Tetradecafluorosexithiophene: The first perfluorinated oligothiophene [22]

Full text of DOI:10.1021/ja015712j

J. Am. Chem. Soc. 2001, 123, 4643-4644
4643
Chart 1
Tetradecafluorosexithiophene: The First
Perfluorinated Oligothiophene
Youichi Sakamoto, Shingo Komatsu, and Toshiyasu Suzuki*
Institute for Molecular Science
Myodaiji, Okazaki 444-8585, Japan
ReceiVed February 23, 2001
ReVised Manuscript ReceiVed April 3, 2001
Scheme 1
Organic semiconductors have attained much attention because
of the recent progress of organic light-emitting diodes (OLEDs)
and field-effect transistors (FETs). It was impressive that FETs
based on organic single crystals exhibited superconductivity,1
lasing,2 and quantum Hall effect.3 In the last paper, we reported
that perfluorinated oligo(p-phenylene)s, such as perfluoro-p-
sexiphenyl (C36F26), were efficient n-type semiconductors for the
electron-transport layer of OLEDs.4a Molecular design of organic
semiconductors for FETs should be different from that for OLEDs.
A FET requires planar and crystalline materials for high carrier
mobility. On the other hand, an OLED prefers nonplanar and
amorphous materials. We designed tetradecafluorosexithiophene
(perfluoro-R-sexithiophene; PF-6T) as a potential n-type semi-
conductor for FETs for the following reasons: (1) The dihedral
angles of perfluorinated oligothiophenes would be much smaller
than those of perfluorinated oligophenylenes5 if thiophene rings
have an all-trans conformation.6 (See Chart 1.) (2) R-Sexithiophene
(6T) is an excellent p-type semiconductor with high hole
mobility.7 (3) Perfluorination is an effective way to convert a
p-type organic semiconductor to an n-type one.4,8,9 We report
herein the synthesis and X-ray structure of PF-6T. Absorption
and emission spectra as well as thermal and electrochemical
properties are also presented in comparison with 6T.
Although tetrafluorothiophene has been prepared by fluorina-
tion of thiophene over KCoF4 and subsequent dehydrofluorina-
tion,10 any perfluorinated oligothiophenes (PF-nT: n > 1) have
not yet been reported. To introduce fluorine atoms to a thiophene
ring, the reaction of thienyllithiums11 with several F+ reagents12
was examined. We found that N-fluoro-N-(phenylsulfonyl)-
benzenesulfonamide, (PhSO2)2NF, gave the most satisfactory
result.13 As shown in Scheme 1, lithiation of TMS-protected 3,4-
dibromothiophene 1 with an equivalent of n-BuLi followed by
treatment with (PhSO2)2NF provided a mixture of 1 and 3,4-
difluoro-2,5-bis(trimethylsilyl)thiophene (2).14 Further lithiation
and fluorination were repeated without isolating the mixture to
afford 2 in 73%. Bromination of 2 with N-bromosuccinimide
(NBS) yielded monobromothiophene 3 in 79%. Tributyltin
derivative 4 was obtained in 62% by lithiation of 3 and quenching
with Bu3SnCl. Dibromide 5, which was prepared by the reaction
of 2 with bromine in 86%, was allowed to react with 2 equiv of
n-BuLi, (PhSO2)2NF, and Bu3SnCl to give stannylated trifluoro-
thiophene 6 in 85%. The Stille coupling of 3 and 6 in the presence
of PdCl2(PPh3)2 afforded bithiophene 7. This was treated with
NBS without purification to give perfluoro-5-bromobithiophene
(8) in 74%. Similarly, the coupling reaction of 4 and 8 followed
by bromination yielded perfluoro-5-bromoterthiophene (10) in
50%. The Ullmann coupling of 10 in DMF provided PF-6T in
57%.
(1) Scho¨n, J. H.; Kloc, Ch.; Batlogg, B. Nature 2000, 406, 702-704.
(2) Scho¨n, J. H.; Kloc, Ch.; Dodabalapur, A.; Batlogg, B. Science 2000,
289, 599-601.
(3) Scho¨n, J. H.; Kloc, Ch.; Batlogg, B. Science 2000, 288, 2338-2340.
(4) (a) Heidenhain, S. B.; Sakamoto, Y.; Suzuki, T.; Miura, A.; Fujikawa,
H.; Mori, T.; Tokito, S.; Taga, Y. J. Am. Chem. Soc. 2000, 122, 10240-
10241. (b) Sakamoto, Y.; Suzuki, T.; Miura, A.; Fujikawa, H.; Tokito, S.;
Taga, Y. J. Am. Chem. Soc. 2000, 122, 1832-1833.
(5) The dihedral angle in crystalline perfluorobiphenyl is 59.6°. Gleason,
W. B.; Britton, D. Cryst. Struct. Commun. 1976, 5, 483-488.
(6) Handbook of Oligo- and Polythiophenes; Fichou, D., Ed.; Wiley-
VCH: Weinheim, 1999.
(7) (a) Garnier, F. Acc. Chem. Res. 1999, 32, 209-215. (b) Dodabalapur,
A.; Torsi, L.; Katz, H. E. Science 1995, 268, 270-271.
PF-6T was purified by train sublimation15 and used for
characterization. It is an orange crystalline solid and slightly
soluble in CHCl3 and aromatic solvents such as toluene. Its
structure was determined by EI-MS, elemental analysis, and X-ray
crystallography.13 PF-6T exhibits bluish-green photoluminescence
in solution and an orange emission in the solid state. Figure 1
shows the absorption and emission spectra of PF-6T and 6T in
CHCl3. The shapes of the spectra are almost identical. The
absorption and emission maxima of PF-6T (421 and 471 nm,
respectively) shifted to higher energies relative to those of 6T
(435 and 508 nm, respectively).
(8) Copper perfluorophthalocyanine as the n-type semiconductor: Bao, Z.;
Lovinger, A. J.; Brown J. J. Am. Chem. Soc. 1998, 120, 207-208.
(9) Some n-type semiconductors have fluorinated alkyl chains. (a) Naph-
thalenetetracarboxylic diimide derivatives: Katz, H. E.; Lovinger, A. J.;
Johnson, J.; Kloc, Ch.; Siegrist, T.; Li, W.; Lin, Y.-Y.; Dodabalapur, A. Nature
2000, 404, 478-481. (b) R,ω-Diperfluorohexylsexithiophene: Facchetti, A.;
Deng, Y.; Wang, A.; Koide, Y.; Sirringhaus, H.; Marks, T. J.; Friend, R. H.
Angew. Chem., Int. Ed. 2000, 39, 4547-4551.
(10) Burdon, J.; Campbell, J. G.; Parsons, I. W.; Tatlow, J. C. Chem.
Commun. 1969, 27-28.
(11) Thienyllithiums with perchloryl fluoride (FClO3): Christiansen, H.;
Gronowitz, S.; Rodmar, B.; Rodmar, S.; Rosen, U.; Sharma, M. K. Ark. Kemi
1969, 30, 561-582.
(14) A small amount of monofluorinated compound, (TMS)2C4SFBr, was
detected by mass spectrometry. The metal-halogen exchange between
(TMS)2C4SBrLi and (TMS)2C4SFBr is faster than the reaction of (TMS)2C4-
SBrLi with (PhSO2)2NF and gives 1 and (TMS)2C4SFLi.
(15) Wagner, H. J.; Loutfy, R. O.; Hsiao, C.-K. J. Mater. Sci. 1982, 17,
2781-2791.
(12) Electrophilic fluorinating agents: Lal, G. S.; Pez, G. P.; Syvret, R.
G. Chem. ReV. 1996, 96, 1737-1755.
(13) See Supporting Information for the experimental details.
10.1021/ja015712j CCC: $20.00 © 2001 American Chemical Society
Published on Web 04/20/2001

4644 J. Am. Chem. Soc., Vol. 123, No. 19, 2001
Communications to the Editor
Figure 3. Molecular packing diagram of PF-6T with a view down the
c axis.
Figure 1. Absorption and emission spectra of PF-6T and 6T in CHCl3.
Figure 2. Molecular structure and labeling of PF-6T.
Figure 4. The short C-C (3.53-3.68 Å) and S-S contacts (3.65 Å) in
the same stack (dotted lines). Dipole moments induced by fluorine atoms
are indicated by arrows.
The thermal properties were investigated by differential scan-
ning calorimetry (DSC). The DSC trace of PF-6T exhibited a
sharp melting endotherm at 286 °C.13 On the second heating, a
lower melting endotherm due to a different crystalline phase was
observed at 258 °C, which was followed by immediate crystal-
lization at 262 °C to the high-temperature phase (Tm ) 286 °C).16
The electrochemical measurements on PF-6T and 6T were
performed in 1,2-dichlorobenzene at 50 °C.13 The differential
pulse voltammogram (DPV) of 6T showed a reduction peak at
-2.31 V and two oxidation peaks at 0.41 and 0.63 V (vs Fc/
Fc+). As expected, the redox potentials of PF-6T shifted positively
relative to 6T: Two reduction peaks at -1.86 and -2.05 V as
well as an oxidation peak at 0.95 V were observed.17 The potential
differences between the first reduction and oxidation peaks are
2.72 V for 6T and 2.81 V for PF-6T. This result is in agreement
with a blue shift in absorption spectra, suggesting that the
HOMO-LUMO gap of PF-6T is larger than that of 6T in
solution.
Single crystals of PF-6T were successfully grown by slow
sublimation at 270 °C under a flow of 1 atm of argon.18 An orange
plate was used for X-ray crystallography.13,19 The structure of
PF-6T is all-trans and planar as observed for 6T (Figure 2).16
The dihedral angles between the adjacent thiophene rings are 0.7
(S1-C4-C5-C6), 1.5 (S2-C8-C9-C10), and 0.8° (S3-C12-
C12′-C11′). The C-C bond distances are 1.33-1.39 Å in the
thiophene rings (6T: 1.34-1.42 Å) and 1.43-1.44 Å between
the rings (6T: 1.44 Å).16a The C-S bond distances range from
1.70 to 1.73 Å (6T: 1.71-1.74 Å). Interestingly, the C-C and
C-S bonds in PF-6T are slightly shorter than the corresponding
bonds in 6T. This is probably because the inductive effect of
fluorine lowers the energy levels of π-bonding orbitals. Figure 3
shows a crystal packing view of PF-6T along the c axis. PF-6T
adopts a π-stack structure with face-to-face molecules.20 This is
quite different from the herringbone structure of 6T,16 in which
π-π interactions between neighboring molecules are minimized
to reduce the repulsion between π-orbitals. The molecules in the
adjacent stack are tilted at an angle of 84.7°. In Figure 4, the
short C-C (3.53-3.68 Å) and S-S contacts (3.65 Å) in the same
stack are shown. The next molecule slides along the long
molecular axis by a thiophene unit to cancel dipole moments
induced by fluorine atoms. We speculate that the dipole-dipole
interactions between neighboring molecules overcome the π-π
repulsions and are responsible for the π-stack structure rather than
the herringbone structure.
In conclusion, we have shown the synthetic method to obtain
perfluorinated oligothiophenes. The X-ray structure of PF-6T
suggests high electron mobility along the π-π stacking direction
(the b axis). Fabrication of n-type FETs with this new material is
currently underway and will be reported elsewhere.
Acknowledgment. This paper is dedicated to Professor Fred Wudl
on the occasion of his 60th birthday. We thank Prof. K. Inoue for the
use of his X-ray diffractometer and Dr. M. Akita for help in X-ray
structural determination. This work was supported by Japan Society for
the Promotion of Science (Grant-in-Aid for Scientific Research B-12554027
and Encouragement of Young Scientists 12740391).
(16) Two polymorphs of 6T have been reported. (a) The low- temperature
form: Horowitz, G.; Bachet, B.; Yassar, A.; Lang, P.; Demanze, F.; Fave,
J.-L.; Garnier, F. Chem. Mater. 1995, 7, 1337-1341. (b) The high-temperature
form: Siegrist, T.; Fleming, R. M.; Haddon, R. C.; Laudise, R. A.; Lovinger,
A. J.; Katz, H. E.; Bridenbaugh, P.; Davis, D. D. J. Mater. Res. 1995, 10,
2170-2173.
(17) The first reduction potential of PF-6T is less positive than we expected.
However, this result in solution may not directly correlate with the electron
affinity of PF-6T in the solid state. A conformational study of PF-2T by
AM1 calculations indicates that the energy minimum is obtained when two
thiophene rings are perpendicular. Ce´sare, N. D.; Belleteˆte, M.; Leclerc, M.;
Durocher, G. Synth. Met. 1998, 94, 291-298.
Supporting Information Available: Experimental details, DSC
traces, DPVs, and X-ray structural determination (PDF); an X-ray
crystallographic data in CIF format. This material is available free of
charge via the Internet at http://pubs.acs.org.
(18) Kloc, Ch.; Simpkins, P. G.; Siegrist, T.; Laudise, R. A. J. Cryst. Growth
1997, 182, 416-427.
JA015712J
(19) Crystal data for PF-6T: monoclinic, space group P21/c, a ) 18.549(6)
Å, b ) 5.323(2) Å, c ) 13.214(4) Å, â ) 108.631(5)°, V ) 1236.3(6) Å3, T
) 15 °C, Z ) 2, R ) 0.060, GOF ) 0.89.
(20) Li, X.-C.; Sirringhaus, H.; Garnier, F.; Holmes, A. B.; Moratti, S. C.;
Feeder, N.; Clegg, W.; Teat, S. J.; Friend, R. H. J. Am. Chem. Soc. 1998,
120, 2206-2207.