Chromophore fluorination enhances crystallization and stability of soluble anthradithiophene semiconductors

Article abstract of DOI:10.1021/ja073235k

We report dramatic improvements in the stability and crystallinity arising from partial fluorination of soluble anthradithiophene derivatives. These fluorinated materials still behave as p-type semiconductors but with dramatic increases in thermal and photostability compared to the non-fluorinated derivatives. The triethylsilyl-substituted material forms highly crystalline films even from spin-cast solutions, leading to devices with maximum hole mobility greater than1.0 cm²/V s. In contrast, the triisopropylsilyl derivative forms large, high-quality crystals that could serve as the substrate for transistor fabrication. For this compound, mobility as high as 0.1 cm²/V s was measured on the free-standing crystal. Copyright

Full text of DOI:10.1021/ja073235k

Published on Web 02/09/2008
Chromophore Fluorination Enhances Crystallization and Stability of Soluble
Anthradithiophene Semiconductors
Sankar Subramanian,† Sung Kyu Park,‡ Sean R. Parkin,† Vitaly Podzorov,§
Thomas N. Jackson,‡ and John E. Anthony*,†
Department of Chemistry, UniVersity of Kentucky, Lexington, Kentucky 40506, Department of Electrical
Engineering, The PennsylVania State UniVersity, UniVersity Park, PennsylVania 16802, and Department of Physics
and Astronomy, Rutgers UniVersity, Piscataway, New Jersey 08854
Received May 7, 2007; E-mail: anthony@uky.edu
The field of organic electronics often struggles with the tradeoff
between the high performance but costly processing of small-
molecule semiconductors and the generally lower performance of
solution-deposited polymers.1 While some small molecules can be
deposited from solution to yield high-performance field-effect
transistors (FETs), they typically require either high-temperature
Figure 1. Substituted anthradithiophenes 1H, 1F, 2H, and 2F and expected
F‚‚‚F and F‚‚‚S interactions.
annealing steps or deposition methods involving slow solvent
evaporation to yield high crystallinity, precluding the use of low-
cost, high-throughput processing techniques.2 To utilize common
fabrication methods such as spin-coating, substitution strategies that
accelerate small-molecule semiconductor crystallization are needed.
Our prior work on anthradithiophenes involved manipulation of
simple geometrical factors to induce the necessary π-stacking for
high carrier mobility, yielding high performance devices from
slowly crystallized films. Spin-coating, however, gave only amor-
phous films with poor device performance.3 This report describes
partial fluorination as a method to accelerate the crystallization and
improve the stability of these soluble semiconductors.
Noncovalent interactions such as H-bonding have been used to
control the growth of organic optoelectronic materials,4 but such
forces might interfere with the weaker π-stacking that drives self-
assembly in silylethynyl heteroacenes. Weaker halogen-based
interactions have been exploited in tetrathiafulvalene materials to
improve crystallization,5 and fluorine interactions in particular are
promising supramolecular synthons in crystal engineering.6 For
functionalized anthradithiophenes, we envisioned that both FsF
and FsS interactions (Figure 1), as well as the known interactions
between fluorinated and non-fluorinated aromatic surfaces,7 should
enhance the crystallization of these soluble materials.
Fluorinated anthradithiophenes 1F and 2F are easily prepared,
like all anthradithiophenes reported to-date, as an inseparable
mixture of syn- and anti-isomers (in Figure 1, only the anti-isomer
is shown for clarity; in Figure 2, the disorder is readily apparent).
Fluorine substitution of this chromophore significantly changed a
number of molecular properties, inducing a blue-shift in absorption
and a 100 mV increase in oxidation potential vs 1H and 2H. More
significant is the dramatic increase in thermal and photostability
of the fluorinated derivatives. 1F and 2F are now stable in the melt,
and while films of 2H bleached under laboratory lighting with t1/2
< 30 min, films of 2F exhibited minimal decomposition over
several weeks of study (Figure S1). Under bright light, 2F
decomposed with t1/2 > 2000 h. This stabilization arises from both
substitution of the most reactive position of the anthradithiophene
and electronic changes induced by the electron-deficient fluorine
substituent.8 Crystallographic analysis of 1F and 2F showed
Figure 2. Crystal packing of 1F (top, some alkyl groups on Si removed
for clarity) and 2F (bottom), along with images of their crystals.
π-stacked arrangements with interplanar spacings ranging from 3.27
to 3.40 Å. In 1F (F‚‚‚F contacts as close as 2.6 Å) the crystal
packing changed dramatically from 1H’s 1-D “slipped” π-stack
(Figure S2) to a 2-D π-stacking arrangement. In 2F (S‚‚‚F contacts
as close as 3.16 Å) the differences from 2H are more subtle but
include a 0.3 Å long-axis shift between neighboring molecules in
the π-stacks (Figure S3). Although this difference is small, such
long-axis movements are predicted to yield significant changes in
transport properties.9
Unlike non-fluorinated 1H, we were unable to prepare uniform
films of 1F, which instead formed large block-like crystals on the
substrate. In contrast, 2F exhibited two-dimensional film growth
yielding uniform, crystalline films even from spin-cast solutions.
The difference in film growth may arise from the longitudinal tilt
of the 2F chromophore in the unit cell allowing interaction between
the fluorinated chromophore and the substrate (compared to 1F,
which would interact strictly through the silyl substituents).10
Successful device fabrication with 2F also required treatment of
the Au electrodes with pentafluorothiophenol. While such treatments
are common in the fabrication of organic FETs, they typically serve
to alter the work function of the electrode to facilitate charge
injection.11 Here, the surface treatment appears to induce nucleation
and rapid crystal growth of 2F on the gold electrodes, yielding
large, plate-like crystallites that grow from the electrodes and span
the channel region (Figure S4). Without this treatment, crystalline
films still formed but with significantly lower uniformity and device
† University of Kentucky.
‡ The Pennsylvania State University.
§ Rutgers University.
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J. AM. CHEM. SOC. 2008, 130, 2706-2707
10.1021/ja073235k CCC: $40.75 © 2008 American Chemical Society

C O M M U N I C A T I O N S
The perfluorination or perfluoroalkyl substitution of aromatic
compounds is a well-known technique to alter the electronic
properties of p-type semiconductors, creating high performance
n-type materials.13 We demonstrate here that the strategic addition
of a few fluorine substituents does not alter p-type behavior but
dramatically improves thermal and photostability and induces solid-
state interactions that accelerate crystallization. Fluorine-substituted
2F in particular is a robust semiconductor that easily and repro-
ducibly forms stable, high quality thin films, allowing detailed
studies not possible with non-fluorinated 2H. This partial fluorina-
tion strategy should also be suitable for use on other heteroaromatic
semiconductors.14 We are currently exploiting the increased volatil-
ity of fluorinated aromatic systems13b to form high quality single
crystals by vapor transport methods, which will allow further study
of the transport properties of these materials.
Figure 3. Log(ID) and xID versus VGS with mobility versus VGS inset (left)
and ID versus VDS for several values of VGS (right) for a FET fabricated
from a spin-cast thin film of 2F. W/L ) 220/10 µm, Tox ) 200 nm.
Acknowledgment. J.E.A. thanks the Office of Naval Research
for support of semiconductor synthesis. V.P. thanks the support of
NSF Grants DMR-0405208 and ECS-0437932.
Supporting Information Available: Synthesis of 1F and 2F, and
their corresponding CIF files, as well as details describing device
fabrication and characterization. This material is available free of charge
via the Internet at http://pubs.acs.org.
Figure 4. Single-crystal FET device performance of 1. The inset is a
photograph of the device, where a rough top surface of the solution-grown
crystal can be seen under the gate.
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