Suraj Kumar Pathak

Columnar self-assembly of star-shaped luminescent oxadiazole and thiadiazole derivatives

A new class of blue light emitting liquid crystalline star-shaped molecules based on 1,3,4-thiadiazoles have been designed and synthesized. These compounds were investigated using polarizing optical microscopy, differential scanning calorimetry, X-ray diffraction, cyclic voltammetry and photophysical studies. In comparison to their 1,3,4-oxadiazole counterparts, these thiadiazole-based molecules are promising as they stabilize the hexagonal columnar phases over a broad thermal range. The thermal behavior and photophysical properties of these new star-shaped molecules are extremely dependent on the number and types of peripheral tails in the molecular structure. 1,3,4-Thiadiazole derivatives exhibit sky-blue emission in solution, unlike the deep blue emission of 1,3,4-oxadiazole derivatives. They also exhibit a lower band gap as compared to their oxadiazole counterparts and offer great potential in organic light emitting diode applications.

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.

Effect of regioisomerism on the self-assembly and photophysical behavior of 1,3,4-thiadiazole-based polycatenars

A new class of polycatenars, where the central benzene ring is connected to two arms derived from substituted 1,3,4-thiadiazoles at the 1,3- and 1,4-positions, was synthesized and characterized. These thiadiazole-based molecules are promising as they stabilize columnar phases over a wide temperature range, in comparison to their oxadiazole analogues. The para-substituted polycatenars exhibited a columnar hexagonal and/or a columnar oblique phase, while the meta-substituted polycatenars exhibited solely a columnar oblique phase. The para-substituted polycatenars exhibited green emission, while the meta-substituted polycatenars exhibited blue emission in solution and film states. Stabilization of a broad range columnar phase and luminescence in the solid state make these new compounds promising from the viewpoint of applications in emissive displays. The self-assembly and luminescence of these regioisomers was greatly influenced by the molecular structure.

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.

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.

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.

Contrasting effects of heterocycle substitution and branched tails in the arms of star-shaped molecules

Herein, star-shaped tris(N-salicylideneaniline)s (TSANs) containing 1,3,4-oxadiazole based and 1,3,4-thiadiazole based arms are synthesized and characterized. The introduction of branched tails at their peripheries has different effects on these tris(N-salicylideneaniline)s, which are dependent on the type of heterocycle. The TSANs bearing 1,3,4-oxadiazole arms with branched tails exhibit a room temperature columnar rectangular phase in comparison to the high temperature columnar hexagonal phase exhibited by their hexadecyloxy chain analogues. In the case of the thiadiazole based TSANs, the compound with hexadecyloxy chains exhibits a columnar rectangular phase over a wide temperature range including room temperature, whereas its branched chain analogue is a liquid. Thus, in the case of star-shaped molecules, the type of peripheral tails not only affects the transition temperature, but also affects the type of self-assembly, which is in contrast to conventional discotic liquid crystals. The introduction of substituted 1,3,4-thiadiazole rings helps in the reduction of their melting points and enhances their mesophase width. The presence of bulky branched tails at their peripheries enhances the intermolecular interactions between the cores of the 1,3,4-oxadiazole based TSANs, which leads to the stabilization of the columnar rectangular phase. The 1,3,4-thiadiazole based TSANs exhibit a columnar rectangular phase even with straight peripheral chains because of the attractive intermolecular interactions of the thiadiazole ring.

Room temperature columnar liquid crystalline self-assembly of acidochromic, luminescent, star-shaped molecules with cyanovinylene chromophores

Two new star-shaped tris(N-salicylideneanilines) (TSANs) incorporated with cyanovinylene chromophores were prepared through a multistep synthesis. The position of the cyano group was altered in the target molecules, to understand its impact on the photophysical and thermotropic behavior, in comparison to the stilbene derivative without a cyano group. The presence of the cyano group enhanced the mesophase range in comparison to the non-cyano stilbene derivative. Further, enhanced intermolecular interactions led to the stabilization of the columnar rectangular phase in comparison to the columnar hexagonal phase of the simple stilbene derivative. These compounds exhibited the freezing of the columnar phase in a glassy state, which is beneficial from the device fabrication point of view. Thus the introduction of the cyano group within the molecular structure of a star-shaped TSAN enhanced the intermolecular interactions and also altered the luminescence behavior. Such ordered luminescent molecular assemblies which stabilize the columnar order over a long range are promising from the viewpoint of emissive displays. These compounds can be utilized for the sensing of volatile acids in solution at very low concentration (in parts per billion levels) or in the thin film state, by either fluorescence switching or quenching.

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.