Maurizio Casarin

The frontier electronic structure of tetraphenylporphyrinato (TPP^2-) and phthalocyaninato (Pc^2-) square planar transition metal complexes (MTPP and MPc; M = V, Cr, Mn, Fe, Co, Ni, Cu, and Zn) has been revisited through DFT calculations. The different symmetry and spin multiplicity between MPc and MTPP of the same M is shown to originate from the different Pc^2- and TPP^2- ligand field, stronger in the former ligand than in the latter. The corresponding spatial localization and symmetry of the unoccupied molecular orbitals postulate unescapable geometric constraints to their overlap with the electron cloud of a crystalline metal surface. From comparison with literature experimental evidence, we show that the adsorption geometry (atomic site and azimuthal orientation) of MTPPs and MPcs on the low index crystal planes of coinage metals (CM = Au, Ag, Cu) may be predicted when two conditions are satisfied: i) evidence of a surface → adsorbate charge transfer, ii) absence of significant distortion of the macrocycle upon adsorption. In this regard, the overall susceptibility to charge transfer is determined by the strength of the molecular ligand field (i.e., charge transfer to MPc is more favoured than to MTPP) and inversely linked to the electronegativity of the surface atoms (being Au the most inert CM substrate thanks to its highest electronegativity).

A precise understanding, at the molecular level, of the massive substrate → adsorbate charge transfer at the NiTPP/Cu(100) interface has been gained through the application of elementary symmetry arguments to the structural determination of the NiTPP adsorption site by photoelectron diffraction (PED) measurements and Amsterdam density functional calculations of the free D_4h NiTPP electronic structure. In particular, the PED analysis precisely determines that, among the diverse NiTPP chemisorption sites herein considered (fourfold hollow, atop, and bridge), the fourfold hollow one is the most favorable, with the Ni atom located at 1.93 Å from the surface and at an internuclear distance of 2.66 Å from the nearest-neighbors of the substrate. The use of elementary symmetry considerations enabled us to provide a convincing modeling of the NiTPP–Cu(100) anchoring configuration and an atomistic view of the previously revealed interfacial charge transfer through the unambiguous identification of the adsorbate π* and σ* low-lying virtual orbitals, of the substrate surface atoms, and of the linear combinations of the Cu 4s atomic orbitals involved in the substrate → adsorbate charge transfer. In addition, the same considerations revealed that the experimentally reported Ni(II) → Ni(I) reduction at the interface corresponds to the fingerprint of the chemisorption site of the NiTPP on Cu(100).

Free-base porphyrins on TiO₂(110) anchor to O_br rows, where they capture diluted hydrogen underneath and favour outdiffusion of Ti interstitials by self-metalation; Ti metalated species may also capture oxygen from hydroxyl adsorbates on Ti_5f rows.

Density functional theory, combined with the molecular cluster model, has been used to investigate the surface trans-effect induced by the coordination of small molecules L (L = CO, NH₃, NO, NO₂ and O₂) on the cobalt electronic structure of cobalt tetraphenylporphyrinato (CoTPP) surface-supported on coinage metal surfaces (Cu, Ag, and Au). Regardless of whether L has a closed- or an open-shell electronic structure, its coordination to Co takes out the direct interaction between Co and the substrate eventually present. The CO and NH₃ bonding to CoTPP does not influence the Co local electronic structure, while the NO (NO₂ and O₂) coordination induces a Co reduction (oxidation), generating a 3d⁸ Co^I (3d⁶ Co^III) magnetically silent closed-shell species. Theoretical outcomes herein reported demonstrate that simple and computationally inexpensive models can be used not only to rationalize but also to predict the effects of the Co–L bonding on the magnetic behaviour of CoTPP chemisorbed on coinage metals. The same model may be straightforwardly extended to other transition metals or coordinated molecules.

Defect interactions and short-range ordering determine the bandgap reduction in linear lead/bismuth iodide chains.

The relative stability of trans -[(η ⁵ -C ₅ H ₅ )Fe(η ¹ -CO)(μ-CO)] ₂ ( trans -I) and cis -I tautomers in a vacuum and in solvents with different dielectric constants ( ε ) has been investigated by exploiting density functional theory (DFT).

A series of Gd³⁺ complexes (Gd1–Gd3) with the general formula GdL₃(EtOH)₂, where L is a β-diketone ligand with polycyclic aromatic hydrocarbon substituents of increasing size (1–3), was studied by combining time-resolved electron paramagnetic resonance (TR-EPR) spectroscopy and DFT calculations to rationalize the anomalous spectroscopic behavior of the bulkiest complex (Gd3) through the series. Its faint phosphorescence band is observed only at 80 K and it is strongly red-shifted (∼200 nm) from the intense fluorescence band. Moreover, the TR-EPR spectral analysis found that triplet levels of 3/Gd3 are effectively populated and have smaller |D| values than those of the other compounds. The combined use of zero-field splitting and spin density delocalization calculations, together with spin population analysis, allows us to explain both the large red shift and the low intensity of the phosphorescence band observed for Gd3. The large red shift is determined by the higher delocalization degree of the wavefunction, which implies a larger energy gap between the excited S₁ and T₁ states. The low intensity of the phosphorescence is due to the presence of C–H groups which favor non-radiative decay. These groups are present in all complexes; nevertheless, they have a relevant spin density only in Gd3. The spin population analysis on NaL models, in which Na⁺ is coordinated to a deprotonated ligand, mimicking the coordinative environment of the complex, confirms the outcomes on the free ligands.