1
766
Macromolecules 2003, 36, 1766-1768
functional groups.1
3-16
Furthermore, ROMP is often a
Design a n d Syn th esis of Alq
P olym er s
3
-F u n ction a lized
living polymerization method resulting in polymers with
controlled molecular weights and low polydispersities
and also allows for the formation of block copoly-
Am y Meyer s a n d Ma r cu s Weck *
1
4,17,18
mers.
School of Chemistry and Biochemistry and the Molecular
Design Institute, Georgia Institute of Technology,
Atlanta, Georgia 30332-0400
The synthesis of the monomer (Scheme 1) began with
the functionalization of the norbornene 1, formed using
a Diels-Alder reaction between allyl bromide and
cyclopentadiene. Attachment of a bromoalkyl chain
using Grignard chemistry followed by the conversion of
the bromide to the nitrile and subsequent reduction of
the nitrile resulted in the precursor 4 in an overall yield
of 51%. Compound 4 was then coupled to 6,19 followed
by the reduction of the resulting imine to yield monomer
8.
Received December 2, 2002
Revised Manuscript Received J anuary 27, 2003
Aluminum tris(8-hydroxyquinoline) (Alq3) is one of
the most stable and fluorescent solid-state materials,
making it the emission and electron-transport layer of
1
-5
choice in organic light-emitting diodes (OLEDs).
One
limitation associated with Alq3 is its poor processability.
The current trend in the fabrication of OLEDs is
solution-processing; however, Alq3 must be vacuum-
The formation of the Alq3-functionalized monomer 9
was achieved by adding monomer 8 to 10 equiv of
triethylaluminum followed by 20 equiv of 8-hydroxy-
quinoline (Scheme 2). This resulted in the formation of
1 equiv of 9 and 9 equiv of nonfunctionalized Alq3. This
procedure was developed to ensure full metalation of
each monomer without coordination of two monomer
units onto the same aluminum center, thereby prevent-
6
,7
deposited. One possible solution to this problem is the
use of polymers containing Alq3 pendant groups. These
polymers could combine the fluorescent properties of
Alq3 while maintaining the processability of a polymer,
allowing for the low-cost manufacturing techniques such
6
20
as solution-processing and possibly ink-jet printing.
ing any cross-linking during the polymerization. The
Herein, we report the design and synthesis of such a
polymer-supported Alq3.
9:1 mixture (Alq3:9) was used directly in the polymer-
izations, which were carried out in chloroform at room
temperature using the ruthenium catalyst 11. A 50:1
monomer-to-catalyst ratio was fully polymerized within
12 h. After complete polymerization, the excess Alq3 was
removed from the polymer through extensive washings
with methanol and methylene chloride, yielding a
polymer without any impurities. Solubility of the poly-
mer proved to be limited. However, solubility could be
increased by copolymerizing 9 with 5-nonylnorbornene
10, a nonfunctionalized monomer, which was synthe-
sized in a similar manner as 2. The optimal ratio of
functional monomer to spacer monomer (9:10) (i.e., the
highest percentage of 9 while retaining full and con-
trolled solubility) was investigated through the synthe-
sis of a series of copolymers (Table 1). All copolymers
with a 9:10 ratio of at least 1:4 could be fully solubilized
in a 0.1% (v/v) chloroform/trifluoroacetic acid mixture.
All resolubilized copolymers were characterized using
gel permeation chromatography and showed polydis-
persities between 1.5 and 1.8. Differential scanning
calorimetry did not show a glass-transition temperature
or a melting temperature, while thermogravimetric
analysis showed the onset of polymer decomposition at
250 °C.
Alq3 is a thermally stable, highly fluorescent material
with excellent electron-transport mobilities.7 Its usual
blue-green luminescence can be either blue- or red-
shifted through (a) the addition of substituents or (b)
,8
the introduction of optically inactive spacer molecules
into the crystalline network of Alq3.5
,7-9
While most
research activities have been focused on manipulating
the optical properties, the problem with the process-
ability of Alq3 has not been solved. One possible solution
that has been utilized to enhance processability is the
introduction of Alq3-doped polymers, where the Alq3
1
0,11
complex is embedded within a polymer matrix.
However, phase separation can occur, leading to poor
optical properties in these systems.12 To circumvent the
phase separation problems, Alq3 can be covalently
attached to the polymer backbone. However, the only
reported Alq3-functionalized polymer to date is a con-
densation polymer that was functionalized with Alq3 in
a postpolymerization step.12 As a result of this postpo-
lymerization step, the probability of having a fully
functionalized polymer without cross-linking is a major
concern. In contrast, our design strategy is based on a
fully functionalized monomer that can be polymerized
in a controlled fashion, thereby eliminating any cross-
linking. Additionally, we are able to control and alter
the polymer structure by using comonomers to tune the
polymeric properties.
Essential for the success of our polymer-supported
Alq strategy is that the copolymers retain the optical
3
properties of Alq3 and show no interference of the
polymer coil with the emission properties. Therefore, we
investigated the photoluminescence of the copolymers
Our monomer design requires two structural motifs:
1) a polymerizable unit that allows for a high degree
2
1
(
and monomer 8 and compared them to Alq3. The UV/
vis absorption spectrum of monomer 8 shows a λmax at
319 nm, corresponding to the low-energy singlet transi-
of control during the polymerization and (2) an alkyl
spacer between the polymerizable unit and the Alq3 to
decouple the backbone from the Alq3 group. Norbornene
was chosen as the polymerizable unit. It can be polym-
erized using ring-opening metathesis polymerization
2
2
tion of the hydroxyquinoline group. The absorption
spectrum of Alq3 as described in the literature and
2
3
determined by us shows peaks at 372 and 316 nm. The
absorption spectra of all copolymers show identical
peaks as that of Alq3, indicating the same transitions
taking place in our copolymer system as the ones known
for Alq3. The emission spectra of Alq3 and the copoly-
(ROMP), a method that has a high tolerance to many
*
Corresponding
author:
e-mail
marcus.weck@
chemistry.gatech.edu.
1
0.1021/ma0259012 CCC: $25.00 © 2003 American Chemical Society
Published on Web 02/19/2003