Benzobisoxazole cruciforms: A tunable, cross-conjugated platform for the generation of deep blue OLED materials

Article abstract of DOI:10.1039/c5tc03622d

Four new cross-conjugated small molecules based on a central benzo[1,2-d:4,5-d′]bisoxazole moiety possessing semi-independently tunable HOMO and LUMO levels were synthesized and the properties of these materials were evaluated experimentally and theoretically. The molecules were thermally stable with 5% weight loss occurring well above 350 °C. The cruciforms all exhibited blue emission in solution ranging from 433-450 nm. Host-guest OLEDs fabricated from various concentrations of these materials using the small molecule host 4,4′-bis(9-carbazolyl)-biphenyl (CBP) exhibited deep blue-emission with Commission Internationale de L'Eclairage (CIE) coordinates of (0.15 ≤ x ≤ 0.17, 0.05 ≤ y ≤ 0.11), and maximum luminance efficiencies as high as ～2 cd A^-1. These results demonstrate the potential of benzobisoxazole cruciforms as emitters for developing high-performance deep blue OLEDs.

Full text of DOI:10.1039/c5tc03622d

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Benzobisoxazole cruciforms: A tunable, cross-conjugated platform
for the generation of deep blue OLED materials
Received 00th January 20xx,
Accepted 00th January 20xx
Ramiro Chavez, a Min Cai b, Brian Tlach,a David L Wheeler,c Rajiv Kaudal,b Ayuna Tsyrenova,b,d
Aimée L. Tomlinson,c Ruth Shinar,e Joseph Shinar,b and Malika Jeffries-ELa*
DOI: 10.1039/x0xx00000x
www.rsc.org/
Four new cross-conjugated small molecules based on a central benzo[1,2-d:4,5-d′]bisoxazole moiety possessing semi-
independently tunable HOMO and LUMO levels were synthesized and the properties of these materials were evaluated
experimentally and theoretically. The molecules were thermally stable with 5% weight loss occurring well above 350 °C.
The cruciforms all exhibited blue emission in solution ranging from 433 – 450 nm. Host-guest OLEDs fabricated from
various concentrations of these materials using the small molecule host 4,4’-bis(9-carbazolyl)-biphenyl (CBP) exhibited
deep blue-emission with Commission Internationale de L'Eclairage (CIE) coordinates of (0.15 ≤ x ≤ 0.17, 0.05 ≤ y ≤ 0.11),
and maximum luminance efficiencies as high as ~2 cd/A. These results demonstrate the potential of benzobisoxazole
cruciforms as emitters for developing high-performance deep blue OLEDs.
consistently synthesized with high purity.4 Initially, studies on
these molecules focused on simple systems comprising one
building block.5 In recent years, combining alternating electron
Introduction
Due to the multitude of everyday applications conceivable for
organic semiconductors, the field of organic electronics is
rapidly developing.1 Research in this area is motivated by the
many benefits of these materials, which include the relative
abundance of carbon, the reduced fabrication cost attained via
solution processing, and the opportunity to optimize the
material properties for specific applications through organic
synthesis. Initially, conjugated polymers were investigated for
organic electronic applications.2 However, since the properties
of conjugated polymers are a function of the materials
structure and morphology, variations in the molecular weight,
dispersity, defects and purity can dramatically impact their
performance.3 Furthermore, current methods for the synthesis
of conjugated polymers make consistency between batches
difficult. As a result the reported performance of conjugated
polymers in organic solar cells (OSC)s, organic light emitting
diodes (OLED)s, and organic field effect transistors can vary
widely. In contrast, mono-disperse conjugated molecules
possess well-defined molecular structures, and can be
donating (D) and electron accepting (A) moieties within the
same (macro)molecule has emerged as an effective way to
manipulate the position of its highest occupied molecular
orbital (HOMO) and lowest unoccupied molecular orbital
(LUMO). To date, a number of different donor-acceptor
molecules have been synthesized and OSCs using small
molecules as the active layer have recently reached record-
breaking efficiencies above 9.0%.6 Small molecules are the
materials of choice in commercial OLEDs due to their superior
efficiency and color purity in comparison to polymers.7
However, until recently, small molecule OLEDs (SMOLEDs),
have suffered from limited solubility.8 Therefore there is a
need for the development of new materials that can be
solution-processed.
While the donor-acceptor approach enables the tuning of
the materials’ frontier molecular orbitals (FMO)s, due to the
extensive delocalization of electrons within conjugated
systems most chemical modifications impact the position of
both the HOMO and LUMO levels. One tactic to circumvent
this phenomenon is through the synthesis of two-dimensional
cross-conjugated molecules comprising two perpendicular π-
conjugated linear units that intersect via a central aromatic
core. As a result of the spatial segregation of the FMOs in
these “cruciforms” it is possible to semi-independently alter
the LUMO or the HOMO level through judicious selection and
strategic positioning of substituents around the central
molecule.9
Benzo[1,2-d:4,5-d′]bisoxazole
(BBO)-based
cruciforms are unique since one of the conjugation pathways
encompasses the two oxazole rings whereas the other goes
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through the central benzene ring.10 Recently, the Miljanić targeted the synthesis of deep blue light-emitting materials, as
group reported the use of BBO cruciform as fluorescent they are essential components in solid-state light based on
sensors.10e-h Their work, along with our own synthetic and white OLEDs.12 These wide band gap materials can also be
11
theoretical studies on cross-conjugated BBOs,10c,
used as hosts for green and red emitters to achieve the
demonstrates that in the case of donor-acceptor systems, the primary color required for displays.13 Unfortunately, the
HOMO level can be localized along perpendicular axis (4,8-) requisite wide band gap and corresponding reduction in
whereas the LUMO level is localized along horizontal axis (2,6- emission quantum yield make it very challenging to attain
). Thus, selective tuning of the FMOs is possible, although to efficient deep blue emission.13b, 14 Thus we envisioned that a
date has not been widely utilized for developing organic small molecule with tunable FMOs could best achieve these
electronics.
In this work we demonstrate, for the first time the utility of
goals.
BBO cruciforms as a tunable platform for the development of
organic semiconductors, Figure 1. Based on theoretical
calculations, we synthesized four novel cross-conjugated
molecules composed of the BBO core and different
arrangements of phenyl- and fluorenyl- substitutents. We
Results and discussion
Synthesis and characterization
The synthetic approach for the 2,4,6,8-tetraarylBBOs is shown
in Scheme 1. All of the molecules were prepared using a simple
two step approach: a condensation reaction to prepare the
4,8-axis
C8H17
2,6-diaryl-4,8-dibromoBBOs and then
a cross-coupling to
2,6-axis
extend conjugation across the central benzene ring. This
approach allows for the synthesis of multiple BBOs from
common intermediates. The condensation reaction of 3,6-
diamino-2,5-dibromohydroquinone (1) with benzoyl chloride
or 9,9-dihexyl-9H-fluorene-2-carbonyl chloride (5), afforded
N
O
O
N
compounds
2 and 6 in yields of 23% and 15%, respectively.
C8H17
SM 1
Although the yields for these steps were low, we were able to
obtain adequate amounts of material for the next step. The
cross-coupling of
2 with 4-octylphenylboronic acid (3) or 2-
C8H17
(9,9-dioctyl-9H-fluoren-2-yl)-4,4,5,5-tetramethyl-1,3,2-
C8H17
dioxaborolane (4) yielded SM 1 and SM 2 in yields of 60% and
75%, respectively. The cross coupling of
6 with phenylboronic
N
O
O
acid or 4 afforded SM 3 and SM 4 in 40% and 62%, yield
N
respectively. Purification of these compounds was easily
accomplished by first passing through a short silica gel plug to
remove non-polar and residual catalyst followed by
precipitation from methanol. Lastly, the small molecules
purified by a subsequent column and precipitation. The
cruciforms were soluble in chlorinated solvents and
characterized by NMR spectroscopy, and high-resolution mass
spectrometry. The thermal properties of the materials were
studied using differential scanning calorimetry (DSC) under
nitrogen and thermogravimetric analysis (TGA) in air with a
heating rate of 20 °C. All the BBOs showed excellent thermal
stability under ambient conditions with 5% weight loss
C8H17
C8H17
SM 2
H13C6
C6H13
O
N
N
O
H13C6
C6H13
SM 3
temperatures of 368, 357, 390 and 409 °C for SM 1
, SM 2, SM
C8H17
C8H17
3
and SM 4 respectively (Figure S2). DSC did not reveal any
transitions pertaining to melting or crystallization before the
decomposition temperature. Thus, the heat generated during
device operation should not alter the film morphology of these
materials.
H13C6
C6H13
N
O
O
N
H13C6
C8H17
C8H17
C6H13
Optical and electrochemical properties
SM 4
The solution UV-Vis absorption and photoluminescence
spectra of the BBOs are shown in Figure S4 (supporting
information) and Figure 1, respectively. The data is
summarized in Table 1. All of the BBOs exhibited strong blue-
emission in solution (433 - 450 nm). SM 1, SM 2 and SM 4 all
Chart 1. Structures of molecules synthesized in this work.
2 | J. Name., 2012, 00, 1-3
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Scheme 1. Synthesis of BBO cruciform.
had broad, featureless peaks, whereas SM 3 had a fairly
shifted shoulder. The fluorescence quantum yields in solution
as SM
1 was the lowest, SM 4 the highest and the
constitutional isomers, SM 2 and SM 3 were identical.were
referenced to Fc/Fc+ and the results are summarized in Table
1. All of the materials exhibited measureable and reproducible
oxidation and reduction processes. The HOMO
values of the BBOs were similar,15 ranging from -6.0 to -6.2 eV,
whereas the LUMO levels ranged from -2.7 to -3.7 eV. SM 1
,
with phenyl substituents in all four positions, had the deepest
HOMO level (-6.2 eV), and the highest-lying LUMO level (-2.7
eV), resulting in the widest band gap. For the mixed systems
possessing both fluorenyl and phenyl groups (SM 2 and SM 3
)
the band gaps were both 2.9 eV. Since both the phenyl and
Table 1 Optical and electronic properties for SM 1- SM 4
Figure 1. Photoluminescence spectra of SM1 – SM4.
d
ꢃꢄꢅꢀ
ꢀꢁꢂ
ꢃꢄꢅꢀ
ꢀꢁꢂ
Abs
λ
PL
λ
HOMOb LUMOc Eꢋꢌꢍ
BBO
ΦPL
ꢆꢇꢈꢉꢊa (eV)
(nm)
355
364
386
395
(nm)
433
441
444
450
(eV)
-6.2
-5.7
-6.0
-6.0
(eV)
-2.7
-3.5
-3.7
-3.5
(eV)
SM 1
SM 2
SM 3
SM 4
0.34
0.51
0.51
0.71
3.1
2.9
2.9
2.8
3.5
2.2
2.3
2.5
fluorenyl substitutents are weakly electron-donating the
difference in the energy levels in these system is likely a result
of the difference in conjugation length.10c This conclusion is
verified by the band gap trend SM 1
> SM 2 = SM 3 > SM 4.
a
b
c
The influence of substitutent selection, and location is further
revealed in frontier orbital diagrams below.
ꢎꢓ
ꢎꢏꢐꢑꢒ
Estimated from the absorption onset of the film. HOMO= -(E
+ 4.8) eV.
+ 4.8) eV. d Eꢋꢌꢍ = LUMO - HOMO.
ꢔꢑꢕ
LUMO = -(E
ꢎꢏꢐꢑꢒ
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The electrochemical properties of the BBOs were aggregation between the BBO cores enhancing their efficiency
investigated by cyclic voltammetry (CV) using a platinum in OLEDs.18 The ranking in order of increasing dihedral angle
working electrode in acetonitrile, with 0.1 M Bu4NPF6 as the was: SM 4
electrolyte and an Ag/Ag+ reference electrode. The onsets for could not be fabricated into a working device with the CBP
the BBOs ranged from 0.34 – 0.71. An increase in quantum host material. SM indicated unfavourable electron
yield was seen upon the addition of fluorene to the BBO core, distribution in the frontier orbitals, thus SM 2 and SM 4, are
< SM 2 < SM 1 < SM 3. As discussed below, SM 1
3
narrow peak-width at half-max with a bathochromically- had
the most viable candidates for use in OLEDs. Based on the
torsion angle data, theory predicts that a SM 2 should produce
better device than SM 4, and this is corroborated by the data
Theoretical calculations
To further evaluate the structure-property relationships within below.
these cross-conjugated BBOs, time-dependent density
functional theory was performed. Initially, all four materials
were optimized and their excited states generated using
B3LYP/6-31G*. Based on previous benchmarking studies,16 it
was determined that the MPW1PBE level with an SV basis set
combined with the polarizable conductor calculation model
(CPCM)17 mimicking the CHCl3 solvation effects provided the
best correlation to experiment. Both sets of computational
results were compared and contrasted to experimental CV and
UV-Vis data and are summarized in Table 2 where the absolute
HOMO and the band gap measurements was 0.2 eV leading to
3.5% and 6.9% errors, respectively. Conversely, the LUMO
comparison suffered from much larger absolute and percent
errors, which was expected based on the error in the CV as
well as given the limitations inherent in DFT computations.15
The HOMO levels predicted by theory demonstrate the impact
substituent identify plays along the 4,8-axis.
Phenyl
substituents (SM 1 and SM 3) lower the HOMO to -6.1 eV,
whereas fluorenyl rings raise the HOMO to -5.9 eV (SM 2 and
SM 4). The band gaps appear to be influenced by conjugation
length as well as the extent of delocalization of electron
density as shown in the frontier orbitals displayed in Figure 3.
There was localization along the donor axis for the HOMOs
with the exception of SM
3. Both SM 2 and SM 4
demonstrated more localization along the acceptor axis than
SM 3 or SM 1. These two trends along with conjugation length
led to both lower band gaps and may play a part in the
improved performance of SM 2 and SM 4 in OLEDs relative to
SM 1 and SM 3. The importance of aryl ring identity was
further examined by measuring the torsion angle produced
between the 4,8-axis and the plane possessing the 2,6-axis,
Figure 3. The larger the torsion suppress the intermolecular
Figure 2: Frontier orbitals for SM 1 – SM 4.
Table 2 Theoretical computations compared to the Experimental Optical and Electronic measurements for SM 1 - SM 4
HOMO (abs diff. from Exp’t) in eV
LUMO (abs diff. from Exp’t) in eV
Eg (abs diff. from Exp’t) in eV
BBO
B3LYP/
6-31G*
MPW1PBE/SV-
CHCl3
B3LYP/
6-31G*
MPW1PBE/SV-
CHCl3
B3LYP/
6-31G*
MPW1PBE/SV-
Exp't
Exp't
Exp't
CHCl3
SM 1
SM 2
SM 3
SM 4
-6.2
-5.7
-6.0
-6.0
-5.3 (0.9)
-5.2 (0.5)
-5.3 (0.7)
-5.1 (0.9)
-6.1 (0.1)
-5.9 (0.2)
-6.1 (0.1)
-5.9 (0.1)
-2.7
-3.5
-3.7
-3.5
-1.9 (0.8)
-1.9 (1.6)
-2.0 (1.7)
-2.0 (1.5)
-2.4 (0.3)
-2.5 (1.0)
-2.5 (1.2)
-2.5 (1.0)
3.1
2.9
2.9
2.8
3.2 (0.1)
2.9 (0.0)
3.0 (0.1)
2.8 (0.0)
3.2 (0.1)
2.9 (0.0)
3.1 (0.2)
2.9 (0.1)
4 | J. Name., 2012, 00, 1-3
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Figure 3. Optimized geometries for SM 1- SM 4
~2 Cd/A and a peak emission at 443 nm, when doped into the
host at 10 wt %. The CIE 1931 coordinates for SM 2 of x= 0.16,
y= 0.05, were well into the deep blue region. Interestingly,
switching the conjugation axis had a detrimental impact on the
Organic light emitting diodes
The EL of the cruciforms was evaluated in host-guest OLEDs
using 4,4’-bis(9-carbazolyl)-biphenyl (CBP) as the small
molecule host. The configuration for these devices was
OLED performance as SM
3 had the lowest efficiency,
ITO/PEDOT:PSS(60
nm)/CBP:BBO/BPhen(40
nm)/LiF(1
luminescence and EQE regardless of the doping level. This may
be a result of the difference in the distribution of the electron
density within the system as seen in the frontier orbitals,
Figure 3. Another contributing factor is that the deep HOMO
level of SM 3 is close to that of the CBP guest. However, since
SM 4 has the same HOMO level as SM 3 and performed
significantly better, energy level misalignment is not likely the
cause of the poor performance. In the future we will utilize a
wider band gap host to further evaluate this phenomena. The
device made using 2% of SM 4 gave the second best
performance an efficiency of ~1.9 Cd/A, with peak emission at
443 nm, and CIE 1931 coordinates x= 0.15, y= 0.09, which are
nm)/Al(100 nm). The energy level diagrams and the EL spectra
of devices comprising the CBP host and 10% of the guest (SM
1, SM 2 or SM 3) are shown in Figures 4 and 5a, respectively.
Due to the poor performance of the device from SM 1, the EL
spectra could not be obtained. This result is not surprising as
both the HOMO and LUMO levels for SM 1 are outside those
for the CBP host. The emission maximum of SM 2 is red-shifted
11 nm from SM 3, demonstrating the influence of the direction
of the extended conjugation axis. Similarly, the emission
maximum of SM 4 was red-shifted 22 nm relative to SM 3 as a
result of extended conjugation. The EL spectrum for SM 3 is
similar in width and shape to its emission spectrum, but the
peak widths at half-maximum for the EL spectra of SM 2 and
SM 4 are considerably narrower than their emission spectra.
The normalized electroluminescent spectra of devices with
different weight % concentrations of the BBO’s in the CBP host
are shown below in Figure 5. The key performance parameters
of these OLEDs are summarized in Table 3. It can be seen from
these plots that the host is quenched upon the addition of ~5%
of the guest molecule, indicating that there is efficient energy
transfer from the host to the emitters. Interestingly, the best
devices were obtained when 10 wt. % of the guest was used.
These results are also an improvement improvement from our
previous work where BBO polymer: CBP host systems were
studied, as emission was not observed with 10% doping
levels.19 This relatively high concentration of the guest
suggests that excellent guest-host mixing is occurring in this
system, as OLEDs based on neat films of the cruciforms
exhibited strong concentration quenching. Such quenching is
often a result of strong π-stacking within conjugated systems.
Based on the optimized geometries in Figure 3, we could
surmise that SM 2, was the least planar and thus least
susceptible to intermolecular aggregation. Indeed, SM 2, gave
the best performance of all the molecules with an efficiency of
close to the National Television System Committee (NTSC) blue
color standard (0.14, 0.08).18,
20
These are very promising
materials considering that the photopic factor is only 0.023 at
~445 nm. Furthermore, considering that the PL quantum yield
is only 0.5, the maximum internal quantum efficiency is only
~0.25 x 0.5 = 0.125 (yield of singlets times PL quantum yield),
Figure 4. Energy level diagram of SM 1 – SM 4 and CBP host.
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Table 3. Device characteristics of SMOLEDs based on BBO cruciforms.
b
Dopant
Devicea
Von
Drive Voltage Current Density Brightness
Efficiency [Cd/A, lm/W
(%EQE)c]
λmaxEL[nm]
CIE 1931
[x,y, y’]
[V]
[V]
J, [mA/cm2]
[Cd/m2]
wt. %
1.0
4.2
4.0
4.4
4.0
5.0
5.2
4.8
4.2
4.0
4.0
3.9
3.6
6.2
6.0
6.6
7.0
8.0
6.4
8.4
8.2
6.3
6.4
6.4
6.9
392
439
489
336
365
74
1155
2152
1141
1051
275
0.93, 0.61, 1.82
1.30, 0.85, 2.33
0.71, 0.45, 1.08
2.05, 1.40, 2.95
0.33, 0.20, 0.60
0.25, 0.13, 0.37
0.45, 0.25, 0.58
0.50, 0.33, 0.76
1.23,0.77, 1.55
1.93, 1.24, 2.22
1.09, 0.67, 1.01
1.29,0.97, 1.20
433, 400 0.16, 0.05, 0.79
436, 400 0.16, 0.05, 0.79
439, 400 0.16, 0.06, 0.78
443, 400 0.16, 0.05, 0.79
425, 400 0.17, 0.06, 0.78
428, 400 0.16, 0.07, 0.77
430, 400 0.17, 0.08, 0.76
430, 400 0.16, 0.06, 0.78
450, 400 0.15, 0.09, 0.76
450, 400 0.15, 0.09, 0.76
450, 400 0.16, 0.11, 0.73
450, 400 0.15, 0.11, 0.74
2.0
SM 2
5.0
10.0
1.0
2.0
148
SM 3
SM 4
5.0
497
583
360
350
404
606
801
10.0
1.0
779
1649
1568
1412
1159
2.0
5.0
10.0
Figure 5.Electroluminescence spectra of SM2 – SM4 (top left), and EL emission at various concentrations as noted.
and the EQE is about ~0.2 x 0.125 = 0.025 = 2.5%, since the fluorescent OLEDs based on SM 2 and SM 4 are close to the
outcoupling factor in these conventional bottom-emitting- maximal possible values.
through-the-glass OLEDs with ITO is < 20%. Hence, based on
the ΦPL of the emitting materials, the EQE of the deep blue
6 | J. Name., 2012, 00, 1-3
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In a dried RBF, compound
1
(1.49 g, 5 mMol) and a fresh
Conclusions
prepared solution of poly-phosphosilylether (PPSE) (2g in 20
mL of DCE) were degassed for 20 minutes and then heated at
reflux for 3 days under argon. The solution was allowed to cool
to room temperature and quenched with 2M sodium
hydroxide and extracted with methylene chloride. The
combined organic layers were washed with 2M hydrochloric
acid, water, brine and dried over sodium sulphate then
concentrated. The resulting residue was filtered a small silica
gel column with CH2Cl2 and concentrated in vacuo. The residue
In summary, we have synthesized four cross-conjugated, blue
light emitting materials based on benzo[1,2-d:4,5-
d′]bisoxazole. The HOMO and LUMO levels of these materials
were readily tuned via substitution pattern to afford materials
with wide-band gaps. Structure-property studies reveal that
such modifications not only impact the energy levels and band
gaps, but they also affect the electron density and planarity
within the systems. As a result, the performance of the OLEDs
varied depending upon structure. Non-optimized OLEDs
achieved EQE approaching 3% and efficiency of ~2 Cd/A with a
relatively narrow EL band peaking at 443 nm and a CIEy ≤ 0.05.
Collectively these results indicate that the synthesis of BBO
cruciforms is a promising strategy for the design of efficient
deep-blue OLEDs. Future work is ongoing in our labs on OLED
optimization and synthesis of new materials. These will be
reported in due course.
was put through
a second column with 80/20 v/v
hexanes/CH2Cl2. The solution was concentrated and
recrystallized from ethyl acetate.
M.P. >260 °C, 1H NMR (400 MHz, CDCl3) 8.36 (2H, d, J=4), 8.30
(2H, s), 7.86 (2H, d, J=8), 7.79 (2H, dd, J=4,4), 7.39 (6H, d, J=4),
2.10 (9H, m), 1.06 (24H, m), 0.75 (12H, t), 0.60 (9H, m); HRMS
(ESI) calcd for C58H66N2O2Br2 981.3564 [M + H]+ , found
981.3559.
4,8-BIS(4-OCTYLPHENYL)-2,6-DIPHENYLBENZO[1,2-
']BIS OXAZOLE) (SM 1).
D:4,5-
Experimental
D
(
In a dried, 3-necked RBF purged with argon, fitted with an
condenser, monomers (117.5 mg, 0.25 mMol) and (174 mg,
Materials and measurements
2
3
Br-DAHQ (1) 21 polyphosphoric acid, silyl ester (PPSE),22 were
,
0.55 mMol) and were dissolved in degassed toluene. One drop
of Aliquot 336 (surfactant) was added to the reaction flask,
followed by sodium carbonate (6 mL, 10 mMol). The reaction
solution was then deoxygenated for 30 min by pumping argon
through the solution. PEPPSI-iPr catalyst (0.0125 mMol) was
added and the reaction refluxed for 3 days under argon. The
solution was then diluted with toluene and layers separated.
Aqueous layer was extracted twice with toluene (2x20 mL
portions). The combined organic layers were washed with 1M
hydrochloric acid, water, brine and dried with sodium sulfate.
After concentration, the residue was purified by column
chromatography (gradient hexanes to 50/50 v/v
hexanes/dichloromethane). The eluents were concentrated
and dissolved in 1.5 mL of chloroform and crashed out into
methanol (-78 °C). The resulting powder was taken through a
second column by dry packing to silica, following the same
gradient then precipitated into MeOH to give the product as a
white powder (87 mg, 50%).
synthesized according to literature procedures. THF and
toluene were dried using an Innovative Technologies solvent
purification system. All other chemical reagents were
purchased from commercial sources and used without further
purification unless otherwise noted. Nuclear magnetic
resonance (NMR) experiments were carried out in CDCl3 at 600
MHz (1H) AND 800 MHZ (13C). 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 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 spectroscopy were
obtained using CHCl3 solutions unless otherwise noted.
Relative solution fluorescence quantum yields were obtained
using 9,10-diphenylanthracene (ΦPL = 0.885 in ethanol)23 as a
standard with excitation at 372 nm.
M.P. 242 °C, 1H NMR (600 MHz, CDCl3) δ 8.34-8.37 (8H, m, J=6,
6 Hz), 7.54 (6H, d, J=6 Hz), 7.47 (4H, d, J=6 Hz), 2.75 (4H, m),
1.75 (4H, m) 1.55 (13H, s) 1.45 (5H, m), 1.38 (4H, m) 1.32(10H,
m) 0.90 (6H, t); 13C (400 MHz, CDCl3) δ 165.68, 152.10, 152.00,
147.37, 146.04, 140.35, 140.21, 128.91, 127.95, 127.53,
124.95, 123.50, 122.84, 121.61, 120.61, 91.86, 56.03, 40.83,
32.01, 30.12, 24.22, 23.03, 14.46; HRMS (ESI) calcd for
C48H52N2O2 689.4102 [M + H]+ , found 689.4087.
4,8-DIBROMO-2,6-DIPHENYLBENZO[1,2-
(2).
D:4,5-D']BIS(OXAZOLE)
In a dried round-bottomed flask (RBF), a fresh prepared
solution of poly-phosphosilylether (PPSE) (5g in 20 mL of o-
DCB) and
1 was degassed for 20 minutes. Benzoyl chloride was
added and the solution was refluxed for 2 days under argon.
The solution was allowed to cool to room temperature and
crashed into cold methanol(-78 °C). The precipitate was
filtered and refluxed in EtOH to remove the red color, resulting
in a tan powder. The resulting product was used without
further purification. M.P. >260 °C Due to the poor solubility of
the product, NMR spectra were not obtained.
4,8-BIS(9,9-DIOCTYL-9
H
-
FLUOREN-2-YL)-2,6-
:4,5- ']BIS OXAZOLE) (SM 2)
SM 2 was synthesized similarly to SM 1 from
DIPHENYLBENZO[1,2-
D
D
(
2
and 4. The
product was obtained as a bright yellow powder (90 mg, 76%).
M.P.149 - 152 °C, 1H NMR (800 MHz, CDCl3) δ 8.50 (2H, m,
J=8,8 Hz), 8.45 (2H, s), 8.37 (4H, dd, J=8,8 Hz), 7.99 (2H, d, J=8
Hz), 7.84 (4H, d, J=8 Hz), 7.75 (2H, d, J=8 Hz), 7.45 (2H, d, J=8
4,8-DIBROMO-2,6-BIS(9,9-DIHEXYL-9
H-FLUOREN-2-
YL BENZO[1,2- :4,5- ']BIS OXAZOLE) (6)
)
D
D
(
This journal is © The Royal Society of Chemistry 20xx
J. Name., 2013, 00, 1-3 | 7
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Journal of Materials Chemistry C
Page 8 of 10
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DOI: 10.1039/C5TC03622D
ARTICLE
Journal Name
Hz) 7.42 (2H, td, J=8 Hz); 7.38 (2H, td, J=8 Hz),2.15 (10H,m), PEDOT:PSS layer in the argon-filled glovebox. The combined
1.17(18H, m), 0.95 (4H, m), 0.82 (4H,m), 0.78 (14H, t, J=8 Hz); concentration of the CBP and guest material was kept constant
13C (800 MHz, CDCl3) δ 163.66, 151.51, 150.88, 146.62, 141.28, at 9 mg mL–1. The solution was spin-coated at 4000 rpm for 60
141.05, 138.60, 131.61, 131.33, 129.29, 129.27, 129.01, s. The fabricated structure was then annealed at 60 °C for 30
127.80, 127.50, 127.39, 127.00, 125.07, 125.05, 123.15, min. Following this annealing step, the samples were
120.16, 119.97, 114.99, 55.37, 40.82, 31.92, 31,90, 30.53, transferred to a thermal evaporator within the glovebox, and
29.54, 29.53, 29,46, 24.20, 22.73, 22.72, 14.19, 14.18 HRMS the 4,7-diphenyl-1,10-phenanthroline (BPhen), LiF, and Al
(ESI) calcd for C78H92N2O2 1089.7232 [M + H]+ , found layers were deposited sequentially by thermal evaporation at a
1089.7228.
base pressure of
∼
1 × 10–6 Torr. The OLEDs were characterized
by monitoring their electroluminescence (EL) spectra and
brightness as a function of the applied voltage, and the
luminous and power efficiencies were determined.
2,6-BIS(9,9-DIHEXYL-9
H
-
FLUOREN-2-YL)-4,8-
:4,5- ']BIS OXAZOLE) (SM 3)
SM 3 was synthesized similarly to SM 1 from
DIPHENYLBENZO[1,2-
D
D
(
6
and phenyl Computational Details
boronic ester. The resulting residue was purified by column All of the calculations on these oligomers studied in this work
chromatography (gradient hexanes to 50/50 v/v were studied using the Gaussian 03W with the GaussView 4
hexanes/dichloromethane). The product was obtained as light- GUI interface program package. All electronic ground states
yellow flakes (97 mg, 40%).
geometries were optimized using density functional theory
1
M.P. 214 °C, H NMR (600 MHz, CDCl3) δ 8.49 (2H, dd, J=6 Hz), (DFT) employing an SVP functional and a 6-31G* basis set.
8.44 (2H,s), 8.37 (4H, m), 7.98 (2H, d, J=6 Hz), 7.82 (2H, d), 7.56 Excited states were generated through time dependent
(6H, m), 6.14 (6H,m), 2.14 (9H, ddq), 1.13 (45H, dd), 0.76 (14H, density functional theory (TD-DFT) applied to the optimized
t); 13C (800 MHz, CDCl3) δ 164.81, 151.88, 151.68, 146.74, ground state for each oligomer. The HOMO, LUMO, band gap,
144.99, 140.37, 138.84, 133.03, 130.69, 130.67, 130.66, first ten excited states, and UV-Vis simulations were generated
129.01, 128.61, 128.44, 127.34, 127.29, 125.89, 123.30, from these excited computations. All computations were
122.38, 120.31, 114.45, 55.72, 40.60, 31.85, 29.95, 24.07, performed using Gaussian 09 using the Marcy cluster provided
14.31; HRMS (ESI) calcd for C70H76N2O2 977.598 [M + H]+
found 977.5982.
,
by the MERCURY consortium.
2,6-BIS(9,9-DIHEXYL-9
DIOCTYL-9 FLUOREN-2-YL
']BIS OXAZOLE) (SM 4)
H
-
FLUOREN-2-YL)-4,8-BIS(9,9-
Acknowledgements
H
-
)
BENZO[1,2- :4,5-
D
We would like to thank Dr. Sarah Cady, Dr. Kamel Harrata and
Mr. Steven Veysey of Iowa State University (ISU) Chemical
Instrumentation Facility for compound analysis. We thank
Eeshita Manna for technical assistance. We also thank the
National Science Foundation (CHE-1413173) for financial
support of this work. RK and JS were partially supported by
Basic Energy Sciences, Division of Materials Science and
Engineering, USDOE. Ames Laboratory is operated by Iowa
State University for the US Department of Energy (USDOE)
under Contract No. DE-AC 02-07CH11358. Computational
resources were provided in part by the MERCURY consortium
D
(
SM 4 was synthesized similarly to SM 1 from
4
and
6
. The
product was obtained as yellow crystals (208 mg, 52%).
M.P. 144-146 °C, 1H NMR (600 MHz, CDCl3) H NMR (600 MHz,
CDCl3) 8.62 (2H, d, J=6 Hz), 8.55 (2H, s), 8.32 (4H, m, J=12 Hz),
7.99 (2H, d, J=6 Hz), 7.81 (4H, t, J=6 Hz), 7.73 (2H, dd), 7.40
(2H, d, J=6 Hz) 7.33 (10H, m, J=6,6 Hz); 13C (800 MHz, CDCl3) δ
164.44, 151.76, 151.53, 151.11, 146.87, 144.99, 141.48,
141.35, 140.45, 138.90, 131.83, 129.53, 129.50, 128.47,
127.56, 127.32, 127.15, 126.16, 125.05, 125.02, 123.37,
123.32, 122.22, 120.69, 120.34, 120.31, 120.18, 114.70, 55.81,
55.66, 41.13, 40.83, 32.11, 31.89, 30.76, 30.04, 29.70, 29.68,
24.39, 24.10, 22.90, 14.33,14.31; HRMS (ESI) calcd for
1
(
http://mercuryconsortium.org/) under NSF grants CHE-
0116435, CHE-0521063, CHE-0849677, and CHE-1229354.
C
116H148N2O2 1602.1614 [M + H] +, found 1602.1583.
Notes and references
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Journal of Materials Chemistry C
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DOI: 10.1039/C5TC03622D
ARTICLE
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Journal of Materials Chemistry C
Page 10 of 10
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DOI: 10.1039/C5TC03622D
Benzobisoxazole cruciforms: A tunable, cross-conjugated platform for the generation of deep blue OLED
materials.
Ramiro Chavez, Min Cai, Brian Tlach, David L Wheeler, Rajiv Kaudal, Ayuna Tsyrenova, Aimée L. Tomlinson,
Ruth Shinar, Joseph Shinar, and Malika Jeffries-EL*
A structure-property investigation of a series of cross-conjugated molecules shows that their performance is
dependent on both the nature of the substituents and their conjugation axis.