Deciphering Intersystem Crossing and Energy Transfer Mechanisms in A Nonacoordinated Ternary Europium(III) Complex: A Combined Spectroscopic and Theoretical Study

A monochromatic red emitting nonacoordinate organoeuropium complex with the formula [Eu(hfaa)3(Ph-TerPyr)] (Eu-1) incorporating hexafluoroacetylacetone (hfaa) primary ligands and a tridentate 4′-phenyl-2,2′:6′,2′′-terpyridine (Ph-TerPyr) ancillary ligand has been synthesized. The complex was characterized by analytical and spectroscopic methods, and its structure was established by single crystal X-ray diffraction (SC-XRD) analysis at low temperature, which explicitly confirms that the coordination sphere is composed of a EuO ₆N ₃ core. Under the UV excitation, Eu-1 displayed typical red emission in solution with a long-excited state lifetime (τ _obs = 1048.06 ± 9.39 μs) with a good photoluminescence quantum yield (Q ^L _Eu = 41.14%). We have utilized pump-probe ultrafast transient absorption spectroscopy in tandem with the time-dependent density functional theory (TD-DFT) and the Lanthanide Luminescence Software Package (LUMPAC) to explore the intricate photophysical event that occurs in the vicinity of the ligands of Eu-1 sensitized photoluminescence (PL).