Jean-Claude Bunzli - Intermolecular Interactions as Actors in Energy-Transfer Processes in Lanthanide Complexes with 2,2'-Bipyridine

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  Puntus,L.N., Lyssenko,K.A., Pekareva,I.S., Bunzli,J.C.G.   Journal of Physical Chemistry B 2009  113 (27) 9265-9277

  Abstract: A series of lanthanide complexes   [LnCl(x)(bpy)(y)(H(2)O)(z)]-Cl(3)-(x)(H(2)O)(n)(EtOH)(m) (Ln =   Eu, Gd, Tb; x = 1, 2; y = 1, 2; z = 2-4) with different numbers   of 2,2'-bipyridine (bpy), chloride ions, and water molecules in   the inner coordination sphere were synthesized and investigated   with the aim of relating their molecular geometry and crystal   packing to the efficiency of ligand-to-metal energy transfer. In   conjunction to the rotation of the pyridine rings upon   coordination to the Ln ion, the high flexibility displayed by bpy   ligands leads to rather unexpected bending of these rings with   respect to the central bond, owing to intermolecular interactions   such as Cl center dot center dot center dot pi and pi-stacking   ones. Deciphering the luminescence properties of the Eu and Tb   complexes needs to take into account both the composition of the   inner coordination sphere and the peculiarities of the crystal   packing. For instance, in addition to the classical ligand ->   Eu charge-transfer state (LMCT), another charge-transfer state   induced by pi-stacking interactions (SICT) could be identified.   These two states, located between the singlet and triplet states   of the bpy ligand(s), provide relays facilitating the energy   migration from the singlet to the triplet states and eventually   to the excited Eu states, improving the overall ligand-to-Eu   energy transfer. Another point is the involvement of the   inner-sphere water molecules in H-bonding with chloride ions,   which considerably lowers their luminescence quenching ability,   so that the adducts remain highly luminescent. For instance, the   terbium chloride with two bpy ligands is an efficient near-UV to   green light converter, with an overall quantum yield equal to 37%   despite the coordinated water molecules. The interpretations   given are substantiated by DFT and TD-DFr theoretical   calculations of the complexes and ligand assemblies

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