Quantum chemistry-based interpretations on the lowest triplet state of luminescent lanthanides complexes. Part 2. Influence of the electrostatic interactions on the triplet state energy of terbium complexes

In part I of this series, we have focused our attention on a theoretical interpretation of the luminescent properties of some recently synthetized lanthanide-hydroxamate complexes (LnL3 stoichiometry, with L−=[CO-N(R)O]−). Following the Jablonski diagram, and considering that sensitization of lanthanide ions from S1 does not seem to occur frequently, the efficiency of the luminescence of terbium complexes is essentially related to the energetic position of the first excited triplet state (T1). Due to the size of the complexes, we have used DFT (density functional theory) and TDDFT (time-dependent density functional theory) based approaches, which allowed us to make calculations on the full systems. The purpose of the present paper is to present new results and interpretations obtained through a modelization of the 1:3 complexes, either by replacing two bidentate hydroxamate ligands by a chlorine atom, or with an even simpler model in which the lanthanide atom itself is replaced by a sodium atom. At first, we have checked that this quite crude modelization is essentially in agreement with the calculations on the full complex, both for the structural properties and for the nature and qualitative position of the T1 state. In the last part, we propose to explain the variation of S0-T1 excitation energies in terms of electrostatic interactions between the metal center and the ligands, related to the variation of the M-L distances. Concluding remarks concern the different possibilities of performing a quick and reliable screening of luminescent properties of new lanthanide complexes, which was the chief goal of the present work. Considering one strategy, i.e. the replacement of TbL3 by NaL, we propose, prior to chemical synthesis, the methyl 1-hydroxypyridin-2-one-5 carboxylate as a relevant ligand for an efficient luminescence of terbium complexes.