Three color random fluorene-based oligomers for fast micrometer-scale photopatterning

Article abstract of DOI:10.1021/cm902254f

In this contribution we show that random fluorene cooligomers with photo reactive acrylate units can be prepared in a simple 1-step Yamamoto synthesis. The acrylate functionalities are preserved quantitatively under Yamamoto conditions.NMRand Maldi-ToF measurements point to an almost statistical incorporation of the comonomers into the oligomer chain. Maldi-ToF analyses give a further insight into the chain compositions, and we found fluorene-only oligomers to be present in low quantities. Thin films of the aromatic amine containing cooligomer show a blue fluorescence, the benzothiadiazole oligomer shows yellow photoluminescence, and the bithiophene oligomer emits orange-red light upon excitation. Compared to pure fluorene oligomers with a HOMO of 5.7 eV the HOMOlevels of the TPD and bithiophene derivatives are increased to 5.25 and 5.31 eV, respectively, whereas theHOMO level of the benzothiadiazole oligomer is decrased to 5.85 eV. Photolithography experiments reveal that a careful optimization of the conditions, for example, the choice of the photoinitiator, temperature, and irradiation wavelength, leads to well-resolved micrometer sized patterns. A minimum feature size of 1 μm was obtained. Thus we showed that with a simple 1-step Yamamoto coupling oligomers with photo-cross-linkable acrylate groups are accessible. UV irradiation leads to densely cross-linked, insoluble networks. Thus these materials are ideal candidates for multilayer as well as patterned semiconducting devices.

Full text of DOI:10.1021/cm902254f

1410 Chem. Mater. 2010, 22, 1410–1419
DOI:10.1021/cm902254f
Three Color Random Fluorene-Based Oligomers for Fast
Micrometer-Scale Photopatterning
Esther Scheler and Peter Strohriegl*
€
Macromolecular Chemistry 1, University of Bayreuth, Universitatsstr. 30, 95440 Bayreuth, Germany
Received July 23, 2009. Revised Manuscript Received December 7, 2009
In this contribution we show that random fluorene cooligomers with photo reactive acrylate units
can be prepared in a simple 1-step Yamamoto synthesis. The acrylate functionalities are preserved
quantitatively under Yamamoto conditions. NMR and Maldi-ToF measurements point to an almost
statistical incorporation of the comonomers into the oligomer chain. Maldi-ToF analyses give a
further insight into the chain compositions, and we found fluorene-only oligomers to be present in
low quantities. Thin films of the aromatic amine containing cooligomer show a blue fluorescence, the
benzothiadiazole oligomer shows yellow photoluminescence, and the bithiophene oligomer emits
orange-red light upon excitation. Compared to pure fluorene oligomers with a HOMO of 5.7 eV the
HOMO levels of the TPD and bithiophene derivatives are increased to 5.25 and 5.31 eV, respectively,
whereas the HOMO level of the benzothiadiazole oligomer is decrased to 5.85 eV. Photolithography
experiments reveal that a careful optimization of the conditions, for example, the choice of the
photoinitiator, temperature, and irradiation wavelength, leads to well-resolved micrometer sized
patterns. A minimum feature size of 1 μm was obtained. Thus we showed that with a simple 1-step
Yamamoto coupling oligomers with photo-cross-linkable acrylate groups are accessible. UV
irradiation leads to densely cross-linked, insoluble networks. Thus these materials are ideal
candidates for multilayer as well as patterned semiconducting devices.
Introduction
the substrate, which is difficult to achieve when it comes
to very small pattern sizes. Thus to obtain a reasonable
separation in the range of e10 μm prepatterned sub-
strates are often used.2 This adds considerable complexity
and costs to the production process.
Conjugated organic materials have been in the focus of
research for more than 20 years due to their various
applications in organic light emitting devices (OLEDs)
or organic field effect transistors (OFETs). The first
commercially available products based on semiconduct-
ing organic molecules were launched in the late 1990s, and
since then constant research and development has opened
up many more applications. A critical feature in the
manufacturing of such devices often is the spatial pattern-
ing of the organic material.1 Furthermore in view of the
ongoing miniaturization in the electronic world it can be
expected that the feature sizes of patterns from organic
semiconductors will become smaller in the future.
In general, there are two different approaches for
lateral patterning: additive and subtractive techniques.
Additive methods include vacuum deposition using sha-
dow masks and printing techniques. The vacuum deposi-
tion technique is only suitable for small molecules, needs
expensive equipment, and the scalability to larger sub-
strate sizes is difficult. In contrast, printing techniques
allow a roll-to-roll production process and are therefore
most suitable for the production of low cost and large
area devices. A critical feature in the printing process is
the spatial resolution. The single droplets must not mix on
Subtractive techniques include lithographic methods,
like photolithography or electron beam lithography. Here,
a chemical modification of the organic material is induced
by light or an electron beam, which leads to different
solubilities of the exposed and nonexposed areas. With
lithographic techniques, very small feature sizes of less
than 100 nm are attainable. To realize these small feature
sizes ultraprecise equipmentand cleanroom conditions are
necessary, which makes the process expensive. For feature
sizes in the range of 1-10 μm the cost argument is less
pronounced. However conventional photolithography is
not well suited to pattern conjugated organic semiconduc-
tors.3 A very promising approach is the use of conjugated
organic polymers with pendant photo-cross-linkable
groups. Such materials can be processed exactly like
negative photoresists. Thus the direct formation of single
pixels or patterns of arbitrary size and shape becomes
possible. A second advantage of such compounds is
their multilayer capability. Upon cross-linking the organic
(2) Sirringhaus, H.; Kawase, T.; Friend, R. H.; Shimoda, T.; Inbasekaran,
M.; Wu, W.; Woo, E. P. Science 2000, 290, 2123.
(3) Huang, J.; Xia, R.; Kim, Y.; Wang, X.; Dane, J.; Hofmann, O.;
Mosley, A.; de Mello, A. J.; de Mello, J. C.; Bradley, D. D. C.
J. Mater. Chem. 2007, 17, 1043.
*Author to whom correspondence should be addressed. Phone: þ49/921/
553296. Fax þ49/921/553206. E-mail peter.strohriegl@uni-bayreuth.de.
(1) Holdcroft, S. Adv. Mater. 2001, 13, 1753.
r
pubs.acs.org/cm
Published on Web 01/12/2010
2010 American Chemical Society

Article
Chem. Mater., Vol. 22, No. 4, 2010 1411
semiconductor becomes insoluble and allows the succes-
sive spincoating of a second layer.
at a heating rate of 40 K/min. The thermogravimetric analyses
(TGA) were performed on a Netsch Simultane Thermoanalysis
apparatus STR 409 C with a heating rate of 10 K/min under N2.
The abosrption spectra were recorded with a Hitachi U-3000
spectrophotometer and the photoluminescence spectra were
measured with a Shimadzu RF-5301 PC spectrofluorometer.
For the solution measurements distilled chloroform was used.
The HOMO energy values were measured using a Riken Keiki
AC-2 apparatus. For scanning electron microscopy (SEM) a
Phenom from Fei Company was used. For the AFM experi-
ments a commercial AFM (Dimension 3100 equipped with a
Nano-Scope IV SPM controller and a XY closed-loop scanner,
all from Veeco) was used. The 1H NMR spectra were obtained
in CDCl3 with a Bruker AC 250. A Finnigan Mat 8500 Mat
112 S Varian with EI iniziation was used for the mass spectra.
The fluorescence microscope used was a Leica DMR-SP with
selective filter systems (here: dichromatic mirror 400 nm, sup-
pression filter LP 420 nm).
It was shown that oxetane-substituted light-emitting
polymers can be directly patterned with lithographic
techniques under mild conditions, and resolutions of 2
μm were obtained.4,5 Photo-cross-linkable acrylates for
example for passive optical elements like polarizers are
also described in the literature.6-11 Further on fluorene
based polymers were intensively studied due to their
strong blue fluorescence properties and are frequently
used as emitting materials in OLEDs.12,13
In recent papers we have described the synthesis
and photopatterning of acrylate functionalized oligo-
fluorenes.14,15 Fluorene copolymers with electron donat-
ing or withdrawing comonomers have become increas-
ingly important in organic electronics. Copolymers
incorporating aromatic amines like TPD (N,N0-bis(3-
methylphenyl)-N,N0-bis(phenyl)-benzidine) are used as
hole conductors16 and bithiophene copolymers such
Film Preparation and Irradiation. For the formation of thin
films, the oligomers and 1 wt % of photoinitiator Irgacure 784
(Ciba Geigy) were mixed. From this mixture 4 wt % solutions in
toluene were prepared and spin coated onto cleaned silicon
wafers at 1000 rpm for 60 s. For the spin coating and develop-
ment procedure purified solvents were used. The film thick-
nesses were approximately 100 nm. The irradiation was per-
formed with a Hg/Xe lamp Ushio UXM 200H with an intensity
of 70mW/cm2 and a selective filter UG5 (Schott).
ꢀ
as F8T2 (poly(9,9-dioctylfluorene-co-bithiophene)) are
known to exhibit high mobilities in organic field effect
transistors.17
We have now extended our studies on photopattern-
able fluorene acrylates to cooligomers with TPD, ben-
zothiadiazole, and bithiophene units. In this paper we
describe the synthesis, photophysical characterization,
and photopatterning of three novel cross-linkable fluor-
ene cooligomers.
Materials. The Yamamoto and Suzuki reagents Ni(COD)2,
COD, bipyridyl, Pd(OAc)2, P(o-tol)3, and Aliquat were used as
received from Aldrich. CCl4, dry DMF and dry toluene were
used as received from Fluka. Na-tert-butylat, N-Bromosuccini-
mide (NBS), 5,50-dibromo-2,20-bithiophene, and tritert-butyl-
phosphin were used as obtained from Aldrich. CuBr2-Al2O3,18
2-(4,4,5,5-tetramethyl-1,3,2-dioxaborolane-2-yl)-9,9-dihexylfluo-
rene19, and 4,7-dibromo-2,1,3-benzothiadiazole20 were prepared
according to literature methods. The acrylate monomers F and E
were synthesized as described elsewhere.15
Experimental Section
Measurements. The molecular weights were determined by a
Waters gel permeation chromatography system (GPC) for
polymers (2 analytical columns: cross-linked polystyrene gel,
length 2 ꢀ 30 cm, width 0.8 cm, particle size 5 μm, eluent THF
(0.5 mL/min), oligofluorene calibration). The Maldi-ToF spec-
tra were recorded with a Bruker Reflex III with a high-mass
detector in the linear mode using POPOP (1,4-Bis(5-phenyl-2-
oxazolyl)benzene) as matrix. The differential scanning calorim-
etry (DSC) analyses were performed on a Perkin-Elmer DSC7
N,N0-bis(4-methylphenyl)-N,N0-diphenyl-benzidine (1). A Sch-
lenk flask was charged with 4 g (12 mmol) N,N0-diphenylbenzi-
dine, 6.1 g (36 mmol) 4-bromotoluene, 0.08 g (0.34 mmol)
Pd(OAc)2 and 3.05 g (31 mmol) sodium-tert-butylat under argon.
75 mL dry freshly distilled THF wereadded. 7.8g (1 mmol) tritert-
butylphosphin were added and the solution was heated to 80 °C
upon stirring. After 3 h the solution was filtered over neutral
alumina and washed with THF. The solvent was removed and
compound 1 precipitated into MeOH. After drying 4.6 g (75%)
€
(4) Muller, C. D.; Falcou, A.; Reckefuss, N.; Rojahn, M.; Wiederhirn,
V.; Rudati, P.; Frohne, H.; Nyken, O.; Becker, H.; Meerholz, K.
Nature 2003, 421, 829.
1
of 1 were obtained as light brown powder. H NMR (CDCl3):
δ 7.4 (s, 5H), 7.07 (s, 21 H), 2.32 (s, 6H). m/z 515 (Mþ, 100%), 258
(M2þ, 80%)
€
(5) Gather, M. C.; Kohnen, A.; Falcou, A.; Becker, H.; Meerholz, K.
Adv. Func. Mater. 2007, 17, 191.
(6) Wu, G.; Yang, C.; Fan, B.; Zang, B.; Chen, X.; Li, Y. J. Appl.
Polym. Sci. 2006, 100, 2336.
(7) Yao, Y.-H.; Kung, L.-R.; Chang, S.-W.; Hsu, C.-S. Liq. Cryst.
2006, 33, 33.
(8) Jandke, M.; Hanft, D.; Strohriegl, P.; Whitehead, K.; Grell, M.;
Bradley, D. D. C. Proc. SPIE 2001, 4105, 338.
(9) O’Neill, M; Kelly, S. M. Adv. Mater. 2003, 15, 1135.
(10) Broer, D. J.; Boven, J.; Mol, G. N.; Challa, G. Makromol. Chem.
1989, 190, 2255.
(11) Broer, D. J.; Lub, J.; Mol, G. N. Nature 1995, 378, 467.
(12) Grell, M.; Knoll, W.; Lupo, D.; Meisel, A.; Miteva, T.; Neher, D.;
Nothofer, H.-G.; Scherf, U.; Yasuda, A. Adv. Mater. 1999, 11, 671.
(13) Whitehead, K. S.; Grell, M.; Bradley, D. D. C.; Jandke, M.;
Strohriegl, P. Appl. Phys. Lett. 2000, 76, 2946.
N,N0-bis(4-methylphenyl)-N,N0-bis(4-bromophenyl)-benzidine
(C1). We weighed 2.5 g (5 mmol) of compound 1 and 1.72 g
(10 mmol) NBS into a flask. Twenty-five mL of chloroform were
added, and the solution was stirred at room temperature (RT)
for 1 h. We added 12.5 mL of acetic acid, and the solution was
stirred for further 6.5 h at RT. The product was extracted with
diethyl ether and the solvent was removed. The crude product
was purified by column chromatography using hexane/toluene
(14) Scheler, E.; Bauer, I.; Strohriegl, P. Macromol. Symp. 2007, 254,
203.
(15) Scheler, E.; Strohriegl, P. J. Mater. Chem. 2009, 19, 3207.
(16) Redecker, M.; Bradley, D. D. C.; Inbasekaran, M.; Wu, W. W.;
Woo, E. P. Adv. Mater. 1999, 11, 241.
(18) Kodomari, M.; Satoh, H.; Yshitomi, S. J. Org. Chem. 1988, 53,
2093.
(19) Thiem, H.; Jandke, M.; Hanft, D.; Strohriegl, P. Macromol. Chem.
Phys. 2006, 207, 370.
(17) Sirringhaus, H.; Wilson, R. J.; Friend, R. H.; Inbasekaran, M.; Wu,
W.; Woo, E. P.; Grell, M.; Bradley, D. D. C. Appl. Phys. Lett. 2000,
77, 406.
(20) Mancilha, F. S.; Da Silveira Neto, B. A.; Lopes, A. S.; Moreira,
P. F., Jr.; Quina, F. H.; Goncalves, R. S.; Dupont, J. Eur. J. Org.
Chem. 2006, 21, 4924.

1412 Chem. Mater., Vol. 22, No. 4, 2010
Scheler and Strohriegl
Results and Discussion
5:1 as eluent. After drying 2.35 g (75%) of C2 were obtained as
1
pale yellow needles. H NMR (CDCl3): δ 7.36 (m, 8H), 7.23
Oligomer Synthesis. Scheme 1 shows the overall reaction
pathway to the acrylate functionalized fluorene cooligo-
mers O1-3. The preparation of the comonomers C1 and
C2 is described in Scheme 2. The TPD-comonomer C1 was
prepared in a two-step synthesis. In the first step diphenyl-
benzidine and p-bromo-toluene were coupled using palla-
dium acetate as catalyst. The resulting TPD derivative 1
was brominated in its two vacant para-positions using
NBS. The overall yield was 56%. The benzothiadiazole
trimer C2 had to be designed because we found that the
strong electron withdrawing 4,7-dibromo-2,1,3-benzothia-
diazole was not incorporated into the polyfluorene chains
in the Yamamoto polymerization. The trimer 2 was pre-
pared from a fluorene borolane and 4,7-dibromo-2,1,3-
benzothiadiazole in a Suzuki coupling with 70% yield
(Scheme 2 right). Compound 2 was then selectively bromi-
nated in the two terminal positions of the fluorene rings
using activated CuBr2 on alumina to yield in 30% C2.21 The
bithiophene comonomer C3 is commercially available.
The Yamamoto condensation was chosen as aryl-aryl
coupling method, because acrylates are tolerated and the
coupling reaction can be carried out without any protecting
groups.14,15 Conjugated polymers often suffer from their
low solubility. In the fluorene cooligomers described here
the comonomers do not contain any solubilizing substitu-
ents. To ensure a good solubility and processability of
the cooligomers we have limited the molecular weights by
the addition of a monofunctional end-capper E during the
polymerization. The end-capper terminates the chain
growth in a controlled way and residual bromine end groups
are avoided.15 Further on, decreased molecular weights also
led to lower glass transition temperatures as we have shown
previously.22 Low glass transition temperatures allow low
processing temperatures during photolithography and
photodegradation upon UV illumination is prevented.
From our experience on fluorene oligomers we chose a ratio
of 6:1 of dibromo monomers (FþC) to the monobromi-
nated end-capper E for oligomers O1 and O3. In the case of
the comonomer C2 with higher molecular weight a ratio
(FþC2)/E of 4:1 was applied. For the cooligomerization
∼70 mol % of the fluorene compound F and ∼30 mol % of
the comonomers C1-3 were used to ensure a good solubi-
lity of the oligomers and to introduce enough acrylate
functionalities for a fast photo-cross-linking reaction.
The molecular weights of O1-3 were determined by gel
permeation chromatography (GPC) and were in the
range of 5200-5800 g/mol (oligofluorene calibration,
see Table 1). The molecular weight distributions are
monomodal and are shown in Supporting Information
(SI) Figure S1. Details about the generation of the oligo-
fluorene GPC calibration are also given in the SI (Figures
S2 and S3). The ratio of fluorene acrylate to comonomers
C1-3 incorporated into the oligomer chains were deter-
mined with NMR analyses (Table 1). We found that the
(m, 2H), 7.11 (m, 14H), 2.34 (s, 6H). m/z 672 (Mþ, 100%),
336 (25%).
4,7-bis(9,90-dihexylfluorene-2-yl)-2,1,3-benzothiadiazole (2). We
weighed 0.22 g (0.75 mmol) 4,7-dibromo-2,1,3-benzothiadiazole
and 0.69 g (1.5 mmol) 2-4,4,5,5-tetramethyl-1,3,2-dioxaboro-
lane-2-yl)-9,9-dihexylfluorene into a Schlenk flask under argon.
Thirty mL of toluene, 15 mL of 2 M K2CO3, and one drop of
Aliquat were added. The mixture was degassed by three freeze-
thaw cycles beforeitwas heated to65 °C for 12 h. The product was
extracted with diethyl ether and the solvent was removed. The
crude product was purified by column chromatography using
hexane/toluene 2:1 as eluent. 0.416 g (70%) of 2 were obtained
as yellow crystalline material. 1H NMR (CDCl3): δ 8.1 (m, 2H),
7.95 (s, 2H), 7.88 (m, 4H), 7.77 (m, 2H), 7.36 (d, 6H), 2.03 (m, 8H),
1.08 (s, 24H), 0.77 (t, 20H). m/z 800 (Mþ, 100%).
4,7-bis[2,7-dibromo-(9,90-dihexylfluorene-2-yl)]-2,1,3-benzothia-
diazole (C2). A flask was charged with 0.219 g (0.27 mmol) 2
and 0.91 g CuBr2-Al2O3. Thirty mL CCl4 were added and the
suspension was stirred at 70 °C for two weeks. The CuBr2-Al2O3
was filtered off and washed with CH2Cl2. The crude product
was purified by recrystallization from hexane. 70 mg (30%)
of C2 as yellow crystals were obtained. 1H NMR (CDCl3): δ 8.1
(m, 2H), 7.95 (s, 2H), 7.88 (m, 4H), 7.64 (m, 2H), 7.50 (d, 4H), 2.00
(m, 8H), 1.10 (s, 24H), 0.77 (t, 20H). m/z 958 (Mþ, 100%).
Oligo[(9-(2-ethylhexyl)-9-((6-acryloyloxy)-hexyl)-fluorene-2,7-
diyl)-co-2,20-bithiophene] (O3). A Schlenk flask was charged
with nickeldicyclooctadiene (Ni(COD)2, 0.45 g, 1.64 mmol),
cyclooctadiene (COD, 0.17 g, 1.64 mmol), 2,20-bipyridyl (0.25 g,
1.64 mmol), and 20 mL dry DMF under argon. The mixture was
degassed by three freeze-thaw cycles before it was heated to 80 °C
for 30 min while stirring. Appropriate amounts of monomer F
(0.203 g, 0.34 mmol), the end-capper E (0.04 g, 0.08 mmol) and C3
(0.048 g, 0.147 mmol) were weighed into a separate flask under
argon. A trace of BHT (2,6-ditert-butyl-p-cresol) and 40 mL of
dry toluene were added and the mixture was degassed by three
freeze-thaw cycles. Subsequently the monomer mixture was
added to the catalyst mixture using a cannula. The reaction
mixture was stirred at 80 °C for five days in the dark. Afterward
it was poured into methanol/HCl(conc.) 1:1 and stirred at room
temperature for two hours. The organic layer was separated
from the HCl layer which was then washed with Et2O. The
combined organic layers were washed with water and the solvent
was evaporated. The crude product was washed with alkaline
EDTA solution (5%) then filtered over a short neutral alumina
column using toluene as eluent. It was reprecipitated twice
from THF into methanol and dried under vacuum, yielding
0.130 g (50%) of O3 as a red powder. 1H NMR (CDCl3): δ 7.72
(m, 7.9H), 6.3 (d, 1H), 6.06 (m, 1H), 5.74 (d, 1H), 4.01 (s, 2.6H),
2.05 (s, 5.6H), 0.887 (m, 36.7H). Mn (GPC, oligofluorene
calibration) 5500 g/mol.
The oligomers O1 and O2 were prepared in an analogous
manner, but the workup of O1 was slightly changed. The crude
product mixture of O1 was directly extracted with Et2O without
pouring it into MeOH/HCl(conc.) before.
Data for O1: 1H NMR (CDCl3): δ 7.63 (m, 10.9H), 7.14
(m, 8.9H), 6.28 (d, 1H), 6.04 (m, 1H), 5.73 (d, 1H), 4.00 (s, 2.1H),
2.35 (s, 2.6H), 2.05 (s, 4.7H), 0.887 (m, 27.8H). Mn (GPC,
polyfluorene calibration) 5200 g/mol, yield 84%.
Data for O2: 1H NMR (CDCl3): δ 7.72 (m, 9.7H), 7.34
(m, 1.2H), 6.33 (d, 1H), 6.04 (m, 1H), 5.74 (d, 1H), 4.01 (m,
2.6H), 2.08 (s, 6.7H), 0.887 (m, 42.6H). Mn (GPC, polyfluorene
calibration) 5800 g/mol, yield 30%.
(21) Kodomari, M.; Satoh, H.; Yoshitomi, S. J. Org. Chem. 1988, 53,
2093.
(22) Scheler, E.; Strohriegl, P. Liq. Cryst. 2007, 34, 667.

Article
Chem. Mater., Vol. 22, No. 4, 2010 1413
Scheme 1. Synthetic Route for the Preparation of the Acrylate Containing Fluorene Cooligomers O1-3. the Photoinitiator Irgacure 784 Is
Displayed below
comonomer feeds correlate well with the contents in the
oligomers. Hence all monomers seem to have similar
reactivities in the Yamamoto coupling and a statistical
linkage is most probable. However a statement about
the monomer sequence in the chains is not possible, but
the probability for the formation of block sequences is
very low. The NMR spectra of O1-3 are given in the SI
(Figures S4-S6).
Thermal Properties. All oligomers were characterized
by both differential scanning calorimetry (DSC) and
thermogravimetric analysis (TGA), and the results are
summarized in Table 1. The decomposition temperatures
are all in the range of 350 °C. The DSC scans of the
oligomers showed no signs of melting or recrystallization.
The glass transition temperatures (Tgs) were 50 and
58 °C °C for O2 and O3, respectively, and 119 °C for

1414 Chem. Mater., Vol. 22, No. 4, 2010
Scheme 2. Synthesis of the Comonomers C1 and C2
Scheler and Strohriegl
Table 1. Composition, Molecular Weights, And Thermal Analyses of O1-3
ratio
(FþC):E
feed C1-3
[mol %]
found C1-3
Mn
[g/mol]b
Mw
[g/mol]b
Td,onset
[°C]d
oligomer
comonomer
[mol %]a
Tg [°C]c
O1
O2
O3
C1
C2
C3
6:1
4:1
6:1
30
20
30
30
25
33
5200
5800
5500
7900
7400
6100
119
50
58
340
355
350
a Comonomer content determined by NMR. b Values determined by GPC, oligofluorene calibration. c Measured with DSC upon heating with a rate
of 40 K/min. d Onset of decomposition determined by thermogravimetry with a heating rate of 10 K/min under N2.
Figure 1. Left: Maldi-ToF spectrum of O1. Right: Magnification and assignment of the signals between 2100 and 3400 g/mol. The spectrum was recorded
in the linear mode using POPOP as matrix. F corresponds to a fluorene and TPD to a C1 unit according to Scheme 1. F4TPD2, for example, is an oligomer
consisting of four fluorene and two TPD repeat units.

Article
Chem. Mater., Vol. 22, No. 4, 2010 1415
Figure 2. Normalized absorption and emission spectra of O1-3 (a-f) in chloroform solutions and as thin films. The photographical insets show the
chloroform solutions and thin films upon excitation at 366 nm.
O1. The transition enthalpies were very small and only
detectable using a heating rate of 40 K/min. They were
determined upon heating and due to the high heating rate
the values are expected to be slightly overestimated. The
value of 119 °C measured for O1 is in the range of the
reported value for TPD containing polyfluorenes.16,23
For an efficient photopolymerization the acrylate units
have to be mobile. Thus photo-cross-linking experiments
with organic semiconductors are usually carried out at
elevated temperatures to provide an appropriate mobility
of the reactive groups. Here the low Tgs of O2 and O3
allow low processing temperatures, which helps to avoid
photodegradation during UV exposure.
Due to the limited molecular weights of the oligomers
Maldi-ToF measurements could be performed and the
Maldi-ToF spectrum of O1 is shown in Figure 1. Oligo-
mer O1 is well suited for Maldi-ToF investigations since
aromatic amines are known to form stable molecular
(23) Sato, H.; Ogino, K.; Hirai, H. Macromol. Symp. 2001, 175, 159.

1416 Chem. Mater., Vol. 22, No. 4, 2010
Table 2. Optical Properties of Chloroform Solutions and Thin Films of O1-3
Scheler and Strohriegl
solution
films
UV λmax (nm)
PL λem. (nm)
UV λmax (nm)
PL λem. (nm)
O1
O2
O3
375
373, 430
373, 430
439 (375)a
375
375, 440
375, 440
442 (375)a
415, 437, 537 (373)a 540 (430)a
534 (375, 440)a
568 (375, 440)a
413, 438, 477, 520 (373)a 521 (430)a
a The values in brackets give the corresponding excitation wavelength in nm.
ions. The spectrum was recorded in the linear mode with
POPOP (1,4-Bis(5-phenyl-2-oxazolyl)benzene) as matrix.
The signals are well resolved in the range of 1000-4500
g/mol. A magnification of the spectrum between 2100
and 3400 g/mol is also shown in Figure 1 right. Every
Table 3. Energy Levels and Optical Bandgaps of O1-3
optical
HOMO
[eV]a
LUMO
[eV]b
bandgap
[eV]c
O1
O2
5.25
5.85
5.31
5.70
2.30
3.29
2.91
2.73
2.95
2.56
2.40
2.97
single peak can be assigned to an oligomer with a distinct
composition of TPD and F (=fluorene) units and a
detailed assignment is included in Figure 1. In general
all expected products are found and different “series” can
be observed. Consequently the reactivities of fluorene
monomers F and E in the Yamamoto condensation seem
to be similar to those of C1-3. In the range of 2100-2500
g/mol all pentamers, between 2600 and 2900 g/mol all
hexamers, and from 3000 to 3400 g/mol all heptamers are
found. In the case of the hexamers, five different signals
are observed. The signal at lowest molecular weight
corresponds to a pure fluorene hexamer named F6 and
with increasing molecular weight the number of incorpo-
rated TPD units increases. The highest molecular weight
hexamer consists of two fluorene units F and four TPD
units (named F2TPD4). The occurrence of fluorene-only
oligomers is not unexpected if mathematical probability
considerations are performed. The probability of adding
a fluorene monomer F to a growing chain end is 70%
according to the monomer feed. If now the two end-capping
molecules E are fixed at both ends of the chain, the prob-
ability of a (hexamer)-chain consisting of six fluorene units
is 24%, assuming an equal reactivity of F and TPD.
Within the pentamers and heptamers pure fluorene
oligomers, named F5 and F7, respectively, are also found.
However the probability for the formation of fluorene-
only compounds decreases with increasing chain lengths
according to reaction statistics.
The existence of these pure fluorene oligomers has
major consequences on the spectroscopic properties in
solution as described later on. In the case of the pentamers
in addition to the expected oligomers a peak correspond-
ing to a monoend-capped pentamer (F1TPD4) is ob-
served. For higher molecular weight oligomers monoend-
capped species are not found. Oligomers with bromine
end groups are not present.
Optical Properties. The absorption and emission spec-
tra of O1-3 were measured in chloroform solution and
thin film and are shown in Figure 2. The solution spectra
are displayed left and the film spectra right. The results are
summarized in Table 2. In solution the TPD containing
oligomer O1 exhibits one absorption maximum at 375 nm.
Upon excitation at 375 nm one photoluminescence maxi-
mum at 439 nm is observed. The film spectra of O1 are
similar to the solution spectra, an absorption maximum
O3
Oligofluorened
a Measured with photoelectron spectrometry. b Calculated from the
HOMO and optical bandgap values. c Determined as intersection of
absorption and emission spectra (film spectra). d Oligofluorene with an
average degree of polymerization of 13 from ref 15.
at 375 nm and an emission maximum at 442 nm are found.
In the case of O2 the solution absorption spectrum shows
two maxima, a dominant maximum at 373 nm and a small
maximum at 430 nm (Figure 2c). Upon excitation at
373 nm a PL spectrum with three maxima is obtained.
The first two maxima at 415 and 437 nm correspond to the
typical oligfluorene emission pattern and result from the
fluorene-only oligomers present in the mixture. The third
maximum at 537 nm originates from the benzothiadiazole
containing cooligomers. If the excitation wavelength is
430 nm the fluorene-only emission is suppressed and only
the benzothiadiazole oligomers emit light with a max-
imum at 540 nm.24 The film spectra of O2 are displayed in
Figure 2d. Here the absorption also shows two maxima,
one dominant maximum at 375 nm and a weak one at
440 nm. An excitation at 375 and 440 nm lead to the same
PL maxima at 534 nm. Thisdemonstrates that in thin films
an intermolecular energy transfer between different oligo-
mer chains becomes possible. The energy is transferred
from the fluorene-only compounds to the benzothiadia-
zole oligomers and a pure yellow emission color indepen-
dent from excitation is obtained. The solution absorption
spectrum of O3 (Figure 2e) also exhibits two maxima, one
at 373 nm and one at 430 nm. An excitation at 373 nm
leads to a complex PL spectrum with four maxima. The
first two maxima at 413 and 438 nm display the emission
pattern of the pure oligofluorenes present in the mixture.
The other maxima at 477 and 520 nm originate from
the bithiophene cooligomers. However an excitation at
430 nm leads to a simpler PL spectrum with one dominant
maximum at 521 nm. Here only the bithiophene cooligo-
mers are excited and emit light, the fluorene-only emission
is suppressed. The thin film spectra of O3 are shown
in Figure 2f. The absorption spectrum again exhibits
two maxima, a dominant one at 375 nm and a weak one
(24) Chuang, C.-Y.; Shih, P.-I.; Chien, C.-H.; Wu, F.-I.; Shu, C.-F.
Macromolecules 2007, 40, 247.

Article
Chem. Mater., Vol. 22, No. 4, 2010 1417
Figure 3. (a) Photolithographic process for a (negative) fluorene resist system. The first step is the spin coating of the oligofluorene followed by the
irradiation with light through a mask (shown in detail in (b)). The exposure is carried out on a hot stage. In the last step the lithographic pattern is developed
by dissolving the nonexposed parts in THF. (b) Experimental setup of the exposure step. The film is irradiated in a closed chamber in vacuum to avoid
degradation processes.
at 440 nm. Upon excitation at both wavelengths (375 and
440 nm) the same PL spectrum with a maximum at 568 nm
is obtained. Thus an intermolecular energy transfer from
the fluorene-only oligomers to the bithiophene oligomers
takes place and pure emission colors independent from the
excitation wavelength are obtained. Here we note that for
O3 the PL maximum in solution is blue-shifted (47 nm)
compared to the film spectrum. This may be due to
well-known packing phenomena described for thiophene
compounds.25
In general the typical fluorene emission pattern found
for solutions of O2 and O3 also indicate that pure-
fluorene compounds are present in the mixtures, which
confirms the Maldi-ToF results. However for practical
applications as emitters in yellow and red OLEDs, the
oligomers are well suited since the blue oligofluorene
fluorescence is not present in thin films.
The HOMO levels were measured by photoelectron
spectrometry (Riken Keiki AC-2) from thin films. From
the optical bandgaps, determined as intersection of the
UV/vis and photoluminescence spectra, the LUMO va-
lues were calculated. All data are listed in Table 3. The
oligomers O1 and O3 with electron donating units have
HOMO energies of 5.25 and 5.31 eV respectively. In
comparison to a fluorene-only oligomer, whose data are
also displayed in Table 3, the HOMO values are in-
creased. For the oligomer O2 with an electron withdraw-
ing comonomer we find a slight decrease of the HOMO
level from 5.70 to 5.85 eV and a LUMO energy level of
3.29 eV, which is 0.59 eV lower than for a fluorene-only
oligomer. Both O2 and O3 have smaller bandgaps (2.56
and 2.40 eV, respectively) and decreased LUMO energy
values compared to a pure fluorene oligomer.
conditions were thoroughly investigated. Concerning the
use of the patterned oligomers as semiconductors it is
very important that, during the cross-linking procedure,
the chemical structure of the material is not altered.26
However, photo-cross-linking often causes a substan-
tial degree of photochemical degradation. To avoid
this, the processing conditions have to be very mild in
terms of temperature and exposure time. The new ma-
terials O1-3 are well suited due to their large number of
acrylate functionalities and their low glass transition
temperatures. To exclude any oxygen we put the samples
into a vacuum chamber during the exposure procedure
(Figure 3b).
The first processing step is the spin coating of the
oligomer onto cleaned silicon wafers (see Figure 3). After
that the films were dried at 115 °C for one minute under
inert atmosphere. The thickness of all films was about
100 nm. The second step is the UV-exposure. The samples
were put in a vacuum chamber as shown in Figure 3b,
annealed at a certain temperature for several minutes in
the chamber and then irradiated using a Hg-Xe lamp
while keeping the temperature. The heating during irra-
diation is essential because the chains have to be mobile to
polymerize. The last lithographic step is the development
of the patterns, which is achieved by dissolving the
nonirradiated areas in THF for 30 s.
Our previous experiments with photopolymerizable
oligofluorenes showed that the use of an additional
filter during exposure is helpful.15 In that case we used
the long pass filter GG400 (Schott) to cut off wavelengths
below 400 nm, which avoids the excitation of the fluorene
backbone. Thus most of the energy is absorbed by the
initiator Irgacure 784 and a very fast cross-linking can
be achieved. Here we note that the initiator Irgacure
784 shows absorption below 360 nm and above 400 nm.
Photolithography. With regard to the use of O1-3
as photopatternable materials the photo-cross-linking
(25) Yasuda, T.; Namekawa, K.; Iijima, T.; Yamamoto, T. Polymer
2007, 48, 4375.
(26) Qiang, L.; Ma, Z.; Zeng, Z.; Yin, R.; Huang, W. Macromol. Rapid
Commun. 2006, 27, 1779.

1418 Chem. Mater., Vol. 22, No. 4, 2010
Scheler and Strohriegl
Figure 4. Fluorescent microstructures of O1-3 observed under a fluorescence microscope with an excitation wavelength of 340-380 nm. Upon cross-
linking the filter UG5 was used and the detailed conditions were: O1: exposure at 40 °C for 2 min; O2: exposure at 50 °C for 6 min; O3: exposure at 50 °C
for 4.5 min. The corresponding SEM images are found in Figure 5. The numbers next to the stripes represent their width in μm.
Figure 5. SEM images of microstructured oligomers O1-3. White corresponds to the cross-linked material, black and dark gray represent the wafer.
For the SEM pictures the same samples were used as for the fluorescence microscopy images from Figure 4. The numbers next to the stripes represent their
width in μm.
Figure 6. Left: AFM height image of the 2.5 μm-lines of oligomer O3. Right: Cross section of the stripes as indicated in the height image.
Its chemical structure is presented in Scheme 1. For O1-3
we also tested the filter GG400, but the irradiation times
needed appeared to be very long (up to 15 min). We
attribute that to the absorption of O1-3 above 400 nm.
Irgacure 784 is not able to decompose into radicals,
because most of the light energy is absorbed by the
oligomers. Therefore we decided to irradiate using
wavelengths below 300 nm. We utilized the Schott filter
UG5, which transmits light between 230 and 400 nm and
blocks shorter and longer wavelengths. Here we find
much faster photo-cross-linking. For O1 an exposure of
2 min at 40 °C was sufficient enough for the formation of
well resolved patterns. In the case of O2 the films had to
be exposed for 6 min at 50 °C and for O3 an irradiation at

Article
Chem. Mater., Vol. 22, No. 4, 2010 1419
50 °C for 4.5 min were found to be most suitable. In each
case minimum resolutions of 1-2 μm were achieved.
Photooxidation was not observed, during the whole
process the fluorescence colors of the oligomers were
preserved, which was checked by fluorescence spectro-
scopy and microscopy.
prepared in a simple 1-step Yamamoto synthesis. The
acrylate functionalities are preserved quantitatively un-
der Yamamoto conditions. NMR and Maldi-ToF mea-
surements point to an almost statistical incorporation of
the comonomers into the oligomer chain. Maldi-ToF
analyses give a further insight into the chain compositions
and we found fluorene-only oligomers to be present in
low quantities. Thin films of the aromatic amine contain-
ing cooligomer show a blue fluorescence, the benzothia-
diazole oligomer shows yellow photoluminescence and
the bithiophene oligomer emits orange-red light upon
excitation. Compared to pure fluorene oligomers with a
HOMO of 5.7 eV the HOMO levels of the TPD and
bithiophene derivatives are increased to 5.25 and 5.31 eV,
respectively, whereas the HOMO level of the benzothia-
diazole oligomer is decrased to 5.85 eV. Photolithography
experiments reveal that a careful optimization of the
conditions, for example, the choice of the photoinitiator,
temperature, and irradiation wavelength, leads to well
resolved micrometer sized patterns. A minimum feature
size of 1 μm was obtained. Thus we showed that with a
simple 1-step Yamamoto coupling oligomers with photo-
cross-linkable acrylate groups are accessible. UV irradia-
tion leads to densely cross-linked, insoluble networks.
Thus these materials are ideal candidates for multilayer as
well as patterned semiconducting devices.
The quality of the patterns was investigated with
fluorescence microscopy and scanning electron micro-
scopy (SEM). In the fluorescence microscope the excita-
tion wavelength was 340-380 nm. Strong blue, yellow-
green, and red fluorescent micrometer-sized stripes are
observed (see Figure 4). A comparison of the fluorescence
colors of irradiated and nonirradiated films using the
fluorescence microscope showed that both films yielded
identical fluorescence colors. That points to the preserva-
tion of the chemical structure and thus the opto-electronic
properties of the oligomers during irradiation. The silicon
wafers appear black, which indicates a complete removal
of the polyfluorene in the nonexposed areas. The resolu-
tion goes down to 1 μm for O1 and O3 and 2 μm for O2.
From the same samples SEM images were taken and are
displayed in Figure 5. Here the same resolutions are
found as observed with fluorescence microscopy. For
O1 and O2 a slight overexposure is observed because
the micrometer sized lines are slightly broader than given
by the mask. To examine the slope and quality of the
patterns we performed AFM experiments. Figure 6 shows
an AFM height image and the corresponding cross-
section of the 2.5 μm lines of O3. The desired rectangular
line shape can not be observed, which points to nonideal
patterning conditions.
Acknowledgment. We thank Evonik Degussa GmbH and
especially Dr. Heiko Thiem for the Riken Keiki AC-2 and
SEM measurements. We thank Markus Hund, Irene Bauer,
Dr. Klaus Kreger for their help and the Bavarian Elite study
program Macromolecular Science for the financial support.
Conclusion
Supporting Information Available: Additional information
including Figures S1-S6. This material is available free of
charge via the Internet at http://pubs.acs.org.
In this publication we show that random fluorene
cooligomers with photo reactive acrylate units can be