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 bea ring amide co-ordinating groups have been performed in order to evaluate th e parameters influencing the co-ordination polyhedron and to assess the imp ortance of the geometric factor in energy transfer processes. In all the in vestigated structures the co-ordination geometry is a mono-capped twisted s quare antiprism, a situation commonly observed for similar compounds. High resolution luminescence spectra of the europium complexes are consistent wi th a tetragonal site symmetry for the metal ion. An analysis of the present ed crystal structures and of previously reported ones indicates that (i) th e relative orientation of the O-4 and N-4 planes is not determined by the c o-ordinated solvent molecule and (ii) the twist angle between them is mainl y dictated by the flexibility of the pendant arms. Interpretation of the lu minescence 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 solu tion (by NMR spectroscopy). Both the energy parameter (i.e. the gap between the ligand triplet state and the metal ion excited state) and the geometri c parameter (i.e. the donor-acceptor distance and the orientation of the ch oromophore) have to be taken into account to explain the results obtained i n terms of the efficiency of the L-->Ln intra-molecular energy transfer. Fu rthermore, the correlative comparison between structural and luminescent pr operties shows how inter-molecular interactions in the solid state can be a prominent factor in the effectiveness of this transfer.