Synthesis and properties of novel optically active poly(thiophenyleneethynylene-phenyleneethynylene)s

Article abstract of DOI:10.1246/cl.150270

The Sonogashira-Hagihara coupling polymerization of L- alanine-derived 2,5-dibromothiophene monomers 1a-1d with p- and m-diethynylbenzenes 2p and 2m gave novel optically active poly(thiophenyleneethynylene-phenyleneethynylene)s poly(1a-2p)-poly(1d-2m). p-Phenylene-linked poly(1b-2p)-poly(1d-2p) bearing amide-amide side chains exhibited CD signals based on chiral aggregates, while the m-phenylene-linked counterparts poly(1b-2m)-poly(1d-2m) exhibited no evidence of the formation of secondary structures. Poly(1a-2p) and poly(1a-2m) bearing amide-ester side chains exhibited no CD signal as well.

Full text of DOI:10.1246/cl.150270

Received: March 24, 2015 | Accepted: April 13, 2015 | Web Released: April 17, 2015
CL-150270
Synthesis and Properties of Novel Optically Active
Poly(thiophenyleneethynylene-phenyleneethynylene)s
Yoshinori Otaki, Yu Miyagi, and Fumio Sanda*
Department of Chemistry and Materials Engineering, Faculty of Chemistry, Materials and Bioengineering,
Kansai University, 3-3-35 Yamate-cho, Suita, Osaka 564-8680
(E-mail: sanda@kansai-u.ac.jp)
R
R
The Sonogashira-Hagihara coupling polymerization of L-
O
O
alanine-derived 2,5-dibromothiophene monomers 1a-1d with p-
and m-diethynylbenzenes 2p and 2m gave novel optically active
poly(thiophenyleneethynylene-phenyleneethynylene)s poly(1a-
2p)-poly(1d-2m). p-Phenylene-linked poly(1b-2p)-poly(1d-
2p) bearing amide-amide side chains exhibited CD signals based
on chiral aggregates, while the m-phenylene-linked counterparts
poly(1b-2m)-poly(1d-2m) exhibited no evidence of the for-
mation of secondary structures. Poly(1a-2p) and poly(1a-2m)
bearing amide-ester side chains exhibited no CD signal as well.
[Pd(PPh3)2Cl2]
PPh3, CuI
N
H
O
N
H
O
n
+
n
Br
1a:
Br
S
DMF/Et3N = 2/3
80 °C, 0.5–24 h
S
R = OMe
2p: p-
2m: m-
n
1b:
R = NHC3H7
poly(1a-2p)–poly(1d-2m)
1c: R = NHC6H13
1d:R = NHC12H25
Scheme 1. Synthesis of L-alanine-based poly(thiophenylenee-
thynylene-phenyleneethynylene)s.
Table 1. Sonogashira-Hagihara coupling polymerization of 1a-
1d with 2p and 2ma
Conjugated polymers gather much attention due to their
excellent photo- and electric functions.1 Among various con-
jugated polymers, polythiophenes have potential applications to
field effect transistors,2 organic thin film solar cells,3 and light-
emitting diodes.4 Polythiophenes are also useful for chemo- and
biosensors originating from the responsiveness of photoelectric
properties to external stimuli.5 The progress of studies on the
structure-property relationship of polythiophenes contributes to
further improvement in applicability to functional materials.
Some conjugated polymers adopt chirally ordered secondary
structures such as a helix, because of their stiff main chain
stabilizing the conformation.6 We have reported that a series of
optically active poly(m-phenyleneethynylene-aryleneethynyl-
ene)s bearing amide groups form folded helical structures,
which are stabilized by π-stacking, amphiphilicity, and intra-
molecular hydrogen bonding between the amide groups at the
side chains.3 In the course of our study on poly(m-phenyl-
eneethynylene-aryleneethynylene)s forming chirally regulated
structures, we designed novel optically active polythiophene
derivatives bearing amide groups. Herein, we wish to report the
synthesis of poly(thiophenyleneethynylene-phenyleneethynyl-
ene)s using an amino acid as a chiral source, and examination
of the secondary structures of the obtained polymers.
c
c
Monomers
Time/h
Yieldb/%
Mn
Mw/Mn
1a + 2p
1a + 2m
1b + 2p
1b + 2m
1c + 2p
1c + 2m
1d + 2p
1d + 2m
0.5
0.5
1.0
0.5
24
0.5
3.0
0.5
10
79
14
73
51
89
8
8600
19000d
3700
5700d
11000
7500
2.1
4.5d
1.4
1.8d
1.7
2.0
1.3
2.7
4600
4800
71
aConditions: [1a-1d]0 = [2p]0 = [2m]0 = 100 mM, [Pd(PPh3)2Cl2] =
[PPh3] = [CuI] = 5 mM, DMF/Et3N = 2/3 (v/v), 80 °C. bInsoluble
part in Et2O. cEstimated by SEC measured in DMF, polystyrene
d
calibration. Data of DMF soluble part.
poly(1a-2p)
poly(1b-2p)
poly(1c-2p)
poly(1d-2p)
The Sonogashira-Hagihara coupling polymerization of 2,5-
dibromothiophene monomers 1a-1d with p- and m-diethynyl-
benzenes 2p and 2m was performed in DMF/Et3N = 2/3 at
80 °C for 0.5-24 h to obtain the corresponding polymers,
poly(1a-2p)-poly(1d-2m), with moderate molecular weights
(Mn = 3700-19000) in 8-89% yields (Scheme 1 and Table 1).
The polymerization stopped in every case when the reaction
mixture became heterogeneous.7 All polymers, except poly(1a-
2m) and poly(1b-2m), were completely soluble in CHCl3 and
DMF.
Figure 1. CD and UV-vis spectra of poly(1a-2p)-poly(1d-2p)
measured in CHCl3 (c = 0.04 mM) at 20 °C.
Figure 1 shows the CD and UV-vis spectra of poly(1a-2p)-
poly(1d-2p) measured in CHCl3 at 20 °C. The extension of
conjugation from the monomers to polymers was confirmed by
the red shift of UV-vis absorption (Table S1). Poly(1a-2p)
bearing amide-ester side chains exhibited no CD signal, while
poly(1b-2p)-poly(1d-2p) bearing amide-amide side chains
exhibited CD signals around 450 nm, assignable to chirally
regulated conjugated main chains. Poly(1b-2p)-poly(1d-2p)
also exhibited CD signals around 320 nm, assignable to the
transition of less conjugated chromophores, presumably thio-
Chem. Lett. 2015, 44, 1013–1015 | doi:10.1246/cl.150270
© 2015 The Chemical Society of Japan | 1013

linked polymers possibly adopt a more extended conformation,
leading to the formation of aggregates. The amide-amide side
chains of poly(1b-2p)-poly(1d-2p) seem to stabilize the
aggregated structures by forming intermolecular hydrogen bonds
together with π-stacking between the conjugated main chains.
The CD and UV-vis spectra of poly(1a-2p)-poly(1d-2p)
and poly(1a-2m)-poly(1d-2m) were also measured in CHCl3/
Et2O = 1/9 (v/v) (Figures S1 and S2), wherein Et2O is a poor
solvent for the polymers, and in CHCl3/DMF = 1/9 (v/v)
(Figures S3 and S4), wherein DMF is a good solvent. No
remarkable difference was observed between the spectroscopic
patterns in CHCl3, CHCl3/Et2O, and CHCl3/DMF mixed
solvents. It is common that the addition of poor solvents to
polymer solutions induces aggregation, and that DMF disturbs
the formation of hydrogen bonds between polymer molecules.
It is proved that the aggregation-induced CD patterns of the
present polymers are only slightly affected by the solvents.
Judging from the difference of signs of the CD signals around
450 nm between the polymers [poly(1b-2p) (R = C3H7): +,
poly(1c-2p) (R = C6H13): +, poly(1d-2p) (R = C12H25): ¹], the
way of van der Waals interaction between the alkyl groups at
the side chains is of key importance for the chirality of the
aggregates. The presence of alkyl group interactions may be the
reason for the negligibly small solvent effect on aggregation.
Namely, once the polymers form aggregates, the external alkyl
groups possibly shield the internal amide groups from solvent
molecules, resulting in the small solvent effect mentioned above.
In summary, we have demonstrated the synthesis of novel
optically active poly(thiophenyleneethynylene-phenyleneethy-
nylene)s bearing amide-amide/amide-ester side chains with p-/
m-phenylene linkages. It was revealed that p-phenylene-linked
poly(1b-2p)-poly(1d-2p) formed chirally regulated aggregated
structures, presumably stabilized by intermolecular hydrogen
bonding between amide-amide side chains together with π-
stacking between the conjugated main chains. Further inves-
tigation regarding the effect of alkyl chain length on the sense of
chirality is now ongoing.
poly(1a-2m)
poly(1b-2m)
poly(1c-2m)
poly(1d-2m)
Figure 2. CD and UV-vis spectra of poly(1a-2m)-poly(1d-2m)
measured in CHCl3 (c = 0.04 mM) at 20 °C.
poly(1a-2p)
poly(1b-2p)
poly(1c-2p)
poly(1d-2p)
Figure 3. CD and UV-vis spectra of poly(1a-2p), poly(1b-2p),
poly(1c-2p), and poly(1d-2p) measured in CHCl3 at 20 °C after
filtration using a membrane filter with a pore size of 0.50 ¯m.
This research was supported by a Grant-in-Aid for Scientific
Research on Innovative Areas “New Polymeric Materials Based
on Element-Blocks (No. 2401)” from the Ministry of Education,
Culture, Sports, Science and Technology, Japan.
phene moieties bearing chiral substituents.8 It is considered that
the terminal amide moieties play an important role in the
formation of chirally ordered structures. On the other hand,
poly(1a-2m)-poly(1d-2m) exhibited no CD signal in CHCl3,
as shown in Figure 2. The p-phenylene linkage is effective
for inducing chirally ordered secondary structures, while the
m-phenylene linkage is not. The band edges of the m-linked
polymers were ca. 100 nm shorter than the p-linked polymers,
indicating the shorter conjugation length of the m-polymers than
the p-polymers.
Figure 3 shows the CD and UV-vis spectra of CHCl3
solutions of poly(1a-2p)-poly(1d-2p) measured after filtration
using a membrane filter (pore size: 0.50 ¯m). The CD signals of
poly(1b-2p)-poly(1d-2p) completely disappeared after filtration.
Since the solutions still exhibited UV-vis absorption signals, it
is assumed that the CD signals of poly(1b-2p) and poly(1d-2p)
observed in Figure 1 do not come from a unimolecular
secondary structure, such as a helix, but from aggregates. This
assumption agrees with the fact that the p-phenylene-linked
polymers showed CD signals while the m-phenylene-linked
polymers did not. Compared with the m-linked polymers, the p-
Supporting Information is available electronically on J-STAGE.
References and Notes
1
2
For reviews, see: a) X. Chen, J.-L. Liao, Y. Liang, M. O.
Ahmed, H.-E. Tseng, S.-A. Chen, J. Am. Chem. Soc. 2003,
125, 636. b) J.-F. Morin, N. Drolet, Y. Tao, M. Leclerc,
Chem. Mater. 2004, 16, 4619. c) J. C. Sanchez, A. G.
DiPasquale, A. L. Rheingold, W. C. Trogler, Chem. Mater.
2007, 19, 6459. d) A. C. Grimsdale, K. L. Chan, R. E.
Martin, P. G. Jokisz, A. B. Holmes, Chem. Rev. 2009, 109,
897.
a) H. G. O. Sandberg, G. L. Frey, M. N. Shkunov, H.
Sirringhaus, R. H. Friend, M. M. Nielsen, C. Kumpf,
Langmuir 2002, 18, 10176. b) A. Pron, P. Rannou, Prog.
Polym. Sci. 2002, 27, 135. c) B. S. Ong, Y. Wu, P. Liu, S.
Gardner, J. Am. Chem. Soc. 2004, 126, 3378. d) A. S. Dhoot,
J. D. Yuen, M. Heeney, I. McCulloch, D. Moses, A. J.
1014 | Chem. Lett. 2015, 44, 1013–1015 | doi:10.1246/cl.150270
© 2015 The Chemical Society of Japan

Heeger, Proc. Natl. Acad. Sci. U.S.A. 2006, 103, 11834.
a) R. D. McCullough, Adv. Mater. 1998, 10, 93. b) I. F.
Perepichka, D. F. Perepichka, H. Meng, F. Wudl, Adv. Mater.
2005, 17, 2281.
685, 15. c) M. Suginome, Y. Ito, in Advances in Polymer
Science, Springer, 2004, Vol. 171, p. 77. doi:10.1007/
b95531. d) T. Aoki, T. Kaneko, M. Teraguchi, Polymer
2006, 47, 4867. e) E. Yashima, K. Maeda, Y. Furusho, Acc.
Chem. Res. 2008, 41, 1166. f) E. Yashima, K. Maeda, H.
Iida, Y. Furusho, K. Nagai, Chem. Rev. 2009, 109, 6102.
g) K. Akagi, Chem. Rev. 2009, 109, 5354. h) J. Liu, J. W. Y.
Lam, B. Z. Tang, Chem. Rev. 2009, 109, 5799. i) M.
Shiotsuki, F. Sanda, T. Masuda, Polym. Chem. 2011, 2,
1044. j) Polymeric Chiral Catalyst Design and Chiral
Polymer Synthesis, ed. by S. Itsuno, Wiley, Hoboken, 2011.
doi:10.1002/9781118063965.
The chiroptical properties of the polymers definitely depend
on the molecular weights, but it is commonly difficult to
control the molecular weights of the polymers obtained
by step-growth polymerization including the Sonogashira-
Hagihara coupling polymerization. In the present study,
extension of polymerization time resulted in the formation
of a large amount of solvent-insoluble polymers. In the case
of poly(1c-2p), shortening of polymerization time resulted
in the formation of a CD-silent polymer.
3
4
a) J. Chen, Y. Cao, Acc. Chem. Res. 2009, 42, 1709. b) S.-L.
Hsu, C.-M. Chen, Y.-H. Cheng, K.-H. Wei, J. Polym. Sci.,
Part A: Polym. Chem. 2011, 49, 603. c) S.-Y. Ku, C. D.
Liman, D. J. Burke, N. D. Treat, J. E. Cochran, E. Amir,
L. A. Perez, M. L. Chabinyc, C. J. Hawker, Macromolecules
2011, 44, 9533. d) M. T. Dang, L. Hirsch, G. Wantz, J. D.
Wuest, Chem. Rev. 2013, 113, 3734. e) P. Morvillo, R.
Diana, C. Fontanesi, R. Ricciardi, M. Lanzi, A. Mucci, F.
Tassinari, L. Schenetti, C. Minarini, F. Parenti, Polym.
Chem. 2014, 5, 2391.
a) D. T. McQuade, A. E. Pullen, T. M. Swager, Chem. Rev.
2000, 100, 2537. b) A. Sundararaman, M. Victor, R.
Varughese, F. Jäkle, J. Am. Chem. Soc. 2005, 127, 13748.
c) M. Sebastian, M. Hissler, C. Fave, J. Rault-Berthelot,
C. Odin, R. Réau, Angew. Chem., Int. Ed. 2006, 45, 6152.
d) S. W. Thomas, III, G. D. Joly, T. M. Swager, Chem. Rev.
2007, 107, 1339.
7
5
6
Fore reviews, see: a) T. Nakano, Y. Okamoto, Chem. Rev.
2001, 101, 4013. b) M. Fujiki, J. Organomet. Chem. 2003,
8
K. Watanabe, I. Osaka, S. Yorozuya, K. Akagi, Chem. Mater.
2012, 24, 1011.
Chem. Lett. 2015, 44, 1013–1015 | doi:10.1246/cl.150270
© 2015 The Chemical Society of Japan | 1015