2 HIGH AND LOW-SYMMETRY EUROPIUM COMPLEXES WITH L-PROLINE - SPECTROSCOPY AND STRUCTURE

Lanthanide ions are widely applied as spectroscopic probes in studies of biomolecular structures. However, in all known crystal structures o f lanthanide complexes with amino acids and peptides, dimers and polym ers are formed with one or more symmetry sites of the metal ions; this strongly affects the spectroscopic properties. In order to explain th e role of the type of amino acid, the Ln/ligand ratio and ligand chira lity in the structure, two types of Eu(III) complexes with L-proline o f formulae [Eu-2(C5H9O2N)(4)(H2O)(8)](ClO4)(6). 4H(2)O (I) and [Eu(C5H 9O2N)(3)(H2O)(3)](ClO4)(3) (II) were obtained and characterised by X-r ay and electron spectroscopic methods. Compound I crystallizes in a te tragonal space group (P42(i)2) and forms dimers with two nonequivalent europium sites, with square antiprism polyhedra. Compound II (triclin ic P1) forms polymeric chains in a crystal lattice, with two coordinat ion modes of the proline molecules (mono- and bridging bidentate) and with dodecahedron polyhedra. Moreover, differences in the Eu-O bond le ngths between two crystallographically nonequivalent metal ions are sm aller in structure II. High resolution absorption, excitation and emis sion spectra of the compounds were measured at 4-293 K and analysed in order to explain how subtle structural changes affect the intensity o f the electronic transitions, splitting of levels and vibronic couplin g. (C) 1998 Published by Elsevier Science S.A.