Rodlike Tetracene Derivatives

Article abstract of DOI:10.1002/chem.201702382

Efficient and versatile synthetic access to rodlike tetracene derivatives was developed by means of Diels–Alder cycloaddition, halogenation, halogen–metal exchange, and transition metal mediated coupling reactions. Herein, the synthesis and structural, electrical, and charge-transport properties of three of the resulting materials, namely, 2-(tetracen-2-yl)tetracene, 1,4-bis(2-tetracenyl)benzene, and 2,5-bis(2-tetracenyl)thiophene, are presented. Good crystallization behavior on SiO₂ substrates, narrowing of the bandgap by 0.2 eV, and a decrease of the ionization potential of more than 0.5 eV compared to tetracene were observed. Charge-carrier field-effect mobilities on the order of 10^?1 cm² V^?1 s^?1, on/off ratios of 10⁵, and threshold voltages V_th<15 V were found in thin-film organic field-effect transistors prepared by standard high-vacuum deposition techniques.

Full text of DOI:10.1002/chem.201702382

A Journal of
Accepted Article
Title: Rod-like Tetracene Derivatives
Authors: Michael Roth, Marcus Ahles, Christian Gawrisch, Thorsten
Schwalm, Roland Schmechel, Christian Melzer, Heinz von
Seggern, and Matthias Rehahn
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To be cited as: Chem. Eur. J. 10.1002/chem.201702382
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Rod-like Tetracene Derivatives
Michael Roth,[a] Marcus Ahles,[b] Christian Gawrisch,[a] Thorsten Schwalm,[a] Roland Schmechel,[b]
Christian Melzer,[c]* Heinz von Seggern,[b]* and Matthias Rehahn[a]*
Abstract: An efficient and versatile synthetic access to rod-like
tetracene derivatives has been developed using Diels-Alder
cycloaddition, halogenation, halogen-metal exchange, and transition-
metal mediated coupling reactions. Here we review the synthesis of
three of the resulting materials, i.e., 2-(tetracen-2-yl) tetracene DT,
the molecules in such a way that controllable electronic coupling
between the individual tetracenes is insured. This is since once
the functional materials are supposed to be used in organic
functional devices the ionization potential of 5.4 eV found with
tetracene [3] needs to be adapted to the workfunction of common
contact materials such as Au. On the other hand, pentacene and
its higher homologues are known to be chemically quite
unstable.[4] High HOMO and LUMO energy levels caused by the
largely delocalized -electron systems are the reason. Thus,
controlled segmentation of the -electron systems might be a
useful concept in the rational molecular design to improve the
materials’ stability.
1,4-bis(2-tetracenyl)benzene
TPT,
and
2,5-bis(2-tetracenyl)-
thiophene TTT, and present their structural, electrical and charge-
transport properties. Good crystallization behaviour on SiO2
substrates, a narrowing of the bandgap by 0.2 eV and a decrease of
the ionization potential of more than 0.5 eV if compared to tetracene
were observed. Charge-carrier field-effect mobilities in the order of
10-1 cm2V-1s-1, on/off ratios of 105, and threshold voltages Vth<15 V
were found in thin-film organic field-effect transistors prepared by
standard high-vacuum deposition techniques.
Following the above considerations, we developed a synthetic
tool-box approach leading to
a variety of bis(tetracenyl)
derivatives.[5] Using this tool-box, fluorinated bis(tetracenyl) as
an n-type material and bis(tetracenyl)benzene as a p-type
material could be already realized, providing complementary
charge-carrier transport for CMOS applications.[6] Here we now
review the synthesis of (i) the employed bis(tetracenyl)benzene
TPT, (ii) the directly linked, unsubstituted bis(tetracenyl) DT, and
(iii) the bis(tetracenyl)thiophene TTT (Figure 1). Moreover, we
present the comparative properties of these materials. While
increasing the aspect ratio of tetracene by directly linking two of
them, we further increased the aspect ratio by additional -
conjugated anchor groups and additionally changed the nature
of the group in order to modify the electronic coupling between
the involved tetracenes.
Introduction
Pentacene together with many of its derivatives show excellent
semiconductor performance in e.g. organic thin-film field-effect
transistors (OFETs), but its shorter homologues like tetracene
and anthracene perform much less excitingly in those devices.[1]
The weak performance of tetracene in OFETs seems to be due
to less appropriate thin-film morphologies, insufficient overall
molecular ordering and inappropriate electronic states not
matching the device requirements.[2] Suboptimal thin-film
structure might be the consequence of an insufficient molecular
aspect ratio, which antagonizes the required crystallization in the
right structure. To circumvent this problem, it might be a useful
approach to embed the small tetracene motif into larger
molecules of higher aspect ratio for getting improved
semiconductor performances. Thereby it seems suitable to link
DT
TPT
[a]
[b]
[c]
Dr. M. Roth, Dr. M. Gawrisch, Dr. T. Schwalm, Prof. Dr., M. Rehahn
Ernst-Berl-Institut für Technische und Makromolekulare Chemie
FB Chemie
S
Technische Universität Darmstadt
TTT
Alarich-Weiss-Straße 4, D-64287 Darmstadt, Germany
E-mail: m.rehahn@mc.tu-darmstadt.de
Dr. M. Ahles, Prof. Dr. R. Schmechel, Prof. Dr. H. von Seggern
Institut für Elektronische Materialien
FB Materialwissenschaft
Technische Universität Darmstadt
Alarich-Weiss-Straße 2, D-64287 Darmstadt, Germany
E-Mail: seggern@e-mat.tu-darmstadt.de
PD Dr. C. Melzer
Figure 1. Chemical structures of the synthesized and characterized
bis(tetracenyl) derivatives DT, TPT, and TTT.
Centre for Advanced Materials and Kirchhoff Institut für Physik
Universität Heidelberg
Im Neuenheimer Feld 252, D-69120 Heidelberg, Germany
E-Mail: melzer@uni-heidelberg.de
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Results and Discussion
aqueous EDTA. Crystallization from o-dichlorobenzene resulted
in pure 1 in 80% yield. Its dehydrogenation proved to be
feasible by treatment with 2,3-dichloro-5,6-dicyano-1,4-
benzoquinone (DDQ) in boiling o-xylene.[11] Pure DT was
obtained as a reddish-orange crystalline powder in 75% yield
after repeated gradient sublimation under slightly reductive
conditions (argon/H2).
Synthesis of ditetracene derivatives
The development of an efficient access to the parent 2-
(tetracen-2-yl)tetracene [bis(tetracenyl), “ditetracene”, DT], and
the characterization of the material properties were the first
issues in the research program presented here. Ditetracene DT
in principle is readily accessible by various synthetic routes,
including those three pathways A – C shown in Scheme 1.
6
1
2
7
8

5
Cl
Br2

3
4
Br
Br
7
9
8
9*
2
+ 2
O
O
Br
2
1
8
5
4
5
Cl
Br
9
2
7
12
BuLi
Route A: SYC
Ni
O
Route C

3
6
B(OR)2
10
4
3
O
10
11
5

Route B: SMC
+ 2, [Pd]
O
HCl
4
5
6
7
6
1
3
8
9
HCl
13
14
15
16
17
18
O
DDQ
2
Br
Br
10
12 13 11
1
2
DT
Scheme 2. Synthesis of starting material 2. The additional numbers shown in
the chemical formulas of molecules 5, 9 and 13 indicate the systematics used
for NMR signal assignment.
Scheme 1. Three successful synthetic routes A – C leading to ditetracene DT.
From the efficiency point of view, the pathway that proceeds via
palladium-catalyzed Suzuki-Miyaura coupling (SMC) of starting
materials 2 and 3 (route B) was judged to be too elaborate for
the preparation of “simple”, i.e., symmetric and unsubstituted
ditetracene molecules like DT. Therefore, it was not investigated
in deeper detail here. The same might apply to route C, which
includes a twofold Diels-Alder reaction of the capped bis-diene 4
with 5 as the dienophile. This latter approach has shown its
specific strengths in the preparation of some substituted
derivatives of DT. Nevertheless, we tested this route C here as
well. Anyway, the Semmelhack-Yamamoto coupling (SYC)
reaction (route A) was applied predominantly for the present
purpose as it proved to open the most direct access to the target
molecule DT.
Starting material 2-bromo-5,12-dihydrotetracene 2 was obtained
following Scheme 2: flash-vacuum pyrolysis of -chloro-o-xylene
7 led to benzocyclobutene 8,[7] which was brominated in 4-
position in the next step to give 9.[8] Diels-Alder [4+2]-
cycloaddition of its ring-opened high-temperature isomer 4-
bromo-o-xylylene 9* with epoxynaphthalene 5 – obtained from 1-
chloro-2-bromobenzene 10 and furan 12 via o-benzyne 11 as
the intermediate – resulted in adduct 13.[9] Cleavage of its epoxy
bridge followed by dehydration resulted in 2-bromo-5,12-
dihydrotetracene 2.[9]
In addition to the highly efficient route A shown in Scheme 1 we
tested the Diels-Alder pathway (route C) as well in order to
analyze its benefits for the synthesis of ditetracene DT. Here the
twofold Diels-Alder cycloaddition reaction of the ring-opened
high-temperature isomer of bis(benzocyclobutene) 4 – i.e. of
bis(o-quinodimethane) derivative 4* – with epoxynaphthalene 5
as the dienophile is the key step. Scheme 3 illustrates three
methods A – C which were tested for the preparation of the
bis(benzocyclobutene) 4. The boronate derivative 14 required
for the SMC approach (method A) was prepared in 71% yield by
the treatment of 4-bromobenzocyclobutene
9 with n-BuLi
followed by adding isopropoxyborolane. Reflux of equimolar
amounts of 9 and 14 in the heterogeneous mixture of THF (or
dioxane) and aq. K2CO3 in the presence of catalytic amounts of
Pd(PPh3)4 for 1 d gave dimer 4 in 70% yield. Alternatively, 4 was
obtained by the Negishi coupling using organozinc intermediate
15 (method B, Scheme 3): the solution of 9 was treated
successively with 0.5 eq. of n-BuLi, the corresponding amount of
ZnCl2, and finally some Pd(PPh3)4 as the catalyst for the
coupling step. Refluxing the mixture for 2 d led to the desired
product 4 in 80% yield as a colorless, slowly crystallizing oil. In
overall comparison, method B proved to be more efficient and
convenient than the SMC approach (method A) in this case.
Finally, we also tested the direct conversion of 9 with its lithiated
counterpart (method C). Rather surprisingly, this most simple
and direct approach proved to be excitingly successful, and
gave the bis(benzocyclobutene) 4 in approx. 80% yield.
The next step was SYC of two molecules of starting material 2 to
give
2-(5,12-dihydrotetracen-2-yl)-5,12-dihydrotetracene
(“bis(dihydrotetracene)”) 1. The active nickel complex needed for
this coupling step[10] was obtained from Ni(COD)2, COD and 2,2’-
bipyridine
(COD:
1,5-cyclooctadiene).[10e]
2-Bromo-5,12-
dihydrotetracene 2 was added to the resulting dark violet
solution, and after 5 d reaction time the conversion was
completed. The nickel residues were removed carefully using
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B(OR)2
[Pd]
2
+
O
(RO)2B
B
Br
ClZn
Br
2
16
17
14
9
15
O
Method C
DDQ
S
S
(CH3)3Sn
Sn(CH3)3
+ 9
[Pd]
+ 9
Method B
Method A
[Pd]
18
4
19
TPT
[Pd]

+2
Br
2
S
S
DDQ
4*
20
TTT
Scheme 3. Three competing synthetic pathways (methods A – C) leading to
starting material 4.
Scheme 4. Synthesis of TPT and TTT.
Diels-Alder reaction of the ring-opened isomer 4* with
epoxynaphthalene 5 (see Scheme 1) proceeds in refluxing
dodecane under ambient pressure and in the presence of a
base like K2CO3, which prevents rearrangement side-reactions
of 5 under the drastic conditions (> 200°C). Intermediate 6 was
isolated in 65% yield. It was transferred into ditetracene DT by
cleavage of the epoxy bridges and dehydration in boiling acetic
anhydride/HCl followed by dehydrogenation using DDQ in
boiling xylene as usual. All in all, pathways A and C were shown
to rank quite similar with respect to efficiency, work-load, up-
scalability and yields.
Last but not least, the arene-bridged ditetracene derivatives TPT
and TTT were prepared. For the synthesis of TPT, a mixture of
two equiv. of 2-bromo-5,12-dihydrotetracene 2 and one equiv. of
the 1,3-propanediol diester of benzene-1,4-diboronic acid 16
were refluxed in the heterogeneous mixture of aq. K3PO4,
xylene, and Pd[P(Ph3)4].[12] 1,4-Bis(5,12-dihydrotetracen-2-
yl)benzene 17 was obtained in 72% yield as a yellow solid after
recrystallization from 1,2-dichlorobenzene. Finally, 17 was
converted into TPT by the common treatment with an excess of
DDQ in boiling xylene. TPT was obtained as an orange solid and
purified further by gradient sublimation (60% yield).
For the synthesis of TTT, on the other hand, 2,5-
bis(trimethylstannyl)thiophene 19 was the required starting
material.[13] It was obtained in 70% yield from thiophene 18 by its
conversion into the dilithiated intermediate using n-BuLi in the
presence of tetramethylethylene diamine (TMEDA), followed by
addition of trimethyltin chloride. 19.[14] In the next step, 19 was
treated with two equiv. of 2-bromo-5,12-dihydrotetracene 2 in
boiling xylene in the presence of catalytic amounts of
[Pd(PPh3)4].[15] After recrystallization from o-dichlorobenzene,
2,5-bis(5,12-dihydrotetracen-2-yl)thiophene 20 was isolated as a
bronze-colored crystalline powder in 70% yield. It was converted
into 2,5-bis(tetracen-2-yl)-thiophene TTT by using DDQ in
boiling xylene. The yield was 50% after purification by gradient
sublimation.
Thin layer properties of ditetracene derivatives
Wide-angle X-ray scattering (WAXS) studies were carried out on
80 nm thin films of ditetracene DT on Si (100) wafers covered
with a SiO2 top layer. Figure 2 shows the resulting  - 2 scans
for films produced by deposition on untreated SiO2 at room
temperature,
and
for
deposition
on
octadecyldimethylchlorosilane (OMS)-treated SiO2 substrate at
elevated temperature (140°C). Comparison with scattering
patterns obtained from analogously prepared thin films of
tetracene, as are given in literature,[16] makes evident that the
crystalline ordering of ditetracene DT is higher, especially when
deposited on OMS-treated SiO2 at increased deposition
temperature. Reflexes up to the 8th order can be detected, all
belonging to the same interplanar d spacing of 24.24 Å. This d
spacing comes close to the molecule length of DT of 22.53 Å.
109
 = 0.4°
10
DT
107
1
DT
15
30
105
103
101
10-1
2 / °
TPT
TTT
0
5
10
15
20
25
30
2 / °
Figure 2. X-ray diffractograms of thin films of TPT and TTT measured in
Bragg-Brentano geometry on top of an OMS-pretreated Si/SiO2 wafer. WAXS
pattern of an 80 nm ditetracene DT film prepared on an untreated and an
OMS-treated SiO2 substrate at room temperature (black) and 140°C (red),
respectively. Inset: WAXS pattern of a thin film of DT, recorded at gracing
incidence ( = 0.4°).
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central 1,4-phenylene moiety does not result in further improved
ordering. Quite similar results were obtained for thiophene-
bridged ditetracene TTT. Reflexes up to the 6th order can be
seen, corresponding to a lattice constant of 28.06 Å. Again, this
corresponds to the molecular length, estimated to be 27.06 Å.
The UV-vis absorption and photoluminescence spectra of the
parent ditetracene DT thin films deposited on quartz wafers are
shown in Figure 4. For comparison purposes, the corresponding
spectra of tetracene, TPT and TTT are shown as well. A distinct
shift of approx. 50 nm toward longer wave lengths is evident for
the absorption bands in the UV-vis absorption spectra of all
ditetracenes compared to tetracene. Significantly larger
expansion of the delocalized -electron systems seems the
major reason. As determined from the absorption edge a
decrease of the bandgap from 2.3 eV of thin films of tetracene to
arround 2.1 eV for the DT derivatives has been found. The sub-
bandgap absorption from light scattering within the film is
indicative for granular film morphology. Likewise, the
photoluminescence spectra show shorter wavelength emission
for tetracene compared to the ditetracene materials, whose
photoluminescence spectra are very similar.
Figure 3. AFM images of 80 nm thick films of ditetracene DT deposited (a) at
room temperature on an untreated Si(100)/SiO2 substrate, and (b) on an OMS-
treated Si(100)/SiO2 substrate heated to 140°C. The average grain diameter is
(a) 40 nm and (b) 200 nm, respectively.
The inset of Figure 2 shows the x-ray diffractogram under
gracing incidence ( = 0.4°). Crystallites appear to be equally
oriented relative to the normal of the DT thin film since only few
reflexes can be observed compared to full WAXS measurement.
The orientation of crystallites is superior in the case of OMS-
treated and heated substrates.
Figure 3 shows atomic-force microscopy (AFM) images of the
same 80 nm thick DT films, once prepared on untreated SiO2 at
room temperature and on an OMS-treated SiO2 at 140°C. The
larger size of the crystallites in the films prepared on OMS-
treated and heated SiO2 substrates is evident. Thus, from WAXS
and AFM measurements a deposition at elevated substrate
temperature on OMS-treated SiO2 seems to be favorable for
charge-carrier transport.
He I 21.22 eV
a)
b)
8
6
4
2
0
Valence band
region
TTT
TPT
tetracene
DT
1.0
a)
TPT
TTT
0.5
0.0
DT
T
1.0
0.5
0.0
b)
16
12
8
4
0
2
0
binding energy / eV
Figure 5. UV photoexcitation (UPS) spectrum of a thin film of tetracene, DT,
TPT and TTT on top of a SnO2 substrate (a). The spectra have been shifted
up for clarity. The ionization potentials were calculated to be 5.3 eV for
tetracene, 4.8 eV for DT and 4.6 eV for TPT and TTT from the spectra onset at
low binding energies (marked by the vertical line for tetracene and the shaded
area for DT, TPT and TTT (b)) and the secondary cut-off energies.
500
600
 / nm
700
800
Figure 4. UV-vis absorption (a) and photoluminescence spectra (b) of
tetracene, DT, TPT and TTT, obtained from thin layers deposited on quartz
wafers. The UV-vis absorption spectra were recorded in transmission mode,
the photoluminescence spectra were recorded using excitation wavelength of
 = 345 nm.
The ionization potential of tetracene and the ditetracene
derivatives
were
estimated
using
UV-photoelectron
spectroscopy (Figure 5). A thin layer of the compound was
deposited by vacuum sublimation on a SnO2 substrate and the
UPS spectra where recorded using the He I line. From the
secondary cut-off and the binding energy an ionization potential
(IP) of 5.3 eV was found for tetracene in accordance with other
reports1.[3] In line with the reduced bandgap, the IP of DT with
4.8 eV was found to be substantially smaller. Accordingly, for
For the 1,4-phenylene-bridged ditetracene TPT, the degree of
ordering was estimated by WAXS studies on samples prepared
at room temperature on OMS-treated substrates. Figure 2
shows the X-ray diffractogram measured in Bragg-Brentano
geometry. Reflexes up to the 6th order are resolved, and a lattice
constant of 28.42 Å can be calculated, implying that the
molecules stay upright on the substrate’s surface. The estimated
molecular length is 27.56 Å. However, the degree of ordering in
thin films of TPT is clearly lower than in the corresponding films
of DT. At the first glance, it seems that the introduction of the
1
One of the special features provided by pentacene is that it offers an
energetic level of its highest occupied molecular orbital (HOMO; 5.07 eV) [17]
which is well-adapted to gold, the commonly used material for source and
drain electrodes.
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TPT and TTT an IP of 4.6 eV has been obtained. For all
materials an increased mesomeric system seems to reduce the
IP. Thus, assuming that no interface effects disturb the band
alignment at the interface to gold contacts, the DT derivatives
appear to be more suited for efficient hole injection than
tetracene in organic devices. In conjunction with the estimated
band gaps all materials are apparently rather p-type than n-type
materials. The energy levels of the respective molecules are
depicted in Figure 6.
To determine the charge-carrier transport properties of holes, p-
type organic field-effect transistors have been realized. Bottom-
contact architectures were constructed on heavily doped n-type
silicon substrates covered by a high-quality thermally grown
oxide layer (thickness  230 nm). A thin chromium adhesive
layer was deposited on top of the entire oxide surface, followed
by a 50 nm gold layer. The gold source and drain electrodes
were photolithographically configured as interdigitated fingers
with a channel length of L = 7 m and a total channel width of
W = 20 cm.[18] In addition, top contact, bottom gate OFETs were
prepared with a W/L = 8 mm/100 µm and a SiO2 thickness of
200 nm. The ditetracene derivatives were thermally deposited at
a pressure of 10-6 mbar and a deposition rate of 1 Å s-1.
Table 1. Summarized device parameters of DT, TPT and TTT based OFETs.
Tdep
µh
Vth
On/off
ratio
S
Nt
[°C]
[cm2/(Vs)]
[V]
[V/dec]
[V-1cm-2]
DT
DT
bg/tc
bg/bc
bg/bc
bg/bc
23
140
30
0.27
0.35
0.24
0.12
-32
-2
105
104
105
105
3.7
1.2
1.8
1.8
1.5x1013
4x1012
6x1012
6x1012
TPT
TTT
-14
-13
23
[a]The device layout comprises OMS treated SiO2 gate dielectrics and Au S/D
contacts.
The characteristics of DT based bottom-gate, top-contact
OFETs deposited at RT is shown in Figure 7. The output
characteristic is indicative of a unipolar p-type field-effect
transistor with good saturation behavior. From the transfer
characteristics in the saturation regime field-effect mobilities of
holes of h = 0.27 cm2V-1s-1 and a threshold voltage of Vth = -
32 V were found for VD = -100 V. An on/off ratio of 105 and a
subthreshold swing at the turn on voltage of 3.7 V/dec were
determined. The subthreshold swing relates to a large areal trap
density of 1.5x1013 cm-2V-1.[19] All results of the transistor
performance are listed in Table 1. The large threshold voltage
and week subthreshold swing go along with an injection
impeded output characteristic. This contradicts the assumed
ohmic contact expected from the UPS measurement but might
be realistic due to weak carrier transport properties in the bulk of
the DT layer hindering the carrier deliverance to the transistor
channel in a top contact configuration. A bottom contact
configuration was thus a possible alternative to reduce the
injection problem.
S
-2
-2.47
-2.47
LUMO
HOMO
-2.73
-3
-4
-5
-3.01
0.035
100
a)
-100V
-80V
-60V
-40V
-20V
b)
-0.8
-0.7
-0.6
-0.5
-0.4
-0.3
-0.2
-0.1
0.0
Vg=-100V
0.030
0.025
0.020
0.015
0.010
0.005
0.000
-4.56
-4.61
10-1
10-2
10-3
10-4
10-5
10-6
EF(Au)
-4.83
-5.30
-80V
Vd=
DT
TTT
T
TPT
-100V
-60V
-40V
Figure 6. Frontier orbital energies of T, DT, TPT and TTT obtained by UPS
and UV/VIS absorption experiments. The dashed line indicates the
workfunction of Au at 4.8 eV.
-20V
-80
-40
Vd / V
0
-120
-80
-40
Vg / V
0
Figure 7. (a) Id versus Vd for a top-contact bottom-gate OFET from RT
deposited DT with pre-treated SiO2 using octadecyldimethylchlorosilane
(OMS); (b) Id versus Vg for a top-contact bottom-gate OFET from RT deposited
DT with pretreated SiO2 using octadecyldimethylchlorosilane; A field-effect
mobility of µh = 0.27 cm2/Vs for holes has been determined in the saturation
regime of the transistor. The inset shows the molecular structure of DT.
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102
101
-20
0.25
a)
b)
a)
b)
-20V
-15V
-10V
-5V
-40V
-30
-25
-20
-15
-10
-5
101
100
Vg=-20V
-30V
0.20
-20V
Vg=-40V
0.16
0.08
0.00
-15
100
-10V
Vd=
Vd=
0.15
10-1
10-2
10-3
10-4
10-5
-20V
-15V
-10
-40V
10-1
10-2
10-3
0.10
0.05
0.00
-30V
-10V
-5
0
-20V
-10V
-5V
0V
-20
0
-10
Vd / V
0
-20
-10
Vg / V
0
-40
-20
0
-40
-20
Vg / V
0
Vd / V
Figure 8. (a) Id versus Vd measured at different gate voltages Vg for a bottom-
contact OFET with OMS-treated SiO2, produced by deposition of DT at 140°C
substrate temperature; (b) Id versus Vg of the same DT-based OFET,
measured at different Vd. At source-drain voltages of –20 V the field-effect
mobility was found from the saturation regime to be µh = 0.35 cm2 V-1 s-1 and
Vth was –2 V. The inset shows the molecular structure of DT.
Figure 9. (a) Output characteristics at different Vg of a 1,4-bis(tetracen-2-
yl)benzene-based bottom-contact OFET on an OMS-treated SiO2 transistor
substrate, prepared by deposition of TPT at 30°C substrate temperature; (b)
transfer characteristics of the same OFET for different Vd; at Vd of -40 V the
field-effect mobility was found to be µh = 0.24 cm2 V-1 s-1 and Vth was -14 V.
The inset shows the molecular structure of TPT.
Further improved device performance could be achieved by
depositing DT at elevated temperature on bottom contact
given in Figure 9, showing almost a linear increase of the drain
current for small values of Vd and a saturation regime at higher
Vd values These curves again are indicative of unipolar p-type
field-effect transistors with good saturation behavior. The
characteristics do not show any nonlinear contact resistance
with respect to the gold electrodes. From the transfer
characteristics, a field-effect hole mobility of h = 0.24 cm2 V-1 s-1
comparable to the one found for DT based OFETs and a
threshold voltage of Vth = -14 V were determined for Vg = -40 V.
A subthreshold swing at the turn-on voltage of 1.8 V/dec was
determined. This corresponds to a trap density of 6x1012 cm-2V-1.
The on/off ratio was with 105 smaller than for DT devices.
Finally, OFET devices were prepared from 2,5-bis(tetracene-2-
yl)-thiophene TTT on OMS treated SiO2 substrates. The
deposition of TTT was carried out at RT. The p-type output
characteristics given in Figure 10 show no injection problem and
tend to saturate at higher VD. From the transfer characteristics of
transistor substrates. DT was deposited at
a substrate
temperature of 140°C on OMS-pretreated SiO2 as suggested
from the structural investigations. The resulting device
characteristics are shown in Figure 8. If compared to the results
discussed before, a substantial improvement in the device
characteristics is obtained. Even though the field-effect mobility
is only slightly increased to 0.35 cm2V-1s-1 the threshold voltage
is substantially decreased to only –2 V. The subthreshold swing
reduced to 1.2 V/dec at the turn-on voltage translating to a
substantially reduced trap density of 4x1012 cm-2V-1. On the one
hand the reduced threshold voltage and subthreshold swing
might be due to the improved thin film morphology as shown in
Figures 2 and 3 providing less deep trap states. On the other
hand, the contact problem found with the output characteristic of
RT deposited DT disappeared, indicating better carrier
deliverance to the transistor channel. The threshold voltage
benefits from such an improved injection. Further improved field-
effect mobilities were determined for those devices, approaching
values of up to 0.65 cm2V-1s-1.
It is reasonable to assume that the good charge-carrier transport
properties compared to tetracene based devices are due to the
molecules’ larger aspect ratio. However, one might argue that a
substantial change of the crystal structure in thin films
determines the device performance instead. Nevertheless, this
seems not to be the case because the WAXS patterns of
ditetracene DT are quite similar to those of tetracene with
respect to the number and position of the reflexes, implying a
quite similar overall solid-state structure.
the TTT based transistor
a hole field-effect mobility of
μh = 0.12 cm2 V-1 s-1 was calculated for Vd = -40 V. This is
slightly lower than was found for TPT and DT under comparable
conditions. The threshold voltage was with Vth = -13 V
comparable to the one of TPT based devices as well. The on/off
ratio was determined to 105. Like for TPT, the subthreshold
swing at the turn-on voltage was found to be 1.8 V/dec relating
to a trap density of 6x1012 cm-2V-1.
We assign the better OFET properties of DT based devices
compared to the devices fabricated from the molecules with
larger aspect ratio to the improved processability of the smaller
molecule. The longer aspect ratio might hinder the molecular
movement during the crystal growth and may thus lead to less
ordered films as mentioned already earlier.
For 1,4-bis(tetracen-2-yl)-benzene TPT, the OFET output and
transfer characteristics were measured in bottom-gate/bottom-
contact transistor geometry, using OMS-treated SiO2 and a
deposition temperature of 30°C. The output characteristics are
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102
-16
Experimental Section
0.16
0.12
0.08
0.04
0.00
a)
b)
-40V
-30V
-20V
-10V
-14
-12
-10
-8
101
100
Materials: All chemicals were purchased from Acros, Aldrich and Strem
Chemical Co. and used without further purification. Solvents were
purified and dried using standard procedures. Throughout, the reactions
were carried out under an atmosphere of nitrogen or argon using
Schlenck technique.
Vg=-40V
Vd=
10-1
10-2
10-3
10-4
10-5
-40V
-6
-30V
Syntheses:
-4
S
Benzocyclobutene 8: -Chloro-o-xylene 7 (30 g, 213.35 mmol) is flash-
pyrolized at 800°C under reduced pressure of 0.3 - 0.7 mbar in an
apparatus composed of a 100 mL round-bottom flask storing the starting
material, a quartz tube (100 cm in length, 3 cm in diameter, filled with
quartz Raschig rings), and two cryo traps. The first cryo trap is cooled
down to –30°C and collects product 8, unchanged starting material 7,
and some side products. The second cryo trap is cooled down to –196°C
by liquid nitrogen and collects the formed HCl. The pyrolysis reaction is
finished when all starting material is evaporated and has passed the
oven. Product 8 collected in the first cooling trap is suspended in
aqueous sodium hydroxide (200 mL, 15 % w/v) to neutralize last traces
of HCl. The suspension is extracted with n-hexane (3 x 100 mL). The
combined organic layers are washed with water, the solvent is removed,
and the oily residue is dissolved in dimethyl sulfoxide (DMSO, 150 mL).
Powdered KOH (85 %, 3.5 g, 53.03 mmol, 0.75 eq) is added, the
suspension is stirred for 4 h and filtered. Water (150 mL) is added to the
solution, and the mixture is extracted with n-hexane (3 x 100 mL). The
combined organic layers are washed with water until the aqueous layer is
neutral in pH. The organic layer is dried (MgSO4), the solvent is removed,
and the yellow oil is distilled in vacuum (12 mbar, b.p. 49°C). Pure 8 is
obtained in 82% yield (18.3 g) as colorless oil.
-2
-20V
-10V
0
-40 -30 -20 -10
0
-40
-20
Vg / V
0
Vd / V
Figure 10. (a) Output characteristics for different Vg of a TTT-based bottom-
contact OFET on an OMS-treated substrate (deposition at room temperature);
(b) transfer characteristics of TTT-based OFET on OMS-treated substrate; at
source-drain voltages of -40 V the field-effect mobility was found to be
µh = 0.12 cm2 V-1 s-1 and Vth was -13 V.
Conclusions
Three rod-like tetracene based molecules have synthesized to
investigate the impact of the aspect ratio on the material
properties of thin films. While one molecule is based on two
tetracene molecules directly linked at their terminal positions,
benzene and thiophene links elongate the molecule even further.
1H NMR (300 MHz, CDCl3, ppm):  = 7.25 (d, 2H, H4,5), 7.10 (d, 2H, H3,6),
3.24 (s, 4H, H7,8);
Diels-Alder
cycloaddition,
halogenation,
halogen-metal
exchange, and transition-metal mediated coupling reactions
have been used successfully. While directly coupled tetracenes
improve the crystallinity of the thin film if compared to parent
tetracene, a further elongation of the molecule by additional
bridging groups did not improve the crystallinity further. The
latter can be addressed to a damped molecular diffusion during
thin-film deposition. Likewise an increased crystallinity found in
DT might result from the increased aspect ratio but also from a
reduced dimerization which could hinder diffusion of molecules
as well. All investigated molecules stand upright on the SiO2
surface. The elongation of the mesomeric system results in a
reduced bandgap and a reduced ionization potential. Already
with DT the ionization potential was reduced by 0.5 eV if
compared to tetracene based thin films. Therefore, in bottom-
gate, bottom-contact organic field-effect transistors good
transistor performance was observed with a lack of substantial
contact problems. Field-effect mobilities in the order of 0.1 cm2V-
1s-1, threshold voltages below 15 V and subthreshold swings
below 1.8 V/dec were found. Largest field-effect mobilities with
lowest threshold voltage and subthreshold swing were obtained
for DT as predicted from the crystallinity of the material.
13C NMR (75 MHz, CDCl3, ppm):  = 146.06 (s; C1,2), 126.68 (d; C3,6),
22.69 (d; C4,5), 29.78 (t; C7,8).
4-Bromobenzocyclobutene 9:
Method 1: Benzocyclobutene 8 (9.00 g, 86.412 mmol, 1 eq) and iodine
(250 mg) are dissolved in glacial acetic acid (80 mL). A solution of
bromine (15.15 g, 94.801 mmol, 1.1 eq) in glacial acetic acid (5 mL) is
added dropwise at room temperature. The mixture is stirred for 3 d at
room temperature under exclusion of light. The brownish-red solution is
poured into water. n-Hexane is added and the layers are separated. The
aqueous layer is extracted with n-hexane (3 x 150 mL). The combined
organic layers are washed consecutively with aqueous KOH (2M; to
remove last traces of bromine and iodine) and water until the aqueous
layer is neutralized. The organic layer is dried (MgSO4), the solvent is
removed in vacuum, and the slightly yellowish oil is purified by column
chromatography (silica gel, n-hexane). The colorless oil is distilled in
vacuum (10 mbar, b.p. 87°C). The yield is 7.4 g (48%).
Method 2: Benzocyclobutene
8
(100 g, 960 mmol) and sodium
dodecylsulfate (1 g, 3.5 mmol) are dispersed in water (600 mL). Under
rigorous exclusion of light, bromine (49.4 mL, 960 mmol, 1 eq.) is added
slowly at 0°C with stirring. After completed addition stirring is continued
for a further 1 h. Aqueous NaOH is added until pH = 7. The organic layer
is separated, the aqueous one extracted with n-hexane, the combined
organic layers are dried (MgSO4) and filtered. After removal of solvent,
the remaining oil is distilled under reduced pressure. The yield is 117 g
(67%) of a colorless liquid.
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1H NMR (300 MHz, CDCl3, ppm):  = 7.35 (d, 1H, H5), 7.21 (s, 1H, H3),
(2d; C6,7,10,11), 127.58 (d; C1), 125.78 (d; C8,9), 125.53, 125.49 (2d; C3,4),
119.94 (s; C2), 36.48, 36.87 (2t; C5,12);
6.94 (d, 1H, H6), 3.23 (m, 4H, H7,8);
13C NMR (75 MHz, CDCl3, ppm):  = 147.44 (s; C1), 144.34 (s; C2),
130.02 (d; C3), 126.07 (d; C6), 124.48 (d; C5), 120.63 (s; C4), 29.61 (t; C7),
29.33 (t; C8).
Mass spectrometry (EI): m/z = 308 (100%, M+; calculated for C18H13Br:
309).
Benzocyclobuten-4-yl boronic acid: 4-Bromobenzocyclobutene 9 (5 g,
27.3 mmol) is dissolved in dry diethyl ether (100 mL). At 20°C a solution
of n-BuLi (1.6 M in n-hexane, 17 mL, 27.2 mmol, 1 eq.) is added
dropwise. After completed addition, the amber-colored solution is stirred
for further 1 h, transferred into a dropping funnel and diluted by dry
diethyl ether (100 mL). It is dropped slowly to a stirred and cooled
(–50°C) solution of B(OCH3)3 (100 mL, 897 mmol, 33 eq.) in dry diethyl
ether (100 mL). After completed addition, the mixture is allowed to warm
up slowly to room temperature and stirred for 12 h. The solvent and the
excess of B(OCH3)3 are removed almost completely under reduced
pressure. Aq. HCl (1 N, 50 mL) is added to hydrolyze the ester. The
formed colorless solid is isolated by filtration, washed excessively with
water, and dried under reduced pressure (P4O10, 3 d, room temperature).
The free boronic acid is obtained in 68% yield (2.75 g);
1,4-Epoxynaphthalene 5:
1-Bromo-2-chlorobenzene
10
(271.8 g,
1.4 mol) is dissolved in furan 12 (1000 mL), and the solution is cooled
down to –80°C. A solution of n-BuLi in n-hexane (2.5 M, 560 mL, 1.4 mol,
1 eq.) is added slowly with stirring. Stirring at –70°C is continued after
completed addition for a further 2 h. At room temperature, the excess of
furan is removed by distillation under reduced pressure. The residue is
washed with water (3 x 250 mL) and recrystallized from n-hexane. Pure
product 5 is obtained as a colorless solid in 75% yield (151 g).
1H NMR (300 MHz, CD2Cl2, ppm):  = 7.17 (dd, 2H, H5,8), 6.94 (d, 2H,
H2,3), 6.88 (dd, 2H, H6,7), 5.63 (d, 2-H, H1,4);
13C NMR (75 MHz, CD2Cl2, ppm):  = 148.96 (s; C9,10), 142.97 (d; C2,3),
124.94 (d; C5,8), 120.21 (d; C6,7), 82.27 (d; C1,4).
1H NMR (300 MHz, DMSO-d6, ppm):  = 7.81 (s, 2H, OH), 7.74 (d, 1H,
H6), 7.58 (s, 1H, H3), 7.07 (d, 1H, H5), 3.12 (m, 4H, H7,8).
2-Bromo-6,11-epoxy-5,12-dihydrotetracene 13:
Method 1:
A
solution of 4-bromobenzocyclobutene
9
(5.50 g,
Benzocyclobuten-4-yl-pinakolborolane 14: 4-Bromobenzocyclobutene 9
(15 g, 82 mmol] is dissolved in dry diethyl ether (200 mL) and cooled
down to –78°C. n-BuLi (1.6 M in n-hexane, 52 mL, 83 mmol) is added
slowly. The color of the solution turns to orange, and a colorless solid
appears. After completed addition, the mixture is stirred for further 1 h at
27.316 mmol, 1 eq) and 1,4- epoxynaphthalene 5 (4.136 g, 28.682 mmol,
1.05 eq) in dry toluene (60 mL) is stirred and heated in an autoclave to
220°C for 20 h. Methanol (100 mL) is added at room temperature. The
formed solid is separated by filtration and dried under reduced pressure.
It is a mixture of endo and exo product 13. The yield is 6.85 g (78%), and
the purity is sufficient for the subsequent conversion.
that temperature and then
a solution of isopropoxy-pinakolborolan
(17 mL, 83 mmol, 1 eq.) in dry THF (6 mL) is added. The solid re-
dissolves, and the mixture is allowed to war up to room temperature
slowly whilst stirring is continued for a further 12 h. The solvent is
removed under reduced pressure, water (50 mL) is added, and the
resulting mixture is extracted three times using methylene chloride
(3 x 100 mL). The combined organic layers are dried (MgSO4), the
solvent is removed under reduced pressure, and the residue is purified
by column chromatography (n-hexane/methylene chloride, 1:1; silica gel).
After removal of the solvent and drying under reduced pressure, pure
product is obtained in 78% yield (14.7 g) as bright yellow oil.
Method 2: 4-Bromobenzocyclobutene 9 (5.0 g, 27.3 mmol, 1 eq.), 1,4-
epoxynaphthalene 5 (4.1 g, 28.7 mmol, 1.1 eq.) and Na2CO3 (500 mg)
are dissolved in dodecane (60 mL). The solution is stirred at 220°C for
12 h. At room temperature, the formed solid is isolated by filtration,
dissolved in dichloromethane (100 mL) and filtered. After removal of the
solvent, the solid is crystallized from toluene and washed with methanol.
The product is obtained as a colorless solid in 92% yield (8.2 g).
1H NMR (300 MHz, CD2Cl2, ppm):  = 7.29, 7.16, 7.02 (3m; 7H, H1,3,4,7-10),
5.11 (m, 2H, H6,11), 3.05, 2.73, 2.01 (3m; 6H, H5,12,15,16);
1H NMR (300 MHz, CD2Cl2, ppm):  = 7.67 (d, 1H, H6), 7.48 (s, 1H, H3),
7.08 (d, 1H, H5), 3.21 (m, 4H, H7,8), 1.34 (s; 12H, CH3).
13C NMR (75 MHz, CD2Cl2, ppm):  = 145.53, 141.65 (2s¸C13,14), 137.97
(s; C17,18), 129.91, 129.17, 128.58 (3d; C1,3,4), 126.80, 126.05 (2d; C7,10),
119.61 (s; C2), 119.09 (d; C8,9), 84.57, 84.61 (2d; C6,11), 42.80, 42.68 (2d;
C15,16), 32.69, 33.01 (2t; C5,12).
4-(Benzocyclobuten-4-yl)benzocyclobutene 4 (via method C): A solution
of n-BuLi in n-hexane (85 mL, 1.6 M, 136 mmol, 0.5 eq.) is added slowly
to a cooled (–80°C) solution of 4-bromobenzocyclobutene 9 (50 g,
273 mmol) in dry tetrahydrofuran (500 mL). The mixture is stirred for 3 h
at –80°C and then is allowed to warm up to room temperature. The
solvent is removed under reduced pressure and the oily residue is
purified by column chromatography (silica gel, n-hexane). Pure product 4
is obtained as colorless oil in 79% yield (22.1 g) which crystallizes slowly.
2-Bromo-5,12-dihydrotetracene 2: At room temperature conc. aq. HCl
(45 mL) is added dropwise to a solution of 13 (13.0 g, 0.04 mmol) in
acetic anhydride (250 mL). The mixture is refluxed for 12 h. At room
temperature, it is poured slowly into ice water. The yellow precipitate is
isolated by filtration, washed successively with water and methanol, dried
in vacuum, and recrystallized from toluene. The yellow crystalline solid 2
is obtained in 86% yield (10.5 g).
1H NMR (300 MHz, CDCl3, ppm):  = 7.49 (d, 2H, H5), 7.25 (s, 2H, H3),
7.11 (d, 2H, H6), 3.25 (m, 8H, H7,8).
1H NMR (300 MHz, CD2Cl2, ppm):  = 7.80 (d, 2H, H7,10), 7.75 (s ,2H,
H6,11), 7.50 (s, 1H, H1), 7.44 (m, 2H, H8,9), 7.35 (d, 1H; H3), 7.22 (d, 1H,
H4), 4.04, 4.01 (2s, 4H, H5,12);
5,12-Dihydrotetracen-2-yl
pinakolborane 3:
2-Bromo-5,12-dihydro-
tetracene (11.72 g, 37.9 mmol), KOAc (5.7 g, 56.8 mmol),
2
bis(pinacolato)diborone (10.547 g, 41.5 mmol) and PdCl2(dppf) (0.82 g,
1.0 mmol, 3 mol%) in dry DMSO (600 mL) are stirred and heated (80°C)
for 12 h under an atmosphere of nitrogen. CH2Cl2 (30 ml) and water
(30 ml) are added, the layers are separated, the organic layer is washed
with brine (20 ml), dried (MgSO4), filtered and concentrated down under
13C NMR (75 MHz, CD2Cl2, ppm):  = 139.97 (s; C14), 136.72 (s; C13),
135.51, 135.29 (2s; C15,16), 132.77, 130.35 (2s; C17,18), 129.41, 129.15
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reduced pressure. Crystallization from n-hexane / methylene chloride
(1:1) results in pure product 3. The yield is 87% (11.75 g). Exposure to
oxygen, especially in combination with daylight, must be avoided during
conversion and work-up in order to minimize formation of side-products
like tetracenequinones;
Mass spectrometry (EI): m/z = 534 (100%, M+; calculated for C42H30:
534.2).
1,4-Bis(tetracen-2-yl)benzene TPT:
1,4-Bis(3,10-dihydrotetracenyl)
benzene 17 (190 mg, 0.355 mmol, 1 eq.) and DDQ (200 mg, 0.9 mmol,
2.5 eq.) in dry xylene (50 mL) are heated to 135°C for 48 h. During that
time, the color of the reaction mixture changes from red to orange. After
cooling down the reaction mixture to room temperature, acetone
(100 mL) is added. The orange precipitate is isolated by filtration, washed
with acetone, dried in high vacuum at room temperature, and purified by
gradient sublimation (for procedure and conditions, see DT). The yield is
125 mg (60%).
1H NMR (300 MHz, CD2Cl2, ppm):  = 7.86 - 7.74 (m, 5H, H1,6,7,10,11), 7.61
(d, 1H, H3), 7.43 (m, 2H, H8,9), 7.36 (d, 1H, H4), 4.10 (s, 4H, H5,12), 1.32 (s,
12H, CH3).
2-(5,12-Dihydrotetracen-2-yl)-5,12-dihydrotetracene 1:
Under
an
atmosphere of nitrogen, [Ni(COD)2] (704 mg, 2.56 mmol), 1,5-
cyclooctadiene (COD, 0.31 mL, 2.5 mmol) and 2,2’-bipyridine (400 mg,
2.56 mmol) are dissolved in dry DMF (10 mL). Dry toluene (10 mL) is
added, and the mixture is heated to 80°C for 30 min. A solution of 2-
Mass spectrometry (EI): m/z = 530.4 (100%, M+; calculated for C42H30:
530.2).
bromo-5,12-dihydrotetracene
2 (1.376 g, 4.4 mmol) in dry toluene
(30 mL) is added to the dark violet solution. Stirring at 80°C is continued
for 3 d. The solid is isolated by filtration, dispersed in aqueous
ethylenediamine tetraacetate (EDTA, 150 mL, 0.1 M) and stirred for 6 h
at room temperature. The slightly yellowish solid is isolated by filtration,
washed with methanol, and recrystallized from 1,2-dichlorbenzene. The
yield is 80% (0.83 g).
2,5-Bis(trimethylstannyl)thiophene 19: A solution of thiophene 18 (1.01 g,
12 mmol), tetramethylethylene diamine (TMEDA, 4 mL, 26.8 mmol), dry
hexane (10 mL) and dry THF (20 mL) is cooled down to –50°C. n-BuLi
(1.6 M in n-hexane, 17.56 mL, 28.1 mmol) is added slowly. After
completed addition, the mixture is allowed to warm up to room
temperature, and it is refluxed for 45 min. At –78°C, a solution of
trimethyltin chloride in n-hexane (1 M, 28.1 mL, 28.1 mmol) is added. The
cooling bath is removed and the reaction mixture is stirred for 2 h at room
temperature. Aqueous NH4Cl (2 M, 50 mL) is added, and the mixture is
extracted with diethyl ether (3 x 100 mL). The combined organic layers
are washed with water (2 x 50 mL), dried (MgSO4), and the solvent is
removed in vacuum. The oily residue is dissolved in a small volume of
ethanol and crystallized at –20°C. Colorless crystals of 19 are obtained in
70% yield (3.4 g).
1H NMR (300 MHz, CD2Cl2, ppm):  = 7.68 (m, 8H,H6,7,10,11), 7.53 (s, 2H,
H1), 7.36 (d, 2H,H3), 7.28(d, 2H, H4), 7.26 (m, 4H, H8,9), 4.01, 4.04 (2s,
8H, H5,12);
Mass spectrometry (EI): m/z = 458.2 (100%, M+; calculated for C36H26:
458.2).
1H NMR (300 MHz, CDCl3, ppm):  = 7.37 (s, 2H, H2,3), 0.37 (s, 18H,
CH3).
2-(Tetracen-2-yl)tetracene DT: Under an atmosphere of nitrogen,
a
solution of 1 (0.388 g, 0.85 mmol), DDQ (0.425 g, 1.9 mmol) and o-
xylene (40 mL) is heated to 135°Cfor 2 d. At room temperature, acetone
(100 mL) is added. The formed solid is isolated by filtration and dried in
high vacuum. Purification of DT is done by gradient sublimation
(T = 390°C, p = 1 mbar): the raw material is placed in a tungsten
cruciblein a 50 cm glass tube and put in an oven having a temperature
gradient from Tmax = 390°C to Tmin = room temperature. The crucible is
located in the hottest zone of the oven. Sublimation proceeds in a mild
argon/H2 stream at a pressure of 0.1 mbar. The materials contained in
the raw product deposit downwards at different places of the tube. For
example, the mono-dehydrated intermediate is orange, and the
unchanged starting material 1 appears as a yellow solid. The red fraction
is the desired product. It is obtained in 75% yield (0.29 g).
2,5-Bis(5,12-dihydrotetracen-2-yl)thiophene 20: A solution of 2-bromo-
5,12-dihydrotetracene
2
(0.60 g,
1.94 mmol,
2 eq.),
2,5-
bis(trimethylstannyl)thiophene 19 (0.3975 g, 0.97 mmol, 1 eq.) and
[Pd(PPh3)4] (60 mg, 2 mol%) in dry xylene (60 mL) is stirred and refluxed
for 2 d. The formed solid is isolated by filtration, washed with aqueous
HCl (2N, 3 x 50 mL), dispersed in methylene chloride, filtered over a pad
of silica gel, and the solvent is removed. The solid is isolated and washed
with THF and methanol. Recrystallization from o-dichlorobenzene
(500 mL) results in 366 mg (70%) of a bronze-colored crystalline powder.
Mass spectrometry (EI): m/z = 540.0 (100%, M+; calculated for C40H28S:
540.2).
Mass spectrometry (EI): m/e = 454.2 (100%, M+; calculated for C36H22:
454.2).
2,5-Bis(tetracen-2-yl)-thiophene TTT:
A
solution of 2,5-bis(5,12-
dihydrotetracen-2-yl)thiophene 20 (300 mg, 0.555 mmol, 1 eq.) and DDQ
(300 mg, 1.4 mmol, 2.5 eq.) in dry xylene (50 mL) is stirred at 125°C for
3 d. Acetone (30 mL) is added at room temperature. The formed solid is
removed by filtration, washed with acetone, dried under reduced
pressure at room temperature, and purified by gradient sublimation (for
procedure and conditions, see DT). The yield is 0.15 g (50%) of orange-
red solid.
1,4-Bis(5,12-dihydrotetracen-2-yl)benzene 17:
A mixture of 2-bromo-
5,12-dihydrotetracene 2 (462 mg, 1.49 mmol, 2.1 eq.), 1,3-propanediol
diester of benzene-1,4-diboronic acid 16 (175 mg, 0.7117 mmol, 1.0 eq.),
K3PO4 (549 mg, 2.587 mmol, 4 eq.), Pd[P(Ph3)4] (35 mg; 2 mol%), xylene
(30 mL) and water (30 mL) is refluxed with vigorous stirring for 2 d. At
room temperature, the formed precipitate is isolated by filtration and re-
crystallized from 1,2-dichlorobenzene. The yield is 530 mg (72%) of a
yellow solid.
Mass spectrometry (EI): m/z = 536.0 (100%, M+; calculated for C40H24S:
536.16).
1H NMR (300 MHz, CD2Cl2, ppm):  = 7.65 (m, 8H, H6,7,10,11), 7.40 (s, 2H,
H1), 7.26 (m, 4H, H8,9), 7.21(d, 2H, H3), 7.13 (d, 2H, H4), 3.87 (s, 8H,
H5,12);
OFET fabrication: Bottom-contact, bottom-gate OFET devices were
fabricated using heavily doped n-type silicon substrates (3-5  cm
resistivity) covered by a high quality thermally grown oxide with a
thickness of about 230 nm, acting as gate electrode and gate oxide,
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10.1002/chem.201702382
Chemistry - A European Journal
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[1]
a) H. Klauk, M. Halik, U. Zschieschang, G. n. Schmid, W. Radlik, W. Weber,
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oxide surface before a 50 nm gold layer was deposited. The gold source
and drain electrodes were structured photolithographically. They are
configured as interdigitated fingers with a channel length of L = 7 m and
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electrodes is displayed in the Figure 11. If indicated, the SiO2 surface
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[2]
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V
W = 100 x
2 mm
DS
Drain
Source
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Au
Au
L = 7 µm
Drain
SiO2
230 nm
n - Si
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Figure 11. Transistor geometry used to test transport properties of the
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NMR spectrometer operating at 300 MHz (1H NMR) and 75 MHz (13
C
NMR), and on a Bruker AM 500 NMR spectrometer working at 500 MHz
(1H NMR) and 125 MHz (13C NMR). For those compounds that show
sufficient solubility for NMR characterization the assignment of the
absorptions was done according to the numbering of hydrogen and
carbon atoms as indicated exemplary in Scheme 2. Mass spectra (EI)
were recorded using a FINNIGAN MAT 95 instrument. X-Ray diffraction
(XRD) measurements were carried out with a Seifert (PTS XRD 3003) X-
ray diffractometer system. XRD patterns were obtained using Bragg-
Brentano geometry (-2) with Cu K radiation as X-ray source. The
electrical characterization of OFETs was performed by means of an
HP 4155 A semiconductor parameter analyzer in inert N2 atmosphere.
The on / off-ration was determined as ratio of the drain currents obtained
at Vg=Vd=-40 V and Vg=Vd=0 V.
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Acknowledgements
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The authors would like to thank the German Research
Foundation (Deutsche Forschungsgemeinschaft, DFG, projects
Re 923/9-1 and Re 923/9-2, Schm1523/5-1 and Schm1523/3-4),
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Schwerpunktprogramm 1121
(Organische
Feldeffekt-
Transistoren), for financial support of this work, and Christian
Quick for support in the synthesis of some of the presented
materials. The patent[21] resulting from this work is a property of
the Dritte Patentportfolio Beteiligungsgesellschaft mbH & Co.
KG.
[21] M. Ahles, M. Rehahn, M. Roth, R. Schmechel, H. von Seggern, patent
DE102005029574 A1, 2005.
Keywords: Tetracene • Pentacene • Transition-metal catalyzed
coupling reaction • Organic field-effect transistor • Charge-carrier
mobility
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10.1002/chem.201702382
Chemistry - A European Journal
FULL PAPER
Entry for the Table of Contents
FULL PAPER
Merging two tetracenes together new
ridged-rod tetracene derivatives with
different aspect ratio were realized.
The coupling between the two
Michael Roth, Marcus Ahles, Christian
Gawrisch, Thorsten Schwalm, Roland
Schmechel, Christian Melzer*, Heinz
von Seggern*, and Matthias Rehahn*
tetracenes reduces the bandgap and
ionization potential. Good crystallinity
on SiO2 substrates is observed. Thus,
decent performance in organic field-
effect transistors was found but an
increased aspect ratio above the one
of directly linked tetracenes did neither
result in an improved crystallinity nor
transport property.
Page No. – Page No.
Rod-like Tetracene Derivatives
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