N -alkyldinaphthocarbazoles, azaheptacenes, for solution-processed organic field-effect transistors

Article abstract of DOI:10.1021/ja3082582

Substituted N-alkyldinaphthocarbazoles were synthesized using a key double Diels-Alder reaction. The angular nature of the dinaphthocarbazole system allows for increased stability of the conjugated system relative to linear analogues. The N-alkyldinaphthocarbazoles were characterized by UV-vis absorption and fluorescence spectroscopy as well as cyclic voltammetry. X-ray structure analysis based on synchrotron X-ray powder diffraction revealed that the N-dodecyl-substituted compound was oriented in an intimate herringbone packing motif, which allowed for p-type mobilities of 0.055 cm² V ^-1 s^-1 from solution-processed organic field-effect transistors.

Full text of DOI:10.1021/ja3082582

Communication
pubs.acs.org/JACS
N‑Alkyldinaphthocarbazoles, Azaheptacenes, for Solution-Processed
Organic Field-Effect Transistors
Toan V. Pho,†,‡ Jonathan D. Yuen,‡ Joshua A. Kurzman,† Braden G. Smith,† Maosheng Miao,§
Wesley T. Walker,∥ Ram Seshadri,†,§ and Fred Wudl*,†,‡,§
†Department of Chemistry and Biochemistry, ‡Center for Polymers and Organic Solids, and §Department of Materials and Materials
Research Laboratory, University of California, Santa Barbara, California 93106, United States
∥Department of Chemistry and Biochemistry, University of California, Los Angeles, California 90095, United States
S
* Supporting Information
Similarly, the introduction of heteroatoms such as sulfur or
nitrogen16−22 into the polyacene backbone allows for added
ABSTRACT: Substituted N-alkyldinaphthocarbazoles
were synthesized using a key double Diels−Alder reaction.
The angular nature of the dinaphthocarbazole system
allows for increased stability of the conjugated system
relative to linear analogues. The N-alkyldinaphthocarba-
zoles were characterized by UV−vis absorption and
fluorescence spectroscopy as well as cyclic voltammetry.
X-ray structure analysis based on synchrotron X-ray
powder diffraction revealed that the N-dodecyl-substituted
compound was oriented in an intimate herringbone
packing motif, which allowed for p-type mobilities of
0.055 cm2 V−1 s−1 from solution-processed organic field-
effect transistors.
stability arising from the loss of linearity of the fused system.
The inclusion of these heterocycles also provides sites where
solubilizing groups or additional functional motifs can readily
be appended. For these reasons, we studied the chemical and
electronic properties of one of the simplest heteroheptacenes,
dinaphthocarbazole (DNC), which is composed of a carbazole
ring laterally fused to two naphthalene substituents.
First reported in 1962,23 unfunctionalized DNC was
synthesized by Elbs pyrolysis of a bis(o-tolyl)ketone precursor.
However, the low yield (ca. 0.5% for the key pyrolysis step) and
insolubility of the target DNC have precluded its application in
organic electronics. Herein we report the synthesis of soluble
substituted DNCs in moderate yields via a double Diels−Alder
reaction24 and our evaluation of their performance in p-type
OFETs.
ince the advent of the organic field-effect transistor
S(OFET), linear polyacenes and their derivatives have
emerged as promising materials for small-molecule hole-
transporting OFETs.1−3 In particular, tetracene and pentacene
have been extensively studied because the combination of their
delocalized π systems and solid-state structure/morphology
provide them with high charge-carrier mobilities. The more
extended acenes, such as hexacene and heptacene, may
demonstrate enhanced transport properties due to their
increased π conjugation, but these acenes have been
underutilized, primarily because of the instability of their π
systems as well as processing difficulties arising from the
insolubility of the unfunctionalized systems.1
The synthesis of the substituted DNCs (Scheme 1) was
adapted from a synthetic procedure for tetrasubstituted
carbazoles.24 Thermolysis of diacetate 124 provided a transient
diene that underwent a double Diels−Alder reaction with 2
equiv of 1,4-naphthoquinone to give octahydrocarbazole 2.
a
Scheme 1. Synthesis of DNCs
Whereas unsubstituted extended acenes (i.e., hexacene
through nonacene) are stable in a polymeric matrix for only
a few hours,4−6 stable and soluble extended acene derivatives
have been prepared via extensive functionalization of the acene
core to provide both steric and electronic protection.7−12 The
stability of extended polyacenes can also be improved by
disrupting the linearity of the catacondensed array, thus
decreasing their proclivity toward dimerization and polymer-
ization. This is best exemplified by the increased stability of the
angular “phenes” relative to their isomeric linear acene
counterparts.13,14 In addition, the shift from a linear acene to
its angular analogue is typically accompanied by a stabilization
of the highest occupied molecular orbital (HOMO) level,15
thereby mitigating the oxidative degradation processes that
plague the linear polyacenes.
a
Conditions: (a) 1,4-naphthoquinone, mesitylene, reflux; (b) reflux;
(c) Al, HgCl2, CBr4, cyclohexanol, reflux.
Received: August 20, 2012
Published: October 17, 2012
© 2012 American Chemical Society
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Figure 1. (a) X-ray crystal structure projected down the b axis and (b) herringbone packing motif of DNC 4b. (c) DFT-optimized crystal structure
of 4b overlaid on the structure determined using powder diffraction.
Whereas the literature protocol required an oxidizing agent
such as 2,3-dichloro-5,6-dicyanobenzoquinone (DDQ) to
aromatize the octahydrocarbazole, we found that excess
naphthoquinone could be used in the one-pot reaction to
oxidize 2 to the corresponding bisanthraquinone 3. Reduction
mediated by aluminum amalgam then afforded the substituted
DNC 4. Despite the modest yields (40−45%) of the bis-
cycloaddition, this synthesis provides a versatile pathway to
various substituted DNCs. The N-benzyl derivative 4a was
synthesized first, but its low solubility provided an impetus for
the synthesis of the significantly more soluble N-dodecyl
analogue 4b.
details regarding the diffraction and DFT methods are available
in the SI. Despite the tendency of the PBE functional to
overestimate lattice constants and bond distances, the DFT-
optimized unit cell volume was less than that determined
experimentally, suggesting that there may have been some
residual solvent trapped in the sample. This is consistent with
the fact that the measured density of the sample (1.20 g/cm3)
was slightly larger than the density calculated from the crystal
structure (1.16 g/cm3).
According to Figure 1a, the fused rings of DNC 4b adopt an
angular configuration due to the central pyrrole unit. The
strong hydrophobic interaction between the dodecyl side chains
promotes segregation of the alkyl groups and the aromatic
cores within the unit cell. Moreover, the closest face-to-face
distance between the π systems is the b-axis cell constant (5.84
Å), which is too large for intermolecular π−π stacking.
However, the molecules are oriented in an intimate
herringbone packing motif (Figure 1b) in which the closest
C−C distance is 3.78 Å, auguring well for facile intermolecular
charge transport.
Because attempts to produce crystals of the DNCs suitable
for single-crystal X-ray diffraction were unsuccessful, the X-ray
crystal structure of DNC 4b was instead modeled from
synchrotron X-ray powder diffraction data. The crystal structure
model was obtained using the parallel tempering algorithm of
the Monte Carlo-simulated annealing method implemented in
FOX.25 The input molecular structure was optimized within the
framework of density functional theory (DFT) using Gaussian
03.26 The 6-31G(d) basis set was used, and the exchange−
correlation potential and energy were described using the
B3LYP hybrid functional. In the structure optimization
simulation, the molecules were allowed to translate and rotate
around the center of mass and their dihedral angles were
allowed to change, subject to the constraints imposed by the
unit cell dimensions and space group symmetry. The ring
system was constrained as a rigid body, leaving the dodecyl
chain bound through the carbazole nitrogen as the only
fragment in which the torsion angles and bond distances were
allowed to vary. Soft restraints were applied to the bond
distances in the alkyl chain. Because of the large number of
atoms in the asymmetric unit and the modest crystallinity of the
sample, attempts to refine the atom positions were
unsuccessful. Thus, the model presented in Figure 1 was
obtained directly from the Monte Carlo simulation without
further optimization. Nonetheless, the visual agreement
between the observed and calculated diffraction intensities is
impressive [Figure S1 in the Supporting Information (SI)], and
the statistical figures of merit [χ2 (GoF) = 3.8; Rwp = 11.7%; Rp
= 7.5%] also support the plausibility of the proposed model.
To assess the validity of the model further, the crystal
structure was also optimized by periodic DFT within the
generalized gradient approximation (GGA) of Perdew, Burke,
and Ernzerhof (PBE) using VASP.27 In Figure 1c, the DFT-
optimized crystal structure of DNC 4b is overlaid on the
structure determined using powder diffraction. Additional
UV−vis absorption studies of the DNCs (Figure 2a) revealed
absorption onsets at ca. 570 nm, with the N-substitution
exerting a negligible influence on the photophysical properties.
Although the DNCs are colored red, the actual absorption in
■
●
○
Figure 2. (a) UV−vis absorption ( and ) and fluorescence ( and
□
) spectra of the DNCs in chloroform. (b) Magnified (higher-gain)
view of the absorption spectrum in the 400−550 nm region, showing
the typical acene “fingerlike” absorption.
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the visible region from 450 to 570 nm is quite minimal (Figure
2b). Strong vibronic structure was observed throughout the
UV−vis spectrum, which is typical of similar aromatic
hydrocarbons.28 The UV−vis spectra of the DNCs underwent
a slight bathochromic shift (ca. 30 nm) relative to the
absorption profile of their angular arene analogue, hepta-
phene.29 This red shift is most likely due to the electron-
donating character of the nitrogen atom. The DNCs also
displayed a yellow-orange fluorescence in organic solvents such
as toluene and chloroform (λmax = 559 and 570 nm for 4a and
4b, respectively, in chloroform with an excitation wavelength of
510 nm). The fluorescence quantum yields of the N-benzyl and
N-dodecyl derivatives in chloroform were determined to be 30
and 25%, respectively, relative to a standard of rhodamine B in
ethanol.
Figure 3. (a) Transfer and (b) output curves in p-type operation
mode of the OFETs using DNC 4b.
The electrochemical properties of the DNCs were studied by
cyclic voltammetry (CV) (Table S1 in the SI). The DNCs
exhibited a reversible oxidation wave (Figure S2), whereas no
reduction behavior was observed. The HOMO level of −5.0 eV
suggests that the DNCs should be relatively air-stable,
supporting the empirical observations of air stability.
overlap between heteroatoms has been shown to enhance the
mobility of heteroacenes.32 DFT calculations of two neighbor-
ing DNC molecules from the crystal structure revealed that the
HOMO extends onto only the anthracene portion of the
nearest neighbor (Figure S5). Finally, given that solution-
processed small molecules tend to exhibit lower charge-carrier
mobilities because of the presence of numerous grain
boundaries, we expect that vacuum-processed devices may
exhibit better performance due to their larger crystalline
domains and the resulting more facile charge transport. Further
investigations to optimize the charge-carrier mobility are in
progress.
In summary, substituted DNCs have been synthesized and
utilized in solution-processed OFETs. The enhanced stability of
the DNCs relative to linear heptacene derivatives allows for the
study of the charge transport characteristics of these angular
heteroheptacenes. Hole mobilites of 0.055 cm2 V−1 s−1 were
obtained, and additional studies to determine the optimal
morphology and long-range ordering for increased charge-
carrier mobility are currently underway.
The atomic and electronic structures of isolated DNC
molecules were probed by DFT calculations at the B3LYP/6-
31G(d) level as implemented in Gaussian 03 (Figure S3). The
HOMOs and LUMOs of DNCs 4a and 4b were computed to
be completely delocalized over the entire dinaphthocarbazole
core, and the HOMO and LUMO energies of 4b were
calculated to be −4.7 and −1.9 eV, respectively. The calculated
HOMO level is 0.3 eV higher than the experimental value,
whereas the LUMO level is 1 eV higher than the value of −2.9
eV estimated from the experimental HOMO level and the
optical gap. The calculated gap therefore is 0.7 eV greater than
that obtained from the optical measurements; this discrepancy
is likely caused by neglect of the exciton effect, which may be
quite severe for this molecule since the HOMO and LUMO
orbitals overlap significantly. Time-dependent DFT (TD-DFT)
calculations at the B3LYP/6-31G(d) level on an N-methyl-
substituted DNC subsequently provided a theoretical band gap
of 2.38 eV, which is significantly closer to the experimental
band gap of 2.1 eV.
ASSOCIATED CONTENT
* Supporting Information
■
S
Synthetic details, CV data, molecular orbital diagrams, X-ray
structure determination, device fabrication details, crystallo-
graphic data (CIF), and complete ref 26. This material is
available free of charge via the Internet at http://pubs.acs.org.
Thermogravimetric analysis (TGA) revealed that both DNCs
exhibited good thermal stability, with 5 wt % loss occurring at
ca. 370 °C (Table S1 and Figure S4). Differential scanning
calorimetry (DSC) showed that benzyl-substituted DNC 4a
decomposed without melting, whereas dodecyl-substituted
DNC 4b melted at 235 °C.
AUTHOR INFORMATION
Corresponding Author
wudl@chem.ucsb.edu
■
Because of its improved solubility relative to the benzyl
derivative 4a, dodecyl-substituted DNC 4b was chosen for use
in solution-processed OFETs. Bottom-gate, Au bottom-contact
OFETs were fabricated on heavily doped n-type Si wafers with
a thermally grown 200 nm thick SiO2 layer. A chloroform
solution of 4b (10 mg mL−1) was then spin-cast onto the
decyltrichlorosilane (DTS)-treated SiO2 substrates. A more
detailed description of the fabrication process is provided in the
SI. Annealing the devices at temperatures ranging from 80 to
200 °C resulted in small changes in the charge-carrier mobility,
and the highest average mobility (0.055 cm2 V−1 s−1) was
achieved at an annealing temperature of 160 °C. The transfer
and output curves of the device annealed at 160 °C are shown
in Figure 3. The mobility was somewhat lower than those of
other (particularly spin-cast) seven-ring organic semiconduc-
tors,30,31 which may be attributed to the lack of intermolecular
HOMO overlap between neighboring heteroatoms, as orbital
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
T.V.P., J.A.K., B.G.S., M.M., R.S., and F.W. acknowledge
support from the ConvEne IGERT Program (NSF-DGE
0801627). Use of the Advanced Photon Source at Argonne
National Laboratory was supported by the U.S. Department of
Energy, Office of Science, Office of Basic Energy Sciences,
under Contract DE-AC02-06CH11357. We acknowledge
support from the Center for Scientific Computing at the
California NanoSystems Institute and the MRL, funded by NSF
MRSEC (DMR-1121053) and NSF CNS-0960316. J.A.K.
acknowledges partial support from the IMI Program of the
National Science Foundation under Award DMR-0843934 and
thanks Jean-Noel Chotard for guidance in using FOX.
̈
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