Luminescent boron-contained ladder-type π-conjugated compounds

Article abstract of DOI:10.1021/ic900673s

Four diboron-contained ladder-type π-conjugated compounds 1-4 were designed and synthesized. Their thermal, photophysical, electrochemical properties, as well as density functional theory calculations, were fully investigated. The single crystals of compo

Full text of DOI:10.1021/ic900673s

7230 Inorg. Chem. 2009, 48, 7230–7236
DOI: 10.1021/ic900673s
Luminescent Boron-Contained Ladder-Type π-Conjugated Compounds
Zuolun Zhang,† Hai Bi,† Yu Zhang,† Dandan Yao,† Hongze Gao,† Yan Fan,† Hongyu Zhang,*,† Yue Wang,*,†
Yanping Wang,‡ Zhenyu Chen,‡ and Dongge Ma‡
†State Key Laboratory of Supramolecular Structure and Materials, College of Chemistry, Jilin University,
Changchun 130012, P.R. China, and ‡State Key Laboratory of Polymer Physics and Chemistry, Changchun
Institute of Applied Chemistry, Graduate School of the Chinese Academy of Sciences, Chinese Academy of
Sciences, Changchun 130022, P.R. China
Received April 6, 2009
Four diboron-contained ladder-type π-conjugated compounds 1-4 were designed and synthesized. Their thermal,
photophysical, electrochemical properties, as well as density functional theory calculations, were fully investigated.
The single crystals of compounds 1 and 3 were grown, and their crystal structures were determined by X-ray diffraction
analysis. Both compounds have a ladder-type π-conjugated framework. Compounds 1 and 2 possess high thermal
stabilities, moderate solid-state fluorescence quantum yields, as well as stable redox properties, indicating that they
are possible candidates for emitters and charge-transporting materials in electroluminescent (EL) devices. The double-
layer device with the configuration of [ITO/NPB (40 nm)/1 or 2 (70 nm)/LiF (0.5 nm)/Al (200 nm)] exhibited good EL
performance with the maximum brightness exceeding 8000 cd/m2.
Introduction
proved to be an efficient way to modulate their electronic
and optical properties.2-6
Four-coordinate boron compounds reported by other
groups and us have proved to be promising emitters in
electroluminescent (EL) devices.7-9 In addition, some of
these compounds exhibit certain electron-transporting
Ladder-type π-conjugated compounds are a class of ma-
terials with flat and rigid skeletons that lead to a set of
desirable properties such as a strong luminescence and a high
carrier mobility, which are important for high-performance
organic electronic and optoelectronic devices including or-
ganic light-emitting diodes (OLEDs), transistors, and lasers.1
Recently, the ladder π-conjugated systems bearing main
group elements such as silicon, sulfur, selenium, phosphorus,
boron, and nitrogen have attracted much attention and
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*To whom correspondence should be addressed. E-mail: hongyuzhang@
jlu.edu.cn (H.Z.), yuewang@jlu.edu.cn (Y.W.).
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r
pubs.acs.org/IC
Published on Web 06/25/2009
2009 American Chemical Society

Article
Inorganic Chemistry, Vol. 48, No. 15, 2009 7231
properties, so that they were employed to fabricate simple
double-layer EL devices; however, the performance of these
devices was usually not satisfactory. To achieve high EL
performance, the devices based on this type of materials
usually need additional electron-transporting materials,
which usually make the fabrication processes tedious and
thus increase the cost of the production of OLEDs.8a-8c,9d
Therefore, it is a very important issue to develop boron
materials appropriate for fabricating structurally simple and
high-performance EL devices. Recently, Yamaguchi and co-
workers reported thatthe diboron-containedladderπ-system
can increase not only the electron affinity but also the
fluorescence efficiency compared with the monoboron spe-
cies.10 This result indicates that incorporating multiboron
centers in a rigid conjugated π-system is an ideal synthetic
strategy to endow this system with strong electron-accepting
character and high fluorescence quantum yield. However,
only few rigid mutiboron-contained π-systems, such as ladder-
type boron-contained systems, have been achieved to date
probably because of the lack of efficient synthetic routes.
In this study, we design and synthesize four ladder-type
compounds containing two boron atoms in the π-conjugated
skeleton. We suspect that compounds with such a molecular
structure would be the light-emitting materials with trans-
porting properties. The X-ray single crystal structures, ther-
mal stabilities, photophysical and electrochemical properties
of these compounds have been investigated. To demonstrate
their application potentials in OLEDs, the EL properties of
some of these compounds are also presented.
vacuum, and 50 mL of petroleum ether was added. The resulting
suspension was filtered, and the solid was purified by vacuum
sublimation to give compound 1 as orange solid (215 mg, 64%
yield). 1H NMR (DMSO, ppm): δ 8.51 (d, J=8.0 Hz, 2 H), 8.34
(t, J=7.5 Hz, 2 H), 8.14 (d, J=5.5 Hz, 2 H), 7.73-7.71 (m, 4 H),
7.16-7.13 (m, 20 H). Ms m/z: 593.0 [M]þ (calcd: 592.2). Anal.
Calcd (%) for C40H30B2N2O2: C, 81.11; H, 5.11; N, 4.73. Found:
C, 81.13; H, 4.81; N, 4.69.
Ph2B(μ-L2)BPh2 (2). In the same manner as described for 1,
the reaction of H2L2 (80 mg, 0.28 mmol) and BPh3 (144 mg, 0.60
mmol) provided 2 as an orange solid (92 mg, 54% yield). H
1
NMR (DMSO, ppm): δ 8.41 (d, J=8.5 Hz, 2 H), 8.18 (d, J=
8.0 Hz, 2 H), 7.91 (s, 2 H), 7.67 (s, 2 H), 7.16-7.10 (m, 20 H), 2.35
(s, 6 H). Ms m/z: 620.3 [M]þ (calcd: 620.3). Anal. Calcd (%) for
C42H34B2N2O2: C, 81.32; H, 5.52; N, 4.52. Found: C, 81.14; H,
5.77; N, 4.33.
(C2H5)2B(μ-L1)B(C2H5)2 (3). In the same manner as de-
scribed for 1, the reaction of H2L1 (200 mg, 0.76 mmol) and B
(C2H5)3 (1.67 mL, 1 M in THF, 1.67 mmol) provided 3 as an
1
orange solid (173 mg, 57% yield). H NMR (CDCl3, ppm): δ
8.31 (d, J=6.0 Hz, 2 H), 8.00-7.95 (m, 4 H), 7.44 (t, J=6.5 Hz, 2
H), 7.35 (s, 2 H), 0.75 (t, J=7.5 Hz, 12 H), 0.64-0.53 (m, 8 H).
Ms m/z: 400.1 [M]þ (calcd: 400.3). Anal. Calcd (%) for
C24H30B2N2O2: C, 72.04; H, 7.56; N, 7.00. Found: C, 71.93;
H, 7.76; N, 6.86.
(C2H5)2B(μ-L2)B(C2H5)2 (4). In the same manner as de-
scribed for 1, the reaction of H2L2 (100 mg, 0.34 mmol) and B
(C2H5)3 (0.75 mL, 1 M in THF, 0.75 mmol) provided 4 as red-
orange solid (77 mg, 53% yield). 1H NMR (CDCl3, ppm): δ 8.09
(s, 2 H), 7.85 (d, J=8.5 Hz, 2 H), 7.78 (d, J=8.0 Hz, 2 H), 7.30 (s,
2 H), 2.45 (s, 6 H), 0.75 (t, J=7.5 Hz, 12 H), 0.61-0.53 (m, 8 H).
Ms m/z: 428.3 [M]þ (calcd: 428.3). Anal. Calcd (%) for
C26H34B2N2O2: C, 72.93; H, 8.00; N, 6.54. Found: C, 72.72;
H, 7.73; N, 6.56.
Experimental Section
Ph2BL3 (5). A solution of BPh3 (139 mg, 0.57 mmol) in THF
(10 mL) was added to a solution of HL3 (82 mg, 0.48 mmol) in
THF (20 mL) at room temperature, and then the reaction
mixture was stirred overnight at the same temperature. The
solvent was removed by vacuum, and the residue was purified by
column chromatography (silica gel, CH2Cl2/cyclohexane 3:1) to
General Information. All starting materials were purchased
from Aldrich Chemical Co. and used without further purification.
The solvents for syntheses were freshly distilled over appropriate
drying reagents. All experiments were performed under a nitrogen
atmosphere by using standard Schlenk techniques. 1H NMR
spectra were measured on Bruker AVANCE 500 MHz spectro-
meter with tetramethylsilane as the internal standard. Mass
spectra were recorded on a Shimadzu AXIMA-CFR MALDI-
TOF mass spectrometer. Elemental analyses were performed on a
flash EA 1112 spectrometer. Differential scanning calorimetric
(DSC) measurements were performed on a NETZSCH DSC204
instrument. Thermogravimetric analyses (TGA) were performed
on a TA Q500 thermogravimeter. UV-vis absorption spectra
were recorded using a PE UV-vis lambdazo spectrometer. The
emission spectra were recorded using a Shimadzu RF-5301 PC
spectrometer. Tri-(8-hydroxyquinolino)aluminum (Alq3) was
purchased from Aldrich and purified by vacuum sublimation.
N,N0-di(1-naphthyl)-N,N0-diphenyl-(1,10-biphenyl)-4,40-diamine
(NPB),11 2,5-bis(2-pyridyl)-1,4-hydroquinone (H2L1), 2,5-bis(5-
methyl-2-pyridyl)-1,4-hydroquinone (H2L2),12 and 2-(2-pyridyl)
phenol (HL3)13 were synthesized according to the literature
procedures.
1
give compound 5 as light green solid (129 mg, 80% yield). H
NMR (DMSO, ppm): δ 8.40 (d, J=8.5 Hz, 1 H), 8.32 (t, J=7.5
Hz, 1 H), 8.10 (d, J=5.5 Hz, 1 H), 7.94 (d, J=8.0 Hz, 1 H), 7.67
(t, J=6.5 Hz, 1 H), 7.40 (t, J=7.5 Hz, 1 H), 7.17-7.11 (m, 10 H),
7.06 (d, J=8.0 Hz, 1 H), 6.88 (t, J=7.5 Hz, 1 H). Ms m/z: 335.2
[M]þ (calcd: 335.1). Anal. Calcd (%) for C23H18BNO: C, 82.41;
H, 5.41; N, 4.18. Found: C, 82.40; H, 5.41; N, 4.07.
X-ray Diffraction. Diffraction data were collected on a Rigaku
RAXIS-PRID diffractometer using the ω-scan mode with
graphite-monochromator Mo KR radiation. The structures
3
were solved with direct methods using the SHELXTL programs
and refined with full-matrix least-squares on F2. Non-hydrogen
atoms were refined anisotropically. The positions of hydrogen
atoms were calculated and refined isotropically.
Photoluminescent (PL) Quantum Yield Measurements. The
fluorescence quantum yields in solution were measured relative
to quinine sulfate in sulfuric acid aqueous solution (λexc
=
Ph2B(μ-L1)BPh2 (1). A solution of BPh3 (303 mg, 1.25 mmol)
in THF (10 mL) was added to a solution of H2L1 (150 mg,
0.57 mmol) in THF (20 mL) at room temperature, and then the
reaction mixture was stirred overnight at the same temperature.
The reaction mixture was concentrated to about 5 mL by
365 nm, ΦF = 0.546) at room temperature. For compounds
1-5, the longest absorption maximum was chosen as the
excitation wavelength. The quantum yields were calculated
using previously known procedures.14 The solid-state quantum
yields were measured from the freshly evaporated film using an
integrating sphere with the excitation wavelength of 409 nm for
all the compounds.
(10) Zhao, Q.; Zhang, H.; Wakamiya, A.; Yamaguchi, S. Synthesis 2009,
127.
Electrochemical Measurements. Cyclic voltammetry was per-
formed on a BAS 100W instrument with a scan rate of 100 mV/s.
(11) Koene, B. E.; Loy, D. E.; Thompson, M. E. Chem. Mater. 1998, 10,
2235.
(12) Shu, W.; Valiyaveettil, S. Chem. Commun. 2002, 1350.
(13) Li, Y.; Liu, Y.; Bu, W.; Lu, D.; Wu, Y.; Wang, Y. Chem. Mater. 2000,
12, 2672.
(14) Ye, K.; Wang, J.; Sun, H.; Liu, Y.; Mu, Z.; Li, F.; Jiang, S.; Zhang, J.;
Zhang, H.; Wang, Y.; Che, C. M. J. Phys. Chem. B. 2005, 109, 8008.

7232 Inorganic Chemistry, Vol. 48, No. 15, 2009
Zhang et al.
A three-electrode configuration was used for the measurement:
a platinum electrode as the working electrode, a platinum wire
as the counter electrode, and an Ag/Agþ electrode as the
reference electrode. A 0.1 M solution of tetrabutylammonium
perchlorate (TBAP) in DMF or CH2Cl2 was used as the
supporting electrolyte.
Theoretical Calculations. The ground state geometries were
fully optimized by the density functional theory (DFT)15 meth-
od with the Becke three-parameter hybrid exchange and the
Lee-Yang-Parr correlation functional16 (B3LYP) and 6-31G*
basis set using the Gaussian 03 software package.17
Fabrication of EL Devices. Indium-tin oxide (ITO) coated
glass was used as the substrate. It was cleaned by sonication
successively in a detergent solution, acetone, methanol, and
deionized water before use. The devices were prepared in
vacuum at a pressure of 5 ꢀ 10-6 Torr. Organic layers were
Figure 1. ORTEP drawing of 3 with 30% thermal ellipsoids.
˚
deposited onto the substrate at a rate of 1-2 A/s. After the
organic film deposition, LiF and aluminum were thermally
evaporated onto the organic surface. The thicknesses of the
organic materials and the cathode layers were controlled using a
quartz crystal thickness monitor. The electrical characteristics
of the devices were measured with a Keithley 2400 source meter.
The EL spectra and luminance of the devices were obtained on a
PR650 spectrometer.
Scheme 1. Synthetic Procedures for Compounds 1-5
Results and Discussion
Syntheses and Crystal Structures. The synthetic proce-
dures for 1-4 are shown in Scheme 1. Simple mixing of
the ligands (H2L1 or H2L2) and boron reagents in THF
and subsequent purification by vacuum sublimation gave
the targets 1-4 in moderate yields (53-64%). For com-
parison purposes, the monoboron compound 5 was also
synthesized in 80% yield.
The single crystal structures of compounds 1 and 3 were
determined by X-ray crystallography. The crystal lattices
of 1 and 3 belong to triclinic P1 space group, in which
both molecules have a center of symmetry. Figure 1 shows
the Oak Ridge thermal ellipsoid plot (ORTEP) drawing
of 3, and the ORTEP structure of 1 is shown in Support-
ing Information, Figure S6 [We failed in the attempt to
obtain the better single crystals of compound 1 using
several methods. The crystal was always too small to be
used in the X-ray diffraction analysis. Although we finally
got a small crystal suitable for the X-ray analysis, the
diffractions were weak, which led to the high R indices.].
Boron atoms adopt a typical tetrahedral geometry, and
the boron-contained six-membered rings take a chair
conformation in which the boron atoms deviate from
the central benzene plane by 0.69 A for 1 and 0.72 A for 3.
The B-N, B-O, and B-C bond lengths (Supporting
Information, Table S2) of 1 and 3 are similar to those of
the organoboron quinolates reported previously.7a The
dihedral angles between the terminal pyridine plane and
the central benzene plane are 14.53° in 1 and 16.83° in 3,
indicating that the ladder skeletons have certain distor-
tions from the coplanar framework. There is no inter-
facial π-π interaction in the packing structure of 1
probably because of the large steric hindrance of the
peripheral phenyl rings, and this structural feature should
play an important role to reduce the excimer formation in
the solid state as already observed in ladder-type struc-
tures.3c,18 In contrast, 3 forms a slipped face-to-face
packing structure in which the interfacial π-π distance
˚
of the adjacent molecules is about 3.5 A (Supporting
Information, Figure S7).
Thermal Analyses. We used thermogravimetric analysis
(TGA) and differential scanning calorimetry (DSC) to
characterize the thermal properties of 1-5 under a nitro-
gen atmosphere (Figure 2). The decomposition tempera-
tures with a 5% weight loss (Td5) for 1-5 are 378, 350,
268, 234, and 247 °C, respectively. Compounds 1 and 2
having aryl groups on the boron centers show much
higher Td5 values compared with compounds 3 and 4 in
which boron centers are substituted by flexible alkyl
chains. This comparison clearly shows the significant
effect of the substituents attached to the boron centers.
We also note that the thermal stability of diboron com-
pound 1 is greatly improved when compared with that of
the monoboron compound 5, indicative of the effect of
the extended π-conjugation. It is believed that com-
pounds 1-4 had already partially decomposed before
melting in the DSC examinations considering that the
˚
˚
(15) Runge, E.; Gross, E. K. U. Phys. Rev. Lett. 1984, 52, 997.
(16) Becke, A. D. J. Chem. Phys. 1993, 98, 5648.
(17) Frisch, M. J. et al. Gaussian 03, Revision C.02; Gaussian, Inc.:
Pittsburgh, PA, 2003.
(18) Wong, K.-T.; Chi, L.-C.; Huang, S.-C.; Liao, Y.-L.; Liu, Y.-H.;
Wang, Y. Org. Lett. 2006, 8, 5029.

Article
Inorganic Chemistry, Vol. 48, No. 15, 2009 7233
Table 1. Photophysical Properties of Compounds 1-5, H2L1, and H2L2
λem (nm)
ΦF
λ
abs, nma,b
(log ε)
compd
solutiona
filmc
solutiona
filmc
1
2
3
4
5
461 (3.94)
455 (4.00)
479 (3.84)
473 (3.95)
361 (3.80)
398 (4.15)
394 (4.17)
584
578
611
604
481
584
579
562
587
448
0.29
0.30
0.08
0.08
0.34
0.18
0.22
0.13
0.14
0.39
H2L1
H2L2
a Data were collected in THF (3 ꢀ 10-6 M). b Only the longest
absorption maxima are shown. c Thin film on quartz substrate with
the thickness of about 100 nm.
Figure 3. UV-vis absorption spectra of 1-5, H2L1, and H2L2 in THF.
Figure 2. TGA (a) and DSC (b) curves of 1-5 at a heating rate of 10 °C/
min.
measured melting points of these compounds (425, 393,
301, and 246 °C for 1-4, respectively) are higher than
their decomposition temperatures. The second cycles of
the DSC measurement were not carried out because the
melting of compounds 1-4 was accompanied by decom-
position in the first cycles, and thus whether these com-
pounds possess a glass transition temperature (Tg) could
not be determined. The TGA and DSC data indicate that
the ladder compounds 1 and 2 are highly thermally stable
and would be capable of enduring the vacuum thermal
sublimation process during OLED fabrication.
Optical Properties. The absorption and fluorescence
spectra of 1-4 were recorded in THF. Their data are
summarized in Table 1, together with the spectra data of
5, H2L1, and H2L2 for comparison. In the UV-vis
absorption spectra (Figure 3), all compounds 1-4 exhibit
broad peaks around 450-480 nm, which are greatly red-
shifted compared with those of the ligands. Such a red-
shifted absorption caused by boron chelation was often
observed in four-coordinate boron systems.19 In compar-
ison with 5 (λmax = 361 nm), compound 1 with the
absorption maximum of 461 nm exhibits an obvious red
shift, which might be due to the increased effective con-
jugation length. The absorption maximum of ethyl-sub-
stituted derivative 3 is red-shifted by about 18 nm
Figure 4. Emission spectra of 1-5 in THF.
compared with that of phenyl-substituted derivative 1,
and the same red shift is observed in the comparison
between 4 and 2, indicating that the electron-donating
substituents on boron centers have great influence on the
photophysical characteristics. Methyl groups on the li-
gand (compounds 2 and 4) only have a subtle influence on
the longest absorption maxima, leading to a blue shift of
about 6 nm from the non-substituted to the methyl-
substituted compounds.
Although the ligands H2L1 and H2L2 are non-emissive,
compounds 1-4 show fluorescence in both solution and
solid state. Figure 4 shows the fluorescence spectra of 1-5
in tetrahydrofuran (THF). The emission maxima of
compounds 1-4 are located in the orange to red region,
(19) (a) Ren, Y.; Liu, X.; Gao, W.; Xia, H.; Ye, L.; Mu, Y. Eur. J. Inorg.
Chem. 2007, 1808. (b) Liddle, B. J.; Silva, R. M.; Morin, T. J.; Macedo, F. P.;
Shukla, R.; Lindeman, S. V.; Gardinier, J. R. J. Org. Chem. 2007, 72, 5637.

7234 Inorganic Chemistry, Vol. 48, No. 15, 2009
Zhang et al.
being 584, 578, 611, and 604 nm, respectively. The emis-
sion maximum of 1 is obviously red-shifted by about
103 nm in comparison with that of 5, which reflects the
influence of the extended π-conjugation. In addition, the
emission maxima of compounds 1-4 show a trend con-
sistent with that of the observed energy gaps determined
by UV-vis spectra. The fluorescence quantum yields of
1 and 2 determined by using quinine sulfate as reference
are 0.29 and 0.30, respectively. These values are compar-
able to that of 5 (ΦF = 0.34). In contrast, compounds
3 and 4 show very weak emission in solution (quantum
yield for both 3 and 4 is 0.08). In the solid state, the
emission peaks of compounds 1-4 are found at 584, 579,
562, and 587 nm, respectively. The emission spectra of
1 and 2 in the solid state are almost identical to those
observed in solution. This data indicates that although
the π-systems have ladder-type framework, the bulky
phenyl groups attached to boron centers prevent effi-
cient π-stacking in their aggregated state, which was also
explained by the X-ray structure analysis. Moreover, in
view of the application as emitting materials, com-
pounds 1 and 2 might be promising because this feature
makes it possible for these compounds to maintain the
moderate solid-state fluorescence quantum yields in the
solid state. Indeed, the quantum yields of the thin films
reach 0.18 and 0.22 for 1 and 2, respectively, which are
relatively higher than those of the reported diboron-
contained π-conjugated systems.20 Compounds 3 and 4
display blue-shifted emissions in the solid state as com-
pared with the solutions, which is probably due to the
luminescence rigidchromism.21
Electrochemical Properties. The electrochemical prop-
erties of compounds 1-5 and the ligands H2L1 and H2L2
were studied by means of cyclic votammetry. Their cyclic
voltammograms are shown in Figure 5 and the data are
summarized in Table 2. These data are discussed from the
viewpoints of (1) effect of conjugation length; (2) effect of
boron chelation; and (3) effect of substituents attached to
boron atom.
Figure 5. Cyclic voltammograms of 1-5, H2L1, and H2L2: (a) anodic
runs; (b) cathodic runs.
Table 2. Electrochemical Data a and HOMO/LUMO Energy Levels
experimental data
DFT calculations
1/2
1/2
Eox
(V)b
Ered
(V)c
HOMO LUMO HOMO LUMO
(eV)
compd
(eV)
(eV)
(eV)
While the monoboron compound 5 displays an irrever-
sible reduction wave with peak potential (Epc) at -2.24 V,
all diboron compounds 1-4 exhibit two sequential re-
versible reduction waves with the first half-wave poten-
tials (Ered1/2) at -1.81 ∼ -2.03 V (vs Fc/Fcþ). This result
demonstrates that extension of the π-system from mono-
boron to diboron not only make the cathodic reductions
easier but also stabilizes the produced radical anions. The
consecutive peaks observed in the reductions of all ladder
compounds are likely associated to the two boron chelat-
ing parts considering that there is only one reduction peak
appearing in the cyclic voltammogram of the monoboron
compound. For the free ligands H2L1 and H2L2, only one
well resolved quasi-reversible reduction wave can be
observed within the electrochemical window of N,
N-dimethylformamide (DMF). The first reduction po-
tentials of boron compounds 1-4 are obviously less
negative than those of the corresponding free ligands,
indicating that the boron chelation can greatly lower the
1
2
3
4
5
þ0.75
þ0.70
-1.81
-1.89
-5.55
-5.50
-5.06e
-5.01e
-2.99
-2.91
-2.86
-2.77
-5.35
-5.25
-4.99
-4.88
-5.80
-2.46
-2.34
-2.31
-2.19
-1.96
þ0.40d -1.94
þ0.36d -2.03
þ1.20d -2.24d
þ0.30d -2.32
þ0.37d -2.40
H2L1
H2L2
-5.21e
-5.17e
-2.48
-2.40
a Potentials are given against ferrocene/ferrocenium (Fc/Fcþ). b Mea-
sured in CH2Cl2. c Measured in DMF. d Peak potential. e From the
optical bandgap and the LUMO energy.
lowest unoccupied molecular orbital (LUMO) level of the
π-system. Notably, the first reduction potentials of com-
pounds 1-4 are less negative than that of Alq3 (Epc
=
-2.3 V),22 one of the most widely used electron-trans-
porting materials, which might be indicative of the good
electron-transporting properties of these compounds in
EL devices. By replacing the phenyl groups in compounds
1 and 2 with ethyl groups, in compounds 3 and 4, the
reduction potentials show a slight negative shift.
All these diboron compounds 1-4 have a large
negatively shifted oxidation wave compared with the
(20) Cui, Y.; Wang, S. J. Org. Chem. 2006, 71, 6485.
(21) (a) Wu, J.; Abu-Omar, M. M.; Tolbert, S. H. Nano Lett. 2001, 1, 27.
(b) Nicola, A. D.; Liu, Y.; Schanze, K. S.; Ziessel, R. Chem. Commun. 2003, 288.
(c) Lees, A. J. Coord. Chem. Rev. 1998, 177, 3.
(22) Shinar, J. Organic light-emitting devices: a survey; Springer-Verlag
New York, Inc.: New York, 2003.

Article
Inorganic Chemistry, Vol. 48, No. 15, 2009 7235
Figure 7. EL spectra of the devices and PL spectra of thin films of 1 and
2.
Table 3. Data of Electroluminescent Devices
λmax turn-on
device (nm) voltage (V)
maximum luminance maximum efficiency
(Cd/m2)
(Cd/A)
Figure 6. Calculated frontier orbitals for 1-4.
A1
A2
A5
B1
B2
B5
588
580
485
588
580
484
2.6
3.2
14.0
2.4
2.5
10.0
8156
8505
9
8438
9754
264
2.03
2.56
0.07
2.18
4.04
0.69
monoboron compound, for instance, 3 and 4 exhibit an
irreversible oxidation wave with peak potentials at þ0.40
and þ0.36 V, respectively, which are negatively shifted by
about 0.80 V as compared with that of 5 (Epa=þ1.20 V).
This result shows the significant effect of the diboron-
contained ladder π-conjugated skeleton. The oxidation
potentials of these ladder-type boron compounds are in
general more positive than those of the corresponding
free ligands. Compounds 3 and 4 with ethyl groups on
boron atoms have a decreased oxidation potential com-
pared with 1 and 2, respectively, which might be due to the
electron-donating properties of the ethyl groups. Worth
noting is that 1 and 2 show reversible oxidation waves.
This is in contrast to the cases of 3 and 4, which only show
irreversible oxidation waves. The stability of the oxidized
species of these diboron compounds is dependent on the
substituents on boron centers, namely, the phenyl groups
stabilize the produced radical cation while the ethyl
groups do not.
Molecular Orbital Calculations. To further under-
stand the electronic structures of compounds 1-4, we
carried out density functional theory (DFT) calcula-
tions for these compounds, together with that of com-
pound 5 for comparison. All diboron compounds 1-4
have a higher highest occupied molecular orbital
(HOMO) level and a lower LUMO level compared with
the monoboron compound 5 (Table 2), which is con-
sistent with the electrochemical data. Figure 6 shows
the HOMO and LUMO plots of compounds 1-4. The
HOMOs are mainly donated by the π-orbitals of quinol
moiety, while the LUMOs are predominated by the π*-
orbitals of the whole ligand (L1 or L2). The orbital
components of HOMO and LUMO are shown in
Supporting Information, Table S3. Compounds 3 and
4 with ethyl groups on the boron atom show a signifi-
cantly higher HOMO level and a slightly higher LUMO
level compared with the phenyl-substituted compounds
1 and 2, respectively. This result also agrees well with
the electrochemical data. The increased HOMO and
LUMO levels of the ethyl-substituted compounds
should be attributed to the electron-donating proper-
ties of the ethyl group, which will lead to an increased
electron density at the ligand skeleton. A similar effect
of substituents attached to boron on the HOMO and
LUMO levels was observed in a class of borondipyrro-
methene dyes.23
Electroluminescent Properties. Considering the high
thermal stabilities, intensive emissions, and strong
electron-accepting properties of compounds 1 and 2,
we thus fabricated two types of devices to evaluate their
EL characteristics. The first one was a double-layer
device with the configuration of [ITO/NPB (40 nm)/1
or 2 (70 nm)/LiF (0.5 nm)/Al (200 nm)] (devices A1 and
A2) in which NPB was used as the hole-transporting
material, and compound 1 or 2 acted as both the emitter
and the electron-transporting material. These devices
produced orange electroluminescence with peaks cen-
tered at 588 and 580 nm for A1 and A2, respectively.
The EL spectra of both devices matched well with the
thin-film PL spectra of corresponding boron com-
pounds, indicating that the EL luminescence originated
from the intrinsic emissions of these compounds, as
shown in Figure 7. It is worth noting that only a few
non-doped orange or red emission OLEDs fabricated
by using boron-contained emitting materials have been
reported previously.7d,8b,8d The turn-on voltages were
quite low and reached 2.6 and 3.2 V for devices A1 and
A2, respectively. Device A1 could reach a maximum
brightness of 8156 cd/m2 at 11 V, and a maximum
current efficiency of 2.03 cd/A at 6.5 V. The brightness
and current efficiency of A2, summarized in Table 3,
are comparable to those of A1. The EL performance of
(23) Goze, C.; Ulrich, G.; Mallon, L. J.; Allen, B. D.; Harriman, A.;
Ziessel, R. J. Am. Chem. Soc. 2006, 128, 10231.

7236 Inorganic Chemistry, Vol. 48, No. 15, 2009
Zhang et al.
compounds 1 and 2 are at high level in comparison with
those of the previously reported four-coordinate boron
compounds.7-9 The second type of devices we fabri-
cated was the typical three-layer device with the con-
figuration of [ITO/NPB (40 nm)/1 or 2 (60 nm)/Alq3
(10 nm)/LiF (0.5 nm)/Al (200 nm)] (devices B1 and B2),
in which an additional Alq3 material was used as the
electron-transport layer. The EL spectra of devices B1
and B2 were nearly the same as those of the double layer
devices A1 and A2 (Figure 7), respectively, indicating
that the EL emissions of the three-layer devices also
originated from the boron compounds. The EL perfor-
mance of B1 and B2 are summarized in Table 3. Device
B1 exhibited similar performance to the double-layer
device A1, while device B2 showed higher brightness
and efficiency than device A2. In fact, device B2 with
the maximum brightness of 9754 cd/m2 is one of the
brightest EL devices fabricated by using boron-con-
tained emitting materials. Worth noting is that
although compound 5 has higher PL efficiency com-
pared with 1 and 2, the double-layer (A5) and three-
layer (B5) devices based on 5 with the same configura-
tions as devices A1 and B1, respectively, have much
lower EL brightness and efficiencies (Table 3). Devices
A5 and B5 display blue-green color emission at around
485 nm. This comparison demonstrates the superiority
of boron compounds with ladder structure as EL
materials.
Conclusion
We have designed and synthesized four boron-contained
ladder-type compounds 1-4. Incorporating two boron
atoms into the ladder-type π-conjugated skeletons endows
these materials with high thermal stability and strong elec-
tron affinity. For compounds 1 and 2, the PL quantum yield
in the solid-state does not show significant decrease as
compared with that in solution, which is attributed to the
steric hindrance of the bulky BPh2 groups. Simple double-
layer EL devices fabricated using materials 1 and 2 as both
emitters and electron-transporting layers displayed good
performance. We suggest that the ladder-type boron com-
pounds are a class of promising candidates for applications in
OLEDs. Further modification on this boron-contained lad-
der-type π-conjugated system is ongoing in our laboratory.
Acknowledgment. This work was supported by the
National Natural Science Foundation of China (50733002),
the Major State Basic Research Development Program
(2009CB623600), the 863 Project (2006AA03A162) and 111
Project (B06009).
Supporting Information Available: 1H NMR spectra of
compounds 1-5, ORTEP drawing of 1, J-V and L-V char-
acteristics of the EL devices, crystallographic data for 1 and 3 in
PDF and CIF formats, and the orbital components of HOMO
and LUMO for compounds 1-4. This material is available free
of charge via the Internet at http://pubs.acs.org.