AVS 52nd International Symposium
    Surface Science Wednesday Sessions
       Session SS+EM-WeA

Paper SS+EM-WeA7
Conformational Changes and Chiral Ordering in Adsorbed Molecular Layers Investigated by Time-resolved STM

Wednesday, November 2, 2005, 4:00 pm, Room 202

Session: Organic Film Growth and Characterization
Presenter: S. Weigelt, University of Aarhus, Denmark
Authors: S. Weigelt, University of Aarhus, Denmark
C. Busse, University of Aarhus, Denmark
L. Petersen, University of Aarhus, Denmark
T.R. Linderoth, University of Aarhus, Denmark
E. Rauls, University of Aarhus, Denmark
B. Hammer, University of Aarhus, Denmark
K.V. Gothelf, University of Aarhus, Denmark
F. Besenbacher, University of Aarhus, Denmark
Correspondent: Click to Email
Detailed understanding of intermolecular interactions and molecular dynamical processes is required to control, and ultimately exploit, molecular self-assembly on surfaces. Recently, self-assembled structures formed from molecules that become chiral once confined to the surface plane have received particular attention. In this contribution we investigate a family of organic molecules that surprisingly can switch chirality as well as switch between chiral and non-chiral forms after adsorption by undergoing spontaneous conformational changes. The molecules (oligo-(phenylene-ethynylene)'s) consist of a central benzene ring with two or three ethynylene spokes each terminating in a tert-butyl substituted salicylaldehyde moiety. Upon vapour deposition onto the Au(111) surface under UHV conditions, the molecules assume different surface conformers, distinguishable in STM images by the positions of the tert-butyl groups relative to the molecular backbone. Some of these surface conformers are chiral and the chirality of the conformers and the chirality of the assumed molecular tiling patterns are highly correlated. The correlation is enabled by an intra-molecular switching mechanism, allowing the adsorbed molecules to flip between different surface conformers (and hence between different chiral forms) by rotating their end groups around the axis of the ethynylene spoke. This chiral switching enable the molecules to form extended homo-chiral domains by dynamically accommodating to the chiral template found at domain perimeters. We have performed a detailed investigation of this intra-molecular dynamic process by monitoring the surface with time-resolved STM at substrate temperatures in the interval 150-220K. The rate for the conformational change follows an Arrhenius dependence on temperature with activation energy ~0.3 eV. Theoretical modeling is currently being performed to illuminate this novel intra-molecular dynamical process.