Jean-Claude Bunzli - Strong enhancement of the lanthanide-centred luminescence in complexes with 4-alkylated 2,2 ';6 ',2 ''-terpyridines

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      Murner,H.R., Chassat,E., Thummel,R.P., Bunzli,J.C.G. Journal   of the Chemical Society-Dalton Transactions  2000 (16) 2809-2816

      Abstract: 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

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