Two-Step Vapochromic Luminescence of Proton-Conductive Coordination Polymers Composed of Ru(II)-Metalloligands and Lanthanide Cations.

Two proton-conductive and phosphorescent porous coordination polymers, La3-[H5.5RuP]2 and Pr3-[H5.5RuP]2 (H12RuP = [Ru(H4dpbpy)3]2+, H4dpbpy = 2,2'-bipyridine-4,4'-bis(phosphonic acid), composed of Ru(bpy)3-type metalloligands (bpy = 2,2'-bipyridine) functionalized with six phosphonate groups and lanthanide cations (Ln3+ = La3+ or Pr3+) were successfully synthesized. X-ray diffraction studies revealed that six to seven protons of the H12RuP metalloligand were removed in the coordination polymerization reaction to form the porous coordination framework (Ln3-[H5.5RuP]2) with Ln3+ cations (Ln3+ = La3+ or Pr3+). Although their porous structures collapsed on the removal of water molecules from the porous channels, the original porous structures were reconstructed by water adsorption. Interestingly, the triplet metal-to-ligand charge-transfer (3MLCT) emission of Ln3-[H5.5RuP]2 was blue-shifted on increasing the relative humidity (RH) in the low RH region, whereas the inverse red shift was observed in the high RH region, resulting in the highest-energy 3MLCT emission at medium RH. The origin of this two-step vapochromic luminescence (that is, the blue and red shifts of the 3MLCT emission) is ascribable to the water-adsorption-triggered reconstruction of the porous structure and the proton release from the H5.5RuP metalloligand to the water filled channels, respectively. The proton conductivity of Ln3-[H5.5RuP]2 is about 1000-times higher at 20% RH and 10-times higher at 95% RH than that of the carboxylate analog, La7-[RuC]4 (H6RuC = [Ru(H2dcbpy)3]2+; H2dcbpy = 2,2'-bipyridine-4,4'-bis(carboxylic acid)), probably because of the highly acidic phosphonic acid groups.