Synthesis and properties of substituted polyacetylenes containing pyrene moieties in the side group

Article abstract of DOI:10.1246/bcsj.20150294

A novel phenylacetylene derivative containing a pyrene group was synthesized and polymerized with various Rh complex catalysts. The obtained polymers were soluble in common organic solvents such as chloroform and THF. The M_w and M_w/M_n were estimated as ca. 12000-18000 and 1.31.7 respectively, and the polymer yields were estimated as around 5772%. The characteristic absorptions of ≡ C-H (3200 cm^-1) and C≡C (2100 cm^-1) stretchings seen in the monomer spectra have completely disappeared in the polymer spectra from IR spectrum. ¹HNMR analysis confirmed that the acetylene triple bonds transformed to polyene double bonds. These polymers were found to be thermally stable based on TGA data. The synthesized polymers exhibited blue fluorescence. The fluorescence quantum yield in solution was high at 0.37-0.41. Furthermore, the obtained polymer had a long fluorescence lifetime.

Full text of DOI:10.1246/bcsj.20150294

Synthesis and Properties of Substituted Polyacetylenes Containing Pyrene
Moieties in the Side Group
Shou Sugano and Hiroaki Kouzai*
Department of Applied Chemistry, College of Science and Engineering, Kanto Gakuin University,
1-50-1 Mutsuura-higashi, Kanazawa-ku, Yokohama, Kanagawa 236-8501
E-mail: kouzai@kanto-gakuin.ac.jp
Received: August 25, 2015; Accepted: October 1, 2015; Web Released: October 9, 2015
Shou Sugano
Shou Sugano was born in Kanagawa, Japan, in 1991. He obtained his bachelor’s degree (2014) from
Department of Applied Chemistry, College of Science and Engineering, The Kanto Gakuin University under
the supervision of Professor Hiroaki Kouzai. In 2014, he began his master’s degree studies at Department of
Applied Chemistry, College of Science and Engineering, The Kanto Gakuin University under the supervision
of Professor Hiroaki Kouzai.
Abstract
substituted polyacetylenes can be obtained by the polymeriza-
tion of the corresponding acetylenic monomers with group 5
(Nb, Ta), 6 (Mo, W), and 9 (Rh) transition metal catalysts.
Being sensitive to air and moisture, Nb, Ta, Mo, and W cata-
lysts cannot be applied for the polymerization of substituted
acetylenes bearing polar groups due to their deactivation. On the
other hand, Rh catalysts display remarkable activity for the
polymerization of monomers containing polar groups owing to
their low oxophilicity. In addition, Rh catalysts exhibit activity
even in polar and protic solvents. However, they suffer from
the disadvantage of being effective only for monosubstituted
acetylenes. Attracted by these properties and the corresponding
potential applications, many research groups have been working
on the synthesis of novel substituted PPAs. Acetylene mono-
mers could only be polymerized by Rh-based catalysts and the
obtained polymers were found to have unusually high cis-
polyene contents (90%). Pyrene is a fluorescent substance
emitting blue light, is photoconductive,23 and has been widely
studied as an organic EL material.24,25 However, since the solu-
bility is a problem with pyrene alone, the introduction of sub-
stituents, such as an ethynyl group,26 which has been reported
to have high fluorescence characteristics, has been considered.27
In this article, we describe a synthetic design different from
previous reports, that is, synthesis of an acetylene monomer
containing an amide bond with a pyrene group at the end.
Introduction of a pyrene23 to the side chain of acetylene extends
the conjugated chain and would impart excellent heat resistance
and fluorescence.26 These monomers were polymerized with Rh
complexes. The synthesis and characterization of these novel
fluorescent substituted polyacetylenes are described herein.
A novel phenylacetylene derivative containing a pyrene
group was synthesized and polymerized with various Rh com-
plex catalysts. The obtained polymers were soluble in common
organic solvents such as chloroform and THF. The Mw and Mw/
Mn were estimated as ca. 12000-18000 and 1.3-1.7 respective-
ly, and the polymer yields were estimated as around 57-72%.
The characteristic absorptions of ¸C-H (3200 cm¹1) and C¸C
(2100 cm¹1) stretchings seen in the monomer spectra have
completely disappeared in the polymer spectra from IR spec-
trum. 1H NMR analysis confirmed that the acetylene triple
bonds transformed to polyene double bonds. These polymers
were found to be thermally stable based on TGA data. The syn-
thesized polymers exhibited blue fluorescence. The fluorescence
quantum yield in solution was high at 0.37-0.41. Furthermore,
the obtained polymer had a long fluorescence lifetime.
In the past few decades, conjugated polymers have been
investigated intensively due to their promising electrical and
optical properties, making them useful materials for electronic
and photonic devices such as field-effect transistors and light
emitting diodes (LEDs).1-3 Polyacetylene (PA), as the simplest
conjugated polymer, exhibits high conductivity when doped
with an appropriate dopant, such as iodine.4 However, the
application of PA in electronic devices is not feasible due to its
lack of solution processability and air stability.5-7 Through the
introduction of various groups, such as alkyl and aryl groups, to
the polymer chain, the solubility and stability of polyacetylenes
(PAs) can be greatly improved.8,9 Among the various PAs,
poly(phenylacetylene)s (PPAs) have been found to possess
photoconductivity,10,11 third-order nonlinear optical properties,
light emission,12-14 light emissivity,15,16 helical chirality,17,18
liquid crystallinity,19,20 and gas permeability.21,22 In general,
Experimental
Materials.
2-Methyl-3-butyn-2-ol (Tokyo Kasei), bis-
(triphenylphosphine)palladium(II) chloride (Tokyo Kasei),
Bull. Chem. Soc. Jpn. 2016, 89, 27–32 | doi:10.1246/bcsj.20150294
© 2016 The Chemical Society of Japan | 27

1
copper(I) iodide, ethyl 3-iodobenzoate (Tokyo Kasei), 1-
aminopyrene (Tokyo Kasei), N,N¤-dicyclohexylcarbodiimide
(DCC, Tokyo Kasei), N-(4-pyridyl)dimethylamine (DMAP,
Wako Pure Chemical), (bicyclo[2.2.1]hepta-2,5-diene)chloro-
rhodium(I) dimer ([Rh(nbd)Cl]2, Aldrich), chloro(1,5-cyclo-
octadiene)rhodium(I) dimer ([Rh(cod)Cl]2, Aldrich), and 2,5-
norbornadiene (Tokyo Kasei) were all purchased and used
without further purification. The solvents were treated as
follows: tetrahydrofuran (THF) was dried over calcium
hydride, triethylamine (Et3N) over potassium hydride, and
chloroform under nitrogen. Other chemicals were obtained
from Kanto Kagaku Co. and were used as received.
(C¸C), 3200 (C¸CH), 3400 (-COOH). H NMR (400 MHz,
DMSO-d6, ppm): ¤ 4.5 (s, C¸CH, 1H), 7.6, 8.0 (d, Ar-H, 4H),
13.1 (s, -COOH, 1H). 13C NMR (100 MHz, CDCl3, ppm): ¤
82.2, 83.1, 126.8, 139.7, 131.1, 132.5, 167.0.
Synthesis of 4-Ethynyl-N-pyrenylbenzamide (EPBA) (2).
First, (0.217 g 1.0 mmol) of 4-ethynylbenzoic acid, (0.146 g,
1.0 mmol) of 1-aminopyrene, (0.040 g, 3.2 mmol) of DMAP,
and (0.068 g, 0.4 mmol) of p-toluenesulfonic acid monohydrate
were dissolved in 10 mL of dry dichloromethane. To the di-
chloromethane solution, 0.82 g (4 mmol) of DCC was added
dropwise with stirring. The reaction mixture was stirred for
24 h at room temperature. After the solvent was removed, the
crude product was washed with saturated sodium hydrogen
carbonate aqueous solution, 0.1 M HCl aqueous solution, and
water. The organic phase was dried over Na2SO4 and con-
centrated by rotary evaporation. The residue was purified
by recrystallization from MeOH and acetone to afford solid
EPBA 2 in 85% yield. The data are as follows. Yield: 85.0,
brown powder (Figure 1). IR (cm¹1, KBr): 650 (C¸CH), 1750
(C=O), 2200 (C¸C), 3200 (C¸CH), 3300 (-NH). 1H NMR
(400 MHz, CDCl3, ppm): ¤ 3.0 (s, C¸CH, 1H), 4.5 (s, -NH
1H), 7.6, 7.8 (d, Ar-H, 4H), 8.0-8.5 (m, 9H). 13C NMR
(100 MHz, CDCl3, ppm): ¤ 51.2, 82.2, 83.1, 126.8, 125-135
(Pyrene), 139.7, 131.1, 132.5, 180.0.
Monomer and Catalyst Synthesis.
Synthesis of 4-
Ethynylbenzoic Acid (1): The reaction procedures are similar
to these reported elsewhere.28 To a 200-mL three-necked flask
were added methyl ethyl-4-iodebenzoate (5.0 g, 25.7 mmol),
2-methyl-3-butyn-2-ol (2.51 mL, 25.7 mmol), Ph3P (0.075 g,
0.28 mmol), CuI (23 mg, 0.12 mmol) in 93.2 mL of dry Et3N
and 7 mL of dry pyridine. [PdCl2(Ph3P)2] (326 mg, 3.8 mmol)
was then added to the above mixture under nitrogen. After
refluxing for 4 h at ca. 80 °C, the mixture was cooled to room
temperature and filtered to remove the insoluble trimethylamine
hydrobromide. The salt was washed with trimethylamine and
ethyl ether until the ether washings were clear. The combined
filtrates were reduced to dryness under reduced pressure, and
the crude product was washed with water. The residue was
dried under vacuum to yield 3.12 g of methyl 4-(3-hydroxy-3-
methylbut-1-ynyl)benzoate. The above solid was dissolved in
300 mL of refluxing solution of n-butanol with 3.2 g of KOH.
After refluxing for 10 min, the mixture was cooled in an ice
bath. After filtration, the collected residue was acidified to pH 2
by HCl. The mixture was extracted with ethyl acetate. After
removing the solvent, the residue was dried under vacuum to
yield 2.31 g of 4-ethynylbenzoic acid 1 as a yellow solid (yield
76.2%) (Figure 1). IR (KBr): 650 (C¸CH), 1750 (C=O), 2200
Synthesis of [Rh(nbd){B(C6H5)4}] Catalyst. Synthesis of
this catalyst was carried out according to the method reported
by Schrock et al.29 RhCl3¢3H2O (250 mg) was dissolved in
water (1 mL), and Na[B(C6H5)4] (1 g) in methanol (20 mL) was
added. 2,5-Norbornadiene (nbd; 1 mL) was then added and the
solution was allowed to stand until crystals were deposited.
The yellow crystalline product was filtered and air-dried. The
desired complex was obtained in a yield of ca. 68.2%.
Polymer Synthesis (3).
All the polymerizations were
carried out in a glass tube equipped with a three way stopcock
under nitrogen. The polymerization conditions were as follows:
Synthesis of EA
O
O
O
[Pd], CuI
TEA, THF
O
+
HO
I
HO
O
KOH
BuOH
OH
ꢀ
Synthesis of PEBA
NH2
O
O
DCC, DMAP
TsOH, CHCl3
N
+
OH
H
ꢁ
Figure 1. The synthetic route of 4-ethynylbenzoic acid (1) and 4-ethynyl-N-pyrenylbenzamide (EPBA) (2).
28 | Bull. Chem. Soc. Jpn. 2016, 89, 27–32 | doi:10.1246/bcsj.20150294
© 2016 The Chemical Society of Japan

Polymerization
O
O
Rh Catalysts
N
H
H
N
H
n
Figure 2. Synthesis of poly(EPBA) (3).
Table 1. Synthesis of polymers
Monomer
Cat.
Yieldb)
c)
c)
c)
Mn
Mw
Mw/Mn
State
/g
/mmol
/g
/%
nbdd)
codd)
Bd)
0.1
0.1
0.1
0.29
0.29
0.29
0.065
0.057
0.072
65
57
72
1.5 © 104
1.2 © 104
1.8 © 104
2.4 © 104
1.6 © 103
3.0 © 104
1.6
1.3
1.7
Orange powder
Orange powder
Orange powder
a) [M] = 0.29 mmol L¹1, [Rh] = 2.9 © 10¹3 mmol L¹1, [TEA] = 0.5 mmol L¹1, solvent THF. b) Acetone-
insoluble part. c) Determined from GPC calibrated by polystyrene standard. d) nbd: [Rh(norbornadiene)-
Cl]2, cod: [Rh(cyclooctadiene)Cl]2, B: [Rh(nbd){B(C6H5)4}].
A solution of EPBA (0.1 g, 0.29 mmol), [Rh(nbd)Cl]2 (1.3 mg,
2.9 © 10¹3 mmol) and trimethylamine (0.045 ml, 0.5 mmol) in
distilled dry tetrahydrofuran (2.0 ml) was kept at room temper-
ature for 24 h in a glass tube under nitrogen. The polymers were
isolated by precipitation in a large amount of methanol with
yield. 4-Ethynyl-N-pyrenylbenzamide (EPBA) was condensed
using DCC with 1-aminopyrene and 4-ethynylbenzoic acid to
give the product 2 in 85.0% yield. The synthetic procedures
have not yet been optimized. Figure 2 shows the results of the
polymer synthesis. All polymers were obtained in yields of
around 57-72% as orange powders using various Rh catalysts
(Table 1). The molecular weights were measured by GPC
(using the polystyrene standard). All the polymers possessed
moderate molecular weights (Mw = 12000-18000), and their
polydispersities (Mw/Mn) were in the range of ca. 1.3-1.7.
Thus, it is concluded that these monomers gave polymers
having relatively high molecular weights in good yields. At the
optimal ratio between initiator and catalyst of 100:1, high
monomer conversions were observed. These results are some-
what better than those of polyacetylenes by Rh catalysts.30 The
characteristic absorptions of ¸C-H (3200 cm¹1) and C¸C
(2100 cm¹1) stretchings seen in the monomer spectra have
completely disappeared in the polymer spectra (Figure 3a).
stirring. The precipitate was collected by filtration and dried
¹1
under vacuum to give poly(EPBA) 3 (Figure 2). IR (cm
,
KBr): 650 (C¸CH), 1750 (C=O), 2200 (C¸C), 3200 (C¸CH),
3300 (-NH). 1H NMR (400 MHz, CDCl3, ppm): ¤ 3.0 (s,
C¸CH, 1H), 4.5 (s, -NH 1H), 7.6, 7.8 (d, Ar-H, 4H), 8.0-8.5
(m, 9H).
Results and Discussion
Measurement.
Infrared absorption spectra (FT-IR)
measurements were carried out using a JASCO FT-IR-615
spectrometer. Raman measurements were carried out using a
JASCO NRS-3000 Laser Raman Spectrophotometer. Thermo-
gravimetric analysis (TGA) was performed with a Rigaku
¹1
1
Thermoplus instrument at a heating rate of 10 °C min in
Figure 3b shows the H NMR spectrum of poly(EPBA). In the
nitrogen or air atmosphere. Gel permeation chromatography
(GPC) was carried out on a TOSOH HLC-8220 instrument
using THF as an eluent at 25 °C. The GPC calibration curve was
obtained with linear polystyrene standards. UV-vis absorption
measurements were performed with a V-670 UV-vis spectrom-
eter. The dry THF solvent was used as the reference. Fluores-
cence spectrum measurements were carried out using a JASCO
FP-6500. The dry THF solvent was used as the reference.
Fluorescence lifetime measurements were carried out using a
HORIBA modular fluorescence spectrophotometer Fluorolog-3.
1H NMR (400 MHz) and 13C NMR (100 MHz) spectra were
recorded on a Varian Mercury plus spectrometer (Varian Tech-
nologies Co., Ltd., Japan) at room temperature using deuterated
chloroform (CDCl3) or dimethyl sulfoxide (DMSO-d6) as the
solvent and tetramethylsilane (TMS) as the internal standard.
Synthesis and Characterization. The starting acetylene
compounds based on 2-methyl-3-butyn-2-ol were obtained
according to Figure 1 using a Sonogashira cross-coupling reac-
tion. The cross coupling of ethyl-4-iodebenzoate with excess 2-
methyl-3-butyn-2-ol afforded 4-ethynylbenzoic asid in 76.2%
1H NMR spectra of the polymers, the signals at around 3.1 ppm
assigned to the acetylenic proton (¸CH) of the monomer were
undetectable. The polymer obtained with the Rh catalyst in the
present study also showed a singlet peak around 6.1 ppm,
corresponding to the cis structure observed. The obtained
polymers were soluble in organic solvents such as toluene,
chloroform and THF. We used Raman technique to study the
stereostructure of the polymer backbones. The Raman spectra
revealed of poly(EPBA) possesses a cis-polyene structure
¹1
(Figure 3c). The peaks at 1537, 1340, and 962 cm can be
assigned to the stretching C=C band in cis-polyacetylene
(although it is partially overlapped with that of the phenyl
ring), the stretching vibration of cis C-C bond coupled with the
single bond connecting the main chain and phenyl ring, and the
C-H deformation band of the cis main chain, respectively.
On the other hand, the scattering peaks corresponding to
the trans form, which appear at around 1190 cm ,
¹1 31,32 are very
weak. Therefore, the resulting polymers are cis-rich poly-
acetylenes. This result is consistent with that usually found
in the monosubstituted polyacetylenes obtained from Rh-
Bull. Chem. Soc. Jpn. 2016, 89, 27–32 | doi:10.1246/bcsj.20150294
© 2016 The Chemical Society of Japan | 29

D
-NH-
disappear
disappear
-C=O
HC=C
HC≡C
HC≡C
㻠㻜㻜㻜
㻟㻡㻜㻜
㻟㻜㻜㻜
㻞㻡㻜㻜
㻞㻜㻜㻜
㻝㻡㻜㻜
㻝㻜㻜㻜
㻡㻜㻜
㼃㼍㼢㼑㼚㼡㼙㼎㼑㼞㻛㼏㼙㻙㻝
E
Temperature/°C
Figure 4. TGA results of poly(EPBA) and PPA measured
under nitrogen atmosphere and air.
Table 2. Thermal properties of poly(EPBA)
a)
b)
c)
T5%
T10%
Yc at
Polymer
/°C
/°C
600 °C/%
Poly(EPBA)-N2
Poly(EPBA)-Air
PPA-N2
414.2
405.0
296.3
232.7
431.2
414.5
301.4
266.6
67.7
60.5
4.1
δ/ppm
F
PPA-Air
2.3
a) T5%: The temperature for which the weight loss is 5%.
b) T10%: The temperature for which the weight loss is 10%.
c) Yc: Char yields.
㻝㻡㻟㻣
㻝㻟㻠㻜
nm. Furthermore, absorption peaks at 243 and 273 nm, can be
observed, considered to be the derived from the -NH- and
benzene ring absorption peaks, respectively.34
㻥㻢㻞
㻝㻝㻥㻜
Additionally, the maximum wavelength of the blue wave-
length 420 nm was subjected to fluorescence spectroscopy,
confirming the broad fluorescence peak. The peak of the exci-
mer fluorescence derived from pyrene was not observed. This
emission was observed as blue light by the naked eye. The
fluorescence quantum yield in solution was high at 0.37-
0.41.35,36 Additionally, the fluorescent assessment was sub-
jected to fluorescence lifetime measurements, and two compo-
nents were observed as shown in Figure 6, with fluorescence
lifetime of 24 and 107 ns. In general, organic compounds have
lifetime of 1 ns,37 so these lifetimes are considered to be long.38
This is because the preventing excimer fluorescence. Thus, the
lifetime is long because there in no energy loss. From the above
results, highly efficient fluorescence nature is observed, which
is expected to be available to the new fluorescent materials.
Figure 6. Fluorescence decay curves of poly(EPBA) in THF.
㻝㻤㻜㻜
㻝㻢㻜㻜
㻝㻠㻜㻜
㻝㻞㻜㻜
㻝㻜㻜㻜
㻤㻜㻜
㻾㼍㼙㼍㼚㻌㻿㼔㼕㼒㼠㻛㼏㼙㻙㻝
Figure 3. (a) FT-IR spectra of the corresponding poly-
(EPBA). (b) H NMR spectrum of poly(EPBA) in CDCl3.
(c) Raman spectra recorded from the poly(EPBA).
1
catalyzed polymerization, of which the main-chain is stereo-
regularly cis-transoid.
Thermal stability of the poly(EPBA) was investigated by
TGA and compared with pristine PPA. The TGA curves are
presented in Figure 4 and weight loss behaviors of the species
are tabulated at Table 2. The TGA results show that the
temperatures for 10% weight loss of the samples are 414 and
405 °C in the air and nitrogen atmosphere, respectively. The
thermal stability of the polymers is considerably improved in
comparison with PPA which degrades at about 290 °C.33
On the other hand, the thermal data also reveals that the
char yield of the poly(EPBA) is enhanced approximately fifteen
fold compared to PPA because of the presence of more rigid
and bulky pyrene group. Another noticeable feature is that the
weight loss difference at 600 °C between the polymer before
and after modification.
Conclusion
In summary, we designed and synthesized a novel acetylene
monomer and polymer containing pyrene units and investigated
the chemical and physical properties of the macromolecules.
Our results can be summarized as follows:
1. The monomer was prepared by a three step reaction route.
At each step, products were obtained in at least 60% yields, and
their structures were confirmed by various measurements.
2. Rh catalysts initiate the polymerization of the monomers,
furnishing polymers in high yields. The [Rh(nbd){B(C6H5)4}]
catalyst was the most effective in our polymerization.
Photophysical Properties.
The optical absorption and
photoluminescence (PL) emission properties of poly(EPBA) in
dilute solution (1 © 10¹5 M) are shown in Figure 5. In dilute
solution, poly(EPBA) has a rather broad absorption in the
320-375 nm region with an absorption maximum (-max) at 348
30 | Bull. Chem. Soc. Jpn. 2016, 89, 27–32 | doi:10.1246/bcsj.20150294
© 2016 The Chemical Society of Japan

3
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