Insights into Quantum Cutting and Downshifting Contributions to Near-Infrared YbIII Luminescence in 1D Coordination Polymers

Abstract

YbIII-doped phosphors are promising near-infrared (NIR) emitters for telecommunications and energy conversion. The quantum cutting mechanism, where one high-energy photon yields two (or more) lower-energy photons, is often used to improve the YbIII luminescence. While YbIII quantum cutting has mainly been studied in matrices such as oxides, this work aims to investigate its contribution to molecular systems. For that, the 1D coordination polymer [Ln(tfa)3(μ-dppeo)]n (tfa− = trifluoroacetylacetonate, dppeo = [(diphenylphosphoryl)ethyl](diphenyl)phosphine oxide, Ln = YbIII/TbIII) was synthesized. Luminescence was investigated by varying the TbIII/YbIII ratio, analysing emission under different excitation powers, and modelling excited-state dynamics. The materials exhibit visible TbIII luminescence (quantum yield: 45%) and NIR YbIII emission (quantum yield: ~1%). Upon ligand excitation, TbIII-to-YbIII energy transfer (ET) competes with ligand-to-YbIII ET to populate the YbIII 2F5/2 level, favouring its luminescence. The sub-unit slope in the log−log dependence of the YbIII emission intensity on the excitation power, supported by simulations, indicates a two-photon emission process. This behaviour aligns with a cooperative quantum cutting mechanism, which competes with the conventional downshifting luminescence driven by ligand-to-YbIII ET. The quantum cutting contribution is initiated by the ligand-centred absorption, followed by ET to TbIII and subsequently to two YbIII centres. These findings offer insights into enhancing YbIII luminescence in complexes and provide a design framework for advanced materials aimed at energy conversion.