Santanu Kumar Pal

Novel conducting nanocomposites: synthesis of triphenylene-covered gold nanoparticles and their insertion into a columnar matrix

Gold nanoparticles fully covered with triphenylene-based discotic liquid crystals (DLCs) were synthesized and dispersed in a columnar matrix. The thermophysical properties of these nanocomposites, studied using polarizing optical microscopy, differential scanning calorimetry and small angle X-ray diffraction studies (SAXS), confirm their insertion into the columnar matrix. The presence of the gold nanoparticles in the triphenylene-based DLCs does not disturb the nature of the mesophase other than altering the transition temperatures. We propose that the gold nanoparticles are distributed between the domain gaps of the DLCs in random disordered manner. Interestingly the DC conductivity measurements show an enhancement of the electrical conductivity by more than a million times upon doping of the discotic liquid crystals with the triphenylene-capped nanoparticles under ambient conditions.

Novel triphenylene‐based ionic discotic liquid crystalline polymers

The photopolymerization of triphenylene‐substituted 1‐vinylimidazole yielded a novel ionic discotic liquid crystalline polymer. The thermotropic liquid crystalline properties of both the monomer and polymer were investigated by polarizing optical microscopy and differential scanning calorimetry. The mesophase structure has been characterized by X‐ray diffractometry. Both the monomer and polymer displayed columnar rectangular mesophase over a wide temperature range. However, the columnar order in the polymer was found to be small compared to the monomer. Such ionic discotic polymers are useful for unidirectional transport of ion and energy at nanoscale.

The first examples of terminally thiol‐functionalized alkoxycyanobiphenyls

In this communication, we report the synthesis of the first examples of terminally thiol‐functionalized alkoxycyanobiphenyls. The thermal behaviour of these mesogens and their intermediates was investigated by polarizing optical microscopy, differential scanning calorimetry and X‐ray diffractometry.

Phase transitions in novel disulphide‐bridged alkoxycyanobiphenyl dimers

The synthesis, characterization and mesomorphic properties of the first examples of novel disulphide‐bridged alkoxycyanobiphenyl dimers are reported. The thermal behaviour of these mesogens was investigated by polarizing optical microscopy, differential scanning calorimetry and X‐ray diffractometry. The dimers with a shorter spacer exhibit only the nematic phase while dimers with a longer spacer display nematic as well as smectic phases. X‐ray diffraction experiments reveal the intercalated structure of the SmA phase of these dimers and the presence of short range SmA‐like order in the N phase (cybotactic nematic) of all the compounds, except the one with the shortest spacer.

Synthesis and characterisation of novel rod–disc oligomers

Microwave‐promoted synthesis of novel rufigallol‐based liquid crystalline oligomers, containing six rod‐like cyanobiphenyl moieties connected to the rufigallol core via flexible alkyl spacers, is reported. The synthesis of the target compounds was challenging since classical reactions failed to produce these oligomers. Differential scanning calorimetry and polarising optical microscopy analysis revealed the existence of a nematic phase with a shorter spacer and a smectic A (SmA) phase with a longer spacer. The compound with medium alkyl spacer exhibits both nematic and SmA phases at higher temperature and a re‐entrant nematic phase at lower temperature.

Discotic-Decorated Gold Nanoparticles

ABSTRACT Several mixtures of hexanethiolate capped gold nanoparticles and discotic liquid crystals were prepared. The thermophysical properties of these mixtures were investigated using polarizing optical microscopy, differential scanning calorimetry and DC conductivity. Formation of columnar mesophase and about 250 times enhancement in the DC conductivity indicate inclusion of gold nanoparticles into the matrix of triphenylene based discotic liquid crystals.

Twist viscoelastic coefficient of novel thiol terminated alkoxy-cyanobiphenyl nematic liquid crystals

We report our studies on the refractive indices and twist viscoelastic coefficient of novel thiol terminated alkoxy-cyanobiphenyl nematic liquid crystals. Dynamic light scattering (DLS) was employed to measure the twist viscoelastic coefficient as a function of temperature and the length of alkoxy chain. Our studies show that the twist viscoelastic coefficient is about three times smaller than that of the ordinary alkoxy-cyanobiphenyl nematic liquid crystals. DLS experiments carried out on this system with applied electric field show that the director relaxation frequency increases. These measurements yield the ratio of dielectric anisotropy to twist viscosity. We have also estimated the activation energy corresponding to twist viscosity.

Green Chemistry Approach to the Synthesis of Liquid Crystalline Materials

This article presents a simple, rapid, economic and environmentally acceptable microwave promoted “green chemistry” approach to the synthesis of some conventional and non-conventional liquid crystalline materials. Alkylation of 4′-hydroxy-4-biphenylcarbonitrile with various alkyl halides under microwave irradiation furnished alkoxycynobiphenyl-based calamitic liquid crystals in very high yield within one minute. Similarly, terminally-functionalized alkoxycyanobiphenyls and alkoxycyanobiphenyl dimers can also be prepared. Microwave irradiation of hexaalkoxytriphenylene in the presence of ionic liquids gives monohydroxy-pentaalkoxytriphenylene in about 30% yield. Novel imidazolium-based ionic liquid crystalline dimers can be easily prepared via quaternization reaction using microwaves. Classical reaction conditions failed to produce these dimers.