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Spontaneous Achiral Symmetry Breaking in Organic Fluids by Professor David...

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San Diego State University

5500 Campanile Dr

Department of Chemistry and Biochemistry, GMCS 333

San Diego, CA 92182

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ABOUT THE SPEAKER

Currently Professor in the CU Department of Chemistry, Dr. Walba also holds an appointment in the CU Department of Chemical and Biological Engineering and serves as the Associate Director of the Soft Materials Research Center.

After receiving a B. S. degree in Chemistry from the University of California at Berkeley (1971), he studied at the California Institute of Technology under the direction of Robert E. Ireland, receiving his Ph.D. on the total synthesis of natural products in 1975. After two years of postdoctoral research on host-guest chemistry with Donald J. Cram at UCLA, Walba started his academic career as Assistant Professor of Chemistry at CU Boulder in 1977, where he has worked since.

Professor Walba's teaching and research programs emphasize organic chemistry, and in particular stereochemistry. For his work in teaching and research, he has been made a Fellow of the American Association for the Advancement of Science (1999), a Camille and Henry Dreyfus Teacher Scholar (1984-1986), and a Fellow of the A.P. Sloan Foundation (1982-1984). In addition, Dr. Walba was a co-founder and Vice President for Chemical Research (1984-1994), and member of the Board of Directors (1994-2001), of Displaytech, Inc.

ABOUT THE PRESENTATION

Liquid crystals (LCs) are organic materials exhibiting spontaneous symmetry-breaking in the liquid phase, producing structurally (and optically) anisotropic fluids. Since the discovery of LCs in the late 1880s, chirality has played a key role in the field. Not surprisingly, throughout most of the long history of the study of LCs, the formation of chiral LC phases required enantiomerically enriched chiral molecules – either the LC molecules themselves, or through the addition of chiral dopants.

However, beginning in 1997 the discovery of chiral LC phases formed by achiral or racemic molecules led to the characterization of several variations of spontaneous achiral symmetry- breaking in fluids. These represent fluid analogs of the achiral symmetry breaking occurring upon crystallization of sodium ammonium racemate (racemic tartaric acid) discovered by Pasteur 171 years ago.

LCs showing achiral symmetry breaking are most often (perhaps always) of the “bent-core” variety – the classic example being formation of chiral smectic (layered) phases from a class of resorcinol diesters possessing two “mesogenic” wings. In that system, four diastereomeric phases are formed, two d,l pairs, and two “meso” LC phases. Since the smectic layering is “long-range,” these materials can be considered crystalline in one dimension, though they show fluid rheology.

Then, in 2009, two variants of “dark conglomerates” were described. In these phases, the smectic layering is only short-range, with fluid order giving rise to chiral, optically isotropic fluids (dark between polarizer and analyzer) with optical rotatory power for visible light on the order of 0.1°/μm. Finally, in 2013 bent-core materials possessing fluid order in three dimensions were proven to exist. In these phases, known as “twist-bend nematics,” packing in the fluid phase favors a spontaneous twist, affording an optically anisotropic conglomerate lacking any layering.

These liquid manifestations of Pasteur’s crystal conglomerates will be described.

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San Diego State University

5500 Campanile Dr

Department of Chemistry and Biochemistry, GMCS 333

San Diego, CA 92182

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