Upconversion Luminescence with Bis-pyclen Yb(III) Chelates: Crown vs. Linear Polyether Linkers in Discrete...

Ligands combining two lateral bis-pyridyl-phosphonated-pyclens were synthesized, using a flexible linear pegylated linker (L2) or a bulkier K22 crown-ether (L3). A functionalized pyridyl-phosphonated-pyclen (L1) was also prepared as a mononuclear analogue. Coordination behavior of lanthanide cations was studied via NMR titration with Lu for L1, and UV/Vis and luminescence spectroscopy with Yb for L2/L3. Strong coordination of two Yb atoms enabled isolation and spectroscopic characterization of dinuclear complexes in H₂O and D₂O. Excited state lifetime analysis at 980 nm revealed strong protection of Yb cations, with no coordinated water molecule. Upon titration of the isolated dinuclear Yb complexes with Tb cations, cooperative upconversion (UC) sensitization of Tb in the visible was observed upon excitation of Yb at 980 nm in D₂O. In the absence of Tb, the Yb complexes also exhibited cooperative luminescence with a weak emission band around 500 nm upon NIR Yb excitation. Efficient UC with Tb was only observed after thermal treatment, suggesting a slow kinetic of formation of the UC species. [Yb₂TbL3] showed weak Tb centered UC emission, while the dinuclear complex of L2 displayed more intense UC emission up to two equivalents of Tb, forming [(Yb₂L2)Tb_x] (x=1-2), with the tetranuclear heterometallic complex being the most intense emitter. Log-Log plot analysis confirmed the two-photon nature of the UC process.