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

Materials capable of emitting through two distinct radiative channels can generate more than one optical output from a single system, offering opportunities in areas such as sensing, multilevel encryption and optical data storage, imaging, and electroluminescence. Most dual-channel emitters reported to date are closed-shell systems, in which emission generally involves singlet fluorescence, triplet phosphorescence, or contributions from both. By comparison, dual-channel emission from open-shell systems remains much less explored, despite their distinctive spin configurations and excited-state properties.

Cerium(III) complexes provide an appealing molecular platform for investigating open-shell emission. Ce(III) has a 4f¹ ground-state electronic configuration, and its spin- and parity-allowed 5d-4f transitions typically give rise to nanosecond-scale luminescence. Unlike the more shielded 4f orbitals, the spatially extended 5d orbitals are sensitive to the surrounding ligand field, allowing the emission properties of Ce(III) complexes to be tuned through coordination chemistry. Nevertheless, many well-studied Ce(III) complexes exhibit conventional single-channel emission from the lowest 5d excited state in accordance with Kasha’s rule. Achieving competitive radiative emission from a higher-lying 5d state while maintaining efficient luminescence therefore remains a challenging goal.

In a new study published in *Light: Science & Applications*, Zhiwei Liu and colleagues at the Beijing National Laboratory for Molecular Sciences, College of Chemistry and Molecular Engineering, Peking University, report a series of Ce(III) complexes with systematically varied coordination numbers. The series comprises a seven-coordinate complex, an eight-coordinate complex, and three nine-coordinate complexes, providing 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 with increasing coordination number, from 2.585 Å in the seven-coordinate complex to 2.612 Å in the eight-coordinate complex and approximately 2.67 Å in the nine-coordinate complexes. The percent buried volume around the Ce(III) center also increased from 89.3% to 92.0% and then to approximately 94%. These structural trends indicate that increasing the coordination number generates a more crowded coordination environment around the Ce(III) center while also producing longer Ce–N coordination bonds.

The photophysical measurements revealed a clear dependence of the emission behavior on coordination environment. In dichloromethane at room temperature, the seven-coordinate Ce(Tp^4Me)(Bp^4Me)₂, eight-coordinate Ce(Tp^4Me)₂(Bp^4Me), and nine-coordinate Ce(Tp^4Me)₃ complexes all exhibit blue emission with maximum wavelength near 445 nm, with photoluminescence quantum yields of 97%, 98%, and 97%, respectively. Upon cooling, however, their behaviors diverge. The seven- and eight-coordinate complexes retain conventional single-channel emission associated with the lowest 5d excited state. In contrast, Ce(Tp^4Me)₃ develops an additional higher-energy emission at lower temperatures. And between 200 and 160 K, emissions from both a higher-energy doublet state (D₁^H) and a lower-energy doublet state (D₁^L) are simultaneously observed.

The distinction becomes particularly evident in the solid state. The seven- and eight-coordinate reference complexes retain single-channel doublet emission, whereas all three nine-coordinate Ce(Tp^4R)₃ complexes (R = Me, Cl, I) exhibit excitation-dependent dual-channel doublet emission at room temperature as well as at 77 K. Importantly, this additional radiative pathway does not necessarily require a substantial loss of luminescence efficiency: the solid-state photoluminescence quantum yields of Ce(Tp^4Me)₃ and Ce(Tp^4Cl)₃ reach approximately 100%.

Time-resolved photoluminescence measurements provide further insight into how the two emissive pathways compete. At 77 K, Ce(Tp^4Me)₃ exhibits lifetimes of approximately 33 ns for the higher-energy D₁^H emission and 45 ns for the lower-energy D₁^L emission. From these values, the intrinsic radiative rate of D₁^H is estimated to be approximately 3.0 × 10⁷ s⁻¹. At 180 K, where both emission channels coexist, analysis of the transient decay gives an estimated D₁^H-to-D₁^L energy-transfer rate of 1.7 × 10⁷ s⁻¹. Because this estimated relaxation rate is lower than the radiative rate from D₁^H, radiative decay from the higher-energy state can compete with relaxation to D₁^L. Consequently, part of the excited-state population can emit directly from D₁^H, while another fraction relaxes to D₁^L before emission.

Electronic-structure calculations further suggest a structural origin for this kinetic competition. For the seven- and eight-coordinate complexes, geometry optimization predicts that all Ce–N bonds become shorter in the first excited doublet state, while the overall coordination number remains unchanged. Ce(Tp^4Me)₃ behaves differently: eight Ce–N bonds shorten, whereas one Ce–N bond undergoes pronounced elongation. The calculations further suggest that the associated pyrazolyl fragment can rotate toward a metastable conformation, consistent with a tendency of the crowded nine-coordinate structure to undergo local coordination rearrangement toward an eight-coordinate-like excited-state geometry.

Calculated emission spectra also support the presence of two emissive excited-state configurations in Ce(Tp^4Me)₃. The calculated separation between the higher- and lower-energy emission channels is 0.33 eV, close to the experimentally determined value of approximately 0.40 eV. Although the calculations do not by themselves constitute direct structural observation of the excited-state rearrangement, together with the crystallographic and spectroscopic results they support a picture in which coordination relaxation modifies the excited-state energy landscape and allows two 5d-derived doublet states to become radiatively accessible.

Overall, the study identifies a close relationship between coordination environment, excited-state structural relaxation, and dual-channel emission in molecular Ce(III) complexes. The results suggest that two factors are 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, these findings demonstrate 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 therefore provide a useful molecular-design strategy for developing open-shell luminescent materials with multiple optical outputs, with potential relevance to sensing, information encoding, and photonic and optoelectronic applications.