Synthesis and characterization of novel azobenzene-based mesogens and their organization at the air–water and air–solid interfaces

Abstract

Eight new oligomeric mesogens are reported, consisting of an azobenzene-based core attached to which are four 4-cyanobiphenyl units via flexible alkyl spacers (n = 5–12). Their chemical structures were determined by ¹H NMR, ¹³C NMR, IR, UV-Vis and Raman spectroscopy, and elemental analysis. The thermotropic liquid crystalline properties of these materials were investigated by POM, DSC and XRD. The oligomers containing n = 8 and n = 10 were found to exhibit a monotropic nematic (N) phase while others were non-mesomorphic. Langmuir monolayers and Langmuir–Blodgett films of the nematic compounds (n = 10) were studied at the air–water interface (Langmuir film) and the air–solid interface. Surface manometry studies on the Langmuir monolayer showed that the film had nearly zero surface pressure at a large area per molecule (A_m ≥ 0.55 nm²). The film on compression showed a gradual increase in surface pressure and finally collapsed at an A_m of about 0.15 nm² with a collapse pressure of about 60 mN m⁻¹. Brewster angle microscopy (BAM) images (during compression) showed that dark regions coexisting with grey spots in large areas transformed to a uniform grey region with an increase in surface density and finally collapsed exhibiting bright regions. Atomic force microscopy studies (AFM) on the LB films, transferred onto freshly cleaved hydrophilic mica substrates, exhibited a network of thin fibers with the height of fibers varying between ∼4 nm to 80 nm which could be due to π–π stacking and dipolar interactions associated with the cyanobiphenyl units. On a hydrophobic silicon substrate, the LB transfer yielded a multilayer film which dewetted to form nanodroplets. We carried out temperature dependent AFM studies of the nematic compounds which showed the reversible formation of aligned fibers (∼20 to 40 μm) in the mesophase. In brief, our study provides new approaches for the realization of controlling the anisotropic properties of an ordered phase.

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Introduction

Molecular ordering and structuring is often different in thin films compared to the bulk.¹ In recent years, considerable attention has been paid towards films of organized assemblies such as Langmuir–Blodgett films due to the intrinsic control of the internal layer structure, down to the molecular level (monolayer), and the precise control of the resulting film thickness.^2,3 The study of such monolayer films at the air–water interfaces is of significant interest because it helps us to understand the packing of molecules based on their structures and different intermolecular interactions, which are difficult to study in the bulk.⁴ Stable films can be transferred onto various substrates, which can be easily controlled by the hydrophilicity or hydrophobicity of the solid support. This offers routes to tune the layer architecture according to the demands of the desired molecularly engineered organic thin-film devices.^5–7 In this paper, we report synthesis of a new series of mesogenic azobenzene based materials and their self-organization at the air–water and air–solid interfaces. In contrast to the previous studies, the azobenzene-based mesogens reported in this paper show a reversible formation of aligned fibers in the mesophase, making them suitable for devices involving photomechanics, optical switching and others.

Photoactive molecules with different molecular architectures have been used for thin film devices. Among them, azobenzene is the most widely studied photoactive molecule and holds a special place in molecular devices because of its photosensitivity. For example, utilizing the optical alignment of the azobenzene groups, anisotropic gels⁸ and copolymers⁹ have been prepared which can be used for a number of potential applications. Azobenzene-containing amphiphilic complexes, which contain complementary hydrogen bond pairs were shown to form a thermally stable bilayer membrane with varied chromophore orientations.¹⁰ The photochromism of azobenzene has been used for the study of the photomodulation of DNA-based molecules in the regulation of various bio-reactions.^11,12 In addition, azobenzene serves as a unique core for liquid crystals (LCs) as a photo-responsive trigger. The trans-form of azobenzene derivatives stabilizes a LC phase when dispersed in a LC host because of its rod-like molecular shape which is similar to that of the host LC molecule, while the cis-isomer shows a bent shape and destabilizes the LC phase.^13–16 The self-organization ability of LC materials and the photo-orientation ability of the azo-group mutually influence each other, which means that the change in molecular alignment is amplified due to the cooperative motion.¹⁷ The LC phase becomes light sensitive (when azobenzene is incorporated into LC systems) on photo-isomerization of the azobenzene moiety. Although it creates difficulty in measuring the clearing temperatures of the azobenzene-containing LC, it provides a new opportunity for switching & controlling the anisotropic properties of the ordered phase.¹⁸

Numerous examples are reported on the photo-responsive nature of the azobenzene moiety in thin films.¹⁹ Photo-isomerization can bring in situ changes in the molecular structure of the azobenzene chromophore, which makes it ideal for many photo-devices, such as information storage devices,^20,21 molecular switching devices,²² and sensors.²³ Photo-isomerization kinetics at the air–water interface of the Langmuir monolayers of mesogenic azobenzene dimers have been thoroughly studied by Kumar et al.²⁴ Thin films of LC mesogens, containing the azobenzene group form promising candidates for the fabrication of devices because the LCs can offer high sensitivity and high control over the architecture using the LB technique.^24,25 Ozaki and his co-workers have reported phase transitions in the LB films of azobenzene-containing long chain fatty acids based on the spectroscopic studies of H-aggregates.^26,27

In this study, we synthesized new oligomeric mesogens consisting of an azobenzene-based core attached to which are four 4-cyanobiphenyl units via flexible alkyl spacers. The study represents a simple but useful method in the design of a molecular system that enables fundamental insights into the unconventional structure-mesophase morphology relationship. Monolayer properties of one of the mesogenic oligomers were studied at the air–water and air–solid interfaces. The thin film at the air–water interface was studied using the techniques of surface manometry and Brewster angle microscopy (BAM). The film was transferred onto hydrophilic and hydrophobic solid substrates by the vertical LB technique and the wetting behavior was studied using atomic force microscopy (AFM). Our analysis showed that the film transferred onto a hydrophobic silicon substrate dewets to yield nanodroplets associated with the mechanism of spinodal dewetting. The temperature dependent AFM topography indicated that the film showed a reversible formation of aligned fibers in the mesophase. Identification of the temperature dependent structural morphology, leading to the formation of aligned microstructures is not only fundamentally significant, but also practically important because it enables a rational design and controlling of the anisotropic properties of the ordered phase for various applications.

Experimental section

Materials and reagents

All chemicals and solvents were of AR quality and used without further purification. 5-Nitroisophthalic acid, 4′-hydroxy-4-biphenylcarbonitrile, sodium hydroxide, potassium hydroxide, dextrose, hydrochloric acid and tetraoctyl ammonium bromide were all purchased from Sigma-Aldrich (Bangalore, India). Chloroform of HPLC grade was used for the thin film study. Column chromatographic separations were performed on silica gel (60–120 & 230–400 mesh). Thin layer chromatography (TLC) was performed on aluminium sheets pre-coated with silica gel (Merck, Kieselgel 60, F254).

Synthesis and characterization of azobenzene-based oligomers (4a–h)

The starting material, azobenzenetetracarboxylic acid was synthesized using a procedure reported in the literature.²⁸ For the synthesis of the target material, azobenzenetetracarboxylic acid (1 equivalent) was dissolved in KOH solution to increase the pH of the solution to about 8–9. To that solution, the corresponding ω-bromo terminated cyanobiphenyl precursor (6 equivalents) and 100 mg of tetraoctyl ammonium bromide (TOAB) were added. The reaction mixture was refluxed under vigorous stirring for 5–6 hours. After that the reaction mixture was cooled to room temperature and extracted with chloroform. The organic layer was separated, washed with brine and dried over anhydrous sodium sulphate. The chloroform was removed by rotary evaporation and the resulting residue was purified by repeated column chromatography over silica gel using chloroform and hexane as the eluent. The synthesized compounds were characterized by proton nuclear magnetic resonance (¹H NMR), carbon-13 nuclear magnetic resonance (¹³C NMR), infra-red (IR), ultraviolet-visible (UV-Vis) and Raman spectroscopy, and elemental analysis. (Details of the analytical data of 4a–h are given in the ESI†).

Instrumental

Structural characterization. Structural characterization of the compounds was carried out through a combination of infrared spectroscopy (Perkin Elmer Spectrum AX3), ¹H NMR and ¹³C NMR (Bruker Biospin Switzerland Avance-iii 400 MHz and 100 MHz spectrometers, respectively), UV-Vis spectrophotometry (LABINDIA UV-Vis Spectrophotometer 3000+) and elemental analysis (Thermo Flash 2000). IR spectra were recorded on a Diamond ATR for the target compounds. NMR spectra were recorded using deuterated chloroform (CDCl₃) as the solvent and tetramethylsilane (TMS) as an internal standard.

Differential scanning calorimetry. DSC measurements were performed on a Perkin Elmer DSC 8000 coupled to a controlled liquid nitrogen accessory (CLN 2) with a scan rate of 10 °C min⁻¹.

Polarized optical microscopy. Textural observations of the mesophase were performed with a Nikon Eclipse LV100POL polarising microscope provided with a Linkam heating stage (LTS 420). All images were captured using a Q-imaging camera.

X-ray diffraction. X-ray diffraction (XRD) was carried out on powder samples using Cu Kα (λ = 1.54 Å) radiation from DY 1042-Empyrean XRD with a Polymer Control System and Pixel system (Diffractometer System-Empyrean, Measuring program-Focusing mirror, scans axis-gonioKAlpha-1.54060, Goniometer Radius-240 mm and Modification editor-Panalytical).

Studies on ultrathin films at air–water and air–solid interfaces

Surface manometry. Ultra-thin films of compound 4f were studied using surface manometry, Brewster angle microscope (BAM) and atomic force microscopy (AFM). The surface manometry experiments were carried out using an APEX LB-2007 and a NIMA 611M trough. The subphase used was the ultrapure deionized water obtained from a Millipore Milli-Q system. The stock solution of 0.591 mM concentration was prepared using chloroform (HPLC grade, Merck). After spreading it on the air–water interface, the film was left for 20 min, allowing the solvent to evaporate. The π–A_m isotherms were obtained by the symmetric compression of barriers with a constant barrier speed of 6.95 × 10⁻³ nm² per molecule per minute. The time period for a compression cycle was around 72 minutes. The surface pressure (π) was measured using the standard Wilhelmy plate technique.

Brewster angle microscopy. A Brewster angle microscope (BAM), MiniBAM (NFT, Nanotech, Germany) was employed to observe the films at the air–water interface.

Film deposition. LB technique was employed to transfer various layers of the films onto hydrophilic and hydrophobic substrates at the target surface pressure (π_t) with a dipping speed of 1 mm min⁻¹. For hydrophilic surfaces, freshly cleaved mica was used. To obtain hydrophobic surfaces, freshly etched silicon wafers were dipped in hexamethyldisilazane (HMDS) for 12 h and then rinsed with HPLC grade chloroform. Silicon wafers were etched by treating polished silicon wafers for about 5 minutes in a hot acidic piranha solution (3 : 1 ratio of H₂SO₄ and H₂O₂) which were then repeatedly rinsed with ultrapure deionized water and dried.

Atomic force microscopy. The atomic force microscope (AFM) studies on these LB films were performed using Asylum Research MFP 3D. We used silicon tips (radius: 9 ± 2 nm) with a spring constant of 42 N m⁻¹ and a resonance frequency of 300 kHz. Non-contact mode was used to obtain the topography of the film. Surface manometry, BAM and LB deposition were carried out at room temperature (25 ± 0.5 °C). AFM was carried out at different temperatures.

Results and discussion

Compounds 4 were synthesized by the route shown in Scheme 1. Azobenzenetetracarboxylic acid 2 and ω-bromo substituted alkoxycyanobiphenyl were prepared following the literature methods.²⁸ The target compounds 4 were prepared by reacting 2 with ω-bromo terminated alkoxycyanobiphenyl 3 in presence of potassium hydroxide and tetraoctylammonium bromide (see details in the Experimental section) under reflux for 4–5 hours.

Scheme 1 Synthesis of the target compounds 4. Reagents and conditions: (i) NaOH, dextrose, 80 °C, HCl. yield: 70% (ii) KOH, H₂O, TOAB, reflux, 4 h. Yield: 15–40%.

All the compounds (4a–h, n = 5–12) gave satisfactory elemental analyses and showed similar ¹H NMR, ¹³C NMR, IR, Raman and UV-Vis spectra (see ESI for details, a representative spectra of compound 4f is provided in Fig. S1–S5, see ESI†).

The thermal behavior (phase transition temperatures and associated enthalpy values) of all the new materials (listed in Table 1) was investigated by polarizing optical microscopy (POM) and differential scanning calorimetry (DSC).

Table 1 Phase behavior of the target compounds^ab

TM	Heating scan	Cooling scan
a Phase transition temperatures in °C and latent heat values in kJ mol^−1 (in parentheses).b Phase assignments: Cr = crystal, N = nematic phase, I = isotropic.
4a	Cr 212.4 (82.4) I	I 158.9 (83.4) Cr
4b	Cr 162.7 (75.3) I	I 127.8 (76.8) Cr
4c	Cr 188.5 (109.1) I	I 166.9 (119.4) Cr
4d	Cr 187.5 (159.4) I	I 120 (6.6) N 101 (61) Cr
4e	Cr 164.72 (135.9) I	I 148.3 (132.2) Cr
4f	Cr 125.9 (131.5) I	I 114.6 (6.5) N 102(98.3)Cr
4g	Cr 150 (189) I	I 134.7 (137.5) Cr
4h	Cr 132.4 (142.8) I	I 108.9 (145.7) Cr

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Except for 4d and 4f, all other compounds displayed high energy transitions from crystal to isotropic on heating, while isotropic to crystal on cooling. Compound 4a, having a pentyloxy spacer (n = 5) melted (ΔH = 82 kJ mol⁻¹) at 212 °C to the isotropic phase and on cooling solidified at 159 °C (ΔH = 83 kJ mol⁻¹). Similarly, compounds 4b (n = 6) and 4c (n = 7) on heating melted at about 163 °C and 188 °C with a transition enthalpy of 75 kJ mol⁻¹ and 109 kJ mol⁻¹, respectively. On cooling, they showed the crystallization peak centered at 128 °C and 167 °C, respectively. Compounds 4d and 4f having an octyloxy and decyloxy spacer showed a monotropic N phase. On heating, they showed a melting transition centered around 188 °C and 126 °C with a transition enthalpy of 159 kJ mol⁻¹ and 131 kJ mol⁻¹, respectively. The decrease in melting temperature can be attributed to the increase in spacer length in the ω-bromo terminated alkoxycyanobiphenyl connected with the azobenzene core. On cooling from isotropic phase for both 4d and 4f, DSC showed the appearance of an N phase at about 120 °C (ΔH = 6.6 kJ mol⁻¹) and 115 °C (ΔH = 6.5 kJ mol⁻¹), respectively. The DSC traces obtained on the heating and cooling runs of compound 4f (Fig. 1a) are shown in Fig. 1b. Under a microscope, a typical schlieren texture of the N phase is displayed, as shown in Fig. 1c and d.

Fig. 1 (a) Energy-minimized structure of compound 4f. (b) The DSC trace of compound 4f on heating and cooling (scan rate 10 °C min⁻¹). Optical photomicrograph of compounds (c) 4d and (d) 4f at 119.8 °C and 110.6 °C, respectively (on cooling, crossed polarizers, scale bar 10 μm).

Compounds 4e, 4g and 4h were prepared to examine the effect of the spacer length. Compound 4e exhibited a melting transition at around 165 °C (ΔH = 135 kJ mol⁻¹) which then crystallized at about 148 °C (ΔH = 132 kJ mol⁻¹) on cooling. Similarly, compounds 4g and 4h did not display any mesophase transition either on heating or cooling. On heating they showed a crystal to isotropic transition at around 150 °C (ΔH = 189 kJ mol⁻¹) and 132 °C (ΔH = 142 kJ mol⁻¹), respectively. On cooling, the crystallization peak appeared at 134 °C (ΔH = 137 kJ mol⁻¹) and 109 °C (ΔH = 146 kJ mol⁻¹), respectively.

The supramolecular organization of these disc-rod oligomers in the mesophase was further investigated by X-ray diffraction studies (XRD). In the N phase of the compound 4f, two diffuse reflections were observed in the wide and the small angle region (Fig. 2a). These indicate the absence of any positional order in the nematic mesophase, and thus exclude the possibility of the existence of a smectic and columnar phase structure of these hybrids, consistent with their microscopy textures. The broad small angle reflection for 4f showed a d-spacing of 15.50 Å in the mesophase (at 110 °C). This corresponds to the average length of the azobenzene core (9 Å) and rod-like cyanobiphenyl units (23 Å), indicating a molecularly mixed N phase. In fact, the considerably smaller reflection at a smaller angle than that of the total length of the hybrid, confirms the compatibility (homogeneously mixed) of both the components in the mesophase and no nanophase segregation occurs between them. A significantly diffused peak at around 4.46 Å was observed which can be attributed to the average lateral separation of the molecules in the mesophase. Based on the X-ray diffraction studies and microscopy textures, we propose a sketch of the order of the N phase (Fig. 2b).

Fig. 2 (a) An intensity versus 2θ graph derived from the X-ray diffraction pattern of compound 4f in the mesophase. (b) Sketch of the order in the mixed nematic phase.

Our next attempt was to study the formation of mesogenic azobenzene molecules films at air–water and air–solid interfaces. Because of the presence of strong polar groups (such as cyano) and long alkyl chain these molecules are expected to show monolayer properties (amphiphilic character). We chose the compound 4f, which showed a monotropic nematic phase on cooling (114 °C) from the isotropic phase (125.9 °C). Fig. 3 shows the surface pressure (π, mN m⁻¹)–area per molecule (A_m, nm²) isotherm for the compound 4f. At a large area per molecule (A_m ≥ 0.55 nm²) the isotherm shows zero surface pressure. At an A_m of around 0.55 nm² the surface pressure starts increasing on compressing. The film collapses at an A_m of 0.15 nm² with a collapse pressure of 60.55 mN m⁻¹. The high collapse pressure can be attributed to the presence of strong polar groups and strong chain–chain interactions associated with the 4-cyanobiphenyl units via flexible alkyl spacers. After the collapse, there was a gradual increase in the surface pressure.

Fig. 3 Surface pressure (π)–area per molecule (A_m) isotherm of compound 4f synthesized in the study.

The stability of the film was checked (i) by measuring the equilibrium surface pressure at a constant area and (ii) by compressing the film to a target pressure of 35 mN m⁻¹ and then monitoring the change in the A_m as a function of time. In both ways, the film was found to be stable for over 30 minutes with no change in the A_m as a function of time or surface pressure (Fig. S6 and S7, see ESI†). The BAM (Fig. 4) during the compression shows that a uniform film was formed from the co-existence phase of dark and grey regions for A_m ≥ 0.55 nm² (Fig. 4a). For an A_m < 0.55 nm², the grey domains came closer and formed a uniform grey region (Fig. 4b). On further compression, when the area per molecule was significantly less, there occurred a collapse at an A_m of about 0.15 nm², where the BAM image shows bright regions growing from the grey background.

Fig. 4 The Brewster Angle Microscopy images of compression for compound 4f at air–water interface. The area per molecule in each case is (a) 0.6 nm² (b) 0.4 nm² (c) 0.1 nm² (collapsed state). The scale bar in each image represents 500 μm.

The film was transferred onto a hydrophilic mica substrate and studied using AFM. Topography images of the LB film transferred onto a mica substrate at a surface pressure of 35 mN m⁻¹ is shown in Fig. 5.

Fig. 5 AFM topography for the film of compound 4f transferred by the LB technique onto hydrophilic mica substrate at target surface pressure (π_t) of 35 mN m⁻¹ showing dense fibers in (a), and aligned fibers in the less dense area in (b). The respective height profiles corresponding to the lines drawn on the images are shown below.

The film shows a network of thin fibres with the height of fibres varying between 4 nm and 80 nm. The fibre-like patterns formed on the mica substrate were observed due to the π–π stacking of the molecules, which leads to column-like structures. These individual fibres bundle up to form thick fibres. These studies along with the BAM studies show that the film at the air–water interface is not a monolayer. Studies on Langmuir monolayers of 4′-n-octyl-4-cyanobiphenyl (8CB) molecules²⁹ have shown that the monolayer collapses at an A_m of about 0.42 nm² per molecule. Below 0.42 nm² per molecule, the monolayer transforms into a trilayer, and further at an A_m below 0.15 nm² per molecule, the film has a coexistence phase consisting of L₁, trilayer, and multilayers. The formation of trilayers and multilayers in 8CB films at the air–water interface was attributed to the dipole–dipole interactions between the nitrile groups. These studies on the 8CB film indicate that the molecules of the compound 4f form an intercalated structure at the air–water interface resulting in a multilayer film. The exact thickness of the film at air–water interface between the A_m of 0.55 nm² per molecule and 0.15 nm² per molecule is not known. On compression the film forms a more condensed phase before forming a multilayer of higher thickness at about 0.15 nm² per molecule, as indicated by the bright regions in the BAM images. As the density of the film at the interface increases, the water surface is covered by a thick layer of oil (compound 4f) and the oil–water interface is stabilized by the polar nitrile groups of the 4f molecules. Thus, the interfacial energy decreases thereby increasing the collapse pressure to 60.55 mN m⁻¹. Langmuir monolayers of lipids molecules, such as dipalmitoylphosphatidylcholine (DPPC), exhibit similar high collapse pressures.³⁰ The BAM images taken during expansion further shows that the fibre-network pattern (Fig. S8, see ESI†) is retained, and the isotherm corresponding to the expansion shows high hysteresis (Fig. S9, see ESI†). We assumed that the strong dipole moment of ∼3.8 Debye of the cyano groups lead to substantial dipole–dipole interactions between them, which is responsible for the intercalation of the molecules showing fibre-like patterns and aggregates. The size and shape of the aggregates are also guided by the π–π interactions between the azobenzene-core. We infer that these π–π interactions along with the dipolar interactions are responsible for the observed fibre-like patterns at the air–water and air–solid interface.

In the case of hydrophobic silicon substrate coated with HMDS, two layers of the film (bilayer) get coated in one dipping cycle (i.e. deposition takes place in both the downward and upward stroke). Interestingly, the AFM topography of the films showed the formation of nanodroplets (Fig. 6a and b) with the height of nanodroplets varying between 20–80 nm. The droplet formations with larger heights were likely to be associated with the dewetting of the film on the hydrophobic silicon. The formation of such kinds of nanodroplets has been observed by other groups in past. For example, Kumar et al. demonstrated that the formation of nanodroplets is due to the post-transfer reorganization of the film.⁴ Their study revealed that the transfer of a thin liquid film to a non-wettable solid substrate leads to the formation of an unstable film that eventually ruptures to form the droplets, through spinodal dewetting.⁴ Hashimoto and coworkers reported the micrometer sized droplet (with controlled diameter and height) formation by dewetting.³¹ We believe a similar mechanism is associated for the formation of droplets of larger height, with the oligomeric mesogens synthesized in our study.

Fig. 6 AFM topography of the ultra-thin film of compound 4f transferred by the LB technique onto hydrophobic HMDS coated silicon substrate at a target surface pressure (π_t) of 35 mN m⁻¹ (a) liquid crystal nanodroplet formation at room temperature as shown in (b). (c) shows the formation of well aligned rod like structures formed when compound is in liquid crystalline (nematic) phase at 115 °C. (d) shows that on cooling back to room temperature, it reverts back to nanodroplets. (e) and (f) show the co-existence of rod like structures and nanodroplets at an onset.

Our next aim was to study the film supported on an HMDS-coated silicon with respect to temperature. The study was motivated by two aims. First, we sought to characterize the film at a temperature in which compound 4f showed the mesophase. Because the most intriguing properties of LC materials are exhibited at mesophase, we thought that observing the film topography as a function of temperature would lead to significant conclusions. Second, by varying the temperature, we sought to provide additional insight into the physicochemical phenomena, underlying the formation of nanodroplets. Fig. 6c shows the AFM images of the LB films of 4f (shown in Fig. 6a and b) at 114 °C. This was achieved by heating the film to the isotropic temperature (126 °C) followed by cooling it to the mesophase temperature. Interestingly, we observed well-defined aligned fibers (in the same direction) of length ∼20 to 60 μm at 114 °C under AFM. The film was cooled back to room temperature which again showed the formation of small droplets as was seen earlier, prior to heating the sample in the film. This result demonstrated that the LC mesophase plays an important role in the formation of aligned fibers. We assumed that because of the inherent orientational order present in the mesophase, molecules are able to self-organize themselves through supramolecular non-covalent interactions. At room temperature, because of the liquid-crystalline nature of the molecules the thin film on hydrophobic silicon develops surface fluctuations of various wavelengths (spinodal dewetting as discussed above), and thus become unstable and ruptures in the form of droplets. Similar observations are also reported in the past studies.⁴ Moreover, on further heating to the isotropic temperature and cooling back to the onset temperature revealed the coexistence of both the aligned fibers and the droplets. This result suggested the reversible transformation of the droplets to fibers and vice versa in the mesophase and at room temperature, respectively. We believe that there are strong supramolecular non-covalent interactions in the N mesophase and in order to prevent steric hindrance from each other, these fibers align in the same direction.

Conclusions

In conclusion, the study reported in this paper is three-fold. Firstly, we synthesized eight new oligomeric mesogens, consisting of an azobenzene-based core attached to which are four 4-cyanobiphenyl units via flexible alkyl spacers. Among them, compounds having octyloxy and decyloxy spacers showed a monotropic N phase. Second, the novel mesogenic molecules are amphiphilic and form stable monolayers at the air–water interface, which was characterized through surface manometry and BAM. Third, LB films transferred onto the freshly cleaved hydrophilic mica substrates showed domains with a height of about 4 nm to 80 nm that could be due to π–π stacking and strong dipoles associated with the cyanobiphenyl molecules. On a hydrophobic silicon substrate, the LB transfer obtained a multilayer film which could be evidenced through the dipolar interactions associated with the cyanobiphenyl units. The film dewets to form nanodroplets that can be attributed to spinodal dewetting. Temperature dependent AFM topography showed the reversible formation of aligned fibers in the mesophase, which provides new approaches to the realization of controlling the anisotropic properties of the ordered phase.

Acknowledgements

The authors would like to thank IISER Mohali for providing research infrastructure and financial support. This work was carried out with the financial support from IISER Mohali and Department of Science and Technology, Key Project diary number SERB/F/4407/2013-14 “Liquid Crystal Nanocrystal – A new resource of functional soft materials for nanosciences”. We are grateful to NMR research facility at IISER Mohali for recording NMR spectrum. BK and GS acknowledge support from DST-INSPIRE faculty and DST Ramanujan Fellowship research grant, respectively. M. Gupta acknowledges the receipt of a graduate fellowship from IISER Mohali. N. Agarwal acknowledges DST for INSPIRE fellowship.

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Footnote

† Electronic supplementary information (ESI) available: ¹H NMR, ¹³C NMR, IR, UV-Vis, Raman spectrogram, equilibrium surface pressure measurement and BAM images of the compound 4f used in the study. See DOI: 10.1039/c4ra05572a

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