Influence of urea N-H acidity on receptor-anionic and neutral analyte binding in a ruthenium(II)-polypyridyl-based colorimetric sensor

Abstract

A new ruthenium(II)-polypyridyl complex {[Ru(bpy)₂L]²⁺; bpy = 2,2'-bipyridyl, L = 1-(6-nitro-1,10-phenanthrolin-5-yl)-3-phenylurea} having a urea functionality as a receptor fragment for anionic analytes was synthesized. Its binding affinity towards various oxy anions and halides was studied. This complex was found to act as a selective colorimetric sensor for F- among halides and CH₃COO^-/H₂PO₄^- among oxy anions. The relative binding affinity of different anions towards this receptor was examined by using quantum chemical calculations. This complex was also found to act as a colorimetric sensor for neutral molecules like DMSO and DMF, though the binding affinity was weaker than that of the three anions mentioned above. The relative acidity of two HN_urea atoms was compared with that of one from the related complex by using pK_a calculations, and its influence on binding affinities towards different analytes is discussed. Results of the time-resolved fluorescence studies reveal that two nonequilibrated excited states exist involving two different ³MLCT transitions, namely Ru_dΠ → bpy_Π* and Ru_dΠ → L_Π*.