The influence of imidazolium counterions on the luminescence properties of Cnmim[Eu(tta)4] tetrakis complexes in solid-state and ionic liquid solutions

A series of lanthanide tetrakis complexes Cnmim[Ln(tta)4] where Ln3+: Eu3+ and Gd3+, Cnmim: imidazolium-based counterions (n from 3 to 8) and tta: thenoyltrifluoroacetonate were synthesized by a one-pot method. The Cnmim[Ln(tta)4] complexes are thermostable up to 210 °C and present an isomorphic-like character for the complexes with n = 5, 6, and 7. The phosphorescence spectral profile of the Cnmim[Gd(tta)4] complexes showed intense emission broadbands, in which the barycenters are slightly shifted to energies from the 18,950 to 18,450 cm−1 range. The Cnmim[Eu(tta)4] complexes in solutions of the corresponding [Cnmim]Br ionic liquids show similar emission spectral profiles as their corresponding in the solid state, indicating that the coordination polyhedron does not undergo a significant variation when the chemical environment is changed. Relatively high values of the intensity parameter Ω2 in both media are presented. The Ω4 values of the complexes in solutions of ionic liquids are also similar, reinforcing that there are small changes in the polyhedral coordination from the solid state to the ionic liquid solution. The series presents high intrinsic emission quantum efficiencies, 90%≥QEuEu≥53%. In addition, theoretical calculations on the intramolecular energy transfer (IET) reveal the participation of the 7F0→5D1 and 7F1→5G2 transitions for the IET via S1 while the IET via T1 state is dominated by the 7F0→5D1 and 7F1→5D0 transitions. Therefore, due to their high luminescent features, these Eu3+ tetrakis complexes are potential candidates to be applied as optical markers in the solid-state or even in ionic liquid solutions. •Lanthanide tetrakis complexes with imidazolium as counterions are stable and have high thermal stability.•A shift in the T1 state was found in the [Cnmim][Gd(tta)4] series (n = 3 to 8).•Intramolecular energy transfer rates from T1 to Eu3+ were calculated and show different behaviors according to the shift in the T1 state.•The Eu3+ tetrakis complexes have high intrinsic emission quantum efficiencies in both solid-state and ionic liquid solutions.