Jean-Claude Bunzli - Photophysical properties of lanthanide dinuclear complexes with p-nitro-calix[8]arene

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  Bunzli,J.C.G., Ihringer,F. Inorganica Chimica Acta  1996 246 (1-2) 195-205

  Abstract: The ligand p-nitrocalix[8]arene,   n-LH(8), was synthesized and its acidity constants were   determined at 25 degrees C in H2O/THF 70/30 v/v by a   potentiometric method: pK(a1) < 0, pK(a2) = 2.6 +/- 0.1,   pK(a3) = 7.2 +/- 0.2, pK(a4) = 10.2 +/- 0.2, and PKa5-8 > 12.   Upon reaction of n-LH(8) with lanthanide nitrates in DMF   containing an excess of triethylamine, lanthanide dinuclear   complexes are isolated whose elemental analyses correspond to the   formula [Ln(1)Ln(2)(n-LH(2))(DMF)(x)](DMF)(y)(EtOH)(z). Solvation   is difficult to control and different crystallization or drying   conditions yield compounds with different solvation. When two   Ln(III) ions are added to the solution, compounds are isolated   which contain a mixture of the ****- and heterodinuclear species.   A small size discriminating effect is evidenced, larger Ln(III)   ions being apparently favoured. Analysis of the luminescence of   the Eu(D-5(0)) level reveals the following features: (i) the   series of complexes with Ln(1) = Eu, and Ln(2) = Nd, Gd, Tb, Ho   appears to be approximately isostructural, (ii) the two metal ion   sites are similar and possess a low site symmetry, and (iii) the   Eu(III) environment is not well defined, either because the   compounds behave like 'glasses', with ligand molecules adopting   several comparable configurations, or because the solvent   molecules completing the coordination polyhedron of the Eu(III)   ion occupy statistical sites rendering the structure somewhat   disordered. A photophysical study of the ligand n-LH(8), of the   homodinuclear complexes with Ln(1) = Ln(2) = Eu, Gd, Tb, Lu and   of the heterodinuclear complexes has been performed. Compared to   the situation for complexes with p-tert-butylcalix[8]arene, the   ligand excited states are shifted to lower energy, henceforth the   ligand-to-Tb(III) energy transfer no more occurs while the   transfer to the Eu(III) ion is favoured, the Eu(D-5(0)) level and   the ligand states being almost in resonance. The replacement of   the p-tert-butyl groups by the electron-attracting nitro groups   shifts the ligand-to-metal charge-transfer (LMCT) state of the   Eu-containing compounds to higher energy, which reduces the   amount of mixing between the Eu(F-7) and the LMCT states,   resulting in a less efficient quenching of the Eu(D-5(0))   luminescence and in a less enhanced D-5(0) <-- F-7(0)   transition. Nevertheless, the latter still exhibits an unusually   large oscillator strength (ca. 10(-7), epsilon = 0.84 l mol(-1)   cm(-1)). Europium-to-lanthanide (Ln = Nd, Ho) energy transfer   processes in heterodinuclear compounds allow one to estimate the   intermolecular Eu-Ln distance to 10.3-10.5 Angstrom, a value   close to that reported for the p-tert-butylcalix[8]arene   complexes. In conclusion, this study demonstrates the potential   of calixarenes as host molecules for spectroscopically active   metal ions since a simple modification of the para substituents   induces large differences in the photophysical properties of the   dinuclear lanthanide complexes

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