π-Extended thiadiazoles fused with thienopyrrole or indole moieties: Synthesis, structures, and properties

Article abstract of DOI:10.1021/jo301458m

We report the syntheses, structures, photophysical properties, and redox characteristics of donor-acceptor-fused π-systems, namely π-extended thiadiazoles 1-5 fused with thienopyrrole or indole moieties. They were synthesized by the Stille coupling reacti

Full text of DOI:10.1021/jo301458m

Article
pubs.acs.org/joc
π‑Extended Thiadiazoles Fused with Thienopyrrole or Indole
Moieties: Synthesis, Structures, and Properties
Shin-ichiro Kato,† Takayuki Furuya,† Atsushi Kobayashi,† Masashi Nitani,‡ Yutaka Ie,‡,§ Yoshio Aso,‡
Toshitada Yoshihara,† Seiji Tobita,† and Yosuke Nakamura*,†
†Department of Chemistry and Chemical Biology, Graduate School of Engineering, Gunma University, 1-5-1 Tenjin-cho, Kiryu,
Gunma 376-8515, Japan
‡The Institute of Scientific and Industrial Research (ISIR), Osaka University, 8-1 Mihogaoka, Ibaraki, Osaka 567-0047, Japan
§PRESTO-JST, 4-1-8 Honcho, Kawaguchi, Saitama 333-0012, Japan
S
* Supporting Information
ABSTRACT: We report the syntheses, structures, photo-
physical properties, and redox characteristics of donor−
acceptor-fused π-systems, namely π-extended thiadiazoles 1−
5 fused with thienopyrrole or indole moieties. They were
synthesized by the Stille coupling reactions followed by the
PPh3-mediated reductive cyclizations as key steps. X-Ray
crystallographic studies showed that isomeric 1b and 2b form
significantly different packing from each other, and 1a and 4a
afford supramolecular networks via multiple hydrogen bonding
with water molecules. Thienopyrrole-fused compounds 1b and 2b displayed bathochromically shifted intramolecular charge-
transfer (CT) bands and low oxidation potentials as compared to indole-fused analog 3b and showed moderate to good
fluorescence quantum yields (Φf) up to 0.73. In 3b−5b, the introduction of electron-donating substituents in the indole moieties
substantially shifts the intramolecular CT absorption maxima bathochromically and leads to the elevation of the HOMO levels.
The Φf values of 3−5 (0.04−0.50) were found to be significantly dependent on the substituents in the indole moieties. The
OFET properties with 1b and 2b as an active layer were also disclosed.
nitrogen,9 silicon,10 and boron,11 have been synthesized, and
their potential applications as π-functional materials have been
recently investigated by taking advantage of the characteristic
electronic properties derived from heteroatoms.
INTRODUCTION
■
The development of organic π-conjugated molecules for
organic electronics, such as organic light-emitting diodes
(OLEDs), organic field-effect transistors (OFETs), and organic
Intramolecular charge-transfer (CT) interactions are a
photovoltaics (OPVs), remains a key objective.1,2 Organic
fundamental property of π-systems consisting of strong electron
materials have distinct advantage of their structural diversity,
which can be achieved by versatile synthetic protocols in
organic chemistry. It is important to tailor the electronic
donors (D) and acceptors (A) connected via a π-spacer.12
Donor−acceptor-substituted compounds have a potential to
possess the small HOMO−LUMO gap and show the
structure of π-conjugated molecules and control the solid-state
electrochemically amphoteric behavior, and thereby they are
structure to obtain appropriate intermolecular interactions in an
of current interest as attractive materials.13 Therefore, the D−
intelligent way. Consequently, recent advances in exploring π-
conjugated molecules involve the development of multifused
A-fused heterocycles would be also an interesting class of π-
electronic systems. In addition to the prominent features of the
aromatic and heteroaromatic compounds, because it is expected
D−A-substituted compounds, they are expected to form
that their flat and rigid frameworks provide the effective
extension of π-conjugation and allow for forming a dense
densely packed structures in the solid state due to the
multifused π-framework. As for D−A-fused molecules,
Yamashita and co-workers synthesized tetrathiafulvalene
(TTF)−quinoxaline-fused systems and demonstrated their
considerably high stability and good carrier mobility.14 Liu
and co-workers extensively studied analogous compounds,
which resulted from fusing TTF and strong acceptors, such as
phenazine and perylenebisimide units.15 Very recently, Xiao
molecular packing and thereby, lead to spectacular properties
such as strong fluorescence and high carrier mobility. Over the
years, polycyclic aromatic hydrocarbons (PAHs), such as acene
derivatives with collinearly fused benzene rings and 2D
extended fully fused compounds, were extensively studied.3,4
Thienoacenes5 and azaacenes6,7 have been also synthesized,
and the effects of the replacement of benzene rings in acenes
with thiophene and pyrazine rings on the optical and electronic
properties have been discussed. Furthermore, various ladder-
type π-conjugated systems incorporating heteroatoms,8 such as
Received: July 11, 2012
Published: August 8, 2012
© 2012 American Chemical Society
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and co-workers have synthesized carbazole−perylenebisimide-
fused derivatives and disclosed the remarkable third-order
nonlinear susceptibility reflecting the intramolecular CT
interactions.16 Siri and co-workers17 and Miao and co-
workers18 independently reported the efficient synthetic
pathways of dihydrotetraazapentacenes, namely dihydrophena-
zine−quinoxaline-fused molecules, as interesting D−A-fused
heterocycles without exocyclic double bond, and their
optoelectronic and electrochemical properties and solid-state
structures were studied.
It is likely that the choice of building blocks in the molecular
design of the D−A-fused systems is critical because they are
anticipated to dominate the properties in many cases.
Benzothiadiazole (BTD) unit is a leading candidate as a strong
electron-acceptor and an important component of narrow band
gap π-conjugated oligomers and copolymers for organic solar
cells.19 The BTD unit has been combined with carbazoles,
fluorenes, pyrroles, thiophenes, and other electron-rich building
blocks.20 Hence, we became interested in π-extended
thiadiazoles 1 and 2 fused with thienopyrrole moieties and
thiadiazoles 3−5 fused with indole moieties (Chart 1). We
means of electronic absorption spectroscopy, cyclic voltamme-
try (CV), and theoretical calculations. The photophysical
properties were further elucidated by fluorescence spectrosco-
py, fluorescence lifetime measurements, and time-resolved
photoacoustic measurements. The X-ray crystallographic
structures of 1a, 1b, 2b, 4a, and 5b were determined and
examined in detail. The p-type semiconducting nature of 1b
and 2b was also disclosed.
RESULTS AND DISCUSSION
■
Synthesis. The synthetic route to 1−5 is outlined in
Scheme 1. First, we subjected 6 to Stille coupling reaction with
tributyl(2-thienyl)tin to prepare 7.23 Then, we carried out
double-reductive cyclization of 7 with PPh3 in o-dichloroben-
zene (o-DCB) at the refluxing temperature24 and isolated 1a in
a remarkably high yield of 97%. This result prompted us to
synthesize isomeric compound 2a. Using 8 as a precursor which
was synthesized by Stille coupling of 6 with tributyl(3-
thienyl)tin, the double cyclization with PPh3 proceeded
smoothly, and noticeably 2a was obtained as the sole product
in a yield of 93%. We next examined the synthesis of indole
analogs 3a−5a with H-, t-Bu-, and OMe-substituents,
respectively, by the PPh3-mediated reductive cyclization
reactions. As a result, the reactions of 9−11 with PPh3 in o-
DCB at the refluxing temperature furnished the desired 3a−5a
in 97%, 70%, and 92% yields, respectively. It was recently
reported that 3a21b and 5a21f were prepared by P(OEt)3-
mediated reactions in 51% and 61%, respectively (Supple-
mentary Scheme S1 in the Supporting Information). Overall,
the reactions with PPh3 appear to give better yields than those
with P(OEt)3, and the yields were readily reproducible.24b In
most cases, almost the only byproduct, triphenylphosphine
oxide, was easily removed by standard chromatography and/or
washing with appropriate solvents. We also attempted the
cyclization reaction of 4,7-di(2-furyl)-5,6-dinitro-2,1,3-benzo-
thiadiazole by the treatment with PPh3, however, only
decomposed materials were recovered presumably due to
instability of the cyclized product (Supplementary Scheme S2).
N-Ethyl products 1b−5b were readily obtained by the reaction
of the corresponding 1a−5a with iodoethane in the presence of
NaOH as a base in DMF. Compounds 1b−5b are more soluble
than 1a−5a, respectively, in common organic solvents without
the loss of the tendency for crystallization upon introduction of
the ethyl groups. Compounds 1a and 2a were also converted to
N-benzyl products 1c and 2c, respectively. N-Benzylation of
3a−5a was not investigated on the basis of the results that the
photophysical and electrochemical properties of 1c and 2c are
not significantly different from those of 1b and 2b, respectively
(vide infra). All compounds were fully characterized by various
spectroscopic methods.
Chart 1
envisaged that the ring fusion of highly electron-donating
thienopyrrole or indole moieties to an electron-accepting BTD
unit brings about not only the intramolecular CT interactions
but also the efficient intermolecular interactions in the solid
state.21 Very recently, Cheng and Hsu’s group independently
reported the synthesis of 1a.21g,22 A few thiadiazole derivatives
fused with indole moieties were sporadically reported;21b,f
however, the elucidation of the structure−property relation-
ships still remain to give the guideline for the molecular design
of the D−A-fused systems as π-functional materials. Here we
report the synthesis of 1−5 by the alternative synthetic method
and their structure−property relationships in detail. The
electronic properties were systematically investigated by
Structural Properties. We determined the molecular
structures by X-ray analysis of single crystals of 1a, 1b, 2b,
4a, and 5b. Single crystals suitable for X-ray diffraction analysis
were obtained by slow diffusion of hexane into a solution of the
molecules in CH2Cl2 or acetone. Extensive X-ray analyses
confirmed that all of the π-extended thiadiazoles have highly
planar frameworks (Figure 1). Especially, the planarity of 1a
and 4a is still high relative to that of 1b, 2b, and 5b, in which
the steric hindrance between the ethyl groups on the nitrogen
atoms occurs. In all the molecular structures in the solid state,
no noticeable bond-length alternation is observed and the bond
angles are inconspicuous.
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a
Scheme 1. Synthesis of π-Extended Thiadiazoles 1−5 Fused with Thienopyrrole or Indole Moieties
a
Reagents and conditions: (a) Pd(PPh3)4, tributyl(2-thienyl)tin for 7, tributyl(3-thienyl)tin for 8, tributylphenyltin for 9, tributyl(4-tert-
butylphenyl)tin for 10, and tributyl(4-methoxyphenyl)tin for 11, THF, reflux. (b) PPh3, o-dichlorobenzene (o-DCB), reflux. (c) Iodoethane, NaOH,
DMF, 50 °C. (d) benzyl bromide, NaOH, DMF, 60 °C.
For the potential usefulness of π-conjugated molecules in
organic electronics, the π-stacking motif with large overlap of π-
orbitals with the neighboring molecules in the solid state is a
critical aspect. As shown in Figure 2a and b, isomeric 1b and 2b
exhibit good π−π stacking with a distance of about 3.35−3.4 Å,
however, they differ much in the packing motif reflecting the
structural difference. Compound 1b is packed in a head-to-tail
manner to form a 1D column in a parallel and slanted
arrangement and the columns assemble together by weak S···H
interactions with a distance of 2.84 Å to give a sheet-like
network (Figure 2a). The thienopyrrole moieties are over-
lapped with the BTD moieties, suggesting the presence of
intermolecular CT interactions between the thienopyrrole
donor and BTD acceptor parts. On the other hand, 2b forms
a stacked dimer structure with a close C···C distance of 3.36 Å,
which is aligned in a 1D array by efficient S···S interactions with
a distance of 3.59 Å (Figure 2b). Again, the dimer is packed in a
sandwich-herringbone motif by C−H···π interactions. Com-
pound 5b forms offset stacks in the single crystal with a longer
distance of 3.49 Å than those in the crystals of 1b and 2b, and a
brickwork motif runs parallel to the b,c plane (Supplementary
Figure S1, Supporting Information).
Thus, in the crystal packing of 1a, the indole sites prefer to trap
the water molecules by bifurcated N−H···O hydrogen bonding
(2.05 and 2.26 Å), and the neighboring molecules interact with
the water molecules by CN···H hydrogen bonding with the
thiadiazole sites (2.04 Å). The electron-rich pyrrole rings are
also involved in favorable O−H···π interactions. Likewise, in
the crystal of 4a, the combination of the N−H···O hydrogen
bonding (2.38 and 1.94 Å) between the water molecules and
the indole sites and the CN···H hydrogen bonding between
the water and the thiadiazole sites yields a sheet-like structure
(Figure 4a). The sheets are assembled in a brickwork motif
through the combination of O−H···π (2.40 and 2.45 Å) and
π···π interactions (Figure 4b). It is noteworthy that both 1a and
4a accommodate the water molecules without losing the
efficient S···π and/or π···π interactions.
Photophysical Properties. To investigate the photo-
physical properties in detail, we measured the UV−vis and
fluorescence spectra of 1−5 in various solvents, and the
representative data are summarized in Table 1. These data are
mainly discussed from the viewpoints of (1) the effect of the
orientation of fused heterorings in 1a−c and 2a−c and (2) the
effect of the substituents in indole moieties in 3a,b−5a,b.
Figure 5a shows the UV−vis spectra of compounds 1b−5b in
CH2Cl2. The spectra of 1a−c and 2a−c feature broad CT
absorptions of moderate intensity (ε ≈ 5000−7000 M−1 cm−1)
with the longest absorption maxima (λmaxabs) between 415 and
436 nm in CH2Cl2 (Supplementary Figure S2 and S3 and Table
S1 and S2, Supporting Information). Introduction of the ethyl
or benzyl groups on the nitrogen atoms in 1b,c and 2b,c results
in slightly bathochromic shifts of the λmaxabs values as compared
Interestingly, the water molecules, which apparently came
from the solvent of wet acetone, were included in the single
crystals of both 1a and 4a in a 1:1 stoichiometry, whereas the
crystals of 1b, 2b, and 5b with the Et-substituents described
above did not contain solvent molecules. As shown in Figures 3
and 4, both 1a and 4a interact with water molecules through a
combination of N−H···O, CN···H, and O−H···π inter-
actions, which led to the formation of supramolecular networks.
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Figure 3. Arrangement of neighboring molecules in the crystal packing
of 1a.
(ε = 6200 M−1 cm−1), respectively. Similar bathochromic shifts
abs
are observed when going from 2a (λmax = 415 nm, ε = 5100
M−1 cm−1) to 2b (λmax = 425 nm, ε = 7000 M−1 cm−1) and
abs
2c (λmax = 424 nm, ε = 6400 M−1 cm−1). The λmax values
and the absorption edges of 1a−c are slightly red-shifted by
about 10 nm as compared to those of the corresponding 2a−c.
According to the density functional theory (DFT) calculations
(B3LYP/6-311G**//B3LYP/6-31G*) by the Gaussian 03 suit
of program,25 1b has a larger dipole moment of 2.608 D than
that for 2b to be 2.053 D, which may account for the difference
abs
abs
of the λmax values between 1a−c and 2a−c.26 The broad
bands of 1a−c in the region of 400−500 nm are quite similar to
those of 2a−c, whereas the absorption curves in the region of
300−350 nm significantly differ from each other.
abs
Figure 1. ORTEP plots of (a) 1a, (b) 1b, (c) 2b, (d) 4a, and (e) 5b
with displacement ellipsoids at the 50% probability level. Water
molecules in the crystals of 1a and 4a, and H-atoms are omitted for
clarity.
To investigate the solvent effects on the photophysical
properties, we measured the UV−vis and fluorescence spectra
in various media from nonpolar to polar solvents.27 While the
UV−vis spectra of 1a−c and 2a−c are slightly changed between
nonpolar toluene and polar DMF, remarkable fluorescence
solvatochromism for 1a−c and 2a−c was observed depending
on the polarity of the solvents. The color of fluorescence varies
from green in toluene to yellow in DMF (Supplementary
Figure S4, Supporting Information). Figure 5b shows the
fluorescence spectra of 2b in different solvents as a typical
example. Upon increasing the solvent polarity from toluene to
DMF,28 the fluorescence maxima (λmaxfl) of 2b shift to longer
wavelength region by 44 nm: 508 nm in toluene and 552 nm in
DMF (see Supplementary Figure S5−S10 for 1a−c and 2a−c).
The observed solvatochromic effects in the emission strongly
suggest a more polar electronic structure in the excited state
than in the ground state in 1a−c and 2a−c upon the
intramolecular CT interactions. We determined the absolute
fluorescence quantum yields (Φf) of 1a,b and 2a,b in various
solvents by using an integrating sphere system.29 In nonpolar
toluene, THF, and ethyl acetate, the Φf values of 1a,b are
almost 1.5 times as high as those of the corresponding 2a,b:
0.68 (1a), 0.73 (1b), 0.42 (2a), and 0.45 (2b) in toluene. On
the contrary, in CH2Cl2 and polar DMF, the Φf values of 1a,b
are almost the same as those of the corresponding 2a,b,
although the reason for these findings is unclear at present
(Table 1, and Supplementary Table S1 and S2).
Figure 2. Arrangement of neighboring molecules in the crystal packing
of (a) 1b and (b) 2b.
One peak and one shoulder were observed for 3a,b over 350
nm in contrast to 1a,b and 2a,b which show distinctive two
peaks, probably reflecting the structural difference between the
indole and thienopyrrole moieties (Figure 5a and Supple-
mentary Figure S11 and S12, Supporting Information). A
comparison of 3−5 shows the effects of the substituents in the
abs
to 1a and 2a, respectively. For example, 1a shows the λmax
value of 423 nm (ε = 5800 M−1 cm−1), while 1b and 1c feature
the λmaxabs values of 436 nm (ε = 7100 M−1 cm−1) and 434 nm
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Figure 4. Arrangement of neighboring molecules in the crystal packing of (a) top view and (b) side view of 4a.
a
Table 1. Summary of Photophysical Data of 1b−5b
kr [107 knr [107
abs
fl
λmax
λmax
τf
b
c
d
e f
s−1
]
solv.
[nm (eV)]
[nm] [ns]
Φf
s−1
]
1b
toluene
THF
432 (2.91)
436 (2.85)
433 (2.87)
436 (2.85)
441 (2.82)
423 (2.94)
424 (2.93)
422 (2.94)
425 (2.92)
523
539
538
565
573
508
521
521
548
552
470
497
500
489
515
515
517
553
557
19.4
0.73
0.68
0.63
0.33
0.42
0.45
0.51
0.47
0.34
0.43
0.07
0.04
0.04
0.08
0.07
0.07
0.41
0.44
0.49
3.8
2.9
2.6
2.0
1.9
3.5
3.4
3.0
2.3
2.5
6.9
4.3
4.1
7.0
3.9
4.2
5.5
3.1
3.2
1.4
1.4
1.5
4.0
2.6
4.2
3.3
3.4
4.5
3.4
96
23.7
24.2
16.6
22.1
13.0
15.0
15.6
14.6
16.8
0.97
1.00
1.07
1.18
1.81
1.57
7.5
EtOAc
CH2Cl2
DMF
2b
toluene
THF
EtOAc
CH2Cl2
DMF
430 (2.89)
g
3b
4b
5b
toluene
CH2Cl2
DMF
420 (2.96)
g
420 (2.96)
95
g
420 (2.96)
89
g
toluene
CH2Cl2
DMF
425 (2.92)
78
g
430 (2.89)
51
g
430 (2.89)
59
toluene
CH2Cl2
DMF
439 (2.83)
440 (2.82)
445 (2.79)
7.9
4.5
3.3
12.6
15.3
a
Complete data for 1−5 are included in the Supporting Information.
b
c
d
Only the longest absorption maxima are shown. Lifetime. Absolute
e
quantum yields determined by an integrating sphere system. Radiative
decay constant. Nonradiative decay constant. Shoulder.
f
g
Figure 5. (a) UV−vis absorption spectra of 1b−5b in CH2Cl2. (b)
Fluorescence spectra of 2b in different solvents.
indole moieties on the photophysical properties.30 Introduction
of the substituents results in the longer wavelength shifts of
comparable to those of 2a,b (Table 1 and Supplementary Table
S3). However, 3a,b and 4a,b with the H- and t-Bu-substituents,
respectively, display unexpectedly low Φf values of 0.04−0.12
by about 1 order of magnitude compared to 5a,b.
abs
fl
both λmax and λmax (Table 1, Figure 5, and Supplementary
abs
Table S3), and thus the λmax values for 3b, 4b, and 5b in
CH2Cl2 were observed at 420, 430, and 440 nm, respectively, as
shown in Figure 5a. On the contrary, the absorption maxima of
3−5 around 380 nm, which are probably related to the S0→S2
transitions, are nearly independent of the substituents. The
To gain insight into the photophysical properties of 1−5, we
determined their fluorescence lifetimes (τf) in various solvents
with the time-correlated single-photon counting method
(Figure 6), and calculated the radiative (kr) and nonradiative
(knr) decay rate constants from the singlet excited state,31 based
on the equations of kr = Φf/τf and knr = (1 − Φf)/τf (Table 1
and Supplementary Table S1−S3, Supporting Information).
Compounds 1a,b exhibit slightly longer τf values (14.4−24.2
ns) than those of 2a,b (13.0−16.8 ns). Compounds 1a,b and
2a,b exhibit the similar kr values in many cases, however, in
toluene, THF, and ethyl acetate the knr values for 1a,b are
somewhat smaller than those for 2a,b. These findings correlate
with the trend of their Φf values as described above.
fl
abs
λmax values of 3−5 follow the trend of their λmax values.
Thus, as summarized in Table 1, compounds 3b, 4b, and 5b
feature the λmax values of 497, 515, and 553 nm in CH2Cl2,
fl
respectively (see Supplementary Table S3 for 3a−5a). As is the
case with 1 and 2, compounds 3−5 also exhibited characteristic
fluorescence solvatochromism (Supplementary Figure S11−
S16). Notably, the Φf values were dramatically affected by the
substituents. Compounds 5a,b with the OMe-substituents
exhibit the Φf values of 0.41−0.50 in toluene, CH2Cl2, and
DMF, which are slightly lower than those of 1a,b and almost
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the experimental errors in all cases. Based on the equation Φf +
Φisc + Φic = 1, where Φic is the quantum yield of internal
conversion, one can consider that the internal conversion
hardly occurs in 1b−5b and hence the knr values almost
correspond to the kisc values. Thus, the fast nonradiative
processes for 3a,b and 4a,b are ascribed predominantly to the
fast intersystem crossing. The heavy-atom effect of the sulfur
atom in the BTD moiety may be one of the reasons for
somewhat fast intersystem crossing to the triplet state in 1−5
(vide infra).
Electrochemistry. The electrochemical properties of 1a−c,
2a−c, and 3b−5b were examined by cyclic voltammetry (CV;
Table 2) in o-DCB (0.1 M n-Bu4NPF6, standard Fc+/Fc), and
Table 2. Oxidation and Reduction Potentials of 1−5 by CV
Figure 6. Fluorescence decay curves (top) and residual (bottom) of
1b−5b measured by the time-correlated single-photon counting
method.
a
in o-DCB (0.1 M n-Bu4NPF6), Theoretically Calculated
b
HOMO and LUMO Levels, and Optical Energy Gap
ox
red
Eonset
Eonset
HOMO LUMO
ΔEredox
ΔEopt
d
c
Compounds 5a,b display the τf values of 6.9−16.1 ns which are
slightly shorter than those of 2a,b. The kr and knr values of 5a,b
are calculated to be 2.9 × 107 − 5.3 × 107 s−1 and 3.3 × 107 −
7.9 × 107 s−1, respectively, both of which are roughly
comparable to those of 1a,b and 2a,b. In sharp contrast, 3a,b
and 4a,b show remarkably short lifetimes of 0.97−2.61 ns
(Figure 6), which give the values kr = 3.8 × 107 − 7.3 × 107 s−1
and knr = 34 × 107 − 96 × 107 s−1. Consequently, whereas
compounds 3a,b−5a,b show similar kr values, the knr values of
3a,b and 4a,b are significantly larger than those of the
corresponding 5a,b by more than 1 order of magnitude in
many cases. Thus, the extremely fast nonradiative processes
occur in 3a,b and 4a,b as compared to 1a,b, 2a,b, and 5a,b,
which can be attributed to the internal conversion and/or
intersystem crossing. The phosphorescence for 1b−5b was
observed in the region of about 600−800 nm at 77 K in 2-
methyltetrahydrofuran, illustrating that the intersystem crossing
in 1−5 is involved in the nonradiative processes (Figure 7 and
Supplementary Figure 17−21).
(Epa) [V]
(Epc) [V]
[eV]
[eV]
[V]
[eV]
f
e
1a
1b
+0.23
−2.19
−2.23
2.42
2.54
2.62
2.54
e
g
+0.31
−5.18
−2.00
(+0.46)
(−2.42)
e
g
1c
+0.39
−2.24
2.63
2.55
(+0.51)
(−2.38)
f
e
2a
2b
+0.29
−2.15
−2.25
2.44
2.65
2.69
2.59
h
g
+0.40
−5.24
−1.91
(+0.59)
(+1.12)
(−2.51)
g
e
2c
+0.44
−2.25
2.69
2.61
(+0.59)
(+1.01)
(−2.36)
e
g
3b
4b
5b
+0.62
−2.22
−5.52
−5.36
−5.06
−2.13
−2.04
−2.08
2.84
2.71
2.48
2.72
2.63
2.50
(+0.77)
(−2.39)
g
g
+0.48
−2.23
(+0.50)
(−2.48)
g
h
+0.26
−2.22
(+0.42)
(+0.86)
(−2.46)
a
All potentials are given versus the Fc+/Fc couple used as external
b
standard. Scan rate: 100 mV s−1. B3LYP/6-311G**//B3LYP/6-
c
31G*. Electrochemical gap, ΔEredox, is defined as the potential
d
difference between Eonsetox and Eonsetred. Optical gap, ΔEopt, is defined
as the energy corresponding to the λonset, which is defined as the
longest energy absorption wavelength with a molar absorptivity ε =
e
f
1000 M−1 cm−1 in CH2Cl2. Irreversible wave. Unresolved wave.
g
h
Reversible wave. Quasi-reversible wave.
all of the compounds were found to display amphoteric
behavior within the available potential window (Figure 8 and
Supplementary Figure S22−S26, Supporting Information).33
Compounds 1−5 exhibit a one BTD-centered 1e− reduction
step. The reduction onsets (Eonsetred) for 1−5 are observable in
the region of −2.25 to −2.19 V, and the effect of the structural
variation in 1−5 on the reduction potentials is insignificant. It is
Figure 7. (a) UV−vis, (b) total emission, (c) phosphorescence, and
(d) phosphorescence excitation (λem = 620 nm) spectra of 4b in 2-
methyltetrahydrofuran matrix at 77 K.
red
In order to determine the relative contribution of the internal
conversion and intersystem crossing to the nonradiative energy
dissipation, we performed time-resolved photoacoustic meas-
urements for 1b−5b in toluene at room temperature and
estimated their quantum yields of intersystem crossing (Φisc).32
The Φisc values of 1b, 2b, 3b, 4b, and 5b are 0.26, 0.56, 0.94,
0.92, and 0.61, respectively. It is noteworthy that the sums of
the Φf and Φisc values for 1b−5b are almost equal to 1 within
noted that their Eonset values are anodically shifted as
compared to that for the parent BTD (−2.00 V). Thus, the
reduction of 1−5 is clearly rendered more difficult by the
effective conjugation with the strong thienopyrrole or indole
donors. Whereas 1b−5b and 1c showed reversible or quasi-
reversible waves, compounds 1a, 2a, and 2c showed irreversible
waves. This indicates that the functional groups on the nitrogen
atoms in π-extended thiadiazoles play an important role in the
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Theoretical Calculations. We carried out the electronic
transition analysis of 1b−5b by time-dependent (TD) DFT at
the B3LYP/6-31G* level of theory with the ground-state
geometry optimized at the B3LYP/6-31G* level without any
symmetry restriction (Supplementary Table S4−S8, Supporting
Information). The absorption maxima in the low-energy region
of 2b, 4b, and 5b are related to the HOMO−LUMO
transitions, whereas those of 1b and 3b are attributed to the
weak transitions from the HOMO−LUMO+1 as well as the
abs
strong HOMO−LUMO transitions. The calculated λmax
values are red-shifted with respect to the experimental values
by 5−35 nm. The calculated λmax value (458 nm) of 1b is
longer than that of 2b (441 nm), which compares well with the
abs
abs
experimental data. The calculated λmax values of 3b, 4b, and
Figure 8. Cyclic voltammograms of (a) 1b, (b) 2b, (c) 3b, (d) 4b, and
(e) 5b measured in o-DCB (0.1 M n-Bu4NPF6) at a scan rate of 100
mV s−1.
5b are 425, 436, and 476 nm, respectively. Thus, 3b−5b follow
the observed trend for the λmaxabs in the experiments, where the
introduction of the functional groups in the indole moieties
resulted in the more red-shifted absorption bands (Figure 2a).
The effects of the orientation of fused thiophene rings in 1b
and 2b and the substituents in the indole moieties of 3b−5b on
their electronic structures were studied by molecular orbital
calculations. The frontier molecular orbitals (FMOs) and the
HOMO and LUMO levels of 1b−5b were obtained by the
single-point calculations at the B3LYP/6-311G**//B3LYP/6-
31G* level of theory. The results are summarized in Table 2.
The calculated HOMO levels of 1b and 2b are higher than that
of 3b, and the HOMO levels apparently rise from 3b, 4b, to 5b.
The LUMO levels of 1b−5b are less changed than the HOMO
levels. Thus, the calculated HOMO−LUMO gaps become
smaller from 3b, 2b, to 1b, and compound 5b displays the
smallest gap in the series of 1b−5b. These findings qualitatively
agree well with the results in the CV and UV−vis data. The
HOMOs and LUMOs of 1b−5b are shown in Figure 9. The
HOMOs are delocalized over the whole π-system. The HOMO
density of 1b is found on the CC bonds in the thiophene
moieties similar to typical thiophene derivatives, whereas that of
electrochemical stability and the ethyl groups more effectively
stabilize the radical-anionic species than the benzyl groups.
Compounds 1a−c and 2a−c exhibited thienopyrrole-
centered oxidation steps. Likewise, compounds 3b−5b showed
indole-centered oxidation steps. In sharp contrast to the
reduction process, the potentials and the reversibility in the
oxidation and back-oxidation processes are significantly
dependent on the donor moieties. The oxidation onsets
(Eonsetox) for 1a−c are cathodically shifted as compared to the
corresponding 2a−c by 50−90 mV. The oxidation peak
amplitude of 1a and 2a was quite large after the first oxidation,
indicative of their electrochemical polymerization and/or
decomposition after the oxidation (Supplementary Figure S22
and S24, Supporting Information). Compounds 1b,c also
display irreversible thienopyrrole-centered oxidation waves,
however, the isomeric compounds 2b,c show different
oxidation behavior from that of the corresponding 1b,c.
Thus, 2b displays distinct, two 1e− oxidation steps, and the
first and second oxidation waves are quasi-reversible and
irreversible, respectively. Compound 2c having benzyl groups
experienced two reversible 1e− oxidation steps (Supplementary
Figure S26). These results confirm that the orientation of the
fused thiophene rings in 1 and 2 affects the kinetic stability of
ox
electrochemically generated cationic species. The Eonset value
of 3b shifts anodically relative to those of 1b and 2b by 310 mV
and 220 mV, respectively, clearly reflecting the difference of
electron-donating ability between thiophene and benzene.
Introduction of the electron-donating substituents in the indole
ox
moieties brings about the cathodic shifts of the Eonset values
(+0.62 V (3b), +0.48 V (4b), and +0.26 V (5b)), that is, the
elevating of the HOMO levels. The Hammett σp values of t-Bu
and OMe groups are −0.20 and −0.27, respectively,34 and thus
ox
the observed trend for the Eonset values follows the electron-
donating ability of the substituents. The two well-separated,
reversible 1e− oxidation steps for 5b were observable as in the
ox
case with 2b and 2c. The Eonset value of +0.26 V for 5b is
more cathodic than those for 1b and 2b by 50 mV and 140 mV,
respectively, and thus 5b is found to possess the strongest
donor ability in a series of 1b−5b. The oxidation wave for 3b
was irreversible, while those for 4b and 5b were almost
reversible. This suggests that the functional groups in the indole
moieties in 4b and 5b play an important role in the stability of
electrochemically generated cationic species as well as the
donor ability.
Figure 9. Molecular orbital plots of (a) 1b, (b) 2b, (c) 3b, (d) 4b, and
(e) 5b calculated by B3LYP/6-311G**//B3LYP/6-31G*. The upper
plots represent the LUMOs, and the lower plots represent the
HOMOs.
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2b is found on the C−S bonds as well as the CC bonds,
which may be responsible for their different properties. The
LUMOs of 1b−5b have large contribution from the electron-
accepting BTD moiety. This implies that the sulfur atom in the
BTD moiety promotes triplet formation by intersystem
crossing. The FMOs of 3b−5b readily explain the reason
why the introduction of the electron-donating substituents in
4b and 5b affects the oxidation potentials than the reduction
potentials. The considerably larger HOMO densities than the
LUMO densities in the indole moieties should lead to the
destabilization of the HOMOs through the introduction of the
electron-donating substituents. Indeed, the HOMO density of
5b has substantial contribution from the OMe groups, whereas
its LUMO has almost no contribution.
OFET Properties. We performed thermal gravimetric
analysis (TGA) of 1b−5b, which revealed that they are
thermally stable up to ca. 300 °C (Supplementary Table S16,
Supporting Information). Again, 1b−5b can be readily
sublimed at about 130 °C by using a high vacuum pump
under laboratory conditions, which offers the potential for thin
film preparation by vapor deposition techniques.35 We became
interested in the relationship between the solid-state structures
of 1b and 2b and their semiconducting nature. On the basis of
the crystal structures of 1b and 2b, we estimated the
intermolecular transfer integrals (t) of the HOMOs and
LUMOs between the neighboring molecules by Amsterdam
density functional (ADF) calculations.36,37 The calculations
were independently performed for the HOMOs and LUMOs of
the molecules. The calculated t values of the HOMOs are
significantly larger than those of the LUMOs, and large transfer
integral values of 151.4 and 102.7 meV for 1b and 2b,
respectively, were observed along the π-stacked directions (a-
axis (1b), b-axis (2b)) (Supplementary Table S14 and S15).
This observation implies that efficient hole-transport along the
a-axis and b-axis for 1b and 2b, respectively, is favorable. In the
crystal structure of 2b, the moderate t value of 27.0 meV was
also observable in the a-axis, whereas the value in 1b in the b-
axis is almost negligible (6.4 meV), which indicates that the
electronic structure of 1b with a sheet-like network is more
anisotropic than that of 2b.
We carried out to fabricate OFET devices with a bottom-
contact configuration with both 1b and 2b to evaluate the field-
effect carrier mobility. At first, the thin films of 1b and 2b acting
as the active layer were spin-coated from CHCl3 solutions (1.0
wt %), and the electrical characteristics were measured under
vacuum (10−2 Pa) at room temperature after annealing at 100
°C for 0.5 h. As expected from the high oxidation potentials of
1b and 2b, the devices exhibited the typical p-channel FET
behavior and the n-channel behavior was not observed. The
mobility of 1b-based device was too low to be determined. The
mobility of 2b-based device was found to be 0.9 × 10−5 cm2
V−1 s−1 with an on/off current ratio of 106 and a threshold of 13
V from the plot of the square root of the drain current versus
the gate voltage. Next, the active layer was prepared by vapor
deposition. The mobility of 1b-based device was again too low
to be estimated. The 2b-based device showed the mobility of
2.7 × 10−5 cm2 V−1 s−1, which is better than that of the device
prepared by a spin-coating method, with an on/off current ratio
of 105 and a threshold voltage of 17 V (Figure 10).
Figure 10. (a) Output and (b) transfer (VSD = −80 V) characteristics
of a 2b-based OFET device prepared by vapor deposition. ISD, VSD
,
and VGS denote source-drain current, source-drain voltage, and gate
voltage, respectively.
coupling reactions of 6 with the corresponding aryltin
derivatives, followed by PPh3-mediated reductive cyclization
reactions as key steps. The X-ray crystallographic analyses of 1a,
1b, 2b, 4a, and 5b revealed that they form the dense crystal
packing and compounds 1a and 4a construct the unique
supramolecular networks via multiple hydrogen bonding with
water molecules. The electronic structure of 1−5 was
approached by the UV−vis and fluorescence spectral measure-
ments, CV, and DFT calculations. Their intramolecular CT
interactions were clearly confirmed. The thienopyrrole-fused 1b
and 2b display bathochromically shifted absorption spectra,
namely small HOMO−LUMO gaps, and low oxidation
potentials as compared to the indole-fused 3b. The
introduction of the t-Bu- and OMe-substituents in 4b and 5b,
respectively, brings about the decrease of HOMO−LUMO
gaps and the increase of donor ability. We have presented that
the functional groups in the indole moieties dramatically affect
the fluorescence behavior: 3b and 4b display significantly lower
Φf values than 5b due to the significantly fast intersystem
crossing. Using thienopyrrole-fused compounds 1b and 2b as
the active layer, we fabricated OFETs by vapor deposition and
spin coating, and found that the orientation of the fused
thiophene rings is an important factor in the device
performance, which may reflect the difference of their packing
structures as demonstrated by the X-ray diffraction analyses.
We believe that the results in the present study provide the
useful guideline to alter the photophysical and electrochemical
properties and solid-state structures of π-extended thiadiazoles
for their application into π-functional materials. Further
structural functionalization of π-extended thiadiazoles is
currently underway in our group.
CONCLUSION
In conclusion, we have synthesized π-extended thiadiazoles 1−
5 fused with thienopyrrole or indole moieties by Stille cross-
■
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415 (5100) nm; HR-FAB-MS (NBA, positive): m/z calcd for
EXPERIMENTAL SECTION
■
+
C14H6N4S3 325.9755, found 325.9743 (M+).
General Procedure of PPh3-Mediated Reductive Cyclization
for Preparation of π-Extended Thiadiazoles Fused with
Thienopyrrole or Indole Moieties. A mixture of dinitro precursor
(1 equiv) and PPh3 (10 equiv) in o-DCB (∼20 mM) was bubbled by
argon for 15 min and stirred at 180 °C for 8−14 h. The mixture was
cooled to room temperature, and the solvent was removed in vacuo.
The residue was subjected to column chromatography and the
collected material was subsequently washed with CHCl3 or CHCl3/
hexane.
General Procedure for N-Ethylation. A mixture of π-extended
thiadiazole derivatives (1 equiv), iodoethane (∼6.5 equiv), and sodium
hydroxide (∼10 equiv) in DMF (∼20 mM) was stirred at 50 °C for
2−6 h. After the mixture was poured into water, the resulting
suspension was extracted with toluene/ethyl acetate (1:1). The
combined organic phase was washed with water, and evaporated in
vacuo after dried over anhydrous magnesium sulfate. The residue was
purified by column chromatography.
Preparation of 2b. Compound 2a (432 mg, 1.32 mmol) was
allowed to react with iodoethane (1.26 g, 8.05 mmol) in the presence
of sodium hydroxide (531 mg, 13.3 mmol) for 7 h according to the
general procedure for N-ethylation. The crude product was purified by
column chromatography (silica gel; toluene) to give 2b (370 mg, 0.97
mmol, 73%) as orange solids. Mp 187−190 °C; 1H NMR (300 MHz,
CDCl3): δ 7.74 (d, J = 4.8 Hz, 2H), 7.14 (d, J = 4.8 Hz, 2H), 4.38 (q, J
= 7.2 Hz, 4H), 1.62 (t, J = 7.2 Hz, 6H); 13C NMR (125 MHz,
CDCl3): δ 148.0, 143.0, 133.0, 128.0, 120.6, 118.7, 110.3, 46.4, 14.4;
UV−vis (CH2Cl2): λmax (ε) 278 (22700), 366 (17200), 425 (7000)
+
nm; HR-FAB-MS (NBA, positive): m/z calcd for C18H14N4S3
382.0381, found 382.0378 (M+); Elemental analysis calcd (%) for
C18H14N4S3: C 56.52, H 3.69, N 14.65; found C 56.48, H 3.73, N
14.53.
Preparation of 2c. A mixture of 2a (81 mg, 0.24 mmol), benzyl
bromide (155 mg, 0.90 mmol), and sodium hydroxide (70 mg, 1.72
mmol) in DMF (12 mL) was stirred at 60 °C for 7 h. After the mixture
was poured into water (30 mL), the resulting suspension was extracted
with toluene/ethyl acetate (1:1, 50 mL × 5). The combined organic
phase was washed with water (100 mL × 3), and evaporated in vacuo
after dried over anhydrous magnesium sulfate. The residue was
purified by column chromatography (silica gel; toluene) to give 2c (38
Preparation of 1a.21g Compound 7 (735 mg, 1.88 mmol) was
allowed to react with PPh3 (4.92 g, 18.8 mmol) for 13 h according to
the general procedure of PPh3-mediated reductive cyclization. The
crude product was subjected to column chromatography (silica gel;
toluene/ethyl acetate 2:1) and the collected material was subsequently
washed with CHCl3 to give 1a (598 mg, 1.82 mmol, 97%) as orange
solids. Mp 137−139 °C; 1H NMR (300 MHz, acetone-d6): δ 11.23 (br
s, 2H), 7.54 (d, J = 5.1 Hz, 2H), 7.31 (d, J = 5.1 Hz, 2H); 13C NMR
(150 MHz, acetone-d6): δ 148.5, 142.3, 130.7, 127.4, 120.9, 113.1,
108.6; UV−vis (CH2Cl2): λmax (ε) 265 (21400), 311 (26900), 325
(28700), 351 (15200), 423 (5800) nm; HR-FAB-MS (NBA, positive):
1
mg, 0.07 mmol, 31%) as pale yellow solids. Mp 227−230 °C; H
NMR (300 MHz, CDCl3): δ 7.78 (d, J = 5.4 Hz, 2H), 7.33−7.29 (m,
6H), 7.07 (d, J = 5.4 Hz, 2H), 7.03 (dd, J = 5.4 Hz, 2.1 and 3.6 Hz,
4H), 5.36 (s, 4H); 13C NMR: Not available due to low solubility;
UV−vis (CH2Cl2): λmax (ε) 278 (21200), 365 (16600), 424 (6400)
+
nm; HR-FAB-MS (NBA, positive): m/z calcd for C28H18N4S3
+
m/z calcd for C14H6N4S3 325.9755, found 325.9759 (M+).
506.0694, found 506.0691 (M+).
Preparation of 3a.21b Compound 9 (50 mg, 0.13 mmol) was
allowed to react with PPh3 (347 mg, 13.2 mmol) for 12 h according to
the general procedure of PPh3-mediated reductive cyclization. The
crude product was subjected to column chromatography (silica gel;
toluene/ethyl acetate 8:1) and the collected material was subsequently
washed with CHCl3/hexane to give 3a (38 mg, 0.12 mmol, 93%) as
yellow solids: The 1H NMR data are in agreement with those
Preparation of 1b. Compound 1a (50 mg, 0.15 mmol) was
allowed to react with iodoethane (156 mg, 1.00 mmol) in the presence
of sodium hydroxide (60 mg, 1.50 mmol) for 4 h according to the
general procedure for N-ethylation. The crude product was purified by
column chromatography (silica gel; toluene/hexane 10:1) to give 1b
(54 mg, 0.14 mmol, 95%) as brown solids. Mp 210−213 °C; 1H NMR
(300 MHz, CDCl3): δ 7.46 (d, J = 5.1 Hz, 2H), 7.23 (d, J = 5.1 Hz,
2H), 4.56 (q, J = 7.2 Hz, 4H), 1.54 (t, J = 7.2 Hz, 6H); 13C NMR (125
MHz, CDCl3): δ 147.5, 145.6, 132.0, 127.1, 121.6, 111.7, 111.0, 45.0,
15.4; UV−vis (CH2Cl2): λmax (ε) 263 (23300), 313 (31200), 330
(27700), 367 (19500), 436 (7100) nm; HR-FAB-MS (NBA, positive):
1
previously reported. H NMR (300 MHz, acetone-d6): δ 11.25 (br s,
2H), 8.61 (dd, J = 7.5 and 2.1 Hz, 2H), 7.75 (dd, J = 7.8 and 2.1 Hz,
2H), 7.45−7.38 (m, 4H).
Preparation of 3b. Compound 3a (51 mg, 0.16 mmol) was
allowed to react with iodoethane (148 mg, 0.95 mmol) in the presence
of sodium hydroxide (51 mg, 1.27 mmol) for 2 h according to the
general procedure for N-ethylation. The crude product was purified by
column chromatography (silica gel; hexane/toluene 1:2) to give 3b
(51 mg, 0.14 mmol, 87%) as yellow solids. Mp 179−181 °C; 1H NMR
(300 MHz, CDCl3): δ 8.76 (d, J = 6.9 Hz, 2H), 7.63 (d, J = 6.9 Hz,
2H), 7.54−7.44 (m, 4H), 4.66 (q, J = 7.2 Hz, 4H), 1.37 (t, J = 7.2 Hz,
6H); 13C NMR (125 MHz, CDCl3): δ 149.4, 141.8, 132.3, 126.0,
125.0, 122.4, 122.3, 112.9, 111.8, 43.4, 14.7; UV−vis (CH2Cl2): λmax
(ε) 273 (27000), 300 (27500), 312 (37500), 337 (7000), 387 (22600)
nm; MALDI-TOF-MS (Dith, positive): m/z 370.09 (M+); Elemental
analysis calcd (%) for C22H18N4S: C 71.32, H 4.90, N 15.12; found C
71.34, H 4.96, N 15.08.
+
m/z calcd for C18H14N4S3 382.0381, found 382.0393 (M+);
Elemental analysis calcd (%) for C18H14N4S3: C 56.52, H 3.69, N
14.65; found C 56.37, H 3.61, N 14.46.
Preparation of 1c. A mixture of 1a (40 mg, 0.12 mmol), benzyl
bromide (98 mg, 0.57 mmol), and sodium hydroxide (49 mg, 1.23
mmol) in DMF (6 mL) was stirred at 60 °C for 10 h. After the mixture
was poured into water (30 mL), the resulting suspension was extracted
with toluene/ethyl acetate (1:1, 50 mL × 5). The combined organic
phase was washed with water (100 mL × 3), and evaporated in vacuo
after dried over anhydrous magnesium sulfate. The residue was
purified by column chromatography (silica gel; toluene/hexane 5:1) to
give 1c (26 mg, 0.05 mmol, 43%) as pale orange solids. Mp 201−204
°C; 1H NMR (300 MHz, CDCl3): δ 7.34−7.26 (m, 8H), 6.98 (dd, J =
2.1 and 6.0 Hz, 4H), 6.77 (d, J = 5.1 Hz, 2H), 5.39 (s, 4H); 13C NMR:
Not available due to low solubility; UV−vis (CH2Cl2): λmax (ε) 262
(19700), 313 (25700), 330 (23800), 366 (18900), 434 (6200) nm;
Preparation of 4a. Compound 10 (152 mg, 0.31 mmol) was
allowed to react with PPh3 (1.00 g, 3.81 mmol) for 8 h according to
the general procedure of PPh3-mediated reductive cyclization. The
crude product was subjected to column chromatography (silica gel;
toluene) and the collected material was subsequently washed with
CHCl3/hexane to give 4a (90 mg, 0.21 mmol, 70%) as yellow solids:
+
HR-FAB-MS (NBA, positive): m/z calcd for C28H18N4S3 506.0694,
found 506.0693 (M+).
Preparation of 2a. Compound 8 (302 mg, 0.77 mmol) was
allowed to react with PPh3 (2.09 g, 7.97 mmol) for 12 h according to
the general procedure of PPh3-mediated reductive cyclization. The
crude product was subjected to column chromatography (silica gel;
toluene/ethyl acetate 8:1) and the collected material was subsequently
washed with CHCl3 to give 2a (231 mg, 0.71 mmol, 93%) as pale
1
Mp > 300 °C; H NMR (300 MHz, acetone-d6): δ 11.03 (br s, 2H),
8.50 (d, J = 8.4 Hz, 2H), 7.74 (d, J = 1.8 Hz, 2H), 7.53 (dd, J = 8.4 and
1.8 Hz, 2H), 1.45 (s, 18H); 13C NMR (150 MHz, acetone-d6): δ
150.1, 148.8, 139.5, 130.1, 122.9, 121.5, 120.4, 109.5, 109.2, 35.7, 32.1;
UV−vis (CH2Cl2): λmax (ε) 271 (33500), 299 (29700), 312 (46800),
368 (18400), 412 (10200) nm; HR-FAB-MS (NBA, positive): m/z
calcd for C26H26N4S+ 426.1878, found 426.1894 (M+).
1
brown solids. Mp 115−118 °C; H NMR (300 MHz, acetone-d6): δ
11.40 (br s, 2H), 7.66 (d, J = 5.4 Hz, 2H), 7.24 (d, J = 5.4 Hz, 2H);
13C NMR (150 MHz, acetone-d6): δ 149.0, 138.7, 131.3, 128.3, 121.1,
118.6, 107.9; UV−vis (CH2Cl2): λ max (ε) 274 (19300), 352 (12000),
Preparation of 4b. Compound 4a (51 mg, 0.12 mmol) was
allowed to react with iodoethane (115 mg, 0.74 mmol) in the presence
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of sodium hydroxide (38 mg, 0.94 mmol) for 6 h according to the
general procedure for N-ethylation. The crude product was purified by
column chromatography (silica gel; hexane/toluene 1:3) to give 4b
(51 mg, 0.10 mmol, 90%) as yellow solids. Mp 200−204 °C; 1H NMR
(300 MHz, CDCl3): δ 8.65 (d, J = 8.1 Hz, 2H), 7.64 (d, J = 1.2 Hz,
2H), 7.56 (dd, J = 8.1 and 1.2 Hz, 2H), 4.73 (q, J = 7.2 Hz, 4H), 1.50
(s, 18H), 1.37 (t, J = 7.2 Hz, 6H); 13C NMR (125 MHz, CDCl3): δ
149.4, 148.6, 142.0, 132.5, 123.9, 121.6, 120.6, 112.6, 108.1, 43.3, 35.4,
32.0, 14.6; UV−vis (CH2Cl2): λmax (ε) 275 (28400), 303 (29400), 316
(42300), 335 (7200), 387 (19700) nm; HR-FAB-MS (NBA, positive):
m/z calcd for C30H34N4S+ 482.2504, found 482.2502 (M+).
positive): m/z calcd for C26H26N4O4S+ 490.1675, found 490.1676
(M+).
Preparation of 11.21f A mixture of 6 (201 mg, 0.516 mmol) and
tributyl(4-methoxyphenyl)tin (511 mg, 1.28 mmol) in THF (12 mL)
was bubbled with argon for 30 min. Pd(PPh3)4 (31 mg, 0.027 mmol)
was added to the mixture. The mixture was heated at the refluxing
temperature for 17 h, and the solvent was removed in vacuo. The
residue was subjected to column chromatography (silica gel; toluene)
and the collected material was subsequently washed with hexane to
1
give 11 (148 mg, 0.395 mmol, 77%) as orange solids: H NMR (300
MHz, CDCl3): δ 7.53 (d, J = 9.0 Hz, 4H), 7.08 (d, J = 9.0 Hz, 4H),
3.90 (s, 6H); MALDI-TOF-MS (Dith, positive): m/z 438.01 (M+).
Device Fabrication. The field-effect mobility was measured using
bottom-contact thin-film field-effect transistor (FET) geometry. The
n-doped silicon substrate functions as the gate electrode. A thermally
grown silicon oxide (SiO2) dielectric layer on the gate substrate has
300 nm thick and a capacitance of 10.0 nF cm−2. Interdigital source
and drain electrodes were constructed with gold (30 nm) that were
formed on the SiO2 layer. The channel width (W) and channel length
(L) are 38 nm and 5 μm, respectively. The silicon oxide surface was
first washed with acetone, water, and 2-propanol, and then activated by
UV−ozone treatment. For the vacuum deposition, the substrate was
pretreated with hexamethyldisilazane (HMDS) and washed with again
with toluene, acetone, and 2-propanol. The semiconductor layer was
vacuum deposited on the Si/SiO2 at a rate of 1 Å/s under a pressure of
10−6 Pa to a thickness of 10 nm determined by a quartz crystal
monitor or spin-coated from a solution in CHCl3 (1.0 wt.%) at 1500
rpm for 1 min onto the substrate in air. The characteristics of the
OFET devices were measured at room temperature under a pressure
of 10−2 Pa without exposure by using a semiconductor parameter
analyzer after the substrate was annealed at 100 °C for 30 min under a
vacuum condition (10−2 Pa). The filed-effect mobility (μ) was
calculated in the saturated region at the VSD of −80 V by using the
following equation. The current on/off ratio was determined from the
ISD at VGS = 0 V (Ioff) and VGS = −80 V (Ion).
Preparation of 5a.21f Compound 11 (101 mg, 0.23 mmol) was
allowed to react with PPh3 (707 mg, 2.69 mmol) for 14 h according to
the general procedure of PPh3-mediated reductive cyclization. The
crude product was subjected to column chromatography (silica gel;
toluene/ethyl acetate 4:1) and the collected material was subsequently
washed with CHCl3 to give 5a (79 mg, 0.21 mmol, 92%) as orange
1
solids. The H NMR data are not included in the reported literature:
1
Mp > 300 °C; H NMR (300 MHz, acetone-d6): δ 11.13 (br s, 2H),
8.44 (d, J = 8.7 Hz, 2H), 7.26 (d, J = 2.4 Hz, 2H), 7.06 (dd, J = 8.7 and
2.4 Hz, 2H), 3.92 (s, 6H); 13C NMR: Not available due to low
solubility; UV−vis (CH2Cl2): λmax (relative intensity) 271 (0.71), 308
(0.63), 320 (1.0), 365 (0.41), 429 (0.23) nm; HR-FAB-MS (NBA,
positive): m/z calcd for C20H14N4O2S+ 374.0837, found 374.0837
(M+).
Preparation of 5b. Compound 5a (50 mg, 0.13 mmol) was
allowed to react with iodoethane (126 mg, 0.80 mmol) in the presence
of sodium hydroxide (44 mg, 1.07 mmol) for 6 h according to the
general procedure for N-ethylation. The crude product was purified by
column chromatography (silica gel; hexane/CHCl3 1:2) to give 5b (44
mg, 0.10 mmol, 78%) as orange solids. An analytical sample for the
elemental analysis was obtained by recrystallization from toluene. Mp
1
214−216 °C; H NMR (300 MHz, CDCl3): δ 8.59 (d, J = 8.7 Hz,
2H), 7.10 (s, 2H), 7.09 (d, J = 8.7, 2H), 4.63 (q, J = 7.2 Hz, 4H), 3.98
(s, 6H), 1.34 (t, J = 7.2 Hz, 6H); 13C NMR (150 MHz, CDCl3): δ
158.4, 149.2, 143.1, 132.1, 122.7, 120.4, 112.3, 110.8, 96.4, 56.0, 43.5,
14.5; UV−vis (CH2Cl2): λmax (ε) 253 (46000), 277 (25100), 322
(36000), 384 (17600), 440 (10000) nm; HR-FAB-MS (NBA,
positive): m/z calcd for C24H22N4O2S+ 430.1463, found 430.1463
(M+); Elemental analysis calcd (%) for C24H22N4O2S·0.1toluene
(439.73): C 67.46, H 5.23, N 12.74; found C 67.31, H 5.30, N 12.57.
Preparation of 8. A mixture of 6 (406 mg, 1.05 mmol) and
tributyl(3-thienyl)tin (948 mg, 2.53 mmol) in THF (14 mL) was
bubbled with argon for 30 min. Pd(PPh3)4 (60 mg, 0.052 mmol) was
added to the mixture. The mixture was heated at the refluxing
temperature for 13 h, and the solvent was removed in vacuo. The
residue was dissolved in CH2Cl2, and the resulting solution was filtered
through a bed of silica gel. After the filtrate was evaporated in vacuo,
the residue was subjected to column chromatography (silica gel;
hexane/toluene 1:1) and the collected material was subsequently
washed with hexane to give 8 (369 mg, 0.95 mmol, 87%) as yellow
solids. Mp 254−255 °C; 1H NMR (300 MHz, CDCl3): δ 7.86 (dd, J =
3.0 and 1.5 Hz, 2H), 7.56 (dd, J = 5.0 and 3.0 Hz, 2H), 7.41 (dd, J =
5.0 and 1.5 Hz, 2H); 13C NMR (150 MHz, CDCl3): δ 152.8, 142.1,
129.6, 128.9, 127.9, 127.1, 123.4; UV−vis (CH2Cl2): λmax (relative
intensity) 298 (0.57), 396 (0.44) nm; HR-FAB-MS (NBA, positive):
ISD = WCiu(VGS − V )2/2L
th
ASSOCIATED CONTENT
■
S
* Supporting Information
General experimental methods, X-ray data, cif files, UV−vis and
fluorescence data, CV data, theoretical data, thermal gravimetric
analysis data, and 1H and 13C NMR spectra of all new
compounds. This material is available free of charge via the
Internet at http://pubs.acs.org.
AUTHOR INFORMATION
■
Corresponding Author
*E-mail: nakamura@gunma-u.ac.jp. Tel: +81 277 30 1310. Fax:
+81 277 30 1314.
Notes
The authors declare no competing financial interest.
+
m/z calcd for C14H6N4O4S3 389.9551, found 389.9548 (M+).
ACKNOWLEDGMENTS
Preparation of 10. A mixture of 6 (153 mg, 0.38 mmol) and
tributyl(4-tert-butylphenyl)tin (821 mg, 1.94 mmol) in THF (15 mL)
was bubbled with argon for 15 min. Pd(PPh3)4 (23 mg, 0.020 mmol)
was added to the mixture. The mixture was heated at refluxing
temperature for 19 h, and the solvent was removed in vacuo. The
residue was subjected to column chromatography (silica gel; hexane/
toluene 2:3) and the collected material was subsequently washed with
hexane to give 10 (165 mg, 0.38 mmol, 98%) as orange solids. Mp >
300 °C; 1H NMR (300 MHz, CDCl3): δ 7.58 (d, J = 8.7 Hz, 4H), 7.52
(d, J = 8.7 Hz, 4H), 1.40 (s, 18H); 13C NMR (75 MHz, CDCl3): δ
153.8, 153.2, 142.6, 129.0, 128.9, 127.6, 126.2, 35.1, 31.4; UV−vis
(CH2Cl2): λmax (ε) 295 (9000), 380 (7100) nm; HR-FAB-MS (NBA,
■
This work was supported by a Grant-in-Aid for Scientific
Research from the Ministry of Education, Culture, Sports,
Science and Technology, Japan, and performed under the
Cooperative Research Program of “Network Joint Research
Center for Materials and Devices” (Kyushu University, Osaka
University). We thank Prof. Dr. Soichiro Kyushin (Gunma
University) for generous permission to use an X-ray
diffractometer, Prof. Dr. Teruo Shinmyozu (Kyushu Univer-
sity) for mass spectrometry measurements, and Dr. Mikio
Yamasaki (Rigaku) for helpful suggestions for X-ray analyses.
7604
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The Journal of Organic Chemistry
Article
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