A simple route to unsymmetric cyano-substituted Oligo(p-phenylene-vinylene)s

Article abstract of DOI:10.1246/cl.180345

We report here a synthetic route to unsymmetric cyano-substituted oligo(p-phenylene-vinylene)s (COPVs), which have not been reported so far. These molecules were synthesised in high yield through a simple pathway of step-wise Knoevenagel condensation. The

Full text of DOI:10.1246/cl.180345

Advance Publication Cover Page
A Simple Route to Unsymmetric Cyano-substituted Oligo(p-phenylene-vinylene)s
Shotaro Hayashi,* Ryosuke Hirai, Shin-ichi Yamamoto, and Toshio Koizumi
Advance Publication on the web June 9, 2018
doi:10.1246/cl.180345
© 2018 The Chemical Society of Japan
Advance Publication is a service for online publication of manuscripts prior to releasing fully edited, printed versions. Entire
manuscripts and a portion of the graphical abstract can be released on the web as soon as the submission is accepted. Note that the
Chemical Society of Japan bears no responsibility for issues resulting from the use of information taken from unedited, Advance
Publication manuscripts.

1
A Simple Route to Unsymmetric Cyano-substituted Oligo(p-phenylene-vinylene)s
Shotaro Hayashi,* Ryosuke Hirai, Shin-ichi Yamamoto and Toshio Koizumi
Department of Applied Chemistry, National Defense Academy, 1-10-20, Hashirimizu, Yokosuka, Kanagawa 239-8686, Japan
E-mail:shayashi@nda.ac.jp
We report here the synthetic route to unsymmetric
knowledge, few reports on the synthesis of unsymmetric
COPVs have been reported.7 These report on the step-wise
synthesis of COPVs, but the synthestic phathway is not
efficient7a and the purification procedure is not simple7b.
Oxidative-coupling of arylacetonitriles and the Knoevenagel
condensation between arylacetonitriles and arylaldehydes
yield both symmetric and unsymmetric CSs (s-CSs and u-CSs,
Scheme 1a).8,9 Symmetric COPVs (s-COPVs) can be easily
obtained by the condensation of aryl dialdehydes with
arylacetonitriles (Scheme 1a).10
cyano-substituted oligo(p-phenylene-vinylene)s (COPVs),
which are not reported so far. These molecules were
synthesised in high yield through a simple pathway of step-
wise Knoevenagel condensation. The proposed study
improves the molecular design strategies of COPVs towards
solid fluorescent materials. This synthetic pathway gave
various novel COPVs. The properties of the obtained COPVs
were also discussed.
π-Conjugated Molecules • Knoevenagel Condensation •
Fluorescence • Unsymmetric Structures • Donor-Acceptor
Cyano-substituted stilbenes (CSs) and oligo(p-phenylene-
vinylne)s (COPVs) have attracted great interest owing to their
unique physical properties and applications in organic
optoelectronics.1 These molecules possess unique self-
assembly characteristics for nanostructures with enhanced
emission and/or efficient carrier transport.2 Moreover,
fluorescent π-conjugated molecules often show aggregation-
caused fluorescence quenching; however, cyano-substituted
molecules, such as CSs and COPVs, generally exhibit
aggregation-induced emission (AIE) or aggregation-induced
enhanced emission (AIEE). This is due to the changing of the
electric interactions involved into through-bond and through-
space interactions.3 The ristriction of intramolecular rotation
(RIR) of the twisted phenylene-vinylene frameworks in the
aggregated state offers through-bond and/or through-space
interactions. In addition to the efficient solid-state fluorescent
behaviour, almost all these molecules possess different and
unique fluorescence characteristics. CSs and COPVs have
played a crucial role in the recent advances in organic
mechanofluorochromic materials.4 These compounds are
synthesised via Knoevenagel condensation of an
arylacetonitrile with an arylaldehyde in the presence of a base,
usually in high yield. Recent efforts in the molecular design
and synthesis of such molecules are focused on the properties
that are useful for organic device applications. Since this
condensation is an easy synthetic procedure that uses
commercially available reagents, various CSs and COPVs
have been reported (Scheme 1a). However, so far, the
synthesis of some simple CSs and COPVs has been limited.
Regioregularity of π-conjugated oligomers and polymers
is important for self-assembly and physical properties.5 An
unsymmetric π-conjugated molecule offers the advantages of
good solubility, tuneable properties, and assembly.6 In
comparison with the simple symmetric π-conjugated
molecules, unsymmetric π-conjugated molecules require
selective synthetic strategies. Accordingly, there are several
reports on symmetric COPVs, but to the best of our
Scheme 1. (a) Retrosynthesis of various CSs and symmetric
COPVs from previous reports. (b) Retrosynthesis of cyano-
substituted stilbene carbardehyde and unsymmetric COPVs from
the proposed work.
The combination of the two arene units tunes and
controls the physical properties and self-assembly of COPVs.
For unsymmetric COPVs (u-COPVs), the molecular design is
flexible and physical properties can be tuned by choosing the
aryl moieties, but there are currently no reports of such a
synthetic strategy. CSs with an aldehyde group (CSAs) are
key molecules for u-COPVs synthesis (Scheme 1b). They are
also important building blocks because the aldehyde group is
useful not only for the Knoevenagel condensation but also for
various synthetic procedures, such as imination, and the
Wittig, Aldol and redox reactions.11 The synthesis of CSAs
would allow to obtain various CS-based molecules and u-
COPVs, which can function as organic emitters and building
blocks of oligomeric and polymeric materials (Scheme 1b). In
this study, we report the synthesis of CSAs and their
Knoevenagel condensation to yield u-COPVs.
Scheme 2 displays the synthetic procedure for the novel
COPVs. The target compounds can be prepared from a half
protected terephthalaldehyde that is employed during the first
step of the synthesis. The condensation of a p-arylacetonitrile,
1a-d, with terephthalaldehyde mono(diethylacetal), 2, was
carried out in methanol in the presence of NaOH at room
temperature for 10 min. The intermediate, acetal-protected 3,

2
was easily isolated by filtration because acetal-protected 3 is
insoluble in methanol/water (1:1 vol.). Then, the deprotection
of the acetal group afforded the CSs with an aldehyde group
(CSA), 3a-d, in good yield (> 89%). The second Knoevenagel
condensation, 3a-d with various arylacetonitriles (1a-e) in
ethanol in the presence of NaOH, resulted in symmetric (4aa,
4bb, 4cc, 4dd) and several unsymmetric (4ab, 4ac, 4ad, 4ae,
4bc, 4bd, 4be, 4cd, 4ce, 4de) COPVs in high isolated yield
(91%–99%). Also, the condensation of 3a-c with 4-
hydroxyphenylacetonitrile, 1f, was performed by acid
treatment after the reaction, affording monohydroxylated
unsymmetric COPVs (4af, 4bf, 4cf) in good isolated yield
(84%–96%). However, the reaction of 3d with 1f was not
successful, as it resulted in a complicated reaction mixture.
Symmetric COPVs (4aa, 4bb, 4cc, 4dd, 4ee, 4ff) were also
synthesised by the condensation of terephthalaldehyde, 2’,
with 1 in high isolated yield. It is worth noting that the
purification was very easy (filtering, washing and drying) as
COPVs are insoluble in ethanol and methanol.
a multichannel spectrometer. The Φ values of 3c and 3d in
CH2Cl2 were 0.26 and 0.25, respectively. The solid state Φ
values were higher than those in CH2Cl2 (Table 1). The
methoxy group effectively increased the quantum yield.
Table 1. Photophysical data of compounds
In CH2Cl2
Solid b
λabs (nm) λfl (nm)
Φ a
λabs (nm)
λfl (nm) Φ a
c
3a
331
333
354
363
347
372
381
380
363
373
-
-
333
342
367
396
496
497
540
517
517
544
500
547
548
550
523
558
619
547
528
547
559
583
533
592
594
534
-
5
c
3b
-
-
23
56
33
96
45
99
38
50
< 1
9
3c
485
448
-
26
25
3d
c
e
4aa
4ab
4ac
4ad
4ae
4af
-
-
c
e
-
-
-
e
500
-
38
-
-
e
-
c
e
-
-
-
e
496
-
21
-
CN
CN
O
OEt NaOH HClaq
c
e
Ar
4bb 373
4bc 383
4bd 404
-
-
Ar
+
OEt
MeOH CHCl3
e
O
492
496
-
29
28
-
82
36
20
< 1
97
12
98
< 1
15
28
50
-
3
1
1
2
3
e
-
c
e
CN
4be
4bf
4cc
4cd
4ce
4cf
370
377
377
400
382
376
-
-
NaOH
Ar
e
+
+
497
491
502
521
492
492
500
-
70
36
66
57
65
10
24
-
EtOH
Ar
e
NC
-
a-4
s-4
e
-
CN
e
O
NaOH
EtOH
-
Ar
1
e
-
O
Ar
NC
e
2'
4dd 392
-
e
4de
4ee
4ff
382
360
392d
-
c
e
-
-
Br
:
OMe
OH
Ar
489d
84d
-
e
1a
1b
1c
a
Absolute fluorescence quantum yields determined with a calibrated
integrating sphere system (average Φ value measured 5 times, errors <
3%).b Cast courted on glass plate.c <0.1 or = 0.d In DMSO.e The spectrum
was not clearly obtained.
S
N
1e
1f
1d
Scheme 2. Synthesis of CSA, 3, unsymmetric COPVs, a-4 and
symmetric COPVs, s-4, via Knoevenagel condensation.
UV-vis absorption and fluorescence spectroscopies and
quantum yield measurements were performed to investigate
the fundamental optical properties of these new compounds
(Figures 1, 2, S47-57 and Table 1). All photophysical data are
summarised in Table 1.
The UV-vis absorption spectra of 3 in CH2Cl2 are
presented in Figure 1a. The band energy of 3b was slightly
lower than that of 3a. Compounds 3c and 3d, which have
electron-donating 4-methoxyphenyl and 2-thienyl units,
respectively, were red-shifted in comparison with 3a and 3b.
The solid thin-film spectra of 3c and 3d also showed red-
shifted bands in comparison with 3a and 3b (Figure 1b). It is
worth noting that 3c and 3d showed fluorescence in CH2Cl2
(Figure S48), but this was not observed for 3a and 3b. This is
probably due to the charge transfer (CT) fluorescence based
on the donor-acceptor structures.12 Moreover, these
compounds 3 were emitted in the solid state. The quantum
yields (Φ) of the solutions and solids were measured based on
an absolute method using an integrating sphere equipped with
Figure 1. (a) UV-vis absorption in CH2Cl2 and (b) solid-state
fluorescence spectra of 3.

3
(JSPS). T. K. also acknowledged a KAKENHI (Grant-in-Aid
for Scientific Research C: no. 17K05891) of JSPS. This work
was performed under the Cooperative Research Program of
"Network Joint Research Center for Materials and Devices”.
References and Notes
†
Present address: Department of Applied Chemistry, National
Defense Academy, 1-10-20, Hashirimizu, Yokosuka, Kanagawa
239-8686.
1
2
B.-K. An, J. Gierschner, S. Y. Park, Acc. Chem. Res. 2012, 45,
544–554.
(a) H.-J. Kim, D. R. Whang, J. Gierschner, C. H. Lee, S. Y. Park,
Angew. Chem. Int. Ed. 2015, 54, 4330–4333; (b) F. Aparicio, S.
Cherumukkil, A. Ajayaghosh, L. Sánchez, Langmuir 2016, 32,
284–289; (c) S. Shin, S. H. Gihm, C. R. Park, S. Kim, S. Y. Park,
Chem. Mater. 2013, 25, 3288–3295; (d) J. W. Chung, B.-K. An, S.
Y. Park, Chem. Mater. 2008, 20, 6750–6755; (e) B.-K. An, D.-S.
Lee, J. S. Lee, Y.-S. Park, H.-S. Park, H.-S. Song, S. Y. Park, J. Am.
Chem. Soc. 2004, 126, 10232–10233.
(a) Y. Hong, J. W. Y. Lam, B. Z. Tang, Chem. Soc. Rev. 2011, 40,
5361–5388; (b) H. Wang, E. Zhao, J. W. Y. Lam, B. Z. Tang,
Materials Today 2015, 18, 365–377.
Z. Chi, X. Zhang, B. Xu, X. Zhou, C. Ma, Y. Zhang, S. Liua, J. Xu,
Chem. Soc. Rev. 2012, 41, 3878–3896.
Figure 2. (a) UV-vis absorption in CH2Cl2 and (b) solid-state
fluorescence spectra of 4bx.
We focused on the absorption spectra of 4bx in CH2Cl2
and solid-state fluorescence spectra of 4bx to analyse the
properties and differences in symmetric and unsymmetric
COPVs (Figure 2). The spectra of 4bx showed lower energy
bands than 3b due to the extension of the π-conjugated system
(Figure 2b). The peaks of 4bx were located between 372–401
nm. The spectra of u-COPVs (4ba and 4be), which have
phenyl and 3-pyridyl groups, showed peaks at ca. 372 nm.
These were similar to that of s-COPVs (4bb). Furthermore, u-
COPVs with electron-donor units (c, d, f) showed slightly
red-shifted bands due to the intramolecular CT interaction12
between the electron-donating units and the electron-acceptor
cyano-substituted vinylene.
Compounds 4bx showed fluorescence in the solid state
(Figure 2b). In addition, 4bc, 4bd and 4bf showed
fluorescence in solution (CH2Cl2). Introduction of the
electron-donor units into the molecules resulted in
fluorescence in both solution (isolated state) and solid
(aggregated state). The solid-state spectra of 4bx (518–631
nm) exhibited lower energy fluorescence bands than those in
CH2Cl2 (Figure S50) owing to the intramolecular CT
interaction in solids. The spectra of u-COPVs exhibited lower
energy than that of u-COPV (4bb) due to CT fluorescence.
Moreover, the compounds with a p-methoxyphenyl unit had
relatively high Φ value in solid. A strong electron-donor unit,
thienyl, dramatically decreases the energy band due to an
3
4
5
(a) L. Ying, F. Huang, G. C. Bazan, Nat. Commun. 2017, 8, 14047;
(b) S. Sato, K. Tajima, K. Hashimoto, Macromolecules 2009, 42,
1785−1788.
6
(a) M. L. Tang, M. E. Roberts, J. J. Locklin, M. M. Ling, H. Meng,
Z. Bao, Chem. Mater. 2006, 18, 6250−6257; (b) A. Y. Amin, A.
Khassanov, K. Reuter, T. Meyer-Friedrichsen, M. Halik, J. Am.
Chem. Soc. 2012, 134, 16548−16550; (c) H. Tian, Y. Han, C. Bao,
D. Yan, Y. Geng, F. Wang, Chem. Commun. 2012, 48, 3557−3559;
(d) H. Iino, T. Kobori, J.-I. Hanna, J. Non-Cryst. Solids 2012, 358,
2516−2519; (e) T. K. An, S. H. Jang, S.-O. Kim, J. Jang, J. Hwang,
H. Cha, Y. R. Noh, S. B. Yoon, Y. J. Yoon, L. H. Kim, D. S.
Chung, S.-K. Kwon, Y.-H. Kim, S.-G. Lee, C. E. Park, Chem. Eur.
J. 2013, 19, 14052−14060; (f) K. He, W. Li, H. Tian, J. Zhang, D.
Yan, Y. Geng, F. Wang, ACS Appl. Mater. Interfaces 2017, 9,
35427–35436.
(a) P. Xue, Y. Zhang, J. Jia, D. Xu, X. Zhang, X. Liu, H. Zhou, P.
Zhang, R. Lu, M. Takafuji, H. Ihara, Soft. Mater 2011, 7, 8296-
8304. (b) K. Nii, Jpn. Kokai Tokkyo Koho 2001-234159, 2001.
H.-C. Yeh, W.-C. Wu, Y.-S. Wen, D.-C. Dai, J.-K. Wang, C.-T.
Chen, J. Org. Chem. 2004, 69, 6455–6462.
(a) Y. Zhang, G. Zhuang, M. Ouyang, B. Hu, Q. Song, J. Sun, C.
Zhang, C. Gu, Y. Xu, Y. Ma, Dyes Pigm. 2013, 98, 486–492; (b) S.
Shin, S. H. Gihm, C. R. Park, S. Kim, S. Y. Park, Chem. Mater.
2013, 25, 3288–3295; (c) C. Dou, L. Han, S. Zhao, H. Zhang, Y.
Wang, J. Phys. Chem. Lett. 2011, 2, 666–670.
(a) Y. Sagara, A. Lavrenova, A. Crochet, Y. C. Simon, K. M.
Fromm, C. Weder, Angew. Chem. Int. Ed. 2016, 22, 4374–4378;
(b) K. A. N. Upamali, L. A. Estrada, P. K. De, X. Cai J. A. Krsuse,
D. C. Neckers, Langmuir 2011, 27, 1573–1580.
7
8
9
effective CT interaction. The compound with
a p-
hydroxyphenyl unit (4bf) showed higher Φ in CH2Cl2 than in
solid state, although the other compounds showed AIE or
AIEE 13. Therefore, incorporation of a hydroxyl group results
in an opposite trend.
10
In summary, we synthesised cyano-substituted stilbenes
with an aldehyde group, 3. Their Knoevenagel condensation
with different arylacetonitriles afforded symmetric and
unsymmetric cyano-substituted oligo(p-phenylene-vinylne)s,
4. π-conjugated compounds were successfully synthesised.
The absorption and fluorescence properties of the molecules
can be tuned based on the synthetic strategy. Further studies
based on the synthesis of various unsymmetric cyano-
substituted oligo(p-phenylene vinylne)s and reactions of 3
with various reagents are currently in progress. Moreover, the
possible applications of COPVs (such as flexible crystal
11
12
13
V. Rajeshkumar, Y. T. Lee, M. C. Stuparu, Eur. J. Org. Chem.
2016, 36–40.
Y. Hong, J. W. Y. Lam, B. Z. Tang, Chem. Soc. Rev. 2011, 40,
5361–5388.
(a) R. Matsidik, J. Martin, S. Schmidt, J. Obermayer, F. Lombeck,
F. Nübling, H. Komber, D. Fazzi, M Sommer, J. Org. Chem. 2015,
80, 980-987; (b) S. Hayashi, S. Yamamoto, T. Koizumi, Chem. Lett.
2017, 46, 200-203; (c) S. Hayashi, T. Koizumi, Polym. Chem. 2012,
3, 613-616; (d) O. Atwani, C. Baristiran, A. Erden, G. Sonmez,
Synth. Met. 2008, 158, 83-89.
(a) S. Hayashi, T. Koizumi, Angew. Chem. Int. Ed. 2016, 55, 2701–
2704; (b) S. Hayashi A. Asano, N. Kamiya, Y. Yokomori, T.
Maeda, T. Koizumi, Sci. Rep. 2017, 7, 9453; (c) S. Hayashi, T.
Koizumi, N. Kamiya, Cryst. Grow. Des. 2017, 17, 6153–6162.
Z. Chi, X. Zhang, B. Xu, X. Zhou, C. Ma, Y. Zhang, S. Liua, J. Xu,
Chem. Soc. Rev. 2012, 41, 3878–3896.
14
engineering of π-conjugated molecules14 and chromism15,16
are also under investigation.
)
15
16
S. H. acknowledged a KAKENHI (Grant-in-Aid for
Scientific Research B: no. 18H02052 and Grant-in Aid for
Scientific Research on Innovative Areas ‘π-figuration’: no.
17H05171) of the Japan Society for the Promotion of Science
(a) Y. Niko, S. Kawauchi, G. Konishi, Chem. Eur. J. 2013, 19,
9760–9765; (b) S. Ito, T. Yamada, T. Taguchi, Y. Yamaguchi, M.
Asami, Chem. Asian J. 2016, 11, 1963–1970; (c) U. Balijapalli, S.
Manickam, M. D Thiyagarajan, RSC Adv. 2016, 6, 58549–58560.