Contorted tetrabenzoacenes of varied conjugation: Charge transport study with single-crystal field-effect transistors

Article abstract of DOI:10.1039/c7tc02254a

A series of contorted and polyfused aromatic tetrabenzoacene derivatives differing in conjugation length were synthesized and characterized. X-ray diffraction revealed the contorted molecular shape, as well as the packing arrangement of these molecules. Thus unsubstituted tetrabenzoacenes showed a shifted or perfect face-to-face π-stacking depending on their conjugation length. The single crystals of these tetrabenzoacenes were used as conducting channels in fabricating field-effect transistors (SCFETs). Tetrabenzotetracene (TBT) exhibited the highest measured mobility, approaching 0.81 cm² V^-1 s^-1 (average 0.64 cm² V^-1 s^-1) among these molecules. In contrast, theoretical calculation showed that the tetrabenzooctacene (TBO) crystal has large-area, face-to-face π-packing, with the highest intermolecular coupling in the series. The lower charge mobility (average 0.32 cm² V^-1 s^-1, highest 0.55 cm² V^-1 s^-1) observed was rationalized as a result of possible involvement of delocalized polaron formation due to comparable electronic coupling and reorganization energy in TBO, as supported by the Monte Carlo simulation with this delocalized effect taken into account.

Full text of DOI:10.1039/c7tc02254a

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PAPER
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Nguyên T. K. Thanh, Xiaodi Su et al.
Fine-tuning of gold nanorod dimensions and plasmonic properties using
the Hofmeister effects
rsc.li/materials-c

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DOI: 10.1039/C7TC02254A
Contorted Tetrabenzoacenes of Varied Conjugation: Charge Transport
Study with Single-Crystal Field-Effect Transistors
Ding-Chi Huang,† Chi-Hsien Kuo,‡ Man-Tzu Ho,§ Bo-Chao Lin,‡ Wei-Tao Peng,∥ Ito Chao,‡ and
Chao-Ping Hsu,*,‡ Yu-Tai Tao*,†, ‡
† Department of Chemistry, National Tsing-Hua University, Hsin-chu, Taiwan
‡ Institute of Chemistry, Academia Sinica, Taipei, Taiwan
§ Department of Chemistry, National Central University, Chung-Li, Taiwan
∥Department of Chemistry, Michigan State University, East Lansing, MI, USA 48824
*Email: ytt@gate.sinica.edu.tw
KEYWORDS: tetrabenzoacenes, single crystal field-effect transistor
ABSTRACT: A series of contorted and polyfused aromatic tetrabenzoacenes derivatives differing in
conjugation length were synthesized and characterized. X-ray diffraction revealed the contorted molecu-
lar shape, as well as the packing arrangement of these molecules. Thus unsubstituted tetrabenzoacenes
showed a shifted or perfect face-to-face π-stacking depending on their conjugation length. The single
crystals of these tetrabenzoacenes were used as conducting channels in fabricating field-effect transis-
tors (SCFETs). Tetrabenzotetracene (TBT) exhibited the highest measured mobility, approaching 0.81
cm2V-1s-1 (average 0.64 cm2V-1s-1) among these molecules. In contrast, theoretical calculation showed
that the tetrabenzooctacene (TBO) crystal has a large-area, face-to-face π-packing, with the highest in-
termolecular coupling in the series. The lower charge mobility (average 0.32 cm2V-1s-1, highest 0.55
cm2V-1s-1) observed was rationalized as a result of possible involvement of delocalized polaron for-
mation due to comparable electronic coupling and reorganization energy in TBO, as supported by the
Monte Carlo simulation with such delocalized effect taken into account.
INTRODUCTION
Incorporation of organic field-effect transistors (OFETs) into organic electronic devices is being con-
sidered as the future trend in the realization of flexible electronics, like organic light-emitting diodes,
memory devices, and sensors and etc.1 An essential property for organic materials to be used in OFET
as conducting channel is a high charge transport ability between neighbouring molecules. It is generally
accepted that the rate of intermolecular charge transport relates to the molecular reorganization energy
as well as the intermolecular transfer integral,2 so that a small reorganization energy and large electronic
coupling will benefit the charge mobility. Crystal engineering of organic semiconductors to improve the
intermolecular π-π overlap has been an ensuing effort.3 One of the strategies is the expansion of a con-
jugate system to increase the possible π-π overlap. For example, the extension of fused benzene rings in
linear acene was found to increase the coupling and mobility (Figure 1a)4, so is the extension of
[1]benzothineno[3,2-b]benzothiophene derivatives (Figure 1b).5 Another idea is to play with substitu-
ents on the conjugate core framework to induce a face-to-face packing behaviour, which is considered to
produce effective electronic coupling.6 For example, chlorine atoms substituted on linear acenes resulted
in face-to-face packing.7 In the case of rubrene, the four bulky phenyl rings substituted on the periphery
of tetracene backbone change the herringbone packing to co-facial herringbone packing and produced a
high electronic coupling.8 The substitution of triisopropylsilylethynyl (TIPS) groups on the 6- and 13-
positions of the pentacene skeleton induces a brick-wall-type stacking.9 The similar idea is applied to
other polycyclic aromatic systems with the choice of fitting trialkylsilylethynyl group to modulate the
crystal packing.10

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Molecular geometry is yet another factor that influences the electronic coupling. In particular, the
contorted polyaromatics have a high tendency to pack face-to-face due to self-complementarity in
shape.11 Thus contorted hexabenzocoronene (cHBC, Figure 1c) with the six annulated benzene rings
twisted away from the central coronene core shows significant π-π stacking after post-deposition proꢀ
cessing.12 The p-type OFET mobility based on alkoxy-substituted cHBC thin films was about 0.02 cm2
V-1 s-1.13 Contorted octabenzocircum biphenyl (cOBCB) is a a larger π-conjugated extension of cHBC,
which gave an OFET mobility about 0.002 cm2 V-1 s-1.14 The 1, 2, 3, 4, 7, 8, 9, 10-tetrabenzocoronene
(TBC) is a smaller yet contorted π-conjugated system whose single crystal gave co-facial or shifted π-π
stacking, depending on the size of the substituent groups.15 A high hole-mobility of 0.7 cm2 V-1 s-1 was
obtained for symmetric tetra-chlorinated TBC and 1.19 cm2 V-1 s-1 was achieved for asymmetric hexa-
fluorinated TBC based on their single-crystal field-effect transistors (SCFETs). The tetraoctyl-
substituted TBC exhibited self-assembling behavior in solution to give fibrous microstructures with a
space-charge limited charge mobility of 0.61 cm2 V-1 s-1.16 Pyreno[4,5-a]coronene (PRC), is yet another
non-planar molecule giving a perfect on-top co-facial π-stacking in its needle-like crystal. The theoretiꢀ
cal calculation suggests a large electronic coupling of 160 meV for the co-facial dimer.17
Figure 1. Structures of linear (a) linear acenes; (b)thienoacenes; contorted (c)TBC, cHBC and cOBCB;
(d) PRC, TBN and Tetrabenzoacenes.
The crystal packing of TBN shows a partial face-to-face packing with an electronic coupling of 38.4
meV for dimer along the b-axis.18 Although TBN has the same twisted conformation and similar crystal
packing with PRC mentioned above, the electronic coupling, which relates to the extent of π-π overlap,
is significantly low, and this indicates a possible correlation in the size of π-conjugated skeletons with
the extent of π-π overlap for contorted polyaromatics. The mobility improvement by extending π-
aromatic skeleton of linear acenes (ex. tetracene, pentacene, hexacene4) or thienoacenes (BTBT,
DNTT, DATT)5 was observed. But there is little systematic study on contorted polyaromatics as a funcꢀ
tion of conjugation length.19
In this work, we studied a series of contorted yet linear tetrabenzoacene derivatives containing tetraꢀ
benzonaphthalene (TBN) unit as the central core to introduce a twist in the molecule. These tetrabenzoꢀ
acenes differ in their π-conjugation lengths, from naphthalene (TBN) to tetracene (TBT), hexacene
(TBH), and octacene (TBO). Single crystal structures of these derivatives were determined and anaꢀ
lysed. The physical properties, the performance of SCFETs were measured and theoretical calculation
were conducted in an attempt to understand the structure/property correlation in this series of derivatives
and their potential as transistor materials.
2

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EXPERIMENTAL METHODS
All the tetrabenzoacene derivatives were synthesized in the laboratory and fully characterized by
NMR, Mass and X-ray crystallography (the synthetic details are provided in Supporting Information,
SI). UV-Visible spectra were recorded by using a Jasco V-530 double beam spectrophotometer with the
sample solution prepared in dichloromethane (~10-5 M). For oxidation potential measurements, the sam-
ples were dissolved in anhydrous and degassed dichloromethane (~10-3 M) containing 0.1 M tetrabu-
tylammonium hexafluorophosphate (TBAPF6) as the supporting electrolyte. The cyclic voltammograms
were recorded on a CHI 620 analyzer with glassy carbon as the working electrode, platinum wire as the
counter electrode and Ag/Ag+ as the reference electrode respectively. The HOMO values for the solid
samples were measured on an AC-2 photoelectron spectrometer (Riken Keiki) under ambient conditions.
Single crystals were grown by vapor phase transfer method with argon or helium as the carrier gas.20
The X-ray diffraction was measured on a Bruker X8APEX or Kappa APEXII X-ray diffractometer with
Mo Kα radiation (λ = 0.71073 Å) and the structure was solved by SHELX 97 program.
The fabrication procedure of top-contact, top-gate SCFET device was reported earlier.15 The selected
single crystal was placed on an n-octadecyltrichlorosilane(ODTS)-modified glass slide. Colloidal graph-
ite was used as the source, drain and gate electrode materials. Parylene-N thin film (~2 - 4 µm thick)
was used as the insulating dielectric layer. The channel length and width of the devices depended on the
crystals chosen and were determined for each crystal. The electrical characteristics of the devices were
measured in ambient and in the dark by an Agilent 4156C Semiconductor Parameter Analyzer. The
field-effect mobility of the OFET devices in the saturation region was calculated by using equation 1:21
ISD,sat= (WCi/2L) ꢁsat(VG-Vth)2 (1)
where W is the channel width, L is the channel length, and Ci is the capacitance per unit area of the
parylene-N gate dielectric, VG and Vth are the gate-source and threshold voltage respectively.
The quantum ab initio calculations were performed by a developmental version of Q-Chem program.22
The reorganization energy was calculated following the commonly used scheme in the literature,23 at the
level of B3LYP/6-31G*.24 The transfer integral (t) between neighboring dimer pair was calculated with
the directing coupling method by the LC-ωPBE scheme (ω = 0.2)25 and the Dunning’s double-ζ basis
sets.26 The dimer configurations were obtained from the crystal structure, with the hydrogen coordinates
optimized at the level of B3LYP/6-31G*. To see the thermal fluctuation of the electronic coupling, the
configurations from molecular dynamics (MD) simulations were also sampled in the canonical ensem-
ble (NVT) at 298 K. The MD simulation was performed using the Tinker program with the MM3 force
field.27 Charge-transfer rates for pairs of neighboring molecules were calculated based on Marcus theo-
ry.28 The carrier transport rate at 300 K was obtained from three different approaches: (I) the Einstein
relationship29 where the carrier diffusion coefficient was estimated by the Marcus’ charge transfer rates,
together with the intermolecular distances analyzed from the crystal structure, (II) a Monte Carlo simu-
lation30 with disorder in charge-transfer couplings derived from a molecular dynamics sampling, and
(III) similar Monte Carlo simulation but with delocalized states for sites with large coupling values, sim-
ilar to that described in reference.31
RESULTS and DISCUSSION
Synthesis.
The syntheses of various tetrabenzoacenes are shown in Scheme 1 and the key step in each is the
formation of tetrabenzonaphthalene (TBN) moiety as the twisted core. The fluorenone-type precursors
3

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(1, 3, 5, 7) of various conjugation lengths were commercially available or prepared inDtOhI:e10l.a10b3o9/rCa7tToCr0y22a54nAd
then underwent thesuperacid-catalyzed dehydration/cyclization to give the desired products through a
pinacol rearrangement, as reported by Klumpp and Olah.32
Scheme 1 Synthetic routes of various tetrabenzoacenesa
aReagent and condition: (a) Zn/ZnCl2, THF/H2O; (b) triflic acid, benzene; (c) NaOH, Ba(OH)2, then
400°C; (d) NaOH; 20% alcoholic potash, then methanesulfonic acid, P2O5; (e) n-BuLi, oxygen, -78°C.
The fluorenone-type compound 3 was prepared from vacuum pyrolysis of the barium salt of 4,5-
phenanthrene dicarboxylic acid33 (2) at 400°C through decarboxylation and cyclization, in 35 % yield.34
After hydrolysis of methoxycarbonyl-substituted triphenylene 4, cyclodehydration with Eaton's reagent
led to compound 5 in 95% yield. The longest analogue 7 was prepared from compound 6 via abstraction
of the bridge hydrogen at 9-position by n- butyllithium, followed by oxidation with oxygen.35 The flu-
orenone derivatives was transformed to corresponding diols by pinacol coupling in the presence of
zinc/zinc2+.36 Without further purification, these crude pinacol derivatives were dissolved in anhydrous
benzene with triflic acid to yield tetrabenzoacenes smoothly in acceptable or moderate yields (45-70%
for two steps). All steps were carried out in gram‐scale. The final products were purified by column
chromatography and vacuum-sublimation before crystal growth by vapor-phase transfer method and
physical measurement.
Characterization
The TBN appears white, yet TBT, TBH and TBO appear orange. Figure 2a shows the UV-Vis absorp-
tion spectra of these compounds, except for TBO due to its poor solubility in common solvents. The
UV-spectra of TBN and TBT conform to that reported in the literature.37 The UV-spectra of TBT and
4

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TBH show a significant red-shift relative to that of TBN, indicating the effecDtOoI: f10.1i0n3c9r/Ce7aTsCi0n2g254πA -
conjugation in TBT and TBH than in TBN. The electrochemical property of these compounds was
studied by cyclic voltammetry (CV) experiment in dichloromethane solution. One oxidation peak was
observed for each of these compounds, as illustrated in Figure 2b. That for TBO was not determined
due to solubility problem. The solid state HOMO levels of these compounds were also measured by AC-
2 photoelectron spectrometer. The HOMO values from both methods and the energy gaps are summaꢀ
rized in Table 1. The calculated frontier orbitals are shown in Figure 3. The increased conjugation length
from TBN to TBT and TBH raises the HOMO level from 5.7 to 5.5 eV (by CV) as expected. It is interꢀ
esting to note that the energy gap and HOMO values of TBT and TBH were the same, and the calculatꢀ
ed HOMO orbitals of TBT, TBH and TBO were also similar. The experiments in dilute solution and
calculated HOMO orbitals based on single molecule implied the analogous π-delocalized extensions of
these annulated π-skeletons, that is, the HOMO orbitals do not extend to the extra rings. However, the
HOMO values obtained from AC-2 for TBT, TBH and TBO were 5.5, 5.2 and 5.1 respectively, deꢀ
creasing with longer extension. This different HOMO values (by AC-2) between TBT, TBH and TBO
may imply the packing interaction can have additional influence on HOMO level in the aggregated state
compare to isolated molecules.
Figure 2. (a) Normalized UV absorption spectra; (b) plots of cyclic voltammetry of tetrabenzoacenes in
dry CH2Cl2 with a scan rate at 0.1 V/s; (c) Thermal gravimetric analyses of tetrabenzoacenes.
5

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Figure 3. Results of frontier molecular orbital and the calculated energy of TBN, TBT, TBH, and
TBO, respectively. The calculating level is B3LYP/6-31G*, and the isovalue is 0.02 au.
Table 1. Energy levels of tetrebenzoacene derivatives
Item
Egap (eV)a
TBN
TBT
TBH
TBO
3.2
2.9
2.9
--
E HOMO (eV) by CV
E HOMO ( 0.1eV) by AC-2
Td (°C)
5.7
5.7
327
5.5
5.5
369
5.5
5.2
486
--
5.1
491
aThe gap was estimated from the onset of their UV absorption (Egap= 1241/λonset).
Thermal gravimetric analysis was performed in a nitrogen atmosphere and the decomposition temper-
atures (at 5% weight losses) are illustrated in Figure 2(c) and Table 1. The result showed the decomposi-
tion temperature (Td) correlates with the molecular weight of the aromatic system in the situation where
the Td increases with increasing conjugation. Below their decomposition temperature, these compounds
survived well under vacuum sublimation and during PVT crystal growth condition.
Crystal structure analyses and theoretical results
The compounds TBN, TBH, TBO give needle-like or fiber-like crystals, whereas TBT exhibits long
and flaky crystals (see ESI). The structures and crystal packing were clearly revealed by X- ray analyses
and are shown in Figure 4-7. Selected crystal data are summarized in Table 2. All molecules have very
similar twisted geometries. A high torsion angle between 36° and 43° was observed, depending on the
conjugation length. According to the optimized structures, the two torsion angles in the molecule are the
same. Yet in reality, the two torsion angles in the fjord regions are slightly different, which may be due
to the shifted packing of neighboring molecules in the crystalline state. The degree of twist of TBT is
the smallest in this series, and that of TBN, TBH and TBO are all larger than 40°.
These contorted acenes are chiral, with P and M enantiomers both present in the crystal, like other
twisted molecules.11(a)-(c), 15(e), 18 No achiral anti-folded conformer was observed presumably due to high-
er strain energy. Based on the enantiomer stacking behavior in these tetrabenzoacenes, two kinds of
packing models could be categorized. For TBN, TBT and TBO, their stackings are similar in that all
crystallized in a monoclinic system, such that the same enantiomer (P-P or M-M) stacks in a column
along the b-axis, forming π-π contacts with different overlap ratio, as seen in Fig. 4, 5 and 7. TBO has a
perfect co-facial packing like previously reported PRC,17 and the π-π overlap in TBT packing is larger
than that of TBN. Nevertheless, the crystal structure of TBH is orthorhombic and has a dense π-π stack-
ing taking place between the P and M enantiomers and shows a complicated packing mode. The inter-
molecular face-to-face stacking in TBH crystal is not obvious.
In Table 3, we summarized the reorganization energy (λ) and interaction of important neighbors from
one reference molecule in the crystal, together with the calculated electronic couplings, and the charge
mobility estimated from the Einstein relationship (ꢁE).29 From Table 3, it is seen that the reorganization
energy (λ) of TBN is 181 meV, and that of TBT, TBH and TBO is 114, 112 and 103 meV, respective-
ly. The reorganization energy decreases by 67 meV in going from TBN to TBT. However, in going
from TBT to TBH and TBO, the reorganization energy does not decrease much. HOMO distributions
of TBT, TBH and TBO are very similar, that is, mostly concentrated on the “TBT” core of the struc-
tures. The geometry relaxation mainly occurs in the “TBT” portions of these three compounds, so that a
similar reorganization energy is obtained.
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The couplings for π-π stacking conformations are 75.6, 84.0 and 179 meV for TBDNO,I:T10B.10T39/aCn7TdC0T22B54OA ,
respectively, as they are related to the degree of π-π overlap. The calculated mobility also follows the
trend, with a very high mobility expected for TBO (17.9 cm2V-1s-1). On the other hand, other inter-
column couplings also exist, providing a charge-hopping network. However, such intercolumn couꢀ
plings are weaker in TBO than those in TBN and TBT. In other words, the charge hopping network in
TBO is essentially 1-dimensional. With higher inter-column couplings, TBN and TBT have 3-
dimensional charge-hopping networks. The π -π stacking offers a fast charge hopping path, and it is
aligned with the crystal axis in this series. In terms of the coupling of the π-π stacking along the b-axis,
TBH has the smallest, 41meV, compared to the other three tetrabenzoacene derivatives. Nevertheless,
the charge conduction in TBH crystal packing can have multiple channels if one particular path is
blocked. In this case, the fast charge hopping paths do not align in a single direction, as seen in Figure 6.
Therefore, the contribution of these charge hopping pathways to the charge mobility will be smaller, deꢀ
pending on how the hopping path projected along the electric field.
Figure 4. Crystal packing of TBN by viewing along b-axis and a-axis (C-H bonds are omitted for clariꢀ
ꢁ
ꢁ
ty). Unit cell exhibiting long b-axis along the (0ꢀ0), short a-axis along (ꢀ00) and c-axis along (001).
The crystal shown has a dimension of 0.28mm x 0.14mm x 0.08 mm.
Figure 5. Crystal packing of TBT by viewing along b-axis and a-axis (C-H bonds are omitted for clariꢀ
ꢂ
ty). Unit cell exhibiting long b-axis along the (010), short a-axis along (100) and c-axis along (001). The
crystal shown has a dimension of 0.36 mm x 0.20 mm x 0.10 mm.
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Figure 6. Crystal packing of TBH by viewing along b-axis and a-axis (C-H bonds are omitted for clari-
ꢂ
ꢂ
ꢂ
ty). Unit cell exhibiting long b-axis along the (010), short axis along (101) and c-axis along (001). The
crystal shown has a dimension of 0.44 mm x 0.16 mm x 0.04 mm.
Figure 7. Crystal packing of TBO by viewing along b-axis and a-axis (C-H bonds are omitted for clariꢀ
ty)
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Table 2. Selected structural parameters of single crystal of tetrabenzoacene derivatives
Compound
a (Å)
TBN
12.14
7.56
17.64
90
TBT
13.18
7.49
18.15
90
TBH
18.87
7.65
31.44
90
TBO
31.94
3.90
23.95
90
b (Å)
c (Å)
α (deg.)
β (deg.)
γ (deg.)
97.62
90
100.99
90
90
116.35
90
90
space group
P21/n
P21/n
Pbca
C2/c
40.65
&
35.92
&
40.69
&
41.21
&
Exp. and calc.(in parentheses)
torsion angle°
43.29
(43.14)
1.388
(1.397)
38.99
(38.85)
1.395
(1.400)
41.95
(41.82)
1.386
(1.393)
41.21
(42.25)
1.397
(1.392)
A-B-C-D & E-F-G-H
Exp. and calc.(in parentheses)
central C=C bond length
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Table 3. Calculated the reorganization energy and electronic coupling for hole transfer
c
c
c
f
Molecule
λin Distance b
pa
pb
pc
Character d
Coupling e
ꢁE
4.815
3.0 3.8 0.1 PP, π-π
75.64
TBN
181 10.249
10.207
4.3 2.1 9.6 PM, parallel 28.72
1.9 5.4 8.1 PM, parallel 11.91
0.99
2.85
4.927
2.7 3.7 1.2 PP, π-π
2.2 3.3 6.6 PM, CH-π
2.2 4.2 6.6 PM, CH-π
0.9 0.7 6.4 PM, π-π
84.0
38.8
16.9
41.44
TBT
114 8.152
8.579
6.491
TBH
TBO
112 10.553
7.648
0.9 8.3 6.4 PM, parallel 50.57
2.81
17.9
0.0 7.6 0.0 PP, parallel
29.28
178.5
13.74
3.898
103
0.0 3.9 0.0 PP, π-π
11.987
5.3 0.6 12
PM
aReorganization Energy, in units of meV.
bDistance (in units of Å) of the neighbor to the reference molecule, which is centered in the origin.
cProjection of the center of the neighboring molecule to the lattice axes a, b, and c, in units of Å.
dDescription of the conformation of the pair. “P” and “M” indicate the corresponding enantiomers.
π-π and CH-π describe the major feature of the interaction. “Parallel” indicates that the two π frag-
ments from the two molecules are parallel to each other, but they are shifted and there is no π or-
bital overlap.
eListed are coupling of the reference molecule to the neighbor, in units of meV.
fCharge mobility estimated from the Einstein’s relationship, in the units of cm2 V-1 s-1. For TBN,
TBT, TBH and TBO we report the value for the b-axis.
SCFET device property
Crystallites grown by the vapor phase transfer method were carefully examined under an optical micro-
scope for their quality and integrity and then chosen for device fabrication. The single crystal field-
effect transistors were prepared only along the long axis (indexed to be b-axis) in the same fashion for
all compounds, as this is also the direction with significant π-πstacking and thus electronic couplings.
Four to ten samples for each compound were prepared and measured in the dark under ambient condi-
tion. All of the devices exhibited typical p-type FET characteristics. The device characteristics are sum-
marized in Table 4. The typical output and transfer characteristics for TBT are shown in Figure 8. The
rests are shown in ESI, Figure S13-16.
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Table 4. SCFET performance and Simulated charge mobilities of tetrabenzoacene derivatives
Mobility, ꢁ,
(cm2 V-1 S-1)
Average mobility
(ꢁavg)
Threshold voltage
(Vth, V)
Compound
On/off ratio
0.28– 0.55
0.46– 0.81
0.044– 0.074
0.11– 0.55
0.41± 0.07
0.62± 0.14
0.061± 0.011
0.32± 0.15
104– 105
105– 106
104– 105
102– 104
(-9) – (-3)
(0) – (-10)
(-11) – (-35)
(3) – (-16)
TBN
TBT
TBH
TBO
Figure 8. (a) Output characteristics and (b) transfer characteristics of TBT.
Table 5. Simulated charge mobilities of tetrabenzoacene derivatives
Simulated mobility (cm2V-1s-1)
TBN
0.72
0.72
TBT
2.24
0.45
TBH
1.24
1.24
TBO
5.64
0.35
ꢁKMC
ꢁdeloc
The SCFET devices based on tetrabenzoacenes, TBN, TBT, TBH and TBO gave on/off current ratios
of 102- 106, and an average mobility of 0.41, 0.62, 0.061 and 0.32 cm2 V-1s-1 respectively. Compound
TBT exhibited the best performance among the SCFET devices, with the highest measured mobility of
0.81 cm2 V-1s-1. In contrast to the calculation results, TBO has a rather moderate mobility of 0.32 cm2V-
1s-1, and TBH has the lowest mobility in this series. In an attempt to understand the discrepancy, other
models are being considered. Thus in Table 5 we also include simulated charge mobilities from two difꢀ
ferent Monte Carlo simulation schemes: a standard Monte Carlo simulation with disorder in site enerꢀ
gies and charge-transfer couplings (ꢁKMC), and a modified Monte Carlo simulation with delocalized
states for sites with large coupling values (ꢁdeloc),30 (additional details are in the supplementary inforꢀ
mation accompanying this work).
Following TBN, TBT and TBO series, the P-P and M-M stacking offers an increased level of π-π
overlap, and so is the corresponding electronic coupling values, which are 75.6, 84.0 and 178.5 meV,
respectively. It is also seen that the measured mobility of TBN and TBT are quite close, with that of
TBT being larger. Depending on the theoretical model, TBT’s mobility is larger than TBN, with simple
11

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Einstein’s relationship, or with kinetic Monte Carlo model. But TBO is still the highest at 5.64 cm2V-1s-
1
.
In an earlier report delocalized polaron formation was suggested to form when the electronic coupling
is comparable to reorganization energy so that the mobility can be diminished. It is noticed that among
this series of compound, TBO is the only one that has a rather high electronic coupling larger than its
reorganization energy (179 meV and 103 meV respectively). Thus delocalized polaron effects in the
simulation (ꢁdeloc) is considered. Namely, when the sampled site has a neighboring coupling larger than
the reorganization energy, the two (or more) sites are replaced by a delocalized state, which is lower in
energy and the overall coupling to external sites are slightly weaker.30 When this delocalized polaron is
included in the kinetic Monte Carlo simulation, the mobility ꢁdeloc of TBO drops dramatically from 5.64
to 0.34 cm2 V-1s-1, which appears to be close to experimental results. Similar effect is also seen on TBT,
whose largest coupling from the crystal structure is 84 meV, pretty close to its reorganization energy of
114 meV. With a sampled disorder from a molecular dynamics simulation, a small fraction of the mole-
cules is with coupling being larger than the reorganization energy (Figure S6 in the supplementary ma-
terial). The corresponding simulated mobility is decreased from 2.24 cm2 V-1s-1 to 0.45 cm2 V-1s-1 when
low-lying charge-delocalized state is considered. We note that the latter result is close to experimental
results.
Our simulation predicted large charge mobilities for TBH. TBH’s largest electronic coupling is only
about 50 meV, much smaller than the reorganization energy of 112 meV, implying that the effect of de-
localized polaron does not exist. The crystal structure of TBH indicates several intermolecular interac-
tions with good electronic coupling, leading to a rather isotropic connection network, which would al-
low easy escape should a blockage in the pathway be formed. However, the averaged and highest ex-
perimental mobility were only 0.061 and 0.074 cm2 V-1s-1. The source of this discrepancy is not clear
yet.
CONCLUSIONS
In conclusion, a series of linear contorted tetrabenzoacenes were synthesized via a convenient route
using triflic acid-catalyzed rearrangement reaction of pinacol derivatives, which were prepared from
benzannulated fluorenone in acceptable yields. The photo-physical properties and the calculated frontier
molecular orbitals suggest that linear extension of aromatic skeletons in contorted tetrabenzoacenes did
not enlarge the π-electron delocalization.
The single crystals were grown by PVT technique for X-ray analyses. For contorted tetrabenzo-
acenes, not only face-to-face packing exists, but other packing modes were obtained. The increased con-
jugated length of unsubstituted tetrabenzoacenes derivatives leads to improved intermolecular π- π over-
lap except for compound TBH, which had a slightly inferior π- π overlap but a noteworthy edge-to-face
packing. Compound TBO showed a perfect cofacial stacking.
Among the SCFETs with these tetrabenzoacenes as the conducting channel, compound TBT gave the
best field-effect hole-mobility of 0.8 cm2 V-1 s-1. Theoretical calculation is used to rationalize the trends
observed in measured mobilities with reasonable agreement, except in the case of TBO, whose electron-
ic coupling is much larger than its reorganization energy. It is rationalized by the involvement of delo-
calized polarons, which potentially trap the charges by their lower energy state.
AUTHOR INFORMATION
Corresponding Author
* Yu-Tai Tao, ytt@gate.sinica.edu.tw and Chao-Ping Hsu, cherri@gate.sinica.edu.tw
Conflicts of Interest
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There are no conflicts of interest to declare
ACKNOWLEDGMENT
Financial support from the Ministry of Science and Technology, Taiwan (Grant Number:101-2113-M-
001 -006 -MY3) is gratefully acknowledged.
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Table of Content
A series of non-planar tetrabenzo-fused acenes exhibited a hole mobility
from 0.044 cm2V-1s-1 up to 0.81 cm2V-1s-1 in their single crystal
field-effect transistor.