Study of the correlations of the MLCT Vis absorption maxima of 4-pentacyanoferrate-4′-arylsubstituted bispyridinium complexes with the Hammett substituent parameters and the solvent polarity parameters ETN and AN

In this work six new 4-aryl substituted bispyridinium salts have been synthesized using 2,4-dinitrophenyl-bispyridinium chloride as the starting material, which was arenamine exchanged via the Zincke reaction. These aromatic heterocyclic salts were used as ligands, acting as Lewis bases to form a series of 4-pentacyanoferrate-4'-aryl substituted bispyridinium complex salts in high yields. The complex salts were characterized using several spectroscopic techniques including H-1 as well as C-13 NMR spectroscopy, UV-Vis and FTIR spectrophotometry. These materials present a number of interesting electronic and optical properties. Herein we mainly focused on the electronic absorption of these materials. The visible metal-to-ligand-charge-transfer (MLCT) band of these materials assigned as d pi(Fe-II) -> pi*(L) was proved to be affected by substituent changes (of the benzene ring of the complexes) but mainly by solvent polarity changes. Band shifts of even 4000 cm(-1) were induced by small solvent polarity changes (e.g., water to methanol). The Vis absorption spectra were investigated in four protic solvents (water, 2,2,2-trifluoroethanol, ethylene glycol, and methanol). The substituent effects were then quantified using the Hammett equation which correlates the MLCT absorption wavenumbers with the Hammett constant (sigma(x)). Furthermore, two of the most successful solvent polarity parameters (the acceptor number AN and the normalized solvent polarity scale E-T(N)) were used to quantify the solvent polarity effects on the MLCT absorption wavenumbers. The correlations obtained in all cases proved to be satisfactory. The dominant interaction responsible for the solvent polarity effects proved to be the hydrogen-bond formation between the cyano groups of the complex salts and the protic solvents. Copyright (C) 2009 John Wiley & Sons, Ltd.