The team led by Zhiwei Liu at the College of Chemistry and Molecular Engineering, Peking University discovered a new photoluminescence mechanism-delayed doublet emission (DDE) in a rare earth cerium(III) complex Ce(CzPhTp)3, which also represents the first example with metal-centered delayed photoluminescence. The energy gap between the doublet and triplet excited states of Ce(CzPhTp)3 is reduced by the management of the inner and outer coordination spheres, thereby promoting efficient energy transfer between the two excited states and activating the delayed emission. The research results were published online in the journal Angewandte Chemie International Edition on May 3, 2023 (https://doi.org/10.1002/anie.202302192), with the title “Delayed Doublet Emission in a Cerium(III) Complex”.
Due to the unique electronic structures and abundant transition energy levels, luminescent rare earth complexes have important applications in fields such as display, lighting, and communication. The luminescent mechanisms of rare earth complexes can be mainly divided into f-f transition and d-f transition. Compared with the widely studied f-f transition complexes with the characteristics of forbidden transition, narrow emission peaks, and long excited state lifetimes, the less studied d-f transition complexes exhibit the characteristics of allowed transition, broad emission peaks, and short excited state lifetimes. Such differences may expand the application of luminescent rare earth complexes in more fields.
The electron configurations of the ground and excited states of rare earth cerium(III) ion are 4f15d0 and 4f05d1, respectively. The spin multiplicity is 2 and the emission of cerium(III) complexes originates from the doublet d-f transition. When designing luminescent cerium(III) complexes, the energy level of the first triplet excited state (T1) of the ligand is usually significantly higher than that of the first doublet excited state (D1) of the metal center to avoid energy transfer from D1 to T1, so as to ensure efficient radiative transition from excited state D1 to ground state D0. In this work, the authors regulated the D1 and T1 energy levels independently and achieved a small energy gap ETD (0.220 eV) in the cerium(III) complex Ce(CzPhTp)3, which exhibits blue emission with photoluminescence quantum yields of 84% and 70% in the solid powder state and dichloromethane solution, respectively. The high photoluminescence quantum yield in the case of small ETD benefits from the activation of the delayed doublet emission.
Fig. 1 Energy diagram and molecular structure. (a) Energy diagram of doublet emission in a conventional Ce(III) complex. (b) Molecular structure of Ce(CzPhTp)3 and electron distribution (represented by blue iso-surfaces) of excited states calculated from hole-electron analysis. (c) Energy diagram of delayed doublet emission in Ce(CzPhTp)3.
The excited Ce(CzPhTp)3 in dichloromethane solution decays with a double-exponential lifetime. The prompt component with a short lifetime (32 ns) originates from doublet emission, and the delayed component with a longer lifetime (1498 ns) originates from delayed doublet emission. The variable temperature spectroscopy measurements show that the energy transfer from D1 to T1 is inhibited at low temperature, and the energy transfer efficiency from D1 to T1 increases as the ambient temperature increases. Additionally, the proportion of prompt doublet emission decreases, and that of delayed doublet emission increases as the ambient temperature increases. Based on the Arrhenius plot, the fitted value of ETD is 0.220 eV.
Fig. 2 Photoluminescence characteristics of Ce(CzPhTp)3. (a) Ultraviolet–visible absorption and photoluminescence spectra of Ce(CzPhTp)3 in 10-3 M dichloromethane solution (inset is the photograph of the solution under 365 nm excitation and the magnification of the absorption spectrum around 380 nm). (b) The transient photoluminescence of Ce(CzPhTp)3 in 10-3 M dichloromethane solution under a nitrogen or oxygen atmosphere. (c) Photoluminescence decay curves of Ce(CzPhTp)3 in 10-3 M 2-methyltetrahydrofuran solution at different temperatures. The excitation wavelength is 370 nm. (d) Arrhenius plot of the energy transfer rate from the doublet excited state D1 to the triplet excited state T1 of Ce(CzPhTp)3.
In addition, by modifying the aryl substituents, the authors also designed cerium(III) complexes Ce(FPhTp)3 and Ce(NapTp)3 with ETD of 0.80 eV and –0.21 eV, respectively. The former exhibits conventional doublet emission asit has a large energy difference between the D1 and T1 excited states, showing blue emission in dichloromethane solution at room temperature with an excited state lifetime of 63 ns and a photoluminescence quantum yield as high as 100%. The latter exhibits no emission in dichloromethane solution at room temperature, but its polymethyl methacrylate film shows phosphorescent emission of ligands with a photoluminescence quantum yield of 19%. The energy transfer from D1 to T1 in Ce(NapTp)3 quenches the doublet emission, which is the first case of intramolecular energy transfer from doublet to triplet excited states, i.e. doublet excited state sensitized phosphorescence.
Fig. 3 Photophysical properties of Ce(ArTp)3 and photoluminescence mechanism of Ce(NapTp)3. (a) Photoluminescence spectra and (b) Photoluminescence decay curves of Ce(ArTp)3 in 10−3 M 2-methyltetrahydrofuran solution at 77 K. (c) Photoluminescence mechanism of Ce(NapTp)3.
This work indicates the flexibility of energy level regulation and the variety of luminescence mechanisms in cerium(III) complexes. The new luminescence mechanisms of delayed doublet emission and doublet excited state sensitized phosphorescence are reported for the first time, which will enrich our understandings of doublet emission and may shed new light on rational structure design and excited state management in d-f transition rare earth cerium(III) complexes.
Prof. Zhiwei Liu and Prof. Jianglan Qu are co-corresponding authors of the paper, while Peiyu Fang, a doctoral student at Peking University, and Jiawen Liu, a master student at Beijing University of Agriculture, are co-first authors of the paper. This work has been funded and supported by the National Natural Science Foundation of China, the Ministry of Science and Technology, the Beijing Municipal Education Commission, and the Beijing National Laboratory for Molecular Sciences in China.
Original link for the paper: https://doi.org/10.1002/anie.202302192
