Preparation, Structure, and spectral properties of cyclophanes consisting of oligothiophene units

Article abstract of DOI:10.1246/cl.2008.870

Small-sized cyclophanes consisting of oligothiophene units as a component have been prepared. Correlation between the cyclophane structure and fluorescence spectral properties has been examined. It has been found that emission from intramolecular excimer-formation is confirmed for the cyclophanes having two or three thiophene units, but not for those having four or five thiophene units with the β-hexyl chain. Copyright

Full text of DOI:10.1246/cl.2008.870

870
Chemistry Letters Vol.37, No.8 (2008)
Preparation, Structure, and Spectral Properties of Cyclophanes Consisting
of Oligothiophene Units
Akihiko Tsuge,ꢀ1;2 Takeshi Hara,1 Tetsuji Moriguchi,1 and Minoru Yamajiꢀ3
1Department of Applied Chemistry, Kyushu Institute of Technology, 1-1 Sensui-cho, Tobata-ku, Kitakyushu 804-8550
2Institute for Materials Chemistry and Engineering, Kyushu University, 6-10-1 Hakozaki, Higashi-ku, Fukuoka 812-8581
3Department of Chemistry and Biochemistry, Gunma University, 1-5-1 Tenjin-cho, Kiryu 376-8515
(Received May 15, 2008; CL-080501; E-mail: tsuge@che.kyutech.ac.jp)
Small-sized cyclophanes consisting of oligothiophene units
CH
CH
CH Br
CH SH
2
3
3
2
S
S
S
as a component have been prepared. Correlation between the cy-
clophane structure and fluorescence spectral properties has been
examined. It has been found that emission from intramolecular
excimer-formation is confirmed for the cyclophanes having
two or three thiophene units, but not for those having four or five
thiophene units with the ꢀ-hexyl chain.
Br
Br
CH Br
CH SH
2
2
1a
1b
1c
1) n-BuLi/THF
Cs2CO3, NaBH4
EtOH/C6H6
1b + 1c
2a
3a
2) H3O+
S
Pd2(dba)3, (o-Tol)3P, CuI
Bu Sn
3
2a +
2a +
3b
Toluene, reflux
Over the past decades well-defined ꢁ-conjugated oligothio-
phenes have gained great importance as advanced molecular
electronic materials, including for example, organic field
effect transistors (OFETs)1 and organic light-emitting diodes
(OLEDs)2 because of the feasibility in manipulation of their
chemical structures. Indeed, in order to improve electronic and
optical properties of oligothiophenes, introduction of end-cap-
ping groups,3 insertion of various functional groups,4 and chang-
ing oligomer lengths5 have been carried out. These strategies
mainly involve creation of novel ꢁ systems.
Meanwhile, we have reported that ꢁ systems based on the
cyclophane skeleton give rise to specific electronic properties
owing to strong transannular ꢁ-electronic interactions.6 Some
cyclophane-derived oligothiophenes consisting of a [2.2]meta-
cyclophane skeleton have been reported.3a,7 Cyclophane-type
compounds in which two oligothiophene units are bridged with
alkylene chains have been also reported.8 Among a number of
cyclophanes, the [3.3]dithiametacyclophane skeleton seems to
provide an appropriate platform to arrange two oligomer chains
side by side in stacked form since it is known9 that this kind of
cyclophane assumes a syn structure.
1d
O
S
B
S
Bu Sn
3
O
3c
X
S
X
X = C H
1f
6
13
1e
S
2a: n = 0, R = Br, R = H
1 2
S
R
R
1
S
2b: n = 1, R = H, R = C H
1
2
6 13
n
2c: n = 1, R = Br, R = C H
1
2
6 13
R
2
2d: n = 1, R = Br, R = H
S
S
1
2
S
1
2e: n = 2, R = H, R = C H
1
2
6 13
n
2f: n = 2, R = Br, R = C H
1
2
6 13
R
2
S
S
S
S
S
S
S
R
1
S
m
n
n
R
2
S
S
R
1
m
R
R = C H
6 13
2
2
3a : n = 1, m = 0, R = H 3b : n = 2, m = 0, R = H
1
1
3c : n = 3, m = 0, R = C H 3d: n = 1, m = 1, R = C H
1
6
13
13
1
6
13
Thus, we describe the preparation of metacyclophanes to
which two oligothiophene units are introduced, their structure
and spectral properties.
Bromination of compound 1a gave bromomethyl compound
1b in 65% yield. The sulfanylmethyl compound 1c was obtained
by the reaction of 1b and thiourea in 95% yield.
3e: n = 2, m = 1, R = C H
1
6
H C
3
S
S
S
S
R
m
1
n
H C
3
R
R = C H
6 13
2
2
4a : n = 1, m = 0, R = H 4b : n = 2, m = 0, R = H
Cyclization of 1b and 1c using Cs2CO3 as a base under high-
ly dilute conditions afforded the corresponding dithiacyclophane
2a in 57% yield. Treatment of 2a with n-butyllithium followed
by quenching with hydrochloric acid afforded the desired cyclo-
phane 3a in 83% yield. The cyclophane having two thiophene
units 3b was obtained from 2a and 2-tributylstannylthiophene
(1d) in 43% yield. Although the cyclophane having three thio-
phene units was prepared by similar reaction, the product
showed poor solubility in conventional organic solvents.
Thus, the coupling of 2a and tributylstannyl-2,20-bithio-
phene carrying hexyl group 1e gave cyclophane having three
thiophene units 3c in 35% yield. As expected 3c showed good
solubility. This result implies more than two alkyl groups should
1
1
4c : n = 3, m = 0, R = C H 4d: n = 1, m = 1, R = C H
1
6
13
13
1
6
13
4e: n = 2, m = 1, R = C H
1
6
Scheme 1.
be necessary to obtain good solubility for cyclophanes carrying
four or five thiophene units. Thus, cyclophane 2b was prepared
from 2a and 1f in 80% yield.
Bromination of 2b gave cyclophane 2c (89%), which react-
ed with 1e to afford cyclophane having four thiophene units 3d
in 37% yield. After 3b was brominated to give cyclophane
2d (96%), followed by coupling with 1f, cyclophane 2e was
obtained in 73% yield. The treatment of 2e with NBS afforded
Copyright Ó 2008 The Chemical Society of Japan

Chemistry Letters Vol.37, No.8 (2008)
871
no essential differences in shape of the absorption spectra be-
tween the cyclophanes and the corresponding thiophenes. Cyclo-
phane 3a exhibits a weak excimer fluorescence band accompa-
nied with a locally excited (LE) fluorescence band. For 3b and
3c, intense excimer fluorescence bands are observed together
with the corresponding weak LE fluorescence bands. No such
excimer fluorescence bands were obtained when increasing
the concentrations of 4a–4c. These results strongly suggest that
thiophene moieties of 3a–3c have a sandwich-like structure in
the ground state, which corresponds to the syn conformation
as seen for 3b.
Conversely, cyclophanes 3d and 3e show fluorescence
spectra with vibronic structures. These observations indicate that
the fluorescence of 3d and 3e is not excimeric, but from the
corresponding monomer moieties. In 3c the ꢂ-hexyl chain seems
to be no hindrance to excimer formation. From these results,
it is inferred that elongation of thiophene units accompanied
with introduction of a ꢀ-hexyl chain could hamper the
conformation suitable for excimer formation.
Figure 1. Perspective view of 3b.
Table 1. Maximum wavelength (ꢅmax/nm)a of absorption and
fluorescence spectra and quantum yields (ꢁf) of total fluores-
cence for 3a–3e and 4a–4e
Compound
Absorption
Fluorescenceb
ꢁf
3a
3b
3c
3d
3e
4a
4b
4c
4d
4e
284
341
383
409
415
284
340
384
408
423
342 (435)c
418 (483)
445 (508)
506
528
337
414
445
491
513
0.016
0.094
0.044
0.068
0.083
0.083
0.080
0.120
0.349
0.431
References and Notes
1
R. Hajlaoui, G. Horowitz, F. Garnier, A. Arce-Brouchet, L.
Laigre, A. E. Kassmi, F. Demanze, F. Kouki, Adv. Mater.
1997, 9, 389.
2
3
T. Noda, H. Ogawa, N. Noma, Y. Shirota, J. Mater. Chem.
1999, 9, 2177.
a) L. Guyard, C. Dumas, F. Miomandre, R. Pansu, R.
´
Renault-Meallet, P. Audebert, New J. Chem. 2003, 27,
1000. b) V. Promarak, A. Pankvuang, S. Ruchirawat.
Tetrahedron Lett. 2007, 48, 1151.
V. Promarak, A. Pankvuang, D. Meunmat, T. Sudyoadsuk,
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N. Sumi, H. Nakanishi, S. Ueno, K. Takimiya, Y. Aso, T.
Otsubo, Bull. Chem. Soc. Jpn. 2001, 74, 979.
a) A. Tsuge, Y. Tanba, T. Moriguchi, K. Sakata, Chem. Lett.
2002, 384. b) A. Tsuge, M. Otsuka, T. Moriguchi, K. Sakata,
Org. Biomol. Chem. 2005, 3, 3590.
L. Guyard, P. Audebert, W. R. Dolbier, Jr., J.-X. Duan, J.
Electroanal. Chem. 2002, 537, 189.
aIn cyclohexane, T ¼ 295 K. bExcitation of the samples was
performed at the wavelength of the corresponding absorption
maximum. Values in parentheses are for excimer fluores-
cence.
c
4
5
6
cyclophane 2f (61%), which reacted with 1e to give cyclophane
having the five thiophene units 3e in 44% yield. The
corresponding referential oligothiophenes 4a–4e were also
prepared by similar methods (Scheme 1). All cyclophanes
3a–3e10 were identified by their 1H NMR spectra, mass spectro-
metric measurements, and/or elemental analyses.
The protons of the bridges in cyclophanes 3a–3e showed
one singlet at room temperature in the 1H NMR spectra. No
significant signal changes were observed when the temperature
was lowered to ꢁ80 ꢂC, suggesting that they are basically a
flexible structure.
The X-ray analysis of 3b11 shows the syn conformation
(Figure 1). It has been reported9,12 that these kinds of dithiacy-
clophanes tend to assume a syn conformation.
Obviously this syn structure could be essential for creating
vertically stacked ꢁ-conjugated systems based on the cyclo-
phane structure.
7
8
9
T. Sakai, T. Satou, T. Kaikawa, K. Takimiya, T. Otsubo, Y.
Aso, J. Am. Chem. Soc. 2005, 127, 8082.
T. Moriguchi, K. Sakata, A. Tsuge, J. Chem. Soc., Perkin
Trans. 2 2001, 934.
10 Supporting Information is available electronically on the
CSJ-Journal Web site, http://www.csj.jp/journals/chem-lett/
index.html.
.
11 Crystal data for 3b: C32H24S6 C6H6, Mr ¼ 679:01, triclinic,
ꢀ
space group P1 (No. 2), a ¼ 9:8263ð3Þ, b ¼ 13:5119ð3Þ, c ¼
ꢂ
ꢂ
˚
14:1160ð4Þ A, ꢂ ¼ 104:9451ð7Þ , ꢀ ¼ 101:9353ð9Þ , ꢃ ¼
3
110:110ð1Þꢂ,
V ¼ 1607:71ð8Þ A ,
Z ¼ 2,
Dcalcd
¼
˚
The electronic absorption and fluorescence spectral data of
cyclophanes 3a–3e, together with the corresponding oligothio-
phenes 4a–4e, are summarized in Table 1. In the electronic
absorption spectra the ꢁ–ꢁꢀ transitions are shifted to longer
wavelengths with increasing chain length. There were observed
1:403 g cmꢁ3, Rigaku AFC7R diffractometer, R1 ¼ 0:106
(for 6356 reflections with I > 3ꢄðIÞ), wR2 ¼ 0:214 (for
all data (15124 reflections)). CCDC reference No 664968.
12 W. Anker, G. M. Bushnell, R. H. Mitchell, Can. J. Chem.
1979, 57, 3080.
Published on the web (Advance View) July 12, 2008; doi:10.1246/cl.2008.870