Synthesis and properties of terthiophene and bithiophene derivatives functionalized by BF2 chelation: A new type of electron acceptor based on quadrupolar structures

Article abstract of DOI:10.1002/chem.201001185

Terthiophene and bithiophene derivatives functionalized by BF₂ chelation were synthesized as a new type of electron acceptor, and their properties were compared to those of bifuran and biphenyl derivatives. These new compounds are characterized

Full text of DOI:10.1002/chem.201001185

FULL PAPER
DOI: 10.1002/chem.201001185
Synthesis and Properties of Terthiophene and Bithiophene Derivatives
Functionalized by BF2 Chelation: A New Type of Electron Acceptor Based
on Quadrupolar Structures
Katsuhiko Ono,*[a] Akihiro Nakashima,[a] Yujiro Tsuji,[a] Takatoshi Kinoshita,[a]
Masaaki Tomura,[b] Jun-ichi Nishida,[c] and Yoshiro Yamashita[c]
Abstract: Terthiophene and bithio-
phene derivatives functionalized by
BF2 chelation were synthesized as a
new type of electron acceptor, and
their properties were compared to
those of bifuran and biphenyl deriva-
tives. These new compounds are char-
acterized by quadrupolar structures
due to resonance contributors generat-
ed by BF2 chelation. The bithiophene
derivative has a strong quadrupolar
character compared with the bifuran
and biphenyl derivatives because their
hydrolytic analyses indicated that the
bithiophene moiety has a larger on-site
Coulomb repulsion than the others.
absorptions and according to measure-
ments of ionization potentials and ab-
sorption edges they have energetically
low-lying HOMOs and LUMOs. The
crystal structure of the bithiophene de-
rivative is of the herringbone type, with
short F···S and F···C contacts affording
dense crystal packing. n-Type semicon-
ducting behaviour was observed in or-
ganic field-effect transistors based on
these BF2 complexes.
The terthiophene derivative has
a
smaller on-site Coulomb repulsion than
the bithiophene derivative due to the
addition of a thiophene spacer. These
BF2 complexes exhibit long-wavelength
Keywords: boron · chelates · elec-
tron-deficient compounds · hetero-
cycles · semiconductors
Introduction
ic heterocyclic ligands, such as 2-(2-hydroxyphenyl)pyri-
dines,[1] 2-(2-pyridyl)indoles,[2] and 2-(2-pyrrolyl)quinolines,[3]
were applied as fluorescent emitters in studies on organic
light-emitting diodes (OLEDs). Organoboron quinolates
with various functional groups were also synthesized and
their electro- and photoluminescence properties were stud-
ied.[4] These units were integrated into polymers to produce
light-emitting materials.[5] The known boradiazaindacene
(BODIPY) fluorophores are used as light-sensing units and
fluorescent probes in sensor materials.[6–8] In particular, they
have been studied as electron-accepting units of photoin-
duced charge-transfer materials for application in dye-sensi-
tized solar cells (DSCs).[9] Recently, boron complexes with
subporphyrins were synthesized and their optical properties
investigated.[10]
Boron complexes are highly attractive targets in the re-
search and development of materials for organic devices be-
cause they have pronounced electron-accepting properties
and strong fluorescence with high quantum yields. Their
properties vary depending on the ligand structures and sub-
stituents on the boron atoms. Boron complexes with aromat-
[a] Prof. Dr. K. Ono, A. Nakashima, Y. Tsuji, Prof. Dr. T. Kinoshita
Graduate School of Engineering
Nagoya Institute of Technology
Gokiso, Showa-ku, Nagoya 466-8555 (Japan)
Fax : (+81)52-735-5407
E-mail: ono.katsuhiko@nitech.ac.jp
Boron diketonates have also attracted considerable atten-
tion as fluorophores and electron-accepting units.[11–16]
Among these compounds, difluoroboron complexes have
been synthesized as potential electron-transporting materi-
als. For example, a BF2 complex with 1,6-diphenyl-1,3,4,6-
hexanetetrone exhibited n-type semiconducting behaviour
in organic field-effect transistors (OFETs).[15] Recently, we
reported the n-type semiconducting properties of BF2 com-
[b] Dr. M. Tomura
Institute for Molecular Science
Myodaiji, Okazaki 444-8585 (Japan)
[c] Dr. J.-i. Nishida, Prof. Dr. Y. Yamashita
Interdisciplinary Graduate School of Science and Engineering
Tokyo Institute of Technology
Nagatsuta, Midori-ku, Yokohama 226-8502 (Japan)
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.201001185.
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plexes
with
6,11-dihydroxy-5,12-naphthacenediones
(DHNDs) as a new type of electron-deficient acene com-
pounds.[16] Chelation of BF2 enhanced the electron affinity
and p-electron delocalization of the tetracene moieties. To
investigate p-conjugated systems functionalized by BF2 che-
lation, we synthesized terthiophene 1a and bithiophene 1b
with two difluoroboron diketonate units, as well as bifuran
derivative 1c and biphenyl derivative 1d (Scheme 1) be-
Scheme 2. Synthesis of BF2 complexes 1.
form and n-hexane. Compounds 1b–d were prepared as red,
orange and yellow crystals, respectively, in 30–86% yield
after sublimation. Their structures were determined by
means of spectral data and elemental analyses. Compounds
1 were immediately hydrolyzed in solution to afford ligands
3, whereas they were relatively stable in dry acetonitrile,
chloroform and dichloromethane. In the solid state, com-
pounds 1 were stable in air. The stability of 1 in the pres-
ence of moisture is considerably higher than that of the BF2
complex of 1,4-phenylene, which decomposed in air within a
day, even in the solid state. Compounds 1 were thermally
stable under inert atmosphere. Thermogravimetry (TG)
measurements on 1 revealed thermal stability up to 2008C;
5% weight loss was observed at 261, 253, 236 and 2358C for
1a–d, respectively.
Scheme 1. Structure of BF2 complexes 1 with quadrupolar contributions,
in which two negative charges are localized on the boron atoms and two
positive charges are delocalized over the arylene moieties.
cause various electron-deficient compounds with oligothio-
phene skeletons are of interest as n-type semiconductors in
OFETs.[17] These compounds have quadrupolar character, as
represented by the resonance contributors generated by BF2
chelation. Thus, arylene moieties such as terthiophene and
bithiophene become electron-deficient p-electron systems.
When the p-conjugated systems have a larger on-site Cou-
lomb repulsion, the difluoroboron diketonates become un-
stable. We studied the instability of BF2 chelation by using
hydrolytic analysis. Furthermore, molecular arrangements of
BF2 complexes in the crystal have attracted considerable at-
tention because they form dense crystal packings with short
heteroatom contacts.[11,14,16b] Herein we report the synthesis,
electron-accepting properties and crystal structure of BF2
complexes 1 and their application to OFETs.
Spectroscopic properties of BF2 complexes: The UV/Vis ab-
sorption spectra of
1 were measured in acetonitrile
(Figure 1). The red-shifted absorption maxima (Table 1)
compared with ligands 3 (lmax =453, 421, 412 and 362 nm for
Results and Discussion
Synthesis and stability of BF2 complexes: The synthesis of
BF2 complexes 1 is illustrated in Scheme 2. Diacetyl com-
pounds 2a–d reacted with ethyl trifluoroacetate in the pres-
ence of lithium bis(trimethylsilyl)amide or NaOMe to afford
ligands 3 in 64–82% yields. BF2 complexes 1 were synthe-
sized by treatment of ligands 3a–d with BF3·OEt2. Com-
pound 1a was obtained as dark purple crystals in 86% yield
after removal of excess BF3·OEt2 and washing with chloro-
Figure 1. UV/Vis absorption spectra of BF2 complexes 1 in acetonitrile;
c: 1a, b: 1b, g: 1c, À··À: 1d.
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Oligothiophenes Functionalized by BF2 Chelation
FULL PAPER
Table 1. UV/Vis absorption and fluorescence properties of BF2 com-
plexes 1.[a]
labs [nm] (e [molÀ1 dm3 cmÀ1])
Egap [eV][b]
lem [nm]
1a
1b
1c
1d
517 (59400)
2.09
2.38
2.42
2.89
608
518
516
448
488 (61500), 466 (59900)
482 (74800), 457 sh (57900)
389 (65500)
[a] In acetonitrile. [b] Estimated from the absorption edge.
3a, 3b, 3c and 3d, respectively) indicate significant contri-
butions of quadrupolar structures generated by BF2 chela-
tion (Scheme 1). The absorption maximum of 1a is red-shift-
ed by about 30 nm relative to 1b,c, which are red-shifted by
about 100 nm in comparison to 1d. This indicates that the
shift of the absorption bands depends on the arylene moiet-
ies linking the two difluoroboron diketonate units. The max-
imum molar absorption coefficients of 1a–d lie in a similar
range. The absorption maximum of 1a is red-shifted by
165 nm compared to that of terthiophene (lmax =352 nm)
and the molar absorption coefficient of 1a is three times
larger than that of terthiophene (e=19500 molÀ1 dm3 cmÀ1
;
Figure S4 in the Supporting Information). This indicates ex-
tension of the p-electron system in 1a by introduction of the
difluoroboron diketonate moieties. Furthermore, the broad
absorption band of 1a is due to the terthiophene skeleton.
When the number of thiophene rings increased, the band
shape became broader and longer wavelength absorption
edges resulted. The HOMO–LUMO energy gaps (Egap) for
1a–d were found to be in the range of 2.09–2.89 eV
(Table 1).
Boron complexes 1 exhibit photoluminescence (PL) in so-
lution; emission maxima are listed in Table 1. The Stokes
shifts of 1a–d of 30–91 nm are larger than that of DHND–
BF2 (6 nm),[16] that is, compounds 1 have flexible geometries.
Only compound 1d exhibited PL in the solid state, and its
emission maximum (lmax =519 nm) was red-shifted by 71 nm
compared to that observed in solution.
Figure 2. Time-dependent absorption spectra of a) 1a (c=1.1ꢁ10À5 m)
and b) 1b (c=2.0ꢁ10À5 m) in acetonitrile at 1 h intervals and c) 1b at
10 min intervals for the initial 70 min.
ure S9a in the Supporting Information). The ratio of 1a,
mono-BF2 complex and ligand 3a as a function of time
(Figure 3) indicates that 1a was consumed primarily over
the period of 0–20 h. This rate constant k1 for the hydrolysis
of 1a to give the mono-BF2 complex was determined to be
8.2ꢁ10À5 sÀ1 (Figure S9b in the Supporting Information),
Intramolecular interaction between the two difluoroboron
diketonate moieties: To investigate intramolecular interac-
tions between the two difluoroboron diketonate moieties,
we studied hydrolysis of 1 in acetonitrile. The UV/Vis ab-
sorption spectra of 1a were monitored as a function of time
over 3 d (Figure 2a). The maximum absorption intensity at
l=517 nm gradually decreased and an absorption band due
to ligand 3a emerged. This decay consisted of two types of
reaction with first-order kinetics (Figure S6a in the Support-
ing Information). The fast reaction is hydrolysis of 1a to
generate a metastable mono-BF2 complex with a rate con-
stant of k1 =5.0ꢁ10À5 sÀ1, measured over the period of 0–
16 h. The slow reaction is hydrolysis of the mono-BF2 com-
plex to give ligand 3a with a rate constant of k2 =1.7ꢁ
10À5 sÀ1, obtained over the period of 22–67 h. Thus, the first
step of the hydrolysis proceeded three times faster than the
second step. Hydrolysis of 1a was also monitored by time-
dependent 1H NMR spectroscopy in [D3]acetonitrile (Fig-
Figure 3. Progress in the hydrolysis of 1a monitored by time-dependent
1
&
*
H NMR spectra in [D3]acetonitrile: 1a ( ), mono-BF2 complex ( ) and
~
ligand 3a ( ).
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K. Ono et al.
which is similar to the k1 value
measured by analysis of the
UV/Vis absorption spectrum.
The hydrolysis of 1b was also
elucidated by analysis of the
UV/Vis absorption spectrum
(Figure 2b and c). For the initial
40 min, the intensity of the ab-
sorption maximum at l=
466 nm increased (Figure 2c)
and thus indicated generation
of a mono-BF2 complex. After
that, the intensity of the absorp-
tion bands gradually decreased
with an isosbestic point, and the
spectrum of ligand 3b resulted
(Figure 2b).
This
spectral
change indicates that the hy-
drolysis of 1b generating its
mono-BF2 complex is much
faster than that of the mono-
BF2 complex to give ligand 3b.
The k1 and k2 values were
found to be 1.2ꢁ10À3 and 6.7ꢁ
10À5 sÀ1,
respectively
(Fig-
ure S6b in the Supporting Infor-
mation).
The time-dependent absorp-
tion spectra of 1c and 1d are
shown in Figure S7 and S8 in
Scheme 3. First-order rate constants [sÀ1] for the hydrolysis of 1 in acetonitrile from time-dependent absorption
spectral analysis.
the Supporting Information, respectively. In the hydrolysis
of 1c, we observed an increase in absorbance in the initial
stage and then gradual decay of the absorption bands, re-
sulting in the spectrum of ligand 3c. This spectral change is
similar to that of 1b. On the other hand, the hydrolysis of
1d exhibited a spectral change similar to that of 1a. Thus,
the spectral changes depended on the nature of the arylene
moieties. Complexes 1a and 1d exhibited a single absorp-
tion maximum, whereas 1b and 1c revealed two absorption
maxima (Table 1). These absorption properties may be re-
sponsible for the differences in spectral changes.
Scheme 3 summarizes the rate constants (k1 and k2) for
the hydrolysis of 1. The k1 values of 1c,d were similar to
each other and smaller than that of 1b. This suggests that
1b has a larger on-site Coulomb repulsion on the bithio-
phene moiety, which indicates a significant contribution of
the quadrupolar structure (Scheme 1). Thus, the k1 value of
1b was 18 times greater than the k2 value. Furthermore, the
k1 value of 1a was smaller than those of 1b–d, that is, 1a
has a smaller on-site Coulomb repulsion than 1b due to the
longer distance between the difluoroboron diketonate moi-
eties because of the additional thiophene spacer. Thus, the
hydrolytic analysis indicates that the two difluoroboron di-
ketonate units interact with each other through the linking
arylene moieties.
Electron-accepting properties of BF2 complexes: To investi-
gate the electron affinities of 1, cyclic voltammetry (CV)
measurements were performed. However, the reproducibili-
ty of reduction waves was poor because of the instability of
1 in solution. The ionization potentials of 1b–d were mea-
sured in air by using a photoelectron spectrometer (Riken
Keiki AC-3), and their HOMO energy levels were deter-
mined to be À6.55 (1b), À6.50 (1c) and À6.56 eV (1d). Ac-
cording to their absorption band edges (Table 1), their
LUMO energy levels were estimated to be À4.17 (1b),
À4.08 (1c) and À3.67 eV (1d). These LUMO energies are
comparable to those of a,w-diperfluorohexylquaterthio-
phene (DFH-4T, À3.31 eV) and 5,5’’’-diperfluorohexylcar-
bonyl-2,2’:5’,2’’:5’’,2’’’-quaterthiophene
(DFHCO-4T,
À3.96 eV)[17a,b] and indicate that compounds 1 have strong
electron-accepting abilities.
Molecular orbital (MO) calculations on 1 were performed
at the B3LYP/6-31G(d) level,[18] and the energy diagram of
their HOMOs and LUMOs is summarized in Figure 4. The
energy diagram supports the experimental data for 1b–d.
The HOMO and LUMO energies of 1a are higher than
those of 1b due to addition of a thiophene spacer. The
HOMO–LUMO energy gap of 1a is smaller than that of 1b,
because of the extended p conjugation of the thiophene
moiety. The HOMOs and LUMOs of 1a,b are attributed to
the aromatic and quadrupolar structures of their resonance
contributors, respectively (Figure S11 in the Supporting In-
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Oligothiophenes Functionalized by BF2 Chelation
FULL PAPER
À
bouring bithiophene moiety. The B F bonds are almost
symmetrically located on the molecular plane. The mole-
cules are stacked into a columnar structure in an overlap
mode that decreases steric hindrance (Figure 5b). The face-
to-face stacking is induced by short F···S (3.15 and 3.22 ꢂ)
and F···C (3.11–3.14 ꢂ) contacts, which are shorter than the
sum of the van der Waals radii (F···S 3.27, F···C 3.17 ꢂ). The
crystal structure is of the herringbone type (Figure 5c). The
À
molecular sheets interact with each other through short C
H···F contacts to form a three-dimensional (3D) network.
These heteroatom contacts result in dense packing in the
crystal (1calcd =1.86 gcmÀ3).
Figure 4. Energy diagram of the HOMOs and LUMOs of BF2 complexes
1 obtained from B3LYP/6-31G(d) calculations; values given in eV.
n-Type semiconducting properties of BF2 complexes: Field-
effect mobilities were measured on OFET devices with a
bottom-contact configuration. The organic layers were
formed on SiO2 dielectric layers by vacuum deposition
(10À5 Pa) and the measurements were performed in vacuum.
The OFET devices with the films of 1b–d exhibited n-type
semiconducting behaviour with electron mobilities of 6.3ꢁ
10À5 (1b), 1.2ꢁ10À6 (1c) and 7.0ꢁ10À5 cm2 VÀ1 sÀ1 (1d),
whereas p-type semiconducting behaviour was not observed
because these complexes have low HOMO energies. Thus,
these BF2 complexes are useful as electron acceptors in
OFET devices. Therefore, we synthesized BF2 complex 1e,
in which perfluorohexyl groups are introduced to improve
the molecular assembly in an OFET film. Relatively good n-
type semiconducting behaviour was observed in a film of 1e
deposited on a hexamethyldisilazane (HMDS)-treated SiO2
substrate at 908C. The output and transfer characteristics
are shown in Figure 6. The electron mobility and on/off
ratio were calculated to be 2.0ꢁ10À3 cm2 VÀ1 sÀ1 and 1.0ꢁ
106, respectively.[19] The device also exhibited n-type semi-
conducting behaviour in air and in vacuum in the gate volt-
age region of 10–40 V. Further modification of 1e by lower-
formation). The MOs exist on the whole thiophene moieties,
that is, compounds 1a,b are effective p-conjugated systems.
X-ray crystallographic analysis of BF2 complex: The molecu-
lar and crystal structures of 1b were investigated by X-ray
crystallographic analysis. Single crystals suitable for X-ray
analysis were grown by slow sublimation at 1608C under ni-
trogen. The molecular structure is shown in Figure 5a. Six-
À
membered rings containing boron atoms were found and B
O distances in the range of 1.46–1.53 ꢂ were observed.
These rings are almost planar and coplanar with the neigh-
Figure 6. a) Drain current (Id) vs. drain voltage (Vd) as a function of gate
voltage (Vg) for a bottom-contact OFET with 1e deposited on a HMDS-
1=2
~
*
Figure 5. X-ray crystallographic analysis of 1b: a) molecular structure (el-
lipsoids were drawn at the 50% probability level), b) overlap mode,
c) crystal structure.
treated SiO2 substrate at 908C. b) Id ( ) and Id ( ) vs. Vg plots at Vd =
100 V for the same device. The field-effect mobility calculated in the sat-
uration regime is 2.0ꢁ10À3 cm2 VÀ1 sÀ1
.
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13543

K. Ono et al.
to provide ligand 3a as dark red crystals (0.45 g, 71%). M.p. 196–1978C;
1H NMR (600 MHz, CDCl3): d=6.42 (s, 2H), 7.28 (d, J=4.1 Hz, 2H),
7.33 (s, 2H), 7.75 ppm (d, J=4.1 Hz, 2H); IR (KBr): n˜ =1578, 1435,
ing its LUMO energy is expected to give an air-stable n-
type OFET.
1424, 1281, 1271, 1246, 1194, 1150, 1113, 1076, 795 cmÀ1
; UV/Vis
(MeCN): lmax (e)=453 nm (56200 molÀ1 dm3 cmÀ1); MS (70 eV): m/z (%):
524 (100) [M]+, 455 (37); elemental analysis calcd (%) for C20H10F6O4S3:
C 45.80, H 1.92; found: C 46.04, H 1.64.
Ligand 3e: Obtained as yellow crystals (0.65 g, 86%). M.p. 207–2088C;
1H NMR (600 MHz, CDCl3): d=6.47 (s, 2H), 7.40 (d, J=4.1 Hz, 2H),
7.79 ppm (d, J=4.1 Hz, 2H); IR (KBr): n˜ =1632, 1572, 1302, 1235, 1198,
1144,
793,
725 cmÀ1
;
UV/Vis
(CH2Cl2):
lmax
(e)=430 nm
(44400 molÀ1 dm3 cmÀ1); MS (70 eV): m/z (%): 944 (82) [M+2]+, 623
(100); elemental analysis calcd (%) for C26H8F26O4S2: C 33.14, H 0.86;
found: C 33.17, H 0.92.
Ligands 3b–d: Similar reaction conditions were applied but with NaOMe
as a base. Ethyl trifluoroacetate (4.0 mL, 34 mmol) was added dropwise
to a solution of NaOMe (Na 0.40 g, 17 mmol) in dry methanol (10 mL)
and dry diethyl ether (20 mL) at RT under nitrogen. 5,5’-Diacetyl-2,2’-bi-
thiophene (2b)[21] (0.33 g, 1.3 mmol) was added in portions to this solu-
tion and the mixture was stirred at RT overnight. Ethyl acetate (20 mL)
was added, and the solution washed with hydrochloric acid (2m, 10 mLꢁ
2) and water (10 mL). The organic solution was dried over MgSO4 and
concentrated. The residue was filtered and washed with n-hexane to
afford an orange solid (0.58 g). The crude product was purified by subli-
mation at 1808C and 10À2 Torr to provide ligand 3b as orange crystals
(0.48 g, 82%). M.p. 193–1948C; 1H NMR (600 MHz, CDCl3): d=6.44 (s,
2H), 7.40 (d, J=4.1 Hz, 2H), 7.77 ppm (d, J=4.1 Hz, 2H); IR (KBr): n˜ =
1576, 1426, 1263, 1235, 1198, 1155, 1117, 799, 683, 635, 581 cmÀ1; UV/Vis
(MeCN): lmax (e)=421 nm (45300 molÀ1 dm3 cmÀ1); MS (70 eV): m/z (%):
442 (100) [M]+, 373 (93), 331 (31); elemental analysis calcd (%) for
C16H8F6O4S2: C 43.44, H 1.82; found: C 43.45, H 1.66.
Conclusion
We have synthesized terthiophene and bithiophene deriva-
tives functionalized by BF2 chelation as well as bifuran and
biphenyl derivatives. These compounds have quadrupolar
structures due to their resonance contributors generated by
BF2 chelation. The bithiophene derivative has a stronger
quadrupolar character than the bifuran and biphenyl deriva-
tives because it has a larger on-site Coulomb repulsion ac-
cording to hydrolytic analysis. The on-site Coulomb repul-
sion of the terthiophene derivative was smaller than that of
the bithiophene derivative. These BF2 complexes exhibited
long-wavelength absorptions and low-lying HOMOs and
LUMOs, as revealed by the spectroscopic studies and MO
calculations. The crystal structure of the bithiophene deriva-
tive is of the herringbone-type with short F···S and F···C
contacts that result in a 3D network. n-Type semiconducting
behaviour was observed in OFET devices based on films of
the BF2 complexes. Noteably, a bithiophene derivative with
perfluorohexyl groups exhibited relatively high n-type semi-
conductor performance. Thus, BF2 functionalization of oli-
gothiophenes gives a new type of electron acceptor that is
useful for application in OFETs.
Ligand 3c: Obtained as yellow crystals (0.42 g, 75%). M.p. 223.5–2248C;
1H NMR (600 MHz, CDCl3): d=6.52 (s, 2H), 7.06 (d, J=3.8 Hz, 2H),
7.44 ppm (d, J=3.8 Hz, 2H); IR (KBr): n˜ =1642, 1601, 1561, 1422, 1314,
1263, 1181, 1144, 1103, 1071, 1022, 802, 666 cmÀ1; UV/Vis (MeCN): lmax
(e)=412 nm (44600 molÀ1 dm3 cmÀ1); MS (70 eV): m/z (%): 410 (100)
[M]+, 341 (95), 299 (62); elemental analysis calcd (%) for C16H8F6O6: C
46.85, H 1.97; found: C 46.76, H 1.78.
Ligand 3d:[22] Obtained as yellow crystals (0.35 g, 64%). M.p. 177.5–
1788C; 1H NMR (600 MHz, CDCl3): d=6.62 (s, 2H), 7.78 (d, J=8.6 Hz,
4H), 8.06 ppm (d, J=8.6 Hz, 4H); IR (KBr): n˜ =1601, 1445, 1271, 1213,
1154, 1111, 795, 718, 627, 577 cmÀ1; UV/Vis (MeCN): lmax (e)=362 nm
(54300 molÀ1 dm3 cmÀ1); MS (70 eV): m/z (%): 431 (96) [M+1]+, 362
(100), 319 (54); elemental analysis calcd (%) for C20H12F6O4: C 55.83, H
2.81; found: C 55.87, H 2.74.
Experimental Section
Synthesis of BF2 complex 1a:
A mixture of ligand 3a (115 mg,
0.22 mmol) and BF3·OEt2 (2.0 mL, 16 mmol) was heated at reflux for 4 h
under nitrogen. After removal of excess BF3·OEt2 under reduced pres-
sure, the residue was collected by filtration and washed with chloroform
and n-hexane to afford 1a as dark purple crystals (117 mg, 86%). Be-
cause an elemental analysis revealed that the crystals had sufficient
purity, further purification was not performed. 1H NMR (600 MHz,
CDCl3): d=6.74 (s, 2H), 7.48 (d, J=4.6 Hz, 2H), 7.55 (s, 2H), 8.15 ppm
(d, J=4.6 Hz, 2H); IR (KBr): n˜ =1603, 1505, 1422, 1356, 1277, 1206,
General: Melting points were measured by using a Yanaco micro melting
point apparatus and are uncorrected. 1H NMR spectra were recorded by
using a Bruker Avance 600 spectrometer (600 MHz). IR, UV/Vis and PL
spectra were obtained by using Jasco FT/IR-5300, Hitachi U-3500 and
Otsuka Electronics PTI-5100S spectrometers, respectively. Mass spectra
(EI) were determined by using a Hitachi M-2000S mass spectrometer. El-
emental analyses were performed by using a Perkin-Elmer 2400II ana-
lyzer. Compounds 2a–c were synthesized by procedures reported in the
literature (see the Supporting Information). Compound 2d was pur-
chased from Wako Pure Chemical Industries.
1032,
802 cmÀ1
;
UV/Vis
(MeCN):
lmax
(e)=517 nm
(59400 molÀ1 dm3 cmÀ1); MS (70 eV): m/z (%): 620 (93) [M]+, 187 (58),
139 (100); elemental analysis calcd (%) for C20H8B2F10O4S3: C 38.74, H
1.30; found: C 38.68, H 1.10.
Synthesis of ligand 3a: Lithium bis(trimethylsilyl)amide in THF (1.6m,
2.0 mL, 3.2 mmol) was added dropwise to a solution of 5,5’’-diacetyl-
2,2’:5’,2’’-terthiophene (2a)[20] (0.40 g, 1.2 mmol) in dry THF (30 mL) at
08C under nitrogen. Ethyl trifluoroacetate (0.40 mL, 3.4 mmol) was
added dropwise to the solution and the mixture was stirred at 08C for
30 min and at RT overnight. The reaction mixture was treated with hy-
drochloric acid (2m, 10 mL) and ethyl acetate (20 mL) was added. The
organic layer was separated, then the solution washed with water
(20 mLꢁ3) and dried over Na2SO4. After removal of the solvent, the res-
idue was filtered and washed with n-hexane to afford a red solid (0.67 g).
The crude product was purified by sublimation at 1908C under 10À3 Torr
Complexes 1b–e: Similar reaction conditions to that described for 1a
were applied to the synthesis of these complexes. The crude products
were sublimed at 215 (1b,c,e) or 2408C (1d) at 10À3 Torr to provide pure
products.
Complex 1b: Obtained as red crystals (38 mg, 30%). IR (KBr): n˜ =1595,
1510, 1427, 1350, 1263, 1202, 1038, 953, 822, 681 cmÀ1; UV/Vis (MeCN):
lmax (e)=488 (61500), 466 nm (59900 molÀ1 dm3 cmÀ1); MS (70 eV): m/z
(%): 538 (100) [M]+, 469 (66), 379 (41); elemental analysis calcd (%) for
C16H6B2F10O4S2: C 35.72, H 1.12; found: C 35.75, H 0.86.
13544
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Chem. Eur. J. 2010, 16, 13539 – 13546

Oligothiophenes Functionalized by BF2 Chelation
FULL PAPER
´
Complex 1c: Obtained as orange crystals (56 mg, 47%). IR (KBr): n˜ =
1605, 1584, 1524, 1451, 1410, 1370, 1329, 1273, 1217, 1159, 1053, 1026,
972, 820, 679 cmÀ1; UV/Vis (MeCN): lmax (e)=482 (74800), 457 nm sh
(57900 molÀ1 dm3 cmÀ1); MS (70 eV): m/z (%): 506 (33) [M]+, 369 (100);
elemental analysis calcd (%) for C16H6B2F10O6: C 37.99, H 1.20; found: C
37.92, H 1.13.
Complex 1d: Obtained as yellow crystals (0.32 g, 86%). 1H NMR
(600 MHz, CDCl3): d=7.06 (s, 2H), 7.92 (d, J=8.7 Hz, 4H), 8.34 ppm (d,
J=8.7 Hz, 4H); IR (KBr): n˜ =1593, 1350, 1321, 1271, 1206, 1169, 1047,
812 cmÀ1; UV/Vis (MeCN): lmax (e)=389 nm (65500 molÀ1 dm3 cmÀ1); MS
(70 eV): m/z (%): 526 (100) [M]+, 457 (79), 367 (72); elemental analysis
calcd (%) for C20H10B2F10O4: C 45.68, H 1.92; found: C 45.83, H 1.72.
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Complex 1e: Obtained as orange crystals (70 mg, 63%). IR (KBr): n˜ =
1580, 1530, 1510, 1427, 1354, 1231, 1204, 1148, 1030 cmÀ1
; UV/Vis
(CH2Cl2): lmax (e)=490 sh (50000), 464 nm (57400 molÀ1 dm3 cmÀ1); MS
(70 eV): m/z (%): 1040 (100) [M+2]+, 1021 (39), 720 (87), 629 (31); ele-
mental analysis calcd (%) for C26H6B2F30O4S2: C 30.08, H 0.58; found: C
29.98, H 0.55.
X-ray crystallography for 1b: A single crystal was obtained by slow subli-
mation at 1608C under nitrogen. Crystal data for 1b: 0.10ꢁ0.02ꢁ
0.01 mm; C16H6B2F10O4S2; Mr =537.97; orange needle; orthorhombic;
space group Pc21b; a=4.836(3), b=12.485(9), c=31.90(2) ꢂ; V=
1926(2) ꢂ3; Z=4; 1calcd =1.855 gcmÀ3; m=0.40 mmÀ1; F
ACHTNUTRGNEUNG(000)=1064. Re-
flection data were collected by using a Rigaku/MSC Mercury CCD dif-
fractometer equipped with a confocal monochromator with MoKa radia-
tion (l=0.71070 ꢂ) at 150(1) K. No absorption correction was applied.
The structure was solved by direct method using SHELXS97.[23] All non-
hydrogen atoms were refined anisotropically by full-matrix least-squares
technique on F2 by using SHELXL97.[23] All hydrogen atoms were posi-
tioned geometrically and refined by using a riding model. The final
values of R1 =0.115, GOF=0.86 and max./min. residual electron density
1.29/À0.56 eꢂÀ3 were obtained for 2761 unique reflections [I>2s(I)].
CCDC-763409 contains the supplementary crystallographic data for this
paper. These data can be obtained free of charge from The Cambridge
Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/cif.
OFET fabrication with 1e: A highly doped n+-Si wafer was used as sub-
strate. An SiO2 film (300 nm) grown by thermal oxidation was used as
the gate dielectric layer. Source and drain electrodes were photolitho-
graphically delineated by evaporating Cr (10 nm)/Au (20 nm). The chan-
nel length and width of source-drain contacts were 25 mm and 294 mm
(6 mmꢁ49), respectively. The substrate was immersed in hexamethyldisi-
lazane at RT overnight. A thin film (70 nm) of 1e was deposited on the
channel region at 908C by vacuum evaporation at
a rate of 0.02–
0.03 nmsÀ1 under a pressure of 10À5 Pa. The OFET measurements were
performed at RT in a vacuum chamber in the absence of air. The output
and transfer characteristics are shown in Figure 6.
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Acknowledgements
This work was supported by a Grant-in-Aid for Scientific Research (no.
20550037) from the Ministry of Education, Culture, Sports, Science and
Technology, Japan. We thank The Mazda Foundation and ADEKA
CORP. for their financial support and Riken Keiki Co. for the measure-
ment of ionization potentials by using AC-3. We thank the Instrument
Center of the Institute for Molecular Science for the X-ray crystallogra-
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Received: May 4, 2010
Published online: October 11, 2010
13546
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Chem. Eur. J. 2010, 16, 13539 – 13546