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  
[PERSON]  
1 Department of Solid State Electronics, V.A. Fock Institute of Physics, St. Petersburg State University,
In the past two decades, all these problems have been the subject of large-scale research on redox poly-[M(Salen)] and their composites [1; 2; 11; 12; 13; 14; 15; 16; 17; 18; 19; 20; 21; 22; 23; 24; 25; 26; 27; 28; 29; 30; 31; 32; 33; 34; 35; 36; 37; 38; 39; 40; 41; 42; 43; 44]. Until now, the most popular methods for experimentally characterizing the properties of these new materials remain scanning electron microscopy (SEM), _in situ_ UV-visible and IR spectroscopy, _in situ_ ellipsometry, and electrochemical methods (cyclic voltammetry, chronoamperometry, impedance spectroscopy, etc.) [3; 4; 5; 6; 7; 8; 9; 10; 11; 12; 13; 14; 15; 16]. However, the methods used provide far from a complete understanding and description of the formation processes of these systems. First of all, this concerns the lack of important information about the spatial localization and atomic orbital composition of the upper occupied and lower unoccupied molecular orbitals (MOs) of monomer complexes, since electronic transitions between these MOs determine the chemical bonding of monomers during their polymerization. At the same time, these transitions often occur between MOs localized on different metal and ligand atoms of the complex and are therefore associated with the charge transfer between them. All of this makes it difficult to fully understand and correctly describe the electronic properties of these complexes, which are of particular importance for their practical use. Thus, for the widespread use in modern technologies of electrically conductive redox poly-[M(Salen)] polymers and their composites with carbon nanostructures, detailed knowledge is required about the properties of occupied and empty valence MOs of complexes, which are responsible for processes of electrochemical polymerization of monomers and the mechanisms of redox conductivity in these polymer compounds. The X-ray transitions of inner-shell electrons are different from valence electronic transitions probed by optical absorption spectroscopy since they are element-specific, atomically localized transitions. 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; 57; 58; 59; 60; 61; 62; 63; 64; 65; 66; 67; 68; 69; 70; 71; 72; 73; 74; 75; 76; 77; 78; 79; 80; 81; 82; 83; 84; 85; 86;  
of two strong intramolecular N\(\cdots\)H hydrogen bonds. Furthermore, the out-of-plane distortion of benzene moieties in all isomers is negligible. Let us consider the structure of [M(Salen)] complexes (M = transition metal such as V, Cr, Mn, Fe, Co, Ni, Cu, Pd) using the example of the monomer N,N\({}^{\prime}\)-ethylene-bis(salicylaldiminato)nickel(II), NiO\({}_{2}\)N\({}_{2}\)C\({}_{16}\)H\({}_{14}\) (hereafter [Ni(Salen)]). The [Ni(Salen)] is considered a standard representation of complex 3\(d\)-atom monomers with Schiff bases [M(Schiff)] (Figure 1) [18; 45; 58; 59]. The molecule consists of a nickel atom bound to a tratendentate N\({}_{2}\)O\({}_{2}\) ligand of the salen type forming an almost planar coordination center [NiN\({}_{2}\)O\({}_{2}\)] with C\({}_{2v}\) symmetry.