Influence of upper rim dibromo-substitution in bis-1,3-diketone calix[4]arenes on spectral properties of ligands and their lanthanide complexes

Abstract

The present work introduces the synthesis and characterization of upper rim dibromo-substituted bis-1,3-diketone calix[4]arenes with hydroxy and propyloxy groups at the lower rim as ligands for Gd3+ and Tb3+ complexes. The dibromo-substitution does not affect 1:1 complex formation with the lanthanide ions arising from their coordination via two 1,3-diketonate groups incorporated at the upper rim of the calix[4]arene. The analysis of the time-resolved luminescence spectra of the Gd3+ complexes clearly demonstrates the notable enhancement of the decay time of the ligand triplet state, while the energy of the triplet state for the new dibromo-bis-1,3-diketones remains practically unchanged versus its non-substituted analogues. The latter tendency points to the heavy-atom effect on the energy transfer from the excited state to vibration levels of the dibromo-substituted ligand resulting in a decreased radiationless decay contribution. Measurements of Tb3+-centered luminescence indicate a better antenna effect (∼1.7) of dibromo-calix[4]arenes compared with its non-substituted counterpart.