The Valence Band Structure of the [Ni(Salen)] Complex: An Ultraviolet, Soft X-ray and Resonant Photoemission Spectroscopy Study

Abstract

The valence band photoemission (VB PE) spectra of the [Ni(Salen)] molecular complex were measured by ultraviolet, soft X-ray and resonant photoemission (ResPE) using photons with energies ranging from 21.2 eV to 860 eV. It was found that the Ni 3d atomic orbitals’ (AOs) contributions are most significant for molecular orbitals (MOs), which are responsible for the low-energy PE band at a binding energy of 3.8 eV in the VB PE spectra. In turn, the PE bands in the binding energies range of 8–16 eV are due to the photoionization of the MOs of the [Ni(Salen)] complex with dominant contributions from C 2p AOs. A detailed consideration was made for the ResPE spectra obtained using photons with absorption resonance energies in the Ni 2p3/2, N 1s, and O 1s Near-Edge X-ray Absorption Fine Structure (NEXAFS) spectra. A strong increase in the intensity of the PE band ab was found when using photons with an energy 854.4 eV in the Ni 2p3/2 NEXAFS spectrum. This finding is due to the high probability of the participator-Auger decay of the Ni 2p3/2−13d9 excitation and confirms the relationship between the PE band ab with the Ni 3d-derived MOs.

Atomic and Electronic Structure of Metal–Salen Complexes [M(Salen)], Their Polymers and Composites Based on Them with Carbon Nanostructures: Review of X-ray Spectroscopy Studies

“…This finding is explained by an increase in the effective charges of carbon and nitrogen atoms and a corresponding decrease in the effective charge of oxygen atoms in the ligand during the formation of the [Ni(Salen)] complex. An equally interesting result was obtained in the work [53], where VB PE spectra of the molecular complex [Ni(Salen)] were measured using ultraviolet, soft X-ray, and resonance photoemission methods using photons with energies from 21.2 to 848 eV. As a result, it was established that electronic states at a binding energy of 3.8 eV are associated specifically with the Ni 3d-derived MOs of the complex (see Figure 8).…”

“…As noted above in this review, studies of occupied electronic states of the valence band for monomers [M(Salen)] and their polymers using valence photoemission (VB PE) have so far been limited to a few works [45,46,53]. These works are devoted to studying the electronic structure of only the H 2 (Salen) molecule and the [Ni(Salen)] complex.…”

“…Therefore, the X-ray absorption and photoelectron spectroscopy methods are among the most informative experimental techniques for studying the local atomic and electronic structures of various polyatomic systems: molecules, complexes, polymers, etc. However, recent studies are characterized by only separate attempts to use the methods of X-ray core-level photoelectron spectroscopy (XPS) [8,26,35,37,39,40,[42][43][44][45][46][47][48][49][50][51][52] and valence-band photoemission spectroscopy (VB PES) [45,46,53], as well as X-ray absorption spectroscopy (near-edge X-ray absorption fine structure, NEXAFS, and extended X-ray absorption fine structure, EXAFS) [27,29,30,40,45,46,[52][53][54][55][56] to obtain information about the features of the electronic structure of [M(Salen)] complexes, their polymers, and composites based on them, as well as about the atomic structure of these polyatomic systems.…”

A Comparative XPS, UV PES, NEXAFS, and DFT Study of the Electronic Structure of the Salen Ligand in the H2(Salen) Molecule and the [Ni(Salen)] Complex

“…They are normalized to the intensity of the PE band d and plotted on the scale of the valence electron BE’s relative to the Fermi level ( E F ). A preliminary discussion of these spectra was carried out in the framework of a study devoted to a consideration of the structural evolution of the VB PE spectra of the [Ni(Salen)] complex with a change in the energy of its exciting photons in the range of 21.2–848.0 eV [ 34 ]. Here, the qualitative results of this discussion are supplemented with a detailed comparative analysis of the UV PE spectra of H 2 (Salen) and [Ni(Salen)], using the results of the DFT calculations of their valence bands.…”

“…It has previously been shown that combined studies based on NEXAFS, CL, and VB PES measurements are very effective in characterizing the electronic properties and chemical bonding of similar complexes of nickel porphyrins and phthalocyanine [ 28 , 29 , 30 , 31 , 32 ]. Recently, similar studies have been carried out for the molecular complex [Ni(Salen)], in order to derive information about the features of its local electronic structure [ 33 , 34 ]. As a result, for the [Ni(Salen)] monomer, new knowledge of the energy positions and AO composition of the valence MOs responsible for its high-lying occupied and low-lying vacant electronic states has been obtained.…”