Electronic states and the influence of oxygen addition on the optical absorption behaviour of manganese phthalocyanine

Tuning the Magnetic Coupling of a Molecular Spin Interface via Electron Doping

Mastering the magnetic response of molecular spin interfaces by tuning the occupancy of the molecular orbitals, which carry the spin magnetic moment, can be accomplished by electron doping. We propose a viable route to control the magnetization direction and magnitude of a molecular spin network, in a graphene-mediated architecture, achieved via alkali doping of manganese phthalocyanine (MnPc) molecules assembled on cobalt intercalated under a graphene membrane. The antiparallel magnetic alignment of the MnPc molecules with the underlying Co layer can be switched to a ferromagnetic state by electron doping. Multiplet calculations unveil an enhanced magnetic state of the Mn centers with a 3/2 to 5/2 spin transition induced by alkali doping, as confirmed by the steepening of the hysteresis loops, with higher saturation magnetization values. This new molecular spin configuration can be aligned by an external field, almost independently from the hard-magnet substrate effectively behaving as a free magnetic layer.

Properties and degradation of manganese(III) porphyrin thin films formed by high vacuum sublimation

Manganese porphyrins are of interest due to the optical, electronic and magnetic properties of the central metal ion, coupled to the low bandgap of the polyaromatic ring. These attractive characteristics are harnessed in solutions or in ultra-thin films, such as, for example, self-assembled monolayers. However, for devices, thicker films deposited using a controlled and reproducible method are required. Here we present the morphological, structural, chemical and optical properties of manganese(III) tetraphenylporphyrin chloride (MnTPPCl) thin films deposited using organic molecular beam deposition, typically employed to process analogue molecules for applications such as organic photovoltaics. We find, using a combination of UV-vis and X-ray photoelectron spectroscopies, that the sublimation process leads to the scission of the Mn–Cl bond. The resultant film is a Mn(II)TPP:Mn(III)TPPCl blend where approximately half the molecules have been reduced. Following growth, exposure to air oxidizes the Mn(II)TPP molecule. Through quantitative analysis of the time-dependent optical properties, the oxygen diffusion coefficient (D) [Formula: see text] is obtained, corresponding to a slow bulk oxidation following fast oxidation of a 8-nm-thick surface layer. The bulk diffusion D is lower than for analogous polycrystalline films, suggestion that grain boundaries, rather than molecular packing, are the rate-limiting steps in oxidation of molecular films. Our results highlight that the stability of the axial ligands should be considered when depositing metal porphyrins from the vapor phase, and offer a solvent-free route to obtain reproducible and smooth thin films of complex materials for engineering film functionalities.

Synthesis and purification of metallooctachlorophthalocyanines

A detailed synthetic procedure based on the use of urea, dichlorophthalic acid, respective transition metal halides and [NH4]2[MoO4] as a catalyst in the melt or by using 1,2,4-trichlorobenzene as a high-boiling inert solvent is described to gain 2,3,9,10,16,17,23,24-metallooctachlorophthalocyanines (MPcCl8 compounds with M=Mn, Fe, Co, Ni, Cu). In cases that a first purification by subsequent treatment of the crude materials with HCl, NaOH and HCl would not give rise to analytically pure compounds, a second novel purification by using pyridine is described. The degree of purity, exceeding always 98%, is determined by thermogravimetric analysis. Comparative IR, UV/Vis and PXRD studies of the MPcCl8 compounds are reported.

Charge transfer from and to manganese phthalocyanine: bulk materials and interfaces

Manganese phthalocyanine (MnPc) is a member of the family of transition-metal phthalocyanines, which combines interesting electronic behavior in the fields of organic and molecular electronics with local magnetic moments. MnPc is characterized by hybrid states between the Mn 3d orbitals and the π orbitals of the ligand very close to the Fermi level. This causes particular physical properties, different from those of the other phthalocyanines, such as a rather small ionization potential, a small band gap and a large electron affinity. These can be exploited to prepare particular compounds and interfaces with appropriate partners, which are characterized by a charge transfer from or to MnPc. We summarize recent spectroscopic and theoretical results that have been achieved in this regard.

Spin Polarization Inversion at Benzene-Absorbed Fe4N Surface

We report a first-principle study on electronic structure and simulation of the spin-polarized scanning tunneling microscopy graphic of a benzene/Fe(4)N interface. Fe(4)N is a compound ferromagnet suitable for many spintronic applications. We found that, depending on the particular termination schemes and interface configurations, the spin polarization on the benzene surface shows a rich variety of properties ranging from cosine-type oscillation to polarization inversion. Spin-polarization inversion above benzene is resulting from the hybridizations between C p(z) and the out-of-plane d orbitals of Fe atom.

Optical and magneto-optical properties of metal phthalocyanine and metal porphyrin thin films

The optical constants together with the magneto-optical Voigt constants of several phthalocyanine (Pc) and methoxy functionalized tetraphenylporphyrin (TMPP) thin films prepared on silicon substrates are presented. The materials investigated are MePc with Me = Fe, Co, Ni, Cu, Zn and MeTMPP with Me = Cu, Ni. We also compared our results to the metal-free H2Pc, H2TPP and H2TMPP. The experimental results will be supported by electronic structure calculations based on density functional theory (DFT) and interpreted using the perimeter model initially proposed by Platt. The model allows for qualitative understanding of the forbidden character of transitions in planar, aromatic molecules, and is able to qualify differences between Pc and TMPP type materials.