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) 4 ] crystal (M 2+ = Ca 2+ , Sr 2+ , and Ba 2+ ; bpy = 2,2'-bipyridine). The X-ray crystallographic study revealed that the crystals are constructed from linear-chains of {[Ru(bpy)(CN) 4 ][Ca(H 2 O) 5 ]} n , {[Ru(bpy)(CN) 4 ][Sr(H 2 O) 6 ]} n , and {[Ru(bpy)(CN) 4 ] 2 [Ba(H 2 O) 5 ] 2 (μ-H 2 O) 2 } n , respectively. Ru(II) complex linear chains and the hydrophilic channels composed of M 2+ ion and water along them enable reversible water sorption/desorption without collapse of crystals responsible for the colour change. The emission spectra of Ca 2+ and Sr 2+ salts are remarkably shifted to the red side when the temperature was increased from 296 to 500 K, while Ba 2+ salt shows a slight shift in the emission spectrum during the heating. The change in the interaction of M 2+ ion to the equatorial CN ligand depending on the number of hydrated water molecules effectively contributes to the luminescence colour change for Ca 2+ and Sr 2+ salts. FT-IR spectra after heating at 473 K show the high-frequency shifts in the CN stretching mode for Sr 2+ salt, while no remarkable peak shifts are observed for Ca 2+ and Ba 2+ salts. Thermogravimetry results indicate that heating over 470 K leads to the desorption of 5H 2 O from all salts, resulting in {[Ru(bpy)(CN) 4 ][Ca]} n , {[Ru(bpy)(CN) 4 ][Sr(H 2 O)]} n , and {[Ru(bpy)(CN) 4 ] 2 [Ba] 2 (μ-H 2 O) 2 } n for linear chains. The change in the hydration structure for M 2+ ions regulates the shift of CN stretching modes.