Importance of the chromophore orientation to the ligand-to-metal energy transfer in lanthanide complexes with pendant-arm fitted cyclen derivatives

Comparative solid state structural studies of three lanthanide macrocyclic complexes derived from 1,4,7,10-tetraazacyclododecane with pendant arms bearing amide co-ordinating groups have been performed in order to evaluate the parameters influencing the co-ordination polyhedron and to assess the importance of the geometric factor in energy transfer processes. In all the investigated structures the co-ordination geometry is a mono-capped twisted square antiprism, a situation commonly observed for similar compounds. High resolution luminescence spectra of the europium complexes are consistent with a tetragonal site symmetry for the metal ion. An analysis of the presented crystal structures and of previously reported ones indicates that (i) the relative orientation of the O4 and N₄ planes is not determined by the co-ordinated solvent molecule and (ii) the twist angle between them is mainly dictated by theflexibility of the pendant arms. Interpretation of the luminescence properties of the complexes of Sm(III), Eu(III) and Tb(III) can be made from the structural parameters found in the solid state and in solution (by NMR spectroscopy). Both the energy parameter (i.e. the gap between the ligand triplet state and the metal ion excited state) and the geometric parameter (i.e. the donor-acceptor distance and the orientation of the choromophore) have to be taken into account to explain the results obtained in terms of the efficiency of the L→Ln intra-molecular energy transfer. Furthermore, the correlative comparison between structural and luminescent properties shows how inter-molecular interactions in the solid state can be a prominent factor in the effectiveness of this transfer.