Four ruthenium(II) complexes, [Ru(bpy)2L1](PF6)2, [Ru(bpy)2L2](PF6)2, [Ru(dmb)2L1](PF6)2, and [Ru(dmb)2L2](PF6)2, where bpy = 2,2'-bipyridine, dmb = 4,4'-dimethyl-2,2'-bipyridine, L1 = 4-methyl-2,2'-bipyridine-4'-carboxaldehyde, and L2 = 4,4'-diformyl-2,2'-bipyridine, have been characterized for sensing cyanide based on the well-known formation of cyanohydrins. The structure of [Ru(dmb)2L2](PF6)2 is also determined by single crystal X-ray diffraction. Their cyanide anion binding properties in pure and aqueous acetonitrile solution were thoroughly examined by UV-Vis absorption and photoluminescence (PL) spectroscopy. Upon the addition of cyanide to the solution of ruthenium(II) complexes at 298 K, a color change from orange to yellow and a luminescent variation from dark red to red-orange can be observed by the naked eye. The large blue shift of the absorption and emission spectra and significant enhancement of the emission intensity upon the addition of cyanide could be attributed to the formation of a cyanohydrin group by the selective reaction of the aldehyde group of the ruthenium(II) complexes with cyanide. Importantly, these kinds of ruthenium(II) complexes show a unique recognition of cyanide ions over other anions. The results reveal that this kind of ruthenium(II) complex containing an aldehyde group is an eligible sensor for cyanide anions.

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