Implication of dispersion interactions on geometric, adsorption and electronic properties of porphyrin monolayer on conductive surfaces using density functional theory.

A cationic tris(ethylene)copper(I) and tris(ethylene)silver(I) adduct has been isolated using hexafluoroantimonate counterion. X-ray crystallographic analysis shows that the metal atom in these adducts coordinates to three ethylene molecules in a trigonal planar fashion, forming a spoke-wheel arrangement. In [M(C₂H₄)₃][SbF₆] adducts (M = Cu, Ag, Au), the average M–C distances of 2.189 Å for Cu and 2.410 Å for Ag can be compared with the corresponding distance of 2.268 Å for Au and show that the copper(I) complex has the shortest while the silver adduct displays the longest M–C distances. These agree well with the computed values (2.172, 2.408, and 2.308 Å for Cu, Ag, and Au analogues, respectively) and follow the same trend as the covalent radii of M(I). Structural data on [μ₃-Cl{Cu(C₂H₄)₂}₃][SbF₆]₂, which features bis(ethylene)copper(I) moieties, are also presented. It has a trigonal planar Cu₃Cl core with a triply bridging chloride ion bonded to three trigonal planar copper centers. Ethylene molecules are bonded in an η²-fashion. The data indicate that while these [M(C₂H₄)₃]⁺ complexes may be primarily electrostatic in their metal–ethylene binding, there remains an important covalent contribution, particularly for the copper and gold congeners.

Porphyrins are fascinating molecules with applications spanning various scientific fields. In this review we present the use of periodic density functional theory (PDFT) calculations to study the structure, electronic properties, and reactivity of porphyrins on ordered two dimensional surfaces and in the formation of nanostructures. The focus of the review is to describe the application of PDFT calculations for bridging the gaps in experimental studies on porphyrin nanostructures and self-assembly on 2D surfaces. A survey of different DFT functionals used to study the porphyrin-based system as well as their advantages and disadvantages in studying these systems is presented.

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.

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 ΔG_c = −15.8 kJ mol⁻¹ and the standard enthalpy of adsorption is estimated to be ΔH_c ≈ −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.