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

A luminescent Tb 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) complex, which was shown to affect the activation energy (ΔE ) and frequency factor (A) in the BEnT process between Tb ion and hfa ligands. According to temperature-dependent emission-lifetime measurements of mononuclear Tb(hfa) complexes with monodentate phosphine oxides and polynuclear Tb(hfa) complexes with bidentate phosphine oxides, the ΔE and A values of polynuclear Tb complexes were smaller than those of mononuclear Tb complexes. Phosphorescence spectra and lifetimes of each Gd(hfa) complex revealed that excited states of hfa ligands in Tb complexes differed from those of the polynuclear Tb complexes and mononuclear Tb complexes. The differences in the ΔE and A values between polynuclear and mononuclear Tb complexes were caused by the formation of different excited states, such as delocalization of the excited state in the polynuclear Tb complexes and localization of excited states in the mononuclear Tb complexes. In particular, small ΔE and A values of polynuclear Tb 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.