Near-Infrared Sm2+ Complexes with Temperature-Dependent Emission Mechanisms: Thermally Activated Delayed d-f Transition and f-f Transition.

Abstract

Due to the instability of divalent samarium, studies of luminescent Sm²⁺ compounds are mainly focused on doping Sm²⁺ ions into inorganic solids, with very limited work on molecular complexes. In this work, three Sm²⁺ complexes SmI₂-O₅, SmI₂-O₄, and SmI₂-O₆ (O₅ = 15-crown-5, O₆ = 18-crown-6, and O₄ = 12-crown-4) were synthesized, showing near-infrared emissions with maximum wavelengths at 765, 728, and 730 nm in solid powder at room temperature, respectively. Interestingly, these Sm²⁺ complexes exhibit temperature-dependent emission mechanism transformation from a thermally activated delayed d-f transition at room temperature to f-f transition at low temperature, which is proven to be a fast thermal equilibrium between the two excited states 5d* and 4f*. This phenomenon results in an obvious excited state lifetime change upon temperature, and the three complexes are demonstrated as the most efficient lifetime-readout luminescence thermometer among divalent samarium compounds, showing the highest maximum temperature sensitivity of 5.9% K^-1 at 75 K. Further organic ligand extension from crown ether to azacrown ether results in redshifted emission with a maximum wavelength around 900 nm. These results demonstrate the interesting photophysical properties of molecular Sm²⁺ complexes and will inspire their studies as well as potential applications.