A Self-Assembled Covalently Linked Porphyrin as a High-Density Reaction Center

Metal porphyrins find wide use in catalysis, photoelectronics,
sensing, and other applications. The application of covalently linked
porphyrins is a new area that deserves attention. The driving forces
are the desire for a high density of single metal ions on a surface,
the potential to design in both electronic and steric intramolecular
cooperativity, and chemical tunability. One method for creating surfaces
with a high density of covalently linked porphyrins, self-assembly
from fluid solution, is almost unexplored. In this study we give a
detailed analysis of the self-assembled monolayer formed from an alkyne
linked porphyrin dimer, 1,4-Bis{[5,10,15,20-tetra(n-hexyl)porphyrinato]zinc(II)}butadiyne, that forms well-ordered structures
upon adsorption on highly oriented pyrolytic graphite (HOPG) from
1,2,4-trichlorobenzene. The adlayer is studied by scanning tunneling
microscopy (STM) and by density functional theory (DFT). By combining
experiment with computation, the detailed structure of the adlayer
is determined. It is found that it has a higher density of single
metal atoms (0.66 sites/nm2) than adlayers of well-known
dimeric porphyrins or monomeric zinc octaethylporphyrin. DFT is key
to understanding the alkane chain disposition and its possible role
in cooperativity.