Jean-Claude Bunzli - Design of Luminescent Building-Blocks for Supramolecular Triple-Helical Lanthanide Complexes

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  Publication Details (including relevant citation   information):

  Piguet,C., Bunzli,J.C.G., Bernardinelli,G., Bochet,C.G.,   Froidevaux,P. Journal of the Chemical Society-Dalton   Transactions 1995 (1) 83-97

  Abstract: The ligand   2,6-bis(1'-ethyl-5'-methylbenzimidazol-2'-yl)pyridine (L5) reacts   with lanthanide perchlorate in acetonitrile to give the   mononuclear triple-helical complexes [Ln(L5)3]3+ (Ln = Eu, Gd or   Tb). The crystal structure of [Eu(L5)3][ClO4]3.4MeCN has been   determined, which shows three unco-ordinated perchlorate anions   and an [Eu(L5)3]3+ cation where the three tridentate ligands are   wrapped around a pseudo-C3 axis. The co-ordination sphere around   Eu(III) may be best described as a slightly distorted   trigonal-tricapped prism where the six benzimidazole nitrogen   atoms occupy the vertices of the prism and the three pyridine   nitrogen atoms occupy the capping positions. A detailed   geometrical analysis showed that the ethyl groups in L5 produce a   slide of the strands which is responsible for the distortion of   the triple-helical structure as exemplified by the low symmetry   for the Eu(III) site in the luminescence spectra of [Eu(L5)3]3+.   Proton NMR spectra in acetonitrile indicate that the   triple-helical structure is maintained for [Ln(L(i))3]3+ {Ln = Eu   or Tb; L = 2,6-bis(1'-R-benzimidazol-2'-yl)pyridine [R = Me L1,   Et L2, Pr L3 or CH2C6H3(OMe)2-3,5 L4] or L5} on the NMR   time-scale, but the stability of the complexes together with the   structural arrangement of the ligands depend on the size of the   substituents bound to the benzimidazole nitrogen atoms.   Photophysical studies of [Eu(L(i))3]3+ show that these steric   effects affect the quantum yield in solution and that methyl   groups bound to the 5 positions of the benzimidazole rings in L5   shift the pi --> pi* transitions centred on the ligand, but do   not strongly modify the emission properties of [Eu(L5)3]3+.   Extended Huckel calculations give a qualitative insight into the   factor controlling the pi --> pi* transitions of the ligands   and complexes

  Address (URL): WOS:A1995QB24600013