Subrata Nath

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.

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.

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.

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.

A sensitive and selective sensor for picric acid detection with a fluorescence switching response

A low molecular weight organogelator containing 3,5-substituted-1,3,4-oxadiazole and tetrazole units was synthesized and characterized. This compound is only soluble in DMSO and forms a stable gel. The solution and gel exhibit a blue light emission. The gel was characterized by atomic force microscopy, field-emission scanning electron microscopy, 1H NMR and fluorescence measurements. The gel to solution interconversion was reversible for many cycles of heating and cooling. The compound in solution exhibited a high selectivity for the detection of picric acid, a common explosive and water pollutant. Fluorimetric titration studies with nitro explosive compounds revealed that the emission of the compound was red shifted in response to the addition of picric acid, and exhibited a shifting of fluorescence from blue to green. Theoretical and experimental studies revealed that the sensing is due to the complexation of the picrate anion with the protonated fluorophore. The shifting of emission in response to picric acid in the visible region is ideal for the naked eye detection of explosives and therefore it is promising in comparison to the detection methods based on fluorescence quenching.

The effect of regioisomerism on the mesomorphic and photophysical behavior of oxadiazole-based tris(N-salicylideneaniline)s: synthesis and characterization

Two new regioisomeric star-shaped tris(N-salicylideneaniline)s are synthesized and characterized. The arms of these star-shaped mesogens differ from each other with respect to the substitution on the 3,5-positions of the central 1,2,4-oxadiazole moieties. The unsymmetrical nature of substitution leads to a change in the distribution of electron density, which will have an effect on the type of columnar self-assembly. One of these molecules stabilizes the columnar hexagonal phase, while the other one stabilizes the columnar rectangular phase. The columnar rectangular phase, which requires enhanced intermolecular interactions, is observed in the case of the star-shaped molecule, where the trialkoxy phenyl group is connected at the 5-position of the heterocycle, whereas the columnar hexagonal phase is observed in the case of the star-shaped molecule, where the trialkoxy phenyl group is connected at the 3-position of the heterocycle. These compounds showed reduced melting points, clearing points and an enhanced mesophase range with respect to their symmetric counterpart (1,3,4-oxadiazole derivative) reported earlier. All the molecules exhibited green light emission in solution, with good quantum yield. The emission of 1,2,4-oxadiazole derivatives was considerably red-shifted in comparison to those of 1,3,4-oxadiazole derivatives. This study emphasizes how a minor change in the molecular structure brings about a beneficial change in the self-assembly characteristics of star-shaped molecules.

Star-shaped π-gelators based on oxadiazole and thiadiazoles: a structure–property correlation

Star-shaped and tetracatenar molecules based on 1,3,4-oxadiazole and thiadiazole derivatives were synthesized and their liquid crystallinity and gelation behavior were studied. The self-assembly and photophysical properties of these molecules are sensitive to the type of the heteroatom present in the molecule and the pattern of peripheral substitution. Only the star-shaped molecule with substituted oxadiazole arms exhibited a columnar hexagonal phase, while the tetracatenars were crystalline. This compound exhibited a supergelation behavior that is mainly supported by attractive π–π interactions. This is notable because usually supergelation is supported by H-bonding interactions. Further, this compound exhibited aggregation-induced emission with a several-fold increase in the luminescence intensity upon gelation. Surprisingly its thiadiazole counterpart was crystalline and did not gelate. The corresponding oxadiazole and thiadiazole star-shaped molecules, with peripheral 3,4-substitution, were liquid crystalline and stabilized gelation. This shows that in addition to π–π interactions, nanosegregation of incompatible molecular subunits like flexible tails plays a major role in organogelation and liquid crystalline self-assembly. Microscopy studies revealed a fibrillar network of several micrometers length confirming the long range molecular self-assembly. Electrochemical studies helped to understand the effect of peripheral substitution on the HOMO–LUMO levels and the band gaps.