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.

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.

Mechanical and structural properties of ionically self-assembled nanostructures of meso-tetra(4-sulfonatophenyl)porphyrin (TSPP) and meso-tetra(4-pyridyl)porphyrin (TPyP) are presented. This is the first time that elastic modulus of an ionic porphyrin nanostructure has been reported. X-ray photoelectron spectroscopy (XPS), UV-visible spectra, and elemental analysis all support a stoichiometric 1:1 TSPP to TPyP composition. Atomic force microscopy (AFM) revealed that the porphyrin nanostructure is composed of stacked ribbons about 20 nm tall, 70 nm wide, and several microns in length. High resolution transmission electron microscopy (HRTEM) images showed clear lattice fringes 1.5 ± 0.2 nm in width aligned along the length of the nanorod. Selected area electron diffraction (SAED) and powder X-ray diffraction patterns of TSPP:TPyP are consistent with an orthorhombic system and space group Imm2 with lattice parameters a = 26.71 Å, b = 20.16 Å, and c = 8.61 Å. Crystallographic data is consistent with an arrangement of alternating face-to-face TSPP and TPyP molecules forming ordered columns along the length of the nanorods. The structural integrity of the solid is attributed to combined noncovalent interactions that include ionic, hydrogen bonding, and π–π interactions. The values of Young's modulus obtained for the crystalline TSPP:TPyP nanorods averaged 6.5 ± 1.3 GPa. This modulus is comparable to those reported for covalently bonded flexible polymeric systems. The robust bonding character of the TSPP:TPyP nanostructures combined with their mechanical properties makes them excellent candidates for flexible optoelectronic devices.

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 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.

For the first time, accurate quantitative data on the temperature evolution of a surface monolayer formed at the solution solid interface are reported. In addition, a detailed analysis is provided of the structures of three different monolayers formed when coronene in heptanoic acid is in contact with Au(111). All three monolayer structures are well-defined epitaxial structures that are extremely stable for temperature variations between 0 and 60 °C. At high concentrations, a dense hexagonal structure with molecular separation of 1.19 ± 0.04 nm is observed. At reduced concentration, the most often observed structure is an open hexagonal epitaxial structure with one molecule per unit cell and a molecular separation of 1.45 ± 0.04 nm. This structure is stabilized by solvent molecule adsorption. If the dense phase is exposed to pure solvent, or occasionally with low concentration direct adsorption, then a different hexagonal phase is formed with three molecules per unit cell but exactly the same density (lattice length of 2.46 ± 0.04 nm). Under some conditions, all three phases can be simultaneously present. It is notable that even when the least stable triangular phase is present on a large fraction of the surface, the low-density hexagonal phase is often observed decorating the reconstruction lines. The energy difference between the two low density phases is due to surface–solvent and coronene-adsorbed solvent interactions as the coronene–gold interactions in the two phases are essentially the same. The barrier to thermal conversion between the two low density phases must be several kT or greater than 2 kcal/mol.

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 P1/2298K = 3200 Torr. Over the temperature range of 10–40 °C, it was found that ΔHP = −68 ± 10 kJ/mol and ΔSP = −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 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 <1 solutions reveal ordered branched web-like assemblies hundreds of nanometers in length, generally 2 nm tall and having variable widths. High-resolution UHV-STM images of TSPc aggregates on Au(111) reveal detailed coherent columnar architecture with the phthalocyanine macrocycles orientated parallel to the substrate surface. OMTS was used to identify high-energy occupied orbitals, the LUMO of the TSPc aggregates, and the results are contrasted with the same molecular states in unsubstituted metalated phthalocyanines (MPc’s). The positions of the filled and the empty states of the TSPc are comparable to those of other unsubstituted MPc’s, indicating that the electronegative sulfonate substituents have minimal effect on the electronic properties of the macrocycle in this aggregated state on Au. The HOMO–LUMO separation of the TSPc is slightly >2 eV, a value consistent with the literature assignments for the Pc ring band gap.

Nanorods produced from the sodium salt of tetrakis(4-sulfonatophenyl) porphyrin, dissolved in acidic aqueous solutions, were deposited onto Au(111) substrates and imaged by atomic force microscopy (AFM) and scanning tunneling microscopy (STM). The AFM and STM images revealed individual rods with a diameters of 25−40 nm and lengths of hundreds of nanometers. Bundles of individual rods fashioned larger structures. We report for the first time high resolution STM images of TSPP on Au(111) which reveal that the rods are composed of disk-like building blocks approximately 6.0 nm in diameter. We speculate that the disks are formed by a circular J-aggregation of 14−16 overlapping electronically coupled porphyrin chromophores and that this circular porphyrin organization is driven by nonplanar distortions of the porphyrin diacid. The resonance Raman spectra of the solution phase aggregate and the surface-enhanced resonance Raman spectra of the aggregate on gold films were obtained at an excitation wavelength coincident with the exchange-narrowed J-band and found to be similar in peak frequencies and relative intensities. The UV−visible absorption spectrum of the solution phase aggregate was also found to be similar to that of the aggregate deposited on quartz. These comparisons confirm similar ground and excited electronic state structures of the excitonically coupled chromophores which comprise the aggregate in solution and on gold. Our results shed light on a number of previous experimental observations that could not be rationalized within the typical presumed staircase model of J-aggregation.

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 unit cell (2 molecules/cell and α = 84.7°) with lattice vector lengths of 1.48 and 2.99 nm. As a second layer begins to form, it preferentially adds to the large unit cell, keeping those lattice parameters, but with only four molecules per cell. The H2PcPhO adlayer achieves an oblique unit cell (α = 74° and 1 molecule/cell) with 1.58 and 1.68 nm length lattice vectors. On the basis of combined experimental results and theoretical calculations, we propose models for the observed molecular organizations.

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.

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.