Our group is interested in the preparation of chiral materials in which small quantities of chiral “dopants” can control or affect the properties of bulk materials.  In particular, we have been studying Chiral mesoporous materials; organic/inorganic composites (PMOs) that have nanosized pores. By using a mixture of functionally achiral bulk material and chiral enantiomerically pure dopant molecules, we have been able to prepare a material that is nanoporous and has chirality built into the walls of the material. Analysis of these materials by CD spectroscopy indicates the formation of new chiral aggregates in the solid state consistent with charge delocalization from adjacent aryl rings in close contact. These aggregates are only observed when the biphenyl and enantiomerically pure biaryl are both present.[1]

chiral materials 1

The use of chiral dopants to affect the chiral morphology of mesoporous materials was also studied and it was found that as the chiral dopant increased, the twisting of the resulting material also increased![2]

 

chiral materials 2

A similar effect was observed when dopants were added that were either (1) able to cross link and polymerize within the material since they each have two reactive sites or (2) had only one siloxane site so acted at end groups.  In the former case, twisted structures were observed and in the latter, only spherical morphology.[3]

chiral materials3

In more recent work, we were able to elucidate some of the key features in chirality transmission. As might be expected, the point of attachment of the siloxanes has a dramatic effect on the transmission of chirality.  Since the polymerization “freezes” the dihedral angle of the chiral dopant, chirality transfer is much more effective if the polymerization points are on the same axis as the chirality in atropisomeric dopants.  This is especially important since a wide range of angles are accessible at room temperature but not all are optimal for transmission of chirality.  Interestingly, although the optimal dihedral angle is about 90°, as shown in green, a significant range of other angles are all accessible at room temperature (yellow to red).

chiral materials 4

We are currently working to expand our knowledge of the factors that control transmission of chirality and also examining these materials in catalysis.

 

[1]S.L. MacQuarrie, M.P. Thompson, A. Blanc, N.J. Mosey, R.P. Lemieux, and C.M. Crudden “Chiral Mesoporous Materials Based on Axial Chirality” Journal of the American Chemical Society, 2008, 130, 14099-14101.

[2]X. Wu, T. Blackburn, J. D. Webb, A. E. Garcia-Bennett, C.M. Crudden, “A Novel Method for the Synthesis of Chiral Periodic Organo-Silica Materials with Ordered Supermicropores”, Angewandte Chemie International Edition, 2011, 35, 8095-8099.

[3] X. Wu, C.M. Crudden, “Chiral Hybrid Mesoporous Silicas: Assembly of Uniform Hollow Nanospheres and Helical Nanotubes with Tunable Diameters” Chemistry of Materials,2012, 24, 3839-3846