Correlation of mobility and molecular packing in organic transistors based on cycloalkyl naphthalene diimides

Article abstract of DOI:10.1039/c3tc30920g

A series of cycloalkyl-substituted naphthalene tetracarboxylic diimides (Cyn-NTCDIs, n = 3-8) are prepared and the transistor properties and molecular packings are systematically investigated. Cy5-Cy8-NTCDIs have similar brickwork structures, where two mo

Full text of DOI:10.1039/c3tc30920g

Journal of
Materials Chemistry C
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PAPER
Correlation of mobility and molecular packing in
organic transistors based on cycloalkyl naphthalene
diimides†
Cite this: J. Mater. Chem. C, 2013, 1,
395
5
a
a
a
a
Tomoyuki Kakinuma, Hirotaka Kojima, Minoru Ashizawa, Hidetoshi Matsumoto
ab
and Takehiko Mori*
A series of cycloalkyl-substituted naphthalene tetracarboxylic diimides (Cyn-NTCDIs, n ¼ 3–8) are prepared
and the transistor properties and molecular packings are systematically investigated. Cy5–Cy8-NTCDIs have
similar brickwork structures, where two molecules are located on the top of a molecule with large
displacement along the molecular short axis. The intermolecular overlap is maximized when the small
slipping along the molecular long axis makes the molecules nearly perpendicular to the substrate.
Consequently, the mobility decreases exactly in the same order as the interlayer d-spacings.
Received 16th May 2013
Accepted 6th July 2013
DOI: 10.1039/c3tc30920g
www.rsc.org/MaterialsC
forming the usual stacking structure, two molecules are
equivalently located on the top of a molecule. In this face-to-
Introduction
Naphthalene tetracarboxylic diimide (NTCDI, Scheme 1) is a face overlap, the molecules are slipped largely along the
1
–11
representative n-type organic semiconductor,
and in
molecular short axis, and the resulting intermolecular inter-
actions run in two diagonal directions. Molecular orbital
calculation has revealed that this diagonal interaction is
particular the cyclohexyl derivative (Cy6-NTCDI, Scheme 1)
2
ꢀ1 ꢀ1 11
shows an electron mobility as high as 6.2 cm V
s , which
is one of the highest among the n-type organic eld-effect important in the brickwork structure.20
transistors. In contrast, the n-hexyl derivative (n6-NTCDI,
Scheme 1) shows an electron mobility of only 0.1 cm V
Among the NTCDI derivatives, long alkyl substituted NTCDI
2
ꢀ1 ꢀ1 11
21,22
s
.
derivatives have been investigated extensively,
but cycloalkyl
In order to achieve high performance in organic transistors, substituted NTCDI derivatives are limited. In the present work,
control of the molecular packing and the thin-lm morphology we have prepared various cycloalkyl NTCDIs (Cyn-NTCDIs,
1
1–14
is important.
PTCDI) adopt p-stacking structures, where the slips along
the molecular long axis (longitudinal offset) and the molecular
short axis (transverse offset) determine the intermolecular bone structure by continuously changing the intermolecular
Many perylene tetracarboxylic diimides
n ¼ 3–8, Scheme 1) and investigated the crystal structures, thin-
lm properties, and transistor characteristics. We have previ-
ously investigated intermolecular interaction in the herring-
15
(

2,16
transfer.
It has been pointed out that not only the simple geometry, and demonstrated that the dihedral angle has crucial
molecular overlap but also the molecular orbital nodes inu- importance in determining the charge transport. From a
23
ence the band broadening and the resulting crystallochrom- similar systematic investigation, here we demonstrate that the
16
ism. In contrast, halogenated PTCDIs form a large variety of crucial factor that determines the transistor performance in the
molecular packings from the p-stacking to the herringbone
structure. Many halogenated NTCDIs have a p-stacking
brickwork structure is the molecular tilt angle, which is gov-
erned by the slipping distance along the molecular long axis in
the overlapped molecules.
17
structure, and the intermolecular interaction is essentially one-
18
dimensional. In contrast to these PTCDIs and NTCDIs,
2,19
cyclohexyl NTCDI has a brickwork structure (see Fig. 4),
where all molecules are parallel to each other, but instead of
a
Department of Organic and Polymeric Materials, Tokyo Institute of Technology,
O-okayama 2-12-1, Meguro-ku, Tokyo 152-8552, Japan. E-mail: mori.t.ae@m.titech.
ac.jp; Fax: +81-3-5734-2427; Tel: +81-3-5734-2427
b
ACT-C, JST, Honcho, Kawaguchi, Saitama 332-0012, Japan
†
Electronic supplementary information (ESI) available: Additional information
on synthesis, redox properties, UV-vis spectra, single crystal structures, device
fabrication, theoretical calculations, and packing of the cycloalkyl group. CCDC
9
39277–939280. For ESI and crystallographic data in CIF or other electronic
format see DOI: 10.1039/c3tc30920g
Scheme 1 Chemical structure of NTCDI derivatives.
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unoccupied molecular orbital (LUMO) levels are located around
Results and discussion
Synthesis
ꢀ
4.1 eV, which is low enough to achieve the electron
25
transport.
The Cyn-NTCDIs were prepared following the reported proce-
dure (Scheme 2) by direct condensation of naphthalene dia-
Crystal structures
24
nhydride with the appropriate amines. 1,4,5,8-Naphthalene-
tetracarboxylic acid anhydride, cycloalkylamine, and a catalytic
Single crystals of Cyn-NTCDIs were obtained by slow evapora-
tion of toluene solutions. Cy3-NTCDI showed poor solubility,
and good quality crystals were not obtained. Therefore, single-
crystal X-ray structure analyses were carried out for Cy4–Cy8-
NTCDIs. Crystallographic data are listed in Table 2. Cy4-NTCDI
takes a packing different from the other compounds, but other
Cy5–Cy8-NTCDIs show similar packing motifs.
ꢁ
amount of zinc acetate were heated in quinoline at 140–150 C
for four hours. The mixture was cooled and diluted with several
volumes of methanol. The resulting slurry was ltered, and the
collected solid was washed with methanol and dried in air. The
crude product was puried by sublimation.
The crystal structure of Cy4-NTCDI is shown in Fig. 2. Cy4-
NTCDI has one crystallographically independent molecule in a
Electrochemical property
In order to estimate the energy levels of the present semi- unit cell. All molecular planes are parallel and the stack is
conductors, cyclic voltammograms (CV) and the ultraviolet- strongly dimerized. The directions of the long axes in the dimer
visible (UV-vis) spectra were measured. The optical band gaps of molecules are, however, not parallel to each other (Fig. 2(b)). A
˚
Cyn-NTCDIs were determined from the absorption edge of the short intermolecular C–O contact of 3.14 A is observed in the
UV-vis absorption spectra (Fig. 1). The electrochemical data are dimer. The transfer integral exists only along the stacking (c)
summarized in Table 1. Although alkyl groups are different, axis, indicating one-dimensional conduction. The transfer
their electrochemical properties are very similar. The lowest integral c1 (c1 ¼ 127 meV) is much larger than the other one (c2
¼
7.7 meV) to indicate remarkable dimerization.
The crystal structure of Cy6-NTCDI has been reported
11,19
previously.
The crystal structure analyses are carried out for
Cy5, Cy7, and Cy8-NTCDIs. The packing motifs are depicted in
Fig. 3–6. Cy5–Cy8-NTCDI molecules are located on inversion
centers so that half the molecule is crystallographically inde-
pendent. Except for Cy8-NTCDI, the NTCDI planes are parallel
to each other. The molecules, however, do not make the usual
stacking structure. Another molecule located on the side of a
molecule exists on the same plane, but the next “stacked”
molecule is located in between these two molecules. Although
the usual stacking structure forms a face-to-face overlap using
nearly the whole molecular plane, the brickwork structure
makes a face-to-face overlap using half the molecular plane, and
the remaining half is used to construct the interaction with
another molecule. Since the interplanar spacings in these two
face-to-face overlaps are nearly the same (see Table 5), the two
molecules are located in the same plane in a good approxima-
tion. Cy6-NTCDI shows high symmetry, and the geometry to
these two molecules in the next layer is exactly the same (Fig. 4).
Consequently, the interaction p forms a square lattice in the
two-dimensional conducting layer. Cy5- and Cy7-NTCDIs have
non-equivalent interactions, a and c (Fig. 3 and 5), but these two
construct a similar two-dimensional network. The transfer
integrals are later discussed in connection with Table 5. Cy8-
NTCDI shows a distorted brickwork structure (Fig. 6), where the
(eV) molecular planes are not parallel to each other, but tilted with a
Scheme 2 Synthesis of NTCDI derivatives.
Fig. 1 UV-vis spectra of Cyn-NTCDIs.
2
6
Table 1 Electrochemical properties of Cyn-NTCDIs
a
b
c
d
e
Cyn
Ered (eV)
ELUMO (eV)
EHOMO (eV)
l
abs (nm)
E
g
ꢁ
dihedral angle of 16.8 .
Cy4
Cy5
Cy6
Cy7
Cy8
ꢀ1.11
ꢀ1.15
ꢀ1.18
ꢀ1.14
ꢀ1.17
ꢀ4.13
ꢀ4.09
ꢀ4.06
ꢀ4.10
ꢀ4.07
ꢀ7.33
ꢀ7.27
ꢀ7.24
ꢀ7.28
ꢀ7.25
387
390
390
390
390
3.20
3.18
3.18
3.18
3.18
Thin-lm properties
Atomic force microscopy (AFM) images of the thin lms evap-
orated on n-octyltrichlorosilane (OTS)-treated SiO /Si are shown
in Fig. 7. The thin-lm of Cy3-NTCDI consists of small domains
with typical dimensions of 0.3 ꢂ 1 mm (Fig. 7a), while the
2
a
From cyclic voltammogram using Bu
4
NPF
6
in CH
Cl
2 2
3
vs. AgNO .
b
LUMO levels are estimated by the relationship, ELUMO ¼ ꢀ5.24 eV ꢀ
2
2
7
c
d
E
red
.
Estimated by EHOMO ¼ ELUMO ꢀ E
g
.
From optical absorption
e
data in CH
Cl
2 2
.
E
g
: optical gap.
Cy8-NTCDI lm is composed of long microcrystals with typically
5
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Table 2 Crystallographic data of Cyn-NTCDIs
a
Cyn
Cy4
Cy5
Cy6
Cy7
Cy8
Chemical formula
Crystal system
Molecular weight
Space group
C
22
H
18
N
2
O
4
C
24
H
22
N
2
O
4
C
26
H
26
N
2
O
4
C
28
H
30
N
2
O
4
30 34 2 4
C H N O
Monoclinic
486.61
Monoclinic
374.40
C2/c
27.567
9.156
13.752
—
97.93
—
3437.86
8
Triclinic
402.45
P1ꢀ
Monoclinic
—
Triclinic
458.56
P1ꢀ
C2/m
8.541
6.678
18.4279
—
102.479
—
1026.19
2
1
P2 /c
˚
a (A)
6.0407
16.75
5.2404
98.72
104.53
74.8
493.223
1
1.355
6.64
6.211
17.454
5.236
90.85
100.77
94.3
555.809
1
1.370
12.09
19.385
7.905
8.96
—
97.38
—
1222.12
2
1.322
6.69
˚
b (A)
˚
c (A)
ꢁ
a ( )
ꢁ
b ( )
ꢁ
g ( )
3
˚
V (A )
Z
ꢀ3
D (g cm
)
1.447
5.09
—
6.65
R
1
(%)
a
11,19
The crystallographic data are taken from the previous report.
Fig. 4 Crystal structure of Cy6-NTCDI, (a) viewed along the molecular long axis,
19
(
b) viewed from the top of the molecular plane, and (c) the overlap modes.
Fig. 2 Crystal structure of Cy4-NTCDI, (a) viewed along the b axis, (b) overlap
mode viewed from the top of the molecular plane, and (c) the packing motif.
Fig. 5 Crystal structure of Cy7-NTCDI, (a) viewed along the molecular long axis,
b) viewed from the top of the molecular plane, and (c) the overlap modes.
(
Fig. 3 Crystal structure of Cy5-NTCDI, (a) viewed along the molecular long axis,
(
b) viewed from the top of the molecular plane, and (c) the overlap modes.
2
0
.5 ꢂ 3 mm size (Fig. 7f). For large n-cycloalkanes, the grain size
tends to increase. Cy6-NTCDI is an exception, in which at
homogeneous regions with clear stripes are obtained.
Thin lms of Cyn-NTCDIs show clear X-ray diffraction (XRD)
peaks as shown in Fig. 8, from which the d-spacings are
extracted as summarized in Table 3. The d-spacing of Cy6-
Fig. 6 Crystal structure of Cy8-NTCDI, (a) viewed along the molecular long axis,
NTCDI is in good agreement with the crystallographic c axis and (b) viewed from the top of the molecular plane, and (c) the overlap modes.
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Transistor characteristics
Aer vacuum deposition of Cy3–Cy8-NTCDIs on OTS-treated
SiO /Si substrates, transistors with top-contact Au electrodes
2
were fabricated. The transistor characteristics were measured
both under vacuum and in air. The transfer characteristics are
shown in Fig. 9. The mobility (m) and threshold voltage (VTH) are
summarized in Table 4 and Fig. 10. All compounds including
Cy3-NTCDI and Cy8-NTCDI show n-type transistor characteris-
tics, among which Cy6-NTCDI shows the highest mobility of
2
ꢀ1 ꢀ1
0
.52 cm V
some extent (0.11 cm V
1 V. Comparing the measurements under the same conditions,
the mobility of these materials makes a peak at Cy6-NTCDI
Fig. 12), whereas the VTH values are approximately the same
except for Cy7-NTCDI. It is relatively difficult to attain small VTH
s
under vacuum. In air, the mobility drops to
2
ꢀ1 ꢀ1
s
) and VTH increases from 44 to
6
(
Fig. 7 AFM images of Cyn-NTCDIs.
11
values in these materials, but we have achieved VTH values
around 40 V under vacuum. These transistors are operated even
in air, though the mobilities are reduced, and VTH increases
typically from about 40 V to 60 V.
Intermolecular interactions
In Cy5–Cy8-NTCDIs with a brickwork structure, all molecules
are parallel, and as shown in Fig. 11 the intermolecular geom-
etry is dened by the displacements along the long and short
axes within the molecular plane. We consider that a nitrogen
atom represents the position of a molecule, and the nitrogen
atom of the nearby molecule is projected onto the molecular
plane of the reference molecule as indicated by the blue lines.
Within the reference molecular plane, the direction of the long
axis is dened as the x axis, and the perpendicular direction as
the y axis. Then the positions of the P and Q points are repre-
Fig. 8 XRD profiles of Cyn-NTCDIs.
Table 3 XRD peaks and d-spacings of Cyn-NTCDIs
x y
sented by the x and y coordinates (D and D ).
ꢁ
˚
d (A)
Cyn-NTCDI
2q ( )
Cy3
Cy4
Cy5
Cy6
Cy7
Cy8
6.93
6.23
5.49
4.91
5.09
5.19
12.77
14.20
16.10
18.00
17.36
17.03
the conducting ab plane is parallel to the substrate (Fig. 4).
Accordingly, the molecular long axis of Cy6-NTCDI is perpen-
dicular to the substrate. This is because the molecular long axis
of Cy6-NTCDI coincides with the crystallographic c axis
(Fig. 4b). The d-spacings of Cy5 and Cy7-NTCDIs also agree well
with the crystallographic b axes (Table 2), and the conducting ac
planes are nearly parallel to the substrate. However, the
molecular long axes are not exactly perpendicular to the
substrate, and tilted to some extent. This is because
the molecular long axis is considerably tilted from the crystal-
lographic b axis (Fig. 3b and 5b). Cy8-NTCDI is also tilted
considerably (Fig. 6). As a consequence, Cy6-NTCDI exhibits the
largest d-spacing, whereas other materials show smaller
d-spacings (Table 3).
Fig. 9 Transfer characteristics of (a) Cy3-, (b) Cy4-, (c) Cy5-, (d) Cy6-, (e) Cy7-, and
f) Cy8-NTCDI transistors measured at V
¼ 60 V under vacuum (solid curves) and
in air (dashed curves).
(
D
5
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Table 4 Transistor properties of Cyn-NTCDIs
Table 5 Displacements, transfer integrals, and interplanar spacings in Cyn-
NTCDIs
2
ꢀ1 ꢀ1
Cyn-NTCDI
Cy3
Condition
m (cm V
s
)
VTH (V)
On/off ratio
Interplanar
distance
D ( A˚ )
Transfer
integral
(meV)
ꢀ
3
4
4
Vacuum
Air
Vacuum
Air
Vacuum
Air
Vacuum
Air
Vacuum
Air
Vacuum
Air
5.0 ꢂ 10
6.3 ꢂ 10
0.013
43
76
48
90
38
62
44
61
89
122
20
61
2 ꢂ 10
Cyn-
NTCDI
Dx
Dy
ꢀ
ꢀ
ꢀ
3
1 ꢂ 10
Direction
( A˚ )
( A˚ )
3
Cy4
5 ꢂ 10
3
4
3
2.1 ꢂ 10
1 ꢂ 10
Cy5
c
1.12
2.85
3.97
0.82
1.64
0.98
2.93
3.91
1.41
1.95
3.36
3.99
4.02
8.01
4.19
8.38
4.06
3.95
8.01
3.43
4.3
3.22
3.48
—
3.34
—
3.22
3.80
—
35
55
1.5
19
3
25
26
1.5
39
39
0
4
Cy5
0.011
5 ꢂ 10
a
p
4
a
9.1 ꢂ 10
0.52
2 ꢂ 10
6
Cy6
1 ꢂ 10
Cy6
Cy7
p
a
6
a
0.11
0.019
1 ꢂ 10
5
Cy7
3 ꢂ 10
c
ꢀ
ꢀ
3
4
4
1.9 ꢂ 10
1 ꢂ 10
a
p
3
a
Cy8
0.018
3 ꢂ 10
3
9.4 ꢂ 10
1 ꢂ 10
Cy8
p1
p2
—
—
—
a
c
7.73
a
Interactions between the molecules located on approximately the
same molecular plane. Others are slipped face-to-face interactions.
Two-dimensional map of the transfer integral
In order to estimate the overlap between two parallel molecules,
we have carried out density functional theory (DFT) calculation.
The molecular orbital (MO) of methyl NTCDI is calculated by a
28
Gaussian 09 program at the B3LYP/6-31G(d,p) level. The
LUMO spreads over the whole NTCDI core (inset in Fig. 12), but
does not have population on the atoms located on the long axis
because of the orbital symmetry.
Fig. 10 Alkyl dependence of mobility and VTH in Cyn-NTCDIs. Closed symbols
represent measurements under vacuum and open symbols are those in air.
x y
The overlap integral is calculated as a function of (D , D ) in
the two-dimensional plane (Fig. 12) for a xed interplanar
˚
spacing, D ¼ 3.4 A. Since the interplanar spacings in the actual
crystals are slightly different from this value, the individual
transfer integrals in Table 5 are somewhat different from the
values in the two-dimensional map. Positive transfer integral
regions are designated in green, and negative regions in red, but
the absolute magnitude is important to make an energy band.
The displacements in the actual Cyn-NTCDIs are plotted in the
x y
two-dimensional map. When D or D increases, the transfer
integral decreases with oscillation. The periodicity of the
Fig. 11 Intermolecular geometry with the definition of the long and short axes.
The displacements, the transfer integrals, and the inter-
planar spacings are summarized in Table 5. It is characteristic
˚
that D is about 4 A for all NTCDI face-to-face interactions, but
y
D changes largely. In particular, D of Cy6-NTCDI is compara-
x
x
tively small. The interplanar spacings D are typically 3.3–3.4 A˚
but slightly dependent on the crystals.
As shown in Table 5, transfer integrals between the mole-
cules located on approximately the same molecular plane are
less than one tenth of the face-to-face interactions. The main
interactions are mediated by the face-to-face interactions,
which make a two-dimensional network owing to the brickwork
19,20
molecular arrangement (Fig. 3–6).
When there are two
interactions t and t , the bandwidth is obtained from 4t +
a
p
a
2
4
4t
p
.
The calculated transfer integrals of 20–50 meV lead to a
Fig. 12 Two-dimensional map of the transfer integral (eV) in NTCDIs (Cyn and
n6) and PTCDIs (n4-P and n5-P).
bandwidth of 150–300 meV.
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˚
˚
11
oscillation in the D
y
direction is about 2.4 A, which is equal to D ¼ 3.20 A, but the second face-to-face interaction is so largely
˚
the width of an aromatic ring. All Cyn-NTCDI compounds are displaced (D ¼ 6.29 A) that a one-dimensional p-stacking
x
˚
located around D ¼ 4 A. This corresponds to the displacement structure is realized instead of the brickwork structure.
y
of one and half aromatic rings; the typical overlap is depicted in Butyl(n4)-PTCDI and pentyl(n5)-PTCDI have stacking structures
˚
˚
˚
˚
Fig. 11. The oscillation along D
is stripe-like in this direction (Fig. 12). At D
bond is located approximately on the top of an aromatic ring of small with the relatively large long-axis displacement D
x
is less clear because the LUMO with (D
¼ 0, the side C]C with D ¼ 3.40 A. Therefore, the short-axis displacement D
. As a
x
, D
y
) ¼ (3.07 A, 1.26 A) and (3.12 A, 1.02 A), respectively,
15
˚
x
y
is
x
the adjacent molecule (Fig. 4b). This is a ring-over-bond consequence, the interaction is large only in the stacking
geometry, which is known to maximize the intermolecular direction, and the molecular interaction is one dimensional.
overlap by minimizing the intermolecular repulsion at the same High mobility has been reported in uorocarbon (C F H –)
4
7
2
29
2
time. The calculated transfer integral has a maximum around substituted dicyano-PTCDI not only in the thin lms (0.6 cm
ꢀ
1
ꢀ1
2
ꢀ1 ꢀ1 30,31
˚
D ¼ 2.8 A, at which a C]C bond is placed on the top of another
V
s
) but also in the single crystals (6 cm V
s
).
In this
x
C]C bond. This is an eclipsed geometry. Although the calcu- compound, two molecules are located on the top of a molecule,
˚
˚
lated overlap becomes large, this is not an actually favored where one molecule has a geometry (D
x
, D
y
) ¼ (0.47 A, 4.08 A)
˚
geometry. The periodicity of D
x
corresponds to the length of an with D ¼ 3.24 A that is very close to the present brickwork
˚
aromatic ring, that is about 2.8 A.
structure. However, another molecule has a considerably dis-
˚
˚
˚
placed geometry (D
x
, D
y
) ¼ (3.65 A, 6.41 A) with D ¼ 3.29 A. This
is an intermediate between the brickwork and p-stacking
Correlation between mobility and molecular arrangement
structures, and the relatively small anisotropy (transfer inte-
As shown in Fig. 8, Cy6-NTCDI shows the smallest q value, and grals: 35 and 19 meV) is related to the reported high mobility.
Cy7- and Cy8-NTCDIs are the next, followed by Cy5-NTCDI. This There is a large degree of freedom in the stacking manner of
order is exactly the same as the order of the mobility. Cy6- NTCDI and PTCDI molecules because these molecules have
NTCDI has the largest d-spacing, because the molecular long large aromatic planes, but high mobility is attained in cyclo-
axis is perpendicular to the substrate, but other NTCDIs have alkyl-NTCDIs due to the two-dimensional brickwork structure.
smaller d-spacings, in which the direction of the molecular long
axis is tilted from the perpendicular direction. Here the ring size In-plane geometry
does not have a principal importance particularly from Cy5 to
Cy7-NTCDIs, but the tilt angle is mainly important. If the
Fig. 13 depicts two parallel molecules on the same plane moved
along D
x
by xing D
y
. A geometry with no displacement (D
x
¼
˚
molecule is perpendicular to the substrate, D
increases as the molecule is more tilted. Accordingly, Cy6-
NTCDI with the smallest D shows the highest mobility, and the
x x
is 0 A, and D
0
A˚ ) is not favorable because of the repulsion of the hydrogen
atoms (Fig. 13a). When the NTCDI molecule is moved half the
x
˚
benzene ring, namely by D ¼ 1.4 A (Fig. 13b), the geometry is
x
x
mobility seems to be a simple decreasing function of D . This
contrasts with the calculated transfer integral shown in Fig. 12,
favorable due to the reduced hydrogen–hydrogen repulsion as
well as the interaction between the carbonyl oxygen and the
˚
which makes a maximum around D
x
¼ 2.8 A. Different from the
˚
hydrogen. The actual hydrogen–oxygen distance is 2.6 A, which
stacking structure, the intermolecular overlap in the brickwork
is not recognized as a hydrogen bond, but the weak interaction
is expected. Further translation with a benzene ring distance
˚
structure is small due to the large slip of D
y
¼ 4 A. The charge
transport, expected from the mobility, seems to be determined
by the simple molecular overlap without considering the
molecular orbitals. Consequently, the mobility is a simple
decreasing function of D , which is simply related to the d-
spacing and the tilt angle.
˚
(D
x
¼ 2.8 A) is unfavorable (Fig. 13c), and translation with one
˚
and a half rings (D ¼ 4.2 A) is favorable (Fig. 13d). The
x
16
displacement (D ) of two molecules in the same plane is
x
x
x
obtained from the sum of two face-to-face D values as shown in
Table 5. The interaction a in Cy6-NTCDI is located around D
x
¼
In the stacking structure, the standing geometry has been
frequently reported to be most favorable for charge transport as
well. In the tilted arrangement, mist on the domain boundary
has been pointed out to be the origin of the reduced charge
transport. The densely packed characteristic morphology in
Cy6-NTCDI observed by AFM (Fig. 7d) may be related to this
scenario. In such a case, the difference of the transfer integral is
not the principal factor to determine the mobility. The perfectly
two-dimensional square-lattice-like arrangement is another
favorable factor to the charge transport in Cy6-NTCDI.
˚
1
.6 A. The interactions p in Cy5- and Cy7-NTCDIs are located
˚
around D
x
¼ 4 A. The actual system takes the stable geometries
˚
of either Fig. 13b or 13d. This is the reason that D
not exactly zero even in Cy6-NTCDI. Since this in-plane D
x
¼ 0.82 A is
x
determines the face-to-face D 's and consequently the molec-
x
ular tilt angles as well as the d-spacings, the small slip shown in
It is, however, notable that all Cy5–Cy8-NTCDIs basically
adopt the brickwork structure, though the molecular packing is
slightly distorted from the ideal symmetry in Cy6-NTCDI. In
general, alkyl-NTCDI and PTCDI tend to have p-stacking
structures. For example, the n-hexyl derivative (n6-NTCDI) has
˚
˚
a similar face-to-face stack with (D
x
, D
y
) ¼ (1.18 A, 3.52 A) with Fig. 13 Possible geometries of two NTCDI molecules in the same plane.
5
400 | J. Mater. Chem. C, 2013, 1, 5395–5401 This journal is ª The Royal Society of Chemistry 2013

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Paper
Journal of Materials Chemistry C
Fig. 13b is considered to be the origin of the high mobility. For 11 D. Shukla, S. F. Nelson, D. C. Freeman, M. Rajeswaran,
Cy8-NTCDI, however, the molecular planes are not exactly
W. G. Ahearn, D. M. Meyer and J. T. Carey, Chem. Mater.,
2008, 20, 7486.
12 C. D. Dimitrakopoulos and P. R. L. Malenfant, Adv. Mater.,
˚
parallel to each other, and the so obtained D ¼ 3.36 A does not
x
exactly correspond to the stable geometry.
2002, 14, 99.
13 M. Mas-Torrent and C. Rovira, Chem. Rev., 2011, 111, 4833.
Conclusions
14 S. E. Fritz, S. M. Martin, C. D. Frisbie, M. D. Ward and
M. F. Toney, J. Am. Chem. Soc., 2004, 126, 4084.
Although the stacking manner of NTCDI and PTCDI molecules
has a large degree of freedom owing to the large aromatic
planes, the molecular packing is, in many cases, a one-dimen-
sional p-stacking structure. However, the present cycloalkyl-
NTCDIs realize the two-dimensional brickwork structure, where
two molecules are equivalently located on the top of a molecule.
Among the brickwork packings, the displacement along the
1
1
1
1
1
5 E. Hadicke and F. Graser, Acta Crystallogr., Sect. C: Cryst.
Struct. Commun., 1986, 42, 189.
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x
molecular long axis (D ) is the crucial factor that determines the
transistor performance. The mobility shows a maximum at Cy6-
˚
NTCDI near D ¼ 0 A, and the order of the mobility is exactly the
x
same as that of the d-spacing in XRD. Cy6-NTCDI has the largest
2
2
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d-spacing, which is associated with the small displacement D
and the molecular long axis nearly perpendicular to the
substrate. Small displacement D is also important because
x
x
2
the perpendicular arrangement minimizes the mist between
the domain boundaries.
2
2
3 H. Kojima and T. Mori, Bull. Chem. Soc. Jpn., 2011, 84, 1049.
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The authors are grateful to Prof. Kakimoto for NMR, UV-vis and
AFM measurement and to Tokyo Institute of Technology Center
for Advanced Materials Analysis for XRD measurement.
2
2
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