Synthesis and spectroscopic investigations of a crystalline humidity sensor: Bis(triphenylphosphine) iminium 2,2 '-bipyridyltetracyanoruthenate

We have prepared a new salt of the solvatochromic [Ru(bpy)(CN)(4)](2-) anio n bis(triphenylphosphine)iminium 2,2'-bipyridyltetracyanoruthenate, ((PPN)( 2)[Ru(bpy)(CN)(4)]), that reversibly sorbs water and changes color from pur ple to yellow when it is exposed to humid air. Solid samples of anhydrous ( PPN)(2)[Ru(bpy)(CN)(4)] reversibly hydrate in humid air, first to (PPN)(2)[ Ru(bpy)(CN)(4)] . H2O and then to (PPN)(2)[Ru(bpy)(CN)(4)]. 15H(2)O. A sing le-crystal X-ray study of(PPN)(2)[Ru(bpy)(CN)(4)]. 2CH(3)CN . 2(CH3CH2)(2)O . 2H(2)O shows that this novel humidity sensor has large channels in the s tructure that allows sorbed water molecules to reversibly interact with the cyanide ligands of the chromophore. The hydration changes that result from exposure of the solid in the 0 to 66% relative humidity range cause the ML CT visible absorption bands to shift dramatically to higher energy. These h ydration changes also result in large shifts in the v(CN) region of the inf rared spectrum; these changes indicate significant H-bonding interactions b etween sorbed water molecules and the dianionic chromophore occur in the cr ystal lattice. The hydration changes in the 66 to 100% relative humidity ra nge cause large intensity changes in a visible photoluminescence band. This emission intensity increase is accompanied by small wavelength shifts in a bsorption and emission and in the v(CN) region of the infrared spectrum. Th ese changes are consistent with further oligomerization of the water molecu les that initially formed hydrogen-bonding interactions to the cyanide liga nds.