Rare-earth coordination polymers with multimodal luminescence on the nano-, micro-, and milli-second time scales

We present a coordination polymer based on rare-earth metal centers and carboxylated 4,4′-diphenyl-2,2′-bipyridine ligands. We investigate Y$^{3+}$, Lu$^{3+}$, Eu$^{3+}$, and a statistical mixture of Y$^{3+}$ with Eu$^{3+}$ as metal centers. When Y$^{3+}$ or Lu$^{3+}$ is exclusively present in the coordination polymer, biluminescence from the ligand is observed: violet emission from the singlet state (417 nm, 0.9 ns lifetime) and orange emission from the triplet state (585 nm, 76 ms (Y$^{3+}$) and 31 ms (Lu$^{3+}$)). When Eu$^{3+}$ is present in a statistical mixture with Y$^{3+}$, red emission from the Eu$^{3+}$ (611 nm, ∼500μs) is observed in addition to the ligand emissions. We demonstrate that this multi-mode emission is enabled by the immobility of singlet and triplet states on the ligand. Eu$^{3+}$ only receives energy from adjacent ligands. Meanwhile, in the broad inhomogeneous distribution of ligand energies, higher energy states favor singlet emission, whereas faster intersystem crossing in the more stabilized ligands enhances their contribution to triplet emission.