The role of an LMCT state on luminescence quenching in dimeric lanthanide dipivaloylmethanate compounds

A series of four dimeric lanthanide dipivaloylmethanate (dpm) compounds with bis(diphenylphosphine)ethane oxide (dppeO₂) as an ancillary ligand having the formula [Ln₂(dpm)₆(dppeO₂)] (Ln³⁺: Sm³⁺, Eu³⁺, Gd³⁺, and Tb³⁺) have successfully been synthesized. Single-crystal X-ray diffraction analysis shows that dimeric compounds crystallize in the <i>P</i>1̄ space group. Furthermore, the first coordination sphere {LnO₇} of each Ln³⁺ ion can be described as a capped octahedron with a distorted <i>C</i>_3v symmetry site. Photoluminescence data show that the Tb³⁺ compound exhibits high emission intensity consistent with an efficient intramolecular energy transfer from the dpm organic ligand to the metal ion. However, the analogous Eu³⁺ and Sm³⁺ compounds display only weak emission intensities under excitation upon the ligand S₀ → S₁ transition, reflecting efficient luminescence quenching <i>via</i> a low-energy ligand-to-metal charge transfer (LMCT) state and metal-to-ligand energy back transfer, respectively. For the [Eu₂(dpm)₆(dppeO₂)] compound, the photophysical properties based on the radiative rate (<i>A</i>_rad), the nonradiative rate (<i>A</i>_nrad), and the Judd-Ofelt intensity parameters (<i>Ω</i>_2,4) were also investigated. Besides, theoretical studies were performed in the DFT and TD-DFT levels in conjunction with the JoySpectra platform, yielding a deeper understanding of the intramolecular energy transfer processes. The highest value of the energy transfer rate (<i>W</i>) assigned to the S₁ → LMCT deactivation pathway compared to the rate values from the ⁵D₀ or ⁵D₁ → LMCT channels agrees well with the experimental values of the intrinsic (<i>Q</i>EuEu) and extrinsic (<i>Q</i>LEu) quantum yields. These optical results indicated that the LMCT state plays the most critical role in Eu-dpm luminescence quenching by depopulating the excited S₁ ligand state.