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.