Synthesis, structure and luminescence behaviour of complexes of type [Cd(dpa)₂X₂] (dpa = 2,2′-dipyridylamine; X = N ₃^-, NCS^-, NCO^-)

The synthetic details and characterization data of three mononuclear complexes of the type [Cd(dpa)₂X₂] (dpa = 2,2'-dipyridylamine; X = N₃^-, 1; X = NCS ^-, 2 and X = NCO^-, 3) are described. In one case, [Cd(dpa)₂(NCS)₂] (2), X-ray structure determination has been made for the exact definition of the coordination sphere. Structural analysis reveals that the cadmium(II) centre is in a distorted octahedral environment with a CdN₅S chromophore with two dpa units roughly perpendicular to one another and the two thiocyanates in mutual cis orientation with one in S-bound and the other in N-bound states. The mononuclear unit packs through N-H...S and N-H...N hydrogen bonding to have a one-dimensional (ID) chain. Spectroscopic and other physicochemical data show that compounds 1 and 3 have the cognate structure. The compounds display high energy intraligand ¹(p-p*) fluorescence and intraligand ³(p- p*) phosphorescence in glassy solutions (DMF at 77 K). A band (near 375 nm) observed for all the complexes in their solid state emission spectra is an excimeric emission. The role of dpa, a tailored diimine with active amine hydrogen and aromatic pyridine ring, is discussed towards functionalisation on crystal engineering and luminescence behaviour.