(a) Molecular structures for DTPA and pyridinyl-substituted DTPA (termed as DTTA-PyM). (b) Luminescence lifetime decays for DTTA-PyM with Eu(III) and Cm(III) in aqueous solution with different pC_H. For all solutions, C_Eu(III) = 2.0 mM, C_Cm(III) = 50 μM, C_DTTA-PyM = 10.0 mM, I = 2.0 M Na(H)ClO₄, and T = 25.0 ± 0.1 ^oC. (c) Comparison of π-back-bonding interactions between the representative singly occupied 5f orbital of Am (n_Am, top) and 4f orbital of Eu (n_Eu, bottom) and the Rydberg orbitals on the amine (Ry*_N2) and pyridine (Ry*_N4) nitrogen atoms, respectively. Panel (b) and (c) were replotted according to Ref [117].