Energy-transfer process in crystals of chiral and racemic double complex salts of [Co(ethylenediamine)₃][Tb(2,6-pyridinedicarboxylate) ₃]

Luminescence and energy-transfer reactions in double complex crystals of [Co(en)₃][Tb(dpa)₃] (en = ethylenediamine, dpa = 2,6-pyridinedicarboxylate) were investigated and compared to single crystals of rac-[Co(en)₃]-rac-[Tb(dpa)3] (double complexes salt of racemic [Co(en)₃]³⁺ and racemic [Tb(dpa)₃] ^3-) and A-[Co(en)₃]-rac-[Tb(dpa)₃] (chiral [Co(en)₃]³⁺ and racemic [Tb(dpa)₃]^3- salt). The energy-transfer rate constants from Tb³⁺ to Co ³⁺ complexes were determined from the time profile of emission intensity of photo-excited Tb^III ion in the double complex crystals. The emission decay profiles of rac-[Co(en)₃]-rac-[Tb(dpa) ₃] were analyzed with single exponential curves. On the other hand, the profiles of δ-[Co(en)₃]-rac-[Tb(dpa)3] show double-exponential curves. From the X-ray analysis of crystal structures, it was found that there is only one site for [Tb(dpa)₃]^3- in rac-[Co(en)₃] rac-[Tb(dpa)₃] crystal, and there are two sites in the δ-[Co(en)₃]-rac[Tb(dpa)₃] crystal, i.e., δ-[Tb(dpa)₃]^3- and A-[Tb(dpa) ₃]^3- are put in different sites in the crystal. The two rate constants obtained from the double-exponential curve of A-[Co(en) ₃]-rac-[Tb(dpa)₃] are assigned to energy-transfer rates from A-[Tb(dpa)₃]^3- and A-[Tb(dpa)₃] ^3- in the crystal. A distance dependence was determined from the obtained energy-transfer rate constants and the Tb-Co distances in the crystals according to a Dexter type electron-exchange mechanism of energy-transfer model. The energy-transfer rate constants in the crystals are comparable in magnitude with energy-transfer rates in quenching experiments in the [Tb(dpa) ₃]^3--[Co(en)₃]³⁺ aqueous solution.