Biscalix[4]arene ligands for dinuclear lanthanide ion complexation

Three types of lower-lower rim linked biscalix[4]arenes that contain carboxylic ester (1) and/or amide functions (2 and 3) at their remaining phenolic oxygen atoms were synthesized. The homo- and heterodinuclear lanthanide ion complexes based on these ligands were used to study the energy transfer between different lanthanide ions. Photophysical studies comparing the luminescence properties of the homodinuclear Eu3+ complex and the heterodinuclear Eu3+-Nd3+ complex of 2 indicated that energy transfer is likely to occur from Eu3+ to Nd3+ with an efficiency of > 50%. The luminescence properties turned out to be strongly solvent dependent, which is attributed to structural changes leading to different positions of the lanthanide ions in the cavities provided by the biscalix[4]arene