The Enhanced Intramolecular Energy Transfer and Strengthened ff Luminescence of a Stable Helical Eu Complex in Ionic Liquids

The luminescence of a Eu complex (EuL) is enhanced by stabilization of the coordination structure in highly viscous ionic liquids. The EuL was found to maintain a stable single helical structure both in organic solvents and in the ionic liquids [BMIM][PF(6)] and [EMIM][PF(6)]. A colorless solution of EuL dissolved in [BMIM][PF(6)] exhibits bright red luminescence with a quantum yield of 32.3%, a value that is much higher than that in acetonitrile (12%). Estimated rate constants for the energy relaxation pathway indicate that the energy transfer efficiency is enhanced in [BMIM][PF(6)] as a result of the suppression of molecular fluctuations in the ligands. Additionally, a highly luminescent helical structure is preserved in [EMIM][PF(6)] up to 120 °C.