Impact of substituent nature in diformylpyridyl Schiff base on photophysical and electrochemical properties of ruthenium- and iron-based complexes

Abstract

New complexes of ruthenium and iron based on the bis-azomethine condensation product of 2,6-diformyl-4R-pyridine and para-aminocinnamic acid have been theoretically studied. A widely studied ruthenium-based “black dye” sensitizer (N719) based on a tridentate ligand: 2,2′:6′,2′′-terpyridyl-4,4′,4′′-tricarboxylic acid was chosen as the reference complex for calculation protocol tests, followed by the study of complexes with much simpler ligand systems from a synthetic point of view. The changes in geometry and FTIR spectra were evaluated to analyze the stability of complexes to redox processes occurring in dye-sensitized solar cells. The coordination environment did not change significantly, which reflects the low values of the energy of reorganization and indicates the stability of the system in the course of redox processes. The electronic absorption spectra of all studied compounds were modeled using the CAM-B3LYP functional. For the M(II) forms, there are no significant changes in the position of the band, in contrast to its intensity, which can be explained by the difference in the dipole moments. In the case of the reduction form (M(III)) of the complexes, a bathochromic shift is observed with an increase in the Hammett constant of the substituent in the benzene ring. A linear relationship between the redox potential and the donor strength of a substituent in the pyridine ring was also found, and the reorganization energy of the inner shell turned out to be very low compared to cobalt polypyridyl complexes. The study performed makes it possible to reveal the quantitative effect of substituents of organic ligands in complexes based on iron and ruthenium on the electrochemical and photophysical properties, especially on the standard reduction potentials and the reorganization energies of the M(III/II) complexes.