From mononuclear to pyrazine-bridged dinuclear samarium(iii) complexes: role of fluxidentate coordination in luminescent and thermal behavior

Abstract

Three Sm(III)-complexes featuring β-diketone ligand 4,4,4-trifluoro-1-thienyl-1,3-butadionate (TTBD) were studied for their potential as visible-light emitters. Utilizing pyz (pyrazine) as an ancillary ligand, two mononuclear compounds i.e. binary (SmA) and ternary (SmM) along with a dinuclear compound (SmD) were synthesized. Elemental (CHN), IR and NMR analyses confirmed that two complexes (SmA and SmM) are mononuclear with identical structural frameworks, whereas SmD forms a dinuclear species with pyrazine acting as a bridging ligand. The optical band gaps were measured from UV-vis absorption using Tauc's relation corresponding to HOMO–LUMO (Highest Occupied Molecular Orbital–Lowest Unoccupied Molecular Orbital) transitions. Photophysical investigations in solution phase using dichloromethane (DCM) solvent revealed characteristic emissions of Sm(III) ions under UV excitation, due to ⁴G_5/2 → ⁶H_j (j = 5/2, 7/2, 9/2, 11/2) transitions in visible region. All complexes displayed intense luminescence with notably high lifetimes (0.063 ms), similar to the most efficient Sm(III)-based emitters reported. Radiative lifetimes and qualitative ligand-to-metal energy transfer (ET) efficiencies were deduced from time-resolved measurements, confirming that TTBD and pyz function effectively as sensitizing ligands. Colorimetric analysis based on emission spectra yielded CIE coordinates indicative of orange-red luminescence, typical of Sm(III) ions. Correlated color temperature (CCT) values further suggest their suitability as warm-light emitters. thermogravimetric analysis (TGA) revealed thermal stability up to ∼224–231 °C, emphasizing their promise in thermally stable optoelectronic devices.