Building blocks for n-type organic electronics: Regiochemically modulated inversion of majority carrier sign in perfluoroarene-modified polythiophene semiconductors

Article abstract of DOI:10.1002/anie.200351253

A new family of perfluoroarene-modified thiophene semiconductors 1-3 has been synthesized to assess the influence of perfluoroarene introduction and regiochemistry on molecular and thin-film transistor properties. Compound 1 is an n-type semiconductor wit

Full text of DOI:10.1002/anie.200351253

Communications
Semiconductors
Building Blocks for n-Type Organic Electronics:
Regiochemically Modulated Inversion of Majority
Carrier Sign in Perfluoroarene-Modified
Polythiophene Semiconductors**
Antonio Facchetti, Myung-Han Yoon,
Charlotte L. Stern, Howard E. Katz, and
Tobin J. Marks*
Electron-transporting (n-type) organic semiconductors[1]are
relatively rare, but of great interest for “plastic electronics”
such as thin-film transistors (TFTs), organic light-emitting
diodes (OLEDs), and photovoltaics.[2] The most prominent
examples of TFT n-type materials have been obtained by
modifying known p-type cores with strongly electronegative
fluoro/fluoroalkyl substituents. However, despite these
advances, those factors underlying fluoro stabilization of the
injected electrons are not fully understood, and have often
been attributed to the reduction in LUMO energies. Of the
known thiophene-based semiconductors, the vast majority[1a,b]
exhibit hole-transporting (p-type) TFT activity. However, n-
type thiophene-based oligomers and polymers would be
extremely useful considering the stability and ready functio-
nalizability of oligothiophene cores.[3]
We report here on the surprising properties of the first
fluoroarene-modified thiophene semiconductors 1–3, which
can be viewed as p-type a6T derivatives drastically recon-
stituted according to the following criteria: 1) Electronic:
electron-deficient perfluoroarene substitution for electron-
rich thiophene rings should lower LUMO energies, thus
allowing more favorable electron injection. 2) Steric: non-
bonded interactions accompanying five!six-membered ring
substitution should destabilize ring coplanarity and addition-
ally modulate orbital energies.[4] 3) Regiochemistry: system-
atic displacement of fluoroarene moieties from the periphery
to embedded in the molecular core (1!2!3) can be achieved
while preserving rodlike molecular architectures. This is
impossible in other classes of molecular semiconductors.
4) Molecular packing: mixing electron-rich and electron-
deficient p rings should favor close cofacial p-stacking
versus slipped-stacking (herringbone) motifs, as suggested
by packing in many 1:1 p-excessive/p-deficient molecular
crystals.[5] The net result is a new family of closely packed
polythiophene–fluoroarene conductors in which carrier mobi-
lity and sign can be drastically modulated by changing the
molecular regiochemistry.
Compounds 1–3 were prepared in 60–80% yield by
Pd[PPh3]4-catalyzed Stille coupling (Scheme 1), purified by
gradient sublimation, and characterized by elemental analy-
sis, mass spectrometry, multinuclear NMR, differential scan-
ning calorimetry, thermogravimetric analysis, and single-
crystal X-ray diffraction.[6]
Interestingly, the crystal structures of 1–3 (Figure 1)[7]
exhibit the common feature of close cofacial packing of
electron-rich and electron-deficient subunits, in contrast to all
previously reported unsubstituted oligothiophenes (terthio-
phene to octathiophene) in which a herringbone motif with
twist angles of 60–708 between adjacent molecular planes is
found. The latter arrangement reduces intermolecular orbital
overlap and therefore should increase the barrier to charge
transport versus an ideal, p–p overlapped structure. The
structures of 1 and 2 reveal substantially planar cores with
maximum thiophene–fluoroarene torsional angles between
adjacent outer rings of 17.68 and 7.98, respectively, versus 4.18
for a6T.[8] These angles are much smaller than the limiting
ꢀ 308 value required for the intramolecular p overlap of an
efficient conduction band.[4a,9] Molecules of 1 exhibit an
uncommon syn conformation between outer pairs of thio-
phene rings and are stacked in parallel layers along the b axis.
System 2 packs along c in a zig-zag double layer motif, the first
[*] Prof. Dr. T. J. Marks, Dr. A. Facchetti, M.-H. Yoon, Dr. C. L. Stern
Department of Chemistry and
the Materials Research Center
Northwestern University
2145 Sheridan Road, Evanston, IL, 60208-3113 (USA)
Fax: (+1)847-491-2290
E-mail: tjmarks@casbah.acns.nwu.edu
Dr. H. E. Katz
Bell Laboratories
Lucent Technologies
Murray Hill, NJ 07974 (USA)
[**] We thank DARPA (N00421-98-1187), ONR (N00014-02-1-0909), and
the NSF-MRSEC programthrough the Northwestern Materials
Research Center (DMR-0076097) for support of this research.
layer having an 18.31 Š (1= c) repeat distance. Within each
Supporting information for this article is available on the WWW
under http://www.angewandte.org or fromthe author.
2
layer, the molecular units form slipped p–p stacks. To our
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DOI: 10.1002/anie.200351253
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Angewandte
Chemie
Scheme 1. Synthetic routes to compounds 1–3.
(solution) or one (50 nm films) chemically reversible reduc-
tive peaks. Electron affinity increases upon introduction of
fluoroaryl rings as evidenced by anodic E1/2(THF)/E1/2(film)
shifts from a6T to 1 (+ 0.20/ + 0.33 V), 2 (+ 0.17/ + 0.31 V),
and 3 (+ 0.16/ + 0.17 V), respectively. Reversible oxidations
are observed in solution for a6T, 1, and 2, but not for 3. The
film oxidative waves are irreversible as all films dissolve upon
cation formation. LUMO energies were calculated directly
from reductive E1/2 values whereas HOMO energies were
estimated from EHOMO = ELUMOꢁEgap(optical).[11] The derived
energy level orderings (Figure 2) demonstrate that fluoroar-
ene substitution strongly lowers solution/film HOMO and
LUMO energies. However, solution EHOMO values strongly
decrease on going from 1!2!3 (DEHOMO(max) = 0.33 eV),
whereas film EHOMO and ELUMO values remain in a narrower
range (DE(max) = 0.16 eV). Therefore, relative stabilization
at the molecular level of injected charge should be similar for
all of these fluoroarene oligothiophenes, especially for 1
and 2.
Film microstructure and morphology were investigated by
XRD and SEM for 500 nm films vacuum-deposited on 608C
Si/SiO2 substrates. qꢁ2q X-ray diffraction spectra of 1 and 3
exhibit only one set of Bragg reflections from the same family
and a single strong peak, respectively, whereas 2 exhibits two
sets of reflections with one being predominant. From the first
reflection, interlayer spacings (d) of 25.07, 18.38 (14.96 for the
second, less intense reflection set), and 5.98 Š are deduced for
the films of 1, 2, and 3, respectively,
Figure 1. Crystal structures of fluoroarene-thiophene compounds 1 (a),
2 (b), and 3 (c). (C=gray, F=green, S=yellow,).
knowledge, the minimum cofacial p–p distances in 1 (3.20 Š)
and 2 (3.37 Š) are the shortest so far reported for thiophene-
based oligomers. In contrast, the structure of 3 exhibits a large
intramolecular torsional angle (53.98) between adjacent
fluoroarene rings, hence interrupted p conjugation. The
packing of 3 is also more complex, with waved layers having
the long molecular axes extending along c.
Cyclic voltammetry[10] (Table 1) of a6T and 1–3 as THF
solutions or as thin films on Au/glass reveal either two
Table 1: Solution and FilmFirst Reduction Potentials ( E1/2),[a] Derived LUMO and HOMO Energies, and
Transistor Data[b] for Oligomers a6T[c] and 1–3.
versus 23.3 Š for a6T.[12] From the
1–3 single-crystal cell parameters,[7]
it can be surmised that preferential
growth of 1–3 crystallites is with
the ab plane parallel to the sub-
strate surface. Since the molecular
long axes are in the c direction for 1
and 2 and in the a direction for 3,
this corresponds to a preferential
orientation of 1 and 2 molecules
Oligomers
E
1/2 [V]
ELUMO [eV]
FilmTHF
EHOMO (eV)
Film
m (type)
Ion/Ioff
THF
FilmTHF
a6T
1
2
ꢁ1.71
ꢁ1.51
ꢁ1.54
ꢁ1.55
ꢁ2.00
ꢁ1.67
ꢁ1.69
ꢁ1.83
ꢁ2.65
ꢁ2.36
ꢁ2.69
ꢁ2.67
ꢁ2.53
ꢁ5.20
ꢁ4.78
ꢁ5.27
ꢁ5.32
ꢁ5.40
0.03 (p)
>106
>105
104
ꢁ2.85
ꢁ2.82
ꢁ2.81
ꢁ5.48
ꢁ5.64
ꢁ5.81
0.08 (n)
0.01 (p)
3
4”10ꢁ5(p)
102
[a] Versus SCE. Ferrocene used as internal standard. [b] m=mobility (cm2 Vꢁ1 sꢁ1), Ion/Ioff=current on/
off ratio. [c] Optimized device. See Ref. [2b].
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Communications
conducting channel. As shown above, fluoroarene ring
displacement along the molecular core dramatically affects
packing characteristics and should therefore affect intermo-
lecular orbital overlap.[13] This in turn modulates electronic
coupling (bandwidth) between HOMO (for hole transport)
and LUMO (for electron transport) levels of adjacent
molecules and consequently may greatly affect the corre-
sponding carrier mobilities. Another important factor may be
localization of charge in polaronic structures. Maximum
charge separation would result in charge localization on the
end rings, fluoroarene for 1, and thiophene for 2 and 3. The
former should preferentially accommodate a negative charge
whereas the latter a positive charge. Finally, n- versus p-type
transport may be influenced by the superior core screening by
the end-fluoroarene groups in 1 against environmental
electron traps (O2, H2O), probably at the grain boundaries.[1e]
Since the 1–3 film morphologies are similar, mobility
trends within the series (m (cm2 Vꢁ1 sꢁ1): 1 (0.08) > 2 (0.01) @ 3
(4 ” 10ꢁ5)) can be explained by considering the single-crystal
and film microstructural data. Closely p–p stacked planar 1
and 2 are more favorably organized for charge transport in
the plane of the substrate than the poorly conjugated
structure of 3. In addition, the highly crystalline, textured
films of 1 provide more favorable microstructures for charge
transport than less ordered films of 2 having two sets of
reflections, which presumably correspond to two crystallite
growth orientations. However, both crystal and film micro-
structures are clearly superior to that of 3.
In summary, a new polythiophene–fluoroarene conductor
family allowing direct scrutiny of regiochemical effects on
crystal structure, orbital energies, and charge transport has
been synthesized. For the first time, n- and p-type activities
have been achieved within the same series by manipulating
relative building-block connectivity. Proper LUMO energy
and molecular regiochemistry results in n-type activity only
for 1, with m = 0.08 cm2 Vꢁ1 sꢁ1—the highest mobility yet
reported in the thiophene series. Enclosing the fluoroaryl
fragments within the thiophene(s) core results in more (2) or
less (3) efficient p-type structures, depending on the corre-
sponding film microstructure and molecular self-organiza-
tion. This study therefore provides fundamental insight into
the improved design and understanding of fluoroaromatic n-
type semiconductors.
Figure 2. Molecular orbital energy (E) level diagram(solution =fine
line; film=thick line) for molecules a6T and 1–3.
with the long molecular axes along the substrate normal,
while 3 molecules orient parallel to the oxide surface. SEM
images of 1 and 2-based films reveal similar morphologies,
consisting of highly interconnected crystallites. Smoother and
larger grains are found for films of 3.
Electrical measurements (Table 1) were performed in top
source, drain TFT configurations under moderate vacuum
( ꢀ 20 mTorr). No attempts were made to optimize perform-
ance through exhaustive variation in film morphology.
Transistor responses are observed only for positive bias for
1, and for negative bias for 2 and 3 (Figure 3), which means
that these materials are n- and p-type semiconductors,
respectively. These results combined with the solution/film
CVand optical data clearly demonstrate that reduction of the
LUMO energy is necessary but not sufficient to promote
majority electron transport. The regiochemistry of fluoroar-
ene electron-withdrawing moieties with respect to the p
backbone plays a fundamental role in establishing the active
Received: February 24, 2003
Revised: May 16, 2003 [Z51253]
Keywords: conducting materials · electron transport · fluorine ·
.
semiconductors · thiophene oligomers
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Figure 3. Output characteristics of TFT devices based on films of 1 (a)
and 2 (b) at different gate bias.
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1, and 2 EHOMO values calculated from reversible oxidation
potentials (ꢁ5.34, ꢁ5.55, and ꢁ5.64 eV, respectively) and optical
Egap data (see Table 1) are in excellent agreement. Solution
(THF) optical Egap (eV): 3.00 (3), 2.82 (2), 2.63 (1), 2.55 (a6T).
Film optical Egap (eV): 2.87 (3), 2.65 (2), 2.58 (1), 2.42 (a6T).
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micrographs and I–V plot for compound 3, cyclic voltammo-
grams, and details of device fabrication.
[7] Single crystals of 1–3 were grown by either vacuum sublimation
or cooling of saturated solutions. All measurements were made
on a Bruker SMART CCD diffractometer with graphite mono-
chromated MoKa (0.71073 Š) radiation. The data were collected
at a temperature of 153(2) K and the structures were solved by
direct methods and expanded using Fourier techniques using
SHELXTL. All non-hydrogen atoms were refined anisotropi-
cally. Hydrogen atoms were included in idealized positions and
not refined. Intensities were corrected for absorption. 1. Crystal
¯
size: 0.160 ” 0.054 ” 0.032 mm. Triclinic, P1, Z = 1. Cell dimen-
sions: a = 6.3854(15) Š; b = 7.6862(18) Š; c = 12.502(3) Š; a =
89.412(4)8; b = 78.660(4)8; g = 80.760(4)8. V= 593.7(2) Š3,
2qmax = 56.82, 1calcd = 1.853 gcmꢁ3. Of 5530 reflections, 2751
were independent (Rint = 0.064), 190 parameters, R1 = 0.066
(for reflections with I > 2s(I)), wR2 = 0.160 (for all reflections).
2. Crystal size: 0.600 ” 0.136 ” 0.044 mm. Monoclinic, P2(1)/n,
Z = 2. Cell dimensions: a = 6.258(2) Š; b = 5.0431(14) Š; c =
36.618(11) Š; a = 90.008; b = 90.71(3)8 g = 90.008. V=
1155.6(6) Š3, 2qmax = 57.64, 1calcd = 1.801 gcmꢁ3. Of 10121 reflec-
tions, 2835 were independent (Rint = 0.073), 181 parameters,
R1 = 0.086 (for reflections with I > 2s(I)), wR2 = 0.272 (for all
reflections). 3. Crystal size: 0.512 ” 0.076 ” 0.062 mm. Ortho-
rhombic, Pbcn, Z = 4. Cell dimensions: a = 25.754(5) Š; b =
7.625(2) Š; c = 11.842(2) Š; a = 90.008; b = 90.008; g = 90.008.
V= 2325.5(9) Š3, 2qmax = 57.92, 1calcd = 1.790 gcmꢁ3. Of 20218
reflections, 2931 were independent (Rint = 0.047), 188 parame-
ters, R1 = 0.048 (for reflections with I > 2s(I)), wR2 = 0.124 (for
all reflections). CCDC 207396 (1), CCDC 207397 (2), CCDC
207398 (3) contain the supplementary crystallographic data for
this paper. These data can be obtained free of charge via
www.ccdc.cam.ac.uk/conts/retrieving.html (or from the Cam-
bridge Crystallographic Data Centre, 12, Union Road, Cam-
bridge CB21EZ, UK; fax: (+ 44)1223-336-033; or deposit@
ccdc.cam.ac.uk).
[8] D. Fichou, J. Mater. Chem. 2000, 10, 571.
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[10] Measurements performed in O2- and moisture-free n-Bu4NPF6/
THFin a one-compartment cell with a C disk working electrode,
a bare Ag reference, and Pt wire counter electrodes.
[11] A. J. Bard, L. R. Faulkner, Electrochemical Methods—Funda-
mentals and Applications, Wiley, New York, 1984. Note that a6T,
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