Origin of Concentration Quenching in Ytterbium Coordination Polymers: Phonon-Assisted Energy Transfer

Phonons are important in energy transfer for compensating the energy mismatch between a donor and an acceptor. In inorganic hosts doped with lanthanides, phonon-assisted energy transfer can lead to quenching by a direct transfer of the energy to the phonon mode of the acceptor lanthanide. We demonstrate that this also applies to lanthanide coordination polymers and is the reason for the weak concentration quenching in this type of material. [Yb_xGd_1–x(hfa)₃dpbp]_n [x = 0.001, 0.005, 0.01, 0.05, 0.1, 0.5, 1; hfa: hexafluoroacetylacetonate, dpbp: 4,4′-bis(diphenylphosphoryl)biphenyl] coordination polymers have been synthesized and their photophysical properties investigated. The single-exponential emission lifetimes of Yb³⁺ gradually decreased with increasing concentration of Yb³⁺. Qualitative agreement was achieved between the experimental and theoretical lifetimes by using the phonon-assisted energy-transfer-induced quenching model, which indicates that the origin of the concentration quenching in the coordination polymers is phonon-assisted energy transfer.

Lanthanide coordination polymers with various concentrations of Yb³⁺ have been synthesized to investigate the concentration-quenching mechanism. Experimental and theoretical considerations showed that concentration quenching occurs by nonradiative phonon-assisted energy-transfer relaxation.

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