Photoluminescence, optical absorption and hypersensitivity in mono- and dinuclear lanthanide (TbIII and HoIII) β-diketonate complexes with diimines and bis-diimine bridging ligand

The photoluminescence properties of three Tb(III) complexes of the form [Tb2(fod)6(μ-bpm)], [Tb(fod)3(phen)] and [Tb(fod)3(bpy)] and optical absorption properties of their Ho(III) analogues (fod=anion of 6,6,7,7,8,8,8-heptafluoro-2,2-dimethyl-3,5-octanedione, bpm=2,2′-bipyrimidine, phen=1,10-phenanthroline and bpy=2,2′-bipyridyl) in a series of solvents are presented. The luminescence of the complexes is sensitive to changes in environment (ligand/solvent) around Tb(III) and co-sensitization of the ancillary ligands. The enhancement of the luminescence intensity in coordinating solvents is attributed to the transformation of eight-coordination into less symmetric nine-coordination structure around Tb(III). Among phen and bpy, the phen is better co-sensitizer while bpm has been observed as poor co-sensitizer. The enhancement of the oscillator strength of 5G6←5I8 hypersensitive transition in the 4f–4f absorption in some coordinating solvents is attributed to decrease in the symmetry of the field around Ho(III) ion. The [Ho(fod)3(phen)] is inert towards the solvents and retains its bulk structure and composition in solution. The transformation of the holmium complexes in DMSO into [Ho(fod)3(DMSO)2] species is found. The results reveal that the luminescence and 4f–4f absorption properties of lanthanide complexes in solution can be modulated by tuning the coordination structure through ancillary ligands and donor solvents.