Enhanced Deep-Red Luminescence of Tris(hexafluoroacetylacetonato)samarium(III) Complex with Phenanthroline in Solution by Control of Ligand Coordination

The effect of solvent molecule on the emission properties of Sm(hfa)3(phen)2 (hfa = hexafluoroacetylacetonato, phen = phenanthoroline) was investigated using acetone, acetonitrile, and pyridine. 5G5/2 → 5H9/2 transition intensities in pyridine were found to be larger than those in corresponding acetone and acetonitrile. The radiative rate constant in pyridine (4.8 × 102 s-1) was 2 times larger than those in acetonitrile (2.6 × 102 s-1) and acetone (2.3 × 102 s-1), although the nonradiative transition via vibrational relaxation (knr = 1.7 × 104 s-1) in pyridine was the same as those in acetone and acetonitrile (knr = 1.8 × 104 s-1), resulting in the enhanced emission quantum yield of Sm(III) complex in pyridine (2.7%). The coordination structures of Sm(hfa)3(phen)2 in acetonitrile, acetone, and pyridine were estimated by X-ray single-crystal analyses. These results indicate that enhancement of the emission properties in pyridine is due to faster radiative rate related to formation of asymmetrical nine-coordinated structure, Sm(hfa)3(phen)(py) (py = pyridine).