X-ray photoelectron spectroscopy: Charge transfer in Fe 2p peaks and inner-sphere reorganization of ferrocenyl-containing chalcones

A systematic analysis of the XPS Fe 2p(3/2) core-level photoelectron lines and their satellite structures gave insight into the electronic properties of a series of ferrocenyl-containing chalcones with the structure Fc-CO-CH=CH-C6H4R, where R is in the para-position on the phenyl ring and R=OCH3 (1), CH3 (2), C6H5 (3), Bu-t (4), H (5), Br (6) and CF3 (7). A V-shaped correlations obtained between the binding energy of the Fe 2p(3/2) photoelectron line and Hammett constants (sigma(R)) of the R-groups as well as the oxidation potential of the Fe-II/Fe-III couple, E-pa, indicates that there are different modes of zwitterion formation, of electron-donating and electron-withdrawing R-substituents, after photoemission of an electron during the XPS measurements. The intensity of the satellite peak (I-ratio) within the substructure of the photoelectron line is an indication of the amount of charge being transfer from the donor (ferrocenyl-fragment) to the acceptor fragments. The link between the Gordy group electronegativities, lambda(R), and I-ratio show that more charge is being transfer from Fc in chalcone with more electron withdrawing R-groups. During electron transfer processes (like electrochemical oxidation or photoemission) the chalcone undergoes inner-sphere reorganisation to compensate for the positively charged species which is formed. The I-ratio is also an indication of the degree of inner-sphere reorganisation, while the carbonyl stretching frequency (vCO as measured by ATR FTIR) is a measure of the amount of energy of required for this inner-sphere reorganisation. The link between I-ratio and vCO showed that as the degree of inner-sphere reorganisation increases, more energy is required for this inner-sphere reorganisation. (C) 2018 Elsevier B.V. All rights reserved.