Synthesis and properties of thiophene-fused benzocarborane

Article abstract of DOI:10.1002/chem.201201513

The o-carborane-based π-conjugated compound, benzocarborano- [2,1-b:3,4-b']dithiophene was synthesized. Its crystal structure revealed high coplanarity for the two thiophene rings of the 2,2'-bithiophene skeleton, which is fixed in the cisoid structure by the o-carborane unit. Theoretical calculations indicated non-aromaticity for its center C₆ ring moiety as well as decreased HOMO and LUMO levels. The o-carborane moiety provides an electron-withdrawing character to the 2,2'-bithiophene unit through an inductive effect. Fused carboranes: The o-carborane-based π-conjugated compound, benzocarborano[2,1-b:3,4-b']dithiophene, was synthesized (see scheme). The 2,2'-bithiophene unit is fixed in the cisoid structure with high planarity by the o-carborane unit. The o-carborane moiety provides an electron-withdrawing character to the 2,2'-bithiophene unit through an inductive effect. Copyright

Full text of DOI:10.1002/chem.201201513

FULL PAPER
DOI: 10.1002/chem.201201513
Synthesis and Properties of Thiophene-Fused Benzocarborane
Yasuhiro Morisaki,* Masato Tominaga, and Yoshiki Chujo*[a]
Abstract: The o-carborane-based p-conjugated compound, benzocarborano-
[2,1-b:3,4-b’]dithiophene was synthesized. Its crystal structure revealed high copla-
narity for the two thiophene rings of the 2,2’-bithiophene skeleton, which is fixed
in the cisoid structure by the o-carborane unit. Theoretical calculations indicated
non-aromaticity for its center C6 ring moiety as well as decreased HOMO and
LUMO levels. The o-carborane moiety provides an electron-withdrawing character
to the 2,2’-bithiophene unit through an inductive effect.
Keywords: carboranes · conjuga-
tion · electronic structure · fused-
ring systems · sulfur heterocycles
Introduction
Carboranes are a class of polyhedral boron cluster com-
pounds containing two carbon atoms in the cluster cage.
Representative examples of this class of compound are the
icosahedral boron–carbon clusters o-, m-, and p-carborane
(o-, m-, and p-C2B10H12, respectively) shown in Scheme 1.
They exhibit three-center two-electron bonds as well as
three dimensionally conjugated electrons that confer high
thermal and chemical stability.[1] Because of these unique
properties, such compounds have been widely used in vari-
ous areas ranging from, for example, heat-resistant poly-
mers[2] for industrial use to boron-neutron capture therapy
(BNCT) in the field of health and medical care.[3] Several p-
conjugated compounds and polymers in which carboranes
are incorporated into the extended p-conjugation systems
have been reported. For example, Yamamoto and co-work-
ers synthesized carborane–fullerene as well as carborane–
Scheme 1. Structures of o-, m-, p-carboranes, and o-carborane-based
ferrocene conjugated dyads by taking advantage of the
strong electron-withdrawing character of carborane, which
leads to considerable hyperpolarizability.[4] Tour and co-
workers employed carborane as a wheel for their nanocars
by exploiting the highly symmetric structural features of car-
borane rather than the electronic features.[5] A series of car-
borane-appended 1,3,5-triphenylbenzene and 1,3,5-tris(bi-
phenyl-4-yl)benzene derivatives were synthesized, and their
emission behavior was investigated.[6] Moreover, conjugated
polymers containing carboranes in their main chains have
compounds.
also been prepared; carboranes have been incorporated into
poly(p-arylene)[7a] and polythiophene[7b,c] skeletons. In addi-
tion, carborane-based luminescent conjugated polymers
have been reported independently by two research groups.
Coughlin, Carter, and co-workers synthesized o- and p-car-
borane-containing polyfluorenes[8] by Yamamoto coupling
polymerization[9] and described their application as reversi-
ble colorimetric sensors for volatile nitrogen-containing mol-
ecules.[8c] Our group has also synthesized o- and m-carbor-
ane-containing poly(p-arylene-ethynylene) derivatives[10] by
Sonogashira–Hagihara coupling.[11] We found that the o-car-
borane-containing poly(p-arylene-ethynylene) materials ex-
hibited aggregation-induced-emission (AIE)[12] in the film or
aggregated form, whereas their emission was quenched in
solution. This emission behavior stems from the variable
carbon–carbon bond in the phenyl-substituted o-carborane
moiety. Very recently, it was reported that p-electron sys-
tems with a o-carborane core act as electron acceptors by
[a] Prof. Y. Morisaki, M. Tominaga, Prof. Y. Chujo
Department of Polymer Chemistry
Graduate School of Engineering, Kyoto University
Katsura, Nishikyo-ku, Kyoto 615-8510 (Japan)
Fax: (+81)75-383-2607
E-mail: ymo@chujo.synchem.kyoto-u.ac.jp
chujo@chujo.synchem.kyoto-u.ac.jp
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.201201513.
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photo-excitation, and their charge-transfer (CT) behavior in
the excited state was investigated in detail.[13] Thus, the car-
borane skeleton is an attractive framework for constructing
new high-performance p-conjugated compounds.
(4) in 52% yield. The bromine–lithium exchange reaction of
4 with nBuLi proceeded, and successive treatment with
ZnBr2 and CuCl2 provided the target compound 5 in 50%
isolated yield.
Inspired by these results, we extended our work to create
o-carborane-based p-conjugated molecules. In this study, we
focused on benzocarboranes (Scheme 1), in which a benzene
ring and the carbon–carbon bond of the o-carborane are
fused to form a benzenoid ring structure on the carborane
cage.[14] Specifically, we prepared benzocarborano[2,1-b:
3,4-b’]dithiophene (Scheme 1), in which the thiophene rings
are further fused on the benzenoid unit. Note that the for-
mation of the benzocarborano[1,2-b:4,3-b’]dithiophene skel-
eton in the conjugated polymer backbone was implied by
the intramolecular b-b’ electrochemical cyclization of di-
(2-thienyl)-o-carborane.[7c,15] However, there have been no
reports on the synthesis or isolation of a series of benzocar-
boranodithiophene derivatives, and their fundamental prop-
erties remain unclear. Here, we report a detailed synthetic
procedure for and characteristics of benzocarborano-
[2,1-b:3,4-b’]dithiophene, and we discuss its potential use as
a building block for various o-carborane-based p-conjugated
molecules.
Compound 5 was obtained as an air- and moisture-stable
white solid, which was characterized by 1H, 13C, and
11B NMR spectroscopic analysis (Figure S6–8 in the Sup-
porting Information), high-resolution mass analysis, and ele-
mental analysis. Single crystals were readily formed from
CHCl3 and MeOH solution, and the molecular structure was
determined by X-ray crystallography. As shown in Figure 1,
Figure 1. Molecular structures of 5. Thermal ellipsoids are drawn at the
À
30% probability level. Selected bond lengths (ꢁ) are: C1 C2 1.376(9);
Results and Discussion
À
À
À
À
C2 C3 1.419(10); C3 C4 1.351(10); C4 C5 1.494(9); C5 C6 1.634(8);
À
C6 C1 1.483(8).
The synthetic procedure used to access benzocarborano-
[2,1-b:3,4-b’]dithiophene (5) is shown in Scheme 2. 3-Iodo-
thiophene (1) was brominated at the 2-position by N-bromo-
succinimide (NBS) to obtain 2-bromo-3-iodothiophene (2)
in 44% yield. Sonogashira–Hagihara coupling reaction of 2
with trimethylsilylacetylene was carried out by using a
the crystal structure of 5 revealed the high planarity of the
2,2’-bithiophene skeleton with a torsion angle (S1-C2-C3-
C4) of 179.268 between the two thiophene rings, which was
fixed with a cisoid structure. From the structure of 5, it
looks as if the 3- and 3’-positions in bithiophene are fas-
[PdCl
G
2ACHTUNGTRENNUNG(PPh3)2]/CuI catalytic system with 1,8-diazabicyclo-
[5.4.0]undec-7-ene (DBU) in benzene and H2O,[16] and the
tened with a bulldog clip. The C C bond lengths of the
À
À
desilylation also proceeded simultaneously to afford 3 in
48% yield. Treatment of 3 and decaborane with N,N-di-
center C6 ring are found to be 1.376(9) (C1 C2), 1.419(10)
À
À
À
(C2 C3), 1.351(10) (C3 C4), 1.494(9) (C4 C5), 1.634(8)
(C5 C6), and 1.483(8) ꢁ (C6 C1). In particular, the results
show that the C5 C6 bond length (1.634(8) ꢁ) of the car-
À
À
A
N
À
borane cage in 5 is much longer than that in benzene
(1.397 ꢁ) and is similar to that for benzocarborane
(1.651 ꢁ).[17] In addition, the C1 C2 and C3 C4 bond
lengths in 5 (1.376(9) and 1.351(10) ꢁ, respectively) are
À
À
À
shorter than the C2 C3 bond length (1.419(10) ꢁ), indicat-
ing that the C1-C2-C3-C4 butadiene skeleton consists of lo-
calized double bonds and a single bond.[17]
From the molecular structure as well as from reports by
Matteson[14] and Wade,[17] aromaticity is not expected for the
center C6 ring in 5. To obtain further insight into the aroma-
ticity, the nucleus-independent chemical shift (NICS) values
were calculated at the B3LYP/6-31G
ACHTUNGTREN(NUNG d,p)//B3LYP/6-31G-
AHCTUNGTREG(NNNU d,p) level, together with values for related compounds
benzo[2,1-b:3,4-b’]dithiophene and 4,5-dihydrobenzo[2,1-b:
3,4-b’]dithiophene. As shown in Figure 2, the NICS(1)
values of the thiophene rings were estimated to be less than
À8.0 for all the compounds, suggesting their aromaticity.
The NICS(1) value of the benzene ring in benzo[2,1-b:
Scheme 2. Synthesis of 5.
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Thiophene-Fused Benzocarborane
FULL PAPER
benzo[2,1-b:3,4-b’]dithiophene (À5.76 and À0.97 eV for
HOMO and LUMO, respectively) and 4,5-dihydroben-
zo[2,1-b:3,4-b’]dithiophene (À5.21 and À1.09 eV for HOMO
and LUMO, respectively), indicating the electron-withdraw-
ing character of the carborane unit. Both the HOMO and
LUMO of 5 were centered on the bithiophene moiety, and
no overlap of orbitals was observed between bithiophene
and o-carborane units. For the present fused system, charge-
transfer (CT) transition is not expected, and the o-carborane
moiety acts as an electron-withdrawing group operating
through an inductive effect rather than a conjugation effect.
We also constructed extended p-conjugation systems by
Figure 2. NICS values (ppm) of 5, benzo[2,1-b:3,4-b’]dithiophene, and
4,5-dihydrobenzo[2,1-b:3,4-b’]dithiophene calculated at the B3LYP/6-
31GACHTUNGTRENNUNG(d,p)//B3LYP/6-31GACHTUNGTRENNUNG(d,p) level.
3,4-b’]dithiophene was À11.9, whereas those of the C6 rings
in and 4,5-dihydrobenzo[2,1-b:3,4-b’]dithiophene were
5
À2.0 and À0.4, respectively. Although the NICS(1) value
(À2.0) for the center C6 ring in 5 is smaller than that in 4,5-
dihydrobenzo[2,1-b:3,4-b’]dithiophene (À0.4), this result
À
does not confirm that the C C bond of the carborane cage
in 5 possesses double bond character to express the aroma-
ticity of the center C6 ring moiety.
The highest occupied molecular orbitals (HOMOs) and
the lowest unoccupied molecular orbitals (LUMOs) of 5,
benzo[2,1-b:3,4-b’]dithiophene, and 4,5-dihydrobenzo[2,1-b:
3,4-b’]dithiophene were estimated (Figure 3). The HOMO
and LUMO levels of 5 were calculated to be À6.13 and
À1.99 eV, respectively. These values are lower than those of
Scheme 3. Synthesis of 7 and 8.
using compound 5 as a scaf-
fold. As shown in Scheme 3,
bromination of 5 by NBS pro-
ceeded smoothly to afford 6 in
67% yield.[18] Treatment of 6
with
2-(tributylstannyl)thio-
phene and 5-(tributylstannyl)-
2,2’-bithiophene in the pres-
ence of [Pd
(2-furyl)phosphane (TFP)[19] or
[Pd(PPh3)4] afforded the corre-
sponding oligothiophenes
2ACHTUNGTREN(NUNG dba)3] with tri-
AHCTUNGTRENNUNG
7
and 8 in 45 and 39% isolated
yields, respectively, as air-
stable powders.[20] UV/Vis ab-
sorption spectra and photo-
AHCTUNGTERGlNNUN uminescence spectra of com-
pounds 5, 7, and 8 in CHCl3
(1.0ꢂ10À5 m) are shown in
Figure 4,[21] and their optical
properties are summarized in
Figure 3. The HOMO and LUMO orbitals and their energy levels calculated at the B3LYP/6-31GACHTNUTRGNE(NUG d,p) level of
theory for compound 5, benzo[2,1-b:3,4-b’]dithiophene, and 4,5-dihydrobenzo[2,1-b:3,4-b’]dithiophene.
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Y. Morisaki, Y. Chujo and M. Tominaga
Table 1. Optical and electrochemical data.
Compound
UV/Vis[a]
PL[a,d]
CV[f]
LUMO
[eV]
[e]
red
ox
labs,max
e
labs,edge
Eg
lpl,max
Fpl
Eonset
[V]
Eonset
[V]
HOMO
[eV]
[b]
[nm]
[MÀ1 cmÀ1
]
[nm]
[eV][c]
[nm]
5
310, 323, 337
305
261, 315, 414
250, 392
265, 320, 380, 458
255, 324, 422
15000
11000
47000
39000
–
350
350
475
460
530
520
3.5
3.5
2.6
2.7
2.3
2.4
365
366
460, 489
459, 488
519, 555
507, 549
0.01
0.11
0.21
0.20
0.45
0.34
À2.1
À2.7
À2.3[g]
À3.0
À2.5[g]
–
1.1
0.8
0.6
0.4
–
À5.9
À5.6
À5.4
À5.2
–
BT[h]
7
–
À1.8
–
QT[i]
8
–
–
ST[j]
–
–
–
–
[a] In CHCl3 solution (1.0ꢂ10À5 m) at room temperature. Solubility of 8 and ST was less than 1.0ꢂ10À5 m in CHCl3; therefore, the data were collected
from highly diluted CHCl3 solutions. [b] Molar extinction coefficient. [c] Band gap energy: Eg =1240/labs,edge. [d] Photoluminescence (excited at each
labs,max). [e] Absolute photoluminescence quantum efficiency. [f] In solution (1.0 mm) at room temperature, vs. ferrocene/ferrocenium external reference.
See Figure S26 and S27 in the Supporting Information. [g] LUMO=HOMO+Eg. [h] Bithiphene. [i] Quaterthiophene. [j] Sexithiophene.
Scheme 4. Structures of bithiophene (BT), quaterthiophene (QT), and
sexithiophene (ST).
motion of the two thiophene rings by the o-carborane skele-
ton in 5, 7, and 8, which can also explain the smaller Stokes
shifts of 5, 7, and 8 compared with those of the series of
oligoACHTNUTRGENNUGthioACHTUNGTRENpNNGU henes (Table 1). As shown in Figure 4b, com-
pounds 5, 7, and 8 emitted by photo-excitation (in CHCl3;
1.0ꢂ10À5 m) absolute photoluminescence quantum efficien-
cies (Fpls) of 0.01, 0.21, and 0.45, respectively. Their spectra
also exhibited vibrational structures, as observed in the spec-
tra of the corresponding oligothiophenes BT, QT, and ST
(Figure S21–23 in the Supporting Information). In addition,
AIE was not observed in 5, 7, and 8. These results imply
that the photoluminescence of benzocarborano[2,1-b:
3,4-b’]dithiophene and its derivatives is the common S1–S0
emission rather than CT emission due to suppression of the
Figure 4. a) UV/Vis absorption spectra of 5, 7, and 8 in CHCl3 (1.0ꢂ
10À5 m). b) Photoluminescence spectra of 5, 7, and 8 in CHCl3 (1.0ꢂ
10À5 m) excited at each absorption maximum.
À
mobility of the carbon carbon bond in the o-carborane
cage.
Table 1 together with data[22] for bithiphene (BT), quater-
thiophene (QT), and sexithiophene (ST) (Scheme 4). As
shown in Figure 4a, the longest absorption bands of 5, 7,
and 8, which are ascribed to the p–p* transition, are batho-
chromically shifted with a concomitant increase in the molar
extinction coefficient (e) (Table 1) in accordance with the
extension of the p-conjugation systems.[22,23] The absorption
maxima (labs,max) of 5, 7, and 8 were observed at longer
wavelengths than those of the corresponding oligothio-
phenes BT, QT, and ST (Table 1). This result is rationalized
as being partly the result of suppression of the rotary
The HOMO–LUMO band gaps for all compounds were
estimated from their absorption edges, which are listed in
Table 1. In addition, the HOMO and LUMO energy levels
were also estimated from the cyclic voltammetry peak onset
potentials as well as the band gap energy (Figure 5). Com-
pounds 5 and 7 exhibited both oxidation and reduction
peaks, whereas only oxidation peaks were observed in BT
and QT. For example, the energy band gaps of 5 and BT
were almost identical (3.5 eV according to the absorption
spectra as well as 3.2 eV according to the cyclic voltammo-
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Thiophene-Fused Benzocarborane
FULL PAPER
ference between 2Tf and 5 is
their conjugation system. The
À
C F bond orbital is conjugated
with the bithiophene p-or
bit
N
in the LUMO of 2Tf, indicating
that the contribution of the
conjugation effect decreases
the energy level, in contrast to
the inductive effect of 5. The
labs,max of 2Tf is 348 nm, which
is longer than that of
5
(labs,max =323 nm) due to the
conjugation effect.
From the structural view-
point, the bithiophene unit of 5
adopts the fixed cisoid form, in
common with, for example,
benzo[2,1-b:3,4-b’]dithiophene,
4,5-dihydrobenzo[2,1-b:3,4-b’]
dithiophene, methylene-bridg-
ed bithiophene, difluoro
ylene-bridged bithiophene
2Tf,[24] dithienosilole,[25] and
dithieno-fused
1,2-diborin[26]
ACHTUNGTRENNUNGmeth-
AHCTUNGTRENNUNG
(Scheme 6).[27] However, the
conjugation systems of the bi-
thiophene units in benzo-
[2,1-b:3,4-b’]dithiophene, 2Tf,
dithienosilole, and dithieno-
fused 1,2-diborin are conjugat-
ed with their bridge units
(-CH=CH-, -CF2-, -SiR2-, and
-BACHTNURTGENNUG(NMe2)-BHCATUNGTREN(NGUN NMe2)-, respec-
tively). In addition, the bithio-
phene units in 4,5-dihydroben-
zo[2,1-b:3,4-b’]dithiophene and
dithieno-fused 1,2-diborin are
Figure 5. a) Cyclic voltammograms of 5 and BT. In CH3CN (1.0 mm) containing NBu4BF4 (0.1m) for oxidation
and DMF (1.0 mm) containing NBu4ClO4 (0.1m) for reduction using a Pt working electrode, a Pt wire counter
electrode, a Ag/AgCl reference electrode, and a ferrocene/ferrocenium external standard at room temperature
with a scan rate of 0.1 VsÀ1 under N2. b) Cyclic voltammograms of 7 and QT. In THF (1.0 mm) containing
NBu4ClO4 (0.1m) for oxidation and DMF (1.0 mm) containing NBu4ClO4 (0.1m) for reduction using a Pt work-
ing electrode, a Pt wire counter electrode, a Ag/AgCl reference electrode, and a ferrocene/ferrocenium exter-
nal standard at room temperature with scan rate of 0.1 VsÀ1 under N2.
À
À
twisted by C C and B B
bridge chains, respectively.
Thus, o-carborane is regarded
as
a molecular bulldog clip
that fastens biaryls to achieve
the high planarity without elec-
tronic perturbations in the con-
jugation systems.
grams in Table 1); however, the HOMO and LUMO levels
of 5 were approximately 0.3 eV lower than those of BT.
Thus, the carborane moiety acts as an electron-withdrawing
group acting through an inductive effect rather than a conju-
gation effect.
For comparison, we consider the difluoromethylene-bridg-
ed bithiophene 2Tf, in which bithiophene is fixed with the
electron-withdrawing -CF2- group (Scheme 5).[24] Compound
2Tf is electronegative with an anodic peak potential Epa of +
0.99 V and cathodic peak potential Epc of À2.42 V. The Epc
value of 5 (À2.3 V in Figure S26 in the Supporting Informa-
tion) is slightly larger than that of 2Tf, indicating that 5 is
Scheme 5. Structures of compound 5 and difluoromethylene-bridged bi-
comparable to 2Tf in terms of its electronegativity. The dif-
thiophene (2Tf).
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Y. Morisaki, Y. Chujo and M. Tominaga
stirring, anhydrous benzene (53 mL) was added under an Ar atmosphere
by using a syringe. DBU (9.62 g, 63.2 mmol) was added by using a sy-
ringe, followed by a purge of the reaction flask with Ar. Ice-chilled trime-
thylsilylacetylene (0.745 mL, 5.27 mmol) was added by using a syringe,
and H2O (76.0 mL, 40 mol%) was immediately added.[4] The reaction
flask was covered in aluminum foil and the reaction mixture was heated
to reflux for 13 h, then poured into H2O and extracted with Et2O. The or-
ganic layer was washed with diluted aqueous HCl, brine, and dried over
MgSO4. After MgSO4 was removed, the solvent was reduced under
vacuum. The residue was purified by silica gel column chromatography
(hexane) to give 3 as a yellow solid (0.885 g, 48%). 1H NMR (400 MHz,
CDCl3): d=7.21 (d, J=5.60 Hz, 2H), 7.03 ppm (d, J=5.60 Hz, 2H);
13C NMR (100 MHz, CDCl3): d=129.8, 125.9, 124.2, 117.4, 86.6 ppm;
1H NMR chemical shifts were in agreement with reported values.[28]
Synthesis of 4: A mixture of 3 (734 mg, 2.11 mmol) and decaborane
(296 mg, 2.43 mmol) was dissolved in anhydrous toluene (21 mL) at RT
under an Ar atmosphere. N,N-Dimethylaniline (481 mL, 3.80 mmol) was
added and the mixture was heated to reflux for 24 h. After cooling to
RT, the mixture was decanted from the solid residue and evaporated.
The crude residue was subjected to silica gel column chromatography
with hexane as an eluent (Rf =0.30). Recrystallization from CHCl3 and
MeOH provided
4
as colorless crystals (593 mg, 52%). 1H NMR
(400 MHz, CDCl3): d=7.03 (d, J=6.09 Hz, 2H), 6.92 (d, J=6.09 Hz,
2H), 4.60–1.60 ppm (br, 10H; B-H); 13C NMR (100 MHz, CDCl3): d=
131.2, 127.6, 125.6 (Ar), 116.5, 81.0 ppm (carborane-C); 11B NMR
(128 MHz, CDCl3): d=À2.15 (d,
J
N
Synthesis of 5: A solution of 4 (187 mg, 0.400 mmol) in THF (4.0 mL)
was added slowly to a stirred solution of nBuLi (1.60m in hexane,
530 mL) under an Ar atmosphere at À788C by using a syringe, and the
mixture was stirred at À788C for 40 min. A solution of anhydrous ZnBr2
(180 mg, 0.800 mmol) in THF (1.0 mL) was added dropwise to the stirred
solution under an Ar atmosphere at À788C by using a syringe, and the
mixture was stirred at À788C for 1 h. Anhydrous CuCl2 (53.1 mg,
0.400 mmol) was added in one portion with vigorous stirring and the mix-
ture was stirred at À788C for 3 h. The reaction mixture was allowed to
slowly warm to RT and, after stirring at RT for 20 h, the reaction was
quenched by the addition of aqueous ammonia and extracted with
CHCl3. The CHCl3 solution was washed with aqueous ammonia and
brine, and dried over MgSO4. MgSO4 was removed, and the solvent was
evaporated. The residue was purified by silica gel column chromatogra-
phy with hexane as an eluent (Rf =0.46) to obtain 5 as a white solid
(56.8 mg, 0.20 mmol, 50%). Single crystals of 5 were obtained by recrys-
Scheme 6. Structures of cisoid-fixed bithiophenes and transoid-fixed bi-
thiophene.
Conclusion
This work introduces an o-carborane-based p-conjugated
compound, benzocarborano[2,1-b:3,4-b’]dithiophene, in
which the 2,2’-bithiophene unit is fixed in the cisoid struc-
ture by the o-carborane unit. Its crystal structure revealed
high coplanarity for the two thiophene rings of the 2,2’-bi-
thiophene skeleton and non-aromaticity for the center C6
ring moiety. The o-carborane unit provides an electron-with-
drawing character to the 2,2’-bithiophene unit through an in-
ductive effect rather than a conjugation effect; according to
UV/Vis absorption spectroscopy and CV, it lowers both the
HOMO and LUMO levels of the 2,2’-bithiophene unit with-
out changing the band-gap energy, which was supported by
DFT calculations. In the aromatic-ring-fused benzocarbor-
ane, the CT state is not formed by photo-excitation, in con-
trast to the p-electron-system-substituted o-carboranes. Fi-
nally, the facile transformation of benzocarborano[2,1-b:
3,4-b’]dithiophene enables us to prepare various p-conjugat-
ed oligomers and polymers, offering great promise for use as
an electronegative p-conjugated building block.
1
tallization from CHCl3 and MeOH (CCDC-874952). H NMR (400 MHz,
CDCl3): d=7.29 (d, J=5.12 Hz, 2H), 7.22 (d, J=5.12 Hz, 2H), 3.60–
0.70 ppm (br, 10H; B-H); 13C NMR (100 MHz, CDCl3): d=131.6, 129.1,
126.7, 124.5, 73.2 ppm (carborane-C); 11B NMR (128 MHz, CDCl3): d=
À9.02 (d, J(B,H)=151.0 Hz, 2B), À12.9 ppm (m, overlapping signals,
G
8B); HRMS (EI+): m/z calcd for C10H14B10S2: 308.1468; found: 308.1479;
elemental analysis calcd (%) for C10H14B10S2: C 39.19, H 4.60, S 20.93;
found: C 38.90, H 4.44, S 20.94. CCDC-874952 (5) and CCDC-874593 (6)
contain the supplementary crystallographic data for this paper. These
data can be obtained free of charge from The Cambridge Crystallograph-
ic Data Centre via www.ccdc.cam.ac.uk/data_request/cif.
Acknowledgements
Financial support from The Japan Securities Scholarship Foundation is
gratefully acknowledged. M. T. appreciates Research Fellowships from
the Japan Society for the Promotion of Science for Young Scientists.
Experimental Section
Synthesis of 3: A 100 mL round-bottom flask with a Teflon-coated mag-
netic stir bar was fitted with a reflux tube, and dried under vacuum. The
flask was purged with dry Ar and charged with [PdCl
2ACHTUNGTREN(UNNG PPh3)2] (0.444 g,
0.632 mmol), CuI (0.200 g, 1.05 mmol), and 2 (3.04 g, 10.5 mmol). While
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Thiophene-Fused Benzocarborane
FULL PAPER
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[18] A single crystal of 6 suitable for X-ray crystallography was obtained
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future.
[21] Their spectra are shown together with the corresponding oligothio-
phenes BT, QT, and ST in Figure S23–25 in the Supporting Informa-
tion.
ˇ
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Received: May 1, 2012
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Y. Morisaki, Y. Chujo and M. Tominaga
Carboranes
Y. Morisaki,* M. Tominaga,
Y. Chujo* ...................... &&&& — &&&&
Synthesis and Properties of Thio-
phene-Fused Benzocarborane
Fused carboranes: The o-carborane-
based p-conjugated compound, benzo-
carborano[2,1-b:3,4-b’]dithiophene was
synthesized (see scheme). The 2,2’-
bithiophene unit is fixed in the cisoid
structure with high planarity by the o-
carborane unit. The o-carborane
moiety provides an electron-withdraw-
ing character to the 2,2’-bithiophene
unit through an inductive effect.
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These are not the final page numbers!