p-tert-Butylcalix[4]arene Functionalised with Bipyridyl Carboxylates for Lanthanide Complexation: Synthesis, Photophysical Properties, Solution and Solid State Behavior

A new ligand based on a p-tert-butylcalix[4]arene functionalised with three 6-carboxylato-2,2'-bipyridine arms reacted with lanthanide(III) cations to form 2:2 complexes in the solid state, whereas in solution, a concentration-dependent equilibrium is observed between 2:2 and 1:1 species, as evidenced by ES-MS and metal luminescence measurements. In the X-ray molecular structure of the terbium complex two branched calixarene platforms share one pendant arm in order to provide a neutral dimeric structure which is held together by a strong hydrogen bonded network together with efficient pi-pi stacking of two pyridine rings belonging to each ligand.