Bisimidazole-based phosphorescent thiocyanatocadmates

Abstract

Simple room-temperature self-assemblies between Cd²⁺ salts, SCN⁻ and bisimidazole molecules at pH = 2 created three new organically templated thiocyanatocadmates [H₂(L1)][Cd(SCN)₄]·H₂O (L1 = 1,4-bis(1H-imidazol-1-yl)benzene) 1, [H₂(L2)][Cd(SCN)₄] (L2 = 1,3-bis(2-methylimidazol-1-yl)propane) 2, and [H₂(L3)][Cd₂(SCN)₆] (L3 = 1,4-bis(2-methyl-1H-imidazol-1-yl)butane) 3. X-ray single-crystal diffraction analysis reveals that (i) in 1–3, the SCN⁻ groups doubly bridge the Cd²⁺ centers to form different thiocyanatocadmates: a linear chain in 1; a zigzag chain in 2; and a 2-D layer network (6³ net) in 3; and (ii) in 1, via N_base–H⋯N_SCN interactions, the L1 molecules extend the thiocyanatocadmate chains into a 2-D supramolecular layer, whereas in 2, the zigzag thiocyanatocadmate chains self-assemble into a 3-D supramolecular network via weak S⋯S interactions. Photoluminescence analysis indicates that the three title compounds all emit light: blue light for 1 and 2 and green light for 3. At low temperatures, the emission positions of the three compounds hardly change, but the emission intensities are largely enhanced. Interestingly, after turning off the UV lamp, 1 and 2 still briefly emit light (ca. 2 s), which means that 1 and 2 possess phosphorescence properties. Phosphorescence lifetimes at 77 K are 1619 ms for 1 and 247 ms for 2.