LUMINESCENCE DYNAMICS AND C-13 NMR CHARACTERISTICS OF DINUCLEAR COMPLEXES EXHIBITING COUPLED LANTHANIDE(III) CATION PAIRS

Luminescence and cross-polarization magic angle spinning (CP-MAS) C-13 NMR properties of lanthanide dinuclear macrocyclic complexes of a compartmental Schiff base chelate (1) derived from the condensation of 2,6-diformyl-p-cresol and 3,6-dioxa-1,8-octanediamine are reported. The Schiff base chromophore in 1 is aa strong light absorber and an efficient sensitizer for intense Tb3+ (5D4) and Eu3+(5D0) (T < 110 K) emission which does not exhibit self-quenching effects. Emission from Tb3+ is sensitized by the ligand singlet state; in striking contrast, Eu3+ emission is sensitized by the triplet state and reveals an unusual nonradiative quenching process at T > 110 K with aa thermal barrier of almost-euqal-to 2300 cm-1. Weak emission is observed from Dy3+(4F9/2), Sm3+(4G5/2), and Pr3+(1D2) diluted in Gd3+ (i.e., from Gd3+-Ln3+ heteropairs, Ln = Pr, Sm, Dy). Intramolecular metal-metal (Ln-Ln = 4 angstrom) interactions account for the greatly quenched emission from Sm3+-Sm3+ and Dy3+-Dy3+ homopairs compared to Gd3+-Ln3+ heteropairs (Ln = Sm, Dy). Gd3+-Ln3+ emission lifetimes at 77 K are 1610 (Tb3+), 890 (Eu3+), 14 (Dy3+) and almost-equal-to 13-mu-s (Sm3+). Nonradiative relaxation processes at 77 K in dilute Ln3+:Gd21(NO3)4.H2O, being temperature independent for Sm3+ and Eu3+ but temperature dependent for Tb3+, follow the energy gap law with alpha almost-equal-to -10(-3) cm and B almost-equal-to 2 x 10(8) s-1. CP-MAS data show paramagnetic broadening of C-13 resonances which increases with the magnetic moment of Ln3+. Surprisingly, no significant shifts in resonance positions corresponding to the changing nature of paramagnetic Ln3+ ions are observed.