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Kinetic analysis of surface reactions at the single molecule level is important for understanding the influence of the substrate and solvent on reaction dynamics and mechanisms, but it is difficult with current methods. Here we present a stochastic kinetic analysis of the oxygenation of cobalt octaethylporphyrin (CoOEP) at the solution/solid interface by monitoring fluctuations from equilibrium using scanning tunneling microscopy (STM) imaging. Image movies were used to monitor the oxygenated and deoxygenated state dwell times. The rate constants for CoOEP oxygenation are

Abstract: Metalloporphyrins have been shown to bind axial ligands in a variety of environments, including the vacuum/solid and solution/solid interfaces. Understanding the dynamics of such interactions is a desideratum for the design and implementation of next generation molecular devices which draw inspiration from biological systems to accomplish diverse tasks such as molecular sensing, electron transport, and catalysis to name a few. In this article, we review the current literature of axial ligand coordination to surface-supported porphyrin receptors. We will focus on the coordination process as monitored by scanning tunneling microscopy (STM) that can yield qualitative and quantitative information on the dynamics and binding affinity at the single molecule level. In particular, we will address the role of the substrate and intermolecular interactions in influencing cooperative effects (positive or negative) in the binding affinity of adjacent molecules based on experimental evidence and theoretical calculations.

Substituents present on a molecule are known to significantly control the assembly, adsorption, and orientation behavior on solid surfaces. Using scanning tunneling microscopy (STM), self-assembly ...

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.

Cooperative interactions play a critical role in the stability and reactivity of biological systems and an increasingly important consideration in the synthesis of functional materials, but quantit...

This study explores directed noncovalent bonding in the self-assembly of nonplanar aromatic carboxylic acids on gold and graphite surfaces. It is the first step in developing a new design strategy to create two-dimensional surface metal-organic frameworks (SURFMOFs). The acid molecules used are tetraphenylethene-based and are typically employed in the synthesis of three-dimensional (3D) MOF crystalline solids. They include tetraphenylethene tetracarboxylic acid, tetraphenylethene bisphenyl carboxylic acid, and tetraphenylethene tetrakis-phenyl carboxylic acid. The two-dimensional structures formed from these molecules on highly ordered pyrolytic graphite (HOPG) and Au(111) are studied by scanning tunneling microscopy in a solution environment. The process of monolayer formation and final surface linker structures are found to be strongly dependent on the combination of the molecule and substrate used and are discussed in terms of intermolecular and molecule-substrate interactions, bonding geometry, and symmetry of the acid molecules. In the case of linker self-assembly on HOPG, the molecule-substrate interactions play a significant role in the resulting surface structure. When the acid molecules are adsorbed on Au(111), the intermolecular interactions tend to dominate over the weaker molecule-substrate bonding. Additionally, the interplay of π-π interactions and hydrogen bonding that directs the surface self-assembly on different supports can be modified by varying the linker concentration. This is particularly applicable for the case of the acid molecules adsorbing on the Au(111) substrate. Precise control over predesigned surface structures and orientation of the nonplanar aromatic carboxylic linkers open up an exciting prospect for manipulating the direction of SURFMOF growth in two dimensions and potentially in 3D.

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.

We present a quantitative study comparing the binding of 4-methoxypyridine, MeOPy, ligand to Co(ii)octaethylporphyrin, CoOEP, at the phenyloctane/HOPG interface and in toluene solution. Scanning tunneling microscopy (STM) was used to study the ligand binding to the porphyrin receptors adsorbed on graphite. Electronic spectroscopy was employed for examining this process in fluid solution. The on surface coordination reaction was completely reversible and followed a simple Langmuir adsorption isotherm. Ligand affinities (or ΔG) for the binding processes in the two different chemical environments were determined from the respective equilibrium constants. The free energy value of -13.0 ± 0.3 kJ mol-1 for the ligation reaction of MeOPy to CoOEP at the solution/HOPG interface is less negative than the ΔG for cobalt porphyrin complexed to the ligand in solution, -16.8 ± 0.2 kJ mol-1. This result indicates that the MeOPy-CoOEP complex is more stable in solution than on the surface. Additional thermodynamic values for the formation of the surface ligated species (ΔHc = -50 kJ mol-1 and ΔSc = -120 J mol-1) were extracted from temperature dependent STM measurements. Density functional computational methods were also employed to explore the energetics of both the solution and surface reactions. At high concentrations of MeOPy the monolayer was observed to be stripped from the surface. Computational results indicate that this is not because of a reduction in adsorption energy of the MeOPy-CoOEP complex. Nearest neighbor analysis of the MeOPy-CoOEP in the STM images revealed positive cooperative ligand binding behavior. Our studies bring new insights to the general principles of affinity and cooperativity in the ligand-receptor interactions at the solution/solid interface. Future applications of STM will pave the way for new strategies designing highly functional multisite receptor systems for sensing, catalysis, and pharmacological applications.

Synthetic ionic porphyrin self-assembled structures possess remarkable optoelectronic properties that are useful for energy and sensor related technological application, and there is a continued interest in tailoring their molecular structure and interactions in order to control their various functions. Central to these efforts is the fundamental understanding of the structure, the chemical and physical properties, and the relationship between the structural organization of the chromophores and their functions. In this Feature Article, we demonstrate the capability of a nucleation and growth algorithm to reproducibly synthesize nano- to millimeter size single crystals of several different ionic binary porphyrin (BP) systems under chemically controlled conditions. Single crystal X-ray diffraction analysis, morphology, sensing, and mechanical measurements are presented. UV–vis and X-ray photoelectron spectroscopy of these materials is also discussed. The crystalline porphyrin assemblies have varying degrees...

The effect of selective metallation of free-base ionic porphyrin tectons on the structural, electronic, and optical properties of their crystalline self-assemblies is presented. Rod-like crystals were prepared under neutral pH conditions by combining meso-tetra(N-methyl-pyridyl)porphyrin, H2TMPyP, and meso-tetra(4-sulfonatophenyl)porphyrin, H2TSPP, with either a nickel or a copper ion contained in one of the synthons. These materials were characterized by optical microscopy, X-ray diffraction methods, thermogravimetric analysis, diffuse reflectance UV-visible and luminescence spectroscopies, and conductivity and photoconductivity measurements. All the porphyrin assemblies formed monoclinic P21/c crystals with pseudo-hexagonal cross-sections. Thermogravimetric experiments indicate that water molecules associated with crystals desorb at two different rates. In addition, temperature dependent XRD showed that the dehydration of the porphyrin solids causes modification in the crystals which is completely reversible up to 100 °C annealing (i.e., crystals return to their original structural geometry upon rehydration). All the metallated porphyrin crystals exhibit dark conductivity at moderately high temperatures and become more conductive upon photoexcitation. The photoresponse of the H2TMPyP:CuTSPP-substituted crystals is significantly higher than that of the CuTMPyP:H2TSPP and the Ni-substituted crystals. The Cu-substituted systems, like the parent free-base, exhibit persistent photoconductivity resulting from excitations into the Q and Soret bands. The primary charge carriers in these solids upon photoexcitation are electrons and the charge recombination mechanism follows monomolecular kinetics. Quantum mechanical calculations provide the electronic band structure and the density of states and explain the experimental prompt photoconductivity measurements of the porphyrin self-assemblies. This work provides evidence that optoelectronic properties of organic semiconductors can be effectively tuned by introducing transition metals into their crystal structures.

Porphyrins and phthalocyanines are ubiquitous in modern science and technology. Their stability, redox properties, and photoresponse make them candidates for numerous applications. Many of these applications rely on thin films, and these are critically dependent on the first monolayer. In this article, we focus on noncovalently bound self-assembled monolayers of porphyrins and phthalocyanines at the solution-solid interface with special emphasis on the kinetic and thermodynamic processes that define the films and their reaction chemistry. We first discuss the difference between film-formation kinetics and desorption kinetics from fully formed films. We then present evidence that many of these monolayers are controlled by adsorption kinetics and are not in thermodynamic equilibrium. Measurement of the solution-solid interface desorption energy by scanning tunneling microscopy is discussed, and data is presented for cobalt, nickel, and free base octaethylporphyrin. The activation energy for the desorption of these compounds into phenyloctane is about half of the computed desorption energy in vacuum, and this is discussed in terms of the role of the solvent. Preexponential factors are very low compared to desorption into vacuum, and this is attributed to a reduction in the entropy of activation due to the participation of solvent in the transition state. An example of the use of relative desorption kinetics to create a new binary surface structure is given. It is suggested that this is a synthesis route that may have been missed because of the large difference in solution concentrations required to drive binary film formation. Attention then turns to the axial reaction chemistry of metalloporphyrins and metallophthalocyanines supported on conducting surfaces. We show several examples of chemistry unique to the supported complexes: cases where the metal binds ligands more readily and cases where the substrate induces ligand loss. Understanding this new axial coordination chemistry is of great importance in catalysis, sensing, and the growth of 3D materials from a self-assembled template.

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.

We present the first detailed structure-function study of a photoconducting ionic porphyrin supermolecular assembly, fabricated from tetra(N-methyl-4-pyridyl)porphyrin (TMPyP) and tetra(4-sulfonatophenyl)porphyrin (TSPP) in a 1:1 stoichiometric ratio. Rod like crystals large enough for single crystal diffraction studies were grown by utilizing a nucleation and growth model described in our previous work. The unit cell of the TMPyP:TSPP crystals is monoclinic P21/c and the cell constants are a = 8.3049(11) A, b = 16.413(2) A, c = 29.185(3) A, β = 92.477(9)°. These crystals have smooth well defined facets and their internal structure consists of highly organized molecular columns of alternating porphyrin cations and anions that are stacked face to face. For the first time crystal morphology (habit) of an ionic porphyrin solid is predicted by using the crystal structure data and applying attachment energy (AE) model. The predicted habit is in good agreement with the experimental structural morphology observed in AFM and SEM images of the TMPyP:TSPP crystalline solid. The TMPyP:TSPP crystals are non-conducting in the dark and are photoconducting. The photoconductive response is significantly faster with excitation in the Q-band (Red) than with excitation in the Soret band (blue). DFT calculations were performed to determine their electronic band structure and density of states. The TMPyP:TSPP crystalline system is a useful model structure that combine the elements of molecular organization and morphology along with theory and correlate them with electronic and optical electronic properties.

One of the common practices in the literature of molecular desorption is the comparison of theoretically (mostly using DFT) calculated single molecule adsorption energies with experimental desorption energies from studies like temperature programmed desorption (TPD) etc. Comparisons like those do not consider that the experimental desorption energies are obtained via ensemble techniques while theoretical values are calculated at the single molecule level. Theoretical values are generally based upon desorption of a single molecule from a clean surface, or upon desorption of an entire monolayer. On the other hand, coverage dependent molecule–molecule interactions add to and modify molecule–substrate interactions that contribute to the experimentally determined desorption energies. In this work, we explore the suitability of an additive nearest neighbor model for determining general coverage dependent single molecule desorption energies in non-covalent self-assembled monolayers (SAMs). These coverage dependent values serve as essential input to any model attempting to reproduce coverage dependent desorption or for understanding the time dependent desorption from a partially covered surface. This method is tested using a case study of coronene adsorbed on Au(111) and HOPG substrates with periodic DFT calculations. Calculations show that coronene exhibits coverage and substrate dependence in molecular desorption. We found that intermolecular contact energies in the coronene monolayer are not strongly influenced by the HOPG substrate, while coronene desorption on Au(111) exhibits strong cooperativity where the additive model fails.

We present a quantitative study comparing the binding of 4-methoxypyridine, MeOPy, ligand to Co(II)octaethylporphyrin, CoOEP, at the phenyloctane/HOPG interface and in toluene solution. Scanning tunneling microscopy (STM) was used to monitor ligand binding to the porphyrin receptors adsorbed on graphite with single-molecule resolution. Electronic spectroscopy was employed for examining this process in fluid solution. The on-surface coordination reaction was completely reversible and followed a simple Langmuir adsorption isotherm. Ligand affinities (or ΔG) for the binding processes in the two different chemical environments were determined from the respective equilibrium constants. The free energy value of -13.0±0.3 kJ/mol for the ligation reaction of MeOPy to CoOEP at the solution/HOPG interface is less negative than the ΔG for cobalt porphyrin complexed to the ligand in solution, -16.8±0.2 kJ/mole. This result indicates that the MeOPy-CoOEP complex is more stable in solution than on the surface. Additional thermodynamic values for the formation of the surface ligated species (ΔHc = -50 kJ/mol and ΔSc = -120 J/mol) were extracted from temperature dependent STM measurements. Density functional computational methods were also employed to explore the energetics of both the solution and surface reactions. At high concentrations of MeOPy the monolayer was observed to be stripped from the surface. Computational results indicate that this is not because of a reduction in adsorption energy of the MeOPy-CoOEP complex. Nearest neighbor analysis of the MeOPy-CoOEP in the STM images revealed positive cooperative ligand binding behavior. Our studies bring new insights to the general principles of affinity and cooperativity in the ligand-receptor interactions at the solution/solid interface. Future applications of STM will pave the way for new strategies designing highly functional multisite receptor systems for sensing, catalysis, and pharmacological applications.

Substituents present on a molecule are known to significantly control the assembly, adsorption, and orientation behavior on solid surfaces. Using scanning tunneling microscopy (STM), self-assembly of the Y[C6S-Pc]2and Y[C4O-Pc]2 double-decker complexes were investigated at a solution-solid interface. At concentrations above 1 µM, Y[C6S-Pc]2 formed well-defined monolayers with low defect density on highly oriented pyrolytic graphite (HOPG). On Au(111), on the other hand, it formed dense groups of small islands with some isolated molecules. There was a clear preference for multiples of 15 degrees between the orientations of the islands. A clear visualization of the Y[C6S-Pc]2 inner molecular structure including the sulfur atoms was achieved. At concentrations below 1 µM, stable isolated single molecules were observed only on Au(111), not on HOPG. The thiol linked side chains favor strong adsorption on Au(111) surface, with isolated single molecules being easily visualized and stable over several image scans. On both HOPG and Au(111) isolated molecules of Y[C4O-Pc]2 are never observed, Even at very low solution concentrations small rafts of molecules on an otherwise open substrate surfaces are observed. On HOPG the structure of the Y[C4O-Pc]2 monolayer is complex with both an open and filled cubic structure occurring as a mixed structure. We suggest that core moieties with peripheral S-linked alkanes may generally be particularly strong adsorbates on gold. Density functional calculations suggest that the S-linked alkane substituent system may be as much as 1 eV more stable on Au than a similar O-linked system. This work demonstrates the important role of molecule-substrate interactions, bias voltage, tunneling current, and STM tip on controlling and stabilizing molecular assembly.

Persistent conductivity is observed following Ar+ bombardment of meso-tetra(4-pyridyl)porphyrin:meso-tetra(4-sulfonatophenyl)porphyrin (TPyP:TSPP) nanorods. The lifetime of the persistent conductivity in ultrahigh vacuum (UHV) is exceptionally long at room temperature, between 106 and 107 s. Ion beam currents can be used to both increase and decrease the level of persistent conductivity in these nanorods. Initial Ar+ bombardment of a sample causes an increase in the persistent current. Subsequent bombardment with low-energy Ar+ can cause a rapid decrease in the persistent current. A model is presented which presumes that persistent conductivity is carried by metastable defects with rates of excitation and relaxation following the Arrhenius relationship. Energy conservation suggests that ion bombardment introduces a thermal gradient across the nanorod which quickly quenches when ion bombardment ceases. This quick quenching results in a population of metastable defects which decay very slowly at room temper...

We provide a structure–function relationship study of an organic crystalline photoconductor composed of oppositely charged ionic porphyrins. Nano to millimeter size crystals with well-defined morphology composed of stoichiometric amounts of meso-tetra(N-methyl-4-pyridyl)porphyrin (TMPyP) and meso-tetra(4-sulfonatophenyl)porphyrin (TSPP) were grown in a controlled and reproducible manner. The rod shaped TMPyP:TSPP monoclinic P21/c crystals have a pseudo-hexagonal cross section and their internal structure consists of highly organized molecular columns of alternating porphyrin cations and anions. Experimental characterization of the TMPyP:TSPP solid was performed using powder-XRD, AFM, SEM, DRS UV-visible, and photoconductivity measurements. For the first time the morphology of an ionic porphyrin solid is predicted. The TMPyP:TSPP crystals are non-conducting in the dark but become conductive with illumination. The n-type photoconductive response is significantly faster with excitation in the Q-band than with excitation in the Soret band. Quantum mechanical calculations were performed to determine the electronic band structure and density of states and to explain the photoconduction in TMPyP:TSPP. Based on these results we propose a model in which two types of photoconductivity occur: (1) band conduction which occurs at all excitation wavelengths and (2) hopping conductivity caused by metastable photoinduced defects that form primarily at higher energy excitations. This work combines the results from structural and theoretical studies and correlates them with electronic and optoelectronic properties thereby opening the road to the engineering of highly-organized functional materials from organic π-conjugated molecules.

In biology, key molecules for life processes rely on cooperation between metal containing porphyrins to function effectively. In technology, (catalysis and sensing) cooperative interactions between the support and metal porphyrins play a key role in their efficiency. In this study, we present a molecular level study of cooperative surface reactions between nitrogen based ligands and Co(II) porphyrin adsorbates as a function of their spatial proximity at the solution/solid interface. Central metal ion reactivity as a function of surface coverage is measured at varying temperatures and ligand concentrations. Reaction rates and thermodynamic parameters are extracted from measurements made on single molecules.

A two-component self-assembly process that results in three different compositional phases is observed and explained. Solutions containing various molar ratios of cobalt octaethylporphyrin (CoOEP) and coronene in phenyloctane were brought into contact with a clean Au(111) surface. At no relative concentration was coexistence of the CoOEP and coronene phase observed. Rather, at intermediate concentrations a new 1–1 surface structure (1 coronene to 1 CoOEP) is formed with sharp compositional boundaries. This behavior is unusual in that only weak van der Waals forces, slight variations in surface charge exchange, and steric effects stabilize the 1–1 structure and that it occurs where the solution concentration ratio is an order of magnitude different than the surface concentration. The nature of this 1–1 structure and the transitions between the three phases (pure CoOEP, 1–1, and pure coronene) were studied by variable temperature STM and by density functional theory (DFT). Adsorption energies for CoOEP and ...

Synthesis, crystallographic characterization, and molecular self-assembly of two novel cyclotrimeric gold(I) complexes, Au3[3,5-(COOEt)2Pz]3 (Au3Pz3) and Au3[(n-Pr–O)C═N(Me)]3 (Au3Cb3) was studied. Single crystal X-ray crystallography data reveal that both gold(I) complexes have one-dimensional stacking patterns caused by intermolecular Au(I)···Au(I) aurophilic interactions. The Au3Pz3 trimer units stack with two alternate and symmetrical Au(I)···Au(I) interactions while the Au3Cb3 units have three alternating and nonsymmetrical Au(I)···Au(I) interactions. Molecular self-assembly of the gold(I) complexes on the 1-phenyloctane/highly ordered pyrolytic graphite (HOPG) (0001) solution–solid interface is studied with scanning tunneling microscopy (STM). The gold(I) cyclotrimers form epitaxial nanostructures on the HOPG surface. At a concentration of ∼1 × 10–4 M, Au3Pz3 complexes exhibit a single morphology, while Au3Cb3 complexes exhibit polymorphology. Two polymorphs, one nonporous and the other porous, are ...

Temperature-dependent desorption rates and desorption energies are determined from a monolayer assembly at the solution–solid (SS) interface. Scanning tunneling microscopy (STM) was used to measure molecular-scale temperature-dependent desorption of cobalt(II) octaethylporphyrin (CoOEP) at the phenyloctane solution–highly ordered pyrolytic graphite (HOPG) interface. At lower temperatures, monolayer formation of metal(II) octaethylporphyrin (MOEP) on HOPG from solution was found to be completely controlled by kinetics, and the adlayer formed was stable up to 70 °C. Significant desorption of CoOEP from the HOPG surface was observed above 80 °C on a time scale of hours. CoOEP desorbs from HOPG into phenyloctane at a rate of 0.0055 ± 0.0007 min–1 at 90 °C, 0.013 ± 0.001 min–1 at 100 °C, and 0.033 ± 0.003 min–1 at 110 °C. From these temperature- and time-dependent measurements, assuming an Arrhenius rate law, the activation energy of molecular desorption at the SS interface was determined using studies solely ...

A focused review is presented on the evolution of our understanding of the kinetic and thermodynamic factors that play a critical role in the formation of well ordered organic adlayers at the solution-solid interface. While the current state of knowledge is in the very early stages, it is now clear that assumptions of kinetic or thermodynamic control are dangerous and require careful confirmation. Equilibrium processes at the solution-solid interface are being described by evolving thermodynamic models that utilize concepts from the thermodynamics of micelles. A surface adsorption version of the Born-Haber cycle is helping to extract the thermodynamic functions of state associated with equilibrium structures, but only a very few systems have been so analyzed. The kinetics of surface phase transformation, especially for polymorphic phases is in an early qualitative stage. Adsorption and desorption kinetics are just starting to be measured. The study of kinetics and thermodynamics for organic self-assembly at the solution-solid interface is experiencing very exciting and rapid growth.

Crystallization of a binary porphyrin nanostructure (BPN) of TSPP (meso-tetra(4-sulfonatophenyl)porphyrin) and TMPyP (meso-tetra(N-methyl-4-pyridyl)porphyrin) was studied. The morphology and crystallinity of the BPN was investigated using transmission electron (TEM) and atomic force microscopies (AFM). The composition of the BPN was analyzed using X-ray photoelectron spectroscopy (XPS), elemental analysis, and UV–visible spectroscopy. These techniques revealed a 1:1 composition of anionic to cationic porphyrins in the structure. Our initial studies on the synthesis of these materials revealed that the average size of these crystals increases monotonically with synthesis temperature and decreasing monotonically with initial concentration (supersaturation) of the mother solution. In this work we have developed a model to simulate the growth of these organic monocrystalline materials for the first time. This model encompasses all the major kinetic and thermodynamic steps of crystallization including homogene...

Scanning tunneling microscopy was used to make the first molecular scale measurements of the temperature dependence of composition of an adlayer at the solution-solid interface. We conclusively demonstrate that metal porphyrins adsorb very strongly on Au(111) at the solution solid interface such that the monolayer composition is entirely kinetically controlled below about 100 °C. The barrier for desorption is so great in fact that a temperature of 135 °C is required to induce desorption over a period of hours. Moreover, cobalt(II) octaethylporphyrin (CoOEP) and NiOEP desorb at different rates from different sites on the surface. We have measured the rate constant for desorption of CoOEP into phenyloctane to be 6.7 × 10(-5)/s at 135 °C. On the basis of these measurements, an upper bound can be set for the desorption rate of NiOEP into phenyloctane as 6.7 × 10(-4)/s at 135 °C. For solutions of the order of 100 μM in NiOEP or CoOEP, a dense monolayer is formed within seconds, and the adsorption rate constants fall within 40% of each other. The structures of NiOEP and CoOEP monolayers are essentially identical, and the molecular spacing for both can be described by A = 1.42 ± 0.02 nm, B = 1.32 ± 0.02 nm, and α = 57° ± 2°. The solubility of CoOEP and NiOEP in phenyloctane at room temperature was measured to be 0.228 and 0.319 g/L, respectively.

In physisorption from the vapor phase, one generally finds that the activation energy is negligible. Further, because of their molecular mass and strong non-covalent interactions with conductors, the desorption energies into vacuum from a gold surface are very high, of the order of 3-4 eV. Thus, at room temperature the desorption rates of porphyrins and phthalocyanines from gold are essentially zero. Such is not the case for the solution-gold interface. In adsorption there are several possible processes that lead to a significant activation energy for adsorption. These include partial desolvation of the adsorbate and partial desolvation of the surface. In the case of adsorption, the adsorption energies measured in the vapor phase are significantly diminished by solvent-adsorbate and solvent-surface interactions. Thus, the desorption rates at the solid solution interface can be orders of magnitude larger than at the vapor-solid interface. Unfortunately, there is very little data on the adsorption and desorption rates of non-ionic porphyrins and phthalocyanines from solution onto gold. In this talk we will present new measurements covering several organic solvents and a selection of porphyrins and phthalocyanines. Preliminary models for understanding these rates will also be presented.

In biology, key molecules for life processes rely on cooperation between metal containing porphyrins to function effectively. In technology, (catalysis and sensing) cooperative interactions between the support and metal porphyrins play a key role in their efficiency. In this study, we present a molecular level study of cooperative surface reaction between O2 ligand and Co(II) porphyrin adsorbates as a function of their spatial proximity at the solution/solid interface. Interactions of note include through-support, through-intramolecular-pi structure, and intermolecular communication. Central metal ion reactivity as a function of surface coverage is measured at varying temperatures and ligand concentrations. Reaction rates and thermodynamic parameters are extracted from measurements made on single molecules. DFT calculations are used to provide electronic insights into the observed results.

Scanning tunneling microscopy (STM) is used to study for the first time the reversible binding of imidazole (Im) and nickel(ii) octaethylporphyrin (NiOEP) supported on highly oriented pyrolytic graphite (HOPG) at the phenyloctane/NiOEP/HOPG interface at 25 °C. The ligation of Im to the NiOEP receptor while not observed in fluid solution is readily realized at the solution/HOPG interface. The coordination process scales with increasing Im concentration and can be effectively modeled by the Langmuir isotherm. At room temperature it is determined that the standard free energy of adsorption is ΔGc = -15.8 kJ mol(-1) and the standard enthalpy of adsorption is estimated to be ΔHc ≈ -80 kJ mol(-1). The reactivity of imidazole toward NiOEP adsorbed on HOPG is attributed to charge donation from the graphite stabilizing the Im-Ni bond. This charge transfer pathway is supported by molecular and periodic modeling calculations which indicate that the Im ligand behaves as a π-acceptor. DFT calculations also show that the nickel ion in the Im-NiOEP/HOPG complex is in a singlet ground state. This is surprising since both our calculations and previous experimental studies find a triplet ground state for the five and six coordinated Im-nickel(ii) porphyrins in the gas-phase or in solution. Both the experimental and the theoretical findings provide information that is useful for better understanding of chemical sensing/recognition and catalytic processes that utilize metal-organic complexes adsorbed on surfaces where the reactivity of the metal is moderated by the substrate.

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.

For the first time, the pressure and temperature dependence of a chemical reaction at the solid/solution interface is studied by scanning tunneling microscopy (STM), and thermodynamic data are derived. In particular, the STM is used to study the reversible binding of O(2) with cobalt(II) octaethylporphyrin (CoOEP) supported on highly oriented pyrolytic graphite (HOPG) at the phenyloctane/CoOEP/HOPG interface. The adsorption is shown to follow the Langmuir isotherm with P(1/2)(298K) = 3200 Torr. Over the temperature range of 10-40 °C, it was found that ΔH(P) = -68 ± 10 kJ/mol and ΔS(P) = -297 ± 30 J/(mol K). The enthalpy and entropy changes are slightly larger than expected based on solution-phase reactions, and possible origins of these differences are discussed. The big surprise here is the presence of any O(2) binding at room temperature, since CoOEP is not expected to bind O(2) in fluid solution. The stability of the bound oxygen is attributed to charge donation from the graphite substrate to the cobalt, thereby stabilizing the polarized Co-O(2) bonding. We report the surface unit cell for CoOEP on HOPG in phenyloctane at 25 °C to be A = (1.46 ± 0.1)n nm, B = (1.36 ± 0.1)m nm, and α = 54 ± 3°, where n and m are unknown nonzero non-negative integers.

Highly ordered assemblies prepared from tetra (4-sulfonatophenyl) phthalocyanine (TSPc) by employing very acidic aqueous solutions were deposited onto Au(111) substrates and studied in UHV using X-ray photoelectron spectroscopy (XPS), scanning tunneling microscopy (STM), and orbital mediated tunneling spectroscopy (OMTS). XPS of the TSPc aggregates shows that the ratio of protonated to unprotonated nitrogens does not change with decreasing solution pH. STM images of TSPc deposited from pH

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.

Well-defined monolayer islands of protonated meso-tetrakis(4-sulfonatophenyl)porphine (H2[H4TSPP]) self-assembled on highly oriented pyrolytic graphite (HOPG) from 0.75 M HCl solutions are studied in ultrahigh vacuum using scanning tunneling microscopy (STM), orbital-mediated tunneling spectroscopy (OMTS), ultraviolet photoelectron spectroscopy (UPS), and X-ray photoelectron spectroscopy (XPS). HOPG proved to be a superior substrate because it does not form a thin surface chloride in the presence of HCl, as does Au(111). Unlike meso-tetrakis(4-carboxyphenyl)porphine (Hx[HyTCPP]), the carboxylate analog, monolayers are stable on HOPG and can be studied at room temperature without the addition of a second stabilizing compound. Thus, the surface organizational energy for H2[H4TSPP] is significantly stronger than that of Hx[HyTCPP]. This increased interaction is also apparent in the fact that the H2[H4TSPP] monolayer has a much smaller unit cell. It is suggested that sulfonate groups may be more useful than c...

We report polarized resonance Raman data of tetrakis(4-sulfonato)phenyl porphyrin (TSPP) aggregates in solution and deposited on Au(111) at wavelengths resonant with the red-shifted (J-band) and blue-shifted (H-band) components of the split Soret (B) band. We also report scanning tunneling microscopy (STM) images which reveal that the aggregate on Au(111) is a nanotube with a 2 nm wall thickness which tends to flatten on the substrate. Relative Raman intensities and their dependence on polarization of the incident and scattered light are found to vary greatly for H- and J-band excitation, revealing a much greater degree of coherence for the J-band, in agreement with the resonance light scattering spectrum. The J-band transition is found to have transition moment components both parallel and perpendicular to the long axis of the nanotube, consistent with a helical nanotube structure. The intensity increase of the Q-band on aggregation and the weak intensity of the H-band in both the absorption and the reso...

The effect of selective metallation of free-base ionic porphyrin tectons on the structural, electronic, and optical properties of their crystalline self-assemblies is presented. Rod-like crystals were prepared by combining meso-tetra(N-methyl-pyridyl)porphyrin, H2TMPyP, and meso-tetra(4-sulfonatophenyl)porphyrin, H2TSPP, with either a nickel or copper ion contained in one of the synthons. These materials were characterized by optical microscopy, X-ray diffraction methods, thermogravimetric analysis, diffuse reflectance UV-visible and luminescence spectroscopies, and conductivity and photoconductivity measurements. All the porphyrin assemblies formed monoclinic P21/C crystals with pseudo-hexagonal cross sections. Thermogravimetric experiments indicate that water molecules associated with crystals desorb at two different rates. In addition, the temperature dependent XRD showed that the dehydration of the porphyrin solids causes modification in the crystals which is completely reversible (i.e., crystals return to their original structural geometry upon rehydration). All the metallated porphyrin crystals exhibit dark conductivity at moderately high temperatures and become more conducting upon photoexciation. The primary charge carriers in these solids upon photoexcitation are electrons and the charge recombination mechanism follows monomolecular kinetics. Quantum mechanical calculations provide the electronic band structure, the density of states and explain the experimental prompt photoconductivity measurements of the porphyrin self-assemblies.

The growth and ordering of copper-substituted and free-base 2,9,16,23-tetra-tert-butyl-phthalocyanine on a graphite surface at the solution−solid interface can be controlled by solvent choice. One-dimensional ordered molecular layers having the molecular plane lying parallel to the graphite substrate were obtained at the octylbenzene solution−graphite interface at room temperature. The arrangement was changed to two different two-dimensionally ordered lattices by replacing the solvent with bromine-substituted octylbenzenes. The free base and copper complexes gave the same surface structures in octylbenzene and in 1-bromo-4-n-octylbenzene, indicating that the central metal ion was not playing a critical role in determining the surface structure. These structures were determined from the analysis of the images with scanning tunneling microscopy.

Scanning tunneling microscopy (STM) is employed to demonstrate that the presence of the vanadyl ion (VO2+) in the 2,9,16,23-tetraphenoxy-29H,31H-phthalocyanine complex (VOPcPhO) has a pronounced effect on the organization of the complex at the highly oriented pyrolytic graphite (HOPG)−n-alkyl benzene (n = 6, 7, 8, and 10) interface. VOPcPhO forms at least three different stable architectures, as compared to only one type of structure resulting from the adsorption of the metal-free tetraphenoxyphthalocyanine analog (H2PcPhO) on HOPG under conditions similar to VOPcPhO. All three observed VOPcPhO monolayer structures have high packing density and are commensurate with the underlying graphite lattice, whereas H2PcPhO forms a lower packing density incommensurate adlayer. The VOPcPhO structures display a small rectangular unit cell (1 molecule/cell) with lattice vector lengths of 1.28 and 1.48 nm, a larger rectangular unit cell (6 molecules/cell) with lattice vector lengths of 4.43 nm 2.98 nm, and an oblique u...

We investigate a strategy for producing organic NLO materials with high chromophore densities by assembling Langmuir monolayers and Langmuir–Blodgett films containing mixtures of two hyperpolarizable chromophore amphiphiles. When deposited at an air–water interface, the amphiphilic molecules are oriented with their molecular hyperpolarizabilities aligned, and their dipole moments antialigned. We find that mixed chromophore Langmuir monolayers are more stable than pure ones, suggesting that electrostatic interactions aid self-assembly and ordering. Upon transfer of the monolayers to glass and silicon substrates, aggregates with well-defined topological features appear. We characterize aggregate formation using fluorescence microscopy, atomic force microscopy, and nonlinear optical ellipsometry, and we propose a mechanism for aggregate formation. Understanding molecular interactions between strong chromophores should enable fabrication strategies that prevent aggregation and optimize chromophore alignment. ...

A focused review is presented on the evolution of our understanding of the kinetic and thermodynamic factors that play a critical role in the formation of well ordered organic adlayers at the solution–solid interface. While the current state of knowledge is in the very early stages, it is now clear that assumptions of kinetic or thermodynamic control are dangerous and require careful confirmation. Equilibrium processes at the solution–solid interface are being described by evolving thermodynamic models that utilize concepts from the thermodynamics of micelles. A surface adsorption version of the Born–Haber cycle is helping to extract the thermodynamic functions of state associated with equilibrium structures, but only a very few systems have been so analyzed. The kinetics of surface phase transformation, especially for polymorphic phases is in an early qualitative stage. Adsorption and desorption kinetics are just starting to be measured. The study of kinetics and thermodynamics for organic self-assembly at the solution–solid interface is experiencing very exciting and rapid growth.

Self-assembled crystalline nanostructures with sheaf-like morphology fabricated from tetra(4-aminophenyl)porphyrin and tetra(4-sulfonatophenyl)porphyrin are reported for the first time. The hierarchical sheaf-like growth of the assemblies exhibits Arrhenius behaviour. The observed morphology results from crystal splitting during initial oriented attachment growth followed by Ostwald ripening.

A dense monolayer of cobalt(II) phthalocyanine, CoPc, can be formed on a gold surface by spontaneous redox deposition followed by vacuum annealing at about 110 °C. CsCoPc(CN)2 or KCoPc(CN)2 in dilute (∼10-6 M) ethanol solution rapidly forms a dense adlayer that, when washed with ethanol, can be imaged by scanning tunneling microscopy, STM, in air. This adlayer is converted to a monolayer of CoPc by vacuum annealing as confirmed by STM and X-ray photoelectron spectroscopy (XPS). This spontaneous surface redox adsorption process represents a novel method for depositing metallorganic complexes, which are ultimately only physisorbed on gold.

The mechanical properties of monolayer and multilayer structures of several symmetrically substituted alkoxy copper phthalocyanine, CuPc(OR)8, Langmuir–Blodgett (LB) films were evaluated. One-dimensional compliance of the monolayers was determined from the slopes of the surface pressure–area isotherms. Multilayers were examined using a SPM and a commercial nanoindentation system. The nanomechanical studies show a distinct relationship between the position and length of alkoxy substituents on the Pc macrocycle and the LB film elasticity and hardness. The phthalocyanine complexes with peripheral octabutoxy and octaoctyloxy substituents form stiff LB monolayers and multilayers. The nonperipheral alkoxy derivetized CuPc films were less stiff. For monolayer films, it appears that the extent of π-π interaction determines the strength of the film. In multilayers, significant additional stability can be imparted through interdigitation of long paraffinic chains which play a significant role in determining the interlayer structure of the LB layers.

Langmuir–Blodgett (LB) films of copper (II) 1,4,8,11,15,18,22,25-octabutoxyphthalocyanine, nCuPc(OBu) 8 , (non-peripheral substitution) and copper (II) 2,3,9,10,16,17,23,24-octabutoxyphthalocyanine, pCuPc(OBu) 8 , (peripheral substitution), were fabricated and characterized by optical spectroscopy and scanning probe microscopy. The LB films were transferred onto hydrophilic substrates by vertical dipping. Although they posses relatively short polar substituents both compounds form smooth, uniform, dense, and highly stable LB monolayers composed of linear arrays of cofacial oligomers. The long range discotic assemblies of LB and spun cast films of pCuPc(OBu) 8 and nCuPc(OBu) 8 posses physical and chemical properties favorable for molecular electronic device application.