Crowded cerium complexes unlock two glowing pathways at once

A single molecule that can shine through two separate light-emitting channels at nearly perfect efficiency has been demonstrated by chemists at Peking University, and the secret turns out to be a crowded coordination sphere. In a study published in Light: Science & Applications, Zhiwei Liu and colleagues report a family of open-shell cerium(III) complexes in which raising the coordination number from seven to nine transforms conventional single-channel emission into dual-channel doublet emission, with solid-state photoluminescence quantum yields approaching 100 percent for the nine-coordinate members of the series.

Dual-channel emitters are prized because they can produce more than one optical output from a single system, a capability with potential uses in sensing, multilevel encryption, optical data storage, imaging and electroluminescence. Most dual-channel emitters reported so far, however, are closed-shell systems, in which the emission generally involves singlet fluorescence, triplet phosphorescence, or a mixture of the two. Open-shell systems, with their distinctive spin configurations and excited-state properties, have remained far less explored as platforms for this kind of behavior, despite the unusual opportunities their electronic structures offer.

Cerium(III) complexes are an appealing molecular platform for investigating open-shell emission. Ce(III) carries a 4f1 ground-state electronic configuration, and its spin- and parity-allowed 5d–4f transitions typically produce luminescence on the nanosecond scale. Unlike the more shielded 4f orbitals, the spatially extended 5d orbitals are sensitive to the surrounding ligand field, which means the emission properties of these complexes can be tuned through coordination chemistry. Yet many well-studied Ce(III) complexes follow Kasha’s rule and emit conventionally from the lowest 5d excited state through a single channel. Persuading a higher-lying 5d state to radiate competitively, while keeping the luminescence efficient, has been a difficult goal.

The new study tackles that goal by assembling a series of Ce(III) complexes with systematically varied coordination numbers: one seven-coordinate complex, one eight-coordinate complex, and three nine-coordinate complexes. This ladder of structures gave the team a molecular platform for examining how changes in the coordination environment influence excited-state relaxation and emission. Single-crystal X-ray diffraction revealed a progressive increase in the average Ce–N bond length as the coordination number rose, from 2.585 angstroms in the seven-coordinate complex to 2.612 angstroms in the eight-coordinate complex and approximately 2.67 angstroms in the nine-coordinate complexes. The percent buried volume around the cerium center climbed in step, from 89.3 percent to 92.0 percent and then to roughly 94 percent. In other words, adding ligands made the coordination sphere more crowded while simultaneously lengthening the individual Ce–N bonds.

The photophysical measurements showed a clear dependence of emission behavior on that coordination environment. In dichloromethane at room temperature, the seven-coordinate Ce(Tp4Me)(Bp4Me)2, the eight-coordinate Ce(Tp4Me)2(Bp4Me), and the nine-coordinate Ce(Tp4Me)3 all emit blue light with a maximum wavelength near 445 nanometers, and with photoluminescence quantum yields of 97 percent, 98 percent and 97 percent respectively. The divergence appears on cooling. The seven- and eight-coordinate complexes keep their conventional single-channel emission from the lowest 5d excited state, but Ce(Tp4Me)3 develops an additional higher-energy emission at lower temperatures. Between 200 and 160 kelvin, emissions from both a higher-energy doublet state, labeled D1H, and a lower-energy doublet state, labeled D1L, are observed simultaneously.

The contrast becomes especially striking in the solid state. The seven- and eight-coordinate reference complexes retain single-channel doublet emission, whereas all three nine-coordinate Ce(Tp4R)3 complexes, with R equal to Me, Cl or I, show excitation-dependent dual-channel doublet emission both at room temperature and at 77 kelvin. Crucially, this extra radiative pathway does not necessarily come at the cost of efficiency: the solid-state photoluminescence quantum yields of Ce(Tp4Me)3 and Ce(Tp4Cl)3 reach approximately 100 percent. A molecule that emits through two channels while wasting almost none of the absorbed energy is an unusual combination in luminescent materials.

Time-resolved photoluminescence measurements reveal how the two emissive pathways compete. At 77 kelvin, Ce(Tp4Me)3 shows lifetimes of approximately 33 nanoseconds for the higher-energy D1H emission and 45 nanoseconds for the lower-energy D1L emission. From these values, the intrinsic radiative rate of D1H is estimated at approximately 3.0 × 10^7 per second. At 180 kelvin, where both channels coexist, analysis of the transient decay gives an estimated D1H-to-D1L energy-transfer rate of 1.7 × 10^7 per second. Because this relaxation rate is lower than the radiative rate from D1H, radiative decay from the higher-energy state can compete with relaxation down to D1L. Part of the excited-state population therefore emits directly from D1H, while another fraction relaxes to D1L before radiating, producing the two simultaneous outputs.

Overall, the study draws a close connection between coordination environment, excited-state structural relaxation and dual-channel emission in molecular Ce(III) complexes. The authors identify two factors as important for achieving efficient dual-channel doublet emission: a rigid multidentate ligand framework that suppresses nonradiative decay, and a sufficiently adaptable coordination environment that permits access to distinct emissive excited-state configurations. More broadly, the findings show that coordination chemistry can regulate not only the emission energy of Ce(III) complexes but also their excited-state relaxation pathways. Photoinduced local coordination rearrangement may thus offer a molecular-design strategy for open-shell luminescent materials with multiple optical outputs, with potential relevance to sensing, information encoding, and photonic and optoelectronic applications.

Subject of Research: Dual-channel doublet emission in coordination-tuned cerium(III) complexes

Article Title: Highly efficient dual-channel doublet emission in lanthanide cerium(III) complex

Article References: Highly efficient dual-channel doublet emission in lanthanide cerium(III) complex. Original publication (Bibliographic details unavailable.)

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Keywords: cerium(III) complexes, dual-channel emission, open-shell luminescence, 5d–4f transitions, coordination chemistry, photoluminescence quantum yield, excited-state relaxation, lanthanide, doublet emission, Light: Science & Applications, photoinduced rearrangement, multichannel emitters