Dr. Torsten Hegmann

“Chiral Nanoparticles in Liquid Crystal Phases”

Transfer of chirality from an adsorbed molecule to a surface of a metal nanoparticle (NP) depends on the structure of the chiral adsorbate and its interactions with the surrounding,¹ but is often very difficult to elucidate² and hence the major focus of this research project.  Liquid crystals (LCs) known from liquid crystal displays (LCDs) are powerful materials to detect, measure and visualize chirality.  Our group was the first to decorate metal NPs with chiral molecules known to be effective inducers of chiral nematic (N*) liquid crystalline phases. We demonstrated that (S)-naproxen-capped gold NPs induce N* phases in achiral nematic hosts more effectively in comparison to the pure organic chiral dopant, remarkably with the opposite helical sense.^{3, 4} This unexpected result prompted us to take this research into several new directions using more powerful, LC phase-specific chiral inducers, focusing on establishing structural as well as NP size and shape effects.⁵  In due course, we also determined that chiral ligand-capped metal NPs induce chirality much further into the bulk of N-LC phases than their free organic chiral ligand counterparts.⁶  In addition, we are currently testing more complex 2D smectic and 3D-ordered LC phases such as helical nanofilament (HNF)⁷ and twist-bent nematic (N_TB) phases.

REU students in our group will investigate synergies between NPs and LC phases and test new combinations of chiral NPs and LCs (Fig. 1). Chiral NPs are promising for asymmetry amplification at different length scales with examples ranging from chiral catalysts in chemical syntheses to chiral metal materials.  In particular, HNF, N_TB and tilted SmC phases⁸ will be tested for their ability to detect, analyze and tune subtle changes in and effects of chirality at the nanoscale.

Several undergraduate students have contributed to NP-doped LC research in the Hegmann group, producing 20+ undergraduate co-authored papers and 2 patents since 2006. REU students in Dr. Hegmann’s laboratory will acquire hands-on training in the synthesis of chiral organic molecules and metal NPs^{9, 10} and be exposed to a wide-array of characterization tools including, but not limited to NMR, UV-vis, TEM, CD, POM and electro-optic tests.

1. C.J. Baddeley, N.V. Richardson, Chirality at metal surfaces, In Scanning Tunneling Microscopy in Surface Science, Nanoscience and Catalysis, M. Bowker and P.R. Davies, Eds., Wiley-VCH, Weinheim (2010).
2. C. Gautier, T. Bürgi, Chiral gold nanoparticles, ChemPhysChem 10, 483-492 (2009).
3. H. Qi, T. Hegmann, Formation of periodic stripe patterns in nematic liquid crystals doped with functionalized gold nanoparticles, J. Mater. Chem. 16, 4197-4205 (2006).
4. H. Qi, J. O’Neil, T. Hegmann, Chirality transfer in nematic liquid crystals doped with (S)-naproxen-functionalized gold nanoclusters: an indiced circular dichroism study, J. Mater. Chem. 18, 374-380 (2008).
5. A. Sharma, T. Mori, H.-C. Lee, M. Worden, E. Bidwell, T. Hegmann, Detecting, visualizing, and measuring gold nanoparticle chirality using helical pitch measurements in nematic liquid crystal phases, ACS Nano 8, 11966-11976 (2014).
6. T. Mori, A. Sharma, T. Hegmann, Significant enhancement of the chiral correlation lengths in nematic liquid crystals by gold nanoparticle surfaces featuring axially chiral binaphthyl ligands, ACS Nano 10, 1552-1564 (2016).
7. L. Li, M. Salamonczyk, A. Jakli, T. Hegmann, A dual modulated homochiral helical nanofilament phase with local columnar ordering formed by bent core liquid crystals: Effects of molecular chirality, Small 12, 3944-3955 (2016).
8. R. K. Shukla, A. Sharma, T. Mori, T. Hegmann, W. Haase, Effect of two different size chiral ligand-capped gold nanoparticle dopants on the electrooptic and dielectric dynamics of a ferroelectric liquid crystal mixture, Liq. Cryst. 43, 695-703 (2016).
9. H. Qi, B. Kinkead, T. Hegmann, Unprecedented dual alignment mode and Freedericksz transition in planar nematic liquid crystal cells doped with gold nanoclusters, Adv. Funct. Mater. 18, 212-221 (2008).
10. (a) V. M. Marx, H. Girgis, P. A. Heiney, T. Hegmann, Bent-core LC decorated gold nanoclusters: Synthesis, self-assembly, and effects in mixtures with bent-core LC hosts, J. Mater. Chem. 18, 2983-2994 (2008); (b) H. Qi, T. Hegmann, Post-synthesis racemization and place exchange reactions. Another step to unravel the origin of chirality for chiral ligand-capped gold nanoparticles, J. Am. Chem. Soc. 130, 14201-14206 (2008); (c) U. Shivakumar, J. Mirzaei, X. Feng, A. Sharma, P. Moreira, and T. Hegmann, Nanoparticles: complex and multifaceted additives for liquid crystals, Liq. Cryst. 38, 1495-1514 (2011).