Invited paperPhotophysical properties of Lanthanide(III) 1,1,1-trifluoro-2,4-pentanedione complexes with 2,2′-Bipyridyl: An experimental and theoretical investigation

- A series of the complexes [Ln(tfaa)3bpy] are described.
- Distorted square-antiprism structure is proposed for ternary complexes.
- Luminescence, in the visible region, from the ternary complexes were studied.
- The radiative decay rate, energy transfer and back energy transfer rate are discussed.

Semi-empirical Sparkle/RM1 model is employed to elucidate the ground state geometry of [Ln(tfaa)3bpy] complexes [Ln = Pr (1), Eu (2), Tb (3), Dy (4) and Tm (5); tfaa = 1,1,1-trifluoro-2,4-pentanedione and bpy = 2,2′-bipyridyl], which were synthesized in high yield by a one-step method and thoroughly characterized. The room-temperature photoluminescence study confirms the sensitization of Pr (III), Eu (III), Tb (III), Dy (III) and Tm (III) ions by the antenna effect, leading to characteristic red, brilliant red, green, yellow and blue emissions, respectively. Replacement of the water molecule from the coordination sphere of [Ln(tfaa)3H2O] by bpy results in appreciable enhancement of the quantum yields [(i.e., 3.2% vs. 35% for Eu(III); 21.0% vs 24.0% for Tb(III); 0.3% vs.1.2% for Dy(III) and 0.03% vs. 0.14% for Tm(III)) and lifetimes (330 vs. 870 μs for 5D0; and 220 vs 330 μs for 5D4). The Judd-Ofelt parameters (Ω2 and Ω4) were calculated for the Eu(III) complexes and theoretical values of these parameters (Ω2 and Ω4) were obtained by using Sparkle/RM1 structures. The higher value of the Ω2 parameter (14.37 × 10−20 cm2) shows a highly polarizable chemical environment around Eu(III) ion and suggests that the dynamic-coupling mechanism is dominant. The Judd-Ofelt parameters were used to calculate the radiative decay rate, energy transfer rate (WET) and back energy transfer rate (WBT) in the case of (2). The energy transfer processes show that energy transfer occurs from the triplet state of the ligands (T) to the first emissive level 5D1 of Eu(III) and immediate next lower energy level 5D0 of Eu(III) ion i.e., T → 5D1 (WET = 5.33 × 109 s−1) and T→ 5D0 (WET = 4.91 × 109 s−1) levels. Furthermore, the lower value of energy transfer rate of S1 → 5D4 (WET = 4.61 × 103 s−1) reflects that exchange mechanism is dominant. The theoretical emission quantum yield (31%) obtained from the Sparkle/RM1 structure is in good agreement with the experimental emission quantum yield (35%), which reflects that the present theoretical approach could be a great means for the a priori design of highly luminescent lanthanide complexes.