Synthesis of polydiacetylenes with pyridyl groups directly bound to the main chain

Article abstract of DOI:10.1080/15421406.2012.710191

N-Substituted carbamic acid esters of 8-pyridyl-5,7-octadiyn-1-ol were synthesized and all the monomers showed solid-state polymerizabilities by UV irradiation to give polydiacetylenes with pyridyl groups directly bound to the -conjugated backbone. Hydrogen-bond complexes of these monomers with dodecanoic acid (DA) or perfluorododecanoic acid (PFDA), in which the pyridyl nitrogen atom formed a hydrogen bond with the hydrogen atom of the carboxyl group, were also prepared. Although complex formability of DA with the monomers in this study was not so high, many PFDA complexes were obtained because of the higher acidity. All complexes could be polymerized in the solid state and the conversion increased by complexation compared with the original monomers. More favorable monomer stacking for solid-state polymerization could be achieved by combination of intermolecular hydrogen bonds between urethane groups and alkyl (or perfluoroalkyl) chain packing. Copyright

Full text of DOI:10.1080/15421406.2012.710191

Mol. Cryst. Liq. Cryst., Vol. 568: pp. 93–97, 2012
Copyright © Taylor & Francis Group, LLC
ISSN: 1542-1406 print/1563-5287 online
DOI: 10.1080/15421406.2012.710191
Synthesis of Polydiacetylenes with Pyridyl Groups
Directly Bound to the Main Chain
TADAHIRO SHIBUYA, YOKO TATEWAKI,
AND SHUJI OKADA∗
Graduate School of Science and Engineering, Yamagata University, 4-3-16
Jonan, Yonezawa Japan
N-Substituted carbamic acid esters of 8-pyridyl-5,7-octadiyn-1-ol were synthesized and
all the monomers showed solid-state polymerizabilities by UV irradiation to give polydi-
acetylenes with pyridyl groups directly bound to the π-conjugated backbone. Hydrogen-
bond complexes of these monomers with dodecanoic acid (DA) or perfluorododecanoic
acid (PFDA), in which the pyridyl nitrogen atom formed a hydrogen bond with the
hydrogen atom of the carboxyl group, were also prepared. Although complex forma-
bility of DA with the monomers in this study was not so high, many PFDA complexes
were obtained because of the higher acidity. All complexes could be polymerized in the
solid state and the conversion increased by complexation compared with the original
monomers. More favorable monomer stacking for solid-state polymerization could be
achieved by combination of intermolecular hydrogen bonds between urethane groups
and alkyl (or perfluoroalkyl) chain packing.
Keywords Solid-state polymerization; conjugated polymer; polydiacetylene; pyridyl
group; hydrogen-bond complex
1. Introduction
Polydiacetylenes (PDAs) can be obtained by solid-state polymerization of butadiyne
monomers as π-conjugated polymer single crystals [1]. Since third-order nonlinear optical
susceptibilities of PDAs have been reported in 1969 for the first time among the conjugated
polymers [2], a lot of researches on PDAs as third-order nonlinear optical materials have
been performed. We have been studying about extension of the π-conjugated structure of
PDA for improved optical and electronic properties. One of the strategies is introduction of
conjugated side groups such as aromatic rings [3–5]. Among the aromatic groups, pyridyl
groups have some interesting properties, i.e., hydrogen-bond formability by acting as a
proton acceptor, salt formability by reacting with acid, and metal-complex formability.
These characteristics may be used for adjustment of monomer arrangement in crystals and
changes in electronic structures of π-conjugation systems. In our previous communication,
we prepared 4-pyridylbutadiyne derivatives with a urethane group and their conversions
were generally improved by complexation with dodecanoic acid (DA) [6]. In this study, we
extended the system into 2-pyridyl and 3-pyridyl derivatives. These monomers and their
∗Address correspondence to Shuji Okada, Department of Organic Device Engineering, Graduate
School of Science and Engineering, Yamagata University, 4-3-16 Jonan, Yonezawa 992-8510, Japan.
E-mail: okadas@yz.yamagata-u.ac.jp
[509]/93

94/[510]
T. Shibuya et al.
hydrogen-bond complexes with DA or perfluorododecanoic acid (PFDA) were prepared
and their solid-state polymerization behaviors were compared.
2. Experimental
Pyridylbutadiyne derivatives 2a–2f, 3a–3f and 4f were synthesized according to Fig. 1.
Compound’s numbers 2, 3 and 4 indicate 2-pyridyl, 3-pyridyl and 4-pyridyl derivatives,
respectively, and f is for bifunctional monomers. A urethane group was introduced to
align monomers in a polymerizable stack by the intermolecular hydrogen bonding be-
tween adjacent monomers. Their chemical structures were confirmed by 1H- and 13C-NMR
spectra, IR spectra and elemental analyses. Melting points of the monomers measured
by a SII DSC 220 differential scanning calorimeter are included in Table 1. For prepara-
tion of the hydrogen-bond complexes of 2a–2e and 3a–3e with DA or PFDA, equimolar
solutions of a pyridyl butadiyne and DA or PFDA were mixed and solvent was evapo-
rated. For 2f, 3f and 4f, two equivalents of DA or PFDA were mixed. Solvent used were
ethyl acetate for DA and chloroform-ethyl acetate mixture for PFDA. Solid-state poly-
merization was confirmed by irradiating UV light at 254 nm from a 4-W lamp (UVP,
UVG-11). Conversion of monomers to the corresponding polymers was calculated from
decrease in IR peak intensity corresponding to νC≡C or from heat of melt of the monomer
remaining in the polymerized sample. UV-visible diffuse reflectance spectra were mea-
sured using a Jasco V-570 spectrophotometer with an integrated sphere (ILN-472). Powder
X-ray diffraction patterns were recorded on Rigaku RINT-Ultimate III and Ultimate IV
diffractometers.
Figure 1. Synthesis scheme of pyridylbutadiyne derivatives in this study. 2-Pyridyl, 3-pyridyl and
4-pyridyl derivatives are indicated by compound’s numbers 2, 3 and 4, respectively.

Synthesis of Polydiacetylenes with Pyridyl Groups
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Table 1. Melting point of monomers, polymer excitonic absorption wavelength (λex), poly-
mer absorption cutoff wavelength (λex) and conversion (c) by UV irradiation of monomers
and complexes
Original monomer
DA complex
PFDA complex
Mp/oC λex/nm λco/nm c/% λex/nm λco/nm c/% λex/nm λco/nm c/%
a
2a
2b
2c
2d
2e
2f
53
85
43
–
–
–
–
–
800
695
710
610
550
800
620
650
550
610
620
750
800
17b
10b
22b
8b
NCd
NCd
NCd
NCd
NCd
NCd
NCd
NCd
31b
625
785
670
770
40b
28b
33b
NCd
NCd
87c
30b
32b
48b
41b
15b
81c
58c
a
a
a
a
a
–
–
a
68
134
123
70
15b
76c
12b
26b
8b
a
610
585
570
–
800
690
560
550
570
580
710
770
3a
575
a
3b 112
3c 83
3d 100
3e
3f
4f
–
–
–
–
–
–
a
a
–
–
–
610
650
a
a
7b
NCd
NCd
87c
a
a
149
123
120
12b
34c
40c
a
608
575
612
610
720
795
81c
a
aAbsorption maximum was not observed although the absorbance in the visible region increased.
bConversion was calculated from decrease in IR peak intensity corresponding to νC≡C
.
cConversion was calculated from heat of melt of the monomer remaining in the polymerized sample
since the no peak corresponding to νC≡C was observed.
dNC means no complexation and conversion was not obtained.
3. Results and discussion
When we monitored the UV-visible spectral change depending on time of UV irradiation to
the monomers, all the monomers synthesized showed absorption increase in visible region
suggesting formation of π-conjugated backbones. Especially, 3a and 3b showed excitonic
absorption bands. Monomers 2f, 3f, and 4f with two pyridylbutadiynyl groups at both ends
also showed clear excitonic absorption bands at longer wavelengths compared with the
monomers with one pyridylbutadiynyl group. For the bifunctional derivatives, increase in
hydrogen-bond number within a monomer may assist to form more appropriate alignment
of monomers for the solid-state polymerization. Other monomers without showing exci-
tonic absorption peaks seemed to have less regular structures. The cutoff wavelengths of
absorption bands, the peak wavelengths of the excitonic band, and the conversions of the
original monomers are summarized in Table 1 together with those of the complexes with
DA or PFDA (see below).
By mixing pyridylbutadiyne derivatives with DA or PFDA, hydrogen-bond complex
formation was investigated. Complexation was confirmed by the following three points.
First, the IR spectrum of the mixture was not mere summation of the spectra of the two
components especially in the case of DA. The peak at around 920 cm−1 of DA originated
from O–H out-of-plane bending vibration of dimerized carboxyl groups was weakened by
hydrogen-bond complex formation of DA with pyridylbutadiyne derivatives although such
clear changes were hardly observed in the case of PFDA. Second, the melting point of the
mixture was different from that of the original two components. Third, the X-ray diffraction

96/[512]
T. Shibuya et al.
patterns of the mixture were not summation of the patterns of the original two components.
When the monomers were mixed with DA, production of the hydrogen-bond complexes
was only confirmed for few compounds, i.e., 3c, 3f and 4f. However, when more acidic
PFDA was used, all monomers except 2d and 2e could form the hydrogen-bond complexes.
When a conjugated substituent such as vinyl or phenyl group is attached to pyridine, pKa
order of the corresponding conjugate acids is 4-pyridyl > 3-pyridyl > 2-pyridyl [7]. Even
in the case of butadiynyl-substututed pyridines 2–4, similar tendency is expected, and order
of basicity is considered to be 4 > 3 > 2. Accordingly, order of hydrogen-bond-complex
formability should be 4-pyridyl > 3-pyridyl > 2-pyridyl. Actually, DA complex formation
of the corresponding 4-pyridyl derivatives has been found for 4a, 4b, 4c and 4e [6], in
which alphabets indicate the substituents in Figure 1, and 4-pyridyl derivatives gave five
DA complexes including 4f in this study. In Table 1, 3-pyridyl and 2-pyridyl derivatives
gave two and no DA complexes, respectively. More PFDA hydrogen-bond complexes were
obtained for 3-pyridyl derivatives than 2-pyridyl derivatives.
Figure 2 displays the results obtained for 3c as an example. In UV-visible diffuse
reflectance spectra after UV irradiation shown in Figure 2(a), the cutoff wavelength of the
DA complex of 3c (3c-DA) shifts to the longer wavelength (∼650 nm) compared with that
of 3c (∼550 nm). It is interesting that 3c-DA showed weak excitonic absorption bands at
around 610 nm although there were no excitonic bands for 3c. In the case of 3f and 4f,
excitonic bands were also retained in the corresponding DA complexes. The PFDA complex
Figure 2. (a) Change in UV-visible diffuse reflectance spectra of 3c, 3c-DA and 3c-PFDA during
UV irradiation. For 3c and 3c-PFDA, UV-irradiation spans for the lower, middle and upper curves
were 10 min, 1 h and 4 h, respectively, while those were 1 min, 5 min and 1 h, respectively, for
3c-DA. (b) Powder X-ray diffraction patterns of 3c, 3c-DA, DA, 3c-PFDA and PFDA. Magnified
diffractograms are inserted for 3c, 3c-DA and DA. Asterisks indicate obviously new peaks appeared
by complexation.

Synthesis of Polydiacetylenes with Pyridyl Groups
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of 3c (3c-PFDA) showed a cutoff wavelength similar to 3c. Among PFDA complexes, only
2a-PFDA and 3a-PFDA showed excitonic absorption bands, and other complexes showed
only absorption increase in visible region without maximum. Composition of the hydrogen-
bond complexes were confirmed by measuring the powder X-ray diffractograms, and those
of 3c are shown in Figure 2(b). When the diffraction patters of 3c-DA were compared
with those of 3c and DA, many peaks observed for 3c-DA coincided with those of 3c and
DA suggesting that the complexation partly occurred. Partial complexation with DA was
also found in 3f while no diffraction peaks of the original compounds were observed for
4f-DA. For the PFDA complexes, all of them showed new diffraction peaks without peaks
assigned to the original pyridylbutadiyne compounds and PFDA, i.e., the complexes were
obtained in quantitative yield. In the complexes irrespective of DA or PFDA, the conversions
from a monomer to the corresponding PDA increased. Namely, by forming hydrogen-bond
complexes with long-chain carboxylic acid, alignment of the pyridylbutadiyne derivatives
was tuned to more favorable one for the solid-state polymerization.
In conclusion, original monomers synthesized could be all solid-state polymerizable
because the introduced urethane group formed intermolecular hydrogen bonds to align
monomers in a polymerizable stack. Dipyridyl derivatives 2f, 3f and 4f showed relatively
higher conversion and clear excitonic absorption, and two arrays of intermolecular hydrogen
bonds seemed to be more effective to form a polymerizable stack. Complex formability of
PFDA with the pyridyl monomers in this study was higher than that of DA. More acidic
PFDA is apt to form hydrogen-bond complexes. All complexes could be polymerized and
the conversion increased by complexation. Both intermolecular hydrogen bonds between
urethane groups and alkyl (or perfluoroalkyl) chain packing resulted in more favorable
stacking for the solid-state polymerization.
Acknowledgment
This work was supported by a Grant-in-Aid for Scientific Research on Priority Areas (No.
17067001) from the Ministry of Education, Culture, Sports, Science and Technology and
a Grant-in-Aid for Challenging Exploratory Research (No. 21655048) from Japan Society
for the Promotion of Science.
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