Construction of ratiometric optical thermometers by linking chiral cyclometalated dicyanidoiridates(III) with europium(III) luminophores into cluster compounds

Abstract

Luminescent thermometers, in particular those relying on the emission intensity ratio, attract broad interest due to vital applications in bioimaging, catalysis, plasmonics, and microelectronics. Typically, two lanthanide ions, e.g., Tb(III) and Eu(III), are combined in solids to induce a crucial temperature variation of the intensity ratio between their emission signals. Here, we present an alternative approach, which employs two broadly explored luminophores, namely green-to-yellow emissive Ir(III) complexes exhibiting tunable photoluminescence (PL) of a metal-to-ligand charge transfer (MLCT) character and Eu(III) complexes showing red PL due to their f-f electronic transitions. These luminophores were combined into cluster compounds with bonding by cyanido bridges, enabling efficient energy transfer (ET) processes. We report two materials, (TBA)₂{[Eu^III(NO₃)₃(H₂O)_0.5]₂[Ir^III(CN)₂(R,R-pinppy)₂]₂}⋅4H₂O (Eu₂Ir₂, R,R-pinppy = carbanion of (R,R)-2-phenyl-4,5-pinenopyridine; TBA = tetrabutylammonium) and (TBA)₂{[Eu^III(NO₃)₃(H₂O)_0.5]₂[Ir^III(CN)₂(R,R-Fpinppy)₂]₂}⋅6H₂O (Eu₂FIr₂, R,R-Fpinppy = carbanion of (R,R)-2-(4-fluorophenyl)-4,5-pinenopyridine) bearing two different pinppy derivatives. They exhibit dual emission combining red f-f-PL of Eu(III) with yellowish-green MLCT-PL of Ir(III) centers. Due to the thermally activated back-ET (BET) process, the ratio between these emission signals is T-dependent, leading to high-performance ratiometric optical thermometry with the maximal relative thermal sensitivity, S_r, of 2.92% K^–1 at 240 K and the broadest operating T-range (i.e., S_r > 1% K^–1) of 140–280 K for Eu₂Ir₂. Moreover, its fluorinated analog, Eu₂FIr₂, exhibits a higher-energy MLCT excited state, which results in the higher energy needed to activate the BET, leading to the shift of optical thermometry towards higher temperatures. This effect can be further enhanced using the optimized UV-vis excitation of lower energy. Therefore, we report a novel type of ratiometric optical thermometer based on Eu(III)–Ir(III) molecules that can be tuned by structural modulation on Ir(III) sides. Our strategy also offers the pathway for extended multitunctionality due to the chiral character of the obtained materials related to the usage of enantiopure pinppy-type ligands. Here, chirality was explored as a convenient route to the removal of the crystals’ inversion center, leading to the non-linear optical activity represented by the second-harmonic generation.