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

Abstract

We report colour/luminescence colour changes of M[Ru(bpy)(CN)₄] crystal (M²⁺ = Ca²⁺, Sr²⁺, and Ba²⁺; bpy = 2,2′-bipyridine). The X-ray crystallographic study revealed that the crystals are constructed from linear-chains of {[Ru(bpy)(CN)₄][Ca(H₂O)₅]}_n, {[Ru(bpy)(CN)₄][Sr(H₂O)₆]}_n, and {[Ru(bpy)(CN)₄]₂[Ba(H₂O)₅]₂(μ-H₂O)₂}_n, respectively. Ru(II) complex linear chains and the hydrophilic channels composed of M²⁺ ion and water along them enable reversible water sorption/desorption without collapse of crystals responsible for the colour change. The emission spectra of Ca²⁺ and Sr²⁺ salts are remarkably shifted to the red side when the temperature was increased from 296 to 500 K, while Ba²⁺ salt shows a slight shift in the emission spectrum during the heating. The change in the interaction of M²⁺ ion to the equatorial CN ligand depending on the number of hydrated water molecules effectively contributes to the luminescence colour change for Ca²⁺ and Sr²⁺ salts. FT-IR spectra after heating at 473 K show the high-frequency shifts in the CN stretching mode for Sr²⁺ salt, while no remarkable peak shifts are observed for Ca²⁺ and Ba²⁺ salts. Thermogravimetry results indicate that heating over 470 K leads to the desorption of 5H₂O from all salts, resulting in {[Ru(bpy)(CN)₄][Ca]}_n, {[Ru(bpy)(CN)₄][Sr(H₂O)]}_n, and {[Ru(bpy)(CN)₄]₂[Ba]₂(μ-H₂O)₂}_n for linear chains. The change in the hydration structure for M²⁺ ions regulates the shift of CN stretching modes.