On-Surface Chemistry of Helicenes

Department Online Seminar
Date: May 2, 2022
Time: 11:00 AM (Local Time Germany)
Speaker: Karl Heinz Ernst
EMPA, Dübendorft, Switzerland
Host: Martin Wolf

Surfaces functionalized with helicenes are of interest for chiroptical electronic devices or for electron-spin filtering. While self-assembled layers facilitate studying interesting phenomena, covalently linked chiral modified materials would be much more robust and therefore better suited for applications.

We report stereoselective chemistry of different helicene species on noble metal surfaces. The stereochemical analysis of diastereoisomers formed on the surface is performed by scanning tunneling microscopy (STM) and non-contact atomic force microscopy (ncAFM). Depending on the site of bromination at the helical backbone, the Ullmann coupling reaction of different bromohelicenes proceeds stereospecifically to heterochiral or homochiral products. The diastereoselective chemistry is explained by topochemical effects due to the surface alignment of the helicenes during reaction.
Cyclodehydrogenation of bishelicenes was found to proceed also stereo-specifically Upon annealing the substrate to approximately 670 K and subsequent cooling to 50 K, the characteristic twisted shape of the bis[5]helicenes with protrusions is no longer observed and two-dimensional homochiral domains of planar coronocoronene molecular species can be distinguished. The transformation from helical to planar chiral molecules via dehydrogenation and loss of eight hydrogen atoms was confirmed by secondary ion mass spectrometry (ToF-SIMS). Further STM measurements at 7.5 K affirmed the formation of mirror and rotational 2D homochiral conglomerate domains of planar Sp- and Rp-coronocoronene.

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