Strong enhancement of the lanthanide-centred luminescence in complexes with 4-alkylated 2,2′;6′,2"-terpyridines

The stability and photophysical properties of trivalent lanthanide complexes with 2,2';6',2 "-terpyridines substituted in the 4 position (L-1, t-butyl; L-2, ethyl) have been compared to those with the unsubstituted ligand terpy. The stability constants log beta(3) of complexes with L-1 and terpy are similar and reflect a preference for the harder heavier members of the series. Cyclic voltammetry of the [Eu(L)(3)][ClO4](3) complexes show a considerable cathodic shift of the Eu-III-Eu-II reduction potential on going from terpy to L-2 and L-1. The energy of the LMCT states, indirectly determined from the half peak potentials for ligand oxidation and europium(III) reduction, is too high to allow an effective non-radiative deactivation by this pathway. Complexes of the substituted ligands [Ln(L-i)(3)][ClO4](3) (Ln=Eu or Tb) show a substantial increase in the quantum yields of the metal-centred luminescence in acetonitrile solution compared to the terpy reference systems: Q(Eu)=0.10 (L-1), 0.11 (L-2) vs. 0.013 (terpy), and Q(Tb)=0.67 (L-1), 0.34 (L-2) vs. 0.047 (terpy). The main factor responsible for this enhancement arises from a facilitated intersystem crossing in L-1, L-2 and in their complexes, as demonstrated by the ratio of the fluorescence and phosphorescence intensity of both the "free" ligands and their lanthanum(III) tris complexes. This effect is tentatively assigned to the electron donating substituents in the 4 position affecting the mixing of energetically close singlet and triplet ligand states.