The first inert and photostable encapsulated lanthanide(III) complexes based on dendritic 9,10-diphenylanthracene ligands: Synthesis, strong near-infrared emission enhancement, and photophysical studies

A series of inert and photostable encapsulated lanthanide(III) complexes-based on dendritic anthracene ligands-is shown for the first time to exhibit strong near-IR emission bands via efficient energy transfer from the excited states of the peripheral antenna to the Ln(3+) ions (Er3+; Yb3+, and Nd3+). A significant decrease in the fluorescence of the anthracene ligand is accompanied by a strong increase in the near-IR emission of the Ln(3+) ions. The near-IR emission intensities of Ln(3+) ions in the encapsulated Ln(3+)-dendrimer complexes are dramatically enhanced on increasing the generation number (n) of dendrons, owing to site-isolation and light-harvesting effects. Furthermore, a first attempt is made to distinguish between the site-isolation and light-harvesting effects in the present complexes. Photophysical studies indicate the sensitization of Ln(3+) luminescence by energy transfer through the excited singlet state of the anthracene ligands, and the energy-transfer efficiency between the dendritic anthracene ligands and the Ln(3+) ion is evaluated to be in the range of 90 to 97%. Their energy-transfer efficiency is in good agreement with the result that the biexponential decays contain a radiative decay of anthracene units (< ca. 10%) and an energy-transfer component (> ca: 90%) from the excited state of anthracene ligands to the Ln(3+) ions. Time-resolved luminescence spectra show monoexponential decays with a lifetime of 2 mu s for the Er3+ ion 11 mu s for the Yb3+ ion and 0.7 Its for the Nd3+ ion in thin films, and calculated intrinsic quantum yields of the Ln(3+) ions are in the range of ca. 0.025 to 0.55%.