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Understanding the interactions between molecules on surfaces is crucial for advancing technologies in sensing, catalysis, and energy harvesting. In this study we explore the complex surface chemistry resulting from the interaction of Co(II)octaethylporphyrin (CoOEP) and iodine, I2, both in solution and at the phenyloctane/HOPG interface. In pursuit of this goal, we report results from electrochemistry, NMR and UV-Vis spectroscopy, X-ray crystallography, scanning tunneling microscopy (STM), and density functional theory (DFT). Both spectroscopic methods of analysis confirmed that at and above the stoichiometric ratio of one CoOEP to one I2 the reaction product was metal centered CoIII(OEP)I. X-ray crystallography verified that a single iodine is bonded to each cobalt ion in the triclinic, P-1 system. The surface chemistry of CoOEP and I2 is complicated and remarkably dependent on the iodine concentration. STM images of CoOEP and I2 in phenyloctane on highly oriented pyrolytic graphite (HOPG) at low halogen concentrations (1:<2 Co:I ratios) presented random individual Co(OEP)I molecules weakly adsorbed onto a hexagonal (HEX) CoOEP monolayer. Images of 1:2 Co:I ratio solutions, showed phase segregated HEX CoOEP and pseudo-rectangular (REC) Co(OEP)I incorporating one solvent molecule per Co(OEP)I. The REC structure formed in long parallel rows with the number of rows increasing with increasing solution I2. In this case, the presence of CoOEP on the surface was attributed to the spontaneous reduction of Co(OEP)I by the graphite substrate. DFT calculations indicate that the REC Co(OEP)I:PhO form is energetically more stable than the HEX form of Co(OEP)I on HOPG. Experimental STM images and DFT calculated adsorption energies and STM images support our interpretation of the observed structures. 

Adsorption of organics on surfaces is important in both outdoor and indoor environments. Surfaces can serve as sinks for gas-phase species, act as reservoirs by emitting previously partitioned organics back into the gas phase, and can facilitate heterogeneous chemistry. We report here studies of the uptake and desorption energetics of gas-phase limonene, a volatile and widely-distributed monoterpene, on solid silica nanoparticles using a unique apparatus that allows for temperature programmed desorption (TPD) measurements of surface binding energies as well as Knudsen cell gas uptake measurements. A multiphase kinetic model was applied to these data to provide additional molecular-level understanding of the processes involved. TPD experiments yielded an average desorption energy of 47.5 ± 8.2 kJ mol-1 (±1s, sample standard deviation), the first direct experimental measurement of this parameter over a broad temperature range (150–320 K). Initial net uptake coefficients (0,net) range from (1.7 ± 0.3) ×10-3 (±1s) at 210 K to (2.3 ± 0.4) ×10-4 (±1s) at 250 K, reflecting increased rates of desorption with an increase in temperature combined with increased rates of diffusion and re-adsorption within the pores between adjacent silica nanoparticles. Effective Langmuir constants, which also reflect the effects of pore diffusion and re-adsorption, were determined from the uptake data and vary from (1.8–0.3)×10-13 cm3 molecule-1 over the same temperature range. These results are in excellent agreement with previous studies around room temperature and with theoretical calculations of the energetics of the limonene-silica interaction. The strong attraction between limonene and the polar silica surface shows the importance of including such interactions in models of the atmospheric fates of terpenes both indoors and outdoors. 

 

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 ka = 4.9×10-6 s-1∙torr-1 and kd = 0.018 s-1. This is the first use of stochastic dwell time analysis with STM to study a chemical reaction and the results suggest that it has great potential for application to a wide range of surface reactions. Expanding these stochastic studies to further systems is key to unlocking kinetic information for surface confined reactions at the molecular level -- especially at the solution/solid interface. 

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. 

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 Δ G ) 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 Δ G 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 (Δ H c = −50 kJ mol −1 and Δ S c = −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.