ABSTRACT

Non-technical abstract:

Liquid crystals are compounds that display "phases" (states of matter) that are more ordered than liquids, but less ordered than crystalline arrangements. LCs are the basis of a huge display industry as well as potential game changers as semiconductors, enabling cheap flexible electronics, organic photovoltaics, and sensors. This project, funded by the Solid State and Materials Chemistry Program in the Division of Materials Research at NSF, breaks a paradigm in liquid crystal (LC) molecular design and drives a transformation in the understanding and use of these important materials. Liquid crystal molecules, and especially discotic (disc-like) LCs, are nearly always built of a rigid central "core" surrounded by long, flexible "tails", which constantly change shape. These highly dynamic tails represent a challenge to computer modeling of LCs and also often compromise conduction in LC semiconductors. How can society better reap the benefits of materials like LC semiconductors while avoiding the limitations imposed by tails? A new light-driven synthesis offers a way out of this conundrum, revealing multiple families of tail-free discotic LCs based on simple building blocks with a few added fluorine atoms. Besides offering LCs of technological interest, these materials promise to shine light on a difficult question: Why do some molecules form LCs while other closely related ones do not? To answer this question and gain insights into the design and synthesis of new semiconductors, the researchers tightly couple experiment and theory. Using computational modeling of the molecules to understand the details of their interaction with each other, new chemical syntheses and measurements of structural and electronic properties the researchers identify new tail-free LCs and increase the understanding of what drives liquid crystallinity in these fascinating systems. Additionally, this project supports a public outreach project, "What?s in the Box?", in which everyday electronics will be deconstructed in a guided, fun atmosphere. Through this activity the public gets an opportunity to better understand everyday electronic technology, and children might become more interested in STEM education.


Technical abstract:

The importance of liquid crystals (LCs) in the display industry is well known, and discotic LCs are promising organic semiconductors. Liquid crystals, and especially discotic LCs, are nearly always comprised of a rigid core surrounded by flexible, highly dynamic, often saturated tails. Families of discotic LCs are usually found empirically, with limited theoretical guidance, partly due to the complexity of modeling the tails. Recent work on fluorinated triphenylene and other fused-ring aromatics demonstrates that there are multiple families of tail-free discotic LCs (TFDLCs). These new materials promise to illuminate a difficult question: Why do some molecules form LCs while other closely related ones do not? TFDLCs are structurally rigid, simple model systems much closer to idealized models of LCs than conventional discotic LCs with tails. In this project, funded by the Solid State and Materials Chemistry Program in the Division of Materials Research at NSF, TFDLCs' small size and rigidity are leveraged to perform quantum ab-initio calculations of intermolecular potentials and enable realistic simulations to discover what forces drive liquid crystallinity. These simulations are built on literature methods, enabling efficient Monte Carlo and molecular dynamics for real molecules including effects of anisotropic potentials and steric interactions. Experimentally, all of TFLDCs are comprehensively characterized, including time-of-flight charge transport and x-ray diffraction in the crystal and discotic phases. The researchers correlate experimental results with the simulations. Design principles, resulting from this work are expected to lead to new classes of organic semiconductors to probe the role that disorder plays in transport, the use of fluorination to enhance electron mobility, and the effects of dimensionality due to strong intercolumn interactions in the absence of tails. Larger TFDLCs are studied to explore potential high mobility discotic semiconductors to enhance device design and performance.

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