NCAFM2023 Programme Booklet

Thursday 1120 - 1140

LOCAL REACTIVITY OF PORPHYRINS

Nan Cao 1 , Jonas Björk 2 , Eduardo Corral 1 , Mathias Senge 3 , Johannes V. Barth 1 , Alexander Riss 1,*

1 Physics Department E20, Technical University of Munich, D-85748 Garching, Germany 2 Department of Physics, Chemistry and Biology, IFM, Linköping University, 58183 Linköping, Sweden 3 Institute for Advanced Study (TUM-IAS), Focus Group – Molecular and Interfacial Engineering of Organic Nanosystems, Technical University of Munich, D-85748 Garching, Germany

Email: a.riss@tum.de

Understanding the reactivity of porphyrins is crucial for synthesizing novel molecular compounds, developing new therapeutic agents, and enhancing our understanding of reaction mechanisms in organic chemistry and biology. The reactivity of free-base porphyrins is related to their aromaticity, i.e., the occurrence of an aromatic pathway that gives rise to two different types of carbon atoms at the “corners” of the macrocycle [1,2] (Fig. 1a). However, the effect of aromaticity remains largely unexplored for surface-supported molecules, where the substrate can substantially influence the electronic and structural properties of the adsorbates. In this work, we systematically investigate and quantify how aromaticity impacts the on-surface reactivity of porphyrins using scanning tunneling microscopy (STM) and bond-resolved atomic force microscopy (AFM) (Fig 1b). Additionally, we directly measure the interaction of single porphyrin molecules with functionalized AFM tips (Fig. 1c), providing valuable insights into the forces acting between the porphyrins and potential reactants.

Fig. 1 Effect of aromaticity on the chemical reactivity of surface-supported porphyrins. (a) Aromatic pathway in a free-base porphyrin, giving rise to two chemically different types of ß-carbons at the “corners” of the macrocycle. (b) 3D rendering of STM and AFM measurements of the products of cyclization and coupling reactions of alkyne-substituted porphyrins. These experiments reveal a substantially lower reactivity of the β -carbon atoms within the aromatic pathway. (c) Interaction forces between a free-base porphyrin (atomic positions overlaid in white color) and a functionalized AFM tip, showing a twofold symmetry.

References [1] S. Fatayer et al. Science 365 , 142 (2019) [2] J. I. Wu et al. JACS 135 , 315 (2013)

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