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Synthesis and characterization of azobenzene-based gold nanoparticles for photo-switching properties
MOLLIQ-05349; No of Pages 7 Journal of Molecular Liquids xxx (2015) xxx–xxx
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Synthesis and characterization of azobenzene-based gold nanoparticles for photo-switching properties Tapan Kumar Biswas a,b,⁎, Shaheen M. Sarkar a, Mashitah M. Yusoff a, Md Lutfor Rahman a,⁎ a b
Faculty of Industrial Sciences and Technology, Universiti Malaysia Pahang, 26300, Gambang, Kuantan, Pahang, Malaysia Department of Chemistry, University of Rajshahi, Rajshahi 6205, Bangladesh
a r t i c l e
i n f o
Article history: Received 19 September 2015 Received in revised form 21 December 2015 Accepted 22 December 2015 Available online xxxx Keywords: Photoisomerisation Gold nanoparticles Cis–trans isomerization Photoswitching
a b s t r a c t A series of new azobenzene based thiolated liquid crystals modified with gold nanoparticles were synthesized and characterized using a different mode of delegated tools e.g. FTIR, NMR and FESEM–EDX measurements for the structural properties of synthesized compounds. Polarized optical microscopy studies have revealed all the studied compounds having liquid crystalline properties, as a typical nematic phase. These liquid crystal capped gold nanoparticles' size was determined by TEM experiment. In addition, azobenzene-based gold nanoparticles containing flexible spacers showed photochromic behavior upon UV irradiation. These molecules exhibited strong photoisomerisation behavior in solutions and their trans to cis isomerisation took about 44 s whereas the reverse process almost took place ranging from 82 to 125 min. © 2015 Elsevier B.V. All rights reserved.
1. Introduction Photoswitchable molecules attached to solid surfaces is of a substantial interest for the preparation of advanced nano-systems, leading to a variety of applications, such as information storage, molecular machines, and sensors [1–2]. A photoswitchable molecule can be converted from one form to another with light of one wavelength and can either revert thermally to the original state or can be reverted by irradiation with light of a different wavelength. In recent years, the photo-responsive molecules on metal nanoparticle surface has received significant attention due to the opportunity of using switching devices as photo-responsive components in optical storage, switching, photoswitchable surface wettability and molecular recognition applications [3–16]. Moreover, the hybrid or composite nanomaterials consisting of inorganic nanoparticle and photoactive organic molecules may provide light-controlled nano-devices [17–21]. For optical switching applications, one of the most used organic chromophores is mostly azobenzene or its derivatives. Generally, ‘azobenzene’ refers only to the parent molecule, though the term is now repeatedly used to refer to the entire class of substituted azo molecules. The unique commonality among the azobenzene molecules (azo's) is the clean and efficient photochemical isomerization that can occur about the azo linkage when the chromophore absorbs a photon. Azobenzene, a photochromic T-type system, exhibits a reversible isomerisation process between its trans and cis ⁎ Corresponding authors. E-mail addresses: [email protected] (T.K. Biswas), [email protected] (M.L. Rahman).
isomers of different stability [22]. In addition, azobenzene containing compounds have two geometric isomers (Z/E) around the N N double bond; the trans isomer (E) is more stable than the cis isomer (Z). In this process, without any bond breaking, the photoreaction occurs due to the simple rearrangement of the electronic and nuclear structure of the molecule. The photo isomerism process (cis to trans) can be carried out either by heating or by irradiation with visible light [23]. The energetically more stable trans configuration will turn into the cis configuration when UV light of wavelength 365 nm shines on azobenzene systems and reversible to the original configuration is brought about either by keeping it in the dark (terms as called thermal back relaxation) or by illuminating with white light of higher wavelength (450 nm). According to the literature, gold nanoparticles derived using as biological molecules investigation such as DNA, protein have gained much interest in liquid crystal research as biosensor [24]. The present study focuses on the synthesis and photo-isomerization behavior of four new gold nanoparticles (Gold-NPs) decorated azobased liquid crystals having azobenzene chromophores connected via a flexible–CH2–spacer. In this paper, substituted azo-derivatives capping thiol groups were employed for the preparation of decorated Gold-NPs and the photoisomerization performance of their solution phase have been investigated by UV–vis absorption spectroscopy. Interestingly, thermal back relaxation of the studied compounds were increased with respect to the flexible spacer size (–CH2–) increased. The morphology and photo-responsive properties of the gold nanoparticles modified azobenzene derivatives having different flexible spacers were characterized via FESEM and UV/vis spectroscopy. In this article, we report a preparation of the gold nanoparticles cover with a layer of
http://dx.doi.org/10.1016/j.molliq.2015.12.078 0167-7322/© 2015 Elsevier B.V. All rights reserved.
Please cite this article as: T.K. Biswas, et al., Synthesis and characterization of azobenzene-based gold nanoparticles for photo-switching properties, J. Mol. Liq. (2015), http://dx.doi.org/10.1016/j.molliq.2015.12.078
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T.K. Biswas et al. / Journal of Molecular Liquids xxx (2015) xxx–xxx
photoresponsive azo-based liquid crystals by Au–S covalent bonds and its liquid crystal properties and photo responsive behavior were discussed.
C C stretch), 1458 (N N stretch), 1282 (C–O stretch). δH (500 MHz; CDCl3; Me4Si) 8.37 (d, 2H, Ar–H), 7.98 (d, 4H, Ar–H), 7.03 (d, 2H, Ar– H), 4.12 (t, 2H, OCH2), 3.52 (t, 2H, CH2Br), 2.08–2.13 (m, 2H, CH2), 2.00–2.03 (m, 2H, CH2).
2. Experimental details 2.1. Materials 4-Nitro aniline (Fluka), sodium nitrite (Fluka), phenol (Merck), 1,3- dibromopropane (Fluka), 1,4-dibromobutane (Fluka), 1,5dibromopentane (Fluka), 1,6-dibromohexane (Fluka), potassium carbonate (Aldrich), Gold(III) chloride hydrate (Aldrich), tetrabutylammonium fluoride (Merck), hexamethyldisilathiane (Merck), tetraoctylammonium bromide, silica gel-60 (Merck) were used as received. Phosphorus pentaoxide (Merck) was used to reflux the acetone and distilled out before used. Other solvents and chemicals were used as received. 2.2. Preparation of 4-(4-nitrophenylazo)phenol (1) 4-Nitroaniline (10.0 g, 72.4 mmol) was dissolved in methanol (150 mL) and water (30 mL) with 18 mL of conc. hydrochloric acid and the mixture was cooled to 2–5 °C. Sodium nitrite (7.493 g, 108.6 mmol) in water (30 mL) was added dropwise and the mixture was stirred for 1 h. Then phenol (6.813 g, 72.4 mmol) in acetone (100 mL) and water (50 mL) was added and the pH 8–9 was adjusted using sodium hydroxide solution, which was further stirred for another 2 h. Then, dilute hydrochloric acid (ca. 60 mL, 10%) and water (800 mL) were added and the resulting precipitate was collected by filtration. The product, compound 1 was crystallized twice using methanol and ethanol, respectively. A reddish colored solid; yield: 68%. IR (KBr), νmax/cm− 1 3429 (O–H stretch), 3283 (aromatic C–H stretch), 1618, 1585 (aromatic C C stretch), 1458 (N N stretch), 1282 (C–O stretch). δH (500 MHz; CDCl3; Me4Si) 8.46 (dd, 4H, Ar–H), 8.08 (d, 2H, Ar–H), 7.97 (d, 2H, Ar–H), 9.40 (s, 1H, Ar–OH). 2.3. 1-bromo-4-(4-nitrophenylazo)phenoxypropane (2a) Compound 1 (2.0 g, 8.23 mmol) was dissolved in dry acetone (110 mL) followed by potassium carbonate (9 g, 82.2 mmol), potassium iodide (30 mg) as well as 10-fold excess of 1,3-dibromopropane (16.61 g, 82.27 mmol) were added and the mixture was refluxed for 24 h under argon. The reaction mixture was filtered at hot state and the acetone was removed under reduced vacuumed pressure. Then, hexane was added to remove excess 1,3-dibromopropane, the product was insoluble in hexane. The product was collected by filtration and was dissolved in dichloromethane and water (1:1). The organic phase was washed with dilute hydrochloric acid (ca. 50 mL, 10%), sodium carbonate solution (ca. 50 mL, saturated) and water successively. The organic fraction was dried with anhydrous sodium sulfate and the solvent was removed under reduced pressure. The product (compound 2a) was re-crystallized from ethanol. Compound 2a: yellow-reddish colored solid; yield 60%. IR (KBr), νmax/cm− 1 3283 (aromatic C–H stretch), 1618, 1585 (aromatic C C stretch), 1458 (N N stretch), 1282 (C–O stretch). δH (500 MHz; CDCl3; Me4Si): 8.37 (d, 2H, Ar–H), 7.98 (d, 4H, Ar–H), 7.05 (d, 2H, Ar–H), 4.23 (t, 2H, OCH2), 3.64 (t, 2H, CH2Br), 2.36–2.41 (m, 4H, CH2). 2.3.1. 1-bromo-4-(4-nitrophenylazo)phenoxybutane (2b) Compound 1 (2.0 g, 8.23 mmol) was dissolved in dry acetone (130 mL) and potassium carbonate (9.09 g), potassium iodide (30 mg) and a 10-fold excess of 1,4-dibromobutane (17.9 g) were added and the reaction was carried out by the same method used for synthesis of 2a. Work-up procedure was also followed by the same method of 2a. Solid yellow-reddish colored, yield 65%, mp. 90 °C. IR (KBr), νmax/cm−1 3283 (aromatic C–H stretch), 1618, 1585 (aromatic
2.3.2. 1-bromo-4-(4-nitrophenylazo)phenoxypentane (2c) Compound 1 (2.0 g, 8.23 mmol) was dissolved in dry acetone (110 mL) and potassium carbonate (9 g, 8 equivalent of compound 1), potassium iodide (30 mg) and a 10-fold excess of 1, 5-dibromopentane (18.97 g) were added and the reaction was carried out by the same method used for synthesis of 2a. Compound 2c, yield 65% as a solid yellow-reddish colored, mp. 91 °C. IR (KBr) νmax/cm−1 3283 (aromatic C–H stretch), 1618, 1585 (aromatic C C stretch), 1458 (N N stretch), 1282 (C–O stretch)]. δH (500 MHz; CDCl3; Me4Si) 8.37 (d, 2H, Ar–H), 7.97 (d, 4H, Ar–H), 7.02 (d, 2H, Ar–H), 4.09 (t, 2H, OCH2), 3.46 (2H, t, CH2Br), 1.89–2.00 (m, 2H, CH2), 1.85–1.89 (m, 2H, CH2), 1.66–1.70 (m, 2H, CH2). 2.3.3. 1-bromo-4-(4-nitrophenylazo)phenoxyhexane (2d) Compound 1 (2.0 g, 8.23 mmol) was dissolved in dry acetone (100 mL) and potassium carbonate (8 g, 82.2 mmol), potassium iodide (20 mg) and a 10-fold excess of 1,6-dibromohexane (20 g, 82.3 mmol) were added and the reaction was carried out by the same method used for synthesis of 2a. The product, 2d was re-crystallized from ethanol. Yield 60% as a solid. IR (KBr), νmax/cm−1 3283 (aromatic C–H stretch), 1618, 1585 (aromatic C C stretch), 1458 (N N stretch), 1282 (C–O stretch). δH (500 MHz; CDCl3; Me4Si) 8.30 (d, 2H, Ar–H), 7.95 (d, 2H, Ar–H), 7.88 (d, 2H, Ar–H), 7.03 (d, 2H, ArH), 4.05 (t, 2H, OCH2), 3.38 (t, 2H, CH2Br), 1.91–1.94 (m, 4H, CH2), 1.70 (d, 2H,CH2). 2.4. Synthesis of thiolated azo-derivatives (3a–d) Compound 2a (250 mg) was dissolved in dry THF (5 mL). Then a mixture of tetrabutylammonium fluoride (TBAF, 1.3 equiv.) and hexamethyldisilathiane (HMDST, 1 equiv.) in THF was added slowly by stirring. The mixture was stirred for 12 h, poured onto dilute aqueous sodium chloride (2 mL; 1 M), and extracted with dichloromethane (2 × 20 mL). The organic layer was separated and dried over anhydrous sodium sulfate and the solvent was evaporated under vacuum at room temperature, and the product was purified by column chromatography [silica; hexane: ethyl acetate (9:1)] and to yield the thiolated compound 3a, 150 mg, ~ 60%) as a solid. Compounds 3b, 3c and 3d were synthesized from compound 2b, 2c and 2d, respectively by the same method used for synthesis of 3a. 2.5. Synthesis of gold nanoparticles capped with azobenzene moieties (representative compound 4d) Synthesis begins by dissolving of tetraoctylammonium bromide (TOAB) (0.577 mmol) in 30 mL of toluene. In the second step, the aqueous solution of 40 mg of hydrogen tetrachloroaurate trihydrate (HAuCl4·3H2O) in 20 mL deionized water was prepared at room temperature. The organic solution was poured to the aqueous phase and stirred vigorously. Immediately, a two-layer system was formed. The system was kept stirring until the bottom layer (the aqueous phase) became transparent and the top layer (the organic phase) became orange-brown. This assures the transfer of all Au+3 from the aqueous phase into the organic phase. During this reaction step, the phase transfer reagent (TOAB) was transferred all AuCl− 4 from the aqueous phase to organic layer. The organic phase was washed with distilled water (10 mL) several times, and the organic layer is separated. A solution of thiolated azo-compound, 3d (0.0084 mmol) in toluene (5 mL) was added to the above solution, and the resulting mixture was stirred for 30 min. An aqueous solution of freshly prepared sodium borohydride (NaBH4) (40 mg) was then added drop-wise, and the mixture was stirred for another 3 h that resulted in the reduction of AuCl−4
Please cite this article as: T.K. Biswas, et al., Synthesis and characterization of azobenzene-based gold nanoparticles for photo-switching properties, J. Mol. Liq. (2015), http://dx.doi.org/10.1016/j.molliq.2015.12.078
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into gold nanoparticles (Gold-NPs) and the formation of studied thiol capped Gold-NPs, 4d (Scheme 1). The organic layer was washed with water and diluted with methanol (350 mL) and was kept in a refrigerator (−18 °C). Now the obtained precipitate was further purified by being resuspending in toluene and then centrifuging after the addition of methanol. This process was repeated twice to remove any unbound thiolated azo derivatives liquid crystals. This method was applied to prepare the gold decorated azo-based all liquid crystal compounds (4a-c). This method was subsequently modified in different ways, among which a phasetransfer and widely accepted reduction method introduced by Brust and co-workers [25]. 2.6. Instruments Intermediates and final product structures were confirmed by spectroscopic tools. Perkin Elmer (670) spectrometer was used to record the FTIR spectra. 1H NMR (500 MHz), and 13C NMR (125 MHz) spectra were measured with a Bruker (DMX500) spectrometer. The 1H NMR chemical shifts were reported with reference to tetramethylsilane (TMS, 0.00 ppm). The 13C NMR chemical shifts were reported relative to CDCl3 (77.0 ppm). The photo-switching study was performed by recording UV–vis absorption spectra using a UV–visible spectrophotometer obtained from Ocean Optics (HR2000+). For photo-switching studies in solutions, photo-switchable studied azo-compounds were dissolved in chloroform (concentrations C = 1.1 × 10−5 mol L−1).
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Photoisomerization of these compounds were investigated by illuminating with an OMNI-CURE S2000 UV source that was equipped with a 365 nm filter with 5 mW/cm2 intensity with heat filter to avoid heat radiations, arising from the source to the sample. The phase transition temperatures were measured by differential scanning calorimetry (DSC; NETZSCH DSC214 Polyma) with heating and cooling rates were 10 °C/min. Polarized optical microscopy (POM) investigation was conducted with melt-pressed samples sandwiched between two glass slides on an Olympus BX51 polarizing optical microscope equipped with a Linkam heating stage T95 system controller and Olympus DP26 digital camera. The compounds morphology and composition (elements detection) were studied by field emission scanning microscopy (FESEM) and FESEM–EDX (JSM7800F, FESEM, JEOL, USA) respectively. Transmission electron microscopy (TEM) experiment was performed on Hitachi instrument (HT-7700) at an accelerating voltage of 110 kV. 3. Results and discussion 3.1. Materials synthesis The synthetic approach used to prepare the intermediates and target compounds was presented in Scheme 1. All studied compounds were purified on silica gel by column chromatography and are confirmed by FTIR, 1H and 13C NMR. Fig. 1 represents the proton-NMR analysis of
Scheme 1. Synthesis of azobenzene-based gold nanoparticles liquid crystals.
Please cite this article as: T.K. Biswas, et al., Synthesis and characterization of azobenzene-based gold nanoparticles for photo-switching properties, J. Mol. Liq. (2015), http://dx.doi.org/10.1016/j.molliq.2015.12.078
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Fig. 1. H NMR spectra of compound 3d (–SH) in CDCl3.
synthesized thiol containing azo-compound 3d. After completing the synthesis of compound 2a–d (characteristics peak was observed around 3.40–3.68 ppm due to the presence of –CH2Br into the compound; H NMR data are provided as an evidence of the formation of these compounds, 2a–d in the supporting document), we used all these compounds to further produced thiol (–SH) containing compounds 3a–d. The structure of the compound 3d is represented with protonNMR as shown in Fig. 1. 1H NMR (CDCl3) δ: 8.36 (s, 2H, ArH), 7.96 (t, 4H, ArH), 7.02 (s, 2H, ArH), 4.06 (t, 2H, OCH2), 2.73 (t, 2H, CH2S-), 1.85 (m, 4H, CH2CH2), 1.3-1.7 (m, 5H, −(CH2CH2SH). 3.2. Thiol capped gold nanoparticles characterization The NMR was employed to support the attachment and composition of organic coating and evaluate the mesogenic azo based-thiols attached on the surface of gold nanoparticles. In Fig. 1, a proton signal is around 2.73 ppm, which can be assigned to α-protons (CH2SH), disappeared and the broadening of all proton signals was observed (presented in the supporting information). The disappearance of α-proton indicates the formation of a gold–sulfur bonds, and the broadening of the proton signals demonstrates that all –SH containing molecules are covalently linked to the gold surface [26] i.e., there are no parent-SH molecules
remaining in gold nanocomposites; TLC also supported this result [26, 27]. Compared with the sharp peaks of the azo-thiolate compound, the obvious broadening of proton signals and the disappearance of characteristic peak around 2.73 ppm of methylene near mercapto group (i.e., CH2SH) indicated the attachment of the azo-molecules to gold nanoparticles surface as well as the formation of thiolate, and the aromatic proton peaks around 7.01–7.03 and 7.95–7.98 and 8.35–8.37 ppm of azobenzene and methylene signal 4.05 ppm of alkoxy adjacent to phenyl ring (O2NC6H4N=NC6H4–OCH2)further confirmed that the azobenzene mesogenic compounds being tethered on the gold surface successfully [28]. In addition, FTIR analysis results also revealed that the feature of alkoxyazobenzene thiolate coated gold nanomaterials. The main characteristic peaks such as 2920 and 2850 cm−1 of methylene stretching vibrations, 1600 and 1580 cm−1 of typical vibration absorption of benzene ring, and 1245 cm−1 and 1145 cm−1 of the aryl ring–O vibration and phenyl–N stretching mode were clearly observed in the FTIR spectra of gold supported azo molecules. 13C NMR study also revealed that after attachment of the azo-thioled compounds to form –S–Au bond and retaining the aromatic and other alkynes structure of the attached molecules. In Fig. 2, the following chemical shifts were observed for the prepared gold decorated azo-molecules. A similar result was reported to synthesis of this type of compounds by Rahman et al. [29]. δC
Fig. 2. 13C NMR spectra of compound 4d gold attached with azo-derivatives in CDCl3.
Please cite this article as: T.K. Biswas, et al., Synthesis and characterization of azobenzene-based gold nanoparticles for photo-switching properties, J. Mol. Liq. (2015), http://dx.doi.org/10.1016/j.molliq.2015.12.078
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thermal decomposition of gold-thiolated azo products (for example, cleavage of the S–Au bond), but to a phase transition of the O2NC6H4N = NC6H4OCH2-(CH2)4S–moieties at the gold nanoparticle surface. Badia and his group reported thermodynamic behavior of gold nanoparticles covered with various length alkanethiols [30]. Furthermore, the entire studied gold compound with azo-derivatives showed fluidity at the mesophase and optically isotropic state as evidenced by polarizing microscopy. 3.4. Micrographic images of gold decorated azo-derivatives liquid crystal
Fig. 3. POM image of compound 4d (texture taken at 126 °C).
(500 MHz; CDCl3; Me4Si) 25.69, 29.02, 29.06, 29.71, 38.91, 68.30, 114.90, 123.10, 124.72, 125.63, 146.88, 148.35, 156.05, 162.83. 3.3. Mesomorphic properties Polarized optical microscopy (POM) was used to investigate the crystalline or mesomorphic characteristics of the studied compounds. The POM of the compounds was measured at a rate of 10 °C min−1. Mesophases were observed in all the studied compounds. Therefore, these compounds showed the liquid crystalline behavior. Fig. 3 shows the nematic phase of the compound 4d and texture was taken at 126 °C. All observations of the studied compounds showed nematic phase and the mesophase temperatures are decreasing with increasing alkyl groups between Au and azobenzene moieties. In addition, we examined the phase sequence behavior of all the gold-thiolated azo-derivatives by differential scanning calorimetry (DSC; NETZSCH DSC214 Polyma). Fig. 4 shows DSC thermogram of 4d compound. This compound exhibited phase transition behavior on heating and cooling and showed a mesophase. When heated, only one peak was observed while cooling two peaks were observed, this means that the compound showed monotropic nature. The phase sequences are C 129 I and I 127N 95 I. The thermal behavior of gold supported azo-based liquid crystal was quite different from that of bulk SH containing azo-derivatives. The phase transition temperatures of composite compounds were high and the mesomorphic phase temperature regions were wide when compared with those of without gold attachment means that bulk SH compounds. Since this phase transition behavior occurred reversibly, we assigned these exo-and endo-thermic responses as not due to
Fig. 4. DSC thermograms for gold-thiol capped azo-derivatives (compound 4d). Abbreviation, C = crystal, N = nematic phase and I = isotropic phase.
Fig. 5 a–b shows the image of azo-based liquid crystal decorated gold surfaces (compound 4d), obtained by field emission electron microscopy (FESEM). In Fig. 5 a the domains of gold particle-azo compounds (bright spots) observed which are distributed homogeneously. Elemental analysis (EDX result): EDX results, shown in Fig. 5 a,b confirm that the synthesized nanoparticles are gold. The spectrum also shows a C, N, S, O and Au peaks which indicate that azo-containing synthesized liquid crystal [O2NC6H4N = NC6H4OCH2-(CH2)4S-Au provides the picture only] remains attached to the gold nanoparticles. It is clearly displayed that the prepared gold decorated liquid crystal nanomaterials contained only C, N, O, S and Au elements with the 55.68, 9.85, 16.00, 5.74 and 12.73 wt.% respectively, which is presented in Fig. 5 a,b. No other peak related with any impurity has been detected in the EDX, which confirms that the nanoparticles are composed only with C, N, O, S and Au. A similar result has been reported for Cu nanoparticles [31], gold nanoparticles [32] and other nanocrystals [33]. To further confirm the attached of liquid crystals with gold nanoparticles and size of the nanoparticles, TEM experiments were conducted. TEM photograph of the nanoparticles shows (Fig. 6) that they have
Fig. 5. a: FESEM image of compound 4d (Au-azo derivative). b: EDX spectra for chemical elements detection of compound 4d (Au decorated compound).
Please cite this article as: T.K. Biswas, et al., Synthesis and characterization of azobenzene-based gold nanoparticles for photo-switching properties, J. Mol. Liq. (2015), http://dx.doi.org/10.1016/j.molliq.2015.12.078
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Fig. 6. TEM picture of the thiol derivatised Gold nanoparticles (compound 4d) at low magnification.
diameters in the range of 1.4–2.5 nm and a maximum in the particle size distribution at 2.0–2.5 nm. 3.5. Photo-switching properties The photoisomerization of the studied azo-based compounds were measured in chloroform solutions with UV light illumination of intensity 5 mW/cm2. UV light of a wavelength of 365 nm passing through a heat filter (to avoid heat radiation) was directed onto the samples for different time intervals until they reached the photosaturation. The absorption peak between 300 and 400 nm is due to the π–π* transition, and the absorption peak at 450–500 nm is due to the n–π* transition. The photo saturation of the compound 4a was observed at 44 s (Fig. 7 a) whereas, the thermal back relaxation was observed at 82 min at 376.64 nm (Fig.7 a,b). Photosaturation time of compounds 4b, 4c and 4d (Fig.7 a,c, at 457.57 nm) were almost 44 s, whereas, the back relaxations were found at 91 min, 100 min and 125 min (Fig. 7 a,d, compound 4d,) respectively. Photoisomerisation of compounds 4b and 4c are presented in supporting information. Due to E/Z photo-isomerization, the absorbance decreases with time which
indicates to the transformation of trans isomer to cis isomer (E/Z). The reverse transformation from Z to E was carried out by keeping the solution in the dark (thermal back relaxation). From the observation all the synthesized compounds showed almost similar absorption spectra for photosaturation (44 s) whereas, the thermal back relaxation times are different although their structures are similar and only difference is spacer chain length (n = 3 to 6). Spacer chain lengths are responsible for free rotation, flexibility and elasticity. The synthesized compounds showed different intervals of thermal back relaxation due to the variation of spacers, hence the optical activity of the compounds was changed significantly [34,35]. The clear differences of the thermal back relaxation for compounds 4a, 4b, 4c and 4d with time duration are seen. The possible fact for this behavior must be the effect of spacers. Compounds having long chain aliphatic spacers, (–CH2–)6 exhibited longer time thermal back relaxation when compared to shorter chain aliphatic chain spacer. The aliphatic spacers affect the free movement of the molecules to a significant extent. By forming a coiled geometry, the flexibility of the molecules prevents the system going back easily. Similar effects were observed for long chain alkenes [36] and aliphatic/ aromatic spacers based azo dye dimers earlier [35]. Furthermore, the kinetics of the thermal Z/E and E/Z (trans-to-cis and contrarily) isomerisation process for azo-derivatives, 4a–4d were analyzed by conventional UV–vis spectroscopy. We calculated the first order plot for the photo isomerization behavior for all the studied compounds according to Eq. (1) at 25 °C [37]. ln ðA∞ At Þ ¼ kt A∞ A0
ð1Þ
where, A∞ At, and A0 are the absorbance at infinite time, time t and time zero at 377.57 nm, respectively. Rate constants were calculated for the Z-E isomerization of 4.7 × 10− 2, 1.47 × 10− 2, 5.57 × 10−2 and 3.60 × 10−2 for compounds 4a–d, respectively. Using the above equation a typical first order plot was observed for all the studied four compounds at room temperature (Fig. 8 a–b). It is evident that throughout the photosaturation (UN on) and relaxation curve followed a typical
Fig. 7. a: E–Z Photoisomerism for compound 4a (trans to cis). b: E–Z Photoisomerism for compound 4a (cis to trans). c: E–Z Photoisomerism for compound 4d (trans to cis). d: E–Z Photoisomerism for compound 4d (cis to trans).
Please cite this article as: T.K. Biswas, et al., Synthesis and characterization of azobenzene-based gold nanoparticles for photo-switching properties, J. Mol. Liq. (2015), http://dx.doi.org/10.1016/j.molliq.2015.12.078
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long chain aliphatic spacers like (–CH2–)6 containing compounds needed longer time thermal back relaxation compared to shorter chain aliphatic chain spacer supported with gold surface. The aliphatic spacers affect the free movement of the molecules to a significant extent which may be of significance for promising applications in photochromism (photo storage) and photoresponsive molecular sensing devices. Moreover, this result could be useful for the design of functional hybrid nanoparticles. Acknowledgments This work was supported by FRGS (RDU130121) grant obtained from Ministry of Education, Malaysia (No. 130121). We appreciate Mr. A.R. Yuvaraj for his valuable assistance in the UV–vis measurements. Appendix A. Supplementary data Supplementary data to this article can be found online at http://dx. doi.org/10.1016/j.molliq.2015.12.078. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] Fig 8. a: First order plots for the studied compounds (4a–4d) for trans-cis (E/Z) photosaturation at 25 °C. b: First order plots for the studied compounds (4a–4d) for cistrans (Z/E) thermal isomerization at 25 °C.
first order kinetic. All four compounds, irrespective of their different flexible spacers (–CH2–) present, behaved in the similar way at studied temperature. The reversible photoisomerization of the gold decorated azo derivatives nanomaterials may provide promising photo switching devices and molecular sensor applications. 4. Conclusion In this paper, four new alkoxy azobenzene mesogenic thiols with different length alkyene spacer have been synthesized and employed as capping compounds to prepare gold nanoparticles liquid crystals. All the studied compounds showed nematic phases through polarizing optical microscopy. The photostwiching properties of the prepared thiol capped gold nanomaterials have been investigated by UV–vis spectroscopy and achieved trans to cis isomerization almost 44 s. However, the reverse process took place around 1–2 h in solution. In addition, it is very interesting that the presence of spacer in the molecules plays an important role over the thermal back relaxation in photoisomerization. From the observation we can conclude that having
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Synthesis and characterization of azobenzene-based gold nanoparticles for photo-switching properties Tapan Kumar Biswas a,b,⁎, Shaheen M. Sarkar a, Mashitah M. Yusoff a, Md Lutfor Rahman a,⁎ a b
Faculty of Industrial Sciences and Technology, Universiti Malaysia Pahang, 26300, Gambang, Kuantan, Pahang, Malaysia Department of Chemistry, University of Rajshahi, Rajshahi 6205, Bangladesh
a r t i c l e
i n f o
Article history: Received 19 September 2015 Received in revised form 21 December 2015 Accepted 22 December 2015 Available online xxxx Keywords: Photoisomerisation Gold nanoparticles Cis–trans isomerization Photoswitching
a b s t r a c t A series of new azobenzene based thiolated liquid crystals modified with gold nanoparticles were synthesized and characterized using a different mode of delegated tools e.g. FTIR, NMR and FESEM–EDX measurements for the structural properties of synthesized compounds. Polarized optical microscopy studies have revealed all the studied compounds having liquid crystalline properties, as a typical nematic phase. These liquid crystal capped gold nanoparticles' size was determined by TEM experiment. In addition, azobenzene-based gold nanoparticles containing flexible spacers showed photochromic behavior upon UV irradiation. These molecules exhibited strong photoisomerisation behavior in solutions and their trans to cis isomerisation took about 44 s whereas the reverse process almost took place ranging from 82 to 125 min. © 2015 Elsevier B.V. All rights reserved.
1. Introduction Photoswitchable molecules attached to solid surfaces is of a substantial interest for the preparation of advanced nano-systems, leading to a variety of applications, such as information storage, molecular machines, and sensors [1–2]. A photoswitchable molecule can be converted from one form to another with light of one wavelength and can either revert thermally to the original state or can be reverted by irradiation with light of a different wavelength. In recent years, the photo-responsive molecules on metal nanoparticle surface has received significant attention due to the opportunity of using switching devices as photo-responsive components in optical storage, switching, photoswitchable surface wettability and molecular recognition applications [3–16]. Moreover, the hybrid or composite nanomaterials consisting of inorganic nanoparticle and photoactive organic molecules may provide light-controlled nano-devices [17–21]. For optical switching applications, one of the most used organic chromophores is mostly azobenzene or its derivatives. Generally, ‘azobenzene’ refers only to the parent molecule, though the term is now repeatedly used to refer to the entire class of substituted azo molecules. The unique commonality among the azobenzene molecules (azo's) is the clean and efficient photochemical isomerization that can occur about the azo linkage when the chromophore absorbs a photon. Azobenzene, a photochromic T-type system, exhibits a reversible isomerisation process between its trans and cis ⁎ Corresponding authors. E-mail addresses: [email protected] (T.K. Biswas), [email protected] (M.L. Rahman).
isomers of different stability [22]. In addition, azobenzene containing compounds have two geometric isomers (Z/E) around the N N double bond; the trans isomer (E) is more stable than the cis isomer (Z). In this process, without any bond breaking, the photoreaction occurs due to the simple rearrangement of the electronic and nuclear structure of the molecule. The photo isomerism process (cis to trans) can be carried out either by heating or by irradiation with visible light [23]. The energetically more stable trans configuration will turn into the cis configuration when UV light of wavelength 365 nm shines on azobenzene systems and reversible to the original configuration is brought about either by keeping it in the dark (terms as called thermal back relaxation) or by illuminating with white light of higher wavelength (450 nm). According to the literature, gold nanoparticles derived using as biological molecules investigation such as DNA, protein have gained much interest in liquid crystal research as biosensor [24]. The present study focuses on the synthesis and photo-isomerization behavior of four new gold nanoparticles (Gold-NPs) decorated azobased liquid crystals having azobenzene chromophores connected via a flexible–CH2–spacer. In this paper, substituted azo-derivatives capping thiol groups were employed for the preparation of decorated Gold-NPs and the photoisomerization performance of their solution phase have been investigated by UV–vis absorption spectroscopy. Interestingly, thermal back relaxation of the studied compounds were increased with respect to the flexible spacer size (–CH2–) increased. The morphology and photo-responsive properties of the gold nanoparticles modified azobenzene derivatives having different flexible spacers were characterized via FESEM and UV/vis spectroscopy. In this article, we report a preparation of the gold nanoparticles cover with a layer of
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Please cite this article as: T.K. Biswas, et al., Synthesis and characterization of azobenzene-based gold nanoparticles for photo-switching properties, J. Mol. Liq. (2015), http://dx.doi.org/10.1016/j.molliq.2015.12.078
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photoresponsive azo-based liquid crystals by Au–S covalent bonds and its liquid crystal properties and photo responsive behavior were discussed.
C C stretch), 1458 (N N stretch), 1282 (C–O stretch). δH (500 MHz; CDCl3; Me4Si) 8.37 (d, 2H, Ar–H), 7.98 (d, 4H, Ar–H), 7.03 (d, 2H, Ar– H), 4.12 (t, 2H, OCH2), 3.52 (t, 2H, CH2Br), 2.08–2.13 (m, 2H, CH2), 2.00–2.03 (m, 2H, CH2).
2. Experimental details 2.1. Materials 4-Nitro aniline (Fluka), sodium nitrite (Fluka), phenol (Merck), 1,3- dibromopropane (Fluka), 1,4-dibromobutane (Fluka), 1,5dibromopentane (Fluka), 1,6-dibromohexane (Fluka), potassium carbonate (Aldrich), Gold(III) chloride hydrate (Aldrich), tetrabutylammonium fluoride (Merck), hexamethyldisilathiane (Merck), tetraoctylammonium bromide, silica gel-60 (Merck) were used as received. Phosphorus pentaoxide (Merck) was used to reflux the acetone and distilled out before used. Other solvents and chemicals were used as received. 2.2. Preparation of 4-(4-nitrophenylazo)phenol (1) 4-Nitroaniline (10.0 g, 72.4 mmol) was dissolved in methanol (150 mL) and water (30 mL) with 18 mL of conc. hydrochloric acid and the mixture was cooled to 2–5 °C. Sodium nitrite (7.493 g, 108.6 mmol) in water (30 mL) was added dropwise and the mixture was stirred for 1 h. Then phenol (6.813 g, 72.4 mmol) in acetone (100 mL) and water (50 mL) was added and the pH 8–9 was adjusted using sodium hydroxide solution, which was further stirred for another 2 h. Then, dilute hydrochloric acid (ca. 60 mL, 10%) and water (800 mL) were added and the resulting precipitate was collected by filtration. The product, compound 1 was crystallized twice using methanol and ethanol, respectively. A reddish colored solid; yield: 68%. IR (KBr), νmax/cm− 1 3429 (O–H stretch), 3283 (aromatic C–H stretch), 1618, 1585 (aromatic C C stretch), 1458 (N N stretch), 1282 (C–O stretch). δH (500 MHz; CDCl3; Me4Si) 8.46 (dd, 4H, Ar–H), 8.08 (d, 2H, Ar–H), 7.97 (d, 2H, Ar–H), 9.40 (s, 1H, Ar–OH). 2.3. 1-bromo-4-(4-nitrophenylazo)phenoxypropane (2a) Compound 1 (2.0 g, 8.23 mmol) was dissolved in dry acetone (110 mL) followed by potassium carbonate (9 g, 82.2 mmol), potassium iodide (30 mg) as well as 10-fold excess of 1,3-dibromopropane (16.61 g, 82.27 mmol) were added and the mixture was refluxed for 24 h under argon. The reaction mixture was filtered at hot state and the acetone was removed under reduced vacuumed pressure. Then, hexane was added to remove excess 1,3-dibromopropane, the product was insoluble in hexane. The product was collected by filtration and was dissolved in dichloromethane and water (1:1). The organic phase was washed with dilute hydrochloric acid (ca. 50 mL, 10%), sodium carbonate solution (ca. 50 mL, saturated) and water successively. The organic fraction was dried with anhydrous sodium sulfate and the solvent was removed under reduced pressure. The product (compound 2a) was re-crystallized from ethanol. Compound 2a: yellow-reddish colored solid; yield 60%. IR (KBr), νmax/cm− 1 3283 (aromatic C–H stretch), 1618, 1585 (aromatic C C stretch), 1458 (N N stretch), 1282 (C–O stretch). δH (500 MHz; CDCl3; Me4Si): 8.37 (d, 2H, Ar–H), 7.98 (d, 4H, Ar–H), 7.05 (d, 2H, Ar–H), 4.23 (t, 2H, OCH2), 3.64 (t, 2H, CH2Br), 2.36–2.41 (m, 4H, CH2). 2.3.1. 1-bromo-4-(4-nitrophenylazo)phenoxybutane (2b) Compound 1 (2.0 g, 8.23 mmol) was dissolved in dry acetone (130 mL) and potassium carbonate (9.09 g), potassium iodide (30 mg) and a 10-fold excess of 1,4-dibromobutane (17.9 g) were added and the reaction was carried out by the same method used for synthesis of 2a. Work-up procedure was also followed by the same method of 2a. Solid yellow-reddish colored, yield 65%, mp. 90 °C. IR (KBr), νmax/cm−1 3283 (aromatic C–H stretch), 1618, 1585 (aromatic
2.3.2. 1-bromo-4-(4-nitrophenylazo)phenoxypentane (2c) Compound 1 (2.0 g, 8.23 mmol) was dissolved in dry acetone (110 mL) and potassium carbonate (9 g, 8 equivalent of compound 1), potassium iodide (30 mg) and a 10-fold excess of 1, 5-dibromopentane (18.97 g) were added and the reaction was carried out by the same method used for synthesis of 2a. Compound 2c, yield 65% as a solid yellow-reddish colored, mp. 91 °C. IR (KBr) νmax/cm−1 3283 (aromatic C–H stretch), 1618, 1585 (aromatic C C stretch), 1458 (N N stretch), 1282 (C–O stretch)]. δH (500 MHz; CDCl3; Me4Si) 8.37 (d, 2H, Ar–H), 7.97 (d, 4H, Ar–H), 7.02 (d, 2H, Ar–H), 4.09 (t, 2H, OCH2), 3.46 (2H, t, CH2Br), 1.89–2.00 (m, 2H, CH2), 1.85–1.89 (m, 2H, CH2), 1.66–1.70 (m, 2H, CH2). 2.3.3. 1-bromo-4-(4-nitrophenylazo)phenoxyhexane (2d) Compound 1 (2.0 g, 8.23 mmol) was dissolved in dry acetone (100 mL) and potassium carbonate (8 g, 82.2 mmol), potassium iodide (20 mg) and a 10-fold excess of 1,6-dibromohexane (20 g, 82.3 mmol) were added and the reaction was carried out by the same method used for synthesis of 2a. The product, 2d was re-crystallized from ethanol. Yield 60% as a solid. IR (KBr), νmax/cm−1 3283 (aromatic C–H stretch), 1618, 1585 (aromatic C C stretch), 1458 (N N stretch), 1282 (C–O stretch). δH (500 MHz; CDCl3; Me4Si) 8.30 (d, 2H, Ar–H), 7.95 (d, 2H, Ar–H), 7.88 (d, 2H, Ar–H), 7.03 (d, 2H, ArH), 4.05 (t, 2H, OCH2), 3.38 (t, 2H, CH2Br), 1.91–1.94 (m, 4H, CH2), 1.70 (d, 2H,CH2). 2.4. Synthesis of thiolated azo-derivatives (3a–d) Compound 2a (250 mg) was dissolved in dry THF (5 mL). Then a mixture of tetrabutylammonium fluoride (TBAF, 1.3 equiv.) and hexamethyldisilathiane (HMDST, 1 equiv.) in THF was added slowly by stirring. The mixture was stirred for 12 h, poured onto dilute aqueous sodium chloride (2 mL; 1 M), and extracted with dichloromethane (2 × 20 mL). The organic layer was separated and dried over anhydrous sodium sulfate and the solvent was evaporated under vacuum at room temperature, and the product was purified by column chromatography [silica; hexane: ethyl acetate (9:1)] and to yield the thiolated compound 3a, 150 mg, ~ 60%) as a solid. Compounds 3b, 3c and 3d were synthesized from compound 2b, 2c and 2d, respectively by the same method used for synthesis of 3a. 2.5. Synthesis of gold nanoparticles capped with azobenzene moieties (representative compound 4d) Synthesis begins by dissolving of tetraoctylammonium bromide (TOAB) (0.577 mmol) in 30 mL of toluene. In the second step, the aqueous solution of 40 mg of hydrogen tetrachloroaurate trihydrate (HAuCl4·3H2O) in 20 mL deionized water was prepared at room temperature. The organic solution was poured to the aqueous phase and stirred vigorously. Immediately, a two-layer system was formed. The system was kept stirring until the bottom layer (the aqueous phase) became transparent and the top layer (the organic phase) became orange-brown. This assures the transfer of all Au+3 from the aqueous phase into the organic phase. During this reaction step, the phase transfer reagent (TOAB) was transferred all AuCl− 4 from the aqueous phase to organic layer. The organic phase was washed with distilled water (10 mL) several times, and the organic layer is separated. A solution of thiolated azo-compound, 3d (0.0084 mmol) in toluene (5 mL) was added to the above solution, and the resulting mixture was stirred for 30 min. An aqueous solution of freshly prepared sodium borohydride (NaBH4) (40 mg) was then added drop-wise, and the mixture was stirred for another 3 h that resulted in the reduction of AuCl−4
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into gold nanoparticles (Gold-NPs) and the formation of studied thiol capped Gold-NPs, 4d (Scheme 1). The organic layer was washed with water and diluted with methanol (350 mL) and was kept in a refrigerator (−18 °C). Now the obtained precipitate was further purified by being resuspending in toluene and then centrifuging after the addition of methanol. This process was repeated twice to remove any unbound thiolated azo derivatives liquid crystals. This method was applied to prepare the gold decorated azo-based all liquid crystal compounds (4a-c). This method was subsequently modified in different ways, among which a phasetransfer and widely accepted reduction method introduced by Brust and co-workers [25]. 2.6. Instruments Intermediates and final product structures were confirmed by spectroscopic tools. Perkin Elmer (670) spectrometer was used to record the FTIR spectra. 1H NMR (500 MHz), and 13C NMR (125 MHz) spectra were measured with a Bruker (DMX500) spectrometer. The 1H NMR chemical shifts were reported with reference to tetramethylsilane (TMS, 0.00 ppm). The 13C NMR chemical shifts were reported relative to CDCl3 (77.0 ppm). The photo-switching study was performed by recording UV–vis absorption spectra using a UV–visible spectrophotometer obtained from Ocean Optics (HR2000+). For photo-switching studies in solutions, photo-switchable studied azo-compounds were dissolved in chloroform (concentrations C = 1.1 × 10−5 mol L−1).
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Photoisomerization of these compounds were investigated by illuminating with an OMNI-CURE S2000 UV source that was equipped with a 365 nm filter with 5 mW/cm2 intensity with heat filter to avoid heat radiations, arising from the source to the sample. The phase transition temperatures were measured by differential scanning calorimetry (DSC; NETZSCH DSC214 Polyma) with heating and cooling rates were 10 °C/min. Polarized optical microscopy (POM) investigation was conducted with melt-pressed samples sandwiched between two glass slides on an Olympus BX51 polarizing optical microscope equipped with a Linkam heating stage T95 system controller and Olympus DP26 digital camera. The compounds morphology and composition (elements detection) were studied by field emission scanning microscopy (FESEM) and FESEM–EDX (JSM7800F, FESEM, JEOL, USA) respectively. Transmission electron microscopy (TEM) experiment was performed on Hitachi instrument (HT-7700) at an accelerating voltage of 110 kV. 3. Results and discussion 3.1. Materials synthesis The synthetic approach used to prepare the intermediates and target compounds was presented in Scheme 1. All studied compounds were purified on silica gel by column chromatography and are confirmed by FTIR, 1H and 13C NMR. Fig. 1 represents the proton-NMR analysis of
Scheme 1. Synthesis of azobenzene-based gold nanoparticles liquid crystals.
Please cite this article as: T.K. Biswas, et al., Synthesis and characterization of azobenzene-based gold nanoparticles for photo-switching properties, J. Mol. Liq. (2015), http://dx.doi.org/10.1016/j.molliq.2015.12.078
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Fig. 1. H NMR spectra of compound 3d (–SH) in CDCl3.
synthesized thiol containing azo-compound 3d. After completing the synthesis of compound 2a–d (characteristics peak was observed around 3.40–3.68 ppm due to the presence of –CH2Br into the compound; H NMR data are provided as an evidence of the formation of these compounds, 2a–d in the supporting document), we used all these compounds to further produced thiol (–SH) containing compounds 3a–d. The structure of the compound 3d is represented with protonNMR as shown in Fig. 1. 1H NMR (CDCl3) δ: 8.36 (s, 2H, ArH), 7.96 (t, 4H, ArH), 7.02 (s, 2H, ArH), 4.06 (t, 2H, OCH2), 2.73 (t, 2H, CH2S-), 1.85 (m, 4H, CH2CH2), 1.3-1.7 (m, 5H, −(CH2CH2SH). 3.2. Thiol capped gold nanoparticles characterization The NMR was employed to support the attachment and composition of organic coating and evaluate the mesogenic azo based-thiols attached on the surface of gold nanoparticles. In Fig. 1, a proton signal is around 2.73 ppm, which can be assigned to α-protons (CH2SH), disappeared and the broadening of all proton signals was observed (presented in the supporting information). The disappearance of α-proton indicates the formation of a gold–sulfur bonds, and the broadening of the proton signals demonstrates that all –SH containing molecules are covalently linked to the gold surface [26] i.e., there are no parent-SH molecules
remaining in gold nanocomposites; TLC also supported this result [26, 27]. Compared with the sharp peaks of the azo-thiolate compound, the obvious broadening of proton signals and the disappearance of characteristic peak around 2.73 ppm of methylene near mercapto group (i.e., CH2SH) indicated the attachment of the azo-molecules to gold nanoparticles surface as well as the formation of thiolate, and the aromatic proton peaks around 7.01–7.03 and 7.95–7.98 and 8.35–8.37 ppm of azobenzene and methylene signal 4.05 ppm of alkoxy adjacent to phenyl ring (O2NC6H4N=NC6H4–OCH2)further confirmed that the azobenzene mesogenic compounds being tethered on the gold surface successfully [28]. In addition, FTIR analysis results also revealed that the feature of alkoxyazobenzene thiolate coated gold nanomaterials. The main characteristic peaks such as 2920 and 2850 cm−1 of methylene stretching vibrations, 1600 and 1580 cm−1 of typical vibration absorption of benzene ring, and 1245 cm−1 and 1145 cm−1 of the aryl ring–O vibration and phenyl–N stretching mode were clearly observed in the FTIR spectra of gold supported azo molecules. 13C NMR study also revealed that after attachment of the azo-thioled compounds to form –S–Au bond and retaining the aromatic and other alkynes structure of the attached molecules. In Fig. 2, the following chemical shifts were observed for the prepared gold decorated azo-molecules. A similar result was reported to synthesis of this type of compounds by Rahman et al. [29]. δC
Fig. 2. 13C NMR spectra of compound 4d gold attached with azo-derivatives in CDCl3.
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thermal decomposition of gold-thiolated azo products (for example, cleavage of the S–Au bond), but to a phase transition of the O2NC6H4N = NC6H4OCH2-(CH2)4S–moieties at the gold nanoparticle surface. Badia and his group reported thermodynamic behavior of gold nanoparticles covered with various length alkanethiols [30]. Furthermore, the entire studied gold compound with azo-derivatives showed fluidity at the mesophase and optically isotropic state as evidenced by polarizing microscopy. 3.4. Micrographic images of gold decorated azo-derivatives liquid crystal
Fig. 3. POM image of compound 4d (texture taken at 126 °C).
(500 MHz; CDCl3; Me4Si) 25.69, 29.02, 29.06, 29.71, 38.91, 68.30, 114.90, 123.10, 124.72, 125.63, 146.88, 148.35, 156.05, 162.83. 3.3. Mesomorphic properties Polarized optical microscopy (POM) was used to investigate the crystalline or mesomorphic characteristics of the studied compounds. The POM of the compounds was measured at a rate of 10 °C min−1. Mesophases were observed in all the studied compounds. Therefore, these compounds showed the liquid crystalline behavior. Fig. 3 shows the nematic phase of the compound 4d and texture was taken at 126 °C. All observations of the studied compounds showed nematic phase and the mesophase temperatures are decreasing with increasing alkyl groups between Au and azobenzene moieties. In addition, we examined the phase sequence behavior of all the gold-thiolated azo-derivatives by differential scanning calorimetry (DSC; NETZSCH DSC214 Polyma). Fig. 4 shows DSC thermogram of 4d compound. This compound exhibited phase transition behavior on heating and cooling and showed a mesophase. When heated, only one peak was observed while cooling two peaks were observed, this means that the compound showed monotropic nature. The phase sequences are C 129 I and I 127N 95 I. The thermal behavior of gold supported azo-based liquid crystal was quite different from that of bulk SH containing azo-derivatives. The phase transition temperatures of composite compounds were high and the mesomorphic phase temperature regions were wide when compared with those of without gold attachment means that bulk SH compounds. Since this phase transition behavior occurred reversibly, we assigned these exo-and endo-thermic responses as not due to
Fig. 4. DSC thermograms for gold-thiol capped azo-derivatives (compound 4d). Abbreviation, C = crystal, N = nematic phase and I = isotropic phase.
Fig. 5 a–b shows the image of azo-based liquid crystal decorated gold surfaces (compound 4d), obtained by field emission electron microscopy (FESEM). In Fig. 5 a the domains of gold particle-azo compounds (bright spots) observed which are distributed homogeneously. Elemental analysis (EDX result): EDX results, shown in Fig. 5 a,b confirm that the synthesized nanoparticles are gold. The spectrum also shows a C, N, S, O and Au peaks which indicate that azo-containing synthesized liquid crystal [O2NC6H4N = NC6H4OCH2-(CH2)4S-Au provides the picture only] remains attached to the gold nanoparticles. It is clearly displayed that the prepared gold decorated liquid crystal nanomaterials contained only C, N, O, S and Au elements with the 55.68, 9.85, 16.00, 5.74 and 12.73 wt.% respectively, which is presented in Fig. 5 a,b. No other peak related with any impurity has been detected in the EDX, which confirms that the nanoparticles are composed only with C, N, O, S and Au. A similar result has been reported for Cu nanoparticles [31], gold nanoparticles [32] and other nanocrystals [33]. To further confirm the attached of liquid crystals with gold nanoparticles and size of the nanoparticles, TEM experiments were conducted. TEM photograph of the nanoparticles shows (Fig. 6) that they have
Fig. 5. a: FESEM image of compound 4d (Au-azo derivative). b: EDX spectra for chemical elements detection of compound 4d (Au decorated compound).
Please cite this article as: T.K. Biswas, et al., Synthesis and characterization of azobenzene-based gold nanoparticles for photo-switching properties, J. Mol. Liq. (2015), http://dx.doi.org/10.1016/j.molliq.2015.12.078
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T.K. Biswas et al. / Journal of Molecular Liquids xxx (2015) xxx–xxx
Fig. 6. TEM picture of the thiol derivatised Gold nanoparticles (compound 4d) at low magnification.
diameters in the range of 1.4–2.5 nm and a maximum in the particle size distribution at 2.0–2.5 nm. 3.5. Photo-switching properties The photoisomerization of the studied azo-based compounds were measured in chloroform solutions with UV light illumination of intensity 5 mW/cm2. UV light of a wavelength of 365 nm passing through a heat filter (to avoid heat radiation) was directed onto the samples for different time intervals until they reached the photosaturation. The absorption peak between 300 and 400 nm is due to the π–π* transition, and the absorption peak at 450–500 nm is due to the n–π* transition. The photo saturation of the compound 4a was observed at 44 s (Fig. 7 a) whereas, the thermal back relaxation was observed at 82 min at 376.64 nm (Fig.7 a,b). Photosaturation time of compounds 4b, 4c and 4d (Fig.7 a,c, at 457.57 nm) were almost 44 s, whereas, the back relaxations were found at 91 min, 100 min and 125 min (Fig. 7 a,d, compound 4d,) respectively. Photoisomerisation of compounds 4b and 4c are presented in supporting information. Due to E/Z photo-isomerization, the absorbance decreases with time which
indicates to the transformation of trans isomer to cis isomer (E/Z). The reverse transformation from Z to E was carried out by keeping the solution in the dark (thermal back relaxation). From the observation all the synthesized compounds showed almost similar absorption spectra for photosaturation (44 s) whereas, the thermal back relaxation times are different although their structures are similar and only difference is spacer chain length (n = 3 to 6). Spacer chain lengths are responsible for free rotation, flexibility and elasticity. The synthesized compounds showed different intervals of thermal back relaxation due to the variation of spacers, hence the optical activity of the compounds was changed significantly [34,35]. The clear differences of the thermal back relaxation for compounds 4a, 4b, 4c and 4d with time duration are seen. The possible fact for this behavior must be the effect of spacers. Compounds having long chain aliphatic spacers, (–CH2–)6 exhibited longer time thermal back relaxation when compared to shorter chain aliphatic chain spacer. The aliphatic spacers affect the free movement of the molecules to a significant extent. By forming a coiled geometry, the flexibility of the molecules prevents the system going back easily. Similar effects were observed for long chain alkenes [36] and aliphatic/ aromatic spacers based azo dye dimers earlier [35]. Furthermore, the kinetics of the thermal Z/E and E/Z (trans-to-cis and contrarily) isomerisation process for azo-derivatives, 4a–4d were analyzed by conventional UV–vis spectroscopy. We calculated the first order plot for the photo isomerization behavior for all the studied compounds according to Eq. (1) at 25 °C [37]. ln ðA∞ At Þ ¼ kt A∞ A0
ð1Þ
where, A∞ At, and A0 are the absorbance at infinite time, time t and time zero at 377.57 nm, respectively. Rate constants were calculated for the Z-E isomerization of 4.7 × 10− 2, 1.47 × 10− 2, 5.57 × 10−2 and 3.60 × 10−2 for compounds 4a–d, respectively. Using the above equation a typical first order plot was observed for all the studied four compounds at room temperature (Fig. 8 a–b). It is evident that throughout the photosaturation (UN on) and relaxation curve followed a typical
Fig. 7. a: E–Z Photoisomerism for compound 4a (trans to cis). b: E–Z Photoisomerism for compound 4a (cis to trans). c: E–Z Photoisomerism for compound 4d (trans to cis). d: E–Z Photoisomerism for compound 4d (cis to trans).
Please cite this article as: T.K. Biswas, et al., Synthesis and characterization of azobenzene-based gold nanoparticles for photo-switching properties, J. Mol. Liq. (2015), http://dx.doi.org/10.1016/j.molliq.2015.12.078
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long chain aliphatic spacers like (–CH2–)6 containing compounds needed longer time thermal back relaxation compared to shorter chain aliphatic chain spacer supported with gold surface. The aliphatic spacers affect the free movement of the molecules to a significant extent which may be of significance for promising applications in photochromism (photo storage) and photoresponsive molecular sensing devices. Moreover, this result could be useful for the design of functional hybrid nanoparticles. Acknowledgments This work was supported by FRGS (RDU130121) grant obtained from Ministry of Education, Malaysia (No. 130121). We appreciate Mr. A.R. Yuvaraj for his valuable assistance in the UV–vis measurements. Appendix A. Supplementary data Supplementary data to this article can be found online at http://dx. doi.org/10.1016/j.molliq.2015.12.078. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] Fig 8. a: First order plots for the studied compounds (4a–4d) for trans-cis (E/Z) photosaturation at 25 °C. b: First order plots for the studied compounds (4a–4d) for cistrans (Z/E) thermal isomerization at 25 °C.
first order kinetic. All four compounds, irrespective of their different flexible spacers (–CH2–) present, behaved in the similar way at studied temperature. The reversible photoisomerization of the gold decorated azo derivatives nanomaterials may provide promising photo switching devices and molecular sensor applications. 4. Conclusion In this paper, four new alkoxy azobenzene mesogenic thiols with different length alkyene spacer have been synthesized and employed as capping compounds to prepare gold nanoparticles liquid crystals. All the studied compounds showed nematic phases through polarizing optical microscopy. The photostwiching properties of the prepared thiol capped gold nanomaterials have been investigated by UV–vis spectroscopy and achieved trans to cis isomerization almost 44 s. However, the reverse process took place around 1–2 h in solution. In addition, it is very interesting that the presence of spacer in the molecules plays an important role over the thermal back relaxation in photoisomerization. From the observation we can conclude that having
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Please cite this article as: T.K. Biswas, et al., Synthesis and characterization of azobenzene-based gold nanoparticles for photo-switching properties, J. Mol. Liq. (2015), http://dx.doi.org/10.1016/j.molliq.2015.12.078