Influence of fluoride ions on the absorption and luminescence properties of the [Eu⊂2.2.1]3+ and [Tb⊂2.2.1]3+ cryptates

The luminescence spectra and decay rate constants of the [M⊂2.2.1]3+ cryptates (M = Eu3+ or Tb3+) are strongly affected by addition to the solution of F- anions. In agreement with a previous electrochemical study of Weaver et al., it is found that two distinct species corresponding to the 1:1, [M⊂2.2.1]3+-F-, and 1:2, [M⊂2.2.1]3+-2F-, stoichiometries are formed. It is shown that F- ions replace water in the holes of the cryptate structure, preventing or limiting the radiationless decay of the emitting excited state that occurs via coupling with the O-H oscillators. In the case of M = Eu3+ the coordination of F- to the metal ion shifts the ligand-to-metal charge-transfer bands of the cryptate to higher energies, thereby preventing radiationless decay via low-lying charge-transfer levels. As a consequence, the luminescence quantum yield in D2O solution at 300 K for f-f excitation increases from 0.03 to ∼1 in passing from [Eu⊂2.2.1]3+ to [Eu⊂2.2.1]3+-2F-. The reported results confirm that luminescence studies are a powerful method to elucidate the structure and composition of the coordination sphere of lanthanide ions in solution and show that it is possible to tune the photophysical properties of lanthanide complexes by appropriate perturbations. © 1987 American Chemical Society.