Synthesis, Characterization, and Mesomorphic Properties of New Pyridine Derivatives

Full text of DOI:10.1002/open.201500158

DOI: 10.1002/open.201500158
Synthesis, Characterization, and Mesomorphic
Properties of New Pyridine Derivatives
Ahipa T. N.*[a]
Awarding Institute: National Institute of Technology Karnataka, Surathkal (India)
Date Awarded: September 22nd, 2014
Supervisors: Prof. Airody Vasudeva Adhikari, National Institute of Technology Karnataka, Surathkal,
(India)
Introduction
The field of liquid crystals (LCs) is now well established not only in basic research but also in the devel-
opment of new applications and their commercial use. As LCs represent an intermediate state be-
tween ordinary liquids and solids, the investigation of their physical properties is very complex and
hence their study requires the use of many different tools and techniques. The fundamental require-
ment for any substance to exhibit a liquid crystalline phase is the shape anisotropy of the constituent
molecules.
LCs play important roles in materials science, and they are model materials for organic chemist to in-
vestigate the relationship between the chemical structure and physical properties. In recent years, sig-
nificant attention has been given to the development of new LC materials for novel areas such as
thin-film transistors (TFT), organic light-emitting diodes (OLED), solid organic lasers and photovoltaic
devices; this is mainly due to the self-assembling properties and the various supramolecular structures
of liquid crystals.
Organic LC materials constitute aromatic or heteroaromatic systems carrying different linking groups
and terminal substituents. Generally, these organic LCs can be designed and constructed using various
aromatic or heteroaromatic systems to fulfill the criteria for the desired applications. In this respect,
benzene, biphenyl, anthracenyl and phenanthrenyl moieties have been most commonly explored as
the aromatic core.
Heterocyclic LCs contain a heterocycle as the core moiety. Precisely, a heterocyclic compound is one
that contains a ring made up of one or more heteroatom, such as nitrogen, oxygen or sulfur, in addi-
tion to carbon. Generally, the design of novel thermotropic LCs involves the suitable selection of
a core fragment, linking group and terminal functionality. Over many years, a large number of LCs con-
taining heterocyclic units as core moieties have been synthesized. Modern synthesis techniques allow
researchers to access tailor-made compounds with predictable properties, in the development of new
LC materials. The incorporation of heterocyclic moieties as core units in thermotropic LCs can result in
large changes in their mesophases and physical properties, because they possess more polarizable het-
eroatoms, such as nitrogen, oxygen and sulfur atoms in their structure. In this aspect, the development
of new LC materials bearing heterocyclic rings such as pyridine, pyrimidine, pyrazole, imidazole, oxa-
diazole and benzothiazole are of great interest, as they impart long range mesogenic stability to the
resulting LCs.
[a] Dr. A. T. N.
Centre for Nano and Material Sciences, Jain University
Jakkasandra Post, Kanakapura, Ramanagara District, Karnataka 562 112 (India)
E-mail: tn.ahipa@jainuniversity.ac.in
The full thesis is available as Supporting Information on the WWW under http://dx.doi.org/10.1002/open.201500158.
ꢀ 2015 The Authors. Published by Wiley-VCH Verlag GmbH & Co. KGaA.
This is an open access article under the terms of the Creative Commons Attribution-NonCommercial-NoDerivs License,
which permits use and distribution in any medium, provided the original work is properly cited, the use is non-commer-
cial and no modifications or adaptations are made.
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Amongst various heterocyclic LCs, pyridine-based mesogens have gathered much interest because
the presence of the pyridine moiety not only imparts rigid stable structure but also induces complex
LC phases in the resulting molecules. Also, it adds a wide range of photophysical properties. Given
their importance, the doctoral research described here mainly focused on design and synthesis of new
pyridine-based LCs.
Design of Pyridine Derivatives
On the basis of literature reports, four new series of pyridine derivatives were designed. The first series
was designed by taking an electron-deficient core, 2-methoxy-3-cyanopyridine, as an acceptor unit
and an electron-rich alkyloxyphenyl unit as a donor group to form D-A type molecular architecture
(Series I; LC1–13). In the design of the second series, methoxypyridine/2-methoxy-3-cyanopyridine was
selected as a core and substituted aryl/heteroaryl moieties as terminal groups in order to create mole-
cules with an unsymmetrical “bent” shape (Series II; LC14–33). The third series containing new disc
shaped 2-(4,6-disubstituted aryl-3-cyanopyridyl)oxyacetohydrazones were designed (Series III; LC34–38).
Finally, the fourth series was designed by connecting a 1,4-disubstituted 1,2,3-triazole unit with a lumi-
nescent 3-cyanopyridone ring in order to obtain a novel molecular architecture (Series IV; LC39–48). The
Scheme 1. Summary of new pyridine derivatives LC1–48
.
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hope was that the newly designed compounds (Scheme 1) would exhibit liquid crystalline behavior at
ambient temperature as well as improved photophysical properties.
Synthesis and Characterization
Series I–IV were successfully synthesized following appropriate synthetic routes. The target compounds
of Series I were synthesized according to Scheme 2. The required starting materials alkoxyacetophe-
nones 1a–m and alkoxybenzaldehydes 2a–m were prepared in good yield by O-alkylation of 4-hy-
droxyacetophenone and 4-hydroxybenzaldehyde or meta-substituted 4-hydroxybenzaldehyde by re-
acting with the corresponding alkyl bromides according to a reported procedure. Required dialkoxyar-
ylprop-2-en-1-one 3a–m were prepared from 1a–m and 2a–m using a Claisen–Schmidt reaction. It
was then cyclized to obtain 4,6-dialkoxyaryl-2-methoxy nicotinonitriles LC1–13 by reacting them with
malononitrile in presence of sodium methoxide.
Scheme 2. Synthesis of 4,6-dialkoxyaryl-2-methoxy nicotinonitriles (Series I; LC1–13). Reagents and conditions: a) aq KOH (1.2
equiv), EtOH, rt, 4 h, 68–83%; b) Malononitrile, NaOMe, MeOH, rt, 4 h, 62–82%.
The synthetic route for the preparation of the target compounds of Series II are shown in Scheme 3.
The starting materials (4a–g) were prepared in good yield by reacting various alkyl bromides (m=4, 6,
8, 10, 12, 14) with 4-hydroxyacetophenone according to the reported procedure. The required prop-2-
en-1-ones (6a–t) were prepared from aryl or heteroaryl ketones (4a–g) and aryl or heteroaryl alde-
hydes (5a–g) using a Claisen–Schmidt reaction. Further, it was cyclized to obtain 6-alkoxyaryl/thio-
phenyl-4-substituted aryl-2-methoxy pyridines derivatives (LC14–33) by reacting it with malononitrile in
presence of sodium methoxide at room temperature. It has been well-established that prop-2-en-1-
one upon reacting with malononitrile in basic medium (i.e., sodium methoxide) cyclizes to 2-methoxy-
3-cyanopyridine via dehydrogenation. Further, prop-2-en-1-ones (6a–f, m–s), upon reacting with malo-
nonitrile in basic medium, readily cyclizes to methoxypyridines (LC14–19, 26–32) via dehydrocyanation.
The synthetic route for the preparation of LC34–38 (Series III) is illustrated in Scheme 4. The required
3,4-bis(alkoxy)benzaldehydes (8a–e) were prepared by O-alkylation of 3,4-dihydroxybenzaldehyde (7)
with the appropriate alkyl bromides. On the other hand, compound 11 was prepared by a one-pot re-
action of 4-hydroxyacetophenone (9) and 4-hydroxybenzaldehyde (10) with ethyl cyanoacetate in the
Scheme 3. Synthesis of new 6-alkoxyaryl/thiophenyl-4-substituted aryl-2-methoxy pyridines. Reagents and conditions: a) aq
KOH (1.2 equiv), EtOH, rt, 4 h, 66–85%; b) Malononitrile, NaOMe, MeOH, rt, 4 h, 46–82%.
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Scheme 4. Synthesis of new 2-(4,6-disubstituted aryl-3-cyanopyridyl) oxy acetohydrazones (LC34–38). Reagents and conditions:
a) alkyl bromide, anhyd K2CO3, DMF, 808C, 12 h, 82–88%; b) ethyl cyanoacetate, ammonium acetate, 1,4-dioxane, reflux, 12 h,
62%; c) ethyl chloroacetate, anhyd K2CO3, DMF, 808C, 12 h, 71%; d) NH2NH2·H2O, EtOH, reflux, 12 h, 48%; e) AcOH (few
drops), EtOH, reflux, 12 h, 61–73%.
presence of excess ammonium acetate with 1,4-dioxane as the solvent. Furthermore, the reaction of
intermediate 11 with ethyl chloroacetate and anhydrous potassium carbonate in dry dimethylforma-
mide afforded tri-ester-functionalized compound 12 in good yield (71%). The reaction of compound
12 with hydrazine hydrate in refluxing ethanol gave trihydrazide 13. Finally, trihydrazide derivative 13
was condensed with substituted benzaldehyde 8a–e in the presence of glacial acetic acid as a catalyst
in order to obtain target trihydrazone-functionalized cyanopyridines LC34–38
.
The synthetic route for the preparation of target compounds LC39–48 (Series IV) is shown in
Scheme 5. Acetyl-functionalized 1,4-disubstituted 1,2,3-triazole intermediate 16 was prepared from
a one-pot reaction between 4-propargyloxyacetophenone 14 and 4-substituted benzylbromide 15 via
a little modified Click reaction in ethanol/water. The other intermediate, 3,4-bis(alkoxy)benzaldehyde
(8), was prepared by reacting 3,4-dihydroxybenzaldehyde with various alkylbromides using the Wil-
liamson method. Furthermore, cyanopyridone derivatives LC39–48 were prepared by a one-pot reaction
of acetyl-functionalized 1,2,3-triazole 16 and 3,4-bis(alkoxy)benzaldehyde 8 with ethyl cyanoacetate in
the presence of ammonium acetate in 1,4-dioxane.
All newly synthesized compounds were characterized by Fourier transform infrared spectroscopy
(FTIR), 1H and 13C NMR spectroscopy, mass spectrometry, and elemental analysis. The structures of
a select few compounds were also confirmed by single-crystal X-ray diffraction (SCXRD) analysis.
Mesogenic, Optical, and Optoelectronic Properties
Mesogenic properties of newly synthesized target compounds were investigated by using polarizing
optical microscopy (POM), differential scanning calorimetry (DSC), and powder X-ray diffraction (PXRD)
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Scheme 5. Synthetic route for the synthesis of target compounds LC39–48. Reagents and conditions: a) NaN3, CuI (10 mol%),
EtOH/H2O (1:1), 808C, 12 h, 69–76%; b) 8a–e (see Scheme 4 for synthesis), ethyl cyanoacetate, ammonium acetate, 1,4-diox-
ane, reflux, 12 h, 64–83%.
studies. Amongst the 48 compounds, almost all the compounds exhibit LC phases; LC1, LC14, and
LC20–25 show nematic phases. The observed nematic phase in compounds LC1 and LC14 is due to the
presence of a polar cyano group and lower alkoxy chains (butoxy) in their structure. Compounds
LC20–25 display nematic phase; this is mainly because of the presence of a lateral cyano group as well
as a fluoro or chloro group and variable alkoxy chains as terminal substituents.
Compounds LC2–13 and LC26–33 show rectangular columnar phase. Appearance of ambient tempera-
ture rectangular columnar phase in compounds LC2–13 is attributed to the strong intermolecular inter-
actions and the effect of higher alkoxy chains in them, while the presence of elevated temperature rec-
tangular columnar phase in compounds LC26–33 is mainly due to the effect of nitro, bromo or 4-pyridin-
yl groups and variable alkoxy chains as terminal substituents.
Compounds LC15–19 exhibit orthorhombic columnar phase at an elevated temperature (99–1258C)
owing to the presence of a polar cyano group and higher alkoxy chains. Compounds LC34–38 and
LC39–48 show hexagonal columnar phase at ambient temperature. The formation of mesophase in com-
pounds LC34–38 is due to the existence of intermolecular hydrogen bonds and p–p interactions exerted
by the cyanopyridine core as well as variable alkoxy chain lengths. While, the observed mesophase in
compounds LC39–48 can be attributed to the presence of terminal polar substituents (i.e., CN or NO2)
and variable alkoxy chain lengths. Also, the hydrogen bond ability of cyanopyridone moity together
with the planar 1,2,3-triazole unit contribute significantly in the formation of the observed mesophase.
Photophysical properties of target compounds were evaluated by using UV-visible and fluorescence
spectrometry. UV-visible absorption and fluorescence emission spectra of LC1–48 show a strong absorp-
tion band in the range of 320–370 nm and a strong blue emission band in the range of 380–470 nm.
The blue light emission is mainly due to the presence of p–p* electronic transition. The solvent-depen-
dent fluorescence emission study of LC1 reveals that the blue emission bands shift to higher wave-
length (positive solvatochromism) with the variation of organic solvents (nonpolar to polar). The fluo-
rescence spectra of LC2–13 in solution show blue emission band in the range of 398–415 nm, and these
bands are slightly red shifted by 6–12 nm in the liquid crystalline film state. Also, the compounds ex-
hibit good quantum yields (Ff=28–49%) in the solution state and satisfactory quantum yields (Ff=
10–22%) in the film state also. The fluorescence emission spectra of LC14–19, LC34–38 and LC39–48 display
emission bands in the range of 380–466 nm (Ff=42–60%) in the solution state and emission bands
in the range of 434–480 nm in the film state. Finally, the optoelectronic study was carried out on
a single-layer device with device structure ITO/LC38/LiF (1 nm)/Al (110 nm) under dark as well as UV
and white-light illuminations. The optoelectronic results reveal that LC38 is an active photo-responsive
and electron-transporting material for device applications.
Keywords: cyanopyridines · cyanopyridones · liquid crystals · methoxypyridines · solvatochromism
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Received: June 8, 2015
Published online on July 14, 2015
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