Optical absorption spectroscopic studies on holmium(III) complexes with β-diketone and heterocyclic amines: The environment effect on 4f–4f hypersensitive transitions

The optical absorption spectra of [Ho(acac)3(H2O)2]·H2O, [Ho(acac)3phen] and [Ho(acac)3bpy] (where acac is the anion of acetylacetone; phen is 1,10-phenanthroline and bpy is 2,2′-bipyridyl) complexes in the visible region, in a series of non-aqueous solvents (methanol, ethanol, isopropanol, chloroform, acetonitrile and pyridine.) have been analyzed. The largest intensity variation was observed in the 5G6 ← 5I8 (centered at 450 nm), and 5G5, 3H5, 3H6 ← 5I8 (centered at 360 nm) transition regions. The band shape and oscillator strength of the hypersensitive transitions display pronounced changes as compared to Ho3+ aqua-ion. The band shapes of the hypersensitive transitions show remarkable changes on passing from aqueous solution to various non-aqueous solutions which is the result of change in the environment about the Ho(III) ion in the various solutions and suggests coordination of solvent molecule(s), in some cases. The results clearly show that among the solvents studied pyridine is the most effective in promoting the 4f–4f spectral intensity. It has been inferred from this study that 2,2′-bipyridyl is a stronger ligand for heavier lanthanides. A comparative account of hypersensitivity in the present complexes with those of other adducts of Ho(β-diketoenolate)3 with heterocyclic amines is discussed. The TGA analyses showed that the phen complex is thermally more stable over its bpy analogue.