Luminescent lanthanide complexes with a pyridine-bis(carboxamide)-bithiophene sensitizer showing wavelength-dependent singlet oxygen generation

Abstract

A new pyridine-bis(carboxamide)-based ligand with a bithiophene pendant, 2Tcbx, was synthesized. Its lanthanide ion (Ln^III) complexes, [Ln(2Tcbx)₂]³⁺, were isolated and their photophysical properties were explored. Upon excitation at 360 nm, these complexes display emission in the near-infrared (NIR) with efficiencies of 0.69% for Ln^III = Yb^III, 0.20% for Ln^III = Nd^III, and 0.01% for Ln^III = Er^III, respectively. Concurrent ¹O₂ formation was seen for all complexes, with efficiencies of 19% for the Yb^III complex, 25% for the Nd^III complex, and 9% for the Er^III complex. When exciting at a longer wavelength, 435 nm, only Ln^III emission was observed and larger efficiencies of Ln^III-centered emission were obtained. The lack of ¹O₂ generation indicates that energy pathways involving different ligand conformations, which were investigated by transient absorption spectroscopy, are involved in the sensitization process, and enable the wavelength-dependent generation of ¹O₂.