Invited Paper SS2-TuM9
Conformational Chirality, Chiral Switching and Chiral Induction in Self-Assembled Molecular Structures

Tuesday, November 1, 2011, 10:40 am, Room 110

Chiral self-assembled structures formed from organic molecules have been subject to intense investigation, motivated both by applications such as enantiospecific heterogeneous catalysis as well as by fundamental interest e.g. in relation to the origin of biomolecular homochirality. Chirality on surfaces may arise both for intrinsically chiral molecules and for prochiral molecules that become chiral due to reduced symmetry upon adsorption. However, chiral effects originating from conformational degrees of freedom have received relatively little attention.

Here we use a combination of organic synthesis and UHV-STM experiments to address how rational design of molecular building blocks allows transfer of chirality from the molecular to the supra-molecular level. We investigate a class of custom-designed molecules based on a linear oligo-phenylene-ethynylene backbone and characterize their adsorption structures on the Au(111) surface. Most of these compounds are prochiral and display conformational chirality in the sense that they can adsorb in different chiral conformations distinguished by the positions of two tert-butyl side-groups. A novel chiral switching mechanism, involving a conformational change where the terminal groups rotate around the molecular axis, is directly revealed from time-resolved STM. We demonstrate that it is possible through control of the terminal group functionalization to steer the molecular backbones into surface assemblies that are either mirror symmetric or display pronounced organizational chirality in the form of a characteristic windmill motif. We furthermore achieve control over the absolute chirality of windmill assemblies by synthesizing an intrinsically chiral variant where the tert-butyl side pendant is replaced by a chiral (S)-sec-butyl group. This intrinsically chiral compound is finally used in co-deposition experiments as an induction seed to control the chirality of assemblies formed from the original prochiral compound.

Chiral switching by spontaneous conformational change in adsorbed organic molecules

S. Weigelt, C. Busse, L. Petersen, E. Rauls, B. Hammer, K.V. Gothelf, F.Besenbacher, and T.R. Linderoth, Nature Materials, 5 11 (2006)

Steering organizational and conformational surface chirality by controlling molecular

chemical functionality

C. Bombis, S. Weigelt, M. M. Knudsen, M. Nørgaard, C. Busse, E.Lægsgaard, F. Besenbacher,

K. V. Gothelf, and T. R. Linderoth, ACS NANO 4, 297 (2010).

Controlling chiral organization of molecular rods on Au(111) by molecular design

M. Knudsen, N. Kalashnyk, F. Masini, J. Cramer, E. Lægsgaard, F. Besenbacher, T. R. Linderoth, K. Gothelf, Journ. Am. Chem. Soc. 133 4896 (2011).