Predicting the Size Distribution in Crystallization of TSPP:TMPyP Binary Porphyrin Nanostructures in a Batch Desupersaturation Experiment

Crystallization of a binary porphyrin nanostructure (BPN) of TSPP (<i>meso</i>-tetra­(4-sulfonatophenyl)­porphyrin) and TMPyP (<i>meso</i>-tetra­(<i>N</i>-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 homogeneous nucleation, growth, and Ostwald ripening. The model is then validated by comparing the simulation results with experimental crystallization histograms. The unknown parameters are extracted by fitting the simulation to the experimental data. This investigation will help in better understanding of crystallization and size control in this class of photoactive organic materials. The integration rate constant pre-exponential is found to be (2.9 ± 1.3) × 10⁶ m⁴/(mol s), and the activation energy for the integration rate is determined as 44 ± 2 kJ/mol

Persistent Conductivity in TPyP:TSPP Organic Nanorods Induced by Ion Bombardment

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 temperature

Influence of the Central Metal Ion on the Desorption Kinetics of a Porphyrin from the Solution/HOPG Interface

The changes in desorption kinetics that result from incorporating a metal ion into a porphyrin ring are studied by scanning tunneling microscopy (STM). Desorption studies of cobalt­(II) octaethylporphyrin (CoOEP) and free base octaethylporphyrin (H₂OEP) at the 1-phenyloctane/HOPG interface were performed in the 20–110 °C temperature range. These studies of mixtures of CoOEP and H₂OEP have shown that the resulting monolayer compositions are stable for more than one year at 20 °C, and are controlled by kinetics to above 100 °C. Quantitative temperature and time dependent surface coverage studies were performed on both CoOEP and H₂OEP at 90, 100, and 110 °C. The desorption activation energies for both porphyrins were found to be (1.25 ± 0.05) × 10² kJ/mol. The rate of desorption and the rate of adsorption for CoOEP are similar to the corresponding rates for H₂OEP, indicating that replacing the central protons with a cobalt ion has only a minor influence on adsorption. Thus, the adsorption strength is dominated by the interactions between the porphyrin ring and HOPG. Comparison of these results with previously published work for the NiOEP/CoOEP system suggests the presence of weak cooperativity in the desorption process. We also found that setting the sample potential to ±1.5 V relative to the earth for periods of the order of an hour had no effect on desorption rates at 50 °C. On the other hand, a large potential difference between the tip and sample did produce a significant change in desorption rate

Single Molecule Imaging of Oxygenation of Cobalt Octaethylporphyrin at the Solution/Solid Interface: Thermodynamics from Microscopy

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 O₂ 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 <i>P</i>_1/2^298K = 3200 Torr. Over the temperature range of 10–40 °C, it was found that Δ<i>H</i>_P = −68 ± 10 kJ/mol and Δ<i>S</i>_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 O₂ binding at room temperature, since CoOEP is not expected to bind O₂ 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–O₂ bonding. We report the surface unit cell for CoOEP on HOPG in phenyloctane at 25 °C to be <i>A</i> = (1.46 ± 0.1)<i>n</i> nm, <i>B</i> = (1.36 ± 0.1)<i>m</i> nm, and α = 54 ± 3°, where <i>n</i> and <i>m</i> are unknown nonzero non-negative integers

In Situ Imaging and Computational Modeling Reveal That Thiophene Complexation with Co(II)porphyrin/Graphite Is Highly Cooperative

Scanning tunneling microscopy (STM) was employed to quantitively investigate in situ binding of 3-phenyl thiophene (PhTh) to Co(II)octaethyl porphyrin (CoOEP) supported on highly ordered pyrolytic graphite (HOPG) in fluid solution. To our knowledge, this is the first single-molecule level study of a complexation reaction between a metalloporphyrin and a sulfur base at the solution/solid interface and one of the few examples of thiophene coordination with a d7 transition metal. Real-time imaging experiments revealed that PhTh binds reversibly to HOPG-supported CoOEP at room temperature. The coordination process increases with increasing PhTh concentration. The nearest-neighbor analysis of STM images indicates that the complexation reaction is cooperative. Because PhTh does not bind to CoOEP in solution, the STM results strongly suggest that the presence of HOPG is crucial to observe ligand binding and cooperativity in this system. Periodic plane-wave density functional theory (DFT) computations corroborate that PhTh has low binding affinity toward CoOEP in solution but predict that the ligand can adsorb to CoOEP/HOPG through coordination with S atoms or interact through noncovalent π–π bonding with the porphyrin chromophore. Three possible structures were considered, and DFT theory was used to calculate binding energies and free energies. In solution and on the HOPG surface both a π–π configuration and a η1(S) configuration have similar computed energies. The η1(S) structure shows the largest stabilization in going from the vapor to adsorbed on HOPG. We also show that statistical analysis of nearest neighbors is more sensitive to cooperative binding than is fitting with the Temkin or Langmuir isotherm. The implication is that isotherm fitting alone is insufficient for identifying cooperative binding on surfaces

Polymorphic, Porous, and Host–Guest Nanostructures Directed by Monolayer–Substrate Interactions: Epitaxial Self-Assembly Study of Cyclic Trinuclear Au(I) Complexes on HOPG at the Solution–Solid Interface

Synthesis, crystallographic characterization, and molecular self-assembly of two novel cyclotrimeric gold­(I) complexes, Au₃[3,5-(COOEt)₂Pz]₃ (Au₃Pz₃) and Au₃[(<i>n</i>-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 observed at 22.0 ± 2.0 °C for Au₃Cb₃ complexes. A nonporous, low-surface-density (0.82 molecules/nm²) Au₃Cb₃ nanostructure forms first and then transforms into a high-density (1.43 molecules/nm²) porous nanostructure. This is the first time any porous surface nanostructure is reported for an organometallic system. The porous structure is thought to be stabilized by a combination of hydrogen bonding and monolayer–substrate interactions. These pores are utilized to incorporate pyrene into the film, rendering this the first organometallic host–guest system imaged at the solid–solution interface. Molecular and periodic density functional theory (DFT) calculations shed light on the two-dimensional topography and polymorphic self-assembly revealed by STM; these calculations suggest significant electronic hybridization of the Au₃ trimer orbitals and HOPG. The multiple-technique approach used herein provides insights concerning molecule–substrate and molecule–molecule interactions