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

A luminescent Tb^III 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 (ΔE_a ) and frequency factor (A) in the BEnT process between Tb^III 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 ΔE_a and A values of polynuclear Tb^III complexes were smaller than those of mononuclear Tb^III complexes. Phosphorescence spectra and lifetimes of each Gd(hfa)₃ complex revealed that excited states of hfa ligands in Tb^III complexes differed from those of the polynuclear Tb^III complexes and mononuclear Tb^III complexes. The differences in the ΔE_a and A values between polynuclear and mononuclear Tb^III complexes were caused by the formation of different excited states, such as delocalization of the excited state in the polynuclear Tb^III complexes and localization of excited states in the mononuclear Tb^III complexes. In particular, small ΔE_a and A values of polynuclear Tb^III 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.