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

Abstract

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 P1¯ space group. Furthermore, the first coordination sphere {LnO7} of each Ln3+ ion can be described as a capped octahedron with a distorted C3v 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 via 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 (Arad), the nonradiative rate (Anrad), and the Judd–Ofelt intensity parameters (Ω2,4) 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 (W) 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 (QEu Eu) and extrinsic (QL Eu) 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.