Comparing Cyanophenyl and Pyridyl Ligands in the Formation of Porphyrin-Based Metal–Organic Coordination Networks

Abstract

In recent studies, porphyrin derivatives have been frequently used as building blocks for the fabrication of metal–organic coordination networks (MOCNs) on metal surfaces under ultrahigh vacuum conditions (UHV). The porphyrin core can host a variety of 3d transition metals, which are usually incorporated in solution. However, the replacement of a pre-existing metal atom in the porphyrin core by a different metallic species has been rarely reported under UHV. Herein, we studied the influence of cyanophenyl and pyridyl functional endgroups in the self-assembly of structurally different porphyrin-based MOCNs by the deposition of Fe atoms on tetracyanophenyl (Co-TCNPP) and tetrapyridyl-functionalized (Zn-TPPyP) porphyrins on Au(111) by means of scanning tunneling microscopy (STM). A comparative analysis of the influence of the cyano and pyridyl endgroups on the formation of different in-plane coordination motifs is performed. Each porphyrin derivative formed two structurally different Fe-coordinated MOCNs stabilized by three- and fourfold in-plane coordination nodes, respectively. Interestingly, the codeposited Fe atoms did not only bind to the functional endgroups but also reacted with the porphyrin core of the Zn-substituted porphyrin (Zn-TPyP), i.e., an atom exchange reaction took place in the porphyrin core where the codeposited Fe atoms replaced the Zn atoms. This was evidenced by the appearance of molecules with an enhanced (centered) STM contrast compared with the appearance of Zn-TPyP, which suggested the formation of a new molecular species, i.e., Fe-TPPyP. Furthermore, the porphyrin core of the Co-substituted porphyrin (Co-TCNPP) displayed an off-centered STM contrast after the deposition of Fe atoms, which was attributed to the binding of the Fe atoms on the top site of the Co-substituted porphyrin core. In summary, the deposition of metal atoms onto organic layers can steer the formation of structurally different MOCNs and may replace pre-existing metal atoms contained in the porphyrin core.

Structure and Stability of Metalloporphyrin Networks on Au(111)

“…We have performed calculations for free-standing MOCNs with the FeTPyP S and P conformers and for structures with one and four FeTPyP–Fe c units per cell that are comparable to those published in the literature for the same or related systems. ,, The 2D molecular network has a square periodicity in every arrangement that has been looked at, and the long axes of the FeTPyP molecules are rotated ∼45° with respect to the unit cell vectors. As a result, Fe c and all N py atoms of neighboring molecules can form equidistant coordination bonds.…”

“…We run calculations for free-standing SAMs of the S and P conformers of FeTPyP with structures involving various numbers of molecules per unit cell, similar to those reported in the literature for this one or similar molecules. ,− Because our calculations indicate that the isolated P conformer is only 20 meV less stable than the S conformer, it is important to investigate whether the intermolecular interactions in the SAM might make up for this little energy difference, favoring a conglomerate of P conformers. We consider a structure of the free-standing SAM, inspired by the CoTPP/Au(111) and Ni–DPPyP/Au(111) structures, − with just one molecule per (square) unit cell (hereafter termed 1 × 1­(X), with X = S, P), and the molecular axis containing two opposing pyrrole rings rotated by ∼30° with respect with respect to the horizontal lattice vector defining the unit cell (see Figure a,b).…”

“…It is well known that iron atoms have been observed to form both 3-fold and 4-fold coordination centers . It has been noted that structures with 3-fold coordination are less compact than those with 4-fold coordination.…”

“…In this context, a particularly interesting molecule is metal–tetrapyridylporphyrin (hereafter termed MTPyP) whose four peripheral meso substituents are pyridyl groups containing one terminal nitrogen. Such N atoms can not only strengthen the intermolecular interactions because of the formation of H ···N hydrogen bonds, but also provide an auspicious environment for new (intermolecular) coordination sites for extra metal atoms M′which can be dosed by evaporation over a MTPyP self-assembled monolayer (SAM) , or can proceed from the metal support − to form a metal–organic coordination network (MOCN).…”

Structural Influence on Exciton Migration and Singlet Oxygen Photosensitization in Porphyrinic Metal–Organic Coordination Networks

“…The node is generally a polymetallic zero-dimensional (0D)- or one-dimensional (1D) secondary building unit (SBU). In contrast, metal–organic coordination networks (MOCNs) are a subgroup of MOFs, where the node consists of a single metal atom, for which porphyrin-based ligands turn out to be ideal building blocks. − This versatile linker benefits from intense absorption bands in the visible region that allows porphyrin-based MOFs to perform well in fields such as photo-induced eradication of pollutants and toxins in water, photosensitization of singlet oxygen ( 1 O 2 ) for antibacterial and fungicidal purposes (in solution or in contact with air), photocatalytic transformation of organic molecules, and cancer therapy . Concurrently, porphyrin-based MOCNs also exhibit rich visible-light-driven properties such as degradation of organic molecules and pollutants, − organic synthesis, , generation of solar fuels, , and even solar cells. − These photocatalytic and photosensitizing behaviors are profoundly influenced by the morphology of the solid ,, and are also known to affect the nonlinear optical properties of solid samples. − For instance, a direct link between excitation energy migration and upconversion processes was recently established …”