Thermotropic liquid crystals are a kind of organic molecules which show order and flow under suitable conditions. The design and synthesis of these molecular materials assumed greater significance for past decade owing to numerous applications. The current approach of design of mesogenic materials emphasize on topology and shape. Accordingly, the packing of them in solid, liquid crystalline and liquid state change thereby influencing the properties. Our group has been actively engaged in addressing these issues with a view to understand the interplay of structure-property relationship.
Optical polarizing microscopy(OPM): OPM is an essential toll for identification of Liquid crystals. Since majority of mesogenic molecules show mesophases above room temperature, the microscope requires hot-stage. When viewed under OPM, the mesogen exhibits birefringence with flow. The characteristic patterns are noted for various categories of phases and based on the texture the meosphase identification is carried out. Typical textures of mesophases synthesized in our lab .
Differential scanning calorimetry (DSC): DSC is another important tool for confirming the mesophase of a mesogen. Since crystal to mesophase and one mesophase to another is mostly first order transition, the associated transition enthalpy would provide information about the nature of the phase. The characteristic DSC feature of mesogen is observation of multiple transitions. Accordingly a mesogen may show more than one peak in DSC and based the transition enthalpy values (?H), the phase identification is done. Typical DSC of a mesogen which shows polymesomorphism .
Powder X-ray diffraction (XRD): Small angle and wide angle powder x-ray diffraction is very important tool in assigning the mesophase of liquid crystalline molecules. Using the variable-temperature XRD, one can clearly assign the mesophase structure at molecular level. Usually a sharp reflection at small angle region is associate with lamellar structure while the similar reflection at wide angle indicates three dimensional crystalline lattice. However, In nematic phase broad diffuse pattern at wide angles indicates the dynamics of the terminal chains. Additionally information pertaining to molecular length, layer separation, lamellar versus columnar packing can be obtained from this tool. XRD of a typical mesogen structure in crystalline phase .
Solid-state 13C NMR (SSNMR): In recent years, high-resolution solid state NMR has gaining tremendous importance as a tool for characterization of mesogens. Solution NMR is routinely used in organic chemistry for structural characterization. However, the cross-polarisation and proton-decoupled (SSNMR) NMR of mesogens in mesophase provide wealthy information. Since liquid crystalline molecules possess anisotropic properties in mesophase due to orientational order, the chemical shift anisotropy which vanishes in solution due to molecular tumbling appears partially in the mesophase. The alignment induced chemical shift anisotropy (AIS) can give information about the molecular alignment in the magnetic field. The advent of 2D SSNMR techniques like Separated Local Field spectroscopy (SLF) would provide information about the orientational order and molecular topology. Figure 4A shows the NMR in solution, solid and mesophase of a mesogen. The solution NMR shows sharp lines characteristic of isotopic motion. In the solid state (DD-CP-MAS), the lines are broad and low intense. However in the liquid crystalline phase, the lines are sharp and more intense and the chemical shifts are entirely different indicating the alignment which is characteristic of mesogens. Figure 4B shows typical 2D NMR of mesogen in the phase. The 13C-1H dipolar contours provide information about the orientational order and molecular topology. In other words, this technique facilitates to map the topology of the mesogen. Accordingly, liner, hockey-stick, bent-core, lambda, star and other shapes of the mesogens in the mesophase can be precisely determined. This would enable one to determine the packing of the mesogen molecules in the phase. Additionally, the order and dynamics of the segments of molecule can in principle possible to address using variable-temperature SSNMR.
The broad objectives of our group are as follows
- Synthesis of liquid crystalline molecules using multi-step synthetic approach.
- Structural characterization using FT-IR, solution 1H and 13C 1D and 2D techniques.
- Liquid crystalline mesophase characterization using OPM, DSC, Powder X-ray and Solid State NMR techniques.
- Synthesis and characterization of side chain and main chain liquid crystalline monomers and polymers.
- Advanced studies of thermotropic liquid crystals using High-resolution solid state 13C NMR (1D and 2D) to probe molecular level information using orientational order, topology and dynamics.