Synthesis, assembly, and thin film transistors of dihydrodiazapentacene: An isostructural motif for pentacene

Article abstract of DOI:10.1021/ja036466+

The study below details the synthesis, assembly, and thin film transistors from dihydrodiazapentacenes. These molecules have the same molecular shape as pentacene but are much easier to prepare and have much greater environmental stability. Thin films made from the dihydrodiazapentacene behave as field effect transistors with mobilities and on/off ratios high enough to be useful in certain applications. X-ray diffraction and AFM experiments on these films show that the molecules stack in layers with their long axis upright from the surface. Some of the derivatives synthesized for this study have unexpectedly high solubility in polar solvents such as DMF and DMSO. The crystal structure from DMF reveals self-assembled channels with each of the aniline functionalities forming a hydrogen bond with solvent. In more nonpolar solvents, the solid-state assembly switches to a herringbone motif characteristic of the linear acenes.

Full text of DOI:10.1021/ja036466+

Published on Web 08/01/2003
Synthesis, Assembly, and Thin Film Transistors of
Dihydrodiazapentacene: An Isostructural Motif for Pentacene
†
†
§
‡
Qian Miao, Thuc-Quyen Nguyen, Takao Someya, Graciela B. Blanchet, and
,
†
Colin Nuckolls*
Contribution from Department of Chemistry, Columbia UniVersity, New York, New York 10027,
and DuPont, Central Research and DeVelopment, Wilmington, Delaware 19880
Received June 2, 2003; E-mail: cn37@columbia.edu
Abstract: The study below details the synthesis, assembly, and thin film transistors from dihydrodiaza-
pentacenes. These molecules have the same molecular shape as pentacene but are much easier to prepare
and have much greater environmental stability. Thin films made from the dihydrodiazapentacene behave
as field effect transistors with mobilities and on/off ratios high enough to be useful in certain applications.
X-ray diffraction and AFM experiments on these films show that the molecules stack in layers with their
long axis upright from the surface. Some of the derivatives synthesized for this study have unexpectedly
high solubility in polar solvents such as DMF and DMSO. The crystal structure from DMF reveals self-
assembled channels with each of the aniline functionalities forming a hydrogen bond with solvent. In more
nonpolar solvents, the solid-state assembly switches to a herringbone motif characteristic of the linear
acenes.
2
Introduction
studied, it is not clear if these molecules will ultimately be
useful in devices because of a number of interrelated factors
The study below details the syntheses of a number of
derivatives of dihydrodiazapentacenes (2 and 3) and their
electrical response in thin films. Although some derivatives of
including low thermal stability, difficult derivatization, and facile
2,5,6
Although high field effect mobility5
atmospheric degradation.
is one important criterion for organic FETs, ultimate success
in these organic transistors will require a holistic approach to
1
these compounds have been known for over a century, this
study appears to be the first time that their electrical properties
in thin films has been reported. The important finding is that
some derivatives of these materials behave as organic field-
4
c,6
creating new building blocks.
The most challenging derivatives to synthesize among the
linear acenes are ones that have their ends but not their edges
functionalized. These derivatives are difficult to prepare due to
the lack of mild and efficient methods for aromatization and
2
4
effect transistors (FETs) with on/off ratios greater than >10
and mobilities approaching 10-2 cm V s . Thin-film organic
2
-1 -1
FETs are useful because they can be deposited through a variety
8
annulation. In contrast, the synthesis of linear pentacycles such
3
of techniques on flexible substrates over large area. Some
as the dihydrodiazapentacenes (2 and 3), where nitrogens
replaces two of the carbons of pentacene, are efficiently achieved
through simple condensation reactions between a 1,2-aromatic
organic molecules have a sufficiently good electrical profile that
they can be used as the drivers for individual pixels making
them prime for applications such as electronic paper and large
1
diamine and a 1,2-aromatic diol. Particularly attractive are the
4
area displays. Although certain compounds such as pentacene
derivatives in Figure 1csthe 5,14-dihydrodiazapentaceness
(1) and derivatives of oligothiophenes have been heavily
(
4) (a) Rogers, J. A.; Bao, Z.; Baldwin, K.; Dodabalapur, A.; Crone, B.; Raju,
V. R.; Kuck, V.; Katz, H.; Amundson, K.; Ewing, J.; Drzaic, P. Proc. Nat.
Acad. Sci. (USA). 2001, 98, 4835-4840. (b) Roger, J. A.; Bao, Z. J. Polym.
Sci. Pt. A: Polym. Chem. 2002, 40, 3327-3334. (c) Huitema, H. E. A.;
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Cantatore, E.; Herwig, P. T.; van Breemen, A. J. J. M.; de Leeuw, D. M.
Nature 2001, 414, 599.
†
Department of Chemistry, Columbia University.
DuPont, Central Research and Development.
Current address: School of Engineering, University of Tokyo, 7-3-1
‡
§
Hongo, Bunkyo-ku, Tokyo 113-8656, Japan.
(
1) (a) Hinsberg, O. Ann. Chem. 1901, 319, 260. (b) Manassen, J.; Khalif, S.
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Berichte der Bunsen-Gesellschaft 1967, 71(9-10), 1119. (d) Leete, E.;
Ekechukwu, O.; Delvigs, P. J. Org. Chem. 1966, 31, 3734.
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ref 2b. (h) see ref 6.
(
2) Reviews on organic FET’s from small molecules: (a) Dimitrakopoulos,
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1
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9
10.1021/ja036466+ CCC: $25.00 © 2003 American Chemical Society

Isostructural Motif for Pentacene
A R T I C L E S
7
Figure 1. (a) Molecular structure and crystal structure of pentacene (1);
b) molecular structure and crystal structure of 2, 6,13-dihydrodiazapenta-
(
cene; and (c) derivatives of 5,14-dihydrodiazapentacene (3a-d).
Scheme 1
Figure 2. (a) Crystal structure of 2 from DMF showing two hydrogen
bonds to molecules of solvent; (b) channels through the crystals (hydrogens
have been removed to clarify the view); (c) Crystal structure of 2 from hot
benzophenone showing a herringbone motif; (d) View of the crystal lattice
showing slipping. For the models: gray atoms are carbon, blue atoms are
nitrogen, and red atoms are oxygen.
because end-functionalized derivatives are easily generated
through the readily available benzene diamines.
Results and Discussion
The synthetic scheme that affords the 5,14- and 6,13-
dihydrodiazapentacene derivatives is shown in Scheme 1. Full
experimental details are available in the Supporting Information,
but in general, the products can be crystallized from the reaction
mixture. The crystals vary in color for the different deriva-
tives: yellow for 3a and light green for 2.
Figure 3. X-ray diffraction from a 100 nm film of 3a on Si/SiO2.
Pentacene is only sparingly soluble in high boiling point
aromatic solvents, but 2, 3a, and 3d are soluble in DMF and
DMSO at concentrations higher than ca. 1 mg/mL. Moreover,
solutions of 2 and 3 that are open to air and light resist oxidation
whereas solutions of pentacene are known to degrade in a matter
herringbone arrangement as shown in Figure 2c. This motif is
7
,10
the characteristic of the packing in pentacene,
dihydrodiaz-
1
1
aanthracene, and other electron rich π-systems as predicted
1
2
by the model of Hunter and Sanders. For 2, the stacking is
slipped relative to pentacene with only four overlapping rings
between nearest neighbors in the crystal as shown in Figure
2d. Presumably, this arrangement avoids the energetic penalty
of stacking the central dihydropyrazine ring over itself.
9
,17
of minutes.
The origin of the stability of these derivatives
has to do with breaking of the conjugation in the pentacene
skeleton by interposing the amino functionality. In essence, 2
consists of two naphthalene rings connected by a spacer, and 3
consists of an anthracene ring plus a benzene ring. X-ray
diffraction of crystals grown from saturated solutions of 2 in
DMF reveals why these derivatives are so soluble. Two
molecules of DMF each form a hydrogen bond with the N-H’s
of 2 as shown in Figure 2a. Interestingly, these three-molecule
assemblies further assemble into large nanoscale channels as
shown in Figure 2b. From solvents that cannot form hydrogen
bonds such as benzophenone, the structure for 2, changes to a
Films of these compounds can be grown on oxidized silicon
substrates through thermal evaporation in a vacuum. The X-ray
diffractograms from films of 2, 3a, and 3b (shown in Figure 3)
were similar to each other showing only multiple (00l) reflec-
tions. This is also characteristic of films from linear acenes and
is attributed to the molecules stacking into layers with the
13
molecule’s long axis perpendicular to the surface. The intensity
of the X-ray diffraction peaks indicates that the molecules are
highly aligned.
(
8) Representative syntheses of pentacene and its derivatives: (a) Goodings,
(10) Campbell, R. B.; Robertson, J. M.; Trotter, J. Acta Crystallogr. 1961, 14,
705-711.
E. P.; Mitchard, D. A.; Owen, G. J. Chem, Soc. Perkin Trans. 1 1972, 11,
1
310-1314. (b) Bailey, W. J.; Madoff, M. J. Am. Chem. Soc. 1953, 75,
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2000, 24, 143-147.
5
603. (c) Luo, J.; Hart, H. J. Org. Chem. 1987, 52, 4833-4836. (d) Satchell,
M. P.; Stacey, B. E. J. Chem. Soc. 1971, 468. (e) Laquindanum, J. G.;
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5534.
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G. IEEE Electron DeV. Lett. 1997, 18, 87-89. (b) Dimitrakopoulos, C.
D.; Brown, A. R.; Pomp, A. J. Appl. Phys. 1996, 80, 2501-2508. (c)
Minakata, T.; Imai, H.; Ozaki, M.; Saco, K. J. Appl. Phys. 1992, 72, 5220-
5225.
1
23, 9482. (g) Takahashi, T.; Kitamura, M.; Shen, B.; Nakajima, K. J.
Am. Chem. Soc. 2000, 122, 12876-12877. (h) Wartini, A. R.; Staab, H.
A.; Neugebauer, F. A. Eur. J. Org. Chem. 1998, 1161, 1-1170.
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J. AM. CHEM. SOC.
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VOL. 125, NO. 34, 2003 10285

A R T I C L E S
Miao et al.
Figure 5. (a) Schematic of device configuration for FET’s measurements;
(
b) current/voltage curves for 3a with varying gate bias (0 to -20 V from
red to black in -4 V steps).
The diameter of the grains measured in the AFM micrographs
in Figure 4c is ca. 127 ( 14 nm and increases to ca. 144 ( 23
nm for a 35 nm thick film (not shown).
Thermal evaporation of source/drain electrodes onto these
polycrystalline films allows field-effect transistor structures to
be fabricated with the heavily doped silicon layer functioning
as the gate electrode as shown in Figure 5a. The oxide layer
was 100 nm thick in these devices and the ratio of the width to
the length (W/L) of the channel was 40. Shown in Figure 5b
are I-V curves at different gate biases for a ca. 35 nm thick
film of 3a. The film performs as a p-type transistor with a field-
-
3
2
-1 -1
effect mobility of 6 × 10 cm V s , which is determined
2
in the saturation regime by using the equation : IDS ) (µWCi/
L)(VG-V0) . On/off ratio of the drain current between 0 and
20 V gate bias was greater than 10 . With a thicker oxide
2
2
3
-
layer (300 nm) and higher gate voltage (-100 V) the on/off
4
ratio for 3a could be raised to >10 . With the exception of 3d,
which decomposed upon thermal evaporation, all of the other
derivatives from Figure 1 behave as organic field effect
transistors. Their mobilities and on/off ratios in thermally
evaporated films are shown in Table 1. A remarkable feature
of devices from 2, 3a-c is their atmospheric stability; when
they are operated periodically over several days, open to the
Figure 4. (a) 1 × 1µm AFM height image of ca. 1.5 nm film of 3a. (b)
Step height in the film in Figure 4a. (c) 1 × 1µm AFM height image of ca.
1
5
atmosphere, they show no significant change.
8
nm film of 3a.
Shown in Figure 6a is a comparison between the UV-vis
spectrum of ca. 150 nm films of pentacene and 3a on quartz
substrates. Both materials show sharp band edges suggesting
little structural disorder. The longest wavelength transition that
is attributed to the band gap is around 670 nm (1.85 eV) for
Atomic force microscopy of films of 3a on Si/SiO2, also
support this layer-by-layer growth. Shown in Figure 4a is
micrograph of a film from a very short deposition time, intended
to give a film of approximately one molecular layer in thickness
1
6
pentacene and around 480 nm (2.58 eV) for 3a. In pentacene,
the transitions at 620 nm (1.98 eV) and 670 nm (1.85 eV) are
attributed to Davydov doublet of the 0-0 band and the two
other peaks at 540 nm (2.29 eV) and 585 nm (2.13 eV) to the
(ca. 1.5 nm). As shown in Figure 4b, each of the steps from the
surface in this film are ca. 1.8 nm--roughly the length of the
long axis in 3a. This first layer consists of an intricate set of
interconnections that is remarkably different than the dendritic
morphology seen in pentacene crystallites.14 The difference
1
7
doublet of the 0-1 band. Although the relative intensity of
could arise from the dihydrodiazapentacenes having a different
balance for its surface diffusion rate and intermolecular attrac-
tion. A micrograph of a thicker film (ca. 8 nm) is displayed in
Figure 4c. Distinct terraces can be seen in each grain with
heights that are consistent with the long axis of the molecule.
(
15) (a) Hong, X. M.; Katz, H. E.; Lovinger, A. J.; Wang, B.-O.; Raghavachari,
K. Chem. Mater. 2001, 13, 4686-4691. (b) Horowwitz, G.; Hajlaoui, R.;
Bouchriha, H.; Bourguiga, R.; Hajlaoui, M. AdV. Mater. 1998, 10, 923. (c)
Gundlach, D. J.; Lin, Y.-Y.; Jackson, T. N.; Schlom, D. G. Appl. Phys.
Lett. 1997, 71, 3853-3855.
(
16) Kim, K.; Yoon, Y. K.; Mun, M.-O.; Park, S. P.; Kim, S. S.; Im, S.; Kim,
J. H. J. SuperconductiVity: Incorporating NoVel Magnetism 2002, 15, 595-
598.
(
14) Meyer zu Heringdorf, F.-J.; Reuter, M. C.; Tromp, R. M. Nature 2001,
(17) Siebrand, W.; Zgierski, M. Z. Springer Ser. Solid-State Sci. 1983, 49, 136-
4
12, 517-520.
44.
10286 J. AM. CHEM. SOC.
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VOL. 125, NO. 34, 2003

Isostructural Motif for Pentacene
A R T I C L E S
a
Table 1.
Mobility and On/Off Ratios for 2 and 3a-d for Transistors Prepared at Different Deposition Temperatures for the Substrate (Ts)
T
s
) 17 °C
T
s
) 60 ± 3 °C
T
s
) 90 ± 1 °C
mobility
mobility
mobility
cm V s-1)
2
-1
on/off ratio
(cm V s-1)
2
-1
on/off ratio
(cm V s-1)
2 -1
on/off ratio
(
-
5
-5
3
-6
2
1 × 10
200-400
(2-5) × 10
100
2 × 10
5 × 10
1 × 10
2 × 10
50
-
-
3
4
3
-3
3
-3
3
3a
3b
3c
(3-6) × 10
(3-4) × 10
1 × 10
2 × 10
3 × 10
2 × 10
-3
-4
1 × 10
500-700
(6-8) × 10
100-500
-
4
3
-4
3
no field effect
3 × 10
5 × 10
4 × 10
a
The thickness of the gate dielectric layer (SiO2) was 100 nm and the on/off ratios were measured between gate biases of 0 and -20 V.
applications. Moreover, these films do not show any evidence
for photooxidation that occurs readily in films of pentacene.20
A comparison of the infrared spectra from 3a and pentacene
on KBr substrates is displayed in Figure 6b. The intense IR
-1
bands at 730 and 908 cm in films of pentacene that correspond
to out of plane H-movement shift only slightly to 736 and 889
-1
-1
cm for 3a. The additional strong bands at 3378 and 1500 cm
for 3a correspond to N-H stretching and bending, respectively.
Conclusion
In summary, these studies demonstrate that dihydrodiazo-
pentacenes self-assemble into polycrystalline films that can form
the active layer in a FET device. The on/off ratios are
sufficiently large that some of the derivatives synthesized here
could find utility as drivers for organic displays. These materials
have several other advantages relative to pentacene: they are
easily derivatized, environmentally stable, and also somewhat
soluble. One very promising application of small molecule
organic FETs is in chemical and vapor sensing applications.21
The amine functionality of the dihydrodiazapentacenes films
could serve both as a hydrogen bond donor and a site for
derivatization providing analyte specificity in these sensors.
Figure 6. (a) UV-vis spectrum of 3a (red trace) compared to pentacene
(black trace). (b) Infrared spectrum of 3a (red trace) compared to pentacene
Acknowledgment. This work is supported primarily by the
Nanoscale Science and Engineering Initiative of the National
Science Foundation under NSF Award Number CHE-0117752.
We also acknowledge financial support from the Chemical
Sciences, Geosciences and Biosciences Division, Office of Basic
Energy Sciences, US D.O.E. (No. DE-FG02-01ER15264). C.N.
thanks the Beckman Young Investigator Program (2002) and
the Dupont Young Investigator Program (2002) for financial
support. We thank Prof. Parkin (Columbia) and Dr. Kevin Janak
(black trace).
the peaks is different, 3a also has four long wavelength
transitions that could be assigned as the two sets of Davydov
doublets: the 0-0 band at 445 nm (2.78 eV) and 480 nm (2.58
eV); the 0-1 band at 395 nm (3.14 eV) and 420 nm (2.95 eV).
The Davydov components are polarized in the plane that is
defined by the two short axes of the molecule (the a,b-plane),
indicating that this plane is parallel to the substrate--the
molecule’s long axis is upright from the surface. This conclusion
is agreement with the X-ray diffraction and AFM results in
Figures 4 and 5.
(Columbia) for solving the X-ray structures in Figure 2.
Supporting Information Available: Details for the synthesis
of 2 and 3, the device preparation, the electrical measurements,
and the crystal structure data. This material is available free of
charge via the Internet at http://pubs.acs.org.
The longest wavelength absorption for pentacene is red-
18
19
shifted by ca. 80 nm on going from solution or the gas phase
to thin film due to the association between the molecules in the
solid state.16,17
JA036466+
By contrast, the red shift for 3a is only around
1
0 nm going from a THF solution to a film. This low amount
(
20) Kamura, Y.; Shirotani, I.; Inokuchi, H.; Maruyama, Y. Chem. Lett. 1974,
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of shifting for 3a upon film formation coupled with these films
intense fluorescence makes them viable candidates for LED
(
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37, 123. (c) Guillaud, G.; Simon, J.; Germain, J. P. Coord. Chem. ReV.
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