Luminescent lanthanide complexes with a pyridine-bis(carboxamide)-bithiophene sensitizer showing wavelength-dependent singlet oxygen generation†
A new pyridine-bis(carboxamide)-based ligand with a bithiophene pendant, 2Tcbx, was synthesized. Its lanthanide ion (LnIII) 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 LnIII = YbIII, 0.20% for LnIII = NdIII, and 0.01% for LnIII = ErIII, respectively. Concurrent 1O2 formation was seen for all complexes, with efficiencies of 19% for the YbIII complex, 25% for the NdIII complex, and 9% for the ErIII complex. When exciting at a longer wavelength, 435 nm, only LnIII emission was observed and larger efficiencies of LnIII-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.