Synthesis of new liquid crystals embedded gold nanoparticles for photoswitching properties

Journal of Colloid and Interface Science 478 (2016) 384–393

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Regular Article

Synthesis of new liquid crystals embedded gold nanoparticles for photoswitching properties Md Lutfor Rahman a,⇑, Tapan Kumar Biswas b, Shaheen M. Sarkar b, Mashitah Mohd Yusoff b, A.R. Yuvaraj c, Sandeep Kumar c a b c

Faculty for Science and Natural Resources, Universiti Malaysia Sabah, 88400 Kota Kinabalu, Sabah, Malaysia Faculty of Industrial Sciences & Technology, Universiti Malaysia Pahang, 26300 Gambang, Kuantan, Pahang, Malaysia Raman Research Institute, C.V. Raman Avenue, Sadashivnagar, Bangalore 560080, India

h i g h l i g h t s

g r a p h i c a l a b s t r a c t


A series of azobenzene liquid crystals

New molecules, Au NP azobenzene liquid crystals (texture at middle can be adopted for optical storage devices (ITO cell at right image). Gray color spot in the middle of ITO cell is the UV irradiated area which is disordered isotropic phase whereas greenish area is protected from the light by mask.

decorated gold nanoparticles is synthesized.
LC gold nanoparticles exhibit nematic and smectic A phase with monotropic nature.
These molecules exhibit strong photoisomerization behaviour.
Reversible isomerization repeatable up to 10 cycles.

a r t i c l e

i n f o

Article history: Received 17 March 2016 Revised 12 June 2016 Accepted 14 June 2016 Available online 15 June 2016 Keywords: Liquid crystals Gold nanoparticles Photoswitching Molecular switches Optical storage

a b s t r a c t A new series of liquid crystals decorated gold nanoparticles is synthesized whose molecular architecture has azobenzenes moieties as the peripheral units connected to gold nanoparticles (Au NPs) via alkyl groups. The morphology and mesomorphic properties were investigated by field emission scanning electron microscope, high-resolution transmission electron microscopy, differential scanning calorimetry and polarizing optical microscopy. The thiolated ligand molecules (3a–c) showed enantiotropic smectic A phase, whereas gold nanoparticles (5a–c) exhibit nematic and smectic A phase with monotropic nature. HR-TEM measurement showed that the functionalized Au NPs are of the average size of 2 nm and they are well dispersed without any aggregation. The trans-form of azo compounds showed a strong band in the UV region at 378 nm for the p-p⁄ transition, and a weak band in the visible region at 472 nm due to the n-p⁄ transition. These molecules exhibit attractive photoisomerization behaviour in which trans-cis transition takes about 15 s whereas the cis-trans transition requires about 45 min for compound 5c. The extent of reversible isomerization did not decay after 10 cycles, which proved that

⇑ Corresponding author. E-mail address: [email protected] (M.L. Rahman). http://dx.doi.org/10.1016/j.jcis.2016.06.039 0021-9797/Ó 2016 Published by Elsevier Inc.

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the photo-responsive properties of 5c were stable and repeatable. Therefore, these materials may be suitably exploited in the field of molecular switches and the optical storage devices. Ó 2016 Published by Elsevier Inc.

1. Introduction There have been extensive developments of nanomaterials for the application in high-technology and medical devices. The discovery of new methods for nanoscale building blocks into functional bulk materials is the key challenge to the nanotechnology research [1–3]. Liquid crystalline materials have been considered as promising candidates for generating self-assembled nanomaterials which respond to external fields to influence their structure and properties [4]. Liquid crystals (LCs) have attracted scientific communities for their application in liquid crystal displays (LCDs) devices, however, liquid crystals have also found wide spread use, in sensors, drug delivery vehicles, photonic band gap structures, controllable lenses and lasing devices [5,6]. A number of recent examples of self-assembly of quasi-spherical gold nanoparticles into LC phases have been reported where thermotropic LCs are used as a capping agents which display nematic or smectic phase morphologies in such system [7–10]. Typically, Brust et al. [11] method is used to prepare these nanoparticles where thiolated thermotropic LCs are used to functionalize the gold nanocluster surface. The gold nanoclusters are found to be stable to air and moisture, as well as to temperatures up to ca. 120 °C, if the hydrocarbon chain of the thiol (CnH2n+1SH) is equal or longer than 12 carbon atoms [12]. Hao and Hegmann [13] reported that the gold nanoclusters in the size regime of conventional LC molecules (2 nm) are known to undergo size changes with increasing temperature. The size effect was related with smaller nanocluster have larger chemical potential and as a consequences, have a greater tendency to sinter and increase in size (Ostwald ripening) releasing thiols from the surface [14,15]. The electrically controlled light scattering produced from quasispherical gold nanoparticles (80 nm) in a nematic LC mixture was demonstrated by Müller et al. [16], Park and Stroud [17] reported that the gold nanoclusters embedded in a thin film of a nematic liquid crystals enhance the surface plasmon splitting. A voltage dependent colour tuning device has been fabricated using gold nanodot arrays formed from a sandwiched nematic LC cell [18]. The dispersion of nanoparticles (NP) in LCs (NP-doped LCs or LC/ NP composites) significantly influences LCD-relevant characteristics, such as, threshold voltage, pretilt angle, dielectric behaviour, and contrast ratio [19–27]. The Au-NP properties can be tuned with LCs which initiated changes in the localized surface plasmon resonances [28]. The liquid crystals are unique candidates to manipulate Au-NP assembly [29–31], as well as their electronic and optical properties for novel nanoscale devices [32]. In addition, the liquid crystals properties can also be affected by the Au-NP, such as electro-optic properties [33] and alignment of LC molecules used in the display applications [23]. The textural and alignment effects could be used as a way to control the alignment of LC molecules using functionalized Au-NP [26]. Hao and Hegmann [13] reported an unprecedented dual alignment and electro-optic mode in the same system using planar ITO cells with rubbed polyimide alignment layers. In this mode, LCs with a positive dielectric anisotropy can be initially homeotropically aligned, then tuned to change to planar alignment and switch between optical ON and OFF state by applying an electric field [25]. However, a limited number of functionalized Au-NP are reported to date for commercial display devices.

Alongside with the design and synthesis of Au-NP liquid crystals molecules, a field of research progressing well is the photo-induced phenomenon in which the incident light leads the molecular ordering/disordering of the liquid crystalline system. This controlled light process is proposed as the future technology for high-speed information processing [34]. Prasad et al. [35–41] studied photo-induced isothermal phase transitions in various liquid crystalline systems. The photochromic azo (AN@NA) functional groups can be chemically attached or used as a guest in a liquid crystal host system have been received much attention due to their unique photo-switchable properties induced by light [42–46]. This system is the reversible photo-induced shape transformation of the molecules containing the photochromic azo groups. Upon UV irradiation (365 nm, p-p⁄ band of AN@NA), the energetically more stable E configuration (trans) converts to the Z configuration (cis). The reverse transformation of the Z isomer into the E isomer can be generate by illumination with visible light (range 400– 500 nm, n-p⁄ band). The process is known as thermal back relaxation which can occur in the dark or photo-chemically [47,48]. If the system composed of the photoactive guest–non-photoactive host, the trans form of the azo guest (rod-like) is favourable for the stabilization of the liquid crystalline phase whereas the bent cis form destabilizes the liquid crystalline phase. Therefore, photo-isomerization from the trans to the cis form leading a lowering of the transition temperature such as a nematic–isotropic (N–I) transition by UV irradiation in the nematic phase. Thus, photochemically induced transition is promising for the optical image storing systems [49]. Very recently, Xue et al., reported photoresponsive azo thiol grafted gold nanoparticles for stimuli-directed alignment control of LCs [50]. In this paper, we report a new series of thiolated azobenzene molecules embedded to gold NPs which exhibit nematic and SmA phases irrespective of chain length and parity. Multiple cycles of photoisomerization is performed and result show that the reversible isomerization did not decay after 10 cycles, indicating that the photo-responsive properties is stable and repeatable for possible application to the optical storage devices. 2. Experimental 2.1. Synthesis of intermediate compounds All intermediate compounds 1 and 2a–c were synthesized according to our earlier papers [51–54]. 2.2. Synthesis of thiolated liquid crystals (3a–c) 2.2.1. 1-(4-{(E)-[4-(4-mercaptobutoxy)phenyl]diazenyl}phenyl) ethanone (3a) To a solution of 2a (0.76 mmol) in THF (10 mL) was added a mixture of tetrabutylammonium fluoride (221 mg, 0.84 mmol) and hexamethyldisilathiane (166.8 mg, 0.92 mmol) in THF (5 mL) under stirring [55]. The reaction mixture was stirred for a further period of 12 h. It was diluted with dichloromethane and washed with saturated ammonium chloride (10 mL) and then with distilled water (20 mL). The organic layer is separated and dried over

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anhydrous sodium sulfate, and the solvent was removed under reduced pressure to give a residue that will be recrystallized from a mixture (9:1) of hexane and ethyl acetate to yield the thiolated product of 3a. Yield: 65.5%, mesophase Cr 111 SmA 150 I. IR, mmax/ cm1 2947 (CH2), 2870 (CH2), 1674 (C@O), 1592, 1480 (C@C, aromatic), 1391 (OH), 1362 (CH3), 1245, 1132 (CAO), 844 (CH). dH (500 MHz: CDCl3: Me4Si) 8.02 (d, 2H, J = 8.6 Hz, Ph), 7.88 (d, 2H, J = 8.8 Hz, Ph), 7.82 (d, 2H, J = 8.9 Hz, Ph), 7.01 (d, 2H, J = 8.8 Hz, Ph), 4.02 (t, 2H, J = 6.5 Hz, OCH2A), 2.60 (s, 3H, CH3), 2.64 (t, 2H, J = 5.6 Hz, ACH2SH), 1.83–1.78 (m, 2H, ACH2A), 1.64–1.62 (m, 2H, ACH2A), 1.53 (1H, ASH). dC (125 MHz: CDCl3: Me4Si) 25.94, 26.44, 27.57, 29.15, 68.67, 114.43, 122.48, 125.33, 129.44, 137.77, 146.57, 155.56, 162.37, 197.56.

2.2.2. 1-{4-[(E)-{4-[(5-mercaptopentyl)oxy]phenyl}diazenyl]phenyl} ethanone (3b) Compound 3b was synthesized by the same method use for synthesis of 3a. Yield: 66.2%, mesophase Cr 90 SmA 144 I. IR, mmax/ cm1 2947 (CH2), 2870 (CH2), 1677 (C@O), 1590, 1485 (C@C, aromatic), 1391 (OH), 1362 (CH3), 1243, 1132 (CAO), 848 (CH). dH (500 MHz: CDCl3: Me4Si) 8.02 (d, 2H, J = 8.6 Hz, Ph), 7.87 (d, 2H, J = 8.9 Hz, Ph), 7.82 (d, 2H, J = 8.9 Hz, Ph), 7.01 (d, 2H, J = 8.9 Hz, Ph), 4.02 (t, 2H, J = 6.6 Hz, OCH2A), 2.60 (s, 3H, CH3), 2.64 (t, 2H, J = 5.6 Hz, ACH2SH), 1.85–1.81 (m, 2H, ACH2A), 1.64–1.60 (m, 4H, ACH2A), 1.43 (1H, ASH). dC (125 MHz: CDCl3: Me4Si) d: 25.23, 26.92, 27.36, 29.27, 32.65, 68.55, 114.45, 122.77, 125.44, 129.27, 137.52, 146.27, 155.66, 162.33, 197.77.

2.2.3. 1-{4-[(E)-{4-[(6-mercaptohexyl)oxy]phenyl}diazenyl]phenyl} ethanone (3c) Compound 3c was synthesized by the same method use for synthesis of 3a. Yield: 64.8%, mesophase Cr 94 SmA 146 I. IR, mmax/ cm1 2945 (CH2), 2875 (CH2), 1675 (C@O), 1592, 1482 (C@C, aromatic), 1391 (OH), 1362 (CH3), 1244, 1132 (CAO), 847 (CH). dH (500 MHz: CDCl3: Me4Si) 8.01 (d, 2H, J = 8.8 Hz, Ph), 7.89 (d, 2H, J = 8.6 Hz, Ph), 7.84 (d, 2H, J = 8.9 Hz, Ph), 7.01 (d, 2H, J = 8.8 Hz, Ph), 4.01 (t, 2H, J = 6.4 Hz, OCH2A), 2.60 (s, 3H, CH3), 2.64 (t, 2H, J = 5.7 Hz, ACH2SH), 1.82–1.80 (m, 2H, ACH2A), 1.68–1.64 (m, 2H, ACH2A), 1.55–1.52 (m, 4H, ACH2A), 1.51 (1H, ASH). dC (125 MHz: CDCl3: Me4Si) 25.32, 26.82, 27.67, 29.12, 32.22, 33.86, 68.34, 114.28, 122.26, 125.33, 129.54, 137.55, 146.92, 155.29, 162.23, 197.66.

2.3. Synthesis of 1-decanethiol capped gold nanoparticle (4a-c) To a stirred solution of tetraoctylammonium bromide (5.00 g, 9.17 mmol) in toluene (150 mL) was added an aqueous solution (100 mL) of gold(III)chloride trihydrate (1.45 g, 3.68 mmol) and the mixture was stirred for 30 min [56]. The reaction mixture was washed with distilled water and the organic layer was separated. Then 1-decanethiol (1.40 g, 8.0 mmol) in toluene (10 mL) was added to the above solution and the resulting mixture will be stirred for 30 min. An aqueous solution of sodium borohydride (1.20 g, 31.5 mmol) was added drop-wise and the mixture is stirred for a further period of 3 h. The organic layer was washed with water and dilute with methanol (500 mL). It was kept in a refrigerator, the precipitate obtained is further purified by being resuspending in toluene and then addition of methanol (100 mL). This process was repeated to remove any unbound thiol for pure Au-NP of 4a. The purification procedure repeated until no trace of excess of thiol could be found in the 1H NMR spectrum (signal absence at 2.56 ppm, proton NMR) and TLC.

2.4. Synthesis of thiolated gold nanoparticle (5a–c) by ligand exchange reaction with 4a A solution was prepared by dissolving 85 mg of gold nanoparticles of 4a in 5 mL of toluene [56]. Thiolated liquid crystal 3a (90 mg) was dissolved in 10 mL of toluene and added with vigorous stirring. The mixture was stirred for 24 h, followed by addition of methanol (100 mL) and mixture was kept in a refrigerator, the precipitate obtained is further purified by being re-suspending in toluene and then addition of methanol (100 mL). This process was repeated several times to remove the residual organic ligands. The purification procedure repeated until no trace of excess of thiolated LC is found in the 1H NMR spectrum (signal absence at 2.64 ppm) and TLC of compound 5a. Compounds 5b–c were synthesized by similar procedure of 5a. 2.5. Characterizations The structure of the compounds was confirmed by spectroscopic method. IR spectra were recorded with a Perkin Elmer (670) FTIR spectrometer. 1H NMR (500 MHz) and 13C NMR (125 MHz) spectra were recorded with a Bruker (DMX500) spectrometer. The transition temperatures and their enthalpies were measured by differential scanning calorimetry (Perkin DSC 7) with heating and cooling rates were 10 °C min1. Optical textures were obtained by using Olympus BX51 polarizing optical microscope attached with Olympus DP26 digital camera equipped with a Mettler Toledo FP82HT hot stage and a FP90 central processor unit. FESEM was used to study the morphology with JEOL (JSM-7800F). TEM was measured with Hitachi instrument (HT-7700). Absorption spectra for photochromic study were recorded using an Ocean Optics HR2000+ miniature UV–Vis spectrophotometer. All the solutions were prepared and measured under air in the dark at room temperature (21 ± 1 °C) using 1 cm quartz cells. The cells were closed to avoid the evaporation of the solvent and the solutions were stirred during the irradiation time. The solutions were irradiated at kmax = 365 nm along with heat filter to avoid any extra heat radiation to the sample using Omnicure S2000 UV source. 3. Results and discussion 3.1. Synthesis The synthetic approach used to prepare the intermediates and target compounds is outlined in Schemes 1 and 2. The peripheral rod-like side arms were prepared from 4-aminoacetophenone in which the amino group is diazotized by sodium nitrite in the acid media. The obtained diazonium salt was coupled with phenol to yield 4-(4-hydroxyphenylazo)acetophenone 1 and purified by recrystallization from methanol with 58% yield (Scheme 1). The flexible spacer was introduced by alkylation of 1 with dibromoalkane in the presence of potassium carbonate to give 1-brom o[4-(4-acetylphenylazo)phenoxy]alkane 2a–c (n = 4, 5 and 6), purified by column chromatography on silica and crystallization from methanol/chloroform with 60% yield. The compound having thiol functional groups (3a–c) were synthesized from 2a–c using tetrabutylammonium fluoride and hexamethyldisilathiane (Scheme 1). All compounds were purified on silica gel by column chromatography followed by recrystallization. The synthesized compounds were characterized by 1H and 13C NMR analyses. Spectroscopic data were found to be in good agreement with the structure (see synthetic procedures and analytical data including 1H NMR spectra of 1 and 2a–c in the Supporting Information). Au-NPs having long alkyl chains of 1-decanethiol were prepared following Brust and co-worker method [57]. Then Au-NPs with azo

M.L. Rahman et al. / Journal of Colloid and Interface Science 478 (2016) 384–393

O C

O C

NaNO2

NH2

N2Cl

HCl, 2 oC O C

OH

N N

OH

1 K2CO3/KI

O C

N

Br(CH2)nBr n=4-6 TBAF

N

O

n Br

2a-c O C

N

HMDST

N 3a-c

O

n SH

Scheme 1. Synthesis of thiolated azobenzene liquid crystals.

liquid crystals (5a–c) were synthesized by ligand exchange reaction from thiolated LC compounds 3a–c and thiol-caped Au-NPs (4a) using agitation at room temperature as shown in Scheme 2. The pure Au-NPs did not show any trace of thiol in the 1H NMR spectra (absence of the signal at 2.64 ppm) and representative proton NMR for 3c and 5c are presented in Fig. 1 (top) and Fig. 1 (bottom), respectively. The synthesized compounds were characterized by 1H and 13C NMR analyses. Spectroscopic and analytical data were also found to be in good agreement with the structures (see analytical data including 1H NMR spectra in the Supporting Information). 3.2. Mesomorphic properties 3.2.1. DSC study The phase transition temperatures as well as the phase transition enthalpy changes were determined by differential scanning calorimetry (DSC) and the results of the second heating and cooling scans are summarized in Table 1. There are two peaks observed in both the endothermic and exothermic cycles for all thiolated compounds (3a–c). On heating, for compound 3a, two peaks appeared at 111 (DH = 38 J g1) and 150 °C (DH = 10 J g1) which corresponding to the Cr - SmA and SmA - I transitions, respectively. On cooling, the isotropic to SmA and SmA to crystals phase transitions occurred at 146 (DH = 9 J g1) and 95 °C (DH = 21 J g1), respectively. Similarly, the compound 3b exhibited two peaks on heating at 90 °C (DH = 44 J g1) and 144 °C (DH = 11 J g1), which correspond to the Cr - SmA and SmA - I transitions. On cooling, transitions at 142 °C (DH = 10 J g1) and 76 °C (DH = 22 J g1) corresponding to I - SmA and SmA - Cr were observed. The compound 3c also displayed two transitions at 94 °C (DH = 52 J g1) and 146 °C (DH = 9 J g1) on heating and at 144 °C (DH = 9 J g1) and 73 °C (DH = 16 J g1) on cooling. The DSC thermogram of compound 3c is shown in Fig. S3 at ESI. As can be seen, these transitions occurred at slightly lower temperature compared to 3a but at slightly higher temperature compared to 3b. This is usual behaviour due to the odd even effect of the spacer reported in our earlier paper [53,54]. On the other hand, DSC thermograms showed only one peak at the endothermic and three peaks at exothermic cycles for all AuNP liquid crystals (5a–c). On heating, compound 5c exhibits a broad peak at 153 °C which corresponding to the Cr - I transition.

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On cooling, the isotropic to nematic, nematic to smectic A and smectic A to crystals phase transitions occurred at 152 °C, 138 °C and 101 °C, respectively as shown in Fig. 2. Similarly, compound 5a and 5b showed an endothermic peak at 153 °C and 153 °C, respectively. On cooling, three peaks corresponding to isotropic to nematic, nematic to smectic A and smectic A to crystals phase were observed (Table 1). 3.2.2. Polarizing optical microscopy study The mesophase structures were evaluated by means of polarizing optical microscopy. The mesophases of all compounds 3a–c (n = 4, 5 and 6) were observed upon cooling from the isotropic phase. All the compounds (3a–c) display typical fan shaped texture for smectic A phase upon cooling from the isotropic phase. Photomicrographs of the textures were taken at 135 °C, 127 °C and 130 °C for compounds 3a, 3b and 3c, respectively as shown in Figs. 3a and S4 (ESI) upon cooling from isotropic temperature. Upon shearing, homeotropic alignment was achieved and these homeotropically aligned regions showed complete darkness which confirmed the presence of a uniaxial SmA phases for these compounds 3a–c. All desired compounds, the Au NP liquid crystals (5a–c) showed schlieren texture and typical fan shaped texture of the nematic and smectic A phase, respectively on cooling cycles. Therefore, the Au NP compounds (5a–c) exhibit monotropic mesomorphic nature as the nematic and SmA phases appeared only in the cooling cycle. Photomicrographs of textures were taken at 144 °C and 120 °C for compound 5c as shown in Fig. 3. Textures of compound 5a, 5b are presented in Fig. S5 (ESI). 3.2.3. FESEM and TEM study FESEM measurement was performed with JEOL instrument (JSM-7800F) instrument and photomicrograph of compound 5c is shown in Fig. 4a. A compact flower like (rose) morphology was observed which does not show any metal particles (Au) even in nanosize image. Therefore, we have taken a HRTEM image of compound 5c using Hitachi instrument (HT-7700), compound consist of Au NP functionalized with azobenzene liquid crystals via variable alkyl spacer units. TEM image was obtained by drop-casting a 5 ll solution of the 5c in ethanol on a carbon-coated copper grid and allowing the solvent to slowly evaporate under ambient conditions. This TEM image is shown that the functionalized Au NPs are well dispersed without any aggregation. The average size of Au NPs was estimated to be about 2 nm as shown in Fig. 4b. Corresponding HRTEM image is shown in Fig. 4c. 3.3. Photoswitching studies The photo-switching studies of Au NP liquid crystals (5a–c) were initially carried out in solutions. All Au NPs with liquid crystal molecules (5a–c) show similar absorption spectra due to their similar molecular structure with variable alkyl chain (n = 4–6). Fig. 5 depicts the absorption spectra of 5c (n = 6) before and after UV illumination. The absorption spectra of compound 5c show absorbance maxima at 378 nm. The absorption spectra of all compounds was taken in chloroform solution having same concentration, C = 1.2  105 mol L1. Compound 5c was illuminated with UV light with 365 nm filter at different time intervals and promptly absorption spectra were recorded. The absorption maximum at 378 nm decreases due to E/Z photoisomerization. After 15 s illumination, very little change in the spectrum was observed up to 25 s illumination which confirms the saturation of E/Z isomerization process. Fig. 6 shows the E-Z absorption of compound 5c as a function of exposure time. Data is extracted from Fig. 5. The 378 nm peak wavelength was fixed and absorption values at 364 nm at different exposure time were recorded. Curve shows that photosaturation

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TOAB / NaBH4 HAuCl4 CH3(CH2)9SH S

S

S

Au

S

S

S

4a O C

N N

O

n SH

3a-c

O C

C O

N N

N N

O

O C

N

O

N

O n n S S S S S nO nS Au S

O

S S nS S S n

N

O N

C

O

N N

N N

C O

O C 5a : n = 4 5b : n = 5 5c : n = 6

Scheme 2. Synthesis of gold NPs with azobenzene liquid crystals.

occurs within 15 s for 5c, here 20 and 25 s illumination show very little change, these results are highly significant as compared to literature data [35,51,52]. The thermal back relaxation process where the solution is shined continuously for 20 s (photo stationery state), and kept in the dark and then at subsequent time intervals spectral data were recorded up to 65 min. Thermal back relaxation for the compound 5c is shown in Fig. 7. Fig. 8 shows the time dependence of the Z-E absorption of compound 5c. Peak wavelength at 378 nm as obtained from Fig. 7 is plotted as a function of recovery time. The thermal back relaxation occurs 45 min was time taken to relax back to their original state for the compound 5c. Although data was recorded up to 65 min, almost no change was observed after 45 min. This process is reasonably fast as compared to other known systems [35,51].

Prasad et al. [35] reported that the faster thermal back relaxation is due to their layered structure since changes are confined to in-plane rotation of the molecules as compared with nematic to isotropic phase involve transition. This hypothesis is wellestablished by the fact that a similar feature was observed in another case wherein the two phases involved have a layer structure [49]. We also calculated the rate constant (kt) for the cis-trans isomerization behaviour at room temperature for compound 5c according to the Eq. (1) reported by Lutfor et al. [54]

ln

A1  At ¼ kt A1  A0

ð1Þ

where At, A0 and A1, is the absorbance at 378 nm of time t, time zero and infinite time, respectively. A typical first order plot using

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Fig. 1. 1H NMR spectra of thiolated liquid crystal 3c (top) & corresponding Au NP LC 5c (bottom).

153oC

Table 1 Phase transition temperature (T/°C) and associated transition enthalpy values [DH, J g1] observed for the second heating and cooling DSC scans of 3a-c and 5a-c.a n

Heating

Cooling

3a

4

I 146 [9] SmA 95 [21] Cr

3b

5

3c

6

5a

4

Cr 111 [38] SmA 150 [10] I Cr 90 [44] SmA 144 [11] I Cr 94 [52] SmA 146 [9] I Cr 157 [30] I

5b

5

Cr 155 [27] I

5c

6

Cr 153 [32] I

I 142 [10] SmA 76 [22] Cr

Cr

Endo UP

Compound

Heating

Cooling

Cr

SmA

I 144 [9] SmA 73 [16] Cr I 156 [2.9] N 145 [2.8] SmA 106 [24] Cr I 154 [2.5] N 141 [2.5] SmA 102 [22] Cr I 152 [2.6] N 138 [2.9] SmA 101 [27] Cr

a Peak temperatures from DSC (rate 10 K1); Abbreviation Cr = crystal, SmA = smectic A phase, N = nematic phase, I = isotropic.

I

N 138oC

I 152oC

101oC 40

60

80

100

120

140

160

180

o

Temperature ( C) Fig. 2. DSC heating and cooling traces of compound 5c (n = 6) at 10 °C min1.

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a

b 1.6 2.4

1.8 2.0

c

Fig. 3. Polarizing optical microscope images of compound 3c and 5c. All the optical texture are viewed through cross-polariser on cooling from the isotropic temperature under 200 magnification; (a) SmA phase at 130 °C of 3c, (b) nematic phase at 144 °C of 5c and (c) SmA phase at 120 °C of 5c.

Eq. (1) at room temperature (25 °C) for compound 5c are shown in Fig. 9. A typical first order behaviour for 5c shows throughout the relaxation time. Apparently, compound showed first order exponential decay in solutions. The rate constant was observed for the Z-E isomerization of 2.21  102 s1 for 5c.

Fig. 4. FESEM of (a) gold NP liquid crystals of 5c, (b) TEM micrograph of 5c and (c) HR-TEM micrograph of 5c.

Fig. 10 shows the photo-stability of the light sensitive compound. Multiple cycles of photoisomerization is considered here, irrespective of time of trans–cis and cis-trans isomerization. The compound 5c is dissolved in chloroform with the concentration

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1.0 1.0

NO UV 0.2 sec 0.4 sec 1.2 sec 3.2 sec 6.0 sec 7.5 sec 9.0 sec 12.0 sec 15.0 sec 20.0 sec 25.0 sec

0.8 trans

0.6

0.4

Absorbance (arb.units)

Absorbance (arb.units)

UV ON

UV OF dynmaics of 4c

0.8 0.6 0.4 0.2

0.2 0.0 0

0.0 300

350

400

450

500

10

20

550

λ (nm) Fig. 5. Spectra shows the absorbance behaviour of the compound 5c when UV light is shined on the sample. It is evident that within 15 s of illumination, photo saturation occurs. Data were taken before shining UV light (NO UV) and with subsequent time intervals.

30

40

50

60

70

Time (min) Fig. 8. Photoisomerization curve of Z isomer (5c) as a function of recovery time when UV light is illuminated until it reaches photosaturation and followed by measuring back relaxation time.

0.0 1.0

Absorbance (arb.units)

-0.5 UV ON dynamics of 4c

0.8

-1.0 -1.5

0.6

-2.0 0

0.4

10

20

30

40

50

60

70

Time (min) 0.2 0

3

6

9

12

15

18

21

24

Fig. 9. First order plots for the compounds 5c for cis-trans isomerization. Conditions: c = 1.2  105 mol L1 in chloroform at room temperature (25 °C).

Time (sec)

1.0 10 sec 2 min 3 min 5 min 7 min 10 min 15 min 20 min 25 min 30 min 35 min 40 min 45 min 53 min 65 min

Absorbance (arb.units)

UV OFF

0.8 trans

0.6 0.4 0.2 0.0 300

350

400 450 λ (nm)

500

550

Fig. 7. Thermal back relaxation process for the compound 5c shows that to relax from cis to trans takes around 45 min. Solution is shined for 20 s (photo stationery state) and then at subsequent intervals data were recorded.

1.1  105 mol L1. The UV light of intensity 10 mW cm2 was illuminated on the solution continuously, until photo-stationary state is reach. Immediately, visible light of 10 mW cm2 (450 nm

λmax= 349.95 nm

Absorbance (arb. units)

Fig. 6. Photoisomerization curve (5c) as a function of UV illumination time showing trans to cis behaviour.

0.8 trans

0.6 0.4

trans

cis

0.2 cis

0.0 0

200

400

600

800

1000

1200

1400

Time (sec) Fig. 10. Reversibility of the photoisomerization process of the azobenzene chromophore of Au NP (5c) in in chloroform with the concentration of 1.1  105 mol L1.

wavelength) was irradiated on the solution for back relaxation process after photosaturation state. This phenomenon was continued 10 times to evaluate the photo-stability of the compound. Thus, compound is stable towards light and does not degrade via light illumination. The extent of reversible isomerization did not decay after 10 cycles, indicating that the photo-responsive properties of 5c were stable and repeatable. Therefore, these Au NP liquid

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crystals are suitable in optical storage device industries, due to the excellent light induced characteristics. Spectral investigation on solid films of 5c was used as a representative compound and data were also recorded as a function of UV illumination. Here guest-host effect is employed where 5CB, room temperature nematic liquid crystal, acts as a host and liquid crystals gold nanoparticle 5c act as guest systems. The mixture is used capillary filled into the commercially available cell (Instec) ITO + polyimide coated, unidirectional rubbed, sandwiched cell at isotropic temperature (70 °C). Qualities of the cells were observed under optical polarizing microscope. The guest-host mixture was illuminated with UV light of 10 mW cm2 intensity through a standard mask for 10 min. The light green region is the area which is masked with UV radiation which remains in liquid crystalline state whereas the grey region in the central position is the erasing area which is illuminated with UV radiation which transforms to isotropic state (see Fig. 11). As we know that the trans isomer of the photoactive molecule stabilizes the liquid crystalline phase (e.g. nematic phase) due to the shape difference, the cis isomer destabilizes the respective phase. Therefore, the photo-controlled conversion of one isomer to another can lead to a transition from the nematic to the isotropic phase. Comprehensive studies such as absorption spectra, visual observation, opto-dielectric effect, and random field model, effect of pressures, dynamics and disorder-to-order transition are reported on the photo-induced N–I transition by Prasad et al. [35–41]. By its very nature the UV–vis spectra of the sample provides a simple method to judge the occurrence and extent of photoisomerization. The polarizing microscopy can be used to visually observe the photoinduced N–I isothermal transition due to the finite birefringence of the nematic phase. In the case of polyisocyanate copolymer materials, low concentration of chiral moieties (sergeants) is present, the system with a majority of achiral moieties (soldiers) exhibits a large chiral amplification produced chiral ordering [58]. Sandhya et al. [39] reported that the magnitude of the shift in the transition temperature as a function of the magnitude of the radiation can be explain in terms of a random-field model [59]. During the UV radiation, the photoactive molecules exhibit bent and therefore a local region of higher orientational entropy compared to the regions of the host molecules.

Application of pressure reduces the shift in the transition temperature induced by photoisomerization. At constant temperature have established that a decrease in the transition volumes as one move up to higher transition pressures and temperatures [60]. Such a reduction in volume mean that the intermolecular space available for the azo molecule to take a bent shape decreases as the pressure is increased. Resulted in the photo-induced shift in the transition temperature caused by the cis isomer also becomes smaller as the pressure is increased. This opposition due to reduction in the intermolecular space by increasing the energy of the UV radiation pumped into the system. Virtually, a higher intensity level of the radiation should be force the trans isomer of the azo to transform to the cis isomer leading at least a partial restoration of the photo-induced shift in the transition temperature [41]. By definition, the trans–cis conversion created by photoisomerization always leads to an order-to-disorder transition. Prasad et al. [37,38] found that a system in which photoisomerization leads to an ordering in the medium. The system exhibits a nematic–smectic A–reentrant nematic phase sequence when cooled from the isotropic phase [61]. When irradiated in the SmA and high temperature N phases, the material transforms into N and isotropic phases respectively. This is in agreement with the results that the UV irradiation can leads to a transition to a less-ordered state. Whereas, if the experiment is performed in the Nre phase, they found that the Nre phase transforms into the SmA phase (more ordered state). Lansac et al. [62] explained this behaviour using the photocontrolled nanophase segregation mechanism. The concept of nanophase segregation, when the UV radiation is absent then the azo molecules are in their trans form (a rod-like shape) which lead to easily accommodated into the smectic layers. However, in the photo-induced cis form, the molecules assume a bent-shaped and therefore deport from within the layers and occupy the region between the layers [35]. 4. Conclusion A series of new Au NPs passivized with liquid crystalline molecules were synthesized whose molecular architecture is composed of Au NPs as central core and rod-like azobenzenes as the peripheral units linked through alkyl spacers. All the compounds of Au NPs show nematic and smectic A phases irrespective of chain length and parity. Experimental study suggests that these Au NPs liquid crystal azo molecules exhibit strong photoisomerization properties. A short range of thermal back relaxation (about 45 min) has potential advantage in the creation of molecular switches. The reversible isomerization did not decay after 10 cycles of 5c, therefore, the photo-responsive properties of the compounds were stable and repeatable. The presences of the azo linkage in these liquid crystals molecules are suitable for using as an optical storage and molecular switches. Acknowledgements This research was supported by FRGS Grant (RDU130121). Appendix A. Supplementary material Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.jcis.2016.06.039.

Fig. 11. Optical data storage device based on the principle described in this article observed under the crossed polarizers. The sample 5c was kept at room temperature and illuminated with UV radiation through a photo masks. The colour erasing region in the middle position is the molecules which are exposed to UV radiation and the light green region where the radiation is masked. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

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