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

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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