Synthesis, structures, and luminescent properties of phenol-pyridyl boron complexes

Article abstract of DOI:10.1021/ic051881j

Syntheses of the four mixed phenol-pyridine derivatives 1,6-bis(2-hydroxyphenyl)pyridyl boron naphthalene (1), 1,6-bis(2-hydroxy-5-methylphenyl)pyridyl boron naphthalene (2), 1,6-bis(2-hydroxyphenyl)pyridyl boron 2-methoxylbenzene (3), and 1,6-bis(2-hydro

Full text of DOI:10.1021/ic051881j

Inorg. Chem. 2006, 45, 2788−2794
Synthesis, Structures, and Luminescent Properties of Phenol
Boron Complexes
−Pyridyl
Hongyu Zhang, Cheng Huo, Kaiqi Ye, Peng Zhang, Wenjing Tian, and Yue Wang*
Key Laboratory for Supramolecular Structure and Materials of Ministry of Education, College of
Chemistry, Jilin UniVersity, Changchun 130012, People’s Republic of China
Received October 31, 2005
Syntheses of the four mixed phenol−pyridine derivatives 1,6-bis(2-hydroxyphenyl)pyridyl boron naphthalene (1),
1,6-bis(2-hydroxy-5-methylphenyl)pyridyl boron naphthalene (2), 1,6-bis(2-hydroxyphenyl)pyridyl boron 2-methoxyl-
benzene (3), and 1,6-bis(2-hydroxy-5-methylphenyl)pyridyl boron 2-methoxylbenzene (4) are reported. The structures
of the boron compounds 1, 3, and 4 were determined by single-crystal X-ray diffraction. The molecular packing is
characterized by intermolecular π‚‚‚π and hydrogen-bonding interactions. DSC analysis demonstrates that 1 and
2 have good thermal stability with higher glass transition temperatures (Tg) and melting points (Tm) than 3 and 4.
Boron complexes 1
−4 display bright blue luminescence in solution and the solid state. White and blue
electroluminescent (EL) devices were fabricated successfully using these boron compounds.
Introduction
and electroluminescent (EL) properties of boron-containing
compounds.13 In view of the important potential applications
of boron-containing compounds in OLEDs, we have explored
the synthesis of novel four-coordinate boron complexes that
contain the mixed phenol-pyridyl functional group.14-16
These boron complexes are easy to handle in conventional
vacuum deposition equipment. The following properties are
Polypyridines are widely used as polydentate chelating
ligands that play a major role in the current intense interest
in coordination chemistry. The coordination chemistry of
polydentate chelating ligands that contain mixed pyridine-
phenol donor sets has been a very popular target of study
and is a possible extension to the chemistry of polypyridines.
Ward and co-workers performed early studies of the com-
plexes based on the mixed-donor polydentate ligands.1-4
Recently, much effort has been invested in the design and
synthesis of boron-containing compounds because of their
potential applications in functional materials, including
organic light-emitting devices (OLEDs), nonlinear optics,
fluorescent and fluoride ion sensors, and biomolecular
probes.5-12 Wang and co-workers have systematically re-
ported the syntheses, structures, and photoluminescent (PL)
(6) (a) Wu, Q. G.; Wu, G.; Brancaleon, L.; Wang, S. Organometallics
1999, 18, 2553. (b) Liu, S.-F.; Wu, Q.; Schmider, H. L.; Aziz, H.;
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S. Organometallics 2000, 19, 5709. (d) Jia, W.-L.; Song, D.; Wang,
S. J. Org. Chem. 2003, 68, 701. (e) Jia, W. L.; Moran, M. J.; Yuan,
Y.-Y.; Lu, Z. H.; Wang, S. J. Mater. Chem. 2005, 15, 3326.
(7) (a) Yuan, Z.; Taylor, N. J.; Marder, T. B.; Williams, I. D.; Kurtz, S.
K.; Cheng, L.-T. J. Chem. Soc., Chem. Commun. 1990, 1489. (b) Yuan,
Z.; Collings, J. C.; Taylor, N. J.; Marder, T. B.; Jardin, C.; Halet,
J.-F. J. Solid State Chem. 2000, 154, 5. (c) Entwistle, C. D.; Marder,
T. B.; Smith, P. S.; Howard, J. A. K.; Fox, M. A.; Mason, S. A. J.
Organomet. Chem. 2003, 680, 165.
* To whom correspondence should be addressed. E-mail: yuewang@
jlu.edu.cn.
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2788 Inorganic Chemistry, Vol. 45, No. 7, 2006
10.1021/ic051881j CCC: $33.50
© 2006 American Chemical Society
Published on Web 01/13/2006

Phenol-Pyridyl Boron Complexes
Scheme 1. Synthetic Procedure for Boron Compounds 1-4
strongly required or indispensable for molecular EL materi-
als: They must (a) form a uniform thin film, (b) have a
carrier (hole or electron) transport ability, (c) have a high
fluorescent yield, (d) be stable to heat (have a high glass
transition temperature, Tg), and (e) have suitable HOMO/
LUMO levels for carrier injections. To improve the perfor-
mance of the boron compounds, we modified the boron
molecules by attaching aromatic groups to the boron center.
Herein, we report the details of our investigation on synthetic
efforts; structures; and thermal, PL, and EL properties of a
series of pyridine-phenol boron complexes.
Experimental Section
Materials. All starting materials were purchased from Aldrich
Chemical Co. and used without further purification. Solvents were
freshly distilled over appropriate drying reagents. All experiments
were carried out under a dry nitrogen atmosphere using standard
Schlenk techniques unless otherwise stated. The synthetic proce-
dures of four-coordinate boron compounds 1-4 are shown in
Scheme 1. N,N′-di(1-naphthyl)-N,N′-diphenyl-(1,1′-biphenyl)-4,4′-
diamine) (NPB),17 4,4′-N,N′-dicarbazolebiphenyl (CBP),17 2,6-bis-
(2-hydroxylphenyl)pyridine (H2L1), and 2,6-bis(2-hydroxyl-5-
methylphenyl)pyridine (H2L2)15 were synthesized according to the
literature procedures. Aromatic-substituted boronic acids naphth-
ylboronic acid (NBA) and 2-methoxylphenylboronic acid (MPBA)
were prepared in high yield in a method analogous to the synthesis
of 4-(dimethylamino)-naphthalene boronic acid (DMANBA) and
phenylboronic acid.18 Tri(8-hydroxyquinolino)aluminum (Alq3) was
purchased from Aldrich and purified by vacuum sublimation.
was heated to reflux for 10 h. A light yellow solid precipitated
from the solution. The solid product was collected by filtration and
purified by recrystallization and sublimation to give a light yellow
1
Instrumentation. H NMR spectra were recorded on Bruker
1
solid (1.18 g, yield 80%). H NMR (CDCl3): δ (ppm) 9.12 (d, J
AVANCE 500-MHz spectrometer with tetramethylsilane as the
internal standard. Mass spectra were recorded on a GC/MS mass
spectrometer. UV-vis absorption spectra were obtained on a PE
UV-vis lambdazo spectrometer. Emission spectra were recorded
with a Shimadzu RF-5301 PC spectrometer. Elemental analyses
were performed on a flash EA 1112 spectrometer. The EL spectra,
luminance, and Commission Internationale del l’Eclairage (CIE)
coordinates of the devices were recorded on a PR650 spectrometer.
The melting points were determined on a Fisher-Johns melting point
apparatus. Differential scanning calorimetric (DSC) measurements
were performed on a NETZSCH DSC204 instrument.
) 8.0 Hz, 1H, naph), 8.12 (t, J ) 8.0 Hz, 1H, py), 7.86 (d, J ) 8.0
Hz, 2H, py), 7.64-7.70 (t, J ) 7.0 Hz, 3H, naph, ph), 7.56 (t, J )
8.0 Hz, 1H, naph), 7.49 (d, J ) 8.0 Hz, 1H, naph), 7.39 (t, J ) 7.0
Hz, 1H, naph), 7.19 (t, J ) 7.0 Hz, 2H), 6.94 (t, J ) 8.0 Hz, 3H,
naph, ph), 6.80 (t, J ) 7.5 Hz, 2H), 6.32 (d, J ) 5.0 Hz, 1H, naph).
MS m/z: 399 [M]+. Anal. Calcd (%) for C27H18BNO2: C, 81.22;
H, 4.54; N, 3.51. Found: C, 81.05; H, 4.75; N, 3.47. X-ray-quality
crystals were grown by slow diffusion of diethyl ether vapor into
a CHCl3 solution of 1.
1,6-Bis(2-hydroxy-5-methylphenyl)pyridyl Boron Naphtha-
lene (2). In the same manner as described for 1, the reaction of
NBA (0.80 g, 4.6 mmol), H2L2 (1.38 g, 4.7 mmol), and NEt3 (1.7
1,6-Bis(2-hydroxyphenyl)pyridyl Boron Naphthalene (1).
NBA (0.60 g, 3.5 mmol), H2L1 (0.95 g, 3.6 mmol), and NEt3 (1.5
mL) were dissolved in 30 mL of benzene, and the reaction mixture
1
mL) provided 2 as a light yellow solid (1.05 g, yield 53%). H
NMR (CDCl3): δ (ppm) 9.09 (d, J ) 8.5 Hz, 1H, naph), 8.09 (t,
J ) 8.0 Hz, 1H, py), 7.84 (d, J ) 8.5 Hz, 2H, py), 7.65 (d, J ) 8.0
Hz, 1H, naph), 7.54 (t, J ) 7.5 Hz, 1H, naph), 7.48 (d, J ) 8.5 Hz,
1H, naph), 7.42 (s, 2H, ph), 7.38 (t, J ) 7.5 Hz, 1H, naph), 6.99
(d, J ) 8.5 Hz, 2H, ph), 6.94 (t, J ) 7.5 Hz, 1H, naph), 6.82 (d,
J ) 8.0 Hz, 2H, ph), 6.33 (d, J ) 6.0 Hz, 1H, naph), 2.20 (s, 6 H).
MS m/z: 427 [M]+. Anal. Calcd (%) for C29H22BNO2: C, 81.51;
H, 5.19; N, 3.28. Found: C, 81.35; H, 5.12; N, 3.47.
(12) (a) Yuan, Z.; Taylor, N. J.; Ramachandran, R.; Marder, T. B. Appl.
Organomet. Chem. 1996, 10, 305. (b) Yamaguchi, S.; Shirasaka, T.;
Tamao, K. Org. Lett. 2000, 2, 4129.
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Tao, Y.; Diorio, M.; Wang, S. Chem. Mater. 2000, 12, 79. (b) Liu,
Q.; Mudadu, M. S.; Schmider, H.; Thummel, R.; Tao, Y.; Wang, S.
Organometallics 2002, 21, 4743. (c) Jia, W. L.; Feng, X. D.; Bai, D.
R.; Lu, Z. H.; Wang, S.; Vamvounis, G. Chem. Mater. 2005, 17, 164.
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Funct. Mater. 2005, 15, 143.
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Mater. 2000, 12. 2672. (b) Chen, J. S.; Ma, D. G.; Liu, Y.; Wang, Y.
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2000, 1551.
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Lett. 2001, 78, 3497.
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10, 2235.
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A.; Haag, R. J. Org. Chem. 2002, 67, 9452.
1,6-Bis(2-hydroxyphenyl)pyridyl Boron 2-Methoxylbenzene
(3). In the same manner as described for 1, the reaction of MPBA
(0.50 g, 3.3 mmol), H2L1 (0.89 g, 3.4 mmol), and NEt3 (1.5 mL)
1
provided 3 as a light yellow solid (0.82 g, yield 66%). H NMR
(CDCl3): δ (ppm) 8.04 (t, J ) 7.5 Hz, 1H, py), 7.79 (d, J ) 8.0
Hz, 2H, py), 7.68 (d, J ) 8.0 Hz, 2H, ph), 7.34 (t, J ) 8.5 Hz, 2H,
ph), 7.22 (d, J ) 7.0 Hz, 1H, ph), 7.09 (d, J ) 7.5 Hz, 2H, ph),
7.00 (t, J ) 8.0 Hz, 1H, ph), 6.87 (t, J ) 8.0 Hz, 2H, ph), 6.69 (t,
J ) 7.5 Hz, 1H, ph), 6.55 (d, J ) 8.0 Hz, 1H, ph), 3.28 (s, 3H,
methoxyl). MS m/z: 379 [M]+. Anal. Calcd (%) for C24H18BNO3:
Inorganic Chemistry, Vol. 45, No. 7, 2006 2789

Zhang et al.
Table 1. Crystal Data for Compounds 1, 3, and 4
C, 76.01; H, 4.78; N, 3.69. Found: C, 76.27; H, 4.75; N, 3.57.
X-ray-quality crystals were grown by slow diffusion of diethyl ether
vapor into a CHCl3 solution of 3.
1
3
4
formula
fw
crystal system
space group
a (Å)
b (Å)
c (Å)
R (deg)
â (deg)
γ (deg)
V (Å)
C54H36B2N2O4 C25H19BCl3NO3 C31H27BN2O3
1,6-Bis(2-hydroxy-5-methylphenyl)pyridyl Boron 2-Meth-
oxylbenzene (4). In the same manner as described for 1, the reaction
of MPBA (0.60 g, 3.9 mmol), H2L2 (1.16 g, 4.0 mmol), and NEt3
(1.5 mL) provided 4 as a light yellow solid (0.97 g, yield 61%).
1H NMR (CDCl3): δ (ppm) 7.97 (t, J ) 8.0 Hz, 1H, py), 7.74 (d,
J ) 8.0 Hz, 2H, py), 7.44 (s, 2H, ph), 7.19 (d, J ) 7.5 Hz, 1H,
ph), 7.13 (d, J ) 8.5 Hz, 2H, ph), 6.99 (t, J ) 8.0 Hz, 1H, ph),
6.97 (d, J ) 8.5 Hz, 2H, ph), 6.68 (t, J ) 7.0 Hz, 1H, ph), 6.55 (d,
J ) 8.0 Hz, 1H, ph), 3.30 (s, 3H, methoxyl), 2.28 (s, 6 H, methyl).
MS: m/z: 407 [M]+. Anal. Calcd (%) for C26H22BNO3: C, 76.68;
H, 5.44; N, 3.44. Found: C, 76.59; H, 5.58; N, 3.47. X-ray-quality
crystals were grown by slow diffusion of diethyl ether vapor into
a pyridine solution of 4.
798.47
monoclinic
P21/c
498.57
triclinic
P1h
486.36
monoclinic
P2/n
7.626(2)
25.956(5)
13.249(3)
90
104.34(3)
90
2540.8(9)
4
15.033(3)
27.353(6)
10.260(2)
90
108.50(3)
90
9.309(2)
11.609(2)
12.000(2)
65.96(3)
74.95(3)
75.20(3)
1127.2(4)
2
4000.9(14)
4
Z
Dc (g cm-3
)
1.326
1.469
1.271
27.48
5946
5801
θ
max (deg)
27.48
8247
27.44
5060
no. of reflns meads
no. of reflns used
8084
5060
no. of parameters
559
298
334
R(int)
0.0546
0.02
0.04
Electrochemical Measurements. Redox potentials were mea-
sured by cyclic voltammetry using a three-electrode configuration
on a BAS 100W instrument with scan rates of 100 mV/s. The
working electrode was a platinum wire, and a platinum wire and a
saturated calomel electrode (SCE) served as the counter and
reference electrodes, respectively. A 0.1 M solution of tetrabutyl-
ammonium perchlorate (TBAP) in DMF was used as the supporting
electrolyte and was flushed with N2 prior to the measurements to
avoid oxygen contamination.
final R indices [I > 2d(I)]
R1
wR
R indices (all data)
R1
0.049
0.104
0.043
0.122
0.044
0.091
0.187
0.16
0.715
0.095
0.146
0.807
0.158
0.142
0.767
wR2
GOF on F2
Table 2. Selected Bond Lengths (Å) and Angles (deg) for Compounds
1, 3, and 4
PL Quantum Yield Measurements. Room-temperature lumi-
nescence quantum yields were measured at a single excitation
wavelength (360 nm) referenced to quinine sulfate in sulfuric acid
aqueous solution (Φ ) 0.546). The quantum yields were calculated
using previously known procedures.19 The excitation wavelength
was 346 nm for all boron compounds 1-4.
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, and deionized water,
followed by air-plasma treatment. The devices were prepared in a
vacuum at a pressure of 5 × 10-6 Torr, and the organic materials
were deposited onto indium tin oxide at a deposition rate of 1-2
Å/s. Aluminum electrodes were thermally evaporated onto the
organic surface, resulting in active areas of ∼5 mm2. The thick-
nesses of the organic materials and cathode layers were controlled
using a quartz crystal thickness monitor. The electrical character-
istics of the diode were measured with a Keithley 2400 source
meter.
1
3
4
B(1)-O(1)
1.447(4)
1.459(4)
1.594(5)
1.614(5)
1.339(4)
1.349(4)
1.352(4)
1.376(4)
1.442(3)
1.477(3)
1.605(3)
1.617(4)
1.348(3)
1.343(3)
1.357(3)
1.358(3)
1.478(3)
1.460(3)
1.613(3)
1.624(4)
1.354(3)
1.357(3)
1.371(3)
1.374(3)
B(1)-O(2)
B(1)-N(1)
B(1)-C(18)
O(2)-C(17)
O(1)-C(1)
N(1)-C(11)
N(1)-C(7)
O(1)-B(1)-O(2)
O(1)-B(1)-N(1)
O(2)-B(1)-N(1)
O(1)-B(1)-C(18)
O(2)-B(1)-C(18)
N(1)-B(1)-C(18)
C(17)-O(2)-B(1)
C(1)-O(1)-B(1)
C(11)-N(1)-C(7)
C(11)-N(1)-B(1)
C(7)-N(1)-B(1)
O(1)-C(1)-C(2)
O(1)-C(1)-C(6)
105.0(3)
105.4(2)
105.3(2)
107.9(3)
106.6(3)
113.3(3)
114.3(3)
109.3(3)
116.2(3)
118.8(3)
122.8(3)
118.3(3)
118.8(3)
118.8(4)
120.3(4)
109.1(2)
104.9(2)
114.0(2)
111.3(2)
111.6(2)
114.4(2)
122.6(2)
121.4(2)
116.6(2)
122.2(2)
117.9(2)
122.1(2)
106.2(2)
108.7(2)
111.0(2)
113.2(2)
111.9(2)
122.9(2)
115.9(2)
120.7(2)
122.0(2)
117.3(2)
119.4(3)
121.0(2)
X-ray Diffraction. Diffraction data were collected on a Rigaku
RAXIS-PRID diffractometer using the ω-scan technique with
graphite-monochromated Mo‚KR (λ ) 0.71073 Å) radiation. The
structures were solved by direct methods and refined by the full-
matrix least-squares technique using the SHELXTL programs.
Anisotropic displacement parameters were applied to all nonhy-
drogen atoms, hydrogen atoms were assigned isotropic displacement
coefficients. Crystal data and details of data collection and
refinement for compounds 1, 3, and 4 are summarized in Table 1.
transport properties.20,21 To understand the relationship
between the properties and structures of functional materials,
the crystal structures of the synthesized complexes were
investigated. The crystallographic data for 1, 3, and 4 are
given in Table 1. Selected bond lengths and angles are listed
in Table 2. Even though all of the boron compounds have
similar structures, they crystallize in different space groups.
1 belongs to the monoclinic space group P21/c, 3 belongs to
the triclinic space group P1h, 4 belongs to the monoclinic
Results and Discussion
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(b) Ma, C. Q.; Zhang, L. Q.; Zhou, J. H.; Wang, X. S.; Zhang, B. W.;
Cao, Y.; Bugnon, P.; Schaer, M.; Nuesch, F.; Zhang, D. Q.; Qiu, Y.
J. Mater. Chem. 2002, 12, 3481.
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N.; Sironi, A. J. Am. Chem. Soc. 2000, 122, 5147. (b) Antolini, L.;
Tedesco, E.; Barbarella, G.; Favaretto, L.; Sotgiu, G.; Zambianchi,
M.; Casarini, D.; Gigli, G.; Cingolani, R. J. Am. Chem. Soc. 2000,
122, 9006.
Single-Crystal Structures. The solid-state structures of
molecular functional materials have a pronounced impact on
their physical properties, especially luminescent and charge-
(19) 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 2004, 109,
8008.
2790 Inorganic Chemistry, Vol. 45, No. 7, 2006

Phenol-Pyridyl Boron Complexes
Figure 3. (a) Molecular structure with labeling scheme and (b) stereoview
of the crystal packing of 4.
is chelated in a tridentate fashion by ligand (L1 for 1 and 3
and L2 for 4, respectively) via O1, O2, and N1; the remaining
site is occupied by the aromatic group. In all of the
compounds, each ligand (L1 or L2) is slightly distorted from
planarity with inter-ring torsion angles between the pyridyl
ring and the phenolate rings in the range of 4.46-20.07°
(14.45° and 19.53° for 1, 7.59° and 20.07° for 3, and 4.46°
and 16.53° for 4). The molecular packing of the boron
compounds in the solid state exhibits interesting features.
The crystal packing diagram given in Figure 1b shows
that 1 has a chain arrangement in the solid state. Within each
chain, there is a π-π stacking interaction with a pyridyl/
phenolate ring contact distance of 3.40 Å that is longer than
that in solid (dppy)BF (3.28 Å);15 the increase in the π-π
interaction distance might be due to the large steric hindrance
of the naphthyl group. Two molecular chains are linked by
a C-H‚‚‚O hydrogen bond at a distance of 2.465 Å, resulting
in a chain dimmer. For molecular semiconducting materials,
strong intermolecular aromatic stacking interactions can
enhance the charge-transfer ability.22 Our earlier reports also
demonstrated that the higher electron mobility of tris(8-
Figure 1. (a) Molecular structure with labeling scheme and (b) stereoview
of the crystal packing of 1.
Figure 2. Molecular structure with labeling scheme of 3.
space group P2/n, The molecular structures of 1, 3, and 4
are very similar to each other. The ORTEP drawings of 1,
3, and 4 are given in Figures 1a, 2, and 3a, respectively. For
1, in the asymmetric unit, there are two independent
molecules that display the same structure; the B-N, B-O,
and B-C distances and angles for the two molecules are
very similar to each other. For 3 and 4, the two structures
contain one crystallographically independent molecule. The
boron centers in all of the compounds are four-coordinate
and adopt a typical tetrahedral geometry. Each boron center
hydroxyquinolinato)gallium23 and Be(pp)2 is due to inter-
14
molecular π-π stacking interactions. It is possible that the
strong intermolecular π-π stacking interaction accompanied
by a hydrogen bond in solid 1 is beneficial in terms of charge
(22) (a) Munakata, M.; Wu, L. P.; Sowa, T. K.; Maekawa, M.; Suenaga,
Y.; Ning, G. L.; Kojima, T. J. Am. Chem. Soc. 1998, 120, 8610. (b)
Cornil, J.; Calbert, J-P.; Beljonne, D.; Silbey, R.; Bredas, J. L. AdV.
Mater. 2000, 12, 978.
(23) Wang, Y.; Zhang, W. X.; Li, Y. Q.; Ye, L.; Yang, G. D. Chem. Mater.
1999, 11, 530.
Inorganic Chemistry, Vol. 45, No. 7, 2006 2791

Zhang et al.
Table 3. Thermal and Redox Properties for Compounds 1-4
E
(V)
red/2
EHOMO
(eV)
ELUMO
(eV)
band gap
(eV)
Tg
(°C)
Tm
(°C)
compd
1
2
3
4
-2.01
-2.13
-2.08
-2.07
-5.62
-5.42
-5.54
-5.49
-2.84
-2.72
-2.77
-2.78
2.78
2.70
2.77
2.71
142
139
309
312
227
208
104
mobility. In 3 and 4, the molecular packing modes are very
similar, with two molecules arrayed in a face-to-face fashion
by π‚‚‚π interactions at distances of 3.342 Å for 3 and 3.299
Å for 4, resulting in the formation of molecular “dimmers”.
In 3, the dimmers are isolated from each other, and no other
short intermolecular contacts exist between the dimmers.
However, in 4, the dimmers are linked via hydrogen-bonded
contact C4-H4‚‚‚O3 at a distance of 2.713 Å and an angle
of 156.54° (Figure 3b).
Figure 4. DSC curves of compound 1 at a heating rate of 5 °C/min.
Thermal Behaviors. The glass transition temperatures (Tg)
and melting point temperatures (Tm) of compounds 1-4 were
determined by differential scanning calorimetry (DSC). A
heating rate of 5 °C/min was used after initial melting of
the compounds and subsequent rapid cooling to room
temperature under ambient conditions. These compounds
constitute a new class of amorphous molecular materials with
Tg values of 142, 139, and 104 °C for 1, 2, and 4,
respectively. DSC shows no evidence of a glass transition
for 3. The Tg values significantly increase in the order phenyl
< naphthyl; the results are in agreement with the concept
that the introduction of a rigid moiety into nonplanar
molecules increases the Tg value.24 Tm values of 309, 312,
227, and 208 °C were observed for compounds 1-4,
respectively, and increase in the same order (phenyl <
naphthyl) as Tg. The high Tm and Tg values are promising in
view of the thermal stability of thin films in EL devices.
The Tg and Tm values of all of the compounds are listed in
Table 3.
Among these compounds, 1 and 4 were found to exhibit
crystal polymorphism in addition to the formation of
amorphous glasses. Figure 4 shows the DSC curves of 1.
When the crystalline sample (crystal 1a) obtained by vacuum
sublimation was heated from room temperature, it melted at
321 °C to give an isotropic liquid. When the isotropic liquid
was cooled by standing in air, it changed into an amorphous
glassy state via a supercooled liquid. When the amorphous
glassy sample was again heated, an endothermic phenomenon
was observed, and a glass transition took place at 142 °C.
Then, a broad exothermic peak was observed around 236
°C, probably due to a solid-solid phase transition to form a
different crystal form (crystal 1b).5d Upon further heating,
crystal 1b melted at 309 °C to give an isotropic liquid. No
crystallization was observed on further heating, and the same
DSC trace was observed repeatedly. Figure 5 shows the DSC
curves of 4. When a crystalline sample (crystal 4a) obtained
by vacuum sublimation was heated, it melted at 208 °C and
then instantly crystallized to form another crystal (crystal
4b), which melted at 241 °C to give an isotropic liquid. When
Figure 5. DSC curves of compound 4 at a heating rate of 5 °C/min.
the isotropic liquid was cooled by standing in air, it changed
into an amorphous glassy state via a supercooled liquid.
When the amorphous glassy sample was again heated, an
endothermic phenomenon was observed, and a glass transi-
tion took place at 104 °C. Then, a broad exothermic peak
due to crystallization (crystal 4a) was observed around 183
°C, followed by an endothermic peak due to the melting at
208 °C. No crystallization was observed on further heating,
and the same DSC trace was observed repeatedly.
Redox Properties and Energy Levels. The electrochemi-
cal properties of compounds 1-4 were investigated by cyclic
voltammetry. All of these compounds display a reversible
reduction peak in DMF at about -2.0 V, which is believed
to originate from the reduction of the phenol-pyridyl boron
moiety. No oxidation potentials could be obtained for this
class of compounds in DMF. The half-wave reduction
red
1/2
potentials (E ) are almost all the same, being -2.01,
-2.13, -2.08, and -2.07 V vs Ag/Ag+ (0.01 M) for 1-4,
respectively. The results indicate that the electron-accepting
properties of the compounds are almost the same as those
of Alq3 (Ered ) -2.01 V vs Ag/Ag+), suggesting that 1-4
1/2
might have electron-transport properties. Primary mobility
measurements demonstrated that 1-4, indeed, exhibit electron-
transporting properties. The detailed experimental results will
be published elsewhere. When the scan was repeated in the
region between -2.5 and 1.0 V vs Ag/Ag+ (0.01 M), the
same trace was obtained. Using the reduction potentials of
1-4 and the absorption edges of the UV-vis spectra, we
(24) (a) Higuchi, A.; Inada, H.; Kobata, T.; Shirota, Y. AdV. Mater. 1991,
3, 549. (b) Okumoto, K.; Ohara, T.; Shirota, Y. Synth. Met. 2001,
121, 1655.
2792 Inorganic Chemistry, Vol. 45, No. 7, 2006

Phenol-Pyridyl Boron Complexes
Figure 6. EL cell structures of devices A-C.
obtained the HOMO and LUMO energy levels, and the
resulting data are also summarized in Table 3.
Photoluminescent Properties. The optical properties of
boron compounds 1-4 were measured both in chloroform
solution and in the solid state. The absorbance and emission
spectra of compounds 1-4 in chloroform solution were
measured at room temperature. All of these compounds show
essentially similar absorption bands around 280-289, 338-
352, and 382-393 nm. Upon irradiation by UV light (λext
) 365 nm), compounds 1-4 exhibit bright blue emissions
in chloroform. The emission maxima of compounds 1-4 in
chloroform are at 461, 478, 459, and 479 nm, respectively.
These emissions are attributed to the intraligand π f π*
electron transition of L1 or L2. The emissions of compounds
2 and 4 with two electron-donating substituents (methyl
groups) are red-shifted by 17 and 20 nm compared to those
of 1 and 3, respectively, reflecting the important role played
by the methyl group in the phenol-pyridyl ligand. Emission
quantum yields were measured for compounds 1-4. Among
these compounds, 1 has a quantum yield 0.30, significantly
higher than those of 2 (0.21), 3 (0.16), and 4 (0.14). The
low quantum yields of 3 and 4 are most likely due to the
thermal motion of the conformationally mobile methoxy
group, which results in energy loss via nonradiative decay.13b,13d
In the solid state, the emissions of 1-4 are found at 454,
466, 449, and 462 nm, respectively. The emission spectra
of 1-4 in the solid state exhibit slight blue shifts compared
to those of 1-4 in solution. This phenomenon is probably
due to luminescence rigidochromism.25
Electroluminescent Properties. Three types of devices
with structures of [ITO/NPB (50 nm)/1-4 (50 nm)/LiF (1
nm)/Al (200 nm)] (device A), [ITO/NPB (50 nm)/1-4 (40
nm)/Alq3 (10 nm)/LiF (1 nm)/Al (200 nm)] (device B), and
[ITO/NPB (40 nm)/CBP (10 nm)/1-4 (40 nm)/Alq3 (10 nm)/
LiF (1 nm)/Al (200 nm)] (device C) were fabricated (Figure
6). Figure 7 shows the molecular structures of the materials
used in this study. The EL performance data for the devices
are summarized in Table 4 for comparison.
The EL spectra of device A-1 are broad, ranging from
400 to 680 nm, which contains a blue emission peak and a
orange one and completely covers the whole wavelength
region of the visible spectrum; that is, white-light emission
was obtained. It was demonstrated that high-performance
white-light EL devices can be obtained by the rational
combination of a blue emission and an orange emission.26-28
Figure 7. Molecular structures of Alq3, NPB, and CBP.
Table 4. Performance of OLEDs of Compounds 1-4
b
c
d
Ta
(V)
Lmax
EImax
EPmax
CIEe
(x, y)
device
compd
(Cd m2)
(Cd/A)
(lm/W)
A-1
A-2
A-3
A-4
B-1
B-2
B-3
B-4
C-1
C-2
C-3
C-4
1
2
3
4
1
2
3
4
1
2
3
4
3.0
3.2
3.8
4.7
4.1
4.7
3.5
4.5
4.8
5.7
5.5
6.5
1016
792
813
1.68
1.47
1.35
1.27
2.39
1.71
2.01
1.62
1.55
0.82
1.01
0.71
0.68
0.59
0.53
0.52
1.17
0.79
0.95
0.7
0.87
0.43
0.57
0.31
0.30, 0.37
0.28, 0.39
0.26, 0.36
0.22, 0.33
0.26, 0.38
0.27, 0.44
0.27, 0.37
0.20, 0.33
0.17, 0.19
0.21, 0.27
0.18, 0.19
0.19, 0.29
648
1852
1323
1537
1271
1171
527
494
445
a Turn-on voltage, defined as the voltage needed for a brightness of 1
Cd/m2. b Maximum luminance. c Maximum current efficiency. d Maximum
luminance efficiency. e CIE coordinates at a driving voltage of 8.0 V.
Table 4 shows the color coordinates of devices A and B, for
example, and the values (0.28, 0.39) and (0.26, 0.38) deviate
from the ideal CIE chromaticity coordinates for pure white
light, (0.33, 0.33), this variation is attributed to the lack of
sufficient red emission from these devices. In other words,
these devices exhibit blue-white light emissions. The EL
spectral component around 450 nm can be attributed to 1 or
NPB. The EL band beyond the blue region is due to exciplex
emissions. Electroluminescent devices using four-coordinate
boron compounds as emitters can produce exciplex emissions
and were demonstrated previously by us and other group.13a,16
The EL spectral profile and the CIE coordinates (0.30, 0.38)
remained almost unchanged when the driving voltage was
varied. The EL spectra of device B-1 change dramatically
as a function of the applied bias. At high driving voltage
(g9 V), the device EL spectra resembled that of device A-1.
At low driving voltage (4-8 V), the green emission intensity
was stronger than the blue emission. This observation
suggests that the electron-hole recombination zone in this
device is field-dependent. Clearly, emission from device B-1
is more pronounced with increasing driving voltage, indicat-
ing that CIE coordinates depend on driving voltage. For
example, the CIE coordinates of device B-1 with an Alq3
thickness of 10 nm are (0.27, 0.40) and (0.25, 0.35) at 6 and
10 V, respectively.
(25) Kalyanasundaram, K. Photochemistry of Polypyridine and Porphyrin
Complexes; Academic Press: London, 1992; pp 322-323.
(26) Li, G.; Shinar, J. Appl. Phys. Lett. 2003, 83, 5359.
(27) Shao, Y.; Yang, Y. Appl. Phys. Lett. 2005, 86, 1.
(28) Li, J. Y.; Liu, D.; Ma, C.; Lengyel, O.; Lee, C.-S.; Tung, C.-H.; Lee,
S. AdV. Mater. 2004, 16, 1538.
Inorganic Chemistry, Vol. 45, No. 7, 2006 2793

Zhang et al.
To investigate the exciplex emission properties of the
NPB/1 system, solid thin films deposited on quartz substrates
were employed to record the emissions spectra. The PL
spectra of thin films of NPB and 1 show blue emissions with
peaks at 443 and 465 nm. In contrast, the PL spectrum of a
blend thin film of NPB/1 (ca. 1:1 w/w) composite centered
at 520 nm and red-shifted by 55 and 77 nm relative to those
of neat films of 1 and NPB, respectively. The PL spectra of
solid thin films confirm that the interaction between NPB
and 1 molecules can result in exciplex emission.
that the white-emitting devices in the B series are more
efficient and have higher luminances than the white-emitting
devices in the A series. We attribute the high luminance and
EL efficiency in the three-layer device to enhanced electron
transport and efficient radiative carrier recombination. In the
C series, the blue-emitting device C-1 exhibits a maximum
luminance of 1171 Cd/m2 at 11 V, a maximum current
efficiency of 1.55 Cd/A, and CIE coordinates of (0.17, 0.19).
The present study shows that the synthesized new amorphous
boron compounds 1-4 can function as emitters as well as
electron-transport materials for white- and blue-emitting
materials in organic EL devices. 1 exhibits better EL
performance in all three series of devices than 2-4.
Considering the possibility of exciplex emission between
a layer of 1 and a layer of NPB, a second hole-transporting
layer of CBP (4,4′-N,N′-dicarbazolebiphenyl) was inserted
between NPB and 1. In this case, the exciplex emission
disappeared. Therefore, whereas exciplex emission occurs
at the interface NPB and 1, no exciplex emission takes place
at the interface of CBP and 1. This can be attributed to the
proper HOMO energy level of CBP that is favorable for hole
injection into the 1 layer to give an EL response that is
dominated by emission of 1 at about 460 nm. The CBP layer
provides an intermediate level for holes to transport through
to the 1 layer. Two layers of hole-transporting materials NPB
and CBP were used, as it was demonstrated previously that
CBP serves to provide an intermediate HOMO level by
which the holes can pass to the TPBI layer.29
Conclusions
The novel four-coordinate boron-containing amorphous
molecular materials 1-4 have been synthesized. Single-
crystal X-ray diffraction demonstrates that the molecular
packing is sensitive to the aromatic groups attached to the
boron center, although the molecular structures are very
similar to each other among compounds 1, 3, and 4. They
readily form stable amorphous glasses with high Tg values
and form uniform amorphous thin films by vacuum deposi-
tion. Compounds 1-4 show strong blue photoluminescence
in solution and the solid state, and the luminescence of the
boron compounds is caused by π f π* electronic transitions
centered on the phenol-pyridine ligands (L1 and L2). The
naphthyl-substituted compound 1 appears to be the most
promising as an emitter for EL displays.
The performance of the OLEDs of these materials is
summarized in Table 4. The emissions from the white-
emitting devices started at turn-on voltages of 3.0-4.7 V
for devices of the A and B series. In the A series, the white-
emitting device A-1 exhibits a maximum luminance of 1016
Cd/m2 at a driving voltage of 14 V (202 mA/cm2), a current
efficiency of 1.68 Cd/A at a luminance of 192 Cd/m2, and a
turn-on voltage as low as 3.0 V. With regard to the white-
emitting devices in the B series, B-1 exhibits better perfor-
mance, with a maximum luminance of 1852 Cd/m2 at a
driving voltage of 14 V (415 mA/cm2), a maximum luminous
efficiency of 1.17 lm/W, and a maximum current efficiency
of 2.39 Cd/A at a luminance of 152 Cd/m2. It is obvious
Acknowledgment. This work was supported by the
National Natural Science Foundation of China (50225313
and 50520130316), the Major State Basic Research Develop-
ment Program (2002CB613401), and the Program for
Changjiang Scholars and Innovative Research Team in
University (IRT0422).
Supporting Information Available: Crystallographic data in
CIF and PDF formats, Tables S1-S15, Figures S1-S12. This
material is available free of charge via the Internet at
http://pubs.acs.org.
(29) Tao, Y. T.; Balasubramaniam, E.; Danel, A.; Tomasik, P. Appl. Phys.
Lett. 2000, 77, 933.
IC051881J
2794 Inorganic Chemistry, Vol. 45, No. 7, 2006