Organic light-emitting diode (OLED) technology offers economical and powerful solutions for flexible displays and innovative area lighting. A key challenge in developing high-performance OLEDs is maximizing the fraction of excitons used to produce light. Traditional organic dyes, relying on fluorescence, have a theoretical internal quantum efficiency limit of 25%. Phosphorescent OLEDs (PHOLEDs), using heavy precious metals like Ir, Pt, and Ru, overcome this limitation by emitting light from the triplet excited state, achieving near 100% internal quantum efficiency. However, the high cost of these precious metals hinders widespread adoption, particularly in large-scale applications like area lighting. Recently, copper-based coordination complexes have emerged as a cost-effective alternative, leveraging the temperature-activated delayed fluorescence (TADF) effect. These materials achieve high photoluminescence quantum yields, exceeding 99% in some cases. The bright luminescence in Cu-based OLED materials is linked to the properties of the triplet state. In conventional organic materials, the small spin-orbit coupling restricts light emission primarily to singlet excited state decay, limiting efficiency. PHOLEDs, with strong spin-orbit coupling, enable intersystem crossing from the singlet to the triplet state, enabling triplet emission. Cu-based luminophores follow a different strategy: they have sufficient spin-orbit coupling for singlet-triplet transitions but not for efficient direct triplet emission. Instead, they act as singlet harvesters, with a small singlet-triplet energy gap allowing thermally activated reverse intersystem crossing from the triplet to singlet state, resulting in delayed fluorescence (TADF) and high efficiency. Two factors greatly influence the photoluminescence quantum yield of TADF materials: the energy difference between the lowest excited singlet and triplet states, and the presence of non-radiative decay pathways. The first factor affects the probability of the thermal transition from triplet to singlet state, easily estimated from emission measurements at varying temperatures. Non-radiative decays, which are usually temperature-dependent and can be intermolecular or intramolecular, significantly impact efficiency. A critical intramolecular process in Cu materials involves vibrational coupling to the ground state, influenced by the displacement of the equilibrium excited-state structure from the ground state along vibrational coordinates. Quantifying these non-radiative processes requires advanced quantum calculations, which need experimental verification regarding structural rearrangements and charge redistribution within the complex. Recent advances in pump-probe techniques at X-ray free-electron lasers and synchrotrons allow for such verification. X-ray absorption and emission spectroscopy offer element-specific insights into electronic structure (charge and spin state) of elements, while pump-probe versions are vital for studying excited states. Pump-probe X-ray scattering reveals structural changes, especially concerning heavy atom displacements. Combining these methods provides comprehensive information.

The literature extensively discusses the use of precious metals like iridium, platinum, and ruthenium in phosphorescent OLEDs (PHOLEDs) to achieve high internal quantum efficiency by harvesting triplet excitons. However, the high cost of these metals necessitates the exploration of more cost-effective alternatives. Recent research highlights the potential of copper-based complexes exhibiting thermally activated delayed fluorescence (TADF) as promising candidates for next-generation OLEDs. Studies focusing on the photophysical properties of Cu complexes, particularly their triplet state characteristics, and the influence of ligand design on luminescence efficiency are reviewed. The literature also explores the use of advanced computational techniques like density functional theory (DFT) to predict and understand the excited-state behavior of these complexes. However, a gap exists in experimentally verifying the computational predictions concerning charge transfer and structural rearrangements in multi-nuclear copper complexes. This paper aims to fill this gap by using cutting-edge time-resolved X-ray techniques.

This study employed a combination of time-resolved X-ray techniques to investigate the charge transfer and structural rearrangements in the triplet excited state of the [Cu₄(PCP)₃]BAr₄ complex. The methodology involved three distinct sets of experiments conducted at three different synchrotron and XFEL facilities. **1. Pump-probe X-ray Absorption Spectroscopy (XANES):** XANES measurements at the Cu K-edge were performed at the SuperXAS beamline of the Swiss Light Source (SLS). A pump-sequential-probes mode was used, employing a pulsed laser for excitation and a synchrotron as a semi-continuous X-ray source. The setup allowed simultaneous acquisition of kinetics and absorption spectra with a time resolution of 30 ns. The concentration of [Cu₄(PCP)₃]BAr₄ in anhydrous THF solution was 2 mM. Data were collected to monitor changes in the electronic structure around copper atoms. Theoretical XANES spectra were also calculated using DFT with the ADF code to aid in interpreting experimental results. **2. Pump-probe X-ray Emission Spectroscopy (XES):** XES measurements at the P Kα lines were performed at the Swiss X-ray Free Electron Laser (SwissFEL). The high photon flux of SwissFEL, a low repetition rate facility, facilitated efficient photoexcitation for this experiment. A von Hamos spectrometer, utilizing a cylindrical crystal and a 2D JUNGFRAU detector, enabled the collection of full X-ray emission spectra on a shot-to-shot basis. The sample concentration was 8.3 mM in THF and the experiment was performed under a helium atmosphere to minimize solvent evaporation. This experiment was designed to probe changes in electronic structure around phosphorus atoms. **3. Pump-probe X-ray Solution-State Scattering (WAXS):** WAXS measurements were performed at the ID09 beamline of the European Synchrotron Radiation Facility (ESRF). A fast mechanical X-ray chopper isolated individual X-ray pulses from the synchrotron at a 1 kHz repetition rate. X-ray scattering patterns were collected with a 2D detector placed behind the liquid-jet sample. The experiment targeted structural changes, particularly the relative displacements of Cu atoms, in the excited state. The sample concentration was 2.5 mM in THF, and data were acquired under a helium atmosphere to minimize oxygen interference. **Data Analysis:** Principal component analysis (PCA) was applied to the series of X-ray absorption and emission spectra to separate the ground state and excited state components. The kinetics were fitted using exponential decay functions. The X-ray scattering data were analyzed by subtracting the solvent contribution. DFT calculations (using Gaussian 09 and ADF-2018) were employed to model the ground and excited triplet states of [Cu₄(PCP)₃]BAr₄, allowing for comparison with experimental data and charge analysis using various methods (Mulliken, NBO, Bader).

The combined X-ray techniques provided a detailed picture of the excited-state dynamics in [Cu₄(PCP)₃]BAr₄. **Charge Transfer:** Time-resolved XANES revealed that upon excitation, charge transfer occurs from both P-coordinated and C-coordinated Cu atoms, and also from the phosphorus atoms. This suggests that the charge moves towards the phenyl rings, consistent with DFT calculations predicting increased negative charge on the C atoms of the bridging phenyl ring in the triplet state. **Involvement of Phosphine Ligands:** Time-resolved XES showed that phosphorus atoms participate in charge transfer, with an estimated change in average charge of 0.097 electrons (formal charge) or 0.013 electrons (DFT charge). This confirms the ligands' role beyond structural integrity. **Structural Rearrangements:** Pump-probe WAXS demonstrated that photoexcitation leads to an increase of 0.05 Å in the distance between C-coordinated Cu atoms and a decrease of 0.12 Å in the distance between P-coordinated Cu atoms. The average distance between C-coordinated and P-coordinated Cu atoms also changes, indicating a minor structural rearrangement within the Cu core. This minimal structural change reduces the Huang-Rhys parameters, minimizing non-radiative decay paths. **Comparison with DFT Calculations:** A comparison between the experimental results and DFT calculations showed some discrepancies in the predicted charge shifts depending on the DFT method used. While some calculations showed charge movement primarily from one type of Cu center, the experimental results clearly indicate involvement of both types of Cu atoms and the P atoms. The experimental estimate for the charge change on the phosphorus atoms (0.013 electrons of DFT charges) is in reasonable agreement with some of the DFT calculations. This highlights the importance of experimental validation of computational methods for understanding excited-state dynamics.

The findings of this study address the fundamental question of charge transfer and structural dynamics within a promising copper-based OLED luminophore. The combined use of three complementary X-ray techniques provides direct experimental evidence supporting the charge transfer mechanism and structural rearrangements upon photoexcitation. The results demonstrate the significant participation of both copper and phosphorus atoms in the charge transfer process, contradicting some DFT predictions that only showed charge movement from one type of Cu center. The observed minimal structural changes in the Cu core upon excitation are vital for the high luminescence efficiency of the complex, as these reduce non-radiative decay pathways and enhance radiative transitions. The study's results validate the multi-core approach as an effective strategy to design highly efficient and stable OLED materials. The experimental validation of charge transfer and structural dynamics will help refine computational models for future design and optimization of OLED materials.

This research successfully elucidated the charge transfer mechanism and structural changes in the triplet excited state of the [Cu₄(PCP)₃]BAr₄ complex using a combination of advanced time-resolved X-ray techniques. The results demonstrate charge transfer from both types of Cu atoms and P atoms, accompanied by minimal structural rearrangement within the Cu core. This minimal rearrangement is critical for suppressing non-radiative decay and enhancing the efficiency. The findings highlight the importance of experimental verification of computational predictions for OLED material design. Future research could explore modifications of the ligands (P-donors and phenyl-backbone) to independently tune the electronic densities on Cu and P atoms, further optimizing emission and efficiency. The rigidity and stability of this multi-core Cu complex establish it as a promising material for optoelectronic applications.

The study focuses on a single specific copper complex, [Cu₄(PCP)₃]BAr₄, and its behavior in a specific solvent (THF). Generalizing these findings to other copper-based TADF materials requires further investigation. While the DFT calculations provided valuable insights, there were inconsistencies in predicting the charge shift depending on the method and level of theory employed. The time resolution of the XANES measurements (30 ns) might limit the observation of even faster processes occurring in the picosecond regime. Further experiments could be performed to address these limitations and provide a more comprehensive understanding of the excited-state dynamics of copper-based TADF materials.