Synthesis, liquid crystal characterization and photo-switching studies on fluorine substituted azobenzene based esters

Article abstract of DOI:10.1039/c4ra13700k

A series of fluorinated azobenzene esters have been synthesized and studied by polarized optical microscopy (POM) and UV-Vis spectrophotometry. The -CO₂C₂H₅ group with monofluoro-substituted azobenzene exhibited nematic and smectic phases whereas difluoro-substituted azobenzene showed only the nematic phase. The addition of the electronegative fluorine atom plays an important role in photoisomerization of the azobenzene molecules. The monofluoro-substituted azobenzene gave strong photoisomerization in solution as compared with its diflouro counterparts. In these systems, trans-cis isomerization occurred after 4 minutes and cis-trans isomerization occurred after 22 hours which is much longer than expected for fluorine-substituted azobenzene systems. The presented results might have an influence on creating optical data storage devices.

Full text of DOI:10.1039/c4ra13700k

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Synthesis, liquid crystal characterization and
photo-switching studies on fluorine substituted
azobenzene based esters†
Cite this: RSC Adv., 2015, 5, 6279
a
a
a
a
ab
*
S. M. Gan, A. R. Yuvaraj, M. R. Lutfor, M. Y. Mashitah and Hegde Gurumurthy
A series of fluorinated azobenzene esters have been synthesized and studied by polarized optical
microscopy (POM) and UV-Vis spectrophotometry. The –CO2C2H5 group with monofluoro-substituted
azobenzene exhibited nematic and smectic phases whereas difluoro-substituted azobenzene showed
only the nematic phase. The addition of the electronegative fluorine atom plays an important role in
photoisomerization of the azobenzene molecules. The monofluoro-substituted azobenzene gave strong
photoisomerization in solution as compared with its diflouro counterparts. In these systems, trans–cis
isomerization occurred after 4 minutes and cis–trans isomerization occurred after 22 hours which is
much longer than expected for fluorine-substituted azobenzene systems. The presented results might
have an influence on creating optical data storage devices.
Received 3rd November 2014
Accepted 15th December 2014
DOI: 10.1039/c4ra13700k
www.rsc.org/advances
the dark by a process known as thermal back relaxation, where
the metastable Z isomer relaxes to the thermodynamically more
stable E form.11 This azobenzene based systems allows light-
induced inter-conversion to be used as photo switches for
information processing as well as optical data storage devices
and switches.12,13
1. Introduction
Photonics in which the molecular switching can be controlled
by light as a stimulus has attracted a lot of attention due to its
wide variety of applications ranging from display-electronics to
bio photonics.1,2 Azobenzene molecules have been highly dis-
cussed light sensitive chromophoric groups reported in the
literature, especially for storage devices and molecular switches
because they exhibit very interesting photoisomerization prop-
erties.3,4 Energetically more stable trans conguration converts
into less stable cis conguration with the illumination of light.
Then, come back to original trans conguration form cis
conguration, without any external aid.4 This process is called
Thermal back relaxation. It is basic concept for optical storage
of information.3
Azobenzene is considered as a versatile compound due to
their photo isomerization triggered by the p–p* excitation and
result in inter-conversion between cis (Z) isomer and trans (E)
isomers.4–10 E–Z conversion occurs with UV light (around
365 nm), which corresponds to the p–p* transition while Z–E
conversion occurs with irradiation of visible light (in the range
400–500 nm), which is equivalent to that of the n–p* transition.
Alternatively, the latter change can also occur spontaneously in
One of the most important requirements for azobenzene to
be used for optical data storage devices is long thermal back
relaxation. The macroscopic properties can readily be ne-
tuned with appropriate substitution and molecular geom-
etry.14 Until today, the molecular structure behaviour and also
their light-induced orientations are determined by azobenzene
with variety of substitutions.15 Particularly, uorine has a
unique effect on molecular properties, attributed to its high
electronegativity and lone pair electrons.16 Moreover, uori-
nated azo compounds exhibit greater stability to light than their
non-uorinated homologs.17
On the other hand, the rapid development in the area of
optical storage devices leads to signicant interest in liquid
crystalline materials from uorobenzene derivatives.18–20 It has
been reported recently by M. R. Lutfor et al., on pentauoro-
substituted azobenzene moiety and their liquid crystalline
behaviour along with photoswitching property. They showed
that addition of uorine substituents inuences the thermal
back relaxation.21 Still there is no report on very long thermal
back relaxation on adding uorine substituents to azobenzene
esters. Thus, the photo chemically induced transition is always
a promising approach for the optical data storage device.
With this in mind, we report the synthesis, liquid crystalline
property and photoswitching study of novel azobenzene based
esters with terminal uorobenzene moiety. The addition of
uorine will be expected to increase the thermal back
aFaculty of Industrial Sciences and Technology, University Malaysia Pahang, 26300
Gambang, Kuantan, Malaysia. E-mail: murthyhegde@gmail.com
bBMS R and D Centre, BMSCE, Bull Temple Road, Bangalore, India
† Electronic supplementary information (ESI) available: (i) The similar type of
compounds which do not containing ester linkage and uorine substitution
were studied to justify the effects discussed in the manuscript. (ii) 1H-NMR,
13C-NMR raw data is given to support the synthetic chemistry description. See
DOI: 10.1039/c4ra13700k
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relaxation. Longer the thermal back relaxation, applicability quenched with 1.5 N hydrochloric acid. The compound was
also more for data storage. With this aspect, presented extracted two times with dichloromethane. Then it was washed
compounds may be suitable for the creation of optical data with 1 N sodium hydrogen carbonate, followed by brine wash.
storage devices.
Removed the water content by adding anhydrous sodium
sulfate.27 Finally, collected the solid crude product by evapo-
rating dicholoromethane using rotatory evaporator. Obtained
product was puried by column chromatography using chlor-
oform : methanol (20 : 1) as eluent. The product was recrystal-
lized from methanol : chloroform (2 : 1) to get the target
compounds.
2. Experimental
2.1 Starting materials
Ethyl 4-amino benzoate (Fluka), aniline (Merck), sodium nitrite
(Fluka), phenol (Merck), 1,3-dicyclohexylcarbodiimide (DCC)
(Fluka), 4-(N,N-dimethylamino)pyridine (DMAP) (Fluka), 4-u-
oro benzoic acid (Merck), 3,5-diuoro benzoic acid (Merck), and
silica gel-60 (Merck) were used. Acetone was dried over phos-
phorus pentaoxide (Merck) and dichloromethane was dried
over calcium hydride (Fluka) and distilled out before use. Other
solvents and chemicals were used as such.
D1. A pale yellow coloured solid; yield: 35%; melting point is
151.2 C; IR (KBr pellet) gmax in cmꢁ1: 2922, 2857, 1732, 1605,
ꢀ
1
1508, 1238, 1075, 884, 759, 604; H NMR (400 MHz, CDCl3): d
8.29–8.25 (m, 4H, Ar), 8.06–8.03 (m, 4H, Ar), 7.96–7.94 (m, 5H,
Ar); 13C NMR (100 MHz, CDCl3): d 167.32 (ester C]O), 165.29–
115.8 (Ar–C); MS (FAB+): m/z for C19H13FN2O2, calculated:
320.32. Found: 320.13; elemental analysis: calculated (found)
%: C 71.24 (71.29), H 4.09 (3.97), F 5.93 (5.97), N 8.75 (8.68), O
9.99 (9.91).
2.2 General procedure for the preparation of para
substituted azobenzene
D2. A pale yellow coloured solid; yield: 35%; melting point is
167.35 ꢀC; IR (KBr pellet) gmax in cmꢁ1: 1742, 1706, 1602, 1508,
1278, 1196, 1067, 1015, 875, 755, 684; 1H-NMR (400 MHz,
CDCl3): d 8.22–7.21 (m, 12H, Ar), 4.45 (q, 2H, OCH2), 1.46 (t, 3H,
CH3); 13C NMR (100 MHz, CDCl3): d 166.07 (ester C]O), 163.86–
115.87 (Ar–C), 61.32 (OCH2), 14.35 (CH3).
Compound A (46.00 mmol, 1 equiv.) was dissolved in methanol
(40 ml) cooled the solution to 2 ꢀC. The 25% HCl was added
dropwise (8.672 ml) to the reaction mixture, still maintained the
temperature at 2 ꢀC. NaNO2 was dissolved with water (44.6 mmol,
ꢀ
1 equiv.) and added dropwise at 2 C. The reaction mixture was
stirred for 15 minutes to get compound B. Phenol solution was
prepared with methanol (44.6 mmol, 1 equiv.) and added slowly
at 2 ꢀC. The pH was elevated to 8.5–9.0 by using 1 N NaOH
solution. The reaction mixture was agitated for 4 hours. Diluted
the reaction mixture with methanol (250 ml) and ice. Reduced
the pH up to 4. The reddish yellow precipitate was ltered and
dried. The crude product was recrystallized twice from meth-
anol26 and compound C was obtained.
E1. A pale yellow colored solid; yield: 35%; melting point is
168.7 C; IR (KBr pellet) gmax in cmꢁ1: 1742, 1595, 1442, 1335,
ꢀ
1212, 1116, 983, 864, 802, 687; 1H-NMR (400 MHz, CDCl3): d
8.06–7.39 (m, 12H, Ar); 13C NMR (100 MHz, CDCl3): d 166.08
(ester C]O), 163.95–109.11 (Ar–C); MS (FAB+): m/z for
C
22H17FN2O4, calculated: 392.38. Found: 391.23.
E2.ꢀA pale yellow coloured solid; yield: 35%; melting point is
170.1 C; IR (KBr pellet) gmax in cmꢁ1: 1737, 1705, 1625, 1596,
1447, 1337, 1280, 1222, 1124, 1024, 870, 800, 642; 1H-NMR (400
MHz, CDCl3): d 8.24–7.02 (m, 11H, Ar), 4.45 (q, 2H, OCH2), 1.46
(t, 3H, CH3); 13C NMR (100 MHz, CDCl3): d 166.05 (ester C]O),
161.97–109.38 (Ar–C), 61.34 (OCH3), 14.53 (CH3).
A red coloured solid; Rf ¼ 0.42 (40% CH2Cl2–EtOH); yield:
62%; melting point: 158.5 ꢀC; IR (KBr pellet) gmax in cmꢁ1: 1728,
1
1602, 1484, 1248, 1140, 829; H NMR (400 MHz, CDCl3): d 7.26
(t, J ¼ 7.68 MHz, 1H, Ar), d 7.50 (t, J ¼ 7.52 MHz, 2H, Ar), d 7.88
(t, J ¼ 8.11 MHz, 2H, Ar), d 7.44 (d, J ¼ 8.36 MHz, 2H, Ar), d 6.95
(d, J ¼ 8.32 MHz, 2H, Ar), d 7.88 (s, 1H, OH).
A red coloured solid; Rf ¼ 0.42 (40% CH2Cl2–EtOH); yield:
62%; melting point: 160.2 ꢀC; IR (KBr pellet) gmax in cmꢁ1: 3321,
1728, 1602, 1484, 1248, 1140, 829; 1H NMR (400 MHz, CDCl3): d
8.17 (d, J ¼ 8.2 Hz, 2H, Ar), d 7.92 (d, J ¼ 7.5 Hz, 2H, Ar), d 7.88 (d,
J ¼ 7.5 Hz, 2H, Ar), d 7.01 (d, J ¼ 8.2 Hz, 2H, Ar), d 5.54 (s, 1H,
OH), d 4.42 (q, J ¼ 7.2 Hz, 2H, CH2CH3), d 1.44 (t, 3H, CH2CH3).
2.4 Characterization
The structures of the intermediates and desired products were
conrmed by spectroscopic methods: IR spectra were recorded
1
using a “Perkin Elmer (670) FTIR spectrometer” and H NMR
(400 MHz), 13C NMR (100 MHz) by Bruker. Also, CHN elemental
analyser (Leco & Co) was used. Optical textures were obtained by
using Olympus BX 51 polarizing optical microscope equipped
with a Linkam Hotstage. Sample were prepared on glass slide
and covered with coverslip. D1, D2, E1, E2 were prepared in
2.3 General procedure for the preparation of azobenzene
based esters
The uorine substituted benzoic acid (15.3 mmol, 1 equiv.) was chloroform with xed concentration of ꢂ1.2 ꢃ 10ꢁ5 mol lꢁ1 for
dissolved in 50 ml of dry dichloromethane. DMAP (1.40 mmol, photoswitching study. Photoswitching study was performed by
0.1 equiv.) were added and the mixture was stirred for 30 using UV-Vis spectrophotometer (model: Ocean Optic HR
minutes. A solution of compound C (15.3 mmol, 1 equiv.) in dry 2000+). Quartz cuvettes containing the sample solutions were
dichloromethane (10 ml) was added to the mixture and DCC closed to avoid evaporation of the solvent. The absorption for all
(23.0 mmol, 1.5 equiv.) in 10 ml of dry dichloromethane was the compounds (E-isomers) were measured before UV irradiation.
added slowly. The mixture was stirred for 24 hours. The reaction Omni Cure Series 2000 UV light source at intensity 5 mW cmꢁ2
was monitored by TLC. Aer completion of reaction, the ltered through 365 nm lter and heat lter (to avoid any heat
precipitate was removed by ltration and the ltrate was radiation arising from the source). The irradiation extended
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3. Results and discussion
3.1 Synthesis
The synthesis of compound D1, D2, E1, and E2 were performed
as depicted in Fig. 1 which shows synthesis scheme.
Ethyl-4-amino benzoate (compound A) was diazotised by
using sodium nitrite and hydrochloric acid. The diazotised
nitronium ion (compound B) was coupled with phenol to
produce para substituted azobenzene (compound C).
Compound D1 and D2 having single uorine atom was obtained
by coupling compound C with 4-uoro benzoic acid in the
presence of DCC and DMAP as coupling agent. The synthesis of
compound E1 and E2 with two uorine atoms were performed
by coupling compound C with 3,5-diuoro benzoic acid by DCC
and DMAP. Crude products were further puried by column
chromatography, followed by recrystallization from methanol.
The newly synthesized compounds were characterized by IR, 1H
and 13C NMR, and elemental analyses.
Fig. 1 Chemical structure and synthesis of fluorinated azobenzene
compound.
Table 1 Phase transition temperature (T/ꢀC) for compounds D1, D2, E1
and E2
a
Compound
Scan
Phase transitions
with no disturbance until a photostationary states was attained
and then, the light source was switch off. The changes in the
absorption spectra at the range from 260 to 600 nm were
recorded as a function of time. All these experiments have been
carried out at room temperature.
D1
D2
E1
E2
Cool
Cool
Cool
Cool
I 141.4 Cr
I 235.5 N 172.0 SmA 164.0 Cr
I 135.0 Cr
I 181.0 SmA 106 Cr
a
Abbreviations: Cr ¼ crystalline phase, N ¼ nematic phase, SmA ¼
smectic A phase, I ¼ isotrophic phase.
Fig. 2 Polarized optical micrographs obtained from cooling of isotropic phases of (a) nematic phase of D2 at 225.5 ꢀC (b) SmA phase of D2 at
168.0 ꢀC (c) crystalline phase of D2 at 160.0 ꢀC (d) SmA phase of E2 at 181.0 ꢀC (e) crystalline phase of E2 at 106 ꢀC.
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Optical texture was captured for compound D2 at 225.5 ꢀC
(Fig. 2a) showing typical nematic phase. On further cooling a
broken fan shaped texture was observed as typical for smectic A
phase. It was found at lower temperature 168 ꢀC (Fig. 2b) whereas
crystalline phase (Fig. 2c) observed on further cooling. For
compound E2, a fan shaped texture typical for smectic A phase
was observed as shown in Fig. 2d at 171 ꢀC and at 106 ꢀC it
transforms to crystalline phase (Fig. 2e). Similar phases were
found by us where we used uorine-substituted benzoate ester
(rod-shaped liquid crystals).21 It was successfully incorporated
into azobenzene as side arm linked with terminal double bonds
as polymerizable functional groups. They showed smectic A and
nematic phases of liquid crystal.21 In case of compound D1 and E1
with no –CO2C2H5 functional group, not showed any liquid
crystalline phases. Thus, with –CO2C2H5 group undoubtedly
plays a major role to bring mesogenic features of the azobenzene
derivatives. It should be noted that the uorinated azobenzene
compounds have lower transition temperature compare to non-
uoro compounds. Because, transition temperature decreases
with the number of uorine atom increases.21,22
Fig. 3 Absorption spectra before illumination.
3.2 Polarizing optical microscopy (POM) studies
Liquid crystalline phases were observed in compound D2 using
the polarizing-light optical microscope. A nematic phase
(schlieren texture) was observed upon cooling from the
isotropic at the rate of 2 ꢀC per minute.
Table 1 summarizes the mesophase behavior observed under
the polarizing optical microscope. One can see the nematic and
smectic phase for D2 compound and SmA phase for E2
compound and only crystalline phases for D1 and E1 compounds.
Fig. 4 Normalized absorption spectra with different exposure time of UV light with 365 nm filter for the compounds D1, E1, D2, and E2. No UV
corresponds to the 0 s of UV illumination. Intensity used for the illumination is 5 mW cmꢁ2
.
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molecules which can be attuned synthetically with substituent
groups to the chromophores.4 By all means, azobenzene mole-
cules undergo E–Z isomerization when irradiated with light
attuned to an appropriate wavelength.25 Upon UV illumination
of mono- and di-uoro azobenzene compound, absorption was
observed at their peak wavelengths, 322 nm and 331 nm
respectively. Even so there were notably no changes in their E–Z
isomerization time observed where all four compounds took ꢂ4
minutes to reach photostationary state due to photo isomeric
equilibrium of the E–Z isomers. That is to say the limitation in
wavelength tuning did not inuence the speed of E to Z isomer
transition during illumination. The photo conversion efficiency
(CE) of the E–Z photoisomerization is estimated from eqn (1).26
3.3 Photo switching studies
Photo switching studies were performed on quartz cuvette using
chloroform as solvent which gives an idea of the materials
behaviour with respect to UV light and also these results are
indispensable for creating optical storage devices. The inu-
ence of uorine on the p–p* absorption band position is best
evaluated from the absorption spectra. Prior to illumination, a
strong peak appeared at UV region correspond to p–p* transi-
tion of E isomer and weak peak at visible region correspond to
n–p* transition of Z isomer21,23 as shown in Fig. 3.
The E isomers of D1 and D2, which contain mono uorine
display their absorbance band at 322 nm whereas E1 and E2
which substituted with two uorine produces absorption band
at 331 nm. E1 and E2 showed bathochromic shis with respect
to D1 and D2 in the spectra. The addition of uorine on the
molecular structure of azobenzene derivatives effects the elec-
tronic spectra of the compounds. The lone pair electrons
present in the –F have considerable interaction with the p
system of the aromatic ring.24
Aðt0Þ ꢁ AðtNÞ
CE ¼
ꢃ 100%
(1)
Aðt0Þ
Where A(t0) is absorbance before UV and A(tN) is absorbance
aer UV. Table 2 summarizes the photo conversion efficiency in
these systems.
D2 gives 80.8% of Z fraction which is the highest among all
four compounds whereas E2 gives the lowest CE which is
64.46%. D1 and E1 give 72.93% and 70.26% of CE respectively.
With an increasing number of uorine substituent, the size of
the molecules increases and this change should result in an
increasing free volume required for the photoisomerization.
The reduced Z fraction observed in the case of E1 in comparison
with D1 and also E2 in comparison with D2 may be explicable in
terms of the sterically hindered structure of the Z isomer having
increased number of uorine substitution on phenyl ring. It is
clear that as the Z isomer is sterically crowded and thermody-
namically unstable, leading to the reduced Z fraction in the
photostationary state.27
During UV illumination, using 365 nm lter (UV intensity is
around 5 mW cmꢁ2) along with heat lter to remove any heat
radiation arising from the sample. Absorption peak decreases
due to p–p* transition, followed by slight increase in the peak
around 450 nm as a result of n–p* transition as shown in Fig. 4
(D1, E1, D2 and E2).
The wavelength at which azobenzene photoisomerization
occurs depends on the particular structure of azobenzene
Table 2 Summarised E–Z photoisomerization result results
Compounds
Monouoro
Diuoro
E–Z (minutes)
CE (%)
Thermal back relaxation occurs when molecules aer
attaining photo saturation state has been le in the dark where
Z isomers transform to E isomers. The effect of structural
modication can be observed in their thermal back relaxation
D1
D2
E1
E2
4
4
4
4
72.93
80.80
70.26
64.46
Fig. 5 Thermal back relaxation process of D1 and D2 as function of time after illuminating the material to photostationary state.
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Fig. 6 Thermal back relaxation process of E1 and E2 in the function of time after illuminating the material to photostationary state.
time. The reverse isomerisation process of D1 and D2 as a diuoro-substituted with –CO2C2H5 group, E2 shows 11.55
function of recovery time is as shown in Fig. 5. In case of hours of back relaxation. The presence of COOC2H5 functional
monouoro substitution D1, took about 16.42 hours to convert group in diuoro azobenzene chromophores reduced the
back to stable trans conguration whereas D2 having with thermal back relaxation time to 11.55 hours. Fluorine, the most
–CO2C2H5 group as functional group took around 22.48 hours. electronegative element are thought to withdraw electron
The reason of D2 took longer time to relax back to its original E density from the bonding p-type orbitals of –N]N– double
conguration could be that phases involved on both sides of bond. This electronic effect made the –N]N– double bond
transitions possess layered structure (smectic phases). In fact, a unstable and reduces the barrier for inter-conversion leading to
similar feature was observed in another case wherein the phase fast thermal back relaxation when more uorine was added.1
involved has a layer structure.21
Surprisingly, the –CO2C2H5 group containing monouoro azo-
In the following, D1 and D2 will be compared with azo- benzene gives a longest Z isomer lifetime among the other
benzene chromophore incorporating two uorine atoms at the compounds synthesized and almost double than that of
terminal end. As shown in Fig. 6, diouro-substituted molecule diuoro-substituted azobenzene with –CO2C2H5 group.
E1, shows 13.08 hours of thermal back relaxation whereas
Fig. 7 First-order plots for Z–E thermal isomerization for monofluoro-substituted and difluoro-substituted azobenzene measured at room
temperature 25 ꢀC.
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For Z–E photo isomerization, it is necessary to measure rst
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tting the experimental data to the eqn (2) (ref. 27) at room
ꢀ
temperature 25 C.
AN ꢁ At
ln
¼ ꢁkcꢁt
t
(2)
AN ꢁ A0
Where At, A0, and AN are the absorbance at peak wavelength at
time t, time zero, and innite time, respectively. The reaction
was rst order in the time region indicated. But, deviated from
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out in solution. Here one can mainly observed the change in
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samples with different temperature where room temperature
liquid crystals were mixed with these synthesized compounds is
in progress and will be reported in due course.
The similar series of carboxylic acid derivative and amide
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of ester compounds which are reported in this article. The
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4. Conclusion
Overall, the presence of uorine and –CO2C2H5 group in azo-
benzene proved to be the most important structural factors
determining the liquid crystallinity and thermal back relaxation
time. Based on experimental results, materials with
monouoro-substituted azobenzene esters exhibit long thermal
back relaxation in comparison with diuoro-substituted coun-
terparts. Long thermal back relaxation nearly 22 hours is the
rst report on uorine-substituted esters and is strong
contenders for creation of optical information storage devices.
Acknowledgements
We acknowledge Ministry of Science and Technology Industry
MOSTI for providing e-Science fund RDU 130503 and also
Ministry of Education for providing ERGS grant RDU 130619,
RAGS grants RDU 131408 and 131403.
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