Chromotropism of Iron(II) Complexes with Non-Symmetric Heterocyclic Ligands: Polarity Sensing in Aqueous Urea Solutions

This study reports the synthesis and characterization of two solvatochromic pentacyanoferrate(II) complexes. Their structural design incorporates ligands with flexible xylylene bridges and distinct heterocycles—one combining 4-dimethylaminopyridine (DMAP) with 4,4′-bipyridine, and the other isoquinoline with 4,4′-bipyridine. Their structural diversity enables the complexes to engage in a broad range of solvent–solute interactions, providing valuable insights into the behavior of solvents and media with regard to polarity, through sizable solvatochromic shifts. Their solvatochromism is examined using a set of nine solvents and solvent mixtures. The solvatochromic sensitivities to polarity changes, expressed through a variety of polarity parameters and functions, are determined. Moreover, using a set of four complementary linear solvation energy relationships (LSERs), the roles of solvent polarity, solute–solvent interactions and the molecular responsiveness of the two compounds to different media are investigated. Additionally, their dipole moments in the ground and MLCT-excited states are determined using a suitable model, namely that of Suppan and Tsiamis. As a step further, the polarity sensing aptitude of the two solvatochromic compounds is examined in aqueous urea solutions at varying urea concentrations. The solvatochromic sensitivity of the two compounds is compared with that of a model cyanoferrate(II) complex, FeII(CN)2(phen)2. The two compounds clearly surpass the sensitivity of FeII(CN)2(phen)2 with subtle solvent polarity changes induced by varying the urea concentration. An LSER describing and predicting the solvatochromic effects in aqueous urea is developed and tested.