The Role of Multiple Triplet Excited States in Luminescent Tb(III) Coordination Polymers and Their Triplet Tuning Methods

Abstract

Trivalent terbium (Tb(III)) coordination polymers exhibit photosensitized metal-centered emission via strong ligand absorption. Clarification of the photosensitized emission mechanism is of critical importance for the development of luminescent Tb(III) coordination polymers. Herein, we demonstrate the role of the multiple excited ligand triplet states in photosensitized Tb(III) emission. In the prepared Tb(III) coordination polymers, Tb(III) is coordinated with three hexafluoroacetylacetonate (hfa) and phosphine oxide bridging ligands. The investigation of photophysical properties revealed that the multiple triplet excited states act as quenching sites for Tb(III) emission in the photosensitization process. The lowest and second-lowest energy levels of the triplet excited states are determined by time-resolved emission spectra. Quantum chemical calculations indicate that the triplet electronic structures originate from two types of hfa ligands, each characterized by a different dihedral angle within the π-conjugated plane. The triplet electronic structure tunings were also demonstrated by incorporating different lanthanide ions (Gd(III) or Lu(III)) into the Tb(III) coordination polymer to enhance the photosensitized emission. These findings provide valuable insights into the design strategies for highly luminescent Tb(III) coordination polymers.