Lutfor Rahman

Synthesis and photoswitching properties of bent-shaped liquid crystals containing azobenzene monomers

Three novel bent-shaped monomers, namely 1,3-phenylene bis-{4-[4-(n-allyloxyalkyloxy)phenylazo]benzoate} 5a–c, containing azobenzene as side arms, resorcinol as central units and terminal double bonds as polymerisable functional groups were synthesised and characterised. The mesophase behaviour was investigated by polarising optical microscopy, differential scanning calorimetry and X-ray diffraction measurements and it was found that all three compounds display SmAintercal mesophases. These bent-shaped molecules exhibit strong photoisomerisation behaviour in solutions in which trans to cis isomerisation takes about 50 seconds whereas the reverse process takes almost 31 hours.

Liquid crystalline banana‐shaped monomers derived from 2,7‐naphthalene: synthesis and properties

Three banana‐shaped monomers, i.e. 2,7‐naphthalene bis[4‐(4‐allyloxyphenylazo)‐benzoate], 2,7‐naphthalene bis[4‐(4‐allyloxy‐3‐fluorophenylazo)benzoate] and 2,7‐naphthalene bis{4‐[4‐(10‐undecenyloxy)phenylazo]benzoate}, containing azobenzene as side arms, 2,7‐dihydroxynaphthalene as central units and terminal double bonds as polymerisable functional groups, were synthesised and their mesophase behaviour investigated. Polarizing optical microscopy and DSC measurements reveal that all compounds exhibit nematic mesophases. The absorption spectrum of the trans‐azobenzene groups displays a high‐intensity π–π* transition at about 365 nm and a low‐intensity n–π* transition at around 450 nm for all compounds. Hence, photochromism can be achieved by the introduction of the azo linkage to banana‐shaped liquid crystals molecules.

New pyrimidine-based photo-switchable bent-core liquid crystals

The first examples of liquid crystalline pyrimidine-based photo-switchable bent-core monomers incorporating azobenzene as side arms linked with terminal double bonds as polymerizable functional groups are synthesized and characterized. Polarizing optical microscopy (POM), differential scanning calorimetry (DSC) and X-ray diffraction analysis reveal that the bent-shaped lower homologue compounds are crystalline in nature whereas higher homologue compounds display the stable enantiotropic B6 phase. They exhibit fast photoisomerization effects in solution and relatively slow photoisomerization effects in liquid crystal cells. In solution both trans–cis and cis–trans occur at around 3 s and 200 s, respectively, whereas in solids they occur at around 10 s to 200 min. These are some of the first examples of azobenzene liquid crystals which exhibit very fast switching properties in solutions.

Synthesis and mesomorphic properties of nonsymmetric liquid crystalline dimers containing azobenzene groups

Four novel nonsymmetric dimers containing azobenzene mesogenic groups were synthesized. The nonsymmetric dimers compounds namely, ethyl 4-[(4-{4-(4-((4-nitrophenyl)diazenyl)phenoxy)alkyloxy}phenyl)diazenyl]benzoate were obtained from the alkylation of ethyl 4-[(4-(4-bromoalkyloxy)phenyl)diazenyl]benzoate with 4-[(4-nitrophenyl)diazenyl]phenol. The mesomorphic properties of the compounds were determined by DSC and polarizing optical microscopy. The first member of the series was nonliquid crystalline while all other homologues display nematic and smectic A phases. The trans-azobenzene groups of the dimers display a high-intensity π–π* transition at about 365 nm and a low-intensity n–π* transition at around 465 nm, therefore, photochromism can be achieved by the introduction of the azo linkage to the dimeric liquid crystalline molecules.

Efficient removal of transition metal ions using poly(amidoxime) ligand from polymer grafted kenaf cellulose

A desired copolymer, cellulose-graft-polyacrylonitrile was synthesized by a free radical initiating process and optimum reaction conditions were determined for maximum grafting yield (125%). The nitrile functionalities of the grafted copolymers were converted into the amidoxime ligand by oximation reaction. The kenaf cellulose, cellulose grafted copolymer and poly(amidoxime) ligand were characterized by infrared spectroscopy, field emission scanning electron microscopy, transmission electron microscopy and X-ray photoelectron spectroscopy. The pH of the solution acts as a key factor to achieve the optical detection of metal ions using complexation of this ligand with some transition metal ions. The reflectance spectra of the [M–ligand]n+ complex was found to have a highest absorbance ranging from 97 to 99.9% at pH 6. The reflectance spectra were increased by increasing Cu2+ ion concentration and a broad peak at 700 nm was observed which indicates the charge transfer (π–π transition) complex. The adsorption capacity with copper was found to be superior; 326.6 mg g−1. The adsorption capacities for other transition metal ions were also found to be strong; Fe3+, Co3+, Mn2+, Cr3+, Ni2+ and Zn2+ were 273.6, 271.6, 241.7, 228.2, 204.2 and 224.3 mg g−1, respectively at pH 6. The experimental data of all metal ions fitted significantly with the pseudo-second-order rate equation. The data proved that the transition metal ion sorption onto ligands was well fitted with the Langmuir isotherm model (R2 > 0.99), which suggest that the cellulose-based adsorbent known as poly(amidoxime) ligand surface is homogenous and monolayer. The reusability was checked by the sorption/desorption process for seven cycles and the sorption and extraction efficiency in each cycle was determined. The new type of adsorbent can be reused in many cycles without any significant loss in its original sensing and removal performances.

Synthesis of ion imprinted polymers for selective recognition and separation of rare earth metals

Lanthanide-ion imprinted polymers (L-IIPs) were synthesized by stoichiometric amounts of rare earth ions and the cavities in the polymers were created for the corresponding lanthanide ions. The maximum sorption capacities were estimated to be 125.3, 126.5, 127.6, 128.2 and 129.1 mg/g for Pr, Nd, Sm, Eu and Gd, respectively at pH 6. In the selectivity study, the L-IIPs exhibited good selectivity to the specific rare earth ions in the presence of coexisting cations. The imprinting results were found to be excellent with some rare earth ions over other competitor rare earth ions with the same charges and close ionic radius.

Synthesis of poly(hydroxamic acid) ligand from polymer grafted khaya cellulose for transition metals extraction

A cellulose-graft-poly(methyl acrylate) was synthesized by free radical initiating process and the ester functional groups were converted into the hydroxamic acid ligand. The intermediate and final products are characterized by FT-IR, FE-SEM, HR-TEM and XPS technique. The pH of the solution acts as a key factor in achieving optical color signals of metalcomplexation. The reflectance spectra of the [Cu-ligand]n+ complex was found to be a highest absorbance at 99.8 % at pH 6 and it was increased upon increasing of Cu2+ ion concentrations and a broad peak at 700 nm was observed which indicated the charge transfer (π-π transition) metals-Cu complex. The adsorption capacity of copper was found to be superior (336 mg g−1) rather than other transition metals such as Fe3+, Co3+, Cr3+, Ni2+, Mn2+ and Zn2+ were 310, 295, 288, 250, 248 and 225 mg g-1, respectively at pH 6. The experimental data of all metal ions fitted significantly with the pseudo-second-order rate equation. The transition metal ions sorption onto ligand were well fitted with the Langmuir isotherm model (R2>0.99), which suggested that the cellulose-based adsorbent known as poly(hydroxamic acid) ligand surface is homogenous and monolayer. The reusability of the poly(hydroxamic acid) ligand was checked by the sorption/desorption process up to ten cycles without any significant loss in its original sensing and removal performances.

Synthesis and photoswitching properties of liquid crystals derived from myo-inositol

A new series of unconventional liquid crystal molecules derived from myo-inositol core were synthesized by linking six azobenzene units having terminal double bonds as polymerizable functional groups to myo-inositol. Differential scanning calorimetry, polarizing optical microscopy and X-ray diffraction studies were carried out to determine mesomorphism in these materials. Thus, all compounds showed stable enantiotropic SmA phase which was independent of the chain length and chain parity. The photoswitching behaviour of these molecules in solutions showed E to Z isomerization in 8–10 s, whereas the reverse process took about 270–305 min. In solid film, the E–Z photoisomerization took 5 s, while the reverse transformation from Z to E state took 120 min. The photoswitching behaviour of these materials represents one of the fastest switching times so far observed in such materials and, therefore, they are good candidates for molecular switches.

Synthesis of new U-shaped azobenzene liquid crystals for photoswitching properties

A new series of liquid crystalline compounds comprising a U-shaped unit as central core incorporating azobenzene in the side arms and having terminal alkene functional groups, is synthesized and characterized by differential scanning calorimetry (DSC), polarized-light optical microscopy (POM), X-ray diffraction analysis and UV-vis spectroscopy. In the U-shaped series, all compounds showed stable enantiotropic monolayer SmA phases independent on the chain length and chain parity. These U-shaped molecules exhibit strong photoisomerisation behaviour in solutions and in solid state. The photoswitching properties of compounds showed trans to cis isomerization in about 15 seconds, whereas reverse process required much longer times ranging from 235–380 min in solutions. In case of solid film of 4a, E–Z photoisomerization takes around 4 s and the reverse transformation to original Z–E state takes about 74 min. The kinetic study using UV light irradiation shows that all compounds (4a–e) behave first order rate law throughout the relaxation time in solution. The effect of alkyl chain length for trans to cis is negligible whereas, cis to trans is substantial on the odd–even chain length. The photoisomerization study with variable light intensities shows that the photosaturation and thermal back relaxation times are decreased when intensity of the irradiated light is increased. Compounds did not degrade with light illumination at 10 mW cm−2. The reversible isomerization did not significantly decay after multiple cycles indicating that the photo-responsive properties are stable and repeatable. Thus, the photoswitching behaviour of these materials may be suitably exploited in the field of optical data storage device and in molecular switches for suitable switching times.

Synthesis of New Liquid Crystals Embedded Gold Nanoparticles for Photoswitching Properties

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