexc = 491 nm).
Computational Details. All of the calculations on the oligomers
studied in this work were performed using the Gaussian 03W with the
GaussView 4 GUI interface program package. All electronic ground
states were optimized using density functional theory (DFT), B3LYP/
6-31G*. Excited states were generated through time-dependent
density functional theory (TD-DFT) applied to the optimized ground
1
yield a yellow solid (4.87 g, 74% yield): mp 119−120 °C; H NMR
(400 MHz, CDCl3) δ 1.87 (6H, t, J = 8 Hz), 1.28−1.35 (16H,
overlapping multiplets), 1.36 (12H, J = 8 Hz), 1.46 (4H, m), 1.69 (4H,
quintet, J = 8 Hz), 2.58 (4H, t, J = 8 Hz), 3.64 (4H, d, J = 24 Hz), 4.22
3
(8H, m) ; 13C NMR (100 MHz, CDCl3) δ 14.3, 16.5 (d, J = 6 Hz),
20.4, 22.9, 27.8 (d, 1J = 137 Hz), 28.8, 29.3, 29.35, 29.39, 32.1, 63.3 (d,
2
3J = 6 Hz), 70.6, 98.9, 102.3, 140.0, 149.3, 160.4 (d, J = 25 Hz);
HRMS (ESI) calcd for C58H83N2O8P2 997.5619 (M + H)+, found
997.5631.
8679
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The Journal of Organic Chemistry
Article
state for each oligomer. The HOMO, LUMO, band gap, first 10
excited states, and UV−vis simulations were generated from these
excited computations. Approximations of the molecular orbital
contributions for each excited state were obtained using the GaussSum
2.2 freeware package. Finally, electrostatic potential maps were created
using a coarse setting and an isovalue of 0.03.
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ASSOCIATED CONTENT
■
S
* Supporting Information
1
Experimental procedures, H and 13C NMR spectra for new
compounds, and tables of the calculated atom coordinates and
absolute energies. This material is available free of charge via
the Internet at http://pubs.acs.org.
AUTHOR INFORMATION
■
Corresponding Author
*E-mail: malikaj@iastate.edu.
ACKNOWLEDGMENTS
■
We acknowledge the donors of the American Chemical Society
Petroleum Research Fund for support of this research. We also
thank the 3M Foundation and the National Science Foundation
(DMR-0846607) and Teragrid (TG-CHE100148) for partial
support of this work. We thank Dr. Kamel Harrata and the
Mass Spectroscopy Laboratory of Iowa State University (ISU)
for analysis of our compounds and Dr. Arkady Ellern for X-ray
crystallographic analysis. We thank Atta Gueye, Dr. Elena
Sheina, and Dr. Christopher Brown of Plextronics for providing
UPS measurements. We thank Scott Meester for the synthesis
of 4-(3,7-dimethyloctyloxy)-1-bromobenzene and Andrew
Makowski for the synthesis of 2,5-didodecyloxyterephthalde-
hyde. We also thank Dr. Toby Nelson and Dr. David Yaron
(Carnegie Mellon University) for helpful discussions of this
research.
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