Tuning wavelength: Synthesis and characterization of spiro-DPVF-containing polyfluorenes and applications in organic light-emitting diodes

Article abstract of DOI:10.1021/ma048797s

We have synthesized polyfluorene copolymers containing bis(2,2- diphenylvinyl)fluorene pendent groups attached orthogonally to the C-9 positions of fluorene units. These polymers possess high glass transition temperatures and good thermal stability. The r

Full text of DOI:10.1021/ma048797s

Macromolecules 2004, 37, 7197-7202
7197
Tuning Wavelength: Synthesis and Characterization of
Spiro-DPVF-Containing Polyfluorenes and Applications in Organic
Light-Emitting Diodes
Hu ei-J en Su , F a n g-Iy Wu , a n d Ch in g-F on g Sh u *
Department of Applied Chemistry, National Chiao Tung University, Hsin-Chu, Taiwan 30035
Received J une 17, 2004; Revised Manuscript Received J uly 11, 2004
ABSTRACT: We have synthesized polyfluorene copolymers containing bis(2,2-diphenylvinyl)fluorene
pendent groups attached orthogonally to the C-9 positions of fluorene units. These polymers possess high
glass transition temperatures and good thermal stability. The results from PL studies indicate that most
excitons formed in the polyfluorene backbone by direct photoexcitation are likely to migrate to lower-
energy pendent groups, from which emission occurs. An organic light-emitting device using the copolymer
PF4-DPVF as the emitting layer exhibits a voltage-independent and stable blue emission with color
coordinates (0.15, 0.17) at 11 V, with a maximum brightness of 3137 cd/m2 at 9 V (262 mA/cm2) and a
maximum external quantum efficiency of 1.06%. In addition, we blended PF4-DPVF as the host material
with 0.5 wt % of MEH-PPV to realize a white electroluminescence having CIE coordinates of (0.29,0.34)
and a maximum brightness of 3258 cd/m2 (119 mA/cm2). We demonstrate that both Fo¨rster energy transfer
and direct charge trapping/recombination on the MEH-PPV guest are responsible for the observed EL.
In tr od u ction
bis(2,2-diphenylvinyl)-1,1′-biphenyl (DPVBi), exhibits
intense fluorescence in the blue region (λmax ) ca. 460
nm) and been utilized as an emitting material to achieve
efficient organic blue- and white-light-emitting
diodes.13,14 By attaching DPVF to the polyfluorene
chain, energy transfer from the higher-energy polyfluo-
rene backbone to the lower-energy DPVF pendant
groups may occur, leading to emission solely or pre-
dominantly from the latter. As a result, the photolumi-
nescence (PL) of the copolymer is fine-tuned to a blue
region, which is closer to the maximum of a relative
photopic luminous efficiency function.15 Another special
feature of DPVBi is its nonplanar molecular structure;
i.e., the phenyl rings at both ends of the molecule are
forced to twist as a result of steric hindrance.16 The
presence of the rigid spiro-DPVF side chain may also
reduce interchain interactions and suppress the forma-
tion of long-wavelength aggregates/excimers,9 resulting
in a stable and efficient blue electroluminescence.
Since the discovery of poly(phenylenevinylene)-based
LEDs in 1990,1 considerable efforts have been made
toward the development of new conjugated polymers
and in the performance of their related LEDs.2 Organic
luminescent polymers are attractive because of the
flexibility available for fine-tuning their luminescence
properties through the manipulation of their chemical
structures and because of the feasibility of combining
spin-coating and printing processes for preparing large-
area flat-panel displays. Blue-emitting polymers are of
special interest because they can be used either as a
blue light source in full-color displays or as the host
material for generating other colors through energy
transfer to lower-energy fluorophores.3 Polyfluorenes
(PFs) are among the most promising candidates for blue-
emitting polymers because of their high photolumines-
cence and electroluminescence efficiencies.4 In addition,
the facile process of functionalizing the C-9 position of
the fluorene unit provides the opportunity to improve
both the solubility and processability of the resulting
polymers, while also offering the ability to tune the
optoelectronic properties of the PFs through macro-
molecular engineering.5-8
Exp er im en ta l Section
Mater ials. 2,7-Dibromo-9,9′-spirobifluorene (1), 2,7-dibromo-
9,9-dioctylfluorene (5), 2,7-bis(4,4,5,5-tetramethyl-1,3,2-di-
oxaborolane-2-yl)-9,9-dioctylfluorene (6), poly(2-ethylhexloxy-
5-methoxy-1,4-phenylenevinylene) (MEH-PPV), and the elec-
tron-transport material 1,3,5-tris(N-phenylbenzimidazol-2-yl)-
benzene(TPBI)werepreparedaccordingtoreportedprocedures.17-20
The solvents were dried using standard procedures. All other
reagents were used as received from commercial sources,
unless otherwise stated.
Ch a r a cter iza tion . 1H and 13C NMR spectra were recorded
on either a Varian Unity Yinova 500 MHz or a Bruker DRX
300 MHz spectrometer. Mass spectra were obtained on a J EOL
J MS-SX/SX 110 mass spectrometer. Size exclusion chroma-
tography (SEC) was carried out on a Waters chromatography
unit interfaced to a Waters 410 differential refractometer.
Three 5 µm Waters styragel columns (300 × 7.8 mm) connected
in series in decreasing order of pore size (104, 103, and 102 Å)
were used in conjunction with THF as the eluent. Standard
polystyrene samples were used for calibration. Differential
The application of polyfluorenes in PLEDs has been
hampered, however, because the formation of an un-
desired long-wavelength emission band occurs during
device operation and results in both color instability and
reduced efficiency.9,10 Moreover, the emissions of typical
dialkylpolyfluorenes are located in the deep-blue region
(λmax ) ca. 420 nm) where the human eye is not very
sensitive.11 In this paper, we report the synthesis and
characterization of fluorene-based copolymers contain-
ing bis(2,2-diphenylvinyl)fluorene (DPVF) pendent groups
attached orthogonally to the C-9 positions of fluorene
units. This polymer design has the advantage of permit-
ting the incorporation of a high concentration of DPV
dye without affecting the electronic properties of the
polyfluorene backbone.12 We chose DPVF dye to be the
side chain because its distyrylarylene analogue, 4,4′-
scanning calorimetry (DSC) was performed on
a SEIKO
EXSTAR 6000DSC unit at a heating rate of 20 °C min-1 and
10.1021/ma048797s CCC: $27.50 © 2004 American Chemical Society
Published on Web 08/17/2004

7198 Su et al.
Macromolecules, Vol. 37, No. 19, 2004
a cooling rate of 40 °C min-1. Samples were scanned from 30
to 300 °C, cooled to 0 °C, and then scanned again from 30 to
300 °C. The glass transition temperatures (Tg) were deter-
mined from the second heating scan. Thermogravimetric
analysis (TGA) was undertaken on a DuPont TGA 2950
instrument. The thermal stability of each sample under a
nitrogen atmosphere was determined by measuring its weight
loss while heating at a rate of 20 °C min-1. UV-vis spectra
were measured using an HP 8453 diode-array spectrophotom-
eter. Photoluminescence (PL) spectra were obtained on a
Hitachi F-4500 luminescence spectrometer. Cyclic voltamme-
try (CV) measurements were performed using a BAS 100 B/W
electrochemical analyzer in anhydrous acetonitrile with 0.1
M tetrabutylammonium hexafluorophosphate (TBAPF6) as the
supporting electrolyte at a scan rate of 50 mV s-1. The
potentials were measured against an Ag/Ag+ (0.01 M AgNO3)
reference electrode using ferrocene as the internal standard.
The onset potentials were determined from the intersection
of two tangents drawn at the rising current and background
current of the cyclic voltammogram.
2,7-Dib r om o-2′,7′-b is(b r om om et h yl)-9,9′-sp ir ob iflu o-
r en e (2). 2,7-Dibromo-9,9′-spirobifluorene (378 mg, 800 µmol)
was added to a stirred solution of paraformaldehyde (216 mg,
7.06 mmol) in 30 wt % HBr/AcOH (8 mL) in a medium-
pressure flask at 0 °C. The flask was sealed, and the reaction
mixture was heated at 120 °C for 24 h before being cooled and
added dropwise to water (100 mL). The precipitate was washed
with water and purified by column chromatography (hexane/
CH2Cl2, 5:1) to afford 2 (210 mg, 41%). 1H NMR (300 MHz,
CDCl3): δ 4.35 (s, 4 H), 6.68 (s, 2 H), 6.81 (d, J ) 1.6 Hz, 2 H),
7.45 (dd, J ) 7.9, 1.5 Hz, 2 H), 7.51 (dd, J ) 8.1, 1.8 Hz, 2 H),
7.67 (d, J ) 8.1 Hz, 2 H), 7.79 (d, J ) 7.9 Hz, 2 H). 13C NMR
(75 MHz, CDCl3): δ 33.4, 65.3, 120.8, 121.5, 122.1, 124.5,
127.3, 129.7, 131.5, 138.0, 139.6, 141.2, 147.9, 149.6. HRMS
[M+ + H] calcd for C27H1779Br4, 656.8064; found 656.8062. Anal.
Calcd for C27H16Br4: C, 49.13; H, 2.44. Found: C, 49.22; H,
2.66.
2,7-Dibr om o-2′,7′-bis(d ieth oxyp h osp h or ylm eth yl)-9,9′-
sp ir obiflu or en e (3). A mixture of 2 (310 mg, 470 µmol) and
triethyl phosphite (0.18 mL, 1.03 mmol) was heated at 150 °C
for 15 h. Excess triethyl phosphite was evaporated under
vacuum, and the residue was added to hexane. The precipitate
was washed with hexane to yield a white solid (352 mg, 97%).
1H NMR (300 MHz, CDCl3): δ 1.04 (t, J ) 7.0 Hz, 12 H), 3.00
(d, J H,P ) 21.6 Hz, 4 H), 3.74-3.89 (m, 8 H), 6.57 (s, 2 H), 6.77
(d, J ) 1.6 Hz, 2 H), 7.34 (d, J ) 7.7 Hz, 2 H), 7.46 (dd, J )
8.1, 1.7 Hz, 2 H), 7.65 (d, J ) 8.1 Hz, 2 H), 7.75 (d, J ) 7.8 Hz,
2 H). 13C NMR (75 MHz, CDCl3): δ 16.2 (d, J C,P ) 6.0 Hz),
33.8 (d, J C,P ) 137.1 Hz), 62.2 (d, J C,P ) 6.8 Hz), 65.2, 120.4
(d, J C,P ) 2.6 Hz), 121.5, 121.8, 125.2 (d, J C,P ) 6.4 Hz), 127.1,
130.1 (d, J C,P ) 6.3 Hz), 131.2, 131.6 (d, J C,P ) 9.3 Hz), 139.6,
140.1 (d, J C,P ) 1.9 Hz), 147.2 (d, J C,P ) 2.8 Hz), 150.2. HRMS
[M+] calcd for C35H3679Br2P2O6, 772.0353; found 772.0352.
Anal. Calcd for C35H36Br2P2O6: C, 54.40; H, 4.70. Found: C,
54.25; H, 4.87.
277 µmol) in distilled toluene (3.5 mL). The mixture was
degassed, and tetrakis(triphenylphosphine)palladium (8 mg,
2.5 mol %) was added in one portion under N2. The solution
was then heated at 110 °C for 14 h. The end groups were then
capped by heating under reflux for 12 h with benzeneboronic
acid (70.9 mg, 582 µmol) and then for 12 h with bromobenzene
(91 mg, 582 µmol). The reaction mixture was then cooled to
room temperature and precipitated into a mixture of MeOH
and H2O (1:1 v/v, 100 mL). The crude polymer was collected
and washed with excess MeOH. The resulting polymer was
dissolved in chloroform and reprecipitated into MeOH. The
polymer was then washed with acetone for 24 h using a
Soxhlet apparatus and dried under vacuum to give PF2-DPVF
1
(279 mg, 95%). H NMR (300 MHz, CDCl3): δ 0.69-0.77 (m,
10 H), 0.98-1.08 (m, 20 H), 2.02 (br, 4 H), 6.23 (s, 2 H), 6.83
(d, J ) 7.5 Hz, 4 H), 6.85-6.94 (m, 10 H), 7.11 (d, J ) 7.7 Hz,
2 H), 7.17-7.20 (m, 10 H), 7.41-7.43 (m, 2 H), 7.50-7.53 (m,
2 H), 7.57-7.66 (m, 6 H), 7.77 (d, J ) 7.7 Hz, 2 H). Anal. Calcd
for (C82H74)n: C, 92.96; H, 7.04. Found: C, 92.34; H, 7.61.
P r ep a r a tion of P F 4-DP VF . Following the procedure
described for P F 2-DP VF , the copolymerization of monomers
4 (150 mg, 181 µmol), 5 (99.0 mg, 181 µmol), and 6 (231 mg,
362 µmol) gave PF4-DPVF (259 mg, 78%). 1H NMR (300 MHz,
CDCl3): δ 0.70-0.79 (m, 30 H), 1.05-1.11 (m, 60 H), 2.06 (br,
12 H), 6.24 (s, 2 H), 6.84-6.94 (m, 14 H), 7.14-7.18 (m, 12
H), 7.45-7.81 (m, 24 H). 13C NMR (75 MHz, CDCl3): δ 14.1,
22.52, 22.56, 22.59, 23.9, 29.2, 30.0, 31.6, 31.7, 31.8, 40.4, 55.3,
65.6, 119.5, 120.0, 121.1, 121.5, 122.2, 125.2, 126.1, 126.7,
127.2, 127.4, 128.0, 128.2, 128.8, 129.8, 137.2, 139.7, 140.0,
140.3, 140.5, 140.9, 142.6, 143.1, 148.7, 149.2, 151.7, 151.8.
Anal. Calcd for (C140H154)n: C, 91.55; H, 8.45. Found: C, 91.08;
H, 8.38.
F a r br ica tin g Ligh t-Em ittin g Devices. Polymer LED
devices were fabricated having the configuration ITO/poly-
(styrenesulfonate)-doped poly(3,4-ethylenedioxythiophene)
(PEDOT) (35 nm)/light-emitting layer (50-70 nm)/TPBI (30
nm)/Mg:Ag (100 nm)/Ag (100 nm). To improve hole injection
and substrate smoothness, the PEDOT was spin-coated di-
rectly onto the ITO glass and dried at 80 °C for 12 h under
vacuum. The light-emitting layer was spin-coated on top of
the PEDOT layer using toluene as the solvent and then dried
for 3 h at 60 °C under vacuum. Prior to film casting, the
polymer solution was filtered through a Teflon filter (0.45 µm).
The TPBI layer, which was grown by thermal sublimation in
a vacuum of 3 × 10-6 Torr, was to be used as the electron-
transport layer that would block holes and confine excitons.21
The cathode Mg:Ag (10:1, 100 nm) alloy was deposited onto
the TPBI layer by coevaporation of the two metals, followed
by an additional layer of Ag (100 nm) deposited onto the alloy
as a protection layer. The current-voltage-luminance was
measured under ambient conditions using a Keithley 2400
source meter and a Newport 1835C optical meter equipped
with an 818ST silicon photodiode.
Resu lts a n d Discu ssion
2,7-Dib r om o-2′,7′-b is(2,2-d ip h en ylvin yl)-9,9′-sp ir ob i-
flu or en e (4). Potassium tert-butoxide (343 mg, 2.71 mmol)
was added to a solution of 3 (700 mg, 900 µmol) and benzophe-
none (494 mg, 2.71 mmol) in anhydrous THF (7 mL) under
nitrogen. The mixture was heated under reflux for 9 h, cooled,
and then added to water (100 mL). The precipitate was washed
with hexane and purified by column chromatography (hexane/
EtOAc, 5:1) to give 4 (459 mg, 61%). 1H NMR (500 MHz,
CDCl3): δ 5.98 (s, 2 H), 6.62 (d, J ) 1.5 Hz, 2 H), 6.82 (s, 2 H),
6.90-6.93 (m, 4 H), 6.97-7.02 (m, 6 H), 7.14 (dd, J ) 8.0, 1.4
Hz, 2 H), 7.21-7.24 (m, 10 H), 7.42 (dd, J ) 8.0, 2.0 Hz, 2 H),
7.47 (d, J ) 8.0 Hz, 2 H), 7.56 (d, J ) 8.0 Hz, 2 H). 13C NMR
(75 MHz, CDCl3): δ 64.9, 119.7, 121.0, 121.7, 124.8, 127.1,
127.3, 127.4, 127.7, 128.11, 128.14, 129.8, 130.2, 130.7, 137.4,
139.2, 139.7, 139.8, 142.9, 146.8, 149.8. HRMS [M+ + H] calcd
for C53H3579Br2, 829.1106; found 829.1104. Anal. Calcd for
Syn th esis of th e Mon om er a n d th e P olym er s.
Scheme 1 illustrates the synthetic route we followed to
prepare a spirobifluorene monomer containing bis(2,2-
diphenylvinyl) moieties. Bromomethylation of 117 with
paraformaldehyde in HBr/AcOH afforded compound 2,
which was then reacted with triethyl phosphite to yield
the phosphonate ester 3. The Horner-Emmons conden-
sation of 3 and benzophenone furnished the target
monomer, 2,7-dibromo-2′,7′-bis(2,2-diphenylvinyl)-9,9′-
spirobifluorene (4). The structure of monomer 4 was
characterized by 1H and 13C NMR spectroscopy. The
vinylic protons give rise to a singlet at δ 6.82, while the
signal for a quarternary carbon atom at δ 64.9 (C-9)
indicates the presence of the spiro skeleton. We syn-
thesized two random-fluorene copolymers, PF2-DPVF
and PF4-DPVF, which incorporate 50 and 25 mol % of
bis(2,2-diphenylvinyl) units, respectively, by the Suzuki
C
53H34Br2: C, 76.64; H, 4.13. Found: C, 76.75; H, 4.47.
P r ep a r a tion of P F 2-DP VF . Aqueous potassium carbon-
ate (2.0 M, 2.4 mL) and Aliquat 336 (ca. 34 mg) were added to
a mixture of monomers 4 (230 mg, 277 µmol) and 6 (178 mg,

Macromolecules, Vol. 37, No. 19, 2004
Spiro-DPVF-Containing Polyfluorenes 7199
Sch em e 1. Syn th etic Rou te for DP VF -Con ta in in g Mon om er
Sch em e 2. Syn th etic Sch em e for P olyflu or en es Con ta in in g DP VF P en d a n ts
Ta ble 1. Molecu la r Weigh ts a n d Th er m a l P r op er ties of
P F 2-DP VF a n d P F 4-DP VF
insoluble portion may correspond to higher-molecular-
weight material.
DSC (°C)
TGAc (°C)
5%
We investigated the thermal properties of these spiro-
DPVF-containing copolymers by thermogravimetric
analysis (TGA) and differential scanning calorimetry
(DSC); the results are also tabulated in Table 1. Both
polymers are thermally stable and have 5% weight loss
temperatures in nitrogen >410 °C. We did not detect
any possible phase transition signals during repeated
heating/cooling DSC cycles for PF2-DPVF. This obser-
vation probably results from the stiffness of this poly-
mer’s chains, which again is due to its higher content
of the rigid spiro-DPVF. In the case of PF4-DPVF, we
observed a distinct glass transition temperature at 107
°C, which is higher than that of poly(9,9-dioctylfluorene)
(POF; Tg ) ca. 75 °C).23 The increased value of Tg can
be attributed to the presence of the rigid diphenylvinyl-
9,9′-spirobifluorene unit, which enhances the molecular
rigidity of the polymer and restricts its segmental
mobility. It is important that PLEDs be constructed
from materials having a relatively high value of Tg to
avoid the problems associated with the formation of
aggregates and excimers upon exposure to heat.24
P h otop h ysica l P r op er ties. Because PF2-DPVF ex-
hibits poor solubility in organic solvents, which limits
this material’s use in PLED applications, in this study
we investigated the optical properties of PF4-DPVF
only. Figure 1 presents the absorption and PL spectra
of this fluorene copolymer as dilute solutions; the
spectral data are summarized in Table 2. The absorption
and PL spectra of compound 4, which serves as a model
compound in studying the optical properties of the bis-
(2,2-diphenylvinyl)fluorene (DPVF) pendants, are also
presented in Figure 1. In chloroform solution, PF4-
DPVF exhibits an absorption having its λmax at 387 nm,
which we ascribe to the combined π-π* transitions
originating from the polyfluorene backbone and pendent
groups because POF and compound 4, in chloroform,
a
a
b
polymer
Mn × 104
Mw × 104
Tg
PF2-DPVF
PF4-DPVF
0.86
1.5
2.5
3.6
n.o.d
107
410
423
a
Molecular weights were determined by GPC, eluting with
b
THF, by comparison with polystyrene standards. The value of
T
g was determined by DSC at a heating rate 20 °C min-1 under a
nitrogen atmosphere. c Temperature at which a 5% weight loss
occurred was determined at a heating rate of 20 °C min-1 under
d
a
nitrogen atmosphere. No noticeable phase transition was
observed in the temperature range 30-300 °C.
coupling reaction between dibromides 4 and 5 and the
diboronate 6 (Scheme 2).22 We estimated the composi-
tions of these copolymers by integrating the areas of the
1
peaks in the H NMR spectra. We attribute the reso-
nance at ca. 2.04 ppm to the aliphatic protons (4 H)
adjacent to the quarternary carbon atom (C-9) of the
fluorene unit, whereas the peak at 6.23 ppm represents
the protons (2 H) located in the spirobifluorene unit. The
ratios between the integrals of the former and the latter
are 1.9:1.0 and 5.8:1.0 for PF2-DPVF and PF4-DPVF,
respectively, which reveals that the incorporation ratios
of bis(2,2-diphenylvinyl) units are in good agreement
with the monomer feed ratios.
PF4-DPVF is readily soluble in common organic
solvents, such as toluene, chloroform, and THF, but
PF2-DPVF is only slightly soluble in chloroform and
exhibits poor solubility in other organic solvents as a
result of its higher content of the rigid spiro-DPVF (50
mol %). We determined the molecular weights of these
polymers by size exclusion chromatography (SEC) using
THF as the eluent and calibrating against polystyrene
standards; Table 1 lists the results. For PF2-DPVF, the
molecular weights are probably higher than the values
listed because it is only partially soluble in THF; the

7200 Su et al.
Macromolecules, Vol. 37, No. 19, 2004
F igu r e 2. Current density-voltage-luminance characteris-
tics of ITO/PEDOT/PF4-DPVF/TPBI/Mg:Ag.
F igu r e 1. (a) UV-vis absorption and PL spectra of PF4-DPVF
in chloroform solution and in the solid state. UV-vis absorp-
tion and PL spectra of (b) POF and (c) 4 in chloroform
solutions.
Ta ble 2. Op tica l P r op er ties a n d Qu a n tu m Yield s of
P F 4-DP VF , 4, a n d P OF
solutiona
filmb
abs
(nm)
PL
abs
(nm)
PL
(nm)c
Φfd
(nm)c
Φf
PF4-DPVF
4
POF
387
374
389
(424) 447 0.75
451
418 (442) 0.85
388
387
(426) 450 0.59e
424 (448) 0.55
a
b
Evaluated in chloroform. Prepared by spin-coating from a
toluene solution. c Excited at 380 nm. Determined in toluene
d
relative to 9,10-diphenylanthracence in cyclohexane (Φf ) 0.9) with
excitation at 365 nm. e Relative to the thin-film quantum efficiency
of POF (Φf ) 0.55) with excitation at 380 nm. f Peaks that appear
as shoulders or weak bands are indicated in parentheses.
F igu r e 3. EL spectra of the device ITO/PEDOT/PF4-DPVF/
TPBI/Mg:Ag at different voltages.
Red ox P r op er ties. Cyclic voltammetry (CV) mea-
surements of PF4-DPVF film coated on a glassy carbon
electrode were performed in an electrolyte of 0.1 M
tetrabutylammonium hexafluorophosphate (TBAPF6) in
acetonitrile using ferrocene as the internal standard.
On the basis of the onset potentials of the oxidation and
reduction, which were 0.91 and -2.61 V, we estimated
the HOMO and LUMO energy levels of PF4-DPVF to
be 5.71 and 2.18 eV, respectively, with regard to the
energy level of ferrocene (4.8 eV below vacuum).27 These
results are very close to the data reported for POF (Ip
) 5.8 eV, Ea ) 2.12 eV).28 Because the energy level of
ferrocene is determined by photoelectron spectroscopy
in the solid state, this method can be considered to
provide merely a rough approximation.
E lect r olu m in escen ce P r op er t ies of LE D De-
vices. To evaluate the potential of PF4-DPVF as a blue
emissive material in polymer LED applications, we
fabricated a triple-layered diode with a TPBI electron
injection/transport layer (ETL), introduced between the
light-emitting layer and the Mg:Ag cathode, for hole
blocking and exciton confinement.21 As indicated in
Figure 2, the device reaches a maximum brightness of
3137 cd/m2 at a bias of 9 V (261.5 mA/cm2); the
maximum external quantum efficiency of 1.06% occurs
at 6.5 V (64.7 mA/cm2). The EL spectra presented in
Figure 3 having their peak wavelengths at ca. 455 nm
are quite similar to the PL emission spectrum of PF4-
DPVF (Figure 1), which indicates that both the PL and
EL originate from the same radiative decay process of
the singlet exciton. Moreover, Figure 3 also demon-
strates that the PF4-DPVF-based device exhibits volt-
age-independent and stable EL spectra with Commis-
exhibit absorptions having λmax at 389 and 374 nm,
respectively. Upon excitation at 380 nm, we observed a
broad PL spectrum that has a main peak at 447 nm,
which is contributed from the emission of the DPVF
pendants, together with a minor contribution from the
PF main chain (a shoulder at 424 nm). This result
reveals that an efficient intramolecular energy transfer
occurs from the excited PF backbone to the DPVF side
chain, even though the spectral overlap between the
emission spectrum of the former and the absorption
spectrum of the latter is moderate (Figure 1b,c). PF4-
DPVF exhibits a high quantum efficiency in solution;
we measured its fluorescence yield (Φf) when excited
at 365 nm in toluene, using 9,10-diphenylanthrancene
(Φf ) 0.9) as a standard,25 to be 0.75, which is close to
the fluorescence yield measured for POF (Φf ) 0.85).
In comparison to the dilute solution, the absorption
spectrum of a PF4-DPVF thin film appears slightly
broadened, probably because of aggregate formation, but
without a spectral shift, while the emission spectrum
shows a red shift of 3 nm. It is noteworthy that the
emission from the polyfluorene is almost completely
suppressed in this PF4-DPVF film; instead, the PL
spectrum exhibits emission arising predominantly from
the DPVF pendants. The lack of polyfluorene emission
in the film indicates that an efficient Fo¨rster energy
transfer is facilitated by both intra- and interchain
interactions that result from the shorter polymer chain
distances in the solid state. We estimated the PL
quantum yield of the PF4-DPVF film to be 0.59 by
comparing its fluorescence intensity to that of a thin-
film sample of POF polymer that was excited at 380 nm
(Φf ) 0.55).26

Macromolecules, Vol. 37, No. 19, 2004
Spiro-DPVF-Containing Polyfluorenes 7201
F igu r e 5. Current density-voltage-luminance characteris-
tics of ITO/PEDOT/MEH-PPV:PF4-DPVF/TPBI/Mg:Ag.
F igu r e 4. (a) Absorption and emission spectra of MEH-PPV
in toluene and the solid-state PL spectrum of the binary blend.
(b) EL spectra of the device ITO/PEDOT/MEH-PPV:PF4-
DPVF/TPBI/Mg:Ag at different voltages.
utes predominately to the corresponding EL spectrum
(Figure 4b), which results in the white electrolumines-
cence having CIE coordinates of (0.29, 0.34) at 7 V. The
contribution from the MEH-PPV emission is obviously
more significant in the EL process than it is in the PL
process, which reveals that both Fo¨rster energy transfer
and direct charge trapping/recombination on the MEH-
PPV guest are responsible for the observed EL.30 The
inset of Figure 5 depicts the energy level diagram. The
highest occupied molecular orbit (HOMO) and lowest
unoccupied molecular orbit (LUMO) levels of PF4-DPVF
are 5.7 and 2.2 eV, respectively, while the ionization
potential and LUMO of MEH-PPV are 5.1 and 3.0 eV,31
respectively. These values imply that holes or electrons
can be potentially trapped by MEH-PPV with a depth
of 0.6-0.8 eV and recombined with opposite charges in
the blend device. Table 3 summarizes the performances
of the device based on the blend. The maximum external
quantum efficiency is 1.31% (3.40 cd/A) at a bias of 8 V
(28.2 mA/cm2) with a brightness of 957 cd/m2; the
maximum brightness (Figure 5) is 3258 cd/m2 at 11 V
(118.5 mA/cm2), and the corresponding EL spectrum is
almost identical to that recorded at 7 V. Furthermore,
the turn-on voltage for the binary blend device is 6.1 V,
while that for the host-only device is decreased to 4.6
V. This observation also supports the charge trapping
mechanism previously proposed.30a
sion Internationale de L’Eclairage (CIE) color coordinates
of (0.15, 0.15) at 7 V and (0.15, 0.17) at 11 V, which fall
in the region of pure blue on a CIE chromaticity
diagram.
Wh ite Electr olu m in escen ce fr om a Bin ar y Blen d.
It has been demonstrated that white polymer LEDs can
be achieved by emission from a binary blend composed
of a blue host polymer and a red-orange polymer used
as the guest material.29 In this study, we used PF4-
DPVF as the host material and blended it with 0.5 wt
% of MEH-PPV to realize white electroluminescence.
The PL spectrum of PF4-DPVF and the absorption
spectrum of MEH-PPV (Figure 4a) overlap to a reason-
able extent in the region 400-550 nm, which meets the
requirement for efficient energy transfer. The excitons
formed in the host, PF4-DPVF, by direct photoexcitation
are likely to migrate to the lower-energy dopant, MEH-
PPV. As a result, the PL profile of the blend contains
two peaks: one centered at 450 nm that originates from
the emission of the PF4-DPVF host, while another at
ca. 560 nm corresponds to the emission of MEH-PPV.
The emission peak of MEH-PPV from the blend is blue-
shifted by 30 nm relative to that of neat MEH-PPV and
is similar to that of the dilute solution of MEH-PPV in
toluene (Figure 4a). This observation indicates that the
interchain interactions between MEH-PPV chains are
reduced in the blend as a result of the dilution effect
induced by the host polymer. At a low dopant concen-
tration of 0.5 wt %, the PL spectrum is dominated by
the emission of the host because of incomplete energy
transfer. In contrast, the MEH-PPV emission contrib-
In summary, we have developed DPVF-containing
polyfluorenes having DPVF moieties as pendent groups
attached orthogonally to the 9-positions of some of the
fluorene units. Our PL studies indicate that color tuning
can be achieved through Fo¨rster energy transfer from
the higher-energy polyfluorene backbone to the lower-
energy DPVF pendants, from which the emission occurs.
This moderate red shift (ca. 25 nm) leads to a higher
Ta ble 3. P er for m a n ces of Devices Ha vin g Str u ctu r es ITO/P EDOT/EML/TP BI/Mg:Ag
device
PF4-DPVF
MEH-PPV (0.5 wt %): PF4-DPVF
turn-on voltage (V)a
4.6
6.9
1404
6.1
10.2
3078
voltage (V)b
brightness (cd/m2)b
luminance efficiency (cd/A)b
external quantum efficiency (%)b
max brightness (cd/m2)
max luminance efficiency (cd/A)
max external quantum efficiency (%)
EL max (nm)c
1.41
1.05
3137 (at 9 V)
1.42
1.06
3.08
1.19
3258 (at 11 V)
3.40
1.31
455
0.15, 0.15
561
0.29, 0.34
CIE coordinates, x and yc
a
b
At 1 cd/m2. At 100 mA/cm2. c At 7 V.

7202 Su et al.
Macromolecules, Vol. 37, No. 19, 2004
(9) Lee, J . I.; Klaerner, G.; Miller, R. D. Synth. Met. 1999, 101,
126. (b) Lee, J . I.; Klarner, G.; Miller, R. D. Chem. Mater.
1999, 11, 1083. (c) Yu, W.-L.; Pei, J .; Huang, W.; Heeger, A.
J . Adv. Mater. 2000, 12, 828. (d) Zeng, G.; Yu, W. L.; Chua,
S. J .; Huang, W. Macromolecules 2002, 35, 6907.
(10) Scherf, U.; List, E. J . W. Adv. Mater. 2002, 14, 477. (b) List,
E. J . W.; Gunter, R.; Scandiucci de Freitas, P.; Scherf, U. Adv.
Mater. 2002, 14, 374. (c) Gaal, M.; List, E. J . W.; Scherf, U.
Macromolecules 2003, 36, 4236. (d) Gong, X.; Iyer, P. K.;
Moses, D.; Bazan, G. C.; Heeger, A. J .; Xiao, S. S. Adv. Funct.
Mater. 2003, 13, 325.
luminance efficiency. An EL device based on PF4-DPVF
exhibits a voltage-independent and stable blue emission
having color coordinates of (0.15,0.17) at 11 V, a low
turn-on voltage at 4.6 V, and a maximum brightness of
3137 cd/m2 at 9 V. Moreover, we realized a white
electroluminescent device having CIE coordinates of
(0.29, 0.34) and a maximum brightness of 3258 cd/m2
using a blend of PF4-DPVF with 0.5 wt % of MEH-PPV
as the emitting material.
(11) Setayesh, S.; Marsitzky, D.; Mu¨llen, K. Macromolecules 2000,
33, 2016.
(12) Ego, C.; Marsitzky, D.; Becker, S.; Zhang, J .; Grimsdale, A.
C.; Mu¨llen, K.; MacKenzie, J . D.; Silva, C.; Friend, R. H. J .
Am. Chem. Soc. 2003, 125, 437.
Ack n ow led gm en t. We thank the National Science
Council for financial support. Our special thanks go to
Professor C.-H. Cheng, Dr. J .-P. Duan, and Dr. H.-T.
Shih for their support and cooperation regarding the
preparation and characterization of the light-emitting
devices.
(13) Hosokawa, C.; Higashi, H.; Nakamura, H.; Kusumoto, T.
Appl. Phys. Lett. 1995, 67, 3853. (b) Shaheen, S. E.; J abbour,
G. E.; Morrell, M. M.; Kawabe, Y.; Kippelen, B.; Peygham-
barian, N.; Nabor, M.-F.; Schlaf, R.; Mash, E. A.; Armstrong,
N. R. J . Appl. Phys. 1998, 84, 2324.
(14) Huang, Y.-S.; J ou, J .-H.; Weng, W.-K.; Liu, J .-M. Appl. Phys.
Lett. 2002, 80, 2782. (b) Cheon, K. O.; Shinar, J . Appl. Phys.
Lett. 2002, 81, 1738. (c) Li, G.; Shinar, J . Appl. Phys. Lett.
2003, 83, 5359. (d) Zheng, X. Y.; Zhu, W. Q.; Wu, Y. Z.; J iang,
X. Y.; Sun, R. G.; Zhang, Z. L.; Xu, S. H. Displays 2003, 24,
121. (e) Xie, W.; Hou, J .; Liu, S. Semicond. Sci. Technol. 2003,
18, L42. (f) Xie, W.; Liu, S.; Zhao, Y. J . Phys. D: Appl. Phys.
2003, 36, 1246.
(15) Wyszecki, G.; Stiles, W. S. Color Science: Concepts and
Methods, Quantitative Data and Formulae; J ohn Wiley &
Sons: New York, 1982; p 259.
(16) Tokailin, H.; Higashi, H.; Hosokawa, C.; Kusumoto, T. Proc.
SPIEsInt. Soc. Opt. Eng. 1993, 1910, 38.
(17) Pei, J .; Ni, J .; Zhou, X. H.; Cao, X. Y.; Lai, Y. H. J . Org. Chem.
2002, 67, 4924.
(18) Ranger, M.; Rondeau, D.; Leclerc, M. Macromolecules 1997,
30, 7686.
Refer en ces a n d Notes
(1) Burroughes, J . H.; Bradley, D. D. C.; Brown, A. R.; Marks,
R. N.; Mackay, K.; Friend, R. H.; Burns, P. L.; Holmes, A. B.
Nature (London) 1990, 347, 539.
(2) Kraft, A.; Grimsdale, A. C.; Holmes, A. B. Angew. Chem., Int.
Ed. 1998, 37, 402. (b) Friend, R. H.; Gymer, R. W.; Holmes,
A. B.; Burroughes, J . H.; Marks, R. N.; Taliani, C.; Bradley,
D. D. C.; Dos Santos, D. A.; Bre´das, J . L.; Lo¨gdlund, M.;
Salaneck, W. R. Nature (London) 1999, 397, 121. (c) Bernius,
M. T.; Inbasekaran, M.; O’Brien, J .; Wu, W. Adv. Mater. 2000,
12, 1737. (d) Mitschke, U.; Ba¨urele, P. J . Mater. Chem. 2000,
10, 1471.
(3) Kido, J .; Hongawa, K.; Okuyama, K.; Nagai, K. Appl. Phys.
Lett. 1994, 64, 815. (b) Kido, J .; Shionoya, H.; Nagai, K. Appl.
Phys. Lett. 1995, 67, 2281. (c) Lee, J .-I.; Kang, I.-N.; Hwang,
D.-H.; Shim, H.-K.; J eoung, S. C.; Kim, D. Chem. Mater. 1996,
8, 1925. (d) McGehee, M. D.; Bergstedt, T.; Zhang, C.; Saab,
A. P.; O’Regan, M. B.; Bazan, G. C.; Srdanov, V. I.; Heeger,
A. J . Adv. Mater. 1999, 11, 1349. (e) Chen, F.-C.; Yang, Y.;
Thompson, M. E.; Kido, J . Appl. Phys. Lett. 2002, 80, 2308.
(f) Swanson, S. A.; Wallraff, G. M.; Chen, J . P.; Zhang, W. J .;
Bozano, L. D.; Carter, K. R.; Salem, J .; Villa, R.; Scott, J . C.
Chem. Mater. 2003, 15, 2305.
(19) Neef, C. J .; Ferraris, J . P. Macromolecules 2000, 33, 2311.
(20) Shi, J .; Tang, C. W.; Chen, C. H. U.S. Patent 5,645,948, 1997.
(21) Culligan, S. W.; Geng, Y.; Chen, S. H.; Klubek, K.; Vaeth, K.
M.; Tang, C. W. Adv. Mater. 2003, 15, 1176.
(22) Miyaura, N.; Suzuki, A. Chem. Rev. 1995, 95, 2457.
(4) Pei, Q.; Yang, Y. J . Am. Chem. Soc. 1996, 118, 7416. (b)
Leclerc, M. J . Polym. Sci., Part A: Polym. Chem. 2001, 39,
2867. (c) Neher, D. Macromol. Rapid Commun. 2001, 22,
1365. (d) Becker, S.; Ego, C.; Grimsdale, A. C.; List, E. J . W.;
Marsitzky, D.; Pogantsch, A.; Setayesh, S.; Leising, G.;
Mu¨llen, K. Synth. Met. 2002, 125, 73.
(5) Setayesh, S.; Grimsdale, A. C.; Weil, T.; Enkelmann, V.;
Mu¨llen, K.; Meghdadi, F.; List, E. J . W.; Leising, G. J . Am.
Chem. Soc. 2001, 123, 946. (b) Marsitzky, D.; Vestberg, R.;
Blainey, P.; Tang, B. T.; Hawker, C. J .; Carter, K. R. J . Am.
Chem. Soc. 2001, 123, 6965. (c) Tang, H.-Z.; Fujiki, M.; Zhang,
Z.-B.; Torimitsu, K.; Motonaga, M. Chem. Commun. 2001,
2426. (d) Chou, C.-H.; Shu, C.-F. Macromolecules 2002, 35,
9673.
(6) Lee, J .-H.; Hwang, D.-H. Chem. Commun. 2003, 2836. (b) Wu,
F.-I.; Dodda, R.; J akka, K.; Huang, J .-H.; Hsu C.-S.; Shu, C.-
F. Polymer 2004, 45, 4257.
(7) Ego, C.; Grimsdale, A. C.; Uckert, F.; Yu, G.; Srdanov, G.;
Mu¨llen, K. Adv. Mater. 2002, 14, 809. (b) Pogantsch, A.;
Wenzl, F. P.; List, E. J . W.; Leising, G.; Grimsdale, A. C.;
Mu¨llen, K. Adv. Mater. 2002, 14, 1061.
(23) Grell, M.; Bradley, D. D. C.; Inbasekaran, M.; Woo, E. P. Adv.
Mater. 1997, 9, 798.
(24) Tokito, S.; Tanaka, H.; Noda, K.; Okada, A.; Taga, Y. Appl.
Phys. Lett. 1997, 70, 1929.
(25) Eaton, D. Pure Appl. Chem. 1998, 60, 1107.
(26) Grice, A. W.; Bradley, D. D. C.; Bernius, M. T.; Inbasekaran,
M.; Wu, W. W.; Woo, E. P. Appl. Phys. Lett. 1998, 73, 629.
(27) Pommerehne, J .; Vestweber, H.; Guss, W.; Mahrt, R. F.;
Ba¨ssler, H.; Porsch, M.; Daub, J . Adv. Mater. 1995, 7, 551.
(28) J anietz, S.; Bradley, D. D. C.; Grell, M.; Giebeler, C.;
Inbasekaran, M.; Woo, E. P. Appl. Phys. Lett. 1998, 73, 2453.
(29) Tasch, S.; List, E. J . W.; Ekstro¨m, O.; Graupner, W.; Leising,
G.; Schlichting, P.; Rohr, U.; Geerts, Y.; Scherf, U.; Mu¨llen,
K. Appl. Phys. Lett. 1997, 71, 2883. (b) Liu, J .; Shi, Y.; Yang,
Y. Appl. Phys. Lett. 2001, 79, 578.
(30) Uchida, M.; Adachi, C.; Koyama, T.; Taniguchi, Y. J . Appl.
Phys. 1999, 86, 1680. (b) Niu, Y.-H.; Yang, W.; Cao, Y. Appl.
Phys. Lett. 2002, 81, 2884.
(31) Alam, M. M.; Tonzola, C. J .; J enekhe, S. A. Macromolecules
2003, 36, 6577.
(8) Wu, F.-I.; Reddy, D. S.; Shu, C.-F.; Liu, M. S.; J en, A. K.-Y.
Chem. Mater. 2003, 15, 269. (b) Shu, C.-F.; Dodda, R.; Wu,
F.-I.; Liu, M. S.; J en, A. K.-Y. Macromolecules 2003, 36, 6698.
MA048797S