A highly efficient light-emitting poly{[5-(diphenylamino)-1,3- phenylenevinylene]-alt-(2,5-dihexyloxy-1,4-phenylenevinylene)}: Synthesis and optical properties

Article abstract of DOI:10.1021/ma0494232

The improvement in efficiency of LED's by making a transport layer between emitter and cathode and a hole transporting layer between emitter and anode was discussed. Triphenylamine (TPA) was used as hole transport layer in OLED. For the improvment in the

Full text of DOI:10.1021/ma0494232

3970
Macromolecules 2004, 37, 3970-3972
Sch em e 1. Str u ctu r es of P olym er s 1-3
A High ly Efficien t Ligh t-Em ittin g P oly{[5-
(d ip h en yla m in o)-1,3-p h en ylen evin ylen e]-
a lt-(2,5-d ih exyloxy-1,4-p h en ylen evin ylen e)}:
Syn th esis a n d Op tica l P r op er ties
Lia n g Lia o a n d Yi P a n g*
Department of Chemistry & Center for High Performance
Polymers and Composites, Clark Atlanta University,
Atlanta, Georgia 30314
Lim in g Din g a n d F r a n k E. Ka r a sz
Department of Polymer Science and Engineering,
University of Massachusetts, Amherst, Massachusetts 01003
Received March 24, 2004
Revised Manuscript Received April 14, 2004
In tr od u ction . Since the discovery of polymeric light-
emitting diodes (LEDs)1 in 1990, considerable progress
has been made in the development of π-conjugated
polymers and exploration of their application in display
technologies.2,3 The unique combination of their struc-
tural, mechanical, photonic, and electronic properties
also renders them attractive candidates for use as
plastic lasers4 and chemical sensors.5
Light emission from the polymer LED is produced via
the recombination of electrons and holes injected through
the electrodes. Applying an additional electron injection/
transport layer between the emitter and cathode or/and
a hole-transporting layer between the emitter and the
anode has been successfully used to improve device
efficiency and stability.6-9 Although using blends of
polymers with low molecular weight materials with
electron/hole-transporting properties10-13 remains a
useful strategy, emissive material with built-in charge-
transporting moieties14-20 proves to be more feasible,
since the latter simplifies device fabrication and elimi-
nates problems associated with phase separation of
different components. A challenge is to improve the
charge-transporting properties while simultaneously
preserving or improving the desirable luminescent
properties (both fluorescence efficiency and emission
color).
Triarylamine- or triphenylamine (TPA)-based deriva-
tives, such as TPD (N,N-diphenyl-N,N-di(m-tolyl)-p-
benzidine), have been widely used as the hole-transport
layer (HTL) in organic light-emitting diodes (OLEDs).21
Their applications in polymer light-emitting diodes
(PLEDs), however, are restrained intrinsically by their
physical properties, such as phase separation, crystal-
lization upon aging,22 erosion, and spectrum absorption,
and by the sophisticated vacuum evaporation procedure
required in device fabrication. To circumvent these
problems, TPA fragments have been incorporated into
the polymer main chain13,17,18,23 (e.g., 124), which often
alters the emission color due to the presence of nitrogen
along the π-conjugated backbone. Pendent-arylamine-
based hole-transporting polymers have also been pre-
pared25,26 and shown to be useful as hole-transporting
materials.27
0.56%).32 Although the m-phenylene linkage helps to
confine excitons through π-conjugation interruption, the
EL performance of PmPVpPVs has not yet exceeded
that of PPVs. A fundamental question remains whether
the exciton confinement associated with an m-phenylene
bridge will eventually lead to materials of superior EL
performance. To further improve the EL efficiency of
PmPVpPV, we have now synthesized polymer 3, which
includes TPA as a hole-transporting component. The
m-phenylene in the backbone of 3 belongs to both the
emitting chromophore and TPA fragment, thereby
creating an intimate contact between them for effective
charge injection/migration. In addition, the pendent
diphenylamino group attached to the m-phenylene ring
minimizes its steric impact on the emissive PmPVpPV
backbone.
Resu lts a n d Discu ssion . a . Syn th esis of Mon o-
m er s a n d P olym er . As shown in Scheme 2, the
monomer 5-(N,N-diphenylamino)benzene-1,3-dicarbal-
dehyde (8) was synthesized from 5-aminoisophthalic
acid (4) in four steps. Condensation polymerization
between dialdehyde 8 and diphosphonate 9 was carried
out at 110 °C in toluene using potassium tert-butoxide
as a base, which afforded polymer 3 as a pale yellow
1
resin in 58% yield. H NMR of the polymer in CDCl3
detected no aldehyde groups, indicating that polymer-
ization was complete. Polymer 3 was readily soluble in
common organic solvents such as chloroform, methylene
chloride, and tetrahydrofuran. Uniform thin films could
be cast from its solutions. On the basis of size exclusion
chromatography (SEC) analysis, the weight-average
molecular weight of 3 was estimated to be 4.6 × 104
(polydispersity index ≈ 2.3). Thermogravimetric analy-
sis (TGA) in air showed that the polymer was thermally
stable up to about 300 °C.
b. P h otoa bsor p tion a n d P h otolu m in escen ce of
Solu tion s. The absorption spectrum of polymer 3 in
THF solution exhibited two bands with λmax ) 308 and
406 nm (Figure 1). The longer wavelength (or lower
energy) absorption band of 3 (λmax ) 406 nm) was very
similar to that of the parent polymer 2a (λmax ) 408
nm),33 indicating a small substitution effect from the
diphenylamino group to the electronic band-gap of the
material. Fluorescence of 3 in THF displayed two
Recent studies have shown that poly[(m-phenylenevi-
nylene)-alt-(p-phenylenevinylene)] derivatives (PmPVp-
PVs), such as 2, are highly effective green-emitting
materials.28-31 With a device configuration of ITO/
PEDOT/2a /Ca, these polymers exhibit promising elec-
troluminescence (EL) (external quantum efficiency ≈
10.1021/ma0494232 CCC: $27.50 © 2004 American Chemical Society
Published on Web 04/29/2004

Macromolecules, Vol. 37, No. 11, 2004
Communications to the Editor 3971
Sch em e 2. Syn th esis of Mon om er s a n d P olym er 3
F igu r e 1. UV-vis and photoluminescence of polymer 3 (solid
line) and its parent polymer 2a (dotted line) in THF solution
(bottom) and film (top) states.
emission bands with λmax ) 456 and 483 nm, which are
about 10 nm red-shifted from the fluorescence peaks of
2a (λmax ) 445 and 473 nm). The observed slight
bathochromic shift in the emission of 3 is in agreement
with the anticipated substitution effect. The fluores-
cence quantum efficiency of 3 was measured to be φfl ≈
63%, which is about the same as that of 2a (φfl ≈ 64%).33
In summary, the solution PL characteristics of PmPVp-
PV backbone were essentially unaffected by the attach-
ment of diphenylamino group to the m-phenylene ring.
c. Th in F ilm Op tica l P r op er ties. Polymer films
were prepared by spin-casting from THF solutions onto
quartz plates. Films of 3 exhibited two absorption bands
(λmax ≈ 313 and 408 nm), slightly red-shifted from its
respective solution absorption bands. Although the low-
energy absorption band of 3 in solution was slightly red-
shifted from that of 2a , the absorption of film 3 was
blue-shifted from that of film 2a . The opposite trends
observed from the absorption of the solution and film
states indicated that the chromophore-chromophore
interaction in the ground state of film 3 was weaker
than that in the film 2a , attributed to the bulky
diphenylamino substituent in the former. Fluorescence
F igu r e 2. EL spectrum (top) and current density (b)-
voltage-luminance (O) characteristics (bottom) for device ITO/
PEDOT/polymer 3/Ca.
observed in the absorption, the emission of film 3 was
also blue-shifted from that of film 2a (by ∼11 nm) but
remained in the green region. Since a minor emission
shoulder at ∼465 nm was observed, the major emission
band at 503 nm from 3 was assigned to the 0-1
transition. In contrast to the solution spectrum in which
the fluorescence of 3 was red-shifted from that of 2a ,
the film fluorescence of the former was blue-shifted from
of film 3 exhibited a major peak and a shoulder (λmax
≈
503 and 530 nm, respectively). Consistent with the trend

3972 Communications to the Editor
Macromolecules, Vol. 37, No. 11, 2004
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