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onset wavelength as 2.77 eV. The fluorescence spectrum
exhibits a maximum at 461 nm with a shoulder around 440 nm.
The absolute PL efficiency of the polymer as a neat film was
measured in an integrating sphere as 45 ± 3%. These results
indicate that PBPITP is an efficient blue emissive polymeric PL
material.
energy barrier for electron injection. Also, the improved EL
demonstrates the good hole transporting property of PBPITP.
It can be concluded from the results that PBPITP is a
promising red EL polymeric material.
In conventional conjugated polymers, PL and EL normally
originate from the same molecular excitation, i.e. singlet
excitons. This results in the EL emission spectrum of a
conjugated polymer closely resembling its PL spectrum. The
large difference between the PL and EL spectra here implies
that the recombination mechanisms of charge carriers for PL
and EL in PBPITP are different. A similar phenomenon was
reported most recently in a binaphthalene-containing conju-
gated polymer.16 Excimer emission might be responsible for the
The redox behavior of the polymer was investigated by cyclic
voltammetry (CV) with a standard three-electrode electro-
chemical cell in a 0.10 M tetrabutylammonium perchlorate
solution in acetonitrile at room temperature. The anodically
scanned cyclic voltammogram is shown in Fig. 2. The oxidative
process starts at ca. 0.6 V (vs. standard calomel electrode, SCE)
and gives a sharp oxidative peak at 1.04 V. The oxidation is
highly reversible, and the corresponding reduction peak appears
at 0.77 V. The energy level of the highest occupied molecular
orbital (HOMO) of the polymer, EHOMO, can be estimated from
17
unusual emissive phenomenon. Further investigation is re-
quired in order to understand the origin of the unusual emissive
phenomenon and what mechanisms are responsible for it, and
are actively being pursued.
14
the oxidative onset potential, to be 25.0 eV. In the cathodic
scan, no reduction peak (down to 22.5 V vs. SCE was
observed). The energy level of the lowest unoccupied molecular
orbital (LUMO), ELUMO, can be estimated by subtracting the
In summary, a new soluble triarylamine-containing conju-
gated polymer has been synthesized through a Suzuki coupling
reaction. The polymer in the form of a film exhibits high
absolute PL efficiency and good hole transporting properties.
The PL emission of the polymer is blue whereas its EL emission
is red. The wavelength difference between the PL and EL
spectrum is as large as 157 nm. It is evident that the PL and EL
spectrum correspond to different molecular excitations. Such a
polymer provides a good opportunity to understand the PL and
EL processes in conjugated polymers.
g
optical band gap energy, E , from EHOMO as determined by the
electrochemistry. This leads to an estimate of ELUMO of 22.17
eV.
Part of the work was carried out in the University of
California, Santa Barbara (UCSB). We would like to thank Dr
Vojislav Srdanov (UCSB) for his kind help in the measurement
of PL quantum efficiency.
Notes and references
†
H 3
Selected data for PBPITP: d (CDCl ) d 7.76–7.44 (m, 10H), 7.22–7.01
(m, 6H), 2.75–2.49 (t, 2H), 1.74–1.49 (m, 2H), 1.49–1.24 (m, 2H),
1
c 3
.03–0.86 (t, 3H). d (CDCl ) 147.06, 144.88, 138.91, 138.23, 134.33,
1
29.27, 128.97, 128.16, 127.27, 126.81, 125.06, 123.67, 35.04, 33.61,
Fig. 2 Cyclic voltammogram of PBPITP. Working electrode: PBPITP film-
22.39, 13.95. Anal. Calc. for C28
C, 88.17; H, 6.84; N, 3.46%.
H25N: C, 89.56; H, 6.71; N, 3.73. Found:
2
coated platinum plate (square, ca. 1 cm ); counter electrode: platinum wire;
21
3
reference electrode: Ag/AgNO (0.10 M). Scan rate: 20 mV.s room
1
2
A. J. Heeger, Solid State Commun., 1998, 107, 673.
temperature. The arrows indicate the film color change during scan.
R. H. Friend, R. W. Gymer, A. B. Holmes, J. H. Burroughes, R. N.
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Lögdlund and W. R. Salaneck, Nature, 1999, 397, 121.
Single-layered electroluminescent (EL) cells, ITO|PB-
PITP(1000 –1500 Å)|Ca (1500 Å), were fabricated. The devices
emitted bright red light above ca. 15 V under a forward bias
3
4
J. L. Segura, Acta Polym., 1998, 49, 319.
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(
3
ITO wired positive). The EL spectrum is shown in Fig. 1 (curve
) and exhibits a maximum at 618 nm. The EL spectrum is red-
5
5
shifted by 157 nm (0.7 eV) in comparison with the PL spectrum.
A layer (1000–1500 Å) of poly(9,9-dihexylfluorene-alt-co-
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2
,5-didecyloxy-para-phenylene) (PDHFDDOP), an efficient
10
blue EL polymer, was further deposited on the PBPITP film to
fabricate double polymer-layer ITO|PBPITP|PDHFDDOP|Ca
devices, in which the PBPITP film was kept from direct contact
with the calcium electrode by the PDHFDDOP layer. The
double-layered devices also emitted red light. The EL spectrum
8
9
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(curve 4 in Fig. 1) is almost the same with that recorded from the
single-layered devices. The results reveal that an interface effect
is not the cause of the large red-shift of EL compared with PL.
In addition, the threshold voltages for current flow and light
emission were dramatically decreased to ca. 5 V upon addition
of the PDHFDDOP layer. The external EL quantum efficiency
was increased from ca. 0.01 to ca. 0.025% by the addition of the
PDHFDDOP layer. It is evident that the role of the PDHFDDOP
film is to serve as an electron-transporting rather than as an
emissive layer. The improved EL performance can be attributed
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15
to the lower LUMO energy level of PDHFDDOP (2 2.6 eV)
compared with that of PBPITP, which results in a smaller
682
Chem. Commun., 2000, 681–682