Aliphatic/aromatic spacers based azo dye dimers: Synthesis and application for optical storage devices

Article abstract of DOI:10.1039/c4ra08219b

The photoisomerization effect of new bent-shaped azo dyes in the presence of aliphatic and aromatic spacers is reported for the first time. The synthesized compounds with n-hexane and benzene as central moieties were characterized by different spectral analytical techniques such as ¹H-NMR, ¹³C-NMR, FTIR and UV-Vis. They revealed the photoisomerization effect in solution and on the solid cells as well. In solution, the E-Z and Z-E isomerization occurred in around 17-18 seconds and 7-13 hours, respectively. The dramatic variation in back relaxation is speculated to take place due to the nature of the spacers involved in the system. The synthesized materials are expected to be more useful in optical storage devices.

Full text of DOI:10.1039/c4ra08219b

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PAPER
Aliphatic/aromatic spacers based azo dye dimers:
synthesis and application for optical storage
devices†
Cite this: RSC Adv., 2014, 4, 50811
A. R. Yuvaraj, Gan Siew Mei, Ajaykumar D. Kulkarni, M. Y. Mashitah
and Gurumurthy Hegde*
The photoisomerization effect of new bent-shaped azo dyes in the presence of aliphatic and aromatic
spacers is reported for the first time. The synthesized compounds with n-hexane and benzene as central
1
13
moieties were characterized by different spectral analytical techniques such as H-NMR, C-NMR, FTIR
and UV-Vis. They revealed the photoisomerization effect in solution and on the solid cells as well. In
solution, the E–Z and Z–E isomerization occurred in around 17–18 seconds and 7–13 hours,
respectively. The dramatic variation in back relaxation is speculated to take place due to the nature of
the spacers involved in the system. The synthesized materials are expected to be more useful in optical
storage devices.
Received 6th August 2014
Accepted 18th September 2014
DOI: 10.1039/c4ra08219b
www.rsc.org/advances
chemical structure is because of the system of molecules,
which allows delocalized electronic charge distribution
1
. Introduction
An optical storage device is a system that is used to save infor- between donor and acceptor groups at both sides of the p-
mation by light illumination. There are a number of light system. Moreover, an interesting feature of the azo group is the
sensitive organic compounds that have been employed to trans–cis isomerization by light absorption. In the eld of
fabricate information storage devices. The performance of any liquid crystals, this property might be very useful, especially in
1
0
11
optical storage device depends on the chemical structure of the the area of holographic media, optical storage and photo-
1,2
12
compounds as well as properties of illuminated UV light. The alignment of LC systems. Moreover, these bent molecules are
light sensitive groups, mainly chromophores, are responsible symmetric in nature, and in these bow-shaped molecules,
3
–5
for the photoisomerization and reorientation of molecules.
Among chromophores, azo dyes are good candidates for extent.
studying light-induced properties, which in turn makes it
On the other hand, the photoisomerization of azo dyes
“spacers” affect the photoisomerization to a signicant
1
3
possible to create optical storage devices, due to their highly generally varies with structure, different functional groups and
1
sensitive photoisomerization nature.
spacers. Note that different spacers give different character-
1
4
Azo dyes with different structures have been studied exten- istics to the compounds. It is quite interesting to study the
sively for photoisomerization and liquid crystalline behaviour back relaxation time based on aliphatic/aromatic spacers. To
2
–4
by many groups. The rst examples of bent core azobenzene the best of our knowledge, the dramatic inuence of aliphatic/
6
molecules were studied by Vorl ¨a nder et al. Moreover, there has aromatic spacers on dimeric azo dyes has not been studied to
been a lot of interest in banana-shaped liquid crystal azo dyes date. The energetically more stable trans conguration will
due to their unique photo switching and electro-optic behav- turn into the cis conguration when UV light of wavelength
7,8
iour. Prasad et al. reported quite interesting photo responsive 365 nm shines on azobenzene systems and reversion to the
9
behaviour of banana-shaped azo dyes.
original conguration is brought about either by keeping it in
It is not always true that all banana-shaped azo dyes are the dark (well known as thermal back relaxation) or by illu-
liquid crystalline in nature, but it is interesting to study these minating with white light of higher wavelength (say 450 nm),
azo dyes as guest–host systems in the liquid crystalline media. as shown in Fig. 1.
The main reason for using azobenzene molecules in the
Here, in this study, we investigated the optical properties of
the bent-shaped azo dye compounds, which might induce
dramatic changes in the photoisomerization of dimeric azo
dyes because of the spacer effect. Our newly synthesized
compounds are good candidates for photoswitching studies as
well as information storage devices.
Faculty of Industrial Sciences and Technology, Universiti Malaysia Pahang, 26300,
Gambang, Kuantan, Malaysia. E-mail: murthyhegde@gmail.com
†
Electronic supplementary information (ESI) available: The spectroscopic data
1
13
and characterization are given in SI. They are FTIR, H-NMR, C-NMR, etc. See
DOI: 10.1039/c4ra08219b
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host liquid crystal and the mixture was lled using the capillary
method in previously prepared ITO coated cells with unidirec-
tionally rubbed polyimide layers. Cell thickness was xed at
around 5 mm, and host liquid crystal used was room tempera-
ture nematic liquid crystals MLC 6873-100, exhibiting isotropic
ꢂ
21,22,23
temperature around 80 C.
2.2. Ethyl 4-(4-hydroxyphenyl azo) benzoate (B)
Compound A (46.00 mmol, 1 equiv.) was dissolved in methanol
ꢂ
(
40 ml), and the solution was cooled to 2 C. 25% HCl was added
drop wise (8.672 ml) to the reaction mixture while still main-
ꢂ
taining the temperature at 2 C. NaNO
(
2
was dissolved with water
ꢂ
44.6 mmol, 1 equiv.) and added drop wise at 2 C, and the
Fig. 1 Mechanism of photo-isomerization of azo-dyes with UV illu-
mination. Under normal conditions, azo dyes will be in the E-iso-
reaction mixture was stirred for 15 min. Phenol solution was
prepared with methanol (44.6 mmol, 1 equiv.) and added slowly
at 2 C, and the pH was elevated to 8.5–9.0 by using 1 N NaOH
solution. The reaction mixture was agitated for 4 hours, and
then diluted with methanol (250 ml) and ice. The pH was then
reduced to 4, and the reddish yellow precipitate was ltered and
dried. The crude product was recrystallized twice from
methanol.
ꢂ
merized form at room temperature ꢃ27 C. Once UV light of ꢃ365 nm
ꢂ
shines on the system, the E-configuration of the azobenzene will
convert into the Z-configuration.
2. Experimental details
A red coloured solid; R ¼ 0.42 (40% CH Cl –EtOH); yield:
A detailed synthesis scheme for synthesizing dimeric azo dyes
with aliphatic and aromatic spacers is shown in Fig. 2.
f
ꢂ
2
2
ꢁ1
6
1
2%; melting point: 160.2 C; IR (KBr) cm : 3321, 1728, 1602,
1
484, 1248, 1140, 829; H NMR (400 MHz, acetone-d6): 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
2
.1. Materials and methods
Ethyl 4-amino benzoate (Fluka), sodium nitrite (Fluka), phenol
Merck), 1-bromohexane (Fluka), potassium hydroxide (Fluka),
Hz, 2H, Ar), d 7.01 (d, J ¼ 8.2 Hz, 2H, Ar), d 5.54 (s, 1H, OH), d
13
4
.42 (q, J ¼ 7.2 Hz, 2H, CH
2
CH
3
), d 1.44 (t, 3H, CH
2
CH
3
);
C
(
NMR (100 MHz, acetone-d6): d 160.7, 145.3, 157.0, 132.3, 116.2,
24.4, 122.9, 130.2, 165.9, 60.9, 14.1; MS (FAB+): m/z for
C H N O , calculated: 270.28. Found: 270.08; elemental
potassium iodide (Fluka), potassium carbonate (Aldrich),
hydroquinone (Fluka), aniline (Fluka), 1,3-dicyclohexyl-
carbodiimide (DCC) (Fluka), 4-(N,N-dimethylamino)pyridine
1
1
5
14 2 3
analysis: calculated (found) %: C 66.66 (66.74), H 5.22 (5.16), N
(DMAP) (Fluka), 4-nitroaniline (Fluka), 4-aminoacetophenone,
10.37 (10.31), O 17.61 (17.67).
1
,6-dibromo hexane and silica gel-60 (Merck) were used.
Acetone was dried over phosphorus pentoxide (Merck) and
dichloromethane was dried over calcium hydride (Fluka) and
distilled before use. Other solvents and chemicals were used as
provided.
2
.3. Ethyl 4-[4-(4-hexyloxy)phenylazo] benzoate (C)
Compound B (18.5 mmol, 1 equiv.) and 1-bromohexane (37.1
mmol, 2 equiv.) were dissolved in dry acetone (300 ml). Then,
potassium carbonate (18.5 mmol, 1 equiv.) was added drop wise
by using an addition funnel. A catalytic amount of potassium
iodide (50 mg) was added to the reaction mixture. Then, the
reaction mixture was reuxed for 24 hours under a nitrogen
atmosphere, and the reaction was monitored by TLC to conrm
its completion. The reaction mixture was taken out and the
solvent evaporated inside a fume hood. Note that the dried
reaction mixture was taken for the next step, i.e., ester hydro-
lysis reaction.
The synthetic procedures were obtained from previously
16–20
reported studies
according to reactions.
and the procedures were modied
The structures of the intermediates and desired products
were conrmed by spectroscopic methods: IR spectra were
1
recorded using a Perkin Elmer (670) FTIR spectrometer and H
NMR (400 and 500 MHz) and 13C NMR (100 MHz) using Bruker;
CHN elemental analyser using Leco & Co. Optical textures were
obtained by using Olympus BX 51 polarizing optical microscope
equipped with a Linkam hot stage. The photo-switching study
was performed by recording UV-Vis absorption spectra using a
2.4. 4-[4-(4-hexyloxy)phenylazo] benzoic acid (D)
UV-Visible spectrophotometer obtained from Ocean Optics Compound C was used as such aer conrmation using TLC.
(
HR2000+). For photo-switching studies in solutions, photo- It was dried and dissolved in 100 ml of methanol. The reaction
ꢂ
switchable azo dyes were dissolved in chloroform at concen- mixture was cooled below 5 C by using an ice bath, and a
ꢁ5
ꢁ1
trations of C ¼ 1.1 ꢀ 10 mol L . Photoisomerization of these solution of potassium hydroxide (62.2 mmol, 3 equiv.) in
compounds were investigated by illuminating with an OMNI- water (20 ml) was added dropwise. Then, the reaction mixture
CURE S2000 UV source that was equipped with a 365 nm lter was reuxed for 4 hours, and the reaction was monitored by
and heat lter to avoid heat radiations, arising from the source TLC to conrm its completion. The reaction mixture was
to the sample. Photoswitching studies were also performed in taken in a separating funnel, and then it was washed two
solid cells, where 5% of guest azo dyes were mixed with 95% times with n-hexane to remove nonpolar impurities. Then, the
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Fig. 2 The synthetic design used in this study is depicted here.
aqueous solution was acidied with dilute hydrochloric acid 122.9, 130.6, 68.7, 29.6, 25.6, 31.8, 22.7, 14.1; MS (FAB+): m/z for
and the pH was adjusted exactly to 6.0. Subsequently, the , calculated: 326.39. Found: 326.09; elemental
19 22 2 3
C H N O
compound was extracted with ethyl acetate, followed by analysis: calculated (found) %: C 69.92 (69.98), H 6.79 (6.70), N
washing with brine solution, and anhydrous sodium sulphate 8.58 (8.49), O 14.38 (14.45).
was added to remove the water content. The solid product was
collected from the solution by evaporating ethyl acetate using
2
(E )
1
.5. 1,6-Hexylene bis-{4-[4-(4-hexyloxy) phenylazo]benzoate}
a rotatory evaporator and recrystallizing from ethanol : -
chloroform (2 : 1).
ꢁ
1
A dark yellow coloured solid; yield: 46%; IR (KBr) cm : 2849, Compound D (15.3 mmol, 1 equiv.) was dissolved in 50 ml of dry
2
918, 3004, 1711, 2918, 1220, 1588, 1493, 1248, 1130, 1092, 835; dichloromethane. DMAP (1.40 mmol, 0.1 equiv.) was added and
1
H NMR (400 MHz, DMSO): d 10.46 (s, 1H, COOH), d 8.12 (d, J ¼ the mixture was stirred for 30 min. A solution of 1,6-dibromo-
8
.45 Hz, 1H, Ar), d 7.85 (d, J ¼ 8.80 Hz, 1H, Ar), d 7.15 (d, J ¼ 8.95 hexane (7.6 mmol, 0.5 equiv.) in dry dichloromethane (10 ml)
Hz, 1H, Ar), d 6.97 (d, J ¼ 8.24 Hz, 1H, Ar), d 4.098 (t, J ¼ 13.05 was added to the mixture, and then DCC (23.0 mmol, 1.5 equiv.)
Hz, 2H, OCH ), d 1.82 (t, 2H, CH ), d 1.37 (t, 2H, CH ), d 1.27 (h, in 10 ml of dry dichloromethane was added slowly. The mixture
2
2
2
1
3
2H, CH
2
), d 1.25 (q, 2H, CH
2
3
), d 0.86 (t, 3H, CH ); C NMR (100 was stirred for 24 hours, and the reaction was monitored by
MHz, acetone-d6): d 161.6, 144.3, 157.9, 132.4, 114.7, 123.6, TLC. Aer completion of the reaction, the precipitate was
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removed by ltration and the ltrate was quenched with 1.5 N 3.1. Mesophase characterization
hydrochloric acid. The compound was extracted two times with
Table 1 indicates the transition temperature range of the
compounds in heating and cooling cycles. One can notice from
the table that the synthesized compounds form liquid crystal-
line nematic phases. The nematic textures were observed with a
polarizing optical microscope, as shown in Fig. 3, and the
dichloromethane, and then it was washed with 1 N sodium
hydrogen carbonate, followed by brine. The water content was
removed by adding anhydrous sodium sulphate. Finally, the
solid crude product was collected by evaporating dichloro-
methane using a rotatory evaporator. The obtained product was
ꢂ
ꢁ1
cooling rate is xed at 2 C minute
.
puried
by
column
chromatography
using
chlor-
oform : methanol (20 : 1) as eluent. The product was recrystal-
lized from methanol : chloroform (2 : 1) to obtain the target
compound E1.
3
.2. Photoswitching properties
Photo-switching studies of our two synthesized azo dyes showed
E : a pale yellow coloured solid; yield: 35%; melting point is similar absorption spectra with peaks at ꢃ357 nm, due to their
1
ꢂ
ꢁ1
1
1
54.2 C; IR (KBr) cm : 1730 (C]O), 1240 (C–O–C), around similar molecular structures. The only difference between the
1
500 (Ar C]C) along with other C–C, C–H absorption bands; H two compounds is the spacers at the central moiety, which are
NMR (500 MHz, CDCl
3
): d 0.92–1.85 (m, 30H, aliphatic-H), 4.08 responsible for elasticity, exibility and free rotation. The
), 4.37 (t, 4H, J ¼ 10 Hz, ester–CH ), absorption spectra of compounds E and E are shown in Fig. 4.
.4–6.9 (m, 16H, Ar–H); C NMR (100 MHz, CDCl ): d 14.0–33.9
(t, 4H, J ¼ 6.5 Hz, ether–CH
2
2
1
2
1
3
8
3
Photoswitching studies were initially performed on solutions
(aliphatic C), 114.79–136.12 (Ar–C); MS (FAB+): m/z for and then with solid cells. These studies provide an idea of the
C H N O , calculated: 734.92. Found: 734.40; elemental behaviour of the materials with respect to UV light and these
4
4
54 4 6
24,25
analysis: calculated (found) %: C 71.91 (71.85), H 7.41 (7.22), N results are indispensable for creating optical storage devices.
7.62 (7.57), and O 13.06 (12.99).
Fig. 5a and b depicts the E–Z isomerization absorption
A similar procedure is followed to synthesize the compound spectra of E and E , respectively, before and aer UV illumi-
1
2
E
2
.
nation. The absorption spectra of the compounds show
E
2
: a pale yellow colored solid; yield: 35%; melting point is absorption maxima at 358 nm and 356 nm. The absorption
ꢂ
ꢁ1
1
1
24.3 C; IR (KBr) cm : 1735 (C]O), 1245 (C–O–C), Around spectra of the compounds were obtained in chloroform (CHCl3)
1
500 (Ar C]C) along with other C–C, C–H absorption bands; H and the concentration of each solution was xed at C ¼ 1.1 ꢀ
ꢁ5
ꢁ1
NMR (500 MHz, CDCl ): d 0.84–1.78 (m, 22H, aliphatic-H), 6.93– 10 mol L . The strong absorbance in the UV region at ꢃ357
3
8
(
.28 (m, 20H, Ar–H), 3.985 (t, 4H, J ¼ 8.5 Hz, ether–CH ); MS nm corresponds to p–p* transition of the E isomer (trans
2
FAB+): m/z for C44
elemental analysis: calculated (found) %: C 72.71 (72.78), H 6.38 at around ꢃ450 nm represents n–p* transition of the Z isomer
6.31), N 7.71 (7.66), and O 13.21 (13.17).
(cis isomer).
The photo-switching properties of E
gated with UV light illumination of intensity 5.860 mW cm
46 4 6
H N O , calculated: 726.86. Found: 726.32; isomer), whereas a very weak absorbance in the visible region is
(
1
2
and E were investi-
ꢁ2
.
3. Results and discussion
Azo dyes were prepared by diazotisation, followed by phenol
coupling, and then etherication and hydrolysis by employing
4-ethylamino benzoate as a starting material. Aliphatic and
aromatic spacers were introduced into the molecule by DCC
coupling, in which dimerization of azo dyes takes place. This
gives rise to bent-shaped azo dyes E and E . The synthesized
1
2
1
13
compounds were conrmed by FTIR, H and
C NMR,
elemental analysis and mass spectroscopy studies. The
elemental analysis results are in accordance with the NMR
results.
1 2
Table 1 The POM observation of mesophase present in E and E .
Here, N – nematic mesophase, Cr – crystalline phase and I – isotropic
phase
ꢂ
Compounds
Scan
Phase transition ( C)
E
1
2
Heat
Cool
Heat
Cool
Cr 134.5 N 236.1 I
I 236.1 N 134 Cr
Cr 168.2 N 245.6 I
I 245.6 N 164.3 Cr
Fig.
3
Optical micrograph of compound
E
2
was captured at
ꢂ
E
temperature 200 C. All the compounds showed the nematic texture.
The image was captured in cooling mode at cooling rate of 2 C min
ꢂ
ꢁ1
under the optical polarizing microscope along with hot stage.
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nm, respectively, as a function of exposure time. The absorption
values of these peak wavelengths with different exposure times
were recorded. The curve shows that the phase involving pho-
toisomerization occurs within 17–18 s.
The conversion efficiency (CE), which is also called the extent
of isomerization of the E–Z photoisomerization, was estimated
15
from the following equation:
Aðt Þ ꢁ Aðt
Aðt
N
) is absorbance before UV and A(t ) is absorbance
0
N
Þ
CE ¼
ꢀ100%
0
Þ
where A(t
aer UV.
0
The extent of isomerization in these compounds is shown in
Table 2. In other words, the extent of isomerization varies in the
presence of different spacers: compound E1 has 76% and
compound E2 has only 55% of extent of isomerization. This
effect is due to the nature of the spacers, length of the spacers
and free rotations, and the reason for this behaviour is
explained in the later sections.
1 2
Fig. 4 The absorption spectra of E and E were measured with the
ꢁ5
ꢁ1
L
solutions at
C
¼
1.1
ꢀ
10
mol
by using a UV-Vis
spectrophotometer.
The reverse transformation from Z to E can be brought about
by two methods: rst method is by keeping the solution in the
dark, a process called thermal back relaxation, and the other
method is by shining white light of higher wavelength. Fig. 7a
and b shows the thermal back relaxation process, in which the
solution is illuminated continuously for 30 seconds (much
higher than the photostationary state) and kept in a dark place.
Then, spectral data were recorded at subsequent time inter-
Fig. 5 E to Z conversion of E
light with a 365 nm filter and heat filter.
1 2
(a) and E (b) in solution by shining UV
29,30
vals.
1 2
The back relaxation of compounds E and E are 13.21
hours and 7.25 hours, respectively.
Fig. 8 shows the time dependent Z–E absorption spectra of
the compounds presented here, which is obtained from Fig. 7a
and b.
The compounds were illuminated with UV light having a 365
nm lter and heat lter, at different time intervals and the
26–28
absorption spectra were recorded immediately.
The
absorption maxima decreased, due to E–Z photoisomerization,
which led to the transformation of E isomers into the Z isomers.
The photosaturation of these compounds are 17 s and 18 s,
respectively.
Table 2 The extent of isomerization or conversion efficiency of
E and E
1 2
1 2
Fig. 6 shows the E–Z absorption of compounds E and E as a
function of exposure time. The data were extracted from Fig. 5a Compounds
and b by considering the peak wavelengths of 358 nm and 356
Extent of isomerization (%)
E
E
1
76
55
2
Fig. 7 Thermal back relaxation or Z–E transformation was achieved by
keeping the solution for a long time in a dark place after shining UV
light until it attained photosaturation. Z to E conversion of E
1 2
(a) and E
Fig. 6 The peak absorbance of E and E , extracted from Fig. 5, with (b) is given and one can notice the difference in back relaxation time
1 2
respect to the function of exposure time for E–Z isomerization.
for both aliphatic and aromatic spacers.
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back relaxation was observed at 382 min with intensity 5.860
ꢁ
2
mW cm . Therefore, E shows good optical activity and is a
1
good candidate for optical storage devices.
Optical storage devices based on the above concept are pre-
sented using the abovementioned materials. Thickness of the
cell is xed at around 5 mm, whereas sandwiched and previ-
ously ITO coated rubbed polyimide layers were used for making
the prototype. Guest-host mixture in which liquid crystal host is
mixed with azo-dye guest molecule were lled inside the cell at
isotropic temperature. Dark regions shown in Fig. 10 belong to
the illuminated area where materials change from ordered to
disordered states, whereas bright regions are the masked area
where material remains in the nematic phase.
1 2
Fig. 8 The peak absorbance of E (a) and E (b), extracted from Fig. 7,
with respect to the function of recovery time for Z–E isomerization.
3
.3. Effect of spacer
Peak wavelength was xed at ꢃ357 nm and absorbance
plotted as a function of recovery time. A possible reason for the
difference in thermal back relaxation as well as the UV ON
process could be the properties of the spacers present in the
middle of the molecules, and the change that occurs is conned
to in-plane rotation of the molecules. The optical activity of the
compounds will change signicantly due to the effect of
spacers; hence, the synthesized compounds show different
intervals of photosaturation and thermal back relaxation.
Spectral investigations on solid cells were performed on the
We now discuss the effect of spacers on the dramatic variation
of photoisomerization.
In Table 3, there is a clear description of the difference in
1 2
photosaturation and thermal back relaxation for E and E . The
possible reason for this behaviour must be the effect of spacers.
The dimers show long time back relaxation in photo-
isomerization. Dimers with aromatic spacers exhibited shorter
time thermal back relaxation than aliphatic dimeric azo dyes.
The aromatic/aliphatic spacers affect the free movement of the
molecules to a signicant extent. There are two different cases
4
1
sample E because it showed high conversion efficiency of E–Z
in solution, which can be seen in Fig. 9.
that can explain the material properties of E
1 2
and E .
Commercial liquid crystal was used as the host material and
Case 1: consider the compound E . In between the two azo
1
it was mixed with guest material E
1
. The cell was constructed by
moieties, there is an aliphatic chain (n-hexane) present, as
shown in Fig. 11.
ITO glass plates precoated with polyamide and rubbed unidi-
rectionally. 5% of E (guest molecules) were dissolved in 95% of
1
The molecules can have different structures due to the ex-
ible movement of the carbon chain, which can result in a
restriction of the isomerization. In this case, E–Z isomerization
takes place with illumination of UV light with the wavelength of
ꢃ365 nm and the photoisomerization is a little slower due to
the presence of long chain aliphatic spacers. The time required
for photosaturation is 17 s and thermal back relaxation time is
commercial liquid crystal “MLC6873-100” (host molecules). The
mixture was capillary lled in the previously prepared liquid
crystal cell of thickness 5 mm via capillary action at isotropic
ꢂ
phase (ꢃ100 C). Similar to the solution illumination, solid cells
were also illuminated with UV light of wavelength 365 nm and
the photosaturation was found at 10 s and complete thermal
13.25 hours. The exibility of the molecules prevents the system
from going back easily by forming a coiled geometry, and
Fig. 10 Demonstration of an optical storage device based on the
Fig. 9 E–Z and Z–E isomerization of E
made by commercial liquid crystal “MLC6873-100” as host material polarizers. Dark regions correspond to the illuminated area and bright
and 5% guest E was mixed with it. The guest–host mixture was regions correspond to the masked area. The system changes from
introduced into a polyimide coated cell which was previously rubbed ordered to disordered state with the illumination of UV light of
to get a unique direction. wavelength 365 nm along with heat filter.
1
with the solid cell. The cell was materials described here. Prototype is observed under the crossed
1
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1 2
Table 3 Description of photo-isomerization and nature of spacers in E and E
Photo-isomerization
Photo-
saturation
Thermal back
relaxation
Sl. no. Compound structures & spacer locations
1
17 seconds 13.25 hours
Compound E
1
: here, the aliphatic n-hexane chain is placed at the center, it acts as spacer.
2
18 seconds 7.21 hours
Compound E
2
: the aromatic benzene ring acts as spacer.
2
Fig. 12 Effect of aromatic spacer on photoisomerization of E .
1
Fig. 11 Effect of aliphatic spacer on photoisomerization of E .
structure of E aer the E–Z isomerization. In this case, the
2
photoisomerization is relatively faster due to the presence of
somewhat similar results on long chain alkanes were reported aromatic benzene spacers. Time required for photosaturation is
14
earlier.
Case 2: now, consider the compound E
18 s, whereas for thermal back relaxation, it is 7.21 hours. It is
. In between the two obvious that time taken for the molecules to achieve photo-
2
azo moieties, there is an aromatic benzene ring present, as saturation is almost same in both cases, because they are
shown in Fig. 12. Due to the rigidity of the benzene ring, the radiation-induced processes. Note that thermal back relaxation
molecules are not bending as much as in the case of the is a completely radiation free process and it depends on the
aliphatic spacers. Apart from the E–Z isomerization of the structural modications; therefore, the aromatic spacers are
benzene ring, there is no appreciable change in the molecular giving back relaxation faster due their rigidity, whereas the long
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chain aliphatic spacers relax slower than their aromatic coun-
terparts due their exibility. More investigation is in progress to
study the different kinds of azo dye spacers and their effects
under UV light and will be reported elsewhere.
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1
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Acknowledgements
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We acknowledge the Ministry of Science and Technology
Industry MOSTI for providing e-Science fund RDU 130503 and
also the Ministry of Education for providing ERGS grant RDU
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50818 | RSC Adv., 2014, 4, 50811–50818
This journal is © The Royal Society of Chemistry 2014