Monika Gupta

New perylene-based non-conventional discotic liquid crystals

The synthesis, optical properties and thermal behaviour of three novel non-conventional 3,4,9,10-tetrasubstituted perylene-based discotic oligomers are reported for the first time consisting of a perylene core attached to which are four 4-cyanobiphenyl, triphenylene and cholesteryl units via flexible alkyl spacers. All the oligomers self-assemble into a mesophase and exhibit excellent fluorescence emission properties making them suitable for various opto-electronic applications.

Triphenylene-Based Room-Temperature Discotic Liquid Crystals: A New Class of Blue-Light-Emitting Materials with Long-Range Columnar Self-Assembly

A straightforward synthesis of multialkynylbenzene-bridged triphenylene-based dyad systems (via flexible alkyl spacers) that self-organize into room-temperature columnar structures over a long range is reported. The compounds with spacer lengths (n) of 8 and 10 exhibit a columnar rectangular mesophase whereas a compound with n = 6 shows a columnar rectangular plastic phase. Interestingly, the later compound (n = 6) shows the formation of well-nucleated spherulites of about several hundred micrometers that suggest the existence of a long-range uniform self-assembly of columns. All of these compounds show blue luminescence in solution and in the thin-film state under long-wavelength (365 nm) UV light. These compounds fulfill the described demands such as long-range columnar self-assembly at room temperature, a good yield with high purity, and blue-light emitters under the neat condition for possible potential applications in semiconductor devices. They also match the criteria of facile processing from the isotropic state because of their low isotropization temperature. This new class of materials is promising, considering the emissive nature and stabilization of the columnar mesophase at ambient temperature.

Perylo[1,12-b,c,d] Thiophene Tetraesters: A New Class of Luminescent Columnar Liquid Crystals

Perylo[1,12-b,c,d] thiophene tetraesters exhibiting wide-range hexagonal columnar phase have been synthesized. These compounds also exhibit good homeotropic alignment in the liquid-crystalline phase which is very important for the device fabrication. These compounds showed sky-blue luminescence in solution under the long-wavelength UV light. With high solubility and high quantum yield these compounds can serve as standards to measure quantum yields of unknown samples. This new class of materials is promising, considering the emissive nature and stabilization of hexagonal columnar mesophase over a wide thermal range and ease of synthesis.

Star-shaped fluorescent liquid crystals derived from s-triazine and 1,3,4-oxadiazole moieties

Star-shaped molecules with a central triazine core appended with three 1,3,4-oxadiazole arms have been designed with the variation in the number, length and pattern of peripheral chain substitution. These compounds were investigated for their thermal, electrochemical and photophysical behavior. These nonconventional molecules stabilized wide range columnar phases and demonstrated how one can tune the liquid crystal self-assembly through simple structural modification. The photophysical properties of these star shaped molecules are extremely dependent on the number and pattern of peripheral chain substitution. These compounds exhibit blue and green luminescence in the solid/liquid crystal state. The ability to overcome aggregation induced quenching is due to the favorable packing of these molecules in the solid state. These solid-state emissive materials with good thermal stability and lower band gap may find applications in the construction of emissive displays and organic lasers.

The first examples of room temperature liquid crystal dimers based on cholesterol and pentaalkynylbenzene

In this article, we report the synthesis of the first examples of room temperature liquid crystal dimers based on cholesterol and pentaalkynylbenzene linked via flexible alkyl spacers. The thermal behaviour and mesomorphic properties of the newly synthesised compounds were investigated by differential scanning calorimetry, polarised optical microscopy and X-ray diffraction studies. All the synthesised dimers showed nematic (N*) phase at room temperature and exhibited excellent fluorescent emission properties. The steady-state anisotropy and fluorescence lifetime were measured in order to gain further insights into the supramolecular arrangement of these dimers.

Electroluminescent room temperature columnar liquid crystals based on bay-annulated perylene tetraesters

Room temperature columnar liquid crystals with a wide thermal range, based on bay-annulated perylene tetraesters are reported. Through the incorporation of heteroatoms like N, S and Se in the bay position of the perylene tetraester the emission behavior in the solution state has been effectively tuned, where the N, S and Se-annulated derivatives showed bright green, blue and weak yellowish green fluorescence. The electroluminescence behavior of these compounds has been explored as emissive layers in organic light emitting diodes. Further, a remarkable improvement in the emission was achieved, when these molecules were doped in a matrix of polyvinyl carbazole (PVK), which is due to a combination of Forster resonance energy transfer from the host to the guest and charge carrier trapping in the emissive layer.

Aromatic π–π driven supergelation, aggregation induced emission and columnar self-assembly of star-shaped 1,2,4-oxadiazole derivatives

A new family of star shaped 1,2,4-oxadiazole derivatives with the variation in the number of flexible peripheral tails have been synthesized and characterized. It is interesting to note that the number of flexible tails at the periphery dictated the self-assembly and the photophysical behavior. The compound with three flexible tails stabilized the crystalline state with lamellar packing, and did not show organogelation, but showed blue emission in crystalline and thin film states. The compound with six flexible tails stabilized the hexagonal columnar liquid crystalline state and it showed the ability to gelate in nonpolar solvents at a concentration less than 1 weight percent, qualifying it as a supergelator, where π–π interactions play a major role. This phenomenon is very rare, in comparison to earlier reports where supergelation is supported by H-bonding interactions. Besides its capability to form a self-standing, moldable gel, this compound also exhibited aggregation-induced emission (AIE), which persisted even in the xerogel state. X-ray diffraction studies unraveled the rectangular columnar self-assembly in the gel state. The columnar order and emissive nature in the liquid crystal and xerogel states makes this molecule promising for application in emissive displays. Compounds with nine alkyl tails stabilized a long range columnar hexagonal phase. This report emphasizes the importance of various non-covalent interactions in deciding the nature of self-assembly.

Multifunctional hexacatenar mesogen exhibiting supergelation, AIEE and its ability as a potential volatile acid sensor

A bent shaped hexacatenar mesogen self-assembling into a columnar phase, in liquid crystalline and gel states exhibiting aggregation induced enhanced blue light emission has been synthesized. Its ability to detect volatile acids is demonstrated.

Bay-Annulated Perylene Tetraesters: A New Class of Discotic Liquid Crystals.

Selenium-annulated perylene tetraesters that stabilize the hexagonal columnar phase have been synthesized and characterized, and their thermal and photophysical behavior has been determined. The mesophase range decreased with an increase in chain length. A comparative account of the structure-property relationships of this series of compounds with respect to parent perylene tetraesters, N- and S-annulated perylene tetraesters, in terms of their thermal, photophysical and electrochemical behavior is provided. The bay-annulation of perylene tetraesters is a good option to modify the thermal and photophysical properties of perylene derivatives and it can provide a new avenue for the synthesis of several technologically important self-assembling perylene derivatives.

Microwave-assisted synthesis of novel mixed tail rufigallol derivatives

Microwave-assisted syntheses of five new series of rufigallol-based mesogens are reported with branched alkyl chains at the peripheral positions. The chemical structures of these newly synthesised compounds were determined by 1H nuclear magnetic resonance (NMR), 13C NMR, infrared spectroscopy, ultraviolet spectroscopy and elemental analysis. The thermotropic liquid crystalline properties were investigated by polarising optical microscopy (POM), differential scanning calorimetry and X-ray diffractometry. Most of the derivatives were found to be liquid crystalline over a wide temperature range.