Predicting efficient antenna ligands for Tb(III) emission.

A series of highly luminescent Tb(III) complexes of para-substituted 2-hydroxyisophthalamide ligands (5LI-IAM-X) has been prepared (X = H, CH(3), (CO)NHCH(3), SO(3)(-), NO(2), OCH(3), F, Cl, Br) to probe the effect of substituting the isophthalamide ring on ligand and Tb(III) emission in order to establish a method for predicting the effects of chromophore modification on Tb(III) luminescence. The energies of the ligand singlet and triplet excited states are found to increase linearly with the pi-withdrawing ability of the substituent. The experimental results are supported by time-dependent density functional theory calculations performed on model systems, which predict ligand singlet and triplet energies within approximately 5% of the experimental values. The quantum yield (Phi) values of the Tb(III) complexes increase with the triplet energy of the ligand, which is in part due to decreased non-radiative deactivation caused by thermal repopulation of the triplet. Together, the experimental and theoretical results serve as a predictive tool that can guide the synthesis of ligands used to sensitize lanthanide luminescence.