Chinese Chemical Letters  2014, Vol.25 Issue (12):1611-1614   PDF    
Synthesis and photoswitching properties of azobenzene liquid crystals with a pentafluorobenzene terminal
Md Lutfor Rahman , Gurumuthy Hegde, Shaheen M. Sarkar, Mashitah Mohd Yusoff    
Faculty of Industrial Sciences & Technology, Universiti Malaysia Pahang, Kuantan 26300, Malaysia
Abstract: The fluorine-substituted benzoate ester rod-shaped liquid crystals containing an azobenzene side chain linked with terminal double bonds were synthesized and characterized. The mesophase and photoswitching properties were determined by polarizing optical microscopy (POM), differential scanning calorimetry (DSC) and UV-visible spectrometry. The rod-shaped compound 4a having an odd number of carbon atoms in the alkyl chains exhibits nematic phase and SmA type phase whereas compound 4b having an even number of carbon atoms showed only nematic phase. Both fluorinecontaining compounds 4c and 4d showed only SmA type phases. The photoswitching properties of these compounds showed a rate of trans to cis isomerization ranging 19-20 s, whereas reverse process took around 230 min in solution. These materials may be ideal in the field of optically rewritable applications where both on and off rates should be crucial.
Key words: Liquid crystals     Azobenzene     Photoswitching     Fluorine compounds    
1. Introduction

The unique properties of liquid crystals (LCs) have been utilized in the area of display technologies,optical storage devices,solar cells,ion conductors and templates for nanoparticles [1, 2]. The insertion of a fluorine atom or fluorinated group into the liquid crystals molecules changes the physical characters,which may influence the display and non-display properties [3]. Literature survey reveals that few fluorinated liquid crystals (FLCs) are reported in spite of their potential unique applications. Replacing the fluorobenzene in the ionic LCs by the fluorinated heterocycles and the FLCs bearing fluorinated heterocycles were highly valuable because these materials may have some advantage in applications [1]. The 4-fluorobenzene and 3,4-difluorobenzene derivatives have been used as STN and TFT liquid crystal materials [4].

The field of photoinduced phenomena is growing fast using the molecular ordering/disordering of the liquid crystalline systems [5]. In photonic,the light controlled by stimulus is being proposed as a future potential technology for optical storage devices [6]. The molecules having azobenzene units exhibit reversible isomerization behaviour upon irradiations with UV and visible light [7]. The more stabletransconfiguration converts into theircis configuration upon absorption of UV light [8]. In this case,time required fortranstocisisomerization is faster whereas the thermal back relaxation takes much longer. The UV irradiation is imposed to the nematic phase for the materials exhibit a nematic-isotropic (N-I) transition and the lowering of the transition temperature,TNI, could induce an isothermal N-I transition. The photochemically induced transition is promising to develop an optical imagestoring system and their optical rewritable ability.

In this paper,we report the synthesis,characterization and properties of some rod-shaped molecules containing fluorine substituted phenylenes as the terminal and the azobenzene units with terminal chains having double bonds,and also report the photoswitching properties of compounds in the field of optically rewritable applications. 2. Experimental

The structures of the compounds were confirmed by spectroscopic methods: 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/min. Optical textures were obtained by using Olympus BX51 polarizing optical microscope equipped with a MettlerToledo FP82HT hot stage and a FP90 central processor unit. UV/vis absorption spectra were recorded using UV-visible spectrophotometer obtained from Ocean Optics (HR2000+). For photo-switching studies in solutions,fluorine azobenzene liquid crystalline monomers were dissolved in dimethylformamide at suitable concentrations. Photo-switching behaviour of the azobenzene containing fluorine compounds investigated by illuminating quartz cuvette containing sample with OMNICURE S2000 UV source equipped with 365 nm filter along with heat filter inserted just before the sample to avoid any heat effects arising from the sample.

Compounds 4a-d were synthesized as depicted in Scheme 1. Compound1awas alkylated with allyl bromide or 4-bromo-1-butene in the presence of K2CO3 and catalytic amount of KI to give ethyl 4-{2-(4-allylphenyl)diazenyl}benzoate 2a or ethyl 4-{2-(4-(but-3-enyl)phenyl)diazenyl}benzoate 2b. Similarly,fluorine-containing1bwas alkylated with allyl bromide or 4-bromo-1-butene to give ethyl 4-{2-(4-allyl-3-fluorophenyl)diazenyl}benzoate 2c or ethyl 4-{2-(4-(but-3-enyl)-3-fluorophenyl)diazenyl}benzoate 2d. Therein the ester compounds 2a-d were hydrolyzed under basic conditions to yield 4-{2-(4-allylphenyl)diazenyl}benzoic acid 3a (other 3b-d in Supporting information). Finally,acids 3a-d were esterified with pentafluorophenol in the presence of DCC and DMAP to produce the desired compounds 4a-d (pentaflourophenyl 4-{2-(4-allylphenyl)diazenyl}benzoate is4aand detail experimental procedures of 4b-d are described in Supporting information).

Scheme 1. Synthesis of liquid crystals compounds 4a–d.
3. Results and discussion 3.1. Mesomorphic properties

Polarizing optical microscopy (POM) studies: Under the optical polarizing microscope,compound4ashowed a typical schlieren texture for nematic phase upon cooling from the isotropic phase. Optical texture was taken at 154°C for compound 4a and on further cooling a typical fan shaped texture for smectic A phase was observed before crystallization (Fig. 1b). For compound 4b,a schlieren texture was also observed for nematic phase upon cooling from the isotropic phase (Fig. S1 in Supporting information),on further cooling no mesophase was observed before crystallization. Both fluorine-containing compounds 4c and 4d showed a typical fan shaped texture for smectic A phase at lower temperature than non-fluorine compounds (Fig. S1). All the transition temperatures observed under POM matched with DSC data.

Fig. 1. Polarized micrographs obtained from isotropic phases of (a) nematic phase of 4a at 154°C, (b) smectic A phase of 4a at 128°C, (c) nematic phase of 4b at 142°C, (d) smectic A phase of 4c at 120°C. All textures were captured with 10× objective in Olympus BX51 at horizontal picture width 200mm by Olympus DP26 camera.

Differential scanning calorimetry (DSC) studies confirmed the phase transition temperatures (T/°C) observed by polarizing microscopy and gave the enthalpy changes (DH/J g-1 ) associated with these phase transitions. Compound 4a exhibits three transition peaks on heating,which correspond to the Cr-SmA, SmA-N and N-I transitions,respectively. On cooling,compound 4a also showed the isotropic-nematic,nematic-smectic and smectic- crystal transitions (Table 1). Compound 4b displayed two peaks on heating and on cooling and two transition peaks were observed for only nematic phase (Table 1). Compounds4cand4dshowed two peaks on heating and on cooling,which can be attributed to the Cr- SmA and SmA-I transitions at about 30°C lower transition temperatures compared to 4a and 4b due to fluorine effects (Table 1). The DSC curves representing enthalpy changes are given in Supporting information.

Table 1
Phase transition temperature (T/°C) and associated transition enthalpy values (DH/J g-1) in parentheses given for the second heating and cooling of DSC scans for compounds 4a and 4b.
3.2. Photoswitching study

The rod-shaped molecules 4a-d were considered for a photoisomerization study in DMF solution with a concentration of C= 1.1×10-5 mol/L [9, 10]. The absorption spectra of compound 4a-d showed an absorbance maximum at 364 nm as shown in Fig. S2 (in Supporting information). The strong absorbance in the UV region at 364 nm corresponds to the π-π* transition of the E isomer (trans isomer) and a very weak absorbance in the visible region around 450 nm represents the n-π* transition ofZisomer (cisisomer). The photoswitching property study was performed using UV/visible spectroscopy in the absence and presence of UV light illumination. Compounds 4a-d were illuminated with 365 nm UV light at different time intervals and immediately the absorption spectra were recorded. Heat filter is used just before sampling to avoid any heat radiation arising from the samples. The absorption maximum at 364 nm decreases due to the E/Z photoisomerization. After ~20 s of illumination,lack of change in the absorption spectrum confirms the saturation of E/Z isomerization process (Fig. S2).

Fig. 2 shows the E-Z absorption of compounds 4a-d as a function of exposure time. Data were extracted from Fig. S2 at 364 nm,where peak wavelength was fixed and absorption values at 364 nm at different exposure time were recorded. The curve shows that photosaturation occurs within 19 s,which is faster than the photoisomerization from nematic to isotropic phase [11]. The thermal back relaxation process is shown Fig. S3 (in Supporting information) for compounds4a-dwhose solution was shined continuously for 20 s (photo stationery state) and kept in the dark and then at subsequent time intervals,spectral data were recorded. Fig. 3 shows the time dependence of the Z-E absorption of compounds 4a-d. Peak wavelength at 364 nm as obtained from Fig. S3 is plotted as a function of recovery time. The curve shows that thermal back relaxation occurs within 230 min,which is reasonably fast as compared with the thermal back relaxation from nematic to isotropic phase [11]. A possible reason for the observed relatively fast thermal back relaxation and the UV ON process could be that the phases involved on both sides of transition possess a layered structure and the changes that occurred were confined to the in-plane rotation of the molecules [12].

Fig. 2. Time dependence photoisomerization curve ofEisomer (4a–d) showing effect of UV illumination (364 nm wavelength).

Fig. 3. Time dependence photoisomerization curve ofZisomer (4a–d) showing thermal back relaxation time.

We also calculated the rate constant for thecis-trans isomerization at room temperature for all four compounds using the following equation [13]:

where At,A0,A are the absorbance at 364 nm at timet,time zero and infinite time,respectively. Fig. 4 shows a typical first order plot using the above equation at room temperature for all four compounds. It is evident that throughout the relaxation curve a typical first order kinetic was observed except at the last point, which may be due to the small change in thermal variation that changed absorbance values. All four compounds,irrespective of their terminal substitutions,behaved in the similar way at room temperature.
Fig. 4. First order plots for the compounds4a–dforcis–transthermal isomerization at 25°C.
4. Conclusion

We have successfully synthesized and characterized the fluorine-substituted benzoate ester linked to rod-shaped azobenzene liquid crystals with terminal double bonds as polymerizable functional groups. The alkyl side chain double bonds in the presence of anazomoiety are suitable for photochromism and trans-cis-trans isomerization cycles under UV irradiation. Compound 4a showed both nematic phase and SmA phase whereas 4b exhibited nematic phase only and both fluorine-containing compound 4c and 4d showed only SmA phase. The photoswitching properties of these compounds showed atranstocisisomerization rate ranging 19-20 s,whereas the reverse process took around 230 min in solution. The compounds showed first order exponential decay in solutions. The photoswitching behaviours of these materials may be useful in the field of optically rewritable display applications.


This research was supported by UMP Research (No. RDU 100338) and also RAGS (No. RDU131408).

Appendix A. Supplementary data

Supplementary material related to this article can be found,in the online version,at

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