Semiperfluoroalkyl polyfluorenes for orthogonal processing in fluorous solvents

Article abstract of DOI:10.1021/ma902179s

The challenging synthesis of semiperfluoroalkyl polyfluorenes and characterization of their electro-optical properties has been reported. Solutions of conjugated polymers can be dispensed onto the desired area by inkjet printing or screen printing, or for

Full text of DOI:10.1021/ma902179s

Macromolecules 2010, 43, 1195–1198 1195
DOI: 10.1021/ma902179s
Semiperfluoroalkyl Polyfluorenes for Orthogonal
Processing in Fluorous Solvents
The Suzuki cross-coupling reaction chosen for the synthesis
of target polymers required semiperfluoroalkyl fluorene dibro-
4
,18
mides 5 and 8 and diboronates 6 and 9 (Scheme 1).
While the
†
Jin-Kyun Lee, Hon Hang Fong, Alexander A. Zakhidov,
†
†
dibromide 8 was obtained conveniently through alkylation
reactions of 2,7-dibromofluorene 7 with semifluorinated iodide
3 under phase-transfer catalysis conditions, the same reaction
protocol did not work for the dibromide 5. 2,7-Dibromofluo-
ren-9-one was recovered with a large amount of black viscous
impurities. It is postulated that the semifluorinated iodide 2
undergoes an elimination reaction at elevated temperatures to
the corresponding vinyl compound, which results in the tarry
material. The synthetic pathway was thus modified to permit
‡
Georgia E. McCluskey, Priscilla G. Taylor,
†
§
§
Mitk’el Santiago-Berrios, H ꢀe ctor D. Abru ~n a,
‡
Andrew B. Holmes, George G. Malliaras, and
†
,
†
Christopher K. Ober*
†
Department of Materials Science and Engineering, Cornell
‡
University, Ithaca, New York 14853, School of Chemistry, Bio21
Institute, University of Melbourne, Victoria 3010, Australia, and
§
Department of Chemistry and Chemical Biology, Cornell
n
low temperature synthesis using BuLi as a base. Monodepro-
University, Ithaca, New York 14853
tonation and alkylation of fluorene 1 produced the monoalky-
lated intermediate which was again deprotonated and alkylated
to give the dialkyl compound 4 in 80% yield over two
steps. Subsequent bromination at 60 °C led to the desired
dibromide 5.
Received October 2, 2009
Revised Manuscript Received December 20, 2009
Since the discovery of electroluminescence (EL) in conjugated
polymers, polymeric light-emitting diodes (PLEDs) have been
A similar situation occurred during the synthesis of diboronate
6. The dibromide 8 was transformed into the diboronate 9 in
good yield through a series of lithiation reactions and substitu-
tions with isopropoxydioxaborolane. However, the same set of
reactions did not prove effective in converting the dibromide 5
into the diboronate 6. Only a large amount of the starting
material 5 was recovered after crystallization from acetone. By
1-4
extensively studied for display fabrication.
In order for these
devices to be integrated into displays, it is necessary to pattern the
light-emitting polymers into small, multilayered elements for full-
5,6
color visualization. In principle, PLEDs can access a wide
range of patterning options if suitable processing methods are
7,8
available. Solutions of conjugated polymers can be dispensed
9
,10
19
onto the desired area by inkjet printing or screen printing,
or
using the palladium catalyzed borylation method, the fluorene
form films on regions where a sacrificial photoresist material
defines the target.
diboronate 6 was isolated in 46% yield after double recrystalliza-
tions from acetone.
5
Recently, we have identified hydrofluoroethers (HFEs) as
chemically benign process solvents for nonfluorinated materials.
With all the monomers in hand, Suzuki polycondensation
reactions were attempted in toluene, employing parameters
11,12
2
0
Taking advantage of HFEs and a fluorinated photoresist, we
were able to define functional materials photolithographically
down to 5 μm-size patterns. This success prompted us to consider
an alternative orthogonal combination of materials for device
patterning, i.e., highly fluorinated functional polymers with
conventional photoresists and solvents. Fluorinated polymer
films with sufficient F content remain unaffected when exposed
to organic solvents even without cross-linking. To prove this
concept, we embarked on the synthesis of semiperfluoroalkyl
polyfluorenes. Polyfluorene is a well-known blue-emitting poly-
mer and can be conveniently copolymerized with a variety of
optimized for ordinary dialkyl polyfluorenes (Scheme 1). The
immediate problem was the early stage precipitation of high
F content polymers. Furthermore, the recovered solids were not
soluble in common organic solvents. This limited capability of
nonfluorinated solvents made it necessary to adopt fluorinated
liquids, in particular, fluorinated aromatic solvents. Benzotri-
2
1
fluoride was identified as a suitable alternative to toluene, which
finally enabled the successful syntheses of PR F10 and PR F12.
F
F
It is worth noting that the catalyst system based on Pd(PPh3)4
worked equally well in benzotrifluoride without a fluorinated
phosphine ligand.
1
3,14
monomer units to tune its emission.
The polymers were purified through repeated precipitations
In this communication, we report the challenging synthesis of
semiperfluoroalkyl polyfluorenes (Scheme 1) and characteriza-
tion of their electro-optical properties. Finally, a proof of concept
of successful orthogonal patterning of the fluorinated polymers
under conventional photolithographic conditions is described.
It is known that perfluoroalkyl moieties are strongly electron-
from MeOH, after which they appeared to have M ’s as high
n
-
1
as 25 000 g mol with polydispersity ca. 2 (Table 1). For
comparison, nonfluorinated poly(9,9-didodecylfluorene) (PF12)
was synthesized in toluene, which showed a similar molecular
weight to the fluorinated polymers. In thermogravimetric analysis
(
TGA), the semiperfluoroalkyl polyfluorenes exhibited higher
15
withdrawing, which can perturb the electronic characteristics of
decomposition temperatures (>400 °C) than the nonfluorinated
reference (Table 1 and Supporting Information). Differential
scanning calorimetry (DSC) did not provide information on glass
transition temperatures of the polymers. Only transitions possi-
bly associated with their liquid crystalline phase behavior could
16,17
polyfluorenes.
In the case of monomer synthesis, the alkyl
spacers give the added benefit of enabling S 2 reactions between
N
semiperfluoroalkyl halides [X-(CH ) (CF ) F] and fluorene
2
y
2 z
anions. It is, therefore, essential to insert alkyl spacers between
the polymer backbone and perfluoroalkyl moieties. The initial
2
5,26
be observed (Supporting Information).
Solubility tests con-
synthetic targets were set for polyfluorenes which have (CH2)
2
firmed that all the semiperfluoroalkyl polyfluorenes are only
soluble in fluorinated solvents, including HFE-7500, at least to
the extent that spin-coating from solution is suitable for EL
device fabrication.
Optical properties of the polyfluorenes, including absorption
maxima of the UV-vis spectra and photoluminescence are also
summarized in Table 1. It is evident that semifluorination of the
and (CH ) spacers (PR F10 and PR F12, respectively). The
2
4
F
F
resulting polymers have ca. 60% F content by weight, which is
sufficiently high to make the polymers soluble in HFE-7500 for
processing.
*Corresponding author. E-mail: cko3@cornell.edu.
r 2010 American Chemical Society
Published on Web 01/07/2010
pubs.acs.org/Macromolecules

1
196 Macromolecules, Vol. 43, No. 3, 2010
Scheme 1. Synthesis of Monomers and Polymers PR
Lee et al.
a
F F
F10 and PR F12
a
n
n
Reagents and conditions: (i) BuLi, 2 (1 equiv), THF, -78 °C f room temperature, 1 h, followed by BuLi, 2 (1.2 equiv), THF, -78 °C f room
temperature, 1 h; (ii) Br
Bu
2
with catalytic I , CHCl
2
3
, 60 °C, overnight; (iii) bis(pinacolato)diboron, Pd(dppf)Cl , KOAc, DMF, 80 °C, 2 h; (iv) NaOH,
2
t
4
NBr, toluene þ water, 80 °C, overnight; (v) BuLi, 2-isopropoxy-4,4,5,5-tetramethyl[1,3,2]dioxaborolane, THF, -78 °C f room temperature, 1.5 h;
(vi) Pd(PPh ) , Et and PhBr.
3
4
4
NOH, benzotrifluoride þ water, 90 °C, 2 h, end-capping with PhB(OH)
Table 1. Physical and Optical Properties of Semiperfluoroalkyl Polyfluorenes
2
-1 b
)
c
d
e
f
g
polymer
M
n
(M
w
/M ) (g mol
n
Tdecomp. (°C)
Absmax (nm)
Egap (eV)
EHOMO (eV)
PL/ELmax (nm)
h
PR
PR
PF12
a
F
F10
F12
25 000 (2.0);
29 000 (1.8);
19 000 (1.7);
452
433
418
380
391
395
2.90
2.88
2.80
-5.83
-5.83
-5.66
418
F
422
435
a
b
22 c
Poly(9,9-didodecylfluorene) (PF12) was prepared as a reference. GPC using fluorinated solvent (Supporting Information). Onset temperature
on TGA thermogram. UV absorption was measured using polymer thin films. Energy bandgap was extracted from the onset of absorption spectrum
d
e
f
employing a polymer thin film. Energy level was obtained by photoelectron spectroscopy in air.
23,24 g
h
Photoluminescence in thin film state. PL only.
alkyl side chains influences the energy band gap by lowering the
both the HOMO and LUMO levels of the polymer. In addition,
reducing the length of alkyl spacers from (CH ) to (CH ) results
in enlargement of the band gap. A detailed study into the
structure-property relationships of a more diverse range of
semiperfluoroalkyl polyfluorenes is in progress.
The EL characteristics were then investigated in a device
structure of ITO/PEDOT:PSS/light-emitting polymer/CsF/Al.
Films of the fluorinated polymers were deposited from HFE-
During our effort to improve the EL performances of the
semifluorinated polymers, we found that a small portion of
electron-accepting comonomers, including pyridine or benzotria-
2
4
2 2
2
7
zole, enhanced device performance. An EL device made of the
statistical copolymer P(R F12-R BTz) (Figure 1b, M = 26 000,
F
F
n
-
2
Supporting Information) turned on at 5 V, reached 1000 cd m at
10 V (Figure 1c), and exhibited a sky-blue emission spectrum
(ELmax = 490 nm, Figure 1d). The copolymer showed a current
efficiency of 1 cd A at 0.1 mA cm . The device lifetime was
also highly improved compared to semiperfluoroalkyl homo-
polymers. Currently, we are tuning polymer structures for full
color configuration.
-
1
-2
7
500 solution onto a hole injection layer of poly(3,4-ethylenedio-
xythiophene):poly(styrenesulfonate) (PEDOT:PSS). We observed
different EL behavior in the fluorinated polymers compared to
their corresponding nonfluorinated polymer PF12. In the case of
Finally, we attempted to prove that the highly fluorinated
polymers are patternable under conventional photolithographic
conditions (Figure 2a). A commercial photoresist film was
patterned lithographically on a prepatterned ITO substrate, onto
which an aqueous dispersion of PEDOT:PSS and a solution of
P(R F12-R BTz) in HFE-7500 (3 parts by volume)/benzotri-
PR F10, the emission brightness from the device was below
F
-
2
5
0 cd m . With PR F12, the device exhibited a similar emission
F
spectrum to its PL characteristics (Figure 1a), but the device
operated at a higher voltage (>10 V) than that of PF12 and its
lifetime was of the order of seconds. This difference in device
performance was attributed to the electron-withdrawing nature
F
F
fluoride (1 part by volume) mixed solvent were spin-coated
(Supporting Information). After vacuum deposition of CsF/Al
cathode on top of P(R F12-R BTz) film, lift-off of the photo-
1
5
of semiperfluoroalkyl chains. It is probable that the introduc-
tion of semiperfluoroalkyl groups leads to charge traps in the
polymer film adversely affecting charge transport.
F
F
resist layer in 1-methyl-2-pyrrolidinone produced a patterned EL

Communication
Macromolecules, Vol. 43, No. 3, 2010 1197
Figure 1. (a) EL spectra of PR
F
F12 and the reference polymer PF12. (b) Statistical copolymer P(R
F
F12-R
F
BTz) composed of semiperfluoroalkyl
F12-R BTz). (d) UV-vis absorption
fluorene and semiperfluoroalkyl benzotriazole units. (c) Current-voltage-luminance plot of PR
and EL spectra of P(R F12-R BTz).
F
F12 and P(R
F
F
F
F
F F
Figure 2. (a) Fabrication of a patterned EL device using P(R F12-R BTz) and a conventional photoresist. (b) Operating pixels of the EL device
described in (a).
device. The resulting pixel array operated to give light emission in
a 100 μm scale (Figure 2b).
Murotani, Dr. Yoshitomi Morizawa and Asahi Glass Company
for GPC measurement in fluorous solvents.
In summary, semiperfluoroalkyl polyfluorenes with ca. 60 wt %
F content were synthesizedbySuzuki polycondensation reactions
in benzotrifluoride. Their optical and EL properties were evaluated,
exhibiting distinctive characteristics from their nonfluorinated
Supporting Information Available: Text giving the synthetic
procedure (with structures), thermal analysis data, and photo-
lithographic patterning conditions. A scheme showing the reac-
tions used, and figures showing size exclusion chromatograms,
H NMR spectra, TGA and DSC plots, and UV-vis and PL
spectra. This material is available free of charge via the Internet
at http://pubs.acs.org
analogues. Good performance and processability of P(R F12-
F
1
R BTz) in fluorinated solvents enabled the fabrication of a
F
patterned EL device byconventional photolithographic methods.
This concept of orthogonality between highly fluorinated electronic
polymers and nonfluorinated imaging materials is proposed as a
promising strategy to construct patterned organic electronic
devices in a more efficient way.
References and Notes
(1) Burroughes, J. H.; Bradley, D. D. C.; Brown, A. R.; Marks, R. N.;
Mackay, K.; Friend, R. H.; Burn, P. L.; Holmes, A. B. Nature 1990,
3
47, 539–541.
2) Forrest, S. R.; Thompson, M. E. Chem. Rev. 2007, 107, 923–
25.
Acknowledgment. We thank Australian Research Council
LX0669165)/National Science Foundation (Materials World
(
(
9
Network DMR-0602821) and CSIRO Australia for their gener-
ous support. We also thank Dr. Klaus Kahle for his advice on the
materials synthesis and Dr. Scott E. Watkins (CSIRO) for his
interest in this work. This work was performed in part at the
Cornell NanoScale Facility. We are grateful to Mr. Eisuke
(
(
3) Malliaras, G.; Friend, R. Phys. Today 2005, 58, 53–58.
4) Grimsdale, A. C.; Chan, K. L.; Martin, R. E.; Jokisz, P. G.;
Holmes, A. B. Chem. Rev. 2009, 109, 897–1091.
(5) Lidzey, D. G.; Pate, M. A.; Weaver, M. S.; Fisher, T. A.; Bradley,
D. D. C. Synth. Met. 1996, 82, 141–148.

1
198 Macromolecules, Vol. 43, No. 3, 2010
Lee et al.
(
(
6) Sheats, J. R. J. Mater. Res. 2004, 19, 1974–1989.
7) Gather, M. C.; Kohnen, A.; Falcou, A.; Becker, H.; Meerholz,
K. Adv. Funct. Mater. 2007, 17, 191–200.
(15) Alvey, L. J.; Meier, R.; Soos, T.; Bernatis, P.; Gladysz, J. A. Eur. J.
Inorg. Chem. 2000, 1975–1983.
(16) Kameshima, H.; Nemoto, N.; Endo, T. J. Polym. Sci., Part A:
(
(
8) Menard, E.; Meitl, M. A.; Sun, Y. G.; Park, J. U.; Shir, D. J. L.; Nam,
Y. S.; Jeon, S.; Rogers, J. A. Chem. Rev. 2007, 107, 1117–1160.
9) Hebner, T. R.; Wu, C. C.; Marcy, D.; Lu, M. H.; Sturm, J. C. Appl.
Phys. Lett. 1998, 72, 519–521.
Polym. Chem. 2001, 39, 3143–3150.
(17) Amara, J. P.; Swager, T. M. Macromolecules 2006, 39, 5753–5759.
(18) Miyaura, N.; Suzuki, A. Chem. Rev. 1995, 95, 2457–2483.
(19) Ishiyama, T.; Murata, M.; Miyaura, N. J. Org. Chem. 1995, 60,
7508–7510.
(
(
10) Pardo, D. A.; Jabbour, G. E.; Peyghambarian, N. Adv. Mater.
2
000, 12, 1249–1252.
(20) Cho, S. Y.; Grimsdale, A. C.; Jones, D. J.; Watkins, S. E.; Holmes,
A. B. J. Am. Chem. Soc. 2007, 129, 11910–11911.
(21) Maul, J. J.; Ostrowski, P. J.; Ublacker, G. A.; Linclau, B.; Curran,
D. P. Top. Curr. Chem. 1999, 206, 79–105.
11) Zakhidov, A. A.; Lee, J.-K.; Fong, H. H.; DeFranco, J. A.;
Chatzichristidi, M.; Taylor, P. G.; Ober, C. K.; Malliaras, G. G.
Adv. Mater. 2008, 20, 3481–3484.
(
12) Lee, J.-K.; Chatzichristidi, M.; Zakhidov, A. A.; Taylor, P. G.;
DeFranco, J. A.; Hwang, H. S.; Fong, H. H.; Holmes, A. B.;
Malliaras, G. G.; Ober, C. K. J. Am. Chem. Soc. 2008, 130, 11564–
(22) Isemura, T.; Kakita, R.; Kawahara, K. J. Chromatogr. A 2004,
1026, 109–116.
(23) Kane, E. Phys. Rev. 1962, 127, 131–141.
1
1565.
(24) Kirihata, H.; Uda, M. Rev. Sci. Instrum. 1981, 52, 68–70.
(25) Yang, G. Z.; Chen, X. L.; Wang, W. Z.; Wang, M.; Liu, T. X.;
Li, C. Z. J. Polym. Sci., Part B: Polym. Phys. 2007, 45, 976–987.
(26) Teetsov, J.; Fox, M. A. J. Mater. Chem. 1999, 9, 2117–2122.
(27) Tanimoto, A.; Yamamoto, T. Macromolecules 2006, 39, 3546–3552.
(
13) Bernius, M.; Inbasekaran, M.; Woo, E.; Wu, W. S.; Wujkowski,
L. J. Mater. Sci. - Mater. Electron. 2000, 11, 111–116.
14) Leclerc, M. J. Polym. Sci., Part A: Polym. Chem. 2001, 39, 2867–
(
2
873.