Dresden 2009 – scientific programme

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O: Fachverband Oberflächenphysik

O 44: Metal substrates: Adsorption of organic / bio molecules VI

O 44.1: Talk

Thursday, March 26, 2009, 10:30–10:45, SCH A118

Self-Assembly Induced Chirality in CoTPP on Cu(110) — •Abel Robin, Philip Donovan, Rasmita Raval, Matthew Stephen Dyer, and Mats Persson — University of Liverpool, Liverpool, U.K.

We describe the adsorption of Co(II) tetraphenylporphyrin (CoTPP) on Cu(110) using a combination of STM and periodic DFT. Porphyrins’ electronic, magnetic and catalytic properties make them an attractive target for introducing organised functionality at surfaces.

We show that CoTPP forms highly ordered 2D arrays on Cu(110). Importantly, the self-assembled structures exhibit chirality on both organisational and molecular levels, creating equal and opposite mirror domains. The expression of chirality in adsorbed porphyrin systems has only been observed rarely. Here we delineate important factors driving chiral expression in a per se non-chiral system.

DFT shows a strong energetic preference for molecules adsorbing in bridge sites, accompanied by a significant charge exchange between the macrocycle and the substrate. Optimised intermolecular and molecule-substrate interactions rotate and tilt the phenyls, endowing the porphyrins with a propeller-like, thus chiral, conformation. These propeller-like molecules assemble along two non-symmetry directions, leading to large chiral domains mirrored across the main copper axes.

For the predominant structure we distinguish two sets of diametrically opposite phenyls possessing distinct interactions with the substrate. Because of this anisotropy, assembly along one chiral direction is less flexible than along the other. Our DFT calculated images correspond well with the STM images for this structure.