The spectral properties of Tb(III) complexes with a series of 4-substituted analogues of dipicolinic acid (2,6 pyridinedicarboxylic acid) were compared. Analogues in which the 4-substituent was -H, -OH, -Cl, -Br, -NH2, or -NHAc all sensitized emission from bound terbium ions when excited with ultraviolet light. Molar extinction coefficients of the complexes at the wavelengths of maximum absorbance were in the range (2.2 x 10(3) to 8.3 x 10(3)) M(-1) cm(-1) with magnitudes in the following order : NHAc > NH2 > OH > H > Br > Cl. When excitation spectra of Tb(III) complexes were measured under identical conditions for each ligand, the maximum Tb(mj emission intensities were in the following order : NH2 > NHAc > OH > K > Cl similar to Br. Measurements of Tb(III) emission decay kinetics revealed lifetimes in the range 1.0-2.0 ms. Relative quantum yields for energy transfer were ordered as follows : NH2 > OH > NHAc > Cl > H similar to Br. Energy transfer efficiency was not enhanced when Br was substituted for Cl at the 4-position, consistent with energy transfer from excited singlet, rather than triplet states. The efficient sensitization of Tb(III) emission by the analogues with nitrogen at the 4-position suggests that this class of compounds will be most useful for developing millisecond luminescence probes for bioanalytical studies.