Bis(methylthio)tetracenes: Synthesis, crystal-packing structures, and OFET properties

Article abstract of DOI:10.1021/jo200696a

5,12-Bis(methylthio)tetracene (2) and 5,11-bis(methylthio)tetracene (3) were synthesized. DFT calculations indicate that the HOMO and LUMO energy levels of 2 and 3 are lowered by 0.13-0.24 eV and their HOMO-LUMO energy gaps are reduced by 0.1 eV relative to those of tetracene. X-ray crystallographic data revealed that 2 is arranged as a result of a 1-D slipped-cofacial π-stacking with S-S and S-π interactions, similar to the packing arrangement of 6,13-bis(methylthio)pentacene (1), whereas 3 exhibits a herringbone packing arrangement without S-S interactions. The OFET devices fabricated using spin-coated films of soluble 1 and 2, with a bottom-contact device configuration, exhibited hole mobilities as high as 1.3 × 10^-2 and 4.0 × 10^-2 cm² V^-1 s^-1 with current on/off ratios of over 10⁵ and 10⁴, respectively.

Full text of DOI:10.1021/jo200696a

ARTICLE
pubs.acs.org/joc
Bis(methylthio)tetracenes: Synthesis, Crystal-Packing Structures, and
OFET Properties
Takakazu Kimoto,† Kenro Tanaka,† Masatoshi Kawahata,‡ Kentaro Yamaguchi,‡ Saika Otsubo,§
Yoshimasa Sakai,§ Yuuki Ono,§ Akira Ohno,§ and Kenji Kobayashi*,†
†Department of Chemistry, Faculty of Science, Shizuoka University, 836 Ohya, Suruga-ku, Shizuoka 422-8529, Japan,
‡Faculty of Pharmaceutical Sciences at Kagawa Campus, Tokushima Bunri University, Shido, Sanuki, Kagawa 769-2193, Japan, and
§Mitsubishi Chemical Group Science and Technology Research Center, Inc., 1000 Kamoshida, Aoba-ku, Yokohama 227-8502, Japan
S Supporting Information
b
ABSTRACT: 5,12-Bis(methylthio)tetracene (2) and 5,11-bis(methylthio)-
tetracene (3) were synthesized. DFT calculations indicate that the
HOMO and LUMO energy levels of 2 and 3 are lowered by 0.13ꢀ0.24
eV and their HOMOꢀLUMO energy gaps are reduced by 0.1 eV relative
to those of tetracene. X-ray crystallographic data revealed that 2 is
arranged as a result of a 1-D slipped-cofacial π-stacking with SꢀS and
Sꢀπ interactions, similar to the packing arrangement of 6,13-bis-
(methylthio)pentacene (1), whereas 3 exhibits a herringbone packing
arrangement without SꢀS interactions. The OFET devices fabricated using spin-coated films of soluble 1 and 2, with a bottom-
contact device configuration, exhibited hole mobilities as high as 1.3 ꢁ 10ꢀ2 and 4.0 ꢁ 10ꢀ2 cm2 Vꢀ1 sꢀ1 with current on/off ratios
of over 105 and 104, respectively.
’ INTRODUCTION
cofacial π-stacking leads to significant overlap of areas of high
HOMO density with extrema in the evolution of the transfer
integrals upon sliding.1d,3a,6f
Acenes, which are made up of linearly annelated benzene
rings, and their derivatives are promising candidates as p-type
semiconductors for use in organic field-effect transistors
(OFETs).1 Devices based on highly pure single crystals of
pentacene and rubrene (5,6,11,12-tetraphenyltetracene) have
demonstrated very high field-effect hole mobilities.2,3 Vacuum-
deposited thin film transistors of pentacene also exhibit values up
to 3 cm2 Vꢀ1 sꢀ1.4 However, pentacene is unstable in air and/or
light and is subject to oxidation and photooxidation.1 Pentacene
is also almost insoluble in common organic solvents, which
makes the use of pentacene difficult as a solution-processable
semiconductor.1 Acenes tend to be susceptible to herringbone
packing arrangements with minimal π-stacking;5 however, the
herringbone-packed pentacene still holds the highest hole mo-
bility among organic semiconductors.1,2 In general, good elec-
tronic performance requires strong electronic coupling between
adjacent molecules in the solid state.6 Realization of a 1-D or 2-D
cofacial π-stacked packing arrangement of acenes might achieve
greater charge-carrier transport efficiency because this molecular
ordering would facilitate good overlap of the intermolecular
π-orbitals.1cꢀe,6bꢀ6f Rubrene single crystal presents a 2-D herring-
bone packing in the ab plane, but a 1-D largely slipped-cofacial
π-stacking to form tetracene columns along the a axis, wherein
tetracene rings are slipped relative to each other along the long
molecular axis by 6.13 Å, but not slipped along the short
molecular axis.3a,6f Consequently, mobility in rubrene single
crystal is highest along the molecular π-stacking axis and lowest
perpendicular to this axis (edge-to-face direction) and also the
Thus, improvements in solubility, photooxidative resistance,
and control of molecular orientation and arrangement of acenes
are very important properties for the development of OFETs.1,6,7
To resolve these problems associated with acenes, Anthony and
co-workers have reported the introduction of bulky trialkylsily-
lethynyl groups into acenes at appropriate positions.1,8 It is
believed that herringbone packing arrangements of polycyclic
aromatic hydrocarbons could be prevented by increasing the
ratio of π-faced carbon atoms to π-edged hydrogen atoms.5a,b
Recently, we have demonstrated that SꢀS and Sꢀπ interactions
assist a cofacial π-stacking of 9,10-bis(methylthio)anthracene
(4)9 and 6,13-bis(methylthio)pentacene (1).10,11 It is noted that
the introduction of a small methylthio group into pentacene at
the 6,13-positions changes the packing structure from a herring-
bone to a cofacial π-stacking motif10a and also produces con-
siderable improvements in solubility as well as photooxidative
resistance compared with pentacene.10a,12 Miller and co-workers
have reported that arylthio groups are effective to stabilize more
unstable larger acenes.12b,c The present work is mainly con-
cerned with (1) if SꢀS and Sꢀπ interactions generally induce
cofacial π-stacking of bis(methylthio)acenes and (2) the OFET
properties of bis(methylthio)acenes as a solution-processable
semiconductor. We report here on the synthesis, chemical
Received: April 3, 2011
Published: May 16, 2011
r
2011 American Chemical Society
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Chart 1. Structures of 6,13-Bis(methylthio)pentacene (1),
5,12-Bis(methylthio)tetracene (2), 5,11-Bis(methylthio)-
tetracene (3), and 9,10-Bis(methylthio)anthracene (4)
Scheme 2. Photochemical Reactions of (a) 2, (b) 3, and (c) 1
in Both Room Light and Air
Scheme 1. Synthesis of (a) 2 and (b) 3
Figure 1. UVꢀvis spectra of 1 (green line), 2 (red line), 3 (blue line),
and tetracene (black line) in CH2Cl2.
properties, and crystal-packing structures of 5,12-bis(methylthio)-
tetracene (2) and 5,11-bis(methylthio)tetracene (3) (Chart 1)13ꢀ15
and also describe the OFET properties of spin-coated films of 1ꢀ3
with a bottom-contact device configuration.
Solubility and Stability. Bis(methylthio)tetracenes 2 and 3
are soluble in common organic solvents such as CHCl3, THF,
and toluene (solubility = >20 mM in CD2Cl2). Compounds 2
and 3 are stable in air in the absence of light both in solution and
in the solid state. However, in both air and room light
(fluorescent light), 2 (2 mM) in CD2Cl2 changed to a mixture
of endoperoxide 9 and unknown intermediate after 1.5 h by
photooxidation (the half-life of 2 = ca. 30 min), which predomi-
nantly decomposed to 5 and dimethyl disulfide after 10 h of
photoirradiation (Scheme 2a and Figure S5, Supporting
Information).17 In both air and room light, 3 (2 mM) in CD2Cl2
almost disappeared after 1 h (the half-life of 3 = ca. 15 min) and
changed to a mixture of endoperoxide 10 and semiquinone 11
(Scheme 2b and Figure S6, Supporting Information).17 After 3 h
of photoirradiation, 10 was fully converted to 11. Surprisingly,
the pentacene derivative 1 was more resistant to photooxidation
than 2 and 3,12a although it is not easy to elucidate the reason at
this stage.18 In both air and room light, 1 (saturating concentra-
tion = 0.8 mM) in CD2Cl2 showed a half-life of ca. 3 h and
predominantly decomposed to 6,13-pentacenequinone (13) via
’ RESULTS AND DISCUSSION
Synthesis. 5,12-Bis(methylthio)tetracene (2) was synthe-
sized following a three-step procedure similar to that for the
synthesis of 6,13-bis(methylthio)pentacene (1).10a The reduc-
tion of 5,12-tetracenequinone (5) was carried out with NaBH4,
followed by the ZnI2-mediated reaction of trans-5,12-dihydroxy-
5,12-dihydrotetracene (6) with CH3SH,10a,16 and finally the
dehydrogenative aromatization of the resulting trans-5,12-bis-
(methylthio)-5,12-dihydrotetracene (7) with p-chloranil to give
2 (Scheme 1a).17 5,11-Bis(methylthio)tetracene (3) was synthe-
sized by the dilithiation of a mixture of 5,11- and 5,12-dibromo-
tetracenes (8 and 80) with tert-butyllithium,15c followed by the
reaction of 5,11- and 5,12-dilithiotetracenes with dimethyl
disulfide. Finally, the resulting mixture of 2 and 3 was repeatedly
recrystallized from toluene and then CHCl3ꢀhexane to give pure
3 (Scheme 1b).17
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Table 1. Electrochemical and Optical Properties and HOMOꢀLUMO Data of 1, 2, 3, and Tetracene
compd
1
2
3
tetracene
0.41c
Eoxonset (V)a
Eredonset (V)a
EgEchem (eV)b
λmax (nm)d
0.39
0.56
0.47
ꢀ1.63
2.02
ꢀ1.88
2.44
ꢀ1.89
2.36
ꢀ2.05c
2.46c
613, 566, 527
4.11, 3.95, 3.59
647
506, 473, 444
4.01, 3.95, 3.62
528
503, 471, 442
4.00, 3.95, 3.65
528
474, 444, 419
3.72, 3.70, 3.41
486
log ε
λcutoff (nm)e
EgOptical (eV)f
ELUMODFT (eV)g
EHOMODFT (eV)g
EgDFT (eV)h
1.92
2.35
2.35
2.55
ꢀ2.96
ꢀ2.69
ꢀ2.68
ꢀ2.45
ꢀ5.05
ꢀ5.33
ꢀ5.33
ꢀ5.20
2.09
2.64
2.65
2.75
a Calculated from the onset of cyclic voltammograms measured in 0.1 M solution of n-Bu4NClO4 as supporting electrolyte in CH2Cl2, with a Pt electrode
and ferrocene as the internal standard. b Electrochemical HOMOꢀLUMO gap. c See ref 15d performed in 0.1 M solution of n-Bu4NPF6 in CH2Cl2.
d Low energy absorption maxima measured in CH2Cl2. e Wavelength at which the transmittance is 95%. f Optical HOMOꢀLUMO gap based on λcutoff
g Calculated at B3LYP/6-311þG(d,p)//B3LYP/6-31G(d). h Calculated HOMOꢀLUMO gap.
.
Figure 2. HOMOꢀLUMO diagrams of (a) 1, (b) 2, (c) 3, and (d)
tetracene at the B3LYP/6-311þG(d,p)//B3LYP/6-31G(d) level.
endoperoxide 12 after 6 h (Scheme 2c and Figure S8, Supporting
Information).17
The thermogravimetric differential thermal analyzer (TG-
DTA) analysis showed that the melting points of 2 and 3 are
184 and 256 °C, respectively, and at higher temperatures than
melting points 2 and 3 begin to decompose with sublimation
(Figure S10, Supporting Information).17 Compound 1 thermally
decomposed at 297 °C (Figure S10, Supporting Information).17
Thus, the thermal stability increased in the order 2 < 3 < 1.
Optical and Electrochemical Properties and HOMOꢀ
LUMO Calculations. The UVꢀvis spectra and spectral data of
2 and 3 in CH2Cl2 are shown in Figure 1 and Table 1,
respectively, as well as compound 1 and tetracene. The longest
wavelength absorption maxima (λmax) for 2 and 3 appeared at
506 and 503 nm, respectively, which were blue-shifted by 107
and 110 nm, respectively, relative to λmax of 1 because of
shortening of the π-conjugated acene rings. In contrast, λmax
Figure 3. ORTEP views of (a) 2 and (b) 3 (50% probability thermal
ellipsoids).
for 2 and 3 were red-shifted by 32 and 29 nm, respectively,
relative to λmax of tetracene because of the reduced HOMO
ꢀLUMO gaps induced by the methylthio groups.9a,10a,12a The
magnitudes of the molar extinction coefficients of λmax for 2 and
3 are approximately twice that of tetracene. The reduced
HOMOꢀLUMO gaps of tetracene induced by the introduction
of methylthio groups were supported by experimental data
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Figure 5. (a) 3-D packing diagram viewed down the b axis (the same as
Figure 4a), wherein R and S mean the (R)-enantiomer (red color) and
(S)-enantiomer (blue color) of 2 in the crystal. The S1 and S2 show the
sulfur atoms, and dotted lines indicate the SꢀS interactions. One 2-D
network sheet of 2 from (a): (b) top view along the b axis, (c) front view,
and (d) side view.
Figure 4. (a) 3-D packing structure of 2 in the crystal: perspective view
looking down the b axis, wherein black and red dotted lines indicate the
unit cell and the SꢀS interactions, respectively. One cofacial π-stacked
column of 2: (b) top and (c) side views.
obtained with cyclic voltammetry (CV, Figure S11, Supporting
Information)17 and also DFT calculations (B3LYP/6-311þG(d,
p)//B3LYP/6-31G(d) level)12a of 2 and 3 (Table 1). The intro-
duction of methylthio groups into tetracene lowers both the
HOMO and LUMO energy levels by 0.13ꢀ0.24 eV relative to
those of tetracene. This suggests that 2 and 3 are more susceptible
to reduction and less susceptible to oxidation than tetracene. In
particular, the lowering of the LUMO energy levels is noticeable.
DFT calculations of 2 and 3 also showed that neither the HOMO
nor the LUMO are delocalized between the tetracene ring and the
sulfur atoms of the methylthio groups (Figure 2). This result
suggests that the lowering of both the HOMO and LUMO energy
levels, as well as the reduced HOMOꢀLUMO gaps, of 2 and 3
relative to those of tetracene may be attributed to the electron-
withdrawing inductive effect of the methylthio groups12a
rather than an extension of the π-conjugation through the
methylthio groups.
Figure 6. 3-D packing structure of 3 in the crystal viewed down (a) the
a axis and (b) the c axis.
distance of 3.39 Å. The tetracene rings in one column are slipped
relative to each other along the long molecular axis by 3.54 Å and
along the short molecular axis by 1.07 Å (Figure 4b,c). These
slipped distances are slightly shorter than those of the pentacene
derivative 1 (3.64 and 1.19 Å).10a The presence of such molecular
ordering would permit good overlap of the intermolecular
π-orbitals of the tetracene rings of 2.6 There is no SꢀS interaction
(5.078 Å) in the tetracene column. Instead, there are weak
intermolecular Sꢀπ interactions between the sulfur atoms and
the neighboring tetracene rings in the tetracene column, with
interatomic distances of S1 C30 = 3.566 Å, S1 C40 = 3.546 Å,
X-ray Crystal Structures. Single crystals of 2 and 3 suitable for
X-ray diffraction analysis were obtained by slow diffusion of
hexane into a solution of 2 in THF or by slow cooling to room
temperature of a hot solution of 3 in CHCl3ꢀhexane. Figure 3
shows the ORTEP views of 2 and 3, wherein 2 does not have a
center of symmetry and 3 has a center of symmetry.
The X-ray crystal packing structure of 2 reveals that 2 is
arranged by cofacial π-stacking along the b axis with SꢀS and
Sꢀπ interactions (Figures 4 and 5).9a,10a The tetracene ring of 2
forms the tetracene column through a slipped-cofacial π-stacking
motif along the b axis with a face-to-face tetraceneꢀtetracene
3 3 3
3 3 3
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Figure 7. IꢀV characteristics of bottom-contact FET devices using spin-coated 1 and 2 as the semiconductor layer: output curves at different gate
voltages for (a) 1 and (b) 2; transfer curve in saturated regime at constant sourceꢀdrain voltage of ꢀ40 V and square root of the absolute value of the
drain current as a function of gate voltage for (c) 1 and (d) 2.
S20
C10 = 3.519 Å, and S20
C11 = 3.532 Å.19 They are
Table 2. FET Characteristics of Spin-Coated Bis-
(methylthio)acene-Based Devices Fabricated on Si/SiO2
Substratea and IP Datab
3 3 3
3 3 3
also somewhat shorter than those determined in 1, which are
g3.610 Å for all of these interatomic distances. The SꢀS
interactions exist between the neighboring tetracene columns
compd
μFET (cm2 Vꢀ1 sꢀ1
)
Vth (V)
Ion/Ioff
IP (eV)
in 2, given the interatomic distances of S1 S10 = 3.370 and
3 3 3
S2 S20 = 3.733 Å (Figures 4a and 5).19 Thus, self-assembly of
1
2
3
1.3 ꢁ 10ꢀ2
4.0 ꢁ 10ꢀ2
NAc
ꢀ5.8
ꢀ1.7
>1 ꢁ 105
>1 ꢁ 104
5.3
5.8
3 3 3
bis(methylthio)acenes into 1-D slipped-cofacial π-stacked col-
umns by the assistance of SꢀS and Sꢀπ interactions is proved to
be a general phenomenon.9a,10a The intermolecular S
S
a Bottom-contact configuration. b ITO/glass substrate. c Not available
due to no transistor action because of nonuniform films.
3 3 3
distance of bis(methylthio)acenes decreases in the following
order: pentacene 1 (4.297 Å)10a > anthracene 4 (3.638 Å)9a
g
tetracene 2 (3.370 and 3.733 Å). These values are still in the
range of nꢀσ* and van der Waals interactions between sulfur
atoms.11,20
5a; see also Figure S4, Supporting Information).17 The 3-D
packing structure of 2 is similar to what is called a γ-motif.5a,b,10a
In marked contrast to 2, the regioisomer 3 with a center of
symmetry showed a typical herringbone packing arrangement
(Figures 3b and 6). There is no SꢀS interaction between
molecules of 3, wherein the interatomic distance between the
two closest sulfur atoms is 4.391 Å. However, there are weak
intermolecular Sꢀπ interactions, given the interatomic distances
of S C20 = 3.631 and S C30 = 3.422 Å. Thus, the substitu-
As the molecular symmetry of 2 (Figure 3a) is different from
that of 1 and 4, which both possess a center of symmetry, the 2-D
packing arrangement of 1-D cofacial π-stacked columns in 2 is
somewhat different from those in 1 and 4 (Figure 5),9a,10a
although they are all similar in the sense that acene columns
formed by slipped-cofacial π-stacking and Sꢀπ interactions are
linked by SꢀS interactions to self-assemble into a 2-D network
sheet. The crystal form of 2 is found to be racemic as it exists as a
1:1 mixture of (R)- and (S)-enantiomers. As shown in Figure 5a,
each tetracene column of 2 consists of either an (R)- or (S)-
enantiomer, wherein the (R)- and (S)-columns are shown in red
and blue colors, respectively. As shown in Figure 5bꢀd, the two
3 3 3
3 3 3
tion position of two methylthio groups in bis(methylthio)-
tetracenes is an important factor in controlling the cofacial
π-stacked packing arrangement of the tetracene rings.
OFET Properties. The solubility of bis(methylthio)acenes
allowed us to study the fabrication of OFETs by a solution
process. OFET devices incorporating compounds 1 and 2 with a
bottom-contact configuration were fabricated on a n-doped Si/
SiO2 substrate and goldꢀchromium bilayer source/drain elec-
trodes by a simple spin-coating process from a 1,2,4-trichlor-
obenzene solution of 1 or 2.17,21 The data for FET characteristics
are summarized in Table 2.21 Parts a and c of Figure 7 exhibit
characteristic p-type FET behavior of the device using 1. The
output characteristics display very good saturation behavior. The
adjacent parallel tetracene columns are racemic ((R) (S)) and
3 3 3
are connected by S1 S10 interactions, whereas the two ad-
3 3 3
jacent staggered tetracene columns with a tilt angle of 83.8° are
the same enantiomer ((R) (R) or (S) (S)) and are con-
3 3 3
3 3 3
nected by S2 S20 interactions. Even for the 3-D packing
3 3 3
structure formed by self-assembly of the 2-D network sheets of
2, there is no herringbone packing arrangement (Figures 4a and
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Sꢀπ interactions. In contrast, 3 exhibited a herringbone packing
arrangement without SꢀS interactions. We have also demonstrated
that 1 and 2 serve as solution-processable semiconductor materials
for OFETs. The spin-coated films of 1 and 2, which are soluble, with
a bottom-contact device configuration exhibit field-effect hole
mobilities as high as 1.3 ꢁ 10ꢀ2 and 4.0 ꢁ 10ꢀ2 cm2 Vꢀ1 sꢀ1 with
current on/off ratios of over 105 and 104, respectively, whereas 3,
which is also soluble, showed no transistor behavior because of
nonuniform films. Thus, the substitution position of the two
methylthio groups in bis(methylthio)tetracenes is a very important
factor in controlling the cofacial π-stacked packing arrangement of
the tetracene ring, as well as in fabricating OFET devices by a
solution process.
Studies on the effect of the introduction of multiple methylthio
groups in other polycyclic aromatic hydrocarbons to control their
crystal packing structures and OFET properties are currently in
progress.
’ EXPERIMENTAL SECTION
Figure 8. AFM images of spin-coated films of (a) 1 and (b) 2 on SiO2.
transfer characteristics show a turn-on voltage of þ10 V and a
General Methods. CH2Cl2 and Et2O were distilled from CaH2
and sodiumꢀbenzophenone ketyl, respectively, under an argon atmo-
sphere. The other solvents and all commercially available reagents were
used without any purification. 1H and 13C NMR spectra were recorded
at 400 and 100 MHz, respectively.
threshold voltage (Vth) of ꢀ5.8 V. The carrier mobility (μFET
)
extracted from the saturated regime is as high as 1.3 ꢁ 10ꢀ2 cm2
V
ꢀ1 sꢀ1 with a current on/off ratio (Ion/Ioff) of over 105. The device
using 2 demonstrated μFET = 4.0 ꢁ 10ꢀ2 cm2 Vꢀ1 sꢀ1 with an
Ion/Ioff > 1 ꢁ 104 and Vth = ꢀ1.7 V (Figure 7b,d). It is noted that
both of the devices using compounds 1 and 2 exhibit relatively low
Vth values, even though values of μFET and Ion/Ioff are moderate.1
On the other hand, using 3 as a regioisomer of 2 did not give
uniform films, which led to poor coverage and therefore no
transistor behavior was observed. Thus, the substitution position
of the two methylthio groups in bis(methylthio)tetracenes is also an
important factor in device fabrication for OFETs.
Mobilities of spin-coated thin film transistor of 1 and 2
observed here were 1 order of magnitude lower than that of
unsubstituted spin-cast pentacene,22 probably due to roughness
of thin films of 1 and 2. Figure 8 shows the surface morphologies
of the spin-coated films of 1 from 1,2,4-trichlorobenzene and 2
from chloroform on SiO2 observed by atomic force microscopy
(AFM). Both thin film surfaces are homogeneous but rough,
wherein average film thickness (and height of salient region) are
ca. 40ꢀ45 nm (80 nm) for 1 and 35ꢀ40 nm (90 nm) for 2.
The ionization potential (IP) of spin-coated films of 1 and 2
was measured by photoelectron yield spectroscopy in vacuo
(Figure S13, Supporting Information).17 The IPs of 1 and 2 were
determined to be 5.3 and 5.8 eV, respectively (Table 2). This
result indicates that the HOMO energy difference between 1 and
2 in spin-coated films is approximately comparable with that
based on DFT calculations (Table 1).
trans-5,12-Dihydroxy-5,12-dihydrotetracene (6). To a suspension of
5,12-tetracenequinone (5) (7.03 g, 27.2 mmol) in MeOH (350 mL) at
0 °C under Ar was slowly added NaBH4 (4.12 g, 109 mmol). The
reaction mixture was stirred at 0 °C for 0.5 h and at room temperature for
2 h and then quenched with H2O (100 mL) at 0 °C. The resulting
mixture was filtered and washed with H2O and then cold MeOH. The
resulting solid was purified by reprecipitation with THFꢀCHCl3 to give
1
6 as an off-white solid (4.98 g, 70% yield): mp 184 °C; H NMR
(DMSO-d6) δ 8.08 (s, 2H), 7.93 (dd, J = 6.2 and 3.1 Hz, 2H), 7.65 (dd,
J = 5.5 and 3.1 Hz, 2H), 7.46 (dd, J = 6.2 and 3.1 Hz, 2H), 7.29 (dd, J =
5.5 and 3.1 Hz, 2H), 6.45 (d, J = 7.1 Hz, 2H), 5.56 (d, J = 7.1 Hz, 2H);
13C NMR (DMSO-d6) δ 139.4, 138.4, 131.7, 127.6, 126.0, 125.5, 122.5,
121.1, 66.7. Anal. Calcd for C18H14O2: C, 82.42; H, 5.38. Found: C,
82.26; H, 5.59.
trans-5,12-Bis(methylthio)-5,12-dihydrotetracene (7). To a suspen-
sion of CH3SNa (527 mg, 7.52 mmol) in dry CH2Cl2 (10 mL) at 0 °C
under Ar was added acetic acid (430 μL, 7.51 mmol). This mixture was
used as CH3SH quickly without any purification, which was added by
cannula to a heterogeneous mixture of 6 (489 mg, 1.86 mmol) and ZnI2
(1.19 g, 3.72 mmol) in dry CH2Cl2 (50 mL) at 0 °C under Ar. The
resulting mixture was stirred at room temperature for 20 h and then
quenched with H2O. The mixture was extracted with CH2Cl2. The
organic layer was washed with H2O and brine and dried over Na2SO4.
After evaporation of solvents, the residue was purified by column
chromatography on silica gel eluted with CH2Cl2ꢀhexane (1:3 and
then 1:2) to give 7 as a white solid (382 mg, 64% yield): mp 134 °C; 1H
NMR (CDCl3) δ 7.85 (dd, J = 6.3 and 3.4 Hz, 2H), 7.84 (s, 2H), 7.49
(dd, J = 6.3 and 3.4 Hz, 2H), 7.45 (dd, J = 5.7 and 3.4 Hz, 2H), 7.33 (dd,
J = 5.7 and 3.4 Hz, 2H), 5.18 (s, 2H), 2.31 (s, 6H); 13C NMR (CDCl3) δ
136.7, 134.6, 132.4, 129.1, 127.6, 127.5, 127.4, 126.2, 49.6, 17.6. Anal.
Calcd for C20H18S2: C, 74.49; H, 5.63. Found: C, 74.31; H, 5.86.
5,12-Bis(methylthio)tetracene (2). To a mixture of 7 (323 mg, 1.00
mmol) and tetrachloro-1,4-benzoquinone (493 mg, 2.01 mmol) under
Ar was added Ar-saturated CHCl3 (50 mL). The resulting mixture was
stirred at 60 °C for 43 h under Ar in the dark. Workup and purification
were carried out under air but without room light. After cooling to room
temperature and evaporation of CHCl3, the residue was passed through
short column of silica gel eluted with CH2Cl2ꢀhexane (1:20), and the
’ CONCLUSION
We have synthesized 5,12-bis(methylthio)tetracene (2) and
5,11-bis(methylthio)tetracene (3) and revealed that 2 and 3
lower both HOMO and LUMO energy levels and reduce the
HOMOꢀLUMO gap relative to those of tetracene. We have
presented that SꢀS and Sꢀπ interactions generally induce
cofacial π-stacking of bis(methylthio)acenes; i.e., 9,10-bis-
(methylthio)anthracene (4),9a 2 (this work), and 6,13-bis-
(methylthio)pentacene (1)10a self-assemble into 1-D slipped-
cofacial π-stacked acene columns by the assistance of SꢀS and
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The Journal of Organic Chemistry
ARTICLE
deep red band was collected. After evaporation of solvents, the residue
’ REFERENCES
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solid (203 mg, 63% yield): mp 184 °C; H NMR (CDCl3) δ 9.72
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and 29% yield for 2). This mixture of 3 and 2 was recrystallized from
hot toluene (twice) and then from hot CHCl3ꢀhexane (twice) to give
analytical pure 3 as a red solid (140 mg): mp 256 °C; 1H NMR
(CDCl3) δ 9.74 (s, 2H), 8.95 (d, J = 9.0 Hz, 2H), 8.16 (d, J = 8.5 Hz,
2H), 7.59 (dd, J = 9.0 and 6.6 Hz, 2H), 7.50 (dd, J = 8.5 and 6.6 Hz,
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X-ray Data Collection and Crystal Structure Determina-
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’ ASSOCIATED CONTENT
S
Supporting Information. Additional data and X-ray crys-
b
tallographic data (CIF). This material is available free of charge
via the Internet at http://pubs.acs.org.
’ AUTHOR INFORMATION
Corresponding Author
*E-mail: skkobay@ipc.shizuoka.ac.jp.
’ ACKNOWLEDGMENT
This work was supported in part by PRESTO, JST, and the Asahi
Glass Foundation. We thank Dr. Kengo Suzuki (Hamamatsu
Photonics K.K.) and Mr. Yutaka Fujiwara (Shizuoka University)
for measurements of fluorescence quantum yields and lifetimes
of 1ꢀ3.
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(17) See the Supporting Information.
(18) The fluorescence spectra, fluorescence quantum yields, and
fluorescence lifetimes of 1, 2, 3, and tetracene were measured in Ar-
saturated CH2Cl2 (Figure S9 and Table S3, Supporting Information).17
The fluorescence quantum yields and lifetimes of them were significantly
low and short, respectively. In particular, the fluorescence quantum yield
of 1 (Φf = < 0.001, below measurable limits) was much lower than those
of 2 (Φf = 0.028) and 3 (Φf = 0.127), and the fluorescence lifetime of 1
(τs = < 0.15 ns, below measurable limits) was much shorter than those of
2 (τs = 0.97 ns) and 3 (τs = 3.69 ns). Thus, one reason why 1 is more
resistant to photooxidation than 2 and 3 may be a consequence of a
shorter excited state lifetime of 1 relative to those of 2 and 3.
(19) The sum of van der Waals radii of two atoms is as follows:
S
S = 3.60a (3.70)b Å; S C = 3.50a (3.55)b Å.(a) Elemental Radii
3 3 3
3 3 3
in the Cambridge Structural Database, Cambridge Crystallographic
Data Centre, Cambridge. (b) Pauling, L. The Nature of the Chemical
Bond, 3rd ed.; Cornell University Press: Ithaca, NY, 1973.
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Intermolecular Forces; Clarendon Press: Oxford, 1981.
(21) The TFT devices were fabricated in yellow light and were
measured in the dark. The devices measured two structures (bottom
electrode contact and top electrode contact). These values were the
average of more than three samples.
(22) Minakata, T.; Natsume, Y. Synth. Met. 2005, 153, 1–4.
(23) Sheldrick, G. M. SHELXS-97 and SHELXL-97, University of
G€ottingen, G€ottingen, Germany, 1997.
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