Functionalized pentacene: Improved electronic properties from control of solid-state order [20]

Full text of DOI:10.1021/ja0162459

9482
J. Am. Chem. Soc. 2001, 123, 9482-9483
Functionalized Pentacene: Improved Electronic
Properties from Control of Solid-State Order
John E. Anthony,*,† James S. Brooks,‡ David L. Eaton,† and
Sean R. Parkin†
Department of Chemistry, UniVersity of Kentucky
Lexington, Kentucky 40506-0055
FSU/NHMFL, 1800 East Paul Dirac DriVe
Florida State UniVersity, Tallahassee Florida 32310
ReceiVed May 22, 2001
Molecular order has proven to be a significant factor in the
performance of devices based on organic semiconductors. Recent
studies involving solubilized versus unsubstituted thiophene
oligomers have demonstrated that modifications which increase
orbital overlap in the solid state can improve device performance
by more than an order of magnitude.1 Similar studies on
pentacene, a compound which has already demonstrated remark-
able potential for device applications,2 have also focused on
maximizing orbital overlap by inducing order in films.3 However,
these pentacene studies have thus far relied on substrate modifica-
tion, rather than on pentacene functionalization,4 to achieve the
desired goals. We report here the preparation of two functionalized
pentacene derivatives, and the effect of this functionalization on
both the solid-state ordering and the electronic properties of the
resulting crystals.
Our goal for a functionalized pentacene was two-fold: First,
the substituents should impart solubility to the acene, to simplify
purification and processing. Second, the substituents should induce
some capability for self-assembly of the aromatic moieties into
π-stacked arrays to enhance intermolecular orbital overlap. We
anticipated that both of these goals could be accomplished by
exploiting a rigid spacer to hold the necessarily bulky solubilizing
groups well away from the aromatic core, allowing the closest
possible contact between the aromatic rings.5 Our initial targets
were the bis(triisopropylsilylethynyl)pentacenes 1 and 2. Both of
these compounds are easily prepared in near quantitative yield
in a one-pot reaction from 6,13-pentacenequinone and 5,14-
pentacenequinone, respectively.6
Figure 1. Solid-state ordering of 1. (Left) View of the ac layer (looking
down the b axis). (Right) View of the bc layer (looking down the a axis).
Pentacene 1 is very soluble in most organic solvents. Relatively
large crystals (plates, 1 mm × 1 mm × 0.1 mm) were easily
grown from acetone and analyzed by X-ray diffraction.7
Comparison of the solid-state ordering of substituted pentacene
1 with that of unsubstituted pentacene reveals striking differences
(Figure 1). Most apparently, 1 does not adopt the herringbone
pattern of unsubstituted pentacene. Rather, it stacks in a two-
dimensional columnar array with significant overlap of the
pentacene rings in adjacent molecules. Because of this arrange-
ment, the interplanar spacing of the aromatic rings is significantly
smaller in the substituted system (3.47 Å for 1, compared with.
6.27 Å for unsubstituted pentacene8).
The unique arrangement of molecules in the solid state should
lead to significant anisotropy of resistivity in the crystal. To
determine the magnitude of this effect we performed a series of
4-terminal resistivity measurements on several different crystals
of 1, with leads in the “Montgomery method” configuration.9
Resistivity measured along the b axis, perpendicular to the plane
of the pentacene rings, was the lowest (2.5 × 106 Ω-cm), followed
closely by that measured parallel to the long axis of the molecules
(the a axis, 5 × 108 Ω -cm). The resistivity across the short (c)
axis is particularly high (3 × 1010 Ω-cm) due to the insulating
barrier created by the solubilizing groups. All of these values are
significantly lower than those reported for high-purity pentacene
crystals (1012 Ω-cm).10
We prepared asymmetric derivative 2 because we envisioned
that a near-perfect stacking of this compound could be possible
in the solid state. Crystallographic analysis11 of this material
(Figure 2) supported at least part of this hypothesis: The molecule
does form nearly perfect stacked pairs, with the solubilizing
groups arranged to minimize steric interactions. Unfortunately,
these pairs then pack in a herringbone pattern similar to that
observed for unsubstituted pentacene. As expected, this solid-
† University of Kentucky
‡ Florida State University, FSU/NHMFL
(1) (a) Dodabalapur, A.; Torsi, L.; Katz, H. E. Science 1995, 268, 270. (b)
Horowitz, G.; Fichou, D.; Peng, X.; Xu, Z.; Garnier, F. Solid State Commun.
1989, 72, 381. (c) Garnier, F.; Hajlaoui, R.; Yassar, A.; Srivastava, P. Science
1994, 265, 1684. (d) Garnier, F.; Horowitz, G.; Fichou, D.; Yassar, D. Synth.
Met. 1996, 81, 163. (e) Dimitrakopoulos, C. D.; Furman, B. K.; Graham, T.;
Hegde, S.; Purushothaman, S. Synth. Met. 1998, 92, 47.
(2) (a) Scho¨n, J. H.; Kloc, Ch.; Batlogg, B. Science 2000, 288, 2338. (b)
Scho¨n, J. H.; Berg, S.; Kloc, Ch.; Batlogg, B. Science 2000, 287, 1022. (c)
Scho¨n, J. H.; Kloc, Ch.; Batlogg, B. Science 2000, 406, 702. (d) Scho¨n, J. H.;
Kloc, Ch.; Bucher, E.; Batlogg, B. Synth. Met. 2000, 115, 177. (e) Dimitra-
kopoulos, C. D.; Purushothaman, S.; Kymissis, J.; Callegari, A.; Shaw, J. M.
Science 1999, 283, 822. (f) Dimitrakopoulos, C. D.; Kymissis, J.; Purushotha-
man, S.; Neumayer, D. A.; Duncombe, P. R.; Laibowitz, R. B. AdV. Mater.
1999, 11, 1372.
(5) Anthony, J.; Boldi, A. M.; Boudon, C.; Gisselbrecht, J.-P.; Gross, M.;
Seiler, P.; Knobler, C. B.; Diederich, F. HelV. Chim. Acta 1995, 78, 797.
(6) The purity and structure of all new compounds were confirmed by 1H
and 13C NMR, IR, UV, MS, and EA. Synthetic details are provided as
Supporting Information.
(7) X-ray data for 1; triclinic space group P1h, Dc )1.104 g cm3, Z ) 1, a
) 7.5650(15) Å, b ) 7.7500(15) Å, c ) 16.835(3) Å, R ) 89.15(3)°, â )
92.713(10)°, γ ) 83.63(3)° V ) 960.9(3) Å3. Final R(F) ) 0.0486, wR2(F2)
) 0.1039 for 232 variables and 3375 data. Details provided as Supporting
Information.
(8) Cornil, J.; Calbert, J. Ph.; Bre´das, J. L. J. Am. Chem. Soc. 2001, 123,
1250.
(9) Montgomery, H. C. J. Appl. Phys. 1971, 42, 2971.
(10) Scho¨n, J. H.; Kloc, Ch.; Batlogg, B. Appl. Phys. Lett. 2000, 77, 2473.
(11) X-ray data for 2; monoclinic space group C2/c, Dc )1.094 g cm-3, Z
) 8, a ) 28.222(3) Å, b ) 18.904(2) Å, c ) 14.555(2) Å, â ) 92.713(10)°,
V ) 7756.5(16) Å3. Final R(F) ) 0.0568, wR2(F2) ) 0.1080 for 471 variables
and 6843 data. Details provided as Supporting Information.
(3) Lin, Y. Y.; Gundlach, D. J.; Nelson, S.; Jackson, T. N. IEEE Trans.
Electron DeVices 1997, 44, 1325.
(4) For results using spin-cast films of pentacene with remoVable solubi-
lizing groups: Herwig, P. T.; Mu¨llen, K. AdV. Mater. 1999, 11, 480.
10.1021/ja0162459 CCC: $20.00 © 2001 American Chemical Society
Published on Web 08/30/2001

Communications to the Editor
J. Am. Chem. Soc., Vol. 123, No. 38, 2001 9483
Figure 2. View of the close-packing “dimeric” stacks of 2 (left) and
their arrangement in the solid state (right).
state arrangement leads to very high resistivity along all crystal-
lographic axes. The lowest resistivity measured for this material
was 8 × 109 Ω-cm. This value still compares quite favorably
with that of unsubstituted pentacene and is even lower than the
resistivity of 1 along its least-conductive axis.
An additional demonstration of the remarkably low resistivity
associated with crystalline 1 can be seen in the Arrhenius plot of
resistivity versus inverse temperature for 1 and 2 (Figure 3).
Measurement of resistivity in these samples was complicated by
apparent capacitative charging of the material at low temperatures,
so that the temperature-dependent resistivity was most easily
acquired by first heating the sample and monitoring the resistivity
as the sample cooled. The data thus acquired was used to derive
the band gap along the three crystallographic axes of 1, as well
as the band gap for 2. As expected, the band gaps mirror the
resistivity data. The gap measured down the π-stacking axis of 1
was the smallest (0.6 eV),12 and that measured across the channels
of solubilizing groups (the c axis, Figure 1) was the largest (0.9
eV). The offset pentacene 2 yielded a band gap of 1.83 eVs
essentially identical to the absorption edge found in the optical
spectrum (∼660 nm for a thin film).
Figure 3. Plot of resistivity vs temperature along all three crystallographic
axes of 1, and along one axis of 2.
unit cell length. Thus, the films grow with the silyl groups on
the surface of the glass, leading to π-stacking in the direction of
current flow.
With these two selectively functionalized pentacene derivatives,
we have demonstrated significant conductivity enhancements by
controlling the arrangement of the molecules in the solid state.
This example demonstrates that simultaneous improvement in ease
of processing and in electronic properties is achievable.
More technologically relevant is the resistivity of thin films of
these materials, which were easily prepared by either spin-casting
or vacuum evaporation. While films of 1 proved to be quite robust,
films of 2 decomposed rapidly upon exposure to air. The sheet
resistivity of solvent-cast films of 1 is approximately 1010
Ω/square. Vacuum-evaporated films of 1 had significantly
improved electronic properties, with surface resistivities on the
order of 108 Ω/square. For a 5000 Å thick film, this value
translates to a resistivity of ∼106 Ω-cmsthe same order of
magnitude as the resistivity along the π-stacking axis of crystalline
1simplying the formation of a highly ordered film. X-ray
diffraction analysis of this film gave rise to sharp diffraction peaks
showing a layer separation of 16.83 Å, identical to the c-axis
Acknowledgment. J.E.A. thanks the Office of Naval Research
(N00014-99-1-0859) and the National Science Foundation (CAREER
Award CHE-9875123) for generous support of this research. We thank
Professor Carolyn Brock for helpful discussions and Mr. Matt Krepps
for performing combustion analysis on 1 and 2.
Supporting Information Available: Synthetic procedures, charac-
terization (PDF) and X-ray crystallographic data (CIF) for compounds 1
and 2. This material is available free of charge via the Internet at
http://pubs.acs.org.
(12) Confirmed by examination of the NIR spectrum of powdered 1, which
showed a strong absorption at 4380 cm-1 (0.56 eV).
JA0162459