Tb3+ luminescence is enhanced by complex formation in aqueous phases as its pyridine 2,6 dicarboxylate (dpa2−) complexes by using experimental spectroscopic techniques and theoretical time-dependent density functional theory (TD-DFT) calculations. The fluorimetric titration of Tb3+ ion with dpa2− ion is followed at λext/λems = 310/490 nm and 310/545 nm, the emission intensities of which are graphed against the mol ratios of the ligand to metal ion [moles of dpa2−/mol of Tb3+]. Experimental results denote that the tris complex; [Tb(dpa)3]3− is the most stable form at pH > 5.3. Molecular absorption spectra of tris complex shows a batho-chromic shift of 222 nm of dpa2− band to 232 nm accompanied by the hyper-chromic effect at 272 nm band. The luminescence intensities at 490, 545, 592 and 620 nm are enhanced over 100 times in tris complex. The coordination of complexes calculated by thermodynamic cycles and with supporting the experimental result, the most stable form was found to be nine coordinated tris complex; [Tb(dpa)]3−. The theoretical TD-DFT calculations perfectly matched the experimental absorption and emission bands of tris-complex. The novelty of this study is to present the first theoretical calculation of the phosphorescence results and energy transfer process for emission path of Tb3+ and pda2− aqua complexes.