A covalently bonded AlQ3/SiO2 hybrid material with blue light emission by a conventional sol-gel approach

Article abstract of DOI:10.1039/b515518e

We report the design and synthesis of a covalently bonded AlQ ₃/SiO₂ hybrid material with strong blue light emission by a sol-gel approach, which make AlQ₃ solution-processable and chemically stable. The Royal Society of Chemistry 2006.

Full text of DOI:10.1039/b515518e

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A covalently bonded AlQ3/SiO2 hybrid material with blue light emission
by a conventional sol–gel approach{
Hongyu Zeng, Weimin Huang and Jianlin Shi*
Received (in Cambridge, UK) 1st November 2005, Accepted 20th December 2005
First published as an Advance Article on the web 18th January 2006
DOI: 10.1039/b515518e
dispersion, and leaching and optical quenching of the dopants.
To overcome these shortages, we designed and synthesized an
organic–inorganic hybrid material with AlQ3 molecules covalently
linked to the silica matrix by C–Si bonding through a conventional
sol–gel approach, making the AlQ3 system highly solution
processable while maintaining its excellent luminescent characteri-
stics. Comparing to some AlQ3 containing polymer materials,16,17
this sol–gel route for covalently-bonded organic–inorganic hybrid
materials is of convenience, low cost, and may be adaptable to
many inorganic material hosts.
We report the design and synthesis of a covalently bonded
AlQ3/SiO2 hybrid material with strong blue light emission by a
sol–gel approach, which make AlQ3 solution-processable and
chemically stable.
Tris(8-hydroxyquinoline)aluminium (AlQ3) is one of the most
common light emission and electron transport materials in OLEDs
for next generation panel displays.1–3 Its luminescence can be blue-
or red-shifted through substitution on the 8-hydroxyquinoline
(HQ) ligand or the introduction of optically inactive spacer
molecules into the crystalline network of AlQ3.4,5 Though much
work has been done on the optical properties of AlQ3, it has to be
vacuum sublimed and deposited5,6 in device fabrication due to its
low solubility in solvents. This drawback limits its practical
applications and the problem remains unsolved to date. Another
drawback with the material is its instability in the presence of water
or oxygen. Currently the large scale applications of the materials
require a low cost and simple processing route, such as solution-
processing based ink-print techniques, for the fabrication of
OLEDs. One solution to this problem is to disperse AlQ3 in some
solution-processable matrix. Herein we report the first example of
an organic–inorganic hybrid material with AlQ3 molecules
covalently linked to the silica matrix by a conventional sol–gel
approach.
To make such a covalent bonding of AlQ3 to the inorganic silica
matrix, a new compound (denoted as SiNHQ) is synthesized as a
dual-functional ligand, which not only acts as a bridge to link the
organic and inorganic moieties, but also affects the luminescent
properties of AlQ3. The synthesis mainly contains three steps: 1st,
synthesis of a derivative (5-formyl-8-hydroxyquinoline, denoted as
FHQ{)18 from HQ with a certain active functional group; 2nd,
formation of a silylated HQ derivative ligand by grafting (3-
aminopropyl)triethoxysilane (APTES) onto FHQ; 3rd, processing
of the hybrid material by a conventional sol–gel process. This
synthesis procedure is illustrated in Scheme 1. A physically mixed
AlQ3–SiO2 material sample (PMAlQ) was prepared by the direct
mixing of HQ and Al3+ with TEOS and water in DMF, and then
following the same sol–gel procedure with HSiAlQ.
Organic–inorganic hybrid materials have received great interest
nowadays.7–9 The hybrid system combines the chemical and
thermal stability, the mechanical strength and the process flexibility
of the inorganic matrix, together with the functional characteristics
of active organic molecules. The sol–gel method, due to its low
processing temperature, which is required to retain the activity of
the organic moieties, has been proven to be a suitable approach for
the preparation of hybrid materials.7 Some hybrid materials
with rare earth ion complexes and dye molecules have been
reported.10–13 However, reports on AlQ3 organic–inorganic hybrid
materials are not found as AlQ3 has been reported to be unstable
in the presence of water and oxygen,14,15 while water exists
throughout the common sol–gel process.
The IR spectra for FHQ, SiNHQ, and HSiAlQ samples are
shown in ESI{ (Fig. S3). The silylation of the FHQ ligand is
supported by the following three bands: the sharp band at
1641 cm21 of CLN bond, with a red-shift from the CLO
absorption in FHQ (1662 cm21); a group of bands at 2888–
2971 cm21 of C–H vibration stretching on O–R groups, and the
Physical mixing of hybrid materials is easy to achieve, but in
such a mixture organic and inorganic moieties are linked together
by weak interactions (such as hydrogen bonds and van der Waals
forces), which may cause phase separation, inhomogeneous
State Key Laboratory of High-Performance Ceramics and Superfine
Microstructures. Shanghai Institute of Ceramics, Chinese Academy of
Sciences, Shanghai, 200050, China. E-mail: jlshi@sunm.shcnc.ac.cn;
Fax: +86-21-5241-3122
{ Electronic supplementary information (ESI) available: experimental
details, FT-IR spectra, UV-Vis spectra and the GC-MS spectra of SiNHQ.
See DOI: 10.1039/b515518e
Scheme 1 Synthesis procedure of HSiAlQ.
880 | Chem. Commun., 2006, 880–881
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the energy level of the p orbital, and thus increase the p–p*
transition band gap of the HQ ligands. Furthermore, according to
a previous report,4 the Si–O–Si network linked to Al(SiNHQ)3 in
the hybrid system may act as optically inactive spacer molecules,
reducing the packing density of AlQ3 and the direct intermolecular
interactions between the chromophore molecules, also causing a
blue-shift of the emission. Therefore, it is the silylation of the
ligand and the covalent bonding between the silica matrix and the
organic complex that affect the luminescent properties of AlQ3.
Also we can see a great intensity difference in the PL spectra
between HSiAlQ and PMAlQ. PMAlQ’s emission is negligible,
while HSiAlQ does show a strong blue emission though there is
only one fiftieth of HQ in the hybrid, and the PL intensity shows
no apparent change after being exposed in air for four weeks. The
most probable reason for the luminescent failure of PMAlQ is the
degradation of AlQ3 in the presence of water.14,19 The only
difference between PMAlQ and HSiAlQ is that there is a large
polar alkoxy silane end in the HSiAlQ system, so it can be
confirmed that it is the ligand’s chemical nature that makes
HSiAlQ stable in the presence of water. Furthermore, in HSiAlQ,
the complex is well dispersed due to the covalent bonding between
the matrix and the organic moiety, thus avoiding the leaching and
quenching effects in the system which may occur in PMAlQ. We
can expect the chemically stable HSiAlQ will greatly expand the
application of AlQ3 related materials.
Fig. 1 Photoluminescent spectra of HSiAlQ, PMAlQ and AlQ3 in the
solid state.
Table 1 Photoluminescent data of the samples
Excitation
max/nm
Solutiona
emission lmax/nm
Solid state
emission lmax/nm
Sample
l
SiNHQ
AlQ3
380
385
385
368
475
510
—
—
511
480–520
476
PMAlQ
HSiAlQ
a
470
1024 M DMF solution.
In conclusion, we have designed and synthesized the first
example of organic–inorganic hybrid materials with AlQ3
covalently bonded to a silica matrix through a conventional sol–
gel approach, making it a solution-processable material, and
determined its blue luminescent property together with its chemical
stability against water and oxygen.
Si–O–C stretching band of alkoxy silane located at 1090 cm21
.
While these three band cannot be seen in the FHQ spectrum,
suggesting that APTES has been grafted onto FHQ. Good
agreement between the calculated molecular weight of SiNHQ
(376.5) and the detected value (m/z: 376.1) by mass spectroscopy
also confirms the formation of this intermediate product. The C–H
vibration stretching in O–R groups disappeared from the HSiAlQ
spectrum, together with the emergence of a broad Si–O–Si
absorption band around 1010–1180 cm21, suggesting the full
hydrolysis/condensation reactions between SiNHQ and TEOS.
From the UV-Vis absorption spectra, we can find a slight red-
shift from 327 nm and 410 nm for FHQ to 331 nm and 415 nm for
SiNHQ, respectively, owing to the silylation of the ligand and the
substitution of CLO with CLN bond, and the absorption peak at
375 nm in the spectrum of HSiAlQ may be attributed to the p–p*
transition of the metal complex, indicating the complexation of the
ligands to Al3+.
Notes and references
{ 5-Formyl-8-hydroxyquinoline (FHQ) was synthesized and characterized
following a reported procedure.18
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Fig. 1 shows the photoluminescent spectra of HSiAlQ, PMAlQ
and AlQ3 respectively, and the related data are summarized in
Table 1.The solid state HSiAlQ shows a blue emission at 476 nm,
an apparent blue-shift by 35 nm from the AlQ3 PL spectrum
(511 nm), confirming the formation of the metal complex. On the
other hand, PMAlQ shows very weak light emission in the range
from 480 nm to 520 nm.
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It is known that the luminescence of the AlQ3 complex
molecules originates from the p–p* transitions on the quinolate
ligands. The filled p orbitals (HOMOs) are located on the
phenoxide side of the HQ ligands, and an electron-withdrawing
substitution at C-5 on the HQ ligand will cause a blue-shift.5 So
this blue-shift may be partly due to the CLN bond formation as
well as the silane grafting, which enhance the p conjugation, lower
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