Complexes of p-tert-butylcalix[5]arene with lanthanides: synthesis, structure and photophysical properties†

Abstract

Spectrophotometric pK_a determination for p-tert-butylcalix[5]arene (H₅L) in acetonitrile (pK_a1 = 11.5 ± 0.7, pK_a2 = 15.4 ± 1.0 at 298 K) evidenced both intra- and inter-molecular stabilisation of the deprotonated forms. Dimeric complexes [Ln₂(H₂L)₂(dmso)₄] (Ln = Eu^III, Gd^III, or Tb^III; dmso = dimethyl sulfoxide) were isolated from tetrahydrofuran (thf) in the presence of NaH as base. A single-crystal analysis of [Eu₂(H₂L)₂(dmso)₄]· 10thf showed the deformation of the cone conformation of the calixarene upon complexation and co-ordination of dmso molecules by inclusion through the hydrophobic cavity of the ligand. A photophysical investigation revealed a total quenching of the metal luminescence by a ligand-to-metal charge-transfer state in the case of Eu^III while luminescence of Tb^III is sensitised (quantum yield in thf: 5.1%). The temperature-dependent lifetime of Tb^III is analysed in terms of a potential metal-to-ligand back-transfer process.