Ligand-Assisted Back Energy Transfer in Luminescent TbIII Complexes for Thermosensing Properties.

A luminescent TbIII complex with a hexafluoroacetylacetone (hfa) ligand shows a characteristic back energy transfer (BEnT), which leads to high temperature sensitivity and potential application as a thermosensitive paint. Ligand-assisted BEnT was observed when a phosphine oxide ligand was introduced into Tb(hfa)3 complex, which was shown to affect the activation energy (ΔEa ) and frequency factor (A) in the BEnT process between TbIII ion and hfa ligands. According to temperature-dependent emission-lifetime measurements of mononuclear Tb(hfa)3 complexes with monodentate phosphine oxides and polynuclear Tb(hfa)3 complexes with bidentate phosphine oxides, the ΔEa and A values of polynuclear TbIII complexes were smaller than those of mononuclear TbIII complexes. Phosphorescence spectra and lifetimes of each Gd(hfa)3 complex revealed that excited states of hfa ligands in TbIII complexes differed from those of the polynuclear TbIII complexes and mononuclear TbIII complexes. The differences in the ΔEa and A values between polynuclear and mononuclear TbIII complexes were caused by the formation of different excited states, such as delocalization of the excited state in the polynuclear TbIII complexes and localization of excited states in the mononuclear TbIII complexes. In particular, small ΔEa and A values of polynuclear TbIII complexes provided high, effective activation of the BEnT at low temperature, which resulted in high-sensitive temperature-dependent phosphor materials over a wide temperature range.