Synthesis and electronic investigation of mono- and di-substituted 4-nitro- and 4-amino-pyrazol-1-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-β-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 T1 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.