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

Abstract

A systematic analysis of the XPS Fe 2p3/2 core-level photoelectron lines and their satellite structures gave insight into the electronic properties of a series of ferrocenyl-containing chalcones with the structure FcCOCHCHC6H4R, where R is in the para-position on the phenyl ring and ROCH3 (1), CH3 (2), C6H5 (3), tBu (4), H (5), Br (6) and CF3 (7). A V-shaped correlations obtained between the binding energy of the Fe 2p3/2 photoelectron line and Hammett constants (σR) of the R-groups as well as the oxidation potential of the FeII/FeIII couple, Epa, 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 (Iratio) 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, λR, and Iratio 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 Iratio 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 Iratio and vCO showed that as the degree of inner-sphere reorganisation increases, more energy is required for this inner-sphere reorganisation.