Tb³⁺→Eu³⁺ Energy Transfer in Mixed-Lanthanide-Organic Frameworks

In this work, we report a theoretical and experimental investigation of the energy transfer mechanism in two isotypical 2D coordination polymers, ∞[(Tb1–xEux)(DPA)(HDPA)], where H2DPA is pyridine 2,6-dicarboxylic acid and x = 0.05 or 0.50. Emission spectra of ∞[(Tb0.95Eu0.05)(DPA)(HDPA)] and ∞[(Tb0.5Eu0.5)(DPA)(HDPA)], (1) and (2), show that the high quenching effect on Tb3+ emission caused by Eu3+ ion indicates an efficient Tb3+→Eu3+ energy transfer (ET). The kET of Tb3+→ Eu3+ ET and rise rates (kr) of Eu3+ as a function of temperature for (1) are on the same order of magnitude, indicating that the sensitization of the Eu3+ 5D0 level is highly fed by ET from the 5D4 level of Tb3+ ion. The ηET and R0 values vary in the 67–79% and 7.15 to 7.93 Å ranges. Hence, Tb3+ is enabled to transfer efficiently to Eu3+ that can occupy the possible sites at 6.32 and 6.75 Å. For (2), the ET processes occur on average with ηET and R0 of 97% and 31 Å, respectively. Consequently, Tb3+ ion is enabled to transfer energy to Eu3+ localized at different layers. The theoretical model developed by Malta was implemented aiming to insert more insights about the dominant mechanisms involved in the ET between lanthanides ions. Calculated single Tb3+→ Eu3+ ETs are three orders of magnitude inferior to those experimentally; however, it can be explained by the theoretical model that does not consider the role of phonon assistance in the Ln3+→ Ln3+ ET processes. In addition, the Tb3+→ Eu3+ ET processes are predominantly governed by dipole–dipole (d–d) and dipole–quadrupole (d–q) mechanisms.