Preparation of functionalized polystyrene-block-polyisoprene copolymers and their luminescence properties

Article abstract of DOI:10.1021/ma0107206

A series of block copolymers functionalized with aromatic 1,3,4-oxadiazole and stilbene derivatives have been synthesized by the palladium catalyzed reaction between polystyrene-block-polyisoprene and different functional units. These polymers exhibited different emission properties in solution and in the solid state. In chloroform solution, they showed relatively narrow and featured emission bands while, in the solid state, the emission band was broadened and showed a significant red shift. These observations were attributed to the formation of aggregates between the luminophores. After some oxadiazole functionalized copolymers were annealed at elevated temperature, such aggregation was enhanced and there were further changes in the emission spectra. For the bifunctional copolymers, such a shift in the emission band was not significant because the presence of two different chemical species in the same block may prevent the same type of luminophores from aggregating together.

Full text of DOI:10.1021/ma0107206

850
Macromolecules 2002, 35, 850-856
Preparation of Functionalized Polystyrene-block-polyisoprene
Copolymers and Their Luminescence Properties
Sijia n Hou a n d Wa i Kin Ch a n *
Department of Chemistry, University of Hong Kong, Pokfulam Road, Hong Kong, China
Received April 26, 2001; Revised Manuscript Received November 12, 2001
ABSTRACT: A series of block copolymers functionalized with aromatic 1,3,4-oxadiazole and stilbene
derivatives have been synthesized by the palladium catalyzed reaction between polystyrene-block-
polyisoprene and different functional units. These polymers exhibited different emission properties in
solution and in the solid state. In chloroform solution, they showed relatively narrow and featured emission
bands while, in the solid state, the emission band was broadened and showed a significant red shift.
These observations were attributed to the formation of aggregates between the luminophores. After some
oxadiazole functionalized copolymers were annealed at elevated temperature, such aggregation was
enhanced and there were further changes in the emission spectra. For the bifunctional copolymers, such
a shift in the emission band was not significant because the presence of two different chemical species in
the same block may prevent the same type of luminophores from aggregating together.
In tr od u ction
other photosensitizing functional groups into block
copolymers. Different polycyclic aromatic compounds
such as naphthalene,12 pyrene,13 perylene,14 cinnamoyl,15
and phenanthryl16 groups were used. By studying the
fluorescence properties17 or intramolecular energy mi-
gration process,18 one can obtain information about the
aggregation, morphology, or micelle formation in these
copolymers.
In this paper, we report the synthesis of a series of
polystyrene-block-polyisoprene (PSt-b-PIP) that were
tagged with luminescent aromatic oxadiazole or stilbene
derivatives on the polymer side chain. The functional
groups were generally used as the electron transport
and emission units in light emitting polymers. They
were attached to the polymer side chain by a palladium
catalyzed olefinic coupling reaction. The resulting func-
tionalized block copolymers are expected to have more
defined domain sizes and morphologies, and they can
serve as a good model for studying the aggregation of
luminophores in some light emitting polymers. Some of
the polymers also exhibit electroluminescence activities.
Besides, it is envisaged that an annealing process would
enhance microphase separation between different do-
mains that in turn affects the degree of aggregation. The
effect of annealing on the morphologies and lumines-
cence properties was also studied. This paper reports
the results.
Organic luminescent material has been the subject
of growing interest in past years, particularly polymer-
based materials, because of their processibility and
durability. Many photoactive and electronically active
compounds were blended with commercial polymers
such as poly(methyl methacrylate) and polycarbonates
that served as the supporting matrixes.1,2 However, the
incompatibility between the polymer and dopants may
induce the crystallization of the small molecules and
may also yield a material with poor strength. This
problem can be overcome by attaching the functional
units to the polymer main chain or side chain with
covalent bonding. Nevertheless, functional polymers
may still exhibit microphase separation because of the
fact that functional units with similar structure tend
to form aggregates. The physical properties of the
functional polymers may be altered because of the
presence of such aggregation. For example, it has been
reported that, in some light emitting functional poly-
mers, the emission originated from inter- and intrachain
aggregate complexes.3-5
The aggregation effect of functional units on the
physical properties of functional polymers can be stud-
ied by synthesizing functional block copolymers. Block
copolymers usually exhibit unique and interesting phase
separation properties in the solid state because of the
thermodynamic incompatibility between different blocks.6
Depending upon the volume fraction, block length, and
chemical nature of different blocks, the degree of ag-
gregation and the size and shape of the microdomain
will be different. It has been reported that some block
copolymers can self-assemble into nanoclusters7,8 or
exhibit a specific shape in the solid or in solution.9 In
our previous reports, we synthesized different block
copolymers10 that were functionalized with charge
transport moieties or metal complexes. They exhibit
interesting morphological behaviors, and some rhenium
containing polystyrene-block-poly(4-vinylpyridine)s were
able to form micelles in solution. The polymer morphol-
ogies were dependent on the relative block size and the
solvent system used.11 In the literature, there are
several examples of incorporating fluorescence tags or
Exp er im en ta l Section
Ma ter ia ls. Polystyrene-block-polyisoprene (PS-b-PIP) 1 was
synthesized by the anionic polymerization in THF using sec-
butyllithium as the initiator.19 The molecular weight of the
polymer and the polydispersity were 150 000 and 1.2, respec-
tively (determined by gel permeation chromatography). The
ratio of the PS to PIP block was 6:4. In the polyisoprene block
the content of the pendant vinyl group was above 80% as
determined by 1H NMR spectroscopy. p-Bromobenzonitrile,
benzoyl chloride, p-nitrobenzoyl chloride, benzaldehyde, p-
nitrobenzaldehyde, biphenylcarboxylic acid, p-bromobenzyl
bromide, 4-tert-butylbenzoic acid, 1-cyanonaphthalene, 9-an-
thraldehyde, tributylamine, triphenylphosphine, sodium azide,
ammonium chloride, benzene, toluene, N,N-dimethylforma-
mide (DMF), and pyridine were purchased from Lancaster Ltd.
Tri-o-tolylphosphine and palladium acetate were purchased
10.1021/ma0107206 CCC: $22.00 © 2002 American Chemical Society
Published on Web 12/29/2001

Macromolecules, Vol. 35, No. 3, 2002
Polystyrene-block-polyisoprene Copolymers 851
from Strem Chemicals. Unless otherwise specified, all the
reagents were used as received.
Sch em e 1. Syn th esis of Differ en t Su bstitu ted
Ar om a tic Oxa d ia zole a n d Stilben e Der iva tives for th e
F u n ction a liza tion Rea ction
In str u m en ts.1H and 13C NMR spectra were collected on a
Bruker 300 DPX-NMR spectrometer. FTIR spectra were
collected on
a Bio-Rad FTS-7 FT-IR spectrometer. Mass
spectrometry was performed on a high-resolution Finnigan
MAT-95 mass spectrometer. UV-visible absorption spectra
were recorded in
a Hewlett 8452A Packard diode array
spectrophotometer. The photo- and electroluminescence spec-
tra were collected on an ORIEL MS 257 monochromator
equipped with an ANDOR DV420-BV charge coupled device
(CCD) detector. Thermal transition processes were studied by
using a Perkin-Elmer DSC7 differential scanning calorimeter.
Transmission electron microscope (TEM) photographs of the
samples were recorded on a J EOL-100 electron microscope
operated at 80 kV. The specimen for TEM studies was
prepared by dropping a 0.5% polymer solution in dichloroet-
hane on the surface of water, and the solvent was evaporated
slowly. The thin film was then transferred to the surface of a
copper grid, and then the sample was stained with osmium
tetroxide prior to the TEM studies. For the fabrication of the
light emitting devices, the polymer thin film was prepared by
spin coating onto an ITO glass slide with a typical thickness
of 80-100 nm. A layer of aluminum electrode (thickness )
100 nm) was coated on the polymer film surface by vacuum
deposition under a pressure of 10-6 mbar. The polymer film
thickness was measured with a Veeco Dektak 3ST surface
profiler.
Syn th esis of Su bstitu ted Ar om a tic Oxa d ia zoles 3a -f.
The aromatic oxadiazoles were synthesized according to a
procedure similar to that reported in the literature.20 The
synthesis of 2-(4-bromophenyl)-5-phenyl-1,3,4-oxadiazole (3a )
is described as the general procedure (Scheme 1). 4-Bromophe-
nyltetrazole (2; 4.26 g, 20 mmol) and benzoyl chloride (4.24 g,
30 mmol) were mixed with anhydrous pyridine (40 mL). The
resulting mixture was heated under reflux under a nitrogen
atmosphere for 3 days. The solution was poured into water,
and the solid was filtered. The crude product was recrystallized
with methanol-CHCl3 and dried in a vacuum oven for 24 h.
Yield: 86%. Mp: 180 °C. 1H NMR (CDCl3): δ (ppm) 8.15-
8.12 (d, J ) 7.8 Hz, 2 H), 8.03-8.00 (d, J ) 8.4 Hz, 2 H), 7.70-
7.67 (d, J ) 8.4 Hz, 2 H), 7.50 (m, 3 H). 13C NMR (CDCl3): δ
(ppm) 164.1, 163.3, 132.4, 132, 129.3, 128.5, 126.7, 125.6, 123.1,
122.5. FTIR (KBr, pellet): ν ) 1601 (s, CdN), 1075 (s, CsO),
837 (s, 1,4-substituted benzene). MS (EI): m/z ) 301 {M+}.
2-(4-Br om op h en yl)-5-(4-ter t-bu tylp h en yl)-1,3,4-oxa d ia -
zole (3b). Yield: 73%. Mp: 209 °C. 1H NMR (CDCl3): δ (ppm)
8.19-8.08 (m, 4 H), 7.68 (d, J ) 8.4 Hz, 2 H), 7.55 (d, J ) 8.4
Hz, 2 H), 1.37 (s, CH3, 9 H). 13C NMR (CDCl3): δ (ppm) 164.8,
163.7, 155.6, 132.4, 128.3, 126.8, 126.3, 126.1, 122.9, 120.9,
35.1, 31.1. FT IR (KBr, pellet): ν ) 2960 (m, CH3), 1603 (s,
CdN), 1075 (s, CsO), 839 cm-1 (s, 1,4-substituted benzene).
MS: m/z ) 357 {M+}.
7.70 (m, 3 H), 7.62-7.59 (m, 2 H). 13C NMR (CDCl3): δ (ppm)
164.8, 163.5, 133.9, 132.8, 132.5, 130.1, 128.7, 128.4, 128.3,
126.8, 126.5, 126.1, 124.9, 122.8, 120.3. FTIR (KBr pellet): ν
) 1600 (s, CdN), 1079 (s, CsO), 830 cm-1 (s, 1,4-substituted
benzene). MS: m/z ) 352 {M}+.
2-(9-An th r yl)-5-(4-br om op h en yl)-1,3,4-oxa d ia zole (3f).
1
Yield: 35%. Mp: 261 °C. H NMR (DMSO-d6): δ (ppm) 9.02
(s, 1 H), 8.35-8.25 (d, J ) 8.6 Hz, 2 H), 8.15-8.00 (m, 4 H),
7.95-7.84 (d, J ) 8.6 Hz, 2 H), 7.75-7.60 (m, 4 H). 13C NMR
(DMSO-d6): δ (ppm) 165.6, 163.2, 133.4, 132.6, 131.6, 131.4,
129.7, 129.1, 126.8, 126.6, 125.6, 123.7, 117.3. FTIR (KBr
pellet): ν ) 1602 (s, CdN), 1084 (s, CsO), 843 cm-1 (s, 1,4-
substituted benzene). MS: m/z ) 402 {M+}.
2-(4-Biph en yl)-5-(4-br om oph en yl)-1,3,4-oxadiazole (3c).
Yield: 81%. Mp: 208 °C. 1H NMR (DMSO-d6): δ (ppm) 8.25-
8.20 (d, J ) 8.5 Hz, 2 H), 8.15-8.05 (d, J ) 8.6 Hz, 2 H), 7.98-
7.93 (d, J ) 8.4 Hz, 2 H), 7.90-7.83 (d, J ) 8.5 Hz, 2 H), 7.81-
7.78 (d, J ) 7.2 Hz, 2 H), 7.58-7.42 (m, 3 H). 13C NMR (DMSO-
d6): δ (ppm) 164.7, 163.9, 144.7, 139.8, 132.4, 129, 128.4, 128.2,
127.8, 127.4, 127.2, 126.4, 122.9, 122.5. FTIR (KBr pellet): ν
) 1603 (s, CdN), 1079 (s, CsO), 830 cm-1 (s, 1,4-substituted
benzene). MS: m/z ) 378 {M+}.
Syn th esis of Stilben e Der iva tives. The synthesis of
4-bromostilbene (5a ) is described as the general procedure.
4-Bromophenyltriphenylphosphonium bromide (4; 0.838 g, 1.63
mmol) and benzaldehyde (0.17 g, 1.62 mmol) were dissolved
in dichloromethane (6 mL). Sodium hydroxide (0.7 g, 18 mmol)
dissolved in H2O (2 mL) was added to the solution slowly with
stirring. The solution was stirred at room temperature for 12
h, and the product was extracted with dichloromethane (30
mL). After the solvent was evaporated, the crude product was
purified by silica gel chromatography using chloroform as the
2-(4-Br om op h e n yl)-5-(4-n it r op h e n yl)-1,3,4-oxa d ia z-
1
ole (3d ). Yield: 96%. Mp: 182 °C. H NMR (CDCl3): δ (ppm)
8.48-8.40 (d, J ) 8.4 Hz, 2 H), 8.40-8.33 (d, J ) 7.9 Hz, 2 H),
8.10-8.02 (d, J ) 8.4 Hz, 2 H), 7.80-7.70 (d, J ) 8.4 Hz, 2 H).
13C NMR (CDCl3): δ (ppm) 164.9, 163.0, 149.7, 132.7, 129.2,
128.5, 127.8, 127.2, 124.5, 122. 2. FTIR (KBr pellet): ν ) 1601
(s, CdN), 1534 (s, NO2), 1347 (s, NsO), 1073 (s, CsO), 854 (s,
CsN), 836 cm-1 (s, 1,4-substituted benzene). MS: m/z ) 346
{M+}.
1
eluent. Yield: 26.1%. Mp: 139 °C. H NMR (CDCl3): δ (ppm)
7.53-7.50 (d, J ) 7.9 Hz, 2 H), 7.50-7.47 (d, J ) 8.7 Hz, 2 H),
7.39-7.36 (d, J ) 8.7 Hz, 2 H), 7.35-7.29 (m, 3 H), 7.14-7.00
(d, vinylene proton, J ) 16.4 Hz, 2 H). 13C NMR (CDCl3): δ
(ppm) 136.9, 136.3, 131.8, 129.4, 128.7, 128.0, 127.9, 127.4,
126.5, 121.3. FTIR (KBr pellet): ν ) 968 (s, CHdCH), 812 cm-1
(s, 1,4-substituted benzene). MS: m/z ) 258 {M+}.
2-(4-Br om oph en yl)-5-(1-n aph th yl)-1,3,4-oxadiazole (3e).
Yield: 80%. Mp: 139 °C. 1H NMR (CDCl3): δ (ppm) 9.28-
9.25 (d, J ) 8.5 Hz, 1 H), 8.28-8.26 (d, J ) 7.3 Hz, 1 H), 8.09-
8.06 (d, J ) 8.5 Hz, 3 H), 7.97-7.94 (d, J ) 8.1 Hz, 1 H), 7.73-

852 Hou and Chan
Macromolecules, Vol. 35, No. 3, 2002
Ta ble 1. P h ysica l P r op er ties of th e Block Cop olym er s F u n ction a lized w ith Su bstitu ted Oxa d ia zole or Stilben e
Der iva tives
completeness of
polymer
λ
max,abs (nm) Tg (°C)
λ
max,em(CHCl3) (nm)
λ
max,em(pristine film) (nm)
λ
max,em(annealed film) (nm) functionalization (%)a
P 3a
P 3b
P 3c
P 3d
P 3e
P 3f
P 5a
P 5b
P 5c
310
310
320
325
335
255, 305, 390
320
370
255, 330, 390
88
109
98
103
c
410, 440
410, 440
410, 440
450, 500
420, 445
490
520b
520b
520b
520b
445
560b
560b
560b
560b
445
21
39
32
19
17
31
36
40
30
c
500
500
510
510
108
108
101
450, 490
580b
590b
580, 640
590b
580, 640
540b
a
b
Percentage of functionalized pendant vinyl group in the polyisoprene block, calculated from NMR spectra. Broad peak was observed.
c Not observed.
Ta ble 2. P h ysica l P r op er ties of Som e Block Cop olym er s F u n ction a lized w ith Differ en t Ra tios of Oxa d ia zole a n d Stilben e
Der iva tives
oxadiazole to stilbene oxadiazole to stilbene
completeness of
d
d
polymer oxadiazole stilbene
ratio (feed)a
ratio (experimental)b functionalization (%)c λmax,abs (nm) λmax,em (nm)
T1
T2
T3
T4
T5
T6
T7
T8
T9
T10
T11
3d
3d
3d
3d
3d
3d
3d
3d
3c
3c
3c
5b
5b
5b
5b
5b
5c
5c
5c
5c
5c
5c
12.9
9.0
5.8
3.3
1.1
4.7
3.1
1.3
0.9
2.1
3.3
8.0
2.4
2.0
1.2
0.8
8.6
4.0
0.9
1.1
2.2
3.8
53
23
35
38
33
49
41
33
53
50
44
340
330
340
340
580
580
580
580
580
490
490
490
530
530
530
340
260, 330
260, 330
260, 330
260, 320
265, 320
265, 320
a
Ratio of oxadiazole to stilbene in functionalized copolymers, based on the feeding ratio. b Ratio of oxadiazole to stilbene in functionalized
copolymers, calculated from NMR spectra. c Percentage of functionalized pendant vinyl group in the polyisoprene block, calculated from
NMR spectra. Determined in CHCl3 solution.
d
Sch em e 2. F u n ction a liza tion of
4-Nitr o-4′-br om ostilben e (5b). Yield: 31%. Mp: 207 °C.
P olystyr en e-block-p olyisop r en e by th e P a lla d iu m
1H NMR (CDCl3): δ (ppm) 8.27-8.20 (d, J ) 8.8 Hz, 2 H),
Ca ta lyzed Rea ction
7.64-7.61 (d, J ) 8.8 Hz, 2 H), 7.55-7.50 (d, J ) 8.5 Hz, 2 H),
7.43-7.39 (d, J ) 8.5 Hz, 3 H), 7.22-7.09 (d, vinylene proton,
J ) 16.4 Hz, 2 H). 13C NMR (CDCl3): δ (ppm) 147.1, 143.4,
135.2, 132.1, 132.0, 128.4, 127.0, 126.9, 124.2, 122.8. FTIR
(KBr pellet): ν ) 1507 (s, NO2), 948 (s, CHdCH), 814 cm-1 (s,
disubstituted phenylene). MS: m/z ) 303 {M+}.
9-An th r yl-1-br om ostilben e (5c). Yield: 57.4%. Mp: 167
1
°C. H NMR (CDCl3): δ (ppm) 8.42 (s, 1 H), 8.38-8.28 (m, 2
H), 8.08-7.98 (m, 2 H), 7.98-7.88 (d, vinylene proton, J )
16.5 Hz, 1 H), 7.65-7.45 (m, 8H), 6.95-6.85 (d, vinylene
proton, J ) 16.5 Hz, 1 H). 13C NMR (CDCl3): δ (ppm) 136.2,
136.0, 131.9, 131.4, 129.6, 128.7, 128.0, 126.7, 125.8, 125.7,
125.6, 125.2, 121.8. FTIR (KBr pellet): ν ) 970 (s, CHdCH),
833 cm-1 (s, disubstituted phenylene). MS: m/z ) 358 {M+}.
Syn th esis of F u n ction a lized Cop olym er s. The synthesis
of polymer P 3a is described as the general procedure. A
mixture of PSt-b-PIP copolymer (0.12 g, 0.29 mmol isoprene
unit), 3a (0.11 g, 0.38 mmol), palladium acetate (13 mg, 0.06
mmol), tri-o-tolylphosphine (21 mg, 0.07 mmol), and tributy-
lamine (1.0 mL, 4.2 mmol) was heated with DMF (20 mL) at
100 °C under a nitrogen atmosphere for 3 days. The polymer
was precipitated in methanol, and the product was subse-
quently washed with methanol in a Soxhlet extractor for 2
days. The oxadiazole and stilbene contents in the copolymers
survive in the mild reaction conditions. We have func-
1
were determined by H NMR spectroscopy, and the data are
tionalized the block copolymers with two types of
summarized in Tables 1 and 2.
functional units: aromatic oxadiazoles 3a -3d and
stilbenes 5a -5c which bear different substituents
Resu lts a n d Discu ssion
(Scheme 1). Polymers P 3a -f and P 5a -c were func-
Syn th esis of P olym er s. The functionalization of the
pendant vinyl group in PSt-b-PIP was achieved by the
palladium catalyzed Heck coupling reaction according
to our previously reported procedures.10 Compared to
other functionalization methods such as hydroboration
and hydrosilation, this approach enjoys the advantage
that various types of functional groups are able to
tionalized with either oxadiazole or stilbene only (Table
1), while polymers T1-11 were functionalized with both
functional units with different feed ratios (Scheme 2 and
Table 2). From GPC studies, there has been little change
in the polydispersity of these polymers. The complete-
ness of functionalization was examined by 1H NMR
spectroscopy. It was found that the degrees of function-

Macromolecules, Vol. 35, No. 3, 2002
Polystyrene-block-polyisoprene Copolymers 853
F igu r e 3. UV-vis absorption and emission spectra of the
stilbene functionalized copolymers P 5a -c in CHCl3 solution.
F igu r e 1. NMR spectrum of copolymer T9 in CDCl3.
F igu r e 2. UV-vis absorption and emission spectra of the
oxadiazole functionalized copolymers P 3a and P 3f in CHCl3
solution.
F igu r e 4. UV-vis absorption and emission spectra of the
bifunctional copolymers T1, T3, and T5 in CHCl3 solution.
to the E1 and E2 ethyleneic bands of polynuclear
aromatic, respectively.21 For the stilbene functionalized
copolymers (Figure 3), the absorption peaks of those
polymers with nitro substituted stilbene or anthracene
units (P 5b, P 5c) also show a remarkable red shift
compared to those of the unsubstituted stilbene (P 5a ).
When the copolymers were functionalized with both
substituted oxadiazole and stilbene, the resulting ab-
sorption features were dependent on the ratio of the
chromophores incorporated. Polymers T1-T5 exhibit a
broad absorption band centered at ∼330 nm, which
results from the absorption of oxadiazole 3d and stilbene
5b (Figure 4). Figure 5 shows the absorption spectra of
polymers T9-11 that were functionalized with oxadia-
zole 3c and stilbene 5c in different ratios. Polymer T9
has the highest stilbene content, and its absorption
spectrum is very similar to that of polymer P 5c. When
the oxadiazole content in the copolymer was gradually
increased, the peak at 320 nm corresponding to the
oxadiazole absorption started to dominate. Similar
trends were also observed for other series of polymers
when the oxadiazole-stilbene ratios had been changed.
alization in polymers P 3a -P 5c are approximately 20-
40%. The distribution of the groups attached to the PI
block should be random because there is no control of
which pendant vinyl groups are preferentially function-
alized. The results are summarized in Tables 1 and 2.
Figure 1 shows the NMR spectrum of polymer T11 in
CDCl3. The peak at 8.3 ppm is assigned to three protons
in the anthracene moiety, while those at 7.9-8.1 ppm
correspond to seven protons from both the oxadiazole
and stilbene moieties. In addition, the peaks at 4.6-
5.0 and 5.7 ppm are assigned to be the protons on the
isoprene unit. From the ratio of the integration of the
peaks, the amount of the oxadiazole and stilbene rela-
tive to the free isoprene units can be calculated.
Electr on ic Absor p tion P r op er ties. The UV-vis
spectral absorptions of some oxadiazole functionalized
copolymers are shown in Figure 2, and the data are
summarized in Table 1. In general, polymers P 3a -c
show strong and feature absorption bands in the range
between 290 and 340 nm which is due to the π-π*
transition of the conjugated oxadiazole unit. For poly-
mers P 3d and P 3e, their absorption peaks show further
red shift because of the presence of the electron with-
drawing nitro group and the π-conjugated naphthalene
moieties, respectively. For polymer P 3f, the absorption
spectrum is dominated by the absorption of the an-
thracene unit. The strong absorption peak at 255 nm
and other smaller ones at 370-390 nm are attributed
P h oto- a n d Electr olu m in escen ce P r op er ties. The
photoluminescence (PL) spectra were collected in chlo-
roform solution and in the solid state. Polymers P 3a -c
and P 3e have similar luminescence properties, and
peaks were observed at the vicinities of 370, 410, and
440 nm (Table 1 and Figure 2). There was no significant

854 Hou and Chan
Macromolecules, Vol. 35, No. 3, 2002
F igu r e 6. Emission spectra of copolymer P 3b in CHCl3
solution and in the solid state. The spectra were collected
before and after annealing the polymer at 110 °C for 3 days.
F igu r e 5. UV-vis absorption and emission spectra of the
bifunctional copolymers T9-11 in CHCl3 solution.
substitution effect on the emission frequencies in solu-
tion. Polymer P 3f shows a relatively broad emission
band with a significant red shift at ∼490-520 nm. This
is because polycyclic aromatic hydrocarbons usually
form excimers because of the proximity of different
anthracene units in the pendant chain.22 For polymer
P 3d with the nitro-substituted oxadiazole, the emission
properties are of special interest. For many nitro
aromatics, the existence of the n,π* singlet states would
lead to rapid singlet-triplet intersystem crossing, and
the emission was from the corresponding triplet excited
states.23 For the stilbene functionalized copolymers, the
position of the emission peaks for polymers P 5a -c
showed a gradual bathchromic shift from ∼450 to 600
nm (Figure 3). Such a shift in polymers P 5b and P 5c
to longer wavelength could be attributed to the same
arguments as those for the shifts in polymers P 3d and
P 3f. The excitation spectra of these polymers closely
resemble their absorption spectra, indicating the emis-
sion originated from the excitation of the π conjugated
systems. For the bifunctional copolymers, their emission
bands are dominated by the emission from the stilbene
moieties. The emission spectra of copolymers T1-5
(Figure 4) and T9-11 (Figure 5) are very similar to
those of polymers P 5b and P 5c, respectively. They are
in general dominated by the emission arising from the
stilbene derivatives because of their higher quantum
efficiencies.24 We repeated the luminescence studies of
these polymers in THF, which is another good solvent
for both polymer blocks, and found out that the solvent
had very little effect on the position of the emission
bands.
F igu r e 7. Emission spectra of copolymer P 3c in CHCl3
solution and in the solid state. The spectra were collected
before and after annealing the polymer at 110 °C for 3 days.
When the luminescence spectra were collected in a
polymer film cast on quartz plates, there were dramatic
changes in the luminescence properties. Figures 6 and
7 show the emission spectra of P 3b and P 3c in solution
and in the solid state. In chloroform solution, the width
of the emission band is smaller and the peak can be
resolved into vibronic fine structures. However, the
emission band became broadened and shifted to lower
energy in the solid state. This is due to the aggregation
of luminophores in the solid state which resulted in the
formation of excimers upon excitation. Such a phenom-
enon was commonly observed in various types of lumi-
nescent polymers.25
F igu r e 8. Electroluminescence spectrum of polymer P 3c
under a forward bias of 20 V.
devices were subjected to forward bias, and light emis-
sion was observed when the applied voltage was greater
than 10 V. A typical electroluminescence spectrum of
polymer P 3c was shown in Figure 8. It closely resembles
its PL spectrum, which suggests that the light emission
originated from the same types of excitons. However,
the lifetime of such devices was quite short (<2 min). A
much more detailed study of the energy levels and
Some polymers were fabricated into single layer
electroluminescence devices using indium-tin-oxide
(ITO) as the anode and aluminum as the cathode. The

Macromolecules, Vol. 35, No. 3, 2002
Polystyrene-block-polyisoprene Copolymers 855
Therefore, the effect of the polymer morphology may not
be the only factor that affects the luminescence proper-
ties. When the copolymers were functionalized with both
oxadiazole and stilbene moieties, the interaction be-
tween these species and their effect on the polymer
morphologies became more complicated. For the bifunc-
tional copolymers T1-11, the shift in the emission peak
after annealing was not significant and was barely
observable. There are two possible reasons for this
observation. The first one is that the polyisoprene block
is functionalized with both oxadiazole and stilbene
moieties. The presence of two different chemical species
in the same block may prevent the same type of
luminophores from aggregating together. TEM studies
also confirmed that the domain sizes of both blocks
undergo little change after annealing. Alternatively, the
polymer films might have achieved their optimum
aggregation state immediately after the film casting
process.
Con clu sion s
The syntheses and spectroscopic studies of some block
copolymers functionalized with stilbene and aromatic
oxadiazole moieties were reported. It was found that the
emission properties of these functionalized copolymers
were different in solids and in the solution state. Also,
the emission and morphological properties were also
dependent on the thermal history of the block copoly-
mers. Due to the more defined morphology and the
mocrodomain sizes, these polymers can serve as models
for studying the aggregation of luminophores in light
emitting polymers.
Ack n ow led gm en t. The work described in this paper
was substantially supported by a grant from the Re-
search Grant Council of the Hong Kong Special Admin-
istrative Region, China (Project No. HKU 7090/98P).
Partial financial support from the Committee on Re-
search and Conference Grant (University of Hong Kong)
is also acknowledged.
F igu r e 9. (a) Transmission electron micrograph of an OsO4
stained pristine sample of polymer P 3c (magnification )
100000×). (b) Transmission electron micrograph of an OsO4
stained polymer P 3c which has been annealed at 110 °C for 3
days (magnification ) 150000×).
choice of electrodes is necessary in order to improve the
device performance.
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