A mixed pyridine-phenol boron complex as an organic electroluminescent material

Article abstract of DOI:10.1039/b003774p

A blue light emitting 1,6-bis(2-hydroxyphenyl)pyridine boron complex has been synthesized and used as an emitting material to fabricate electroluminescent devices.

Full text of DOI:10.1039/b003774p

A mixed pyridine–phenol boron complex as an organic electroluminescent
material
Yanqin Li, Ya Liu, Weiming Bu, Jianhua Guo and Yue Wang*
Key Laboratory for Supramolecular Structure and Spectroscopy, Jilin University, Changchun 130023, P. R. China.
E-mail: yuewang@mail.jlu.edu.cn
Received (in Cambridge, UK) 8th May 2000, Accepted 7th July 2000
1
6
A blue light emitting 1,6-bis(2-hydroxyphenyl)pyridine
boron complex has been synthesized and used as an emitting
material to fabricate electroluminescent devices.
gallium is due to intermolecular p…p stacking interactions.
Munakata et al. confirmed that strong intermolecular inter-
actions as well as intermolecular aromatic stacking could assist
charge-transfer pathways.17 We believe that the strong inter-
molecular p…p interactions in solid (dppy)BF are beneficial in
terms of charge mobility.
3
The electroluminescence (EL) of Alq (q = 8-hydroxyquinoli-
nate) and its derivatives and also of other metal chelate
–14
complexes1 have attracted a great deal of attention since the
(dppy)BF exhibits strong blue emission at ca. 445 nm. The
first light-emitting devices (LEDs) fabricated from Alq
3
were
3
PL spectra of (dppy)BF in CHCl solution, the solid state and as
reported by Tang and VanSlyke.1 This report describes the
synthesis and EL properties of a 1,6-bis(2-hydroxyphenyl)pyr-
idine boron complex.
an evaporated film are compared in Fig. 2 while Fig. 3 shows
the PL and EL spectra of devices of structure [ITO/TPD (500
Å)/(dppy)BF (400 Å)/Al (2000 Å)].§ The EL spectrum are
dramatically different from the PL spectra of pure (dppy)BF or
solid TPD films, shifting to longer-wavelength (lmax = 550
nm). This suggests that exciplex formation takes place at the
organic solid-state interface between TPD and (dppy)BF,
resulting in a yellow emission. The devices exhibit a maximum
The pyridine–phenol ligand dppy and its boron complex were
prepared following the synthetic route shown in Scheme 1.†
The dppy ligand was obtained by the reaction of 2-bromopyr-
idine with the Grignard reagent from 2-bromoanisole in THF
with [NiCl
demethylation in molten pyridinium chloride. The reaction of
dppy with 1 equivalent of BF in benzene at 60 °C resulted in
2
(dppe)] as catalyst at room temperature, followed by
15
22
luminance (L) of 500 cd m with an efficiency of 0.0013 lm
2
1
21
3
w
and achieve a maximum efficiency of 0.01 lm W at 18
2
2
the formation of (dppy)BF. An X-ray diffraction study shows
that in the solid state (dppy)BF molecules exhibit inter-
molecular p…p interactions,‡ leading to columnar aromatic
stacks as shown in Fig. 1. The average intermolecular p…p
interaction distance is 3.28 Å. Our earlier report demonstrated
that the higher electron mobility of tris(8-hydroxyquinolinato)-
cd m . In order to investigate the relationship between the
interface state and EL properties of boron complexes, PVK (N-
vinylcarbazole) was employed as hole transport layer to
Scheme 1 Synthetic route towards dppy and (dppy)BF. Reagents and
conditions: i, THF, 25 °C; ii, [NiCl
h, 75%; iii, py·HCl, 200 °C, 3 h, 85%; iv, BF
0–85%.
2
(dppe)], 2,6-dibromopyridine, 25 °C, 12
3
, benzene, 60 °C, 5 h,
7
Fig. 2 The PL spectra of (dppy)BF in CHCl
evaporated film (3).
3
(+), the solid state (5) and
Fig. 3 EL and PL spectra for [ITO/TPD/(dppy)BF/Al] together with the PL
spectrum of an evaporated single layer film of (dppy)BF; the inset shows the
I–V (5) and L–V (+) characteristics for the devices.
Fig. 1 Intermolecular p…p interactions in (dppy)BF.
DOI: 10.1039/b003774p
Chem. Commun., 2000, 1551–1552
This journal is © The Royal Society of Chemistry 2000
1551

Notes and references
1
†
2
6
4
Mass, H NMR Spectroscopic and elemental analysis: dppy: EIMS: m/z
H 3
63. d (CDCl , 80 MHz) 9.41 (s 2H), 7.99–7.53 (m, 5H), 7.36–6.80 (m,
H). Anal. Calc. for C17
.6; N, 5.6%.
2
H13NO : C, 77.6; H, 4.9; N, 5.3. Found: C, 77.2; H
(
bpy)BF: EIMS: m/z 291. d
H 3
(CDCl , 80 MHz) 8.20–7.72 (m, 5H),
7
2
.57–6.88 (m, 6H). Anal.: Calc. for C17H11NO BF: C, 70.1; H, 3.8; N, 4.8.
Found: C, 70.3; H, 3.6; N, 4.5%.
‡
Crystal data: (dppy)BF: C17
H11NO
2
BF, M
r
= 291, monoclinic, space
1
group P2 /c, a = 7.5482(7), b = 7.9581, c = 22.422 Å, b = 98.227(9)°,
3
23
c
V = 1333.0(2) Å , Z = 4, D = 1.450 g cm , F(000) = 600. Reflection
data were collected on a Siemens R3 four-cicle diffractometer using
graphite-monochromated Mo-Ka radiation and w scans at room tem-
perature, giving 3365 unique reflections. The structure was solved by direct
methods (SHELXTL Version 5). All atoms were refined anisotropically
Fig. 4 The EL and PL spectra for [ITO/PVK/(dppy)BF/Al]; the inset shows
the I–V (5) and L–V (+) characteristics for the devices.
using full-matrix least squares to give R
F > 4s(F). CCDC 182/1716.
1
= 0.066 for 2340 reflections with
§
The devices of [ITO/TPD/(dppy)BF/Al] were fabricated by successive
fabricate emitting devices with structure [ITO/PVK (1000 Å)/
vacuum deposition of organic materials onto the ITO coated glass substrate.
Then a layer of aluminum was deposited onto the layer of (dppy)BF. For
devices [ITO/PVK/(dppy)BF/Al], the PVK layer was formed by spin
coating a 10 mg ml chloroform solution, and (dppy)BF was deposited on
the PVK layer by thermal evaporation, followed by deposition of the
aluminum cathode in vacuo.
(
dppy)BF (500 Å)/Al (2000 Å)].§ Fig. 4 presents the EL
spectrum of [ITO/PVK (1000 Å)/(dppy)BF (500 Å)/Al (2000
Å)] together with its PL spectrum. The EL spectrum exhibits a
strong peak at 450 nm accompanied by a weaker shoulder at 500
nm. The above experimental results suggest that there are very
little or no exciplex formation at the interface between PVK and
21
1 C. W. Tang and S. A. VanSlyke, Appl. Phys. Lett., 1987, 51, 913.
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(
dppy)BF. An alternative explanation may be electrolumines-
3
610.
cence of PVK is favored by the fact that (dppy)BF acts as an
electron transport material. When PVK was used as the hole
transport material, the devices showed a dramatic improvement
in performance. Each device exhibited a maximum luminance
of 400–600 cd m with an efficiency of 0.012 lm W at a
driving voltage of ca. 12.5 V and showed a maximum efficiency
of 0.1 lm W at a luminance of 26 cd m . The devices [ITO/
TPD/(dppy)BF/Al] and [ITO/PVK//(dppy)BF/Al] exhibited
obviously different EL properties. Our experimental results also
3
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determine the EL properties of organic light emitting devices.18
At present we have yet to elucidate the mechanism and further
investigation is in progress.
The complexes (dppy)B(OMe) and (dppy)B(OEt) have also
been synthesized and exhibited similar EL behavior as
1
1
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1
intermolecular p…p interactions in the solid state of (dppy)-
B(OEt) and detailed results will be reported in due course.
In conclusion, we have demonstrated that a pyridine–phenol
ligand can be used as to synthesize strong blue phtoluminescent
boron complexes. The pyridine–phenol boron complexes show
novel electroluminescent properties. We also found that the
interface state determines the EL color of the devices based on
the boron complexes.
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This work was supported by the National Natural Science
Foundation of China (No. 597905006).
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Chem. Commun., 2000, 1551–1552
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