Luminescent europium(iii) and terbium(iii) complexes of β-diketonate and substituted terpyridine ligands: synthesis, crystal structures and elucidation of energy transfer pathways

Abstract

In this work, we synthesized and structurally characterized a series of six coordinatively saturated Eu^III and Tb^III complexes: [Ln(R-TPY)(TTA)₃] (1–6), having three β-diketonate ligands i.e.TTA = 1,1,1-trifluoro-3-(2-theonyl)acetone, and one judiciously substituted terpyridine derivative (R-TPY), viz. [Eu(FTPY)(TTA)₃] (1), [Tb(FTPY)(TTA)₃] (2), [Eu(TTPY)(TTA)₃] (3), [Tb(TTPY)(TTA)₃] (4), [Eu(PTPY)(TTA)₃] (5) and [Tb(PTPY)(TTA)₃] (6), where FTPY = 4′-(2-furyl)-2,2′:6′,2′′-terpyridine, TTPY = 4′-(2-thienyl)-2,2′:6′,2′′-terpyridine, and PTPY = 4′-(2-pyrolyl)-2,2′:6′,2′′-terpyridine. The complexes were synthesized and structurally characterized by X-ray crystallography and various other physicochemical and spectroscopic methods to realize their optical properties and energy transfer pathways from dual antennae. The structural characterization of the complexes shows discrete nine-coordinated {LnN₃O₆} geometry originating from six oxygen donors of three monoanioninc β-diketonate ligands and three nitrogens from a tridentate terpyridine derivative (R-TPY). We elucidate the energy transfer (ET) pathways from two coordinating antennae moieties (i.e.R-TPY and TTA) in these complexes using relativistic multiconfigurational methods. For this purpose, a theoretical analysis was performed through a method that consists of a fragmentation scheme, wherein all the constituent fragments (TTA, R-TPY and Ln^III) were treated at the same level of theory. These calculations were based on scalar relativistic time-dependent density functional theory (SR-TDDFT) and the multireference complete active space self-consistent field (CASSCF/PT2) technique to construct the respective energy level diagrams and determine the most probable ET pathways. Possible pathways were elucidated from the optimum energy difference between the ligand-centered triplet (³T) states and the emissive excited states of the Ln^III fragments. These calculations and energy transfer pathways were in good agreement with the experimental photophysical data and explain the involvement of several parallel energy transfer pathways to varying extent in these luminescent Ln^III complexes.