Tuning the Emission of Homometallic Dy^III, Tb^III, and Eu^III 1-D Coordination Polymers with 2,6-Di(1H-1,2,4-triazole-1-yl-methyl)-4-R-phenoxo Ligands: Sensitization through the Singlet State

This work reports the structural characterization and photophysical properties of Dy-III, Tb-III, and Eu-III coordination polymers with two phenoxo-triazole-based ligands [2,6-di(1H-1,2,4-triazole-1-yl-methyl)-4-R-phenoxo, L (R) Tr (R = CH3; Cl)]. These ligands permitted us to obtain isostructural polymers, described as a 1D double chain, with Ln(III) being nona-coordinated. The energies of the ligand triplet (T-1) states were estimated using low-temperature time-resolved emission spectra of Y-III analogues. Compounds with (LTr)-Tr-Cl present higher emission intensity than those with (LTr)-Tr-Me. The emission of Tb-III compounds was not affected by the different excitation wavelengths used and was emitted in the pure green region. In contrast, (DyLTr)-Tr-Me emits in the blue-to-white region, while the luminescence of (DyLTr)-Tr-Cl remains in the white region for all excitation wavelengths. On the other hand, Eu-III compounds emit in the blue (ligand) or red region (Eu-III) depending on the substituent of the phenoxo moiety and excitation wavelength. Theoretical calculations were employed to determine the excited states of the ligands by using time-dependent density functional theory. These calculations aided in modeling the intramolecular energy transfer and rationalizing the optical properties and demonstrated that the sensitization of the Ln(III) ions is driven via S-1 -> Ln(III), a process that is less common as compared to T-1 -> Ln(III).