Joydip De

Applications of liquid crystals in biosensing and organic light-emitting devices: future aspects

ABSTRACT This article summarises recent advances made in our laboratory towards the development of new technological applications, such as biosensors and organic light-emitting diodes (OLEDs) based on liquid crystals (LCs) other than LC displays. The study of biomolecular interaction using LC material relies on the specific interaction between the LC and the biomolecule of interest at interfaces that permit the biomolecular events to be amplified into easily measured signals for various sensing applications. In the first part, we emphases recent studies in the design and modulation of LC-based interfaces based on robust colloidal LC gels for biological amplification, qualitative and quantitative understanding of important biomolecular interactions at LC–aqueous interfaces for diagnostic and laboratory applications and design of LC droplets that hold promise to act as a marker for cells and cell-based interactions. In the second part, we described design of organic materials for application in OLEDs on various discotic monomers, dimers and oligomers. These molecules have the ability to transport charges, holes and electrons. In addition, because of the high conductivity and π–π stacking, they are considered as the advanced materials for practical applications. The technological advances in our laboratory using discotic LCs will be briefly presented in this article. GRAPHICAL ABSTRACT

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.

Heptazine: an Electron-Deficient Fluorescent Core for Discotic Liquid Crystals

Herein, room-temperature discotic liquid crystals based on heptazine, an electron deficient central core, are reported for the first time. Mesomorphic behaviors of the materials are also investigated. Supramolecular assembly of the mesophase derivatives were confirmed by X-ray scattering experiments. Heptazine-based solid thin films are strong blue light emitters, whereas in the solution state, they are weakly emissive or non-emissive. The band gap energy is found to be low in this class of compounds. Formation of room-temperature mesophases, low band-gap behavior, and strong blue-light emission in the solid state are promising attributes for optoelectronic applications of the materials.

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.

Phase behavior of a new class of anthraquinone-based discotic liquid crystals

Five novel columnar liquid crystalline compounds (4.1-4.5) consisting of a central anthraquinone core carrying four alkoxy chains (R = n-C6H13, n-C8H17, n-C10H21, n-C12H25, and 3,7-dimethyl octyl) with two diagonally opposite 1-ethynyl-4-pentylbenzene units were synthesized, and their phase transitions were investigated between changes in the molecular structure and their self-assembly into the columnar mesophases. Small and wide-angle X-ray scattering (SAXS/WAXS) studies were performed to deduce the exact nature of the mesophases, and their corresponding electron density maps were derived from the intensities of the peaks observed in the diffraction patterns. A comparison of compounds with different alkoxy chains indicated that the soft crystal columnar rectangular (Crcolrec) phase was stable at lower temperature for the shortest peripheral alkoxy chain (4.1; R = n-C6H13) and was found to exhibit the columnar hexagonal (Colh) phase and then the discotic nematic (ND) phase with increasing temperature. In contrast, increasing the peripheral chain length to n-C8H17 or the branched one (4.2 and 4.5) stabilized the Colh phase at lower temperature and showed the ND phase at higher temperature. Further increase in chain length (4.3 and 4.4; n-C10H21, n-C12H25) demonstrated the formation of the ND phase. Conductivity measurement in the Colh mesophase was found to be almost 10 times higher in magnitude than the corresponding Crcolrec phase. The HOMO-LUMO band gap of all the compounds was found to be in the range from 2.79 to 2.82 eV, which is quite less and comparable with the optical energy band gap.

Hydrogen-bond mediated columnar liquid crystalline assemblies of C3-symmetric heptazine derivatives at ambient temperature

A new class of hydrogen (H) bonded fluorescent liquid crystals (FLCs) based on a newly discovered s-heptazine fluorophore discotic component have been synthesized. The tendency of the s-heptazine core to form H-bonded LCs has been explored for the first time. Interestingly, the pure heptazine derivatives (non-mesomorphic) on complexation with tri-alkoxy benzoic acids exhibit enantiotropic columnar mesomorphism over a wide range of temperatures including room temperature. This indicates the strength of the resulting H-bonded complexes. The H-bonded supramolecular complexes were studied through FT-IR, temperature dependent FT-IR and NMR studies and H-D exchange studies, and their thermal behaviour was deduced through polarized optical microscopy (POM), differential scanning calorimetry (DSC) and X-ray diffraction (XRD) studies. Because of the inherently fluorescent pure heptazine derivative, the resulting complexes exhibit fluorescent behaviour in the solution state as well as in the solid state.

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.

Aggregation dependent fluorescence switching in benzothiazole derivative based H-bonded mesogen

ABSTRACT In this work, hydrogen-bonded liquid crystal exhibiting phase dependent fluorescence has been reported. The H-bonded materials were prepared using pyridenyl benzothiazole and alkoxy benzoic acid as H-bond acceptor and donor respectively. The formation of the target materials was confirmed via different spectroscopic technique and their mesogenic behaviour has been studied through polarising optical microscope, differential scanning calorimetry and 2D-X-ray diffraction. The H-bonded mesogenic material showed aggregation dependent fluorescence switching when subjected to transformation from isotropic-nematic-crystalline phase sequence. Computational studies were performed to probe into fluorescence switching behaviour of the mesogenic material. GRAPHICAL ABSTRACT