Synthesis, structure, and transport property of perfluorinated oligofluorenes

Article abstract of DOI:10.1002/chem.200800406

A study was conducted to demonstrate the synthesis, characterization, and properties of perfluorinated oligo(9,9-dimethylfluore)s. The study demonstrated that the synthesis starts with 1,2-dibromo3,4,5,6-tetrafluorobenzene, with a yield of 52%. PY-2F to PY-4F were purified by train sublimation and used for characterization in the study. The structures were determined by FNMR spectroscopy, mass spectroscopy, and elemental analysis. It was found that the structures were colorless solids and soluble in CHCL₃, THF₃, and aromatic solvents, such as toluene. It was also found that the UV/Vis absorption and photoluminescence spectra of PF-3F were similar to those of the parent compound. The UV/Vis absorption and photoluminescence spectra was observed to be consistent with the density functional theory (DTF) calculations.

Full text of DOI:10.1002/chem.200800406

DOI: 10.1002/chem.200800406
Synthesis, Structure, and Transport Property of Perfluorinated Oligofluorenes
Kimitaka Ohkubo,[a] Youichi Sakamoto,[a] Toshiyasu Suzuki,*[a] Toshimitsu Tsuzuki,[b]
Daisuke Kumaki,[b] and Shizuo Tokito[b]
A perfluorinated aromatic compound is typically an elec-
tron acceptor and considered as an important n-type semi-
conductor for organic electronics.[1] We have reported the
synthesis and electron-transport properties of perfluorinated
phenylene oligomers. Branched oligomers[2] such as per-
fluoro-1,3,5-tris(p-terphenyl)benzene (C60F42) displayed the
excellent ability to block exitons and holes in organic light-
emitting diodes (OLEDs) with phosphorescent dopants.
Linear oligomers[3] such as perfluoro-p-sexiphenyl (C36F26)
exhibited good electron injection and transport, but they
were not practically useful for OLEDs because of their high
crystallinity.[4] The purpose of this research is to produce a
new n-type semiconductor with 1) a high electron affinity, 2)
a high electron mobility, and 3) a stable amorphous state.
After theoretical calculations on several different structures,
we recognized that perfluorinated oligofluorenes could be
good candidates: The conversion of perfluorinated oligophe-
nylenes to perfluorinated oligofluorenes significantly lowers
the LUMO energy levels.[5] Oligo- and polyfluorenes have
been known as amorphous blue emitters with high carrier
mobilities.[6–8] We report here the synthesis, characterization,
and properties of perfluorinated oligo(9,9-dimethylfluor-
ene)s (PF-nFs, Scheme 1).
Scheme 1. Structures of perfluorinated oligo(9,9-dimethylfluorene)s.
tion of 4 afforded 2-bromoheptafluorofluorene 5 in 47%
yield. The reaction of 5 with the trifluoromethylating agent
6 in the presence of NaH and [15]crown-5 gave 7 in 53%
yield. The dimer PF-2F was synthesized by the Ullmann
coupling of 7 in 72% yield.
Octafluorofluorene 3 was first reported by Filler et al. in
1980.[9] Our improved synthesis starts with 1,2-dibromo-
3,4,5,6-tetrafluorobenzene, and the total yield is 52% as
shown in Scheme 2. The substitution reaction of 3 with hy-
drazine took place at the 2-position to provide 4. Bromina-
Further bromination of 5, which was followed by trifluor-
omethylation, provided the dibromide 9 (Scheme 3). The
copper derivative 10 was prepared from the Grignard re-
agent of 7 and copper(I) bromide without isolation. The re-
action of 10 with 9 gave the trimer PF-3F (C45F38) in 52%
yield. Similarly, the dimeric dibromide 11,[5] prepared from
5, was allowed to react with 10 to afford the tetramer PF-4F
(C60F50) in 46% yield.
PF-2F to PF-4F were purified by train sublimation and
used for characterization. The structures were determined
by 19F NMR spectroscpy, mass spectrometry, and elemental
analyses. They are colorless solids and soluble in CHCl3,
THF, and aromatic solvents such as toluene. The UV/Vis ab-
sorption and photoluminescence spectra[5] of PF-3F were
very similar to those of the parent compound (3F:
C45H38)[10] in shape but shifted by 40 nm to the higher
energy. This is consistent with the DFT calculations on 3F
[a] K. Ohkubo, Dr. Y. Sakamoto, Prof. Dr. T. Suzuki
Institute for Molecular Science and
Department of Structural Molecular Science
The Graduate University for Advanced Studies, Myodaiji
Okazaki 444-8787 (Japan)
Fax : (+81)564-59-5510
E-mail: toshy@ims.ac.jp
[b] Dr. T. Tsuzuki, D. Kumaki, Dr. S. Tokito
NHK Science and Technical Research Laboratories
Kinuta, Setagaya-ku, Tokyo 157-8510 (Japan)
Supporting information for this article is available on the WWW
under http://www.chemistry.org or from the author.
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Chem. Eur. J. 2008, 14, 4472 – 4474

COMMUNICATION
À2.64; PF-3F: À2.82; PF-4F: À2.91 eV). The potential dif-
ference between the first and second reductions (PF-3F:
0.31; PF-4F: 0.20 V) is smaller in PF-4F because of less
electron-electron repulsion in its longer p-conjugation. The
observed reduction potentials are more positive than those
of conventional electron-transport materials such as Alq3
(À2.36 V under the same conditions).[11]
Figure 1. Cyclic voltammograms of PF-3F and PF-4F at 100 mVsÀ1 con-
taining 0.1m (nBu)4NPF6 in THF.
Scheme 2. Synthesis of octafluorofluorene 3 and PF-2F.
We performed the X-ray crystallography of PF-2F to see
the molecular conformation and packing.[12] Interestingly, a
single crystal of PF-2F contains a 1:1 mixture of cis and
trans conformers in the unit cell (Figure 2), and each confor-
mer forms the separated layer.[5] The dihedral angle is slight-
ly larger in the cis conformer (cis: 57.78; trans: 53.28). These
values are typical for perfluorinated oligo(p-phenylene)s.
For example, the dihedral angles of perfluoro-p-quaterphen-
yl (C24F18) and -quinquephenyl (C30F22) are 59.58 and 53.58,
respectively.[13]
The differential scanning calorimetry (DSC) measure-
ments of PF-3F and PF-4F showed high glass transition
temperatures (Tg =133 and 1638C, respectively).[5] No peaks
due to melting or crystallization were observed after the
first cycle, indicating that they are stable amorphous solids.
Scheme 3. Synthesis of PF-3F and PF-4F.
and PF-3F: The HOMO–LUMO gaps are 3.81 and 4.03 eV,
respectively. The wider HOMO–LUMO gap of PF-3F is
probably because of its larger dihedral angles (3F: 37.28;
PF-3F: 55.28 by the DFT calculations). Contrary to oligo-
and polyfluorenes, PF-2F to PF-4F showed only weak emis-
sions with the peaks around 350 nm.[5]
To estimate the LUMO energy levels, we measured the
cyclic voltammetry (CV). Although PF-2F showed only one
reversible couple at À1.80 V (versus the ferrocene/ferroceni-
um couple (Fc/Fc+)), PF-3F and PF-4F exhibited two rever-
sible couples in THF (Figure 1). The reduction potentials
shifted positively from PF-3F (À1.65 and À1.96 V) to PF-
4F (À1.60 and À1.80 V). This is consistent with the LUMO
energy levels obtained by the DFT calculations (PF-2F:
Figure 2. ORTEP drawing of cis-PF-2F (top) and trans-PF-2F (bottom).
Thermal ellipsoids are shown at 50% probability.
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4473

T. Suzuki et al.
The charge carrier mobilities for PF-3F and PF-4F were
measured by the conventional time-of-flight (TOF) tech-
nique with a N2 pulse laser. The samples were prepared on
ITO-coated glass substrates by high-vacuum thermal evapo-
ration of tris(8-quinolinolato)aluminum (Alq3, 30 nm) as the
charge-generation layer, PF-3F (5.6 mm) or PF-4F (3.8 mm),
and aluminum (100 nm). Relatively thick films of PF-3F
and PF-4F were nicely transparent, and no crystallization
was observed. A negative voltage was applied to the ITO
electrode to examine the electron-transport properties. Fig-
ure 3a and b show the representative transient photocur-
rents for PF-3F and PF-4F films, respectively. PF-3F exhib-
ited a current plateau followed by a tail in the double-linear
plot (Figure 3a). This nondispersive transport in the solid
film indicates that the density of electron traps due to ener-
getic disorder is small. Carrier transit times (Tt) were ob-
tained from the intersection point in the double-logarithmic
plots (insets of Figure 3a and 3b). The electron mobilities
can be calculated from the carrier transit times and are plot-
ted against the square of the electric field as shown in
Figure 4. PF-3F and PF-4F displayed weak field depend-
ence and high electron mobilities (2.1”10À4 and 2.0”
10À4 cm2 VÀ1 sÀ1 at 5.8”105 VcmÀ1, respectively) compared
to Alq3 (2.7”10À5 cm2 VÀ1 sÀ1 at 1.2”106 VcmÀ1).
~
&
*
Figure 4. Electron mobilities of PF-3F ( ), PF-4F ( ), and Alq3 ( ) by
the time-of-flight technique.
tron affinity and mobility. The application to OLEDs is cur-
rently underway and will be reported elsewhere.
Acknowledgements
We thank Dr. T. Nagasawa for assistance with X-ray crystallography and
Kanto Denka Kogyo Co., Ltd. for financial support.
In conclusion, we have synthesized perfluorinated
oligo(9,9-dimethylfluorene)s up to the tetramer. PF-3F and
PF-4F are amorphous n-type semiconductors with high elec-
Keywords: amorphous materials
fluorine · oligomerization · semiconductors
· electron transport ·
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[5] See Supporting Information for details.
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[12] Crystal data for PF-2F: C60F52, Mr =1708.58, triclinic, space group
¯
P1, a=10.027(5), b=12.407(6), c=23.003(11) Š, a=92.961(7), b=
93.513(9), g=90.443(8)8, V=2852(2) Š3, T=173 K, Z=2, R=
0.0819, GOF=1.007. CCDC 677181 contains the supplementary
crystallographic data for this paper. These data can be obtained free
of charge from The Cambridge Crystallographic Data Centre via
www.ccdc.cam.ac.uk/data_request/cif
[13] S. Komatsu, PhD thesis, Kyushu University (Japan), 2002.
Received: March 6, 2008
Published online: April 9, 2008
Figure 3. TOF current transients for a) PF-3F at 4.8”105 VcmÀ1 and b)
PF-4F at 5.0”105 VcmÀ1. Insets of a) and b) are the double-logarithmic
plots.
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