Promoting a Significant Increase in the Photoluminescence Quantum Yield of Terbium(III) Complexes by Ligand Modification.

Two discrete mononuclear complexes, [Tb(bbpen)(NO)] () and [Tb(bbppn)(NO)] (), for which = -bis(2-hydroxybenzyl)--bis(pyridin-2-ylmethyl)ethylenediamine and = bis(2-hydroxylbenzyl)-bis(pyridin-2-ylmethyl)-1,2-propanediamine, were synthesized and characterized by FTIR, Raman, and photoluminescence (PL, steady-state and time-resolved modes) spectroscopy. The attachment of a methyl group to the ethylenediamine portion of the ligand backbone differentiates from and acts as a determining feature to both the structural and optical properties of the former. The single-crystal X-ray structure of is described here for the first time, while that of complex has been redetermined in the monoclinic 2 space group in light of new diffraction data. In , selective crystallization leads to spontaneous resolution of enantiomeric molecules in different crystals. Absolute emission quantum yields (ϕ) and luminescence excited-state lifetimes (at room temperature and 11 K) were measured for both complexes. Despite their similar molecular structures, and exhibit remarkably different ϕ values of 21 ± 2% and 67 ± 7%, respectively, under UV excitation at room temperature. Results of quantum-mechanical (DFT and TD-DFT) calculations and experimental PL measurements also performed for and confirmed that both ligands are suitable to work as "antennas" for Tb. Considering the D lifetime profiles and the significantly higher absolute quantum yield of , it appears that thermally active nonradiative pathways present in are minimized in due to differences in the conformation of the ethylenediamine bridge.