P-quaterphenyls laterally substituted with a dimesitylboryl group: A promising class of solid-state blue emitters

Article abstract of DOI:10.1021/jo202574n

A new family of p-quaterphenyls 1-6 laterally substituted with a bulky electron-accepting dimesitylboryl group has been designed and synthesized. These compounds were characterized by X-ray crystallography, UV-vis and fluorescence spectroscopy, and DFT ca

Full text of DOI:10.1021/jo202574n

Article
pubs.acs.org/joc
p-Quaterphenyls Laterally Substituted with a Dimesitylboryl Group:
A Promising Class of Solid-State Blue Emitters
Guang-Liang Fu,† Hong-Yu Zhang,‡ Yi-Qiao Yan,† and Cui-Hua Zhao*,†
†School of Chemistry and Chemical Engineering, Shandong University, Shanda Nanlu 27, Jinan 250100, P. R. China
‡State Key Laboratory of Supramolecular Structure and Materials, College of Chemistry, Jilin University, Changchun 130012, P. R.
China
S
* Supporting Information
ABSTRACT: A new family of p-quaterphenyls 1−6 laterally
substituted with a bulky electron-accepting dimesitylboryl
group has been designed and synthesized. These compounds
were characterized by X-ray crystallography, UV−vis and
fluorescence spectroscopy, and DFT calculations as well as
thermogravimetric analysis (TGA), differential scanning
calorimetry (DSC), and cyclic voltammetry (CV). X-ray
single-crystal analysis revealed that the p-quaterphenyl main
chain framework exhibits a twisted structure due to the steric
effect of the lateral boryl group, and the intermolecular
interactions are effectively suppressed in the solid state.
Despite the significantly twisted main-chain structure, these molecules still display efficient intramolecular charge-transfer
emissions with large Stokes shifts. An intriguing finding is that all these molecules show bright fluorescence with good to
excellent quantum yields in the blue region in the solid state. In addition, the two representative p-quaterphenyls 3 and 4
containing both the electron-accepting boryl group and the electron-donating carbazolyl (3) or diphenylamino group (4) possess
high thermal stability and good oxidation−reduction reversibility, which together with their excellent solid-state fluorescence
efficiency make them promising bipolar transporting blue emitters.
In addition, blue-emitting materials may act as host materials
for green and red dopants. In this context, it is of particular
interest to develop blue emissive materials exhibiting high
fluorescence efficiency in the pure solid state.
INTRODUCTION
■
Since the breakthrough discovery of organic electrolumines-
cence (EL) by Tang and VanSlyke,1 the organic light emitting
diodes (OLEDs) employing organic fluorophores as emitters
have attracted considerable interest because of their potential
applications in flat panel displays2 and solid-state lighting.3 In
OLEDs, organic emitters are used as thin films and thus the
external quantum efficiency of the devices is greatly dependent
on the fluorescence quantum yield of the emissive materials in
the solid state. However, most organic fluorophores are highly
emissive only in the dilute solution and tend to show a decrease
of fluorescence efficiency in the solid state due to the severe
fluorescence quenching, which may result from the intermo-
lecular electronic interactions such as aggregate or excimer
formation and energy migration in the solid state. As a result, in
contrast to the great number of molecules that are known to be
highly emissive in solution, the examples of highly emissive
organic solids with a fluorescence quantum yield close to unity
are still quite limited,4 especially those exhibiting strong solid-
state blue emission. It is even more challenging to achieve
stable and efficient solid-state blue emitters compared with
green and red emitters because of the relatively high energy gap
required between the interactive orbitals, which may cause
either poor photostability or low quantum efficiency.5
However, blue is one of the primary colors (red, green, and
blue) that are essential for the realization of a full-color display.
To achieve an intense solid-state emission, several effective
strategies have been adopted, such as protection with bulky
substituents,6 cross dipole stacking,7 taking advantage of
aggregation-induced emission,8−10 J-aggregate formation,11
spiro concept12 and enhanced intramolecular charge transfer
(CT) transition.13−15 Among them, we and others recently
focused our attention on an interesting molecular design, in
which the bulky electron-accepting dimesitylboryl groups16
were introduced at the lateral positions of electron-donating π-
conjugated framework.13 This effective molecular design is
attributed to two factors of the lateral dimesitylboryl group.
One is the steric bulkiness that can prevent an intermolecular
interaction. The other is the electron-accepting character that
induces an intramolecular CT transition with a large Stokes
shift and thus effectively suppresses self-quenching in the
condensed state. The utility and generality of this molecular
design have been demonstrated by synthesizing two series of
compounds, 2,5-diborylphenylene-cored oligo-
(phenylenethynylene) systems and 3-boryl-2,2′-bithiophene
Received: December 20, 2011
Published: January 18, 2012
© 2012 American Chemical Society
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systems.13 In the above two series of compounds, it was hard to
attain ideal blue emissive molecules, especially those possess
bipolar transporting properties, as a result of the extended
conjugation of the parent π-framework. To continue our effort
in the development of highly solid-state emissive materials
utilizing boron element, we herein disclose another series of π-
conjugated system laterally substituted with a bulky electron-
accepting boryl group, p-quaterpheyls 1−6 containing 2-
dimesitylboryl-4,4′-biphenylene as a core unit (Figure 1). The
catalyzed Suzuki−Miyaura cross-coupling reaction with a
variety of substituted phenylboronic acid proceeded smoothly
to produce the boryl-substituted p-quaterphenyls 1−6 in good
to excellent yields, with the exception of 5. It is generally
preferable to employ alternative methods such as Stille or
Negishi coupling rather than Suzuki coupling when the
substrate contains a triarylboron group,18,13b due to the
association of hydroxide with the boron center, which makes
the triarylboron compound susceptible to react in much the
same manner as traditional aryl boronate to produce coupling
product of its pendant aryl groups.19 The success of the Suzuki
coupling reaction of 8 with a variety of substituted phenyl-
boronic acid is probably ascribed to the significant steric
congestion of the boron center in it, which can prevent the
attack of hydroxide. It is reasonable that the reaction of 8 with
o-(diphenylamino)phenylboronic acid proceeded in a much
lower yield relative to other substituted phenylboronic acids,
considering the significant steric bulk of the diphenylamino
group at the o-position. All of the obtained boryl-substituted π-
conjugated compounds are stable in air and water and can be
purified by silica gel column chromatography. They were fully
1
characterized using H and 13C NMR spectroscopy and high-
resolution mass spectrometry.
Figure 1. Structures of boryl-substituted p-quaterphenyls.
X-Crystal Structure Analysis. Among the newly prepared
molecules, the structure of 2 was determined by single-crystal
X-ray diffraction analysis (XRD). The crystals for XRD were
obtained by recrystallization from a mixed solvent of CH2Cl2
and hexane. The molecular structure and packing structure of 2
are shown in Figure 2. In this compound, the trivalent boron
p-quaterpheyl was chosen as the parent framework because of
its limited conjugation, which was expected to prevent the
emission from red-shifting. It was intriguing to find that these
boryl-substituted p-quaterphenyls exhibit intense solid-state
fluorescence with the quantum yields close to unity in the blue
region, even those containing strong electron-donating
carbazolyl or diphenylamino groups. Their single-crystal X-ray
structure, photophysical properties in solution and the solid
state, theoretical calculations, thermal stabilitie, as well as
electrochemical properties have been comprehensively charac-
terized to investigate the characteristics of the current π-
conjugated system and the impact of the structure modification.
RESULTS AND DISCUSSION
■
Synthesis. The p-quaterphenyl derivatives 1−6 were
prepared in two steps from 4,4′-dibromo-2-iodobiphenyl 7,17
as illustrated in Scheme 1. Thus, the selective monolithiation of
Figure 2. Crystal structure of 2: (a), (b) the molecular structure and
the packing structure; (c) view from a axis; (d) view from c axis.
Hydrogen atoms are omitted for clarity.
a
Scheme 1. Synthesis of Boryl-Substituted p-Quaterphenyls
centers are well protected by the methyl groups at the o-
positions of two mesityl groups, which accounts for its high
stabilities. It is noteworthy that the p-quaterphenyl main chain
framework exhibits a significantly twisted structure, similar to
other π-conjugated compounds laterally substituted with
dimesitylboryl groups.13 All of the benzene rings of the main
chain are nonplanar with each other. The dihedral angle
between benzene rings 1 and 2 is 28.7°. And the benzene ring 3
is twisted by a much larger dihedral angle (63.9°) relative to the
benzene ring 2 due to the higher steric congestion between
them. It is also interesting to note that the benzene ring 4 is
further twisted relative to the benzene 3 with a dihedral angle of
45.7°, which is even larger than that between the benzene rings
1 and 2. Apparently, the significantly twisted conformation of
the main-chain framework arises from the steric hindrance of
the lateral dimesitylboryl group. Presumably due to the
nonplanar structure of the main chain framework and the
a
Reagents and conditions: (a) (i) n-BuLi, −78 °C, 1 h; (ii) Mes2BF,
THF, −78 °C to rt; (b) ArB(OH)2, Pd(PPh3)4, Na2CO3 (2 M),
toluene, reflux.
7 with n-BuLi at −78 °C followed by treatment with
dimesitylfluoroborane afforded the borylated product 8 in
51% yield. With the precursor 8 in hand, the following Pd(0)-
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steric bulkiness of the lateral dimesitylboryl group, the
molecules are far apart from each other. The distance between
the central 2 benzene planes of the two adjacent molecules is
6.39 Å, which is supposedly large enough to prevent
intermolecular electronic interactions. Compounds 1, 3, and
4 would possibly exhibit similar main chain structure to that of
2 because of their similarities in the molecular structure and the
main chains of 5 and 6 would presumably become much more
twisted due to the increased steric hindrance induced by the
lateral diphenylamino groups.
of the intense absorption band at the shorter wavelength. It is
noteworthy the intramolecular CT transition bands were also
not observed in the absorption spectra of 5 and 6, the
regioisomers of 4 with diphenylamino groups at different
positions, indicating that the substituted sites of the electron-
donating groups significantly affect the electronic structure of
the present p-quaterphenyl systems.
In the fluorescence spectra, the boryl-substituted p-
quaterphenyls 1−4 exhibit intense blue fluorescence with
quantum yields ranging 0.51−0.64. The emission maximum
wavelength is red-shifted with the increase in the electron-
donating ability of the terminal p-substituents, from 449 nm for
1 to 469 nm for 4. The bathochromism in the absorption and
emission spectra from nonsubstituted 1 to the electron-
donating Ph2N substituted 4 is much less significant compared
with other π-conjugated systems containing lateral dimesityl-
boryl groups.13a,b This phenomenon is presumably ascribed to
the significant nonplanar main chain framework, which would
prevent the participation of the p-substituents in the π-
conjugation extension. Again the emissions of 5 and 6 are
greatly blue-shifted by ca. 20 nm relative to that 4, which are
almost same to that of 1. Moreover, 5 and 6 display much lower
fluorescence quantum efficiency compared with that of 4 (ΦF =
0.60 for 4, 0.17 for 5, 0.32 for 6), confirming the substitution
pattern has significant influence on the electronic structure,
producing obviously different optical properties. Another
noticeable feature for the present boryl-substituted p-
quaterphenyls is the large Stokes shift for this quaterphenyl
unit with such a limited conjugation length. There are almost
no overlaps between the absorption and emission for all the
derivatives, which may partially arises from significant structure
differences between the twisted structure in the ground state
and the presumably planar structure in the excited state.
Theoretical Studies. To elucidate the influence of the
electronic effect and the substitution pattern of the substituents
on the terminal benzene rings on the electronic structures and
thus the photophysical properties, we conducted theoretical
calculations of the representative derivatives, nonsubstituted 1,
electron-donating Ph2N geometrically substituted 4−6. The
optimizations of the molecular geometry and total energy
calculations were carried out using density functional theory
(DFT) calculations at B3LYP/6-31G(d) level of theory. We
also performed time-dependent density-functional theory (TD-
DFT) calculations of these compounds at the B3LYP/6-
31G(d) theory. The pictorial drawing of their molecular
orbitals and the Kohn−Sham HOMO and LUMO energy levels
are shown in Figure 4.
The nonsubstituted compound 1 has a HOMO delocalized
over the entire p-quaterphenyl framework despite its twisted
main chain structure, and its LUMO is localized on the central
boryl-biphenylene moiety. The HOMO and LUMO energy
levels are −5.60 and −1.64 eV, respectively. The calculated first
excited state, mainly consisting of a HOMO→LUMO intra-
molecular CT transition, has an excitation energy of 3.33 eV
(372 nm) with a small oscillator strength of 0.0801. The
incorporation of the electron-donating Ph2N groups at 4,4′′′-
positions of this skeleton (4) leads to a remarkable increase in
the HOMO energy level by 0.76 eV, whereas the LUMO level
remains almost unchanged. Consequently, the calculated first
excited energy of 4 is reduced to 2.86 eV (434 nm). It is
notable that the HOMO of 4 spreads over the entire
quaterphenyl framework, including the Ph2N unit. This change
leads to a significant increase in the oscillator strength of the
Photophysical Properties in Benzene Solutions. The
UV/vis absorption and emission spectra of 1−6 in solutions
were measured in benzene, which are shown in Figure 3. The
corresponding data are summarized in Table 1.
Figure 3. Absorption and fluorescence spectra of boryl-substituted p-
quaterphenyls 1−6 in benzene.
Table 1. UV/Vis Absorption and Fluorescence Data for
Boryl-Substituted p-Quaterphenyls 1−6 in Benzene
absorption
fluorescence
a
b
c
λabs (nm)
ε (M−1 cm−1
)
λem (nm)
ΦF
1
2
3
4
5
6
290
43000
7280
449
456
451
469
450
449
0.51
0.64
0.62
0.60
0.17
0.32
373 (sh)
370 (sh)
384 (sh)
301
9910
14550
49490
96650
301
a
b
Only the longest maxima are shown. Excited at the longest
c
absorption maxima. Calculated using quinine sulfate as standard.
In the absorption spectra, compounds 2−4 feature a weak
shoulder band at the wavelength longer than that of the intense
band, while this shoulder band is hard to distinguish for 1. In
addition, the shoulder band of 4 is much red-shifted relative to
those of 2 and 3 (λabs = 373 nm for 2, 370 nm for 3, and 384
nm for 4). The shoulder band of 2−4 is presumably assigned to
the intramolecular charge-transfer (CT) transition from the
HOMO delocalized over the whole π-conjugated framework to
the LUMO mainly localized on the borylphenylene moiety.
The trend observed in the absorption may be rationalized by
considering that the HOMO energy levels of the present
systems are highly dependent on electron-donating ability of
the substituents at terminal positions. As the electron-donating
ability of the terminal groups increases, the HOMO energy
levels increase, leading to the intramolecular CT transition at
the longer wavelength. As a result, the intramolecular CT
absorption of the nonsubstituted p-quaterphenyl 1 is presumed
to be blue-shifted relative to those of the electron-donating
groups substituted 2−4 and thus possibly overlaps with the tail
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Figure 4. Plot of the Kohn−Sham HOMO and LUMO energy levels and pictorial drawings of the HOMOs and LUMOs for (a) 1, (b) 4, (c) 5, and
(d) 6. The transition energies and oscillator strengths were calculated at the B3LYP/6-31G(d) level of theory.
first excited state (f = 0.2738). In contrast to the extended
conjugation of the HOMO in 4, the HOMO is mainly localized
on the (diphenylamino)phenyl moiety which is located at the
meta-position of the boryl group when the electron-donating
Ph2N groups are introduced at 2,2′′′-positions (5) or 3,3′′′-
positions (6) of the quaterphenyl framework. As a
consequence, both their HOMO energy levels are lowered by
ca. 0.11 eV and the excitation energy of the first excited state is
increased by ca. 0.11 eV compared to those of 4. In conjunction
with these changes, the oscillator strength of the first excited
state decreases to 0.0156 and 0.0013 for 5 and 6, respectively.
Probably due to the quite low oscillator strength of 1, 5, and 6,
the lowest intramolecular charge-transfer transition bands are
too weak to be distinguished in their absorption spectra.
Whereas the accuracy of the calculated excitation energy is not
sufficiently high by this level of calculations, these calculated
results clearly demonstrate the nature of the transition
significantly depends on electronic effect of the substituents
and the substitution pattern of the molecular structure. The
introduction of the electron-donating groups at 4,4′′′-positions
leads to the most effective conjugation and thus the most
efficient intramolecular CT transition for this boryl-substituted
p-quaterphenyl π-conjugated system, which is consistent with
our experimental results.
Table 2. UV/Vis Absorption and Fluorescence Data of
Boryl-Substituted p-Quaterphenyls 1 and 4−6 in Various
Solvents
a
b
c
Λabs (nm)
λem (nm)
Δν (cm−1
)
1
4
5
6
cyclohexane
benzene
CHCl3
287
290
289
290
289
449
441
451
454
459
457
469
480
493
529
433
449
458
470
506
445
448
459
478
538
12571
11807
12429
12456
12815
4503
THF
MeCN
d
cyclohexane
benzene
CHCl3
379
d
384
4719
d
374
5904
d
THF
381
5962
d
MeCN
380
7412
cyclohexane
benzene
CHCl3
300
301
303
299
295
300
301
303
300
298
10238
10950
11169
12168
14135
10861
10901
11216
12412
14969
THF
MeCN
cyclohexane
benzene
CHCl3
THF
MeCN
Fluorescence Solvatochromism. To gain a deeper insight
into the effect of the substituents on the intramolecular CT
transitions and the excited states, we investigated the solvent
effects in the absorption and emission spectra for 1, 4−6, the
data for which are summarized in Table 2.
a
Only the longest absorption maximum wavelengths are given.
Excited at the longest absorption maxima. Stokes shift. Observed
b
c
d
as shoulder.
be applicable to several quadrupole extended π-conjugated
molecules.20,13d For the present p-quaterphenyl derivatives and
4−6, we indeed obtained linear relationships for the plots Δν as
a function of the Δf, as shown in Figure 5. The slope obtained
for 4 is 8856 cm−1, which is much steeper than that of 1 (2447
cm−1). These results suggest a larger quadrupole moment of 4
in the excited state than that of 1, which is reasonable
considering the significant electron-donating property of Ph2N
group. It is beyond our expectation that the slopes of 5 and 6
are even a little steeper than that of 4 (slope = 10453 cm−1 for
5 and 11671 cm−1 for 6). This may be ascribed to their
localized HOMO contributed from only one (diphenylamino)-
phenyl moiety, which results in a dipole moment rather than a
quadrupole moment. These results suggest that the emissions
The fluorescence spectra of all these molecules show obvious
solvatochromism while the absorption spectra display trivial
solvent dependence (see the Supporting Information),
indicating that they have more polar structures in the excited
state relative to those in the ground state. To compare the
degree of the polarization in the excited state among these
molecules, we employed the Lippert−Mataga equation. (eq 1),
in which C is a constant, μe and μg are the dipole moments in
the excited state and ground state, respectively, and Δv is the
Stokes shift. Δf is the solvent polarity and is given by eq 2, in
which ε is the dielectric constant and n is the optical refractive
constant. Although this equation originally assumes the
presence of a molecular dipole, it has been demonstrated to
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Table 3. UV/Vis Absorption and Fluorescence Data for
Boryl-Substituted p-Quaterphenyls 1−6 in the Spin-Coated
Films
absorption
fluorescence
a
b
b
λabs (nm)
λem (nm)
ΦF
1
2
3
4
5
6
287
446
454
447
473
444
0.94
0.99
0.83
0.99
0.72
0.60
373 (sh)
370 (sh)
391 (sh)
304
301
446
b
Figure 5. Lippert−Mataga plots for boryl-substituted p-quaterphenyls
1 (blue), 4 (red), 5 (green), 6 (magenta).
a
Only the longest maxima are shown. Excited at the longest
absorption maxima. Absolute quantum yields determined by a
c
calibrated integrating sphere system.
of all these molecules come from the highly polarized excited
states induced by the intramolecular CT transition, irrespective
of their different substitution patterns. The intramolecular CT
transition generally leads to the large Stokes shift, which is
supposed to facilitate suppressing the fluorescence quenching
in the solid state.
increased fluorescence intensities in the solid-state compared
with those in benzene solutions. Such fluorescence behavior
may be ascribed to the easily exchangeable multiple
conformations of the nonplanar structure of the main chain,
which facilitate the nonradiative decay of the excited state. In
the solid-state, the exchanges between multiple conformations
are greatly suppressed due to the spatial congestion of the
molecular stacking in the solid-state.8 It is most noteworthy
that all the p-quaterphenyl derivatives display an intense
fluorescence with good to excellent quantum yields (ΦF =
0.60−0.99) for their spin-coated films, even for 5 and 6, the
quantum yields of which in benzene are very low. The high
fluorescence efficiency in the solid state is presumably
attributed to the absence of the intermolecular interaction in
the solid state, as well as the large Stokes shift induced by the
intramolecular CT transition.13−15 In addition, the emission
bands of 1−6 are in the blue light region with the maxima
ranging from 446 to 473 nm. Moreover, compounds 3−6 not
only display intense blue fluorescence in the solid state, but also
contain both electron-accepting boryl group and electron-
donating carbazolyl or diphenylamino group, which may make
them potentially applied as bipolar blue emitters in OLEDs.
Bipolar light-emitting materials would facilitate exciton
formation (via stable cation and anion radicals) and may
improve charge balancing in OLEDs.21
2Δf
hca
2
Δν = ν − ν =
(μ − μ ) + C
A
F
e
g
3
(1)
2
ε − 1
2ε + 1
n − 1
Δf =
−
2
(2)
2n + 1
Photophysical Properties in the Solid State. It is
intriguing to note that all these boryl-substituted p-
quaterphenyls 1−6 exhibit bright blue fluorescence even in
the powder form. This observation prompted us to investigate
their photophysical properties in the solid state in details. Thin
films of compounds 1−6 were prepared from their dichloro-
methane solutions with ca. 3 mg mL−1 concentration on the
quartz plates and their absorption and fluorescence spectra
were directly measured. The absolute fluorescence quantum
yields were determined by a calibrated integrating sphere
system. The corresponding spectra are shown in Figure 6, and
the data are summarized in Table 3.
Thermal Properties. Considering the excellent quantum
yields of the current p-quaterphenyls in the spin-coated films,
we evaluated their thermal stabilities by thermogravimetric
analysis (TGA) and differential scanning calorimetry (DSC) to
explore their potential utility as the blue emitters in OLEDs
using 3 and 4 as the representative molecules. The melting
points of 3 and 4 measured by DSC are 323 and 267 °C,
respectively. Although both compounds showed no mesophase
or glass transition, they displayed very high decomposition
temperatures. The decomposition temperatures for a 5% weight
loss (Td5) of 3 and 4 are 437 and 429 °C, respectively. The
thermal analyses indicate that these two compounds exhibit
excellent thermal stability, which ensures that they are stable
and suitable for the vacuum deposition in the fabrication of
OLEDs.
Electrochemical Properties. To further evaluate the
potential applicability of the present boryl-substituted p-
quaterphenyl system in electronic devices, we also investigated
the electrochemical properties of 3 and 4 by cyclic voltammetry
(CV), which are shown in Figure 7. Both compounds show not
only reversible oxidation waves (3: +0.57, +0.88 V; 4: +0.48 V,
versus ferrocene/ferrocenium (Fc/Fc+)), but also reversible
Figure 6. Absorption and fluorescence spectra of boryl-substituted p-
quaterphenyls 1−6 in the spin-coated films.
In the absorption and fluorescence spectra, compounds 1−6
maintain almost the same spectra as those in benzene solutions,
in terms of the maximum wavelengths as well as the full width
at the half-maximum (fwhm) (Supporting Information). These
results suggest the formation of neither aggregates in the
ground state nor an excimer in the excited state. The lateral
boryl group is bulky enough to prevent the intermolecular
interactions in the solid state. Another noticeable characteristic
for this series of π-conjugated molecules is that they show the
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The Journal of Organic Chemistry
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same temperature for 1 h. A solution of dimesitylboron fluoride (6.43
g, 24 mmol) in anhydrous THF (10 mL) was added to the reaction
mixture via syringe. The reaction mixture was warmed to room
temperature and stirred overnight. The reaction was quenched with
saturated solution of NaCl, and the aqueous layer was extracted with
CH2Cl2. The combined organic layer was dried over anhydrous
Na2SO4, filtered, and concentrated under reduced pressure. The
resulting mixture was subjected to a silica gel column chromatography
(petroleum ether, Rf = 0.20) to afford 2.30 g (4.1 mmol) of 8 in 51%
1
yield as a pale yellow solid: mp 139−141 °C; H NMR (CDCl3, 300
MHz) 7.58 (dd, J = 8.1, 1.8 Hz, 1H), 7.51 (d, J = 1.8 Hz, 1H), 7.15 (d,
J = 8.1 Hz, 1H), 7.08 (d, J = 8.1 Hz, 2H), 6.88 (d, J = 8.1 Hz, 2H),
6.61 (s, 4H), 2.21 (s, 6H), 1.91 (s, 12H); 13C NMR(CDCl3, 75 MHz)
δ 145.3, 140.9, 140.8, 139.7, 138.7, 136.9, 133.1, 130.6, 129.4, 128.8,
129.3, 127.8, 121.6, 120.5, 22.8, 20.6. Anal. Calcd for C30H29BBr2: C,
64.32; H, 5.21. Found: C, 64.41; H, 5.29.
Figure 7. Cyclic voltammograms of 3 and 4 in THF (1 mM),
measured with TBAP (0.1 M) as the supporting electrolyte at a scan
rate of 100 mV/s.
3′′-Dimesityllboryl-[1,1′:4′,1′′:4′′,1′′′]quaterphenyl (1). To a
mixture of 8 (112 mg, 0.2 mmol), phenylboronic acid (73 mg, 0.6
mmol), and Pd(PPh3)4 (12 mg, 0.01 mmol) were added degassed
toluene (20 mL) and aqueous sodium carbonate solution (2 M, 2 mL)
under a stream of nitrogen. The reaction mixture was stirred and
refluxed at 110 °C overnight. The mixture was cooled to room
temperature and then extracted with CH2Cl2. The combined organic
layer was dried over anhydrous Na2SO4, filtered, and concentrated
under reduced pressure. The resulting mixture was subjected to a silica
gel column chromatography (5/1 petroleum ether/CH2Cl2, Rf = 0.32)
to give 107 mg (0.19 mmol) of 1 in 96% yield as a colorless solid: mp
183−185 °C; 1H NMR (CDCl3, 300 MHz) δ 7.72 (dd, J = 8.1, 1.8 Hz,
1H), 7.69 (d, J = 1.8 Hz, 1H), 7.52 (d, J = 7.2 Hz, 2H), 7.47 (d, J = 8.1
Hz, 1H), 7.38−7.43 (m, 6H), 7.29−7.34 (m, 2H), 7.18−7.24(m, 4H),
6.58 (s, 4H), 2.14 (s, 6H), 1.97 (s, 12H); 13C NMR (CDCl3, 75 MHz)
δ 147.3, 146.8, 142.2, 142.0, 140.9, 140.4, 139.8, 139.1, 138.9, 138.0,
133.5, 129.4, 129.0, 128.5, 128.2, 128.0, 127.6, 126.6, 126.6, 126.4,
125.3, 22.8, 20.6; HRMS (ESI) 593.2779 ([M + K]+), calcd for
C42H39BK 593.2782.
4,4′′′-Dimethoxy-2′′-dimesitylboryl-[1,1′:4′,1′′:4′′,1′′′]-
quaterphenyl (2). This compound was prepared essentially in the
same manner as described for 1 using 8 (224 mg, 0.4 mmol) and 4-
methoxylphenylboronic acid (182 mg, 1.2 mmol), Pd(PPh3)4 (23 mg,
0.02 mmol), degassed toluene (20 mL), and aqueous sodium
carbonate solution (2 mL, 2 M). The purification by a silica gel
column chromatography (4/1 petroleum ether/CH2Cl2, Rf = 0.15)
afforded 196 mg (0.32 mmol) of 2 in 80% yield as a colorless solid:
mp 225−227 °C; 1H NMR (CDCl3, 300 MHz) δ 7.68 (dd, J = 8.1, 1.8
Hz, 1H), 7.63 (d, J = 1.8 Hz, 1H), 7.47−7.43 (m, 3H), 7.38 (d, J = 8.7
Hz, 2H), 7.20−7.13 (m, 4H), 6.97−6.93 (m, 4H), 6.57 (s, 4H), 3.85
(s, 3H), 3.83(s, 3H), 2.14(s, 6H), 1.97(s, 12H); 13C NMR(CDCl3, 75
MHz) δ 158.6, 158.4, 147.2, 146.3,142.2, 141.4, 139.7, 138.6, 138.4,
137.9, 133.5, 133.0, 132.9, 129.3, 128.5, 128.4, 127.6, 127.6, 127.5,
124.9, 113.7, 113.5, 54.87, 54.84, 22.8, 20.6; HRMS (ESI) 653.2954
([M + K]+), calcd for C44H43BO2K 653.2993.
reduction waves (3: −2.46 V; 4: −2.54 V). The high
reversibility in their redox process demonstrates the substantial
stability of the produced species, and is indicative of their
potential use as bipolar transporting blue emitters. Our current
molecular design may provide an efficient strategy to achieve
solid-state blue emissive materials with bipolar transporting
properties, which is of great interest for realizing full-color
display of OLEDs.
CONCLUSIONS
■
We have designed and synthesized a series of p-quaterphenyls
1−6 laterally substituted with a boryl group. This series of
boryl-substituted p-quaterphenyls were facilely prepared by
Pd(0)-catalyzed Suzuki−Miyaura cross-coupling reaction from
a common precursor. And thus further functionalization was
easily realized by introducing various electron-donating groups
at different substitution sites. Owing to the influence of steric
bulkiness and the electron-accepting property of the lateral
dimesitylboryl group, these quaterphenyls exhibit a twisted
main-chain structure and efficient intramolecular charge-
transfer transition with a large Stokes shift, facilitating the
suppression of fluorescence quenching in the solid-state. All of
these compounds display intense fluorescence in the solid state
with good to excellent quantum yields in the blue region, even
those derivatives 3−6 containing both the electron-accepting
boryl group and the electron-donating carbazolyl or dipheny-
lamino group. In addition, the two representative compounds 3
and 4 possess high thermal stability and good oxidation−
reduction reversibility, which together with their excellent solid
state fluorescence efficiency make them promising bipolar
transporting blue emitters. We here not only disclosed a new
kind of emissive materials, but also demonstrated the general
utility of the molecular design to obtain organic emissive solids
by introduction of bulky electron-accepting boryl group at the
side position of electron-donating framework.13 The prelimi-
nary results will provide a firm direction for rational design of
such kind of materials. Further application of these obtained
blue emissive materials to OLEDs is in progress.
4 , 4 ′ ′ ′ - B i s ( N , N - c a r b a z o l y l ) -2 ′ ′ - d i m e s i t y l b o r y l -
[1,1′:4′,1′′:4′′,1′′′]quaterphenyl (3). This compound was prepared
essentially in the same manner as described for 1 using 8 (107 mg,
0.19 mmol) and 4-carbazolylphenylboronic acid (163 mg, 0.57 mmol),
Pd(PPh3)4 (12 mg, 0.01 mmol), degassed toluene (20 mL), and
aqueous sodium carbonate solution (2 mL, 2 M). The purification by a
silica gel column chromatography (4/1 petroleum ether/CH2Cl2, Rf =
0.30) afforded 123 mg (0.14 mmol) of 3 in 73% yield as a colorless
1
solid: mp >300 °C; H NMR (CDCl3, 300 MHz) δ 8.18−8.14 (m,
4H), 7.86 (dd, J = 7.8, 2.1 Hz, 1H), 7.82 (d, J = 2.1 Hz, 1H), 7.76 (d, J
= 8.4 Hz, 2H), 7.69−7.56 (m, 7H), 7.52−7.39 (m, 8H), 7.32−7.27 (m,
8H), 6.65 (s, 4H), 2.20 (s, 6H), 2.04 (s, 12H). 13C NMR (CDCl3, 75
MHz) δ 147.5, 147.0, 142.2, 140.4, 140.3, 139.9, 139.3, 138.3, 138.1,
138.0, 136.4, 136.1, 133.4, 129.6, 129.0, 128.6, 128.0, 127.8, 127.7,
126.8, 126.7, 125.4, 125.3, 122.9, 119.8, 119.5, 109.3, 22.9, 20.6;
HRMS (ESI) 883.4353 ([M]+), calcd for C66H53BN2 883.4302.
3′′-Dimesitylboryl-4,4′′′-bis(N,N-diphenylamino)-
[1,1′:4′,1′′:4′′,1′′′]quaterphenyl (4). This compound was prepared
essentially in the same manner as described for 1 using 8 (112 mg, 0.2
EXPERIMENTAL SECTION
■
General Methods. All reactions were carried out under nitrogen
atmosphere. Compound 7 was prepared according to the literature.17
Computational Methods. All calculations were conducted by
using the Gaussian 09 program.22
4,4′-Dibromo-2-(dimesitylboryl)biphenyl (8). To a solution of
4,4′-dibromo-2-iodobiphenyl 7 (3.54 g, 8.0 mmol) in anhydrous THF
(20 mL) was added a hexane solution of n-BuLi (5.0 mL, 1.6 M, 8.0
mmol) dropwise by syringe at −78 °C. The mixture was stirred at the
1988
dx.doi.org/10.1021/jo202574n | J. Org. Chem. 2012, 77, 1983−1990

The Journal of Organic Chemistry
Article
mmol) and 4-diphenylaminophenylboronic acid (173 mg, 0.6 mmol),
Pd(PPh3)4 (11 mg, 0.01 mmol), degassed toluene (20 mL), and
aqueous sodium carbonate solution (2 mL, 2 M). The purification by a
silica gel column chromatography (4/1 petroleum ether/CH2Cl2, Rf =
0.37) afforded 130 mg (0.14 mmol) of 4 in 70% yield as bluish green
ASSOCIATED CONTENT
■
S
* Supporting Information
1H NMR and 13C NMR spectra of all new compounds,
absorption and fluorescence spectra of 1−6 in benzene and the
solid state, ORTEP drawing of 2, Cartesian coordinates and
total energies for the optimized structures, and crystallographic
data of 2 (CIF). These materials are available free of charge via
the Internet at http://pubs.acs.org.
1
solids: mp 273−275 °C; H NMR (CDCl3, 300 MHz) δ 7.69 (d, J =
7.5 Hz, 1H), 7.66 (s, 1H), 7.44 (d, J = 7.8 Hz, 1H), 7.39 (d, J = 8.1 Hz,
2H), 7.31−6.92 (m, 30H), 6.57 (s, 4H), 2.14 (s, 6H), 1.97 (s, 12H).
13C NMR (CDCl3, 75 MHz) δ 147.2, 147.1, 146.6, 146.4, 146.3, 142.2,
141.5, 139.8, 138.4, 138.3, 137.9, 134.9, 134.1, 132.9, 129.3, 128.7,
128.4, 127.6, 127.2, 124.8, 123.9, 123.3, 123.2, 122.4, 122.3, 22.8, 20.6;
HRMS (ESI) 927.4244 ([M + K]+), calcd for C66H57BN2K 927.4252.
3′′-Dimesitylboryl-2,2′′′-(N,N-diphenylamino)-
[1,1′:4′,1′′:4′′,1′′′]quaterphenyl (5). This compound was prepared
essentially in the same manner as described for 1 using 8 (168 mg, 0.3
mmol) and 2-diphenylaminophenylboronic acid (260 mg, 0.9 mmol),
Pd(PPh3)4 (17 mg, 0.015 mmol), degassed toluene (20 mL), and
aqueous sodium carbonate solution (2 mL, 2 M). The purification by a
silica gel column chromatography (5/1 petroleum ether/CH2Cl2, Rf =
0.20) afforded 52 mg (0.058 mmol) of 5 in 19% yield as white solids:
mp 210−212 °C; 1H NMR (CDCl3, 300 MHz) δ 7.41(dd, J = 7.8, 1.8
Hz, 1H), 7.33−7.16 (m, 9H), 7.07−6.96 (m, 10H), 6.86−6.72 (m,
13H), 6.63 (d, J = 8.4 Hz, 2H), 6.49 (s, 4H), 2.15 (s, 6H), 1.76 (s,
12H); 13C NMR (CDCl3, 75 MHz) δ 147.0, 146.9, 145.6, 145.4,
144.3, 144.1, 142.2, 141.3, 140.4, 139.7, 139.6, 137.7, 137.4, 137.1,
135.0, 131.6, 131.1, 130.3, 129.3, 128.7, 128.4, 128.2, 128.1, 128.0,
127.8, 127.4, 127.4, 126.3, 125.3, 124.9, 121.5, 121.4, 120.8, 120.6,
22.7, 20.6; HR-MS (ESI) 927.4272 ([M + K]+), calcd for C66H57BN2K
927.4252.
3′′-Dimesitylboryl-3,3′′′-(N,N-diphenylamino)-
[1,1′:4′,1′′:4′′,1′′′]quaterphenyl (6). This compound was prepared
essentially in the same manner as described for 1 using 8 (168 mg, 0.3
mmol) and 3-diphenylaminophenylboronic acid (260 mg, 0.9 mmol),
Pd(PPh3)4 (34 mg, 0.03 mmol), degassed toluene (20 mL), and
aqueous sodium carbonate solution (2 mL, 2 M). The purification by a
silica gel column chromatography (4/1 petroleum ether/CH2Cl2, Rf =
0.40) afforded 200 mg (0.22 mmol) of 6 in 75% yield as a white solid:
mp 113−115 °C; 1H NMR (CDCl3, 300 MHz) δ 7.57 (dd, J = 8.1, 1.8
Hz, 1H), 7.55 (d, J = 1.8 H, 1H), 7.36 (d, J = 7.8 Hz, 1H), 7.30−7.21
(m, 10H), 7.16−6.98 (m, 22H), 6.50 (s, 4H), 2.10 (s, 6H), 1.89 (s,
12H). 13C NMR (CDCl3, 75 MHz) δ 147.8, 147.6, 147.3, 147.1, 146.7,
142.0, 141.9, 141.4, 139.6, 138.8, 138.6, 137.8, 133.3, 129.1, 128.95,
128.91, 128.77, 128.72, 128.3, 127.5, 125.1, 123.9, 123.8, 122.4, 122.3,
122.2, 121.9, 121.8, 121.7, 120.7, 120.6, 22.7, 20.5; HRMS (ESI)
927.4241 ([M + K]+), calcd for C66H57BN2K 927.4251.
AUTHOR INFORMATION
■
Corresponding Author
*E-mail: chzhao@sdu.edu.cn.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We are grateful for financial support from the National Nature
Science Foundation of China (Grant Nos. 21072117 and
20802041), Nature Science Foundation of Shandong Province
(No. Q2008B02), and Science Foundation of the Ministry of
Education of China (No. 200804221009).
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1990
dx.doi.org/10.1021/jo202574n | J. Org. Chem. 2012, 77, 1983−1990