The lower-rim substituted calix[4]arene, 5,11,17,23-tetra-tert-butyl-25,26,27,28-tetrakis(dimethylphosphinoylmethoxy)calix[4]arene (L) has been synthesized and characterised by the single-crystal structure of its acetonitrile adduct, L·2MeCN (monoclinic, space group P2₁/c). The ligand adopts a cone conformation and a Δ enantiomeric form with the four phosphinoyl arms in a δδδδ configuration. The cone conformation is maintained in organic solution where L displays a time-averaged C₄ symmetry. Trivalent lanthanum ions interact with L in acetonitrile to yield both 1∶1 (log β₁ = 11.4 ± 1.5) and 1∶2 (log β₂ = 19.6 ± 1.8) complexes; two forms of the 1∶1 complex are identified depending on the water content of the solutions. A photophysical study of both the ligand- and metal-centred luminescence of complexes of La, Eu and Tb points to L having a moderate quantum yield (ca. 10%, ligand-centred luminescence) and being a poor sensitiser of europium and terbium ions. It confirms the presence of differently solvated 1∶1 (and 1∶2) complexes, depending on the solvent composition. The lifetimes of the metal-centred luminescence of the unhydrated 1∶1 and 1∶2 complexes are long, in the range 1.4–2.4 ms for Eu and 1.7–7.3 ms for Tb, which indicates that the lanthanide(III) ions are well encapsulated in the cavity formed by the donor groups of the calix[4]arene molecule(s).