Atomic and electronic structure of poly-[Ni(Salen)]: combined study by XPS, UV PES, NEXAFS and DFT methods

Abstract

A detailed study of poly-[Ni(Salen)] polymer in its oxidized (Ox) and reduced (Red) states was conducted using X-ray photoelectron (XPS) and ultraviolet photoemission (UV PES) spectroscopy, near-edge X-ray absorption fine structure (NEXAFS) spectroscopy, and quantum-chemical calculations. XPS analysis revealed significant energy shifts (-1.5 to -1.8 eV) and broadening of the PE lines for all atoms upon polymerization, indicating a major redistribution of valence electron density between the monomer fragments. In the oxidized polymer, new features in the Ni 2p and O 1s PE spectra were associated with the formation of polarons with weakened Ni-O bonds; this effect diminished upon reduction as the number of polarons decreased. Quantum-chemical calculations attributed the valence band broadening to enhanced C 2p contributions from $\pi$-conjugation between monomers. NEXAFS spectroscopy confirmed the stability of the ethylenediamine fragment and the direct involvement of the phenolic rings of the salen ligand in polymerization, also revealing a partial weakening and incomplete restoration of the $\pi$ bonding between O and Ni atoms upon reduction. Furthermore, it was shown that it is the $BF_{4}^-$ anions that weaken the Ni-O bonds during oxidation, which are partially preserved in the reduced state.