Photoresponsive behavior of hydrophilic/hydrophobic-based novel azobenzene mesogens: Synthesis, characterization and their application in optical storage devices

RSC Advances

View Article Online

View Journal | View Issue

PAPER

Photoresponsive behavior of hydrophilic/

hydrophobic-based novel azobenzene mesogens:

synthesis, characterization and their application in

optical storage devices†

Cite this: RSC Adv., 2019, 9, 40588

c

a

Sunil B. N.,^abWan Sinn Yam and Gurumurthy Hegde

*

Three series of alkoxy chain-bearing azobenzene-derived quaternary ammonium iodides with an alkoxy

chain at one end, namely N,N-diethanol-6-(4-((4⁰-alkyloxyphenyl)diazenyl)phenoxy)hexan-1-ammonium

iodides, N-ethyl-N-ethanol-6-(4-((4⁰-alkyloxyphenyl)diazenyl)phenoxy)hexan-1-ammonium iodides and

N,N-diethyl-6-(4-((4⁰-alkyloxyphenyl)diazenyl)phenoxy)hexan-1-ammonium iodides were synthesized

and characterized. Their mesomorphic and photoswitching properties were examined via polarising

optical microscopy (POM), diﬀerential scanning calorimetry (DSC) and UV-vis spectrophotometry. The

liquid crystalline tilted schlieren texture of smectic C, non-tilted natural focal conic texture of smectic A

and smectic B phases were observed in the N,N-diethanol- and N-ethyl-N-ethanol-bearing ammonium

group substituted at the terminal via the alkoxy chain of the azo moiety. In these azo moieties, the

equilibrium time for trans–cis isomerization was about 1 min and cis–trans isomerization occurred at

around 590 min, which had the highest alkoxy chain and no hydroxyl group on their head group. The

absence of a hydroxyl group on the terminal head group resulted in slow thermal back relaxation,

whereas the hydroxyl group-bearing head group showed fast thermal back relaxation. These results

suggest that the inﬂuence of the substituent on the cationic ammonium head group and alkoxy chain

length on the photoisomerization of the azo compounds is vital for optical storage devices. Furthermore,

the device fabricated using these materials demonstrated that they are excellent candidates for optical

image storage applications.

Received 9th October 2019

Accepted 10th November 2019

DOI: 10.1039/c9ra08211e

rsc.li/rsc-advances

p* transition). This reverse transformation can also occur in the

“dark” via a process known as thermal back relaxation.^12,13The

1 Introduction

decoration of diﬀerent functional groups on the azobenzene

chromophore has been studied to enhance its optical storage

properties.^14–21

Optical storage devices have attracted increasing attention in

the eld of information technology over the last few years.^1–5

Organic photochromic compounds are used to evaluate the

photoinduced behaviour of optical storage devices.^6,7This

process arises from the reversible photoinduced isomerization

of a photochromic group such as azobenzene. The azobenzene

chromophore is a good candidate for storing data optically due

to its photosensitivity and good photoisomerization proper-

ties.^8–11Upon UV illumination (ꢀ365 nm related to the p–p*

transition), its thermodynamically more stable rod-like molec-

ular form of E or trans isomer is converted into the bent Z or cis

isomer. Reverse cis–trans isomerization can be achieved via

illumination under visible light (ꢀ400–500 nm, related to the n–

Ionic liquids have been developed in the past two decades

because of their unique physicochemical properties and highly

tunable features with chemical modication.^22–25Thus, it is

possible to design functionalized azobenzene-based ionic

liquids by introducing quaternary ammonium salts into the

photoresponsive unit of azobenzene.^26,27In this context, some

azobenzene-based ionic liquids have been designed and used in

many applications such as catalysis,²⁸sensors,²⁹drug delivery³⁰

and coatings.³¹Based on

a

recent literature survey,^32–34

azobenzene-based ionic liquids are rarely used in the eld of

optical storage devices. In recent years,^35–37some photo-

responsive ionic liquids have been reported. The azobenzene-

based ionic liquid 4-butylazobenzene-40-hexyloxytrimethyl-

ammonium triuoro-acetate ([C₄AzoC₆TMA] [TfO]) with an

azobenzene unit bridged between the alkyl chain has been

synthesized and its reversible micelle-vesicle transformation

under UV-vis illumination studied.³⁸Similarly, the synthesis

^aCentre for Nano-materials and Displays, B.M.S. College of Engineering, Bull Temple

Road, Bengaluru 560019, India. E-mail: murthyhegde@gmail.com

^bDepartment of Chemistry, B.M.S. College of Engineering, Bangalore, India 560019

^cSchool of Chemical Sciences, Universiti Sains Malaysia, 11800 USM Penang,

Malaysia. E-mail: wansinn@usm,my

† Electronic supplementary information (ESI) available. See DOI:

10.1039/c9ra08211e

40588 | RSC Adv., 2019, 9, 40588–40606

This journal is © The Royal Society of Chemistry 2019

View Article Online

Paper

RSC Advances

and reversible transformation of the photoresponsive ionic

liquid

1-(4-methyl

azobenzene)-3-tetradecylimidazolium

bromide ([C₁₄mimAzo] Br) with an azobenzene group were

investigated.³⁹However, to understand the photoresponsive

behavior of azobenzene-based ionic liquids, a detailed investi-

gation of this class of azobenzenes with quaternary ammonium

salts in other positions of the diﬀerent functional groups is

necessary.

In this work, we designed, synthesized and characterized 3

types of photoresponsive compounds, N,N-diethanol-6-(4-((4⁰-

alkyloxyphenyl)diazenyl)phenoxy)hexan-1-ammonium iodides

(hereaer named as compounds 22–24), N-ethyl-N-ethanol-6-(4-

((4⁰-alkyloxyphenyl)diazenyl)phenoxy)hexan-1-ammonium

iodides (hereaer named as compounds 25–27) and N,N-

diethyl-6-(4-((4⁰-alkyloxyphenyl)diazenyl)phenoxy)hexan-1-

ammonium iodides (hereaer named as compounds 28–30). In

the ammonium salts of these ionic liquids, the alkoxy chain is at

one end of the azobenzene group (i.e. the azobenzene and head

group are bridged by the alkoxy chain, as shown in Fig. 1) and

other end has diﬀerent alkoxy chain lengths. The mesomorphic

and photoisomerization behavior modulation of these ionic

liquids in solution and in solid through UV light irradiation

Scheme 1 Reagents and conditions: (i) 1-bromoalkanes (1.1 equiv.), (n

were investigated via UV-vis spectroscopy. The parameters of ¼ 7,8,12), K₂CO₃(3.0 equiv.), reﬂuxing in acetone, 48 h. (ii) Reﬂuxing in

NaOH/EtOH, overnight. (iii) Cooled in HCl_conc(aq); cooled NaNO₂(aq),

the liquid crystalline phase, phase transition temperature,

photoisomerization eﬃciency and thermal back relaxation were

determined.

stirred for 1 h; cooled phenol/EtOH and Na₂CO₃(aq), stirred at 0 ^ꢁC for

1 h. (iv) 1,6-Dibromohexane, (7.0 equiv.), K₂CO₃(3.0 equiv.), reﬂuxing in

acetone, 7 h. (v) Substituted amines (in excess), reﬂuxing in 2-propanol,

This study provides useful information for creating optical

storage devices by understanding the structure–property rela-

tionship of azobenzene-based ionic liquids. The photo-

isomerization and liquid crystalline properties of the

intermediates were also studied for a better understanding

between them.

38 h. (vi) Ethyl iodide (in excess.), reﬂuxing in acetonitrile, 36 h.

were treated with 1,6-dibromohexane, and reuxed overnight

under a nitrogen atmosphere to obtain compounds 10–12, and

then reacted with substituted amines to produce 13–21.

Compounds 22–30 were obtained upon treatment with ethyl

iodide. The crude product was recrystalized from a mixture of

2 Results and discussion

2.1 Synthesis

1

ethanol/n-hexane and characterized using H-NMR, ¹³C-NMR,

HRMS and elemental analyses.

The new azobenzene-based ammonium salts discussed herein

were prepared following the procedure in Scheme 1.

4-Acetamidophenol was O-alkylated using 1-bromoalkanes

to produce 4-acetamidophenoxyalkanes (1–3) and further

2.2 Azobenzene mesogen bearing terminal bromo group via

alkoxy chain

reuxed with alcoholic sodium hydroxide to give 4-alkylox- The bromo group was substituted at the terminal end (via the

yanilines (4–6). The 4-alkyloxyani_ꢁlines were diazotized with alkoxy chain) of azobenzene derivatives 7–8 bearing diﬀerent

sodium nitrite and conc. HCl at 0 C and the diazonium solu- alkoxy chain lengths on the other end of the phenyl ring. The

tion was treated with phenol in the presence of sodium bromo group was xed at one end and the alkoxy chain varied at

carbonate to obtain the coupled products 7–9. Compounds 7–9 the other end of compounds 10–12. The push–pull type eﬀect

Fig. 1 Chemical structures of the azobenzene-based ionic and non-ionic liquids under investigation.

This journal is © The Royal Society of Chemistry 2019

RSC Adv., 2019, 9, 40588–40606 | 40589

View Article Online

RSC Advances

Paper

was observed in these types of molecules due to the presence of a strong absorption peak at the wavelength of 359–360 nm and

the bromo group, which acts as an electron withdrawing group, weak band at ꢀ450 nm, which correspond to the symmetry

whereas, the alkoxy chain acts as an electron donating group. allowed p–p* transition and symmetry forbidden n–p* transi-

The inuence of the alkoxy chain and bromo group on the tion, respectively. There was no notable diﬀerence among the

mesomorphic and photoswitching properties of the azobenzene absorption spectra of compounds 10–12 due to their similar

chromophore were studied in detail.

chemical architecture. Therefore, an increase in the length of

2.2.1 Mesomorphic studies. Mesomorphic studies on 10– the alkoxy chain does not aﬀect the wavelength of the absorp-

12 were performed using polarizing optical microscopy (POM) tion spectrum.

in the crystalline phase upon heating at the rate of 2 ^ꢁC per min.

During UV illumination, the typical maximum absorption

The azobenzene-based mesogen formed diﬀerent types of wavelength at 359 nm disappeared, while a peak appeared at

enantiotropic mesophases with phase sequences depending on ꢀ450 nm due to an increase in the content of cis isomer. The

the alkoxy chain length (see Table 1). Compounds 10 and 12 isomerisation process of 10 was recorded as a function of UV

exhibited a non-tilted smectic (SmA) phase, while compound 11 irradiation time. Compounds 10–12 took around 44 s to reach the

formed tilted an SmC phase along with an SmA phase.

photoequilibrium state and upon further continuous irradiation

Fig. 2 shows the natural focal conic texture, which is typical of UV light up to 60 s, there were no changes in their absorption

of the SmA phase, exhibited by compounds 10–12 at the spectra. This suggests the trans isomer reached its equilibrium

respective temperatures. The phase transition temperature and state of isomerization reaction. Aer reaching the photosta-

enthalpy changes were determined via diﬀerential scanning tionary state, the thermal back relaxation of the cis–trans isom-

calorimetry (DSC) upon heating at 5 C min^ꢂ1. A summary of erization was examined by keeping the samples in the dark.

ꢁ

the phase transition temperatures, mesophase types and tran- Among them, compound 11 showed a good thermal back relax-

sition enthalpies is given in Table 1. The DSC thermograms are ation time of about 590 min, whereas, 10 showed a fast back

presented in the ESI.†

relaxation time of ꢀ100 min. The normalized absorption spectra

2.2.2 Photoswitching properties. The photoswitching upon UV illumination and thermal back relaxation of 10–12 are

studies on intermediates 10–12 were performed using a UV-vis shown in Fig. 3 and 4, respectively. The peak absorbance graph

spectrophotometer with a quartz cuvette and chloroform as was plotted as a function of time by extracting data from the

the solvent. Before UV illumination, compounds 10–12 showed absorption spectra of compounds 10–12. Graphs (a and b) (in

Table 1 Phase transition temperatures, mesophase types, and transition enthalpies [DH/kJ mol^ꢂ1] of compounds 10–12 upon heating^a

Compound code

n

Phase transitions T/^ꢁC [DH/kJ mol^ꢂ1

]

10

11

12

7

12

H: Cr 103.80 [54.46] SmA 111.77 [0.92] Iso

H: Cr 95.69 [50.96] SmC 109.67 (1.90) SmA 113.54 [1.14] Iso

H: Cr 100.03 [77.0] SmA 112.29 (9.73) Iso

a

Transition temperatures and enthalpy values were taken from the DSC heating (H) scans at 5 ^ꢁC min^ꢂ1; abbreviations: Cr ¼ crystalline solid; SmA

¼ smectic phase A phase; SmB ¼ smectic B phase; SmC ¼ smectic C phase; and Iso ¼ isotropic liquid.

Fig. 2 The SmA phase exhibiting the natural focal conic texture as seen in compound 10, 11 and 12 at respective temperatures. Magniﬁcation

used is 10ꢃ.

40590 | RSC Adv., 2019, 9, 40588–40606

This journal is © The Royal Society of Chemistry 2019

View Article Online

Paper

RSC Advances

Fig. 3 Normalized absorption spectra of compounds 10–12 as a function of UV irradiation time and graph (a) represents a peak absorbance plot

for trans–cis isomerization, extracted from the absorption spectra (in this ﬁgure) of 10–12. Intensity of the UV illumination was 1 mW cm^ꢂ2

.

Fig. 3 and 4, respectively) show the peak absorbance graph of UV terminal with the same alkoxy chain length of intermediates 10–

illumination and thermal back relaxation of compounds 10–12 12 at one end, and the other end of the azo ring bearing

with respect to irradiation time and recovery time, respectively.

diﬀerent alkoxy chain lengths. The mesomorphic and photo-

For trans–cis isomerization of 10–12, the equilibrium state of switching properties of intermediates 13–21 were investigated

the E/Z isomer ratio was dependent on the wavelength. Aer here. These types of compounds exhibited a push–push type

exposure to a wavelength of 365 nm, conversion eﬃciency of the eﬀect due to the electron donating nature of both groups

trans isomer was around 90%. Considering the case of sample present in the side chain. The inuence of the hydrophobic and

12, the conversion eﬃciency of trans–cis isomerization was 99%, hydrophilic nature of the substituted tertiary amine on the

whereas, 10 showed 85%. Aer 40 s UV irradiation, the mesomorphism and photoisomerization properties were

conversion eﬃciency of these compounds remained the same. extensively studied.

The photoconversion eﬃciency of the trans–cis isomerisation

2.3.1 Mesomorphic studies. The liquid crystalline proper-

ties of intermediates 13–21 were examined similarly to 10–12

using POM and DSC. Three types of enantiotropic phases were

observed in the N,N-diethanol, N-ethyl-N-ethanol, and N,N-

diethyl substituted amine compounds. The N,N-diethanol

substituted amine 15 exhibited schlieren texture of smectic C

and smectic B phases, whereas, in the case of the N-ethyl-N-

ethanol amine-bearing compound 18, it showed smectic C and

smectic A phases upon heating. The polarizing optical images

are shown in Fig. 5 together with the representative mesophase

of SmC and SmB for compounds 18 and 15 at the respective

temperature. However, the compounds possessing the highest

alkoxy chain (n ¼ 12) showed mesophases, whereas, the other

two lower alkoxy homologues (n ¼ 7 and 8) were non-mesogenic

in nature. The N,N-diethyl amine group-containing compounds

was determined using eqn (1).⁴⁰

A

ꢂ A

ðt₀Þ

A

ðt_NÞ

CE ¼

(1)

ðt₀Þ

where CE is the conversion eﬃciency, and A_{(t )}and A_(tare

)

N

0

absorbance before and aer UV illumination, respectively. A

summary of the trans–cis isomerization time, thermal back

relaxation of cis–trans isomerization and photoconversion eﬃ-

ciency data is shown in Table 2.

2.3 Azobenzene-mesogen bearing a terminal tertiary amine

group

The bromo group was replaced by N,N-diethanol, N-ethyl-N-

ethanol, and N,N-diethyl containing an amine group at the

This journal is © The Royal Society of Chemistry 2019

RSC Adv., 2019, 9, 40588–40606 | 40591

View Article Online

RSC Advances

Paper

Fig. 4 Normalized absorption spectra of compounds 10–12 as a function of recovery time during thermal back relaxation and graph (b)

represents the peak absorbance plot for cis–trans isomerization, which was extracted from the absorption spectra (Fig. 4) of 10–12.

Table 2 A summary of the E/Z isomerisation, thermal back relaxation

and its photoconversion eﬃciency (CE %) data

Fig. 8 shows the peak absorbance plots upon UV illumination

(graph (c)) and thermal back relaxation (graph (d)) with respect

to time. As Fig. 8 suggests, reversible isomerization depends on

the alkoxy chain and substituted groups the on nitrogen atom of

the amine group. The thermal back relaxation time increased as

the length of the alkoxy chain increased. The presence of

a hydroxyl group on amine resulted in fast-thermal back relax-

ation, whereas, it was slow in the case of the alkyl chain

substituted amine. For example, the N,N-diethanol amine-

based azobenzene with a terminal alkoxy chain length of n ¼

8 reached photosaturation at a time of ꢀ80 s, whereas, its

thermal back relaxation was about 240 min. A summary of the

E/Z isomerisation, thermal back relaxation time and its photo-

conversion eﬃciency of intermediates 13–21 are given in

Table 4.

The photoconversion eﬃciency of E/Z isomerisation was

determined using eqn (1). The photoconversion eﬃciency of

intermediates 13–21 was more than 90%, indicating that the

thermally stable trans isomer was converted to the unstable cis

isomer to a great extent. In the case of compounds 13–18,

conversion of the trans isomer to the cis isomer took more time

compared to that for 19–21 due to the presence of a hydroxyl

group on the amine group. The intermolecular hydrogen

bonding eﬀect of the hydroxyl group increased the equilibrium

time for the isomerization reaction.

trans–cis

Thermal back relaxation

Compound code isomerization in s in min

CE %

10

11

12

44

100

590

375

85.67

98.25

99.20

19, 20 and 21 also did not show liquid crystalline phases due to

the hydrophobic nature of the substituted group on the tertiary

amine. This information is vital to achieve mesomorphicity

when tuning the alkoxy homologues.

A summary of the phase transition temperature and transi-

tion enthalpies are given in Table 3. The DSC thermograms for

all the compounds are given in the ESI.†

2.3.2 Photoswitching properties. The UV-vis investigation

showed the inuence of N,N-diethanol, N-ethyl-N-ethanol, N,N-

diethyl containing an amine group on the isomerisation of

intermediates 13–21. The photoisomerization of 13–21 was

examined similarly to 10–12. The normalized absorption

spectra upon UV illumination and thermal back relaxation of

intermediates 13–21 are shown in Fig. 6 and Fig. 7, respectively.

The peak absorbance graph was plotted similarly to 10–12, and

40592 | RSC Adv., 2019, 9, 40588–40606

This journal is © The Royal Society of Chemistry 2019

View Article Online

Paper

RSC Advances

Fig. 5 Schlieren texture of SmC phase observed in compound 18 (left) and SmB phase for compound 15 (right) magniﬁcation 10ꢃ.

2.4 Azobenzene mesogen bearing a terminal quaternary

ammonium group

2.4.1 Mesomorphic studies. The liquid crystalline proper-

ties of compounds 22–30 were examined similarly to the inter-

mediates. Enantiotropic and monotropic phases were observed

in the N,N-diethanol and N-ethyl-N-ethanol quaternary ammo-

nium salts bearing alkoxy azobenzene. The N,N-diethanol

quaternary ammonium salt exhibited enantiotropic phases

such as smectic C and smectic B phases upon heating. In the

case of the N-ethyl-N-ethanol quaternary ammonium salt 24, it

exhibited monotropic phases (schlieren texture of SmC phase)

as shown in Fig. 9a.

The natural focal conic texture exhibiting the SmA phase of

compound 26 and the SmC phase of compound 27 are shown in

Fig. 9b and c, respectively. The N,N-diethyl quaternary ammo-

nium salt-based azobenzenes are non-liquid crystalline in

nature due to the absence of a hydroxyl group on their head

group. Hence, the mesophases changes in their properties may

Azobenzene is the most eﬃcient and extensively studied chro-

mophore, where azobenzene derivatives with the trans isomer

are thermodynamically more stable than that with the cis

isomer. In this section, we present the mesomorphic and pho-

toswitching properties of the quaternary ammonium salt

bearing alkoxy azobenzene. Generally quaternary ammonium

salts are called cationic surfactants, which are prepared by

adding an ethyl group to the nitrogen atom of a tertiary amine.

Similarly, the quaternary ammonium group-based azobenzene

derivatives were prepared by adding an ethyl group to the

terminal tertiary amine of intermediates 13–21. The mesomor-

phic properties and photo-induced isomerization of the

azobenzene-based mesogen-bearing cationic surfactants (22–

30) were examined in detail.

Table 3 Phase transition temperatures, mesophase types, and transition enthalpies [DH/kJ mol^ꢂ1] of compounds 13–21 upon heating^a

Compound code

n

R⁰

R⁰⁰

Phase transitions T/^ꢁC [DH/kJ mol^ꢂ1

]

13

14

15

16

17

18

19

20

21

7

8

12

7

8

12

7

8

–OH

–H

–OH

–H

H: Cr 106.37 [24.18] Iso

H: Cr 107.31 [27.29] Iso

H: Cr 82.92 [21.49] SmC 99.26 [13.82] SmB 115.08 [13.04] Iso

H: Cr 73.28 [18.76] Iso

H: Cr 74.51 [19.97] Iso

H: Cr 87.79 [37.55] SmC 104.46 [4.81] SmA 115.52 [2.72] Iso

H: Cr 81.72 [16.15] Iso

–H

H: Cr 79.55 [13.91] Iso

H: Cr 79.76 [44.32] Iso

12

a

Abbreviations see Table 1.

This journal is © The Royal Society of Chemistry 2019

RSC Adv., 2019, 9, 40588–40606 | 40593

View Article Online

RSC Advances

Paper

Fig. 6 Normalized absorption spectra of compound 13–21 as a function of UV irradiation time. Intensity of the UV illumination is 1 mW cm^ꢂ2

.

be due to the polar group present in the quaternary ammonium phase occurs by increasing the length of the terminal chain. For

salt. The phase transition temperature and transition compounds 24 and 27, which have the highest chain length at

enthalpies were determined via DSC and the summarized data the terminal end (n ¼ 12), evidence was observed for the

given in Table 5.

formation of the smectic C phase and also similar results were

To better understand the mesophases, the proposed model, observed in the case of compounds 15 and 18. In the case of the

as shown in Fig. 10 shows the formation of non-tilted smectic A substituent X ¼ H on the head group, no liquid crystalline

and tilted smectic C phases. The smectic A mesophase was properties were observed because the hydroxyl group plays

observed in all the compounds bearing a hydroxyl group on a major role in the mesomorphism.

their terminal head group. The inuence of the terminal alkoxy

2.4.2 Photoswitching properties. The photoswitching

chain (n > 10) length on the formation of ionic liquid crystals studies of the azobenzene-mesogen-bearing ionic liquids were

with the smectic C phase was previously reported.⁴¹According investigated similarly to the intermediates. Before and aer UV

to the zigzag model by Wulf,⁴²the appearance of the smectic C illumination, the absorption spectra of 22–30 were recorded by

40594 | RSC Adv., 2019, 9, 40588–40606

This journal is © The Royal Society of Chemistry 2019

View Article Online

Paper

RSC Advances

Fig. 7 Normalized absorption spectra of compounds 13–21 as a function of recovery time during thermal back relaxation.

UV-vis spectroscopy. There were no notable changes in the peak absorption spectra of compounds 22–30 are given in Fig. 11.

wavelength, and the characteristic peak absorption wavelength The peak absorbance versus time was plotted by extracting data

at 356–359 nm originated from the p–p* transition of the trans from the absorption spectra of compounds 22–30 upon UV

isomer. Aer UV irradiation, the absorption peak at 356 nm illumination as shown in graph (e) (see Fig. 11). The photo-

gradually decreased due to the transformation of the cis isomer conversion eﬃciency for trans–cis isomerization of the

for compound 22. Aer 25 s of irradiation, the trans isomer azobenzene-based ionic liquids was determined using eqn (1).

reached the photo equilibrium state in the isomerization All the compounds showed good conversion eﬃciency, sug-

reaction.

gesting the ionic liquids exhibit quick photoresponsive behav-

Aer reaching the photosaturation state, the reverse trans- iour in solution. In the case of compound 30, the conversion

formation of the cis isomer occurred in the dark and the eﬃciency was about 94%, whereas, compound 27 showed 66%.

thermal back relaxation was recorded with respect to the The presence of a hydroxyl group on the head group reduced the

recovery time of the trans isomer. Similar results were also cis isomer ratio in the photoequilibrium state during photo-

observed to that of the other compounds, and the normalized isomerization. Therefore, the presence of a hydroxyl group and

This journal is © The Royal Society of Chemistry 2019

RSC Adv., 2019, 9, 40588–40606 | 40595

View Article Online

RSC Advances

Paper

Fig. 8 Peak absorbance with respect to time graph (c) peak absorbance plot for trans–cis isomerization, with the data extracted from the

absorption spectra (Fig. 6) of 13–21 during UV illumination and graph (d) peak absorbance plot for cis–trans isomerization, with the data

extracted from the absorption spectra (Fig. 7) of 13–21 during thermal back relaxation.

Table 4 The summarized data of E/Z isomerisation, thermal back

relaxation and its photoconversion eﬃciency

electron withdrawing group, which can reduce the isomeriza-

tion energy barrier and accelerate the isomerization of azo-

benzene. However, in the case of 22, the presence of a hydroxyl

E/Z

Thermal back relaxation

group reduced the electron withdrawing character of the head

group. As a result, the cis isomer was not fully converted into its

original state and also could be phase involved during thermal

back relaxation due to the molecules being arranged in an

ordered layer structure (smectic phase). Furthermore, the

normalized absorption spectra for the thermal back relaxation

of the azobenzene-based ionic liquids were investigated in

detail (Fig. 12). According to Fig. 12, aer the photoequilibrium

state, the cis isomer of 22–27 did not fully transform back to its

original state due to the formation of intermolecular hydrogen

bonding in the hydroxyl group present on its head group.

Compound code isomerization in s in min

CE %

13

14

15

16

17

18

19

20

21

70

80

85

90

85

56

40

46

52

180

240

330

240

270

320

440

460

440

96.00

96.77

94.47

99.49

98.92

94.47

99.82

95.20

94.96

A

schematic diagram of the light-induced photo-

isomerization of the azo compounds is shown in Fig. 13. The

polar substituent (X ¼ OH) on the head group of the azo

compounds of the cis isomer could not achieve its original state

due to the layered arrangement of molecules (smectic A phase)

restricting the free rotation of the molecule during thermal back

relaxation. In the case of 29 and 30, they possess random

arrangements due to the absence of a hydroxyl group on their

head group and they exhibited long thermal back relaxation.

the alkoxy chain length play a signicant role in photo-

isomerization. A summary of the photoconversion eﬃciency

data for the trans–cis isomerization is shown in Table 6.

Among all the compounds, 30 showed the best thermal back

relaxation time of about 590 min, whereas, the cis isomer of

compound 22 was not fully transformed into the trans isomer.

The terminal head group of the azobenzene unit acts as an

Fig. 9 Polarized optical images. (a) Schlieren texture of SmC phase observed in compound 24, (b) natural focal conic texture typical of SmA for

compound 26 and (c) SmC phase as seen in compound 27. Magniﬁcation used is 10ꢃ.

40596 | RSC Adv., 2019, 9, 40588–40606

This journal is © The Royal Society of Chemistry 2019

View Article Online

Paper

RSC Advances

Table 5 Phase transition temperatures, mesophase types, and transition enthalpies [DH/kJ mol^ꢂ1] of compounds 22–30 upon heating^a

Compound code

n

R⁰

R⁰⁰

Phase transitions T/^ꢁC [DH/kJ mol^ꢂ1

]

22

23

24

25

26

27

28

29

30

7

8

12

7

8

12

7

–OH

–H

–OH

–H

H: Cr 143.04 [38.48] SmA 222.92 [3.57] Iso

H: Cr 141.0 [13.80] SmA 220.92 [2.22] Iso

H: Cr 135.98 [45.11] SmC 132.15 [1.66] SmA 194.17 [3.32] Iso

H: Cr 158.09 [42.26] SmA 185.98 [0.91] Iso

H: Cr 148.91 [32.42] SmA 199.56 [0.89] Iso

H: Cr 52.81 [11.23] SmC 111.71 [6.64] Iso

H: Cr 81.72 [21.53] Iso

8

12

–H

H: Cr 142.92 [11.98] Iso

H: Cr 174.19 [29.18] Iso

a

Abbreviations: see Table 1.

respective cis isomer. The time region in Fig. 14 shows that the

reaction is rst order in the case of 10, whereas in the case of 11

and 12, the reactions are rst order at a certain time and

thereaer, due to the long thermal back relaxation it is deviated

from rst order to second order kinetics. During thermal back

relaxation, temperature may play a signicant role in deviating

from rst order.⁴⁰As shown in Fig. 14, the reaction is rst order

at a certain time and deviates to second order due to the cis

isomer being thermally stable for some time due to the push–

push-type of eﬀect arising in compounds 13–21.

In the case of compounds 22–30, most of the reaction is rst

order except for compound 30, which exhibited a push–pull-

type eﬀect due to the head group of the ionic liquid acting as

an electron withdrawing group. In the case of 22–27, the reac-

tion was rst order due to the presence of a hydroxyl group

exhibiting the pull–pull-type eﬀect, which could not reduce the

isomerization energy barrier and the acceleration of the isom-

erization of azobenzene was negligible.

2.6 Optical storage device

Fig. 10 Proposed model showing the appearance of liquid crystalline

Spectral investigation on a real device was also conducted to

determine the potential of the materials. Fabrication of the cell

involved ITO coated, polyimide rubbed sandwiched glass plates

with the desired thickness of around ꢀ5 mm. The guest–host

mixture was prepared, where azobenzene molecules acted as

the guest and the room temperature liquid crystal acted as the

host material.

As a representative compound, we took compound 30 as our

guest light sensitive molecules since it showed the best prop-

erties during the photoisomerization studies. The mixture

consisted of 5% of compound 30 (which is a light-sensitive

molecule but non-liquid crystalline in nature) mixed with 95%

MLC-6873-100 (room temperature commercial liquid crystal).

This mixture exhibited a room temperature nematic meso-

phase. The prepared mixture was capillary lled in a cell, which

was previously fabricated.

phases (SmA and SmC phases).

2.5 Kinetic studies

For azo compounds 10–30, it is necessary to study the kinetics of

their cis–trans isomerisation, which was carried out thermally

using UV-vis spectroscopy.⁴³The experiment was carried out in

the dark at room temperature of 27 ^ꢁC and chloroform was used

as the solvent. The unimolecular thermal cis–trans isomer-

isation in the dark obeys eqn (2).

A

ðNÞ ꢂ ðtÞ

ꢂK_cꢂtt ¼ ln

(2)

A

ꢂ A

ðNÞ

ð0Þ

where A_t, A₀and A_Nare the change in absorbance at time t, time

zero and innite time, respectively. t is the relaxation time of the

This journal is © The Royal Society of Chemistry 2019

RSC Adv., 2019, 9, 40588–40606 | 40597

View Article Online

RSC Advances

Paper

Fig. 11 Normalized absorption spectra of compounds 22–30 as a function of UV irradiation time. Graph (e) peak absorbance plot for trans–cis

isomerization extracted from the absorption spectra (in this ﬁgure) of 22–30 upon UV illumination. Intensity of the UV light used was 1 mW cm^ꢂ2

.

Table 6 The summarized data of E/Z isomerisation, thermal back

relaxation and its photoconversion eﬃciency

The E/Z and Z/E photoisomerization behaviour of the solid

cell is depicted in Fig. 15. The intensity used for achieving

photosaturation was around 1.2 mW cm^ꢂ2and it took ꢀ80 s

to reach the photosaturation state (Fig. 15a and b). In

contrast, back relaxation, which occurred in the absence of

light, took around ꢀ400 min to reach the original position

(Fig. 15c and d).

To see the eﬀect of thermal back relaxation, we fabricated

an optical storage device with the above mixture. Previously

prepared ITO coated, unidirectionally rubbed polyimide

layers were sandwiched between two glass plates with

a uniform thickness of ꢀ5 mm. The device, as depicted in

E/Z

Thermal back relaxation

Compound code isomerization in s in min

CE %

22

23

24

25

26

27

29

30

25

32

46

32

48

36

44

46

60

160

210

140

240

270

390

590

80.20

80.64

77.42

85.43

80.33

66.18

74.55

94.43

40598 | RSC Adv., 2019, 9, 40588–40606

This journal is © The Royal Society of Chemistry 2019

View Article Online

Paper

RSC Advances

Fig. 12 Normalized absorption spectra of compounds 22–30 as a function of recovery time during thermal back relaxation. Graph (f) peak

absorbance plot for cis–trans isomerization extracted from the absorption spectra (in this ﬁgure) of 22–30 during thermal back relaxation.

Fig. 16, showed bright and dark regions aer illumination

with UV light of intensity 1.2 mW cm^ꢂ2for 10 min using

suitable masks. As can be seen from the gure, the exposed

region where UV light is illuminated transforms from an

ordered nematic state to a disordered isotropic state (black

region), whereas the masked region remains in the nematic

phase (bright region). To transfer back to the original nematic

phase from the isotropic phase, the device took around

250 min, showing the potential of the optical rewriting

capabilities of the materials.

2.7 Proposed model for guest–host eﬀect

When UV light of 365 nm was illuminated on the composite

mixture of azobenzene and liquid crystal molecules, the ener-

getically stable trans isomer (order state) transformed to the cis-

isomer (disorder state). The guest-host eﬀect of azobenzene and

liquid crystal molecules is schematically depicted Fig. 17, which

shows that the reverse transformation of the cis isomer is not easy

to reach its original state due to the presence of hydrophobic and

hydrophilic head groups on the azo molecules.

This journal is © The Royal Society of Chemistry 2019

RSC Adv., 2019, 9, 40588–40606 | 40599

View Article Online

RSC Advances

Paper

Fig. 13 Schematic diagram of the light-induced trans–cis isomerization of compounds 22–30 (if X ¼ OH, layered order arrangements of

molecules and if X ¼ H, molecules are randomly arranged).

3.1.1 General synthetic procedure for 4-acetamidophenox-

yalkanes (1–3). To a 250 mL round-bottom ask containing 4-

acetamidophenol (1.0 equiv.) in dry acetone, anhydrous potas-

3 Experimental

3.1 Materials and methods

N,N-Dimethylethanolamine (99%), ethanol (99.9%), ethyl sium carbonate (3.0 equiv.), a catalytic amount of potassium

iodide (98%), 2-propanol (99%), HCl conc. (35%), NaNO₂(99%), iodide and the corresponding 1-bromoalkane (1.1 equiv.) were

phenol (98%) K₂CO₃(99.5%), Na₂CO₃(99.5%), NaOH (98%), 1- added. The reaction mixture was reuxed for 48 h. The reaction

bromododecane (99%), 1-bromoheptane (99%), 1-bromooctane progress was monitored by TLC. Aer completion of the reac-

(99%) and 1,6-dibromohexane (99%) were purchased from tion, excess solvent was removed using a rotatory evaporator.

Acros Organics and used without further purication. Petro- The crude product was washed with distilled water to remove

leum ether (b.p. 60–90 ^ꢁC), acetonitrile (99.9%), dichloro- excess potassium carbonate and the residue was ltered and

methane (99.9%), acetone (99%) and ethyl acetate (99%) were dried. Excess 1-bromoalkane was removed by washing the

1

commercial products obtained from Acros Organics. H NMR residue with n-hexane. The crude product was puried by

(400 MHz) and ¹³C NMR (100 MHz) were recorded on a Bruker column chromatography using ethyl acetate/hexane (v/v,

400 MHz Ultrashield™ spectrometer. Elemental analysis was 30 : 70) to yield compounds 1–3.

1

performed using a CHN elemental analyser.

(1) (n ¼ 7) yield: 71.5%. H NMR (400 MHz, CDCl₃) d/ppm

7.38–7.32 (m, 2H), 7.13 (s, 1H), 6.88–6.80 (m, 2H), 3.96–3.88

Fig. 14 First-order plot for the thermal back relaxation of compounds 10–30 measured at room temperature.

40600 | RSC Adv., 2019, 9, 40588–40606

This journal is © The Royal Society of Chemistry 2019

View Article Online

Paper

RSC Advances

Fig. 15 E/Z (a and b) and Z/E (c and d) photoisomerization of the solid cell ﬁlled with the mixture of compound 30 and room temperature liquid

crystals as guest–host combination. Intensity used was around 1.2 mW cm^ꢂ2, where the light-sensitive moiety (which is compound 30) acts like

the guest and MLC-6873-100, which is a room temperature liquid crystal, acts as the host.

(m, 2H), 2.14 (s, 3H), 1.80–1.69 (m, 2H), 1.43 (dt, J ¼ 15.1,

(2) (n ¼ 8) yield: 68.82%. ¹H NMR (400 MHz, CDCl₃) d/ppm 7.39–

6.6 Hz, 2H), 1.33 (ddd, J ¼ 15.0, 8.6, 4.6 Hz, 6H), 0.88 (t, 7.33 (m, 2H), 6.89–6.78 (m, 2H), 3.97–3.84 (m, 2H), 1.81–1.68 (m,

J ¼ 6.9 Hz, 3H).

2H), 1.49–1.37 (m, 2H), 1.37–1.20 (m, 8H), 0.87 (t, J ¼ 6.9 Hz, 3H).

Fig. 16 Demonstration of the optical storage device described herein through UV illumination together with a photomask. Bright region

corresponds to the non-illuminated area, whereas, the dark region corresponds to the UV illuminated area. Material transformed from the

nematic to isotropic phase during illumination, showing excellent dark and bright states between them.

This journal is © The Royal Society of Chemistry 2019

RSC Adv., 2019, 9, 40588–40606 | 40601

View Article Online

RSC Advances

Paper

Fig. 17 Schematic diagram of the guest–host eﬀect of the azobenzene-based ionic liquid and liquid crystal molecules.

1

(3) (n ¼ 12) yield: 83.5% H NMR (400 MHz, CDCl₃) d/ppm crude products were ltered, washed with distilled water and

7.37–7.31 (m, 2H), 7.12 (s, 1H), 6.86–6.82 (m, 2H), 3.98–3.88 (m, recrystallized from n-hexane to yield compounds 7–9.

2H), 2.14 (s, 3H), 1.81–1.67 (m, 2H), 1.49–1.38 (m, 2H), 1.38–1.22

(m, 17H), 0.87 (t, J ¼ 6.9 Hz, 3H).

(7) (n ¼ 7) yield: ¹H NMR (400 MHz, CDCl₃) d/ppm 7.87–7.77

(m, 4H), 7.01–6.85 (m, 4H), 4.02 (t, J ¼ 6.6 Hz, 2H), 1.87–1.75 (m,

3.1.2 General synthetic procedure for 4-alkyloxyanilines (4– 2H), 1.47 (dt, J ¼ 14.6, 6.7 Hz, 2H), 1.40–1.23 (m, 6H), 0.89 (t, J ¼

6). To a 250 mL round-bottom ask containing the corre- 6.9 Hz, 3H).

sponding 4-acetamidophenoxyalkanes (1–3), sodium hydroxide

(8) (n ¼ 8) yield: 47%. ¹H NMR (400 MHz, CDCl₃) d/ppm 7.88–

pellets in ethanol were added. The reaction mixture was 7.78 (m, 4H), 7.01–6.95 (m, 2H), 6.95–6.88 (m, 2H), 5.04 (s, 1H),

reuxed for 24 h. The reaction progress was monitored by TLC. 4.02 (t, J ¼ 6.6 Hz, 2H), 1.86–1.74 (m, 2H), 1.51–1.41 (m, 2H),

Aer completion of the reaction, the excess solvent was evapo- 1.41–1.22 (m, 8H), 0.88 (t, J ¼ 6.9 Hz, 3H).

rated using a rotatory evaporator. The crude product was puri-

ed via recrystallization from ethanol.

(9) (n ¼ 12) yield: 73.5%. 400 MHz, CDCl₃) d/ppm 7.87–7.79

(m, 2H), 7.00–6.96 (m, 1H), 6.95–6.91 (m, 1H), 4.02 (t, J ¼ 6.6 Hz,

(4) (n ¼ 7) yield: ¹H NMR (400 MHz, CDCl₃) d/ppm 6.76–6.69 1H), 1.85–1.76 (m, 1H), 1.46 (dd, J ¼ 15.4, 7.0 Hz, 1H), 1.26 (s,

(m, 2H), 6.66–6.58 (m, 2H), 4.06 (t, J ¼ 6.6 Hz, 2H), 1.73 (m, 2H), 9H), 0.87 (t, J ¼ 6.9 Hz, 2H).

1.45–1.36 (m, 2H), 1.36–1.22 (m, 6H), 0.88 (t, J ¼ 6.9 Hz, 3H).

3.1.4 General synthetic procedure for N-[4-(bromohexyloxy)

(5) (n ¼ 8) yield: 80%. ¹H NMR (400 MHz, CDCl₃) d/ppm 6.76– phenyl]-N-(alkyloxyphenyl)diazenes, 10–12. To 250 mL a round-

6.70 (m, 2H), 6.65–6.60 (m, 2H), 4.04 (t, J ¼ 6.6 Hz, 2H), 1.74 (m, bottom ask containing 4-[(4⁰-alkyloxyphenyl)diazenyl]phenols,

2H), 1.43 (m, 2H), 1.30–1.26 (m, 8H), 0.87 (t, J ¼ 6.9 Hz, 3H).

7–9 (1.0 equiv.) in dry acetone, 1,6-dibromohexane (7.0 equiv.),

(6) (n ¼ 12) yield: 70%.¹H NMR (400 MHz, CDCl₃) d/ppm anhydrous potassium carbonate (3.0 equiv.) and a catalytic

6.76–6.70 (m, 2H), 6.65–6.59 (m, 2H), 4.06 (t, J ¼ 6.6 Hz, 2H), amount of potassium iodide were added. The reaction mixture

1.74 (m, 2H), 1.43 (m, 2H), 1.29–1.26 (m, 16H), 0.87 (t, J ¼ was reuxed for 7 h. The reaction progress was monitored by

6.9 Hz, 3H).

TLC. Aer completion of the reaction, excess solvent was

3.1.3 General synthetic procedure for 4-[(4⁰-alkyloxyphenyl) removed using a rotatory evaporator, upon which a yellow

diazenyl]phenols, 7–9. In a 250 mL round-bottom ask, the precipitate was formed. The crude product was washed with

corresponding 4-alkyloxyanilines, 4–6 (1.0 equiv.) were dis- distilled water to remove excess potassium carbonate and the

solved in a mixture of acetone (100 mL) and concentrated residue was ltered and dried. Excess 1,6-dibromohexane was

hydrochloric acid. The reaction mixture was stirred for 1 h with removed by washing the residue with n-hexane and the products

ꢁ

the temperature of the mixture maintained below 0 C. A cold were recrystallized from methanol to yield compounds 10–12.

solution of phenol (1.0 equiv.), sodium hydroxide (1.0 equiv.)

(10) (n ¼ 7) yield: ¹H NMR (400 MHz, CDCl₃) d/ppm 7.89–7.81

and sodium carbonate (1.0 equiv.) was then added dropwise to (m, 4H), 7.01–6.94 (m, 4H), 4.03 (td, J ¼ 6.5, 3.9 Hz, 4H), 3.43 (t, J

the reaction mixture and it was stirred for another hour at 0 ^ꢁC, ¼ 6.8 Hz, 2H), 1.95–1.85 (m, 3H), 1.86–1.75 (m, 4H), 1.50–1.41

thereaer the ice-water bath was removed, and it was stirred at (m, 2H), 1.33 (ddd, J ¼ 11.3, 10.4, 5.2 Hz, 6H), 0.89 (dd, J ¼ 9.0,

room temperature overnight. The reaction mixture was then 4.8 Hz, 3H).

1

neutralized with an aqueous solution of sodium hydroxide,

upon which a reddish-brown precipitated was formed. The 7.88–7.82 (m, 4H), 6.98 (dd, J ¼ 9.0, 1.0 Hz, 4H), 4.02 (td, J ¼ 6.5,

(11) (n ¼ 8) yield: 77.5%. H NMR (400 MHz, CDCl₃) d/ppm

40602 | RSC Adv., 2019, 9, 40588–40606

View Article Online

Paper

RSC Advances

4.5 Hz, 4H), 3.42 (q, J ¼ 6.4 Hz, 2H), 1.90 (dd, J ¼ 13.9, 7.0 Hz, 1.81 (dd, J ¼ 13.2, 6.1 Hz, 16H), 1.53–1.42 (m, 26H), 1.42–1.33 (m,

2H), 1.86–1.71 (m, 4H), 1.51 (dd, J ¼ 9.8, 6.2 Hz, 3H), 1.48–1.40 17H), 1.26 (s, 52H), 1.05–0.98 (m, 11H), 0.87 (t, J ¼ 6.9 Hz, 11H).

(m, 2H), 1.26 (s, 16H), 0.87 (t, J ¼ 6.8 Hz, 3H).

3.1.7 N,N-diethyl-6-(4-((4⁰-alkyloxyphenyl)diazenyl)phenoxy)

1

(12) (n ¼ 12) yield: 50%. H NMR (400 MHz, CDCl₃) d/ppm hexan-1-amines, 19–21. To a 250 mL round-bottom ask con-

7.88–7.82 (m, 4H), 6.98 (dd, J ¼ 9.0, 1.0 Hz, 4H), 4.02 (td, J ¼ 6.5, taining the corresponding N-[4-(bromohexyloxy)phenyl]-N-(alky-

4.5 Hz, 4H), 3.42 (q, J ¼ 6.4 Hz, 2H), 1.90 (dd, J ¼ 13.9, 7.0 Hz, loxyphenyl)diazenes, 10–12, in 2-propanol, N,N-diethyl amine

2H), 1.86–1.71 (m, 4H), 1.51 (dd, J ¼ 9.8, 6.2 Hz, 3H), 1.48–1.40 was added in excess. The reaction mixture was reuxed for 38 h

(m, 2H), 1.26 (s, 16H), 0.87 (t, J ¼ 6.8 Hz, 3H).

and the reaction progress was monitored by TLC. Aer comple-

3.1.5 General synthetic procedure for N,N-diethanol-6-(4- tion of the reaction, excess solvent was removed using a rotatory

((4⁰-alkyloxyphenyl)diazinyl)phenoxy)hexan-1-amines,

13–15. evaporator. The crude products were recrystallized from ethanol

To a 250 mL round-bottom ask containing the corresponding to yield compounds 19–21.

1

N-[4-(bromohexyloxy)phenyl]-N-(alkyloxyphenyl)diazenes, 10–

(19) (n ¼ 7) yield: 60%. H NMR (400 MHz, CDCl₃) d/ppm

12, in 2-propanol, N,N-diethanol amine was added in excess. 7.88–7.82 (m, 4H), 7.00–6.95 (m, 4H), 4.02 (t, J ¼ 6.5 Hz, 4H),

The reaction mixture was reuxed for 38 h. The reaction prog- 2.53 (q, J ¼ 7.1 Hz, 4H), 2.46–2.40 (m, 2H), 1.80 (dt, J ¼ 14.9,

ress was monitored by TLC. Aer completion of the reaction, 5.5 Hz, 4H), 1.49 (dd, J ¼ 12.9, 5.5 Hz, 7H), 1.41–1.27 (m, 9H),

excess solvent was removed using a rotatory evaporator. The 1.02 (t, J ¼ 7.2 Hz, 6H), 0.89 (t, J ¼ 6.9 Hz, 3H).

crude products were recrystallized from ethanol to yield

compounds 13–15.

(20) (n ¼ 8) yield: 93.5%. 7.86–7.82 (m, 4H), 7.00–6.93 (m,

4H), 4.01 (t, J ¼ 6.5 Hz, 4H), 2.52 (q, J ¼ 7.1 Hz, 4H), 2.44–2.40

(13) (n ¼ 7) yield: 73.02%. ¹H NMR (400 MHz, CDCl₃) d/ppm (m, 2H), 1.78 (dt, J ¼ 14.9, 5.5 Hz, 4H), 1.47 (dd, J ¼ 12.9, 5.5 Hz,

7.87–7.81 (m, 4H), 6.98 (t, J ¼ 5.9 Hz, 4H), 4.05–3.98 (m, 4H), 10H), 1.41–1.27 (m, 10H), 1.02 (t, J ¼ 7.2 Hz, 6H), 0.89 (t, J ¼

3.66–3.57 (m, 4H), 2.66 (t, J ¼ 5.4 Hz, 4H), 2.57–2.50 (m, 2H), 6.9 Hz, 3H).

1.86–1.74 (m, 6H), 1.56–1.42 (m, 6H), 1.42–1.20 (m, 9H), 0.88 (q,

(21) (n ¼ 12) yield: 85.71%. ¹H NMR (400 MHz, CDCl₃) d/ppm

7.85 (d, J ¼ 8.9 Hz, 4H), 6.97 (d, J ¼ 9.1 Hz, 4H), 4.02 (t, J ¼

J ¼ 7.0 Hz, 3H).

1

(14) (n ¼ 8) yield: 73.8%. H NMR (400 MHz, CDCl₃) d/ppm 5.9 Hz, 4H), 2.52 (dd, J ¼ 14.4, 7.2 Hz, 4H), 2.45–2.39 (m, 2H),

7.88–7.81 (m, 4H), 6.98 (t, J ¼ 6.0 Hz, 4H), 4.05–3.99 (m, 4H), 1.81 (dd, J ¼ 13.6, 7.0 Hz, 5H), 1.53–1.43 (m, 7H), 1.31 (d, J ¼

3.62 (t, J ¼ 5.4 Hz, 4H), 2.67 (t, J ¼ 5.4 Hz, 4H), 2.54 (d, J ¼ 7.5 Hz, 40.8 Hz, 20H), 1.02 (t, J ¼ 7.2 Hz, 6H), 0.87 (t, J ¼ 6.7 Hz, 3H).

2H), 1.80 (dt, J ¼ 14.6, 5.2 Hz, 5H), 1.49 (dd, J ¼ 13.5, 6.4 Hz, 6H),

1.41–1.23 (m, 11H), 0.88 (t, J ¼ 6.9 Hz, 3H).

3.1.8 N,N-diethanol-6-(4-((4⁰-alkyloxyphenyl)diazenyl)phe-

noxy)hexan-1-ammonium iodides, 22–24. To a 250 mL round-

(15) (n ¼ 12) yield: 52.6%. ¹H NMR (400 MHz, CDCl₃) d/ppm bottom ask containing the corresponding N,N-diethanol-6-(4-

7.87–7.82 (m, 4H), 6.98 (d, J ¼ 9.0 Hz, 4H), 4.02 (td, J ¼ 6.5, ((4⁰-alkyloxyphenyl)diazinyl)phenoxy)hexan-1-amines in aceto-

1.9 Hz, 4H), 3.62 (td, J ¼ 5.4, 2.9 Hz, 4H), 2.66 (dd, J ¼ 9.8, nitrile, 13–15, excess ethyl iodide was added. The reaction

4.5 Hz, 4H), 2.57–2.51 (m, 2H), 1.80 (dd, J ¼ 16.8, 9.0 Hz, 7H), mixture was reuxed for 36 h and the reaction progress was

1.55–1.41 (m, 7H), 1.36 (dd, J ¼ 20.2, 11.4 Hz, 5H), 1.26 (s, 15H), monitored by TLC. Aer completion of the reaction, excess

0.86 (d, J ¼ 7.0 Hz, 3H).

solvent was removed using a rotatory evaporator. The crude

3.1.6 General synthetic procedure for N-ethyl-N-ethanol-6- product was recrystalized from a mixture of ethanol/n-hexane to

(4-((4⁰-alkyloxyphenyl)diazenyl)phenoxy)hexan-1-amines,

16– yield compounds 22–24.

18. To a 250 mL round-bottom ask containing the corre-

sponding N-[4-(bromohexyloxy)phenyl]-N-(alkyloxyphenyl)dia-

(22) (n ¼ 7) yield: 30.8%. Elemental analysis: calculated for

C

₃₁H₅₀IN₃O₄: C ¼ 56.79, H ¼ 7.69, N ¼ 6.41; found: C ¼ 56.184,

zenes, 10–12 in 2-propanol, N-ethyl-N-ethanol amine was added H ¼ 7.601, N ¼ 6.327. HRMS [C₃₁H₅₀N₃O₄]⁺¼ 528.3802. ¹H

in excess. The reaction mixture was reuxed for 38 h and the NMR (400 MHz, CDCl₃) d/ppm 7.86–7.81 (m, 16H), 6.97 (ddd, J

reaction progress was monitored by TLC. Aer completion of ¼ 6.9, 5.0, 2.6 Hz, 16H), 4.10 (s, 16H), 4.05–3.98 (m, 25H), 3.68–

the reaction, excess solvent was removed using a rotatory 3.63 (m, 15H), 3.57 (q, J ¼ 7.1 Hz, 8H), 3.48–3.41 (m, 8H), 1.80

evaporator. The crude products were recrystallized from ethanol (dt, J ¼ 8.0, 6.5 Hz, 17H), 1.60–1.51 (m, 10H), 1.50–1.39 (m,

to yield compounds 16–18.

18H), 1.39–1.25 (m, 40H), 0.90–0.86 (m, 12H). ¹³C NMR (101

(16) (n ¼ 7) yield: 64.6%.¹H NMR (400 MHz, CDCl₃) d/ppm MHz, CDCl₃) d/ppm 161.34, 161.07, 146.94, 124.74 (d, J ¼ 8.0

7.86–7.81 (m, 4H), 6.98 (dd, J ¼ 6.9, 5.0 Hz, 4H), 4.02 (t, J ¼ Hz), 124.41, 124.04, 123.71, 115.06–114.54, 114.40 (d, J ¼ 8.1

6.6 Hz, 4H), 3.55–3.51 (m, 2H), 2.60–2.52 (m, 4H), 2.49–2.44 (m, Hz), 110.98 (d, J ¼ 1.4 Hz), 103.65 (s), 89.59 (d, J ¼ 3.2 Hz), 83.33

2H), 1.85–1.77 (m, 4H), 1.53–1.41 (m, 7H), 1.42–1.25 (m, 10H), (d, J ¼ 12.1 Hz), 82.92 (s), 82.53 (s), 82.15 (s), 82.03–81.83, 81.60,

1.04–0.99 (m, 3H), 0.90–0.85 (m, 3H).

81.37–81.16, 80.95 (dd, J ¼ 14.1, 9.8 Hz), 80.71, 80.24 (d, J ¼ 7.7

1

(17) (n ¼ 8) yield: 52.9%. H NMR (400 MHz, CDCl₃) d/ppm Hz), 79.85 (dd, J ¼ 16.6, 7.4 Hz), 79.09 (s), 78.86–77.71 (m),

7.84 (t, J ¼ 9.0 Hz, 4H), 6.96 (t, J ¼ 9.0 Hz, 4H), 4.02 (t, J ¼ 6.2 Hz, 77.73–77.71 (m), 77.38 (d, J ¼ 11.9 Hz), 77.12, 76.41 (d, J ¼ 77.7

4H), 3.53 (t, J ¼ 5.4 Hz, 2H), 2.61–2.51 (m, 4H), 2.49–2.43 (m, Hz), 75.09 (d, J ¼ 17.3 Hz), 74.72, 74.61–74.09, 73.87–73.45,

2H), 1.86–1.76 (m, 4H), 1.54–1.42 (m, 6H), 1.41–1.23 (m, 10H), 73.30 (d, J ¼ 6.8 Hz), 72.84 (s), 71.45 (s), 68.48 (d, J ¼ 10.6 Hz),

1.01 (t, J ¼ 7.1 Hz, 3H), 0.88 (t, J ¼ 6.8 Hz, 3H).

67.97 (d, J ¼ 13.3 Hz), 66.34 (s), 65.04 (d, J ¼ 14.4 Hz), 60.69 (d, J

(18) (n ¼ 9) yield: 61.3%. ¹H NMR (400 MHz, CHLOROFORM- ¼ 7.2 Hz), 59.93 (s), 56.00 (s), 55.53 (d, J ¼ 17.3 Hz), 53.69–53.28,

D) d 7.88–7.82 (m, 15H), 7.01–6.95 (m, 15H), 4.06–3.99 (m, 15H), 41.42 (d, J ¼ 1.4 Hz), 40.45 (d, J ¼ 5.3 Hz), 39.54–39.25, 32.31–

3.53 (t, J ¼ 5.4 Hz, 8H), 2.60–2.52 (m, 17H), 2.49–2.44 (m, 8H), 31.75, 31.12 (d, J ¼ 19.6 Hz), 29.75, 29.50–28.77, 26.37–25.47,

RSC Adv., 2019, 9, 40588–40606 | 40603

View Article Online

RSC Advances

Paper

24.73, 22.85–22.45, 21.88 (d, J ¼ 52.8 Hz), 14.56, 14.17, 8.41 (d, J 77.47 (d, J ¼ 11.1 Hz), 77.21, 76.89, 68.43, 68.00, 59.51, 58.85,

¼ 11.6 Hz), 4.12 (dd, J ¼ 28.5, 9.0 Hz), ꢂ20.92 to ꢂ21.26. 55.33, 54.81, 32.01–31.67, 29.55–29.18, 29.01, 26.35–25.94,

(23) (n ¼ 8) yield: 40.0%. Elemental analysis: calculated for 25.67, 22.87–22.57 (m), 22.19, 14.18, 8.41.

C

₃₂H₅₂IN₃O₄: C ¼ 57.39, H ¼ 7.83, N ¼ 6.27; found: C ¼ 57.019,

(27) (n ¼ 12) yield: 20.8%. Elemental analysis: calculated for

₃₆H₆₀IN₃O₃: C ¼ 60.92, H ¼ 8.52, N ¼ 5.92; found: C ¼ 58.221,

H ¼ 7.707, N ¼ 6.175. HRMS [C₃₂H₅₂N₃O₄]⁺¼ 542.3959. ¹H

C

NMR (400 MHz, CDCl₃) d/ppm 7.84 (d, J ¼ 8.9 Hz, 6H), 7.00–6.94 H ¼ 8.095, N ¼ 5.558. HRMS [C₃₆H₆₀N₃O₃]⁺¼ 582.4630. ¹H

(m, 6H), 4.10 (s, 8H), 4.01 (q, J ¼ 6.5 Hz, 6H), 3.66 (s, 6H), 3.57 NMR (400 MHz, CDCl₃) d/ppm 7.84 (dd, J ¼ 8.9, 0.7 Hz, 9H),

(dd, J ¼ 14.2, 6.9 Hz, 3H), 3.48–3.41 (m, 4H), 1.80 (dd, J ¼ 14.2, 7.00–6.94 (m, 9H), 4.26 (t, J ¼ 5.8 Hz, 2H), 4.14 (s, 4H), 4.03 (tt, J

7.1 Hz, 6H), 1.76–1.70 (m, 4H), 1.55 (d, J ¼ 7.2 Hz, 4H), 1.45 (d, J ¼ 11.3, 5.9 Hz, 10H), 3.58–3.45 (m, 12H), 3.42–3.34 (m, 4H),

¼ 7.2 Hz, 7H), 1.32 (dd, J ¼ 18.5, 11.3 Hz, 17H), 0.87 (dd, J ¼ 9.2, 1.87–1.69 (m, 14H), 1.53–1.40 (m, 12H), 1.35 (t, J ¼ 7.2 Hz, 17H),

4.5 Hz, 4H). ¹³C NMR (101 MHz, CDCl₃) d/ppm 161.35 (s), 161.08 1.32–1.19 (m, 36H), 0.86 (t, J ¼ 6.8 Hz, 7H).

(s), 146.93 (d, J ¼ 13.1 Hz), 124.42 (s), 114.82 (d, J ¼ 11.1 Hz),

3.1.10 N,N-diethyl-6-(4-((4⁰-alkyloxyphenyl)diazenyl)

77.45 (s), 77.13 (s), 76.81 (s), 68.43 (s), 68.04 (s), 60.64 (s), 59.94 phenoxy)hexan-1-ammonium iodides, 28–30. Compounds 28–

(s), 56.00 (s), 55.60 (s), 31.90 (s), 29.55–29.20 (m), 29.03 (s), 26.10 30 were synthesized following the procedure described for the

(s), 25.67 (s), 22.69 (d, J ¼ 10.1 Hz), 22.15 (s), 14.20 (s), 8.49 (s). preparation of compounds 22–24 using the corresponding N,N-

(24) (n ¼ 12) yield: 36.8%. Elemental analysis: calculated for diethyl-6-(4-((4⁰-alkyloxyphenyl)diazenyl)phenoxy)hexan-1-

C

₃₆H₆₀IN₃O₄: C ¼ 59.58, H ¼ 8.33, N ¼ 5.79; found: C ¼ 59.39, H amines 19–21 as the starting materials.

¼ 8.25, N ¼ 5.709. HRMS [C₃₆H₆₀N₃O₄]⁺¼ 598.4585. H NMR

(400 MHz, CDCl₃) d/ppm 7.84 (dd, J ¼ 9.0, 0.7 Hz, 16H), 7.00–

(28) (n ¼ 7) yield: 15.0%. Elemental analysis: calculated for

₃₁H₅₀IN₃O₂: C ¼ 59.70, H ¼ 8.08, N ¼ 6.74; found: C ¼ 58.677,

1

C

6.94 (m, 16H), 4.11 (s, 16H), 4.01 (dd, J ¼ 14.2, 6.5 Hz, 20H), 3.67 H ¼ 7.889, N ¼ 6.432. HRMS [C₃₁H₅₀N₃O₂]⁺¼ 496.3918. ¹H

(d, J ¼ 3.1 Hz, 15H), 3.58 (q, J ¼ 7.2 Hz, 8H), 3.49–3.42 (m, 8H), NMR (400 MHz, CDCl₃) d/ppm 7.85 (d, J ¼ 8.8 Hz, 7H), 6.97 (d, J

1.86–1.76 (m, 17H), 1.72 (dd, J ¼ 16.3, 9.4 Hz, 10H), 1.45 (dt, J ¼ ¼ 8.9 Hz, 7H), 4.08–3.99 (m, 7H), 3.45 (q, J ¼ 7.3 Hz, 10H), 3.33–

14.1, 7.2 Hz, 19H), 1.34 (t, J ¼ 7.2 Hz, 25H), 1.27 (d, J ¼ 13.8 Hz, 3.27 (m, 4H), 1.89–1.71 (m, 11H), 1.59 (s, 19H), 1.55–1.49 (m,

57H), 0.87 (t, J ¼ 6.9 Hz, 12H). ¹³C NMR (101 MHz, CDCl₃) d/ 4H), 1.45 (dd, J ¼ 15.3, 7.5 Hz, 4H), 1.39 (t, J ¼ 7.2 Hz, 16H), 1.31

ppm 161.35, 161.05, 146.97 (d, J ¼ 14.8 Hz), 124.4, 114.81 (d, J ¼ (s, 7H), 0.89 (t, J ¼ 6.9 Hz, 5H). ¹³C NMR (101 MHz, CDCl₃) d/

9.0 Hz), 77.43, 77.11, 76.79, 68.43, 67.98, 60.69, 59.96, 55.97, ppm 161.34 (s), 161.03 (s), 146.99 (d, J ¼ 13.7 Hz), 124.41 (s),

55.68, 53.52, 32.00, 31.02, 29.86–29.17, 29.02, 26.11, 25.69, 114.80 (d, J ¼ 5.7 Hz), 77.45 (s), 77.13 (s), 76.81 (s), 68.44 (s),

22.77, 22.15, 14.21, 8.39.

67.94 (s), 53.91 (s), 31.85 (s), 29.39–28.87 (m), 26.29 (s), 26.06 (s),

25.77 (s), 22.65 (d, J ¼ 7.1 Hz), 22.31 (s), 14.18 (s), 8.44 (s).

(29) (n ¼ 8) yield: 48.0%. Elemental analysis: calculated for

3.1.9 N-ethyl-N-ethanol-6-(4-((4⁰-alkyloxyphenyl)diazenyl)

phenoxy)hexan-1-ammonium iodides, 25–27. Compounds 25–

17 were synthesized following the procedure described for the

C

₃₂H₅₂IN₃O₂: C ¼ 60.27, H ¼ 8.22, N ¼ 6.59; found: C ¼ 59.338,

preparation of compounds 22–24, using the corresponding N- H ¼ 8.039, N ¼ 6.273. HRMS [C₃₂H₅₂N₃O₂]⁺¼ 510.4064. ¹H

ethyl-N-ethanol-6-(4-((4⁰-alkyloxyphenyl)diazenyl)phenoxy)

hexan-1-amines 16–18 as the starting materials.

NMR (400 MHz, CDCl₃) d/ppm 7.84 (d, J ¼ 8.5 Hz, 8H), 6.97 (dd,

J ¼ 9.0, 1.3 Hz, 8H), 4.02 (dt, J ¼ 13.0, 6.4 Hz, 8H), 3.44 (q, J ¼

(25) (n ¼ 7) yield: 60.0%. Elemental analysis: calculated for 7.3 Hz, 11H), 3.33–3.26 (m, 4H), 1.88–1.71 (m, 12H), 1.70–1.55

C₃₁H₅₀IN₃O₃: C ¼ 58.21, H ¼ 7.88, N ¼ 6.57; found: C ¼ 57.724, H (m, 13H), 1.55–1.41 (m, 9H), 1.38 (t, J ¼ 7.2 Hz, 17H), 1.30 (dt, J

¼ 7.802, N ¼ 6.302. HRMS [C₃₁H₅₀N₃O₃]⁺¼ 512.3839. H NMR ¼ 16.8, 10.5 Hz, 14H), 0.88 (dd, J ¼ 8.6, 5.2 Hz, 6H). ¹³C NMR

1

(400 MHz, CDCl₃) d/ppm 7.84 (dd, J ¼ 9.0, 0.7 Hz, 4H), 7.01–6.94 (101 MHz, CDCl₃) d/ppm 161.33, 161.03, 146.97 (d, J ¼ 12.8 Hz),

(m, 4H), 4.36 (t, J ¼ 6.0 Hz, 1H), 4.12 (s, 2H), 4.02 (dt, J ¼ 9.4, 124.39, 114.80 (d, J ¼ 5.8 Hz), 77.47, 77.15, 76.83, 68.44, 67.95,

6.4 Hz, 4H), 3.58–3.47 (m, 6H), 3.43–3.36 (m, 2H), 1.88–1.70 (m, 57.81, 53.88, 31.88, 29.37 (d, J ¼ 12.8 Hz), 29.01, 26.19 (d, J ¼

6H), 1.63–1.53 (m, 4H), 1.47 (td, J ¼ 14.3, 7.1 Hz, 4H), 1.36 (t, J ¼ 17.3 Hz), 25.75, 22.73, 22.30, 14.19, 8.43.

7.1 Hz, 7H), 1.33–1.27 (m, 4H), 0.89 (t, J ¼ 6.9 Hz, 3H). ¹³C NMR

(101 MHz, CDCl₃) d/ppm 161.34, 161.06, 146.96 (d, J ¼ 12.7 Hz),

(30) (n ¼ 12) yield: 43.2%. Elemental analysis: calculated for

₃₆H₆₀IN₃O₂: C ¼ 62.32, H ¼ 8.72, N ¼ 6.06; found: C ¼ 61.73, H

C

124.40, 114.81 (d, J ¼ 7.5 Hz), 100.00, 77.48, 77.16, 76.84, 75.37, ¼ 8.617, N ¼ 5.913. HRMS [C₃₆H₆₀N₃O₂]⁺¼ 566.4680. ¹H NMR

68.43, 67.97, 59.56, 58.88, 55.35, 54.80, 31.85, 29.15 (t, J ¼ 13.9 Hz), (400 MHz, CDCl₃) d/ppm 7.84 (d, J ¼ 8.6 Hz, 4H), 6.97 (dd, J ¼

26.12 (d, J ¼ 12.5 Hz), 25.70, 22.64 (d, J ¼ 7.9 Hz), 22.21, 14.17, 8.37. 8.9, 1.5 Hz, 4H), 4.08–3.98 (m, 4H), 3.44 (q, J ¼ 7.2 Hz, 6H), 3.33–

(26) (n ¼ 8) yield: 62.5%. Elemental analysis: calculated for 3.26 (m, 2H), 1.89–1.71 (m, 6H), 1.57–1.49 (m, 3H), 1.49–1.43

C

₃₁H₅₀IN₃O₄: C ¼ 58.80, H ¼ 8.02, N ¼ 6.43; found: C ¼ 58.435, (m, 2H), 1.38 (t, J ¼ 7.1 Hz, 10H), 1.25 (s, 15H), 0.87 (t, J ¼ 6.7 Hz,

H ¼ 7.919, N ¼ 6.287. HRMS [C₃₂H₅₂N₃O₃]⁺¼ 526.4027. Yield: 3H). ¹³C NMR (101 MHz, CDCl₃) d/ppm 161.34, 161.04, 146.96

¹H NMR (400 MHz, CDCl₃) d/ppm 7.84 (d, J ¼ 8.7 Hz, 4H), 6.97 (d, J ¼ 12.8 Hz), 124.39, 114.81 (d, J ¼ 7.0 Hz), 77.48, 77.16,

(dd, J ¼ 9.0, 2.2 Hz, 4H), 4.36 (t, J ¼ 6.0 Hz, 1H), 4.12 (s, 2H), 4.02 76.84, 68.45, 67.98, 57.80, 53.88, 31.99, 30.02–29.18 (m), 29.02,

(dt, J ¼ 9.6, 6.4 Hz, 4H), 3.55 (d, J ¼ 8.7 Hz, 2H), 3.53–3.45 (m, 26.19 (d, J ¼ 19.0 Hz), 25.76, 22.76, 22.28, 14.20, 8.43.

3H), 3.43–3.34 (m, 2H), 1.89–1.68 (m, 6H), 1.60–1.53 (m, 2H),

1.53–1.41 (m, 4H), 1.35 (t, J ¼ 7.2 Hz, 7H), 1.29 (d, J ¼ 9.4 Hz,

3.2 Mesomorphic studies

5H), 0.88 (t, J ¼ 6.8 Hz, 3H). ¹³C NMR (101 MHz, CDCl₃) d/ppm

161.35 (d, J ¼ 2.4 Hz), 161.07 (s), 146.92 (d, J ¼ 11.9 Hz), 124.72– The thermal behaviour and liquid crystalline properties of all

124.08, 122.78 (d, J ¼ 14.1 Hz), 114.66 (dd, J ¼ 29.2, 10.1 Hz), the synthesized compounds were studied via polarizing optical

40604 | RSC Adv., 2019, 9, 40588–40606

View Article Online

Paper

RSC Advances

microscopy (POM) and diﬀerential scanning calorimetry (DSC). relaxation. As a result, the presence of a hydroxyl group on the

For polarizing microscopy, a Linkam hot stage and temperature ammonium group decreases the thermal back relaxation time.

observation under polarized light of an Olympus BX 51 polar- In the case of the solid cell, the thermal back relaxation time

izing optical microscope were used. The magnication used to achieved was ꢀ400 min. These results suggest that chemical

capture the optical texture was 10ꢃ and 200 mm. DSC thermo- modication of the photoresponsive behaviour of azobenzene

grams were recorded using a _ꢂP₁erkinElmer DSC-7 with heating derivatives can be useful for the design of molecular switches

and cooling rates of 5 ^ꢁC min

.

and optical storage devices.

Conﬂicts of interest

3.3 Photoswitching studies

The UV-vis spectra of the azobenzene-based ionic liquids and

their intermediates were recorded using an Ocean Optics HR-

2000+ UV-vis spectrophotometer setup. The photoisomerization

of the azo compounds were recorded in a dark room in the

wavelength range of 250 nm to 800 nm, and chloroform was

There are no conicts to declare.

Acknowledgements

This research work was supported by DST-SERB (Department of

Science and Technology-Science and Engineering Research

Board) Govt of India under ECR grant (File No. ECR/2015/

000190) and Universiti Sains Malaysia through the RUI

research grant (Grant No. 1001/PKIMIA/8011094). We also

thank Hima S Reddy for helping us to conduct the photo-

isomerization experiments.

used as the blank. At xed concentration of ꢀ1.0

ꢃ

10^ꢂ5mol L^ꢂ1of solution was prepared in chloroform and irra-

diated with a UV light source (Omni cure series 2000). The

irradiation wavelength was 360 nm and a heat lter was used to

keep heat radiation away from the source. The intensity of the

irradiation light was 1 mW cm^ꢂ2, which was measured using

a UV513AB UV meter. The absorption spectra of compounds 10–

30 were recorded before illumination and aer illumination.

Aer reaching the photosaturation state, the thermal back

relaxation was recorded by keeping the samples in the dark. For

trans–cis–trans photoisomerization, the changes in absorption

spectra upon UV illumination and thermal back relaxation were

investigated with respect to time. For thermal back relaxation,

rst-order plots were plotted for cis (Z)- trans (E) isomerisation

of all the compounds and measured at room temperature at

Notes and references

1 G. Hegde, G. Shankar, S. M. Gan, A. R. Yuvaraj, S. Mahmood

and U. K. Mandal, Liq. Cryst., 2016, 43(11), 1578–1588.

2 M. L. Rahman, M. M. Yusoﬀ and S. Kumar, RSC Adv., 2014,

4(66), 35089–35098.

3 M. L. Rahman, G. Hegde, M. Azazpour, M. M. Yusoﬀ and

S. Kumar, J. Fluorine Chem., 2013, 156, 230–235.

4 L. Rahman, S. Kumar, C. Tschierske, G. Israel, D. Ster and

G. Hegde, Liq. Cryst., 2009, 36(4), 397–407.

ꢁ

ꢀ27 C.

5 M. R. Lutfor, G. Hegde, S. Kumar, C. Tschierske and

V. G. Chigrinov, Opt. Mater., 2009, 32(1), 176–183.

6 S. K. Prasad, G. G. Nair and D. S. Rao, Liq. Cryst., 2009, 36(6–

7), 705–716.

7 J. Sun and V. Chigrinov, Mol. Cryst. Liq. Cryst., 2012, 561(1),

1–7.

8 M. R. Lutfor, M. M. Yusoﬀ, G. Hegde, M. N. Fazli, A. Malek,

N. A. Samah and H. T. Srinivasa, Mol. Cryst. Liq. Cryst., 2013,

587, 41–53.

9 A. R. Yuvraj, W. S. Yam, T. N. Chan, Y. P. Goh and G. Hegde,

Spectrochim. Acta, Part A, 2015, 135, 1115–1122.

4 Conclusions

In summary, the photoresponsive behaviour of N,N-diethanol-

6-(4-((4⁰-alkyloxyphenyl)diazenyl)phenoxy)hexan-1-ammonium

iodides, 22–24, N-ethyl-N-ethanol-6-(4-((4⁰-alkyloxyphenyl)

diazenyl)phenoxy)hexan-1-ammonium iodides, 25–27 and

N,N-diethyl-6-(4-((4⁰-alkyloxyphenyl)diazenyl)phenoxy)hexan-1-

ammonium iodides, 28–30 and their intermediates 10–21 was

characterized and their mesomorphic and photoswitching

properties investigated. The schlieren texture of the smectic

C (tilted) and smectic B phases was observed in case of the

highest substituted carbon atom length in the alkoxy chain 10 Y. Norikane, Y. Hirai and M. Yoshida, Chem. Commun., 2011,

(n ¼ 12) in the side chain of the azo moiety. The natural

47(6), 1770–1772.

focal conic texture of the smectic A (non-tilted) phases was 11 A. Ziegler, J. Stumpe, A. Toutianoush and B. Tieke, Colloids

observed for all the compounds except the compound without Surf., A, 2002, 198, 777–784.

a hydroxyl group in the terminal head group of its azo moieties. 12 A. R. Yuvaraj, G. S. Mei, A. D. Kulkarni, M. Y. Mashitah and

In the case of compound 30, aer 45 s, the photoequilibrium G. Hegde, RSC Adv., 2014, 4(92), 50811–50818.

state was achieved under UV irradiation and its photo- 13 S. M. Gan, A. R. Yuvaraj, M. R. Lutfor, M. Y. Mashitah and

isomerization eﬃciency was greater than 95%. Its thermal H. Gurumurthy, RSC Adv., 2015, 5(9), 6279–6285.

back relaxation time was about ꢀ590 min in solution at room 14 G. Hegde, V. M. Kozenkov, V. G. Chigrinov and H. S. Kwok,

temperature. The photoisomerization reaction rate was

Mol. Cryst. Liq. Cryst., 2009, 507(1), 41–50.

aﬀected by the hydrophobic and hydrophilic head groups, and 15 V. Jayalakshmi, G. Hegde, G. G. Nair and S. K. Prasad, Phys.

it also aﬀected the equilibrium isomerization eﬃciency of the Chem. Chem. Phys., 2009, 11(30), 6450–6454.

compounds. Thus, the azobenzene at the end of the head 16 G. Hegde, A. R. Yuvaraj, W. Sinn-Yam and M. M. Yusoﬀ, in

group bearing a hydroxyl group inuences the thermal back

Macromolecular Symposia, 2015, vol. 353, pp. 240–245.

RSC Adv., 2019, 9, 40588–40606 | 40605

View Article Online

RSC Advances

Paper

17 M. Alaasar, M. Prehm, K. May, A. Eremin and C. Tschierske, 31 X. Chen, J. Gao, B. Song, M. Smet and X. Zhang, Langmuir,

Adv. Funct. Mater., 2014, 24(12), 1703–1717. 2009, 26, 104–108.

18 M. Alaasar, M. Prehm, M. Brautzsch and C. Tschierske, J. 32 K. Stappert, J. Muthmann, E. T. Spielberg and A. V. Mudring,

Mater. Chem. C, 2014, 2(28), 5487–5501. Cryst. Growth Des., 2015, 15(9), 4701–4712.

19 M. M. Naoum, A. A. Fahmi, N. H. Ahmed and G. R. Saad, Liq. 33 L. J. Yu, R. G. Peng, Y. Z. Wang and Y. K. Yang, Adv. Mater.

Cryst., 2015, 42(9), 1298–1308. Res., 2012, 534, 122–125.

20 M. M. Naoum, A. A. Fahmi, A. H. Abaza and G. R. Saad, Liq. 34 H. Tamura, Y. Shinohara and T. Arai, Chem. Lett., 2010, 39,

Cryst., 2014, 41(11), 1559–1568. 240–241.

21 M. Podruczna, A. Hofmanska, I. Niezgoda, D. Pociecha and 35 S. Zhang, S. Liu, Q. Zhang and Y. Deng, Chem. Commun.,

Z. Galewski, Liq. Cryst., 2014, 41(1), 113–125. 2011, 47, 6641–6643.

22 Z. Li, H. Wang, M. Chu, P. Guan, Y. Zhao, Y. Zhao and 36 J. Avo, L. Cunha-Silva, J. Lima and A. Jorge Parola, Org. Lett.,

J. Wang, RSC Adv., 2017, 7(71), 44688–44695. 2014, 16, 2582–2585.

23 R. Hayes, G. G. Warr and R. Atkin, Chem. Rev., 2015, 115, 37 J. Yang, H. Wang, J. Wang, Y. Zhang and Z. Guo, Chem.

6357–6426. Commun., 2014, 50, 14979–14982.

24 J. E. S. J. Reid, F. Agapito, C. E. S. Bernardes, F. Martins, 38 S. Shi, T. Yin, X. Tao and W. Shen, RSC Adv., 2015, 5, 75806–

´

A. J. Walker, S. Shimizu and M. E. Minas da Piedade, Phys.

Chem. Chem. Phys., 2017, 19, 19928–19936.

25 O. Palumbo, F. Trequattrini, M. A. Navarra, J. B. Brubach,

75809.

39 A. Wu, F. Lu, P. Sun, X. Gao, L. Shi and L. Zheng, Langmuir,

2016, 32, 8163–8170.

P. Roy and A. Paolone, Phys. Chem. Chem. Phys., 2017, 19, 40 E. Madiahlagan, B. N. Sunil, Z. Ngaini and G. Hegde, J. Mol.

8322–8329. Liq., 2019, 292, 111328.

26 K. Dong, X. Liu, H. Dong, X. Zhang and S. Zhang, Chem. Rev., 41 N. Trbojevic, J. C. Haenle, T. Wohrle, J. Kirres and S. Laschat,

¨

2017, 117, 6636–6695.

27 D. Andrienko, J. Mol. Liq., 2018, 267, 520–541.

Liq. Cryst., 2016, 43(8), 1135–1147.

42 W. J. Goodby, Handbook of Liquid Crystals Set, 1998, pp. 411–

440.

´

¨

28 D. D. Dıaz, D. Kuhbeck and R. J. Koopmans, Chem. Soc. Rev.,

2011, 40(1), 427–448.

29 M. Ikeda, R. Ochi and A. W. I. Hamachi, Chem. Sci., 2010, 1,

491–498.

43 M. L. Rahman, G. Hegde, M. M. Yusoﬀ, M. N. F. A. Malek,

H. T. Srinivasa and S. Kumar, New J. Chem., 2013, 37(8),

2460–2467.

¨

30 M. A. C. Stuart, W. T. Huck, J. Genzer, M. Muller, C. Ober,

M. Stamm and F. Winnik, Nat. Mater., 2010, 9(2), 101.

40606 | RSC Adv., 2019, 9, 40588–40606

Article Doi

Photoresponsive behavior of hydrophilic/hydrophobic-based novel azobenzene mesogens: Synthesis, characterization and their application in optical storage devices

DOI: 10.1039/c9ra08211e

Source and publish data:

Authors:

Article abstract of DOI:10.1039/c9ra08211e

Full text of DOI:10.1039/c9ra08211e

Products guided by the article

R&D Labs maybe for 1033125-88-0

Relevant to this article

Hot Product