The on-Surface Supramolecular Chemistry of Porphyrins, Phthalocyanines and Sub-Phthalocyanines: How Shape and Functionalization Change the Game

Abstract

By their role in biochemistry, porphyrins have played a central role for “Chemistry beyond the molecule”, long before the term ‘supramolecular chemistry’ has been coined. It is interesting that the bio-chemical functions of porphyrins predominantly derive from the center metal atom, its coordination shell and the coordination-site specific properties. The integration of porphyrins into bio-molecules is safeguarded by covalent chemistry and not ‘supra-molecular’ as such. Nevertheless, square planar porphyrins and pthalocyanines take a decisive role in the development of on-surface supra molecular chemistry: This for their excellent stability and for the many ways they can be functionalized with ligands. Thereby, porphyrin and phthalocyanine chemistry paved the way towards the systematic investigation of many different functional groups known to direct self-assembly and their influence on the formation of on-surface supramolecular layers and chains. I will review the results of on-surface supramolecular assembly experiments performed at the solid-vacuum interface with CN-[1], CN-phenyl, alkoxy [1], fluoro [2] and fluorophenyl [3] and di-methylene-aniline derived functional groups and discuss the supramolecular ordering in terms of estimated binding energies and formation kinetics. It is also interesting to compare the supramolecular ordering in fluids with the constrained ordering of molecules confined to surface substrates. Further insight into the directing power of the individual functional groups derives from a fresh look at the three-fold symmetric pyramidal sub-Phthalocyanines.Fig. 1) Supramolecular ordering of substituted porphyrins in networks fromed by CN-phenyl interaction: a) and b) show higher magnification micrographs of c) and d) in two different imaging modes emphasizing the pi orbitals (HOMO) and the alkyl substituants, respectively. e) shows the dynamicity of a network from sterically more demanding alkyl-phenyl substituents. (for further details see [1] and references therein)[1] N. Wintjes et al. J. Am. Chem. Soc. 2010, 132, 7306–7311[2] J. Girovsky et al. Nature Communications 2017 8:15388[3] S. Nowakowska et al. ACS Nano 2018, 12, 768−778. Figure 1