Synthesis of phenylbutadiynylpyridinium derivatives for nonlinear optics

Article abstract of DOI:10.1080/15421400701548993

1-Methyl-4-{[4-(dimethylamino)phenyl]butadiynyl}pyridinium (PBP) derivatives with iodide or benzenesulfonate anion were synthesized as potential compounds for both second- and third-order NLO applications. PBP shows peculiar absorption spectrum in methano

Full text of DOI:10.1080/15421400701548993

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Synthesis of
Phenylbutadiynylpyridinium
Derivatives for Nonlinear
Optics
Shin-ya Kan-no a & Shuji Okada a
a Department of Polymer Science and Engineering,
Faculty of Engineering , Yamagata University ,
Yonezawa, Yamagata, Japan
Published online: 22 Sep 2010.
To cite this article: Shin-ya Kan-no & Shuji Okada (2007) Synthesis of
Phenylbutadiynylpyridinium Derivatives for Nonlinear Optics, Molecular Crystals and
Liquid Crystals, 471:1, 365-371, DOI: 10.1080/15421400701548993
To link to this article: http://dx.doi.org/10.1080/15421400701548993
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Mol. Cryst. Liq. Cryst., Vol. 471, pp. 365–371, 2007
Copyright # Taylor & Francis Group, LLC
ISSN: 1542-1406 print=1563-5287 online
DOI: 10.1080/15421400701548993
Synthesis of Phenylbutadiynylpyridinium Derivatives
for Nonlinear Optics
Shin-ya Kan-no
Shuji Okada
Department of Polymer Science and Engineering, Faculty of
Engineering, Yamagata University, Yonezawa, Yamagata, Japan
1-Methyl-4-{[4-(dimethylamino)phenyl]butadiynyl}pyridinium (PBP) derivatives
with iodide or benzenesulfonate anion were synthesized as potential compounds
for both second- and third-order NLO applications. PBP shows peculiar absorp-
tion spectrum in methanol, in which molar extinction coefficient at absorption
maximum (kmax) decrease irrespective of bathochromic shift of kmax compared with
the corresponding compound with one triple bond between two aromatic rings.
From semiempirical MO calculations, it was explained by aromatic-ring rotation
of PBP in methanol. Activity on second-harmonic generation (SHG) was found in
four out of thirteen compounds synthesized. Although most of PBP derivatives
were found to show exothermic reaction before melting, pure solid-state polymeri-
zation was not confirmed in these compounds.
Keywords: DAST; ionic dye; nonlinear optics; organic crystal; solid-state polymerization
INTRODUCTION
Ionic p-conjugated crystals such as 1-methyl-4-{2-[4-(dimethylamino)-
phenyl]ethenyl}pyridinium p-toluenesulfonate (DAST) [1] are con-
sidered to be one of the promising second-order nonlinear optical
(NLO) materials used for terahertz-wave generation [2] etc. In this
connection, development of high performance second-order NLO mate-
rials is still an interesting research subject. In our previous study, we
This work was supported by Tokyo Ohka Foundation for the Promotion of Science
and Technology and Grant-in-Aid for Scientific Research on Priority Areas (No. 446,
Super-hierarchical Structures) from the Ministry of Education, Culture, Sports, Science
and Technology.
Address correspondence to Shuji Okada, Department of Polymer Science and
Engineering, Faculty of Engineering, Yamagata University, 4-3-16 Jonan, Yonezawa,
Yamagata 992-8510, Japan. E-mail: okadas@yz.yamagata-u.ac.jp
365

366
S.-Y. Kan-no and S. Okada
FIGURE 1 Synthesis scheme of PBP derivatives 7a through 7m.
have found that increase of double bond number between phenyl and
pyridinium rings of DAST cation is quite effective to enhance hyper-
polarizability b [3]. Even when a triple bond is used as a p-conjugated
bridge between phenyl and pyridinium rings, many compounds with
second-harmonic generation (SHG) activity were found irrespective
of the symmetrical cationic structure compared with stilbazolium
structure [4]. In this study, above two molecular designs were
combined, and 1-methyl-4-{[4-(dimethylamino)phenyl]butadiynyl}pyr-
idinium (PBP) derivatives 7a through 7m in Figure 1 were prepared
and their properties were elucidated. In addition, another interesting
point on the PBP molecular structure is possibility of polymerization
in solid state because some butadiyne derivatives are known to give
polydiacetylenes (PDAs) [5]. This point was also investigated.
EXPERIMENTAL
PBP derivatives were synthesized according to Figure 1. Synthesis
procedure of 4-ethynyl-N,N-dimethylaniline 3 was reported pre-
viously [4]. Cadiot-Chodkiewicz coupling reaction between 3 and 4

367

368
S.-Y. Kan-no and S. Okada
FIGURE 2 Chemical structures of cations with various p-conjugation bridges
between pyridinium and (dimethylamino)phenyl groups. Two conformations of
cations are displayed as Structure A with a planer form and Structure B in
which two aromatic rings are perpendicular.
[6] gave 5 in 92% yield. Elimination of acetone from 5 and the following
Sonogashira coupling reaction with 4-bromopyridine hydrochloride
were performed in one pot to give 6 in 45% yield [7]. Methylation of
6 by iodomethane in chloroform resulted in 7a in 98% yield. Melting
points, spectroscopic data and elemental analyses of 5, 6 and 7a are
summarized in Table 1. Anion exchange of 7a was performed by the
reaction with silver sulfonates [8] to give the corresponding PBP
sulfonates 7b through 7m. The reason why such sulfonates were
selected as counter anions was that they gave many SHG active crys-
tals with stilbazolium and its related cations [3,4]. PBP derivatives
were finally recrystallized from methanol. Since 7d and 7g gave crys-
tals containing water or solvent, they were heated at 120ꢀC for 6 h
under reduced pressure to remove the included compounds. Thermo-
gravimetric and differential thermal analyses (TG-DTA) were per-
formed using a Seiko TG=DTA 220U apparatus with temperature
increasing rate of 10ꢀC=min. SHG activity was verified by irradiation
from a Nd:YAP laser at 1079 nm. Calculations of ionic species 7
through 10 shown in Figure 2 were performed by CAChe WorkSystem
Pro Ver. 6.1.12.33. Structures were optimized by PM3 in MOPAC
2002 (Ver. 2.5.0) and structure A in Figure 2 with planer conformation
was found to be most stable form for all cations. Calculations for struc-
ture B in Figure 2, in which one of the aromatic rings was rotated
by 90ꢀ, were also executed. The b values at zero frequency (b0) and
UV-Vis spectra were obtained by PM3 and ZINDO, respectively.
RESULTS AND DISCUSSION
In order to find difference in the conjugation effect of the bridging
group between pyridinium and (dimethylamino)phenyl groups,
UV-Vis absorption spectra of cations 7 through 10 were compared for
iodide in methanol (Fig. 3). About absorption maximum wavelength

Synthesis of Phenylbutadiynylpyridinium Derivatives
369
FIGURE 3 UV-Vis spectra of cations 7 through 10 in methanol. All cations
were measured in iodide form.
(kmax) in visible region, bathochromic shift of the triple-bond system
(from 8 to 7) is 18 nm, which is the same as the shift in the double-bond
system (from 9 to 10). Namely, similar bathochromic effect originated
from conjugation number was observed for both multiple-bond sys-
tems. On the other hand, molar extinction coefficient at kmax (emax
)
decrease in the triple-bond system (from 8 to 7) by 29% while emax
increase in the double-bond system (from 9 to 10) by 24%. In addition
to this opposite tendency, 7 shows relatively large absorption at 323 nm
whose emax is larger than that at 468 nm. This difference can be
explained by relative stability of the planer structure of the p-conju-
gation systems compared with the non-planer structures. By using
the PM3 method, we calculated difference in heat of formation (DHf)
between the optimized planer structure and the structure with 90ꢀ
rotation of one of the aromatic ring of the optimized structure, i.e.,
structures A and B in Figure 3. The DHf values for 9 and 10 are 28.4
and 21.5 kJ=mol at least, respectively, indicating that planer structure
A is quite stable for the double-bond system. On the other hand, DHf for
8 is reduced to be 7.9 kJ=mol and that for 7 is only 2.1 kJ=mol, which is
comparable to the energy of molecular thermal motion at ambient tem-
perature, i.e., about 2.5 kJ=mol. Since triple-bond carbons have two p-
orbitals crossed at right angles together with two sp-orbitals, aromatic
rings can conjugate with the butadiynyl bridge even in structure B for 7
resulting in small DHf. Accordingly, the structure of 7 at ambient tem-
perature is not fixed as structure A and aromatic rings can rotate each
other. When the absorption spectra of 7 calculated by ZINDO were
compared between structures A and B, HOMO ! LUMO transition at
478 nm for structure A was found to shift to a longer wavelength for
structure B but no oscillator strength resulting in no absorption band
at longer wavelength than 478 nm. Longest-wavelength absorption

370
S.-Y. Kan-no and S. Okada
for structure B of 7 was calculated to be 372 nm. From these calculation
results, peculiar spectral shape of 7 was considered to be due to aro-
matic ring rotation in methanol solution.
The b0 values of 8, 7, 9 and 10 were calculated to be 190 ꢂ 10ꢁ30
,
211 ꢂ 10ꢁ30, 240 ꢂ 10ꢁ30 and 417 ꢂ 10ꢁ30 esu, respectively. The order
of b0 values just corresponds to that of kmax at the longest wavelength.
The longer kmax is, the larger b0 is obtained in these compounds. Emis-
sion from the crystals stimulated by irradiation of a Nd:YAP laser is
summarized in Table 2. Among synthesized thirteen compounds 7a
through 7m, 7c, 7e, 7f and 7k showed clear SHG activity confirmed
by green emission. Most of other crystals showed red emission. Since
these compounds show red one-photon fluorescence and they have
no absorption at 1079 nm of the laser wavelength, we think that the
red emission by the laser irradiation was caused by multiphoton fluor-
escence.
Although PBP has the typical p-conjugation system with a donor and
an acceptor for second-order nonlinear optics as mentioned above, it is
also interesting for 1,4-addition solid-state polymerization to
give polydiacetylenes (PDAs) for third-order NLO materials if PBP
alignment is appropriate for polymerization in crystals. When polymer-
ization occurs in the present PBP derivatives, PDAs with (dimethyla-
mino)phenyl groups as electron donors and pyridinium groups as
electron acceptors directly attached to the backbone will be obtained.
When UV was irradiated to the crystals at ambient temperature, no
color changes were observed indicating that photopolymerization did
not proceed. On the other hand, many crystals showed an exothermic
peak before melting as shown in Table 2, suggesting that thermal poly-
merization occurred. However, partial melting was observed under an
optical microscope in this temperature region and this polymerization
seemed to be not pure solid-state polymerization. Solid-state polymer-
ization was reported to occur at high temperature even if butadiyne
moieties are not aligned in proper conditions for 1,4-addition in some
TABLE 2 Temperature of Exo- or Endo-Thermic Peaks Observed by DTA and
Emission Observed by Nd:YAP Laser Irradiation
Compound
7a 7b 7c 7d 7e 7f 7g 7h 7i
7j 7k 7l 7m
Temperature=ꢀC 211a 234a 228a 185b 228a 196a 213a 219a 223a 246a 180a 232a 211b
Emissionc
–
R
G
R
G
G
R
R
R
R
G
R
R
aOnset of an exothermic peak.
bOnset of an endothermic peak followed by an exothermic peak.
cG: Green emission due to SHG, R: Red emission due to multiphoton fluorescence.

Synthesis of Phenylbutadiynylpyridinium Derivatives
371
cases [9]. Thus, in the present cases, such polymerization was initiated
at high temperature below melting point and exothermic polymeriza-
tion reaction resulted in partial melting.
In conclusion, PBP derivatives, which are potential compounds for
both second- and third-order NLO applications, were successfully
synthesized. Peculiar absorption spectrum of 7a in methanol, in which
emax decrease although kmax is red-shifted compared with iodide of 8,
was explained by aromatic-ring rotation of 7 in methanol. As a
second-order NLO materials, several SHG active crystals were found.
Their crystal growth for crystallographic analysis is being progressed.
From the thermal study, most of PBP derivatives were found to show
exothermic reaction before melting. However, it was not pure solid-
state polymerization because partial melting was observed in the exo-
thermic reaction. Further anion exchange to control PBP arrangement
seems to be interesting to obtain efficient second-order NLO crystals
and solid-state polymerizable crystals. Another possible way to obtain
PDAs is hydrogen-bond complex formation [10] of PBP precursor 6.
These studies are in progress.
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