Ligand triplet energy escape in lanthanide complexes for developing luminescent molecular thermometers

Abstract

Luminescent lanthanide complexes can exhibit temperature-sensitive metal-centered emission due to energy transfer quenching from the lanthanide to the ligand triplet states, which have been promising application in emission lifetime-based thermometers. However, the long-lived ligand triplet state limits the temperature sensitivity of lanthanide emission. This study demonstrates an enhancement in the temperature sensitivity of Tb(III) emission by introducing an energy escape pathway from the ligand triplet state. A dinuclear Tb(III)-Nd(III) complex containing hexafluoroacetylacetonate (hfa) and triphenylene bridging ligands was prepared, which exhibits temperature-dependent energy transfer from the Tb(III)-emitting state to the hfa triplet state. The triplet level of the hfa ligand is similar to that of the triphenylene ligand, inducing effective energy transfer from hfa to Nd(III) via the triphenylene ligands. This energy transfer pathway provides a short-lived excited state of hfa ligands, resulting in the highest temperature sensitivity (4.4% K^-1) among emission lifetime-based thermometers of lanthanide complexes.

Fig. 1. Molecular design strategies.

Schematic images of excited state dynamics for emission with thermal sensitivity in lanthanide complexes (a) previous study, (b) this study). c Molecular design for proof of concept. d The chemical structures of Tb(III), Nd(III) and Gd(III) complexes.

Fig. 2. Crystal structure.

ORTEP drawing (ellipsoids set at 50% probability) of [Tb₂(hfa)₆(dptp)₂] without hydrogen atoms. Carbon, oxygen, fluorine, phosphorus, and terbium atoms are represented by gray, red, light green, orange, and green spheres, respectively.

Fig. 3. Emission properties.

Emission spectra of [Tb₂(hfa)₆(dptp)₂] (black line) and [Nd₂(hfa)₆(dptp)₂] (blue line) in 2-MeTHF (0.1 mM) and Tb-Nd_MeTHF (0.1 mM, red broken line) at 293 K (λ_ex = 360 nm). The spectra in visible (a) and near infrared regions (b) are normalized at 542 nm and 1060 nm, respectively.

Fig. 4. Time-resolved emission intensities.

Tb(III) emission decay curves of [Tb₂(hfa)₆(dptp)₂] in 2-MeTHF (0.5 mM, black line) and Tb-Nd_MeTHF (0.5 mM, red line) at 300 K (λ_ex = 360 nm, λ_em = 542 nm). The emission decay curves are normalized at their maximum intensity.

Fig. 5. Temperature-dependent emission lifetimes.

Temperature-dependent Tb(III) emission lifetimes (λ_ex = 360 nm, λ_em = 542 nm) of [Tb₂(hfa)₆(dptp)₂] in 2-MeTHF (0.5 mM, green dots) and Tb-Nd_MeTHF (0.5 mM, black and red dots). The solid lines are obtained by fitting with the Mott-Seitz model (Eq. 1). Error bar represents standard deviation.

Fig. 6. Thermal sensitivity of Tb(III) emission.

Relative thermal sensitivities of [Tb₂(hfa)₆(dptp)₂] (black line) and [TbNd(hfa)₆(dptp)₂] (red line) derived by their temperature-dependent Tb(III) emission lifetimes.

Fig. 7. Phosphorescence spectroscopy.

Phosphorescence spectra (red lines, λ_ex = 360 nm) of [Gd₂(hfa)₆(dptp)₂] in 2-MeTHF (0.5 mM) at 80 K with different delay times ((a) 10 ms and (b) 200 ms). Blue lines are obtained by band deconvolution analyses with Gaussian functions. Black broken lines are the summations of the (a) five and (b) seven blue lines.

Fig. 8. Excited-state dynamics.

The energy diagram and triplet energy escape for [TbNd(hfa)₆(dptp)₂]. The hfa_Tb and hfa_Nd represent the hfa ligands coordinated to Tb(III) and Nd(III) ion, respectively. The ³LE denotes the locally excited triplet state.