Interaction of a 1,3-Dicarbonyl Toxin with Ru(II)-Biimidazole Complexes for Luminescence Sensing: A Spectroscopic and Photochemical Experimental Study Rationalized by Time-Dependent Density Functional Theory Calculations.

A family of ruthenium(II) complexes containing one 2,2'-biimidazole (bim) ligand and two polypyridyl (NN) ligands has been prepared and their photophysical and photochemical features have been tested in the presence of tenuazonic acid (TeA), a widespread food and feed mycotoxin of current concern. While not tested in in vivo studies, TeA and other secondary metabolites of Alternaria fungi are suspected to exert adverse effects on the human health, so sensors and rapid analytical procedures are required. It is well-known that 1,3-dicarbonyl compounds such as TeA are relatively easy to deprotonate (the p K a of TeA is 3.5), yielding an enolate anion stabilized by resonance. The chelating and hydrogen-donor features of bim allow simultaneous binding to the metal core and to the target β-diketonate delocalized anion. Such a binding induces changes in the blue absorption (40 nm bathochromic shift), red luminescence intensity (>75% quenching), and triplet lifetime (0.2 μs decrease) of the Ru(NN) 2 (bim) 2+ luminophore. Moreover, we have computationally rationalized, by time-dependent density functional theory, the structure of the different adducts of Ru-bim complexes with TeA and the electronic nature of the spectral absorption bands and their change upon the addition of TeA.