Tuning the sensitization pathway T₁→⁵D_J in Eu-based MOF through modification of the antenna ligand. A theoretical approach via multiconfigurational quantum calculations

The current study demonstrates the possibility of improving the efficiency of Eu³⁺ sensitization in an Eu-based MOF, labelled as EuL-R, by modifying the antenna ligand. The electronic structure of seven antenna ligand derivatives with the form R-4-(pyrimidin-5-yl) benzoic acid (L-R) ligand derivates was thoroughly examined in this research. Also, the most likely energy transfer channels for the sensitization pathway of Eu³⁺ ions in the EuL-R systems (R: CH₃, –OH, –SH and –NH₂) was studied. The efficiency in the intersystem crossing (ISC) process from S₁ to T₁ in the antenna make plausible a subsequent energy transfer to the emissive state in the Eu³⁺. This process was studied via multireference CASSCF/NEVPT2 calculations due to the multiconfigurational character of Eu³⁺ ions. Based on Latva and Reindhout's empirical rules, the 4-(2-(thiophen-2-yl)pyrimidin-5-yl)benzoic acid (L6) was chosen as efficient antenna for the sensitization process in this system. In conclusion, it was discovered that including the thiophenyl group at the L antenna (L6), increased the energy gaps S₁→T₁ and T₁→⁵D_J (Eu³⁺) to an optimal range for the ISC and subsequent energy transfer. Finally, a deeply understanding of the sensitization and emission mechanisms was possible from the excited-state dynamics analysis of the L6 antenna, via fluorescence, inter-system crossing (ISC), phosphorescence, and k_F, k_ISC, and k_P, rates calculations. This study therefore highlights the importance of a thorough theoretical procedure via a robust quantum mechanical tool to guide the development of novel luminescent lanthanide-based MOFs.