Theoretical calculations employing time-dependent density functional theory (TDDFT) are used to characterize the excited states of Tb(III) β-diketonate complexes. Calculated results are compared directly with experimental results that together show a correlation between relative quantum yields and the excited-state energies that depend on the electronic properties of the p,p'-substituent group associated with the coordinating N-donor neutral ligand. It is found that changes in the electron donating nature of the neutral ligand structure lead to shifts in the lowest triplet energy level of the complex that consequently change the relative quantum yield. This work provides critical direction for the synthesis of high quantum yield terbium complexes.