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 Tb 3+ 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 A-[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 A-[Co(en) 3 ]·rac-[Tb(dpa)3] crystal, i.e., Δ-[Tb(dpa) 3 ] 3- and Λ-[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]·roc-[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.