Luminescence in Ln3+ dipivaloylmethanate complexes: Spectroscopic and theoretical investigation on the energy transfer and LMCT state

Abstract

Three novel lanthanide dipivaloylmethanate (dpm) complexes of general formula [Ln(dpm)(NO₃)₂(tchpo)₂], where Ln: Eu³⁺, Gd³⁺ and Tb³⁺, and tchpo: tricyclohexylphosphine oxide neutral ligand have been prepared and characterized by elemental analyses, absorption infrared spectroscopy, thermalgravimetric analyses, diffuse reflectance, and luminescence spectroscopies. Furthermore, the [Tb(dpm)(NO₃)₂(tchpo)₂] complex was structurally characterized by the single crystal X-ray diffraction analysis. This complex exhibited high luminescence intensity in the green region. On the other hand, the analogous Eu³⁺-complex exhibited very low luminescence intensity due to an efficient luminescent quenching process via ligand-to-metal charge transfer (LMCT) state. Theoretical studies employing Time-Dependent Density Functional Theory (TD-DFT) calculations along with results obtained from the JOYSpectra platform, support this experimental result. Remarkably, despite the high values of non-radiative intramolecular energy transfer from excited ligand states (S₁ and T₁) to the excited levels of the Ln³⁺ ions, in the [Eu(dpm)(NO₃)₂(tchpo)₂], the highest S₁-LMCT rate (W = 1.2 × 10¹⁰ s⁻¹) emphasizes that the primary luminescence quenching pathway is via depopulation of excited ligand states. Interestingly, the nitrogen atoms from nitrate ions play an essential role in the lanthanide chemical environment, which has been suggested by the analyses of the ligand field parameters charge factors ($g$) and effective polarizabilities ($\alpha \prime$) values.