Balaram Pradhan

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.

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.

Effect of Atomic-Scale Differences on the Self-Assembly of Thiophene-based Polycatenars in Liquid Crystalline and Organogel States

Two series of polycatenars are reported that contain a central thiophene moiety connected to two substituted oxadiazole or thiadiazole units. The number, position, and length of the peripheral chains connected to these molecules were varied. The oxadiazole-based polycatenars exhibited columnar phases with rectangular and hexagonal or oblique symmetry, whereas the thiadiazole-based polycatenars exhibited columnar phases with rectangular and/or hexagonal symmetry. All of the compounds exhibited bright emission in the solution and thin-film states. Two oxadiazole-based molecules and one thiadiazole-based molecule exhibited supergelation ability in hydrocarbon solvents, which is mainly supported by attractive π-π interactions. These gels showed aggregation-induced enhanced emission, which is of high technological importance for applications in solid-state emissive displays. X-ray diffraction studies of the xerogel fibers of oxadiazole-based polycatenars revealed a columnar rectangular organization, whereas a hexagonal columnar arrangement was observed for thiadiazole-based polycatenars. Rheological measurements carried out on the samples quantitatively confirmed the formation of gels and showed that these gels are mechanically robust. The impact of an atomic-scale difference (oxygen to sulfur, <2 % of the molecular weight) on the self-assembly and the macroscopic properties of those self-assembled structures are clearly visualized.

Tuning the self-assembly and photophysical properties of bi-1,3,4-thiadiazole derivatives through electron donor–acceptor interactions and their application in OLEDs

We report several shape anisotropic molecules that contain two centrally placed 1,3,4-thiadiazole units, which vary from each other with respect to the number and length of the flexible chains at the termini. The number, position and length of the peripheral chains connected to the termini showed an impact on the thermal behavior of these compounds. The compounds with two terminal tails exhibited an enantiotropic smectic C phase, whereas the compounds with four terminal tails turned out to be crystalline. Surprisingly, among the compounds with six terminal tails, only the compound with a longer terminal chain exhibited a columnar phase with oblique symmetry. It is also to be noted that only compounds with six terminal chains exhibited gelation in long chain hydrocarbons. The xerogel of the hexacatenar with six n-decyloxy chains showed an entangled network of nanofibers of several micrometers in length. The aggregation behavior of the hexacatenar in the hydrocarbon solvent is mainly supported by the attractive π–π interactions of the aromatic cores and the van der Waals interactions offered by the peripheral flexible tails. The emission behavior is dependent on the number of peripheral tails and not on the length. Furthermore, one of the hexacatenars exhibited solvatochromic emissive behavior. This molecular design helps in the development of long molecular nanowires with a central conducting core and insulating peripheral sheath, which will be helpful for the application in organic electronic devices. The application potential of the columnar liquid crystal material was tested by the fabrication of organic light emitting diodes (OLEDs) either as a single emissive material or as a guest material in a host polymer. Higher efficiency and brightness were noticed in the host guest OLED, which exhibited a technologically important bright blue emission.

Liquid-Crystalline Star-Shaped Supergelator Exhibiting Aggregation-Induced Blue Light Emission

A family of closely related star-shaped stilbene-based molecules containing an amide linkage are synthesized, and their self-assembly in liquid-crystalline and gel states was investigated. The number and position of the peripheral alkyl tails were systematically varied to understand the structure-property relation. Interestingly, one of the molecules with seven peripheral chains was bimesomorphic, exhibiting columnar hexagonal and columnar rectangular phases, whereas the rest of them stabilized the room-temperature columnar hexagonal phase. The self-assembly of these molecules in liquid-crystalline and organogel states is extremely sensitive to the position and number of alkoxy tails in the periphery. Two of the compounds with six and seven peripheral tails exhibited supergelation behavior in long-chain hydrocarbon solvents. One of these compounds with seven alkyl chains was investigated further, and it has shown higher stability and moldability in the gel state. The xerogel of the same compound was characterized with the help of extensive microscopic and X-ray diffraction studies. The nanofibers in the xerogel are found to consist of molecules arranged in a lamellar fashion. Furthermore, this compound shows very weak emission in solution but an aggregation-induced emission property in the gel state. Considering the dearth of solid-state blue-light-emitting organic materials, this molecular design is promising where the self-assembly and emission in the aggregated state can be preserved. The nonsymmetric design lowers the phase-transition temperatures.The presence of an amide bond helps to stabilize columnar packing over a long range because of its polarity and intermolecular hydrogen bonding in addition to promoting organogelation.

Nonsymmetrical cholesterol dimers constituting regioisomeric oxadiazole and thiadiazole cores: an investigation of the structure–property correlation

Three series of chiral nonsymmetrical dimers were prepared by connecting promesogenic cholesterol to a bent structure derived from a substituted 1,3,4-oxadiazole or 1,2,4-oxadiazole or 1,3,4-thiadiazole moiety. These two mesogenic segments are interconnected through spacers of varying lengths and parity. The structures of the bent achiral unit were systematically varied with different central heterocyclic cores to understand the influence of bent angles on the thermal and gelation behavior. The bent angle of the achiral unit, which is determined by the heterocyclic core, has a major role in the stabilization of frustrated phases. Dimers based on the 1,3,4-oxadiazole unit with a more bent structure stabilized frustrated phases like blue phases and twist grain boundary phases. The bent system with a wider bent angle preferred to stabilize chiral nematic and smectic A phases. It is interesting to note that an increased bent structure reduced the mesophase stability as in the case of dimers based on the 1,3,4-oxadiazole unit, where many compounds exhibited monotropic phases. In the case of dimers with a wider bent angle, enantiotropic mesomorphism was observed. All the compounds showed blue light emission in the solution. Among these chiral dimers, only the compounds based on the 1,3,4-oxadiazole unit showed the gelation ability, which emphasizes how small structural changes like bent angle, dipole moment and the type of heteroatom in the heterocyclic unit affect the macroscopic self-assembly.

Columnar self-assembly of luminescent bent-shaped hexacatenars with a central pyridine core connected with substituted 1,3,4-oxadiazole and thiadiazoles

Bent-shaped molecules with a central pyridine core flanked with substituted 1,3,4-oxadiazole and thiadiazole derivatives with a variation in the number and length of terminal tails were synthesized. Thiadiazole based compounds exhibited a wider mesophase range in comparison to oxadiazole derivatives, while the oxadiazole derivatives exhibited a higher gelation tendency. All hexacatenars exhibited supergelation in hydrocarbon solvents along with an ability to form self-standing, moldable gel at higher concentration. Thiadiazole based compounds exhibited bathochromic absorption and emission in comparison to oxadiazole derivatives but a lower quantum yield. Two of the gelators investigated exhibited aggregation induced enhanced emission in gel and thin film state. This study shows that in addition to π–π interactions, nanosegregation of incompatible molecular subunits like flexible tails plays a major role in gelation and liquid crystalline self-assembly. Microscopic studies and X-ray diffraction studies revealed a fibrillar network of several micrometers in length with long range molecular self-assembly. They showed the ability to sense acids with an emission quenching/shifting mechanism, which makes it possible to detect the acids by naked eye. Considering the dearth of solid-state organic blue light emitters that are pivotal to realize the white light emission, these polycatenars are promising due to their wide-range Col phase and aggregation induced blue emission. Further the introduction of the pyridine central unit enhanced the mesophase stability and the acid sensing functionality in comparison to simple benzene-based bent-shaped polycatenars.