Considerable progress has been made in recent years in the search for synthetic methods leading to functionalized porphyrins, especially for modification of either the β‐ or meso positions. For the latter, total synthesis based on condensation methods or partial synthesis through functionalization of preformed porphyrin have emerged as possible methods. The increasing number of possible technical and medicinal applications for unsymmetrically meso‐substituted porphyrins requires straightforward methods for the preparation of the so‐called ABCD‐porphyrins, i.e., porphyrins with up to four different meso substituents. Here, we describe new strategies for the synthesis of ABCD‐type porphyrins based on porphyrin reactions with organolithium reagents and the use of Pd‐catalyzed coupling reactions. With the whole repertoire of contemporary functionalization methods, a comprehensive analysis and comparison of the various strategies for A‐, AB‐, A2B‐, ABC‐, A2BC‐ and ABCD‐type porphyrins is given. In addition, we report on the synthesis of new functionalized derivatives for some of these porphyrin classes. In practical terms and taking an applied‐science‐oriented approach, the synthesis of unsymmetrically meso‐substituted porphyrins is best accomplished by a combination of well‐developed condensation methods with subsequent functionalization by organolithium compounds or transition‐metal‐catalyzed coupling protocols. The methods described are suitable for the preparation of porphyrins for many divergent applications ranging over amphiphilic porphyrins for photodynamic therapy, push‐pull systems for optical applications and chiral systems useful in catalysis to donor–acceptor systems suitable for electron‐transfer studies.

Investigations of chemical bonding and electronic structure features for polycrystalline (porphyrinato)nickel (II) (NiP, the simplest Ni porphyrin), (5,10,15,20-tetraphenylporphyrinato)nickel (II) (NiTPP) and (2-aza-21-carba-5,10,15,20-tetraphenylporphyrinato)nickel (II) (N-confused NiTPP, NiNCTPP) have been performed by means of high-resolution soft x-ray absorption and x-ray photoemission spectroscopy. The Ni 2p(3/2) x-ray absorption spectra show strong π-back-bonding in these compounds leading to a high-energy shift (1.2 eV for the NiP and NiTPP) of the entire absorption structure compared to Ni metal. It has been found that the main absorption line of the Ni 2p(3/2) spectrum of the NiNCTPP is shifted by an additional 0.5 eV to higher energies in comparison with those for other nickel porphyrins. This shift is evidence of stronger back-donation (metal-to-ligand charge transfer) and a smaller effective number of 3d electrons on the central Ni atom in the NiNCTPP as compared to other Ni porphyrins. The confused N atom in the NiNCTPP is of pyrrolic type (protonated nitrogen), which was confirmed by the N 1s absorption and core-level photoemission spectra.

The growth and ordering of {5,10,15,20-tetrakis(4-bromophenyl)porphyrinato}nickel(II) (NiTBrPP) molecules on the Au(111) surface have been investigated using scanning tunnelling microscopy, X-ray absorption, core-level photoemission, and microbeam low-energy electron diffraction. When deposited onto the substrate at room temperature, the NiTBrPP forms a well-ordered close-packed molecular layer in which the molecules have a flat orientation with the porphyrin macrocycle plane lying parallel to the substrate. Annealing of the NiTBrPP layer on the Au(111) surface at 525 K leads to dissociation of bromine from the porphyrin followed by the formation of covalent bonds between the phenyl substituents of the porphyrin. This results in the formation of continuous covalently bonded porphyrin networks, which are stable up to 800 K and can be recovered after exposure to ambient conditions. By controlling the experimental conditions, a robust, extended porphyrin network can be prepared on the Au(111) surface that has many potential applications such as protective coatings, in sensing or as a host structure for molecules and clusters.

A strong molecule-surface interaction between free-base-tetra(4-bromophenyl)-porphyrin and Cu(111) results in a distortion of both the molecule and the underlying copper surface in the vicinity of the molecule. This in turn leads to the formation of an intermediate complex due to bonding between the iminic nitrogens and surface copper atoms.

The growth and ordering of 5-(10,15,20-triphenylporphyrinatonickel(ii))dimer (NiTPP-dimer) molecules on the Ag(111) surface have been investigated using scanning tunnelling microscopy/spectroscopy (STM/STS) and low-energy electron diffraction (LEED). At one monolayer (ML) coverage the NiTPP-dimer forms a well-ordered close-packed molecular layer in which the porphyrin molecules have a flat orientation with the molecular plane lying parallel to the substrate. STM and LEED data obtained from one monolayer of the NiTPP-dimer on the Ag(111) surface show the formation of three domains which grow along the main crystallographic directions of the substrate. Scanning tunnelling spectroscopy data obtained from the NiTPP-dimer on the Ag(111) surface show good agreement with optical band gap measurements and density functional theory calculations.