New bent-shaped azomethine monomers for optical applications

Article abstract of DOI:10.1080/15421406.2016.1257301

It was convenient preparative method of new methacrylic monomers with banana-shaped structure and photoactive azomethine fragments synthesis. To determine the effect of large azomethine fragment on the process of homopolymerization, symmetrical model compounds with simpler structure were synthesized. The kinetic of its thermally initiated radical polymerization in DMF solution was studied and compared with the kinetics of model compounds does not containing bulky photochromic fragment. Peculiarities of photo initiated processes of E,Z-isomerization were studied.

Full text of DOI:10.1080/15421406.2016.1257301

Molecular Crystals and Liquid Crystals
ISSN: 1542-1406 (Print) 1563-5287 (Online) Journal homepage: http://www.tandfonline.com/loi/gmcl20
New bent-shaped azomethine monomers for
optical applications
Valeriia Ovdenko & Aleksey Kolendo
To cite this article: Valeriia Ovdenko & Aleksey Kolendo (2016) New bent-shaped azomethine
monomers for optical applications, Molecular Crystals and Liquid Crystals, 640:1, 113-121, DOI:
10.1080/15421406.2016.1257301
To link to this article: http://dx.doi.org/10.1080/15421406.2016.1257301
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Date: 20 December 2016, At: 02:11

MOLECULAR CRYSTALS AND LIQUID CRYSTALS
, VOL. , –

http://dx.doi.org/./..
New bent-shaped azomethine monomers for
optical applications
Valeriia Ovdenko and Aleksey Kolendo
Taras Shevchenko National University of Kyiv, Kyiv, Ukraine
ABSTRACT
KEYWORDS
azomethines;
banana-shaped;
methacrylate
polyazomethines;
photoisomerization
It was convenient preparative method of new methacrylic monomers
with banana-shaped structure and photoactive azomethine fragments
synthesis. To determine the effect of large azomethine fragment on the
process of homopolymerization, symmetrical model compounds with
simpler structure were synthesized. The kinetic of its thermally initi-
ated radical polymerization in DMF solution was studied and compared
with the kinetics of model compounds does not containing bulky pho-
tochromic fragment. Peculiarities of photo initiated processes of E,Z-
isomerization were studied.
1
. Introduction
Numerous investigations in recent years devoted to the development of polymers with a range
of characteristics that cannot be implemented in one polymer. Polymers with predetermined
properties that can be used in various industrial sectors for the development of devices with
photosensitive elements are one of the most promising for practical application. These poly-
mers can be created on the basis of polymethylmethacrylates with photoactive azomethine
fragments.
Azomethines are of great importance in many areas of application, both in medicine [1–4]
and in branches of industry related to information technologies [5–6]. They are used as a pig-
ments and dyes, catalysts [17], intermediates in organic synthesis [7], in analytical chemistry
[
8], as polymer stabilizers [9]. Also examples of their use for the extraction of metals [10–11]
and to obtain heat-resistant polymers with chelate type of structure [12–14] are well known.
Azomethines-based polymers have many useful properties such as thermal stability [15–
1
7], liquid-crystalline (LC) properties [18–21], ability to fiber formation, nonlinear optical
(
NLO) [22–23] and luminescent properties [24–25], conductivity [26–28] and the ability to
chelate formation [29]. Polyazomethines synthesized from aromatic amines and aldehydes,
can be used as semiconductors [29–31].
Prospects for future research are to study polyazomethines photochromic behaviour for
creating new materials with predictable properties. It is known that the large number of
scientific papers devoted to azomethine complexes with metal ions or azomethine based
polymers with only one photochromic fragment in monomer unit. Synthesis and properties
investigation of polymers, where each monomer unit contains two equivalent photochromic
CONTACT Valeriia Ovdenko
Color versions of one or more of the figures in the article can be found online at www.tandfonline.com/gmcl.
 Taylor & Francis Group, LLC
valeryovdenko@gmail.com
©

114/[300]
V. OVDENKO AND A. KOLENDO
azomethine fragments is a perspective direction in creation of NLO, LC, and other photoac-
tive materials.
It should be noted that the photoinduced transformation of one isomeric form to another
studied for many organic compounds. In literature much attention paid to photo-optical pro-
cesses that are characteristic for dichroic dyes structurally similar to azomethines such as azo-
benzene dyes [32–33] and stilbenes [34–35].
Due to the development and improvement of photonic technologies as the most progres-
sive for the optical information processing, interest for plastic materials which properties can
be controlled by light is rapidly growing. Photochromic transformations, go along with a
changing of the molecular geometry and dipole moment, affect the optical properties of pho-
toactive compounds. This is particularly evident when each molecule contains the number of
photoisomerizable fragments more than one.
Considering to the growing interest in the development of new materials with a range of
specified properties the purpose of this work was a synthesis of symmetric bis-azomethines
and its methacrylate derivatives, study of their photochemical properties and polymerization
ability. Also, it is important to investigate the impact of a large lateral fragment on the poly-
merization ability and rate of polymerization with further comparing of the experimental data
obtained for model compounds.
2
. Experimental
2.1. Instrumentation
NMR experiments in solutions have been carried out on a Varian Mercury 400 (399.88 MHz
1
for H) spectrometer. The measurements were performed in DMSO-d6 solution and tetram-
ethylsilane (TMS) was used as internal standard. Infrared spectra were recorded on Perkin
−1
Elmer Spectrum BX in the frequency range of 4000–400 cm using KBr pellets. Optical
absorption spectra have been recorded at room temperature in quartz cuvettes using Varian
Cary 50 UV-Vis-NIR spectrophotometer. Contractions during the process of thermal initi-
ated free-radical polymerization were measured by KM-8 cathetometer.
2
.2. Materials
All manipulations were performed in air atmosphere using commercial grade solvents. 4-
Hydroxybenzaldehyde, potassium hydroxide (KOH) and all 4-substituted anilines were of
reagent grade quality and used as received. Triethylamine and methacrylic anhydride were
purchased from Sigma-Aldrich and used as received.
2.3. Synthesis
,-diphenoxypropan--ol DPP
In 250 ml flask, equipped with reflux, 40.42 g (0,44 mol) of phenol was dissolved in 45 ml
of dioxane. The mixture was stirred at 95°C with 12.32 g (0.22 mol) of finely grind KOH for
3
1
0 minutes. After heating to 100–105°C, epichlorohydrin (ECH) was dropwisely added over
h. Then solution was intensively stirred at the same temperature for 2 hours. The precipitate
of KCl was filtered out, and solvent was removed by evaporation in vacuum. The crude prod-
uct washed with hot 15% KOH solution and extracted using ethyl acetate. DPP was purified
by recrystallization from hexane and obtained as white vitreous substance. Yield 51%. M.p.:

MOLECULAR CRYSTALS AND LIQUID CRYSTALS
[301]/115
1
8
1
0–81°C. H-NMR (400 MHz, DMSO-d ) δ/ppm: 7.23 (s, 4H, Ar), 6.90 (s, 6H, Ar), 5.28 (s,
6
−1
H, -OH), 4.17 (t, 1H, CH), 4.03 (m, 4H, CH ). Selected IR bands (cm ): ν = 2944 (-CH -
2
2
υ ), 2874 (>CH- υ), 1322 (>CH-. δ), 1118 (Ar, C-H δ).
as
,-Bis(-formylphenoxy)propan--ol FPP was synthesized according to the modified
procedure []
To 300 ml of water the 9.32 g (0.16 mol) of pre-grind in a mortar KOH and 20 g (0.16 mol) of 4-
hydroxybenzaldehyde were added. The mixture was warmed to 70°C and kept under vigorous
stirring for 40 minutes. Then 5.8 ml (0.08 mol) of epichlorohydrin was added over 1 h. The
reaction mixture was stirred at 70°C for an additional 4 h. After cooling, the precipitate was
filtered off, washed with water and dried. The crude product was purified by recrystallization
from ethanol-water (1:1, v/v). 1,3-Bis(formyl-phenoxy)-4-propanol was finally obtained as
1
beige powder. Yield 64%. M.p.: 142–143°C. H-NMR (400 MHz, DMSO-d ) δ/ppm: 9.89 (s,
6
2
H, COH), 7.85–7.83 (d, 4H, Ar), 7.13–7.11 (d, 4H, Ar), 5.44 (d, 1H, -OH), 4.23 (s, 1H, CH),
4
.19 (m, 4H, CH ). Selected IR bands (cm-1): ν = 2936 (-CH - υ.as), 2844 (>CH- υ), 2760
2
2
(
-CHO υ), 1680 (C = O), 1316 (>CH- δ), 1134 (1,4-subs Ar, C-H δ).

,-bis(-((p-tolylimino)methyl)phenoxy)propan--ol FPP-CH3
FPP (2 g, 0,0067 mol), p-toluidine (1.52 g, 0,0138 mol) and formic acid as catalyst in benzene
30 mL) were placed in a an oven-dried two-necked flask fitted with a Dean-Stark trap. After
(
heating at reflux for 2 h, the crude product was filtered off and washed with hot ethanol. Yield
1
8
7
2
5%. M.p.: 208–209°C. H-NMR (400 MHz, DMSO-d ) δ/ppm: 8.42 (s, 2H, CМ = N), 7.84–
6
.82 (d, 4H, Ar), 7.16–7.14 (d, 4H, Ar), 7.07 (t, 8H, Ar), 5.36 (s, 1H, OH), 4.23–4.13 (m, 5H,
CH + CH), 2.36 (s, 6H, CH ). Selected IR bands (cm-1): ν = 2936 (-CH -, υ ), 2914 (Ar-
2
3
2
as
CH , C-H υ ), 2876 (>CH- υ), 1606 (CH = N υ), 1308 (>CH-. δ).
3
as
-Formylphenyl methacrylate M-F
To the solution of 2.4 g (0.02 mol) of 4-hydroxybenzaldehyde in 20 ml of THF and the 3.05 ml
(
0.022 mol) of triethylamine were added. The mixture was cooled to about 0°C and 2.3 g
(0.022 mol) of methacryloyl chloride were added dropwisely to the stirred mixture. After-
wards, the system was stirred at room temperature for 6 hours. The precipitate of triethy-
lamine hydrochloride was filtered out, the solvent was removed in vacuum. The crude product
1
was washed with ethanol and obtained as yellow oil. Yield 88 %. H-NMR (400 MHz, DMSO-
d ) δ/ppm: 9.98 (s, 1H, -CHO), 7.98 (d, 2H, Ar), 7.35 (d, 2H, Ar), 6.34 (s, 1H, = CH2), 5.86
6
(s, 1H, = CH2), 2.05 (s, 3H, -CH3).

,-diphenoxypropan--yl methacrylate M-DPP
DPP (1,5 g, 0,006 mol) with phenothiazine (as polymerization inhibitor) and sulfuric acid
as a catalyst in acylation process) were dissolved in dioxane (5 ml). The solution was
(
heated to a temperature 65°C and after methacrylic anhydride (1.84 ml, 0.012 mol) was
added. Then, solution was heated at 70–75°C for additional 4 hours. After finishing of
the reaction (according to TLC) product was poured into water (70 ml), washed 3 times
with 10% sodium carbonate solution, recrystallized from benzene and dried. Yield 86%.
1
M.p.: 14–15°C. H-NMR (400 MHz, DMSO-d ) δ/ppm: 7.35 (d, 4H, Ar), 7.01 (m, 6H, Ar),
6
6
,26 (s, 1H, = CH ), 5,71 (s, 1H, = CH ), 5,58 (s, 1H, CH), 4,41(s, 4H, CH ), 2,1 (s, 3H,
2
2
2
CH ).
3

1
16/[302]
V. OVDENKO AND A. KOLENDO
,-bis(-formylphenoxy)propan--yl methacrylate M-FPP
M-FPP was synthesized according to previously described methods for M-DPP. Yield 57%.
1
M.p.: 18–19°C. H-NMR (400 MHz, DMSO-d ) δ/ppm: 9.85 (s, 2H, COH), 7.85–7.83 (d, 4H,
6
Ar), 7.14–7.12 (d, 4H, Ar), 6,09 (s, 1H, C = CH ), 5,65 (s, 1H, C = CH ), 5,54 (s, 1H, CH),
2
2
−1
4
(
,43(s, 4H, CH ), 1,93 (s, 3H, CH ). Selected IR bands (cm ): ν = 2930 (-CH - υ.as), 2832
2
3
2
>CH- υ), 2740 (-CHO υ), 1722 (C = O υ), 1690 (C = O), 1428 (CR R = CH , CH δ).
1
2
2
2
,-bis(-((p-tolylimino)methyl)phenoxy)propan--yl methacrylate M-FPP-CH3
The synthesis of 1,3-bis(4-((p-tolylimino)methyl)phenoxy)propan-2-yl methacrylate is
described as a representative case. FPP-CH (1 g, 0.0021 mol) solution in 20 ml of
3
dichloroethane was added dropwisely during 30 minutes to a solution containing methacrylic
acid (0.3 g, 0.0035 mol), dicyclohexylcarbodiimide (DCC, 0.43 g, 0.0021 mol), and dimethy-
laminopyridine (DMAP, 0.19 g, 0.0016 mol) in 10 ml of dichloroethane at about 0°C. The
reaction mixture was stirred for 36 h at room temperature. Then solvent was evaporated until
about 5 ml of solutionremained, the product was filtered and purified by recrystallization
from ethanol to provide M-FPP-CH . Final products were obtained as bright yellow powder.
3
1
Yield 56%. M.p.: 139–140°C. H-NMR (400 MHz, DMSO-d ) δ/ppm: 8.41 (s, 2H, CH = N),
6
7
.84–7.82 (d, 4H, Ar), 7.14–7.12 (d, 4H, Ar), 7.05–7.07 (t, 8H, Ar), 6,12 (s, 1H, = CH ), 5,65
2
(
s, 1H, = CH ), 5,54 (s, 1H, CH), 4.40–4.41 (d, 2H, CH ), 4.17–4.15 (m, 2H, CH ), 2.36 (s,
2
2
2
6
H, CH ), 1,95 (s, 3H, CH ). Selected IR bands (cm-1): ν = 2874 (>CH- υ), 2856 (Ar-CH ,
3
3
3
C-H υ ), 1718 (C = O υ), 1598 (CH = N υ), 1420 (CR R = CH , CH δ)
as
1
2
2
2

... Polymerization
Polymerization of monomers was carried out in dry dimethylformamide (DMF) solution
ꢀ
under argon atmosphere at 80°C in the presence of 1wt % 2,2 -azobisisobutyronitrile (AIBN)
as an initiator in dilatometer. Contractions were determined by KM-8 cathetometer, and con-
versions - by gravimetric method. The resulting polymer solution was cooled and poured into
ethanol to precipitate the polymer. The polymer was purified by reprecipitation. This proce-
dure was repeated until the polymer contained no trace of monomer.

... Calculations
Geometry optimization of the compounds was performed in semi-empirical PM6 approx-
imation [36] (program package MOPAC 2012). The correctness of the optimized structure
in the minimum on the potential energy surface was confirmed by the absence of imaginary
frequencies in the vibration spectrum. The wavelengths of 20 electronic transitions and the
corresponding oscillator strength in the absorption spectra were calculated by INDO/S-CI
[
37] in the ArgusLab 4.0.1 program [38] using the geometry of the ground state. Thus in CI
calculations were applied 20 orbitals (HOMO - 9 - HOMO і LUMO - LUMO +9). Simulated
absorption spectra obtained through expanding of every transition using Gaussian function.
3
. Results and discussion
ꢀ
The synthesis of methacrylate derivatives of 4,4 -substituted 1,3-bis(4(4-phenylimino)
methyl)phenoxy)propan-2-ol was carried out by a multi-step synthetic route as shown in
Scheme 1.
Initial hydroxyl azomethines obtained by condensation of 1,3-bis(4-formylphenoxy)
propan-2-ol with 4-substituted aniline in benzene in the presence of a catalytic amount
of formic acid. Four hours is usually enough to finish this reaction, but the end of the

MOLECULAR CRYSTALS AND LIQUID CRYSTALS
[303]/117
ꢀ
Scheme . Synthesis of the , -substituted ,-bis(-((-phenylimino)methyl)phenoxy)propan--ol
methacrylate derivative.
reaction can be determined by the amount of water in the Dean-Stark trap. Azomethine
methacrylates obtained by acylation reaction of hydroxy derivatives by methacrylic acid
ꢀ
in the presence of N,N -Dicyclohexylcarbodiimide (DCC) and 4-Dimethylaminopyridine
(
DMAP) at 0°C in dichloroethane. DPP and FPP methacrylic derivatives obtained by the
action of methacrylic anhydride in DMF solution in the presence of a catalytic amount of
concentrated sulfuric acid. The structure of all obtained compounds was proved by NMR
spectroscopy.
In order to study the polymerization ability of new methacrylates in radical thermoiniti-
ated polymerization process the kinetics of its polymerization were investigated. For deter-
mination of contraction in definite moment of time dilatometric method was used. All
ꢀ
monomers were studied in 10% dimethylformamide solution in the presence of 1wt% 2,2 -
azobisisobutyronitrile (AIBN) as an initiator in argon atmosphere.
Figure 1 shows the polymerization kinetic curves for methacrylates M-FPP, M-F, M-FPP-
CH , M-DPP (curves 1, 2, 3 and 4, respectively) and methyl methacrylate (MMA) (curve
3
5
).
According to kinetic curves rate of propagation V , reduced rate of propagation V and
p
re
total rate polymerization constant K were calculated (Table 1).
ꢀ
As seen from the data in Table 1, in case of all new methacrylates polymerization rate a
little bit higher than in the case of methyl methacrylate and 1.5–2 times slower than phenyl
Figure . Polymerization kinetic curves for % solution of new monomers in DMF at °C in the presence
of % AIBN (argon):  – m-FPP,  – m-F,  – m-FPP-CH ,  –m-DPP and  - MMA.


1
18/[304]
V. OVDENKO AND A. KOLENDO
Table  Kinetic parameters of thermoinitiated radical polymerization, % solution of methacrylic
monomers and phenylmethacrylate [] in DMF (°C, % AIBN, argon).

−

V × , s
re
−

−
Compound
Mr
Conversion, %, ( min)
V × , mol/l×s
K × , mol/l×s
p
ꢀ
MMA
PhMА
M-F
M-DPP
M-FPP
M-FPP-CH







—




.
.
,
.
.
.
,
,
,
,
.
,
.
.
.
.
.
,
methacrylate polymerization. It should be mentioned, that all kinetic parameters were calcu-
lated for steady state.
Among the new monomers methacrylate derivative with one aldehyde group (M-F) can
be polymerized fastest. Bis-phenyl (M-DPP) polymerization proceeds with a slightly lower
speed, than M-F. Dialdehyde containing derivative (M-FPP) demonstrates least low poly-
merization ability than M-DPP, but only during first 9 minutes, then polymerization rate
of M-DPP decreases, whereas the M-FPP polymerization rate remains constant for another
1
0 minutes. Though for M-FPP polymerization continuing to a larger conversion value than
for M-DPP, but total rate polymerization constant during steady state is larger for M-DPP.
In case of The bis-azomethine (M-FPP-CH ) lowest rate of polymerization possibly observed
3
due to highest molecular weight of monomer unit. The obtained data are in agreement with
the regularities of radical polymerization of such alkyl methacrylates.
Therefore, it was found that new symmetrical monomers containing bis-azomethine frag-
ment are capable to thermoinitiated radical polymerization and it provides the possibility to
obtain polymers with predetermined structure.
Photo-induced isomerization of azomethines and their complexes were have been inves-
tigated for a long time [39–40]. But the process of isomerization of symmetrical bis-
azomethines not yet described in the literature, so requires accurate study.
Figure 2 and 3 shows the changes in absorbance spectra of azomethine solutions in ace-
tonitrile under irradiation by 254 nm light and reversing process of Z,E-isomerization after
turning off the light, respectively. The process of reverse Z-E isomerization more than in 3
∗
−
Figure . Changes in the absorption spectrum of FPP-CH in acetonitrile (C =   mol/l) solution under

irradiation.

MOLECULAR CRYSTALS AND LIQUID CRYSTALS
[305]/119
∗
−
Figure . Changes in the absorption spectrum of previously irradiated FPP-CH in acetonitrile (C =  

mol/l) solution after the end of irradiation.
orders slower than the process of photo-induced trans-cis isomerization (half-reaction time
are 2180 minutes (36 hours) and 2 minutes, respectively). For that reason to increase the
resolution for reverse process spectrum shown in Figure 3 only in absorption maximum
region.
Theoretical absorption spectra of E,E-, E,Z- and Z,Z- isomers of original azomethines in a
vacuum were calculated by INDO/S method after geometry optimization with semi-empirical
PM6 approximation. In INDO/S approximation wavelengths and corresponding oscillator
strengths for 20 first transitions in absorption spectra in a vacuum were calculated. Figure 4
shows calculated absorption spectra of FPP-CH in vacuum:
3
After irradiation of FPP-CH solutions by UV light decreasing of a long-wave adsorption
3
band (corresponding to E,E–isomer) and increasing of a short-wavelength adsorption band
(
corresponding to E,Z– and Z,Z–isomer) can be observed. As a result appearance of isobestic
point takes place. Theoretically calculated spectra excluding the effect of solvent are in good
agreement with the experimental absorption spectra and confirmed experimentally obtained
photo-conversion direction.
Figure . Calculated absorption spectra of FPP-CH in vacuum:  – E,E- isomer,  – E,Z- isomer,  - Z,Z- isomer.


1
20/[306]
V. OVDENKO AND A. KOLENDO
Conclusions
A convenient method of synthesis of new methacrylic monomers containing bis-azomethine
photoactive fragment have been developed. The processes of photoinduced E,Z-isomerization
for original azomethine was investigated. It has been shown that the process is reversible. It
should be noted, that unlike the already known azomethines, bis-azomethines inclined to
continue the process of isomerization even after the termination of irradiation. The kinet-
ics of thermally initiated radical homopolymerization of azomethine monomer and model
methacrylates in solution was studied. Kinetic investigation of radical polymerization process
for new methacrylic monomers show their high polymerization activity that allows getting
different special function copolymers for the photophysical application.
Ultimately, it has been proven that vinyl polymers for special purpose and for use in photo-
physics based on synthesized monomers may be obtained.
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