Synthesis of self-threading bithiophenes and their structure-property relationships regarding cyclic side-chains with atomic precision

Article abstract of DOI:10.1002/asia.201100524

We have recently reported a self-threading polythiophene as a new family of insulated molecular wires. Herein, we focused on the structure-property relationships of the unique three-dimensional architecture of the monomer. We have synthesized nine self-threading bithiophene monomers that have cyclic side-chains of different size and flexibility: i.e., 21-, 22-, 23-, 24-, 26-, and 30-membered rings composed of paraffinic, olefinic, or alkynic chains. To investigate their structure-property relationships, ¹H NMR spectroscopy, UV absorption, and fluorescence spectroscopy measurements were conducted. We found that cyclic side-chains define the movable range of the dihedral angle of the bithiophene backbone, thereby affecting its photophysical properties. Therefore, the ability to design a structure with atomic precision as described herein would lead to the fine-tuning of the electronic properties of insulated molecular wires.

Full text of DOI:10.1002/asia.201100524

DOI: 10.1002/asia.201100524
Synthesis of Self-Threading Bithiophenes and their Structure–Property
Relationships Regarding Cyclic Side-Chains with Atomic Precision
Yuki Ouchi,[a, b] Kazunori Sugiyasu,*[a] Soichiro Ogi,[a] Akira Sato,[c] and
Masayuki Takeuchi*[a, b]
Abstract: We have recently reported a
self-threading polythiophene as a new
family of insulated molecular wires.
Herein, we focused on the structure–
property relationships of the unique
three-dimensional architecture of the
monomer. We have synthesized nine
self-threading bithiophene monomers
that have cyclic side-chains of different
size and flexibility: i.e., 21-, 22-, 23-,
24-, 26-, and 30-membered rings com-
posed of paraffinic, olefinic, or alkynic
chains. To investigate their structure–
property relationships, 1H NMR spec-
troscopy, UV absorption, and fluores-
cence spectroscopy measurements were
conducted. We found that cyclic side-
chains define the movable range of the
dihedral angle of the bithiophene back-
bone, thereby affecting its photophysi-
cal properties. Therefore, the ability to
design a structure with atomic preci-
sion as described herein would lead to
the fine-tuning of the electronic prop-
erties of insulated molecular wires.
Keywords: molecular wires · oligo-
mers · side-chains · structure–prop-
erty relationships · thiophenes
Introduction
property of the IMW-based OLED was found to be affected
by continuously thinning out the insulating cyclodextrin,
which was quantifiable by the so-called “threading ratio”.[5c]
Such a control over the optoelectronic properties of conju-
gated polymers—by taking advantages of the unique three-
dimensional structures of IMWs—enable us to not only opti-
mize the performance of but also to unravel the fundamen-
tals of organic electronics; in this context, understanding the
structure–property relationships of IMWs is particularly im-
portant.
Insulated molecular wires (IMWs) are p-conjugated poly-
mers which are sheathed with an electronically-inactive mo-
lecular layer.[1–3] A representative of the IMWs is polyrot-
axane structures in which a p-conjugated polymer axis is
threaded through many insulating cyclic molecules, such as
cyclodextrins. Their structural analogy with electric power
cords has aroused expectation of the realization of molecu-
lar integrated circuits.[4] In addition, the lack of the p p
À
stacking interactions in the solid state could lead to ad-
vanced functions and configurations in organic electronic
devices.[5] For instance, the insulating layer prevented inter-
wire interactions between the conjugated polymers, which
could improve luminescence efficiency of organic light-emit-
ting diodes (OLEDs).[5a] Furthermore, the luminescence
We have recently reported insulated polythiophenes as a
new family of IMWs (Scheme 1).[6] The unique three-dimen-
sional architecture of the monomer is reminiscent of rotax-
anes; however, the “ring” and “axis” are covalently connect-
ed. In other words, the p-conjugated molecular axis (i.e., bi-
thiophene) is threaded through its own cyclic side-chain; ac-
cordingly, we designated the monomer as a self-threading bi-
thiophene (STB6/6: where the subscript “6/6” denotes that
two hexyl chains (C6H12) are covering both faces of the bi-
thiophene p-platform, see Scheme 1). Bromination at the
5,5’-positions using NBS, followed by Yamamoto reductive
coupling, afforded the polymerized STB6/6 to provide a self-
threaded polythiophene (i.e., PSTB6/6). The striking features
of PSTB6/6 are as follows:
[a] Y. Ouchi, Dr. K. Sugiyasu, Dr. S. Ogi, Prof. Dr. M. Takeuchi
Organic Materials Group, Polymer Materials Unit
National Institute for Materials Science (NIMS)
1-2-1 Sengen, Tsukuba, Ibaraki 305-0047 (Japan)
Fax : (+81)29-859-2101
E-mail: SUGIYASU.Kazunori@nims.go.jp
TAKEUCHI.Masayuki@nims.go.jp
À
1) p p stacking between the adjacent polythiophene
[b] Y. Ouchi, Prof. Dr. M. Takeuchi
backbones is effectively prevented by the defect-free insulat-
ing layer even in the solid film.
Department of Materials Science and Engineering
Graduate School of Pure and Applied Sciences
University of Tsukuba
2) The covalently-connected cyclic side-chains impose a
co-planar conformation on the bithiophene backbone, there-
by extending the effective conjugation length (ECL).
This latter feature has engaged our interest because few
IMWs have such a merit.[3a–d] In the case of previously re-
ported STB6/6, the cyclic side-chain corresponds to 1,8,15,22-
1-1-1 Tennoudai, Tsukuba, Ibaraki 305-8571 (Japan)
[c] A. Sato
Materials Analysis Station
National Institute for Materials Science (NIMS)
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/asia.201100524.
Chem. Asian J. 2012, 7, 75 – 84
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75

FULL PAPERS
we can establish a new molecu-
lar design parameter in the tar-
geted synthesis of IMWs.
Results and Discussion
Synthesis of Self-Threading
Bithiophenes
Our synthetic strategy toward
STB derivatives is based upon
the double ring-closing meta-
thesis (RCM) reaction. In the
previously reported example,
the precursor had four 1-bu-
tenyl moieties, the two pairs of
which ring-close and result in
two 3-hexenyl chains that cross
over the bithiophene platform.
After hydrogenation of the
olefin moieties, STB6/6 is ob-
tained (Scheme 1).[6]
Scheme 1. Synthetic approach toward self-threading polythiophene: a) second-generation Grubbs catalyst,
CH2Cl2, b) H2, Pd/C, CH2Cl2/MeOH, c) NBS, CHCl3/AcOH, d) [Ni
ACHTUNGRTENN(UNG cod)2], cod, bipy, toluene/DMF. “Cyclic
side-chain” (green area in the axial view) corresponds to a 22-membered ring in the case of the previously re-
[6]
ported STB6/6
.
NBS=N-Bromosuccinimide, DMF=N,N-dimethylformamide, cod=1,5-cyclooctadiene,
The main framework of
bipy=2,2’-bipyridine.
STBs (i.e., 1ACHTNUTRGENNG(U OMe)4) was syn-
thesized from 3,3’-dibromo-
2,2’-bithiophene and 2,6-dimethoxyphenylboronic acid
through Suzuki–Miyaura coupling under Buchwald condi-
tetraoxaACHTUNGTRENNUNG[6.6]metacyclophane, which affords an overall 22-
membered ring (Scheme 1). Here, it occurred to us that de-
signing the ring size of the STB should control the planarity
of the bithiophene backbone, which in turn leads to the
tuning of the electronic properties of IMWs. In fact, Mayor
and co-workers have recently reported that the torsion
angle (f) of biphenyl derivatives determines their optoelec-
tronic characteristics, such as band gaps,[7a] NLO respon-
ses,[7b] and conductance,[7c–e] where the variations in the
properties showed proportional relationships to cos2f.[7,8]
Herein, we report on the synthesis of a family of STBs.
By varying the length of the alkyl chains, we have prepared
STB derivatives that have 21, 22 (reported[6]), 23, 24, 26, and
30-membered rings, through which the bithiophene back-
bone is threaded. We found that the ring size defines the
movable range of the dihedral angle and affects the elec-
tronic properties of the bithiophene. This result means that
tions (SPhos, [Pd
thylation of 1(OMe)4 using BBr3 afforded 1(OH)4 in quanti-
(dba)3], and K3PO4 in toluene).[9] Deme-
2ACHTUNGTRENNUNG
AHCTUNGTRENNUNG
tative yield. We first carried out Williamson ether synthesis
for 1(OH)4 to introduce RCM-active olefinic side-chains
(Scheme 2c: 4-bromobutene, K2CO3, DMF, 908C); however,
we mainly obtained di-(104/04)- and tri-(104/44)-substituted in-
termediates, together with 144/44 in poor yield (<20%),
which is probably because of the steric hindrance of 1(OH)4.
We thus applied the modified Mitsunobu reaction reported
by Lepore and He.[10] High-concentration conditions and
sonication in the presence of 3-buten-1-ol, DIAD, and PPh3
in tetrahydrofuran were effective against the sterically hin-
dered reaction sites of 1(OH)4, thus affording 144/44 in good
yield (88%). Likewise, 155/55 and 166/66 were synthesized using
corresponding olefinic alcohols. The precursors obtained
through this approach (i.e., 144/44, 155/55, and 166/66) led only to
symmetrical STB products, of which insulating alkyl chains
were composed of even numbers of carbon atoms (i.e.,
STB6/6, STB8/8, and STB10/10, respectively). In order to syn-
thesize a variety of STBs, we made use of the products from
the abovementioned Williamson ether synthesis (i.e., 104/04
and 104/44 in Scheme 2). Through the modified Mitsunobu
conditions, 104/04 led to 134/34 and 145/45 by reacting with allyl
alcohol and 4-penten-1-ol, respectively. Likewise, 104/44 af-
forded 134/44 and 144/45. Consequently, we have prepared
seven precursors for the double RCM reaction, namely: 134/
34, 134/44, 144/44, 144/45, 145/45, 155/55, and 166/66 (Scheme 2).
Abstract in Japanese:
So far, multiple RCM reactions have successfully been ap-
plied to synthesize three-dimensional molecular architec-
tures such as catenanes,[11] rotaxanes,[12] gyroscopes,[13] and so
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Chem. Asian J. 2012, 7, 75 – 84

Synthesis of Self-Threading Bithiophenes and their Structure–Property Relationships
Scheme 2. Synthesis of precursors for the double RCM: a) SPhos, [Pd2ACTHNUTGRNEUNG(dba)3], K3PO4, 2,6-dimethoxyphenylboronic acid, toluene, b) BBr3, CH2Cl2,
c) K2CO3, 4-bromobutene, DMF, d) corresponding olefinic alcohol, PPh3, DIAD, THF, sonication. dba=dibenzylideneacetone, DIAD=diisopropyl azo-
dicarboxylate, THF=tetrahydrofuran.
forth.[14,15] Particularliy related to our approach are the ex-
amples by Cox and co-workers[16] and Gladysz and co-work-
ers[17] in which they have reported strategies to synthesize
IMWs through double RCM reactions. RCM reaction using
second-generation Grubbs catalyst was doubly applied on
both faces of the bithiophene platform (Scheme 3). By clos-
ing the two pairs of olefinic side-chains that face each other,
mation, Figure S1). A single RCM reaction attempted using
24/4 (Scheme 4) revealed that olefinic side-chains on the
same resorcinol group cannot cyclize but polymerize by acy-
clic diene metathesis (ADMET) polymerization; this obser-
vation implies the selective double RCM reaction among
the four olefinic side-chains in 144/44. Furthermore, it is re-
markable that STBD showed only a single olefinic peak
6/6
1ab/cd affords STBD
(here, the superscript “D”
(He) in the 1H NMR spectrum, which indicates that the
metathesis reaction of 144/44 is regiospecific. We confirmed
from its X-ray crystal structure that only the trans configura-
(a+b)À2/(c+d)À2
denotes the double bonds in the ring). Figure 1 shows
1H NMR spectra taken before and after the double RCM
reaction of 144/44; after 80 min reaction time, 144/44 was exclu-
tion of STBD had formed; thus, we designated this particu-
6/6
sively converted into STBD (also see the Supporting Infor-
lar compound as (E)-STBD (Figure 1c). The downfield
6/6
6/6
shifts of the protons Ha, Hb,
and Hc, and upfield shifts of
the protons Hd and He, induced
by the double RCM reaction,
are consistent when considering
the enhanced ring current
effect by the planarized bithio-
phene
(Figure 1d).[18]
The
quantitative and regiospecific
double RCM reaction of 144/44
and the planar conformation of
the product (aC3C2C2’C3ꢁ of
(E)-STBD is 179.768) suggest
6/6
that the 22-membered ring per-
fectly fits the girth of the bi-
thiophene.
Scheme 3. Synthesis of STBD
CH2Cl2/MeOH.
and STBm/n: a) second-generation Grubbs catalyst, CH2Cl2, b) H2, Pd/C,
With the above successful ex-
ample in mind, we carried out
m/n
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K. Sugiyasu, M. Takeuchi et al.
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Figure 1. 1H NMR spectra taken a) before and b) after 80 min of the double RCM reaction of 144/44. c) X-ray crystal structure of (E)-STBD6/6. d) Ring-cur-
rent effect of the planarized bithiophene backbone; Ha: +0.29 ppm; Hb: +0.43 ppm; Hc: +0.29 and +0.17 ppm; Hd: À0.09 ppm; He: À0.58 ppm shifts
were induced by double RCM reaction. Spectrum (b) was taken after removing the catalyst by filtering through a short silica gel pad.
two isomers were not interconvertible even after stirred at
1
1008C for 24 hours, which was confirmed by H NMR spec-
troscopy (in 1,1,2,2-[D2]tetrachloroethane). This result indi-
cates that the alkyl chains in the ring cannot stride over the
bithiophene framework (like a jumping rope) in case they
are shorter than C10H20. This observation demonstrates the
well-defined structure of PSTB6/6 in our previous study.
These reactions using a systematic series of the precursors
revealed that the available cyclic side-chain of STBm/n is be-
tween 21- and 30-membered rings: here, the ring-size equals
to m+n+10. All the STBD products were subjected to hy-
m/n
drogenation to provide the STBm/n products. As a reference
compound, we also synthesized 16/44 by stopping the double
RCM reaction of 144/44 before the completion followed by
hydrogenation (Scheme 4).
Scheme 4. Structures of the compounds discussed in the text: ones of the
In addition to the double RCM approach, we have ap-
plied a double ring-closing alkyne metathesis reaction for
enantiomers of 16/44 1D5/34, and UTBD
are shown, which were used as
10/10
racemates.
3
55/55, which afforded STBT (Scheme 5, the superscript “T”
6/6
denotes the triple bonds in the ring).[20] Furthermore, semi-
the double RCM reactions of other precursors (Scheme 3).
reduction of the triple bonds of STBT through standard
6/6
In the case of the smallest precursor (i.e., 134/34), STBD was
Lindlar hydrogenation led to (Z)-STBD6/6, a cis isomer of
5/5
[21]
not obtainable; a pair of olefinic side-chains cyclizes but the
other pair seems unable to reach each other, which yielded
(E)-STBD
.
6/6
The influence of the flexiblity of the ring
(i.e., paraffinic, olefinic, and alkynic side-chains in STB6/6,
STBD6/6, and STBT6/6, respectively) on photophysical proper-
ties of bithiophene is also worth considering.
STBD
(Scheme 4). The second-smallest precursor, 134/44,
5/34
afforded STBD5/6. Thus, the structural threshold for the
double RCM reaction is very precise, and the 21-membered
ring appeared to be the smallest in the STB family. It is in-
teresting to note the coincidence of this size with that of the
crown ether [21]crown-7 which is known as the smallest ring
component for rotaxane formation.[19] Using the same proce-
Structure–Property Relationships of Self-Threading
Bithiophenes
1
Figure 2a displays aromatic region of the H NMR spectra
dure, STBD6/7, STBD7/7, and STBD were obtained from 144/
of STBm/n. As mentioned earlier, the double RCM induces
8/8
45, 145/45, and 155/55, respectively. Interestingly, in the case of
the largest precursor (166/66),
peak shifts owing to the deshielding effect by the planarized
STBD
was obtained togeth-
10/10
er with an unthreaded confor-
mational isomer (UTBD
,
10/10
Scheme 4), which is formed via
STBD(a+c)À2/(b+d)À2-type
mis-
matching double RCM. The
ratio of [STBD10/10]/[UTBD
]
10/10
in the reaction mixture was
about 1/0.8, and they are isola-
ble by column chromatograpy
on silica gel. In addition, these
Scheme 5. Synthesis of STBT6/6 and (Z)-STBD6/6: a) 2-fluorophenol, Mo(CO)6, PhCl,[20] b) Lindlar catalyst, quin-
oline, H2 gas.[21]
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Synthesis of Self-Threading Bithiophenes and their Structure–Property Relationships
Figure 3. Axial views of a) STB6/6, b) STB10/10, and c) 1
with the dihedral angles of their bithiophene backbone.
ACHTUNGRTENUN(NG OMe)4, shown
mation, Figure S2). This feature seems to be different from
those reported by Mayor and co-workers, in which dihedral
angles are twisted even in the crystallized form.[7] Hence, in-
stead of f, we hereafter regard the chemical shift of the
proton Ha (dHa) as an index that reflects the movable range
of the dihedral angle of STBs in solution: for reference, dHa
of 144/44 is 7.00 ppm.
Having a family of STBs in hand, we evaluated the photo-
physical properties of STBm/n products in dichloromethane;
Figure 4 compares their UV absorption spectra. All the
STBm/n compounds showed identical absorption maxima at
318 nm which is longer than that of 1ACTHNUTRGENUN(G OMe)4 (285 nm) be-
cause the cyclic side-chains dictate co-planar conformation
to the bithiophene backbone. Small but significant differ-
ence among the STBm/n compounds is that the vibrational
fine structure became less pronounced as the ring-size in-
creased. In addition, IR spectra also became less structured
with increasing ring size (see the Supporting Information,
Figure S3).[23] These results indicate that the molecular
motion is restricted by the cyclic side-chains. Against the
ring size, a plot of the ratio of A318 to A285 showed similar
tendency with that of dHa (compare Figure 2b and 4b). We
thus plotted the A318/A285 as a function of dHa. The plot
showed a linear relationship, which suggests the structure–
property relationship of the STBm/n compounds.
Figure 2. a) 1H NMR spectra of STB5/6 (i), STB6/6 (ii), STB6/7 (iii), STB7/7
(iv), STB8/8 (v), and STB10/10 (vi). b) Plot of the chemical shift of proton
“Ha” (d) versus the ring size.
bithiophene p-system. As shown in Figure 2b, the chemical
shift of the Ha proton moved upfield by letting the ring
become looser. This change plateaued with STB10/10, the
largest family. This result indicates that the population of
the planar conformation decreases with increasing the ring
size.
X-ray crystallographic structures of STB6/6, STB10/10, and
1
(OMe)4 are shown in Figure 3. Although it is known that
Because a potential-energy surface of the excited state is
generally steeper than that of the ground state, more-distinct
differences among the STBs could be observed in their fluo-
rescence spectra. As shown in Figure 5a, the fluorescence
maxima (lem) shifted to longer wavelengths as the ring-size
increased. Excitation spectra of STBs completely traced
their absorption spectra (see the Supporting Information,
Figure S4). In addition, the fluorescence of the STBm/n com-
pounds was independent of the solvent polarity: measured
in dimethyl sulfoxide, tetrahydrofuran, chloroform, dichloro-
methane, and hexane. Therefore, we ruled out the “insula-
ordinary b-substitutions cause the dihedral twist,[22] all the
derivatives have nearly planar bithiophene backbones in
crystallized form (Figure 3). This result implies that a planar
conformation can be allowed by a packing effect in the crys-
tal. In fact, the rotational barrier at f=1808 was calculated
to be<1 kJmolÀ1 for 1
ACTHNUTRGNEN(UG OMe)4, which is small enough to be
overcome at room temperature. Thus, the dihedral angles of
the bithiophene backbone of the STBm/n compounds are not
immobilized but fluctuating within the range determined by
the cyclic side-chains in solution (see the Supporting Infor-
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K. Sugiyasu, M. Takeuchi et al.
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Figure 4. a) Normalized absorption spectra of STB5/6 (i), STB6/6 (ii), STB6/7 (iii), STB7/7 (iv), STB8/8 (v), STB10/10 (vi), and 1
between absorption intensities at 318 nm and 285 nm as a function of the ring size (b) and the dHa (c).
ACHTUNGRTEN(NUNG OMe)4 (vii). Plots of the ratio
tion” effect of the cyclic side-chains on the observed fluores-
cence difference.
bone because aromatic/quinonoid structural rearrangement
is also accompanied throughout the charge-carrier trans-
port.[27]
According to the Franck–Condon principle, all electronic
transitions are vertical; namely, they proceed without a
change in the molecular geometry.[24] In general, in the
ground state, oligoaromatic conjugation is more or less
twisted owing to the steric hindrance between the side-
chains.[7,22] In contrast, co-planar conformation is favored in
the excited state because the lowest unoccupied molecular
orbital (LUMO) is delocalized over the quinonoid-type con-
jugation.[25,26] Thus, after the vertical transition processes
(i.e., absorption and emission), the conjugation backbone
undergoes geometrical rearrangements, which loses excita-
tion energy as much as the so-called reorganization energy
(also referred to as relaxation or equilibration energy). One
can observe this as Stokesꢁ shifts: the energy space between
the absorption and fluorescence maxima. This value can re-
flect the charge carrier mobility along a polythiophene back-
As shown in Figure 5b (filled circles), Stokesꢁ shifts of the
STBm/n bithiophenes decreased as the ring became small-
er.[28] STB6/6 showed the smallest Stokesꢁ shift because ther-
mal fluctuation of its dihedral angle is limited by the sym-
metrical and tight alkyl chains, and the co-planar conforma-
tion is favored even at the ground state.[28] As such, structur-
al similarity between the ground and excited states of STB6/6
results in the small reorganization energy. Importantly, we
confirmed that Stokesꢁ shifts of STBm/n in polystyrene (PS)
matrix—where molecular motion is rather restricted—were
each smaller than those measured in solution (compare the
filled circle and lozenge in Figure 5b). The difference in the
Stokesꢁ shifts between in solution and PS matrix should be
related to the molecular mobility, which is indeed dependent
on the ring size and proportional to dHa (Figure 5c). Thus,
Figure 5. a) Normalized fluorescence spectra of STB5/6 (i), STB6/6 (ii), STB6/7 (iii), STB7/7 (iv), STB8/8 (v), STB10/10 (vi), and 1ACHTNUGTRENUNG(OMe)4 (vii) displayed in a
stacked mode: in CH2Cl2 (0.05 mm) excited at 340 nm. b) Plots of Stokesꢁ shift of STBm/n as a function of dHa in solution (circle) and in PS film (square).
c) Plot of the difference between the Stokesꢁ shifts observed in solution and PS film as a function of dHa.
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Synthesis of Self-Threading Bithiophenes and their Structure–Property Relationships
the smaller the ring size is, the less the geometrical rear-
rangement at the excited state is accompanied. We, there-
Conclusions
fore, expect that tuning of the electronic functions of the
STB family is possible with atomic precision by designing
the movable range of the dihedral twist.[7,29–31]
We have synthesized a family of self-threading bithiophenes
having cyclic side-chains in different size: i.e., 21-, 22-, 23-,
24-, 26-, and 30-membered rings. Structure-property rela-
tionships revealed that STB6/6 has the smallest reorganiza-
tion energy because its conjugated backbone is imposed to
be co-planar by the symmetric and the most fitting alkyl
chains. Hence, the two hexyl chains in STBm/n (i.e., m=n=
6) are optimal in respect of effective conjugation, which sup-
ports our previous results where the PSTB6/6 showed an ex-
cellent hole mobility along the polythiophene wire (0.9 cm2/
Vs measured by a TRMC method).[6,32] The subtle differ-
ence in the size and symmetry of the cyclic side-chains
would therefore enable fine-tuning of the resistivity of the
IMWs.[7,8,33] Furthermore, by using STB6/6, (Z)-STBD6/6, (E)-
STBD6/6, and STBT6/6, we demonstrated that the electronic
characteristics can be preserved intact with a fixed ring-size.
Finally, comparisons among STB6/6, (E)-STBD6/6, (Z)-
STBD6/6, and STBT did not confirm significant differences
6/6
in absorption and fluorescence spectra (Figure 6). In fact,
their dHa values are 7.383, 7.379, 7.376, and 7.370 ppm, re-
spectively, thus indicating that they have more or less the
same movable range of dihedral angle in-between STB5/6
(dHa =7.398 ppm) and STB6/7 (dHa =7.360 ppm); see the
1
Supporting Information, Figure S5 for H NMR spectra of
STB6/6 derivatives. It is interesting to note that the photo-
physical properties of 16/44 (Scheme 4) are different from
those of STB6/6 but rather close to those of STB7/7 (Fig-
ure 6b,c). This result demonstrates the privilege of the
doubly-strapped structure. We conclude that the most-fa-
vored conformation is achieved with 22-membered cyclic
side-chains and flexibility of the ring is less influential in the
structure–property relationships than the size. This finding
should be of importance when designing advanced IMWs
from STB monomers, such as the ones suggested in the fol-
lowing conclusions.
These olefins and alkynes in STBD and STBT6/6, respective-
6/6
ly, can be used as a “buckle” to functionalize the surface of
IMWs through, for example, epoxidation and 1,3-dipolar cy-
cloaddition.[34] As such, this finding motivated us to design
more functional IMWs without disturbing the optimized co-
planar conformation of STB6/6. For instance, coaxial cables,
in which electron- and hole-transporting pathways are sepa-
rated by the insulating layer, would find use in photovoltaics
and ambipolar transistors (see Scheme 6, cartoon). The des-
ignability of the STBm/n family is unique in realization of
such advanced molecular wires; the synthesis is now under-
way in our laboratory.
Scheme 6. A potential advanced molecular wire made by surface-func-
tionalization of our insulated molecular wires derived from STBT
.
6/6
Experimental Section
Air- and water-sensitive synthetic manipulations were performed under
an argon atmosphere using standard Schlenk techniques. All chemicals
were purchased from Aldrich, Kanto Chemical, or Wako and used as re-
ceived. NMR spectra were recorded on Bruker Biospin DRX-600 spec-
trometer, and all chemical shifts are referenced to (CH3)4Si (0 ppm for
1H) or residual CHCl3 (77 ppm for 13C). MALDI-TOF-MS spectra were
obtained with SHIMADZU AXIMA Confidence. UV absorption spectra
and fluorescence spectra were obtained on a Hitachi U-2900 spectropho-
tometer and a Hitachi F-7000 spectrophotometer, respectively. Relative
fluorescence quantum yields were determined using quaterthiophene
(FF =0.18, in benzene[35]) as standards. Melting points were determined
with Yanako NP-500P micro melting point apparatus. Fluorescence life-
time was recoreded on HORIBA FluoroCube UltraFast-3000U.
Figure 6. a) Normalized absorption and fluorescence spectra of STB6/6
(blue), (E)-STBD (green), (Z)-STBD (orange), STBT (red), and 16/44
(dotted black) in CH2Cl2 (0.05 mm): lex =340 nm. Their fluorescence
quantum yields are 0.13, 0.15, 0.14, 0.14, and 0.13, respectively. Plots of
the b) A318/A285 and c) Stokesꢁ shift of STB6/6 brothers as a function of
dHa, which are overlaid with those of STBm/n family.
6/6
6/6
6/6
Chem. Asian J. 2012, 7, 75 – 84
ꢀ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemasianj.org
81

K. Sugiyasu, M. Takeuchi et al.
FULL PAPERS
Synthesis
(E)-STBD
6/6
Synthesis of the self-threading bithiophenes (STBm/n) were performed ac-
cording to the same method reported in our previous paper.[6] Also, see
the Supporting Information for details. Characterization of the final com-
pounds used to evaluate structure–property relationships are shown
below.
m.p.>3008C. 1H NMR (CDCl3, TMS): d=2.14 (m, 8H), 3.80 (m, 4H),
4.00 (m, 4H), 5.11 (dt, J=1.2, 1.8 Hz, 4H), 6.71 (d, J=8.4 Hz, 4H), 6.73
(d, J=4.8 Hz, 2H), 6.98 (d, J=4.8 Hz, 2H), 7.38 ppm (t, J=8.4 Hz, 2H).
13C NMR (CDCl3): d=32.63, 68.45, 107.40, 117.61, 122.62, 128.39, 129.13,
129.97, 130.73, 133.55, 158.79 ppm. MALDI-TOF-MASS: m/z: calcd for
C32H30O4S2: calculated 542.16; found: 542.04. Elemental analysis: calcd
(%) for C32H30O4S2+0.25CH2Cl2: C 68.69, H 5.45. found: C 68.70, H 5.31.
STB5/6
m.p.>3008C. 1H NMR (CDCl3, TMS): d=0.98–1.55 (m, 14H), 3.54 (m,
2H), 3.85 (m, 2H), 4.05 (m, 2H), 4.28 (m, 2H), 6.73 (d, J=8.4 Hz, 2H),
6.77 (d, J=8.4 Hz, 2H), 6.79 (d, J=4.8 Hz, 2H), 7.00 (d, J=4.8 Hz, 2H),
7.40 ppm (t, J=4.8 Hz, 2H) 13C NMR (CDCl3): d=23.32, 26.66, 29.34,
30.49, 70.45, 71.54, 109.07, 109.39, 119.06, 122.83, 129.53, 130.24, 131.42,
133.39, 158.99, 158.99 ppm. MALDI-TOF-MASS: m/z: calcd for
C31H32O4S2: 532.17; found: 532.86 [M+H]+. Elemental analysis: calcd
(%) for C31H32O4S2: C 69.89, H 6.05. found: C 70.21, H 5.82.
Synthesis of STB6/44
Before the completion of the RCM reaction of 144/44 (200 mg, 0.33 mmol)
(15 min of reaction time) the reaction mixture was passed through short
silica gel pad to remove the catalyst. The solution was concentrated and
solid residue was obtained. Pd/C (50 mg) and THF (30 mL) was added
and the mixture was stirred under H2 atmosphere for 5 hrs. The mixture
was passed through Celite and concentrated. The product was purified by
column chromatography on silica gel to yield white powder. m.p. 126.0–
127.68C. 1H NMR (CDCl3, TMS): d=0.75 (t, J=7.5 Hz, 6H), 1.02 (m,
2H), 1.19 (m, 4H), 1.34 (m, 2H), 1.48–1.60 (m, 8H), 3.72 (dd, J=2.4,
9.9 Hz, 2H), 3.90 (m, 4H), 4.00 (m, 2H), 6.61 (d, J=8.4 Hz, 2H), 6.67 (d,
J=8.4 Hz, 2H), 6.69 (d, J=4.8 Hz, 2H), 6.94 (d, J=4.8 Hz, 2H),
7.36 ppm (t, J=8.4 Hz, 2H). 13C NMR (CDCl3): d=13.58, 18.86, 27.68,
29.78, 31.14, 68.38, 69.00, 104.94, 105.93, 115.95, 122.12, 129.62, 129.65,
130.39, 133.68, 158.78, 159.00 ppm. MALDI-TOF-MASS: m/z: calcd for
C34H40O4S2: 576.24; found: 577.05 [M+H]+. Elemental analysis: calcd
(%) for C34H40O4S2: C 70.80, H 6.99. found: C 70.79, H 6.99.
STB6/6
m.p.>3008C. 1H NMR (CDCl3, TMS): d=0.93 (m, 4H), 1.17 (m, 4H),
1.50 (m, 8H), 3.75 (tt, J=4.8, 8.4 Hz, 4H), 4.05 (dt, J=4.8, 6.6 Hz, 4H),
6.70 (d, J=8.4 Hz, 4H), 6.73 (d, J=4.8 Hz, 2H), 6.97 (d, J=4.8 Hz, 2H),
7.38 ppm (t, J=8.4 Hz, 2H). 13C NMR (CDCl3): d=27.23, 30.26, 69.73,
107.10, 117.45, 122.57, 129.36, 130.00, 131.03, 133.35, 158.93 ppm.
MALDI-TOF-MASS: m/z: calcd for C31H32O4S2: 546.19; found: 546.13
[M+H]+. Elemental analysis: calcd (%) for C32H34O4S2+0.2CH2Cl2:
C 68.60, H 6.15. found: C 68.91, H 6.45.
Synthesis of STBT
6/6
A solution of 355/55 (200 mg, 0.31 mmol), Mo(CO)6 (16.3 mg, 20 mol%),
and 2-fluorophenol (69.3 mg, 200 mol%) in chlorobenzene (20 mL) was
refluxed for 6 h under an Ar atmosphere. Upon cooling, the solution was
diluted with CH2Cl2 and passed through silica gel to remove the catalyst.
The solution was concentrated, to which methanol was added to obtain
precipitate of the product. Analytical sample was purified by column
STB6/7
m.p.>3008C. 1H NMR (CDCl3, TMS): d=0.92–1.50 (m, 18H), 3.72 (m,
2H), 3.86 (m, 2H), 4.01 (m, 4H), 6.61 (d, J=8.4 Hz, 2H), 6.68 (d, J=
8.4 Hz, 2H), 6.72 (d, J=5.4 Hz, 2H), 6.96 (d, J=5.4 Hz, 2H), 7.36 ppm
(t, J=8.4 Hz, 2H). 13C NMR (CDCl3): d=26.14, 27.75, 28.312, 29.04,
29.86, 69.08, 69.63, 105.30, 106.85, 116.51, 122.20, 129.73, 129.80, 158.69,
159.28 ppm. MALDI-TOF-MASS: m/z: calcd for C33H36O4S2: 560.21;
found: 561.33 [M+H]+. Elemental analysis: calcd (%) for C33H36O4S2:
C 70.68, H 6.47. found: C 70.66, H 6.33.
1
chromatography on silica gel. m.p.> 3008C. H NMR (CDCl3, TMS): d=
2.25–2.30 (m, 4H), 2.35–2.38 (m, 4H), 3.84–3.87 (m, 4H), 3.95–3.98 (m,
4H), 6.89 (d, J=8.4 Hz, 4H), 6.73 (d, J=5.4 Hz, 2H), 7.02 (d, J=5.4 Hz,
2H), 7.36 ppm (t, J=8.4 Hz, 2H). 13C NMR (CDCl3): d=20.05, 67.55,
77.57, 107.88, 117.88, 122.88, 129.08, 129.98, 158.61 ppm. MALDI-TOF-
MASS: m/z: calcd for C32H26O4S2: 538.13; found: 538.93 [M+H]+. Ele-
mental analysis: calcd (%) for C32H26O4S2+0.3CH2Cl2 : C 68.77, H 4.75.
found: C 68.79,H 5.04.
STB7/7
m.p.>3008C. 1H NMR (CDCl3, TMS): d=1.03–1.10 (m, 8H), 1.19–1.23
(m, 4H), 1.51–1.55 (m, 8H), 3.85 (m, 4H), 3.95 (m, 4H), 6.66 (d, J=
8.4 Hz, 4H), 6.71 (d, J=5.4 Hz, 2H), 6.97 (d, J=5.4 Hz, 2H), 7.35 ppm
(t, J=8.4 Hz, 2H). 13C NMR (CDCl3): d=25.03, 27.15, 28.17, 69.29,
106.49, 117.0, 121.90, 129.57, 130.31, 133.56, 159.00 ppm. MALDI-TOF-
MASS: m/z: calcd for C34H38O4S2: 574.22; found: 575.33 [M+H]+. Ele-
mental analysis: calcd (%) for C34H38O4S2+0.25CH2Cl2: C 69.02, H 6.51.
found: C 68.83, H 6.27.
Synthesis of (Z)-STBD
6/6
A
suspension of Lindlar catalyst (50 mg) and STBT
(100 mg,
6/6
0.19 mmol), and quinoline (0.1 mL) in THF (10 mL) were stirred under
H2 atmosphere for 5 h. The reaction mixture was passed through Celite
and concentrated. The solid residue was purified by column chromatogra-
1
phy on silica gel to give a white powder. m.p.> 3008C. H NMR (CDCl3,
TMS): d=2.07–2.08 (m, 4H), 2.16–2.17 (m, 4H), 3.84–3.87 (m, 4H),
3.98–4.02 (m, 4H), 5.08 (t, J=4.5 Hz, 4H), 6.90 (d, J=5.4 Hz, 2H), 6.40
(d, J=8.4 Hz, 4H), 6.97 (d, J=5.4 Hz, 2H), 7.38 ppm (t, J=8.4 Hz, 2H).
13C NMR (CDCl3): d=27.83, 71.04, 109.77, 122.32, 127.42, 129.77, 129.97,
130.52, 133.94, 159.25 ppm. MALDI-TOF-MASS: m/z: calcd for
C32H30O4S2: 542.16; found: 542.38. Elemental analysis: calcd (%) for
C32H30O4S2+0.15CH2Cl2: C 69.52, H 5.50. found: C 69.71, H 5.74.
STB8/8
m.p. 199.2–200.98C. 1H NMR (CDCl3, TMS): d=1.09–1.21 (m, 16H),
1.43–1.60 (m, 8H), 3.92–3.94 (m, 8H), 6.61 (d, J=8.4 Hz, 4H), 6.68 (d,
J=4.8 Hz, 2H), 6.95 (d, J=4.8 Hz, 2H), 7.34 ppm (t, J=8.4 Hz, 2H)
13C NMR (CDCl3): d=24.11, 27.42, 28.46, 67.70, 104.48, 109.39, 121.88,
129.34, 130.05, 158.73 ppm. MALDI-TOF-MASS: m/z: calcd for
C36H42O4S2: 602.25; found: 603.29 [M+H]+. Elemental analysis: calcd
(%) for C36H42O4S2: C 71.72, H 7.02. found: C 71.60, H 6.81.
STB10/10
m.p. 130.2–131.58C. 1H NMR (CDCl3, TMS): d=1.09–1.33 (m, 24H),
1.43–1.56 (m, 8H), 3.84 (m, 4H), 3.97 (m, 4H), 6.61 (d, J=8.4 Hz, 4H),
6.68 (d, J=4.8 Hz, 2H), 6.95 (d, J=4.8 Hz, 2H), 7.33 ppm (t, J=8.4 Hz,
2H) 13C NMR (CDCl3): d=23.49, 25.07, 27.35, 27.56, 68.16, 104.53,
121.98, 129.27, 129.96, 158.69 ppm. MALDI-TOF-MASS: m/z: calcd for
C40H46O4S2: 658.28; found: 659.32 [M+H]+. Elemental analysis: calcd
(%) for C40H50O4S2: C 72.91, H 7.65. found: C 72.93, H 7.66.
Acknowledgements
This work was supported by KAKENHI (No. 20750097 and No.
23655108), the Shorai Foundation for Science and Technology, and The
Association for the Progress of New Chemistry for K.S. The authors
thank Nanotechnology Network Project of the Ministry of Education,
Culture, Sports, Science, and Technology Japan (MEXT).
82
www.chemasianj.org
ꢀ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Asian J. 2012, 7, 75 – 84

Synthesis of Self-Threading Bithiophenes and their Structure–Property Relationships
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Received: June 9, 2011
Published online: November 16, 2011
84
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