Boron-bridged π-conjugated ladders as efficient electron-transporting emitters

Article abstract of DOI:10.1021/ic102554j

Four diboron-bridged ladder molecules 1-4 have been designed and synthesized. X-ray diffraction analysis revealed that the bulky phenyl substituents on boron centers efficiently prevented π stacking of the luminescent ladder unit. Characterizations of the

Full text of DOI:10.1021/ic102554j

ARTICLE
pubs.acs.org/IC
Boron-Bridged π-Conjugated Ladders as Efficient
Electron-Transporting Emitters
Di Li, Yang Yuan, Hai Bi, Dandan Yao, Xingjia Zhao, Wenjing Tian, Yue Wang, and Hongyu Zhang*
State Key Laboratory of Supramolecular Structure and Materials, College of Chemistry, Jilin University, Changchun 130012,
People's Republic of China
S Supporting Information
b
ABSTRACT: Four diboron-bridged ladder molecules 1ꢀ4 have been designed and
synthesized. X-ray diffraction analysis revealed that the bulky phenyl substituents on
boron centers efficiently prevented π stacking of the luminescent ladder unit. Character-
izations of these complexes demonstrated that the construction of diboron-containing
ladder-type skeletons endowed these materials with good thermal stability, high fluores-
cence quantum yields, and strong electron affinity. The highly efficient nondoped organic
light-emitting diodes using complexes 1 and 2 as electron-transporting emitters exhibited
maximum luminance values of 16 930 and 18 060 cd/m2 with turn-on voltages of 3.5 and
2.5 V as well as maximum luminous efficiencies of 6.4 and 5.4 cd/A, respectively.
’ INTRODUCTION
π-conjugated skeletons by simple synthetic routes has become
a very important issue that arouses our strong interest.
Ladder-type π-conjugated molecules with fully ring-fused
structures have attracted considerable attention in the field of
organic electronics.1 The flat and rigid π-conjugated skeletons of
such molecules give rise to a set of desired properties such as
intense luminescence, good thermal stability, and high carrier
mobility that are important in terms of their applications in high-
performance optoelectronics including organic light-emitting
diodes (OLEDs), organic field effect transistors, and lasers.2
To modulate the electronic structure of ladder-type molecules
and obtain the required optical properties, various bridging
atoms such as silicon, sulfur, selenium, phosphorus, boron, and
nitrogen have been introduced to the π-conjugated skeleton.3ꢀ9
Recently, multiboron-bridged ladder-type π-conjugated com-
pounds reported by other groups and us have proved to be
emissive organic solids with high electron affinity, which enables
them to be used as both emitting and electron-transporting
materials to fabricate structurally simple and high-performance
electroluminescent (EL) devices.10,11 In this kind of system, the
incorporation of boryl groups into the π-conjugated skeleton
allows intramolecular coordination from the Lewis basic nitrogen
atom to the boron atom,12ꢀ14 and the produced fully ring-fused
structures not only constrain the π-conjugated framework to
intensify the emission but also affect the electronic state by
lowering the lowest unoccupied molecular orbital (LUMO) level
to enhance the electron affinity. Although this type of molecule
has promising potential in the fields of OLEDs, only a few ladder-
type boron-containing systems have been achieved to date
probably because of the lack of efficient synthetic routes. In
this regard, constructing new ladder-type multiboron-doped
Recently, we have developed a simple approach to synthesize
luminescent π-conjugated ladders bearing the boron element as
bridges.12 To continue our effort in the development of boron-
containing ladder molecules with enhanced emission and
electron-transporting properties, we herein report the synthesis
and light-emitting properties of a series of novel diboron-
bridged ladders. In the present work, we choose phenol-
substituted thiazolothiazole derivatives as the key building units
and incorporate two phenyl-substituted boron moieties into the
backbones to construct a rigid extended π-conjugated skeleton
(Chart 1). This type of precursor is selected because of its
inherent electron-deficient character as well as its high reactivity
with boron reagents by OꢀBꢀN chelation. This design aims at
producing a novel ladder-type system with intense emission and
high electron affinity to be used as both high-performance
emitters and electron-transporting materials. Further functio-
nalization of the system has been performed on the molec-
ular structure by introducing electron-withdrawing/donating
groups into the π-conjugated plane. The single-crystal X-ray
structures, thermal stabilities, photophysical and electrochemi-
cal properties of these complexes have been investigated to
study the characteristics of the π system and the impact of
modification. Furthermore, the EL properties of this system
are also presented to demonstrate their application potential
in OLEDs.
Received: December 22, 2010
Published: May 10, 2011
r
2011 American Chemical Society
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Inorganic Chemistry
ARTICLE
Chart 1. Design Strategy toward a Diboron-Containing
Ladder
Scheme 1. Synthetic Procedure for Diboron Complexes 1ꢀ4
’ EXPERIMENTAL SECTION
General Procedure. All starting materials were purchased from
Aldrich Chemical Co. and used without further purification. The
solvents for the syntheses were freshly distilled over appropriate drying
reagents. All experiments were performed under a nitrogen atmosphere
using standard Schlenk techniques. 1H NMR spectra were measured on
a Bruker AVANCE 500 MHz spectrometer with tetramethylsilane as the
internal standard. Mass spectra were recorded on a Shimadzu AXIMA-
CFR MALDI-TOF mass spectrometer. Elemental analyses were per-
formed on a flash EA 1112 spectrometer. Differential scanning calorim-
etry (DSC) measurements were performed on a Netzsch DSC204
instrument. Thermogravimetric analyses (TGA) were performed on a
TA Q500 thermogravimeter. UVꢀvis absorption spectra were recorded
using a Perkin-Elmer UVꢀvis Lambdazo spectrometer. The emission
spectra were recorded using a Shimadzu RF-5301 PC spectrometer.
Tris(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),15 2,5-bis(2-hydroxyphenyl)thiazolo-
[5,4-d]thiazole (H2L1), 2,5-bis(2-hydroxy-5-fluorophenyl)thiazolo[5,4-d]-
thiazole (H2L2), 2,5-bis(2-hydroxy-5-methoxyphenyl)thiazolo[5,4-d]thiazole
(H2L3), and 2,5-bis(2-hydroxy-4-methoxyphenyl)thiazolo[5,4-d]thiazole
(H2L4) were synthesized according to the literature procedures.16
Ph2B(μ-L1)BPh2 (1). To a rapidly stirred solution of H2L1 (200 mg,
0.55 mmol) in tetrahydrofuran (THF; 20 mL) was added BPh3 (293 mg,
1.21 mmol) in THF (5 mL), and the solution was refluxed for 12 h. The
volatiles were removed under vacuum, and the residual solid was purified
by vacuum sublimation to give complex 1 as a yellow solid (180 mg, 50%
yield). 1H NMR (CDCl3, ppm): δ 7.44 (t, J = 7.5 Hz, 2 H), 7.36ꢀ7.28
(m, 20 H), 7.17 (d, J = 7.5 Hz, 2 H), 7.14 (d, J = 7.5 Hz, 2 H), 6.78 (t, J =
7.5 Hz, 2 H). MS: m/z 653.1 [M]þ (calcd m/z 654.2). Anal. Calcd for
C40H28B2N2O2S2: C, 73.41; H, 4.31; N, 4.28; S, 9.80. Found: C, 73.35;
H, 4.22; N, 4.28; S, 10.06.
1
provided 4 as a yellow solid (140 mg, 40% yield). H NMR (CDCl3,
ppm): δ 7.35ꢀ7.28 (m, 20 H), 7.01 (d, J = 9 Hz, 2 H), 6.61 (d, J = 2.5
Hz, 2 H), 6.35 (dd, J = 9 and 2.5 Hz, 2 H), 3.82 (s, 6 H). MS: m/z 713.2
[M]þ (calcd m/z 714.2). Anal. Calcd for C42H32B2N2O4S2: C, 70.61; H,
4.51; N, 3.92; S, 8.98. Found: C, 70.33; H, 4.42; N, 3.87; S, 8.88.
X-ray Diffraction. Diffraction data were collected on a Rigaku RAXIS-
PRID diffractometer using the ω-scan mode with graphite-monochro-
mator Mo KR radiation. The structures were solved with direct methods
using the SHELXTL programs17 and refined with full-matrix least
squares on F2. Non-hydrogen atoms were refined anisotropically. The
positions of the hydrogen atoms were calculated and refined isotropically.
Photoluminescent (PL) Quantum Yield Measurements. The solu-
tion fluorescence quantum yields were measured in dilute CH2Cl2
relative to quinine sulfate in an aqueous sulfuric acid solution (λexc
=
365 nm; ΦF = 0.546) at room temperature. The quantum yields were
calculated using previously known procedures.18 The solid-state quan-
tum yields were measured using an integrating sphere with an excitation
wavelength of 325 nm for all of the complexes.
Electrochemical Measurements. Cyclic voltammetry (CV) was
performed using a BAS 100W instrument with a scan rate of 100
mV/s. A three-electrode configuration was used for the measurements:
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 THF
was used as the supporting electrolyte. The ferrocene/ferrocenium
(Fc/Fcþ) couple was used as the internal standard.
Theoretical Calculations. The ground-state geometries were fully opti-
mized by the density functional theory (DFT)19 method with the Becke three-
parameter hybrid exchange, the LeeꢀYangꢀParr correlation functional20
(B3LYP), and the 6-31G* basis set using the Gaussian 03 software package.21
Fabrication of EL Devices. Indiumꢀtin 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 ꢁ 10ꢀ6 Torr.
Organic layers were deposited onto the substrate at a rate of 1ꢀ2 Å/s.
After 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.
Ph2B(μ-L2)BPh2 (2). In the same manner as that described for 1, the
reaction of H2L2 (100 mg, 0.28 mmol) and BPh3 (144 mg, 0.60 mmol)
1
provided 2 as an orange-yellow solid (82 mg, 43% yield). H NMR
(CDCl3, ppm): δ 7.33ꢀ7.29 (m, 20 H), 7.20ꢀ7.14 (m, 4 H), 6.81 (dd,
J = 7.5 and 2.5 Hz, 2 H). MS: m/z 690.0 [M]þ (calcd m/z 690.2). Anal.
Calcd for C40H26B2F2N2O2S2: C, 69.59; H, 3.80; N, 4.06; S, 9.29.
Found: C, 69.55; H, 3.75; N, 4.07; S, 9.57.
Ph2B(μ-L3)BPh2 (3). In the same manner as that described for 1, the
reaction of H2L3 (190 mg, 0.49 mmol) and BPh3 (266 mg, 1.10 mmol)
provided 3 as an orange solid (182 mg, 51% yield). 1H NMR (CDCl3,
ppm): δ 7.36ꢀ7.28 (m, 20 H), 7.13 (d, J = 9 Hz, 2 H), 7.07 (dd, J = 9 and
2.5 Hz, 2 H), 6.51 (d, J = 3 Hz, 2 H), 3.71 (s, 6 H). MS: m/z 714.0 [M]þ
(calcd m/z 714.2). Anal. Calcd for C42H32B2N2O4S2: C, 70.61; H, 4.51;
N, 3.92; S, 8.98. Found: C, 70.38; H, 4.45; N, 3.92; S, 9.23.
’ RESULTS AND DISCUSSION
Syntheses. Scheme 1 shows the synthetic procedures for the
diboron ladders 1ꢀ4. Simple mixing of the ligands and boron
Ph2B(μ-L4)BPh2 (4). In the same manner as that described for 1, the
reaction of H2L4 (190 mg, 0.49 mmol) and BPh3 (266 mg, 1.10 mmol)
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Figure 2. (a) Photographs in solution and the solid state under UV light
(λex = 365 nm) and (b) absorption and emission spectra of diboron
complexes 1ꢀ4.
interactions are found for all of the complexes in their packing
structures because of the large steric hindrance of peripheral
phenyl rings. This structural feature should play an important
role in reducing excimer formation in the solid state, as was
already observed in ladder-type structures.4c,23 In the packing
structure of 1, one molecule is connected with four adjacent
molecules by two types of CꢀH π intermolecular interac-
3 3 3
Figure 1. Molecular structures of complexes 1 (a), 2 (b), and 3 (c) with
50% thermal ellipsoids and packing structures of 1 (d), 2 (e), and 3 (f).
ꢀ
tions: CꢀH(ladder) π(Ph) (d = 2.70 Å; θ = 137.8°) and
3 3 3
ꢀ
CꢀH(Ph) π(ladder) (d = 2.87 Å; θ = 146.6°), as depicted in
3 3 3
Figure 1d. Figure 1e shows the packing structure of 2. Each
molecule interacts with four adjacent molecules by CꢀF-
reagents in THF and subsequent purification by vacuum sub-
limation gave the targets 1ꢀ4 in moderate yields (40ꢀ51%),
which were fully characterized by 1H NMR spectroscopy, mass
spectrometry (MS), element analyses, and/or X-ray crystal
analysis. The skeletons were modified by introducing electron-
withdrawing/donating groups into different sites of the back-
bones. The modification was performed during the syntheses of
the relative ligands before boron chelation, and the ligands were
obtained by condensation of dithiooxamide with the relative
aromatic aldehydes at reflux temperature.
Crystal Structures. Single crystals of complexes 1ꢀ3 were
obtained by vacuum sublimation, and their structures were
determined by X-ray crystallography. The crystal lattices of
1ꢀ3 are all centrosymmetric, in which 1 and 2 belong to
monoclinic space group P21/c and 3 holds triclinic space
group P1. All boron atoms adopt a typical tetrahedral geometry
to form N,O-chelate six-membered rings, which contribute to
form the six-ring-fused ladder-type π-conjugated skeletons
(Figure 1aꢀc). In the ladder-type skeleton of complex 1, the
six-membered rings constructed by boron chelation take a chair
conformation in which the oxygen and boron atoms deviate from
the central thiazolothiazole plane by 0.59 and 0.16 Å, respec-
tively, in opposite directions. The bond lengths of BꢀN, BꢀO,
and BꢀC are 1.658, 1.470, 1.608 Å, respectively, which are
similar to those of the organoboron quinolates reported
previously.22 The dihedral angle between the central thiazo-
lothiazole plane and the terminal phenylene plane is 14.53°,
indicating that the ladder-type skeletons have certain distortions
from the coplanar framework. Parts dꢀf of Figure 1 show the
crystal packing structures of 1ꢀ3. No significant interfacial πꢀπ
ꢀ
(ladder) π(ladder) (d = 2.89 Å; θ = 132.7°) interactions to
3 3 3
form the molecular stacking, which is further stabilized by
ꢀ
CꢀH(Ph) π(Ph) (d = 2.81 Å; θ = 142.1°) interactions. In
3 3 3
complex 3, one-dimensional molecular columns are formed by
ꢀ
CꢀH O (d = 2.71 Å; θ = 140.6°) hydrogen-bonding inter-
3 3 3
actions. These intermolecular interactions observed in the crystals
1ꢀ3 might be of benefit in terms of their charge carrier mobility.
Thermal Analyses. The thermal properties of 1ꢀ4 were
characterized by TGA and DSC under a nitrogen atmosphere
(Figures S9 and S10 in the Supporting Information). The melting
points of 1ꢀ4 measured by DSC examinations are 332, 340, 405,
and 352 °C, respectively. Complex 3 has the highest melting
point among these boron species, which should be a result of
intermolecular interactions of hydrogen bonds that condensed
the molecular packing. Notably, complexes 1ꢀ4 had already
partially decomposed before or during their melt in the DSC
examinations because the measured melting points are close to
their decomposition temperatures. As a result, the second cycles
of the DSC measurements show no endo- or exothermic peaks;
thus, the glass transition temperatures (Tg) of these complexes
cannot be determined. The decomposition temperatures with a
5% weight loss (Td5) of 1ꢀ4 are 346, 341, 378, and 374 °C,
respectively. Methoxyl-substituted complexes 3 and 4 exhibit
higher decomposition temperatures than those of 1 and 2,
probably owing to the intermolecular hydrogen bonds observed
in the crystal structure. Thermal analyses indicate that the
diboron-containing ladders 1ꢀ4 exhibit excellent thermal stabi-
lity, which ensures that they are stable and suitable for vacuum
evaporation in OLEDs.
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Table 1. Photophysical Properties, Electrochemical Data, and Energy Levels of Complexes 1ꢀ4
λem (nm)
ΦF
experimental (eV)
compd
λabs (nm)a,b (log ε)
solutiona
powder
solutiona
powder
Ered1/2 (V)c,d
HOMO
LUMO
1
2
3
4
442 (4.45)
460 (4.44)
481 (4.56)
460 (4.54)
496
518
561
491
523
547
563
517
0.78
0.79
0.38
0.82
0.56
0.31
0.12
0.15
ꢀ1.75, e
ꢀ5.57f
ꢀ5.68f
ꢀ5.45f
ꢀ5.41f
ꢀ3.05
ꢀ3.22
ꢀ3.11
ꢀ2.88
ꢀ1.58, ꢀ2.22
ꢀ1.69, ꢀ2.26
ꢀ1.92, ꢀ2.43
a Data were collected in CH2Cl2 (1 ꢁ 10ꢀ4 M). b Only the longest absorption maxima are shown. c Potentials are given versus Fc/Fcþ. d Measured
in THF. e Not observed. f Calculated from the optical band gap and the LUMO energy.
Figure 3. Solid-state emission spectra of diboron complexes 1ꢀ4.
Figure 4. Cyclic voltammograms of 1ꢀ4 in THF (1 mM), measured
with TBAP (0.1 M) as the supporting electrolyte at a scan rate of
100 mV/s.
Photophysical Properties. All of the boron complexes are
highly emissive in both solution and the solid state, as shown in
Figure 2a. The UVꢀvis absorption and fluorescence spectra of
1ꢀ4 in CH2Cl2 are shown in Figure 2b, and the data are
summarized in Table 1.
compared to that of 1, and the emission band of 4 is blue-shifted
by about 6 nm compared to that of 1, reflecting that the
molecular packing also greatly affects their optical properties.
Moreover, the emission spectra of complexes 1, 2, and 4 in the
solid state are red-shifted compared to those of solutions, which
can be attributed to the CꢀF π and/or CꢀH π inter-
The absorption spectrum of 2 is red-shifted by 18 nm com-
pared to 1. Interestingly, compared with complex 1, the absorp-
tion maximum of 3 with the electron-donating methoxyl groups
on the para position of the oxygen atoms is red-shifted by about
39 nm, while that of 4 with the methoxyl groups on the meta
position of the oxygen atoms is only red-shifted by about 18 nm.
This observation reflects that the substituted sites significantly
affect the electronic structure of this type of ladder skeleton.
In the fluorescence spectra, complexes 1ꢀ4 show intense
green or yellow fluorescence with the maximum wavelengths at
496, 518, 561, and 491 nm, respectively. Compared with complex
1, the emission maxima of 2 and 3 are red-shifted by 22 and
65 nm, respectively, consistent with the trend observed in the
absorption maxima. The emission spectrum of complex 4 is
slightly blue-shifted by 5 nm compared to that of 1. Alkoxylation
of the ladder skeleton on different positions produces obviously
different optical properties, probably because of their different
effects on the electronic structures. For complex 3, alkoxylation
on the para position of the oxygen atoms effectively pushes the
highest occupied molecular orbital (HOMO) level up, thus
narrowing the energy gap, while for complex 4, alkoxylation on
the meta position of the oxygen atoms increases both HOMO
and LUMO levels by a similar extent. Notably, the fluorescence
quantum yields for 1, 2, and 4 are all around 0.80 in CH2Cl2,
which are much higher than most reported ladder-type boron-
doped species.24 The solid-state fluorescence has also been
investigated, as shown in Figure 3. Interestingly, the fluores-
cences of 2 and 3 are red-shifted by 24 and 40 nm, respectively,
3 3 3
3 3 3
molecular interactions, as demonstrated by the crystal structures.
The solid-state fluorescence quantum yields of these ladders are
also remarkable in the reported π-conjugated molecules, espe-
cially for that of 1, with the value reaching 0.56. Complex 1
exhibits relatively high quantum yield both in solution and in the
solid state, which is probably due to the fact that the absence of
fluoro or methoxyl substituents on the ladder increases the
molecular rigidity and weakens the intermolecular interactions.
Actually, although the π systems have extended ladder-type
frameworks, the bulky phenyl groups attached to the boron
centers prevent efficient π stacking in their aggregated state,
which was also demonstrated by the X-ray structure analysis.
This feature allows some of the complexes to maintain high
fluorescence quantum yields in the solid state and enables them
to be further used as emitting materials.
Electrochemical Properties. The electrochemical properties
of 1ꢀ4 were characterized by CV, their cyclic voltammograms
are shown in Figure 4, and the data are summarized in Table 1. All
of the complexes 1ꢀ4 display reversible reduction peaks in the
CV diagrams. Two sequential reduction peaks are observed for
2ꢀ4; the first one is reversible, while the second one is quasi-
reversible with a more negative value. In contrast, complex 1 only
displays one reversible reduction wave. For 1ꢀ4, the first half-
wave potentials (Ered1/2) attributed to the reduction of the
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Figure 5. Molecular orbital diagrams for HOMOs and LUMOs of complexes 1ꢀ4.
Figure 7. EL spectra of devices A and B and fluorescent spectra of thin
films based on complexes 1 and 2.
Figure 6. Calculated and experimental energy levels of diboron com-
plexes 1ꢀ4.
the HOMO and LUMO levels in 2, while the HOMO and
LUMO levels are raised because of the electron-donating effect
of the methoxyl groups in 3 and 4. Moreover, different sub-
stituted sites also influence the energy levels. The LUMOs are
partly donated on the substituted groups of 4 instead of 2 and 3,
just because of the different substituted positions of the intro-
duced moieties on the main skeleton. The degree of variation for
HOMOs/LUMOs and the band gaps of 1ꢀ4 are greatly
influenced by the electronic properties and substituted sites of
the introduced groups. The general trend of the calculated
HOMO/LUMO levels and energy gaps agrees with the experi-
mentally observed trend. The discrepancies of the HOMO/
LUMO levels between calculated and experimental values can be
attributed to the limitation of the ground-state DFT calculations.
EL Properties. Considering that complexes 1 and 2 have
relatively high solid-state fluorescence quantum yields among the
synthesized diboron ladders, we devote our major effort to
studying the EL properties of 1 and 2. Two types of devices
were designed and fabricated to evaluate the electron-transport
property and the electroluminescence of each complex. The first
one was a four-layer device with the configuration of [ITO/NPB
(15 nm)/BA (30 nm)/1 or 2 (40 nm)/Alq3 (15 nm)/LiF
(0.5 nm)/Al (200 nm)] (device A). In device A, NPB was used
as the hole-transport material, BA (Figure S11 in the Supporting
Information) served as an additional hole buffer layer for the
purpose of hindering the formation of exciplex between NPB and
the boron complex, Alq3 acted as an electron-transport layer, and
π-conjugated ladder are located from ꢀ1.58 to ꢀ1.92 V (vs Fc/
Fcþ), in which slight shifts are observed owing to the effect of
modification. These values are similar to those of the diboron-
doped ladder-type molecules reported before, illustrating that
the low LUMO levels of the π system are contributed by boron
chelation, which makes the cathodic reductions easier and
endows the complexes with high electron affinity. Notably, the
first reduction potentials of 1ꢀ4 are less negative than that of
Alq3 (Epc = ꢀ2.3 V),25 one of the most widely used electron-
transporting materials, which might be indicative of the good
electron-transporting properties of these complexes in EL de-
vices. On the basis of the reduction potentials, the LUMO energy
levels are calculated and summarized in Table 1. Because we are
unable to observe any oxidation within the electrochemical
window of CH2Cl2 for 1ꢀ4, their HOMO levels are empirically
calculated from their LUMO energy levels and optical data
(absorption edges in UVꢀvis spectra).
Molecular Orbital Calculations. DFT calculations were
performed for complexes 1ꢀ4 to further understand the electro-
nic structures and energy levels of these ladders. The plots and
data of HOMOs and LUMOs are shown in Figures 5 and 6. The
HOMOs and LOMOs of all of these complexes are mainly
composed of π and π* orbitals of the whole ligand, respectively,
while the boryl moieties give little contribution. Compared with
1, the electron-withdrawing effect of the fluorine atoms decreases
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Figure 8. Current densityꢀbrightnessꢀvoltage characteristics of de-
vice A of complex 1 (f), device B of complex 1 (2), device A of complex
2 (9), and device B of complex 2 (b).
complexes 1 or 2 acted as the emitting material. For complex 1,
this device was turned on at 5.5 V and produced a green color
centered at 516 nm (Figure 7). Figure 8 shows the JꢀV and LꢀV
characteristics of device A. It could reach a maximum brightness
of 15 620 cd/m2 at 16 V, a maximum luminous efficiency of 7.2
cd/A at 8.5 V, and a maximum power efficiency of 3.2 lm/W at
6 V. For complex 2, device A was turned on at 6.5 V with the
electroluminescence center at 532 nm and reached a maximum
brightness of 10 340 cd/m2 at 17 V, a maximum luminous
efficiency of 6.3 cd/A at 9.5 V, and a maximum power efficiency
of 2.1 lm/W at 6.5 V. Another kind of device with the config-
uration of [ITO/NPB (15 nm)/BA (30 nm)/1 or 2 (55 nm)/
LiF (0.5 nm)/Al (200 nm)] (device B) was also fabricated. With
omission of the Alq3 layer, here complexes 1 or 2 served as both
the light-emitting layer and the electron-transport layer. Inter-
estingly, the EL performance of device B, with a turn-on voltage
of 3.5 V, a maximum brightness of 16 930 cd/m2, and a maximum
luminous efficiency of 6.4 cd/A at 5 V for 1 and a turn-on voltage
of 2.5 V, a maximum brightness of 18 060 cd/m2, and a maximum
luminous efficiency of 5.4 cd/A at 5 V for 2, showed a notable
improvement. Device B with 2 as the emitter exhibits lower turn-
on voltage and higher maximum brightness compared to that
with 1 as the emitter. The power efficiencies of device B were
5.1 lm/W for 1 and 4.5 lm/W for 2, which are greatly improved
compared to those of device A (3.2 lm/W for 1 and 2.1 lm/W
for 2). This comparison clearly suggests that this kind of boron
complex is an efficient emitter with excellent electron-trans-
porting character. The EL spectra of devices A and B of each
complex were nearly the same, and they matched well with the
solid-state PL spectrum of the complex, as shown in Figure 7,
demonstrating that the EL luminescence originated from the
intrinsic emission of the complexes. We have not yet studied
the lifetime of the devices, but the unencapsulated devices
were stable and the emission color as well as IꢀLꢀV did not
change during measurement of the EL data in air. It is
noteworthy that the EL performance of complexes 1 and 2 is
at a high level in comparison with those previously reported
for boron-doped ladders,11,26 and the brightness and efficiency
of devices A and B are much higher than those of the reported
EL devices fabricated using other boron-containing emitter
materials.
Figure 9. TOF transients for electrons at room temperature: (a)
complex 1, D = 1.95 μm, E = 1.54 ꢁ 105 V/cm, and Tt = 4.0 μs; (b)
complex 2, D = 1.18 μm, E = 2.12 ꢁ 105 V/cm, and Tt = 0.67 μs. Insets:
Double-logarithmic plots of data in parts a and b.
ITO/complexes 1 or 2 (1ꢀ2 μm)/Al was carried out at room
temperature. Figure 9 displays typical TOF transients of elec-
trons for complexes 1 and 2 under an applied field. The tran-
sient photocurrent signals were dispersive, suggesting the pre-
sence of electron traps. According to the equation μ = D/ETt,
where D is the thickness of the transporting layer and E is the
electrical field strength, the electron mobilities of the complexes
on their vacuum-deposited films were determined to be 3.2 ꢁ
10ꢀ4 cm2/V s for complex 1 and 8.3 ꢁ 10ꢀ4 cm2/V s for
3
3
complex 2, which respectively are 30 and 80 times higher than
that of the commonly used electron-transporting material
Alq3 (μe =1ꢁ10ꢀ5 cm2/V s measured by means of the TOF
3
method).27
’ CONCLUSION
In this study, we have designed and synthesized four novel
diboron-containing π-conjugated ladders 1ꢀ4. The system was
prepared by a simple synthetic procedure, and further functio-
nalization was easily realized by introducing electron-withdraw-
ing/donating groups into different positions of the ladder
skeleton. Single-crystal structures determined by X-ray diffrac-
tion analysis demonstrate that bulky aryl substituents on the
boron centers prevent efficient π stacking. Characterizations of
these complexes demonstrate that the construction of diboron-
containing ladder-type skeletons endows these materials with
good thermal stability, high fluorescence quantum yields, and
strong electron affinity. Simple EL devices fabricated using
complexes 1 and 2 as both emitter and electron-transporting
materials exhibit the highest brightness and efficiency among
boron-containing materials reported so far.
Electron Mobility. To further affirm the electron-transport
ability, time-of-flight (TOF) carrier-mobility measurement
based on complexes 1 and 2 with a device configuration of
4830
dx.doi.org/10.1021/ic102554j |Inorg. Chem. 2011, 50, 4825–4831

Inorganic Chemistry
ARTICLE
’ ASSOCIATED CONTENT
(11) Zhang, Z. L.; Bi, H.; Zhang, Y.; Yao, D. D.; Gao, H. Z.; Fan, Y.;
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S
Supporting Information. Synthesis and characterization
b
of the starting materials, 1H NMR spectra, TGA and DSC curves
of 1ꢀ4, as well as crystallographic data for 1ꢀ3, X-ray crystal-
lographic data for 1ꢀ3 in CIF format, and Cartesian coordinates
of 1ꢀ4 at the S0-optimized geometry. This material is available
free of charge via the Internet at http://pubs.acs.org.
’ AUTHOR INFORMATION
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Corresponding Author
*E-mail: hongyuzhang@jlu.edu.cn.
’ ACKNOWLEDGMENT
This work was supported by the National Natural Science
Foundation of China (Grants 50903037, 20921003, and
50733002) and the Major State Basic Research Development
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