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

Spectrophotometric pK(a) determination for p-tert-butylcalix[5]arene (H5L) in acetonitrile (pK(a1) = 11.5 +/- 0.7, pK(a2) = 154 +/- 1.0 at 298 K) evidenced both intra-and inter-molecular stabilisation of the deprotonated forms. Dimeric complexes [Ln(2)(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 [Eu-2(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.