Influence of conjugation axis on the optical and electronic properties of aryl-substituted benzobisoxazoles

Article abstract of DOI:10.1021/jo4007927

Six different 2,6-diethyl-4,8-diarylbenzo[1,2-d:4,5-d′]bis(oxazoles) and four different 2,4,6,8-tetraarylbenzobisoxazoles were synthesized in two steps: a Lewis acid catalyzed orthoester cyclization followed by a Suzuki or Stille cross-coupling with various arenes. The influence of aryl group substitution and/or conjugation axis variation on the optical and electronic properties of these benzobis(oxazole) (BBO) compounds was evaluated. Structural modifications could be used to alter the HOMO, LUMO, and band gap over a range of 1.0, 0.5, and 0.5 eV, respectively. However, depending on the location and identity of the substituent, the HOMO level can be altered without significantly impacting the LUMO level. This is supported by the calculated frontier molecular orbitals. Our results indicate that the FMOs and band gaps of benzobisoxazoles can be readily modified either jointly or individually.

Full text of DOI:10.1021/jo4007927

Article
pubs.acs.org/joc
Influence of Conjugation Axis on the Optical and Electronic
Properties of Aryl-Substituted Benzobisoxazoles
Brian C. Tlach,† Aimee L. Tomlinson,‡ Alden G. Ryno,‡ Dawn D. Knoble,† Dana L. Drochner,†
́
Kyle J. Krager, and Malika Jeffries-EL*,†
†Department of Chemistry, Iowa State University, Ames, Iowa 50010, United States
‡Department of Chemistry, University of North Georgia, Dahlonega, Georgia 30597, United States
S
* Supporting Information
ABSTRACT: Six different 2,6-diethyl-4,8-diarylbenzo[1,2-d:4,5-d′]bis(oxazoles) and
four different 2,4,6,8-tetraarylbenzobisoxazoles were synthesized in two steps: a Lewis
acid catalyzed orthoester cyclization followed by a Suzuki or Stille cross-coupling with
various arenes. The influence of aryl group substitution and/or conjugation axis
variation on the optical and electronic properties of these benzobis(oxazole) (BBO)
compounds was evaluated. Structural modifications could be used to alter the HOMO,
LUMO, and band gap over a range of 1.0, 0.5, and 0.5 eV, respectively. However,
depending on the location and identity of the substituent, the HOMO level can be
altered without significantly impacting the LUMO level. This is supported by the
calculated frontier molecular orbitals. Our results indicate that the FMOs and band
gaps of benzobisoxazoles can be readily modified either jointly or individually.
Among the aforementioned examples, the benzo[1,2-d:4,5-
d′]bis(oxazole) (BBO)-based cruciforms are particularly
interesting, since these molecules have two different con-
jugation pathways: 2,6-conjugation through the oxazole rings
and 4,8-conjugation through the central benzene ring (Scheme
1). Since one pathway encompasses heterocyclic rings, the
optical and electronic properties can be altered as a function of
the substitution pattern around the central benzene ring.
INTRODUCTION
■
During the past four decades, interest in the development of π-
conjugated materials has increased due to their potential use as
replacements for inorganic materials in a variety of semi-
conducting applications including field effect transistors
(FETs),1−5 organic light emitting diodes (OLEDs),6−8 and
photovoltaic cells (PVCs).9−12 In addition to the ability to be
processed from solution, organic semiconductors can be
modified at the molecular level to optimize the optical and
electronic properties of the materials for specific applications.
Since the characteristics of π-conjugated systems are strongly
influenced by the highest occupied molecular orbital (HOMO)
and the lowest unoccupied molecular orbital (LUMO),
synthetic strategies that can alter these parameters are of
paramount importance. Unfortunately, most chemical mod-
ifications of π-conjugated materials result in changes in the
position of both of the frontier molecular orbitals (FMOs).
This is a result of the extensive delocalization of electrons
within these systems, rendering selective modification of either
the HOMO level or LUMO level difficult. One approach for
independently tuning FMOs within π-conjugated materials is
through the synthesis of two-dimensional molecules that
feature two perpendicular π-conjugated linear “arms” connected
through a central aromatic core.13−23 These so-called “cruci-
forms” possess spatially segregated FMOs, enabling strategic
tuning of either the LUMO or the HOMO by varying the
nature of the substituents and their arrangement around the
central molecule. Representative examples include benzobis-
(oxazole)-based cruciforms,13−15 distyrylbis(arylethynyl)-
benzenes,16−18 tetrakis(arylethynyl)benzenes,19−22 and tetra-
styrylbenzenes.23
Recently, our group24,25 and that of Miljanic13
́
have shown that
the conjugation of BBOs is readily extended via Sonogashira
cross-coupling reactions between 4,8-dibromoBBOs and
various alkynes. The resulting materials have HOMO and
LUMO levels that could readily be tuned by substitution.
However, the synthesized cruciforms that were reported feature
combinations of triple bonds between the arenes along the 4,8-
axis and single bonds between the arenes along the 2,6-axis, so
correlating optical and electronic properties to the conjugation
pathway was not possible.16,26,27 The Nuckolls group previously
synthesized tetraarylBBO cruciforms; however, since their
interest was in developing rigid molecules for self-assembled
molecular electronics, the optical and electronic properties of
these systems were not fully explored.15,28
In order to understand how different substitution patterns
will affect the optical and electronic properties of larger BBO
structures, we performed a more detailed structure−property
relationship study. Modification of the aryl group has previously
been used to vary properties through extension of the π system
and inductive effects of their electron-donating or electron-
withdrawing nature.16,26−32 Furthermore, aryl substituents are
Received: April 24, 2013
© XXXX American Chemical Society
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Scheme 1. Proposed Sequential Functionalization of the 2,6- and 4,8-Axes of BBO
robust, are easily modified, and can be installed through a
variety of methods. First, we evaluated the influence of different
aryl groups at the 4- and 8-positions of the BBO moiety using a
range of spectroscopic techniques along with computational
modeling to gain further insight about the FMO shape and
electron distribution. Then, we assessed the impact of
conjugation pathway by comparing a pair of 2,6-diarylBBOs
to the analogous 4,8-diarylBBOs. Finally, we compared four
BBO cruciforms featuring two different aryl groups to study
how properties change upon expanding to a two-dimensional π
system. These structure−property studies provide basic insight
on BBO behavior in two-dimensional π-systems as a function of
aryl group selection and conjugation axis.
Scheme 3. Synthesis of 2,6-diethylBBOs
are easier and safer to synthesize, the facile protodeboronation
that occurs with thien-2-ylboronic acids37 required the use of a
Stille cross-coupling for the synthesis of 4,8-bis(5-dodecylthien-
2-yl)-2,6-diethylBBO 8. To our knowledge, this is the first
report of aryl−aryl cross-coupling of 4,8-dibromoBBOs.
Synthesis of 2,6-diarylBBOs. The synthetic approach for
the 2,6-diarylBBOs is shown in Scheme 4, and the synthesis of
the requisite orthoesters is shown in Scheme 5. We prepared 5-
dodecyl-2-(triethoxymethyl)thiophene in one pot from 2-
bromo-5-dodecylthiophene using the method reported by
Tschitschibabin.36,38 The synthesis of 4-dodecyl-1-
(triethoxymethyl)benzene proved more difficult, as the
Tschitschibabin method failed to yield the desired orthoester.
In a search for an alternative approach, we found that aryl
orthoesters have been prepared from trithio orthoester
intermediates.39,40 Since the corresponding aldehyde was
more readily available, we decided to approach the synthesis
of the trithio orthoester intermediate starting from 2-(4-
bromophenyl)-1,3-dithiane. This decision proved beneficial, as
the dithiane protecting group allowed for the addition of a
solubilizing alkyl chain via a Kumada cross-coupling reaction,
and the targeted orthoester was obtained in 84% yield. The 2,6-
diarylBBOs 9 and 11 were obtained by the condensation of the
corresponding orthoesters and 2,5-diamino-1,4-hydroquinone
hydrochloride (DAHQ)36,41 in yields of 73% and 65%,
respectively. Due to their rigid-rod nature, compounds 9 and
11 had noticeably higher melting points and decreased
solubility in comparison to their respective 4,8-diarylBBOs 5
and 7.
Synthesis of 2,4,6,8-tetraarylBBOs. The synthetic
approach for the 2,4,6,8-tetraarylBBOs is shown in Scheme 6.
The Nuckolls group accomplished the synthesis of their
tetraarylbenzo[1,2-d:4,5-d′]bis(oxazole) cruciforms via a dou-
ble-Staudinger cyclization of substituted bis(azidoquinones).15
Although the reactions occurred in good yields, in their strategy
the substituents are introduced early in the reaction sequence.
Thus, several steps are repeated to make different molecules. A
more versatile approach is to first synthesize 2,6-diaryl-4,8-
dibromoBBOs and then extend conjugation across the central
benzene ring via cross-coupling chemistry. This approach
allows for the synthesis of several BBOs using common
intermediates. The condensation reaction of 3,6-diamino-2,5-
dibromo-1,4-hydroquinone (Br-DAHQ) with 5-dodecyl-2-
(triethoxymethyl)thiophene or 4-dodecyl-1-(triethoxymethyl)-
RESULTS AND DISCUSSION
Synthesis of 4,8-diarylBBOs. The synthesis of compounds
3−8 is shown in Scheme 2. Initially, we set out to synthesize
■
a
Scheme 2. Synthesis of 4,8-diaryl-2,6-diethylBBOs
a
Reaction conditions: (i, 3−7) Pd(dppf)Cl2, K2CO3, TBAB, boronic
acid or ester, toluene/H2O (10/1); (ii, 8) Pd2(dba)3, P(otolyl)3, 5-
dodecyl-2-(trimethylstannyl)thiophene, toluene.
the 4,8-diarylBBOs using the Suzuki cross-coupling reaction of
various arylboronic acids and 4,8-dibromo-2,6-dimethylBBO.24
Suzuki coupling was chosen, as boronic acids and esters do not
require the use of toxic reagents and are readily purified by
standard techniques. Unfortunately, this approach gave low
yields of 4,8-diarylBBOs. We hypothesized that the labile
methyl protons were being deprotonated under the basic
reaction conditions, resulting in undesirable side reactions. The
Hegedus group previously decreased the reactivity of the 2,6-
position of BBOs by substituting ethyl groups for methyl
groups.33 Therefore, we synthesized 2,6-diethyl BBOs 1 and 2
via the Lewis acid catalyzed condensation reaction between the
corresponding diamino hydroquinone and triethyl orthopropi-
onate, as shown in Scheme 3.34−36 The Suzuki cross-coupling
reaction of 2 and the appropriate boronic acids or esters
afforded 3−7 in yields of 60−83%. Although arylboronic acids
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Scheme 4. Synthesis of 2,6-diarylBBOs
60−94% yield. Purification of these compounds was easily
accomplished by first passing through a short silica gel plug to
remove polar impurities and residual catalyst followed by
recrystallization from an appropriate solvent. The synthesized
cruciforms were soluble in chlorinated solvents and charac-
Scheme 5. Synthesis of Substituted Orthoesters
1
terized by H NMR, 13C NMR, and high-resolution mass
spectrometry.
Spectroscopic and Electronic Characterization. 4,8-
DiarylBBOs. The HOMO levels of the BBOs were investigated
using ultraviolet photoelectron spectroscopy (UPS), which
provides an absolute determination of the HOMO level.42−44
We elected to use UPS instead of electrochemistry, as many of
the compounds were insoluble in acetonitrile and did not have
a reduction cycle within the solvent window for the solvent/
counterion blend used. The HOMO values of the BBOs ranged
from −4.92 to −5.94 eV. The band gaps were estimated from
the intersection of the absorption and emission spectra and
benzene afforded compounds 10 and 12 in yields of 75% and
68%, respectively. The subsequent cross-coupling of 10 and 12
with either 5-dodecyl-2-(trimethylstannyl)thiophene or (4-
dodecylphenyl)boronic acid afforded cruciforms 13−16 in
Scheme 6. Synthesis of BBO Cruciforms
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Table 1. Optical and Electronic Data for BBO Compounds
a
a
b
ac
,
d
e
f
λmax (nm)
λem (nm)
ε (M−1 cm−1
)
ϕ
HOMO (eV)
LUMO (eV)
Egopt (eV (nm))
1
252
314
333
320
384
358
392
369
342
380
374
395
358
314
372
379
381
440
404
404
396
386
416
460
448
19200
42600
44700
46800
56400
72000
59000
78000
74900
85200
59600
67900
71500
−6.50
−5.94
−5.87
−5.96
−4.92
−5.98
−5.71
−5.51
−5.75
−5.78
−5.49
−5.29
−2.3
−2.3
−2.5
−2.5
−2.0
−2.8
−2.6
−2.3
−2.3
−2.7
−2.7
−2.5
−2.7
4.2
3.6
3.4
3.4
3.0
3.0
3.1
3.2
3.4
3.1
2.8
2.8
3.2
3
0.72
0.80
0.59
0.81
0.80
0.34
0.84
0.98
0.72
0.28
0.34
0.74
4
5
6
7
8
9
11
13
14
15
16
422
b
−5.88
c
a
All measurements performed in THF. Extinction coefficients based on absorbance at λmax. Relative ΦPL values were measured using anthracene as
d
e
f
a standard. Measured by UPS. Calculated using HOMO + Egopt. Measured at the intersection of UV−vis and PL spectra.
ranged from 3.1 to 3.6 eV. The LUMO levels ranged from −2.0
to −2.5 eV and were calculated by adding the optical band gap
to the HOMO values; thus, these values have a higher degree of
uncertainty. These results are summarized in Table 1. The
electronic properties of the BBOs varied substantially when aryl
substituents were incorporated in the 4,8-axis. In general, the
extent of the change was dependent on the electron-donating
strength of the aryl substituent. BBO 3, which bears a weakly
donating phenyl substituent, has the deepest HOMO at −5.94
eV, whereas BBO 6, which bears the most electron-rich
substituent, has a HOMO level of −4.92 eV. BBO 6 also had
the highest-lying LUMO (−2.0 eV), indicating that the
incorporation of electron-rich N,N-dibutylaniline reduces the
acceptor strength of the BBO. Overall, the deepest LUMO,
−2.8 eV, was observed on the fluorene-substituted BBO 7. This
is most likely a result of the greater conjugation length, which
increases the acceptor strength of the BBO in the absence of
strong electron-donating groups.
this shift varies as a function of the nature of the aryl
substituent. Within the 4,8-diphenylBBO series, the maximum
absorption (λmax) increases with greater electron-donating
strength of the phenyl substituent (H < alkyl < alkoxy <
dialkylamino), and thus 3 absorbs at the shortest wavelength
(314 nm) and 6 at the longest wavelength (384 nm). Similarly,
BBO 8, which bears thiophene rings, also has absorption at
longer wavelengths, a result of the electron-rich nature of the
substituents and the greater planarity of this molecule relative
to the phenyl-substituted BBOs. Unlike the 4,8-diphenylBBOs,
the main absorption of 8 has visible vibronic coupling likely due
to a more planar structure, whereas the spectra of 3−7 all
exhibit diminished vibronic detail due to the twisting between
the aryl substituent and the BBO core.45 Surprisingly, further
extending the conjugation through the use of fluorene
substituents did not display a large bathochromic shift in
absorption. This is a result of the fluorene substituents being
twisted out of plane due to steric interactions between the BBO
core and the hydrogen atoms on fluorene.
The PL spectra of BBOs 3−8 were obtained by exciting at
their respective λmax values and are shown in Figure S43 in the
Supporting Information and summarized in Table 1. BBOs 3−
8 show strong solution emission with similar profiles, each
containing a main peak and a bathochromically shifted
shoulder. The increased vibronic character in the PL spectra
in comparison to the UV−vis spectra likely indicates that the
excited state is more planar than the ground state.46 The PL
quantum yields (ΦPL) of BBOs 3−8 were calculated using
anthracene as a standard and an excitation wavelength at 325
nm. The incorporation of bulkier side groups or electron-
donating groups results in an increase in ΦPL, as seen previously
in the literature.30,47,48 Furthermore, thiophene substituents
reduce ΦPL, due to the heavy-atom effect of sulfur.
The solution UV−vis absorption of the 4,8-diarylBBOs is
shown in Figure 1, along with unsubstituted BBO 1. All of the
4,8-diarylBBOs exhibit high-energy absorptions between 250
and 275 nm and a second more intense, lower energy
absorption in the range of 312−392 nm. The magnitude of
2,6-DiarylBBOs. The electronic properties of the 2,6-
diarylBBOs are summarized in Table 1. On the basis of the
data obtained from the 4,8-BBOs, we synthesized a pair of 2,6-
analogues bearing alkylphenyl or alkylthienyl groups. These
groups were selected as representative examples of weak and
strong electron-donating substituents, respectively. We did not
further evaluate the (dialkylamino)phenyl group, as it resulted
in a high-lying HOMO on the 4,8-diarylBBO. Additionally,
fluorene was not used due to its longer conjugation length,
which would complicate comparisons. The position of the
HOMO level exhibits a dependence on the nature of the aryl
Figure 1. Solution UV−vis spectra of parent BBO 1 and 4,8-
diarylBBOs 3−8.
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group and is lowered by 0.24 eV when the aryl group is
switched from a thiophene to a phenyl ring. This trend is the
result of the increased donor strength of the thiophene in
comparison to the phenyl substituent. Due to the uncertainty in
the values, we cautiously state that the position of the LUMO
level appears to be independent of the aryl group, as both 9 and
11 have similar LUMO levels. The band gaps vary proportion-
ally to the HOMO as a result of the independent nature of the
LUMO level with respect to the aryl substituent. Additionally,
the UV−vis absorption spectra (Figure 2) are consistent with
S43 in the Supporting Information). The HOMO level is raised
by approximately 0.20 eV upon switching the substituents from
the 4,8-axis to the 2,6-axis, i.e. 5 versus 11 or 8 versus 9,
suggesting that the HOMO preferentially aligns along the 4,8-
axis. This decrease in stability was confirmed by the
computational studies (Table S1 in the Supporting Informa-
tion).
In order to further compare the effect of substituent type and
location, a series of 2,4,6,8-tetraarylBBOs were synthesized and
characterized. The ΦPL values of cruciforms 13 (0.72) and 16
(0.74) are similar, while those of 14 (0.28) and 15 (0.34) were
both significantly lower, which was expected, since ΦPL
commonly decreases with narrowing band gap. This has
previously been observed in 1,4-bis(phenylethynyl)-2,5-bis-
(styryl)benzene cruciforms17 and is a consequence of more
accessible nonradiative processes in compounds with narrower
band gaps.51,52 Additionally, the λem value and vibronic detail in
the PL spectra of the cruciforms display solvatochromism, in
which there is a bathochromic shift in λem as the solvent
polarity is increased (Figure 3 and Figure S46 in the Supporting
Figure 2. UV−vis spectra of 4,8-diarylBBOs 5 and 8 and the related
2,6-diarylBBOs 9 and 11.
this finding, as the λmax value of 9 is bathochromically shifted
relative to that of 11. PL spectra of 9 and 11 both exhibit
vibronic coupling (Figure S43 in the Supporting Information).
The ΦPL values of the 2,6-diarylBBOs (0.84 for 9 and 0.98 for
11) are also considerably higher than those for their
corresponding 4,8-diarylBBOs (0.34 for 8 and 0.59 for 5).
Importance of Substitution Axis. The observed differences
in the shape of the UV−vis spectrum of the BBOs are most
likely a result of varying degrees of planarity, which affects the
conjugation length. The steric effects of the ortho hydrogens on
the phenyl ring result in out-of-plane twisting along the 4,8-axis.
This is supported by DFT calculations, which predict an out-of-
plane twisting between 19 and 28° for 3−7. In contrast,
thiophene-substituted BBOs 8 and 9 are expected to be planar,
regardless of which axis the ring is placed on. Phenyl-
substituted BBO 11 is also planar, since the ring is placed at
the 2,6-axis, minimizing the steric effects. Thus the 2,6-
diarylBBOs 9 and 11 both have similar peak topographies,
containing vibronic progression in the range of 1310−1410
cm−1, which is characteristic of aromatic ring-stretching
modes.49,50 These features are seen to a small extent in 8 and
are not observed in 5. Further analysis of the absorption spectra
reveals that moving the thienyl group from the 4,8-axis (8) to
the 2,6-axis (9) results in a small bathochromic shift in
absorption. However, switching the phenyl group from the 4,8-
axis (5) to the 2,6-axis (11) produces a 22 nm bathochromic
shift in λmax and a slight reduction in peak width at half-
maximum. This red shift is a result of both the longer
conjugation pathway along the 2,6-axis and an increase in
planarity. Compounds 5, 8, 9, and 11 show similar PL spectra
with Stokes shifts of 61, 12, 27, and 44 nm, respectively (Figure
Figure 3. Solvatochromism effect of cruciform 15.
Information). This is more apparent in cruciforms 14 and 15
than in 13 and 16. The vibronic detail in the PL spectra
decreased as the solvent polarity increased for all cruciforms
except 13. This type of behavior has previously been seen in
cruciforms studied by Bunz and co-workers, who correlated the
disappearance of vibronic character along with a bathochromic
shift in λem of the PL spectra in highly polar solvents with
intermolecular charge transfer (ICT) between the two arms of
the cruciform.17,53 Cruciforms 14 and 15 likely have more
charge transfer between the two axes than cruciforms 13 and
16. This is further substantiated by the observation of a lower
energy shoulder in the UV−vis spectra and alignment of the
FMOs in cruciforms 14 and 15 (vide infra).
An evaluation of cruciforms 15 and 16 demonstrates the
influence of the aryl substituents on optoelectronic properties.
Cruciform 16, which bears phenyl substituents in all positions,
has the lowest lying HOMO of all the cruciforms at −5.88 eV
and the widest band gap at 3.2 eV. However, cruciform 15,
which bears electron-rich thiophenes in all positions, has the
highest HOMO of all cruciforms at −5.29 eV and the smallest
band gap at 2.8 eV. This is consistent with our observations
(vide supra) that the addition of electron-donating groups at
any position destabilizes the HOMO level. The impact of the
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Figure 4. FMO diagrams of BBOs 5, 8, 9, 11, and 13−16.
aryl group is substantiated by a comparison of the isomeric
cruciforms 13 and 14. Cruciform 14, which has the strongly
donating thiophene substituents along the 4,8-axis, has a
HOMO that is nearly 0.3 eV higher and a band gap that is 0.2
eV narrower than that of 13. This is likely the result of the
difference in the planarity of the two structures along with the
difference in the electron-donating behavior of the substituents.
The origins of this difference may be further explained by
comparing the FMO diagrams of the two compounds.
Computational studies were carried out to gain further insight
into the effect of structural modification by axis and/or aryl
group variation. Optimized ground-state geometries were
obtained through density functional theory (DFT) employing
a B3LYP functional and a SVP basis set. The resulting FMO
charge distributions of the optimized structures are shown in
Figure 4, and the percent distributions along each axis are
summarized in Table 2. The electron density is focused along
the 2,6-axis in the LUMO of cruciforms 13, 15, and 16, whereas
it is distributed evenly across both axes of 14. A comparison of
the molecular orbital diagrams reveals significant differences in
the HOMOs of 13 and 14. In the case of 13, the electron
density is distributed throughout the entire structure, whereas
in 14 approximately 96% of the electron density is along the
Table 2. Calculated Percent Electronic Distribution along
Each Axis for the HOMO and LUMO of the BBO
Cruciforms
HOMO
LUMO
4,8-axis
2,6-axis
4,8-axis
2,6-axis
13
14
15
16
58.6
95.9
91.6
79.3
41.4
4.1
21.1
45.5
32.5
29.1
78.9
54.5
67.5
70.9
8.4
20.7
4,8-axis. This localization of the HOMO along the more
electron-rich axis is consistent with observations made for other
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Figure 5. UV−vis spectra of BBO cruciforms 13−16 and their corresponding 1-D building blocks 5, 8, 9, and 11 (left) and the corresponding
structures where R = C12H25 (right).
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cruciforms that contain donor−π−donor conjugation path-
ways.,16,19,22,32,53−55 Interestingly, approximately 92% of the
HOMO is also concentrated along the 4,8-axis in the
tetrathiophene-substituted cruciform 15, indicating that there
is a preference for the HOMO to be aligned perpendicular to
the benzobis(oxazole) ring system.14
study, 16 has the lowest HOMO and widest band gap. This is a
result of a decrease in electron-donating strength and
conjugation caused by the phenyl substituents on the 4,8-axis.
The FMOs show a slight localization of electron density along
the 4,8-axis in the HOMO and the 2,6-axis in the LUMO of 16,
consistent with our observations in the other cruciforms.
A comparison of the BBO cruciforms to their 1-D building
blocks gives further insight on the impact of aryl substituent
location and nature. The UV−vis spectra of cruciforms 13−16
and their respective 1-D building blocks are shown in Figure 5,
and the data are summarized in Table 1. Cruciform 13 has
phenyl substituents along the 4,8-axis, analogous to the case for
BBO 5, and thiophene substituents along the 2,6-axis,
analogous to the case for BBO 9. The spectrum of 13 has
high- and low-energy absorptions consistent with 5 and 9,
respectively. Additionally, the HOMO level of 13 is between
that of 5 and 9, indicating that 13 behaves as a hybrid of the
two compounds. The FMOs show delocalization of electron
density along the longest conjugation axis for 5 and 9, whereas
13 shows delocalization throughout the molecule, consistent
with our observations. Cruciform 14 has thiophene substituents
along the 4,8-axis, analogous to the case for BBO 8, and phenyl
substituents along the 2,6-axis, analogous to the case for BBO
11. The overall topography seen in the spectrum of 14 has
absorptions consistent with 8 and 11. However, the spectrum
of 14 is significantly broader and has an additional low-energy
peak around 412 nm, suggesting that ICT is occurring.22,56 As a
result, the HOMO level of 14 is higher than those of 8 and 11.
The fact that the band gap of 14 is smaller than that of its
structural isomer 13 is further evidence of the importance of
aryl substituent placement. The FMO diagram of 14 shows that
there is a large amount of electron density localized along the
4,8-axis in the HOMO, providing a suitable environment for
charge transfer. These results were somewhat contradictory to
those seen for similar 2,6-diaryl-4,8-bis(arylethynyl)BBO cruci-
forms, in which the HOMO, in some cases, is fully localized
along the 2,6-axis. In these tetraarylBBOs, full axial “inversion”
of the HOMOs and LUMOs does not appear to be prompted
by exchanging the aryl groups.14 However, switching the aryl
substituents does appear to have a significant effect on the
extent of delocalization in the FMOs.
Cruciform 15 has thiophene substituents along the 4,8- and
2,6-axes, analogous to the cases for BBO 8 and BBO 9,
respectively. The spectrum of 15 is broad and lacking in
structural detail; thus, direct comparisons to 8 and 9 are
unclear. Similarly to 14, there is significant broadening and the
appearance of a lower energy shoulder in the spectrum of 15,
which is indicative of ICT. Furthermore, the larger Stokes shift
and greater influence of solvent polarity on the λem values of 14
and 15 are also indicative of ICT. Collectively, the higher
HOMO, narrower band gap, and FMOs of 14 and 15 support
the notion that charge transfer is favorable when the 4,8-axis is
substituted with thiophene.
Cruciform 16 has phenyl substituents along the 4,8- and 2,6-
axes, analogous to the cases for BBO 5 and BBO 11,
respectively. As seen in cruciform 15, the spectrum of 16 is
also broad and lacking in structural detail; thus, direct
comparisons to 5 and 11 are indeterminate. The diminished
vibronic detail in 16, and to a lesser extent in 13, is likely a
result of the more nonplanar structures of these cruciforms in
comparison to 14 and 15 (vide supra). This trend also indicates
that changes in the spectral topography may be related to the
symmetry of these molecules. Of all the cruciforms in this
CONCLUSIONS
■
A series of related 4,8-diarylBBOs, 2,6-diarylBBOs, and 2,4,6,8-
tetraarylBBO cruciforms were readily synthesized in a few high-
yielding steps. It was observed that the optical and electronic
properties of these molecules are dependent on both the
electronic nature of the aryl substituent and its location. For
linear molecules, as the electron-donating strength of the
substituent was increased, the HOMO levels of the BBOs were
raised regardless of axis. Additionally, BBOs with an electron-
donating group along the 4,8-axis had lower lying HOMO
levels than the analogous 2,6-BBOs. On the other hand, the
LUMO levels for the related linear BBOs appeared to be
unaffected by structural modifications. Similarly, with the mixed
cruciforms placing the electron-donating group along the 4,8-
axis resulted in a higher HOMO in comparison to that obtained
when the substituent was placed on the 2,6-axis. Changing the
location of the substituent impacted the LUMO levels for the
cruciforms, whereas the band gaps remained unaffected.
Collectively, the UV−vis, PL, and DFT results indicate that
the FMOs and band gaps of benzobisoxazoles can be readily
modified either jointly or independently of each other. This
result is of significance in the design of new materials for use in
a range of organic semiconducting applications. Work is
ongoing in our laboratories to develop new two-dimensional
oligomers and conjugated polymers.
EXPERIMENTAL SECTION
■
General Methods. Nuclear magnetic resonance (NMR) experi-
ments were carried out in CDCl3 at 400 MHz (1H) or 100 MHz (13C).
1H NMR spectra are internally referenced to the 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 δ relative to tetramethylsilane. Coupling constants
are reported in hertz (Hz). High-resolution mass spectra were
recorded on a double-focusing magnetic sector mass spectrometer
using EI, ESI, or APCI. Melting points were obtained on a melting
point apparatus with 260 °C upper limit and are uncorrected. All UV−
vis and fluorescence spectra were obtained using THF solutions unless
otherwise noted. Relative solution fluorescence quantum yields were
obtained using anthracene (ΦPL = 0.27 in ethanol)57 as a standard with
excitation at 325 nm. Ultraviolet photoelectron spectroscopy (UPS)
measurements were performed on sample films spun from CHCl3.
DAHQ,35 Br-DAHQ,58 1-bromo-4-(3,7-dimethyloctyloxy)benzene,58
4-dodecyl-1-iodobenzene,58 4-dodecylphenylboronic acid,59 2-bromo-
9,9-dihexylfluorene,60 2-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)-
9,9-dihexylfluorene,61 N,N-dibutylaniline,62,63 4-bromo-N,N-dibutyla-
niline,63 4-(dibutylamino)phenylboronic acid,64 2-bromo-5-dodeca-
noylthiophene,65 2-bromo-5-dodecylthiophene,65,66 5-dodecyl-2-
(trimethylstannyl)thiophene,67 and 2-(4-bromophenyl)-1,3-dithiane68
were synthesized according to literature procedures.
2,6-Diethylbenzo[1,2-d;4,5-d′]bis(oxazole) (1). A dry round-
bottom flask was placed under an argon atmosphere and charged with
0.054 g (0.05 mmol) Y(OTf)3 and 1.06 g (6.00 mmol) of triethyl
orthopropionate. The flask was fitted with a dry addition funnel and
heated to 55 °C with stirring. In the addition funnel, 0.43 g (2.00
mmol) of DAHQ was dissolved in 0.33 g (4.20 mmol) of pyridine and
2 mL of DMSO and the solution added dropwise to the flask. The
mixture was stirred at 55 °C for 2 h and then cooled to room
temperature and diluted with water. The resulting precipitate was
H
dx.doi.org/10.1021/jo4007927 | J. Org. Chem. XXXX, XXX, XXX−XXX

The Journal of Organic Chemistry
Article
collected by filtration and washed with water. The crude product was
purified by recrystallization from hexanes to give white needles (0.28 g,
65% yield): mp 95−97 °C; 1H NMR (400 MHz, CDCl3) δ 1.45 (6H,
t, J = 8 Hz) 2.96 (4H, q, J = 8 Hz), 7.70 (2H, s, J = 8 Hz); 13C NMR
(100 MHz, CDCl3) δ11.0, 22.5, 100.5, 139.2, 148.3, 169.4; HRMS
(ESI) m/z calcd for C12H13N2O2 217.0972 [M + H]+, found 217.0972.
4,8-Dibromo-2,6-diethylbenzo[1,2-d;4,5-d′]bis(oxazole) (2).
A dry round-bottom flask was placed under an argon atmosphere
and charged with 0.52 g (0.98 mmol) of Y(OTf)3, 21 mL of DMA, and
10.3 g (58.5 mmol) of triethyl orthopropionate. The mixture was
heated to 55 °C with stirring, and 5.81 g (19.5 mmol) freshly prepared
Br-DAHQ was added portionwise over 45 min. The reaction mixture
was stirred at 55 °C for 2 h and then cooled to room temperature and
diluted with water. The resulting precipitate was collected by filtration
and rinsed with water and ethanol. The crude product was further
purified by recrystallization from 1/1 chloroform/ethanol to give small
0.91 (12H, d, J = 8 Hz), 0.98 (6H, d, J = 8 Hz), 1.2−1.37 (12H,
comp), 1.52 (6H, t, J = 8 Hz), 1.75−1.92 (8H, comp), 3.04 (4H, q, J =
8 Hz), 4.11 (4H, m), 7.11 (4H, d, J = 8 Hz, 8.21 (4H, d, J = 8 Hz); 13C
NMR (100 MHz, CDCl3) δ 11.5, 19.9, 22.79, 22.84, 23.0, 25.0, 28.2,
30.1, 36.4, 37.5, 39.5, 66.5, 113.2, 114.8, 125.1, 131.4, 136.8, 146.0,
159.1, 168.6; HRMS (ESI) m/z calcd for C44H61N2O4 681.4626 [M +
H]+, found 681.4636.
4,8-Bis(4-dodecylphenyl)-2,6-diethylbenzo[1,2-d;4,5-d′]bis-
(oxazole) (5). This compound was prepared analogously to BBO 3
from BBO 2 (1.00 mmol) and (4-dodecylphenyl)boronic acid. The
product was purified by column chromatography with hexanes/
chloroform as eluent (3/1 gradient to 1/3) followed by recrystalliza-
tion from hexanes to give a white powder (0.59 g, 83% yield): mp
1
103−105 °C; H NMR (400 MHz, CDCl3) δ 0.89 (6H, t, J = 8 Hz,
1.28 (36H, comp), 1.47 (6H, t, J = 8 Hz), 1.70 (4H, m), 2.70 (4H, t, J
= 8 Hz), 3.03 (4H, q, J = 8 Hz), 7.39 (4H, d, J = 8 Hz), 8.15 (4H, d, J
= 8 Hz); 13C NMR (100 MHz, CDCl3) δ 11.6, 14.4, 22.8, 22.9, 29.6,
29.7, 29.82, 29.87, 29.89, 29.94, 31.7, 32.2, 36.2, 113.9, 128.9, 130.0,
137.0, 143.2, 146.2, 168.8; HRMS (ESI) m/z calcd for C48H69N2O2
705.5354 [M + H]+, found 705.5372.
1
white needles (5.85 g, 80% yield): mp 216−218 °C; H NMR (400
MHz, CDCl3) δ 1.50 (6H, t, J = 8 Hz) 3.06 (4H, q, J = 8 Hz); 13C
NMR (100 MHz, CDCl3) δ 11.3, 22.8, 91.4, 138.6, 146.7, 170.0;
HRMS (ESI) m/z calcd for C12H11Br2N2O2 372.9182 [M + H]+,
found 372.9197.
4,8-Bis(4-(dibutylamino)phenyl)-2,6-diethylbenzo[1,2-d;4,5-
d′]bis(oxazole) (6). This compound was prepared analogously to
BBO 3 from BBO 2 (1.00 mmol) and (4-(dibutylamino)phenyl)-
boronic acid. The product was purified by column chromatography
with 1/1 hexanes/toluene as eluent followed by recrystallization from
acetone to give small yellow needles (0.26 g, 60% yield): mp 103−105
°C. 1H NMR (400 MHz, CDCl3) δ 0.99 (6H, t, J = 8 Hz, 1.40 (4H, p,
J = 8 Hz), 1.51 (4H, t, J = 8 Hz), 1.65 (4H, m), 3.04 (4H, q, J = 8 Hz),
3.36 (4H, t, J = 8 Hz), 6.82 (4H, d, J = 8 Hz), 8.16 (4H, d, J = 8 Hz).
13C NMR (100 MHz, CDCl3) δ 11.7, 14.3, 20.6, 22.8, 29.8, 51.0,
111.7, 113.0, 119.7, 131.0, 136.5, 146.0, 147.8, 168.0; HRMS (ESI) m/
z calcd for C40H55N4O2 623.4320 [M + H]+, found 623.4302.
4,8-Bis(9,9-dihexylfluoren-2-yl)-2,6-diethylbenzo[1,2-d;4,5-
d′]bis(oxazole) (7). This compound was prepared analogously to
BBO 3 from BBO 2 (1.00 mmol) and 2-(4,4,5,5-tetramethyl-1,3,2-
dioxaborolan-2-yl)-9,9-dihexylfluorene. The product was purified by
column chromatography with hexanes/CHCl3 as eluent (3/1 gradient
to 1/1) followed by recrystallization from hexanes to give a white solid
(0.37 g, 61% yield): mp 157−160 °C; 1H NMR (400 MHz, CDCl3) δ
0.78 (12H, t, J = 8 Hz), 0.86 (8H, m), 1.09−1.17 (24H, comp), 1.55
(6H, t, J = 8 Hz), 2.06 (8H, m), 3.09 (4H, q, J = 8 Hz), 7.33−7.42
(6H, comp), 7.79 (2H, d, J = 8 Hz), 7.91 (2H, d, J = 8 Hz), 8.25 (2H,
s), 8.29 (2H, d, J = 8 Hz); 13C NMR (100 MHz, CDCl3) δ 11.3, 13.2,
22.86, 22.90, 24.2, 30.1, 31.8, 40.7, 55.3, 114.6, 120.0, 120.1, 123.1,
125.0, 127.0, 127.4, 129.0, 131.4, 137.2, 141.0, 141.1, 146.4, 150.9,
152.6; HRMS (ESI) m/z calcd for C62H77N2O2 881.5980 [M + H]+,
found 881.5979.
4,8-Diphenyl-2,6-diethylbenzo[1,2-d;4,5-d′]bis(oxazole) (3).
A two-neck round-bottom flask was charged with 0.37 g (1.00
mmol) of 2, 0.10 g (1.00 mmol) of tetrabutylammonium bromide
(TBAB), and 0.33 g (3.0 mmol) of phenylboronic acid. The flask was
equipped with a reflux condenser and placed under an argon
atmosphere. A 25 mL portion of deoxygenated toluene was added
followed by 2 mL of deoxygenated 3 M aqueous K2CO3. The mixture
was further deoxygenated for 10 min, then 0.036 g (0.05 mmol) of
Pd(dppf)Cl2 was added, and the reaction mixture was heated to reflux
with stirring under an argon atmosphere for 36 h. The mixture was
cooled to room temperature and diluted with toluene, and the layers
were separated. The organic layer was washed with 1 M HCl, H2O,
and brine and dried over MgSO4. The solution was filtered and the
solvent removed in vacuo. The crude product was taken up in hot
hexanes and filtered, and the solvent was removed in vacuo. The
product was further purified by recrystallization from acetone to give
1
white needles (0.27 g, 73% yield): mp 178−179 °C; H NMR (400
MHz, CDCl3) δ 1.50 (6H, q, J = 8 Hz), 3.05 (4H, t, J = 8 Hz), 7.45
(2H, m), 7.58 (4H, m), 8.23 (4H, d, J = 8 Hz); 13C NMR (100 MHz,
CDCl3) δ 11.5, 22.8, 114.1, 128.3, 128.8, 130.2, 132.7, 137.2, 146.2,
169.0; HRMS (ESI) m/z calcd for C24H21N2O2 369.1598 [M + H]+,
found 369.1603.
4,8-Bis(4-((3,7-dimethyloctyl)oxy)phenyl)-2,6-diethylbenzo-
[1,2-d;4,5-d′]bis(oxazole) (4). (4-((3,7-Dimethyloctyl)oxy)phenyl)-
boronic Acid. A dry, two-neck round-bottom flask was placed under
an argon atmosphere and charged with 300 mL of dry THF and 9.88 g
(30.0 mmol) of 4-((3,7-dimethyloctyl)oxy)-1-bromobenzene. The
solution was cooled to −78 °C in a dry ice/acetone bath, 15.0 mL
of nBuLi (2.5 M in hexanes) was added dropwise, and the mixture was
stirred at −78 °C for 1 h. A 9.35 g (90.0 mmol) amount of B(OMe)3
was added dropwise to the solution, and the mixture was stirred at −78
°C for 1 h and warmed to room temperature over 3 h. The reaction
was quenched with 2 M HCl (25 mL), and the mixture was extracted
with diethyl ether. The combined organic layers were washed with
water and brine and dried over MgSO4. The solution was filtered and
the solvent removed in vacuo. The resulting solid was dried under
vacuum overnight to give an off-white paste that was used without
further purification (2.90 g, 35% yield): 1H NMR (400 MHz, CDCl3)
δ 0.90 (6H, d, J = 8 Hz), 1.00 (3H, d, J = 8 Hz), 1.21−1.88 (10H,
comp), 4.09 (2H, m), 7.02 (2H, d, J = 8 Hz), 8.17 (2H, d, J = 8 Hz);
13C NMR (100 MHz, CDCl3) δ 19.9, 22.9, 23.0, 24.9, 28.2, 36.4, 36.5,
4,8-Bis(5-dodecylthien-2-yl)-2,6-diethylbenzo[1,2-d;4,5-d′]-
bis(oxazole) (8). A dry, two-neck round-bottom flask was fitted with
a reflux condenser, placed under an argon atmosphere, and charged
with 0.28 g (0.75 mmol) of 2, 0.70 g (1.67 mmol) of 5-dodecyl-2-
(trimethylstannyl)thiophene, and 15 mL of dry, deoxygenated toluene.
The mixture was further deoxygenated for 10 min and then charged
with 9.1 mg (0.03 mmol) of P(otolyl)3 and 13.7 mg (0.015 mmol) of
Pd2(dba)3. The reaction mixture was heated to reflux with stirring
under an argon atmosphere for 24 h, cooled to room temperature, and
concentrated in vacuo. The crude mixture was dissolved in CH2Cl2
and passed through a pad of silica gel with CH2Cl2 as eluent, and the
solution was concentrated in vacuo. The product was further purified
by recrystallization from hexane to give small light yellow needles
1
(0.51 g, 94% yield): mp 97−99 °C; H NMR (400 MHz, CDCl3) δ
0.88 (6H, t, J = 8 Hz), 1.26−1.43 (36H, comp), 1.56 (6H, t, J = 8 Hz),
1.77 (4H, m), 2.91 (4H, t, J = 8 Hz), 3.11 (4H, q, J = 8 Hz), 6.90 (2H,
d, J = 4 Hz), 8.11 (2H,d, J = 8 Hz); 13C NMR (100 MHz, CDCl3) δ
11.4, 14.4, 22.89, 22.92, 29.4, 29.58, 29.64, 29.82, 29.87, 29.90, 30.4,
32.0, 32.1, 107.8, 124.8, 128.8, 131.9, 135.4, 144.5, 148.2, 168.4;
HRMS (ESI) m/z calcd for C44H65N2O2S2 717.4482 [M + H]+, found
717.4488.
37.5, 39.5, 66.4, 114.1, 115.7, 137.6, 162.9.
4,8-Bis(4-((3,7-dimethyloctyl)oxy)phenyl)-2,6-diethylbenzo[1,2-
d;4,5-d′]bis(oxazole) (4). This compound was prepared analogously to
BBO 3 from BBO 2 (1.00 mmol) and (4-((3,7-dimethyloctyl)oxy)-
phenyl)boronic acid. The product was purified by column
chromatography with hexanes/chloroform as eluent (3/1 gradient to
1/3) followed by recrystallization from hexanes to give a white powder
5-Dodecyl-2-(triethoxymethyl)thiophene. A dry, three-neck
round-bottom flask was equipped with a reflux condenser and placed
1
(0.54 g, 79% yield): mp 74−76 °C. H NMR (400 MHz, CDCl3) δ
I
dx.doi.org/10.1021/jo4007927 | J. Org. Chem. XXXX, XXX, XXX−XXX

The Journal of Organic Chemistry
Article
under an argon atmosphere. The flask was charged with 8.28 g (25.0
and then heated to reflux with stirring overnight. The mixture was
cooled to room temperature and poured into cold 1 M HCl, and the
layers were separated. The aqueous layer was extracted with ether, and
the combined organic layers were washed with 1 M HCl, water, and
brine and dried over MgSO4. The solution was filtered, and the solvent
was removed in vacuo. The crude product was purified by
recrystallization from isopropyl alcohol to give ivory-colored needles
mmol) of 2-bromo-5-dodecylthiophene and 25 mL of dry diethyl
i
ether. A 13.8 mL (27.5 mmol) amount of PrMgCl (2.0 M in ether)
was added dropwise, and the reaction mixture was heated to reflux
with stirring for 24 h and then cooled to room temperature. A solution
of 5.53 g (28.8 mmol) of tetraethyl orthocarbonate in 20 mL of dry
diethyl ether was added dropwise and the mixture heated to reflux with
stirring overnight. The mixture was cooled to room temperature and
poured into a saturated aqueous solution of NH4Cl. The layers were
separated, and the aqueous layer was extracted with ether. The
combined organic layers washed with water and brine and dried over
MgSO4. The solution was filtered and the solvent removed in vacuo.
The low-boiling impurities were removed by vacuum distillation, and
the resulting red oil was used without further purification (5.05 g, 51%
1
(9.32 g, 93% yield): mp 52−54 °C; H NMR (400 MHz, CDCl3) δ
0.88 (3H, t, J = 8 Hz), 1.25−1.35 (18H, comp), 1.60 (2H, p, J = 8 Hz),
1.92 (1H, m), 2.16 (1H, m), 2.57 (2H, t, J = 8 Hz), 2.90 (2H, dt), 3.06
(2H, td), 5.15 (1H, s), 7.14 (2H, d, J = 8 Hz), 7.36 (2H, d, J = 8 Hz);
13C NMR (100 MHz, CDCl3) δ 14.3, 22.8, 25.3, 29.49, 29.50, 29.65,
29.73, 29.78, 29.8, 31.5, 32.1, 32.3, 51.4, 127.7, 128.9, 136.4, 143.5;
HRMS (APCI) m/z calcd for C22H37S2 365.2331 [M + H]+, found
365.2341.
1
yield): H NMR (400 MHz, CDCl3) δ 0.89 (3H, t, J = 8 Hz), 1.20
(9H, t, J = 8 Hz), 1.21−1.40 (18H, comp), 1.67 (2H, m), 2.77 (3H, t, J
= 8 Hz), 3.46 (2H, q, J = 8 Hz), 6.64 (1H, d, J = 4 Hz), 6.98 (1H, d, J
= 4 Hz); 13C NMR (100 MHz, CDCl3) 14.3, 15.1, 22.9, 29.3, 29.5,
29.6, 29.84, 29.86, 29.88, 30.4, 31.7, 32.1, 58.1, 113.2, 123.4, 126.7,
139.0, 147.0; HRMS (ESI) m/z calcd for C23H42NaO3S 421.2747 [M
+ Na]+, found 421.2753.
4-Dodecyl-1-(triethoxymethyl)benzene. A dry, two-neck round-
bottom flask was placed under an argon atmosphere and charged with
1.82 g (5.00 mmol) of 2-(4-dodecylphenyl)-1,3-dithiane, 1.16 g (10.0
mmol) of TMEDA, and 50 mL of dry THF. The flask was cooled to
n
−78 °C in a dry ice/acetone bath, and 2.2 mL (5.5 mmol) of BuLi
(2.5 M in hexane) was added dropwise. The flask was stirred for 1 h at
−78 °C, and 0.55 mL (6.00 mmol) of dimethyl disulfide was added in
one portion. The mixture was stirred for 30 min at −78 °C and
warmed to room temperature over 3 h. The reaction mixture was
quenched with water and extracted with ether. The combined organic
layers were washed with water and brine and dried over Na2SO4. The
solution was filtered and concentrated in vacuo. The remaining residue
was dried under vacuum with stirring overnight. The flask was back-
filled with argon and charged with 30 mL of THF, 6 mL of ethanol,
2,6-Bis(5-dodecylthien-2-yl)benzo[1,2-d;4,5-d′]bis(oxazole)
(9). A dry Schlenk flask was placed under an argon atmosphere and
charged with 1.59 g (4.00 mmol) of 5-dodecyl-2-(triethoxymethyl)-
thiophene, 27 mg (0.050 mmol) of Y(OTf)3, and 1 mL of THF. The
solution was deoxygenated for 20 min and then heated to 55 °C with
stirring. Concurrently, a dry 5 mL pear-bottom flask was place under
an argon atmosphere, 1 mL of DMSO was added, and the flask and
contents were deoxygenated for 20 min. The pear-bottom flask was
charged with 0.21 g (1.00 mmol) of DAHQ and 0.164 g (2.10 mmol)
of pyridine, and the resulting solution was added dropwise to the THF
solution at 55 °C. After 3 h, the reaction mixture was diluted with 3
mL of THF and stirred at 55 °C overnight. The mixture was cooled to
room temperature and diluted with methanol. The resulting
precipitate was filtered and rinsed with methanol. The product was
purified by recrystallization from CHCl3 to give a yellow solid (0.48 g,
i
1.05 mL (10.0 mmol) of Pr2NH, and 3.60 mL (27.5 mmol) of 2,4,6-
collidine. A solution of 4.25 g (25.0 mmol) of AgNO3 in dry CH3CN
was added in one portion and the mixture stirred vigorously for 24 h.
The reaction was quenched by stirring with brine for 6 h, and the
solids were filtered and rinsed with large portions of ether. The two
layers of the filtrate were separated, and the organic layer was washed
with water (3×) and brine and dried over Na2SO4. The solution was
filtered, and the solvents were removed in vacuo. Low-boiling
impurities were removed by vacuum distillation, and the dark yellow
oil was used without further purification (1.64 g, 84% yield): 1H NMR
(400 MHz, CDCl3) δ 0.89 (3H, t, J = 8 Hz), 1.78 (9H, t, J = 8 Hz),
1.26−1.35 (18H, comp), 1.62 (2H, m), 2.61, (2H, t, J = 8 Hz), 3.65
(6H, q, J = 8 Hz), 7.16 (2H, d, J = 8 Hz), 7.50 (2H, d, J = 8 Hz); 13C
NMR (100 MHz, CDCl3) δ 14.3, 15.1, 22.8, 29.51, 29.52, 29.65, 29.74,
29.80, 29.81, 29.83, 31.5, 32.1, 35.9, 57.6, 114.0, 127.4, 128.0, 135.5,
143.4; HRMS (ESI) m/z calcd for C25H44NaO3 415.3183 [M + Na]+,
415.3185.
2,6-Bis(4-dodecylphenyl)benzo[1,2-d;3,4-d′]bis(oxazole)
(11). This compound was prepared and isolated analogously to BBO 9
from 0.11 g (0.50 mmol) of DAHQ, 0.63 g (1.60 mmol) of 4-dodecyl-
1-(triethoxymethyl)benzene, and 43.5 mg of Yb(OTf)3. The product
was purified by recrystallization from THF to give white needles (0.21
g, 65% yield): mp >260 °C; 1H NMR (400 MHz, CDCl3) δ 0.90 (6H,
t, J = 8 Hz), 1.28−1.37 (36H, comp 1.69 (4H, m), 2.71 (4H, t, J = 8
Hz), 7.35 (4H, d, J = 8 Hz), 7.89 (2H, s), 8.19 (4H, d, J = 8 Hz); 13C
NMR (100 MHz, CDCl3) 14.2, 22.8, 29.48, 29.50, 29.6, 29.7, 29.81,
29.84, 31.3, 32.1, 36.3, 100.9, 124.9, 127.9, 129.2, 140.7, 147.4, 148.8,
164.7; HRMS (ESI) m/z calcd for C44H61N2O2 649.4728 [M + H]+,
found 649.4730.
1
73% yield): mp 190−192 °C; H NMR (400 MHz, CDCl3) δ 0.89
(6H, t, J = 8 Hz), 1.27−1.41 (36H, comp), 1.75 (4H, p, J = 8 Hz), 2.90
(4H, t, J = 8 Hz), 6.89 (2H, d, J = 4 Hz), 7.75 (2H, d, J = 4 Hz), 7.80
(2H, s); 13C NMR (100 MHz, CDCl3) δ 14.3, 22.9, 29.3, 29.6, 29.76,
29.85, 29.89, 30.7, 31.7, 32.2, 100.6, 125.9, 127.0, 130.4, 140.5, 148.6,
152.6, 160.6; HRMS (ESI) m/z calcd for C38H57N2O2S2 661.3832 [M
+ H]+, found 661.3837.
4,8-Dibromo-2,6-bis(5-dodecylthien-2-yl)benzo[1,2-d;3,4-
d′]bis(oxazole) (10). A dry Schlenk flask was placed under an argon
atmosphere and charged with 4.78 g (12.0 mmol) of 5-dodecyl-2-
(triethoxymethyl)thiophene, 5 mL of dry DMA, and 5 mL of dry
THF. The mixture was deoxygenated for 20 min, 0.12 g (0.20 mmol)
of Yb(OTf)3 was added, and the mixture was heated to 55 °C. A 1.19 g
(4.00 mmol) amount of of freshly prepared Br-DAHQ was added
portionwise, and the reaction mixture was diluted with 5 mL of THF
after 3 h and stirred at 55 °C overnight. The mixture was cooled to
room temperature and diluted with methanol. The resulting
precipitate was filtered and rinsed with methanol. The product was
purified by recrystallization from CHCl3 to give pale yellow needles
(2.45 g, 75% yield): mp 190−192 °C; 1H NMR (400 MHz, CDCl3) δ
0.90 (6H, t, J = 8 Hz), 1.29−1.43 (36H, comp), 1.77 (4H, p, J = 8 Hz),
2.92 (4H, t, J = 8 Hz), 6.90 (2H, d, J = 8 Hz), 7.87 (2H, d, J = 4 Hz);
13C NMR (100 MHz, CDCl3) δ 14.3, 22.9, 29.3, 29.55, 29.57, 29.7,
4,8-Dibromo-2,6-bis(4-dodecylphenyl)[1,2-d;3,4-d′]bis-
(oxazole) (12). This compound was prepared and isolated
analogously to BBO 10 from 0.15 g (0.50 mmol) of Br-DAHQ and
0.59 g (1.50 mmol) of 4-dodecyl-1-(triethoxymethyl)benzene, except
the reaction mixture was heated to 65 °C. The product was purified by
recrystallization from toluene (hot filter, charcoal) to give white
29.85, 29.86, 29.89, 30.7, 31.6, 32.2, 91.3, 125.9, 126.0, 131.8, 139.8,
146.9, 153.9, 160.7; HRMS (ESI) m/z calcd for C40H55Br2N2O2S2
817.2046 [M + H]+, found 817.2066.
4-Dodecyl-1-(triethoxymethyl)benzene. 2-(4-Dodecylphenyl)-
1,3-dithiane. A dry three-neck round-bottom flask was equipped with
a reflux condenser and placed under an argon atmosphere. The flask
was charged with 7.05 g (25.6 mmol) of 2-(4-bromophenyl)-1,3-
dithiane, 60 mL of dry, deoxygenated ether, and 0.21 g (0.256 mmol)
of Pd(dppf)Cl2·CH2Cl2. The flask was placed in an ice/water bath, and
30.7 mL (30.7 mmol) of dodecylmagnesium bromide (1.0 M in ether)
was added dropwise. The mixture was warmed to room temperature
1
needles (0.27 g, 68% yield): mp 154−156 °C; H NMR (400 MHz,
CDCl3) δ 0.88 (6H, t, J = 8 Hz), 1.26−1.36 (36H, comp), 1.67 (4H,
m), 2.70 (4H, t, J = 8 Hz), 7.35 (4H, d, J = 8 Hz), 8.25 (4H, d, J = 8
Hz); 13C NMR (100 MHz, CDCl3) δ 22.8, 29.46, 29.51, 29.6, 29.7,
29.8, 31.2, 32.1, 36.3, 91.8, 123.9, 128.3, 128.4, 129.2, 129.3, 139.9,
J
dx.doi.org/10.1021/jo4007927 | J. Org. Chem. XXXX, XXX, XXX−XXX

The Journal of Organic Chemistry
Article
147.1, 148.2, 164.9; HRMS (APCI) m/z calcd for C44H59Br2N2O2
807.2918 [M + H]+, found 807.2922.
Trestles Cluster. Excited states were generated through time-
dependent density functional theory (TD-DFT) applied to the
optimized ground state for each oligomer. The HOMO, LUMO,
band gap, first 10 excited states, and UV−vis simulations were
generated from these excited computations.
4,8-Bis(4-dodecylphenyl)-2,6-bis(5-dodecylthien-2-yl)-
benzo[1,2-d;3,4-d′]bis(oxazole) (13). This compound was pre-
pared and isolated analogously to BBO 7 from BBO 10 (0.61g, 0.75
mmol) and (4-dodecylphenyl)boronic acid (0.54g, 1.88 mmol). The
crude product was dissolved in CHCl3 and passed through a pad of
silica gel with CHCl3 as eluent, and the solution was concentrated in
vacuo. The product was further purified by recrystallization from
CHCl3 to give a bright yellow solid (0.72 g, 87% yield): mp 94−96 °C;
1H NMR (400 MHz, CDCl3) δ 0.91 (12H, comp), 1.29−1.46 (72H,
ASSOCIATED CONTENT
* Supporting Information
■
S
Text, figures, and tables giving experimental procedures, 1H and
13C NMR spectra for new compounds, PL spectra of selected
compounds, calculated atom coordinates, and absolute
energies. This material is available free of charge via the
Internet at http://pubs.acs.org.
comp), 1.74 (8H, comp), 2.75 (4H, t, J = 8 Hz), 2.88 (4H, t, J = 8 Hz),
6.85 (2H, d, J = 4 Hz), 7.43 (4H, d, J = 8 Hz), 7.76 (2H, d, J = 4 Hz),
8.29 (4H, d, J = 8 Hz); 13C NMR (100 MHz, CDCl3) δ 14.4, 22.92,
22.93, 29.3, 29.57, 29.58, 29.61, 29.75, 29.77, 29.84, 29.86, 29.89,
29.94, 29.95, 30.6, 31.68, 31.73, 32.14, 32.16, 36.2, 113.7, 125.6, 127.1,
128.8, 130.0, 130.3, 138.3, 143.2, 146.2, 152.1, 159.8; HRMS (ESI) m/
z calcd for C76H113N2O2S2 1149.8238 [M + H]+, found 1149.8229.
4,8-Bis(5-dodecylthien-2-yl)-2,6-bis(4-dodecylphenyl)[1,2-
d;3,4-d′]bis(oxazole) (14). This compound was prepared and
isolated analogously to BBO 8 from 0.32 g (0.40 mmol) of BBO 12
and 0.38 g (0.92 mmol) of 5-dodecyl-2-(trimethylstannyl)thiophene.
The product was purified by recrystallization from ethyl acetate to give
AUTHOR INFORMATION
Corresponding Author
*E-mail for M.J.-E.: malikaj@iastate.edu.
■
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
1
■
a yellowish orange powder (0.40 g, 87% yield): mp 97−100 °C; H
We acknowledge the donors of the Petroleum Research Fund
for support of this work. We also thank the 3M Foundation and
the National Science Foundation (DMR-0846607) for partial
support of this work. We thank Kamel Harrata and the Mass
Spectroscopy Laboratory Iowa State University (ISU) for
analysis of our compounds. 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 1-bromo-4-((3,7-dimethyloctyl)oxy)benzene. We also thank
Dr. William Jenks (ISU), Dr. Jared Mike (Texas A&M
University), and Dr. Jeremy Intemann (University of
Washington) for helpful discussions on this research. We
would like to thank the National Foundation Extreme Science
Engineering Discovery Environment (TG-CHE120020) for
providing the resources for all the computational work included
in this article.
NMR (400 MHz, CDCl3) δ 0.89 (12H, comp), 1.25−1.51 (72H,
comp), 1.70 (4H, p, J = 8 Hz), 1.82 (4H, p, J = 8 Hz), 2.73 (4H, t, J =
8 Hz), 2.98 (4H, t, J = 8 Hz), 6.80 (2H, d, J = 3 Hz), 7.39 (4H, d, J = 8
Hz), 8.31 (4H, d, J = 8 Hz), 8.33 (2H, d, J = 4 Hz); 13C NMR (100
MHz, CDCl3) δ 14.2, 22.8, 29.44, 29.47, 29.52, 29.63, 29.68, 29.77,
29.83, 29.85, 29.9, 30.5, 31.3, 32.0, 32.1, 36.3, 108.1, 124.8, 124.9,
128.0, 129.1, 129.2, 132.2, 136.8, 144.6, 147.2, 148.2, 163.6; HRMS
(APCI) m/z calcd for C76H113N2O2S2 1149.8238 [M + H]+, found
1149.8259.
2,4,6,8-Tetrakis(5-dodecylthien-2-yl)[1,2-d;3,4-d′]bis-
(oxazole) (15). This compound was prepared and isolated
analogously to BBO 8 from 0.41 g (0.50 mmol) of BBO 10 and
0.47 g (1.13 mmol) of 5-dodecyl-2-(trimethylstannyl)thiophene. The
product was purified by recrystallization from ethyl acetate to give a
dark yellow powder (0.55 g, 94% yield): mp 78−80 °C; 1H NMR (400
MHz, CDCl3) δ 0.88 (12H, t, J = 8 Hz), 1.27−1.45 (72H, comp), 1.78
(8H, comp), 2.93 (8H, comp), 6.90 (2H, d, J = 4 Hz), 6.94 (2H, d, J =
4 Hz), 7.86 (2H, d, J = 4 Hz), 8.25 (2H, d, J = 4 Hz); 13C NMR (100
MHz, CDCl3) δ 14.3, 22.9, 29.26, 29.4, 29.5, 29.6, 29.7, 29.80, 29.82,
29.85, 30.4, 30.6, 31.7, 31.95, 32.08, 107.6, 124.8, 125.7, 127.0, 129.0,
130.4, 131.9, 136.5, 144.2, 148.2, 152.3, 159.4; HRMS (APCI) m/z
calcd for C72H109N2O2S4 1161.7366 [M + H]+, found 1161.7346.
2,4,6,8-Tetrakis(4-dodecylphenyl)[1,2-d:3,4-d′]bis(oxazole)
(16). This compound was prepared and isolated similarly to 7 from
0.32 g (0.40 mmol) of 12 and 0.29 g (1.00 mmol) of 4-
dodecylphenylboronic acid. The crude product was dissolved in
CHCl3 and passed through a pad of silica gel with CHCl3 as eluent,
and the solution was concentrated in vacuo. The product was purified
by recrystallization from ethyl acetate to give white needles (0.34 g,
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