Carbazolyl-contained phenol-pyridyl boron complexes: Syntheses, structures, photoluminescent and electroluminescent properties

Article abstract of DOI:10.1039/b925608c

A series of phenol-pyridyl boron complexes 1-4 bearing carbazolyl- containing groups on boron centres have been designed and synthesized. The single crystals of complexes 1, 2 and 4 were grown, and the crystal structures were determined by X-ray diffraction analysis. All complexes possess very high melting points (346-385 °C) and decomposition temperatures (T_d5: 397-423 °C), indicative of their high thermal stabilities. The electrochemical and photophysical properties as well as theoretical calculations were also investigated. Typical three-layer electroluminescent (EL) devices based on these organoboron complexes exhibited white electroluminescence. The Royal Society of Chemistry 2010.

Full text of DOI:10.1039/b925608c

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PAPER
www.rsc.org/dalton | Dalton Transactions
Carbazolyl-contained phenol-pyridyl boron complexes: syntheses, structures,
photoluminescent and electroluminescent properties†
Zuolun Zhang, Dandan Yao, Shanshan Zhao, Hongze Gao, Yan Fan, Zhongmin Su, Hongyu Zhang* and
Yue Wang
Received 4th December 2009, Accepted 25th March 2010
First published as an Advance Article on the web 20th April 2010
DOI: 10.1039/b925608c
A series of phenol-pyridyl boron complexes 1–4 bearing carbazolyl-containing groups on boron centres
have been designed and synthesized. The single crystals of complexes 1, 2 and 4 were grown, and the
crystal structures were determined by X-ray diffraction analysis. All complexes possess very high
melting points (346–385 ◦C) and decomposition temperatures (Td5: 397–423 ◦C), indicative of their
high thermal stabilities. The electrochemical and photophysical properties as well as theoretical
calculations were also investigated. Typical three-layer electroluminescent (EL) devices based on these
organoboron complexes exhibited white electroluminescence.
structures of phenol-pyridyl boron complexes with the aim of
developing more candidates for white-light OLEDs.
Introduction
Since the report of Tang and VanSlyke’s pioneering work in 1987,1
Herein, we present our investigations on the syntheses, X-ray
organic light-emitting devices (OLEDs) have attracted much
crystal structures, and thermal, electrochemical, photolumines-
interest because of their potential applications in next-generation
cent and electroluminescent properties of a series of carbazolyl-
full-colour flat-panel displays.2–4 To promote the development of
contained phenol-pyridyl boron complexes. The bulky carbazolyl-
OLEDs, much effort has been devoted to the design and synthesis
containing groups were chosen as the boron-bonded substituents
of various new EL materials.5–9 Among these materials, four-
because carbazole derivatives have been widely used as active
coordinate organoboron complexes have proved to be efficient
materials in the fabrication of various optoelectronic devices
emitters. The devices, based on this type of materials, showing
and the thermal stability or the glass-state durability of organic
emission colours ranging from blue to red, have been extensively
compounds could be greatly enhanced by the introduction of
investigated.10–13 However, the application of boron complexes for
white-light OLEDs was rarely reported. In our previous work, we
carbazole group.
found that some phenol-pyridyl boron complexes could be used
to fabricate bright non-doped white-light OLEDs with two- or
three-layer device structures.14 The white light originated from the
Experimental
General information
combination of the intrinsic blue emission of boron complexes and
the emission of the exciplex formed by the complexes and N,N¢-
di(1-naphthyl)-N,N¢-diphenyl-(1,1¢-biphenyl)-4,4¢-diamine (NPB,
a typical hole-transporting material). Compared with some other
approaches for achieving white-light emission, such as multilayer
device structures,15 polymer blends,16 polymer doped with red (R),
green (G) and blue (B) emitting dyes17 and microcavity structures,18
the adoption of exciplex emission enables the devices to possess
simple structures, and consequently, simple fabrication process
and high reproducibility. Considering the wide applications of
white-light (including serving as backlights in liquid crystal
displays and the light source for fabricating full-color displays
as well as general lighting),19 we tended to modify the molecular
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 ex-
periments were performed under a nitrogen atmosphere by using
standard Schlenk techniques. 1H NMR spectra were measured on
Bruker Avance 500 MHz (for 1, 2, 4 and 7) and Varian 300 MHz
(for 3, 8 and 9) spectrometers 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 NET-
ZSCH DSC204 instrument. Thermogravimetric analyses (TGA)
were performed on a TGA Q500 thermogravimeter. UV-vis ab-
sorption spectra were recorded using a PE UV-vis lambdazo spec-
trometer. The emission spectra were recorded using a Shimadzu
RF-5301 PC spectrometer. Tri-(8-hydroxyquinolino)aluminium
(Alq3) was purchased from Aldrich and purified by vacuum
sublimation. N,N¢-di(1-naphthyl)-N,N¢-diphenyl-(1,1¢-biphenyl)-
4,4¢-diamine (NPB),20 2,6-bis(2-hydroxylphenyl)pyridine (Dppy),
2,6-bis(2-hydroxyl-5-methylphenyl)pyridine (Mdppy),13b 4-(9H-
carbazol-9-yl)phenylboronic acid (5),21 and 4-(9H-carbazol-9-yl)-
4¢-iodobiphenyl (6)20 were synthesized according to the literature
procedures.
State Key Laboratory of Supramolecular Structure and Materials, Col-
lege of Chemistry, Jilin University, Changchun, 130012, P. R. China.
E-mail: hongyuzhang@jlu.edu.cn; Fax: +86-431-85193421; Tel: +86-431-
85168484
† Electronic supplementary information (ESI) available: 1H NMR spectra,
crystal packing structures of 4, DSC thermograms for the second heating
cycle, solid-state emission spectra, calculated frontier orbital energies;
EL spectra, J–V and L–V characteristics, as well as CIE coordinates
of the devices and crystallographic data for 1, 2 and 4. CCDC reference
numbers 757234–757236. For ESI and crystallographic data in CIF or
other electronic format see DOI: 10.1039/b925608c
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4-(3,6-Bis(t-butyl)-9H-carbazol-9-yl)-4¢-iodobiphenyl (7). 1,2-
Dichlorobenzene (30 mL) was added to a mixture of 3,6-bis(t-
butyl)carbazole (968 mg, 3.47 mmol), 4,4¢-diiodobiphenyl (5.63 g,
13.88 mmol), CuI (441 mg, 2.31 mmol), K2CO3 (2.87 g, 20.8 mmol)
and 18-crown-6 (367 mg, 1.39 mmol). After degassing via three
freeze-vacuum-thaw cycles, the reaction mixture was heated under
reflux for 11 h. After cooling to room temperature, the mixture was
filtered. The filtrate was concentrated by vacuum to yield a brown
residue which was purified by column chromatography (silica gel,
cyclohexane/dichloromethane = 4 : 1) to give 7 as a white solid
(1.60 g, 83% yield). 1H NMR (CDCl3, ppm): d 8.14 (s, 2 H), 7.82
(d, J = 8 Hz, 2 H), 7.75 (d, J = 8.5 Hz, 2 H), 7.63 (d, J = 8.5 Hz,
2 H), 7.46 (d, J = 8.5 Hz, 2 H), 7.42 (d, J = 8.5 Hz, 2 H), 7.39
(d, J = 9 Hz, 2 H), 1.47 (s, 18 H). Ms m/z: 557 [M]+ (calcd: 557).
Anal. calcd (%) for C32H32IN: C 68.94, H 5.79, N 2.51. Found: C
68.76, H 5.81, N 2.53.
(calcd: 514). Anal. calcd (%) for C35H23BN2O2: C 81.72, H 4.51, N
5.45. Found: C 81.54, H 4.68, N 5.28.
Complex 2. In the same manner as described for 1. The
reaction of ligand Mdppy (349 mg, 1.20 mmol), compound 5
(379 mg, 1.32 mmol) and triethylamine (0.50 mL, 3.60 mmol)
provided 2 as light-yellow solid (429 mg, 66% yield).1H NMR
(CDCl3, ppm): d 8.09–8.05 (m, 3 H), 7.86 (d, J = 8 Hz, 2 H), 7.52
(s, 2 H), 7.36 (d, J = 8 Hz, 2 H), 7.31–7.27 (m, 4 H), 7.23–7.18 (m,
6 H), 7.05 (d, J = 8.5 Hz, 2 H), 2.34 (s, 6 H). Ms m/z: 542 [M]+
(calcd: 542). Anal. calcd (%) for C37H27BN2O2: C 81.93, H 5.02, N
5.16. Found: C 81.78, H 5.22, N 5.08.
Complex 3. In the same manner as described for 1. The
reaction of Dppy (108 mg, 0.41 mmol), compound 8 (163 mg,
0.45 mmol) and triethylamine (0.17 mL, 1.23 mmol) provided 3 as
light-yellow solid (181 mg, 75% yield).1H NMR (DMSO, ppm): d
8.48–8.37 (m, 3 H), 8.24 (d, J = 7.8 Hz, 2 H), 8.09 (d, J = 7.8 Hz, 2
H), 7.77 (d, J = 8.7 Hz, 2 H), 7.60 (d, J = 8.7 Hz, 2 H), 7.45–7.36
(m, 8 H), 7.28 (t, J = 7.2 Hz, 2 H), 7.13 (d, J = 8.4 Hz, 2 H),
7.01–6.95 (m, 4 H). Ms m/z: 590 [M]+ (calcd: 590). Anal. calcd
(%) for C41H27BN2O2: C 83.40, H 4.61, N 4.74. Found: C 83.52, H
4.50, N 4.55.
4-(9H-Carbazol-9-yl)-4¢-biphenylboronic acid (8). To a solu-
tion of 6 (801 mg, 1.80 mmol) in THF (60 mL) at -78 ◦C, n-BuLi
(1.35 mL, 1.6 M in cyclohexane, 2.16 mmol) was added dropwise.
◦
After addition, the mixture was kept at -78 C for 30 min, and
then warmed to room temperature for 1 h. The mixture was cooled
to -78 ◦C again, and 1.0 mL (9.0 mmol) of B(OCH3)3 was added.
After stirring at -78 ◦C for 30 min, the mixture was warmed
to room temperature and stirred overnight. A saturated aqueous
solution of NH4Cl (100 mL) was added to the mixture, and then
the mixture was extracted by CH2Cl2. The CH2Cl2 extract was
evaporated to dryness, and the resulting residue was separated
by column chromatography (silica gel, CH2Cl2/CH3OH = 50 : 1).
The strongly blue-emissive fraction was collected and evaporated
to dryness. The obtained solid was dissolved in THF (10 mL),
which was then dropwise added to dilute hydrochloric acid (pH 5)
to afford white precipitate. The precipitate was filtered and dried
to give compound 8 (405 mg, 62% yield). 1H NMR (CDCl3, ppm):
d 8.45 (d, J = 8.1 Hz, 2 H), 8.18 (d, J = 7.8 Hz, 2 H), 7.95 (d, J =
8.4 Hz, 2 H), 7.89 (d, J = 8.1 Hz, 2 H), 7.72 (d, J = 8.4 Hz, 2 H),
7.54–7.43 (m, 4 H), 7.32 (t, J = 6.9 Hz, 2 H). Anal. calcd (%) for
C72H48B3N3O3·1.5H2O: C 81.38, H 4.84, N 3.95. Found: C 81.44,
H 5.08, N 3.68.
Complex 4. In the same manner as described for 1. The
reaction of Dppy (252 mg, 0.96 mmol), compound 9 (504 mg,
1.06 mmol) and triethylamine (0.40 mL, 2.88 mmol) provided 4 as
light-yellow solid (425 mg, 63% yield). 1H NMR (CDCl3, ppm): d
8.14–8.11 (m, 3 H), 7.89 (d, J = 8.5 Hz, 2 H), 7.73 (d, J = 8 Hz, 2
H), 7.63 (d, J = 8.5 Hz, 2 H), 7.49 (d, J = 8.5 Hz, 2 H), 7.44–7.36
(m, 6 H), 7.33 (d, J = 8.5 Hz, 2 H), 7.29 (d, J = 8.5 Hz, 2 H), 7.14
(d, J = 8 Hz, 2 H), 6.93 (t, J = 8 Hz, 2 H), 1.45 (s, 18 H). Ms m/z:
702 [M]+ (calcd: 702). Anal. calcd (%) for C49H43BN2O2: C 83.75,
H 6.17, N 3.99. Found: C 83.55, H 6.15, N 3.94.
X-Ray diffraction
Diffraction data were collected on
a Rigaku RAXIS-
PRID diffractometer using the w-scan mode with graphite-
monochromator MoKa radiation. The structures 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.
4-(3,6-Bis(t-butyl)-9H-carbazol-9-yl)-4¢-biphenylboronic acid
(9). In the same manner as described for 8. Compound 7
(650 mg, 1.17 mmol) was treated with n-BuLi (0.88 mL, 1.6 M
in cyclohexane, 1.40 mmol) and subsequent trimethyl borate
(0.65 mL, 5.85 mmol) to provide 9 as white solid (295 mg, 53%
yield). 1H NMR (CDCl3, ppm): d 8.44 (d, J = 7.8 Hz, 2 H), 8.17 (s,
2 H), 7.94–7.88 (m, 4 H), 7.70 (d, J = 8.4 Hz, 2 H), 7.51–7.43 (m,
4 H), 1.49 (s, 18 H). Anal. calcd (%) for C96H96B3N3O3·4.5H2O: C
79.34, H 7.28, N 2.89. Found: C 79.45, H 7.45, N 2.60.
Electrochemical measurements
Cyclic voltammetry was performed on a BAS 100 W instrument
with a scan rate of 100 mV s-1. 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/Ag+ electrode as the reference electrode. A 0.1 M solution
of tetrabutylammonium perchlorate (TBAP) in DMF was used as
the supporting electrolyte.
Complex 1. A mixture of ligand Dppy (334 mg, 1.27 mmol),
compound 5 (401 mg, 1.40 mmol) and triethylamine (0.53 mL,
3.81 mmol) in benzene (30 mL) was heated under reflux for
15 h. The reaction mixture was evaporated to dryness, and the
resulting residue was purified by column chromatography (silica
gel, cyclohexane/dichloromethane = 1 : 3) to give 1 as light-yellow
Fluorescence quantum yield measurements
The fluorescence quantum yields (UF) in solution were measured
1
solid (480 mg, 74% yield). H NMR (CDCl3, ppm): d 8.10–8.05
relative to quinine sulfate in sulfuric acid aqueous solution (lexc
=
(m, 3 H), 7.87 (d, J = 8 Hz, 2 H), 7.73 (d, J = 8 Hz, 2 H), 7.42
(t, J = 8.5 Hz, 2 H), 7.36 (d, J = 8 Hz, 2 H), 7.28 (d, J = 4 Hz, 4
H), 7.21–7.16 (m, 6 H), 6.94 (t, J = 8 Hz, 2 H). Ms m/z: 514 [M]+
365 nm, UF = 0.546) at room temperature. For complexes 1–4,
the longest absorption maximum was chosen as the excitation
wavelength. The quantum yields were calculated using previously
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known procedures.9b The solid-state UF values were measured
using a calibrated integrating sphere in terms of the previously
reported procedures.22
Theoretical calculations
The ground state geometries were optimized by the density
functional theory (DFT)23 method with the Becke three-parameter
hybrid exchange and the Lee–Yang–Parr correlation functional24
(B3LYP) and 6-31G* basis set using the Gaussian 03 software
package.25 The excitation energies were investigated by time-
dependent density functional theory (TD-DFT).
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
-1
˚
layers were deposited onto the substrate at a rate of 1–2 A s . After
the organic film deposition, LiF and aluminium 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.
Results and discussion
Syntheses and crystal structures
The synthetic procedures for 1–4 are shown in Scheme 1. The
boronic acid compounds 5, 8 and 9 could be readily obtained
from phenyl or biphenyl halides by treatment with n-BuLi and
subsequent trimethyl borate. Complexes 1–4 were obtained in a
moderate to high yield by refluxing a mixture of boronic acid
compounds and ligand Dppy or Mdppy in benzene. The molecular
Scheme 1 Synthetic procedures for complexes 1–4. Conditions: (i) Dppy,
triethylamine, benzene; (ii) Mdppy, triethylamine, benzene; (iii) 1. n-BuLi,
THF; 2. B(OCH3)3; 3. NH4Cl (aq).
1
structures of these complexes were characterized by H NMR,
mass spectrometry and elemental analysis.
To further confirm the structures of these complexes and
investigate the solid-state packing features, we tried to get the
single crystals of these complexes by diffusing petroleum ether
vapour into their solutions. We have already obtained the single
crystals of complexes 1, 2 and 4. The ORTEP drawings of
complexes 1 and 2 are shown in Fig. 1. For all these complexes,
the conformation of (Dppy)B or (Mdppy)B moiety is very similar
to that of our previously reported boron complexes bearing
corresponding ligands.14a The boron atoms in all complexes are
four coordinated and adopt a typical tetrahedral geometry, and the
B–N, B–O and B–C bond lengths are very similar to those of our
previously reported boron complexes. The ligand parts exhibit a
slight torsion from co-planar structure, the torsion angles between
pyridine and phenol rings are listed in Table S3 (ESI).† The major
difference in the molecular structures of these complexes lies in the
different torsion angles of C(19)–C(18)–B(1)–N(1) and dihedral
angles between carbazole and benzene rings, as can be seen in
Table S3.†
Fig. 1 ORTEP drawings of 1 and 2 with 30% thermal ellipsoids.
bonding interactions. Two such chains are packed together by
another type of C–H ◊ ◊ ◊ O hydrogen bond, thus a double-chain
structure is formed (Fig. 2). There are no intermolecular p–p
interactions in the packing structures. As for the crystal of 2,
a molecular chain is formed through the C–H ◊ ◊ ◊ O hydrogen
bonding interaction similar to that in the crystal of 1, and the
In the packing structures of 1, the molecules are arranged into
a one-dimensional molecular chain through C–H ◊ ◊ ◊ O hydrogen
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Fig. 2 (a) Crystal packing structures of 1, green line: C(28)–H(28) ◊ ◊ ◊ O(1)
◦
˚
hydrogen bond (C ◊ ◊ ◊ O = 3.46 A, C–H ◊ ◊ ◊ O = 151.3 ), yellow line:
◦
˚
C(2)–H(2) ◊ ◊ ◊ O(1) hydrogen bond (C ◊ ◊ ◊ O = 3.55 A, C–H ◊ ◊ ◊ O = 154.7 );
(b) crystal packing structures of 2, green line: C(31)–H(31) ◊ ◊ ◊ O(2)
◦
˚
hydrogen bond (C ◊ ◊ ◊ O = 3.42 A, C–H ◊ ◊ ◊ O = 147.6 ), yellow line:
˚
C(15)–H(15) ◊ ◊ ◊ p hydrogen bond (C ◊ ◊ ◊ p-ring centroid ◦
=
3.48 A,
˚
C ◊ ◊ ◊ p-ring plane = 3.45 A, C–H ◊ ◊ ◊ p-ring centroid = 146.9 ).
chains are connected together by C–H ◊ ◊ ◊ p hydrogen bonding
interactions (Fig. 2). Intermolecular p–p interactions also could
not be observed. For the crystal of 4, molecules are cross-
packed into a chain structure by C–H ◊ ◊ ◊ p interactions and the
adjacent chains are packed together through another type of
C–H ◊ ◊ ◊ p interactions and very weak p–p interactions between
the ligand moieties (ESI, Fig. S8).† A common feature of all
these crystals is that there are no strong p–p interactions in the
packing structures, which is different from the case in the crystal
of phenol-pyridyl boron complex with 2-methoxyphenyl or 1-
naphthyl substituent.14a Such a feature should be attributed to
the steric bulk of carbazolyl-containing groups, and it would be
effective in preventing fluorescence quenching in the solid state.
Fig. 3 TGA (a) and DSC (b) curves of 1–4 at a heating rate of 10 ◦C min-1.
TGA are 397, 399, 404 and 423 ◦C, respectively. Such a high
thermal stability is very rare for organoboron complexes.
Electrochemical properties
The electrochemical properties of complexes 1–4 were studied
by cyclic voltammetry in DMF. Their cyclic voltammograms are
shown in Fig. 4. Complexes 1–3 exhibited an irreversible oxidation
wave, while complex 4 with tert-butyl groups on carbazolyl
ring showed a quasi-reversible oxidation. The oxidation onset
potentials for complexes 1–4 are +0.78, +0.76, +0.80 and +0.67 V,
respectively. According to our previous research, the (Dppy)B
and (Mdppy)B moieties are very difficult to be oxidized in
DMF,14a thus the oxidations of these complexes are most likely
related to the carbazolyl part. The oxidation potentials of 1
and 3 are in close proximity, indicating that the length of the
linker between carbazolyl and (Dppy)B moieties has little impact
on the oxidation potentials. From the comparison between the
potentials of 3 and 4, it could be observed that the tert-butyl
groups on carbazolyl can lower the oxidation potential by about
0.11 V. This effect of tert-butyl groups should be attributed to their
electron-donating properties that make the oxidation easier. The
cathodic reductions of all these complexes are quasi-reversible.
The reduction onset potentials of 1–4 are -1.95, -1.97, -1.96 and
-1.97 V, respectively, which indicates that the reductions should be
associated with (Dppy)B or (Mdppy)B moieties.14a It is reasonable
Thermal properties
Differential scanning calorimetry (DSC) and thermogravimetric
analysis (TGA) were carried out to check the thermal properties
of 1–4 under a nitrogen atmosphere. The DSC and TGA curves of
these complexes are shown in Fig. 3. The melting points (Tm) of
complexes 1–4 are very high, being 357, 346, 385 and 374 ◦C,
respectively. These values are obviously higher than those of
previously reported phenol-pyridyl boron complexes bearing 2-
methoxyphenyl or 1-naphthyl group on the boron centre.14a The
much higher melting points of these complexes may be related to
their significantly higher molecular weights. On the second heating
cycles in DSC experiments, a glass transition process starting at
◦
the temperatures (Tg) of 156, 125 and 162 C could be observed
for complexes 2, 3 and 4, respectively, and no evidence for a
glass transition of complex 1 was detected (ESI, Fig. S9).† The
high Tm and Tg values might serve as an advantage when these
complexes are used in EL devices since the high Tm and Tg values
could improve the lifetime of the devices.11c,26 The decomposition
temperatures with a 5% weight loss (Td5) for 1–4 determined by
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Table 1 Calculated photophysical data of 1–4
Oscillator
strength f Main contribution
Transition
l
exc/nma
1
2
3
4
S0 → S1
S0 → S2
S0 → S1
S0 → S2
S0 → S1
S0 → S2
S0 → S1
S0 → S2
421
391(385)
420
403 (393) 0.128
406 0.025
391 (383) 0.129
419 0.003
393 (384) 0.150
0.005
0.158
0.019
HOMO → LUMO (69%)
HOMO - 2 → LUMO (67%)
HOMO → LUMO (66%)
HOMO - 1 → LUMO (66%)
HOMO → LUMO (62%)
HOMO - 1 → LUMO (62%)
HOMO → LUMO (69%)
HOMO - 2 → LUMO (67%)
Fig. 4 Cyclic voltammograms of 1–4: (a) anodic runs; (b) cathodic runs.
that the reduction potentials of these complexes are very close,
since the ligand parts of these complexes are very similar.
a Values in parentheses are experimental longest absorption maxima.
Theoretical calculations
Photophysical properties
To further understand the electronic structures of these complexes
and examine the orbitals involved in the electronic transitions, we
carried out TD-DFT calculations. For all these complexes, the
HOMO is localized on the boron-bonded cabazolyl-containing
moiety, while the LUMO is localized on the ligand part (Fig. 6).
Both the calculated HOMO and LUMO energy levels show a
trend consistent with that of the energy levels obtained from the
electrochemical data (ESI, Table S4).† The excitation of these
complexes to S1 state corresponds to the intramolecular charge-
transfer (ICT) transition from HOMO → LUMO (Table 1).
However, the S0 → S1 transitions are weakly allowed with
oscillator strengths (f ) smaller than 0.025, and the calculated
excitation wavelength for this transition is largely red-shifted
compared with the experimental longest absorption maximum. By
contrast, the S0 → S2 transitions are much stronger (f > 0.128),
and the excitation energies agree well with the experimental data.
These results indicate that the experimentally observed lowest-
The absorption and fluorescence spectra of 1–4 in CHCl3 are
shown in Fig. 5. The longest absorption maxima of 1, 3 and 4 are
very close, located at 385, 383 and 384 nm, respectively, indicating
that the carbazolyl-containing substituents on boron centres have
little impact on the lowest-energy absorption band. Methyl groups
on the ligand of complex 2 (labs = 393 nm) cause a slight absorption
red shift of this complex compared with 1.
Fig. 5 UV-vis absorption and emission spectra of 1–4 in CHCl3 (10-5 M).
The emission maxima of complexes 1, 3 and 4 are very close
and located in the blue region, being 456, 455 and 453 nm,
respectively. Complex 2 (lem = 473 nm) with methyl groups on
the ligand has an obviously red-shifted emission compared with 1.
The above variation trend in emission maxima of these complexes
is consistent with that observed in the longest absorption maxima.
The fluorescence quantum yields (UF) of 1–4 in dilute chloroform
solution are 0.13, 0.09, 0.13 and 0.12, respectively. Methyl groups
in complex 2 lead to a decreased quantum yield of this complex
as compared with that of 1. There is no obvious solvent effect
on the fluorescence spectra of these complexes, for example, the
emission maxima of 1 in cyclohexane, chloroform and acetone are
454, 456 and 458 nm, respectively (ESI, Fig. S10).† In the solid
state, complexes 1–4 also show blue emission with the emission
peaks at 451, 468, 450 and 449 nm, respectively (ESI, Fig. S11).†
Their solid-state UF values are 0.12 for 1, 0.07 for 2, 0.10 for 3,
and 0.08 for 4. These values don’t show significant decrease from
corresponding quantum yields in solution, which should be owed
to the nonexistence of strong interchromophore p–p interactions.
Fig. 6 Calculated frontier orbitals for 1–4.
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Table 2 Data of electroluminescent devices
Device
V
turn-on/Va
L
max(Cd/m2)b
E
max(Cd/A)c
V/Vb
E(Cd/A)d
CIEd
A1
A2
A3
A4
5.4
6.0
6.0
4.5
2338 (13)
1507 (14)
1245 (13)
990 (12.5)
1.40 (213, 8.5)
0.68 (454, 11)
0.65 (128, 9)
0.70 (64, 7.5)
10
1.21
0.58
0.47
0.22
0.28, 0.36
0.25, 0.34
0.29, 0.37
0.28, 0.34
12.5
11.5
12.5
a Recorded at 1 Cd/m2. b Values in parentheses are corresponding voltages (V). c Values in parentheses are corresponding EL brightness (Cd/m2) and
voltages (V). d Recorded at about 1000 Cd/m2.
white region. When the driving voltage was further increased,
the EL spectra as well as the CIE coordinates (ESI, Table S5)†
only exhibited a very small variation and the device maintained
white electroluminescence with significantly increased brightness.
Devices A1–A4 could reach a maximum brightness of 2338 Cd/m2
at 13 V, 1507 Cd/m2 at 14 V, 1245 Cd/m2 at 13 V, and 990 Cd/m2
at 12.5 V, respectively. The maximum brightness (2338 Cd/m2)
and current efficiency (1.40 Cd/A) of device A1 were obviously
higher than those of the other devices. In addition, at a brightness
of about 1000 Cd/m2, device A1 had a lower driving voltage and
an obviously higher current efficiency. These results suggest the
better EL properties of complex 1, which may be related to its
relatively higher solid-state UF value.
Fig. 7 Structure of the EL device (left) and photograph of device A1 at a
driving voltage of 11 V (right).
energy absorption band mainly corresponds to the S2 state, which
has a main contribution from HOMO - 2 → LUMO for 1 and 4,
and HOMO - 1 → LUMO for 2 and 3. As can be seen in Fig. 6,
the HOMO - 2 for 1 and 4, as well as HOMO - 1 for 2 and 3, is
localized on the ligand part. According to above results, it could
be concluded that the lowest-energy absorption of complexes 1–4
arises predominantly from the intraligand p → p* transition rather
than the ICT transition. This transition characteristic should be
responsible for the nonexistence of obvious solvent effect on the
emission spectra.
Conclusion
Four carbazolyl-contained phenol-pyridyl boron complexes were
synthesized. X-Ray analysis showed that there are no strong
interchromophore p–p interactions in the solid state. They possess
very high thermal stabilities and show blue emission both in
solution and solid state. Theoretical calculations indicate that
intraligand p → p* transitions contribute predominantly to the
lowest-energy absorption bands of these complexes. White-light
OLEDs were realized through the combination of complexes’
intrinsic blue mission and exciplexes’ yellow emission. Complex
1 showing much better EL properties than the other complexes
and is the most promising material as an emitter.
Electroluminescent properties
To evaluate the EL properties of these complexes, typical three-
layer devices with the configuration of [ITO/NPB (40 nm)/1–
4 (60 nm)/Alq3 (10 nm)/LiF (0.5 nm)/Al (200 nm)] (devices
A1–A4) in which NPB was used as hole-transporting materials,
complexes 1–4 acted as emitters, and Alq3 served as electron-
transporting materials were fabricated (Fig. 7). The device data
are listed in Table 2. The turn-on voltages of devices A1–A4 were
between 4.5 V and 6 V. The EL spectra of these devices were
very broad, ranging from 400 to 780 nm. At a low driving voltage
of 7 V, the EL spectra of all these devices exhibited a peak at
yellow region and a shoulder at blue region, and consequently,
the devices displayed a yellowish-white emission (ESI, Fig. S12).†
According to our previous research on the devices of other phenol-
pyridyl boron complexes, the shoulder at short-wavelength region
should be attributed to the intrinsic emission of boron complexes,
while the emission at long-wavelength region should originate
from the exciplex formed by the boron complexes and NPB.14
When the applied voltage was increased from 7 V to 10 V, the
intensity of the shoulder gradually increased (ESI, Fig. S12),†
and the emission colour of the devices turned white. At 10 V,
the CIE coordinates of (0.28, 0.36) for A1, (0.26, 0.37) for A2,
(0.30, 0.39) for A3, and (0.28, 0.36) for A4 were within the
Acknowledgements
This work was supported by the National Natural Science
Foundation of China (50733002, 50903037), the Major State Basic
Research Development Program (2009CB623600), the 863 Project
(2006AA03A162) and 111 Project (B06009).
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