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

Abstract

The ligand p-nitrocalix[8]arene, n-LH 8, was synthesized and its acidity constants were determined at 25°C in H 2O/THF 70/30 v/v by a potentiometric method: p K a1 < 0, p K a2 = 2.6 ± 0.1, p K a3 = 7.2 ± 0.2, p K a4 = 10.2 ± 0.2, and p K a5–8 > 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 1Ln 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 homo- and heterodinuclear species. A small size discriminating effect is evidenced, larger Ln(III) ions being apparently favoured. Analysis of the luminescence of the Eu( 5D 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( 5D 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( 7F) and the LMCT states, resulting in a less efficient quenching of the Eu( 5D 0) luminescence and in a less enhanced 5D 0 ← 7F 0 transition. Nevertheless, the latter still exhibits an unusually large oscillator strength (ca. 10 −7, ϵ = 0.84 1 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 Å, 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.