Poly(tetraarylindenofluorene)s: New stable blue-emitting polymers

Article abstract of DOI:10.1021/ma034849m

The problem of long wavelength emission from ketone defects in polyindenofluorenes can be overcome by the introduction of aryl substituents which are much less susceptible to oxidation. A new polyindenofluorene with octylphenyl side chains has been prepar

Full text of DOI:10.1021/ma034849m

8240
Macromolecules 2003, 36, 8240-8245
Poly(tetraarylindenofluorene)s: New Stable Blue-Emitting Polymers
J osem on J a cob, J in gyin g Zh a n g, An d r ew C. Gr im sd a le, a n d Kla u s Mu1 llen *
Max-Planck-Institute for Polymer Research, Ackermannweg 10, 55128 Mainz, Germany
Ma r tin Ga a l a n d Em il J . W. List
Christian Doppler Laboratory “Advanced Functional Materials”, Institute of Solid State Physics,
Graz University of Technology, Petersgasse 16, 8010 Graz, Austria, and Institute of Nanostructured
Materials and Photonics, Franz-Pichler-Strasse 30, A-8160 Weiz, Austria
Received J une 23, 2003; Revised Manuscript Received August 25, 2003
ABSTRACT: The problem of long wavelength emission from ketone defects in polyindenofluorenes can
be overcome by the introduction of aryl substituents which are much less susceptible to oxidation. A new
polyindenofluorene with octylphenyl side chains has been prepared via a terphenyl intermediate. Unlike
alkyl-substituted polyindenofluorenes, this displays stable blue emission in the solid state. Blue-emitting
LEDs have been fabricated whose emission is stable at voltages below 10 V, with brightnesses of up to
125 cd/m2 and efficiencies of up to 0.11 cd/A. Furthermore, no emission from ketone defects was detected
even in devices run at voltages above 20 V.
In tr od u ction
For PIFs (2) the long wavelength emission band
appears in the green around 565 nm. We have shown
that the amount of green emission seen from PIFs (2)
correlates with the degree of π-stacking shown by the
detection of fibrillar structures in the film morphologies
by atomic force microscopy (AFM) and with the propor-
tion of n-alkyl side chains on the polymers.8 Thus, a
polymer 2a with all n-alkyl side chains showed green
solid-state photoluminescence (PL) and electrolumines-
cence (EL), while a polymer 2b with branched alkyl side
chains initially produced blue EL, but the emission
rapidly red-shifted. Copolymers showed intermediate
behavior.
Luminescent conjugated polymers are of considerable
importance as the active materials in emerging new
technologies such as organic light-emitting diodes
(LEDs)1,2 and polymer lasers.3 One area of major on-
going effort in academic and industrial research into
these materials is the development of materials which
show efficient, stable blue emission.4 Much research into
blue-emitting materials has centered on phenylene-
based polymers such as polyfluorenes (PFs, 1),5,6 poly-
indenofluorenes (PIFs, 2),7,8 and ladder-type polyphen-
ylenes (LPPPs, 3) (Figure 1).9
All of these materials show blue to blue-green emis-
sion in solution, which red-shifts with increasing rigidity
of the polymer chain with emission maxima of 420 nm
for 1, 430 nm for 2, and 450 nm for 3. In the solid state
the appearance of longer wavelength emission bands
causes the emission color to become green or yellow.5,7-9
The source of this long wavelength emission was
initially believed to be excimer emission from aggregates
formed by π-stacking of the polymer chains.10 It has
been found that polymers 1 with n-alkyl side chains
display a unique packing behavior in the condensed
phase. For such samples, as well as the regular, glassy
(R-)phase, a novel phase, the so-called â-phase, has been
identified which shows a distinct red shift of absorption
and emission peaks.6
Recently, the occurrence of the long wavelength
emission band for 1 has been correlated to the emission
from ketonic defects incorporated in the polymer back-
bone as 9-fluorenone units.6,11,12 Furthermore, the syn-
thesis of fluorene-fluorenone copolymers with con-
trolled amounts of fluorenone and quantum chemical
calculations have established a quantitative correlation
between the 9-fluorenone content and the low-energy
emission band intensity.13 Thus, the evidence favors the
main source of the long wavelength emission in poly-
fluorenes (1) being from ketone defects. It has also been
proposed that the yellow emission seen from LPPP (3)
arises from ketonic defect sites.14
Given the similarity of the structures and of the
observed phase-forming and optical behavior of PIFs
and PFs with the same types of alkyl substituents, it is
likely that the long-wavelength emission in PIFs stems
from similar sources to that in PFs. Moreover, delayed
PL measurements in solution show the presence of an
emission band at 545 nm,15 which is intermediate
between the positions of the corresponding defect emis-
sion bands seen for PFs (1) and LPPPs (3), as would be
expected for defect emission from a PIF. This suggests
that the long wavelength emission band in poly-
indenofluorenes also arises from emission from ketone
defect sites. We currently believe the evidence favors
emission from ketone defects as being the most probable
source of the long wavelength emission in PIFs, and
investigations are underway to test this, the results of
which will be published separately.
We have previously demonstrated that stable blue EL
can be obtained from polyfluorenes with bulky den-
drimer 416 or triphenylamine 517 side chains (Figure 2).
The stability of the blue emission from polymers 4 and
5 arises from their greater resistance to oxidation and
a reduction in excimer diffusion to any ketone defect
sites that might form. It has been shown that exciton
diffusion is affected by the dendrimer side chains in the
stable blue-emitting polyfluorene 4.18
From the above results it can be seen that polyin-
denofluorenes 6 with aryl substituents would be ex-
pected to show stabler blue emission than the alkyl-
substituted polymers 2. In this paper we report the
* To whom correspondence should be addressed: Fax +49 6131
379350; e-mail muellen@mpip-mainz.mpg.de.
10.1021/ma034849m CCC: $25.00 © 2003 American Chemical Society
Published on Web 10/07/2003

Macromolecules, Vol. 36, No. 22, 2003
Poly(tetraarylindenofluorene)s 8241
F igu r e 1. Typical blue-emitting conjugated polymers.
-78 °C gave the diol 12 in 77-81% yield. Displacement
of the silyl groups with bromine and sodium acetate
followed by ring closure of 8 with concentrated sulfuric
acid in hot acetic acid then gave the desired monomers
7 in 87-92% yield (71% overall). These were then
polymerized with bis(cycloocta-1,7-diene)nickel(0) and
2,2′-bipyridine in toluene-DMF to give the polymers 6.
The tert-butyl substituent in 7a was found to be too
poor a solubilizing group to retain the growing polymer
chain of 6a in the reaction medium, so that the product
was determined to be a mixture of oligomers by GPC
(Mn ) 1940, Mw ) 3470), but the octylphenyl groups in
7b allowed formation of a high molar mass polymer 6b
(Mn ) 66 400, Mw ) 257 000). These GPC results were
obtained using polyphenylene standards20 which are
much closer in structure to the rigid-rod polymers 6
than the polystyrene standards commonly used for GPC
calibration and therefore can be expected to give more
accurate mass values. This corresponds to a degree of
polymerization of about 66 units (ca. 200 benzene rings).
The absorption and emission spectra for polymer 6b
are shown in Figure 3. The absorption was strong in
the violet with the maximum at 417 nm. The PL was
blue both in solution (λmax ) 428 nm) and as a thin film
(λmax ) 434 nm) (Figure 3). The small Stokes shift of
only 11 nm indicates the rigidity of the polymer. The
emission from the oligomeric 6a was only slightly blue-
shifted (∼1 nm) compared with the polymer 6b as the
oligomers were close to the effective conjugation length
previously determined for PIFs (n ) 5).6 The solid-state
PL spectra of 6b closely resembles that of the ethylhexyl
polymer 2b with two peaks at 434 and 457 nm and a
small shoulder at about 515 nm. No sign of the broad
green emission band at 560 nm seen for the tetraoctyl
polymer 2a was observed.
P h otoin d u ced Absor p tion a n d Electr olu m in es-
cen ce Mea su r em en ts on P olym er 6b. Figure 4 shows
the absorbance and PL emission spectra for polymer 6b
in the solid state. The PL and absorption spectra are
very similar to the spectra observed in solution, how-
ever, exhibiting a bathochromic shift of a ca. 60 meV.
The PIA spectrum (Figure 4) is characterized by one
broad feature with a maximum at ca. 810 nm. From a
comparison to various polyfluorene derivatives this band
is assigned to a transition from the lower to a higher
lying triplet state (T1-Tn absorption). Note that this
band is similar to the triplet PIA of polymer 3b.
F igu r e 2. Stable blue-emitting polyfluorenes.
synthesis of poly(tetraarylindenofluorene)s and their
emission properties.
Resu lts a n d Discu ssion
As aryl substituents cannot be attached directly to
indenofluorene, an indirect synthesis of the desired
monomerss2,8-dibromo-6,6,12,12-tetraarylindenofluo-
rene (7)swas needed. In such a synthetic route the five-
membered rings would be prepared by ring closure of a
terphenyl 8 with two diarylmethanol substituents. Such
methods have been used in the synthesis of LPPP (3)9
and of the monomer of the dendronized polyfluorene
(4).16a As bromination of indenofluorenes is difficult to
control so as to get only substitution at the 2- and
8-positions,6 we decided to use the electrophilic dis-
placement of a trimethylsilyl group with bromine at
these positions to introduce the bromines without
danger of overbromination. The key intermediate for the
synthesis of the desired monomers 7 was therefore the
bis(trimethylsilyl)terphenyl 9. Addition of 4 equiv of an
aryllithium followed by conversion of the trimethylslilyl
groups to bromines was expected to give 7 in good yield.
The synthesis thus devised of the polymers 6 is shown
in Scheme 1.
LEDs were constructed with structure ITO/PEDOT:
PSS (80 nm)/6b/Ca (50 nm)/Al(200 nm). The thickness
of the active layer was varied by varying the concentra-
tion of the polymer 6b in toluene used for spin-coating.
A solution of 5 g/L gave films of 70-80 nm thickness,
Suzuki coupling of dimethyl 2,5-dibromoterephthalate
(10)19 with 4-(trimethylsilyl)benzeneboronic acid (11)
proceeded smoothly to give the terphenyl 9 in 92% yield.
Addition of 4 equiv of aryllithium in dry THF at

8242 J acob et al.
Macromolecules, Vol. 36, No. 22, 2003
Sch em e 1. Syn th esis of P oly(6,6,12,12-tetr a a r ylin d en oflu or en e)s 6
while a solution of 10 g/L produced films of 150 nm.
LEDs made using the thicker films required operating
voltages above 10 V, while for the devices with thinner
films the emission onset was at 5 V. The emission from
all devices had maxima in the blue at 435 and 455 nm
with a long tail into the red (Figure 5).
under argon. By contrast, the devices operated at higher
bias voltages with driving current densities of 0.5-1.0
A/cm2 showed an increase in emission from the long
The emission from the devices with a 70 nm thick
layer of 6b showed a pure blue emission (Figure 6), with
a maximum luminance of 125 cd/m2 and an efficiency
of 0.11 cd/A for the best devices tested. The long
wavelength emission from devices with thicker emissive
layers was more intense due to greater self-absorption
within the thicker layer, with a maximum luminance
of up to 108 cd/m2 and an efficiency of up to 0.11 cd/A.
The stability of the blue emission was found to be
dependent upon the driving current of the device. The
devices with 70 nm thick emissive layers were stable
below 50 mA/cm2 at ca. 7 V bias and showed no
significant degradation when operated over 10-15 min
F igu r e 4. PL emission (black) and excitation (red) spectra
for a film of polymer 6b at 300 K together with the photo-
induced absorption spectrum (blue) of 6b at 77 K (laser
excitation at 351 and 363 nm).
F igu r e 5. EL emission spectrum and current-voltage and
luminance-voltage graphs for device ITO/PEDOT:PSS/6b(70
nm)/Ca/Al.
F igu r e 3. Absorption (solid) and PL emission in solution
(dashed) and thin film (dots) of 6b.

Macromolecules, Vol. 36, No. 22, 2003
Poly(tetraarylindenofluorene)s 8243
F igu r e 6. Emission from devices operated at 7 V (left) and 16 V (right). The whitish color of the latter is due to saturation of the
CCD camera at high intensities.
clear from these results that poly(tetraarylindenofluo-
rene)s are extremely promising materials for stable blue
emission in LEDs. More recent detailed device studies
on PF and PIF based devices have revealed that the long
wavelength tail between 500 and 650 nm may stem from
defects induced by the deposition of the metal cathodes
as also suggested by the Santa Barbara group.11 How-
ever this mechanism is not correlated with the forma-
tion of oxidation-related emissive bulk defects, i.e., keto-
type defects, but only with the conditions under which
the cathode is deposited. Such defects cannot be over-
come by a synthetic strategy as presented here, but can
only be avoided by choosing proper interface layers to
protect the polymer during deposition of the cathode
metal.11 Identifying the exact nature of this defect as
well as testing and optimizing such protection layers
which may simultaneously act as electron transport
layers will be the subject of further studies.
Con clu sion s
Polyindenofluorenes with aryl substituents on the
methine bridges have been prepared in high yields by
a short, efficient synthetic route. These polymers, unlike
previous indenofluorene-based polymers, show stable
blue PL and EL emission. Efficient, stable blue-emitting
LEDs have been fabricated using the polymer 6b. The
F igu r e 7. (a) EL emission from device ITO/PEDOT:PSS/6b
improved stability is attributed to the greater resistance
(150 nm)/Ca/al at 16 V and j < 0.5 A/cm2 over time. (b) EL
of the polymers to oxidation.
emission from same device driving current densities between
0.5 and 1 A/cm2 (19 and 25 V).
Exp er im en ta l Section
wavelength tail between 500 and 650 nm when operated
Ma ter ia ls a n d Meth od s. Commercial grade reagents were
for several minutes (Figure 7). This is dissimilar to the
used without further purification. Solvents were purified and
change in emission seen for the dendronized polyfluo-
dried using standard procedures. 4-Octylbromobenzene and
rene 4 when run in continuous mode for extended
periods as no clear peak around 520-550 nm appears.15b
Increasing the bias in these devices between 19 and 25
V led to a marked increase in an emission band at 500
nm, but again with no sign of a new band as would be
seen from the formation of a ketone defect due to
oxidation of the bulk polymer. Above 25 V the devices
melted. The green emission band at 565 nm seen for
polyindenofluorenes 2,8 which we propose is due to the
formation of ketone defects analogous to those produced
upon in polyfluorenes 1, was not observed for any device
using 6b. This suggests that 6b is much more resistant
to oxidation than 2.
4-trimethylsilylbenzeneboronic acid were purchased from com-
mercial sources and used as received. 1H and 13C NMR spectra
were recorded in CDCl3 on a Bruker DRX-250 spectrometer.
GPC measurements used PL-gel columns (three columns, 10
µm gel, pore widths: 500, 104, and 105 Å) connected with
ultraviolet/visible (UV/vis) detectors. UV-vis transmission
spectra were measured using a Perkin-Elmer Lambda9 spec-
trophotometer. PL emission and excitation spectra were
recorded using a Shimadzu RF5301 spectrofluorometer or a
SPEX Fluorolog 2 type 212 spectrometer.
The ITO-covered glass substrates for the PLEDs were
thoroughly cleaned in a variety of organic solvents and exposed
to an oxygen plasma dry cleaning step. PEDOT:PSS (Baytron
P from Bayer Inc.) layers were spin-coated under ambient
conditions and dried according to specifications by Bayer Inc.
under inert atmosphere. The emissive polymer films were spin-
Though some further optimization of devices is re-
quired to obtain higher brightness and efficiency, it is

8244 J acob et al.
Macromolecules, Vol. 36, No. 22, 2003
cast from solution and dried at 80 °C overnight in vacuo. Metal
electrodes were thermally deposited in a Balzers MED010
vacuum coating unit at base pressures of below 2 × 10-6 mbar.
EL spectra were recorded using an ORIEL spectrometer
with an attached CCD camera. The current/luminance/voltage
(ILV) characteristics were recorded in a customized setup
using a Keithley 236 source measure unit for recording the
current/voltage characteristics while recording the luminance
using a calibrated photodiode attached to an integrating
sphere (Ulbrich).
For PIA measurements the sample was excited with the 351
and 363 nm pump beam of the Ar+ laser (100 mW), which was
mechanically chopped at 17 Hz, providing the reference for
the lock-in amplifier. The sample was mounted in an optically
accessible cryostat under a dynamic vacuum of less than 10-5
mbar to prevent photooxidation. A 200 W halogen lamp
provided the source for the transmission measurement. All PA
spectra were measured at 77 K and were corrected for the PL
and optical throughput of the setup.
dichloromethane. The extract was washed with sodium bicar-
bonate solution, dried, and then concentrated. The residue was
purified by chromatography on silica using 0-2% ethyl acetate
in hexane as eluent to give 7b as a white solid (751 mg, 87%).
Elemental analysis: Calcd: C, 78.33; H, 7.96%. Found: C,
77.89; H, 7.79%. FDMS m/z ) 1166.9. 1H NMR (CD2Cl2): δ
7.73 (s, 2H) 7.49 (m, 6H) 7.10 (m, 16H) 2.56 (t, J ) 8.5 Hz,
8H) 1.58 (m, 8H) 1.28 (m, 40H) 0.88 (t, J ) 8.5 Hz, 12H). 13C
NMR: δ 154.84, 152.26, 143.10, 142.68, 140.13, 139.70, 131.35,
129.96, 129.20, 128.70, 122.44, 122.00, 118.63, 65.57, 36.24,
32.66, 32.25, 30.23, 30.20, 30.01, 23.45, 14.65.
Monomer 7a was prepared from diol 12a as a white solid
in 92% yield by the same procedure. Elemental analysis:
Calcd: C, 76.59; H, 6.43%. Found: C, 76.61; H, 6.43%. FDMS
m/z 940.60. 1H NMR: δ 7.66 (s, 2H) 7.48 (m, 2H) 7.41 (m, 4H)
7.21 (d, J ) 8.5 Hz, 8H) 7.05 (d, J ) 8.5 Hz, 8H) 1.28 (s, 36H).
The solubility was too poor for 13C NMR.
Syn th esis of P oly(tetr a a r ylin d en oflu or en e)s 6. 397 mg
of bis(cyclooctadiene)nickel(0) (1.44 mmol), 225 mg of 2,2′-
bipyridine (1.44 mmol), and 0.18 mL of cyclooctadiene (1.47
mmol) were added to a Schlenk flask containing 6 mL of
toluene and 6 mL of DMF in a glovebox. The resulting solution
was heated under an argon atmosphere for 30 min at 60 °C in
an oil bath, and then 700 mg of the monomer 7b (0.6 mmol)
was added dropwise. The mixture was heated at 95 °C for 4
days; then 0.1 mL of bromobenzene was added, and the
reaction continued for a further 12 h. The solution was poured
into 300 mL of 1:1 concentrated hydrochloric acid and metha-
nol and stirred for 4 h. The solution was extracted with 500
mL of toluene, and the crude polymer 6b obtained on removal
of solvent was redissolved in 50 mL of THF. Reprecipitation
from methanol gave 595 mg of polymer 6b after drying (98%
yield). GPC (THF, PPP standards): Mn ) 6.64 × 104 g/mol,
Mw ) 2.57 × 105 g/mol; D ) 3.86. Elemental analysis: Calcd:
C, 90.79; H, 9.22%. Found: C, 90.60; H, 9.36%. 1H NMR: δ
7.49 (br, 6H) 7.05 (br, 18H) 2.52(br, 8H) 1.37 (br, 44H) 0.98
(br, 16H).
Syn th esis of th e Tr ip h en ylen e Diester , 9. To 1.50 g of
the dibromo diester 1017 (4.26 mmol) in a 100 mL flask, 2.07
g of 4-(trimethylsilyl)benzeneboronic acid (11) (2.5 equiv) was
added followed by 1.77 g of K2CO3, 30 mL of toluene, and 15
mL of water. The flask was purged with argon for 15 min, and
then 197 mg of (Ph3P)4Pd was added and the mixture was
heated with stirring at 70 °C in an oil bath for 24 h when TLC
analysis showed complete consumption of 11. The crude
reaction mixture was extracted with toluene, and the extract
was concentrated and chromatographed on silica using 2-30%
ethyl acetate in hexane as eluent. The product was isolated
as a white solid. Yield ) 1.91 g (92%). Melting point ) 212-
213 °C. Elemental analysis: Calcd: C, 68.53; H, 6.98%.
1
Found: C, 68.08; H, 7.17%. H NMR (CDCl3): δ 7.85 (s, 2H)
7.64 (d, J ) 8.0 Hz, 4H) 7.39 (d, J ) 8.0 Hz, 4H) 3.73 (s, 6H)
0.36 (s, 18H). 13C NMR: δ 169.38, 142.41, 141.63, 141.29,
134.66, 134.62, 133.43, 129.08, 53.53, -1.23.
Syn th esis of th e Diols 12. To 1.47 mL of 4-octylbromoben-
zene (6 mmol) in 75 mL of dry THF cooled to -78 °C in an
acetone/dry ice bath, 3.95 mL of n-butyllithium (1.6 M, 6.3
mmol) was added dropwise with stirring. After 20 min, 0.50 g
of 9 (1 mmol) dissolved in 20 mL of THF was added dropwise;
the mixture was allowed to slowly warm to room temperature
over 14 h, and then the reaction was quenched with brine.
The crude product was extracted into ether, and the extract
was washed with brine and dried. Removal of solvent followed
by recrystallization from hexane gave 12b as a white solid (975
mg, 81%). Melting point ) 177-178 °C. Elemental analysis:
Calcd: C, 82.91; H, 9.67%. Found: C, 82.86; H, 9.61%. FDMS
Polymer 6a was obtained from monomer 7a in 78% yield
by the same procedure. GPC (THF, PPP standards): Mn ) 1.94
× 103 g/mol, Mw ) 3.74 × 103 g/mol; D ) 1.73.
Ack n ow led gm en t. We acknowledge the financial
support of the Bundesministerium fu¨r Bildung und
Forschung (Projects 13N8165 OLAS and 13N8215
OLED). J .J . acknowledges the Alexander von Humboldt
Stiftung for the grant of a Research Fellowship. The
CDL-AFM is a key member of the long-term AT&S
research strategies.
1
m/z ) 1186.6. H NMR (CD2Cl2): δ 7.23 (d, J ) 7.9 Hz, 4H)
7.07 (m, 16H) 6.72 (d, J ) 7.9 Hz, 4H) 6.60 (s, 2H) 2.84 (s, 2H)
2.60 (t, J ) 7.3 Hz, 8H) 1.61 (m, 8H) 1.31 (m, 40H) 0.89 (t, J
) 7.0 Hz, 12H) 0.23 (s, 18H). 13C NMR: δ 145.28, 144.31,
142.78, 142.31, 139.68, 139.15, 134.45, 133.22, 128.99, 128.19,
128.13, 83.23, 35.82, 32.34, 31.89, 29.90, 29.72, 29.65, 23.10,
14.30, -1.10.
Refer en ces a n d Notes
(1) Kraft, A.; Grimsdale, A. C.; Homes, A. B. Angew. Chem., Int.
Ed. 1998, 37, 402.
Diol 12a was prepared as a white solid in 77% yield by
addition of 4-tert-butylphenyllithium to 9 under the same
conditions. Elemental analysis: Calcd: C, 82.77; H, 8.58%.
Found: C, 81.87; H, 8.53%. FDMS m/z 963.70. 1H NMR: δ
7.26 (d, J ) 8.5 Hz, 8H) 7.18 (d, J ) 8.2 Hz, 4H) 7.06 (d, J )
8.5 Hz, 8H) 6.69 (d, J ) 8.2 Hz, 4H) 6.60 (s, 2H) 2.85 (br s,
2H) 1.29 (s, 36H) 0.32 (s, 18H). The solubility was too poor for
13C NMR.
Syn th esis of th e Mon om er s 7. To 0.88 g of 12b (0.74
mmol) dissolved in 12 mL of THF, 128.0 mg of sodium acetate
(2 equiv) was added, and the mixture was cooled to 0 °C. To
this, 0.16 mL of bromine (4.2 equiv) was added, and the
mixture was stirred for 20 min. Then, 0.87 mL of triethylamine
(8 equiv) was added followed by aqueous sodium sulfite
solution. The crude 8b obtained after extraction with dichlo-
romethane was used without further purification for the next
step.
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containing 3 drops of H2SO4 and heated at 90 °C for 3 h. The
mixture was cooled, and the product was extracted into
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