Contorted tetrabenzocoronene derivatives for single crystal field effect transistors: Correlation between packing and mobility

Article abstract of DOI:10.1021/cm301190c

A series of contorted tetrabenzo[a,d,j,m]coronenes (TBCs) substituted with four fluoro-, chloro-, or methyl groups at 2,7,12,17-positions were synthesized and characterized. Except for the one with methyl substituents, which exhibits a shifted π-π stacking, the rest all show cofacial π-π stacking with small parallel displacements. One-dimensional growth along the stacking direction was observed in the single crystals for all derivatives. A systematic comparison of the crystal packing and the calculated electronic coupling/mobility with the measured field-effect mobility for single-crystal field-effect transistors shows a good correlation.

Full text of DOI:10.1021/cm301190c

Article
pubs.acs.org/cm
Contorted Tetrabenzocoronene Derivatives for Single Crystal Field
Effect Transistors: Correlation between Packing and Mobility
Someshwar Pola,† Chi-Hsien Kuo,† Wei-Tao Peng,† Md. Minarul Islam,‡ Ito Chao,*,† and Yu-Tai Tao*,†,§
‡
†Institute of Chemistry, and Taiwan International Graduate Program, Academia Sinica, Taipei 115, Taiwan
§Department of Chemistry, National Tsing Hua University, Hsinchu 30013, Taiwan
S
* Supporting Information
ABSTRACT: A series of contorted tetrabenzo[a,d,j,m]-
coronenes (TBCs) substituted with four fluoro-, chloro-, or
methyl groups at 2,7,12,17-positions were synthesized and
characterized. Except for the one with methyl substituents,
which exhibits a shifted π−π stacking, the rest all show cofacial
π−π stacking with small parallel displacements. One-dimen-
sional growth along the stacking direction was observed in the
single crystals for all derivatives. A systematic comparison of
the crystal packing and the calculated electronic coupling/
mobility with the measured field-effect mobility for single-crystal field-effect transistors shows a good correlation.
KEYWORDS: tetrabenzocoronenes, single crystal field-effect transistors
correlation and aid the material design, delineate the effect of
dielectric surface on charge transport4 and so on. Among the
SCFETs reported, rubrene yields the highest performance with
a charge carrier mobility up to 20 cm2 V−1 s−1 and exhibits the
expected anisotropy depending on the direction along which
the measurement is made.5
INTRODUCTION
■
Organic field-effect transistors have attracted much attention
lately for their potential applications in cost-effective flexible
organic electronics.1 As a driving component in many device
applications, the field-effect mobility that can be extracted from
the transistor is one of the major concerns in its development.
Important progress has been made in this field because of the
enhancement of charge carrier mobility through the develop-
ment of new organic semiconductor materials and new device
structure designs involving new dielectric materials and
electrode selection and/or modification.2 For organic semi-
conductors, many new material systems emerged besides the
most common oligoacenes in recent years. Yet a clear
structure/property correlation is difficult to reveal, partly
because the varied molecular packing associated with structural
changes and partly because the complicated morphology
normally involved in thin film forms of these materials. A
mere substitution in a molecule can readily change the packing
motif of organic molecules due to different steric requirements
and/or electrostatic interactions. The film morphology depends
not only on the molecular structure, but also the substrate
nature, deposition temperature/rate and etc. A direct
comparison of measured mobilities between devices prepared
from different materials under different conditions and/or from
different laboratories may not generate much insight to the
property correlation. In contrast, organic single-crystal field
effect transistors (SCFETs)3 offer the opportunity to explore
the intrinsic charge transport properties in organic semi-
conductors, since the single crystals in principle are free from
extrinsic effects of grain boundaries, molecular disorder and
charge traps inevitably associated with polycrystalline films.
Comparison of properties obtained with single-crystal devices
would help to reveal the intrinsic structure−property
The charge transport between organic semiconductors has
been shown to relate to the electronic coupling between
neighboring molecules, as well as the reorganization energy
involved during neutral/charged or charged/neutral state
transformation.6 While minimization of the latter is preferred,
a maximization of the former is desirable. It is further
demonstrated by theoretical calculation that if the π-planes of
two pentacene molcules are held parallel at a constant distance,
the electronic coupling is the highest if the two π-faces are
stacked perfectly face-to-face,7 whereas decreasing or vanishing
coupling results when the two molecules shift away from the
perfect cofacial arrangement, due to a smaller or nodal overlap
between the highest occupied molecular orbitals (HOMOs) or
the lowest unoccupied molecular orbitals (LUMOs). Although
perfect face-to-face packing is implied to give higher electronic
coupling, it is in general not a favored packing for aromatic
compounds due to quadrupole repulsion.8 Hence herringbone
packings are commonly observed for most unsubstituted linear
acenes and oligothiophenes. Substitution at the long edges of
pentacene perturbs the packing from herringbone fashion to
either shifted π-stacking or brick-wall type packing depending
on the size of the substituent, which in turn very much affects
the morphology of the crystal that is grown.9
Received: April 18, 2012
Revised: June 11, 2012
Published: June 11, 2012
© 2012 American Chemical Society
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derivatives were measured with a photoelectron spectrometer (AC-2,
Riken Keiki) in ambient conditions with a UV source.
Organic molecules pack based on steric interaction as well as
electrostatic interaction. It is a challenging task to predict a
priori the packing of organic molecules (molecular engineer-
ing). Contorted polyaromatics are conceived to have a high
tendency to pack face-to-face because of the potential of self-
complementary packing.10 A number of nonplanar polycyclic
aromatic hydrocarbons have been shown to exhibit cofacial
stacking in single crystals.11 The nonplanar tetraalkoxy-
substituted cata-hexabenzocoronenes have been shown to
exhibit columnar discotic liquid crystal phase, also suggesting
cofacial π-stacking.11 Fiberlike structure has been fabricated
into transistors. We demonstrated recently that the nonplanar
pyreno[4,5-a]coronene molecules pack exactly face-on. Tran-
sistors based on its single crystal exhibit a high field-effect
mobility of 0.89 cm2/(V s).12 The 1,2,3,4,7,8,9,10-tetrabenzo-
coronene (TBC) is another class of contorted molecules.13 The
2,7,12,17-tetraoctyl-substituted TBC showed strong long-range
ordered π-stacking in solution state by forming nanofibers. A
high hole mobility of 0.61 cm2/(V s) was measured by space-
charge-limited-current method. Nevertheless, no evidence of
cofacial π−π stacking was found in this compound or the
unsubstituted parent compound.13
Top-contact, top-gate SCFET was prepared by placing the single
crystal on a glass substrate, which was modified with a monolayer of n-
octadecyltrichlorosilane(OTS). Painted colloidal graphite was used as
the source and drain. A thin film of parylene (∼2−5 μm thick) was
deposited on top of the crystal in a homemade reactor as insulating
dielectric. Finally, colloidal graphite was painted on top of the parylene
film as the gate electrode. The channel length and width of devices
varied depending on the length and dimensions of crystals chosen.
These parameters, as well as the parylene thickness, were determined
for each individual device. The electrical characteristics of the devices
were measured in ambient in a dark chamber using a computer-
controlled Agilent 4156C Semiconductor Parameter Analyzer. The
field-effect mobility of the OFET device was calculated from currents
in the saturation region according to eq 115
Ids,sat = (WCi/2L)μ (V − V )2
gs
th
(1)
sat
where W and L are the channel width and length respectively, and Ci is
the capacitance per unit area of the parylene dielectric, Vgs is the gate
voltage and Vth is the threshold voltage.
Computational procedure for calculating charge carrier mobility has
been elaborated previously.16 Briefly, the calculations of internal
reorganization energy (λ) and relative energy of conformations of 1a−
d were performed with Gaussian 09 program17 at the B3LYP/6-
31G(d,p) level of theory.7,18 The transfer integral (t) between
neighboring dimer pairs in the crystal structure was calculated with
ADF19 at the PW91PW91/DZP level of theory.20 With λ and t,
Marcus theory for self-exchange electron transfer was used to obtain
the carrier transport rate at 300 K. With the rate and the
intermolecular centroid-to-centroid distance obtained from the crystal
structure, carrier diffusion coefficient was deduced and used in Einstein
relation to derive carrier mobility. Although the Marcus theory has
certain limitations such as the neglect of nuclear tunneling effect and
the nonlocal electron−phonon couplings, it has been quite useful for
the purposes of qualitative analysis and molecular design.21
In this paper, we report the synthesis and structure
characterization of a series of 2,7,12,17-tetrasubstituted TBCs
(1a−d, with R = H, Cl, F, CH3, Figure 1). Single-crystal
Figure 1. Molecular structure of 2,7,12,17-tetra-substituted tetrabenzo-
[a,d,j,m]coronene(1).
Scheme 1. Synthesis of Contorted Tetrabenzocoronenes
structures were grown and determined for all four derivatives.
Cofacial stacking with small displacements was found for all
derivatives, except for 1d with R=CH3, which shows a shifted π-
stacking and the molecular planes are not exactly parallel to
each other. Needlelike or ribbon type crystals along the π-
stacking direction were obtained. Charge conduction along this
direction is the major and probably the only direction in a
single-crystal-based FET device. A systematic comparison of
charge transport properties among various substituted deriva-
tives may be elucidating. Theoretical hole mobility along the π-
stacking direction was calculated based on the single crystal
structures. Single-crystal field-effect transistors were fabricated.
The field-effect mobility was measured and its correlation with
the theoretical results is discussed.
EXPERIMENTAL AND COMPUTATIONAL DETAILS
■
RESULTS AND DISCUSSION
The series of derivatives of 1 were synthesized in the laboratory (the
synthetic details are provided in the Supporting Information).
Compounds 1a, 1b, and 1c, which are sparingly soluble in common
organic solvents, were characterized by mass, elemental analyses and
X-ray crystallography. Compound 1d was additionally characterized by
NMR. Single crystals were grown in a temperature-gradient copper
tube by vapor-phase transfer method with Argon as the carrier gas.14
The X-ray diffraction was carried out on a Bruker X8APEX X-ray
diffractometer with Mo Kα radiation (λ = 0.71073 Å) and the
structure was solved by SHELX 97 program. The HOMO levels of the
■
Synthesis Scheme 1 Shows the Synthetic Route to
Compound 1. Synthesis of contorted coronene derivatives,
such as hexabenzocoronenes (HBCs), has been demonstrated
recently,22,23 starting from pentacene quinone derivatives. For
the four TBC derivatives studied here, the parent compound 1a
was first reported in 197224 with little characterization. Recently
Wu’s group synthesized compound 1a in eight steps starting
from benzophenone.13 Our procedure started from anthraqui-
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none. After Corey-Fuchs25 reaction using triphenylphosphine
and carbon tetrabromide to afford compound 2, the Suzuki-
Miyaura26 reaction between 2 and substituted boronic acid led
to the key intermediate bisolefin 3. Oxidative dehydrogenation
gave a mixture of half-cyclized products which, without
isolation, was converted to fully cyclized products 1a, 1b, and
1c respectively by Scholl cyclization27 using FeCl3 as the
catalyst. However, it is noted that with 1d, the last step yielded
a mixture of chlorinated side products. It was found that with
Cu(OTf)2 as the catalyst, the fully cyclized product was
obtained in good yield. All products were purified by vacuum-
sublimation.
Characterization. The compounds appear yellow (1a and
1d) to orange yellow (1b and 1c). Similar UV−vis absorption
spectra in dichlorobenzene exhibiting multiple λmax were
obtained in the 300−500 nm region (Figure 2). The p-band
Figure 3. Thermal gravimetric analyses of 1a−d.
Crystal structure analyses. All four compounds give
needlelike crystals. X-ray analyses revealed a contorted
geometry as expected. However, the contorted molecules
have two stereoisomers. One has the two benzenoids in the
right side pointing in the same direction and opposite to the
two in the left side (Figure 4a). That is, the molecule possesses
Figure 2. UV−vis spectra for 1.
(π→π* transistion) in long wavelength region for 1a occurs at
426 nm. A small substituent effect on the absorption maximum
can be seen, with chloro and methyl substitution giving a red-
shift (∼6−7 nm) and fluoro substitution giving a blue-shift (3
nm) relative to that of the parent compound. Very weak α-band
(n→π*) was discernible for all four compounds between 457 to
465 nm. Among the derivatives, 1a and 1d exhibit ambient
stability in that their solution UV−vis spectra did not change
and no discoloration occurred after 2 days, whereas slow
degradation was observed with 1b and 1c, whose UV
absorption decayed to some uncharacterized species in 2 days
(see the Supporting Information, Figure S1). The degradation
process was nevertheless much slower compare to pentacene,
which degraded completely within an hour under similar
conditions (see the Suppotying Information, Figure S2).
Thermal gravimetric analyses showed the series of compounds
have excellent thermal stability with decomposition temper-
ature (5−8% weight loss) at ∼460 °C and above in nitrogen
atmosphere (see Figure 3).
The electrochemical measurements of 1 were performed in a
1 × 10−3 M dichloroethane solution containing 0.1 M of tetra-
n-butylammoniumhexafluorophosphate (TBAPF6) as electro-
lyte. While oxidation peaks were observed (see the Supporting
Information, Figure S3), the onset oxidation potentials were
less well-defined, probably because of the low solubility
(incompletely dissolved). The HOMO energy levels for 1a−d
were nevertheless measured by photoelectron spectrometer
(AC2) on the solid samples to be 5.41, 5.85, 5.88, and 5.05 eV,
respectively. Thus the electron-withdrawing groups lower the
HOMO level and electron-donating raises the HOMO level.
Figure 4. (a) The C2 h structure observed for 1a−1c, (b) The D2
structure of 1d, (c) crystal packing structure of 1b viewing down the
long molecular axis (b axis) and from the top of the molecules (a axis)
respectively, (d) crystal packing of 1d viewing from the short axis (a
axis) and from the top of the molecule(b axis) respectively.
a C2h symmetry and is achiral. Compounds 1a−c adopt this
geometry. Compound 1d, on the other hand, has the two
benzenoids in the right side pointing in the opposite direction
so that the molecule has a D2 symmetry and is chiral (Figure
4b). All four compounds show monoclinic packing motif in
crystal packing. Compounds 1a, 1b and 1c pack nearly face-to-
face with π−π stacking distances of 3.61−3.66 Å and centroid-
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to-centroid distances of 3.72−3.77 Å (Figure 4c and Table 1).
The displacements along the short molecular axis are rather
Table 1. π−π Stacking Distances (dπ−π), Centroid-to-
Centroid Distances (dc−c), Short Molecular Axis Shift (ds),
Long Molecular Axis Shift (dl) in the Crystal Structures and
the Calculated Reorganization Energies (λ+), Electronic
Couplings (t+), Mobilities (μ+) for Hole Transport
dπ−π
(Å)
dc‑c
(Å)
λ+
t+
μ+
compd
ds(Å) dl (Å) (meV) (meV) (cm2/(V s))
1a
1b
1c
1d
3.66
3.63
3.61
3.77
3.75
3.72
4.39
0.87
0.76
0.84
0.01
0.55
0.34
132
154
169
136
41
51
41
20
0.58
0.68
0.36
0.18
Figure 5. Diagrams of π−π stacking for contorted TBC derivatives
with (a) C2h and (b) D2 molecular symmetry. These diagrams are
seeing molecules along the long molecular axis. The middle line means
the molecular mean plane. The tilted solid and dotted lines represent
the benzo groups in the front and in the back, respectively.
similar (0.76−0.87 Å). For the displacements along the long
molecular axis (dl) of unsubstituted 1a and halogenated 1b/1c,
the values are 0.01 and 0.55/0.34 Å, respectively. The
methylated 1d, on the other hand, packs somewhat
unparalleled and shifts more prominently along the long
molecular axis (Figure 4d). The centroid-to-centroid distance
increases to 4.39 Å. It is further noticed that for 1a and 1d, the
unit cell contains four molecules at the four corners of a square
(Figure 4d), whereas for 1b and 1c, the unit cell contains five
molecules, with a molecule at the center of the square (Figure
4c). Thus the introduction of chlorine or fluorine atoms had
little effect on the packing motif between layers, except making
small shifts along the long molecular axis. Yet the presence of
chlorine or fluorine atoms does change the packing arrange-
ment within a layer. In a five-molecule-per-cell arrangement,
the halogen−halogen interactions or the C−X/C−X dipole
repulsions present in a four-molecule-per-cell packing can be
alleviated. The cell parameters are listed in Table 2. The
directions of the needlelike crystals are indexed to be along the
π−π stacking direction.
alleviates all the steric interactions. When substituted with
larger atoms such as fluorine or chlorine atoms, shifting slightly
along the long molecular axis is also needed (dl = 0.34 and 0.55
Å, respectively). Thus with a slightly less favored conformation,
these molecules can achieve near cofacial packing. However,
methyl group is probably too big for 1d to pack effectively in a
cofacial manner, so it adopts different packing with the more
stable conformation. It is noted that a D2 structure cannot shift
along the short axis as depicted in Figure 5a. In D2 structure,
the benzo groups on each side are pointing to different
directions, so that moving along a direction alleviates the
congestion of a pair of benzo groups, and yet aggravates the
steric interaction of another pair (Figure 5b).
SCFET Device Property and Theoretical Calculation.
Crystallites of 1a−d were carefully examined under microscope
for their integrity and chosen for device fabrication. The single
crystal field-effect transistors were prepared in otherwise the
same fashion for all four compounds, which exhibited typical p-
type behavior. Three to twelve samples were prepared and
measured. The measured device characteristics are shown in
Table 3. The averaged carrier mobilities are of similar
Table 2. Cell Parameters of Single Crystals of 1
1a
1b
3.75
1c
3.72
1d
a (Å)
16.25
3.77
18.72
90
24.65
7.33
29.87
90
b (Å)
14.39
23.72
90
11.31
27.72
90
Table 3. Device Characteristics of Single-Crystal Field-Effect
Transistors Based on 1
c (Å)
α (deg)
thresh.
β (deg)
106.55
90
91.06
90
92.67
90
101.70
90
mobility μ
averaged mobility
voltage Vth
compd (cm2 V−1 s−1
)
μavg (cm2 V−1 s−1
)
on/off ratio
(V)
γ (deg)
cell density (g/cm3)
1a
1b
1c
1d
0.271−0.501
0.401 0.118
1 × 102 to 1 −27 to −36
1.35
1.42
1.32
1.40
C2/c
× 105
point group
P21/n
P21/c
P21/c
0.385−0.702
0.045−0.165
0.123−0.188
0.564 0.162
0.100 0.0399
0.155 0.032
1 × 103 to 1 −15 to −19
× 105
1 × 103 to 1 −30 to −41
Theoretical calculation shows that the C2h and D2
conformations differ by ∼1 kcal in energy, with D2
conformation as the more stable one for all four derivatives.
The interchange of the two involves a barrier of 9−10 kcal,
which suggests the interchange may proceed even at room
temperature. The reason that 1a−1c and 1d adopt different
molecular symmetry may be understood from the packing.
With C2h structure (1a−1c), a slightly shifted cofacial packing
can be obtained. As shown in Figure 5a, the upward-pointing
benzo groups of the bottom C2h molecule are having close
contacts with the downward-pointing benzo groups of the top
molecule. This congestion problem can be eased if the bottom
molecule shifts to the right (i.e., shifts along the short molecular
axis). For the unsubstituted 1a, shifting along the short axis
× 105
1 × 102 to 1 −1 to −29
× 104
magnitude, but 1a and 1b (0.401−0.564 cm2 V−1 s−1) are
noticeably better than that of 1c and 1d (0.100−0.155 cm2 V−1
s−1). The typical output and transfer characteristics for
compound 1a are shown in Figure 6.
Theoretical calculations have brought insights to exper-
imental SCFET results and been used to screen potential
organic compounds.21a,28 Similar to Sokolov’s recent effort, in
this study we estimate carrier mobilities on the basis of Marcus
theory of charge transfer rate. The calculated mobilities are all
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prohibited by the size of substituent. Compounds 1a−c exhibit
face-to-face π−π stacking and grow into needle-like single-
crystals. A shifted π-stacking in 1d also leads to ribbonlike
single crystal. The small structural perturbation resulted from
the substitution lead to variation in the transfer integral as well
as reorganization energy and thus the mobility, which seems to
be in agreement with the experimental observation. Among
these, the SCFET with chlorinated 1b as channel material and
parylene as gate insulator gave hole mobility as high as 0.7 cm2
V−1 s−1.
Figure 6. (a) Output and (b) transfer characteristics of 1b SCFETs.
in the same order of magnitude (Table 1). As in the
experimental observation, 1a and 1b have mobilities larger
than 1c and 1d. For compounds with the same packing motif
(1a−1c), the mobility order of 1b > 1a > 1c is found both
experimentally and theoretically. Comparing the calculated
reorganization energies, transfer integrals and mobilities to the
device performances, one can rationalize the observed trend of
mobilities for these compounds and achieve some insight of the
structure−property relationship that may be beneficial for
future design of high performance organic semiconductors.
First, the smaller mobility of 1d can be readily attributed to the
large shift stack in the crystal structure and hence the smallest
transfer integral of these compounds in the charge transport
direction. Second, compounds 1a−1c possess the similar value
of transfer integrals, but 1c has the largest reorganization
energy which suppresses the efficient charge transfer. Chao et
al. had demonstrated that fluorination and chlorination would
have an impact on the reorganization energy, which originated
from the antibonding nature of the C-X bond in the frontier
orbital.29 They also demonstrated that chlorination has a
smaller effect on the reorganization energy than fluorination.27b
This statement holds in this study. Third, although the
reorganization energy of 1b is 22 meV larger than 1a, the
slightly higher mobility of 1b can be rationalized by the larger
transfer integral due to the smaller short axis shift (0.76 vs 0.87
Å). By examining the shape of HOMO, one will realize that the
shift along the short axis will have a more significant impact on
the transfer integral than the long axis shift for TBC derivatives
(Figure 7), as the wave function has more nodal planes along
ASSOCIATED CONTENT
■
S
* Supporting Information
Details of synthesis procedure, UV−vis spectra, and CV data
are provided.This material is available free of charge via the
Internet at http://pubs.acs.org.
AUTHOR INFORMATION
■
Corresponding Author
*E-mail: ytt@chem.sinica.edu.tw; ichao@chem.sinica.edu.tw.
Author Contributions
S.P. did materials syntheses and characterizations, C.H.K. and
M.M.I. grew single crystals and fabricated devices, W.T.P. did
the theoretical calculation, Y.T.T. and I.C. cowrote the paper.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
The authors thank the National Science Council, Taiwan,
Republic of China (Grant NSC-98-2119-M-001-007-MY3 and
NSC-100-2113-M-001-005-MY3) and Academia Sinica for the
financial support. They also thank the National Center for
High-Performance Computing and the Computing Center of
Academia Sinica for providing computational resources.
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