Multireference Ab Initio Investigation on Ground and Low-Lying Excited States: Systematic Evaluation of J–J Mixing in a Eu³⁺ Luminescent Complex

Abstract

A theoretical protocol combining density functional theory (DFT) and multireference (CAS) calculations is proposed for a Eu3+ complex. In the complex, electronic levels of the central Eu3+ ion are correctly calculated at the CASPT2 level of theory, and the effect of introducing different numbers of states in the configuration interaction matrices is highlighted as well as the shortcomings of DFT methods in the treatment of systems with high spin multiplicity and strong spin–orbit coupling effects. For the 5D0 state energy calculation, the inclusion of states with different multiplicity and the number of states considered for each multiplicity are crucial parameters, even if their relative weight is different. Indeed, the addition of triplet and singlets is important, while the number of states is relevant only for the quintets. The herein proposed protocol enables a rigorous, full ab initio treatment of Eu3+ complex, which can be easily extended to other Ln3+ ions.

Elucidation of LMCT Excited States for Lanthanoid Complexes: A Theoretical and Solid-State Experimental Framework

“…Examination of methods reported in the literature revealed very few attempts to capture the LMCT character and/or energies of excited states of lanthanoid complexes, in marked contrast to the larger amount of work dedicated to the theoretical treatment of charge transfers in d-block metals complexes. For instance, in a recent ab initio multireference computational study of the 4f–4f excitations responsible for the luminescence of Eu III β-diketonate complexes, it was shown very convincingly that the energies of the emitting 5 D J multiplets in Eu III are much more sensitive than the ground-state 7 F J levels to the effects of spin–orbit coupling, inducing a strong mixing of various multireference Russell–Saunders terms, which necessitates the inclusion of not only septets and quintets in the calculation, but also of relevant triplets and singlets. Remarkably, in the same study, it was also shown that TD-DFT with an asymptotically corrected exchange potential (LB94), despite the complete absence of any spin–orbit coupling description, also appears to provide a reasonable estimate of the 5 D J multiplets excitation energies, in clear contradiction with the basic electronic structure theory at play.…”

“…Remarkably, in the same study, it was also shown that TD-DFT with an asymptotically corrected exchange potential (LB94), despite the complete absence of any spin–orbit coupling description, also appears to provide a reasonable estimate of the 5 D J multiplets excitation energies, in clear contradiction with the basic electronic structure theory at play. This unprecedented study of the role of spin–orbit coupling in the ab initio description of Eu III excited states exposes the risk with DFT methods of obtaining seemingly good results due to accidental cancellation of errors.…”

“…However, the modeling of a transition to a LMCT state has remained a significant challenge because it requires an accurate description of both the ground state, which is usually a magnetic level and the LMCT excited state, which involves a change of (oxidation) state for the metal and one or more ligands. For this reason, while several multireference ab initio studies of 4f–4f excitations in lanthanoid complexes ,− and 4f–5d transitions in doped glasses − have been reported, ab initio studies of LMCT remain scarce in the literature due to their complexity. , This work sets a foundational framework based on both theoretical and solid-state experimental approaches to investigate LMCT excited states for lanthanoid complexes.…”

Magnetism Studies of Bis(acyl)phosphide-Supported Eu3+and Eu2+Complexes

“…The ground-state term symbol for the Eu 3+ ion is 7 F, which is split into seven different J states ( 7 F 0‑6 ) by spin–orbit coupling, λ. The 7 F 0 state is the ground state and the first excited state, 7 F 1 , is typically found to lie approximately 350 cm –1 above the 7 F 0 ground state. − This 7 F 1 excited state can therefore be thermally populated and can also mix, via out-of-state spin–orbit coupling, with the 7 F 0 ground state. − The latter effect leads to TIP. The full spin-Hamiltonian is given by eq .25ex2ex Ĥ = Ĥ Zeeman + Ĥ SO + Ĥ CF with the 7 F 1 excited state being split in zero applied magnetic field by a low-symmetry crystal field described by the following Hamiltonian , .25ex2ex Ĥ CF = ∑ i = 1 N ∑ k = 2 , 4 , 6 ∑ q = − k k σ i k B k i q θ k Ô k i q …”

Nature of the Ligand-Centered Triplet State in Gd3+β-Diketonate Complexes as Revealed by Time-Resolved EPR Spectroscopy and DFT Calculations

“…Among processes involving them, triplets are of paramount importance in the so-called antenna effect for the sensitization of lanthanide (Ln) ion emission, as the energy gap between triplet and Ln 3+ emitter levels is one of the key factors ruling the emission properties . For instance, lanthanide luminescence-based thermometric features are tightly bound to the triplet state energy, in particular when the back-energy transfer is considered. − Besides its energy, the design of novel luminescent systems with tailored properties requires a detailed knowledge of the triplet spin distribution over the molecular skeleton. Indeed, energy transfer pathways are sometimes directly influenced by the specific spatial distribution of the spin density in the sensitizer ligand and by the triplet energy. − Moreover, the delocalization of the triplet state spin density can be related to the phosphorescence quantum yield, whose control is crucial in technological applications such as organic light-emitting diodes …”