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Ligands bind reversibly to metal porphyrins in processes such as molecular recognition, electron transport and catalysis. These chemically relevant processes are ubiquitous in biology and are important in technological applications. In this article, we focus on the current advances in ligand binding to metal porphyrin receptors noncovalently bound at the solution/solid interface. In particular, we restrict ourselves to studies at the single molecule level. Dynamics of the binding/dissociation process can be monitored by scanning tunneling microscopy (STM) and can yield both qualitative and quantitative information about ligand binding affinity and the energetics that define a particular ligation reaction. Molecular and time dependent imaging can establish whether the process under study is at equilibrium. Ligand-concentration-dependent studies have been used to determine adsorption isotherms and thermodynamic data for processes occurring at the solution/solid interface. In several binding reactions, the solid support acted as an electron-donating fifth coordination site, thereby significantly changing the metal porphyrin receptor’s affinity for exogenous ligands. Supporting calculations provide insight into the metalloporphyrin/support and ligand–metalloporphyrin/support interactions and their energetics.

Mechanical properties of six different binary ionic porphyrin crystals with variable morphologies were measured and correlated with their structural properties. These solids were formed from stoichiometric combinations of negatively charged tectons, meso-tetra(4-sulfonatophenyl)porphyrin (TSPP), Cu(II) meso-tetra(4-sulfonatophenyl)porphyrin (CuTSPP), Ni(II) meso-tetra (4-sulfonatophenyl)porphyrin (NiTSPP), and four different cationic tectons, namely, meso-tetra (4-pyridyl)porphyrin (TPyP), tetra([Formula: see text]-methyl-4-pyridyl)porphyrin (TMPyP), Cu(II) meso-tetra([Formula: see text]-methyl-4-pyridyl)porphyrin (CuTMPyP), Ni(II) meso-tetra([Formula: see text]-methyl-4-pyridyl)porphyrin (NiTMPyP), and tetra(4-aminophenyl)porphyrin (TAPP). Crystal structures were determined from single crystal and powder X-ray diffraction patterns. Scanning electron and atomic force microscopes (SEM and AFM) provided topographical information. The common arrangement of the porphyrin tectons within the crystals is consistent with alternating face-to-face molecular arrangement forming coherent columns along the fast-growing long axis which are held together by electrostatic and [Formula: see text]–[Formula: see text] interactions as well as hydrogen bonding. In acquiring the indentation data of the porphyrin crystals using AFM, stress was applied perpendicular to the direction where ionic and [Formula: see text]–[Formula: see text] bonds dominate the packing. At indent loads [Formula: see text]50 nN/nm2, all the porphyrin structures deformed elastically. Young’s modulus ([Formula: see text] values for the different crystals range from 6 to 28 GPa. In a broader perspective, this study highlights the extraordinary mechanical behavior of porphyrin assemblies formed by ionic self-assembly. Judicious selection of charged porphyrin synthons can yield crystalline materials with mechanical properties that combine the elastic characteristics of ‘soft’ polymers with the stiffness of composite materials. Such high-performance materials are excellent candidates for deformable optoelectronic devices.

The mechanism of photoconductivity in a crystalline photoconductor synthesized from 1:1 ratio of meso-tetra(4-pyridyl)porphyrin (TPyP) and meso-tetra(4-sulfonatophenyl)porphyrin (TSPP) ionic tectons was examined. The rod-like crystals of TPyP:TSPP insulate in the dark but become photoconducting on illumination and a portion of the photoinduced current persists after the laser light is turned off. This persistent photoconductivity (PPC) is investigated as a function of laser illumination wavelength, laser power, and sample temperature. The primary charge carriers in the TPyP:TSPP upon photoexcitation are electrons and the charge recombination mechanism follows monomolecular kinetics. The number of electrons contributing to the photocurrent is directly proportional to the number of photons absorbed thus, the mechanisms of the photoconductivity resulting from excitations within the Soret band and the Q-band are the same. The PPC is interpreted to be the result of the formation of photoinduced metastable defects that allow for Miller–Abrahams-like hopping conductivity. The TPyP:TSPP has an incommensurately modulated crystal lattice and its proposed model structure is based on both ionic and neutral porphyrin tectons. The thermogravimetric analysis shows that the porphyrin crystals undergo dehydration on heating (˜50 ∘C) by losing water molecules located in the crystalline channels. Temperature dependent XRD indicates that dehydration causes irreversible changes to the crystal structure. The loss of crystallinity observed with heating the TPyP:TSPP crystals above 90 ∘C causes approximately 25% loss in photoconductivity but has little effect on the lifetime associated with the persistent photoconductivity.

Organic self-assemblies of meso-tetra (4-carboxyphenyl)porphine (TCPP) prepared in trifluoroacetic acid (TFA) and hydrochloric acid solutions at pH < 1 were studied by X-ray photoelectron spectroscopy (XPS) in order to determine the protonation state of the porphyrin building blocks present in the solid self-assembled nanostructures. XPS measurements were conducted both at room and at elevated temperatures. Room temperature N 1s spectra showed two bands with a 3:1 intensity ratio consistent with three protonated and one unprotonated nitrogen in the structures prepared in both TFA and HCl solutions. We attribute this result to TCPP existing as a 50:50 mixture of the free-base and diacid forms of the porphyrin core in the self-assembled state. Upon heating to 150 °C the TCPP/TFA and TCPP/HCl nanomaterials exhibit loss of pyrrolic hydrogens and retain different amounts of their respective counter ions. Atomic force microscopy (AFM) and transmission electron microscopy (TEM) images of the nanostructures showed that these systems have different morphologies depending on the acid employed during fabrication and the post preparation temperature treatment. High resolution transmission electron microscopy (HRTEM) and selected area electron diffraction (SAED) of the TCPP nanostructures indicate that those prepared in HCl are crystalline while the nanoassemblies made in TFA are polycrystalline or amorphous.

Aggregates of tetrakis(4-sulfonatophenyl)phthalocyanine were deposited on Au(111) from solutions of varying pH. The resulting surface species were studied by scanning tunneling microscopy (STM) under ambient and ultra high vacuum conditions. The pH of the solution from which the TSPc is deposited has profound effects on the adsorbed structures. We observed a diversity of surface structures. At low pH web like aggregates composed of vertically stacked coherent columnar assemblies averaging five molecules per stack were observed. Basic solutions yielded dense molecular monolayers.