By Chilla Malla Reddy
Efforts are being made to produce electronics and devices— such as display screens and mobile phones — that are flexible or foldable. There have been some breakthroughs in this direction. My group at IISER Kolkata accidentally got into research which can help in designing similarly flexible materials, though this was not the initial objective of our work.
Solid substances can be described either as crystalline or amorphous. Crystalline solids are typically hard, dense and very brittle. In this group certain crystalline solids show good electrical conductivity. Amorphous substances, on the other hand, are flexible, ductile and malleable, but most often show poor electrical conductivity. The difference comes from the manner in which molecules are organised in these substances.
In 2009, we were studying the mechanical properties of pharmaceutical solids, the compounds that are used in making drugs. In particular, we were interested in knowing the specific properties, including the crystal structures, that allows some of these compounds to bind into good tablets, while others have to be taken in different forms. It is while studying these compounds that we came across caffeine, a crystalline substance that is commonly found in coffee. Caffeine is used in some pharmaceutical applications, for instance as a central nervous system (CNS) stimulant. We discovered that the molecular complex in caffeine, when mixed in certain acidic solutions, produces highly flexible crystals.
We realised that it was possible for molecular materials to have crystallinity as well as flexibility at the same time. It is at this time that we decided to change course and devote ourselves full time to explore how crystalline compounds can be manipulated and induced with some degree of flexibility.
Key to achieving this is to understand the supra-molecular structure of the substances and see how they can be tweaked. In particular, the attempt is to manipulate the hydrogen bond interactions, which is an active research topic, called Crystal Engineering. For our study, we are using perylene, a polycyclic aromatic hydrocarbon with 20 carbon atoms in a molecule. We are trying to make this compound crystalline and mechanically flexible and observe the electrical conductivity in it.
If we are able to do this, and in general are able to design organic crystals that are flexible, it can be put to a variety of uses. They can be used to construct pressure sensors, bio-electronics that can be put inside human bodies etc. They can also be used to make rollable, foldable materials that can be used in electronic devices.