Organic linkers control the thermosensitivity of the emission intensities from Tb(iii) and Eu(iii) in a chameleon polymer

Abstract

Thermometers whose emission color gradually changes with temperature are called chameleon emitters. In this study, we discuss the mechanism of the thermosensitivity of the emission color of polymers that contain two lanthanides (Ln³⁺), e.g., [Tb_0.99Eu_0.01(hfa)₃(linker)]_n, where the Ln³⁺(hfa)₃ complexes (hfa: hexafluoro acetylacetonato) are connected by a phosphine oxide “linker” molecule. First, the difference in the thermosensitivities of the emissions from Tb³⁺ and Eu³⁺ are discussed. With increasing temperature, the green-emission intensity from Tb³⁺ decreases whereas the red-emission intensity from Eu³⁺ does not change. This was found to originate from the different reaction barriers for the quenching of the Ln³⁺ excited state via the intersystem crossing (ISC) between the hfa-centered triplet state and the ground state. Next, the excitation energy transfer (EET) from Tb³⁺ to Eu³⁺ is discussed. Although the direct EET between Ln³⁺ atoms is negligible because of the long distance between them, stepwise EET is found to occur via the linker-centered triplet state with a reasonable barrier. Thus, we propose a new idea—thermosensitivity can be controlled by the linker as well as by the ligand (hfa). To confirm the role of the linker, four phosphine oxides were examined. The thermosensitivity dependence on the linker is validated via experimental measurements.