A substituted pyridazine was used to construct a family of mononuclear heteroleptic complexes [Ln(tta)₃(dppn)], Ln = Pr-Lu; Htta = thenoyltrifluoroacetone; dppn = 3,6-di(2-pyridyl)pyridazine. All the complexes obtained were characterized by CHN elemental analysis, NMR spectroscopy, ESI mass spectrometry, FTIR spectroscopy and single crystal X-ray analysis. The photophysical properties of lanthanide complexes were carefully investigated, and the dppn ligand was found to act as a chromophore centre to sensitize metal-centred emission of the Nd, Sm, Eu, and Yb complexes under UV excitation. It has been shown that a powder sample of the Eu complex can contain two isomers with somewhat different symmetries of the environment of the metallocentre. The mechanism for the emission of luminescent Nd, Sm, Eu, and Yb complexes was investigated by TDDFT calculations and the results confirmed the conclusion made on the basis of experimental data regarding the likely pathway of energy transfer from the chromophore centre to the emissive level of Ln(iii).