Novel fluorescent liquid crystal containing azobenzene and stilbene moieties-synthesis, mesogenic and spectroscopic studies

Article abstract of DOI:10.1016/j.dyepig.2016.11.059

This paper presents synthesis and characterization of new rigid core dye composed of two chromophore moieties such as azobenzene and stilbene parts. During this study liquid-crystalline properties was investigated and found as expected. An additional and

Full text of DOI:10.1016/j.dyepig.2016.11.059

Accepted Manuscript
Novel fluorescent liquid crystal containing azobenzene and stilbene moieties-
synthesis, mesogenic and spectroscopic studies
Izabela Korbecka, Joanna Jaworska, Zbigniew Galewski
PII:
S0143-7208(16)30844-0
10.1016/j.dyepig.2016.11.059
DYPI 5744
DOI:
Reference:
To appear in: Dyes and Pigments
Received Date: 27 September 2016
Revised Date: 18 November 2016
Accepted Date: 29 November 2016
Please cite this article as: Korbecka I, Jaworska J, Galewski Z, Novel fluorescent liquid crystal
containing azobenzene and stilbene moieties-synthesis, mesogenic and spectroscopic studies, Dyes
and Pigments (2017), doi: 10.1016/j.dyepig.2016.11.059.
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to
our customers we are providing this early version of the manuscript. The manuscript will undergo
copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please
note that during the production process errors may be discovered which could affect the content, and all
legal disclaimers that apply to the journal pertain.

ACCEPTED MANUSCRIPT

ACCEPTED MANUSCRIPT
1
2
Novel fluorescent liquid crystal containing azobenzene and stilbene moieties- synthesis,
mesogenic and spectroscopic studies
3
4
Izabela Korbecka1, Joanna Jaworska2 and Zbigniew Galewski1*
5
6
(1) Faculty of Chemistry, University of Wroclaw, Joliot- Curie 14, 50-383 Wroclaw,
Poland
7
(2) Wroclaw University of Science and Technology, Faculty of Chemistry, Advanced
Materials Engineering and Modelling Group, Wybrzeze Wyspianskiego 27, 50-370
Wroclaw, Poland
8
9
10
11
12
13
E-mail addresses:
Izabela Korbecka: izabela.korbecka@chem.uni.wroc.pl
Joanna Jaworska: joanna.jaworska@pwr.edu.pl
Zbigniew Galewski: zbigniew.galewski@chem.uni.wroc.pl
14
15
Corresponding author: Zbigniew Galewski, Faculty of Chemistry, University of Wroclaw,
Joliot- Curie 14, 50-383 Wroclaw, Poland, zbigniew.galewski@chem.uni.wroc.pl,
16
17
18
19
20
21
22
23
1

ACCEPTED MANUSCRIPT
24
25
Novel fluorescent liquid crystal containing azobenzene and stilbene moieties- - synthesis,
mesogenic and spectroscopic studies
26
Abstract
27
28
29
30
31
32
33
This paper presents synthesis and characterization of new rigid core dye composed of two
chromophore moieties such as azobenzene and stilbene parts. During this study liquid-
crystalline properties was investigated and found as expected. An additional and unforeseen
phenomenon of solid state polymorphism was also noticed. Furthermore, fluorescent
properties were examined and confirmed by the fluorescence spectroscopy. This initial study
gave very interesting and promising results which can be basis for further studies of such new
type of azo-stilbene based substances.
34
35
Keywords: liquid crystals, azobenzene, stilbene, fluorescence, smectic, dye
1. Introduction
36
37
38
39
40
41
42
43
44
45
46
47
Liquid crystals containing azo moiety are extensively studied group of compounds.
Main reason of this situation are their interesting physicochemical as well as optical
properties. An important role in the azobenzene derivatives studies plays the photosensitivity
[1], which enables the trans-cis-trans isomerization of -N=N- group. Such compounds can be
used as molecular switches [2], where the UV light causes the transition from trans to cis
isomer. Whereas light from visible range causes back transition. Highly developed
optoelectronic industry is looking for such type of materials, with a number of specific
properties for example luminescent. In order to achieve this property a special group must be
implemented to the molecule structure. For this purpose, commonly used are f-electron
elements such as europium [3] or lanthanum [4,5]. In recent years, heterocyclic components
such those based on oxidazole [6] or coupled ring systems like perylenes became increasingly
popular [7].
2

ACCEPTED MANUSCRIPT
48
49
50
51
52
53
54
Main aim of this study was design and optimization of the synthesis root of rod-like
molecule with two dye groups: azobenzene and stilbene. Such fusion should provide very
interesting properties such as liquid-crystallinity, fluorescent properties and also very
interesting but probably complicated trans-cis isomerization in both photosensitive moieties.
Some of these attributes were investigated and are presented in this communication.
2. Material and method
55
2.1. Measurements Setup
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
Elemental analysis measurements were conducted by the use of Vario EL III elemental
analyzer and the NMR spectra were measured by the use of Bruker Avance III 500 MHz high
resolution spectrometer. As solvent deuterated chloroform was used.
Liquid-crystalline mesophases were investigated by three different methods.
Characteristic textures were observed by the use of polarized optical microscopy (POM) and
phase transitions temperatures were detected by thermo-optical analysis (TOA). Enthalpies of
phase transitions were measured by the use of differential scanning calorimetry (DSC). All
calorimetric measurements were made using Perkin Elmer 8500 calorimeter with scan rate 10
K min-1. During the microscopic observations Olympus polarized light microscope BX61-P
(TRF) with Linkam heating stage and Bresser camera 9 MPix were used. Absorption spectra
were obtained using the UV-Visible spectrophotometer (Varian Cary500 instrument), while
the emission spectra were recorded on a Hitachi F4500 Fluorescence Spectrofluorometer with
an excitation wavelength of 365 nm (mercurial lamp with filter). Scan rate of 1 nm/s. All
measurements were carried out in dichloromethane.
2.2. Synthesis
3

ACCEPTED MANUSCRIPT
72
73
74
75
All starting materials like 4-dodecylaniline, 4-hydroxybenzaldehyde, 4-
nitrophenylacetic acid and all other simple inorganic and organic substances were purchased
and used without additional purification. Detailed synthesis procedures from Figure 1 are
available in appendix.
76
77
78
79
80
81
82
83
84
85
Full synthesis scheme is shown in Figure 1. This project involves synthesis of azo and
stilbene part simultaneously. Moreover, as a spacer between chromophores 6-bromohexanoic
acid to azobenzene part from final molecule was introduced. Synthesis of compound (1) was
done according to the procedure described in [8], whereas compound (3) according to the
modified procedure, which is available in [9]. Compound (2) was obtained by the
esterification reaction of (1) and the above mentioned 6-bromohexanoic acid in the presence
of DCC (N,N'-Dicyclohexylcarbodiimide) and DMAP (4-Dimethylaminopyridine). The last
step of presented scheme is combination of (2) and (3) in etherification process, in which the
1
final product (4) is formed. H NMR and elemental analysis data of final product and
intermediates are available in appendix.
4

ACCEPTED MANUSCRIPT
86
87
88
89
Figure 1. Full synthesis scheme of new azobenzene-stilbene dye.
3. Results and Discussion
90
91
92
93
94
95
96
Synthesized material (4) was designed based on rigid geometry of the molecule, which
is well-known for the formation of liquid-crystalline phases. Studies of mesogenic properties
were started with microscopic observations (POM), which confirmed the presence of one
mesophase - smectic A (Figure 2a) and were continued by the use of thermo-optical analysis
(TOA) and differential scanning microscopy (DSC). Surprisingly, observed mesophase exists
in very wide temperature-range (about 80°C). Detailed data from the above-mentioned
experiments are available in appendix. During cooling cycle in POM measurements two
5

ACCEPTED MANUSCRIPT
97
98
crystalline forms with different textures were observed (Figure 2b and 2c). The occurrence of
two crystalline phases were also confirmed by DSC measurements.
99
100
101
102
103
Figure 2. Textures observed during POM measurements captured in cooling cycle: a)
focal-conic texture of liquid-crystalline smectic A observed at 172 °C b) first crystalline form
(CrI) observed at 108°C. Moreover, rising of second crystalline form (CrII) is slightly visible
c) two crystalline forms at 85°C: CrI- dark area of the picture, CrII- bright area.
104
105
106
107
108
109
110
111
112
Design of presented material involved the introduction of stilbene part to the chemical
structure. Stilbenes are well-known in the literature due to their fluorescent properties.
Therefore, obtained compound (4) was checked if it is able to emit some light. According to
the absorption and emission spectra, which were done in dichloromethane solution (available
in appendix), (4) absorbs light in the UV range (λAmax=350 nm) and emits visible light
(λEmax=550 nm). This very important feature strongly enlarges potential application
capabilities of presented material. This experiment shows that trough the combination two
different dyes we are able to obtain much more interesting material, which combines best
features of applied moieties.
113
4. Conclusion
114
115
116
117
In this study one new dye with two chromophore groups (azo and ethane double bond)
was successfully synthesized and its basic features such as: mesogenic and luminescent
properties were proved. It was discovered that the compound is still sensitive to the light from
both the UV and visible range (azobenzene part), but also exhibits luminescence in the visible
6

ACCEPTED MANUSCRIPT
118
119
120
121
area (stilbene part). Furthermore, the rigid shape of the molecule provides liquid-crystalline
properties (smectic A). According to the obtained results we suppose that presented dye
possess very promising application capabilities and can be used for further more detailed
studies.
122
5. Acknowledgements
123
124
This work was financed by Grant No. 2014/15/B/ST8/00115 from the Polish National Science
Centre.
125
6. References
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
[1] Natansohn A, Rochon P. Photoinduced Motions in Azo-Containging Polymers. Chem Rev
2002;102:4139-4176.
[2] Han M, Ishikawa D, Honda T, Itob E and Hara M. Light-driven molecular switches in
azobenzene self-assembled monolayers: effect of molecular structure on reversible
photoisomerization and stable cis state. Chem. Commun 2010;46:3598-3600.
[3] Driesen K, Binnemans K. Temperature-driven luminescence switching of europium (III)
in a glass dispersed liquid crystal film. Liq Cryst 2004;31:601-605.
[4] Dzhabarov VI, Knyazev AA, Strelkov MV, Molostova EY, Schustov VA, Haase W,
Galyametdinov YG. Tris(β-diketonates) lanthanum nematic adducts. Liq Cryst 2010;37:285-
291.
[5] Channabasaveshwar VY, Prabhu R, Shanker G, Bruce DW. Optically active, mesogenic
lanthanide complexes: design, synthesis and characterisation. Liq Cryst 2009;36:247-255.
[6] Han J, Wang F-L, Zhang F-Y, Zhu L-R. Synthesis, mesomorphic behaviour and photo-
luminescent property of new mesogens containing 1,3,4-oxadiazole fluorophore. Liq Cryst
2010;37:1521-1527.
7

ACCEPTED MANUSCRIPT
141
142
143
144
145
146
147
148
[7] Keuker-Baumann S, Bock H, Sala FD, Benning SA, Haßheider T, Frauenheim T,
Kitzerow H-S. Absorption and luminescence spectra of electroluminescent liquid crystals
with triphenylene, pyrene and perylene units. Liq Cryst 2001;28:1105-1113.
[8] Galewski Z, Hofmańska A, Zielińska K. Liquid crystalline properties of: 4-methoxy,
4-ethoxy, 4-propoxy and 4-buthoxy-4′-alkyloxyazobenzenes. Pol J Chem 1999;73:1357–
1371.
[9] McCulloch IA, Bailey RT. Synthesis and Characterisation of Liquid Crystalline Polymers
for Nonlinear Optical Applications. Mol Cryst Liq Cryst 1991;200:157 — 165.
8

ACCEPTED MANUSCRIPT
•
•
•
•
New dye with two chromophore groups was designed and synthetized
The optimized synthesis root is presented
Novel dye has interesting properties of both intermediates
Short characterization of mesogenic and fluorescent properties is showed