Kerry W. Hipps

Cooperativity involves electronic communication between multiple metal centers as well as a solid support when the active sites reside on a conducting surface. Such electronic messaging is an important characteristic of molecular recognition and is a key factor in controlling the performance of single atom catalysts (SACs).

Metal porphyrins find wide use in optoelectronics, sensing, catalysis, and other important applications. Recently, covalently linked porphyrins have received considerable attention for use in these applications. The driving forces are the desire to have a high density of single metal ions on a surface, the potential to design in both electronic and steric intramolecular cooperativity, and chemical tunability because of the huge number of metal ions that can be incorporated into the porphyrin system. Self-assembly from fluid solution is one method for creating surfaces with a high density of covalently linked porphyrins. It is almost unexplored because of the combined low solubility of the parent dimer and the tendency of known systems to provide low densities of metal ions and to adsorb in a non-coplanar fashion. In this study we give a detailed analysis of the self-assembled monolayer formed from an alkyne linked porphyrin dimer that forms well-ordered structures upon adsorption from 1,2,4-trichlorobenzene onto HOPG. The compound is a new one designed to give insights into design features necessary for ideal cooperative systems. The adlayer is studied by scanning tunneling microscopy (STM) and by density functional theory (DFT). Combining experiment with computation, the detailed structure of the adlayer is determined. It is found that it has a higher density of metal ions than adlayers of well-known monomeric porphyrins (ZnOEP specifically) or previously reported dimeric porphyrins. The energy of adsorption of the dimer is only slightly dependent on the detailed HOPG adsorption site but changes significantly with the angle the dimer long axis makes with the unit cell vector. This angular dependence is shown to be almost entirely due to dimer-dimer interactions.

Nonconventional hydrogen bonds are attractive molecular interactions that can generate self-assembled, dynamic, yet robust, multifunctional hydrogen-bonded organic 2D and 3D frameworks (HOFs). To effectively create and manipulate such HOF structures requires detailed knowledge of their molecular geometrical ordering properties. Here, we employ scanning tunneling microscopy (STM) and X-ray crystallography to probe the differential control of C-H···O═C hydrogen bonding in the ordering of nonplanar tetraphenylethene (TPE)-based methyl ester derivatives at the solution/solid interface and in the solid state. The esters include the core structures of tetraphenylethene (Me₄TPE), tetraphenylethene biphenyl (Me₂TPDC), and tetraphenylethene tetrakis-phenyl (Me₄ETTC), both meta- and para-substituted. At the solution/HOPG (highly ordered pyrolytic graphite) interface, the esters form surprisingly robust monolayer assemblies stabilized by a combination of multiple intermolecular C-H···O═C hydrogen bonds, π-π interactions, and molecule-substrate interactions. The proposed dominant C-H···O═C bonds involve cyclic motifs that show cooperative recognition together with bifurcated or three-centered hydrogen bonding geometries. Mostly similar conformations of C-H···O═C bonds exist in the solid state. Both molecular geometry and energetic parameters are found to complement each other in determining the extent and type of hydrogen bonding present in the tetraphenylethene-based methyl ester organizations at the solution/solid interface and in the solid state.

Scanning tunneling microscopy (STM) and X-ray crystallography were employed to probe the differential control of noncovalent interactions in the ordering of polynuclear porphyrins and phthalocyanines at the solution/solid interface and in the solid environment. The covalently linked porphyrinoid chromophores (dimers, trimers, and pentamers) were substituted with OR groups where R is an aliphatic chain or phenyl group. These n-mers contained different metal ions. At the solution/solid interface, the n-mer arrays were examined by scanning tunneling microscopy (STM). Both highly ordered pyrolytic graphite, HOPG, C(0001) and Au(111) were employed as substrates. The influence of the metal ions and the nature of the external substituents on the porphyrin on-surface assemblies were explored. The single crystal porphyrin geometries were measured and contrasted with the surface structures. The electronic effects of the substrate used in the STM studies were also considered.

Density function calculations at the meta-GGA level were performed on isolated n-mer phthalocyanines and porphyrins adsorbed on C(0001) through application of the VASP periodic boundary code. Adsorption energies per unit area were computed and analyzed to determine enhancement effects relative to the parent phthalocyanine or porphyrin monomer. Effects on internal molecular configuration for the adsorbed molecule relative to the isolated one were considered. STM images were calculated using the p4vasp code that is based on the Tersoff and Hamann method and compared to experimental images.

Well defined monolayers of solvent incorporated cobalt(II) octaethylporphyrin (CoOEP) on Au(111) can be formed in toluene (TOL) and in 1,2,4 trichlorobenzene (TCB). These structures are pseudo rectangular and one-to-one in composition (REC). These structures only form when the initial exposure of Au is to very dilute solutions of CoOEP. When the initial Au exposure is to high CoOEP concentration solution, only the porphyrin adsorbs in the well-known pseudo hexagonal (HEX) structure. Attempts to convert the REC structure to HEX by the addition of concentrated CoOEP at 298K were never observed for the TCB case, and occurred slowly in the TOL case. Density functional theory (DFT) and statistical mechanics are used to investigate the rates of desorption and the equilibrium concentrations for the TOL-REC, TCB-REC, and HEX system. Two different DFT functional types (GGA and meta-GGA), both with corrections for van der Waals forces, were used. In order to address both the energies of the activated states and the kinetics of desorption in contact with liquid solvent, we will use a method proposed by Charlie Campbell for estimating the difference in adsorption energy of a molecule in the presence and absence of a liquid solvent, which assumes pairwise interfacial bond additivity. The results of these calculations indicate that the REC forms only exist because of the rapid adsorption and desorption of the solvents and the slow desorption and diffusion of the porphyrin. We note that computed desorption rates are extremely sensitive to the actual activation energy and, based on the difference in values produced by the two computational methods, are not quantitively reliable. The computed equilibrium state at 298K, on the other hand, is predicted to be the HEX form independent of computational method and solution concentration for all practical values of the concentration. Thus the REC structures are kinetically controlled while the HEX structure is the equilibrium structure.

Highly oxidized metalloporphyrins (MP) exhibit unusual electronic properties and unique reactivities and are known to serve as intermediates in a number of biological reactions. Of special interest are MP systems that can electronically communicate during the oxidative processes. Model synthetic porphyrins substituted with first row transition metals can react with dihalides and undergo metal oxidation or form MP p-cation radicals. MP dimers that are covalently connected by short alkane or alkene bridges exhibit electronic coupling between the macrocycles which can strongly influence the outcome of the oxidation reaction by dihalides. Metal surfaces also can serve as a conduit for electron transfer and promote cooperative behavior during reactions between adsorbed MP and axial ligands. In this contribution, we present STM experimental results and supporting theoretical calculations of the reactions dynamics of I₂ and metal (Co, Zn, and Ni) porphyrins adsorbed on metallic supports. Single molecule level STM imaging was accomplished in-situ under ambient solution conditions. We uncovered unusual surface chemistry and surface modulated cooperativity in the redox reactions. Computations corroborate our experimental findings. The high-resolution images obtained for these reactions at the single molecule level combined with theoretical studies offer insights into surface redox chemistry, surface cooperativity, and surface-driven catalysis.