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, _∞[(Tb_1–<i>x</i>Eu_<i>x</i>)­(DPA)­(HDPA)], where H₂DPA is pyridine 2,6-dicarboxylic acid and <i>x</i> = 0.05 or 0.50. Emission spectra of _∞[(Tb_0.95Eu_0.05)­(DPA)­(HDPA)] and _∞[(Tb_0.5Eu_0.5)­(DPA)­(HDPA)], <b>(1)</b> and <b>(2)</b>, show that the high quenching effect on Tb³⁺ emission caused by Eu³⁺ ion indicates an efficient Tb³⁺→Eu³⁺ energy transfer (ET). The <i>k</i>_ET of Tb³⁺<i>→</i> Eu³⁺ ET and rise rates (<i>k</i>_r) of Eu³⁺ as a function of temperature for <b>(1)</b> are on the same order of magnitude, indicating that the sensitization of the Eu^3+ 5<i>D</i>₀ level is highly fed by ET from the ⁵<i>D</i>₄ level of Tb³⁺ ion. The η_ET and <i>R</i>₀ 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 <b>(2)</b>, the ET processes occur on average with η_ET and <i>R</i>₀ 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⁺<i>→</i> Ln³⁺ ET processes. In addition, the Tb³⁺→ Eu³⁺ ET processes are predominantly governed by dipole–dipole (d–d) and dipole–quadrupole (d<i>–</i>q) mechanisms