Reversible colour/luminescence colour changes of tetracyanoruthenium(II) complexes by sorption/desorption of water molecules in crystals.

We report colour/luminescence colour changes of M[Ru(bpy)(CN)] crystal (M = Ca, Sr, and Ba; bpy = 2,2'-bipyridine). The X-ray crystallographic study revealed that the crystals are constructed from linear-chains of {[Ru(bpy)(CN)][Ca(HO)]}, {[Ru(bpy)(CN)][Sr(HO)]}, and {[Ru(bpy)(CN)][Ba(HO)](μ-HO)}, respectively. Ru(II) complex linear chains and the hydrophilic channels composed of M ion and water along them enable reversible water sorption/desorption without collapse of crystals responsible for the colour change. The emission spectra of Ca and Sr salts are remarkably shifted to the red side when the temperature was increased from 296 to 500 K, while Ba salt shows a slight shift in the emission spectrum during the heating. The change in the interaction of M ion to the equatorial CN ligand depending on the number of hydrated water molecules effectively contributes to the luminescence colour change for Ca and Sr salts. FT-IR spectra after heating at 473 K show the high-frequency shifts in the CN stretching mode for Sr salt, while no remarkable peak shifts are observed for Ca and Ba salts. Thermogravimetry results indicate that heating over 470 K leads to the desorption of 5HO from all salts, resulting in {[Ru(bpy)(CN)][Ca]}, {[Ru(bpy)(CN)][Sr(HO)]}, and {[Ru(bpy)(CN)][Ba](μ-HO)} for linear chains. The change in the hydration structure for M ions regulates the shift of CN stretching modes.