Ground-breaking discoveries in liquid crystals

[Nature India Special Issue: Lighting the way in physics]

Liquid crystal (LC) phase is an intermediate phase between a solid and a liquid phase. Liquid crystals generally have rod-like molecules, but in the 1970s there were attempts to find out whether disc-like molecules, being anisotropic in shape, can exhibit a liquid crystal phase.

The project was taken up by Sivaramakrishna Chandrasekhar in the 1970s who was then the head of the Liquid Crystal Laboratory at the Raman Research Institute (RRI). Chemistry professor Ganugapati Sree Rama Subba Rao at the Indian Institute of Science suggested a few molecules for synthesis including benzene-hexa-n-alkanoates (BHAs). At the beginning of 1976, the synthesis was taken up by Shyam Singh who joined RRI as a scientist after his postdoc in the UK. Since Kattera Appanna Suresh had worked and gained expertise in optics, thermodynamics, texture studies and X-ray diffraction, Chandrasekhar assigned him to study Singh’s BHA samples. Suresh found the samples to be somewhat impure although they did exhibit an LC phase. Singh was trying to purify them, but had to put this on pause to take a month’s leave for his sister's wedding. Chandrasekhar became impatient and asked Bookinkere Kapanipathaiya Sadashiva, then a PhD student, to continue Singh's work. Sadashiva prepared pure samples of BHA. Texture and thermodynamic studies of Suresh found they exhibited the LC phase.

Suresh took X-rays to investigate the structure of BHA in the LC phase. The powder diffraction patterns at room temperature showed features of a crystalline phase which on heating to LC phase exhibited smectic-like features. After much experimentation, Suresh was able to get an X-ray of an aligned sample that showed a hexagonal symmetry which appeared to be more like a crystalline phase. Chandrasekhar thought that the sample had crystallized and was not in LC phase. Suresh repeated the X-rays many times and consistently obtained pictures showing hexagonal symmetry. He was convinced that it was a picture of BHA in the LC phase. He proposed a model of the discs stacked one on top of the other in columns which constitute a hexagonal columnar structure.

In the meantime, Gobbalipur Shamanna Ranganath came up with a theoretical model consisting of sheets, each sheet containing a hexagonal close-packed arrangement of discs. After much deliberation, it was agreed that the X-ray investigations supported Suresh’s model of hexagonal columnar structure, but Ranganath’s model was also stated in the paper as a possibility. After the publication of the findings, the scientific community accepted the model of hexagonal columnar structure with a two dimensional hexagonal order and a liquid-like order in the third dimension as the correct representation of the discotic columnar LC phase.

The paper turned out to be the first report of a thermotropic liquid crystal phase formed with a pure, single component system of disc-like molecules. This opened up an entirely new branch in the field of liquid crystals, namely, discotic liquid crystals (DLC). Hundreds of papers followed, reporting on the synthesis of numerous discotic molecules and their physical properties. Many variants of the discotic columnar phase were found. They have been classified into hexagonal, columnar, rectangular columnar, helical columnar, tetragonal columnar and tilted columnar phases. Most of the DLCs are formed by molecules with aromatic cores. They tend to form the columnar phase due to π-π interactions of the aromatic cores. The aromatic cores facilitate charge transfer along the stacking direction. The DLCs have been used as materials for a new generation of organic electronics, as one-dimensional electrical conductors, materials for organic light emitting diodes, discotic photovoltaic devices, discotic field effect transistors and discotic solar cells.

In addition to columnar phases, it has been found that disc-like molecules can form a discotic nematic (DN) phase. DN phase consists of discotic molecules oriented about their disc’s normal axis or short molecular axis. The molecules possess translational and rotational freedom about their disc’s normal axis.

In the DN phase, the discotic molecules have orientational order but no long-range translational order. Many materials have been synthesized which exhibit DN phase at room temperature. This has paved the way to create new display devices. These displays show significant improvement relating to wide and symmetrical viewing angle profile compared with conventional LC displays.

Doping of nanoparticles (NPs) in DLCs has become a fast developing area of research. These DLC-NPs hybrid systems have potential for significantly improving electronic devices. In a simple procedure, a small amount of functionalized NPs are dispersed in different DLCs. For example, hexanethiol-coated gold NPs, hexadecylamine-coated gold NPs and functionalized carbon nanotubes have markedly improved opto-electronic properties.