Synthesis and electronic investigation of mono-and di-substituted 4-nitro-and 4-amino-pyrazol1-yl bis(pyrazol-1-yl) pyridine-type ligands and luminescent Eu(III) derivatives

Four new disubstituted and monosubstituted nitro-and amino-bis(pyrazol-1-yl) pyridine (bppy) ligands, substituted at the pyrazole 4-position (1, 2, 5, 6) have been synthesized, along with two luminescent Eu(III) tris-beta-diketonate derivatives of the amino substituted ligands (7, 8). The compounds have been studied using UV-Vis absorbance spectroscopy and cyclic voltammetry which has allowed for characterization of the electronic environments of these ligands. The calculated HOMO-LUMO gap values (1: 3.54 eV; 2:3.53 eV; 5:3.01 eV; 6:3.66 eV) differ from that of bppy (3.86 eV) and the range is indicative that tuning of the ligand electronic environment is possible. Additionally, fluorescence spectroscopy studies were employed to determine ligand T-1 energy levels of the amine-bearing ligands 2 and 6, yielding values of T1 of 25 381 cm(-1) and 26 201 cm(-1), respectively. These ligands were employed in the synthesis of Eu(III) complexes 7 and 8, for which the absolute and intrinsic quantum yields, lifetimes and ligand sensitization efficiencies were determined.