Polarized photoluminescence from poly(p-phenylene-ethynylene) via a block copolymer nanotemplate

Article abstract of DOI:10.1021/ja036024y

A new approach based on a conjugated polymer/block copolymer guest/host system for the generation of polarized photoluminescence is reported. Synthetic modification of a poly(p-phenylene-ethynylene) (PPE) conjugated polymer is used for domain-specific inc

Full text of DOI:10.1021/ja036024y

Published on Web 07/26/2003
Polarized Photoluminescence from Poly(p-phenylene-ethynylene) via a Block
Copolymer Nanotemplate
Craig A. Breen,†,§ Tao Deng,‡,§ Thomas Breiner,‡ Edwin L. Thomas,‡,§ and Timothy M. Swager*,†,§
Department of Chemistry, Department of Materials Science and Engineering, and
Institute for Soldier Nanotechnologies, Massachusetts Institute of Technology, 77 Massachusetts AVenue,
Cambridge, Massachusetts 02139
Received May 8, 2003; E-mail: tswager@mit.edu
Polarized luminescent materials1 have attracted attention for
Scheme 1
potential uses in flat panel displays2 and other optoelectronic
devices.3 Conjugated polymers (CPs), such as poly(p-phenylene-
vinylene) (PPV)4 and poly(p-phenylene-ethynylene) (PPE)2,5 de-
rivatives, have been of particular interest for use as polarized
emitters due to the intrinsic anisotropy of their (quasi) one-
dimensional electronic structure and ability to be processed in
uniaxially oriented blends by various techniques such as rubbing,6
friction transfer,7 tensile drawing,4 and Langmuir-Blodgett film
deposition.8 This communication presents a new approach to
achieving polarized emission from CPs, specifically PPEs, by using
a block copolymer host matrix as a nanotemplate for the spatial
orientation of CPs. This technique provides a means of generating
large-area films for polarized photoluminescence.
Self-assembled block copolymers allow for easy access to one-,
two-, and three-dimensional ordered nanostructures.9 The different
morphologies can be selected by tailoring the relative composition
Table 1. Polymer Characterization
photophysical characterization
(solid state)
of each block. Furthermore, with the application of a flow field,
the global orientation of a block copolymer microstructure can be
directed, forming nearly single-crystal structures.10-12 Roll cast
processing is one technique that has been shown to globally orient
block copolymers.12 In this communication, we demonstrate the
ability to orient a cylindrical morphology block copolymer by roll
cast processing and use the resulting nanostructure as a template
for the spatial and orientational ordering of a guest PPE. Sequester-
ing the guest PPE molecules into the cylindrical microdomains of
a block copolymer and globally orienting the host matrix results in
the alignment of the PPEs parallel to the long axis of the cylinders.
The uniaxial orientation of the PPEs within the block copolymer
host gives rise to polarized absorbance and photoluminescence.
Our approach was to synthetically modify a PPE such that it
would have selective miscibility to the cylinder phase of a block
copolymer host. Roll cast processing preferably requires a rubbery
matrix phase, and thus, we used a polystyrene-polyisoprene-
polystyrene (SIS) triblock copolymer system with minority com-
ponent styrene cylinders in an isoprene matrix. Consequently, we
functionalized the PPE with polystyrene (PS) by grafting from a
PPE macroinitiator via atom transfer radical polymerization (ATRP).13
Grafted PPEs were synthesized as shown in Scheme 1. The
diacetylene monomer with terminal hydroxyl side chains, 1, was
polymerized with the diiodide 2 under Pd(0)-catalyzed cross-
coupling conditions to give the PPE backbone (3). The PPE was
then functionalized with 2-bromoisobutyryl bromide to give the
ATRP macroinitiator 4. Subsequent ATRP polymerization of
styrene gave the polystyrene grafted PPE (PPE-g-PS) 5.
GPC analysis
λ
maxabs
λmaxem
polymer
M
n
PDI
(nm)
(nm)
Φ
PPE backbone (3)
PS grafted PPE (5)
cleaved PS grafts
PS/PI/PS
56,000
80,000
5,600
2.59
1.62
1.17
1.05
480
445
-
495
475
-
0.10
0.60
-
-
101,000
-
-
polymers. The polymers were characterized by gel permeation
chromatography (GPC) (Figure 1), nuclear magnetic resonance
(NMR) spectroscopy, UV/vis spectroscopy, and fluorescence
spectroscopy. The success of each step involved in the synthesis
of the grafted polymers could be assessed through 1H NMR analysis.
As shown in Figure 1, the change in the chemical shift of the
methylene protons adjacent to the terminal hydroxyl group on
monomer 1 was monitored to verify the covalent attachment of
the ATRP initiator (4) as well as the successful initiation of PS
polymerization (5). Further confirmation is provided upon observa-
tion of the grafted PPEs photophysical behavior in the solid state.
As expected, the PPE-g-PS displayed both a blue-shift and an
increase in fluorescence quantum efficiency (Table 1) relative to 3
due to the presence of the PS grafts preventing aggregation of the
PPE backbone.13b Finally, the atom transfer polymerization of
styrene was assessed by measuring the polydispersity of the PS
grafts (Table 1). The ester linkages of the ATRP initiator were
cleaved post polymerization by treating the grafted PPE with KOH
in a refluxing THF/MeOH mixture, allowing for GPC analysis of
the free PS grafts.13b
Table 1 contains characterization data for the synthesized
The synthetically modified PPE was then doped into a SIS
triblock copolymer (Dexco, Mn ) 101 000; 29% (w/w) PS) film
with cylindrical morphology. The roll cast films were prepared by
making 40% (w/w) solutions of polymer in cumene. The PPE-g-
† Department of Chemistry.
‡ Department of Materials Science and Engineering.
§ Institute for Soldier Nanotechnologies.
9
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J. AM. CHEM. SOC. 2003, 125, 9942-9943
10.1021/ja036024y CCC: $25.00 © 2003 American Chemical Society

C O M M U N I C A T I O N S
This materials system, combined with roll cast processing,
provides advantages over other techniques for achieving polarized
photoluminescence. Many of the systems and processes reported
to date are not readily amenable to large-scale production.1 In
addition, the emission efficiencies of these systems are often limited
by aggregation phenomena found in highly aligned parallel chains
of conjugated polymer.1 In contrast, we have demonstrated the
ability to process large-area, uniform films that produce polarized
photoluminescence. Furthermore, the grafted PPE, used as the
optically active material, intrinsically eliminates aggregation, and
thus, the aligned films have enhanced emission efficiency in the
solid state.
Figure 1. GPC elugrams of (a) PPE Backbone, (b) PS grafted PPE, and
(c) cleaved PS grafts and 1H NMR of polymers 3, 4, and 5. 1H NMR
normalized on the aromatic methoxy proton signal.
In summary, we have demonstrated that polarized photolumi-
nescence can be obtained from a roll cast CP-block copolymer
guest/host system. Synthetic modification of an optically active PPE
guest was conducted for domain-specific doping of a block
copolymer host. Globally orienting the nanostructure of the host
provided a template for the alignment of the guest PPE. The host/
guest system gives rise to both polarized absorption and photolu-
minescence parallel to the cylinder axis of the host, indicative of
uniaxially oriented PPEs. Our group is currently studying the effects
of grafting length and film thickness on the alignment and
subsequent polarized emission of the templated guest PPEs.
Templating of PPEs in a block copolymer photonic crystal is also
being pursued.
Figure 2. Polarized absorption (a) and photoluminescence (b) of a roll
cast oriented film of 0.1 wt % PPE-g-PS doped in SIS. The spectra were
obtained with polarizers oriented parallel (solid line) and perpendicular
(dashed line) to the average PS cylinder orientation of the block copolymer
host. It should be noted that the emission spectra in (b), measured
perpendicular to the polarizers (dashed line), is corrected for instrumental
polarization dependence. See Supporting Information for details.
Acknowledgment. This research was supported by the U.S.
Army through the Institute for Soldier Nanotechnologies, under
Contract DAAD-19-02-D-0002 with the U.S. Army Research
Office. The content does not necessarily reflect the position of the
Government, and no official endorsement should be inferred.
PS dopant was blended in at a concentration of 0.1 wt % of polymer.
After roll casting, the resultant films were approximately 0.04 mm
thick and 6 × 16 cm2 in area. The polystyrene cylinders were
oriented in the plane of the film, and hence, the films displayed
the expected mechanical anisotropy9 due to the oriented glassy
polystyrene cylinders in an elastic polyisoprene matrix. The film
nanostructure was characterized by transmission electron micros-
copy (TEM).
Supporting Information Available: Experimental procedures,
synthetic preparations for all new compounds, and roll cast film
characterization (PDF). This material is available free of charge via
the Internet at http://pubs.acs.org.
References
Photophysical characterization of the globally oriented roll cast
films was conducted. Figure 2 shows the polarized absorption and
photoluminescence observed from a doped roll cast film. The
measured absorbance dichroic ratio as shown in Figure 2a is 3.0 at
440 nm. The polarized emission in 2b was generated by exciting
the roll cast film with 400 nm radiation normal to the plane of the
film, yielding a polarization ratio of 6.4 at 472 nm. These values
indicate that the transition dipoles of the guest PPE-g-PS are
aligned parallel to the cylinder axis of the block copolymer host
and thus emit polarized light with polarization parallel to the plane
of the film. Furthermore, the difference in the absorbance dichroic
ratio as compared to the emission polarization ratio suggests this
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to 440 nm, i.e. toward the band edge, the observed polarization
ratio increases to 7.3 at 472 nm.14 The lower-energy excitation
directly excites the more aligned, longer-conjugation length seg-
ments.
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