Polyphenylcyclopentadienyl Ligands as an Effective Light-Harvesting pi-Bonded Antenna for Lanthanide+3 Ions

A new approach to design antenna-ligands to enhance the photoluminescence of lanthanide coordination compounds has been developed based on a pi-type ligand the polyphenyl-substituted cyclopentadienyl. The complexes of di-, tri-, and tetraphenyl cyclopentadienyl ligands with Tb and Gd have been synthesized and all the possible structural types from mononuclear to di- and tetranuclear complexes, as well as a coordination polymer were obtained. All types of the complexes have been studied by single-crystal X-ray diffraction and optical spectroscopy. All terbium complexes are luminescent at ambient temperature and two of them have relatively high quantum yields (50 and 60%). Analysis of energy transfer process has been performed and supported by quantum chemical calculations. The role of a low-lying intraligand charge transfer state formed by extra coordination with K+ in the Tb3+ ion luminescence sensitization is discussed. New aspects for design of lanthanide complexes containing pi-type ligands with desired luminescence properties have been proposed.